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Effects of nitrogen partial pressure on titanium oxynitride films deposited by reactive RF magnetron sputtering onto PET substrates
Surface & Coatings Technology 202 (2008) 5440–5443
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Effects of nitrogen partial pressure on titanium oxynitride films deposited by reactive RF magnetron sputtering onto PET substrates M.C. Lin a, L.-S. Chang a,⁎, H.C. Lin b a b
Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan, ROC Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, ROC
A R T I C L E
I N F O
Available online 14 June 2008 Keywords: Titanium oxynitride Magnetron sputtering Polyethylene terephthalate Gas permeation
A B S T R A C T Titanium oxynitride (TiNxOy) films have been deposited onto polyethylene terephthalate (PET) substrates by reactive radio frequency (RF) magnetron sputtering. The influence of the nitrogen (N2) partial pressure in the discharge atmosphere, with a set pressure of 0.133 Pa, was examined. Other deposition conditions were held constant. The deposition rate of the films, which exhibit an island-type morphology, was found to decrease with increasing N2 partial pressure. This concurred with an increase in the surface roughness at higher N2 partial pressure. The TiNxOy films deposited at N2 partial pressures from 0.26 × 10− 1 Pa to 0.8 × 10− 1 Pa possess Ti:N:O ratio of about 1:0.9:0.8 to 1:1.2:0.7. At the lowest N2 partial pressure of 0.26 × 10− 1 Pa, the water vapor (WV) and oxygen transmission rates (OTR) of the TiNxOy films reached values as low as 0.31 g/m2-day-atm and 0.62 cc/m2-day-atm, respectively; these values are about 16 and 50 times lower than those of the uncoated PET substrate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The increased demand in digital display products has led to a great deal of work pertaining to the development and investigation of suitable substrate materials. These substrate materials are required to possess certain properties, such as portability, durability, flexibility and optical transparency. The three most commonly explored groups of flexible substrate materials include the ultra-thin metals, ultra-thin glass [1] and polymer substrates [2–4]. Of these, polymer substrates are considered to be the most attractive due to their more versatile properties. Nevertheless, most polymer substrates have the drawbacks of high thermal expansion, low thermal resistance, low mechanical strength and insufficient resistance to gas permeation. These negative aspects of polymer substrates need to be addressed in order to improve their successful operation in display devices. The transition-metal oxynitride (TMeNxOy) films, which, due to their colorific and optical properties, chemical stability and good adhesion to polymers, has previously been widely employed as a wear resistant, anti-reflective, decorative and/or diffusion barrier coating for polymer components [5–10]. TiNxOy films are the representative transitionmetal oxynitrides and can be deposited onto substrates by various coating techniques [5, 6,9–10]. As deposition temperature and the quality of TiNxOy films can be controlled effectively by varying the magnetron sputtering parameters, it is considered to be advantageous among different deposition techniques.
⁎ Corresponding author. Tel.: +886 4 22840500 405; fax: +886 4 2285 2433. E-mail address: [email protected] (L.-S. Chang). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.066
Polyethylene terepthalate (PET) polymer is a promising material to be used as the flexible substrate due to its excellent optical transmission, low thermal expansion, high toughness and low cost. In previous work by the authors [11], it was found that TiNxOy films could be successfully deposited onto PET substrates by the RF reactive magnetron sputtering technique. In that work, the influences of RF power density and applied substrate bias on the properties of the TiNxOy films were investigated. In the present study, the N2 partial pressure shall be varied to assess its influence on TiNxOy films deposited onto PET by reactive RF magnetron sputtering. The microstructure, composition properties and gas permeation behavior of the TiNxOy films shall be discussed. 2. Experimental details TiNxOy films were deposited by a reactive magnetron sputtering. A 5 cm diameter, 99.999% pure titanium target was used. The PET substrates (size: 4.5 cm × 4.5 cm × 100 μm) were used. The substrate to target distance was kept at 15 cm and the holder was rotated at a speed of 10 rpm. Argon was used as the sputter gas and nitrogen as the reactive gas during sputtering. Meanwhile, oxygen molecules existing residually in chamber or coming from the PET substrate would contribute to form the TiNxOy films during deposition. Table 1 presents the sputtering parameters employed. In essence, the N2 partial pressure, i.e. N2/Ar + N2, was varied while the other deposition parameters were held constant. To clean the substrate surface prior to the deposition and improve film's adhesion, the PET surface was pre-treated by Ar plasma in sputtering chamber for 10 min with a direct current (DC) bias of −300 V.
