Structure and Stability of Patterned Self-Assembled Films of

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Langmuir 1997, 13, 3382-3391

Structure and Stability of Patterned Self-Assembled Films of Octadecyltrichlorosilane Formed by Contact Printing Noo Li Jeon,† Krista Finnie,‡ Kimberly Branshaw,‡ and Ralph G. Nuzzo*,†,‡ Department of Materials Science & Engineering, School of Chemical Sciences, and The Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received February 18, 1997. In Final Form: April 14, 1997X The structures of thin films of octadecyltrichlorosilane (OTS) formed by contact printing and adsorption from solution on Al and SiO2/Si surfaces have been investigated by X-ray photoelectron spectroscopy (XPS), ellipsometry, reflection-absorption infrared spectroscopy (RAIRS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The structures of the OTS films were found to be strongly influenced by the method of preparation used. The films formed by contact printing for 30 s with an OTS “ink” are composed of close-packed, predominantly all-trans alkyl chains aligned nearly perpendicular to the surface, while films formed by adsorption of OTS for 30 s from a solution of comparable concentration are made up of sparsely adsorbed and more randomly oriented chains. Large changes in the film structure (from conformationally disordered, poorly oriented to well-packed, highly oriented hydrocarbon chains) were observed as the reaction time was increased for samples prepared by immersion in an OTS-containing solution (e.g. 15-30 min in a 1 mM solution). On the other hand, films prepared by contact printing reached or exceeded full monolayer mass coverages after only 30 s of substrate contact with an elastomeric stamp inked with a 10 mM solution of OTS in hexane. The OTS films formed by contact printing are stable at the high temperatures and aggressive reaction conditions necessary for their use as molecular resists in directing the selective deposition of metal and ceramic thin films by MOCVD and sol-gel methods, respectively. In this paper we show how such factors as the OTS ink concentration and stamp contact time influence the patterning of OTS thin films by microcontact printing. We find that reactive spreading and island formation of OTS domains in regions of the pattern not in contact with the stamp limit the fidelity of the patterning carried out at dimensions less than a few microns. We discuss several aspects of the physical processing which mediate these effects and propose possible methods for eliminating or greatly reducing them.

Introduction Self-assembled monolayers (SAMs) of organic molecules have evolved as a new class of thin film materials with exciting prospects for applications in materials chemistry.1-4 A significant body of research on SAMs has been motivated by their potential application as building blocks for supramolecular assemblies with engineered chemical5,6 and optical7,8 properties. Even with this continuing interest, SAMs have yet to find significant applications in current commercial device fabrication processes. Nevertheless, there have been several promising recent applications in microfabrication which use SAMs for surface modification and engineering. Calvert and co-workers3,9,10 have lithographically patterned photosensitive silane derivatives and used these latent images †

Department of Materials Science and Engineering. School of Chemical Sciences. X Abstract published in Advance ACS Abstracts, June 1, 1997. ‡

(1) Whitesides, G. M.; Gorman, C. B. In SAMs: Models for Organic Surface Chemistry; Hubbard, A. T., Ed.; CRC Press: Boca Raton, FL, 1995; pp 713-733. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (3) Calvert, J. M. In Lithographically Patterned Self-Assembled Films; Ulman, A., Ed.; Academic: San Diego, CA, 1993; Vol. 20, pp 109-141. (4) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic: San Diego, CA, 1991. (5) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 3191. (6) Schierbaum, K. D.; Weiss, T.; Reinhoudt, D. N.; Goepel, W. Science 1994, 265, 1413-1417. (7) Lin, W.; Yitzchaik, S.; Lin, W.; Malik, A.; Durbin, M. K.; Richter, A. G.; Wong, G. K.; Dutta, P.; Marks, T. Angew. Chem., Int. Ed. Engl. 1995, 34, 1497-1503. (8) Li, D. W.; Ratner, M. A.; Marks, T. J.; Zhang, C. H.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389-7395. (9) Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Fare, T. L.; Stenger, D. A.; Calvert, J. M. Science 1991, 252, 551-554.

S0743-7463(97)00166-2 CCC: $14.00

as both resist and promoter elements in model device fabrication processes. Reports from Whitesides’ group11-14 have extensively documented the utility of alkanethiol SAMs on Au which have been patterned by microcontact printing (µCP) to serve as wet chemical etch resists. Nanopatterning of SAMs by micromachining has also been reported by Crooks et al.15 Other reported applications of SAMs include the use of contact printed alkylsiloxane monolayers to direct selective deposition of metal thin films by CVD16,17 and ceramic thin films by sol-gel methods.18 Microcontact printing (µCP) is a novel method of depositing patterned SAMs of thiols and silane-based molecules.11-13,16,19 Printing offers many potential advantages as a patterning method, not the least of which are high throughput, adaptability to large format substrates, and insensitivity to the optical constraints that significantly limit future developments in lithography.14 Although the ability of µCP to form patterned high-quality SAMs is now well established (at least for laboratory(10) Potochnik, S. J.; Pehrsson, P. E.; Hsu, D. S. Y.; Calvert, J. M. Langmuir 1995, 11, 1841-1845. (11) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188-9189. (12) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 20022004. (13) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1499-1511. (14) Xia, Y.; Zhao, X.; Whitesides, G. M. Microelectron Eng. 1996, 32, 255-268. (15) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632-636. (16) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024-3026. (17) Jeon, N. L.; Lin, W.; Girolami, G. S.; Nuzzo, R. G. Manuscript in preparation. (18) Jeon, N. L.; Clem, P. G.; Payne, D. A.; Nuzzo, R. G. J. Mater. Res. 1995, 10, 2996-2999. (19) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576-9578.

