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Effect of atmospheric-pressure plasma on adhesion characteristics of polyimide film
Journal of Colloid and Interface Science 285 (2005) 267–272 www.elsevier.com/locate/jcis
Effect of atmospheric-pressure plasma on adhesion characteristics of polyimide film Soo-Jin Park ∗ , Hwa-Young Lee Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Daejeon 305-600, South Korea Received 21 July 2004; accepted 29 November 2004
Abstract In this work, the effect of atmospheric-pressure plasma treatments on surface properties of polyimide film are investigated in terms of X-ray photoelectron spectroscopy (XPS), contact angles, and atomic force microscopy (AFM). The adhesion characteristics of the film are also studied in the peel strengths of polyimide/copper film. As experimental results, the polyimide surfaces treated by plasma lead to an increase of oxygen-containing functional groups or the polar component of the surface free energy, resulting in improving the adhesion characteristics of the polyimide/copper foil. Also, the roughness of the film surfaces, confirmed by AFM observation, is largely increased. These results can be explained by the fact that the atmospheric-pressure plasma treatment of polyimide surface yields several oxygen complexes in hydrophobic surfaces, which can play an important role in increasing the surface polarity, wettability, and the adhesion characteristics of the polyimide/copper system. 2004 Elsevier Inc. All rights reserved. Keywords: Polyimide; Atmospheric-pressure plasma; Surface properties; Surface morphology; Adhesion characteristics
1. Introduction Polyimides (PIs) have been widely used as protective overcoats and dielectric layers for semiconductor devices because of their good properties, such as low dissipation factors, excellent thermal and mechanical properties, inherent surface inertness, and low dielectric constants [1–3]. The materials have been considered for use in numerous applications that require robust organic materials, including composites and precursors for high-performance aerospace materials, as well as membranes for gas separation [4,5]. However, in spite of the extensive usage as well as the detailed characterization of the PIs, applications of the materials are limited by their hydrophobic surfaces, which result in poor wettability and adhesion [6,7]. The adhesion of a metal to the PI film is generally poor, unless the PI surface is pretreated prior to metalization. A number of surface * Corresponding author. Fax: +82-42-861-4151.
E-mail address: [email protected] (S.-J. Park). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.11.062
modification techniques, such as wet-chemical treatment [8], plasma treatment [9–11], UV excimer irradiation [12], and pulse ion irradiation [13], have been applied to PI substrates to enhance their adhesion to metals. Plasma treatments are generally thought of as operating under low-pressure conditions to achieve a stable plasma with a variety of gases doped into the reaction chamber. The use of different gases allows increased control of the final surface chemical and physical properties. Low-pressure plasma (LPP) treatments are well known for their ability to impart a variety of surface chemistries to polymers through the use of various gases [14,15]. The equipment for plasma treatment of continuous polymer webs at low pressure is costly and requires the upkeep of large vacuum systems [16]. Atmospheric-pressure plasma (APP) is a new technology for plasma surface treatments in ambient atmospheric, not in vacuum chambers. Nonthermal APPs having temperatures of 50 to 200 ◦ C are used to enhance hydrophilicity and adhesiveness of surfaces or to clean them. High-energy electrons exiting in plasma produce ions and reactive species that interact with surfaces to modify the surface characteristics.
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S.-J. Park, H.-Y. Lee / Journal of Colloid and Interface Science 285 (2005) 267–272
Nonthermal APP systems have structures simpler than those of vacuum systems and are capable of continuous treatments in a single step and a simple process. Possibly, the APP system is one of the most suitable ways to improve the wettability and very attractive for their high productivity and low costs [17,18]. In this work, we attempt to investigate the effect on surface properties of the PI films treated by different treatment power of APP. The influences of surface properties on the adhesion strength of PI films are also studied by a peel test with a metal layer.