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Table 1 Deposition parameters Target Substrate RF power density Sputter gas N2 partial pressure, i.e. N2/(N2 + Ar) Base pressure Working pressure Deposition time
Ti target with diameter of 5 cm PET 7 W/cm2 Ar = 10 sccm N2 = 2.5–15 sccm 0.26–0.8 × 10− 1 Pa 1.33 × 10− 3 Pa 1.33 × 10− 1 Pa 60 min
The plasma was analyzed using an optical emission spectrometer. The optical emission spectrum was recorded over a wavelength range of 400 to 800 nm. Water vapor (WV) and oxygen transmission rates (OTR) were measured by a Permatran-w 3/61 and an Ox-Tran 2/ 61model system, respectively. Both transmission rate measurements were carried out at atmospheric pressure and at 40 °C, and under relative humidities of 100 and 0%, respectively. A multi-function scanning probe microscope was used to analyze the film's roughness. The thickness was measured using a surface profile analyzer. The film thickness was determined from TiNxOy films deposited on pure silicon substrates, placed alongside the PET substrates. The field-emission scanning electron microscope, used for studying coating morphology and microstructure, was operated with a 3 keV accelerating voltage. The chemical compositions of the deposited TiNxOy films were analyzed by X-ray photoelectron spectroscopy (XPS). The XPS spectra were obtained using an Al Kα X-ray operated at 15 kV and 400 Watt. The Ti, N and O concentrations in film were quantified from the areas under the Ti 2p, N 1s and O 1s characteristic signals in the photoelectron spectrum, after carrying out the Shirley background subtraction. Relative sensitivity factors provided by the manufacturer were employed. The sample size and surface etching time were 1 cm2 and 30 s, respectively. The pressure in the analysis chamber was 8 × 10− 8 Pa. 3. Results and discussion 3.1. Plasma analysis and chemical composition The peak intensities of Ti (735 nm), N+2 (661 nm), and Ar (738 nm) are plotted as a function of the N2 partial pressure, i.e. N2/ (Ar + N2), in
Fig. 1. The intensity of the Ar, Ti and N+2 OES peak as a function of nitrogen partial pressure.
Fig. 2. Chemical compositions of TiNxOy films deposited on PET as a function of nitrogen partial pressure.
Fig. 1 [12]. It can be seen that the intensity of N+2 emission peak increases with the nitrogen partial pressure. On the contrary, the intensities of Ti and Ar elements are found to decrease. It appears reasonable that there will be an increase in the number of N+2 compared to Ar for discharges of higher nitrogen concentrations. These variations of OES peak intensities for various species imply that the electron temperature and the corresponding film's surface temperatures will also be varied with nitrogen partial pressure. This will affect significantly the properties of deposited films. Based on quantitative XPS analysis, shown in Fig. 2, the TiNxOy films deposited at N2 partial pressure from 0.26 × 10− 1 Pa to 0.8 × 10− 1 Pa possess Ti:N:O ratio of about 1:0.9:0.8 to 1:1.2:0.7. This is reasonable in view of the increase in the N+2 peak intensity as a function of N2 partial pressure in Fig. 1. In addition, the sputtering yield of titanium target to Ar+ is higher than that to N+2 [13,14]. At higher N2 partial pressure, the N+2/Ar+ ratio increases and hence the sputtering yield of Ti
Fig. 3. Deposition rates of TiNxOy films on PET as a function of nitrogen partial pressure.
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M.C. Lin et al. / Surface & Coatings Technology 202 (2008) 5440–5443
Fig. 4. FE-SEM plane view and cross-section micrographs of TiNxOy films deposited on PET at nitrogen partial pressure of (a) 0.26 × 10− 1 Pa, (b) 0.66 × 10− 1 Pa, (c) 0.8 × 10− 1 Pa.
