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Nonwetting Process for Achieving Surface Functionalization of Chemically Stable Poly(tetrafluoroethylene) Yongsok Seo,*,† Sehyun Kim,‡ Hyongjun Kim,‡ and Jinyeol Kim§ School of Materials Science and Engineering and School of Chemical Engineering, Seoul National University, Shillimdong 56-1, Kwanak-ku, Seoul, Korea 151-744, and School of Advanced Materials Engineering, Kookmin University, Chongnung, Sungbukku, Seoul, Korea 133-791 Received September 16, 2004. In Final Form: November 21, 2004 Using a low-energy Ar+ ion-beam with and without a reactive gas, chemically stable poly(tetrafluoroethylene) (PTFE) films were modified to have special surface features. The adhesion strength between the PTFE and the copper was significantly improved because of both changes in the surface topography and chemical interactions due to PTFE functionalization (oxidation and amination). The surface modification altered the failure mode from adhesive failure for the unmodified PTFE/Cu interface to cohesive failure for the surface-modified PTFE/Cu layer interface.
Introduction Introduction of polar functional groups and in particular amine functional groups on polymer surfaces is extremely important toward the study of various processes involving polymer surfaces. Surface amine or polar functional groups play an important role in processes such as promotion of adhesion, electroless plating of metals, biofouling, and biomedical applications.1 It has been reported that nitrogen- or oxygen-containing polymers can form complexes with metals, which may significantly enhance the adhesive strength between a polymer and a metal.3,4 While poly(tetrafluoroethylene) (PTFE) is an ideal material for microelectronic applications because of its many desirable properties,2 the chemical stability of PTFE makes it difficult to form a chemical bond with a functional group on the surface, which is definitely needed for the deposition of metal films, such as copper, onto a PTFE film to be used in such applications. Many experimental studies have been done to improve the adhesion between PTFE and copper metal.3 Though they have their own merits, wet processes are not always the choice for the PTFE surface functionalization because of the complex processes and pollution and solvent recycling problems. Since vacuum technologies can change the chemical or the physical properties only at the polymer surface without affecting the bulk properties, these technologies have been intensively investigated. * To whom correspondence should be addressed. E-mail:
[email protected]. † School of Materials Science and Engineering. ‡ School of Chemical Engineering. § School of Advanced Materials Engineering. (1) Dhamodharan, R.; Nisha, A.; Puskala, K.; McCarthy, T. J. Langmuir 2001, 17, 3368. (2) (a) Drobny, J. G. Technology of Fluoropolymers; CRC Press: Boca Raton, FL, 2001. (b) Yamada, Y.; Yamada, T.; Tasaka, S.; Inagaki, N. Macromolecules 1996, 29, 4331. (3) (a) Inagaki, N.; Tasaka, S.; Umehara, T. J. Appl. Polym. Sci. 1999, 71, 2191. (b) Wu, S.; Kang, E. T.; Neoh, K. G.; Tan, H. S.; Tan, K. L. Macromolecules 1999, 32, 186. (c) Zhang, Y.; Yang, G. H.; Kang, E. T.; Neoh, K. G.; Huang, W.; Huan, A. C. H.; Wu, S. Y. Langmuir 2002, 18, 6373. (d) Li, H.; Sharma, R.; Zhang, Y.; Tay, A. A. O.; Kang, E. T.; Neoh, K. G. Langmuir 2003, 19, 6845. (e) Inagaki, N.; Tasaka, S.; Narushima, K.; Mochizuki, K. Macromolecules 1999, 32, 8566. (4) (a) Paik, K. W.; Ruoff, A. L. Mater. Res. Soc. Symp. Proc. 1988, 119, 271. (b) Kim, D. H.; Jo, W. H. Macromolecules 2000, 33, 3050. (c) Kim, D. H.; Kim, K. H.; Jo, W. H.; Kim, J. Macromol. Chem. Phys. 2000, 201, 2699.
