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Microstructure analysis of Ag films deposited by low-voltage sputtering
Thin Solid Films 520 (2012) 4139–4143
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
Microstructure analysis of Ag films deposited by low-voltage sputtering☆ Kazuhiro Kato a,⁎, Hideo Omoto b, Atsushi Takamatsu a a b
Glass Research Center, Central Glass Co. Ltd., 1510 Ohkuchi-cho, Matsusaka-city, Mie Prefecture, 515–0001, Japan Glass Business Planning and Development Department, Central Glass Co. Ltd., 3-7-1 Kandanishiki-cho, Chiyoda-ku, Tokyo, 101–0054, Japan
a r t i c l e
i n f o
Available online 28 June 2011 Keywords: Ag Film Low-emissivity coating Low-voltage sputtering Resistivity
a b s t r a c t The microstructure of Ag films was investigated as a function of the cathode voltage during sputter deposition. It was found that the resistivity of the Ag films decreased when the Ag film was deposited at low cathode voltage using a magnetron cathode with high-magnetic flux density. X-ray diffraction measurement revealed that the Ag films deposited at low cathode voltages exhibited higher crystallization degree and larger crystallites. Besides, it was confirmed from glancing incident X-ray reflectivity measurement that the density of the Ag films increased with decreasing in the cathode voltage. It can be concluded from these results that the improvements in the resistivity and microstructure of Ag films result from the low-voltage sputtering. It can be concluded that the kinetic energy of the Ar gas particles decreased with decreasing the cathode voltage; as a result, the microstructure of Ag films should be improved. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Low-emissivity coatings are used for energy efficient windows to reduce excessive heating inside buildings [1,2]. The energy efficient windows are constructed of at least a pair of glass panes and lowemissivity coatings are formed on one side of glass panes. Lowemissivity coatings consisting of glass/under dielectric/Ag/over dielectric or the repetition structure are usually deposited by magnetron sputtering. The electrical resistivity of conductive thin films affects their optical and thermal properties [3,4]; hence, the Ag films used in low-emissivity coatings are strongly required to exhibit low resistivity. T. Ohmi et al. revealed that the kinetic energy of Ar + ions has a great influence on the microstructure in substrate bias sputtering technique [5]. Previous studies have revealed that sputtered metal oxide films are highly crystallized by controlling cathode voltage using a magnetron cathode with high magnetic flux density [6,7]. Besides, we have reported that low-voltage sputtering is one of the best deposition techniques to obtain the Ag films in the low-e coatings with low resistivity [8,9]. However, it has been known that the microstructure and spices of the under dielectrics also have great influences on the properties of the Ag films used in lowemissivity coatings [10–14]. In this study, hence, we prepared Ag
☆ Submitted to Thin Solid Films (Special issue of ICCG8, the 8th International Conference on Coatings on Glass and Plastics, 13th–17th June 2010, held in Braunschweig, Germany). ⁎ Corresponding author. Tel.: + 81 598 53 3160; Fax: + 81 598 53 3180. E-mail address: [email protected] (K. Kato). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.06.051
single layers in order to avoid the complication due to the presence of the under and over dielectrics in low-emissivity coatings; moreover, the resistivity and microstructure of the Ag films were investigated as a function of cathode voltage during sputter deposition. The correlation among the resistivity, microstructure of Ag films and the cathode voltage during sputter deposition is discussed.
2. Experimental 2.1. Deposition conditions Ag films were deposited on soda–lime–silicate glass substrates by magnetron sputtering. Before sputter deposition, ultrasonic cleaning was performed with soaking substrates into alkali detergent and then ultra-pure water. Bottom side (tin side) was selected for the deposition to avoid abnormal film growth due to partial tin drops on the air side. The substrates were not heated during sputter deposition. Besides, the
Table 1 Changes in cathode voltage, cathode current and deposition rate using sputtering target with different magnetic flux density. Magnetic flux density (mT)
Cathode voltage (V)
Cathode current (A)
DC power (W)
Deposition rate (nm/min)
91 154
449–471 360–387
0.41–0.44 0.50–0.55
200 (constant) 200 (constant)
57.9 67.2
4140
K. Kato et al. / Thin Solid Films 520 (2012) 4139–4143
30
5
Crystallite size of Ag film [nm]
Cathode voltage (V) 471 379
4
3
20
10
0 0
10
20
30
40
50
60
Cathode voltage (V) 471 379 10
0
20
30
40
50
60
Ag film thickness [nm]
Ag film thickness [nm] Fig. 1. Dependence of resistivity on thickness of Ag films deposited at different cathode voltages.
