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HETEROGENEOUS PROCESSES 38 including possibly frozen sidewall layers. The work of Szabo and Engel on the etching of Si in C12 environments is noteworthy. These authors report surprising phenomena at 77 K and just above that.11 Angle Dependence Winters and Coburn's work12 showed that the angular dependence of the etching rate in plasma etching environments is very small and does not show the characteristic angular peak at about 60Â° from normal that is observed for physical sputtering. Often, the angular distributions encountered in practical systems are distributed narrowly about the normal. The dependence of etching rate on angle may be secondary compared to other effects. On the other hand, anisotropy requirements are becoming so stringent that even small effects should be considered. Surface corrugation or roughness can mask angular dependencies. The reaction mechanism, especially for nonthermal pathways, may depend strongly on the angle of incidence. On the other hand, Barklund and Blom showed a very pronounced angular dependence of the nitride etching rate in CHF3/O2.13 This was not seen for SiO2. They explained it by the angular dependence of the fluorocarbon passivation layer produced by the CHF3/O2 plasma. Computer Simulations A methodology is required to link the fundamental studies of carefully prepared surfaces to real conditions found in reactors in terms of etching rates, selectivities, anisotropies, etc. This can be done by (a) molecular dynamics (MD) simulations, (b) Monte Carlo simulations, and (c) statistical analysis (multivariate analysis) that relates input or intermediate process conditions to etching rates and other variables. For (c) to be of use the correlation need not be made to the input conditions but to intermediate conditions, such as densities of neutrals and ions, bias voltages that develop, and so on. This would permit sensitivity analysis of different conditions. Molecular dynamics calculations appear useful.14 Currently, they are applicable primarily to the silicon halides because of the availability of relatively good interatomic potential energy surfaces. Application to other systems such as SiO2 and photoresist requires determining realistic potentials for those systems. A large barrier to the use of these techniques may also be the disparity between the time scales used in molecular dynamics (picoseconds) and the time scales associated with neutral and ion fluxes (microseconds to seconds) or product desorption rates. These techniques should be used in conjunction with other techniques borrowed from the vast literature on statistical mechanics (e.g. Metropolis Monte Carlo, lattice gas models, and so on). In principle, combinations of these techniques could be used together with surface diagnostic experiments to shed light on surface reaction pathways and rates. Ultimately, information from the MD calculations could be combined with Monte Carlo or continuum methods for predicting profile evolution. FINDINGS 1. Processes occurring at surfaces exposed to plasmas are, in general, poorly understood. Even the proper variables characterizing the state of the surface (the state variables) are not fully known. Knowledge of the dependence on the state variables of the rates of chemical and physical processes is correspondingly sketchy. 2. Experimental diagnostics and modeling of plasma-surface processes based on first principles are rudimentary and require much development. Surfaces exposed to plasmas are often strongly modified by the intense bombardment by ions, photons, and radicals. Therefore, not only are the chemical and physical processes themselves strongly perturbed by plasma exposure, but in addition, the surfaces on which the processes take place axe unconventional in their structure and composition.