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Spatially Defined Surface Modification of Poly(methyl methacrylate) Using 172 nm Vacuum Ultraviolet Light Atsushi Hozumi,*,† Tomoko Masuda,‡ Kazuyuki Hayashi,§ Hiroyuki Sugimura,§ Osamu Takai,§ and Tetsuya Kameyama† National Institute of Advanced Industrial Science & Technology (AIST), 2266-98, Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan, Department of Industrial Chemistry, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501, Japan, and Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received May 20, 2002. In Final Form: August 6, 2002 Hydrophilization of poly(methyl methacrylate) (PMMA) substrates has been demonstrated using a Xe/2 excimer lamp radiating vacuum ultraviolet (VUV) light of 172 nm in wavelength. In this study, we have particularly focused on the effects of atmospheric pressure during VUV irradiation. Each of the substrates was photoirradiated with VUV light under a pressure of 10, 103, or 105 Pa. Although in each case the hydrophobic PMMA surface became hydrophilic, the water-contact angle and photooxidation rate markedly depended on the atmospheric pressure. The sample treated at 10 Pa was less wettable than the samples treated at 103 and 105 Pa due to the shortage of oxygen molecules in the atmosphere. The minimum water-contact angles of the samples treated at 10, 103, and 105 Pa were about 40, 25, and 24°, respectively. Microfabrication of the PMMA substrates was also demonstrated employing a simple mesh-mask contact method using the same excimer lamp. As confirmed by atomic force microscopy, a photoetched groove composed of 25 × 25 µm2 features was successfully fabricated on the PMMA substrates. Both the spatial resolution and photoetch depth of the microstructures depended on the atmospheric pressure. At 10 and 103 Pa, we achieved finely grooved microstructures at etching rates of 13 and 13.2 nm/min, respectively. In comparison, when the pressure was further increased to 105 Pa, the etching rate decreased to 6.9 nm/min and patterning resolution became significantly worse. The pressure of 103 Pa was determined to be optimum for accurately defining PMMA surfaces both chemically and geometrically.
Introduction The surface modification of polymeric materials through photoinduced chemical processes has attracted much attention because of its wide variety of applications, including microstructure fabrication, photochemical etching, and improvement of the wettability, biocompatibility, and coating adhesion of the surfaces.1-5 Among the various light sources available, vacuum ultraviolet (VUV) light, whose wavelength is much shorter than 200 nm, is of particular interest for polymer surface processing. Shortwavelength radiation in this range can be applied to the surface modification of fluorocarbon polymers,3,6-11 which * To whom correspondence should be addressed: E-mail:
[email protected]. Phone: +81-52-736-7175. Fax: +81-52-7367182. † National Institute of Advanced Industrial Science & Technology. ‡ Chubu University. § Nagoya University. (1) Uchida, T.; Shimo, N.; Sugimura, H.; Masuhara, H. J. Appl. Phys. 1994, 76, 4872. (2) Zhang, J.-Y.; Esrom, H.; Kogelschatz, U.; Emig, G. Appl. Surf. Sci. 1993, 69, 299. (3) Cezeaux, J. L.; Romoser, C. E.; Benson, R. S.; Buck, C. K.; Sackman, J. E. Nucl. Instrum. Methods Phys. Res. B 1998, 141, 193. (4) Huang, F.; Lou, Q.; Dong, J.; Wei, Y. Appl. Surf. Sci. 2001, 174, 1. (5) Haack, L. P.; Straccia, A. M.; Holubka, J. W.; Bhurke, A. B.; Xie, M.; Drzal, L. T. Surf. Interface Anal. 2000, 29, 829. (6) Matienzo, L. J.; Zimmerman, J. A.; Egitto, F. D. J. Vac. Sci. Technol. A 1994, 12, 2662. (7) Costela, A.; Garcia-Moreno, I.; Florido, F.; Figuera, J. M.; Sastre, R.; Hooker, S. M.; Cashmore, J. S.; Webb, C. E. J. Appl. Phys. 1995, 77, 2343. (8) Heittz, J.; Niino, H.; Yabe, A. Jpn. J. Appl. Phys. 1996, 35, 4110. (9) Heittz, J.; Niino, H.; Yabe, A. Appl. Phys. Lett. 1996, 68, 2648. (10) Zhang Q.; Gomi, M.; Itoh, T.; Abe, M. Jpn. J. Appl. Phys. 1996, 35, 665.
