- Email: [email protected]
Growth of Hf(C,N) thin films on Si(100) and D2 steel substrates by plasma assisted MOCVD
Surface and Coatings Technology 131 Ž2000. 73᎐78
Growth of Hfž C,N/ thin films on Siž 100/ and D2 steel substrates by plasma assisted MOCVD J.G. Hana,U , Y.K. Cho a , J.S. Yoona , K.T. Rie b, W.C. Rohb, D. Jung c , S.W. Lee d, J.-H. Boo d,U a
b
Department of Metallurgical Engineering, Sungkyunkwan Uni¨ ersity, Suwon 440-746, South Korea Institute fur und Plasmatechnisch Werkstoffentwicklung, Technische Uni¨ ersitat ¨ Oberflachentechnik ¨ ¨ ¨ Braunschweig, Braunschweig D-38108, Germany c Department of Physics, Sungkyunkwan Uni¨ ersity, Suwon 440-746, South Korea d Department of Chemistry, Sungkyunkwan Uni¨ ersity, Suwon 440-746, South Korea
Abstract We have deposited HfŽC,N. thin films on SiŽ100. and STD11 steel substrates by pulsed d.c. plasma assisted metal᎐organic chemical vapor deposition ŽPA-MOCVD. in the temperature range of 200᎐300⬚C. Tetrakis diethylamido hafnium, HfwNŽCH 2 CH 3 . 2 x 4 ŽTDEAH., was used as the hafnium precursor. A mixture of He Ž90%. and H 2 Ž10%. in volume ratio was used as the carrier gas, and the N2 was used as the reactive gas. During CVD, radical formation and ionization behaviors in plasma were analyzed in situ by optical emission spectroscopy ŽOES. at various pulsed bias voltages and N2 flow rates. The deposition rate with the change of the flow rate ratio of N2 reactive gas to He q H 2 carrier gas was also increased to 0.1. With increasing bias voltage, moreover, the film growth rate was continuously increased resulting in carbon-rich films. Highly oriented oxygen-free, polycrystalline HfŽC,N. thin films in the 1114 direction were successfully grown on the STD11 steel substrate at 300⬚C. The hardness of film changed from 1000 to 2500 HK, depending on N2 gas flow rate ratio and bias voltage. Higher hardness can be obtained for a N2 gas flow rate ratio of 0.1 and bias voltage of 600 V. The film composition was confirmed with XPS and RBS analysis, and the N2 reactive gas makes low carbon contents in the films. SEM revealed that surfaces of as-deposited HfŽC,N. thin films were smooth and featureless. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: HfŽC,N. thin film; Pulsed d.c. plasma assisted MOCVD; Plasma diagnostics
1. Introduction Thin films of transition-metal nitrides such as titanium nitride ŽTiN., zirconium nitride ŽZrN., and hafnium nitride ŽHfN. have a wide variety of technological applications such as wear-resistant and protective coatings on cutting tools, the decorative coatings on watches and frames, and the diffusion barrier coatings in the metallization process for high density semiconU
Corresponding author. Tel.: q82-31-2907354; fax: q82-312907371. E-mail addresses: [email protected] ŽJ.G. Han., jhboo@ chem.skku.ac.kr ŽJ.-H. Boo..
ductor devices w1,2x. Among many deposition methods of transition-metal nitride thin films, plasma assisted metal᎐organic chemical vapor deposition ŽPAMOCVD. can be considered as the most useful mass production technique. PA-MOCVD can deposit thin films of metal nitrides with large area uniformity at low temperatures without using toxic andror corrosive chemicals such as metal-halides w3,4x. While there have been a lot of research on TiN thin films by PAMOCVD, the research on the PA-MOCVD deposition of HfN thin films is very rare. Moreover, high carbon incorporation into the deposited film was sometimes observed in the case of TiN thin film deposition by both conventional d.c. and r.f. plasma assisted MOCVD
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 8 3 3 - 1
74
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
w3᎐9x. Pulsed d.c. plasma assisted MOCVD method was, therefore, used in this work to deposit low carbon content HfN thin films with high adhesion and hardness. Pulsed d.c. does not require complicated matching network as in the case of r.f.-plasma, and since it is pulsed, the pulsed d.c. plasma has less chance of arc generation than the conventional d.c. plasma. In this paper, we report deposition of HfŽC,N. thin films by pulsed d.c. plasma assisted MOCVD and material characteristics of the deposited films. Since all deposited thin films have always contained carbon, which is attributed to being from the metal᎐organic precursor, in this work, we called the deposited films hafnium carbonitride wHfŽC,N.x films rather than hafnium nitride ŽHfN. films.
