Langmuir 2007, 23, 12811-12816
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Characterization of a Molecular Sieve Coating Using Ellipsometric Porosimetry Salvador Eslava,†,‡ Mikhail R. Baklanov,† Christine E. A. Kirschhock,§ Francesca Iacopi,† Steliana Aldea,§ Karen Maex,†,‡ and Johan A. Martens*,§ IMEC, Kapeldreef 75, 3001 LeuVen, Belgium, ESAT-INSYS, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 10, 3001 LeuVen, Belgium, and Centrum Voor OpperVlaktechemie en Katalyse, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 23, 3001 LeuVen, Belgium ReceiVed September 12, 2007. In Final Form: NoVember 5, 2007 Ellipsometric porosimetry was used to determine the adsorption isotherms of toluene, methanol, and water on b-oriented Silicalite-1 coatings with a thickness of less than ca. 250 nm and to obtain adsorption kinetics. The adsorption isotherms are of sufficient quality to reveal several aspects of the pore structure such as the adsorbate capacity and the adsorbate/framework affinity. The use of a combination of different molecular probes in ellipsometric porosimetry to elucidate the molecular accessibility of Silicalite-1 pores is demonstrated. It is shown that ellipsometric porosimetry is an appropriate technique for probing the influence of aging of the Silicalite-1 coating and of planarization polishing on the porosity, pore accessibility, and adsorbate/framework affinity.
Introduction Molecular sieve coatings and their applications in molecular separation processes,1-5 catalysis,6 sensor devices,7 and integrated circuits8-10 have attracted the attention of many research groups. In such studies, a variety of molecular sieves, including ZSM5,11 Silicalite-1,12 Silicalite-2,13 and AlPO4-5,14 have been grown as polycrystalline coatings on porous and nonporous supports. The applicability of molecular sieve coatings critically depends on the nature of the porosity and especially on properties such as pore volume, pore size distribution, and pore accessibility.2 The porosity of a molecular sieve coating is complicated because it comprises interparticle and intraparticle porosity. Several techniques can be used to characterize the porosity of a material on the nanoscale.15 The most widely used technique is based on the experimental determination of the adsorption isotherm of an inert gas and, in particular, nitrogen or argon at their boiling temperatures.16 Such a determination of the porosity * Corresponding author. E-mail:
[email protected]. † IMEC. ‡ ESAT-INSYS, Katholieke Universiteit Leuven. § Centrum voor Oppervlaktechemie en Katalyse, Katholieke Universiteit Leuven. (1) Coronas, J.; Santamaria, J. Sep. Purif. Methods 1999, 28, 127-177. (2) Coronas, J.; Falconer, J. L.; Noble, R. D. AIChE J. 1997, 43, 1797-1812. (3) Coronas, J.; Falconer, J. L.; Noble, R. D. Ind. Eng. Chem. Res. 1998, 37, 166-176. (4) Yu, M.; Hunter, J. T.; Falconer, J. L.; Noble, R. D. Microporous Mesoporous Mater. 2006, 96, 376-385. (5) Sommer, S.; Melin, T.; Falconer, J. L.; Noble, R. D. J. Membr. Sci. 2003, 224, 51-67. (6) Saracco, G.; Specchia, V. Catal. ReV. Sci Eng. 1994, 36, 305-384. (7) Yan, Y. A.; Bein, T. J. Am. Chem. Soc. 1995, 117, 9990-9994. (8) Wang, Z. B.; Mitra, A. P.; Wang, H. T.; Huang, L. M.; Yan, Y. S. AdV. Mater. 2001, 13, 1463-1466. (9) Eslava, S.; Iacopi, F.; Baklanov, M. R.; Kirschhock, C. E. A.; Maex, K.; Martens, J. A. J. Am. Chem. Soc. 2007, 129, 9288-9289. (10) Eslava-Fernandez, S.; Baklanov, M. R.; Iacopi, F.; Brongersma, S. H.; Kirschhock, C. E. A.; Maex, K. Mater. Res. Soc. Symp. Proc. 2006, 914, 0914F03-F08. (11) Lai, R.; Yan, Y. S.; Gavalas, G. R. Microporous Mesoporous Mater. 2000, 37, 9-19. (12) Wang, Z. B; Yan, Y. S. Chem. Mater. 2001, 13, 1101-1107. (13) Dong, J. P.; Xu, Y. J.; Long, Y. C. Microprous Mesoporpous Mater. 2005, 87, 59-66. (14) Mintova, S.; Mo, S.; Bein, T. Chem. Mater. 1998, 10, 4030-4036. (15) Julbe, A.; Ramsay, J. D. F. Fundamentals of Inorganic Science and Technology; Burggraaf, A. J., Cot, L., Eds.; Elsevier Science: Amsterdam, 1996; Chapter 4, p 67.