are barely sensitive to the ultraviolet (UV) light used in conventional photochemistries. Because the photon energy of VUV light is high enough to excite and dissociate various chemical bonds, including C-F bonds, free radicals form on VUV-irradiated polymer surfaces. Such radicals successively react with activated oxygen species simultaneously generated through the photoexcitation of atmospheric oxygen molecules.12 Consequently, oxidation and etching of polymer surfaces proceeds efficiently under VUV irradiation,13,14 even on inert fluorocarbon polymers.12,15 Fluorocarbon polymer modification has been conducted until now by employing UV excimer lasers.4,7,16-21 Although laser processing offers great advantages for etching, patterning, and applications to microelectronics,22 it is not convenient for practical surface modification. Because laser beams have relatively small cross-sections, only small (11) Vasilets, V. N.; Hirata, I.; Iwata, H.; Ikada, Y. J. Appl. Polym. Sci. Part A: Polym. Chem. 1998, 36, 2215. (12) Ho¨llander, A.; Klemberg-Sapieha, J. E.; Wertheimer, M. R. J. Polym. Sci. Part A: Polym. Chem. 1995, 33, 2013. (13) Ponomarev, A. N.; Maksimov, A. I.; Vasilets, V. N.; Menagarishvily, V. M. Khim. Vys. Energ. 1989, 23, 286. (14) Vasilets, V. N.; Kovalchuk, A. V.; Ponomarev, A. N. J. Polym. Sci. and Technol. 1994, 7, 165. (15) Skurat, V. E.; Dorofeev, Yu. I. Angew. Makromol. Chem. 1994, 216, 205. (16) Hiraoka H.; Lazare S. Appl. Surf. Sci. 1990, 46, 342. (17) Okoshi, M.; Murahara, M.; Toyoda, K. J. Mater. Res. 1992, 7, 1912. (18) Dickinson, J. T.; Shin, J.-J.; Jiang, W.; Norton, M. G. J. Appl. Phys. 1993, 74, 4729. (19) Niino, H.; Yabe, A. Appl. Phys. Lett. 1993, 63, 3527. (20) Ichinose, N.; Maruo, M.; Kawanishi, S.; Izumi, Y.; Yamamoto, T. Chem. Lett. 1995, 943. (21) Ku¨per, S.; Stuke, M. Appl. Phys. Lett. 1989, 54, 4. (22) Burrell, M. C.; Liu, Y. S.; Cole, H. S. J. Vac. Sci. Technol. A 1986, 4, 2459.
10.1021/la020478b CCC: $22.00 © 2002 American Chemical Society Published on Web 10/17/2002
Surface Modification of Poly(methyl methacrylate)
areas of the surface can be uniformly treated. Moreover, in most cases, thermal damage is caused on the surface because of the highly intense irradiation.4,16,18,23 To overcome these shortcomings, the use of VUV light generated from a plasma discharge6,24-31 or a synchrotron32 has been proposed. These processing techniques, however, require expensive apparatus or special environments such as ultrahigh vacuum. Among the numerous photochemical approaches toward polymer surface processing, the use of an incoherent VUV excimer lamp is one of the most promising, because it can treat a relatively large area on a polymer substrate at a single time with a moderate light intensity. Accordingly, the characteristic penetration depth of VUV light into polymers is only several hundreds of nanometers because of the high absorption coefficients (104∼105/cm).12 Finally, the VUV excimer lamp radiates no infrared rays and, accordingly, does not heat the sample. Thermal damage to polymer surfaces is thus negligibly small, and they can be arbitrarily modified at relatively low temperatures while retaining their bulk properties intact. Although extensive research has been reported on polymer surface modification with VUV light using plasma,6,24-31 or synchrotron radiation,32 as well as on modification using coherent UV4,16-21 or VUV excimer lasers,7 studies on surface processing employing an incoherent VUV excimer lamp have been few.8-10,33-36 In this article, we report on the surface modification of poly(methyl methacrylate) (PMMA) employing a VUV light 172 nm in wavelength radiated from a Xe/2 lamp. To optimize VUV-induced surface modification, it is crucial to elucidate the photochemistries which proceed when the PMMA surface is exposed to the VUV light. As described above, atmospheric oxygen molecules play key roles in the oxidation and etching which proceed after the bond cleavage of the polymer chains. We have, therefore, focused particularly on the