Tungsten hexafluoride
Tungsten(VI) fluoride, also known as tungsten hexafluoride, is an inorganic compound with the formula Template:Chem2. It is a toxic, corrosive, colorless gas, with a density of about Template:Convert (roughly 11 times heavier than air).<ref>Template:Cite book</ref><ref>Gas chart (Wayback Machine archive 7 September 2022)</ref> It is the densest known gas under standard ambient temperature and pressure (298 K, 1 atm) and the only well characterized gas under these conditions that contains a transition metal.<ref name=b1/><ref>See the English Wikipedia list of gases for a comprehensive list of all compounds with measured boiling points at or below 298 K at 1 atm.</ref> Template:Chem2 is commonly used by the semiconductor industry to form tungsten films, through the process of chemical vapor deposition. This layer is used in a low-resistivity metallic "interconnect".<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is one of seventeen known binary hexafluorides.
PropertiesEdit
The Template:Chem2 molecule is octahedral with the symmetry point group of Oh. The W–F bond distances are Template:Val.<ref name=CRC>Template:RubberBible86th p. 4-93.</ref> Between Template:Val, tungsten hexafluoride condenses into a colorless liquid having the density of Template:Val at Template:Val.<ref name="syn" /> At Template:Val it freezes into a white solid having a cubic crystalline structure, the lattice constant of 628 pm and calculated density Template:Val. At Template:Val 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 Template:Val. In this phase, the W–F distance is 181 pm, and the mean closest molecular contacts are Template:Val. Whereas Template:Chem2 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 Template:Chem2 in the liquid and solid state is rather moderate.<ref>Template:Cite journal</ref> The vapor pressure of Template:Chem2 between Template:Val can be described by the equation
where the P = vapor pressure (bar), T = temperature (°C).<ref>Cady, G.H.; Hargreaves, G.B, "Vapour Pressures of Some Fluorides And Oxyfluorides of Molybdenum, Tungsten, Rhenium, and Osmium," Journal of the Chemical Society, APR 1961, pp. 1568-& DOI: 10.1039/jr9610001568</ref><ref>Template:Cite journal</ref>
History and synthesisEdit
Tungsten hexafluoride was first obtained by conversion of tungsten hexachloride with hydrogen fluoride by Otto Ruff and Fritz Eisner in 1905.<ref>Template:Cite Q</ref><ref>Template:Cite journal</ref>
The compound is now commonly produced by the exothermic reaction of fluorine gas with tungsten powder at a temperature between Template:Val:<ref name="syn">Template:Cite book</ref>
The gaseous product is separated from [[Tungsten oxytetrafluoride|Template:Chem2]], a common impurity, by distillation. In a variation on the direct fluorination, the metal is placed in a heated reactor, slightly pressurized to Template:Convert, with a constant flow of Template:Chem2 infused with a small amount of fluorine gas.<ref name="synth1">Template:Cite patent</ref>
The fluorine gas in the above method can be substituted by ClF, [[Chlorine trifluoride|Template:Chem2]] or [[Bromine trifluoride|Template:Chem2]]. An alternative procedure for producing tungsten fluoride is to treat tungsten trioxide (Template:Chem2) with HF, Template:Chem2 or [[Sulfur tetrafluoride|Template:Chem2]]. And besides HF, other fluorinating agents can also be used to convert tungsten hexachloride in a way similar to Ruff and Eisner original method:<ref name=b1>Template:Cite book</ref>
ReactionsEdit
On contact with water, tungsten hexafluoride gives hydrogen fluoride (HF) and tungsten oxyfluorides, eventually forming tungsten trioxide:<ref name=b1/>
Unlike some other metal fluorides, Template:Chem2 is not a useful fluorinating agent nor is it a powerful oxidant. It can be reduced to the yellow Template:Chem2.<ref>Template:Cite book</ref>
Template:Chem2 forms a variety of 1:1 and 1:2 adducts with Lewis bases, examples being Template:Chem2.<ref>Template:Cite journal</ref>
Applications in semiconductor industryEdit
The dominant application of tungsten fluoride is in semiconductor industry, where it is widely used for depositing tungsten metal in a chemical vapor deposition (CVD) process. The expansion of the industry in the 1980s and 1990s resulted in the increase of Template:Chem2 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. Template:Chem2 is favored over related compounds, such as [[Tungsten hexachloride|Template:Chem2]] or [[Tungsten hexabromide|Template:Chem2]], because of its higher vapor pressure resulting in higher deposition rates. Since 1967, two Template:Chem2 deposition routes have been developed and employed, thermal decomposition and hydrogen reduction.<ref name=Aigueperse>Template:Cite encyclopedia</ref> The required Template:Chem2 gas purity is rather high and varies between 99.98% and 99.9995% depending on the application.<ref name=b1/>
Template:Chem2 molecules have to be split up in the CVD process. The decomposition is usually facilitated by mixing Template:Chem2 with hydrogen, silane, germane, diborane, phosphine, and related hydrogen-containing gases.
