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Notes Formation and Characterization of Patterned Heterogeneous Silane Films Janine Buseman-Williams and John C. Berg* University of Washington, Department of Chemical Engineering, Box 351750, Seattle, Washington 98195-1750 Received June 16, 2003. In Final Form: December 12, 2003
Introduction Silane-modified primers have long been used to promote adhesion of virtually any organic thermoplastic or thermosetting polymer to metals, glass, and other mineral surfaces.1-4 However, those silanes that provide the best adhesion are often susceptible to moisture, which degrades the bond and reduces the effectiveness of the coupling agent. Water will diffuse through any plastic when exposed to a humid environment, but it is harmless unless it clusters into a liquid phase.1 Due to van der Waals interactions, n-alkyltrichlorosilanes will form self-assembled monolayers (SAMs) on silica surfaces when n is greater than 10.5-7 These densely packed, ordered films not only create a hydrophobic surface but also appear to impede the diffusion of water to the silane/substrate interface, which enhances durability under wet conditions.8 It is proposed that both good initial adhesion and durability toward moisture intrusion can be achieved by using micropatterned multicomponent silane primers containing a hydrophobic silane and a coupling agent. Because hydrophobic silanes are not considered coupling agents, it is important to form primers containing hydrophobic silanes in sufficient amount to prevent moisture intrusion but not in great enough amounts to interfere with the effectiveness of the adhesion enhancement of the second silane. This paper describes the formation and characterization of patterned heterogeneous silane films containing one adhesion promoting and one hydrophobic silane. Silanetreated surfaces have been characterized using a variety of analytical techniques such as ellipsometry, contact angle goniometry, infrared spectroscopy, and atomic force microscopy (AFM).9-12 Recently, the pulsed force mode * Corresponding author. E-mail:
[email protected]. (1) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1982. (2) Petrie, E. Handbook of Adhesives and Sealants; McGraw-Hill: New York, 2000. (3) Paek, S.-H.; Lee, K.-W.; Durning, C. J. Adhes. Sci. Technol. 1999, 13, 423. (4) Greenblatt, J.; Araps, C. J.; Anderson, H. R. In Polyimides: Synthesis, Characterization and Application; Mittal, K. L., Ed., Plenum Press: New York, 1982; p 573-588. (5) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (6) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (7) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. Netzer, L.; Isovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67. Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986, 112, 457. (8) Cave, N. G.; Kinloch, A. J. Polymer 1992, 22, 1162. (9) Silberzan, P.; Le´ger, L.; Ausserre´, D.; Benattar, J. J. Langmuir 1991, 7, 1647.
(PFM) of AFM has been developed to extend the capabilities of AFM. By simultaneously mapping the topography, adhesion, and stiffness, PFM enables the characterization of heterogeneous surfaces that cannot be classified by their topographic changes alone.13 Different functional groups on a surface will show distinct differences in the PFM images based on the sample-tip interactions. PFM has been used with heterogeneous hydrophobic silane surfaces13-15 and should provide additional information about heterogeneous patterned silane films with one adhesion promoting and one hydrophobic silane. Experimental Methods Materials. Octadecyltrichlorosilane (ODTS), 3-aminopropyltriethoxysilane (ATS), and 3-aminopropyldimethylethoxysilane (AMS) were purchased from Gelest, Inc. (Tullytown, PA). A silicone elastomer (Sylgard 184) was obtained from Dow Corning (Midland, MI). All solvents were obtained from commercial sources and used without further purification. Polished n-type and p-type silicon wafers with 〈111〉 orientation were obtained from Silicon Valley Microelectronics (San Jose, CA). The silicon wafers were cleaned by sonication in a mixture of Nochromix and sulfuric acid for 45 min using an ultrasonic cleaner (Cole Parmer, Chicago, IL). The wafers were then rinsed in water and dried in a stream of air just prior to use. Silane Deposition. A solvent mixture containing hexadecane, carbon tetrachloride, and chloroform (30:5:3 by volume, respectively) was used to make a 0.2 vol % ODTS solution. The solvent mixture was cooled to 10 °C. Clean substrates were immersed into solution 1 min after the addition of silane. Deposition times shorter than 1 min created partial monolayers. The substrates were rinsed in chloroform at 10 °C to terminate the deposition and dried in a stream of air. They were then cured for 10 min at 125 °C. These substrates were then immersed in a 1 mM solution of AMS in water for 1.5 h. The solution was allowed to hydrolyze for 1 h prior to substrate immersion. The deposition was terminated by rinsing with distilled water and followed by a 6 h cure at 125 °C. The cured substrates were rinsed in chloroform and dried in a stream of air. Partial films of ATS were created using microcontact printing (µCP). Poly(dimethylsiloxane) (PDMS) stamps were made from a master generated from photolithography.16 Patterns of regular straight lines with 20 µm width, 20 µm spacing, and 1 µm depth were used for stamping. Solutions of 0.5 vol % ATS in water were applied to the stamp using a cotton swab, and the solvent was allowed to evaporate. The substrate was placed in contact with the stamp for 0.5-2 min and subsequently rinsed with pure solvent. The treated substrates were then cured and rinsed as previously described. The substrates were then enclosed in a glass container for 30 min at 145 °C with about 1 mL of pure ODTS. The substrates did not directly contact the silane. The elevated temperature allowed the silane to vaporize. The (10) Duchet, J.; Gerard, J. F.; Chapel, J. P.; Chabert, B. Compos. Interfaces 2001, 8, 177. (11) Reiniger, M.; Basnar, B.; Friedbacher, G.; Schleberger, M. Surf. Interface Anal. 2002, 33, 85. (12) Ulman, A., Ed. Characterization of Organic Thin Films; Butterworth-Heinemann: Stoneham, MA, 1995. (13) Kortil, H. U.; Stifter, T.; Waschipky, H.; Weishaupt, K.; Hild, S.; Marti, O. Surf. Interface Anal. 1999, 27, 336. (14) Okabe, Y.; Akiba, U.; Fujihira, M. Appl. Surf. Sci. 2000, 157, 398. (15) Tribology Issues and Opportunities in MEMS; Bhushan, B., Ed.; Kluwer Academic Publishers: Boston, 1998. (16) Obtained from Prof. Younan Xia (University of Washington, Department of Chemistry).
