- Email: [email protected]
Zeolite micromembrane fabrication on magnetoelastic material using electron beam lithography
Microporous and Mesoporous Materials 197 (2014) 213–220
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Zeolite micromembrane fabrication on magnetoelastic material using electron beam lithography Vassiliki Tsukala a,b, Dimitris Kouzoudis b,⇑ a b
Foundation for Research and Technology Hellas, Institute of Chemical Engineering and High-Temperature Chemical Process, P.O. Box 1414, GR 265 04 Patras, Greece Department of Chemical Engineering, University of Patras, GR-26504 Patras, Greece
a r t i c l e
i n f o
Article history: Received 1 February 2014 Received in revised form 8 May 2014 Accepted 2 June 2014 Available online 26 June 2014 Keywords: Micromembrane LTA Electron beam lithography Secondary growth Magnetoelastic
a b s t r a c t In the present study, electron beam lithography (EBL) is employed in the manufacturing of patterns of continuous Linde Type A (LTA) zeolite membranes. The patterns are down to the micrometer scale and they are composed of LTA micromembranes having all three dimensions in the micrometer scale. The control of the size and location of zeolite films or membranes onto specific substrates, will lead to new aspects for their use in microsensing, microelectronics and microreactor applications. Our focus is on the microsensing field, where a magnetoelastic ribbon (Metglas) is used as the sensing platform, since the Metglas/zeolite film composite has been successfully used for the detection of gases and VOCs in the past. Here we report on the first LTA zeolite micromembrane attached onto a previously EBL patterned PMMA coated Metglas substrate. The sensing ability of such a sensor could be significantly improved by using lower amount of zeolite film since it could lead to shorter response and recovery times. The conditions for the manufacturing of the LTA micromembranes onto the Metglas substrate are investigated and discussed in terms of EBL, seeding and hydrothermal synthesis parameters. Ó 2014 Published by Elsevier Inc.
1. Introduction Because of their interesting and unique properties [1], zeolites and their continuous films have attracted great scientific interest [2] over the past decades, which is demonstrated on their various industrial applications such as catalysts in fine chemical and petrochemical industry [3], adsorbents [4], and membranes for separations or as reactors [5]. Their molecular sieving ability, controllable surface properties, well-defined porosity, large surface area along with chemical, thermal and mechanical stability can also be utilized in new advanced applications like microreactors [6], chemical or gas sensors [7], microseparators [8], low dielectric constant materials for microelectronic devices [9], electrodes [10], biomedical science [11] and fuel cells [12]. Nevertheless, the exploitation of zeolites in such advanced applications is often prevented due to the powder form of synthetic zeolites, directing efforts in the development of continuous zeolite films of small size so as to be implemented in the so-called microchemical systems that also include other microdevices such as micromixers, microheat exchangers, microactuators and microsensors [13]. Tailoring and controlling zeolite membranes size in the microscale range offers numerous advantages in many of the above ⇑ Corresponding author. Tel.: +30 2610996880; fax: +30 2610996846. E-mail address: [email protected] (D. Kouzoudis). http://dx.doi.org/10.1016/j.micromeso.2014.06.017 1387-1811/Ó 2014 Published by Elsevier Inc.
advanced applications [14]. Among the benefits of micromembranes, the most obvious one is the elimination of defects (cracks and holes), as the probability of a defect-free surface increases for smaller membrane areas [15]. Other significant aspects, especially for microreactor applications, are the elimination of temperature gradients thus minimizing hot spots, enhancing heat and mass transfer properties and finally, improving yield, conversion and selectivity through the reaction-permeation mechanism of the desired product through the channels [16]. In the field of zeolite membranes used for sensing applications, micromembranes are of great demand, since lower mass could result in shorter response time and subsequently shorter recovery [7]. Thus, the sensitivity of the sensor could increase, especially when used as microbalance (cantilever-based or Quartz Crystal Microbalance, QCM) but also the selectivity of specific components can be enhanced allowing for molecular recognition [17] (as ‘‘electronic nose’’ sensors). Important efforts have been made to fabricate patterned zeolite films or layers of crystals on solid substrates by combining both zeolite-substrate adhesion techniques and pattern definition methods adopted from microelectronics technology. The most common substrates that have been used for patterned zeolite membranes are silicon and glass, while siliceous zeolites are preferred, mainly because of the chemical affinity. In order to attach zeolites onto them, two adhesion techniques are commonly
214
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
used: casting and secondary growth. For the first approach, the most facile method is the direct attachment method, where the zeolite crystals are assembled through covalent or ionic bonding. The most widely used way of ionic bonding is simply by rubbing [18] zeolite crystals on a patterned smooth substrate resulting in uniformly, fully-packed layers of crystals. In a similar manner, ultrasound agitation [19] and dip or spin coating methods of colloidal seed solutions have also been used but with less success in terms of uniformity and coverage. The second approach is the secondary growth method, where zeolite crystals are intergrown to form continuous films attached onto the patterned substrates under hydrothermal conditions. The zeolite type is controlled by the seed type and synthesis conditions. In order to pattern the substrate’s surface for zeolite crystals attachment, three pattern definition methods have been mostly employed that are also compatible with sensor technology: microcontact printing, photoetching and lithography (photolithography or electron beam lithography), resulting in different resolution and structure sizes [20]. Since the resolution of the classic photolithography is limited by the wavelength of the light used, electron beam lithography (EBL) has provided scientists with a powerful tool for creating nano-scale structures. EBL utilizes a highly focused electron beam to expose a resist-coated substrate thus making it soluble (positive tone resist) or insoluble (negative tone resist) to a photoresist developer. As the trend for everincreasing levels of miniaturization needs to be met also for zeolite technology, in order to be integrated with sophisticated microdevices, the demand for high-throughput screening and decreased sample mass [21] can be achieved using EBL technology. Other advantages of EBL are the precise and easy pattern generation using software tools and direct exposure onto the semiconductor substrate [22] without the tedious and often expensive mask preparation. The majority of the studies on patterned layers of crystals or films have focused on siliceous MFI type zeolites (ZSM-5 and silicalite-1) and employed the previously mentioned methods and techniques. The most common substrates used are glass and silicon wafers due to their smooth surface and chemical affinity with the above siliceous zeolites. Using microcontact printing with poly (dimethylsiloxane) (PDMS) stamp, Yoon and co-workers covalently attached b-oriented ZSM-5 crystals on Si wafer. The crystals were self-assembled (from their ethanol solution) after stamp-pressing onto the wafer surface, forming monolayers with approximately 5 lm width [23]. Similarly, Yeung and co-workers [24] fabricated ordered silicalite micromembranes and monolayers on Si wafers, through secondary or direct growth and using photolithography and etching techniques, in order to produce catalytic microreactors and membrane microseparators, while later, they managed to photopattern silicalite-1 membranes on Si wafer reaching 5 lm small zeolite arrays [25]. Another technique was first introduced by Yoon and co-workers employing patterning through photochemical degradation of organic functional groups on glass, covalent bonding of ZSM-5 crystals and subsequent secondary growth of continuous ZSM-5 membrane with approximately 200 lm feature [26]. A different approach, using TEM grid and chromium-gold depositions, was employed by Yan and co-workers to fabricate continuous, b-oriented silicalite membrane onto patterned Si wafer, exploiting the weak interaction of gold surface and colloidal zeolites [27]. Photoetching (both dry and wet) techniques were investigated by Pellejero et al. for the fabrication of patterned silicalite film on Si wafer, after the hydrothermal synthesis, achieving 10 lm structures [28]. More recently, Ozturk et al. [29,30] and Kirdeciler et al. [31] fabricated monolayers of ordered individual crystals of LTA and BEA type zeolites, through the manual assembly method, onto Si wafers patterned using electron beam lithography, with 500 nm features.
