- Email: [email protected]
Influence of substrate bias voltage on the microstructure and residual stress of CrN films deposited by medium frequency magnetron sputtering
Materials Science and Engineering B 176 (2011) 850–854
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Influence of substrate bias voltage on the microstructure and residual stress of CrN films deposited by medium frequency magnetron sputtering Qinghua Kong a,b , Li Ji a , Hongxuan Li a,∗ , Xiaohong Liu a , Yongjun Wang a,b , Jianmin Chen a , Huidi Zhou a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 13 January 2011 Received in revised form 28 March 2011 Accepted 10 April 2011 Keywords: CrN films Medium frequency magnetron sputtering Substrate bias voltage Microstructure Residual stress
a b s t r a c t In this study, CrN films were deposited on stainless steel and Si (1 1 1) substrates via medium frequency magnetron sputtering under a systematic variation of the substrate bias voltage. The influence of the substrate bias voltage on the structural and the mechanical properties of the films were investigated. It is observed that there are two clear regions: (1) below −300 V, and (2) above −300 V. For the former region, the (1 1 1) texture is dominated as the substrate bias voltage is increased to −200 V. The lattice ˚ and the as-deposited films parameter is smaller than that of CrN reported in the ICSD standard (4.140 A) exhibit tensile stress. Meanwhile, the surface roughness decreases and the N concentration show a slow increase. For the latter region, the (2 0 0)-oriented structure is formed. However, the lattice parameter is larger as compared with the value reported in the ICSD standard, and the surface roughness increases and the N concentration decreases obviously. In this case, the compressive stress is obtained. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, chromium nitride coatings are widely used not only for tribological applications but also for protective coatings and diffusion barriers due to their superior corrosion resistance, oxidation resistance and wear resistance [1–4]. These coatings are generally prepared by various physical vapor deposition techniques, among which medium frequency magnetron sputtering has attracted considerable interest [5–8]. However, during the reactive direct current (DC) sputtering process, the vacuum chamber can be deposited by an insulative layer which causes “anode vanished”. This problem may contribute to discharge process unstable and makes the structures and composition of thin film uncontrollable [9,10]. In the present years, dual magnetrons are usually used to solve this problem. By arranging two magnetron sources energized by a bipolar power supply unit, within one cycle each magnetron acts alternately as an anode and cathode so that the anode can be cleaned automatically on every cycle, which may not only avoids the anode disappearing problem but also provides a stable sputtering process [10]. Furthermore, it can increase the degree of ionization and the ratio of ion to neutron in plasma which can enhance the quality of coatings [11]. It is well known that the structure and properties of magnetron sputtered CrN coatings strongly depend on the deposition parameters such as relative Ar/N2 flow rate, substrate bias, sub-
strate temperature and target power etc. [12–14]. In particular, bias voltage is a key parameter which obviously affects the quality of the coating due to enhancement of adatom mobility and the ion bombardment effect. It has been reported that bias magnetron sputtering is an effective method for controlling the microstructural evolution of transition-metal nitride films, namely, film texture and grain size, in order to enhance their properties [15]. Scheerer et al. have reported that increasing applied bias voltage causes preferably (2 0 0) crystallization lattice and CrN coatings with (2 0 0)-oriented lattice orientation seem to cover surface imperfections due to a platelet like grain structure [16]. Phase fraction and preferred orientation in CrNx coatings vary with substrate bias, exerting an effective influence on the properties of the films [12,17]. However, although the influence of substrate bias voltage has been investigated by some authors, coating microstructure and residual stress under different substrate bias voltages are worth further investigation. In the present work, CrN films were deposited using medium frequency magnetron sputtering under various pulsed substrate bias voltages. Coating characteristics such as texture, chemical composition, microstructure, residual stress, and deposition rate were analyzed, respectively. 2. Experimental details 2.1. Deposition of the CrN films
∗ Corresponding author. Tel.: +86 931 4968150; fax: +86 931 8277088. E-mail address: [email protected] (H. Li). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.04.015
CrN films were deposited on stainless steel (1Cr18Mn8Ni5N) and p-type silicon (1 1 1) wafers via a twin target medium frequency
Q. Kong et al. / Materials Science and Engineering B 176 (2011) 850–854
851
(40 kHz) magnetron sputtering technique under different substrate bias voltages. A pair of magnetron planar Cr targets (99.8%) were set in cylindrical vacuum chamber wall. The substrates were ultrasonically cleaned in acetone for 30 min and then blown dry with N2 before fixed in the deposition chamber with a distance of 100 mm from the substrate to target. A base pressure of less than 5.0 × 10−3 Pa was achieved in the chamber using a turbomolecular pumping system before sputtering. Prior to deposition, substrates were firstly etched with Ar+ plasma at a bias voltage of −600 V for 30 min in order to remove contaminants and ensure good adhesion of the deposited films. The working pressure was 0.3 Pa by the inlet of Ar and N2 (Ar: N2 = 3/5) for the deposition of CrN films. During deposition, the target power was kept at 5.0 kW and the substrate temperature was 200 ◦ C. A pulse power supply was employed to provide a substrate bias voltage with a frequency of 40 kHz and duty cycle was kept at 40% during deposition. In order to study the effect of the substrate bias voltage, different substrate bias voltages ranging from −100 to −500 V were applied. 2.2. Characterization of the CrN films
Fig. 1. The deposition rate of CrN films as a function of substrate bias voltage.
