Electroless Deposition of Copper on Surface Modified Poly

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Langmuir 2000, 16, 5192-5198

Electroless Deposition of Copper on Surface Modified Poly(tetrafluoroethylene) Films from Graft Copolymerization and Silanization Shaoyu Wu, E. T. Kang,* and K. G. Neoh Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 119260

K. L. Tan Department of Physics, National University of Singapore, Kent Ridge, Singapore 119260 Received October 27, 1999. In Final Form: February 28, 2000 To improve the adhesion of electrolessly deposited copper on poly(tetrafluoroethylene) (PTFE) film surface, Ar plasma treatment, graft copolymerization, and silanization of PTFE film surface were carried out. The surface of PTFE film was first treated with Ar plasma, followed by UV-induced graft copolymerization with 2-hydroxyethyl acrylate (HEA) (the HEA-g-PTFE surface), or N-(hydroxymethyl)methacrylamide (HMMAAm) (the HMMAAm-g-PTFE surface). The surface-hydroxylated PTFE film was then reactively silanized with N-[3-(trimethoxysilyl)propyl]diethylenetriamine. The surface-modified PTFE films were subsequently activated for the electroless deposition of copper via sensitization with SnCl2 and immobilization of the palladium catalyst. The surface compositions of the PTFE films at various stages of surface modification were studied by X-ray photoelectron spectroscopy. The adhesion strength of the electrolessly deposited copper on the silanized PTFE film was affected by the concentration of the grafted HEA and HMMAAm polymers. The T-peel adhesion strength of the electrolessly deposited copper on graft-copolymerized and silanized PTFE surface could reach about 7 N/cm. This adhesion strength involving the additional silanization step represented a more than 10-fold and 6-fold increase, respectively, over those obtained when the PTFE films were modified by Ar plasma treatment and by graft copolymerization. The mechanism of the adhesion strength enhancement and the cohesive failure of the polymer-metal interface were also investigated.

* To whom correspondence should be addressed. Fax: (65) 7791936. Tel: (65)874-2189. E-mail: [email protected].

plasma treatment have also been employed to activate the surface of PTFE.13 Gas plasma treatment, under various glow discharge conditions, has been used extensively in the surface modification of fluoropolymers. Recently, Shi et al.14 reported improvement in adhesion between evaporated Cu and PTFE modified by N2, O2, H2, and mixed-gas plasmas and proposed that Cu reacted with both oxygen and nitrogen to form, respectively, Cu-O and Cu-N moieties at the interface. Modification of polymer surface by graft copolymerization with specific functional monomers also improves the adhesion properties.15,16 The adhesion between two PTFE films could be substantially improved by graft copolymerization with certain functional monomers. Vargo et al.17-20 reported that silanization of the fluoropolymer surface could be carried out by chemisorbing N-(2-aminoethyl)-3-aminopropyltrimethoxysilane onto a plasma hydroxylated fluoropolymer surface. The so-modified fluoropolymer surface exhibited good adhesion to copper.

(1) Park, J. M.; Matienzo, L. J.; Spencer, D. F. J. Adhes. Sci. Technol. 1991, 5, 153. (2) Granneman, E. H. A. Thin Solid Films 1993, 228, 1. (3) Silvain, J. F.; Ehrhardt, J. J.; Picco, A.; Lutgen, P. ACS Symp. Ser. 1990, No. 440, 453. (4) Vopel, S. L.; Schowhom, H. J. Appl. Polym. Sci. 1979, 23, 495. (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) Sacher, E. Prog. Surf. Sci. 1994, 47, 273. (8) Celerier, A.; Machet, J. Thin Solid Films 1987, 148, 323. (9) Mathieson, I.; Brewis, D. M.; Sutherland, I.; Cayless, R. A. J. Adhes. 1994, 46, 49. (10) Costello, C. A.; MacCarthy, T. J. Macromolecules 1984, 17, 2940. (11) Huang, M.; Burch, R. R. J. Appl. Polym. Sci. 1995, 55, 549. (12) Badey, J. P.; Espuche, E. U.; Jugnet, Y.; Sage, Tran Minn Due, D.; Chabent, B. Polymer 1994, 35, 2472.

