Langmuir 1994,10, 1171-1176
1171
X-ray and Ellipsometric Studies of Self-Assembled Monolayers of Fluorinated Chlorosilanes R. E. Geer, D. A. Stenger, M. S. Chen, J. M. Calvert, and R. Shashidhar' Center for BiolMolecular Science and Engineering, Code 6900, Naval Research Laboratory, Washington, D. C. 20375-5348
Y. H. Jeong and P. S. Pershan Department of Physics and Division of Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 Received November 2, 1993. I n Final Form: January 31,1994' Results of X-ray and ellipsometric studies on self-assembled monolayer films of three fluorinated chlorosilanesare presented. By inferring an index of refraction, the monolayer thicknesses obtained from both these types of studies are self-consistent. The tilt of the molecules with respect to the layer normal is found to be strongly dependent on the functionality of (i.e. the number of chlorine atoms attached to) the terminal Si atom of the molecule.
Introduction Chemisorbed organosilane monolayer films, formed by molecular self-assembly, are widely used for modifying surfaces with organic functional g r o ~ p s . l *Lithographic ~ patterning of Qrganosilane self-assembled monolayer (SAM) films3*' has been demonstrated to be useful in providing reactive templates for selective attachment and buildup of various materials from the surface. This allows the creation of surfaces with tailored physical, chemical, and biological properties that are useful for microelectronics, sensors, and many other applications. Detailed knowledge of the thickness and structure of the SAM films is therefore of considerable importance. A number of studies have been performed to determine the thickness and structure of SAMs; most of these structuralstudies have involved alkanethiols adsorbed on gold6 or long chain alkylsiloxane films on silica- and golds surfaces. Wasserman et al.Io and Tidswell et al." have used X-ray reflectivity to studythe electron density profiles of several organosilane SAMs, including one that was partially fluorinated and one that was terminated with bromine. Stenger et al.I2 have used ellipsometry to evaluate the thickness of organosilane SAMs with amine ~
Abstract published in Advance ACS Abstracts, March 15,1994. (1) Ulman, A. An Introduction to Ultrathin Organic Films: Fron Langmuir-Blodgett to Self-Assembly;Academic Press: San Diego, CA, 1991. (2) Leyden, D. E., Ed. In Silanes, Surfaces and Interfaces; Gordon, Breech and Harwood: New York, 1986; Vol. 1. (3) Dulcey, C. 5.;Georger, J. H.; Krauthamer, V.; Fare, T. L.; Stenger, D. A.; Calvert, J. M. Science 1991,262,551. (4) Calvert, J. M. In Organic Thin Films and Surfaces; Ulman, A., Ed.; Academic Press: San Diego, CA, 1993. (5) A recent review of alkanethiola on gold is given in ref 1, p 288. (6) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (7) S i l b e m , P.; Leger, L.; Aussere,D.;Benattar,J. J. Langmuir 1991, 7, 1647. (8) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100,465. (9) Finklea, H. 0.; Robinson, L. R.; Blackburn,A.; Richter, B.;Allara, D.; Bright, T. Langmuir 1986,2, 239. (10) Tidswell, I. M.; Ocko, B. M.; Perahan, P. S.; Wassennan, S. R.; Whitesides, G. M.; Axe, J. D. Phys. Reu. B 1992,41,1111. Tidswell, 1. M.; Rabedeau, T. A.; Perahan, P. S.; Koeowsky, S. D.; Folkers, J. P.; Whiteaides, G. M. J. Chem. Phys. 1991,96,2854. (11) Wasserman,S. R.;Whitesides,G.M.;Tidewell,I. M.;Ocko,B.M.; Perahan, P. S.; Axe, J. D. J. Am. Chem. SOC.1989,111,5852. (12)Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph,A. 5.;Nielaen, T. B.;McCort, S. M.; Calvert,J. M. J. Am. Chem. SOC. 1992, 14, 8435. @
and fluorinated R groups. We present here detailed X-ray reflectivity and ellipsometry measurements for SAMs of three different fluorinated organosilane films. We show that for the SAMs being investigated here, the structure strongly depends on the functionality of the organosilane molecule. The trifunctional materials, which have the molecular composition R-SiC13 (where R is the fluoroalkyl group), produce SAM films whose thickness and density imply little molecular tilting from the layer normal. In contrast, the monofunctional silanes, of composition R-Si(CH&Cl produce SAMs for which the density and thickness are consistent with a molecular tilt of approximately 30°. We also present a detailed discussion concerning the proper choice of refractive index for ellipsometric measurements on ultrathin films.
