Photochemistry and Patterning of Self-Assembled Monolayer Films

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Photochemistry and Patterning of Self-Assembled Monolayer Films Containing Aromatic Hydrocarbon Functional Groups Charles S. Dulcey,*,† Jacque H. Georger, Jr.,‡,§ Mu-San Chen,‡ Stephen W. McElvany,⊥ C. Elizabeth O’Ferrall,‡ Valarie I. Benezra,† and Jeffrey M. Calvert*,† Center for Bio/Molecular Science and Engineering, Code 6950, and Chemistry Division, Code 6110, Naval Research Laboratory, Washington, D.C. 20375-5348, and Geo-Centers, Inc., Ft. Washington, Maryland 20744 Received October 30, 1995X The deep ultraviolet (λ < ∼250 nm) photochemistry of chemisorbed organosilane self-assembled films of the type R(CH2)nSiO-surface where n ) 0, 1, 2 and R ) phenyl, naphthyl, or anthracenyl is explored. Photochemistry is examined using 193 and 248 nm laser irradiation as well as deep ultraviolet lamp sources. It is demonstrated for a variety of systems, including single and multiple rings as well as heterocycles, that the primary photochemical mechanism is cleavage of the Si-C bond. Photocleavage of the organic group generates a polar, wettable silanol surface that is amenable to subsequent remodification by organosilane chemisorption, allowing the fabrication of high-resolution patterns of chemical functional groups in a single molecular plane. The use of patterned monolayers as templates of reactivity for subsequent selective chemical reactions is demonstrated.

Introduction Self-assembled monolayer (SAM) films formed from materials such as alkanethiols and organosilanes may be used to control the physical and chemical properties of surfaces, including wettability, adhesion, orientation, and chemical reactivity.1-3 Organosilane precursors are particularly useful for such surface modification strategies because SAM films of these materials can be formed on a wide range of polar substrates (such as glass, silicon, sapphire, metals) by self-assembly onto, and then condensation with, surface hydroxyl groups.4 Materials that do not inherently possess high densities of surface OH groups (such as diamond,5 fluoropolymers,6 and many other polymer substrates) are readily made amenable to silanization by various oxidation or hydrolysis treatments that create a sufficient density of surface reactive groups for chemisorption to proceed. Thus, robust chemisorbed organosilane films can be formed on a wide range of substrates. For many applications it is necessary to produce patterns of surface functional groups on a desired substrate material. Various strategies have been recently reported for patterning SAM films, such as stamping,7 mechanical * To whom correspondence should be addressed. † Center for Bio/Molecular Science and Engineering, Code 6950, Naval Research Laboratory. ‡ Geo-Centers, Inc. § Current address: Shipley Co., Marlborough, MA 01752. ⊥ Chemistry Division, Code 6110, Naval Research Laboratory. X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R. W.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Dubois, L. H. Annu. Rev. Mater. Sci. 1991, 21, 373. (3) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (4) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1982. (5) Calvert, J. M.; Pehrsson, P. E.; Dulcey, C. S.; Peckarar, M. C. Proc. Mat. Res. Soc. 1992, 260, 905. (6) Vargo, T. G.; Gardella, J. A.; Calvert, J. M.; Chen, M. S. Science 1993, 262, 1711.

