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Investigation of atmospheric-pressure plasma deposited SiOx films on polymeric substrates
Thin Solid Films 517 (2009) 5141–5145
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
Investigation of atmospheric-pressure plasma deposited SiOx films on polymeric substrates Chun Huang a,⁎, Chi-Hung Liu b, Chun-Hsien Su b, Wen-Tung Hsu b, Shin-Yi Wu a a b
Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, 135 Yuan-Tung Road, Chung-Li, Taiwan 32003, ROC Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan 310, ROC
a r t i c l e
i n f o
Available online 16 March 2009 Keywords: Plasma chemical vapor deposition Hardness Transparency Silicon oxide Surface characteristics
a b s t r a c t The novel technique of plasma chemical vapor deposition without using vacuum chamber was investigated to deposit SiOx films on polymeric substrates through tetraethoxysilane (TEOS)/Air atmospheric-pressure plasma (APP) glow discharge. Depending on the proper deposition parameters, thin and smooth SiOx films on polycarbonate substrates were prepared. The atmospheric-pressure plasma deposited SiOx films obtained the desirable transparency in the visible and increased absorption in UV region. The surface characteristics of APP deposited SiOx films were examined by various surface analysis methods including FTIR, XPS, and SEM. It is shown that SiOx films exhibited low porosity and admirable hardness for optical applications. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon oxide (SiOx) films have attracted much attention because of their specific chemical structure and admirable electrical insulation characteristics [1,2]. The unique properties of SiOx films make it applicable as an inter-metal dielectric [3], gas barrier-layer [4], and protective coating [5]. In general, the SiOx film can be obtained by thermal chemical vapor deposition or sol-gel methods [6,7]. These conventional coating methods have been well-studied, well-documented, and are being extensively utilized in both academic and industrial fields. It has been recognized; however, that these conventional coating methods have their limitations or disadvantages. For instance, the inherently high processing temperatures (typically over 400 °C) of these methods are unsuitable for articles made of heat sensitive materials such as polymers and fabrics. Transparent polymeric substrates are widely used for optical applications in recent years owing to their lightweight, low cost, and similar light transmission characteristics to glass. However, the components of polymeric substrates are intrinsically soft, and thus the usage of plastic is limited to moderately mild applications. To improve the inherent restrictions of polymeric substrates, a variety of the coating methods are now being investigated. The major requirement of the coating methods is to achieve low temperature processing on polymeric substrates. The maximum coating temperature is limited by the thermal stability of the polymeric substrates. To overcome thermal stability issue, low temperature plasma enhanced chemical vapor deposition (PECVD) under vacuum system was used to perform SiOx film on polymeric substrates with several advantages such as dry process and thickness control. Because of the existence of these ⁎ Corresponding author. E-mail address: [email protected] (C. Huang). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.054
merits, low temperature plasma enhanced chemical vapor deposition technique has proven to be an effective coating method [8]. In spite of the excellent coating efficiency, the currently available plasma enhanced chemical vapor deposition processes with regard to coating process have several major limitations, such as the restricted volume of the plasma reactor and one or more vacuum and chemical cycles required. If it is possible to be stably operated under atmosphericpressure, not only decreasing in processing costs but also increasing in productivity could be obtained. To address these demands, we present new atmospheric-pressure plasma (APP) jet system and the deposition of SiOx films utilizing this system. In this investigation, we proposed the remote atmosphericpressure plasma deposition. The deposition of remote APP jet to form SiOx films on polymeric substrates simultaneously achieves the advantages that the film structures are kept relatively constant and low temperature nature for coating processing. The influence of process parameters on the characteristics of SiOx films deposited in TEOS/Air plasma at atmospheric-pressure was studied in this investigation. Since the chemical structure of atmospheric-pressure plasma deposited SiOx film determined its properties such as the transmittance and hardness, FTIR, XPS, and SEM were used to examine the effect of process parameters on film structure and its surface characteristics, respectively. Furthermore, the relationship between the hardness, the atomic oxygen to silicon ratio (the O/Si ratio), and the porosity degree of the SiOx films were also studied. 2. Experimental 2.1. Atmospheric pressure plasma deposition system SiOx films were deposited in an atmospheric-pressure plasma jet system as shown in Fig. 1. This system contains discharge chamber with
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2.2. Film characterization and analysis The thickness of the atmospheric-pressure plasma deposited SiOx film was measured by the optical thin-film thickness detector (Mission Peak Optics MP100-S) at a wavelength of 632.8 nm. The chemical structures of the plasma deposited SiOx films deposited were characterized by the Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer LX 20000G). Each spectrum was obtained from an average of 32 scans in the range of 400–4000 cm− 1 at a resolution of 4 cm− 1. The surface composition of APP deposited SiOx film was investigated by X-ray photoelectron spectroscope (XPS) with Mg Kα source (1253.6 eV). UV–VIS transmission detection was carried out using a GBC Cintra 202 UV spectrophotometer to determine the transmittance of APP deposited SiOx film. The surface morphology and roughness of the plasma deposited SiOx film were observed by scanning electron microscope (SEM), respectively. The hardness of atmospheric-pressure plasma deposited SiOx film was determined by pencil test (loading 500 g) according to ASTM D3363-05 and meantime the scratch sample seen with an optical microscope. The pencil hardness is from 9H to 6B (hard to soft). 3. Results and discussion 3.1. Deposition rate of atmospheric plasma chemical vapor deposition process Fig. 1. Schematic diagram of the atmospheric-pressure plasma jet deposition system.