Monolayer Preparation Native oxide Si(l00) wafers were n-type (P-doped; 5.07.3 cm) 2 in. diameter from Wafernet, Inc. These wafers were found to have an average surface roughness of 2-3 A as determined by atomic force micro~copy.~3 Substrates were cleaned prior to film formation using the following procedure: The wafers were soaked for a t least 30 min in a 1:l (v/v) mixture of HC1-methanol, removed from the acid, and rinsed copiously with deionized water (18 Ma resistivity from a Nanopure still). The wafers were then soaked in concentrated sulfuric acid for a t least 30 min, removed from H2S04, rinsed thoroughly with DI water, and transferred to a boiling water bath until required for film formation. Handling of wafers during cleaning and film formation was performed in a Class 100 clean room facility to the greatest extent possible. Organosilane precursors (tridecafluoro-1,1,2,2-tetrahydroocty1)dimethylchlorosilane (13F) and (tridecafluoro1,1,2,2-tetrahydrooctyl)trichlorosilane(13F-T) for selfassembled monolayer film formation were obtained from Huls America and were used as received. lH,lH,W,WPerfluorodecyldimethylchlorosilane(17F) was obtained from PCR, Inc. Chemical structures of the fluorinated organosilanes are shown in Figure 1. Films of 13F were prepared as follows: A 1%(v/v) solution of the 13F (13) Fare, T. L.; Palmer, C. A.; Silvestre, C. G.; Cribbs, D. C.; Turner, D. C.; Brandow, S. L.; Gaber, B. P. Langmuir 1992,8, 3116.
0743-746319412410-1171$04.50/0 0 1994 American Chemical Society
Geer et al.
1172 Langmuir, Vol. 10, No. 4,1994
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Figure 1. Chemical structures of the fluorinated organosilanes. organosilane in anhydrous toluene (Aldrich SureSeal grade) was prepared in a He-filled drybox. The cleaned substrates described above were removed from the hot water bath and dried in a stream of filtered Nz to remove excess water from the hydrophilic surface. Substrates were placed on a quartz wafer carrier rack and then both the substrates and silane solution were transferred to an Arpurged glovebag (12R Industries). The substrates were then immersed in the 13F solution for 60 min. The substrates were removed from the 13F solution, rinsed twice with fresh toluene, and baked for 4 min at 120 "C on a VWR Series 400 HPS programmable hot plate to complete siloxane bond formation to the substrate. Films of 13F-Twere prepared using a procedure similar to that for 13F films, except that the clean, dry substrates were ' solution of 13F-T in toluene for 5 min immersed in a 1% for the silanization step. 17Ffilms were obtained following the same procedure as for 13F films. Contact angle measurements of the SAM films were obtained using the sessile water drop method on a NRL Zisman-type contact angle goniometer. Values of 91°, 103", and 91" were obtained for 13F,13F-T,and 17F films, respectively. These values are consistent with previously reported values for fluorinated organosilane filmsS4 Ellipsometry Measurements All ellipsometric thickness measurements were performed using a Gaertner Model L115C ellipsometer equipped with a helium-neon laser (632.8nm) light source. The angle of incidence was 70" and the compensator set at -45". Because silicon substrates have a native oxide layer, a three-layer model could conceivably be used to provide the most accurate description of the oxide and organic thin film thicknesses. In this work, we determined the oxide and film thicknesses separately, using a twolayer model in each case. In both cases, the thickness was measured using Gaertner Waferscan software, which executes a computational enhancement14of the algorithm originally described by McCrackin et al.15 Ellipsometric measurements can be used to simultaneously determine the thickness and refractive index of layers having a thickness of >200 A. However, measurement of thinner films requires that one of these values be somewhat arbitrarily assumed in order to determine the other.2J6 Thicknesses determined by arbitrarily fixing the index of refraction have an inherent limitation in accuracy of f2 A.l0J6 All ellipsometric thicknesses reported in this paper are assumed to be limited to this accuracy. (14) Reinberg, A. R. Appl. Opt. 1972,11, 1273. (15) McCrackin, F. L.; Passaglia, E.; Stromberg, R. R.; Steinberg, H. L. J. Res. Natl. Bur. Stand., Sect. A 1963,67, 363. (16) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North Holland Publishing Co.: Amsterdam,1977, and references therein.
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Figure 2. Schematicof X-ray scatteringapparatus. Path lengths between the various components are identical to Figure 2 of ref 10.
The thickness of the native oxide on the wafers was determined using values for the real (Ita)and imaginary ( K S ~ )components of the silicon substrate refractive index, v s i = 3.85 and KSi = -0.02. The thickness of the surface oxide was determined by assuming a corresponding refractive index of 1.5. Individual substrates had measured oxide thicknesses of 19-22 f 1A. The thicknesses of the chemisorbed monolayers were also determined using a two layer model where the single refractive index measured for the substrate accounted for contributions from both the bulk Si and the native oxide layers.1°J6 These optical constants were determined for each substrate following cleaning but prior to film deposition. The majority of work done so far on the ellipsometric determination of the thickness of ultrathin organic films has been performed on well-ordered SAMs formed from long chain (110 carbons) alkanethiols and alkylsiloxanes, which lend themselves to the assumption that the monolayers are both uniform and homogeneous. In these cases, the assumption of a refractive index (n)value of 1.45-1.50 (as found for bulk paraffins) has repeatedly proven justifiable, as confirmed by a variety of t e c h n i q u e ~ . l ~ J ~ - ~ Few ellipsometric thickness measurements have been performed on SAMs formed from shorter chain (
