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Uniformity and stoichiometry of large-area multicomponent oxide thin films deposited by sputtering
Thin Solid Films 485 (2005) 47 – 52 www.elsevier.com/locate/tsf
Uniformity and stoichiometry of large-area multicomponent oxide thin films deposited by sputtering B.W. Taoa,b,*, J.J. Chena, X.Z. Liua, Y.R. Lia, R. Fromknechtb, J. Geerkb a
School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, 610054 Chengdu, China b Institute of Festphysica, Forschungzentrum Karlsruhe, D-76021, Karlsruhe, Germany Received 16 July 2004; accepted in revised form 18 March 2005 Available online 27 April 2005
Abstract Some experimental results about thickness uniformity, composition uniformity, and stoichiometry of large-area multicomponent oxide thin films deposited by inverted cylindrical sputtering from a compound target were reported. Rutherford backscattering had been employed to determine the thickness distribution, the composition distribution, and the stoichiometry of NdBa2Cu3O7x thin films. For the film deposited on stationary substrate, the thickness and composition deviate severely. But using suitable substrate movements, we could reduce both the thickness and composition deviation simultaneously without the decrease in deposition rate. Uniform 3-in. thin films could be obtained at a relatively high deposition rate on substrate with displaced tilted in-plane rotation or biaxial rotation. D 2005 Elsevier B.V. All rights reserved. PACS: 8115Cd; 7476; 7462Bf Keywords: Multicomponent thin film; Homogeneity; Sputtering
1. Introduction Multicomponent oxides are widely studied for their fascinating properties for microelectronic applications. These materials, including superconductor, conductor, semiconductor, insulator, and magnet, could be applied in many devices, such as ferroelectric devices, piezoelectric devices, magnetic devices, optical devices, infrared devices, and microwave devices [1– 5], in the form of thin film. Following the discovery of high temperature superconductor, ReBa2Cu3O7x (or ReBCO, Re means rear earth elements) thin films have been extensively studied with a profusion of deposition techniques [6– 9] owing to high critical current density and low microwave surface resistance.
* Corresponding author. School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, 610054 Chengdu, China. Tel.: +86 28 8320 2140; fax: +86 28 8320 2569. E-mail address: [email protected] (B.W. Tao). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.03.013
For large-scale applications, uniform thickness of largearea films is required. There have been many results about deposition of uniform large-area thin films [10 – 13]. Additionally, both composition uniformity and stoichiometry of large-area multicomponent thin films are also essential for high quality films, besides the thickness uniformity. Sputtering is one of the physical vapor deposition methods for fabricating thin films. Due to a relatively large source, it is suitable for large-area thin films. However, because of the intrinsic characters of atom emission [10,14 –16] in sputtering, it is not easy to deposit thin films with uniform thickness, uniform composition, and stoichiometry at the same time. In sputtering, there are three main origins of inhomogeneity of multicomponent oxide films: (1) Different angular distribution of atoms [10,15]. (2) Bombardment of negative ions originating from the cathode [17]. (3) Possible difference of deposition parameters. The composition variation of sputtered multicomponent thin films, such as Pb(Zrx Ti1x )O3 [18], Bax Sr1x TiO3 [19,20], SrBi2Ta2O9 [21], YBa2Cu3O7x
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B.W. Tao et al. / Thin Solid Films 485 (2005) 47 – 52
[10,22], and Lax Sry CoO3 [23], results in deviations from stoichiometry and inhomogeneous distributions. Usually, the fabrication of homogeneous films needs a long target-to-substrate distance (D t – s) and a large size target. To improve composition uniformity of multicomponent oxide films, modified sputtering methods, such as high sputtering gas pressure [24] and off-axis electrodes [25], had been developed. Cylindrical target is one of the developments [19,26]. Because of its special geometry, there are less negative ions that can bombard the film than in planar target sputtering. Thus selective resputtering can be reduced. However the different points on a substrate might still have different cation concentrations and deposition rates [27]. It seems to be inevitable for the large-area films. In this article, we report on the composition and thickness distribution of large-area NdBa 2 Cu 3 O 7x (NBCO) films deposited with inverted cylindrical target. NBCO thin film has a slightly higher transition temperature and a flatter surface [28] in ReBCO. With a suitable substrate movement, high-speed deposition of films with uniform thickness and composition distributions is possible. Meanwhile stoichiometric composition can also be maintained.
2. Experimental details Fig. 1 schematically illustrates the single compound target DC sputtering system built for film deposition. A sintered inverted cylindrical stoichiometric compound target, which is 25 mm high and 40 mm in diameter, was sputtered with a pressure as high as 30 Pa (Ar + 33% O2) at 75 W for deposition of more stoichiometric film [29 –32]. The substrate could be stationary, or rotating driven by a motor. The rotation mode could be in-plane (with the substrate normal being rotation axis), or out-ofplane (with x-axis being rotation axis), or biaxial (with both x-axis and y-axis being rotation axes), as shown in
Fig. 1. Sketch of target and substrate.
Fig. 2. RBS spectra of NBCO film deposited on stationary aluminum sheet corresponding to nine different radiuses (r, in mm, increases along the arrow).
