Nitrogen trifluoride

Nitrogen trifluoride
IUPAC name Nitrogen trifluoride
Other names Nitrogen fluoride Trifluoramine Trifluorammonia
Identifiers
CAS number	7783-54-2 Y
PubChem	24553
ChemSpider	22959 Y
UN number	2451
ChEBI	CHEBI:30231 Y
RTECS number	QX1925000
Jmol-3D images	Image 1
SMILES FN(F)F
InChI InChI=1S/F3N/c1-4(2)3 Y Key: GVGCUCJTUSOZKP-UHFFFAOYSA-N Y InChI=1/F3N/c1-4(2)3 Key: GVGCUCJTUSOZKP-UHFFFAOYAA
Properties
Molecular formula	NF3
Molar mass	71.00 g/mol
Appearance	colorless gas
Density	3.003 kg/m3 (1 atm, 15 °C) 1.885 g/cm3 (liquid at b.p.)
Melting point	−207.15 °C, 66 K, -341 °F
Boiling point	−129.1 °C, 144 K, -200 °F
Solubility in water	0.021 vol/vol (20 °C)
Structure
Molecular shape	trigonal pyramidal
Dipole moment	0.234 D
Thermochemistry
Std enthalpy of formation ΔfHo298	-31.4 kJ/mol[1]
Hazards
MSDS	AirLiquide
EU Index	Not listed
NFPA 704	0 1 0 OX
Flash point	Non-flammable
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Nitrogen trifluoride is the inorganic compound with the formula NF₃. This nitrogen-fluorine compound is a colorless, toxic, odourless, nonflammable gas. It finds increasing use as an etchant in microelectronics.

Applications

Nitrogen trifluoride is used in the plasma etching of silicon wafers. Today nitrogen trifluoride is predominantly employed in the cleaning of the PECVD chambers in the high volume production of liquid crystal displays and silicon-based thin film solar cells. In these applications NF₃ is initially broken down in situ, by a plasma. The resulting fluorine atoms are the active cleaning agents that attack the polysilicon, silicon nitride and silicon oxide. Nitrogen trifluoride can be used as well with tungsten silicide, and tungsten produced by CVD. NF₃ has been considered as an environmentally preferable substitute for sulfur hexafluoride or perfluorocarbons such as hexafluoroethane.^[2] The process utilization of the chemicals applied in plasma processes is typically below 20 %. Therefore some of the PFCs and also of the NF₃ always escape into the atmosphere. Modern gas abatement systems can decrease such emissions.

Elemental fluorine has been introduced as an environmentally friendly replacement for nitrogen trifluoride in the manufacture of flat panel displays and solar cells.^[3]

Nitrogen trifluoride is also used in hydrogen fluoride and deuterium fluoride lasers, which are types of chemical lasers. It is preferred to fluorine gas due to its convenient handling properties, reflecting its considerable stability.

It is compatible with steel and Monel, as well as several plastics.

Synthesis and reactivity

Nitrogen trifluoride is a rare example of a binary fluoride that can prepared directly from the elements only at very uncommon conditions, such as electric discharge.^[4] After first attempting the synthesis in 1903, Otto Ruff prepared nitrogen trifluoride by the electrolysis of a molten mixture of ammonium fluoride and hydrogen fluoride.^[5] It proved to be far less reactive than the other nitrogen trihalides nitrogen trichloride, nitrogen tribromide and nitrogen triiodide, all of which are explosive. Alone among the nitrogen trihalides it has a negative enthalpy of formation. Today, it is prepared both by direct reaction of ammonia and fluorine and by a variation of Ruff's method.^[6] It is supplied in pressurized cylinders.

Reactions

NF₃ is slightly soluble in water without undergoing chemical reaction. It is nonbasic with a low dipole moment of 0.2340 D. By contrast, ammonia is basic and highly polar (1.47 D).^[7] This difference arises from the fluorine atoms acting as electron withdrawing groups, attracting essentially all of the lone pair electrons on the nitrogen atom. NF₃ is a potent yet sluggish oxidizer.

