Tungsten hexafluoride

Tungsten hexafluoride

Names
IUPAC names Tungsten hexafluoride Tungsten(VI) fluoride
Identifiers
CAS Number	7783-82-6
Jmol 3D image	Interactive graph
PubChem	522684
InChI InChI=1S/6FH.W/h6*1H;/q;;;;;;+6/p-6
SMILES F[W](F)(F)(F)(F)F
Properties
Chemical formula	WF6
Molar mass	297.830 g/mol
Appearance	Colorless gas
Density	12.4 g/L (gas) 4.56 g/cm^3 (−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
Structure
Molecular shape	Octahedral
Dipole moment	zero
Hazards
Main hazards	Toxic, corrosive; gives HF on contact with water
Flash point	Non-flammable
Related compounds
Other anions	Tungsten hexachloride Tungsten hexabromide
Other cations	Chromium(VI) fluoride Molybdenum(VI) fluoride
Related compounds	Tungsten(IV) fluoride Tungsten(V) fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Tungsten(VI) fluoride, also known as tungsten hexafluoride, is the inorganic compound of tungsten and fluorine with the formula WF₆. This corrosive, colorless compound is a gas under standard conditions, with a density of about 13 g/L (roughly 11 times heavier than air.^[1][2][3]), WF₆ is one of the heaviest known gases under standard conditions.^[4] WF₆ gas is most commonly used in the production of semiconductor circuits and circuit boards through the process of chemical vapor deposition – upon decomposition, molecules of WF₆ leave a residue of metallic tungsten. This layer serves as low-resistive metallic "interconnect".^[5]

Properties

At ambient pressure and temperatures above 17 °C, tungsten hexafluoride is a colorless^[4] diamagnetic gas. The WF₆ molecule is octahedral with the symmetry point group of O_h. The W–F bond distances are 183.2 pm.^[6] 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 heaviest 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.^[7] The vapor pressure of WF₆ between −70 °C 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).^[8][9]

Synthesis

Tungsten hexafluoride is commonly produced by the exothermic reaction of fluorine gas with tungsten powder at a temperature between 350 and 400 °C:^[10]

W + 3 F₂ → WF₆

The gaseous product is condensed and separated by distillation from WOF₄, a common impurity. 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.^[11]

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:^[4]

WCl₆ + 6 HF → WF₆ + 6 HCl or

WCl₆ + 2 AsF₃ → WF₆ + 2 AsCl₃ or

WCl₆ + 3 SbF₅ → WF₆ + 3 SbF₃Cl₂

Reactions

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

WF₆ + 3 H₂O → WO₃ + 6 HF

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.^[12]

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 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. Whereas 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.^[13] The required WF₆ 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 mixing WF₆ with hydrogen, silane, germane, diborane, phosphine, and related hydrogen-containing gases.

Silicon

WF₆ reacts upon contact with a silicon substrate.^[4] The WF₆ decomposition on silicon is temperature-dependent:

2 WF₆ + 3 Si → 2 W + 3 SiF₄    below 400 °C and

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 Si only, but not on silicon oxide or 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 WF₆ 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 but in an oxygen containing atmosphere (air) then instead of tungsten, a tungsten oxide layer is produced.^[14]

Hydrogen

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

WF₆ + 3 H₂ → W + 6 HF

The crystallinity of the produced tungsten layers can be controlled by altering the WF₆/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 the HF vapor is very aggressive and etches away most materials. Also, the deposited tungsten shows poor adhesion to the silicon dioxide which 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]

Silane and germane

The characteristic features of tungsten deposition from the WF₆/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.^[4]

Deposition from WF₆/GeH₄ mixture is similar to that of WF₆/SiH₄, 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.^[4]

Other applications

WF₆ can be used for the production of tungsten carbide.

As a heavy gas, WF₆ can be used as a buffer to control gas reactions. For example, it slows down the chemistry of the Ar/O₂/H₂ flame and reduces the flame temperature.^[15]

Safety

Tungsten hexafluoride is an extremely corrosive compound that attacks any tissue. Exposure of humans to the gas first affects the eyes and respiratory tracts causing irritation, loss of vision, cough, and excessive formation of saliva and sputum. Upon mixing with the body fluids, the gas transforms into hydrofluoric acid which burns the skin and mucous tissues of the respiratory tract. Upon prolonged exposure, this results in pneumonitis and pulmonary edema, and could be fatal. Because of the formation of hydrofluoric acid upon reaction of WF₆ with humidity, WF₆ storage vessels have Teflon gaskets.^[16]

Related compounds

Most related compounds share the octahedral symmetry of the WF₆ molecule. However, hexamethyltungsten (W(CH₃)₆) and tungsten hexahydride (WH₆) adopt a trigonal prismatic structure.^[17][18] Numerous hexafluorides are known with other metals and metalloids. Such MF₆ compounds characteristically form dense gases, however, when element M is heavier than tungsten, the compound is either liquid or solid at room temperature.

References

1. Roucan, J.-P.; Noël-Dutriaux, M.-C. Proprietes Physiques des Composes Mineraux. Ed. Techniques Ingénieur. p. 138.
2. Gas chart
3. "Tungsten Hexafluoride MSDS" (pdf).
4. 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.
5. "Tungsten and Tungsten Silicide Chemical Vapor Deposition". CVD Fundamentals. TimeDomain CVD.
6. 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.
7. 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.
8. 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
9. http://webbook.nist.gov/cgi/cbook.cgi?ID=C7783826&Mask=4&Type=ANTOINE&Plot=on
10. 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.
11. US patent 6544889, "Method for tungsten chemical vapor deposition on a semiconductor substrate", issued 2003-04-08 
12. Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4.
13. 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.
14. 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.
15. 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 3-86727-816-4.
16. "Tungsten hexafluoride MSDS" (pdf). Linde Gas.
17. Haaland, A.; Hammel, A.; Rypdal, K.; Volden, H. V. (1990). "The coordination geometry of gaseous hexamethyltungsten is not octahedral". Journal of the American Chemical Society 112 (11): 4547–4549. doi:10.1021/ja00167a065.
18. Weinhold, F.; Landis, C. R. (2005). Valency and bonding: a natural bond orbital donor-acceptor perspective. Cambridge University Press. p. 427. ISBN 0-521-83128-8.