Nitrogen trifluoride

Names
IUPAC name Nitrogen trifluoride
Other names Nitrogen fluoride Trifluoramine Trifluorammonia
Identifiers
CAS Number	7783-54-2 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30231 Y
ChemSpider	22959 Y
ECHA InfoCard	100.029.097
EC Number	232-007-1
Gmelin Reference	1551
PubChem CID	24553
RTECS number	QX1925000
UNII	4402F4X0RH Y
UN number	2451
CompTox Dashboard (EPA)	DTXSID4040181
InChI InChI=1S/F3N/c1-4(2)3 Y Key: GVGCUCJTUSOZKP-UHFFFAOYSA-N Y InChI=1/F3N/c1-4(2)3 Key: GVGCUCJTUSOZKP-UHFFFAOYAA
SMILES FN(F)F
Properties
Chemical formula	NF3
Molar mass	71.00 g/mol
Appearance	colorless gas
Odor	moldy
Density	3.003 kg/m3 (1 atm, 15 °C) 1.885 g/cm3 (liquid at b.p.)
Melting point	-207.15 °C (-340.87 °F; 66.00 K)
Boiling point	-129.06 °C (-200.31 °F; 144.09 K)
Solubility in water	0.021 g/100 mL
Vapor pressure	44.0 atm[1](-38.5 °F or -39.2 °C or 234.0 K)[a]
Refractive index (nD)	1.0004
Structure
Molecular shape	trigonal pyramidal
Dipole moment	0.234 D
Thermochemistry
Heat capacity (C)	53.26 J/(mol·K)
Std molar entropy (So298)	260.3 J/(mol·K)
Std enthalpy of formation (?fH?298)	-31.4 kJ/mol[2] -109 kJ/mol[3]
Gibbs free energy (ΔfG?)	-84.4 kJ/mol
Hazards
Safety data sheet	AirLiquide
NFPA 704 (fire diamond)	0 1 0 OX
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LC50 (median concentration)	2000 ppm (mouse, 4 h) 9600 ppm (dog, 1 h) 7500 ppm (monkey, 1 h) 6700 ppm (rat, 1 h) 7500 ppm (mouse, 1 h)[5]
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 10 ppm (29 mg/m3)[4]
REL (Recommended)	TWA 10 ppm (29 mg/m3)[4]
IDLH (Immediate danger)	1000 ppm[4]
Related compounds
Other anions	nitrogen trichloride nitrogen tribromide nitrogen triiodide ammonia
Other cations	phosphorus trifluoride arsenic trifluoride antimony trifluoride bismuth trifluoride
Related binary fluoro-azanes	tetrafluorohydrazine
Related compounds	dinitrogen difluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
Infobox references

Nitrogen trifluoride is the inorganic compound with the formula NF₃. This nitrogen-fluorine compound is a colorless, nonflammable gas with a slightly musty odor. It finds increasing use as an etchant in microelectronics. Nitrogen trifluoride is an extremely strong greenhouse gas.

Synthesis and reactivity

Nitrogen trifluoride is a rare example of a binary fluoride that can be prepared directly from the elements only at very uncommon conditions, such as electric discharge.^[6] 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.^[7] 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.^[8] 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).^[9] 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

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.^[10] The process utilization of the chemicals applied in plasma processes is typically below 20%. Therefore some of the PFCs and also some of the NF₃ always escape into the atmosphere. Modern gas abatement systems can decrease such emissions.

F₂ gas (diatomic fluorine) has been introduced as a climate neutral replacement for nitrogen trifluoride in the manufacture of flat-panel displays and thin-film solar cells.^[11]

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.

Greenhouse gas

Growth in atmospheric concentration of NF₃ since the 1990s is shown in right graph, along with a subset of similar man-made gases. Note the log scale.

is a greenhouse gas, with a global warming potential (GWP) 17,200 times greater than that of when compared over a 100-year period.^[12][13][14] Its GWP place it second only to in the group of Kyoto-recognised greenhouse gases, and was included in that grouping with effect from 2013 and the commencement of the second commitment period of the Kyoto Protocol. It has an estimated atmospheric lifetime of 740 years,^[12] although other work suggests a slightly shorter lifetime of 550 years (and a corresponding GWP of 16,800).^[15]

Although 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 routinely break it down, while in the past previously used regulated compounds such as 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 use .^[15][16]

Nitrogen trifluoride timeseries at various latitudes.

Since 1992, when less than 100 tons were produced, production has grown to an estimated 4000 tons in 2007 and is projected to increase significantly.^[15] World production of NF₃ is expected to reach 8000 tons a year by 2010. By far the world's largest producer of is the US industrial gas and chemical company Air Products & Chemicals. An estimated 2% of produced is released into the atmosphere.^[17][18] 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.^[19] The mean global tropospheric concentration of NF₃ has risen from about 0.02 ppt (parts per trillion, dry air mole fraction) in 1980, to 0.86 ppt in 2011, with a rate of increase of 0.095 ppt yr^-1, or about 11% per year, and an interhemispheric gradient that is consistent with emissions occurring overwhelmingly in the Northern Hemisphere, as expected. This rise rate in 2011 corresponds to about 1200 metric tons/y NF₃ emissions globally, or about 10% of the NF₃ global production estimates. 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.^[20] One study co-authored by industry representatives suggests that the contribution of the NF₃ emissions to the overall greenhouse gas budget of thin-film Si-solar cell manufacturing is clear.^[21]

The UNFCCC, within the context of the Kyoto Protocol, decided to include nitrogen trifluoride in the second Kyoto Protocol compliance period, which begins in 2012 and ends in either 2017 or 2020. Following suit, the WBCSD/WRI GHG Protocol is amending all of its standards (corporate, product and Scope 3) to also cover NF₃.^[22]

Safety

Skin contact with is not hazardous, and it is a relatively minor irritant to mucous membranes and eyes. It is a pulmonary irritant with a toxicity considerably lower than nitrogen oxides, and overexposure via inhalation causes the conversion of hemoglobin in blood to methemoglobin, which can lead to the condition methemoglobinemia.^[23] The National Institute for Occupational Safety and Health (NIOSH) specifies that the concentration that is immediately dangerous to life or health (IDLH value) is 1,000 ppm.^[24]

See also

• Nitrogen pentafluoride
• Tetrafluorohydrazine

Notes

References

External links

• National Pollutant Inventory - Fluoride and compounds fact sheet at the Wayback Machine (archived December 22, 2003)
• WebBook page for NF₃
• CDC - NIOSH Pocket Guide to Chemical Hazards