effects of atmospheric pressure during VUV irradiation on the surface wettability, chemical structure, and morphology of the PMMA surfaces. Finally, we demonstrate the microfabrication of PMMA substrates using the same light source based on a simple meshmask contact method. Experimental Section Sample substrates 10 × 10 × 1.1 mm3 were cut from a commercial PMMA sheet (Asahi KASEI Co., DelaglasA; number-average and weight-average molecular weights were 10644 and 154177, respectively). Each of the (23) Watanabe, H.; Takata, T.; Tsuga, M. Polym. Int. 1993, 31, 1993. (24) Kudo, K.; Iwabuchi, T.; Mutoh, K.; Miyata, T.; Sano, R.; Tanaka, K. Jpn. J. Appl. Phys. 1990, 29, 2572. (25) Ho¨llander, A.; Klemberg-Sapieha, J. E.; Wertheimer, M. R. Macromolecules 1994, 27, 2893. (26) Ho¨llander, A.; Klemberg-Sapieha, J. E.; Wertheimer, M. R. Surf. Coat. Technol. 1995, 74-75, 55. (27) Ho¨llander, A.; Klemberg-Sapieha, J. E.; Wertheimer, M. R. J. Polym. Sci. Part A: Polym. Chem. 1996, 34, 1511. (28) Fozza, A. C.; Roch, J.; Klemberg-Sapieha, J. E.; Kruse, A.; Ho¨llander, A.; Wertheimer, M. R. Nucl. Instrum. Methods Phys. Res. B 1997, 131, 205. (29) Ho¨llander, A.; Behnisch, J. Surf. Coat. Technol. 1998, 98, 855. (30) Ho¨llander, A.; Behnisch, J. Surf. Coat. Technol. 1998, 78, 855. (31) Ho¨llander, A.; Wilken, R.; Behnisch, J. Surf. Coat. Technol. 1999, 116-119 788. (32) Okudaira, K. K.; Morikawa, E.; Hasegawa, S.; Sprunger, P. T.; Saile, V.; Seki, K.; Harada, Y.; Ueno, N. J. Electron Spectrosc. Relat. Phenom. 1998, 89-91, 913. (33) Esrom, H.; Kogelschatz, U. Thin Solid Films 1992, 218, 231. (34) Bergonzo, P.; Boyd, I. W. J. Appl. Phys. 1994, 76, 4372. (35) Zhang, J.-Y.; Esrom, H.; Kogelschatz, U.; Emig, G. J. Adhes. Sci. Technol. 1994, 8, 1179. (36) Fuchs, C.; Goetzberger, O.; Henck, R.; Fogarassy, E. Appl. Phys. A 1995, 60, 505.
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samples was placed in a vacuum chamber evacuated by a rotary pump. The pressure in the chamber was controlled at 10 or 103 Pa by introducing air through a variable leak valve. When experiments were conducted at atmospheric pressure, the chamber was not evacuated. In this case, the pressure was indicated as 105 Pa. The PMMA substrate was then irradiated for 300∼3000 s with VUV light generated from a Xe/2 excimer lamp (Ushio Electric UER20-172V, λ ) 172 nm, 10 mW/cm2) at a distance of about 20 mm from the lamp window to the sample surface. Light intensities at the surface under the pressures of 10, 103, and 105 Pa were measured using a photometer for 172 nm (Ushio Electric, VUV-S172). At the reduced pressures of 10 and 103 Pa, the light intensities were measured to be about 10 mW/cm2 for both. These values were almost equal to that just outside the lamp window. These results indicate that under the reduced pressure conditions the VUV light had penetrated the atmosphere without attenuation. On the other hand, in the case of 105 Pa, the light intensity weakened considerably, because 172 nm VUV light is greatly absorbed by atmospheric oxygen molecules. In this experiment, the transparency of the 20 mm air layer at 105 Pa was measured to be about 8% (the light intensity at the sample surface was about 0.8 mW/cm2). Substrate temperature was measured by a thermocouple and remained at less than 40 °C, even when irradiation was prolonged up to 3000 s. After VUV irradiation for the appropriate period, static water-contact angles of the sample surfaces were measured at 25 °C in air using a contact angle goniometer (CA-X, Kyowa Interface Science) based on the sessile drop method. All of the contact angles were determined by averaging values measured at five different points on each sample surface. The water-contact angle error was about (2°. The VUV-modified PMMA surfaces were characterized by X-ray photoelectron spectroscopy (XPS; Shimadzu, ESCA3400) using Mg KR (E ) 1253.6 eV) radiation. The binding energy (BE) scale was calibrated to provide Au4f7/2 ) 83.9 eV and Cu2p3/2 ) 932.8 eV. The X-ray source was operated at 10 mA and 12 