A laser direct‐write process has been applied to structure silicon on a nanometer scale. In this process, a silicon substrate, placed in a chlorine ambience, is locally heated above its melting point by a continuous‐wave laser and translated by high‐resolution direct‐current motor stages. Only the molten silicon reacts spontaneously with the molecular chlorine, resulting in trenches with the width of the laser‐generated melt. Trenches have been etched with a width of less than 70 nm. To explain the functional dependence of the melt size on absorbed power, the calculations based on a two‐phase steady state heat model are presented, taking the temperature‐dependent thermal conductivities and optical parameters into account. ©1995 American Institute of Physics.