practically involves the confinement of the porous sample in vacuo and exposing it to an inert gas at increasing partial pressure and fixed temperature. Adsorption isotherms are currently determined using either volumetric or gravimetric methods. In the case of powdery samples of molecular sieves, typically 100 mg quantities are needed to permit the accurate determination of a nitrogen adsorption isotherm. With supported molecular sieve coatings, however, the typically required quantity of material is problematic because of the tiny amount of molecular sieve material present in the sample. As a consequence, an accurate determination of an adsorption isotherm of a molecular sieve coating via gravimetric or volumetric determination necessitates an extremely sensitive device.17 A surface acoustic wave (SAW) device17 and quartz crystal microbalance (QCM)15 are examples of such highly sensitive equipment. In both SAW and QCM, the thin coating has to be applied to the measuring device itself. The drawback of these techniques is that the characterization of the porosity of the thin coating is done on a support other than the actual one. Properties such as the orientation of the crystals, the presence of grain boundaries, and the film thickness could be different. The problem of limited sample size sometimes can be overcome by scraping off and collecting several coatings from their supports to obtain a sufficient quantity of powdery sample for a conventional determination. This drastic intervention entails the risk of destroying the coating, of altering the porosity in an uncontrolled manner, and of including some support material in the sample. The problem of accurate characterization of the porosity of thin coatings is particularly critical in the area of films with low dielectric constants18 (so-called low-k films). Information about the porosity of low-k films has been obtained using positron annihilation lifetime spectroscopy (PALS), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) combined with specular X-ray reflectivity (XRR).18-19 Although these methods yield significant insight into the porosity by (16) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity, 2nd ed.; Academic Press: London, 1982; Chapters 1 and 5. (17) Hietala, S. L.; Smith, D. M.; Hietala, V. M.; Frye, G. C.; Martin, S. J. Langmuir 1993, 9, 249-251. (18) Maex, K.; Baklanov, M. R.; Shamiryan, D.; Iacopi, F.; Brongersma, S. H.; Yanovitskaya, Z. S. J. Appl. Phys. 2003, 93, 8793-8841. (19) Baklanov, M. R.; Maex, K. Philos. Trans. R. Soc. London, Ser. A 2006, 364, 201-215.
10.1021/la7028388 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2007
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determining some parameters that are indirectly related to it, they do not follow a well-established recommendation,20 which is to use molecular probes to assess molecular sieving properties directly. The determination of the adsorption isotherms of an adsorbate on a film via spectroscopic ellipsometry, termed ellipsometric porosimetry (EP), is a convenient means to determine the total porosity and pore size distribution of a thin film of low-k material.9,18,19,21,22 Further applications on low-k materials are also known, such as the evaluation of diffusion barriers23 and the quantification of processing damage.24 Outside the field of low-k films, Boissiere et al. investigated ordered mesoporous silica thin films by EP,25 and Bjorklund et al. applied EP to thin Silicalite-1 zeolite films to determine the adsorption isotherms of water, toluene, 1-propanol, and hexane.26 The latter experiments on zeolites were performed at room temperature using a flow cell and a stream of nitrogen carrier gas into which adsorbate vapors were dosed. In the investigated partial pressure range up to ca. 0.7, the Silicalite-1 film exhibited I-type isotherms typical of microporous material. The experiment revealed the lower adsorption capacity for water vapor compared to that of organic adsorbates. Here we report on the experimental determination of adsorption isotherms of molecular sieve films using spectroscopic ellipsometry in a vacuum system. Adsorption isotherms of water, toluene, and methanol were determined on b-oriented Silicalite-1 films synthesized by in-situ crystallization. We used the technique to reveal the porosity changes with crystallization time, to evaluate the impact of planarization polishing on pore accessibility, and to monitor the changes upon aging of the film in consecutive wetting and drying cycles. Ellipsometric porosimetry is shown to be a convenient technique enabling