Research Articles Reactive DC Magnetron Sputtering Deposition of Copper Nitride Thin Film

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1 468 J. Mater. Sci. Technol., Vol.23 No.4, 2007 Research Articles Reactive DC Magnetron Sputtering Deposition of Copper Nitride Thin Film Xing ao LI 1,2), Zuli LIU 1) and Kailun YAO 1,3) 1) Department of Physics, Huazhong University of Science and Technology, Wuhan , China 2) School of Science, Hubei Institute for Nationalities, Enshi , China 3) International Center of Material Physics, Chinese Academy of Science, Shenyang , China [Manuscript received August 28, 2006, in revised form January 9, 2007] Copper nitride thin film was deposited on glass substrates by reactive DC (direct current) magnetron sputtering at a 0.5 Pa N 2 partial pressure and different substrate temperatures. The as-prepared film, characterized with X-Ray diffraction, atomic force microscopy, and X-ray photoelectron spectroscopy measurements, showed a composed structure of Cu 3 N crystallites with anti-reo 3 structure and a slight oxidation of the resulted film. The crystal structure and growth rate of Cu 3 N films were affected strongly by substrate temperature. The preferred crystalline orientation of Cu 3 N films were (111) and (200) at RT, 100 C. These peaks decayed at 200 C and 300 C only Cu (111) peak was noticed. Growth of Cu 3 N films at 100 C is the optimum substrate temperature for producing high-quality (111) Cu 3 N films. The deposition rate of Cu 3 N films estimated to be in range of nm/min increased while the resistivity and the microhardness of Cu 3 N films decreased when the temperature of glass substrate increased. KEY WORDS: DC magnetron sputtering; Copper nitride thin film; Resistivity; Microhardness 1. Introduction Copper nitride (Cu 3 N), one kind of excellent semiconductors with many extraordinary properties, has obtained considerable attention in recent years as a new material applicable for optical storage devices and high-speed integrated circuits. For instance, its optical reflectivity in visible and infrared range is far smaller than that of pure Cu [1]. Cu 3 N is also stable at room temperature whereas it can decompose into Cu and N 2 as the temperature is over 300 C. The low decomposition temperature and discriminating optical properties of the compound compared to those of Cu are applicable for optical read-only memory disks by generating microscopic Cu-metal spots on Cu 3 N film by performing local laser heating [2]. Cu 3 N films were also used as buffer layers for depositing Cu-metal lines on Si wafers to achieve higher signal speed than existing Al-metal lines in integrated-circuit fabrication processes. The crystal structure of Cu 3 N is also interesting it has the cubic anti-reo 3 structure in which Cu atoms do not occupy the fcc close-packing sites. Therefore, another metallic atom (e.g., Pd [3], Cu [4], Li [5] ) can be inserted into the body center of the cubic unit cell, inducing significant changes in the electrical properties. Few methodology has been reported for the growth of Cu 3 N films, mainly covering RF (radio frequency) [6 10], DC (direct current) [11] sputtering, molecular beam epitaxy [12,13] and reactive pulsed laser deposition [14]. It also has been known that variations of sputtering parameters such as gas pressure, substrate temperature, and sputtering power allow alerting the structure of the deposited films. Therefore, the variations of these experimental parameters are promising to affect the crystalline structure and growth rate of deposited films on substrate. Prof., to whom correspondence should be addressed, zlliu@hust.edu.cn. With an aim to find the optimum conditions for growing well-oriented and defect-free Cu 3 N films on glass substrates, we reported here the preparation of Cu 3 N films by reactive DC magnetron sputtering of a Cu target at different substrate temperatures and in 1.0 Pa Ar/N 2 gas mixture and at 0.5 Pa N 2 partial pressure. The investigation of the crystalline structure and growth rate of as-prepared Cu 3 N films on glass substrate was also proposed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). 