Tunable Optoelectronic Devices - University of California, Berkeley

Tunable Optoelectronic Devices

by

Carlos Fernando Rondina Mateus

Engineer (Aeronautics Institute of Technology) 1996 Grad (Aeronautics Institute of Technology) 1997

A dissertation submitted in partial satisfaction of the requirements for the degree of

Doctor of Philosophy

in

Engineering – Electrical Engineering and Computer Sciences

in the

GRADUATE DIVISION of the

UNIVERSITY of CALIFORNIA at BERKELEY Committee in charge:

Prof. Constance J. Chang-Hasnain, Chair Prof. Nathan Cheung Prof. Yuri Suzuki

Spring 2004

Tunable Optoelectronic Devices

Copyright Spring 2004

by


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Abstract

Tunable Optoelectronic Devices by


Doctor of Philosophy in Engineering – Electrical Engineering and Computer Sciences

University of California, Berkeley

Professor Constance J. Chang-Hasnain, Chair

Tunable devices, specifically filter, detector and laser, are key components for a

variety of applications such as communications, spectroscopy, biosensing, inter and intra-

chip connection, infrared (IR) imaging, and biometrics. Monolithically tuned devices are

even more attractive because of compactness, robustness, easiness of integration, and low

price inherent to semiconductor batch fabrication and testing.

This dissertation discusses design, fabrication and applications of monolithically

tuned optoelectronic devices. The devices have vertical optical cavity with respect to the

substrate, which uses micromaching techniques to provide the largest monolithic tuning

range among all options.

Design is described in separate for optical cavity and mechanical beams. A novel

grating mirror, with both high bandwidth and reflectivity, is theoretically and

experimentally demonstrated as an alternative to the conventional distributed Bragg

reflectors. General design rules and scaling laws with wavelength are presented in a

flowchart as function of system requirements such as tuning range, central wavelength,

tuning voltage, tuning speed and resolution. By following the various steps, all

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To my wife, Anna Paula,

who always gave support and strength for me to pursue my dreams.

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Table of Contents

CHAPTER 1 INTRODUCTION ............................................................................................................1 1.1 Introduction......................................................................................................... 1 1.2 Applications of tunable devices .......................................................................... 4

1.2.a Communications – Wavelength Division Multiplexing (WDM) ............... 4 1.2.b Spectroscopy............................................................................................... 8 1.2.c Biosensing................................................................................................. 10 1.2.d Infrared (IR) imaging................................................................................ 11 1.2.e Biometrics ................................................................................................. 13

1.3 Overview of the dissertation ............................................................................. 14 CHAPTER 2 DESIGN...........................................................................................................................17

2.1 Introduction....................................................................................................... 17 2.2 Optical Cavity ................................................................................................... 18

2.2.a Distributed Bragg Reflector (DBR) .......................................................... 23 2.2.a.1 Transmission Matrix ................................................................................. 24

2.2.b Sub-Wavelength Grating (SWG).............................................................. 27 2.2.b.1 Rigorous Coupled Wave Analysis (RCWA) ............................................ 34

2.2.c Comparison between DBR and SWG....................................................... 35 2.3 Mechanical Structure ........................................................................................ 38 2.4 General Design Rules ....................................................................................... 44

2.4.a Torsional Design....................................................................................... 51 2.5 Summary ........................................................................................................... 54

CHAPTER 3 FABRICATION..............................................................................................................55 3.1 Introduction....................................................................................................... 55 3.2 Subwavelength Waveguide Grating (SWG) Fabrication.................................. 55 3.3 AlGaAs Micromachining Fabrication............................................................... 61

3.3.a Method ...................................................................................................... 62 3.3.b Epitaxial Growth....................................................................................... 63 3.3.c Optical Lithography.................................................................................. 66 3.3.d Metal deposition – liftoff .......................................................................... 68 3.3.e Vertical Etch ............................................................................................. 70

3.3.e.1 Wet Isotropic Etch..................................................................................... 70 3.3.e.2 Dry Isotropic Etch ..................................................................................... 72

3.3.f Oxidation................................................................................................... 76 3.3.g Selective Etch............................................................................................ 78

3.3.g.1 Wet Selective Etch.................................................................................... 79 3.3.g.2 Dry Selective Etch .................................................................................... 82

3.3.h Critical Point Drying (CPD) ..................................................................... 86 3.3.i Inspection.................................................................................................. 87

3.4 Summary ........................................................................................................... 89 CHAPTER 4 TUNABLE FILTER .......................................................................................................90

4.1 Introduction....................................................................................................... 90

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4.2 Design ............................................................................................................... 90 4.2.a Torsional Filter.......................................................................................... 91 4.2.b Folded beam filter ..................................................................................... 98

4.3 Fabrication ...................................................................................................... 106 4.3.a Torsional ................................................................................................. 106 4.3.b Folded beam............................................................................................ 118

4.4 Applications .................................................................................................... 125 4.4.a Communications ..................................................................................... 125 4.4.b Infrared Imaging ..................................................................................... 136

4.5 Summary ......................................................................................................... 143 CHAPTER 5 TUNABLE DETECTOR..............................................................................................144

5.1 Introduction..................................................................................................... 144 5.2 Design ............................................................................................................. 144

5.2.a Fabry-Pérot tunable detector................................................................... 146 5.2.b P-I-N detector.......................................................................................... 151 5.2.c Optical cavity and wafer design.............................................................. 153

5.3 Fabrication ...................................................................................................... 158 5.3.a Detector separation ................................................................................. 165 5.3.b Oxidation................................................................................................. 167

5.4 Application...................................................................................................... 168 5.4.a Biosensing............................................................................................... 169

5.4.a.1 Biosensor description.............................................................................. 170 5.4.a.2 Tunable detector characterization ........................................................... 175

5.5 Summary ......................................................................................................... 181 CHAPTER 6 TUNABLE VCSEL.......................................................................................................183

6.1 Introduction..................................................................................................... 183 6.2 Sensor Description − tunable VCSEL and GMR biosensor ........................... 184 6.3 Sensor characterization ................................................................................... 188 6.4 Protein Binding Assays................................................................................... 194 6.5 Summary ......................................................................................................... 201

CHAPTER 7 CONCLUSION .............................................................................................................203 7.1 Summary ......................................................................................................... 203 7.2 Potential Future Work..................................................................................... 204

BIBLIOGRAPHY .....................................................................................................................................207

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List of Figures Figure 1.1 – Scheme of and typical dimensions of (a) edge emitting laser (EEL) and (b) vertical cavity surface emitting laser (VCSEL). ................................................................. 3 Figure 1.2 – Schematics of an add/drop using tunable devices. ......................................... 6 Figure 1.3 – The two revolutions in fiber optic communications. In the 90’s, erbium doped fiber amplifier (EDFA) and wavelength division multiplexing (WDM) transformed the system with several fibers and repeaters into 1 fiber that can carries more than 100 channels (> 200 in dense WDM). Recently, tunable devices were launched with the capability to shrink transmitters, receivers and respective spares into multifunctional boards.................................................................................................................................. 7 Figure 1.4 – Water vapor spectroscopy using a tunable VCSEL. After [27]. ................... 9 Figure 1.5 – Change of resonance in a micromachined Fabry-Pérot filter due to the presence of hydrogen sulfide in the air gap. After [28]. .................................................. 10 Figure 1.6 – Multispectral infrared images from Bay Area taken by satellite LANDSAT 7 and composed by NASA. Picture to the left has green and red spectral components only while picture to the right is the composite of the picture to the left plus long ultra-violet, blue and short infrared wavelength bands. ....................................................................... 12

Figure 2.1 - Fabry-Pérot cavity: two parallel partial mirrors separated by a distance l. Index of refraction may be different inside (n2) and outside (n1) the cavity. A plane wave is incident from the left (Ei) and, after multiple reflections inside the cavity, partially transmitted (Et) and reflected (Er). .................................................................................... 18 Figure 2.2 – Characteristic theoretical transmission of a Fabry-Pérot cavity. Total transmission is allowed at some wavelengths (for the case where both mirrors have the same reflectivity) and the linewidth of the transmission decreases for increasing reflectivity. ........................................................................................................................ 21 Figure 2.3 - Schematic of a DBR. Constructive interference of all reflections, achieved by the proper arrangement of layer thickness and refractive index sequence, builds the high reflectivity of a DBR. In this illustration, the input layer has a high index and the output layer has a low index. ............................................................................................ 23 Figure 2.4 – Simulated power and phase reflectivity spectra of a DBR stack of AlxGa1-

xAs centered at 1.55µm. (a) Power and (b) phase for 20.5 pairs of Al0.2Ga0.8As/ Al0.7Ga0.3As. (c) Power reflectivity for 30.5 pairs of Al0.2Ga0.8As/ Al0.7Ga0.3As. Reflectivity increases by increasing the number of pairs but the full width at half maximum (FWHM) band slightly shrinks. (d) Power reflectivity for 20.5 pairs of GaAs/AlAs. Reflectivity and FWHM band increases by increasing the difference in index.................................................................................................................................. 25 Figure 2.5 – Transmission Matrix theory. (a) Single element. (b) Cascaded elements. The input and output from one port of one element are the output and input, respectively, to the next element and the overall effect can be calculated by simply multiplying the cascaded element matrices................................................................................................ 26 Figure 2.6 - Scheme of the sub-wavelength grating reflector. The low index material under the grating is essential for the broadband mirror effect. ......................................... 28 Figure 2.7 - Reflected power for light polarized perpendicularly to the grating lines. (a) Thick line was obtained based on Rigorous Coupled Wave Analysis (RCWA) Error!

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Reference source not found. while dashed with TEMPEST© Error! Reference source not found.. (b) A simple scaling factor (6.5) applied to the dimensions gives completely overlapped traces. Thick line is centered at 1.55µm while dashed at 10 µm. ................. 30 Figure 2.8 - Effect of the low index layer under the grating. (a) Reflectivity as function of wavelength and tL. There is no reflection band when tL <0.1µm, and above this value, the structure has low sensitivity to this parameter. (b) Reflectivity as function of wavelength and nL. The mirror also does not exist if nL >2.5. ............................................................ 31 Figure 2.9 - Reflectivity as function of wavelength and Λ. The reflection band shifts to longer wavelengths proportionally to the period and for Λ = 0.7 the band is the broadest............................................................................................................................................ 32 Figure 2.10 - Reflectivity as function of wavelength and tg. The optimized bandwidth occurs for tg = 0.45 and it gets sharper it is further increased. This parameter can be precisely controlled by epitaxial growth or plasma deposition techniques. ..................... 32 Figure 2.11 - Reflectivity as function of wavelength and duty cycle. When duty cycle is increased, two reflection peaks merge to form one broad and flat reflection band. ......... 33 Figure 2.12 – Representation of the field expansion inside the grating. A plane wave is refracted into the grating and then diffracted into space harmonic components. These components are coupled and phase matched to propagating and evanescent waves outside the grating. ........................................................................................................................ 35 Figure 2.13 – Comparison between SWG and DBR made out of Si/SiO2 system. (a) 3 pairs of Si/SiO2 DBR spectrum superimposed to Figure 2.7. (b) Reflectivity in Log scale shows that at least 6 pairs would be required to yield comparable reflectivity between SWG and DBR.................................................................................................................. 37 Figure 2. 14 - Comparison between SWG and DBR made out of GaAs/AlOx and GaAs/AlAs, respectively. The same thickness as the SWG (2µm) would correspond to 8 pairs of DBR, which has considerable less reflectivity than the SWG. Same level of reflectivity would require 40 DBR pairs........................................................................... 37 Figure 2.15 – Schematic of the cantilever held mirror under a concentrated load. .......... 39 Figure 2.16 – Overcoming the 1/3 rule with a torsion actuated device. The leveraging effect of the torsional device allows a movement of the head 1.3 times larger than the counterweight.................................................................................................................... 45 Figure 2.17 - Design flow chart for any type of deflection beam MEMS filter. Grey boxes show the requirements. ........................................................................................... 46 Figure 2.18 - Tuning range versus wavelength for different material systems. ............... 47 Figure 2.19 – Definition of the terms used in the torsional structure. .............................. 52

Figure 3.1 – SEM picture of the fabricated sub-wavelength grating. Grating is formed by poly-silicon and air on top of silicon dioxide. .................................................................. 56 Figure 3.2 – Calculated contour plot showing reflectivity as function of wavelength and duty cycle. The broadband effect is achieved for a duty cycle of (68±2) %. ................... 57 Figure 3.3 – Optical measurement setup for the SWG characterization........................... 58 Figure 3.4 – Reflectivity as function of wavelength and duty cycle for light polarized perpendicularly to the grating lines (Λ=0.7µm). (a) Duty cycle of 66% gives very broad bandwidth, 1.12-1.62 um, with R>98.5%. (b) Duty cycle of 48%. (c) Duty cycle of 83%............................................................................................................................................ 60

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Figure 3.5 – Reflectivity as function of wavelength for light polarized parallel to the grating lines (duty cycle=66%, Λ=0.7µm). There is no broadband mirror effect in excellent agreement with simulations............................................................................... 61 Figure 3.6 – SEM picture from a mesa etched by wet solution. Note the slope of the walls due to mask undercut............................................................................................... 71 Figure 3.7 - SEM picture from a cantilever etched by RIE plasma. Note the verticality of the walls. ........................................................................................................................... 73 Figure 3.8 – In situ etch monitor setup. A horizontal HeNe laser beam is deflected to have vertical incidence on the die being etched. The reflected beam is deviated, through a beam splitter, to a broad area detector and its current is recorded by a plotter.............. 74 Figure 3.9 – Typical recorded trace from the in situ monitoring system during the etch of a tunable detector wafer (to be described in chapter 5). High and low reflectivity peaks correspond to a DBR pair. Layers thicker than λHeNe/2n have multiple peaks. ............... 75 Figure 3.10 – In situ monitoring trace for a die with high density of devices on the surface. Note the spatial interference resultant from the topography. ............................. 76 Figure 3.11 – (a) Etch rates of AlxGa1-xAs (x < 0.5) as a function of volume ratio of citric acid/H2O2 solution at room temperature. (b) Turning volume ratio of the solution, at which etch starts, as a function of x. After [114]. ............................................................. 81 Figure 3.12 – (a) Etch rates of AlyGa1-yAs (y > 0.7) as a function of volume ratio of DI H2O/buffered oxide etch (10:1) solution at room temperature. (b) Etch rates as a function of y at a volume ratio of 25. After [114]. .......................................................................... 81 Figure 3.13 – Test structures to calibrate dry etching rate. Square sizes vary from 6µm to 90µm. Some cantilevers were also included.................................................................... 85 Figure 3.14 – Critical point drying cycle: temperature is lowered to ~ -15ºC, pressure is raised to ~80atm, temperature is increased to ~40ºC, and, finally, pressure returns to 1atm. The chamber conditions move around the critical point of CO2 (31.5ºC and 73atm). .............................................................................................................................. 87

Figure 4.1 - (a) Top view and (b) side view along the filter arm direction of the torsional structure. The sacrificial layer under the black region remains and is removed everywhere else. ................................................................................................................................... 91 Figure 4.2 – Layer composition for the tunable filter proposed structure. First column shows Al content in AlxGa1-xAs........................................................................................ 95 Figure 4.3 – Reflectivity spectrum for the top and bottom mirrors designed to have reflectivity of 99.4% @1550nm........................................................................................ 96 Figure 4.4 – Linewidth as function of wavelength for the structure shown in Figure 4.2.97 Figure 4.5 – Transmitted wavelength through the Fabry-Pérot cavity as function of the gap size between the mirrors............................................................................................. 98 Figure 4.6 – Top view of a single pixel of a tunable filter with folded-beams................. 99 Figure 4.7 – (a) Optical transmissivity of the tunable filter designed for MWIR. (b) Transmitted wavelength as a function of ai rgap size. Choosing to work with the 2nd mode, we can tune the entire MWIR range with an initial gap of 5 um......................... 101 Figure 4.8 – (a) Optical transmissivity of the tunable filter designed for LWIR. (b) Transmitted wavelength as a function of airgap size. Choosing to work with the 2nd mode, we can tune the entire LWIR range with an initial gap of 9 um. ......................... 101

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Figure 4.9 – Calculated maximum drive voltage as a function of filter head size for the cases of 1-3 folded springs supports, 5x5µm2 beam cross-section, and a 50µm pixel size.......................................................................................................................................... 102 Figure 4.10 – Finite element simulation of the folded-beam structure. It is interesting to point that the simulation allows a complete analysis, even of the small torsion of the beams. ............................................................................................................................. 104 Figure 4.11 – (a) Schematic of 2-Band filter design. The top DBR is centered at 9.4µm and the bottom one at 4µm. (b) Calculated optical transmissivity spectra to show two-band tuning. Only the LWIR gap is varied. ................................................................... 105 Figure 4.12 – SEM picture from a device fabricated from the first wafer [80-82]......... 106 Figure 4.13 – SEM pictures showing (a) incomplete selective etching and (b) excessive selective etching.............................................................................................................. 107 Figure 4.14 – SEM pictures showing redeposition problem........................................... 107 Figure 4.15 – Points of preferential redeposition around the device geometry. ............. 108 Figure 4.16 – Reflectivity spectrum for the fabricated wafer as grown. (a) Calculated and (b) measured.................................................................................................................... 108 Figure 4.17 – Fabrication sequence of the torsional tunable filter. ................................ 110 Figure 4.18 – In situ monitoring trace for vertical etch of the wafer. The lateral small peaks are due to the fact that the layers of DBR are larger than λHeNe/2 and not to interference. The structure from Figure 4.2 can be clearly identified. .......................... 110 Figure 4.19 – SEM picture from a device fabricated from the second wafer. Note the larger gap, both contacts on top and vertical etch of both mirrors. Mirrors are also thicker for this wafer. .................................................................................................................. 111 Figure 4.20 – Different topologies fabricated on the same second wafer: (a) cantilever, (b) bridge, and (c) multiple beams.................................................................................. 112 Figure 4.21 – Photoresist thickness had to be increased because of the long vertical etch. (a) Cantilever surface after etch and thickness of 1.8µm. (b) Thickness of 2.3µm....... 112 Figure 4.22 – Critical length as function of strain for different thicknesses of beams. The picture to the right illustrates the increased buckling of beams for increased lengths. .. 114 Figure 4.23 – White light interferometry image of the bridge structure. The top graph shows the cross section of the beam with a buckle of almost 7µm. ............................... 114 Figure 4.24 – White light interferometry image of the cantilever released in one step. The beam had an upward droop of more than 1.8µm..................................................... 115 Figure 4.25 – White light interferometry image of the cantilever released in two steps. The beam droop was reduced to 0.16µm........................................................................ 116 Figure 4.26 – Hole at the optical path. Top view from a device with broken cantilever.......................................................................................................................................... 117 Figure 4.27 – Remaining post under the head after 15min of etch. The cantilever was totally released by then. .................................................................................................. 117 Figure 4.28 – Two steps release. The head is released first by protecting the cantilever with photoresist (a), which is removed later to complete the selective etch (b). ............ 118 Figure 4.29 - Picture from a fabricated device with folded beams and open laterals..... 119 Figure 4.30 – Buried devices. (a) Picture of die designed to have large anchors for direct probing. (b) Schematics of the corner of the die designed to have surface wires.......... 120 Figure 4.31 - SEM picture from a fabricated cluster with 70µm head devices .............. 121

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Figure 4.32 – White light interferometry image of the folded beam structure. The surface of the heads of released devices is very flat. No considerable buckling was noticed (<40nm). ......................................................................................................................... 123 Figure 4.33 – Processing sequence showing redeposition after vertical etch. Redeposited material was not uniform and was removed further during selective etch, wet and dry. 124 Figure 4.34 – SEM images from under the head after release. (a) Redeposition under the head after release of a die with heavy redeposition from vertical etch. (b) Clean release from a die with almost no redeposition from vertical etch. ............................................ 125 Figure 4.35 – Measured displacements of the torsional filter. The measurements were performed using white light interferometry and show the upward head movement corresponding to double the counterweight downward movement. ............................... 127 Figure 4.36 – Transmitted wavelength as function of gap size. Stars indicate the expected modes and squares represent measured transmitted modes. Continuous tuning through the mode to the right covers around 100nm. ........................................................................ 127 Figure 4.37 – Measured transmitted wavelength at the filter head as function of voltage. Measurement was performed by sweeping the TTF tuning voltage and recording its value at peak transmission for each wavelength. ..................................................................... 128 Figure 4.38 – Setup used for spectral characterization of the tunable filter. .................. 129 Figure 4.39 – Measured transmission spectrum for the TTF. 100nm tuning were achieved and this result were limited by our tunable source range................................................ 130 Figure 4.40 – Transmission spectrum for the torsional filter at 1525 nm showing an extinction ratio greater than 20dB and 1nm linewidth. Optimized coupling has eliminated the side mode at longer wavelength.............................................................. 130 Figure 4.41 – Detail of the two topologies used to compare the satellite peaks. Torsional has smaller head and larger gap than cantilever. ............................................................ 131 Figure 4.42 – Short wavelength spectrum for torsional and cantilever topologies. Cantilever shows two satellite peaks while torsional shows only one. The first peak is around -4dB for torsional and -3dB for cantilever.......................................................... 132 Figure 4.43 – Side mode spacing as function of wavelength for torsional and cantilever topologies........................................................................................................................ 132 Figure 4.44 – Optical circuit for bit error rate (BER) measurements performed using (a) Arrayed Waveguide Grating (AWG) from Lightwave Microsystems and (b) torsional tunable filter (TTF). ........................................................................................................ 134 Figure 4.45 – Schematics of the setup used for transmission measurements in a data link.......................................................................................................................................... 134 Figure 4.46 – Picture of the setup used for transmission measurements. Bit error rate measurements require the output from single mode (SM) optical fiber to be transmitted through the device and coupled back to SM fiber. ......................................................... 135 Figure 4.47 – BER plots using either (a) commercial AWG from Lightwave Microsystems or (b) TTF in the optical circuit from Figure 4.44................................... 136 Figure 4.48 – Setup designed for optical characterization of the device at MWIR........ 138 Figure 4.49 – Gap size as function of voltage for the 60m device from Figure 4.32. .... 139 Figure 4.50 – Gap size as function of voltage for the 60µm device. Simulation was done based on the spring constant of the trampoline structure................................................ 140 Figure 4.51 – Simulated reflectivity in the short and middle IR ranges as function of wavelength and gap size. ................................................................................................ 141

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Figure 4.52 – Simulated transmission characteristic for the tunable filter at short IR wavelengths. If a laser at 1.53µm is used, the contrast between on and off positions can be significant. .................................................................................................................. 142 Figure 4.53 – Measured transmitted power through the folded beam filter as function of time for variable applied voltage. ................................................................................... 142

Figure 5.1 – Schematic of two possible tunable detector designs: (a) resonant cavity detector (RCD) and (b) Fabry-Pérot filter detector (FP). ............................................... 145 Figure 5.2 – Schematic drawing of the Fabry-Pérot tunable detector. ........................... 148 Figure 5.3 – Pull-in voltage as function of filter head size for different beam lengths. . 150 Figure 5.4 – Picture from a fabricated die showing a unit cell. Three devices were fabricated on each anchor and a total of twelve different head sizes were fabricated. Left head of the top left anchor (F6A) has 12µm and size increases in steps of 2µm (and anchor sequence goes to bottom left – F6B, bottom right – F6C, and top right – F6D ). Right head of the top right anchor (F6D) is the largest, with 34µm............................... 151 Figure 5.5 – Detector speed as function of absorption layer thickness. Speed is RC limited for thin detectors and transit time limited for thick ones.................................... 153 Figure 5.6 – Layer composition for the tunable detector proposed structure. First column shows Al content in AlxGa1-xAs...................................................................................... 155 Figure 5.7 – Power distribution inside the optical cavity. Even for only one pair of separation between the cavity and the air gap, power inside the gap is much lower than in the cavity......................................................................................................................... 156 Figure 5.8 – Reflectivity spectrum for the top and bottom mirrors designed to have reflectivity of 99.9% @850nm. The insert shows a zoom at the center of the band. .... 156 Figure 5.9 – Linewidth as function of wavelength for the structure shown in Figure 5.6.......................................................................................................................................... 157 Figure 5.10 – Transmitted wavelength through the Fabry-Pérot cavity as function of the gap size between the mirrors. Because the gap is moved into the top mirror, the transmission mode is not linear with gap size and side modes may appear for a given gap.......................................................................................................................................... 158 Figure 5.11 – Reflectivity spectrum for the fabricated wafer as grown. (a) Calculated and (b) measured. Vertical scale of measured results was not calibrated. ........................... 159 Figure 5.12 – Fabrication sequence of the tunable detector. Each step displays both top view and cross-section along the cantilever beam. 1: top and bottom metal; 2: device mesa vertical etch; 3: wet etch for metal deposition; 4: ground metal; 5: detector separation; 6: oxidation; 7: selective etch....................................................................... 161 Figure 5.13 – Vertical etch in situ monitoring trace for the tunable detector wafer. The structure from Figure 5.6 can be clearly identified. The top DBR has a slight non-uniformity when compared to the bottom DBR, which can in part explain the broadening of the transmission peaks. ............................................................................................... 162 Figure 5.14 – SEM top view from a fabricated device. Dark regions were protected with photoresist for the release etch. Note the small mesas around the device head and anchor.......................................................................................................................................... 162 Figure 5.15 – Irregular and asymmetric etch. SEM pictures were taken after head release etch. Both devices shown have the same head size and were in the same wafer die. The

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one to the left was completely released while the one to the right had etching happening from right to left.............................................................................................................. 163 Figure 5.16 – Another die with irregular and asymmetric etch. ..................................... 164 Figure 5.17 – SEM of cantilever, broken on purpose by a probe, showing the top mirror (bottom), layers in between gap and cavity (center) and bottom mirror (top). GaAs was not totally etched and there was still some remaining material on top of the center beam. AlAs cavity was almost totally etched. Head has 20µm of side.................................... 165 Figure 5.18 – In situ monitoring trace showing 6 peaks................................................. 166 Figure 5.19 – Depth of etching as function of the number of peaks present in the in situ monitoring trace. Measurements were done using a surface profiler. ........................... 167 Figure 5.20 – Guided mode resonant (GMR) filter. The device is made on a plastic (polyethylene terephthalate - PET) substrate by imprinting a master grating into epoxy and curing. High index dielectric is further deposited. Surface is very sensitive to attachment of biological material with thickness tbio and index nbio. After [136]. ......... 171 Figure 5.21 – Simulation to illustrate the GMR filter spectrum for different thicknesses of material on top. ............................................................................................................... 172 Figure 5.22 – Schematic of the sensor using white light and spectrometer configuration.......................................................................................................................................... 172 Figure 5.23 – Schematic of the sensor using LED and tunable detector configuration. The broadband signal from the LED is depleted by the resonant reflection from the GMR; the tunable detector can keep track of the dip................................................................. 174 Figure 5.24 – Tunable detector equivalent circuit with wavelength tracking. A load is inserted in between the p-contact and ground so that the potential Vf floats with the detector current and causes the tuning potential to change accordingly......................... 174 Figure 5.25 – White light interferometry image of the tunable detector. This device shows a small negative droop of less than 0.2µm........................................................... 175 Figure 5.26 – Setup used for the optical characterization of the tunable detector integrated to the biosensor filter. ..................................................................................................... 177 Figure 5.27 – Measured spectral characteristics for the tunable detector....................... 178 Figure 5.28 – Transmission spectrum for the torsional filter at 856 nm. Linewidth is very sharp and a lorentzian fit shows 0.4nm of FWHM. Optimized coupling has eliminated the side mode at longer wavelengths but broadening on shorter wavelengths may have been caused by high order modes. .................................................................................. 179 Figure 5.29 – Typical current-voltage (IV) characteristic for the tunable detector. ....... 180

Figure 6.1 – White light trace showing the trade-off between resolution and signal strength............................................................................................................................ 185 Figure 6.2 – Biosensor with VCSEL based measurement system. A tunable VCSEL and two p-i-n detectors work as a readout system for a plastic guided-mode resonant (GMR) filter that is the binding surface. Peak reflectivity from the GMR is detected by correlating maximum normalized detector current with laser bias current. ................... 187 Figure 6.3 – Measured spectral shift of the resonance as function of index of refraction of the solution on top of the GMR. ..................................................................................... 189 Figure 6.4 – Wavelength shift as function of index of refraction of the solution on top of the GMR. Index changes ∆n < 0.001 can be easily detected by this system. ................ 189

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Figure 6.5 – White light measurements of the spectral shift of the resonance as function of index of refraction of the solution on top of the GMR............................................... 190 Figure 6.6 – Schematic of the film deposition process using slides and beakers. (A) Steps 1 and 3 represent the adsorption of a polyanion and polycation, respectively, and steps 2 and 4 are washing steps. (B) Simplified molecular picture of the first two adsorption steps, depicting film deposition starting with a positively charged substrate. After [143].......................................................................................................................................... 191 Figure 6.7 – Spectral shift of the resonance due to changes in polymer thickness on top of the GMR.......................................................................................................................... 192 Figure 6.8 – Wavelength shift as function of polymer thickness on top of the GMR. The system can resolve much less than 10Å of thickness variation. ..................................... 193 Figure 6.9 – Simulation of partial coverage of the grating surface with same percentages on top and bottom of the grating..................................................................................... 194 Figure 6.10 – Simulation of partial coverage of the grating surface with no deposited material (0%), deposition only on top (45%), only on bottom (55%) or full coverage (100%)............................................................................................................................. 194 Figure 6.11 – The configuration of VCSEL based biosensor and the sequence of protein layers. The protocol follows a standard mouse IgG capture immunoassay, as shown on the right. .......................................................................................................................... 195 Figure 6.12 – Optical density at 560nm as given from the ELISA reader for both GMR and standard polystyrene ELISA plate. .......................................................................... 197 Figure 6.13 – Dynamic measurement: surface binding versus time for different antigen concentrations. Most of the protein binding occurred rapidly at the beginning of the reaction, followed by a gradual saturation. The 80% surface binding time is about 300s, with a slight dependence on the mouse IgG concentration............................................. 199 Figure 6.14 – Static measurement: average resonant wavelength shift as function of the antigen concentration. The high sensitivity of the VCSEL based measurement system is shown from its ability to detect the smallest concentration of 1pg/ml or 6.7 fM. .......... 200

Figure 7.1 – Tunable VCSEL and resonant cavity detector array integrated to the GMR sensor. The sensor will be on top of the laser and separate locations of the sensor can analyze different proteins simultaneously. ..................................................................... 205 Figure 7.2 – Picture of the 96 well plate with GMR and a prototype of an electronic driver for VCSEL and detectors. Optics and optoelectronic devices can be inserted in between and use a XY positioning system to allow inspection of all 96 wells. ............. 206

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List of Tables

Table 2-1 – Photonic MEM tunable devices and respective spring constants.................. 42 Table 2-2 – Dimensions (µm) for the torsional test structures. ........................................ 53 Table 4-1 Proposed material system for each band of operation.................................... 100

Table 5–1 Summary of tunable detector average characteristics.................................... 180

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Acknowledgments

I want to start by giving my deepest appreciation to Prof. Connie J. Chang-

Hasnain for her advising during the past almost four years. Her knowledge, experience,

dedication, insights, vision and passion has not only stimulated my enthusiasm for

research but also taught me how to improve as an individual looking to make a difference

for society.

I also want to give my special thanks to Prof. Nathan Cheung, Prof. Yuri Suzuki

and Prof. David Attwood for giving me a very easy time scheduling my qualifying exam,

and also for their valuable time spent on reading and commenting this dissertation.

Collaboration with other groups, inside and outside university, had enormous

contribution to the several branches of the work presented here. Prof Andrew R.

Neureuther and his students provided the insights to a better understanding of the grating

mirror. Prof. Yuri Suzuki and Lu Chen fabricated the first grating mirror ever. Prof. P.

Robert Beatty and Jonathan Foley have taught me about preparation and analysis of

protein assays. Drs. Steven Yang, Decai Sun, Rajiv Pathak, Peter Kneer and Wupen

Yuen, from Bandwidth9, Inc., were invaluable for the development of the torsional

tunable filter. Drs. Brian Cunnigham and Peter Li, from SRU Biosystems, were our

partners on developing the VCSEL based biosensor. Drs. Philip Worland, Jim Chiu and

Paul Cornelius, from VueMetrix, provided a prototype of a miniature electronic

driver/reader for the VCSEL based biosensor. Drs. Henry Yao, Ghulam Hasnain,

Claudio Marinelli, Chihping Kuo and Andy Liao, from LuxNet Corporation, provided the

tunable detector wafer and also helped me with some of its characterization. Dr. Leonard

Chen, from Raytheon Corporation, was a great supporter to the long and middle infrared

filters. Dr. Yuh-Ping Tong, from Industrial Technology Research Institute (ITRI -

Taiwan), has inspired many new ideas for future developments on biosensing.

UC Berkeley staff has definitely provided strong grounds for my work. I would

like to specially thank Microlab/IML people for keeping the equipments running, ERL

for managing our finances and graduate matters personnel for all the support.

Our research group was always a great environment to work, not only for the

resourceful people but also for the friendly and supportive atmosphere. I have to thank

xiv

former members from Prof. Chang- Hasnain’s group, specially Drs. Dan Francis, Ed

Vail, Gabriel Li (in memoriam), Wupen Yuen, Melissa Li, Marianne Wu and Mr. Steve

Chase. Even though we were not graduate students at the same time, they left a

wonderful inheritance in form of their valuable thesis, not to mention the informal

advising for several times. I enjoyed a lot having had discussions, brain-storming (and

laughing) sessions and a shared learning experience with all my group member buddies:

Dr. Jacob Hernandez, Dr. Chih-Hao Chang, Dr. Pei-Cheng Ku, Dr. Lukas Chrostowski,

Dr. Yuh-Ping Tong; Jeff Waite, Mike Huang, Paul Hung, Forrest Sedgwick, Shanna

Crankshaw, Michael Moewe, Bala Pesala, Wendy Xiaoxue Zhao, Mervin Ye Zhou,

Eiichi Sakaue, Kathy Buchheit, Kerry Maize, Karen Lee, Devang Parekh, Happy Hsin

and Heart Hsin. Special thanks go to Jeff Waite, who introduced me to the first steps of

device fabrication, and Mike Huang, my research partner through some of the most

exciting developments presented here. I also would like to thank our visiting professors,

Dr. Elsa Garmire, Dr. John Strand, Dr. Ghulam Hasnain and Dr. Ivan Kaminow, for

teaching us from the stand point of people who decisively contributed to the history of

semiconductor devices and optical communications.

My Brazilian friends have also contributed a lot by encouraging me to pursue my

degree. Prof. José Edimar Barbosa Oliveira and Maj.-Eng. André César da Silva first

introduced me to research in the field of optoelectronics, and Maj.-Av. André Luiz Pierre

Mattei gave me the incentive to go after my chances. Dr. Jean Paul Jacob has always

been present, fostering my steps in Berkeley and giving me his example.

I acknowledge CAPES Foundation and Brazilian Air Command for the financial

support.

My family always gave me the necessary strength. I am forever indebted to my

wife, Anna Paula, who has carried a load much larger than mine and always gave me

unconditional love, support and peace of mind to work. Our daughter Isadora brought

joy to our home and gave me new reasons to be a better person. My parents taught me

how to have endurance and face long term projects.

Most of all, I thank God for being blessed with health and surrounded by love.

1

Chapter 1 Introduction

1.1 Introduction

Lasers have generated enormous impact for humankind since its invention, in

applications ranging from eye surgery to barcode reading. More and more practical use

had appeared when the frequency of the devices started to be controlled over some range.

The first application of these large devices with complex tuning control was into

spectroscopy [1], but tunability would still open the doors for even more amazing new

possibilities if devices could be made cheap and portable. This is the only way that this

technology would leave the laboratory.

Through semiconductor technology, lasers evolved from the large table-top

devices into transistor size ones costing only a few dollars. This breakthrough has made

lasers a commodity, which have found their way into everyday life in CD and DVD

players, laser printers and scanners.

The first semiconductor lasers developed were the edge emitting lasers (EEL).

The devices have many different designs but the basic structure is shown in Figure 1.1(a).

It consists of a medium that can provide gain in between a pn junction. Current flows in

the vertical direction, with electrons and holes being injected in the active region and

generating emission of photons. The edges of the semiconductor die are cleaved and the

semiconductor-air interfaces work as mirrors to provide optical feedback and create a

resonant optical cavity, which is described in more detail in the next chapter. The active

region usually has a smaller bandgap than the p and n regions, which provides

confinement for the electrical carriers in such a way that most of the radiative

2

recombination occurs in the active region. This topology is called double heterostructure

and a further advantage is that the large bandgap semiconductor has a lower refractive

index than that in the active region, also giving index guiding in the transverse direction.

Thus, the two conditions for optical oscillation, population inversion and optical

feedback, are provided by carrier injection and cleaved mirrors, respectively. As Figure

1.1 suggests, the output beam is not symmetric due to different sizes of the optical

aperture in the vertical and horizontal directions, which makes it difficult to couple light

into optical fibers.