Fig. 1. The details of substrate rotation mode were described elsewhere [31,33]. The in-plane rotation has three different cases according to the substrate posture. The substrate could be perpendicular to (tilting angle, u = 0) or parallel to (u = k/2) the target, or else it is put obliquely (0 < u < k/2), as shown in Fig. 1. Parallel inplane rotation is a valid movement [34] to improve the uniformity of large-area thin films only if the D t – s is large enough (low R dep) and the substrate is loaded with off-axis displacement (lower R dep). Due to the poor thickness distribution of perpendicular in-plane rotation and out-of-plane rotation, only tilted in-plane rotation and biaxial rotation have been discussed and compared with stationary substrate deposition. In order to minimize composition variation of target surface in a series experiments, an NBCO target that had been sputtered for hundreds of hours with constant pressure, Ar/O2 ratio, and power was employed. Conventional Rutherford backscattering (RBS) was carried out with 2 MeV helium ion beam to determine the composition and thickness distribution of films deposited on an aluminum sheet at room temperature. The relative
Fig. 3. Composition variation with substrate radius of thin film deposited on stationary aluminum sheet. (a) Radial distribution of C Cu. (b) Radial distribution of C Ba + Nd. (c) Radial distribution of ratios of C Cu / C Ba + Nd.
B.W. Tao et al. / Thin Solid Films 485 (2005) 47 – 52
Fig. 4. Radial distribution of relative thickness of NBCO thin film deposited on stationary substrate. Solid circle for results from DEKTAK and open circle for those from RBS.
thickness distribution was determined using the peak width of RBS spectra.
3. Results and discussion 3.1. Deposition on stationary substrates Fig. 2 shows the RBS spectra of NBCO film deposited on a stationary aluminum sheet for 5 h with a 50 mm D t – s. On account of the lateral central symmetry and continuity of variation, data of several points on different substrate radiuses can represent the distributions of thickness and composition. From the peak intensities of Cu, it is obvious that the central film is Cu-rich and (Ba + Nd)-poor. Because the atom number of Ba is very close to that of Nd, it is difficult to distinguish one from the other in RBS spectrum. Thus the (Ba + Nd) concentration, C Ba + Nd, was taken for concentration distribution. The concentration ratio, C Cu / C Ba + Nd, represents the stoichiometry of films. The ratio is equal to 1 for stoichiometric NBCO film. The maximum
Fig. 5. RBS spectra of NBCO film deposited on displaced tilted rotated aluminum sheet corresponding to eight different radiuses (r, in mm).
49
Fig. 6. Composition variation with substrate radius of thin film deposited with displaced tilted rotation. (a) Radial distribution of C Cu. (b) Radial distribution of C Ba + Nd. (c) Radial distribution of ratios of C Cu / C Ba + Nd.
difference, (max min) / mean, of C Cu is 28% (seen in Fig. 3a), while that of C Ba + Nd is 21% (Fig. 3b, the error bars are limited in the uncertain C Ba / C Nd, from 0 to V). As shown in Fig. 3c, the ratio of C Cu / C Ba + Nd range from 1.3 to 2.2, indicating that there are much more Cu than stoichiometric composition for the entire film, especially at center. Fig. 4 shows the relative thickness distributions determined by RBS and DEKTAK. The relative thickness distribution simulated with RBS peak width by RUMP is normalized with centric thickness. By measuring the height of the chemical etched step along the diameter, DEKTAK profiler was also used to directly measure absolute thickness of films deposited on silicon wafer. In addition, we measured the aluminum peak shift to investigate the thickness distribution. It is also valid. Thus all the distributions obtained with different methods are consistent with each other, even though the absolute value from RBS spectrum is about 10% lower than that from DEKTAK because of the unknown film density. The maximum film thickness deviation from center to edge is 75%. The results mentioned above show that it is
Fig. 7. Radial distribution of relative thickness of thin film deposited with displaced tilted rotation.
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B.W. Tao et al. / Thin Solid Films 485 (2005) 47 – 52
Fig. 8. RBS spectra of NBCO film deposited on biaxial rotated aluminum sheet corresponding to nine different radiuses (r, in mm).
Fig. 10. Radial distribution of relative thickness of NBCO film deposited with biaxial rotation.
difficult to obtain homogeneous films using stationary substrates.
3.3. Deposition with biaxial rotation
3.2. Deposition with tilted in-plane rotation Adjustment of target composition and change of the substrate position were commonly employed in deposition of compound thin films to improve homogeneity of thickness and composition [34,35]. To improve film uniformity and stoichiometry, we tilted the substrate (u = k/4) and displaced it 10 mm along the y-axis. The D t – s is fixed at 50 mm. Fig. 5 shows RBS spectra of the film. For a substrate radius of 45 mm, the maximum difference of C Cu is 18%, while that of C Ba + Nd is only 5%. The ratios of C Cu / C Ba + Nd range from 0.97 to 1.18, which are very close to stoichiometric composition, as shown in Fig. 6. It indicates that the composition distributions are much better than that of stationary substrate. Fig. 7 shows the relative thickness distribution. The maximum deviation is more than 40%. But if a limited r sub within 30 mm was chosen, namely 1.5 times of target radius, the maximum thickness deviation could be limited within 10%.
