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.
High aspect ratio single crystal silicon resonant beams oriented with angles of 0, 11.25, 22.5, 33.75, and 45 degrees to the Si 100 direction were fabricated by using high density plasma etching and dissolved wafer techniques. The silicon was doped with boron with a concentration of >= 5 X 1019 cm-3. The moduli of elasticity of the boron doped silicon were calculated from the measured resonant frequencies of the fabricated cantilever beams. They were approximately 1.56, 1.60, 1.66, 1.81 and 1.92 X 1012 dyne/cm2 with respect to the five different orientations mentioned above. These values are smaller than those calculated theoretically for intrinsic single crystal silicon. The built-in stresses in the clamped-clamped bridges were also found to be orientations. The stress variation is attributed to the variation of the modulus of elasticity and the difference in thermal expansion coefficients between silicon and the glass substrate to which the beam anchors are bonded.
High volume silicon micromachining has been employed by the automotive industry for 20 years. This paper examines past, current and future applications of MEMS to the automobile. Both sensor and the application of micromachining to other automotive areas are covered. Technologies such as wet and plasma etching, wafer bonding, LIGA, circuit integration and packaging are discussed.
Nickel structures were electroformed from a commercially available nickel sulfamate bath. The plating molds were made of thick photoresist (approximately 20 micrometers) and delineated by a UV lithographic method. For mold cavities with very small aspect ratios (minimum planar dimension/mold thickness), the deposition rate is higher for those with smaller feature sizes than those with larger feature sizes. For mold cavities with large aspect ratios, no such correlation was observed. X-ray and transmission electron microscopy results show that the electroformed nickel is polycrystalline and in columnar form. For current density less than or equal to 8 mA/cm2, the nickel deposits orient preferably with <220> crystallographic direction normal to the substrate surface. For current density greater than or equal to 12 mA/cm+2), the nickel cantilevers tend to curl downward, and the nickel deposits orient preferably with <200> crystallographic direction normal to the substrate surface. There are only minor differences in the relative intensities of the (111), (200) and (220) x-ray peaks of the nickel deposits electroplated on gold, copper and chromium, implying that the effect of the plating base material on the nickel structure is minimal. The relative intensities of the (111), (200) and (220) x-ray peaks vary throughout the thickness of nickel structures. However, the variations are random, and therefore no correlation between the crystallinity and the built-in stress can be established at this point.
Electroformed nickel resonators were constructed and tested in the temperature range of - 40 to 110 degrees C. The temperature sensitivities of the resonant frequencies are - 150 ppm/degrees C, - 200 ppm/degrees C, and - 3000 ppm/degrees C for cantilever beams, ring structures, and clamped-clamped bridges, respectively. The built-in stress for the bridge was estimated to be approximately 2 X 109 dyne/cm2. No resonant frequency shift was detected after long-term (over 60 days), large amplitude vibration. This implies that the electroformed nickel is a viable material for the construction of resonant mechanical sensors.
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