2. Experimental 2.1. Materials and sample preparation The PI film used in this study was purchased from the Du Pont Chemical Co. as Kapton 50 µm in thickness. The surface of the PI film was cleaned with methanol in an ultrasonic water bath for 30 min and then dried in a vacuum oven at a room temperature. The atmospheric-pressure plasma surface treatment system was manufactured by PLASMART, Korea (ATMOSMulti). A schematic diagram of atmospheric plasma treatment apparatus is shown in Fig. 1. The gases used for the plasma treatments were mixture of Ar (99%) and O2 (1%). The nominal treatment time was fixed to 1 min and the treatment power was varied between 0 and 300 W at radio frequency 13.56 MHz, namely PI-0, PI-150, PI-200, and PI300. 2.2. FT-IR spectroscopy FT-IR ATR (attenuated total reflection) spectra of APPtreated PI films were obtained using a Digilab FTS-165 spec-
trometer with a KRS-5 reflection element. The scans were shown from 400 to 4000 cm−1 and required 40 s to complete. 2.3. X-ray photoelectron spectroscopy The X-ray photoelectron spectroscopy (XPS) measurement of PI surfaces was performed using a VG Scientific ESCA LAB MK-II spectrometer equipped with a MgKα X-ray source. The base pressure in the sample chamber was controlled in the range of 10−8 to 10−9 Torr. 2.4. Surface free energy The surface free energy of the APP-treated PI films was determined at 20 ± 1 ◦ C using a sessile drop method on a Surface & Electro-Optic Co. SEO 300A. Two different wetting liquids, distilled water and diiodomethane, were selected. For each sample, every calculated contact angle was an average of 10 measurements with a standard deviation below 1◦ . For this work, the surface tension and its components for the wetting liquids are shown in Table 1. 2.5. Surface morphology The surface morphologies of the APP-treated PI films were studied by atomic force microscopy (AFM), using a Digital Instruments Inc. Nanoscope III microscope. In each case, an area of 10 × 10 µm was scanned using the tapping mode. The AFM observation was carried out at ambient pressure and room temperature. An arithmetic mean of the surface roughness (Ra ) was calculated from the roughness profile determined by AFM. 2.6. Adhesion characteristics The adhesion strength of the electrolessly deposited copper with the various APP-treated PI substrates was determined by measuring the T-peel adhesion strength. The copper foil used as a metal layer for the T-peel test was supplied by Fukuda with a thickness of 50 µm and the epoxy resin was the diglycidyl ether of bisphenol A (YD-128, supplied by the Kukdo Chemical Co. of Korea). The APP-treated PI films were adhered to a copper foil using an epoxy resin for the subsequent and then cured for 2 h at 120 ◦ C of cure time. The T-peel test (ASTM 1876-72) was measured at a peel rate of 254 mm/min using Lloyd LR5K. Table 1 London dispersive (γLL ) and specific (γLSP ) components of surface free energy (γL ) in wetting liquids (subscript L), measured at 20 ◦ C
Fig. 1. Schematic diagram of APP treatment apparatus.
Wetting liquid
γLL (mJ m−2 )
γLSP (mJ m−2 )
γL (mJ m−2 )
Water Diiodomethane
21.8 50.42
51.0 0.38
72.8 50.8
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269
Fig. 2. FT-IR spectra of APP-treated PI films. Fig. 3. XPS survey scan spectra of APP-treated PI films.
