Sealing technique for wafer-level integrated cavity using In-Ag multilayers

The performance of MOEMS (Micro-Optical-Electronic-Micro-Systems) may be significantly improved and their service life-time extended by packaging under vacuum. For numerous applications the bonding process temperature has to be below 200°C and sealed joints must withstand a reflow temperature (about 250-280°C) without debonding and unsealing. The Ag-In system has been selected for developing such a bonding process. In this study, the kinetics and sequence of intermetallic phase growth in a Ti/Ag/In/Ag multilayer structure with various ratios of thicknesses, deposited on a silicon substrate, was investigated by DSC (Differential Scaning Callorimetry), SEM and XRD analysis. The joints were examined by DSC to determine the re-melting temperatures and the quality of a cavity sealing was evaluated using a helium leak detection system. It was established that annealing a Ag/In multilayer with a total Ag to In layer thickness ratio equal to 3 and an overall thickness of about 10μm under vacuum of 10^-7torr at 190°C for 40 min allows achieving a void-free joint consisting of two intermetallic phases Ag₂In (γ) and AgIn₂ (φ). A further appropriate thermal treatment at a relatively low temperature in air leads to the formation of the joint with a re-melting temperature higher than 300°C. The helium leak rate of the integrated cavity was experimentally estimated in the 5-10×10^-9 mbar 1 s^-1 range.