622
Chem. Mater. 1995, 7, 622-630
Pyrolysis Behavior of Titanocene Dichloride Used as a Precursor for the Chemical Vapor Deposition of Titanium Carbide at Atmospheric Pressure Joel Slifirski and Gildas Huchet Etablissement Technique Central de I'Armement, 16 bis Av. Prieur de la C6te d'Or, F-94114 Arcueil Cedex, France
Alex Reynes Laboratoire de Cristallographie, Rkactivitk, Protection des Matkriaux, Ecole Nationale Superieure de Chimie de Toulouse, 118 route de Narbonne, F-31077 Toulouse Cedex, France
Alain Marty and Francis Teyssandier" Institut de Science et de Gknie des Matkriaux et Prockdks, Universitk de Perpignan, 52 Av. de Villeneuve, F-66860 Perpignan Cedex, France Received July 20, 1994. Revised Manuscript Received August 28, 1994@
In a n attempt to lower the temperature of depositing titanium carbide on steel by chemical vapor deposition, titanocene dichloride (CpzTiCl2) was used a s a n organometallic precursor. The thermochemistry of the precursor was characterized by several techniques. Both isothermal and dynamic thermogravimetric analysis showed that congruent sublimation of the precursor occurs below 160 "C. Above this temperature, the precursor progressively decomposed to leave a solid residue and a volatile compound which was observed by gas chromatography. According to mass spectroscopy results, the lower temperature decomposition mechanism was interpreted as the loss of a Cp group. Melting of CpzTiCl2 was observed to occur at 264 "C, and the transformation previously reported at 290 "C corresponds to the total decomposition of the molecule rather than melting. This conclusion was supported by the detection of HC1 by mass spectroscopy and gas-phase infrared spectroscopy. Chemical vapor deposition of T i c was then performed by three different experimental procedures. The composition of the layers measured by electron probe microanalysis with wavelengthdispersive spectroscopy indicated a n excess of carbon in the film (18 at. % < carbon in excess < 34 at. %). The amounts of both chlorine and oxygen contained in the films a s well a s the deposition rate was strongly dependent on the vaporization temperature of Cp2TiClz.
Introduction Titanium carbide's remarkable thermal stability (mp 3067 "C), high abrasion resistance, and low friction behavior make it an attractive candidate as a protective coating against abrasion, corrosion, or frictional wear. Tic has been used in numerous technological applications including cutting or milling tools and inserts, cold extrusion nozzles and punches, forming or stamping tools, and ball-bearing elements in which both the ball and the rolling element are coated. Traditionally, these substrates have been industrially coated from an initial gas mixture composed of hydrogen, methane, and titanium tetrachloride. Due to the high thermal stability of these compounds, a high deposition temperature is required (1200-1500 K). Such a high temperature maintained throughout the process enhances solid-state reactions between the film and the substrate, increases @
Abstract published in Advance ACS Abstracts, February 15,1995.
0897-4756/95/2807-0622$09.00/0
residual stresses due to thermal expansion mismatch between the substrate and coating,l promotes grain growth in the substrate structure, and increases the extent of decarburization of carbon-containing substrates.2 To minimize these problems, it is helpful to deposit the coatings at a lower temperature. This can be achieved by conventional thermal CVD using less stable molecules such as organometallic compounds. In 1975, Sugiyama3 et al. used Ti(NR2)4 (R = methyl, ethyl, n-propyl, n-butyl) to deposit TiN(C) materials under atmospheric pressure at a temperature as low as 250 "C with a maximum yield at 400 "C. The films char(1) Takahashi, T.; Sugiyama, K.; Tomita, K. J . Electrochem. SOC. __ 1967,114,1230. (2) Van der Straten, P. J. M.; Michorius, M. M.; Verspui, G. Proc. N Euro. Conf. CVD: Bloem, J., Versuui, G., Wolff, L. R., Eds.: Eindhoven, The Netherlands, 1983; p 553. (3) Sugyama, K.; Pac, S.; Takahashi, Y.; Motojima. S. J. Electrochem. Soc. 1975, 122, 1545.
0 1995 American Chemical Society
Pyrolysis Behavior of Titanocene Dichloride
acterized by XRD revealed crystallized materials with small grain size. Morancho415et al. obtained Ti(C,N,H) amorphous films in the temperature range 370-520 "C using the precursor tris(2,2'-bipyridine)titanium a t a reduced pressure (1.3 x lop3 Pa). Tic coatings were also grown from tetraneopentyltitanium at even lower deposition temperatures (150-300 "C) under similar reduced total pressures ((0.133-1.3) x low3 In this work, a relatively low deposition rate was observed: 0.33 p m h . More recently, FixgJoet al. studied several organometallic compounds in the form of amidoor imidotitanium(IV1 complexes. The deposition temperature ranged between 150 and 450 "C and the coatings were deposited at atmospheric pressure using various carrier gases. The presence of NH3 was conspicuously influential on the amount of carbon and oxygen measured in the films. The use of NH3 lowered the contamination to less than 1at. %, but attempts to grow films thicker than 2000 A resulted in film cracking and powder formation. Fine-grained crystalline TiN was also deposited by Ishiharall et al. from Ti(N(CH3)2)4 and NH3 in the temperature range 300-500 "C at 40 Pa total pressure. To develop a CVD process which is appropriate for industrial applications, a precursor was sought which had a reasonable vapor pressure and would be commercially available at a reasonable price. The selected precursor, titanocene dichloride: CppTiCl2 (Cp = h5C5H5) fulfilled these requirements. This compound is a bright red powder with a melting point reported t o be 287-29012 or 230 OC.13 Very few reports have discussed the pyrolysis of organometallic compounds14J5or their influence on the properties of the coatings. This paper focuses on the effect of the organometallic precursor thermal history on the CVD of Tic.
