Tungsten hexafluoride

Solid WF6 melting into liquid WF6

| Tungsten(VI) fluoride | Ball-and-stick model of tungsten hexafluoride

|

Names
IUPAC name

* Tungsten hexafluoride
* Tungsten(VI) fluoride

Identifiers

CAS Number

* 7783-82-6 Y

3D model (JSmol)

* Interactive image

ChemSpider |

* 455941

EC Number |

* 232-029-1

PubChem CID

* 522684

UNII |

* 45B0AG2C4S Y

UN number | 2196

InChI

* InChI=1S/6FH.W/h6*1H;/q;;;;;;+6/p-6

SMILES

* F[W](F)(F)(F)(F)F

Properties

Chemical formula

| WF

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)

Solubility in water

| Hydrolyzes

Magnetic susceptibility (χ)

| −40.0·10−6 cm3/mol
Structure

Molecular shape

| Octahedral

Dipole moment

| zero
Hazards
Main hazards | Toxic, corrosive; gives HF on contact with water
Safety data sheet | ChemAdvisor
GHS pictograms |
GHS Signal word | Danger

GHS hazard statements

| H301+311Script error: No such module "Preview warning".Category:GHS errors, H314, H330

GHS precautionary statements

| P260, P264, P264+265Script error: No such module "Preview warning".Category:GHS errors, P270, P271, P280, P284, P301+316Script error: No such module "Preview warning".Category:GHS errors, P301+330+331, P302+352, P302+361+354Script error: No such module "Preview warning".Category:GHS errors, P304+340, P305+354+338Script error: No such module "Preview warning".Category:GHS errors, P316Script error: No such module "Preview warning".Category:GHS errors, P317Script error: No such module "Preview warning".Category:GHS errors, P320, P321, P330, P361+364Script error: No such module "Preview warning".Category:GHS errors, P363, P403+233, P405, P410+403, P501
NFPA 704 (fire diamond)  |

~~W~~

Flash point | Non-flammable
Related compounds

Other anions

* Tungsten hexachloride
* Tungsten hexabromide

Other cations

* Chromium(VI) fluoride
* Molybdenum(VI) fluoride
* Uranium hexafluoride

Related compounds

* Sulfur hexafluoride
* Tungsten(IV) fluoride
* Tungsten(V) fluoride
* Tungsten oxytetrafluoride

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Y verify (what is YN ?)
Infobox references

Tungsten(VI) fluoride, also known as tungsten hexafluoride, is an inorganic compound with the formula WF
6. 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 under standard ambient temperature and pressure (298 K, 1 atm) and the only well-characterized gas under these conditions that contains a transition metal.[4][5] WF
6 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".[6] It is one of seventeen known binary hexafluorides.

## Contents

* 1 Properties
* 2 History and synthesis
* 3 Reactions
* 4 Applications in the semiconductor industry
* 4.1 Silicon
* 4.2 Hydrogen
* 4.3 Silane and germane
* 5 Other applications
* 6 Safety
* 7 References

## Properties

The WF
6 molecule is octahedral with the symmetry point group of Oh. The W–F bond distances are 183.2 pm.[7] Between 2.3 and 17 °C, tungsten hexafluoride condenses into a colorless liquid having the density of 3.44 g/cm3 at 15 °C.[8] At 2.3 °C it freezes into a white solid having a cubic crystalline structure, a 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 a density of 4.56 g/cm3. In this phase, the W–F distance is 181 pm, and the mean closest molecular contacts are 312 pm. Whereas WF
6 gas is one of the densest gases, with a density exceeding that of the heaviest elemental gas radon (9.73 g/L), the density of WF
6 in the liquid and solid state is rather moderate.[9] The vapor pressure of WF
6 between −70 and 17 °C can be described by the equation

log10 P = 4.55569 − ⁠1021.208/ T \+ 208.45⁠,

where the P = vapor pressure (bar), T = temperature (°C).[10][11]

## History and synthesis

Tungsten hexafluoride was first obtained by conversion of tungsten hexachloride with hydrogen fluoride by Otto Ruff and Fritz Eisner in 1905:[12][13]

