Surface Graft Copolymerization of Poly(tetrafluoroethylene) Films with

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Langmuir 2001, 17, 211-218

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Surface Graft Copolymerization of Poly(tetrafluoroethylene) Films with N-Containing Vinyl Monomers for the Electroless Plating of Copper G. H. Yang, E. T. Kang,* and K. G. Neoh Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 119260

Yan Zhang and K. L. Tan Department of Physics, National University of Singapore, Kent Ridge, Singapore 119260 Received July 10, 2000. In Final Form: October 17, 2000 Argon plasma-pretreated poly(tetrafluoroethylene) (PTFE) films were subjected to UV-induced surface graft copolymerization, under atmospheric conditions, with several N-containing vinyl monomers, including 4-vinylpyridine (4VP), 2-vinylpyridine (2VP), 1-vinylimidazole (VIDz), 1-vinylpyridinone (VPD), and acrylamide (AAm). Electroless plating of copper could be carried out effectively on the 4VP or 2VP graftcopolymerized PTFE surface after PdCl2 activation and in the absence of SnCl2 sensitization. The surface compositions of the modified PTFE films were studied by X-ray photoelectron spectroscopy (XPS). The catalytic processes for the electroless plating of copper in the presence and absence of sensitization by SnCl2 were compared. The adhesion strength of the electrolessly deposited copper on the graft-modified PTFE surface was affected by the type of the monomers used for graft copolymerization, the graft concentration, and the activation time of the PTFE surface in PdCl2 solution. The T-peel adhesion strength between the electrolessly deposited copper and the graft-modified PTFE film was further improved in the absence of the SnCl2 sensitization step and could reach as high as 7 N/cm. However, the electroless deposition of copper could not be carried out on the pristine or Ar plasma-treated PTFE surface in the absence of SnCl2 sensitization.

1. Introduction Poly(tetrafluoroethylene) (PTFE) possesses many outstanding properties, such as low dielectric constant, good thermal stability, and inherent surface inertness, which make it important for applications in printed circuit technology and in microelectronics packaging.1-3 Numerous studies on the metallization of PTFE have been carried out.4-6 However, due to the surface inertness of the polymer, the adhesion strength between the metal and the pristine PTFE film was inadequate for practical applications. Thus, pretreatment of the PTFE surface is a necessary and essential step. Various methods for modifying the PTFE surfaces, such as ion beam treatment, chemical etching with sodium naphthalenide, plasma treatment, and X-ray irradiation,7-11 have been developed * To whom correspondence should be addressed. Fax (65) 7791936; E-mail [email protected]. (1) Park, J. M.; Matienzo, L. J.; Spencer, D. F. J. Adhes. Sci. Technol. 1991, 5, 153. (2) Chang, C. A.; Kim, Y. K.; Lee, S. S. In Metallized Plastics 5&6: Fundamentals And Application; Mittal, K. L., Ed.; Marcel Dekker: New York, 1998; p 345. (3) Silvain, J. F.; Ehrhardt, J. J.; Picco, A.; Lutgen, P. ACS Symp. Ser. 1990, No. 440, 453. (4) Mittal, K. L., Ed. Metallized Plastics 2: Fundamental And Applied Aspects; Plenum Press: New York, 1991. (5) Chang, C. A.; Kim, Y. K.; Schrott, A. G. J. Vac. Sci. Technol. 1990, A8, 3304. (6) Shi, M. K.; Lamentagne, B.; Selmani, A.; Martinu, L.; Sacher, E.; Wertheimer, M. R.; Yelon, A. J. Vac. Sci. Technol. 1994, A12, 29. (7) Zhang, Y.; Huan, A. C. H.; Tan, K. L.; Kang, E. T. Nucl. Instrum. Methods B 2000, 168, 29. (8) Marchesi, J. T.; Keith, H. D.; Garton, A. J. Adhes. 1992, 39, 185. (9) Liston, E. M.; Martinn, L.; Wertheiner, M. R. J. Adhes. Sci. Technol. 1993, 7, 1091. (10) Inagaki, N.; Tasaka, S.; Umehara, T. J. Appl. Polym. Sci. 1999, 71, 2191.