2. Experimental HfŽC,N. thin films were deposited on SiŽ100. and D2 steel substrates using a home-made PA-MOCVD system. Biasing of the susceptor and the ground, the plasma was generated by the pulsed d.c. After cleaning the samples, the pre-cleaned substrates were treated with Ar and N2 plasma sequentially to make an oxygen-free surface and buffer layer of HfN, respectively. The process of plasma surface cleaning using N2 gas prior to deposition may increase the adhesion of films on cold forming steel. The deposition was carried out for 0.5᎐1 h at deposition temperatures in the range of 200᎐300⬚C. The temperatures of the substrate were detected by an optical pyrometer. The general process conditions for film deposition are y500 to y700 V discharge voltage, 30᎐50% d.c. pulse duration, 10᎐30 kHz d.c. pulse frequency, and 100 sccm He q H 2 carrier gas with 0᎐60 sccm N2 reactive gas at 133 Pa working pressure, respectively. Tetrakis diethyl amido hafnium ŽHfwNŽC 2 H 5 . 2 x 4 : TDEAH. was used as the Hf precursor. Mixture of He 90% and H 2 10% in volume ratio was used to carry the vaporized TDEAH molecules into the reaction reactor. N2 gas was fed into the reactor as the reactive gas. To understand the gas-phase reaction chemistry of the MO-compounds in the plasma, moreover, the plasma diagnostics were carried out using single Langmuir probe and optical emission spectroscopy at various bias voltages and gas flow rate ratios. The films produced by pulsed d.c. enhanced MOCVD were characterized by X-ray photoelectron spectroscopy ŽXPS., X-ray diffraction ŽXRD., Rutherford backscattering spectroscopy ŽRBS., scanning electron microscopy ŽSEM., and Fourier transform infrared spectroscopy ŽFTIR.. The microhardness was also checked with micro Knoop hardness tester. Resistivities of the films were measured by the standard four-point probe method.
3. Results and discussion The measurement of plasma density is important to understand the coating process and gas phase reaction. It is generally well known that the plasma density is not dependent on the pressure and the bias voltage but it can be influenced by the electron temperature. The plasma mainly consists of two particles such as ions and electrons. In this study, the value of electron density Ž n e . for the general PACVD process was changed in the range of 1.2= 10 10 ᎐6.3= 10 9 cmy3 , depending on the various gas flow rate ratio wi.e. N2rŽHe q H 2 .x and the bias voltages. Optical emission spectroscopy ŽOES. is a powerful tool for controlling the coating process and for optimizing both the layer properties and the coating process. In this study, therefore, the OES was used as a qualitative in situ diagnostic method. By measuring the intensity of the spectral lines, it is possible to identify the density of the excited state. Fig. 1a shows an experimental example of the emission spectra obtained in situ during a coating process of HfŽC,N. growth. These typical OE spectra were obtained from HfŽC,N. coating processes under the same gas flow rate ratio with different bias voltages between y500 and y700 V. In Fig. 1a, several excited lines occurred in the discharge, e.g. at wavelengths of 382.7, 396.7, 431.4 nm, etc. Comparing the experimentally observed lines with a theoretical spectroscopic data, we identify that the 396.7 nm line is attributed to A2 ⌸᎐X 2 ⌺ transition of CN II species, that of 431.4 nm due to A2⌬᎐X 2 ⌸ transition of CH I species, and that of 382.7 nm of Hf I species in the plasma, respectively. Figs 1b,c show a variation of relative plasma intensities obtained from Fig. 1a and other OES data Žnot shown. as a function of bias voltage and gas flow rate ratio. Fig. 1b shows that the plasma intensity increased with increasing bias voltage. Above y600 V, the intensity of CH species appears two times higher than that of CN and Hf species, suggesting a possibility of large amounts of carbon in the deposited films. However, the gas rate ratio was more affected by the carbon contents in the films rather than the bias voltage. In Fig. 1c, the intensities of Hf and CH species were increased by as much as factors 8 and 5, respectively, whereas that of CN remains constantly. This indicates that much higher carbon amounts in the films could be obtained when N2 gas flow rate was increased above 0.1. The best deposition condition of this study was y600 V of bias voltage and 0.1 of gas flow rate ratio. Noticeably, the OES results have to be correlated with the layer properties such as surface topography and microhardness, etc. The deposited films of HfŽC,N. were first characterized by XRD and RBS. Fig. 2 shows a typical X-ray diffraction patterns and RBS data of a HfŽC,N. thin films grown on D2 steel and SiŽ100. surfaces at 300⬚C
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
75
using TDEAH under a gas flow rate of 0.1 with various bias voltages. XRD patterns of Fig. 2a showed a hint of HfŽC,N. film formation, since the as-grown films exhibited a relatively intense HfŽC,N.Ž111. diffraction peak, which appeared at 2 s 33⬚. Other detectable phases in the diffraction pattern were also observed at 2 s 40⬚ attributed to the HfŽC,N.Ž200. phase, indicating that a polycrystalline films could be deposited on the D2 steel surface at temperature as low as 300⬚C. With increasing the bias voltage, however, the intensity of the diffraction peak was gradually increased whereas the FWHM of HfŽC,N.Ž111. diffraction peak was decreased, suggesting that the growth rate is increased and the film quality are affected by the bias voltage. RBS was also applied to study the changes of stoichiometry, film thickness, and microhardness of our
Fig. 2. X-Ray diffraction patterns and Rutherford backscattering spectra of HfŽC,N. thin films grown on D2 steel and SiŽ100. substrates at 300⬚C with TDEAH with various bias voltages: Ža. XRD; and Žb. RBS.