(13) Liston, E. M.; Martinn, L.; Wertheiner, M. R. J. Adhes. Sci. Technol. 1993, 7, 1091. (14) Shi, M. K.; Selmani, A.; Martinu, L.; Sacher, E.; Wertheimer, M. R.; Yelon, A. Polymer surface Modifications: Relevance to Adhesion; Mittal, K. L., Ed.; VSP: Zeist, 1995; p 73. (15) Kang, E. T.; Tan, K. L.; Kato, K.; Uyama, Y.; Ikada, Y. Macromolecules 1996, 29, 6872. (16) Wang, T.; Kang, E. T.; Neoh, K. G.; Tan, K. L.; Cui, C. Q.; Lim, T. B. J. Adhes. Sci. Technol. 1997, 11, 679. (17) Vargo, T. G.; Gardella, J. A., Jr.; Calvert, J. M.; Chen, M. Science 1993, 262, 1711. (18) Hook, D. J.; Vargo, T. G.; Gardella, J. A., Jr.; Litwiler, K. S.; Bright, F. V. Langmuir 1991, 7, 142. (19) Bening, R. C.; Mccarthy, T. J. Macromolecules 1990, 23, 2648. (20) Vargo, T. G.; Thompson, P. M.; Gerenser, L. J.; Valentini, R. F.; Aebischer, P.; Hook, D. J.; Gardella, J. A. Langmuir 1992, 8, 130.

Introduction Poly(tetrafluoroethylene) (PTFE) has many desirable properties, such as thermal stability, low dielectric constant, and surface inertness, which make it ideal for microelecronic applications.1-3 Many studies on metalization of PTFE have been carried out.4-6 However, due to the surface inertness of PTFE, the adhesion between the polymer and various metals fails to satisfy many of the industry requirements.7 In many occasions, activation of polymer surfaces prior to metalization has been shown to enhance metal adhesion. The most widely used method for activating the PTFE surface is the treatment with a reducing etchant,8,9 such as sodium naphthalenide10 and potassium t-butoxide/benzoin/dimethyl sulfoxide.11 The etchants affect not only the surface aspects of the polymer but also its bulk properties.12 Apart from the wet chemical treatment, ion bombardment, X-ray irradiation, and cold

10.1021/la9914211 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000

Copper on PTFE Films

In the present work, further surface hydroxylation of the Ar-plasma-pretreated PTFE film has been carried out via UV-induced graft copolymerization with 2-hydroxyethyl acrylate (HEA) and N-(hydroxymethyl)methacrylamide (HMMAAm). The hydroxylated PTFE film surface is subsequently treated with N-[3-(trimethoxysilyl)propyl]diethylenetriamine (TMS) to enhance the adhesion of PTFE surface with copper metal from electroless deposition. Physicochemical parameters affecting the adhesion between surface-silanized PTFE and deposited copper, such as the concentration of grafted polymers and the thermal post-treatment, are studied. The chemical compositions of the PTFE films at various stages of surface modification are characterized by X-ray photoelectron spectroscopy (XPS). Experimental Section Materials. PTFE film with a thickness of about 0.01 cm and a density of 2.18 g/cm3 was used in this study and was obtained from Goodfellow Ltd. of Cambridge, U.K. The surface of the film was cleaned by Soxhlet extraction with acetone for 6 h before use. The 2-hydroxyethyl acrylate (HEA), N-(hydroxymethyl)methacrylamide (HMMAAm), N-[3-(trimethoxysilyl)propyl]diethylenetriamine (TMS), and the solvent, 1,4-dioxane, used for surface graft copolymerization were obtained from Aldrich Chemical Co. of Milwaukee, WI. The chemical structures of HEA, HMMAAm, and TMS are as follows: H2C dCHCO2CH2CH2OH (HEA); H2CdC(CH3)CONHCH2OH (HMMAAm); (CH3O)3Si(CH2)3NHCH2CH2NHCH2CH2NH2 (TMS). Graft Copolymerization. The PTFE films were cut into strips of about 2 cm × 6 cm in size. They were pretreated with Ar plasma before 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 30 W at a radio frequency of 40 kHz. The film was placed between the two electrodes and subjected to glow discharge for a predetermined period of time at an Ar pressure of about 0.05 Torr. The Ar-plasma-pretreated polymer films were then exposed to the atmosphere to effect the formation of peroxides and hydroperoxides at the surface21 before UVinduced graft copolymerization. Peroxide concentration of the Ar-plasma-pretreated PTFE surface was determined by reaction with SO2.22-24 The Ar-plasma-pretreated PTFE film was exposed to SO2 for 30 min. The peroxide species reacted with SO2 via the following redox mechanism:

