Hermetic Package for MEMS Mirror

Abstract

Applications for optical scanning and 3D sensing, mostly related to the consumer electronics and automotive markets, are becoming more and more demanding in terms of performance and power consumption. When using a MEMS based Laser Beam Scanning, enclosing the mirrors into a hermetic package helps to address these requirements. Two MEMS mirrors with different sizes and performance have been selected for this study. A unique package, suitable for both mirrors, has been properly designed. The package is based on a ceramic chip carrier with a soldering ring on the top surface. A metal cap with a sintered optical window is attached to the ceramic substrate. The optical window is tilted with respect to the mirror to avoid spurious reflections. The MEMS mirrors have been integrated onto the ceramic substrate by standard die attach and wirebonding technologies. Hermetic sealing has been developed under nitrogen atmosphere to guarantee the MEMS stable operation against humidity and contaminants. Flux-less SnAgCu solder preforms have been selected as sealing material. The solder reflow profile has been optimized in order to obtain a uniform solder fillet at the sealing region while minimizing the thermal stress to the mirrors. The hermeticity of the assemblies has been proved by fine and gross leak testing. The packages show a leak rate < 1.5 E-8 atm cc/s and pass the fine and gross leak test, defined accordingly to MIL-STD-883 TM 1014.18.