View source for Tungsten hexafluoride

{{chembox
| Watchedfields = changed
| verifiedrevid = 440139630
| Name = 
| ImageFile = Tungsten hexafluoride liquid.png
| ImageSize = 
| ImageCaption = Solid WF<sub>6</sub> melting into liquid WF<sub>6</sub>
| ImageFileL1 = Tungsten-hexafluoride-3D-balls.png
| ImageNameL1 = Tungsten(VI) fluoride
| ImageFileR1 = Wolframhexafluorid.svg
| ImageNameR1 = Ball-and-stick model of tungsten hexafluoride
| ImageClassL1 = bg-transparent
| ImageClassR1 = skin-invert-image
| IUPACName = {{ubl|Tungsten hexafluoride|Tungsten(VI) fluoride}}
| OtherNames = 
| SystematicName = 
| Section1 = {{Chembox Identifiers
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 7783-82-6
| ChemSpiderID = 455941
| EC_number = 232-029-1
| UNNumber = 2196 
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 45B0AG2C4S
| PubChem = 522684
| InChI = 1S/6FH.W/h6*1H;/q;;;;;;+6/p-6
| SMILES = F[W](F)(F)(F)(F)F
}}
| Section2 = {{Chembox Properties
| Formula = {{chem2|WF6}}
| MolarMass = 297.830 g/mol
| Appearance = Colorless gas
| Density = 12.4 g/L (gas) <br> 4.56 g/cm<sup>3</sup> (−9 °C, solid)
| Solubility = Hydrolyzes
| MeltingPtC = 2.3
| BoilingPtC = 17.1
| MagSus = &minus;40.0·10<sup>−6</sup> cm<sup>3</sup>/mol
}}
| Section3 = {{Chembox Structure
| MolShape = Octahedral
| Dipole = zero
}}
| Section4 = 
| Section5 = 
| Section6 = 
| Section7 = {{Chembox Hazards
| MainHazards = Toxic, corrosive; gives HF on contact with water
| FlashPt = Non-flammable
| GHS_ref=<ref>{{cite web |title=Tungsten hexafluoride |url=https://pubchem.ncbi.nlm.nih.gov/compound/522684#section=Safety-and-Hazards |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref>
| GHSPictograms = {{GHS05}}{{GHS06}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|301+311|314|330}}
| PPhrases = {{P-phrases|260|264|264+265|270|271|280|284|301+316|301+330+331|302+352|302+361+354|304+340|305+354+338|316|317|320|321|330|361+364|363|403+233|405|410+403|501}}
| NFPA-H = 3
| NFPA-F = 0
| NFPA-I = 2
| NFPA-S = W
| ExternalSDS = [https://www.chemadvisor.com/matheson/database/msds/mat24560000800003.pdf ChemAdvisor]
}}
| Section8 = {{Chembox Related
| OtherAnions = {{ubl|[[Tungsten hexachloride]]|[[Tungsten hexabromide]]}}
| OtherCations = {{ubl|[[Chromium(VI) fluoride]]|[[Molybdenum(VI) fluoride]]|[[Uranium hexafluoride]]}}
| OtherCompounds = {{ubl|[[Sulfur hexafluoride]]|[[Tungsten(IV) fluoride]]|[[Tungsten(V) fluoride]]|[[Tungsten oxytetrafluoride]]}}
}}
}}

'''Tungsten(VI) fluoride''', also known as '''tungsten hexafluoride''', is an [[inorganic compound]] with the [[chemical formula|formula]] {{chem2|WF6|auto=1}}. It is a toxic, corrosive, colorless gas, with a density of about {{convert|13|kg/m3|abbr=on}} (roughly 11 times heavier than air).<ref>{{cite book |url= https://books.google.com/books?id=BpPmFsA4yn4C&pg=PA138 |page= 138 |title= Proprietes Physiques des Composes Mineraux |author1=Roucan, J.-P. |author2=Noël-Dutriaux, M.-C. |publisher= Ed. Techniques Ingénieur}}</ref><ref>[https://web.archive.org/web/20140104205601/http://www.qualiflow.com/pdf/faq_gas_chart.pdf 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> {{chem2|WF6}} is commonly used by the [[Semiconductor device fabrication|semiconductor industry]] to form tungsten films, through the process of [[chemical vapor deposition]]. This layer is used in a low-[[resistivity]] metallic "[[Semiconductor device fabrication#Interconnect|interconnect]]".<ref>{{cite web |url= http://www.timedomaincvd.com/CVD_Fundamentals/films/W_WSi.html |title= Tungsten and Tungsten Silicide Chemical Vapor Deposition |work= CVD Fundamentals |publisher= TimeDomain CVD}}</ref> It is one of seventeen known binary [[hexafluoride]]s.