kV. Samples were introduced into the XPS system immediately after VUV irradiation. The pressure in the analysis chamber was about 4∼6 × 10-6 Pa during measurements. The core-level signals were obtained at a photoelectron takeoff angle of 90° (with respect to the sample surface). We confirmed that there was no X-ray damage to the samples by repetitive measurements. Data acquisition and processing were performed by a SUN Microsystems ULTRA 5 computer, using the VISION 2.0 processing package. To compensate for surface charging effects, all of the BEs were referenced to the maximum peaks on the C1s spectra of hydrocarbon moieties in the PMMA backbone at 285.0 eV. The accuracy of the BE determined with respect to this standard value was within (0.3 eV. The overlapping peaks were resolved by the peak synthesis method, employing linear-type background subtraction and peak profiles mixed with 70% of Gaussian and 30% of Lorentzian contributions. Surface compositions were determined by the corresponding C1s and O1s core-level spectral area ratios calculated using the relative sensitivity factor method. The VUV-modified surfaces were observed by an atomic force microscope (AFM; Seiko Instruments, SPA300HV + SPI3800N) using a Si probe (Park Scientific Instruments, Ultralever, force constant ) 2.8 N/m). To fabricate microstructures on the PMMA substrates, some of the samples were irradiated with VUV light through a mesh-mask contacting the substrate surface. This VUV irradiation was conducted for 1800 s at 10, 103, or 105 Pa. A 10 mm thick quartz glass plate (ASAHI
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Figure 1. Change in water-contact angle of the PMMA substrates after VUV irradiation; O, 10 Pa; 4, 103 Pa; and b, 105 Pa.
GLASS, Synthetic silica glass AQX for Xe2 172 nm excimer lamp) served as a weight on top of the mesh-mask in order to attain satisfactory contact between the mask and the sample surface. The transparency of the quartz plate at 172 nm was about 90%. At 105 Pa, the transparency of the 10 mm air layer was measured to be about 18%. Thus, the total light intensity at the PMMA substrate surface at 10, 103, or 105 Pa was estimated to be about 9, 9, or 1.6 mW/ cm2, respectively. The microstructures fabricated on the PMMA substrates were similarly observed using the AFM system. Results and Discussion Surface Wettability and Photooxidation Rate. VUV irradiation of the PMMA substrates resulted in the sample surfaces becoming hydrophilic. Changes in the water-contact angles of the PMMA substrates VUVirradiated under various atmospheric pressures are plotted in Figure 1. The water-contact angles of the samples treated at 10 and 103 Pa rapidly decreased during the first 900 s and stopped decreasing at around 1200 s. The contact angles remained unchanged at about 40° and 25°, respectively, even when irradiation extended up to 3000 s. On the other hand, at the pressure of 105 Pa, the water-contact angle of the PMMA substrate reached its minimum of about 24° at the irradiation time of 1800 s and increased slightly at 2400 and 3000 s. This gradual increase in water-contact angle at irradiation times longer than 1800 s indicates that the surface had been overtreated.37 This increase may be attributed to partial removal of the oxidized region from the PMMA substrate. Highly hydrophilic surfaces were successfully prepared with VUV irradiation conducted at 103 and 105 Pa. In contrast, the VUV-irradiated surface obtained in the 10 Pa atmosphere containing only a small amount of oxygen molecules was less hydrophilic. The photooxidation rate markedly accelerated when pressure was increased from 10 to 103 Pa. However, when the pressure was further increased to 105 Pa, the rate decreased slightly. As described in the Experimental Section, at this pressure, the intensity of the VUV light reaching the sample surface was considerably weaker than that at the reduced pressures of 10 and 103 Pa, because the 172 nm VUV light was greatly absorbed by atmo(37) Liston, E. M. J. Adhesion 1989, 30, 199.
