Diffusion-reaction modeling of step coverage in low pressure hydrogen reduction of tungsten hexafluoride has assumed multiple roles in blanket tungsten process development. The model provides a framework for understanding the underlying phenomena which control step coverage, and has allowed the development of qualitative processing rules of thumb for the impact of process parameters on step coverage. These rules suggest that high pressure operation should yield both conformation step coverage in high aspect ratio features and the high deposition rates required in modern single wafer reactors, a prediction subsequently borne out by experiment. In addition, the model has been used to predict the optimum hydrogen to tungsten hexafluoride partial pressure ratio to maximize step coverage; this ratio is significantly lower than that typically employed commercially. Lastly, the model can be used to synthesize novel process concepts.
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