∼C-OOH + SO2 f ∼C-O-SO2-OH or

∼C-OO-C∼ + SO2 f ∼C-O-SO2-OC∼ The peroxides concentration, expressed as the [peroxide]/[F] ratio or the equivalent [S]/[F] ratio, was determined from the sensitivity-factors-corrected S2p and F1s X-ray photoelectron spectroscopy (XPS) core-level spectral area ratio. The plasma-pretreated PTFE films were immersed in 30 mL of 1,4-dioxane solutions of HEA or HMMAAm in a Pyrex tube. The concentrations of each monomer were varied from 2 to 20 vol %. Each reaction mixture was thoroughly degassed and sealed under a nitrogen atmosphere. It was then subjected to UV irradiation for 30-60 min in a Riko Rotary (model RH 400-10W) photochemical reactor, manufactured by Riko Denki Kogyo of Chiba, Japan. The reactor was equipped with a 1000 W highpressure Hg lamp and a constant-temperature water bath. All UV-induced graft copolymerization reactions were carried out at a constant temperature of 28 °C. After each grafting experi(21) Suzuki, M.; Kishida, A.; Iwata, H.; Ikada, Y. Macromolecules 1986, 19, 1804. (22) Mitchell, J., Jr.; Perkins, L. R. Appl. Polym. Symp. 1967, 4, 167. (23) Briggs, D.; Kendall, C. R. Int. J. Adhes. Adhesives 1982, 2, 13. (24) Zhang, J.; Kato, K.; Uyama, Y.; Ikada, Y. J. Polym. Sci.: Part A: Polym. Chem. 1995, 33, 2629.