It oxidizes hydrogen chloride to chlorine:

2 NF₃ + 6 HCl → 6 HF + N₂ + 3 Cl₂

It converts to tetrafluorohydrazine upon contact with metals, but only at high temperatures:

2 NF₃ + Cu → N₂F₄ + CuF₂

NF₃ reacts with fluorine and antimony pentafluoride to give the tetrafluoroammonium salt:

NF₃ + F₂ + SbF₅ → NF+
4SbF−
6

Greenhouse gas

NF₃ is a greenhouse gas, with a global warming potential (GWP) 17,200 times greater than that of CO₂ when compared over a 100 year period.^[8][9][10] Its GWP would place it second only to SF₆ in the group of Kyoto-recognised greenhouse gases, although NF₃ is not currently included in that grouping. It has an estimated atmospheric lifetime of 740 years,^[8] although other work suggests a slightly shorter lifetime of 550 years (and a corresponding GWP of 16,800).^[11]

Although NF₃ has a high GWP, for a long time its radiative forcing in the Earth's atmosphere has been assumed to be small, spuriously presuming that only small quantities are released into the atmosphere. Industrial applications of NF₃ routinely break it down, while in the past previously used regulated compounds such as SF₆ and PFCs were often released. Research has questioned the previous assumptions. High-volume applications such as DRAM computer memory production, the manufacturing of flat panel displays and the large-scale production of thin-film solar cells in regions with insufficient ecological awareness continues to increase the emissions of NF₃.^[11][12]

Since 1992, when less than 100 tons was produced, production has grown to an estimated 4000 tons in 2007 and is projected to increase significantly.^[11] World production of NF₃ is expected to reach 8000 tons a year by 2010. By far the world's largest producer of NF₃ is the US chemical company Air Products & Chemicals. An estimated 2% of produced NF₃ is released into the atmosphere.^[13][14] Robson projected that the maximum atmospheric concentration is less than 0.16 parts per trillion (ppt) by volume, which will provide less than 0.001 Wm⁻² of IR forcing.^[15] The mean global tropospheric concentration of NF₃ has risen quasi-exponentially from about 0.02 ppt (parts per trillion, dry air mole fraction) at the beginning of the measured record in 1978, to a July 1, 2008 value of 0.454 ppt, with a rate of increase of 0.053 ppt yr⁻¹, or about 11% per year, and an interhemispheric gradient that is consistent with these emissions occurring overwhelmingly in the Northern Hemisphere, as expected. This rise rate corresponds to about 620 metric tons/y of 2009 NF₃ emissions globally, or about 16% of the NF₃ production estimate of 4000 metric tons yr⁻¹. This is a significantly higher percentage than has been estimated by industry, and thus strengthens the case for inventorying NF₃ production and for regulating its emissions.^[16] One study suggests that the contribution of the NF₃-Emissions to the overall greenhouse gas budget of thin-film Si-solar cell manufacturing is overestimated. Instead, the contribution of the nitrogen trifluoride to the CO₂-budget of thin film solar cell production is compensated already within a few months by the CO₂ saving potential of the PV technology.^[17]

Safety

Skin contact with NF₃ is not hazardous, and it is a relatively minor irritant to mucous membranes and eyes. It is a pulmonary irritant with a toxicity comparable with nitrogen oxides, and overexposure via inhalation causes the conversion of hemoglobin in blood to methemoglobin, which can lead to the condition methemoglobinemia.^[18]

