This entry is a brief review of semiconductor etching with sulfur hexafluoride (SF6) as applied to fabricate advanced devices such as microelectromechanical systems (MEMS). Since silicon (Si) and germanium (Ge) are widely used in MEMS, this entry focuses on etching process of Si and Ge. The basis of reactive ion etching (RIE) is presented because it is an important and widespread technique for fabrication. The RIE mechanics of Si and Ge in SF6 and SF6–O2 are examined. The issue of surface roughness is investigated as a mean to improve the quality of the etching process. Because RIE depends on many parameters, this entry focuses on gas flow, radio-frequency power, and pressure. The applications of RIE are discussed.