| Revision as of 16:06, 18 July 2011 editLavateraguy (talk | contribs)Autopatrolled, Extended confirmed users11,600 editsm →Applications in semiconductor industry: mot juste← Previous edit |
Latest revision as of 04:35, 7 January 2025 edit undoParagon64552 (talk | contribs)52 edits →Silicon: consistent usage of silicon instead of Si, and fix grammar |
| (124 intermediate revisions by 75 users not shown) |
| Line 1: |
Line 1: |
|
{{chembox |
|
{{chembox |
|
|
| Watchedfields = changed |
|
| verifiedrevid = 405951132 |
|
|
|
| verifiedrevid = 440139630 |
|
| ImageFileL1 = Wolframhexafluorid.svg |
|
|
| ImageSizeL1 = 100px |
|
| Name = |
|
|
| ImageFile = Tungsten hexafluoride liquid.png |
|
|
| ImageSize = |
|
|
| ImageCaption = Solid WF<sub>6</sub> melting into liquid WF<sub>6</sub> |
|
|
| ImageFileL1 = Tungsten-hexafluoride-3D-balls.png |
|
| ImageNameL1 = Tungsten(VI) fluoride |
|
| ImageNameL1 = Tungsten(VI) fluoride |
|
| ImageFileR1 = Tungsten-hexafluoride-3D-balls.png |
|
| ImageFileR1 = Wolframhexafluorid.svg |
|
| ImageSizeR1 = 100px |
|
|
| ImageNameR1 = Ball-and-stick model of tungsten hexafluoride |
|
| ImageNameR1 = Ball-and-stick model of tungsten hexafluoride |
|
| IUPACName = Tungsten hexafluoride<br/>Tungsten(VI) fluoride |
|
| IUPACName = Tungsten hexafluoride<br/>Tungsten(VI) fluoride |
|
| OtherNames = |
|
| OtherNames = |
|
|
| SystematicName = |
|
| Section1 = {{Chembox Identifiers |
|
| Section1 = {{Chembox Identifiers |
|
|
| CASNo_Ref = {{cascite|correct|CAS}} |
|
| CASNo = 7783-82-6 |
|
| CASNo = 7783-82-6 |
|
|
| EC_number = 232-029-1 |
|
| CASNo_Ref = {{cascite}} |
|
|
| PubChem = 522684 |
|
| UNNumber = 2196 |
|
|
| UNII_Ref = {{fdacite|correct|FDA}} |
|
|
| UNII = 45B0AG2C4S |
|
|
| PubChem = 522684 |
|
|
| InChI = 1S/6FH.W/h6*1H;/q;;;;;;+6/p-6 |
|
|
| SMILES = F(F)(F)(F)(F)F |
|
}} |
|
}} |
|
| Section2 = {{Chembox Properties |
|
| Section2 = {{Chembox Properties |
|
| Formula = WF<sub>6</sub> |
|
| Formula = {{chem2|WF6}} |
|
| MolarMass = 297.83 g/mol |
|
| MolarMass = 297.830 g/mol |
|
| Appearance = Colorless gas |
|
| Appearance = Colorless gas |
|
| Density = 13.0 g/L, gas |
|
| Density = 12.4 g/L (gas) <br> 4.56 g/cm<sup>3</sup> (−9 °C, solid) |
|
| Solubility = Hydrolyzes |
|
| Solubility = Hydrolyzes |
|
| MeltingPt = 2.3 °C (275.45 K) |
|
| MeltingPtC = 2.3 |
|
| BoilingPt = 17.1 °C (290.25 K) |
|
| BoilingPtC = 17.1 |
|
|
| MagSus = −40.0·10<sup>−6</sup> cm<sup>3</sup>/mol |
|
}} |
|
}} |
|
| Section3 = {{Chembox Structure |
|
| Section3 = {{Chembox Structure |
| Line 27: |
Line 38: |
|
| Dipole = zero |
|
| Dipole = zero |
|
}} |
|
}} |
|
|
| Section4 = |
|
|
| Section5 = |
|
|
| Section6 = |
|
| Section7 = {{Chembox Hazards |
|
| Section7 = {{Chembox Hazards |
|
| MainHazards = |
|
| MainHazards = Toxic, corrosive; gives HF on contact with water |
|
| EUIndex = Not listed |
|
|
| FlashPt = Non-flammable |
|
| FlashPt = Non-flammable |
|
|
| GHS_ref=<ref>{{cite web |title=Tungsten hexafluoride |url=https://pubchem.ncbi.nlm.nih.gov/compound/522684#section=Safety-and-Hazards |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref> |
|
|
| GHSPictograms = {{GHS05}}{{GHS06}} |
|
|
| GHSSignalWord = Danger |
|
|
| HPhrases = {{H-phrases|301+311|314|330}} |
|
|
