Nanoporous Organosilicate Thin Films Prepared with Covalently

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Chem. Mater. 2006, 18, 378-385

Nanoporous Organosilicate Thin Films Prepared with Covalently Bonded Adamantylphenol Pore Generators Bong Jun Cha,† Suhan Kim,† Kookheon Char,*,† Jin-Kyu Lee,‡ Do Y. Yoon,‡ and Hee-Woo Rhee§ School of Chemical and Biological Engineering and NANO Systems Institute-National Core Research Center and School of Chemistry and Molecular Engineering, Seoul National UniVersity, Seoul 151-744, Korea, and Department of Chemical Engineering, Sogang UniVersity, Seoul 121-742, Korea ReceiVed August 25, 2005. ReVised Manuscript ReceiVed October 28, 2005

A new class of nanoporous organosilicate thin films with balanced mechanical and low dielectric properties has been designed and prepared using covalently bonded adamantylphenols as pore-generating (porogen) materials. The adamantylphenol groups were grafted or bridged to poly(methyl silsesquioxane) (PMSSQ) polymers through propyl linkers and the thermal decomposition of such porogens through the cleavage of covalent bonds during curing was confirmed by FT-IR, TGA, and GC-MS. One of the nanoporous thin films examined in this study contains nanopores, less than 10 nm, within the films with 18% porosity, as characterized by X-ray reflectivity, ellipsometry, and nitrogen sorption analysis. Elastic modulus of a nanoporous film measured by a nanoindenter was significantly increased to 5.5 GPa, while maintaining the dielectric constant of 2.3, which is due to the partial formation of silica structure by the decomposition of residual propenyl groups after the bond cleavage of porogens and also due to the enhanced cross-linking density in the case of bridge-PMSSQ.

Introduction Nanoporous organic and inorganic thin films provide a variety of potential applications such as intermetallic dielectric materials, optical components, sensor elements, and substrates for biological applications.1 The most straightforward way to realize thin films containing nanometer-size pores is to introduce sacrificial porogens (or pore-generating materials) into the thin films, where the porogens can be removed by physical or chemical means leaving behind airfilled nanopores. In the semiconductor industry, this approach is particularly valuable since ultra-low-k dielectrics with dielectric constant (k) value as low as 1.5 can only be achieved with porosity.2 However, nanoporous materials have inherently poor mechanical stability owing to their reduced density. Consequently, the preparation of low-k thin films with improved mechanical properties is an important issue in the integration of low-k materials in the semiconductor industry.3 More recently, as an alternative to the incorporation of porogens into a host film by simple blending, a new * Corresponding author. Tel.: +82-2-880-7431. Fax: +82-2-888-7295. E-mail: [email protected]. † School of Chemical and Biological Engineering and NANO Systems Institute-National Core Research Center, Seoul National University. ‡ School of Chemistry and Molecular Engineering, Seoul National University. § Sogang University.

(1) Kim, H.-C.; Wallraff, G.; Kreller, C. R.; Angelos, S.; Lee, V. Y.; Volksen, W.; Miller, R. D. Nano Lett. 2004, 4, 1169 and references therein. (2) (a) Yang, S.; Mirau, P. A.; Pai, C.-S.; Nalamasu, O.; Reichmanis, E.; Pai, J. C.; Obeng, Y. S.; Seputro, J.; Lin, E. K.; Lee, H.-J.; Sun, J.; Gidley, D. W. Chem. Mater. 2002, 14, 369. (b) Ro, H. W.; Char, K.; Lee, J. K.; Min, S. K.; Rhee, H. W.; Yoon, D. Y. Polym. Prepr. 2001, 42 (2), 889. (c) Ro, H. W.; Char, K.; Chu, S. H.; Jin, M. Y.; Kim, W. C.; Lee, J. K.; Min, S. K.; Rhee, H. W.; Yoo, D. Y.; Yoon, D. Y. Polym. Prepr. 2002, 43 (2), 1166. (d) Kim, S.; Toivola, Y.; Cook, R. F.; Char, K.; Chu, S.-H.; Lee, J.-K.; Yoon, D. Y.; Rhee, H. W. J. Electrochem. Soc. 2004, 151, F37.