and applied to improve the adhesion property of the PTFE substrate. Apart from these methods, surface modification of PTFE via graft copolymerization with functional monomers has also been explored.12-14 An earlier study had found that the adhesion strength between a copper foil and a PTFE film could be significantly enhanced through the surface modification of the Ar plasmapretreated PTFE film via thermal graft copolymerization with 4-vinylpyridine.15 The charge-transfer interaction between the nitrogen atom of the grafted polymer and the Cu atom accounts for the observed high adhesion strength. The electroless plating technique has been widely used in the automotive and electronics industries for the metallization of plastics.16 The metal, usually Cu or Ni, was deposited on the catalytic sites formed on the polymer surface. These sites usually contain palladium nuclei chemisorbed from solution. Different methods have been proposed to perform this chemisorption. Historically, the most widely used methods were the “two-step” process and “one-step” process. In the two-step method, the polymer surface was immersed successively in SnCl2 and then in PdCl2 solution.17 The one-step process, on the other (11) Rye, R. R.; Chi, K. M.; Hampdensmith, M.; Kodas, T. T. J. Electrochem. Soc. 1992, 139, L60. (12) Tan, K. L.; Woon, L. L.; Wong, H. K.; Kang, E. T.; Neoh, K. G. Macromolecules 1993, 26, 2832. (13) Kang, E. T.; Neoh, K. G.; Shi, J. L.; Tan, K. L.; Liaw, D. J. Polym. Adv. Technol. 1999, 10, 20. (14) Kang, E. T.; Neoh, K. G.; Tan, T. L.; Senn, B. C.; Pigram, P. J.; Liesegang, J. Polym. Adv. Technol. 1997, 8, 683. (15) Kang, E. T.; Liu, Y. X.; Neoh, K. G.; Tan, K. L.; Cui, C. Q.; Lim, T. B. J. Adhes. Sci. Technol. 1999, 13, 293. (16) Yosi, S. D.; Valery, D.; Matthew, A. Thin Solid Films 1995, 262, 93. (17) Muller, G.; Baudrand, D. W. Plating on Plastics: a Practical Handbook; Robert Draper Ltd.: Teddington, 1971.

10.1021/la0009689 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/09/2000

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2.1. Materials. PTFE films having a thickness of about 0.1 mm and a density of 2.18 g/cm3 were used in this study and were obtained from Goodfellow Ltd., of Cambridge, UK. The surfaces of the PTFE films were cleaned with reagent grade acetone in an ultrasonic water bath for 45 min before use. The monomers 4-vinylpyridine (4VP), 2-vinylpyridine (2VP), 1-vinylimidazole (VIDz), 1-vinylpyridinone (VPD), and acrylamide (AAm) used for the surface graft copolymerization were obtained from Aldrich Chemical Co. of Milwaukee, WI. The chemical structures of these vinyl monomers are shown in Figure 1. 2.2. Argon Plasma Pretreatment and Graft Copolymerization. The PTFE films were cut into strips of about 20 ×

40 mm in size. They were pretreated with Ar plasma before the UV-induced graft copolymerization. A cylindrical type glow discharge cell, model SP 100, manufactured by Anatech Ltd. of Springfield, VA, was used for the plasma treatment. The plasma power applied was kept at 35 W at a radio frequency of 40 kHz. The film was placed between the two parallel electrodes and subjected to the glow discharge for a predetermined period of time at an Ar flow rate of 50 sccm and a pressure of about 0.5 Torr. The Ar plasma-pretreated polymer films were then exposed to the atmosphere at room temperature (25 °C) and 60% relative humidity for at least 10 min to facilitate the formation of surface peroxides and hydroperoxides for the subsequent UV-induced graft copolymerization experiments.22 Prior to the UV-induced graft copolymerization experiment, a drop of pure 4VP, 2VP, VIDz, VPD, or the 50 wt % aqueous solution of AAm was introduced onto the surface of the plasmapretreated PTFE film. The film was then sandwiched between two quartz plates. The assembly was subjected to UV irradiation in a Riko RH 400-10W rotary photochemical reactor, manufactured by Riko Denki Kogyo of Chiba, Japan. The reactor was equipped with a 1000 W high-pressure Hg lamp and a constanttemperature bath. All UV-induced graft copolymerization were carried out under atmospheric conditions at a constant temperature of 28 °C. After each grafting experiment, the AAm, VIDz, and VPD graft-copolymerized PTFE films (AAm-g-PTFE, VIDz-g-PTFE, and VPD-g-PTFE films) were washed thoroughly with doubly distilled water. The 4VP and 2VP graft-copolymerized PTFE films (4VP-g-PTFE and 2VP-g-PTFE films), on the other hand, were washed thoroughly with copious amounts of ethanol to remove the residual monomers and adsorbed homopolymers. 2.3. Electroless Deposition of Copper on Surface-Modified PTFE Films. The graft-modified PTFE films were activated, in either the presence or absence of prior sensitization by SnCl2, through the immobilization of palladium catalyst for the subsequent electroless copper plating. For surface activation in the absence of SnCl2 sensitization, the graft-modified PTFE film was immersed directly in an aqueous solution containing 0.1 wt % PdCl2 and 1.0 wt % HCl (37%) for a fixed period of time and then rinsed with copious amounts of distilled water. The surfaceactivated PTFE film was then immersed in an electroless plating bath for 10-20 min, and a copper layer of about 200 nm in thickness was electrolessly deposited. The composition of the plating bath was as following: 7 g/L of CuSO4‚5H2O, 25 g/L of potassium sodium tartrate, 4.5 g/L of sodium hydroxide, and 9.5 g/L of formaldehyde.23 The Cu-deposited PTFE films were then rinsed thoroughly with copious amounts of doubly distilled water. The metallized PTFE films were adhered to a copper sheet backing (0.1 mm in thickness), using an epoxide adhesive (Araldite Stand, from Ciba-Geigy Chemical Co. of Switzerland), for the subsequent T-peel adhesion strength measurement. The assembly was then thermally cured in a vacuum oven at 140 °C for 4 h. For comparison purposes, activation of the graft-modified PTFE films was also carried out via the conventional two-step process.23 In this method, the surface-modified PTFE films were first sensitized by soaking in an aqueous solution containing 0.3 wt % SnCl2 and 2.5 wt % HCl (37%) for 2 min, followed by rinsing with doubly distilled water. The subsequent activation in PdCl2 solution, electroless copper plating, and thermal posttreatment were similar to those described above. 2.4. Surface Characterization. X-ray photoelectron spectroscopy (XPS) was used to determine the surface composition of the samples. The XPS measurements were made on an Axis HSi 165 spectrometer (Kratos Analytical Ltd., England) using the Mg KR X-ray source (1253.6 eV photons) at a constant dwell time of 100 ms and a pass energy of 40 eV. The anode voltage was 15 kV. The anode current was 15 mA. The pressure in the analysis chamber was maintained at 5.0 × 10-8 Torr or lower during each measurement. The PTFE films were mounted on the standard sample studs by means of double-sided adhesive tape. The core-level signals were obtained at a photoelectron

