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Fluoroalkylsilane Monolayers Formed by Chemical Vapor Surface Modification on Hydroxylated Oxide Surfaces Atsushi Hozumi,*,† Kazuya Ushiyama, Hiroyuki Sugimura, and Osamu Takai Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received July 21, 1998. In Final Form: June 17, 1999 Water-repellent surfaces have been prepared by exposing Si substrates with a hydroxylated surface oxide to fluoroalkyl silane (FAS) vapor. Since this chemical vapor surface modification (CVSM) is based on the chemical reaction between organosilane molecules and hydroxyl groups at the oxide surface, prior to CVSM, the substrate surface was completely hydroxylated by irradiating in air with a 172-nm ultraviolet light until the water contact angle of the surface became almost 0°. Under atmospheric pressure, the substrate was then exposed to vapor of an FAS precursor, that is, one of three types of FAS having different perfluoroalkyl chain lengths [CF3(CF2)nCH2CH2Si(OCH3)3, where n ) 0, 5, or 7, referred to as FAS-3, FAS-13, and FAS-17, respectively]. The FAS molecules chemically reacted with the hydroxyl groups on the substrate surface and adsorbed onto it, forming a thin layer of less than 2 nm in thickness. The water repellency of the substrate surface increased with an increase in perfluoroalkyl chain length. The maximum water-contact angles of the substrates treated with FAS-3, -13, and -17 were ca. 86°, 106°, and 112°, respectively.
Introduction Water-repellent coatings have attracted much attention because of their application in a wide variety of engineering fields. There have been many reports describing waterrepellent coatings and surface treatments by many means, including the sol-gel method,1-3 plasma surface modification and polymerization,4-6 physical vapor deposition,7 chemical adsorption,8 and dispersion plating.9 We have also reported on the preparation of transparent waterrepellent films based on plasma-enhanced chemical vapor deposition (PECVD) using various organosilane compounds and fluoroalkyl silanes (FASs) as source materials.10,11 The films we obtained were transparent and showed excellent water repellency with water-contact angles up to 107°. Furthermore, through the control of surface morphology, we have succeeded in fabricating ultra-water-repellent films showing water-contact angles of ca. 160°.12 It is well-known that water repellency is governed by the surface chemical properties of the topmost layer. * To whom all correspondence should be addressed: tel, +8152-789-2796; fax, +81-52-789-3260; e-mail, hozumi@ otakai.numse.nagoya-u.ac.jp. † Present address: National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462-8510, Japan. Tel, +81-52-911-2491; fax, +81-52-916-6993; e-mail,
[email protected]. (1) Izumi, K.; Tanaka, H.; Murakami, M.; Deguchi, T.; Morita, A.; Toge, N.; Minami, T. J. Non-Cryst. Solids 1993, 121, 344. (2) Tsuchiya, T.; Inoguchi, G. J. Surf. Sci. Jpn. 1993, 14, 540. (3) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 1040. (4) Wydeven, T.; Kubacki, R. Appl. Opt. 1976, 15, 132. (5) Goto, T.; Chen, J.; Wakida, T. Chem. Express 1991, 6, 711. (6) Kogoma, M.; Kasai, H.; Takahashi, K.; Moriwaki, T.; Okazaki, S. J. Phys. D: Appl. Phys. 1987, 20, 147. (7) Hopkins, R. H. Appl. Opt. 1975, 14, 2831. (8) Takada, Y.; Soga, M.; Ogawa, K.; Ozaki, S. Polym. Prepr. Jpn. 1993, 42, 1719. (9) Watanabe, N.; Tei, Y. Chemistry 1991, 46, 477. (10) Takai, O.; Hozumi, A.; Sugimoto, N. J. Non-Cryst. Solids 1997, 218, 280. (11) Hozumi, A.; Kakinoki, N.; Asai, Y.; Takai, O. J. Mater. Sci. Lett. 1996, 15, 675. (12) Hozumi, A.; Takai, O. Thin Solid Films 1997, 303, 222.
