VOLUME 4, NUMBER 7, JULY 2004 © Copyright 2004 by the American Chemical Society
Photopatterned Nanoporous Media Ho-Cheol Kim,* Gregory Wallraff, Cortney R. Kreller, Sarah Angelos, Victor Y. Lee, Willi Volksen, and Robert D. Miller IBM Research DiVision, Almaden Research Center, 650 Harry Road, San Jose, California 95120 Received January 21, 2004; Revised Manuscript Received May 12, 2004
ABSTRACT We report a direct photopatterning method to produce patterns of thin films containing nanometer-sized pores. A photoacid generator (PAG) was used as a photosensitive agent to initiate the cross-linking reaction of poly(methylsilsesquioxane) (PMSSQ) while an amphiphilic polymeric nanoparticle templates porous structure. Deep UV exposure on the films of PMSSQ/nanoparticle mixtures through a photolithographic mask and subsequent solvent rinsing give well-defined hybrid patterns where the polymeric nanoparticles are entrapped in the cross-linked PMSSQ matrix. The porous structure was generated by subsequent heating the hybrid patterns above the thermal decomposition temperature of the polymeric nanoparticle. Patterns of nanoporous film with controlled hydrophilicity were obtained by a simple UV/ozone treatment which mitigates the intrinsic hydrophobicity of the PMSSQ matrix. The patterned nanoporous films could find a variety of potential applications ranging from ultralow dielectric constant (ULK) materials to high surface area substrates for catalysis and biotechnology.
Organic or inorganic thin films containing nanoscopic pores are very attractive for a variety of applications due to their unique physicochemical properties. Potential applications include ultralow dielectric constant (ULK) material for interlayer (or intermetal) dielectrics,1-3 optical components,4 supporting materials for catalysts,5 sensors,6 biosubstrates,7 and so on. A simple and effective method to generate patterns of porous thin films on substrates would broaden areas of applications significantly. In the semiconductor industry, for example, a simpler process to generate patterns of porous dielectric materials can reduce the cost for current back-endof-line (BEOL) integration scheme that constitutes about 75% of the total wafer process cost. Arrays of patterns of porous films are also useful as substrates for biological applica* Corresponding author:
[email protected] (email), 408-927-3725 (phone), 408-927-3310 (fax). 10.1021/nl0498738 CCC: $27.50 Published on Web 05/29/2004
© 2004 American Chemical Society
tions.8,9 High surface area resulting from porous structure would allow for the overall size of an array to be reduced by maximizing the number of reaction sites within the patterns which contain different bioactivities and enable rapid evaluations of complex bioevents.10 To generate thin films containing pores of nanometer sizes is, however, not trivial. Numerous methods including templating pores using self-assembly of surfactants and inorganic precursors,11 catalyzed sol-gel polymerization of orthosilicate esters,12 synthesizing pure silica zeolite,13 generating periodic mesoporous organosilica through self-assembly of silsesquioxane precursors,14 etc. have been proposed and studied extensively. An approach originally designed for producing ULK materials is known to be a facile route to nanoporous thin films: nanoscopic pores are generated by removing sacrificial organic polymers, denoted as porogens,
from a hybrid of organic polymers and matrix materials where the phase-separated organic polymer forms domains of nanometers within the cross-linked matrix. The size, size distribution, and morphology of pores generated by this approach strongly depend on the interaction between matrix and porogens, composition of the hybrid, and the molecular architecture of the porogens.15 Thin organic or inorganic films prepared by this approach are known to be viable material candidates that meet the requirements for ULK materials in interconnects of future microchips. In this letter, we report a direct photopatterning method to generate patterns of