(18) Meek, R. L. J. Electrochem. Soc. 1975, 122, 1478. (19) Jackson, R. L. J. Electrochem. Soc. 1990, 137, 95. (20) Yen, P. C. Polymer 1995, 36, 3399. (21) Charbonnier, M.; Alami, M.; Romand, M. J. Electrochem. Soc. 1996, 143, 472.

(22) Suzuki, M.; Kishida, A.; Iwata, H.; Ikada, Y. Macromolecules 1986, 19, 1804. (23) Ebneth, H. In Metalllizing of Plastics, Handbook of Theory and Practice; Suchentrunk, R., Ed.; 30th ASM International: Materials Park, OH, 1993; p 30.

Figure 1. Chemical structures of (a) 4-vinylpyridine, (b) 2-vinylpyridine, (c) 1-vinylimidazole, (d) vinylpyridinone, and (e) acrylamide.

hand, used a mixed SnCl2/PdCl2 solution.18 Since SnCl2 was used in both methods, the palladium particle was surrounded by a tin chloride shell after the Pd2+ ion was reduced to Pd0 by SnCl2.19 However, tin chloride is not an active catalyst for electroless plating. As a result, the growth of the copper deposit was inhibited.17,19 Therefore, it is important to develop a tin-free process in the electroless plating industry to avoid the side effect of the tin atom. However, palladium chemisorption onto most polymer substrates cannot be performed directly due to their surface inertness. It is thus necessary to subject the polymer surfaces to an effective pretreatment. It has been shown that palladium can coordinate with the chromic acid-etched acrylonitrile-butadiene-styrene (ABS) surface.20 Charbonnier et al.,21 on the other hand, have reported that palladium can be adsorbed directly on the nitrogen functional groups of the polymer surfaces generated from N2 or NH3 plasma treatment. In the present study, surface modification of Ar plasmapretreated PTFE film is carried out via UV-induced surface graft copolymerization with several N-containing monomers, including 4-vinylpyridine (4VP), 2-vinylpyridine (2VP), 1-vinylimidazole (VIDz), 1-vinylpyridinone (VPD), and acrylamide (AAm). Electroless copper plating on the graft-copolymerized PTFE surfaces in the absence of SnCl2 sensitization is explored. The adhesion of the electrolessly deposited copper on the graft-modified PTFE surfaces is also evaluated. 2. Experimental Section

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Figure 2. Schematic diagram illustrating the processes of Ar plasma treatment, graft copolymerization, surface activation, and electroless deposition of copper on the PTFE film. takeoff angle of 90° (with respect to the sample surface). All binding energies (BE’s) were referenced to the C 1s hydrocarbon peak at 284.6 eV. In the peak synthesis, the line width (full width at half-maximum or fwhm) of the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from peakarea ratios, after correcting with the experimentally determined sensitivity factors, and were reliable to (5%. The elemental sensitivity factors were determined using stable binary compounds of well-established stoichiometries. 2.5. Adhesion Strength Measurement. The adhesion strengths of the electrolessly deposited copper on the graftmodified PTFE films were determined by measuring the T-peel adhesion strengths on an Instron model 5544 tensile tester from the Instron Corp. of USA. All measurements were carried out at a crosshead speed of 1.0 cm/min. Each adhesion strength reported was the average of at least three sample measurements which did not vary by more than (0.5 N/cm.

