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Surface modification of polyimide film by coupling reaction for copper metallization
Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Surface modification of polyimide film by coupling reaction for copper metallization Hwa Jin Kim a,b, Yun Jun Park a, Jong-Ho Choi a, Hak Soo Han b, Young Taik Hong a,* a b
Energy Materials Research Center, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yuseong-gu, Daejon 305-600, Republic of Korea Electronic Materials Research Lab, Yonsei University, 134 Sinchon, Seodaemun, Seoul 120-749, Republic of Korea
A R T I C L E I N F O
Article history: Received 13 July 2008 Accepted 3 August 2008 Keywords: Polyimide Surface modification Adhesion Copper Coupling reaction Flexible copper clad laminate
A B S T R A C T
In this study, Upilex-S [poly(biphenyl dianhydride-p-phenylene diamine)], one of polyimide films, was modified by coupling reactions with N,N-carbonyldiimidazole (CDI) to increase adhesion to copper for flexible copper clad laminate (FCCL). Imidazole groups show strong interaction with copper metal to make charge transfer complexes. Because polyimide film did not have active site with coupling agent, the film surfaces were modified by aqueous KOH solutions and reacted with dilute HCl solutions. Surface modified Upilex-S was analyzed by X-ray photoelectron spectroscopy (XPS) to examine the surface chemical composition and film morphology and investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Changes in the wettability were evaluated by measuring contact angle with the sessile drop method. After deposition of copper on surface modified Upilx-S, the adhesion strength of the copper/polyimide system was measured by a 908 peel test using the Instron tensile strength tester. The peel strength of the copper/polyimide system increased from 0.25 to 0.86 kgf/ cm by surface modification. This result confirmed that the CDI coupling reaction is an effective treatment method for the improvement of the adhesion property between copper metal and polyimide film. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction The high density electronics such as notebooks, cellular phones, and personal digital assistants (PDAs) are increasingly in great demands and the high density of electronic packages are also required. For the packaging on driver integrated circuit (IC), a flexible design package is needed. In order to interconnect device components or layer on flexible substrates, copper is used for metallization on the flexible substrate such as polyimide. Polyimide is employed as dielectric layers in microelectronics applications such as flexible printed circuit board (FPCB) and multichip module, since they have low dielectric constant, high thermal stability, low moisture absorption, good mechanical properties and good processability [1–5]. But the adhesion property between polyimide film and copper was poor due to the inactivity of polyimide surfaces. Therefore, three-layer process, which is constituted by copper, adhesion promoter and polyimide, had been studied to enhance adhesion [6,7]. Organic adhesive such as epoxy resin, or metal, like Cr or Ni, was used as adhesion promoter. But this three-layer process had crucial drawbacks in
* Corresponding author. Tel.: +82 42 860 7292; fax: +82 42 861 4151. E-mail address: [email protected] (Y.T. Hong).
that they were easily delaminated because each component has different coefficient of thermal expansion (CTE) and residual stress. In addition, weak thermal stability of organic adhesive was another problem [8,9]. In contrast, two layer-flexible copper clad laminate (FCCL), without any adhesion promoter, is more excellent than three layer-FCCL for several reasons, which are miniature of circuit, good insulation characteristics of the matrix wafer, good durability against a moisture absorption and high thermal stability. Therefore, the FPCB which demands a precision is mainly made by using the two layer-FCCL. In two layer-FCCL, the polyimide surface is usually modified by dry process such as plasma, corona discharge, X-ray, laser, ion beam, or flame treatments [10,11]. Recently, interest in wet-process for surface modification of polyimide has increased because of its simplicity and low cost [12,13]. In a typical wet process for polyimide surface modification, the polyimide substrate is immersed into chemical solution or chemical solution is sprayed on polyimide film. In this study, Upilex-S film surfaces were modified by aqueous KOH solutions at various conditions to yield potassium polyamates. Subsequently, they reacted with dilute HCl solutions to convert it into polyamic acids. The resulting polyimide films are expected to have active sites to react with coupling agents to improve the adhesion properties. It was reported that imidazole functional groups contained N,N0 -carbonyldiimidazole (CDI)
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.08.016
24
H.J. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
Fig. 1. Schematic diagrams illustrating the processes of surface modification and copper deposition.
showed strong interactions with copper metal to form charge transfer complexes [14] and carboxylic acid groups were efficiently activated with CDI [15]. We report that the CDI-coupled polyimide could be formed on the polyimide surface and they adhere strongly to the copper metal by complexing between the copper atoms and
imidazole functional groups. Surface chemistry and morphological changes of films were analyzed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Adhesion properties were investigated by contact angle measurement and tensile strength tester.
Fig. 2. XPS high-resolution spectra of Upilex-S (C1s component); (a) pristine Upilex-S; (b) KOH treatment followed by acidification; (c) CDI-coupling treatment.