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Figure 2. C1s XPS spectra of VUV-irradiated PMMA substrates; (a) initial, (b) 10 Pa, (c) 103 Pa, and (d) 105 Pa.
spheric oxygen molecules. These results are evidence that oxygen molecules play an important role in VUV-induced surface modification. An irradiation time of 1800 s was hereafter chosen, because at 1800 s the water-contact angle of each sample was at its minimum, as shown in Figure 1. Surface Analyses by XPS. We investigated the relation between the chemical structure of the VUVmodified PMMA substrates and their surface wettability. C1s and O1s XPS spectra were acquired for all of the samples. C1s spectra of the PMMA substrates before and after VUV irradiation are shown in Figure 2. Deconvolution divided the C1s spectra into 3 or 4 features which were identified according to the reported chemical shifts.38 Spectrum a shows a typical C1s spectrum of the initial PMMA substrate, consisting of three components centered at BEs of 285.0, 286.7, and 289.0 eV, corresponding to C-C, C-O, and OdC-O groups, respectively. The intensity ratios of these components are in good agreement with the expected values.38 C1s spectra of the VUVirradiated PMMA substrates are shown in Figure 2 parts b-d. These samples were treated for 1800 s at 10, 103, and 105 Pa, respectively. Markedly different from spectrum a, their spectrum profiles consist of four components centered at BEs of 285.0, 286.5-286.8, 287.7-288.0, and 289.0-289.3 eV. These correspond to C-C, C-O, CdO, and OdC-O groups, respectively. It is noteworthy that oxygen-containing functional groups, that is, CdO groups, were newly formed for all of the modified samples. In addition, the relative intensities of the C-O and OdCsO groups in spectra b and c have decreased markedly. This indicates that the photodecomposition of ester side groups in the PMMA unit was enhanced39 at the reduced pressures of 10 and 103 Pa. In contrast, the intensities of the C-O and OdCsO groups changed little at 105 Pa. O1s spectra were similarly altered by VUV-irradiation. Figure 3 parts a∼d shows O1s spectra of the same samples as shown in Figure2 parts a∼d. The spectra consist of at least two components with BEs at 532.2-532.5 and 533.7534.0 eV, which are assigned to CdO and C-O groups, respectively. The locations of these two components in (38) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers; Wiley: New York, 1992. (39) France, R. M.; Short, R. D. Langmuir 1998, 14, 4827.
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Table 1. Chemical Compositions of the PMMA Substrates before and after VUV Irradiation
water-contact angle (degree) C (a/o) composition O (a/o) O/C
initial
10 Pa × 1800 s
103 Pa × 1800 s
105 Pa × 1800 s
80 75 25 0.33
40 79 21 0.27
25 68 32 0.47
24 64 36 0.56
10 Pa × 1800 s
initial
C1s components C-C C-O CdO CdO-C O1s components CdO C-O
103 Pa × 1800 s
103 Pa × 1800 s
position (eV)
fwhm (eV)
area ratio
position (eV)
fwhm (eV)
area ratio
position (eV)
fwhm (eV)
area ratio
position (eV)
fwhm (eV)
area ratio
285.0 286.7
1.74 1.68
0.62 0.22
289.0
1.61
0.16
285.0 286.6 287.9 289.3
1.77 1.64 1.62 1.65
0.72 0.12 0.10 0.06
285.0 286.8 288.0 289.3
1.74 1.62 1.69 1.67
0.63 0.14 0.12 0.11
285.0 286.5 287.7 289.0
1.76 1.63 1.65 1.61
0.57 0.17 0.09 0.17
532.2 533.7
1.93 1.96
0.51 0.49
532.5 533.9
1.96 1.94
0.68 0.32
532.5 534.0
1.96 1.97
0.66 0.34
532.4 533.8
1.94 1.94
0.56 0.44
spectrum a are almost equal to each other, corresponding to the CdO-C groups in the PMMA unit. However, in spectra b and c the intensities of the C-O groups have decreased markedly, whereas in spectrum d, there is little change. The chemical compositions of the VUV-irradiated PMMA substrates shown in Figure 2 are summarized in Table 1, as well as the full width at half-maximum (fwhm), peak position and area ratio of each of the individual components of the core-level spectra. The C concentrations of the samples treated at 103 and 105 Pa decreased from 75 a/o to 68 and 64 a/o, respectively. This demonstrates that the PMMA surfaces were oxidized by the VUV irradiation. On the contrary, the sample treated at 10 Pa has a slightly larger C concentration of 79 a/o. The O/C ratio of the initial PMMA substrate (O/C ) 0.33) was smaller than that of the PMMA unit (O/C ) 0.40). This is most likely due to an increase in surface carbon concentration caused by adventitious carbon contaminants.40 It is noteworthy that the O/C ratios of the samples treated at 103 and 105 Pa increased to 0.47 and 0.56, respectively, whereas that of the VUV-treated sample at 10 Pa decreased to 0.27. Although the O/C ratio of 10 Pa decreased markedly, the sample surface became relatively hydrophilic with a water-contact angle of about 40°. The photochemical reactions which proceeded when atmospheric oxygen molecules were insufficient differed form
Figure 3. O1s XPS spectra of VUV-irradiated PMMA substrates; (a) initial, (b) 10 Pa, (c) 103 Pa, and (d) 105 Pa.
