Wafer-Level Vacuum-Packaged Translatory MEMS Actuator with Large Stroke for NIR-FT Spectrometers

Abstract

We present a wafer-level vacuum-packaged (WLVP) translatory micro-electro-mechanical system (MEMS) actuator developed for a compact near-infrared-Fourier transform spectrometer (NIR-FTS) with 800–2500 nm spectral bandwidth and signal-nose-ratio (SNR) > 1000 in the smaller bandwidth range (1200–2500 nm) for 1 s measuring time. Although monolithic, highly miniaturized MEMS NIR-FTSs exist today, we follow a classical optical FT instrumentation using a resonant MEMS mirror of 5 mm diameter with precise out-of-plane translatory oscillation for optical path-length modulation. Compared to highly miniaturized MEMS NIR-FTS, the present concept features higher optical throughput and resolution, as well as mechanical robustness and insensitivity to vibration and mechanical shock, compared to conventional FTS mirror drives. The large-stroke MEMS design uses a fully symmetrical four-pantograph suspension, avoiding problems with tilting and parasitic modes. Due to significant gas damping, a permanent vacuum of ≤3.21 Pa is required. Therefore, an MEMS design with WLVP optimization for the NIR spectral range with minimized static and dynamic mirror deformation of ≤100 nm was developed. For hermetic sealing, glass-frit bonding at elevated process temperatures of 430–440 °C was used to ensure compatibility with a qualified MEMS processes. Finally, a WLVP MEMS with a vacuum pressure of ≤0.15 Pa and Q ≥ 38,600 was realized, resulting in a stroke of 700 µm at 267 Hz for driving at 4 V in parametric resonance. The long-term stability of the 0.2 Pa interior vacuum was successfully tested using a Ne fine-leakage test and resulted in an estimated lifetime of >10 years. This meets the requirements of a compact NIR-FTS.

An Experimental and Numerical Study on Glass Frit Wafer-to-Wafer Bonding

“…Gf bonding, together with all the other deposition steps, can be still delicate, as it was also recently shown in [ 20 ], where voids of unclear origin were observed in a batch of isolation gf trenches.…”

“…Gf is nowadays also adopted to seal other ceramic materials, see, e.g., [ 22 , 23 ], and, in its lead-free version or in a mix with other materials such as graphene, for solar cells [ 24 ]; however, here we focus on its traditional application in the microsystem industry [ 20 , 25 ], and we aim to carefully characterize the material as obtained from the foundry, so that the subsequent calculations regarding wafer warpage can be more accurate.…”

Wafer-level vacuum packaged translatory MEMS mirror with large stroke for compact NIR-FT spectrometers

“…Now, vacuum MEMS packaging can be performed at wafer level instead of the previous cost-intensive hybrid package. In this work, we update our previous work 18,19 and contribute new results concerning the following four topics:…”

Non-resonant MOEMS mirror for laser cavity-length tuning

“…In the past years there are numerous reports on translational micromechanics suited for various optical applications as Fourier transform spectroscopy (FTS), [15][16][17] time-domain optical coherence tomography (TD-OCT) 18 or cavity length tuning, e.g., for cavity ring-down spectroscopy (CRDS) 8 or tunable mode-hop free external cavity quantum cascade lasers. 19 Different drive concepts have been issued to control the position of optical elements, e.g., electrothermal bimorphs, 15, 20-22 magnetic [23][24][25] or piezoelectric drives [26][27][28] and electrostatic comb drives.…”

“…19 Different drive concepts have been issued to control the position of optical elements, e.g., electrothermal bimorphs, 15, 20-22 magnetic [23][24][25] or piezoelectric drives [26][27][28] and electrostatic comb drives. 17,29 All of them have their individual assets and drawbacks. In order to assess the system performance, all parameters have to be considered, i.e., mirror size, dynamic properties, maximum displacement and the extent of tilt.…”