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Introduction to Micro-Electro-Mechanical Systems (MEMS)with Emphasis on Optical Applications

Adisorn Tuantranont and Victor M. Bright

NSF Center for Advanced Manufacturing and Packagingof Microwave, Optical, and Digital Electronics (CAMPMODE),

University of Colorado at Boulder, CO, USA 80309-0427Adisorn.Tuantranont@colorado.edu (e-mail); Tel: (303) 735-1763; Fax: (303) 492-3498

ABSTRACT -- Micro-Electro-Mechanical Systems, or MEMS, are integrated micro devices orsystems combining electrical and mechanical components. They are fabricated using integrated circuit (IC)batch processing techniques and can range in size from micrometers to millimeters. These systems cansense, control and actuate on the micro scale, and function individually or in arrays to generate effects onthe macro scale. This paper presents an overview of MEMS technology with emphasis on opticalapplications. Applications of MEMS devices vary in many fields from automotive transducers, biomedicaltechnologies, communication systems, robotics, aerospace, micro-optics, industrial sensors and actuators.The applications of MEMS in optics include display systems, optical switching, optical communication,optical data storage, optical processing and interconnection, and adaptive optics. Examples of micro-opticalcomponents and systems are described in this paper.

KEY WORDS: MEMS, MOEMS, micromachining, micro-optics, sensor, actuator

บทคัดยอ -- ระบบจุลภาคไฟฟา - เครื่องกล (MEMS) คือประดิษฐกรรมขนาดไมโครมิเตอรซึ่งรวมสวนประกอบของระบบไฟฟาและเครื่องกลเขาดวยกัน ระบบจุลภาคไฟฟาเครื่องกลสรางโดยอาศัยเทคโนโลยีการผลิตวงจรทรานซิสเตอรซึ่งมีขนาดตั้งแตไมโครมิเตอรถึงมิลลิมิเตอร ระบบดังกลาวนี้สามารถตรวจวัด,ควบคุมและกระตุนในระดับตํ่ าและสามารถทํ างานทั้งในแบบอิสระหรือแบบขบวนเพื่อใหมีผลในระดับสูงขึ้น บทความนี้กลาวถึงภาพรวมของเทคโนโลยีระบบจุลภาพไฟฟา-เครื่องกลและการนํ าไปใชประโยชนโดยเนนการใชประโยชนทางดานแสง การใชประโยชนของระบบนี้สามารถใชไดในหลายสาขา อาทิ เทคโนโลยียนตกรรม เทคโนโลยีการแพทยชีวภาพเทคโนโลยีระบบสื่อสาร ระบบหุนยนต ระบบอากาศยาน เปนตน การใชประโยชนทางดานแสงสามารถนํ าไปสรางระบบการแสดงผล ระบบการสับเปล่ียนทางแสง การสื่อสารดวยแสง การเก็บขอมูลดวยแสง การประมวลและเชื่อมตอขอมูลดวยแสง รวมท้ังการปรับปรุงภาพทางดาราศาสตร ในบทความนี้ไดอธิบายถึงตัวอยางของอุปกรณและระบบขนาดเล็กเกี่ยวกับแสง

คํ าสํ าคัญ -- ระบบจุลภาคไฟฟา - เครื่องกล, ระบบจุลภาคไฟฟาเครื่องกลทางแสง, กลจักรขนาดเล็ก, อุปกรณขนาดเล็กเกี่ยวกับแสง, อุปกรณตรวจวัด, อุปกรณกระตุน

1. IntroductionTrend toward smaller size, higher performance, andgreater functionality for electronic devices is madepossibly by the success of solid-state microelec-tronics technology. In the late 1980s, the siliconVery-Large-Scale-Integrated (VLSI) design andmanufacturing was developed for use in field of

Micro-Electro-Mechanical System (MEMS)[1].This field is called by a wide variety of names indifferent parts of the world: micro-electro-mechanical systems (MEMS), microsystem techno-logy (MST), micromechanics, and micro totalanalysis systems (µ-TAS) etc. These systemsinterface with both electronic and non-electronicsignals and interact with non-electrical physical