Biaxial rotation is designed especially for simultaneous deposition of double-sided large-area thin films. Driven with a single axis, it combines out-of-plane rotation and in-plane rotation, and makes every point run on a spherical surface. In this case, substrate edge can go very near to the target and catches more atoms in the high particle density region. Deposition with this rotation, uniform film thickness could be obtained. Fig. 8 shows overlapped RBS spectra of film deposited with 60 mm D t – s for 10 h with biaxial rotation. For a substrate radius of 42 mm, the maximum difference of C Cu is only 6%, while that of C Ba + Nd is only 8%. The ratios of C Cu / C Ba + Nd range from 1.32 to 1.52 (see Fig. 9). The thickness distribution of biaxial rotation is much better than that of stationary substrate and tilted rotation (see Fig. 10). The maximum thickness deviation is about 11%. 3.4. Discussion The total particle density in center is higher than that at edge. The deposition rate (R dep) will rapidly decrease with extending of the D t – s. Displacement from the target center can improve the composition and thickness distributions in parallel in-plane rotation. Enlarging D t – s and displacing the substrate can produce more homogeneous films with a lower R dep. Table 1 Comparison of homogeneities of films deposited on stationary, displaced tilted rotated, and biaxial rotated aluminum sheet Rotation mode
D t – s, R dep, DT, DC Cu, DC Ba + Nd, D(C Cu / C Ba + Nd), mm % % nm/min % %
Stationary 50 Displaced 50 and tilted Biaxial* 60 Fig. 9. Composition variation with substrate radius of thin film deposited with biaxial rotation. (a) Radial distribution of C Cu. (b) Radial distribution of C Ba + Nd. (c) Radial distribution of ratios of C Cu / C Ba + Nd.
1.74 1.36 0.69
29.7 10.9 15.9 6.8 3.8
2.5
8.5 1.8
32.9 8.8
2.7
7.0
All of the DC (composition deviation) and DT (thickness deviation) are expressed by variation (standard deviations/mean) here. The R dep of biaxial rotation mode (marked with asterisk) should be multiplied by 2 for comparison because it is that of single side.
B.W. Tao et al. / Thin Solid Films 485 (2005) 47 – 52
However, deposition rate means product efficiency, target utility, and film cost. With different substrate positions and movements, R dep could be greatly different. For a single point, R dep is roughly in inverse proportion to the square of the distance to the target, l t – s. R dep on substrate center is 1.74 nm/min for stationary sheet, 1.36 nm/min for displaced tilted in-plane rotation, and 1.38 nm/ min for biaxial rotation, respectively, as shown in Table 1. R dep with displaced tilted rotation or biaxial rotation is lower than that on stationary substrate because of larger l t – s, incident particle angles, and displacement. For the latter two cases, every point, except center on the substrate, moves close to and far away from the target alternatively. The effect can be simply expressed with R¶dep ”
= circle
1 ðlts þ d Þ
2
/Rdep
=
1
2 circle lts
;
da½ r; r;
where r being the radius of the point in spherical coordinate and d being its projection. When r increases, the improvement effect is more noticeable. This causes the thicker edge than that on stationary substrate. Moreover, the films have more uniform thickness distributions when tilted or biaxial rotation is employed. On a stationary substrate, the thickness variation (standard deviation/mean), T, is 29.7% for an r sub = 43 mm, while it becomes 15.9% for an r sub = 45 mm in displaced tilted rotating deposition and 3.8% for an r sub = 42 mm in biaxial rotating deposition, respectively. The average deposition rate, R dep¶, on the entire substrate, is more accurate for comparison of deposition efficiency. They are 0.49, 0.72, and 0.68 nm/min for films deposited with stationary substrate, displaced tilted rotation, and biaxial rotation, respectively. Therefore, deposition with tilted or biaxial rotation has higher rate and target utility factor because substrates go closer to target. Table 1 also gives cation concentration variation for films deposited with different substrate movements. It is clear that biaxial rotation brings most uniform C Cu distribution, and displaced tilted rotation brings best C Ba + Nd distribution and most stoichiometric composition, as seen in Figs. 3c, 6c, and 9c). Considering the deposition with displaced tilted inplane rotation, if the atoms transfer directly, the C Cu / C Ba + Nd at substrate center should be about 1.8, just near to that of film on stationary substrate at r sub = 10 mm. But the measured ratio is only 1.18 (Fig. 6c). This reveals that the difference of traveling characters [36] between Cu and (Ba + Nd) had been weakened to a very low level on tilted substrate center. Deposited with biaxial rotation, because the tilted angle varies continuously in every out-of-plane revolution, the value C Cu / C Ba + Nd of the films is also lower than that of film deposited on stationary substrate, 1.5 at center. Stoichiometry of those films is not as good as that of displaced tilted in-plane rotation deposition. Furthermore, the stoichiometry must be better if the substrate is loaded with off-axis displacement in biaxial rotation, just similar to that in displaced tilted in-plane rotation.