H.J. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
2. Experimental 2.1. Materials The polyimide films used in this study was Upilex-S (25 mm thick, Ube Industries). The films were cleaned prior to use in ethanol at room temperature for 5 min under ultrasonication and dried. Potassium hydroxide and hydrochloric acid were purchased from Samchun Chemical Co. and Showa Chemical Co., respectively. N,N0 carbonyldiimidazole was obtained from Aldrich Chemical Co. and was used without further purification as a reagent for the surface modification by coupling reactions. Deionized water was used for the preparation of all aqueous solutions and washing steps. 2.2. Surface modification by coupling reactions The surface modification of the polyimide films was carried out in three steps. First, the polyimide films were immersed in different concentrations (1 or 3 M) of KOH aqueous solutions at room temperature for various times (1, 5, and 10 min). After this step, the resulting polyimide film surfaces were changed to potassium polyamate. Second, the potassium polyamate samples were immersed in 0.2 M HCl aqueous solution at room temperature for 5 min in order to be formed as polyamic acid. This step makes Upilex-S film wettable to form carboxylic acid groups, which could
25
immobilize new functional group on its film surface through coupling reaction. Third, the acidified samples were dipped into the ethanol solution containing 0.05 M N,N0 -carbonyldiimidazole at room temperature for 5 min, and then heated at 110 8C for 30 min in air oven to induce the coupling reactions. After the coupling reaction, the polyimide films were washed with ethanol using an ultrasonic washer and finally dried. The schematic diagram of hydrolysis and coupling process are shown in Fig. 1. 2.3. Copper metallization on Upilex-S film After surface modification on Upilex-S, it is kept in a plasma sputtering chamber for further coating with copper. Before applying copper coating on Upilex-S, the copper target is precleaned by argon plasma sputtering for 5 min. After the surface cleaning on the copper target, the copper coating was applied onto Upilex-S. The average thickness of sputtered copper for this study was 200 nm and sputtered film was subsequently electroplated with a copper metal layer of 8–10 mm thickness. The electroplating process was carried out at a constant DC current density of 3 104 A/m2 at room temperature in aqueous sulfuric acid solution (90 g/L) containing copper sulfate (0.5 M), hydrochloric acid (50 ppm) and a glossy reagent (5 mL). The electroplated Upilex-S films were washed with deionized water and dried at 60 8C for 30 min in air oven.
Fig. 3. XPS high-resolution spectra of Upilex-S (O1s component); (a) pristine Upilex-S; (b) KOH treatment followed by acidification; (c) CDI-coupling treatment.
H.J. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
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2.4. Measurements
energies at the solid–vapor and solid–liquid interface by the following expression [16]:
2.4.1. X-ray photoelectron spectroscopy XPS spectra were obtained with a Kratos Axis NOVA (Manchester, U.K.) instrument, employing a monochromatic excitation radiation of Al Ka at 1486.6 eV. A pass-energy of 160 eV was used for the wide scans at a constant dwell time of 100 ms and 40 eV for the high-resolution scans at a constant dwell time of 250 ms. The X-ray source was run at a reduced power of 150 W. The pressure in the analysis chamber was maintained at 1.7 109 Torr or lower during each measurement. AFM measurement were performed with a NanoScope IV (Digital Instrument, USA) using the tapping mode in air at room temperature. A triangular-pyramidal silicon nitride tip was used and an area of 1 mm 1 mm was scanned under a probe pressure of 8.7 1011 N/m2. A root mean square (RMS) of the surface roughness was calculated from the roughness profile determined by AFM. The surface morphology of the films was observed using a FSM-670F (JEOL, Japan) and specimens were coated with a thin layer of platinum to eliminate charging effects. Contact angle and wetting energy between the surface and deionized water were measured and automatically calculated using a contact angle meter with a phoenix-450 (SEO, Korea). An average value was determined from five measurements. Contact angle was known to correlate with wetting energy, so that the wetting energy may be calculated by using the surface
W e ¼ g SV þ g SL where We is the wetting energy, and g is interfacial free energy with subscript SL referring to solid–liquid interface and SV to solid–vapor interface. Surface energy of the films also can be calculated by measuring contact angle [17]
g ¼ gd þ g p where g is total surface energy which is sum of dispersive interaction (gd) and non-dispersive, i.e. polar interaction (gp). Specially, the solid surface energy can be estimated by the following expression [17]: qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi g L ð1 þ cos uÞ ¼ 2 g dS ldL þ 2 g Sp lLp where gS and gL are the surface energy of solid and liquid, respectively, and u is the contact angle between the solid and liquid. In this study, the surface energy of the films was determined by using two liquid, water and diiodomethane, with known g dL and g Lp . The 908 peel strength (using 3 mm wide strips) of copper metal/Upilex-S film systems was evaluated at a peel rate of 50 mm/ min using Instron 4482-tensile strength tester (Shimadzu, Japan). The peel strength was determined from the average of five specimens.
Fig. 4. XPS high-resolution spectra of Upilex-S (N1s component); (a) pristine Upilex-S; (b) KOH treatment followed by acidification; (c) CDI-coupling treatment.
