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Characterization of the silicon oxide thin films deposited on polyethylene terephthalate substrates by radio frequency reactive magnetron sputtering
Thin Solid Films 515 (2007) 4596 – 4602 www.elsevier.com/locate/tsf
Characterization of the silicon oxide thin films deposited on polyethylene terephthalate substrates by radio frequency reactive magnetron sputtering M.-C. Lin, C.-H. Tseng, L.-S. Chang ⁎, D.-S. Wuu Department of Materials Engineering, National Chung Hsing University, 250, Kuo Kuang Road, 40227, Taichung, Taiwan, ROC Received 11 August 2005; received in revised form 22 October 2006; accepted 17 November 2006 Available online 26 December 2006
Abstract Transparent silicon oxide films were deposited on polyethylene terephthalate substrates by means of reactive magnetron sputtering with a mixture of argon and oxygen gases. The influences of process parameters, including the oxygen flow ratio, work pressure, radio frequency (RF) power density and deposition time, on the film properties, such as: deposition rate, morphology, surface roughness, water vapor/oxygen transmission rate and flexibility, were investigated. The experimental results show that the SiOx films deposited at RF power density of 4.9 W/cm2, work pressure of 0.27 Pa and oxygen flow ratio of 40% have better performance in preventing the permeation of water vapor and oxygen. Cracks are produced in the SiOx films after the flexion of more than 100 cycles. The minimum transmission rates of water vapor and oxygen were found to be 2.6 g/m2 day atm and 15.4 cc/m2 day atm, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Silicon oxide; Sputtering; Polyethylene terephthalate (PET); Gas transmission rate
1. Introduction Silicon oxide films have been widely used in industry as coating materials due to their excellent electrical, optical and chemical properties. They are used as passivation layers [1], antireflection layers [2], gas barriers for food and medical packages [3] and recently developed organic flat panel displays [4]. It has also been forecasted that the usage of metal oxide films coated on polyethylene terephthalate (PET) will increase rapidly in the near future [5]. Various methods have been used to deposit these metal oxide films on the polymer substrates, for instance, electron beam evaporation, magnetron sputtering [6], other physical vapor deposition methods [7], plasma enhanced chemical vapor deposition [8], sol–gel [9] and roll-to-roll coating processes [10,11]. Due to its low deposition temperature, simple operation and lesser environmental hazard, magnetron sputtering has become one of the most popular choices among these coating techniques. The magnetron sputtering can produce excellent films as gas barrier layers, in spite of its low deposition rate. The properties ⁎ Corresponding author. E-mail address: [email protected] (L.-S. Chang). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.039
of deposited films are quite dependent on the process parameters, including: oxygen flow ratio, work pressure, radio frequency (RF) power density and deposition time. Therefore, in this study, SiOx films were deposited on PET substrates by means of RF reactive magnetron sputtering. The microstructure, surface roughness of the deposited films and their water vapor/ oxygen transmission rates (WV/OTR) were investigated. Meanwhile, the effects of process parameters during the RF reactive magnetron sputtering were also discussed. 2. Experimental details 2.1. Preparation of SiOx thin films The SiOx films were deposited in an in-house built RF reactive magnetron sputtering system in which a two-inch diameter and 99.999% pure silicon disk was used as the target. PET substrates commercially sold in market (Nan Ya Plastics Corporation, Taiwan) with thickness of 125 μm were employed. The sputtering chamber was pumped to 2.7 × 10− 3 Pa using a high vacuum system with a turbo pump. The substrate holder was rotated around the face-centered axis during sputtering deposition to increase the film uniformity. Table 1 presents the
M.-C. Lin et al. / Thin Solid Films 515 (2007) 4596–4602 Table 1 Sets of sputtering parameters Parameters
Set 1
Oxygen flow ratio (%) Work pressure (Pa) RF power density (W/cm2) Deposition time (min)
10, 20, 40, 40 40 60 1.06 0.27, 0.53, 0.80, 0.27 1.06 3.7 3.7 2.5, 3.7, 4.9, 5.7 120 120 120
Set 2
Set 3
Set 4 40 0.27 4.9 120, 180, 240, 360
sputtering parameters and their values used in this study. The experiments were divided into 4 sets, in which one of the four parameters, the oxygen flow ratio, the work pressure, the RF power density and the deposition time, was varied and the others were fixed, respectively. Before the deposition, the PET surface was plasma pre-treated in the sputtering chamber with the direct current (DC) bias of 300 V generated by the 1 kW MDX low-power DC magnetron driver system for 10 min to clean the substrate surface and improve film adhesion. The substrate temperature during sputtering must be lower than the temperature of glass transition of PET because all specimens were not deformed after the deposition, although no water cooling was introduced in the sample stage and the substrate temperature was not measured. 2.2. Characterization of SiOx thin films The water vapor and oxygen transmission rates were measured using the MOCON instruments, including: an Ox-