approach with porogens covalently bonded to the matrix precursors receives more attention since this approach can possibly avoid the massive phase separation that is often encountered in the blending approach and thus can have potential to realize attractive low-k materials with balanced controlled porous structure and improved mechanical properties. Various thermally labile moieties ranging from nanoscopic molecules such as alkyls or norbornenes to macromolecules such as PAMAM dendrimers and poly(methyl methacrylate)s (PMMAs) were covalently bonded to the silsesquioxane matrix and it has been reported that the reduction in dielectric constant is realized after the decomposition of the porogens.4 In the present study, we describe the formation of nanoporous organosilicate thin films with robust mechanical and low dielectric properties using adamantylphenols grafted or bridged to poly(methyl silsesquioxane) (PMSSQ). Adamantane is a small, bulky, and caged molecule and its axial length is smaller than ca. 1 nm, also known as nicely fitted into a β-cyclodextrin cavity.5 To be applied as intermetallic dielectrics, nanoporous films with desirable electrical and mechanical properties can be achieved by the thermal (3) (a) Braun, A. E. Semicond. Int. 2003, May. (b) Braun, A. E. Semicond. Int. 2003, August. (4) (a) Mikoshiba, S.; Hayase, S. J. Mater. Chem. 1999, 9, 591. (b) Zhong, B.; Spaulding, M.; Albaugh, J.; Moyer, E. Polym. Mater. Sci. Eng. 2002, 87, 440. (c) Padovani, A. M.; Rhodes, L.; Allen, S. A. B.; Kohl, P. A. J. Electrochem. Soc. 2002, 149, F161. (d) Padovani, A. M.; Riester, L.; Rhodes, L.; Allen, S. A. B.; Kohl, P. A. J. Electrochem. Soc. 2002, 149, F171. (e) Yu, S.; Wong, T. K. S.; Hu, X.; Pita, K.; Ligatchev, V. J. Electrochem. Soc. 2004, 151, F123. (f) Ro, H. W.; Kim, K. J.; Theato, P.; Gidley, D. W.; Yoon, D. Y. Macromolecules 2005, 38, 1031. (h) Su, K.; Bujalski, D. R.; Eguchi, K.; Gordon, G. V.; Ou, D.; Chevalier, P.; Hu, S.; Boisvert, R. P. Chem. Mater. 2005, 17, 2520. (5) Eftink, M. R.; Andy, M. L.; Bystrom, K.; Perlmutter, H. D.; Kristol, D. S. J. Am. Chem. Soc. 1989, 111, 6765.

10.1021/cm051916d CCC: $33.50 © 2006 American Chemical Society Published on Web 12/15/2005

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decomposition of adamantylphenol groups and the partial formation of silica structure. Although low-k films using chemically bonded porogens have previously been reported,4 a systematic study on the structural change during the decomposition of chemically bonded porogens from the PMSSQ matrix and its subsequent effects on both electrical and mechanical properties has not been performed in detail, which is the main objective of the present study. Experimental Section Characterization. 1H NMR spectra were obtained at room temperature using a Bruker (AVANCE) 500 MHz. The NMR spectra were taken in deuterated chloroform for porogen-bonded monomers and in acetone-d6 for the copolymers. Gel permeation chromatographic (GPC) analyses were carried out using a Spectra System P1000 with a LR125 Laser Refractometer as a detector. THF was used as an eluent at a flow rate of 1.0 mL/min and the GPC results were calibrated with respect to polystyrene standards (Polymer Standard Service, USA), ranging from 500 to 1000000 in molecular weight. Thermal gravimetric analysis (TGA) was performed using a TA model 2050 TGA instrument under N2 flow at a heating rate of 10 °C/ min. The solid samples were heated from 50 °C up to 900 °C. The weight loss of the samples (TG curves) and their temperature derivatives (DTG curves) were collected. Volatile compounds evolved during the curing under N2 were also recovered in bulk and analyzed by GC-MS (GC System, HP 6890). 29Si CP-MAS NMR spectra were obtained on a spectrometer (UnityINFINITYplus, Varian) at 6 kHz. All the samples for the measurements were annealed for 2 h at a set temperature under N2 purge. Nitrogen sorption analyses were performed on a Micromeritics ASAP 2010 gas sorption porosimeter. FT-IR measurements were performed on a JASCO FT/IR 200 spectrometer; 10 accumulations were signal-averaged at a resolution of 4 cm-1. Baseline-corrected infrared spectra were obtained for the films on silicon wafers in transmission mode at room temperature. The refractive indices of thin films were measured with a variable-angle multiwavelength ellipsometer (Gaertner, L2W16C830) with two wavelengths at 633 and 834.5 nm and 49 points wafer scanning setup. Mechanical properties of all the films were measured by a nanoindenter (Nano Indenter XP, MTS Corp.) in the CSM (Continuous Stiffness Measurement) with a load of 0.3 mN. All the modulus and hardness values reported here were averaged over eight indents. The dielectric constants were measured in the metal-insulator-metal (MIM) configuration, with evaporated aluminum electrodes, at 1 MHz using a HP4284 LCR meter. Field emission scanning electron microscopy (JEOL 6330F) was employed to investigate the cross-sectional texture of the thin films. Spin-coated films were cut into small pieces and fixed vertically on a sampling holder to analyze the internal structure. To minimize the film damage due to the electron beam and also to obtain clear images, gold was sputtered onto the thin films. X-ray reflectivity measurements were also carried out in the 3C2 X-ray beam line at Pohang Light Source, Korea. Computational Methods. All the geometries for the porogenbonded monomers were fully optimized and characterized by frequency calculations with all positive frequencies at the B3LYP/ 6-31G(d) level of theory. To obtain more accurate energies, singlepoint energies were obtained at the B3LYP/6-311+G(3df,2p) level using the optimized geometries at the B3LYP/6-31G(d) level, B3LYP/6-311+G(3df,2p)/B3LYP/6-31G(d). All the calculations were performed with the Gaussian 98 Program. Materials. All the reagents used in the present study were purchased from Aldrich or TCI and used as received unless otherwise specified.

