An Ar‐ion laser has been used to activate surface pyrolytic deposition of polycrystalline Si in a several micrometer reaction zone. Controlled doping with boron during deposition allows one‐step direct writing of highly conducting Si lines with linewidths as small as 1 &mgr;m. An unexpectedly fast deposition rate is associated with changes in the reaction kinetics as the dimensions of the reaction volume are reduced to microscopic lengths.