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Tran 2/61 and a Permatran-w 3/61 model system with a sample size of 4.5 × 4.5 cm2. The WVTR data were measured at atmospheric pressure and 40 °C with 100% relative humidity, and the OTR data were measured at 40 °C with 0% relative humidity. A Model CP, Seiko Instruments Inc (SPI 3800N) atomic force microscope (AFM) was operated in contact mode to provide topographical images of the SiOx coatings, as well as surface roughness measurements. The Si tips with a tip radius of 10 nm were used for this analysis. The scanned sample area and scanning rate were 9 μm2 and 1 Hz, respectively. One area of each sample was taken. Field-emission scanning electron microscopy (FE–SEM) was used to observe the microstructure. The FE–SEM experiments were conducted using a JEOL JSM6700F operated at 1 keV accelerating voltage. The SiOx surface was coated with a thin platinum layer to prevent excessive charging. The chemical compositions of deposited SiOx films were analyzed by means of X-ray photoelectron spectroscopy (ESCA, PHI 1600). XPS spectra were obtained by using the magnesium anti-cathode of the X-ray tube operated at 15 kV and producing characteristic Al Kα at 1.4866 keV. The composition was calculated from the Si 2p/O 1s peak area in the photoelectron spectrum in which the background was subtracted. Prior to XPS measurements, the 1 cm2 large sample surface was cleaned by ion sputtering with an accelerating voltage of 3 kV and a power of 250 W for 30 s. The photoelectronic spectra were obtained using Al Kα X-rays source operated at 15 kV and 400 W. Estimates of Si and O surface concentrations were calculated from the area of the Si 2p and O 1s peaks after Shirley background subtraction, while no obvious carbon signal was observed. Relative sensitivity factors provided by the manufacturer were used. The pressure in the
Fig. 1. Deposition rates of SiOx films prepared at various sputtering parameters: (a) oxygen flow ratio, (b) work pressure, (c) RF power density and (d) deposition time.
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values was obtained from the XPS analysis of mode compounds. In particular, 1.14 eV and 1.40 eV were adopted as FWHM standard values for Si 2p and O 1s, respectively. The XPS analysis of model compounds provided also an initial estimate for the binding energy values of the different Si 2p and O 1s peaks. The thickness of SiOx films on pure silicon substrates which were put together with the PET substrates into the sputtering chamber was measured by a surface profile analyzer (Veeco Dektak3 ST). The persistency of SiOx thin films on PET against flexion was evaluated by means of an inhouse designed apparatus. Samples were fixed at two opposite sides on stainless rods which were driven by a motor and moved forwards and backwards oppositely. SEM observations and vapor/gas permeation tests were made after the flexion test. 3. Results and discussion 3.1. Deposition rate measurement
Fig. 2. The O/Si atomic ratio of SiOx films deposited on PET substrate as a function of various parameters: (a) oxygen flow ratio and (b) RF power density. The error bars represent the measurement error caused by the signal noise in the photoelectron spectrum.
analysis chamber 6.65 × 10− 8 Pa during spectra acquisition. Unless otherwise specified, all XPS spectra were acquired at a 90° takeoff angle. The full width at half maximum (FWHM)
Fig. 1 shows the deposition rates of SiOx films at various sputtering parameters. It can be found that the deposition rate decreases with increasing oxygen flow ratio from 10% to 60% (Fig. 1(a)). This phenomenon is ascribed to the poisoning of the target surface at higher oxygen flow ratio. The oxide film formed on the target surface will significantly reduce the deposition rate. Meanwhile, one can also find that the deposition rate decreases with increasing work pressure (Fig. 1(b)). At lower work pressure, particles have lower collision probability and hence they can have a longer mean free path. Therefore, the particles retain sufficient energy to impact the substrate and hence increase the deposition rate.
Fig. 3. The surface roughness of SiOx films prepared at various sputtering parameters: (a) oxygen flow ratio, (b) work pressure, (c) RF power density and (d) deposition time.
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Fig. 4. The SEM micrographs of SiOx films deposited at 40% oxygen flow ratio, 0.27 Pa work pressure for 120 min at various RF power densities: (a) 2.5 W/cm2, (b) 3.7 W/cm2, (c) 4.9 W/cm2 and (d) 5.7 W/cm2.
