Tungsten(VI) fluoride
Encyclopedia
Tungsten fluoride, also known as tungsten hexafluoride, 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,...

 of 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...

 and 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...

 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....

 WF6. This corrosive, colorless compound is a gas under standard conditions. With a density of about 13 g/L (roughly 11 times heavier than air.), WF6 is one of the heaviest known gases under standard conditions. WF6 gas is most commonly used in the production of semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 circuits
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

 and circuit boards through the process of chemical vapor deposition
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...

 – upon decomposition, molecules of WF6 leave a residue of metallic tungsten. This layer serves as low-resistive metallic "interconnects."

Properties

At ambient pressure and temperatures above 17 °C, tungsten hexafluoride is a colorless diamagnetic gas. The WF6 molecule is octahedral with the symmetry point group
Symmetry group
The symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...

 of Oh. The W–F bond distances are 183.2 pm. Between 2.3 and 17 °C, tungsten hexafluoride condenses into a pale yellow liquid having the density of 3.44 g/cm3 at 15 °C. At 2.3 °C it freezes into a white solid having a cubic crystalline structure, the lattice constant of 628 pm and calculated density 3.99 g/cm3. At −9 °C this structure transforms into an orthorhombic solid with the lattice constants of a = 960.3 pm, b = 871.3 pm, and c = 504.4 pm, and the density of 4.56 g/cm3. In this phase, the W–F distance is 181 pm, and the mean closest intermolecular contacts are 312 pm. Whereas WF6 gas is one of the heaviest gases, with the density exceeding that of the heaviest elemental gas radon
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...

 (9.73 g/L), the density of WF6 in the liquid and solid state is rather moderate.

Synthesis

Tungsten hexafluoride is commonly produced by the exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...

 reaction of 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...

 gas with 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...

 powder at a temperature between 350 and 400 °C:
W + 3 F2 → WF6

The gaseous product is condensed and separated by distillation from WOF4, a common impurity. In a variation on the direct fluorination, the metal is placed in a heated reactor, slightly pressurized to 1.2 to 2 psi (8.3 to 13.8 kPa), with a constant flow of WF6 infused with a small amount of 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...

 gas.

The fluorine gas in the above method can be substituted by ClF, ClF3 or BrF3. An alternative procedure for producing tungsten fluoride is to react tungsten trioxide
Tungsten trioxide
Tungsten oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce WO3...

 (WO3) with HF, BrF3 or SF4. Tungsten fluoride can also be obtained by conversion of tungsten hexachloride
Tungsten hexachloride
Tungsten hexachloride is the chemical compound of tungsten and chlorine with the formula WCl6. This dark violet blue species exists as a volatile solid under standard conditions. It is an important starting reagent in the preparation of tungsten compounds. WCl6 is a rare example of a...

:
WCl6 + 6 HF → WF6 + 6 HCl or
WCl6 + 2 AsF3 → WF6 + 2 AsCl3 or
WCl6 + 3 SbF5 → WF6 + 3 SbF3Cl2

Reactions

On contact with water, tungsten fluoride gives 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...

 (HF) and tungsten oxyfluorides, eventually forming tungsten trioxide
Tungsten trioxide
Tungsten oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce WO3...

:
WF6 + 3 H2O → WO3 + 6 HF


Unlike some other metal fluorides, WF6 is not a useful fluorinating agent nor is it a powerful oxidant. It can be reduced to the yellow tetramer WF4.

Applications in semiconductor industry

The dominant application of tungsten fluoride is in semiconductor industry, where it is widely used for depositing tungsten metal in a chemical vapor deposition
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...

 process. The expansion of the industry in the 1980s and 1990s resulted in the increase of WF6 consumption, which remains at around 200 tonnes per year worldwide. Tungsten metal is attractive because of its relatively high thermal and chemical stability, as well as low resistivity (5.6 µΩ·cm) and electromigration
Electromigration
Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in...

. Whereas WF6 is favored over related compounds, such as WCl6 or WBr6, because of its higher vapor pressure resulting in higher deposition rates. Since 1967, two WF6 deposition routes have been developed employed, thermal decomposition and hydrogen reduction. The required WF6 gas purity is rather high and varies between 99.98% and 99.9995% depending on the application.

WF6 molecules have to be split up in the CVD process. The decomposition is usually facilitated by mixing WF6 with hydrogen, silane
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...

, germane
Germane
Germane is the chemical compound with the formula GeH4, and the germanium analogue of methane. It is the simplest germanium hydride and one of the most useful compounds of germanium. Like the related compounds silane and methane, germane is tetrahedral. It burns in air to produce GeO2 and...

, diborane
Diborane
Diborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...

, phosphine
Phosphine
Phosphine is the compound with the chemical formula PH3. It is a colorless, flammable, toxic gas. Pure phosphine is odourless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphine...

, and related hydrogen-containing gases.

Silicon

WF6 reacts upon contact with silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 substrate. The WF6 decomposition on silicon is temperature-dependent:
2 WF6 + 3 Si → 2 W + 3 SiF4    below 400 °C and
WF6 + 3 Si2 → W + 3 SiF2     above 400 °C.