Chem. Mater., Vol. 18, No. 2, 2006 379 Preparation of Precursors. 4-(Hydroxyphenyl)adamantane (2). A solution of 1-bromoadamantane (1) (10.0 g, 46.5 mmol) and phenol (35 g, 372 mmol) was refluxed overnight at 120 °C under N2 atmosphere. The mixture was poured into 1000 mL of hot water and washed three times to remove excess phenol. The product was filtered and dried under vacuum to yield 2 as a white solid. Yield: 10.2 g (96%). 1H NMR (CDCl3): δ 7.24 (d, -ArH-, 2H), 6.78 (d, -ArH-, 2H), 4.61 (s, -OH, 1H), 2.08 (br, adamantyl, -CH-, 3H), 1.88 (adamantyl, -CH2-, 6H), 1.76 (adamantyl, -CH2-, 6H). GC-MS (m/z): 228. 1-(4-Allyloxyphenyl)adamantane (3). A solution of 2 (2.0 g, 8.8 mmol), K2CO3 (3.6 g, 26 mmol), and allyl bromide (2.3 mL, 26 mmol) was refluxed in acetone (25 mL) at 80 °C for 12 h. The mixture was allowed to cool to room temperature (rt), K2CO3 was filtered off, and the filtrate was concentrated under reduced pressure. The product was further dried under vacuum for 24 h in order to yield 2.4 g of 3 in light yellow solid. Yield: 2.4 g (89%). 1H NMR (CDCl3): δ 7.27 (d, -ArH-, 2H), 6.87 (d, -ArH-, 2H), 6.07 (m, -CHd, 1H), 5.44 (m, CH2d, 2H), 5.26 (m, CH2d, 2H), 4.52 (d, -CH2-, 2H), 2.08 (s, adamantyl, -CH-, 3H), 1.88 (adamantyl, -CH2-, 6H), 1.71 (adamantyl, -CH2-, 6H). GC-MS (m/z): 268. 1-(4-Trimethoxysilylpropoxyphenyl)adamantane (4). A 25 mL three-necked flask equipped with a condenser, N2 inlets, and a stir bar was charged with 0.15 g of 1% Pt on carbon (7.7 µmol). The flask was initially flushed with nitrogen for a few minutes and trimethoxysilane (2.85 mL, 22.4 mmol) and 3 (2 g, 7.5 mmol) were then added by syringes. The mixture was stirred at 80 °C for 24 h. After cooling the reaction mixture down to room temperature, ethanol (20 mL) was added and the solution was filtered by a 0.2 µm membrane filter. The filtrate was evaporated under reduced pressure to yield a transparent oil of 2.8 g. Yield: 2.75 g (94.5%). 1H NMR (CDCl ): δ 7.26 (d, -ArH-, 2H), 6.83 (d, -ArH-, 2H), 3 3.93 (t, -CH2-, 2H), 3.82 (s, -OCH3-, 9H), 2.05 (br, adamantyl, -CH-, 3H), 1.88 (adamantyl, -CH2-, 6H), 1.74 (adamantyl, -CH2-, 6H), 0.75 (t, -CH2-, 2H). GC-MS (m/z): 390. 1,3-Bis(4-hydroxyphenyl)adamantane (6). From 1,3-dibromoadamantane (5), compound 6 was prepared, analogous to compound 2 and isolated as a white solid with a yield of 70%. Yield: 6.1 g (70%). 1H NMR (d-acetone): δ 7.95 (s, -OH, 2H), 7.23 (d, -ArH-, 4H), 6.77 (d, -ArH-, 4H), 2.28 (br, ter-CH-, 2H), 1.95 (adamantyl, -CH2-, 2H), 1.91 (adamantyl, -CH2-, 8H), 1.78 (adamantyl, -CH2-, 2H). GC-MS (m/z): 320. 1,3-Bis(4-allyloxyphenyl)adamantane (7). From 3.3 g of compound 6, compound 7 was prepared similarly to compound 3 to yield 3.6 g of a light yellow solid. Yield: 3.6 g (85%). 1H NMR (CDCl3): δ 7.30 (d, -ArH-, 4H), 6.85 (d, -ArH-, 4H), 6.07 (m, -CHd, 2H), 5.44 (d, CH2d, 4H), 5.26 (d, CH2d, 4H), 2.33 (br, ter-CH-, 2H), 2.12 (adamantyl, -CH2-, 2H), 1.97 (adamantyl, -CH2-, 8H), 1.76 (adamantyl, -CH2-, 2H). GC-MS (m/z): 400. 1,3-Bis-(4-trimethoxysilylpropoxyphenyl)adamantane (8). Compound 8 was prepared by hydrosilylation with trimethoxysilane similarly to compound 4. Starting from 3.0 g of compound 7, compound 8 was isolated as a light yellow oil (4.3 g). Yield: 4.3 g (88%). 1H NMR (CDCl3): δ 7.26 (d, -ArH-, 4H), 6.82 (d, -ArH-, 4H), 3.88 (d, -CH2-, 4H), 3.56 (s, -OCH3-, 18H), 2.25 (br, ter-CH-, 2H), 1.94 (br, -CH2-, 4H), 1.12 (adamantyl, -CH2-, 2H), 1.88 (adamantyl, -CH2-, 10H), 1.73 (adamantyl, -CH2-, 2H), 0.75 (t, -CH2-, 4H). Preparation of graft- and bridge-PMSSQ Copolymers. As shown in Scheme 1, compound 4 (2.0 g, 5.1 mmol), methyltrimethoxysilane (6.3 g, 46.2 mmol), and THF (19.3 g) were added to a 50 mL three-neck, round-bottom flask. At 25 °C, HCl dissolved in H2O was added dropwise into the vigorously stirred reaction mixture over 30 min. After 30 min of stirring, the reaction mixture

