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.

Keywords: NIR Fourier-transform spectrometer; glass-frit wafer bonding; micro-opto-electro-mechanical system (MOEMS); optical wafer-level vacuum package (WLVP); translatory micro mirror.

Figure 1

MEMS-based IR-FTS using a large-stroke four-pantograph MEMS mirror: (a) schematic optical FTS set-up designed for a broad MIR (mid-infrared) spectral range of 2.5…16 µm using a translatory MEMS [27]; (b) hybrid optical vacuum package of MEMS using a ZnSe window due to broad spectral MIR range [25]: 1, MEMS chip; 2, cover spacer; 3, ceramic board; 4, MEMS chip soldered on ceramic board; 5, Al wire bonds; 6, ZnSe window with broad-band anti-reflex coating (BB-ARC); 7, final hybrid vacuum package; BW, bottom wafer; DW, device wafer; TSW, top spacer wafer; TGW, top glass wafer); (c) wafer-level vacuum package (WLVP) of MEMS for NIR-FTS with a spectral range of 0.8–2.5 µm.

Figure 2

Examples of state-of-the-art wafer-level MEMS vacuum packages: (a) WLVP of a MEMS resonator using alternating layers of evaporated Au and Sn [39]; (b) schematic set-up of wafer-level vacuum-packaged micro scanning mirrors using anodic bonding of a glass cover wafer to a polished Epipoly sealing frame and eutectic AuSn bonding of the bottom wafer to the MEMS backside [40].

Figure 3

Pantograph mirror suspension for large-stroke translatory MEMS: (a) SEM detail of single pantograph (pre-deflected, three parallel torsional spring axes are obvious); (b) translatory MEMS device designed for NIR with full symmetric suspension of 5 mm mirror using four pantographs (FEA geometry model of movable elements); (c) details of pantograph suspension of NIR-MEMS design.

Figure 4

SEM details of single pantograph suspension: mirror moved (a) up and (b) down.

Figure 5

SEM pantograph details of deflectable spring system: (a) spring 1; (b) spring 2; (c) spring 3.

Figure 6

Reduction of dynamic mirror deformation: (a) FEA results of topology at 350 µm deflection; (b) SEM photograph of outer ring-shaped support structure with mechanical decoupling springs.

Figure 7

Further options to minimize the mirror deformation of pantograph MEMS: (a) initial design with direct coupling of pantographs and mirror plate; (b) conceptual design for dynamic self-compensation of dynamic mirror deformation by means of additional outer inert masses and locally thinned mirror membrane.

Figure 8

FEA results of minimization of mirror deformation of 5 mm pantograph mirrors of 75 µm thick c-Si; translatory MEMS designs were developed for MIR-FTS for 500 µm amplitude at 500 Hz and λ_min = 2.5 µm: (a) initial MEMS design with direct mechanical coupling of pantograph suspensions and mirror plate; (b) with ring-shaped pantograph support structure and mirror held by mechanical decoupling structure; (c) dynamic self-compensation by means of additional outer inert masses and locally thinned mirror membrane.

Figure 9

Results of FEA modal analysis: eigenmodes and separation to used translation mode 1 at 257 Hz.

Figure 10

SEM photographs of electrostatic comb drives (visible are also the filled isolation trenches): (a) basic comb drives at ring-like support structure; (b) comb drive variant at pantograph levers.

Figure 11

Fabrication of device wafer: (a) MEMS process AM75; (b) backend integration of Au coating.

Figure 12

Translatory MEMS device after fabrication of device wafer: (a) chip photograph of MEMS chip without WLVP; (b) microscopic detail of pantograph suspension and comb drive; (c) MEMS chip topography measured with WLI (white light interferometry); metal lines add significant height profile.

Figure 13

Schematic set-up of the MEMS WLVP optimized for NIR-FTS: (a) schematic cross-section of WLVP stack; total thickness of WLVP stack is 3476 μm + thicknesses of glass-frit layers; (b) overall dimensions of encapsulated cavity; (c) MEMS WLVP chip.