nanoporous poly(methylsilsesquioxane) (PMSSQ) thin films on substrates. We employ a photosensitive acid generator (PAG) which initiates a crosslinking reaction of PMSSQ in the mixtures of PMSSQ and porogens. Pattern-wise radiation of deep UV through a photomask followed by wet development with solvent produces patterned hybrid films. Subsequently, porous structure is created by removing the organic polymer phase. The hydrophilicity of patterns of nanoporous material was controlled by subsequent UV/ozone treatment, which provides arrays of patterned nanoporous films with a variety of hydrophilicity. Figure 1 depicts schematically the pattern generation method of this study. Thin films of PMSSQ, porogen and PAG mixture were prepared by spin coating 20 wt % 1-methoxy-2-propanol acetate solutions on to a clean silicon wafer (Figure 1A). An amphiphilic polymeric nanoparticle containing a polystyrene (PS) core and a poly(ethylene oxide) (PEO) corona was used as the porogen. A combination of anionic polymerization and atom transfer polymerization was used to synthesize the porogen. It is known that living PS anionic chains produce star polymers in the presence of a cross-linking reagent such as divinyl benzene.16 As shown in Figure 2, the initial reaction using 3-tert-butyldimethylsiloxy-2,2-dimethylpropyl lithium (FMC Corporation) as an initiator produces a polyanionic star polymer with linear PS arms emanating from a small polyanionic core. Subsequent addition of more styrene (ST) monomer leads to the growth of additional arms from the core, thus a star polymer containing half of the total arms decorated with tertbutyldimethylsiloxy functionality is produced. Static and dynamic light scattering were used to obtain the molecular weight (Mw) and hydrodynamic radius (Rh) in THF (Mw ) 269 000 g/mol, Rh ) 7.8 nm). The tert-butyldimethylsilyl protecting groups were removed with tetrabutylammonium fluoride (1 M in THF) and the pendant hydroxy functionality esterified with 2-bromo isobutyryl chloride in methylene chloride in the presence of 4-N,N-dimethylaminopyridine (DMAP) and triethylamine (TEA). The material so functionalized constitutes a multiarm macroinitiator for ATRP processes. A polar corona was grown by the polymerization of poly(ethylene glycol) methacrylate using bis-triphenylphosphine nickel(II) bromide as the catalyst in toluene (95 °C, 40% solids, initiator/catalyst ) 0.23, nickel/monomer ) 0.019). The product was isolated by precipitation. From 1H NMR, the degree of polymerization per arm was estimated at ∼14 and the measured Rh (THF) ) 10.1 nm. A photoacid 1170
Figure 1. Schematic diagram of the patterning process in this study.
generator (PAG), bis(tert-butyl phenyl)iodonium triflate (TBIT), was used as the photosensitive initiator for crosslinking of PMSSQ. Portions of the thin film were exposed to a deep UV radiation source (Optical Associations Inc., wavelength ) 254 nm, dose ) 145 mJ/cm2) through a standard photolithographic mask (Figure 1A). The PAG within each selectively exposed area produces acid and initiates the cross-linking reaction of PMSSQ (Figure 1B) as reported in the literature.17,18 Similar to the process of standard negative tone photoresists, patterns on the layer were developed by rinsing with 1-methoxy-2-propanol acetate (Figure 1C). The patterned films were heated to 450 °C at a rate of 5 °C/min under argon atmosphere to generate a porous structure by removing the organic porogen phase (Figure 1D). Subsequent UV/ozone treatment with a commercial UV/ozone stripper (SAMCO, UV-300H) was used to control the hydrophilicity of the patterned porous materials. As shown in the optical micrographs in Figure 3, welldefined checkerboard patterns of hybrid film were obtained by the pattering method of this study. The feature sizes of patterns in Figure 3 are 50 µm (A) and 100 µm (B). Effort to generate patterns of porous media has been devoted by several research groups. Hozumi et al. used area-selective Nano Lett., Vol. 4, No. 7, 2004
Figure 2. Schematic synthetic route for the porogen.