3. Results and Discussion The processes of Ar plasma pretreatment, UV-induced graft copolymerization, surface activation, and electroless copper plating on the PTFE surface are shown schematically in Figure 2. The details for each process are described below. 3.1. Surface Modification of PTFE Films by Ar Plasma Treatment and Graft Copolymerization. The effect of Ar plasma treatment and subsequent air exposure on the surface [O]/[C] ratio and [F]/[C] ratio of the PTFE film, as determined from the O 1s, C 1s, and F 1s corelevel spectral peak area ratios, is shown in Figure 3. An increase in the [O]/[C] ratio and a decrease in the [F]/[C] ratio are observed with the increase in Ar plasma treatment time of the PTFE film. The observations are in agreement with the results generally reported in the literature for the Ar plasma treatment of the PTFE surface.24,25 The [O]/[C] ratio approaches an asymptotic value at Ar plasma treatment times above 60 s. This observation suggests that prolonged Ar plasma treatment does not introduce more oxygen species on the PTFE surface, probably due to the onset of the etching effect of the plasma. The Ar plasma treatment causes the breakage of the C-F bonds, resulting in the defluorination of the (24) Da, Y. X.; Griesser, H. J.; Mau, A. W. H.; Schmidt, R.; Liesegang, J. Polymer 1991, 32, 1126. (25) Wu, S. Y.; Kang, E. T.; Neoh, K. G.; Han, H. S.; Tan, K. L. Macromolecules 1999, 32, 186.

Figure 3. [O]/[C] ratio and [F]/[C] ratio of the PTFE film surface as a function of Ar plasma pretreatment time.

PTFE surface and the formation of active species on the PTFE surface. The subsequent exposure to air causes the oxygen to be incorporated onto the PTFE surface, leading to surface oxidation and the formation of peroxide and hydroperoxide species on the PTFE surface.25 These peroxide and hydroperoxide species can be used to initiate the subsequent UV-induced surface free radical graft copolymerization.13,22 Figure 4a-e shows the respective N 1s and C 1s corelevel spectra for the 90 s Ar plasma-pretreated PTFE films after having been subjected to the UV-induced graft copolymerization with 4VP, 2VP, VIDz, VPD, and AAm for 2 h completely under atmospheric conditions. The respective PTFE surfaces after modification by graft copolymerization are referred to as the 4VP-g-PTFE, 2VPg-PTFE, VIDz-g-PTFE, VPD-g-PTFE, and AAm-g-PTFE surfaces. The presence of surface-grafted 4VP and 2VP polymers can be deduced from the appearance of the N 1s core-level spectra at the binding energy (BE) of about 398.5 eV for the imine (-N)) species (Figure 4, a and b, respectively).26 The N 1s spectrum of the VIDz-g-PTFE film shows two peak components of about equal size at the BE’s of 398.2 and 399.8 eV (Figure 4c). The two peak components are attributed respectively to the imine (-N)) (26) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. J. Mol. Electron. 1990, 6, 5.

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Figure 4. N 1s and C 1s core-level spectra of the modified PTFE surfaces from graft copolymerization with 4VP, 2VP, VIDz, VPD, and AAm (Ar plasma pretreatment time of PTFE film ) 90 s, UV graft copolymerization time ) 2 h).

Yang et al.

Figure 5. Pd 3d core-level spectra of the (a) 4VP-g-PTFE surface, (b) 2VP-g-PTFE surface, (c) VIDz-g-PTFE surface, (d) VPD-g-PTFE surface, and (e) AAm-g-PTFE surface after activation in PdCl2 solution for 10 min. (The graft concentrations correspond to those shown in Figure 4.)

and the amine (-N〈) species of the imidazole ring.27 The presence of surface-grafted VPD polymer and AAm polymer on the PTFE surfaces, on the other hand, can be deduced from the N 1s peak component with BE’s at about 400.4 eV for the -N〈 group of VPD (Figure 4d) and at about 399.6 eV for the -NH2 group of AAm (Figure 4e).28 The C 1s peak component at the binding energy (BE) of about 291.4 eV is attributable to the CF2 species of the PTFE substrate.15 The presence of only a residual CF2 component in the C 1s core-level spectra of Figures 4 suggests that the thickness of the grafted polymer layer is approaching the probing depth of the XPS technique (∼7.5 nm in an organic matrix12). The graft concentration in each case can be expressed simply as the surface [N]/[F] ratio. The ratio can be used to determine the number of repeat units of the N-containing polymer per repeat unit of the PTFE substrate within the probing depth of the XPS technique by taking into account of the respective N and F stoichiometries in the graft and the substrate polymers. 3.2. Activation of the Pristine and Graft-Modified PTFE Surfaces. The pristine, the Ar plasma-treated, and the surface graft-copolymerized PTFE films were activated in the PdCl2 solution, in the absence of prior sensitization by SnCl2 (the Sn-free process). The presence of surface adsorbed Pd species is revealed by the Pd 3d core-level signal on the activated PTFE surface. No Pd 3d signal was detected on the pristine PTFE surface and the 90 s Ar plasma-treated PTFE surface, even in the presence