the characterization of various properties of a molecular sieve coating such as the pore volume, free apertures of the pore system, and sorbate/framework affinity. Experimental Section We followed the procedure reported by Wang et al. to synthesize a b-oriented Silicalite-1 coating by in-situ crystallization for 2 h (referred to as SIL-2h).27 Pieces of silicon wafer, each with a top layer of 50 nm of silicon carbide, served as supports. The presence of a SiC top layer is essential to avoid the etching of the silicon wafer top layer in the alkaline medium used for the synthesis of the Silicalite-1 film. After the coating step, the sample was heated for 1 day at 450 °C in air to evacuate the tetrapropylammonium template from the pore structure. The coating was introduced into a 12 L vacuum chamber in which the spectroscopic ellipsometer (Sentech SE801, 350-850 nm wavelength) was mounted, and a system for dosing adsorbates was foreseen. Room temperature was controlled at 20 °C. Methanol, toluene, and doubly deionized water served as adsorbates. The sample was pretreated at 300 °C in air to evacuate the pores. Two types of experiments were performed. For the (20) Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. H.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Pure Appl. Chem. 1994, 66, 1739-1758. (21) Baklanov, M. R.; Mogilnikov, K. P.; Polovinkin, V. G.; Dultsev, F. N. J. Vac. Sci. Technol., B 2000, 18, 1385-1391. (22) Baklanov, M. R.; Mogilnikov, K. P. Microelectron. Eng. 2002, 64, 335349. (23) Shamiryan, D.; Baklanov, M. R.; Maex, K. J. Vac. Sci. Technol., B 2003, 21, 220-226. (24) Baklanov, M. R.; Mogilnikov, K. P.; Le, Q. T. Microelectron. Eng. 2006, 83, 2287-2291. (25) Boissiere, C.; Grosso, D.; Lepoutre, S.; Nicole, L.; Bruneau, A. B.; Sanchez, C. Langmuir 2005, 21, 12362-12371. (26) Bjorklund, R. B.; Hedlund, J.; Sterte, J.; Arwin, H. J. Phys. Chem. B 1998, 102, 2245-2250. (27) Wang, Z. B.; Wang, H. T.; Mitra, A.; Huang, L. M.; Yan, Y. S. AdV. Mater. 2001, 13, 746-749.
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Figure 1. SEM images of the Silicalite-1 coating synthesized on SiC using crystallization times of (a) 2 h, SIL-2h sample, and (b) 4 h. (c) Cross section of the Silicalite-1 coating crystallized for 4 h after polishing with a slurry of silica particles, SIL-4h-P sample. determination of adsorption isotherms, the adsorbate pressure was slowly increased until the saturation pressure was reached. The desorption was performed by slowly evacuating the chamber. The toluene adsorption-desorption cycle was performed in approximately 2 h. The water adsorption-desorption isotherms were realized in 1.5 to 3.5 h, depending on the sample. For the determination of adsorption kinetics, the film was evacuated, and the adsorbate pressure was suddenly increased to the saturation pressure. Ellipsometric angles Ψ and ∆ over the whole spectral range of 350-850 nm, the pressure, and the time were continuously recorded. After the ellipsometric porosimetry characterization of the SIL-2h coating, both the adsorption of methanol and water and the respective evacuation pretreatment were repeated five times to imitate the aging of the coating. This coating is referred to as SIL-2h-aged. Another coating was crystallized for 4 h instead of 2 h and polished with slurry of silica particles to improve planarity. Then, the template was eliminated from the zeolite pores by heating to 450 °C in air for 1 day (SIL-4h-P coating). Further characterization of the coatings was done by scanning electron microscopy (SEM, W-filament Philips X-30), Fourier transform infrared spectroscopy (Biorad FTS-40), and X-ray diffractometry (XRD) with standard incidence (Siemens D5000 Bragg Brentano geometry).
Results and Discussion The Silicalite-1 coatings deposited by in-situ crystallization are polycrystalline, as SEM inspection shows (Figure 1). The coating obtained after 2 h of crystallization, SIL-2h, consists of one polycrystalline layer formed by the intergrowth and coalescence of insular crystals (Figure 1a) with a thickness of ca. 250 nm. Silicalite-1 crystals have a coffin shape. When the crystallization was prolonged to 4 h, the coating was covered by a less uniform and incomplete second layer (Figure 1b). This two-layer Silicalite-1 coating was not flat, but polishing with a slurry of silica particles restored the planarity (SIL-4h-P sample, Figure 1c) and left a film of ca. 210 nm. The b orientation of
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Figure 2. XRD pattern of the b-oriented Silicalite-1 coating crystallized for 2 h (SIL-2h).