Therefore, detailed knowledge of the adsorption behavior of FAS is vital in order to elucidate the water repellency of PECVD films. Although the chemisorption of FAS molecules at the solid/liquid interface has been studied extensively and has been widely applied to the modification of solid surfaces using perfluoroalkyl groups,8,13,14 there have been few reports on preparing FAS monolayers from the vapor phase.15-20 Tada et al. have prepared FAS monolayers by chemical vapor surface modification (CVSM) in a vacuum.15-18 The preparation of FAS monolayers under atmospheric pressure conditions, however, has not been reported, despite its practical advantages. In this paper, we report the formation of FAS monolayers through the chemisorption of FAS molecules onto hydroxylated oxide surfaces by CVSM under atmospheric pressure. We have investigated the relations between the fluoroalkyl chain lengths of the FAS molecules and the properties of the films produced, including thickness, water repellency, chemical properties, and morphology. The surface properties of these FAS monolayers are compared with those of water-repellent films fabricated by PECVD. Experimental Section Sample substrates were cut from n-type Si (100) wafers (Shinetsu Handoutai). These substrates were photochemically cleaned by exposure for 3 min to vacuum ultraviolet light (excimer lamp, Ushio Electric, UER 20-172 V, λ ) 172 nm) in air. Shortwavelength radiation in this range dissociatively excites the carbon-carbon and carbon-hydrogen bonds of organic molecules. Furthermore, the light produces oxygen atoms and ozone (13) Geer, R. E.; Stenger, D. A.; Chen, M. S.; Calvert, J. M.; Shashidhar, R.; Jeong, Y. H.; Pershan, P. S. Langmuir 1994, 10, 1171. (14) Banga, R.; Yarwood, J.; Morgan, A. M.; Evans, B.; Kells, J. Langmuir 1995, 11, 4393. (15) Tada, H.; Nagayama, H. Langmuir 1994, 10, 1472. (16) Tada, H.; Shimoda, K.; Goto, K. J. Electrochem. Soc. 1995, 142, L230. (17) Tada, H. J. Electrochem. Soc. 1995, 142, L11. (18) Tada, H.; Nagayama, H. Langmuir 1995, 11, 136. (19) Hoffmann, P. W.; Stelzle, M.; Rabolt, J. F. Langmuir 1997, 13, 1877. (20) Sugimura, H.; Nakagiri, N. J. Vac. Sci. Technol., A 1996, A 14(3), 1223.
10.1021/la9809067 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/26/1999
Fluoroalkylsilane Monolayer Formation molecules because of the photoexcitation of O2 molecules in the atmosphere. Consequently, organic contaminants on the sample surface are decomposed by direct photoexcitation and by the following oxidation with activated oxygen species. The photoirradiated substrates became clean because of this photochemical decomposition of surface contamination. The substrate surfaces became completely hydrophilic, with their water-contact angles changing from 40° before cleaning to almost 0° afterward. Because of this cleaning, a thin oxide layer formed on the Si substrate. At room temperature, such a hydrophilic silica surface is most likely terminated with hydroxyl (OH) groups with a concentration of 5 or 6 OH groups/nm2.21,22 These samples with hydroxylated surfaces were placed in a 65 cm3 vessel together with a glass container filled with ca. 0.2 cm3 of FAS liquid. The vessel was sealed with a cap and then heated in an oven maintained at a temperature of either 100 or 150 °C. In each case, one of three types of FAS [CF3(CF2)nCH2CH2Si(OCH3)3; n ) 0, 5, or 7] was used as a precursor, referred to hereafter as FAS-3, FAS-13, and FAS-17, in that order. Their vapor pressures are 1.0 × 105 Pa at 143-145 °C, 133 Pa at 50-52 °C, and 133 Pa at 85-87 °C, respectively. The FAS liquid in the vessel vaporized and reacted with the OH groups on each sample surface. The molecules were chemisorbed onto the sample, resulting in the formation of an FAS monolayer. The thickness of the monolayer was estimated by ellipsometry. Measurements were made using an ellipsometer (PLASMOS, SP2300) with a He-Ne laser as a light source. The angle of incidence for the