J. MATER. CHEM., 1991, 1(4), 551-554 55I Low-temperature Vapour Deposition of High-purity Iridium Coatings from Cyclooctadiene Complexes of Iridium Synthesis of a Novel Liquid Iridium Chemical Vapour Deposition Precursor Jeffrey B. Hoke,* Eric W. Stern and H. H. Murray Engelhard Corporation, Menlo Park, CN 40, Edison, NJ 08818, USA High-purity, crystalline iridium coatings have been prepared by low-temperature chemical vapour deposition (CVD) from three volatile cyclooctadiene (COD) iridium precursors. One of these, (MeCp)lr(COD) (MeCp = methylcyclopentadienyl), is a novel complex which melts at low temperature (40 "C) and therefore can be used as a liquid iridium source for CVD. When hydrogen is used as a carrier gas, iridium coatings containing < 1 atm.% carbon are generated at ca.120 "C using (MeCp)lr(COD) and Cplr(C0D) (Cp =cyclopentadienyl). Likewise, deposition under low oxygen pressure in partial vacuum also generates coatings free of carbon and oxygen. However, when the deposition is carried out in vacuo (no carrier gas), ca. 80% carbon is incorporated into the films. When [(COD)lr(p-OAc)], (OAc =acetate) is used as the metal-organic CVD (MOCVD) precursor, films containing c1% carbon and oxygen are obtained at ca. 250 "C in vacuo without the need for a carrier gas. Keywords: Metal-organic chemical vapour deposition; Iridium; Thin film In recent years there has been growing interest in the develop- ment of volatile organometallic precursors to low temperature (<300 "C) vapour-deposited metal and metal oxide coat-ings.'.2 This interest has resulted largely from the demand for new thin-film materials in the electronics industry and from the benefits of the chemical vapour deposition process itself which allows for the deposition of dense, high-purity coatings onto complex shapes at high growth rates.The availability of suitable precursors which have sufficient volatility and which can be cleanly decomposed to the desired metal or oxide, however, has been a common hindrance to further development. In this paper we describe the preparation of pure iridium coatings at low temperature (5250 "C) from one novel and two known cyclooctadiene (COD) complexes of iridium. Iridium is of current interest as a high-temperature (>1600 "C) oxidation barrier for carbon-carbon composite^,^ and it also has potential applications in the microelectronics and optics ind~stries.~ Excluding the iridium halide^,^"-^*' only four CVD precur- sors to metallic iridium have been reported in the literature.These include Ir(acac), (acac =acetylacetonate),6 [(COD)Ir (p-OMe)I2 (OMe =meth~xide),~Ir(acac) (COD),7 and Ir(a1- lyl),.' While all reportedly produce metallic iridium coatings, these sources have their drawbacks. They require high (>500 "C) deposition temperatures (Ir(acac),, Ir(acac)(COD), [(COD)Ir(p-OMe)],, halides}, or they yield coatings contain- ing measurable impurity levels ([(COD)Ir(p-OMe)],, Ir(acac),, Ir(all~l)~}.None of these compounds combines the beneficial features of low deposition temperature and clean decompo- sition in addition to air stability and high volatility.However, not only do the complexes described in this paper extend the number of available iridium precursors, they also satisfy these basic requirements. Experimental (MeCp)Ir(COD), a novel compound, was prepared using an improvement to published procedures for (Cp)Ir(COD). (Reac- tion of NaCp with [(COD)Ir(p-Cl)], at -78 "C increased product yield by 50% over the reported reactions of NaCp with [(COD)IrHCl(p-Cl)], or [(COD)Ir(p-C1)I2 at room tem- perature.)