Nitrogen trifluoride

Nitrogen trifluoride

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Nitrogen trifluoride is the inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 NF3. This nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

-fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

 compound is a colorless, toxic, odourless, nonflammable gas. It finds increasing use as an etchant in microelectronics
Microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...

.

Applications


Nitrogen trifluoride is used in the plasma etching
Plasma etching
Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge of an appropriate gas mixture being shot at a sample. The plasma source, known as etch species, can be either charged or neutral...

 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
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...

 are the active cleaning agents that attack the polysilicon, silicon nitride
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...

 and silicon oxide
Silicon oxide
Silicon oxide may refer to either of the following:*Silicon dioxide, SiO2, very well characterized*Silicon monoxide, SiO, not very well characterized...

. Nitrogen trifluoride can be used as well with tungsten silicide, and tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

 produced by CVD
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...

. NF3 has been considered as an environmentally preferable substitute for sulfur hexafluoride
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...

 or perfluorocarbons such as hexafluoroethane
Hexafluoroethane
Hexafluoroethane is a fluorocarbon counterpart to the hydrocarbon ethane. It is a non-flammable gas negligibly soluble in water and slightly soluble in alcohol.-Physical properties:...

.
The process utilization of the chemicals applied in plasma processes
Plasma processing
Plasma processing is a plasma-based material processing technology that aims at modifying the chemical and physical properties of a surface.Plasma processing techniques include:*Plasma activation*Plasma etching*Plasma modification*Plasma functionalization...

 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
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

 has been introduced as an environmentally friendly
Green chemistry
Green chemistry, also called sustainable chemistry, is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances...

 replacement for nitrogen trifluoride in the manufacture of flat panel displays and thin film solar cells.

Nitrogen trifluoride is also used in hydrogen fluoride and deuterium fluoride lasers
Hydrogen fluoride laser
The hydrogen fluoride laser is an infrared chemical laser. It is capable of delivering continuous output power in the megawatt range.Hydrogen fluoride lasers operate at the wavelength of 2.7-2.9 µm. This wavelength is absorbed by the atmosphere, effectively attenuating the beam and reducing its...

, which are types of chemical laser
Chemical laser
A chemical laser is a laser that obtains its energy from a chemical reaction. Chemical lasers can achieve continuous wave output with power reaching to megawatt levels...

s. It is preferred to fluorine gas due to its convenient handling properties, reflecting its considerable stability.

It is compatible with steel and Monel
Monel
Monel is a trademark of Special Metals Corporation for a series of nickel alloys, primarily composed of nickel and copper, with some iron and other trace elements. Monel was created by David H. Browne, chief metallurgist for International Nickel Co...

, 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. After first attempting the synthesis in 1903, Otto Ruff
Otto Ruff
-Life:Otto Ruff was born in Schwäbisch Hall, Württemberg. After becoming an pharmacist under the supervision of Carl Magnus von Hell at the University of Stuttgart he joined the group of Hermann Emil Fischer at the University of Berlin. Fischer was noted for his work on carbohydrates and so Ruff...

 prepared nitrogen trifluoride by the electrolysis of a molten mixture of ammonium fluoride
Ammonium fluoride
Ammonium fluoride is the inorganic compound with the formula NH4F. It crystallizes as small colourless prisms, having a sharp saline taste, and is exceedingly soluble in water.-Crystal structure:...

 and hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...

. It proved to be far less reactive than the other nitrogen trihalides nitrogen trichloride
Nitrogen trichloride
Nitrogen trichloride, also known as trichloramine, is the chemical compound with the formula NCl3. This yellow, oily, pungent-smelling liquid is most commonly encountered as a byproduct of chemical reactions between ammonia-derivatives and chlorine .In pure form, NCl3 is highly reactive...

, nitrogen tribromide
Nitrogen tribromide
Nitrogen tribromide is a chemical compound with the formula NBr3. It is extremely explosive in its pure form, even at −100°C, and was not isolated until 1975...

 and nitrogen triiodide
Nitrogen triiodide
Nitrogen triiodide is the inorganic compound with the formula NI3. It is an extremely sensitive contact explosive: small quantities explode with a gunpowder-like snap when touched even lightly, releasing a purple cloud of iodine vapor...

, 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. 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
Bond dipole moment
The bond dipole moment uses the idea of electric dipole moment to measure the polarity of a chemical bond within a molecule. The bond dipole μ is given by:\mu = \delta \, d....

 of 0.2340 D. By contrast, ammonia is basic and highly polar (1.47 D). 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
Tetrafluorohydrazine
Tetrafluorohydrazine, , is a colourless, reactive inorganic gas. It is a fluorinated analog of hydrazine. It is a highly hazardous chemical. In presence of organic materials it explodes....

 upon contact with metals, but only at high temperatures:
2 NF3 + Cu → N2F4 + CuF2

NF3 reacts with fluorine and antimony pentafluoride
Antimony pentafluoride
Antimony pentafluoride is the inorganic compound with the formula SbF5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, the strongest known acid...

 to give the tetrafluoroammonium
Tetrafluoroammonium
The tetrafluoroammonium cation is a positively-charged polyatomic ion with chemical formula . It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine....

 salt:
NF3 + F2 + SbF5 → NFSbF

Greenhouse gas


is a greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

, with a global warming potential
Global warming potential
Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time...

 (GWP) 17,200 times greater than that of
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 when compared over a 100 year period. Its GWP would place it second only to
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...

 in the group of Kyoto-recognised
Kyoto Protocol
The Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change , aimed at fighting global warming...

 greenhouse gases, although is not currently included in that grouping. 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).

Although has a high GWP, for a long time its radiative forcing
Radiative forcing
In climate science, radiative forcing is generally defined as the change in net irradiance between different layers of the atmosphere. Typically, radiative forcing is quantified at the tropopause in units of watts per square meter. A positive forcing tends to warm the system, while a negative...

 in the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 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
Dram
Dram or DRAM may refer to:As a unit of measure:* Dram , an imperial unit of mass and volume* Armenian dram, a monetary unit* Dirham, a unit of currency in several Arab nationsOther uses:...

 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 .

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. 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 chemical company Air Products & Chemicals
Air Products & Chemicals
Air Products and Chemicals, Inc. is an international corporation whose principal business is selling gases and chemicals for industrial uses. Air Products' headquarters is in Allentown, Pennsylvania, in the Lehigh Valley region of Pennsylvania, in the United States. Air Products is the Lehigh...

. An estimated 2% of produced is released into the atmosphere. 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. 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.
One study suggests that the contribution of the NF3-Emissions to the overall greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 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.

Safety


Skin contact with is not hazardous, and it is a relatively minor irritant to mucous membrane
Mucous membrane
The mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs...

s and eyes. It is a pulmonary irritant with a toxicity
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

 comparable with nitrogen oxides, and overexposure via inhalation causes the conversion of hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 in blood to methemoglobin, which can lead to the condition methemoglobinemia
Methemoglobinemia
Methemoglobinemia is a disorder characterized by the presence of a higher than normal level of methemoglobin in the blood. Methemoglobin is an oxidized form of hemoglobin that has an increased affinity for oxygen, resulting in a reduced ability to release oxygen to tissues. The oxygen–hemoglobin...

.

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