The crystallographic structure of the films was studied by conventional Bragg-Brentano X-ray diffraction (XRD) using a Philips X’Pert-MRD type diffractometer with a Cu tube operated at 40 kV and 60 mA. The chemical composition of the as-deposited films was determined by energy dispersive X-ray spectroscopy (EDS) analysis attached to a JSM-6701F type field emission scanning electron microscope. The thickness of the deposited films was measured using a Micro XAM 3D non-contact surface profilometer, and the deposition rate was calculated via film thickness and the corresponding deposition time. The residual stress of the films was evaluated using the substrate curvature method, where the thickness of the films and the curvature of the Si wafer before and after the deposition were measured using a Micro XAM 3D non-contact surface profilometer. Then the residual stress was calculated applying Stoney’s equation [18]. A digital instruments Nanoscope IIIa multimode atomic force microscope (AFM) operating with a Si3 N4 probe in a “constant force” mode was performed to observe the surface topographies of the CrN film and to determine the surface roughness.
concentration first slightly increases as the bias voltage varies from −100 V to −300 V, which may be attributed to the increase of the ionized N atoms. However, N concentration decreases dramatically with further increasing substrate bias voltage, with a minimum nitrogen content of 31%. Two effects may contribute to this behavior. On the one hand, the atomic mass of N atoms is lighter than Cr atoms. Thus, N atoms is prone to being sputtered by impinging ions with high energy, especially at higher energy as the bias voltage is increased to −500 V. On the other hand, the texture of CrN films changes from (1 1 1) to (2 0 0) when the substrate bias voltage is increased to −300 V (as shown in Fig. 3b). As CrN has the fcc structure, pure nitrogen layers and pure chromium layers are alternately presented in the (1 1 1) planes, whereas the (2 0 0) planes consist of both nitrogen and chromium atoms [11], suggesting (2 0 0) plane is not a favorable accommodation for nitrogen atoms. It is argued that the N concentration would decrease as the (2 0 0)-oriented structure is dominated in the film. In addition, although the N content is as low as 31%, there no Cr2 N phase is observed in the as-deposited films (as shown in Fig. 3a). Similar result has been obtained by Forniés et al. [19]. They showed that the only phase present is the cubic CrN when nitrogen concentration in the films is clearly below the concentration corresponding to stoichiometric CrN. Obviously, there is an excess of chromium in the films which is not detected
3. Results and discussion 3.1. Deposition rate The deposition rate of CrN films as a function of substrate bias voltage is presented in Fig. 1. It is clearly observed that the substrate bias voltage has an important effect on the deposition rate of the as-deposited films, the higher the absolute value of the substrate bias voltage, the lower the deposition rate (Fig. 1). It is observed that the deposition rate changes from 16 to 11 nm/min as the substrate bias voltage is changed from −100 V to −500 V. It is well known that the positive ions of the plasma are attracted toward the substrate and transfer their kinetic energy to the surface atoms when the bias voltage is applied to the substrate [19]. It is a matter of common knowledge that the incident ion energy becomes higher as the substrate bias voltage increases, and as a result, more atoms from the growing films will be re-sputtered. Therefore, it is assumed that the re-sputtering effect on growing films with high bias value is responsible for the decrease of deposition rate in the present study. 3.2. Chemical composition Fig. 2 shows the N concentration of the as-deposited films obtained under various substrate bias voltages. It can be seen that N
Fig. 2. The N concentration of CrN films deposited under various substrate bias voltages.
852
Q. Kong et al. / Materials Science and Engineering B 176 (2011) 850–854
Fig. 3. XRD patterns (a) and texture coefficients (b) of CrN films deposited under various substrate bias voltages.
in the diffraction pattern (as shown in Fig. 3a). However, it is difficult for the Cr atoms to insert into the octahedral positions of CrN structure because Cr atom is bigger than N atom. Meanwhile, it seems impossible to develop amorphous form due to the high deposition temperature (200 ◦ C). Therefore, it is assumed that the N atoms incorporated into CrN lattice is not adequate, forming an N-deficient CrN.
3.3. Structure and residual stress The XRD patterns of CrN films deposited with various substrate bias voltages are shown in Fig. 3a. It is clear that the crystalline orientations of CrN films strongly depend on the applied bias voltage. Diffraction lines corresponding to the (1 1 1), (2 0 0), and (2 2 0) peaks can be observed from the XRD pattern that confirms the formation of cubic CrN films. However, the variation of substrate bias voltage affects the crystallographic texture of CrN films. It can be observed that, at the bias voltage of −100 V, structure features with mixed (1 1 1), (2 0 0) and (2 2 0) reflection peaks are seen. Films exhibit a preferred (1 1 1) orientation as the substrate bias voltage is increased to −200 V. However, with further increasing substrate bias voltage, the preferred orientation changes from CrN (1 1 1) to (2 0 0).
Fig. 4. Lattice parameters of CrN films deposited under different substrate bias voltages.
