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Notes Photoinduced Cleavage of Alkyl Monolayers on Si Naoto Shirahata,*,† Tetsu Yonezawa,‡,§ Won-Seon Seo,| and Kunihito Koumoto† Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Nagoya 464-8603, Japan, Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, PRESTO “Structure Ordering and Physical Properties”, Japan Science and Technology Corporation, Bunkyo-ku, Tokyo 113-0033, Japan, and Next Generation Enterprise Group, Advanced Materials Analysis & Evaluation Center, 233-5, Gasan-Dong, Guemcheon-Gu, Seoul, 153-801, Korea Received July 2, 2003. In Final Form: November 26, 2003
1. Introduction Well-controlled modification of surface properties on Si substrate is the starting stage for the development of chemically based molecular and semiconductor devices.1-4 One particularly notable approach is surface modification with highly functional monolayers. Increasing attention has focused on alkyl monolayers (or Si-C linked monolayers) directly bonded to Si surfaces5 since they offer several striking features: (1) the electron-transfer rate through the alkyl monolayer can be controlled by its thickness;3 (2) the monolayer can be flat at the angstrom level;5 (3) the Si-C bonds formed at the interface between the Si surface and the alkyl monolayer show high stability against chemicals, including HF;6,7 (4) the terminated groups of the monolayer can be easily modified into highly functional groups.8,9 These features suggest that such Si-C linked monolayers could be applied to fabricate new integrated systems on Si, such as molecular sensors or molecular-based p-n junction devices, by exploiting * To whom correspondence may be addressed. Present address: National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan. E-mail:
[email protected]. † Nagoya University. ‡ The University of Tokyo. § PRESTO “Structure Ordering and Physical Properties,” Japan Science and Technology Corporation. | Next Generation Enterprise Group, Advanced Materials Analysis & Evaluation Center. (1) Xia, Y.; Roger, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (2) Shirahata, N.; Masuda, Y.; Yonezawa, T.; Koumoto, K. Langmuir 2002, 18, 10379. (3) Cheng, J.; Robinson, D. B.; Cicero, R. L.; Eberspacher, T.; Barrelet, C. J.; Chidsey, C. E. D. J. Phys. Chem. B 2001, 105, 10900. (4) Wagener, P.; Nock, S.; Spudich, J. A.; Volkmuth, W. D.; Chu, S.; Cicero, R. L.; Wade, C. P.; Linford, M. R.; Chidsey, C. E. D. J. Struct. Biol. 1997, 119, 189. (5) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (6) Ara, M.; Graaf, H.; Tada, H. Jpn. J. Appl. Phys. 2002, 41, 4894. (7) Ashurst, W. R.; Yau, C.; Carraro, C.; Lee, C.; Kluth, G. J.; Howe, R. T.; Maboudian, R. Sens. Actuators, A 2001, 91, 239. (8) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513. (9) Sieval, A. B.; Linke, R.; Heij, C.; Meijer, G.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 2001, 17, 7554.
