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Investigations on the Influence of Adhesion, Cohesion, and External Forces on the Formation of Ultrathin Polymeric Films T. Gesang,* W. Possart, O.-D. Hennemann, and J. Petermann Fraunhofer Institut fu¨ r angewandte Materialforschung, Neuer Steindamm 2, D-28719 Bremen, Germany Received November 28, 1995. In Final Form: March 1, 1996X The birth and growth of ultrathin polymer films out of solution were studied by means of atomic force microscopy (AFM). The system investigated consisted of a model substance for a curable high-temperature adhesive (a prepolymer of the dicyanate of bisphenol A) and as received silicon wafers or aluminum evaporation coatings. The application techniques, dip-coating and spin-coating, allow film formation either in nearly equilibrated conditions or with the influence of centrifugal acceleration, respectively. AFM reveals significant differences between the films depending on the application technique and the substrate, but in some instances there are surprising similarities. The comparative interpretation of the findings results in the proposal of a model for the film formation of the investigated systems.
Introduction There is a 2-fold interest in investigating the formation of ultrathin polymeric films on application-oriented substrates. Firstly, adhesion science and technology requires microscopic knowledge of the development of interface and interphase of adhesive joints. Secondly, thoroughly controlled fabrication of highly defined ultrathin polymer films is of increasing importance in some technical fields such as microelectronics and sensors. Adsorption and formation of ultrathin films have been successfully studied for greatly idealized substrate/ adsorbate systems in ultrahigh vacuum (UHV). Since the advent of atomic force microscopy (AFM), a growing number of studies have been performed for more application-oriented systems in ambient or liquid environment with both inorganic1 and organic films. Organic molecules can be applied to substrate surfaces in various ways. Application processes such as dip-coating, allowing unperturbed adsorption out of solution, can result in selfassembly monolayers (SAMs). Characteristic features of this process are that the film formation happens spontaneously without external driving forces and that the solute molecules possess a high mobility.2 Competing influences are solute-solute, solute-solvent, and solutesubstrate interactions plus entropic effects. If the contact time of the solution with the substrate is sufficiently long, the resulting films can represent a molecular equilibrium arrangement and exhibit good thermal, chemical, and mechanical stability.3 For example, the investigation of SAMs of n-octadecyltrichlorosilane, n-propyltrichlorosi* Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) George, M.; Burger, A.; Collins, W. E.; Davidson, J. L.; Barnes, A. V.; Tolk, N. H. J. Appl. Phys. 1994, 76 (7), 4099. Kanniainen, T.; Lindroos, S.; Prohaska, T.; Friedbacher, G.; Leskela, M.; Grasserbauer, M.; Niinisto, L. J. Mater. Chem. 1995, 5 (7), 985. Sum, R.; Lang, H. P.; Gu¨ntherodt, H. J. Physica C. 1995, 242 (1-2), 174. Cotell, C. M.; Horwitz, J. S.; Sprague, J. A.; Auyeung, R. C. Y.; Dantonio, P.; Konnert, J. Mater. Sci. Eng., B 1995, 32 (3), 221. Hu, J.; Xiao, X. D.; Ogletree, D. F.; Salmeron, M. Science 1995, 268 (5208), 267. (2) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Acedemic Press: San Diego, CA, 1991. (3) Ulman, A. In Characterization of Organic Thin Films; Ulman, A., Ed.; Butterwoth-Heinemann: Manning, Boston/Greenwich, 1995; Chapter 1, p 21. Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. Shnidman,Y.; Ulman, A.; Eilers, J. E. Langmuir 1993, 9, 1071.
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lane, and n-triacontyltrichlorosilane films on hydroxylated Si(100) revealed details of the respective growth mechanisms and their dependence on the organic molecule’s chain length.4 Adsorption of octadecanethiol on a cleaved GaAs(110) surface was found to protect the surface from being oxidized by the solvent (ethanol).5 A photo reorientation of azobenzene molecules on solid supports resulting in two-dimensional polycrystals was observed, with unpolarized irradiation probably involving material transport over micrometer distances.6 Gelatin films on mica can show two distinct phases with characteristic frictional, morphological, and adsorptive signatures which change upon heating.7 Films of N-octyl-D-gluconamide on mica and graphite were found to grow independently of substrate topography indicating relatively strong intermolecular interactions. Film thickness and molecular packing differ from the one in the corresponding organic crystal.8 However, the fundamental understanding of self-organization phenomenon is still very limited3seven on the single crystalline substrates primarily investigated. More complex adsorbate systems result in more intricate film formation processes. A polystyrene/ poly(vinyl methyl ether) blend on hydrophilic SiO substrates exhibited a film thickness dependent phaseseparation temperature.9 A polystyrene-block-polybutadiene on silicon wafers shows different structural phases as a function of surface coverage.10 Another widely used fabrication technique for organic films is spin-coating.11,12 There are a number of empirical investigations on the formation of spin-coating films. However, the microscopic mechanisms are particularly complex due to the additional presence of the centrifugal (4) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11 (6), 2143. (5) Ohno, H.; Motomatsu, M.; Mizutani, W.; Tokumoto, H. Jpn. J. Appl. Phys. Part 1 1995, 34 (2B), 1381. (6) Schonhoff, M.; Chi, L. F.; Fuchs, H.; Losche, M. Langmuir 1995, 11 (1), 163. (7) Haugstad, G.; Gladfelter, W. L. Langmuir 1994, 10 (11), 4295. (8) Tuzov, I.; Cra¨mer, K.; Pfannemu¨ller, B.; Kreutz, W.; Magonov, S. N. Adv. Mater. 1995, 7 (7), 656. (9) Tanaka, K.; Yoon, J. S.; Takahara, A.; Kajiyama, T. Macromolecules 1995, 28 (4), 934. (10) Siqueira, D. F.; Kohler, K.; Stamm, M. Langmuir 1995, 11 (8), 3092. (11) Meyerhofer, D. J. Appl. Phys. 1978, 49, 3993. (12) Weill, A.; Dechenaux, E. Polym. Eng. Sci. 1988, 28, 945.
© 1996 American Chemical Society
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Figure 1. AFM image of an as received silicon wafer: scan size, left 500 nm2, right 200 nm2; gray scale, 1.0 nm (depressions, dark; peaks, bright).
acceleration and the according knowledge is severely confined.13 To gain understanding on a microscopic level, research on the initial stages of adsorption and growth is of major importance. As can be expected, they depend both on the system solute/solvent/substrate and on the application parameters. Spin-coating films often represent a glasslike nonequilibrium state.14 For different polymer films, such as poly(methyl methacrylate), polystyrene, natural rubber, and poly(styrene-b-methyl methacrylate) block copolymer, spun onto silicon wafers, the mean film thickness to achieve continuous films was found to vary from 2 to 30 nm.15,16 Discontinuous films may be comprised of islands or network structures.17 In between the detected islands or structures there may be further, small adsorbates. Studies on other growth processes are also reported. To name but a few results, the growth kinetics of electrochemically deposited poly(phenylene oxide) films on gold was characterized to commence with a rapid deposition followed by mass transport controlled growth.18 Plasma polymer films of monomers, such as hexamethylcyclotrisiloxane, hexamethyldisiloxane, and pyrrole, can be smooth and pinhole-free with only 2-10 nm thickness.19,20 AFM inherently has no access to the actual interface between film and substrate since it images the surface. Approaches to image the “interface” after removal of substrate or film21 bear the uncertainty of how the original interface was modified due to the removal process. The focus of this paper is on AFM investigations on the initial stages of organic film formation on inorganic substrates for application-oriented systems. To this end, films of increasing thickness (
