A wide frequency range, rugged silicon micro accelerometer with overrange stops

A rugged and wide frequency range silicon micro accelerometer with integral overrange stops has been developed for use in anthropomorphic test dummies and on vehicles for crash and sled safety testing. The accelerometer utilizes a monolithic piezoresistive cantilever type sensor crafted on (110) silicon wafer. Taking advantage of the unique vertical wall etching capability on (110) silicon, the geometry of the hinge, mass, and stop-gap can be defined precisely. It is limited only by the photolithographic accuracy instead of the etching time control. The piezoresistive gages are etch freed from the silicon wafer. Their cross sectional area is very small, 0.6 μm by 4.2 μm, thus needing very little strain energy to produce a useful signal. By locating them at an appropriate distance apart from the centrally support hinge, the gages are very efficient in picking up the acceleration force. A high sensitivity (nominally 0.2 mV/g) yet wide frequency range (nominally resonate at 28 kHz) sensor can be obtained. The etch freed gages enable the accelerometer to stabilize (or warm up) in less than 15 μs after power on. It also has minimal thermal zero shifts of ±6 mV in a wide temperature range from -54 to +100°C. Furthermore, the vertical wall etching capability on (110) silicon allows very narrow gaps between the mass tip and surrounding rim to “stop” the mass tip from traveling too far. These “overrange stops” protect the sensor from excessive g shock. The accelerometer demonstrates survivability under repeated high g shock in excess of 10000 g