a proprietary design. An electrical field was applied to the two electrodes located inside the chamber to ignite the plasma glow discharge by a 16 kHz AC power supply. Polycarbonate (PC 4.0 × 4.0 cm) films were used as substrates and cleaned ultrasonically in deionized water for 5 min to remove surface contamination before loading into the atmospheric-pressure plasma system. The high speed gas flow rate air (30 slm) was introduced from the upside of the plasma system and passed through the discharge chamber as the ionization gas. Argon gas used as the carrying gas was passed to the precursor system and the flow rate was fixed at 1 slm. The tetraethoxysilane monomer was introduced into mixing system and the temperature was maintained at 150 °C. The monomer gas flow rate was kept at 5 g/h by a liquid mass flow controller. The distance of nozzle-to-sample varied from 15 to 25 mm. The applied plasma power, stage moving velocity, and deposition cycle time were set at 555 W, 100 mm/s, and 5 times, respectively.
Fig. 2. Thickness of SiOx film as a function of nozzle-to-sample distance.
Fig. 2 shows the thickness of atmospheric-pressure plasma deposited SiOx film as a function of nozzle-to-sample distance. The thickness of the atmospheric-pressure plasma deposited SiOx film was measured by the optical thin-film thickness detector (Resolution: 2 nm, Precision: 0.1 nm). To examine the deposition effects of remote APP jet, the substrates were subjected to plasma exposure in the down stream at a position 15 to 25 mm away from the visible glow. The longlife plasma species from the plasmas were allowed to diffuse and get in contact with the substrates to form the SiOx film. As the distance of nozzle-to-sample is increasing, the thickness seems slightly decreased. In atmospheric-pressure plasmas, the deposition forming plasma species were mainly generated by electron impact and penning dissociation. These plasma species could lose its energy or reactivity in a very short time due to the much higher collision frequency among the plasma particles. The lifetime of the reactive plasma species in atmospheric-pressure plasmas is much shorter as
Fig. 3. Thickness of SiOx film as a function of substrate temperature. (Nozzle-to-sample distance was fixed at 20 mm; Tsub represents substrate temperature.)
C. Huang et al. / Thin Solid Films 517 (2009) 5141–5145
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compared to that in low-pressure plasmas [9]. As a result, the plasmas could lose their reactivity dramatically in a remote position (away from the glow). From this aspect, Fig. 2 determined the APP deposited SiOx film growth dependence of the deposition forming species of remote atmospheric-pressure jet plasma exposure. It was observed that the increasing nozzle to sample distance causes much slower deposition rate on substrates. The dependence of the atmospheric-pressure plasma deposited SiOx film growth rate on deposition substrate temperature (Tsub.) is shown in Fig. 3. It was found that the thickness of plasma deposited SiOx film drastically decreases with increasing deposition substrate temperature. This finding is possibly owing to the decreasing sticking coefficient of deposition forming plasma species with the increasing substrate temperature [10]. In other words, the major part of film growth rate-controlling step is the adsorption of deposition forming plasma species in atmospheric-pressure plasma. Moreover, this result is consistent with several researches [3,5] that the substrate temperature plays a significant part in atmospheric-pressure plasma deposition processing. 3.2. Chemical structure analysis of atmospheric plasma deposited SiOx film The chemical components and structure analysis results of atmospheric plasma deposited SiOx films are shown in Figs. 4 and 5, respectively. Fig. 4 shows the FTIR analysis of APP deposited SiOx film with different nozzle-to-sample distances. The main features of the spectrum including several groups [11–13] are: a peak in the region of 801–845 cm− 1 corresponding to SiRx (R represents methyl group); a peak in the region of 980–1230 cm− 1 corresponding to Si–O–Si; a weaker peak absorbance at 1700–1850 cm− 1 corresponding to either C = C or C = O and a peak in the region of 3000–3600 cm− 1 assigned to OH groups. It is obvious that SiOx films deposited at longer nozzleto-sample distance (e.g. 25 mm) retained the structure of the starting silicon carbon monomers. It should be noted that obvious peaks are detected at 2900 cm− 1 and 3000–3600 cm− 1 due to C–H and C = O
Fig. 5. FTIR spectra of SiOx film under various substrate temperatures. (Nozzle-tosample distance was fixed at 20 mm.)
