Tungsten hexafluoride
Tungsten hexafluoride
- Non-flammable gas with a pungent odor
- Below 17 ° C: pale yellow liquid
- Below 2 ° C, white crystals
Gaseous
- Gas: 12.4 g · l-1
- Fixed: 4.56 g · cm -3 ( -9 ° C)
2.3 ° C
17.1 ° C
113.2 kPa ( 20 ° C)
Risk
1 mg · m-3
411.7 ± 0.5 kcal · mol -1
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Tungsten ( VI ) fluoride ( WF6 ), often tungsten hexafluoride is a colorless, gaseous compound of tungsten and fluorine, and belongs to the group of hexafluoride. It is the heaviest known gas under standard conditions, the fumes in moist air due to hydrolysis and has a pungent odor. It is most commonly used in the production of semiconductor circuits and printed circuit boards in the process of chemical vapor deposition; it decomposes a residue of metallic tungsten remains.
- 3.1 In the semiconductor industry
- 3.2 Other Uses
Representation
Tungsten hexafluoride is obtained by reaction of the tungsten fluoride stream at temperatures between 350 and 400 ° C:
The gaseous product is condensed and separated from WOF4 impurities by distillation.
Instead of fluorine gas can also chlorine fluoride ( ClF ), chlorine trifluoride ( ClF 3 ) or bromine trifluoride ( BrF 3 ) are used. An alternative method for the production of tungsten hexafluoride is the conversion of tungsten trioxide ( WO3 ) with hydrogen fluoride (HF), BrF3 or sulfur tetrafluoride ( SF4 ). Tungsten hexafluoride may also be obtained by conversion of tungsten hexachloride ( WCl6 ):
Due to the chemical similarity of molybdenum and tungsten is contained as an impurity molybdenum hexafluoride. This can be by reduction of WF6 - MoF6 mixture with any item, including molybdenum, are removed at a moderately elevated temperature.
Properties
Physical Properties
At atmospheric pressure and temperatures above 17.1 ° C tungsten hexafluoride is a colorless diamagnetic gas. In the temperature range from 2.3 to 17.1 ° C, it is a pale yellow liquid with a density of 3.44 g/cm3 at 15 ° C.
At 2.3 ° C, it freezes to a white solid in the cubic crystal system with the lattice parameters a = 628 pm and two formula units per unit cell with a calculated density of 3.99 g · cm -3. At -9 ° C, a solid phase transition is observed. Below this temperature, it crystallizes in the orthorhombic crystal system, space group Pnma and four formula units per unit cell with a calculated density of 4.56 g · cm -3. The fluorine atoms take on the Hexagonal a densest sphere packing.
The gas has a relatively high density of about 12.4 g / L, about 10.3 times denser than air and approximately half as large as the foam polystyrene. While WF6 gas is the heaviest known gas (density of the heaviest elementary gas radon: 9.73 g / l), however, the density moves in the liquid or solid state rather in central regions.
The critical point is 179.6 ° C, 45.7 bar and 1.28 kg / l; the triple point at 2.4 ° C and 0.5597 bar.
The WF6 molecule is octahedral ( Oh); the W -F bond length is 182.6 pm. This high degree of symmetry is observed in most of the related compounds. However, it is noted that Wolframhexahydrid ( WH6 ) and hexamethyltungsten (W ( CH3 ) 6) adopt a trigonal prismatic structure.
Chemical Properties
Tungsten hexafluoride is highly toxic and corrosive, as it forms on contact with water under vigorous reaction of hydrofluoric acid. This produces Wolframoxyfluoride or tungstic acid:
The high susceptibility to hydrolysis, in contrast to the at first glance similarly constructed sulfur hexafluoride can be explained by the significantly larger covalent radius of the central tungsten atom which is thus less sterically hindered.
Use
In the semiconductor industry
Tungsten hexafluoride is used in the process of chemical vapor deposition in semiconductor manufacturing. In the decomposition of WF6 molecules, a residue of metallic tungsten remains. This layer serves as a low- resistive metallic interconnect.
The development of the semiconductor industry in the 1980s and 1990s led to an increase in the consumption of WF6, which is run at about 200 tons per year. Tungsten metal is attractive because of its relatively high thermal and chemical stability as well as low resistance ( 5.6 μΩ · cm) and electromigration. The use of WF6 is compared with its related compounds (such as WCl6 or WBr6 ) cheaper, due to its higher vapor pressure, it leads to higher deposition rates. Since 1967, two deposition methods have been developed, the thermal decomposition and hydrogen reduction. The required WF6 gas purity is quite high and ranges between 99.98 % and 99.9995 % depending on the application.
Other Uses
As a heavy gas WF6 can be used as a buffer to control gas phase reactions. So it slows example, the chemistry of Ar/O2/H2-Flamme and reduces its temperature.
Safety
Tungsten hexafluoride is a very aggressive substance that attacks every tissue. Upon contact of the gas with body fluids to hydrofluoric acid burns on the skin and the mucous membranes of the respiratory tract forms. The exposure of humans to the gas acts first on the eyes and respiratory tract and causes irritation, loss of vision, cough and excessive formation of saliva and sputum. After prolonged exposure, this leads to pneumonitis and pulmonary edema or death. Because of these features storage containers are provided with Teflon seals.