Acceleration Sensor in Surface Micromachining for Airbag Applications with High Signal/Noise Ratio 960758

Employing novel surface micromachining techniques, a highly miniaturized, robust device has been fabricated. The accelerometer fulfills all requirements of state-of-the-art airbag systems. The present paper reports on the manufacturing and assembly process as well as the performance of the sensor.
The capacitive sensing element consists of a moveable proof mass of polysilicon on a single crystalline silicon substrate. A lateral acceleration displaces the proof mass and a capacitive signal is generated at a comb electrode configuration. An external IC circuit provides the signal evaluation and conditioning in a closed loop mode, resulting in low temperature dependency of sensor characteristics and a wide frequency response.
The sensor is fabricated by standard IC processing steps combined with additional surface micromachining techniques. A special deposition process in an epitaxial reactor allows the fabrication of moveable masses of more than 10 µm thickness.
As a result, working capacitances are up to 10 times higher as compared with conventionally fabricated surface micromachined accelerometers. Measurement ranges of 50gn with mgn resolution have been realized.
Secondly, the larger thickness results in an increased mechanical stiffness of the mechanical structure normal to the wafer surface. Hence, cross sensitivities of this sensing element are reduced by more than one order of magnitude. Also, fewer problems are encountered with the more rugged sensor structure during mounting and encapsulation.


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