A symmetric surface micromachined gyroscope with decoupled oscillation modes

This paper reports a new symmetric gyroscope structure that allows both matched resonant frequencies for the drive and sense vibration modes for better sensitivity, and also decoupled drive and sense oscillation modes for preventing unstable operation due to mechanical coupling and achieving a low zero-rate output drift. The symmetry and decoupling features are achieved at the same time with a new suspension beam design. The gyroscope structure is designed using a standard three-layer polysilicon surface micromachining process (MUMPS) and simulated using the MEMCAD software. The drive and sense mode resonant frequencies of the fabricated device are measured as 28,535 and 30,306 Hz, respectively, which are in agreement with the finite element simulations. The small mismatch is due to the unsymmetric distribution of the etch holes, which can be eliminated with a proper design. When the resonant frequencies are closely matched, the rate sensitivity of the gyroscope is amplified by the mechanical quality factor of the sense resonant mode. The new suspension beam structure also provides very high quality factors for the drive and sense modes, such as 10,400, when operated under 10 mTorr vacuum level. Even though the performance of the fabricated sensor is limited due to large parasitic capacitances between the mechanical structure and the substrate, measurements, and calculations show that the sensor can still sense angular rates as small as 1.6degrees/s under vacuum. This sensitivity can be enhanced by at least an order of magnitude if the parasitic capacitances could have effectively been eliminated. The advantage of the new structure can be combined with advanced, high-aspect ratio fabrication processes to obtain very sensitive micromachined gyroscopes. (C) 2002 Elsevier Science B.V. All rights reserved.