Single-mask SOI fabrication process for linear and angular piezoresistive accelerometers with on-chip reference resistors

Piezoresistive accelerometers usually require separate photomasks for the fabrication of the proof mass, suspension, lightly doped piezoresistors, and highly doped conductors. This paper introduces a fabrication process that defines all components simultaneously using a single mask. By dry-etching SOI wafers that have a specific resistivity, piezoresistors can be defined and isolated from each other without the pn-junction normally required in piezoresistive sensors. The conductors are designed to be much wider than the piezoresistors and a free-standing proof mass is obtained by selectively removing the buried silicon oxide layer. Furthermore, reference resistors that provide the second half of a Wheatstone bridge are included on the silicon chip, eliminating the need for external resistors. Both single-axis and angular accelerometers have been fabricated and linear output characteristics are demonstrated when the single-axis devices are subjected to acceleration