H.J. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
3. Results and discussion 3.1. XPS analysis Upilex-S polyimide film is only composed of C, O, and N. But, small amounts of silicon are observed in the commercially available Upilex-S film. Silicon incorporation could be intentional (e.g. as part of an additive to reduce frictional drag) or unintentional (as a contaminant, e.g. from silicon rubber rollers uses in the curing process). In the curve-fitting routine, the contributions of silicon were ignored (i.e. a pure BPDA–PDA structure assumed). Three different carbon environments are found; carbon atoms from the aromatic rings that are not directly attached to an imide ring atoms or to nitrogen atoms (282.1 eV), carbon atoms bonded to nitrogen (283.3 eV), and carbon atoms in the imide rings (285.5 eV). In addition, a shake satellite arising from p–p* transition in the aromatic ring appears at 287.68 eV. In Fig. 2(a), the first three peaks have approximate areas of 71.4%, 5.4%, and 12.9%, respectively. In Fig. 2(b), the two carbonyl peaks at 286.3 eV (carboxylate carbon) and 285.2 eV (amide carbon) are separated in polyamic acid. In the XPS spectrum of CDI-modified Upilex-S film, the separated peaks turned back in previous condition according to disappearance of carboxylate carbon peak as shown in Fig. 2(c). The hydrolysis reactions can be further proven by the O1s XPS spectra. Based on the general structure of BPDA–PDA, a single oxygen signal is expected for the carbonyl groups in the imide ring
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(in addition to p–p* sake-up satellite). However, peak asymmetry suggests the presence of an additional contribution at higher binding energy. Fig. 3(a) shows two O1s peaks with binding energies of 529.3 and 530.7 eV. The additional oxygen peak has approximately 20% of the total area of the O1s peak. This peak is located in the range of ether group of polyimide such as PMDA– ODA. The presence of this peak has been reported for the Upilex-S film samples [18] and other thermally cured polyimides [19], which is probably due to oxygen contaminations during production. A shake-up satellite is located at about 534.5 eV. In Fig. 3(b), the O1s XPS spectrum of polyamic acid is fitted with two peaks at 528.8 and 530.5 eV. The low binding energy peak is assigned to the carbonyl oxygen in both carboxylic acid [–C(O)OH] and amide groups [–C(O)NH–]. The high binding energy peak is assigned to the hydroxyl oxygen in the –C(O)OH groups. The appearance of high binding energy peak in the polyamic acid is probably due to the change of the chemical environment of the oxygen atoms from carbonyl oxygen to hydroxyl oxygen in Fig. 3(c). The N1s spectrum of pristine polyimide in Fig. 4 consists of a major component with a weak shoulder on the low binding energy side. Since there is only one nitrogen environment in polyimde, only one peak is expected in this spectrum. However, the results are consistent with other investigation. This shoulder has been attributed to several possible sources including residual amine, isoimide, and the formation of interchain imine linkages. However, amide formation could also account for this feature. The shoulder makes up about 4.4% of the total N1s emission. If the coupling
Fig. 5. AFM images of pristine and modified Upilex-S; (a) pristine; (b) 1 M KOH treatment for 5 min; (c and d) 3 M KOH treatment for 5 and 10 min; (e and f) 0.05 M CDI treatment at RT and 110 8C.
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H.J. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
reactions occurred, some change in the N1s spectrum should be observed. However, the N1s spectrum did not show any reliable evidence that imidazole groups were incorporated onto the polyimide film surface. This may be due to very small difference in the chemical shift between nitrogen atoms in imidazole groups in CDI and imino groups in the Uplex-S.
the surface which is torn and visible like the crater. Excessive microetching may degrade the physical properties of the surface, causing the solvent to diffuse into the bulk, leading to poor adhesion. After CDI-coupling reaction, the surface becomes very smooth and homogeneous as well as AFM images. 3.3. Wettability
3.2. Morphology analysis 3.2.1. Atomic force microscopy The roughness of polyimde films increased by mechanical interlocking which is macroscopic scale, not related to molecular interaction. In Fig. 5, pristine Upilex-S polyimide surface was generally smooth with a RMS roughness slightly greater than 2.0 nm. After KOH treatment, the surface roughness was slightly increased but distinct features. Recent studies found that the amorphous regions of the polyimide are much more easily etched away by the basic solutions than the crystalline regions [20,21]. As the treatment time goes over 5 min, however, the roughness was decreased. The protrusive regions are more exposed to the basic solutions, resulting more attacked and etched. After CDI-coupling reaction, the surface becomes very smooth and homogeneous. 3.2.2. Scanning electron microscopy The surface topography of the pristine and modified Upilex-S films is shown in Fig. 6. The water droplets grow up on the surface of the polyimide films treated KOH treatment. The best adhesion property to metal without any loss of its bulk physical properties occurred only at a critical treatment time. By increasing the treatment intensity, some of the modified regions break away from
The modified Upilex-S films by KOH treatment followed by acidification became wettable. As the treatment time increased from 0 to 5 min in 1 M KOH treatment, the contact angles decreased from 67.418 to 46.628 and the wetting energy increased from 28.30 to 49.98 mN/m shown in Fig. 7(a) and (b). In addition, surface energy also increased from 50.01 to 59.66 mN/N with increasing treatment time. From these results, there is no doubt that KOH treatment followed by acidification could modify the Upilex-S film surface from water-repellent to wettable. Although the Upilex-S film surfaces were wettable, their surface could lose wettability a little in more severe condition. From the SEM image, the wettability decrease could be interpreted that the tissues containing the hydrophilic groups were removed from the surface by excessive microetching treatment. It would be able to confirm from the SEM image. The hydrophilic fragments were removed from the surface by the excessive microetching, as a result, the film wettability decreased. 3.4. Adhesion properties The effect of the surface modification by coupling treatment on the adhesion with copper metal was investigated. Not only
Fig. 6. SEM images of pristine and modified Upilex-S; (a) pristine; (b) 1 M KOH treatment for 5 min; (c and d) 3 M KOH treatment for 5 and 10 min; (e and f) 0.05 M CDI treatment at RT and 110 8C.
