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HETEROGENEOUS PROCESSES 37 studies that indicate that reaction probabilities on the surface can be a function of the flux. This may be the case if, for example, the flux is so high that the surface does not get a chance to relax to an equilibrium state. Most UHV experiments are done under low-flux conditions where this effect may not be seen often. Sticking Coefficients The "sticking coefficient" S (or more precisely, the "loss coefficient") is a lumped parameter that describes the loss rate of a certain species on the surface. The sticking coefficient does not say anything about how that species is lost at the surface. There may be many reaction pathways that result in the loss of the incident species. The most important parameter affecting S is probably the coverage, and, in particular, this involves the determination of the coverage that would be encountered in a plasma processing system. However, the effective surface coverage probably depends on the composition and structure of the surface reaction layer. For example, a Si surface may be covered with F atoms alone, or the fluorine may be present as part of a fluorocarbon film layer. The total F coverage (in atoms/cm2) may be the same in the two cases, but it is probable that effective sticking coefficients would differ. Sticking coefficient dependence on temperature is in general unknown, although evidence suggests that in high density plasma reactors etching silicon dioxide, surface temperature can play a key role in polymer deposition. Sticking coefficients also probably depend on the species incident energy. Energetic species may impact surfaces either due to ion bombardment or through the formation of fast neutrals via charge exchange near the surface. For energetic impacting species, additional unknown factors include the angle of impact and the number and composition of atoms in molecular species. In addition, surface roughness (corrugation) effects may enhance trapping of the incident reactive species. The underlying mechanisms can be very complex and are in general not known. For instance, F2 can stick at a crystalline Si surface via either dissociative or abstractive chemisorption. These processes have been shown to depend on incident angle, flux, translational energy, and (for molecular species) rovibrational energy. Pure beams, both atomic and molecular, must be studied as a function of these parameters before the mixed beam effect can be understood. A distinction between thermal and nonthermal reaction pathways must be made here. Surface temperature affects Langmuir-Hinshelwood channels whereas nonthermal channels may not exhibit a temperature dependence.9 Most studies have focused on thermal channels. Synergistic Effects Understanding of synergistic effects, e.g. the ion/neutral synergism or the enhancement of polymer removal by O atoms in the presence of F atoms (a few percent), relates to the essence of "reactive ion etching." The effects of non-ground state species, in particular metastables or electronically excited species, need to be evaluated. For instance, O(1D) atoms appear to be much more reactive than O(3P) atoms (possibly by one or two orders of magnitude). The role of site occupation by other species is unknown. We would like to know if the ions are both reagent and energy source and how that affects the process. Ultraviolet (UV) photons may play a role in promoting surface reactions, either directly by photolytic processes or indirectly by molecular excitation. The nature of the internal excitation in promoting a reaction is not known. Substrate Temperature Dependence Tachi's data on cryogenic etching10 are a good survey of several materials of interest to the microelectronics industry, e.g. Si, SiO2, Si3N4, photoresist, tungsten, tungsten silicide, and aluminum. In these data all effects are convoluted, but they can be used as a basis against which to compare other work. It is not clear whether low- temperature etching results in better feature profile anisotropy because of the reduction of surface reaction rate coefficients or because ions scatter less from cold feature sidewalls,