Langmuir, Vol. 16, No. 11, 2000 5193 ment, the PTFE film was washed thoroughly with copious amounts of acetone to remove the residual monomer and physically adsorbed homopolymer. Silanization of PTFE Film Surface. After surface hydroxylation by graft copolymerization, the PTFE films were immersed in 1.0 wt % 1,4-dioxane solution of N-[3-(trimethoxysilyl)propyl]diethylenetriamine (TMS) for 1 min. The films were then rinsed with copious amounts of the solvent to remove the physically adsorbed TMS. The TMS became chemisorbed via Si-O bonding17 on the graft-modified PTFE film surface. XPS Measurement. XPS measurements were made on a VG ESCALAB MkII spectrometer with a Mg KR X-ray source (1253.6 eV photons) at a constant retard ratio of 40. The polymer films were mounted on the standard sample studs by means of doublesided adhesive tape. The core-level signals were obtained at a photoelectron takeoff angle of 75° (with respect to the sample surface). The X-ray source was run at a reduced power of 120 W. The pressure in the analysis chamber was maintained at 7.5 × 10-9 Torr or lower during each measurement. All binding energies (BEs) were referenced to the C1s neutral carbon peak at 284.6 eV. In the peak synthesis, the line width (full width at halfmaximum, fwhm) for the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from peak-area 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 wellestablished stoichiometries. Electroless Deposition of Copper on Surface-Modified PTFE Film and T-Type Peel Strength Measurement. The pristine and surface-modified PTFE films were activated by a two-step process to immobilize the Pd catalyst for the electroless deposition of copper. The film surface was first sensitized by soaking in 0.3 wt % aqueous solution of SnCl2, containing also 2.5 wt % HCl, for 1 min, followed by rinsing. It was subsequently placed in 0.1 wt % aqueous solution of PdCl2, containing 1.0 wt % HCl, for 5 min and rinsed thoroughly with doubly distilled water. The surface-activated PTFE film was then immersed in an electroless plating bath for 5-20 min.25-27 The composition of the plating solution was as follows: 0.7 wt % CuSO4‚5H2O, 2.5 wt % potassium sodium tartrate, 0.4 wt % sodium hydroxide, and 0.4 wt % formaldehyde.27 The Cu-deposited PTFE films were then rinsed thoroughly with copious amounts of doubly distilled water. The plating rate was determined gravimetrically after the electroless plating for 5, 10, and 20 min. For each type of the PTFE surface, the plating rates for the three time periods were averaged. In each case, a copper layer of up to 2000 Å in thickness was electrolessly deposited onto the surface of the PTFE film. The metalized samples were post-treated in a vacuum oven at different temperatures. It was then adhered to a copper sheet backing (0.1 mm in thickness) using an epoxy adhesive (Araldite Stand, from Ciba-Geigy Chem Co. of Switzerland.). The assembly was cured at 100 °C for 2 h or at room temperature for 24 h. The assembly was then subjected to T-peel adhesion test in an Instron model 5544 materials tester. All measurements were carried out at a crosshead speed of 1.0 cm/min. For each T-type peel strength reported, at least three sample measurements were averaged.

Results and Discussion The processes of graft copolymerization, silanization, surface sensitization, and electroless deposition on a PTFE film are shown schematically in Figure 1. The details for each process are described below. Surface Modification of PTFE Films by Plasma Treatment, Graft Copolymerization, and Reactive Silanization. The dependence of the surface [O]/[C] ratio and [peroxide]/[F] ratio (derived from the equivalent [S]/[F] (25) Charbonnier, M.; Alami, M.; Romand M. J. Appl. Electrochem. 1998, 28, 449. (26) Mance, A. M.; Waldo R. A.; Dow, A. A. J. Electrochem. Soc. 1989, 136, 1667 (27) Ebneth, H. Metallizing of Plastics-Handbook of Theory and Practice; Suchentrunk, R., Ed.; ASM International: Materials Park, OH, 1993; p 30.

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Figure 1. Schematic diagram illustrating the processes of graft copolymerization, silanization, sensitization, and electroless metal deposition on the PTFE film.

ratio) of the PTFE film, as determined from the corrected O1s, C1s, S2p, and F1s core-level spectral peak-area ratios, on the Ar plasma treatment time is shown in Figure 2. Both the [O]/[C] and [peroxide]/[F] ratios increase with increasing Ar plasma treatment time of the PTFE film, in agreement with the results generally reported in the literature.16,28 The Ar plasma treatment causes the breakage of some C-F bonds, resulting in the defluorination of the film surface. The subsequent exposure of the activated surface to air causes oxygen to be incorporated onto the PTFE surfaces, leading to surface oxidation and the formation of peroxide and hydroperoxide species.21 The peroxide species can be utilized to initiate the surface free radical polymerization in a mechanism generally proposed for the UV-induced graft copolymerization.21 Figure 3 shows the C1s core-level spectra for a 60-s Ar-plasma-pretreated PTFE film after having been subjected to UV-induced graft copolymerization in different concentrations (5 vol % and 10 vol %) of HEA solution for 20 min (the HEA-g-PTFE surfaces). The presence of surface-grafted HEA polymer can be deduced from the three C1s peak components with binding energies (BEs) at 284.6 eV for the CH species, 286.2 eV for the CO species,