of the incorporated oxygen species in the latter. The Pd 3d core-level signal is discernible in the PTFE films graftcopolymerized with one of the five N-containing monomers. These results indicate that the palladium species are chemisorbed on the nitrogen groups and not on the oxygen groups. The conclusion is consistent with that reported earlier.21 Figure 5a-e shows the respective Pd 3d corelevel spectra of the 4VP-g-PTFE surface, the 2VP-g-PTFE surface, the VIDz-g-PTFE surface, the VPD-g-PTFE surface, and the AAm-g-PTFE surface, after activation in PdCl2 solution for 10 min. The copolymer compositions of the five surfaces correspond to those shown in Figure 4. The Pd 3d core-level spectra can be curve-fitted with three spin-orbit-split doublets. The lowest BE doublet with the BE’s for the Pd 3d5/2 and Pd 3d3/2 peak components lying at about 335 and 340 eV, respectively, are assigned to the Pd0 species.29 The Pd 3d5/2 and Pd 3d3/2 peak components with BE’s at about 338 and 343 eV, respectively, are assigned to the Pd2+ ions.29 The remaining doublet with the BE’s for the 3d5/2 and 3d3/2 components at about 336.5 and 341.5 eV, respectively, are assigned to the Pd complex (Pd*).30 The amount of Pd uptake, via the Sn-free process, on the respective surface graft-copolymerized PTFE film, defined as the [Pd]/[N] ratio and determined from the corresponding Pd 3d and N 1s core-level spectral peakarea ratio, is also indicated in Figure 5. A substantial

(27) Zhang, M. C.; Kang, E. T.; Neoh, K. G.; Tan, K. L. J. Adhes. Sci. Technol. 1999, 13, 819. (28) Muilenberg, G. E., Ed. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1978; p 228.

(29) Mance, A. M.; Waldo, R. A.; Dow, A. A. J. Electrochem. Soc. 1989, 136, 1667. (30) Dressick, W. J.; Dulcey, C. S.; George, J. H.; Calabrese, G. S.; Calvert, J. M. J. Electrochem. Soc. 1994, 141, 210.

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amount of Pd is adsorbed on the 4VP-g-PTFE, 2VP-gPTFE, VIDz-g-PTFE, and AAm-g-PTFE surfaces, while the amount of adsorbed Pd on the VPD-g-PTFE surface is rather low. The adsorption of Pd probably involves the formation of various Pd complexes. The nitrogen and oxygen atoms of the grafted polymers have strong electronegativity and thus can attract the Pd ions from the PdCl2 acid solution. For example, the nitrogen atoms can impart their lone pair electrons to the adsorbed Pd ions. As a result, Pd atoms are complexed to the nitrogen atoms according to the following plane-square model:21

Because of the presence of the alkyl group and the neighboring carbonyl group, the nitrogen atom of VPD is severely sterically hindered. Thus, even in the presence of the polar CdO group, the amount of palladium chemisorbed on the VPD-g-PTFE surface is much lower than those chemisorbed on the other four graft-modified PTFE surfaces. Nevertheless, other factors, such as the missing molecular orbital on the N atom of VPD, may also contribute to the observed effect. It is obvious that the Pd complex is the predominant species on all the five PTFE films graft copolymerized with the N-containing monomers. The [Pd0]/[Pd*] ratios for the 4VP-g-PTFE, 2VP-g-PTFE, VIDz-g-PTFE, VPDg-PTFE, and AAm-g-PTFE surfaces are about 0.2, 0.2, 0.1, 0.4, and 0.3, respectively. Palladium metal is a wellknown catalyst for initiating the electroless plating of copper.10,19,20 The catalytic effect of the Pd complex, however, is less well-understood. To investigate the catalytic process involved in the electroless copper plating, the five surface-activated PTFE films were immersed in the Cu plating bath for about 40 s. Almost no Cu reduction in the plating solution or Cu uptake by the modified-PTFE films were observed during this period. In fact, an incubation time of about 3 min is generally required for the initiation of Cu uptake by these Pd-laden PTFE surfaces obtained for the Sn-free process. The Pd 3d corelevel spectra of the corresponding five PTFE surfaces are shown in Figure 6. The relative proportions of Pd0 on the 4VP, 2VP, and VIDz graft-copolymerized PTFE surfaces increase after immersion in the copper plating bath. The respective [Pd0]/[Pd*] ratios increase substantially to about 0.8, 2.6, and 0.5 from 0.2, 0.2, and 0.1, while the corresponding [Pd]/[N] ratios decrease only moderately to about 0.5, 0.3, and 0.2 from 0.7, 0.6, and 0.3. Even if only Pd* was lost in the dissolution, the dissolution of Pd* does not account for observed change in the [Pd0]/[Pd*] ratio in each case. The decrease in [Pd*]/[N] ratio for each surface is substantially larger than the decrease in the corresponding [Pd]/[N] ratio (compare the data insert in Figure 5 to those in Figure 6). The assertion that Pd* is reduced to Pd0 is supported by the fact that the corresponding [Pd0]/[N] ratios for the three surfaces has increased from 0.08, 0.08, and 0.03 to 0.2, 0.17, and 0.05. This result suggests that the coordinated Pd complex has