Fig. 3. Change in crystallite size of Ag films deposited at different cathode voltages.
substrate was rotated on the sputter target at the speed of 5 rotations per minute. The distance between the substrate and sputter target was adjusted to 90 mm. The Ag films were deposited on the glass substrates in 0.5 Pa Ar gas using Ag metal target with 3 inch diameter; the dc power applied to the target was maintained at 200 W; the density of perpendicular magnetic flux on the center of Ag target surface of 91 or 154 mT was used to control the cathode voltage. The thicknesses of Ag films were adjusted to be about 10–55 nm by varying the deposition time.
The crystallite size of the Ag films was calculated using the Scherrer's equation and measuring the full width at half maximum (FWHM) of the XRD peak obtained in a narrow scan mode [15]. Glancing incident X-ray reflectivity (GIXR) measurement was performed to evaluate the density, surface roughness and thickness of the Ag films [12,16]. For selected samples, surface morphology of Ag films was observed as topography with an atomic force microscope (AFM) in a dynamic scan mode. Surface roughness (Ra) was determined through AFM analysis. 3. Results and discussion
2.2. Evaluation Sheet resistance (Rs) of the Ag films was measured using a fourpoint probe method to estimate the resistivity (ρs) using the following equation: ρs = Rs × tAg ;
ð1Þ
where the thickness of Ag films is expressed with tAg. X-ray diffraction (XRD) measurement with out-of-plane arrangement was performed to evaluate the crystal structure in a wide scan mode, where X-ray diffraction on lattice planes parallel to film surface can be obtained.
The cathode voltage and current during Ag film deposition are listed in Table 1. This table indicates that the cathode voltage decreased; on the other hand, the cathode current increased using the sputter target with higher magnetic flux density. This effect has already been elucidated as the result of the change in the electron density trapped into the magnetic field [7]. The deposition rate is also shown in Table 1 and it was confirmed that the deposition rate increased with increasing magnetic flux density. It can be concluded that the electron density increased using the sputter target with high magnetic flux density as a results, the amount of Ar + ions accelerated to the sputter target increased, hence, the deposition rate is increased.
(a) 471 V
(b) 379 V
Thickness Ag(111)
(nm) Intensity [a.U.]
Intensity [a.U.]
Ag(111)
Thickness
10.4 22.8
(nm) 11.4 25.8 40.3
34.8
53.8
46.3 10
20
30
40
50
60
70
10
20
30
40
50
60
Fig. 2. XRD patterns of Ag films deposited at different cathode voltages of (a) 471 V and (b) 379 V.
70
K. Kato et al. / Thin Solid Films 520 (2012) 4139–4143
11.2
Density of Ag film [g cm-3]
6 5 4 3
4141
Cathode voltage (V) 471 379
2 10
20
11.0 10.9 10.8 10.7 10.6
30
Cathode voltage (V) 471 379
11.1
Bulk Ag = 10.5 g cm -3 0
Ag film crystallite size [nm]
10
20
30
40
50
60
Thickness of Ag film [nm]
Fig. 4. Resistivity of Ag films against Ag crystallite size.
Fig. 6. Density of Ag films deposited at different cathode voltages as a function of Ag thickness, both derived from GIXR analysis.
