Abstract: An apparatus and method for producing films of silicon nitride whose index of refraction varies continuously with film depth by preselected amounts between n=3.9 and n=1.99. This is done by producing an amorphous film of silicon nitride, Si.sub.1-x N.sub.x, of pre-selected stoichiometry between x=0 and x=0.57. In a vacuum-chamber, a target substrate is exposed to vaporized silicon while being simultaneously bombarded with an ion beam of relatively high kinetic energy, ionized, nitrogen particles. The nitrogen embeds in the silicon film deposited on the substrate to form amorphous silicon nitride, the stoichiometry of which depends on the intensity of the ion beam. Instruments measure during the deposition the relative rate of arrival at the target for silicon and nitrogen, and, with pre-generated calibration data for the apparatus, enable an operator to selectively control the film's stoichiometry by controlling the ion beam's intensity response to the measured rate of silicon deposition.