On the other hand, the deposition rate increases with increasing RF power density (Fig. 1(c)). This tendency can be reasonably explained as follows. At higher RF power density, more Ar+ ions impact the silicon target. Therefore, more silicon ions are sputtered out, and then deposit on the PET substrates reactively. Hence, the deposition rate of SiOx films increases with increasing RF power density. The rapid increase of deposition rate at the first time period between 120 and 180 min, as evidenced in Fig. 1(d), is probably due to more stable adsorptions of molecules on surface sites relatively unoccupied at the initial stage. 3.2. Chemical composition of deposition films Fig. 2(a) shows the O/Si atomic ratio of SiOx films deposited on PET substrate as a function of oxygen flow ratio at the work pressure of 1.06 Pa, RF power density of 3.7 W/cm2 and deposition time of 120 min. The O/Si atomic ratio increases rapidly with increasing oxygen flow ratio, especially within the range of 0–40%. Fig. 2(b) shows the O/Si atomic ratio as a function of RF power density. As can be seen, the O/Si atomic ratio decreases with increasing RF power density. This feature can be explained as follows. Higher oxygen flow ratio means higher partial pressure of oxygen in chamber, and has two
effects. First, more oxygen ions are excited and participate in the reaction. Second, fewer argon ions are excited and sputter fewer silicon ions. These two effects lead to high O/Si ratio. Therefore, the O/Si atomic ratio of SiOx films decreases with increasing RF power density at a constant oxygen flow ratio. It must be remarked that the O/Si atomic ratio of SiOx films does not change much with work pressure varying between 1.5 and 1.06 Pa. 3.3. Surface morphology and roughness of deposition films Fig. 3 shows the surface roughness of SiOx films at various sputtering parameters. The surface roughness is found to increase with increasing oxygen flow ratio, work pressure and deposition time, but decrease with increasing RF power density. This is in agreement with the results found by A. G. Erlat et al. [12]. The surface roughness of thin film is strongly influenced by the kinetic behavior of depositing particles. Particles with higher kinetic energy move faster on the surface and occupy the energetically unstable sites more easily. This reduces the surface roughness. Both oxygen flow ratio and work pressure reduce the kinetic energy of sputtered atoms, while the RF power density has a positive effect.
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The FE–SEM micrographs of SiOx films deposited at various RF power densities are shown in Fig. 4. In examining the surface microstructure qualitatively, the SiOx films deposited at higher RF power density seem to have a denser structure with finer columnar, and to possess shorter and less narrow fissures. The denser structure is usually thought to have better barrier properties, and this will be discussed in the following section. 3.4. Permeation behavior of deposition films The WVTR and OTR of SiOx coated PET in dependence of various sputtering parameters are plotted in Fig. 5. All tendencies of WVTR with parameters are similar to that of OTR for these four parameters. However, the dependences between WVTR/OTR and sputtering parameters are quite dividing. In Fig. 5(a), WVTR/OTR reaches the minima at 40% oxygen flow ratio. The composition of SiOx film deviates widely from silica at low oxygen ratio. The depletion of oxygen is accomplished with more unsatisfied bonds at silicon atom. This enhances the penetration of molecules containing oxygen. At high oxygen flow ratio, the decrease of film thickness results in the raising of WVTR/OTR. Because the O/Si atomic ratio of SiOx films does not change much with work pressure mentioned in Section 3.2, the increase of WVTR/OTR with work pressure as shown in Fig. 5(b) is mainly caused by the decrease in the deposition rate and probably also by the increase of roughness.
As can be seen in Fig. 5(c), the SiOx films deposited at higher RF power density have better performance to prevent the permeation of water vapor and oxygen. Higher RF power density results in thicker and denser films, as previously discussed. However, at much higher power density (5.7 W/cm2), the thermal mismatch between SiOx and PET may induce cracks in nanometer scale at the interface. This inhibits the further reduction of WVTR/OTR. In the recent research by A.G. Erlat et al. [7], the permeation mechanism of oxygen through the composite films was studied. They reported that both the nanoscale defects and sparsely distributed large defects were the main pathways of oxygen through the composite films. In Fig. 5(d), the values of WVTR and OTR are found to decrease with increasing deposition time and then approach steady values when the deposition time is longer than 240 min. This indicates that if the thickness of deposition films reaches a critical level, the WVTR and OTR will have saturated values. This result is consistent with that reported by A.P. Roberts et al. [13]. 3.5. Flexion tests Fig. 6 shows the SEM micrographs of SiOx films deposited at 40% oxygen flow ratio, 0.27 Pa and 4.9 W/cm2 for 240 min and subjected to various flexion cycles. Parallel patterns of cracks of the SiOx films after the flexion test, are apparent. The greater the number of flexion cycles, the more cracks are produced. The distance between cracks decreases
Fig. 5. WVTR and OTR of SiOx coated PET in dependence of various sputtering parameters: (a) oxygen flow ratio, (b) work pressure, (c) RF power density and (d) deposition time. The WVTR and OTR of uncoated PET are 7.2 g/m2 day and 34.6 cc/m2 day, respectively.
