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Langmuir 1993,9, 249-251
Pore Structure Characterization of Thin Films Using a Surface Acoustic Wave/Volumetric Adsorption Technique S.L.Hietala and D.M.Smith* UNMINSF Center for Micro-engineered Ceramics, University of New Mexico, Albuquerque, New Mexico 87131
V. M.Hietala, G.C.Frye, and S.J. Martin Sandia National Laboratories, Albuquerque, New Mexico 87185 Received June 25,1992. In Final Form:September 10,1992 A surface acoustic wave/volumetric adsorption technique is employed to elucidate pore size, pore size distribution, and surface area of inorganic thin films. The technique has been adapted to a commercial bulk sample adsorption analyzer in order to span the characterization of nonporous, microporous, and mesoporous thin films. Nitrogen adsorption was measured on an uncoated surface-acousticwave device, a memporous silica thin film, and a microporous silica thin fiilm. In addition, carbon dioxide adsorption at several temperatures (196 K, 273 K) was used to probe the microporous silica thin film. Data are analyzed by commonly-used gas adsorption data reduction techniques including the BET, Kelvin/BJH, and Dubinin-Radushkevich methods.
Introduction The pore structure of thin films is of interest for many applications such as asymmetric membranes (inorganic and organic), low permeabilitybarrier (i.e., oxygen,water, etc.) coatings, optical films (reflective/antireflective),and sensor coatings. For most applications, the thickness of the layer of interest is on the order of micrometersor less. Because of the small total pore volume/surface area contained in these pores, pore structure characterization techniques such as gas adsorptionlcondensation and mercury porosimetry are inadequate without significant modification. For example, in a 1-cm2film which is 1pm thick with a porosity of l o % ,the total pore volume would be cm3. The isotherm resolution would have to be 1/100 to 1/1OOO of this volume (10-8 to 10-7 cm3) to accurately obtain pore Structure information such as surface area, whereas typical resolution limits for conventional instruments are on the order of lP cm3. It is clear that conventional commercial volumetric or gravimetric adsorption analyzers cannot approach the required level of resolution for thin film analysis. Adsorption uptake on bulk porous materials is usually determined via volumetric techniques using calibrated volumes and a pressuretransducer,using a flow or dynamic method, or gravimetrically (with a microbalance). For film analysis,one can cast a large area of a thick film on a solid substrate. &r the film has dried or cured, it is scraped off the substrate and a sufficient volume is obtainedto employ bulk adsorptiontechniques. However, the pore structure of samples made via this approach may be quite different than the film in ita native state because of vastly different drying and curing kinetics. A~so,a comparably large sample volume is required. For thin filme, measurements of adsorption uptake have been reported using surface acoustic wave (SAW) devices' or quartz crystalmicrobalances (QCM).213These techniques have been used with nitrogen adsorption at 77 K and BET analysis to study film texture by Krim and co-workers2 (1) Ricco, A. J.; Frye, G. C.; Martin, S. J. Langmuir 1989,5,273. (2).Krim, J.; Panelk, V. In Characterization of Poroue Solide I& Rodnguez-Remm, Rouquerol, Sing,Unger, Eds.; Elsevier Press: New York, 1991; p 217. (3) Lu, C.; L e h , 0. J. J . Appl. Phys. 1972,43,4386.
and pore size/surfacearea by Frye and co-worker~.~ Krim and co-workers employed a QCM contained in a vacuum chamber which allowed measurements over theP/Po range of O.o001~.9. Frye and co-workers4used gas mixing of nitrogen and helium streams to vary the relative pressure range of their SAW measurements from 0.01 to 0.9. Both approachessuffer from certain limitations. Of particular interest in the study of microporous materiale is (1) measurement of adsorption over a very wide relative pressure range with the ability to accurately measure uptake in the 1o-Sto 10-1PIP0 range, (2) measurement of relative pressures very close to 1.0 to assess for possible mesoporosity, (3)ability to use a large variety of gases and temperatures, and (4) access to the large base of commercially availablesoftwarefor data reduction of isotherms to pore structure information. In order to achieve them capabilities, we modified a commercial volumetric adsorptionanalyzer (MicromeriticeASAP-2000) by installing a SAW device in order to enhance measurement sedtivity for use with thin films.