2. Experimental Reactive DC magnetron sputtering was performed on a more function magnetron sputtering system, JZCK-III (Shenyang Juzhi Tech. Com. Shenyang, China) to prepared Cu 3 N films on glass substrates. The target was a pure Cu (99.999%) with a diameter of 50 mm and a thickness of 5 mm. The distance between substrate and target was kept at 60 mm and the DC power was 50 W during the sputtering. The base pressure of the vacuum chamber was less than Pa before the sputtering. The flow rates of the working and the reactive gases (99.99% pure Ar and N 2 ) were adjusted by independent mass-flow controller. During the sputtering the total sputtering pressure was maintained at 1.0 Pa by setting the total gas-flow rate to be 40 sccm. The partial pressures of Ar and N 2 in the chamber were estimated to be 0.5 Pa as the respective flow rates was 20 sccm from the mass-flow controllers. Prior to deposition, the glass substrates were cleaned by ultrasonic waves in acetone and alcohol, and then dried by blowing air. Pre-sputtering was performed for 3 min before each deposition process in order to remove the oxidized surface layer of the target and also to maintain the stability of copper target during the sputtering. The crystalline properties of the obtained films were analyzed by XRD measurements with CuKα

2 J. Mater. Sci. Technol., Vol.23 No.4, in agreement with the reported results [6,7]. Theoretically the temperature of glass substrate controls the diffusion of N atoms towards Cu surface during the crystal growth, which, on the other hand, is proportional to exp( E/k B T ) according to the Arrhenius law. Here E is the activation energy, the characteristic parameter for the respective diffusion pathway and k B is the Boltzmann constant [16]. Therefore, the diffusion of Cu and N atoms is more efficient at lower substrate temperatures for producing appreciable density of Cu-N bonds necessary for the growth along the (100) direction. However, at very high substrate temperature, it is difficult to from the Cu-N bonds, and hence the Cu atoms grow in crystal alone. Fig.1 XRD spectra of Cu 3N films deposited reactive DC magnetron sputtering at 0.5 Pa N 2 partial pressure at the substrate temperature of temperature of RT (curve T 0 ), 100 C (curve T 1 ), 200 C (curve T 2 ) and 300 C (curve T 3 ) line (D/max-γA, Japan). To determine the chemical binding state and composition of the films, XPS (Kratos, XSAM800) was performed with MgKα radiation. The surface morphology of copper nitride films was characterized using atomic force microscopy (AFM, Molecular Imaging USA). The thickness of films was measured with a profilometer (DEKTAKII). The resistivities of films were measured using the fourprobe method. The microhardness measurements of films were carried out using MICROMET2104 apparatus. 3. Results and Discussion 3.1 Structure of films Figure 1 shows the XRD spectra of Cu 3 N films deposited at the substrate temperature of RT (curve T 0 ), 100 C (curve T 1 ), 200 C (curve T 2 ), and 300 C (curve T 3 ) with a fixed N 2 partial pressure of 0.5 Pa. At RT and 100 C, the XRD spectra (curve T 0 and T 1 ) show a strong (111) preferred orientation and large integrated intensity of the diffraction peak, which is corresponded to a Cu 3 N (111) reflection. The XRD spectrum also shows grains with other orientations like (200) and (210) along with those (111) at low substrate temperature. These results suggest that the deposited films are composed of Cu 3 N crystallites with the anti-reo 3 structure and preferred to be (111) peak at relatively low substrate temperature. As the temperature of the glass substrate increased to 200 C, both the peak of (100) and (211) of CuN 3 films were noticed while the (111), (200) peaks of Cu 3 N were very weak, and then totally disappeared when the temperature of the glass substrate increased to 300 C, where only Cu (111) peak was observed. These results indicate that too high temperature of the substrate was not suitable for the nitrification of Cu, in consistent with that in literature [15]. Subsequently, the temperature of glass substrates affects the crystal growth of Cu 3 N films on the glass substrate remarkably. The