© 1997 American Chemical Society

Patterned SAMs of Octadecyltrichlorosilane

scale demonstrations), the structures of the films formed by contact printing have not been studied extensively. In order for these organic thin films to be successfully exploited as surface modifiers in applications involving the patterned deposition of thin films by CVD and sol-gel processing (both of which are strong interests of our current research), detailed understandings of this structure and other properties (especially viz. thermal stability) is essential. In addition, the effects of the various processing parameters governing film formation by printing (such as concentration, solvent, and time of contact) remain poorly understood and thus need to be thoroughly investigated. SAMs of organosulfur compounds on metal surfaces such as Au and Ag have been extensively studied.1,2,4,20-23 This class of SAM, one based on a fairly simple chemical adsorption process, has been studied by numerous physical and spectroscopic techniques, and their structure is now well understood. The structures of monolayers of organosilanes (such as octadecyltrichlorosilane (OTS)) adsorbed on oxide surfaces like glass, Al2O3, SiO2, and ITO also have been investigated by numerous groups,4,22,24-47 but due to their extreme sensitivity to the methods of preparation used, their character remains less well understood. In general, it is found that the OTS chains can pack with densities approaching those found in bulk hydrocarbon crystals but that even the “highest quality” phases lack the long-range translational ordering found for thiolate SAMs on Au and Ag. It thus is well appreciated that the reproducible formation of alkyltrichlorosilane monolayers is a signifi(20) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45-52. (21) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (22) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (23) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (24) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (25) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852-5861. (26) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120-1126. (27) Tripp, C. P.; Hair, M. L. Langmuir 1994, 11, 1215-1219. (28) Thompson, W. R.; Perberton, J. E. Langmuir 1995, 11, 17201725. (29) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647-1651. (30) Ruhe, J.; Novotny, V. J.; Kanazawa, K. K.; Clarke, T.; Street, G. B. Langmuir 1993, 9, 2383-2388. (31) Ohtake, T.; Norihisa, M.; Kazufumi, O. Langmuir 1992, 8, 20812083. (32) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607-3614. (33) Le-Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749-1753. (34) Guyot-Sionnest, P.; Superfine, R.; Hunt, J. H.; Shen, Y. R. Chem. Phys. Lett. 1988, 144, 1-7. (35) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201-213. (36) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236-2242. (37) Tillmann, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136-6140. (38) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143-2150. (39) Bierbaum, K.; Kinzler, M.; Woll, C.; Grunze, M.; Hahner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512-518. (40) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304-1312. (41) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. Vib. Spectrosc. 1995, 8, 151-157. (42) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. J. Mol. Struct. 1995, 349, 305-308. (43) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577-7590. (44) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357-2360. (45) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054-3056. (46) Garoff, S.; Hall, R. B.; Deckman, H. W.; Alvarez, M. S. Proc. Electrochem. Soc. 1985, 85-8, 112. (47) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532-538.

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cant problem given that the quality of the film varies sensitively with the reaction conditions (e.g. temperature, adsorption time, solvent, water content of adsorbate solutions, and laboratory ambient) and varies widely from one laboratory to another.24-36,43,48 Despite extensive study, relatively little is known about the dependence of the structure of the alkylsilane monolayer on its formation mechanism. It is generally believed that the adsorption of alkyltrichlorosilanes and other alkylsilanes with hydrolyzable bonds proceeds on hydrated surfaces via the formation of silanols as intermediates, which then react in turn laterally or with surface OH groups to form a network polymer which is covalently bound (in some degree) to the surface.26,27,33,35,36,47,48 The resulting films have significant mechanical, thermal, and chemical stability. Infrared spectroscopy, ellipsometry, and contact angle measurements indicate a high degree of structural organization in these films. Recently, Allara et al. reported that OTS monolayers on SiO2 and oxidized gold surfaces have identical film structures, ones in which the largely all-trans alkyl chains tilted at ∼10 ( 2° from the surface normal direction.43,44 They suggest that surface hydration is responsible for decoupling the film formation (polymerization) from the surface chemistry and results in comparable high-quality films being formed on these widely disparate substrates. Hoffmann et al.40-42 have reported structural information about alkylsiloxane monolayers formed on silicon surfaces. Using reflection absorption infrared spectroscopy (RAIRS), they observed highly oriented, crystallinelike structures for long-chain molecules, including OTS, in which the hydrocarbon chains are tilted by an average of ∼8° from the surface normal direction. They also demonstrated that RAIRS on Si has a unique sensitivity for differentiating between liquid-like regions and oriented, crystalline-like domains in partially disordered hydrocarbon thin films. In this study we concentrate our characterization efforts on OTS films formed by contact printing. The structures of these OTS films are compared to those of OTS films prepared by dipping the substrate in a dilute OTS solution. We have also examined the effects of other processing variables such as the concentration of the OTS solution, solvent effects, and deposition time. The relationship between the OTS film quality and its suitability for potential applications involving patterning of thin films by CVD and sol-gel methods is also discussed. Experimental Section Materials and OTS Film Formation. The substrates used for the experiments were Si(100) wafers (test grade, p-type, resistivity between 15 and 50 µΩ) purchased from Silicon Sense (Nashua, NH). The Al substrates (2000 Å) were prepared by DC magnetron sputtering onto Si(100) wafers. The substrates were cut into 2 × 2 cm2 pieces for further modification by either adsorption or contact printing. The Al substrates were used immediately after deposition. For the preparation of OTS SAMs on Si, the substrates were cleaned by first removing the native oxide by dipping it in a dilute HF solution (2% in deionized water) for 1 min (the oxide subsequently regrowsssee below). Then, after repeated cleaning with purified deionized water, acetone, and isopropyl alcohol (2-3 times each in sequence), the samples were dried in a stream of high-purity Ar (99.99%) gas. Both the Al and the HF-treated Si substrates were exposed in a UV/ozone chamber for 30 min immediately prior to OTS adsorption and contact printing. The UV/ozone generator was a home-built minichamber equipped with a low-pressure mercury lamp (λ ) (48) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367-4373.

