Replicas of a mask are etched in Si wafers with a micrometer lateral resolution and typical depths of 200 nm by irradiation with filtered synchrotron radiation using cutoff wavelengths of 105, 122, and 150 nm. An excellent selectivity and anisotropy is obtained by suppressing the spontaneous etching of the XeF2etch gas (typical 10−2mbar) with O2or Ar buffer gas (typical 1 mbar). The efficiency of etching increases by more than two orders of magnitude by reducing the wavelength from longer than 150 nm to the spectral range of 105–122 nm. The number of removed Si atoms per incident photon reaches a value above unity for the short wavelengths. This very high quantum efficiency, which exceeds that in the visible spectral range by more than four orders of magnitude, is attributed to selective electronic excitation of a thin fluorosilyl layer on top of the Si wafer. The low probability of absorption in this layer implies a reaction efficiency far above unity. ©1995 American Institute of Physics.