Tungsten(VI) fluoride

Tungsten(VI) fluoride
General
Systematic name	Tungsten(VI) fluoride
Other names	Tungsten hexafluoride
Molecular formula	WF₆
Molar mass	297.83 g/mol
Appearance	Colorless gas.
CAS number
Properties
Density and phase	13.1 g/L^-1, gas.
Solubility in water	Hydrolyzes.
Melting point	2.3°C (275.45 K)
Boiling point	17.1°C (290.25 K)
Structure
Molecular shape	Octahedral
Dipole moment	0 D
Hazards
MSDS	External MSDS
Main hazards	Corrosive, highly toxic.
NFPA 704
Flash point	Non-flammable.
R/S statement	R: ? S: ?
RTECS number	?
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Tungsten(VI) chloride, Tungsten(VI) bromide
Other cations	Chromium(V) fluoride, Molybdenum(VI) fluoride
Related compounds	Tungsten(IV) fluoride, Tungsten(V) fluoride, Uranium hexafluoride
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Chemical infobox

Tungsten(VI) fluoride, also known as tungsten hexafluoride, is a colorless gas. It is nonflammable, but highly corrosive and very toxic. The molecule itself is octahedral with the symmetry point group of O_h. The gas is most commonly used in the production of semiconductor circuits and circuit boards, through the process of chemical vapor deposition.

Industrial Synthesis

Tungsten hexafluoride of a purity high enough for semiconductor CVD is produced by the reaction of fluorine gas with tungsten metal. The metal is placed in a heated reactor, slightly pressurized to 1.2 to 2.0 psi, with a constant flow of WF₆ infused with a small amount of fluorine gas.

Reactions

WF₆(g) + H₂(g) + Al(s) → W(s) + HF(g) + AlF₃(s)

This reaction occurs when tungsten is deposited on an aluminum. One application of this would be in the production of studs for semiconductor circuits.

Tungsten can also be deposited on a silicon wafer or other semiconducting material via chemical vapor deposition, or CVD, as follows:

WF₆(g) + W* → WF₆*

2 SiH₄(g) + WF₆* → W(s) + 2 SiHF₃ (g) + 3 H₂(g)

The silane (SiH₄ reduces the tungsten from (VI) to its elemental state of zero.

Safety

On contact with water tungsten(VI) fluoride forms hydrofluoric acid (HF), which can penetrate the skin and cause damage to the subdermal tissues and bone. Inhalation burns the respiratory tract and can be toxic. WF₆ is a lachrymator which causes tearing and irritation of the eyes. Contact causes burns to the eyes, skin and mucous membranes.

Trivia

In Memories of a Chemical Boyhood, Oliver Sacks comments on how he wanted WF₆-filled ballons for his 65^th birthday, but the gas was too reactive. Had one of the balloons popped, the tungsten(VI) fluoride would have reacted with moisture in the air to form hydrogen fluoride.

References

Kirss, Rein U., Lamartine Meda. "Chemical Vapor Deposition of Tungsten Oxide." Applied Organometallic Chemistry 12 (1998): 155–160.

Patent Storm

Matheson Gas

Ip.com

James Clark School of Engineering

External links

National Pollutant Inventory - Flouride and compounds fact sheet

Tungsten compounds | Inorganic compounds | Fluorides | Metal halides | lachrymatory agents