In the present study, a magnetoelastic material (Metglas) was chosen as a substrate, which was successfully utilized by our group as a sensing platform together with zeolite films for the detection of CO2 [32], VOCs [33] and stress[34]. Another reason for using Metglas in the present study is the excellent adhesion of continuous LTA films on it. A major drawback in these sensing applications was the relatively slow response time, of the order of tens of seconds, resulting from the slow adsorption and diffusion of the detected species through the zeolite layer. A smaller size zeolite feature, down to micro scale, could probably result in much shorter diffusion time which is one of the objectives of micropatterning LTA films on a Metglas substrate. Three novel ideas are introduced in the present work: (a) electron beam lithography is employed for the first time for the in situ synthesis of micropatterned, continuous zeolite films through the secondary growth. (b) It is also the first time that an LTA film is synthesized in the micrometer scale, revealing new aspects of its future use in microsensing, microelectronics and microreactor applications. Most of the zeolite micro-patterning in the literature involves either MFI membranes or LTA microcrystals, not LTA membranes. It should also be noted that LTA has a number of industrial applications thus its micro-manipulation is of paramount importance. And c) this is the first time that a metallic magnetoelastic material is used as a substrate for zeolite micromembrane patterning applications. Usually conventional substrates such as Silicon wafers or glass are used. 2. Experimental The e-beam lithography and secondary hydrothermal synthesis procedure that was used to develop the micro-patterned LTA film is schematically represented in Fig. 1. The steps can be summarized in the following list: (a) Cleaning of the Metglas substrate (b) PMMA spin-coating
Fig. 1. Schematic representation of the experimental procedure for the fabrication of zeolite micromembranes on Metglas using EBL technology and secondary growth.
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
(c) (d) (e) (f) (g)
E-beam exposure PMMA development LTA seeding LTA synthesis Ultrasound Cleaning
Each individual step is described in detail in the following paragraphs. 2.1. PMMA coating on Metglas Several Metglas 2826 MBA (Fe40Ni38Mo4B18) ribbons from Allied Signal with dimensions 6 mm 20 mm 28 lm were cut and thoroughly washed with trichloroethylene (C2HCl3, Sigma– Aldrich) and methanol (CH3OH, Sigma–Aldrich) in ultrasonic bath (SONOREX DIGITEC DT 100) for several 15-min cycles and were finally dried at 80 °C (Fig. 1a). For the e-beam lithography process, a solution of 5% wt. of poly(methyl methacrylate) (PMMA, MW 350 K, Sigma–Aldrich), in propylene glycol methyl ether acetate (PGMEA, Sigma–Aldrich) solution was prepared after 36 h of stirring at room temperature. Any un-dissolved PMMA particles were removed by filtration (Chromafil Xtra PTFE-45/25) and approximately 0.5 lL was spin-coated (Spin150, SPS) on the Metglas surface at 4000 rpm for 60 s (Fig. 1b). The PMMA-coated Metglas was then hot-baked at 130 °C for 30 min in order to remove the PGMEA solvent. The PMMA film thickness was measured using White Light Reflectance Spectroscopy (FR-Basic, ThetaMetrisis) to be approximately 450 nm. 2.2. Electron beam lithography The system for the electron beam lithography (EBL), Xedraw2 (XENOS Semiconductor Technologies GmbH), consists of the 3-axis movable stage (XeMove), the beam blanking system (XeSwitch), equipped with a Faraday cup, and the pattern generator software (Exposure Control Program, ECP). The XeSwitch is mounted on a JSM-6610LV (JEOL) scanning electron microscope for the beam control and the beam current is measured through the Faraday cup connected to a Keithley 6430 multimeter. The PMMA-coated Metglas was attached on the XeMove stage and the exposure was performed after appropriate adjustment of the SEM’s accelerating voltage, spot size, beam focus and astigmatism correction and suitable pattern definition, where the dwell time is calculated for given: field size, beam current and exposing dose (Fig. 1c). After the exposure, the patterned surfaces were developed by isopropanol/water (IPA/H2O, 7/3) solution for 60 s and dried with nitrogen (Fig. 1d). All tests were performed using accelerating voltage of 10 kV, the beam load current was kept constant at 40 mA, the tension of the beam blanker was adjusted at 35 V and the spot size was set to 50 which resulted in a Keithley reading of approximately 200 pA. For the determination of the most suitable dose for the PMMA resist exposure, several doses in the range of 10–200 lC/cm2 were tested. 2.3. LTA seed synthesis Typically the LTA film synthesis begins with seeding, i.e. the use of previously prepared crystals which later develop, during the synthesis, and form a continuous film. Following the lithography and development processes, seeding was applied by LTA seeds of two different sizes: micro-sized and nano-sized one. The crystals were prepared via the hydrothermal method using the conditions described by: (1) Thompson et al. [35] and (2) by Jafari et al. [36] resulting in micro-sized and nano-sized crystals respectively. For micro-sized ones, sodium hydroxide (NaOH pellets, 98%,
215
Sigma–Aldrich) was dissolved in distilled water (Millipore, MilliQ8) and divided in two equal parts in polypropylene bottles. The silicon source (TEOS 98%, Sigma–Aldrich) was added to one part, while aluminum source (sodium aluminate, Al2O3 55%, Na2O 45%, Sigma–Aldrich) was added to the other one. Both solutions were stirred in room temperature until clear for approximately 4 h. Then, the aluminum solution was added to the silicate one under stirring to form the synthesis gel with molar ratio: 3.165 Na2O/Al2O3/1.926 SiO2/128 H2O. The gel was aged for 1 h under stirring at room temperature and then heated at 80 °C for 24 h in sealed polypropylene bottle. Following the synthesis, the crystals were collected using vacuum filtration (Whatman No. 2) and washed with distilled water until pH reached 9. Afterwards, the crystals were dried at 100 °C for several hours, characterized (XRD and SEM), pulverized and re-dispersed in water (1% wt.) to form the seeding solution. Slightly different conditions were used for the synthesis of nano-sized crystals. Briefly, sodium hydroxide (NaOH pellets, 98%, Sigma–Aldrich) was dissolved in distilled water (Millipore, Milli-Q8) and the organic template (tetra-methyl-ammonium hydroxide, TMAOH, 25%, Sigma–Aldrich) was added. This solution was divided in two equal parts in polypropylene bottles and the silicon source (LUDOX SM-30, Sigma–Aldrich) was added to one part, while aluminum source (aluminum isopropoxide, Al(OiPr)3, 98%, Sigma–Aldrich) was added to the other one. Both solutions were stirred in room temperature until clear for approximately 2 h. Then, the silicate solution was added to the aluminum one under stirring to form the synthesis gel with molar ratio: 0.32 Na2O/Al2O3/6 SiO2/7.27 (TMA)2O/350 H2O. The gel was aged for 4 days under stirring at room temperature and then heated at 80 °C for 24 h. Following the synthesis, the crystals were collected after several cycles of: (i) centrifugation (8000 rpm for 15 min, Eppendorf Centrifuge 5804), (ii) re-dispersion in water and (iii) ultrasonication (Elmasonic S30H), until pH reached 9. Afterwards, the crystals were dried at 100 °C for several hours, pulverized and characterized. After their characterization, the nanocrystals were re-dispersed in water (1% wt.) to form the seeding solution. Then, the entire patterned PMMA-coated Metglas was drop-casted (2 times) with one of the above solutions and dried for approximately 10 min at 85 °C (Fig. 1e) in order to promote crystallization of homogeneous zeolite film and to accelerate the growth rate. 2.4. LTA membrane synthesis Following the seeding, the zeolite membrane was hydrothermally synthesized on the patterned substrate by the secondary growth. The seeded and patterned PMMA-coated Metglas was transferred on a polypropylene bottle with the seeded side facing downwards in order to avoid agglomeration from the bulk. The sample was kept elevated in the horizontal position with the aim of two exterior placed magnets (Metglas is iron rich so it is attracted by magnets). Two basic (NaOH) aqueous solutions were prepared separately under stirring at 50 °C: one containing the silica source (Sodium Silicate Solution, Sigma–Aldrich) and one with the aluminum source (Sodium Aluminate, Sigma– Aldrich). The aluminum solution was filtered (Whatman No. 2) and slowly added to the silica solution under stirring resulting in the final synthesis gel with molar ratio: 2.68 Na2O/Al2O3/2.53 SiO2/150 H2O. Subsequently, the synthesis gel was transferred in the polypropylene bottle containing the Metglas sample and the hydrothermal synthesis was carried out at 100 °C for various crystallization times (Fig. 1f). At the end of the synthesis, the sample and the bulk crystals (for further examination) were separated from the mother liquor under vacuum filtration (Fig. 1g), washed with distilled water several times using ultrasound bath and finally dried at 100 °C overnight so as to reveal
216
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
LTA micromembranes on the exposed Metglas surface and dry crystals correspondingly.
