Chemical Vapor Deposition of Tungsten Oxide Rein U
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APPLIED ORGANOMETALLIC CHEMISTRY, VOL. 12, 155–160 (1998) Chemical Vapor Deposition of Tungsten Oxide Rein U. Kirss* and Lamartine Meda Department of Chemistry, Northeastern University, Boston, MA 02115, USA Crystalline and amorphous thin films of tung- literature contains several reports of electro-opti- sten(VI) oxide can be prepared by chemical cally inactive WO3 films prepared by sputtering, vapor deposition using a variety of volatile evaporation or spray methods,3 although later work precursors below 500 °C. Deposition parameters has yielded electrochromic WO3 films by these 1 for preparation of WO3 films from tungsten methods. No attempt is made here to review the hexacarbonyl [W(CO)6], tungsten hexafluoride literature pertaining to WO3 films prepared by these (WF6), tungsten ethoxides [W(OEt)x, x =5,6] methods. 3 and tetra(allyl)tungsten [W(h -C3H5)4] are sum- The present review focuses on CVD of amor- marized. The electrochromic behavior of these phous and crystalline tungsten oxide films, in- films is comparable with that observed for WO3 cluding thermal CVD, plasma-enhanced CVD films prepared by evaporation, sputtering and (PECVD) and photo-assisted CVD (PACVD). The electrodeposition. # 1998 John Wiley & Sons, principle advantage of chemical vapor deposition Ltd. of electronic materials over other methods is in step Appl. Organometal. Chem. 12, 155–160 (1998) coverage, the absence of radiation damage, throughput and the possibility for selective 4 Keywords: tungsten trioxide; electrochromism; growth. For example, CVD of WO3 occurs at thin films; chemical vapor deposition (CVD) temperatures significantly below those for evapora- ° 5 Received 5 December 1996; accepted 3 March 1997 tion (1300 C). FILM GROWTH INTRODUCTION CVD of WO3 from tungsten hexafluoride (WF6) and tungsten carbonyl [W(CO)6] precursors has The electrochromic behavior of tungsten(VI) oxide received the greatest attention. Both compounds are makes WO3 an attractive material for electronic volatile and commercially available. Tungsten 1 devices. Partial reduction of colorless WO3 trioxide films have also been prepared from generates an intense blue color assigned to inter- tungsten alkoxides, W(OC2H5)n (n = 5, 6). Re- valence charge transfer in partially reduced WO3. cently, we have prepared thin films of WO3 from 3 The electrochemical reduction of tungsten(VI) to tetra(allyl)tungsten, W(Z -C3H5)4. A summary of tungsten(V) is accompanied by uptake of cations deposition conditions and film composition is to be (e.g. H,Lietc.) to maintain electrical neutrality, found in Table 1. and is reversible. Applications for thin films of WO3 and other electrochromic materials include ‘smart windows’, electrochromic displays and other electronic devices.1 TUNGSTEN HEXAFLUORIDE Both amorphous and crystalline tungsten oxide films have been prepared. The electrochromic Tungsten hexafluoride, WF6, is widely applied as a properties are strongly dependent on the prepara- precursor for the deposition of tungsten metal and tive method and conditions. For example, sol–gel- has been used in the deposition of tungsten 4,6 derived WO3 films do not exhibit electrochromism oxides. Deposition of pure tungsten from WF6 2 unless they are annealed or laser-fired. The early under H2 or SiH4 results in inclusion of some crystalline W20O58 and possibly other tungsten oxide impurities in body-centered cubic (bcc) 7 *Correspondence to: Rein U. Kirss, Department of Chemistry, tungsten films. Northeastern University, Boston, MA 02115, USA. Tungsten oxide films have been intentionally # 1998 John Wiley & Sons, Ltd. CCC 0268–2605/98/030155–06 $17.50 156 R. U. KIRSS AND L. MEDA Table 1 Comparison of film deposition conditions and composition Growth temp. Growth rate Precursor (°C) (nm/min) Film thickness Structure O:W ratio Ref. a W(CO)6/O2 anneal 500 — 185–1500 Crystalline 3:1 15–17 a W(CO)6/O2/O2 anneal 500 — 185–1500 Crystalline 3.12:1 15–17 W(CO)6/air 200–400 30–40 250 Amorphous 1.83:1 21–24 W(CO)6/air/hn 150–250 200–400 250 Amorphous 2.92:1 25 W(CO)6/O2 (PECVD) 60 8–9 Amorphous 10 WF6/O2 (PECVD) 50–60 28 150–650 Amorphous 3.7:1 10 WF /H /air 50–60 18–24 250 3:1 11 6 Á W(OEt)x (X = 5, 6) 100–300 0.75–30 Amorphous 2.6–3.2:1 6 a W(allyl)4/O2 anneal 250–450 — 250 Crystalline 32 a For these samples, the oxide is generated by oxygen annealing of a previously prepared WOxCy film, so direct comparisons of growth rates are meaningless. deposited from WF6 using pure oxygen or air. A tungsten(VI) in ‘black tungsten’ films. Films were 1 growth rate of 28 