==Properties==
The {{chem2|WF6}} molecule is octahedral with the [[symmetry group|symmetry point group]] of O<sub>h</sub>. The W–F bond distances are {{val|183.2|u=pm}}.<ref name=CRC>{{RubberBible86th}} p. 4-93.</ref> Between {{val|2.3|and|17|u=degC}}, tungsten hexafluoride condenses into a colorless liquid having the density of {{val|3.44|u=g/cm3}} at {{val|15|u=degC}}.<ref name="syn" /> At {{val|2.3|u=degC}} it freezes into a white solid having a cubic crystalline structure, the lattice constant of 628 pm and calculated density {{val|3.99|u=g/cm3}}. At {{val|−9|u=degC}} 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 {{val|4.56|u=g/cm3}}. In this phase, the W–F distance is 181 pm, and the mean closest molecular contacts are {{val|312|u=pm}}. Whereas {{chem2|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 {{chem2|WF6}} in the liquid and solid state is rather moderate.<ref>{{cite journal |last1= Levy |first1= J. |title= The structures of fluorides XIII: The orthorhombic form of tungsten hexafluoride at 193 K by neutron diffraction |journal= Journal of Solid State Chemistry |year= 1975 |volume= 15 |issue= 4 |pages= 360–365 |doi= 10.1016/0022-4596(75)90292-3|bibcode= 1975JSSCh..15..360L }}</ref>
The vapor pressure of {{chem2|WF6}} between {{val|−70|and|17|u=degC}} [[Antoine equation|can be described]] by the equation
:{{math|1=[[Common logarithm|log<sub>10</sub>]] ''P'' = 4.55569 − {{sfrac|1021.208 | ''T'' + 208.45}}}},
where the ''P'' = vapor pressure ([[bar (unit)|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>{{Cite journal|url=http://webbook.nist.gov/cgi/cbook.cgi?ID=C7783826&Mask=4&Type=ANTOINE&Plot=on|doi = 10.1021/ie50448a022|title = Vapor Pressure of Pure Substances. Organic and Inorganic Compounds|year = 1947|last1 = Stull|first1 = Daniel R.|journal = Industrial & Engineering Chemistry|volume = 39|issue = 4|pages = 517–540|url-access = subscription}}</ref>

==History and synthesis==
Tungsten hexafluoride was first obtained by conversion of [[tungsten hexachloride]] with [[hydrogen fluoride]] by [[Otto Ruff]] and Fritz Eisner in 1905.<ref>{{Cite Q|Q56639371}}</ref><ref>{{Cite journal |last=Ruff |first=Otto |author-link=Otto Ruff |last2=Eisner |first2=Fritz |last3=Heller |first3=Wilhelm |date=1907 |title=Über die Darstellung und Eigenschaften von Fluoriden des sechswertigen Wolframs |url=https://onlinelibrary.wiley.com/doi/10.1002/zaac.19070520122 |journal=Zeitschrift für anorganische Chemie |language=de |volume=52 |issue=1 |pages=256–269 |doi=10.1002/zaac.19070520122 |issn=1521-3749|url-access=subscription }}</ref>

:{{chem2|WCl6 + 6 HF → WF6 + 6 [[hydrogen chloride|HCl]]}}

The compound is now commonly produced by the [[exothermic]] reaction of [[fluorine]] gas with [[tungsten]] powder at a temperature between {{val|350|and|400|u=degC}}:<ref name="syn">{{cite book |author1=Priest, H. F. |author2=Swinehert, C. F. |editor= Audrieth, L. F. |chapter= Anhydrous Metal Fluorides |title= Inorganic Syntheses |volume= 3 |pages= 171–183 |isbn= 978-0-470-13162-6 |year= 1950 |publisher= Wiley-Interscience |doi= 10.1002/9780470132340.ch47}}</ref>
:{{chem2|W + 3 F2 → WF6}}
The gaseous product is separated from [[Tungsten oxytetrafluoride|{{chem2|WOF4}}]], a common impurity, by distillation. In a variation on the direct fluorination, the metal is placed in a heated reactor, slightly pressurized to {{convert|1.2|to|2.0|psi|kPa|abbr=on}}, with a constant flow of {{chem2|WF6}} infused with a small amount of [[fluorine]] gas.<ref name="synth1">{{cite patent |country= US |status= patent |number= 6544889 |title= Method for tungsten chemical vapor deposition on a semiconductor substrate |gdate= 2003-04-08}}</ref>

