Micro-effects of resputtering due to negative ion bombardment of growing thin films

Negative ion bombardment of an evolving thin film can cause changes in the film's surface due to resputtering of the already deposited material. Through the study of rf-sputtered perovskite (BaTiO₃) thin films, we have found that surface micro-effects, i.e., changes in the surface morphology of the films at the μm-scale level, are dependent on the deposition conditions. Ripples, cones, ridges, and etch pits of various shapes and sizes were all observed on growing films. A transformation of the morphology of the top surface of the film as a function of both deposition time and location on the substrate has been observed. The type of surface morphology found at any point was found to be dependent on a number of factors, including deposition rates, flux and energy of bombarding ions, and the average angle of incidence of the bombarding ions. We have developed a qualitative model for the formation of these surface features, based on the resputtering yield as a function of the average angle of incidence of the bombarding ions. The model suggests that surface nonuniformities, often ripples, initiate the development of etch pits. Other mechanisms of the surface morphology development (such as clustering) are used to explain the formation of surface features other than etch pits.