3384 Langmuir, Vol. 13, No. 13, 1997 185 and 254 nm). This treatment removes residual hydrocarbon impurities and yields hydrophilic surfaces.49 The OTS films were formed by immersing the freshly cleaned substrates in freshly prepared solutions of octadecyltrichlorosilane (Aldrich, Milwaukee, WI) dissolved in either hexane, hexadecane, or toluene (Aldrich, Milwaukee, WI). Contactprinted OTS films were obtained by spin casting the OTS solution onto a polydimethylsiloxane (PDMS) stamp using a conventional photoresist spinner (Headway Research, Garland, TX) at 3000 rpm for 30 s. A spinner was used to apply the solution uniformly without the risk of contaminating the surface with particles. The wetted ("inked") PDMS stamp was then dried with a stream of Ar for 30 s, after which it was brought in contact with the substrate for 10-60 s. The PDMS stamps used for both wide area planar and microcontact printing were fabricated according to a previously reported procedure.11-13 X-ray Photoelectron Spectroscopy (XPS) Experiments. XPS data for OTS samples were obtained with a Perkin-Elmer/ PHI XPS 5400 spectrometer using Mg KR radiation (15 kV, 400 W). Photoelectrons were energy analyzed using a hemispherical analyzer at a constant pass energy of either 35.7 or 17.9 eV. Ellipsometry. Ellipsometry measurements were performed with a Gaertner Scientific (Chicago, IL) Model L116C ellipsometer employing a He-Ne laser, set at an angle of incidence of 70°. Measurements were made at 5-10 points randomly picked across the sample and averaged. A two-layer transparent film model was used for the thickness calculations. The refractive index of the OTS film was fixed at 1.5. Reflection Absorption Infrared Spectroscopy (RAIRS) Experiments. A grazing angle, reflection absorption infrared spectroscopy setup was used with the IR beam from a conventional Fourier transform spectrometer (Bio-Rad FTS-60) incident on the sample at a grazing angle of ∼84°. The reflection optics were optimized at ∼f/12, and the incident radiation was ppolarized. The signal was detected by a liquid-nitrogen-cooled, narrow band MCT detector. The spectra were recorded at a resolution of 4 cm-1, requiring 512 (OTS/Al) to 4096 (OTS/SiO2/ Si) scans to obtain acceptable S/N ratios. Freshly cleaned substrates without the OTS film were used to take the reference spectra, and the data are reported in the absorbance format (-log R/R0). Secondary Electron Microscopy (SEM) Experiments. SEM micrographs of OTS films were acquired using a Hitachi S-800 microscope. Standard operating conditions involved a sample stage tilt angle of ∼30° and an accelerating voltage of 20 kV. Atomic Force Microscopy (AFM) Experiments. A Topometrix Explorer scanning probe microscope was used in acquiring AFM images of the OTS films on silicon. A standard (narrow) Si3N4 tip was used, and scanning was carried out in the contact mode (the contact force was minimized to values just sufficient to maintain feedback).

Results and Discussion X-ray Photoelectron Spectroscopy of ContactPrinted OTS Films. Figure 1 shows representative XPS survey spectra of OTS thin films formed by contact printing on SiO2/Si substrates bearing a thick (1000 Å) thermal oxide. The contact printing was effected using a planar stamp in contact with the substrate for 30 s; varying concentrations of OTS in hexane, as indicated in the legend, were used to ink the stamp. Across this series one notes that the increase in the C(1s) peak intensity (284.5 eV) seen with increasing OTS concentration correlates directly with the increasing attenuation of the O(1s) core level signal. The data thus suggest that the mass coverage of the OTS transferred by the stamp varies directly with concentration of the ink. This result agrees qualitatively with the ellipsometry data discussed below, which also suggest the formation of a thicker OTS film when a higher concentration of OTS is used in the contact printing. As expected, the intensities of all core levels associated with the substrate [i.e. Si(2s) (155 eV), Si(2p) (49) Vig, J. R. J. Vac. Sci. Technol., A 1985, 3, 1027-1033.

Jeon et al.

Figure 1. Survey XPS spectra of OTS films formed by contact printing on SiO2/Si (1000 Å thick thermal oxide) substrates. The contact printing was performed with a planar stamp in contact with the substrate for 30 s using varying concentration of OTS in hexane, as indicated (the intensities are shown on the same scale).