10.1021/la030246s CCC: $27.50 © 2004 American Chemical Society Published on Web 02/06/2004
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Figure 1. (top, left to right) Topography, adhesion, and stiffness PFM-AFM images of 1 s ODTS solution deposition of 0.2 vol %. (bottom, left to right) Topography, adhesion, and stiffness AFM images of the same sample backfilled with 0.1 vol % AMS in water. Scan size: 5 × 5 µm2. container cooled for 2-3 min before the substrates were removed from the glass container. The substrates were not rinsed in solvent after the deposition. Characterization. Atomic force microscopy was performed using an Autoprobe CP (Park Scientific Instruments, Sunnyvale, CA). An external module (WITec, Ulm, Germany) was added to the AFM feedback circuit for pulsed force mode. Silicon tips (Veeco, Sunnyvale, CA) with a force constant of 2.1 N/m were used. At least three different images were acquired for each sample to obtain a characteristic image of the surface. The surface area coverage and height differences were calculated using the PSI ProScan Image Processing software (Park Scientific Instruments). A contact angle goniometer (Rame´-Hart, Mountain Lakes, NJ) was used to measure contact angles with triply distilled water. The contact angle values of at least three different areas were averaged. For heterogeneous surfaces, the contact angle can be used to calculate the fractional coverage of each component using the Cassie-Baxter equation:
cos(θmix) ) f1 cos(θ1) + f2 cos(θ2)
(1)
where θmix is the contact angle of the heterogeneous surface, θ1 and θ2 are the contact angles of the pure components 1 and 2, respectively, and f1 and f2 are the fractional coverages of components 1 and 2.
Results and Discussion Sequential Adsorption. Figure 1 shows an ODTStreated substrate that has been backfilled with AMS using solution deposition. In all AFM-PFM images presented, the lighter colors refer to areas of higher topography, higher adhesion, and higher stiffness. The islands of higher topography are depositions of ODTS, which have a height of 16.8 Å. Since the length of a fully stretched ODTS molecule is 24.1 Å, it is likely that the ODTS molecules are slightly disordered or tilted at an angle. Because the ODTS islands have lower adhesion and stiffness than the surrounding areas, these surrounding areas can be identified as bare substrate and not disordered silane lying on the surface. The hydrophobic ODTS reduces the effect of capillary forces on the tip as it measures the adhesion. These strong capillary forces increase the attraction between the cantilever and substrate, which results in a higher adhesion force on the bare substrate. After backfilling, the mean height difference between the islands and surrounding area is reduced by 9 to 7.8 Å. The AMS has a maximum dimension
Table 1. Contact Angles before and after Backfilling (Predicted ODTS Coverage) sample
static
advancing
receding
ODTS islands ODTS islands with AMS
83 ( 3 (63%) 94 ( 4 (61%)
96 ( 1 99 ( 3
71 ( 4 83 ( 1
of 9.6 Å, and the AMS does not form multilayers, so this result is reasonable. The adhesion images show that the ODTS islands have lower adhesion than the surrounding areas both before and after backfilling. Even though an AMS-treated surface is a partial wetting surface, the AMS-treated surface is not hydrophobic and does not reduce the capillary forces as much as the ODTS. The capillary forces increase the interaction between the tip and the AMS causing greater adhesion in the AMS region than in the ODTS region. Stiffness images show a reduction in contrast between the islands and surrounding areas after backfilling. Changing from bare silicon oxide to aminosilane should decrease the stiffness because the long chain of the silane provides some flexibility. After backfilling, the ODTS islands are now more similar to their surroundings, so the stiffness data produce a more monochromatic image. The contact angle measurements also indicate that the surface has been modified with aminosilane. Table 1 compares the contact angle measurements of the samples shown in Figure 1. The ODTS coverage for this sample was measured to be 59%, which corresponds well to the Cassie-Baxter approximations of 63% before backfilling and 61% after backfilling based on measured contact angle. After backfilling, the ODTS coverage should remain the same, and surrounding areas should be functionalized by AMS. The contact angle should increase as the bare SiO2 regions are filled in with the lower-energy AMS. The advancing and receding contact angles also increase after backfilling. The advancing contact angle, which is dominated by the low-energy regions, increases only slightly. Because the ODTS is the lowest-energy region both before and after backfilling, the advancing contact angle would not be greatly influenced by the addition of AMS. Microcontact Printing with Vapor Deposition. Figure 2 shows the result of ODTS vapor deposited onto a silicon oxide substrate and microcontact printed ATS on a silicon oxide substrate. The vapor deposits of ODTS are small islands but not fractal islands like solutiondeposited ODTS. These islands are about 0.2 µm tall, which
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Figure 2. (top, left to right) Topography, adhesion, and stiffness PFM-AFM images of ODTS vapor deposited on SiO2. Scan size: 20 × 20 µm2. (bottom, left to right) Topography, adhesion, and stiffness AFM images of microcontact-printed ATS on SiO2. Scan size: 50 × 50 µm2.