WCl
6 \+ 6 HF → WF
6 \+ 6 HCl

The compound is now commonly produced by the exothermic reaction of fluorine gas with tungsten powder at a temperature between 350 and 400 °C:[8]

W + 3 F
2 → WF

The gaseous product is separated from WOF
4, 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 WF
6 infused with a small amount of fluorine gas.[14]

The fluorine gas in the above method can be substituted by ClF, ClF
3, or BrF
3. An alternative procedure for producing tungsten fluoride is to treat tungsten trioxide (WO
3) with HF, BrF
3, or SF
4. 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]

WCl
6 \+ 2 AsF
3 → WF
6 \+ 2 AsCl
3 or
WCl
6 \+ 3 SbF
5 → WF
6 \+ 3 SbF
3Cl

## Reactions

On contact with water, tungsten hexafluoride gives hydrogen fluoride (HF) and tungsten oxyfluorides, eventually forming tungsten trioxide:[4]

6 \+ 3 H
2O → WO
3 \+ 6 HF

Unlike some other metal fluorides, WF
6 is not a useful fluorinating agent, nor is it a powerful oxidant. It can be reduced to the yellow WF
4.[15]

6 forms a variety of 1:1 and 1:2 adducts with Lewis bases, examples being WF
6(S(CH

2), WF
6(S(CH

2, WF
6(P(CH

3), and WF
6(py)
2.[16]

## Applications in the semiconductor industry

The dominant application of tungsten fluoride is in the 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 an increase in WF
6 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. WF
6 is favored over related compounds, such as WCl
6 or WBr
6, because of its higher vapor pressure resulting in higher deposition rates. Since 1967, two WF
6 deposition routes have been developed and employed: thermal decomposition and hydrogen reduction.[17] The required WF
6 gas purity is rather high and varies between 99.98% and 99.9995% depending on the application.[4]

6 molecules have to be split up in the CVD process. The decomposition is usually facilitated by mixing WF
6 with hydrogen, silane, germane, diborane, phosphine, and related hydrogen-containing gases.

### Silicon

6 reacts upon contact with a silicon substrate.[4] The WF
6 decomposition on silicon is temperature-dependent:

2 WF
6 \+ 3 Si → 2 W + 3 SiF
4 below 400 °C and

6 \+ 3 Si → W + 3 SiF
2 above 400 °C.

This dependence is crucial, as twice as much silicon is 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 WF
6 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]

### Hydrogen

The deposition process occurs at temperatures between 300 and 800 °C and results in formation of hydrogen fluoride vapors:

6 \+ 3 H
2 → W + 6 HF

The crystallinity of the produced tungsten layers can be controlled by altering the WF
6/H
2 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 the silicon dioxide which is the main passivation material in semiconductor electronics. Therefore, SiO
2 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]

### Silane and germane

The characteristic features of tungsten deposition from WF
6/SiH
4 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 a WF
6/GeH
4 mixture is similar to that of WF
6/SiH
4, but the tungsten layer becomes contaminated with relatively (compared to Si) heavy germanium up to concentrations of 10–15%. This increases tungsten resistivity from about 5 to 200 μΩ·cm.[4]

## Other applications

6 can be used for the production of tungsten carbide.

As a heavy gas, WF
6 can be used as a buffer to control gas reactions. For example, it slows down the chemistry of the Ar/O
2/H
2 flame and reduces the flame temperature.[19]

## Safety

Tungsten hexafluoride is an extremely corrosive compound that attacks any tissue. Because of the formation of hydrofluoric acid upon reaction of WF
6 with humidity, WF
6 storage vessels have Teflon gaskets.[20]