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world as well as the electronic world by mergingsignal processing with sensing and/or actuation.Instead of dealing only electrical signals, MEMSalso deals with moving-part mechanical elements,making miniature systems possible such asaccelerometers, fluid-pressure and flow sensors,gyroscopes, and micro-optical devices. MEMS aredesigned using computer-aided design (CAD)techniques based on VLSI and mechanical CADtools and typically batch-fabricated using VLSI-based fabrication process [1]. The commercialavailable surface and bulk micromachining such asMulti-User-MEMS process (MUMPs) at MCNCand MOSIS service respectively are widely used tofabricate prototype MEMS devices due to their lowcost and short turn-around time. Post-processingsuch as cavity etching, silicon bonding and flipchip soldering can be applied to produce the morecomplex mechanical structures for suitableapplications. An early application of MEMS was inthe field of microsensor and microactuator formeasuring or driving position, pressure, velocity,acceleration, force, torque, flow, magnetic field,temperature, gas composition, humidity, pH, fluidionic concentration, and biological gas/liquid-/Molecular concentration. Some applications havebeen successfully commercialized in market suchas thermal inkjet printer, automotive accelerometer,and high-resolution display projector.

Figure 1. Schematic diagram showsinterdisciplinary field of MOEMS

MEMS is also widely used to fabricate microoptical components or optical systems such asdeformable micromirror array for adaptive optics,optical scanner for bar code scanning, optical

switching for fiber optical communication etc. Thisspecial field of MEMS is called ‘Micro-Opto-Electro-Mecha-nical Systems (MOEMS)’. Thisinterdisciplinary field has to combine knowledge inoptics, electronics, and mechanics to design andfabricate devices as shown in Fig.1.

2. Fabrication TechnologyBatch IC fabrication technology is used tofabricate mechanical microstructures suchas beam, spring, diaphragms, orifices,grooves, gears, linkages, and complexmechanical suspended flexure. MEMSdevices can be divided into two fabricationclasses: Surface micromachining and bulkmicromachining.

2.1 Surface micromachiningSurface micromachining is an additive fabricationtechnique that involves the building of devices onthe top surface of substrates. Specific structureparts of a device are encased in layers of asacrificial material during fabrication process. Thestructural parts are released by chemical etchantdissolving of the sacrificial layers. Surfacemicromachining can be used to fabricate not onlyrelatively conventional mechanical structure suchas beams or diaphragms, but also moresophisticated ones such as gears, linkages, andmicro-motors. Commercial polycrystalline siliconsurface micromachining processes such as MUMPs(Multi-User-MEMS-Process) and SUMMiT(Sandia Ultra-planar Multi-level MEMSTechnology) are available for prototyping MEMSdesigns. Multi-levels of doped or undopedpolysilicon layers are used to form the mechanicalstructures and silicon dioxide layers are used as thesacrificial material. The schematic layout diagramsof MUMPs and SUMMiT surface micromachiningfabrication were shown in Fig. 2 [2]. Electrostaticmicromotor fabricated by surface micromachiningwas shown in Fig.3.

2.2 Bulk micromachiningBulk micromaching is a subtractive fabricationtechnique that uses the substrate to formmechanical structure of MEMS devices. The singlecrystal substrate is etched in anisotropic

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Figure 2. Surface micromachining fabrication(MUMPs and SUMMiT)

Figure 3. Electrostatic micromotor fabricated byMUMPs: diameter is 80 µm. (Courtesy of MCNC)(For reference, human hair diameter is 70-80 µm)

etchants such as potassium hydroxide (KOH) orethylene-diamine pyrocatecol (EDP) along givencrystal planes. The process is based on the fact thatanisotropic etchants etch the <100> and <110>crystal plane significantly faster than the <111>crystal planes. In a <100> silicon substrate,anisotropic etching proceeds along <100> plane butpractically stop etching at <111> plane, making a54.7 degrees angle slanted wall in the etched

cavity. The final size of etched cavity is controlledby the etch-mask opening or heavily boron-dopedsilicon etch-stop. Under-etching occurring whereetch masks are misaligned with <110> directioncan be used to fabricate suspended microstructure.The Miller indices indicated the plane of siliconcrystal was shown in Fig.4. Fig. 5 shows the cavityanisotropic-etched by EDP.