3. Results and discussion In order to achieve precise and accurate exposure of the desired patterns using e-beam lithography, several factors need to be considered: type of resist, substrate, developer, e-beam energy, dose, development time and temperature [37]. Since the most commonly used positive resist is PMMA, the appropriate development conditions (developer type, time and temperature) are well documented [38]. High e-beam energy (accelerating voltage) creates numerous backscattered electrons thus giving rise to significant proximity effect (developed pattern is wider than the scanned pattern). On the other hand, low e-beam energy results in divergence of the beam from mutual electrostatic repulsion by the electrons. Therefore, an intermediate value of 10 kV accelerating voltage has been chosen in order to minimize those effects. What still needs to be determined is the dose (energy deposited per unit area that is actually measured in terms of current deposited per unit area, lC/cm2) that results in maximum pattern accuracy and minimum remaining PMMA film thickness. For this purpose, several doses in the range of 10–200 lC/cm2 were tested on the same sample consecutively for features of 10 lm and accelerating voltage of 10 kV. In Fig. 2, light grey areas show the Metglas’ exposed area while the darker ones show the intact PMMA film. XeDraw2 has the ability of exposing consecutive features with increasing or decreasing dosage when a dose scale factor is implemented in the source code. The most satisfactory results were obtained in the range of 20–90 lC/cm2, as patterns deviate considerably outside this range. This is because at higher doses the proximity effect raises significantly resulting in wider features and at lower doses the PMMA is underexposed thus the features are narrower. The actual width of the structures was determined using
SEM’s measurement tool. The mean width of all the stripes at each dose was calculated and the percent error from the 10 lm programmed value is plotted against exposure dose, as shown in Fig. 3. The relation is almost linear in the range of 20–50 lC/cm2 while the percent error minimizes at about 40 lC/cm2 and thus this optimum dose was chosen for the rest of the current work. Since seeding is critical for the membrane formation, the crystals collected (using the above mentioned methodologies for micro and nano-sized seeds) were examined and characterized with SEM and XRD. Their average size was estimated using SEM’s software tool to be about 3.5 lm for the micro-sized crystals and about 90 nm for the nano-sized crystals, as can be seen in Fig. 4a and b, respectively. Their crystallinity and anticipated chemical formula were confirmed using XRD measurements (Fig. 5a and b) through the database PDF-2 (01-073-2340). The effect of seeding was examined on three samples treated under the same hydrothermal conditions for 3 h at 100 °C: (a) without any seeds, (b) with micro-sized seeds and (c) with nanosized seeds. As can be seen in Fig. 6, the effect of seeding is a determining factor for the membrane synthesis, since no membrane is grown onto the unseeded sample (Fig. 6a) after the hydrothermal synthesis. Nevertheless, in the sample seeded with the micro-sized crystals (Fig. 6b), the membrane is quite inhomogeneous and consists of several uneven features where zeolite film is grown, probably covering the larger seed crystals or agglomerates. In the sample seeded with the nano-sized crystals (Fig. 6c), the membrane synthesized is considerably more uniform. All subsequent studies were therefore performed using nano-sized seeds, taking also into account that smaller features can be achieved with those seeds. The duration of the hydrothermal synthesis is another significant factor for the membrane formation, since it affects the thickness, uniformity and purity of the membrane. In order to investigate this effect, several samples were prepared via EBL patterning of the PMMA coating and seeded with nano-sized LTA
Fig. 2. SEM images of the patterns generated with exposure dose ranging from 20 to 90 lC/cm2.
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
Fig. 3. The relationship between the percent error and the exposure dose.
217
crystals. Indicative SEM images after the seeding are shown in Fig. 7a for a square pattern with dimensions of 44.75 lm. The coverage of the exposed Metglas surface with seeds is significantly higher than in the PMMA surface, as can clearly be seen from the magnification of the marked area (Fig. 7b). This observation is in accordance with the studies of Yilmaz et al. [39] showing negative charge of developed PMMA films, thus repulsing the LTA seeds from its surface. Following the seeding, all samples were hydrothermally treated for 1–5 h and indicative SEM images are shown in Fig. 8 (a–f) for six different times. After 1 h of synthesis (Fig. 8a) the crystallization has already begin and the membrane is covering most of the exposed Metglas surface. In the second hour of synthesis (Fig. 8b), the membrane is almost uniform and well-defined, while large crystals from the bulk try to incorporate. Fully grown and uniform membrane is observed after 3 h (Fig. 8c) having embodied large zeolites from the bulk. Those phenomena become more important for longer synthesis duration (Fig. 8d–f):
Fig. 4. SEM images of the micro-sized (a) and the nano-sized (b) LTA seed crystals.
Fig. 5. XRD spectra of the micro-sized (a) and the nano-sized (b) LTA seed crystals.
Fig. 6. SEM images after 3 h of hydrothermal synthesis at 100 °C for the un-seeded sample (a), micro-seeded sample (b) and nano-seeded sample (c).
218
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
Fig. 7. SEM images of the patterned surface seeded with nano-sized crystals (a) and higher magnification of the exposed Metglas/PMMA interface (b).
Fig. 8. SEM images of the LTA micromembranes synthesized after: (a) 1 h, (b) 2 h, (c) 3 h, (d) 3.5 h, (e) 4 h and (f) 5 h.
as the membrane grows, more crystals are incorporated and since those crystals protrude, the film growth over them produces a nonuniform membrane with uneven height. Subsequently, optimum hydrothermal synthesis duration should be around 3 h, at the conditions examined. Preliminary observation of the continuity of the films, in terms of micro-cracks and holes, was performed using the back-scattered electrons detector attached on the JSM-6610LV microscope. The image analysis using this detector is very helpful when examining different materials since the contrast depends on the average atomic number of each material. Thus for different materials (zeolite/Metglas), cracks give a very sharp contrast. Down to our SEM resolution limit (about 90 nm for these samples) no cracks were observed. Nevertheless, defects in the nano-scale may be present, as the accuracy of this technique is limited to only a surface observation and the SEM’s specifications. It is noteworthy that in all cases, the LTA film was synthesized only onto the exposed parts of the substrate, where the PMMA film was minute or absent. This remarkable effect, the selective synthesis of LTA on the exposed Metglas areas and not on the PMMA unexposed areas, can perhaps be explained in terms of repulsive forces. LTA is known to be a highly polar material with extra negative ions in its structure, due to its high aluminum content [40] thus its precursors are also negatively charged in alkali media [41]. On the other hand, as already mentioned, developed PMMA
has a slight negative charge, thus repelling the LTA seeds and precursors during seeding and synthesis respectively. Therefore, the traditional chemical lift-off process, typically needed to all lithographic procedures in order to totally remove the PMMA film, can thus be eliminated. Only an ultrasound-aided cleaning with water is needed so as to remove the extra ‘‘debris’’ of LTA precipitates left from the synthesis solution. This process leaves the PMMA layer intact, available for a second lithographic step without any further coating or pattern alignment for a second exposure. For example, the left over PMMA can be utilized to develop micro channels in order to connect the LTA micromembranes in our samples, a study that will be examined in the future. Characteristic example of the unnecessary PMMA removal step in order to reveal the micro-structures, is shown in Fig. 9. The images show a micromembrane on the same sample (hydrothermally treated for 3, 5 h at 100 °C) before (Fig. 9a) and after (Fig. 9b) the lift-off in acetone ultrasound bath. However, PMMA film must be removed if no further exposure is needed, since it would affect the sensor’s performance and in addition, undesired sediments are washed away. In order to exploit those LTA micromembranes in sensing applications, the entire surface of the Metglas must be uniformly covered with them. As an example, since the typical dimensions of a Metglas strip are 20 mm 6 mm, a 5% coverage with LTA
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
219
Fig. 9. SEM images of the LTA micromembranes before (a) and after (b) the lift-off process.
figure, the reproducibility of the patterned features is satisfactory. Nevertheless, statistically minor defects, like the absence of a micromembrane in one of the features, cannot always be eliminated.