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Scheme 1. Chemical Structure of Comonomers Containing Covalently Bonded Adamantylphenol Porogens and Their Copolymerization with Methyltrimethoxysilanea

a The graft- or bridge-comonomer is synthesized by the hydrosilylation of either 1-(4-allyloxyphenyl)adamantane or 1,3-bis(4-allyloxyphenyl)adamantane with trimethoxysilane.

was refluxed at 60 °C for 8 h and cooled to room temperature and 30 mL of diethyl ether was then added. The organic layer was washed with H2O (3 × 30 mL) and was dried over Na2SO4. The solvent was evaporated off under reduced pressure to yield 1.34 g in white solid form. 1H NMR (d-acetone): δ 7.26 (d, -ArH-, 2H), 6.85 (d, -ArH-, 2H), 5.76 (br, -OH, 1H), 3.96 (br, -CH2-, 2H), 1.88 (br, adamantyl, -CH2-, 6H), 1.78 (br, adamantyl, -CH2-, 6H), 0.78 (br, -CH2-, 2H), 0.12 (br, Si-CH3, 3H). bridge-PMSSQ was prepared similarly to the graft-PMSSQ with compound 8 to yield 1.5 g in white solid form. 1H NMR (d-acetone): δ 7.33 (d, -ArH-, 4H), 6.86 (d, -ArH-, 4H), 3.96 (d, -CH2-, 4H), 2.25 (br, adamantyl, -CH-, 2H), 1.92 (br, -CH2-, 4H), 1.12 (adamantyl, -CH2-, 2H), 1.80 (adamantyl, -CH2-, 10H), 1.78 (adamantyl, -CH2-, 10H), 0.75 (br, -CH2-, 2H), 0.12 (br, Si-CH3, 3H). All the workup procedures in the preparation of copolymers were performed below 50 °C to suppress the gelation of the copolymers. As a reference, PMSSQ homopolymer was also prepared using the same reaction condition described above. 1H NMR (d-acetone): δ 5.72 (br, -OH-, 1H), 3.50 (br, -OCH3-, 3H), 0.12 (br, -SiCH3-, 3H), Preparation of Copolymer Thin Films. Copolymer thin films were prepared by spin-coating from solutions dissolved in MIBK with a concentration of 20 wt %. The silicon wafers pretreated with piranah solution (H2SO4/H2O2 70/30 by v/v) were typically spun at 2000 rpm for 30 s to yield film thicknesses around 400 nm and the coated films were then cured at a desired temperature for 2 h after preannealing the film at 250 °C for 30 min with a homemade furnace under N2 purge.

Results and Discussion Scheme 1 shows a schematic on the copolymer synthesis where comonomers containing adamantylphenol as a porogen are prepared by the hydrosilylation between trimethoxysilane and (allyloxyphenyl)adamantane and the content of chemically bonded porogens in the copolymers is varied by the sol-gel copolymerization with methyltrimethoxysilane (MTMS) for a given amount of comonomers containing the adamantylphenol. The prepared copolymers are readily spincoatable and the molecular weight of the graft-PMSSQ copolymer (Mw ) 3300) is found to be lower than that of the bridge-PMSSQ copolymer (Mw ) 11400) from the GPC analysis. Figure 1 shows representative 1H NMR spectra for two different PMSSQ-based copolymers as well as the PMSSQ homopolymer prepared. For the copolymers containing chemically bonded adamantylphenol groups, the chemical shifts of tertiary and secondary carbon of the adamantane moieties appear at 2.3 and 1.9 ppm, respectively. The

Figure 1. 1H NMR spectra of (a) PMSSQ, (b) graft-PMSSQ, and (c) bridge-PMSSQ.

