A reaction chamber assembly suitable for forming thin film deposition layers onto solid substrates includes a reaction chamber and an input plenum for receiving source material from gas source containers and delivering a flow of source material into the reaction chamber uniformly distributed across a substrate support width. An output plenum connected between the reaction chamber and a vacuum pump uniformly removes an outflow of material from the reaction chamber across the substrate support width. The input plenum is configured to expand a volume of the source material and deliver the source material to the substrate support area with uniform source material flow distribution across the substrate support width. The output plenum is configured to remove the outflow material across the entire substrate support width and to compress the volume of outflow material prior to the outflow material exiting the output plenum. The resulting source material flow over substrates supported in the substrate support area is uniformly distributed across the substrate support width and unidirectional with a uniform flow velocity. The configuration of the reaction chamber assembly reduces pump down times.