Film morphology and reaction rate for the CVD of tungsten by the WF₆-SiH₄ reaction

Low pressure chemical vapor deposition (LPCVD) of tungsten (W) by SiH₄ reduction of WF₆ on Si(100) surfaces was studied in a single-wafer, cold-wall reactor over a temperature range of 137-385°C and a pressure range of 1-10 Torr at a SiH₄/WF₆ ratio of 1.0. Rate data were obtained in the absence of gas-phase mass transport limitations and were measured using gravimetric techniques. The amount of tungsten that was deposited varied between 5.79 × 10^-5 and 1.70 × 10^-2 g/cm² (∼300-88,000Å based on a tungsten density of 19.3 g/cm³), and the rates were between 1.02 × 10^-4 and 1.74 × 10^-3 g/cm² min (∼500-9,000Å/ min). The apparent overall activation energy increased with pressure; 0.12 eV/ atom at 1 Torr, and 0.40 eV/atom at 10 Torr for short reaction times (0.5-1.5 min). The overall rate was dependent on reaction time (film thickness). Better film morphologies were obtained at higher temperatures and lower pressures. A W(110) preferential orientation was observed at the Si-W interface. Tungsten orientation switched from (110) to (100) as the films grew thicker. Higher apparent activation energies observed at higher pressures were attributed to gas phase reactions and/or by-product readsorption. The interdependence of rate and film morphology was attributed to a reconstruction of W(100) surfaces on which reactant diffusion/surface reaction is favored.