chemical shifts attributed to the phenyl groups are assigned at 7.25 and 6.82 ppm, respectively. The chemical shifts of Si-CH3, Si-OCH3, and Si-OH for the copolymers are also shown at 0.1, 3.48, and 5.7 ppm, which are similar to the 1 H NMR spectrum of PMSSQ homopolymer reported in the literature.6 Based on the area of the peak assigned to the phenyl groups at about 6.8 ppm, it is estimated that the graftPMSSQ and bridge-PMSSQ copolymers contain 8 and 10 mol % adamantylphenol porogens, respectively. Figure 2 shows the structural changes, measured with FTIR, of the graft-PMSSQ and bridge-PMSSQ copolymer films cured under N2 purge. The infrared spectra of the graftPMSSQ films show that the peak intensity at 2910 cm-1 due to the asymmetric stretching of CH in the adamantane moieties notably decreases above 300 °C together with the peaks at 1514 and 1256 cm-1 assigned to the phenyl groups attached to the adamantane moieties and ether groups, respectively, and completely disappears at 450 °C. On the other hand, a small amount of residual adamantane is left even after the curing at 450 °C in the case of the bridgePMSSQ. This spectroscopic evidence indicates the thermal decomposition of chemically bonded porogens through the cleavage of ether bonds, which has the lowest dissociation energy in the comonomer structure. In fact, the calculation of bond dissociation energy (BDE) by the density functional theory (DFT) shows that the C6H5O-C bond has the lowest BDE of 53.3 kcal/mol, which is in good agreement with the (6) (a) Lee, J.-K.; Char, K.; Rhee, H.-W.; Ro, H. W.; Yoo, D. Y.; Yoon, D. Y. Polymer 2001, 42, 9085. (b) Lee, L. H.; Chen, W.-C.; Liu, W.C. J. Polym. Sci., Polym. Chem. 2002, 40, 1560.

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Figure 3. TGA scans of graft- and bridge-PMSSQ copolymers measured under N2 with a heating rate of 10 °C/min.

Figure 2. FT-IR spectra showing the structural change of (a) graft-PMSSQ and (b) bridge-PMSSQ films with curing temperature.

previous result.7 With the increase in temperature, the intensity of the vibrational band assigned to the ladder-like Si-O stretching at around 1040 cm-1 increases while the intensity of the band assigned to the cage-like Si-O stretching at 1140 cm-1 decreases. This spectroscopic result suggests that a structural rearrangement from a cage-like structure to a ladder-like network structure occurs by the additional polycondensation of alkoxysilane matrix during heat treatment, implying the increased dense matrix structure.8 It is also noted that the peak intensity of Si-O-Si (i.e., ladder-like structure) in the bridge-PMSSQ is stronger than that for the graft-PMSSQ since more Si-O-Si bonds are formed from the bridge-PMSSQ compared with the graftPMSSQ during the hydrolytic condensation of alkoxysilanes. It is thus expected that both thermal and mechanical stabilities of the bridge-PMSSQ are higher than those of the graft-PMSSQ due to the formation of dense structure in the bridge-PMSSQ. In addition, the peak ratio of the cage to the ladder Si-O bonds in the bridge-PMSSQ is higher than the value for the graft-PMSSQ, which is, in turn, related to more enhanced porosity in the bridge-PMSSQ than in the graft-PMSSQ.9 (7) Jursic, B. S. J. Chem. Soc., Perkin Trans. 1999, 2, 369. (8) (a) Liu, W.-C.; Yang, C.-C.; Chen, W.-C.; Dai, B.-T.; Tasi, M.-S. J. Non-Cryst. Solids 2002, 311, 233. (b) Chua, C. T.; Sarkar, G.; Hu, X. J. Electrochem. Soc. 1998, 145, 4007. (c) Huang, Q. R.; Volksen, W.; Huang, E.; Toney, M.; Frank, C. W.; Miller, R. D. Chem. Mater. 2002, 14, 3676.

Figure 3 shows the thermal decomposition of organosilicates containing chemically bonded porogens measured with thermal gravimetric analysis (TGA). The weight loss up to 350 °C is mainly associated with the loss of water originating from the polycondensation reaction as well as the removal of residual solvent. A significant weight loss in the TGA scans of graft- and bridge-PMSSQ is observed in the temperature range from 350 to 680 °C, showing the weight loss of 16.5% and 17.4%, respectively. In contrast, no such weight loss is detected in the same temperature range for the PMSSQ homopolymer. Consequently, the weight loss above 350 °C indicates the decomposition of the adamantylphenol porogens. It is also important to note from Figure 3 that the decomposition temperature of the bridge-PMSSQ is higher than that of the graft-PMSSQ, implying that the mode of porogen decomposition in the case of the bridgePMSSQ is somewhat different from that of graft-PMSSQ. Further evidence on the different mode of decomposition can be found from the analysis of adamantyl compounds recovered from bulk samples after thermal decomposition. The GC-MS data, shown in Figure 4, indicate that the recovered component from the graft-PMSSQ is dominantly adamantylphenol, indicating that the chemically bonded porogens are removed immediately after the cleavage of ether bonds, resulting in nanoporous structure. On the other hand, the decomposition compounds recovered from the bridgePMSSQ at higher degradation temperature are phenyladamantane or 1,3-diphenyladamantane compounds, which are clearly different from expected adamantlyphenol or adamantyldiphenol. Although it is quite difficult to provide definitive evidence for the different mode of decomposition for the bridge-PMSSQ,10 one possible explanation for this difference in recovered compounds is that it is almost impossible to cleave both phenyl ether linkages in the bridge-PMSSQ at the same time according to the principle of microreversibility. While one phenyl ether group remains bonded to the matrix, (9) (a) Grill, A.; Patal, V. Appl. Phys. Lett. 2001, 79, 803. (b) Maex, K.; Baklanov, M. R.; Shamiryan, D.; Iacopi, F.; Brongersma, S. H.; Yanovitskaya, Z. S. J. Appl. Phys. 2003, 93, 8793.

