Apparatus and process for conditioning a generally planar substrate, contained in a chamber isolatable from the ambient environment and fed with a conditioning gas which includes reactive gas. The apparatus includes a support for supporting the substrate in the chamber, the substrate being in a lower pressure reaction region of the chamber. A gas inlet is provided for feeding conditioning gas into a gas inlet region of the chamber which is at a higher pressure than the lower pressure reaction region so that the pressure differential causes the conditioning gas to flow toward the surface of the substrate wherein the conditioning gas component will chemically react with and condition the substrate surface, both said higher and lower pressure regions operating in a viscous flow regime. The substrate is supported such that a pressure bias is created across the surface of this substrate so that the gas, after it has chemically reacted with the substrate surface, flows outward from where it has reacted, off the substrate toward the periphery of the chamber and out a peripheral or central underside exhaust outlet. Gas feed may be provided to one or both sides the substrate and light activation of the substrate or conditioning gas may be provided on one or both sides.