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Structural characterization of chromium carbide coatings deposited at low temperature by low pressure chemical vapour decomposition using dicumene chromium
Surface and Coatings Technology, 41 (1990) 51
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61
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STRUCTURAL CHARACTERIZATION OF CHROMIUM CARBIDE COATINGS DEPOSITED AT LOW TEMPERATURE BY LOW PRESSURE CHEMICAL VAPOUR DECOMPOSITION USING DICUMENE CHROMIUM F. MAURY, D. OQUAB, J. C. MANSE and R. MORANCHO Cristallochimie, Reactivité et Protection des Materiaux, UA-CNRS 445, ENSCT 118 route de Narbonne, 31077 Toulouse Cédex (France) J. F. NOWAK and J. P. GAUTHIER UNIREC, Centre de Recherche d’Unieux, BP 50, 42702 Firminy Cédex (France) (Received May 22, 1989)
Summary The growth of chromium carbide coatings by pyrolysis of dicumene chromium in a hot-wall low pressure chemical vapour deposition reactor has been investigated between 300 and 550 °C. Amorphous chromium carbide films were obtained in the low temperature range 300 500 °Cwhereas a textured crystalline Cr7C3 phase was grown above 500 °C.The total carbon -
content was independent of the deposition temperature and amounted to a carbon excess of about 30% compared with the Cr7C3 stoichiometry. Electron spectroscopy for chemical analysis of both amorphous and crystalline coatings confirms the presence of this carbon excess since about 30% 40% of free carbon was found. A heterogeneous structural model composed of the Cr7C3 phase and free carbon is proposed both for crystalline and amorphous coatings. -
1. Introduction Research on low temperature chemical vapour deposition (CVD) processes is carried out using either a means of physical enhancement such as a plasma or a laser, or, for thermal CVD, more reactive chemicals such as organometallic precursors. In the field of hard metallurgical coatings, chromium carbide is an important material which is deposited industrially by pack cementation at around 1050 °C[1]. In order to avoid the dimensional and the structural changes generally related to these high temperatures, a considerable emphasis has been placed on the preparation and application of protective coatings produced by organometallic chemical vapour deposition (OMCVD). Moreover, physical deposition processes do not seem suitable for deposition of chromium carbide in a high capacity 0257-8972/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
52
reactor or onto samples with a complex shape, and the material produced may have a multiphase structure, depending on the experimental conditions [2, 3]. The microstructure of CVD chromium carbide coatings also depends on the deposition conditions, but the use of organometallic compounds often leads to amorphous films with a carbon content which depends on the nature of the alkyl groups bonded to the chromium atom [4]. Since the thermal decomposition of chromium hexacarbonyl was reported [5], many organochromium compounds have been used as starting materials for CVD chromium deposition at low temperatures [6 15]. In order to prepare the hard phase Cr7C3 at low temperature, without -
oxygen and halogen contamination, we chose, in accordance with the literature, dicumene chromium (DCC) as the single precursor. This compound has been used in various CVD processes [6 9], but the experimental data required to obtain a layer both of uniform thickness and with a homogeneous composition in an industrial-type CVD reactor are insufficient. Thus growth rate results as a function of the deposition temperature are only given by Anantha et al. [9] and results on the influence of the other experimental parameters are rather sparse. Furthermore, the microstructure of these low temperature chromium coatings is not known precisely, since they are described either as a mixture of free chromium, Cr7C3 and Cr3C2 [6] or as Cr1C3 with evidence for free chromium and free carbon [8]. This paper deals with the OMCVD growth of chromium carbide layers using DCC over a wide range of temperature. Their microstructure, investigated by diffraction and spectroscopic techniques, is discussed in relation to previous work. -
2. Ex~erimenta1details The commercially available DCC used in this study was a liquid at room temperature. Because of the discrepancy in melting points found in the literature [7, 16] its chemical homogeneity was in doubt. This suspicion was confirmed by chromatographic analysis using the acid-decomposition method [17]. The substance was a mixture of various bis-arene complexes of chromium differing in the number of isopropyl groups bonded to the benzene ring. This mixture contained only about 70% DCC. After the failure of attempts at vacuum distillation, the DCC was used without further purification. A standard horizontal hot-wall low pressure CVD quartz reactor 2.8 cm in diameter and 30 cm in length was used. Since the chromium atom of the starting material is in the zero-valence state, thermal decomposition of the DCC was carried out under helium with no reducing agent such as hydrogen. The low pressure allowed both compensation for the low vapour pressure of the precursor (2 X 10-2 Torr at 110 °C) and enhancement of the mass transfer rate relative to the surface reaction rate, in order to make the thick-
53
ness more uniform throughout the reactor tube. An optimum growth rate was obtained using a total pressure of 20 Torr, a total flow rate of 143 standard cm3 min’ and a DCC molar fraction of io~.The main structural features of the coatings were investigated by X-ray diffraction, as well as scanning (SEM) and transmission electron microscopy (TEM). The carbon content of the films was determined using either an electron microprobe with reference samples or a combustion method, measuring the amount of the resulting CO 2 gas by chromatography. Since the free carbon was oxidized more easily than the carbon bound as Cr—C, the total amount of carbon was determined following the addition of Pb304 to the oxidizing flow [18]. The electron spectroscopy for chemical analysis (ESCA) spectra were obtained with a spectrometer (VG ESCALAB MKII) operating with a monochromatic Al Ka X-ray source. The data were taken with a constant analyser pass energy of 50 eV in a vacuum system with a base pressure of 10b0 Torr. The binding energies measured were referred to the Au 4f712 level at 84.0 eV from a gold standard film.
