H2O → WO3 + 6 HF Unlike some other metal fluorides, WF6 is not a useful fluorinating agent nor is it a powerful oxidant. It can be reduced to the yellow WF4. Applications in semiconductor industry The dominant application of tungsten fluoride is in semiconductor industry, where it is widely used for depositing tungsten metal in a chemical vapor deposition process. The expansion of the industry in the 1980s and 1990s resulted in the increase of WF6 consumption, which remains at around 200 tonnes per year worldwide. Tungsten metal is attractive because of its relatively high thermal and chemical stability, as well as low resistivity (5.6 μΩ·cm) and very low electromigration. WF6 is favored over related compounds, such as WCl6 or WBr6, because of its higher vapor pressure resulting in higher deposition rates. Since 1967, two WF6 deposition routes have been developed and employed, thermal decomposition and hydrogen reduction. The required WF6 gas purity is rather high and varies between 99.98% and 99.9995% depending on the application. WF6 molecules have to be split up in the CVD process. The decomposition is usually facilitated by mixing WF6 with hydrogen, silane, germane, diborane, phosphine, and related hydrogen-containing gases. Silicon WF6 reacts upon contact with a silicon substrate. The WF6 decomposition on silicon is temperature-dependent: 2 WF6 + 3 Si → 2 W + 3 SiF4 below 400 °C and WF6 + 3 Si → W + 3 SiF2 above 400 °C. This dependence is crucial, as twice as much silicon is being consumed at higher temperatures. The deposition occurs selectively on pure Si only, but not on silicon oxide or nitride, thus the reaction is highly sensitive to contamination or substrate pre-treatment. The decomposition reaction is fast, but saturates when the tungsten layer thickness reaches 10–15 micrometers. The saturation occurs because the tungsten layer stops diffusion of WF6 molecules to the Si substrate which is the only catalyst of molecular decomposition in this process. If the deposition occurs not in an inert but in an oxygen containing atmosphere (air) then instead of tungsten, a tungsten oxide layer is produced. Hydrogen The deposition process occurs at temperatures between 300 and 800 °C and results in formation of hydrogen fluoride vapors: WF6 + 3 H2 → W + 6 HF The crystallinity of the produced tungsten layers can be controlled by altering the WF6/H2 ratio and the substrate temperature: low ratios and temperatures result