A Speciality gas used in the semiconductor industry, tungsten hexafluoride is in slightly declining demand as the downturn hits the electronics sector.

Microchips have become so ubiquitous in our day-to-day life, they can be found virtually everywhere in modern society. They are in cars and washing machines, but also in mobile phones and DVD players.

They also form the basis for such new developments as intelligent textiles and wrist watches with built-in GPS systems.

But the production of these tiny units requires large amounts of specialty gases – and they must meet the stringent purity standards of the semiconductor industry.

Our profile this month features one of the important gases used in semiconductor the industry, Tungsten Hexafluoride.

Tungsten (VI) fluoride, also known as tungsten hexafluoride, is the inorganic compound with the formula WF6. This corrosive, colourless compound is the densest known gas at a pressure of 1 atm and room temperature of 25°C.

The gas is most commonly used in the production of semiconductor circuits and circuit-boards through the process of chemical vapor deposition. It is a pale yellow, toxic, non-flammable, odourless and extremely corrosive liquefied gas, reacting violently with water to form hydrofluoric acid.

Production
Tungsten hexafluoride is produced by the exothermic reaction of fluorine gas with tungsten powder.

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, with a constant flow of WF6 infused with a small amount of fluorine gas.

Regarding its applications in microelectronics, WF6 is used to deposit tungsten films. Two routes are employed, thermal decomposition and hydrogen reduction. Both processes rely on CVD technology.

Major manufacturing facilities
Air products went ahead with a major expansion of WF6 in its Hometown facility in the US, at the start of last year.

The burgeoning demand around that time led the company to increase its production by 60% in one go. Asia, particularly Taiwan, China, the Philippines and Japan accounts for a major chunk of WF6 production, on account of the concentration of the semiconductor industry in that region.

Applications
The most common application of WF6 is in etching processes in the semiconductor industry. In the etching facility, this gas is decomposed in plasma, releasing an active component which etches the surfaces.

The photoresist is then removed without damaging the silicon or poly-silicon layers below it.

Tungsten hexafluoride is particularly suitable for depositing the metal layers that form the electrical connections between the various layers. To create the high structural and functional density on a microchip, deposition, lithography and etching processes often have to be repeated to manufacture the large number of different layers – one on top of the other.

A reliable supply of high purity gases is critical to advanced semiconductor manufacturing, as producing an integrated circuit requires over 30 different process gases for etching, deposition, oxidation, doping, and inerting applications.

The range of gases used is broader than in virtually any other industry. Trace impurities at the parts-per-million, parts-per-billion, and even parts-per-trillion levels must be measured and eliminated.

Health & Safety
Tungsten hexafluoride is an extremely toxic, corrosive, water-reactive, non-flammable gas, or pale-yellow liquid. It is a powerful, corrosive irritant to skin, eyes and mucous membranes, and contact by all routes of exposure can lead to burns.

The on-set of over-exposure symptoms may be delayed, while inhalation over-exposure can lead to potentially fatal lung disorders.

Inhalation of tungsten hexafluoride can cause severe health effects, even at relatively low concentrations. This gas causes severe irritation of the nose and throat, with other symptoms of over-exposure including coughing, excessive salivary and sputum formation, laboured breathing, and a sore throat.

Demand, Prices and Future Outlook
Demand for WF6, which was quite healthy for a long time, has shown signs of decline in the last two quarters of the last calendar year, the signs of which were most evident in the last quarter.

Our assessment of the last quarterly results of the major four gases companies has shown either a negative or single low digit growth for tungsten hexafluoride, due to curtailment in the semiconductor industry.

Prices too, which appreciated in tandem with the growth of the semiconductor industry, have largely cooled down due to slackening of demand.

We will have to wait for the economic revival of Asian economies and its semiconductor industry to see the healthy growth of this niche product.