growth of Cu 3 N (111)-preferred film on the glass substrate is pronounced at low substrate temperature and low N 2 partial pressures, which is 3.2 Surface morphology of films The surface morphology of Cu 3 N films was studied in air by AFM in a scanning areas of 5 µm 5 µm. Figure 2 shows the AFM images taken from films deposited at the substrate temperature of RT (image T 0 ), 100 C (image T 1 ), 200 C (image T 2 ), and 300 C (image T 3 ) at a N 2 partial pressure of 0.5 Pa during the sputtering. Image T 0 shows that the Cu 3 N film has various hills, which is possible due to the small diffusivity of atoms at low substrate temperature. Image T 1 and T 2 show an unknit surface and the grain distributing uniformity. This morphology is similar to a compactness structure. However, the crystal grains size in image T 2 is bigger than that in image T 1. Moreover, the crystal structure in image T 2 is more complex than that in image T 1. Image T 3 shows an individual and uniform crystal structure, which is actually the crystal structure of Cu. These results agree with those obtained from XRD experiments. The AFM results are summarized in Table 1. From the data of the AFM images, one can see that the surface roughness of Cu 3 N films decreased from 28.4 to 5.1 nm at the temperature from RT to 100 C. However, their surface roughness then increased to 8.0 and 11.9 nm when the temperature of glass substrate increased to 200 C and 300 C, respectively. The enhancement in surface smoothness with substrate temperature from RT to 100 C (may be 150 C [17] ) can be assigned to the higher surface atom diffusion at higher substrate temperature [1], which predicted [18] theoretically smoothening film surface. When the temperature of glass substrate was 200 C or 300 C, the surface was rougher than that deposited at 100 C. This change may be due to the different crystal structures of Cu 3 N films at different substrate temperatures. Within these four samples, the smoothest Cu 3 N films were obtained when the temperature of substrate was 100 C (image T 1 ), and hence the temperature of glass substrate of 100 C is the optimum temperature for producing high-quality and (111)- preferred Cu 3 N films. 3.3 Chemical composition of films X-ray photoelectron spectroscopy (XPS) was performed with MgKα radiation to characterize the chemical binding state and composition of the films. Figure 3(a), (b), and (c) show the XPS spectra of Cu 3 N films for Cu2p, N1s, O1s, respectively. Figure 3(a) shows the peaks in the domain of ev and of ev as well as the peak at

3 470 J. Mater. Sci. Technol., Vol.23 No.4, 2007 Fig.2 AFM images of Cu3 N films deposited at 0.5 Pa N2 partial pressure and at the substrate temperature of temperature of RT (image T0 ), 100 C (image T1 ), 200 C (image T2 ), and 300 C (image T3 ) Table 1 Overview of the Cu3 N films and investigated with AFM Sample T0 T1 T2 T3 Substrate Temp./ C RT Mean Height/nm Surface roughness/nm Thickness/nm Fig.3 XPS spectra of Cu3 N films: (a) Cu2p, (b) N1s, (c) O1s about 944 ev. The peak at around ev and ev result from Cu2p3/2 and Cu2p1/2 peaks of Cu2+, respectively. The peak at 944 ev is the shakeup peak. The spin-orbit coupling energy gap of Cu2p was 20 ev. These results match those published in literature [19] and also support the concurrence of Cu1+ and Cu2+ for the Cu3 N films deposited at RT substrate temperature. Cu1+ exists in the form of Cu3 N while Cu2+ in the form of CuO. The XRD spectrum of Cu3 N films unfortunately did not show the CuO information. However, the XPS spectra of O1s in Fig.3(c) prove the idea that the Cu2+ comes from Cu atoms oxidation in the films surface. Figure 3(b) presents that the main peak of N1s is centered at about 397 ev, which results from N1s peak of Cu3 N phase[7]. For all samples, no obvious shoulder peaks appeared and the line shape remained constant with affiliation to substrate temperatures although the height and the area of the peaks for different samples have a little change. The results reveal that the nitrogen bond exists mainly in the films and the nitrogen concentrations are disparate in different samples. 3.4 Thickness and deposition rates of films The thickness of films was measured by a profil-

