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Influence of N2 flow rate on the mechanical properties of SiNx films deposited by microwave electron cyclotron resonance magnetron sputtering
Thin Solid Films 518 (2010) 2077–2081
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Influence of N2 flow rate on the mechanical properties of SiNx films deposited by microwave electron cyclotron resonance magnetron sputtering Wanyu Ding a,b,⁎, Jun Xu b, Wenqi Lu b, Xinlu Deng b, Chuang Dong b a b
Institute of Optoelectronic Materials and Device, Dalian Jiaotong University, Dalian, 116028, China State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 21 August 2008 Received in revised form 29 July 2009 Accepted 30 July 2009 Available online 8 August 2009 Keywords: Silicon nitride Plasma processing and deposition Fourier-transform infrared spectroscopy Mechanical properties
a b s t r a c t Hydrogen-free amorphous silicon nitride (SiNx) films were deposited at room temperature by microwave electron cyclotron resonance plasma-enhanced unbalance magnetron sputtering. Varying the N2 flow rate, SiNx films with different properties were obtained. Characterization by Fourier-transform infrared spectrometry revealed the presence of Si–N and Si–O bonds in the films. Growth rates from 1.0 to 4.8 nm/min were determined by surface profiler. Optical emission spectroscopy showed the N element in plasma mainly existed as N+ species and N+ 2 species with 2 and 20 sccm N2 flow rate, respectively. With these results, the chemical composition and the mechanical properties of SiNx films strongly depended on the state of N element in plasma, which in turn was controlled by N2 flow rate. Finally, the film deposited with 2 sccm N2 flow rate showed no visible marks after immersed in etchant [6.7% Ce(NH4)2(NO3)6 and 93.3% H2O by weight] for 22 h and wear test for 20 min, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the artificial synthesis of silicon nitride (SiNx) thin film by Morosanu [1], SiNx films have been widely used as mechanical protective layer, dielectric interlayer, and passivation layer, owing to their good performance in mechanical, chemical, electronic, and thermal properties [1–5]. Recently, SiNx films have become an ideal protective layer for magnetic disk drives because of their high hardness, wear resistance, and chemical inertness, especially their dense film structure, which can effectively prevent the diffusion of water and oxygen [6]. SiNx films have been prepared by various techniques, such as lowpressure chemical vapor deposition (CVD) [7,8], plasma-enhanced CVD [9], ion beam deposition [10,11], and sputtering deposition [12,13]. By controlling deposition parameters, SiNx films can have a wide range of nitrogen compositions, including the stoichiometry Si3N4. Studies have shown that the properties of SiNx films deposited by CVD critically depended on the H content, which in turn, was influenced by the deposition temperature [7–11]. It was at least 400 °C that the dense SiNx films could be deposited by CVD, which the temperatures were considered too high to deposit SiNx film on magnetic disks. Hence sputtering was the only viable way of depositing dense SiNx films on magnetic disks while a few studies have been published on sputtered SiNx [15–17]. Sugimoto et al. have ⁎ Corresponding author. Institute of Optoelectronic Materials and Device, Dalian Jiaotong University, Dalian, 116028, China. Tel.: +86 411 84105696; fax: +86 411 84105118. E-mail address: [email protected] (W. Ding). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.07.191
found that near stoichiometric SiNx films contained significant amounts of H and O contaminants, which were deposited by radio frequency (RF) reactive sputtering at pressures of 4 mTorr [15]. Such contamination could be effectively minimized by applying a substrate bias and assisted plasma source during sputtering [16,17]. It is necessary to understand the relationship between plasma properties and film mechanical properties during the sputtering process, in order to effectively use SiNx films as a magnetic disk overcoat. However, except for the work of Ding et al. that diagnosed the properties of N2/ Ar mixture plasma during the deposition of SiNx films [18], most studies mentioned above focused primarily on structure and composition of SiNx films. In this paper, hydrogen-free amorphous SiNx films were deposited at room temperature by microwave electron cyclotron resonance (MW-ECR) plasma-enhanced unbalance RF magnetron sputtering. The aim of this work was to systematically study the influence of the state of N element in plasma on the mechanical properties of SiNx films, which the state of N element was controlled by N2 flow rate. 2. Experimental details In our experiments, the MW-ECR plasma-enhanced unbalance RF magnetron sputtering was employed to prepare the SiNx films. This method used high purity Ar (99.999%) and N2 (99.999%) as sputtering gas and reactive gas, respectively. The RF sputtering process was carried out using Si targets (99.99% purity) in N2/Ar mixture plasma. Polished single crystal silicon (100) wafers were used as substrates. The substrates were firstly ultrasonically pre-cleaned in acetone, ethanol, and deionized water respectively, and then dried with N2 gas.