Fig. 1. Plasma diagnostics by optical emission spectroscopy. Ža. OE spectra obtained during HfŽC,N. thin film growth under various bias voltages. Žb. and Žc. show the changes of plasma intensities of Hf, CN, and CH radicals as a function of plasma bias voltage and gas flow rate ratio.
films. In the RBS analysis, nitrogen resonance method was used for the clear distinction of the nitrogen peak in spectra. A 3.6-MeV He 2q ion-beam was bombarded to the sample and detected at backsacttered angle of 165⬚. Rutherford universal manipulation program ŽRUMP. was used for the simulation of the RBS spectra. Fig. 2b shows a representative RBS spectra for the HfŽC,N. films deposited on SiŽ100. at 300⬚C with TDEAH under the gas flow rate ratio of 0.1 with different bias voltages. In contrast to the XPS data, very small signals of carbon were detected with increasing bias voltages due to small sensitivity factor of carbon and the contents of Hf were increased. The variation of composition ratio and growth rate has close relation as bias voltage increases the growth rate increases and the carbon contents increase rapidly Žsee Fig. 4.. The surface morphologies of the as-grown HfŽC,N. films deposited on D2 steel at 300⬚C under the gas rate ratio of 0.1 with various bias voltages showed a very smooth appearance as shown in Fig. 3b. The surface morphology of the HfŽC,N. layers deposited under different bias voltages, linked with a domed appearance of
76
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
Fig. 3. SEM images of HfŽC,N. thin films grown on D2 steel substrates at 300⬚C with TDEAH with various bias voltages.
the surface topography, were changed from rough surface to a very smooth surface topography at higher discharge voltages of y500 to y600 V. This is in agreement with data reported previously w8,10x and indicates that the plasma contains the deposition such as particle energies and densities which can be determined during the nucleation and growth of the layer. The most ultra-fine surface structure was obtained from a HfŽC,N. film deposited at bias voltage of y600 V. Moreover, the sputtering effect due to high
bias voltage might be an important role of growing a more rougher surface than that grown at y600 V. Fig. 3a,c showed that the investigated morphology shapes of our as-grown HfŽC,N. layers have micron-sized clusters or powers of HfŽC,N. particles. The thickness of the as-grown film of y600 V is approximately 500 nm according to the cross-sectional SEM measurement. The changes of plasma, including radicals and film properties, are illustrated in terms of carrier and reactive gases as well as pulsed power variation. The effect of the plasma bias voltage is one of the important factors to obtain high quality film. Fig. 4a shows the change of deposition rates with various plasma bias voltages. Flow rates of He q H 2 and N2 were kept to be 100 sccm and 10 sccm Ži.e. N2rHe q H 2 s 0.1., respectively. Fig. 4a shows that deposition rate was increased with increasing the bias voltages. Above y600 V, however, the slope of growth rate significantly decreased from y600 V. This indicates that the film thickness might be decreased with increasing the bias voltage due to higher sputtering rate which is induced by molecular ions with relatively high kinetic energy. The influence of the ratio of N2 reactive gas to He q H 2 carrier gas on the deposition of HfŽC,N. thin films was quite noticeable. Fig. 4a also shows the variation of the deposition rate with the change of the ratio of N2 reactive gas to He q H 2 carrier gas. The flow rate of N2 was varied in the range of 0᎐60 sccm, and the flow rate of He q H 2 was kept at 100 sccm. The bias voltage was kept at y600 V. The deposition rate increased as the flow rate ratio was increased to 0.1. For the flow gas rate ratio higher than 0.1, the deposition rate decreased drastically with increasing the gas flow rate ratio. As the N2 flow rate ratio was increased from 0 to 0.6, plasma current increased from 10 to 80 mA. This indicates that below flow rate ratio of 0.1 the number of ionized molecules andror atoms increases, making more plasma decomposition of hafnium and nitrogen precursors and, thus, giving higher growth rates. On the other hand, as N2 flow rate ratio was increased to 0.6, it is thought that a sputtering on the growing surface due to the ionized molecules was increased, and the deposition rate was decreased. Plasma enhanced decomposition of precursors dominated at the flow rate ratio - 0.1, while sputtering of the growing surface dominated at the flow rate ratio ) 0.1, giving a maximum deposition rate at the flow rate ratio of 0.1. Fig. 4b shows the resistivity of the deposited films vs. the flow rate ratio. The resistivity shows a minimum value at the flow rate ratio of 0.1. This is in good agreement with our OES results shown in Figs 1b,c. RBS data show a variation of composition ratio of HfrNrC with the bias voltage and gas flow rate ratio change. The compositions of N and C have been increased especially as the gas flow rate ratio was increased Žsee Fig. 4c.. It is thought that the number of plasma activated
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
77
Fig. 4. Variations of deposition rate, electrical resistivity, composition ratio, and microhardness of the as-grown HfŽC,N. thin films as a function of plasma bias voltage and gas flow rate ratio: Ža. deposition rate; Žb. electrical resistivity; Žc. composition ratio; and Žd. microhardness.
