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Synthesis and characterization of tantalum nitride films prepared by cathodic vacuum arc technique
Applied Surface Science 253 (2007) 5223–5227 www.elsevier.com/locate/apsusc
Synthesis and characterization of tantalum nitride films prepared by cathodic vacuum arc technique Erwu Niu *, Li Li, Guohua Lv, Wenran Feng, Huan Chen, Songhua Fan, Size Yang, Xuanzong Yang Institute of Physics, Chinese Academy of Science, Beijing 100080, China Received 13 October 2006; received in revised form 22 November 2006; accepted 22 November 2006 Available online 27 December 2006
Abstract Tantalum nitride films were deposited on silicon wafer and steel substrates by cathodic vacuum arc in N2/Ar gas mixtures. The chemical composition, crystalline microstructure and morphology of the films were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. According to the results, film composition and microstructure depends strongly on the N2 partial pressure and the applied negative bias (Vs). # 2006 Elsevier B.V. All rights reserved. Keywords: Cathodic vacuum arc; Tantalum nitride
1. Introduction Tantalum nitride (TaN) thin films have been extensively used as the key elements of mask absorbers of X-ray lithography, hard protective coatings, magnetic multilayers of recording heads and diffusion barrier layers for Cu wiring of Si semiconductor device in integrated circuits (ICS) [1–5]. There have been several techniques applied for the fabrication of TaN thin films, such as reactive magnetron sputtering [5,6], chemical vapor deposition (CVD) [7], low-energy nitrogen implantation [8] and metalorganic chemical vapour deposition (MOCVD) [9]. Among these techniques, reactive magnetron sputtering is the primary candidate for application in microelectronic devices and hard coatings. However, due to the complex phase of the Ta–N system, the experimental results of different research groups are not consistent satisfactorily with each other, and the growth mechanism of TaN films has not been fully understood yet. The growth mechanism and controllability of TaN films are still challenging tasks. Due to the characterization of high ionization ratio, high ion energy (50–150 eV), high deposition rate and flexibility of target arrangements, cathodic vacuum arc (CVA) deposition is * Corresponding author. Tel.: +86 10 82649458 17. E-mail address: [email protected] (E. Niu). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.11.042
the most commonly used technique for deposition of transition metal nitride thin films. Moreover, the inherent high energy can improve the film pack density and adhesion to the substrate. This technique is also characterized by low temperature synthesis owing to the reaction between energetic ions and metal plasma [10,11]. Little or no work, to the best of our knowledge, has been taken on the fabrication of tantalum nitride films by CVA technique. In this paper, we synthesized tantalum nitride films by this technique. Meanwhile, the composition and structure evolution with different N2 partial pressure and applied negative bias were investigated. 2. Experimental procedure Tantalum nitride films were deposited by cathodic vacuum arc technique. The Ta plasma was generated from the arc spot on a tantalum target with purity of 99.9%. The arc current was set to 60 A. Mirror-like polished stainless steels and silicon wafers were used as substrates. The specimens were ultrasonically cleaned in acetone and then in de-ionized water. Prior to deposition, the chamber was evacuated to a base pressure of 4 103 Pa, then Ar+ ions were introduced into the chamber for sputter cleaning the substrates. Typical conditions for sputtering were 600 V bias, 0.8 Pa pressure and 20 min.
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Subsequently, the chamber was evacuated to 4 103 Pa again. During the deposition of films, nitrogen and argon gases were introduced into the deposition chamber, and the total pressure was kept constant at 0.6 Pa. The thickness of the films was determined by measuring the cross-section of the as-deposited films with SEM. And the deposition rate was defined as the film thickness deposited per minute. The phase and crystal structure of the as-deposited films were identified by XRD. The chemical state of the films was tested by XPS after bombarded by Ar+ ion for 5 min to eliminate the negative oxide during the exposure time to air. However, this cleaning procedure introduces changes in the surface composition of the tantalum nitride films because of preferential sputtering effects. In particular, it is well established that nitrogen loss in tantalum nitride [12]. So the XPS can only give qualitative results of the films. Also the surface morphology of the films was investigated by atomic force microscopy (AFM) with the tapping mode.
