Vacuum Packaging of MEMS by Self-Assembly

Abstract

The development of a solder-based vacuum bonding technique for micro-electro mechanical systems (MEMS) applications is presented. A chip with a micro-sensor was bonded to a cover plate to form a sealed cavity. The method relies on a solder-based hybridization comprising a self-assembly process that takes advantage of the surface tension and viscous forces of the solder. A model of the assembly was developed to predict the capillary instability of the solde, and the dynamic behavior of the assembled chip. Experimental results showed that a molten bead with parallel contact lines is stable when the ratio between the solder height and solder width is less than one half. Misalignment attributed to the self-assembly process was within a few microns. Fractographic analysis and leak and shear tests confirmed the predicted sealing and mechanical characteristics of the bonding. This method is especially suitable for bonding wafers in a vacuum for MEMS and other micro-devices, at low manufacturing temperatures (∼250 °C).