4. Conclusions
Fig. 10. SEM image of a Metglas sample showing a small area with 48 LTA micromembranes.
micromembranes would translate into approximately 3000 square micromembranes with 44.75 lm side. In Fig. 10, a characteristic small fraction of such a sensor, showing 48 of the 3000 micromembranes, is given, prior the lift-off process. The corresponding XRD spectrum of the sensor is given in Fig. 11, where the amorphous background is characteristic of the Metglas, confirming the crystallization of LTA type membrane. As it is evident from this
In the present study, electron beam lithography was employed in the manufacturing of micropatterned, continuous LTA zeolite membranes on a Metglas magnetoelastic substrate. This substrate was chosen because it has been successfully used in the past coated with a continuous LTA membrane as the chemically active layer for sensing applications. Even though the technique needs further improvement, the patterns are composed of continuous LTA membranes and are down to micrometer scale, which is a first step towards developing faster contactless sensors. In addition, this methodology could be applied in the manufacturing of micromembranes of different zeolite types. The optimization of the LTA micromembranes manufacturing process, in order to control their thickness and uniformity should be further examined, by using different seeding methods and especially by improving the secondary growth parameters like duration, temperature or even synthesis gel composition. The final step will be the use of the patterned structures as sensors for the detection of different gasses and VOCs and determination of the response times. Finally, as it was mentioned above, another
Fig. 11. XRD spectrum of the Metglas sample with the LTA micromembranes.
220
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
direction will be the development of microchannels between the micromembranes using a second lithography process on the unexposed PMMA film. Acknowledgments The present study is financially supported by the Research Committee of the University of Patras under the ‘‘K. Karatheodori’’ grant (Project Code D-153). References [1] S. Mintova, C. Cejka, Stud. Surf. Sci. Catal. 168 (2007) 301. [2] C.M. Lew, R. Cai, Y. Yan, Acc. Chem. Res. 43 (2) (2010) 210–219. [3] E.G. Derouane, G. Crehan, C.J. Dillon, D. Bethell, H. He, S.B.D.A. Hamid, J. Catal. 194 (2000) 410. [4] J. Kuhn, K. Yajima, T. Tomita, J. Gross, F. Kapteijn, J. Membr. Sci. 321 (2008) 344. [5] L.O. Connor, Zeolites – Industry Trends and Worldwide Markets to 2010, Hewin International Report, John Willey and Sons, 2000. [6] N.C. Pérez, E.E. Miró, J.M. Zamaro, Appl. Catal. B Environ. 129 (2013) 416. [7] M. Vilaseca, J. Coronas, A. Cirera, A. Cornet, J.R. Morante, J. Santamaria, Sens. Actuators B Chem. 133 (2008) 435. [8] Z. Lai, G. Bonilla1, I. Diaz1, J.G. Nery, K. Sujaoti, M.A. Amat, E. Kokkoli, O. Terasaki, R.W. Thompson, M. Tsapatsis, D.G. Vlachos, Science 300 (2003) 456. [9] Z. Li, M.C. Johnson, Y. Yan, Angew. Chem. Int. 45 (2006) 6329. [10] J.C. Yang, P.K. Dutta, Sens. Actuators B Chem. 123 (2007) 929. [11] G. Zhang, T. Tanii, T. Zako, T. Funatsu, I. Ohdomari, Sens. Actuators B Chem. 97 (2004) 243. [12] W. Han, S.M. Kwan, K.L. Yeung, Top. Catal. 53 (2010) 1394. [13] W. Ehrfeld, V. Hessel, H. Löwe, Microreactors: New Technology for Modern Chemistry, VCH, Weinheim, 2000. [14] Z. Ming, Z. BaoQuan, L. XiuFeng, Chin. Sci. Bull. 53 (2008) 801. [15] R. Mallada, M. Menendez (Eds.), Membrane Science and Technology, Series 13, Inorganic Membranes, Synthesis, Characterization and Applications, Elsevier, 2008. [16] J. Coronas, J. Santamaria, Chem. Eng. Sci. 59 (2004) 4879. [17] J.R. Huang, G.Y. Li, Z.Y. Huang, X.J. Huang, J.H. Liu, Sens. Actuators B Chem. 1059 (2006) 114.
[18] J.S. Lee, J.H. Kim, Y.J. Lee, N.C. Jeong, K.B. Yoon, Angew. Chem. Int. Ed. 46 (2007) 3087. [19] J.S. Lee, K. Ha, Y-J Lee, K.B. Yoon, Adv. Mater. 17 (2005) 837. [20] K.B. Yoon, Acc. Chem. Res. 40 (2007) 29–40. [21] G.J. Zhang, T. Tanii, T. Zako, T. Funatsu, I. Ohdomari, Sens. Actuators B Chem. 97 (2004) 243. [22] S.M. Sze, Semiconductor Devices: Physics and Technology, second ed., Wiley, New York, 2001. p. 404. [23] K. Ha, Y.J. Lee, D.Y. Jung, J.H. Lee, K.B. Yoon, Adv. Mater. 12 (2000) 1614. [24] Y.S.S. Wan, J.L.H. Chau, A. Gavriilidis, K.L. Yeung, Micropor. Mesopor. Mater. 42 (2001) 157. [25] J.L.H. Chau, Y.S.S. Wan, A. Graviilidis, K.L. Yeung, Chem. Eng. J. 88 (2001) 187. [26] K. Ha, Y.J. Lee, Y.S. Chun, Y.S. Park, G.S. Lee, K.B. Yoon, Adv. Mater. 13 (2001) 594. [27] S. Li, C. Demmelmaier, M. Itkis, Z. Liu, R.C. Haddon, Y. Yan, Chem. Matter 15 (2003) 2687. [28] I. Pellejero, M. Urbiztondo, M. Villarroya, J. Sese, M.P. Pina, J. Santamaria, Micropor. Mesopor. Mater. 114 (2008) 110. [29] S. Ozturk, B. Akata, Micropor. Mesopor. Mater. 126 (2009) 228. [30] S.K. Kirdeciler, C. Ozen, B. Akata, Micropor. Mesopor. Mater. 191 (2014) 59. [31] S H. Kirdeciler, Preparation of functional surfaces using zeolite nanocrystals for biosensor and biomedical applications (Thesis), Middle East Technical University, July 2012. [32] I.G. Giannakopoulos, D. Kouzoudis, C.A. Grimes, V. Nikolakis, Adv. Funct. Mater. 15 (2005) 1165. [33] T. Baimpos, L. Gora, V. Nikolakis, D. Kouzoudis, Sens. Actuators A Phys. 186 (2012) 21. [34] T. Baimpos, V. Tsukala, V. Nikolakis, D. Kouzoudis, Sens. Lett. 10 (2012) 879. [35] R.W. Thompson, M.J. Huber, J. Cryst. Growth 56 (1982) 711. [36] M. Jafari, A. Nouri, M. Kazemimoghadam, T. Mohammadi, Powder Technol. 237 (2013) 442. [37] M.A. Mohammad, M. Muhammad, S.K. Dew, M. Stepanova, Fundamentals of electron beam exposure and development, in: Nanofabrication, SpringerVerlag, Wien, 2012. [38] A. Olzierski, I. Raptis, Microelectron. Eng. 73–74 (2004) 244. [39] E. Yilmaz, H. Sezen, S. Suzer, Angew. Chem. Int. Ed. 51 (2012) 5488. [40] M. Moliner, Basic principles of zeolite synthesis, in: C. Martinez Sanchez, J. Pérez Pariente (Eds.), Zeolites and Ordered Porous Solids: Fundamentals and Applications, Universitat Politècnica de València, 2011, pp. 37–65. [41] V. Valtchev, S. Mintova, Micropor. Mesopor. Mater. 43 (2001) 41.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Zeolite micromembrane fabrication on magnetoelastic material using electron beam lithography Vassiliki Tsukala a,b, Dimitris Kouzoudis b,⇑ a b
Foundation for Research and Technology Hellas, Institute of Chemical Engineering and High-Temperature Chemical Process, P.O. Box 1414, GR 265 04 Patras, Greece Department of Chemical Engineering, University of Patras, GR-26504 Patras, Greece
a r t i c l e
i n f o
Article history: Received 1 February 2014 Received in revised form 8 May 2014 Accepted 2 June 2014 Available online 26 June 2014 Keywords: Micromembrane LTA Electron beam lithography Secondary growth Magnetoelastic
a b s t r a c t In the present study, electron beam lithography (EBL) is employed in the manufacturing of patterns of continuous Linde Type A (LTA) zeolite membranes. The patterns are down to the micrometer scale and they are composed of LTA micromembranes having all three dimensions in the micrometer scale. The control of the size and location of zeolite films or membranes onto specific substrates, will lead to new aspects for their use in microsensing, microelectronics and microreactor applications. Our focus is on the microsensing field, where a magnetoelastic ribbon (Metglas) is used as the sensing platform, since the Metglas/zeolite film composite has been successfully used for the detection of gases and VOCs in the past. Here we report on the first LTA zeolite micromembrane attached onto a previously EBL patterned PMMA coated Metglas substrate. The sensing ability of such a sensor could be significantly improved by using lower amount of zeolite film since it could lead to shorter response and recovery times. The conditions for the manufacturing of the LTA micromembranes onto the Metglas substrate are investigated and discussed in terms of EBL, seeding and hydrothermal synthesis parameters. Ó 2014 Published by Elsevier Inc.