Experimental Section Characterization and experiments were undertaken on a powder sample provided by Aldrich Chemical Co. with a purity of 97% and granule size ranging between 0.1 and 0.4 mm. Thermal Behavior. The thermal behavior of titanocene dichloride was investigated by both thermogravimetry analysis (TGA Perkin-ElmerTGS-2) and differential scanning calorimetry (DSC Setaram TG-DSC 111). The CpzTiClz compound was loaded under argon atmosphere using standard Schlenk techniques or in an inert-atmosphere glovebox. Thermogravimetric studies (Perkin-Elmer TGS-2) were performed under both isothermal and variable-temperature conditions on samples (4)Morancho, R.; Petit, J. A,; Dabosi, F.; Constant, G. J . Electrochem. Soc. 1982,129,854. ( 5 ) Morancho, R.; Constant, G.; Ehrhardt, J. J. Thin Solid Films 1981,77,155. (6) Kaloyeros, A. E.; Williams, W. S.; Alloca, C. M.; Pollina, D. B.; Girolami, G. S.Adu. Ceram. Mater. 1987,2,257. (7)Kaloyeros, A.E.;Williams, W. S.; Constant, G. Reu. Sci. Instrum. 19M. _ _ _ _ 59. 1209. ~ ~ (8) Alloca, C. M.; Williams, W. S.; Kaloyeros, A. E. J . Electrochem. Soc. 1987,134,3170. (9) Fix, R. M.; Gordon, R. G.; Hoffman, D. M. Chem. Mater. 1990, 2,235. (10) Fix, R.M.; Gordon, R. G.;Hoffman, D. M. Chem. Mater. 1991, 3,1138. (11)Ishihara, K.;Yamazaki, K.; Hamada, H.; Kamisako, K.; T a d , Y. Jpn. J . Appl. Phys. 1990,29,2103. (12)Wilkinson, G.; Birmingham, J. M. J . Am. Chem. Soc. 1954, 76, 4281. (13)Cotton, F.A.;Wilkinson, G. Advanced inorganic chemistry, 5th ed.; Interscience: New York, 1988. (14) Waters, J. A,; Vickroy, V. V.; Mortimer, G. A. J . Oganomet. Chem. 1971,33,41. (15) Margitfalvi, J.; Koltai, L. J . Thermal Anal. 1979,15, 251. , - - I
Chem. Mater., Vol. 7, No. 4, 1995 623 with an initial mass of 3.5 mg. All the experiments were performed under inert nitrogen carrier gas (50 sccm), and the system was evacuated and purged with nitrogen before each run. In the isothermal TGA experiments, a constant temperature was reached using the maximum heating rate (120 "C/ min) and maintained for 2-8 h depending on the transformation rate of CpzTiClz. In the scanning TGA experiments, variable heating rates (between 5 and 120 "C/min) were used in the range 20-940 "C. The DSC scans were taken on samples in sealed in aluminium crucibles. Optical Observation of the Melting Process. Melting of CpzTiClz was optically observed with the following apparatus. A glass capillary tube (Lindeman tube i.d. 0.35 mm) was filled with titanocene dichloride and sealed in a glovebox. The capillary was heated by a resistor wrapped around it. The temperature was controlled by a regulator with an input from a chromel-alumel thermocouple placed between the capillary and the resistor. The morphological modifications of CpzTiClz were observed by means of an optical microscope (Nikon 104, x 100 enlargement). Mass Spectroscopy. The CpzTiClz compound was introduced through a load lock-chamber directly into the ionization chamber of the mass spectrometer (Nermag R.lO-10 quadrupole spectrometer). Electron impact spectra using 70 eV electrons were recorded as a function of temperature for heating at a rate of 120 "C/min from 50 to 350 "C. Gas-Phase Chromatography. The precursor was placed in a pyrex U tube heated with a variable-resistance heater whose temperature was monitored by a chromel-alumel thermocouple. Helium (168 sccm) transported the vaporized compounds to the chromatograph through an ice-cooled trap. The CPG 121 FL Intersmat Fid chromatograph was equipped with a 2 m long stainless steel column having an i.d. of 3 mm and coated with 10% SE30 on chromosorb W-AW, 80/100. The carrier gas in the column was nitrogen (flow rate 15 sccm), and the oven temperature was controlled at 60 "C. Infrared Spectroscopy. Fourier-transformed infrared spectroscopy (FTIR) was performed, using a Perkin-Elmer 1600, as a function of temperature on both solid and gas samples. The solid samples were made of 300 mg of KBr mixed with 2 mg of CpzTiClz and pressed at 10 W/cm2 for 1 min. They were then placed in the slot of a stainless steel tube (i.d. 10 mm, I = 60 mm) heated by a resistor and regulated in temperature. The gas-phase investigations were performed in a stainless steel commercial cell (1 = 58 mm) equipped with KBr windows (diameter 15 mm) and heated by a resistor (Thermocoax) tightly wrapped around the lateral surface and regulated by a PID controller. A 0.9 mg sample of CpzTiClz was placed inside the