growth of silica-organic nanocomposite films and the subsequent elimination of the organic component to generate micorpatterned silica films with ordered nanopores.19 Doshi et al. reported a method for forming a patterned mesoporous material by selective UV exposure on the self-assembly of the photosensitive silica/surfactant mesophase.20 More recently, Li et al. demonstrated a patterning method of oriented zeolite monolayer films using a substrate having prepatterned gold islands.21 However, to the best of our knowledge, generating patterns containing isotropic pores of nanometer sizes without a prepatterned template surface has not been reported in the literature. Figure 4 shows FESEM images of 50 µm feature sized patterns of porous film prepared with 20 wt % loadings of porogen. Porous structure is generated by thermolysis of the porogen. The cross-sectional image in Figure 4B clearly shows the porous structures of the patterned film with randomly distributed spherical pores of approximately 20 nm in diameter. The refractive index (n) of the patterns measured with a UV-visible interferometer (model F20, Filmetrics, Inc.) was 1.23, which also indicates that the patterns have porous structure since for dense PMSSQ, n ) 1.37. It should be noted that generating patterns containing nanoporous structure as shown in Figure 4 was only possible with the star-shaped nanoparticle porogen used in this study. Other Nano Lett., Vol. 4, No. 7, 2004
porogens of linear or branched molecular architectures were able to generate patterns in hybrid films but not to template porous structure within the patterns. This is attributable to the different phase separation mechanisms resulting from the molecular architectures of porogens. The nanoparticle used here as a porogen is found to be immiscible with PMSSQ in as-cast film but templates pores effectively without matrix collapse during the pore generation process. Detailed discussion on the phase separation mechanism is beyond this letter’s perspective and will be explored in more detail in a future publication.15 The combination of UV irradiation and ozone has been used as an effective, low damage cleaning technique for organic materials.22,23 Although the precise mechanism of UV/ozone treatment has remained unclear, 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. Changes in surface hydrophilicity of the PMSSQ matrix with the UV/ozone treatment were measured by static water contact angles using a video contact angle system (AST, 2500 XE). UV/ozone treatments were performed using a UV/ozone stripper which has a low1171
Figure 4. FESEM images of patterned nanoporous film: (A) top view; (B) cross-sectional image of patterned area.
Figure 3. Reflection optical micrographs of patterns of nanohybrids. Feature size: (A) 50 µm; (B) 100 µm.
pressure Hg lamp and an ozone generator. As shown in Figure 5, a cross-linked dense PMSSQ surface shows intrinsic hydrophobicity showing a water contact angle of 102°. UV/ozone treatment at 30 °C mitigates hydropbobicity of the surface, which results in decreasing water contact angle with treatment time. The decrease in water contact angle is accelerated at higher temperatures (not shown here) and porous films prepared with various porogen loadings give substantially similar results. This suggests that patterned nanoporous films having a wide range of hydrophilicity can be easily prepared by the patterning process in this study followed by UV/ozone treatment. Figure 6 shows optical microscope images of patterns having different feature sizes. The line width of the patterns ranges from 8 to 1 µm. Clearly, the images show that the patterning method in this study can generate patterns with a spatial resolution of ∼1 µm. It will require further work to generate submicron patterns by using a mask of smaller feature size and optimizing the pattering process. Mechanical properties such as modulus, hardness, fracture toughness, adhesion, and coefficient of thermal expansion (CTE) are critical parameters for porous materials to achieve successful fabrication for given applications. However, measurement of these properties has been challenging and time-consuming. In the semiconductor industry, Young’s 1172
Figure 5. Static water contact angles of dense PMSSQ surfaces as a function of UV/ozone treatment time at 30 °C.
modulus is a commonly accepted parameter for presenting mechanical property of porous materials. Traditionally, nanoindentation has been used to determine Young’s moduli of thin solid films but is challenged by soft materials, especially those of submicron thickness or those which exhibit significant viscoelastic behavior.24 Recently, surface acoustic wave spectroscopy (SAWS) has been introduced as an alternative method to determine Young’s modulus of soft, porous materials based on the fact that Nano Lett., Vol. 4, No. 7, 2004
Figure 6. Reflection optical micrographs of patterns of different feature sizes. Line width: (A) 8 µm; (B) 4 µm; (C) 2 µm; (D)1 µm.