scraping,8,9 locally-confined plasma surface modification,10 and photo-11-15 (or beam-induced)12,13,16,17 transformations using lithographic exposure tools. Photochemical patterning is an attractive approach for producing such surfaces for many reasons; for example, high-resolution (sub-micrometer line width) features can be readily produced with excellent control of feature critical dimensions; very high placement accuracy of features is achieved with respect to both each other and a specific location on the substrate, and exposure of a large fieldsor the entire substratesoccurs as a parallel process. Furthermore, standard industrial exposure tools, such as deep UV (λ < ∼250 nm) contact aligners or projection exposure systems, are designed for high-quality, high-resolution pattern formation. It is therefore useful to develop novel photoimaging chemistries that can exploit the capabilities of these machines for high-resolution patterning of SAM films. We have previously shown that organosilane SAM films are sensitive to, and can be patterned with, various forms of radiation, including deep UV,11-13,18-22 soft X-rays,22,23 ions,22,24 and electrons.16,25 Upon irradiation, properties such as the wettability or reactivity of a film are modified, to an extent controlled by the dose delivered to the surface. Patterned irradiation of organosilane SAMs may be used (7) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (8) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380. (9) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632. (10) Vargo, T. G.; Thompson, P. M.; Gerenser, L. J.; Valentini, R. F.; Aebischer, P.; Hook, D. J.; Gardella, J. A. Langmuir 1992, 8, 130. (11) Dulcey, C. S.; Georger, J. H.; Kleinfeld, V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551. (12) Calvert, J. M. J. Vac. Sci. Technol. 1993, B11 (6), 2155. (13) Dressick, W. J.; Calvert, J. M. Jpn. J. Appl. Phys. 1994, 32, 5829. (14) Wollman, E. W.; Frisbie, C. D.; Wrighton, M. S. Langmuir 1993, 9, 1517. (15) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398. (16) Perkins, F. K.; Dobisz, E. A.; Brandow, S. L.; Koloski, T. S.; Calvert, J. M.; Rhee, K. W.; Kosakowski, J. E.; Marrian, C. R. K. J. Vac. Sci. Technol. 1994, B12, 3725. (17) Tiberio, R. C.; Craighead, H. G.; Lercel, M.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476. (18) Calvert, J. M.; Chen, M. S.; Dulcey, C. S.; Georger, J. H.; Peckerar, M. C.; Schnur, J. M.; Schoen, P. E. J. Vac. Sci. Technol. 1991, B9, 3447.

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to create laterally modulated patterns of chemical functionality on a surface. Patterns formed in this manner have been demonstrated to serve as high-resolution reactivity templates that direct the spatially-defined attachment of metals, colloids, biological moieties, and various other materials on the surface of a substrate. Organosilane SAM films with aromatic hydrocarbon groups are an interesting class of materials for study with regard to their photochemistry. The chromophoric groups are relatively simple ring systems which absorb strongly in the deep UV. A variety of materials of this general class are available with different numbers of rings, spacer groups, and numbers of surface reactive groups. For the phenyltrichlorosilane (PTCS) SAM film, our initial studies have reported that deep UV radiation cleaves the Si-C bond, and a lithographic process can be obtained by selective metal deposition onto the unexposed phenyl groups.11 Our goal in this paper is to examine in detail the deep UV photochemical mechanisms of a series of organosilane SAM films with aromatic hydrocarbon functional groups (i.e., R(CH2)nSiO-surface where n ) 0, 1, 2 and R ) phenyl, naphthyl, anthracenyl) and to evaluate the molecular and environmental factors that control the efficiency of the photochemical process for this class of materials. We then use photochemical patterning techniques to demonstrate positional control of various surface properties, including wetting, topography on the molecular scale, and selective metal deposition. Experimental Section Substrates. Fused silica slides (25 mm square × 1 mm thick), obtained from Dell Optics, were used for UV spectroscopy of SAM films. Two- to four-inch-diameter Si wafers with a native oxide surface layer (15-20 Å thick) were obtained from Wafernet (San Jose, CA) and were typically (100) cut and p-doped, with resistivity ranging between 1 and 10 Ω cm. Si wafers with a 35 nm thick thermally grown oxide were prepared at the NRL Nanoelectronics Processing Facility (NPF). Borosilicate glass microscope slides (1 × 3 in.) were obtained from Fisher Scientific. For XPS studies, Si substrates with a 50 nm thick layer of electron beam evaporated platinum, prepared at the NRL NPF, were employed. Organosilane Precursors and Other Reagents. Phenyltrichlorosilane (PTCS), benzyltrichlorosilane (BZTCS), phenethyltrichlorosilane (PETCS), tolyltrichlorosilane (TTCS), 2-[(trimethoxysilyl)ethyl]-2-pyridine (PYRTMS), 4-biphenylyltrimethoxysilane (BPTMS), and octadecyltrichlorosilane (OTS) were obtained from Hu¨ls America (Piscataway, NJ). 