Hydrofluoric-Acid-Resistant and Hydrophobic Pure-Silica-Zeolite MEL

Apr 3, 2009 - In order to test corrosive resistance, hydrofluoric acid (HF) is often used as a proxy for a cleaning chemistry against silica-based mat...
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Hydrofluoric-Acid-Resistant and Hydrophobic Pure-Silica-Zeolite MEL Low-Dielectric-Constant Films Christopher M. Lew,† Yan Liu,† Brandon Day,†,^ Grant M. Kloster, Hugo Tiznado,‡ Minwei Sun,† Francisco Zaera,‡ Junlan Wang,§ and Yushan Yan*,† Department of Chemical and Environmental Engineering, ‡Department of Chemistry and §Department of Mechanical Engineering, University of California, Riverside, Riverside, California and Components Research, Intel Corporation, Hillsboro, Oregon. ^ Current address: Air Liquide Electronics US LP, Dallas, Texas )

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Received December 1, 2008. Revised Manuscript Received February 10, 2009 A new technique for the silylation of pure-silica-zeolite MEL low-k films has been developed in which the spinon films are calcined directly in trimethylchlorosilane or 1,1,1,3,3,3-hexamethyldisilazane (HMDS) in order to protect the films against corrosive wet etch chemicals and ambient moisture adsorption. In an alternative procedure, HMDS is also added to the zeolite suspension before film preparation. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, water-soak tests, and HF etch tests are performed to characterize the films. The dielectric constant is as low as 1.51, and the films resist HF attack up to 5.5 min. These properties are highly desirable by the semiconductor industry for next-generation microprocessors.

Introduction Gordon Moore first proposed Moore’s Law in 1965 and predicted that the number of transistors on a microprocessor would double every 2 years.1 The semiconductor industry has done a remarkable job in keeping pace with Moore’s Law and has since increased the rate to doubling every 18 months. Recently, however, as the feature sizes continue to decrease, the problem of cross-talk noise and resistance-capacitance (RC) delay has become a significant issue.2-4 The semiconductor industry has reduced the resistance by changing the metal wires from aluminum to copper, but the capacitance from the lowdielectric-constant (low-k) material continues to plague the semiconductor industry. The traditional dielectric material has been dense silicon dioxide (k ∼ 4), but according to the International Technology Roadmap for Semiconductors 2007 edition - Interconnect, target interlevel metal insulator effective k values for 2020 are between 2.3 and 2.6 for a DRAM 1/2 pitch of 14 nm, and manufacturable materials are currently only known for k values of 2.5-2.8.5 Besides a low k value, several other properties of low-k materials must also meet minimum requirements.3 The mechanical stiffness, as measured by the elastic modulus, should be at least 6 GPa so that the material can survive the chemical *To whom correspondence should be addressed. E-mail: yushan. [email protected]. (1) Moore, G. E. Electronics 1965, 38(8), 114–117. (2) Ho, P. S.; Lee, W. W.; Leu, J. J. Low dielectric constant materials for IC applications; Springer: New York, 2003; Vol. 9. (3) Maex, K.; Baklanov, M. R.; Shamiryan, D.; Iacopi, F.; Brongersma, S. H.; Yanovitskaya, Z. S. J. Appl. Phys. 2003, 93(11), 8793–8841. (4) Morgen, M.; Ryan, E. T.; Zhao, J. H.; Hu, C.; Cho, T. H.; Ho, P. S. Annu. Rev. Mater. Sci. 2000, 30, 645–680. (5) International Technology Roadmap for Semiconductors 2007 EditionInterconnect; The International Technology Roadmap for Semiconductors, 2007. (6) Wang, Z. B.; Wang, H. T.; Mitra, A.; Huang, L. M.; Yan, Y. S. Adv. Mater. 2001, 13(10), 746–749.

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mechanical polishing steps.6 The porosity and pore structure of a porous low-k material are also important parameters to characterize and optimize, and the thermal conductivity needs to be evaluated so that the issue of heat dissipation can be understood. A hydrophilic material is problematic because water adsorption from ambient moisture can significantly increase the k value (kwater ∼ 80). Finally, low-k materials need to survive several wet etch processes, and thus, the resistance to corrosive chemical dissolution must be high. In order to test corrosive resistance, hydrofluoric acid (HF) is often used as a proxy for a cleaning chemistry against silicabased materials, and at least 5 min of HF resistance is required for demonstrative purposes. Zeolites are a class of crystalline and microporous aluminosilicates, and synthetic zeolites MFI and MEL are commonly used catalysts in the oil refining industry. (Each zeolite framework type is assigned a three-letter code by the International Zeolite Association.) Both zeolite types have also been studied as separation membranes for organic compounds.7,8 MFI and MEL structures contain 10-membered rings, and while MFI contains straight and sinusoidal channels along the b- and a-axes, respectively, MEL has straight channels along the a- and b-axes.9-11 The pure-silica forms were both first synthesized in the 1970s10,12 and have since been used in thin film form for low-k materials. Although initial zeolite low-k studies used MFI films,6 recent efforts have focused (7) Tuan, V. A.; Li, S. G.; Noble, R. D.; Falconer, J. L. Chem. Commun. 2001, 6, 583–584. (8) Lai, Z. P.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Science 2003, 300(5618), 456–460. (9) Baerlocher, C.; McCusker, L. B.; Olson, D. H. Atlas of Zeolite Framework Types., 6th revised ed.; Elsevier: Amsterdam, 2007. (10) Bibby, D. M.; Milestone, N. B.; Aldridge, L. P. Nature 1979, 280 (5724), 664–665. (11) Kokotailo, G. T.; Chu, P.; Lawton, S. L.; Meier, W. M. Nature 1978, 275(5676), 119–120. (12) Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature 1978, 271(5645), 512–516.