Figure 4. The Miller indices of silicon crystalplane

Figure 5. The etched cavity from anisotropicetching: cavity size ranges from 10-100 µm. (Bulk

micromachining)

The design of MEMS device is limited by planargeometry of IC fabrication process. Three-dimensional structure or high aspect ratio (height towidth) is difficult to fabricate by conventional ICprocess. The x-ray photolithography technique(LIGA) was developed to fabricate high aspectratio structure over 100. Microgear and motorfabricated by LIGA were shown in Fig.6.

Post-fabrication processes such as bonding or flipchip soldering permit a silicon substrate to beattached to another substrate to provide addeddesign flexibility and packaging possibility.

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Figure 6. Micromotor components fabricated byhigh aspect ratio LIGA. Stator (upper figure) and

rotor(lower figure). (Courtesy of MCNC)

3. Computer-Aided Design forMEMS

Computer-Aided Design and Engineering programs(CAD and CAE) have been recently developed toassist MEMS design easier. In MEMS technology,the design complexity is compounded by intimacybetween mechanical and electronic performance.Several commercially available software tools suchas L-EDIT, Cadence, and MEMCAD can providethe layout tool for MEMS design. The MEMS-specific tools that are integrated into anenvironment where complete structural, as well asoperational, analysis such as MEMCAD,CAEMEMS, and IntelliSuite have used to bedesign verification tools (e.g., solid modeling,finite element analysis, discretization, andvisualization). These software tools have alsoproven useful for modeling a variety of parameters(e.g., displacement, stress, electric field, magneticfield, temperature, and fluid velocity) under a widevariety of conditions. Fig. 7 shows computer-usersinterface of MEMS CAD tool available in market.

Finite-element analysis (FEA) is one very powerfultechnique to model a variety of static and dynamicphenomena for a complex microstructure. Amongthem are mechanical stress-strain distribution,thermal distribution, frequency response, fluidflow, electromagnetic field and resonant frequency.Several commercially available software packages

such as Ansys, Nastran, Cosmo, and Abaqusprovide sophisticated modeling capabilities.

Figure 7. MEMS CAD tool for design, layout, andsimulation (Courtesy of Tanner EDA)

4. Microactuators, Microsensorsand Microsystems

4.1 MicroactuatorMicroactuators are component that converts energyinto appropriate action capable of producingmicron-scale motion. Microactuators can be classi-fied into two classes: rigid microstructure anddeformable microstructures. Rigid-type micro-actuators such as micromotors provide displace-ment and force through rigid-body motion [1].

Deformable microstructures such as beams anddiaphragms provide displacement and forcethrough mechanical deformation. Currently, themicroactuation methods in common use areelectrostatic, electromagnetic, piezoelectric andthermal. Fig.8 shows deformable mirror driven byelectrostatic mechanism.

Figure 8. Electrostatic deformable micromirrorarray [4]

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The electrostatic actuation uses the nature ofelectrostatic force provided by parallel platecapacitor structures or comb-finger structures. Theattractive and repulsive forces generated by electriccharge distribution are used to convert electrical tomechanical energy. The electrostatic actuated de-vices (e.g., micromirror array, microswitch,scanner, microshutter, micromotor) are widely usedin varieties of fields. Fig.9 shows the commerciallyavailable micromirror array for display projectiondeveloped by Texas Instrument, USA.