Fig. 2. XRD diffraction patterns of films deposited with different N2 partial pressure.
3. Results and discussion The dependence of deposition rate for tantalum nitride films on the nitrogen partial pressure was shown in Fig. 1. It is necessary to point out that these films were deposited under total pressure of about 0.6 Pa and applied negative bias 50 V. It can be seen that deposition rate monotonously decreases from 15.4 to 7.6 nm/min with the increase of N2 partial pressure. The reason for the decrease could be attributed to a decreasing number of opening sites on the Ta target surface for sputtering. Many sites of the target are occupied by nitrogen atoms, N2, or TaN compounds when the N2 partial pressure increases [13,14]. Fig. 2 shows the X-ray diffraction (XRD) patterns of films deposited with different N2 partial pressure. While the deposition pressure and applied negative bias were kept about
Fig. 1. Deposition rate of various films vs. N2 partial pressure.
0.6 Pa and 50 V, respectively. It reveals that the crystal structure of the films is sensitive to the deposition condition. The pure tetragonal b-Ta films deposited in argon atmosphere exhibit a strong (3 3 0) orientation. With the increase of N2 partial pressure from 0 to 10%, the preferred orientation of bTa changes from (3 3 0) to (2 2 1). It is interesting to note that there is almost no distinctive TaN peak detected under 10% partial pressure, except for a less pronounced and broad peak corresponding to hexagonal e-TaN (1 1 0). This may be attributed to the low content of nitrogen in the background gas or nitrogen deficiency, which results in the formation of amorphous structure. Another reason for this is that the N atoms may occupy the interstitial site as solid solution and cause little deformation of tantalum lattice. With the nitrogen content increased from 15 to 25%, the diffraction peak of eTaN (1 1 0) becomes distinct and the b-Ta (2 2 1) peak vanishes completely. However, the preferred orientation of the e-TaN phase changes from (1 1 0) to (3 0 0) when the N2 partial pressure increases to 50%. As for the change of preferred orientation of TaN crystalline grain with nitrogen partial pressure, the deposition species and ion energy may be the important factors. Sun et al. [15] have reported that the Nrich phases, e.g. Ta3N5, Ta5N6 were found with sputterdeposition technique under high N2 partial pressure. Although a preferred orientation transition is obvious, there is no sign of N-rich phases observed in XRD pattern even with 50% N2 partial pressure. Nie et al. [16] also found the variation of preferred orientation of fcc-TaN with different nitrogen pressure deposited by magnetron sputtering technique. Chhowalla [17] has reported that the species and ion energy of cathodic arc zirconium (Zr) plasma in nitrogen atmosphere change more compared with that in argon atmosphere. Therefore, further work is necessary to study and verify this phenomenon. The crystallite size of the films was estimated from XRD data using the full width at half maximum (FWHM) of the
E. Niu et al. / Applied Surface Science 253 (2007) 5223–5227
(1 1 0) reflection peak, which is quantified by the Scherrer equation: D¼
0:9l B cos u
where B is the FWHM in 2u units, l the X-ray wavelength and u is the Bragg diffraction angle. Note that the formula does not involve peak broadening arising from stress and defects in the film as well as from instrument itself. Consequently, the crystallite size calculated from the formula is smaller than the actual value. The calculated crystallite size of our films ranges from about 17 to 42 nm when the N2 partial pressure increases from 15 to 50%. Generally, growth of large crystallite is linked with high surface mobility of adatoms and the deposition rate [18,19]. In our case, the deposition rate also decreases when nitrogen partial pressure increases, as mentioned before. Such a reduction leads to a relative lack of condensing species and possibly affects the film nucleation and crystallite growth. Applied negative bias has influence on the ion energy, which is an important factor to determine the composition and structure of the films. Fig. 3 demonstrates the influence of applied bias on the phase and preferred orientation of the asdeposited films under the N2 partial pressure of 20%. In this series of experiments, deposition pressure and substrate temperature remained constant at 0.6 Pa and below 100 8C, respectively. It shows that the phase composition and preferred orientation of the films are strongly dependent on the applied negative bias. With increasing applied negative bias, diffraction peaks of the films converts from B1-NaCl-structure d-TaN to hexagonal e-TaN, and eventually to Ta3N5. It has been reported previously by Lee et al. [20] that TaN structure changed from B1-NaCl-structure d-TaN to hexagonal e-TaN mixture with increase of ion energy. It is possible that the ion energy contributes not only to the preferred orientation but also to the crystalline structure. As we know, films deposited under high bias possess high residual stress, which can lead to deviation of diffraction peaks.