3. Results Chromium carbide coatings were obtained between 300 and 550 °C with growth rates as high as 4 pm h~ on steel and silicon substrates. The growth rate seems to be independent of the nature of the substrate. Its variation with the deposition temperature reveals two temperature ranges with two maxima, centred at about 400 and 525 °C.Such a variation in the growth rate as a function of the deposition temperature is unusual in the CVD literature. However, similar behaviour has been reported for epitaxial Ga As~P1 layers [19] and OMCVD titanium nitride [20]. In each case, a change in the reaction mechanism was assumed. Investigations into the thermal decomposition of bis-arene chromium and aromatic ligands led us to the same conclusion and this will be discussed in a later paper [21]. Coatings deposited at low temperature, i.e. 300 450 °C,have a smooth mirror-like surface (Fig. 1(a)). SEM examination of cross-sections reveals good homogeneity, high compactness and uniform thickness in the layers, as well as good step coverage on rough samples. On steel or silicon substrates, the interface is regular and well defined (Fig. 1(b)). In the intermediate temperature range 450 500 °C,the morphology is more heterogeneous since the films assume a mixed structure composed of both a uniform amorphous thin layer and chromium carbide crystallites embedded in and spread over the film. The number of these superficial crystallites increases with temperature and the film roughness is characterized by a peak-to-valley height of 3 5 pm. Above 500 °Cthe coatings have a polycrystalline structure with a mean grain size in the range 5 10 pm (Figs. 1(c) and 1(d)). In the entire temperature range investigated, a uniform chromium distribution was observed by X-ray imaging produced using Cr Ka radiation. Structural analysis by X-ray diffraction indicated that the films deposited -
-
-
-
.
54
_
2Opm
5
1
—4
c15O
~
d 5
Fig. 1. Typical SEM micrographs of chromium carbide coatings deposited at (a) and (b) T<450°C;(c)and(d)T •500°C.
below 500 °Cwere amorphous. The sparse microcrystallites grown on the layer deposited between 450 and 500 °C were not detectable by X-ray diffraction. Indexing of the X-ray diffraction patterns of polycrystalline films deposited above 500 °C revealed that the single Cr 7C3 phase had been obtained. These polycrystalline Cr7C3 films had a hard texture which has not yet been fully characterized. The amorphous state of the low temperature coatings (T < 500 °C)has been confirmed by means of electron diffraction patterns (Fig. 2(a)). The two broad halos characteristic of short-range amorphous order are similar to those in X-ray patterns, i.e. centred around d spacings of 2.10 and 1.20 A. Polycrystalline films deposited above 500 °C are composed of polygonal crystals with a mean size of a few micrometres which have been easily identified as Cr7C3 from indexing of some simple crystallographic planes. The high dislocation density along the grain boundaries is probably the result of stresses during the growth of the layer (Fig. 2(b)). The electron micro-
O5pm
f~
j
r
55
~
‘O.5prn
~
(
~
_
Fig. 2. TEM micrographs and selected area electron diffraction patterns from chromium carbide coatings deposited at (a) T < 500 °Cand (b) T> 500 °C;(c) layer—substrate interface showing the amorphous chromium carbide buffer layer precoated at 350 °Cand the Cr 7C3 coating grown at 525 °C.
graph in Fig. 2(b) also shows linear fringes in the crystal carbides caused by stacking faults. The streaks in the corresponding electron diffraction pattern are caused by these faults in the crystal structure which are characteristic of M7C3 compounds [221 and especially of Cr7C3 [23]. The crystalline Cr7C3 phase was also obtained from amorphous samples prepared at T < 500 °Cafter annealing at 550 °Cfor 5 h, but in this case no preferential orientation was found and the Cr~C3crystals were smaller than
56
those grown at 525 °C.The texture of Cr7C3 films deposited at high temperature (T> 500 °C)seems to be independent of the nature of the substrate since similar X-ray diffraction patterns were obtained for coatings grown on steel substrates and on steel substrates coated with an amorphous chromium carbide intermediate layer deposited at 350 °C.This low temperature chromium precoating seemed to improve the film adhesion on a steel substrate, and TEM analysis at various depths in the film, up to the layer—substrate interface, revealed, near this interface, the expected thin amorphous chromium carbide layer, with a thickness of a few hundreds of ângstrbms. Thereafter, increasing the temperature up to the value during deposition led to nucleation from this amorphous thin layer and a growth of small acicular Cr7C3 crystallites (Fig. 2(c)). Beyond this zone toward the surface, typical polygonal Cr7C3 crystals were observed (Fig. 2(b)). This indicates that the texture was due to the growth conditions rather than to the structure of the substrate. The carbon content of a number of samples is shown as a function of the deposition temperature in Fig. 3. Although there was a temperature threshold near 500 °C,above or below which the structure of any film was crystalline or amorphous respectively, the carbon content of the films was quite independent of the deposition temperature. For the coatings deposited under a total pressure of 20 Torr, the proportion of carbon was in the range 12 13 wt.%, which is significantly higher than the stoichiometric composition of Cr7C3 (9 wt.% carbon) and close to that of Cr3C2 (13.3 wt.% carbon). However no evidence for the existence of this last phase has been found by diffraction analysis. This carbon excess seems to be reduced at lower pressures since in four experiments, two at 425 °Cand two at 510 °C,the carbon content was reduced by about 20% when the pressure was decreased from 20 to 5 Torr, leading to a composition of the layers close to the Cr7C3 stoichiometry -
~2O I
I
I
16 H
z
.
12
z o
-
r.