* To [(COD)Ir(p-C1)],9 (0.89 g, 1.32 mmol) dissolved under argon in THF (30 cm3) at -78 "C was added sodium methylcyclopentadienide (1.30 cm3, 2.8 mmol; 2.1 5 mol dm-, in THF) by syringe. An immediate reaction ensued with the formation of a clear yellow solution. After it had been stirred at -78 "C for 10 min, the reaction mixture was warmed to room temperature and stirred an additional 15 min.Removal of the solvent in U~CUOleft a yellow oil. Extraction with hexane (3 x 10 cm3), filtration under argon, and removal of solvent in U~CUOleft a pale-brown solid. Sublimation at 95-100 "C and 0.05 mmHg gave (MeCp)Ir(COD) as a white, air-stable solid, yield 0.82 g, 2.16 mmol, 8l%, m.p. 38.5-40.0 "C. (Found: C, 44.51; H, 5.06. Calc. for CI4Hl9Ir C, 44.31; H, 5.05%) dH (solvent CD2C12; 200 MHz) 1.80 (m, 4 H, CH2), 1.89 (s, 3 H, CH3), 2.00 (m, 4 H, CH2), 3.53 (br s, 4 H, cyclooctadiene CH), 4.95 (t, 2 H, cyclopentadienyl CH, J=1.9 Hz), 5.16 (br s, 2 H, cyclopentadienyl CH).hC (solvent CD,Cl,; 50.4 MHz) 12.1 1 [q, CH3, J(C-H)= 127 Hz], 34.26 [t, CH2, J(C-H)= 125 Hz], 47.49 [d, cyclooctadiene CH, J(C-H) = 153 Hz], 80.44 [d, cyclo- pentadienyl CH, J(C-H) = 176 Hz], 83.24 [d, cyclopentadienyl CH, J(C-H)= 175 Hz]. CpIr(C0D) was prepared similarly from [(COD)Ir(p-Cl)], and NaCp at -78 "C in 65% sublimed yield (1 10 "C ca. 0.01 Torrl.) and identified by comparison of NMR and melting- point data with literature values.8 Depositions onto readily available fused silica substrates were accomplished using a 1 in$ 0.d. externally thermostatted hot-walled quartz reactor. The iridium precursor was sublimed into the hot zone of the reactor under a stream of hydrogen (1.6 cfh), under vacuum with a low-pressure oxygen bleed [p(02)1.3 Torr], or under full vacuum with no carrier gas.A typical run lasted 4 h. Thermogravimetric and mass spectrometric data were obtained using an Omnitherm high-temperature TG-DTA 1500 and a Dycor quadrapole mass spectrometer operating in the electron impact (EX) mode. Freshly sublimed samples of (Cp)Ir(COD) and (MeCp)Ir(COD) were heated under flowing hydrogen to 120 "C at 10 "C min-' and then held at 120 "C. [(COD)Ir(p-OAc)], was heated under flowing helium t 1 Torr z133.322 Pa. $ 1 in=2.54 cm. to 250°C at lO"Cmin-' and then held at 250 "C. Mass spectral analysis of volatile by-products was accomplished in real time. Film composition and thickness data were obtained by X-ray photoelectron spectroscopy using an SSI model 206 electron spectrometer with a monochromatized A1 X-ray source.The instrument was operated with a 600 p X-ray spot and a pass energy of 150 eV. Depth profiling was accomplished with a Leybold-Hereaus IQ 12/38 argon ion gun operating at 5 kV with a current density of ca. 100 pA cm-2. Film thicknesses were based on the etching rate for SO2. The pressure during data collection was 2 x 10-Torr; data were collected using the Ir 4f line (binding energy =60.6 eV). Quantification was accomplished using linear background subtraction and Scofield sensitivity factors. lo Results and Discussion The three cyclooctadiene iridium precursors chosen for this study were (MeCp)Ir(COD) (MeCp =methylcyclopen-tadienyl), CpIr(C0D) (Cp=cyclopentadienyl),8 and [(COD)Ir(p-OAc)], (OAc =acetate)." R Ir \\ACOzMe Of these, (MeCp)Ir(COD) is a novel compound and was prepared from [(COD)Ir(p-Cl)]2 and sodium methylcyclopen- tadienide as described above.All are air stable and sufficiently volatile for CVD (Table 1). All produce impurity-free coatings of iridium at low temperature [120 "C for both CpIr(C0D) and (MeCp)Ir(COD)] and are thus superior to other iridium precursors described in the literature. (MeCp)Ir(COD) is