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Proximity X-ray Lithography Using Self-Assembled Alkylsiloxane Films: Resolution and Pattern Transfer Xiao M. Yang,† Richard D. Peters,† Tae K. Kim,† and Paul F. Nealey,*,† Department of Chemical Engineering and Center for Nanotechnology, University of Wisconsin, Madison, Wisconsin 53706
Susan L. Brandow,‡ Mu-San Chen,‡ Loretta M. Shirey,§ and Walter J. Dressick*,‡ Center for Bio/Molecular Science & Engineering (Code 6950) and Nanofabrication Process Facility (Code 6804), Naval Research Laboratory, Washington, D.C. 20375-5348 Received August 15, 2000. In Final Form: October 18, 2000 Self-assembled films of octadecyltrichlorosilane (OTS) on Si/SiO2 were patterned with proximity X-rays (λ ) 1.0 nm) in air, resulting in the incorporation of oxygen-containing functional groups, that is, hydroxyl and aldehyde, into the film. Unexposed and exposed OTS exhibited sufficient chemical contrast for patterning processes based on differences in wetting behavior and chemical reactivity. Latent images of features as small as ∼70 nm, defined by the X-ray mask, were successfully fabricated in the OTS with high fidelity over areas of ∼1 cm2. Patterned OTS was imaged directly with lateral force microscopy and indirectly through atomic force microscopy of three-dimensional structures formed on the surface of thin films of diblock copolymers after deposition and annealing on the patterned OTS. Pattern transfer of features with dimensions as small as ∼150 nm into the underlying silicon substrate was achieved by reactive ion etching using thin films of nickel selectively deposited onto the exposed areas of the OTS as etch masks.
Introduction Many applications in electronics, sensor, and display technologies require the two-dimensional patterning of materials on surfaces.1 A general approach to creating patterns of differing chemical reactivity, wettability, or surface charge utilizes a surface-deposited polymeric layer that is patterned through exposure to actinic radiation. Siloxane films, formed by the self-assembly and chemisorption of organosilanes of general structure RSiCl3 (R ) alkyl or aromatic organofunctional group) to surface hydroxyl sites, are one example of an ultrathin polymeric system.2 The resulting Si-O-substrate covalent linkage imparts a high degree of stability to the organosiloxane films, as compared to other self-assembled films such as Langmuir-Blodgett or coordinatively bound alkanethiols on the coinage metals (Au, Ag, Cu).2 In particular, the stability of siloxane films, their ability to form films on technologically relevant substrates such as silicon or oxidized polymeric planarizing layers, and their ability to deposit uniformly on nonplanar substrates makes them of interest for patterning applications.3 We and others have previously demonstrated that siloxane films can be molecularly engineered to be highly * Corresponding authors:
[email protected] and wjd@ cbmse.nrl.navy.mil. † Department of Chemical Engineering and Center for Nanotechnology, University of Wisconsin. ‡ Center for Bio/Molecular Science & Engineering, Naval Research Laboratory. § Nanofabrication Process Facility, Naval Research Laboratory. (1) Thompson, L. F.; Willson, G. G.; Bowden, M. J. Introduction to Microlithography, 2nd ed.; American Chemical Society: Washington, DC, 1994. (2) Ulman, A. Introduction to Ultrathin Organic Films from Langmuir-Blogett to Self-assembly, 2nd ed.; Academic Press: New York, 1991. (3) Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551.
