Xenon difluoride (XeF2) has been used for fabrication of MEMS devices since 1990s. XeF2 isotropically etches silicon in a gas phase at room temperature with extremely high selectivity to numerous materials including silicon dioxide, many metals including aluminum, copper and gold, many polymers including photo resists and polyimides. Compared to wet etching of silicon, the XeF2 etching has the advantage of gentle etching by eliminating undesired capillary forces of liquids. Compared to plasma etching, the XeF2 etching has the advantage of extremely high selectivity.