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

A translatory MOEMS actuator is presented, which enables a precise out-of-plane translatory oscillation of a 5mm mirror with 700 μm large stroke at 267Hz, when driven at 4V in parametric resonance. Due to significant gas damping operation in vacuum is needed. The minimum requirements on vacuum pressure (pmax ≥ 3.21 Pa, Q ≥ 1177) were determined experimentally. Therefore, the MOEMS are permanently encapsulated by means of a wafer-level-vacuum package. The hermetic sealing of MEMS WLVP (stack of 4 wafer 6”) was realized by glass-frit bonding (i) to be compatible to MEMS process AME75 and (ii) to avoid any (vertical) TWI. The ductile glass frit bond layer allows hermetic sealing also on non-ideal wafer topologies with height differences of several 100 nm. But high process temperatures of 435°C are required. Despite the high process temperatures (430°C needed for glass frit bonding) a sufficient static mirror planarity of ≤ λ/10 was achieved. The paper will discuss details of VWLP development and MEMS system integration. The longterm stability of 0.1 Pa inner vacuum pressure was successfully tested to be > 10a using a Ne fine leakage test. For system integration into a miniaturized FT-NIR spectrometer selected MEMS with minimal tilt were used. The NIR-FTS achieved a spectral resolution of 8.3 cm-1 and SNR ≤ 8000 (with co-adding of 1000 spectra). The new translatory MEMS are very promising for compact FTS. The versatility and ruggedness of a MOEMS-FTS makes it ideal for process control in harsh environments (e.g. surveillance of fast chemical reactions).