Published on Web 4/3/2009

DOI: 10.1021/la803956w 5039

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on the MEL structure because its lower framework density (17.4 T atoms/1000 A˚3 versus 18.4 for MFI)13 gives the films more microporosity and a lower k value.14,15 Their intrinsic crystalline nature gives zeolites several of the desired characteristics for new low-k materials, such as high mechanical strength and high heat conductivity.15-17 Puresilica-zeolite (PSZ) is also chemically compatible with the existing dielectric infrastructure that utilized silicon dioxide materials. Initial studies on PSZ MFI showed that the fully crystalline structure is intrinsically hydrophobic.12 Later research by Li et al. has shown that zeolites offer better mechanical properties over amorphous porous silicas at any given k value.18 Specifically, at ultralow-k values of less than 2, PSZs have elastic moduli well above the 6 GPa threshold value. Recent 3ω measurements on PSZ MFI show that its thermal conductivity is between 1.0 and 1.3 W/m 3 K, depending on the crystal orientation.16,17 Furthermore, the use of a spin-on deposition procedure with zeolite nanoparticle suspension adds mesoporosity between the nanocrystals that further decreases the k value.19 The as-synthesized spin-on PSZ MFI and PSZ MEL films previously reported by Wang et al.19 and Li et al.15 are hydrophilic because they are composites of nanocrystalline zeolite and possibly amorphous silica. Although large PSZ crystals are hydrophobic,6,12 the spin-on films from the nanoparticle suspension have a high concentration of terminal hydroxyl groups.20 As a result, the k value of untreated PSZ MFI films has been shown to increase by 70% within the first 20 min of exposure to ambient conditions.21 Furthermore, hydrophilic untreated PSZ MEL films dissolve in 100:1 HF in less than 5 s (Figure 1). Previous studies on porous methylsilsesquioxane (MSQ) films have shown that one way to impart HF resistance to a material is to increase its hydrophobicity.22 Several publications on MSQ and SiOCH-containing films show that silylation with trimethylchlorosilane (TMCS), 1,1,1,3,3,3-hexamethyldisilazane (HMDS), and other hydrophobic silanes can repair and protect the materials from wet etching processes after they have been damaged from plasma treatments.23-25 Thus, rendering PSZ spin-on films hydrophobic serves two purposes: a reduction in moisture adsorption and protection (13) IZA Database of Zeolite Structures, http://www.iza-structure.org/ databases/ (accessed Sept 11, 2008). (14) Lew, C. M.; Sun, M. W.; Liu, Y.; Yan, Y. S. Pure-Silica-Zeolite LowDielectric Constant Materials. In Ordered Nanoporous Solids: Recent Advances and Prospects; Valtchev, V., Mintova, S., Tsapatsis, M., Eds; Elsevier: Amsterdam, The Netherlands, 2008. (15) Li, Z. J.; Lew, C. M.; Li, S.; Medina, D. I.; Yan, Y. S. J. Phys. Chem. B 2005, 109(18), 8652–8658. (16) Greenstein, A. M.; Graham, S.; Hudiono, Y. C.; Nair, S. Nanoscale Microscale Thermophys. Eng. 2006, 10(4), 321–331. (17) Hudiono, Y.; Greenstein, A.; Saha-Kuete, C.; Olson, B.; Graham, S.; Nair, S. J. Appl. Phys. 2007, 102(5), 053523. (18) Li, Z. J.; Johnson, M. C.; Sun, M. W.; Ryan, E. T.; Earl, D. J.; Maichen, W.; Martin, J. I.; Li, S.; Lew, C. M.; Wang, J.; Deem, M. W.; Davis, M. E.; Yan, Y. S. Angew. Chem., Int. Ed. 2006, 45(38), 6329–6332. (19) Wang, Z. B.; Mitra, A. P.; Wang, H. T.; Huang, L. M.; Yan, Y. S. Adv. Mater. 2001, 13(19), 1463–1466. (20) Li, S.; Wang, X.; Beving, D.; Chen, Z. W.; Yan, Y. S. J. Am. Chem. Soc. 2004, 126(13), 4122–4123. (21) Li, S.; Li, Z. J.; Medina, D.; Lew, C.; Yan, Y. S. Chem. Mater. 2005, 17(7), 1851–1854. (22) Rebiscoul, D.; Puyrenier, B.; Broussous, L.; Louis, D.; Passemard, G. Microelectron. Eng. 2006, 83(11-12), 2319–2323. (23) Gorman, B. P.; Orozco-Teran, R. A.; Zhang, Z.; Matz, P. D.; Mueller, D. W.; Reidy, R. F. J. Vac. Sci. Technol., B 2004, 22(3), 1210–1212. (24) Xie, B.; Muscat, A. J. Microelectron. Eng. 2004, 76(1-4), 52–59. (25) Chaabouni, H.; Chapelon, L. L.; Aimadeddine, M.; Vitiello, J.; Farcy, A.; Delsol, R.; Brun, P.; Fossati, D.; Arnal, V.; Chevolleau, T.; Joubert, O.; Torres, J. Microelectron. Eng. 2007, 84(11), 2595–2599.

