Growth of TiC thin films by pulsed laser evaporation

Growth of TiC thin films by pulsed laser evaporation

Volume IO,number 7,s MATERIALS LETTERS January 1991 Growth of TIC thin films by pulsed laser evaporation Oskar Rist ’ .biLlukv-ials Laborator!: ...

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Volume

IO,number 7,s

MATERIALS LETTERS

January 1991

Growth of TIC thin films by pulsed laser evaporation Oskar

Rist ’

.biLlukv-ials Laborator!:

U’rrght Research and Development

Center,

UGight-Patterson

Air

Fwce

Base. OH

45433, L’S:1

and P. Terrence

Murray

Receised 24 August 1990

Thin films of TiC have been grown on Si( 100) substrates by pulsed laser evaporation. Analysis by X-ray photoelectron spectroscopy and by Auger sputter depth profiles indicates that the films grown between RT and 5OO’C are stoichiometric Tic. Film/ substrate interdiffusion is observed at higher substrate temperatures.

1. Introduction Titanium carbide is a semimetallic ceramic, combining some unusual properties [ 1,2] such as thermal stability, high hardness, anticorrosion stability. and good electrical and thermal conductivity. Therefore. titanium carbide is used in a number of applications, e.g., as a protective coating to increase the wear life of steel parts under extreme chemical, thermal. and mechanical conditions. It is also being studied as a low friction thermal barrier coating for cylinder walls in the adiabatic diesel engine [ 31. In the United States and in Japan TIC has been identified as a candidate material for the “first wall” coating for the fusion reactor [ 31. Fu~hermore. thin films of TiC are considered an excellent diffusion barrier between metal silicides and aluminum and are used in very large scale integration (VLSI) semiconductor technology [ 4,5 1. Until now, thin films of titanium carbide have been grown by several techniques, usually by chemical vapor deposition (CVD) [ 1,6] or by some kind of reactive physical vapor deposition (PVD). Georgiev ’ Permanent address: Wehrwissenschafttiches terialuntersuchungen Erding, Germany.

Institut ftir Ma(WIM), Landshuterstrasse 70, W-8058

et al. [ 1 ] and Eisenberg and Murarka [ 5 ] deposited TIC by reactive magnetron sputtering of titanium in an Ar+CH, atmosphere. The film deposition by reactive sputtering is dependent on several parameters such as the sputtering pressure, partial pressure of argon and methane, rf power and the bias voltages on the Ti target and the substrate. Sundgren et al. [ 71 reported that in order to produce dense TiC films, the substrate temperature must exceed 730’ C. However, these high substrate temperatures can cause problems for many applications; e.g.. the temperature should not exceed the annealing temperature if depositions on high speed steels are to be performed. Among other things, film/substrate interdiffusion and reaction as well as film stress by different thermal temperature coefficients may occur. Clearly, the development of a lower-temperature growth process would help to solve this problem. The purpose of the work described here was to determine the feasibility of growing stoichiometric thin films of TIC by pulsed laser evaporation (PLE). PLE is an emerging film deposition process which possesses a number of advantages over more conventional growth methods. Among these advantages are the potential for congruent target evaporation, the capability of growing high-purity films, the relative ease with which most materials, even very hard ma-

0 167-577x/9 i/$03.50 0 199 1- Elsevier Science Publishers B.V. (Nosh-Holland)

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MATERIALS LETTERS

terials, can be evaporated, the inherent simplicity of the process, and the potential for film growth at lower substrate temperatures. PLE grown films can also have superior structural properties due to the presence of ions and other excited species in the evaporant plume. The additional electronic energy present in such excited species has been postulated to be a source of non-thermal energy for enhancing adatom migration during film growth. PLE has been used previously to deposit a number of semiconductor [ 8 1, dielectric [ 9 1, high-temperature superconductor [ lo], and tribologicai films [ 111. Cheung [ 12 ] has recently reviewed the technique.

