BCB-based wafer-level packaging of integrated CMOS/SOI piezoresistive MEMS sensors

We present a two-staged BCB- and anodic-bonding-based packaging approach used to package both a 3-axis piezoresistive accelerometer and an absolute pressure sensor both based on the same CMOS/SOI process with integrated on-chip amplification. A number of electrical connections run from the sensing element and the integrated amplification circuitry to the bond-pads, crossing the area used for the bond. Therefore, the bonding technique used for the top surface needs to provide good conformance over non-planar structures in order to create a sealed cavity. Zero-level packaging is achieved using anodic bonding of a pyrex wafer on the back-side and a BCB-based bonding approach to attach another pyrex wafer to the front-side. As the seismic mass of the accelerometer is formed by both the SOI-handle and -device layers, recesses to allow upwards and downwards movement of the mass are crucial for the performance of this device. Due to the heat-induced reflow process and the relative softness of the BCB material good conformance over non-planar connection tracks is achieved in the bonding process.