measurements was set at 70°. We calculated each sample’s monolayer thickness using the refractive index of Si (3.875-0.023 i) and assuming that the monolayer and the native oxide on the Si substrate were transparent at this wavelength and had the same refractive index of 1.46. Because the resulting value was the sum of the thickness of the monolayer and the native oxide, the actual monolayer thickness was determined by subtracting the oxide thickness from the total. Before monolayer coating, the thickness of the native oxide layer of each sample was measured. We estimated the oxide thickness on the silicon substrate to be 2.0 ( 0.2 nm through measurement of more than 100 samples. The error of thickness for an identical sample was about ( 0.05 nm. The water repellency of the treated substrates was evaluated by measuring their water-contact angles at 25 °C in air using an automatic contact anglemeter (CA-X150, Kyowa Interface Science) using the sessile-drop method. Substrate surfaces were studied by X-ray photoelectron spectroscopy (XPS) (AXIS HS, Shimadzu) using monochromated AlKR radiation. The X-ray source was operated at 12 mA and 15 kV. The binding energy (BE) scales were referenced to 285.0 eV as determined by the locations of the maximum peaks on the C1s spectra of hydrocarbon (CHx), associated with adventitious contamination. The accuracy of the BE determined with respect to this standard value was within ( 0.3 eV. Morphology of the sample surfaces was observed by atomic force microscope (AFM) (Autoprobe-LS, Park Scientific Instruments).
Results and Discussion Growth Rate and Water Repellency. Organosilane molecules are known to react with OH groups on an oxide surface, resulting in the formation of a self-assembled monolayer.23 In the present experiment, thin layers consisting of FAS molecules were expected to form similarly on the FAS-treated substrates. Indeed, extremely thin films were found to have grown on the substrates, as demonstrated in Figure 1 a. These FAS monolayers were fabricated at a preparation temperature (Tp) of 100 °C. In the cases of FAS-13 and -17, the film thicknesses increased rapidly during the first 0.5 h and stopped increasing at around 1 h. Thickness remained unchanged at ca. 0.6 and 1.1 nm, respectively, even when the CVD time was (21) Zhuravlev, L. T. Langmuir 1987, 3, 316. (22) Sneh, O.; George, S. M. J. Phys. Chem. 1995, 99, 4639. (23) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074.
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(a)
(b)
(c)
Figure 1. Growth of FAS monolayers fabricated at Tp ) 100 °C: (a) Ellipsometric thickness, (b) Water-contact angle, and (c) Surface fluorine concentration. O, FAS-3; 0, FAS-13; and 4, FAS-17.
extended up to 7 h. The saturated thickness of the FAS-3 monolayer was ca. 0.2 nm and required 3 h to reach this point. In our case, the error of ellipsometric thickness for an identical sample is about ( 0.05 nm. Although the thickness error was not small enough to allow us to clearly indicate the growth curve for the FAS-3 monolayer, after considering the experimental results shown in Figure 1b,c, we concluded that the film became denser with an increase in the CVD time. However, from only the ellipsometric data, the growth curve for the FAS-3 monolayer cannot be determined exactly. Thus, the curve was drawn with a broken line, indicating less confidence than for the other curves. The saturated thicknesses of the FAS monolayers clearly depended on the perfluoroalkyl chain length. The molecular lengths of FAS-3, -13, and -17 (from the Si atom at one end to the C atom at the other), calculated using MOPAC (Molecular Orbital Package, Version 7) are ca. 0.42, 1.06, and 1.33 nm, respectively. We optimized the geometries of these FASs using Pople’s bond lengths and
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Figure 2. C1s-XPS spectra of Si substrates treated with FASs for 5 h at Tp ) 150 °C: (a) FAS-3, (b) FAS-13, and (c) FAS-17.
