Nitrogen trifluoride is the inorganic compound with the formula (NF3). It is a colorless, non-flammable, toxic gas with a slightly musty odor. In contrast with ammonia, it is nonbasic. It finds increasing use within the manufacturing of flat-panel displays, photovoltaics, LEDs and other microelectronics. 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.
Synthesis and reactivity Nitrogen trifluoride can be prepared from the elements in the presence of an electric discharge. In 1903, Otto Ruff prepared nitrogen trifluoride by the electrolysis of a molten mixture of ammonium fluoride and hydrogen fluoride. It is 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. It is prepared in modern times both by direct reaction of ammonia and fluorine and by a variation of Ruff's method. 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). This contrast reflects the differing electronegativities of H vs F. Similar to dioxygen, NF3 is a potent yet sluggish oxidizer. It oxidizes hydrogen chloride to chlorine:
2 NF3 + 6 HCl → 6 HF + N2 + 3 Cl2 However, it only attacks (explosively) organic compounds at high temperatures. Consequently it is compatible under standard conditions with several plastics, as well as steel and Monel. Above 200-300 °C, NF3 reacts with metals, carbon, and other reagents to give tetrafluorohydrazine:
2NF3 + Cu → N2F4 + CuF2 NF3 reacts with fluorine and antimony pentafluoride to give the tetrafluoroammonium salt:
NF3 + F2 + SbF5 → NF+4SbF−6 NF3 and B2H6 react vigorously even at cryogenic temperatures to give nitrogen gas, boron trifluoride, and hydrofluoric acid.
Applications 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 NF3.
Etching
Nitrogen trifluoride is primarily used to remove silicon and silicon-compounds during the manufacturing of semiconductor devices such as LCDs, some thin-film solar cells, and other microelectronics. In these applications NF3 is initially broken down within a plasma. The resulting fluorine radicals are the active agents that attack polysilicon, silicon nitride and silicon oxide. They can be used as well to remove tungsten silicide, tungsten, and certain other metals. In addition to serving as an etchant in device fabrication, NF3 is also widely used to clean PECVD chambers. NF3 dissociates more readily within a low-pressure discharge in comparison to perfluorinated compounds (PFCs) and sulfur hexafluoride (SF6). The greater abundance of negatively-charged free radicals thus generated can yield higher silicon removal rates, and provide other process benefits such as less residual contamination and a lower net charge stress on the device being fabricated. As a somewhat more thoroughly consumed etching and cleaning agent, NF3 has also been promoted as an environmentally preferable substitute for SF6 or PFCs such as hexafluoroethane. The utilization efficiency of the chemicals applied in plasma processes varies widely between equipment and applications. A sizeable fraction of the reactants are wasted into the exhaust stream and can ultimately be emitted into Earth's atmosphere. Modern abatement systems can substantially decrease atmospheric emissions. NF3 has not been subject to significant use restrictions. The annual reporting of NF3 production, consumption, and waste emissions by large manufacturers has been required in many industrialized countries as a response to the observed atmospheric growth and the international Kyoto Protocol. Highly toxic fluorine gas (F2, diatomic fluorine) is a climate neutral replacement for nitrogen trifluoride in some manufacturing applications. It requires more stringent handling and safety precautions, especially to protect manufacturing personnel. Nitrogen trifluoride is also used in hydrogen fluoride and deuterium fluoride lasers, which are types of chemical lasers. There it is also preferred to fluorine gas due to its more convenient handling properties
Greenhouse gas
The GWP of NF3 is second only to SF6 in the group of Kyoto-recognised greenhouse gases, and NF3 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, although other work suggests a slightly shorter lifetime of 550 years (and a corresponding GWP of 16,800).
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