The texture coefficients of the CrN T (h k l) are calculated from their respective XRD peaks with the following formula: T (h k l) =
I(h k l) I(1 1 1) + I(2 0 0) + I(2 2 0)
(1)
where (h k l) represents the (1 1 1), (2 0 0) or (2 2 0) orientations. The texture development of CrN films deposited under various substrate bias voltages is depicted in Fig. 3b. It can be seen that films grown under weak ion-irradiation conditions at −100 V contain predominantly a mixture of (1 1 1) and (2 0 0) orientations with a small volume fraction of (2 2 0) grains. For the films deposited with medium substrate bias voltage (−200 V), the dominant (1 1 1) texture is developed. In contrast, at higher energies of incident ions the grain growth is forced to form highly (2 0 0)-oriented texture. Two theoretical predictions of the growth mechanisms may explain these trends in the texture of the as-deposited films. First, it is well known that the texture development during film growth can be discussed by the competition between the surface and strain energy [14,20]. According to Pelleg et al. [20], the texture evolution is caused by the driving force to lower the overall energy of films consisting of surface energy and strain energy. It has been reported that CrN would grow toward the orientation of the (2 0 0) plane with the lowest surface energy when the surface energy is dominant, whereas the orientation of the (1 1 1) plane with the lowest strain energy would be expected when the strain energy is dominant. At low bias voltage (in absolute values), films grow with (1 1 1) grains at large thickness, where the strain energy dominates [21]. Further increasing the substrate bias voltage, however, the film thickness and the grain size are decreased, where the surface energy dominates. Therefore, the CrN films show preferred orientation of (2 0 0) as bias voltages ranging from −300 V to −500 V were applied to the substrate. Another growth mechanism is the ion channeling effect [12,22]. It is well known that the structure of CrN is built by an interpenetration of two fcc lattices of Cr and N atoms. The (2 0 0) planes are the densest planes (4 atoms/a2 , a is the lattice parameter of CrN) and have the largest distance between crystal planes (0.5 a) in CrN. For the (1 1 1) planes of CrN, the planes density and the distance between the crystal planes are 2.31 atoms/a2 , and 0.289 a, respectively [12]. In the light of the structural features, the nuclear stopping cross section for (1 1 1) oriented grains will be the largest and will be the smallest for (2 0 0) grains in CrN [23]. According to sputtering theory, the sputter yield changes with nuclear stopping cross section. Therefore, the preferential sputtering of the atoms in (1 1 1) planes is intensified with increasing substrate bias voltage, namely, the growth of CrN grains along (1 1 1) is suppressed at high bias voltage. In contrast, for grains with a (2 0 0) orientation having
Q. Kong et al. / Materials Science and Engineering B 176 (2011) 850–854
Fig. 5. Residual stresses of CrN films deposited under different substrate bias voltages.
the smallest nuclear stopping cross section and the lowest sputtering yield will develop. Thus, the preferred orientation of CrN films changes from (1 1 1) to (2 0 0) as a function of the substrate bias voltage. The lattice parameters were calculated from the (2 0 0) and (1 1 1) planes. It has been reported that the lattice of chromium nitride films is distorted due to the residual stress and defects in the films [24]. The evolution of the lattice parameter and residual stress of CrN films deposited under different substrate bias voltages are shown in Figs. 4 and 5. As it can be seen, there are two regions: (1) below −300 V, where the lattice parameter is smaller ˚ and (2) than that of CrN reported in the ICSD standard (4.140 A), above −300 V, exhibiting higher lattice parameter than the standard value. The lattice parameter increases as the bias voltages ranging from −100 V to −400 V are applied to the substrate. This may be attributed to the enhanced flux and energy of the bombarding ions, whereby Frenkel pairs and anti-Schottky defects are induced by the “ion peening effect” [25,26]. However, the lattice parameter shows a slight decrease when the substrate bias voltage
853
Fig. 6. Grain sizes of CrN films deposited under different substrate bias voltages.
is increased to −500 V, which appears to be related to the deficiency in N atoms relative to the stoichiometric CrN [27]. The negative deviation from the lattice parameter as compared with the ICSD standard values for CrN indicates a tensile stress [28]. Increasing the substrate bias voltage leads to a change to positive deviation values of lattice parameter indicating the change from tensile stress to compressive stress, which is consistent with the evolution of the residual stress (Fig. 5). It can be observed that residual stresses are not isotropic in our study. With the bias voltage ranging from −100 V to −300 V, the films show tensile stresses which decreases with the increase of bias voltage. However, as the bias voltage is increased from −300 V to −500 V, the films exhibit compressive stress. Furthermore, the compressive stress increases with increasing the substrate bias voltage, which is attribute to high defect densities induced by ion bombardment [29]. The grain sizes of the CrN films deposited under different substrate bias voltages are shown in Fig. 6. As it is seen from the picture, the grain sizes tend to decrease on the whole, although the grain size show a little increase as the substrate bias voltage ranges from −200 V to −300 V. The film deposited at the bias voltage of
Fig. 7. The AFM images of CrN films deposited under different substrate bias voltages: (a) −100 V, (b) −200 V, (c) −300 V, (d) −400 V, and (e) −500 V.
854
Q. Kong et al. / Materials Science and Engineering B 176 (2011) 850–854
−100 V has the largest grain size and the grain sizes decrease as the bias voltage is increased to −200 V. It has been reported that more defects will be generated on the surface of growing film with the increase of the energy of incident ions [30]. Therefore, the nucleation sites will increase, and as a result the grain sizes of the films decrease. However, the grain sizes increase slowly when the bias voltage is increased from −200 V to −300 V. It is well known that adatom mobility will increases due to the higher ion energies. Consequentially, higher adatom mobility promotes the migration of atoms to the grain boundaries and consequently the grain sizes show a small increase. With increasing the substrate bias voltages from −300 V to −500 V, it is interesting to note that the grain sizes decrease again. In this case, the effect of the ion bombardment is stronger than the effect of adatom mobility. Thus, the grain sizes decrease again. The AFM images of microstructure of CrN films as a function of bias voltage are shown in Fig. 7a–e. As it is seen from the pictures, the RMS roughness decreases as the bias voltage is increased to −200 V, which is related to an increase in the atomic movement and densification of the film material as a result of the increased flux and energy of the ions flux [25]. However, the RMS roughness shows a linear increase with the substrate bias voltage varying from −300 V to −500 V. Increasing bias voltage leads to high-energy bombardment which will produce too many surface defects to roughen the film surface. In addition, it also can be observed that the surface particle sizes of the as-deposited films firstly decrease and then increase with the increase of the substrate bias voltage. During the low bias voltage range (in absolute values), with the increase of substrate bias voltage, the ion bombardment increases and causes the surface particle sizes reduction. However, the surface particle sizes increase obviously during the high substrate bias voltage range, which is possible attributed to the increase of surface energy resulted from the higher ion energies. As discussed above, adatom mobility will increase because of the high surface energy. Thus, high adatom mobility promotes the migration of small grains to the grain boundaries and consequently the surface particle sizes increase.