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Finally the substrates were loaded into the processing chamber. The base vacuum was less than 3.5 × 10− 3 Pa. Before deposition, Ar plasma was used for sputtering cleaning the substrates in order to remove the native silicon oxide layer on the substrate surface and improve the adhesion of SiNx films onto the substrates. The detailed sputtering cleaning conditions are listed in Table 1. Subsequently, the N2 was introduced into the processing chamber to deposit SiNx films. SiNx films were deposited on Si substrates without intentional heating to characterize their intrinsic physical and chemical properties. The detailed deposition conditions are listed in Table 2. The film analytical techniques used in this study include Fouriertransform infrared spectroscopy (FT-IR), surface profiler, wear test system, and scanning electron microscopy (SEM). FT-IR was performed using Nicolet AVATAR 360 FT-IR spectrometer, with 10 scans and resolution of 2 cm− 1 from 400 cm− 1 to 4000 cm− 1 for each spectrum. Wear characteristic of SiNx films was obtained using a standard UMT-2 pin-plate wear test system in the atmosphere at room temperature, with the condition of 400 μN load, 6 mm length, 2 mm/min velocity, and moving to and fro for 20 min. The Si wafer was partly covered to prepare a step during the SiNx deposition process. By this way, the thickness of the films was obtained by Surfcorder ET 4000 M System, with 1 nm resolution. Therefore, the growth rate of films could be calculated. The film surface morphology was obtained by JEOL JSM-5600LV SEM, with 15 kV accelerating voltage. A home-made Langmuir emission probe was used to diagnose the plasma properties, such as the plasma potential, the density of plasma, and the kinetic energy of electrons in plasma. Element states in plasma were measured using a SP-305 optical emission spectroscopy (OES) with a raster of 1200 grooves/mm, electron-multiplier phototube voltage of 900 V, and system resolution ratio of 0.01– 1.0 nm. 3. Results and discussion 3.1. FT-IR analysis FT-IR spectroscopy is the most popular non-destructive technique for the characterization of SiNx films. Fig. 1 shows FT-IR spectra of SiNx films deposited at different N2 flow rates, which the other deposition conditions are set constant. A typical strong absorption peak centered at around 860 cm− 1, attributed to Si–N stretching vibration, appears in the spectrum of SiNx film deposited at 2 sccm N2 flow rate, which corresponds well with the results of other researchers [7–17]. And there is a weaker shoulder at around 1100 cm− 1 in the spectrum, which is resulted from Si–O stretching vibration [9]. Compared with FT-IR spectra of SiNx films deposited by CVD, Fig. 1 notably shows that all spectra display little hydrogen impurity, which usually appears as N–H bending vibration peak at 1160 cm− 1, N–H stretching vibration peak at 3340 cm− 1 [6–22], and Si–H stretching vibration peak at 2200 cm− 1, respectively. Fig. 1 also indicates that with increasing the N2 flow rate to 40 sccm from 1 sccm, the integrated intensity of Si–N stretching vibration peak decreases while that of Si–O stretching vibration peak increases. Especially, when N2 flow rate is more than 20 sccm, the Si–O stretching vibration peak turns into the main absorption peak and that of Si–N turns into the weaker shoulder. Such changing trend could be explained as follows: firstly, on the Si target surface, the target-poisoning effect becomes more and more serious with increasing the N2 flow rate. Therefore, less silicon particles are sputtered into plasma, which
Table 2 Experimental parameters in deposition process. Parameter
Value
Sputtering power/RF Microwave power/2.45 GHz Substrate bias/RF Deposition time Ar flow rate N2 flow rate Substrate–target distance Deposition pressure
350 W 850 W − 100 V 60 min 20 sccm 1–40 sccm 110 mm 0.15–0.22 Pa
decreases the growth rate of SiNx films, just as shown in Fig. 2. Secondly, the lower growth rate makes the film growth surface bombarded by more Ar ions (Ar+). Then more defects and nano-pores form in the films, which cause more O2 and water vapor chemical adsorption on/in films when the films are exposed to the open air, just as mentioned in our previous reports [17,19,20]. It is chemical adsorption in air that makes the main compositions of films transform from Si–N bond to Si–O bond, which results in the main FT-IR absorption peak shifting from Si–N stretching vibration peak to Si–O stretching vibration peak. 3.2. Plasma properties For the purpose of obtaining more factors causing the transformation of film compositions, the plasma density and electron kinetic energy (electron temperature, Te/eV) were measured by Langmuir emission probe, as shown in Fig. 3. From Fig. 3, it can be seen that Te decreases monotonously with increasing the N2 flow rate. This phenomenon can be easily explained by the following reasons: on the condition of keeping other parameters constant, the collision between electrons and other particles becomes more frequent in unit time with increasing the N2 flow rate, which causes electrons loss more kinetic energy. So Te of plasma displays such decreasing trend. However, with increasing the N2 flow rate from 1 sccm, the plasma density firstly increases and reaches the maximum at 5 sccm N2 flow rate, then the plasma density decreases monotonously. The original increasing of plasma density is attributed to the Penning collision processes of Ar* (metastable Ar atoms)–N2 and Ar*–N, since Ar* is superfluous in the collision processes when N2 flow rate is less than 5 sccm [18]. With all the above considerations, the density of N+ 2 and N+ ionized through Penning collisions increases with increasing N2 flow rate from 1 sccm to 5 sccm, because of excessive Ar* in the proceeding. So the total density of plasma increases and reaches the maximum 3.15 × 1010/cm3 with 5 sccm N2 flow rate. When N2 flow rate is more than 5 sccm, the relative content of Ar* in plasma decreases with continually decreasing the Ar partial pressure, which results in Ar* lacking in Penning collisions of Ar*–N2 and Ar*–N. It is
Table 1 Experimental parameters in sputtering cleaning process. MW power
Ar flow rate
Substrate bias/RF
Sputtering pressure
Sputtering time
850 W
20 sccm
− 400 V
0.1 Pa
10 min
Fig. 1. The FT-IR spectra of SiNx films deposited at different N2 flow rates.