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Figure 1. 100:1 HF resistance of PSZ MEL (b), Traditionally Silylated (9), TMCS (2), HMDS (1), and Presilylated MEL films (f). from chemical corrosion, and our silylation method accomplishes both. Numerous studies on silica-based materials have investigated the replacement of terminal hydroxyl groups by methyl groups using TMCS24,26-33 and HMDS,23,24,26,34,35 and a few papers have reported results for zeolites.36,37 For PSZ low-k films, several techniques have been developed to render the films hydrophobic. Wang et al. used a postcalcination vaporphase silylation step with TMCS,19 and both Li et al.21 and Lew et al.38 added hydrophobic silanes to the zeolite precursor solution to functionalize the zeolites with hydrophobic groups. Also, Eslava et al. used ultraviolet-assisted curing to functionalize the zeolites during the removal of the organic template.39 All three methods have been effective in reducing the moisture adsorption. However, previous work has shown that reactive vapors similar to those used in the vapor-phase silylation step of Wang et al.19 lead to pore blocking.40,41 Furthermore, most silylation techniques are performed after annealing, which can be a costly and time-consuming step in semiconductor production, and any possible reduction in processing steps is desirable. (26) Capel-Sanchez, M. C.; Barrio, L.; Campos-Martin, J. M.; Fierro, J. L. G. J. Colloid Interface Sci. 2004, 277(1), 146–153. (27) Castricum, H. L.; Mittelmeijer-Hazeleger, M. C.; Sah, A.; ten Elshof, J. E. Microporous Mesoporous Mater. 2006, 88(1-3), 63–71. (28) Nitta, S. V.; Pisupatti, V.; Jain, A.; Wayner, P. C.; Gill, W. N.; Plawsky, J. L. J. Vac. Sci. Technol., B 1999, 17(1), 205–212. (29) Prakash, S. S.; Brinker, C. J.; Hurd, A. J.; Rao, S. M. Nature 1995, 374 (6521), 439–443. (30) Zhao, X. S.; Lu, G. Q. J. Phys. Chem. B 1998, 102(9), 1556–1561. (31) Antochshuk, V.; Jaroniec, M. Chem. Commun. 1999, 23, 2373–2374. (32) Yuan, P.; Yang, D.; Lin, Z. Y.; He, H. P.; Wen, X. Y.; Wang, L. J.; Deng, F. J. Non-Cryst. Solids 2006, 352(36-37), 3762–3771. (33) Huang, K. Y.; He, Z. P.; Chao, K. J. Thin Solid Films 2006, 495(1-2), 197–204. (34) Chen, J. Y.; Pan, F. M.; Lin, D. X.; Cho, A. T.; Chao, K. J.; Chang, L. Electrochem. Solid-State Lett. 2006, 9(6), G215–G218. (35) Gun’ko, V. M.; Turov, V. V.; Bogatyrev, V. M.; Charmas, B.; Skubiszewska-Zieba, J.; Leboda, R.; Pakhovchishin, S. V.; Zarko, V. I.; Petrus, L. V.; Stebelska, O. V.; Tsapko, M. D. Langmuir 2003, 19(26), 10816–10828. (36) Corbin, D. R.; Herron, N. J. Mol. Catal. 1994, 86(1-3), 343–369. (37) Bein, T.; Carver, R. F.; Farlee, R. D.; Stucky, G. D. J. Am. Chem. Soc. 1988, 110(14), 4546–4553. (38) Lew, C. M.; Li, Z. J.; Li, S.; Hwang, S. J.; Liu, Y.; Medina, D. I.; Sun, M. W.; Wang, J. L.; Davis, M. E.; Yan, Y. S. Adv. Funct. Mater. 2008, 18(21), 3454–3460. (39) Eslava, S.; Iacopi, F.; Baklanov, M. R.; Kirschhock, C. E. A.; Maex, K.; Martens, J. A. J. Am. Chem. Soc. 2007, 129, 9288–9289. (40) Impens, N. R. E. N.; van der Voort, P.; Vansant, E. F. Microporous Mesoporous Mater. 1999, 28(2), 217–232. (41) , C. T.; Moller, K. P.; Manstein, H. J. Mol. Catal. A: Chem. 2002, 181 (1-2), 15–24.