~,
Cr3C2
~..Cr7C3
08
-
z o
-~—-—~~23~6
0
0 300 350 400 450 500 550 DEPOSITION TEMPERATURE (°C) Fig. 3. Carbon content of chromium carbide films as a function of the deposition temperature: data include chemical analysis of samples prepared under a total pressure of (•) 20 Torr and (0) 5 Torr as well as (A) electron microprobe results obtained for coatings deposited at 20 Torr.
57
(Fig. 3). Similar values of the elemental composition have been found previously [4, 8] and no variation in the composition with the deposition temperature has been observed. However, nothing has been said about the influence of the total pressure. Nevertheless a carbon content of 8 wt.% has been reported for a total pressure as low as 2 Torr [6], which is in good agreement with our results. In order to obtain further structural information, analysis by JR spectrometry was attempted on some amorphous samples but no absorption band was observed. In fact, in searching in the literature, one finds that carbides of some transition metals in groups IV and V have characteristic JR vibrational frequencies in the range 400 600 cm’ [24] but that chromium carbides, especially Cr3C2, have no specific JR absorption band above 1 [25]. Nevertheless, these JR spectra indicate the absence of 400 cmin the coatings since there is no absorption near 660 cm~,as would oxygen be expected for Cr—O bond vibrations, in contrast with the films produced by thermal decomposition of the organometallic compound Cr(CO) 6 [26]. Figure 4 shows ESCA spectra of amorphous and crystalline samples 4~sputtering (4 keV) for a few minutes in order to eliminate the after Ar surface contamination. The binding energy of the C is peak and of the Cr 2p doublet is the same for the two coatings. The shape of the C is peak gives evidence for at least two forms of carbon in the material at both low and high temperature. The C is main peak at 283.5 eV is assigned to carbon bonded to chromium atoms (chromium carbide phase) whereas the shoulder at higher binding energy (285.2 eV) is most probably due to free carbon -
280
283
286 BINDING
572 ENERGY
578
584
1eV)
Fig. 4. ESCA spectra of OMCVD chromium carbide layers deposited at (a) 510 ~C and (b) 400 °Cin the C is and Cr 2p energy region recorded after 15 mm of Ar~sputtering.
58
[27]. The energy difference between the two forms of carbon is in good agreement with that found for amorphous electrodeposited chromium films, i.e. 1.7 eV and 1.6 eV respectively [28]. The higher energy shift of the Cr 2P3/2 level at 575.1 eV is consistent with a chromium carbide structure and no evidence of metallic chromium, which might be expected near 574.0 eV, has been found. The full width at half-maximum of the Cr 2p~2 peak is slightly higher in the amorphous sample than in the crystalline one, which is probably an effect of the disorder. A gaussian—lorentzian curve-fitting computer program was used to resolve the measured C is peak. Then the relative amount of free carbon could be estimated from the intensity ratio, Icc/(Icc + ICCr), where ~ and ‘CCr are the intensities of the peaks due to C—C and C—-Cr bonds respectively. The relative proportion of free carbon was typically in the range 30% 40%. Furthermore, using the intensity of the C is component ‘CCJ and that of Cr 2P2’3 as well as the standard atomic sensitivity factors of carbon and chromium, a quantitative determination gave about 30 at.% of carbon bonded to the chromium i.e. close to the Cr 7C3 stoichiometry for both amorphous and crystalline coatings. -
—
4. Discussion CVD processes using bis-arene chromium may be represented by the simplified chemical equation Cr(arene)2
—~
Cr
+
2 arene
However, in OMCVD the mechanism is more complicated, and while it may be true that the decomposition mechanism is based on selective breaking of the less stable chemical bonds (chromium—ligand in this case), there are often side reactions in competition with the main process, their importance depending on the experimental conditions. Therefore the thermal decomposition of DCC produces chromium carbide films with a carbon content and a microstructure which depend on the deposition process. These coatings have been variously described as a mixture of chromium metal, Cr7C3 and Cr3C2 [6], as chromium metal supersaturated with carbon [8] and often as pure metallic coatings, ignoring the incorporated carbon [7, 9]. In fact, the carbon content in the films may be reduced by the addition of inhibitors in the gas phase [15] or by catalytic decomposition of the DCC [29]. From a thermodynamic point of view, Cr7C3 is the most stable chromium carbide compound among the three possible phases, as shown by its lower standard free energy of formation. X-ray and electron diffraction patterns of the coatings grown above 500 °Cexhibit the Cr7C3 structure and no evidence for crystallites of chromium metal or other chromium carbide phases has been found.
