Abstract: A large cross-section molecular beam source for the controlled delivery of a reactant species to provide deposition or etching over a generally large cross-sectional surface area of a substrate is described. The apparatus includes a source of the reactant species, typically as a gaseous material, a microchannel array for providing a directionalized, low divergence molecular beam of the reactant species having a cross-sectional dimension directly comparable to that of the substrate. The apparatus further includes a substrate mount that maintains the substrate so as to allow the direct impingement of the columnarized molecular beam on the wide area surface of the substrate. The reactant species source, microchannel array and substrate mount are housed within a vacuum chamber adapted to maintain a vacuum sufficient to enable the formation of the directionalized molecular beam of the reactant species upon its passing through the microchannel array.