Fig. 3. XRD diffraction patterns of films deposited with different negative bias.
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It is possible for us to get inaccurate or even wrong information just from XRD result because of its complexity of Ta–N system [21]. XPS measurement is a sensitive method to characterize the chemical states of the films. Therefore, X-ray photoelectron spectra were recorded using a non-monochromatised Mg Ka source for scan. Peak positions were then calibrated with respect to the C1s peak at 284.6 eV from the hydrocarbon contamination. Fig. 4 presents the photoelectron spectra of films deposited with different negative bias. It can be observed from Fig. 4(a) that the Ta 4f core level shifts to higher value with the increase of applied negative bias. Arranz and Palacio [8,22] reported that Ta 4f core level shifts to higher binding energy value with the increase of N content in TaNx compound fabricated by low-energy nitrogen implantation. It is worth noticing that the Ta 4f core level shows a broad shape that is indicative of several Ta–N phases. The insert presents the result by Gaussian Fitting technique. The Ta 4f spectra can be fitted to two sets of Ta 4f doublets (a1, a2 and b1, b2). As is shown, one
Fig. 4. XPS spectra of tantalum nitride films with different negative bias: (a) Ta 4f and (b) N 1s.
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of the doublets consists of a1 (Ta 4f7/2 = 23.6 eV) and a2 with a spin-orbit splitting of about 2.0 eV, which shows binding energy values closing to the chemical state of the Ta in cubic/ hexagonal TaN compound [23,8]. Combining the XRD and XPS results, the binding energy value 23.6 eV of Ta 4f corresponds to cubic TaN. As for the other doublet, the binding energy values of b1 (Ta 4f7/2 = 25.3 eV) and b2 can be attributed to N-rich Ta3N5 films [24]. With the increase of bias, the binding energy value shift to higher value can be attributed to the increase of N-rich TaN content in the films. This is consistent with XRD pattern shown in Fig. 3, although there may be some error induced by preferential sputtering. The evolution of N 1s spectra with bias is presented in Fig. 4(b). It can be observed that the N 1s core level at 398 eV overlaps with the Ta 4p 3/2 at 402 eV. It also means that the peak at 398 eV is associated with N in tantalum nitride, and the peak at 403 eV is associated with the overlapping Ta 4p core level. As shown in Fig. 4(b), the binding energy of N 1s shifts to lower value with the increase of bias. This shift means the increase of N content in tantalum
films [22]. Therefore, this also gives the evidence that N-rich phase is formed in tantalum nitride films deposited with high bias as shown in Fig. 3. The AFM images of films deposited at various applied negative biases are presented in Fig. 5. It indicates that the surface morphology of the films depends strongly on the applied negative bias. Loose and random grains characterize film deposited at a bias of 0 V. The density and smoothness of films deposited at the bias of 100 V are improved. Under the applied bias of 200 V, the average size of grain increases and the grain boundary becomes ambiguous. When the substrate bias is increased to 300 V, large clusters are observed on the surface, which leads to the increase of surface roughness. At Vs = 0 V, the energy of the depositing species, as well as the mobility of the surface atoms is very low. Thus, the asdeposited films show a random orientation because of the lower structural relaxation ability. With the increase of negative bias voltage, the depositing ions can be effectively accelerated. Under the bombardment of the energetic depositing ion flux,
Fig. 5. AFM images of tantalum nitride films deposited at the negative bias: of (a) 0 V, (b) 100 V, (c) 200 V and (d) 300 V.