(103 eV), and O(1s) (531 eV)] decrease with the increasing mass coverages of the OTS films transferred across the series. These changes also mirror those seen in the intensities of the carbon KLL and oxygen KVV Auger transitions. Ellipsometry Results. Optical ellipsometry is a convenient technique for measuring the thickness of very thin layers and has been used extensively to study the nature of SAMs and Langmuir-Blodgett films. Figure 2 shows the thickness of SAMs on Si/SiO2 as a function of the OTS concentration in hexane for films prepared either by immersion in solution or by contact printing with a planar stamp for 30 s. It is a striking difference between the two methods of OTS film preparation that the films formed by adsorption from solution are much thinner than those prepared by contact printing. As judged from the literature, the incomplete formation of the adsorbed OTS monolayer in the former case is expected, since the reaction time was short compared to those of standard reported protocols (where samples were placed into the solution for 1-12 h).24,35,43,47 These low-coverage films are expected to be less coherently organized than would be a full monolayer.30,31 RAIRS experiments described in the following section clearly reveal the nature of the structural organization in OTS films formed by contact printing and solution immersion, respectively. As shown in Figure 2, the thickness of the films prepared by contact printing (here using a planar stamp as a transfer media) increases nonlinearly with the OTS concentration (in hexane) of the ink used. It is striking to note that the film formed by contact printing (i.e. for 30 s using a 10 mM OTS solution in hexane as an ink) has a thickness

Patterned SAMs of Octadecyltrichlorosilane

Langmuir, Vol. 13, No. 13, 1997 3385

Figure 2. Ellipsometrically determined thicknesses of OTS films on SiO2/Si formed by contact printing and solution immersion for 30 s at different OTS concentrations in hexane: b, contact printing; 9, solution immersion.

Figure 4. RAIR spectra of OTS films on Al in the C-H stretching mode region (2800-3000 cm-1) as a function of OTS concentration: (A) contact-printed and (B) solution-immersed films. The reaction time was held at 30 s for each. Mode assignments are described in the text.

Figure 3. Ellipsometrically determined thicknesses of OTS films formed by contact printing for 10, 30, and 60 s using 10 mM OTS in (b) hexane and (9) toluene.

(∼25 Å) close to that reported for a densely packed monolayer of OTS.4,24,25,43 RAIRS data discussed below confirm that these OTS films consist of highly oriented hydrocarbon chains. At higher concentrations, the films obtained by contact printing are clearly multilayers. We found that the material present in the multilayer is not strongly bound, since the thickness decreased markedly when the sample was washed with hexane. The films left after washing had mass coverages closer to that expected for a monolayer. The thickness of the OTS SAMs obtained by immersion of the SiO2/Si substrate in hexane solutions of varying concentrations (again for 30 s, Figure 2) also vary nonlinearly with the silane concentration. In this case however, the mass coverages measured never exceeded that of a monolayer. These layers, as judged by data presented below, consist of conformationally disordered OTS chains. The data shown in Figure 3 reveal that the mass coverage of OTS varies strongly with the reaction time for

either method of preparation. For purposes of comparison, an OTS concentration of 10 mM was used in each experiment. The thicknesses of OTS films prepared by contact printing increase significantly with the stamp contact time. For samples printed with a stamp contact time longer than 30 s, the thickness measured clearly indicates the formation of multilayers. As before, the material present in the multilayer is not strongly bound, since the thickness decreased markedly when the sample was washed with hexane; the films left after washing, again, had mass coverages closer to that expected for a monolayer. Structure of OTS Films Formed by Contact Printing and Immersion in Solution, As Deduced by RAIRS. Figure 4 shows RAIRS spectra in the C-H stretching region for OTS films on Al surfaces formed by contact printing (A) and immersion in solution (B) as a function of the OTS concentration used.50 The preparation time for each was held at 30 s. Both groups of spectra are characterized by the presence of three distinct sets of peaks, falling in the ranges 2851-2858, 2921-2830, and 2962-2967 cm-1. The bands seen in these regions are most reasonably attributed to symmetric methylene [υs(CH2)], antisymmetric methylene [υas(CH2)], and antisymmetric methyl [υas(CH3)] stretching vibrations, respectively. Samples prepared by contact printing in addition show a peak at ∼2980 cm-1, a frequency which corresponds to symmetric methyl [υs(CH3)] stretching (50) Because microcontact printing was carried out under ambient, laboratory conditions, with varying humidity, slight variations in thicknesses and structure were observed. (51) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85-116.

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Jeon et al.

Table 1. Mode Assignments for C-H Stretching Vibrations in the RAIRS Spectra of Octadecyltrichlorosilane (OTS) on Al peak frequencies (cm-1) contact printed mode

assignmenta

νas(CH3)ip νs(CH3) νas(CH2) νs(CH2) νs(CH3)FRb a

1 mM 2967 2927 2855 2937 (sh)

immersed in solution

10 mM

50 mM

1 mM

10 mM

50 mM

2867 2879 2919 2851 2937 (sh)

2966 2879 2919 2851 2937 (sh)

2967

2967

2967

2930 2857

2929 2858

2929 2859

Taken from ref 43. b Fermi resonance.

vibrations. The shoulders (2937 cm-1) evident on the highfrequency side of υas(CH2) peaks for OTS films prepared by contact printing correspond to Fermi resonances of υs(CH3). A closer examination of the spectra reveals that peak position, peak shape, and the relative intensities of the CH2 and CH3 stretching vibrations are markedly different between the two groups of samples (Table 1). The most dramatic and important differences in the data shown above for the two classes of samples are those seen in the methylene C-H symmetric (d+)52 and antisymmetric (d-)52 stretching modes. Using the samples obtained with 10 mM OTS solution/ink as a benchmark, we see that the bands for the sample prepared by adsorption from solution appear at much higher frequencies (near 2858 and 2929 cm-1) than those of the stamped samples (near 2851 and 2921 cm-1). The peak frequencies of the d+ and d- modes of alkyl chains are reported to fall in the ranges 2846-2850 and 2915-2920 cm-1 for alltrans extended chains while those for highly conformationally disordered chains appear as broad bands centered near 2856 and 2928 cm-1.55 The data thus suggest that the films prepared by contact printing (Figure 4A) are primarily made up of densely packed and presumably nearly all-trans extended chains, while the films prepared by adsorption of OTS from solution are chiefly composed of disordered chains with a more “liquid-like” structure. Frequency changes in (CH2) vibrations are not the only spectroscopic indicator of the degree of conformational order present in the various SAMs. The mass coverages measured by ellipsometry are also suggestive of the packing which may be present. Again using the 10 mM samples as a benchmark, ellipsometry data for OTS films formed by a 30 s immersion suggest that the chains are sparsely adsorbed. The RAIRS data, which show a nearly isotropic pattern of relative band intensities, are certainly consistent with a loosely packed structure containing a significant fraction of conformationally disordered chains. In contrast, the OTS film formed by contact printing for 30 s shows close packing and a more uniform orientation of the chains. This is most strongly argued by the significant dichroism seen in this spectrum. For example, the relatively weak intensities of the CH2 absorptions (as compared to those of the CH3 absorptions) can only be rationalized by a structure involving a nearly vertical orientation of the chains (an orientation which only weakly projects the d+ and d- transition moments along the Al surface normal direction and hence lowers their intensity in RAIR spectra). The concentration dependence of the data shown in Figure 4 is also quite marked, and we comment on this point briefly. For both methods of preparation, the mass (52) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395-406. (53) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334-341. (54) Hill, I. R.; Lewin, I. W. J. Chem. Phys. 1979, 70, 842-581. (55) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150.

