- Email: [email protected]
X-Ray photoelectron spectroscopy analysis of plasma–polymer interactions for development of low-damage plasma processing of soft materials
Thin Solid Films 518 (2010) 6492–6495
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
X-Ray photoelectron spectroscopy analysis of plasma–polymer interactions for development of low-damage plasma processing of soft materials Yuichi Setsuhara a,d,⁎, Ken Cho a,d, Masaharu Shiratani b,d, Makoto Sekine c,d, Masaru Hori c,d a
Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Department of Electronics, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan Plasma Nanotechnology Research Center, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan d Japan Science and Technology Agency, CREST, Japan b c
a r t i c l e
i n f o
Available online 16 February 2010 Keywords: Inductively-coupled plasma Low-inductance antenna Polymer Low-damage process X-ray photoelectron spectroscopy
a b s t r a c t Plasma–polymer interactions have been investigated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) of polyethyleneterephthalate (PET) films, which have been exposed to argon plasmas driven by low-inductance antenna modules as a parameter of ion energy. The AFM images indicated that the argon plasma exposure exhibited a significant change in surface roughness. The XPS analyses suggested that the degradation of chemical bonding structure and/or bond scission of PET could be effectively suppressed in the plasma exposures with ion energies below 6 eV. However, significant degradations of O = C–O bond, C–O bond and phenyl group were observed with increasing ion energy above 6 eV. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Next-generation devices including thin-film photovoltaic cells [1], flexible displays [2], advanced ULSIs (low-k interlayer and EUV lithography) [3] and advanced medical devices [4] are composed of organic–inorganic hybrid materials. For successful development of the organic–inorganic hybrid materials for advanced devices, lowtemperature formation of high-quality inorganic functional films on polymer surface and controlling nano-surface/interface structure are required to avoid considerable degradation of organic materials. In this respect, plasma processing technologies are considered as a promising candidate to perform high-quality material processes at low temperature. However, it is of great significance to suppress process damage via plasma exposure to organic materials due to bombardment with charged particles and chemical reactions by radicals and photons, because the bond-dissociation energies of organic soft materials are comparatively lower than those of inorganic materials. For example, the bond-dissociation energies are, in general, about 4 eV for the C–CH3, about 8 eV for C = O bond and about 3 eV for the O–C(=O) bond [5]. In the plasma process, it is considered that irradiation with radicals [6,7], ions [8], photons [9] and electrons [10] from plasma may cause significant degradation in chemical bonding structures of organic materials. Therefore it is of great significance to control the internal
⁎ Corresponding author. Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. Tel./fax: + 81 6 6879 8641. E-mail address: [email protected] (Y. Setsuhara). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.01.057
plasma parameters based on understandings of plasma–polymer interactions to prevent degradations of the bonding structures. As one of the effective plasma sources for polymer processing, inductivity-coupled plasma (ICP) sources have been developed with multiple low-inductance antenna (LIA) modules [11]. In previous studies, we have reported that the plasmas sustained with LIA modules allow high-density plasma production with active control of power deposition profiles over large area and low sheath-edge potential [12–17]. Ion-energy distribution measured with a massseparated ion-energy analyzer showed significantly reduced ion energy at the sheath edge as low as 5 eV [19]. In this study, the plasma–polymer interactions have been investigated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) of polyethyleneterephthalate (PET) films, which have been exposed to argon plasmas driven by low-inductance antenna modules as a parameter of ion energy. 