Figure 9. Texas Instrument’s micromirror displayprojector (Courtesy of TI DSP)

Electromagnetic actuator has been demonstrated inconjunction with both mechanical classes ofmicroactuators but electro-magnetic systems provedifficult to micromachine using planar ICprocesses. The high aspect ratio micromachiningmethod such as LIGA is required to build the three-dimensional structure of magnet and solenoid.Magnetostatic micromotor was shown in Fig.10.Electrostatic actuator dominates in the developmentof actuators in microworld because of its simplerand more compatible with IC fabrication.

Figure 10. Magnetostatic micromotor. (Courtesy ofUW Madison)

Another form of microactuation is based on thepiezoelectric effect [5]. This material exhibitsdeformation of crystal when voltage is applied.Therefore, piezoelectric films can be used toprovide actuation in a variety of applications suchas valves, pumps, and positioning devices. Typicalpiezoelectric thin films now used in microactuatortechnology are zinc oxide (ZnO), lead zirconatetitanate (PZT), and polyvinylidene fluoride. Ofthese materials, PZT has the largest piezoelectriccoefficient. Thermal actuator uses bimorphstructure that there is thermal coefficient ofexpansion mismatch between two layers ofmaterials to generate force or motion. The bimorphstructure can provide deformations in the lateral ornormal to the plane of substrate. In general, thermalmicroactuators have a slow response time (on theorder of tens of milliseconds) and high powerconsumption (on the order of tens of milliwatts).Electrostatic microactuators can be much faster(with response time measured in microseconds),and consume far less power but less force ormotion generated.

4.2 MicrosensorMicrosensors are component that converts oneform of energy into another and provides a usableenergy output in response to a specific measurableinput. Due to the micro-scale size of microsensors,less invasive, high accurate/sensitive, and lowcost/weight sensor can be achieved. The smartsensor fabricated by IC processes can integratewith CMOS electronic circuits on the chip tohandle, switch, or amplify the signal. Several kindsof microsensors are successful in the market suchas pressure sensor, flow sensor, thermal sensor,gas/chemical sensor, accelerometer, andimmunosensor. The commercially available inte-grated accelerometer for automotive air bag systemdeveloped by Analog Devices was shown inFig.11.

Figure 11. Integrated accelerometer and packageddevice. (Courtesy of Analog Devices)

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5. MEMS for Optical Applications

Micro-Opto-Electro-Mechanical System (MOEMs)is a specific field of MEMS that includes theknowledge in fields of optoelectronics and optics tocreate the micro-optical components and systems.Some micro-optical components recently fabricatedat University of Colorado, Boulder, Colorado havebeen described and demonstrated as follow [6].

5.1 Piston micromirrorsOne of the most useful MOEM components is theelectrostatically actuated piston mirror. This devicetakes advantage of the planar nature of the surfacemicromachining process and the ease to formparallel plate capacitor structures by sacrificiallayers releasing. The segmented or membranedeformable micromirrors have been fabricated. Theupper plate of the structure can be metallized tocreate a moving mirror. The lower plate was usedas an underlying addressable electrode. When avoltage is applied between the two plates, anattractive electrostatic force is developed andbalanced by the restoring mechanical force of theflexures that suspend the mirror over electrode. Thephase of incoming light modulated by mirrordeflection can be controlled by applied voltagebetween two electrodes. Fig.12 shows one elementof micromirror array.

Figure 12. Element of segmented deformable

micromirror

Bulk micromachining process is another way tofabricate micromirror array that micromirror andCMOS circuit can be integrated on the same chip.Thermal bimorph actuator was used instead ofelectrostatic actuator to actuate the mirror invertical direction. The larger deflection stroke canbe achieved with trade off in lower modulatedfrequency. The bulk micromachining micromirrorarray was shown in Fig.13. Applications ofdeformable micromirror array include activeaberration correction for atmosphere’s turbulence

compensation or free space optical communicationsystems. The schematic diagram of wavefrontcompensation for aberration correction was shownin Fig.14. The incoming wavefront was phase-modulated by up/down movement of micromirror

element.

