A high aspect-ratio high-performance polysilicon vibrating ring gyroscope.

Abstract

Design, fabrication and testing of a micromachined, high aspect-ratio high-performance polysilicon vibrating ring gyroscope are presented. The polysilicon vibrating ring is 1mm in diameter, 80mum thick and 4mum wide. It is supported by eight semi-circular springs to a center post that is 120mum in diameter. Sixteen electrodes are evenly located around the structure to drive, sense and electronically tune the ring structure. To sense rotation, the ring is electrostatically vibrated into its first in-plane flexural vibration mode and the position of the node lines are capacitively monitored. In this dissertation, a detailed analysis has been performed to determine the overall sensitivity of the vibrating ring gyroscope and identify its scaling limits. Several aspects of this analysis apply to other gyroscope sensing structures. Based on this analysis, a new single-wafer, high aspect ratio, dry-release poly-silicon microfabrication technology has been developed to implement sensor structures that provide all the features required for achieving the desired high performance. This process utilizes polysilicon as the structural element with its superior and homogenous material properties and is capable of producing very thick vertical polysilicon and silicon structures using Deep Reactive Ion Etching of silicon. Various size capacitive air-gaps ranging from sub-micron to tens of micron can be realized in this technology. This process is capable of producing silicon electrodes as tall as the main body structure with a simple fabrication process that eliminates the limitations of the previous technologies. Using a bent-beam strain gauge sensor, residual stress in 80mum thick, 4mum wide trench-refilled vertical polysilicon beams has been measured to be zero. 300mum long clamped-clamped beam resonators have shown quality factors as high as 85,000 in 1mTorr vacuum. A prototype 1.7 x 1.7 mm<super>2</super> Polysilicon Ring Gyroscope (PRG) fabricated using the above technology has been tested in hybrid format. An open-loop sensitivity of 200muV/deg/sec in a dynamic range of +/-250 deg/sec was measured under low vacuum level for a prototype device. The resolution of the sensor is currently limited by the noise from the circuitry and for a prototype sensor with a quality factor (Q) of 1150 and parasitic capacitances of 2pF was measured to be less than 1 deg/sec in 1Hz bandwidth. Elimination of the parasitic capacitances and improvement in the quality factor of the ring structure are expected to reduce the resolution to 0.01 deg/sec in 1Hz bandwidth. By increasing the drive amplitude to 1mum in an asymmetric single ring gyroscope design, a minimum detectable signal of 5 x 10<super> -3</super> deg/sec (18 deg/h) in a 10Hz bandwidth can be achieved. Recommendations for improving the sensor performance to tactical and inertial grade levels are discussed at the end.