| PPhrases = {{P-phrases|260|264|264+265|270|271|280|284|301+316|301+330+331|302+352|302+361+354|304+340|305+354+338|316|317|320|321|330|361+364|363|403+233|405|410+403|501}} |
|
|
| NFPA-H = 3 |
|
|
| NFPA-F = 0 |
|
|
| NFPA-I = 2 |
|
|
| NFPA-S = W |
|
|
| ExternalSDS = |
|
}} |
|
}} |
|
| Section8 = {{Chembox Related |
|
| Section8 = {{Chembox Related |
|
| OtherAnions = ]<br/>] |
|
| OtherAnions = ]<br/>] |
|
| OtherCations = ]<br/>] |
|
| OtherCations = ]<br/>] |
|
| OtherCpds = ]<br/>] |
|
| OtherCompounds = ]<br/>] |
|
}} |
|
}} |
|
}} |
|
}} |
|
|
|
|
|
'''Tungsten(VI) fluoride''', also known as '''tungsten hexafluoride''', is the ] of ] and ] with the ] WF<sub>6</sub>. This corrosive, colorless compound is a gas under ]. With a density of about 13 g/L (roughly 11 times heavier than air.<ref>{{cite book|url=http://books.google.com/?id=BpPmFsA4yn4C&pg=PT138|page=138|title=Proprietes Physiques des Composes Mineraux|author=Jean-Pierre Roucan, Marie-Christine Noel-Dutriax|publisher=Ed. Techniques Ingénieur}}</ref><ref></ref><ref></ref>), WF<sub>6</sub> is one of the heaviest known gases under standard conditions.<ref name=b1/> WF<sub>6</sub> gas is most commonly used in the production of ] ] and ] through the process of ] – upon decomposition, molecules of WF<sub>6</sub> leave a residue of metallic tungsten. This layer serves as low-resistive metallic "interconnects."<ref>.</ref> |
|
'''Tungsten(VI) fluoride''', also known as '''tungsten hexafluoride''', is an ] with the ] {{chem2|WF6|auto=1}}. It is a toxic, corrosive, colorless gas, with a density of about {{convert|13|kg/m3|abbr=on}} (roughly 11 times heavier than air).<ref>{{cite book |url= https://books.google.com/books?id=BpPmFsA4yn4C&pg=PA138 |page= 138 |title= Proprietes Physiques des Composes Mineraux |author1=Roucan, J.-P. |author2=Noël-Dutriaux, M.-C. |publisher= Ed. Techniques Ingénieur}}</ref><ref> (Wayback Machine archive 7 September 2022)</ref> It is the only known gaseous ] (or d-block) compound and the densest known gas under ] (298 K, 1 atm).<ref name=b1/> {{chem2|WF6}} is commonly used by the ] to form tungsten films, through the process of ]. This layer is used in a low-] metallic "]".<ref>{{cite web |url= http://www.timedomaincvd.com/CVD_Fundamentals/films/W_WSi.html |title= Tungsten and Tungsten Silicide Chemical Vapor Deposition |work= CVD Fundamentals |publisher= TimeDomain CVD}}</ref> It is one of seventeen known binary ]s. |
|
|
|
|
|
== Properties == |
|
==Properties== |
|
At ambient pressure and temperatures above 17 °C, tungsten hexafluoride is a colorless<ref name=b1/> ] gas. The WF<sub>6</sub> molecule is octahedral with the ] of O<sub>h</sub>. The W–F bond distances are 183.2 pm.<ref name=CRC>{{RubberBible86th}} p. 4-93.</ref> Between 2.3 and 17 °C, tungsten hexafluoride condenses into a pale yellow liquid having the density of 3.44 g/cm<sup>3</sup> 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/cm<sup>3</sup>. At −9 °C this structure transforms into an ] 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/cm<sup>3</sup>. In this phase, the W–F distance is 181 pm, and the mean closest intermolecular contacts are 312 pm. Whereas WF<sub>6</sub> gas is one of the heaviest gases, with the density exceeding that of the heaviest elemental gas ] (9.73 g/L), the density of WF<sub>6</sub> in the liquid and solid state is rather moderate.<ref>{{cite journal|doi=10.1016/0022-4596(75)90292-3|title=The structures of fluorides XIII: The orthorhombic form of tungsten hexafluoride at 193 K by neutron diffraction|year=1975|last1=Levy|first1=J|journal=Journal of Solid State Chemistry|volume=15|pages=360}}</ref> |