Figure 13. Micromirror array fabricated by bulkmicromachining

Figure 14. Aberration correction

5.2 GratingsOptical grating is another optical element that canbe easily fabricated in surface micromachinedpolysilicon. The optical grating is an opticalelement that serves to periodically modulated thephase or the amplitude of the incident wave. Thegrating consists of repetitive arrays of polysiliconline suspended over substrate by flexure whichindividual line was electrostatically movedperpendicular to the plane of substrate to diffractlight of a particular wavelength at a designed angle.The electrostatic grating, shown in Fig.15, has anactive area of 500 µm × 500 µm with 2 µm linesspaced 4 µm center-to-center. The array of gratingline is moved perpendicular to the plane of thesubstrate to change the phase relationship between

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light reflected off the grating lines and substrate.This grating is designed to be able to modulatedoptical intensity by shifting power from the zero

diffracted order to the ±1st diffracted orders.

Figure 15. Micro-optical grating

5.3 Fresnel LensesSince surface micromachining uses materials withuniform layer thickness, it is not possible to designcurved refracting lenses: however, Fresnel lensescan be fabricated easily. Fresnel lens consists of anarray of polysilicon circular rings increasing inwidth and spacing toward the center. An exampleof a 7 order Fresnel lens is shown in Fig.16. Theplate is 200 µm tall and locks into place. The sliderattached at the left lifts the plate. The bottom oflens plate is hinged so it can be flipped up into thelight path. The Fresnel lens on substrate can beused to collimate laser light from laser diode. Theschematic diagram of microhinge is shown inFig.17

.

Figure 16. Micro-Fresnel lenses.

5.4 Optical ScannerOptical scanner is another optical componentsuccessfully demonstrated for commercial market.The scanner consists of hinged mirror platesconnected to thermal actuator arrays, which canmove 10 µm laterally on substrate. The lateralthermal actuator consists of a narrower polysiliconhot arm connected to a wider polysilicon cold arm.

Figure 17. Schematic diagram of micro-hinge forflip-up structure

When the current is applied to them, the highercurrent density in the hot arm causes it to heat andexpand more than the cold arm. This causes theactuator tip to move laterally in an arcing motiontowards the cold arm side as shown in Fig.18.Typical dimensions of thermal actuator are: ‘hot’arm 2.5 µm wide, 240 µm long; ‘cold’ arm 16 µmwide, 200 µm long and gap between both arms 2 µm wide.

Figure 18. Thermal actuator (Heatuator).

The coated gold mirror surface is 75µm square (forreference , the diameter of human hair is about 70-80 µm). An etch hole has been cut in the center of

Hot arm

Cold arm

motion

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the mirror to ensure that the mirror is completelyreleased during the sacrificial oxide etch. Themirror is connected to the substrate with twosubstrate hinge, and to the actuator array with aself-locking tether. The lock and key mechanismconsists of a key hole in the bottom of the mirrorplate with awide opening at the top and narrowopening at the bottom. The tether end is taperedinto a triangularshape which slightly overhangs thebottom of the keyhole. Below the triangular tip,slots are cut into both sides of tether, correspondingto the size of the narrow bottom of the keyhole. Asa hinged plate is rotated off the substrate, the tetherslide into the keyhole. The actuator array is used toset the angle of the mirror plate for beam steering,or it could be used to move continuously, to createa scanning mirror with a large scan angle. The15.7° maximum deflection angle is observed [6].Micro-optical scanner for bar code scanning ordisplay projection was shown in Fig.19.

Figure 19. Micro-optical scanner

Rotate mirror plate is another mechnism forscanning the optical beam. Fig.20 shows a flip-upmirror attached to rotary stepper motor, which isdriven by an array of thermal actuator. The 200 µmsector gear on the base allows the 180° ofpositioning. Mirror is 185 µm wide and 200 µmtall.