atoms reduces. Therefore, Ti and O elements decrease with increasing N2 partial pressure in the deposited TiNxOy film. 3.2. Deposition rate and surface morphology Fig. 3 shows the deposition rate of the TiNxOy films as a function of N2 partial pressure. In Fig. 3, one can find that the deposition rate decreases significantly with increasing nitrogen partial pressure. This feature is explained as following. It is known that the sputtering yield of titanium target to argon is higher than to nitrogen [13]. Therefore, the higher nitrogen partial pressure (from 0.66 × 10− 1 Pa to 0.8 × 10− 1 Pa) is increased, more current is carried by nitrogen ion in the plasma, as shown by Fig. 1, thus the depositing rate is reduced. Fig. 4 shows the FE-SEM plane view and cross-section micrographs of TiNxOy films deposited on PET substrates at various N2 partial pressures. It can be seen that the deposited TiNxOy films exhibit a dense, island-type morphology. Fig. 4(a–c) shows cross-sectional FESEM micrographs of the TiNxOy films deposited on PET at N2 partial pressure of 0.26, 0.66 and 0.8 × 10− 1 Pa. By examining Fig. 4(a–c), one can see that the thickness of TiNxOy films indeed decreases with increasing nitrogen partial pressure; from about 76 to 46 nm for 0.26 to 0.8 × 10− 1 Pa. This observation is consistent with the results of Fig. 3.
Fig. 5. RMS roughness of TiNxOy film deposited onto PET with nitrogen partial pressure at a RF power density of 7 W/cm2.
M.C. Lin et al. / Surface & Coatings Technology 202 (2008) 5440–5443
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WVTR and OTR of PET uncoated with TiNxOy films as a function of nitrogen partial pressure are plotted in Fig. 6. It can be seen that both the WVTR and OTR of the PET substrates are reduced significantly after the deposition of the TiNxOy films. From further examination of this figure, one can see that the WVTR and OTR of the TiNxOy films increase with increasing nitrogen partial pressure. This phenomenon indicates that thickness and quality of the film may have an effect on its resistance against gas permeation. As discussed in Sections 3.2, both the deposition rate of the TiNxOy films decreases and the resultant films are rougher at higher nitrogen partial pressures. The thinner, rougher characteristics of TiNxOy films produced at higher nitrogen partial pressures will be expected to reduce the film's resistance against permeation of water vapor and oxygen. The thickest and most uniform TiNxOy films, i.e. those deposited at the lowest nitrogen partial pressure, can reach values as low as 0.31 g/m2-day-atm and 0.62 cc/m2-day-atm, respectively. These values of WVTR and OTR are about 16 and 50 times lower than those of the uncoated PET substrate. 4. Conclusions TiNxOy films have been successfully deposited on PET substrates by RF reactive magnetron sputtering. The deposition rate decreases significantly with increasing nitrogen partial pressure. This was attributed to the decrease in the number of Ar ions in the plasma at higher nitrogen partial pressures. The maximum deposition rate in this study was found to be 1.3 nm/min, and occurred at the lowest nitrogen partial pressure of 0.26 × 10− 1 Pa. The TiNxOy films deposited on PET substrate exhibited an island-type morphology. The surface roughness and nitrogen content of the deposited films increased with an increased nitrogen partial pressure. The Ti and O contents of TiNxOy films decrease with nitrogen partial pressure. Both the thickness and quality of the TiNxOy films were found to have important effects on their resistance against gas permeation. The WVTR and OTR of TiNxOy films in this study were up to about 16 and 50 times lower than those of the uncoated PET substrate. Acknowledgement This work was financially supported by the National Science Committee (NSC) of Taiwan/R.O.C., under the auspices of the Targeted Project (No. NSC94-2216-E-005-005). References Fig. 6. (a) WVTR and (b) OTR of TiNxOy films deposited as a function of nitrogen partial pressure.