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from 60 μm after 100 cycles, down to 30 μm after 1000 cycles. It can also be found that the thicker SiOx films will produce more cracks after the flexion tests. This feature will significantly degrade the permeation behavior of water vapor and oxygen, as shown in Fig. 7. Namely, the transmission rates of water vapor and oxygen increase dramatically with
Fig. 7. (a) The WVTR and (b) OTR of SiOx films deposited at 40% oxygen flow ratio, 0.27 Pa work pressure and 4.9 W/cm2 RF power density for various deposition time, as a function of flexion cycle.
increasing number of flexion cycles, especially in the early 100 cycles. 4. Conclusions The SiOx films have been deposited on PET substrates by RF reactive magnetron sputtering technique. The deposition rate decreases with the increase of both oxygen flow ratio and work pressure, but increases with increasing RF power density. The oxygen flow ratio has more influence on O/Si atomic ratio of films than do work pressure and RF power density. The SiOx films deposited at higher RF power density (4.9 W/cm 2 ), lower work pressure (0.27 Pa) and moderate oxygen flow ratio (40%) have denser structure which performs better in preventing the permeation of water vapor and oxygen gas. Cracks are produced in the SiOx films after the flexion tests for more than 100 cycles. These cracks will severely degrade the film's permeation behavior of water vapor and oxygen. The minimum transmission rates of water vapor and oxygen were found to be 2.6 g/m2 day atm and 15.4 cc/m2 day atm, respectively. Acknowledgment Fig. 6. The SEM micrographs of SiOx film deposited at sputtering parameters of 40% oxygen flow ratio, 0.27 Pa work pressure and 4.9 W/cm2 RF power density for 240 min and subjected to the flexion tests for: (a) 100, (b) 500, and (c) 1000 cycles.
This work was financially supported by the National Science Committee (NSC) of Taiwan/R.O.C., under the auspices of the Targeted Project (No. NSC 92 -2261-E-005017).
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References [1] H. Bartzsch, D. Glob, B. Bocher, P. Frach, K. Goedicke, Surf. Coat. Technol. 174/175 (2003) 774. [2] C. Martinet, V. Paillard, A. Gagnaire, J. Joseph, J. Non-Cryst. Solids 216 (1997) 77. [3] A.L. Brody, Packing Technol. Eng. (Feb. 1994) 44. [4] A. Yializis, M.G. Mikhael, R.E. Ellwanger, Proceedings of the 43rd Annual Technical Conference, Society of Vacuum Coaters, Denver, U.S.A., Apr 15–20 2000, p. 404. [5] B.M. Henry, F. Dinelli, K.-Y. Zhao, C.R.M. Grovenor, O.V. Kolosov, G.A.D. Briggs, A.P. Roberts, R.S. Kumar, R.P. Howson, Thin Solid Films 355–356 (1999) 500. [6] A.F. Jankowski, J.P. Hayes, T.E. Felter, C. Evans, A.J. Nelson, Thin Solid Films 420–421 (2002) 43. [7] Y. Leterrier, L. Boogh, J. Andersons, J.A.E. Manson, J. Polym. Sci., B, Polym. Phys. 35 (1997) 1449.
[8] K. Teshima, Y. Inoue, H. Sugimura, O. Takai, Vacuum 66 (2002) 353. [9] K. Tadanaga, K. Iwashita, T. Minami, J. Sol–Gel Sci. Technol. 6 (1996) 107. [10] F. Casey, A.W. Smith, G. Ellis, Proceedings of the 42nd Annual Technical Conference, Society of Vacuum Coaters, Chicago, U.S.A., Apr 17–22 1999, p. 415. [11] S. Yokoyama, K. Iseki, T. Ohya, S. Konmeda, Y. Yamada, H. Ishihara, Proceedings of the 42nd Annual Technical Conference, Society of Vacuum Coaters, Chicago, U.S.A., Apr 17–22 1999, p. 450. [12] A.G. Erlat, B.-C. Wang, R.J. Spontak, Y. Tropsha, K.D. Mar, D.B. Montgomery, E.A. Vogler, J. Mater. Res. 15 (2000) 704. [13] A.P. Roberts, B.M. Henry, A.P. Sutton, C.R.M. Grovenor, G.A.D. Briggs, T. Miyamoto, M. Kano, Y. Tsukahara, M. Yanaka, J. Membr. Sci. 208 (2002) 75.