## References

1. ↑ "Tungsten hexafluoride" (in en). https://pubchem.ncbi.nlm.nih.gov/compound/522684#section=Safety-and-Hazards.
2. ↑ Roucan, J.-P.; Noël-Dutriaux, M.-C.. Proprietes Physiques des Composes Mineraux. Ed. Techniques Ingénieur. p. 138. https://books.google.com/books?id=BpPmFsA4yn4C&pg=PA138.
3. ↑ Gas chart (Wayback Machine archive 7 September 2022)
4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Lassner, E.; Schubert, W.-D. (1999). Tungsten - Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds. Springer. pp. 111, 168. ISBN 0-306-45053-4. https://books.google.com/books?id=foLRISkt9gcC.
5. ↑ 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.
6. ↑ "Tungsten and Tungsten Silicide Chemical Vapor Deposition". CVD Fundamentals. TimeDomain CVD. http://www.timedomaincvd.com/CVD_Fundamentals/films/W_WSi.html.
7. ↑ Lide, D. R., ed (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5. p. 4-93.
8. ↑ 8.0 8.1 Priest, H. F.; Swinehert, C. F. (1950). "Anhydrous Metal Fluorides". in Audrieth, L. F.. Inorganic Syntheses. 3. Wiley-Interscience. pp. 171–183. doi:10.1002/9780470132340.ch47. ISBN 978-0-470-13162-6.
9. ↑ Levy, J. (1975). "The structures of fluorides XIII: The orthorhombic form of tungsten hexafluoride at 193 K by neutron diffraction". Journal of Solid State Chemistry 15 (4): 360–365. doi:10.1016/0022-4596(75)90292-3. Bibcode: 1975JSSCh..15..360L.
10. ↑ 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
11. ↑ Stull, Daniel R. (1947). "Vapor Pressure of Pure Substances. Organic and Inorganic Compounds". Industrial & Engineering Chemistry 39 (4): 517–540. doi:10.1021/ie50448a022. http://webbook.nist.gov/cgi/cbook.cgi?ID=C7783826&Mask=4&Type=ANTOINE&Plot=on.
12. ↑ , Wikidata Q56639371
13. ↑ Ruff, Otto; Eisner, Fritz; Heller, Wilhelm (1907). "Über die Darstellung und Eigenschaften von Fluoriden des sechswertigen Wolframs" (in de). Zeitschrift für anorganische Chemie 52 (1): 256–269. doi:10.1002/zaac.19070520122. ISSN 1521-3749. https://onlinelibrary.wiley.com/doi/10.1002/zaac.19070520122.
14. ↑ "Method for tungsten chemical vapor deposition on a semiconductor substrate" US patent 6544889, issued 2003-04-08
15. ↑ Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4.
16. ↑ Benjamin, Sophie L.; Levason, William; Reid, Gillian (2013). "Medium and high oxidation state metal/Non-metal fluoride and oxide–fluoride complexes with neutral donor ligands". Chem. Soc. Rev. 42 (4): 1460–1499. doi:10.1039/C2CS35263J. PMID 23014811.
17. ↑ Aigueperse, J.; Mollard, P.; Devilliers, D.; Chemla, M.; Faron, R.; Romano, R.; Cuer, J.-P. (2005). "Fluorine Compounds, Inorganic". in Ullmann. Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH.
18. ↑ Kirss, R. U.; Meda, L. (1998). "Chemical vapor deposition of tungsten oxide". Applied Organometallic Chemistry 12 (3): 155–160. doi:10.1002/(SICI)1099-0739(199803)12:3<155::AID-AOC688>3.0.CO;2-Z. https://deepblue.lib.umich.edu/bitstream/2027.42/38321/1/688_ftp.pdf.
19. ↑ Ifeacho, P. (2008). Semi-conducting metal oxide nanoparticles from a low-pressure premixed H2/O2/Ar flame: Synthesis and Characterization. Göttingen: Cuvillier Verlag. p. 64. ISBN 978-3-86727-816-4. https://books.google.com/books?id=0B5HI9TNmakC&pg=PT64.
20. ↑ "Tungsten hexafluoride MSDS". Linde Gas. http://www.orcbs.msu.edu/msds/LINDE_MSDS/pdf/154.pdf.