advanced ultra-large-scale integration (ULSI) technology, including lithographic processes. Moreover, a patterned monolayer surface fabricated by ULSI lithographic processes could be applied as the sole template to build chemically based semiconductor devices, such as high-k gate insulator and insulators, at any desirable site on the Si without an intervening SiO2 interlayer, since additional HF treatment of the patterned substrate has no adverse effect on the resolution of the template.6 Several techniques have been reported for patterning Si-C linked monolayers. Wojtyk et al.10 formed a methyl-terminated monolayer on a patterned Si(111)-H substrate prepared by the photolithography of Si(111)-H through a Cu grid. Ara et al.11 fabricated a nanoscale-patterned Si-C linked monolayer on Si(111) by atomic force microscopy (AFM) lithography. However, since these techniques cannot be readily applied for commercial mass production, they are not very suitable for building chemically based devices. Among the variety of optical and nonoptical lithographic techniques currently employed in ULSI technology, optical lithography is the most realized.12 Saito13 recently used vacuum ultraviolet irradiation to micropattern a 1-alkene monolayer, which had been prepared on Si(111) through reduced pressure chemical vapor deposition. We recently prepared a Si-C linked carbohydrate monolayer and patterned its surface in order to capture a protein molecule. To effectively fabricate chemically based molecular and semiconductor device systems, it is necessary to clarify the photodegradation mechanism of Si-C linked monolayers through detailed investigation of the photochemical changes occurring in the interfacial bonding state during optical lithography. However, no report has yet been published in which this interfacial change was directly observed. We report here on the photolithography of alkyl monolayers on Si(001), since atomically flat Si(001) is the most important surface applied for ULSI technology.15 To advance the development of new chemically based device technology, we have particularly investigated in detail the photochemical changes occurring in the chemical bonding states due to photolithographic processing. 2. Experimental Section We selected 1-octadecene as the reagent for the alkyl monolayer. H:Si(001) substrates (P-type, 5-10 Ω cm) were prepared by immersing a Si(001) wafer, freshly covered with SiO2, into HF solution (1.0 vol %) for 1 min. 1-Octadecene monolayer (ODM) was formed on the Si(001) surface by placing each H:Si substrate in one of four different solutions of 1-octadecene (1.0 vol %, degassed by N2 bubbling (>1 h)) at boiling temperature for 30 min under N2 atmosphere. We employed n-hexane, toluene, n-octane, and n-decane as solvents for the 1-octadecene solutions in order to form the ODM at thermal equilibrium. After immersion at boiling temperature, each substrate was carefully rinsed (10) Wojtyk, J. T. C.; Tomietto, M.; Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 2001, 123, 1535. (11) Ara, M.; Graaf, H.; Tada, H. Appl. Phys. Lett. 2002, 80, 2565. (12) Ito, T.; Okazaki, S. Nature 2000, 406, 1027. (13) Saito, N.; Kadoya, Y.; Hayashi, K.; Sugimura, H. Takai, O. J. J. Appl. Phys. 2003, 42, 2534. (14) Shirahata, N.; Yonezawa, T.; Miura, Y.; Kobayashi, K.; Koumoto, K. Langmuir 2003, 19, 9107. (15) Morita, Y.; Tokumoto, H. Appl. Phys. Lett. 1995, 67, 2654.
10.1021/la035179g CCC: $27.50 © 2004 American Chemical Society Published on Web 12/30/2003
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several times with the solvent used and then ultrasonically rinsed in pure ethanol for 10 min in order to remove any physically adsorbed molecules. X-ray photoelectron spectroscopy (XPS; Shimadzu Kratos Axis; 1.0 × 10-7 Torr) with the X-ray line at Mg KR (1253.6 eV) was conducted at 12 kV and 10 mA. Ar sputtering was carried out in order to directly observe Si-C bonds at the substrate surface before and after UV irradiation. With an analyzer pass energy of 20 eV, elemental compositions were calculated from the XP spectra of C 1s and Si 2p photoemission lines, corrected along the surface normal. All peak positions analyzed by XPS were normalized to that of the Si 2p peak appearing at 99.3 eV, which was referenced to a p-type Si wafer.9 UV irradiation was performed under ambient air with a low-pressure Hg lamp, producing 1.885 mW/cm2 irradiance at 50 mm from the ODM substrate. The primary wavelength of the lamp was 254 nm. The ratio of 254 to 185 nm was 100 to 18 in terms of emission energy. To examine the formation of the alkyl monolayer by evaluating the hydrophobicity at its surface, the surface water drop contact angles (θc) of the samples were measured using a sessile drop method. The UV stability of the ODM was compared with that of an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) on SiO2/Si, the preparation method of which we reported in a previous paper.2 The roughness of the H:Si and ODM/Si surfaces was evaluated by root-mean-square (rms). The thickness of the monolayers was estimated by ellipsometry in the following manner.5,16 Measurements