3. Results and discussion 3.1. Chemical compositions and structures of APP-treated PI surfaces
Table 2 Results of the O1s /C1s ratio of APP-treated PI films O1s /C1s ratio (%)
The chemical structures of APP-treated PI films are determined with FT-IR. Fig. 2 shows the FT-IR ATR spectra of the APP-treated PI films. The characteristic absorption bands of PIs appear near 1780 (C=O in phase), 1720 (C=O out of phase), 1502 (C=C in C6 H5 ), 1370 (C–N in imide), and 1100 ((OC)2 NC in imide) cm−1 . As seen in Fig. 2, the absorption at 1780, 1720 (C=O), 1502 (C=C in aromatic ring), and 1100 ((OC)2 NC in imide) cm−1 of PI-150, 200, 300 is increased, while there are not enough differences in the absorption at 1370 (C–N in imide) cm−1 . Especially, the absorption bands at 3660–3200 (O–H stretching peaks in carboxyl acids) cm−1 of APP-treated PI films are largely increased compared with PI-0. These results indicate that reactive free radicals, formed by APP treatments, colloid with the surface of PIs, which introduce the oxygen-containing functional groups. The changes of chemical compositions of APP-treated PI are also studied by XPS measurements. Fig. 3 shows XPS survey scan spectra of the APP-treated PI films. The XPS spectra of PI films show carbon, nitrogen, and oxygen (binding energy, 285, 404, and 532 eV, respectively) peaks [19]. From the results of XPS spectra, Table 2 represents the O1s /C1s ratios of the PI before and after APP treatments. As experimental results, the O1s /C1s ratios of the PI are largely increased with increasing treatment power, which is probably due to the increase of oxygen-containing functional groups of PI surfaces by the APP treatments. The increase of the O1s /C1s ratios is attributable to the C=O species, which is different from the carbonyl structure of the imide group in the pyromellitic dianhydride chain, and to the carboxyl species. The presence of the carbonyl and carboxyl species on the APP-treated PI surface is attributable to the oxida-
PI-0
PI-150
PI-200
PI-300
23.10
36.7
37.6
38.9
tion in air of the active species on the PI surface induced by the APP treatments. In agreement with the results generally reported in many previous works, an increase in the O1s /C1s ratios is observed with increasing input RF power used for the treatment of the PI surfaces. It is noted that APP treatment of PI results in the generation of radicals on the surface with the subsequent exposure to air resulting in the formation of various oxygen-containing species, such as peroxides and hydroperoxides, which can be utilized to initiate the surface free-radicals [9]. 3.2. Surface free energies of APP-treated PI films The concept of surface free energy, γ , for physical interactions which can be resolved into a London dispersive component (superscript: L) and specific (or polar, SP) component [20–24] is γ = γ L + γ SP .
(1)
Fig. 4 shows the results of surface free energies of the APP-treated PI films. As a result, it is observed that the polar component of surface free energy is largely increased with increasing APP treatment power. These behaviors can be interpreted as showing that the APP treatment of the PI films surfaces produces various oxygen complexes in hydrophobic surfaces, which can be attributed to the increase of the surface polarity and wettability, resulting in improving the total surface free energies. It is also found that dispersive components of the APP-treated PI films are slightly increased up to PI-200 and then decreased at PI-300. This result indicated that APP treatment at suitable treatment power can
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Fig. 4. Surface free energies of APP-treated PI films (γS , surface free energy; γSSP , specific component; γSL , London dispersive component).
Fig. 5. Aging effects on the polar component of surface free energy of APP-treated PI films.
cause the increase of not only the polar component but also the dispersive component, due to the scratching effects of the plasma treatment itself, attributed to the increasing specific surface area of the PI films, resulting in a slight increase of hydrophobicity in APP-treated PI films [25]. Meanwhile, the surface free energy of the film decreases as the aging time increases, as shown in Fig. 5. These results can be discussed in the formation of surface functional groups or the deterioration of reactive sites of PI film in the presence of APP treatment with aging time. It is also found that the APP-treated PI films at high treatment power show less aging effect than PI-150. This result indicates that the films at high APP power probably have enough energy to form intermolecular bonds between the newly produced oxygen complex and the film surfaces [26].
5.1 nm, respectively. Earlier studies [28] have shown that plasma treatments have generated more excited species in the plasma chamber. The excited species, in turn, give rise to the formation of more crosslinked aggregates on the PI surfaces and account for the observed increase in high surface roughness. Also, it seems that the excited species, such as oxygen complexes, exists as a distinctive overlayer on the PI surfaces, while the surface of the pristine PI film is comparatively flat.