Characterization of the silicon oxide thin films deposited on polyethylene terephthalate substrates by radio frequency reactive magnetron sputtering M.-C. Lin, C.-H. Tseng, L.-S. Chang ⁎, D.-S. Wuu Department of Materials Engineering, National Chung Hsing University, 250, Kuo Kuang Road, 40227, Taichung, Taiwan, ROC Received 11 August 2005; received in revised form 22 October 2006; accepted 17 November 2006 Available online 26 December 2006
Abstract Transparent silicon oxide films were deposited on polyethylene terephthalate substrates by means of reactive magnetron sputtering with a mixture of argon and oxygen gases. The influences of process parameters, including the oxygen flow ratio, work pressure, radio frequency (RF) power density and deposition time, on the film properties, such as: deposition rate, morphology, surface roughness, water vapor/oxygen transmission rate and flexibility, were investigated. The experimental results show that the SiOx films deposited at RF power density of 4.9 W/cm2, work pressure of 0.27 Pa and oxygen flow ratio of 40% have better performance in preventing the permeation of water vapor and oxygen. Cracks are produced in the SiOx films after the flexion of more than 100 cycles. The minimum transmission rates of water vapor and oxygen were found to be 2.6 g/m2 day atm and 15.4 cc/m2 day atm, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Silicon oxide; Sputtering; Polyethylene terephthalate (PET); Gas transmission rate
1. Introduction Silicon oxide films have been widely used in industry as coating materials due to their excellent electrical, optical and chemical properties. They are used as passivation layers [1], antireflection layers [2], gas barriers for food and medical packages [3] and recently developed organic flat panel displays [4]. It has also been forecasted that the usage of metal oxide films coated on polyethylene terephthalate (PET) will increase rapidly in the near future [5]. Various methods have been used to deposit these metal oxide films on the polymer substrates, for instance, electron beam evaporation, magnetron sputtering [6], other physical vapor deposition methods [7], plasma enhanced chemical vapor deposition [8], sol–gel [9] and roll-to-roll coating processes [10,11]. Due to its low deposition temperature, simple operation and lesser environmental hazard, magnetron sputtering has become one of the most popular choices among these coating techniques. The magnetron sputtering can produce excellent films as gas barrier layers, in spite of its low deposition rate. The properties ⁎ Corresponding author. E-mail address: [email protected] (L.-S. Chang). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.039
of deposited films are quite dependent on the process parameters, including: oxygen flow ratio, work pressure, radio frequency (RF) power density and deposition time. Therefore, in this study, SiOx films were deposited on PET substrates by means of RF reactive magnetron sputtering. The microstructure, surface roughness of the deposited films and their water vapor/ oxygen transmission rates (WV/OTR) were investigated. Meanwhile, the effects of process parameters during the RF reactive magnetron sputtering were also discussed. 2. Experimental details 2.1. Preparation of SiOx thin films The SiOx films were deposited in an in-house built RF reactive magnetron sputtering system in which a two-inch diameter and 99.999% pure silicon disk was used as the target. PET substrates commercially sold in market (Nan Ya Plastics Corporation, Taiwan) with thickness of 125 μm were employed. The sputtering chamber was pumped to 2.7 × 10− 3 Pa using a high vacuum system with a turbo pump. The substrate holder was rotated around the face-centered axis during sputtering deposition to increase the film uniformity. Table 1 presents the
M.-C. Lin et al. / Thin Solid Films 515 (2007) 4596–4602 Table 1 Sets of sputtering parameters Parameters
Set 1
Oxygen flow ratio (%) Work pressure (Pa) RF power density (W/cm2) Deposition time (min)
10, 20, 40, 40 40 60 1.06 0.27, 0.53, 0.80, 0.27 1.06 3.7 3.7 2.5, 3.7, 4.9, 5.7 120 120 120
Set 2
Set 3
Set 4 40 0.27 4.9 120, 180, 240, 360
sputtering parameters and their values used in this study. The experiments were divided into 4 sets, in which one of the four parameters, the oxygen flow ratio, the work pressure, the RF power density and the deposition time, was varied and the others were fixed, respectively. Before the deposition, the PET surface was plasma pre-treated in the sputtering chamber with the direct current (DC) bias of 300 V generated by the 1 kW MDX low-power DC magnetron driver system for 10 min to clean the substrate surface and improve film adhesion. The substrate temperature during sputtering must be lower than the temperature of glass transition of PET because all specimens were not deformed after the deposition, although no water cooling was introduced in the sample stage and the substrate temperature was not measured. 2.2. Characterization of SiOx thin films The water vapor and oxygen transmission rates were measured using the MOCON instruments, including: an Ox-