The vertical cavity surface emitting laser (VCSEL) is shown in Figure 1.1(b). In

this device, both carriers and photons fluxes are in the vertical direction. The overall

VCSEL technology is now mature and detailed description has been subject of several

textbooks [2-6]. The cavity length of VCSELs is very short typically 1-3 wavelengths of

the emitted light. As a result, VCSELs have only one longitudinal mode while EELs are

intrinsically multimode. However, in a single pass of the cavity, a photon has a small

chance of triggering a stimulated emission event at low carrier densities. Therefore,

VCSELs require highly reflective mirrors to be efficient. In EELs, the reflectivity of the

facets is about 30%. For VCSELs, the reflectivity required for low threshold currents is

greater than 99.9%. Such a high reflectivtiy can not be achieved by the use of metallic

mirrors, and VCSELs make use of distributed Bragg reflectors (DBRs). These are

formed by laying down alternating layers of semiconductor or dielectric materials with a

difference in refractive index and will be described in detail in the next chapter.

Semiconductor materials used for DBRs have a small difference in refractive index

therefore many periods are required. Since the DBR layers also carry the current in the

3

device, more layers increase the resistance of the device and, therefore, dissipation of

heat and growth may become a problem if the device is poorly designed. The symmetry

of the output aperture makes the optical beam circular and mode matched to optical

fibers, increasing coupling efficiency with very simple optics.

> 100λ~3λ

> 100λ~3λ

(a) (b)

Figure 1.1 – Scheme of and typical dimensions of (a) edge emitting laser (EEL) and (b) vertical cavity surface emitting laser (VCSEL).

VCSEL has a shorter history than EEL, but it has surpassed the former in all

applications where they have competed. Main advantages are compatibility to 2D arrays;

device completion and testing at the wafer level, which also make them cheaper; lower

temperature sensitivity; low numerical aperture output; single longitudinal mode; fast

response time; circular shaped beam output; and high power conversion efficiency. The

natural path of facts leads to the development of tunable semiconductor devices, filters,

detectors and lasers, with vertical cavities, which still have a whole universe of

applications barely explored on the surface.

The optical cavities that are described in this thesis are all vertical with respect to

the substrate, which can make use of micromaching techniques and provide the largest

monolithic tuning range among all options. The devices have a movable mirror, usually

the top DBR, which changes the resonance of the optical cavity and provides tunability.

4

Even though in this work only filter and detector were fabricated using monolithic

movable parts, design and fabrication methods described for these devices can be readily

extended to a laser [7-9].

Compact tunable optical devices are not only extremely useful tools but also

enabling technologies for a variety of applications such as wavelength multiplexing in

communications, reading of absorption spectrum in spectroscopy, lab-on-a-chip in

biosensing, board to board interconnects in computers, real time hyperspectral images in

infrared (IR) imaging and astronomy, and facial and fingerprint reading in biometrics.

Monolithically tuned optoelectronic devices are even more attractive because of

compactness, robustness, easiness of integration, and low price inherent to semiconductor

batch fabrication and testing. However, before jumping into the research, it is instructive

to show more about the vast area of applications and provide more perspective to the

work described later.

1.2 Applications of tunable devices

Applications for tunable devices are vast. In the following, a brief discussion

about some of the areas that can be largely benefited from tunable optoelectronic devices.

1.2.a Communications – Wavelength Division Multiplexing (WDM)

Communications has been the “holy grail” for optoelectronic applications, even

after the burst of the bubble in the year 2001. A lot of dark fiber and idle equipment are

still available for potential growth and very small investment in infrastructure is being

done at this time. However, new services such as video and music on-demand are being

added and can increase the need for bandwidth very fast after their popularization.

5

Current deployed technology utilizes WDM systems which are basically several

different optical carriers (channels) inside the same optical fiber. It is much more cost-

effective to add channels to a fiber than to dig trenches in the ground to lay new fibers.

Dense WDM (DWDM) has ongoing development of up to 256 channels in the same

fiber. These systems are used both for long-haul as well for local communications and

indicate that there is a huge potential market for optoelectronics devices in the horizon.

Sources for WDM systems are usually handpicked to match the wavelength to

each channel. Each laser has to be extensively characterized and wavelength as function

of temperature and injected current has to be mapped out in order to precisely control the

spectrum with the aid of a thermoelectric (TE) cooler. The final cost to package 256 of

these lasers into a box is huge and most of the link cost is on the transceivers, not on

devices or software. The enormous inventory necessary to handpick each source and its

spares has still to be added to this cost.

Thus, among the several reasons to embrace tunable lasers, the most immediate

advantage is to purchase just one type of laser instead of many versions with different

wavelengths, helping to reduce inventory costs. Tunable devices, in general, can also be

used to build more flexible networks where traffic can be redirected by switching the

wavelength. Current systems are only manually reconfigurable in the field to add or

change wavelengths as network routes are added or dropped. Figure 1.2 shows the

schematics for an add/drop element consisting of a tunable detector and a tunable

VCSEL. Virtually any channel within the tuning range can be detected from or added to

the optical backbone. A system similar to Figure 1.2 can also be used for wavelength

conversion, a necessary feature for flexible and reconfigurable networks.

6

λλλ

Tunable Filter + Detector to DROP

Tunable VCSEL to ADD

circulator circulatorλ

λλ

Tunable Filter + Detector to DROP

Tunable VCSEL to ADD

circulator circulator

Figure 1.2 – Schematics of an add/drop using tunable devices.

In general, tunable devices enable metro carriers to maintain and modify their

networks much more quickly and cost-effectively, because they support multiple

wavelengths with fewer devices and they can be reconfigured remotely. Tunable filters

and detectors can also be added along the link to monitor each one of the channels with

just one device at each monitoring point.

Figure 1.3 shows the two recent revolutions in fiber optic communications. In the

90’s, erbium doped fiber amplifier (EDFA) and WDM transformed the system with

several fibers and repeaters into 1 fiber that carries more than 100 channels (> 200 in

DWDM). Recently, tunable devices were launched with the capability to shrink

transmitters, receivers and respective spares into one multifunctional board.

7

TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTermTermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm

TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTermTermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm TermReptr Reptr Reptr Reptr Reptr Reptr ReptrTerm

16 Fibers, Single wavelength: 40Gb/s16 Fibers, Single wavelength: 40Gb/s

OC-48OC-48

EDFAEDFA EDFAEDFA

OC-48OC-48OC-48OC-48OC-48OC-48

OC-48OC-48OC-48OC-48OC-48OC-48OC-48OC-48OC-48



1 Fiber, 100 wavelength WDM: 40Gb/s1 Fiber, 100 wavelength WDM: 40Gb/s

16 Fibers 16 Fibers 1 Fiber1 Fiber~48 regenerators ~48 regenerators 1 Optical Amplifier 1 Optical Amplifier

EDFAEDFA EDFAEDFATunable Laser

+Tunable detector

Tunable Laser+

Tunable detector

200 200 TxTx/Rx + 200 spares /Rx + 200 spares 4 tunable boards4 tunable boards

… …

……

WDM +EDFA

Tunable devices





16 Fibers, Single wavelength: 40Gb/s16 Fibers, Single wavelength: 40Gb/s

OC-48OC-48

EDFAEDFAEDFAEDFA EDFAEDFAEDFAEDFA





1 Fiber, 100 wavelength WDM: 40Gb/s1 Fiber, 100 wavelength WDM: 40Gb/s

16 Fibers 16 Fibers 1 Fiber1 Fiber~48 regenerators ~48 regenerators 1 Optical Amplifier 1 Optical Amplifier

EDFAEDFAEDFAEDFA EDFAEDFAEDFAEDFATunable Laser

+Tunable detector

Tunable Laser+

Tunable detector

200 200 TxTx/Rx + 200 spares /Rx + 200 spares 4 tunable boards4 tunable boards

… …

……

WDM +EDFA

Tunable devices

Figure 1.3 – The two revolutions in fiber optic communications. In the 90’s, erbium doped fiber amplifier (EDFA) and wavelength division multiplexing (WDM) transformed the system with several fibers and repeaters into 1 fiber that can carries more than 100 channels (> 200 in dense WDM). Recently, tunable devices were launched with the capability to shrink transmitters, receivers and respective spares into multifunctional boards.

A lot of research has been done on the specific development of tunable VCSELs

[7-13], filters [14-20] and detectors [21-23] for WDM applications. Most of the

development in this market has migrated into companies and university research cannot

compete with them. However, some space was left for innovative structures that can

enhance the tuning range, such as the torsional filter to be described in chapters 2 and 4.

Finally, the use of conventional electrical interconnections limits the achievable

bandwidth and the noise immunity of signal transmission in information systems. This

8

restriction in performance can be overcome by the use of the optical interconnection

technology. The concept of WDM can also be extended to communication among boards

inside a computer or some other equipment [24, 25], with huge potential application for

tunable devices.

1.2.b Spectroscopy

Absorption spectroscopy is as simple as shining light through the material and

measuring the reduction in its intensity. However, strong, incoherent light sources and

abundant target material are needed to achieve high sensitivity with this technique.

Lasers and resonant cavities have enabled new spectroscopic techniques including laser

spectroscopy and multipass cell absorption spectroscopy, and have been used for long

time for this purpose [1, 26] . Because of their higher sensitivity, high signal to noise

ratio and high resolution, lasers play important roles in measuring weak transitions and

low concentrations in the chemical species. These advances have enabled a wide range

of applications in the fields of air pollution control and hazardous gas detection.

However, in order to identify different materials, several absorption lines have to be

characterized and tunable lasers are the devices for the task.

Optoelectronic devices are tiny in volume and are inexpensive to fabricate in large

arrays. They also have low power requirements and higher efficiencies. These features

make tunable semiconductor devices suitable for low-cost spectroscopy equipment and

wide area, low power distributed gas-detection systems.

Figure 1.4 shows a high resolution absorption spectroscopy trace obtained with a

VCSEL tuned with injection current [27]. Single frequency VCSELs show a variety of

advantages compared to edge emitting semiconductor lasers, such as wide current tuning

9

range, very high tuning speeds (MHz) and therefore very good time resolution, little

susceptibility to optical feedback and low manufacturing costs.

0 .0 0 0 0 .0 0 1 0 .0 0 2 0 .0 0 3 0 .0 0 4 0 .0 0 5 0 .0 0 60 .9 5

0 .9 6

0 .9 7

0 .9 8

0 .9 9

1 .0 0

1 .0 1

1 .0 2

V c s e lS p e k h 2 o _ g _ S t ro m .O P J

1 .8 0 3 9 41 .8 0 0 4 4Tran

smis

sion

W a v e le n g th [a .u . ]

1.800 1.801 1.802 1.803 1.804 1.8050.980

0.985

0.990

0.995

1.000

1.80394

1.802751.802231.80119

h2oSpek.OPJ

Tra

nsm

issi

on

Wavelength [µm]

calculated

measured

0 .0 0 0 0 .0 0 1 0 .0 0 2 0 .0 0 3 0 .0 0 4 0 .0 0 5 0 .0 0 60 .9 5

0 .9 6

0 .9 7

0 .9 8

0 .9 9

1 .0 0

1 .0 1

1 .0 2

V c s e lS p e k h 2 o _ g _ S t ro m .O P J

1 .8 0 3 9 41 .8 0 0 4 4Tran

smis

sion

W a v e le n g th [a .u . ]

1.800 1.801 1.802 1.803 1.804 1.8050.980

0.985

0.990

0.995

1.000

1.80394

1.802751.802231.80119

h2oSpek.OPJ

Tra

nsm

issi

on

Wavelength [µm]

calculated

measured

Figure 1.4 – Water vapor spectroscopy using a tunable VCSEL. After [27].

Micromachined devices have been fabricated for spectroscopy purposes [28].

Figure 1.5 shows the transmittance change of a Fabry-Pérot filter due to the presence of

hydrogen sulfide in the optical cavity. When there is vacuum in the cavity, incident light

at 970nm will be transmitted through the filter. Once there is a tiny amount of hydrogen

sulfide in the air gap, the change in index of refraction and absorption will move the

resonant wavelength away from 970 nm and greatly reduce the transmission of light at

970 nm. However, the required level of resolution for this kind of monitoring is very

high and tight control is required over the ambient conditions.

10

Figure 1.5 – Change of resonance in a micromachined Fabry-Pérot filter due to the presence of hydrogen sulfide in the air gap. After [28].

1.2.c Biosensing

Two recent developments are true milestones that have stimulated the application

of optoelectronics into biosensing to an exponential growth on research: the burst of the

communications bubble and the 09/11 terrorist attacks in New York, both in 2001. The

first one caused a fast migration of workers from one application without market and

flooded with industry lay offs to other emerging areas. Several professionals came back

to university to do research by the time that biosensing was being boosted by the

preventive actions against biological attacks taken by the newly created Agency of

Homeland Security. On top of this, the steady and large pharmaceutical and medical

diagnostics markets added even more value to a large area of new applications for

optoelectronic devices.

Several techniques are used for biosensing but the label-free ones are the most

attractive because of assay simplicity and no direct interference with the species being

measured. Furthermore, among all available label-free techniques, optical methods are

very advantageous over the others because of high signal-to-noise ratio, which reflects on

accuracy; in situ real-time process monitoring; and high sensitivity to surface changes,

11

where most of the bioprocesses take place [29-31]. This justifies a worldwide effort to

transfer optical techniques out of the laboratory and into clinical, industrial and battlefield

settings.

Tunable devices are attractive for several different topologies of label-free

sensors. Laser tuning through evanescent field has been proposed as a monolithic sensor

of index modifications such as the ones due to protein binding [32]. Measurement of

absorption spectra of harmful species, similarly to spectroscopy, also has been making

use of tunable lasers [33, 34]. Many techniques have been proposed for the detection of

gas and spores at very low concentration after threats of biological contamination using

anthrax, for example. Some of them make use of tunable filter [35], tunable laser [36]

and tunable detector [37]. Techniques that involve the detection of resonant wavelength

[38, 39] can advantageously use tunable devices as will be shown in chapters 5 and 6.

1.2.d Infrared (IR) imaging

Broadband detectors at cryogenic temperatures can achieve very high sensitivity

but resolution of the images is usually poor because of low contrast. Details can be

largely enhanced by acquiring data at several different wavelengths and composing them

in what is called multi and hyperspectral images.

High resolution IR imaging is usually related to military applications such as

target recognition and day and night surveillance. With the recent appearance of

commercial airborne hyperspectral imaging systems and the launch of satellite-based

sensors, hyperspectral imaging is also entering in the mainstream of remote sensing.

Remote sensing technology, developed for Earth resources monitoring, is

composed of creative techniques that combine and integrate spectral with spatial

12

methods. Such techniques are also finding use in medicine, agriculture, manufacturing,

forensics, and an expanding list of other applications. The technique uses a variety of

electronically tunable filters that are mounted in front of a monochrome camera to

produce a stack of images at a sequence of wavelengths, forming an “image cube”. The

combined spectral/spatial analysis offered by such image cubes takes advantage of tools

borrowed from spatial image processing, chemometrics, spectroscopy, and new custom

exploitation tools developed specifically for these applications [40]. For example,

images from a filter-based system can be visualized as shown in Figure 1.6. Each

acquisition is of the same FOV in a sequence of wavelength bands. Complex data

processing software then constructs the image as a function of the wavelength data. Even

in black and white is possible to see the huge difference between the two pictures, where

the one to the right is the composite image of three slices of spetrum (blue + short UV,

red + green, short IR) and the other has only the central band (red + green).

Figure 1.6 – Multispectral infrared images from Bay Area taken by satellite LANDSAT 7 and composed by NASA. Picture to the left has green and red spectral components only while picture to the right is the composite of the picture to the left plus long ultra-violet, blue and short infrared wavelength bands.

Data processing such as the one done above can generate very rich range of

13

information. Military would be able to detect camouflaged targets [41], remote sensing

would easily discover new mineral reservoirs [42], and medicine would locate and

dimension tumors. Furthermore, any of the images can be adjusted with respect to high

background noise in real time if tunable devices are integrated to the detector.

Commercial cameras, designed for remote gas leak detection are versatile enough

that can be applied to numerous other applications such as homeland security,

chemical/biological agent detection, medical and pharmaceutical applications, as well as

standard research and development [43, 44]. However, those cameras are still big mainly

because of the filtering system.

Ideally, a monolithically tunable filter array would be integrated to a focal plane

array and each pixel or small cluster would have independent addressability for real time

adjustments. This is where tunable optoelectronic devices can play a major role and a

filter design for this purpose will be shown in chapter 4.

1.2.e Biometrics

Biometrics is a methodology for recognizing and identifying people based on

individual and distinct physiological or behavioral characteristics. Identification methods

include fingerprints, facial shape, iris patterns, retina patterns, hand geometry, speech,

handwriting, and even wrist vein patterns. Biometric identification can be used to

prevent unauthorized access to buildings, ATM machines, desktop PCs, laptop PCs,

workstations, cellular telephones, wireless devices, computer files and databases, and

both closed and open computer networks.

Biometric security is more robust than methods such as passwords, personal

identification numbers, and smart cards or tokens because biometrics identifies

14

individuals rather than devices. These other security methods can be lost or stolen and

therefore get into the hands of unauthorized users. A biometric, such as a fingerprint, is a

key that can never be lost.

The system identifies a person by comparing the code created from the fingerprint

image captured at access attempt (livescan template) to one or more pre-registered codes

(reference templates). This comparison is based on a number of characteristic points of

the fingerprint. Most of the technology relies on processing the acquired data and

comparing to the database. However, security can be greatly enhanced if more than one

wavelength is used in the scanning process, allowing more details such as surface

roughness or presence of liquid and powders to be revealed [45]. The idea behind the

method is the same as multispectral imaging. If more spectral information is available,

more detail can be revealed and more robust is the identification method.

1.3 Overview of the dissertation

This dissertation covers design, fabrication technology, characterization and

applications of tunable filter, detector and VCSEL. The devices are fabricated in GaAs

and are all monolithically tuned. Filter and detector are micro-mechanically tuned while

VCSEL is tuned through injection current and temperature. However, any of the

proposed micro-mechanical structures can be readily applied to the laser in order to

extend the tuning range of the device.

Chapters 2 and 3 are organized to provide the background to the development of

tunable devices. Chapter 2 is specifically dedicated to design, which is divided in two

separate topics: optical cavity and mechanical structure. Then the two topics are

combined into general and systematic design rules for tunable filters ranging from short

15

(850nm) to long (10µm) infrared ranges. Details of the design of torsional filter close the

chapter. Chapter 3 describes all processes utilized to fabricate the devices and

adjustments done to accommodate design variations. The chapter starts with fabrication

and performance of a new subwavelength waveguide grating (SWG) mirror, which was

done in silicon for the purpose of demonstration and in excellent agreement with theory.

Then general methods that are utilized in III-V compounds micromachining techniques

are described. This way the following chapters can flow more easily into the specific

developed structures.

Chapters 4, 5 and 6 report tunable filter, detector and VCSEL, respectively.

Chapter 4, tunable filter, specifically describes two different devices: a torsional

filter and a folded trampoline structure, which were developed for applications in

communications and infrared imaging, respectively. All details inherent to the structures

are given in the first two sections and the third one describes the characterizations that

were done looking into each of the applications. A record tuning range of more than

100nm around the central wavelength of 1550nm was achieved with the torsional

structure.

Chapter 5 shows the development of a novel a tunable detector, which consists on

a tunable filter integrated to a broadband detector designed to be applied in biosensing.

The optical design involves lateral optical confinement to enhance linewidth and avoid

side modes. The proposed biosensor, integration of the detector with a guided mode

resonator (GMR), is described and the device is characterized with respect to electrical,

optical and mechanical characteristics.

Chapter 6 reports the usage of tunable VCSEL in a novel approaches for compact

16

and portable label-free optical biosensor. It uses a tunable VCSEL based measurement

system integrated to a GMR. This approach can provide high sensitivity, low power

consumption and low cost. Protein concentrations as low as 6.7femto-Mol/l has been

detected.

Finally, in the last chapter, the dissertation is summarized and some directions for

future work are pointed.

17

Chapter 2 Design

2.1 Introduction

Tunable optical devices are key components for various applications, such as

dense wavelength division multiplexed (DWDM) optical networks, optical interconnects,

spectroscopy, infrared imaging and optical sensing. Various structures and designs have

been reported for these different applications [14]. The micromechanically actuated

devices have intrinsic advantages such as large dynamic range, fast tuning speed, a

simple control mechanism, small size, cost effectiveness and process compatibility with

other optoelectronic devices. A wide and continuous tuning range was first demonstrated

by Chang-Hasnain’s group, at Stanford University and now University of California,

Berkeley, in a family of micro-electro-mechanical (MEM) photonic devices using a

simple cantilever structure. The devices included tunable filters [46], detectors [22] and

vertical cavity surface emitting lasers (VCSELs) [47]. Other research teams have also

reported tunable photonic devices using similar cantilever [16], bridge [18, 19, 48, 49] or

membrane structures [17, 50, 51].

Design variations affect the final performance and may require very elaborate

fabrication steps but the design theory can be divided in two separate topics: optical

cavity and mechanical structure. I will start describing these two topics and then show

how they can be combined to give continuous tuning throughout the desired range. I will

also establish general and systematic design rules for tunable filters ranging from short

(850nm) to long (10µm) infrared ranges, which will be presented by the end of the

18

chapter.

2.2 Optical Cavity

An optical cavity, in the simplest case, consists of two parallel mirrors that

provide feedback for electromagnetic waves, also named Fabry-Pérot cavity or etalon

(Figure 2.1). The interaction among the multiple reflections, which is intrinsically

dependent on the separation between the mirrors, determines what wavelengths can

resonate in this cavity and they are called Fabry-Pérot wavelengths. Thus, like in

microwaves resonators, an optical cavity traps an optical beam that becomes the mode of

the cavity.

n1 n2 n1

Ei

Er

Et

n1 n2 n1

Ei

Er

Et

l

E0+

E0-

E1+

E1-

E2+ …

E0+

E0-

E1+

E1-

E2+ …

Figure 2.1 - Fabry-Pérot cavity: two parallel partial mirrors separated by a distance l. Index of refraction may be different inside (n2) and outside (n1) the cavity. A plane wave is incident from the left (Ei) and, after multiple reflections inside the cavity, partially transmitted (Et) and reflected (Er).

The modes of the cavity can be easily solved if one considers the incidence of a

plane wave produced by an external source. For simplicity, consider all waves, incident

on the cavity from the left, reflected to the left, inside the cavity, or transmitted through it

to the right, to be uniform plane waves of limited spatial extent transverse to the direction

of propagation. An infinite number of partial waves is produced by reflections at the two

19

mirrors. The phase delay between two partial waves (one additional round trip) is given

by:

22

42 n ll πδ βλ

= = (2.1)

where n2 is the index of refraction inside the cavity, l is the separation between mirrors

and λ is the wavelength in vacuum of the incident wave.

If the complex amplitude of the incident wave is taken as Ei (Figure 2.1), then the

partial reflections Er0, Er1, and so forth are given by:

0' ' ' ' '

1 0 0'3 ' '3 ' '2 ' ' ' '

2 0 0 1 1

r ii i i

r ii i i i i

r i

E E r

E E tr t e E r t e E t e

E E tr t e E r t e E r t e E r t e E t e

δ δ δ

δ δ δ δ δ

+ −

+ − + −

=

= = =

= = = = = (2.2)

where r is the reflection coefficient from n1 to n2, t is the transmission coefficient for

waves incident from n1 toward n2, and r’ and t’ are the corresponding quantities for waves

traveling from n2 toward n1. Of course, in this case r’= -r, and for lossless mirrors r2 +

tt’ = 1. Thus, the fraction of optical intensity transmitted and reflected at each interface

can be given by:

2 '2 ' and R r r T tt≡ = ≡ (2.3)

The complex amplitude of the total reflected wave is Er = Er0 + Er1 + ⋅⋅⋅, or

( ) 21i i ir iE E R T Re Re R eδ δ δ= + + + +… (2.4)

and, analogously, for the transmitted wave:

( )2 / 21 i i it iE E T Re R e eδ δ δ= + + +… (2.5)

The terms inside parenthesis in (2.4) and (2.5) form an infinite geometric

progression that can be added to give:

20

( )1

1

i

r i i

R eE E

Re

δ

δ

−=

− (2.6)

1t i i

TE ERe δ=

− (2.7)

Finally, the optical intensities can be solved if the incident intensity is taken as

EiEi*:

( )

( ) ( )

2*

2* 2

4 sin 2

1 4 sin 2r r r

i i i

RI E EI E E R R

δδ

= =− +

(2.8)

( )( ) ( )

2*

2* 2

1

1 4 sin 2t t t

i i i

RI E EI E E R R δ

−= =

− + (2.9)

Thus, transmission is unity (and reflection is zero for this lossless model)

wheneverδ, as given by (2.1), is an even multiple of π (δ = 2mπ), where m is an integer.

This same condition can be written as:

22m

cmn l

ν = (2.10)

where c = νλ is the speed of light in vacuum and ν is the optical frequency. For a fixed

separation l between mirrors, (2.10) defines the frequencies with unity transmission.

These frequencies determine the longitudinal cavity modes. The separation between two

consecutive of these frequencies is the so called free spectral range.

Figure 2.2 shows the theoretical transmission plots of a Fabry-Pérot cavity. The

maximum transmission is unity, as previously stated, and the minimum transmission

approaches zero as R approaches unity. Another important fact from R approaching unity

is that the transmitted linewidth becomes sharper.

21

0.0

0.2

0.4

0.6

0.8

1.0

R=0.7R=0.9

R=0.2

R=0.1

δ2(m+1)π2mπ

It/Ii

Figure 2.2 – Characteristic theoretical transmission of a Fabry-Pérot cavity. Total transmission is allowed at some wavelengths (for the case where both mirrors have the same reflectivity) and the linewidth of the transmission decreases for increasing reflectivity.

The transmission peaks in Figure 2.2 form the basis of a Fabry-Pérot tunable

filter. If one can change the cavity length, l, the transmitted wavelength also changes.

The filter would be limited to operate within one free spectral range or it would be

transmitting at more than one wavelength. Finally, the way to achieve a small passband

is by increasing mirror reflectivity. However, the linewidth shape is always constrained

to a lorentzian.

In case the mirrors do not have the same reflectivity, (2.9) is modified to:

( )( )

( ) ( )1 2

2 21 2 1 2

1 1

1 4 sin 2t

i

R RII R R R R δ

− −=

− + (2.11)

where R1 and R2 are the two mirrors reflectivities. One can see that total transmission

only occurs if the mirrors have the same reflectivity, condition usually referred to as

mirror matching. In the same way, if the cavity has a power gain G (or loss A), meaning

22

that the field is amplified by G , (2.8) and (2.11) are modified to [52]:

( ) ( )

( ) ( )

2 21 2 1 2

2 21 2 1 2

4 sin 2

1 4 sin 2r

i

R R G R RI netI G R R G R R

δ

δ

− +=

− + (2.12)

( )( )( ) ( )

1 22 2

1 2 1 2

1 1

1 4 sin 2t

i

G R RII G R R G R R δ

− −=

− + (2.13)

and these equations can be used to model optical amplifiers or resonant cavity detectors.

The optical cavities that are described in this thesis are vertical with respect to the

substrate. One unique characteristic of devices such as VCSELs is that the optical wave

propagates in a direction perpendicular to the active region, so that the interaction length

between the optical wave and the gain medium is very short, resulting in a small amount

of gain per pass. This condition requires the mirrors to provide extremely high

reflectivity (>99%) to compensate for the insufficiency of gain. Filters and detectors also

require high reflectivity in order to effectively select wavelength.

Metal mirrors have larger reflection bandwidths but lower reflectivities (R),

limited by absorption loss. As a result, they are not suitable for fabricating transmission-

type optical devices such as etalon filters. Dielectric mirrors have a low loss and, thus,

can achieve a higher reflectivity. Mirrors for vertical cavities can be fabricated in two

different ways. In the first and most used one, they consist of alternating layers of high

and low index of refraction, called distributed Bragg reflector (DBR). In the second,

which were recently invented [53] and is still under development [54], a simple grating

can be designed to provide a broadband spectrum and high reflective mirror. The

description of these two different techniques is the objective of the next two subsections.

23

2.2.a Distributed Bragg Reflector (DBR)

A DBR is a stack of dielectric layers designed for a particular wavelength (λ0).

The dielectric materials used to compose a DBR can be either amorphous as formed by

using evaporation or deposition techniques, such as SiNX, ZnSe etc., or epitaxial as

formed by molecular beam epitaxi (MBE) or metalorganic epitaxial chemical vapor

deposition (MOCVD). Each layer in the stack has a physical thickness of λ0/(4n), with n

being the refractive index of the material. In a DBR, the adjacent layers have

alternatively high and low refractive indices. As shown in Figure 2.3, the quarter-

wavelength layer thickness allows the reflected waves from each interface to add up in

phase, resulting high reflectivity. Since perfect phase matching is only achieved at the

designed wavelength, λ0, the maximum reflectivity is also obtained only at λ0. However,

as long as the number of mirror pairs is sufficiently large, optical waves with wavelength

sufficiently close to λ0 will still experience high reflectivity although not as high as λ0.

nH nL nH nL nH nL

π/2

π/2

π/2

π/2 π/2

π/2

π/2

π/2 π/2

π/2

π

-π

π

-π

π

nH

π/2

π/2-π

π

nL

0 0 0 0 0 0

λ/4nL λ/4nH λ/4nL λ/4nH λ/4nL λ/4nH

nH nL nH nL nH nL

π/2

π/2

π/2

π/2 π/2

π/2

π/2

π/2 π/2

π/2

π

-π

π

-π

π

nH

π/2

π/2-π

π

nL

0 0 0 0 0 0

λ/4nL λ/4nH λ/4nL λ/4nH λ/4nL λ/4nH

Figure 2.3 - Schematic of a DBR. Constructive interference of all reflections, achieved by the proper arrangement of layer thickness and refractive index sequence, builds the high reflectivity of a DBR. In this illustration, the input layer has a high index and the output layer has a low index.

24

The absolute reflectivity of the mirror is function of the number of mirror pairs

and the index of refraction difference between the quarter wave layers. Figure 2.4(a) and

(b) shows reflected power and phase, respectively, for 20.5 pairs of

Al0.2Ga0.8As/Al0.7Ga0.3As centered at 1.55µm (thicknesses of the DBR layers are

1.55µm/4n). A quasi-linear phase exists for almost the entire stop band width. The

linear phase shift, as derived from the theory of Fourier transform, is quickly recognized

as a time delay in the time domain. This means that the incident optical wave stays for a

finite amount of time within the DBR before it is reflected back from the DBR,

equivalent to the notion that there is a certain penetration depth in the DBR that the

lightwave has to go through. The concept of penetration depth is very useful in

determining the optical properties of a VCSEL [55].

Figure 2.4(c) shows power reflectivity for 30.5 pairs of Al0.2Ga0.8As/Al0.7Ga0.3As.

Reflectivity increases by increasing the number of pairs but the full width at half

maximum (FWHM) band slightly shrinks. Finally, Figure 2.4(d) shows power

reflectivity for 20.5 pairs of GaAs/AlAs. Reflectivity and FWHM band increases by

increasing the difference in index.

2.2.a.1 Transmission Matrix

DBR simulation is done by using Transmission Matrix Theory. This theory

considers a two port element and expresses the input and output at a given port as

function of those at the other port. So, it is straight forward to do this calculation in the

sense that all layers can be cascaded and the multiple reflections are taken care by simple

matrix multiplication. Implementation of such codes is easy and fast and not only

reflected or transmitted power can be obtained but also field distribution inside the DBR.

25

1.4 1.45 1.5 1.55 1.6 1.65 1.7wavelength HumL

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

1.4 1.45 1.5 1.55 1.6 1.65 1.7wavelength HumL-3

-2

-1

0

1

2

3

RBDesahp

HnaidarL

(a) (b)

1.4 1.45 1.5 1.55 1.6 1.65 1.7wavelength HumL

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

1.4 1.45 1.5 1.55 1.6 1.65 1.7wavelength HumL

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

(c) (d)

Figure 2.4 – Simulated power and phase reflectivity spectra of a DBR stack of AlxGa1-xAs centered at 1.55µm. (a) Power and (b) phase for 20.5 pairs of Al0.2Ga0.8As/ Al0.7Ga0.3As. (c) Power reflectivity for 30.5 pairs of Al0.2Ga0.8As/ Al0.7Ga0.3As. Reflectivity increases by increasing the number of pairs but the full width at half maximum (FWHM) band slightly shrinks. (d) Power reflectivity for 20.5 pairs of GaAs/AlAs. Reflectivity and FWHM band increases by increasing the difference in index.

=

2

2

2221

1211

1

1

BA

TTTT

BA

TA1 A2

B2B1

=

2

2

2221

1211

1

1

BA

TTTT

BA

TA1 A2

B2B1 TA1 A2

B2B1 T’ B’2B’

1

A’1 A’

2

′′

′′′′

=

=

2

2

2221

1211

2221

1211

2

2

2221

1211

1

1

BA

TTTT

TTTT

BA

TTTT

BA

TA1 A2

B2B1 T’ B’2B’

1

A’1 A’

2

TA1 A2

B2B1 T’ B’2B’

1

A’1 A’

2

′′

′′′′

=

=

2

2

2221

1211

2221

1211

2

2

2221

1211

1

1

BA

TTTT

TTTT

BA

TTTT

BA

(a) (b) Figure 2.5 illustrates the Transmission Matrix Theory. In the cascaded

representation, the output from the right port of the first element, A2, works as input for

26

the left port of the second element, A1’. So, by multiplying the element matrices, A1 and

B1 can be calculated as function of A2’and B2’. Note that all calculations have to be done

by using field, not intensity. Thus, once the matrices have been multiplied, normalized

reflected and transmitted fields can be determined from:

2

1 21

1 110B

B TrA T

=

= = (2.14)

2

2

1 110

1

B

AtA T

=

= = (2.15)

=

2

2

2221

1211

1

1

BA

TTTT

BA

TA1 A2

B2B1

=

2

2

2221

1211

1

1

BA

TTTT

BA

TA1 A2

B2B1 TA1 A2

B2B1 T’ B’2B’

1

A’1 A’

2

′′

′′′′

=

=

2

2

2221

1211

2221

1211

2

2

2221

1211

1

1

BA

TTTT

TTTT

BA

TTTT

BA

TA1 A2

B2B1 T’ B’2B’

1

A’1 A’

2

TA1 A2

B2B1 T’ B’2B’

1

A’1 A’

2

′′

′′′′

=

=

2

2

2221

1211

2221

1211

2

2

2221

1211

1

1

BA

TTTT

TTTT

BA

TTTT

BA

(a) (b) Figure 2.5 – Transmission Matrix theory. (a) Single element. (b) Cascaded elements. The input and output from one port of one element are the output and input, respectively, to the next element and the overall effect can be calculated by simply multiplying the cascaded element matrices.

In order to implement an optical cavity, two DBR mirrors have to be cascaded,

but separated by some distance that has to be of the order of magnitude of λ0/2. This

way, wavelengths in the neighborhood of λ0, and thus inside the mirror stop band, can

have a standing wave in between the mirrors and resonate. The entire final structure can

be represented by a simple and exact matrix, resultant of the cascaded multiplication.

The only design parameters are the materials that constitute the DBR and the

number of DBR pairs. Once the material is defined (constrained by epitaxial growth,

doping, processing, etc.), the designer can only choose the number of pairs and determine

the total reflectivity that ultimately determines the filter passband.

27

2.2.b Sub-Wavelength Grating (SWG)

Even though DBR is the commonly used mirror for vertical cavity devices, the

deposition methods are often not precise enough to lead to very high reflectivity.

Furthermore, the typical material combinations often have a rather small bandwidth,

which is limited by the small refractive index difference of the materials used. Usually,

different dielectric materials are needed for different wavelength regimes (e.g. near vs.

far infared). For tunable etalon type devices, such as micro-electro-mechanical (MEM)

filters [15-17, 19, 46, 51, 56], detectors [21, 22] and VCSELs [22, 47, 50, 57, 58], DBRs

are desirable because of higher thermal conductivity and possibility to make electrically

conducting mirrors. However, the tuning range is often limited to ∆λ/λ ≈ 3-9%.

Moreover, when scaling devices to longer wavelengths, the DBR thickness to be grown

may become prohibitive (~20µm per mirror at λ = 10µm). The challenge of designing a

mirror with broadband reflection, low loss and compatibility with optoelectronic

processing has not been overcome yet.

Sub-wavelength gratings have been used to create structures with either sharp

highly reflective peaks at resonance wavelengths [38, 59] or broad antireflective bands

[60]. The final spectral characteristic of the grating can be further tailored by the

materials used and parameters chosen. A novel application has been recently devised

with very broad reflection spectrum (∆λ/λ > 30%) and very high reflectivity (R>99%)

[53]. The design is scalable for different wavelengths, facilitates monolithic integration

of optoelectronic devices at a wide range of wavelengths from visible to far infrared and

has potential application on micro-electro-mechanical tunable devices [15-17, 19, 21, 22,

46, 47, 50, 51, 56-58], visible and infrared VCSELs [61, 62] and reconfigurable focal

28

plane arrays [63]. Another interesting characteristic that the SWG exhibits is its

insensitivity to lateral position on cascaded structures [64-66], facilitating fabrication of

vertical (orthogonal to the substrate) optical cavities.

