| Nitrogen trifluoride | |
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Nitrogen trifluoride |
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Other names
Nitrogen fluoride |
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| Identifiers | |
| CAS number | 7783-54-2 |
| PubChem | 24553 |
| ChemSpider | 22959 |
| UN number | 2451 |
| ChEBI | CHEBI:30231 |
| RTECS number | QX1925000 |
| Jmol-3D images | Image 1 |
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| 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 ΔfH |
-31.4 kJ/mol[1] |
| Hazards | |
| MSDS | AirLiquide |
| EU Index | Not listed |
| NFPA 704 |
0
1
0
OX
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| Flash point | Non-flammable |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| 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.
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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 thin film 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.
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.
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:
It converts to tetrafluorohydrazine upon contact with metals, but only at high temperatures:
NF3 reacts with fluorine and antimony pentafluoride to give the tetrafluoroammonium salt:
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]
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]