H.J. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
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Fig. 7. (a) Contact angle, (b) wetting energy, and (c) surface energy of pristine and modified Upilex-S.
the coupling treatment conditions, but also the KOH treatment conditions influenced the peel strength of the copper/Upilex-S system. Figs. 8 and 9 show the typical results for the peel strength as a function of the KOH concentration and treatment time. The increase of the treatment time from 0 to 5 min in 1 M KOH caused the peel strength to increase from 0.25 to 0.48 kgf/cm. However, in more excessive conditions, the peel strength started to decrease because excessive microetching treatment weakened the physical properties of the surface. The temperature in the coupling treatment also influenced the peel
strength for the copper/Upilex-S system. The peel strength increased slightly in 0.56 kgf/cm when coupling reaction occurred at room temperature, however, at the elevated temperature (110 8C), the peel strength increased conspicuously in 0.86 kgf/cm as shown in Fig. 10.
Fig. 8. Peel strength of the copper/1 M KOH treated Upilex-S system.
Fig. 9. Peel strength of the copper/3 M KOH treated Upilex-S system.
3.5. Reliability test It is important to maintain initial adhesion in various conditions. Thermal and chemical stabilities were confirmed by
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Japanese industrial standard (JIS). In heat resistant test (288 8C, 10 s), pristine, polyamic acid and coupled polyimide surfaces showed 68%, 33% and 19% adhesion decrease, respectively, as shown in Fig. 11. In the NaOH/HCl chemical resistance test, pristine, polyamic acid and coupled polyimide surfaces showed 28/ 20%, 10/6% and 5/4% decrease of adhesion property as shown in Fig. 12. Hereby it can be ascertained that the surface modification of Upilex-S film, particularly by coupling reaction, is effective in thermal and chemical resistance. 4. Conclusions
Fig. 10. Peel Strength of the copper/CDI treated Upilex-S system.
The purpose of this research was to develop chemical procedures for the surface modification of polyimide to improve their adhesion to copper thin layers deposited by sputtering. The polyimide film is required to have active sites to react with coupling agent which improves adhesion with copper metal. The surface modification of Upilex-S polyimide with KOH aqueous solution was successfully suggested. The reaction initially gives a potassium polyamate surface which is acidificated with HCl to yield a polyamic acid surface. As the surface modification was progressed, the hydrophilicity of polyimide surface increased. However, the hydrophilic fragments were removed from the surface and the physical properties of the surface degraded in the excessive microetching treatment. After coupling reaction, the peel strength of the copper/polyimide system increased from 0.25 to 0.86 kgf/cm, and thermal resistance and chemical resistance was also highly improved. References
Fig. 11. Heat resistant test at 288 8C for 10 s of (a) pristine Upilex-S, (b) KOH treatment followed by acidification, and (c) CDI-coupled treatment.
Fig. 12. Chemical resistant test at 288 8C for 10 s of (a) pristine Upilex-S, (b) KOH treatment followed by acidification, and (c) CDI-coupled treatment.