1. Introduction Because of their interesting and unique properties [1], zeolites and their continuous films have attracted great scientific interest [2] over the past decades, which is demonstrated on their various industrial applications such as catalysts in fine chemical and petrochemical industry [3], adsorbents [4], and membranes for separations or as reactors [5]. Their molecular sieving ability, controllable surface properties, well-defined porosity, large surface area along with chemical, thermal and mechanical stability can also be utilized in new advanced applications like microreactors [6], chemical or gas sensors [7], microseparators [8], low dielectric constant materials for microelectronic devices [9], electrodes [10], biomedical science [11] and fuel cells [12]. Nevertheless, the exploitation of zeolites in such advanced applications is often prevented due to the powder form of synthetic zeolites, directing efforts in the development of continuous zeolite films of small size so as to be implemented in the so-called microchemical systems that also include other microdevices such as micromixers, microheat exchangers, microactuators and microsensors [13]. Tailoring and controlling zeolite membranes size in the microscale range offers numerous advantages in many of the above ⇑ Corresponding author. Tel.: +30 2610996880; fax: +30 2610996846. E-mail address: [email protected] (D. Kouzoudis). http://dx.doi.org/10.1016/j.micromeso.2014.06.017 1387-1811/Ó 2014 Published by Elsevier Inc.
advanced applications [14]. Among the benefits of micromembranes, the most obvious one is the elimination of defects (cracks and holes), as the probability of a defect-free surface increases for smaller membrane areas [15]. Other significant aspects, especially for microreactor applications, are the elimination of temperature gradients thus minimizing hot spots, enhancing heat and mass transfer properties and finally, improving yield, conversion and selectivity through the reaction-permeation mechanism of the desired product through the channels [16]. In the field of zeolite membranes used for sensing applications, micromembranes are of great demand, since lower mass could result in shorter response time and subsequently shorter recovery [7]. Thus, the sensitivity of the sensor could increase, especially when used as microbalance (cantilever-based or Quartz Crystal Microbalance, QCM) but also the selectivity of specific components can be enhanced allowing for molecular recognition [17] (as ‘‘electronic nose’’ sensors). Important efforts have been made to fabricate patterned zeolite films or layers of crystals on solid substrates by combining both zeolite-substrate adhesion techniques and pattern definition methods adopted from microelectronics technology. The most common substrates that have been used for patterned zeolite membranes are silicon and glass, while siliceous zeolites are preferred, mainly because of the chemical affinity. In order to attach zeolites onto them, two adhesion techniques are commonly
214
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
used: casting and secondary growth. For the first approach, the most facile method is the direct attachment method, where the zeolite crystals are assembled through covalent or ionic bonding. The most widely used way of ionic bonding is simply by rubbing [18] zeolite crystals on a patterned smooth substrate resulting in uniformly, fully-packed layers of crystals. In a similar manner, ultrasound agitation [19] and dip or spin coating methods of colloidal seed solutions have also been used but with less success in terms of uniformity and coverage. The second approach is the secondary growth method, where zeolite crystals are intergrown to form continuous films attached onto the patterned substrates under hydrothermal conditions. The zeolite type is controlled by the seed type and synthesis conditions. In order to pattern the substrate’s surface for zeolite crystals attachment, three pattern definition methods have been mostly employed that are also compatible with sensor technology: microcontact printing, photoetching and lithography (photolithography or electron beam lithography), resulting in different resolution and structure sizes [20]. Since the resolution of the classic photolithography is limited by the wavelength of the light used, electron beam lithography (EBL) has provided scientists with a powerful tool for creating nano-scale structures. EBL utilizes a highly focused electron beam to expose a resist-coated substrate thus making it soluble (positive tone resist) or insoluble (negative tone resist) to a photoresist developer. As the trend for everincreasing levels of miniaturization needs to be met also for zeolite technology, in order to be integrated with sophisticated microdevices, the demand for high-throughput screening and decreased sample mass [21] can be achieved using EBL technology. Other advantages of EBL are the precise and easy pattern generation using software tools and direct exposure onto the semiconductor substrate [22] without the tedious and often expensive mask preparation. The majority of the studies on patterned layers of crystals or films have focused on siliceous MFI type zeolites (ZSM-5 and silicalite-1) and employed the previously mentioned methods and techniques. The most common substrates used are glass and silicon wafers due to their smooth surface and chemical affinity with the above siliceous zeolites. Using microcontact printing with poly (dimethylsiloxane) (PDMS) stamp, Yoon and co-workers covalently attached b-oriented ZSM-5 crystals on Si wafer. The crystals were self-assembled (from their ethanol solution) after stamp-pressing onto the wafer surface, forming monolayers with approximately 5 lm width [23]. Similarly, Yeung and co-workers [24] fabricated ordered silicalite micromembranes and monolayers on Si wafers, through secondary or direct growth and using photolithography and etching techniques, in order to produce catalytic microreactors and membrane microseparators, while later, they managed to photopattern silicalite-1 membranes on Si wafer reaching 5 lm small zeolite arrays [25]. Another technique was first introduced by Yoon and co-workers employing patterning through photochemical degradation of organic functional groups on glass, covalent bonding of ZSM-5 crystals and subsequent secondary growth of continuous ZSM-5 membrane with approximately 200 lm feature [26]. A different approach, using TEM grid and chromium-gold depositions, was employed by Yan and co-workers to fabricate continuous, b-oriented silicalite membrane onto patterned Si wafer, exploiting the weak interaction of gold surface and colloidal zeolites [27]. Photoetching (both dry and wet) techniques were investigated by Pellejero et al. for the fabrication of patterned silicalite film on Si wafer, after the hydrothermal synthesis, achieving 10 lm structures [28]. More recently, Ozturk et al. [29,30] and Kirdeciler et al. [31] fabricated monolayers of ordered individual crystals of LTA and BEA type zeolites, through the manual assembly method, onto Si wafers patterned using electron beam lithography, with 500 nm features.