* v
* t
* e

Binary hexafluorides

Known binary hexafluorides|

| Chalcogen binary hexafluorides|

* SF6
* SeF6
* TeF6
* PoF6

|

Noble gas binary hexafluorides|

* XeF6

Transition metal binary hexafluorides|

* MoF6
* TcF6
* RuF6
* RhF6
* WF6
* ReF6
* OsF6
* IrF6
* PtF6

Actinide binary hexafluorides|

* UF6
* NpF6
* PuF6

Predicted binary hexafluorides|

| Noble gas binary hexafluorides|

* KrF6
* RnF6

|

Transition metal binary hexafluorides|

* CrF6
* PdF6

Actinide binary hexafluorides|

* AmF6

* v
* t
* e

Tungsten compounds

Tungsten(0)|

* W(CO)6

Tungsten(II)|

* WSi2
* WCl2
* W(OH)2

Tungsten(III)|

* W2O3

Tungsten(IV)|

* WC
* WO2
* WS2
* WSe2
* WTe2
* WF4
* WCl4

Tungsten(V)|

* W2O5
* WBr5
* W2Cl10

Tungsten(VI)|

* H3PW12O40
* (NH4)10(H2W12O42)
* WO2Cl2
* WCl6
* WF6
* WN2
* WO3
* WS3
* WOBr4
* WOCl4
* WOF4
* H2WO4

| Organotungsten(VI) compounds|

* W(CH3)6

|

* v
* t
* e

Salts and covalent derivatives of the fluoride ion

HF |  |  |  | He

|
|
|
|

LiF | BeF2 | BF
BF3
B2F4 | CF4
other compounds | NF3
FN3
N2F2
N2F4
NF5§  | F2O
F2O2
other compounds | F2 | Ne
NaF | MgF2 | AlF
AlF3 | SiF4 | P2F4
PF3
PF5 | S2F2
SF2
SF4
SF6 | ClF
ClF3
ClF5 | Ar
KF | CaF
CaF2 | ScF3 | TiF2
TiF3
TiF4 | VF2
VF3
VF4
VF5 | CrF2
CrF3
CrF4
CrF5
CrF6§  | MnF2 | FeF2
FeF3 | CoF2
CoF3 | NiF2 | CuF
CuF2 | ZnF2 | GaF2
GaF3 | GeF2
GeF4 | AsF3
AsF5 | Se2F2
SeF4
SeF6 | BrF
BrF3
BrF5 | KrF2
RbF | SrF
SrF2 | YF3 | ZrF3
ZrF4 | NbF4
NbF5 | MoF2
MoF3
MoF4
MoF5
MoF6 | TcF4
TcF6 | RuF3
RuF5
RuF6 | RhF3
RhF5
RhF6 | PdF2 | AgF | CdF2 | InF
InF2
InF3 | SnF2
SnF4 | SbF3
SbF5 | Te3F2
TeF4
TeF6 | IF
IF3</br>IF5</br>IF7 | XeF2
XeF4
XeF6
CsF | BaF2 |  | HfF4 | TaF5 | WF2
WF3
WF4
WF5
WF6 | Re3F9
ReF4
ReF5
ReF6
ReF7 | OsF4
OsF5
OsF6 | IrF2
IrF3
IrF4
IrF5
IrF6 | PtF2
PtF4
PtF5
PtF6 | AuF
AuF3
Au2F10
AuF5•F2 | Hg2F2
HgF2
HgF4 | TlF | PbF2
PbF4 | BiF3
BiF5 | PoF2
PoF4
PoF6 | AtF  | RnF2
FrF | RaF2 |  | Rf  | Db  | Sg  | Bh  | Hs  | Mt  | Ds  | Rg  | Cn  | Nh  | Fl  | Mc  | Lv  | Ts  | Og
| ↓  |  |
LaF3 | CeF3
CeF4 | PrF3 | NdF2,
NdF3 | PmF3 | SmF2,
SmF3 | EuF2,
EuF3 | GdF3 | TbF3 | DyF2,
DyF3 | HoF3 | ErF3 | TmF2
TmF3 | YbF2
YbF3 | LuF3
AcF3 | ThF4 | PaF5 | UF3
UF4
UF5
UF6 | NpF4
NpF6 | PuF3
PuF6 | AmF2
AmF3 | CmF3 | BkF3 | CfF3 | EsF3 | Fm  | Md  | No  | LrF3

§ means the substance has not been made.

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