51
4. Conclusions We have comparatively studied thickness distribution, composition uniformity, and stoichiometry of large-area NBCO thin films deposited with different substrate movements. Though with a high pressure, the different elements still have different transfer characters. When the substrate was displaced and in-plane rotating with k/4 tilted angle, the very uniform and stoichiometric film was obtained on a substrate with r sub = 35 mm. When the film deposited with biaxial rotation, the size of uniform film enlarged to r sub = 40 mm. If combining displacement and biaxial rotation, the stoichiometry should be better. References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
D. Damjanovic, Rep. Prog. Phys. 61 (1998) 1267. D.P. Norton, Mater. Sci. Eng., R. Rep. 43 (2004) 139. P. Muralt, Rep. Prog. Phys. 64 (2001) 1339. S. Thakoor, Appl. Phys. Lett. 60 (1992) 3319. K. Sreenivas, M. Sayer, D.J. Baar, M. Nishioka, Appl. Phys. Lett. 52 (1988) 709. M. Lorenz, H. Hochmuth, D. Natusch, M. Kusunoki, V.L. Svetchnikov, V. Riede, G. Stanca, D. Hesse, IEEE Trans. Appl. Supercond. 11 (2001) 3209. T. Schurig, S. Menkel, Z. Quan, et al., J. Eng. Appl. Sci. 262 (1996) 89. J. Azoulay, J. Am. Ceram. Soc. 79 (1996) 568. U. Schmatz, C. Dubourdieu, O. Lebedev, G. Delabouglise, F. Weiss, J.P. Senateur, J. Low Temp. Phys. 105 (1996) 1301. L. Philippe, M. Bernard, M. Hugues, J. Vac. Sci. Technol., A, Vac. Surf. Films 13 (1995) 2221. R.S. Upadhye, E.J. Hsieh, J. Vac. Sci. Technol., A, Vac. Surf. Films 8 (1990) 1348. T. Serikawa, A. Okamoto, J. Vac. Sci. Technol., A, Vac. Surf. Films 3 (1985) 1784. S. Swann, S.A. Collett, I.R. Scarlett, J. Vac. Sci. Technol., A, Vac. Surf. Films 8 (1990) 1299. P.K. Petrov, V.A. Volpyas, R.A. Chakalov, Vacuum 52 (1999) 427. J. Cook, Thin Solid Films 338 (1999) 81. Y. Konishi, K. Harada, Y. Yonezawa, Y. Kawamura, Jpn. J. Appl. Phys. 39 (2000) 5379. S.I. Shah, P.F. Garcia, Appl. Phys. Lett. 51 (1987) 2146. I. Kanno, H. Kotera, K. Wasa, T. Matsunaga, T. Kamada, R. Takayama, J. Appl. Phys. 93 (2003) 4091. W.Y. Park, K.H. Ahn, C.S. Hwang, Appl. Phys. Lett. 83 (2003) 4387. E.J. Cukauskas, S.W. Kirchoefer, J.M. Pond, J. Appl. Phys. 88 (2000) 2830. S.H. Lee, J.K. Lee, K.H. Yoon, J. Mater. Res. 17 (2002) 1455. R.A. Rao, Q. Gan, C.B. Eom, Y. Suzuki, A.A. Macdanel, J.W.P. Hsu, Appl. Phys. Lett. 69 (1996) 3911. D.O. Klenov, W. Donner, B. Fran, S. Stemmer, Appl. Phys. Lett. 82 (2003) 3427. R.J. Lin, L.J. Chen, J. Vac. Sci. Technol., A, Vac. Surf. Films 15 (1997) 2605. C.B. Eom, J.Z. Sun, K. Yamamoto, A.F. Marshall, K.E. Luther, T.H. Geballe, S.S. Laderman, Appl. Phys. Lett. 55 (1989) 595. J. Geerk, G. Linker, O. Meyer, Mater. Sci. Rep. 4 (1989) 193. D.E. Pugel, L.H. Greene, Appl. Phys. Lett. 75 (1999) 1589. S. Bals, G. Van Tendeloo, M. Salluzzo, I. Maggio-Aprile, Appl. Phys. Lett. 79 (2001) 3660. J. Geerk, G. Linker, O. Meyer, J. Supercond. 5 (1992) 345.
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[30] V.N. Tsaneva, T. Donchev, T.K. Popov, F.M. Dias, Z.H. Barber, E.J. Tarte, M.G. Blamire, M. Hogg, F. Kahlmann, J.E. Evets, Physica C 372 – 376 (2002) 1087. [31] B.W. Tao, Y.R. Li, X.Z. Liu, S.M. He, J. Geerk, J. Vac. Sci. Technol., A, Vac. Surf. Films 20 (2002) 1898. [32] R.A. Rao, C.B. Eom, M. Santer, S.M. Anlage, IEEE Trans. Appl. Supercond. 7 (1997) 1278.
[33] B.W. Tao, X.Z. Liu, J.J. Chen, Y.R. Li, R. Fromknecht, J. Geerk, J. Vac. Sci. Technol., A, Vac. Surf. Films 21 (2003) 431. [34] J. Geerk, A. Zaitsev, G. Linker, G. Aidam, R. Schneider, R. Ratzel, F. Fromknecht, R. Scheerer, B. Reiner, H. Gagnidze, E. Schwab, IEEE Trans. Appl. Supercond. 11 (2001) 3856. [35] Y. Kageyama, Y. Taga, Appl. Phys. Lett. 55 (1989) 1035. [36] W.D. Westwood, J. Vac. Sci. Technol. 15 (1978) 1.