SAW Technique The surface acoustic wave (SAW) is generated and detected by interdigital transducers deposited directly on a piezoelectric substrate (ST-quartz in this case),with the device used as a feedback element in an oscillator circuit.6-6 Surface acoustic waves, which are sensitive to surface changes, are especially sensitive to mam loading and theoreticallyorders of magnitude more sensitivethan bulk acoustic waves.6 Adsorption of gas onto the device surface causes a perturbation in the propagation velocity of the surface acoustic wave. Additionally, film coatings of a SAW device alter the velocity due to both mass loading and film stiffness. However, the contribution from the film stiffness is generally negligible? The fractionalchange in the Rayleigh wave velocity, v, as a function of vapor mass loading on a SAW device may be expressed as (4) Frye, G. C.; Ricco, A. J.; Martin, S. J.; Brinker, C. J. Materials Research Society Proceedings V;Materials Reear& Society: Pittabwgh, PA, 1988, p. 349. (5) Martin, S. J.; Ricco, A. J.; Ginley, D.S.; Zipperinn, T. E. IEEZ Trans. Ultrason. Ferroelectr. Freq. Control 1987, UFE-34 (No. 21,142. (6) Wohltjen, H. S e w . Actuators 1984,5,307.
0743-746319312~O9-0249$04~OO/O (8 1993 American Chemical Society
260 Langmuir, Val. 9, No.1, 1993
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Avlvo = -cJo Am * Aflf, (1) where cm is the calculated mass sensitivity (1.29X 10-9 cm2 slg for ST-quart~),~ f is the SAW frequency, uo and fo are the unperturbed SAW velocity and frequency, respectively, and Am is the change in surfacemass density (mase/area). For small perturbations, this is approximately equal to the fractional change in the oscillation frequency when the mass is uniformly distributed over the entire acousticpath. Measurements in the change in oscillation frequencyasa functionof relative preeeure allow observations of changes in mass density of 10-12glcm3. SAW Electronics A stand-alone unit, with the addition of a frequency counter, was built and incorporated into a Micromeritics ASAP 2000 volumetric adsorption analyzer. Figure 1 showea block diagram of the radio-frequency (rf)oscillator circuitu88d to measure the frequencyshift (causedby the SAW velocity change) due to the mass loading on the SAW sensor. T h e large electrical length of the SAW sensor (operating at 97 MHz) dominates the phase shift in the oscillator loop; therefore frequency shifts are due almost entirely to the velocity changes of the surface acoustic wave. The loop gain is automaticallyset by sampling the signal at the oscillator's output using asimple peak detector that provides a dc output to an automatic gain control (AGC) circuit. The AGC circuit forma a second control loop that seta the electronic gain to the precise opposite of the insertion lose of the SAW sensor and other electricallosses in the loop. Consequently, the AGC circuit's dc output ie directly related to the insertion loss of the SAW sensor and therefore allowe for a simpletest of sensorquality and monitors attenuation occurring in the SAW device. The rf loop also containe a 150-MHz low-pass filter to avoid spuriouslooposcillation at higher-order SAW modes. Also, a 10-dB coupler was inserted into the rf loop to provide a signal for frequency measurement. Experimental Section
The SAW Substrates are made from a piezoelectric ST-cut q w r b wafer with Cr-Au evaporated interdigital transducer
f i e r e at a 32-pmperiod. The device measures 8 X 11 mm with an active area -20 mm2. The devices were held in futures deemed to fit on the regular ASAP 2000 outgas and analysis ports. Complete detaila of the the ASAP 2000 modifications for SAW operation may be obtained by contacting the authors. Filmsampleawereprepared byspincoatingprecureorsolutiona onto SAW substrates a t 2000 rpm. However, any coating
technique (e.g., dip coating, spraying) may be used to coat the devices. Dense silica particlea with a diameter of -200 A were prepared via a Stdber method by reactingtetraethyl orthosilicate (TEOS) in an ethanol solution with saturated ammonium hydroxide.' When formed into a f i b , these particles form mesoporosity with pore dimensions of 20-100 A. Microporous silica coatings were prepared by a two-step acid/acid (A2) catalyzed hydrolysis and condensation of TEOS before spinning the sol onto the substrates.8 Films were dried at 353 K for 15 min, then fired at 673 K for 15 min before analysis. After mounting in the sample cell, the f i e were outgassed at 443 K for 2 h under vacuum using the ASAP outgassing station. Adsorption measurementswere made over the preesure range of 0-800 Torr using nitrogen at 77 K and carbon dioxide at 196 and 273K. Standard gas adsorptiondata reduction techniques (BET (Nd, KelvinlBJH (Nz),and Dubinin-Raduehkevich (COP))were used to calculate surface area, pore volume, and pore size distribution in the f h .
