Wafer-level vacuum-packaged triaxial accelerometer with nano airgaps

This paper reports on the design, implementation and characterization of triaxial capacitive accelerometers operating in a low-pressure environment (~1 Torr). Small form-factor devices, with proof-mass area of less than 1mm², were fabricated on a 40 μm-thick SOI substrate using the HARPSS™ process to attain in-plane and out-of-plane nanoscale capacitive airgaps (~300 nm). Increased sensitivity and stable open-loop operation were simultaneously achieved by using additional damping electrodes. Large electromechanical coupling provided by the deep sub-micron airgaps allow for the design of high-frequency accelerometers (~15 kHz) that yield better shock and vibration immunity. Scale factors of 7.5 mV/g and 8.7 mV/g were measured for the in-plane (X/Y-axis) and out-of-plane (Z-axis) accelerometers, respectively, with measured cross-axis sensitivity of less than 0.5 % for accelerations of up to ±6g.