4 J. Mater. Sci. Technol., Vol.23 No.4, from that more matters will be formed and deposited during chemical reaction at higher substrate temperature within a certain temperature limited range. Fig.4 Deposition rates of Cu 3 N films deposited at different substrate temperatures at N 2 partial pressure of 0.5 Pa Fig.5 Resistivity of Cu 3N films deposited at different substrate temperatures and N 2 partial pressure of 0.5 Pa Fig.6 Microhardness of Cu 3N films ometer (DEKTAKII) and the deposition rates of Cu 3 N films were then calculated correspondingly using the thickness obtained and the deposition time (the sputtering process of each sample was completed for 5 min). The deposition rates of Cu 3 N films were estimated to be in the range of nm/min. This is notably higher than the deposition rates (10 20 nm/min) of Cu 3 N films reported by Nosaka et al. [15] who employed RF magnetron sputtering for Cu 3 N film growth. Figure 4 shows the variation of the deposition rates of Cu 3 N films as a function of the substrate temperatures at a fixed N 2 partial pressure of 0.5 Pa. The deposition rates of Cu 3 N films increased almost linearly with increasing the substrate temperature, which presumably results 3.5 Resistivity of films Figure 5 is the variation of the resistivity of Cu 3 N films as a function of the temperature of the glass substrate used for the deposition of Cu 3 N films. The resistivity was obtained using a four-probe method at room temperature. The curve shows the resistivity of Cu 3 N films was in the range from 4.0 to 1.0 Ω cm, a big difference from the values ( Ω cm) reported [6,11,15]. The resistivity of Cu 3 N films decreased with increasing substrate temperature. It is again noteworthy that films grown as mixtures of Cu and Cu 3 N will produce rather high electrical resistivity values. Moreover, the conduction in mixed sample might follow the Percolation Theory, (wherein two conduction processes coexist: (one), a metallic conduction process through the Cu path, and (two), the semiconductor conduction process). We tried to analyze this result qualitatively. At low N 2 partial pressure (the Ar/N 2 gas mixture at 1/2 N 2 partial pressure), the Cu 3 N films were deposited mainly by the insert of N atoms into the lattice of Cu atoms and the center of the Cu 3 N lattice contains Cu atoms. This insertion of Cu atoms into the body center of the anti-reo 3 structure was similar to WO 3, which made conductive tungsten bronzes by inserting some metal atoms into the ReO 3 structure. The inserted Cu atoms act as a donor and release free electrons as a carrier, and thus the resistivity of Cu 3 N films decreased. This is due to the released free electrons of insertion Cu atoms that was localized and scattered by crystal boundaries and crystal defects. 3.6 Microhardness of films The microhardness of the deposited films was measured using the MICROMET2104 apparatus. The microhardness of Cu 3 N films measured were GPa. This result approximates the results in literature [20]. When the N 2 flow rate is small, the microhardness of Cu 3 N films will be a maximum value (4.25 GPa); while the N 2 flow rate increases, the microhardness of Cu 3 N films will be a constant (3.7 GPa). Figure 6 reflects that the microhardness of samples was decreased slightly as the substrate temperature increased (and the N 2 partial pressure was fixed at 0.5 Pa). The reason is that the microhardness of Cu films are lower than Cu 3 N films. These facts indicated that the substrate temperature not only affected the structures of Cu 3 N films, but also affected the combined composition and compactness of Cu 3 N films. 4. Conclusions Cu 3 N films were prepared by reactive DC magnetron sputtering of a pure Cu target on glass using a 0.5 Pa N 2 partial pressure (the Ar/N 2 gas mixture at N 2 partial pressure of 0.5 Pa) at different substrate temperatures. X-ray diffraction measurements show that the films were composed of Cu 3 N crystallites with anti-reo 3 structure (except for at substrate temperature of 300 C). The crystal growth of the Cu 3 N films was affected strongly by substrate temperature.