Results A nitrogen adsorption isotherm (adsorption and desorption branches) of an uncoated SAW device is shown in Figure 2 as well as a 'BET plot" of the low relativepreeeure portion of the same data, The type 2 isotherm, relatively low uptake, and lack of hysteresisis typical of a nonporous material. The high sensitivityof the uptakemeasurement is illustrated by the excellent fit of the BET equation despite the fact that the active area of the SAW device ie only -20 mm2,which implies that the volumetric uptake corresponding to a monolayer is only 106 c m 3 STP. The calculatedsurface area of 1.4 m21m2is only slightly higher than theoretical (1m2/m2),a result of small-scale surface roughness on the uncoated SAW device. We report all adsorption results on a per square meter of film area basis since the conversion of film results to the more common basisfor bu& materials (pergram) impliesthat one knowns the relative mass of the film as compared to the substrate. This conversion is nontrivial when the substrate ie on the order of millimeters in thicknees and the film is less than 1 p m in thickness. Also, from a practical viewpoint, adsorption properties per area of film are typically the most important. With an estimate of the film thickness and bulk density, the area basis may be convertedto a per gram of film basis. The measured BET C constant of 46 is in good agreement with previous nitrogen adsorption results on nonmicroporous silica (the SAW device has an oxide surface 1ayerI.O The ability of the SAWIASAP to
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(7) Stbbsr, W.;Fink, A.; B o b , E.J. Colloid Interface Sei. 1W,26, 62.
(8) Brinker, C. J.; Keefer, K.D.; Schaefer,D.W.;Ashley, C . S. J. NonCryst. Solids 1982,48,47. (9) Gregg, S . J.;Sing, K . 5. W .Adsorption, Surface Area andPorouity, 2nd ed.;Academic Press: Snn Diego, CA, 1982.
Langmuir, Vol. 9, No. 1, 1993 261
Pore Structure Characterization of Thin Film
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Figure 6. Carbon dioxide adsorption isotherm (273 K)and DR isotherm (inset) for an A2 silica gel coated SAW device.
accurately controlpressure and m e a " uptakes at relative pressure near 1 is illustrated by the rapid increase in the isotherm and the lack of hysteresis. In contrast to the uncoated SAW device shown in Figure 2, results for the masoporous Stdber film are shown in Figure 3. Thie isotherm is a type 4 isotherm with considerable hysteresis. BET analysis yields a surface area of 20.2 d / m 2and a BET C constant of 52. The small inset graph in Figure 3 shows the pore size diatribution calculated from the desorptim branch of the isotherm using the Kelvin equation and the BJH method. The hydraulic radius (twice the pore volume to surface area) ia 59 A, which is in good preement withthe particle radius of 100A and the mnllmumin the pore size distribution
surface area of 54.8 m2/m2calculated using the DubininRadushkevich equation (Figure 6 inset) was dramatically different than the nitrogen results (- 1 m2/m2). Since the kinetic diameters of nitrogen (3.4A) and carbon dioxide (3.6 A) are very similar, it appears that the nitrogen may be activated diffusionlimited for entrance into micropores at the lower temperature associated with the nitrogen adsorption, asis commonly obaerved for cOale.8 This result is somewhat surpriaii given the short diffusion lengths associated with these thin films (