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Figure 4. GC-MS data showing porogens recovered in bulk after the thermal decomposition of (a) graft-PMSSQ and (b) bridge-PMSSQ films cured at 450 °C.

the cleaved adamantylphenol group may have a chance to recombine or rearrange with other chemical groups in the silsesquioxane matrix, as in the case of the formation and decomposition of phenolic resin. It is noted that the 1H NMR spectrum of the recovered product from the graft-PMSSQ is almost similar to that of the adamantylphenol precursor showing that a peak at 4.6 ppm corresponding to the hydroxyl group of adamantylphenol is clearly observed from the recovered product (data not shown here). In contrast, the 1H NMR spectrum from the bridge-PMSSQ is considerably different from that of adamantyldiphenol precursor showing that a peak at 7.9 ppm assigned to the hydroxyl group of adamantyldiphenol is absent in the recovered product. Consequently, this experimental evidence suggests the possibility of rearrangement of cleaved adamantyldiphenol groups within the silsesquioxane matrix for the bridgePMSSQ, retaining adamantyl residue to some extent even after thermal treatment at higher temperature. 29 Si CP-MAS NMR spectra, shown in Figure 5, of the graft- and bridge-PMSSQ demonstrate that the Q4 resonances (tetracoordinated Si-O structure) at around -110 ppm that are not observed for uncured copolymers as well as for cured PMSSQ homopolymers start to emerge after curing at 400 °C at the expense of the reduction in T2 resonance at -56 ppm and the Q4 intensity increases with the curing temperature. This result indicates that the decomposition of porogens is accompanied by the formation of tetracoordinated Si-O structure that enhances mechanical properties of the matrix. The origin of the Q4 resonance is believed to be due to the decomposition of residual propenyl groups and the nucleophilic attack by silanol groups after the decomposition of porogens, as previously reported.11 In contrast, we also note that ethyladamantane groups grafted to the silsesquioxane matrix without ether linkages are not decomposed until (10) (a) Morterra, C.; Low, M. J. D. Carbon 1985, 23, 525. (b) Johnson, S. A.; Brigham, E. S.; Ollivier, P. J.; Mallouk, T. E. Chem. Mater. 1997, 9, 2448. (c) Costa, L.; Montelera, R. D.; Camino, G.; Weil, E. D.; Pearce, E. M. Polym. Degrad. Stab. 1997, 56, 23. (d) Kim, Y. J.; Kim, M. I.; Yun, C. H.; Chang, J. Y.; Park, C. R.; Inagaki, M. J. Colloid Interface Sci. 2004, 274, 555. (e) Loy, D. A.; Beach, J. V.; Baugher, B. M.; Assink, R. A.; Shea, K. J.; Tran, J.; Small, J. H. Chem. Mater. 1999, 11, 3333. (11) Shimada, T.; Aoki, K.; Shinoda, Y.; Nakamura, T.; Tokunaga, N.; Inagaki, S.; Hayashi, T. J. Am. Chem. Soc. 2003, 125, 4688.

Figure 5. 29Si CP-MAS NMR spectra of (a) graft-PMSSQ and (b) bridgePMSSQ cured at different temperatures. The asterisks in the spectra denote the spinning sidebands.

450 °C and thus not showing the Q4 peak. This finding suggests that the bond chemical structure and the number of bonds linked between the porogen and the matrix play important roles in the decomposition of porogens. It is also interesting to note that the Q4 peak intensity for the bridgePMSSQ is higher than that for the graft-PMSSQ, probably due to the more extensive depropenylation in the bridgePMSSQ compared with that in the graft-PMSSQ. As a result, the higher intensity of the Q4 resonance in the case of the bridge-PMSSQ implies the significant formation of SiO2 bonds. To gain information on the porosity of copolymer films, specular X-ray reflectivity (SXR) measurements were performed for both graft- and bridge-PMSSQ films in both air

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Figure 7. Changes in (a) dielectric constant and (b) refractive index as a function of curing temperature for graft-PMSSQ and bridge-PMSSQ films.