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the mobility of atoms on the growing film surface can be greatly increased. As a result, the pack density and smoothness are improved. With further increase of ion energy, physical sputtering of the films occurs and becomes more and more significant. Thus, the deposition rate decreases and clusters are formed. The preferred orientation can be explained by Bradley’s model known as channeling effect [25], which is based on the fact that different crystallographic orientations have different sputter yield and the lower sputter yield orientation can be reserved. 4. Conclusion Tantalum nitride films were synthesized by cathodic vacuum arc technique. The deposition rate, microstructure and composition depend strongly on the N2 partial pressure and applied negative bias. With the increase of nitrogen partial pressure, the phase of the films changes from b-Ta to e-TaN. Meanwhile, different preferred orientations of the films were observed under different partial pressures. With increasing applied negative bias, the phase composition of the asdeposited films converts from B1-NaCl-structure d-TaN to hexagonal e-TaN, and eventually to Ta3N5. Acknowledgement
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
This study is financially supported by the National Nature Science Foundation, China.
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S.Y. Lee, C.M. Park, Jpn. J. Appl. Phys. 39 (2000) 6919. Y. Qin, L. Lin, L. Chen, J. Alloys Compd. 269 (1998) 238. E.O. Ezugwu, J. Wallbank, Mater. Sci. Technol. 3 (1987) 881. S. Li, P.P. Freitas, M.S. Rogalski, J. Appl. Phys. 81 (1997) 4501. G.S. Chen, P.Y. Lee, S.T. Chen, Thin Solid Films 353 (1999) 264. G.S. Chen, S.C. Huang, S.T. Chen, Appl. Phys. Lett. 76 (2000) 2895. H. Park, K. Byun, W. Lee, Jpn. J. Appl. Phys. 41 (2002) 6153. A. Arranz, C. Palacio, Appl. Phys. A81 (2005) 1405. M.H. Tsai, S.C. Sun, H.T. Chiu, C.E. Tsai, S.H. Chuang, Appl. Phys. Lett. 67 (1995) 1128. G.Q. Yu, B.K. Tay, Z.W. Zhao, Appl. Phys. A81 (2005) 405. P.J. Martin, A. Bendavid, Thin Solid Films 394 (2001) 1. C. Palacio, J.M. Sanz, J.M. Martinez-Duart, Thin Solid Films 124 (1985) 243. J.C. Lin, G. Chen, J. Lee, J. Electrochem. Soc. 146 (1999) 1835. G.Q. Yu, B.K. Tay, Z.W. Zhao, Appl. Phys. A81 (2004) 405. X. Sun, E. Kolawa, J.S. Chen, M.A. Nicolet, Thin Solid Films 236 (1993) 347. H.B. Nie, S.Y. Xu, S.J. Wang, C.K. Ong, J. Li, T.Y.E. Liew, Appl. Phys. A73 (2001) 229. M. Chhowalla, Appl. Phys. Lett. 83 (2003) 1542. G.Q. Yu, B.K. Tay, S.P. Lau, Chem. Phys. Lett. 373 (2003) 264. A.S. Kao, G.L. Gorman, Appl. Phys. Lett. 67 (1990) 3826. G.R. Lee, J.J. Lee, C.S. Shin, Thin Solid Films 475 (2005) 45. C. Stampfl, A.J. Freeman, Phys. Rev. B71 (2005) 024111. A. Arranz, C. Palacio, Surf. Interface Anal. 29 (2000) 653. J.F. Moulder, W.F. Stickle, P.E. Sobol, Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronic, Eden Praine, 1995. J. Liu, J. Bao, M. Scharnberg, W.C. Kim, P.S. Ho, J. Vac. Sci. Technol. A23 (2005) 1107. G.R. Lee, J.J. Lee, C.S. Shin, Thin Solid Films 475 (2005) 45.
