22 January 2005 Thick single-crystal silicon MEMS with high-aspect-ratio vertical air gaps
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This paper presents recent advances in the HARPSS micromachining technology, which enables implementation of movable Single Crystal Silicon (SCS) structures with high aspect ratio vertical air gaps on low-resistivity silicon substrate. This is suitable for applications of micro-gravity accelerometers, low voltage tunable capacitors, and high-resolution gyroscopes that require aspect ratios as large as 100:1 to achieve high sensitivity and wide tuning range. The single-sided HARPSS process eliminates the need for double-sided processing, and wafer bonding to subsequently package the device. The device thickness and gap spacing can be varied in a wide range, 30-150mm and 0.2-2mm respectively, to select the performance range. The movable MEMS elements are made of bulk silicon substrate, resulting in higher mass, higher quality factor (Q), and better shock resistance, compared to using polysilicon (poly) as the movable structure. This process provides a mechanism for creating corrugation in SCS electrodes to reduce the Brownian noise of sensors. This technique realxes the need to reduce the noise by using the maximum available mass and through-wafer etch. The corrugations are created by a DRIE technique for etching poly surrounded by oxide inside the isolation trenches. Also, uniform capacitive gap spacings are created by growing sacrificial oxide inside the trenches.
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Pejman Monajemi, Farrokh Ayazi, "Thick single-crystal silicon MEMS with high-aspect-ratio vertical air gaps", Proc. SPIE 5715, Micromachining and Microfabrication Process Technology X, (22 January 2005); doi: 10.1117/12.590750; https://doi.org/10.1117/12.590750

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