coverage achieved varies directly (albeit not linearly) with the concentration of OTS in the ink/solution. For films made by contact printing, the structures formed always exhibit significant optical anisotropies (that is to say, the samples are characterized by spectra which suggest some significant degree of chain organization). This latter observation is most striking given that the mass coverages present in the 1 and 50 mM samples do not correspond to a single monolayer (less for the former and greater for the latter). We defer comment on this point to later. The interpretation of the data for films prepared by immersion is more straightforward. It is clear that the coverage of OTS increases with increasing OTS concentration in solution, as judged by ellipsometry, never exceeding a full monolayer of coverage. The changes seen in RAIR spectra thus reflect a varying coverage of conformationally disordered chains. We should note, at this point, that extending the time that the substrate remains in solution (e.g. from 30 s to 1 h or longer) leads to an increased coverage of OTS and a densification of the chain packing in the film. The coverage increases rapidly at first, but after about 15-30 min, it is almost complete and only a slight increase in the packing of the chains occurs after that. We should point out that this latter analysis can be argued on a more quantitative basis using the absolute intensities of the various C-H stretching modes measured for the OTS films. We find, for example, that the band intensities measured here for a saturation monolayer prepared by contact printing are in good agreement with those reported by other groups, who also have inferred a near perpendicular orientation of the hydrocarbon chains in a well-formed SAM.40,43 The disordered structure obtained by brief immersions in solution yields spectra which cannot be fit within these same quantitative constraints. The data, when taken together, suggest that some significant aspect of the mechanism of monolayer formation by contact printing differs from that involved in the interfacial polymerization of OTS, which occurs via adsorption from solution. In contact printing, the solvent is allowed to evaporate, at least in part. This should concentrate the OTS at the ambient (contacting) interface region of the stamp. It may well be, then, that the effects seen are simply ones due to mass-transfer differences. In any event, the net result is that printing is a much faster method to prepare high-coverage OTS SAMs than is adsorption from solution. To examine the substrate dependence of the SAM formation, one other surface was examined. Figure 5 shows RAIR spectra of OTS films on silicon substrates as prepared either by contact printing (A) or adsorption from solution (B) using OTS solutions/inks of varying concentration (in hexane). The reaction time was again held at 30 s. The most striking difference between these spectra and those shown in Figure 4 (both of which were obtained at grazing angle of incidence) is the appearance of

Patterned SAMs of Octadecyltrichlorosilane

Langmuir, Vol. 13, No. 13, 1997 3387 Table 2. Mode Assignments for C-H Stretching Vibrations in the RAIRS Spectra of Octadecyltrichlorosilane (OTS) on SiO2/Si peak frequencies (cm-1) mode assignmenta νas(CH3)ip νs(CH3) νas(CH2) νs(CH2) a

Figure 5. RAIR spectra in the C-H stretching region of OTS films on SiO2/Si substrates as a function of OTS concentration. Samples were prepared by (A) contact printing and (B) solution immersion. The reaction time was held at 30 s for each. Mode assignments are described in the text.

“negative” intensity peaks, especially for the various methyl group vibrations seen at frequencies above 2955 cm-1 [υas(CH3), ra- and rb-].40 These latter complex line shapes arise as a direct consequence of the optical properties of the Si substrates used.56 Such negative absorption band intensities have been analyzed theoretically57 and observed experimentally58-62 for external reflection spectra measured on nonmetallic substrates. Hoffmann et al.40-42 have developed a detailed structural understanding of alkylsiloxane monolayers on silicon surfaces based on such RAIRS data. They observed highly organized, “crystalline” structures for SAMs formed by longer-chain silane molecules (e.g. OTS), ones in which the hydrocarbon chains are tilted by an average of 8 ( 3° (56) On aluminum surfaces, the metal surface selection rule applies: p-polarized IR beams at grazing incidence can only excite vibrational modes with transition dipole moments perpendicular to the surface. Modes of dipole moments parallel to the surface cannot be excited and therefore are not detected. The band intensities in the reflection spectrum are proportional to the perpendicular components of the molecules’ vibrational transition dipole moment. Therefore, for alkanethiol SAMs spectra on gold, the hydrocarbon chain tilt angle can only be determined from the absolute intensities of the υ(CH2) absorption. On silicon surfaces, a sizable electric field exists both normal and parallel to the surface; thus, both the parallel and perpendicular components of the transition dipole moments contribute to the overall absorption with different relative strengths and different signs. Unlike the metal surfaces, the tilt angle of the hydrocarbons in an OTS film on silicon can be determined from the relative υ(CH2) intensities. (57) McIntyre, J. D. E.; Aspnes, D. E. Surf. Sci. 1971, 24, 417-428. (58) Mielczarski, J. A. J. Phys. Chem. 1993, 97, 2649-2663. (59) Wong, J. S.; Yen, Y. S. Appl. Spectrosc. 1988, 42, 598-604. (60) Yen, Y. S.; Wong, J. S. J. Phys. Chem. 1989, 93, 7208-7216. (61) Udagawa, A.; Matsui, T.; Tanaka, S. Appl. Spectrosc. 1986, 40, 794-799. (62) Gericke, A.; Michailov, A. V.; Huhnerfuss, H. Vib. Spectrosc. 1993, 4, 335-340.