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Most importantly for this study, HF dip tests performed on previously published silylated zeolite films15 show that they dissolve in 100:1 HF in less than 1 min. Here, we report on the preparation of spin-on PSZ MEL films that are functionalized by a new method and are able to resist both moisture adsorption and HF etching. Our new silylation method results in zeolite films with k values as low as 1.5 and etch resistance in 100:1 HF of more than 5 min, and no extra processing steps are required. Three new films were prepared: (1) PSZ MEL calcined in TMCS vapor, (2) PSZ MEL calcined in HMDS vapor, and (3) PSZ MEL calcined in HMDS vapor with HMDS also added to the nanoparticle suspension before the spin-on process. (These three films are hereafter referred to as TMCS, HMDS, and Presilylated films, respectively. Presilylated films with TMCS were not prepared because TMCS is not soluble in the zeolite nanoparticle suspension, and a uniform solution for spin coating could not be made.) They are characterized and compared to spinon PSZ MEL films with no silylation (hereafter referred to as PSZ MEL films) and Traditionally Silylated films (i.e., TMCS silylation after annealing, as developed by Li et al.15).

Experimental Section The zeolite films under investigation were prepared by a previously published spin-on method.15 First, a nanoparticle suspension was synthesized by adding 19.97 g of tetrabutylammonium hydroxide (TBAOH, 55% aqueous solution, Sachem) to 30 g of tetraethylorthosilicate (TEOS, 98%, Aldrich) in a polypropylene bottle with stirring. Then, 21.49 g of double-deionized water was added, and the solution was stirred at room temperature for 1 day. The final molar ratio was 0.3 TBAOH/1 SiO2/4 ethanol/10 H2O. The mixture was moved into a preheated convection oven kept at 80 C with stirring for 2 days and was subsequently transferred to Teflon-lined autoclaves. The autoclaves were placed into a preheated convection oven at 114 C for 24 h with no stirring. The resulting zeolite nanoparticle suspension had crystals with sizes of about 80 nm. Butanol (1-butanol, anhydrous, 99.8%, Sigma-Aldrich) was added at a mass ratio of 1:1 to the nanoparticle suspension, and the resulting solution was spun onto low-resistivity (0.005-0.02 Ω-cm) silicon substrates at 3000 rpm for 30 s with an acceleration of 1275 rpm/s on a Laurell spin coater (WS-400A-6NPP/LITE). After the spin-on deposition, the films were further calcined and/or subjected to silylation processes as outlined below for each type of film. All silylation and calcination processes were carried out in a tubular furnace. PSZ MEL. The as-spun-on films were baked at 80 C for 6 h in air and calcined in air at 500 C for 2 h with a heating rate of 1 C/min. Traditionally Silylated. The as-spun-on films were first baked in air at 80 C for 6 h and then calcined in air at 400 C for 2 h with a heating rate of 1 C/min. They were subsequently held in a nitrogen environment for 2 h at 400 C and then in a vacuum for 2 h at 400 C before subjecting them to a vapor-phase silylation at 320 C for 4 h. For the silylation, nitrogen was used as the carrier gas and flowed through a bubbler at room temperature containing 1:1 (by mass) toluene (99.8%, EMD Chemicals, Inc.)/TMCS (98%, Acros Organics). The flow rate through the bubbler was approximately 3 L/h. Finally, the films were exposed to forming gas (95% N2/5% H2) at 400 C for 2 h to remove unreacted TMCS. This silylation procedure was published previously.15,19,42,43 TMCS. The as-spun-on films were calcined in a TMCS environment by flowing nitrogen through a bubbler containing (42) Li, Z. J.; Li, S.; Luo, H. M.; Yan, Y. S. Adv. Funct. Mater. 2004, 14 (10), 1019–1024. (43) Johnson, M. C.; Lew, C. M.; Yan, Y. S.; Wang, J. L. Scr. Mater. 2008, 58(1), 41–44.