|
The {{chem2|WF6}} molecule is octahedral with the ] of O<sub>h</sub>. The W–F bond distances are {{val|183.2|u=pm}}.<ref name=CRC>{{RubberBible86th}} p. 4-93.</ref> Between {{val|2.3|and|17|u=degC}}, tungsten hexafluoride condenses into a colorless liquid having the density of {{val|3.44|u=g/cm3}} at {{val|15|u=degC}}.<ref name="syn" /> At {{val|2.3|u=degC}} it freezes into a white solid having a cubic crystalline structure, the lattice constant of 628 pm and calculated density {{val|3.99|u=g/cm3}}. At {{val|−9|u=degC}} this structure transforms into an ] solid with the lattice constants of ''a'' = 960.3 pm, ''b'' = 871.3 pm, and ''c'' = 504.4 pm, and the density of {{val|4.56|u=g/cm3}}. In this phase, the W–F distance is 181 pm, and the mean closest molecular contacts are {{val|312|u=pm}}. Whereas {{chem2|WF6}} gas is one of the densest gases, with the density exceeding that of the heaviest elemental gas ] (9.73 g/L), the density of {{chem2|WF6}} in the liquid and solid state is rather moderate.<ref>{{cite journal |last1= Levy |first1= J. |title= The structures of fluorides XIII: The orthorhombic form of tungsten hexafluoride at 193 K by neutron diffraction |journal= Journal of Solid State Chemistry |year= 1975 |volume= 15 |issue= 4 |pages= 360–365 |doi= 10.1016/0022-4596(75)90292-3|bibcode= 1975JSSCh..15..360L }}</ref> |
|
|
The vapor pressure of {{chem2|WF6}} between {{val|−70|and|17|u=degC}} ] by the equation |
|
|
:{{math|1=] ''P'' = 4.55569 − {{sfrac|1021.208 | ''T'' + 208.45}}}}, |
|
|
where the ''P'' = vapor pressure (]), ''T'' = temperature (°C).<ref>Cady, G.H.; Hargreaves, G.B, "Vapour Pressures of Some Fluorides And Oxyfluorides of Molybdenum, Tungsten, Rhenium, and Osmium," Journal of the Chemical Society, APR 1961, pp. 1568-& DOI: 10.1039/jr9610001568</ref><ref>{{Cite journal|url=http://webbook.nist.gov/cgi/cbook.cgi?ID=C7783826&Mask=4&Type=ANTOINE&Plot=on|doi = 10.1021/ie50448a022|title = Vapor Pressure of Pure Substances. Organic and Inorganic Compounds|year = 1947|last1 = Stull|first1 = Daniel R.|journal = Industrial & Engineering Chemistry|volume = 39|issue = 4|pages = 517–540}}</ref> |
|
|
|
|
|
|
==History and synthesis== |
|
== Synthesis == |
|
|
|
Tungsten hexafluoride was first obtained by conversion of ] with ] by ] and Fritz Eisner in 1905.<ref>{{Cite Q|Q56639371}}</ref><ref>{{Cite journal |last=Ruff |first=Otto |author-link=Otto Ruff |last2=Eisner |first2=Fritz |last3=Heller |first3=Wilhelm |date=1907 |title=Über die Darstellung und Eigenschaften von Fluoriden des sechswertigen Wolframs |url=https://onlinelibrary.wiley.com/doi/10.1002/zaac.19070520122 |journal=Zeitschrift für anorganische Chemie |language=de |volume=52 |issue=1 |pages=256–269 |doi=10.1002/zaac.19070520122 |issn=1521-3749}}</ref> |
|