3.3. Surface morphology of the APP-treated PI films The surface morphology is of great importance for the surface wettability of the polymer films. In this work, AFM is employed to examine the morphological changes induced on the APP-treated-PI films. It is known that polymer surface treatments made with coronas or plasmas can result in both surface crosslinking and chain scission and these treatments have also been shown to lead to changes in the polymer surface morphologies [27]. Based on observations from these results, it has been hypothesized that the lowmolecular-weight organic materials, attributed to the chain scission, coalesce to form mounds or droplets on the surfaces. Figs. 6 and 7 show the 2-D and 3-D AFM images of the APP-treated PI films, respectively. The differences in morphology after surface treatment further support variation in the relative amounts of ablation, chain scission, and other functionalization. The root-mean-square surface roughness (Ra ) of the pure PI surface is about 2.7 nm. The Ra values of APP-treated PI films increase up to 8.9, 12.9, and
3.4. Adhesion strength of APP-treated PI/copper joints Chou and Tang [29] investigate the cause of poor adhesion between polyimide and copper and they postulate that a metal–oxygen–carbon complex is formed at the polyimide film/metal interface in the metalization process and this complex dictated the adhesion strengths. This investigation suggests the importance of special functional groups that can strongly interact with copper metal. The ultimate adhesion strengths between the metal and the polymer substrate is one of the primary concerns in the actual electronic applications. Fig. 8 represents the peel strength values of PI/copper samples with different APP-treatment powers. These adhesion strengths are much higher in APP-treated PI/copper systems than that of the pristine PI/copper system. The strong adhesion of the electrolessly deposited copper to the APPtreated PI film is attributed to the strong interaction between the oxygen complexes or the spatial distribution of the crosslinked aggregation on the PI surfaces and the metal matrix.
4. Conclusions In this work, the surface properties and adhesion characteristics of the atmospheric-pressure plasma (APP)-treated
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Fig. 6. Two-dimensional AFM spectra of APP-treated PI films.
Fig. 7. Three-dimensional AFM spectra of APP-treated PI films.
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Fig. 8. T-peel strengths of APP-treated PI film/copper foil adhesive joints as a function of APP treatment power.
polyimide (PI) films were investigated in terms of XPS, contact angles, and AFM. It was found that the atmosphericpressure treatments introduce organic functional groups onto PI surfaces, resulting in improving the O1s /C1s ratios and the surface free energies of the PI surfaces. Also, the roughness (Ra ) of the film surfaces, confirmed by AFM observation, was greatly increased by APP treatments. The adhesion strengths at interfaces between the treated PI film and the copper matrix showed the superior increments, which were attributed to the strong interaction between the oxygencontaining groups of PI and copper film. These results can be explained by the increment of the polarity and hydrophilicity of the PI surfaces or the slight increase of the dispersive component and roughness, due to the increase of specific surface area during plasma treatment.
References [1] K.L. Mittal, Polyimides: Synthesis, Characterization, and Applications, Plenum, New York, 1984.