[1] K.W. Lee, A. Viehbeck, in: M.K. Ghosh, K.L. Mittal (Eds.), Polyimides: Fundamentals and Applications, Marcel Dekker, New York, 1996, p. 505. [2] M.W. Kim, S.H. Hyun, J.Y. Kong, L.S. Park, W.S. Kim, J. Ind. Eng. Chem. 12 (2006) 261. [3] B.T. Jung, H.W. Choi, K.S. Park, D.H. Suh, J. Ind. Eng. Chem. 9 (2003) 134. [4] S.B. Oh, B.S. Kim, J.H. Kim, J. Ind. Eng. Chem. 12 (2006) 275. [5] C.E. Stoog, Prog. Polym. Sci. 16 (1991) 561. [6] M. Kent, R. Saunders, J. Emerson, M. Hurst, International SAMPE Technology Conference, vol. 27, 1995, p. 1019. [7] J. Meng, G. Liang, L. Zhao, Compos. Sci. Technol. 62 (2002) 783. [8] (a) F.K. LeGoues, B.D. Silverman, P.S. Ho, J. Vac. Sci. Technol. A 6 (1988) 2200; (b) C.K. Hong, I. Hwang, N. Kim, D.H. Park, B.S. Hwang, C. Nah, J. Ind. Eng. Chem. 14 (2008) 71. [9] F. Paupel, C.H. Yang, S.T. Chen, P.S. Ho, J. Appl. Phys. 65 (1989) (1911). [10] M.M.D. Ramos, Vacuum 64 (2002) 255. [11] S.N. Hwang, B.J. Jeon, I.H. Jung, J. Korean Ind. Eng. Chem. 9 (1998) 383. [12] N.J. Chou, D.W. Dong, J. Kim, A.C. Liu, Electrochem. Soc. 131 (1984) 2335. [13] Z.N. Sanjana, J. Valentich, J.R. Marchetti, R.D. Cent, National SAMPE Symposium and Exhibition, vol. 28, 1983, p. 1240. [14] K.G. Schmitt-Thomas, S. Wege, Brazing Soldering 11 (1986) 27. [15] N. Inagaki, S. Tasaka, K. Hibi, J. Polym. Sci. A: Polym. Chem. 30 (1992) 1425. [16] A.B. Ponter, A.P. Boyer, Can. J. Chem. 50 (1972) 1613. [17] X.D. Huang, S.M. Bhangale, P.M. Moran, N.L. Yakovlev, J. Pan, Polym. Int. 52 (2003) 1064. [18] F.D. Egitto, L.J. Matienzo, in: M.K. Ghosh, K.L. Mittal (Eds.), Poyimides: Fundamentals and Applications, Marcel Dekker, New York, 1996, p. 409. [19] A. Chentie, J. Vac. Sci. Thecnol. A 6 (2003) 815. [20] D. Kim, Y.R. Shen, Appl. Phys. Lett. 74 (1999) 3314. [21] D. Boese, H. Lee, D.Y. Yoon, J.D. Swalen, J.F. Rabolt, J. Polym. Sci. B: Polym. Phys. 30 (1992) 1321.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Surface modification of polyimide film by coupling reaction for copper metallization Hwa Jin Kim a,b, Yun Jun Park a, Jong-Ho Choi a, Hak Soo Han b, Young Taik Hong a,* a b
Energy Materials Research Center, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yuseong-gu, Daejon 305-600, Republic of Korea Electronic Materials Research Lab, Yonsei University, 134 Sinchon, Seodaemun, Seoul 120-749, Republic of Korea
A R T I C L E I N F O
Article history: Received 13 July 2008 Accepted 3 August 2008 Keywords: Polyimide Surface modification Adhesion Copper Coupling reaction Flexible copper clad laminate
A B S T R A C T
In this study, Upilex-S [poly(biphenyl dianhydride-p-phenylene diamine)], one of polyimide films, was modified by coupling reactions with N,N-carbonyldiimidazole (CDI) to increase adhesion to copper for flexible copper clad laminate (FCCL). Imidazole groups show strong interaction with copper metal to make charge transfer complexes. Because polyimide film did not have active site with coupling agent, the film surfaces were modified by aqueous KOH solutions and reacted with dilute HCl solutions. Surface modified Upilex-S was analyzed by X-ray photoelectron spectroscopy (XPS) to examine the surface chemical composition and film morphology and investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Changes in the wettability were evaluated by measuring contact angle with the sessile drop method. After deposition of copper on surface modified Upilx-S, the adhesion strength of the copper/polyimide system was measured by a 908 peel test using the Instron tensile strength tester. The peel strength of the copper/polyimide system increased from 0.25 to 0.86 kgf/ cm by surface modification. This result confirmed that the CDI coupling reaction is an effective treatment method for the improvement of the adhesion property between copper metal and polyimide film. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction The high density electronics such as notebooks, cellular phones, and personal digital assistants (PDAs) are increasingly in great demands and the high density of electronic packages are also required. For the packaging on driver integrated circuit (IC), a flexible design package is needed. In order to interconnect device components or layer on flexible substrates, copper is used for metallization on the flexible substrate such as polyimide. Polyimide is employed as dielectric layers in microelectronics applications such as flexible printed circuit board (FPCB) and multichip module, since they have low dielectric constant, high thermal stability, low moisture absorption, good mechanical properties and good processability [1–5]. But the adhesion property between polyimide film and copper was poor due to the inactivity of polyimide surfaces. Therefore, three-layer process, which is constituted by copper, adhesion promoter and polyimide, had been studied to enhance adhesion [6,7]. Organic adhesive such as epoxy resin, or metal, like Cr or Ni, was used as adhesion promoter. But this three-layer process had crucial drawbacks in
* Corresponding author. Tel.: +82 42 860 7292; fax: +82 42 861 4151. E-mail address: [email protected] (Y.T. Hong).
that they were easily delaminated because each component has different coefficient of thermal expansion (CTE) and residual stress. In addition, weak thermal stability of organic adhesive was another problem [8,9]. In contrast, two layer-flexible copper clad laminate (FCCL), without any adhesion promoter, is more excellent than three layer-FCCL for several reasons, which are miniature of circuit, good insulation characteristics of the matrix wafer, good durability against a moisture absorption and high thermal stability. Therefore, the FPCB which demands a precision is mainly made by using the two layer-FCCL. In two layer-FCCL, the polyimide surface is usually modified by dry process such as plasma, corona discharge, X-ray, laser, ion beam, or flame treatments [10,11]. Recently, interest in wet-process for surface modification of polyimide has increased because of its simplicity and low cost [12,13]. In a typical wet process for polyimide surface modification, the polyimide substrate is immersed into chemical solution or chemical solution is sprayed on polyimide film. In this study, Upilex-S film surfaces were modified by aqueous KOH solutions at various conditions to yield potassium polyamates. Subsequently, they reacted with dilute HCl solutions to convert it into polyamic acids. The resulting polyimide films are expected to have active sites to react with coupling agents to improve the adhesion properties. It was reported that imidazole functional groups contained N,N0 -carbonyldiimidazole (CDI)
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.08.016
24
H.J. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
Fig. 1. Schematic diagrams illustrating the processes of surface modification and copper deposition.