5.5 Corner Cube RetroreflectorSo far only single device have been described;however, more complex systems can be developed.One of the most interesting optical components is a

corner-cube retroreflector (CCR). A CCR has threemutually hinged, stand-up gold plated mirrors. Onehinged mirror is positioned and modulated with athermal actuator array, in the same manner as thescanning mirror. The other hinged mirror is held bya slotted locking plates to fix the position of mirror.

Figure 20. 180 degree scannong mirror

As shown in Fig.21, CCR system has a static goldmirror on the substrate and two perpendicular,mirrored walls. This mirror arrangement willreflect light back in the direction of its incomimgpath, and is commonly used in roadside reflectors.The CCR can be used for line-in-sight opticalcommunication

Figure 21. Micro-corner cube retroreflector

The previously described optical components canbe combined to form a micro optical bench asshown in Fig.22. A vertical cavity surface emittinglaser (VCSEL) is used as a laser light source. Thelaser beam is emitted perpendicular to the substrateand reflected by 135° mirror. The beam is

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collimated by Fresnel lens and scanned in twodimensions by rotating micromirror and fan mirror.This micro optical system is proposed as low costlaser scanner or bar code scanner [7].

6. ConclusionMEMS proves to be a promising technology forfuture sensors and actuators. Trend to decreasesize, enhance performance, and lower the cost oftransducer in market is made possibly by thesuccess of MEMS technology. Fabricationtechnique has to be developed further to supportthe increasing MEMS industry. MEMS hasmerged several fields of knowledge to create amicro-scale device by using today available ICfabrication technology. Discussions of MEMStechnology, fabrication tool, MEMSCAD tool, andMEMS applications for sensor and actuatorconcentrated on micro-optics applications havebeen presented in this paper.

AcknowledgementThank you M. A. Michalicek for nice MEMSpictures and V. Thiantamrong for typing and Thaitranslation.

Reference[1] M. Mehregany, “Micro-Electro-Mechanical

Systems,” IEEE Circuit and Devices, July1993 pp. 14-22.

[2] M. A. Michalicek, “Introduction to MEMS,”CU MEMS web 1998

[3] US National research council, “Micro-Electro-Mechanical System: Advancedmaterials and fabrication method,” NationalAcademic Press, Washington, D.C. 1997.

[4] A. Tuantranont, V. M. Bright, W. Zhang andY.C. Lee, “Flip chip integration of lensletarrays on segmented deformable micro-mirrors,” SPIE Vol. 3680, pp. 668-678, 1999.

[5] M. Madou, “Fundamentals of microfabri-cation,” CRC Press, New York, 1997.

[6] V. M. Bright, J. H. Comtois, J. R. Ried, andD. E. Sene, “Surface micromachined micro-opto-electro-mechanical systems,” IEICETrans. Electron, Vol.E80-C, No.2 February1997, pp.206-213.

[7] V. M. Bright, “Surface micro-machinedoptical systems,” Course material 1998.

Figure 22. Micro-optical bench for display projection system

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Adisorn Tuantranont received the B.S. inElectrical Engineering (Control Eng.) from KingMongkut Institute of Technology Ladkrabang(KMITL), Bangkok, Thailand, in 1995 and theM.S. in Electrical Engineering (Lasers & Optics)from University of Colorado at Boulder in 1998.He is pursuing the Ph.D. degree in ElectricalEngineering at University of Colorado at Boulderand working in Optoelectronic Computing SystemsCenter (OCS) and Center for AdvancedManufacturing and Packaging of Microwave,Optical and Digital Electronics (CAMPmode).During his master degree, he was working for Prof.Kristina M. Johnson and Prof. Y. C. Lee. Researchinterests include the Alignment of Liquid Crystalby Obliquely Evaporated SiOx method, flat screenliquid crystal display and optical modulator. For hisPh.D. degree, he is currently working for Prof.Victor M. Bright in Micro-Electro-MechanicalSystem Laboratory (CU-MEMs Lab). His currentresearches specify in MEM DeformableMicromirror for optical beam steering and shaping,microlens array for optical interconnect, andmicromirror for laser resonator and high powerapplication.