angles as starting geometries.24 The thicknesses of the FAS-13 and -17 monolayers, determined by ellipsometry, were about 57% and 83% of their calculated molecular lengths, respectively. If the FAS molecules are assumed to be inclined in the monolayers, their angles of inclination are calculated to be 55 and 34° to normal, respectively. These angles are larger than the reported inclination angles of alkanethiolate self-assembled monolayers (SAMs) (20 ∼30°).25 Therefore, molecular inclination alone is insufficient to explain why the monolayer thickness is shorter than the molecular length. The FAS molecules making up the monolayers produced in this experiment are thought to be both inclined and disordered to some extent. Such a structure would most likely result from a combination of both conformational and thermal disorder in the perfluoroalkyl chains due to the presence of gauche kinks and weak interchain interactions, respectively.25 The water-contact angles of the Si substrates treated with FAS vapor at the Tp of 100 °C are summarized in Figure 1(b). The substrates became more water-repellent with longer CVD times. However, the water-contact angles became constant at a certain CVD time depending on the precursor. The maximum contact angles were ca. 86°, 106°, and 112° for FAS-3, -13, and -17, respectively. The water repellency of the substrates treated with FAS-13 was almost equivalent to that of poly(tetrafluoroethylene) (PTFE), which is reported to be 108°.26 The FAS-17 monolayer showed a slightly larger water-contact angle than that of the FAS-13 monolayer. Precursor molecules having a longer perfluoroalkyl chain more sufficiently cover the hydrophilic substrate surface, resulting in higher water repellency. The water-contact angles of the FAS-13 and -17 monolayers were almost the same as those of SAMs prepared from CF3(CF2)5CH2CH2SiCl3 and CF3(CF2)7CH2CH2SiCl3 solutions (reported as ca. 105° and 112°, respectively 8,27). Despite FAS-3 having the highest vapor pressure, growth saturation time as reflected by both thickness and water-contact angle was longer than with FAS-13 and -17. FAS-3 seems to require a larger number of adsorbed molecules to complete monolayer formation. These results indicate that, due to the smaller steric hindrance of FAS-3 compared with FAS-13 and -17, the number of reactable adsorption sites for FAS-3 was larger. The saturated film thicknesses and water-contact angles of the FAS-13 and -17 monolayers prepared at the Tp of 150 °C were almost the same as those of the monolayers prepared at 100 °C. The monolayers grew rapidly during the first 0.5 h and stopped increasing in thickness at (24) Pople, J. A.; Beveridge, D. L. Approximate Molecular Orbital Theory; McGraw-Hill: New York, 1970; p 111. (25) Marc, D. P.; Thomas, B. B.; David, L. A.; Christopher, E. D. C. J. Am. Chem. Soc. 1987, 109, 3559. (26) Yano, H.; Mori, K.; Koshiishi, K.; Masuhara, K. J. Surf. Fin. Soc. Jpn. 1989, 40, 110. (27) DePalma, V.; Tillman, N. Langmuir 1989, 5, 868.