cell in direct contact with its wall. The cell was initially loaded with either Ar or Hz. CVD Configuration and Procedure. Deposition of Tic was performed in an inverted vertical cold-wall reactor operating at atmospheric pressure under hydrogen flow (Figure 1). The flow of purified HZcarrier gas was controlled at 500 sccm with a mass-flow controller. The steel substrates (AIS14135, 0.d. 16 mm and 5 mm thick) were polished metallographically using 1ym diamond paste, sonicated, and rinsed with Freon. The temperature of the substrate, which was inductively heated by a RF coil (250 kHz generator), was measured by a sheathed chromel-alumel thermocouple (1 mm diameter) inserted in a blind hole located in the middle of the upper face. The organometallic precursor handling system consisted of a steel crucible of cylindrical shape (i.d. 20 mm, height 25 mm), containing 4 g of precursor for each experiment. The crucible was heated by a resistor (Thermocoax)tightly wrapped around the lateral surface and regulated by a PID controller. The temperature was monitored by a chromel-alumel thermocouple in contact with the inner surface of the crucible. To minimize radiative heating of the precursor by the substrate, the crucible was covered by a lid with a centered 1 mm diameter hole. The CpzTiClz compound was loaded under an argon atmosphere using standard Schlenk techniques. The CVD system, loaded with the substrate and the precursor, was then evacuated for 8 h and purged with hydrogen for 30 min (1slm). At the end of each experiment, heating of the crucible
Slifirski et al.
624 Chem. Mater., Vol. 7, No. 4,1995
Cold wall reactor
4Chromel-alumel thermocouple
Steel substrate
Vaporization crucible
1
Endothermic
I
I
'
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I
f
'
I "
260
'
I
1
' I "
I ' f " l
'
270 280 290 Temperature ("C)
'
'
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Heating wire thermocouple
-b
Figure 1. (a, top) Schematic of the experimental CVD apparatus. (b, bottom) Enlarged view of the crucible. was stopped, and the temperature of the substrate was decreased gradually, with the hydrogen flow rate maintained until the substrate had reached room temperature. Film Characterization. X-ray diffraction studies were conducted using a Philips PW 1729 X-ray diffractometer. Electron probe microanalysis with wavelength-dispersive spectroscopy (EPMA-WDS) was performed on a Cameca SX50 at 15 kV excitation voltage using the PAP correction program.16 The standards were pure and stoichiometric T i c for titanium and carbon, FeaOa for oxygen, and NaCl for chlorine. Scanning electron micrographs (SEM) were obtained on a Hitachi S520 electron microscope, equipped with a Tracor 5500 energydispersive spectrometer (EDS). The thickness of the deposits was determined by abrasion through the coating, obtained by the rotation of a 15 mm diameter stainless steel ball together with an abrasive agent (1or 3 pm diamond). The thickness was calculated from the diameters of the ring prints measured by optical microscopy. Vickers microhardness measurements were performed on a Shimadzu type M tester. For each load, five indentations were made on the polished surface of the coating with a dwell time of 15 s. The microhardness measurements were made to the maximum load available without the occurrence of cracks around the Vickers print. All data were corrected according to the method proposed by Farges and Degout.l7 The following empirical relationship allows the determination of the intrinsic hardness, H,, that is not dependent on the applied load: H, = H, Pd-l, where d is the diagonal of the indentation print. H , is thus deduced from the extrapolation to infinite load of the linear regression applied to the data when plotting the measured Vickers hardness H,, versus lld.
+
Results and Discussion Thermal Behavior of the Precursor. It is normal practice to heat solid (or liquid) precursors to the required temperature as quickly as possible and maintain that temperature during deposition. Due to the poor thermal stability of organometallic compounds, their decomposition generally interferes with the sub(16) Pouchou, J. L.; Pichoir, F. Rech. Aerospatiale 1984,3,167. (17) Farges, G.; Degout, D. Thin Solid Films 1989,181, 365.
263 264 265 266 267 268 269 Temperature ("C)
Figure 2. (a, top) DSC spectrum of titanocene dichloride. (b, bottom) Enlargement of the peak at 266 "C, and extrapolation of the onset temperature.