Furthermore, from Fig. 4(a–c), one can see that all of the deposited films exhibit a columnar structure. The SPM determined RMS roughness, also provided in Fig. 5, is found to increase from 2.7 to 4.8 nm for the nitrogen partial pressures of 0.26 to 0.8 × 10− 1 Pa. Fig. 5 demonstrate that the TiNxOy films deposited on PET substrates possess rougher surface morphologies at higher nitrogen partial pressures. This phenomenon can be explained by the higher nitrogen concentration that the sputtering yield of Ti atoms is more reduced. This feature will decrease the deposition of TiNxOy on the substrate and abate the transverse movement of TiNxOy to a more stable position. Hence, this feature, and the shielding effect will make TiNxOy films exhibit less uniform clusters and rougher surface morphologies at higher nitrogen partial pressure. 3.3. Gas permeation The WVTR and OTR of the uncoated PET substrate were measured to be 5.53 g/m2-day-atm and 28.09 cc/m2-day-atm, respectively. The
[1] M.D. Joong Auch, O.K. Soo, G. Ewald, C.S. Jin, Thin Solid Film 417 (2002) 47. [2] A.G. Eralt, B.M. Henry, J.J. Ingram, D.B. Mountain, A. McGuigan, R.P. Howson, C.R.M. Grovenor, G.A.D. Briggs, Y. Tsukahara, Thin Solid Film 388 (2001) 78. [3] D.S. Wuu, W.C. Lo, L.S. Change, R.H. Horng, Thin Solid Film 468 (2004) 105. [4] J.G. Lee, Y.G. Seol, N.-E Lee, Thin Solid Film 515 (2006) 805. [5] F. Vaz, P. Cerqueira, L. Rebouta, S.M.C. Nascimento, E. Alves, Ph. Goudeau, J.P. Riviere, K. Psichow, J. de Rijk, Thin Solid Film 447–448 (2004) 449. [6] Mehdi H. Kazemeini, Alexander A. Berezin, Nobuhiko Fukuhara, Thin Solid Film 372 (2000) 70. [7] N. Martin, O. Banakh, A.M.E. Santo, S. Springer, R. Sanjines, J. Takadoum, F. Levy, Appl. Surf. Sci. 185 (2001) 123. [8] F. Vaz, P. Cerqueira, L. Rebouta, S.M.C. Nascimento, E. Alves, Ph. Goudeau, J.P. Riviere, Surf. Coat. Technol. 174–175 (2003) 197. [9] A. Bittar, D. Cochrane, S. Caughley, I. Vickeridge, J. Vac. Sci. Technol. A. 15 (2) (1997) 223. [10] M. Lazaror, P. Raths, H. Metager, W. Spirkl, J. Appl. Phys. 77 (5) (1995) 2133. [11] M.-C. Lin, L.-S. Chang, H.C. Lin, Appl. Surf. Sci. 254 (2008) 3059. [12] A.N. Saidel, V.K. Prokofiev, S.M. Raiski, Tables of Spectrum Lines, 1955 Berlin. [13] Brian Chapman’s, Glow Discharge Processes, John Wiley & Sons, Inc, New York, 1980. [14] Rajiv Ranjan, J.P. Allain, M.R. Hendricks, D.N. Ruzic, J. Vac. Sci. Technol. A. 19 (2001) 1004.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Effects of nitrogen partial pressure on titanium oxynitride films deposited by reactive RF magnetron sputtering onto PET substrates M.C. Lin a, L.-S. Chang a,⁎, H.C. Lin b a b
Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan, ROC Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, ROC
A R T I C L E
I N F O
Available online 14 June 2008 Keywords: Titanium oxynitride Magnetron sputtering Polyethylene terephthalate Gas permeation
A B S T R A C T Titanium oxynitride (TiNxOy) films have been deposited onto polyethylene terephthalate (PET) substrates by reactive radio frequency (RF) magnetron sputtering. The influence of the nitrogen (N2) partial pressure in the discharge atmosphere, with a set pressure of 0.133 Pa, was examined. Other deposition conditions were held constant. The deposition rate of the films, which exhibit an island-type morphology, was found to decrease with increasing N2 partial pressure. This concurred with an increase in the surface roughness at higher N2 partial pressure. The TiNxOy films deposited at N2 partial pressures from 0.26 × 10− 1 Pa to 0.8 × 10− 1 Pa possess Ti:N:O ratio of about 1:0.9:0.8 to 1:1.2:0.7. At the lowest N2 partial pressure of 0.26 × 10− 1 Pa, the water vapor (WV) and oxygen transmission rates (OTR) of the TiNxOy films reached values as low as 0.31 g/m2-day-atm and 0.62 cc/m2-day-atm, respectively; these values are about 16 and 50 times lower than those of the uncoated PET substrate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The increased demand in digital display products has led to a great deal of work pertaining to the development and investigation of suitable substrate materials. These substrate materials are required to possess certain properties, such as portability, durability, flexibility and optical transparency. The three most commonly explored groups of flexible substrate materials include the ultra-thin metals, ultra-thin glass [1] and polymer substrates [2–4]. Of these, polymer substrates are considered to be the most attractive due to their more versatile properties. Nevertheless, most polymer substrates have the drawbacks of high thermal expansion, low thermal resistance, low mechanical strength and insufficient resistance to gas permeation. These negative aspects of polymer substrates need to be addressed in order to improve their successful operation in display devices. The transition-metal oxynitride (TMeNxOy) films, which, due to their colorific and optical properties, chemical stability and good adhesion to polymers, has previously been widely employed as a wear resistant, anti-reflective, decorative and/or diffusion barrier coating for polymer components [5–10]. TiNxOy films are the representative transitionmetal oxynitrides and can be deposited onto substrates by various coating techniques [5, 6,9–10]. As deposition temperature and the quality of TiNxOy films can be controlled effectively by varying the magnetron sputtering parameters, it is considered to be advantageous among different deposition techniques.