59
However, both chemical analysis and ESCA spectra reveal a carbon excess of about 30% compared with the stoichiometry of Cr7C3. This carbon excess is found in both the amorphous and the crystalline samples and it has the structural features of free carbon, which would segregate during the growth. The incorporation of this carbon excess would be the result of the thermal decomposition of both isopropylbenzene ligands and organic byproducts by side reactions. This assumption is supported by the influence of the total pressure, since decreasing the pressure in the reactor decreases the carbon content, probably because the elimination of the organic byproducts is facilitated and the coatings are less contaminated. This assumption holds if an increase in the total flow rate has the same effect as a decrease in the total pressure, as has been effectively observed using related bis-arene chromium precursors [30]. Furthermore, the relative proportion of free carbon estimated from ESCA spectra is found to be slightly higher in a film deposited at 510 °C(38%) than in another prepared at 400 °C(31%). This is consistent with the presence of side reactions, particularly pyrolysis, since their importance increases generally with the temperature. The microstructure of the amorphous coatings appears to be related to that of the crystalline films. Indeed, amorphous and crystalline coatings have the same elemental composition (12 13 wt.% of carbon) and almost the same ratio of free carbon (30% 40%). In the amorphous coatings, most of the carbon is then bonded to the chromium. Moreover, the similarity of the C is and Cr 2p transitions in the ESCA spectra of the amorphous and crystalline films indicates that the local atomic environments are probably similar. Compared with the stoichiometry of Cr7C3, the carbon excess found in amorphous films by chemical analysis is about 30%, which fits fairly well with the 30% 40% estimated by ESCA, taking into account the uncertainty in each method. This investigation shows that the crystalline films consist mainly of the crystalline Cr7C3 single phase with an excess of free carbon. It also suggests for amorphous coatings a heterogeneous microstructural model composed of amorphous free carbon zones embedded in an amorphous Cr7C3-like phase. This presupposes that in this amorphous chromium carbide phase, there is preferential bonding of chromium and carbon in such a way that the short-range chemical order of the Cr7C3 is preserved. At low temperatures the non-crystalhinity would then be due mainly to variations in dihedral bond angles and bond lengths rather than to a random distribution of carbon and chromium in the network. In the temperature range just below the crystallization temperature, the short-range order is enhanced and extended and the structure tends towards randomly oriented microcrystallites mixed with some larger Cr7C3 crystallites which start to grow from these microcrystalline nuclei. This is supported on the one hand by the fact that the amount of carbon bonded to the chromium is quite close to the Cr7C3 composition and on the other hand by the observation that annealing of amorphous coatings at 550 °Ceasily produces Cr7C3 crystallites. This structural analysis confirms the presence of free carbon which had previously -
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been indirectly observed mixed with the chromium carbide phase and metallic chromium [8], but no evidence for free chromium has been found. 5. Conclusion Chromium carbide coatings have been obtained by pyrolysis of DCC in a hot-wall low pressure CVD process, between 300 and 550 °C.The chromium carbide films obtained in the low temperature range 300 500 °C are amorphous and give, after annealing at 550 °C, the crystalline Cr7C3 -
phase. The coatings deposited between 500 and 550 °Cexhibit a textured crystalline Cr7C3 structure which appears to be independent of the substrate. The proportion of carbon in these films is about 12 13 wt.% and is not dependent on the deposition temperature. This represents a carbon excess of 30 at.% compared with the Cr7C3 stoichiometry. This carbon excess has been confirmed by ESCA analysis in both amorphous and crystalline samples. Since no evidence for chromium metal or other chromium carbide phases has been found, the material may be described as a mixture of Cr7C3 and free carbon. This heterogeneous structural model also agrees with data for amorphous coatings. -
Acknowledgments The authors gratefully acknowledge the help of Dr. A. Lebugle and Mr. M. Provincial in providing X-ray photoelectron spectroscopy measurements. References 1 R. Knoche, S. Audisio and H. Mazille, in J. M. Blocher, G. E. Vuillard and G. Wahl (eds.), Proc. 8th mt. Conf. on Chemical Vapour Deposition, Electrochemical Society, Pennington, NJ, 1981, p. 653. 2 G. Cholvy and J. L. Derep, J. Vac. Sci. Technol. A, 3 (1985) 2378. 3 S. Komiya, S. Ono, N. Umezu and T. Narusawa, Thin Solid Films, 45 (1977) 433. 4 K. H. Bloss, H. Lukas and W. Kissing, in J. M. Blocher, H. E. Hintermann and L. H. Hall (eds.), Proc. 5th mt. Conf. on Chemical Vapour Deposition, Electrochemical Society, Pennington, NJ, 1975, p. 136. 5 B. B. Owen and R. T. Webber, Trans. Metall. Soc. AIME, 175 (1948) 693. 6 W. H. Metzger, Jr., Plating, 49 (1962) 1176. 7 R. Tomono, E. Yagi and Y. Togashi, Kinsoku Hyomen Gijutsu (J. Met. Finish. Soc. Jpn.), 16(1965) 210. 8 J. E. Knap, B. Pesetsky and F. N. Hill, Plating, 53 (1966) 772. 9 N. G. Anantha, V. Y. Doo and D. K. Seto, J. Electrochem. Soc., 118 (1971) 163. 10 B. D. Nash, T. T. Campbell and F. E. Block, U.S. Bur. Mines, Rep. Invest., 7112 (1968). 11 G. A. Razuvaev, G. G. Petukhov and A. N. Artemov, Zh. Obshch. Khim., 39 (1969) 2494. 12 T. J. Truex, R. B. Saillant and F. M. Monroe, J. Electrochem. Soc., 122 (1975) 1396.