molecules incorporating into the growing films tightly bonded to each other andror to the substrates increases, resulting in the increased strength of atomic network of the films as plasma current increased with increase of the gas flow rate ratio. This explains the decrease of resistivity with the increasing flow rate ratio in the range of 0᎐0.1. For the gas flow rate ratios higher than 0.1, resistivity is thought to increase due to higher incorporation, during the deposition, of nitrogen and carbon into the growing film. Nitrogen and carbon are believed to be from TDEAH andror N2 and from TDEAH, respectively. Higher ratio of N2rŽHe q H 2 . and, thus, higher plasma current are considered to have enhanced the detachment of nitrogen-containing and carbon-containing molecules from the precursors. Resistivity values of the HfŽC,N. films were scattered, and no meaningful relationship between the resistivity and the plasma voltage could be seen. The composition of nitrogen and carbon increased with increasing the bias voltage showing that higher bias voltage is considered to enhance the decomposition of nitrogen-containing and carbon-containing molecules form the precursors, resulting in higher nitrogen and carbon incorporation into the growing HfŽC,N. film. The increase of carbon composition was quite rapid above y700 V, and the carbon composition in the film was three times larger than that of hafnium. In this study, however, under a condition of below y700 V the gas flow rate ratio can
mostly be influenced on the carbon contents in the films rather than that of plasma bias voltage. This is well in agreement with our OES data that within y700 eV, the total intensity of CH radicals is maintained with the same level as in the case of 0.1 of gas flow rate ratio. As shown in Fig. 4c, the amounts of carbon were rapidly increased with increasing the flow rate ratio to 0.6, suggesting that if we grew the HfŽC,N. film under over 0.6 and y700 V a film with a much higher carbon content than that grown at a rather low flow rate ratio and bias voltage could be obtained. As mentioned previously, our OES experiments showed that only some species, such as CN and CH among all observed lines, are proved to be the most important as the main indicator determining the layer properties during the coating process. Good layer properties were achieved only for a deposition temperature of 300⬚C and the bias voltage of y600 V under the gas flow ratio of 0.1. The highest hardness value obtained for the as-grown HfŽC,N. film at this condition was 2500 HK 0.01. As shown in Fig. 4d, however, with increasing the intensities of CN and CH radicals dissociated of MO compounds in the plasma, the carbon contents in the films are highly increased as the gas flow ratio increases. The CH radical increases the microhardness but the CN radical decreases the hardness. This is in good agreement with previous reports w11᎐14x that a reduction of the microhardness of the
78
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
films is in situ monitored by the intensity of the plasma induced optical emission of the CN radical. Moreover, the intensity of CN radical is proportional to the amounts of carbon contents in the film layer. Thus, the importance of the CH and CN lines are proven for the correlation with the layer properties in our study.
4. Conclusions HfŽC,N. thin films were deposited by pulsed d.c. plasma assisted metal᎐organic chemical vapor deposition ŽPA-MOCVD.. Highly oriented polycrystalline films in the w111x direction have been deposited on the D2 steel surface at temperatures as low as 300⬚C. The flow rate ratio of N2 reactive gas and the He q H 2 carrier gas was an important deposition parameter. At N2rŽHe q H 2 . ratio of 0.1, the deposition rate and the resistivity of the HfŽC,N. thin film showed maximum and minimum values, respectively. These are thought to be related to the fact that molecules excited andror decomposed by plasma make the growing thin films fast growing and dense, while they cause high sputtering of the growing surface and high incorporation of nitrogen and carbon from the precursors into the growing films. Good layer properties were achieved only for deposition temperature of 300⬚C and the bias voltage of y600 V under the gas flow ratio of 0.1. Surfaces of as-deposited HfŽC,N. films were smooth and featherless and the highest hardness value obtained for the as-grown HfŽC,N. film at this condition was 2500 HK 0.01. Radical formation and ionization behaviors in plasma are analyzed by OES at various pulsed bias and gases conditions. Moreover, the changes of plasma including radicals and film properties are illustrated in
terms of carrier and reactive gases as well as pulsed power variation.
Acknowledgements Support of this research by the Korea Science and Engineering Foundation through the Grant No. 9810307-2 and by the Ministry of Commerce, Industry, and Energy in Korea, is gratefully acknowledged. References w1x O. Knotek, A. Schrey, in: D.A. Glocker, S.I. Shah ŽEds.., Handbook of Thin Films Process Technology, Institute of Physics, Bristol and Philadelphia, 1995, p. X1.0:1. w2x A. Sherman, Jpn. J. Appl. Phys. 30 Ž1991. 3553. w3x K.T. Rie, J. Whole, A. Gebauer, J. Phys. ŽParis. II Colloq. C2 Ž1. Ž1991. 397. w4x K.T. Rie, J. Whole, A. Gebauer, Surf. Coat. Technol. 59 Ž1993. 202. w5x S.W. Kim, H. Jimba, A. Sekiguchi, O. Okada, N. Hosokawa, Appl. Surf. Sci. 100r101 Ž1996. 546. w6x C.K. Wang, L.M. Liu, M. Liao, H.C. Cheng, M.S. Lin, Jpn. J. Appl. Phys. 35 Ž1996. 4274. w7x F.H.M. Sanders, G. Verspui, Thin Solid Films 161 Ž1988. L87. w8x N.H. Hoang, D.R. McKenzie, Y. Yin, J. Appl. Phys. 80 Ž1996. 6279. w9x K.-T. Rie, J. Wohle, A. Gebauer, Surf. Coat. Technol. 98 Ž1998. 1534. w10x K.-T. Rie, J. Whole, Plasma Chem. Plasma Process 13 Ž1993. 93. w11x B.H. Weiller, J. Am. Chem. Soc. 118 Ž1996. 4975. w12x T.R. Cundari, J.M. Morse, Jr., Chem. Mater. 8 Ž1996. 189. w13x S. Peter, R. Pintaske, G. Hecht, F. Richter, J. Nucl. Mater. 200 Ž1993. 412. w14x X.-Y. Zhu, M. Wolf, T. Huett, J.M. White, J. Chem. Phys. 97 Ž1992. 5856.