The proposed structure has a large refractive index difference among materials,

resulting in a very broadband reflector. Figure 2.6 shows the scheme of such a mirror

that consists of lines of high/low index material surrounded by two low index layers. The

larger the difference between high and low indices is, the larger the reflection band. The

low index layer under the grating is critical for the mirror effect. Design parameters for

the structure include the materials involved (index of refraction), thickness of the low

index layer under the grating (tL), grating period (Λ), grating thickness (tg) and duty

cycle. Duty cycle is defined as the ratio of the width of the high index material to Λ.

substrate

low index material (nL, tL)

high index material (nh) Λ tgair

substrate

low index material (nL, tL)

high index material (nh) Λ tgair

Figure 2.6 - Scheme of the sub-wavelength grating reflector. The low index material under the grating is essential for the broadband mirror effect.

Figure 2.7 shows reflected power for light polarized perpendicular to the grating

lines. The simulation was based on Rigorous Coupled Wave Analysis (RCWA) [67] (a

brief overview is given at the end of this section) and confirmed by finite difference time-

domain electromagnetic propagation using TEMPEST© [68]. The two methods are in

excellent agreement. A very broadband mirror ∆λ/λ > 30%, with R > 0.99, was obtained

around 1.55µm, over the range 1.33µm to 1.80µm, as depicted by Figure 2.7(a). The

mirror is also very broad for R > 0.999 (1.40µm to 1.67µm or ∆λ/λ > 17%). The

29

parameters used in the simulation were: Si substrate (n=3.48), Λ = 0.7µm, nh = 3.48

(Poly-Silicon), low index material in and above the grating = 1 (air), nL = 1.47 (SiO2), tL

= 0.83µm, tg = 0.46µm and duty cycle = 0.75. The index of refraction was considered

constant along the covered range, which is a very good approximation since most

semiconductor materials such as Si, GaAs and ZnSe have index of refraction practically

independent of wavelength in the considered ranges. By recursive analysis, all grating

parameters were optimized to maximize both reflectivity and spectral coverage.

It is interesting to note that the broadband reflectivity does not result from a

resonance, as the period of the grating is sub-wavelength but not half-wavelength.

Furthermore, the reflectivity spectrum can be scaled with wavelength, as shown in Figure

2.7(b). By simply multiplying the dimensions by a constant, in this case 6.5, while

keeping the other parameters, the reflection band shifts to the 8.6-11.7µm wavelength

range with all features and values being identical. Hence, any different wavelength

regime can use the same design. Note that the same constant has also multiplied the

horizontal scale in order to make the comparison easier. Although it may be obvious that

any periodic structure should be wavelength scalable, the scaling here is easily

manufacturable since it only requires changing the layer dimensions

30

1.0 1.2 1.4 1.6 1.8 2.0 2.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Ref

lect

ivity

Wavelength (µm)

RCWA TEMPEST

1.0 1.2 1.4 1.6 1.8 2.0 2.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

mirror centered at 1.55µm

Ref

lect

ivity

Wavelength (µm)

7 8 9 10 11 12 13 14

mirror centered at 10µm (dimensions multiplied by 6.5)

(a) (b)

Figure 2.7 - Reflected power for light polarized perpendicularly to the grating lines. (a) Thick line was obtained based on Rigorous Coupled Wave Analysis (RCWA) Error! Reference source not found. while dashed with TEMPEST© Error! Reference source not found.. (b) A simple scaling factor (6.5) applied to the dimensions gives completely overlapped traces. Thick line is centered at 1.55µm while dashed at 10 µm.

The low index material layer under the grating is essential to obtain the high

broadband reflection. This is shown in Figure 2.8, which consists of contour plots of

reflectivity as a function of wavelength, tL and nL. Keeping all the other parameters the

same, there is no reflection band for tL <0.1 µm. Above this thickness, the structure has

low sensitivity to the low index layer, but this parameter can be used to optimize the

reflection band. The mirror also does not exist if nL >2.5. If Si3N4 (n ≈ 2) is used instead

of SiO2, the result would be a much smaller reflection band, ranging from 1.7µm to

1.8µm.

31

Low

Inde

x Th

ickn

ess

(t L) (

µ m)

t L) (

µ m)

Fig. 2Lo

w In

dex

Thic

knes

s (

Wavelength (µm)Wavelength (µm)

Low

Inde

x Th

ickn

ess

(t L) (

µ m)

t L) (

µ m)

Fig. 2Lo

w In

dex

Thic

knes

s (


Low

Inde

x (n

L)

Wavelength (µm)

Fig. 2

Low

Inde

x (n

L)

Wavelength (µm)

Fig. 2

(a) (b)

Figure 2.8 - Effect of the low index layer under the grating. (a) Reflectivity as function of wavelength and tL. There is no reflection band when tL <0.1µm, and above this value, the structure has low sensitivity to this parameter. (b) Reflectivity as function of wavelength and nL. The mirror also does not exist if nL >2.5.

The various design parameters play interesting roles on the final reflectivity

spectrum. Any material system with a large difference in index of refraction can be used

as a base for this broadband mirror, and the larger this difference, the larger the band.

The simulations show results for Poly-Silicon/air/SiO2, but GaAs/Al2O3, GaN/air or

ZnSe/CaF2 would be comparable. In the following discussion, we show design tolerance

by varying one of the parameters, while keeping the others constant.

The grating period determines the location of the center wavelength of the

reflection band, and this effect is shown in Figure 2.9. The band shifts to longer

wavelengths proportionally to Λ, and for Λ = 0.7 µm the band is the broadest. The period

can be controlled very accurately by lithographic methods and thus, the reflection band

can be precisely fabricated.

32

Fig. 2

Gra

ting

Per

iod

(Λ) (

µm)


Fig. 2

Gra

ting

Per

iod

(Λ) (

µm)


Figure 2.9 - Reflectivity as function of wavelength and Λ. The reflection band shifts to longer wavelengths proportionally to the period and for Λ = 0.7 the band is the broadest.

Grating thickness and fill factor determine the intensity of modulation, or grating

strength. However, this strength cannot increase indefinitely and there is an optimum

point where the grating effect is the strongest with respect to reflectivity.

Figure 2.10 shows the effect of tg. For a very thin grating, the mirror is sharp and

the optimized bandwidth occurs for tg = 0.46µm. Above this value, the mirror gets sharp

again. As this parameter can be precisely controlled by epitaxial growth or plasma

deposition techniques, the optimized design can be easily fabricated.

Low

Inde

x Th

ickn

ess

(t L) (

µ m)

t L) (

µ m)

Fig. 2

Gra

ting

Thic

knes

s (


Low

Inde

x Th

ickn

ess

(t L) (

µ m)

t L) (

µ m)

Fig. 2

Gra

ting

Thic

knes

s (


Figure 2.10 - Reflectivity as function of wavelength and tg. The optimized bandwidth occurs for tg = 0.45 and it gets sharper it is further increased.

33

This parameter can be precisely controlled by epitaxial growth or plasma deposition techniques.

Figure 2.11 shows the effect of duty cycle. There are two reflection peaks for a

duty cycle of 0.5, one at 1.1µm and the other at 1.6µm. As duty cycle increases, the two

peaks merge to form one broad and flat reflection band. This parameter is probably the

most critical in fabrication as small variations in lithography can change the final value.

It may slightly affect the flatness of the band (if the duty cycle gets smaller, the two peaks

tend to separate) or its coverage (if duty cycle gets larger, mirror bandwidth decreases).

Fill

Fact

or


Fig. 2

Fill

Fact

or


Fig. 2

duty cycle

Figure 2.11 - Reflectivity as function of wavelength and duty cycle. When duty cycle is increased, two reflection peaks merge to form one broad and flat reflection band.

In this design, where lines are used, reflection is polarization dependent. This can

be advantageous to control the polarization on a VCSEL, e.g., if the grating design is

used for the mirrors. If a 2D grating is chosen instead, reflectivity would be polarization

independent. The grating sensitivity to all these parameters can be optimized iteratively.

If the application has a less stringent requirement on reflectivity, i.e. <99%, most of the

parameters have a large tolerance range, sometimes up to 10% variation.

34

2.2.b.1 Rigorous Coupled Wave Analysis (RCWA)

All SWG simulation was done by using RCWA [67]. In brief, this theory

considers an incident plane wave that refracts into the grating. The refracted plane wave

inside the grating is diffracted into an infinite set “i”, of coupled plane waves that

propagates toward the other boundary. These waves are represented through the

expansion of electric field into its space harmonic components. This expansion is

inhomogeneous and the individual components do not satisfy the wave equation, only the

infinite sum of these components does. Inside the grating, TE field expansion has the

form:

2( , ) ( ) exp[ ( ) ]y ii

E x z S z j k iK r= − −∑ (2.16)

where Si(z) is the normalized amplitude of ith wave field, K is the “grating

vector”, with direction along the index variation and amplitude 2K π= Λ , and 2k is the

wave vector inside the grating (region 2). By plugging (2.16) back into the wave

equation results:

22

0 02 2 2

1 11 12 2

sin1 sin22

cos cosexp 2 exp 2 0

ii

i i

d S i Sdz

j z S j z S

ε θ εθπ λ λ

ε εφ φπ πλ λ− +

− − − + Λ + + − = Λ Λ

(2.17)

and the components are coupled. Finally, the waves inside the grating are phase

matched into propagating and evanescent waves in the outside regions. Figure 2.12

shows a visual representation of the expansion.

35

θ

2σ

0σ

1σ

Λ

θ’

0

-1

+1

φ

+2

K

K

K

1−σ

K

θ

2σ

0σ

1σ

Λ

θ’

0

-1

+1

φ

+2

K

K

K

1−σ

K

Figure 2.12 – Representation of the field expansion inside the grating. A plane wave is refracted into the grating and then diffracted into space harmonic components. These components are coupled and phase matched to propagating and evanescent waves outside the grating.

The total electromagnetic problem has then to be solved. The differential

equations of the expansion are written in linear system format which can be solved in

terms of eigenvalues and eigenvectors. The coefficients matrix is infinite, as it results

from an infinite expansion, and should be truncate until convergence. The eigenvalues

represent the propagation constants of the various orders in the grating and the

eigenvectors determine the relative amplitude of the orders. The solution is finished by

applying Maxwell’s boundary conditions.

2.2.c Comparison between DBR and SWG

The comparison between DBR and SWG can be done under several different

parameters. However, as the goal is to have two mirrors to form an optical cavity, the

priority has to rely on growing these mirrors. Thus, constraints brought by epitaxial

growth or other deposition techniques should be taken into account and the very first one

is the thickness to be grown. Moreover, as tunable devices are the target, the mechanism

36

to tune is through electrostatic attraction against the restoration force of the beams. These

beams usually have the same thickness as the top mirror and the thinner they are, the

easier to tune. Therefore, the comparison parameter will be mirror thickness.

Using the same parameters that originated Figure 2.7, the SWG thickness would

have 1.29µm (tl=0.83µm and tg=0.46µm). A comparable DBR, made out in the same

material system and having pairs of Si/SiO2, would require 3 pairs and the two spectra are

superimposed in Figure 2.13(a). When the same data is shown with reflectivity in

logarithmic scale, it reveals that the SWG reflectivity is much higher and that it would

require at least double the DBR thickness to achieve the same reflectivity. However, the

DBR bandwidth is larger and smoother with wavelength.

In the case that we would like to integrate optical functions such as detection or

lasing, III-V compounds such as GaAs would be desirable. However, the low index

material for this system, AlOx, starts to peel off when integrated to the thin DBR layers

with mechanical actuation. Thus, if we compare a grating made out of GaAs/AlOx with

a DBR made out of GaAs/AlAs, the last one would have considerable less reflectivity for

the same thickness (2µm), as shown in Figure 2. 14. Comparable reflectivity would

require 40 DBR pairs or about 10µm of epitaxial growth at λ = 1.55µm.

37

1.0 1.2 1.4 1.6 1.8 2.0 2.20.0

0.2

0.4

0.6

0.8

1.0

Ref

lect

ivity

Wavelength (µm)

Grating DBR (3 pairs)

1.0 1.2 1.4 1.6 1.8 2.0 2.2

1

0.1

0.01

1E-3

1E-4

Grating DBR (3 pairs) DBR (6 pairs)

99.99

99.9

99

90

Ref

lect

ivity

(%)

Wavelength (µm)

(a) (b)

Figure 2.13 – Comparison between SWG and DBR made out of Si/SiO2 system. (a) 3 pairs of Si/SiO2 DBR spectrum superimposed to Figure 2.7. (b) Reflectivity in Log scale shows that at least 6 pairs would be required to yield comparable reflectivity between SWG and DBR.

1.0 1.2 1.4 1.6 1.8 2.0 2.21

0.1

0.01

1E-3

1E-4

1E-599.999

99.99

99.9

99

90

0

Ref

lect

ivity

(%)

Wavelength (µm)

Grating DBR - 8 pairs DBR - 30 pairs DBR - 40 pairs

Figure 2. 14 - Comparison between SWG and DBR made out of GaAs/AlOx and GaAs/AlAs, respectively. The same thickness as the SWG (2µm) would correspond to 8 pairs of DBR, which has considerable less reflectivity than the SWG. Same level of reflectivity would require 40 DBR pairs.

This comparison shows that even for short wavelengths, as in the examples, the

DBR may be prohibitive, depending on the desired reflectivity. The situation gets even

worse when scaling to longer wavelengths such as 10µm. The DBR just cannot be

38

realizable. If we take into account some other parameters such as loss, the DBR gives an

even poorer performance due to the longer optical path. Some other advantages of SWG

over DBR would be easy fabrication (easier growth), easy integration (any material

system can be used), scalability and smaller spring constant (thinner for the same

reflectivity). SWG can even replace DBR in static VCSELs.

Even though the SWG shows several advantages over DBR, it is still under

development and not integrated into devices. All work developed in this thesis still has

DBR mirrors only. In the next chapter, I will present fabrication data of the SWG.

2.3 Mechanical Structure

As seen in the previous section, a Fabry-Pérot optical cavity consists of two

mirrors separated by a distance l (Figure 2.1). If one mirror is displaced with respect to

the other, the transmitted wavelength also changes, according to (2.1) and Figure 2.2.

Thus, the mechanical structure has to be able to do this job: move the mirrors.

The simplest structure to move the mirror is a cantilever, such as the one in Figure

2.15. An air gap is formed in-between the mirrors (and the junction) by removal of a

sacrificial layer. The anchor is either protected during the sacrificial etching or has

dimensions much larger than the others involved, so that it can have some undercut

without compromising the mechanical structure. Mirror actuation is achieved

electrostatically by reverse biasing a pn-junction sandwiched between a movable mirror

and a stationary mirror. The distance between mirrors changes when voltage is applied

across the device, due to electrostatic attraction of opposite charges, accordingly

changing the transmitted wavelength.

39

V

d

Az

Fe

Fr

z

anchor

V

d

Az

Fe

Fr

z

anchor

Figure 2.15 – Schematic of the cantilever held mirror under a concentrated load.

Modeling of the electrostatic actuator is done through analogy to the parallel plate

capacitor. Because of the opposite charges on the two plates, there is an electrostatic

force of attraction between them. In fixed-plate electrical capacitors, we do not ever

think about this force, but it is always present whenever the capacitor is charged. This

force can be calculated by taking the derivative of the stored energy as function of

distance between the plates. Energy stored in a capacitor is given by:

2 2

2 2CV AVU

dε= = (2.18)

where C is the capacitance, V is the voltage across the plates, ε is the dielectric constant

of the interleaving material, A is the area of the plates and d is the distance between them.

Thus, the applied electrostatic force, Fe, due to an applied voltage V is

2

22eAVFd

ε= − (2.19)

The beam restoration force keeps the structure under equilibrium and is given by

Hooke’s law:

rF kz= − (2.20)

where k is the spring constant of the beam and “z” origin was chosen to be at the bottom

of the upper plate.

40

The net force on the upper plate at voltage V is

2

22( )net e rAVF F F kz

d zε= + = − −

+ (2.21)

At a point of equilibrium, Fnet is zero. If we now ask how Fnet varies with a small

perturbation to the gap, we can write

( ) ( )

2 2

3 3net

netV

F AV AVF z k zkz d z d z

ε εδ δ δ ∂ = = − > ∂ + +

(2.22)

If netFδ is positive for positive zδ , then the gap is an unstable equilibrium point,

because a small increase zδ creates a force tending to increase it further. If netFδ is

negative, then the gap is a stable equilibrium point. Hence, the expression in parenthesis

in (2.22) must be negative, which means that:

( )

2

3AVk

d zε>

+ (2.23)

and clearly, since the equilibrium gap decreases with increasing voltage, there will be a

specific voltage at which the stability of the equilibrium is lost. This is called the pull-in

voltage. At pull-in, there are two equations that must be satisfied: the original

requirement that netF is zero and the new requirement that

2

3( )pull in

pull in

AVk

d zε −

−

=+

(2.24)

Substituting (2.24) in (2.21) and solving for netF =0 gives:

3pull indz − = − (2.25)

Therefore, the movement of the top mirror is limited to 1/3 of the initial gap

(known as the “1/3 rule”). In practice, the movement will be slightly larger than 1/3

41

because of the distributed nature of the electrostatic force.

The parallel plate capacitor model can be further refined. The concentrated load

on the plate can be maintained and a distributed load can be added to the beam. This may

be convenient, and even required, for devices with small plate area and long beams, in

which case the area of the beam is comparable to the area of plate. However, as the beam

is always farther from the substrate than the plate, this effect is still not important even

for comparable areas, being significant only for beams with larger areas than the plates

[69]. Depending on the plate/beam geometry, it may be required that a distributed load is

considered on the plate.

The application of distributed forces has also to take into account that the distance

from the beam and plate to the substrate is not uniform. This distance varies with

position, what means that each point of the beam plus plate system experiences a

different force. Once this distance is allowed to vary with tuning voltage, there is no

analytical way to solve the displacement problem [70]. The numerical solution has to be

evaluated until convergence of the plate end.

Fringe effects can also be added to the model. However, these effects are only

important when the considered area is small when compared to its perimeter (or has one

dimension much smaller that the other). If the effects are included, the complexity

brought to the calculation does not compensate the added accuracy with respect to the

distributed force case (as the solution also has to be done numerically) [71].

In practice, the parallel plate capacitor model is used, so that the solutions are

analytical and give physical insight of how to adjust parameters. After the structure is

defined (in terms of its mechanical components such as beam length or width), a finite

42

element method software, such as FEMLAB, ANSY, MEMCAD, etc. is used to fine

adjust the parameters. All these softwares are very friendly to use and run fast enough on

PCs in order to justify the procedure. The one that I have used is FEMLAB.

Table 2-1 – Photonic MEM tunable devices and respective spring constants.

Structure Top view Cross section Spring Constant (rectangular beams)

Cantilever

3

3

14c

Et wkl

=

Bridge

3

316bEt wk

l=

Trampoline 3

332tEt wk

l=

Torsional 42.25t Gk

lθ =

(t ≈ w)

Parameters l = supporting beam length; t = supporting beam thickness; w = supporting beam width; E = material Young’s modulus; and G = material shear modulus.

For the purpose of defining the final structure, beams must be anchored

somewhere and the difference among the different proposed topologies reside mainly on

how to anchor them and on the used materials. Table 2-1 gives the top view, cross

section and spring constant for four different anchor types. The spring constants were

derived by applying elementary theory of beam bending together with the approximation

for small deflections [70]. A concentrated load, following the parallel plate capacitor

model, were considered and located at the filter optical path (circles in the figures of

43

Table 2-1) for the first three cases and at the counterweight for the torsional topology

(opposite to the optical path with respect to the anchors). This

approximation/simplification is the only way to yield analytical solutions for the spring

constants, as mentioned before.

The tuning range for all these photonic MEM devices is limited by the actual

allowed movement, which equals approximately 1/3 of the gap size. If the applied

voltage is increased beyond that point, the mechanical structure is pulled onto the

substrate and an electrical catastrophic discharge may occur upon physical contact [72].

This effectively limits the usable tuning range and may result in device damage if the

optical path of the device is located very close to the physical contact point.

Recently, there have been many publications addressing alternatives to overcome

the mechanical limitation of the 1/3 rule. The techniques include the use of parallel

external capacitor [73, 74], feedback control circuit [75], multiphase piecewise-linear

mechanical flexure [76] and restricting the charge distribution of the mechanical structure

[77, 78]. Lateral electrostatic actuators such as comb drives [79] can also implement

large travel distances The best result reported thus far shows that a mechanical movement

of 3/5 gap size can be achieved [78]. However, none of these alternatives is compatible

to III-V micromachining fabrication techniques and some of them even require the

fabrication of additional structures or electronics, with increased complexity in

fabrication.

In order to overcome the problem, a novel torsional tunable filter (TTF) is

proposed (schematics in Table 2-1) [15, 56, 80-82]. Under an applied tuning voltage, the

filter head moves away from the substrate rather than toward it in contrast to

44

conventional electrostatic MEMS. Hence, the torsional MEM structure eliminates the

catastrophic discharge problem that may occur when the micromechanical arm collapses

onto the substrate. Of course the TTF is also subject to the 1/3 rule1 but if any discharge

occurs it will not cause any damage to the optical path, because it will not occur on the

head. Because the discharge points are not in the optical path it is possible to fabricate

stops or holes at the critical positions in order to avoid any kind of discharge.

Furthermore, the tuning range can be made much wider due to the leveraging effect of the

torsional mechanical arm that allows the head to move up more than 1/3 of the gap. This

structure can provide an enhanced continuous tuning range.

The leveraging effect of the torsional device is shown in Figure 2.16 which was

obtained with FEMLAB. The movement of the device’s head is 1.3 times that of the

counterweight (leveraging effect = 1.3). Design details will be discussed in the next

section.

2.4 General Design Rules

In spite of the fact that research has being conducted on tunable devices for

several years, modeling was presented only for specific designs [50, 83, 84]. Moreover,

this modeling was limited to the description of what was done, not applied to a general

situation in order to serve as a guide for future applications. The aim of this section is to

show how to calculate filter parameters and scaling laws with wavelength for any of the

structures in Table 2-1.

1 Torsion electrostatic devices have their movement actually limited to an angle θ=0.44θmax. Nonetheless, I will keep using 1/3 rule as this expression is widely known and used even for this kind of device.

45

Figure 2.16 – Overcoming the 1/3 rule with a torsion actuated device. The leveraging effect of the torsional device allows a movement of the head 1.3 times larger than the counterweight.

First of all, most of the characteristics of AlxGa1-xAs are linear with x. Index of

refraction at the wavelength λ = 850nm varies from 3.60 for x = 0 to 2.99 for x = 1.

Bandgap varies from 1.43eV for x = 0 to 1.98eV for x = 0.45. Bandgap is indirect for x >

0.45. Mechanical characteristics can be considered uniform to a good degree of

approximation, and bulk GaAs values were used for FEMLAB simulations.

The design flow chart for any type of deflection beam MEMS filter is shown on

Figure 2.17. The desired application gives the requirements (grey boxes): central

wavelength (λc), tuning range (∆λtuning), resolution (∆λFP), maximum voltage (Vmax) and

tuning speed (resonant frequency, f0).

∆λtuning is ultimately limited by the DBR reflectivity bandwidth, as discussed in

section 2.2.a. The designer can choose among the various material systems in order to

have the necessary mirror stop band (step 1). The stop band determines the available

tuning range and the larger the difference in index of refraction between the materials

46

forming the DBR layers, the broader the stop band. The available tuning range as

function of central wavelength for different material systems is shown in Figure 2.18,

which were calculated using Transmission Matrix theory. Index of refraction for

different materials is listed by Palik [85]. AlGaAs system, which is very convenient for

optoelectronic devices, has very low stop band across the entire considered range. SiO2

has absorption peaks around 7µm and 10µm, being limited to short and middle range IR

applications. ZnSe system is usually deposited through evaporation and thickness control

is very difficult. MOCVD deposition of ZnSe in GaAs has been reported [86-92] but

surface roughness still causes excessive scattering. AlOx, obtained through wet

oxidation of AlAs, has poor mechanical properties and starts to peel off in time.

∆λtuning

∆λfp

λc

Vmax

fo

Width(w)

DBRpairs

Gap Size(d)

MaterialSystem

1

3 Thickness(t)4

FilterRadius (r)5

Length(l)6

TransmissionMatrix2

Mechanicalexcursion7

8

9

∆λtuning

∆λfp

λc

Vmax

fo

Width(w)

DBRpairs

Gap Size(d)

MaterialSystem

1

3 Thickness(t)4

FilterRadius (r)5

Length(l)6

TransmissionMatrix2


8

9

∆λtuning

∆λfp

λc

Vmax

fo

Width(w)

DBRpairs

Gap Size(d)

MaterialSystem

1

MaterialSystem

1

3 Thickness(t)4

Thickness(t)4

FilterRadius (r)5

FilterRadius (r)5

Length(l)6

Length(l)6

TransmissionMatrix2

TransmissionMatrix2



8

9

Figure 2.17 - Design flow chart for any type of deflection beam MEMS filter. Grey boxes show the requirements.

47

2 4 6 8 100

1

2

3

4

5

6

Wavelength (µm)

Tuni

ng R

ange

(µm

)

GaAs/AlOx

ZnSe/MgF2

ZnSe/CaF2

Si/SiO2

Si/TiO2

AlGaAs

Figure 2.18 - Tuning range versus wavelength for different material systems.

Next step (2) is to calculate the overall effect of the DBR mirrors using

Transmission Matrix theory. This calculation determines the number of DBR pairs (step

3) needed to achieve the required linewidth, which is function of mirror reflectivity and

then function of the number of DBR pairs. A different number of pairs may be required

for top and bottom mirrors, as the top mirror has two interfaces with air and the bottom

mirror has one interface with air and another one with the semiconductor substrate. This

gives a smaller reflectivity for the bottom mirror and it needs larger number of pairs than

the top one for the purpose of matching reflectivity. If the top and bottom reflectivities

are not matched, the filter transmission will be substantially smaller than theoretical

unity, as given by (2.11).

The filter beam thickness (t) can then be determined (step 4) based on the number

of DBR pairs. The minimum value is the sum of all the layers above the gap. However,

bulk material can be added to the beam or the DBRs can be etched out of the optical path

if the beam needs to be stiffer or softer. This would make the fabrication process much

48

more complex and the designing freedom may not compensate. The discussion that

follows assumes the anchor beam with the same thickness as the filter to make processing

easier.

The filter radius (r) is dependent on λc as different wavelengths have different

spot sizes (step 5). At 1.55µm, the radius can be determined based on direct coupling

from a diffracted beam coming from an optical fiber. If the fiber is placed 50µm away

from the filter and has a mode field diameter of 9µm, the spot at the filter would have a

diameter of approximately 15µm and then a 20µm diameter for the filter is prudent.

Similar calculations can always be performed having in mind the desired spot size to be

filtered and the optical system to be used. Scaling is limited by diffraction loss. In the

case of circular areas and same optical system, the radius, r, should increase to keep the

ratio r2/λd constant [93], where d is the effective gap size between the mirrors. A first

good approximation for the gap size is λ, which has a good order of magnitude for an

initial guess.

Voltage and speed, for most applications, are not as stringent requirements as the

optical ones. In many cases they may not be a requirement at all. However, they can

always provide good parameters to the designer. The beam length (l) can be determined

based on f0 (step 6), as f0 for deflecting or torsion beams is independent of width (w):

3

20 3

0 0 0 42

Et wkk Etlf km twl l

ω πρ ρ

= = = = (2.26)

where k is the beam spring constant, k0 is a factor dependent on topology (1/4 for

the cantilever, see Table 2-1), m is the mass of the beam, E is the material’s Young’s

modulus and ρ is the density. As length is proportional to the inverse of square root of

49

frequency, this calculation will give an upper bound and beams smaller than the

calculated will always be faster to the expense of larger voltages.

The last parameters to be determined are d and w. The designer has some degree

of freedom to choose them. At λc, the gap should be an integer multiple, n, of λc/2. The

gap is also dependent on ∆λtuning, which determines the mechanical excursion that the

filter's top DBR needs in order to cover the entire range. The total excursion can be at

most 1/3 of the total gap, as given by the 1/3 rule (2.25). So, the gap should be a little

larger than a multiple of λc/2 and this amount is determined by ∆λtuning (excursion). At

zero applied voltage, for all closing gap actuators, the larger extreme wavelength (λmax)

will be transmitted and at 1/3 of the gap, the shorter (λmin) will be transmitted. An

intermediary position will transmit λc and this position has to have a gap multiple of λc/2.

The way this mechanical range is determined is by solving the matrix equation given by

the transmission matrix theory for the layer structure previously determined (material

system and number of DBR pairs on each mirror) and calculating Fabry-Perot

wavelength as function of gap.

The gap d is also dependent on the filter radius because the larger this radius, the

more difficult to proceed the undercut etch during fabrication. Gap sizes smaller than

1µm may limit etchant and by products diffusion. A good parameter in this case is to

keep the ratio r/d ≤ 5. This will facilitate the diffusion of reactants and guarantee that the

selective etch is reaction limited.

Finally, Vmax determines how large the integer n can be. To keep the voltages as

low as possible, what is always preferable, n should also be the smallest value allowed by

the ratio r/d ≤ 5 (small gap provides larger electrostatic force). For large gap sizes,

50

besides the inconvenience of large voltages, multimodes can appear across ∆λtuning.

Based on this, the designer can choose the gap size (step 8).

After the gap is chosen, the filter area should be revised to make sure the

diffraction loss scaling was correctly done. If the gap is different from the one used

before, the designer may want to change the filter radius and also the beam length. If the

newly determined gap is larger, by increasing the beam length the required voltage may

be decreased.

Vmax and geometrical dimensions determine the width (step 9) solving (2.21) for

0netF = and max - ( / 3) pull inV V z d V= = − = :

32

2 0 32

2 20 0

/3/3

2 ( )2 ( )( ) ( )4 4

pull in

z dz d

Et wk z d zkz d z lV wl wlr rε π ε π−

=−=−

++= − = −+ +

(2.27)

where the coordinates are still referred to Figure 2.15 and ε0 is air permittivity. The terms

inside parenthesis in the denominator relate the effect of the beam and head areas to the

deflection due to an applied voltage but the spring constant was calculated by considering

a force applied only at the head. Note that if the head area is too large when compared to

the beam area, the deflection will be independent of the beam width. If the value

obtained for w is too small for the available lithography, a trade-off among ∆λFP, f0 and

voltage may be necessary to increase w.

The device can then be simulated through a finite element method software and

fine adjust the mechanical parameters.

By following the above rules, the different topologies of mechanical structures

can be easily compared. If we take the first three types from Table 2-1, we readily see

51

that the simple cantilever is the easiest to fabricate. It is also the less affected by residual

tension among the epitaxial layers because it has one end totally released. Looking at the

spring constants in Table 2-1 and to (2.27) we can conclude that the simple cantilever is

the one that requires the smallest voltage for a similar layer structure. The advantage of

the other two types is to keep the surfaces parallel while the top mirror is moving.

However, the tilt angle is very small across the head and has been proved not to affect the

simple cantilever [94].

2.4.a Torsional Design

Everything that was developed in the previous section is also valid for the

torsional structure. However, it has some particularities that have to be addressed.

First of all, the design has much more parameters than discussed before. The

schematics shown in Table 2-1 and Figure 2.16 reflect our particular approach to the

problem and the arrival at this design happened only after careful consideration of other

alternatives. The following discussion utilizes parameters as defined in Figure 2.19.

The counterweight has to have large enough area so that it has much larger

attraction to the substrate than the rest of the frontal beam (and the head diameter has to

be small for the opposite reason). However, it has to be released and thus one has to

follow the r/d ≤ 5 rule of thumb. Etch holes are used to allow etchant a path underneath

the large structure to be released and allow for quicker and more complete undercut.

These holes soften the counterweight and decrease the twisting moment.

52

head

head beam

torsion beams

counterweight beam

counterweight

anchoranchor

x

y

z

head

head beam

torsion beams

counterweight beam

counterweight

anchoranchor

head

head beam

torsion beams

counterweight beam

counterweight

anchoranchor

x

y

z

Figure 2.19 – Definition of the terms used in the torsional structure.

The beam that connects the counterweight to the torsion beams is another design

parameter with a lot of freedom. This beam has to balance a trade-off between

leveraging effect and twisting moment. If the beam is too long, the 1/3 rule limits the

movement of the counterweight and the leveraging effect to the head may be not seen.

Increasing the head beam does not work because that softens its spring constant and it

bends into the substrate direction. If the beam is too short, the twisting moment may be

too small and high voltages may be required (or the device may even not work at all).

Finally, the torsion beams can be softened by being thin and long. This also has

the counterpart of allowing the entire structure to be pushed into the substrate when

voltage is applied. Those beams need bending stiffness and torsion softness. Of course

all other beams also need bending stiffness.

Thus, the design flow chart of Figure 2.17 is still valid for the torsional structure.

However, the designer has to keep in mind that the mechanical excursion is related to the

head (and wavelength shift is from λmin at 0V to λmax at Vpull-in) and the gap is related to

53

the counterweight. Also, length and width in the diagram are related to the torsion

beams. The other parameters have to be determined by a finite element method software

because the bending of the entire structure has to be taken into account. When voltage is

applied, the torsion beams and counterweight bend down, along x direction, and the beam

that connects the head to the counterweight gets curved along y direction.

After optimization of the parameters, using FEMLAB, two test structures were

defined and the dimensions are given in Table 2-2, according to the definition given in

Figure 2.19. Leveraging effect is defined as the ratio of head to counterweight

displacement. Optimization was carried out with the objective of maximizing the

leveraging effect. For example, as the head beam length is increased, at first the

leveraging increases due to a longer lever arm causing larger total deflection. As the

mirror length is increased further it begins to bend towards the substrate. Thus an

optimal value lies somewhere in between these lengths.

Table 2-2 – Dimensions (µm) for the torsional test structures.

Parameter Structure A Structure B

Thickness (all beams) 4.8 4.8

Head diameter 15 15

Head beam 185x5 200x5

Torsion beams 80x5 90x3

Counterweight 300x44 300x44

Counterweight beam 40x13 30x13

Counterweight holes 30x20 30x20

Leveraging effect 1.3 2.0

Structure A in Table 2-2 is on the “safe side” of the design, with dimensions that

very likely will successfully get to the end of the fabrication process. However, it does

54

not give the same leveraging as Structure B, which has longer and narrower torsion

beams, shorter counterweight beam and longer head beam. “B” is more to the

“challenging side”, mainly due to the small width of the torsion beams.

2.5 Summary

This chapter has covered the basic concepts in the design of micromachined

tunable optoelectronic devices. This has included Fabry-Pérot optical cavity and its

realization in compatibility with optoelectronic fabrication. Two mirror options were

described to form the optical cavity: DBR or SWG. The discussion on mechanical

modeling has presented the 1/3 rule and described the actuation of the devices as well as

refinements to the parallel plate capacitor model. In the sequence, general design rules,

in the form of a flux diagram, were presented to guide the way that devices should be

realized. Particularities of the torsional structure were described at the end of the chapter.

After the design has been explained, we can move deeper into the subject of fabrication.

55

Chapter 3 Fabrication

3.1 Introduction

Fabrication of tunable devices turned out to be an elaborate process when

compared to the fabrication of the non-tunable counterparts. In order to document the

fabrication process itself as well as the challenge of making good devices, I dedicate an

entire chapter to the description of the various steps involved in the realization of the

final device.

Design variations affect the final performance and may require very laborious

fabrication. As an example, extremely low actuation voltage may be required for a

specific application (requiring soft beams) that also requires very sharp transmission

linewidth (requiring very high reflectivity thick mirrors). Obviously, the beams and

mirror cannot have the same thickness and the alternatives may be to etch part of the top

mirror that is not in the optical path and even deposit metal on the mirror to increase its

reflectivity [50].

This chapter also describes the fabrication and optical performance of the

subwavelength waveguide grating (SWG) mirror, which was done in silicon for the

purpose of demonstration only. Then we jump into the general methods that utilize III-V

compounds micromachining techniques, describing the various processes that we have

used and adjusted in order to accomplish device fabrication.

3.2 Subwavelength Waveguide Grating (SWG) Fabrication

This fabrication was done in order to experimentally demonstrate the mirror

56

characteristic and to validate the theoretical simulations presented in Chapter 2. The

fabrication was carried out by Lu Chen at the Cornell Nanofabrication Facility.

The grating was fabricated on a bare silicon wafer. First, a wet silicon dioxide

layer was grown at 1100°C. Undoped poly-silicon was deposited on top of the oxide at

600°C and a second oxide layer was grown by plasma enhanced chemical vapor

deposition (PECVD) on top of poly to serve as mask for posterior etching of the grating.