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Tran 2/61 and a Permatran-w 3/61 model system with a sample size of 4.5 × 4.5 cm2. The WVTR data were measured at atmospheric pressure and 40 °C with 100% relative humidity, and the OTR data were measured at 40 °C with 0% relative humidity. A Model CP, Seiko Instruments Inc (SPI 3800N) atomic force microscope (AFM) was operated in contact mode to provide topographical images of the SiOx coatings, as well as surface roughness measurements. The Si tips with a tip radius of 10 nm were used for this analysis. The scanned sample area and scanning rate were 9 μm2 and 1 Hz, respectively. One area of each sample was taken. Field-emission scanning electron microscopy (FE–SEM) was used to observe the microstructure. The FE–SEM experiments were conducted using a JEOL JSM6700F operated at 1 keV accelerating voltage. The SiOx surface was coated with a thin platinum layer to prevent excessive charging. The chemical compositions of deposited SiOx films were analyzed by means of X-ray photoelectron spectroscopy (ESCA, PHI 1600). XPS spectra were obtained by using the magnesium anti-cathode of the X-ray tube operated at 15 kV and producing characteristic Al Kα at 1.4866 keV. The composition was calculated from the Si 2p/O 1s peak area in the photoelectron spectrum in which the background was subtracted. Prior to XPS measurements, the 1 cm2 large sample surface was cleaned by ion sputtering with an accelerating voltage of 3 kV and a power of 250 W for 30 s. The photoelectronic spectra were obtained using Al Kα X-rays source operated at 15 kV and 400 W. Estimates of Si and O surface concentrations were calculated from the area of the Si 2p and O 1s peaks after Shirley background subtraction, while no obvious carbon signal was observed. Relative sensitivity factors provided by the manufacturer were used. The pressure in the
Fig. 1. Deposition rates of SiOx films prepared at various sputtering parameters: (a) oxygen flow ratio, (b) work pressure, (c) RF power density and (d) deposition time.
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values was obtained from the XPS analysis of mode compounds. In particular, 1.14 eV and 1.40 eV were adopted as FWHM standard values for Si 2p and O 1s, respectively. The XPS analysis of model compounds provided also an initial estimate for the binding energy values of the different Si 2p and O 1s peaks. The thickness of SiOx films on pure silicon substrates which were put together with the PET substrates into the sputtering chamber was measured by a surface profile analyzer (Veeco Dektak3 ST). The persistency of SiOx thin films on PET against flexion was evaluated by means of an inhouse designed apparatus. Samples were fixed at two opposite sides on stainless rods which were driven by a motor and moved forwards and backwards oppositely. SEM observations and vapor/gas permeation tests were made after the flexion test. 3. Results and discussion 3.1. Deposition rate measurement
Fig. 2. The O/Si atomic ratio of SiOx films deposited on PET substrate as a function of various parameters: (a) oxygen flow ratio and (b) RF power density. The error bars represent the measurement error caused by the signal noise in the photoelectron spectrum.
analysis chamber 6.65 × 10− 8 Pa during spectra acquisition. Unless otherwise specified, all XPS spectra were acquired at a 90° takeoff angle. The full width at half maximum (FWHM)
Fig. 1 shows the deposition rates of SiOx films at various sputtering parameters. It can be found that the deposition rate decreases with increasing oxygen flow ratio from 10% to 60% (Fig. 1(a)). This phenomenon is ascribed to the poisoning of the target surface at higher oxygen flow ratio. The oxide film formed on the target surface will significantly reduce the deposition rate. Meanwhile, one can also find that the deposition rate decreases with increasing work pressure (Fig. 1(b)). At lower work pressure, particles have lower collision probability and hence they can have a longer mean free path. Therefore, the particles retain sufficient energy to impact the substrate and hence increase the deposition rate.
Fig. 3. The surface roughness of SiOx films prepared at various sputtering parameters: (a) oxygen flow ratio, (b) work pressure, (c) RF power density and (d) deposition time.
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Fig. 4. The SEM micrographs of SiOx films deposited at 40% oxygen flow ratio, 0.27 Pa work pressure for 120 min at various RF power densities: (a) 2.5 W/cm2, (b) 3.7 W/cm2, (c) 4.9 W/cm2 and (d) 5.7 W/cm2.