2. Experimental Argon plasmas were generated via inductive coupling of RF power at 13.56 MHz with a set of eight LIA units [11–21]. The LIA unit consisted of U-shaped antenna conductor, which was fully covered with dielectric tubing for complete isolation from the plasma [11]. Eight LIA units with an 80 mm height and a 70 mm width were mounted on the top flange of the discharge chamber and were coupled to an RF power generator at 13.56 MHz via a matching network. Here it is noted that each LIA unit was connected in parallel to the matching network. The chamber had a 500 mm inner diameter and a 200 mm height, which was connected to a diffusion chamber
Y. Setsuhara et al. / Thin Solid Films 518 (2010) 6492–6495
made of stainless-steel vessel with a 500 mm inner diameter and a 400 mm height [17,19]. The chamber was pumped to 3 × 10− 4 Pa with a turbomolecular pump. Plasma parameters were measured with a Langmuir probe, which was inserted radially at an axial position of 280 mm below the top flange. Kinetic energy distributions impinging onto the ground potential (ion energy at the sheath edge) from the plasmas were measured using a mass-separated ion-energy analyzer (Hiden, EQP500), which was mounted beside the substrate holder [19]. For investigations of the plasma–polymer interactions due to plasma exposures, PET films (0.1 mm thick) were exposed to argon plasmas sustained at an RF power of 1000 W at Ar pressures of 0.67–26 Pa. Here it is noted that the PET films were set on water-cooled substrate holder when they were exposed to the plasmas. For investigation of surface morphological changes, surface roughness was observed with AFM with the measured area of 10 μm × 10 μm for all AFM images in this paper. Chemical bonding states at nano-surface layer of polymers exposed to argon plasmas were analyzed using XPS (AXIS-165x SHIMADZU Corp., Japan) with non-monochromatized MgKα radiation (photon energy of 1253.6 eV). The inelastic mean free path (IMFP) of 1200 eV electrons in polymers is approximately 4 nm [20], which corresponds to probing depth of about 10 nm. Here it is noted that the XPS analyses were carried out with charging-suppression function of the spectrometer in order to avoid anomalous charging of the sample surface due to photoelectron emission during the measurements.
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3. Results and discussion Ion-energy distribution of the argon ions impinging to the ground potential through the sheath edge of the argon plasmas were measured using the mass-separated ion-energy analyzer (the spectra not shown). Peak values of ion-energy distributions decreased from 15 eV to 6 eV with increasing Ar pressure from 0.67 Pa to 26.6 Pa (15 eV at 0.67 Pa, 13 eV at 1.3 Pa, 9 eV at 6.7 Pa, 7 eV at 13 Pa, and 6 eV at 26 Pa). Furthermore, the ion-bombardment energy onto nonconductive substrate such as polymers and glass is almost equivalent to potential drop from the plasma potential to the floating potential. In the previous publication, the potential drop of argon plasma was reported to be in the range of 3–15 eV at Ar pressures of 0.26–13 Pa (15 eV at 0.26 Pa, 8 eV at 1.3 Pa, and 3 eV at 13 Pa) [21], which demonstrated that ICPs driven by LIA units were considered to be one of the plasma sources suitable for low-damage and high-quality processes of polymers. For investigations of the surface morphological change of PET films due to plasma exposures, PET films were exposed to the argon plasma sustained at an RF power of 1000 W and Ar pressure of 13 Pa (with ion energy of 7 eV) and 0.67 Pa (with ion energy of 15 eV) with an ion dose of 2.6 × 1018 ions/cm2. Fig. 1 shows typical AFM images and line profiles of the PET surface (a) before and after exposure to the plasmas sustained at Ar pressures of (b) 13 Pa and (c) 0.67 Pa. Surface roughness Ra (arithmetic average values) was evaluated from the AFM images. The surface roughness of the original PET film was about
Fig. 1. Typical AFM images and line profiles of the PET surface, (a) before and after exposure to the plasmas sustained at Ar pressures of (b) 13 Pa and (c) 0.67 Pa.