nm min was reported for an successfully deposited on quartz, aluminum, and tin amorphous WO3 grown from WF6 in air using oxide-coated glass. plasma-enhanced CVD.8,9 Films ranging from 150 XRD, RHEED and XPS data for ‘black’ or to 650 nm in thickness were deposited at low ‘reflective’ tungsten films annealed in oxygen substrate temperature (50–60 °C) on indium tin between 400 and 600 °C were consistent with oxide-coated sodalime glass substrates. Analysis of formation of polycrystalline WO3 containing 12 fluorine-free films prepared in this fashion gave an monoclinic WO3. The oxygen tungsten ratio in O/W ratio of 3.7:1 by Auger spectroscopy. the transparent films ranged from 2.77:1 to 3.57:1, WO3 films have also been grown on a wide depending on annealing temperature and the partial 15 variety of substrates from WF6 and room air in the pressure of oxygen. Oxidation rates ranged from 1 presence of radiofrequency-generated hydrogen 1 to 2 nm min , producing films from 185 to atoms.10 Growth rates ranged from 18 to 24 nm 1500 nm thick.16,17 1 min producing amorphous, producing films Decomposition of W(CO)6 in air between 200 250 nm thick with a 3:1 ratio of oxygen to tungsten. and 400 °C produces amorphous, transparent WO3 Fluorine impurities were not detected in these films, films with O/W 1.83:1. The films contain metal 1 18–24 but fluorine-doped WO3 films can be prepared from and grow at a rate of 30–40 nm min . Growth pyrolysis of WF6 in the presence of difluoroethane temperatures were further reduced to 150 °C and 1 and isopropanol as the fluorine and oxygen growth rates (200–400 nm min ) were signifi- sources.11 cantly enhanced using photo-assisted CVD (PACVD).21–24 The oxygen/tungsten ratio in the PACVD films ( 250 nm thick) increased to 2.92:1. Growthrates for plasma-enhanced CVD TUNGSTEN HEXACARBONYL using W(CO)6 in O2 at 60 °C are in the range of 8– 1 10 9 nm min . Thermal decomposition of W(CO)6 above 400 °C in the absence of oxygen produces tungsten metal contaminated with carbon and oxygen.12–14 As- deposited films of this type have been referred to as TUNGSTEN ALKOXIDES ‘reflective tungsten’ to distinguish them from ‘black tungsten’ films deposited from W(CO)6 in Tungsten alkoxides are attractive precursors for the presence of oxygen under the same conditions. CVD of tungsten oxide thin films avoiding, in X-ray diffraction (XRD) and Reflection High principle, the need for oxygen annealing steps. Energy Electron Diffraction (RHEED) data on There is one report of CVD of tungsten oxide using 25 black tungsten films were consistent with W18O49 tungsten ethoxides, W(OC2H5)n (n = 5, 6). Penta- and either tetragonal WO2.9 or monoclinic kis(ethoxy)tungsten is a volatile liquid and hex- 11 W20O58. X-ray photo-electron spectroscopy akis(ethoxy)tungsten is a solid. Amorphous, (XPS) confirmed the presence of tungsten(V) and adherent films of WO3 (O:W ratio 2.7–3.2:1) were # 1998 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 12, 155–160 (1998) CHEMICAL VAPOR DEPOSITION OF TUNGSTEN OXIDE 157 Table 2 Summary of electrochromic properties of CVD WO3 films Response time a 2 1 Deposition method Precursor Structure n (cm C ) Coloration Bleaching Ref. Thermal CVD W(CO)6 Amorphous 230 30 s 30 min 18–20 b W(CO)6 Crystalline 40 0.330 s 13 s 13,14 25.5 60–120 s 13–16 c W(OEt)x Amorphous n.r. n.r. n.r. 25 W(allyl)4 Crystalline n.r. n.r. n.r. 31 PACVD W(CO)6 Amorphous 222 2 min 10 min 21–24 PECVD WF6 Amorphous 100 30 s 8,9 a b In 1M LiClO4 or LiBF4 unless otherwise noted. 1M H2SO4. c n.r., not reported. grown on quartz, sodalime glass, indium tin oxide- for application in electrochromic devices can be coated glass and silicon. Growth rates ranged from assessed through measurement of several para- 1 0.75 to 30 nm min over the temperature range meters, including coloration efficiency, response 100–300 °C. Carbon contamination was below time and cycle life1,32 The electrochromic proper- 2 atom%. Powdery blue films were deposited above ties of CVD WO3 films are summarized in Table 2. 350 °C. The color change is described by Eq. [1] and in Fig. 1: WO3 x M x e → MxW(1 x)WxO3 [1] colorless blue ORGANOMETALLIC TUNGSTEN bleached colored COMPOUNDS M = H, Li A number of organometallic tungsten compounds have been investigated as precursors for the CVD of tungsten metal,6 including bis(cyclopentadienyl)- 5 26 tungsten dihydride, [(Z -Cp)2WH2] tris(methyl- COLORATION EFFICIENCY 27 vinylketone)tungsten, [W(C4H6O)3], tris(buta- 27 diene)tungsten [W(C4H8)3] and tetra(allyl)tung- The coloration efficiency, Z, of an electrochromic 28 sten, [W(Z3-C3H5)4 Tungsten carbide films have thin film is determined from Eq.