2. Further increasing the substrate bias voltage, the texture of CrN changes from (1 1 1) to (2 0 0). Meanwhile, the N concentration decreases obviously and the surface roughness increases. The compressive stresses are observed in the films and the lattice parameters are larger than the ICSD standard values. 3. The grain size of the CrN films decreases on the whole and the deposition rate decreases with the increase of the substrate bias voltage. Furthermore, the residual stress is consistent with the evolution of the lattice parameter. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant no. 50705093) and the Innovative Group Foundation from NSFC (Grant no. 50421502) for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusions
[20] [21]
The pure CrN thin films have been prepared successfully using medium frequency magnetron sputtering under various negative substrate bias voltages. The texture, microstructure and residual stress of the as-deposited films are strongly affected by the substrate bias voltage.
[22]
1. When the substrate bias voltage is raised from −100 V to −300 V, a strong CrN (1 1 1) texture is found and the N concentration presents a slight increase and the surface roughness decrease dramatically. The tensile stresses are generated in the films and the lattice parameters exhibit a negative deviation from the ICSD standard values.
[26] [27] [28]
[23] [24] [25]
[29] [30]
L. Hultman, Vacuum 57 (2000) 1. B. Navansek, P. Panjan, I. Milosev, Surf. Coat. Technol. 97 (1997) 182. Y. Chiba, T. Omura, H. Ichimura, J. Mater. Res. 8 (1993) 1109. J.L. Mo, M.H. Zhu, Appl. Surf. Sci. 255 (2009) 7627. T. Elangovan, P. Kuppusami, R. Thirumurugesan, V. Ganesan, E. Mohandas, D. Mangalaraj, Mater. Sci. Eng. B 167 (2010) 17. K.H. Nam, M.J. Jung, J.G. Han, Surf. Coat. Technol. 131 (2000) 222. J.A. Sue, T.P. Chang, Surf. Coat. Technol. 76–77 (1995) 61. C. Liu, A. Leyland, S. Lyon, A. Mathews, Surf. Coat. Technol. 76–77 (1995) 623. P.J. Keely, R.D. Arnell, J. Vac. Sci. Technol. A 17 (1999) 945–953. S. Schiller, K. Goedicke, J. Reschke, V. Kirchhoff, S. Schneider, F. Milde, Surf. Coat. Technol. 61 (1993) 331–337. W.D. Spoul, J. Vac. Sci. Technol. A 21 (2003) S222. X.-M. He, N. Baker, B.A. Kehler, K.C. Walter, M. Nastasi, Y. Nakamura, J. Vac. Sci. Technol. A 18 (2000) 30. C. Gautier, J. Machet, Thin Solid Films 295 (1997) 43. J.-W. Lee, S.-K. Tien, Y.-C. Kuo, J. Electron. Mater. 34 (2005) 1484. R. Daniel, K.J. Martinschitz, J. Keckes, C. Mitterer, J. Phys. D: Appl. Phys. 42 (2009) 075401. H. Scheerer, E.M. Slomski, T. Troßmann, C. Berger, Surf. Coat. Technol. 205 (2010) 547. M. Zhang, M.-K. Li, K.H. Kim, F. Pan, Appl. Surf. Sci. 255 (2009) 9200. M. Larsson, M. Bromark, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 76–77 (1995) 202. E. Forniés, R. Escobar, O. Sánchez, J.M. Albella, Surf. Coat. Technol. 200 (2006) 6047. J. Pelleg, L.Z. Zevin, S. Lungo, Thin Solid Films 197 (1991) 117. N. Schell, J.H. Petersen, J. Bøttiger, A. Mücklich, J. Chevallier, K.P. Andreasen, F. Eichhorn, Thin Solid Films 426 (2003) 100. W. Ensinger, B. Rauschenbach, Nucl. Instrum. Methods Phys. Res. B80/81 (1993) 1409. M. Nastasi, J.W. Mayer, J.K. Hirvonen, Ion-Solid Interaction: Fundamentals and Applications, Cambridge University Press, Cambridge, 1996 (Chapter 9). F. Elstner, C. Gautier, H. Moussaoui, O. Piot, J. Machet, Phys. Stat. Sol. A 158 (1996) 505. H.-W. Chang, P.-K. Huang, J.-W. Yeh, A. Davison, C.-H. Tsau, C.-C. Yang, Surf. Coat. Technol. 202 (2008) 3360. S.J. Bull, A.M. Jones, A.R. McCabe, Surf. Coat. Technol. 54 (1992) 173. Z.B. Zhao, Z.U. Rek, S.M. Yalisove, J.C. Bilello, Surf. Coat. Technol. 185 (2004) 329. T. Hurkmans, D.B. Lewis, H. Paritong, J.S. Brooks, W.D. Münz, Surf. Coat. Technol. 114 (1999) 52. I. Petrov, P. Losbichler, D. Bergstrom, J.E. Greene, W.-D. Münz, T. Hurkmans, T. Trinh, Thin Solid Films 302 (1997) 179. J.W. Lee, S.-K. Tien, Y.-C. Kuo, Thin Solid Films 494 (2006) 161.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Influence of substrate bias voltage on the microstructure and residual stress of CrN films deposited by medium frequency magnetron sputtering Qinghua Kong a,b , Li Ji a , Hongxuan Li a,∗ , Xiaohong Liu a , Yongjun Wang a,b , Jianmin Chen a , Huidi Zhou a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 13 January 2011 Received in revised form 28 March 2011 Accepted 10 April 2011 Keywords: CrN films Medium frequency magnetron sputtering Substrate bias voltage Microstructure Residual stress