E-beam lithography on poly(methyl methacrylate) (PMMA) was used for lift-off the

metal (200Å Cr/80Å Au) that served as mask to pattern the top oxide, which was then

etched by reactive ion etching (RIE). Metal mask was removed and poly was also etched

by RIE. Final measured thicknesses were tg = 0.40µm and tL = 0.58µm. Different

periods and duty cycles were fabricated to also validate the theoretical simulation of the

previous chapter (off course, oxide and grating thickness was the same for all devices)

Figure 3.1 shows the scanning electron microcospy (SEM) picture of one fabricated

grating.

Figure 3.1 – SEM picture of the fabricated sub-wavelength grating. Grating is formed by poly-silicon and air on top of silicon dioxide.

Several designs were fabricated. We had three different grating periods, 0.7µm,

0.8µm and 0.9µm and eight grating duty cycles, ranging from 40% to 83%. Only one of

the gratings (out of 24) gave the broadband mirror effect. This shows that the mirror only

57

works for a set of optimized parameters, which in this case was determined by tg, nh, tL

and nL for all fabricated structures. Once these parameters were the same for all of

devices, only a specific combination of Λ and duty cycle gives the desired effect. This

confirms the previous theoretical prediction from chapter 2 [53].

Figure 3.2 shows the theoretical contour plot of reflectivity as a function of

wavelength and duty cycle. The parameters in this simulation are: Si substrate (n=3.48),

Λ = 0.7µm, nh = 3.48 (Poly-Si.), low index material in and above the grating = 1 (air), nL

= 1.47 (SiO2), tg = 0.4µm and tL = 0.58µm. Note that both tg and tL are the measured

values. Variation of index of refraction with wavelength [85] is also included and the

above numbers are average values, for reference only. The simulation is based on

RCWA [67]. Broadband is achieved for a duty cycle of (68±2) %. The reflection band is

still broad outside this range but with slightly smaller reflectivity.

Dut

y C

ycle

(%)

Wavelength (µm)

Fig. 3.4b

Fig. 3.4a

Fig. 3.4c

Dut

y C

ycle

(%)

Wavelength (µm)

Fig. 3.4b

Fig. 3.4a

Fig. 3.4c

Figure 3.2 – Calculated contour plot showing reflectivity as function of wavelength and duty cycle. The broadband effect is achieved for a duty cycle of (68±2) %.

The optical characteristics of the gratings were measured and the optical

measurement setup includes a tungsten halogen light source, bifurcated fiber bundle,

collimation lens, Glan-Thomson polarizer, focusing lens (NA = 0.1) and an optical

58

spectrum analyzer (OSA). Figure 3.3 shows the optical setup. The output of the light

source is coupled into the bifurcated fiber bundle that has the common end aligned with

the polarizer, focusing lens and grating, respectively. Reflected light from the grating is

coupled back into the bundle and fed to the OSA.

SWG wafer

Bifurcated fiber bundle

Collimation lens

Optical Spectrum Analyzer

Polarizer

Focusing lens

White light

SWG wafer

Bifurcated fiber bundle

Collimation lens

Optical Spectrum Analyzer

Polarizer

Focusing lens

White light

Figure 3.3 – Optical measurement setup for the SWG characterization.

Figure 3.4 shows the reflectivity as a function of wavelength for three different

duty cycles (other parameters as given above). The SWG traces were normalized by the

reflection of a silver coated mirror (rated ≥ 98.5% reflectivity from 1.1µm to 20µm) in

order to eliminate the influence of the blackbody spectrum from both optics and source.

The dB scale is with respect to the silver coated mirror (R=98.5% ≡ 0dB). Figure 3.4a

shows a very broad bandwidth, 1.12-1.62 um, with R>98.5% (0dB would be the same

reflectivity as the silver coated mirror), achieved with the duty cycle of 66% for light

polarized perpendicularly to the grating lines. This is the widest bandwidth (∆λ/λ>35%),

high reflectivity mirror reported by a grating structure. Furthermore, this measurement is

59

currently limited by the OSA spectra and low power density of the source (bulb color

temperature is 2,960K).

Excellent agreement was achieved between simulation and experimental results.

Longer wavelengths have an increase in the noise level due to the fast decay of the power

density of the source. Small variations on parameters, such as index of refraction or

grating uniformity, slightly shift and flatten the curves. The values for thicknesses and

duty cycle are different from optimal design and slightly alter the performance but

reflectivity is still high and very broad. Roughness of the poly-silicon layer (see Figure

3.1) is another factor of optical performance degradation. These small variations can be

easily improved. Figure 3.4b and c show spectral reflectivity for light polarized

perpendicularly to the grating lines and duty cycles of 48% and 83%, respectively. All

experimental results agree with theory and the trend shown in Figure 3.2 can be easily

identified.

Figure 3.5 shows spectral reflectivity for light polarized parallel to the grating

lines and duty cycle of 66%. There is no broadband mirror for this polarization and

agreement between simulation and experimental results is also excellent.

Analyzing the results we confirm that the spectral position of the reflection band

can be precisely located as it is determined by Λ [53], which can be controlled very

accurately by lithographic methods. Duty cycle is probably the most critical parameter as

small variations in lithography, etching or surface roughness can change the final value.

Both grating and oxide thicknesses are also critical but once these values are known, as

given by the growth method, the other parameters can be designed a posteriori and

comply.

60

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8-8

-6

-4

-2

0

Ref

lect

ivity

(dB

)Wavelength (µm)

Experimental

Simulation

-8

-6

-4

-2

0

(a)

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8-8

-6

-4

-2

0

Ref

lect

ivity

(dB

)

Wavelength (µm)

-8

-6

-4

-2

0

Simulation

Experimental

(b)

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8-8

-6

-4

-2

0

Ref

lect

ivity

(dB)

Wavelength (µm)

-8

-6

-4

-2

0

Experimental

Simulation

(c)

Figure 3.4 – Reflectivity as function of wavelength and duty cycle for light polarized perpendicularly to the grating lines (Λ=0.7µm). (a) Duty cycle of 66% gives very broad bandwidth, 1.12-1.62 um, with R>98.5%. (b) Duty cycle of 48%. (c) Duty cycle of 83%.

61

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8-8

-6

-4

-2

0

Ref

lect

ivity

(dB)

Wavelength (µm)

-8

-6

-4

-2

0

Experimental

Simulation

Figure 3.5 – Reflectivity as function of wavelength for light polarized parallel to the grating lines (duty cycle=66%, Λ=0.7µm). There is no broadband mirror effect in excellent agreement with simulations.

This demonstration is the first experimental result of a sub-wavelength grating

that under normal incident light has very high reflectivity and broad reflection spectrum

(R>98.5% and ∆λ/λ > 35%), and is in excellent agreement with theory [53, 67]. This

mirror has potential application for several active and passive devices such as micro-

electro-mechanical tunable devices, visible and infrared wavelength VCSELs and

reconfigurable focal plane arrays. The next step along this line is to develop devices,

initially with a bottom DBR and a top grating mirror and further with two grating mirrors.

3.3 AlGaAs Micromachining Fabrication

Optoelectronic fabrication is a mature technology for III-V compounds, from

which AlxGa1-xAs is the basic ternary compound of the work shown here. GaAs is one of

the most common compound semiconductors. Its good optical properties (including

direct bandgap) make it useful for optical devices. It is also used in specialized

applications in which high electronic speed is required.

62

GaAs has a zincblende crystal structure, which differs from the diamond structure

only in that it has two types of atoms while diamond has only one type (silicon has

diamond structure). The important feature of the structure is that the atoms are joined

together to form a tetrahedron. Each Ga has four As nearest neighbors and vice-versa. In

the ternary compound, AlxGa1-xAs, Al replaces Ga atoms in the proportion given by x and

the lattices constant is practically unchanged (GaAs has lattice constant of 5.65Å and

AlAs of 5.66Å)2.

Mechanical and piezoelectrical characteristics are very good [95, 96]. Even

though GaAs has lower fracture toughness than silicon (about one half), it is still 3 times

better than most construction steel. Piezoelectrical effect is close to quartz. Thermal

resistivity is about 14 times higher than Si.

3.3.a Method

Even though most of the ideas of micromachining are derived from the now

mature area of surface silicon MEMS (micro-electro-mechanical systems), there are some

fundamental differences between Si and GaAs processing. First of all, GaAs is grown

epitaxially, resulting in single crystal, while Si is usually deposited by chemical vapor

deposition, resulting in poly-crystals. Internal stress in Si can then be controlled by

annealing the deposited layers while GaAs can only control stress through lattice

matching. Another fundamental difference is that there is no good mechanical oxide

compatible to GaAs as there is SiO2 compatible to Si. AlOx, which can be easily formed

by wet oxidation of AlAs, has very poor mechanical qualities, builds stress after

oxidation and starts to delaminate when actuated. Surface Si MEMS utilizes the 2 Even this small lattice constant variation may give rise to built-in stress that may cause structures to buckle after release.

63

paradigm of repeating cycles of deposition, patterning, etching and mask removal to form

the structures. This paradigm allows complex structures such as hinges and rotors to be

realized. GaAs, as to be described below, is still lacking a reliable regrowth technique

and the entire structure, optical and mechanical, has to be grown at once. Hence, GaAs

processing has to be done from top to bottom. Thus, wafer design has to incorporate all

layers that make the necessary processing sequence feasible and the final structure has

horizontal uniformity. Once a wafer is grown, it is dedicated to that processing sequence.

The great advantage that GaAs has over Si is that it can incorporate optical

functions, what is impossible for Si due to its indirect bandgap.

The processes presented in this section are all the ones necessary to carry on III-V

micromachining fabrication. I carried on the fabrication at the Berkeley Microfabrication

Laboratory (Microlab) and at Bandwidth9, Inc. The SWG, previously described was

fabricated at the Cornell Nanofabrication Facility, by Lu Chen.

After this brief introduction on some properties and particularities of AlGaAs

processing, the remainder of this section is dedicated to describe all processes used for

III-V compound micromachining fabrication. Cleaning before processing is done by

dipping the die in BOE 10:1 for 10s (removing any surface oxide), rinsing in DI water

and drying by blowing with N2. By the end of each subsection, the recipe used for the

particular process will be provided.

3.3.b Epitaxial Growth

AlxGa1-xAs is grown through epitaxial growth, which is a process where a thin,

single crystal layer is grown on the surface of a single crystal substrate. In the epitaxial

process, the single crystal substrate acts as the seed, although the process takes place far

64

below the melting temperature. Deposition of AlxGa1-xAs on GaAs substrate is termed

heteroepitaxy as the materials are not the same, but single crystal growth results and the

number of defects at the epitaxial-substrate interface can be very small.

The growth task is very complex. Recently, it has turned to be an independent

science and the final quality of the results is directly proportional not only to the

knowledge but also of to exclusive dedication to this task. Thus, it is very common to

have the wafers acquired from the market.

Epitaxial growth is done mainly by molecular beam epitaxy (MBE) or metal

organic chemical vapor deposition (MOCVD). The wafers that were used in this

dissertation were all grown by the first method. MBE has the capability of growing

device quality layers of semiconductors with atomic resolution of the growth thickness.

Basic requirements for MBE systems are ultrahigh vacuum (~10-10torr), in situ heating

and cleaning, and independently controlled sources for all materials and dopants. The

substrate is rotated during growth to ensure good uniformity. Sources have to have

on/off control capability, necessary to the atomic resolution, what is usually realized

through shutters in front of the cells.

The growth rate of an MBE system is simply determined by the flux of atoms

leaving the source and the fraction of those atoms that strike the wafer and stick to it.

Arsenic source is usually kept at high temperatures so that the growth is limited by the

arrival of Gallium atoms. Under normal growth conditions, where the growth

temperature is less than approximately 640°C, the surface sticking coefficient of Ga

atoms is essentially 100% [97]. However, it is a common practice to grow AlGaAs bulk

materials at elevated temperatures, significantly higher than 640ºC. The theory behind

65

this practice is that, at high growth temperatures, oxide impurities become more volatile

so that their sticking to the growth front becomes less probable. This way, it is possible

to minimize oxygen concentration at the interfaces [98] and reduce non-radiative

recombination currents [99].

High growth temperature has direct implications on how to grow a structure that

is very sensitive to the exact thickness and composition of the material. Vertical cavities

are certainly one of the applications where accurate growth rate calibrations are critical in

this growth regime. GaAs growth rate is a very sensitive function of the exact substrate

temperature, which can vary by several degrees throughout the cavity growth. Thus,

monitoring by reflectivity turns to be very poor, which is also allied to the interference

effect created by the DBR’s [97]. Of course, thicker structures are worse to control and

there is a limitation to which the designer has to comply.

GaAs doping scheme also has its limitations. Beryllium is usually used as p-

dopant and it has low solubility in high Al content material. By decreasing the growth

temperature Be solubility can be increased (because it reduces diffusion and atoms stay

where they are adsorbed) but oxide incorporation also increases. Carbon is an alternative

to Be, combining high doping density and low diffusivity. Although C is a group IV

atom, carbon actually acts as a p-type dopant in AlGaAs. Silicon is the most widely used

n-type dopant in MBE. Silicon can actually be either an n-type or a p-type dopant (so-

called amphoteric dopant), depending on the growth condition, the surface crystal

orientation and V/III ratio, etc. Under normal growth conditions and low-to-medium

doping densities in AlGaAs, Si atoms occupy the group III lattice sites and serve as

donors. At high doping densities, however, the probability of Si occupying group V sites

66

becomes higher, and some Si atoms start to become acceptors. This process limits the

highest Si doping density achievable to be in the mid 1018cm-3 in GaAs.

Finally, regrowth is a very difficult task. Whenever the processed surface is

exposed to air, aluminum reacts with oxygen to form a stable oxide that is very difficult

to remove. Subsequential growth will eventually have excessive number of defects. A

lot of effort has been spent on different approaches to realize DFB gratings but no

technique has yet proven to succeed. Thus, the entire structure, optical and mechanical,

has to be grown at once and processed later (top to bottom processing as mentioned

before).

The wafers that we have used were all acquired from the market. However, the

ideas presented can provide the designer with some basic knowledge of the method, so

that the limitations are respected and the best cost/benefit can be achieved. The high

prices (~US$5k/wafer) do not leave margin for trial and error.

3.3.c Optical Lithography

Lithography is the most critical process in semiconductor device fabrication. As

dimensions get smaller and machines and masks more sophisticated, lithography costs

increase and now accounts for about one third of total fabrication cost in the IC industry.

Hopefully, dimensions involved on III-V micromachining are not so critical. The

optical characteristics impose vertical dimensions usually larger than 5µm (optical

confinement layers, mirrors, etc.) and an aspect ratio around 1 is usually adopted. This

way is possible to strength mechanical characteristics and make processing and

manipulation more robust.

The smallest dimension that I have used was 3µm, and conventional projection or

67

contact methods are both suitable to give this precision. The exception is the SWG

mirror that had dimensions of 0.2µm and had to use e-beam lithography.

I have used both projection (Canon Fine Pattern Projection Mask Aligner - FPA-

141F, 4X reduction from mask) and contact (Quintel Q4000 Mask Aligner) aligners.

Either one has advantages and disadvantages. The most uniform patterns were generated

with Canon, because of the more convenient wafer chuck that can easily hold small dies

(usually 1x1cm2). Quintel can be adapted to hold small dies, but the contact is usually

nonuniform as it is designed for 4 inches wafers. Small dies do not give the necessary

contact area and the usage of dummies around the die under processing can improve but

not solve the problem. Tiny gaps between mask and the wafer surface during exposition

are sufficient to modify the pattern. Projection aligners have better tolerance to this

eventual tilt, limited by the depth of focus.

Both aligners are able to resolve features as small as 1µm. However, Quintel has

a friendlier, less tedious alignment system but requires an extra exposure to remove the

edge beads.

Two different positive photoresists were used, S1818 and SJR5740, both from

Shipley. The first one is for standard usage and the second is thicker, designed for high

aspect ratio surfaces. Spinning at 4krpm for 30s, S1818 has a final thickness of 1.8µm

and SJR5740 of 7.5µm, both measured after softbaking. SJR5740 requires much longer

exposure and development times.

The recipe used for lithography is given below:

1. Bake @90ºC, 5min (dehydration).

2. Spin photoresist (either S1818 or SJR5740) @4000rpm, 30sec.

3. Softbake S1818@90ºC, SJR5740@110ºC, 5min.

68

# 4 to 8: Quintel only.

4. Edge bead removal: exposition time to yield 450mJ/cm2 for S1818 and 2250mJ/cm2 for SJR5740.

5. Development: S1818: MP Dev (Microposit Developer, from Shipley):H2O (1:1), 30sec, shaking;

SJR5740: pure MP Dev, 5min, shaking.

6. Dip inside water for at least 10 sec.

7. Blow dry with N2 thoroughly.

8. Bake @90ºC, 5min (dehydration).

9. Exposure: Quintel: exposition time to yield 150mJ/cm2 for S1818 and 750mJ/cm2 for SJR5740;

Canon: intensity of 6.1 for S1818 and 11.9 for SJR5740.

10. Development: S1818: MP Dev (Microposit Developer, from Shipley):H2O (1:1), 30sec, shaking;

SJR5740: pure MP Dev, 5min, shaking.

11. Dip inside water for at least 10 sec.

12. Blow dry with N2 thoroughly.

13. Hardbake S1818@110ºC, SJR5740@120ºC, 10min.

3.3.d Metal deposition – liftoff

Unlike silicon, which uses implanted contacts almost exclusively, ohmic contacts

in GaAs and other compound semiconductors are alloyed [100]. The layer where the

contacts are going to be deposited is always heavily doped (>1018cm-3). To form an

ohmic contact to n-type GaAs, Ni/Au/Ge is the most common alloy [101]. Ge diffuses

into GaAs, occupying the Ga sites and doping the GaAs heavily n-type. Au reacts with

substrate Ga to form various alloys, leaving behind a large concentration of Ga vacancies

[102]. Annealing is done at 450ºC for 10 minutes. However, diffusion of Ge into GaAs

is fast and difficult to control. Thus, a thin Ni barrier is deposited in between the Ge

layer and substrate in order to keep Ge diffusion limited to the surface. Another Ni

barrier is deposited on top of Au to separate the alloyed metal from the surface metal that

69

will be wire-bonded. Ohmic p-contact is done by standard evaporation of Ti/Au. As the

junctions are deep enough, no diffusion barriers are required between Ti and Au [103].

Contacts are deposited through the technique known as liftoff. A photoresist

layer, thick enough to cover all features on the wafer surface, is deposited and patterned.

The patterning is done in such a way to remove the photoresist from the places where the

metal is supposed to be deposited. The rest of the wafer is left covered and metal is

deposited using evaporation. If a nearly vertical wall profile is obtained in the photoresist

“holes” or the profile is reentrant, a break in the metal is virtually assured. Next, the

wafer is immersed in a solution capable of dissolving the photoresist (usually acetone).

The metal that was deposited directly on the semiconductor remains, while the metal

deposited on the resist lifts off.

A reentrant profile on the photoresist can be obtained by soaking (~2min for

S1818 and ~3min for SJR5740) the photoresist in chlorobenzene before development

[104]. The soak process reduces the dissolution rate of the upper surface of the resist and

after developing the pattern a ledge appears. The thickness of the ledge can be controlled

but the process is very complex and depends on soak time, temperature of the

chlorobenzene bath and resist prebake cycle [105].

Metal evaporation in the Microlab has been recently done using Edwards e-beam

evaporator. Special care has to be taken with excessive heating inside the chamber

during operation; otherwise the resist may be superhardbaked and will not dissolve

anymore. Some steps that has proven to help the process are: i) do not hardbake the resist

before evaporation (this avoids resist reflow and keep vertical walls and ledges from

chlorobenzene soak); ii) place the die to the sides of the holder in the evaporator chamber

70

and do not rotate it (this will create an angle between the source and the die and hide

some of the resist walls by shadowing – bad uniformity is good!); and iii) control the

temperature inside the chamber, stopping the process whenever it reaches the softbake

temperature of the resist in use.

The metallization used for the contacts is given below, starting from the substrate:

N-contact: Ni (150Å), Ge (250Å), Au (500Å), Ni (150Å) and Au (1500Å).

P-contact: Ti (200Å) and Au (2000Å).

3.3.e Vertical Etch

Vertical etch can be done either through wet or dry techniques. The desired

feature is defined through lithography and photoresist mask (and also metal already on

the wafer) is used to realize the mesas. Areas that are not protected are etched, leaving

the protected areas as mesas. Important control parameters are side-wall verticality, etch

uniformity (across the wafer), repeatability, material dependence, and monitoring

capability.

3.3.e.1 Wet Isotropic Etch

Wet etch is simply done by immersing the sample into the chemical solution.

Exposed material is removed by a chemical reaction between the sample and solution.

AlGaAs compounds are isotropically etched by a solution of sulfuric acid, hydrogen

peroxide, and DI water. Water is added to weaken the solution and slow down the

etching rate. Usual composition is H2SO4:H2O2:H2O ≡ 1:8:40, by volume, and

sometimes even 80 parts of water were used in order to make monitoring easier.

Side walls are not vertical and the slope is function of the isotropic nature of the

71

process. Etching is reaction limited and non-directional on exposed surfaces, occurring at

the same rate both laterally and vertically. Hence, the upper portion of the side-walls is

always etched more with respect to the lower portion and results in undercut of the

masks. The undercut has the exact same amount of the vertical etch. The sizes from

lithography are also reduced and the walls are not vertical. The size reduction is not

important on large features. However, beams that are 5µm wide may be severely

affected as this dimension is smaller than typical vertical etchs (7-8µm). Masks should

be designed to take the undercut into account if this method is going to be used.

Figure 3.6 – SEM picture from a mesa etched by wet solution. Note the slope of the walls due to mask undercut.

Etch uniformity is very good but goes faster at the corners of the die, leaving a

slightly oval surface behind. The non uniformity for 1x1cm2 dies is usually of the order

of 2 DBR pairs, meaning that the corners have two pairs less than the center of the die.

Repeatability is not very good. Etching rate is strongly dependent on temperature

and peroxide strength. Temperature is easy to control and special care should be taken

not to etching close to hot plates, which can increase etching rates. Peroxide strength is

also very difficult to control as it can change during a batch process of several dies.

Finally, etch only stops after the etchant is diluted in DI water, when some precious

72

seconds of overetch can take place.

AlGaAs compounds etch essentially at the same rate, what is very important for

the formation of mesas with layered structure. Otherwise the mesas would have

undercuts across different layers.

This etch has some monitoring capability that is strongly dependent on the

operator. The color of the substrate changes as different materials are being exposed. As

the structure is known from the epitaxial growth, the operator has to count the number of

changes and to correlate this to the DBR pairs. The initial gray color changes to red-

purple-green-gray during etching of one pair.

Typical etching rates were 2.0µm/min for solution H2SO4:H2O2:H2O ≡ 1:8:40,

and 1.25µm/min for 1:8:60.

3.3.e.2 Dry Isotropic Etch

Dry etch is done by reactive ion etching (RIE), which uses both chemical and

mechanical effects. The pressure is usually around 30-50mTorr and the mean free path is

at least in the millimeter range providing enough energy for ion bombardment of the

surface. This has two important effects: mechanical etch and reactive species on the

surface (damage) for chemical reactions. The etchant that we have used is SiCl4, but

BCl3 is also extensively used [106, 107]. Chlorine ions react with group III atoms, which

have very high vapor pressure and are pumped out of the chamber, providing chemical

etch. Etching rate is essentially the same for any ternary compound composition.

Anisotropy is most probably due to mechanical effects than to sidewall passivation [108].

Side-walls are very vertical due to ion bombardment. Ions can be very energetic

and the RIE chamber have the electrodes designed to have most of the potential drop at

73

the electrode where the substrate is placed, enhancing the local field strength. Figure 3.7

shows a SEM picture from a mesa etched by RIE plasma, resulting very vertical walls.

Figure 3.7 - SEM picture from a cantilever etched by RIE plasma. Note the verticality of the walls.

Etch uniformity is about the same as for wet etch (2 DBR pairs for 1x1cm2 die).

There is a local field enhancement at the edges of the die and its effect can be

counteracted by surrounding the die with pieces of wafer or by placing it inside a ceramic

rod with a hole slightly larger than the die. This way, an almost flat etch is possible to be

achieved but to the expense of a much slower etching rate (about half of the isolated die).

Etching is extremely repeatable, if and only if the in-situ monitoring system is

operating. As to be described below, it provides a precise control of the vertical position

along the wafer. Otherwise, repeatability is very poor for the required level of precision

that some structures have and wet etch may be preferred.

Plasma etching has some advantages over wet etching. Plasmas are much easier

to start and stop than simple immersion wet etching. Furthermore, the etch is much less

sensitive to small changes in the temperature of the wafer [109] (even though a heated

substrate causes the etch to become more reactive and less mechanical). These two

factors allied to a much more precise in situ monitoring capability, to be described below,

and to the verticality of the walls make plasma etch much more repeatable and

convenient than wet etch.

74

In situ monitoring allows precise real time control of dry etch depth. Provided

there is a view port throughout the top electrode of the plasma etcher, a laser beam can be

introduced into the chamber and acquire depth information. Figure 3.8 shows the optical

setup used. The beam reflected from the die impinges on the beam splitter and is

redirected to a broad area detector, whose current is recorded by a plotter. Figure 3.9

shows the typical trace recorded by the plotter during etching of a tunable detector wafer

(to be described in chapter 5). High and low reflectivity peaks correspond to a DBR pair.

Layers that are thicker than λHeNe/2n, such as sacrificial layer and optical cavity, have

multiple peaks.

Device

mirror

beam splitter

detector

Plotter

HeNe laser

RIE chamber Device

mirror

beam splitter

detector

Plotter

HeNe laser

RIE chamber

Figure 3.8 – In situ etch monitor setup. A horizontal HeNe laser beam is deflected to have vertical incidence on the die being etched. The reflected beam is deviated, through a beam splitter, to a broad area detector and its current is recorded by a plotter.

75

Top DBR

1 DBR pair

Bottom DBRSacrificial layer

(GaAs)Cavity

Top DBR

1 DBR pair

Bottom DBRSacrificial layer

(GaAs)Cavity

Figure 3.9 – Typical recorded trace from the in situ monitoring system during the etch of a tunable detector wafer (to be described in chapter 5). High and low reflectivity peaks correspond to a DBR pair. Layers thicker than λHeNe/2n have multiple peaks.

If the index of refraction contrast is more than ~0.4, the precision of the in situ

monitoring system is ½ DBR layer. Initially, a piece of bare wafer (without devices) is

etched and its signature is obtained. Subsequent etchings, with devices, are then done

with respect to the first signature. If the device coverage is too dense, it may not provide

meaningful information. The depth information may be hidden and the operator may not

identify it in time to get the precise stop point. Figure 3.10 shows a very nice trace for a

die with high density of devices on the surface. However, note the spatial interference

resultant from the topography. In particular, in the first etch with this trace I lost the stop

point because I did not interpret the small bumps as one pair. Actually, in the second

etch my partner still did not believe that these bumps were a DBR pair (I should say that

we were both senior grad students at the time). The desired precision on vertical etch

imposes a limit on device density across the wafer.

76

1 DBR pair

Top DBR Cavity Bottom DBR

Interference

1 DBR pair

Top DBR Cavity Bottom DBR

Interference

Figure 3.10 – In situ monitoring trace for a die with high density of devices on the surface. Note the spatial interference resultant from the topography.

Typical etching rates were 0.25µm/min for an isolated die and 0.13µm/min for a

die inside a ceramic rod. Photoresist is etched at a rate of 100Å/min.

The plasma etching recipe is as follows:

SiCl4 flow: 20sccm;

Pressure: 20mTorr;

RF power: 65W;

Substrate temperature: 20ºC; and

Chamber temperature: 20ºC.

3.3.f Oxidation

Aluminum-oxide (AlOx) is desirable in many applications due to its high

selectivity on Al composition, electrical insulating properties, and low refractive index.

It can even be buried and fabricated on nonplanar substrates [110]. AlOx is formed by

thermal wet selective oxidation of high x (x > 0.9) in AlxGa1-xAs. Oxidation is performed

in a furnace under temperature >400ºC and steady flow of water vapor. The process

makes use of the high reactivity between Al atoms and water molecules.

77

The oxidant is transported by the carrier gas, usually dry nitrogen, to the

gas/oxide interface. Gas transport is important only when spacing among devices is

smaller than 200nm [111]. Initial oxidation rate is limited by the reaction kinetics at the

oxidizing interface and not the gas transport for normal oxidation conditions. After the

oxidant reaches the gas/oxide interface, it is absorbed into the oxide and diffuses through

the oxide region to the oxide–AlGaAs interface. Reaction of the oxidant with AlGaAs

transforms the last into oxide, which has a smaller volume. Oxidation by-products,

which are AsH3 and H2, then diffuses out [112].

The process has many variables and is proportional to temperature, water steam

flow rate, operator repeatability, thickness, crystal orientation, composition profile, mesa

size, mesa geometry, and distance between devices. However, it is very repetitive to the

extent that the operator is able to repeat the open/load/close/open/unload sequence in the

exact same way and time. Two different operators usually get two different rates.

Accurate apertures with diameters of 1um can be achieved with good repeatability but the

device mask usually has 3 or 4 different mesa sizes so that every run yields the desired

aperture.

The oxidation of long straight mesas has a thickness threshold of ~15nm [113].

Typical values of oxidation layers are around 250nm. The process is initially reaction

limited and as the oxide thickness increases it becomes diffusion limited. Thus, the

oxidation rate decreases with time.

Small mesas, circular or squared, suffer acceleration on the oxidation rate when

oxide gets thicker because of larger supply of oxidant for smaller volume to be oxidized.

This effect influences directly the devices that are described in this thesis as they all have

78

small area and the goal are apertures with diameter of ~1um, which means that the mesa

is almost entirely oxidized.

A second run of the oxidation process on the same sample should be avoided.

When the AlOx is again exposed to the wet and hot environment, it usually delaminates

and becomes with even poorer mechanical strength. Thus, the process should be

completed at once and not repeated for fine tuning the oxidation depth. Calibration is

usually, and preferentially, done on a die from the same wafer as the batch to be

processed. Several structures similar to the final devices are defined on the test wafer and

two or three runs are enough to get the rate.

Characterization can be done by looking into the top of the mesa with a CCD able

to capture images in the 900 nm to 1700 nm near infrared wavelength spectrum. The

oxide front can be seen even through the top DBR mirror.

Oxidation rate also varies from wafer to wafer. We have found very high rates of

5µm/min for a VCSEL wafer grown by Bandwidth9, Inc., and as low as 0.2µm/min for

another VCSEL wafer grown by LuxNet Corp. Both wafers had comparable composition

and layer thickness.

I have used AlOx for high order modes filtering on tunable detector and both

current and optical field confinement on VCSELs. The process was carried out at 425ºC.

3.3.g Selective Etch

As the vertical etch, selective etch can be done either through wet or dry

techniques. However, creating tunable devices, or III-V MEMS in general, is far more

difficult than Si MEMS fabrication due to the lack of a robust, well studied selective etch

with large undercutting capabilities. This was by far the bottleneck of the entire

79

fabrication process.

The most important factor is selectivity among the sacrificial layer to be etched

and the rest of the structure. As the structure has a DBR mirror with layers of high and

low Al content, selectivity has to be high to both materials. The material to be etched

would have to be either GaAs or AlAs. However, AlAs is not stable at room

temperature. It tends to slowly oxidize when exposed to air and delaminates after a

couple of days. Moreover, some of the devices would need an oxidized layer, which

would also be etched if the sacrificial layer was to be AlAs. The alternative found was to

etch GaAs and leave behind AlGaAs compounds.

Another important factor is the undercut capability, as the etchant has to penetrate

inside the gap and by-products have to be removed from there. Dry etch is preferable

mostly because of the selectivity. It does not bring the advantage of avoiding adhesion

due to capillary forces, to be described next, as the structure usually has photoresist

protecting the anchor sidewalls, which has to be wet cleaned after the selective etch.

3.3.g.1 Wet Selective Etch

Wet selective etch can be achieved for different values of x in AlxGa1-xAs.

However, high selectivity, larger than 500:1, can only be achieved between high (>0.7)

and low (<0.4) x. Low Al content is etched by a solution of citric acid and hydrogen

peroxide while high Al compound is etched by a solution of buffered oxide etchant

(BOE) 10:1. Thus, high and low Al content layers in DBR stacks can be successfully

etched layer by layer.

Wet technique is attractive because it does not require any process machine. The

process is cheap, portable and repeatable at any site. It is also interesting for industrial

80

production lines as it is much cheaper than install, ramp up and maintain a dry etcher.

Etch selectivity and rates are detailed in the literature [114, 115] and reproduced

in Figure 3.11, for x < 0.4, and Figure 3.12, for x > 0.7. Figure 3.11 shows that there is a

turning volume ratio of the citric acid and hydrogen peroxide solution, depending on x. It

would be possible to selectively etch GaAs with respect to Al0.27Ga0.73As by using a

volume ratio of 2:1 (citric acid to hydrogen peroxide). Al0.27Ga0.73As would start to be

etched only above a ratio of 4:1. In fact, the etch rate of Al0.27Ga0.73As for 2:1 has been

found to be 7Å/s what would give a selectivity smaller than 20:1. Thus, if the lowest Al

content in the DBRs is chosen to be larger than x = 0.25, it would be possible to release

structures with GaAs sacrificial layer and limited mirror undercut. However, this may

not give enough contrast between the DBR pairs and a large number of pairs may be

required. AlOx has been proposed as low index material in the DBR in order to

selectively etch GaAs with respect to Al0.5Ga0.5As with very limited success. Main

problems encountered were low etch selectivity, etch inhibition by gold contacts, and

poor mirror reflectivity due to low oxidation selectivity [8].

Figure 3.12 shows that sacrificial etch of high Al content would not work. Any

concentration of Al above 0.7 is etched by BOE, even for very low concentration in

aqueous solution.

81

Figure 3.11 – (a) Etch rates of AlxGa1-xAs (x < 0.5) as a function of volume ratio of citric acid/H2O2 solution at room temperature. (b) Turning volume ratio of the solution, at which etch starts, as a function of x. After [114].

Figure 3.12 – (a) Etch rates of AlyGa1-yAs (y > 0.7) as a function of volume ratio of DI H2O/buffered oxide etch (10:1) solution at room temperature. (b) Etch rates as a function of y at a volume ratio of 25. After [114].

Wet etch technique is very useful for removing specific DBR layers and for wafer

planarization after a long vertical etch (wet or dry). Because this is the main usage in the

fabrication process, etching rates are not as important as an effective monitoring of the

surface during etch. For DBR layers the etching rate is 50Å/s, in average as the rate is

different for different solutions and DBR composition. The numbers in Figure 3.11 and

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Figure 3.12 are a good guide and are in agreement with our results.

3.3.g.2 Dry Selective Etch

Selective etch of GaAs over AlxGa1-xAs can be obtained with dry etch, even for x

as low as 0.1 with selectivity better than 1000:1. Etching is also done by RIE but adding

SF6 to SiCl4 and with low DC bias. There are two counteracting mechanisms: chlorine

etches all AlGaAs compounds but fluoride passivates surfaces with Al preventing further

etch of these materials [116]. Successful selective etch requires very careful balance of

these two mechanisms. Low DC bias avoids that the passivation film is removed. This

film is composed of AlF3 and is about 20Å thick [117]. No traces of GaF3 have been

found, leading to the conclusion that Ga reacts exclusively with chlorine ions.

The most important figure of merit is, of course, selectivity. It depends on several

parameters such as Al content, SF6/SiCl4 ratio, DC bias, pressure, temperature, surface

quality, and mesa geometry. Thus, systematic calibration has to be done in order to

optimize the parameters. However, as calibration is complex, takes a lot of time and

requires a dedicated epitaxial structure to characterize the etching rates, it is usually done

only to ramp up the etcher. Afterwards, the parameters are only fine adjusted and no

more than one is changed at a time.

Selectivity is better against high Al content materials. Increasing the SF6/SiCl4

ratio also improves selectivity. Emission spectra shows an increase in chlorine ions with

the increase of SF6/SiCl4 ratio, which explains the increase in GaAs etching rate [116].

However, the mechanism responsible for this phenomenon is unknown. Increasing the

SF6/SiCl4 ratio also enhances the formation of AlF3, but this is probably not the primary

factor of selectivity enhancement as the AlF3 film is very thin. Redeposition has been

83

noticed in our samples for higher ratio of SF6/SiCl4 (up to 0.425 was reasonably clean

and 0.625 had a slight granular aspect), probably AlF3.

DC bias does not significantly improve the etching rate of GaAs but enhances

AlGaAs etching. More energetic ions remove the passivation film faster than it can be

formed and net etch of AlGaAs occurs. Sufficiently low DC bias leaves the film almost

untouched. Hence, the very low rate of AlGaAs etching. Low DC bias is also required

for diffusion of etchant into the gaps.

Increasing the pressure also improves selectivity. Higher pressure increase the

number of collisions among ions in the plasma (decreases the mean free path of the ions)

and less energetic ions not only preserve the passivation film but also are more likely to

penetrate in the gaps, increasing GaAs etch. However, it has been noticed that the

process at higher pressures (20mTorr x 70mTorr) also increases redeposition of by-

products on the wafer.

Temperature effects are not reported in the literature. However, it is intuitive that

higher chamber temperature would produce the same effect of increased pressure.