W. Ding et al. / Thin Solid Films 518 (2010) 2077–2081
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Fig. 2. SiNx film growth rate as a function of the N2 flow rate.
well known that plasma discharge is mainly caused by Penning collision. Combining the above reasons, the plasma density displays such decreasing trend when N2 flow rate is more than 5 sccm [18,23,24]. In order to detect the detailed transformation of Ar/N2 mixed plasma, the plasma characteristics are measured by OES and the results are shown in Fig. 4. From Fig. 4, it can be seen that with increasing the N2 flow rate from 2 sccm to 20 sccm, there are slight increases in the relative intensities of N2 and N+ emission peaks. However, the relative intensity of N+ 2 emission peak increases dominantly. These phenomena indicate that the N element mainly exists as N+ species with lower N2 flow rate (2 sccm), but as N+ 2 species with higher N2 flow rate (20 sccm). The transformation of N element state is mainly caused by the changing of Te, as well as the relative content of Ar* in plasma [18]. At the same time, Si particles sputtering yield decreases monotonously with increasing the N2 flow rate, as discussed in Section 3.1. Combining the above analyses, in case of N2 flow rate less than 5 sccm, more Si particles react with N+, which results in SiNx film containing high percent of Si–N bond, as
+ Fig. 4. The OES peak intensities of N2, N+ at different N2 flow rates. 2 , and N
Fig. 3. Electron temperature, Te, and plasma density as a function of N2 flow rate.
discussed in Section 3.1. Therefore, SiNx films deposited at 2 sccm N2 flow rate display good mechanical properties, just as the results in Sections 3.3 and 3.4. On the contrary, when N2 flow rate is more than 5 sccm, the relative content of Ar* strongly decreases and N+ 2 is the main component in plasma [18]. In consideration of the serious target-poisoning effect, most Si particles chemically bond with N+ 2 species and more Si–N≡N structures form in the films [18,23]. Combining with lower growth rate, Si–N≡N bonds can be easily broken by serious Ar+ bombardment and Si– dangling bond forms in the films, which results in the SiNx films with uncompact structure. It is well known that Si–N≡N structure is unstable, which –N2 bond in/ on films could be replaced by O element in the atmosphere, just as confirmed in our previous papers [17–20,23,25]. On the other hand, Si– dangling bond in/on films can react with O element in the atmosphere. So the SiNx film deposited at high N2 flow rate is an Orich structure and displays poor mechanical properties, as discussed in Sections 3.3 and 3.4.
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3.3. Eroding analysis In order to confirm the supposition about the uncompacted structure at higher N2 flow rate, the films deposited at 2 and 20 sccm N2 flow rates are immersed in an etchant [6.7% Ce(NH4)2(NO3)6 and 93.3% H2O by weight] [6]. By this way, the nano-pores are etched to micron-pores and could be measured easily. Fig. 5 shows the SEM images of film surface morphology after the films were immersed into the etchant for 22 h at room temperature. The white spots are enlarged nano-pores, which the etchant attacks the underlying Si wafer substrate. There are no visible nano-pores on Sample a, while the surface of Sample b is covered by enlarged nano-pores (~1037/mm2), as shown in Fig. 5. This phenomenon confirms the supposition of uncompact structure directly. In consideration of these results, the films deposited at higher N2 flow rate (20 sccm) contain more nano-pores. However, the films deposited at lower N2 flow rate (2 sccm) display compact structure and can effectively prevent the diffusion of water and oxygen. 3.4. Wear analysis In order to know more about the influence of N2 flow rate on the mechanical properties of SiNx films, the wear property of films is tested and the results are shown in Fig. 6. From Fig. 6(a), it can be seen clearly that for the film deposited at 2 sccm N2 flow rate, the friction coefficient keeps at 0.1 for 20 min, which means the film is good at anti-wear property. On the contrary, for the film deposited at 20 sccm N2 flow rate, the friction coefficient keeps at 0.1 for only 3 min. Then the friction coefficient fluctuates and rises to 0.8, same as that of silicon wafer. Finally, the friction coefficient keeps at 0.8 after 10 min, just as shown in Fig. 6(b). Fig. 7 gives the SEM image of film surface morphology after the wear test. In Fig. 7(a), there is no visible wear marks on the film surface, because of high percent of Si–N bond as discussed in Section 3.1. On the contrary, Fig. 7(b) shows obvious wear marks, which means that the film has been worn out, caused by the higher density of nano-pores as well as higher percent of Si–O bond in films, as discussed above [17–20,23,25]. So the friction coefficient curve has such fluctuant trend. The wear test results
Fig. 5. SEM images of SiNx film surfaces deposited using 2 and 20 sccm N2 after the erosion test.
