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. AW(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.