Figure 6. Pd 3d core-level spectra of the (a) 4VP-g-PTFE surface, (b) 2VP-g-PTFE surface, (c) VIDz-g-PTFE surface, (d) VPD-g-PTFE surface, and (e) AAm-g-PTFE surface after activation in PdCl2 solution for 10 min, followed by immersion in the electroless copper plating bath for 40 s.

been reduced to Pd metal in alkaline solution of the plating bath. The reduction of Pd complex to Pd metal is also indicated by the prompt color change of the films from yellow to black upon immersed in the electroless plating bath. Thus, the process of electroless copper plating is initiated predominantly by the Pd metal. Similar results have also been suggested for the electroless plating of copper on the chromic acid-etched ABS surface,20 in which the Pd2+ ions from PdSO4 are coordinated to the polar groups, such as the -COOH, -SO3H, and -CONH2 groups, on the etched ABS surface. Figure 7 shows the Pd 3d core-level spectra for the PdCl2 powders (Figure 7a), the Pd 3d and the N 1s core-level spectra of the 4VP-g-PTFE surface after activation in PdCl2 solution for 10 min via the Sn-free process (Figure 7b), and via the conventional two-step process (Figure 7c). The two-step process involves the initial sensitization of the 4VP-g-PTFE surface in SnCl2 solution. The Pd complex is the predominant species immobilized in the Sn-free process. The [Pd0]/[Pd*] ratio is about 0.2. On the other hand, palladium metal is the predominant species immobilized in the two-step process. The [Pd0]/[Pd*] ratio is about 6.6. This result helps to account for the observed incubation time of about 3 min for copper deposition to start on the graft-copolymerized PTFE surfaces after activation in PdCl2 solution in the Sn-free process. The time delay allows the reduction of Pd complex to Pd metal. For the surface activated via the two-step process, immediate copper deposition is observed, as Pd0 is the predominant species on these surfaces.

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Figure 8. Effect of the activation time in PdCl2 solution on the amount of adsorbed Pd and on the T-peel adhesion strength of electrolessly deposited copper (from the Sn-free process) on the 4VP-g-PTFE and 2VP-g-PTFE surfaces (Ar plasma pretreatment time of the PTFE film ) 90 s, UV graft copolymerization time ) 2 h).

Figure 7. Pd 3d core-level spectra of (a) the PdCl2 powders, and Pd 3d and N 1s core-level spectra of the activated 4VPg-PTFE surface via (b) the Sn-free process and (c) the two-step process (activation time in PdCl2 solution ) 10 min for both processes, and sensitization time in SnCl2 solution ) 2 min for the two-step process).

The N 1s core-level spectra of the 4VP-g-PTFE surface consists of only one peak component at the BE of about 398.5 eV, attributable to the imine species (Figure 4a). A distinct new peak component at the higher BE of about 399.8 eV is discernible in the N 1s core-level spectrum of the activated 4VP-g-PTFE surface obtained via the Snfree process (Figure 7b). The appearance of this high BE component is consistent with the formation of Pd-N complex. On the other hand, the N 1s core-level spectrum of the activated 4VP-g-PTFE surface obtained via the twostep process (Figure 7c) shows negligible changes from that of the 4VP-g-PTFE surface. Thus, the complex formation between the palladium and the nitrogen atoms in the Sn-free process is ascertained. For the 4VP, 2VP, and VIDz graft-copolymerized PTFE films, a slight loss in the adsorbed Pd species is observed after immersion in the electroless copper plating bath (compare Figure 6 and Figure 5). The [Pd]/[N] ratios for the three PTFE films decrease to about 0.5, 0.3, and 0.2, respectively, from 0.7, 0.6, and 0.3. However, in the cases of the Pd-activated VPD-g-PTFE and AAm-g-PTFE surfaces, a complete loss of palladium is observed after immersion in the electroless copper plating bath. The difference in desorption behavior may be due to the difference in the chemical environment of the Pd-N complex formed on the PTFE surface. The conjugated bonds in the 4VP, 2VP, and VIDz molecules can probably stabilize the Pd-N complex. As a result, the palladium species are strongly adsorbed on these PTFE surfaces. Jackson19 has reported that a higher palladium concentration is required to initiate the metallization of Cu on the poly(acrylic acid)-coated printed circuit boards (PCBs) in the absence of SnCl2 sensitization. Therefore, copper cannot be electrolessly deposited on the present VPD-gPTFE and AAm-g-PTFE surfaces via the Sn-free process. Furthermore, due to the presence of lower [Pd]/[N] and [Pd*]/[Pd] ratios on the VIDz-g-PTFE surface than those