Fig. 6. Friction coefficients of SiNx films deposited using 2 and 20 sccm N2.
indicate that the film deposited at lower N2 flow rate (2 sccm) displays good anti-wear property and can effectively prevent the protected layer from the wear damage. 4. Conclusion High quality hydrogen-free SiNx films have been deposited at room temperature by MW-ECR enhanced unbalance RF magnetron sputtering. The influence of Ar/N2 mixed plasma properties on the structure and properties of SiNx films has been investigated. It is found that the plasma characteristics and the chemical composition of SiNx films, as well as the mechanical properties of films, strongly depend on the N2 flow rate. When N2 flow rate is less than 5 sccm, N element mainly exists as N+ species and SiNx films show higher growth rate, which results in the films that contain high percent of Si–N bonds and display good mechanical properties, such as no visible eroding marks and wear marks appearing after immersed in the etchant for 22 h and wear test for 20 min, respectively. When the N2 flow rate is more than
Fig. 7. SEM images of SiNx film surfaces deposited using 2 and 20 sccm N2 after the wear test.
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5 sccm, the main state of N element transforms into N+ 2 species and the films contain more Si–N≡N bonds, which results in SiNx films with uncompact structure and more O2 and water vapor chemical adsorption on/in films. Finally, SiNx films change into an O-rich structure and show poor mechanical properties. Clear eroding marks (~1037/mm2) and wear marks appear after eroding and wear test, respectively. Combining all the results above, by our deposition technique, SiNx films deposited at 2 sccm N2 flow rate can effectively prevent the protected layer from the wear damage and the eroding of water and oxygen. Acknowledgements We appreciate the Major Program of the National Natural Science Foundation of China (Grant No. 50390060) and National Natural Science Foundation of China (Grant Nos. 60576022 and 50572012) for financial support. References [1] C.E. Morosanu, Thin Solid Films 65 (1980) 171. [2] W.P. Eaton, J.H. Smith, Smart Mater. Struct. 6 (1997) 530. [3] P.J. French, P.M. Sarro, R. Mallée, E.J.M. Fakkeldij, R.F. Wolffenbuttel, Sens. Actuators A 58 (1997) 149. [4] G. Nisato, C. Mutsaers, H. Buijk, P. Duineveld, E. Janssen, J. de-Goede, P. Bouten, H. Zuidema, in: B.R. Chalamala, B.E. Gnade, N. Fruehauf, J. Jang (Eds.), Flexible Electronics 2004—Materials and Device Technology, Warrendale, U.S.A., January 15–18, 2004, Materials Research Society Symposium Proceedings, vol. 814, 2004, p. I8.1.1.
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[5] H. Akira, M. Toshiharu, N. Toshikazu, M. Shigehira, M. Atsushi, U. Hironobu, M. Naoto, M.T. Hideki, Thin Solid Films 516 (2008) 3000. [6] B.K. Yen, R.L. White, R.J. Waltman, C.M. Mate, Y. Sonobe, B. Marchon, J. Appl. Phys. 93 (2003) 8704. [7] J. Yota, J. Hander, A.A. Saleh, J. Vac. Sci. Technol. A 18 (2000) 372. [8] G.R. Yang, Y.P. Zhao, Y.Z. Hu, T.P. Chow, R.J. Gutmann, Thin Solid Films 333 (1998) 219. [9] S. Sitbon, M.C. Hugon, B. Agius, F. Abel, J.L. Courant, M. Puech, J. Vac. Sci. Technol. A 13 (1995) 2900. [10] D. Xu, H. Zhu, Acta Metall. Sin. 31 (1995) B164 in Chinese. [11] K.D. Vargheese, G.M. Rao, J. Vac. Sci. Technol. A 19 (2001) 1336. [12] M. Vila, D. Caceres, C. Prieto, J. Appl. Phys. 94 (2003) 7868. [13] Y.C. Liu, K. Furukawa, D.W. Gao, H. Nakashima, K. Uchino, K. Muraoka, Appl. Surf. Sci. 121/122 (1997) 233. [14] C. Savall, J.C. Bruyère, J.P. Stoquert, Thin Solid Films 260 (1995) 174. [15] I. Sugimoto, K. Yanagisawa, H. Kuwano, J. Vac. Sci. Technol. A 12 (1994) 2859. [16] J.H. Kim, K.W. Chung, J. Appl. Phys. 83 (1998) 5831. [17] W.Y. Ding, J. Xu, Y. Piao, Y.Q. Li, P. Gao, X.L. Deng, C. Dong, Chin. Phys. Lett. 22 (2005) 2332. [18] W.Y. Ding, J. Xu, W.Q. Lu, X.L. Deng, C. Dong, Phys. Plasmas 16 (2009) 053502. [19] W.Y. Ding, J. Xu, W.Q. Lu, X.L. Deng, C. Dong, Chin. Phys. B 18 (2009) 1570. [20] W.Y. Ding, J. Xu, W.Q. Lu, X.L. Deng, C. Dong, Acta Phys. Sin. 58 (2009) 4109 in Chinese. [21] W.T. Li, D.R. Mckenzie, W.D. Mcfall, Q.C. Zhang, Thin Solid Films 384 (2001) 46. [22] Z. Lu, P.S. Fiho, G. Stevens, M.J. Williams, G. Lucovsky, J. Vac. Sci. Technol. A 13 (1995) 607. [23] W. Y. Ding, Ph.D. Thesis, Dept. Physics, Dalian University of Technology, China, 2007 (in Chinese). [24] W.Z. Tang, The Application, Theory, and Technology of the Thin Film Production, Metallurgical Industry Press, Beijing, 2003 (in Chinese). [25] W.Y. Ding, J. Xu, Y.Q. Li, Y. Piao, P. Gao, X.L. Deng, C. Dong, Acta Phys. Sin. 55 (2006) 1363 in Chinese.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Influence of N2 flow rate on the mechanical properties of SiNx films deposited by microwave electron cyclotron resonance magnetron sputtering Wanyu Ding a,b,⁎, Jun Xu b, Wenqi Lu b, Xinlu Deng b, Chuang Dong b a b