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1:1 (by mass) toluene/TMCS at room temperature. The flow rate through the bubbler was approximately 3 L/h. The furnace was heated at 1 C/min from room temperature to 400 C and then held at 400 C for 2 h. Afterward, the vapor flow was stopped, and the films were allowed to cool down to room temperature in the TMCS environment. HMDS. The as-spun-on films were calcined in a HMDS (98%, Alfa Aesar) environment by flowing nitrogen through a bubbler containing 1:1 (by mass) toluene/HMDS at room temperature. The flow rate through the bubbler was approximately 3 L/h. The furnace was heated at 1 C/min from room temperature to 400 C and then held at 400 C for 2 h. Afterward, the vapor flow was stopped, and the films were allowed to cool down to room temperature in the HMDS environment. Presilylated. HMDS was added to the butanol/nanoparticle suspension mixture until reaching 5% (by mass) for the Presilylated films before the spin-on process. The Presilylated spun-on films were then calcined in the same manner as the HMDS films. Toluene was used as a solvent in the original publication that was followed to reproduce the Traditionally Silylated films.19 The decision was made to keep the toluene as originally developed to reduce the number of unknown variables. The more important effect of when the silylation agent was introduced into the silylation process was studied, and all other variables were kept as constant as possible. Thermogravimetric analysis on powders silylated using the same process as the TMCS and HMDS films do not give evidence of coking or any other side products, and there was no black coloring of either the powders or films. To measure the capacitance of the films, 1.62 mm diameter aluminum dots were deposited on top of the prepared films on a PAC-1 Pelco Advanced Coater 9500 instrument. A layer of aluminum was also deposited on the backside of the silicon substrate, thus creating a metal-insulator-metal structure. Film thickness measurements were performed on a Jobin Yvon UVISEL spectroscopic phase-modulated ellipsometer and were further verified by scanning electron microscopy (SEM) using a Philips XL30-FEG instrument operated at 10 kV. The k values were calculated from capacitance measurements taken on an Agilent 4285A precision LCR meter combined with a Signatone S-1160 probe station in 44 ( 3% relative humidity under a nitrogen blanket. Particle size measurements on the nanoparticle suspension were performed by dynamic light scattering on a Brookhaven Instruments Corporation ZetaPALS instrument and were further confirmed by SEM. X-ray diffraction (XRD) patterns of the PSZ MEL powder were taken with a Bruker D8 Advance diffractometer using Cu KR radiation. Nitrogen adsorption/desorption measurements were taken on powder samples on a Micromeritics ASAP 2010 instrument. Water contact angle measurements were performed on an AST Products, Inc. VCA Optima instrument. Transmission Fourier transform infrared spectroscopy (FTIR) was performed on films prepared on high-resistivity silicon wafers (10 Ω-cm) using a Bruker Equinox 55 FTIR instrument. X-ray photoelectron spectroscopy (XPS) data were collected on a Leybold EA11-MCD system kept under ultrahigh vacuum (UHV, P < 1  10-9 Torr) and equipped with a Al-KR X-ray source (hν = 1486.6 eV), a 100 mm concentric hemispherical analyzer, and an 18-channel detector. A constant band-pass energy of 31.5 eV, corresponding to an overall spectral resolution of 0.8 eV, was used to collect the spectra. To compensate for any charging effects, all of the XPS peak positions were referred to the adventitious hydrocarbon C 1s peak at 284.6 eV.44 Spectra for the regions of interest were fitted using Gaussian-Lorentzian lines with XPSPEAK software. HF resistance tests were performed by measuring the film thickness before and after exposing the films to 100:1 (by mass; (44) Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Muilenberg, G. E., Eds.; Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corporation: Eden Prairie, MN, 1978.

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Lew et al. Table 1. Nitrogen Adsorption/Desorption Data of the Traditionally Silylated PSZ MEL Powders

silylation agenta

second stage synthesis time (h)

BET surface area (m2/g)

micropore area (m2/g)

total pore volume (cm3/g)

micropore volume (cm3/g)b

mesopore volume (cm3/g)c

none 20 839 142 0.80 0.057 0.74 TMCS 20 521 121 0.30 0.055 0.25 HMDS 20 563 67 0.37 0.034 0.34 a The calcined MEL powders were silylated with either TMCS or HMDS following the Traditionally Silylated method outlined in the Experimental Section. b The micropore volume was calculated by the t-plot method. c Mesopore volume = total volume - micropore volume.

0.48% final HF solution concentration) DDI water/HF (48%, EMD Chemicals, Inc.) in a static beaker, in a fashion similar to other HF dip tests.45-47 Water-soak tests were performed to probe the internal and surface hydrophobicity of the films by measuring the k value and contact angle after each of the following steps: (1) as-synthesized, (2) drying overnight (>12 h) in air at 110 C, (3) soaking in DDI water overnight (>12 h) at room temperature, (4) drying overnight (>12 h) in air at 110 C, and (5) exposing to ambient conditions (44 ( 3% relative humidity and 20 ( 0.6 C) for 24 h.

Results and Discussion The type and crystallinity of the MEL zeolite were confirmed by XRD (not shown); the diffraction pattern matches well with previously published results.15,48,49 (This zeolite may be an MFI-MEL intergrowth50,51 but will be referred to simply as MEL for simplicity.) The resistance of TMCS, HMDS, and Presilylated films to HF exposure is shown in Figure 1. The three films that were calcined in silane vapor withstand HF for 5 min or more and are clearly more resistant than the PSZ MEL and Traditionally Silylated films. The film etch rate between 5.5 and 10 min of HF exposure time may shed light onto the pore blocking effects of the organosilanes. After 5.5 min, the TMCS films are etched at a faster rate than the films calcined in HMDS. One possible explanation is that the micropores of the films calcined in HMDS are more thoroughly silylated and are thus more protected from HF attack than the films calcined in TMCS. Nitrogen adsorption/desorption experiments performed on Traditionally Silylated PSZ MEL powders provide insight into the pore penetration of TMCS and HMDS; the data are presented in Table 1. The micropore area of the powder silylated in TMCS remains similar to that of the unsilylated powder, while the HMDS silylated sample loses more than half its micropore area. Furthermore, after silylation with TMCS, the micropore volume remains relatively constant. By contrast, silylation with HMDS leads to a 40% drop in micropore volume. These results indicate that TMCS does not penetrate the micropores, while HMDS does. The reason why TMCS does not penetrate the micropores may be due to micropore blocking,40,41 in which the high reactivity of TMCS52 causes it to quickly deposit trimethylsilane groups at the mouth of the micropores and obstruct the entrances to further silylation. (45) Broussous, L.; Puyrenier, W.; Rebiscoul, D.; Rouessac, V.; Ayral, A. Microelectron. Eng. 2007, 84(11), 2600–2605. (46) Le, Q. T.; Baklanov, M. R.; Kesters, E.; Azioune, A.; Struyf, H.; Boullart, W.; Pireaux, J. J.; Vanhaelemeersch, S. Electrochem. Solid-State Lett. 2005, 8(7), F21–F24. (47) Puyrenier, W.; Rouessac, V.; Broussous, L.; Rebiscoul, D.; Ayral, A. Microelectron. Eng. 2006, 83(11-12), 2314–2318. (48) Dong, J. P.; Sun, Y. J.; Long, Y. C. Chem. Lett. 2001, 6, 516–517. (49) Dong, J. P.; Zou, J.; Long, Y. C. Microporous Mesoporous Mater. 2003, 57(1), 9–19. (50) Jablonski, G. A.; Sand, L. B.; Gard, J. A. Zeolites 1986, 6(5), 396–402. (51) Kokotailo, G. T.; Meier, W. M. Spec. Publ. Chem. Soc. 1980, 33, 133. (52) McMurtrey, K. D. J. Liq. Chromatogr. 1988, 11(16), 3375–3384.