Plasma treatment is one of them, but its effect is not so remarkable.3 Recently, we developed a novel process, lowenergy ion-beam irradiation under a reactive gas environment, for polymer surface modification.5 This lowenergy ion-beam irradiation reduces chain degradation and the cross-linking of irradiated polymers, and added gas molecules, such as oxygen or ammonia, are then chemically adsorbed onto the surface to form functional groups. These functional groups can then react or interact with other functional groups or metals evaporated onto the polymer surface. In addition to surface functionalization, the PTFE surface can be also roughened with interactive ion-beams prior to deposition of the overlaying copper.6 Due to the anchor-lock effect, surface roughening is helpful in providing strong adhesion between the PTFE and the evaporated copper. Therefore, for a strong adhesion between the copper metal layer deposited by thermal evaporation and the PTFE surface, functionalization of the PTFE surface and control of the surface topography are two crucial factors. In this study, we investigated the feasibility of using a novel method of reactive low-energy ion-beam etching for PTFE surface modification to improve the adhesion between PTFE and copper, which will be useful for many microelectronic applications. The PTFE surface modification was done by using an Ar+ ion-beam irradiation method with and without a reactive gas (O2 or NH3). Experimental Section Materials. A commercial PTFE sheet and a copper sheet with a thickness of 0.05 mm were used. Argon, ammonia, and oxygen gases of 99.99% purity were used. Ion-Beam Apparatus. The reactive low-energy ion-beam irradiation system is fully described elsewhere.3 It was composed of a conventional ion-beam system, a reactive gas feeding system, and a stand for the polymer samples. The working pressure in the reaction chamber was kept under 10-4 Torr. The Ar+ ionbeam was generated from a 5-cm cold, hollow cathode ion source, and its potential energy was maintained at less than 1 keV. The (5) (a) Kim, H. J.; Lee, K.; Seo, Y.; Kwak, S.; Koh, S. Macromolecules 2001, 34, 2546. (b) Kim, H. J.; Lee, K.; Seo, Y. Macromolecules 2002, 35, 1267. (c) Kim, S.; Lee, K.; Seo, Y. Langmuir 2002, 18, 6185. (6) (a) Kim, S. Korea Polym. J. 1999, 7, 250. (b) Mittal, K. L. J. Vac. Sci. Technol. 1976, 13, 19.
10.1021/la047690v CCC: $30.25 © 2005 American Chemical Society Published on Web 03/08/2005
Surface Functionalization of PTFE
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Figure 1. Surface roughness change with irradiation time (b ) Ar+ only, 0 ) Ar+ + O2, 4 ) Ar+ + NH3) and AFM pictures of the ion-beam irradiated PTFE surfaces. Table 1. Atomic Ratio of Ion-Beam Irradiated PTFE Film
Vergin Ar+ Ar+ + O2 Ar+ + NH3
irradiation time (min)
F/C
1 2 5 1 2 5 1 2 5 10
2.038 1.579 1.493 1.435 1.827 1.665 1.611 1.521 1.417 1.241 0.565
atomic ratio O/C
N/C
0.018 0.033 0.047 0.029 0.043 0.054 0.126
ion current was controlled by the discharge voltage and the ionbeam potential. The discharge current was 0.4 A, and the ion fluence was varied between 1.6 × 1018 and 1.3 × 1020 ions/cm2. The flow rate of Ar gas, which was ionized to Ar+ by an ion source, was fixed at 2 sccm. Reactive gas was constantly injected from the bottom of the chamber. The flow rate of the reactive gas was 3 sccm, which was controlled by a mass flow controller (MassFlo 9121). Surface Characterization. Atomic force microscopy (AFM) images were obtained using a multimode scanning probe microscope (Digital Instruments, Inc.). The tapping mode was used to obtain height imaging data with 125 µm long cantilevers. The lateral scan frequency was about 1.0 Hz. The electron spin resonance (ESR) spectrometer used was a Bruker ESP300 operating in the X-band microwave frequency range. The X-ray photoelectron spectrum was recorded using a Surface Science 2803-S spectrometer (hν ) 1.5 keV). A pressure of 2 × 10-10 Torr was maintained during analysis. An energy resolution of 0.48 eV was kept. Scanning electron microscopy (SEM) observations of the samples were performed on a Hitachi S-2500C. Fractured surfaces were coated with gold in an SPI sputter coater. The morphology was determined using an accelerating voltage of 15 keV. Interfacial Adhesion. A copper layer of about 200 nm in thickness was thermally evaporated onto the PTFE film by using a vacuum thermal evaporation technique. The deposition was carried out under a pressure of 10-6 Torr and at a deposition rate of about 2 Å/s. The copper deposited surface was adhered to a copper sheet by using an epoxy adhesive. The assembly was cured at 100 °C for 6 h and was then subjected to a T-peel adhesion test using an Instron Universal Testing Machine (model 4204) at room temperature. A crosshead speed of 10 mm/min was used. All the reported results are averages of at least 10 measurements.