1-Naphthyltrimethoxysilane (NAPTMS), 1-[(trimethoxysilyl)methyl]naphthalene (MNAPTMS), and (9-methylanthracenyl)trimethoxysilane (MANTMS) were obtained from Solarelectronics (Bellingham, MA). Although detailed procedures for the synthesis of multiring silanes were not available, the general procedure involved Grignard coupling; e.g., NAPTMS was synthesized by Grignard coupling of 1-bromonaphthalene with Si(OCH3)4. Silanes were used as received without further purification. Chloro- and (19) Calvert, J. M.; Dressick, W. J.; Dulcey, C. S.; Chen, M. S.; Georger, J. H.; Stenger, D. A.; Koloski, T. S.; Calabrese, G. S. In Polymers for Microelectronics; Thompson, L. F., Willson, C. G., Tagawa, S., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993; Vol. 537, p 210. (20) Bhatia, S. K.; Teixeira, J. L.; Anderson, M.; Shriver-Lake, L. C.; Calvert, J. M.; Georger, J. H.; Hickman, J. J.; Dulcey, C. S.; Schoen, P. E.; Ligler, F. S. Anal. Biochem. 1993, 208, 197. (21) Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielson, T. B.; McCort, S. M.; Calvert, J. M. J. Am. Chem. Soc. 1992, 114, 8435. (22) Calvert, J. M.; Koloski, T. S.; Dressick, W. J.; Dulcey, C. S.; Peckerar, M. C.; Cerrina, F.; Taylor, J. W.; Suh, D.; Wood, O. R., II; MacDowell, A. A.; D’Souza, R. Opt. Eng. 1993, 32 (10), 2437. (23) Suh, D.; Simons, J. K.; Taylor, J. W.; Calvert, J. M.; Koloski, T. S. J. Vac. Sci. Technol. 1993, B11 (6), 2850. (19) Ada, E. T.; Hanley, L.; Etchin, S.; Melngailis, J.; Dressick, W. J.; Chen, M. S.; Calvert, J. M. J. Vac. Sci. Technol. B 1995, B13, 2189. (25) Marrian, C. R. K.; Perkins, F. K.; Brandow, S. L.; Koloski, T. S.; Dobisz, E. A.; Calvert, J. M. J. Appl. Phys. Lett. 1994, 64, 1.

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Figure 1. Molecular structure of organosilane SAM film precursors. alkoxysilanes were stored in a He atmosphere drybox (Vacuum Atmospheres). Film formation from chlorosilane precursors was performed in a glovebag (I2R) under an Ar blanket; alkoxysilane SAM films were prepared in a fume hood under ambient atmosphere conditions. Chemical structures of the ring-containing organosilanes are shown in Figure 1. Anhydrous toluene was Aldrich Sure Seal grade; acetonitrile was Burdick and Jackson HPLC grade; other solvents and acids were reagent grade or better, and were used as received. SAM Film Formation. Si and fused silica substrates were cleaned by sequential 30 min immersions in 1:1 HCl/CH3OH and 18 M sulfuric acid, followed by multiple rinses with 18 MΩ deionized (DI) water from a Barnstead Nanopure filtration system. The final rinse consisted of immersion in boiling DI water for several minutes. Residual bulk water was removed from the substrates by placing them under a stream of N2 that had passed through a 0.22 µm filter. For the Pt-coated Si wafers it was observed that the HCl/MeOH immersion lifted off the deposited Pt film. Therefore, these substrates were cleaned using only the sulfuric acid step. The cleaned substrates were all highly wettable by water. Using the sessile drop method, contact angles for the various substrates were typically 10° or less. Films of PTCS, BZTCS, PETCS, TTCS, and OTS were formed by immersion of the clean substrates, arranged on a quartz rack, into a 1% (v/v) solution of the organosilane in anhydrous toluene for 5-7 min. The treated substrates were then rinsed in toluene and baked on a programmable hot plate (VWR Series 400 HPS) at a surface temperature of 120 °C for 5 min. Preparation of pyridine silane (PYRTMS) films has been described in detail previously.26 Briefly, a 1% (v/v) solution of PYRTMS in toluene containing 1 mM acetic acid was prepared in the drybox. Clean substrates were immersed in the PYRTMS solution, and the temperature was ramped from 23 to 60 °C during the course of the 1 h treatment using a hot plate. The substrates were removed from the solution, rinsed with toluene, and baked 5 min at 120 °C. Films of the multiring aromatic hydrocarbon silane NAPTMS and MNAPTMS were prepared by immersing clean substrates into a solution containing 1% (v/v) of the silane precursor in acidic toluene (1 mM acetic acid), and the temperature was ramped from 23 to 50 °C during the course of the 1 h treatment. Substrates were then rinsed with toluene and baked 5 min at 120 °C. BPTMS and MANTMS films were prepared using essentially the same procedure, except the precursors are solids and the deposition solutions were prepared as 1% (w/v) mixtures. The MANTMS was not completely soluble in toluene at this concentration, even with extended ultrasonication, and a slight amount of particulate matter was always observable in the deposition solution. However, a 1% concentration was nevertheless used to prepare the MANTMS SAM films to maintain (26) Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calabrese, G. S.; Calvert, J. M. J. Electrochem. Soc. 1994, 141, 210.

