A fundamental feature scale model for low pressure deposition processes

The integro-difFerential equations which govern free molecular flow and low pressure chemical vapor deposition in long rectangular trenches are reviewed. The model equations are used to simulate the deposition of tungsten by hydrogen reduction of tungsten hexafluoride, with reactive sticking coefficients determined by local deposition conditions. Numerical solution of the governing equations provides film profiles and deposition rate profiles as a function of position in the trench at any time until the trench mouth closes. The impact of the operating conditions on step coverage is discussed in relation to reactive sticking factors. Calculated tungsten step coverages for three selected realistic initial trench shapes highlight the importance of establishing a consistent method for reporting step coverages. We introduce the percentage of feature fill as a measure of step coverage. To allow evaluation of the quantitative predictive ability of a model, cross-sectional scanning electron micrographs are required in general.