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Nanoporous Thin Films with Hydrophilicity-Contrasted Patterns Ho-Cheol Kim,* Cortney R. Kreller, Kiet A. Tran, Vikram Sisodiya, Sarah Angelos, Gregory Wallraff, Sally Swanson, and Robert D. Miller IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120 Received January 23, 2004. Revised Manuscript Received July 15, 2004
We report a simple and effective method to generate hydrophilicity-contrasted patterns on porous thin films containing nanometer sized pores. The porous thin films were prepared by removal of the organic polymer phase, denoted as porogen, from the nanohybrid of poly (methylsilsesquioxane) (PMSSQ) and the porogen where nanoscopic porogen domains are phase separated and entrapped in cross-linked PMSSQ matrix. Selective masking the UV radiation during UV/ozone treatment produces patterns of contrasted hydrophilicity. Hydrophilic patterns with resolution of ∼4 µm surrounded by hydrophobic regions were obtained by this patterning process. The nanoporous structure of patterns provides higher number density of reaction site, which was proved by the enhanced intensity of a fluorescent dye attached on patterns.
Introduction Arrays of patterns containing different bioactivities enable rapid evaluation of complex bioevents and have quickly developed into an important tool in life science research.1-3Because of the aqueous environment, patterned media having hydrophilicity contrast (i.e., hydrophilic regions surrounded by hydrophobic areas and vice versa) are ideal for confining bioactivity to within discrete regions. A few methods including traditional lithographic methods, imprinting, and contact printing have been previously used to generate hydrophilicitycontrasted patterns on substrates.4-6 The common feature of these methods is to deliver and tether hydrophilic (or hydrophobic) molecules to the predefined regions (by mask, template, or positioned pipet) that is surrounded by hydrophobic (or hydrophilic) regions. These methods generally involve, however, a series of materials (such as photoresists, elastomers, etc.) and process steps. A simple and more effective route to generating arrays of patterns with hydrophilicity contrast is highly desirable. Further, to reduce the overall size of an array while maximizing the number of * To whom correspondence should be addressed. E-mail: hckim@ us.ibm.com. Phone: (408) 927-3725. Fax: (408) 927-3310. (1) Niemeyer, C. M.; Blohm, D. Angew. Chem., Int. Ed. Engl. 1999, 38, 2865. (2) Pirrung, M. C. Angew. Chem., Int. Ed. 2002, 41, 1276. (3) MacBeath, G.; Schreiber, S. L. Science 2000, 289, 1760. (4) Butler J. H.; Croninc, M.; Anderson, K. M.; Biddison, G. M.; Chatelain, F.; Cummer, M.; Davi, D. J.; Fisher, L.; Frauendorf, A. W.; Frueh, F. W.; Gjerstad, C.; Harper, T. F.; Kernahan, S. D.; Long, D. Q.; Pho, M.; Walker, J. A., II; Brennan, T. M. J. Am. Chem. Soc. 2001, 123, 8887. (5) Bernard, A.; Renault J. P.; Michel, B.; Bosshard, H. R.; Delamarche, E. Adv. Mater. 2000, 12, 1067. (6) U.K. Patent Application GB 2340298A; U.K. Patent Application GB 2332273A.
reaction sites within the pattern and minimizing the required reagent and sample volume, substrates containing high surface area are also desirable. Porous materials containing nanometer sized pores are very attractive due to their potential for numerous applications including optical components, low-dielectricconstant (low-k) interlayer materials for interconnects in semiconductors, nanoscopic chemical reactors for catalysis, biotechnology, and so on. In the microelectronics area, effort has been devoted for developing nanoporous dielectrics to achieve better insulation between the Cu wires in back-end-of-line (BEOL) structures for next generation semiconductors. A simple and effective route to thin films containing nanometer-sized pores has been developed at IBM, which utilizes phase separation of a two-component system where one component (e.g., an organosilicate matrix) cross-links into a network effectively limiting domain growth and coarsening of the porogen phase (an organic, polymeric component) that is ultimately removed from the film by thermolysis.7-9 For this approach, the morphology of the nanohybrid, where the phase-separated porogen domains are entrapped within matrix materials, and hence the final morphology of pores, is strongly dependent on the interaction between porogen and matrix and the molecular weight, molecular architecture, and loading level of porogens. Poly (methylsilsesquioxane) (PMSSQ) has been recognized as a promising matrix material for low-k dielectrics and studied extensively. Thermally cross(7) Hedrick, J. L.; Labadie, J.; Russell, T. P.; Hofer, D.; Wakharker, V. Polymer 1993, 34, 4717. (8) Miller, R. D. Science 1999, 286, 412. (9) Hawker, C. J.; Hedrick, J. L.; Miller, R. D.; Volksen, W. MRS Bull. 2000, 25, 54.
