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Photoreactivity of Alkylsilane Self-Assembled Monolayers on Silicon Surfaces and Its Application to Preparing Micropatterned Ternary Monolayers Lan Hong, Hiroyuki Sugimura,* Takeshi Furukawa, and Osamu Takai Department of Materials Processing Engineering, Nagoya University, Nagoya, Japan Received June 4, 2002. In Final Form: December 29, 2002 The photoreactivity of an n-octadecyltrimethoxysilane self-assembled monolayer (ODS-SAM) on a Si substrate was investigated under the irradiation of vacuum ultraviolet (VUV) light of 172 nm in wavelength. The results of water contact angle, film thickness, and X-ray photoelectron spectroscopy analysis showed that the ODS-SAM decomposed gradually due to the VUV light exposure. Oxidized products, that is, -COOH groups, had formed before the ODS-SAM was completely decomposed and removed from the substrate. Coplanar ternary organosilane SAM microstructures have been successfully fabricated by employing the reaction between -OCH3 functional groups with -COOH groups on the VUV-modified ODS-SAM.
Introduction Organosilane self-assembled monolayers (SAMs) show great potential in microfabrication.1 Conventional lithographic methods have been frequently used to micropattern homogeneously prepared organosilane SAMs. In particular, much attention has been paid to photopatterning of alkylsilane SAMs,2-5 since this technique can transfer an entire pattern on a photomask to a SAMcovered substrate at a single time and is practical. However, few studies have been carried out to clarify the photodegradation mechanism although the knowledge of the mechanism may lead to optimization of micropatterning processes of SAMs. Almost all of these studies have attributed the primary photodegradation pathway of alkylsilane or alkanethiol SAMs to ozonolysis.6-10 Furthermore, some researchers concluded that a combination of ultraviolet (UV) irradiation and oxygen is crucial for degradation of alkylsiloxane SAMs to proceed.11-13 However, the state of the irradiated SAMs remaining on the substrate during the photodegradation process was not mentioned in detail. In this paper, we report photoreactivity of an alkylsilane SAM, that is, octadecyltrimethoxysilane, to convert the SAM to one terminated with carboxyl groups, since carboxyl-terminated surfaces are of special interest in micro/nanofabrication processes.14-16 We applied the resulting SAM to the preparation of ternary SAMs. (1) Dulcey, C. S.; Georger, J. H., Jr.; Krauthamer, V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551. (2) Sugimura, H.; Nakagiri, N. Appl. Phys. A 1998, 66, S427. (3) Lee, B.; Clark, N. A. Langmuir 1998, 14, 5495. (4) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Langmuir 2000, 16, 885. (5) Moser, A. E.; Eckhardt, C. J. Thin Solid Films 2001, 382, 202. (6) Ferris, M. M.; Rowlen, K. L. Appl. Spectrosc. 2000, 54, 664. (7) Lewis, M.; Tarlov, M.; Carron, K. J. Am. Chem. Soc. 1995, 117, 9574. (8) Zhang, Y. M.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654. (9) Norrod, K. L.; Rowlen, K. L. J. Am. Chem. Soc. 1998, 120, 2656. (10) Poirier, G. E.; Herne, T. M.; Miller, C. C.; Tarlov, M. J. J. Am. Chem. Soc. 1999, 121, 9703. (11) Sugimura, H.; Shimizu, T.; Takai, O. J. Photopolym. Sci. Technol. 2000, 13, 69. (12) Moon, D. W.; Kurokawa, A.; Ichimura, S.; Lee, H. W.; Jeon, I. C. J. Vac. Sci. Technol., A 1999, 17, 150. (13) Ye, T.; Wynn, D.; Dudek, R.; Borguet, E. Langmuir 2001, 17, 4497. (14) Brandow, S. L.; Chen, M.-S.; Aggarwal, R.; Dulcey, C. S.; Calvert, J. M.; Dressick, W. J. Langmuir 1999, 15, 5429.
