AIRBAG APPLICATION - A MICROSYSTEM INCLUDING A SILICON CAPACITIVE ACCELEROMETER, CMOS SWITCHED-CAPACITOR ELECTRONICS AND TRUE SELF-TEST CAPABILITY

A low cost, high reliability accelerometer microsystem designed for cr ash 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 x 3.5 mm x 1.15 mm), and high design fle xibility thanks to a two-chip construction. The sensitive part is a su rface micromachined capacitive interdigitated structure realized from a SIMOX SOI substrate. The accelerometer operates in a closed loop mod e using electrostatic feedback with conditioning circuitry realized in a 2 mu m CMOS process. A high performance readout circuit using switc hed capacitors has been developed. Behavioural simulation results show a bandwidth of 630 Hz at +/- 50g with 5 V power supply. The fabricati on process includes the realization of a free-standing seismic mass by means of reactive ion etching and sacrificial oxide etching, the mech anical protection of the sensing element with a thin silicon cap bonde d onto the structured SOI wafer, and eventually the electrical connect ion with the ASIC by flip-chip bonding. Preliminary results are very e ncouraging: dynamic actuation of the sensing elements is optically tes ted, with a yield of 70% at a prototype level. Excellent shock resista nce and low internal stress are observed.