contact printed immersed in solution 1 mM 10 mM 50 mM 1 mM 10 mM 50 mM 2966

2866

2917.5 2917.5 2855 2851

2966 2879 2917.5 2850

2965

2965

2965

2930 2850

2930 2850

2930 2850

Taken from ref 40.

from the surface normal direction. They also demonstrated that RAIRS on Si is highly sensitive to the degree of disorder in these thin organic films, as judged from a detailed analysis of the various C-H stretching mode line shapes (especially via the peak positions and the sign of absorption mode intensities). For example, a thin isotropic film of liquid paraffin oil on a silicon substrate (a largely “isotropic” phase) yields an “inverted” absorbance for all absorption modes at an 80° incidence angle using ppolarized radiation. Anisotropic films, depending on the degree of order, give very complex reflection spectra which can be interpreted on the basis of superimposed positive and negative intensity components in their complex line shapes. These latter components can be related to varying projections of the transition moments, and thus groups, along the optical axis of the reflection experiment. The RAIR spectra of so-called high-quality OTS films, consisting of well packed, highly oriented hydrocarbon chains, show strong positive intensity peaks (at ∼2920 and ∼2851 cm-1) for the various methylene modes and strong negative intensity bands (∼2968 and ∼2879 cm-1) for the respective (antisymmetric and symmetric) methyl modes. Films made of isotropic (disordered) hydrocarbon chains exhibit very complex line shapes, whose intensity patterns diverge significantly from this latter simple trend. The data shown in Figure 5 correspond closely with those seen for the two extreme limiting structures reported in the literature. The OTS films obtained by contact printing show a higher degree of conformational ordering for all concentrations of the OTS ink as compared to those prepared by adsorption from solution. The RAIR spectra of the former materials show sharp peaks at 2851 and 2917 cm-1 (corresponding to the υs(CH2) and υas(CH2) absorptions, with υs(CH3) and υas(CH3) bands at 2879 and 2966 cm-1 appearing as negative intensity features). These latter frequencies (which are summarized in Table 2) and line widths are ones characteristic of the organization of hydrocarbon chains in bulk, solid-state phase of the alkanes. The films formed by contact printing thus appear to be very similar to the densely packed film structure reported by Hoffmann et al. (one obtained by an extended immersion of the substrate in an OTS solution). In comparison, the films prepared by adsorption at these much shorter times of immersion appear to contain a significant population of conformationally disordered hydrocarbon chains. We conclude from this that the substrate, so long as it is highly hydrophilic, appears to have little effect on the phase formation. In each case (Al and Si) the SAM obtained is comprised of largely all-trans chains aligned very close to the surface normal direction (i.e. with cants of no more than ∼10°). Such structures are essentially indistinguishable from those obtained by solution immersion. We point out that SAMs of OTS prepared by the latter method are not ordered (translationally). There is no aspect of the present data which suggests a different status of those prepared by contact printing.

3388 Langmuir, Vol. 13, No. 13, 1997

Figure 6. RAIR spectra of OTS films on SiO2/Si as a function of preparation time as prepared by (A) contact printing (using 10 mM OTS in hexane as ink) and (B) adsorbed from solution (1 mM OTS in toluene).

Effects of Reaction Times on OTS Film Structure. The structure of the OTS films prepared by contact printing using a 10 mM ink in hexane were qualitatively very similar across a wide range of stamp contact times. Figure 6A shows the RAIR spectra of the OTS films on SiO2/Si surfaces formed by contact printing with 10 mM OTS “ink” as a function of stamp contact time. The spectroscopic features seen in these data are ones highly characteristic of (nearly) vertically oriented alkyl chains in the OTS film. An interesting point to note, however, is that even when the stamp contact time is shorter than the time needed to reach a full monolayer coverage (for a 10 s contact stamp, we found a mass coverage of OTS ∼ 8 Å), the alkyl chains in the film are still highly oriented. This is very unlike the growth characteristics of adsorption from solution (Figure 6B), where, at low coverages, the chains show significant conformational disorder. These data thus suggest that the film growth effected by contact printing must proceed via islands of OTS, ones in which the lateral interactions of the chains are sufficient to yield highly organized structures. The intensities of the peaks increase with increasing stamp contact time, indicating a higher mass coverage is reached via a growth in the size of the island domains (or at least until saturation of the monolayer is reached). Strong support of this latter interpretation was obtained in AFM studies of patterned depositions and is discussed below. We close this section by commenting briefly on the structural changes we observed in the OTS film as a function of adsorption time for samples prepared using a protocol reported in the literature (1 mM OTS solution in toluene). Figure 6B shows the RAIRS spectra obtained as a function of adsorption time (30 s and 15 and 60 min). The sample with the shortest immersion time has a

Jeon et al.