Uniformity and stoichiometry of large-area multicomponent oxide thin films deposited by sputtering B.W. Taoa,b,*, J.J. Chena, X.Z. Liua, Y.R. Lia, R. Fromknechtb, J. Geerkb a
School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, 610054 Chengdu, China b Institute of Festphysica, Forschungzentrum Karlsruhe, D-76021, Karlsruhe, Germany Received 16 July 2004; accepted in revised form 18 March 2005 Available online 27 April 2005
Abstract Some experimental results about thickness uniformity, composition uniformity, and stoichiometry of large-area multicomponent oxide thin films deposited by inverted cylindrical sputtering from a compound target were reported. Rutherford backscattering had been employed to determine the thickness distribution, the composition distribution, and the stoichiometry of NdBa2Cu3O7x thin films. For the film deposited on stationary substrate, the thickness and composition deviate severely. But using suitable substrate movements, we could reduce both the thickness and composition deviation simultaneously without the decrease in deposition rate. Uniform 3-in. thin films could be obtained at a relatively high deposition rate on substrate with displaced tilted in-plane rotation or biaxial rotation. D 2005 Elsevier B.V. All rights reserved. PACS: 8115Cd; 7476; 7462Bf Keywords: Multicomponent thin film; Homogeneity; Sputtering
1. Introduction Multicomponent oxides are widely studied for their fascinating properties for microelectronic applications. These materials, including superconductor, conductor, semiconductor, insulator, and magnet, could be applied in many devices, such as ferroelectric devices, piezoelectric devices, magnetic devices, optical devices, infrared devices, and microwave devices [1– 5], in the form of thin film. Following the discovery of high temperature superconductor, ReBa2Cu3O7x (or ReBCO, Re means rear earth elements) thin films have been extensively studied with a profusion of deposition techniques [6– 9] owing to high critical current density and low microwave surface resistance.
* Corresponding author. School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, 610054 Chengdu, China. Tel.: +86 28 8320 2140; fax: +86 28 8320 2569. E-mail address: [email protected] (B.W. Tao). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.03.013
For large-scale applications, uniform thickness of largearea films is required. There have been many results about deposition of uniform large-area thin films [10 – 13]. Additionally, both composition uniformity and stoichiometry of large-area multicomponent thin films are also essential for high quality films, besides the thickness uniformity. Sputtering is one of the physical vapor deposition methods for fabricating thin films. Due to a relatively large source, it is suitable for large-area thin films. However, because of the intrinsic characters of atom emission [10,14 –16] in sputtering, it is not easy to deposit thin films with uniform thickness, uniform composition, and stoichiometry at the same time. In sputtering, there are three main origins of inhomogeneity of multicomponent oxide films: (1) Different angular distribution of atoms [10,15]. (2) Bombardment of negative ions originating from the cathode [17]. (3) Possible difference of deposition parameters. The composition variation of sputtered multicomponent thin films, such as Pb(Zrx Ti1x )O3 [18], Bax Sr1x TiO3 [19,20], SrBi2Ta2O9 [21], YBa2Cu3O7x
48
B.W. Tao et al. / Thin Solid Films 485 (2005) 47 – 52
[10,22], and Lax Sry CoO3 [23], results in deviations from stoichiometry and inhomogeneous distributions. Usually, the fabrication of homogeneous films needs a long target-to-substrate distance (D t – s) and a large size target. To improve composition uniformity of multicomponent oxide films, modified sputtering methods, such as high sputtering gas pressure [24] and off-axis electrodes [25], had been developed. Cylindrical target is one of the developments [19,26]. Because of its special geometry, there are less negative ions that can bombard the film than in planar target sputtering. Thus selective resputtering can be reduced. However the different points on a substrate might still have different cation concentrations and deposition rates [27]. It seems to be inevitable for the large-area films. In this article, we report on the composition and thickness distribution of large-area NdBa 2 Cu 3 O 7x (NBCO) films deposited with inverted cylindrical target. NBCO thin film has a slightly higher transition temperature and a flatter surface [28] in ReBCO. With a suitable substrate movement, high-speed deposition of films with uniform thickness and composition distributions is possible. Meanwhile stoichiometric composition can also be maintained.
2. Experimental details Fig. 1 schematically illustrates the single compound target DC sputtering system built for film deposition. A sintered inverted cylindrical stoichiometric compound target, which is 25 mm high and 40 mm in diameter, was sputtered with a pressure as high as 30 Pa (Ar + 33% O2) at 75 W for deposition of more stoichiometric film [29 –32]. The substrate could be stationary, or rotating driven by a motor. The rotation mode could be in-plane (with the substrate normal being rotation axis), or out-ofplane (with x-axis being rotation axis), or biaxial (with both x-axis and y-axis being rotation axes), as shown in
Fig. 1. Sketch of target and substrate.
Fig. 2. RBS spectra of NBCO film deposited on stationary aluminum sheet corresponding to nine different radiuses (r, in mm, increases along the arrow).