limation (or evaporation) process. Thus, the thermal behavior and especially the melting and decomposition temperatures are of special interest. Two different values of the melting temperature are mentioned in the literature. A value of 289 f 2 "C is referenced by Wilkinson and Birmingham,12 while the value 230 "C is quoted in Cotton and Wi1kin~on.l~ Neither value was in agreement with the results of our experimental observations. Two endothermic peaks a t 266 "C (264 "C for the extrapolated onset temperature, Figure 2b) and 288 "C were revealed from microcalorimetric investigations performed in the range 250-300 "C (Figure 2a). Further microcalorimetric experiments were undertaken in the range 253-315 "C to determine the nature of the transformations encountered, on account of the reversible feature of the melting transition and the irreversible feature of the decomposition reaction. Three temperature cycles (253-275-253, 253-286-253, 253-315253 "C) were successively applied to the organometallic compound which had first been heated to 253 "C at a rate of 10 "C/min. Each cycle was composed of two linear ramps of 4 "C/min from 253 "C to the intermediate temperature and back to 253 "C. A blank curve, recorded in a final run after total decomposition of the compound, was subtracted from the original curve to limit artifacts resulting from the apparatus. During the first cycle, only two small peaks a t 266 "C (endothermic on heating and exothermic on cooling) were detected (Figure 3). During the second cycle, an endothermic peak was moreover observed above 275 "C, but the corresponding transformation was interrupted at 286 "C which was the maximum temperature of the cycle. The exothermic peak at 266 "C was still observed on
Chem. Mater., Vol. 7, No. 4, 1995 625
Pyrolysis Behavior of Titanocene Dichloride 350 315'C 286'C
250
250 "C 500 OC
F
200 300 400 500 Time (min) Figure 4. TGA at constant temperature. Variation of the weight of CpzTiClz as a function of time for different temperature levels. The curves are presented as a percentage of the initial amount of CpzTiClz but do not include the heating transition domain. 0
100
lo00
200 O C
2000
3000 Time (s)
4000
5ooo
Figure 3. Reversible and irreversible thermal effects observed by microcalorimetric investigation. The arrows correspond to the melting and the solidification of titanocene dichloride.
cooling. During the third cycle, the endothermic peak at 290 "C revealed the complete transformation of the compound above 275 "C. The nonappearance of the peak at 266 "C on cooling was an indication of the total decomposition of CpzTiCl2. We can conclude from this experiment that the phenomenon at 266 "C is reversible as long as CpzTiClz is not heated above 290 "C and could thus be assignable to the melting of the compound. Consequently, and in contrast with the previous assessment,12 the thermal event at 290 "C is not assignable to melting but decomposition of the organometallic compound. This point was confirmed by the formation of HC1, as observed from mass-spectroscopy measurements at that temperature.ls To confirm the melting temperature, we optically observed the morphological transformations of CpzTiCl2 around 266 "C. The product was heated from 20 to 220 "C at a rate of 20 "C/min and subsequently at a rate of 5 "C/min. At 270 "C, the first drops of titanocene dichloride began to wet the wall of the capillary tube. At temperatures higher than 280 "C, the melt turned to black. Temperature variations (&5 "C) around 270 "C alternatively induced melting and solidification of titanocene dichloride. We conclude that the melting temperature of titanocene dichloride is 264 "C. The above characterizations have revealed a total decomposition of CpzTiClz above 290 "C. Partial decomposition may nevertheless occur at a lower temperature, and further thermogravimetric investigations were accordingly carried out. The thermal decomposition of CpzTiClz was first studied under isothermal conditions at various temperatures between 150 and 500 "C. The level temperatures were reached using the maximum heating rate (120 "C/min). The curves presented in Figure 4 reveal three basic behaviors according to the temperature range. Note that in Figure 4, time 0 corresponds to the beginning of the isothermal period. (18)Slifirski, J.; Huchet, G.; Teyssandier, F. Proc. VIII Euro. Conf. CVD; Glasgow, U. K., Hitchman, M. L., Archer, N. J., Eds.; Colloque C2, J . Phys. Suppl. No. 7,1991,625.
100
(i)From 150 to 180 "C,an almost constant vaporization rate at the beginning of the curves indicates zeroorder or very slow first-order kinetics which is indicative of a preponderant sublimation process. The departure from such behavior after a certain time, as observed at 200 "C, reveals the occurrence of a decomposition process even at low temperature. The exponential variation of the TGA curve in that case is typical of a first-order reaction. (ii) From 200 to 300 "C the vaporization rates are rapid at the beginning of the isothermal period, and a constant level is quickly reached. The exponential departure from the linear variation, which is observed just before the constant level is reached, once more provides evidence of a decomposition process. The main feature of this temperature domain is the increase of the residue amount with temperature as a consequence of the decomposition process, which yields compounds that do not vaporize at those temperatures. (iii) Above 300 "C the solid products resulting from the first decomposition are no longer thermally stable and the amount of residue decreases with temperature. The vaporization rate constants calculated from the linear part at the beginning of the curves are presented in Figure 5. The rate constants from 150 t o 250 "C can be assignable to the same vaporization mechanism. In this temperature range, the values of both the preexponential factor and the activation energy can be calculated according t o the Arrhenius equation ( K = Ae-EA/RT);with EA = 112 kJ-molw1and A = 7.9 x lo9 min-1. Such an activation energy is too high for a pure sublimation process and suggests that a decomposition process is superposed in the temperature range 150250 "C. Above 300 "C the vaporization process becomes almost independent of temperature (Figure 5). The competition between a sublimation and a decomposition process is supported by the direct observation of the change in appearance of CpzTiClz, induced by the temperature increase. The initially bright red crystals were almost unchanged up t o 180 "C. This observation and the linear vaporization rate of CpzTiClz with temperature up to 180 "C (Figure 4)tend to attest that sublimation prevails over decomposition in this temperature range. The red color became darker above 200 "C and turned black in the neighborhood of the melting
626 Chem. Mater., VoL. 7, No. 4, 1995
Slifirski et al.