Higher substrate temperature enhances the etch rate without any increase on redeposition.

The reason for this is believed to be the chaotic enhancement on fluid dynamics

(turbulence) that favors the flow of etchant into the gaps and by-products out of the gap.

While a temperature of 20ºC was kept at the chamber walls, the substrate was heated up

to 70ºC.

Dehydration is usually advised before selective etch. Water vapor on the surface

of the wafer may also favor redeposition [8]. This is usually taken care when hardbaking

the photoresist before etching.

84

The worst possible variable of the process is the dependence on chamber

conditions, in such a way that the previous process run in the chamber affects the current

one. Etching was not repetitive even for subsequent runs. This was a big problem as the

plasma etcher is a multi-user multi-process machine. The main problem was redeposition

on the wafer, which was not consistent. The wafer was getting really dirty after etch and

sometimes etch did not occur at all. The procedure to selectively etch devices had to be

improved and it was actually starting at least one day in advance. It consisted of chamber

baking (70ºC for at least 4hs) and oxygen plasma cleaning followed by at least two

dummy GaAs etches of 30min using the selective etch recipe. The problem was

alleviated by creating this GaAs friendly environment but etching never got to be

consistent at the Berkeley Microlab. Some releases done at Bandwidth9, Inc., resulted in

the best fabricated devices. The company had a dedicated etcher only for release and this

was the way that they got their process to be consistent.

Etching rate was not reproducible. Sometimes etching did not happen at all.

Rates have varied from 0 to 200Å/s. Whenever etching rate had to be calibrated, the test

structures shown in Figure 3.13 were used. Square sizes vary from 6µm to 90µm, in

steps of 6µm, and some cantilevers were also included in the mask. This way, the

selective etch can be inspected by simple optical microscope because the released top

portion of the squares usually goes away. In case it does not go away, it is usually

misplaced, as 36µm and 42µm in Figure 3.13, and always at a different focal plane. A lot

of redeposition can be seen on the die from the picture.

85

12µm 42µm12µm 42µm

Figure 3.13 – Test structures to calibrate dry etching rate. Square sizes vary from 6µm to 90µm. Some cantilevers were also included.

The recipes used have slightly changed during time as a result of fine adjustments

to adapt to changes in chamber conditions. Problems encountered were mainly slow etch

and redeposition. Final values used, which consistently released squares with side of

70µm with a gap of 1.8µm, were:

SiCl4 flow: 30sccm;

SF6 flow: 8.5 sccm;

Pressure: 50mTorr;

RF power: 42W;

Substrate temperature: 70ºC; and

Chamber temperature: 20ºC.

The above results are valid for the Oxford Instruments Plasma Lab System 100,

which has been used at the Berkeley Microlab. Different machines always have different

electrodes sizes and distances that change the ultimate performance. A plasma etcher

from Bandwidth9, Inc. was also used. Some of the good results on tunable filter, to be

presented on chapter 4, are from etches done with this last one. This was the only time

that reproducible results were achieved.

86

3.3.h Critical Point Drying (CPD)

Adhesion is among the main causes of MEMS failure [118]. It may occur during

processing, right after a wet release or cleaning procedure, during normal operation in

highly humid environments or when suspended parts get in contact with the substrate

(which can be divided in two separate processes: mechanical collapse and adhesion). The

first two happen as the liquid is removed due to a dehydration cycle, when a liquid bridge

is formed between the suspended member and the substrate yielding an attractive

capillary force which may be sufficiently strong to collapse it. The last type is usually

catastrophic for electrostatic MEMS as they are always accompanied by an electrical

discharge.

“Wet” adhesion can and must be avoided. Critical point drying is the most

popular method. The technique has been available for decades and extensively applied in

biology [119]. The usage is highly successful in MEMS, with nearly 100% yields [120].

Among the alternate proposed techniques are freeze-drying method (solution is frozen

and sublimated – can undergo a huge volume change), dry etch without further cleaning

(cannot protect anchors), liquid bridge cleavage (concentrate the remainder liquid close to

anchors), hydrophobic coating (capillary pull becomes a push) and external force to

release (a probe, e.g.).

Critical CO2 drying process allows samples to be dried without any surface

tension, thus reducing the likelihood of stiction. This process is typically used to dry

samples that have been rinsed in de-ionized (DI) water. The DI water is replaced by

methanol prior to the drying process, and then the methanol is displaced by liquid CO2 as

part of the drying process. Then, the cycle shown in Figure 3.14 is performed inside the

drying chamber: temperature is lowered to ~ -15ºC, pressure is raised to ~80atm,

87

temperature is increased to ~40ºC, and, finally, pressure returns to 1atm. With this, the

chamber conditions move around the critical point of CO2 (31.5ºC and 73atm) in the form

of a supercritical fluid that goes from liquid to vapor phase without formation of interface

between the different phases. This process may also be used to clean samples.

Figure 3.14 – Critical point drying cycle: temperature is lowered to ~ -15ºC, pressure is raised to ~80atm, temperature is increased to ~40ºC, and, finally, pressure returns to 1atm. The chamber conditions move around the critical point of CO2 (31.5ºC and 73atm).

3.3.i Inspection

Inspection is carried out several times over the device fabrication. Optical

microscopes are extensively used during all time while processing. Some softwares can

acquire images from CCD cameras and improve the fabrication documentation and

control.

Surface profiler is also quite useful after several fabrication steps. After vertical

etch it can confirm if the desired layer has been reached and confirm the result from in-

situ monitoring, which may be not reliable if wet etch was used or if features on the wafer

88

are very dense, as shown in Figure 3.10. After lithography, photoresist thickness can be

checked to have the necessary thickness to cover the mesas. Finally, after metal

deposition the operator can double check the result from the crystal monitor in the

evaporator. It has also been used to calibrate the trace from in-situ etch monitoring and

correlate number of peaks with etch depth.

Scanning electron microscopy (SEM) is the only nondestructive tool that can

show under the beams, after selective etch. It is also used to acquire information about

the side walls. SEM can also provide limited information about beam bending, stress and

even electrostatic actuation if a dedicated system can apply voltage to the device [8].

White light interferometry is also used to characterize the released surfaces.

Measurements were done using the optical profiler Wyko NT 3300, from Veeco. In

contrast to classical interferometry, this method can be used for measurement of objects

with rough surface even in the case of speckle imaging. The white light interferometer is

in principle a Michelson interferometer with a broad-band light source and a CCD

camera as a detector. The measured object is placed in one arm of the Michelson

interferometer instead of one of the mirrors. Due to the reflection on the rough surface, a

speckle pattern arises in the detector plane. This pattern is superimposed to the reference

wave. White light interference is observable if the optical path lengths of the two arms

differ less than the coherence length. If the object is translated in the longitudinal

direction, different mesa heights fulfill the optical path difference requirement.

Resolution is higher than in monochromatic interference as in this case it is proportional

to the coherence length, which is inversely proportional to the spectral bandwith of the

source. Moreover, the technique is non-destructive and non-contact.

89

3.4 Summary

This chapter has described all processes necessary to carry on the fabrication of

tunable devices. Sub-wavelength grating mirror was fabricated in silicon in excellent

agreement with theory. Processes described for GaAs system were general in the sense

that they can be applied to any III-V fabrication and specifically to all types of tunable

devices. Specific details involved on the fabrication of each one of these devices will be

described in their respective chapter, starting next with the tunable filter.

90

Chapter 4 Tunable Filter

4.1 Introduction

Tunable optical filters are widely used in various applications, such as dense

wavelength division multiplexed (DWDM) optical networks, optical interconnects,

spectroscopy, infrared imaging, biometrics and optical sensing. Various structures and

designs have been reported for these different applications [14]. The major advantages of

MEMS-based tunable filters include a very large tuning range, continuous tuning with

high precision, a narrow passband, and a fast response (1-10 microseconds).

Previous chapters have described general design rules (chapter 2) and fabrication

(chapter 3). This chapter will go deeper into filter specific details in both design, such as

epitaxial layer structure, and fabrication, such as processing sequence. Finally, the

targeted application defines the best applicable structure and two different applications

will be covered in this chapter: communications, through a novel torsional tunable filter,

and infrared imaging, through a large surface area coverage design.

4.2 Design

Filter design follows the rules described in chapter 2. However, every device has

its own particularities that will be addressed here for a torsional filter designed for

communications and a large surface area design for IR imaging.

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4.2.a Torsional Filter

The torsional topology is shown in Figure 4.1. When a voltage is applied between

the top (p-doped) and bottom (n-doped) DBRs, the entire structure experiences an

electrostatic force pulling it toward the substrate. However, by design, the position of the

center of mass lies in the counterweight; the filter head moves upward whereas the

counterweight moves down. Figure 4.1(b) illustrates a side view of the device along the

filter arm direction. With the filter located at the head, the filter wavelength increases

with voltage (red-shift).

A

B

incident light

transmittedlight

AB

(a) (b)

A

B

incident light

transmittedlight

AB

(a) (b)

Figure 4.1 - (a) Top view and (b) side view along the filter arm direction of the torsional structure. The sacrificial layer under the black region remains and is removed everywhere else.

This design has two major advantages. First, by appropriately choosing the length

ratio of the cantilever arm and the counterweight, a leveraging effect can be achieved, as

illustrated by Figure 4.1(b). Thus, the design allows the movement of the head portion

for more than 1/3 of the gap and larger tuning can be achieved. Second, the design can

prevent electrical discharges. If any discharge occurs it will not cause any damage to the

filter head, because it will occur on the counterweight. Because the discharge points are

not in the optical path it is possible to fabricate stops or holes at the critical positions in

order to avoid any kind of discharge.

92

The design of the torsional filter was done according to the communications

market. Wavelength division multiplexing (WDM) systems, which basically consist of

several different carrier wavelengths in the same optical fiber, can benefit from the

versatility of the devices that can both monitor this system and to add and drop signals to

the network. However, requirements are very stringent. Extinction ratio has to be large

between adjacent channels (30dB – measured at -1dB, even though nobody explains why

this large), and linewidth has to be sharp as the channels are very dense.

WDM channels are typically spaced by 1.6nm and DWDM (dense WDM) by

0.4nm. Thus, linewidth requirement is usually of the order of 1nm for WDM and 0.1nm

for DWDM. However, Fabry-Pérot cavities are constrained to have lorentzian shaped

transmission and 30dB of extinction ratio with 0.1nm of linewidth is a challenge for

micromechanical structures. Linewidth of 1nm is feasible [15, 17].

WDM long-distance systems operate around the wavelength 1550nm, where there

is minimum loss for optical fibers. The spectrum there is divided in 3 bands: C

(1492.25nm to 1529.55nm), L (1530.33nm to 1569.59nm) and S (1570.42nm to

1611.79nm). A device that is able to tune from the beginning of C band to the end of L

band would be extremely attractive. Therefore, more than 100nm of tuning would be

required.

Another restriction would apply to response time. Network latency in case of

failure cannot be longer than 15ms. Moreover, transmission is being done at 10Gbits/s

and the add/drop operation has to be compatible; otherwise there is no gain in speed.

Devices should be fast enough to comply with those requirements and less than

millisecond response would be required. All mechanical structures shown in Table 2-1

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have resonant frequency on the order of tens to hundreds of kHz and millisecond

response time is not a constraint for these devices. They can be made stiff enough (high

spring constant) to have very high resonant frequency but to the expense of higher

actuation voltages. Maximum operation voltage is usually not a constraint if less than

100V.

Based on the above requirements, the filter can be designed. A first wafer was

designed long ago [80] but the results were very poor and fabrication very difficult [81,

82]. However, it served as learning basis for the second wafer, which gave some very

good results [15, 56]. The problems were mostly related to fabrication and will be

described in the next section.

The main modifications/improvements on the second wafer were targeting

fabrication. First, the gap size was increased. Previous wafer had a gap of 0.8µm, in

order to have low voltage actuation, and the selective etch had problems of low rate,

redeposition and incomplete release. Some of the places of the structure had a radius/gap

(r/d) ratio >10, which may have caused etching to be diffusion limited. So, the gap was

increased to 2.7µm in order to keep all places with r/d<5. The doping scheme was also

modified. Before, both mirrors were completely doped and electrical discharges were

very frequent during actuation. In the new design, only half of each mirror was doped so

that any contact or proximity happened only between semi-insulating surfaces. Selective

etch was also causing a lot of redeposition and the probable cause was that the wafer

surface defined by the vertical etch was right at the end of the sacrificial layer. So, a

vortex was probably being formed (ions are still moving toward the substrate even for the

low bias of the selective etch recipe) and both etchants and by-products were having

94

trouble to get in and out the gap, respectively. To correct this problem, the vertical etch

was designed to be done down to the end of the bottom DBR. Thus, the gap is located

above the surface level, free from the vortex. This new design also incorporates two

contacts on the top of the processed wafer, so that the substrate can be semi-insulating

GaAs and optical insertion loss is reduced. Finally, the design was shifted from 850nm

to 1550nm and the reason is obviously for WDM application.

Following the flow chart from Figure 2.17, the material system is already defined

as AlxGa1-xAs, but the Al content on each mirror should be chosen. The low Al content

material (high index) is chosen to be x=0.1 and high Al content x=0.85. The first took

into account selectivity to GaAs for the release etch and the second was chosen to be

smaller than x=0.9 in order to avoid spontaneous oxidation. A third composition with

x=0.51 is chosen for the parts exposed to air, including top mesa, gap and exposed

substrate.

The next step is the design of the optical cavity. The linewidth is required to be

below 1nm for the region of interest and this would require large mirror reflectivity.

Designing both mirrors to have reflectivity of 99.4% at the center (λ=1550nm) would

require 18 and 22 DBR pairs in the top and bottom mirrors, respectively. Then, the

structure has the layer composition shown in Figure 4.2. In fact, the high-low index

profile requires that the top mirror has 18 pairs plus a low index layer, as it has high

index on both ends. A smaller number of DBR pairs is required in the top mirror because

it has two semiconductor-air interfaces, while the bottom has one interface of

semiconductor-air (gap) and the other semiconductor- semiconductor (substrate).

Semiconductor-air interface is the largest index contrast, which provides larger

95

reflectivity. A thicker layer of Al0.1Ga0.9As (7x λ/4) works as reference to stop the

vertical etch, as this thickness is easily identifiable with the in-situ monitoring system.

The remaining of this layer and the next one, Al0.85Ga0.15As, will be removed with wet

selective etch in order to land into the n++ layer for the bottom contact. The entire

thickness of the epitaxial layers is 14.13µm, which is a challenge for epi-growth. The

reflectivity spectrum is shown in Figure 4.3. Both mirrors match reflectivity in the

vicinity of 1550nm but the top mirror has a slightly larger stop band also due to the two

semiconductor-air interfaces.

x thickness(Å) λ # function doping doping amount DBR pairs Cumulative thickness (µm)0 1000 − contact p++ 1x1019 14.134537

0.51 1241.14 λ/4 release protection p 1x1018 14.0345370.85 1308.32 λ/4 DBR p 1x1018 9 13.9104230.1 1165.05 λ/4 DBR p 1x1018

0.85 1308.32 λ/4 DBR 9 11.684390.1 1165.05 λ/4 DBR0.85 1308.32 λ/4 low index 9.5891890.51 1241.14 λ/4 release protection 9.458357

0 27000 − air gap 9.3342430.51 1241.14 λ/4 release protection 6.6342430.85 1308.32 λ/4 low index 6.5101290.1 1165.05 λ/4 DBR 10 6.3792970.85 1308.32 λ/4 DBR0.1 1165.05 λ/4 DBR n 1x1018 11 3.9059270.85 1308.32 λ/4 DBR n 1x1018

0.1 8155.35 7 λ/4 dry etch stop n 1x1018 1.185220.85 1308.32 λ/4 wet etch n 1x1018 0.369685

0 1147.39 λ/4 contact n++ 5x1018 0.2388530.51 1241.14 λ/4 etch stop 0.124114

GaAs semi-insulating substrate

Figure 4.2 – Layer composition for the tunable filter proposed structure. First column shows Al content in AlxGa1-xAs.

96

1.45 1.475 1.5 1.525 1.55 1.575 1.6 1.625wavelength HnmL

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

(µm)

bottom

top

1.45 1.475 1.5 1.525 1.55 1.575 1.6 1.625wavelength HnmL

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

(µm)1.45 1.475 1.5 1.525 1.55 1.575 1.6 1.625

wavelength HnmL0.2

0.4

0.6

0.8

1

rewopytivitcelfer

(µm)

bottom

top

Figure 4.3 – Reflectivity spectrum for the top and bottom mirrors designed to have reflectivity of 99.4% @1550nm.

Reflectivity defines linewidth and the simulated results are shown in Figure 4.4.

The FWHM is below 1nm for the range 1510nm to 1596nm, which can be considered

good enough. Further improvement would require an even larger number of DBR pairs

and consequent epitaxial growth, which is already a challenge. An alternative would be

to insert the bottom contact layer into the bottom mirror, instead of underneath it, and use

larger index contrast for the DBR pairs under the contact layer, such as GaAs/AlAs. This

would not be a problem for processing because those layers would be buried. By doing

this, it would be possible to increase reflectivity of the bottom mirror without increasing

the total thickness. However, the top mirror would require larger number of pairs to

match the reflectivity of the bottom mirror and the selective etch would have problems

with formation of vortex close to the gap.

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1480 1500 1520 1540 1560 1580 1600 16200

1

2

3

4

Line

wid

th (n

m)

Wavelength (nm)

Figure 4.4 – Linewidth as function of wavelength for the structure shown in Figure 4.2.

The above design also allows the choice of the sacrificial layer thickness. Figure

4.5 shows the simulated transmitted wavelength as function of gap size between mirrors.

Each one of the curves is a transmission mode of the cavity. These modes are around

multiples of λ/2 of the central wavelength, 1550nm in this case. So, the first mode to the

left in Figure 4.5 is actually the 3rd cavity mode. The chosen mode was the 4th (center of

Figure 4.5), in order to have a large sacrificial layer that can guarantee a reaction limited

sacrificial etch. A gap of 3.5µm would be required to transmit 1620nm and 2.8µm to

transmit 1490nm. So, the filter is able to tune across C, L and S bands if the initial gap is

chosen to be 2.70µm. This choice would also allow the same wafer to be used with

closing gap topologies, such as cantilever, which would tune across the 3rd mode.

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2.0 2.5 3.0 3.5 4.0 4.51480

1500

1520

1540

1560

1580

1600

1620

FP w

avel

engt

h (n

m)

Gap size (µm)

Figure 4.5 – Transmitted wavelength through the Fabry-Pérot cavity as function of the gap size between the mirrors.

The mechanical design was already discussed on chapter 2. Dimensions used for

test structures are also given in chapter 2. This ends the specific portion of the torsional

filter and, before moving to fabrication, design for large area coverage will be discussed.

4.2.b Folded beam filter

This topology was developed for infrared imaging. The filter was designed to be

coupled to a focal plane array (FPA) and to provide specific spectral information,

enabling multispectral imaging on a chip. Modern imaging sensors can achieve high

detection, but such broadband mid-wave infrared (MWIR – 3µm -5µm) and long-wave

infrared (LWIR – 8µm -12µm) sensors are limited for remote sensing, because of low

contrast, and for defense applications, such as in their ability to search and detect

camouflaged targets in the presence of low background noise. Ideally, a filter array

would be integrated to the FPA and would have independent addressability.

Given the wavelength range of interest is very wide, from 3µm to 12µm, it is

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impractical to design one single filter with optimized performance for the entire range.

The most challenging problems are the tuning range, independent addressability, and the

voltage required for the tuning range.

The most stringent requirement is large surface area coverage. The filter has to sit

in front of the FPA and area not covered cannot provide spectral information. As clearly

depicted in Table 2−1, chapter 2, the various existing topologies all suffer from the

relative small fill factor. The trade-off in these designs is voltage and

membrane/cantilever length. To restrict the tuning voltage to <100 V, the arms are

typically several hundreds of micron long.

We propose a folded-beam filter structure to overcome the small fill factor issue

while maintaining a reasonable operating voltage. Figure 4.6 shows the schematic of this

design. For the design of the dimensions, the spring constant of the folded-spring is

considered equivalent to a membrane of the same dimension having its full length

extended out. The voltage required to actuate the device is actually smaller than that for

the trampoline because of the torsion that all beams suffer due to the momentum applied

by electrostatic force from the head [121]. However, for a first approximation, it is

enough to consider the same spring constant as the trampoline structure, knowing that the

voltage is overestimated.

Anchor

Folded-Beam

Tunable Filter Head

Anchor

Folded-Beam

Tunable Filter Head

Figure 4.6 – Top view of a single pixel of a tunable filter with folded-beams.

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Final design/fabrication would be done with a different material system than

GaAs as the tuning range is small (chapter 2 – section 2.4) and fabrication more complex

(chapter 3). Although Aluminum has been used widely used as movable mirrors, the

optical loss is significant for 1-12 um wavelength and hence cannot be used for

transmission purpose. A range of materials are suitable for short wavelength infrared

(SWIR) and medium wavelength infrared (MWIR), whereas the materials are very

limited for long wavelength infrared (LWIR) because of significant absorption in oxides

and SiNx for >10 um. For the maximum DBR bandwidth and processing compatibility,

we propose to use the following:

Table 4-1 Proposed material system for each band of operation.

Band SWIR MWIR LWIR λ (um) 1.5-2.5 3-5 8-12 Material Choice SiO2/Si or

GaAs/AlOx CaF2/ ZnSe

Refractive index (nhigh and nlow)

3.48/1.47 3.43/1.45 2.4/1.3

Figure 4.7 and Figure 4.8 show the calculated tuning range and passband for a

single-band MWIR and LWIR filters using Si/SiO2 and CaF2/ZnSe, respectively. As this

is preliminary, it merely shows feasibility rather than optimized performance. The

passband for a MWIR filter is 24 nm. Full 3-5 um range can be achieved by designing

the gap to be 5 um initially. Using the folded-spring design from Figure 4.6 and

techniques described in [76, 78] to move beyond the limitation of the 1/3 rule, the gap

can be reduced to 3 um, which thus covers the full MWIR range. The LWIR passband is

52 nm, which can be reduced by increasing DBR pairs. Full range tuning can be

achieved by choosing a 9 um gap to start with. Both figures show a thicker line close to

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the edges of the tuning range which is due to a slight linewidth broadening in those

regions, which have smaller reflectivity than the central portion.

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.00.0

0.2

0.4

0.6

0.8

1.0

∆λ=24nm

trans

mis

sivi

ty

wavelength (µm)1 2 3 4 5 6 7 8

3.0

3.5

4.0

4.5

5.0

3rd mode

4th mode

2nd mode1st mode

Fabr

y-Pe

rót w

avel

engt

h (µ

m)

gap size (µm)

Figure 4.7 – (a) Optical transmissivity of the tunable filter designed for MWIR. (b) Transmitted wavelength as a function of ai rgap size. Choosing to work with the 2nd mode, we can tune the entire MWIR range with an initial gap of 5 um.

7 8 9 10 11 12 13 140.0

0.2

0.4

0.6

0.8

1.0

∆λ=52nm

trans

mis

sivi

ty

wavelength (µm)4 6 8 10 12

8

9

10

11

12

3rd mode

2nd mode1st mode

Fabr

y-Pe

rót w

avel

engt

h (µ

m)

gap size (µm)

Figure 4.8 – (a) Optical transmissivity of the tunable filter designed for LWIR. (b) Transmitted wavelength as a function of airgap size. Choosing to work with the 2nd mode, we can tune the entire LWIR range with an initial gap of 9 um.

Figure 4.9 shows the calculated maximum drive voltage, defined as the voltage

required to move the top DBR to reach maximum tuning (1/3 of initial gap), as a function

of filter head size for the cases of 1-3 folded springs supports and a 50um pixel size. The

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beams are considered with squared cross-section with 5µm of side. For a fixed pixel size,

there is a trade-off between head size and number of folds that the beams can have. We

see that for a 1-pair folded spring design, the filter head can be as large as 28um with

25V of maximum voltage. This figure also shows that even for a “harder” beam, with

only one fold, the voltage is not excessive. Furthermore, the voltage is overestimated as

discussed before. Fewer folds allow larger area in the head and larger electrostatic force

for the same voltage. Thus, the fill factor can be greatly improved and pixel size greatly

reduced without the compromise of a very high voltage. Figure 4.9 also shows that it

may be advantageous to increase the pixel size and make one filter covers four FPA

pixels instead of only one.

0 5 10 15 20 25 300

20

40

60

80

100

Vol

tage

(V)

Filter head side (µm)0 5 10 15 20 25 30

0

20

40

60

80

100

Vol

tage

(V)

Filter head side (µm)

Figure 4.9 – Calculated maximum drive voltage as a function of filter head size for the cases of 1-3 folded springs supports, 5x5µm2 beam cross-section, and a 50µm pixel size.

Two different sets of test structures were designed. The first one had open areas,

squared anchors, long beams and small heads, for covering only one pixel. The objective

was to probe the structure and test its movement. The second set had larger heads, for

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covering more than one pixel, slightly shorter beams, and the released parts were buried

into the surrounding anchors, for matrix addressability to be discussed below,. Thus, the

first set had heads of 30 and 40µm, and the second 60, 70, 80 and 90µm.

Even though I was not planning to fabricate these devices in AlGaAs, because of

the limited tuning range around wavelengths of 4 and 10um, this development was used

as proof of concept for the folded beam trampoline structure. A wafer used was designed

for another application, tunable detector to be described in the next chapter. The

absorption layer is GaAs, which is transparent to the wavelengths of interest.

Top DBR was defined from the other application and is 2.6µm thick. Gap size is

also given: 1.8µm. There is no voltage limitation but it is desired that it is as small as

possible, so the beams were designed to be 4µm wide. Previous experience with the

torsional filter has shown that 3µm was too narrow and 5µm worked well, so we tried

4µm. Beam length was defined based on a fabrication limitation: provide an opening of

25µm on both sides of the buried beams for the release. So, beam length is 135µm for all

head sizes (110µm coming from the anchor and more 25µm to the head) and the internal

pixel side is also the same for all devices, 190µm. What is different among different

head sizes is the point where the beam connects to the head: at the corner for 60µm heads

and steps of 5µm away from the corner as the size increases. The open structure had

beams 185µm long. Beam lengths were limited to about the same size as the torsion

beam from the torsional design in order to avoid buckling. Even though this is a wafer

related problem and a different wafer is being used for the folded beam structure, it is

reasonable to expect similar internal strain among the various layers.

Figure 4.10 shows the finite element simulation of the folded-beam structure. It is

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interesting to point that the simulation allows a complete analysis, even of the small

torsion of the beams. This is shown only as an illustration and fine adjustment of the

parameters was not done. As mentioned before, the design was for the smallest voltage

but also based on some other considerations. As long as this analysis shows that it works,

it is enough for the proof of concept.

Figure 4.10 – Finite element simulation of the folded-beam structure. It is interesting to point that the simulation allows a complete analysis, even of the small torsion of the beams.

Another important characteristic of this design is that it can be easily placed into a

2D row-column matrix addressable array. A typical column-row shift-register circuitry

can be applied. In this case, the control circuits can be placed on the same platform and

wire bonded to the row and column bond pads. This will not adversely influence the

detector fill factor or the pixel size.

Designing a tunable filter that can tune across both MWIR and LWIR band is

important for the applications discussed here. We have also designed a double optical

cavity structure to achieve this capability. The two filters may be controlled by one

single electrode or two independent electrodes. The former makes the design of the

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control circuits and MEMS addressability much easier.

Figure 4.11 shows the preliminary simulation of a 2-band filter design. Tuning

range for LWIR and MWIR are 8−12µm and 3.2−4.8µm, respectively. Figure 4.11(b)

shows the optical spectra as the LWIR filter gap is varied. Note that the MWIR filter was

not tuned at the same time, showing the independence of the two cavities.

3½ pairs λ/4 (ZnSe/CaF2)

3½ pairs λ/4 (ZnSe/CaF2)λ Cavity (air)

4 pairs λ/4 (ZnSe/CaF2)λ Cavity (air)

4 pairs λ/4 (ZnSe/CaF2)Substrate

DBR at 9.4µm

DBR at 4µm

3½ pairs λ/4 (ZnSe/CaF2)

3½ pairs λ/4 (ZnSe/CaF2)λ Cavity (air)

4 pairs λ/4 (ZnSe/CaF2)λ Cavity (air)

4 pairs λ/4 (ZnSe/CaF2)Substrate

DBR at 9.4µm

DBR at 4µm

4 6 8 10 120.0

0.2

0.4

0.6

0.8

1.0 LWMW

10.5µm

10.0µm

9.5µm

tuned voltagefixed voltage

trans

mis

sivi

ty

wavelength (µm)

(a) (b)

Figure 4.11 – (a) Schematic of 2-Band filter design. The top DBR is centered at 9.4µm and the bottom one at 4µm. (b) Calculated optical transmissivity spectra to show two-band tuning. Only the LWIR gap is varied.

Moreover, this double cavity design is also generic for tunable lasers and

detectors and can also leverage the design of these devices.

Finally, it is very important to state that dimensions for the implementation of

these filters are large, as can suggest the gap sizes obtained from Figure 4.7 and Figure

4.8. So, the invention of SWG mirror has been inspired based on applications for MWIR

and LWIR and this is the way that its potential can be explored the most.

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4.3 Fabrication

Fabrication was extensively described in the previous chapter. However, any

device has specific details that are related to the design or even for the application. This

section will describe the fabrication sequence of the torsional and folded beam filters.

4.3.a Torsional

The first wafer used for the torsional filter gave very poor results [80-82]. Figure

4.12 shows a SEM picture of one device fabricated on this wafer. The picture can show

the very small gap (0.8µm) and that the gap was at the surface level, favoring the

appearance of vortex during etching.

Figure 4.12 – SEM picture from a device fabricated from the first wafer [80-82].

The main problems on fabrication with this wafer were incomplete or excessive

etch and redeposition. The first two are more a consequence of poor repeatability of the

etching chamber than the wafer design. However, the small gap favors the problem.

Figure 4.13 shows both problems on different dies etched for the same amount of time.

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remaining materialremaining material

etched anchoretched anchor

(a) (b)

Figure 4.13 – SEM pictures showing (a) incomplete selective etching and (b) excessive selective etching.

Figure 4.14 shows the redeposition problem. The picture to the right shows the

clear pattern related to the device geometry (circular under the head and line under the

beam), which can only be determined by fluid dynamics. Redeposited material is very

likely to be Al3F. Unsuccessful attempts were made to remove the debris with BOE,

NH4F and citric acid. Figure 4.15 shows the places with redeposition problem, which

was also observed underneath both corners of the squares in the counterweight and the

torsion connector. Redeposition occurs in places where formation of vortex can cause the

problem. These places are also places where etch could have been incomplete.

However, the uneven topography of the material clearly shows that it is the result of

redeposition.

circularline

circularline

Figure 4.14 – SEM pictures showing redeposition problem.

108

Figure 4.15 – Points of preferential redeposition around the device geometry.

Hence, the second wafer was designed as described in the previous section.

Figure 4.16 shows both calculated and measured reflectivity spectrum of the wafer as

grown. Agreement with theory is excellent but a small shift to shorter wavelengths can

be noticed on the experimental values.

1.46 1.48 1.50 1.52 1.54 1.56 1.58 1.60 1.62 1.640.0

0.2

0.4

0.6

0.8

1.0

Ref

lect

ivity

Wavelength (µm)

(a) (b)

Figure 4.16 – Reflectivity spectrum for the fabricated wafer as grown. (a) Calculated and (b) measured.

Fabrication sequence is shown in Figure 4.17. Starting from the bare wafer, the

top metal contact is deposited through liftoff technique on GaAs. Next, the device is

defined by simple lithography and photoresist mask is used for vertical dry etch. It is

important to hardbake the photoresist before this etch, which is very long (around 45min

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without using the ceramic tool and 1h15min with it). Figure 4.18 shows the in-situ

monitoring trace for this wafer. The lateral small peaks are due to the fact that the layers

of DBR are larger than λHeNe/2 and not to interference. The structure from Figure 4.2 can

be clearly identified, including number of DBR pairs and even Al content. Note the

uniformity of the DBR pairs, which translates very high quality and well controlled

epitaxial growth. Etching is interrupted at the etch stop layer and wet etch is used to

remove both Al0.1Ga0.9As and Al0.85Ga0.15As layers. The etch stop layer is not made of

Al0.85Ga0.15As because this material oxidize very easily when exposed to wet

environment at room temperature. So, it can have a small lateral area in the DBR

exposed but cannot have the entire surface in contact with DI water, which is necessary

for cleaning and during the following lithography. It is also important that the surface of

the wafer does not dry while proceeding the wet etch, always keeping a liquid bubble

covering the die until GaAs is exposed. Wet etch leaves behind a step which works as a

cover for the GaAs layer where the bottom metal contact is deposited. The reason for

this cover is to avoid excessive undercut during selective etch and also to provide

uniform potential for the bottom part of the device when voltage is applied. In fact, metal

is deposited in such a way that it covers the step and avoids undercut. Next step is the

selective etch, which removes all exposed GaAs. After selective etch, the die is cleaned,

photoresist is removed and critical point drying ends the fabrication sequence. Figure

4.19 shows a SEM picture from a fabricated device.

110

Figure 4.17 – Fabrication sequence of the torsional tunable filter.

Top DBR(18 pairs)

1 DBR pair

Bottom DBR(22 pairs)

Sacrificial layer(GaAs)

top GaAs Al0.51Ga0.49As

stop layer

Top DBR(18 pairs)

1 DBR pair



top GaAs Al0.51Ga0.49As

stop layer

Figure 4.18 – In situ monitoring trace for vertical etch of the wafer. The lateral small peaks are due to the fact that the layers of DBR are larger than λHeNe/2 and not to interference. The structure from Figure 4.2 can be clearly identified.

111

Figure 4.19 – SEM picture from a device fabricated from the second wafer. Note the larger gap, both contacts on top and vertical etch of both mirrors. Mirrors are also thicker for this wafer.

Actually, the described sequence is essentially the same for any different topology

to be fabricated on this wafer. This sequence is function of the epitaxial layers. Different

topologies would only use a different set of masks for lithography.

Other tunable filter topologies were fabricated on the same wafer. As the critical

step of fabrication is the selective etch, any different topology with comparable

dimensions can be used to characterize the process and the wafer design. Figure 4.20

shows 3 other designs.

The fabrication sequence had some processing issues. To start with, photoresist

thickness had to be increased in order to stand to the very long etch. When the ceramic

tool was used, etching of the almost 13µm vertical structure was taking around 1h15min.

When hardbaked, photoresist is supposed to etch at an average rate of 100Å/min and the

thickness of 1.8µm was supposed to be enough. However, the rate was much larger than

this, and the final surface was rough, without optical quality. To correct the problem, the

same photoresist was used but spun at 3500rpm. The final thickness was 2.3µm. Figure

4.21 shows the cantilever surface after etch and for photoresist thicknesses of 1.8µm (a)

and 2.3µm (b).

112

(a) (b)

(c)

Figure 4.20 – Different topologies fabricated on the same second wafer: (a) cantilever, (b) bridge, and (c) multiple beams.

(a) (b)

Figure 4.21 – Photoresist thickness had to be increased because of the long vertical etch. (a) Cantilever surface after etch and thickness of 1.8µm. (b) Thickness of 2.3µm.

The others processing issues on this wafer had mainly one cause: stress and strain

among the various layers on the wafer. This problem is related to epitaxial growth and

113

may have several small factors contributing to the final large effect. First of all, there is

some lattice mismatch between GaAs and AlAs. Even though this mismatch is small

(GaAs has lattice constant of 5.65Å and AlAs of 5.66Å) and ternary compounds AlxGa1-

xAs have a lattice constant that changes linearly with x, it is enough to create different

stress among the several layers.

Another factor is the growth time for this wafer that was very large (~10hs).

Control inside the chamber may have been jeopardized during the top mirror deposition

(in-situ monitoring trace in Figure 4.18 shows a slight non-uniformity on the top DBR)

and defect concentration may have increased, modifying the lattice constant.

Temperature variations during deposition may also change stress from compressive to

tensile [122].

Finally, high doping levels may cause lattice contraction [123]. As only the top

portion of the top mirror was doped this may have caused differential strain along the

transversal direction of the structure.

The first consequence of strain and stress is bending of beams after release. If the

beams have one free end, such as in the cantilever topology, they droop. Clamped-

clamped beams will buckle if the length is larger than the critical length as given by

Euler’s buckling criteria:

( )min ,3

crr

L t wπε

= (4.28)

where εr is the internal strain in the beams. Figure 4.22 shows the critical length as

function of strain for different thicknesses of beams. The torsional filter has its torsion

beams clamped-clamped and a total length of 160µm and 180µm for test structures A and

B, respectively. Thus, it would be necessary strain larger than 0.25% to buckle the

114

structure. However, the bridge structure was designed to have its lithography mask set

compatible to the cantilever structure and the beam had 500µm of length. This structure

suffered a huge buckle of almost 7µm, as shown in Figure 4.23, which is the white light

interferometry image of the bridge structure.