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consistency in the deposition conditions of the multiring silanes. After preparation, SAM film coated substrates were stored in a clean room under yellow lights in Fluoroware (Fluoroware, Inc.) containers. Solutions of chloro- and methoxysilanes react with hydroxyl groups on the surface of silicon native or thermal oxide substrates to form covalent siloxane (Si-O-Si) bonds, liberating HCl and CH3OH, respectively, as byproducts.4 Trifunctional silanes, such as those used in this study, can polymerize and form bulk films or deposits if reaction conditions are not carefully controlled. In this paper, using UV spectroscopic and ellipsometric data, we show that the film preparation procedures described above produce SAM films that are consistent with thicknesses of a single molecular layer. We have shown elsewhere, by combined ellipsometric and X-ray reflectivity studies,27 as well as by atomic force microscopy,28,29 that these procedures produce homogeneous monolayer films and do not form multilayer deposits. However, complete adsorption isotherms have not been determined for all of the materials used in this study. Wettability Measurements. Advancing water contact angle measurements were obtained with an NRL Zisman-type contact angle goniometer under ambient atmosphere conditions. A micropipettor was used to dispense 10-20 µL drops of DI water which had a resistivity g17.6 MΩ cm. The contact angle hysteresis for aromatic hydrocarbon SAM films has been observed to be e5°. Multiple contact angle measurements on a given substrate were found to have a precision of ∼3°; the small variation of contact angle across a particular substrate is a qualitative indicator of homogeneous film coverage. Reported data are the average of at least three measurements per substrate for multiple substrates. Batch-to-batch reproducibility of contact angles for a given film was generally better than (5°. UV Spectroscopy. All UV spectra were obtained on a Varian CARY 2400 double-beam spectrophotometer, with data acquisition and analysis software developed in-house at NRL. The CARY 2400 is a high-performance instrument with a detection limit of 0.0004 absorbance units that is of critical importance for accurate measurement of SAM films with moderately- to weakly-absorbing chromophores. Solution spectra of silane film precursors were obtained in CH3CN solution using fused silica cells of 1 cm pathlength. The typical concentration of the organosilane precursor for solution spectroscopy was 2 × 10-5 M. Spectra of organosilane SAM films were obtained on fused silica substrates with samples referenced to freshly cleaned fused silica slides from the same lot. The film formation procedure produces SAM films on both sides of the fused silica slides; thus, recorded spectra correspond to both layers of the film. Spectra were acquired using a spectral bandwidth of 1 nm and a scan rate of 1 nm/s. Deep (DUV) Irradiation and Patterning. DUV exposure of SAM films was performed with a Cymer CX2 excimer laser operating at 248 nm (KrF) or a Questek 2430 excimer operating at 193 nm (ArF). Pulse widths for both lasers are nominally between 10 and 20 ns. For UV spectroscopy, a small homogeneous portion of the laser output was expanded with a negative focal length quartz lens and apertured to an area of ∼4 cm2 to expose the central portion of a film-treated slide, which was then exposed to typical energy densities of 0.5-4.0 mJ/cm2 per shot. The full dose to expose a given sample typically required well in excess of 100 shots. For patterning experiments, the beam was usually expanded to pattern larger areas, with correspondingly lower single shot energy densities. Patterning experiments were conducted by placing a substrate (such as a Si wafer) in tight mechanical contact with the metalized side of a lithographic mask (Cr patterns on a fused silica substrate) and then irradiating the mask/film assembly. Irradiation doses for patterning were increased by ∼10% from the target exposure value to compensate for transmission losses in the mask substrate. DUV irradiation of SAMs was also performed with lamp sources, including low-pressure HgAr lamps (Oriel Model 6035) and a Suss MJB3 DUV contact aligner. The HgAr lamp is a line source with strong outputs at 185, 195, and 254 nm, as well as (27) Geer, R. E.; Stenger, D. A.; Chen, M. S.; Calvert, J. M.; Shashidhar, R.; Jeong, Y. H.; Pershan, P. S. Langmuir 1994, 10, 1171. (28) Durfor, C. N.; Turner, D. C.; Georger, J. H.; Peek, B. M.; Stenger, D. A. Langmuir 1994, 10, 148. (29) Brandow, S. L.; Dressick, W. J.; Dulcey, C. S.; Marrian, C. R. K.; Chow, G. M.; Calvert, J. M. J. Electrochem. Soc. 1995, 142, 2233.