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Recent work by Eslava et al. indicates that all the isolated hydroxyl groups throughout the entire depth of Traditionally Silylated PSZ MFI films react with TMCS, while geminal and vicinal silanols do not.53 In these films, the surfaces of the mesopores, the external surface of the zeolite film, and the micropores close to the external film surface are the most likely surfaces functionalized with methyl groups. However, the micropores deep in the film may not be methylated, making the film vulnerable to HF attack at those points. To better illustrate this point, Figure 2 shows a schematic representation of the pore structure in the zeolite films. (In a real film, the pore system is much more tortuous, and the micropores and mesopores are highly intertwined.) As seen in the figure, the higher reactivity of TMCS over HMDS52 may cause pore blockage of the micropores when the zeolite is silylated with TMCS. The pore-size engineering of zeolites with different silanes has been documented in previous studies.54,55 For the mesopores, the mesopore volume of the sample silylated in TMCS drops by 66%, and in HMDS it decreases by 54%. The mesopores of the TMCS powders may be more fully silylated as a result of two complementary mechanisms. First, because of its lower boiling point (57 C versus 125 C for HMDS), the TMCS concentration in the reaction chamber is higher than the concentration of HMDS during the silylation (they were performed under the same conditions), and that may lead to a more complete silylation in the mesopores with the TMCS. Second, the reactivity of TMCS is higher than HMDS;52 either effect would certainly explain why the mesopores of the TMCS powders are better silylated than those in the HMDS powders. Altogether, the nitrogen adsorption/desorption data indicate that the TMCS only reacts with the mesopore void space between the zeolite nanocrystals, while HMDS penetrates both the mesopores and the micropores. (The maximum lateral extension of a trimethylsilane group is 3.7 A˚,56 while the smallest channel width of MEL zeolite is 5.3 A˚.9) If this assumption is true, then the micropores of the HMDS films are more hydrophobic than the TMCS films, thus resulting in the slower etch rate of the HMDS films. While the nitrogen adsorption/desorption measurements were not performed on films, positronium annihilation lifetime spectroscopy measurements on zeolite films57 measured strikingly similar results to powder measurements19 for both the micropores (0.55 nm versus 0.55 nm, respectively) and mesopores (2.3-2.6 nm versus 2.6-2.8 nm, respectively). Moreover, the pore size measurements in the current study are (53) Eslava, S.; Delahaye, S.; Baklanov, M. R.; Iacopi, F.; Kirschhock, C. E. A.; Maex, K.; Martens, J. A. Langmuir 2008, 24(9), 4894–4900. (54) Chudasama, C. D.; Sebastian, J.; Jasra, R. V. Ind. Eng. Chem. Res. 2005, 44(6), 1780–1786. (55) Lu, D. L.; Kondo, J. N.; Domen, K.; Begum, H. A.; Niwa, M. J. Phys. Chem. B 2004, 108(7), 2295–2299. (56) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1982, 86(26), 5208–5219. (57) Li, S.; Sun, J. N.; Li, Z. J.; Peng, H. G.; Gidley, D.; Ryan, E. T.; Yan, Y. S. J. Phys. Chem. B 2004, 108(31), 11689–11692.

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Article Table 2. Ratio of Intensities of the O-Si-CH3 Peaks at 102.0 eV to the O-Si-O Peak at 103.7 eV for PSZ MEL, Traditionally Silylated, TMCS, HMDS, and Presilylated Films film

ratio of intensities, O-Si-CH3/O-Si-O

PSZ MEL Traditionally Silylated TMCS HMDS Presilylated

0 0.04 0.31 0.30 0.35

Figure 2. Simplified schematic of the pore system with micropores and mesopores silylated by TMCS or HMDS.

Figure 4. FTIR spectra of (a) PSZ MEL, (b) Traditionally Silylated, (c) TMCS, (d) HMDS, and (e) Presilylated films. The inset is a close-up of the spectra between 1240 and 1300 cm-1.