Tungsten hexafluoride is commonly produced by the ] reaction of ] gas with ] powder at a temperature between 350 and 400 °C:<ref>{{cite journal|author = Homer F. Priest|title = Anhydrous Metal Fluorides|journal = ]|volume = 3|pages = 171–183|isbn = 978-0-47013162-6|year =1950|publisher = Wiley-Interscience|doi = 10.1002/9780470132340.ch47|first2 = Carl F.|last2 = Swinehert}}</ref> |
|
|
:W + 3 F<sub>2</sub> → WF<sub>6</sub> |
|
|
The gaseous product is condensed and separated by distillation from WOF<sub>4</sub>, a common impurity. In a variation on the direct fluorination, the metal is placed in a heated reactor, slightly pressurized to {{convert|1.2|to|2.0|psi|kPa|abbr=on}}, with a constant flow of WF<sub>6</sub> infused with a small amount of ] gas.<ref name="synth1">{{US Patent|6544889}} "Method for tungsten chemical vapor deposition on a semiconductor substrate" Issued on April 8, 2003</ref> |
|
|
|
|
|
|
|
:{{chem2|WCl6 + 6 HF → WF6 + 6 HCl}} |
|
The fluorine gas in the above method can be substituted by ClF, ClF<sub>3</sub> or BrF<sub>3</sub>. An alternative procedure for producing tungsten fluoride is to react ] (WO<sub>3</sub>) with HF, BrF<sub>3</sub> or SF<sub>4</sub>. Tungsten fluoride can also be obtained by conversion of ]:<ref name=b1>{{cite book|url=http://books.google.com/?id=foLRISkt9gcC&pg=PA168#v=onepage&q=|pages=111, 168|title=Tungsten: properties, chemistry, technology of the element, alloys, and chemical compounds|author=Erik Lassner, Wolf-Dieter Schubert|publisher=Springer|year=1999|isbn=0-30645053-4}}</ref> |
|
|
:WCl<sub>6</sub> + 6 HF → WF<sub>6</sub> + 6 HCl or |
|
|
:WCl<sub>6</sub> + 2 AsF<sub>3</sub> → WF<sub>6</sub> + 2 AsCl<sub>3</sub> or |
|
|
:WCl<sub>6</sub> + 3 SbF<sub>5</sub> → WF<sub>6</sub> + 3 SbF<sub>3</sub>Cl<sub>2</sub> |
|
|
|
|
|
|
|
The compound is now commonly produced by the ] reaction of ] gas with ] powder at a temperature between {{val|350 |and|400|u=degC}}:<ref name="syn">{{cite book |author1=Priest, H. F. |author2=Swinehert, C. F. |editor= Audrieth, L. F. |chapter= Anhydrous Metal Fluorides |title= Inorganic Syntheses |volume= 3 |pages= 171–183 |isbn= 978-0-470-13162-6 |year= 1950 |publisher= Wiley-Interscience |doi= 10.1002/9780470132340.ch47}}</ref> |
|
== Reactions ==<!--needs an expert on the CVD part--> |
|
|
|
:{{chem2|W + 3 F2 → WF6}} |
|
On contact with water, tungsten fluoride gives ] (HF) and tungsten oxyfluorides, eventually forming ]:<ref name=b1/> |
|
|
|
The gaseous product is separated from ], a common impurity, by distillation. In a variation on the direct fluorination, the metal is placed in a heated reactor, slightly pressurized to {{convert|1.2|to|2.0|psi|kPa|abbr=on}}, with a constant flow of {{chem2|WF6}} infused with a small amount of ] gas.<ref name="synth1">{{cite patent |country= US |status= patent |number= 6544889 |title= Method for tungsten chemical vapor deposition on a semiconductor substrate |gdate= 2003-04-08}}</ref> |
|
:WF<sub>6</sub> + 3 H<sub>2</sub>O → WO<sub>3</sub> + 6 HF |
|
|
|
|
|
|
|
The fluorine gas in the above method can be substituted by ], ] or ]. An alternative procedure for producing tungsten fluoride is to treat ] ({{chem2|WO3}}) with ], {{chem2|BrF3}} or ]. And besides HF, other fluorinating agents can also be used to convert tungsten hexachloride in a way similar to Ruff and Eisner original method:<ref name=b1>{{cite book |url= https://books.google.com/books?id=foLRISkt9gcC |pages= 111, 168 |title= Tungsten - Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds |author1=Lassner, E. |author2=Schubert, W.-D. |publisher= Springer |year= 1999 |isbn= 0-306-45053-4}}</ref> |
