A model is presented to calculate the step coverage of blanket tungsten low pressure chemical vapor deposition(W-LPCVD) from tungsten hexafluoride (WF6). The model can calculate tungsten growth in trenches and circular contactholes, in the case of the WF6 reduction by H2, SiH4, or both. The step coverage model predictions have been verified experimentallyby scanning electron microscopy (SEM). We found that the predictions of the step coverage model for the H2 reductionof WF6 are very accurate, if the partial pressures of the reactants at the inlet of the trench or contact hole areknown. To get these reactant inlet partial pressures, we used a reactor model which calculates the surface partial pressuresof all the reactants. These calculated surface partial pressures are used as input for our step coverage model. In this studywe showed that thermodiffusion plays a very important role in the actual surface partial pressure. In the case where SiH4was present in the gas mixture trends are predicted very well but the absolute values predicted by the step coveragemodel are too high. The partial pressure of HF, which is a by-product of the H2 reduction reaction, may be very high insidetrenches or contact holes, especially just before closing of the trench or contact hole. We found no influence of the calculatedHF partial pressure on the step coverage. Differences between step coverage in trenches and contact holes, as predictedby the step coverage model, were found to agree with the experiments. It is shown that the combination of the stepcoverage and reactor model is very useful in the optimization towards high step coverage, high throughput, and low WF6flow. We found a perfect step coverage (no void formation) in a 2 µm wide and 10 µm deep (2 × 10 µm) trench using anaverage WF6 flow of only 35 sccm, at a growth rate of 150 nm/min. In general, it is shown that the reduction of WF6 by SiH4offers no advantages over the reduction by H2 as far as step coverage is concerned.