Ion beam technology applied to thin film deposition

Two main methods of depositing a thin film is vacuum are by evaporation under high vacuum conditions or by a sputtering process in a softer vacuum in which the substrate is generally exposed to a plasma. Scanning electron microscope examination with a resolution of 3 nm of the early stages of deposition showed that the films produced by either technique had fine cracks and a granular structure. These structures may be caused by surface migration: in evaporation, a relatively low density of nucleation centres permits freedom of movement over the substrate surface; in sputtering, the nucleation density is high but energy from the plasma may be sufficient to cause grain growth. Thin films were deposited by sputtering with beams of ions or energetic neutrals. No structure was visible in such films under the scanning electron microscope. Transmission microscopy indicated grain sizes of about 1 nm. With this technique the nucleation density is high but as the plasma is confined within the source there is little energy transfer to promote surface migration. Very fine adherent films are produced by this technique, but ion beam sputtering is relatively slow. Fast rates of deposition are achieved with a Plasmax source in which the plasma is mechanically separated from the substrate. Fine adherent films are obtained and the adhesion is further improved by cleaning the substrate with an energetic neutral beam from a saddle-field source operating at the same chamber pressure as the Plasmax. (Auth.)