Micro-Opto-Mechanical Pressure, Sound, and Ultrasound Sensors in Silicon-Nitride Photonic Technology

W. J. Westerveld, B. Figeys, H. Gao, C. H. Huang, F. Verhaegen, B. Troia, R. Haouari, P. Neutens, J. O’Callaghan, S. Lenci, X. Rottenberg, R. Jansen, V. Rochus

Imec, Kapeldreef 75, 3001 Leuven, Belgium e-mail: veronique.rochus@imec.be

ABSTRACT Micro-electro-mechanical systems (MEMS) are used in applications ranging from consumer electronics to

medical diagnostics. Alternatively, optical sensors offer low-noise, remote read-out via optical fiber, and are insensitive to electromagnetic interference. We demonstrate micro-opto-mechanical sensors (MOMS) in silicon-nitride photonic technology. Experimental results and design trade-offs of pressure, sound, and ultrasound sensors are reported. Transparency for visible light offers prospective applications in life sciences, such as photo-acoustics.

Keywords: optical pressure sensor, optical microphone, ultrasound sensor, silicon photonics, silicon-nitride

1. INTRODUCTION Micro-electro-mechanical systems (MEMS) are increasingly being used for sensing of pressure, sound, and

ultrasound in applications ranging from consumer electronics to medical diagnostics. E.g. pressure sensors in wearable technology [1], microphones in laptops [2] or hearing aids [3], and micromachined ultrasound sensors (MUTs) in medical imaging [4]. Simultaneously, fiber optical sensors have demonstrated high sensitivity and offer remote readout, but for a less integrated system [5,6]. Micro-opto-mechanical sensors offer the advantages of both worlds, with prospects of high sensitivity, small footprint, and on-chip passive optical multiplexing. Imec has developed silicon-nitride visible and near-infrared integrated photonic technology in their 200 mm CMOS pilot line [7]. This technology is nowadays also offered as multi-project wafer (MPW) service via the Pix4Life photonic pilot line (www.pix4life.eu). Silicon-nitride photonics has particular advantages over silicon. The lower index contrast of SiN photonics requires a larger footprint but is more tolerant to fabrication-induced variations, which cause phase noise and hamper wavelength-division-multiplexing. For photo-acoustic imaging, SiN waveguides can be used to guide photo-acoustic excitation pulses in the visible spectrum. Alternatively, free-space pulses can pass the circuit in the out-of-plane direction [8]. The possible use of visible light allows for cost-effective silicon detectors, including imagers for massively parallel read-out. In this work, we describe the operation principles of these devices and the results of the research ongoing at Imec [9-15].

2. SENSOR PRINCIPLES AND FABRICATION The sensors are based on a flexible membrane that deforms upon applied static, acoustic, or ultrasonic pressure

(Fig 1a). A photonic Mach-Zehnder interferometer (MZI) is integrated [16], with the MZI sensing arm in the moving membrane and the reference arm on the fixed substrate (Fig 1b). Both arms are folded into a spiral to have a long sensing path. A deflection of the membrane stretches the MZI sensing arm, which causes a phase difference between the arms, which results in a change in optical intensity at the MZI output, which is measured. The change in MZI waveguide length contributes one order of magnitude stronger than the change in effective refractive index for these SiN waveguides [10]. Alternatively, ring resonators can be used as sensing element, but they are less robust to laser wavelength noise. Devices are fabricated in Imec’s CMOS pilot line and use silicon-nitride photonic technology (waveguide thickness 300 nm, wavelength 852 nm) [7]. First, the SiN-on-insulator photonic circuitry is fabricated (Fig. 2a) [7]. Second, an additional 4.5 m thick silicon-dioxide layer with low tensile stress is deposited. This layer is essential to the mechanical characteristics of the membrane, as the 2.3 m thick SiO2 bottom cladding layer has high compressive stress causing buckling of the membrane [10,15]. Third, the membrane is formed back-side etching (DRIE) to locally remove the wafer substrate (Fig. 2b). Each application requires a specific design for the mechanical membrane characteristics and optimized photonic circuitry. These are the topics of the following sections.

Figure 1. (a) Sketch of device cross-section with flexible SiO2 membrane and integrated SiN waveguide (single loop). Without (top) and with

(bottom) applied pressure. (b) Sketch of photonic Mach-Zehnder Interferometer (MZI) for sensing the membrane deformation.

Figure 2. Fabricated devices. (a) Cross-section SEM image showing an unclad grating coupler in SiN. (b) SEM image of membrane back-side etch. (c) Microscope picture of one of the arms of an MZI with a spiral loop and also with a single loop. (d) Microscope image of the

new micro-opto-mechanical sensor. From [13].

3. PRESSURE SENSORS For pressure sensing it is desired to have a high precision over large pressure range. However, the optical output

of the interferometer has a sinusoidal dependence on the phase difference, such that the unambiguous pressure sensing range is limited to half a period (Fig. 3b, blue curve). The sensitivity can be engineered by choosing the number of spiral loops of the interferometer arms, more loops give a higher sensitivity, however, with smaller sensing range [13]. To overcome this trade-off, our designs combine a sensitive output based on a many-loop spiral interferometer with a low-sensitive output from an additional interferometer with only a single loop over the membrane (Fig 3a). Another problem is that the interferometer is sensitive around the steep flank of the sinusoidal output, but much less sensitive at the flat tops and bottoms of the sinusoidal output. This has been overcome by using two interferometers on the same membrane in which one of the interferometers has a fixed quarter-period offset between the two arms, such that the two interferometers are sensitive in different pressure ranges within a period. For large pressures, the membrane deflection is nonlinear [10].

