A technique to selectively etch silicon with respect to germanium is described. The method relies on an observed small difference in the effects of polymeric etch-inhibiting layers on the two materials. In a CF 4/H2 plasma, the observed polymer point for Ge is 1-3% lower than for Si. This produces a narrow process window in which Si is etched while etching of Ge is suppressed. This technique has applications to etching of pure Si layers over Ge as well as Si-Ge alloys for device applications.