|
Unlike some other metal fluorides, WF<sub>6</sub> is not a useful fluorinating agent nor is it a powerful oxidant. It can be reduced to the yellow tetramer WF<sub>4</sub>.<ref>Greenwood, N. N.; & Earnshaw, A. (1997). ''Chemistry of the Elements'' (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.</ref> |
|
|
|
:{{chem2|WCl6 + 2 AsF3 → WF6 + 2 AsCl3}} or |
|
|
:{{chem2|WCl6 + 3 SbF5 → WF6 + 3 SbF3Cl2}} |
|
|
|
|
|
|
==Reactions==<!--needs an expert on the CVD part--> |
|
== Applications in semiconductor industry == |
|
|
|
On contact with ], tungsten hexafluoride gives ] (HF) and tungsten oxyfluorides, eventually forming ]:<ref name=b1/> |
|
The dominant application of tungsten fluoride is in semiconductor industry, where it is widely used for depositing tungsten metal in a ] process. The expansion of the industry in the 1980s and 1990s resulted in the increase of WF<sub>6</sub> 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 ]. Whereas WF<sub>6</sub> is favored over related compounds, such as WCl<sub>6</sub> or WBr<sub>6</sub>, because of its higher vapor pressure resulting in higher deposition rates. Since 1967, two WF<sub>6</sub> deposition routes have been developed employed, thermal decomposition and hydrogen reduction.<ref name=Aigueperse>{{Cite book|first = Jean|last = Aigueperse|coauthors = Paul Mollard, Didier Devilliers, Marius Chemla, Robert Faron, Renée Romano, Jean Pierre Cuer|contribution = Fluorine Compounds, Inorganic|editor-last = Ullmann|title = Encyclopedia of Industrial Chemistry|year = 2005|publisher = Wiley-VCH|place = Weinheim}}</ref> The required WF<sub>6</sub> gas purity is rather high and varies between 99.98% and 99.9995% depending on the application.<ref name=b1/> |
|
|
|
:{{chem2|WF6 + 3 H2O → WO3 + 6 HF}} |
|
|
|
|
WF<sub>6</sub> molecules have to be split up in the CVD process. The decomposition is usually facilitated by mixing WF<sub>6</sub> with hydrogen, ], ], ], ], and related hydrogen-containing gases. |
|
|
|
|
|
|
|
Unlike some other metal fluorides, {{chem2|WF6}} is not a useful fluorinating agent nor is it a powerful oxidant. It can be reduced to the yellow {{chem2|WF4}}.<ref>{{cite book |author1=Greenwood, N. N. |author2=Earnshaw, A. |year= 1997 |title= Chemistry of the Elements |edition= 2nd |location= Oxford |publisher= Butterworth-Heinemann |isbn= 0-7506-3365-4}}</ref> |
|
=== Silicon === |
|
|
WF<sub>6</sub> reacts upon contact with ] substrate.<ref name=b1/> The WF<sub>6</sub> decomposition on silicon is temperature-dependent: |
|
|
:2 WF<sub>6</sub> + 3 Si → 2 W + 3 SiF<sub>4</sub> below 400 °C and |
|
|
:WF<sub>6</sub> + 3 Si<sub>2</sub> → W + 3 SiF<sub>2</sub> above 400 °C. |
|
|
|
|
|
|
|
{{chem2|WF6}} forms a variety of 1:1 and 1:2 ]s with ]s, examples being {{chem2|WF6(]), WF6(S(CH3)2)2, WF6(]), and WF6(])2}}.<ref>{{cite journal |doi=10.1039/C2CS35263J |title=Medium and high oxidation state metal/Non-metal fluoride and oxide–fluoride complexes with neutral donor ligands |year=2013 |last1=Benjamin |first1=Sophie L. |last2=Levason |first2=William |last3=Reid |first3=Gillian |journal=Chem. Soc. Rev. |volume=42 |issue=4 |pages=1460–1499 |pmid=23014811 }}</ref> |