⁎ Corresponding author. Tel.: +886 4 22840500 405; fax: +886 4 2285 2433. E-mail address: [email protected] (L.-S. Chang). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.066
Polyethylene terepthalate (PET) polymer is a promising material to be used as the flexible substrate due to its excellent optical transmission, low thermal expansion, high toughness and low cost. In previous work by the authors [11], it was found that TiNxOy films could be successfully deposited onto PET substrates by the RF reactive magnetron sputtering technique. In that work, the influences of RF power density and applied substrate bias on the properties of the TiNxOy films were investigated. In the present study, the N2 partial pressure shall be varied to assess its influence on TiNxOy films deposited onto PET by reactive RF magnetron sputtering. The microstructure, composition properties and gas permeation behavior of the TiNxOy films shall be discussed. 2. Experimental details TiNxOy films were deposited by a reactive magnetron sputtering. A 5 cm diameter, 99.999% pure titanium target was used. The PET substrates (size: 4.5 cm × 4.5 cm × 100 μm) were used. The substrate to target distance was kept at 15 cm and the holder was rotated at a speed of 10 rpm. Argon was used as the sputter gas and nitrogen as the reactive gas during sputtering. Meanwhile, oxygen molecules existing residually in chamber or coming from the PET substrate would contribute to form the TiNxOy films during deposition. Table 1 presents the sputtering parameters employed. In essence, the N2 partial pressure, i.e. N2/Ar + N2, was varied while the other deposition parameters were held constant. To clean the substrate surface prior to the deposition and improve film's adhesion, the PET surface was pre-treated by Ar plasma in sputtering chamber for 10 min with a direct current (DC) bias of −300 V.
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Table 1 Deposition parameters Target Substrate RF power density Sputter gas N2 partial pressure, i.e. N2/(N2 + Ar) Base pressure Working pressure Deposition time
Ti target with diameter of 5 cm PET 7 W/cm2 Ar = 10 sccm N2 = 2.5–15 sccm 0.26–0.8 × 10− 1 Pa 1.33 × 10− 3 Pa 1.33 × 10− 1 Pa 60 min
The plasma was analyzed using an optical emission spectrometer. The optical emission spectrum was recorded over a wavelength range of 400 to 800 nm. Water vapor (WV) and oxygen transmission rates (OTR) were measured by a Permatran-w 3/61 and an Ox-Tran 2/ 61model system, respectively. Both transmission rate measurements were carried out at atmospheric pressure and at 40 °C, and under relative humidities of 100 and 0%, respectively. A multi-function scanning probe microscope was used to analyze the film's roughness. The thickness was measured using a surface profile analyzer. The film thickness was determined from TiNxOy films deposited on pure silicon substrates, placed alongside the PET substrates. The field-emission scanning electron microscope, used for studying coating morphology and microstructure, was operated with a 3 keV accelerating voltage. The chemical compositions of the deposited TiNxOy films were analyzed by X-ray photoelectron spectroscopy (XPS). The XPS spectra were obtained using an Al Kα X-ray operated at 15 kV and 400 Watt. The Ti, N and O concentrations in film were quantified from the areas under the Ti 2p, N 1s and O 1s characteristic signals in the photoelectron spectrum, after carrying out the Shirley background subtraction. Relative sensitivity factors provided by the manufacturer were employed. The sample size and surface etching time were 1 cm2 and 30 s, respectively. The pressure in the analysis chamber was 8 × 10− 8 Pa. 3. Results and discussion 3.1. Plasma analysis and chemical composition The peak intensities of Ti (735 nm), N+2 (661 nm), and Ar (738 nm) are plotted as a function of the N2 partial pressure, i.e. N2/ (Ar + N2), in
Fig. 1. The intensity of the Ar, Ti and N+2 OES peak as a function of nitrogen partial pressure.