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linked PMSSQ is intrinsically hydrophobic (typical static water contact angles are >100°) and has less than 1 wt % moisture sorption, which is an ideal property for low-k applications.10,11 However, as the results shown here indicate, the surface hydrophilicity can be easily tailored by a simple UV/ozone treatment. The combination of UV irradiation and ozone has been used as an effective, low-damage cleaning technique for organic materials.12,13 Although the precise mechanism of UV/ozone treatment has remained unclear in the literature, it has been widely used for a variety of etching/cleaning applications in the microelectronics industry. It is known that ozone is dissociated by absorption of 253.7 nm radiation or thermal heating into atomic oxygen, which is postulated to be the predominant etchant species. Over the temperature range from room temperature to ∼300 °C, organic materials are broken down into simple volatile oxidation products such as carbon dioxide, water, etc. Here, we demonstrate generating hydrophilicity-contrasted patterns on nanoporous PMSSQ thin films using a simple UV/ozone treatment. High-resolution patterns with hydrophilicity contrast can be generated by the method reported in this article. The quality of patterns was evaluated by attaching a fluorescent dye to hydrophilic areas. Patterns on nanoporous films show approximately 10 times enhanced signal intensity compare to flat surfaces, which suggests a potential use of the patterned porous films as a high density, 3-D substrate in biosciences.14
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Figure 1. Schematic diagram of the patterning process in this study.
Experimental Procedures For this study, PMSSQ (GR650F, TechneGlas) was used as the matrix, and a block copolymer of ethylene oxide and propylene oxide (P123, Pluronics, BASF) was used as the porogen, which eventually generates pores by thermal decomposition. Thin films of PMSSQ/porogen mixtures containing varying amounts of porogen were prepared by spin casting 1-methoxy-2-propanol acetate (PMAc) solutions on clean silicon wafers. Samples were heated to 250 and 450 °C under an argon atmosphere to obtain nanohybrids and porous films, respectively. Refractive index and film thickness were determined by a UV-vis interferometer (Filmetrics, Inc., F-20 thin film measurement system). Static water contact angle was measured using a video contact angle system (AST, 2500 XE). A FT-IR (Nicolet, Nexus 670) with a single reflection horizontal ATR accessory (GATR) was used to obtain IR spectra on the surfaces. UV/ozone treatments were performed using a UV/ozone stripper (SAMCO, UV-300H), which has a low-pressure Hg lamp and an ozone generator. Patterns with hydrophilicity contrast were generated on both nanohybrids and nanoporous films by exposing selective regions to UV/ ozone. A metal mask with openings (like a shadow mask) or a standard photolithographic mask was used (10) Lee, W. W.; Ho, P. S. MRS Bull. 1997, 22, 19. (11) Kim, H.-C.; Wilds, J. B.; Hinsberg, W. D.; Johnson, L. R.; Volksen, W.; Magbitang, T.; Lee, V.; Hedrick, J. L.; Hawker, C. J.; Miller, R. D.; Huang, E. Chem. Mater. 2002, 14, 4628. (12) Wood, P. C.; Wydeven, T.; Tsuji, O. Mater. Res. Soc. Symp. Proc. 1993, 315, 237. (13) Keene, M. T. J.; Denoyl, R.; Llewellyn, P. L. Chem. Commun. 1998, 20, 2203. (14) Lei, C.; Shin, Y.; Liu, J.; Ackerman, E. J. J. Am. Chem. Soc. 2002, 124, 11242.