In our earlier study [9], it has already been reported that not deposition rate but cathode voltage had a great influence on the resistivity of Ag films used in low-emissivity coatings. The resistivity changes of the Ag films with various thicknesses are shown in Fig. 1 in relation to the cathode voltage during sputter deposition. Here, the thickness was determined from GIXR analysis for calculating the resistivity using Eq. (1). It can be seen from Fig. 1 that the Ag films deposited at low cathode voltages exhibited lower resistivity. In order to clarify the reasons of this resistivity improvement due to low-voltage sputtering, the microstructure of Ag films was investigated. Fig. 2 shows the XRD patterns of Ag films deposited at different cathode voltages. It can be concluded from Fig. 2 that the Ag films with (111)preferential orientation and higher crystallization degree were obtained when they were deposited at low cathode voltage. Fig. 3 shows the crystallite size estimated from the FWHM of the Ag(111) peaks measured in a narrow scan mode. It was found from Fig. 3 that the crystallites became larger when the low cathode voltage was applied. The resistivity of Ag films is plotted in Fig. 4 with taking the crystallite size as X-axis. Fig. 4 indicates that the resistivity decreased as the crystallite became large. It can be concluded as one possible explanation from Fig. 4 that the improvement in resistivity of Ag films deposited by low-voltage sputtering should come from the large crystallites as classical size effect. The GIXR measurement was performed to investigate the microstructure of Ag films further and their curves are
shown in Fig. 5. Fig. 5 proves that the GIXR curves of Ag films showed enough clear amplitude to calculate the density, surface roughness and thickness. Fig. 6 shows the density of Ag films deposited at different cathode voltages estimated from the GIXR profiles. The GIXR analysis revealed that the density of Ag films became higher when low cathode voltage was applied. This improvement in the density of Ag films by applying low cathode voltage should relate to the larger crystallite growth as mentioned above. Although the density determined for Ag films exceeded the bulk value for Ag, calibration of the density is required in order to clear this uncertainty. Surface roughness of Ag films deposited at different cathode voltages is presented in Fig. 7 as a function of the thickness from GIXR analysis. It can be seen from Fig. 7 that the surface of Ag films is rougher for Ag films deposited at low cathode voltage. This increase in surface roughness observed in Fig. 7 might be caused by the better crystal growth of Ag films deposited at low cathode voltages, although it has been reported that the surface morphology of Ag films influences their resistivity [17], in particular, the Ag films heteroepitaxially grown on ZnO under dielectrics exhibited smooth surface and low resistivity due to small distribution of Ag film thickness [18]. Fig. 8 shows the AFM images of Ag films with a thickness of about 44 nm deposited at different cathode voltages. The clear change in the grain size of Ag films was not confirmed from Fig. 8. This fact indicates that the grain shape exposed on Ag film surface does not reflect on the crystallite size in Ag films, because the AFM results
(a) 471 V
(b) 379 V
(nm)
(nm)
10.4
11.4
Intensity [a. U.]
Thickness
Intensity [a. U.]
Thickness
22.8 34.8
25.8 40.3 53.8
46.3
0
1
2
3
4
5
0
1
2
3
4
Fig. 5. GIXR curves of Ag films deposited at different cathode voltages of (a) 471 V and (b) 379 V.
5
4142
K. Kato et al. / Thin Solid Films 520 (2012) 4139–4143
Surface roughness of Ag film from GIXR [nm]
4
Table 2 Kinetic energy of Ar gas particles estimated using Eq. (2) and Kevin–Meyer's equation.
Cathode voltage (V) 471 379
3
Cathode voltage (V)
Maximum kinetic energy of Ar gas particles (eV)
Final kinetic energy of Ar gas particles (eV)
459 387
96.9 81.7
3.09 2.61
2
1
0
10
20
30
40
50
60
Ag film thickness [nm] Fig. 7. Surface roughness of Ag films for various thicknesses deposited at different cathode voltages, roughness and thickness determined from GIXR analysis.
