Airbag application: a microsystem including a silicon capacitive accelerometer, CMOS switched capacitor electronics and true self-test capability

Abstract

A low cost, high reliability accelerometer microsystem designed for crash sensing in automotive airbag electronic control units is presented. The proposed microsystem offers high level output, on-line self-test function, small size (3.5 mm × 3.5 mm × 1.15 mm), and high design flexibility thanks to a two-chip construction. The sensitive part is a surface micromachined capacitive interdigitated structure realized from a SIMOX SOI substrate. The accelerometer operates in a closed loop mode using electrostatic feedback with conditioning circuitry realized in a 2 μm CMOS process. A high performance readout circuit using switched capacitors has been developed. Behavioural simulation results show a bandwidth of 630 Hz at ±50g with 5 V power supply. The fabrication process includes the realization of a free-standing seismic mass by means of reactive ion etching and sacrificial oxide etching, the mechanical protection of the sensing element with a thin silicon cap bonded onto the structured SOI wafer, and eventually the electrical connection with the ASIC by flip-chip bonding. Preliminary results are very encouraging: dynamic actuation of the sensing elements is optically tested, with a yield of 70% at a prototype level. Excellent shock resistance and low internal stress are observed.