Synthesis and characterization of tantalum nitride films prepared by cathodic vacuum arc technique Erwu Niu *, Li Li, Guohua Lv, Wenran Feng, Huan Chen, Songhua Fan, Size Yang, Xuanzong Yang Institute of Physics, Chinese Academy of Science, Beijing 100080, China Received 13 October 2006; received in revised form 22 November 2006; accepted 22 November 2006 Available online 27 December 2006
Abstract Tantalum nitride films were deposited on silicon wafer and steel substrates by cathodic vacuum arc in N2/Ar gas mixtures. The chemical composition, crystalline microstructure and morphology of the films were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. According to the results, film composition and microstructure depends strongly on the N2 partial pressure and the applied negative bias (Vs). # 2006 Elsevier B.V. All rights reserved. Keywords: Cathodic vacuum arc; Tantalum nitride
1. Introduction Tantalum nitride (TaN) thin films have been extensively used as the key elements of mask absorbers of X-ray lithography, hard protective coatings, magnetic multilayers of recording heads and diffusion barrier layers for Cu wiring of Si semiconductor device in integrated circuits (ICS) [1–5]. There have been several techniques applied for the fabrication of TaN thin films, such as reactive magnetron sputtering [5,6], chemical vapor deposition (CVD) [7], low-energy nitrogen implantation [8] and metalorganic chemical vapour deposition (MOCVD) [9]. Among these techniques, reactive magnetron sputtering is the primary candidate for application in microelectronic devices and hard coatings. However, due to the complex phase of the Ta–N system, the experimental results of different research groups are not consistent satisfactorily with each other, and the growth mechanism of TaN films has not been fully understood yet. The growth mechanism and controllability of TaN films are still challenging tasks. Due to the characterization of high ionization ratio, high ion energy (50–150 eV), high deposition rate and flexibility of target arrangements, cathodic vacuum arc (CVA) deposition is * Corresponding author. Tel.: +86 10 82649458 17. E-mail address: [email protected] (E. Niu). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.11.042
the most commonly used technique for deposition of transition metal nitride thin films. Moreover, the inherent high energy can improve the film pack density and adhesion to the substrate. This technique is also characterized by low temperature synthesis owing to the reaction between energetic ions and metal plasma [10,11]. Little or no work, to the best of our knowledge, has been taken on the fabrication of tantalum nitride films by CVA technique. In this paper, we synthesized tantalum nitride films by this technique. Meanwhile, the composition and structure evolution with different N2 partial pressure and applied negative bias were investigated. 2. Experimental procedure Tantalum nitride films were deposited by cathodic vacuum arc technique. The Ta plasma was generated from the arc spot on a tantalum target with purity of 99.9%. The arc current was set to 60 A. Mirror-like polished stainless steels and silicon wafers were used as substrates. The specimens were ultrasonically cleaned in acetone and then in de-ionized water. Prior to deposition, the chamber was evacuated to a base pressure of 4 103 Pa, then Ar+ ions were introduced into the chamber for sputter cleaning the substrates. Typical conditions for sputtering were 600 V bias, 0.8 Pa pressure and 20 min.
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Subsequently, the chamber was evacuated to 4 103 Pa again. During the deposition of films, nitrogen and argon gases were introduced into the deposition chamber, and the total pressure was kept constant at 0.6 Pa. The thickness of the films was determined by measuring the cross-section of the as-deposited films with SEM. And the deposition rate was defined as the film thickness deposited per minute. The phase and crystal structure of the as-deposited films were identified by XRD. The chemical state of the films was tested by XPS after bombarded by Ar+ ion for 5 min to eliminate the negative oxide during the exposure time to air. However, this cleaning procedure introduces changes in the surface composition of the tantalum nitride films because of preferential sputtering effects. In particular, it is well established that nitrogen loss in tantalum nitride [12]. So the XPS can only give qualitative results of the films. Also the surface morphology of the films was investigated by atomic force microscopy (AFM) with the tapping mode.
Fig. 2. XRD diffraction patterns of films deposited with different N2 partial pressure.
3. Results and discussion The dependence of deposition rate for tantalum nitride films on the nitrogen partial pressure was shown in Fig. 1. It is necessary to point out that these films were deposited under total pressure of about 0.6 Pa and applied negative bias 50 V. It can be seen that deposition rate monotonously decreases from 15.4 to 7.6 nm/min with the increase of N2 partial pressure. The reason for the decrease could be attributed to a decreasing number of opening sites on the Ta target surface for sputtering. Many sites of the target are occupied by nitrogen atoms, N2, or TaN compounds when the N2 partial pressure increases [13,14]. Fig. 2 shows the X-ray diffraction (XRD) patterns of films deposited with different N2 partial pressure. While the deposition pressure and applied negative bias were kept about
Fig. 1. Deposition rate of various films vs. N2 partial pressure.