Institute of Optoelectronic Materials and Device, Dalian Jiaotong University, Dalian, 116028, China State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 21 August 2008 Received in revised form 29 July 2009 Accepted 30 July 2009 Available online 8 August 2009 Keywords: Silicon nitride Plasma processing and deposition Fourier-transform infrared spectroscopy Mechanical properties
a b s t r a c t Hydrogen-free amorphous silicon nitride (SiNx) films were deposited at room temperature by microwave electron cyclotron resonance plasma-enhanced unbalance magnetron sputtering. Varying the N2 flow rate, SiNx films with different properties were obtained. Characterization by Fourier-transform infrared spectrometry revealed the presence of Si–N and Si–O bonds in the films. Growth rates from 1.0 to 4.8 nm/min were determined by surface profiler. Optical emission spectroscopy showed the N element in plasma mainly existed as N+ species and N+ 2 species with 2 and 20 sccm N2 flow rate, respectively. With these results, the chemical composition and the mechanical properties of SiNx films strongly depended on the state of N element in plasma, which in turn was controlled by N2 flow rate. Finally, the film deposited with 2 sccm N2 flow rate showed no visible marks after immersed in etchant [6.7% Ce(NH4)2(NO3)6 and 93.3% H2O by weight] for 22 h and wear test for 20 min, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the artificial synthesis of silicon nitride (SiNx) thin film by Morosanu [1], SiNx films have been widely used as mechanical protective layer, dielectric interlayer, and passivation layer, owing to their good performance in mechanical, chemical, electronic, and thermal properties [1–5]. Recently, SiNx films have become an ideal protective layer for magnetic disk drives because of their high hardness, wear resistance, and chemical inertness, especially their dense film structure, which can effectively prevent the diffusion of water and oxygen [6]. SiNx films have been prepared by various techniques, such as lowpressure chemical vapor deposition (CVD) [7,8], plasma-enhanced CVD [9], ion beam deposition [10,11], and sputtering deposition [12,13]. By controlling deposition parameters, SiNx films can have a wide range of nitrogen compositions, including the stoichiometry Si3N4. Studies have shown that the properties of SiNx films deposited by CVD critically depended on the H content, which in turn, was influenced by the deposition temperature [7–11]. It was at least 400 °C that the dense SiNx films could be deposited by CVD, which the temperatures were considered too high to deposit SiNx film on magnetic disks. Hence sputtering was the only viable way of depositing dense SiNx films on magnetic disks while a few studies have been published on sputtered SiNx [15–17]. Sugimoto et al. have ⁎ Corresponding author. Institute of Optoelectronic Materials and Device, Dalian Jiaotong University, Dalian, 116028, China. Tel.: +86 411 84105696; fax: +86 411 84105118. E-mail address: [email protected] (W. Ding). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.07.191
found that near stoichiometric SiNx films contained significant amounts of H and O contaminants, which were deposited by radio frequency (RF) reactive sputtering at pressures of 4 mTorr [15]. Such contamination could be effectively minimized by applying a substrate bias and assisted plasma source during sputtering [16,17]. It is necessary to understand the relationship between plasma properties and film mechanical properties during the sputtering process, in order to effectively use SiNx films as a magnetic disk overcoat. However, except for the work of Ding et al. that diagnosed the properties of N2/ Ar mixture plasma during the deposition of SiNx films [18], most studies mentioned above focused primarily on structure and composition of SiNx films. In this paper, hydrogen-free amorphous SiNx films were deposited at room temperature by microwave electron cyclotron resonance (MW-ECR) plasma-enhanced unbalance RF magnetron sputtering. The aim of this work was to systematically study the influence of the state of N element in plasma on the mechanical properties of SiNx films, which the state of N element was controlled by N2 flow rate. 2. Experimental details In our experiments, the MW-ECR plasma-enhanced unbalance RF magnetron sputtering was employed to prepare the SiNx films. This method used high purity Ar (99.999%) and N2 (99.999%) as sputtering gas and reactive gas, respectively. The RF sputtering process was carried out using Si targets (99.99% purity) in N2/Ar mixture plasma. Polished single crystal silicon (100) wafers were used as substrates. The substrates were firstly ultrasonically pre-cleaned in acetone, ethanol, and deionized water respectively, and then dried with N2 gas.