par- ticularly attractive since its low melting point (38.5-40 "C) permits its use as a liquid source of iridium near room temperature. This is a significant advantage as many commer- cial CVD systems are designed for the use of liquid precursors.Reduction of (MeCp)Ir(COD) and CpIr(C0D) vapours (sublimation temperature 95 "C)in flowing hydrogen (1.6 cfht) at ca. 120 "C and ambient pressure resulted in the deposition of pure, crystalline coatings of iridium (Table 2). Scanning electron microscopy photographs of coated and uncoated fused silica substrates are shown in Fig. 1. ESCA depth-profile analysis detected <1 atom% carbon within the bulk of the coatings, which were up to 5300 A thick (Fig. 2). Thin-film X-ray analysis (Cu-Ka radiation) confirmed film crystallinity (Fig. 3). The line-broadening evident in the diffraction peaks is consistent with the presence of microcrystalline iridium particles of ca. 50 8, diameter." In contrast to hydrogen reduction, decomposition of (MeCp)Ir(COD) and CpIr(C0D) vapours in uucuo at ca.590 and 680 "C, respectively, generated black films which incorporated ca. 80 atom% carbon as 1 cfhz2.83 xlOP2m3 h-', Table 1 Melting-point and sublimation data for iridium CVD precursors precursor m.p./ "C Tub/ "C (MeCp)Ir(COD) CpIr(C0D) C(COD)Ir(CL-OAc)l, 38.5-40.0 125.5-128.5 135" 95 (0.05 Torr) 110 (0.01 Torr) 125 (0.07 Torr) "Decomposed. J. MATER. CHEM., 1991, VOL. 1 determined by ESCA (Table 2). [X-Ray analysis of the coating derived from (MeCp)Ir(COD) showed the iridium to be mic- rocrystalline.] Clearly, hydrogen is required to cleave the organic ligands from iridium and concurrently reduce the metal to the zero oxidation state.'H NMR and TG-MS were utilized to deduce a possible mechanism for the reduction of (MeCp)Ir(COD) and (Cp)Ir(COD) at 120 "C. For both complexes, 'H NMR anal- ysis of the volatiles collected during vapour deposition showed only the presence of saturated hydrocarbon. No alkene species (methylcyclopentadiene, cyclopentadiene, methylcyclopentene, cyclopentene, cyclooctadiene, or cyclooctene) were detected. Likewise, TG-MS analysis of the decomposition of both complexes in flowing hydrogen at 120 "C confirmed the formation of cyclooctane but no cyclooctadiene, cyclooctene, cyclopentadiene, or methylcyclopentadiene. These results would indicate that the primary reduction mechanism pro- ceeds via a multistep process involving oxidative additions of H2 to Ir' with subsequent reductive eliminations back to Ir' and formation of uncomplexed hydr~carbon.'~ The 1r'-H species which would remain from such a process may then undergo homolytic bond cleavage (Ir-H+Ir +H.) or a bimolecular process (2Ir-H-Ir +H2) to yield metallic iridium. Owing to the gradual decomposition of the source materials under hydrogen during sublimation, the observed deposition rates were slow.However, this problem can be alleviated by directing the organometallic vapours onto the substrate prior to hydrogen exposure.2a Consequently, the precursor will not see hydrogen until the point of deposition, and a subsequent increase in sublimation temperature should significantly improve the rate.Oxidation of (MeCp)Ir(COD) and CpIr(C0D) vapours (sublimation temperature 80 "C) in partial vacuum [p(O,)1.3 Torr] at ca. 270 "C also yielded pure, crystalline iridium coatings. In each case, ESCA depth-profile analysis detected <1 atom% carbon and oxygen through the bulk, and thin- film X-ray analysis confirmed film crystallinity. In agreement with the low oxophilicity of iridium, the presence of oxygen during CVD was not a problem. Since iridium does not form an oxide