Figure 3. New micro-opto-mechanical pressure sensor with 4 outputs. Two high-sensitive outputs with a quarter period phase shift, one low-

sensitive output with large sensing range, and a power tap to compensate for laser intensity noise. High sensitive outputs share spiral MZI arms with 60 loops. Membrane diameter 1.2 mm. (left sketch) Schematics of photonic circuit. (right plot) Measured output signals. Blue

curve: high-sensitive output. Magenta curve: high-sensitive phase shifted output. Red curve: low-sensitive output. From [13].

4. MICROPHONES Microphones require a flat frequency response for acoustical frequencies in the hearing range (20 Hz –

20 kHz). This is achieved by designing the membrane resonance frequency well above 20 kHz. Typical acoustical pressures are much lower than typical static pressures, the pain threshold is about 200 Pa. The resonance frequency of the silicon-dioxide membrane depends on its diameter, thickness, and residual initial stress (Fig. 4a). The residual stress in the fabricated membranes was measured by fitting simulations to measured resonance frequencies for different membrane diameters (Fig. 4b).

Figure 4. Acoustical characterization. (a) Membrane resonance spectra measured with a laser vibrometer and shaker. (b) Measured and

simulated membrane resonance frequencies, showing agreement for 50 MPa tensile stress. (c) Optical sensor response of micro-opto-mechanical microphone. A 1 kHz sound wave with pressure of 1 Pa is impinged on the membrane and optical output is recorded. From [9].

4 4.5 5 5.5 6 6.5 7Pressure [Pa] 104

0

0.2

0.4

0.6

0.8

1

The sensor with diameter 1.7 mm and first resonance at 80 kHz was characterized as microphone with measured SNR of 33 dB (Fig. 4c). We see large room for improvement, e.g. an optimal design would have its first resonance much closer to the hearing range (~20 kHz).

5. ULTRASOUND SENSORS Optical micro-machined ultrasound sensors (OMUS) based on silicon photonic ring resonators showed very

high sensitivity [17]. However, the device reported in [17] has high compressive initial stress and a buckled membrane [15]. We believe that reliable devices are based on neutral or slightly tensile membranes. To harvest the benefits of silicon-nitride photonics for ultrasonic and especially photo-acoustic imaging [8], current research efforts also focus on ultrasound sensing. Our theoretical and experimental studies focus on both thicker membranes as well as smaller membrane diameters (Fig 5).

Figure 5. OMUS for photo-acoustic imaging and spectroscopy. (a) Design, GDS layout. (b) Working principle with optical laser pulses generating acoustical waves due to photo-acoustics.

6. CONCLUSION We demonstrated micro-opto-mechanical pressure sensors and microphones and presented designs for

ultrasound sensors, in silicon-nitride photonic technology. Important design trade-offs and solutions for these membrane-based devices were presented for the different applications. MEMS sensors are widely used and MOMS sensors have significant benefits for applications. For example, the optical technology does not suffer from electromagnetic interference which allows these devices to be used inside medical MRI systems or near heavy industrial machinery.

REFERENCES [1] ST Microelectronics, LPS25HB High-robustness pressure sensor, flyer, Dec. 2014. [2] ST Microelectronics, MEMS analog and digital microphones, flyer, Dec. 2014. [3] Sonion, Data Sheet Microphone O8AC03, Mar. 2017. [4] Szabo, T.L. Diagnostic ultrasound imaging: inside out, 2nd edn, Ch. 5, 122–160, Academic Press, 2014. [5] Wang et al, Miniature all-silica optical fiber pressure sensor with an ultrathin uniform diaphragm, Optics

Express, Vol. 18, pp. 9006-9014, 2010. [6] J.A. Guggenheim et al, "Ultrasensitive plano-concave optical microresonators for ultrasound sensing",

Nature Photonics, Vol. 11, pp. 714–719, 2017. [7] A. Z. Subramanian et al., "Low-Loss Singlemode PECVD Silicon Nitride Photonic Wire Waveguides for

532–900 nm Wavelength Window Fabricated Within a CMOS Pilot Line," IEEE Phot. J., vol 5, 2013. [8] Wang & Yao, “A practical guide to photoacoustic tomography in the life sciences”, Nature Methods, 2016. [9] H. Gao et al., "Simulation and characterization of a high-sensitive micro-opto-mechanical Microphone,"

EuroSimE, Toulouse, 2018. [10] V. Rochus et al, “Fast analytical model of MZI micro-opto-mechanical pressure sensor”, Journal of

Micromechanics and Microengineering, vol. 28 p. 064003, 2018. [11] B. Troia et al, Coupled multiphysics circuital modelling of micro-opto-mechanical pressure sensor systems,

Proc. SPIE 10545, MOEMS and Miniaturized Systems XVII, 1054518, 22 February 2018. [12] C. H. Huang et al., "Frequency response and mode shape characterization for acoustic sensor in micro-

opto-mechanical technology," IEEE International Ultrasonics Symposium (IUS), Washington DC, 2017. [13] V. Rochus et al., "Double MZI Micro-Opto-Mechanical Pressure Sensors for increased sensitivity and

pressure range," TRANSDUCERS, Kaohsiung, pp. 954-957, 2017. [14] V. Rochus et al., "Design of a MZI Micro-Opto-Mechanical Pressure Sensor for a SiN photonics platform,"

EuroSimE, Montpellier, 2016. [15] Westerveld et al, Optical micro-machined ultrasound sensors with a silicon photonic resonator in a buckled

acoustical membrane, EuroSimE, Hannover, March 2019, accepted. [16] W.J. Westerveld and H.P. Urbach, Silicon Photonics: Electromagnetic theory, IOP Publishing, 2017. [17] S.M. Leinders et al, “A sensitive optical micro-machined ultrasound sensor (OMUS) based on a silicon

photonic ring resonator on an acoustical membrane”, Scientific Reports, Vol. 5, Article 14328, 2015.