Abstract: It has been discovered for the practice of this disclosure that a particular ion radiation treatment of amorphous SiO.sub.2 thin film, with a subsequent annealing procedure, improves the dielectric breakdown property of the film. The treated SiO.sub.2 film is found to be substantially more dense than a comparable untreated SiO.sub.2 film. It is theorized for the practice of this disclosure that the physical mechanism which produces the densification of the SiO.sub.2 film may be responsible for the enhanced dielectric properties of the film. Such an improved film is especially useful as the gate insulator layer in an insulated-gate electrode field-effect transistor device, and as an insulating layer for electrically separating two metallic films in a thin film integrated circuit. Such SiO.sub.2 thin films are useful in integrated circuit technology because the electrical insulation property thereof is considerably improved, e.g.