Growth of Hfž C,N/ thin films on Siž 100/ and D2 steel substrates by plasma assisted MOCVD J.G. Hana,U , Y.K. Cho a , J.S. Yoona , K.T. Rie b, W.C. Rohb, D. Jung c , S.W. Lee d, J.-H. Boo d,U a
b
Department of Metallurgical Engineering, Sungkyunkwan Uni¨ ersity, Suwon 440-746, South Korea Institute fur und Plasmatechnisch Werkstoffentwicklung, Technische Uni¨ ersitat ¨ Oberflachentechnik ¨ ¨ ¨ Braunschweig, Braunschweig D-38108, Germany c Department of Physics, Sungkyunkwan Uni¨ ersity, Suwon 440-746, South Korea d Department of Chemistry, Sungkyunkwan Uni¨ ersity, Suwon 440-746, South Korea
Abstract We have deposited HfŽC,N. thin films on SiŽ100. and STD11 steel substrates by pulsed d.c. plasma assisted metal᎐organic chemical vapor deposition ŽPA-MOCVD. in the temperature range of 200᎐300⬚C. Tetrakis diethylamido hafnium, HfwNŽCH 2 CH 3 . 2 x 4 ŽTDEAH., was used as the hafnium precursor. A mixture of He Ž90%. and H 2 Ž10%. in volume ratio was used as the carrier gas, and the N2 was used as the reactive gas. During CVD, radical formation and ionization behaviors in plasma were analyzed in situ by optical emission spectroscopy ŽOES. at various pulsed bias voltages and N2 flow rates. The deposition rate with the change of the flow rate ratio of N2 reactive gas to He q H 2 carrier gas was also increased to 0.1. With increasing bias voltage, moreover, the film growth rate was continuously increased resulting in carbon-rich films. Highly oriented oxygen-free, polycrystalline HfŽC,N. thin films in the 1114 direction were successfully grown on the STD11 steel substrate at 300⬚C. The hardness of film changed from 1000 to 2500 HK, depending on N2 gas flow rate ratio and bias voltage. Higher hardness can be obtained for a N2 gas flow rate ratio of 0.1 and bias voltage of 600 V. The film composition was confirmed with XPS and RBS analysis, and the N2 reactive gas makes low carbon contents in the films. SEM revealed that surfaces of as-deposited HfŽC,N. thin films were smooth and featureless. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: HfŽC,N. thin film; Pulsed d.c. plasma assisted MOCVD; Plasma diagnostics
1. Introduction Thin films of transition-metal nitrides such as titanium nitride ŽTiN., zirconium nitride ŽZrN., and hafnium nitride ŽHfN. have a wide variety of technological applications such as wear-resistant and protective coatings on cutting tools, the decorative coatings on watches and frames, and the diffusion barrier coatings in the metallization process for high density semiconU
Corresponding author. Tel.: q82-31-2907354; fax: q82-312907371. E-mail addresses: [email protected] ŽJ.G. Han., jhboo@ chem.skku.ac.kr ŽJ.-H. Boo..
ductor devices w1,2x. Among many deposition methods of transition-metal nitride thin films, plasma assisted metal᎐organic chemical vapor deposition ŽPAMOCVD. can be considered as the most useful mass production technique. PA-MOCVD can deposit thin films of metal nitrides with large area uniformity at low temperatures without using toxic andror corrosive chemicals such as metal-halides w3,4x. While there have been a lot of research on TiN thin films by PAMOCVD, the research on the PA-MOCVD deposition of HfN thin films is very rare. Moreover, high carbon incorporation into the deposited film was sometimes observed in the case of TiN thin film deposition by both conventional d.c. and r.f. plasma assisted MOCVD
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 8 3 3 - 1
74
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
w3᎐9x. Pulsed d.c. plasma assisted MOCVD method was, therefore, used in this work to deposit low carbon content HfN thin films with high adhesion and hardness. Pulsed d.c. does not require complicated matching network as in the case of r.f.-plasma, and since it is pulsed, the pulsed d.c. plasma has less chance of arc generation than the conventional d.c. plasma. In this paper, we report deposition of HfŽC,N. thin films by pulsed d.c. plasma assisted MOCVD and material characteristics of the deposited films. Since all deposited thin films have always contained carbon, which is attributed to being from the metal᎐organic precursor, in this work, we called the deposited films hafnium carbonitride wHfŽC,N.x films rather than hafnium nitride ŽHfN. films.
