Micro-effects of resputtering due to negative ion bombardment of growing thin films
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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 /i,m-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.
I. INTRODUCTION In the sputtering process, a target material is bombarded by high energy ions, causing atoms to be ejected from the material through momentum transfer. These atoms can be deposited on a substrate material, leading to the formation of a thin film. Similarly, if these atoms deposited on the substrate are bombarded by high energy particles, they can also be ejected from the substrate. This is the process normally referred to as resputtering. In addition, a real surface can have considerable roughness and, due to geometrical considerations, can recapture a significant amount of the primary resputtered material. This local redistribution of material is a second factor in the overall resputtering process. Resputtering is likely to occur when negative ions are formed at the target and accelerated away from the negatively biased target toward the growing film.1 Two of the effects resputtering can have on a growing thin film we have termed the macro-effects and the micro-effects. The macro-effects are the effects resputtering has on the overall thickness distribution of a film; the micro-effects (on which this paper will concentrate) are changes in the surface morphology of a film. The macro-effects, covered in detail in a related paper,2
^Present address: Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695. 1938
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 8, Aug 1993
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manifest themselves through a decrease in the growth rate of a film and, in some cases, a total inhibition of film growth coupled with a subsequent etching of the substrate material. A third effect resputtering can have on a film is compositional changes. Different compone
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