A reaction apparatus for a semiconductor fabrication apparatus, wherein the reaction apparatus includes at least two adjustable outlet ports for withdrawing reactant gases from the reaction chamber. Adjustment of the flow rate through each of the outlet ports selectively modifies the flow pattern of the reactant gases within the reaction chamber to maintain a desired flow pattern therewithin, such as a substantially uniform flow over the surface of a substrate being processed, and/or minimization of turbulence within the reactor.