below 550 "C,14 oxygen is free to consume the organic ligands leaving behind only pure iridium. Unlike the cyclopentadienyl complexes of iridium, the acet- ate bridged dimer, [(COD)Ir(p-OAc)],, does not require a reactive gas (either hydrogen or oxygen) to produce clean coatings of iridium.Deposition at ca. 250 "C in uucuo (subli-mation temperature 130 "C) produced a thin (400 A) coating containing <1 atom% carbon or oxygen within the bulk as determined by ESCA depth-profile analysis. (In this instance, deposition was hampered by the simultaneous decomposition of the precursor at the sublimation temperature.) Thin-film X-ray analysis confirmed the presence of crystalline iridium. 'H NMR analysis of volatiles trapped during the CVD experiment showed the presence of cyclooctadiene, cyclooctene, and cyclooctane in roughly equal amounts. TG mass spectral analysis during the decomposition of [(COD)Ir(p-OAc)], in flowing helium at 250 "Cgave molecu- lar ions corresponding to carbon dioxide, cyclooctadiene, cyclooctene, cyclooctane and possibly ethane.These data suggest that the primary decomposition pathway involves the simultaneous liberation of carbon dioxide from the acetate bridge and cyclooctadiene from iridium. Formation of cyclo- octene and cyclooctane may result from further reaction of the resulting acetate derived methyl radicals with either bound or liberated cyclooctadiene. Dimerization of the methyl rad- icals would also produce ethane. Pure Iro is thus obtained. Although the sublimation temperature of [(COD)Ir(p-OAc)], is somewhat high, its greatest advantage for CVD is its ability J. MATER. CHEM., 1991, VOL. I Fig. 1 (a)SEM of an iridium coating on fused silica produced from the hydrogen reduction of (MeCp)Ir(COD);(b)SEM of an uncoated fused silica substrate Table 2 Data summary for CVD iridium coatings precursor co-reactant &ec/ "C 120 120 285 270 680 590 250 100 1 Ir f 60 1 C ._:40 20 0 25 50 75 100 sputter time/min Fig.2 ESCA depth profile of an iridium coating produced from the hydrogen reduction of (MeCp)Ir(COD); sputter rate 70 A min-' to produce iridium coatings free from contamination without the need for a reactive carrier gas. Such a result is remarkable since most chemical vapour depositions require reactive gases (e.g.hydrogen) to produce coatings completely free of carbon or oxygen. The authors express their gratitude to the following individ- uals: A. Amundsen for supplying [(COD)Ir(p-OAc)] 2; N.Brungard and D. Anderson for providing the ESCA analyses; Professor K. Unruh (University of Delaware) for providing the thin-film X-ray analyses; Earl Waterman for conducting depth/A composition deposition rate/A h- 5300 <1% c 900 460 <I% c 100 500 950 <1O/O c, 0 <1% c,0 150 250 47000 82% C, 18% Ir 9000 1400 80% C, 20% Ir 350 400 <1% c,0 80 10 Fig. 3 Thin-film X-ray scan of an iridium coating produced from the hydrogen reduction of (MeCp)Ir(COD); peaks at ca. 40, 47 and 70" correspond to those in JCPD file for iridium (Ir6-598) the TG-MS experiments; and M. Gashasb for helpful dis- cussions on CVD processing. References 1 (a)J. E. Gozum, D. M. Pollina, J. A. Jensen and G. S. Girolami, J.Am. Chem. Soc., 1988, 110, 2688; (b)Y. Chen, H. D. Kaesz, H. Thridandam and R. F. Hicks, Appl. Phys. Lett., 1988, 53, 1591; (c) Z. Xue, M. J. Strouse, D. K. Shuh, C. B. Knobler, H. D. Kaesz, R. F. Hicks and R. S. Williams, J. Am. Chem. SOC., 1989, 111, 8779; (d) E. Feuer, S. Kraus and H. Suhr, J. Vac. Sci. Technol. A, 1989, 7,2799; (e) M. J. Rand, J. Electrochem. 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Paper 1/OO 1 59K; Received 14th January, 1991