Sub-10 cm3 interferometric accelerometer with nano-g resolution

A high-resolution accelerometer with a bulk-micromachined silicon proof mass and an interferometric position sensor was developed for measuring vibratory accelerations. The interferometer consists of interdigitated ringers that are alternately attached to the proof mass and support substrate. Illuminating the fingers with coherent light generates a series of diffracted beams. The intensity of a given beam depends on the out-of-plane separation between the proof mass fingers and support fingers. Proof masses with mechanical resonances ranging from 80 Hz to 1 kHz were fabricated with a two mask process involving two deep reactive ion etches, an oxide etch stop, and a polyimide protective layer. The structures were packaged with a laser diode and photodiode into 8.6-cm³ acrylic housings. The 80-Hz resonant proof mass has a noise equivalent acceleration of 40 ng/rt Hz and a dynamic range of 85 dB at 40 Hz