New silicon-based micro-electro-mechanical systems for photo-acoustic trace-gas detection

Abstract

The achievable sensitivity level of photo-acoustic trace-gas sensors essentially depends on the performances of the acoustic transducer. In this work, the mechanical response of different silicon-based micro-electro-mechanical systems (MEMS) is characterized, aiming at investigating both their mechanical properties, namely the resonance frequency and the quality factor, and the minimum detection limit (MDL) achievable when they are exploited as an acoustic-to-voltage transducer in a trace-gas photoacoustic setup. For this purpose, a 4.56 µm Continuous-Wave (CW) quantum cascade laser (QCL) is used to excite a strong N₂O roto-vibrational transition with a line strength of 2.14 × 10^-19 cm/molecule, and the detection of MEMS oscillations is performed via an interferometric readout. As a general trend, the minimum detection limit decreases when the resonance frequency investigated increases, achieving a value of 15 parts per billion with a 3 dB cut-off lock-in bandwidth equal to 100 mHz, around 10 kHz.

Keywords: Acoustic-to-voltage transducer; Micro-electro-mechanical systems (MEMS); Minimum detection limit; Parts-per-billion sensitivity; Photo-acoustic spectroscopy; Quality factor; Resonant frequency; Trace-gas sensing.

Fig. 1

Schematic bi-dimensional representation of the five investigated MEMS structures, depicted in orange. The vertical and horizontal scales (length and height, respectively) allow the reader to have an idea of their dimensions. All of them are characterized by a thickness of 10 $\mathrm{\mu }$m. Segments highlighted in blue represent the parts where they are anchored to the substrate. The gray blurred line represents the approximated location of the mid-infrared laser beam used for the pressure wave generation. The red spot indicates the position where the He-Ne laser, used for the interferometric readout, hits the structures. Inset: a three-dimensional sketch of the custom-made aluminum anchoring system. The ensemble is placed within the sample such that the QCL beam passes as close as possible to the MEMS without ever touching it.

Fig. 2

Schematic representation of the photo-acoustic spectroscopy trace-gas setup implemented to characterize our MEMS structures. TEC: temperature Controller. CD: current driver. QCL: Quantum Cascade Laser. ISO: optical isolator. $\lambda$/2: half wave-plate. AOM: acousto-optic modulator. HeNe: Helium-Neon laser. BS: beam splitter. PD: photo-diode detector. RM: reflective mirror.

Fig. 3

Normalized ${\mathrm{R}}^2$-component signals versus modulation frequency at a fixed working pressure of 20 mbar for (a) M1, (b) M2, (c) M3, (d) M4, (e) M5a, and (f) M5b. For each of them, the colored points represent the experimental data, while the dashed black line is the associated Lorentzian fit. Given the FWHM $\mathrm{\Delta }\mathrm{f}$ and the central frequency ${\mathrm{f}}_0$ as the fit results, then the Q-factor can be calculated as $\mathrm{Q}={\mathrm{f}}_0/\mathrm{\Delta }\mathrm{f}$.

Fig. 4

PA signals versus laser current for M4 at different working pressure regimes. The spectra represent the R-component of the 1-f photo-acoustic signal acquired from the Michelson interferometer output and demodulated via the lock-in amplifier at the MEMS resonant frequency. The 3 dB cut-off lock-in bandwidth is set to 100 mHz. The current driver used to scan the laser current is characterized by a driver with a voltage-to-current conversion factor of 2 mA/V. The ramp for the acquisition of the N${}_2$O absorption line is characterized by the following settings: frequency 2.5 mHz, amplitude 2 VPP (peak-to-peak voltage), phase 0 degrees, and symmetry 50%.

Fig. 5

Normalized photo-acoustic peak signal (red dots) and photo-acoustic noise (blue dots) as a function of the working pressure for (a) M1, (b) M2, (c) M3, (d) M4, (e) M5a, and (f) M5b. Each blue data point represents the peak of the photo-acoustic spectrum at a specific pressure, while the red data point is the computed 1-$\sigma$ noise.

Fig. 6

(a): MDL as a function of the working pressure for the different structures investigated acquired over 10 s timescale. As reported in the legend: blue dots for M1, green dots for M2, red dots for M3, gray dots for M4, purple dots for M5a, and orange dots for M5b. (b): minimum detection limit (MDL) reachable for each MEMS structure sorted in ascending order with the resonant frequency. The two graphs share the same color map.

Fig. A.7

Graphical description of the routine used to fabricate the investigated Micro-Electro-Mechanical structures.

Fig. B.8

Resonance frequency versus working pressure for: (a) M1, (b) M2, (c) M3, (d) M4, (e) M5a, and (f) M5 7b. As it is possible to see, the resonant frequency decreases when increasing the pressure.

Fig. B.9

Q-factor versus working pressure for: (a) M1, (b) M2, (c) M3, (d) M4, (e) M5a, and (f) M5b. For each graph, the colored dots represent the experimental data obtained as already explained in Section 4.1, while the black dashed line is the fit on the data.

Fig. C.10

1f photo-acoustic traces at different pressures for: (a) M1, (b) M2, (c) M3, (d) M4, (e) Ma, and (f) M5b.