Gas phase pulse etching of silicon for MEMS with xenon difluoride

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.