Figure 3. XPS Si2p core-level spectra of (a) PSZ MEL, (b) Traditionally Silylated, (c) TMCS, (d) HMDS, and (e) Presilylated films. Raw data were fit and split into the two contributing components. comparing the effectiveness of the silylation agents on only one medium (powder) and not between powder and film media. The films were also characterized by XPS. Figure 3 shows the resulting data together with the envelope from peak fitting of the experimental XPS Si2p traces. The larger peak at 103.7 eV results from Si bonded to the oxygens in SiO2, while a secondary peak that emerges at a lower binding energy of 102.0 eV corresponds to the Si coordinated with oxygen and a methyl group.58 The ratio of the O-Si-CH3 peak at 102.0 eV to the O-Si-O peak at 103.7 eV indicates the relative amounts of methylation, and these values are shown in Table 2. It can be seen from these data that the PSZ MEL film does not exhibit any methyl group functionalization, and that the Traditionally Silylated films show a slight peak at 102.0 eV. In contrast, spectra in Figure 3c-e all have larger contributions at 102.0 eV, indicating a higher amount of methyl functionalization in the TMCS, HMDS, and Presilylated films. Furthermore, the ratio in the Presilylated film is the largest at 0.35. This result is understandable, since the presilylation process exposes the nanoparticle suspension to more silylation agent than the TMCS, HMDS, and Traditionally Silylated films. The intermediate value of the Traditionally Silylated film ratio at 0.04 explains why the Traditionally Silylated film lasts slightly longer in HF than PSZ MEL but more than 4 min less than the TMCS, HMDS, and Presilylated films. FTIR was performed to further examine the methylation that was evident in the XPS spectra; the results are shown in (58) Wagner, C. D. J. Vac. Sci. Technol. 1978, 15(2), 518–523.

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Figure 4. Isolated hydroxyl groups are seen in the PSZ MEL film at 3742-3746 cm-1 (Figure 4a).24,59,60 The Traditionally Silylated films show a reduced hydroxyl peak (Figure 4b) but no total elimination, which indicates that the silylation does not fully methylate the films. Isolated hydroxyl groups are eliminated in the TMCS, HMDS, and Presilylated films, as shown by the absence of the corresponding peak in Figure 4c-e. (While other binding sites were not further studied in this work, previous studies on the silylation of zeolite films53 show that there is no significant binding to geminal or vicinal silanols, and a similar chemistry is envisioned for the current films.) Also, corroborating the information extracted from the OH IR data, the asymmetric and symmetric -CH3 stretching features are seen at 2965-2967 cm-1 24 and 2901-2902 cm-1,24,59-61 respectively, and the Si-CH3 deformation and rocking appear as peaks between 1255 and 1280 cm-1 and 840-850 cm-1,24,59-61 respectively. As expected, the IR traces for PSZ MEL indicate that that film does not contain any methyl groups. In contrast, the Traditionally Silylated film shows small methyl group peaks, and the TMCS, HMDS, and Presilylated films all display significantly more intense methyl IR absorptions. The CH3 stretching bands and the SiCH3 rocking bands are more intense for the TMCS films than the HMDS films, and this may be explained by the increased silylation of the zeolite mesopores by TMCS over HMDS, as shown by the N2 adsorption/ desorption measurements. Nonetheless, in agreement with the XPS results, the Presilylated films contain the most intense methyl peaks, while the intensities of the hydroxyl and methyl group peaks for the Traditionally Silylated films are (59) Wang, C. Y.; Shen, Z. X.; Zheng, J. Z. Appl. Spectrosc. 2000, 54(2), 209–213. (60) Wang, C. Y.; Shen, Z. X.; Zheng, J. Z. Appl. Spectrosc. 2001, 55(10), 1347–1351. (61) Grill, A.; Neumayer, D. A. J. Appl. Phys. 2003, 94(10), 6697–6707.

DOI: 10.1021/la803956w 5043

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Table 3. k Values and Contact Angles after Water-Soak Tests on PSZ MEL, Traditionally Silylated, TMCS, HMDS, and Presilylated Films PSZ MEL condition as-synthesized after drying >12 h (110 C) after soaking in H2O > 12 h (27 C) after drying >12 h (110 C) after ambient conditions for 24 h

k

contact angle ()

Traditionally Silylated k

contact angle ()

contact angle ()

k

contact angle ()

k

Presilylated k

contact angle ()

92.3 ( 4.6

3.18 ( 0.35 2.0 ( 1.5 2.83 ( 0.46

36.9 ( 3.4 1.69 ( 0.13 121.2 ( 15.4 1.64 ( 0.06 122.9 ( 9.1 1.59 ( 0.15 132.0 ( 7.3

2.49 ( 0.53

1.94 ( 0.18

3.00 ( 0.62 2.3 ( 1.4 2.15 ( 0.21

1.53 ( 0.10 42.5 ( 5.6

film

Er (GPa)

H (GPa)