Figure 6. SXR curves of porous (a) graft-PMSSQ thin film cured at 400 °C and (b) bridge-PMSSQ thin films cured at 450 °C under N2.

and toluene-saturated environment, as shown in Figure 6. Since it is well-established that the square of a critical angle, at which the reflectivity of a film drops sharply, is proportional to the average electron density of the film, the shift in the critical angle after capillary condensation of toluene inside accessible pores reflects the porosity of film.12 Accordingly, the shift in critical angles after the toluene treatment with reference to the film in air and model fits with two equations allow us to have information on the absolute values of film density, porosity, and wall density. X-ray reflectivity profiles shown in Figure 6 indicate that the graft- and bridge-PMSSQ films have absolute porosities of 14.9% and 18.4%, respectively. It is also noted that the bridge-PMSSQ film has lower wall density (1.30 g/cm3) than the PMSSQ homopolymer (1.35 g/cm3).12b (12) (a) Lee, H.-J.; Soles, C. L.; Liu, D.-W.; Bauer, B. J.; Wu, W.-L. J. Polym. Sci., Polym. Phys. 2002, 40, 2170. (b) Lee, H.-J.; Soles, C. L.; Liu, D.-W.; Bauer, B. J.; Lin, E. K.; Wu, W.-L.; Grill, A. J. Appl. Phys. 2004, 95, 2355. (c) Grill, A.; Patel, V.; Rodbell, K. P.; Huang, E.; Baklanov, M. R.; Mogilnikov, K. P.; Toney, M.; Kim, H.-C. J. Appl. Phys. 2003, 94, 3427.

Figures 7 a and 7b show the changes in dielectric constant (k) and refractive index (nf) of graft-PMSSQ and bridgePMSSQ films cured under N2 purge. As a reference, the dielectric constant (k) of the PMSSQ homopolymer film obtained in the present study is 2.75, which is in good agreement with the previously reported value.13 For all the films tested, k-values and refractive indices initially decrease with curing temperature due to the formation of porous structure by the decomposition of chemically bonded porogens and the concomitant removal of silanol groups through polycondensation. The k-values of bridge- and graft-PMSSQs reduce to 2.26 ( 0.08 and 2.57 ( 0.08 after curing at 450 and 400 °C, respectively. Above 400 °C, it is interesting to note that both k-value and refractive index of the graftPMSSQ increases after going through the minimum value. It is also noted that the measured dielectric constant for the bridge-PMSSQ is lower than the value (k ) 2.43) estimated from the Maxwell-Garnett equation using the measured porosity, which is believed to originate from the underestimated porosity probably by hindering the toluene vapor infiltration into pores with organic residues. There are also additional contributions to the reduction in k-value of the bridge-PMSSQ. The removal of phenyl groups chemically bonded to the matrix and the dominant formation of caged Si-O bonds can also lower both the k-value and the refractive index by decreasing the molecular polarizability of the matrix as well as increasing molecular pore or free volume.12,14 Cross-sectional FE-SEM photographs for the graft-PMSSQ and bridge-PMSSQ films are shown in Figure 8. It has (13) Yoon, D. Y.; Ro, H. W.; Park, E. S.; Lee, J.-K.; Kim, H.-J.; Char, K.; Rhee, H.-W.; Kwon, D.; Gidley, D. W. Mater. Res. Soc. Proc. 2003, 766, E651.

384 Chem. Mater., Vol. 18, No. 2, 2006

Cha et al. Table 1. Mechanical Characteristics of Cured Thin Filmsa film thickness (nm)b

modulus (GPa)

hardness (GPa)

PMSSQ

423 ( 34c

3.42 ( 0.08c

0.51 ( 0.05c

graft-PMSSQ

438 ( 16c 387 ( 36d

2.71 ( 0.05c 5.50 ( 0.07d

0.32 ( 0.01c 0.64 ( 0.01d

bridge-PMSSQ

537 ( 22c 512 ( 55d

5.61 ( 0.05c 5.53 ( 0.05d

0.52 ( 0.01c 0.72 ( 0.01d

films

a All the values were taken at 50 nm of indenter displacement. b Film thicknesses were measured with an ellipsometer. c Measured for the films cured at 400 °C. d Measured for the films cured at 450 °C.

Figure 8. Cross-sectional FE-SEM micrographs of (a) graft-PMSSQ and (b) bridge-PMSSQ films cured at 400 and 450 °C for 2 h, respectively.

Figure 9. N2 adsorption isotherms of (O) graft- and (0) bridge-PMSSQ samples cured at 400 and 450 °C under N2, respectively.

previously been known that no cross-sectional image of a PMSSQ homopolymer film is textured. In contrast, the crosssectional images of both graft-PMSSQ and bridge-PMSSQ films clearly show the cross-sectional textures due to the difference in electron density between pores and the matrix within the resolution of FE-SEM, which is about 10 nm, implying the formation of nanoporous structure. Figure 9 shows the nitrogen adsorption/desorption isotherms for the (14) (a) Thomas, M. E.; Iwamoto, N. Semicond. Int. 2002, June. (b) Kim, H.-C.; Wilds, J. B.; Hinsberg, W. D.; Johnson, L. R.; Volksen, W.; Magbitang, T.; Lee, V. Y.; Hedrick, J. L.; Hawker, C. J.; Miller, R. D. Chem. Mater. 2002, 14, 4628. (c) Shamiryan, D. G.; Baklanov, M. R.; Vanhaelemeersch, S.; Maex, K. Electrochem. Solid-State Lett. 2001, 4, F3. (d) Fisher, I.; Kaplan, W. D.; Eizenberg, M. J. Appl. Phys. 2004, 95, 5762. (e) Kim, Y.-H.; Hwang, M. S.; Kim, H. J.; Kim, J. Y.; Lee, Y. J. Appl. Phys. 2001, 90, 3367. (f) Kim, J. Y.; Hwang, M. S.; Kim, Y.-H.; Kim, H. J.; Lee, Y. J. Appl. Phys. 2001, 90, 2469.