a b s t r a c t In this study, CrN films were deposited on stainless steel and Si (1 1 1) substrates via medium frequency magnetron sputtering under a systematic variation of the substrate bias voltage. The influence of the substrate bias voltage on the structural and the mechanical properties of the films were investigated. It is observed that there are two clear regions: (1) below −300 V, and (2) above −300 V. For the former region, the (1 1 1) texture is dominated as the substrate bias voltage is increased to −200 V. The lattice ˚ and the as-deposited films parameter is smaller than that of CrN reported in the ICSD standard (4.140 A) exhibit tensile stress. Meanwhile, the surface roughness decreases and the N concentration show a slow increase. For the latter region, the (2 0 0)-oriented structure is formed. However, the lattice parameter is larger as compared with the value reported in the ICSD standard, and the surface roughness increases and the N concentration decreases obviously. In this case, the compressive stress is obtained. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, chromium nitride coatings are widely used not only for tribological applications but also for protective coatings and diffusion barriers due to their superior corrosion resistance, oxidation resistance and wear resistance [1–4]. These coatings are generally prepared by various physical vapor deposition techniques, among which medium frequency magnetron sputtering has attracted considerable interest [5–8]. However, during the reactive direct current (DC) sputtering process, the vacuum chamber can be deposited by an insulative layer which causes “anode vanished”. This problem may contribute to discharge process unstable and makes the structures and composition of thin film uncontrollable [9,10]. In the present years, dual magnetrons are usually used to solve this problem. By arranging two magnetron sources energized by a bipolar power supply unit, within one cycle each magnetron acts alternately as an anode and cathode so that the anode can be cleaned automatically on every cycle, which may not only avoids the anode disappearing problem but also provides a stable sputtering process [10]. Furthermore, it can increase the degree of ionization and the ratio of ion to neutron in plasma which can enhance the quality of coatings [11]. It is well known that the structure and properties of magnetron sputtered CrN coatings strongly depend on the deposition parameters such as relative Ar/N2 flow rate, substrate bias, sub-
strate temperature and target power etc. [12–14]. In particular, bias voltage is a key parameter which obviously affects the quality of the coating due to enhancement of adatom mobility and the ion bombardment effect. It has been reported that bias magnetron sputtering is an effective method for controlling the microstructural evolution of transition-metal nitride films, namely, film texture and grain size, in order to enhance their properties [15]. Scheerer et al. have reported that increasing applied bias voltage causes preferably (2 0 0) crystallization lattice and CrN coatings with (2 0 0)-oriented lattice orientation seem to cover surface imperfections due to a platelet like grain structure [16]. Phase fraction and preferred orientation in CrNx coatings vary with substrate bias, exerting an effective influence on the properties of the films [12,17]. However, although the influence of substrate bias voltage has been investigated by some authors, coating microstructure and residual stress under different substrate bias voltages are worth further investigation. In the present work, CrN films were deposited using medium frequency magnetron sputtering under various pulsed substrate bias voltages. Coating characteristics such as texture, chemical composition, microstructure, residual stress, and deposition rate were analyzed, respectively. 2. Experimental details 2.1. Deposition of the CrN films
∗ Corresponding author. Tel.: +86 931 4968150; fax: +86 931 8277088. E-mail address: [email protected] (H. Li). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.04.015
CrN films were deposited on stainless steel (1Cr18Mn8Ni5N) and p-type silicon (1 1 1) wafers via a twin target medium frequency
Q. Kong et al. / Materials Science and Engineering B 176 (2011) 850–854
851
(40 kHz) magnetron sputtering technique under different substrate bias voltages. A pair of magnetron planar Cr targets (99.8%) were set in cylindrical vacuum chamber wall. The substrates were ultrasonically cleaned in acetone for 30 min and then blown dry with N2 before fixed in the deposition chamber with a distance of 100 mm from the substrate to target. A base pressure of less than 5.0 × 10−3 Pa was achieved in the chamber using a turbomolecular pumping system before sputtering. Prior to deposition, substrates were firstly etched with Ar+ plasma at a bias voltage of −600 V for 30 min in order to remove contaminants and ensure good adhesion of the deposited films. The working pressure was 0.3 Pa by the inlet of Ar and N2 (Ar: N2 = 3/5) for the deposition of CrN films. During deposition, the target power was kept at 5.0 kW and the substrate temperature was 200 ◦ C. A pulse power supply was employed to provide a substrate bias voltage with a frequency of 40 kHz and duty cycle was kept at 40% during deposition. In order to study the effect of the substrate bias voltage, different substrate bias voltages ranging from −100 to −500 V were applied. 2.2. Characterization of the CrN films
Fig. 1. The deposition rate of CrN films as a function of substrate bias voltage.