[2] D. Wilson, H.D. Stenzenberger, P.M. Hergenrother, Polyimides, Chapman & Hall, New York, 1990. [3] W.C. Wang, R.H. Vora, E.T. Kang, K.G. Neoh, Macromol. Mater. Eng. 288 (2003) 152. [4] T. Liang, Y. Makita, S. Kimura, Polymer 42 (2001) 4867. [5] M.K. Ghosh, K.L. Mittal, Polyimides: Fundamentals and Applications, Dekker, New York, 1996. [6] A.K.S. Ang, E.T. Kang, K.G. Neoh, K.L. Tan, C.Q. Cui, B.T. Lim, Polymer 41 (2000) 489. [7] A. Weber, A. Dietz, R. Pockelmann, C.P. Klages, J. Electrochem. Soc. 3 (1997) 1131. [8] K.W. Lee, S.P. Kowalczyk, J.M. Shaw, Langmuir 7 (1991) 2450. [9] N. Inagaki, S. Tasaka, K. Hibi, J. Polym. Sci. Polym. Chem. 30 (1992) 1425. [10] G. Rozovskis, J. Vinkevicius, J. Jaciauskiene, J. Adhes. Sci. Technol. 10 (1996) 399. [11] Z.J. Yu, E.T. Kang, K.G. Neoh, Polymer 34 (2002) 4137. [12] J.Y. Zhang, H. Esrom, U. Kogelschartz, G. Eming, J. Adhes. Sci. Technol. 8 (1994) 1179. [13] M. Celina, H. Kudoh, T.J. Renk, K.T. Gillen, R.L. Clough, Radiat. Phys. Chem. 10 (1998) 243. [14] M. Tatoulian, J. Adhes. Adhes. 15 (1995) 177. [15] R. Foerch, J. Izawa, G. Spears, J. Adhes. Sci. Technol. 5 (1991) 549. [16] S. Kanazawa, M. Kogoma, T. Moriwaki, S. Okazako, J. Phys. D Appl. Phys. 21 (1988) 838. [17] L.J. Ward, W.C.E. Schofield, J.P.S. Badyal, A.J. Goodwin, P.J. Merlin, Chem. Mater. 15 (2003) 1466. [18] Q. Zhang, C. Wang, Y. Babukutty, T. Ohyama, M. Kogoma, M. Kodama, J. Biomed. Mater. Res. 60 (2002) 502. [19] S.J. Park, Y.S. Jang, J. Colloid Interface Sci. 237 (2001) 91. [20] F.M. Fowkes, J. Phys. Chem. 67 (1963) 2538. [21] D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. [22] S. Wu, Polymer Interface and Adhesion, Dekker, New York, 1982, p. 133. [23] S.J. Park, Encyclopedia of Surface and Colloid Science: Van der Waals Interactions at Surfaces, Dekker, New York, 2002, p. 5570. [24] S.J. Park, J.S. Kim, J. Colloid Interface Sci. 232 (2000) 311. [25] S.J. Park, K.S. Cho, S.K. Ryu, Carbon 41 (2003) 1437. [26] C. Girardeaux, E. Druet, P. Demoncy, M. Delamar, J. Electron Spectrosc. Relat. Phenom. 74 (1995) 57. [27] O.D. Greenwood, R.D. Boyd, J. Hopkins, J.P. Badyal, J. Adhes. Sci. Technol. 9 (1995) 311. [28] Y. Zhang, K.L. Tan, B.Y. Liaw, D.J. Liaw, E.T. Kang, G.K. Neoh, Langmuir 17 (2001) 2265. [29] N.J. Chou, C.H. Tang, J. Vac. Sci. Technol. A 2 (1984) 751.
Effect of atmospheric-pressure plasma on adhesion characteristics of polyimide film Soo-Jin Park ∗ , Hwa-Young Lee Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Daejeon 305-600, South Korea Received 21 July 2004; accepted 29 November 2004
Abstract In this work, the effect of atmospheric-pressure plasma treatments on surface properties of polyimide film are investigated in terms of X-ray photoelectron spectroscopy (XPS), contact angles, and atomic force microscopy (AFM). The adhesion characteristics of the film are also studied in the peel strengths of polyimide/copper film. As experimental results, the polyimide surfaces treated by plasma lead to an increase of oxygen-containing functional groups or the polar component of the surface free energy, resulting in improving the adhesion characteristics of the polyimide/copper foil. Also, the roughness of the film surfaces, confirmed by AFM observation, is largely increased. These results can be explained by the fact that the atmospheric-pressure plasma treatment of polyimide surface yields several oxygen complexes in hydrophobic surfaces, which can play an important role in increasing the surface polarity, wettability, and the adhesion characteristics of the polyimide/copper system. 2004 Elsevier Inc. All rights reserved. Keywords: Polyimide; Atmospheric-pressure plasma; Surface properties; Surface morphology; Adhesion characteristics
1. Introduction Polyimides (PIs) have been widely used as protective overcoats and dielectric layers for semiconductor devices because of their good properties, such as low dissipation factors, excellent thermal and mechanical properties, inherent surface inertness, and low dielectric constants [1–3]. The materials have been considered for use in numerous applications that require robust organic materials, including composites and precursors for high-performance aerospace materials, as well as membranes for gas separation [4,5]. However, in spite of the extensive usage as well as the detailed characterization of the PIs, applications of the materials are limited by their hydrophobic surfaces, which result in poor wettability and adhesion [6,7]. The adhesion of a metal to the PI film is generally poor, unless the PI surface is pretreated prior to metalization. A number of surface * Corresponding author. Fax: +82-42-861-4151.