those which occurred in the presence of enough oxygen molecules. Indeed, at 10 Pa, the C concentration and the intensity of the C-C groups in the C1s spectra increased. This was probably due to cross-linking reactions, including the formation of new carbon-carbon bonds.1,11 However, we cannot identify such carbon bonds in the C1s spectra shown in Figure 2 because XPS measurements cannot resolve C-H, C-C or CdC bonds.41 Because, in general, a carbon-rich surface is relatively hydrophobic,1 the surface treated at 10 Pa would be less hydrophilic than the surfaces modified at 103 and 105 Pa. Considering the water-contact angle results shown in Figure 1, it is almost certain that the formation of CdO groups40 and the decrease in the intensity of C-C groups increased surface wettability. Here, we discuss the mechanism of the photooxidation which took place during the VUV irradiation of the PMMA substrates. As mentioned above, VUV light of less than 175 nm wavelength dissociatively excites chemical bonds (e.g., C-C, C-H, and C-O) and decomposes polymer chains.12,15 Moreover, VUV light of 133∼200 nm dissociates O2 and generates oxygen atoms in several states of excitation [i.e., O(1D) or O(1S)], as well as ground-stated atoms [O(3P)].42,43 When PMMA substrates are irradiated with VUV light in the presence of atmospheric oxygen, the two distinct photochemical reactions described above proceed simultaneously. Because these activated oxygen species have strong oxidative reactivities to organic molecules, free radicals which have formed due to the direct VUV excitation of the polymer chains further react with these species. Consequently, some of the radicals are incorporated together with the activated oxygen species resulting in the formation of oxygen-containing functional groups, as shown in Figure 2. Parts of the polymer are eliminated as volatile products such as CO, CO2, and H2O.44 In the case of 105 Pa, the photoinduced cleavage of the polymer chains by direct photoexcitation did not proceed efficiently, because at this pressure the intensity of the VUV light at the polymer surface was only 8%. Thus, the photochemical reactions which occurred at 105 Pa are considered to have been governed primarily by the oxidation reaction with photoexcited oxygen species. This is supported by our XPS results shown in Table 1. (40) Lim, H.; Lee, Y.; Han, S.; Cho, J.; Kim, K.-J. J. Vac. Sci. Technol. A 2001, 19, 1490. (41) Burrell, M. C.; Liu, Y. S.; Cole, H. S. J. Vac. Sci. Technol. A 1986, 4, 2459. (42) Amimoto, S. T.; Force, A. P.; Gulotty, R. G., Jr.; Wiesenfeld, J. R. J. Chem. Phys. 1979, 71, 3640. (43) Inoue, K.; Michimori, M.; Okuyama, M.; Hamakawa, Y. Jpn. J. Appl. Phys. 1987, 26, 805. (44) Ishikawa, Y.; Yoshima, H.; Hirose, Y. J. Surf. Finish Soc. Jpn. 1996, 47, 74.