61 13 G. G. Devyatykh, S. M. Vlasov, B. A. Andreev, S. M. Mazavin and V. I. Faerman, Russ. J. Phys. Chem., 49 (1975) 355. 14 V. M. Koleshko, B. S. Reznikov and E. A. Utkina, Izvest. Akad. Nauk SSSR, Neorg. Mater., 15 (1979) 782. 15 V. B. Polikarpov, A. S. Luzin, V. A. Dodonov and E. K. Klement’ev, Izv. Akad. Nauk. SSSR, Neorg. Mater., 20 (1984) 1839. 16 V. Graves and J. J. Lagowski, Inorg. Chem., 15 (1976) 577. 17 Yu. A. Sorokin and G. G. Petukhov, Zh. Obshch. Khim., 35 (1966) 2135. 18 Y. Avigal, M. Schieber and R. Levin, J. Cryst. Growth, 24—25 (1974) 188. 19 V. S. Ban, H. F. Gossenberger and J. J. Tietjen, J. AppI. Phys., 43 (1972) 2471. 20 K. Sugiyama, S. Pac, Y. Takahashi and S. Motojima, J. Electrochem. Soc., 122 (1975) 1545. 21 F. Maury, D. Oquab and J. F. Nowak, to be published. 22 D. J. Dyson and K. W. Andrews, J. Iron Steel Inst., London, 207 (1969) 208. 23 W. Dudzinski, J. P. Morniroli and M. Gantois, J. Mater. Sci., 15 (1980) 1387. 24 E. Siegel, Semicond. Insul., 5 (1979) 61. 25 0. Kammori, K. Sato and F. Kurosawa, Jpn. Analyst, 17 (1968) 1270. 26 E. Erben, R. Bertinger, A. Muhlratzer, B. Tihanyi and B. Cornils, Sol. Energy Mater., 12 (1985) 239. 27 A. Mestari, F. Maury and R. Morancho, J. Phys. (Paris), Suppi. 5, 50 (1989) C5-765. 28 G. B. Hoflund, D. A. Asbury, S. J. Babb, A. L. Grogan, Jr., H. A. Laitinen and S. Hoshino, J. Vac. Sci. Technol. A, 4 (1986) 26. 29 F. N. Hill, U.S. Patent 3,203,827, 1965. 30 G. G. Devyatykh, S. M. Vlasov and S. M. Mazavin, Izv. Akad. Nauk. SSSR., Met., 2 (1974) 87.
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STRUCTURAL CHARACTERIZATION OF CHROMIUM CARBIDE COATINGS DEPOSITED AT LOW TEMPERATURE BY LOW PRESSURE CHEMICAL VAPOUR DECOMPOSITION USING DICUMENE CHROMIUM F. MAURY, D. OQUAB, J. C. MANSE and R. MORANCHO Cristallochimie, Reactivité et Protection des Materiaux, UA-CNRS 445, ENSCT 118 route de Narbonne, 31077 Toulouse Cédex (France) J. F. NOWAK and J. P. GAUTHIER UNIREC, Centre de Recherche d’Unieux, BP 50, 42702 Firminy Cédex (France) (Received May 22, 1989)
Summary The growth of chromium carbide coatings by pyrolysis of dicumene chromium in a hot-wall low pressure chemical vapour deposition reactor has been investigated between 300 and 550 °C. Amorphous chromium carbide films were obtained in the low temperature range 300 500 °Cwhereas a textured crystalline Cr7C3 phase was grown above 500 °C.The total carbon -
content was independent of the deposition temperature and amounted to a carbon excess of about 30% compared with the Cr7C3 stoichiometry. Electron spectroscopy for chemical analysis of both amorphous and crystalline coatings confirms the presence of this carbon excess since about 30% 40% of free carbon was found. A heterogeneous structural model composed of the Cr7C3 phase and free carbon is proposed both for crystalline and amorphous coatings. -
1. Introduction Research on low temperature chemical vapour deposition (CVD) processes is carried out using either a means of physical enhancement such as a plasma or a laser, or, for thermal CVD, more reactive chemicals such as organometallic precursors. In the field of hard metallurgical coatings, chromium carbide is an important material which is deposited industrially by pack cementation at around 1050 °C[1]. In order to avoid the dimensional and the structural changes generally related to these high temperatures, a considerable emphasis has been placed on the preparation and application of protective coatings produced by organometallic chemical vapour deposition (OMCVD). Moreover, physical deposition processes do not seem suitable for deposition of chromium carbide in a high capacity 0257-8972/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
52
reactor or onto samples with a complex shape, and the material produced may have a multiphase structure, depending on the experimental conditions [2, 3]. The microstructure of CVD chromium carbide coatings also depends on the deposition conditions, but the use of organometallic compounds often leads to amorphous films with a carbon content which depends on the nature of the alkyl groups bonded to the chromium atom [4]. Since the thermal decomposition of chromium hexacarbonyl was reported [5], many organochromium compounds have been used as starting materials for CVD chromium deposition at low temperatures [6 15]. In order to prepare the hard phase Cr7C3 at low temperature, without -
oxygen and halogen contamination, we chose, in accordance with the literature, dicumene chromium (DCC) as the single precursor. This compound has been used in various CVD processes [6 9], but the experimental data required to obtain a layer both of uniform thickness and with a homogeneous composition in an industrial-type CVD reactor are insufficient. Thus growth rate results as a function of the deposition temperature are only given by Anantha et al. [9] and results on the influence of the other experimental parameters are rather sparse. Furthermore, the microstructure of these low temperature chromium coatings is not known precisely, since they are described either as a mixture of free chromium, Cr7C3 and Cr3C2 [6] or as Cr1C3 with evidence for free chromium and free carbon [8]. This paper deals with the OMCVD growth of chromium carbide layers using DCC over a wide range of temperature. Their microstructure, investigated by diffraction and spectroscopic techniques, is discussed in relation to previous work. -
2. Ex~erimenta1details The commercially available DCC used in this study was a liquid at room temperature. Because of the discrepancy in melting points found in the literature [7, 16] its chemical homogeneity was in doubt. This suspicion was confirmed by chromatographic analysis using the acid-decomposition method [17]. The substance was a mixture of various bis-arene complexes of chromium differing in the number of isopropyl groups bonded to the benzene ring. This mixture contained only about 70% DCC. After the failure of attempts at vacuum distillation, the DCC was used without further purification. A standard horizontal hot-wall low pressure CVD quartz reactor 2.8 cm in diameter and 30 cm in length was used. Since the chromium atom of the starting material is in the zero-valence state, thermal decomposition of the DCC was carried out under helium with no reducing agent such as hydrogen. The low pressure allowed both compensation for the low vapour pressure of the precursor (2 X 10-2 Torr at 110 °C) and enhancement of the mass transfer rate relative to the surface reaction rate, in order to make the thick-
53
ness more uniform throughout the reactor tube. An optimum growth rate was obtained using a total pressure of 20 Torr, a total flow rate of 143 standard cm3 min’ and a DCC molar fraction of io~.The main structural features of the coatings were investigated by X-ray diffraction, as well as scanning (SEM) and transmission electron microscopy (TEM). The carbon content of the films was determined using either an electron microprobe with reference samples or a combustion method, measuring the amount of the resulting CO 2 gas by chromatography. Since the free carbon was oxidized more easily than the carbon bound as Cr—C, the total amount of carbon was determined following the addition of Pb304 to the oxidizing flow [18]. The electron spectroscopy for chemical analysis (ESCA) spectra were obtained with a spectrometer (VG ESCALAB MKII) operating with a monochromatic Al Ka X-ray source. The data were taken with a constant analyser pass energy of 50 eV in a vacuum system with a base pressure of 10b0 Torr. The binding energies measured were referred to the Au 4f712 level at 84.0 eV from a gold standard film.