In the present study, a magnetoelastic material (Metglas) was chosen as a substrate, which was successfully utilized by our group as a sensing platform together with zeolite films for the detection of CO2 [32], VOCs [33] and stress[34]. Another reason for using Metglas in the present study is the excellent adhesion of continuous LTA films on it. A major drawback in these sensing applications was the relatively slow response time, of the order of tens of seconds, resulting from the slow adsorption and diffusion of the detected species through the zeolite layer. A smaller size zeolite feature, down to micro scale, could probably result in much shorter diffusion time which is one of the objectives of micropatterning LTA films on a Metglas substrate. Three novel ideas are introduced in the present work: (a) electron beam lithography is employed for the first time for the in situ synthesis of micropatterned, continuous zeolite films through the secondary growth. (b) It is also the first time that an LTA film is synthesized in the micrometer scale, revealing new aspects of its future use in microsensing, microelectronics and microreactor applications. Most of the zeolite micro-patterning in the literature involves either MFI membranes or LTA microcrystals, not LTA membranes. It should also be noted that LTA has a number of industrial applications thus its micro-manipulation is of paramount importance. And c) this is the first time that a metallic magnetoelastic material is used as a substrate for zeolite micromembrane patterning applications. Usually conventional substrates such as Silicon wafers or glass are used. 2. Experimental The e-beam lithography and secondary hydrothermal synthesis procedure that was used to develop the micro-patterned LTA film is schematically represented in Fig. 1. The steps can be summarized in the following list: (a) Cleaning of the Metglas substrate (b) PMMA spin-coating
Fig. 1. Schematic representation of the experimental procedure for the fabrication of zeolite micromembranes on Metglas using EBL technology and secondary growth.
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
(c) (d) (e) (f) (g)
E-beam exposure PMMA development LTA seeding LTA synthesis Ultrasound Cleaning
Each individual step is described in detail in the following paragraphs. 2.1. PMMA coating on Metglas Several Metglas 2826 MBA (Fe40Ni38Mo4B18) ribbons from Allied Signal with dimensions 6 mm 20 mm 28 lm were cut and thoroughly washed with trichloroethylene (C2HCl3, Sigma– Aldrich) and methanol (CH3OH, Sigma–Aldrich) in ultrasonic bath (SONOREX DIGITEC DT 100) for several 15-min cycles and were finally dried at 80 °C (Fig. 1a). For the e-beam lithography process, a solution of 5% wt. of poly(methyl methacrylate) (PMMA, MW 350 K, Sigma–Aldrich), in propylene glycol methyl ether acetate (PGMEA, Sigma–Aldrich) solution was prepared after 36 h of stirring at room temperature. Any un-dissolved PMMA particles were removed by filtration (Chromafil Xtra PTFE-45/25) and approximately 0.5 lL was spin-coated (Spin150, SPS) on the Metglas surface at 4000 rpm for 60 s (Fig. 1b). The PMMA-coated Metglas was then hot-baked at 130 °C for 30 min in order to remove the PGMEA solvent. The PMMA film thickness was measured using White Light Reflectance Spectroscopy (FR-Basic, ThetaMetrisis) to be approximately 450 nm. 2.2. Electron beam lithography The system for the electron beam lithography (EBL), Xedraw2 (XENOS Semiconductor Technologies GmbH), consists of the 3-axis movable stage (XeMove), the beam blanking system (XeSwitch), equipped with a Faraday cup, and the pattern generator software (Exposure Control Program, ECP). The XeSwitch is mounted on a JSM-6610LV (JEOL) scanning electron microscope for the beam control and the beam current is measured through the Faraday cup connected to a Keithley 6430 multimeter. The PMMA-coated Metglas was attached on the XeMove stage and the exposure was performed after appropriate adjustment of the SEM’s accelerating voltage, spot size, beam focus and astigmatism correction and suitable pattern definition, where the dwell time is calculated for given: field size, beam current and exposing dose (Fig. 1c). After the exposure, the patterned surfaces were developed by isopropanol/water (IPA/H2O, 7/3) solution for 60 s and dried with nitrogen (Fig. 1d). All tests were performed using accelerating voltage of 10 kV, the beam load current was kept constant at 40 mA, the tension of the beam blanker was adjusted at 35 V and the spot size was set to 50 which resulted in a Keithley reading of approximately 200 pA. For the determination of the most suitable dose for the PMMA resist exposure, several doses in the range of 10–200 lC/cm2 were tested. 2.3. LTA seed synthesis Typically the LTA film synthesis begins with seeding, i.e. the use of previously prepared crystals which later develop, during the synthesis, and form a continuous film. Following the lithography and development processes, seeding was applied by LTA seeds of two different sizes: micro-sized and nano-sized one. The crystals were prepared via the hydrothermal method using the conditions described by: (1) Thompson et al. [35] and (2) by Jafari et al. [36] resulting in micro-sized and nano-sized crystals respectively. For micro-sized ones, sodium hydroxide (NaOH pellets, 98%,
215
Sigma–Aldrich) was dissolved in distilled water (Millipore, MilliQ8) and divided in two equal parts in polypropylene bottles. The silicon source (TEOS 98%, Sigma–Aldrich) was added to one part, while aluminum source (sodium aluminate, Al2O3 55%, Na2O 45%, Sigma–Aldrich) was added to the other one. Both solutions were stirred in room temperature until clear for approximately 4 h. Then, the aluminum solution was added to the silicate one under stirring to form the synthesis gel with molar ratio: 3.165 Na2O/Al2O3/1.926 SiO2/128 H2O. The gel was aged for 1 h under stirring at room temperature and then heated at 80 °C for 24 h in sealed polypropylene bottle. Following the synthesis, the crystals were collected using vacuum filtration (Whatman No. 2) and washed with distilled water until pH reached 9. Afterwards, the crystals were dried at 100 °C for several hours, characterized (XRD and SEM), pulverized and re-dispersed in water (1% wt.) to form the seeding solution. Slightly different conditions were used for the synthesis of nano-sized crystals. Briefly, sodium hydroxide (NaOH pellets, 98%, Sigma–Aldrich) was dissolved in distilled water (Millipore, Milli-Q8) and the organic template (tetra-methyl-ammonium hydroxide, TMAOH, 25%, Sigma–Aldrich) was added. This solution was divided in two equal parts in polypropylene bottles and the silicon source (LUDOX SM-30, Sigma–Aldrich) was added to one part, while aluminum source (aluminum isopropoxide, Al(OiPr)3, 98%, Sigma–Aldrich) was added to the other one. Both solutions were stirred in room temperature until clear for approximately 2 h. Then, the silicate solution was added to the aluminum one under stirring to form the synthesis gel with molar ratio: 0.32 Na2O/Al2O3/6 SiO2/7.27 (TMA)2O/350 H2O. The gel was aged for 4 days under stirring at room temperature and then heated at 80 °C for 24 h. Following the synthesis, the crystals were collected after several cycles of: (i) centrifugation (8000 rpm for 15 min, Eppendorf Centrifuge 5804), (ii) re-dispersion in water and (iii) ultrasonication (Elmasonic S30H), until pH reached 9. Afterwards, the crystals were dried at 100 °C for several hours, pulverized and characterized. After their characterization, the nanocrystals were re-dispersed in water (1% wt.) to form the seeding solution. Then, the entire patterned PMMA-coated Metglas was drop-casted (2 times) with one of the above solutions and dried for approximately 10 min at 85 °C (Fig. 1e) in order to promote crystallization of homogeneous zeolite film and to accelerate the growth rate. 2.4. LTA membrane synthesis Following the seeding, the zeolite membrane was hydrothermally synthesized on the patterned substrate by the secondary growth. The seeded and patterned PMMA-coated Metglas was transferred on a polypropylene bottle with the seeded side facing downwards in order to avoid agglomeration from the bulk. The sample was kept elevated in the horizontal position with the aim of two exterior placed magnets (Metglas is iron rich so it is attracted by magnets). Two basic (NaOH) aqueous solutions were prepared separately under stirring at 50 °C: one containing the silica source (Sodium Silicate Solution, Sigma–Aldrich) and one with the aluminum source (Sodium Aluminate, Sigma– Aldrich). The aluminum solution was filtered (Whatman No. 2) and slowly added to the silica solution under stirring resulting in the final synthesis gel with molar ratio: 2.68 Na2O/Al2O3/2.53 SiO2/150 H2O. Subsequently, the synthesis gel was transferred in the polypropylene bottle containing the Metglas sample and the hydrothermal synthesis was carried out at 100 °C for various crystallization times (Fig. 1f). At the end of the synthesis, the sample and the bulk crystals (for further examination) were separated from the mother liquor under vacuum filtration (Fig. 1g), washed with distilled water several times using ultrasound bath and finally dried at 100 °C overnight so as to reveal
216
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
LTA micromembranes on the exposed Metglas surface and dry crystals correspondingly.