stretching vibrations. The OH peaks of APP plasma deposited SiOx film FTIR spectra could be owing to OH groups formed from the remote atmospheric-pressure plasma interaction with water vapor of ambient air. Therefore, the APP deposited SiOx films obtain inorganic characteristics at shorter nozzle-to-sample distance of 15 to 20 mm. Furthermore, the strong intensity of Si–O absorption peak was obtained at lower deposition substrate temperature as shown in Fig. 5. FTIR spectrum implies that reasonably more Si–O–Si groups occurred in atmosphericpressure plasma chemical vapor deposition process at lower deposition substrate temperature. Consequently, Si–O–Si groups are indicated as the major groups in the decomposition of TEOS and Air mixture in atmospheric-pressure plasma chemical vapor deposition process, and a relatively high inorganic characteristic was detected by FTIR analysis. Table 1 represents the atomic percentages of the silicon, oxygen, carbon, and nitrogen present in the atmospheric-pressure plasma deposited SiOx films. The APP deposited SiOx films were comprised of 28.8 to 31.9 at.% silicon, 59.7 to 62.2 at.% oxygen, and 5.7 to 11.3 at.% carbon, from higher to lower distance of nozzle-to-sample. The presence of carbon in the film is believed to be due to incomplete decomposition of the TEOS precursor. The ratio of the shoulder area of Si–O stretching mode (between 1140 and 1160 cm− 1) to the primary peak area (between 1070 and 1080 cm− 1) from the FTIR spectrum has been Table 1 Elemental composition and porosity degree of SiOx film under various conditions. Factor Nozzle-to-sample
Fig. 4. FTIR spectra of SiOx film under various nozzle-to-sample distances. (Film thickness is approximately 100 nm.)
Parameter 15 mm 20 mm 25 mm
Porosity degree⁎
C
N
Si
O
(%)
(%)
(%)
(%)
C/O ratio
O/Si ratio
0.66 0.77 1.15
5.7 12.2 11.3
0.2 0.2 0.2
31.9 28.8 28.8
62.2 58.8 59.7
0.09 0.21 0.19
1.95 2.04 2.07
⁎Porosity degree of SiOx film is calculated from the shoulder area of Si–O stretching mode (between 1140 and 1160 cm− 1) to the primary peak area (between 1070 and 1080 cm− 1) by FTIR analysis. The substrate temperature was about 25 °C (room temperature).
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correlated with porosity degree of SiOx film [5] and the estimation result was shown in Table 1. It was found that the film with higher porosity degree and C/O ratio was obtained at longer nozzle-to-sample distance (e.g. 25 mm). These results suggested that the TEOS monomer penetrated through afterglow region of atmospheric-pressure plasma (downstream process) and could be dissociated by electron impact and penning dissociation. The dissociated TEOS monomer formed the porous plasma deposited film structure. In contrast, the dense as-deposited film was bombarded by higher energy particles from the plasma region at lower nozzle-to-sample distance condition. In addition, the C–C (3.8 eV) band energy is much lower than Si–O band energy (5.0 eV) [14]; the C–C on the as-deposited film surface is more easily broken with higher energy particles bombardment. This could be the major reason why few carbon atoms and lower porosity degree of APP deposited SiOx film can be obtained under lower distance condition. The trends reported in Figs. 4 and 5 are consistent with this assumption, where we can note the obvious SiOx characteristic at shorter nozzle-to-sample distance in FTIR analysis. 3.3. Surface morphology of atmospheric plasma deposited film SEM analyses were used to examine the surface morphologies of atmospheric-pressure plasma deposited SiOx films as shown in Fig. 6, respectively. The cross-section view of the films in Fig. 6 shows that APP deposited SiOx films contain smooth and continuous surface. It was also noticed that the surface of the cleaved edge of the atmospheric-pressure plasma deposited SiOx film at lower nozzle-to-sample distance condition is relatively rough, and suggests that it could be bombarded by higher energy particles.
