Nitrogen trifluoride - Cleaning up in electronic applications
Nitrogen trifluoride, NF3, a binary compound of nitrogen and fluorine, is a colourless, toxic, nonflammable, corrosive gas shipped in cylinders at high pressure.
It has a characteristically mouldy odour. It is a pulmonary irritant with toxicity comparable with nitrogen oxides. It may damage liver and kidneys, and is slightly soluble in water without undergoing chemical reaction.
NF3 is used as a chamber cleaning gas in the manufacture of semiconductors, flat panel displays and other electronic devices. When compared with competing products, NF3 offers customers significant reductions in emissions, throughput increases of up to 30%, longer chamber life and faster clean rates.
NF3 is also stable and non-flammable, making it a safe gas to transport, store and deliver to customers around
Nitrogen trifluoride is also used in the plasma and thermal cleaning of CVD reactors, while it is used as a source of fluorine radicals for plasma etching of polysilicon, silicon nitride, tungsten silicide and tungsten for example.
Although semiconductors remain the principal driver for electronic specialty gases, increased interest in photovoltaics is adding to the push.
Electronic gases are needed in thin-film deposition, such as chemical vapour deposition (CVD) or physical vapour deposition (PVD), technologies used to make a semiconductor or a photovoltaic cell.
The three major gases used in semiconductors, liquid crystal displays (LCDs) and photovoltaics are nitrogen trifluoride (NF3), silane (SiH4) and ammonia.
Demand for electronic gases in the semiconductor and solar cell industries continues to outpace global GDP growth by more than two times. In traditional semiconductor segments such as microchips and flat panel displays, market researchers expect sales growth of around 8% per year between now and 2010 – and for the solar segment, the annual forecast lies at around 30%.
Experts anticipate that from 2012, photovoltaic producers will spend more on gases than flat-screen manufacturers, and from 2017 they are even set to overtake the chip sector.
Although only a handful of different gases are used in solar-cell manufacturing – in comparison with more than 20 for semiconductors – the volumes required are significantly greater.
New facilities on stream
Air Products has been routinely expanding its NF3 capacity since the late 1990’s to meet demand, sometimes by as much as 50% at a time.
In late 2007, the company said it would double NF3 capacity at its facility at Ulsan, South Korea, to 1,000 tonnes/year by 2009. The rest of Air Products’ global capacity – about 2,200 tonnes – is at its plant in Hometown, Pennsylvania, US.
Air Products announced in January that it has signed an agreement with Signet Solar Inc. to supply turnkey installation of gas delivery systems and services and related gas needs for a new, thin-film photovoltaic (PV) module production facility in Mochau, Germany.
The long-term agreement includes the supply of hydrogen, helium, nitrogen and argon, as well as specialty gases such as silane, nitrogen trifluoride and dopant gases. The new plant will be on stream in the summer of 2008.
Demand for NF3, coupled with increased interest in solar energy, means the market for electronic gases will keep growing. But are shortages looming?
Emerging thin-film solar cells will be based on thin-film deposition technologies including CVD processing, says industry analyst Mike Corbett, Managing Partner of Linx Consulting, based in Boston, Massachusetts, US.
“Basically, tandem-cell thin-film solar cell production uses similar CVD tool sets as those used in the LCD industry. So as thin-film solar cells become more popular, there will be a high volume-growth potential for these gases,” says Corbett.
According to US-based industrial gas supplier Air Products, solar capacity is growing at more than 30% per year.
“With photovoltaics using many of the same raw materials as semiconductor manufacturers, we would expect to see strong growth in the products Air Products supplies to the photovoltaics industry,” says Dave Tavianini, Photovoltaics Business Development manager for the company.
But thanks to a lot of facilities coming on-stream the demand should exceed supply in the short run.
Asia, which is a hub of LCD industry and slowly becoming the manufacturing hub of solar panels is likey to be the most interesting market for NF3 in coming days.
NF3 is not hazardous by skin contact and is a relatively minor irritant to the eyes and mucous membranes. Overexposure to NF3 via inhalation induces the conversion of haemoglobin to methemoglobin.
The formation of methemoglobin reduces the amount of oxygen available to body tissues. This can lead to chemical cyanosis, headache, dizziness, weakness, confusion and other manifestations associated with a reduced oxygen supply.
Hemolytic anemia, enlargement of the spleen, and pathologic changes in the liver, kidneys and heart muscle may occur as secondary effects of methemoglobinemia.
At the cessation of NF3 exposure, methemoglobin spontaneously reverts to haemoglobin. While methemoglobinemia clears over several hours, hemolytic anemia may take several weeks to resolve.