complex spectrum, one similar to those shown earlier for samples prepared using a different solvent system (Figure 5). After approximately 15 min of reaction, the ellipsometrically determined thickness (25 Å), as well as the RAIR spectrum, indicates the presence of a film whose coverage is close to that of a full monolayer. There is no significant structural change noted for films prepared with longer reaction times. The spectroscopic features seen at this point are essentially identical to the reported RAIR spectrum of fully formed OTS films.40 Thus, if the growth in this case involves the intermediacy of island domains, as has been reported by others,38 these limited RAIRS data do not argue the case as strongly as is done for films prepared by contact printing. Thermal Stability of Contact-Printed OTS Films. In earlier reports, we have shown that patterned OTS SAMs (formed by microcontact printing, µCP), can be utilized as molecular resists and templates for directing the deposition of thin films. We have successfully deposited patterned metal thin films (e.g. Cu,16 Pt,17 and Pd17) by CVD and a number of metal oxide thin films [including LiNbO3,18 (Pb,La)TiO3,18 Ta2O5,63 and Pb(Zr,Ti)O364] by sol-gel methods. Because these thin film deposition techniques often involve process steps carried out at elevated temperatures, we have examined the thermal stability of OTS SAMs prepared by contact printing. In this paper, RAIRS was used to study the thermal stability of printed OTS films (using a stamp contact time of 30 s and a 10 mM OTS in hexane ink) on Al and SiO2/Si substrates; the samples were subjected to thermal treatments similar to those experienced under CVD and solgel process conditions. Figure 7A shows RAIR spectra taken of an OTS film on Al after it was annealed in air for 15 min at the indicated temperatures. These latter conditions are more severe than those experienced in CVD processing, since the latter is carried out in the absence of air. All the spectra shown were taken after cooling to room temperature. The reference spectra were taken using an Al mirror subjected to identical heat treatment. After annealing at 150 °C, the RAIR spectrum shows slight but significant changes. First, the υs(CH2) and υas(CH2) modes show pronounced shifts to higher frequencies (2853 and 2924 cm-1) and have become more intense and slightly broadened. The absolute intensities of both the υ(CH2)as and υ(CH3)s stretching modes have also increased. All of these changes strongly suggest that the packing of the hydrocarbon chains becomes increasingly disordered upon annealing. This agrees with previous AFM65 and IR45 studies that suggested an irreversible order-disorder transition occurs in OTS films near 150 °C. When the Al-supported OTS film is annealed to 250 °C, the C-H vibration peak frequencies and shapes change drastically and strongly resemble the characteristics of disordered, liquid-like hydrocarbon thin films. We see, for example, that the υs(CH2) and υas(CH2) bands have moved to significantly higher frequencies (2859 and 2930 cm-1, respectively) and are considerably broader than those observed for unannealed films. The data are not consistent with the occurrence of a simple phase transition because the effects seen are clearly irreversible. It may well be that some fraction of the OTS volatilizes or, if not, that the sheet-like character of the OTS film is being degraded progressively as the temperature is raised. (63) Clem, P. G.; Jeon, N. L.; Nuzzo, R. G.; Payne, D. A. J. Am. Ceram. Soc., in press. (64) Jeon, N. L.; Clem, P. G.; Jung, D. Y.; Lin, W.; Payne, D. A.; Girolami, G. S.; Nuzzo, R. G. Submitted to Adv. Mater. (65) Calistri-Yeh, M.; Kramer, E. J.; Sharma, R.; Zhao, W.; Rafailovich, M. H.; Sokolov, J.; Brock, J. D. Langmuir 1996, 12, 2747-2755.

Patterned SAMs of Octadecyltrichlorosilane

Figure 7. RAIR spectra of OTS films on (A) Al and (B) SiO2/Si after annealing at different temperatures. The reference and OTS/Al sample were both annealed in air for 15 min under identical conditions at the indicated temperatures. All spectra were taken after cooling to room temperature. The OTS/SiO2/ Si sample and its reference were subjected to thermal conditions typical of those used for selective deposition of metal films by CVD, though without a precursor flow (350 °C for 15 min, at 0.5 Torr of H2 pressure).

Further annealing at 350 °C in air results in the complete loss of the Al-supported OTS film. It is possible that, at such high temperatures, the hydrocarbon chains pyrolyze and/or are removed from the substrate by volatilization and thermal decomposition. The nature of the substrate and the contacting ambient appear to have some effect on the stability of the OTS films. By way of example, Figure 7B shows the RAIR spectra of an OTS film on Si before and after a 30 min heat treatment at 350 °C. Both the reference and the sample were loaded into the CVD reactor and exposed to conditions identical to those of a CVD experiment (except exposure to the organometallic precursor was omitted). The CVD chamber was evacuated to 1 × 10-6 Torr, and the temperature of the susceptor was slowly increased to 350 °C. The substrates were then exposed to Ar and H2 at a chamber pressure of 0.5 Torr. As is evident in the spectra shown, some fraction of the OTS film remains on the surface after annealing, but this material clearly is conformationally disordered. We are uncertain of the exact nature of these latter thin films but do know with certainty that they still function as sufficiently good resists so as to mediate the patterned deposition of metals (e.g. Cu)16 by CVD. It is intriguing to note that this latter spectrum very closely resembles that of an OTS film prepared by dipping in a dilute solution for a short time (Figure 5A). The robustness of the OTS film (albeit with loss of conformational ordering) is consistent with our ability to carry out the selective CVD of metal films at temperatures as high as 350 °C.17

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Figure 8. Schematic flow chart of the steps used in selective CVD of metal thin films directed by microcontact-printed OTS SAMs (A). An example of selectively deposited 3 µm wide platinum lines on a SiO2/Si substrate prepared using this procedure (B).