DEPOSITION CHAMBER ANALYSIS CHAMBER

2. Experimental A schematic diagram of the experimental apparatus is presented in fig. 1. The system consists of a deposition chamber which is directly connected to a Perkin-Elmer (PHI ) model 550 surface analysis system. The base pressure in the deposition chamber was 2 x 10e6 Pa; this pressure increased to 2 x 1Oe5 Pa during film growth. The base pressure in the analysis chamber was 2x lo-’ Pa. The specimen introduction chamber and transfer arm allow film growth in the deposition chamber and subsequent XPS analysis of the deposited films, without air exposure. The Si ( 100) substrates, 5x 10 mm (Sb-doped, ntype) were rinsed with trichloroethylene, acetone, and methanol before being loaded into the deposition chamber. The substrates were resistively heated during deposition; the substrate temperature was determined by use of a calibrated infrared pyrometer. The TiC target was a hot isostatic pressed (HIP) sample purchased from Sylvania. The frequencydoubled output (A=532 nm) of a Q-switched Nd : YAG laser ( 15 ns pulse duration) was used to evaporate the target. The laser beam was focused to a 0.9 mm spot at the target, and the target-substrate distance was 5 cm. The laser was operated at a repetition rate of 5 Hz, and the beam was stepped across the target at a rate of 0.5 mm s- ‘. Firing of the laser as well as scanning the beam across the target were carried out under computer control. The target was irradiated by 2000 laser pulses prior to lilm growth in order to remove contaminants originally present 324

Fig. I. Schematic diagram of the experimental apparatus.

on the target surface. The TIC films were grown by using 15000 laser shots per film. XPS analysis was performed on the deposited films as well as on specimens of the target material. Analysis of the latter was carried out after the target specimens had been cleaned, within the analysis chamber, by several hundred pulses of YAG radiation. This approach was adopted to avoid unnecessary alteration of the surface chemistry of the target. Second, laser cleaning of the target prior to analysis produced a surface which was more representative of the material exposed to subsequent laser pulses. Laser irradiation was carried out in the analysis chamber, as depicted by the broken lines in fig. 1. XPS spectra were obtained using an Al Ka X-ray source operating at 600 W. The pass energy was 25 eV.

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3. Results XPS results are represented in figs. 2 and 3 and compiled in tables 1 and 2. Fig. 2 shows the C Is peaks. Spectrum 2a exhibits two carbon peaks, a carbidic peak at 28 1.9 eV and a graphitic peak at 284.8 eV. The peak height of the graphitic peak is approximately half that of the carbidic peak. It was not possible to reduce the peak intensity of the graphitic component by repeated laser-cleaning. So it would appear that graphitic carbon was present in the target itself, presumably as a binder in the sintered TIC target material. The observed difference in binding energy between the graphitic and the carbidic peak is in excellent agreement with the value reported by lhara et al. [ 131. Figs. 2b, 2c. 2d and 2e present the results of the filmsgrown at RT. 3OO’C, SOO”C, and 6OO’C. Overall, the four spectra show about the same features. These four spectra are very similar to the target spectrum, and they suggest that the carbon chemistry of the films grown as high as 6OO’C is virtually identical to that of the target.

January

LETTERS

199

I

Shown in figs. 2f and 2g are the XPS results for the films grown at 750 and 900°C respectively. As with the lower-temperature films, there is a graphitic component in both spectra. however. the carbidic peak has almost disappeared at 900°C. Shown in fig. 3 are the Ti 2p peaks for the lasercleaned TIC target and the films grown at temperatures from RT up to 900°C. The observed binding energies for the Ti 2p,,? peaks are listed in table 1. With the exception of the 900°C film the binding energies agree with the values reported by Ihara et al. [ 131. The results are also in good agreement with the reported shifts by Ivanovskii et al. [ 141. The Ti 2p binding energies of the films grown up to 600°C are nearly identical to that of the laser-cleaned target. In addition, the peak shapes of the abovementioned spectra are almost superimposable. These results indicate that the Ti chemistry of the PEE grown films up to 6OO’C is the same as that of the target material. The spectrum of the film grown at 750°C (fig. 3f) also exhibits a binding energy (BE) of 454.9 eV, however, the spectrum shows a shoulder at the

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Fig. 2. C 1s photoelectron peaks from (a) TiC target and films grown at substrate temperatures of (b) RT, (c) 300°C. (df 500°C. (e) 600°C (f) 750°C and (g) 900°C.