Experimental Section Three types of organosilane precursors, that is, n-octadecyltrimethoxysilane [ODS, CH3(CH2)17Si(OCH3)3,], heptadecafluoro1,1,2,2-tetrahydro-decyl-1-trimethoxysilane [a kind of fluoroalkylsilane (FAS), CF3(CF2)7(CH2)2Si(OCH3)3], and (p-chloromethyl)phenyltrimethoxysilane [CMPhS, ClCH2C6H4Si(OCH3)3], were purchased from Gelest Inc. and used as received for SAMs. A substrate of an n-type silicon wafer (100) was photochemically cleaned before forming an alkylsilane SAM onto its surface so that it was terminated thoroughly with OH groups. First, an ODS-SAM was formed onto the cleaned Si substrate through a vapor phase method as described elsewhere.17 The film thickness of the ODS-SAM was estimated as 1.7 nm using ellipsometry. The water contact angle was measured to be 108°. The ODSSAM-covered Si substrate was irradiated by vacuum ultraviolet (VUV) light of 172 nm in wavelength through a photomask using the apparatus for which details have been described elsewhere.18,19 An excimer lamp (Ushio Electric, UER20-172V, 10 mW/cm2) was used as a light source. In our VUV exposure system, a sample was placed in air, while the space between the photomask and the lamp window was purged with nitrogen to avoid the absorption of VUV light with oxygen molecules. Namely, the photomask works as a separation wall. Furthermore, a proximity gap between the photomask and the sample is controlled precisely using a mechanical stage. The light intensity at the sample surface was estimated to be 9.2 mW/cm2 from the absorbance of a 2.4 mm thick quartz plate consisting of the photomask, that is, ca. 8%. The proximity gap between the photomask and the substrate was set to be about 0.1 µm throughout this study. In some experiments, a quartz glass plate was used instead of the photomask in order to irradiate the ODS-SAM entirely and to investigate surface states of the irradiated ODS-SAM based on contact angle measurements, ellipsometry, and X-ray photoelectron spectroscopy (XPS). The micropatterned ODS-SAM/Si substrate was further modified with a second SAM precursor, FAS, through the vapor phase method at a substrate temperature of 423 K to form a binary SAM structure. This binary SAM sample was aligned to a desired position under the photomask and irradiated with VUV light again. Finally, a third SAM precursor, CMPhS, was (15) Saito, N.; Hayashi, K.; Sugimura, H.; Takai, O. J. Mater. Chem. 2002, 12, 2684. (16) Hoeppener, S.; Maoz, R.; Cohen, S. R.; Chi, L.; Fuchs, H.; Sagiv, J. Adv. Mater. 2002, 14, 1036. (17) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. Surf. Interface Anal. 2002, 34, 550. (18) Hong, L.; Hayashi, K.; Sugimura, H.; Takai, O.; Nakagiri, N.; Okada, M. Surf. Coat. Technol., in press. (19) Sugimura, H.; Hayashi, K.; Saito, N.; Hong, L.; Takai, O.; Hozumi, A.; Nakagiri, N.; Okada, M. Trans. Mater. Res. Soc. Jpn. 2002, 27, 545.
10.1021/la0205194 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/11/2003
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Figure 1. Schematic illustration for the preparation process of ternary ODS-FAS-CMPhS SAMs.
Figure 2. Changes in contact angle and thickness of ODSSAMs with irradiation time. deposited on the substrate at 373 K. These procedures are schematically illustrated in Figure 1. The ODS-FAS-CMPhS coplanar samples were observed using a field emission scanning electron microscope (FE-SEM; JEOL, JSM-6300F) at an acceleration voltage of 1 kV.