3. Results and discussion In order to achieve precise and accurate exposure of the desired patterns using e-beam lithography, several factors need to be considered: type of resist, substrate, developer, e-beam energy, dose, development time and temperature [37]. Since the most commonly used positive resist is PMMA, the appropriate development conditions (developer type, time and temperature) are well documented [38]. High e-beam energy (accelerating voltage) creates numerous backscattered electrons thus giving rise to significant proximity effect (developed pattern is wider than the scanned pattern). On the other hand, low e-beam energy results in divergence of the beam from mutual electrostatic repulsion by the electrons. Therefore, an intermediate value of 10 kV accelerating voltage has been chosen in order to minimize those effects. What still needs to be determined is the dose (energy deposited per unit area that is actually measured in terms of current deposited per unit area, lC/cm2) that results in maximum pattern accuracy and minimum remaining PMMA film thickness. For this purpose, several doses in the range of 10–200 lC/cm2 were tested on the same sample consecutively for features of 10 lm and accelerating voltage of 10 kV. In Fig. 2, light grey areas show the Metglas’ exposed area while the darker ones show the intact PMMA film. XeDraw2 has the ability of exposing consecutive features with increasing or decreasing dosage when a dose scale factor is implemented in the source code. The most satisfactory results were obtained in the range of 20–90 lC/cm2, as patterns deviate considerably outside this range. This is because at higher doses the proximity effect raises significantly resulting in wider features and at lower doses the PMMA is underexposed thus the features are narrower. The actual width of the structures was determined using
SEM’s measurement tool. The mean width of all the stripes at each dose was calculated and the percent error from the 10 lm programmed value is plotted against exposure dose, as shown in Fig. 3. The relation is almost linear in the range of 20–50 lC/cm2 while the percent error minimizes at about 40 lC/cm2 and thus this optimum dose was chosen for the rest of the current work. Since seeding is critical for the membrane formation, the crystals collected (using the above mentioned methodologies for micro and nano-sized seeds) were examined and characterized with SEM and XRD. Their average size was estimated using SEM’s software tool to be about 3.5 lm for the micro-sized crystals and about 90 nm for the nano-sized crystals, as can be seen in Fig. 4a and b, respectively. Their crystallinity and anticipated chemical formula were confirmed using XRD measurements (Fig. 5a and b) through the database PDF-2 (01-073-2340). The effect of seeding was examined on three samples treated under the same hydrothermal conditions for 3 h at 100 °C: (a) without any seeds, (b) with micro-sized seeds and (c) with nanosized seeds. As can be seen in Fig. 6, the effect of seeding is a determining factor for the membrane synthesis, since no membrane is grown onto the unseeded sample (Fig. 6a) after the hydrothermal synthesis. Nevertheless, in the sample seeded with the micro-sized crystals (Fig. 6b), the membrane is quite inhomogeneous and consists of several uneven features where zeolite film is grown, probably covering the larger seed crystals or agglomerates. In the sample seeded with the nano-sized crystals (Fig. 6c), the membrane synthesized is considerably more uniform. All subsequent studies were therefore performed using nano-sized seeds, taking also into account that smaller features can be achieved with those seeds. The duration of the hydrothermal synthesis is another significant factor for the membrane formation, since it affects the thickness, uniformity and purity of the membrane. In order to investigate this effect, several samples were prepared via EBL patterning of the PMMA coating and seeded with nano-sized LTA
Fig. 2. SEM images of the patterns generated with exposure dose ranging from 20 to 90 lC/cm2.
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
Fig. 3. The relationship between the percent error and the exposure dose.
217
crystals. Indicative SEM images after the seeding are shown in Fig. 7a for a square pattern with dimensions of 44.75 lm. The coverage of the exposed Metglas surface with seeds is significantly higher than in the PMMA surface, as can clearly be seen from the magnification of the marked area (Fig. 7b). This observation is in accordance with the studies of Yilmaz et al. [39] showing negative charge of developed PMMA films, thus repulsing the LTA seeds from its surface. Following the seeding, all samples were hydrothermally treated for 1–5 h and indicative SEM images are shown in Fig. 8 (a–f) for six different times. After 1 h of synthesis (Fig. 8a) the crystallization has already begin and the membrane is covering most of the exposed Metglas surface. In the second hour of synthesis (Fig. 8b), the membrane is almost uniform and well-defined, while large crystals from the bulk try to incorporate. Fully grown and uniform membrane is observed after 3 h (Fig. 8c) having embodied large zeolites from the bulk. Those phenomena become more important for longer synthesis duration (Fig. 8d–f):
Fig. 4. SEM images of the micro-sized (a) and the nano-sized (b) LTA seed crystals.
Fig. 5. XRD spectra of the micro-sized (a) and the nano-sized (b) LTA seed crystals.
Fig. 6. SEM images after 3 h of hydrothermal synthesis at 100 °C for the un-seeded sample (a), micro-seeded sample (b) and nano-seeded sample (c).
218
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
Fig. 7. SEM images of the patterned surface seeded with nano-sized crystals (a) and higher magnification of the exposed Metglas/PMMA interface (b).
Fig. 8. SEM images of the LTA micromembranes synthesized after: (a) 1 h, (b) 2 h, (c) 3 h, (d) 3.5 h, (e) 4 h and (f) 5 h.
as the membrane grows, more crystals are incorporated and since those crystals protrude, the film growth over them produces a nonuniform membrane with uneven height. Subsequently, optimum hydrothermal synthesis duration should be around 3 h, at the conditions examined. Preliminary observation of the continuity of the films, in terms of micro-cracks and holes, was performed using the back-scattered electrons detector attached on the JSM-6610LV microscope. The image analysis using this detector is very helpful when examining different materials since the contrast depends on the average atomic number of each material. Thus for different materials (zeolite/Metglas), cracks give a very sharp contrast. Down to our SEM resolution limit (about 90 nm for these samples) no cracks were observed. Nevertheless, defects in the nano-scale may be present, as the accuracy of this technique is limited to only a surface observation and the SEM’s specifications. It is noteworthy that in all cases, the LTA film was synthesized only onto the exposed parts of the substrate, where the PMMA film was minute or absent. This remarkable effect, the selective synthesis of LTA on the exposed Metglas areas and not on the PMMA unexposed areas, can perhaps be explained in terms of repulsive forces. LTA is known to be a highly polar material with extra negative ions in its structure, due to its high aluminum content [40] thus its precursors are also negatively charged in alkali media [41]. On the other hand, as already mentioned, developed PMMA
has a slight negative charge, thus repelling the LTA seeds and precursors during seeding and synthesis respectively. Therefore, the traditional chemical lift-off process, typically needed to all lithographic procedures in order to totally remove the PMMA film, can thus be eliminated. Only an ultrasound-aided cleaning with water is needed so as to remove the extra ‘‘debris’’ of LTA precipitates left from the synthesis solution. This process leaves the PMMA layer intact, available for a second lithographic step without any further coating or pattern alignment for a second exposure. For example, the left over PMMA can be utilized to develop micro channels in order to connect the LTA micromembranes in our samples, a study that will be examined in the future. Characteristic example of the unnecessary PMMA removal step in order to reveal the micro-structures, is shown in Fig. 9. The images show a micromembrane on the same sample (hydrothermally treated for 3, 5 h at 100 °C) before (Fig. 9a) and after (Fig. 9b) the lift-off in acetone ultrasound bath. However, PMMA film must be removed if no further exposure is needed, since it would affect the sensor’s performance and in addition, undesired sediments are washed away. In order to exploit those LTA micromembranes in sensing applications, the entire surface of the Metglas must be uniformly covered with them. As an example, since the typical dimensions of a Metglas strip are 20 mm 6 mm, a 5% coverage with LTA
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
219
Fig. 9. SEM images of the LTA micromembranes before (a) and after (b) the lift-off process.
figure, the reproducibility of the patterned features is satisfactory. Nevertheless, statistically minor defects, like the absence of a micromembrane in one of the features, cannot always be eliminated.