0.6 Pa and 50 V, respectively. It reveals that the crystal structure of the films is sensitive to the deposition condition. The pure tetragonal b-Ta films deposited in argon atmosphere exhibit a strong (3 3 0) orientation. With the increase of N2 partial pressure from 0 to 10%, the preferred orientation of bTa changes from (3 3 0) to (2 2 1). It is interesting to note that there is almost no distinctive TaN peak detected under 10% partial pressure, except for a less pronounced and broad peak corresponding to hexagonal e-TaN (1 1 0). This may be attributed to the low content of nitrogen in the background gas or nitrogen deficiency, which results in the formation of amorphous structure. Another reason for this is that the N atoms may occupy the interstitial site as solid solution and cause little deformation of tantalum lattice. With the nitrogen content increased from 15 to 25%, the diffraction peak of eTaN (1 1 0) becomes distinct and the b-Ta (2 2 1) peak vanishes completely. However, the preferred orientation of the e-TaN phase changes from (1 1 0) to (3 0 0) when the N2 partial pressure increases to 50%. As for the change of preferred orientation of TaN crystalline grain with nitrogen partial pressure, the deposition species and ion energy may be the important factors. Sun et al. [15] have reported that the Nrich phases, e.g. Ta3N5, Ta5N6 were found with sputterdeposition technique under high N2 partial pressure. Although a preferred orientation transition is obvious, there is no sign of N-rich phases observed in XRD pattern even with 50% N2 partial pressure. Nie et al. [16] also found the variation of preferred orientation of fcc-TaN with different nitrogen pressure deposited by magnetron sputtering technique. Chhowalla [17] has reported that the species and ion energy of cathodic arc zirconium (Zr) plasma in nitrogen atmosphere change more compared with that in argon atmosphere. Therefore, further work is necessary to study and verify this phenomenon. The crystallite size of the films was estimated from XRD data using the full width at half maximum (FWHM) of the
E. Niu et al. / Applied Surface Science 253 (2007) 5223–5227
(1 1 0) reflection peak, which is quantified by the Scherrer equation: D¼
0:9l B cos u
where B is the FWHM in 2u units, l the X-ray wavelength and u is the Bragg diffraction angle. Note that the formula does not involve peak broadening arising from stress and defects in the film as well as from instrument itself. Consequently, the crystallite size calculated from the formula is smaller than the actual value. The calculated crystallite size of our films ranges from about 17 to 42 nm when the N2 partial pressure increases from 15 to 50%. Generally, growth of large crystallite is linked with high surface mobility of adatoms and the deposition rate [18,19]. In our case, the deposition rate also decreases when nitrogen partial pressure increases, as mentioned before. Such a reduction leads to a relative lack of condensing species and possibly affects the film nucleation and crystallite growth. Applied negative bias has influence on the ion energy, which is an important factor to determine the composition and structure of the films. Fig. 3 demonstrates the influence of applied bias on the phase and preferred orientation of the asdeposited films under the N2 partial pressure of 20%. In this series of experiments, deposition pressure and substrate temperature remained constant at 0.6 Pa and below 100 8C, respectively. It shows that the phase composition and preferred orientation of the films are strongly dependent on the applied negative bias. With increasing applied negative bias, diffraction peaks of the films converts from B1-NaCl-structure d-TaN to hexagonal e-TaN, and eventually to Ta3N5. It has been reported previously by Lee et al. [20] that TaN structure changed from B1-NaCl-structure d-TaN to hexagonal e-TaN mixture with increase of ion energy. It is possible that the ion energy contributes not only to the preferred orientation but also to the crystalline structure. As we know, films deposited under high bias possess high residual stress, which can lead to deviation of diffraction peaks.
Fig. 3. XRD diffraction patterns of films deposited with different negative bias.