SiliconEdit
Template:Chem2 reacts upon contact with a silicon substrate.<ref name=b1/> The Template:Chem2 decomposition on silicon is temperature-dependent:
- Template:Chem2 below 400 °C and
- Template:Chem2 above 400 °C.
This dependence is crucial, as twice as much silicon is being consumed at higher temperatures. The deposition occurs selectively on pure silicon only, but not on silicon dioxide or silicon 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 Template:Chem2 molecules to the Si substrate which is the only catalyst of molecular decomposition in this process.<ref name=b1/>
If the deposition occurs not in an inert atmosphere but in an oxygen-containing atmosphere (air), then instead of tungsten, a tungsten oxide layer is produced.<ref name="chemvap">Template:Cite journal</ref>
HydrogenEdit
The deposition process occurs at temperatures between 300 and 800 °C and results in formation of hydrogen fluoride vapors:
The crystallinity of the produced tungsten layers can be controlled by altering the Template:Chem2/[[hydrogen|Template:Chem2]] ratio and the substrate temperature: low ratios and temperatures result in (100) oriented tungsten crystallites whereas higher values favor the (111) orientation. Formation of HF is a drawback, as the HF vapor is very aggressive and etches away most materials. Also, the deposited tungsten shows poor adhesion to the silicon dioxide which is the main passivation material in semiconductor electronics. Therefore, [[silicon dioxide|Template:Chem2]] has to be covered with an extra buffer layer prior to the tungsten deposition. On the other hand, etching by HF may be beneficial to remove unwanted impurity layers.<ref name=b1/>
Silane and germaneEdit
The characteristic features of tungsten deposition from the Template:Chem2/[[silane|Template:Chem2]] are high speed, good adhesion and layer smoothness. The drawbacks are explosion hazard and high sensitivity of the deposition rate and morphology to the process parameters, such as mixing ratio, substrate temperature, etc. Therefore, silane is commonly used to create a thin tungsten nucleation layer. It is then switched to hydrogen, that slows down the deposition and cleans up the layer.<ref name=b1/>
Deposition from Template:Chem2/[[germane|Template:Chem2]] mixture is similar to that of Template:Chem2/Template:Chem2, but the tungsten layer becomes contaminated with relatively (compared to Si) heavy germanium up to concentrations of 10–15%. This increases tungsten resistance from about 5 to 200 μΩ·cm.<ref name=b1/>
Other applicationsEdit
Template:Chem2 can be used for the production of tungsten carbide.
As a heavy gas, Template:Chem2 can be used as a buffer to control gas reactions. For example, it slows down the chemistry of the Ar/[[oxygen|Template:Chem2]]/[[hydrogen|Template:Chem2]] flame and reduces the flame temperature.<ref>Template:Cite book</ref>
SafetyEdit
Tungsten hexafluoride is an extremely corrosive compound that attacks any tissue. Because of the formation of hydrofluoric acid upon reaction of Template:Chem2 with humidity, Template:Chem2 storage vessels have Teflon gaskets.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
ReferencesEdit
Template:Hexafluorides Template:Tungsten compounds Template:Fluorides