disagreed with the XRD results shown in Fig. 2. Therefore, it might be concluded that the resistivity change of Ag films can be attribute to the slight change of crystallite size which cannot be observed through the AFM analysis. Surface roughness (Ra) of Ag films was determined through AFM analysis and it is also shown in Fig. 8. It can be seen that the Ag film deposited at low cathode voltage exhibited lower Ra. It has been reported that there was good agreement between surface roughness determined from AFM and GIXR analysis [12,16]. In our experiments, however, the surface roughness of Ag films determined from AFM did not agree well with that from GIXR. This difference in surface roughness might be because the surface roughness does not exhibit a Gaussian like roughness distribution as assumed in GIXR theory [19]. According to AFM analysis, it might be more reasonable that the Ag films with high density and smooth surface can be grown using low-voltage sputtering. Here, in order to further clarify the improvement mechanism of Ag film microstructure, the kinetic energy of Ar gas particles backscattered on sputter target surface was calculated as follows [20]: 2 2 0 þ EAr = MAg −MAr = MAg + MAr × EAr ;
ð2Þ
where E0Ar is the maximum kinetic energy of the Ar gas particles backscattered on the Ag sputter target surface at the reflection angle of 180°. MAg and MAr are the mass of Ag and Ar, respectively. The initial energy of Ar ions incident on the Ag sputter target is expressed with E+Ar, corresponding to the cathode voltage. The estimated maximum
kinetic energy of the backscattered Ar gas particles is listed in Table 2 in relation to cathode voltage. Besides, the final kinetic energy of Ar gas particles reaching the substrate surface was calculated using Kevin– Meyer's equation shown in previous studies [21–23] and also listed in Table 2. It was found from Table 2 that the kinetic energy of Ar gas particles backscattered on the Ag sputter target surface decreased with decreasing the cathode voltage. It can be concluded as one possible explanation that the Ag films deposited at low cathode voltage are better crystallized because the kinetic energy of Ar gas particles bombarding the film surface decreased, hence, the resistivity of Ag films decreased. 4. Conclusions The resistivity and microstructure of the Ag films were investigated as a function of cathode voltage during sputter deposition. The correlation among the resistivity, microstructure of Ag films and the cathode voltage during sputter deposition has been clarified. The following results were obtained: (1) The Ag films deposited at low cathode voltages exhibited lower resistivity. (2) The Ag films with lower resistivity exhibited larger crystallites and higher density. (3) The crystallite size of Ag films appeared to be one of the dominant factors to determine the resistivity. In addition, the kinetic energy of Ar gas particles backscattered on Ag sputter target surface was clarified as follows. (4) The kinetic energy of Ar gas particles decreased with decreasing cathode voltage. It can be concluded from these results that the Ag films deposited at low-voltage showed better crystallinity because the Ar gas particles bombarding the film surface had a smaller influence on the film
(a) 449 V
(b) 371 V
Ra = 2.0 nm
Ra = 1.3 nm 13 nm
0 nm 1000 nm
1000 nm
Fig. 8. AFM images of Ag films deposited at different cathode voltages.
K. Kato et al. / Thin Solid Films 520 (2012) 4139–4143
growth, hence, the Ag films with larger crystallites and higher density are obtained; as a result, the resistivity of Ag films is improved. References [1] R.J. Hill, S.J. Nadal, Coated Glass Applications and Markets, BOC Coating Technology, 1999, p. 68. [2] T. Otsuki, S. Omi, H. Nakashima, J. Jpn. Soc. Infrared Sci. Technol. 7 (1997) 125 (in Japanese). [3] F. Simonis, M.V. Leij, C.J. Hoogendoom, Sol. Energy Mater. 1 (1979) 221. [4] J. Szczyrbowski, G. Bräuer, M. Ruske, H. Schilling, A. Zmelty, Thin Solid Films 351 (1999) 254. [5] T. Ohmi, T. Saito, T. Shibata, T. Nitta, Appl. Phys. Lett. 52 (1988) 2236. [6] K. Ishibashi, Y. Shiokawa, Proc. of the 3rd International Symposium on Sputtering and Plasma Processes (3rd ISSP), Tokyo, Japan, 1995, p. 471. [7] Y. Shigesato, Proc. of the 5th International Conference on Coatings on Glass (5th ICCG), Saarbruecken, Germany, 2004, p. 315.