Fig. 7. UV–VIS transmittance spectra of atmospheric-pressure plasma deposited SiOx coatings with PC substrates.
3.4. UV–VIS and hardness examination of atmospheric plasma deposited film Since the transparency of protective coating is essential to the advanced plastic development for optical applications, UV–VIS transmission analysis of atmospheric-pressure plasma deposited SiOx films on PC substrates was conducted in this study. Fig. 7 presents the UV–VIS transmission spectra of atmospheric-pressure plasma deposited SiOx coatings with PC substrates; they are colorless and have a transmission of above 80% in the visible region measured on PC substrates. As the results, UV–VIS transmission analysis evidently confirmed that the atmospheric-pressure plasma deposited SiOx coatings have an excellent transparency on polymeric substrates. In addition, the hardness of APP deposited SiOx coatings on PC substrate improved above HB from the pencil hardness method as shown in Table 2. 4. Conclusion The present study demonstrated the new type of atmosphericpressure plasma deposition process in producing SiOx coating on the polymeric substrates. In contrast to common plasma chemical vapor deposition methods, it consists of remote plasma deposition and low temperature feature for the polymeric surface coating. Decreasing of nozzle-to-sample distance and deposition substrate temperature has appreciable effects on the film growth rate and on film composition. In particular, the increasing of nozzle-to-sample distance results in the formation of methyl group species and of higher hydroxyl species in the atmospheric-pressure plasma deposited film. The major Si–O–Si absorbance observed in the FTIR spectra showed that plasma deposited silicon oxide coatings obtain relatively more inorganic features at shorter distance of remote plasma exposure. The improved transmittance and hardness of APP deposited PC substrates was examined in this investigation. Regarding the efficiency and the qualities of atmospheric-pressure plasma deposited SiOx coatings, this remote atmospheric-pressure plasma jet deposition technique is
Table 2 Pencil hardness of SiOx film on PC substrates.
Fig. 6. SEM images of SiOx film under different nozzle-to-sample distances: (a) 15 mm; (b) 25 mm.
Factor
Parameter
Pencil hardness
Nozzle-to-sample
Untreated 15 mm 20 mm 25 mm
6B HB HB HB
C. Huang et al. / Thin Solid Films 517 (2009) 5141–5145
comparable to the traditional techniques. Therefore, we conclude that this remote atmospheric-pressure plasma jet deposition method is a promising technique for coating SiOx layer on polymeric materials, especially when taking into accounts its simplicity and suitability to coat large-scale substrate size for industrial needs. Acknowledgments The authors are thankful for the support of the National Science Council of Republic of Chain under grant NSC 96-2218-E-155-005 and Industrial Technology Research Institute, Taiwan Republic of China, through Project No. 7301XS3C10. References [1] Y. Sawada, S. Orgawa, M. Kogoma, J. Phys. D: Appl. Phys. 28 (1995) 1661. [2] N. Tomozeiu, E.E. van Faassen, W.M. Arnoldbik, A.M. Vredenberg, F.H.P.M. Habraken, Thin Solid Films 420 (2002) 382.
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[3] S.E. Babayan, J.Y. Jeong, A. Schutze, V.J. Tu, M. Moravej, G.S. Selwyn, R.F. Hicks, Plasma Sources Sci. Technol. 10 (2001) 573. [4] K. Teshima, Y. Inoue, H. sugimura, O. Takai, Surf. Coat. Technol. 169 (2003) 583. [5] G.R. Nowling, M. Yajima, S.E. Babayan, M. Moravej, X. Yang, W. Hoffman, R.F. Hicks, Plasma Sources Sci. Technol. 14 (2005) 477. [6] T. Yamazaki, Y. Uraoka, T. Fuyuki, Thin Solid Films 487 (2005) 26. [7] J.J. Pérez-Bueno, R. Ramírez-Bon, Y.V. Vorobiev, F. Espinoza-Beltrán, J. GonzálezHernández, Thin Solid Films 379 (2000) 57. [8] A. Grill, Cold Plasma in Materials fabrication, IEEE Press, New York, NY, 1994. [9] Q.S. Yu, C. Huang, F.-H. Hsieh, H. Huff, Y. Duan, J. Biomed. Mater. Res. 80 (2007) 211. [10] K. Endo, T. Tatsumi, J. Appl. Phys. 78 (1995) 1370. [11] D. Trunec, Z. Narvratil, P. Stahel, L. Zajickova, V. Bursikova, J. Cech, J. Phys. D: Appl. Phys. 37 (2004) 2112. [12] S.E. Alexandrov, N. McSporran, M.L. Hitchman, Chem. Vapor Depos. 11 (2005) 481. [13] L.J. Ward, W.C.E. Schofield, J.P.S. Badyal, Langmuir 19 (2003) 2110. [14] A. Sonnenfeld, T.M. Tun, L. Zajíčková, K.V. Kozlov, H.E. Wagner, J.F. Behnke, R. Hippler, Plasmas Polym. 6 (2002) 237.
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
Investigation of atmospheric-pressure plasma deposited SiOx films on polymeric substrates Chun Huang a,⁎, Chi-Hung Liu b, Chun-Hsien Su b, Wen-Tung Hsu b, Shin-Yi Wu a a b
Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, 135 Yuan-Tung Road, Chung-Li, Taiwan 32003, ROC Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan 310, ROC
a r t i c l e
i n f o
Available online 16 March 2009 Keywords: Plasma chemical vapor deposition Hardness Transparency Silicon oxide Surface characteristics
a b s t r a c t The novel technique of plasma chemical vapor deposition without using vacuum chamber was investigated to deposit SiOx films on polymeric substrates through tetraethoxysilane (TEOS)/Air atmospheric-pressure plasma (APP) glow discharge. Depending on the proper deposition parameters, thin and smooth SiOx films on polycarbonate substrates were prepared. The atmospheric-pressure plasma deposited SiOx films obtained the desirable transparency in the visible and increased absorption in UV region. The surface characteristics of APP deposited SiOx films were examined by various surface analysis methods including FTIR, XPS, and SEM. It is shown that SiOx films exhibited low porosity and admirable hardness for optical applications. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon oxide (SiOx) films have attracted much attention because of their specific chemical structure and admirable electrical insulation characteristics [1,2]. The unique properties of SiOx films make it applicable as an inter-metal dielectric [3], gas barrier-layer [4], and protective coating [5]. In general, the SiOx film can be obtained by thermal chemical vapor deposition or sol-gel methods [6,7]. These conventional coating methods have been well-studied, well-documented, and are being extensively utilized in both academic and industrial fields. It has been recognized; however, that these conventional coating methods have their limitations or disadvantages. For instance, the inherently high processing temperatures (typically over 400 °C) of these methods are unsuitable for articles made of heat sensitive materials such as polymers and fabrics. Transparent polymeric substrates are widely used for optical applications in recent years owing to their lightweight, low cost, and similar light transmission characteristics to glass. However, the components of polymeric substrates are intrinsically soft, and thus the usage of plastic is limited to moderately mild applications. To improve the inherent restrictions of polymeric substrates, a variety of the coating methods are now being investigated. The major requirement of the coating methods is to achieve low temperature processing on polymeric substrates. The maximum coating temperature is limited by the thermal stability of the polymeric substrates. To overcome thermal stability issue, low temperature plasma enhanced chemical vapor deposition (PECVD) under vacuum system was used to perform SiOx film on polymeric substrates with several advantages such as dry process and thickness control. Because of the existence of these ⁎ Corresponding author. E-mail address: [email protected] (C. Huang). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.054
merits, low temperature plasma enhanced chemical vapor deposition technique has proven to be an effective coating method [8]. In spite of the excellent coating efficiency, the currently available plasma enhanced chemical vapor deposition processes with regard to coating process have several major limitations, such as the restricted volume of the plasma reactor and one or more vacuum and chemical cycles required. If it is possible to be stably operated under atmosphericpressure, not only decreasing in processing costs but also increasing in productivity could be obtained. To address these demands, we present new atmospheric-pressure plasma (APP) jet system and the deposition of SiOx films utilizing this system. In this investigation, we proposed the remote atmosphericpressure plasma deposition. The deposition of remote APP jet to form SiOx films on polymeric substrates simultaneously achieves the advantages that the film structures are kept relatively constant and low temperature nature for coating processing. The influence of process parameters on the characteristics of SiOx films deposited in TEOS/Air plasma at atmospheric-pressure was studied in this investigation. Since the chemical structure of atmospheric-pressure plasma deposited SiOx film determined its properties such as the transmittance and hardness, FTIR, XPS, and SEM were used to examine the effect of process parameters on film structure and its surface characteristics, respectively. Furthermore, the relationship between the hardness, the atomic oxygen to silicon ratio (the O/Si ratio), and the porosity degree of the SiOx films were also studied. 2. Experimental 2.1. Atmospheric pressure plasma deposition system SiOx films were deposited in an atmospheric-pressure plasma jet system as shown in Fig. 1. This system contains discharge chamber with
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2.2. Film characterization and analysis The thickness of the atmospheric-pressure plasma deposited SiOx film was measured by the optical thin-film thickness detector (Mission Peak Optics MP100-S) at a wavelength of 632.8 nm. The chemical structures of the plasma deposited SiOx films deposited were characterized by the Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer LX 20000G). Each spectrum was obtained from an average of 32 scans in the range of 400–4000 cm− 1 at a resolution of 4 cm− 1. The surface composition of APP deposited SiOx film was investigated by X-ray photoelectron spectroscope (XPS) with Mg Kα source (1253.6 eV). UV–VIS transmission detection was carried out using a GBC Cintra 202 UV spectrophotometer to determine the transmittance of APP deposited SiOx film. The surface morphology and roughness of the plasma deposited SiOx film were observed by scanning electron microscope (SEM), respectively. The hardness of atmospheric-pressure plasma deposited SiOx film was determined by pencil test (loading 500 g) according to ASTM D3363-05 and meantime the scratch sample seen with an optical microscope. The pencil hardness is from 9H to 6B (hard to soft). 3. Results and discussion 3.1. Deposition rate of atmospheric plasma chemical vapor deposition process Fig. 1. Schematic diagram of the atmospheric-pressure plasma jet deposition system.
a proprietary design. An electrical field was applied to the two electrodes located inside the chamber to ignite the plasma glow discharge by a 16 kHz AC power supply. Polycarbonate (PC 4.0 × 4.0 cm) films were used as substrates and cleaned ultrasonically in deionized water for 5 min to remove surface contamination before loading into the atmospheric-pressure plasma system. The high speed gas flow rate air (30 slm) was introduced from the upside of the plasma system and passed through the discharge chamber as the ionization gas. Argon gas used as the carrying gas was passed to the precursor system and the flow rate was fixed at 1 slm. The tetraethoxysilane monomer was introduced into mixing system and the temperature was maintained at 150 °C. The monomer gas flow rate was kept at 5 g/h by a liquid mass flow controller. The distance of nozzle-to-sample varied from 15 to 25 mm. The applied plasma power, stage moving velocity, and deposition cycle time were set at 555 W, 100 mm/s, and 5 times, respectively.
Fig. 2. Thickness of SiOx film as a function of nozzle-to-sample distance.
Fig. 2 shows the thickness of atmospheric-pressure plasma deposited SiOx film as a function of nozzle-to-sample distance. The thickness of the atmospheric-pressure plasma deposited SiOx film was measured by the optical thin-film thickness detector (Resolution: 2 nm, Precision: 0.1 nm). To examine the deposition effects of remote APP jet, the substrates were subjected to plasma exposure in the down stream at a position 15 to 25 mm away from the visible glow. The longlife plasma species from the plasmas were allowed to diffuse and get in contact with the substrates to form the SiOx film. As the distance of nozzle-to-sample is increasing, the thickness seems slightly decreased. In atmospheric-pressure plasmas, the deposition forming plasma species were mainly generated by electron impact and penning dissociation. These plasma species could lose its energy or reactivity in a very short time due to the much higher collision frequency among the plasma particles. The lifetime of the reactive plasma species in atmospheric-pressure plasmas is much shorter as
Fig. 3. Thickness of SiOx film as a function of substrate temperature. (Nozzle-to-sample distance was fixed at 20 mm; Tsub represents substrate temperature.)
