The processes governing direct‐write etching of semiconductors with halogen gases are studied by concentrating on a specific materials system: the etching of silicon using the output from an argon‐ion laser and chlorine gas to form high‐aspect ratio trenches. Specifically, the effects of laser power, laser polarization, laser scan velocity, Cl2‐gas pressure, and the addition of buffer gases on the trench formation process have been studied. A simple model has been developed which relates trench depth and these parameters. Model results are presented and compared with experimental data.