The crystallographic structure of the films was studied by conventional Bragg-Brentano X-ray diffraction (XRD) using a Philips X’Pert-MRD type diffractometer with a Cu tube operated at 40 kV and 60 mA. The chemical composition of the as-deposited films was determined by energy dispersive X-ray spectroscopy (EDS) analysis attached to a JSM-6701F type field emission scanning electron microscope. The thickness of the deposited films was measured using a Micro XAM 3D non-contact surface profilometer, and the deposition rate was calculated via film thickness and the corresponding deposition time. The residual stress of the films was evaluated using the substrate curvature method, where the thickness of the films and the curvature of the Si wafer before and after the deposition were measured using a Micro XAM 3D non-contact surface profilometer. Then the residual stress was calculated applying Stoney’s equation [18]. A digital instruments Nanoscope IIIa multimode atomic force microscope (AFM) operating with a Si3 N4 probe in a “constant force” mode was performed to observe the surface topographies of the CrN film and to determine the surface roughness.
concentration first slightly increases as the bias voltage varies from −100 V to −300 V, which may be attributed to the increase of the ionized N atoms. However, N concentration decreases dramatically with further increasing substrate bias voltage, with a minimum nitrogen content of 31%. Two effects may contribute to this behavior. On the one hand, the atomic mass of N atoms is lighter than Cr atoms. Thus, N atoms is prone to being sputtered by impinging ions with high energy, especially at higher energy as the bias voltage is increased to −500 V. On the other hand, the texture of CrN films changes from (1 1 1) to (2 0 0) when the substrate bias voltage is increased to −300 V (as shown in Fig. 3b). As CrN has the fcc structure, pure nitrogen layers and pure chromium layers are alternately presented in the (1 1 1) planes, whereas the (2 0 0) planes consist of both nitrogen and chromium atoms [11], suggesting (2 0 0) plane is not a favorable accommodation for nitrogen atoms. It is argued that the N concentration would decrease as the (2 0 0)-oriented structure is dominated in the film. In addition, although the N content is as low as 31%, there no Cr2 N phase is observed in the as-deposited films (as shown in Fig. 3a). Similar result has been obtained by Forniés et al. [19]. They showed that the only phase present is the cubic CrN when nitrogen concentration in the films is clearly below the concentration corresponding to stoichiometric CrN. Obviously, there is an excess of chromium in the films which is not detected
3. Results and discussion 3.1. Deposition rate The deposition rate of CrN films as a function of substrate bias voltage is presented in Fig. 1. It is clearly observed that the substrate bias voltage has an important effect on the deposition rate of the as-deposited films, the higher the absolute value of the substrate bias voltage, the lower the deposition rate (Fig. 1). It is observed that the deposition rate changes from 16 to 11 nm/min as the substrate bias voltage is changed from −100 V to −500 V. It is well known that the positive ions of the plasma are attracted toward the substrate and transfer their kinetic energy to the surface atoms when the bias voltage is applied to the substrate [19]. It is a matter of common knowledge that the incident ion energy becomes higher as the substrate bias voltage increases, and as a result, more atoms from the growing films will be re-sputtered. Therefore, it is assumed that the re-sputtering effect on growing films with high bias value is responsible for the decrease of deposition rate in the present study. 3.2. Chemical composition Fig. 2 shows the N concentration of the as-deposited films obtained under various substrate bias voltages. It can be seen that N
Fig. 2. The N concentration of CrN films deposited under various substrate bias voltages.
852
Q. Kong et al. / Materials Science and Engineering B 176 (2011) 850–854
Fig. 3. XRD patterns (a) and texture coefficients (b) of CrN films deposited under various substrate bias voltages.
in the diffraction pattern (as shown in Fig. 3a). However, it is difficult for the Cr atoms to insert into the octahedral positions of CrN structure because Cr atom is bigger than N atom. Meanwhile, it seems impossible to develop amorphous form due to the high deposition temperature (200 ◦ C). Therefore, it is assumed that the N atoms incorporated into CrN lattice is not adequate, forming an N-deficient CrN.
3.3. Structure and residual stress The XRD patterns of CrN films deposited with various substrate bias voltages are shown in Fig. 3a. It is clear that the crystalline orientations of CrN films strongly depend on the applied bias voltage. Diffraction lines corresponding to the (1 1 1), (2 0 0), and (2 2 0) peaks can be observed from the XRD pattern that confirms the formation of cubic CrN films. However, the variation of substrate bias voltage affects the crystallographic texture of CrN films. It can be observed that, at the bias voltage of −100 V, structure features with mixed (1 1 1), (2 0 0) and (2 2 0) reflection peaks are seen. Films exhibit a preferred (1 1 1) orientation as the substrate bias voltage is increased to −200 V. However, with further increasing substrate bias voltage, the preferred orientation changes from CrN (1 1 1) to (2 0 0).
Fig. 4. Lattice parameters of CrN films deposited under different substrate bias voltages.