|
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 ]. The saturation occurs because the tungsten layer stops diffusion of WF<sub>6</sub> molecules to the Si substrate which is the only catalyst of molecular decomposition in this process.<ref name=b1/> |
|
|
|
|
|
|
|
==Applications in semiconductor industry== |
|
If the deposition occurs not in inert but in oxygen containing atmosphere (air) then instead of tungsten, a tungsten oxide layer is produced.<ref name="chemvap">{{cite journal|doi=10.1002/(SICI)1099-0739(199803)12:3<155::AID-AOC688>3.0.CO;2-Z|title=Chemical vapor deposition of tungsten oxide|author=Rein U. Kirss and Lamartine Meda|journal=Applied Organometallic Chemistry|volume=12|issue=3|pages=155|year=1998}}</ref> |
|
|
|
The dominant application of tungsten fluoride is in semiconductor industry, where it is widely used for depositing tungsten metal in a ] (CVD) process. The expansion of the industry in the 1980s and 1990s resulted in the increase of {{chem2|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 very low ]. {{chem2|WF6}} is favored over related compounds, such as {{chem2|WCl6}} or {{chem2|WBr6}}, because of its higher vapor pressure resulting in higher deposition rates. Since 1967, two {{chem2|WF6}} deposition routes have been developed and employed, thermal decomposition and hydrogen reduction.<ref name=Aigueperse>{{cite encyclopedia |author1=Aigueperse, J. |author2=Mollard, P. |author3=Devilliers, D. |author4=Chemla, M. |author5=Faron, R. |author6=Romano, R. |author7=Cuer, J.-P. |title= Fluorine Compounds, Inorganic |editor-last= Ullmann |encyclopedia= Encyclopedia of Industrial Chemistry |year= 2005 |publisher= Wiley-VCH |location= Weinheim}}</ref> The required {{chem2|WF6}} gas purity is rather high and varies between 99.98% and 99.9995% depending on the application.<ref name=b1/> |
|
|
|
|
|
|
{{chem2|WF6}} molecules have to be split up in the CVD process. The decomposition is usually facilitated by mixing {{chem2|WF6}} with hydrogen, ], ], ], ], and related hydrogen-containing gases. |
|
=== Hydrogen === |
|
|
The deposition process occurs at temperatures between 300 and 800 °C and results in formation of ] vapors: |
|
|
:WF<sub>6</sub> + 3 H<sub>2</sub> → W + 6 HF |
|
|
|
|
|
|
|
===Silicon=== |
|
The crystallinity of the produced tungsten layers can be controlled by altering the WF<sub>6</sub>/H<sub>2</sub> ratio and the substrate temperature: low ratios and temperatures result in ] 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, SiO<sub>2</sub> 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.<ref name=b1/> |
|
|
|
{{chem2|WF6}} reacts upon contact with a ] substrate.<ref name=b1/> The {{chem2|WF6}} decomposition on silicon is temperature-dependent: |
|
|
:{{chem2|2 WF6 + 3 Si → 2 W + 3 SiF4}} below 400 °C and |
|
|
:{{chem2|WF6 + 3 Si → W + 3 SiF2}} above 400 °C. |
|
|
|
|
|
|