Fig. 2. Chemical compositions of TiNxOy films deposited on PET as a function of nitrogen partial pressure.
Fig. 1 [12]. It can be seen that the intensity of N+2 emission peak increases with the nitrogen partial pressure. On the contrary, the intensities of Ti and Ar elements are found to decrease. It appears reasonable that there will be an increase in the number of N+2 compared to Ar for discharges of higher nitrogen concentrations. These variations of OES peak intensities for various species imply that the electron temperature and the corresponding film's surface temperatures will also be varied with nitrogen partial pressure. This will affect significantly the properties of deposited films. Based on quantitative XPS analysis, shown in Fig. 2, the TiNxOy films deposited at N2 partial pressure from 0.26 × 10− 1 Pa to 0.8 × 10− 1 Pa possess Ti:N:O ratio of about 1:0.9:0.8 to 1:1.2:0.7. This is reasonable in view of the increase in the N+2 peak intensity as a function of N2 partial pressure in Fig. 1. In addition, the sputtering yield of titanium target to Ar+ is higher than that to N+2 [13,14]. At higher N2 partial pressure, the N+2/Ar+ ratio increases and hence the sputtering yield of Ti
Fig. 3. Deposition rates of TiNxOy films on PET as a function of nitrogen partial pressure.
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Fig. 4. FE-SEM plane view and cross-section micrographs of TiNxOy films deposited on PET at nitrogen partial pressure of (a) 0.26 × 10− 1 Pa, (b) 0.66 × 10− 1 Pa, (c) 0.8 × 10− 1 Pa.
atoms reduces. Therefore, Ti and O elements decrease with increasing N2 partial pressure in the deposited TiNxOy film. 3.2. Deposition rate and surface morphology Fig. 3 shows the deposition rate of the TiNxOy films as a function of N2 partial pressure. In Fig. 3, one can find that the deposition rate decreases significantly with increasing nitrogen partial pressure. This feature is explained as following. It is known that the sputtering yield of titanium target to argon is higher than to nitrogen [13]. Therefore, the higher nitrogen partial pressure (from 0.66 × 10− 1 Pa to 0.8 × 10− 1 Pa) is increased, more current is carried by nitrogen ion in the plasma, as shown by Fig. 1, thus the depositing rate is reduced. Fig. 4 shows the FE-SEM plane view and cross-section micrographs of TiNxOy films deposited on PET substrates at various N2 partial pressures. It can be seen that the deposited TiNxOy films exhibit a dense, island-type morphology. Fig. 4(a–c) shows cross-sectional FESEM micrographs of the TiNxOy films deposited on PET at N2 partial pressure of 0.26, 0.66 and 0.8 × 10− 1 Pa. By examining Fig. 4(a–c), one can see that the thickness of TiNxOy films indeed decreases with increasing nitrogen partial pressure; from about 76 to 46 nm for 0.26 to 0.8 × 10− 1 Pa. This observation is consistent with the results of Fig. 3.
Fig. 5. RMS roughness of TiNxOy film deposited onto PET with nitrogen partial pressure at a RF power density of 7 W/cm2.
M.C. Lin et al. / Surface & Coatings Technology 202 (2008) 5440–5443
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WVTR and OTR of PET uncoated with TiNxOy films as a function of nitrogen partial pressure are plotted in Fig. 6. It can be seen that both the WVTR and OTR of the PET substrates are reduced significantly after the deposition of the TiNxOy films. From further examination of this figure, one can see that the WVTR and OTR of the TiNxOy films increase with increasing nitrogen partial pressure. This phenomenon indicates that thickness and quality of the film may have an effect on its resistance against gas permeation. As discussed in Sections 3.2, both the deposition rate of the TiNxOy films decreases and the resultant films are rougher at higher nitrogen partial pressures. The thinner, rougher characteristics of TiNxOy films produced at higher nitrogen partial pressures will be expected to reduce the film's resistance against permeation of water vapor and oxygen. The thickest and most uniform TiNxOy films, i.e. those deposited at the lowest nitrogen partial pressure, can reach values as low as 0.31 g/m2-day-atm and 0.62 cc/m2-day-atm, respectively. These values of WVTR and OTR are about 16 and 50 times lower than those of the uncoated PET substrate. 4. Conclusions TiNxOy films have been successfully deposited on PET substrates by RF reactive magnetron sputtering. The deposition rate decreases significantly with increasing nitrogen partial pressure. This was attributed to the decrease in the number of Ar ions in the plasma at higher nitrogen partial pressures. The maximum deposition rate in this study was found to be 1.3 nm/min, and occurred at the lowest nitrogen partial pressure of 0.26 × 10− 1 Pa. The TiNxOy films deposited on PET substrate exhibited an island-type morphology. The surface roughness and nitrogen content of the deposited films increased with an increased nitrogen partial pressure. The Ti and O contents of TiNxOy films decrease with nitrogen partial pressure. Both the thickness and quality of the TiNxOy films were found to have important effects on their resistance against gas permeation. The WVTR and OTR of TiNxOy films in this study were up to about 16 and 50 times lower than those of the uncoated PET substrate. Acknowledgement This work was financially supported by the National Science Committee (NSC) of Taiwan/R.O.C., under the auspices of the Targeted Project (No. NSC94-2216-E-005-005). References Fig. 6. (a) WVTR and (b) OTR of TiNxOy films deposited as a function of nitrogen partial pressure.