iF E
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250 350 450 550 Temperature ("C) Figure 6. Variations of the weight percentage lost through the various heating regimes. (a) At the end of the heating 150
0.0012
0.0016
0.0020
0.0024
1/T( 1/K) Figure 5. Logarithm of the rate constants plotted as a function of the inverse of the temperature level. The rate constants were calculated from the initial linear segment of the vaporization curves recorded during TGA at constant temperature.
period at 120 "C/min; (b) during the isothermal period calculated as a percentage of the remaining amount at the end of the heating period; (c) at the end of the experiment as a percentage of the initial amount of CpzTiClz. h
Y
. I
temperature. These slight modifications observed in the aspect of CpzTiClz below the melting temperature are indicative of minor transformations in the solid precursor but did not induce any modification of the X-ray pattern. Above 300 "C, the compound was no longer crystalline but had the muddy appearance of tar. This tarry consistency may be responsible for the weak dependence of the vaporization rate on temperature above 300 "C, since diffusion of the molecules through the tar before vaporization could be rate limiting. After cooling, CpzTiClz heated in the temperature range 300350 "C had the appearance of a compact gray solid with a smooth and glassy surface. At 360 "C, the residue became a black divided solid with a granular aspect and only white ashes remained when the compound was heated above 500 "C. Though the formation of ashes a t 500 "C suggests the end of the decomposition process, subsequent dynamic heating at 20 "C/min of the residue collected after heating at 500 "C for 164 min showed that 10% of the remaining compound further vaporized above 700 "C. The weight percentages lost during either the heating sequence at 120 "C/min or the isothermal period are shown in Figure 6. The losses recorded at the end of heating sequence (Figure 6, curve a) were not reproducible. They ranged between 5 and 10% a t 250 "C and then increased rapidly, so that more than 50% of the initial amount was vaporized when the imposed temperature was greater than 400 "C. The weight lost during the isothermal period is presented as a percentage of the remaining amount at the end of the heating period (curve b), in order to eliminate the influence of the transition regime. In that case, the weight loss decreases with temperature when the isothermal level is below 300 "C. Above that temperature, the weight loss remains close to 40%. This weight loss value could be interpreted as the loss of one C1 and one C5H5 ligand in Cp2TiClz to form titanium 111molecules (CpTiCls and CpaTiC1). Nevertheless, the tarry consistency above 300 "C and the liberation of HC1 discussed below suggest a much more complex liquid mixture of undetermined composition. Due to the very low vaporization rates observed below 200 "C, it was not possible to reach the end of the vaporization process within a reasonable
10 Temperature ("C) Figure 7. Gas-phase chromatography. Variation of the peak area corresponding to the second retention time (3.2 min) as a function of the vaporization temperature of CpzTiClz.
time. The temperature corresponding to the formation of a nonvaporizable residue, that is to say the temperature a t which decomposition of CpaTiCla starts, can nevertheless be estimated from the weight loss variation displayed as a percentage of the initial amount of CpzTiClz (curve c). A temperature of 150 "C was thus extrapolated using data below 200 "C (curve c). The temperature of 150 "C, a t which decomposition starts, corresponds to the beginning of the formation of a volatile compound as observed by gas-phase chromatography. Two peaks with respective retention times 2.2 and 3.2 min were recorded. The first appeared a t low temperature (50 "C), and the area under it decreased with temperature. The vaporized compound associated with this peak is interpreted as an impurity detected by mass spectroscopy. This point will be further discussed with the mass spectroscopy investigations. In contrast, the area of the second peak increased with temperature, but it was detectable only above 160 "C (Figure 7). A cold trap at 0 "C, placed between the evaporation cell and the chromatograph, prevented interference by sublimated Cp2TiClz. Thus, only volatile compounds were detected. The preferential elimination of a cyclopentadienyl group observed by mass spectro-
Pyrolysis Behavior of Titanocene Dichloride scopic investigation^'^ is an indication of its probable weaker bond strength with titanium. The loss of a cyclopentadienyl group and the subsequent vaporization of the highly volatile cyclopentadiene may then be responsible for the second peak. On the basis of this assumption, the remaining compound that constitutes the residue could be CpTiCl2. As a matter of fact, the dimeric or polymeric structures that have been proposed20 for CpTiCl2 can account for its nonvolatility. Thermogravimetric measurements were also carried out under a dynamic heating mode. Several heating rates were used: 5,10,15,18,20,120"Clmin. A typical curve of the thermal behavior of Cp2TiClz is presented in Figure 8 for a heating rate of 5 "Clmin. Two distinct rates of vaporization were successively observed from 30 to 275 "C and from 275 to 520 "C. If we assume that the thermal phenomenon observed from 30 to 264 "C corresponds to the first-order decomposition process already observed under isothermal conditions, it is then possible to model the weight loss with the following firstorder reaction equation: ----exp{--}dT da- A a P
EA
RT
where a is the fraction of initial amount vaporized at time t, EAis the activation energy, A is the preexponential factor, R is the universal ideal gas constant, and ,l= ? dTldt = 5 "Clmin is the heating rate. Though exp(-EA/RT) is not analytically integrable, an approximation is given for high E d R T values.21 The EAand A values that best fit the experimental curve are EA=
124 k J mol-'
A = 2 x lo1' min-'
Owing to the large variation range of EAand A values that could reasonably fit the experimental curve, these values are in relatively good agreement with those determined at constant temperature and could be attributed to the same decomposition process. The second thermal phenomenon observed from 275 to 520 "C in Figure 8 must be attributed to the total decomposition of CpsTiCl2. Mass Spectroscopy. Several publication^'^,^^,^^ have already been devoted to the mass spectroscopy of n-complexes of transition metals and especially to Cp2TiC12. A detailed fragmentation scheme has been established from observation of metastable transitions together with clastogram data.lg Two initial unimolecular decomposition reactions involve competitive loss of C5H5 and C1, but the greater abundance of Ti(C5H5)C12+ suggests the preferential elimination of the cyclopentadienyl group. This mechanism is followed by ion removal and fragmentation of the cyclopentadienyl group. The most abundant ions are formed according to the scheme:I9 (19) Dillard, J. G.;Kiser, R. W. J. Organomet. Chem. 1969,16,265. (20) Wailes, P. C.; Coutts, R. S. P.; Weigold, H. Organometallic chemistry of titanium, zirconium, and hafnium; Academic Press: New York, 1974. (21) Coats, A. W.; Redfern, J. P. Nature 1964,201, 68. (22) Hunt, D. F.; Russell, J. W.; Torian, R. L. J. Organomet. Chem. 1972, 43, 175. (23) Nesmeyanov, A. N.; Nekrasov, Y. S.; Sizoi, V. F.; Nogina, 0. V.; Dubovitskii, V. A.; Sirotkina, Y. E. J. Organomet. Chem. 1973, 61, 225.