BINDING

I

I

I

1

/

450

465

ENERGY

(eV)

Fig. 3. Ti 2p photoelectron peaks from (a) TiC target and films grown at substrate temperatures of (b) RT, (c) 300°C. (d) 500°C. (e) 600°C. (f) 750°C. and (g) 9OO’C.

325

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Table 1 Results of XPS analysis

LETTERS

January

1991

of TiC target and PLE grown films

carbidic CIS

graphitic Cls

Ti ~Pw

Kinetic energy (eV) Ti(LMM)

TiC target

281.9

284.8

455.1

417.1

871.2

530.9

-

(laser-cleaned) RT film 300°C film 500°C film 600°C film 750°C film 900°C film

281.8 281.7 281.6 281.7 282.0/282.8 283.5

285.1 284.5 284.9 285.2 285.1 285.1

455.0 454.9 455.0 454.9 454.9 453.8

417.1 417.2 417.4 417.6 417.8 419.0

872.1 872.2 872.4 872.5 872.7 872.8

530.9 529.9 530.8 532.3 531.8 532.4

99.2/101.6 99.4/ 100.7 99.0/101.2

Binding energy, BE (eV)

Sample

lower-energy side, which means there is an additional titanium compound formed at 750°C. At 900°C (fig. 3g) the shoulder at 453.8 eV develops into the dominant peak in the spectrum. It is interesting to note that this binding energy is in good agreement with the BE of metallic titanium [ 1.51.As seen with the C 1s results, the Ti chemistry of the films grown at 750°C and higher is significantly different than that of the films grown at lower temperatures. In addition to the abovementioned photoelectron peaks, the kinetic energy (KE) of the Ti( LMM) peak was determined for each film as well as for the lasercleaned TIC target. The results are also shown in table 1. These results, in conjunction with the previously discussed binding energies of the Ti 2p peaks, allow a determination to be made of the modified Auger parameter (a’ ) for each sample. This parameter is independent of any static charging and is Table 2 Quantitative

BE (eV)

parameter 01’

01s

Si 2p

characteristic of a particular chemical state. Therefore, the modified Auger parameter can provide additional insight into the Ti chemical state. It can be seen that the modified Auger parameters a’ also listed in table 1 for the target and the films at RT, 3OO”C, and 500°C are essentially identical. These results indicate again that the surface chemistry of the PLE produced TiC films at RT up to 500°C are very similar to that of the laser-cleaned stoichiometric TiC target. The fy’ values at higher temperatures indicate a difference in the Ti chemistry. A quantitative determination of the Ti/C ratio was performed by measuring the respective XPS peak heights and by using sensitivity factors reported by Briggs and Seah [ 15 1. The results are shown in table 2. The ‘Ii/C ratio is based on the Ti 2p,,, peak and the carbidic part of the C 1s peak. The Ti/C ratio of 0.96 was determined for the laser-cleaned target. Nearly identical values were found for PLE grown

ESCA results

TiC target (laser-cleaned RT film 300°C film 500°C film 600°C film 750°C film 900°C tilm

Ti/C carbidic

Atom of0

Sample

326

Auger

C carbidic

C graphitic

Ti

0

34.2

17.0

32.9

15.8

0.96

33.5 33.8 34.2 31.2 26.5 25.9

25.6 12.2 24.6 18.2 23.8 19.4

27.5 31.9 32.0 29.5 11.4 to.1

13.4 22.2 9.4 11.7 12.3 9.2

0.82 0.94 0.93 0.95 0.66 0.39

Si

)

9.4 19.9 30.9

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films at substrate temperatures from 300 to 600°C. These results indicate that the films grown at the above temperatures have the same stoichiometric Ti/ C ratio as the TiC target. The film grown at RT shows a somewhat smaller Ti/C ratio of 0.82, meaning it is nearly a stoichiometric TIC compound. Films grown at 750 and 900°C show a Ti/C ratio of 0.66 and 0.39, respectively, indicating an explicit deviation from the stoichiometric compound. Further insight into the nature of the 750 and 900°C films was produced by XPS survey scans, which indicated the presence of silicon on these films. The atomic percentages of silicon filed in table 2 show an increasing amount of silicon and a concomitant decrease in titanium, at higher temperatures. A=10

