Single proof-mass tri-axial pendulum accelerometers operating in vacuum

This paper reports on the design, fabrication and characterization of single proof-mass tri-axial capacitive accelerometers coexisting in a low-pressure environment with high-frequency gyroscopes, for the implementation of monolithic 6-degree-of-freedom inertial measurement units. The accelerometers are designed to operate as quasi-static devices (i.e. non-resonant sensors) in mid vacuum levels (1-10 Torr) by increasing squeeze-film air damping through the use of capacitive nano-gaps (<; 300 nm). Reduced die area is achieved utilizing a pendulum-like structure composed of a 450×450×40 μm³ proof-mass anchored to the substrate by a cross-shaped polysilicon spring. The small capacitive gaps, allow for the design of devices with high resonance frequency (~ 15 kHz) that provide large shock and vibration immunity.