4143
[8] K. Kato, K. Fiji, T. Kobayashi, Proc. of the 6th International Conference on Glass and Plastics (6th ICCG), Dresden, Germany, 2006, p. 361. [9] K. Kato, H. Omoto, A. Takamatsu, Vacuum 84 (2009) 587. [10] M. Arbab, Thin Solid Films 381 (2001) 15. [11] Y. Tsuda, H. Omoto, K. Tanaka, H. Ohsaki, Thin Solid Films 502 (2006) 223. [12] S. Ulrich, A. Pflug, K. Schiffmann, B. Szyszka, Phys. Status Solidi C 5 (2008) 1235. [13] K. Kato, H. Omoto, A. Takamatsu, Vacuum 84 (2008) 606. [14] K. Kato, H. Omoto, A. Takamatsu, Trans. Mater. Res. Soc. Jpn. 35 (2010) 181. [15] A.L. Patterson, Phys. Rev. 56 (1939) 978. [16] I. Kojima, B. Li, Rigaku J. 16 (1999) 31. [17] E.Z. Luo, S. Heun, M. Kennedy, J. Wollschlager, M. Henzler, Phys. Rev. B 49 (1994) 4858. [18] K. Kato, H. Omoto, A. Takamatsu, Adv. Mater. Res. 117 (2010) 69. [19] L. Névot, P. Croce, Rev. Phys. Appl. 15 (1980) 761 (in French). [20] P.K. Song, Y. Shigesato, M. Kamei, I. Yasui, Jpn. J. Appl. Phys. 38 (1999) 2921. [21] K. Meyer, I.K. Schuller, C.M. Faico, J. Appl. Phys. 52 (1981) 5803. [22] M. Yamagishi, S. Kuriki, P.K. Song, Y. Shigesato, Thin Solid Films 442 (2003) 227. [23] T. Sasabayashi, N. Ito, E. Nishimura, M. Kon, P.K. Song, K. Utsumi, A. Kaijo, Y. Shigesato, Thin Solid Films 445 (2003) 219.
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
Microstructure analysis of Ag films deposited by low-voltage sputtering☆ Kazuhiro Kato a,⁎, Hideo Omoto b, Atsushi Takamatsu a a b
Glass Research Center, Central Glass Co. Ltd., 1510 Ohkuchi-cho, Matsusaka-city, Mie Prefecture, 515–0001, Japan Glass Business Planning and Development Department, Central Glass Co. Ltd., 3-7-1 Kandanishiki-cho, Chiyoda-ku, Tokyo, 101–0054, Japan
a r t i c l e
i n f o
Available online 28 June 2011 Keywords: Ag Film Low-emissivity coating Low-voltage sputtering Resistivity
a b s t r a c t The microstructure of Ag films was investigated as a function of the cathode voltage during sputter deposition. It was found that the resistivity of the Ag films decreased when the Ag film was deposited at low cathode voltage using a magnetron cathode with high-magnetic flux density. X-ray diffraction measurement revealed that the Ag films deposited at low cathode voltages exhibited higher crystallization degree and larger crystallites. Besides, it was confirmed from glancing incident X-ray reflectivity measurement that the density of the Ag films increased with decreasing in the cathode voltage. It can be concluded from these results that the improvements in the resistivity and microstructure of Ag films result from the low-voltage sputtering. It can be concluded that the kinetic energy of the Ar gas particles decreased with decreasing the cathode voltage; as a result, the microstructure of Ag films should be improved. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Low-emissivity coatings are used for energy efficient windows to reduce excessive heating inside buildings [1,2]. The energy efficient windows are constructed of at least a pair of glass panes and lowemissivity coatings are formed on one side of glass panes. Lowemissivity coatings consisting of glass/under dielectric/Ag/over dielectric or the repetition structure are usually deposited by magnetron sputtering. The electrical resistivity of conductive thin films affects their optical and thermal properties [3,4]; hence, the Ag films used in low-emissivity coatings are strongly required to exhibit low resistivity. T. Ohmi et al. revealed that the kinetic energy of Ar + ions has a great influence on the microstructure in substrate bias sputtering technique [5]. Previous studies have revealed that sputtered metal oxide films are highly crystallized by controlling cathode voltage using a magnetron cathode with high magnetic flux density [6,7]. Besides, we have reported that low-voltage sputtering is one of the best deposition techniques to obtain the Ag films in the low-e coatings with low resistivity [8,9]. However, it has been known that the microstructure and spices of the under dielectrics also have great influences on the properties of the Ag films used in lowemissivity coatings [10–14]. In this study, hence, we prepared Ag
☆ Submitted to Thin Solid Films (Special issue of ICCG8, the 8th International Conference on Coatings on Glass and Plastics, 13th–17th June 2010, held in Braunschweig, Germany). ⁎ Corresponding author. Tel.: + 81 598 53 3160; Fax: + 81 598 53 3180. E-mail address: [email protected] (K. Kato). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.06.051
single layers in order to avoid the complication due to the presence of the under and over dielectrics in low-emissivity coatings; moreover, the resistivity and microstructure of the Ag films were investigated as a function of cathode voltage during sputter deposition. The correlation among the resistivity, microstructure of Ag films and the cathode voltage during sputter deposition is discussed.