The texture coefficients of the CrN T (h k l) are calculated from their respective XRD peaks with the following formula: T (h k l) =
I(h k l) I(1 1 1) + I(2 0 0) + I(2 2 0)
(1)
where (h k l) represents the (1 1 1), (2 0 0) or (2 2 0) orientations. The texture development of CrN films deposited under various substrate bias voltages is depicted in Fig. 3b. It can be seen that films grown under weak ion-irradiation conditions at −100 V contain predominantly a mixture of (1 1 1) and (2 0 0) orientations with a small volume fraction of (2 2 0) grains. For the films deposited with medium substrate bias voltage (−200 V), the dominant (1 1 1) texture is developed. In contrast, at higher energies of incident ions the grain growth is forced to form highly (2 0 0)-oriented texture. Two theoretical predictions of the growth mechanisms may explain these trends in the texture of the as-deposited films. First, it is well known that the texture development during film growth can be discussed by the competition between the surface and strain energy [14,20]. According to Pelleg et al. [20], the texture evolution is caused by the driving force to lower the overall energy of films consisting of surface energy and strain energy. It has been reported that CrN would grow toward the orientation of the (2 0 0) plane with the lowest surface energy when the surface energy is dominant, whereas the orientation of the (1 1 1) plane with the lowest strain energy would be expected when the strain energy is dominant. At low bias voltage (in absolute values), films grow with (1 1 1) grains at large thickness, where the strain energy dominates [21]. Further increasing the substrate bias voltage, however, the film thickness and the grain size are decreased, where the surface energy dominates. Therefore, the CrN films show preferred orientation of (2 0 0) as bias voltages ranging from −300 V to −500 V were applied to the substrate. Another growth mechanism is the ion channeling effect [12,22]. It is well known that the structure of CrN is built by an interpenetration of two fcc lattices of Cr and N atoms. The (2 0 0) planes are the densest planes (4 atoms/a2 , a is the lattice parameter of CrN) and have the largest distance between crystal planes (0.5 a) in CrN. For the (1 1 1) planes of CrN, the planes density and the distance between the crystal planes are 2.31 atoms/a2 , and 0.289 a, respectively [12]. In the light of the structural features, the nuclear stopping cross section for (1 1 1) oriented grains will be the largest and will be the smallest for (2 0 0) grains in CrN [23]. According to sputtering theory, the sputter yield changes with nuclear stopping cross section. Therefore, the preferential sputtering of the atoms in (1 1 1) planes is intensified with increasing substrate bias voltage, namely, the growth of CrN grains along (1 1 1) is suppressed at high bias voltage. In contrast, for grains with a (2 0 0) orientation having
Q. Kong et al. / Materials Science and Engineering B 176 (2011) 850–854
Fig. 5. Residual stresses of CrN films deposited under different substrate bias voltages.
the smallest nuclear stopping cross section and the lowest sputtering yield will develop. Thus, the preferred orientation of CrN films changes from (1 1 1) to (2 0 0) as a function of the substrate bias voltage. The lattice parameters were calculated from the (2 0 0) and (1 1 1) planes. It has been reported that the lattice of chromium nitride films is distorted due to the residual stress and defects in the films [24]. The evolution of the lattice parameter and residual stress of CrN films deposited under different substrate bias voltages are shown in Figs. 4 and 5. As it can be seen, there are two regions: (1) below −300 V, where the lattice parameter is smaller ˚ and (2) than that of CrN reported in the ICSD standard (4.140 A), above −300 V, exhibiting higher lattice parameter than the standard value. The lattice parameter increases as the bias voltages ranging from −100 V to −400 V are applied to the substrate. This may be attributed to the enhanced flux and energy of the bombarding ions, whereby Frenkel pairs and anti-Schottky defects are induced by the “ion peening effect” [25,26]. However, the lattice parameter shows a slight decrease when the substrate bias voltage
853
Fig. 6. Grain sizes of CrN films deposited under different substrate bias voltages.
is increased to −500 V, which appears to be related to the deficiency in N atoms relative to the stoichiometric CrN [27]. The negative deviation from the lattice parameter as compared with the ICSD standard values for CrN indicates a tensile stress [28]. Increasing the substrate bias voltage leads to a change to positive deviation values of lattice parameter indicating the change from tensile stress to compressive stress, which is consistent with the evolution of the residual stress (Fig. 5). It can be observed that residual stresses are not isotropic in our study. With the bias voltage ranging from −100 V to −300 V, the films show tensile stresses which decreases with the increase of bias voltage. However, as the bias voltage is increased from −300 V to −500 V, the films exhibit compressive stress. Furthermore, the compressive stress increases with increasing the substrate bias voltage, which is attribute to high defect densities induced by ion bombardment [29]. The grain sizes of the CrN films deposited under different substrate bias voltages are shown in Fig. 6. As it is seen from the picture, the grain sizes tend to decrease on the whole, although the grain size show a little increase as the substrate bias voltage ranges from −200 V to −300 V. The film deposited at the bias voltage of
Fig. 7. The AFM images of CrN films deposited under different substrate bias voltages: (a) −100 V, (b) −200 V, (c) −300 V, (d) −400 V, and (e) −500 V.
854
Q. Kong et al. / Materials Science and Engineering B 176 (2011) 850–854
−100 V has the largest grain size and the grain sizes decrease as the bias voltage is increased to −200 V. It has been reported that more defects will be generated on the surface of growing film with the increase of the energy of incident ions [30]. Therefore, the nucleation sites will increase, and as a result the grain sizes of the films decrease. However, the grain sizes increase slowly when the bias voltage is increased from −200 V to −300 V. It is well known that adatom mobility will increases due to the higher ion energies. Consequentially, higher adatom mobility promotes the migration of atoms to the grain boundaries and consequently the grain sizes show a small increase. With increasing the substrate bias voltages from −300 V to −500 V, it is interesting to note that the grain sizes decrease again. In this case, the effect of the ion bombardment is stronger than the effect of adatom mobility. Thus, the grain sizes decrease again. The AFM images of microstructure of CrN films as a function of bias voltage are shown in Fig. 7a–e. As it is seen from the pictures, the RMS roughness decreases as the bias voltage is increased to −200 V, which is related to an increase in the atomic movement and densification of the film material as a result of the increased flux and energy of the ions flux [25]. However, the RMS roughness shows a linear increase with the substrate bias voltage varying from −300 V to −500 V. Increasing bias voltage leads to high-energy bombardment which will produce too many surface defects to roughen the film surface. In addition, it also can be observed that the surface particle sizes of the as-deposited films firstly decrease and then increase with the increase of the substrate bias voltage. During the low bias voltage range (in absolute values), with the increase of substrate bias voltage, the ion bombardment increases and causes the surface particle sizes reduction. However, the surface particle sizes increase obviously during the high substrate bias voltage range, which is possible attributed to the increase of surface energy resulted from the higher ion energies. As discussed above, adatom mobility will increase because of the high surface energy. Thus, high adatom mobility promotes the migration of small grains to the grain boundaries and consequently the surface particle sizes increase.