E-mail address: [email protected] (S.-J. Park). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.11.062
modification techniques, such as wet-chemical treatment [8], plasma treatment [9–11], UV excimer irradiation [12], and pulse ion irradiation [13], have been applied to PI substrates to enhance their adhesion to metals. Plasma treatments are generally thought of as operating under low-pressure conditions to achieve a stable plasma with a variety of gases doped into the reaction chamber. The use of different gases allows increased control of the final surface chemical and physical properties. Low-pressure plasma (LPP) treatments are well known for their ability to impart a variety of surface chemistries to polymers through the use of various gases [14,15]. The equipment for plasma treatment of continuous polymer webs at low pressure is costly and requires the upkeep of large vacuum systems [16]. Atmospheric-pressure plasma (APP) is a new technology for plasma surface treatments in ambient atmospheric, not in vacuum chambers. Nonthermal APPs having temperatures of 50 to 200 ◦ C are used to enhance hydrophilicity and adhesiveness of surfaces or to clean them. High-energy electrons exiting in plasma produce ions and reactive species that interact with surfaces to modify the surface characteristics.
268
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Nonthermal APP systems have structures simpler than those of vacuum systems and are capable of continuous treatments in a single step and a simple process. Possibly, the APP system is one of the most suitable ways to improve the wettability and very attractive for their high productivity and low costs [17,18]. In this work, we attempt to investigate the effect on surface properties of the PI films treated by different treatment power of APP. The influences of surface properties on the adhesion strength of PI films are also studied by a peel test with a metal layer.
2. Experimental 2.1. Materials and sample preparation The PI film used in this study was purchased from the Du Pont Chemical Co. as Kapton 50 µm in thickness. The surface of the PI film was cleaned with methanol in an ultrasonic water bath for 30 min and then dried in a vacuum oven at a room temperature. The atmospheric-pressure plasma surface treatment system was manufactured by PLASMART, Korea (ATMOSMulti). A schematic diagram of atmospheric plasma treatment apparatus is shown in Fig. 1. The gases used for the plasma treatments were mixture of Ar (99%) and O2 (1%). The nominal treatment time was fixed to 1 min and the treatment power was varied between 0 and 300 W at radio frequency 13.56 MHz, namely PI-0, PI-150, PI-200, and PI300. 2.2. FT-IR spectroscopy FT-IR ATR (attenuated total reflection) spectra of APPtreated PI films were obtained using a Digilab FTS-165 spec-
trometer with a KRS-5 reflection element. The scans were shown from 400 to 4000 cm−1 and required 40 s to complete. 2.3. X-ray photoelectron spectroscopy The X-ray photoelectron spectroscopy (XPS) measurement of PI surfaces was performed using a VG Scientific ESCA LAB MK-II spectrometer equipped with a MgKα X-ray source. The base pressure in the sample chamber was controlled in the range of 10−8 to 10−9 Torr. 2.4. Surface free energy The surface free energy of the APP-treated PI films was determined at 20 ± 1 ◦ C using a sessile drop method on a Surface & Electro-Optic Co. SEO 300A. Two different wetting liquids, distilled water and diiodomethane, were selected. For each sample, every calculated contact angle was an average of 10 measurements with a standard deviation below 1◦ . For this work, the surface tension and its components for the wetting liquids are shown in Table 1. 2.5. Surface morphology The surface morphologies of the APP-treated PI films were studied by atomic force microscopy (AFM), using a Digital Instruments Inc. Nanoscope III microscope. In each case, an area of 10 × 10 µm was scanned using the tapping mode. The AFM observation was carried out at ambient pressure and room temperature. An arithmetic mean of the surface roughness (Ra ) was calculated from the roughness profile determined by AFM. 2.6. Adhesion characteristics The adhesion strength of the electrolessly deposited copper with the various APP-treated PI substrates was determined by measuring the T-peel adhesion strength. The copper foil used as a metal layer for the T-peel test was supplied by Fukuda with a thickness of 50 µm and the epoxy resin was the diglycidyl ether of bisphenol A (YD-128, supplied by the Kukdo Chemical Co. of Korea). The APP-treated PI films were adhered to a copper foil using an epoxy resin for the subsequent and then cured for 2 h at 120 ◦ C of cure time. The T-peel test (ASTM 1876-72) was measured at a peel rate of 254 mm/min using Lloyd LR5K. Table 1 London dispersive (γLL ) and specific (γLSP ) components of surface free energy (γL ) in wetting liquids (subscript L), measured at 20 ◦ C
Fig. 1. Schematic diagram of APP treatment apparatus.
