Tungsten hexafluoride

Tungsten(VI) fluoride, also known as tungsten hexafluoride, is an inorganic compound with the formula WF6. It is a toxic, corrosive, colorless gas, with a density of about 13 g/L (roughly 11 times heavier than air.) It is one of the densest known gases under standard conditions. WF6 is commonly used by the semiconductor industry to form tungsten films, through the process of chemical vapor deposition. This layer serves as a low-resistivity metallic 'interconnect'. It is one of seventeen known binary hexafluorides. Tungsten(VI) fluoride, also known as tungsten hexafluoride, is an inorganic compound with the formula WF6. It is a toxic, corrosive, colorless gas, with a density of about 13 g/L (roughly 11 times heavier than air.) It is one of the densest known gases under standard conditions. WF6 is commonly used by the semiconductor industry to form tungsten films, through the process of chemical vapor deposition. This layer serves as a low-resistivity metallic 'interconnect'. It is one of seventeen known binary hexafluorides. The WF6 molecule is octahedral with the symmetry point group of Oh. The W–F bond distances are 183.2 pm. Between 2.3 and 17 °C, tungsten hexafluoride condenses into a pale yellow liquid having the density of 3.44 g/cm3 at 15 °C. At 2.3 °C it freezes into a white solid having a cubic crystalline structure, the lattice constant of 628 pm and calculated density 3.99 g/cm3. At −9 °C this structure transforms into an orthorhombic solid with the lattice constants of a = 960.3 pm, b = 871.3 pm, and c = 504.4 pm, and the density of 4.56 g/cm3. In this phase, the W–F distance is 181 pm, and the mean closest intermolecular contacts are 312 pm. Whereas WF6 gas is one of the densest gases, with the density exceeding that of the heaviest elemental gas radon (9.73 g/L), the density of WF6 in the liquid and solid state is rather moderate.The vapor pressure of WF6 between −70 °C and 17 °C can be described by the equation where the P = vapor pressure (bar), T = temperature (°C). Tungsten hexafluoride is commonly produced by the exothermic reaction of fluorine gas with tungsten powder at a temperature between 350 and 400 °C: The gaseous product is separated from WOF4, a common impurity, by distillation. In a variation on the direct fluorination, the metal is placed in a heated reactor, slightly pressurized to 1.2 to 2.0 psi (8.3 to 13.8 kPa), with a constant flow of WF6 infused with a small amount of fluorine gas. The fluorine gas in the above method can be substituted by ClF, ClF3 or BrF3. An alternative procedure for producing tungsten fluoride is to react tungsten trioxide (WO3) with HF, BrF3 or SF4. Tungsten fluoride can also be obtained by conversion of tungsten hexachloride: On contact with water, tungsten hexafluoride gives hydrogen fluoride (HF) and tungsten oxyfluorides, eventually forming tungsten trioxide: 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. 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 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.