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Langmuir 2008, 24, 1648-1653
Additive Route to Lithographically Defined Nanoporous Silica, Polymer, and Carbon Films Lingyan Song, Xinxin Li, and Bryan D. Vogt* Department of Chemical Engineering, Arizona State UniVersity, Tempe, Arizona 85284 ReceiVed NoVember 12, 2007. In Final Form: January 9, 2008 A facile method was investigated for patterning microporous and mesoporous silica, polymer, and carbon films using a combination of lithography and solid-state chemistry. This process exploits the difference in chemical reactivity between the lithographically exposed and unexposed regions to control the reaction of a target precursor from the vapor phase. A block copolymer film loaded with a photoacid generator is utilized as a preformed template, and tetraethylorthosilicate (TEOS) and furfuryl alcohol (FA) are the silica and carbon precursors, respectively. Following UV exposure and reaction with vaporized precursors, thermal decomposition of the polymeric template yields a mesoporous film in the exposed regions. Dense line-space patterns down to 1.5 µm features were resolved with I-line lithography. Sharper features were formed using FA; this behavior is attributed to the requirement for water in the system during TEOS condensation. Moisture in the system appears to lead to enhanced diffusion of the photoacid and a small decrease in the feature resolution. This methodology provides a simple etch-free route to patterning mesoporous films using commercially available materials.
Introduction Patterning in microelectronics is generally a subtractive approach with the features initially defined within a sacrificial photoresist layer and subsequently transferred to the underlying layer through a reactive etch.1,2 To maintain Moore’s law, new materials are required in device integration, including low-k and ultralow-k dielectric films. Porous organosilicates are promising materials for obtaining the desired dielectric constant and physical properties for ultralow k. One problem with integrating low-k dielectric materials is that etch damage during subtractive processing degrades the device performance.3 Additive processing would intrinsically avoid etch damage, but most additive approaches utilize serial processes4,5 whose limited throughput is prohibitive for high-volume production. Direct chemical stamping through soft lithography provides a route for some materials,6,7 but the yield, although high, for academic laboratories could be problematic (100 µm) structures, contact lithography with UVA radiation (Spectroline SB-100P flood lamp) was used. For smaller features, I-line radiation and a mask aligner (EVG620) were used. Following exposure, the wafer was broken into approximately 1.5 cm × 3 cm pieces. Each of these was individually loaded along with the reactants into a 25 mL sealed vessel (Thar Technologies) at 60 °C. The reaction was allowed to proceed for 30 min. For selective solvent removal of the starting materials, the film was liberally rinsed with isopropyl alcohol and then dried under a stream of nitrogen. The silica films were calcined at 450 °C for 5 h in air. Inorganic carbon was formed by heating in a He atmosphere to 400 °C for 3 h. Characterization. Changes in the optical properties and thickness of the film throughout the processing were quantified using spectroscopic ellipsometry (M-2000, J. A. Woollam). The data were modeled using WVASE and consisted of a silicon wafer, a native oxide layer, and the film of interest. The optical properties of the film were determined using a Cauchy formalism with an Urbach adsorption. The angle of incidence was fixed at 75° with the full (19) Hult, A.; MacDonald, S. A.; Willson, C. G. Macromolecules 1985, 18, 1804. (20) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024.
Figure 1. Schematic depicting the methodology for creating micropatterned materials combining lithography and solid-state chemistry. (a) A polymeric film containing a photoacid generator is deposited on the substrate. (b) The film is exposed to light through a mask, selectively generating acid molecules within the exposed regions. (c) The film is placed in a closed container with precursors and is heated to 40-80 °C. Selective deposition occurs as a result of the acid-catalyzed reaction. (d) The polymer template is removed either through decomposition or selective solvent removal, yielding the desired patterned material. wavelength range from 245 to 1700 nm used to fit the thickness and optical constants. X-ray diffraction (XRD) measurements were conducted at the Materials for Separation Laboratory using a Bruker D8 diffractometer with Cu KR radiation (λ ) 0.15418 nm). The data were collected with 2θ varying from 0.5 to 5°, with an increment of 0.01°. Transmission electron microscopy (TEM) was performed on the calcined mesoporous films using a JEOL 2000FX microscope operating at 200 kV. The samples were prepared by scraping the patterned regions of film off the wafer, crushing it, and dispersing the fragments in acetone or isopropanol to form a slurry. Samples for examination were prepared by evaporating drops of the slurry onto carbon-coated copper grids.
Results and Discussion The patterning procedure is illustrated schematically in Figure 1. To begin the process, a polymeric “photoresist” film is prepared via spin coating. The film is then exposed through a lithographic mask with UV light. Adsorption in the exposed regions by a photoactive species generates a catalytic species (such as an acid or base). No readily observable changes in the film are ascertained during the exposure process. Unlike traditional chemically amplified photoresists that use the generated photoacid to catalyze a scission reaction with the polymeric resin,1,2 no direct reaction with the photoresist is used here. Instead, precursors to the materials of interest are exposed to the film through the vapor phase. Selective reaction is ensured through the spatial distribution of photocatalyst. Here, acid-catalyzed reactions are used in the patterning to create silica, polymer, and inorganic carbon structures. The pattern formation is an intricate balance between the transport of precursor into the film and the reaction rate. If the catalyst loading is too large, then the reaction occurs too quickly and forms a skin layer that prevents the transport of the precursors throughout the film. Conversely, if the catalyst loading is too small, then incomplete reaction occurs, yielding poorly defined structures. The reaction temperature and time also significantly influence the quality of the final structures. Following the reaction, the polymer resin is removed; either a selective solvent is used to remove the polymer or it is simply degraded thermally at elevated temperatures. The pattern quality is primarily determined by the exposure step. The background exposure must be kept to a minimum to achieve high reaction selectivity; significant reaction in the nominally unexposed regions can create undesired bridges between the lithographically defined features. However, diffusion of the catalyst can lead to smearing of the feature. In
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Figure 2. Patterning nanoporous materials using PHOSt as a template using (a) a line-space mask. Optical microscope images of (b) silica, (c) poly(furfuryl alcohol), and (d) carbon formed from the pyrolysis of poly(furfuryl alcohol). The negative of the line-space mask is faithfully reproduced in each case.
microelectronics, controlled diffusion of the catalyst is desired to deprotect the resist in exposed regions.1,2 In this process, no diffusion is necessary, thus minimizing diffusion is desired. From fundamental knowledge in microelectronics, the diffusion of the acid catalyst can be minimized by (1) selecting a large counterion and (2) using a polymer resin that allows for hydrogen bonding with the photoacid.21 Here, we use 5-octylsulfonyloxyimino5H-thiophen-2-ylidene-2-methylphenyl acetonitrile (CGI-1325) as the photoacid generator, which yields an n-octyl sulfonium counterion upon photolysis. To address design criterion 2, we use poly(4-hydroxystyrene) (PHOSt) as the polymer resin because of the extremely slow diffusivities (