on the 4VP-g-PTFE and 2VP-g-PTFE surfaces, detailed studies on the electroless plating of copper are carried only on the latter two surfaces. The effect of the activation time on the amount of palladium adsorbed on the 4VP-g-PTFE and 2VP-g-PTFE surfaces, in the Sn-free process, is shown in Figure 8. The [Pd]/[N] ratios increase with the activation time up to about 10 min and then level off. The Pd2+ ions are complexed initially with the nitrogen atoms, as discussed above. When the available sites are saturated, other Pd2+ ions are chemisorbed on the immobilized ones through the Cl- ions which are attracted by the polarized charge (δ+) of the palladium ions previously chemisorbed.21 These additional Pd2+ ions are less tightly bonded on the PTFE surface and are more susceptible to removal by rinsing or extraction in aqueous media. Thus, prolonged activation time does not result in the adsorption of more palladium species on the surface-modified PTFE film. The additional Pd2+ ions presumably are also more readily removed in the Cu plating bath. 3.3. Adhesion Characteristics of the Electrolessly Deposited Copper on the 4VP and 2VP GraftCopolymerized PTFE Surfaces. Irrespective of the methods used (the Sn-free process or the two-step process) for surface activation and subsequent electroless deposition of copper, the ultimate adhesion strength between the metal and the polymer substrate is one of the primary concerns for the microelectronics applications. Thus, the adhesion strengths obtained via the two activation methods are evaluated first as a function of the respective 4VP and 2VP polymer graft concentrations, expressed as the [N]/[F] ratio, and the corresponding Ar plasma pretreatment time of the PTFE substrate (Figure 9). It is obvious that the adhesion strength of the electrolessly deposited copper on the 4VP-g-PTFE surface obtained via the Sn-free process is higher than that obtained via the conventional two-step process (Figure 9a). The same result is also observed in the case of 2VP-g-PTFE film (Figure 9b). The amounts of Sn adsorbed on the 4VP-g-PTFE and 2VP-g-PTFE surfaces generally range from [Sn]/[F] ratios of about 0.1 to 0.5. These results indicate that the presence of Sn species not only poisons the palladium catalyst19,20 but also reduces the bondability of activated PTFE surface to the electrolessly deposited copper. The phenomenon is in general agreement with that reported earlier for the electrolessly deposited Ni-P on the NH3 plasma-treated

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Figure 10. Effect of UV-induced copolymerization time on the graft concentration and on the T-peel adhesion strength of the electrolessly deposited copper (from the Sn-free process) on the 4VP-g-PTFE and 2VP-g-PTFE surfaces (Ar plasma pretreatment time ) 90 s, activation time in PdCl2 solution ) 10 min).

Figure 9. Effect of Ar plasma pretreatment time on the graft concentration and the T-peel adhesion strength of the electrolessly deposited copper on (a) the 4VP-g-PTFE surface and (b) the 2VP-g-PTFE surface (UV graft copolymerization time ) 2 h, activation time in PdCl2 solution ) 10 min, and sensitization time in SnCl2 solution ) 2 min for the two-step process). Table 1. Effect of Surface Modification of PTFE on the Adhesion Strength of Electrolessly Deposited Copper on the PTFE Film PTFE treatment condition

activation method

T-peel adhesion strength (N/cm)

1. pristine PTFE film 2. PTFE film with 90 s of Ar plasma treatment 3. 4VP-g-PTFEa 4. 4VP-g-PTFEa 5. 2VP-g-PTFEa 6. 2VP-g-PTFEa

two-step processb two-step process

0.1 1.0

two-step process Sn-free processc Sn-free process two-step process

4.7 7.0 4.7 4.2

a Ar plasma pretreatment of PTFE for 90 s and UV graft copolymerization for 2 h. b Sensitization in SnCl2 solution, followed by activation in PdCl2 solution. c Direct activation in PdCl2 solution only.

PTFE surface.2 On the other hand, however, regardless of the activation method used, the adhesion of the electrolessly deposited copper on the graft-modified PTFE surface is always much higher than that of the electrolessly deposited Cu on the pristine and the Ar plasma-treated PTFE films, as shown in Table 1. The Ar plasma pretreatment time of the PTFE surface affects both the graft concentration and the adhesion strength of the resulting Cu/PTFE assembly, as shown in Figure 9. The graft concentrations of the 4VP and 2VP polymers are expressed simply as the [N]/[F] ratio and deduced from the N 1s to F 1s core-level spectral peakarea ratios. Taking into account the fact that the [peroxide]/[F] ratio increases with the Ar plasma treatment

