Fabrication options and operation principle for single-crystal silicon vibratory ring gyroscope

Abstract

Four different approaches for the construction of a single crystal silicon vibratory ring gyroscope were investigated. All of them require deep trench etching (DRIE)of silicon, anodic bonding of silicon structures to a glass wafer, and a dissolve silicon wafer process. In the first approach, regular single crystal silicon was used as the starting sensor structural material. Heavy boron diffused silicon (B⁺⁺Si) was formed followed by anodic bonding to the glass plate. The undoped silicon was then dissolved and the device structure was fabricated by deep trench etching of the B++Si. Due to the slow boron diffusion process, this approach severely limits the attainable thickness of the device structure. In the second approach, deep trench etching was carried out first followed by boron diffusion. In order to reduce RIE lag and boron diffusion time, the larger features were subdivided into smaller ones before DRIE process. We found that RIE lag still existed which had a detrimental effect on the sensor performance. In the third approach, silicon-on-insulator (SOI) wafer was used and the sensor structures were built in the Si-epi layer by DRIE. Because of the very slow etch rate of SiO₂ in the DRIE process, RIE lag can be avoided. However, the associated footing problem makes the device dimensional control difficult. In the fourth approach, a layer of epi-GeB⁺⁺Si on silicon wafer was used to build the sensor structures. The fabrication process was similar to that used in the third approach. Both RIE lag and footing problems were avoided. In the operation of the vibratory ring gyroscope, it is highly desirable of having the resonant frequencies around the ring to be isotropic. In this work, theoretical and experimental studies were conducted aiming at achieving the isotropy of the resonant frequency of the single crystal silicon ring, which has an orientation dependent modulus of elasticity. We found that for odd flexural vibration modes, the resonant frequencies of the ring were isotropic, whereas, for even modes they became anisotropic.