were made using an ellipsometer (PZ2000, Philips) with a He-Ne laser as a light source and the angle of incidence set at 70°. The thickness of the monolayer on each sample was evaluated using the refractive index of Si (3.875 - 0.023i) and assuming that both the monolayer and the chemical oxide on the Si substrate were transparent at this wavelength and had the same refractive index of 1.46. The absolute accuracy of ODM thickness measurements remains questionable.5 Therefore the thickness at at least 10 different points on the ODM-covered substrate was measured. By use of our ellipsometric method, the margin of error for our ODM thickness measurements was found to be within (0.18 nm. Since our obtained values constituted the sum of the thickness of the OTS monolayer and the oxide, the actual thickness of the OTS monolayer could be determined by subtracting the oxide thickness from the total. With the same method, the error of thickness for an identical OTS SAM sample has been found to have been about (0.05 nm.16 The interface structure of an ODM/Si substrate was observed by high-resolution electron microscopy (HR-TEM). The ODM/Si substrate was sliced to a thickness of 0.2 mm and mechanically thinned using diamond paste to a thickness of 100 µm or less. Finally, ion thinning was performed to produce electron transparency. The observations were conducted using a HR-TEM (JEOL, JEM-4010) with a spherical aberration coefficient of 0.7 mm and an optimum defocus of -40 nm. When operated at 400 kV, the instrument had a point resolution of 0.15 nm. To observe a patterned ODM substrate, a micropatterned SnO2 film was fabricated on Si-O area of an ODM template utilizing the following technique which we reported previously.2 An ODM template was fabricated by UV irradiation of the ODM/ Si surface for 2 h in ambient air through a photomask. The ODM template was then immersed in anhydrous toluene solution containing 0.05 mol/L of butyl tin trichloride (BTT, C4H9SnCl3) at room temperature under N2 atmosphere. After the desired immersion time, residual solvent on the sample surface was immediately rinsed away using fresh toluene and then wiped off. The as-deposited thin film was then heated at 400 °C under ambient air.
3. Results and Discussion The θc increased gradually with increasing reaction temperature finally reaching to over 110°, which was as high as the θc of an ODM prepared using neat 1-octadecene.5 This value indicates the dense, perfect formation of a methyl-terminated Si surface.5 Figure 1a shows a cross-sectional HR-TEM image of the interface between (16) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600-7604.
Notes
Figure 1. Cross-sectional HR-TEM images of (a) a resin/ODM/ Si(001) substrate formed at 174 °C for 30 min in N2 atmosphere and (b) a SiO2/Si substrate. The selected area diffraction (SAD) patterns shown inset above and below in (a) correspond to resin and Si(001), respectively.
Figure 2. UV-irradiation time dependence of the surface water contact angle of ODMs on Si(001) and OTS SAMs on SiO2covered Si under ambient air. The open and closed circles correspond to the surface angles of the ODMs and OTS SAMs, respectively.
the ODM and Si for an ODM substrate formed at 174 °C. There is no evidence of interfacial oxide, which is consistent with the XPS data shown in Figure 2. Figure 1b shows a cross-sectional HR-TEM image of a SiO2/Si substrate. In Figure 1a, a lattice contrast can be observed extending continuously from the bulk to the top of the Si surface. Thus, an extremely flat, linear Si interface can be observed. In contrast, Figure 1b shows a rough interface similar to a wave line generated at the interface between Si and SiO2. This difference between the interfacial images indicates that no SiO2 interlayer existed between the ODM and the Si. The close correspondence between the rms values of the ODM-covered Si (rmsODM ) 0.21 ( 0.07 nm)) and the H:Si surfaces (rmsH:Si ) 0.15 ( 0.03 nm), as measured by AFM, strongly indicates the formation of an ODM surface with atomic level flatness. The surface roughness of the ODM fabricated here by our process was much lower than that of ODM fabricated by an evaporation
Notes
Figure 3. XP spectra of (a) Si 2p and (b) C 1s regions of -C18H37terminated Si(001) surfaces formed at 174 °C and of (c) Si 2p and (d) C 1s regions of -C18H37-terminated Si(001) surfaces after UV irradiation for 2 h in ambient air. To investigate the bonding mode in both substrates, the peaks were resolved by pseudo-Voigt functional fitting. The solid lines with open circles (-O-) show the experimental data, the dashed lines show the deconvolutions, and the solid lines show the resulting fits to the spectra. In the Si 2p narrow scan XP spectra shown in both Figures 2a and 2c, the main peak appearing at 99.3 eV was composed of Si 2p3/2 and Si 2p1/2 in the expected 2:1 area ratio, with 0.6 eV separation, which is in good agreement with results reported by Tufts et al. (Tufts, B. J.; Kumar, A.; Bansal, A.; Lewis, N. S. J. Phys. Chem. 1992, 96, 4581.) The samples for XPS measurement were Ar sputtered before their spectra were taken.