0.00 0.05 0.10 0.15 0.20 0.250

100

200

300

400

500

600

Crit

ical

leng

th (µ

m)

Strain (εr) (%)

t=1µm t=2µm t=4µm t=4.83µm

0.00 0.05 0.10 0.15 0.20 0.250

100

200

300

400

500

600

Crit

ical

leng

th (µ

m)

Strain (εr) (%)

t=1µm t=2µm t=4µm t=4.83µm

Figure 4.22 – Critical length as function of strain for different thicknesses of beams. The picture to the right illustrates the increased buckling of beams for increased lengths.

Figure 4.23 – White light interferometry image of the bridge structure. The top graph shows the cross section of the beam with a buckle of almost 7µm.

115

The cantilever structure had one end of the beam free and suffered droop. Figure

4.24 shows the white light interferometry image of the structure. An upward droop of

more than 1.8µm suggests that there was tensile stress on the top part of the beam and

compressive stress on the bottom. After release, the stress can relax and the structure

bends up. This will require very high voltage to actuate the beam, as force is proportional

to V2/d2, with d being the gap between the mirror and substrate and V the applied voltage.

Excessive voltage may cause an increased leak current through the anchors, as the

isolation is not done with an insulating layer but intrinsic GaAs. Hence, the increase in

voltage will not totally be translated into applied force and the device will eventually not

move all desired range.

Figure 4.24 – White light interferometry image of the cantilever released in one step. The beam had an upward droop of more than 1.8µm.

A successful corrective measure for the problem was to complete the release in

two steps. Stress relaxation effects can be minimized by releasing first the head of the

116

filter and later the beam. If selective etch is done in only one step, the beam is released

before the head and the stress cannot relax until the head is completely released. By

releasing in two steps, the large area under the head is removed first and in the second

step the beam can relax smoothly. Figure 4.25 shows the white light interferometry

image of the structure released in two steps. The droop was reduced to 0.16µm.

Figure 4.25 – White light interferometry image of the cantilever released in two steps. The beam droop was reduced to 0.16µm.

Another issue that consistently repeated itself was a hole that was being dug right

at the optical path. This hole is shown in Figure 4.26, taken from the top view of a device

with broken cantilever. The origin for such hole was also related to the internal stress in

the wafer during release. Figure 4.27 shows the remaining post under the head after

15min of etch. The cantilever was totally released by then and the internal stress was

pushing the post. As etching proceeds and the post area decreases, the surface resistance

was low enough so that the top layers of the bottom mirror were ripped out. When

117

etching is completed, there is no trace left from such event because the post is totally

etched.

Figure 4.26 – Hole at the optical path. Top view from a device with broken cantilever.

Figure 4.27 – Remaining post under the head after 15min of etch. The cantilever was totally released by then.

The hole issue was also solved by releasing the device in two steps. Figure 4.28

shows the two step release sequence. The head is released first by protecting the

cantilever with photoresist, which is removed later to complete the selective etch. One

restriction of this method is that, after the first release, the head may be too fragile to

stand another spinning of photoresist, which has to be thick to cover the topography of

the devices and probably has high viscosity. Thus, the anchors cannot be protected and

will have undercut. However, the undercut is small, proportional to the beam width, and

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the anchors cannot be protected only when the beams are thin and narrow.

After 1st releaseAfter 1st release

After 2nd releaseAfter 2nd release

Figure 4.28 – Two steps release. The head is released first by protecting the cantilever with photoresist (a), which is removed later to complete the selective etch (b).

Fabrication took a longer way than expected from conception to good working

devices. The overall results were very good but before discussing them, fabrication of

the folded beam filter will also be addressed.

4.3.b Folded beam

The fabrication of this device was done on the same wafer designed to work as a

tunable detector. In fact, the tunable detector design has a tunable filter on top of a

broadband detector. This wafer will be described in detail in the next chapter and for

now the important thing is that the filter has a structure very similar to the wafer designed

for the torsional filter (two contacts on top, etch stop layer, etc.). Fabrication description

will be limited to topology related details.

Masks for lithography were designed for the two sets of test structures. However,

they had to be adapted to a wafer that was actually designed to work with another type of

devices. Both masks included simple cantilevers, which are the simplest structure that

can be fabricated. The open area topology was easy to be adapted and devices were

119

designed similar to the trampoline topology but with one fold per beam.

The two test structures had slightly different fabrication sequences. The open

area topology was processed essentially in the same way as the torsional filter: top metal,

device vertical etch, wet selective etch to reach contact layer, bottom metal, release and

CPD. Figure 4.29 shows a picture from a fabricated device.

Figure 4.29 - Picture from a fabricated device with folded beams and open laterals.

The buried topology had to sacrifice the array addressability to conform to the

impossibility of having a matrix addressable bottom contact. For array addressing, the

top contacts would have to be separated by rows and the bottom ones by column (or vice-

versa). By enabling a specific row and a specific column, the device at the intersection is

actuated. The top to bottom processing allows the top contact to be isolated in one

dimension and connected into the other. Once this has been done, there is no way that

the bottom contact can have isolation orthogonal to the top one. This addressability

requires a different wafer design. The solution was to have a common ground for the

entire die. The top contact had two different test structures. The first one had cells where

devices with same size were located in a column and with large anchors, which would be

directly probed. The other one had clusters of filters with same head size that had surface

wires running to contact pads at the border. These contact pads can be easily probed or

120

wire bonded for testing with the focal plane array. The first approach did not require

protection of the anchors for release but because of this had much worse surface

coverage. Figure 4.30 shows the corner of the die designed to have buried devices. The

bottom contact would surround the entire die, which can be diced later.

Bottom contactTop contacts

Surface wires

Bottom contactTop contacts

Surface wires

(a) (b)

Figure 4.30 – Buried devices. (a) Picture of die designed to have large anchors for direct probing. (b) Schematics of the corner of the die designed to have surface wires.

Fabrication sequence of buried devices encompasses the same processes as the

previous devices but in a different order. Starting from the bare wafer, the first step is the

bottom contact deposition. Photoresist protects the center of the die and the edges are

etched up to the etch stop layer. The width of the etched edge has to be wide enough to

allow the collimated laser beam from in situ monitoring to strike the surface. Wet etch is

used to remove both low and high Al content layers, as before, building a step to protect

the GaAs contact layer during selective etch. Bottom contact is deposited through liftoff

technique. Photoresist is removed and top contact is also deposited using liftoff. Next,

the device is defined by simple lithography and photoresist mask (hardbaked) is used for

vertical dry etch. This etch does not need to go all the way to the etch stop layer as the

contacts are already deposited. It is enough to etch up to half of the bottom DBR. A

121

small aperture was left at the center of the die to accommodate the laser beam for in situ

monitoring. Next step is the selective etch, which removes all exposed GaAs. After

selective etch, the die is cleaned, photoresist is removed and CPD ends the fabrication

sequence. Figure 4.31 shows a SEM picture from a fabricated cluster with 70µm head

devices.

Figure 4.31 - SEM picture from a fabricated cluster with 70µm head devices

Open area devices were all released. Buried devices had some trouble to be

released. A tentative of etching small holes on the top mirror in order to help etchant to

penetrate during selective etch was unsuccessful. Fist the holes cannot be too large or the

incoming wave to be measured will see these holes. Second, the holes cannot be too

small or the etchant/by-products flux will not go on in an orderly way. Also, if the holes

are too small, vertical etching rate will be much smaller than the rest and they may not

reach the sacrificial layer. Squared holes with sides of 3µm and 5µm were etched,

targeting application at LWIR, and etch was happening only ~2µm around the hole for

both sizes. The holes were probably too small. Unfortunately, pictures that can show

this limitation were lost on a computer crash. The only alternative left was to proceed

small adjustments to the selective etch recipe and try a one time release of large areas.

Selectivity of the dry etch was good as layers with Al0.2Ga0.8As were not being

122

attacked. So, SF6/SiCl4 ratio was not changed. Etching was limited probably due to

redeposition or diffusion in the gap. Three parameters were adjusted, each one at a time.

The first one was to decrease pressure from 70mTorr to 50mTorr, as increased pressure

had shown to also increase redeposition. The change did not show any appreciable

improvement but pressure of 50mTorr was kept from this point on. Second factor to be

changed was substrate temperature, increased from 50ºC to 70ºC, the maximum that our

equipment could take. With this increment, probably combined with pressure, the first

60µm head was released and sometimes even 70µm. The last parameter to be changed

was RF power, decrease from 45W to 40W, which made the plasma unstable, turning on

and off. RF power of 42W has resulted on stable plasma and consistent etch of 70µm

and sometimes even 80µm.

This process of adjusting parameters and testing etch results consumes a lot of

wafer and the above results were considered satisfactory under the limited material that

we have available to demonstrate this device. The above results and adjustments were

valid for the Oxford Instruments Plasma Lab System 100, which was being used at the

Berkeley Microlab.

The surface of the heads of released devices is very flat. Figure 4.32 shows the

white light interferometry image of a released 60µm structure. The beams are short

enough and only negligible buckling was noticed (<40nm).

123

Figure 4.32 – White light interferometry image of the folded beam structure. The surface of the heads of released devices is very flat. No considerable buckling was noticed (<40nm).

The fabrication sequence also had some processing issues. To start with, selective

etch was not repetitive for same structure and etching time on different dies. This

problem was persistent and discussed before in chapter 2. The solution was to protect all

anchors and etch for very long time.

The critical problem, however, was redeposition. This problem had increased

from previous runs, such as the torsional filter, and this was attributed to some other users

etching GaN in the chamber. This etch requires very high power (~250W) and the top

electrode has suffered some sputtering. As a result, redeposition was noticed not only

after selective but also after vertical etch. Figure 4.33 shows a sequence of pictures of an

open area device during processing. Redeposition was critical after vertical etch. The

second vertical etch was going up to the abruption layer of the tunable detector wafer.

124

Redeposited material was not uniform and was removed further during selective etch, wet

and dry. Note also that redeposition was not repetitive, sometimes being light and

sometimes being heavy, strongly dependent on chamber conditions.

metal Vertical etch Wet etch

Vertical etch Release etch

metalmetal Vertical etchVertical etch Wet etchWet etch

Vertical etchVertical etch Release etchRelease etch

Figure 4.33 – Processing sequence showing redeposition after vertical etch. Redeposited material was not uniform and was removed further during selective etch, wet and dry.

Redeposited material after vertical etch may have caused flux of this material to

the gap and further redeposition under the head during selective etch. This problem was

previously solved, by increasing the gap and favoring flux dynamics, and came back as a

result of another cause. The only reproducible fact happened when the die had heavy

redeposition from the vertical etch: a lot of debris under the head. Figure 4.34 shows the

SEM images from under the head after release. Severe redeposition is shown in Figure

4.34(a) for a die with heavy redeposition from vertical etch. Figure 4.34(b) shows a clean

release from a die with almost no redeposition from vertical etch.

Attempts to clean the chamber consisted of baking (70ºC for at least 4hs) and

oxygen plasma cleaning followed by at least two dummy GaAs etches of 30min using the

selective etch recipe. This partially solved the problem but even the low RF power (when

compared to GaN etch) that was used for vertical etch may have sputtered the upper

125

electrode after it has been damaged. By the time that the top electrode got fixed we had

already considered this development as finished.

(a) (b)

Figure 4.34 – SEM images from under the head after release. (a) Redeposition under the head after release of a die with heavy redeposition from vertical etch. (b) Clean release from a die with almost no redeposition from vertical etch.

4.4 Applications

This section will show the characterization and performance results from the

torsional and the large area filters. The practical use for these devices is very broad but

both characterizations were done looking to the targeted applications of communications

and IR imaging, respectively.

4.4.a Communications

The torsional filter was designed looking to the solution for two major drawbacks

on micromachined electrostatic tunable devices. First, making possible the movement of

the head portion for more than 1/3 of the gap and achievement of larger tuning for a

given initial gap size. Second, prevention of electrical discharges. Both problems were

successfully solved by this design as to shown next.

126

The mechanical performance, shown in Figure 4.35, was measured using white

light interferometry. The gap size at the head position increases with voltage, whereas

that of the counterweight decreases, as expected. The nonlinear relation between

displacement and voltage is a consequence of the larger increase in electrostatic force (∝

V2/gap2) compared to the spring restoration force (∝ displacement). The device moves

until the counterweight is pulled onto the substrate as a consequence of the 1/3 rule. As,

the counterweight bends and the values were measured at its center, the shown

displacement is slightly less than 1/3 of the initial gap. The changes of the gap size are

larger at the head than at the counterweight as a result of the leverage effect, allowing a

broader tuning range for a given initial gap. In this case the head moves up twice the

distance of the counterweight downward movement. This translates a leveraging effect

of 2, which is much larger than the predicted by finite method analysis. Probably, the

beams are much stiffer than expected and some of the material parameters, such as

Young’s modulus or Poisson’s ratio, are not right for the thin multilayered structure.

This directly affects the beam bending and torsion, which ultimately change leveraging

ratio. The initial gaps (0V) were slightly different showing a small negative droop of the

head (less than 0.1µm), which is probably result of the stress among the epitaxial layers.

The optical performance of the filter was extensively measured. Figure 4.36

shows both theoretical and experimental tunings as a function of the gap size. The results

agree very well with theory. The tuning occurred in two different cavity modes. The

small shift of the experimental curves is in part due to the shift of the entire DBR stop

band during wafer growth, as shown in Figure 4.16. Even with the tuning starting on the

undesired mode, continuous tuning through the entire targeted mode was still possible at

127

higher voltages.

0 5 10 15 20 25

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

2x

filter counterweight

filter head

gap

size

(µm

)

Voltage (V)

x

Figure 4.35 – Measured displacements of the torsional filter. The measurements were performed using white light interferometry and show the upward head movement corresponding to double the counterweight downward movement.

2000 2200 2400 2600 2800 3000 3200 34001480

1500

1520

1540

1560

1580

1600

1620

theory

measurement

mode 1 mode 2

gap size (nm)

Tran

smitt

ed W

avel

engt

h (n

m)

Figure 4.36 – Transmitted wavelength as function of gap size. Stars indicate the expected modes and squares represent measured transmitted modes. Continuous tuning through the mode to the right covers around 100nm.

Figure 4.37 shows the relation between wavelength and voltage. As voltage

increases, the gap size varies faster and so does the transmitted wavelength. By changing

the voltage from 18V to 22V the Fabry-Pérot wavelength is tuned more than 100nm.

128

From 0V up to 16V the cavity mode one order below is being transmitted with 50nm of

tuning (Figure 4.36). Only recently, a cavity using DBR pairs of air/InP achieved

comparable tuning range [51].

0 5 10 15 20 251500

1520

1540

1560

1580

1600

1620

mode 2

Tr

ansm

itted

Wav

elen

gth

(nm

)

Voltage (V)

mode 1

Figure 4.37 – Measured transmitted wavelength at the filter head as function of voltage. Measurement was performed by sweeping the TTF tuning voltage and recording its value at peak transmission for each wavelength.

Figure 4.38 shows the setup used to measure the spectral response. Light is

focused into the filter head and the position of the beam waist has to be coincident to the

cavity waist, otherwise mode mismatch broadens the transmitted linewidth and increases

optical losses [124]. The filter is very sensitive to this optical coupling. Careful

optimization can be done in real time by applying a sawtooth voltage to the filter beam

while keeping the incoming wavelength unchanged. The signal from the reflection

detector is coupled into an oscilloscope and the filter position is optimized (x, y and z)

until the best lorentzian like shape is obtained on the scope. Frequency of the biasing

signal has to high enough only to have the signal on the scope, but 10 kHz was used. If a

frequency smaller than 10Hz is used, it allows the operator to see the beam moving even

for the very small movement involved. After this fine adjustment, the voltage of the

129

beam can be kept constant at the desired value and wavelength is scanned in order to

measure the real linewidth of the filter transmission. However, optimum points are

function of wavelength and dependent on the verticality of the laser beam.

Tunable Source

White light

BS1

BS2

20X Lens

50X Lens

BS3

Cube cage

CCD (top view)

Reflection Detector +Transimpedance Amplifier

Rotation Mount

Tunable Filter

Collimation lens

Saw-tooth bias voltage

Scope

Transmission Detector

Tunable Source

White light

BS1

BS2

20X Lens

50X Lens

BS3

Cube cage

CCD (top view)

Reflection Detector +Transimpedance Amplifier

Rotation Mount

Tunable Filter

Collimation lens

Saw-tooth bias voltage

Scope

Transmission Detector

Figure 4.38 – Setup used for spectral characterization of the tunable filter.

Figure 4.39 shows the transmission spectra for different tuning voltages.

Transmission was measured with a broad area detector under the device. This tuning

range is very broad and it was limited by the tunable source used for testing. The

linewidth broadens for longer wavelengths due to the proximity to the edge of the DBR

stop band, which has smaller reflectivity. The still sharp spectrum for short wavelengths

together with Figure 4.36 and Figure 4.37 shows that if we had a tunable laser able to go

below 1513nm, transmission of shorter wavelengths would have been measured. Thus,

the device can cover C, L and S bands with more than 100nm of tuning.

In some cases, a satellite peak is observed at wavelengths ~7nm longer than the

main modes what is attributed to mismatch in optical coupling. By carefully optimizing

the mode matching for each wavelength, using the above real time adjustment, linewidth

can be reduced to <1nm and the extinction ratio can be above 20 dB, as can be seen in

130

Figure 4.40. However, this linewidth is still almost double than the theoretical prediction

from Figure 4.4. This may be explained by scattering from the rough surfaces and also

the limited area of the head that causes diffraction loss [93]. Sharper linewidths can also

be achieved by increasing the number of DBRs, which increases the mirrors reflectivity.

1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600

0.0

0.2

0.4

0.6

0.8

1.022.1V18.5V

Wavelength (nm)

Nor

mal

ized

Tra

nsm

issi

on 22.1V 21.6V 21.2V 20.8V 20.4V 20.0V 19.8V 19.7V 19.2V 18.8V 18.5V

Figure 4.39 – Measured transmission spectrum for the TTF. 100nm tuning were achieved and this result were limited by our tunable source range.

1500 1520 1540 1560 1580 1600-35

-30

-25

-20

-15

-10

-5

0

>20dB

∆λ=1nm

Tran

smis

sion

(dB

)

Wavelength (nm)

Figure 4.40 – Transmission spectrum for the torsional filter at 1525 nm showing an extinction ratio greater than 20dB and 1nm linewidth. Optimized coupling has eliminated the side mode at longer wavelength.

The satellite peaks were object of a closer look. The fact that they are positioned

131

at longer wavelengths and the effect can be minimized with careful optical adjustment is

not compatible with high order (transverse) cavity modes. Transversal modes are located

at higher frequencies than the fundamental longitudinal mode [52, 124] and have been

experimentally demonstrated at shorter wavelengths for micromachined tunable devices

[17]. In order to investigate the phenomenon, satellite peaks from two different

topologies have been compared. Figure 4.41 shows the two topologies. While torsional

has small head and large gap, cantilever has large head and small gap.

TorsionalSmall head (15µm), large gap

A

A’

A

incident light

AA’

DBRs

air gap

GaAs substrate

transmitted light

A’

incident light

AA’

DBRs

air gap

GaAs substrate

transmitted light

CantileverLarge head (25µm), small gap

TorsionalSmall head (15µm), large gap

AA

A’

A

incident light

AA’

DBRs

air gap

GaAs substrate

transmitted light

A’

incident light

AA’

DBRs

air gap

GaAs substrate

transmitted light

CantileverLarge head (25µm), small gap

Figure 4.41 – Detail of the two topologies used to compare the satellite peaks. Torsional has smaller head and larger gap than cantilever.

Figure 4.42 shows a typical short wavelength spectrum for torsional and

cantilever topologies. Cantilever shows two side modes while torsional shows only one.

The first mode is at about the same level for both devices: around -4dB for torsional and -

3dB for cantilever. However, the spacing is very different. Figure 4.43 shows the side

mode spacing as function of wavelength for both topologies. The larger the cavity, the

farthest the peaks are apart and the spacing with respect to the cavity mode is almost

constant with wavelength. This is also different from what would be expected from

transversal modes. Shorter cavities, in this case cantilever, should have the modes more

132

spaced as free spectral range is inversely proportional to cavity length.

1515 1520 1525 1530 1535 1540 1545 1550 1555-10

-8

-6

-4

-2

0 ∆λ1

∆λ2

norm

aliz

ed tr

ansm

issi

on (d

B)

wavelength (nm)

CantileverTorsional

1515 1520 1525 1530 1535 1540 1545 1550 1555-10

-8

-6

-4

-2

0N

orm

aliz

ed T

rans

mis

sion

(dB

)

Wavelength (nm)

∆λ1

1515 1520 1525 1530 1535 1540 1545 1550 1555-10

-8

-6

-4

-2

0 ∆λ1

∆λ2

norm

aliz

ed tr

ansm

issi

on (d

B)

wavelength (nm)

CantileverTorsional

1515 1520 1525 1530 1535 1540 1545 1550 1555-10

-8

-6

-4

-2

0N

orm

aliz

ed T

rans

mis

sion

(dB

)

Wavelength (nm)

∆λ1

Figure 4.42 – Short wavelength spectrum for torsional and cantilever topologies. Cantilever shows two satellite peaks while torsional shows only one. The first peak is around -4dB for torsional and -3dB for cantilever.

1515 1520 1525 1530 1535 1540 15450

1

2

3

4

5

6

7

8

9 large gap

small gap

torsional only mode

cantilever 2nd mode

cantilever 1st mode

Sid

e m

ode

dist

ance

(nm

)

Wavelength (nm)

Figure 4.43 – Side mode spacing as function of wavelength for torsional and cantilever topologies.

Two possibilities can be drawn from these observations. The hypothesis is that

these side modes are a result of poor optical coupling into the cavity, which originates

other weaker oscillations with respect to the main mode. The first possible cause for the

difference between the spectra is that the smaller head of torsional filters the upper

coupling modes by loss, while cantilever can still confine a second mode. The second

133

possibility is that larger gaps allow optical beams to diffract more inside the cavity and

the beam expansion also filters modes by loss. The first cause is more likely as the side

mode spacing is almost constant with gap. However, this is still not conclusive. The

only certain thing is that these modes are not transverse cavity modes. It would be better

to have several different head sizes for the same type of device. So, the design for the

tunable detector wafer, to be described in the next chapter, has incorporated a cavity with

controllable aperture to make possible a deeper study of this phenomenon. So, high order

modes can be filtered, the transmitted linewidth can be controlled and several different

apertures may allow different behavior of coupling modes.

This filter was characterized in a data link for transmission performance. The

experiment compared the performances of a commercial arrayed waveguide grating

(AWG), from Lightwave Microsystems, and the torsional tunable filter (TTF), as shown

in Figure 4.44. The optical link consisted of a 1550 nm tunable VCSEL directly

modulated (DM) at 2.5 Gbps, an EDFA, either AWG or TTF, and an APD receiver. The

AWG has 40 channels, each with 0.41 nm linewidth and almost flat passband. Both

devices have the same insertion loss, ~7dB. A more optimized optical coupling and

antireflection coating may further reduce the insertion loss for the TTF to around 1dB

[17].

The characterization setup got very complicated and coupling is probably far from

ideal. The complication comes from coupling light from single mode fiber through the

device and back to single mode fiber. Figure 4.45 shows a schematics of the set up. A

CCD camera is used to position the device at the right place and intercept the beam.

134

EDFATunable VCSEL

APD Detector

AWG

or

TTF

(a)

(b)EDFATunable

VCSELAPD

Detector

AWG

or

TTF

(a)

(b)

Figure 4.44 – Optical circuit for bit error rate (BER) measurements performed using (a) Arrayed Waveguide Grating (AWG) from Lightwave Microsystems and (b) torsional tunable filter (TTF).

BS

20x Magnification Objective

Movable mirror

IR CCD (top view)

High SpeedReflection Detector

High SpeedTransmissionDetector

Tunable Filter Holder Set

Fiber from Tunable Source or WDM Fiber

Collimation lens

Collimation lens

Fiber

White lightbundle

Focusing lens

BS

20x Magnification Objective

Movable mirror

IR CCD (top view)

High SpeedReflection Detector

High SpeedTransmissionDetector

Tunable Filter Holder Set

Fiber from Tunable Source or WDM Fiber

Collimation lens

Collimation lens

Fiber

White lightbundle

Focusing lens

Figure 4.45 – Schematics of the setup used for transmission measurements in a data link.

Figure 4.46 shows a picture of the set up built for the experiment. Alignment

starts from the top to bottom by collimating light and making it vertical. The fiber plus

lens have tilt adjustment and are controlled by XYZ1. Collimated light goes through a

10/90 beam splitter and a focusing lens. The lens is positioned in such a way that its

focal point is roughly at the level of the device, and it must still keep the beam vertical.

This lens will later couple light into the filter cavity. The focusing lens is controlled by

135

XYZ2. A CCD camera monitors the position of the focused beam in order to move the

filter head to the right spot and the device position is controlled by XYZ3. Light

transmitted through the device has to be collimated again and another lens is required.

This lens has to have its focal point coincident with the focal point of the lens that

focuses light onto the device (XYZ4 controls the lens). Finally, another focusing lens is

inserted (in the same mount is the previous one) to focus collimated light back into the

single mode fiber (controlled by XYZ5). Then the device is inserted into the optical path,

by looking into the CCD image. The device is moved in 3D until loss is minimized. Fine

adjustment of the position is made using the same method as before, scanning the filter

beam and optimizing the transmitted pattern. After all this, 7dB of insertion loss was a

big victory.

beam splitter

input fiber + collimator (XYZ 1)

focusing lens (XYZ 2)

collimation and focusing lenses(XYZ 4)

output fiber (XYZ 5)

XYZ 1

XYZ 4XYZ 5

XYZ 3

tunable filter (XYZ 3)

tunable filterprobes

CCD

XYZ 2

beam splitter

input fiber + collimator (XYZ 1)

focusing lens (XYZ 2)

collimation and focusing lenses(XYZ 4)

output fiber (XYZ 5)

XYZ 1

XYZ 4XYZ 5

XYZ 3

tunable filter (XYZ 3)

tunable filterprobes tunable filterprobes

CCD

XYZ 2

Figure 4.46 – Picture of the setup used for transmission measurements. Bit error rate measurements require the output from single mode (SM) optical fiber to be transmitted through the device and coupled back to SM fiber.

Figure 4.47 shows BER as a function of received power for the link with AWG

and with the tunable filter. There is virtually no difference between the two cases,

136

indicating the passband design of the single Fabry-Perot torsional filter is suitable for

direct modulated lasers at 2.5 Gbps.

-40 -38 -36 -34 -32 -30

1E-3

1E-4

1E-5

1E-6

1E-7

1e-81E-9

1E-101E-111E-12

(a) AWG Filter (b) Tunable Filter

(a)

(b)

Bit

Err

or R

ate

(BE

R)

Received Optical Power (dBm)

Figure 4.47 – BER plots using either (a) commercial AWG from Lightwave Microsystems or (b) TTF in the optical circuit from Figure 4.44.

During characterization of the filter, no discharges happened at all. The devices

have been exhaustively tested and under no condition an electrical discharge has

occurred. However, this was attributed more to the half doping of the DBRs than to the

structure itself.

Finally, the results presented here are among the best in the literature, not only for

the record tuning range but also for the good transmission characteristics over the

covered spectrum. The device has extended the 1/3 rule and no electrical discharges have

happened.

4.4.b Infrared Imaging

The folded beam filter was designed looking to the solution for two major

challenges on IR imaging. The first one would be to have large area coverage in order to

137

be integrated to a focal plane array. The integration would happen in a pixel wise manner

or covering small clusters of pixels. The second challenge would be individual

addressability. The fabricated devices have targeted large area coverage but individual

addressability would be impossible for the available wafer. Thus, the major challenge

was on fabrication and the characterization done was only to prove that the device can

work in accordance with section 4.2.b.

The developments previously described, design and fabrication, were specifically

for this application. While discussing main issues inherent to those tasks, most of the

application details have been presented and will not be repeated here.

Optical testing and integration to a focal plane array was supposed to be done at

Raytheon Company’s facility in Santa Barbara. The main reason for this is that the

detector usually requires cooling at 77K. Testing would be complete only if done

coupled to a focal plane array. However, this would only be possible if the device was

fabricated on a dedicated wafer.

The setup shown in Figure 4.48 has been designed to accomplish optical

characterization at MWIR. An optical probe with sharp spectral distribution has to be

used to confirm that the filter is working. This probe can be either a laser or a light

emitting diode (LED) coupled to a passband filter. The second option was chosen based

on price. Light has to be focused back to the device and transmission is measured with a

broad area detector. By changing the cantilever voltage from a low to high voltage, the

filter can transmit or block the incoming beam. If the filter is working properly it will

provide spectral information to the focal plane array.

138

4.2 4.4 4.6 4.8 5 5.2 5.4

0.2

0.4

0.6

0.8

1

LED Source (Boston Electronics)

Collimating lens

Bandpass filter(Janos Technology)

λ=4.8um

∆λ=0.5um

P=10uW (CW)

λ=4.8um

∆λ=0.15um

P~1.5uW

Focusing lens

Tunable Filter

V1V2

V1V2

λ=4.8um

∆λ=0.07umP~0.8uW

Detector

or

λ=4.8um

P~0.1uW

Transmitted Spectrum

V1V2

4.2 4.4 4.6 4.8 5 5.2 5.4

0.2

0.4

0.6

0.8

1

LED Source (Boston Electronics)

Collimating lens

Bandpass filter(Janos Technology)

λ=4.8um

∆λ=0.5um

P=10uW (CW)

λ=4.8um

∆λ=0.15um

P~1.5uW

Focusing lens

Tunable Filter

V1V2

V1V2 V1V2

λ=4.8um

∆λ=0.07umP~0.8uW

Detector

or

λ=4.8um

P~0.1uW

Transmitted Spectrum

V1V2

Figure 4.48 – Setup designed for optical characterization of the device at MWIR.

The devices were fabricated on a tunable detector wafer, which were designed to

have the tuning range centered at 850nm. The absorption layer is GaAs and for

transmission measurements of the filter, photon energy has to be below GaAs bandgap (E

< 1.42eV or λ > 873nm).

The mechanical characteristics of the device were shown in section 4.3.b, when

fabrication was discussed. Flat surface and negligible buckling promises good results.

Figure 4.32 shows Gap size as function of voltage for the 60m device from Figure 4.32.

The gap closes well up to ~5V when current across the anchor starts to increase causing

an inflexion to the curve, which was supposed to have parabolic shape. This wafer had

poor electrical isolation between mirrors, probably due to high concentration of defects in

the sacrificial layer. If better isolation was provided, voltage to move the device would

be even smaller.

139

0 1 2 3 4 5 6 7 81.3

1.4

1.5

1.6

1.7

1.8

Gap

siz

e (µ

m)

Voltage (V)

Figure 4.49 – Gap size as function of voltage for the 60m device from Figure 4.32.

However, actual voltage to move a 60µm device is quite small when compared to

the numbers calculated by considering the same spring constant as in the trampoline

structure. Figure 4.50 shows these results. Comparing simulation to experimental data

from Figure 4.49 shows an overestimation of almost 10X. This means that the beams are

much softer than in the trampoline case due to torsion caused by the momentum applied

to the beams due to the fold. Addition of this momentum to the calculation provides a

better approximation to the experimental results [121] but brings no gain with respect to

finite element analysis.

140

0 10 20 30 40 50 60

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Gap

siz

e (µ

m)

Voltage

Figure 4.50 – Gap size as function of voltage for the 60µm device. Simulation was done based on the spring constant of the trampoline structure.

Once it is known that the mechanical structure works, there is not much more to

be done at wavelengths different than the designed for the DBR mirrors. Measurements

at 850nm would be possible but would only provide results similar to the ones obtained

for the tunable detector, to be shown in the next chapter. However, transmission

measurements can be at wavelengths above 900nm, where negligible absorption would

be expected. Some simulations were run at short and middle IR wavelengths and the

results are shown in Figure 4.51 as a contour plot of reflectivity as function of gap size

and wavelength. Good contrast for on-off actuation can be achieved in both ranges.

Short wavelengths of 1.55µm, 1.6µm and 1.75µm can provide contrast on a broadband

detector for gaps going from 1.8 to 1.4µm. The same for middle wavelengths of 2.6µm,

2.7µm, 3µm and 3.5µm.

141

Wavelength (µm)

Gap

(µm

)

Wavelength (µm)

Gap

(µm

)

Wavelength (µm)

Gap

(µm

)

Wavelength (µm)

Gap

(µm

)

Figure 4.51 – Simulated reflectivity in the short and middle IR ranges as function of wavelength and gap size.

It is fortunate that the testing can be done at 1.55µm so the same setup used for

BER measurements, Figure 4.46, can be used. Figure 4.52 shows power reflectivity as

function of wavelength, around 1.53µm for two different gap sizes. By switching the

voltage between 0 (gap of 1.8µm) and 7V (gap of 1.4µm) we would expect the

transmitted power to increase from 30% to 90% of the incident power, respectively, if the

insertion loss is 0. For an insertion loss of 7dB, which was the number for the torsional

filter, transmitted light would increase from 0.6% to 0.18%, giving a contrast of ~5dB,

more than reasonable for the testing.

Measurements were performed using a distributed feedback (DFB) laser at

1.53µm. The results are shown in Figure 4.53 as function of time. When voltage is

increased (smaller gaps) the transmitted power also increases (as reflectivity decreases),

as expected. However, the extinction ratio is much smaller than expected probably due to

degradation of the isolation between mirrors that may have increased the current across

the sacrificial layer and limited the excursion of the filter head.

142

1.48 1.5 1.52 1.54 1.56 1.58wavelength HmmL

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

Gap=1.80µm

Gap=1.45µm

1.48 1.5 1.52 1.54 1.56 1.58wavelength HmmL

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

Gap=1.80µm

Gap=1.45µm

Figure 4.52 – Simulated transmission characteristic for the tunable filter at short IR wavelengths. If a laser at 1.53µm is used, the contrast between on and off positions can be significant.

0 200 400 600 800-31.1

-31.0

-30.9

-30.8

-30.7

-30.6

-30.5

-30.4

-30.3

appl

ied

volta

ge (V

)

trans

mitt

ed o

ptic

al p

ower

(dB

m)

time (a.u.)

0

2

4

6

8

10

12

14

Figure 4.53 – Measured transmitted power through the folded beam filter as function of time for variable applied voltage.

Finally, the device has demonstrated to be a very promising approach to the

integration of tunable filters to focal plane arrays, provided that the wafer is designed to

be dedicated to this fabrication.

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4.5 Summary

This chapter presented specific details on the design and fabrication of tunable

filters applied to communications and IR imaging.

A torsional micromechanical tunable optical filter has been shown to be applied to

communications, with potentially far large tuning range and free of damages resulting

from capacitor discharge. The tuning range is above 100nm and was limited by the

tunable source available for testing. The insertion of the filter in an optical data link does

not cause any power penalty.

A folded beam filter structure with large surface area coverage was presented for

application on IR imaging. A novel double-cavity filter design, covering two different IR

bands, has also been presented. The proposed device has large ratio of detector to pixel

area and requires a low actuation voltage simultaneously. Furthermore, it is scalable and

facilitates an easy implementation of 2D array with matrix addressability.

Both innovative designs presented in this chapter are generic and can be applied

to other tunable optoelectronic devices, such as tunable lasers and detectors, to be

presented in the next chapter.

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Chapter 5 Tunable Detector

5.1 Introduction

In the previous chapter, tunable filters with wide tuning range and innovative

mechanical structures have been presented. Independent of the structure, either a

broadband detector or a delicate coupling system to fiber was used to detect the filtered

signal. This chapter discusses the development of a tunable filter integrated to a detector.

The approach has several advantages such as size reduction due to the integration,

flexibility of closed or open loop operation between filter and detector, shielding to

ambient light, and increased sensitivity due to small dark current.

The chapter starts by presenting particular aspects of design and fabrication. The

device has been designed to be applied in biosensing, integrated to a platform where

biological reactions occur, so next, the biosensor system will be described. Finally, the

characterization results close the chapter.

5.2 Design

The design of a monolithically tunable detector would have basically two

different choices, both related to the position of the absorption layer: resonant cavity

detector (RCD) or Fabry-Pérot filter detector (FP). Figure 5.1 shows the schematics for

both structures. In the first one, an absorption layer, usually quantum wells, is placed

inside the cavity and the standing wave intensity increases sharply at resonance, and so

does the detector response. In the Fabry-Pérot filter detector the structure can be

analyzed as two independent devices brought together: filter and detector. Wavelength

145

selectivity comes from the filter and the detector is a broadband p-i-n.

Input

Mirror 1

Mirror 2

Mirror 1

Mirror 2Absorption

region

Absorptionregion

Input

Mirror 1

Mirror 2

Mirror 1

Mirror 2Absorption

region

Absorptionregion

(a) (b)

Figure 5.1 – Schematic of two possible tunable detector designs: (a) resonant cavity detector (RCD) and (b) Fabry-Pérot filter detector (FP).