2. Experimental 2.1. Deposition conditions Ag films were deposited on soda–lime–silicate glass substrates by magnetron sputtering. Before sputter deposition, ultrasonic cleaning was performed with soaking substrates into alkali detergent and then ultra-pure water. Bottom side (tin side) was selected for the deposition to avoid abnormal film growth due to partial tin drops on the air side. The substrates were not heated during sputter deposition. Besides, the
Table 1 Changes in cathode voltage, cathode current and deposition rate using sputtering target with different magnetic flux density. Magnetic flux density (mT)
Cathode voltage (V)
Cathode current (A)
DC power (W)
Deposition rate (nm/min)
91 154
449–471 360–387
0.41–0.44 0.50–0.55
200 (constant) 200 (constant)
57.9 67.2
4140
K. Kato et al. / Thin Solid Films 520 (2012) 4139–4143
30
5
Crystallite size of Ag film [nm]
Cathode voltage (V) 471 379
4
3
20
10
0 0
10
20
30
40
50
60
Cathode voltage (V) 471 379 10
0
20
30
40
50
60
Ag film thickness [nm]
Ag film thickness [nm] Fig. 1. Dependence of resistivity on thickness of Ag films deposited at different cathode voltages.
Fig. 3. Change in crystallite size of Ag films deposited at different cathode voltages.
substrate was rotated on the sputter target at the speed of 5 rotations per minute. The distance between the substrate and sputter target was adjusted to 90 mm. The Ag films were deposited on the glass substrates in 0.5 Pa Ar gas using Ag metal target with 3 inch diameter; the dc power applied to the target was maintained at 200 W; the density of perpendicular magnetic flux on the center of Ag target surface of 91 or 154 mT was used to control the cathode voltage. The thicknesses of Ag films were adjusted to be about 10–55 nm by varying the deposition time.
The crystallite size of the Ag films was calculated using the Scherrer's equation and measuring the full width at half maximum (FWHM) of the XRD peak obtained in a narrow scan mode [15]. Glancing incident X-ray reflectivity (GIXR) measurement was performed to evaluate the density, surface roughness and thickness of the Ag films [12,16]. For selected samples, surface morphology of Ag films was observed as topography with an atomic force microscope (AFM) in a dynamic scan mode. Surface roughness (Ra) was determined through AFM analysis. 3. Results and discussion
2.2. Evaluation Sheet resistance (Rs) of the Ag films was measured using a fourpoint probe method to estimate the resistivity (ρs) using the following equation: ρs = Rs × tAg ;
ð1Þ
where the thickness of Ag films is expressed with tAg. X-ray diffraction (XRD) measurement with out-of-plane arrangement was performed to evaluate the crystal structure in a wide scan mode, where X-ray diffraction on lattice planes parallel to film surface can be obtained.
The cathode voltage and current during Ag film deposition are listed in Table 1. This table indicates that the cathode voltage decreased; on the other hand, the cathode current increased using the sputter target with higher magnetic flux density. This effect has already been elucidated as the result of the change in the electron density trapped into the magnetic field [7]. The deposition rate is also shown in Table 1 and it was confirmed that the deposition rate increased with increasing magnetic flux density. It can be concluded that the electron density increased using the sputter target with high magnetic flux density as a results, the amount of Ar + ions accelerated to the sputter target increased, hence, the deposition rate is increased.
