High resolution patterns have been fabricated in 〈100〉 silicon by doping selected areas with gallium utilizing an ion microprobe. These doped regions are used as an etch mask for subsequent anisotropic etching of silicon. The etching was performed in a KOH:IPA solution at 80–90 °C. The resulting etch rate of the doped silicon is approximately inversely proportional to the gallium impurity concentration. At high doping concentrations an etch rate difference of greater than 1000:1 has been measured between the virgin silicon and the doped regions. Using this technique features as small as 30 nm have been produced.