showed strong interactions with copper metal to form charge transfer complexes [14] and carboxylic acid groups were efficiently activated with CDI [15]. We report that the CDI-coupled polyimide could be formed on the polyimide surface and they adhere strongly to the copper metal by complexing between the copper atoms and
imidazole functional groups. Surface chemistry and morphological changes of films were analyzed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Adhesion properties were investigated by contact angle measurement and tensile strength tester.
Fig. 2. XPS high-resolution spectra of Upilex-S (C1s component); (a) pristine Upilex-S; (b) KOH treatment followed by acidification; (c) CDI-coupling treatment.
H.J. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
2. Experimental 2.1. Materials The polyimide films used in this study was Upilex-S (25 mm thick, Ube Industries). The films were cleaned prior to use in ethanol at room temperature for 5 min under ultrasonication and dried. Potassium hydroxide and hydrochloric acid were purchased from Samchun Chemical Co. and Showa Chemical Co., respectively. N,N0 carbonyldiimidazole was obtained from Aldrich Chemical Co. and was used without further purification as a reagent for the surface modification by coupling reactions. Deionized water was used for the preparation of all aqueous solutions and washing steps. 2.2. Surface modification by coupling reactions The surface modification of the polyimide films was carried out in three steps. First, the polyimide films were immersed in different concentrations (1 or 3 M) of KOH aqueous solutions at room temperature for various times (1, 5, and 10 min). After this step, the resulting polyimide film surfaces were changed to potassium polyamate. Second, the potassium polyamate samples were immersed in 0.2 M HCl aqueous solution at room temperature for 5 min in order to be formed as polyamic acid. This step makes Upilex-S film wettable to form carboxylic acid groups, which could
25
immobilize new functional group on its film surface through coupling reaction. Third, the acidified samples were dipped into the ethanol solution containing 0.05 M N,N0 -carbonyldiimidazole at room temperature for 5 min, and then heated at 110 8C for 30 min in air oven to induce the coupling reactions. After the coupling reaction, the polyimide films were washed with ethanol using an ultrasonic washer and finally dried. The schematic diagram of hydrolysis and coupling process are shown in Fig. 1. 2.3. Copper metallization on Upilex-S film After surface modification on Upilex-S, it is kept in a plasma sputtering chamber for further coating with copper. Before applying copper coating on Upilex-S, the copper target is precleaned by argon plasma sputtering for 5 min. After the surface cleaning on the copper target, the copper coating was applied onto Upilex-S. The average thickness of sputtered copper for this study was 200 nm and sputtered film was subsequently electroplated with a copper metal layer of 8–10 mm thickness. The electroplating process was carried out at a constant DC current density of 3 104 A/m2 at room temperature in aqueous sulfuric acid solution (90 g/L) containing copper sulfate (0.5 M), hydrochloric acid (50 ppm) and a glossy reagent (5 mL). The electroplated Upilex-S films were washed with deionized water and dried at 60 8C for 30 min in air oven.
Fig. 3. XPS high-resolution spectra of Upilex-S (O1s component); (a) pristine Upilex-S; (b) KOH treatment followed by acidification; (c) CDI-coupling treatment.
H.J. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 23–30
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2.4. Measurements
energies at the solid–vapor and solid–liquid interface by the following expression [16]:
2.4.1. X-ray photoelectron spectroscopy XPS spectra were obtained with a Kratos Axis NOVA (Manchester, U.K.) instrument, employing a monochromatic excitation radiation of Al Ka at 1486.6 eV. A pass-energy of 160 eV was used for the wide scans at a constant dwell time of 100 ms and 40 eV for the high-resolution scans at a constant dwell time of 250 ms. The X-ray source was run at a reduced power of 150 W. The pressure in the analysis chamber was maintained at 1.7 109 Torr or lower during each measurement. AFM measurement were performed with a NanoScope IV (Digital Instrument, USA) using the tapping mode in air at room temperature. A triangular-pyramidal silicon nitride tip was used and an area of 1 mm 1 mm was scanned under a probe pressure of 8.7 1011 N/m2. A root mean square (RMS) of the surface roughness was calculated from the roughness profile determined by AFM. The surface morphology of the films was observed using a FSM-670F (JEOL, Japan) and specimens were coated with a thin layer of platinum to eliminate charging effects. Contact angle and wetting energy between the surface and deionized water were measured and automatically calculated using a contact angle meter with a phoenix-450 (SEO, Korea). An average value was determined from five measurements. Contact angle was known to correlate with wetting energy, so that the wetting energy may be calculated by using the surface
W e ¼ g SV þ g SL where We is the wetting energy, and g is interfacial free energy with subscript SL referring to solid–liquid interface and SV to solid–vapor interface. Surface energy of the films also can be calculated by measuring contact angle [17]
g ¼ gd þ g p where g is total surface energy which is sum of dispersive interaction (gd) and non-dispersive, i.e. polar interaction (gp). Specially, the solid surface energy can be estimated by the following expression [17]: qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi g L ð1 þ cos uÞ ¼ 2 g dS ldL þ 2 g Sp lLp where gS and gL are the surface energy of solid and liquid, respectively, and u is the contact angle between the solid and liquid. In this study, the surface energy of the films was determined by using two liquid, water and diiodomethane, with known g dL and g Lp . The 908 peel strength (using 3 mm wide strips) of copper metal/Upilex-S film systems was evaluated at a peel rate of 50 mm/ min using Instron 4482-tensile strength tester (Shimadzu, Japan). The peel strength was determined from the average of five specimens.
