An inlet gas manifold (11) for a vacuum deposition chamber (10) incorporates inlet apertures (31) which increase in diameter or cross-section transverse to the direction of gas flow (22). The aperture configuration increases the dissociation gases such as nitrogen and, thus increases the rate of silicon nitride deposition provided by nitrogen gas chemistry, without requiring the use of reactants such as ammonia. While one could use ammonia in the deposition gas chemistry if desired, the process provides the option of completely eliminating ammonia The inlet manifold (11) containing the increasing-diameter gas inlet holes (31) provides enhanced control of the process and the deposited film, and is also useful for forming other dielectrics such as silicon oxide and silicon oxynitride. In particular, silicon oxynitride films are characterized by low hydrogen content and by compositional uniformity.