Nature of OTS Films Patterned by Microcontact Printing Which Mediate the Directed Chemical Deposition of Metal and Ceramic Thin Films. Patterned SAMs of OTS formed by contact printing have been successfully used in directing the selective deposition of metal thin films by CVD16,17 and in patterning ceramic thin films deposited by the sol-gel method.18,63 In both of these cases, the OTS film acts as a molecular resist, either by hindering the nucleation of metals or the adhesion of ceramics on a substrate. The ability to pattern metal and ceramic thin films depends on the availability of highquality, high-resolution patterns of OTS. Microcontact printing produces such high-resolution patterns of OTS in thin film form. The OTS film must be nearly defectfree in order to maintain selectivity in CVD and sol-gel processing. In order to probe the fidelity and quality of the OTS film, we used a µCP patterned surface as a substrate for the deposition of platinum by CVD from the precursor bis(hexafluoroacetyl acetonate)platinum(II), Pt(hfac)2 (Figure 8). Many variables had to be optimized to yield patterned OTS films suitable for this application. The most notable of these were the stamp contact time and the concentration of the OTS ink. The stamp contact time is a factor which influences both the quality of the OTS film and the fidelity of the pattern. Short stamp contact times (∼10 s) were found to produce an unacceptable number of defects in the platinum thin film. Long stamp contact times (∼60 s) also adversely affected the patterning either by reducing the lateral dimensions of the features deposited or eliminating them altogether. The best results were obtained with a 30 s stamp contact time. These results

3390 Langmuir, Vol. 13, No. 13, 1997

Figure 9. SEM micrographs of patterned OTS films formed by contact printing on a SiO2/Si substrate. The stamp used was patterned with 3 µm wide lines separated by 3 µm spaces. Distinct 3 µm lines of OTS film (light regions) separated by 3 µm wide underivatized SiO2/Si regions are visible on the sample that was stamped for 30 s (A). A longer stamp contact time (60 s) results in a reduction in the lateral dimensions of the underivatized regions from 3 µm to less than 2 µm due to reactive spreading of OTS from the edge of the stamp (B).

suggest a mechanistic basis for the OTS-mediated patterning related to its mass coverage. At short stamp contact times, the OTS film is not completely coherent and selectivity is lost. At long contact times, OTS must bleed and/or be transported into regions not in direct contact with the stamp surface. Closer examinations with SEM and AFM gave direct support for this hypothesis. Figure 9 shows SEM micrographs of OTS on SiO2/Si surfaces that were patterned by µCP with a stamp bearing arrays of 3 µm lines separated by 3 µm spaces. The image contrast results from differences in the secondary electron emission. An ink with 10 mM OTS in hexane was used. For the sample stamped for 30 s (Figure 9A), sharp boundaries between OTSderivatized (light regions) and underivatized (dark regions) areas were observed. Similar to the reported behavior of alkanethiols on gold,66 a stamp contact time of 60 s is sufficient to promote the reactive spreading of alkylsilanes on SiO2, which in turn leads to the reduction of the lateral dimension of the underivatized regions (from 3 µm to less than 2 µm, Figure 9B). (66) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 32743275.

Jeon et al.

Figure 10. AFM (lateral force) images of small regions between stamped (OTS-derivatized) and unstamped (SiO2/Si) regions which reveal that small islands of OTS have nucleated in the unstamped areas. A 30 s stamp contact time with (A) a 10 mM ink in hexane or (B) a 20 mM ink in hexane.

An essential (and complicating feature) of the pattern is not discernible by SEM alone. Figure 10A shows an AFM (lateral force) micrograph of a small region of the sample shown in Figure 9A (a 30 s stamp contact time using a 10 mM OTS ink). As is easily seen, there are numerous small islands of OTS present in the unstamped regions. Control studies involving underivatized SiO2 substrates show a flat, featureless surface, confirming that the observed islands are due to the reactive spreading of OTS. The exact mechanism involved in formation of these islands is not clear at this time. When a higher concentration OTS ink (20 mM) is used, more islands are seen (Figure 10B). Other than this limited visual comparison, we were not able to more completely quantify the ink concentration dependence of this island formation. This might be better done, we believe, using stamps of greatly varied pitch. A sufficiently high density of these islands leads to a significant inhibition of growth on all regions of the substrate. It is imperative, for design rules more demanding than a few microns, to eliminate this type of island growth altogether. From experiments conducted to date, we have come to believe that the island growth is significantly impacted by mass transport effects, a point on which we comment below. We note here that, for display metallization and printed circuit board applications, the design rules are much less stringent and thus the patterning technique described here can be used effectively. The problems discussed above pose potential limitations in cases employing submicron

Patterned SAMs of Octadecyltrichlorosilane

to micron level design rules, such as are found in very large scale integrated circuit (VLSI) designs. How then can these island domains be eliminated as complications of the µCP-mediated deposition process? We believe it likely that the dominant transport mechanism involved (of OTS into noncontacting regions) is one of vapor-phase transport. Process parameters such as temperature, but more importantly the chain molecular weight, should greatly influence the kinetic competency of this latter transport mechanism. Patterning using printed polymers or high molecular weight amphiphiles should demonstrate

Langmuir, Vol. 13, No. 13, 1997 3391

significant process improvements. We will report on this in greater detail in subsequent publications. Acknowledgment. This work was supported by the National Science Foundation (Grant CHE 96-26871). XPS studies were carried out at the Center for Microanalysis of Materials, University of Illinois, which is supported by the Department of Energy under Contract DEFG02-91ER45439. LA970166M