sensitive to various types of radiation, including deep UV,3,4 e-beam,5-9 ion beam,10 X-ray,11,12 low energy electrons,13 and the low energy electrons from a STM or the electric field from a conducting atomic force microscopy (AFM) tip.14-17 Organosiloxane films, of monolayer thickness, can be particularly effective imaging layers when used in conjunction with high-energy exposure tools because their use avoids depth-of-focus and transparency issues generally associated with conventional thick (i.e., ∼1 µm) photoresist polymer films. For example, recently we demonstrated that siloxane films bearing benzyl chloride functional groups were photooxidized upon exposure to proximity X-rays.18 Aldehyde groups formed (4) Dressick, W. J.; Calvert, J. M. Jpn. J. Appl. Phys. 1993, 32, 5829. (5) Carr, D. W.; Lercel, M. J.; Whelan, C. S.; Craighead, H. G.; Seshadri, K.; Allara, D. L. J. Vac. Sci. Technol., A 1997, 15, 1446. (6) Lercel, M. J.; Whelan, C. S.; Craighead, H. G.; Seshadri, K.; Allara, D. L. J. Vac. Sci. Technol., B 1996, 14, 4085. (7) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. Appl. Phys. Lett. 1996, 68, 1504. (8) Seshadri, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. A 1996, 100, 15900. (9) Whelan, C. S.; Lercel, M. J.; Craighead, H. G.; Seshadri, K.; Allara, D. L. Appl. Phys. Lett. 1996, 69, 4245. (10) Ada, E. T.; Hanley, L.; Etchin, S.; Melngailis, J.; Dressick, W. J.; Chen, M. S.; Calvert, J. M. J. Vac. Sci. Technol., B 1995, 13, 2189. (11) Calvert, J. M.; Koloski, T. S.; Dressick, W. J.; Dulcey, C. S.; Peckerar, M. C.; Cerrina, F.; Taylor, J. W.; Suh, D. W.; Wood, O. R.; Macdowell, A. A.; Dsouza, R. Opt. Eng. 1993, 32, 2437. (12) Suh, D.; Simons, J. K.; Taylor, J. W.; Koloski, T. S.; Calvert, J. M. J. Vac. Sci. Technol., B 1993, 11, 2850. (13) Hild, R.; David, C.; Muller, H. U.; Volkel, B.; Kayser, D. R.; Grunze, M. Langmuir 1998, 14, 342. (14) Brandow, S. L.; Calvert, J. M.; Snow, E. S.; Campbell, P. M. J. Vac. Sci. Technol., A 1997, 15, 1455. (15) Perkins, F. K.; Dobisz, E. A.; Brandow, S. L.; Calvert, J. M.; Kosakowski, J. E.; Marrian, C. R. K. Appl. Phys. Lett. 1996, 68, 550. (16) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1995, 13, 1139. (17) Delamarche, E.; Hoole, A. C. F.; Michel, B.; Wilkes, S.; Despont, M.; Welland, M. E.; Biebuyck, H. J. Phys. Chem. B 1997, 101, 9263.
10.1021/la001176h CCC: $20.00 © 2001 American Chemical Society Published on Web 12/08/2000
Self-Assembled Alkylsiloxane Films
on the irradiated portions of the film were successfully converted to primary amines, which could be selectively metallized. The patterned metal films formed functioned as effective plasma etch masks for transfer of the latent image into a Si substrate. However, resolution was unimpressive (approximately several microns), presumably because of a combination of factors, including poor yields of surface aldehyde under our exposure conditions and low packing densities usually associated with aromatic siloxane films.19 Long-chain alkylsilanes are known to form close-packed films as a result of chain interaction,20 thereby implying a potentially higher resolution than that of the aromatic films. We have recently reported that self-assembled (SA) films of octadecyltrichlorosilane (OTS) and other alkylsiloxanes can be chemically modified by exposure to X-ray radiation in the presence of air.21 The chemical contrast between exposed and unexposed regions results from the difference between the polar, hydrophilic functional groups present on exposed OTS regions and the nonpolar, hydrophobic functional groups remaining on the unexposed regions. The extent of chemical modification was found to be a function of exposure dose and air pressure. These results guided our efforts to optimize chemical contrast and sensitivity in OTS imaging layers using ionizing radiation. In this paper, OTS SA films were patterned by proximity X-ray lithography to create high-resolution latent images of differing surface wettability and chemical reactivity. Features with dimensions as small as 70 nm (limited by the availability of X-ray masks) were successfully printed on the SA films. The current work demonstrates a substantial increase in resolution compared to previous reports in which self-assembled monolayers (SAMs) of alkylsiloxanes were patterned with parallel processes such as deep ultraviolet projection lithography (features of ∼400 nm)4 and X-ray lithography (features of ∼250 nm).11 Better resolution is due in part to our increased knowledge of the chemical modification of SAMs of alkylsiloxanes upon exposure to high-energy photons in the presence of oxygen. SAMs of alkylsiloxanes have been patterned with features of smaller dimensions (5 nm dots) using serial processes such as electron beam lithography.5-7,16 The latent image in these reports, however, relates to physical differences (reduction in thickness) rather than chemical differences between exposed and unexposed regions. Chemical patterns with dimensions greater than 100 nm were imaged directly with lateral force microscopy (LFM). LFM has been used previously to image SAMs of alkanethiols on gold and SAMs of alkylsilanes on silicon at micron and submicron dimensions.22-26 To test the sub-100-nm resolution of our process, we developed a method to map and effectively amplify latent images in the patterned OTS using thin films of diblock copolymers. The block copolymers exhibited symmetric wetting on unexposed regions (18) Dressick, W. J.; Dulcey, C. S.; Brandow, S. L.; Witschi, H.; Nealey, P. F. J. Vac. Sci. Technol., A 1999, 17, 1432. (19) Dressick, W. J.; Chen, M. S.; Brandow, S. L. J. Am. Chem. Soc. 2000, 122, 982. (20) Ulman, A. Introduction to Ultrathin Organic Films from Langmuir-Blogett to Self-assembly, 2nd ed.; Academic Press: New York, 1991; pp 143. (21) Kim, T. K.; Yang, X. M.; Peters, R. D.; Sohn, B. H.; Nealey, P. F. J. Phys. Chem. B 2000, 104, 7403. (22) Wilbur, J. L.; Biebuyck, H. A.; Macdonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825. (23) Bar, G.; Rubin, S.; Parikh, A. N.; Swanson, B. I.; Zawodzinski, T. A.; Whangbo, M. H. Langmuir 1997, 13, 373. (24) Zhou, Y. Q.; Fan, H. Y.; Fong, T.; Lopez, G. P. Langmuir 1998, 14, 660. (25) Lee, B. W.; Clark, N. A. Langmuir 1998, 14, 5495. (26) Sugimura, H.; Nakagiri, N. Appl. Phys. A 1998, 66, S427.
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and asymmetric wetting on exposed regions. The film thickness was quantized in units of nL0 and (n + 1/2)L0 for symmetric and asymmetric wetting, respectively (L0 is the bulk lamellar period of the diblock copolymer).27-29 The topography of the polymer film, with height differences of approximately 1/2L0 on adjacent exposed and unexposed regions, allowed us to visualize the latent image in patterned OTS using AFM. This imaging technique greatly improved our ability to test the resolution of the patterned SAMs. Latent images in the SA films as small as ∼150 nm were transferred into the silicon substrates by selectively depositing nickel films onto the exposed OTS regions to act as etch masks for reactive ion etching (RIE). Experimental Section Materials. Polished 100-mm-diameter silicon 〈100〉 wafers were purchased from Tygh Silicon and were used as substrates for the deposition of SA films. Octadecyltrichlorosilane (CH3(CH2)17SiCl3, 95%) was purchased from Gelest and was used as received. Symmetric poly(styrene-block-methyl methacrylate) (P(S-b-MMA)) was purchased from Polymer Source, Inc. The number average molar mass was 51 200 g/mol, the polydispersity was 1.06, and the styrene volume fraction was 0.48. The bulk lamellar period (L0) of the diblock copolymer was ∼30 nm. Toluene (99.8%, anhydrous) and chloroform (99+%, anhydrous) were purchased from Aldrich and were used without further purification. Preparation and Characterization of SA Films. The silicon wafers were cleaned by immersion in a piranha solution (7:3 (v/v) of 98% H2SO4/30% H2O2) at 90 °C for 30 min.30 The silicon wafers were immediately rinsed with deionized water (F g 18 MΩ/cm) several times and were blown dry with nitrogen. The cleaned substrates were immersed in a 0.1% (v/v) solution of OTS in toluene in a glovebox with a nitrogen atmosphere.21 The typical immersion times used ranged from 24 to 30 h. After the substrates were removed from the silane solution, they were rinsed with chloroform for approximately 30 s, and excess chloroform was allowed to evaporate. The films were rinsed with absolute ethanol and were dried under a stream of nitrogen. Advancing contact angles of deionized water on OTS were measured at five different locations on each wafer using a Rame´Hart goniometer. The typical values of the advancing contact angles were 105 ( 3°. Fabrication of X-ray Mask for Patterning of OTS. A