time of the PTFE film,31 the [N]/[F] ratios for the 4VPg-PTFE and 2VP-g-PTFE surfaces also increase with the Ar plasma pretreatment time of the PTFE film. The corresponding increase in adhesion strength for the resulting Cu/4VP-g-PTFE and Cu/2VP-g-PTFE assemblies with the Ar plasma pretreatment time suggests that a graft chain-induced adhesion mechanism is operative. The adhesion of the electrolessly deposited metal on the graftmodified PTFE surface can be described in terms of the microscopic interactions at the metal/graft polymer interface and is related to the sum of all the intermolecular interactions.32 The increase in adhesion strength with the Ar plasma pretreatment time is attributable to the increase in graft concentration. In the case of the Cu/ 4VP-g-PTFE assembly, the nitrogen atoms in the pyridine rings of the grafted 4VP polymer can interact directly with the electrolessly deposited copper to form the Cu-N bonds,33 which account for the stronger adhesion of the metal to the polymer. The nitrogen atoms in the grafted 2VP polymer are more stericaly hindered due to the presence of the neighboring alkyl back-bond. The extent of charge-transfer interaction is thus reduced. As a result, a lower adhesion strength is observed for the Cu/2VP-gPTFE assembly than that for the corresponding Cu/4VPg-PTFE assembly. Similar steric effects on the chargetransfer interactions have also been observed in other complexes involving Cu and vinylpyridine polymers.33 The UV graft copolymerization time affects both the graft concentration and the adhesion strength of the electrolessly deposited copper on the resulting graftcopolymerized PTFE surface, as shown in Figure 10. At the Ar plasma pretreatment time of 90 s for the PTFE substrate, the graft concentrations of both the 4VP polymer and 2VP polymer increase with the UV graft copolymerization time. With the increase in the graft concentration, the surface roughness may also increase.10 All these effects account for the observed increase in adhesion strength of the electrolessly deposited copper from the Sn-free process on these graft-modified PTFE films. The effect of activation time in PdCl2 solution, in the Sn-free process, on the adhesion strength of the electrolessly deposited copper on the surface-modified PTFE film (31) Wu, S. Y.; Kang, E. T.; Neoh, K. G.; Tan, K. L. Langmuir 2000, 16, 5192. (32) Pritchart, H. W. Acta Polym. 1971, 34, 1132. (33) Lyons, A. M.; Vasile, M. J.; Pearce, E. M.; Waszeza, J. V. Macromolecules 1988, 21, 305.

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free process is briefly investigated by XPS. Figure 11a-c shows the respective wide scan spectra of the pristine PTFE surface and the delaminated Cu and PTFE surface from a Cu/4VP-g-PTFE assembly having a T-peel adhesion strength of about 7 N/cm. The wide scan spectra of the delaminated Cu and PTFE surfaces are grossly similar to that of the pristine PTFE surface. This observation, as well as the fact that no Cu or Pd signals are detected on both of the delaminated surfaces, readily suggests that the metal/polymer assembly must have delaminated by cohesive failure inside the PTFE substrate. The strong adhesion between the electrolessly deposited copper and the 4VP-g-PTFE film is attributable to the strong interaction of the pyridine functional groups of the grafted 4VP polymer with Pd and Cu, the spatial distribution of the graft chains on and above the PTFE surface, and the fact that the grafted 4VP chains are covalently tethered on the PTFE surface.

Figure 11. XPS wide scan spectra of (a) the pristine PTFE surface and the delaminated (b) PTFE surface and (c) Cu surface from a Cu/4VP-g-PTFE assembly having a T-peel adhesion strength of about 7 N/cm.

is shown in Figure 8. The adhesion strength increases with the activation time up to about 10 min and coincides with the increase in the amount of Pd adsorbed on the PTFE surface (Figure 8). The large differences in chemical, physical, and mechanical properties between the deposited metal and the polymer substrate create strains and spatial incompatibility at the interface, which can result in stresses inside the metal film and thus poor adhesion.21 The metal/polymer interface having a higher density of palladium has a lower stress and gives rise to a more adhering Cu film.21 Thus, the amount of adsorbed palladium plays an important role in improving the adhesion strength. Prolonged activation time in PdCl2 solution above 10 min does not result in the adsorption of more palladium species on the PTFE surface. As a result, the adhesion strength is not improved further. 3.4. Failure Mode. The failure mode of the Cu/PTFE interfaces obtained from the electroless deposition of copper on the graft-modified PTFE surfaces via the Sn-

4. Conclusion Argon plasma-pretreated PTFE films were subjected to further surface modification via UV-induced graft copolymerization with nitrogen-containing vinyl monomers under atmospheric conditions. Electroless plating of copper on the 4VP and 2VP graft-copolymerized PTFE films was carried out after surface activation by PdCl2 but in the absence of surface sensitization by SnCl2. The “Sn-free” process involved initially the chemisorption of palladium, in complex form, on the nitrogen sites of the grafted polymer. The Pd complex underwent a reduction to Pd metal in the electroless copper plating bath prior to the initiation of the electroless deposition of copper. The adhesion strength between the electrolessly deposited copper and the surface-modified PTFE film increased with the surface graft concentration and the activation time in PdCl2 solution. The Sn-free activation process was beneficial to the adhesion of the electrolessly deposited copper. An optimum T-peel adhesion strength of about 7 N/cm was achieved for the electrolessly deposited copper on the 4VP-g-PTFE surface. The mode of adhesion failure of the electrolessly deposited copper on the graft-modified PTFE film was cohesive in nature. The strong adhesion was attributed to the strong interaction of the pyridine functional groups of the grafted polymer with Pd and Cu, the spatial distribution of the graft chain on and above the PTFE surface, and the fact that the graft chains were covalently tethered on the PTFE surface. LA0009689