This dependence is crucial as twice more silicon is being consumed at higher temperatures. The deposition occurs selectively on pure Si only, but not on silicon oxide or nitride, thus the reaction is highly sensitive to the contamination or substrate pre-treatment. The decomposition reaction is fast, but saturates when the tungsten layer thickness reaches 10–15 micrometers. The saturation occurs because the tungsten layer stops diffusion of WF6 molecules to the Si substrate which is the only catalyst of molecular decomposition in this process.

If the deposition occurs not in inert but in oxygen containing atmosphere (air) then instead of tungsten, a tungsten oxide layer is produced.

Hydrogen

The deposition process occurs at temperatures between 300 and 800 °C and results in formation of hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....

 vapors:
WF6 + 3 H2 → W + 6 HF


The crystallinity of the produced tungsten layers can be controlled by altering the WF6/H2 ratio and the substrate temperature: low ratios and temperatures result in (100) oriented
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....

 tungsten crystallites whereas higher values favor the (111) orientation. Formation of HF is a drawback as the HF vapor is very aggressive and etches away most materials. Also, the deposited tungsten shows poor adhesion to the silicon dioxide which the main passivation material in semiconductor electronics. Therefore, SiO2 has to be covered with an extra buffer layer prior to the tungsten deposition. On the other hand, etching by HF may be beneficial to remove unwanted impurity layers.

Silane and germane

The characteristic features of tungsten deposition from the WF6/SiH4 are high speed, good adhesion and layer smoothness. The drawbacks are explosion hazard and high sensitivity of the deposition rate and morphology to the process parameters, such as mixing ratio, substrate temperature, etc. Therefore, silane is commonly used to create a thin tungsten nucleation layer. It is then switched to hydrogen, that slows down the deposition and cleans up the layer.

Deposition from WF6/GeH4 mixture is similar to that of WF6/SiH4, but the tungsten layer becomes contaminated with relatively (compared to Si) heavy germanium up to concentrations of 10–15 %. This increases tungsten resistance from about 5 to 200 µΩ·cm.

Other applications

WF6 can be used for the production of tungsten carbide
Tungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...

.

As a heavy gas, WF6 can be used as a buffer to control gas reactions. For example, it slows down the chemistry of the Ar/O2/H2 flame and reduces the flame temperature.

Safety

Tungsten hexafluoride is an extremely corrosive compound that attacks any tissue. Exposure of humans to the gas first affects the eyes and respiratory tracts causing irritation, loss of vision, cough and excessive formation of saliva
Saliva
Saliva , referred to in various contexts as spit, spittle, drivel, drool, or slobber, is the watery substance produced in the mouths of humans and most other animals. Saliva is a component of oral fluid. In mammals, saliva is produced in and secreted from the three pairs of major salivary glands,...

 and sputum
Sputum
Sputum is mucus that is coughed up from the lower airways. It is usually used for microbiological investigations of respiratory infections....

. Upon mixing with the body fluids, the gas transforms into hydrofluoric acid which burns the skin and mucous tissues of the respiratory tract. Upon prolonged exposure, this results in pneumonitis
Pneumonitis
Pneumonitis or pulmonitis is a general term that refers to inflammation of lung tissue.Pneumonia is pneumonitis combined with consolidation and exudation...

 and pulmonary edema
Pulmonary edema
Pulmonary edema , or oedema , is fluid accumulation in the air spaces and parenchyma of the lungs. It leads to impaired gas exchange and may cause respiratory failure...

 and could be fatal. Because of the formation of hydrofluoric acid upon reaction of WF6 with humidity, WF6 storage vessels have Teflon gaskets.

Related compounds

The high symmetry of the WF6 molecule is seen in most related compounds. However, it is interesting to note that tungsten hexahydride (WH6) and hexamethyltungsten (W(CH3)6) adopt a trigonal prismatic structure. Numerous hexafluoride
Hexafluoride
A hexafluoride is a chemical compound with the general formula XF6. Sixteen elements are known to form stable hexafluorides. Nine of these elements are transition metals, three are actinides, and four are nonmetals or metalloids.- Physical properties :...

s are known with other metals and metalloids. Such MF6 compounds characteristically form dense gases, however, when element M is heavier than tungsten, the compound is either liquid or solid at room temperature.

Trivia

In his book Uncle Tungsten
Uncle Tungsten: Memories of a Chemical Boyhood
Uncle Tungsten: Memories of a Chemical Boyhood is a memoir by Oliver Sacks about his childhood published in 2001. The book is named for Sacks's Uncle Dave, owner of a business named Tungstalite, which made incandescent lightbulbs with a tungsten filament, whom Oliver nicknamed Uncle Tungsten. Uncle...

, Oliver Sacks
Oliver Sacks
Oliver Wolf Sacks, CBE , is a British neurologist and psychologist residing in New York City. He is a professor of neurology and psychiatry at Columbia University, where he also holds the position of Columbia Artist...

describes how his uncle—an avid fan of tungsten and tungsten chemistry—told him about the very high-density of gaseous WF6. Later, for his 65th birthday, Sacks wanted to have a WF6-filled balloon, but gave up, knowing that WF6 produces hydrofluoric acid on contact with atmospheric humidity. (Moreover, WF6 would react with most organic materials, including rubber, but not Teflon.)
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK