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High-rate HMDSO-based coatings in open air using atmospheric-pressure plasma jet
Journal of Non-Crystalline Solids 358 (2012) 2462–2465
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
High-rate HMDSO-based coatings in open air using atmospheric-pressure plasma jet H. Kakiuchi a,⁎, K. Higashida a, T. Shibata b, H. Ohmi a, T. Yamada a, K. Yasutake a a b
Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Production Engineering Research Laboratory, Panasonic Electric Works Co., Ltd., 1048, Kadoma, Osaka 571-8686, Japan
a r t i c l e
i n f o
Article history: Received 17 August 2011 Received in revised form 5 December 2011 Available online 13 January 2012 Keywords: Atmospheric-pressure plasma; Plasma jet; Silicon-related coatings; High-rate deposition
a b s t r a c t This work deals with the high-rate and dust-free formation of carbon-containing silicon oxide (SiOC) coatings in open air without substrate heating using an atmospheric-pressure (AP) plasma jet. The AP plasma was excited by a 13.56-MHz radio frequency (RF) power. Hexamethyldisiloxane and oxygen (O2) were used as the source gases. By optimizing the O2 flow rate and RF power, SiOC films were readily fabricated at deposition rates higher than 100 nm/s without suffering from particulate contaminations of the film surface. Additionally, an inorganic SiO2-like film exhibiting O/Si atomic ratio of approximately 2 was obtained at a deposition rate of ~ 13 nm/s, the value of which is still greater than those obtained in other AP plasma sources. Further systematic studies are needed to see if good-quality inorganic SiO2-like films can be obtained with higher rates. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Plasma-enhanced chemical vapor deposition (PECVD) has turned out a very attractive fabrication method for functional thin films, since it is compatible with most materials and also enables to control thickness, chemical composition and properties. PECVD processes at low pressures, however, are expensive owing to the complexity of equipment, particularly in the case that large-sized substrates have to be coated, which is a crucial factor limiting the industrial use of the technology. As a result, there has been a steady increase in the use of plasma generated at atmospheric pressure (AP) in PECVD processes during the last decades, because it allows the exposure of substrate material to the plasma without the requirement of expensive vacuum devices. Various PECVD systems using AP plasma sources have been reviewed in the literatures [1,2]. Recently, organic and inorganic silicon (Si)-related coatings, such as carbon-containing silicon oxide (SiOC) and SiO2-like films, at low temperatures using AP plasma have drawn much interest in advanced industries for use in many applications, such as scratch-resistant and anti-reflection coatings [3-9], food and pharmaceutical packaging [10-12], and corrosion protection [13,14]. Several types of AP plasma sources, including dielectric barrier discharges, radio frequency glow discharges and AP plasma jets, have successfully been used to fabricate Si-related coatings [13-24]. In particular, AP plasma jet is a simple plasma source that is usable in open air conditions, the plasma of which can be generated with argon (Ar), and thus, it is one of the most promising techniques for Si-related coatings in a less expensive,
⁎ Corresponding author. Tel./fax: + 81 6 6879 7269. E-mail address: [email protected] (H. Kakiuchi). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.081
more flexible and continuous manner of treatment. It is, however, recognized that, one of the technical issues to overcome is the contamination of substrate surface by dusty particles [9,16,17,23], especially in the case that high deposition rates are needed. Thus, to achieve a highly efficient Si-related coatings using AP plasma jet, it is very important to react the source gas molecules sufficiently with the flow of plasma jet, while suppressing the dust formation by controlling the gas flow, as well as to understand the reaction process both in gas phase and on the surface. In this paper, we report the SiOC deposition in open air at room temperature using an AP plasma jet system we have developed. Hexamethyldisiloxane (HMDSO, O[Si(CH3)3]2) was used as the source molecules. As a first step to the high-rate and dust-free formation of inorganic SiO2-like films, we discuss the deposition rate, morphology and bond structure of the films prepared under various conditions with and without using O2 as an oxidizer. 2. Experimental details Fig. 1 schematically illustrates the AP plasma jet system used in this study. It consisted of two concentric quartz tubes of 8 and 4 mm diameter, leaving an annular gap of 1 mm, and two copper ring electrodes attached with 5 mm separation to the outer quartz tube. The upper electrode was capacitively coupled to a 13.56-MHz radio-frequency (RF) generator through a matching network, and the lower one was connected to the ground potential. The working gas mixtures (Ar + O2) were introduced into the annular space. By supplying a RF power, nonequilibrium AP plasma was stably confined in the annular space, and the plasma jet was directed toward the Si wafer substrates that were located at a distance d from the end of the outer quartz tube and placed on a non-grounded susceptor as
H. Kakiuchi et al. / Journal of Non-Crystalline Solids 358 (2012) 2462–2465
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3. Results
Fig. 1. Schematic illustration of the experimental setup.
shown in Fig. 1. The source gas mixture (HMDSO vapor diluted with Ar) was separately added downstream of the plasma without disturbing the flow of plasma jet via the inner quartz tube by guiding Ar through a bubbler containing liquid HMDSO monomer of 99.99% purity at a constant liquid temperature of 30 °C. The concentration of HMDSO vapor in the source gas mixture was approximately 3%, which was estimated by measuring the weight of HMDSO carried within the defined time interval. The flow rate of the working Ar (QAr) was fixed at 3000 SCCM (cubic centimeter per minute at STP), while the flow rate of O2 (QO2), that of the source gas mixture (Qsource) and RF power were varied as the principal parameters. The film thickness was determined by a profilometer. The measured film thickness was in good agreement with that observed by cross sectional observations by a scanning electron microscope (SEM) operated at 10 keV. The resultant films had an almost symmetrical cone shape with a gentle slope and were the thickest at the central point of impact of the plasma jet, although the film area depended on each parameter (distance d, QO2 and Qsource). The deposition rate described in this paper has been calculated based on the maximum value of the film thickness to simplify the discussion because it has been found that the maximum thickness is not proportional but strongly related to the overall volume of the film deposited on the substrate. The bond structure was analyzed by infrared (IR) absorption spectroscopy. The spectra were taken in the wavenumber range of 400–4000 cm − 1 in absorbance mode using a Fourier transform IR spectrometer. The atomic composition of the films was determined by X-ray photoelectron spectroscopy (XPS).
Fig. 2. Length of the AP plasma jet (L) observed from the end of the outer quartz tube as a function of QO2. The RF power was fixed at 200 W.
In the case of using an AP plasma jet for thin film deposition, the substrate position (distance d in Fig. 1) is a crucial experimental parameter, because it dominates the residence time of the source molecules in the plasma. From the viewpoint of increasing efficiency in transforming HMDSO into useful film, the shorter distance is more preferable. Indeed, both the deposition rate and film area increased with the decrease in d. However, if the distance d is too short, the resultant films may exhibit a strong organic character due to the insufficient decomposition and oxidation of the source HMDSO monomers. In contrast, if the distance d is too long, the particle growth occurs in gas phase due to the polymerization of precursor molecules, causing the poorer film properties and lower deposition rate [9,16,17,23]. On the other hand, increasing QO2 and Qsource caused the decrease in length of the AP plasma jet (L), resulting in the decrease in film area. Fig. 2 shows the dependence of length of the AP plasma jet (L) on QO2 under a fixed RF power of 200 W. As clearly observed in the figure, L monotonously decreases from 23 to 5 mm with increasing QO2 from 0 to 90 SCCM. This indicates that the decomposition of O2 molecules consumes the input power to generate atomic oxygen, resulting in the decrease in the plasma volume. A similar tendency was observed for the increase in Qsource, while L was almost constant for the change in RF power in the range of 140–200 W (data not shown here). The effects of RF power, QO2 and Qsource on the deposition rate and bond structure of the films are described below. Fig. 3 shows the RF power dependence of deposition rate, in which QO2 and Qsource are fixed at 20 and 200 SCCM, respectively (O2/ HMDSO ≈ 3). The substrate was located at d = 15 mm. The photographs of the films after 10 s of deposition are also shown in the figure. With increasing RF power from 140 to 180 W, the deposition rate proportionally increases from 97 to 125 nm/s. This indicates that RF power principally dominates the reaction of the HMDSO monomers to generate the precursor molecules. Since the deposition rate tends to saturate at around 200 W, it is suggested that the RF power of 200 W is sufficient for the full activation of the available HMDSO monomers in the plasma jet. Fig. 4 shows the QO2 dependences of deposition rate of the films prepared with Qsource = 200 and 500 SCCM and with RF power of 200 W at the substrate position of d = 15 mm, together with the photographs. It is observed for Qsource = 200 SCCM that the deposition
Fig. 3. Dependence of deposition rate on RF power. QO2 and Qsource were 20 and 200 SCCM, respectively, and the distance d (substrate position) was 15 mm. The photographs of the films deposited with 140 and 200 W are also shown.
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Fig. 4. Dependence of deposition rate on QO2 for Qsource = 200 and 500 SCCM, and RF power of 200 W. The distance d was 15 mm. The photographs of the films deposited with QO2 = 30 and 40 SCCM for Qsource = 200 SCCM are also shown.
rate is markedly improved by adding O2 to the working gas and increases with increasing QO2. This indicates that atomic oxygen also contributes to the generation of the precursor molecules by oxidizing HMDSO monomers and/or the fragments of HMDSO. For Qsource = 500 SCCM, although the deposition rate slightly increases by the addition of O2, it is almost independent of the variation of QO2 and is far lower than that for Qsource = 200 SCCM, which is due to the shortage of RF power for the supplied HMDSO monomers. Note that for both Qsource, the deposition rates greatly drop at QO2 = 40 SCCM. These decreases in deposition rate are caused by the generation of dusty particles that have actually covered the entire film surface as shown in the photograph. From Fig. 2, the L value at QO2 = 40 SCCM is ~ 10 mm, and thus, the substrate surface (d = 15 mm) is not exposed to the plasma jet. As a result, it is very likely that the precursor molecules are rapidly deactivated in the outer space of the plasma jet, resulting in their condensation in gas phase to form dusty particles, and do not contribute to the film growth. Thus, the substrate surface must be exposed to the plasma jet in order to supply the reactive precursor molecules efficiently to the substrate. IR absorption spectroscopy measurements were performed to obtain information on the effects of QO2 and RF power on the bonding
Fig. 5. IR absorption spectra of the films prepared with Qsource = 200 SCCM and RF power of 200 W. QO2 was (a) 0, (b) 10, (c) 20, and (d) 30 SCCM. The distance d was 15 mm.
characteristics of the deposited films. The IR absorption spectra of the films prepared with Qsource = 200 SCCM and RF power of 200 W are collected in Fig. 5 as a function of QO2. The spectra were taken at the position with the maximum thickness. The films have three characteristic absorption peaks due to Si―O―Si asymmetric stretching (1050–1070 and ~1150 cm − 1), bending (800 cm − 1), and rocking (450 cm − 1) vibrations. In the 750–900 cm − 1 region, the absorptions related to Si―C rocking vibrations in Si―(CH3)2 and Si―(CH3)3 were observed at 796 and 840 cm − 1, respectively. There was also the absorption assigned to Si―(CH3)x symmetric bending at around 1260 cm − 1, and CHx and Si―OH absorptions were present in the region of 2850–3000 and 3000–3600 cm − 1, respectively [24-29]. From the XPS analyses, it was found that the carbon concentration of the same films was in the range of 10–15%. By investigating a significant number of samples prepared under various conditions, it appeared difficult to drastically reduce the organic character of the film without being contaminated by dusts only by adjusting the combination of QO2 and RF power, as far as the other parameters were maintained constant. However, it is observed in Fig. 5 that the bond structure is almost independent of QO2, although the intensity of the absorption bands slightly varies with QO2. No significant change in bond structure was also shown for the variation of RF power level. Moreover, by comparing the IR absorption spectra taken at three different points across the film, it was confirmed that the bond structure was almost identical in the radial direction despite the gradient thickness distribution (data not shown here). Note that the very high deposition rates (>100 nm/s) without the contamination of film surface by dusts and the good uniformity of bond structure of the films are unique features of the AP plasma jet system from the viewpoint of fabricating a large and uniform film by scanning the plasma jet.
Fig. 6. (a) Depth profile of atomic concentration of the film deposited with Qsource = 50 SCCM, QO2 = 80 SCCM, and RF power of 150 W. (b) SEM image of a cross section of the same film. The distance d was 5 mm.
H. Kakiuchi et al. / Journal of Non-Crystalline Solids 358 (2012) 2462–2465
4. Discussion It is reported that the IR absorption spectrum of a HMDSO film polymerized in plasma without using an oxidizer typically exhibit absorptions due to Si―O―Si stretching vibrations at 1020–1040 cm − 1, Si―C rocking in Si―(CH3)x at 800 and ~840 cm − 1, symmetric bending of Si―(CH3)x at 1260 cm − 1, and CHx absorptions at 2850–3000 cm − 1, while it does not usually show absorptions related to Si―OH groups in the 3000–3600 cm − 1 region [22,24]. From the spectra in Fig. 5, although the absorption peaks due to Si―(CH3)x are seen, intense Si–O–Si stretching bands can be observed at around 1060 cm − 1 and the absorptions owing to the presence of Si–OH bonds are clearly visible, which are characteristic for SiO2-like films [16,22-24]. Namely, the films obtained in this study have both features of plasma-polymerized HMDSO films and inorganic SiO2-like ones. This suggests that the source gas mixture is not well mixed with the flow of plasma jet and that a part of the HMDSO monomers are not sufficiently oxidized, which limits the elimination of organic moieties of the resultant films. In fact, the increase in the O2/HMDSO ratio was shown effective to obtain an inorganic SiO2-like film as demonstrated in Fig. 6. In the experiment, Qsource was decreased from 200 to 50 SCCM, while QO2 was increased from 20 to 80 SCCM (O2/HMDSO ≈ 50), aiming at the complete oxidation of HMDSO molecules introduced into the plasma jet. In addition, to avoid the particulate contamination of the film surface, the distance d was shortened from 15 to 5 mm to expose the substrate surface to the plasma jet, and simultaneously, RF power was reduced from 200 to 150 W. In this condition, although the efficiency in transforming HMDSO into the film estimated roughly from the film volume was as low as approximately 1%, the deposition rate of approximately 13 nm/s was obtained, the value of which is still greater than those obtained in other PECVD methods using AP plasma (0.2–5 nm/s) [16,21-23]. From Fig. 6(a), it is observed that the O/Si atomic ratio is approximately 2 and that the carbon concentration is less than 1%. Besides, the cross sectional morphology [Fig. 6(b)] shows that the film grows homogeneously and is not contaminated by dusts. These suggest that the AP plasma jet used in this study is essentially capable of fabricating inorganic SiO2-like films at very high rates without substrate heating. 5. Conclusion
transforming HMDSO into the film is limited at a low level (~ 1%). To see if inorganic SiO2-like coatings can be obtained with higher rates, future study will focus on investigating the geometry of AP plasma jet system suitable for the efficient decomposition and oxidation of the source molecules, together with further systematic researches to understand the reaction processes in the AP plasma jet. Acknowledgements This work was carried out in Ultra Clean Room of Department of Precision Science and Technology, Osaka University. This work was funded by KAKENHI (20676003) and a Grant-in-Aid for the Global COE Program from the Ministry of Education, Culture, Sports, Science and Technology. The authors would like to thank A. Takeuchi of Osaka University for his technical assistance. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
We have studied the deposition and structural properties of SiOC films prepared from HMDSO and O2 as the source gases without substrate heating using an AP plasma jet system. It is demonstrated that the dust-free film formation with extremely high rates (>100 nm/s) is readily achieved by optimizing the combination of O2 flow rate (O2/HMDSO ratio) and RF power, although the films have 10–15% of organic moieties in their matrix. On the other hand, the deposition of an inorganic SiO2-like film is confirmed at a reduced rate of ~ 13 nm/s under a condition of an excessively large O2/HMDSO ratio of ~50 and a relatively low RF power, although the efficiency in
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[23] [24] [25] [26] [27] [28] [29]
S.E. Alexandrov, M.L. Hitchman, Chem. Vap. Deposition 11 (2005) 457. M. Moravej, R.F. Hicks, Chem. Vap. Deposition 11 (2005) 469. Y.-H. Han, A. Taylor, K.M. Knowles, Surf. Coat. Technol. 203 (2009) 2871. W. Boentoro, A. Pflug, B. Szyszka, Thin Solid Films 517 (2009) 3121. Y.-S. Lin, Y.-H. Liao, M.-S. Weng, Thin Solid Films 517 (2009) 5224. Y.-S. Lin, Y.-H. Liao, C.-H. Hu, J. Non-Cryst. Solids 355 (2009) 182. I.M. Thomas, Appl. Opt. 31 (1992) 6145. H. Nagel, A. Metz, R. Hezel, Sol. Energy Mater. Sol. Cells 65 (2001) 71. H. Kakiuchi, H. Ohmi, Y. Yamaguchi, K. Nakamura, K. Yasutake, Thin Solid Films 519 (2010) 235. M. Deilmann, M. Grabowski, S. Theib, N. Bibinov, P. Awakowicz, J. Phys. D: Appl. Phys. 41 (2008) 135207. D.G. Howells, B.M. Henry, J. Madocks, H.E. Assender, Thin Solid Films 516 (2008) 3081. L. Korner, A. Sonnenfeld, Ph. Rudolf von Rohr, Thin Solid Films 518 (2010) 4840. C. Petit-Etienne, M. Tatoulian, I. Mabille, E. Sutter, F. Arefi-Khonsari, Plasma Processes Polym. 4 (2007) S562. J. Bour, J. Bardon, H. Aubriet, D. Del Frari, B. Verheyde, R. Dams, D. Vangeneugden, D. Ruch, Plasma Processes Polym. 5 (2008) 788. A. Pfuch, R. Cihar, Surf. Coat. Technol. 183 (2004) 134. F. Massines, N. Gherardi, A. Fornelli, S. Martin, Surf. Coat. Technol. 200 (2005) 1855. N. Jidenko, C. Jimenez, F. Massines, J.-P. Borra, J. Phys. D: Appl. Phys. 40 (2007) 4155. Q. Chen, Y. Zhang, E. Han, Y. Ge, J. Vac. Sci. Technol., A 24 (2006) 2082. T.P. Kasih, S. Kuroda, H. Kubota, Chem. Vap. Deposition 13 (2007) 169. J. Schafer, R. Foest, A. Quade, A. Ohl, K.-D. Weltmann, J. Phys. D: Appl. Phys. 41 (2008) 194010. M.H. Han, J.H. Noh, T.I. Lee, J.H. Choi, K.W. Park, H.S. Hwang, K.M. Song, H.K. Baik, Plasma Processes Polym. 5 (2008) 861. R. Morent, N. De Geyter, S. Van Vlierberghe, P. Dubruel, C. Leys, E. Schacht, Surf. Coat. Technol. 203 (2009) 1366. V. Raballand, J. Benedikt, S. Hoffmann, M. Zimmermann, A. von Keudell, J. Appl. Phys. 105 (2009) 083304. F. Fanelli, S. Lovascio, R. d'Agostino, F. Arefi-Khonsari, F. Fracassi, Plasma Processes Polym. 7 (2010) 535. M. Creatore, F. Palumbo, R. d'Agostino, Plasmas Polym. 7 (2002) 291. G. Borvon, A. Goullet, A. Granier, G. Turban, Plasmas Polym. 7 (2002) 341. A. Grill, D.A. Neumayer, J. Appl. Phys. 94 (2003) 6697. A. Milella, J.L. Delattre, F. Palumbo, F. Fracassi, R. d'Agostino, J. Electrochem. Soc. 153 (2006) F106. A. Milella, F. Palumbo, J.L. Delattre, F. Fracassi, R. d'Agostino, Plasma Processes Polym. 4 (2007) 425.
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
High-rate HMDSO-based coatings in open air using atmospheric-pressure plasma jet H. Kakiuchi a,⁎, K. Higashida a, T. Shibata b, H. Ohmi a, T. Yamada a, K. Yasutake a a b
Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Production Engineering Research Laboratory, Panasonic Electric Works Co., Ltd., 1048, Kadoma, Osaka 571-8686, Japan
a r t i c l e
i n f o
Article history: Received 17 August 2011 Received in revised form 5 December 2011 Available online 13 January 2012 Keywords: Atmospheric-pressure plasma; Plasma jet; Silicon-related coatings; High-rate deposition
a b s t r a c t This work deals with the high-rate and dust-free formation of carbon-containing silicon oxide (SiOC) coatings in open air without substrate heating using an atmospheric-pressure (AP) plasma jet. The AP plasma was excited by a 13.56-MHz radio frequency (RF) power. Hexamethyldisiloxane and oxygen (O2) were used as the source gases. By optimizing the O2 flow rate and RF power, SiOC films were readily fabricated at deposition rates higher than 100 nm/s without suffering from particulate contaminations of the film surface. Additionally, an inorganic SiO2-like film exhibiting O/Si atomic ratio of approximately 2 was obtained at a deposition rate of ~ 13 nm/s, the value of which is still greater than those obtained in other AP plasma sources. Further systematic studies are needed to see if good-quality inorganic SiO2-like films can be obtained with higher rates. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Plasma-enhanced chemical vapor deposition (PECVD) has turned out a very attractive fabrication method for functional thin films, since it is compatible with most materials and also enables to control thickness, chemical composition and properties. PECVD processes at low pressures, however, are expensive owing to the complexity of equipment, particularly in the case that large-sized substrates have to be coated, which is a crucial factor limiting the industrial use of the technology. As a result, there has been a steady increase in the use of plasma generated at atmospheric pressure (AP) in PECVD processes during the last decades, because it allows the exposure of substrate material to the plasma without the requirement of expensive vacuum devices. Various PECVD systems using AP plasma sources have been reviewed in the literatures [1,2]. Recently, organic and inorganic silicon (Si)-related coatings, such as carbon-containing silicon oxide (SiOC) and SiO2-like films, at low temperatures using AP plasma have drawn much interest in advanced industries for use in many applications, such as scratch-resistant and anti-reflection coatings [3-9], food and pharmaceutical packaging [10-12], and corrosion protection [13,14]. Several types of AP plasma sources, including dielectric barrier discharges, radio frequency glow discharges and AP plasma jets, have successfully been used to fabricate Si-related coatings [13-24]. In particular, AP plasma jet is a simple plasma source that is usable in open air conditions, the plasma of which can be generated with argon (Ar), and thus, it is one of the most promising techniques for Si-related coatings in a less expensive,
⁎ Corresponding author. Tel./fax: + 81 6 6879 7269. E-mail address: [email protected] (H. Kakiuchi). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.081
more flexible and continuous manner of treatment. It is, however, recognized that, one of the technical issues to overcome is the contamination of substrate surface by dusty particles [9,16,17,23], especially in the case that high deposition rates are needed. Thus, to achieve a highly efficient Si-related coatings using AP plasma jet, it is very important to react the source gas molecules sufficiently with the flow of plasma jet, while suppressing the dust formation by controlling the gas flow, as well as to understand the reaction process both in gas phase and on the surface. In this paper, we report the SiOC deposition in open air at room temperature using an AP plasma jet system we have developed. Hexamethyldisiloxane (HMDSO, O[Si(CH3)3]2) was used as the source molecules. As a first step to the high-rate and dust-free formation of inorganic SiO2-like films, we discuss the deposition rate, morphology and bond structure of the films prepared under various conditions with and without using O2 as an oxidizer. 2. Experimental details Fig. 1 schematically illustrates the AP plasma jet system used in this study. It consisted of two concentric quartz tubes of 8 and 4 mm diameter, leaving an annular gap of 1 mm, and two copper ring electrodes attached with 5 mm separation to the outer quartz tube. The upper electrode was capacitively coupled to a 13.56-MHz radio-frequency (RF) generator through a matching network, and the lower one was connected to the ground potential. The working gas mixtures (Ar + O2) were introduced into the annular space. By supplying a RF power, nonequilibrium AP plasma was stably confined in the annular space, and the plasma jet was directed toward the Si wafer substrates that were located at a distance d from the end of the outer quartz tube and placed on a non-grounded susceptor as
H. Kakiuchi et al. / Journal of Non-Crystalline Solids 358 (2012) 2462–2465
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3. Results
Fig. 1. Schematic illustration of the experimental setup.
shown in Fig. 1. The source gas mixture (HMDSO vapor diluted with Ar) was separately added downstream of the plasma without disturbing the flow of plasma jet via the inner quartz tube by guiding Ar through a bubbler containing liquid HMDSO monomer of 99.99% purity at a constant liquid temperature of 30 °C. The concentration of HMDSO vapor in the source gas mixture was approximately 3%, which was estimated by measuring the weight of HMDSO carried within the defined time interval. The flow rate of the working Ar (QAr) was fixed at 3000 SCCM (cubic centimeter per minute at STP), while the flow rate of O2 (QO2), that of the source gas mixture (Qsource) and RF power were varied as the principal parameters. The film thickness was determined by a profilometer. The measured film thickness was in good agreement with that observed by cross sectional observations by a scanning electron microscope (SEM) operated at 10 keV. The resultant films had an almost symmetrical cone shape with a gentle slope and were the thickest at the central point of impact of the plasma jet, although the film area depended on each parameter (distance d, QO2 and Qsource). The deposition rate described in this paper has been calculated based on the maximum value of the film thickness to simplify the discussion because it has been found that the maximum thickness is not proportional but strongly related to the overall volume of the film deposited on the substrate. The bond structure was analyzed by infrared (IR) absorption spectroscopy. The spectra were taken in the wavenumber range of 400–4000 cm − 1 in absorbance mode using a Fourier transform IR spectrometer. The atomic composition of the films was determined by X-ray photoelectron spectroscopy (XPS).
Fig. 2. Length of the AP plasma jet (L) observed from the end of the outer quartz tube as a function of QO2. The RF power was fixed at 200 W.
In the case of using an AP plasma jet for thin film deposition, the substrate position (distance d in Fig. 1) is a crucial experimental parameter, because it dominates the residence time of the source molecules in the plasma. From the viewpoint of increasing efficiency in transforming HMDSO into useful film, the shorter distance is more preferable. Indeed, both the deposition rate and film area increased with the decrease in d. However, if the distance d is too short, the resultant films may exhibit a strong organic character due to the insufficient decomposition and oxidation of the source HMDSO monomers. In contrast, if the distance d is too long, the particle growth occurs in gas phase due to the polymerization of precursor molecules, causing the poorer film properties and lower deposition rate [9,16,17,23]. On the other hand, increasing QO2 and Qsource caused the decrease in length of the AP plasma jet (L), resulting in the decrease in film area. Fig. 2 shows the dependence of length of the AP plasma jet (L) on QO2 under a fixed RF power of 200 W. As clearly observed in the figure, L monotonously decreases from 23 to 5 mm with increasing QO2 from 0 to 90 SCCM. This indicates that the decomposition of O2 molecules consumes the input power to generate atomic oxygen, resulting in the decrease in the plasma volume. A similar tendency was observed for the increase in Qsource, while L was almost constant for the change in RF power in the range of 140–200 W (data not shown here). The effects of RF power, QO2 and Qsource on the deposition rate and bond structure of the films are described below. Fig. 3 shows the RF power dependence of deposition rate, in which QO2 and Qsource are fixed at 20 and 200 SCCM, respectively (O2/ HMDSO ≈ 3). The substrate was located at d = 15 mm. The photographs of the films after 10 s of deposition are also shown in the figure. With increasing RF power from 140 to 180 W, the deposition rate proportionally increases from 97 to 125 nm/s. This indicates that RF power principally dominates the reaction of the HMDSO monomers to generate the precursor molecules. Since the deposition rate tends to saturate at around 200 W, it is suggested that the RF power of 200 W is sufficient for the full activation of the available HMDSO monomers in the plasma jet. Fig. 4 shows the QO2 dependences of deposition rate of the films prepared with Qsource = 200 and 500 SCCM and with RF power of 200 W at the substrate position of d = 15 mm, together with the photographs. It is observed for Qsource = 200 SCCM that the deposition
Fig. 3. Dependence of deposition rate on RF power. QO2 and Qsource were 20 and 200 SCCM, respectively, and the distance d (substrate position) was 15 mm. The photographs of the films deposited with 140 and 200 W are also shown.
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Fig. 4. Dependence of deposition rate on QO2 for Qsource = 200 and 500 SCCM, and RF power of 200 W. The distance d was 15 mm. The photographs of the films deposited with QO2 = 30 and 40 SCCM for Qsource = 200 SCCM are also shown.
rate is markedly improved by adding O2 to the working gas and increases with increasing QO2. This indicates that atomic oxygen also contributes to the generation of the precursor molecules by oxidizing HMDSO monomers and/or the fragments of HMDSO. For Qsource = 500 SCCM, although the deposition rate slightly increases by the addition of O2, it is almost independent of the variation of QO2 and is far lower than that for Qsource = 200 SCCM, which is due to the shortage of RF power for the supplied HMDSO monomers. Note that for both Qsource, the deposition rates greatly drop at QO2 = 40 SCCM. These decreases in deposition rate are caused by the generation of dusty particles that have actually covered the entire film surface as shown in the photograph. From Fig. 2, the L value at QO2 = 40 SCCM is ~ 10 mm, and thus, the substrate surface (d = 15 mm) is not exposed to the plasma jet. As a result, it is very likely that the precursor molecules are rapidly deactivated in the outer space of the plasma jet, resulting in their condensation in gas phase to form dusty particles, and do not contribute to the film growth. Thus, the substrate surface must be exposed to the plasma jet in order to supply the reactive precursor molecules efficiently to the substrate. IR absorption spectroscopy measurements were performed to obtain information on the effects of QO2 and RF power on the bonding
Fig. 5. IR absorption spectra of the films prepared with Qsource = 200 SCCM and RF power of 200 W. QO2 was (a) 0, (b) 10, (c) 20, and (d) 30 SCCM. The distance d was 15 mm.
characteristics of the deposited films. The IR absorption spectra of the films prepared with Qsource = 200 SCCM and RF power of 200 W are collected in Fig. 5 as a function of QO2. The spectra were taken at the position with the maximum thickness. The films have three characteristic absorption peaks due to Si―O―Si asymmetric stretching (1050–1070 and ~1150 cm − 1), bending (800 cm − 1), and rocking (450 cm − 1) vibrations. In the 750–900 cm − 1 region, the absorptions related to Si―C rocking vibrations in Si―(CH3)2 and Si―(CH3)3 were observed at 796 and 840 cm − 1, respectively. There was also the absorption assigned to Si―(CH3)x symmetric bending at around 1260 cm − 1, and CHx and Si―OH absorptions were present in the region of 2850–3000 and 3000–3600 cm − 1, respectively [24-29]. From the XPS analyses, it was found that the carbon concentration of the same films was in the range of 10–15%. By investigating a significant number of samples prepared under various conditions, it appeared difficult to drastically reduce the organic character of the film without being contaminated by dusts only by adjusting the combination of QO2 and RF power, as far as the other parameters were maintained constant. However, it is observed in Fig. 5 that the bond structure is almost independent of QO2, although the intensity of the absorption bands slightly varies with QO2. No significant change in bond structure was also shown for the variation of RF power level. Moreover, by comparing the IR absorption spectra taken at three different points across the film, it was confirmed that the bond structure was almost identical in the radial direction despite the gradient thickness distribution (data not shown here). Note that the very high deposition rates (>100 nm/s) without the contamination of film surface by dusts and the good uniformity of bond structure of the films are unique features of the AP plasma jet system from the viewpoint of fabricating a large and uniform film by scanning the plasma jet.
Fig. 6. (a) Depth profile of atomic concentration of the film deposited with Qsource = 50 SCCM, QO2 = 80 SCCM, and RF power of 150 W. (b) SEM image of a cross section of the same film. The distance d was 5 mm.
H. Kakiuchi et al. / Journal of Non-Crystalline Solids 358 (2012) 2462–2465
4. Discussion It is reported that the IR absorption spectrum of a HMDSO film polymerized in plasma without using an oxidizer typically exhibit absorptions due to Si―O―Si stretching vibrations at 1020–1040 cm − 1, Si―C rocking in Si―(CH3)x at 800 and ~840 cm − 1, symmetric bending of Si―(CH3)x at 1260 cm − 1, and CHx absorptions at 2850–3000 cm − 1, while it does not usually show absorptions related to Si―OH groups in the 3000–3600 cm − 1 region [22,24]. From the spectra in Fig. 5, although the absorption peaks due to Si―(CH3)x are seen, intense Si–O–Si stretching bands can be observed at around 1060 cm − 1 and the absorptions owing to the presence of Si–OH bonds are clearly visible, which are characteristic for SiO2-like films [16,22-24]. Namely, the films obtained in this study have both features of plasma-polymerized HMDSO films and inorganic SiO2-like ones. This suggests that the source gas mixture is not well mixed with the flow of plasma jet and that a part of the HMDSO monomers are not sufficiently oxidized, which limits the elimination of organic moieties of the resultant films. In fact, the increase in the O2/HMDSO ratio was shown effective to obtain an inorganic SiO2-like film as demonstrated in Fig. 6. In the experiment, Qsource was decreased from 200 to 50 SCCM, while QO2 was increased from 20 to 80 SCCM (O2/HMDSO ≈ 50), aiming at the complete oxidation of HMDSO molecules introduced into the plasma jet. In addition, to avoid the particulate contamination of the film surface, the distance d was shortened from 15 to 5 mm to expose the substrate surface to the plasma jet, and simultaneously, RF power was reduced from 200 to 150 W. In this condition, although the efficiency in transforming HMDSO into the film estimated roughly from the film volume was as low as approximately 1%, the deposition rate of approximately 13 nm/s was obtained, the value of which is still greater than those obtained in other PECVD methods using AP plasma (0.2–5 nm/s) [16,21-23]. From Fig. 6(a), it is observed that the O/Si atomic ratio is approximately 2 and that the carbon concentration is less than 1%. Besides, the cross sectional morphology [Fig. 6(b)] shows that the film grows homogeneously and is not contaminated by dusts. These suggest that the AP plasma jet used in this study is essentially capable of fabricating inorganic SiO2-like films at very high rates without substrate heating. 5. Conclusion
transforming HMDSO into the film is limited at a low level (~ 1%). To see if inorganic SiO2-like coatings can be obtained with higher rates, future study will focus on investigating the geometry of AP plasma jet system suitable for the efficient decomposition and oxidation of the source molecules, together with further systematic researches to understand the reaction processes in the AP plasma jet. Acknowledgements This work was carried out in Ultra Clean Room of Department of Precision Science and Technology, Osaka University. This work was funded by KAKENHI (20676003) and a Grant-in-Aid for the Global COE Program from the Ministry of Education, Culture, Sports, Science and Technology. The authors would like to thank A. Takeuchi of Osaka University for his technical assistance. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
We have studied the deposition and structural properties of SiOC films prepared from HMDSO and O2 as the source gases without substrate heating using an AP plasma jet system. It is demonstrated that the dust-free film formation with extremely high rates (>100 nm/s) is readily achieved by optimizing the combination of O2 flow rate (O2/HMDSO ratio) and RF power, although the films have 10–15% of organic moieties in their matrix. On the other hand, the deposition of an inorganic SiO2-like film is confirmed at a reduced rate of ~ 13 nm/s under a condition of an excessively large O2/HMDSO ratio of ~50 and a relatively low RF power, although the efficiency in
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[23] [24] [25] [26] [27] [28] [29]
S.E. Alexandrov, M.L. Hitchman, Chem. Vap. Deposition 11 (2005) 457. M. Moravej, R.F. Hicks, Chem. Vap. Deposition 11 (2005) 469. Y.-H. Han, A. Taylor, K.M. Knowles, Surf. Coat. Technol. 203 (2009) 2871. W. Boentoro, A. Pflug, B. Szyszka, Thin Solid Films 517 (2009) 3121. Y.-S. Lin, Y.-H. Liao, M.-S. Weng, Thin Solid Films 517 (2009) 5224. Y.-S. Lin, Y.-H. Liao, C.-H. Hu, J. Non-Cryst. Solids 355 (2009) 182. I.M. Thomas, Appl. Opt. 31 (1992) 6145. H. Nagel, A. Metz, R. Hezel, Sol. Energy Mater. Sol. Cells 65 (2001) 71. H. Kakiuchi, H. Ohmi, Y. Yamaguchi, K. Nakamura, K. Yasutake, Thin Solid Films 519 (2010) 235. M. Deilmann, M. Grabowski, S. Theib, N. Bibinov, P. Awakowicz, J. Phys. D: Appl. Phys. 41 (2008) 135207. D.G. Howells, B.M. Henry, J. Madocks, H.E. Assender, Thin Solid Films 516 (2008) 3081. L. Korner, A. Sonnenfeld, Ph. Rudolf von Rohr, Thin Solid Films 518 (2010) 4840. C. Petit-Etienne, M. Tatoulian, I. Mabille, E. Sutter, F. Arefi-Khonsari, Plasma Processes Polym. 4 (2007) S562. J. Bour, J. Bardon, H. Aubriet, D. Del Frari, B. Verheyde, R. Dams, D. Vangeneugden, D. Ruch, Plasma Processes Polym. 5 (2008) 788. A. Pfuch, R. Cihar, Surf. Coat. Technol. 183 (2004) 134. F. Massines, N. Gherardi, A. Fornelli, S. Martin, Surf. Coat. Technol. 200 (2005) 1855. N. Jidenko, C. Jimenez, F. Massines, J.-P. Borra, J. Phys. D: Appl. Phys. 40 (2007) 4155. Q. Chen, Y. Zhang, E. Han, Y. Ge, J. Vac. Sci. Technol., A 24 (2006) 2082. T.P. Kasih, S. Kuroda, H. Kubota, Chem. Vap. Deposition 13 (2007) 169. J. Schafer, R. Foest, A. Quade, A. Ohl, K.-D. Weltmann, J. Phys. D: Appl. Phys. 41 (2008) 194010. M.H. Han, J.H. Noh, T.I. Lee, J.H. Choi, K.W. Park, H.S. Hwang, K.M. Song, H.K. Baik, Plasma Processes Polym. 5 (2008) 861. R. Morent, N. De Geyter, S. Van Vlierberghe, P. Dubruel, C. Leys, E. Schacht, Surf. Coat. Technol. 203 (2009) 1366. V. Raballand, J. Benedikt, S. Hoffmann, M. Zimmermann, A. von Keudell, J. Appl. Phys. 105 (2009) 083304. F. Fanelli, S. Lovascio, R. d'Agostino, F. Arefi-Khonsari, F. Fracassi, Plasma Processes Polym. 7 (2010) 535. M. Creatore, F. Palumbo, R. d'Agostino, Plasmas Polym. 7 (2002) 291. G. Borvon, A. Goullet, A. Granier, G. Turban, Plasmas Polym. 7 (2002) 341. A. Grill, D.A. Neumayer, J. Appl. Phys. 94 (2003) 6697. A. Milella, J.L. Delattre, F. Palumbo, F. Fracassi, R. d'Agostino, J. Electrochem. Soc. 153 (2006) F106. A. Milella, F. Palumbo, J.L. Delattre, F. Fracassi, R. d'Agostino, Plasma Processes Polym. 4 (2007) 425.