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Xenon difluoride is an isotropic gas-phase etchant of silicon which has advantages in some aspects over anisotropic etchants such as KOH, EDP, and TMAH. An inexpensive, small, vacuum system is described for 'pulse etching' single crystal silicon wafers (standard CMOS substrates) for releasing microelectromechanical systems (MEMS) devices. Results are presented of etching protocols from several silicon substrates which had been previously patterned with a thin layer of silicon dioxide to produce etch openings ranging in size from 410 to 2250 microns. Etch depth rates of 1-2 microns/minute and lateral undercutting rates of 3-4 microns/minute were observed. The effects of partial thin film oxide growth on surface roughness was also investigated: a short air-bake at 140/spl deg/C equalized etch depth and lateral undercutting.