4. Conclusions
Fig. 10. SEM image of a Metglas sample showing a small area with 48 LTA micromembranes.
micromembranes would translate into approximately 3000 square micromembranes with 44.75 lm side. In Fig. 10, a characteristic small fraction of such a sensor, showing 48 of the 3000 micromembranes, is given, prior the lift-off process. The corresponding XRD spectrum of the sensor is given in Fig. 11, where the amorphous background is characteristic of the Metglas, confirming the crystallization of LTA type membrane. As it is evident from this
In the present study, electron beam lithography was employed in the manufacturing of micropatterned, continuous LTA zeolite membranes on a Metglas magnetoelastic substrate. This substrate was chosen because it has been successfully used in the past coated with a continuous LTA membrane as the chemically active layer for sensing applications. Even though the technique needs further improvement, the patterns are composed of continuous LTA membranes and are down to micrometer scale, which is a first step towards developing faster contactless sensors. In addition, this methodology could be applied in the manufacturing of micromembranes of different zeolite types. The optimization of the LTA micromembranes manufacturing process, in order to control their thickness and uniformity should be further examined, by using different seeding methods and especially by improving the secondary growth parameters like duration, temperature or even synthesis gel composition. The final step will be the use of the patterned structures as sensors for the detection of different gasses and VOCs and determination of the response times. Finally, as it was mentioned above, another
Fig. 11. XRD spectrum of the Metglas sample with the LTA micromembranes.
220
V. Tsukala, D. Kouzoudis / Microporous and Mesoporous Materials 197 (2014) 213–220
direction will be the development of microchannels between the micromembranes using a second lithography process on the unexposed PMMA film. Acknowledgments The present study is financially supported by the Research Committee of the University of Patras under the ‘‘K. Karatheodori’’ grant (Project Code D-153). References [1] S. Mintova, C. Cejka, Stud. Surf. Sci. Catal. 168 (2007) 301. [2] C.M. Lew, R. Cai, Y. Yan, Acc. Chem. Res. 43 (2) (2010) 210–219. [3] E.G. Derouane, G. Crehan, C.J. Dillon, D. Bethell, H. He, S.B.D.A. Hamid, J. Catal. 194 (2000) 410. [4] J. Kuhn, K. Yajima, T. Tomita, J. Gross, F. Kapteijn, J. Membr. Sci. 321 (2008) 344. [5] L.O. Connor, Zeolites – Industry Trends and Worldwide Markets to 2010, Hewin International Report, John Willey and Sons, 2000. [6] N.C. Pérez, E.E. Miró, J.M. Zamaro, Appl. Catal. B Environ. 129 (2013) 416. [7] M. Vilaseca, J. Coronas, A. Cirera, A. Cornet, J.R. Morante, J. Santamaria, Sens. Actuators B Chem. 133 (2008) 435. [8] Z. Lai, G. Bonilla1, I. Diaz1, J.G. Nery, K. Sujaoti, M.A. Amat, E. Kokkoli, O. Terasaki, R.W. Thompson, M. Tsapatsis, D.G. Vlachos, Science 300 (2003) 456. [9] Z. Li, M.C. Johnson, Y. Yan, Angew. Chem. Int. 45 (2006) 6329. [10] J.C. Yang, P.K. Dutta, Sens. Actuators B Chem. 123 (2007) 929. [11] G. Zhang, T. Tanii, T. Zako, T. Funatsu, I. Ohdomari, Sens. Actuators B Chem. 97 (2004) 243. [12] W. Han, S.M. Kwan, K.L. Yeung, Top. Catal. 53 (2010) 1394. [13] W. Ehrfeld, V. Hessel, H. Löwe, Microreactors: New Technology for Modern Chemistry, VCH, Weinheim, 2000. [14] Z. Ming, Z. BaoQuan, L. XiuFeng, Chin. Sci. Bull. 53 (2008) 801. [15] R. Mallada, M. Menendez (Eds.), Membrane Science and Technology, Series 13, Inorganic Membranes, Synthesis, Characterization and Applications, Elsevier, 2008. [16] J. Coronas, J. Santamaria, Chem. Eng. Sci. 59 (2004) 4879. [17] J.R. Huang, G.Y. Li, Z.Y. Huang, X.J. Huang, J.H. Liu, Sens. Actuators B Chem. 1059 (2006) 114.
[18] J.S. Lee, J.H. Kim, Y.J. Lee, N.C. Jeong, K.B. Yoon, Angew. Chem. Int. Ed. 46 (2007) 3087. [19] J.S. Lee, K. Ha, Y-J Lee, K.B. Yoon, Adv. Mater. 17 (2005) 837. [20] K.B. Yoon, Acc. Chem. Res. 40 (2007) 29–40. [21] G.J. Zhang, T. Tanii, T. Zako, T. Funatsu, I. Ohdomari, Sens. Actuators B Chem. 97 (2004) 243. [22] S.M. Sze, Semiconductor Devices: Physics and Technology, second ed., Wiley, New York, 2001. p. 404. [23] K. Ha, Y.J. Lee, D.Y. Jung, J.H. Lee, K.B. Yoon, Adv. Mater. 12 (2000) 1614. [24] Y.S.S. Wan, J.L.H. Chau, A. Gavriilidis, K.L. Yeung, Micropor. Mesopor. Mater. 42 (2001) 157. [25] J.L.H. Chau, Y.S.S. Wan, A. Graviilidis, K.L. Yeung, Chem. Eng. J. 88 (2001) 187. [26] K. Ha, Y.J. Lee, Y.S. Chun, Y.S. Park, G.S. Lee, K.B. Yoon, Adv. Mater. 13 (2001) 594. [27] S. Li, C. Demmelmaier, M. Itkis, Z. Liu, R.C. Haddon, Y. Yan, Chem. Matter 15 (2003) 2687. [28] I. Pellejero, M. Urbiztondo, M. Villarroya, J. Sese, M.P. Pina, J. Santamaria, Micropor. Mesopor. Mater. 114 (2008) 110. [29] S. Ozturk, B. Akata, Micropor. Mesopor. Mater. 126 (2009) 228. [30] S.K. Kirdeciler, C. Ozen, B. Akata, Micropor. Mesopor. Mater. 191 (2014) 59. [31] S H. Kirdeciler, Preparation of functional surfaces using zeolite nanocrystals for biosensor and biomedical applications (Thesis), Middle East Technical University, July 2012. [32] I.G. Giannakopoulos, D. Kouzoudis, C.A. Grimes, V. Nikolakis, Adv. Funct. Mater. 15 (2005) 1165. [33] T. Baimpos, L. Gora, V. Nikolakis, D. Kouzoudis, Sens. Actuators A Phys. 186 (2012) 21. [34] T. Baimpos, V. Tsukala, V. Nikolakis, D. Kouzoudis, Sens. Lett. 10 (2012) 879. [35] R.W. Thompson, M.J. Huber, J. Cryst. Growth 56 (1982) 711. [36] M. Jafari, A. Nouri, M. Kazemimoghadam, T. Mohammadi, Powder Technol. 237 (2013) 442. [37] M.A. Mohammad, M. Muhammad, S.K. Dew, M. Stepanova, Fundamentals of electron beam exposure and development, in: Nanofabrication, SpringerVerlag, Wien, 2012. [38] A. Olzierski, I. Raptis, Microelectron. Eng. 73–74 (2004) 244. [39] E. Yilmaz, H. Sezen, S. Suzer, Angew. Chem. Int. Ed. 51 (2012) 5488. [40] M. Moliner, Basic principles of zeolite synthesis, in: C. Martinez Sanchez, J. Pérez Pariente (Eds.), Zeolites and Ordered Porous Solids: Fundamentals and Applications, Universitat Politècnica de València, 2011, pp. 37–65. [41] V. Valtchev, S. Mintova, Micropor. Mesopor. Mater. 43 (2001) 41.