Fig. 1. The details of substrate rotation mode were described elsewhere [31,33]. The in-plane rotation has three different cases according to the substrate posture. The substrate could be perpendicular to (tilting angle, u = 0) or parallel to (u = k/2) the target, or else it is put obliquely (0 < u < k/2), as shown in Fig. 1. Parallel inplane rotation is a valid movement [34] to improve the uniformity of large-area thin films only if the D t – s is large enough (low R dep) and the substrate is loaded with off-axis displacement (lower R dep). Due to the poor thickness distribution of perpendicular in-plane rotation and out-of-plane rotation, only tilted in-plane rotation and biaxial rotation have been discussed and compared with stationary substrate deposition. In order to minimize composition variation of target surface in a series experiments, an NBCO target that had been sputtered for hundreds of hours with constant pressure, Ar/O2 ratio, and power was employed. Conventional Rutherford backscattering (RBS) was carried out with 2 MeV helium ion beam to determine the composition and thickness distribution of films deposited on an aluminum sheet at room temperature. The relative
Fig. 3. Composition variation with substrate radius of thin film deposited on stationary aluminum sheet. (a) Radial distribution of C Cu. (b) Radial distribution of C Ba + Nd. (c) Radial distribution of ratios of C Cu / C Ba + Nd.
B.W. Tao et al. / Thin Solid Films 485 (2005) 47 – 52
Fig. 4. Radial distribution of relative thickness of NBCO thin film deposited on stationary substrate. Solid circle for results from DEKTAK and open circle for those from RBS.
thickness distribution was determined using the peak width of RBS spectra.
3. Results and discussion 3.1. Deposition on stationary substrates Fig. 2 shows the RBS spectra of NBCO film deposited on a stationary aluminum sheet for 5 h with a 50 mm D t – s. On account of the lateral central symmetry and continuity of variation, data of several points on different substrate radiuses can represent the distributions of thickness and composition. From the peak intensities of Cu, it is obvious that the central film is Cu-rich and (Ba + Nd)-poor. Because the atom number of Ba is very close to that of Nd, it is difficult to distinguish one from the other in RBS spectrum. Thus the (Ba + Nd) concentration, C Ba + Nd, was taken for concentration distribution. The concentration ratio, C Cu / C Ba + Nd, represents the stoichiometry of films. The ratio is equal to 1 for stoichiometric NBCO film. The maximum
Fig. 5. RBS spectra of NBCO film deposited on displaced tilted rotated aluminum sheet corresponding to eight different radiuses (r, in mm).
49
Fig. 6. Composition variation with substrate radius of thin film deposited with displaced tilted rotation. (a) Radial distribution of C Cu. (b) Radial distribution of C Ba + Nd. (c) Radial distribution of ratios of C Cu / C Ba + Nd.
difference, (max min) / mean, of C Cu is 28% (seen in Fig. 3a), while that of C Ba + Nd is 21% (Fig. 3b, the error bars are limited in the uncertain C Ba / C Nd, from 0 to V). As shown in Fig. 3c, the ratio of C Cu / C Ba + Nd range from 1.3 to 2.2, indicating that there are much more Cu than stoichiometric composition for the entire film, especially at center. Fig. 4 shows the relative thickness distributions determined by RBS and DEKTAK. The relative thickness distribution simulated with RBS peak width by RUMP is normalized with centric thickness. By measuring the height of the chemical etched step along the diameter, DEKTAK profiler was also used to directly measure absolute thickness of films deposited on silicon wafer. In addition, we measured the aluminum peak shift to investigate the thickness distribution. It is also valid. Thus all the distributions obtained with different methods are consistent with each other, even though the absolute value from RBS spectrum is about 10% lower than that from DEKTAK because of the unknown film density. The maximum film thickness deviation from center to edge is 75%. The results mentioned above show that it is
Fig. 7. Radial distribution of relative thickness of thin film deposited with displaced tilted rotation.
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B.W. Tao et al. / Thin Solid Films 485 (2005) 47 – 52
Fig. 8. RBS spectra of NBCO film deposited on biaxial rotated aluminum sheet corresponding to nine different radiuses (r, in mm).
Fig. 10. Radial distribution of relative thickness of NBCO film deposited with biaxial rotation.
difficult to obtain homogeneous films using stationary substrates.
3.3. Deposition with biaxial rotation
3.2. Deposition with tilted in-plane rotation Adjustment of target composition and change of the substrate position were commonly employed in deposition of compound thin films to improve homogeneity of thickness and composition [34,35]. To improve film uniformity and stoichiometry, we tilted the substrate (u = k/4) and displaced it 10 mm along the y-axis. The D t – s is fixed at 50 mm. Fig. 5 shows RBS spectra of the film. For a substrate radius of 45 mm, the maximum difference of C Cu is 18%, while that of C Ba + Nd is only 5%. The ratios of C Cu / C Ba + Nd range from 0.97 to 1.18, which are very close to stoichiometric composition, as shown in Fig. 6. It indicates that the composition distributions are much better than that of stationary substrate. Fig. 7 shows the relative thickness distribution. The maximum deviation is more than 40%. But if a limited r sub within 30 mm was chosen, namely 1.5 times of target radius, the maximum thickness deviation could be limited within 10%.