and 288.5 eV for the COO species.29 The component area ratios for surfaces with low graft concentration (Figure 3a) and high graft concentration (Figure 3b) are all in good agreement with the theoretical ratio of 2:2:1, as dictated by the chemical structure of HEA. Figure 3 also shows the C1s core-level spectra for a 60-s Ar-plasmapretreated PTFE film after having been subjected to UVinduced graft copolymerization in two different concentrations (2 vol. % and 10 vol %) of HMMAAm solution for 5 min (the HMMAAm-g-PTFE surfaces). The presence of surface-grafted HMMAAm polymer can be deduced from the low BE C1s core-level spectrum with peak components centered at about 284.6 eV for the CH species, 286.6 eV for O-C-N species, and 287.7 eV for the CONH species. The component area ratios for PTFE surfaces with low graft concentration (Figure 3c) and high graft concentration (Figure 3d) of the HMMAAm polymer are again in good agreement with the theoretical ratio of 3:1:1.29 The presence of surface-grafted HMMAAm polymer is also confirmed by the appearance of the N1s core-level signal centered at about 399.0 eV and associated with the amide species. The concentrations of the grafted HEA and HMMAAm polymers as a function of the respective monomer concentration used for graft copolymerization are shown in Figure 4. The graft concentration for the HEA polymer is expressed simply as the [COO]/[F] ratio as one repeat unit of HEA polymer contains one COO group. The graft concentration for HMMAAm polymer, on the other hand, is expressed as the [N]/[F] ratio as one repeat unit of HMMAAm polymer contains a N atom. In both cases, the graft concentration increases with the monomer concentration up to the monomer concentration of about 10 vol %, after which the graft concentration tends to decrease. The decrease probabaly has resulted from excess homopolymerization in the concentrated reaction mixture which attenuates the intensity of the UV source reaching the PTFE surface, as is also indicated by the rapid rise in solution viscosity during the UV-induced graft copolymerization process. Parts a-d of Figure 5 show the respective C1s and Si2p core-level spectra, before and after silanization by TMS, for a 60-s Ar-plasma-pretreated PTFE film which has been

(28) Inagaki, N.; Tasaka, S.; Goto, Y. J. Appl. Polym. Sci. 1997, 66, 77.

(29) Wang, T. PhD Thesis, Department of Chemical Engineering, National University of Singapore, 1998.

Figure 2. Effect of Ar plasma treatment time on the [O]/[C] and [peroxide]/[F] ratios of the PTFE film.

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Figure 3. C1s core-level spectra of the PTFE surfaces from the UV-induced graft copolymerization of the 60 s Ar-plasma-pretreated PTFE films in (a) 5 vol % and (b) 10 vol % HEA monomer solutions for 20 min and in (c) 2 vol % and (d) 10 vol % HMMAAm monomer solutions for 5 min.

Figure 4. Effect of monomer concentration on the graft concentration of (a) HEA polymer (Ar plasma pretreatment time ) 60 s, UV time ) 20 min) and (b) HMMAAm polymer (Ar plasma pretreatment time ) 60 s, UV time ) 5 min).

