Tungsten(VI) fluoride

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.^[1]

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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.^[2]

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.^[3]

Tungsten can also be deposited on a silicon wafer or other semiconducting material via chemical vapor deposition, or CVD, as follows:^[4]

WF₆(g) + W* → WF₆*

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

The silane SiH₄ reduces the tungsten from an oxidation state of 6+ (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.^[5]

In Uncle Tungsten, Oliver Sacks comments on how he wanted WF₆-filled balloons 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