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Finally the substrates were loaded into the processing chamber. The base vacuum was less than 3.5 × 10− 3 Pa. Before deposition, Ar plasma was used for sputtering cleaning the substrates in order to remove the native silicon oxide layer on the substrate surface and improve the adhesion of SiNx films onto the substrates. The detailed sputtering cleaning conditions are listed in Table 1. Subsequently, the N2 was introduced into the processing chamber to deposit SiNx films. SiNx films were deposited on Si substrates without intentional heating to characterize their intrinsic physical and chemical properties. The detailed deposition conditions are listed in Table 2. The film analytical techniques used in this study include Fouriertransform infrared spectroscopy (FT-IR), surface profiler, wear test system, and scanning electron microscopy (SEM). FT-IR was performed using Nicolet AVATAR 360 FT-IR spectrometer, with 10 scans and resolution of 2 cm− 1 from 400 cm− 1 to 4000 cm− 1 for each spectrum. Wear characteristic of SiNx films was obtained using a standard UMT-2 pin-plate wear test system in the atmosphere at room temperature, with the condition of 400 μN load, 6 mm length, 2 mm/min velocity, and moving to and fro for 20 min. The Si wafer was partly covered to prepare a step during the SiNx deposition process. By this way, the thickness of the films was obtained by Surfcorder ET 4000 M System, with 1 nm resolution. Therefore, the growth rate of films could be calculated. The film surface morphology was obtained by JEOL JSM-5600LV SEM, with 15 kV accelerating voltage. A home-made Langmuir emission probe was used to diagnose the plasma properties, such as the plasma potential, the density of plasma, and the kinetic energy of electrons in plasma. Element states in plasma were measured using a SP-305 optical emission spectroscopy (OES) with a raster of 1200 grooves/mm, electron-multiplier phototube voltage of 900 V, and system resolution ratio of 0.01– 1.0 nm. 3. Results and discussion 3.1. FT-IR analysis FT-IR spectroscopy is the most popular non-destructive technique for the characterization of SiNx films. Fig. 1 shows FT-IR spectra of SiNx films deposited at different N2 flow rates, which the other deposition conditions are set constant. A typical strong absorption peak centered at around 860 cm− 1, attributed to Si–N stretching vibration, appears in the spectrum of SiNx film deposited at 2 sccm N2 flow rate, which corresponds well with the results of other researchers [7–17]. And there is a weaker shoulder at around 1100 cm− 1 in the spectrum, which is resulted from Si–O stretching vibration [9]. Compared with FT-IR spectra of SiNx films deposited by CVD, Fig. 1 notably shows that all spectra display little hydrogen impurity, which usually appears as N–H bending vibration peak at 1160 cm− 1, N–H stretching vibration peak at 3340 cm− 1 [6–22], and Si–H stretching vibration peak at 2200 cm− 1, respectively. Fig. 1 also indicates that with increasing the N2 flow rate to 40 sccm from 1 sccm, the integrated intensity of Si–N stretching vibration peak decreases while that of Si–O stretching vibration peak increases. Especially, when N2 flow rate is more than 20 sccm, the Si–O stretching vibration peak turns into the main absorption peak and that of Si–N turns into the weaker shoulder. Such changing trend could be explained as follows: firstly, on the Si target surface, the target-poisoning effect becomes more and more serious with increasing the N2 flow rate. Therefore, less silicon particles are sputtered into plasma, which
Table 2 Experimental parameters in deposition process. Parameter
Value
Sputtering power/RF Microwave power/2.45 GHz Substrate bias/RF Deposition time Ar flow rate N2 flow rate Substrate–target distance Deposition pressure
350 W 850 W − 100 V 60 min 20 sccm 1–40 sccm 110 mm 0.15–0.22 Pa
