Maximizing step coverage during blanket tungsten low pressure chemical vapor deposition

A significant range of step coverages can be realized in the low pressure chemical vapor deposition of blanket tungsten films by either hydrogen reduction or silane reduction of tungsten hexafluoride, at a specified deposition rate. The step coverage realized in a single-wafer reactor (SWR) depends on the operating conditions used, e.g. feed flow rate ratios and reactant conversion level. The pseudosteady state approximation to the diffusion-reaction model is used to select the partial pressure ratios at the wafer surface which lead to a balance between reactant availability and consumption. The complete transient model is then used to demonstrate that the reactant pressure ratio obtained using this "availability analysis" maximizes the step coverage at a specified deposition rate. The reactant pressure ratio at the feature mouth is related to the inlet feed rate ratio and reactor conversion level for an SWR, assuming a well-mixed reactor. Guidelines for SWR operation are given to obtain the maximum step coverage for each system. In well-mixed reactors, the optimum feed ratio is a linear function of conversion, and approaches the stoichiometric ratio as the conversion approaches unity.