Fig. 4. XPS high-resolution spectra of Upilex-S (N1s component); (a) pristine Upilex-S; (b) KOH treatment followed by acidification; (c) CDI-coupling treatment.
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3. Results and discussion 3.1. XPS analysis Upilex-S polyimide film is only composed of C, O, and N. But, small amounts of silicon are observed in the commercially available Upilex-S film. Silicon incorporation could be intentional (e.g. as part of an additive to reduce frictional drag) or unintentional (as a contaminant, e.g. from silicon rubber rollers uses in the curing process). In the curve-fitting routine, the contributions of silicon were ignored (i.e. a pure BPDA–PDA structure assumed). Three different carbon environments are found; carbon atoms from the aromatic rings that are not directly attached to an imide ring atoms or to nitrogen atoms (282.1 eV), carbon atoms bonded to nitrogen (283.3 eV), and carbon atoms in the imide rings (285.5 eV). In addition, a shake satellite arising from p–p* transition in the aromatic ring appears at 287.68 eV. In Fig. 2(a), the first three peaks have approximate areas of 71.4%, 5.4%, and 12.9%, respectively. In Fig. 2(b), the two carbonyl peaks at 286.3 eV (carboxylate carbon) and 285.2 eV (amide carbon) are separated in polyamic acid. In the XPS spectrum of CDI-modified Upilex-S film, the separated peaks turned back in previous condition according to disappearance of carboxylate carbon peak as shown in Fig. 2(c). The hydrolysis reactions can be further proven by the O1s XPS spectra. Based on the general structure of BPDA–PDA, a single oxygen signal is expected for the carbonyl groups in the imide ring
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(in addition to p–p* sake-up satellite). However, peak asymmetry suggests the presence of an additional contribution at higher binding energy. Fig. 3(a) shows two O1s peaks with binding energies of 529.3 and 530.7 eV. The additional oxygen peak has approximately 20% of the total area of the O1s peak. This peak is located in the range of ether group of polyimide such as PMDA– ODA. The presence of this peak has been reported for the Upilex-S film samples [18] and other thermally cured polyimides [19], which is probably due to oxygen contaminations during production. A shake-up satellite is located at about 534.5 eV. In Fig. 3(b), the O1s XPS spectrum of polyamic acid is fitted with two peaks at 528.8 and 530.5 eV. The low binding energy peak is assigned to the carbonyl oxygen in both carboxylic acid [–C(O)OH] and amide groups [–C(O)NH–]. The high binding energy peak is assigned to the hydroxyl oxygen in the –C(O)OH groups. The appearance of high binding energy peak in the polyamic acid is probably due to the change of the chemical environment of the oxygen atoms from carbonyl oxygen to hydroxyl oxygen in Fig. 3(c). The N1s spectrum of pristine polyimide in Fig. 4 consists of a major component with a weak shoulder on the low binding energy side. Since there is only one nitrogen environment in polyimde, only one peak is expected in this spectrum. However, the results are consistent with other investigation. This shoulder has been attributed to several possible sources including residual amine, isoimide, and the formation of interchain imine linkages. However, amide formation could also account for this feature. The shoulder makes up about 4.4% of the total N1s emission. If the coupling
Fig. 5. AFM images of pristine and modified Upilex-S; (a) pristine; (b) 1 M KOH treatment for 5 min; (c and d) 3 M KOH treatment for 5 and 10 min; (e and f) 0.05 M CDI treatment at RT and 110 8C.