Wetting liquid
γLL (mJ m−2 )
γLSP (mJ m−2 )
γL (mJ m−2 )
Water Diiodomethane
21.8 50.42
51.0 0.38
72.8 50.8
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269
Fig. 2. FT-IR spectra of APP-treated PI films. Fig. 3. XPS survey scan spectra of APP-treated PI films.
3. Results and discussion 3.1. Chemical compositions and structures of APP-treated PI surfaces
Table 2 Results of the O1s /C1s ratio of APP-treated PI films O1s /C1s ratio (%)
The chemical structures of APP-treated PI films are determined with FT-IR. Fig. 2 shows the FT-IR ATR spectra of the APP-treated PI films. The characteristic absorption bands of PIs appear near 1780 (C=O in phase), 1720 (C=O out of phase), 1502 (C=C in C6 H5 ), 1370 (C–N in imide), and 1100 ((OC)2 NC in imide) cm−1 . As seen in Fig. 2, the absorption at 1780, 1720 (C=O), 1502 (C=C in aromatic ring), and 1100 ((OC)2 NC in imide) cm−1 of PI-150, 200, 300 is increased, while there are not enough differences in the absorption at 1370 (C–N in imide) cm−1 . Especially, the absorption bands at 3660–3200 (O–H stretching peaks in carboxyl acids) cm−1 of APP-treated PI films are largely increased compared with PI-0. These results indicate that reactive free radicals, formed by APP treatments, colloid with the surface of PIs, which introduce the oxygen-containing functional groups. The changes of chemical compositions of APP-treated PI are also studied by XPS measurements. Fig. 3 shows XPS survey scan spectra of the APP-treated PI films. The XPS spectra of PI films show carbon, nitrogen, and oxygen (binding energy, 285, 404, and 532 eV, respectively) peaks [19]. From the results of XPS spectra, Table 2 represents the O1s /C1s ratios of the PI before and after APP treatments. As experimental results, the O1s /C1s ratios of the PI are largely increased with increasing treatment power, which is probably due to the increase of oxygen-containing functional groups of PI surfaces by the APP treatments. The increase of the O1s /C1s ratios is attributable to the C=O species, which is different from the carbonyl structure of the imide group in the pyromellitic dianhydride chain, and to the carboxyl species. The presence of the carbonyl and carboxyl species on the APP-treated PI surface is attributable to the oxida-
PI-0
PI-150
PI-200
PI-300
23.10
36.7
37.6
38.9
tion in air of the active species on the PI surface induced by the APP treatments. In agreement with the results generally reported in many previous works, an increase in the O1s /C1s ratios is observed with increasing input RF power used for the treatment of the PI surfaces. It is noted that APP treatment of PI results in the generation of radicals on the surface with the subsequent exposure to air resulting in the formation of various oxygen-containing species, such as peroxides and hydroperoxides, which can be utilized to initiate the surface free-radicals [9]. 3.2. Surface free energies of APP-treated PI films The concept of surface free energy, γ , for physical interactions which can be resolved into a London dispersive component (superscript: L) and specific (or polar, SP) component [20–24] is γ = γ L + γ SP .
(1)
Fig. 4 shows the results of surface free energies of the APP-treated PI films. As a result, it is observed that the polar component of surface free energy is largely increased with increasing APP treatment power. These behaviors can be interpreted as showing that the APP treatment of the PI films surfaces produces various oxygen complexes in hydrophobic surfaces, which can be attributed to the increase of the surface polarity and wettability, resulting in improving the total surface free energies. It is also found that dispersive components of the APP-treated PI films are slightly increased up to PI-200 and then decreased at PI-300. This result indicated that APP treatment at suitable treatment power can
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Fig. 4. Surface free energies of APP-treated PI films (γS , surface free energy; γSSP , specific component; γSL , London dispersive component).