This dependence is crucial, as twice as much silicon is being consumed at higher temperatures. The deposition occurs selectively on pure silicon only, but not on silicon oxide or silicon nitride, thus the reaction is highly sensitive to contamination or substrate pre-treatment. The decomposition reaction is fast, but saturates when the tungsten layer thickness reaches 10–15 ]. The saturation occurs because the tungsten layer stops diffusion of {{chem2|WF6}} molecules to the Si substrate which is the only catalyst of molecular decomposition in this process.<ref name=b1/> |
|
=== Silane and germane === |
|
|
The characteristic features of tungsten deposition from the WF<sub>6</sub>/SiH<sub>4</sub> 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.<ref name=b1/> |
|
|
|
|
|
|
|
If the deposition occurs not in an inert atmosphere but in an oxygen-containing atmosphere (air), then instead of tungsten, a tungsten oxide layer is produced.<ref name="chemvap">{{cite journal |author1=Kirss, R. U. |author2=Meda, L. |title= Chemical vapor deposition of tungsten oxide |journal= Applied Organometallic Chemistry |year= 1998 |volume= 12 |issue= 3 |pages= 155–160 |doi= 10.1002/(SICI)1099-0739(199803)12:3<155::AID-AOC688>3.0.CO;2-Z|hdl=2027.42/38321 |url= https://deepblue.lib.umich.edu/bitstream/2027.42/38321/1/688_ftp.pdf |hdl-access= free }}</ref> |
|
Deposition from WF<sub>6</sub>/GeH<sub>4</sub> mixture is similar to that of WF<sub>6</sub>/SiH<sub>4</sub>, 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.<ref name=b1/> |
|
|
|
|
|
|
|
===Hydrogen=== |
|
== Other applications == |
|
|
|
The deposition process occurs at temperatures between 300 and 800 °C and results in formation of ] vapors: |
|
WF<sub>6</sub> can be used for the production of ]. |
|
|
|
:{{chem2|WF6 + 3 H2 → W + 6 HF}} |
|
|
|
|
|
|
The crystallinity of the produced tungsten layers can be controlled by altering the {{chem2|WF6}}/{{chem2|H2}} ratio and the substrate temperature: low ratios and temperatures result in ] 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 is the main passivation material in semiconductor electronics. Therefore, {{chem2|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.<ref name=b1/> |
|
As a heavy gas, WF<sub>6</sub> can be used as a buffer to control gas reactions. For example, it slows down the chemistry of the Ar/O<sub>2</sub>/H<sub>2</sub> flame and reduces the flame temperature.<ref>{{cite book|url=http://books.google.com/?id=0B5HI9TNmakC&pg=PT64|page=64|title=Semi-conducting metal oxide nanoparticles from a low-pressure premixed H2/O2/Ar flame: Synthesis and Characterization|publisher=Cuvillier Verlag|isbn=3-86727816-4}}</ref> |
|
|
|
|
|
|
== Safety == |
|
===Silane and germane=== |
|
|
The characteristic features of tungsten deposition from the {{chem2|WF6}}/{{chem2|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.<ref name=b1/> |
|
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 ] and ]. 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 ] and ] and could be fatal. Because of the formation of hydrofluoric acid upon reaction of WF<sub>6</sub> with humidity, WF<sub>6</sub> storage vessels have ] gaskets.<ref>, Linde Gas</ref> |
|
|
|
|
|
|
|
Deposition from {{chem2|WF6}}/{{chem2|GeH4}} mixture is similar to that of {{chem2|WF6}}/{{chem2|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.<ref name=b1/> |
|
== Related compounds == |
|
|