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Because the relative intensities of the C-O and CdOsC groups in the C1s spectra, and the C-O and CdO groups in the O1s spectra changed little, photocleavage of the ester side groups in the PMMA unit is likely to have proceeded to a lesser extent. In contrast, the photooxidation reaction is thought to have preferentially occurred, probably in the main chain of the PMMA unit, because the intensity of the C-C groups decreased markedly and a new component of CdO groups formed. On the other hand, when the oxygen concentration was extremely low at 10 Pa, photocleavage of the ester side groups was probably enhanced39 as indicated by the marked decrease of the relative intensities of the C-O and CdO-C groups in the C1s spectra. However, the oxidation reactions at this pressure did not proceed efficiently. Therefore, the free radicals may have recombined and cross-linked with each other, resulting in the increase of C-C groups observed at the polymer surface. Our XPS results are clear evidence that the degree of hydrophilicity of the modified polymer surface is primarily determined by the amount of oxygen molecules in the atmosphere. The moderate oxygen concentration in the atmosphere at 103 Pa both accelerated the photooxidation rate and increased hydrophilicity. Surface Morphology. The surface morphology of samples identical to those in Figure 2 a-d was observed by AFM (images are not shown). All of the VUV-irradiated sample surfaces appeared to be very smooth over their entire area of 50 × 50 µm2. The average root-mean-square roughness (Rrms) of each sample was estimated from the AFM image to be only 2 nm or less. The VUV-irradiated surfaces became much smoother than the initial PMMA substrate (Rrms ) 11.2 nm) because small impurities existing on the initial surface were decomposed and eliminated completely by the VUV irradiation. Furthermore, even when irradiation time was prolonged to 3000 s, the Rrms value of each VUV-irradiated sample remained unchanged. No thermal damage, e.g., cracking, melting, or bubble formation, was observed on the modified surfaces. This is a clear advantage of our process being conducted at low temperatures, i.e., less than 40 °C. The PMMA substrates were modified uniformly without any marked change in morphology. However, our results presented here are quite different from those of several recent reports describing morphology changes in VUVirradiated polymer surfaces. Zhang et al.35,45 have demonstrated the surface modification of PMMA substrates employing VUV light of 222 nm in wavelength generated from a KrCl* lamp (light intensity of 30 mW/cm2). They observed morphological changes on the PMMA surfaces during VUV irradiation at 102 Pa and confirmed that the roughness increased with irradiation time. Such differences in obtained surface roughness must be attributed to the materials employed and to experimental conditions, such as wavelength, light intensity, molecular weight, and sample thickness. Microfabrication of PMMA Substrates. We fabricated microstructures on a PMMA substrate using the same excimer lamp. Figure 4 a-c shows AFM images of the microfabricated PMMA substrates under the pressures of 10, 103, and 105 Pa, respectively. As can be seen in Figure 4 parts a and b, grooved microstructures composed of 25 × 25 µm2 features were clearly imaged. These AFM images demonstrate that the VUV-irradiated regions were photoetched, whereas the masked regions remained unetched. Although the surface morphologies of the VUV-
irradiated surfaces remained unchanged as demonstrated in the previous section, the PMMA substrates were undoubtedly photoablated because of the VUV irradiation. On the other hand, when the sample was treated at 105 Pa, spatial resolution of the AFM image became significantly worse, as shown in Figure 4c. In this case, the contact between the mask and the sample surface may not have conformed completely. Therefore, some of the excited oxygen species could have diffused into gaps between the surfaces resulting in damage to the PMMA surface even in the masked region.46 As with pattern resolution, the photoetching rate also depended on the atmospheric pressure. The depths of the etched regions as estimated from the AFM images shown in Figure 4 parts a, b, and c were about 390, 397, and 207 nm, respectively. Accordingly, the photoetching rate of each was calculated to be about 13, 13.2, and 6.9 nm/min, respectively. Photoetching of the PMMA substrate proceeded more efficiently under the reduced pressures of 10 and 103 Pa. In contrast, at 105 Pa, the photoetching rate of the PMMA substrate was extremely low. However, although the exposure dose of VUV light for the PMMA surfaces at the reduced pressures (16.2 J/cm2) was about 5.6 times larger than that at 105 Pa (2.88 J/cm2), the actual etching rates at 10 and 103 Pa were only about twice as fast as that at 105 Pa. Furthermore, the etching rate obtained at 103 Pa was almost equivalent to that at 10 Pa. This can possibly be explained as follows. During VUV irradiation conducted at the pressures of 10 and 103 Pa, where the ratios of the amount of oxygen molecules were 10-4 and 10-2, respectively, to