Abstract: Method and apparatus are set forth for providing a reactive radical species at a substrate surface. A wall structure defines an interior chamber. A substrate is mounted in the chamber at a target position therein with a surface region of the substrate oriented in a selected direction. A first vaporous chemical is introduced to the chamber with the first vaporous chemical flowing toward the surface region of the substrate. A first radical species is generated from the first vaporous chemical intermediate the position of introduction of the first vaporous chemical and the substrate surface region and at a pressure of no more than about 0.1 Torr without producing significant amounts of ionized species. The first radical species flows toward the surface region of the substrate. The chamber is evacuated and maintained at a pressure of .ltoreq.10.sup.-7 Torr during the introduction of the first vaporous chemical.