 Solid WF6 melting into liquid WF6 | |||
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| Names | |||
|---|---|---|---|
IUPAC name
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| Identifiers | |||
3D model (JSmol) | |||
| ChemSpider | |||
| ECHA InfoCard | 100.029.117 | ||
| EC Number |
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| UNII | |||
| UN number | 2196 | ||
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| Properties | |||
| WF6 | |||
| Molar mass | 297.830 g/mol | ||
| Appearance | Colorless gas | ||
| Density | 12.4 g/L (gas) 4.56 g/cm3 (−9 °C, solid) | ||
| Melting point | 2.3 °C (36.1 °F; 275.4 K) | ||
| Boiling point | 17.1 °C (62.8 °F; 290.2 K) | ||
| Hydrolyzes | |||
| −40.0·10−6 cm3/mol | |||
| Structure | |||
| Octahedral | |||
| zero | |||
| Hazards | |||
| Occupational safety and health (OHS/OSH): | |||
Main hazards | Toxic, corrosive; gives HF on contact with water | ||
| GHS labelling:[1] | |||
  | |||
| Danger | |||
| H301+H311,H314,H330 | |||
| P260,P264,P264+P265,P270,P271,P280,P284,P301+P316,P301+P330+P331,P302+P352,P302+P361+P354,P304+P340,P305+P354+P338,P316,P317,P320,P321,P330,P361+P364,P363,P403+P233,P405,P410+P403,P501 | |||
| NFPA 704 (fire diamond) | |||
| Flash point | Non-flammable | ||
| Safety data sheet (SDS) | ChemAdvisor | ||
| Related compounds | |||
Otheranions | |||
Othercations | |||
Related compounds | |||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Tungsten(VI) fluoride, also known astungsten hexafluoride, is aninorganic compound with theformulaWF6. It is a toxic, corrosive, colorless gas, with a density of about 13 kg/m3 (22 lb/cu yd) (roughly 11 times heavier than air).[2][3] It is the densest known gas understandard ambient temperature and pressure (298 K, 1 atm) and the only well-characterized gas under these conditions that contains a transition metal.[4][5]WF6 is commonly used by thesemiconductor industry to form tungsten films, through the process ofchemical vapor deposition. This layer is used in a low-resistivity metallic "interconnect".[6] It is one of seventeen known binaryhexafluorides.
TheWF6 molecule is octahedral with thesymmetry point group of Oh. The W–F bond distances are183.2 pm.[7] Between2.3 and 17 °C, tungsten hexafluoride condenses into a colorless liquid having the density of3.44 g/cm3 at15 °C.[8] At2.3 °C it freezes into a white solid having a cubic crystalline structure, a lattice constant of628 pm, and calculated density3.99 g/cm3. At−9 °C, this structure transforms into anorthorhombic solid with the lattice constants ofa =960.3 pm,b =871.3 pm, andc =504.4 pm, and a density of4.56 g/cm3. In this phase, the W–F distance is181 pm, and the mean closest molecular contacts are312 pm. WhereasWF6 gas is one of the densest gases, with a density exceeding that of the heaviest elemental gasradon (9.73 g/L), the density ofWF6 in the liquid and solid state is rather moderate.[9] The vapor pressure ofWF6 between−70 and 17 °Ccan be described by the equation
where theP = vapor pressure (bar),T = temperature (°C).[10][11]
Tungsten hexafluoride was first obtained by conversion oftungsten hexachloride withhydrogen fluoride byOtto Ruff and Fritz Eisner in 1905:[12][13]
The compound is now commonly produced by theexothermic reaction offluorine gas withtungsten powder at a temperature between350 and 400 °C:[8]
The gaseous product is separated fromWOF4, 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 ofWF6 infused with a small amount offluorine gas.[14]
The fluorine gas in the above method can be substituted byClF,ClF3, orBrF3. An alternative procedure for producing tungsten fluoride is to treattungsten trioxide (WO3) withHF,BrF3, orSF4. And besides HF, other fluorinating agents can also be used to convert tungsten hexachloride in a way similar to Ruff and Eisner's original method:[4]
On contact withwater, tungsten hexafluoride giveshydrogen fluoride (HF) and tungsten oxyfluorides, eventually formingtungsten trioxide:[4]
Unlike some other metal fluorides,WF6 is not a useful fluorinating agent, nor is it a powerful oxidant. It can be reduced to the yellowWF4.[15]
WF6 forms a variety of 1:1 and 1:2adducts withLewis bases, examples beingWF6(S(CH3)2), WF6(S(CH3)2)2, WF6(P(CH3)3), and WF6(py)2.[16]
The dominant application of tungsten fluoride is in the semiconductor industry, where it is widely used for depositing tungsten metal in achemical vapor deposition (CVD) process. The expansion of the industry in the 1980s and 1990s resulted in an increase inWF6 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 lowelectromigration.WF6 is favored over related compounds, such asWCl6 orWBr6, because of its higher vapor pressure resulting in higher deposition rates. Since 1967, twoWF6 deposition routes have been developed and employed: thermal decomposition and hydrogen reduction.[17] The requiredWF6 gas purity is rather high and varies between 99.98% and 99.9995% depending on the application.[4]
WF6 molecules have to be split up in the CVD process. The decomposition is usually facilitated by mixingWF6 with hydrogen,silane,germane,diborane,phosphine, and related hydrogen-containing gases.
WF6 reacts upon contact with asilicon substrate.[4] TheWF6 decomposition on silicon is temperature-dependent:
This dependence is crucial, as twice as much silicon is consumed at higher temperatures. The deposition occurs selectively on pure silicon only, but not onsilicon dioxide orsilicon 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 10–15micrometers. The saturation occurs because the tungsten layer stops diffusion ofWF6 molecules to the Si substrate, which is the only catalyst of molecular decomposition in this process.[4]
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.[18]
The deposition process occurs at temperatures between 300 and 800 °C and results in formation ofhydrogen fluoride vapors:
The crystallinity of the produced tungsten layers can be controlled by altering theWF6/H2 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 HF vapor is very aggressive and etches away most materials. Also, the deposited tungsten shows poor adhesion to thesilicon dioxide which is the main passivation material in semiconductor electronics. Therefore,SiO2 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.[4]
The characteristic features of tungsten deposition fromWF6/SiH4 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, which slows down the deposition and cleans up the layer.[4]
Deposition from aWF6/GeH4 mixture is similar to that ofWF6/SiH4, but the tungsten layer becomes contaminated with relatively (compared to Si) heavy germanium up to concentrations of 10–15%. This increases tungstenresistivity from about 5 to200 μΩ·cm.[4]
WF6 can be used for the production oftungsten carbide.
As a heavy gas,WF6 can be used as a buffer to control gas reactions. For example, it slows down the chemistry of theAr/O2/H2 flame and reduces the flame temperature.[19]
Tungsten hexafluoride is an extremely corrosive compound that attacks any tissue. Because of the formation ofhydrofluoric acid upon reaction ofWF6 with humidity,WF6 storage vessels haveTeflon gaskets.[20]
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