2. Experimental HfŽC,N. thin films were deposited on SiŽ100. and D2 steel substrates using a home-made PA-MOCVD system. Biasing of the susceptor and the ground, the plasma was generated by the pulsed d.c. After cleaning the samples, the pre-cleaned substrates were treated with Ar and N2 plasma sequentially to make an oxygen-free surface and buffer layer of HfN, respectively. The process of plasma surface cleaning using N2 gas prior to deposition may increase the adhesion of films on cold forming steel. The deposition was carried out for 0.5᎐1 h at deposition temperatures in the range of 200᎐300⬚C. The temperatures of the substrate were detected by an optical pyrometer. The general process conditions for film deposition are y500 to y700 V discharge voltage, 30᎐50% d.c. pulse duration, 10᎐30 kHz d.c. pulse frequency, and 100 sccm He q H 2 carrier gas with 0᎐60 sccm N2 reactive gas at 133 Pa working pressure, respectively. Tetrakis diethyl amido hafnium ŽHfwNŽC 2 H 5 . 2 x 4 : TDEAH. was used as the Hf precursor. Mixture of He 90% and H 2 10% in volume ratio was used to carry the vaporized TDEAH molecules into the reaction reactor. N2 gas was fed into the reactor as the reactive gas. To understand the gas-phase reaction chemistry of the MO-compounds in the plasma, moreover, the plasma diagnostics were carried out using single Langmuir probe and optical emission spectroscopy at various bias voltages and gas flow rate ratios. The films produced by pulsed d.c. enhanced MOCVD were characterized by X-ray photoelectron spectroscopy ŽXPS., X-ray diffraction ŽXRD., Rutherford backscattering spectroscopy ŽRBS., scanning electron microscopy ŽSEM., and Fourier transform infrared spectroscopy ŽFTIR.. The microhardness was also checked with micro Knoop hardness tester. Resistivities of the films were measured by the standard four-point probe method.
3. Results and discussion The measurement of plasma density is important to understand the coating process and gas phase reaction. It is generally well known that the plasma density is not dependent on the pressure and the bias voltage but it can be influenced by the electron temperature. The plasma mainly consists of two particles such as ions and electrons. In this study, the value of electron density Ž n e . for the general PACVD process was changed in the range of 1.2= 10 10 ᎐6.3= 10 9 cmy3 , depending on the various gas flow rate ratio wi.e. N2rŽHe q H 2 .x and the bias voltages. Optical emission spectroscopy ŽOES. is a powerful tool for controlling the coating process and for optimizing both the layer properties and the coating process. In this study, therefore, the OES was used as a qualitative in situ diagnostic method. By measuring the intensity of the spectral lines, it is possible to identify the density of the excited state. Fig. 1a shows an experimental example of the emission spectra obtained in situ during a coating process of HfŽC,N. growth. These typical OE spectra were obtained from HfŽC,N. coating processes under the same gas flow rate ratio with different bias voltages between y500 and y700 V. In Fig. 1a, several excited lines occurred in the discharge, e.g. at wavelengths of 382.7, 396.7, 431.4 nm, etc. Comparing the experimentally observed lines with a theoretical spectroscopic data, we identify that the 396.7 nm line is attributed to A2 ⌸᎐X 2 ⌺ transition of CN II species, that of 431.4 nm due to A2⌬᎐X 2 ⌸ transition of CH I species, and that of 382.7 nm of Hf I species in the plasma, respectively. Figs 1b,c show a variation of relative plasma intensities obtained from Fig. 1a and other OES data Žnot shown. as a function of bias voltage and gas flow rate ratio. Fig. 1b shows that the plasma intensity increased with increasing bias voltage. Above y600 V, the intensity of CH species appears two times higher than that of CN and Hf species, suggesting a possibility of large amounts of carbon in the deposited films. However, the gas rate ratio was more affected by the carbon contents in the films rather than the bias voltage. In Fig. 1c, the intensities of Hf and CH species were increased by as much as factors 8 and 5, respectively, whereas that of CN remains constantly. This indicates that much higher carbon amounts in the films could be obtained when N2 gas flow rate was increased above 0.1. The best deposition condition of this study was y600 V of bias voltage and 0.1 of gas flow rate ratio. Noticeably, the OES results have to be correlated with the layer properties such as surface topography and microhardness, etc. The deposited films of HfŽC,N. were first characterized by XRD and RBS. Fig. 2 shows a typical X-ray diffraction patterns and RBS data of a HfŽC,N. thin films grown on D2 steel and SiŽ100. surfaces at 300⬚C
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
75
using TDEAH under a gas flow rate of 0.1 with various bias voltages. XRD patterns of Fig. 2a showed a hint of HfŽC,N. film formation, since the as-grown films exhibited a relatively intense HfŽC,N.Ž111. diffraction peak, which appeared at 2 s 33⬚. Other detectable phases in the diffraction pattern were also observed at 2 s 40⬚ attributed to the HfŽC,N.Ž200. phase, indicating that a polycrystalline films could be deposited on the D2 steel surface at temperature as low as 300⬚C. With increasing the bias voltage, however, the intensity of the diffraction peak was gradually increased whereas the FWHM of HfŽC,N.Ž111. diffraction peak was decreased, suggesting that the growth rate is increased and the film quality are affected by the bias voltage. RBS was also applied to study the changes of stoichiometry, film thickness, and microhardness of our
Fig. 2. X-Ray diffraction patterns and Rutherford backscattering spectra of HfŽC,N. thin films grown on D2 steel and SiŽ100. substrates at 300⬚C with TDEAH with various bias voltages: Ža. XRD; and Žb. RBS.