(a) 471 V
(b) 379 V
Thickness Ag(111)
(nm) Intensity [a.U.]
Intensity [a.U.]
Ag(111)
Thickness
10.4 22.8
(nm) 11.4 25.8 40.3
34.8
53.8
46.3 10
20
30
40
50
60
70
10
20
30
40
50
60
Fig. 2. XRD patterns of Ag films deposited at different cathode voltages of (a) 471 V and (b) 379 V.
70
K. Kato et al. / Thin Solid Films 520 (2012) 4139–4143
11.2
Density of Ag film [g cm-3]
6 5 4 3
4141
Cathode voltage (V) 471 379
2 10
20
11.0 10.9 10.8 10.7 10.6
30
Cathode voltage (V) 471 379
11.1
Bulk Ag = 10.5 g cm -3 0
Ag film crystallite size [nm]
10
20
30
40
50
60
Thickness of Ag film [nm]
Fig. 4. Resistivity of Ag films against Ag crystallite size.
Fig. 6. Density of Ag films deposited at different cathode voltages as a function of Ag thickness, both derived from GIXR analysis.
In our earlier study [9], it has already been reported that not deposition rate but cathode voltage had a great influence on the resistivity of Ag films used in low-emissivity coatings. The resistivity changes of the Ag films with various thicknesses are shown in Fig. 1 in relation to the cathode voltage during sputter deposition. Here, the thickness was determined from GIXR analysis for calculating the resistivity using Eq. (1). It can be seen from Fig. 1 that the Ag films deposited at low cathode voltages exhibited lower resistivity. In order to clarify the reasons of this resistivity improvement due to low-voltage sputtering, the microstructure of Ag films was investigated. Fig. 2 shows the XRD patterns of Ag films deposited at different cathode voltages. It can be concluded from Fig. 2 that the Ag films with (111)preferential orientation and higher crystallization degree were obtained when they were deposited at low cathode voltage. Fig. 3 shows the crystallite size estimated from the FWHM of the Ag(111) peaks measured in a narrow scan mode. It was found from Fig. 3 that the crystallites became larger when the low cathode voltage was applied. The resistivity of Ag films is plotted in Fig. 4 with taking the crystallite size as X-axis. Fig. 4 indicates that the resistivity decreased as the crystallite became large. It can be concluded as one possible explanation from Fig. 4 that the improvement in resistivity of Ag films deposited by low-voltage sputtering should come from the large crystallites as classical size effect. The GIXR measurement was performed to investigate the microstructure of Ag films further and their curves are
shown in Fig. 5. Fig. 5 proves that the GIXR curves of Ag films showed enough clear amplitude to calculate the density, surface roughness and thickness. Fig. 6 shows the density of Ag films deposited at different cathode voltages estimated from the GIXR profiles. The GIXR analysis revealed that the density of Ag films became higher when low cathode voltage was applied. This improvement in the density of Ag films by applying low cathode voltage should relate to the larger crystallite growth as mentioned above. Although the density determined for Ag films exceeded the bulk value for Ag, calibration of the density is required in order to clear this uncertainty. Surface roughness of Ag films deposited at different cathode voltages is presented in Fig. 7 as a function of the thickness from GIXR analysis. It can be seen from Fig. 7 that the surface of Ag films is rougher for Ag films deposited at low cathode voltage. This increase in surface roughness observed in Fig. 7 might be caused by the better crystal growth of Ag films deposited at low cathode voltages, although it has been reported that the surface morphology of Ag films influences their resistivity [17], in particular, the Ag films heteroepitaxially grown on ZnO under dielectrics exhibited smooth surface and low resistivity due to small distribution of Ag film thickness [18]. Fig. 8 shows the AFM images of Ag films with a thickness of about 44 nm deposited at different cathode voltages. The clear change in the grain size of Ag films was not confirmed from Fig. 8. This fact indicates that the grain shape exposed on Ag film surface does not reflect on the crystallite size in Ag films, because the AFM results
(a) 471 V
(b) 379 V
(nm)
(nm)
10.4
11.4
Intensity [a. U.]
Thickness
Intensity [a. U.]
