MEMS Wafer-Level Packaging with Conductive Vias and Wafer Bonding

Micromachined accelerometers were packaged at wafer-level using both via-last and via-first approaches. In the via-last approach, a through-hole etched cap wafer was bonded to a micromachined device wafer using glass frit. Interconnections from the bond pads on the device wafer to the top of the cap wafer were made through the holes using sputter-deposition of metals. The bonded pair was then solder bumped, and diced for individually packaged devices. In the via-first approach, the cap wafer vias were filled by poly-crystalline silicon before bonding. After the Al to Al thermo-compression bonding of the cap and MEMS wafers, the vias were exposed by back-grinding. The exposed vias were then redistributed and solder-bumped. The wafer-level packaged three-axis accelerometer demonstrated the same functionality as production accelerometers when it was probed in wafer form as well as when it was mounted on a board.