5 472 J. Mater. Sci. Technol., Vol.23 No.4, 2007 The preferred crystalline orientation of the films were (111) and (200) at RT and 100 C. The (111), (200) peaks of Cu 3 N at 200 C are very weak and decayed to almost zero when the temperature of substrate is approaching to 300 C, indicating that too high substrate temperature is not suitable for the nitrification of Cu. The film showed only Cu (111) at the substrate temperature of 300 C. The surface morphology of Cu 3 N films was studied by AFM. The smoothest film in the four samples was obtained at 100 C and this temperature was selected as the optimum substrate temperature for producing high-quality and (111)-preferred growth Cu 3 N films. Good agreement between XRD and AFM analysis was obtained in characterizing the film structures. XPS measurements revealed that the films were slightly oxidized, especially at RT substrate temperature. The substrate temperature not only affected the crystal structures and the surface morphology of the Cu 3 N films, but also affected its deposition rate, resistivity, and microhardness. The deposition rate of Cu 3 N films was evaluated in our experimental condition to be nm/min. The deposition rate increased while the resistivity and the microhardness of Cu 3 N films decreased with increasing substrate temperatures. Acknowledgement This work was supported by the Key Program of the Education Branch of Hubei Province, China (2003A001 and D ). REFERENCES [1 ] T.Maruyama and T.Morishita: Appl. Phys. Lett., 1996, 69, 890. [2 ] M.Asano, K.Umeda and A.Tasaki: Jpn. J. Appl. Phys., 1990, 29, [3 ] U.Hahn and W.Weber: Phys. Rev. B, 1996, 53, [4 ] M.G.Moreno-Armenta, A.Martinez-Ruiz and N.Takeuchi: Solid State Sci., 2004, 6, 9. [5 ] F.Gulo, A.Simon, J.Kohler and R.K.Kremer: Angew. Chem. Int. Edit., 2004, 43, [6 ] D.Wang, N.Nakamine and Y.Hayashi: J. Vac. Sci. Technol. A, 1998, 16, [7 ] Z.Q.Liu, W.J.Wang, T.M.Wang, S.Chao and S.K.Zheng: Thin Solid Films, 1998, 325, 55. [8 ] G.H.Yue, P.X.Yan and J.Wang: J. Cryst. Growth, 2005, 274, 464. [9 ] J.Wang, J.T.Chen, X.M.Yuan, Z.G.Wu, B.B.Miao and P.X.Yan: J. Cryst. Growth, 2006, 286, 407. [10] X.M.Yuan, P.X.Yan and J.Z.Liu: Mater. Lett., 2006, 60, [11] L.Maya: J. Vac. Sci. Technol. A, 1993, 11, 603. [12] S.Terada, H.Tanaka and K.Kubota: J. Cryst. Growth, 1989, 94, 567. [13] D.M.Borsa and D.O.Boerma: Surf. Sci., 2004, 548(1-3), 95. [14] G.Soto, J.A.Diaz and W.Cruz: Mater. Lett., 2003, 57, [15] T.Nosaka, M.Yoshitake, A.Okamoto, S.Ogawa and Y.Nakayama: Thin Solid Films, 1999, 348, 8. [16] P.Hones, C.Zakri, P.E.Schmid, F.Levy and O.R.Shojaei: Appl. Phys. Lett., 2000, 76, [17] S.Ghosh, F.Singh, D.Choudhary, D.K.Avasthi, V.Ganesan, P.Shah and A.Guptab: Surf. Coat. Technol., 2001, , [18] W.M.Tong and R.S.Williums: Annu. Rev. Phys. Chem., 1994, 45, 401. [19] Zhiguo WU, Weiwei ZhANG, Lifeng BAI, Jun WANG and Pengxun YAN: Acta Phys. Sin., 2005, 54, (in Chinese) [20] J.F.Pierson: Vacuum, 2002, 66, 59.