5225
It is possible for us to get inaccurate or even wrong information just from XRD result because of its complexity of Ta–N system [21]. XPS measurement is a sensitive method to characterize the chemical states of the films. Therefore, X-ray photoelectron spectra were recorded using a non-monochromatised Mg Ka source for scan. Peak positions were then calibrated with respect to the C1s peak at 284.6 eV from the hydrocarbon contamination. Fig. 4 presents the photoelectron spectra of films deposited with different negative bias. It can be observed from Fig. 4(a) that the Ta 4f core level shifts to higher value with the increase of applied negative bias. Arranz and Palacio [8,22] reported that Ta 4f core level shifts to higher binding energy value with the increase of N content in TaNx compound fabricated by low-energy nitrogen implantation. It is worth noticing that the Ta 4f core level shows a broad shape that is indicative of several Ta–N phases. The insert presents the result by Gaussian Fitting technique. The Ta 4f spectra can be fitted to two sets of Ta 4f doublets (a1, a2 and b1, b2). As is shown, one
Fig. 4. XPS spectra of tantalum nitride films with different negative bias: (a) Ta 4f and (b) N 1s.
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of the doublets consists of a1 (Ta 4f7/2 = 23.6 eV) and a2 with a spin-orbit splitting of about 2.0 eV, which shows binding energy values closing to the chemical state of the Ta in cubic/ hexagonal TaN compound [23,8]. Combining the XRD and XPS results, the binding energy value 23.6 eV of Ta 4f corresponds to cubic TaN. As for the other doublet, the binding energy values of b1 (Ta 4f7/2 = 25.3 eV) and b2 can be attributed to N-rich Ta3N5 films [24]. With the increase of bias, the binding energy value shift to higher value can be attributed to the increase of N-rich TaN content in the films. This is consistent with XRD pattern shown in Fig. 3, although there may be some error induced by preferential sputtering. The evolution of N 1s spectra with bias is presented in Fig. 4(b). It can be observed that the N 1s core level at 398 eV overlaps with the Ta 4p 3/2 at 402 eV. It also means that the peak at 398 eV is associated with N in tantalum nitride, and the peak at 403 eV is associated with the overlapping Ta 4p core level. As shown in Fig. 4(b), the binding energy of N 1s shifts to lower value with the increase of bias. This shift means the increase of N content in tantalum
films [22]. Therefore, this also gives the evidence that N-rich phase is formed in tantalum nitride films deposited with high bias as shown in Fig. 3. The AFM images of films deposited at various applied negative biases are presented in Fig. 5. It indicates that the surface morphology of the films depends strongly on the applied negative bias. Loose and random grains characterize film deposited at a bias of 0 V. The density and smoothness of films deposited at the bias of 100 V are improved. Under the applied bias of 200 V, the average size of grain increases and the grain boundary becomes ambiguous. When the substrate bias is increased to 300 V, large clusters are observed on the surface, which leads to the increase of surface roughness. At Vs = 0 V, the energy of the depositing species, as well as the mobility of the surface atoms is very low. Thus, the asdeposited films show a random orientation because of the lower structural relaxation ability. With the increase of negative bias voltage, the depositing ions can be effectively accelerated. Under the bombardment of the energetic depositing ion flux,
Fig. 5. AFM images of tantalum nitride films deposited at the negative bias: of (a) 0 V, (b) 100 V, (c) 200 V and (d) 300 V.
E. Niu et al. / Applied Surface Science 253 (2007) 5223–5227
the mobility of atoms on the growing film surface can be greatly increased. As a result, the pack density and smoothness are improved. With further increase of ion energy, physical sputtering of the films occurs and becomes more and more significant. Thus, the deposition rate decreases and clusters are formed. The preferred orientation can be explained by Bradley’s model known as channeling effect [25], which is based on the fact that different crystallographic orientations have different sputter yield and the lower sputter yield orientation can be reserved. 4. Conclusion Tantalum nitride films were synthesized by cathodic vacuum arc technique. The deposition rate, microstructure and composition depend strongly on the N2 partial pressure and applied negative bias. With the increase of nitrogen partial pressure, the phase of the films changes from b-Ta to e-TaN. Meanwhile, different preferred orientations of the films were observed under different partial pressures. With increasing applied negative bias, the phase composition of the asdeposited films converts from B1-NaCl-structure d-TaN to hexagonal e-TaN, and eventually to Ta3N5. Acknowledgement
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
This study is financially supported by the National Nature Science Foundation, China.
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