Figure 3. (top, left to right) Topography, adhesion, and stiffness PFM-AFM images of microcontact-printed ATS on SiO2 with a 20 µm stamp and ODTS vapor deposited. Scan size: 50 × 50 µm. (bottom, left to right) Zoomed to the ATS/SiO2 border. Scan size: 20 × 20 µm (cropped).
is significantly greater than monolayer coverage. The contact angle of 93.3 ( 2.4° indicates there is enough ODTS coverage to render the surface partially nonwetting. The adhesion image also shows an interesting trend. The left part of the ODTS is darker than the surroundings, which is the expected result, but the right part of the ODTS spots shows higher adhesion. This result could be explained by the direction of the scan. The cantilever moves across the substrate from left to right. Since the left side of the island is the first area encountered by the cantilever, this region could show the correct trend. After scanning over the high peaks of the islands, the cantilever could be slightly out of alignment or unable to correctly image the sharp, downward slope of the islands, resulting in the high-adhesion areas that were observed. Aminosilanes were microcontact printed on a surface; the horizontal lines in the adhesion and stiffness images are artifacts of the AFM and should not be considered features. The aminosilane regions have lower adhesion and stiffness than the bare substrate. The mean height difference between the ATS region and the surrounding area is about 36.3 Å, which would correspond to three layers of aminosilane. Figure 3 shows a heterogeneous surface created by vapor depositing ODTS on a microcontact-printed ATS substrate.
The 20 µm ATS pattern can be seen in all three AFM images, although it is hard to distinguish the ATS pattern in the adhesion image. It was expected that the ODTS would deposit as islands as shown in Figure 2. In the magnified images, many little dots, which are the ODTS, can be seen on the surface. The ODTS deposited over the entire surface and did not show a preference for the bare SiO2 area. However, in terms of moisture resistance and adhesion promotion, it may not be detrimental that the ODTS deposited on top of the ATS. If the ODTS does deposit in these discrete blobs while leaving the rest of the ATS exposed, then the exposed ATS could still bind to the substrate and polymer to promote adhesion. The ODTS would not be bound to the substrate since it is on top of the ATS, but since it does not add to the adhesion, this bond is not necessary. Therefore, the technique might work for creating the desired surface. Full monolayers of ATS could be created instead of the stamped patterns to create full coverage and stronger adhesion. Conclusions In this work, we have created heterogeneous patterned silane films using the methods of sequential adsorption from solution and microcontact printing with vapor deposition. These films were characterized using AFM-
Notes
PFM. Contact angle goniometry was used to provide additional verification of surface coverage for surfaces with low ODTS coverage. AFM-PFM was used to image the heterogeneous surfaces. Islands of ODTS have lower adhesion and lower stiffness than the surrounding areas, which confirms previous research that the surrounding areas are bare substrate. After solution backfilling with AMS, the height of the ODTS regions decreased, and the stiffness image was monochromatic, which indicates that the surface was functionalized by a second silane. The vapor deposition of ODTS resulted in large islands which were significantly greater than monolayer height.
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The height of these islands resulted in mixed adhesion measurements, but the stiffness data showed that the ODTS region has lower stiffness than the surrounding areas. After vapor backfilling on a stamped ATS surface, both the adhesion and stiffness showed little distinction between the silanes because the vapor-deposited ODTS covered both the bare substrate and amino-treated regions. Acknowledgment. The authors gratefully acknowledge the financial support of the Boeing Airplane Company. LA030246S