Figure 14

Schematic process flow of wafer-level vacuum package for translatory MEMS: (a) 1 mm thick borofloat-glass wafer with NIR BB-ARC on interior surface; (b) screen printing and pre-bake of first glass-frit layer on TSW (top spacer wafer) front-side; (c) device wafer (DW) with symmetric Au coating on Si mirror plate; (d) screen printing and pre-bake of third glass-frit layer on bottom wafer (BW) front-side; (e) first bonding of top glass wafer (TGW)–TSW stack using flat-to-flat alignment; (f) screen printing and pre-bake of second glass-frit layer on TSW backside; (g) second bonding of TGW–TSW–DW stack; (h) deposition of Zr-based thin-film getter, using the extern PageWafer process of SAES Getters; (i) third bonding for final hermetic vacuum sealing of WLVP.

Figure 15

Screen-printed glass-frit bonding frames: (a) first glass-frit layer on TSW front-side; (b) after drying process at 120 °C; (c) after glazing process at 425 °C; (d) third glass-frit layer on BW alignment; (e) bond frame after drying at 120 °C; (f) after getter deposition, using PageWafer process of SAES Getters.

Figure 16

Final wafer-level vacuum package (WLVP) of translatory MEMS: (a) 6” wafer of MEMS WLVP before dicing; (b) infrared microscopic image of bond interface; details of bonding frame before and after third bonding process used for final hermetic vacuum sealing of WLVP; a good homogeneity of the bond is evident; (c) details of bond frame (bond islands are still encapsulated).

Figure 17

Frequency–amplitude characteristics (experiment without WLVP using external vacuum chamber): (a) varied pressure of 4–100 Pa @ 12 V; (b) voltage dependency at 0.1 Pa; (c, insert) at 500 Pa.

Figure 18

Experimental determination of minimum requirements for vacuum pressure inside the WLVP cavity: (a) results on vacuum pressure dependency of Q factor; obvious is the final calibration characteristic for determination of the vacuum pressure inside the WLVP; shown are also intermediate experimental results on the influence of cavity size; (b) exemplary result of determination of the Q factor from freely damped oscillation measured with laser vibrometer; (c) new set-up with small cavity and 1 mm gap.

Figure 19

Experimental results of translatory MEMS with WLVP: (a) MEMS sample with WLVP; (b) frequency–amplitude characteristic in parametric resonance (sample of stack 03; initial WLVP run).

Figure 20

Experimental results on influence of process temperature on Au mirror coating: (a) defect images of mirror coating on wafer backside of stack 04 (before final vacuum bonding); (b) comparison of diffusion defects (white-light-interferometry (WLI), microscopy) on mirrors front- and backside; (c) comparison of static mirror deformation before and after the WLVP process.

Figure 21

Improvement of the Au mirror coating and reduction of the influence of the process temperature on the Au mirror coating; experimental results of mirror planarity of simulated WLVP processes using MEMS dummy wafers, comparison of samples with and without diffusion barrier layer of 40 nm thick Al₂O₃ deposited by ALD. Kx and Ky denotes the curvature in x- and y-directions, respectively.

Figure 22

Defect images of potentially parasitic effects on yield and long-term stability of inner vacuum: microscopic images of (a) local cracks and spalling of BW glass-frit frame after getter deposition at SEAS; (b) small cracks inside glass-frit bonding frame of BW before getter deposition; (c) SEM photograph of open void inside filled trench isolation.

Figure 23

Final results of WLVP: (a) process change of third glass-frit bond frame on backside of DW; (b) boxplot of inner vacuum pressure for WLVP stacks (left) and normalized conductance of leakage channel measured for WLVP stack #10.

Figure 24

Frequency amplitude characteristics in parametric resonance at 4 V: (a) influence of Q factor on frequency characteristic, shown are WLVP MEMS of the initial and final WLVP run; (b) exemplary comparison of two WLVP MEMS devices (from different wafers); good reproducibility is obvious.

Figure 25

Exemplary results of (a) parasitic mirror tilt within a full translational oscillation of 350 µm amplitude, and (b) static mirror deformation of the Au-coated mirror plate after WLVP process.