Chem. Mater., Vol. 7, No. 4, 1995 627
L.
\
\
\ L-----
200 400 600 800 Temperature ("C) Figure 8. TGA with a dynamic heating mode of 5 "C/min. Variation of the weight percentage of CpzTiClz as a function of temperature, experimental results (dashed line), and model results (solid line). 0
-
Ti(C5H5)2C12+
Ti(C5H5)CI2+
Ti(C5H5)2CI+
-
Ti(C&)CI+ C5H5+
-
-
Ti(C3H&CI+
C3H3+
Mass spectroscopy of CpzTiCl2 has also been studied with the chemical ionization (CI) technique.22 In that case, the reagent gas was methane which forms CH5+ and C2H5+ ions. Protonation first occurs at the metalchlorine bond to form Ti(CbH5)2Cl+ HC1 and then on the C5H5 ring to form Ti(C5H5)Cl+ C5Hs. Dimeric ions corresponding to (CsH5)3Ti2C13+ were also observed which probably resulted from collisions in the gas phase. In a previous paper,18 we focused on the decomposition of CpzTiClz. A small amount of the compound was introduced into the ionization chamber and was heated at a rate of 12 "Clmin. The occurrence of HC1 (mle = 36) revealed its total decomposition at a temperature which was estimated to be 310 "C but was probably lower. As a matter of fact, the temperature extrapolated from the spectra was not very reliable due to both the limited number of spectra recorded during heating and the shift toward higher temperatures caused by the heating rate. We undertook new mass spectroscopic investigations in order to check for the presence of impurities in our organometallic compound and look at the formation of dimeric structures. From a first run, focused on species with low mle values, we found fragments that did not belong to the initial compound. The spectrum presented in Figure 9 clearly shows that the most abundant ions (mle = 43,57,71 or 83,971 differ in mle value by multiples of 14. This suggests the presence of an alkane compound. The nature of the impurity, which was probably also responsible for the ion at mle = 149 was not determined. The ionic current data shown in Figure 10 as a function of time (heating rate 120 "Clmin) for several mle values (43, 57, 71) furthermore reveals the occurrence of these fragments in a temperature domain lower than that corresponding to the occurrence of fragments belonging to CpaTiClz (mle = 248). The spectrum presented in Figure 11 was obtained in a different run, and was recorded at the time (t = 200 s) of the maximum of the total ionic current which was also the time at which maximum ionic current for the parent ion (248) occurred. The fragments that were observed at lower temperatures were no longer detect-
+
+
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628 Chem. Mater., Vol. 7, No. 4, 1995 108
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Figure 12. Typical infrared spectrum recorded during heating at T = 300 "C of solid CpzTiClz in KBr. 58
38
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Figure 9. Mass spectrum, observed at low temperature of vaporization, that corresponds t o a nondetermined impurity.
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Figure 13. Typical infrared spectrum recorded from the vapors over CpzTiClz at T = 330 "C, which shows the rotational spectrum of HC1.
---' 1 ----lp----
I
Mi
I20
240
180
100
Time (s)
Figure 10. Ionic current corresponding t o mle = 43, 57, 71, and 248 plotted as a function of time (heating rate of 120 "CI min) .
7
268
318
410
360
468
d e
Figure 11. Mass spectrum observed at the maximum ionic current recorded at mle = 248. The signal above mle = 260 is multiplied by 128.
able. In addition to the species previously observed by other authors,lg two other peaks at mle = 433 and 461, presenting a far lower intensity, were also detected. These two groups may, respectively, be attributed to (C5H&TizC14+ and (C~jH&TizC13+. The calculated frequency of isotopes is, in both cases, in complete agreement with the experimental observation. Such structures would be obtained from a shared chlorine atom, and involve two valence states for titanium (111and IV). Infrared Spectroscopy. Two special devices were made to measure the infrared spectra resulting from the pyrolysis of CpzTiClz, from either the solid state or in the gas phase. In the range 500-4000 cm-l several peaks can be used for such a purpose: vibrations relative (i)to the aromatic structure of Cp at 3100,1740, and 820 cm-l or (ii)t o alkane groups at 2910 cm-l. The vibration modes relative to the Ti-C1 bond which appear below 500 cm-l could not be seen with our infrared spectrometer. The spectrum presented in Figure 12 is representative of the spectra recorded from the solid state as a function of time a t different temperatures (270, 300, 330 "C).