The fluorine gas in the above method can be substituted by [[Chlorine monofluoride|ClF]], [[Chlorine trifluoride|{{chem2|ClF3}}]] or [[Bromine trifluoride|{{chem2|BrF3}}]]. An alternative procedure for producing tungsten fluoride is to treat [[tungsten trioxide]] ({{chem2|WO3}}) with [[Hydrogen fluoride|HF]], {{chem2|BrF3}} or [[Sulfur tetrafluoride|{{chem2|SF4}}]]. 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>{{cite book |url= https://books.google.com/books?id=foLRISkt9gcC |pages= 111, 168 |title= Tungsten - Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds |author1=Lassner, E. |author2=Schubert, W.-D. |publisher= Springer |year= 1999 |isbn= 0-306-45053-4}}</ref>
:{{chem2|WCl6 + 2 [[arsenic trifluoride|AsF3]] → WF6 + 2 [[arsenic trichloride|AsCl3]]}} or
:{{chem2|WCl6 + 3 [[antimony pentafluoride|SbF5]] → WF6 + 3 SbF3Cl2}}

==Reactions==<!--needs an expert on the CVD part-->
On contact with [[water]], tungsten hexafluoride gives [[hydrogen fluoride]] (HF) and tungsten oxyfluorides, eventually forming [[tungsten trioxide]]:<ref name=b1/>
:{{chem2|WF6 + 3 H2O → WO3 + 6 HF}}

Unlike some other metal fluorides, {{chem2|WF6}} is not a useful fluorinating agent nor is it a powerful oxidant. It can be reduced to the yellow {{chem2|WF4}}.<ref>{{cite book |author1=Greenwood, N. N. |author2=Earnshaw, A. |year= 1997 |title= Chemistry of the Elements |edition= 2nd |location= Oxford |publisher= Butterworth-Heinemann |isbn= 0-7506-3365-4}}</ref>

{{chem2|WF6}} forms a variety of 1:1 and 1:2 [[adduct]]s with [[Lewis base]]s, examples being {{chem2|WF6([[Dimethyl sulfide|S(CH3)2]]), WF6(S(CH3)2)2, WF6([[Trimethyl phosphine|P(CH3)3]]), and WF6([[Pyridine|py]])2}}.<ref>{{cite journal |doi=10.1039/C2CS35263J |title=Medium and high oxidation state metal/Non-metal fluoride and oxide–fluoride complexes with neutral donor ligands |year=2013 |last1=Benjamin |first1=Sophie L. |last2=Levason |first2=William |last3=Reid |first3=Gillian |journal=Chem. Soc. Rev. |volume=42 |issue=4 |pages=1460–1499 |pmid=23014811 }}</ref>

==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]] (CVD) process. The expansion of the industry in the 1980s and 1990s resulted in the increase of {{chem2|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]]. {{chem2|WF6}} is favored over related compounds, such as [[Tungsten hexachloride|{{chem2|WCl6}}]] or [[Tungsten hexabromide|{{chem2|WBr6}}]], because of its higher vapor pressure resulting in higher deposition rates. Since 1967, two {{chem2|WF6}} deposition routes have been developed and employed, thermal decomposition and hydrogen reduction.<ref name=Aigueperse>{{cite encyclopedia |author1=Aigueperse, J. |author2=Mollard, P. |author3=Devilliers, D. |author4=Chemla, M. |author5=Faron, R. |author6=Romano, R. |author7=Cuer, J.-P. |title= Fluorine Compounds, Inorganic |editor-last= Ullmann |encyclopedia= Encyclopedia of Industrial Chemistry |year= 2005 |publisher= Wiley-VCH |location= Weinheim}}</ref> The required {{chem2|WF6}} gas purity is rather high and varies between 99.98% and 99.9995% depending on the application.<ref name=b1/>

{{chem2|WF6}} molecules have to be split up in the CVD process. The decomposition is usually facilitated by mixing {{chem2|WF6}} with hydrogen, [[silane]], [[germane]], [[diborane]], [[phosphine]], and related hydrogen-containing gases.