Figure 3. Changes in ellipsometric angles Ψ and ∆ during the adsorption of methanol vapor in the SIL-2h Silicalite-1 coating (O) at a wavelength of 632.8 nm. The theoretical trajectory of Ψ and ∆ for a fixed thickness and a variable refractive index (n) is represented by the solid line. The measurements were made over the range of methanol vapor pressures until the saturation point was reached.
the Silicalite-1 coatings was confirmed by XRD (Figure 2). The (0k0) Bragg reflections predominate in the diffractogram. This implies that the straight Silicalite-1 channels are perpendicular and the sinusoidal channels are in plane with the support.27 FTIR spectra of the Silicalite-1 films show an absorption band at ca. 550 cm-1 that is a fingerprint of the pentasil zeolite family to which Silicalite-1 belongs (results not shown).28 Ellipsometric porosimetry involves the determination of ellipsometric angles Ψ and ∆. With the incident electromagnetic wave decomposed in two orthogonal planes, tan Ψ relates to the amplitude change and ∆ relates to the phase shift upon reflection.29 The refractive index is the variable of interest. It is obtained by applying the Fresnel equations to a layer model that describes the system. The measured values of Ψ and ∆ are approximated by regression of the model parameters. The evolution of the refractive index of the film upon adsorption of probe molecules was obtained by regression on the continuously measured Ψ and ∆. The Lorentz-Lorentz theory describes the relation between the refractive indices and the material composition in a multicomponent system21
n2 - 1 ) n2 + 2
ni2 - 1
∑ Vin 2 + 2 i
(1)
Figure 4. Adsorption of (a) methanol, (b) toluene, and (c) water on a SIL-2h-aged film with time after sudden exposure to the saturation vapor pressure.
where n is the refractive index of the multicomponent system, Vi is the volume fraction of component i, and ni is the refractive index of component i. Considering the porous coating during adsorption to be a dicomponent system of porous zeolite and liquid adsorbate, the refractive index measured during the adsorption cycle is directly related to the volume fraction Vi of the liquid adsorbate. The free pore volume is calculated from the initial refractive index before adsorbate uptake, considering the evacuated porous coating to be a dicomponent system of the zeolite framework and void volume. The adsorption of methanol is shown in Figure 3 as a representative example of the correlation between Ψ, ∆, the refractive index, and the film thickness. Figure 3 illustrates how the experimental Ψ and ∆ values follow the theoretical trajectory of Ψ and ∆ for a fixed thickness, a fixed wavelength, and a variable refractive index. By applying the Fresnel equation to a layer model that describes the system in terms of the condensed adsorbate, Silicalite-1, and SiC and Si contributions from the support, the refractive indices that approximate the measured Ψ (28) Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Nature 1978, 272, 437-438. (29) Azzam, R. M. A.; Bashara, N. M., Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1977.