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0.5 nm. After exposure to the plasma at Ar pressures of 13 Pa and 0.67 Pa, the roughness increased to 0.9 nm and 1.1 nm. Chemical bonding states of nano-surface layer of polymers exposed to argon plasmas were analyzed using XPS. The PET films were exposed to argon plasmas sustained at an RF power of 1000 W and Ar pressures of 0.67 Pa–26 Pa with an ion dose of 4.3 × 1018 ions/cm2. The XPS C1s spectra for PET films without and with exposure to argon plasmas were shown in Fig. 2; (a) before plasma exposure, (b) after exposure to the plasma sustained at Ar pressures of 26 Pa (with ion energy of 6 eV), (c) 6.7 Pa (with ion energy of 9 eV) and (d) 0.67 Pa (with ion energy of 15 eV). The C1s spectra of the PET samples can be deconvoluted into four peaks, i.e., the following components: C–C bond at 284.6 eV, C–O bond at 286.1 eV, O = C–O bond at 288.6 eV and π–π* shake-up satellite at 291.2 eV [22,23]. The C1s spectra shown in Fig. 2 exhibit that the exposure with plasmas sustained at decreasing Ar pressure (i.e., increasing ion energy) tended to degrade the O = C–O bond and the C–O bond with slight increase of the C = O bond. These tendencies of the variation of the chemical bonding states with decreasing discharge pressure (increasing ion energy) indicate that the scission of the O = C–O bond and the C–O bond are significantly enhanced by increasing energy of the ion bombardment and the scission of the O = C–O bond may result in formation of the C = O bond. Furthermore, it is remarkable that the degradation of the O = C–O bond and the C–O bond for the plasma exposure at 26 Pa was insignificant, indicating that the suppression of
the plasma process damage can be feasible via controlling the ionbombardment energy during plasma processing of polymers. The peak-area ratio of the C–O bond to the C–C bond (IC–O / IC–C), the C = O bond to the C–C bond (IC = O / IC–C) and the O = C–O bond to the C–C bond (IO = C–O / IC–C) evaluated from the XPS C1s spectra are summarized in Fig. 3 as a function of the peak ion energy measured
Fig. 2. XPS C1s spectra for PET films; (a) without plasma exposure and with exposure to argon plasmas sustained at an RF 1000 W at Ar pressures of (b) 0.67 Pa, (c) 6.7 Pa, and (d) 26 Pa.
Fig. 4. XPS C1s spectra to highlight the phenyl group for PET films; (a) without plasma exposure and with exposure to argon plasmas sustained at an RF 1000 W at Ar pressures of (b) 0.67 Pa, (c) 6.7 Pa, and (d) 26 Pa.
Fig. 3. Variation of the peak-area ratio of C–O bond to C–C bond (IC–O / IC–C), C = O bond to C–C bond (IC = O / IC–C) and O = C–O bond to C–C bond (IO = C–O / IC–C) evaluated from XPS C1s spectra.
Y. Setsuhara et al. / Thin Solid Films 518 (2010) 6492–6495
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er. The peak values of ion-energy distributions decreased from 15 eV to 6 eV as the Ar pressure increased from 0.67 Pa to 26.6 Pa. The AFM images indicated that the argon plasma exposure resulted in insignificant change in surface morphology. The XPS analyses exhibited that the degradation of the O = C–O bond, the C–O bond and the phenyl group could be effectively suppressed in the plasma exposures with ion energy below 6 eV. Acknowledgements
Fig. 5. Variation of the peak-area ratio of π → π* shake-up satellite peak to C–C bond (Iπ → π* / IC–C) evaluated from XPS C1s spectra.