Traditionally Silylated TMCS HMDS Presilylated

9.2 ( 0.3 2.0 ( 0.2 3.3 ( 0.2 3.8 ( 0.2

0.75 ( 0.02 0.16 ( 0.01 0.25 ( 0.01 0.32 ( 0.02

intermediate between the PSZ MEL and the TMCS, HMDS, and Presilylated films. The critical process for this work occurs during the hightemperature annealing of the amorphous silica matrix. For the Traditionally Silylated films, we hypothesize that the annealing step cross-links the amorphous silica matrix and prevents easy diffusion of the TMCS into the pores during the postcalcination silylation step. On the other hand, when the silylation agents are introduced immediately after the spin-on process in the TMCS, HMDS, and Presilylated films, the solvents in the zeolite nanoparticle suspension allow the TMCS and HMDS to quickly diffuse throughout the uncross-linked, “loose”, silica matrix and react with all hydroxyl groups. Moreover, both the cross-linking and the silylation occur during the same processing step, thus making more hydroxyl sites accessible for methylation before significant cross-linking occurs. In this way, methyl groups are incorporated throughout the silica matrix, which then cross-links at high temperatures. As a result, there is more methyl content in the TMCS, HMDS, and Presilylated films than in the Traditionally Silylated films. This hypothesis has been confirmed here via characterization studies with XPS and FTIR, and also with the macroscopic results obtained from the HF dip and water-soak tests, as discussed next. The internal hydrophobicity of the films was probed using water-soak tests. The k value and contact angle results from these studies are presented in Table 3. The k values of the PSZ MEL and Traditionally Silylated films were 2.67 and 2.29, respectively, after film synthesis, whereas those of the TMCS, HMDS, and Presilylated films were all about 1.6. After drying at 110 C, the k values of the PSZ MEL and Traditionally Silylated films decreased by 0.35 and 0.57, respectively, while the values for TMCS, HMDS, and Presilylated films remained about the same. This result suggests that more ambient water was adsorbed on the PSZ MEL and Traditionally Silylated films than on the other three. The k values of both the PSZ MEL and Traditionally Silylated films increased dramatically by 0.86 and 1.11, respectively, after immersion of the films in water for over 12 h, indicating that the internal surfaces of these two films are relatively hydrophilic. The external surface of the Traditionally Silylated film also became more hydrophilic as shown by the decrease in the contact angle from 92.3 DOI: 10.1021/la803956w

1.55 ( 0.09 1.58 ( 0.10

HMDS

2.67 ( 0.22 4.8 ( 5.0 2.29 ( 0.27 2.32 ( 0.20 1.72 ( 0.22

Table 4. Reduced Modulus and Hardness Values of Traditionally Silylated, TMCS, HMDS, and Presilylated Films

5044

TMCS

135.0 ( 7.4 1.56 ( 0.21 137.3 ( 4.0 1.65 ( 0.12 140.7 ( 2.9 1.52 ( 0.16 1.63 ( 0.07

1.60 ( 0.12

1.68 ( 0.11

1.54 ( 0.10 122.3 ( 13.3 1.51 ( 0.14 126.5 ( 8.4 1.63 ( 0.12 133.3 ( 5.7

to 36.9 after soaking the film. The surface of the PSZ MEL film stayed hydrophilic throughout the water-soak tests. Conversely, both the surfaces and the interiors of the TMCS, HMDS, and Presilylated films preserved their hydrophobic nature after the soaking test: the k value increase of all three films was no more than 0.12, and the contact angles remained above 121. After a cycle of drying and exposure to ambient conditions, the k values of the PSZ MEL and the Traditionally Silylated films remained high at 3.00 and 2.15, respectively. The TMCS, HMDS, and Presilylated films, however, maintained k values that were similar to their initial as-synthesized values. These water-soak tests illustrate that the TMCS, HMDS, and Presilylated films do not absorb much water and are all much more stable than the PSZ MEL and Traditionally Silylated films even after complete immersion in water. Furthermore, the results of the water-soak tests show that the internal structure of the TMCS, HMDS, and Presilylated films are more hydrophobic than the PSZ MEL and Traditionally Silylated films, thus further confirming the XPS and FTIR measurements. The reduced modulus and hardness values for the films are presented in Table 4. The mechanical properties for the TMCS, HMDS, and Presilylated films all decreased from the Traditionally Silylated values. The influence of the silylation process on the mechanical properties is worthy of a separate study.

Conclusions We have successfully synthesized PSZ low-k films with improved moisture and HF resistance by calcining the films in TMCS or HMDS vapor and by adding HMDS directly to the as-synthesized nanoparticle suspension. This silylation technique also eliminates the extra postcalcination silylation step that is needed to synthesize Traditionally Silylated films. In real device fabrication, the number of processing steps should be minimized to keep production costs low. A stable k value between 1.5 and 1.7 and HF resistances of 5 min or more are shown through water-soak tests and HF dip tests. XPS and FTIR data show that the improved properties come from higher methyl group content. With hydrophobicities now on par with similar SiCOH low-k materials and k values as low as 1.5, these PSZ films stand to be extremely competitive new low-k materials. Acknowledgment. Financial support was provided by the National Science Foundation (CTS-0404376) and the Semiconductor Research Corporation (Task 1576.001 - Intel Custom Funding). We also thank Boyan Boyanov at Components Research, Intel Corporation for his helpful suggestions and comments. Langmuir 2009, 25(9), 5039–5044