decreases the growth rate of SiNx films, just as shown in Fig. 2. Secondly, the lower growth rate makes the film growth surface bombarded by more Ar ions (Ar+). Then more defects and nano-pores form in the films, which cause more O2 and water vapor chemical adsorption on/in films when the films are exposed to the open air, just as mentioned in our previous reports [17,19,20]. It is chemical adsorption in air that makes the main compositions of films transform from Si–N bond to Si–O bond, which results in the main FT-IR absorption peak shifting from Si–N stretching vibration peak to Si–O stretching vibration peak. 3.2. Plasma properties For the purpose of obtaining more factors causing the transformation of film compositions, the plasma density and electron kinetic energy (electron temperature, Te/eV) were measured by Langmuir emission probe, as shown in Fig. 3. From Fig. 3, it can be seen that Te decreases monotonously with increasing the N2 flow rate. This phenomenon can be easily explained by the following reasons: on the condition of keeping other parameters constant, the collision between electrons and other particles becomes more frequent in unit time with increasing the N2 flow rate, which causes electrons loss more kinetic energy. So Te of plasma displays such decreasing trend. However, with increasing the N2 flow rate from 1 sccm, the plasma density firstly increases and reaches the maximum at 5 sccm N2 flow rate, then the plasma density decreases monotonously. The original increasing of plasma density is attributed to the Penning collision processes of Ar* (metastable Ar atoms)–N2 and Ar*–N, since Ar* is superfluous in the collision processes when N2 flow rate is less than 5 sccm [18]. With all the above considerations, the density of N+ 2 and N+ ionized through Penning collisions increases with increasing N2 flow rate from 1 sccm to 5 sccm, because of excessive Ar* in the proceeding. So the total density of plasma increases and reaches the maximum 3.15 × 1010/cm3 with 5 sccm N2 flow rate. When N2 flow rate is more than 5 sccm, the relative content of Ar* in plasma decreases with continually decreasing the Ar partial pressure, which results in Ar* lacking in Penning collisions of Ar*–N2 and Ar*–N. It is
Table 1 Experimental parameters in sputtering cleaning process. MW power
Ar flow rate
Substrate bias/RF
Sputtering pressure
Sputtering time
850 W
20 sccm
− 400 V
0.1 Pa
10 min
Fig. 1. The FT-IR spectra of SiNx films deposited at different N2 flow rates.
W. Ding et al. / Thin Solid Films 518 (2010) 2077–2081
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Fig. 2. SiNx film growth rate as a function of the N2 flow rate.
well known that plasma discharge is mainly caused by Penning collision. Combining the above reasons, the plasma density displays such decreasing trend when N2 flow rate is more than 5 sccm [18,23,24]. In order to detect the detailed transformation of Ar/N2 mixed plasma, the plasma characteristics are measured by OES and the results are shown in Fig. 4. From Fig. 4, it can be seen that with increasing the N2 flow rate from 2 sccm to 20 sccm, there are slight increases in the relative intensities of N2 and N+ emission peaks. However, the relative intensity of N+ 2 emission peak increases dominantly. These phenomena indicate that the N element mainly exists as N+ species with lower N2 flow rate (2 sccm), but as N+ 2 species with higher N2 flow rate (20 sccm). The transformation of N element state is mainly caused by the changing of Te, as well as the relative content of Ar* in plasma [18]. At the same time, Si particles sputtering yield decreases monotonously with increasing the N2 flow rate, as discussed in Section 3.1. Combining the above analyses, in case of N2 flow rate less than 5 sccm, more Si particles react with N+, which results in SiNx film containing high percent of Si–N bond, as
+ Fig. 4. The OES peak intensities of N2, N+ at different N2 flow rates. 2 , and N
Fig. 3. Electron temperature, Te, and plasma density as a function of N2 flow rate.
discussed in Section 3.1. Therefore, SiNx films deposited at 2 sccm N2 flow rate display good mechanical properties, just as the results in Sections 3.3 and 3.4. On the contrary, when N2 flow rate is more than 5 sccm, the relative content of Ar* strongly decreases and N+ 2 is the main component in plasma [18]. In consideration of the serious target-poisoning effect, most Si particles chemically bond with N+ 2 species and more Si–N≡N structures form in the films [18,23]. Combining with lower growth rate, Si–N≡N bonds can be easily broken by serious Ar+ bombardment and Si– dangling bond forms in the films, which results in the SiNx films with uncompact structure. It is well known that Si–N≡N structure is unstable, which –N2 bond in/ on films could be replaced by O element in the atmosphere, just as confirmed in our previous papers [17–20,23,25]. On the other hand, Si– dangling bond in/on films can react with O element in the atmosphere. So the SiNx film deposited at high N2 flow rate is an Orich structure and displays poor mechanical properties, as discussed in Sections 3.3 and 3.4.
