Xenon Difluoride Etching of Silicon for MEMS

Xenon difluoride (XeF2) is a dry, isotropic gas-phase etchant which gently etches silicon at room temperature. Silicon dioxide and photoresist can be used as mask materials for XeF2 etching. XeF2 is also extremely selective to many metals, including aluminum, gold, and titanium-nickel. Silicon pits etched in XeF2 exhibit an average surface roughness around 500nm, and tend to be convex. The convex curvature of the bottom of the pit increases with increasing etch window diameter and distance from the center of the wafer. It is believed that exposed silicon areas of under 0.01mm^2 experience diffusion-limited etching, while exposed areas of greater than 100mm^2 experience reaction-limited etching. Experiments were conducted to determine the thermal processes present during XeF2 etching. The substrate experiences a steady rise in temperature during the course of the etch. Etching at greater pressures does not appear to affect the etch rate significantly. The use of XeFz to create MEMS devices allows the fabrication of ultra-thin, ultra- compliant structures. One application is discussed.