On the other hand, the deposition rate increases with increasing RF power density (Fig. 1(c)). This tendency can be reasonably explained as follows. At higher RF power density, more Ar+ ions impact the silicon target. Therefore, more silicon ions are sputtered out, and then deposit on the PET substrates reactively. Hence, the deposition rate of SiOx films increases with increasing RF power density. The rapid increase of deposition rate at the first time period between 120 and 180 min, as evidenced in Fig. 1(d), is probably due to more stable adsorptions of molecules on surface sites relatively unoccupied at the initial stage. 3.2. Chemical composition of deposition films Fig. 2(a) shows the O/Si atomic ratio of SiOx films deposited on PET substrate as a function of oxygen flow ratio at the work pressure of 1.06 Pa, RF power density of 3.7 W/cm2 and deposition time of 120 min. The O/Si atomic ratio increases rapidly with increasing oxygen flow ratio, especially within the range of 0–40%. Fig. 2(b) shows the O/Si atomic ratio as a function of RF power density. As can be seen, the O/Si atomic ratio decreases with increasing RF power density. This feature can be explained as follows. Higher oxygen flow ratio means higher partial pressure of oxygen in chamber, and has two
effects. First, more oxygen ions are excited and participate in the reaction. Second, fewer argon ions are excited and sputter fewer silicon ions. These two effects lead to high O/Si ratio. Therefore, the O/Si atomic ratio of SiOx films decreases with increasing RF power density at a constant oxygen flow ratio. It must be remarked that the O/Si atomic ratio of SiOx films does not change much with work pressure varying between 1.5 and 1.06 Pa. 3.3. Surface morphology and roughness of deposition films Fig. 3 shows the surface roughness of SiOx films at various sputtering parameters. The surface roughness is found to increase with increasing oxygen flow ratio, work pressure and deposition time, but decrease with increasing RF power density. This is in agreement with the results found by A. G. Erlat et al. [12]. The surface roughness of thin film is strongly influenced by the kinetic behavior of depositing particles. Particles with higher kinetic energy move faster on the surface and occupy the energetically unstable sites more easily. This reduces the surface roughness. Both oxygen flow ratio and work pressure reduce the kinetic energy of sputtered atoms, while the RF power density has a positive effect.
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The FE–SEM micrographs of SiOx films deposited at various RF power densities are shown in Fig. 4. In examining the surface microstructure qualitatively, the SiOx films deposited at higher RF power density seem to have a denser structure with finer columnar, and to possess shorter and less narrow fissures. The denser structure is usually thought to have better barrier properties, and this will be discussed in the following section. 3.4. Permeation behavior of deposition films The WVTR and OTR of SiOx coated PET in dependence of various sputtering parameters are plotted in Fig. 5. All tendencies of WVTR with parameters are similar to that of OTR for these four parameters. However, the dependences between WVTR/OTR and sputtering parameters are quite dividing. In Fig. 5(a), WVTR/OTR reaches the minima at 40% oxygen flow ratio. The composition of SiOx film deviates widely from silica at low oxygen ratio. The depletion of oxygen is accomplished with more unsatisfied bonds at silicon atom. This enhances the penetration of molecules containing oxygen. At high oxygen flow ratio, the decrease of film thickness results in the raising of WVTR/OTR. Because the O/Si atomic ratio of SiOx films does not change much with work pressure mentioned in Section 3.2, the increase of WVTR/OTR with work pressure as shown in Fig. 5(b) is mainly caused by the decrease in the deposition rate and probably also by the increase of roughness.
As can be seen in Fig. 5(c), the SiOx films deposited at higher RF power density have better performance to prevent the permeation of water vapor and oxygen. Higher RF power density results in thicker and denser films, as previously discussed. However, at much higher power density (5.7 W/cm2), the thermal mismatch between SiOx and PET may induce cracks in nanometer scale at the interface. This inhibits the further reduction of WVTR/OTR. In the recent research by A.G. Erlat et al. [7], the permeation mechanism of oxygen through the composite films was studied. They reported that both the nanoscale defects and sparsely distributed large defects were the main pathways of oxygen through the composite films. In Fig. 5(d), the values of WVTR and OTR are found to decrease with increasing deposition time and then approach steady values when the deposition time is longer than 240 min. This indicates that if the thickness of deposition films reaches a critical level, the WVTR and OTR will have saturated values. This result is consistent with that reported by A.P. Roberts et al. [13]. 3.5. Flexion tests Fig. 6 shows the SEM micrographs of SiOx films deposited at 40% oxygen flow ratio, 0.27 Pa and 4.9 W/cm2 for 240 min and subjected to various flexion cycles. Parallel patterns of cracks of the SiOx films after the flexion test, are apparent. The greater the number of flexion cycles, the more cracks are produced. The distance between cracks decreases
Fig. 5. WVTR and OTR of SiOx coated PET in dependence of various sputtering parameters: (a) oxygen flow ratio, (b) work pressure, (c) RF power density and (d) deposition time. The WVTR and OTR of uncoated PET are 7.2 g/m2 day and 34.6 cc/m2 day, respectively.