Biaxial rotation is designed especially for simultaneous deposition of double-sided large-area thin films. Driven with a single axis, it combines out-of-plane rotation and in-plane rotation, and makes every point run on a spherical surface. In this case, substrate edge can go very near to the target and catches more atoms in the high particle density region. Deposition with this rotation, uniform film thickness could be obtained. Fig. 8 shows overlapped RBS spectra of film deposited with 60 mm D t – s for 10 h with biaxial rotation. For a substrate radius of 42 mm, the maximum difference of C Cu is only 6%, while that of C Ba + Nd is only 8%. The ratios of C Cu / C Ba + Nd range from 1.32 to 1.52 (see Fig. 9). The thickness distribution of biaxial rotation is much better than that of stationary substrate and tilted rotation (see Fig. 10). The maximum thickness deviation is about 11%. 3.4. Discussion The total particle density in center is higher than that at edge. The deposition rate (R dep) will rapidly decrease with extending of the D t – s. Displacement from the target center can improve the composition and thickness distributions in parallel in-plane rotation. Enlarging D t – s and displacing the substrate can produce more homogeneous films with a lower R dep. Table 1 Comparison of homogeneities of films deposited on stationary, displaced tilted rotated, and biaxial rotated aluminum sheet Rotation mode
D t – s, R dep, DT, DC Cu, DC Ba + Nd, D(C Cu / C Ba + Nd), mm % % nm/min % %
Stationary 50 Displaced 50 and tilted Biaxial* 60 Fig. 9. Composition variation with substrate radius of thin film deposited with biaxial rotation. (a) Radial distribution of C Cu. (b) Radial distribution of C Ba + Nd. (c) Radial distribution of ratios of C Cu / C Ba + Nd.
1.74 1.36 0.69
29.7 10.9 15.9 6.8 3.8
2.5
8.5 1.8
32.9 8.8
2.7
7.0
All of the DC (composition deviation) and DT (thickness deviation) are expressed by variation (standard deviations/mean) here. The R dep of biaxial rotation mode (marked with asterisk) should be multiplied by 2 for comparison because it is that of single side.
B.W. Tao et al. / Thin Solid Films 485 (2005) 47 – 52
However, deposition rate means product efficiency, target utility, and film cost. With different substrate positions and movements, R dep could be greatly different. For a single point, R dep is roughly in inverse proportion to the square of the distance to the target, l t – s. R dep on substrate center is 1.74 nm/min for stationary sheet, 1.36 nm/min for displaced tilted in-plane rotation, and 1.38 nm/ min for biaxial rotation, respectively, as shown in Table 1. R dep with displaced tilted rotation or biaxial rotation is lower than that on stationary substrate because of larger l t – s, incident particle angles, and displacement. For the latter two cases, every point, except center on the substrate, moves close to and far away from the target alternatively. The effect can be simply expressed with R¶dep ”
= circle
1 ðlts þ d Þ
2
/Rdep
=
1
2 circle lts
;
da½ r; r;
where r being the radius of the point in spherical coordinate and d being its projection. When r increases, the improvement effect is more noticeable. This causes the thicker edge than that on stationary substrate. Moreover, the films have more uniform thickness distributions when tilted or biaxial rotation is employed. On a stationary substrate, the thickness variation (standard deviation/mean), T, is 29.7% for an r sub = 43 mm, while it becomes 15.9% for an r sub = 45 mm in displaced tilted rotating deposition and 3.8% for an r sub = 42 mm in biaxial rotating deposition, respectively. The average deposition rate, R dep¶, on the entire substrate, is more accurate for comparison of deposition efficiency. They are 0.49, 0.72, and 0.68 nm/min for films deposited with stationary substrate, displaced tilted rotation, and biaxial rotation, respectively. Therefore, deposition with tilted or biaxial rotation has higher rate and target utility factor because substrates go closer to target. Table 1 also gives cation concentration variation for films deposited with different substrate movements. It is clear that biaxial rotation brings most uniform C Cu distribution, and displaced tilted rotation brings best C Ba + Nd distribution and most stoichiometric composition, as seen in Figs. 3c, 6c, and 9c). Considering the deposition with displaced tilted inplane rotation, if the atoms transfer directly, the C Cu / C Ba + Nd at substrate center should be about 1.8, just near to that of film on stationary substrate at r sub = 10 mm. But the measured ratio is only 1.18 (Fig. 6c). This reveals that the difference of traveling characters [36] between Cu and (Ba + Nd) had been weakened to a very low level on tilted substrate center. Deposited with biaxial rotation, because the tilted angle varies continuously in every out-of-plane revolution, the value C Cu / C Ba + Nd of the films is also lower than that of film deposited on stationary substrate, 1.5 at center. Stoichiometry of those films is not as good as that of displaced tilted in-plane rotation deposition. Furthermore, the stoichiometry must be better if the substrate is loaded with off-axis displacement in biaxial rotation, just similar to that in displaced tilted in-plane rotation.
