Resonant Microibeam Accelerometers

Resonant microbeam strain-sensing elements have been combined with a highly symmetric multiwafer silicon microstructure to fonn the first micromachined accelerometer with a polysilicon resonant microbeam pickoff. The resonant microbeam accelerometers uniquely combine bulk micromachining with advanced surface micromachining techniques and a multiple-wafer structure to produce an all-silicon acceleration sensor capsule with a direct-to-digital frequency output. The microbeams are fabricated from thin films of mechanical-grade polysilicon with integral polysi1:icon vacuum encapsulation, electrostatic drive and piezoresistive sense. The silicon microstructure features a dual open-web suspension system formed monolithically with the silicon proof mass and frame. Silicon caps provide squeeze film gas damping, overrange protection, and environmental protection. Initial devices have been fabricated with scale factors greater than 700 Hz./g on a +20-g device with a base microbeam frequency of 524 kHz. Root Allan variances below 0.1 Hz and seven-day stability measurements less than 2 ppm of the tiase frequency indicate that milli-g stability and micro-g sensitivities are attainable.