C. Huang et al. / Thin Solid Films 517 (2009) 5141–5145
5143
compared to that in low-pressure plasmas [9]. As a result, the plasmas could lose their reactivity dramatically in a remote position (away from the glow). From this aspect, Fig. 2 determined the APP deposited SiOx film growth dependence of the deposition forming species of remote atmospheric-pressure jet plasma exposure. It was observed that the increasing nozzle to sample distance causes much slower deposition rate on substrates. The dependence of the atmospheric-pressure plasma deposited SiOx film growth rate on deposition substrate temperature (Tsub.) is shown in Fig. 3. It was found that the thickness of plasma deposited SiOx film drastically decreases with increasing deposition substrate temperature. This finding is possibly owing to the decreasing sticking coefficient of deposition forming plasma species with the increasing substrate temperature [10]. In other words, the major part of film growth rate-controlling step is the adsorption of deposition forming plasma species in atmospheric-pressure plasma. Moreover, this result is consistent with several researches [3,5] that the substrate temperature plays a significant part in atmospheric-pressure plasma deposition processing. 3.2. Chemical structure analysis of atmospheric plasma deposited SiOx film The chemical components and structure analysis results of atmospheric plasma deposited SiOx films are shown in Figs. 4 and 5, respectively. Fig. 4 shows the FTIR analysis of APP deposited SiOx film with different nozzle-to-sample distances. The main features of the spectrum including several groups [11–13] are: a peak in the region of 801–845 cm− 1 corresponding to SiRx (R represents methyl group); a peak in the region of 980–1230 cm− 1 corresponding to Si–O–Si; a weaker peak absorbance at 1700–1850 cm− 1 corresponding to either C = C or C = O and a peak in the region of 3000–3600 cm− 1 assigned to OH groups. It is obvious that SiOx films deposited at longer nozzleto-sample distance (e.g. 25 mm) retained the structure of the starting silicon carbon monomers. It should be noted that obvious peaks are detected at 2900 cm− 1 and 3000–3600 cm− 1 due to C–H and C = O
Fig. 5. FTIR spectra of SiOx film under various substrate temperatures. (Nozzle-tosample distance was fixed at 20 mm.)
stretching vibrations. The OH peaks of APP plasma deposited SiOx film FTIR spectra could be owing to OH groups formed from the remote atmospheric-pressure plasma interaction with water vapor of ambient air. Therefore, the APP deposited SiOx films obtain inorganic characteristics at shorter nozzle-to-sample distance of 15 to 20 mm. Furthermore, the strong intensity of Si–O absorption peak was obtained at lower deposition substrate temperature as shown in Fig. 5. FTIR spectrum implies that reasonably more Si–O–Si groups occurred in atmosphericpressure plasma chemical vapor deposition process at lower deposition substrate temperature. Consequently, Si–O–Si groups are indicated as the major groups in the decomposition of TEOS and Air mixture in atmospheric-pressure plasma chemical vapor deposition process, and a relatively high inorganic characteristic was detected by FTIR analysis. Table 1 represents the atomic percentages of the silicon, oxygen, carbon, and nitrogen present in the atmospheric-pressure plasma deposited SiOx films. The APP deposited SiOx films were comprised of 28.8 to 31.9 at.% silicon, 59.7 to 62.2 at.% oxygen, and 5.7 to 11.3 at.% carbon, from higher to lower distance of nozzle-to-sample. The presence of carbon in the film is believed to be due to incomplete decomposition of the TEOS precursor. The ratio of the shoulder area of Si–O stretching mode (between 1140 and 1160 cm− 1) to the primary peak area (between 1070 and 1080 cm− 1) from the FTIR spectrum has been Table 1 Elemental composition and porosity degree of SiOx film under various conditions. Factor Nozzle-to-sample
Fig. 4. FTIR spectra of SiOx film under various nozzle-to-sample distances. (Film thickness is approximately 100 nm.)
Parameter 15 mm 20 mm 25 mm
Porosity degree⁎
C
N
Si
O
(%)
(%)
(%)
(%)
C/O ratio
O/Si ratio
0.66 0.77 1.15
5.7 12.2 11.3
0.2 0.2 0.2
31.9 28.8 28.8
62.2 58.8 59.7
0.09 0.21 0.19
1.95 2.04 2.07
⁎Porosity degree of SiOx film is calculated from the shoulder area of Si–O stretching mode (between 1140 and 1160 cm− 1) to the primary peak area (between 1070 and 1080 cm− 1) by FTIR analysis. The substrate temperature was about 25 °C (room temperature).
