Characterization of High-Pressure $\hbox {XeF}_{2}$ Vapor-Phase Silicon Etching for MEMS Processing

Typical release for structures in microelectromechanical systems (MEMS) devices requires the use of sacrificial layers and wet etchants. As an alternative, bulk Si can be utilized for nonsilicon MEMS or structures as the sacrificial material when exposed to vapor-phase XeF₂ . This paper presents the results of using relatively high pressures (> 3.0 torr) for the purpose of MEMS processing, while characterizing the physical etching mechanism and its effects on the working Si substrate in relation to the allowed processing time. The observed etch rates for high-pressure release varied from 1.6 to 1.9 mum/min for applied pressures of 4.5-5.5 torr. The resulting roughness is shown to be primarily dependent on time, where the maximum average roughness is approximately 1.4 mum after 3000 s at 5.5 torr. Slightly anisotropic results are produced by the increased pressures, showing a 0.7 : 1.0 (vertical : lateral) etch rate, as well as some detrimental effects to the released structures. Furthermore, the use of etch windows are investigated in relation to etch rate when subjected to these high pressures.