Low temperature wafer level packaging technology of bulk-micromachined MEMS device

Abstract

New low temperature, low cost, small size packaging technology of novel bulk-micromachined MEMS sensor for mobile applications was developed. The sensor was fabricated with the bulk-micromachining process of SOI substrates and composed with a proof mass, membrane and electrodes for capacitance sensing. The sensor device was capped with very thin (130um-thickness) top and bottom silicon cap wafers which have a 80um-depth cavity. Top and bottom cap wafers were bonded with the sensor wafer with a low temperature curing polymer adhesive lower than 200°C. It is needed that the low temperature packaging technology and the passivation of top and bottom sides of the sensor for keeping the sensor performances and preventing stiction of the proof-mass during the molding processes. After bonding the three substrates, the top cap silicon was dry etched to expose bonding pads for the signal interconnection. The ASIC chip was polished to 75um-thickness, diced and bonded on a half-etched 200um-thick lead-frame with a DAF. The diced wafer-level-capped sensor was stacked on the ASIC, wire bonding was accomplished between the sensor and the ASIC, and the ASIC and the lead-frame and finally transfer molding process was done. The developed package is 24-leads QFN and the dimension is 4.0mm×4.0mm×1.1/1.2mm.