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reactions occurred, some change in the N1s spectrum should be observed. However, the N1s spectrum did not show any reliable evidence that imidazole groups were incorporated onto the polyimide film surface. This may be due to very small difference in the chemical shift between nitrogen atoms in imidazole groups in CDI and imino groups in the Uplex-S.
the surface which is torn and visible like the crater. Excessive microetching may degrade the physical properties of the surface, causing the solvent to diffuse into the bulk, leading to poor adhesion. After CDI-coupling reaction, the surface becomes very smooth and homogeneous as well as AFM images. 3.3. Wettability
3.2. Morphology analysis 3.2.1. Atomic force microscopy The roughness of polyimde films increased by mechanical interlocking which is macroscopic scale, not related to molecular interaction. In Fig. 5, pristine Upilex-S polyimide surface was generally smooth with a RMS roughness slightly greater than 2.0 nm. After KOH treatment, the surface roughness was slightly increased but distinct features. Recent studies found that the amorphous regions of the polyimide are much more easily etched away by the basic solutions than the crystalline regions [20,21]. As the treatment time goes over 5 min, however, the roughness was decreased. The protrusive regions are more exposed to the basic solutions, resulting more attacked and etched. After CDI-coupling reaction, the surface becomes very smooth and homogeneous. 3.2.2. Scanning electron microscopy The surface topography of the pristine and modified Upilex-S films is shown in Fig. 6. The water droplets grow up on the surface of the polyimide films treated KOH treatment. The best adhesion property to metal without any loss of its bulk physical properties occurred only at a critical treatment time. By increasing the treatment intensity, some of the modified regions break away from
The modified Upilex-S films by KOH treatment followed by acidification became wettable. As the treatment time increased from 0 to 5 min in 1 M KOH treatment, the contact angles decreased from 67.418 to 46.628 and the wetting energy increased from 28.30 to 49.98 mN/m shown in Fig. 7(a) and (b). In addition, surface energy also increased from 50.01 to 59.66 mN/N with increasing treatment time. From these results, there is no doubt that KOH treatment followed by acidification could modify the Upilex-S film surface from water-repellent to wettable. Although the Upilex-S film surfaces were wettable, their surface could lose wettability a little in more severe condition. From the SEM image, the wettability decrease could be interpreted that the tissues containing the hydrophilic groups were removed from the surface by excessive microetching treatment. It would be able to confirm from the SEM image. The hydrophilic fragments were removed from the surface by the excessive microetching, as a result, the film wettability decreased. 3.4. Adhesion properties The effect of the surface modification by coupling treatment on the adhesion with copper metal was investigated. Not only
Fig. 6. SEM images of pristine and modified Upilex-S; (a) pristine; (b) 1 M KOH treatment for 5 min; (c and d) 3 M KOH treatment for 5 and 10 min; (e and f) 0.05 M CDI treatment at RT and 110 8C.
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Fig. 7. (a) Contact angle, (b) wetting energy, and (c) surface energy of pristine and modified Upilex-S.
the coupling treatment conditions, but also the KOH treatment conditions influenced the peel strength of the copper/Upilex-S system. Figs. 8 and 9 show the typical results for the peel strength as a function of the KOH concentration and treatment time. The increase of the treatment time from 0 to 5 min in 1 M KOH caused the peel strength to increase from 0.25 to 0.48 kgf/cm. However, in more excessive conditions, the peel strength started to decrease because excessive microetching treatment weakened the physical properties of the surface. The temperature in the coupling treatment also influenced the peel
strength for the copper/Upilex-S system. The peel strength increased slightly in 0.56 kgf/cm when coupling reaction occurred at room temperature, however, at the elevated temperature (110 8C), the peel strength increased conspicuously in 0.86 kgf/cm as shown in Fig. 10.
Fig. 8. Peel strength of the copper/1 M KOH treated Upilex-S system.
Fig. 9. Peel strength of the copper/3 M KOH treated Upilex-S system.
3.5. Reliability test It is important to maintain initial adhesion in various conditions. Thermal and chemical stabilities were confirmed by
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Japanese industrial standard (JIS). In heat resistant test (288 8C, 10 s), pristine, polyamic acid and coupled polyimide surfaces showed 68%, 33% and 19% adhesion decrease, respectively, as shown in Fig. 11. In the NaOH/HCl chemical resistance test, pristine, polyamic acid and coupled polyimide surfaces showed 28/ 20%, 10/6% and 5/4% decrease of adhesion property as shown in Fig. 12. Hereby it can be ascertained that the surface modification of Upilex-S film, particularly by coupling reaction, is effective in thermal and chemical resistance. 4. Conclusions
Fig. 10. Peel Strength of the copper/CDI treated Upilex-S system.
The purpose of this research was to develop chemical procedures for the surface modification of polyimide to improve their adhesion to copper thin layers deposited by sputtering. The polyimide film is required to have active sites to react with coupling agent which improves adhesion with copper metal. The surface modification of Upilex-S polyimide with KOH aqueous solution was successfully suggested. The reaction initially gives a potassium polyamate surface which is acidificated with HCl to yield a polyamic acid surface. As the surface modification was progressed, the hydrophilicity of polyimide surface increased. However, the hydrophilic fragments were removed from the surface and the physical properties of the surface degraded in the excessive microetching treatment. After coupling reaction, the peel strength of the copper/polyimide system increased from 0.25 to 0.86 kgf/cm, and thermal resistance and chemical resistance was also highly improved. References
Fig. 11. Heat resistant test at 288 8C for 10 s of (a) pristine Upilex-S, (b) KOH treatment followed by acidification, and (c) CDI-coupled treatment.
Fig. 12. Chemical resistant test at 288 8C for 10 s of (a) pristine Upilex-S, (b) KOH treatment followed by acidification, and (c) CDI-coupled treatment.
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