Fig. 1. Plasma diagnostics by optical emission spectroscopy. Ža. OE spectra obtained during HfŽC,N. thin film growth under various bias voltages. Žb. and Žc. show the changes of plasma intensities of Hf, CN, and CH radicals as a function of plasma bias voltage and gas flow rate ratio.
films. In the RBS analysis, nitrogen resonance method was used for the clear distinction of the nitrogen peak in spectra. A 3.6-MeV He 2q ion-beam was bombarded to the sample and detected at backsacttered angle of 165⬚. Rutherford universal manipulation program ŽRUMP. was used for the simulation of the RBS spectra. Fig. 2b shows a representative RBS spectra for the HfŽC,N. films deposited on SiŽ100. at 300⬚C with TDEAH under the gas flow rate ratio of 0.1 with different bias voltages. In contrast to the XPS data, very small signals of carbon were detected with increasing bias voltages due to small sensitivity factor of carbon and the contents of Hf were increased. The variation of composition ratio and growth rate has close relation as bias voltage increases the growth rate increases and the carbon contents increase rapidly Žsee Fig. 4.. The surface morphologies of the as-grown HfŽC,N. films deposited on D2 steel at 300⬚C under the gas rate ratio of 0.1 with various bias voltages showed a very smooth appearance as shown in Fig. 3b. The surface morphology of the HfŽC,N. layers deposited under different bias voltages, linked with a domed appearance of
76
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
Fig. 3. SEM images of HfŽC,N. thin films grown on D2 steel substrates at 300⬚C with TDEAH with various bias voltages.
the surface topography, were changed from rough surface to a very smooth surface topography at higher discharge voltages of y500 to y600 V. This is in agreement with data reported previously w8,10x and indicates that the plasma contains the deposition such as particle energies and densities which can be determined during the nucleation and growth of the layer. The most ultra-fine surface structure was obtained from a HfŽC,N. film deposited at bias voltage of y600 V. Moreover, the sputtering effect due to high
bias voltage might be an important role of growing a more rougher surface than that grown at y600 V. Fig. 3a,c showed that the investigated morphology shapes of our as-grown HfŽC,N. layers have micron-sized clusters or powers of HfŽC,N. particles. The thickness of the as-grown film of y600 V is approximately 500 nm according to the cross-sectional SEM measurement. The changes of plasma, including radicals and film properties, are illustrated in terms of carrier and reactive gases as well as pulsed power variation. The effect of the plasma bias voltage is one of the important factors to obtain high quality film. Fig. 4a shows the change of deposition rates with various plasma bias voltages. Flow rates of He q H 2 and N2 were kept to be 100 sccm and 10 sccm Ži.e. N2rHe q H 2 s 0.1., respectively. Fig. 4a shows that deposition rate was increased with increasing the bias voltages. Above y600 V, however, the slope of growth rate significantly decreased from y600 V. This indicates that the film thickness might be decreased with increasing the bias voltage due to higher sputtering rate which is induced by molecular ions with relatively high kinetic energy. The influence of the ratio of N2 reactive gas to He q H 2 carrier gas on the deposition of HfŽC,N. thin films was quite noticeable. Fig. 4a also shows the variation of the deposition rate with the change of the ratio of N2 reactive gas to He q H 2 carrier gas. The flow rate of N2 was varied in the range of 0᎐60 sccm, and the flow rate of He q H 2 was kept at 100 sccm. The bias voltage was kept at y600 V. The deposition rate increased as the flow rate ratio was increased to 0.1. For the flow gas rate ratio higher than 0.1, the deposition rate decreased drastically with increasing the gas flow rate ratio. As the N2 flow rate ratio was increased from 0 to 0.6, plasma current increased from 10 to 80 mA. This indicates that below flow rate ratio of 0.1 the number of ionized molecules andror atoms increases, making more plasma decomposition of hafnium and nitrogen precursors and, thus, giving higher growth rates. On the other hand, as N2 flow rate ratio was increased to 0.6, it is thought that a sputtering on the growing surface due to the ionized molecules was increased, and the deposition rate was decreased. Plasma enhanced decomposition of precursors dominated at the flow rate ratio - 0.1, while sputtering of the growing surface dominated at the flow rate ratio ) 0.1, giving a maximum deposition rate at the flow rate ratio of 0.1. Fig. 4b shows the resistivity of the deposited films vs. the flow rate ratio. The resistivity shows a minimum value at the flow rate ratio of 0.1. This is in good agreement with our OES results shown in Figs 1b,c. RBS data show a variation of composition ratio of HfrNrC with the bias voltage and gas flow rate ratio change. The compositions of N and C have been increased especially as the gas flow rate ratio was increased Žsee Fig. 4c.. It is thought that the number of plasma activated
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
77
Fig. 4. Variations of deposition rate, electrical resistivity, composition ratio, and microhardness of the as-grown HfŽC,N. thin films as a function of plasma bias voltage and gas flow rate ratio: Ža. deposition rate; Žb. electrical resistivity; Žc. composition ratio; and Žd. microhardness.