process.7 The film thickness of the ODM and the OTS SAM was dODM ) 1.95 nm and dOTS ) 2.31 nm, respectively. Both measured values are very close to the referenced values.5,17 Figure 2 shows the UV stability of ODMs and OTS SAMs under ambient air. The θc at the ODM surface decreased with increasing UV irradiation time and saturated after 2 h. Interestingly, the photodecomposition rate observed for the ODM was much lower (ca. one-sixth) than that for the OTS SAM. Figure 3 shows XP Si 2p and C 1s spectra of ODM substrates before and after UV irradiation for 2 h under ambient air. The C 1s spectrum seen in Figure 3b showed two components. The smaller component observed at about 285 eV was assigned to emissions from alkyl chains.18 The larger component detected close to 283 eV was assigned to Si-C bonds.19 This roughly 2 eV20 shift to lower binding energy is reasonable, since Si atoms are more electropositive than carbon atoms. As shown in Figure 3d, after UV treatment only the C-C peak at about 285 eV18 referenced to alkyl chains was detected and the Si-C bonds had disappeared. This disappearance of Si-C bonds due to UV irradiation was confirmed in both Arsputtered and unsputtered samples. On comparison of the Si 2p spectra in parts a and c of Figure 3, after irradiation the Si-C peak (100.3 eV19) disappeared and a Si-O peak (103.3 eV19) appeared. In monolayer formation and patterning studies, there have been few reports on the successful observation of (17) Sieval, A. B.; Demirel, A. L.; Nissink, J. W.; Linford, M. R.; Mass, J. H. Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759. (18) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056. (19) Kalomiros, J. A.; Paloura, E. C.; Ginoudi, A.; Kennou, S.; Ladas, S.; Lioutas, Ch.; Vouroutzis, N.; Voutsas, G.; Girginoudi, D.; Georgoulas, N.; Thanailakis, A. Solid State Commun. 1995, 96, 735. (20) Yang, C. S.; Oh, K. S.; Ryu, J. Y.; Kim, D. C.; Yong, J. S.; Choi, C. K.; Lee, H. J.; Um, S. H.; Chang, H. Y. Thin Solid Films 2001, 390, 113.
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Figure 4. SEM images showing (a) the photomask, used and (b) the micropatterned SnO2 thin film selectively deposited on the Si-O areas of the ODM template on Si(001). In (a), the darker region is the UV-irradiated area.