Results and Discussion Figure 2 shows that both water contact angle and thickness of the ODS-SAM decrease monotonically with increasing irradiation time. These results imply that the ODS-SAM is gradually removed due to the VUV irradiation in the presence of oxygen. Changes in the binding
Figure 3. XPS spectra of C 1s for (a) an undegraded ODSSAM and (b) an ODS-SAM irradiated for 100 s.
states of the ODS-SAM initiated by the VUV irradiation were investigated using XPS. Figure 3 shows C1s XPS spectra of ODS-SAMs undegraded and irradiated for 100 s. The C1s peak of the undegraded ODS-SAM is composed of mainly -CH2- bonding. After VUV irradiation for 100 s, as presumed, the intensity of -CH2- has decreased since ODS molecules were removed gradually. Moreover,
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Figure 4. SEM images of the ODS-FAS coplanar microstructure.
a signal from -COOH groups appeared. This is due to the oxidation of ODS molecules with activated oxygen species generated upon VUV irradiation. This result is in contrast to our previous results, which were obtained through VUV irradiation of an ODS-SAM in a vacuum.20 There were no apparent -COOH peaks in the C 1s XPS spectra of the irradiated SAMs. Here we discuss the mechanism of VUV photodegradation of ODS-SAMs. There have been a few reports on photochemical reactions from chloromethylphenylsilane SAMs to COOH-terminated SAMs.14,15 In these cases, -CH2Cl groups absorbed UV light and subsequently oxidized with oxygen. On the other hand, in the case of VUV degradation of ODS-SAMs, VUV light dissociatively excites chemical bonds, for example, C-C, C-H, and C-Si, and forms radicals,21 as in the case of soft X-ray irradiation.22 These radicals may further react with oxygen and water molecules in the atmosphere. Such reactions proceed more efficiently in our present experiments, since the VUV irradiation to ODS-SAMs was conducted in air and, thus, there were more than 1000 times as many oxygen molecules on the sample surface than in our previous study conducted in a vacuum.20 Furthermore, the VUV light was simultaneously absorbed with the oxygen molecules and generated atomic oxygen species.23 Since these activated oxygen atoms have strong oxidative reactivity, the organic radicals formed due to the direct VUV excitation of ODS-SAMs further react with the activated oxygen atoms, resulting in the formation of -COOH groups. When the VUV irradiation was further prolonged, the ODS-SAM was finally converted to volatile species such as H2O, CO, and CO2 and accordingly removed from the substrate. Next, the second SAM, that is, the FAS-SAM, was deposited onto the ODS-SAM/Si substrate micropatterned with VUV irradiation through a photomask for 100 s. At this irradiation time, the VUV-irradiated region of the ODS-SAM is assumed to be covered with -COOH groups as expected from Figures 2 and 3. The result is shown in Figure 4. The dark and bright regions in these FE-SEM images correspond to the regions deposited with the FASSAM and covered with the ODS-SAM, respectively. Due to the large electron negativity of F atoms in the FASSAM, the FAS-SAM is observed as dark in the FE-SEM (20) Sugimura, H.; Hayashi, K.; Amano, Y.; Takai, O.; Hozumi, A. J. Vac. Sci. Technol., A 2001, 19, 1281. (21) Holla¨nder, A.; Klemberg-Sapieha, J. E.; Wertheimer, M. R. Macromolecules 1994, 27, 2893. (22) Kim, T. K.; Yang, X. M.; Peters, R. D.; Sohn, B. H.; Nealey, P. F. J. Phys. Chem. B 2000, 104, 7403. (23) Inoue, K.; Michimori, M.; Okuyama, M.; Hamakawa, Y. Jpn. J. Appl. Phys. 1987, 26, 805.