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from 60 μm after 100 cycles, down to 30 μm after 1000 cycles. It can also be found that the thicker SiOx films will produce more cracks after the flexion tests. This feature will significantly degrade the permeation behavior of water vapor and oxygen, as shown in Fig. 7. Namely, the transmission rates of water vapor and oxygen increase dramatically with
Fig. 7. (a) The WVTR and (b) OTR of SiOx films deposited at 40% oxygen flow ratio, 0.27 Pa work pressure and 4.9 W/cm2 RF power density for various deposition time, as a function of flexion cycle.
increasing number of flexion cycles, especially in the early 100 cycles. 4. Conclusions The SiOx films have been deposited on PET substrates by RF reactive magnetron sputtering technique. The deposition rate decreases with the increase of both oxygen flow ratio and work pressure, but increases with increasing RF power density. The oxygen flow ratio has more influence on O/Si atomic ratio of films than do work pressure and RF power density. The SiOx films deposited at higher RF power density (4.9 W/cm 2 ), lower work pressure (0.27 Pa) and moderate oxygen flow ratio (40%) have denser structure which performs better in preventing the permeation of water vapor and oxygen gas. Cracks are produced in the SiOx films after the flexion tests for more than 100 cycles. These cracks will severely degrade the film's permeation behavior of water vapor and oxygen. The minimum transmission rates of water vapor and oxygen were found to be 2.6 g/m2 day atm and 15.4 cc/m2 day atm, respectively. Acknowledgment Fig. 6. The SEM micrographs of SiOx film deposited at sputtering parameters of 40% oxygen flow ratio, 0.27 Pa work pressure and 4.9 W/cm2 RF power density for 240 min and subjected to the flexion tests for: (a) 100, (b) 500, and (c) 1000 cycles.
This work was financially supported by the National Science Committee (NSC) of Taiwan/R.O.C., under the auspices of the Targeted Project (No. NSC 92 -2261-E-005017).
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References [1] H. Bartzsch, D. Glob, B. Bocher, P. Frach, K. Goedicke, Surf. Coat. Technol. 174/175 (2003) 774. [2] C. Martinet, V. Paillard, A. Gagnaire, J. Joseph, J. Non-Cryst. Solids 216 (1997) 77. [3] A.L. Brody, Packing Technol. Eng. (Feb. 1994) 44. [4] A. Yializis, M.G. Mikhael, R.E. Ellwanger, Proceedings of the 43rd Annual Technical Conference, Society of Vacuum Coaters, Denver, U.S.A., Apr 15–20 2000, p. 404. [5] B.M. Henry, F. Dinelli, K.-Y. Zhao, C.R.M. Grovenor, O.V. Kolosov, G.A.D. Briggs, A.P. Roberts, R.S. Kumar, R.P. Howson, Thin Solid Films 355–356 (1999) 500. [6] A.F. Jankowski, J.P. Hayes, T.E. Felter, C. Evans, A.J. Nelson, Thin Solid Films 420–421 (2002) 43. [7] Y. Leterrier, L. Boogh, J. Andersons, J.A.E. Manson, J. Polym. Sci., B, Polym. Phys. 35 (1997) 1449.
[8] K. Teshima, Y. Inoue, H. Sugimura, O. Takai, Vacuum 66 (2002) 353. [9] K. Tadanaga, K. Iwashita, T. Minami, J. Sol–Gel Sci. Technol. 6 (1996) 107. [10] F. Casey, A.W. Smith, G. Ellis, Proceedings of the 42nd Annual Technical Conference, Society of Vacuum Coaters, Chicago, U.S.A., Apr 17–22 1999, p. 415. [11] S. Yokoyama, K. Iseki, T. Ohya, S. Konmeda, Y. Yamada, H. Ishihara, Proceedings of the 42nd Annual Technical Conference, Society of Vacuum Coaters, Chicago, U.S.A., Apr 17–22 1999, p. 450. [12] A.G. Erlat, B.-C. Wang, R.J. Spontak, Y. Tropsha, K.D. Mar, D.B. Montgomery, E.A. Vogler, J. Mater. Res. 15 (2000) 704. [13] A.P. Roberts, B.M. Henry, A.P. Sutton, C.R.M. Grovenor, G.A.D. Briggs, T. Miyamoto, M. Kano, Y. Tsukahara, M. Yanaka, J. Membr. Sci. 208 (2002) 75.