LOW-KNUDSEN-NUMBER TRANSPORT AND DEPOSITION

We present our model for transport and deposition in features for situ ations in which intermolecular collisions dominate the species transpo rt. Species transport in this ''low''-Knudsen-number regime is modeled using continuum diffusion. Our model allows both homogeneous reaction s and heterogeneous (surface) reactions. We use the Galerkin finite el ement method to estimate reactant species concentration profiles for i nfinite trenches with arbitrary cross section, for which two-dimension al profile evolution is appropriate. We simulate deposition processes using a solution domain which includes the trench as well as a specifi ed region above the surface of the wafer. As an example application, w e present results for the deposition of tungsten using the hydrogen re duction of tungsten hexafluoride in trenches of rectangular cross sect ion with initial aspect ratio 4. The model predicts that step coverage increases with decreasing temperature and increasing tungsten hexaflu oride partial pressure, while keeping hydrogen partial pressure and th e height of the solution domain constant. These trends are in agreemen t with experimental observations for tungsten deposition processes. Tr ends in film conformality with deposition conditions are explained in terms of the ''step coverage modulus'' and reactant concentration rati os for given initial trench dimensions. The step coverage modulus is t he ratio of a characteristic deposition rate to a characteristic trans port rate.