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C. Huang et al. / Thin Solid Films 517 (2009) 5141–5145
correlated with porosity degree of SiOx film [5] and the estimation result was shown in Table 1. It was found that the film with higher porosity degree and C/O ratio was obtained at longer nozzle-to-sample distance (e.g. 25 mm). These results suggested that the TEOS monomer penetrated through afterglow region of atmospheric-pressure plasma (downstream process) and could be dissociated by electron impact and penning dissociation. The dissociated TEOS monomer formed the porous plasma deposited film structure. In contrast, the dense as-deposited film was bombarded by higher energy particles from the plasma region at lower nozzle-to-sample distance condition. In addition, the C–C (3.8 eV) band energy is much lower than Si–O band energy (5.0 eV) [14]; the C–C on the as-deposited film surface is more easily broken with higher energy particles bombardment. This could be the major reason why few carbon atoms and lower porosity degree of APP deposited SiOx film can be obtained under lower distance condition. The trends reported in Figs. 4 and 5 are consistent with this assumption, where we can note the obvious SiOx characteristic at shorter nozzle-to-sample distance in FTIR analysis. 3.3. Surface morphology of atmospheric plasma deposited film SEM analyses were used to examine the surface morphologies of atmospheric-pressure plasma deposited SiOx films as shown in Fig. 6, respectively. The cross-section view of the films in Fig. 6 shows that APP deposited SiOx films contain smooth and continuous surface. It was also noticed that the surface of the cleaved edge of the atmospheric-pressure plasma deposited SiOx film at lower nozzle-to-sample distance condition is relatively rough, and suggests that it could be bombarded by higher energy particles.
Fig. 7. UV–VIS transmittance spectra of atmospheric-pressure plasma deposited SiOx coatings with PC substrates.
3.4. UV–VIS and hardness examination of atmospheric plasma deposited film Since the transparency of protective coating is essential to the advanced plastic development for optical applications, UV–VIS transmission analysis of atmospheric-pressure plasma deposited SiOx films on PC substrates was conducted in this study. Fig. 7 presents the UV–VIS transmission spectra of atmospheric-pressure plasma deposited SiOx coatings with PC substrates; they are colorless and have a transmission of above 80% in the visible region measured on PC substrates. As the results, UV–VIS transmission analysis evidently confirmed that the atmospheric-pressure plasma deposited SiOx coatings have an excellent transparency on polymeric substrates. In addition, the hardness of APP deposited SiOx coatings on PC substrate improved above HB from the pencil hardness method as shown in Table 2. 4. Conclusion The present study demonstrated the new type of atmosphericpressure plasma deposition process in producing SiOx coating on the polymeric substrates. In contrast to common plasma chemical vapor deposition methods, it consists of remote plasma deposition and low temperature feature for the polymeric surface coating. Decreasing of nozzle-to-sample distance and deposition substrate temperature has appreciable effects on the film growth rate and on film composition. In particular, the increasing of nozzle-to-sample distance results in the formation of methyl group species and of higher hydroxyl species in the atmospheric-pressure plasma deposited film. The major Si–O–Si absorbance observed in the FTIR spectra showed that plasma deposited silicon oxide coatings obtain relatively more inorganic features at shorter distance of remote plasma exposure. The improved transmittance and hardness of APP deposited PC substrates was examined in this investigation. Regarding the efficiency and the qualities of atmospheric-pressure plasma deposited SiOx coatings, this remote atmospheric-pressure plasma jet deposition technique is
Table 2 Pencil hardness of SiOx film on PC substrates.
Fig. 6. SEM images of SiOx film under different nozzle-to-sample distances: (a) 15 mm; (b) 25 mm.
Factor
Parameter
Pencil hardness
Nozzle-to-sample
Untreated 15 mm 20 mm 25 mm
6B HB HB HB
C. Huang et al. / Thin Solid Films 517 (2009) 5141–5145
comparable to the traditional techniques. Therefore, we conclude that this remote atmospheric-pressure plasma jet deposition method is a promising technique for coating SiOx layer on polymeric materials, especially when taking into accounts its simplicity and suitability to coat large-scale substrate size for industrial needs. Acknowledgments The authors are thankful for the support of the National Science Council of Republic of Chain under grant NSC 96-2218-E-155-005 and Industrial Technology Research Institute, Taiwan Republic of China, through Project No. 7301XS3C10. References [1] Y. Sawada, S. Orgawa, M. Kogoma, J. Phys. D: Appl. Phys. 28 (1995) 1661. [2] N. Tomozeiu, E.E. van Faassen, W.M. Arnoldbik, A.M. Vredenberg, F.H.P.M. Habraken, Thin Solid Films 420 (2002) 382.
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