with the mass-separated ion-energy analyzer. As elucidated above for Fig. 2, the degradations of the O = C–O bond and the C–O bond are observed to be effectively suppressed in the plasma exposure with ion energy below 6 eV, whereas, degradations of these bonds are significantly enhanced with increasing ion energy. Additionally, the degradation of these bonds are encountered with increase of the C = O bond function, which is considered to be due to the scission of the O = C–O bond. Furthermore, the XPS C1s spectra to highlight the phenyl group (π → π* shake-up satellite peak) are shown in Fig. 4, in which the spectra in Fig. 2 are expanded with the intensities for the C–C bond normalized. Peak-area ratio of π → π* shake-up satellite peak to C–C bond (Iπ → π* / IC–C) evaluated from XPS C1s spectra is summarized in Fig. 5 as a function of the peak ion energy. The π → π* shake-up satellite peak clearly decreased after the plasma exposure, which was sustained at an Ar pressure lower than 13 Pa, then the values of Iπ → π* / IC–C decreased from 0.03 to 0.01. These results of chemical bonding analyses indicate that it is possible to prevent the decomposition of C–O bond, O = C–O bond and phenyl group via controlling the Ar pressure to suppress the ion energy less than 6 eV. These findings are considered to provide significant knowledge to establish the bond engineering for plasma processes of soft materials. 4. Summary The plasma–polymer interactions were investigated using AFM and XPS of PET films, which were exposed to argon plasmas driven by LIA modules. The ion-energy distribution of the argon ions impinging onto the ground potential through the sheath edge of the argon plasmas was measured using the mass-separated ion-energy analyz-
This work was supported partly by The Global COE Program “Center of Excellence for Advanced Structural and Functional Materials Design” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and the Grant-in-Aid for Cooperative Research Project of Nationwide Joint-Use Research Institutes on Development Base of Joining Technology for New Metallic Glasses and Inorganic Materials from MEXT, Japan. References [1] K.L. Chopra, P.D. Paulson, V. Dutta, Prog. Photovoltaics Res. Appl. 12 (2004) 69. [2] M.-C. Choi, Y. Kim, C.-S. Ha, Prog. Polym. Sci. 33 (2008) 581. [3] International technology roadmap for semiconductors, 2007 edition, Executive summary. [4] T. Nakatani, K. Okamoto, Y. Nitta, A. Mochizuki, H. Hoshi, A. Homma, 2J. Photopolym. Sci. Technol. 21 (2008) 225–230. [5] S.I. Stoliarova, P.R. Westmorelanda, M.R. Nydenb, G.P. Forney, Polymer 44 (2003) 883. [6] I. Gancarz, G. Pozniak, M. Bryjak, Eur. Polym. J. 35 (1999) 1419. [7] Y. Setsuhara, K. Cho, K. Takenaka, A. Ebe, M. Shiratani, M. Sekine, M. Hori, E. Ikenaga, H. Kondo, O. Nakatsuka, S. Zaima, Thin Sold Films, in press. [8] T. Steckenreiter, E. Balanzat, H. Fuess, C. Trautmann, Nucl. Instr. Meth. Phys. Res. B 131 (1997) 159. [9] M. Day, D.M. Willes, J. Appl. Polym. Sci. 16 (1972) 203. [10] S. Masseya, P. Cloutierb, L. Sancheb, D. Roy, Rad. Phys. Chem. 77 (2008) 889. [11] Y. Setsuhara, T. Shoji, A. Ebe, S. Baba, N. Yamamoto, K. Takahashi, K. Ono, S. Miyake, Surf. Coat. Technol. 174–175 (2003) 33. [12] K. Takenaka, Y. Setsuhara, K. Nishisaka, A. Ebe, Jpn. J. Appl. Phys. 45 (2006) 8046. [13] O. Tsuda, M. Ishihara, Y. Koga, S. Fujiwara, Y. Setsuhara, N. Sato, J. Phys. Chem. B 109 (2005) 4917. [14] H. Deguchi, H. Yoneda, K. Kato, K. Kubota, T. Hayashi, K. Ogata, A. Ebe, K. Takenaka, Y. Setsuhara, Jpn. J. Appl. Phys. 45 (2006) 8042. [15] Y. Setsuhara, K. Takenaka, A. Ebe, K. Nishisaka, Plasma Process. Polym. 4 (2007) S628. [16] Y. Setsuhara, K. Takenaka, A. Ebe, K. Nishisaka, Solid State Phenomena 127 (2007) 239. [17] K. Takenaka, Y. Setsuhara, K. Nishisaka, A. Ebe, Plasma Process. Polym. 4 (2007) S1009. [18] E. Takahashi, Y. Nishigami, A. Tomyo, M. Fujiwara, H. Kaki, K. Kubota, T. Hayashi, K. Ogata, A. Ebe, Y. Setsuhara, Jpn. J. Appl. Phys. 46 (2007) 1280. [19] K. Takenaka, Y. Setsuhara, K. Nishisaka, A. Ebe, Jpn. J. Appl. Phys. 47 (2008) 6900. [20] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface. Anal. 21 (1994) 165. [21] K. Takenaka, Y. Setsuhara, K. Nishisaka, A. Ebe, Trans. Mat. Res. Soc. Jpn. 32 (2006) 493. [22] T. Miyayama, N. Sanada, S.-I. Iida, J.S. Hammondb, M. Suzuki, Appl. Surf. Sci. 255 (2008) 951. [23] N.-Y. Cui, D.J. Upadhyay, C.A. Anderson, B.J. Meenan, N.M.D. Brown, Appl. Surf. Sci. 253 (2007) 3865.