to define the UV/ozone exposure areas for nanohybrids or porous films, respectively. Surface and cross-sectional morphology were observed using a tapping mode atomic force microscope (AFM, DI Dimension 3100) and a fieldemission scanning electron microscope (FESEM, Hitachi S-4700). A fluorescent dye, 6-FAM amidite, was attached to the patterned hydrophilic areas using a silane linker, 3-bis(2-hydroxyethyl)(aminopropyl)triethoxysilane (HE-APTS). The linker was tethered by soaking the patterned substrate in 60% ethanol solution of HEAPTS. 6-FAM amidite was attached using a DNA synthesizing machine (Applied Biosystems, 8909 Expedite Synthesizer) in DMT-thyminine solution. The fluorescent dye was activated by soaking the ethylenediamine and ethanol mixture solution. Fluorescent images were obtained using a fluorescent microscope (Olympus, BX51) with a wide blue filter cube, and the image intensity captured by a digital camera (Optronics, Macrofire) was quantified using an image analysis software (Image-Pro Plus). Results and Discussion The patterning process in this study is shown schematically in Figure 1. Spin casting the mixture solution of PMSSQ and porogen forms a homogeneous, thin film (typical thickness of a several hundred nanometers) on a silicon wafer (Figure 1A). PMSSQ undergoes a thermal cross-linking reaction above approximately 150 °C, which induces the phase separation between porogen and PMSSQ. The phase separation results in generation of a nanohybrid of PMSSQ and porogen where the organic porogen molecules are entrapped within crosslinked PMSSQ matrix (Figure 1B). It should be noted
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Figure 2. Effect of UV/ozone treatment temperature and time on static water contact angles of porous PMSSQ films.
that the cross-linked PMSSQ is hydrophobic, while oligomeric PMSSQ is hydrophilic. Both the increase in molecular weight by thermal cross-linking reaction and the dramatic change in hydrophilicity that alters the interaction between porogen and PMSSQ cause the phase separation between organic porogen phase and PMSSQ. Further heating the nanohybrid above 450 °C decomposes the thermally labile organic porogen molecules and produces a porous structure (Figure 1C). Patternwise UV/ozone treatment using a standard photolithographic mask generates hydrophilic areas within a hydrophobic matrix (Figure 1D). Alternatively, porous hydrophilic areas can be generated by the UV/ ozone treatment on the nanohybrid using a shadowmask type mask. In this case, UV/ozone treatment not only removes organic porogen molecules within the exposed area but also changes hydrophilicity of matrix PMSSQ (Figure 1E); hence, both pore generation and pattern formation can be achieved by a low temperature, single-step process. As mentioned previously, pore morphology is strongly dependent on the interaction between porogen and PMSSQ and the loading level of porogen. The porogen phases nucleate and grow during the phase separation and form domains of random shapes and broad size distribution. The amount of porogen (i.e., loading level) determines pore morphology ranging from a closed cell to highly interconnected bicontinuous structures. The interconnected pore morphology is desirable for biosubstrate applications, while the closed cell structure is ideal for low-k materials. For this study, we mainly focus on 80 wt % porogen loading, which shows highly interconnected pore morphology. Static water contact angles were measured to monitor the changes in surface hydrophilicity by the UV/ozone treatments. Figure 2 shows the water contact angle of porous PMSSQ, prepared with 80 wt % porogen loading, as functions of UV/ozone treatment time and temperature. As shown in Figure 2, the water contact angle decreases rapidly over time, indicating that the surface is becoming more hydrophilic. This phenomenon is accelerated at higher temperatures. The water contact angle decreases from around 120° (marked with an asterisk) to 10° or less with the treatments. The contact angle data indicate that the surface of the porous PMSSQ film becomes hydrophilic with the UV/ozone treatment and that the
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Figure 3. FT-IR spectra of porous PMSSQ as a function of UV/ozone treatment time at 30 °C.