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3.3. Eroding analysis In order to confirm the supposition about the uncompacted structure at higher N2 flow rate, the films deposited at 2 and 20 sccm N2 flow rates are immersed in an etchant [6.7% Ce(NH4)2(NO3)6 and 93.3% H2O by weight] [6]. By this way, the nano-pores are etched to micron-pores and could be measured easily. Fig. 5 shows the SEM images of film surface morphology after the films were immersed into the etchant for 22 h at room temperature. The white spots are enlarged nano-pores, which the etchant attacks the underlying Si wafer substrate. There are no visible nano-pores on Sample a, while the surface of Sample b is covered by enlarged nano-pores (~1037/mm2), as shown in Fig. 5. This phenomenon confirms the supposition of uncompact structure directly. In consideration of these results, the films deposited at higher N2 flow rate (20 sccm) contain more nano-pores. However, the films deposited at lower N2 flow rate (2 sccm) display compact structure and can effectively prevent the diffusion of water and oxygen. 3.4. Wear analysis In order to know more about the influence of N2 flow rate on the mechanical properties of SiNx films, the wear property of films is tested and the results are shown in Fig. 6. From Fig. 6(a), it can be seen clearly that for the film deposited at 2 sccm N2 flow rate, the friction coefficient keeps at 0.1 for 20 min, which means the film is good at anti-wear property. On the contrary, for the film deposited at 20 sccm N2 flow rate, the friction coefficient keeps at 0.1 for only 3 min. Then the friction coefficient fluctuates and rises to 0.8, same as that of silicon wafer. Finally, the friction coefficient keeps at 0.8 after 10 min, just as shown in Fig. 6(b). Fig. 7 gives the SEM image of film surface morphology after the wear test. In Fig. 7(a), there is no visible wear marks on the film surface, because of high percent of Si–N bond as discussed in Section 3.1. On the contrary, Fig. 7(b) shows obvious wear marks, which means that the film has been worn out, caused by the higher density of nano-pores as well as higher percent of Si–O bond in films, as discussed above [17–20,23,25]. So the friction coefficient curve has such fluctuant trend. The wear test results
Fig. 5. SEM images of SiNx film surfaces deposited using 2 and 20 sccm N2 after the erosion test.
Fig. 6. Friction coefficients of SiNx films deposited using 2 and 20 sccm N2.
indicate that the film deposited at lower N2 flow rate (2 sccm) displays good anti-wear property and can effectively prevent the protected layer from the wear damage. 4. Conclusion High quality hydrogen-free SiNx films have been deposited at room temperature by MW-ECR enhanced unbalance RF magnetron sputtering. The influence of Ar/N2 mixed plasma properties on the structure and properties of SiNx films has been investigated. It is found that the plasma characteristics and the chemical composition of SiNx films, as well as the mechanical properties of films, strongly depend on the N2 flow rate. When N2 flow rate is less than 5 sccm, N element mainly exists as N+ species and SiNx films show higher growth rate, which results in the films that contain high percent of Si–N bonds and display good mechanical properties, such as no visible eroding marks and wear marks appearing after immersed in the etchant for 22 h and wear test for 20 min, respectively. When the N2 flow rate is more than
Fig. 7. SEM images of SiNx film surfaces deposited using 2 and 20 sccm N2 after the wear test.
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5 sccm, the main state of N element transforms into N+ 2 species and the films contain more Si–N≡N bonds, which results in SiNx films with uncompact structure and more O2 and water vapor chemical adsorption on/in films. Finally, SiNx films change into an O-rich structure and show poor mechanical properties. Clear eroding marks (~1037/mm2) and wear marks appear after eroding and wear test, respectively. Combining all the results above, by our deposition technique, SiNx films deposited at 2 sccm N2 flow rate can effectively prevent the protected layer from the wear damage and the eroding of water and oxygen. Acknowledgements We appreciate the Major Program of the National Natural Science Foundation of China (Grant No. 50390060) and National Natural Science Foundation of China (Grant Nos. 60576022 and 50572012) for financial support. References [1] C.E. Morosanu, Thin Solid Films 65 (1980) 171. [2] W.P. Eaton, J.H. Smith, Smart Mater. Struct. 6 (1997) 530. [3] P.J. French, P.M. Sarro, R. Mallée, E.J.M. Fakkeldij, R.F. Wolffenbuttel, Sens. Actuators A 58 (1997) 149. [4] G. Nisato, C. Mutsaers, H. Buijk, P. Duineveld, E. Janssen, J. de-Goede, P. Bouten, H. Zuidema, in: B.R. Chalamala, B.E. Gnade, N. Fruehauf, J. Jang (Eds.), Flexible Electronics 2004—Materials and Device Technology, Warrendale, U.S.A., January 15–18, 2004, Materials Research Society Symposium Proceedings, vol. 814, 2004, p. I8.1.1.
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