3. Results Chromium carbide coatings were obtained between 300 and 550 °C with growth rates as high as 4 pm h~ on steel and silicon substrates. The growth rate seems to be independent of the nature of the substrate. Its variation with the deposition temperature reveals two temperature ranges with two maxima, centred at about 400 and 525 °C.Such a variation in the growth rate as a function of the deposition temperature is unusual in the CVD literature. However, similar behaviour has been reported for epitaxial Ga As~P1 layers [19] and OMCVD titanium nitride [20]. In each case, a change in the reaction mechanism was assumed. Investigations into the thermal decomposition of bis-arene chromium and aromatic ligands led us to the same conclusion and this will be discussed in a later paper [21]. Coatings deposited at low temperature, i.e. 300 450 °C,have a smooth mirror-like surface (Fig. 1(a)). SEM examination of cross-sections reveals good homogeneity, high compactness and uniform thickness in the layers, as well as good step coverage on rough samples. On steel or silicon substrates, the interface is regular and well defined (Fig. 1(b)). In the intermediate temperature range 450 500 °C,the morphology is more heterogeneous since the films assume a mixed structure composed of both a uniform amorphous thin layer and chromium carbide crystallites embedded in and spread over the film. The number of these superficial crystallites increases with temperature and the film roughness is characterized by a peak-to-valley height of 3 5 pm. Above 500 °Cthe coatings have a polycrystalline structure with a mean grain size in the range 5 10 pm (Figs. 1(c) and 1(d)). In the entire temperature range investigated, a uniform chromium distribution was observed by X-ray imaging produced using Cr Ka radiation. Structural analysis by X-ray diffraction indicated that the films deposited -
-
-
-
.
54
_
2Opm
5
1
—4
c15O
~
d 5
Fig. 1. Typical SEM micrographs of chromium carbide coatings deposited at (a) and (b) T<450°C;(c)and(d)T •500°C.
below 500 °Cwere amorphous. The sparse microcrystallites grown on the layer deposited between 450 and 500 °C were not detectable by X-ray diffraction. Indexing of the X-ray diffraction patterns of polycrystalline films deposited above 500 °C revealed that the single Cr 7C3 phase had been obtained. These polycrystalline Cr7C3 films had a hard texture which has not yet been fully characterized. The amorphous state of the low temperature coatings (T < 500 °C)has been confirmed by means of electron diffraction patterns (Fig. 2(a)). The two broad halos characteristic of short-range amorphous order are similar to those in X-ray patterns, i.e. centred around d spacings of 2.10 and 1.20 A. Polycrystalline films deposited above 500 °C are composed of polygonal crystals with a mean size of a few micrometres which have been easily identified as Cr7C3 from indexing of some simple crystallographic planes. The high dislocation density along the grain boundaries is probably the result of stresses during the growth of the layer (Fig. 2(b)). The electron micro-
O5pm
f~
j
r
55
~
‘O.5prn
~
(
~
_
Fig. 2. TEM micrographs and selected area electron diffraction patterns from chromium carbide coatings deposited at (a) T < 500 °Cand (b) T> 500 °C;(c) layer—substrate interface showing the amorphous chromium carbide buffer layer precoated at 350 °Cand the Cr 7C3 coating grown at 525 °C.
graph in Fig. 2(b) also shows linear fringes in the crystal carbides caused by stacking faults. The streaks in the corresponding electron diffraction pattern are caused by these faults in the crystal structure which are characteristic of M7C3 compounds [221 and especially of Cr7C3 [23]. The crystalline Cr7C3 phase was also obtained from amorphous samples prepared at T < 500 °Cafter annealing at 550 °Cfor 5 h, but in this case no preferential orientation was found and the Cr~C3crystals were smaller than
56
those grown at 525 °C.The texture of Cr7C3 films deposited at high temperature (T> 500 °C)seems to be independent of the nature of the substrate since similar X-ray diffraction patterns were obtained for coatings grown on steel substrates and on steel substrates coated with an amorphous chromium carbide intermediate layer deposited at 350 °C.This low temperature chromium precoating seemed to improve the film adhesion on a steel substrate, and TEM analysis at various depths in the film, up to the layer—substrate interface, revealed, near this interface, the expected thin amorphous chromium carbide layer, with a thickness of a few hundreds of ângstrbms. Thereafter, increasing the temperature up to the value during deposition led to nucleation from this amorphous thin layer and a growth of small acicular Cr7C3 crystallites (Fig. 2(c)). Beyond this zone toward the surface, typical polygonal Cr7C3 crystals were observed (Fig. 2(b)). This indicates that the texture was due to the growth conditions rather than to the structure of the substrate. The carbon content of a number of samples is shown as a function of the deposition temperature in Fig. 3. Although there was a temperature threshold near 500 °C,above or below which the structure of any film was crystalline or amorphous respectively, the carbon content of the films was quite independent of the deposition temperature. For the coatings deposited under a total pressure of 20 Torr, the proportion of carbon was in the range 12 13 wt.%, which is significantly higher than the stoichiometric composition of Cr7C3 (9 wt.% carbon) and close to that of Cr3C2 (13.3 wt.% carbon). However no evidence for the existence of this last phase has been found by diffraction analysis. This carbon excess seems to be reduced at lower pressures since in four experiments, two at 425 °Cand two at 510 °C,the carbon content was reduced by about 20% when the pressure was decreased from 20 to 5 Torr, leading to a composition of the layers close to the Cr7C3 stoichiometry -
~2O I
I
I
16 H
z
.
