A high deposition rate amorphous-silicon process for use as a thick sacrificial layer in surface-micromachining

Amorphous and polycrystalline silicon are commonly used as sacrificial layers in the surface micromachining of microelectromechanical systems (MEMS) devices, because they have high thickness uniformity over a large wafer area, and a similar coefficient of thermal expansion to suspended structural materials such as silicon nitride and silicon oxide. However, the low deposition rate of amorphous-silicon hinders its application in devices that require a suspension gap greater than several micrometres, and chemical stability can be an issue. This paper addresses these issues through the development of a high deposition rate hydrogenated amorphous silicon thin film process. We have demonstrated two unique processing regimes, which can support either a low or high temperature process. The low-temperature processes can be used to deposit silicon thin films at temperatures fromroom temperature up to 100°C, with deposition rates as high as 0.2 μm/min. The high-temperature recipes, deposited at temperatures at and above 200°C, have a slightly lower deposition rate of 0.14 μm/min, but are found to be chemically resistant to etching in positive photoresist developer.