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HETEROGENEOUS PROCESSES 33 4 Heterogeneous Processes INTRODUCTION In plasma processing, the term "heterogeneous processes" includes all chemical and physical reactions that occur when a flux (or fluxes) of species is (are) incident on a surface and the interaction of the incident species with the species residing on the surface results in either the gasification of the material or the formation of a new material. It has been pointed out in earlier chapters of this report that surface processes are of central importance in plasma processing. The entire purpose of the technology is to modify a surface, to etch or deposit material, or perhaps to clean or treat the surface in some way. A low-pressure plasma turns out to be an extraordinarily powerful medium within which to effect surface chemical and physical modifications with little impact on bulk material, at low cost, and over large areas. However, plasma-surface interactions are among the most complex and least well understood aspects of plasma processing technology. This chapter presents an introduction to the key issues and the current level of understanding of plasma-surface interactions. Several techniques, experimental and computational, are suggested as most promising in order to advance the state of the database and to improve treatments of plasma-surface interactions in tool scale and feature scale models. Heterogeneous processes that are of interest to plasma processing include the following: 1. Adsorption of radicals at specific surfaces; 2. Reactions to form certain intermediate or stable products; 3. Desorption (etching) or incorporation into a growing film (deposition) of the products formed under 2 above; 4. Ion enhancement (or suppression) of the above processes; 5. Sputtering; 6. Particle and energy reflection; 7. Ion implantation and production of defects; 8. Diffusion effects (on the surface, through the reaction layer, and ion-enhanced diffusion effects); 9. Redeposition of desorbed products on the sidewalls of structures, the walls of the reactor, and elsewhere; 10. The mutual interaction of etching and deposition processes that occur in parallel, e.g. in fluorocarbon-based silicon dioxide etching; 11. Surface roughening; 12. Electron-reduced desorption; and 13. Electron emission. The data necessary to characterize or simulate these processes should be available as a function of the relevant parameters, e.g., temperature, crystallinity and coverage of the substrate, kinetic or internal energy, and angle of incidence of the incident species. In many cases, the functional form relating these relevant parameters and the processes listed above are not known. Synergistic effects between ions and neutrals that strike the surface simultaneously are often essential and also need to be characterized. STATE OF THE DATABASE Very little exists in terms of organized compilations of heterogeneous process data. This may be explained by the fact that the heterogeneous processes relevant to a plasma process are intimately related to the actual application of the plasma process and thus are highly specific. The actual surfaces that are