The RCD main advantage is high detection efficiency at resonance. The field

inside the cavity can be orders of magnitude higher than the input signal for high Q factor

resonators such as the vertical cavities using distributed Bragg reflectors (DBRs) or

subwavelength grating (SWG) mirrors, with reflectivity > 99% and sharp linewidth of

about 0.5nm (Q ≈ 3000). However, the absorber has to be placed at the center of the

cavity and the air gap for tuning has to be moved to the top mirror [21]. Thus, mirror

reflectivity changes when the gap is moved, changing the cavity Q factor and this change

can be drastic. Energy inside the cavity decreases with changes in gap and the detector

response is strongly non-uniform over wavelength tuning, even though it can be very

efficient at some discrete wavelengths.

The FP filter detector is much less sensitive to the problem pointed before. First,

it can still have the air gap as the cavity. Second, even if it does not, responsivity is not

so strongly dependent on the cavity Q factor but on the matching between mirrors, as

given by equation (2.11). So, responsivity does not change significantly over wavelength

146

tuning. Moreover, design is much easier as filter and detector can be decomposed into

two different devices that can be optimized independently.

Based on the above discussion, the tunable detector was chosen to have the Fabry-

Pérot design. So, the filter is designed essentially in the same manner as before and the

detector will be discussed later.

General design changes are required to realize the device and two of them are

straight forward. First, a third electrical contact has to be added, so the device has a

common ground, filter bias and detector bias. Also, a second pn junction is required for

the detector. Some more specific details are discussed next.

5.2.a Fabry-Pérot tunable detector

Tunable detector has design details that are particular to the new structure. Some

of the details are intrinsic for detectors and some are brought from the lessons learned

from the tunable filter.

Characterization of the tunable filter, designed for communications, had shown

lateral satellite modes at longer wavelengths, as described in section 4.4.a. Previous

investigation of such modes was not conclusive but spatial filtering is the most likely

cause of different results for cantilever and torsional topologies. Further study on the

origin of such modes has inspired the design of an optical cavity with variable aperture

for the tunable detector. Thus, the air gap has been moved to the top DBR and an AlAs

layer has been added as the controllable optical cavity. This layer is later oxidized, in

such a way to leave only a small aperture of AlAs surrounded by AlOx, so that it can

confine only the fundamental longitudinal mode of the cavity. This approach may

provide not only more conclusive results for the side modes but also transverse mode

147

filtering, enhancing transmitted linewidth.

The insertion of the oxide layer to form the optical cavity requires that the air gap

is inserted into the top DBR. This is the drawback pointed to the RCD but, as also

pointed before, the FP filter detector does not show the same dependence on Q factor as

RCD.

Oxidation is a mature technique developed for vertical cavity surface emitting

laser (VCSEL) fabrication. It is used primarily for electrical confinement, but also

provides some limited level of optical confinement. Usually, a very thin layer (~250-

400Å) of AlAs (or AlxGa1-xAs with x > 0.97) is grown very close to the active region and

lateral oxidation of the mesa is carried out until a small aperture is left. As the oxide is an

insulator, electrical current will be confined to this small aperture.

Optical confinement in VCSELs is a secondary effect that brings huge benefits.

One of the effects is index confinement on the exposed portion of the mirrors, as high Al

content layers of the DBRs are also oxidized, although in a much smaller extension. This

index confinement reduces diffraction loss inside the cavity, decreasing threshold current

and increasing efficiency. Another benefit is the reduction of transverse modes

occurrence due to the decrease in current spread and spatial hole burning, not to optical

confinement [125-127].

The purpose of an oxidation layer in the tunable detector is primarily for optical

confinement and mode filtering, as there is no current flow to be optimized in the reverse

biased filter portion. The entire optical cavity is made of AlAs and oxidation is carried

out until the aperture is small enough that only the fundamental longitudinal mode can

resonate. The transverse modes are optically filtered in contrary to VCSEL where their

148

occurrence decreases due to current confinement. The optical cavity is much thicker (λ)

than the oxide layer used in VCSEL technology and the optical effect is also much

stronger. Outside the cavity the index is much smaller and the optical thickness is such

that no resonance would occur for higher order modes, which all have larger spatial

distribution.

Figure 5.2 shows the schematic drawing of the Fabry-Pérot tunable detector.

Both filter and detector are reversed biased and the total structure is designed to have npn

doping profile. The p-contact is the common ground and both top and bottom contacts

are n-type. The substrate is also n-doped and the bottom contact is actually placed on the

back of the wafer. The air gap is part of the top mirror and the optical cavity is formed

by AlAs with AlOx confinement.

Optical cavity

Movable partial top DBR

Bottom DBRDetector

Stationary partial top DBROxide layer Optical cavity

Movable partial top DBR

Bottom DBRDetector

Stationary partial top DBROxide layer

Figure 5.2 – Schematic drawing of the Fabry-Pérot tunable detector.

Central wavelength is required to be 850nm because the biosensor surface, which

is going to be integrated to the detector, is designed to operate around this wavelength.

Tuning for the desired application is not required to be wide. Around 40nm of tuning

would be desirable and the mirrors should have high reflectivity to yield sharp

transmitted linewidth (~0.5nm to allow the control by the aperture).

149

The cantilever structure has been chosen for this development. This topology is

not only the mechanically most simple and easy to fabricate but also the one that takes

less space on the wafer surface so that devices can be placed very close to each other.

There are no voltage requirements but it should be limited to less than 50V in order to

facilitate testing and characterization. Frequency response is much higher than required

to monitor any biological reaction. A cantilever with length = 100µm, width = 5µm and

thickness = 3µm has a resonant frequency of almost 600kHz.

The specific optical cavity design will be described in the next section. For now,

let me borrow the resultant beam thickness of 2.6µm. The beam width is arbitrarily

chosen to be 5µm, based on previous results from lithography and handling during

processing. Several different head sizes have been included so that different apertures

will be available after oxidation, as oxidation comes from the lateral of the mesa and

closes into the center. Figure 5.3 shows the simulated pull-in voltage (or maximum) as

function of beam length for different head sizes. The smallest head size will require the

maximum actuation voltage. If the smallest head is chosen to have 12µm, 100µm beams

are a good choice, with required voltages within desirable limits.

150

10 12 14 16 18 20 22 24 26 28 30 32 34 36

10

15

20

25

30

35

40

45

50

80µm

Pul

l-in

Vol

tage

(V)

Head size (µm)

100µm

125µm

150µm

Figure 5.3 – Pull-in voltage as function of filter head size for different beam lengths.

Figure 5.4 shows the unit cell that was designed and fabricated. Each anchor has

three devices, giving a total of twelve different head sizes. Left head of the top left

anchor (F6A) has 12µm and size increases in steps of 2µm (and anchor sequence goes to

bottom left – F6B, bottom right – F6C, and top right – F6D ). Right head of the top right

anchor (F6D) is the largest, with 34µm. This way, oxidation can be carried out until

12µm or even 14µm are totally oxidized and 10 or 11 devices will have different

apertures running from ~2µm up to ~20µm. From VCSEL fabrication, monomode lasers

have usually 2µm to 5µm apertures. A trivial question that may arise is: why not make

the heads with this size and save the oxidation step? The answer is not so trivial but is

basically due to two factors. First, it is not reasonable to couple light into a head as small

as 2µm because very high numerical aperture lens would be required and the beam would

have large diffraction angle. This is not desirable as, from the discussion on chapter 4,

linewidth is strongly sensitive to spatial alignment (gaussian beam waist position,

151

incidence angle, etc.). The other reason is that very small head would have unavoidable

damages from processing and would generate too much scattering.

Figure 5.4 – Picture from a fabricated die showing a unit cell. Three devices were fabricated on each anchor and a total of twelve different head sizes were fabricated. Left head of the top left anchor (F6A) has 12µm and size increases in steps of 2µm (and anchor sequence goes to bottom left – F6B, bottom right – F6C, and top right – F6D ). Right head of the top right anchor (F6D) is the largest, with 34µm.

5.2.b P-I-N detector

The design of the p-i-n detector takes basically two parameters: efficiency and

speed. However, there is a trade off between these two parameters.

Efficiency of a detector is defined as the ratio of generated electron-hole pairs that

contribute to the detector current due to the flux of photons. Not all photons will produce

electron-hole pairs in the absorption layer because not all incident light is absorbed there.

Processing generates scattering that translates into insertion loss. This can be minimized

with anti-reflection coating but not eliminated. Also, the filter portion is a pn junction

and doped AlGaAs has considerable absorption (GaAs absorption coefficient is 29cm-1

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for 1018cm-3 p-type doping and 4cm-1 for 1018cm-3 n-type doping [128]). Quantum

efficiency can then be written as [129]:

(1 ) filter filter a ad dIL e eα αη − − = − − (5.29)

where IL is the device insertion loss, filterα is the absorption coefficient of the filter

(combines all layers and different doping levels), filterd is the filter thickness, aα is the

absorption coefficient of the absorption layer and ad is the thickness of the absorption

layer. Eventually, some of the light will not be absorbed and will be transmitted through

the device.

Equation (5.29) shows readily that the thicker the absorption layer, the more

efficient is the detector. It also shows that efficiency can be increased by decreasing the

thickness of the filter or at least the thickness of doped regions (or the top contact layer

for an isolated detector).

Speed of a p-i-n detector is function of the finite diffusion time of carriers

produced in the p and n regions, drift of carriers across the depletion layer and shunting

effect of the junction capacitance [130]. The first factor can be minimized by proper

choice of the length of the depletion region, so the speed of the detector is given by:

1( )RC transit

Speed Hzτ τ

=+

(5.30)

where RCτ is simply given by R•C and transitτ is the drift time of holes, which have

mobility much lower than electrons [128]. transitτ is given by ad / hv , with h hv Eµ= being

hole drift velocity and E the electric field across the i region.

Figure 5.5 shows the simulation of speed as function of absorption layer thickness

153

for a detector with area (200x400) µm2 (a rectangle surrounding the device), hole

mobility of 400cm2(Vs)-1, reverse bias of 2V and resistance of 100Ω. Speed is RC

limited for thin detectors and transit time limited for thick ones. Moreover, reasonable

speeds (GHz range) are achieved even in case that the design is really “bad”. This means

that the device can be thin in order to save growth time and still be fast enough to monitor

any biological reaction. Furthermore, the resonance frequency of the filter is less than

GHz and the tunable detector is not expected to operate above the filter resonance.

However, the absorption layer thickness was chosen to be 2.5µm in order to get closer to

the optimum value and maximize sensitivity [130].

1 2 3 4 5 6 7 8absorption layer thickness HmmL

1

2

3

4

deepSHzHGL

Figure 5.5 – Detector speed as function of absorption layer thickness. Speed is RC limited for thin detectors and transit time limited for thick ones.

5.2.c Optical cavity and wafer design

Because of the insertion of cavity aperture control in the design, the air gap had to

be moved to the top mirror. However, the position in the top mirror is not arbitrary. The

resonant cavity with DBRs has an exponential power distribution along the mirrors and

confines most of the power in the cavity layer. If the gap itself is the cavity, very high

electric field “sees” the gap changes. On the other hand, if the gap is half way in the top

154

mirror, the electric field is very low, in comparison to the one in the cavity, and tuning is

not as effective. So, the gap has to be placed as close as possible to the cavity layer.

Otherwise, tuning will be very limited [7].

The cavity layer thickness should be a multiple of λ/2 but the thicker it is, more

optical confinement will be provided. Hence, AlAs layer was chosen to have λ @850nm.

Some other fabrication issues also influence the layered design and are very

similar to the isolated tunable filter. Contact layers are made of GaAs and the same wet

procedure is done to reach the p-contact layer. So, a thicker layer with low Al content is

included as a reference to stop the dry vertical etch, and another layer with high Al

content is placed in between the dry etch stop and contact layers. All layers in contact

with air (after release) have intermediary Al concentration.

Several devices fabricated on the same die should have the detector layer

separated. Otherwise, dark current would be large (proportional to detector area) and

testing would suffer huge influence from ambient light. Thus, an etch stop layer has to be

added under the absorption layer. This layer has to have bandgap close enough to GaAs

so that it does not constitute on a potential barrier. The heterojunction was designed to

use Al0.2Ga0.8As. The etching approach in this case was to use plasma etch until 4/5 of

the absorption layer and the remaining GaAs is removed during selective etch. Wet etch

would not give enough selectivity between the two materials and depth cannot be

controlled as accurately as for dry etch.

Figure 5.6 shows the layer composition for the tunable detector. First column

shows Al content in AlxGa1-xAs. The Al content was chosen based on advice from the

company that was going to grow the wafer. The top DBR has 16 pairs above the gap and

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1.5 pair in between gap and cavity. The cavity is not placed right under the release

protection layer in order to avoid spontaneous oxidation through defects or diffusion. As

the top DBR has an air layer, which provides high index contrast, it requires smaller

number of pairs. The bottom DBR has 29 pairs plus dry etch stop and wet etch layers,

which constitute a 30th pair. The entire thickness of the epitaxial layers is 12.62µm,

almost 2.5µm less than the wafer for torsional filter, besides the larger reflectivity and

absorption layer.

x thickness(Å) λ # function doping doping amount DBR multiple Cumulative thickness0 2000 − contact n++ 5x1018 12.6261945

0.45 1285.722 λ/2 release protection n 1x1018 12.42619450.2 611.477 λ/4 DBR n 1x1018 12.29762230.9 698.221 λ/4 DBR n 1x1018 16 12.23647460.2 611.477 λ/4 DBR n 1x1018

0.45 1285.722 λ/2 release protection n 1x1018 10.14095780 18000 − air gap 10.0123856

0.45 642.861 λ/4 release protection 8.21238560.9 698.221 λ/4 low index 8.14809950.2 611.477 λ/4 high index 8.07827741 2842.633 λ oxide layer/cavity 8.0171297

0.2 611.477 λ/4 DBR 15 7.73286640.9 698.221 λ/4 DBR0.2 611.477 λ/4 DBR p 1x1018 14 5.76831940.9 698.221 λ/4 DBR p 1x1018

0.2 7949.201 13 λ/4 dry etch stop p 1x1018 3.93474220.9 698.221 λ/4 wet etch p 1x1018 3.13982210 200 − p contact p++ 5x1018 3.07

0.2 500 − heterojunction p 1x1018 3.050 25000 − absorption 3

0.2 5000 − heterojunction n 1x1018 0.5n givenn givenGaAs n-substrate

Figure 5.6 – Layer composition for the tunable detector proposed structure. First column shows Al content in AlxGa1-xAs.

Figure 5.7 shows power distribution inside the optical cavity. Even for this

design with only one and a half pair separating cavity and air gap, power inside the gap is

much lower than in the cavity. This makes the tuning control not as effective as if the

gap itself was the cavity, as discussed before.

156

Power distributionRefractive index

Top DBR Gap Cavity Bottom DBR





Figure 5.7 – Power distribution inside the optical cavity. Even for only one pair of separation between the cavity and the air gap, power inside the gap is much lower than in the cavity.

The reflectivity spectrum for the design from Figure 5.6 is shown in Figure 5.8

and is matched @850nm for R = 99.9%. Both mirrors have a good reflectivity match in

the range 835nm to 865nm, as shown in the insert. The top mirror, as before, has a

slightly larger stop band due to the two semiconductor-air interfaces.

800 820 840 860 880 900wavelength HnmL0

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

bottom

top

820 830 840 850 860 870 880wavelength HnmL0.94

0.95

0.96

0.97

0.98

0.99

1

rewopytivitcelfer

800 820 840 860 880 900wavelength HnmL0

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

bottom

top

800 820 840 860 880 900wavelength HnmL0

0.2

0.4

0.6

0.8

1

rewopytivitcelfer

bottom

top

820 830 840 850 860 870 880wavelength HnmL0.94

0.95

0.96

0.97

0.98

0.99

1

rewopytivitcelfer

Figure 5.8 – Reflectivity spectrum for the top and bottom mirrors designed to have reflectivity of 99.9% @850nm. The insert shows a zoom at the center of the band.

157

Figure 5.9 shows linewidth of the transmitted peak as function of central

wavelength. Because reflectivity is very high at the center of the band, linewidth is very

sharp there. In the range where the matching between mirrors is good (835nm to

865nm), linewidth is below 0.25nm. For almost the entire stop band it is below 1nm.

810 820 830 840 850 860 870 880 8900.0

0.5

1.0

1.5

2.0

2.5

3.0

Wavelength (nm)

Line

wid

th (n

m)

Figure 5.9 – Linewidth as function of wavelength for the structure shown in Figure 5.6.

Figure 5.10 shows the transmitted wavelength through the Fabry-Pérot cavity as

function of the gap size between the mirrors. Because the gap is moved into the top

mirror, the transmission mode is not linear with gap size anymore and different orders of

modes overlap for a given gap. However, the desired tuning range of 835nm to 865nm

shows monomode characteristics.

158

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

810

820

830

840

850

860

870

880

890

Fabr

y-P

érot

wav

elen

gth

(nm

)

Gap Size (µm)

Figure 5.10 – Transmitted wavelength through the Fabry-Pérot cavity as function of the gap size between the mirrors. Because the gap is moved into the top mirror, the transmission mode is not linear with gap size and side modes may appear for a given gap.

5.3 Fabrication

Fabrication was extensively described in the two previous chapters. However,

this device has specific details that are related to the design or to the application. This

section will describe only particular aspects of fabrication sequence of the tunable

detector.

The wafer was grown by LuxNet Corporation. Figure 5.11 shows both calculated

and measured reflectivity spectrum of the wafer as grown. Agreement with theory is

excellent. Note that the vertical scale of the measured reflectivity was not calibrated but

the positions of the dips agree well with simulation, even though they are not as sharp.

159

780 800 820 840 860 880 9000.0

0.2

0.4

0.6

0.8

1.0

Ref

lect

ivity

Wavelength (nm)

(a) (b)

Figure 5.11 – Reflectivity spectrum for the fabricated wafer as grown. (a) Calculated and (b) measured. Vertical scale of measured results was not calibrated.

Fabrication sequence is sketched in Figure 5.12, showing both top view and cross-

section along the cantilever beam. Starting from the bare wafer, the top and back metal

contacts are deposited through liftoff technique on GaAs. Next, the device is defined by

simple lithography and photoresist mask is used for vertical dry etch. As in the filter

case, this etch is also very long and it is important to hardbake the photoresist

beforehand. Figure 5.13 shows the in-situ monitoring trace for this wafer. The structure

from Figure 5.6 can be clearly identified, including number of DBR pairs and even Al

content. The top DBR has a slight non-uniformity when compared to the bottom DBR,

which can in part explain the broadening of the transmission peaks. This may be

expected from a growth as long as this one, as discussed in chapter 3. Etching is

interrupted at the etch stop layer and wet etch is used to remove both Al0.2Ga0.8As and

Al0.9Ga0.1As layers. Same care has to be taken as before so that the surface of the wafer

does not dry while proceeding the wet etch, always keeping a bubble of liquid covering

the die until GaAs is exposed. Wet etch leaves behind a step to cover the GaAs layer

160

where the ground metal p-contact is deposited through liftoff. This plateau avoids

excessive undercut during selective etch, providing uniform potential for the bottom part

of the device. Next, the absorption layer across the wafer die have to be separated into

individual detectors and plasma etch is used until 1/5 of the absorption layer is left. The

etching control is also done with in situ monitoring and the calibration will be explained

in the next section. The remaining GaAs is removed during selective etch. Then,

oxidation is performed to provide lateral optical confinement of the AlAs cavity. Finally,

selective etch removes all exposed GaAs. Some undercut is shown in the figure but

anchors are usually protected. After selective etch, the die is cleaned, photoresist is

removed and critical point drying ends the fabrication sequence. Figure 5.14 shows a

SEM top view from a fabricated device. Dark regions were protected with photoresist for

the release etch. Note the small mesas done by wet etch around the device head and

anchor.

161

1 2

34

5 6

7

1 2

34

5 6

7

Figure 5.12 – Fabrication sequence of the tunable detector. Each step displays both top view and cross-section along the cantilever beam. 1: top and bottom metal; 2: device mesa vertical etch; 3: wet etch for metal deposition; 4: ground metal; 5: detector separation; 6: oxidation; 7: selective etch.

162

Top DBR(16 pairs)

1 DBR pair



Al0.45Ga0.55As

stop layer

Optical cavity(AlAs)

Top DBR(16 pairs)

1 DBR pair



Al0.45Ga0.55As

stop layer

Optical cavity(AlAs)

Figure 5.13 – Vertical etch in situ monitoring trace for the tunable detector wafer. The structure from Figure 5.6 can be clearly identified. The top DBR has a slight non-uniformity when compared to the bottom DBR, which can in part explain the broadening of the transmission peaks.

Figure 5.14 – SEM top view from a fabricated device. Dark regions were protected with photoresist for the release etch. Note the small mesas around the device head and anchor.

The folded beam filter was developed in this same wafer and the two different

devices were processed almost at the same time, with the filter lagging the detector by a

couple of months. Thus, some of the fabrication issues described in the previous chapter

are also related to this development. Redeposition has been always there, as described

before and shown in Figure 5.4. It really got worse through time, as seen in the pictures

from the previous chapter, but was not so critical for the detector. Non-repetitive etch

163

was also a problem but already discussed. However, some of the issues, also related to

selective etch, were solved before the folded beam filter processing has started.

An irregular and asymmetric etch has happened. Figure 5.15 shows SEM pictures

taken after head release etch. Both devices shown have the same size and were located in

the same wafer die. The one to the left was completely released while the one to the right

had etching happening from right to left. Figure 5.16 is from another die with the same

problem. This was a very strange fact and possible reasons are local plasma flow or

photoresist left on the walls. The last possibility was not very likely, based on optical

inspection and non-uniformity. If photoresist was not totally developed, this would

happen everywhere. Then the substrate temperature was changed for the first time. The

idea of increasing gas turbulence by heat is well known, specially by glider pilots.

Temperature substrate was increased from 20ºC to 50ºC, keeping the walls at 20ºC, and

the problem was not seen anymore. However, the problem was not considered entirely

explained.

Figure 5.15 – Irregular and asymmetric etch. SEM pictures were taken after head release etch. Both devices shown have the same head size and were in the same wafer die. The one to the left was completely released while the one to the right had etching happening from right to left.

164

Figure 5.16 – Another die with irregular and asymmetric etch.

The figures above show only the device head in order to investigate etching of the

sacrificial layer. However, the absorption layer is also selectively etched (as shown in

Figure 5.12, number 7) and it has thickness comparable to the sacrificial layer but much

larger perimeter (all area under the wet etch step). This causes a depletion of etchant and

selective etch took considerable longer time (~45 minutes) than expected from previous

results (~25 minutes).

Another problem that has occurred at the very beginning of the development was

etching of the AlAs cavity layer. This was happening during regular cleaning and also

during wet etch for ground metal deposition. Cleaning procedure, done 4 or 5 times

during the entire fabrication process, includes a dip into BOE 10:1 for 10s in order to

remove possible oxide film from the surface to be processed. AlAs oxidizes

spontaneously in air, although at very low rates [112], and successive cleaning steps do

expect to etch this layer by some extent. Wet etch of Al0.9Ga0.1As layer for ground metal

deposition takes about 1 minute and is also expected to etch the cavity. This undercut

was supposed to be negligible and this etch was not even considered for fabrication

sequence compatibility. Moreover, Figure 3.11 shows an extrapolated rate of ~230Å/s

for Al0.94Ga0.06As in pure BOE 10:1 what would mean 1.4µm of undercut in 1 minute.

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However, Figure 5.17 shows a different result. It was taken while inspecting release etch

and shows 3 layers from a cantilever, broken on purpose by a probe: top mirror (bottom),

layers in between gap and cavity (center) and bottom mirror (top). GaAs was not totally

etched and there was still some remaining material on top of the center beam. AlAs

cavity was almost totally etched. This fact had generated a change in the cleaning

procedure after the first vertical etch, which exposes the cavity: do not clean with BOE

anymore, only with water. Figure 5.15 shows no undercut at all and Figure 5.16 shows a

small one which is negligible. Both pictures were taken after this correction.

Figure 5.17 – SEM of cantilever, broken on purpose by a probe, showing the top mirror (bottom), layers in between gap and cavity (center) and bottom mirror (top). GaAs was not totally etched and there was still some remaining material on top of the center beam. AlAs cavity was almost totally etched. Head has 20µm of side.

Specific processing steps, related to the nature of the device, such as detector

separation, or to the design, such as oxidation, are described next.

5.3.a Detector separation

There are two main reasons why the absorption layer has to be etched across the

die, both related to noise reduction. The first is to minimize dark current, which is

originated from thermal generation of electron-hole pairs inside the device and is

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proportional to area. The second is to decrease background noise as the devices are not

shielded from ambient light for testing.

Dry plasma etch is used to realize these individual detectors and etch is done until

only ~1/5 of the absorption layer is left. The etching control is also done with in situ

monitoring. The remaining GaAs is removed during selective etch.

Calibration of etching was done by correlating the in situ monitoring trace to the

etched depth measured with a surface profiler. The in situ trace is different from the

sacrificial layer in Figure 5.13, where a predominant thin film effect causes the increase

in amplitude. In this case, all contacts are deposited and the topography effect causes

spatial interference resulting on a sinusoidal shape with constant amplitude. The number

of interference peaks can be related to the amount of etch. Figure 5.18 shows an in situ

trace with 6 peaks. Figure 5.19 shows the calibration curve used. The established

procedure was to etch until five peaks were present in the trace, leaving ~1/5 (0.5µm) of

the absorption layer to be removed by selective etch.

Figure 5.18 – In situ monitoring trace showing 6 peaks.

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3 4 5 6 71.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

Etc

h de

pth

(µm

)

Number of peaks

Figure 5.19 – Depth of etching as function of the number of peaks present in the in situ monitoring trace. Measurements were done using a surface profiler.

5.3.b Oxidation

The purpose of the oxidation layer in the tunable detector is primarily for optical

confinement and mode filtering. The process is not critical and is straight forward for

such a thick layer of AlAs [112]. Even though it has many variables, as described in

chapter 3, it is very repetitive. However, it is important to remember that any different

furnace will give different oxidation rate. Moreover, repeatability is strongly related to

the operator and two different operators usually get two different rates because of slightly

different procedures on the open furnace/load sample/close furnace/open furnace /unload

sample sequence. Every time that the furnace is opened, temperature drops very fast and

this influences the oxidation rate to the extent that apertures with precision of 1µm cannot

be achieved.

The temperature used was 425ºC and open furnace/load sample/close furnace

sequence was timed to last 20s. Open furnace /unload sample was being done as fast as

possible so that it would return fast to 425ºC. The next open furnace/load sample/close

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furnace sequence waited for the operation temperature to be achieved. The furnace used

was from Bandwidth9, Inc.

After furnace characterization, oxidation rate was consistently at 0.9µm/minute.

Oxidation has been carried out until heads with 18µm had their apertures totally closed.

The reason for this is that previous experience with 15µm heads required high numerical

aperture objectives to focus light into the device. However, optical coupling is less

sensitive if a small numerical aperture objective is used, as discussed in chapter 4. After

consistent oxidation runs of 20 minutes, only some of the 18µm heads were open and

none of the 20µm were closed, showing excellent repeatability.

Characterization of the aperture size was done with an IR CCD camera able to

“see” in the 900-1700nm. Even for this high reflective top mirror device, the aperture

could be clearly defined.

Unfortunately, the oxidation step was done too late in order to characterize the

effect of aperture on optical coupling. General detector characterization will be described

next together with the concept of a very compact biosensor.

5.4 Application

This section will show the characterization and performance results of the tunable

detector. The device can be used for several different applications such as biosensing,

spectroscopy, biometrics, optical interconnects and communications as part of drop or

monitoring systems. Because of this broad usage, characterization was done in a general

sense. However, the tunable detector was designed to biosensing, integrated to a

platform where biological reactions occur, what will be described next.

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5.4.a Biosensing

Bioassays are important tools used to detect interactions of various biomolecular

complexes for pharmaceutical and biomedical applications. In addition, such analysis

methods can provide a deep understanding on how proteins, encoded by DNA, interact

with enzymes, inhibitors or other proteins. This is one of the most important next grand

challenges after the completion of the sequencing of the human genome [131-135].

In general, bioassay techniques can be put into two categories: labeling with

compounds (such as fluorescent, radioactive or colorimetric) and direct molecule

identification. For the majority of bioassays currently performed for life science research

and pharmaceutical screening, fluorescent or colorimetric chemical labels are commonly

attached to the molecules under study [31]. Labels can be readily visualized and the

measurement technique is simple. However, the attachment of labels may substantially

increase the assay complexity. It may also alter the functionality of molecules through

conformational modification or epitope blocking, which ultimately leads to errors in data

interpretation.

A label-free sensor is a bioassay tool that enables direct molecule detection, and it

is generally desirable due to its non-intrusive nature of detection. The sensor typically

consists of two parts. The first part is the binding surface, which is activated (coated)

with a known receptor molecule that has a high affinity to the molecules to be detected.

The second part is the detection mechanism that converts a recognizable molecular

binding event into a quantifiable signal. The activation step is always done a priori.

Assays using this method are much faster than the compound labeled ones, since no

additional incubation and activation steps for the attachment of labels are required. Thus,

the reduction in assay complexity results in faster screening or developing time.

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Optical biosensors are a type of label-free biosensors that utilizes light as their

detection mechanism. They have several advantages, such as in situ real-time process

monitoring and high sensitivity to surface modifications, where most of the bioprocesses

take place [29-31]. Optical methods can be measured by different quantities, such as

angle, polarization, phase, amplitude, and frequency. The versatility of optical methods

is noticeable by the success of several devices such as surface plasmon resonance, output

grating couplers, ellipsometry, evanescent wave devices and reflectance interference

spectroscopy. Although bioassays using those methods are fairly sensitive, they are still

quite slow, bulky and expensive.

The proposed biosensor has been conceived to use novel integrated optoelectronic

devices in conjunction with a unique colorimetric guided-mode resonant (GMR)

diffractive grating structure as a surface binding platform. This approach can meet

stringent requirements of small size, high resolution, low power consumption, low cost

and the advantage of fabrication in 2D arrays.

5.4.a.1 Biosensor description

A guided mode resonant (GMR) filter has been optimized to work as a label-free

biosensor[38, 59, 136]. The GMR consists on a sub-wavelength grating surrounded by a

region with smaller index of refraction, as shown in Figure 5.20. The device is fabricated

by SRU Biosystems on a plastic (polyethylene terephthalate - PET) substrate by

imprinting a master grating into epoxy and curing [136]. High index dielectric is further

deposited. Thus, it is not only low cost but also disposable with minimum environmental

impact. Light incident onto the GMR in the normal direction can couple to the grating

structure under a certain resonance condition. This coupling results in a reflectance peak

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with sharp spectral distribution. The resonant wavelength (λpeak) is varied by the

attachment of biomolecules, cells, and bacteria to its surface and depends critically on the

thickness (tbio) and refractive index (nbio) of those specimens. Figure 5.21 shows the

simulation for a typical GMR optical reflectance spectra using rigorous coupled wave

analysis (RCWA). Continuous deposition changes the tbio thickness, which in turn

changes λpeak. This system yields a very high resolution of 0.1 nm in protein thickness

change [38]. Hence, it is a high sensitive tool to track molecular/protein interactions once

desirable receptors are coated on the top layer. Since the wavelength regime is

determined by the grating period, it has been conveniently chosen to be around 850nm

based on available low cost optoelectronic components. The rest of the readout system,

including the optoelectronic components, is obviously reusable and measurements can be

repeated to improve accuracy.

Figure 5.20 – Guided mode resonant (GMR) filter. The device is made on a plastic (polyethylene terephthalate - PET) substrate by imprinting a master grating into epoxy and curing. High index dielectric is further deposited. Surface is very sensitive to attachment of biological material with thickness tbio and index nbio. After [136].

The previously reported detection system consists of a white light source that

illuminates the GMR sensor through an optical fiber that also collects the reflected light

to couple it to a spectrometer. Figure 5.22 shows the schematic of such arrangement.

The spectrometer signal is then monitored by a computer to detect wavelength shifts [38,

Polycarbonate Sheet

Cured Epoxy

PET substrate

cured epoxy

high n dielectric

nbiotbio

incident reflected

Polycarbonate Sheet

Cured Epoxy

PET substrate

cured epoxy

high n dielectric

nbiotbio

incident reflected

172

136]. Despite its high sensitivity, resolution will always be limited by the spectrometer

pixel wise nature and the trade-off between resolution and signal strength can limit

further improvements. Major challenges remain on how to make a low-cost, compact

and portable system that has high resolution and throughput.

846 848 850 852 854 856 858 8600.0

0.2

0.4

0.6

0.8

1.0

Ref

lect

ivity

Wavelength (nm)

tbio, nbio

Figure 5.21 – Simulation to illustrate the GMR filter spectrum for different thicknesses of material on top.

White Light

Mini-SpectrometerSi-APD Array

LiquidChamber

GratingRegion

Substrate

CollimationLens

Optical FiberProbe

Fiber

Polarizer

White Light

Mini-SpectrometerSi-APD Array

LiquidChamber

GratingRegion

Substrate

CollimationLens

Optical FiberProbe

Fiber

Polarizer

Figure 5.22 – Schematic of the sensor using white light and spectrometer configuration.

The tunable detector has been conceived to replace the spectrometer/computer in

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an integrated way. The final sensor would be much smaller and still have high

resolution, low power consumption, low fabrication cost and the advantage of fabrication

in 2D arrays. Figure 5.23 shows the schematic of the sensor after integration of tunable

detector and GMR. A light emitting diode (LED) and tunable detector would replace

white light source and spectrometer, respectively. The transmission of the broadband

signal from the LED is depleted by the resonant reflection from the GMR and the tunable

detector can keep track of the dip. However, the area of the tunable detector head still

has to be increased in order to couple broadband signals. Current sizes would be able to

couple only the tunable laser for testing. Anyway, the system can be made very compact

and the resonant wavelength turns out to be irrelevant. Only shifts matter. The analysis

of the surface reaction can be made through the cantilever bias voltage to track the dip.

Processing such kind of data is extremely simple and done in the electrical domain.

Measurements can be completed in ms, much faster than “fast” biological reactions,

which occur in the units of second range. Thus, the built in circuitry for driving LED,

tunable detector and signal processing plus optical assembly can be made as small as

desirable, with the system size limited by the interface to the user. The interface would

have to provide the user with a visual display (output) and access to the GMR surface for

proteins input. The final system can have wrist-watch size or even smaller. Overall

power consumption would be very low what makes it easily portable.

There are essentially two ways to monitor the tuning voltage: lock to the dip or

continuous cantilever scanning. The latter is straight forward: the bias voltage has a

sawtooth format and the dip in detection is correlated to tuning voltage. Shifts in

resonance would change the position of the dip and also the bias voltage that corresponds

174

to the minimum detected current. Figure 5.24 shows the equivalent circuit for the tunable

detector with tracking capability. The p-contact is connected to a load and then to ground

so that the potential Vf floats with the detector current and causes the tuning potential to

change accordingly [21], locking the cantilever to the dip.

Biosensor Chip Plastic

High Index Dielectric

LEDCollimated Light(all wavelengths)

Transmitted light with resonant wavelength removed

Forward Diffracted Light

Backward Diffracted Light

Berkeley MEMSTunable DetectorIntegration of MEMS-Tunable Filter with a broad band detector

λ

ITrans

λλ p

ITrans

Idet

VtunVp

SRU Biosystems’

Plastic




detector

Biosensor Chip Plastic

High Index Dielectric

LEDCollimated Light(all wavelengths)






Berkeley MEMSTunable DetectorIntegration of MEMS-Tunable Filter with a broad band detector

λ

ITrans

λ

ITrans

λλ p

ITrans

λλ p

ITrans

Idet

VtunVp

Idet

VtunVp

SRU Biosystems’

Plastic




detector

Figure 5.23 – Schematic of the sensor using LED and tunable detector configuration. The broadband signal from the LED is depleted by the resonant reflection from the GMR; the tunable detector can keep track of the dip.

Vt

Vf

Vd

Vt

Vf

Vd

Figure 5.24 – Tunable detector equivalent circuit with wavelength tracking. A load is inserted in between the p-contact and ground so that the potential Vf floats with the detector current and causes the tuning potential to change accordingly.

175

5.4.a.2 Tunable detector characterization

The characterization can be divided in three separate performances: mechanical,

optical and electrical. Mechanical characteristics show released parts performance such

as static droop and bending as function of voltage. Most of the mechanical

characteristics were shown in the previous chapter when discussing the folded beam

filter. Figure 5.25 shows the white light interferometry image of the tunable detector.

The device in the image shows a small negative droop (bending up) of less than 0.2µm

while the folded beam did not show any considerable buckle. This is probably because

the length of the beams is still below the critical length according to equation (4.1), and

that other structure has more rigidity with no free ends. However, the beam was still

released in two steps in this development.

Figure 5.25 – White light interferometry image of the tunable detector. This device shows a small negative droop of less than 0.2µm.

