Diffusion as the Rate Controlling Step in the Reaction of Tungsten Hexafluoride with Tungsten and the Role of Tungsten Pentafluoride

Abstract

Estimated thermodynamic properties of the molecule tungsten pentafluoride indicate that it may be the reactive intermediate formed at high temperatures when tungsten hexafluoride etches tungsten. The calculated concentration of tungsten pentafluoride that can form at equilibrium is adequate to explain the etching reaction between 1500°–2680°K, and it becomes sufficiently unstable above 2300°K to explain the transition from etching to deposition at higher temperatures. The differences in reaction rates in the three carrier gases argon, nitrogen, and helium, point to a diffusion controlled reaction. The reaction rates from diffusion calculations of tungsten hexafluoride into, and tungsten pentafluoride out of the Langmuir sheath are in good agreement with experiment for helium, and are reasonable for argon and nitrogen. Convective distortion of the Langmuir sheath is suspected as the cause of the higher than expected etching rates in the latter two gases as well as the higher than expected rates as the temperature increases.