Modeling and optimization of the step coverage of tungsten LPCVD in trenches and contact holes

In this paper a model is presented to calculate the step coverage of blanket tungsten low pressure chemical vapor deposition (W-LPCVD) from tungsten hexafluoride (WF{sub 6}). The model can calculate tungsten growth in trenches and circular contact holes, in the case of the WF{sub 6} reduction by H{sub 2}, SiH{sub 4}, or both. The step coverage model predictions have been verified experimentally by scanning electron microscopy (SEM). The authors found that the predictions of the step coverage model for the H{sub 2} reduction of WF{sub 6} are very accurate, if the partial pressures of the reactants at the inlet of the trench or contact hole are known. To get these reactant inlet partial pressures, the authors used a reactor model which calculates the surface partial pressures of all the reactants. These calculated surface partial pressures are used as input for the authors' step coverage model. In this study the authors showed that thermodiffusion plays a very important role in the actual surface partial pressure. In the case where SiH{sub 4} was present in the gas mixture trends are predicted very well but the absolute values predicted by the step coverage model are too high. The partial pressure of HF, which is a by-product of the H{sub 2} reduction reaction, may be very high inside trenches or contact holes, especially just before closing of the trench or contact hole. The authors found no influence of the calculated HF partial pressure on the step coverage.