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Table 1. Kinetic Diameter of Adsorbates, Maximum Volume Adsorbed on Silicalite-1 Coatings, and Reversibility of Adsorption maximum volume adsorbed (vol %) methanol kinetic diameter 3.8-4.132 (Å) SIL-2h SIL-2h-aged SIL-4h-P
toluene 5.833
water 2.632
23 (reversible) 21 (reversible) 17 (irreversible) 24 (reversible) 22 (reversible) 19 (irreversible) 21 (reversible) 14 (irreversible) 11 (reversible)
and ∆ are obtained. Figure 3 shows the results obtained at a wavelength of 632.8 nm. Actually, the whole spectral range of 350-850 nm was exploited to minimize the error in the determination of the refractive index. The adsorption isotherm was derived from the variation of the refractive index with the relative adsorbate pressure by applying the Lorentz-Lorentz equation (eq 1). To evaluate the kinetics of the adsorption of methanol, toluene, and water, the evacuated SIL-2h-aged film was exposed to the saturation vapor pressure, and the adsorbate uptake was recorded (Figure 4). Methanol uptake was achieved within 15 min. With toluene, the adsorption plateau was reached in ca. 27 min. In determinations of adsorption isotherms of methanol and toluene, the time needed to scan the entire partial pressure range during the adsorption and desorption was greater than 1 h each. Water adsorption was much slower and lasted more than 90 min. For the investigation of the porosity of the films, methanol and toluene served as adsorbates. Table 1 reports the kinetic diameter of these adsorbate molecules and the volume adsorbed in the three coatings and indicates whether the adsorption is reversible. The pore filling of the SIL-2h sample with methanol
is complete at ca. 23 vol %. The adsorption is reversible because desorption and adsorption traces coincide (Figure 5a). It is encouraging to note that the present methanol adsorption isotherm obtained with EP is in good agreement with the data reported by Dubinin et al.30 obtained on micrometer-sized Silicalite-1 powder samples. It demonstrates the applicability of EP for thin zeolite coatings. Toluene at 21 vol % was needed to saturate the SIL-2h coating, which is slightly less than with methanol. Such a discrepancy of the pore volume of Silicalite-1 determined with different adsorbates has already been observed elsewhere.31 Upon aging of the coating (i.e., in SIL-2h-aged), the maximum methanol and toluene uptakes both increase by ca. 1%. The refractive index of the polished Silicalite-1 coating (SIL4h-P) was 1.354. Considering a refractive index for the zeolite framework of 1.460, corresponding to that of fused quartz, the total free pore volume of the SIL-4h-P film is 21 vol %. Figure 5c displays the adsorption isotherm of methanol and toluene on this polished coating. Methanol occupied ca. 21 vol % of the saturated SIL-4h-P film, corresponding to the total free pore volume. In the low-pressure region, the methanol uptake rose more slowly with pressure on SIL-4h-P than on SIL-2h. Provided equilibria were established at all times, the lower slope of the isotherm should be indicative of a weaker adsorbate/framework interaction.16,34 The hydrophilicity of the pores was probed with water adsorption and will be presented below. Another EP experiment with toluene (Figure 5d) taught that the uptake kinetics in SIL-4h-P are much slower than in SIL-2h. After the polishing of the coating, toluene molecules encountered an impediment to diffusion into the pore structure. This manifested itself in the toluene uptake curve by the sluggish increase in toluene adsorption under conditions where a nonpolished film readily adsorbed toluene (Figure 4b). The total volume finally occupied by toluene
Figure 5. Adsorption isotherms of (a) methanol and (b) toluene in a Silicalite-1 coating crystallized for 2 h, SIL-2h. Adsorption of (c) methanol and (d) toluene in a Silicalite-1 coating crystallized for 4 h and polished, SIL-4h-P.
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was ca. 14 vol %. When the pressure was decreased after saturation, toluene remained adsorbed. Only after raising the temperature to 300 °C did the toluene completely desorb (result not shown). This experiment revealed that polishing modifies the zeolite pore structure and affects the free aperture of the pore entrances in a systematic way. The maximum free diameter of the Silicalite-1 channels according to the crystallographic structure is 5.8-6 Å.33 A slight restriction of the pore apertures is dramatic for toluene because of its kinetic diameter of 5.8 Å but is not significant for methanol because of its smaller kinetic diameter of 3.8-4.1 Å (Table 1). The present EP investigation of SIL4h-P illustrates how pore accessibility in a molecular sieve coating can be probed with EP and a combination of molecular probes. Beside molecular sieving, the adsorption of organic molecules in Silicalite-1 can be governed by several additional phenomena.16,34 They are (i) the hydrophobicity/organophilicity of the micropore surfaces, (ii) the occurrence of phase transitions of