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
X-Ray photoelectron spectroscopy analysis of plasma–polymer interactions for development of low-damage plasma processing of soft materials Yuichi Setsuhara a,d,⁎, Ken Cho a,d, Masaharu Shiratani b,d, Makoto Sekine c,d, Masaru Hori c,d a
Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Department of Electronics, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan Plasma Nanotechnology Research Center, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan d Japan Science and Technology Agency, CREST, Japan b c
a r t i c l e
i n f o
Available online 16 February 2010 Keywords: Inductively-coupled plasma Low-inductance antenna Polymer Low-damage process X-ray photoelectron spectroscopy
a b s t r a c t Plasma–polymer interactions have been investigated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) of polyethyleneterephthalate (PET) films, which have been exposed to argon plasmas driven by low-inductance antenna modules as a parameter of ion energy. The AFM images indicated that the argon plasma exposure exhibited a significant change in surface roughness. The XPS analyses suggested that the degradation of chemical bonding structure and/or bond scission of PET could be effectively suppressed in the plasma exposures with ion energies below 6 eV. However, significant degradations of O = C–O bond, C–O bond and phenyl group were observed with increasing ion energy above 6 eV. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Next-generation devices including thin-film photovoltaic cells [1], flexible displays [2], advanced ULSIs (low-k interlayer and EUV lithography) [3] and advanced medical devices [4] are composed of organic–inorganic hybrid materials. For successful development of the organic–inorganic hybrid materials for advanced devices, lowtemperature formation of high-quality inorganic functional films on polymer surface and controlling nano-surface/interface structure are required to avoid considerable degradation of organic materials. In this respect, plasma processing technologies are considered as a promising candidate to perform high-quality material processes at low temperature. However, it is of great significance to suppress process damage via plasma exposure to organic materials due to bombardment with charged particles and chemical reactions by radicals and photons, because the bond-dissociation energies of organic soft materials are comparatively lower than those of inorganic materials. For example, the bond-dissociation energies are, in general, about 4 eV for the C–CH3, about 8 eV for C = O bond and about 3 eV for the O–C(=O) bond [5]. In the plasma process, it is considered that irradiation with radicals [6,7], ions [8], photons [9] and electrons [10] from plasma may cause significant degradation in chemical bonding structures of organic materials. Therefore it is of great significance to control the internal
⁎ Corresponding author. Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. Tel./fax: + 81 6 6879 8641. E-mail address: [email protected] (Y. Setsuhara). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.01.057
plasma parameters based on understandings of plasma–polymer interactions to prevent degradations of the bonding structures. As one of the effective plasma sources for polymer processing, inductivity-coupled plasma (ICP) sources have been developed with multiple low-inductance antenna (LIA) modules [11]. In previous studies, we have reported that the plasmas sustained with LIA modules allow high-density plasma production with active control of power deposition profiles over large area and low sheath-edge potential [12–17]. Ion-energy distribution measured with a massseparated ion-energy analyzer showed significantly reduced ion energy at the sheath edge as low as 5 eV [19]. In this study, the plasma–polymer interactions have been investigated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) of polyethyleneterephthalate (PET) films, which have been exposed to argon plasmas driven by low-inductance antenna modules as a parameter of ion energy. 2. Experimental Argon plasmas were generated via inductive coupling of RF power at 13.56 MHz with a set of eight LIA units [11–21]. The LIA unit consisted of U-shaped antenna conductor, which was fully covered with dielectric tubing for complete isolation from the plasma [11]. Eight LIA units with an 80 mm height and a 70 mm width were mounted on the top flange of the discharge chamber and were coupled to an RF power generator at 13.56 MHz via a matching network. Here it is noted that each LIA unit was connected in parallel to the matching network. The chamber had a 500 mm inner diameter and a 200 mm height, which was connected to a diffusion chamber
Y. Setsuhara et al. / Thin Solid Films 518 (2010) 6492–6495
made of stainless-steel vessel with a 500 mm inner diameter and a 400 mm height [17,19]. The chamber was pumped to 3 × 10− 4 Pa with a turbomolecular pump. Plasma parameters were measured with a Langmuir probe, which was inserted radially at an axial position of 280 mm below the top flange. Kinetic energy distributions impinging onto the ground potential (ion energy at the sheath edge) from the plasmas were measured using a mass-separated ion-energy analyzer (Hiden, EQP500), which was mounted beside the substrate holder [19]. For investigations of the plasma–polymer interactions due to plasma exposures, PET films (0.1 mm thick) were exposed to argon plasmas sustained at an RF power of 1000 W at Ar pressures of 0.67–26 Pa. Here it is noted that the PET films were set on water-cooled substrate holder when they were exposed to the plasmas. For investigation of surface morphological changes, surface roughness was observed with AFM with the measured area of 10 μm × 10 μm for all AFM images in this paper. Chemical bonding states at nano-surface layer of polymers exposed to argon plasmas were analyzed using XPS (AXIS-165x SHIMADZU Corp., Japan) with non-monochromatized MgKα radiation (photon energy of 1253.6 eV). The inelastic mean free path (IMFP) of 1200 eV electrons in polymers is approximately 4 nm [20], which corresponds to probing depth of about 10 nm. Here it is noted that the XPS analyses were carried out with charging-suppression function of the spectrometer in order to avoid anomalous charging of the sample surface due to photoelectron emission during the measurements.
