Hybrid pulsed valve for thin film coating and method

Abstract

An apparatus for coating a substrate with a relatively thin, uniform film of a liquid on a substrate with a minimum of waste. The apparatus includes a droplet generator capable of generating droplets of the liquid at a predetermined rate and of a predetermined size, a pulsed gas jet and a computer control which sequentially triggers the droplet generator and gas jet such that droplets are generated, then contacted by the gas jet, which accelerates the droplets and propels the droplets to impact and coat the substrate. In a preferred embodiment of the invention, the droplet size and velocity are selected such that the droplets break up upon impact with the substrate without splashing or rippling. The apparatus and method are ideal for coating semiconductor wafers with a photoresist solution. Droplets with uniform size and controlled trajectories are generated, in one embodiment, with a piezoelectric droplet generator, and then accelerated to precisely defined velocities by the pulsed gas jet toward the surface to be coated. The kinetic energy of the droplets is adjusted to overcome the free energy associated with surface tension on impact. The collision of the droplet thus results in a uniform, thin coating of photoresist or other coating solution which may then be further processed by conventional techniques.