degree of this hydrophilicity can be controlled over the broad range from more than 100 to 10° or less. Porous films prepared with 10, 30, and 50 wt % porogen loadings were examined as well and gave substantially similar results. Figure 3 shows FT-IR spectra of porous PMSSQ as a function of UV/ozone treatment time at 30 °C. A FT-IR with the GATR, which is a Ge ATR crystal having 65° incidence angle, was used to evaluate the changes in surface functional groups of PMSSQ. As shown in Figure 3, characteristic peaks of Si-CH3 at 780, 1272, and 2973 cm-1 decrease with the UV/ozone treatment, while Si-OH characteristic peaks at 952 and 3430 cm-1 increase. The FT-IR data confirm the conversion of methyl groups (-CH3) to hydroxyl groups (-OH), which makes the porous surface hydrophilic, thereby generating lower water contact angles shown in Figure 2. Heating the mixture films of PMSSQ and porogen to 250 °C generates nanohybrids. In the nanohybrid state, organic porogen molecules that are phase separated from cross-linked PMSSQ exist within cross-linked PMSSQ matrix. UV/ozone treatment on the nanohybrids is found to remove the organic porogen phase and simultaneously change the hydrophilicity of PMSSQ matrix. Figure 4 shows the changes in refractive index (n), film thickness (t), and contact angle of the nanohybrid by UV/ozone treatment at 30 °C. Refractive index decreases with treatment time (Figure 4A) as the UV/ ozone treatment removes organic porogen phase from the hybrid and produces porous structure. The nanohybrid has a n value of 1.44 prior to any treatment. It decreases with the UV/ozone treatment and reaches eventually about 1.20. Higher treatment temperature accelerates the rate of decrease in n values. Film thickness of the nanohybrid decreases with the treatment (Figure 4B), which resulted from removal of the major component of the hybrid (i.e., 80 wt % of organic porogen). It is noted that even though the film shrinks by UV/ozone treatment, the film is porous after UV/ ozone treatment as indicated by the lower refractive index. Figure 4C shows the changes in static water contact angle by UV/ozone treatment on the nanohybrid. Because of the hydrophilicity of porogen and lower curing temperature of PMSSQ (water contact angle for PMSSQ cured at 250 and 450 °C are 89 and 107°,
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Figure 5. Images of water drops on hydrophilicity contrasted patterns of porous PMSSQ films. Diameter: (A) 6.35 mm and (B) 250 µm.
Figure 4. Changes in refractive index (A), film thickness (B), and static water contact angle (C) of nanohybrids by UV/ozone treatment at 30 °C.
respectively), nanohybrid shows a water contact angle of 78° initially, and it decreases with the treatment, indicating that the UV/ozone treatment also affects hydrophilicity of matrix PMSSQ. The results on nanohybrid suggest that the UV/ozone treatment on the nanohybrid can be used to generate areas of porous and hydrophilic surrounded by nonporous nanohybrid matrix as shown schematically in Figure 1E. The ability to control surface hydrophilicity by the simple UV/ozone treatment is applicable for pattern generation. Patterns with hydrophilicity contrast can be generated on both nanohybrid and porous PMSSQ. Images in Figure 5 indicate the formation of hydrophilic patterns surrounded by hydrophobic porous PMSSQ. Images are obtained after spreading water on the patterned surfaces containing different pattern sizes.
