Characterization of intermediate In/Ag layers of low temperature fluxless solder based wafer bonding for MEMS packaging

Low temperature fluxless solder for wafer bonding has received a lot of attention due to its great potential in hermetic MEMS packaging. Previous research activities mainly deploy solder alloy of eutectic composition to achieve low bonding temperature. We proposed new intermediate bonding layers (IBLs) of rich Ag composition in In-Ag materials systems. In this study, we investigated the intermetallic compounds (IMCs) at the bonding interface with respect to the bonding condition, post-bonding room temperature storage and post-bonding heat treatment. With this IBL, the IMCs of Ag2In and Ag9In4 with high temperature resist to post-bonding process are derived under process condition of wafer bonding at 180 degrees C, 40 min and subsequent 120-130 degrees C annealing for 24h. Low melting temperature IMC phase of AgIn2 is formed in the interface after long term room temperature storage or 70 degrees C aging treatment. This low melting temperature IMC phase can be completely converted into high melting temperature IMCs of Ag2In and Ag9In4 after 120 degrees C additional annealing. Based on our results, we can design the packaging process flow so as to get reliable hermetic packaged MEMS devices by using low temperature fluxless In-Ag wafer bonding. (C) 2009 Published by Elsevier B.V.