Dulcey et al. other Hg lines at longer wavelengths. The DUV aligner employs a broadband high-pressure HgXe source with output mainly between 220 and 300 nm. Surface Spectroscopy. X-ray photoelectron spectroscopy (XPS) surface analysis was performed on a Surface Science SSX100-03 X-ray photoelectron spectrometer. The X-ray source was a monochromatized Al KR source with a 35° takeoff angle; operating pressure was ∼15 mJ/cm2 per shot at 193 nm and would be expected to be much larger at 248 nm.31 The size of the laser spot was determined by measuring the ablated region in the polymer film; areas of 0.2 cm2 were typically observed. For the LD-FTMS experiments on SAM films, the laser energy was reduced by ∼90%, using wire mesh screens; energy densities at the wafer plane were estimated to be 1-3 mJ/cm2 at 193 nm and 5-10 mJ/cm2 at 248 nm. However, other factors contribute to uncertainty in the energy density at the sample, such as spatial inhomogeneities in the laser beam, attenuation of the beam by the ion trap mesh before reaching the sample, and the spatial arrangement of the superconducting magnet. The single shot laser energy was maintained in a regime where only neutral species were desorbed from the surface; no ions were observed to desorb from laser irradiation alone, as is characteristic of ablative or multiphoton processes. Desorbing neutral species were post-ionized by electron impact (with electron energies ranging from 10 to 70 eV), and the entire mass spectrum was obtained by broadband ion excitation and detection. All spectra reported here correspond to 20 eV electron ionization in an attempt to minimize the fragmentation of molecular species. Spectra were typically recorded as the result of averaging 10-30 laser shots. For some of the more sensitive films, noticeable depletion of signal from desorbed photoproducts could be observed (30) Amster, I. J.; Land, D. P.; Hemminger, J. C.; McIver, R. T. Anal. Chem. 1989, 61, 184. (31) Feldmann, D.; Kutzner, J.; Laukemper, J.; MacRobert, S.; Welge, K. H. Appl. Phys. 1987, B44, 81.

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Photochemistry of Self-Assembled Monolayer Films within 100 shots at 193 nm. To avoid signal depletion, the sample was rotated to a new spot before acquisition of each mass spectrum. Ellipsometry. Ellipsometric film thickness measurement was performed using a Gaertner Scientific Model L110C scanning ellipsometer with a HeNe (632 nm) laser source. For qualitative visualization of patterned SAM films on Si (native oxide) substrates, data was analyzed employing a single film model and treating the silane film and Si native oxide layer as one layer. Optical constants of n ) 1.45 and k ) 0.0082 were used for Si. For quantitative measurements of SAM film thickness, the optical constants of the “effective” substrate (i.e., silicon + native oxide) were determined before SAM film modification, and these optical constants were then used to derive film thickness with the single film model. The SAM films are too thin to determine both index of refraction and thickness;32 therefore, the index of refraction was input into the calculation of thickness. For the single ring precursors, the values of refractive index (nD) used were obtained from the manufacturer’s technical literature: nD (PTCS) ) 1.525, nD (BZTCS) ) 1.526, nD (PETCS) ) 1.519.33 For the multiring systems, refractive indices have not been determined, and an estimated value of n ) 1.50 was used for thickness determination. The calculated thickness for this class of SAM films is most sensitive to the k value, which corresponds to the real part of the complex refractive index. However, these films have no absorbance at the HeNe laser wavelength, and therefore k can accurately be set to 0. The thickness calculation is not particularly sensitive to small variations in refractive index: using n ) 1.45 or 1.55 instead of 1.50 results in a thickness change of