around a CVD time of 1 h, similar to the results shown in Figure 1a,b. However, in the case of FAS-3, the film thickness and water-contact angle increased with the higher Tp. At 150 °C, the FAS-3 monolayer stopped growing at around 2 h. Its thickness reached a constant value of ca. 0.6 nm, and its maximum water-contact angle was about 91°. These values were greater than those of the FAS-3 monolayers prepared at 100 °C. Thus, the growth saturation time was reduced due to an increase in the concentration of FAS-3 molecules in the atmosphere. Surface Analyses by XPS. Using XPS we investigated the relations between the chemical properties of the film surfaces and their water repellency described in the previous section. Spectra of O (1 s), Si (2 p), C (1 s), and F (1 s) were detected for all the samples treated with FASs. As shown in Figure 1(c), F concentrations of the same sample surfaces as those shown in Figure 1(a)-(b) increased markedly. After 1 h of deposition, F concentration saturated in the case of FAS-13 and -17. When Tp was 150 °C, F concentrations of the FAS-13 and -17 monolayers were almost the same as those of the monolayers prepared at 100 °C. On the other hand, for FAS-3, the F concentration of the monolayers was greater at 150 °C than at 100 °C. There is a mutual relationship between the water repellency and the F concentrations of the FAS films, as can be seen in Figure 1(b)-(c). Si and O concentrations of the films prepared with FAS17 at the Tp of 150 °C for 5 h (25.4 and 24.1 at %, respectively) were smaller than those of the films fabricated under the same conditions with FAS-13 (32.8 and 27.6 at %, respectively) and FAS-3 (39.6 and 34.3 at %, respectively), while the atomic percentage of Si-C groups in the Si2p spectra remained nearly constant. The atomic concentrations corresponding to Si and O decreased with perfluoroalkyl chain length. These results indicate that the Si substrate was more completely covered by the FAS17 monolayer than by the FAS-3 and -13 ones. This finding is consistent with the results of the film thicknesses shown in Figure 1(a). Figure 2a-c shows C1s spectra of the Si substrates treated with FAS-3, -13, and -17, respectively, at the Tp of 150 °C for 5 h. The profile of spectrum (c) is similar to that of spectrum (b), except that the signal intensity of the CF2 group is larger than that of adventitious carbon. The profile of spectrum (a), however, is different from those of both spectra (b) and (c) because FAS-3 has only a single CF3 group in its molecule and no CF2 group. The deconvolution divided the C1s spectra into five or six features which were identified according to the reported chemical shifts.5,28 Spectrum (a) consisted of five components centered at BEs of 283.6, 285.0, 286.4, 289.5, and 292.6 eV and corresponding to Si-C, C-C, C-O, CF3CH2-, and CF3-CH2- groups, respectively. Spectra (b) and (28) Clark, D. T.; Shuttleworth, D. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 27.
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Table 1. CF3/CF2, CF2/Si, and CF3/Si Ratios of FAS Monolayers Determined from C1s Spectraa reagent
CF3/CF2 in the C1s spectra
CF3/CF2 in FAS molecules
CF2/Si
CF3/Si
FAS-13 FAS-17
0.32 (1.6 times) 0.31 (2.2 times)
0.20 0.14
0.13 0.19
0.03 0.06
a
CVD time, 5 h; Tp ) 150 °C.
(c) consisted of six components centered at BEs of 283.5283.6, 285.0, 286.6-286.7, 290.5, 291.7-291.9 and 294.1 eV. These correspond to Si-C, C-C, C-O, CF2-CH2-, CF2-CF2-, and CF3-CF2- groups, respectively. The CF3 and CF2 peaks in spectra (b) and (c) could be resolved clearly from the other components because of fluorine’s great electronegativity.19 There was no CF2 peak in spectrum (a), as FAS-3 has no CF2 group. In addition, the CF3 group BE in spectrum (a) was ca. 1.5 eV smaller than those in spectra (b) and (c), because of the shorter molecular length of the FAS-3 molecule.19 Table 1 summarizes the intensity ratios of CF3/CF2, CF2/Si, and CF3/Si in the C1s spectra shown in Figure 2b,c. For comparison, the CF3/CF2 ratios of FAS molecules are also shown. The CF3/CF2 ratios of the FAS-13 and -17 monolayers were ca. 1.6 and 2.2 times larger than those of the FAS-13 and -17 molecules, respectively. The CF3/ CF2 ratios of the FAS-13 and -17 monolayers increased compared with those of the original precursor molecules. This result indicates that FAS molecules are oriented so that their CF3 headgroups are aligned toward the outer surfaces. However, from only these XPS results, we cannot conclude the difference in molecular ordering between the FAS-13 and -17 monolayers. However, considering that the FAS-17 monolayer was more