Thickness
22.8 34.8
25.8 40.3 53.8
46.3
0
1
2
3
4
5
0
1
2
3
4
Fig. 5. GIXR curves of Ag films deposited at different cathode voltages of (a) 471 V and (b) 379 V.
5
4142
K. Kato et al. / Thin Solid Films 520 (2012) 4139–4143
Surface roughness of Ag film from GIXR [nm]
4
Table 2 Kinetic energy of Ar gas particles estimated using Eq. (2) and Kevin–Meyer's equation.
Cathode voltage (V) 471 379
3
Cathode voltage (V)
Maximum kinetic energy of Ar gas particles (eV)
Final kinetic energy of Ar gas particles (eV)
459 387
96.9 81.7
3.09 2.61
2
1
0
10
20
30
40
50
60
Ag film thickness [nm] Fig. 7. Surface roughness of Ag films for various thicknesses deposited at different cathode voltages, roughness and thickness determined from GIXR analysis.
disagreed with the XRD results shown in Fig. 2. Therefore, it might be concluded that the resistivity change of Ag films can be attribute to the slight change of crystallite size which cannot be observed through the AFM analysis. Surface roughness (Ra) of Ag films was determined through AFM analysis and it is also shown in Fig. 8. It can be seen that the Ag film deposited at low cathode voltage exhibited lower Ra. It has been reported that there was good agreement between surface roughness determined from AFM and GIXR analysis [12,16]. In our experiments, however, the surface roughness of Ag films determined from AFM did not agree well with that from GIXR. This difference in surface roughness might be because the surface roughness does not exhibit a Gaussian like roughness distribution as assumed in GIXR theory [19]. According to AFM analysis, it might be more reasonable that the Ag films with high density and smooth surface can be grown using low-voltage sputtering. Here, in order to further clarify the improvement mechanism of Ag film microstructure, the kinetic energy of Ar gas particles backscattered on sputter target surface was calculated as follows [20]: 2 2 0 þ EAr = MAg −MAr = MAg + MAr × EAr ;
ð2Þ
where E0Ar is the maximum kinetic energy of the Ar gas particles backscattered on the Ag sputter target surface at the reflection angle of 180°. MAg and MAr are the mass of Ag and Ar, respectively. The initial energy of Ar ions incident on the Ag sputter target is expressed with E+Ar, corresponding to the cathode voltage. The estimated maximum
kinetic energy of the backscattered Ar gas particles is listed in Table 2 in relation to cathode voltage. Besides, the final kinetic energy of Ar gas particles reaching the substrate surface was calculated using Kevin– Meyer's equation shown in previous studies [21–23] and also listed in Table 2. It was found from Table 2 that the kinetic energy of Ar gas particles backscattered on the Ag sputter target surface decreased with decreasing the cathode voltage. It can be concluded as one possible explanation that the Ag films deposited at low cathode voltage are better crystallized because the kinetic energy of Ar gas particles bombarding the film surface decreased, hence, the resistivity of Ag films decreased. 4. Conclusions The resistivity and microstructure of the Ag films were investigated as a function of cathode voltage during sputter deposition. The correlation among the resistivity, microstructure of Ag films and the cathode voltage during sputter deposition has been clarified. The following results were obtained: (1) The Ag films deposited at low cathode voltages exhibited lower resistivity. (2) The Ag films with lower resistivity exhibited larger crystallites and higher density. (3) The crystallite size of Ag films appeared to be one of the dominant factors to determine the resistivity. In addition, the kinetic energy of Ar gas particles backscattered on Ag sputter target surface was clarified as follows. (4) The kinetic energy of Ar gas particles decreased with decreasing cathode voltage. It can be concluded from these results that the Ag films deposited at low-voltage showed better crystallinity because the Ar gas particles bombarding the film surface had a smaller influence on the film
(a) 449 V
(b) 371 V
Ra = 2.0 nm
Ra = 1.3 nm 13 nm
0 nm 1000 nm
1000 nm
Fig. 8. AFM images of Ag films deposited at different cathode voltages.
K. Kato et al. / Thin Solid Films 520 (2012) 4139–4143
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