For circular patterns of larger diameter (6.35 mm), water is confined as drops in only the patterned hydrophilic areas (Figure 5A). As shown in Figure 5B, the surface tension of water cannot break water to drops for circular patterns of 250 µm diameter but only shows wavy shapes at the water/surface/air contact line. The penetration depth of UV/ozone treatment used for this study is not clear at the moment. It deserves more study to elucidate depth profile of hydrophilicity using neutron reflectivity with D2O.16 As mentioned previously, UV/ ozone treatment generates porous structure in nanohybrid. AFM images show differences in surface topography between patterns and surrounded areas. Figure 6B is a height-contrasted AFM image of patterns generated on the nanohybrid with 80 wt % porogen loading. Compared to the AFM image of surrounded area shown in Figure 6B, the UV/ozone treated area shows very rough surfaces resulting from the removal of porogen molecules. FESEM image of pattered area shown in Figure 6C clearly shows that the UV/ozone treatment generates porous structure in PMSSQ matrix. The results in Figures 5 and 6 suggest that a probe molecule can be bonded covalently to the patterned area in an aqueous environment using the surface hydroxyl groups generated by UV/ozone treatment. This was demonstrated by attaching a fluorescent dye selectively within the hydrophilic patterns. Figure 7 shows a fluorescence micrograph of a patterned nanoporous film where 6-FAM amidite was attached within the patterns using a silane linker, 3-bis(2-hydroxyethyl)aminopropyl triethoxysilane (HE-APTS). Well-defined arrays of circular patterns were observed, which confirms selective tethering of fluorescent dye molecules to the patterns. It should be noted that the fluorescent intensity from the patterns is significantly higher than from the hydrophobic background. The fluorescence intensity was quantified by a histogram analysis using image analysis software. As shown in Figure 8, the fluorescent signal intensity was approximately 10 times higher from porous PMSSQ than from a native oxide layer of a flat silicon wafer that was not treated by UV/ozone and (15) Benoit, V.; Steel, A.; Torres, M.; Yu, Y.-Y.; Yang, H.; Cooper, J. Anal. Chem. 2001, 73, 2412. (16) Kim, H.-C.; Volksen, W.; Miller, R. D.; Huang, E.; Yang, G.; Briber, R. M.; Shin, K. W.; Satija, S. K. Chem. Mater. 2003, 15, 609.
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Figure 8. Green light intensities of 6-FAM dye attached to silicon wafer, dense PMSSQ, and porous PMSSQ.
Figure 6. Height contrasted AFM images and cross-sectional FESEM image of UV/ozone patterned nanohybrid: (A) AFM image of nontreated area; (B) AFM image of treated area; and (C) FESEM image of treated area.
Figure 7. Fluorescence micrographs of patterns attached with 6-FAM dye. Diameter: 250 µm.
about 7 times higher than the signal from a nonporous PMSSQ surface exposed to the same UV/ozone treatment. This is attributed to the 3-D porous structure of the substrate: the number density of -OH groups useful for fluorescent attachment is much higher than that of a flat surface due to high surface area of the nanoporous substrate. The quantitative data shown in Figure 8 are clear evidence of a volumetric effect, namely, that porous PMSSQ surfaces allow for a higher number density of attached molecules than do nonporous PMSSQ, indicating that -OH groups are formed throughout the porous samples.
Figure 9. Fluorescence micrographs of patterns of different feature sizes. Line width: (A) 32 µm; (B) 16 µm; (C) 8 µm; and (D) 4 µm.
The spatial density of this patterning process has been evaluated using a mask having different feature sizes. As shown in Figure 9, well-defined patterns were observed down to 8 µm feature sizes, while 4 µm features show smeared boundaries. Considering feature sizes of current pattern arrays in biotechnologies are on the order of several tens of micrometers, porous substrate-containing patterns generated by the method in this study promise to provide much higher number density of arrays. Conclusions We demonstrated generating hydrophilicity-contrasted patterns on nanoporous PMSSQ thin films using a simple UV/ozone treatment. The UV/ozone treatment converts methyl groups of PMSSQ to hydroxyl groups; hence, patternwise exposure of UV irradiation generates hydrophilic regions within hydrophobic areas. The hydroxyl groups generated by this method can be used for further reactions including tethering a fluorescent dye. Because of the porous, 3-D structure of the matrix, the patterned substrate has a much higher number density
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of functional groups than flat surface, which results in 10 times higher fluorescence intensity than the corresponding flat silicon wafer surface. The combination of high surface area and facile patterning process makes the nanoporous PMSSQ films promising for a variety of potential applications including biosubstrates.
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Acknowledgment. We acknowledge the Center for Polymer Interfaces and Macromolecular Assemblies (CPIMA) at IBM Almaden Research Center sponsored by NSF-MRSEC (Grant DMR-9808677) and NSF-REU (Grant DMR-9820149). CM049866K
