This invention relates to coating of substrates in a vacuum system. A beam of molecules incident upon a molecular beam converter is transformed into a molecular beam flowing from the converter toward a substrate to be coated, or on which a layer is to be grown epitaxially. The incident beam is directed onto a heated impingement surface. In most embodiments the impingement surface generally faces the substrate to be coated, and the incident beam strikes the surface from the substrate side. A heating means maintains the impingement surface at a designated temperature. The heating means is separated and shielded from the impingement surface to avoid introducing contaminants from the heating means into the converted molecular beam, and also to avoid adverse physical and chemical effects on the heating means caused by the incident beam and its dissociation products. The incident beam may consist of gaseous compounds of normally solid materials which are at least partially dissociated at the heated impingement surface; the converted beam will then differ from the incident beam in direction, angular distribution, and molecular species. Alternatively, the incident beam may consist of atoms or molecules of elemental solids, in which case reflection and angular redistribution, with or without further dissociation, may occur at the heated impingement surface. Coating uniformity and step coverage can be improved by controlling the size and shape of the incident beam.