These spectra indicate that an increase in temperature results in a decrease of the peak heights characteristic of the aromatic structure and a correlative formation and enhancement of the peak heights belonging to the alkane groups. This behavior is indicative of the degradation of the Cp group to form linear decomposition products. As the peak a t 2910 cm-l was not detected in the initial spectrum of the organometallic compound at room temperature, the amount of the impurity detected by mass spectroscopy must be rather low. The small amount of gaseous molecules vaporized was not sufficient for detection of peaks in the gas phase spectra recorded below 300 "C. When maintained a t or above 300 "C, the peaks seen in the solid-state spectra were detected. Their intensities increased with time, but the poor reproducibility of the deduced kinetics prevented further analysis of these results. It is interesting to note that the rotational spectrum of HC1 was also identified from the results shown in Figure 13, which confirmed the mass spectroscopic investigations. Conclusions on Thermal Behavior. From the various characterizations of the thermal behavior of CpzTiC12, several conclusions can be drawn. Heating of this compound results in melting a t 264 "C. As found from TGA, the thermal decomposition onset temperature is 160 "C. Chromatography revealed that the first decomposition process that occurs above 160 "C was accompanied by the loss of a gaseous product which may be CbH6. The loss of a Cp group appears to be the preferential elimination mechanism deduced from clastogram data,21in competition with the loss of a chlorine atom. The chlorine atom liberated in the presence of alkane groups, which infrared spectroscopy revealed to form from the decomposition of Cp groups at a temperature as low as 270 "C, can form HC1. As this species is not observed below 290 "C, we can assume that the decomposition mechanism that involves the loss of a chlorine atom, leading to the total decomposition of the initial molecule, does not take place below that temperature. Consequently, the endothermic peak, observed both by DSC and DTA at 290 "C should be attributed to total decomposition of CpzTiClz.
Pyrolysis Behavior of Titanocene Dichloride
b
5.0 pm Figure 14. Cross-sectional SEM of a film grown with CpzTic12 heated from 25 to 200 "C at a rate of 10 "C/min and subsequently at a rate of 1 "C/min up to 360 "C. The steel substrate was first heated to 740 "C.
Organometallic Chemical Vapor Deposition (OMCVD). From a CVD point of view the decomposition of the precursor is a step required by the process. In the case of a solid precursor the atoms necessary to grow the coating can be transported toward the substrate by means of an initially sublimated molecule or by means of byproducts resulting from its thermal decomposition. To check the influence of the vaporization temperature, CVD experiments were undertaken in both cases. In the first set of experiments, the steel substrate was first heated to 740 "C. CpeTiC12 was then heated from 25 to 200 "C at a rate of 10 "C/min and subsequently at a rate of 1 "C/min up to 360 "C. Two types of layers are clearly seen on the polished cross section of the sample in Figure 14. The thin layer (=2pm) adjacent to the substrate had a black appearance, and was composed of many sublayers (10.5pm). Due to the thinness of these sublayers, only a mean composition of Ti0.34Co.s700.~9was determined in that layer by EPMA-WDS measurements. These laminated structures have already been observed and were presumed to result from oscillating reactions by other author^.^^^^^ The thicker ( 11 pm) homogeneous upper layer, had the gray-metallic appearance of titanium carbide and a (24) Aggour, L.; Fitzer, E.; Schlichting, J. P m . V. Int. Canf CVD; Blocher, J. M., Hintermann, H. E., Hall, L. H., Eds.; Fulmer Grange: England, 1975, 600. (25) Taschner, C. H.; Leonhardt, A.; Schonherr, M.; Wolf, M.; Henke, J. Mater. Sci. Eng., A 1991, 139, 67.