Figure 2. Radial concentration change on the ion-beam irradiated PTFE surfaces with irradiation time (b ) Ar+ only, 0 ) Ar+ + O2, 4 ) Ar+ + NH3).
Results and Discussion The surface morphology of PTFE was substantially changed by the ion-beam irradiation (Figure 1). An AFM image of a virgin PTFE surface shows a smooth and relatively flat surface. The surface modification occurs first through ion bombardment that initiates etching and radical production. Then, addition of reactive gas molecules or gas radicals follows, which is significantly affected by the concentration of radicals on the PTFE surface. The etching pits, which were seen at short irradiation times, evolved to grasslike structures, which became deeper and wider with the increasing irradiation time as a result of the physical bombardment. When Ar+ ion-beam irradiation was conducted without a reactive gas environment, the surface roughness rapidly increased with irradiation time. When reactive oxygen gas was added, the roughness increase became less steep, and it was even slower when NH3 gas was added. Under the NH3 reactive gas environment, the surface roughness almost reached a plateau value after 10 min of irradiation. From the ion fluence data measured by using a Faraday cup placed slightly above the polymer specimen, we infer that some ions collided with the gas molecules to dissociate or activate them; thus, less of the ion-beam reached the PTFE surface, resulting in a smoother surface.
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Figure 3. (a) Peel strength for the adhesion joint vs irradiation time, (b) the peel strength due to the surface roughness, and (c) the peel strength due to chemical interactions (b ) Ar+ only, 0 ) Ar+ + O2, 4 ) Ar+ + NH3).
Figure 4. SEM micrographs of the PTFE-side layers peeled off from the PTFE/copper adhesive joints.
X-ray photoelectron spectroscopy (XPS) analysis showed that the C 1s peak of the untreated PTFE was symmetric with a narrow full width at half-maximum whereas that of the ion-beam irradiated surfaces was asymmetric because of the surface functionalization (see Figure 5). The relative concentrations of these components, estimated from the relative peak areas, are listed in Table 1. Defluorination occurred most for the system with NH3 added, less for the Ar+ ion-beam irradiated system, and least for the system with oxygen added. A large portion of the CF2 units in PTFE were defluorinated for the NH3added system (almost 70% were defluorinated after a 10min irradiation) to produce a large number of carbon radicals, some of which combined with hydrogen or other nitrogen-containing radicals. The system with reactive NH3 gas added showed some nitrogen concentration. The amount of adsorbed oxygen molecules for the system with reactive oxygen gas added was less than that of adsorbed nitrogen for the system with reactive NH3 gas added. This indicates that fewer surface radicals were generated on the PTFE surface for the former system. Figure 2 shows the radical concentration on the ion-beam irradiated PTFE surface which was measured using an electron spin resonance spectrometer.7 The radical concentration increased with irradiation time. Under the same ion dose, the surface irradiated with NH3 gas showed the highest radical concentration while the surface irradiated with O2 gas had the lowest. Fluoropolymers are resistant to oxygen attack. Both surface degradation and oxygen incorporation were found to be small for O2-plasma-treated PTFE.8 (7) Kim, S. Ph.D. Dissertation, Seoul National University, Seoul, Korea, 2003. (8) Shi, M. K.; Selmani, A.; Martinu, L.; Sacher, E.; Wertheimer, M. R.; Yelo, A. In Polymer Structure Modifications: Relevance to Adhesion; Mittal, K. L., Ed.; VSP: Leiden, The Netherlands, 1995.
Figure 3 shows the peel strength for the adhesive joint between the PTFE film and the copper metal as a function of the irradiation time. Obviously, the adhesion strength is affected by the irradiation time. Some facts are worthy of note: (1) Neat PTFE resin has a very low adhesive strength (