molecules incorporating into the growing films tightly bonded to each other andror to the substrates increases, resulting in the increased strength of atomic network of the films as plasma current increased with increase of the gas flow rate ratio. This explains the decrease of resistivity with the increasing flow rate ratio in the range of 0᎐0.1. For the gas flow rate ratios higher than 0.1, resistivity is thought to increase due to higher incorporation, during the deposition, of nitrogen and carbon into the growing film. Nitrogen and carbon are believed to be from TDEAH andror N2 and from TDEAH, respectively. Higher ratio of N2rŽHe q H 2 . and, thus, higher plasma current are considered to have enhanced the detachment of nitrogen-containing and carbon-containing molecules from the precursors. Resistivity values of the HfŽC,N. films were scattered, and no meaningful relationship between the resistivity and the plasma voltage could be seen. The composition of nitrogen and carbon increased with increasing the bias voltage showing that higher bias voltage is considered to enhance the decomposition of nitrogen-containing and carbon-containing molecules form the precursors, resulting in higher nitrogen and carbon incorporation into the growing HfŽC,N. film. The increase of carbon composition was quite rapid above y700 V, and the carbon composition in the film was three times larger than that of hafnium. In this study, however, under a condition of below y700 V the gas flow rate ratio can
mostly be influenced on the carbon contents in the films rather than that of plasma bias voltage. This is well in agreement with our OES data that within y700 eV, the total intensity of CH radicals is maintained with the same level as in the case of 0.1 of gas flow rate ratio. As shown in Fig. 4c, the amounts of carbon were rapidly increased with increasing the flow rate ratio to 0.6, suggesting that if we grew the HfŽC,N. film under over 0.6 and y700 V a film with a much higher carbon content than that grown at a rather low flow rate ratio and bias voltage could be obtained. As mentioned previously, our OES experiments showed that only some species, such as CN and CH among all observed lines, are proved to be the most important as the main indicator determining the layer properties during the coating process. Good layer properties were achieved only for a deposition temperature of 300⬚C and the bias voltage of y600 V under the gas flow ratio of 0.1. The highest hardness value obtained for the as-grown HfŽC,N. film at this condition was 2500 HK 0.01. As shown in Fig. 4d, however, with increasing the intensities of CN and CH radicals dissociated of MO compounds in the plasma, the carbon contents in the films are highly increased as the gas flow ratio increases. The CH radical increases the microhardness but the CN radical decreases the hardness. This is in good agreement with previous reports w11᎐14x that a reduction of the microhardness of the
78
J.G. Han et al. r Surface and Coatings Technology 131 (2000) 73᎐78
films is in situ monitored by the intensity of the plasma induced optical emission of the CN radical. Moreover, the intensity of CN radical is proportional to the amounts of carbon contents in the film layer. Thus, the importance of the CH and CN lines are proven for the correlation with the layer properties in our study.
4. Conclusions HfŽC,N. thin films were deposited by pulsed d.c. plasma assisted metal᎐organic chemical vapor deposition ŽPA-MOCVD.. Highly oriented polycrystalline films in the w111x direction have been deposited on the D2 steel surface at temperatures as low as 300⬚C. The flow rate ratio of N2 reactive gas and the He q H 2 carrier gas was an important deposition parameter. At N2rŽHe q H 2 . ratio of 0.1, the deposition rate and the resistivity of the HfŽC,N. thin film showed maximum and minimum values, respectively. These are thought to be related to the fact that molecules excited andror decomposed by plasma make the growing thin films fast growing and dense, while they cause high sputtering of the growing surface and high incorporation of nitrogen and carbon from the precursors into the growing films. Good layer properties were achieved only for deposition temperature of 300⬚C and the bias voltage of y600 V under the gas flow ratio of 0.1. Surfaces of as-deposited HfŽC,N. films were smooth and featherless and the highest hardness value obtained for the as-grown HfŽC,N. film at this condition was 2500 HK 0.01. Radical formation and ionization behaviors in plasma are analyzed by OES at various pulsed bias and gases conditions. Moreover, the changes of plasma including radicals and film properties are illustrated in
terms of carrier and reactive gases as well as pulsed power variation.
Acknowledgements Support of this research by the Korea Science and Engineering Foundation through the Grant No. 9810307-2 and by the Ministry of Commerce, Industry, and Energy in Korea, is gratefully acknowledged. References w1x O. Knotek, A. Schrey, in: D.A. Glocker, S.I. Shah ŽEds.., Handbook of Thin Films Process Technology, Institute of Physics, Bristol and Philadelphia, 1995, p. X1.0:1. w2x A. Sherman, Jpn. J. Appl. Phys. 30 Ž1991. 3553. w3x K.T. Rie, J. Whole, A. Gebauer, J. Phys. ŽParis. II Colloq. C2 Ž1. Ž1991. 397. w4x K.T. Rie, J. Whole, A. Gebauer, Surf. Coat. Technol. 59 Ž1993. 202. w5x S.W. Kim, H. Jimba, A. Sekiguchi, O. Okada, N. Hosokawa, Appl. Surf. Sci. 100r101 Ž1996. 546. w6x C.K. Wang, L.M. Liu, M. Liao, H.C. Cheng, M.S. Lin, Jpn. J. Appl. Phys. 35 Ž1996. 4274. w7x F.H.M. Sanders, G. Verspui, Thin Solid Films 161 Ž1988. L87. w8x N.H. Hoang, D.R. McKenzie, Y. Yin, J. Appl. Phys. 80 Ž1996. 6279. w9x K.-T. Rie, J. Wohle, A. Gebauer, Surf. Coat. Technol. 98 Ž1998. 1534. w10x K.-T. Rie, J. Whole, Plasma Chem. Plasma Process 13 Ž1993. 93. w11x B.H. Weiller, J. Am. Chem. Soc. 118 Ž1996. 4975. w12x T.R. Cundari, J.M. Morse, Jr., Chem. Mater. 8 Ž1996. 189. w13x S. Peter, R. Pintaske, G. Hecht, F. Richter, J. Nucl. Mater. 200 Ž1993. 412. w14x X.-Y. Zhu, M. Wolf, T. Huett, J.M. White, J. Chem. Phys. 97 Ž1992. 5856.