subjected to UV-induced graft copolymerization in a 10 vol % HEA solution for 20 min (graft concentration or [COO]/[F] ) 0.18). The corresponding C1s and Si2p corelevel spectra, before and after silanization by TMS, for a 60-s Ar-plasma-pretreated PTFE film which has been subjected to UV-induced graft copolymerization in a 10 vol % HMMAAm solution for 5 min (graft concentration of [N]/[F] ) 0.15) are shown in parts e-h of Figure 5. From the changes in the C1s core-level line shape and the appearance of the Si2p core-level signal after silanization, the TMS must have successfully chemisorbed onto the hydroxylated PTFE surfaces (the TMS-HEA-g-PTFE and TMS-HMMAAm-g-PTFE surfaces). The effects of the graft concentrations of HEA and HMMAAm polymer on the amounts of chemisorbed silane on the respective PTFE surfaces, expressed as the [Si]/[F] ratios and determined from the Si2p and F1s core-level spectral area ratios, are shown in Figure 6. The [Si]/[F] ratios increase with increasing graft concentrations of the HEA and HMMAAm polymer. The phenomenon suggests that the amount of chemisorbed silane depends on the concentration of hydroxyl groups arising from the grafted HEA and HMMAAm polymer.

Figure 5. C1s and Si2p core-level spectra of PTFE surfaces which have been salinized by TMS after graft copolymerization with HEA (Ar plasma pretreatment time ) 60 s, UV time ) 20 min, HEA concentration ) 10 vol %) and HMMAAm (Ar plasma pretreatment time ) 60 s, UV time ) 5 min, HMMAAm concentration ) 10 vol %).

Activation of the PTFE Surfaces by SnCl2 and PdCl2. The effects of surface modification on the amounts of adsorbed Sn and Pd on the PTFE surfaces, defined as [Sn]/[F] and [Pd]/[F] ratios and determined from the Sn3d, Pd3d, and F1s core-level spectral area ratios, are summarized in Table 1. The amounts of adsorbed Sn and Pd on the PTFE surface increase after Ar plasma treatment and subsequent air exposure, due to the surface oxidation and hydroxylation of PTFE film.25,30 The largest increase in Sn and Pd adsorption is observed for the graft-modified

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Table 1. Effect of Surface Modification of PTFE Film on the Amount of Adsorbed Sn and Pd during the Two-Step Activation Process and on the Subsequent Rate of Electroless Deposition of Copper sample

[Sn]/[F]

[Pd]/[F]

plating rate ((mg/h) cm2)

1. pristine PTFE film 2. PTFE film with Ar plasma treatment for 60 s 3. sample 2 graft-copolymerized with HMMAAma 4. sample 3 after treatment with TMS 5. sample 2 graft-copolymerized with HEAb 6. sample 5 after treatment with TMS

0.001 0.003 0.53 0.13 0.22 0.17

0.001 0.002 0.35 0.09 0.16 0.09

2.2 2.5 2.0 2.1 2.4 2.2

a UV-induced surface graft copolymerization in 10 vol % HMMAAm solution for 5 min. 10 vol % HEA solution for 20 min.

b

UV-induced surface graft copolymerization in

Figure 6. Effect of graft concentration of (a) HEA polymer and (b) HMMAAm polymer on the chemisorption of TMS.

PTFE film. The effects of the graft concentrations of HEA and HMMAAm polymer on the adsorption of Sn and Pd, defined as the [Sn]/[F] and [Pd]/[F] ratios, are shown in Figure 7. The amounts of adsorbed Sn and Pd increase significantly in the presence of surface grafted HEA and HMMAAm polymer due to the large increase in surface concentration of the functional groups.25,30 The dependence of the amounts of adsorbed Sn and Pd on the respective [Si]/[F] ratios of the HEA and HMMAAm graft-copolymerized PTFE surfaces after silanization is shown in Figure 8. The amounts of adsorbed Sn and Pd increase with increasing [Si]/[F] ratio of each graftcopolymerized surface. Nevertheless, comparison of the data in Figure 8 with the amounts of adsorbed Sn and Pd on the HEA and HMMAAm graft-copolymerized PTFE surfaces in the absence of silanization (Figure 7) indicates that the silanization of the PTFE surfaces has resulted in a decrease in the amounts of adsorbed Sn and Pd. Spectra a and b of Figure 9 show, respectively, the Sn3d core-level spectra of SnCl2 adsorbed on HEA graftcopolymerized PTFE film surface before and after having (30) Charbonnier, M.; Alami, M.; Romand, M. J. Electrochem. Soc. 1996, 143, 113.