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3. Results and discussion Ion-energy distribution of the argon ions impinging to the ground potential through the sheath edge of the argon plasmas were measured using the mass-separated ion-energy analyzer (the spectra not shown). Peak values of ion-energy distributions decreased from 15 eV to 6 eV with increasing Ar pressure from 0.67 Pa to 26.6 Pa (15 eV at 0.67 Pa, 13 eV at 1.3 Pa, 9 eV at 6.7 Pa, 7 eV at 13 Pa, and 6 eV at 26 Pa). Furthermore, the ion-bombardment energy onto nonconductive substrate such as polymers and glass is almost equivalent to potential drop from the plasma potential to the floating potential. In the previous publication, the potential drop of argon plasma was reported to be in the range of 3–15 eV at Ar pressures of 0.26–13 Pa (15 eV at 0.26 Pa, 8 eV at 1.3 Pa, and 3 eV at 13 Pa) [21], which demonstrated that ICPs driven by LIA units were considered to be one of the plasma sources suitable for low-damage and high-quality processes of polymers. For investigations of the surface morphological change of PET films due to plasma exposures, PET films were exposed to the argon plasma sustained at an RF power of 1000 W and Ar pressure of 13 Pa (with ion energy of 7 eV) and 0.67 Pa (with ion energy of 15 eV) with an ion dose of 2.6 × 1018 ions/cm2. Fig. 1 shows typical AFM images and line profiles of the PET surface (a) before and after exposure to the plasmas sustained at Ar pressures of (b) 13 Pa and (c) 0.67 Pa. Surface roughness Ra (arithmetic average values) was evaluated from the AFM images. The surface roughness of the original PET film was about
Fig. 1. Typical AFM images and line profiles of the PET surface, (a) before and after exposure to the plasmas sustained at Ar pressures of (b) 13 Pa and (c) 0.67 Pa.