===Silicon===
{{chem2|WF6}} reacts upon contact with a [[silicon]] substrate.<ref name=b1/> The {{chem2|WF6}} decomposition on silicon is temperature-dependent:
:{{chem2|2 WF6 + 3 Si → 2 W + 3 [[silicon tetrafluoride|SiF4]]}} below 400 °C and
:{{chem2|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 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 [[micrometer (unit)|micrometers]]. The saturation occurs because the tungsten layer stops diffusion of {{chem2|WF6}} 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">{{cite journal |author1=Kirss, R. U. |author2=Meda, L. |title= Chemical vapor deposition of tungsten oxide |journal= Applied Organometallic Chemistry |year= 1998 |volume= 12 |issue= 3 |pages= 155–160 |doi= 10.1002/(SICI)1099-0739(199803)12:3<155::AID-AOC688>3.0.CO;2-Z|hdl=2027.42/38321 |url= https://deepblue.lib.umich.edu/bitstream/2027.42/38321/1/688_ftp.pdf |hdl-access= free }}</ref>

===Hydrogen===
The deposition process occurs at temperatures between 300 and 800&nbsp;°C and results in formation of [[hydrogen fluoride]] vapors:
:{{chem2|WF6 + 3 H2 → W + 6 HF}}

The crystallinity of the produced tungsten layers can be controlled by altering the {{chem2|WF6}}/[[hydrogen|{{chem2|H2}}]] ratio and the substrate temperature: low ratios and temperatures result in [[cubic crystal system|(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|{{chem2|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.<ref name=b1/>

===Silane and germane===
The characteristic features of tungsten deposition from the {{chem2|WF6}}/[[silane|{{chem2|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, that slows down the deposition and cleans up the layer.<ref name=b1/>

Deposition from {{chem2|WF6}}/[[germane|{{chem2|GeH4}}]] mixture is similar to that of {{chem2|WF6}}/{{chem2|SiH4}}, but the tungsten layer becomes contaminated with relatively (compared to Si) heavy germanium up to concentrations of 10–15%. This increases tungsten [[Electrical resistivity and conductivity|resistance]] from about 5 to 200 μΩ·cm.<ref name=b1/>

==Other applications==
{{chem2|WF6}} can be used for the production of [[tungsten carbide]].

As a heavy gas, {{chem2|WF6}} can be used as a buffer to control gas reactions. For example, it slows down the chemistry of the [[argon|Ar]]/[[oxygen|{{chem2|O2}}]]/[[hydrogen|{{chem2|H2}}]] flame and reduces the flame temperature.<ref>{{cite book |author= Ifeacho, P. |title= Semi-conducting metal oxide nanoparticles from a low-pressure premixed H<sub>2</sub>/O<sub>2</sub>/Ar flame: Synthesis and Characterization |year= 2008 |page= 64 |publisher= Cuvillier Verlag |location= Göttingen |isbn= 978-3-86727-816-4 |url= https://books.google.com/books?id=0B5HI9TNmakC&pg=PT64}}</ref>

==Safety==
Tungsten hexafluoride is an extremely corrosive compound that attacks any tissue. Because of the formation of [[hydrofluoric acid]] upon reaction of {{chem2|WF6}} with humidity, {{chem2|WF6}} storage vessels have [[Teflon]] gaskets.<ref>{{cite web |url= http://www.orcbs.msu.edu/msds/LINDE_MSDS/pdf/154.pdf |title= Tungsten hexafluoride MSDS |publisher= Linde Gas |url-status= dead |archive-url= https://web.archive.org/web/20100212202223/http://www.orcbs.msu.edu/msds/LINDE_MSDS/pdf/154.pdf |archive-date= 2010-02-12 }}</ref>

==References==
{{reflist}}

{{Hexafluorides}}
{{Tungsten compounds}}
{{fluorides}}
{{DEFAULTSORT:Tungsten Hexafluoride}}
[[Category:Tungsten halides]]
[[Category:Hexafluorides]]
[[Category:Lachrymatory agents]]
[[Category:Octahedral compounds]]
[[Category:Industrial gases]]