Low Temperature Hermetically Sealed 3-D MEMS Device for Wireless Optical Communication

Novel processes were developed that resulted in a self-packaged device during the system integration, along with a transparent lid for inspection or optical probing. A new process was developed for improving the verticality in Micro Electro Mechanical Systems (MEMS) structures using Deep Reactive Ion Etching (DRIE). A self-pattered, mask-less photolithography technique was developed to metallize these vertical structures while maintaining a transparent window, for packaging of various MEMS devices. The verticality and metallization coverage were evaluated by incorporating the MEMS structures into an optical Corner Cube Retroreflector (CCR). A low temperature, hermetic sealing technique was also developed using In-Au thermo-compression bonding at 160°C. Cross-shaped 550um deep vertical mirrors, with sidewall angles of 90.08° were etched with this new DRIE technique. This is the best reported sidewall angle for such deep structures. The typical scalloped DRIE sidewall roughness was reduced to 40nm using wet polishing. A bonded Pyrex wafer was used as the handle wafer during DRIE; it eventually forms the package window after DRIE. The metallized, vertical mirrors were bonded to a MEMS device chip to assemble and package the CCR. The MEMS device chip consisted of an array of torsion mirrors. The mirrors were designed to modulate at 6Vp-p - 20Vp-p, with the resonant frequencies ranging from 25 KHz - 50 KHz. The design and simulation results are presented. To test the hermetic seal, helium leak tests were performed on the packaged device. Leak rates of as low as 2.8x10-8 atm cc/s air were detected, which is better than the MIL-STD-883G of 5x10-8 atm cc/s air for a package volume of 7.8x10-3 CC. A microprocessor and temperature/humidity sensor was then integrated with the CCR to assemble a passive optical digital data communicator. A flexible circuit design and a folded packaging scheme were utilized to minimize the overall form factor. Flat, flexible polymer batteries were incorporated to reduce the thickness of the package to a few millimeters. The fully packaged sensor system was about 30mmx30mmx6mm. Recorder sensor data was transmitted to a remote location using the CCR, and those results are presented