Figure 7. Modulus vs porogen loading for a series of nanoporous PMSSQ films. Open circles and dashed line present measured data and the best fit, respectively.
the phase velocity of the acoustic wave depends on the Young’s modulus, density, thickness, and Poisson’s ratio of thin films on substrates.25-27 To evaluate mechanical stability of our nanoporous PMSSQ thin films, we measured the Young’s moduli of nanoporous films as a function of porosity using a laser-acoustic thin film analyzer (LaWave, FraunNano Lett., Vol. 4, No. 7, 2004
hofer, USA). Acoustic waves were generated by a nitrogen pulse laser (wavelength 337 nm, pulse duration 0.5 ns) and detected using a transducer employing a piezoelectric polymer film as a sensor. The measured surface wave velocity as a function of frequency was fitted with the theoretical dispersion curve to deduce Young’s modulus. Poisson’s ratio was assumed as 0.25, and density and thickness were determined from X-ray reflectivity and UVvisible interferometery, respectively. These measurements show that the modulus decreases rapidly with increasing the loading amount of porogen, as shown in Figure 7. The data fit well to an exponential decay function, E (GPa) ) 0.27 + 3.94 exp(-P/15.84), where P is the porogen loading wt %. The exact requirements for the modulus of nanoporous materials could vary depending on specific applications. The porogen loading (20 wt %) for this study gives modulus of 1.61(0.09 GPa, which is lower than the required modulus (2 GPa, by SAWS28) to display resilience to chemical mechanical polishing (CMP) and planarization process in chip fabrication. In summary, we demonstrated a simple photopatterning process to generate nanoporous PMSSQ patterns. Welldefined arrays of patterns with nanoscopic pores were obtained by this method. Hydrophilicity of the pattern surface was controllable by using a simple UV/ozone treatment. A variety of potential applications in the fields of microelectronics, optics and biotechnology are envisioned with the patterns of nanoporous PMSSQ of this study. Acknowledgment. We acknowledge the Center for Polymer Interfaces and Macromolecular Assemblies (CPI1173
MA) at IBM Almaden Research Center sponsored by NSFMRSEC (Grant DMR-9808677) and NSF-REU (Grant DMR-9820149). References (1) Hedrick, J. L.; Labadie, J.; Russell, T. P.; Hofer, D.; Wakharker, V. Polymer 1993, 34, 4717. (2) Miller, R. D. Science 1999, 286, 412. (3) Hawker, C. J.; Hedrick, J. L.; Miller, R. D.; Volksen, W. MRS Bull. 2000, 25, 54. (4) Walheim, S.; Schaffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520. (5) Gin, D. L.; Gu, W. AdV. Mater. 2001, 13, 1407. (6) Lu, Y.; Han, L.; Brinker, J.; Niemczyk, T. M.; Lopez, G. P. Sens. Actuators 1996, B35-36, 517. (7) Lei, C.; Shin, Y.; Liu, J.; Ackerman, E. J. J. Am. Chem. Soc. 2002, 124, 11242. (8) Niemeyer, C. M.; Blohm, D. Angew. Chem., Int. Ed. Engl. 1999, 38, 2865. (9) Pirrung, M. C. Angew. Chem., Int. Ed. 2002, 41, 1276. (10) Glazer, M.; Frank, C.; Vinci, R. P.; McGall, G.; Fidanza, J.; Beecher, J. Mater. Res. Soc. Symp. Proc. 1999, 576, 371. (11) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (12) Hrubesh, L. W.; Keene, L. E.; LaTorre, V. R. J. Mater. Res. 1993, 8, 1736. (13) Wang, Z.; Wang, H.; Mitra, A.; Huang, L.; Yan, Y. AdV. Mater. 2001, 13, 746. (14) Landskron, K.; Hatton, B. D.; Perovic, D. D.; Ozin, G. A. Science 2003, 302, 266.
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