highcontrast X-ray mask was fabricated at the Center for Nanotechnology (CNT) at the University of WisconsinsMadison.31 A schematic of the mask is shown in the top part of Figure 1. A 2-µm-thick silicon nitride membrane coated with a ∼25-nm-thick layer of Cr/Au was used as the supporting membrane of the mask. A ∼500-nm-thick photoresist (APEX-E, Shipley) was deposited on the silicon nitride/Au membrane by spin coating, and the resist was patterned by e-beam lithography using a Leica EBMF 10.5 system (30 keV) at CNT. After the patterned resist was developed, gold was plated onto the membrane between the patterned resist structures, and then the remaining resist was removed with an O2 plasma. To obtain high contrast, the electroplated gold structures on the membrane were ∼0.50 µm thick. The need for a high-contrast mask for patterning OTS will be discussed in Results and Discussion. The transmission coefficient of the mask was 49.7% in transmissive regions (membrane only) and 8.0% in absorber regions (membrane plus (27) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; F., M. C. Phys. Rev. Lett. 1989, 62, 1852. (28) Coulon, G.; Russell, T. P.; Deline, V. R.; Green, P. F. Macromolecules 1989, 22, 2581. (29) Russell, T. P.; Coulon, G.; Deline, V. R.; Miller, D. C. Macromolecules 1989, 22, 4600. (30) Ulman, A. Introduction to Ultrathin Organic Films from Langmuir-Blogett to Self-assembly, 2nd ed.; Academic Press: New York, 1991; pp 108. (31) Gentili, M.; Giovannella, C.; Selci, S. Nanolithography: a borderland between STM, EB, IB, and X-ray lithographies; Gentili, M., Giovannella, C., Selci, S., Eds.; Kluwer Academic Publishers: Norwell, MA, 1994; p 103.
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0.5 µm Au). The contrast of the mask was therefore approximately 1:6.2. The different shapes and feature sizes allow us to test the feasibility and limitations of our patterning technique. Inspection and metrology of the mask were performed with scanning electron microscopy (SEM) (Hitachi 6180 CD). Patterning of OTS SA Films by Proximity X-ray Exposure. X-ray exposures were performed in the ES-1 beamline at CNT at the University of WisconsinsMadison. OTS-covered silicon wafers and the X-ray mask were clamped together with a gap of ∼2 µm by a foil spacer and placed in the exposure chamber. The exposures were carried out in a chamber with a pressure of 1 Torr of air with a relative humidity of ∼20%. The samples were irradiated at an incident angle of 90°. The wavelength (λ) of the broadband radiation was centeredat ∼1.0 nm with ∆λ/λ ≈ 3. The intensity of the incident radiation at the surface of OTS was 17-38 mW/cm2 and varied with the beam current of the synchrotron ring. The beamline intensity was calibrated using calorimetry (Scientech, http//:www.scientech.com). Deposition and Annealing of Diblock Copolymer Films on Patterned OTS. Thin films of P(S-b-MMA) were deposited onto patterned OTS substrates by spin coating at 2500 rpm for 60 s from dilute solutions (2% w/w) of the copolymer in toluene. The surface of the polymer film after spin coating is featureless. The initial thicknesses of the films were typically 66 ( 2 nm (∼2.2L0) and were determined by scraping some of the polymer away from the surface with a razor blade and measuring the difference in height between the substrate and the surface of the film using an Alpha Step 200 profilometer (0.5 nm resolution). The polymer films were then annealed at 180 °C in a vacuum oven for 24 h to induce the formation of topography on the surface of the film. Deposition of Electroless Metal on Patterned OTS. Following exposure, an amine ligand group was selectively bound via covalent reaction to the irradiated regions of the SA film of OTS. Aldehyde (ketone) groups formed on the irradiated surface were converted to primary amines by reaction with a methanol solution containing ammonium acetate and sodium cyanoborohydride according to literature procedures.18,32 Upon removal from the grafting solution, samples were rinsed three times with deionized water and three times with methanol and then blown dry with filtered nitrogen. Samples were then treated for 1 h with a colloidal Pd(II) catalyst, PD1,33,34 followed by a 10% strength NIPOSIT 468 electroless Ni metal bath (Shipley Co.) using a procedure described previously in the literature.18,32,35 Metallization times were kept short (