interface structures.18,21 Figure 4 shows scanning electron microscopy (SEM) images of (a) the photomask we used and (b) the micropatterned SnO2 thin film. The micropatterned thin film grew selectively on the Si-O areas of the ODM template. The BTT precursor hydrolyzed with small amounts of H2O molecules dissolved in even the anhydrous toluene solution. It then attached onto the Si-O areas due to dehydration.2 As immersion time increased, tin hydroxide thin film grew due to the formation of SnO-Sn networks in the BTT solution.2 In contrast, the BTT precursor could not attach to the methyl-terminated areas, because of an absence of hydrophilic groups, such as Si-O-Si and Si-OH.2,22 Fourier transform infrared spectroscopic profiles showed that the tin hydroxide thin film deposited selectively onto the Si-O areas. At 400 °C, the tin hydroxide thin film was transformed into SnO2 thin film. The above experimental results verify the cleavage of the Si-C bonds at the interface between the ODMs and the Si surface due to UV irradiation in ambient air, and the surface modification of the hydrophilic (Si-O) areas which followed. The cleavage of the Si-C bonds of organosilane-based SAMs on SiO2/Si substrate by UV irradiation under ambient air has been well reported,23,24 but no reports to date have directly observed this cleavage. A combination of UV irradiation and oxygen is generally necessary for monolayer decomposition.25 Oxygen plays a central role in the photoinduced cleavage mechanism of organic monolayers.23-26 Oxygen was also observed to play a similar role in the photodecomposition behavior of our ODMs, as there was no change in the θc on the ODM surface even after UV irradiation for 19 h under N2 atmosphere. A decrease in the θc indicates a gradual conversion from hydrophobic methyl headgroups to hydrophilic headgroups. Organosilane-based monolayers are decomposed by photooxidation, and the methyl groups at their surface are modified into CHO and COOH groups due to the shortening of their chain length with, finally, a silanol-terminated surface remaining.23-27 Judging from (21) Yamada, T.; Inoue, T.; Yamada, K.; Takano, N.; Osaka, T.; Harada, H.; Nishiyama, K.; Taniguchi, I. J. Am. Chem. Soc. 2003, 125, 8039. (22) Masuda, Y.; Seo, W. S.; Koumoto, K. Thin Solid Films 2001, 382, 153. (23) Dulcey, C. S.; Georger, J. H.; Krauthamer, J. V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551. (24) Ye, T.; Wynn, D.; Dudek, R.; Borguet, E. Langmuir 2001, 17, 4497. (25) Moon, D. W.; Kurokawa, A.; Ichimura, S.; Lee, H. W.; Jeon, I. C. J. Vac. Sci. Technol., A 1999, 17, 150. (26) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Langmuir 2000, 16, 885.
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the gradual decrease of the θc on the ODM surface, a similar surface conversion might occur on both the ODM and the OTS SAM substrates. However, as shown in Figure 2, the photodecomposition rates of the two monolayer substrates consistently and appreciably differed. Quayum et al. have indicated that the formation density of an OTS SAM is slightly larger than that of an ODM.28 This report conflicts with the lower decomposition rate we observed for ODMs compared to OTS SAMs. While similar monolayer densities accumulated on both substrates, the considerable difference between the UVinduced decomposition rate of the monolayers may depend on the difference in the rupture rate of the Si-C bonds on the substrate surface. In the case of the ODM, carbon is directly bonded to the surface Si atoms of the Si wafer. On the contrary, with the OTS SAM carbon is bonded to the Si atoms of organosiloxane. The difference between the electron negativity of the environmental Si atoms (27) Saito, N.; Hayashi, K.; Sugimura, H.; Takai, O. Unpublished. (28) Quayum, M. E.; Kondo, T.; Nihonyanagi, S.; Miyamoto, D.; Uosaki, K. Chem. Lett. 2002, 208.
Notes
probably affects the cleavage rate of the Si-C bonds. We are currently investigating the exact photochemical mechanism accounting for such different UV stability. In conclusion, we observed the cleavage of the Si-C bonds of Si-C linked monolayer substrates due to UV irradiation under ambient air, and this cleavage was verified by XPS measurements. These new findings pave the way to develop advanced monolayer resists to build integrated systems on hydrogen-terminated surfaces, such as Si, diamond, and SiC, for application in molecular electronics, biochemistry, and the micropatterned fabrication of chemically based semiconductor devices. Acknowledgment. The authors wish to thank Dr. A. Hozumi of AIST for his warm hospitality. This work was partly supported by a Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS). LA035179G