{{seealso|Hexafluoride}} |
|
|
The high symmetry of the WF<sub>6</sub> molecule is seen in most related compounds. However, it is interesting to note that ] (WH<sub>6</sub>) and ] (W(CH<sub>3</sub>)<sub>6</sub>) adopt a trigonal prismatic structure.<ref>{{cite journal|doi=10.1021/ja00167a065|title=The coordination geometry of gaseous hexamethyltungsten is not octahedral|year=1990|last1=Haaland|first1=Arne|first2=Andreas|first3=Kristin|first4=Hans V.|journal=Journal of the American Chemical Society|volume=112|pages=4547|last2=Hammel|last3=Rypdal|last4=Volden}}</ref><ref>{{cite book|url=http://books.google.com/?id=6153Kt2ikggC&pg=PA427|page=427|title=Valency and bonding: a natural bond orbital donor-acceptor perspective|author=Frank Weinhold, Clark R. Landis|publisher=Cambridge University Press|year=2005|isbn=0-52183128-8}}</ref> Numerous ]s are known with other metals and metalloids. Such MF<sub>6</sub> compounds characteristically form dense gases, however, when element M is heavier than tungsten, the compound is either liquid or solid at room temperature. |
|
|
|
|
|
|
== Trivia == |
|
==Other applications== |
|
|
{{chem2|WF6}} can be used for the production of ]. |
|
In his book '']'', ] describes how his uncle—an avid fan of tungsten and tungsten chemistry—told him about the very high-density of gaseous WF<sub>6</sub>. Later, for his 65th birthday, Sacks wanted to have a WF<sub>6</sub>-filled balloon, but gave up, knowing that WF<sub>6</sub> produces hydrofluoric acid on contact with atmospheric humidity.<ref>{{cite book|isbn=0-37540448-1|title=Uncle Tungsten: Memories of a Chemical Boyhood|author=Oliver Sacks|publisher=Knopf|year=2001}}</ref> (Moreover, WF6 would react with most organic materials, including rubber, but not Teflon.) |
|
|
|
|
|
|
|
As a heavy gas, {{chem2|WF6}} can be used as a buffer to control gas reactions. For example, it slows down the chemistry of the Ar/{{chem2|O2}}/{{chem2|H2}} flame and reduces the flame temperature.<ref>{{cite book |author= Ifeacho, P. |title= Semi-conducting metal oxide nanoparticles from a low-pressure premixed H<sub>2</sub>/O<sub>2</sub>/Ar flame: Synthesis and Characterization |year= 2008 |page= 64 |publisher= Cuvillier Verlag |location= Göttingen |isbn= 978-3-86727-816-4 |url= https://books.google.com/books?id=0B5HI9TNmakC&pg=PT64}}</ref> |
|
== References == |
|
|
{{reflist|2}} |
|
|
|
|
|
|
|
==Safety== |
|
{{Tungsten compounds}} |
|
|
|
Tungsten hexafluoride is an extremely corrosive compound that attacks any tissue. Because of the formation of hydrofluoric acid upon reaction of {{chem2|WF6}} with humidity, {{chem2|WF6}} storage vessels have ] gaskets.<ref>{{cite web |url= http://www.orcbs.msu.edu/msds/LINDE_MSDS/pdf/154.pdf |title= Tungsten hexafluoride MSDS |publisher= Linde Gas |url-status= dead |archive-url= https://web.archive.org/web/20100212202223/http://www.orcbs.msu.edu/msds/LINDE_MSDS/pdf/154.pdf |archive-date= 2010-02-12 }}</ref> |
|
|
|
|
|
|
==References== |
|
|
{{reflist}} |
|
|
|
|
|
{{Hexafluorides}} |
|
|
{{Tungsten compounds}} |
|
|
{{fluorides}} |
|
{{DEFAULTSORT:Tungsten Hexafluoride}} |
|
{{DEFAULTSORT:Tungsten Hexafluoride}} |
|
] |
|
] |
|
] |
|
] |
|
] |
|
|
] |
|
] |
|
] |
|
] |
|
|
] |
|
|
|
|
] |
|
|
] |
|
|
] |
|
|
] |
|