that at 105 Pa, the amount of activated oxygen species supplied must have been lower than that when conducted at 105 Pa. These active species might not have diffused deeply into gaps, because they are readily deactivated when they collide with a solid surface. Thus, the diffusion of the activated oxygen species into gaps can be considered to have been too slow to be involved in our photoetching process. This is most certainly the reason the photoetching rates at 10 and 103 Pa were relatively slow and almost identical. On the contrary, at the pressure of 105 Pa, the photocleavage reaction of the polymer chains did not proceed efficiently because the light intensity weakened before it reached the PMMA surface, as described above. Furthermore, because photodecomposed products, such as CO, CO2, and H2O, could not diffuse out efficiently and evacuate, such products might have remained in the system.35 Thus, the photoetching rate obtained at 105 Pa was observed to be extremely low. In contrast with our present results, in experiments performed by Zhang et al.,35 PMMA substrates were photoetched at a relatively higher etching speed of about 40 nm/min at the pressure of 102 Pa using a similar excimer lamp of λ ) 172 nm (light intensity of 10 mW/cm2). Although we cannot compare these results directly, their reported rate was about 3 times faster than our best. This marked difference in photoetching rate is probably due to the photoetching method. As described above, we placed a quartz glass plate on the mesh-mask in order to attain satisfactory contact between the mask and the sample surface. Because of the presence of this glass plate, diffusion of activated oxygen species and decomposed products was very limited compared to irradiation conducted without such a glass plate. Indeed, one of the authors has recently reported that the photodecomposition rate of organic monolayers increased markedly with an increase in the gap distance between
(45) Zhang, J.-Y.; Boyd, I.; Esrom, H. Surf. Interface Anal. 1996, 24, 718.
(46) Hozumi, A.; Yokogawa, Y.; Kameyama, T.; Sugimura, H.; Hayashi, K.; Shirayama, H.; Takai, O. J. Vac. Sci. Technol. A 2001, 19, 1812.
Surface Modification of Poly(methyl methacrylate)
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Figure 4. AFM images of microfabricated PMMA substrates; (a) 10 Pa, (b) 103 Pa, and (c) 105 Pa.
the sample surface and a glass plate.47 We conclude that the atmospheric pressure during VUV irradiation is of primary importance in our microfabrication mechanism. Conclusion The hydrophilization of PMMA substrates was successfully demonstrated using a Xe/2 excimer lamp radiating 172 nm VUV light under various atmospheric pressures. Our experimental results presented here offer clear evidence that atmospheric pressure plays a key role in VUV-induced surface modification. Hydrophobic PMMA surfaces became hydrophilic during VUV irradiation conducted over the pressure range of 10∼105 Pa. However, the final water-contact angle and the photooxidation rate depended significantly on the atmospheric pressure. The hydrophilicity of surfaces VUV-irradiated at 103 and 105 Pa was superior to that of samples treated at 10 Pa. In addition, the photooxidation rate increased with an increase in pressure from 10 to 103 Pa, whereas the rate decreased at the pressure of 105 Pa because the light intensity at the sample surface decreased markedly because of the increase in light absorption by atmospheric oxygen molecules. Furthermore, the microfabrication of PMMA was successfully demonstrated using the same light source through a simple mask-contacting method. Again, the spatial resolution and etching rate depended on the (47) Sugimura, H.; Shimizu, T.; Takai, O. J. Photopolym. Sci. Technol. 2000, 13, 69.
atmospheric pressure. Resolution became significantly worse at 105 Pa, probably because of diffusion of the activated oxygen species through gaps between the mask and the sample surface. The etching rate at 105 Pa was approximately one-half as fast as those attained at 10 and 103 Pa. Considering these results, the reduced pressures of 10 and 103 Pa are favorable for the microfabrication of PMMA substrates. However, the surface wettability of the sample treated at 10 Pa was insufficient, as demonstrated in Figure 1. We therefore conclude 103 Pa to be the optimum atmospheric pressure for both the hydrophilization and microfabrication of PMMA substrates. Because the surface wettability of the VUVmodified region is completely different from that of the unirradiated region, microstructured PMMA substrates could be fabricated so as to possess chemically reactive and inert regions in the micrometer scale. Such chemically and geometrically defined polymer substrates could be applied to the spatial arrangement of a variety of functional materials on polymer surfaces, including proteins, fluorescent molecules, metals and biological cells, and to a wide variety of advanced polymer devices. Acknowledgment. This study was partially supported by the “Biomimetic Materials Processing” Research Project (JSPS-RFTF No.99R13101) of the Research for the Future (RFTF) Program of the Japan Society for the Promotion of Science. LA020478B
