A new design and a fabrication approach to realize a high performance three axes capacitive MEMS accelerometer

Abstract

This paper presents a new fabrication approach and design for a three axis capacitive MEMS accelerometer that is capable of measuring externally applied accelerations in three orthogonal axes. Individual lateral and vertical axis accelerometers are fabricated in the same die on an SOI wafer which is anodically bonded to a glass substrate. Handle layer of the SOI wafer is used as the top electrode for the vertical axis accelerometer. This accelerometer has a 2mm2 perforated electrode area anchored to the glass substrate by four beams. The lateral axis accelerometers on the other hand, have comb finger structures with a 2.7×4.2mm device size and anchored to the glass substrate by six folded beams. Rest capacitance of the vertical axis accelerometer is designed to be 8.8pF, and it is 10.2pF for the lateral axis accelerometers. The system level performance results are obtained using analog readout circuitry integrated to each axis separately. The x- and y-axis accelerometers show a noise floor and bias instability equal or better than 13.9μg/√Hz and 17μg, respectively, while the z-axis accelerometer shows 17.8μg/√Hz noise floor and 36μg bias instability values.