Figure 7. Effect of graft concentration of (a) HEA polymer and (b) HMMAAm polymer on [Sn]/[F] and [Pd]/[F] ratios during the two-step activation process.

been subjected to activation by PdCl2. Spectra c and d of Figure 9 show the respective Pd3d core-level spectra of pure PdCl2 and PdCl2 after having been immobilized on HEA graft-copolymerized and the SnCl2-sensitized PTFE surface. The peak components at the BEs of 338 and 343 eV are assigned to Pd3d5/2 and Pd3d3/2 core-level spectra of Pd2+, respectively.25 The new peak components at the BEs of 335 and 340 eV in Figure 9d are assigned to the Pd3d5/2 and Pd3d3/2 core-level spectra of the Pd metal.25 The appearance of the Pd metal after the two-step activation process indicates that some of the Pd2+ ion has been reduced to Pd metal. The broadening of the Sn3d core-level spectrum after the adsorption of PdCl2, as well as the changes in line shape of the Pd3d spectrum, suggest that a redox reaction has taken place during the two-step activation process. The reduced Pd is utilized subsequently to catalyze the electroless plating of copper.25 Adhesion Characteristics of Electrolessly Deposited Copper to the Modified PTFE Surfaces. The effects of surface modification of PTFE films on the rates

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Figure 9. Sn3d core-level spectra of SnCl2 adsorbed on the HEA graft copolymerized PTFE surface (a) before and (b) after PdCl2 treatment, and Pd3d core-level spectra of PdCl2 (c) before and (b) after immobilization on the SnCl2-sensitized surface.

Figure 8. Effect of [Si]/[F] ratios of the (a) HEA and (b) HMMAAm graft-copolymerized PTFE surfaces on [Sn]/[F] and [Pd]/[F] ratios during the two-step activation process.

of copper deposition are summarized in Table 1. The vast differences in compositions of the PTFE surfaces as a result of the various surface modification processes appear to have little effect on the electroless plating rate of copper metal. Nevertheless, the nature of the modified surface has a marked effect on the subsequent adhesion characteristics of the electrolessly deposited copper metal. Simple Ar plasma treatment of the PTFE surface can enhance the T-peel adhesion strength of the electrolessly deposited copper from negligible to about 0.6 N/cm at 60-80 s of plasma treatment time. Figure 10 shows the effect of graft concentration on the T-peel adhesion strength of the electrolessly deposited copper to the HEA and HMMAAm graft-copolymerized PTFE surfaces. The adhesion strength in each case increases with the graft concentration. The charge-transfer interaction between the copper metal and the hydroxyl or nitrogen moieties of the grafted polymer must have contributed to the increase in adhesion strength. The effect of the [Si]/[F] ratio of the graft-copolymerized and silanized PTFE surface on the T-peel adhesion strength of the electrolessly deposited copper is shown in Figure 11. The adhesion strength for each graft-modified and silanized PTFE surface increases with increasing [Si]/[F] ratio, especially after the thermal post-treatment of the metal/polymer interface at higher temperature. A number of mechanisms are probably operative to account for the enhanced adhesion strength. During the electroless deposition of copper in an alkaline solution, the presence of covalently bonded aminosilane groups on the PTFE surface, the coordination bonding of the Pd catalyst to the surface ligand, and the subsequent interaction of the silane

