X-Y and Z-axis capacitive accelerometers packaged in an ultra-clean hermetic environment

Abstract

We present the first ultra-miniature (<;1 mm2) capacitive accelerometers built in the epitaxial polysilicon encapsulation (`epi-seal') process. Lateral (X/Y-axis) and out-of-plane (Z-axis) accelerometers were fabricated using a variant of the epi-seal process that utilizes a separate electrode defined in the encapsulation layer. Sensitivities of up to 10.2 fF/g and 0.24 fF/g were achieved for lateral and out-of-plane accelerometers respectively. Resolution down to 0.47 mg/rt-Hz is realized, approaching the theoretical Brownian limit for a combined accelerometer and off-the-shelf capacitance-measuring chip (AD7746) system. This enables singlechip integration of accelerometers, gyroscopes, pressure sensors, thermometers, and clocks all in an established commercial manufacturing process.