Optical characteristics were measured using the setup from Figure 5.26, also

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designed to measure the integration with the GMR sensor. With this setup it is possible

to measure reflected and transmitted power as a function of input wavelength. A tunable

Ti:Sapphire laser is used and it can tune wavelength from 835nm to 985nm. Light is

collimated and tapped (beam splitter – BS1) to the optical spectrum analyzer in order to

keep track of wavelength. An achromatic lens is inserted after the beam splitter,

functioning in a similar way to a microscope eyepiece and minimizing chromatic

aberrations of the system. The insertion of this lens has allowed a much more stable

coupling while tuning, even though the tuning range is very short. This was an upgrade

from the tunable filter setup where coupling was unstable when tuning wavelength. All

measurements were normalized to the input power by splitting the input beam through

BS2 and measuring this reference signal at the same instant as the other signals (reflected

or detector current). A polarizer and the GMR sheet were supposed to be inserted in the

optical path by the time of integration. A third beam splitter, BS3, was required to use a

CCD for probing and placing the device in the beam path. The insertion of BS4 allows

reflected light to be measured, in complement to the current of the tunable detector.

Objective lenses with numerical aperture of 0.28 and 0.42 were used; however the larger

one has resulted in a linewidth spread in the measurements because of the varying angles

of incidence.

Careful optimization of coupling in real time was done again by applying a

sawtooth voltage to the cantilever beam while keeping the incoming wavelength

unchanged, similarly to what has been done with the torsional filter. The signal from the

tunable detector was coupled into an oscilloscope and the filter position was optimized

(x, y and z) until the best lorentzian like shape was obtained on the scope. After this fine

177

adjustment, the voltage on the cantilever could be kept constant at the desired value and

wavelength was scanned in order to measure the real linewidth of the filter transmission.

Ti:Saphire Tunable Laser

Collimation lens

BS1

Spectrum AnalyzerFiber

BS2

Reference Detector

Achromaticlens

M1

SRU filter

Polarizer

BS3BS4

CCD (top view)

Reflection Detector

Focusing Lens

Tunable Detector

P1

P3P2

M = mirrorBS = beam splitterP = probe

Ti:Saphire Tunable Laser

Collimation lens

BS1

Spectrum AnalyzerFiber

BS2

Reference Detector

Achromaticlens

M1

SRU filter

Polarizer

BS3BS4

CCD (top view)

Reflection Detector

Focusing Lens

Tunable Detector

P1

P3P2

M = mirrorBS = beam splitterP = probe

Figure 5.26 – Setup used for the optical characterization of the tunable detector integrated to the biosensor filter.

Figure 5.27 shows the measured typical spectral characteristics for the tunable

detector. Optimized coupling has eliminated the side mode at longer wavelengths but the

broadening shown on shorter wavelengths may have been caused by high order modes.

The achromatic lens is the element that contributes to this almost constant coupling as

function of wavelength. Tuning range was around 20nm, on average, and limited by poor

electrical isolation between mirrors. This was already discussed in the characterization of

the folded beam filter. When voltage is increased above ~6V the current across the

sacrificial layer increases and the device practically stops moving. If the electrical

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isolation was better or softer beams have been designed, the device would move longer,

and consequently increase the tuning range.

849 852 855 858 861 864 867-0.2

0.0

0.2

0.4

0.6

0.8

1.0

norm

aliz

ed d

etec

tion

wavelength (nm)

6.3V 5.9V 5.7V 5.4V 4.5V 2.8V

Figure 5.27 – Measured spectral characteristics for the tunable detector.

Figure 5.28 shows the transmission spectrum for the torsional filter at 856 nm.

Linewidth is very sharp and a lorentzian fit shows 0.4nm of FWHM. Average value of

linewidth was 0.8nm, without optimized coupling at all wavelengths. Higher resolution

in wavelength for a better trace was limited by the step motor used to tune the

Ti:Sapphire laser.

Electrical characteristics of the detector were measured in a standard way. Figure

5.29 shows a typical Current-Voltage (IV) curve for the tunable detector. This curve is

obtained in the dark, at room temperature, and the current at -5V is usually defined as

dark current (Id). In this case, Id was 4µA but typical range was from 1µA to 50µA. P-i-

n diodes can have dark current as low as 10pA, as the case for the commercial device

from LuxNet Corporation that grew the wafer used for the tunable detector. Id is

originated from thermal generation of electron-hole pairs inside the device and is

proportional to the square root of the applied voltage (of course it is also function of

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temperature). Id is then proportional to the device area and the high values measured are

related to the still large area, even after the separation of the detectors in the die. The

value mentioned above for the commercial device is for a circular detector with diameter

of 100µm while in the tunable detector case the entire mesa, defined by wet etch plus

contacts (>5X larger), contributes to Id. Finally, based on the IV information, the device

was reversed biased with 2V during operation. Noise level during normal operation

(lights on) was around 25µA.

852 854 856 858 860

0.0

0.2

0.4

0.6

0.8

1.0

∆λ=0.4nm

norm

aliz

ed d

etec

tion

wavelength

5.7V Lorentzian fit

Figure 5.28 – Transmission spectrum for the torsional filter at 856 nm. Linewidth is very sharp and a lorentzian fit shows 0.4nm of FWHM. Optimized coupling has eliminated the side mode at longer wavelengths but broadening on shorter wavelengths may have been caused by high order modes.

Capacitance as function of reverse bias was not measured as this value is only

relevant for high speed measurements. Forward resistance can also be derived from the

IV curve and the average value is 40Ω, slightly larger than commercial products (~10Ω).

Detector responsivity was measured in the dark and found to be 0.2A/W, which is a good

value facing the GaAs limit of 0.65A/W (commercial products usually achieve 0.5A/W).

180

-8 -6 -4 -2 0 2

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

Abs

olut

e cu

rren

t (A

)

Voltage (V)

Figure 5.29 – Typical current-voltage (IV) characteristic for the tunable detector.

Table 5–1 Summary of tunable detector average characteristics.

Tuning range 20nm

Tuning voltage 0 - 8V

Linewidth 0.8nm

Responsivity 0.2A/W

Forward resistance 40Ω

Dark current @5V <50µA

While the characterization and fabrication development was still proceeding, with

successive improvements on final devices, the Ti:Sapphire tunable laser went out of

service. Some other options, that not a tunable laser, would be white light or LED as

light source. However, these low coherence sources can not be collimated or focused to

the spot size required to couple light into the tunable cavity. Another change would be

required: that the sizes (head and eventually gap size) were increased accordingly to the

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broadband source. Moreover, it would mean to change the entire lithography mask set

and also the characterization optics. The choice was to wait until the system comes back

to work.

In the mean time, a conceptual alternative approach was launched. The light

source would be replaced by a tunable VCSEL and tunable detector would be replaced by

a broadband p-i-n detector. This turned out to be a very fortunate fact as the tunable laser

has shown to be much more attractive than the tunable detector. In the end, this change

of gears condemned the tunable detector integration to the GMR never to happen.

The optical characterization of the variable apertures was not done either, as the

focus was on biosensing not on coupling. The oxidation development was done in

parallel to the tunable laser integration and its importance was decreasing in the same

proportion that the laser approach was succeeding. However, this optical confinement

and linewidth enhancement of filters would be a very good topic for further study.

5.5 Summary

This chapter presented specific details about the design and fabrication of a

tunable detector that can be applied not only to biosensing but also to several other

applications such as WDM communications systems or spectrosocpy. The topology

chosen was the cantilever Fabry-Pérot tunable filter integrated to a p-i-n detector, which

can have uniform responsivity across the tuning range. Fabrication is similar to the

tunable filter but the absorption layer has further to be vertically etched to separate

devices in the same die and oxidation was performed to control the aperture of the optical

cavity.

The detector was to be integrated to a guided mode resonant (GMR) filter, by

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replacing white light and spectrometer/computer, and providing a portable biosensor with

high resolution, low power consumption, low cost and the advantage of fabrication in 2D

arrays. The GMR serves as a surface binding platform and tunable detector/LED as the

reading mechanism.

The detector was characterized according mechanical, optical and electrical

properties. Final performance yielded smaller tuning range (20nm) than designed (30nm,

in the range 835-865nm) due to a leak current in between mirrors. Electrical and

mechanical characteristics were very good.

The problem with the Ti:Sapphire laser cut short the characterization and opened

space to another biosensor approach using a tunable VCSEL, which has shown to be a

much more attractive option, to be presented in the next chapter.

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Chapter 6 Tunable VCSEL

6.1 Introduction

In the previous chapters, passive tunable micromachined devices with wide tuning

range and innovative mechanical structures have been presented. This chapter discusses

the usage of an active device, vertical cavity surface emitting laser (VCSEL), into the

development of a very sensitive optical biosensor. It makes directly usage of tunable

VCSELs but tunability of the devices is achieved through bias current and temperature

[137, 138], differently than the devices previously discussed, which use micromachining

techniques. However, micro-electro-mechanical (MEM) VCSELs can readily replace the

ones used here and substantially increase the dynamic range of the biosensor [47, 57, 58].

Preceding chapters have described design and fabrication in detail. The same

methods are applied to VCSELs but special design has to be done to the active region.

DBR mirrors also have to suffer some modifications, such as design for current

confinement and graded transition between different material compositions, in order to

improve thermal and electrical characteristics of the device [97]. The overall technology

is now mature and detailed description has been subject of several textbooks [2-6].

Hence, this chapter will limit the discussion to integration of devices to the biosensing

systems.

Even though some devices have been designed and fabricated, the ones that have

been used are commercial. Several reasons would justify this option but the main one

was timing. As mentioned in the last chapter, the systems here described were born as an

alternate approach to the tunable detector and it can only be justified if it can be

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implemented faster than the previous. However, the development of a reliable fabrication

process, from epitaxial growth (which can always be contracted, as before) to final

characterization, may take months of academic research and the final result may still be

uncertain. Price is another factor, as market price for good quality single-mode VCSELs

is around US$35.00 each, which is much less than would take to fabricate such devices.

Moreover, commercial devices have fairly good characteristics and are already packaged

in a very convenient way for current and temperature control. Furthermore, the lasing

wavelength of the fabricated devices did not match the one required for integration to the

guided mode resonator (GMR).

The focused characteristics for the sensor presented in this chapter are label-free

detection, compactness and portability. Besides, it will be shown that the approach used

here also provides high sensitivity, low power consumption and low cost. The ultimate

goal, however, is to generate new systems that can enable new benefits to society.

6.2 Sensor Description − tunable VCSEL and GMR biosensor

The guided mode resonator (GMR) has been already described in the previous

chapter. Although the biosensor that integrates the GMR with white light/spectrometer

reading system, also described before, has high sensitivity through computational signal

processing, its resolution will always be limited by the spectrometer pixel wise nature.

The signal-to-noise ratio is low and the trade-off between resolution and signal strength

can limit further improvements. Figure 6.1 illustrates this trade-off, where increased

resolution of the spectrum analyzer is always together with a lower overall detected

power due to constant power density. The tunable detector approach from last chapter is

a very good alternative to make a low-cost, compact and portable system that has high

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resolution and throughput. However, it may require complicated optical coupling

mechanism, similar to what has been shown in chapter 4 for the tunable filter, and

linewidth may still suffer from nonuniform coupling as function of wavelength.

840 842 844 846 848 850 852 854 856

Ref

lect

ed P

ower

(a.u

.)

Wavelength (nm)

OSA resolution 1nm OSA resolution 0.1nm

Figure 6.1 – White light trace showing the trade-off between resolution and signal strength.

The idea behind the sensor is still the same: monitor the shift of the GMR

resonance. Even though the tunable detector integration was not completed, early

experiments have shown that measurements cannot go through the GMR because of the

resultant lens effect from the surface of the liquid inside the wells. The wells have about

8mm of diameter and the curvature of the liquid surface deviates the light beam and

contributes to poor transmission and coupling. Similar problem happens to dry samples,

where deposited material works as a scattering source.

Here, the detection system of the proposed VCSEL based biosensor utilizes a

tunable single-mode 850nm VCSEL and two p-i-n detectors, as shown in Figure 6.2.

Optoelectronic devices are placed under the GMR and biological reactions occur on the

top. Light output from the VSCEL is already polarized and there is no need for a

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polarizer if the cavity is previously aligned to the grating lines. In order to scan for the

peak resonant position λpeak, the VCSEL wavelength is tuned by sweeping its bias

current, causing a rapid thermal effect that shifts the lasing wavelength [137]. Detector 1

is positioned to measure the reflected light from the GMR substrate, while detector 2

provides the normalization current for the incident power, since the VCSEL output power

also varies with the bias current. Thus, the ratio of the two detector signals is used to

monitor the reflection peak. Varying the laser bias current provides a fast but narrow

range (2−3 nm) tuning of the VCSEL wavelength. Temperature tuning is also used to

augment the tuning range, extending it to 8−9nm. The typical wavelength dependence

for temperature and current tuning for a VCSEL is 0.08nm/ºC and 0.4nm/mA,

respectively. Even though our current design of the VCSEL based biosensor may be

limited to those biomolecular interactions that cause small wavelength shifts, a larger

dynamic range can also be obtained by means of two different approaches: tunable

detector and MEM tunable VCSEL. In the first one, the system from the previous

chapter can be implemented (LED as source and tunable detector as monitoring

mechanism). In the second approach, the current-tuned VCSEL can be replaced by a

micro-electro-mechanical tunable VCSEL [58, 139]. Both approaches can easily enable

about 30nm of peak wavelength scanning.

187

PIN Detector 2(Reference)

Beam ExpansionCollimation

Tunable VCSEL

PET Substrate

Beam Splitter

PIN Detector 1 (Reflection)

Cured Epoxy

high “n” dielectricbiobio , nt

GMR

Figure 6.2 – Biosensor with VCSEL based measurement system. A tunable VCSEL and two p-i-n detectors work as a readout system for a plastic guided-mode resonant (GMR) filter that is the binding surface. Peak reflectivity from the GMR is detected by correlating maximum normalized detector current with laser bias current.

Besides the advantage in compactness, sensitivity is greatly enhanced by using a

laser rather than a broadband source (white light or LED). Since the sharp reflection

peak is due to the resonance from the GMR and it is very sensitive to incident angle [59],

a coherent light source with smaller diffraction and a higher signal-to-noise ratio can

improve the sensitivity of the system.

Furthermore, the analysis of the surface reaction is all done in the electrical

domain, thus simplifying data processing. Using the one-to-one correspondence between

the laser bias current and lasing wavelength, at a given temperature, the normalized

detected current versus wavelength can be directly mapped. From the mapping, the

spectral response of the GMR biosensor as a function of wavelength can be deduced, and

the peak resonant wavelength can be determined. Moreover, the electrical circuitry, for

signal processing and driving the VCSEL and p-i-n detector, and optical assembly can be

made very compact.

188

Finally, as all technologies being used are naturally 2D, the system can be

extended to an array format, where different cells can be activated to respond to different

proteins. In this configuration, laser and detector can be fabricated in the same wafer

[140] and this array topology can also be made even smaller and portable, something

similar to a cellular phone.

6.3 Sensor characterization

In order to characterize the biosensor system apart from the context of a particular

biomolecular assay, two experiments were performed. The purpose of them was to

investigate resonant wavelength shifts due to changes in nbio and tbio respectively. With

the setup previously described, a 3x5in2 GMR plastic sheet was cut and bonded to the

bottom of a bottomless 96-wells microplate [136] so that the liquid solutions can be

easily deposited on top of the GMR sheet.

The first experiment measured the wavelength shift as function of the refractive

index variation of the specimen deposited on top of the grating. Fluids with various

refractive indices were deposited into the wells and the corresponding GMR spectral

response was measured. The liquids used were standard matching fluids from Cargille

Laboratories with the precision of ±0.0002 in index. A very low index of 1.280 (Galden

fluid), from Solvay Solexis, was also used. The measurements were all performed in the

same well. The procedure consisted on pipetting 100µl of index matching fluid into the

well and scanning the laser bias several times. The fluid was pipetted out of the well and

the well was rinsed with isopropanol and water before pipetting in the next fluid. Figure

6.3 shows the average spectral shifts of the resonance for four different index matching

fluids, and the wavelength scale is used in order to compare traces acquired at different

189

operation temperatures. The VCSEL based biosensor system shows a very high

sensitivity to small changes in the surface refractive index, as illustrated in Figure 6.4.

The wavelength dependence is linear and an index change ∆n < 0.001 can be easily

resolved.

846 848 850 852 854 856 858 860Wavelength (nm)

Tran

smitt

ed P

ower

(a.u

.)

n=1.280 n=1.292 n=1.298 n=1.305

Figure 6.3 – Measured spectral shift of the resonance as function of index of refraction of the solution on top of the GMR.

1.275 1.280 1.285 1.290 1.295 1.300 1.305 1.310850.0

850.5

851.0

851.5

852.0

852.5

853.0

853.5

854.0

Wav

elen

gth

(nm

)

Index of refraction

Figure 6.4 – Wavelength shift as function of index of refraction of the solution on top of the GMR. Index changes ∆n < 0.001 can be easily detected by this system.

Measurements of the GMR spectral response were also performed using white

190

light and spectrum analyzer. Figure 6.5 shows the result and comparison with Figure 6.3

confirms an enhanced sensitivity because of the coherence of the light source. The GMR

response is very sensitive to incident angle and even small angles with respect to the

normal broaden the linewidth of the resonance [59, 141, 142]. White light can be seen as

the combination of monochromatic sources with a wide distribution of k vectors in

different amplitudes and directions, which causes the various incident angles. On the

other hand, VCSEL is basically a monochromatic source with a well defined k , yielding

a sharper resonance and increasing sensitivity.

846 848 850 852 854 856 858 860

n=1.280 n=1.292 n=1.298 n=1.305

Tran

smitt

ed P

ower

(a.u

.)

Wavelength (nm)

Figure 6.5 – White light measurements of the spectral shift of the resonance as function of index of refraction of the solution on top of the GMR.

The second experiment measured thickness variations of the specimen deposited

on top of the grating. The experiment consisted of the deposition of successive layers of

polyelectrolyte polymers and the measurement of the GMR spectral response for each

layer. Polyelectrolytes are long molecules that can be either positively (polycations) or

negatively (polyanions) charged and bind through electrostatic attraction. In principle,

the adsorption of molecules carrying equal charges leads to charge reversal on the

191

surface. This has two important consequences: (i) the repulsion of equally charged

molecules and thus self-regulates the adsorption to a single layer; and (ii) the ability for

an oppositely charged molecule to be adsorbed in the second step, on top of the first one.

Cyclic repetition of adsorption/wash steps (positively/negatively charged) leads to the

formation of multilayer structures. Figure 6.6 illustrates the basic buildup sequence. The

linear increase of film thickness is a polyion property, independent of the surface [143].

Figure 6.6 – Schematic of the film deposition process using slides and beakers. (A) Steps 1 and 3 represent the adsorption of a polyanion and polycation, respectively, and steps 2 and 4 are washing steps. (B) Simplified molecular picture of the first two adsorption steps, depicting film deposition starting with a positively charged substrate. After [143].

The polyelectrolytes used in this experiment were cationic poly-ethyleneimine

(PEI), anionic poly-sodium styrenesulfonate (PSS) and cationic poly-allylamine

hydrochloride (PAH), all obtained from Aldrich. The polyelectrolytes were dissolved in

a buffer solution of 0.9M of NaCl, at a concentration of 5mg/ml. The same buffer

solution was also used to rinse the well surface after the adsorption of each monolayer.

An initial base layer of cationic PEI was deposited on the GMR surface by

pipeting 100µl of PEI solution into a microplate well. The PEI polymer has very large

192

charge density and is suitable for this first layer binding to the sensor surface. After 5

minutes of incubation period, the PEI solution was removed, the well was rinsed with the

buffer solution, and the spectral response was measured with buffer inside the well by

scanning the VCSEL bias current. The same deposition/wash procedure was repeated for

alternating cycles of PSS and PAH polymers. Since the growth of the polyelectrolyte

polymers is self-limiting, the thickness of each layer is found to be ~50Å (10% error

expected due to molecular nonuniformity) [136, 144].

842 844 846 848 850 852 854 856

PSS 2PAHPSS 1

PEI

Ref

lect

ivity

(a.u

.)

Wavelength (nm)

PEI PSS 1 PAH PSS 2

Figure 6.7 – Spectral shift of the resonance due to changes in polymer thickness on top of the GMR.

The spectral shifts of the resonant wavelength, as layers of polyelectrolyte

polymers were being deposited, is shown in Figure 6.7, where one-to-one correspondence

between the bias current and wavelength is again used. The peak resonance shift can be

plotted as a function of deposited polymer thickness, as shown in Figure 6.8. The

±0.1nm error bars on wavelength are due to the resolution limit in the measurement

system. Thus, the biosensor has the high sensitivity to resolve much less than 10Å of

thickness variation on top of the GMR surface, as can be seen from the linearity of the

193

curve obtained in Figure 6.8.

-2 0 2 4 6 8 10 12 14 16

847

848

849

850

851

852

Peak

Wav

elen

gth

(nm

)

layer thickness (nm)

Figure 6.8 – Wavelength shift as function of polymer thickness on top of the GMR. The system can resolve much less than 10Å of thickness variation.

The above results had the property that the testing sample resulted in complete

coverage of the grating surface, even for the polymers [144]. However, simulations show

that GMR will respond even for partial coverage, which is the case for real proteins.

Figure 6.9 shows the simulation of partial coverage of the grating surface with same

percentages on top and bottom of the grating. The simulation takes into account

deposition of 100nm of material with index of refraction n=1.5. The response (peak

shift) will be proportionally smaller, with linear dependence until 50% of coverage and

parabolic below this value. Linewidth of the resonance will be almost constant for any

level of coverage. Figure 6.10 shows the simulation for 4 different cases: no deposited

material (0%), deposition only on top of the grating (45%), only on bottom (55%) or full

coverage (100%). These cases are not very likely to happen but illustrate that the device

will work even when the bottom lines of the grating are narrower than the surface

activation molecules (a virus, e.g.).

194

0 20 40 60 80 1000

2

4

6

8

10

12

nm

Surface coverage (%)

Wavelength shift Peak linewidth

Figure 6.9 – Simulation of partial coverage of the grating surface with same percentages on top and bottom of the grating.

0 20 40 60 80 1000

2

4

6

8

10

12

nm

Surface coverage (%)

Wavelength shift Peak linewidth

Figure 6.10 – Simulation of partial coverage of the grating surface with no deposited material (0%), deposition only on top (45%), only on bottom (55%) or full coverage (100%).

6.4 Protein Binding Assays

Following the previously sensor characterization, the application of the VCSEL

based label-free biosensor measurement system for protein-protein binding

immunoassays was investigated. The characterization of protein interactions consists of

the detection of the binding between an antibody – goat anti-mouse immunoglobulin

195

(IgG), and an antigen – mouse immunoglobulin (IgG), as shown in Figure 6.11. The

proteins were obtained from Jackson ImmunoResearch, each being affinity-purified and

chromatography-purified, respectively. Three different experiments were performed, in

order to study the performance of the biosensor system: ELISA, dynamic binding and

static concentration. ELISA stands for enzyme-linked immunosorbent assay, which is a

standard clinical method that requires additional labeling used to determine the antigen

concentration in unknown samples. The dynamic experiment enables the quantification

of the antibody-antigen binding as a function of time. Meanwhile, the static experiment

quantifies the resonant wavelength shift as a function of mouse IgG concentration,

similarly to ELISA but faster because requires a small number of incubations.



Tunable VCSEL

PET Substrate

Beam Splitter


Cured Epoxy

high “n” dielectric

GMR

Antibody Block Antigen



Tunable VCSEL

PET Substrate

Beam Splitter


Cured Epoxy

high “n” dielectric

GMR

Antibody Block Antigen

SENSOR

Goat Anti-Mouse IgG(Antibody)

Goat Serum (Block)

Mouse IgG (Antigen @ various concentrations)

SENSOR


Goat Serum (Block)


SENSOR


Goat Serum (Block)


SENSOR


Goat Serum (Block)


Figure 6.11 – The configuration of VCSEL based biosensor and the sequence of protein layers. The protocol follows a standard mouse IgG capture immunoassay, as shown on the right.

The protocol for all three experiments followed a standard mouse IgG capture

immunoassay. First, the antibody protein was deposited by applying 100µl/well of goat

anti-mouse IgG (1.8µg/ml), which was diluted in phosphate-buffered saline (PBS)

solution, and it was incubated for 10-12 hours at 4ºC. The incubation period allowed

196

sufficient time for the mouse IgG protein to bind to the GMR sensor surface and the low

temperature favors the binding. Then the excess solution was removed and the wells

were washed with PBS-Tween. Next, normal goat serum (200µl/well of 3% goat serum

in PBS) was applied to block the vacant spots of the sensor surface not covered by the

antibody and it was incubated for 2 hours at room temperature. This blocking step is

required to minimize the false-positive errors from the measurements, since the antigen

protein might bind to the exposed sensor surface as well as to the antibody protein.

Following the removal of the block solution and wash of the wells, the spectral response

of the GMR sensor was measured with PBS inside the wells, and this peak resonant

wavelength became the reference of comparison for the subsequent antigen

measurements, on dynamic and static experiments. Lastly, the antigen protein was

deposited by applying 100µl/well of mouse IgG at various concentrations and it was

incubated at 4ºC for ELISA and at room temperature for the others. Measurements were

taken during and after the incubation period for the dynamic and static experiments,

respectively.

Two ELISA assays were performed in parallel, one in a GMR plate and another in

a standard polystyrene plate. This was to compare both responses and use the

comparison to calibrate further results, if needed. It was also used to verify binding of

the first antibody to the GMR surface and test the surface activation. ELISA requires two

additional steps than those described above and the protocol also recommends that mouse

IgG (antigen) is incubated for 10-12hs at 4ºC. The additional required steps are a second

antibody (which has the labels) incubation and development of the labels. This second

labeled antibody incubation is done after removing and washing the previous solution

197

that had antigen, and it specifically binds to the antigen. More than one binding per

antigen may occur, resulting in amplification. This double antibody binding is also called

sandwich ELISA. The label of the secondary antibody, usually an enzyme, converts the

colorless substrate to a colored product. In this case, the enzyme is p-nitrophenyl-

phosphate (pNPP) which is converted to the yellow p-nitrophenol by alkaline

phosphatase. This colored product is measured on an ELISA plate reader as function of

the optical density at some specific wavelength, in this case yellow. Figure 6.12 shows

optical density at λ = 560nm, as given from the ELISA reader for both GMR and

standard polystyrene ELISA plate. The GMR has more sensitivity than the polystyrene

plate as seen from the larger range of optical density. However, it had a strong

background noise due to interference of the grating with the read out system. The results

shown were normalized with respect to the background for both plates. The nearly

similar response of the two plates shows that further calibration of the GMR results is not

necessary.

1E-9 1E-8 1E-7 1E-6 1E-50.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

Opt

ical

den

sity

(a.u

.)

Antigen (mouse IgG) concentration (g/ml)

ELISA plate GMR

Figure 6.12 – Optical density at 560nm as given from the ELISA reader for both GMR and standard polystyrene ELISA plate.

198

In the dynamic measurement, the kinetics of the antibody-antigen binding for

three different mouse IgG concentrations, with each repeated for three different wells,

was continuously monitored during the incubation period with the time resolution of 5

seconds. Figure 6.13 shows the averaged normalized surface binding curves for different

concentrations of mouse IgG as a function of time, obtained by integrating the difference

of the GMR spectral response for the starting time and that of a given time. It should be

noted that the saturation level is antigen concentration dependant, and the normalization

here enables the comparison of surface binding time for different antigen concentrations.

If required, this technique allows for ultrafast, dynamic monitoring in the ns range, as the

VCSEL can be modulated in the GHz range. As expected from results of typical kinetic

binding experiments, most of the protein binding occurred rapidly at the beginning of the

incubation, followed by a gradual saturation. As indicated in Figure 6.13, the time for

80% (of the saturation level for each concentration) surface binding time for goat anti-

mouse IgG and mouse IgG proteins is around 300s, with dependence on the mouse IgG

concentration. Low concentrations take more time to saturate than high concentrations.

These results can also be used to characterize the time needed for an antibody to trigger

its immuno-response to the respective antigen.

199

0 400 800 1200 1600 20000

20%

40%

60%

80%

100%

Time (sec)

Nor

mal

ized

Sur

face

Bin

ding

(a.u

.) 1 ng/ml: 345 sec 10 ng/ml: 304 sec100 ng/ml: 245 sec

1 ng/ml 10 ng/ml

100 ng/ml

Time for 80% Surface Binding

Figure 6.13 – Dynamic measurement: surface binding versus time for different antigen concentrations. Most of the protein binding occurred rapidly at the beginning of the reaction, followed by a gradual saturation. The 80% surface binding time is about 300s, with a slight dependence on the mouse IgG concentration.

In the static measurement, the resonance shift resultant from the antibody-antigen

binding was monitored for eight wells in the microplate. After the removal of the block

solution, mouse IgG solution was applied to each of the wells with the smallest

concentration of 1pg/ml, and it was incubated for 1.5 hour at room temperature. The

solution was removed and the GMR biosensor’s spectral response was finally measured

with PBS inside the well. Thus, any signal masking related to variation of index of

refraction due to concentration of the solution was avoided and only the binding effect

was considered. Then PBS was removed and the next mouse IgG solution of 10pg/ml

was added to the wells. The incubation-measurement-removal sequence was repeated for

different mouse IgG solutions with their concentrations increased by 10X each time, up

to 10µg/ml. Figure 6.14 shows the average of the peak resonant wavelength shifts for the

eight wells, as a function of mouse IgG concentration. The high sensitivity of the

VCSEL based measurement system is shown from its ability to detect the smallest

200

concentration of 1pg/ml. As both the antibody and antigen IgG proteins have their

molecular weight approximately 150 kDalton, a sensitivity of 1 pg/ml is equivalent to 6.7

fM/l (femto-Mol/liter). The nearly saturation observed above 10ng/ml was caused by the

depletion of the available antibody-antigen binding sites, and it is expected that the

saturation concentration can be increased by depositing higher concentration of the

antibody at the first step or by simply using different wells for each concentration. This

last procedure would rely on very good uniformity of the GMR grating.

10-12 10-10 10-8 10-6 10-40

0.1

0.2

0.3

0.4

0.5

0.6

Concentration (g/ml)

Wav

elen

gth

Shi

ft (n

m)

Figure 6.14 – Static measurement: average resonant wavelength shift as function of the antigen concentration. The high sensitivity of the VCSEL based measurement system is shown from its ability to detect the smallest concentration of 1pg/ml or 6.7 fM.

Finally, the proposed structure from Figure 6.11 can be built into a very compact

and cheap biosensor without sacrificing sensitivity. Operation would have a reusable

platform, including optical, electrical and display parts, and the GMR sheets can be easily

replaced for sequential analysis. The overall cost for this system would be very low,

residing much more on packaging than on parts.

The results presented are not only encouraging but also fantastic. The level of

detection (6.7fM/l) is more than one order of magnitude better than state of art methods

201

that employ techniques well known by their high sensitivity such as surface plasmon

resonance (SPR) and reflectance interference spectroscopy [31, 145]. Recently, portable

sensors using SPR have been reported to achieve sensitivity of 100fM/l with binding

amplification and 70pM/l without amplification [146, 147]. Another candidate technique

for compact sensors with multidetection capability, diffractive gratings, has not reported

better sensitivity results than 0.5ng/ml [148]. Moreover, the VSCEL biosensor is much

more sensitive and faster than ELISA, which is the most used method for clinical analysis

[149].

6.5 Summary

This chapter presented a label-free and compact biosensor using VCSEL. The

devices used were commercial VCSELs, tuned by both current and temperature.

The sensor has a very simple configuration, consisting of a tunable VCSEL, a

plastic guided-mode resonant (GMR) surface, and two pin detectors. The device has

shown to be highly sensitive to surface modifications, with the ability to detect thickness

variations 10Å and <0.001 of change in the index of refraction. Protein binding tests,

consisting in standard mouse IgG capture immunoassay, were done for dynamic and

static experiments and the system has shown extremely high sensitivity, detecting even

an extreme low concentration of 1pg/ml (6.7femto-Mol/l).

All technologies being used on the sensor are naturally 2D and the system can be

readily extended to an array format. Then, different cells can be activated to respond to

different proteins and throughput can be very high. Thus, these compact, label-free, high

sensitivity, low power, low cost and potentially portable biosensors systems can enable

enormous new benefits to society such as virus detection in isolated communities in

202

Africa, fast screening on drug discovery and air/water quality monitoring in disaster

areas.

203

Chapter 7 Conclusion

7.1 Summary

This dissertation described design, fabrication technology, characterization and

applications of tunable filter, detector and VCSEL, all fabricated in GaAs and

monolithically tuned. Each one of the chapters in this dissertation had a summary at the

end. Thus, it does not make sense to repeat the same ideas again and again but, however,

the main achievements should be pointed as they figure among the best around the world.

Chapter 2 covered the basic concepts in the design of micro-mechanical tunable

optoelectronic devices. A new sub-wavelength grating mirror, with clear advantages for

long wavelength applications, was presented as an option to the conventional distributed

Bragg reflectors. General design rules, valid form short to long wavelengths, were

organized in the form of a flux diagram.

Chapter 3 gave all processes used on the fabrication of GaAs devices in general.

Sub-wavelength grating mirror was fabricated in silicon in excellent agreement with

theory.

Chapter 4 presented specific details on the design and fabrication of tunable filters

applied to communications and IR imaging. A record tuning range of more than 100nm

was achieved with a torsional micro-mechanical tunable optical filter, which is also free

of damages resulting from capacitor discharge and does not cause any power penalty into

optical links. A folded beam filter structure, which has large ratio of detector to pixel

area and convenient topology for implementation of 2D array with matrix addressability,

was demonstrated for application on IR imaging.

204

Chapter 5 described the conception of a tunable detector to be integrated to a

guided mode resonant (GMR) filter and provide a portable biosensor with low cost, high

resolution, low power consumption, and the advantage of fabrication in 2D arrays. The

detector topology consisted on a Fabry-Pérot tunable filter with controllable aperture

integrated to a broadband p-i-n detector. A tool problem prevented the full integration

and an alternate approach with tunable VCSEL turned to be much more attractive.

Chapter 6 presented a novel label-free compact and portable biosensor using

VCSEL. The system uses a tunable VCSEL based measurement system integrated to a

GMR. The approach can provide high sensitivity, dynamic and static measurements, low

power consumption and low cost. The sensor is particularly sensitive to low

concentrations, detecting protein concentration as low as 6.7femto-Mol/l.

7.2 Potential Future Work

The work presented here is far from being complete and there are still many

exciting topics that are worth investigating. The SWG approach for mirrors has proved

to be effective, easy to fabricate and more advantageous for middle and long wavelength

infrared applications. The next logical step is to fabricate devices that incorporate SWG

mirrors and are specifically designed for the wavelengths of interest. These devices

would then be integrated to a focal plane array in order to provide real-time hyperspectral

images.

Another interesting experiment would be to conclude the characterization of the

optical confinement and linewidth enhancement of filters with respect to the cavity

aperture. The exact same structure designed for the tunable detector can be used for this

purpose.

205

The most exciting perspectives, however, are in the direction of the VCSEL

biosensor. The development of 2D arrays is a natural step as all technologies involved

are naturally 2D. Figure 7.1 illustrates a top view of an array of tunable VCSELs and p-i-

n detectors under the GMR sensor, with the lasers and detectors on the same chip and

normal incidence is used. The wafer can be designed for the laser and detector to be

made in the same chip. The diffraction of the laser beam illuminating the sensor and

reflecting back is sufficient to hit the detector. The final assembly would be extremely

compact and different cells can be activated to respond to different proteins and

throughput can be very high. The simultaneous measurement of a larger biomaterial area

is a new functionality that no other existing technique can provide.

SensorChip(above)

VCSEL/detectorChip (below)

Receptor Molecule Spots

VCSEL

detector

VCSEL/detectorChip (below)VCSEL detector

Focusing lens

SensorChip(above)

(a)

(b)

SensorChip(above)



VCSEL

detectorSensorChip(above)



VCSEL

detector


Focusing lens

SensorChip(above)


Focusing lens

SensorChip(above)

(a)

(b)

Figure 7.1 – Tunable VCSEL and resonant cavity detector array integrated to the GMR sensor. The sensor will be on top of the laser and separate locations of the sensor can analyze different proteins simultaneously.

Experiments with real virus, such as HIV and dengue, are very likely to happen in

the near future. The sensor has proved to be suitable for fast label-free clinical analysis

206

(similar to an ELISA without labels) and specific reagents for the protocol are under

development in several places, including Public Health and Immunology Departments at

UC Berkeley.

Another interesting breakthrough would be the integration of all electronics,

optics and optoelectronics devices into a compact mount. Figure 7.2 shows a picture of

the 96-wells plate with GMR and a prototype of an electronic driver for VCSEL and

detectors. Optics and optoelectronic devices can be inserted in between and use a XY

positioning system to allow inspection of all 96 wells.

Figure 7.2 – Picture of the 96 well plate with GMR and a prototype of an electronic driver for VCSEL and detectors. Optics and optoelectronic devices can be inserted in between and use a XY positioning system to allow inspection of all 96 wells.

After all the exciting results shown in this dissertation, the novel proposed

applications, and the various experiments for future investigation, I expect that this is just

the beginning of a stimulating expedition into fully develop and apply the technique of

monolithic tunable optoelectronic devices for future impact in our everyday lives and

overall benefit of humankind.

207

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