Figure 5. C1s XPS spectra of the FAS-SAM formed on a Si substrate and the VUV-modified ODS-SAM.
compared with the ODS-SAM.24 This proves that the FAS-SAM was formed area-selectively onto the VUVirradiated and, therefore, -COOH-covered region through the chemical reaction between -COOH groups and -OCH3 groups of FAS molecules or their hydrolysis form of -OH. To confirm the coverage of the FAS-SAM on the VUVmodified ODS-SAM, we measured a C1s XPS spectrum of an FAS-SAM formed on the ODS-SAM sample uniformly modified with VUV irradiation, as well as that of a control sample in which the FAS-SAM was directly formed on a Si substrate without inserting the VUV-modified ODSSAM. As shown in Figure 5, spectrum B was obtained from FAS-SAM/Si while spectrum A was from FAS-SAM/ VUV-modified ODS-SAM/Si. In these spectra, there are two peaks corresponding to CFx and CHx peaks. The intensity of CHx certainly increases from spectrum B to spectrum A, due to the underlying monolayer in the FASSAM/VUV-modified ODS-SAM/Si sample. However, both CFx peaks show similar shapes and intensities. Thus we assumed that a FAS-SAM nearly comparable to that on the Si substrate was formed on the VUV-modified ODSSAM. Almost all of the coplanar microstructures of alkylsilane SAMs of more than two phases have been prepared by removing the first SAM thoroughly until -OH groups appeared and, subsequently, self-assembling a second (24) Wu, Y.; Hayashi, K.; Saito, N.; Sugimura, H.; Takai, O. Surf. Interface Anal. 2003, 35, 94.
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Figure 6. SEM images of the ODS-FAS-CMPhS coplanar microstructure.
SAM area-selectively on the OH-terminated surface, so far. An inevitable shortcoming of photolithography is that its patterning resolution is degraded by penumbral blurring effects together with diffraction and reflection of light. When the ODS-SAM is completely degraded in VUVirradiated regions, the ODS-SAM in the masked regions adjacent to the irradiated regions is degraded partly as well, since activated oxygen species diffuse into the regions to some extent. Accordingly, -COOH groups appear on the regions. This leads to the resolution degradation because the second precursors are able to react with both -OH and -COOH groups. Hence, we propose a new process for preparation of coplanar ternary SAM microstructures as illustrated in Figure 1. The ODS-SAM/Si substrate was exposed to VUV light for 100 s at a proximity gap of 0.1 µm so that the exposed surface of the ODS-SAM was terminated by -COOH groups. Then, FAS precursors were area-selectively self-assembled onto the exposed area of the ODS-SAM through the reaction between -COOH and -OCH3 groups. Next, the coplanar ODS-FAS binary SAM was VUV-irradiated again at the predetermined position by aligning the photomask precisely above the substrate under the same irradiation conditions, followed by the deposition of the third SAM precursor, CMPhS, onto the substrate. Figure 6 shows FE-SEM images of the coplanar ternary ODS-FAS-CMPhS SAM microstructures. The contrast in brightness is in the order of FASSAM, CMPhS-SAM, and ODS-SAM. This is consistent with the binding energy of fluorine, chlorine, and carbon with electrons. Less effective secondary electron emission leads to darker imaging of the FAS-SAM surface, and
vice versa. This molecular-dependent FE-SEM contrast has been discussed in more detail elesewhere.25 It is obvious that the present process can successfully arrange coplanar ternary SAMs. Furthermore, by repeating this process, we can fabricate more complicated SAM structures. Conclusions The photoreactivity of ODS-SAMs under VUV light exposure was investigated based on water contact angle, film thickness, and XPS. Oxidized products, -COOH groups, appeared before the ODS-SAM was completely removed from the Si substrate due to photoreaction between ODS molecules and oxygen in the presence of VUV light exposure. We have prepared coplanar ODSFAS-CMPhS microstructures by means of VUV micropatterning of ODS-SAM/Si with a 0.1 µm proximity gap between photomask and Si substrate for 100 s when enough -COOH groups appeared, followed by areaselective self-assembly of FAS and CMPhS precursors in sequence. Acknowledgment. This research has been supported by the Japan Society for the Promotion of Science (Research Project “Biomimetic Materials Processing” (No. JSPS-RFTF 99R13101), Research for the Future (RFTF) Program) and the Tatematsu Foundation, Japan. LA0205194 (25) Saito, N.; Wu, Y.; Hayashi, K.; Sugimura, H.; Takai, O. J. Phys. Chem. B 2003, 107, 664.