o=

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A=T,

DEPTH

I

=st

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OZC

I99

Shown in figs. 4a and 4b are Auger sputter depth profiles for films grown at 500 and 900°C. respectively. The film grown at 5OO’C shows a relatively sharp interface, with no evidence of interdiffusion. The approximate film thickness is 65 nrn. The depth profile of the 900°C film is significantly different. There is a considerable Ti diffusion into the silicon substrate. There is also a Si diffusion from the substrate into the film. These results are consistent with the XPS results (table 2) which also show Si on the surface of the films deposited at temperatures of 600°C and higher. This depth profile indicates that TIC films can be grown on silicon at temperatures as high as 5OO’C with no interdiffusion. At higher temperatures it would appear that film/substrate interdiffusion, and possibly a reaction, occurs.

4. Discussion

DEPTH

January

LETTERS

tnm)

Fig. 4. Auger sputter depth profiles of films grown at (a) 500°C and (b) 900°C.

and summary

The XPS results as well as the Auger sputter depth profiles identify the films grown at 500’ C and lower as stoichiometric TIC. Pulsed laser evaporation is clearly a feasible technique for growing stoichiometric TIC films on Si ( 100 ). Such films can be grown at substrate temperatures as low as RT, albeit with a graphitic carbon component. At higher substrate temperatures, there is presumably a smaller graphitic carbon component. The growth of TIC films with even smaller graphitic components would require a better target material. The growth of TIC films by PLE may help to overcome a number of problems common to the conventional (high-temperature) methods. Among these problems are film/substrate interdiffusion as well as film stress. The growth of TIC at lower temperatures may also help to avoid exceeding the annealing temperatures of high speed steel. PLE growth of TiC has a number of potential applications. One of these is use as very hard, low wear coatings. The crystallinity and the tribological properties of such films will be explored in future studies.

Acknowledgement The excellent technical support of Dave Dempsey and Rick Bach is gratefully acknowledged. James S. 327

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Solomon is greatly appreciated for providing the depth profiles. This work was supported by the Materials Laboratory, Wright Research and Development Center, W~~t-Patte~on Air Force Base, Ohio.

References [ I] G. Georgiev, N. Feschiev, D. Popov and Z. Uzuuov, Vacuum 36 (1986) 595. [ 21 P.K. Ashwini Kumar, V. Kumar and S.K. Sarkar, J. Vacuum Sci. Technol. A 7 ( 1989 ) 1488. [3] A.E. Kaloyeras, W.S. Williams, EC. Brown, A.E. Greene and J.B. Woodhouse, Phys. Rev. B 37 ( 1988) 77 1. [4) A. AppeIbaum and S.P. Murarka, J. Vacuum Sci. Technol. A4 (1986)637. [ 51M. Eizenberg and S.P. Murarka, J. Appt. Phys. 54 ( 1983) 3190. [ 61J.E. Sundgren, B.-O. Johansson and S.E. Karlsson, Thin Solid Films 105 (1983) 353.

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[7] J.E. Sundgren, B.-O. Johansson, H.T.G. Hentzell and S.-E. Karisson, Thin Solid Films 105 ( 1983) 385. [S] J.B. Dubowski, D.F. Williams, P.B. Sewell and P. Norman, Appl. Phys. Letters 46 ( 1985) 108 1. [9] P.T. Murray, J.D. Wolf, J.A. Mescher, J.T. Grant and N.T. McDevitt, Mater. Letters 5 (1987) 250. [ 101 X.D. Wu, A. Inam, T. Venkatesan, C.C. Chang, E.W. Chase, P. Barboux, J.M. Tarascon and B. Wilkens, Appt. Phys. Letters 52 ( 1988) 754. [ 111MS. Do&y, N.T. McDevitt, T.W. Haas, P.T. Murray and J.T. Grant, Thin Solid Films 168 (1989) 335. [ 121 J.T. Cheung, CRC Reviews in Materials Science, in press. [ 131 H. Ihara, Y. Kumashiro, A. Itoh and K. Meada, Japan. J. Appl. Phys. 12 (1973) 1462. t14 1 A.L. Ivanovskii and V.A. Gubanov, J. Electron Spectrosc. 16 (1979) 415. [I5 ] D. Briggs and M.P. Seah. Practical surface analysis by Auger and X-ray photoelectron spectroscopy (Wiley, New York, 1983) App. 5, p. 493.