and amino groups with Cu are likely to have contributed to the high adhesion strength of the electroless deposits on the substrate.17 In addition, it has been reported that Sn can interact strongly with the oxygenated species.31,32 Meanwhile, the polymer chain, which contains the silane group, will cross-link via Si-O bonds to form a network structure in the interphase between the copper and polymer.17 Such cross-linking in the interphase, coupled with the strong interactions between the aminosilane functioned groups and the metals, must have contributed to the observed adhesion strength enhancement. The thermal treatment further increases the extents of crosslinking and interaction, resulting in the further improvement in adhesion strength. Finally, the high density of cross-linking in the interphase must have significantly reduced the hydrolytic instability of the C-O-Si bonds. As a result, the Cu/TMS-HEA-g-PTFE and Cu/TMSHMMAAm-g-PTFE assemblies exhibit excellent adhesion reliability under ambient conditions. The failure mode of the Cu/PTFE interfaces obtained from electroless plating of copper metal onto the graftmodified and silanized PTFE surfaces was briefly investigated. Figure 12 shows the wide scan spectra of the pristine PTFE surface and the delaminated surfaces of a Cu/TMS-HEA-g-PTFE assembly having a peel strength of about 5 N/cm. The fact that the wide scan spectrum of the delaminated Cu surface resembles that of the pristine PTFE surface, as well as the fact that no Cu, Pd, or Si signals are detected on both of the delaminated surfaces, suggests that the metal/polymer interface has delaminated by a cohesive failure inside the PTFE substrate. In fact, for all the Cu/PTFE assemblies from electroless deposition of copper studied in the present work, clean cohesive failure (31) Charbonnier, M.; Alami, M.; Romand, M. In Metallized Plastics: Fundamentals and Applications; Mittal, K. L., Ed.; Marcel Dekker: New York, 1998; pp 3-23. (32) Romand, M.; Charbonnier, M.; Alami, M. J. Electrochem. Soc. 1996, 143, 472.

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Figure 10. Effect of the graft concentration on the T-peel adhesion strength of the electrolessly deposited copper on the (a) HEA graft-polymerized PTFE surface (Ar plasma pretreatment time ) 60 s, UV time ) 20 min) and on (b) HMMAAm graft-polymerized PTFE surface (Ar plasma pretreatment time ) 60 s, UV time ) 5 min).

Wu et al.

Figure 11. Effect of [Si]/[F] on the T-peel strength of the electrolessly deposited copper on the silanized and graftmodified PTFE surface.

in the polymer is always observed when the T-peel adhesion strength of the assembly is about 5 N/cm. Conclusion Ar-plasma-pretreated PTFE films were subjected to surface modification via UV-induced graft copolymerization with HEA and HMMAAm. The graft-modified PTFE surfaces were silanized via chemisorption of TMS. The chemical compositions of the graft copolymerized and silanized PTFE surfaces were analyzed by XPS. In general, the graft yield increased with the concentration of monomer used during graft copolymerization. The pristine and modified PTFE films were subjected to electroless plating of copper after sensitization and activation by the SnCl2-PdCl2 system. Studies on the initiation mechanism of the two-step activation process showed that the Pd2+ ion was partially reduced to Pd metal during the activation of the SnCl2-sensitized surface. The Pd metal catalyzed the electroless deposition of copper. The uptake of both Sn and Pd increased with increase in the graft concentration of HEA and HMMAAm polymers. Nevertheless, the rate of electroless deposition of copper did not appear to be affected by the various surface modification processes. Nevertheless, the adhesion strength between the electrolessly deposited copper and the PTFE film was significantly enhanced by surface graft copolymerization and silanization and by thermal treatment of the interface after the electroless deposition of copper. A T-peel adhesion strength in excess of 7 N/cm was achieved for the electrolessly deposited Cu on the silanized and graftcopolymerized PTFE surface and the assembly delami-

Figure 12. XPS wide scan spectra of (a) the pristine PTFE and the delaminated (b) PTFE and (c) Cu surfaces from a Cu/ TMS-HEA-g-PTFE assembly having a T-peel adhesion strength of about 5 N/cm.

nated by clean cohesive failure inside the PTFE substrate. This adhesion strength also represented a 10-fold increase over that of the assembly involving PTFE surface treated by plasma alone. LA9914211