the zeolite and the adsorbate during filling or emptying, and (iii) the formation of adsorbate associates.34 Figure 6 reports the adsorption of water in the three films in the study: SIL-2h, SIL-2h-aged, and SIL-4h-P. For water adsorption in the SIL-2h coating (Figure 6a), contrary to toluene and methanol adsorption, the adsorption branch of the isotherm resembles a type III isotherm (little adsorption at low pressures and increasing adsorption at high pressures). The maximum water uptake is ca. 17 vol %. Water adsorption and desorption show strong hysteresis. Water occupying some 13 vol % of the film could not be evacuated at 20 °C. Only upon heating was the initial porosity restored. This hysteretic behavior can be interpreted as follows. The fresh Silicalite-1 film (SIL-2h) is hydrophobic, but a small number of silanols at defects on intracrystalline and extracrystalline surfaces permit enough wetting to activate the surface. Such activation may be the result of water chemisorption and hydrolysis of siloxane bridges,16 which has been shown to have autocatalytic character.24,35 When the water vapor pressure is decreased during the desorption step, the chemisorbed water cannot be desorbed if the temperature is not increased. When the SIL-2h coating is aged (SIL-2h-aged), the shape of the water isotherm reveals that the film is much less hydrophobic (Figure 6b). There is less hysteresis. This difference in water adsorption between fresh and aged films suggests that the hydrophilicity of the film increases upon aging. This is attributed to the increasing formation of defects under the substantial stress that the coating is subjected to during the five cycles of water vapor adsorption-desorption and, especially, during sample pretreatment36 at 300 °C. Different behavior was encountered in SIL-4h-P (Figure 6c). In the polished coating, no hysteresis was observed, and a lower quantity of water was adsorbed (11 vol %). At low relative pressure, there is a stronger water uptake than in the SIL-2h nonpolished sample (Figure 6a). The polishing must have created defects on the top surface of the coating,which are responsible for the water uptake at low pressure. The internal (30) (a) Gupta, A.; Clark, L. A.; Snurr, R. Q. Langmuir 2000, 16, 3910-3919. (b) Dubinin, M. M.; Rakhmatkariev, G. U.; Isirikyan, A. A. IzV. Akad. Nauk, S.S.S.R., Ser. Khim. 1989, 9, 2117-2120 (Bull. Acad. Sci. U.S.S.R., Chem. Sci. 1989, 38, 1950-1953). (31) Rouquerol, F.; Rouquerol, J; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999; Chapter 11. (32) ten Elshof, J. E.; Abadal, C. R.; Sekulic, J.; Chowdhury, S. R.; Blank, D. H. A. Microporous Mesoporous Mater. 2003, 65, 197-208. (33) Roque-Malherbe, R.; Wendelbo, R.; Mifsud, A.; Corma, A. J. Phys. Chem. 1995, 99, 14064-14071. (34) Long, Y. C.; Jiang, H. W.; Zeng, H. Langmuir 1997, 13, 4094-4101. (35) Prigogine, M.; Fripiat, J. J. Bull. Soc. R. Sci. Liege 1974, 7-10, 449-458. (36) Julbe, A. Zeolites and Ordered Mesoporous Materials: Progress and Prospects; C ˇ ejka, J., van Bekkum, H., Eds.; Studies in Surface Science and Catalysis: Elsevier: Amsterdam, 2005; Vol. 157, Chapter 7.
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Figure 6. Adsorption of water in Silicalite-1 coatings: (a) crystallized for 2 h, SIL-2h, (b) after aging, SIL-2h-aged, and (c) crystallized for 4 h and polished, SIL-4h-P.
volume of the zeolite film, however, remains more hydrophobic than in the SIL-2h coating. This corroborates the differences at low pressure in the methanol adsorption isotherms (Figure 5). The absence of silanols in the internal volume of the SIL-4h-P film leads to a lower affinity for methanol adsorption, confirming the “phobicity” of defect-free Silicalite-1 to alkyl alcohols.34 The absence of hysteresis in the water adsorption isotherm of this film reveals that there is little water chemisorption. Note that in SIL-4h-P the crystallization was prolonged to 4 h and the polishing was done before the removal of the template to avoid the penetration of the damaging slurry in the pore structure. The creation of defects in the surface of the film that narrow the pore entrances and provide silanol sites for water adsorption explains all of the observations made. Damage to the top layer of low-k films upon polishing has been reported elsewhere.37
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Conclusions Ellipsometric porosimetry with different adsorbates as molecular probes is shown to be a powerful tool for characterizing thin molecular sieve coatings. This technique is able to describe (37) Ishikawa, A.; Shishida, Y.; Yamanishi, T.; Hata, N.; Nakayama, T.; Fujii, N.; Tanaka. H.; Matsuo, H.; Kinoshita, K.; Kikkawa, T. J. Electrochem. Soc. 2006, 153, G692-G696.
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many features of the pore structure of the coatings such as the pore free volume, pore size, pore accessibility, and adsorbate/ framework affinity. Acknowledgment. C.E.A.K. and J.A.M. acknowledge the Flemish government for a concerted research action (GOA). LA7028388