Furthermore, from Fig. 4(a–c), one can see that all of the deposited films exhibit a columnar structure. The SPM determined RMS roughness, also provided in Fig. 5, is found to increase from 2.7 to 4.8 nm for the nitrogen partial pressures of 0.26 to 0.8 × 10− 1 Pa. Fig. 5 demonstrate that the TiNxOy films deposited on PET substrates possess rougher surface morphologies at higher nitrogen partial pressures. This phenomenon can be explained by the higher nitrogen concentration that the sputtering yield of Ti atoms is more reduced. This feature will decrease the deposition of TiNxOy on the substrate and abate the transverse movement of TiNxOy to a more stable position. Hence, this feature, and the shielding effect will make TiNxOy films exhibit less uniform clusters and rougher surface morphologies at higher nitrogen partial pressure. 3.3. Gas permeation The WVTR and OTR of the uncoated PET substrate were measured to be 5.53 g/m2-day-atm and 28.09 cc/m2-day-atm, respectively. The
[1] M.D. Joong Auch, O.K. Soo, G. Ewald, C.S. Jin, Thin Solid Film 417 (2002) 47. [2] A.G. Eralt, B.M. Henry, J.J. Ingram, D.B. Mountain, A. McGuigan, R.P. Howson, C.R.M. Grovenor, G.A.D. Briggs, Y. Tsukahara, Thin Solid Film 388 (2001) 78. [3] D.S. Wuu, W.C. Lo, L.S. Change, R.H. Horng, Thin Solid Film 468 (2004) 105. [4] J.G. Lee, Y.G. Seol, N.-E Lee, Thin Solid Film 515 (2006) 805. [5] F. Vaz, P. Cerqueira, L. Rebouta, S.M.C. Nascimento, E. Alves, Ph. Goudeau, J.P. Riviere, K. Psichow, J. de Rijk, Thin Solid Film 447–448 (2004) 449. [6] Mehdi H. Kazemeini, Alexander A. Berezin, Nobuhiko Fukuhara, Thin Solid Film 372 (2000) 70. [7] N. Martin, O. Banakh, A.M.E. Santo, S. Springer, R. Sanjines, J. Takadoum, F. Levy, Appl. Surf. Sci. 185 (2001) 123. [8] F. Vaz, P. Cerqueira, L. Rebouta, S.M.C. Nascimento, E. Alves, Ph. Goudeau, J.P. Riviere, Surf. Coat. Technol. 174–175 (2003) 197. [9] A. Bittar, D. Cochrane, S. Caughley, I. Vickeridge, J. Vac. Sci. Technol. A. 15 (2) (1997) 223. [10] M. Lazaror, P. Raths, H. Metager, W. Spirkl, J. Appl. Phys. 77 (5) (1995) 2133. [11] M.-C. Lin, L.-S. Chang, H.C. Lin, Appl. Surf. Sci. 254 (2008) 3059. [12] A.N. Saidel, V.K. Prokofiev, S.M. Raiski, Tables of Spectrum Lines, 1955 Berlin. [13] Brian Chapman’s, Glow Discharge Processes, John Wiley & Sons, Inc, New York, 1980. [14] Rajiv Ranjan, J.P. Allain, M.R. Hendricks, D.N. Ruzic, J. Vac. Sci. Technol. A. 19 (2001) 1004.