Surface-micromachining processes for electrostatic microactuator fabrication

Etch characteristics are discussed of three forms of hydrofluoric acid, used as the micromachining etchant for fabricating a rotary variable-capacitance micromotor structure from polycrystalline silicon (poly-Si) thin films. Low-pressure chemically vapor deposited SiO/sub 2/ (LPCVD LTO) is used to form the sacrificial layers and LPCVD Si/sub 3/N/sub 4/ is used as an electrical isolation layer for the micromachined structure. 7:1, NH/sub 4/F:HF buffered HF (BHF), 48 wt.% concentrated hydrofluoric acid (LHF), and HF vapor in air (VHF) are evaluated at 20 degrees C for etch rate and selectivity, and attack of four types of LPCVD poly-Si films; POCl/sub 3/-doped or undoped, with or without 1150 degrees C annealing. Both BHF and VHF are found to damage POCl/sub 3/-doped poly-Si films, possibly by attack of grain boundaries. VHF is found to be effective for micromachining suspended structures since it avoids deformation of microstructures due to surface tension effects during drying after wet etching with LHF or BHF.<>