In-plane micromachined accelerometer and bridge circuit having same

Abstract

A micromachined accelerometer for measuring acceleration in a direction parallel with the plane of the accelerometer substrate. The accelerometer has a strain-isolation pedestal, a flexure attached to the pedestal, and a proof mass attached to the flexure. The pedestal is wider than the flexure and does not bend when the device is under acceleration. The pedestal serves to isolate the flexure from substrate strain which may be caused by device packaging or temperature variations. Preferably, the joint between the pedestal and flexure, and the joint between the flexure and proof mass are smoothed to prevent stress concentration. The joints have a radius of curvature of at least 1 micron. A piezoresistor is located in one sidewall of the flexure. Alternatively, two piezoresistors are located on the flexure, with one on each sidewall. In this embodiment, a center-tap connection is provided to the point where the two piezoresistors are connected. The present invention includes devices having two accelerometers with Large proof masses and two reference accelerometers having very small proof masses. The four accelerometers are electrically connected in a wheatstone bridge circuit. Also, in the embodiment having two piezoresistors on one flexure, two accelerometers (with a total of four piezoresistors) are electrically connected in a Wheatstone bridge circuit to provide accurate acceleration sensing.