chemically bonded copolymers after thermal treatment. For the sorption isotherm measurements, spin-coated graft- and bridge-PMSSQ films were initially cured at 400 and 450 °C, respectively, and scraped off the substrates to yield powder specimens. All the isotherms show the typical Type I isotherms with unclosed hysteresis, implying microporous materials.15 In addition, the amount of nitrogen adsorption for bridge-PMSSQ is higher than that of graft-PMSSQ, indicating that the bridge-PMSSQ has a porosity higher than the graft-PMSSQ, which is again in good agreement with the specular X-ray reflectivity results. Mechanical properties of both graft-PMSSQ and bridgePMSSQ thin films measured with a nanoindenter in dynamic contact mode (DCM) are summarized in Table 1. Since all the reported values in Table 1 were read at 50 nm of the displacement depth, which is about one-tenth of the total film thickness, the substrate effect could be safely neglected. Moreover, the elastic modulus and surface hardness of the PMSSQ homopolymer film, which is the reference in this study, are also identical to the values previously reported.13,16 After curing at 400 °C, the modulus of graft-PMSSQ is lower than the dense, nonporous PMSSQ while the modulus of bridge-PMSSQ is higher than that of PMSSQ homopolymer. On the other hand, moduli of all the films are significantly increased after curing at 450 °C. Despite the decomposition of porogens in the bridge-PMSSQ, the modulus increases up to 5.53 GPa, which is believed to be due to the high crosslinking density and the partial formation of the silica structure, maintaining the nanoporous structure as evidenced by the low dielectric constant. From the fact that the hardness value is even increased from 0.61 ( 0.02 GPa with 2 mol % of chemically bonded porogens to 0.72 ( 0.01 GPa with 10 mol % porogens, we believe that there might be an additional effect, which we think is due to the organic residues left in the pores, to enhance mechanical properties. In the case of the graft-PMSSQ, however, the large increase in modulus for the sample treated at 450 °C in comparison with the value treated at 400 °C is related to the collapse of pore structure, resulting in the large increase in the k value as shown in Figure 7a. (15) (a) Barrie, P. J.; Carr, S. W.; Ou, D. L.; Sullivan, A. C. Chem. Mater. 1995, 7, 265. (b) Adeogun, M. J.; Hay, J. N. Chem. Mater. 2000, 12, 767. (c) Sun, T.; Wong, M. S.; Ying, J. Y. Chem. Commun. 2000, 2057. (d) Kim, J.-H.; Lyu, Y.-Y.; Jeong, H.-D.; Song, S. A.; Hwang, I.-S.; Lee, J. H.; Mah, S. K.; Chang, S.; Park, J.-G.; Hu, Y. F.; Sun, J. N.; Gidley, D. W. AdV. Funct. Mater. 2003, 13, 382. (16) Cook, R. F.; Liniger, E. G. J. Electrochem. Soc. 1999, 146, 4439. (17) Shamiryan, D.; Weidner, K.; Gray, W. D.; Baklanov, M. R.; Vanhaelemeersch, S.; Maex, K. Microelectron. Eng. 2002, 64, 361.

Nanoporous Organosilicate Thin Films

Chem. Mater., Vol. 18, No. 2, 2006 385

Conclusion

porogen and the matrix can be controlled, this strategy to design nanoporous thin films from organosilicate precursors containing covalently bonded porogens can be further optimized by changing the porogen structure, the number and type of covalent bonds, and the bond dissociation of the chemical structure linking the porogen and the matrix.

By introducing covalently bonded adamantylphenol porogens into a silsesquioxane-based matrix, nanoporous thin films with balanced low dielectric constant (k ∼ 2.3) and superior mechanical properties (E ∼ 5.5) were realized by the formation of nanoporous organosilicate structure caused by the decomposition of porogens and the high cross-linking density based on the bridged precursor structure. The decomposition of chemically bonded porogens also led to the partial formation of silica structure, as evidenced by 29Si NMR, by depropenylation followed by the nucleophilic attack by silanol groups, which is also in good agreement with the DFT calculations. In contrast, the incorporation of porogens without covalent bonds to the precursors yields a collapsed film during the curing step. Since the number of bonds as well as the bond structure linked between the

Acknowledgment. Financial support from the Ministry of Education of Korea through the Brain Korea 21 Program, the National Research Laboratory Fund (M1-0104-00-0191), and Thin Film Solutions are greatly acknowledged. We thank Ms. S.-H. Kim of Korea Basic Science Institute (Daegu) for 29Si CP-MAS NMR and Prof. C. K. Kim of Inha University for his assistance in the DFT calculation. The beamtime allocation from the Pohang Light Source (PLS) for the synchrotron X-ray measurements is also greatly acknowledged. CM051916D