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0.5 nm. After exposure to the plasma at Ar pressures of 13 Pa and 0.67 Pa, the roughness increased to 0.9 nm and 1.1 nm. Chemical bonding states of nano-surface layer of polymers exposed to argon plasmas were analyzed using XPS. The PET films were exposed to argon plasmas sustained at an RF power of 1000 W and Ar pressures of 0.67 Pa–26 Pa with an ion dose of 4.3 × 1018 ions/cm2. The XPS C1s spectra for PET films without and with exposure to argon plasmas were shown in Fig. 2; (a) before plasma exposure, (b) after exposure to the plasma sustained at Ar pressures of 26 Pa (with ion energy of 6 eV), (c) 6.7 Pa (with ion energy of 9 eV) and (d) 0.67 Pa (with ion energy of 15 eV). The C1s spectra of the PET samples can be deconvoluted into four peaks, i.e., the following components: C–C bond at 284.6 eV, C–O bond at 286.1 eV, O = C–O bond at 288.6 eV and π–π* shake-up satellite at 291.2 eV [22,23]. The C1s spectra shown in Fig. 2 exhibit that the exposure with plasmas sustained at decreasing Ar pressure (i.e., increasing ion energy) tended to degrade the O = C–O bond and the C–O bond with slight increase of the C = O bond. These tendencies of the variation of the chemical bonding states with decreasing discharge pressure (increasing ion energy) indicate that the scission of the O = C–O bond and the C–O bond are significantly enhanced by increasing energy of the ion bombardment and the scission of the O = C–O bond may result in formation of the C = O bond. Furthermore, it is remarkable that the degradation of the O = C–O bond and the C–O bond for the plasma exposure at 26 Pa was insignificant, indicating that the suppression of
the plasma process damage can be feasible via controlling the ionbombardment energy during plasma processing of polymers. The peak-area ratio of the C–O bond to the C–C bond (IC–O / IC–C), the C = O bond to the C–C bond (IC = O / IC–C) and the O = C–O bond to the C–C bond (IO = C–O / IC–C) evaluated from the XPS C1s spectra are summarized in Fig. 3 as a function of the peak ion energy measured
Fig. 2. XPS C1s spectra for PET films; (a) without plasma exposure and with exposure to argon plasmas sustained at an RF 1000 W at Ar pressures of (b) 0.67 Pa, (c) 6.7 Pa, and (d) 26 Pa.
Fig. 4. XPS C1s spectra to highlight the phenyl group for PET films; (a) without plasma exposure and with exposure to argon plasmas sustained at an RF 1000 W at Ar pressures of (b) 0.67 Pa, (c) 6.7 Pa, and (d) 26 Pa.
Fig. 3. Variation of the peak-area ratio of C–O bond to C–C bond (IC–O / IC–C), C = O bond to C–C bond (IC = O / IC–C) and O = C–O bond to C–C bond (IO = C–O / IC–C) evaluated from XPS C1s spectra.
Y. Setsuhara et al. / Thin Solid Films 518 (2010) 6492–6495
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er. The peak values of ion-energy distributions decreased from 15 eV to 6 eV as the Ar pressure increased from 0.67 Pa to 26.6 Pa. The AFM images indicated that the argon plasma exposure resulted in insignificant change in surface morphology. The XPS analyses exhibited that the degradation of the O = C–O bond, the C–O bond and the phenyl group could be effectively suppressed in the plasma exposures with ion energy below 6 eV. Acknowledgements
Fig. 5. Variation of the peak-area ratio of π → π* shake-up satellite peak to C–C bond (Iπ → π* / IC–C) evaluated from XPS C1s spectra.
with the mass-separated ion-energy analyzer. As elucidated above for Fig. 2, the degradations of the O = C–O bond and the C–O bond are observed to be effectively suppressed in the plasma exposure with ion energy below 6 eV, whereas, degradations of these bonds are significantly enhanced with increasing ion energy. Additionally, the degradation of these bonds are encountered with increase of the C = O bond function, which is considered to be due to the scission of the O = C–O bond. Furthermore, the XPS C1s spectra to highlight the phenyl group (π → π* shake-up satellite peak) are shown in Fig. 4, in which the spectra in Fig. 2 are expanded with the intensities for the C–C bond normalized. Peak-area ratio of π → π* shake-up satellite peak to C–C bond (Iπ → π* / IC–C) evaluated from XPS C1s spectra is summarized in Fig. 5 as a function of the peak ion energy. The π → π* shake-up satellite peak clearly decreased after the plasma exposure, which was sustained at an Ar pressure lower than 13 Pa, then the values of Iπ → π* / IC–C decreased from 0.03 to 0.01. These results of chemical bonding analyses indicate that it is possible to prevent the decomposition of C–O bond, O = C–O bond and phenyl group via controlling the Ar pressure to suppress the ion energy less than 6 eV. These findings are considered to provide significant knowledge to establish the bond engineering for plasma processes of soft materials. 4. Summary The plasma–polymer interactions were investigated using AFM and XPS of PET films, which were exposed to argon plasmas driven by LIA modules. The ion-energy distribution of the argon ions impinging onto the ground potential through the sheath edge of the argon plasmas was measured using the mass-separated ion-energy analyz-
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