2. Further increasing the substrate bias voltage, the texture of CrN changes from (1 1 1) to (2 0 0). Meanwhile, the N concentration decreases obviously and the surface roughness increases. The compressive stresses are observed in the films and the lattice parameters are larger than the ICSD standard values. 3. The grain size of the CrN films decreases on the whole and the deposition rate decreases with the increase of the substrate bias voltage. Furthermore, the residual stress is consistent with the evolution of the lattice parameter. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant no. 50705093) and the Innovative Group Foundation from NSFC (Grant no. 50421502) for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusions
[20] [21]
The pure CrN thin films have been prepared successfully using medium frequency magnetron sputtering under various negative substrate bias voltages. The texture, microstructure and residual stress of the as-deposited films are strongly affected by the substrate bias voltage.
[22]
1. When the substrate bias voltage is raised from −100 V to −300 V, a strong CrN (1 1 1) texture is found and the N concentration presents a slight increase and the surface roughness decrease dramatically. The tensile stresses are generated in the films and the lattice parameters exhibit a negative deviation from the ICSD standard values.
[26] [27] [28]
[23] [24] [25]
[29] [30]
L. Hultman, Vacuum 57 (2000) 1. B. Navansek, P. Panjan, I. Milosev, Surf. Coat. Technol. 97 (1997) 182. Y. Chiba, T. Omura, H. Ichimura, J. Mater. Res. 8 (1993) 1109. J.L. Mo, M.H. Zhu, Appl. Surf. Sci. 255 (2009) 7627. T. Elangovan, P. Kuppusami, R. Thirumurugesan, V. Ganesan, E. Mohandas, D. Mangalaraj, Mater. Sci. Eng. B 167 (2010) 17. K.H. Nam, M.J. Jung, J.G. Han, Surf. Coat. Technol. 131 (2000) 222. J.A. Sue, T.P. Chang, Surf. Coat. Technol. 76–77 (1995) 61. C. Liu, A. Leyland, S. Lyon, A. Mathews, Surf. Coat. Technol. 76–77 (1995) 623. P.J. Keely, R.D. Arnell, J. Vac. Sci. Technol. A 17 (1999) 945–953. S. Schiller, K. Goedicke, J. Reschke, V. Kirchhoff, S. Schneider, F. Milde, Surf. Coat. Technol. 61 (1993) 331–337. W.D. Spoul, J. Vac. Sci. Technol. A 21 (2003) S222. X.-M. He, N. Baker, B.A. Kehler, K.C. Walter, M. Nastasi, Y. Nakamura, J. Vac. Sci. Technol. A 18 (2000) 30. C. Gautier, J. Machet, Thin Solid Films 295 (1997) 43. J.-W. Lee, S.-K. Tien, Y.-C. Kuo, J. Electron. Mater. 34 (2005) 1484. R. Daniel, K.J. Martinschitz, J. Keckes, C. Mitterer, J. Phys. D: Appl. Phys. 42 (2009) 075401. H. Scheerer, E.M. Slomski, T. Troßmann, C. Berger, Surf. Coat. Technol. 205 (2010) 547. M. Zhang, M.-K. Li, K.H. Kim, F. Pan, Appl. Surf. Sci. 255 (2009) 9200. M. Larsson, M. Bromark, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 76–77 (1995) 202. E. Forniés, R. Escobar, O. Sánchez, J.M. Albella, Surf. Coat. Technol. 200 (2006) 6047. J. Pelleg, L.Z. Zevin, S. Lungo, Thin Solid Films 197 (1991) 117. N. Schell, J.H. Petersen, J. Bøttiger, A. Mücklich, J. Chevallier, K.P. Andreasen, F. Eichhorn, Thin Solid Films 426 (2003) 100. W. Ensinger, B. Rauschenbach, Nucl. Instrum. Methods Phys. Res. B80/81 (1993) 1409. M. Nastasi, J.W. Mayer, J.K. Hirvonen, Ion-Solid Interaction: Fundamentals and Applications, Cambridge University Press, Cambridge, 1996 (Chapter 9). F. Elstner, C. Gautier, H. Moussaoui, O. Piot, J. Machet, Phys. Stat. Sol. A 158 (1996) 505. H.-W. Chang, P.-K. Huang, J.-W. Yeh, A. Davison, C.-H. Tsau, C.-C. Yang, Surf. Coat. Technol. 202 (2008) 3360. S.J. Bull, A.M. Jones, A.R. McCabe, Surf. Coat. Technol. 54 (1992) 173. Z.B. Zhao, Z.U. Rek, S.M. Yalisove, J.C. Bilello, Surf. Coat. Technol. 185 (2004) 329. T. Hurkmans, D.B. Lewis, H. Paritong, J.S. Brooks, W.D. Münz, Surf. Coat. Technol. 114 (1999) 52. I. Petrov, P. Losbichler, D. Bergstrom, J.E. Greene, W.-D. Münz, T. Hurkmans, T. Trinh, Thin Solid Films 302 (1997) 179. J.W. Lee, S.-K. Tien, Y.-C. Kuo, Thin Solid Films 494 (2006) 161.