Chem. Mater., Vol. 7, No. 4, 1995 629
composition of Tio.3*Co.590a.a2Clo.al. These two layers contained a high carbon content (60 at. 96)as expected from the composition of the initial precursor molecule (Cmi = 10). According to the Ti-C phase diagram the deposits should be two-phased materials, Tic + C. As graphitic carbon was not detected by XRD,the excess carbon is probably amorphous. Several unsuccessful attempts were made to reveal either carbon (amorphous or graphitic) or titanium carbide by Raman spectroscopy. As a matter of fact, the very important background signal induced by luminescence effects, prevented any detection of the corresponding peaks. The main difference between the two types of layers lies in the percentages of oxygen and chlorine. As chlorine is suspected to lower the adhesion of films on steel substrate26 or corrode them, i t is important to minimize its content in the layers. At least 1 at. 96 chlorine is always present in the upper layer, whereas less than 0.5 at. 96 is detected in the black one. This feature indicates a change in the precursor molecules and, consequently, in the growth mechanism of each layer. Large differences in the oxygen content were also measured. That amount never exceeded 2 at. % in the upper layer, whereas 20 at. % was reached in the black layer of some samples. A second set of experiments was performed with a modified experimental procedure. Vaporization temperatures higher than 300 "C were used in that case corresponding to a total decomposition of CpzTiCl2 in the handling crucible, and a deposit from only the volatile byproducts of the decomposition process. The substrate was first heated and maintained at 280 "C while Cp2Tic12 was heated to the vaporization temperature level (300 or 360 "C) at a rate of 10 "C/min. The moderate heating of the substrate was aimed at preventing condensation of the vaporized compounds while avoiding surface reactions and subsequent growth of a layer. When the crucible temperature reached 300 "C (or 360 "C), the substrate was heated to the deposition temperature level of 740 "C, at which it was maintained during the 2 h of the deposition process. The 10 pm thick coating, obtained under such a procedure (Figure 15) is similar to the upper layer described previously and has a mean composition of Ti~.44Co.ssOo.o2Clo.~2. The slightly lower amount of carbon obtained in that case could not be explained. Maximum thicknesses of 10 and 16 pm were obtained for vaporization temperature levels of respectively 300 and 360 "C. The layer stopped growing when decomposition of the initial amount of precursor (4 g) in the crucible was complete. The thicker layer obtained at 360 "C resulted from the increased amount of volatile byproducts produced by the decomposition, as compared with that at 300 "C. As already observed in Figure 4, above 300 "C, the amount of residue decreases when the vaporization temperature increases. The Vickers microhardness measurements that were performed on the polished surface of a 15 pm thick layer deposited from precursor vaporization at 360 "C yielded Hvo.05 = 30 GPa. This value is in good agreement with the microhardness values mentioned in the literature for titanium carbide deposited at high temperature (29 GPa). Owing to cracks which appeared for loads higher than 50 g, it was not possible to deduce the corrected ~~
(26) Kurtz, S. R.; Gordon, R. G. Thin Satid Films 1986,140, 277.
Slifirski et al.
630 Chem. Mater., Vol. 7, No. 4, 1995
3.8 pm Figure 15. Cross-sectional SEM of a film grown with the following procedure: the substrate was first heated and maintained at 280 "C while CpsTiC1.r was heated to 300 "C at a rate of 10 "C/min. When the crucible temperature reached 300 "C,the substrate was heated to the deposition temperature level.
hardness. Such cracks obtained at low load are indicative of the low toughness of these coatings. A third set of experiments with a procedure similar to the first one was undertaken. The main difference was in the final temperature of the crucible, which was 240 "C. At this temperature only a partial decomposition of the precursor, corresponding to the loss of a Cp group, was expected from the described thermal behavior of CpzTiC12. The layers obtained under these experimental conditions are homogeneous with the gray metallic aspect of titanium carbide. Nevertheless, their composition is very similar to that of the black layer deposited in the first set of experiments (Ti0.39~iC0.4300.17Clo.~o~i), and the films contain a large amount of oxygen. The Vickers microhardness measurements performed on a 25 ,um thick layer yielded Hv0.05 = 39 GPa (corrected microhardness H , = 12.8 GPa) which is somewhat higher than the value measured on coatings obtained from precursor vaporization at 360 "C. Due to the rather high temperature of the substrate used for these experiments, the titanium carbide deposited was in every case well crystallized. No other peaks than those belonging to T i c or the steel substrate
are detected by XRD. All the peaks attributed to T i c are in agreement with the JCPDS reference (No. 6-0614) and indicated no preferential orientation. From these three sets of experiments, we conclude that the temperature of vaporization of Cp2TiCl2 has a dramatic influence on both the oxygen and chlorine content in the titanium carbide film. Deposition conditions which involve sublimation and transport of the precursor in its initial structure lead to deposits with high oxygen content. In contrast, the predecomposition of the precursor molecules followed by the transport of the byproducts allows the release of oxygen from the precursor. The origin of large amounts of oxygen measured in the layers adjacent to the substrate in cases one and three is not clearly established. The presence of oxygen up to 10 or 20 at. % is frequent in OMCVD of ceramic compounds and moisture adsorbed at the surface of the initial granular precursor is a likely source of the oxygen. In particular, this could explain the preponderance of contamination when the precursor is sublimated. Thus, the adsorbed water may be transported toward the substrate and afterwards react at the surface. In contrast, preliminary decomposition of the precursor may release the absorbed water in the vaporization crucible which would tend to dilute its influence. We were not able to check this hypothesis because of the difficulty of determining the quantity of oxygen in the initial solid precursor by chemical analysis in the presence of titanium. Nevertheless, the Ti, C, H and C1 quantities were determined and revealed an atomic composition in agreement with the stoichiometry of Cp2TiC12, even for titanocene dichloride exposed to air for 1 month. These results stress the influence of the vaporization process when solid precursors are used in OMCVD. Deposits carried out with preliminary decomposition of the precursor were almost free of oxygen. The use of titanucene dichloride as precursor for the chemical vapor deposition of titanium carbide significantly lowers the deposition temperature when compared with the traditional industrial process. The main drawback is the excess of carbon in the film.
Acknowledgment. The authors are very grateful to Ms. S. Richelme (Toulouse UPS, URA470), who performed the mass spectroscopy studies. CM9302643