Fig. 5. Aging effects on the polar component of surface free energy of APP-treated PI films.
cause the increase of not only the polar component but also the dispersive component, due to the scratching effects of the plasma treatment itself, attributed to the increasing specific surface area of the PI films, resulting in a slight increase of hydrophobicity in APP-treated PI films [25]. Meanwhile, the surface free energy of the film decreases as the aging time increases, as shown in Fig. 5. These results can be discussed in the formation of surface functional groups or the deterioration of reactive sites of PI film in the presence of APP treatment with aging time. It is also found that the APP-treated PI films at high treatment power show less aging effect than PI-150. This result indicates that the films at high APP power probably have enough energy to form intermolecular bonds between the newly produced oxygen complex and the film surfaces [26].
5.1 nm, respectively. Earlier studies [28] have shown that plasma treatments have generated more excited species in the plasma chamber. The excited species, in turn, give rise to the formation of more crosslinked aggregates on the PI surfaces and account for the observed increase in high surface roughness. Also, it seems that the excited species, such as oxygen complexes, exists as a distinctive overlayer on the PI surfaces, while the surface of the pristine PI film is comparatively flat.
3.3. Surface morphology of the APP-treated PI films The surface morphology is of great importance for the surface wettability of the polymer films. In this work, AFM is employed to examine the morphological changes induced on the APP-treated-PI films. It is known that polymer surface treatments made with coronas or plasmas can result in both surface crosslinking and chain scission and these treatments have also been shown to lead to changes in the polymer surface morphologies [27]. Based on observations from these results, it has been hypothesized that the lowmolecular-weight organic materials, attributed to the chain scission, coalesce to form mounds or droplets on the surfaces. Figs. 6 and 7 show the 2-D and 3-D AFM images of the APP-treated PI films, respectively. The differences in morphology after surface treatment further support variation in the relative amounts of ablation, chain scission, and other functionalization. The root-mean-square surface roughness (Ra ) of the pure PI surface is about 2.7 nm. The Ra values of APP-treated PI films increase up to 8.9, 12.9, and
3.4. Adhesion strength of APP-treated PI/copper joints Chou and Tang [29] investigate the cause of poor adhesion between polyimide and copper and they postulate that a metal–oxygen–carbon complex is formed at the polyimide film/metal interface in the metalization process and this complex dictated the adhesion strengths. This investigation suggests the importance of special functional groups that can strongly interact with copper metal. The ultimate adhesion strengths between the metal and the polymer substrate is one of the primary concerns in the actual electronic applications. Fig. 8 represents the peel strength values of PI/copper samples with different APP-treatment powers. These adhesion strengths are much higher in APP-treated PI/copper systems than that of the pristine PI/copper system. The strong adhesion of the electrolessly deposited copper to the APPtreated PI film is attributed to the strong interaction between the oxygen complexes or the spatial distribution of the crosslinked aggregation on the PI surfaces and the metal matrix.
4. Conclusions In this work, the surface properties and adhesion characteristics of the atmospheric-pressure plasma (APP)-treated
S.-J. Park, H.-Y. Lee / Journal of Colloid and Interface Science 285 (2005) 267–272
Fig. 6. Two-dimensional AFM spectra of APP-treated PI films.
Fig. 7. Three-dimensional AFM spectra of APP-treated PI films.
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Fig. 8. T-peel strengths of APP-treated PI film/copper foil adhesive joints as a function of APP treatment power.
polyimide (PI) films were investigated in terms of XPS, contact angles, and AFM. It was found that the atmosphericpressure treatments introduce organic functional groups onto PI surfaces, resulting in improving the O1s /C1s ratios and the surface free energies of the PI surfaces. Also, the roughness (Ra ) of the film surfaces, confirmed by AFM observation, was greatly increased by APP treatments. The adhesion strengths at interfaces between the treated PI film and the copper matrix showed the superior increments, which were attributed to the strong interaction between the oxygencontaining groups of PI and copper film. These results can be explained by the increment of the polarity and hydrophilicity of the PI surfaces or the slight increase of the dispersive component and roughness, due to the increase of specific surface area during plasma treatment.
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