12
z o
-
r.
~,
Cr3C2
~..Cr7C3
08
-
z o
-~—-—~~23~6
0
0 300 350 400 450 500 550 DEPOSITION TEMPERATURE (°C) Fig. 3. Carbon content of chromium carbide films as a function of the deposition temperature: data include chemical analysis of samples prepared under a total pressure of (•) 20 Torr and (0) 5 Torr as well as (A) electron microprobe results obtained for coatings deposited at 20 Torr.
57
(Fig. 3). Similar values of the elemental composition have been found previously [4, 8] and no variation in the composition with the deposition temperature has been observed. However, nothing has been said about the influence of the total pressure. Nevertheless a carbon content of 8 wt.% has been reported for a total pressure as low as 2 Torr [6], which is in good agreement with our results. In order to obtain further structural information, analysis by JR spectrometry was attempted on some amorphous samples but no absorption band was observed. In fact, in searching in the literature, one finds that carbides of some transition metals in groups IV and V have characteristic JR vibrational frequencies in the range 400 600 cm’ [24] but that chromium carbides, especially Cr3C2, have no specific JR absorption band above 1 [25]. Nevertheless, these JR spectra indicate the absence of 400 cmin the coatings since there is no absorption near 660 cm~,as would oxygen be expected for Cr—O bond vibrations, in contrast with the films produced by thermal decomposition of the organometallic compound Cr(CO) 6 [26]. Figure 4 shows ESCA spectra of amorphous and crystalline samples 4~sputtering (4 keV) for a few minutes in order to eliminate the after Ar surface contamination. The binding energy of the C is peak and of the Cr 2p doublet is the same for the two coatings. The shape of the C is peak gives evidence for at least two forms of carbon in the material at both low and high temperature. The C is main peak at 283.5 eV is assigned to carbon bonded to chromium atoms (chromium carbide phase) whereas the shoulder at higher binding energy (285.2 eV) is most probably due to free carbon -
280
283
286 BINDING
572 ENERGY
578
584
1eV)
Fig. 4. ESCA spectra of OMCVD chromium carbide layers deposited at (a) 510 ~C and (b) 400 °Cin the C is and Cr 2p energy region recorded after 15 mm of Ar~sputtering.
58
[27]. The energy difference between the two forms of carbon is in good agreement with that found for amorphous electrodeposited chromium films, i.e. 1.7 eV and 1.6 eV respectively [28]. The higher energy shift of the Cr 2P3/2 level at 575.1 eV is consistent with a chromium carbide structure and no evidence of metallic chromium, which might be expected near 574.0 eV, has been found. The full width at half-maximum of the Cr 2p~2 peak is slightly higher in the amorphous sample than in the crystalline one, which is probably an effect of the disorder. A gaussian—lorentzian curve-fitting computer program was used to resolve the measured C is peak. Then the relative amount of free carbon could be estimated from the intensity ratio, Icc/(Icc + ICCr), where ~ and ‘CCr are the intensities of the peaks due to C—C and C—-Cr bonds respectively. The relative proportion of free carbon was typically in the range 30% 40%. Furthermore, using the intensity of the C is component ‘CCJ and that of Cr 2P2’3 as well as the standard atomic sensitivity factors of carbon and chromium, a quantitative determination gave about 30 at.% of carbon bonded to the chromium i.e. close to the Cr 7C3 stoichiometry for both amorphous and crystalline coatings. -
—
4. Discussion CVD processes using bis-arene chromium may be represented by the simplified chemical equation Cr(arene)2
—~
Cr
+
2 arene
However, in OMCVD the mechanism is more complicated, and while it may be true that the decomposition mechanism is based on selective breaking of the less stable chemical bonds (chromium—ligand in this case), there are often side reactions in competition with the main process, their importance depending on the experimental conditions. Therefore the thermal decomposition of DCC produces chromium carbide films with a carbon content and a microstructure which depend on the deposition process. These coatings have been variously described as a mixture of chromium metal, Cr7C3 and Cr3C2 [6], as chromium metal supersaturated with carbon [8] and often as pure metallic coatings, ignoring the incorporated carbon [7, 9]. In fact, the carbon content in the films may be reduced by the addition of inhibitors in the gas phase [15] or by catalytic decomposition of the DCC [29]. From a thermodynamic point of view, Cr7C3 is the most stable chromium carbide compound among the three possible phases, as shown by its lower standard free energy of formation. X-ray and electron diffraction patterns of the coatings grown above 500 °Cexhibit the Cr7C3 structure and no evidence for crystallites of chromium metal or other chromium carbide phases has been found.
