Nitrogen trifluoride as an anthropogenic-greenhouse-forcing gas.
A couple of articles in the media captured my interest this evening:
I must say, this looks like more biased “You’ve got a TV? You’re guilty of climate change!” baloney from the “green” fanatics in the press who like spinning scientific papers out of context.
Nitrogen trifluoride is used in the plasma and thermal cleaning of chemical vapor deposition (CVD) reactors in the semiconductor industry. It is also used as a source of fluorine radicals for plasma etching of polysilicon, silicon nitride, tungsten silicide, and tungsten, in which application it can replace perfluorocarbons such as hexafluoroethane and sulfur hexafluoride, resulting in both ecological advantage and improved process efficiency. NF3 is an alternative to these other potent greenhouse gases and its usage has increased markedly over the last decade.
This has got nothing to do specifically with manufacturing plasma TVs, and everything to do with manufacturing semiconductor devices and materials such as polycrystalline silicon.
One has to wonder what the emissions of sulfur hexafluoride, perfluorocarbons and/or nitrogen trifluoride are for the manufacturing of a typical plasma TV, and how it compares to the emissions of sulfur hexafluoride, perfluorocarbons and/or nitrogen trifluoride over the manufacturing of, say, one typical solar photovoltaic panel. Obviously a photovoltaic panel has got much more polycrystalline silicon in it than your TV – yet, do we hear anything about manufacturing of solar photovoltaic cells in this regard?
In fact, worldwide interest in sustainable energy systems and the ensuing growth of the solar photovoltaics industry is one of the main forces driving increased industrial demand for nitrogen trifluoride and other gases employed in the processing of semiconductor materials:
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%/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.
Looking ahead, Tavianini adds, demand for specialty gases will continue to accelerate as second-generation thin-film siliconphotovoltaics proliferate.
The use of such materials applies to basically everything containing semiconductors – essentially all modern electronic technologies are equally relevant, from your PC to your TV to your solar panels to your PC to your cellphone. Semiconductor technology is a fundamentally important cornerstone of our modern civilisation.
Nitrogen trifluoride is a potent greenhouse gas, with a global warming potential (GWP) of 17,200 over a 100 year timescale. This places it second only to sulfur hexafluoride in the group of Kyoto-recognised greenhouse gases. It has an estimated atmospheric lifetime of 740 years, though newer research suggests a slightly shorter lifetime of 550 years and a GWP of 16,800.
With 2008 production equivalent to 67 million metric tons of CO2, based on estimated emissions for 2008, it has been calculated that NF3 may play a more significant role than emissions of the industrialized nations of perfluorocarbons or sulfur hexafluoride, which are included in the Kyoto protocol.
Increased wafer size and reduced critical dimensions demand higher process stability and often new processes. With new production lines being built newer and generally more severe environmental statutes apply. On top of that, there is the worldwide goal of perfluorocarbon emissions reductions.
Regarding the consumption of perfluorocarbon etch gases, chamber cleaning processes are the major contributor. Since the utilisation of etch gas in these processes is usually less than 50%, the remaining gas has to be destroyed and removed by a waste gas abatement system. Generally, for CVD and etch processes, waste gas abatement is necessary for several reasons, certainly including but not limited to environmental concern and legal restrictions on emissions of greenhouse-forcing fluorinated gases.
Tetrafluoromethane is used in the microelectronics industry alone or in combination with oxygen as a plasma etchant for silicon, silicon dioxide, and silicon nitride. Tetrafluoromethane is a gas that contributes to the greenhouse effect. It is very stable, lasts a long time in the atmosphere, and is a powerful greenhouse gas. Its atmospheric lifetime is 50,000 years and it has a global warming potential of 6500.
Hexafluoroethane is also used as a versatile etchant in semiconductor manufacturing. It can be used for selective etching of metal silicides and oxides versus their metal substrates and also for etching of silicon dioxide over silicon. Hexafluoroethane is very stable in the atmosphere and thus acts as an extremely potent greenhouse gas, with an atmospheric lifetime of 10,000 years and a global warming potential (GWP) of 9200.
Sulfur hexafluoride is also used as a plasma etchant in the semiconductor industry, along with other technological applications. It is the most potent greenhouse gas known, with a global warming potential of 22,800 over a 100 year time horizon – SF6 is very stable. Its mixing ratio in the atmosphere is lower than that of CO2; about 6.5 parts per trillion in 2008, compared to 380 ppm of carbon dioxide.
Recently, the use of NF3 as an etch gas for chamber cleaning processed has been reported to give promising results, and use of this gas has been increasing. Aside from the advantage of less wear on the chamber, gas consumption is lower, since the utilisation of NF3 is very high, at 85 to 99\%. At the same time NF3 has a far smaller atmospheric lifetime of 550 years than standard etch gases like tetrafluoromethane and hexafluorethane, with estimated atmospheric lifetimes of 50,000 and 10,000 years, respectively.
When comparing the global warming potentials of these gases, a 100-year integrated time horizon is used, and the benefit of using nitrogen trifluoride to replace the alternative reagents with regards to anthropogenic greenhouse effect forcing is obfuscated for that reason.
Global Warming Potentials over an 100-year integrated time horizon:
Nitrogen trifluoride: 16,800
Sulfur hexafluoride: 22,800
Nitrogen trifluoride: 550 years
Sulfur hexafluoride: 3200 years
Hexafluoroethane: 10,000 years
Tetrafluoromethane: 50,000 years
The fact is, nitrogen trifluoride presents an environmentally friendlier alternative to sulfur hexafluoride, and arguably an environmentally friendlier alternative to perfluorocarbon gases.
Whilst these inorganic fluorine compounds and perfluorocarbons have large global warming potentials, which make for dramatic media headlines, their atmospheric abundances and mixing ratios are very small, and hence their contributions to radiative forcing in the atmosphere and hence to anthropogenic forcing of climate processes are very small by comparison to carbon dioxide, methane and water vapor.
Carbon dioxide is responsible for an increased radiative forcing term of 1.66 W/m2, according to up-to-date IPCC data, along with 0.5 W/m2 for methane and 0.16 W/m2 for nitrous oxide. For comparison, sulfur hexafluoride is associated with a far smaller increased radiative forcing term of 0.002 W/m2, along with 0.001 W/m2 for perfluoroethane. We can reasonably expect that the contribution from nitrogen trifluoride is similar, at around 0.001 to 0.002 W/m2. Whilst nitrogen trifluoride is certainly worthy of inclusion under the Kyoto protocol, along with perfluorocarbons and the like, especially as worldwide consumption of the gas grows, it is however nothing worth making a huge irrational fuss in the media about.
Robson, J. I., L. K. Gohar, M. D. Hurley, K. P. Shine, and T. J. Wallington (2006), Revised IR spectrum, radiative efficiency and global warming potential of nitrogen trifluoride, Geophys. Res. Lett., 33, L10817, doi:10.1029/2006GL026210.
Prather, M. J., and J. Hsu (2008), NF3, the greenhouse gas missing from Kyoto, Geophys. Res. Lett., 35, L12810, doi:10.1029/2008GL034542.
Reichardt, H., Frenzel, A. and Schober, K., Environmentally friendly wafer production: NF3 remote microwave plasma for chamber cleaning. doi:10.1016/S0167-9317(00)00505-0