hydrophobic than the FAS-13 monolayer and that the thickness of the FAS17 monolayer was closer to its molecular length than the FAS-13 monolayer, the FAS-17 monolayer is likely to be more highly oriented than the FAS-13 monolayer. The AFM images shown in Figure 3a,b demonstrate typical morphologies of an untreated Si substrate (a) and a Si substrate on which a monolayer was prepared with FAS-17 at a Tp of 150 °C for 3 h (b). The surface after deposition appears to be very smooth and similar to that of the untreated Si substrate. The surface roughnesses of both were less than 1 nm. No defects or aggregation can be seen in the monolayer. The surface morphology has no influence upon the water repellency of the CVSM film, which is determined only by the surface chemical properties. FAS Films Prepared by PECVD. Here we compare the water repellency of the FAS films prepared in this experiment by CVSM with those prepared by PECVD. Details regarding experimental conditions and results of PECVD were described in our previous papers.10,11 The thickness of the films deposited by PECVD depended on preparation conditions. The average film thicknesses prepared using FAS-3, -13, and -17 after 30 min deposition at a Tp of less than 70 °C were about 3.0, 1.4, and 1.2 µm, respectively. Their water-contact angles were ca. 92°, 98°, and 107°, respectively.10,11 The water repellency of the substrates treated with FAS-13 and -17 by CVSM was superior to that of the films prepared with the same precursors by PECVD. These results can be explained as follows. First, in PECVD, highly reactive atomic fluorine forms as a result of the decomposition of FAS molecules in plasma. These fluorine atoms decompose the deposited film to a certain degree.29 Second, during PECVD, radicals (29) Matsuoka, T.; Yasuda, H. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 2633.
Figure 3. AFM images of Si surfaces: (a) before treatment, and (b) after treatment with FAS-17 for 5 h at Tp ) 150 °C. Lateral and vertical scales of each image are 2 × 2 µm2 and 1 nm, respectively.
are generated beneath or at the film surface. These radicals react with oxygen in the air, resulting in the formation of polar groups which decrease the hydrophobic properties of the film.30 Finally, the FAS molecules on the surface of films prepared by PECVD may be disordered, resulting in lower water repellency. The water-contact angle of the FAS-3 films prepared by CVSM at the Tp of 150 °C was equivalent to that of the FAS-3 films fabricated by PECVD. The effect of the plasma-generated F atoms was negligible because the amount of fluorine in FAS-3 is small. Despite having lower water repellency, F concentrations of the films prepared with FAS-17 by PECVD (ca. 42 at %) were larger than those of the films fabricated by CVSM using the same reactant (ca. 34 at %). In addition, the FAS-17 films prepared by PECVD contained another chemical species, that is, CF, and the CF3 and CF2 peaks could not be resolved clearly compared with those of the CVSMprepared FAS-17 monolayer. With PECVD, FAS molecules were decomposed in plasma and polymerized, resulting in the formation of disordered FAS films. Because of these factors, the films prepared by PECVD were less waterrepellent than those prepared by CVSM. Conclusions We have fabricated FAS monolayers onto a hydroxylated oxide surface by CVSM under atmospheric pressure. These films were very water-repellent, with maximum watercontact angle reaching ca. 112°. The thickness and water repellency of the films depended on the perfluoroalkyl chain length of the precursor molecules. Their water (30) Sar, C.; Valentin, M.; Bui, A. J. Appl. Polym. Sci. 1979, 24, 503.
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repellency was superior to that of films deposited by PECVD using the same FASs as source materials. Highly oriented FAS monolayers were successfully fabricated by simple exposure to FAS vapor under atmospheric pressure. Their water repellency was similar to that of SAMs deposited from FAS solution. CVSM is advantageous because there is no need for a solvent, compared with a liquid-phase process requiring a large amount of organic solvent for both the monolayer deposition and substrate rinsing steps. For practical application, water-repellent
Hozumi et al.
treatment by CVSM under atmospheric pressure is simple and effective. Acknowledgment. The authors would like to express their gratitude to Dr. Masanobu Futsuhara, Nippon Paint Corporation, for the XPS measurements and his fruitful suggestions. This study was supported in part by a Grantin-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan. LA9809067