51
4. Conclusions We have comparatively studied thickness distribution, composition uniformity, and stoichiometry of large-area NBCO thin films deposited with different substrate movements. Though with a high pressure, the different elements still have different transfer characters. When the substrate was displaced and in-plane rotating with k/4 tilted angle, the very uniform and stoichiometric film was obtained on a substrate with r sub = 35 mm. When the film deposited with biaxial rotation, the size of uniform film enlarged to r sub = 40 mm. If combining displacement and biaxial rotation, the stoichiometry should be better. References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
D. Damjanovic, Rep. Prog. Phys. 61 (1998) 1267. D.P. Norton, Mater. Sci. Eng., R. Rep. 43 (2004) 139. P. Muralt, Rep. Prog. Phys. 64 (2001) 1339. S. Thakoor, Appl. Phys. Lett. 60 (1992) 3319. K. Sreenivas, M. Sayer, D.J. Baar, M. Nishioka, Appl. Phys. Lett. 52 (1988) 709. M. Lorenz, H. Hochmuth, D. Natusch, M. Kusunoki, V.L. Svetchnikov, V. Riede, G. Stanca, D. Hesse, IEEE Trans. Appl. Supercond. 11 (2001) 3209. T. Schurig, S. Menkel, Z. Quan, et al., J. Eng. Appl. Sci. 262 (1996) 89. J. Azoulay, J. Am. Ceram. Soc. 79 (1996) 568. U. Schmatz, C. Dubourdieu, O. Lebedev, G. Delabouglise, F. Weiss, J.P. Senateur, J. Low Temp. Phys. 105 (1996) 1301. L. Philippe, M. Bernard, M. Hugues, J. Vac. Sci. Technol., A, Vac. Surf. Films 13 (1995) 2221. R.S. Upadhye, E.J. Hsieh, J. Vac. Sci. Technol., A, Vac. Surf. Films 8 (1990) 1348. T. Serikawa, A. Okamoto, J. Vac. Sci. Technol., A, Vac. Surf. Films 3 (1985) 1784. S. Swann, S.A. Collett, I.R. Scarlett, J. Vac. Sci. Technol., A, Vac. Surf. Films 8 (1990) 1299. P.K. Petrov, V.A. Volpyas, R.A. Chakalov, Vacuum 52 (1999) 427. J. Cook, Thin Solid Films 338 (1999) 81. Y. Konishi, K. Harada, Y. Yonezawa, Y. Kawamura, Jpn. J. Appl. Phys. 39 (2000) 5379. S.I. Shah, P.F. Garcia, Appl. Phys. Lett. 51 (1987) 2146. I. Kanno, H. Kotera, K. Wasa, T. Matsunaga, T. Kamada, R. Takayama, J. Appl. Phys. 93 (2003) 4091. W.Y. Park, K.H. Ahn, C.S. Hwang, Appl. Phys. Lett. 83 (2003) 4387. E.J. Cukauskas, S.W. Kirchoefer, J.M. Pond, J. Appl. Phys. 88 (2000) 2830. S.H. Lee, J.K. Lee, K.H. Yoon, J. Mater. Res. 17 (2002) 1455. R.A. Rao, Q. Gan, C.B. Eom, Y. Suzuki, A.A. Macdanel, J.W.P. Hsu, Appl. Phys. Lett. 69 (1996) 3911. D.O. Klenov, W. Donner, B. Fran, S. Stemmer, Appl. Phys. Lett. 82 (2003) 3427. R.J. Lin, L.J. Chen, J. Vac. Sci. Technol., A, Vac. Surf. Films 15 (1997) 2605. C.B. Eom, J.Z. Sun, K. Yamamoto, A.F. Marshall, K.E. Luther, T.H. Geballe, S.S. Laderman, Appl. Phys. Lett. 55 (1989) 595. J. Geerk, G. Linker, O. Meyer, Mater. Sci. Rep. 4 (1989) 193. D.E. Pugel, L.H. Greene, Appl. Phys. Lett. 75 (1999) 1589. S. Bals, G. Van Tendeloo, M. Salluzzo, I. Maggio-Aprile, Appl. Phys. Lett. 79 (2001) 3660. J. Geerk, G. Linker, O. Meyer, J. Supercond. 5 (1992) 345.
52
B.W. Tao et al. / Thin Solid Films 485 (2005) 47 – 52
[30] V.N. Tsaneva, T. Donchev, T.K. Popov, F.M. Dias, Z.H. Barber, E.J. Tarte, M.G. Blamire, M. Hogg, F. Kahlmann, J.E. Evets, Physica C 372 – 376 (2002) 1087. [31] B.W. Tao, Y.R. Li, X.Z. Liu, S.M. He, J. Geerk, J. Vac. Sci. Technol., A, Vac. Surf. Films 20 (2002) 1898. [32] R.A. Rao, C.B. Eom, M. Santer, S.M. Anlage, IEEE Trans. Appl. Supercond. 7 (1997) 1278.
[33] B.W. Tao, X.Z. Liu, J.J. Chen, Y.R. Li, R. Fromknecht, J. Geerk, J. Vac. Sci. Technol., A, Vac. Surf. Films 21 (2003) 431. [34] J. Geerk, A. Zaitsev, G. Linker, G. Aidam, R. Schneider, R. Ratzel, F. Fromknecht, R. Scheerer, B. Reiner, H. Gagnidze, E. Schwab, IEEE Trans. Appl. Supercond. 11 (2001) 3856. [35] Y. Kageyama, Y. Taga, Appl. Phys. Lett. 55 (1989) 1035. [36] W.D. Westwood, J. Vac. Sci. Technol. 15 (1978) 1.