59
However, both chemical analysis and ESCA spectra reveal a carbon excess of about 30% compared with the stoichiometry of Cr7C3. This carbon excess is found in both the amorphous and the crystalline samples and it has the structural features of free carbon, which would segregate during the growth. The incorporation of this carbon excess would be the result of the thermal decomposition of both isopropylbenzene ligands and organic byproducts by side reactions. This assumption is supported by the influence of the total pressure, since decreasing the pressure in the reactor decreases the carbon content, probably because the elimination of the organic byproducts is facilitated and the coatings are less contaminated. This assumption holds if an increase in the total flow rate has the same effect as a decrease in the total pressure, as has been effectively observed using related bis-arene chromium precursors [30]. Furthermore, the relative proportion of free carbon estimated from ESCA spectra is found to be slightly higher in a film deposited at 510 °C(38%) than in another prepared at 400 °C(31%). This is consistent with the presence of side reactions, particularly pyrolysis, since their importance increases generally with the temperature. The microstructure of the amorphous coatings appears to be related to that of the crystalline films. Indeed, amorphous and crystalline coatings have the same elemental composition (12 13 wt.% of carbon) and almost the same ratio of free carbon (30% 40%). In the amorphous coatings, most of the carbon is then bonded to the chromium. Moreover, the similarity of the C is and Cr 2p transitions in the ESCA spectra of the amorphous and crystalline films indicates that the local atomic environments are probably similar. Compared with the stoichiometry of Cr7C3, the carbon excess found in amorphous films by chemical analysis is about 30%, which fits fairly well with the 30% 40% estimated by ESCA, taking into account the uncertainty in each method. This investigation shows that the crystalline films consist mainly of the crystalline Cr7C3 single phase with an excess of free carbon. It also suggests for amorphous coatings a heterogeneous microstructural model composed of amorphous free carbon zones embedded in an amorphous Cr7C3-like phase. This presupposes that in this amorphous chromium carbide phase, there is preferential bonding of chromium and carbon in such a way that the short-range chemical order of the Cr7C3 is preserved. At low temperatures the non-crystalhinity would then be due mainly to variations in dihedral bond angles and bond lengths rather than to a random distribution of carbon and chromium in the network. In the temperature range just below the crystallization temperature, the short-range order is enhanced and extended and the structure tends towards randomly oriented microcrystallites mixed with some larger Cr7C3 crystallites which start to grow from these microcrystalline nuclei. This is supported on the one hand by the fact that the amount of carbon bonded to the chromium is quite close to the Cr7C3 composition and on the other hand by the observation that annealing of amorphous coatings at 550 °Ceasily produces Cr7C3 crystallites. This structural analysis confirms the presence of free carbon which had previously -
-
-
60
been indirectly observed mixed with the chromium carbide phase and metallic chromium [8], but no evidence for free chromium has been found. 5. Conclusion Chromium carbide coatings have been obtained by pyrolysis of DCC in a hot-wall low pressure CVD process, between 300 and 550 °C.The chromium carbide films obtained in the low temperature range 300 500 °C are amorphous and give, after annealing at 550 °C, the crystalline Cr7C3 -
phase. The coatings deposited between 500 and 550 °Cexhibit a textured crystalline Cr7C3 structure which appears to be independent of the substrate. The proportion of carbon in these films is about 12 13 wt.% and is not dependent on the deposition temperature. This represents a carbon excess of 30 at.% compared with the Cr7C3 stoichiometry. This carbon excess has been confirmed by ESCA analysis in both amorphous and crystalline samples. Since no evidence for chromium metal or other chromium carbide phases has been found, the material may be described as a mixture of Cr7C3 and free carbon. This heterogeneous structural model also agrees with data for amorphous coatings. -
Acknowledgments The authors gratefully acknowledge the help of Dr. A. Lebugle and Mr. M. Provincial in providing X-ray photoelectron spectroscopy measurements. References 1 R. Knoche, S. Audisio and H. Mazille, in J. M. Blocher, G. E. Vuillard and G. Wahl (eds.), Proc. 8th mt. Conf. on Chemical Vapour Deposition, Electrochemical Society, Pennington, NJ, 1981, p. 653. 2 G. Cholvy and J. L. Derep, J. Vac. Sci. Technol. A, 3 (1985) 2378. 3 S. Komiya, S. Ono, N. Umezu and T. Narusawa, Thin Solid Films, 45 (1977) 433. 4 K. H. Bloss, H. Lukas and W. Kissing, in J. M. Blocher, H. E. Hintermann and L. H. Hall (eds.), Proc. 5th mt. Conf. on Chemical Vapour Deposition, Electrochemical Society, Pennington, NJ, 1975, p. 136. 5 B. B. Owen and R. T. Webber, Trans. Metall. Soc. AIME, 175 (1948) 693. 6 W. H. Metzger, Jr., Plating, 49 (1962) 1176. 7 R. Tomono, E. Yagi and Y. Togashi, Kinsoku Hyomen Gijutsu (J. Met. Finish. Soc. Jpn.), 16(1965) 210. 8 J. E. Knap, B. Pesetsky and F. N. Hill, Plating, 53 (1966) 772. 9 N. G. Anantha, V. Y. Doo and D. K. Seto, J. Electrochem. Soc., 118 (1971) 163. 10 B. D. Nash, T. T. Campbell and F. E. Block, U.S. Bur. Mines, Rep. Invest., 7112 (1968). 11 G. A. Razuvaev, G. G. Petukhov and A. N. Artemov, Zh. Obshch. Khim., 39 (1969) 2494. 12 T. J. Truex, R. B. Saillant and F. M. Monroe, J. Electrochem. Soc., 122 (1975) 1396.
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