References

1. ^ G.C. Sinke, "The enthalpy of dissociation of nitrogen trifluoride", J. Phys. Chem. 1967,71 (2), pp 359–360, DOI: 10.1021/j100861a022 [1]
2. ^ H. Reichardt , A. Frenzel and K. Schober (2001). "Environmentally friendly wafer production: NF₃ remote microwave plasma for chamber cleaning". Microelectronic Engineering 56 (1–2): 73–76. doi:10.1016/S0167-9317(00)00505-0. 
3. ^ J. Oshinowo, A. Riva, M Pittroff, T. Schwarze and R. Wieland (2009). "Etch performance of Ar/N₂/F₂ for CVD/ALD chamber clean". Solid State Technology 52 (2): 20–24. 
4. ^ (Russian)Lidin, P. A.; Molochko, V. A.; Andreeva, L. L.. Химические свойства неорганических веществ. pp. 442–455. ISBN 156700041X. 
5. ^ Otto Ruff, Joseph Fischer, Fritz Luft (1928). "Das Stickstoff-3-fluorid". Zeitschrift für anorganische und allgemeine Chemie 172 (1): 417–425. doi:10.1002/zaac.19281720132. 
6. ^ Philip B. Henderson, Andrew J. Woytek "Fluorine Compounds, Inorganic, Nitrogen" in Kirk‑Othmer Encyclopedia of Chemical Technology, 1994, John Wiley & Sons, NY. doi:10.1002/0471238961.1409201808051404.a01 Article Online Posting Date: December 4, 2000
7. ^ Thomas M. Klapötke “Nitrogen–fluorine compounds” Journal of Fluorine Chemistry Volume 127, 2006, pp. 679–687. doi:10.1016/j.jfluchem.2006.03.001
8. ^ ^{a b} Climate Change 2007: The Physical Sciences Basis. IPCC. http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf. Retrieved 2008-07-03 
9. ^ Robson, J.I.; Gohar, L.K., Hurley, M.D., Shine, K.P. and Wallington, T. (2006). "Revised IR spectrum, radiative efficiency and global warming potential of nitrogen trifluoride". Geophys. Res. Lett. 33 (10): L10817. Bibcode 2006GeoRL..3310817R. doi:10.1029/2006GL026210. http://cat.inist.fr/?aModele=afficheN&cpsidt=17893800. 
10. ^ Richard Morgan (2008-09-01). "Beyond Carbon: Scientists Worry About Nitrogen’s Effects". New York Times. http://www.nytimes.com/2008/09/02/science/02nitr.html?ref=science. Retrieved 2008-09-07.  mirror
11. ^ ^{a b c} Prather, M.J.; Hsu, J. (2008). "NF₃, the greenhouse gas missing from Kyoto". Geophys. Res. Lett. 35 (12): L12810. Bibcode 2008GeoRL..3512810P. doi:10.1029/2008GL034542. http://www.agu.org/journals/gl/gl0812/2008GL034542/. 
12. ^ Tsai, W.-T. (2008). "Environmental and health risk analysis of nitrogen trifluoride (NF₃), a toxic and potent greenhouse gas". J. Hazard. Mat. 159 (2–3): 257. doi:10.1016/j.jhazmat.2008.02.023. PMID 18378075. 
13. ^ M. Roosevelt (2008-07-08). "A climate threat from flat TVs, microchips". Los Angeles Times. http://www.latimes.com/news/nationworld/nation/la-na-climate8-2008jul08,0,7460950.story. 
14. ^ Hoag, Hannah (2008-07-10). "The Missing Greenhouse Gas". Nature Reports Climate Change (Nature News). doi:10.1038/climate.2008.72. http://www.nature.com/climate/2008/0808/full/climate.2008.72.html. 
15. ^ Robson, Jon. Nitrogen trifluoride (NF3). Royal Meteorological Society. http://www.rmets.org/activities/awards/scholarships/sch_2.php. Retrieved 2008-10-27 
16. ^ Weiss, Ray F.; J. Mühle, P.K. Salameh, C.M. Harth (2008-10-31). "Nitrogen trifluoride in the global atmosphere". Geophys. Res. Lett. (AGU) 35 (20): L20821. Bibcode 2008GeoRL..3520821W. doi:10.1029/2008GL035913. http://www.agu.org/pubs/crossref/2008/2008GL035913.shtml. 
17. ^ Fthenakis, Vasilis; D.O. Clark, M. Moalem M, P. Chandler, R.G. Ridgeway, F.E. Hulbert, D.B. Cooper, P.J. Maroulis PJ (2010-10-25). "Life-Cycle Nitrogen Trifluoride Emissions from Photovoltaics". Environ. Sci. Technol. (American Chemical Society) 44 (22): 8750. doi:10.1021/es100401y. 
18. ^ Malik, Yogender (2008-07-03). "Nitrogen trifluoride – Cleaning up in electronic applications". Gasworld. http://www.gasworld.com/news.php?a=2896. Retrieved 2008-07-15. 

External links

• National Pollutant Inventory – Fluoride and compounds fact sheet
• WebBook page for NF3