ZEOLITE-ENCLOSED MICRO-CAVITIES ON SILICON WAFER FOR CHEMICAL STORAGE K.F. Lam 1, 2*, W.Y. Lai 1, N.W. Chan 1 and K.L. Yeung 1 1Hong Kong University of Science and Technology, HONG KONG and 2University College London, UNITED KINGDOM

ABSTRACT Zeolitic self-enclosed micro-cavities were successfully fabricated. In the process, a zeolite film was grown on silicon wafer followed by pore activation and wet etching. Etchant molecules diffused through the zeolite pores and etched the silicon underneath. Nitric acid and pyridine were used for the feasibility study of chemical storages and releases using the microcavities. The release of nitric acid was observed by a continuous decrease in pH of DI water for 10minutes whereas the pyridine release was monitored by UV/vis measurement. A control experiment without cavities showed that there was no pH change or measured pyridine in the solution.

KEYWORDS: zeolite membrane, microcavity, nanoliter storage, controlled-release

INTRODUCTION Mechanical and electronic components for actuation and sensing [1], and their assembly and integration into a func- tioning micro-systems have seen significant progress over the past years [2]. However, critical issues of on-chip chemical storage and handling remained problematic. Garcia et al. [3] employed etched micro-cavities in micro-channel to contain chemicals-bound in anhydrous carbohydrate matrix. When liquid passed through the micro-channel, the stored chemical dissolved and released to the solution. Kang et al. [4] used a new method to store chemical in micro-chip. The authors en- capsulated phospholipid bilayer containing single protein nanopore, α-HL in Teflon-polyester composite. Small mole- cules readily accessed the pores through the thin porous agarose layer. The lipid remained functional after storage for weeks. Jackman et al. [5] produced PDMS micro-wells by moulding method. After loading chemicals to the micro-wells, a flat cover-slip was used to cover the micro-wells. NH 4OH diffusion through the PDMS was investigated. Similar me- thod was also applied to micro-wells in silicon wafers [6]. The sample was used for combinatorial studies of enzymatic reactions. The drawback is that the cover-slip was just lid on the micro-wells and chemical leakage is possible. Santini et al. [7] improved on this method by covering the micro-reservoirs in silicon chip by a gold membrane and chemical re- lease is achieved by dissolution of the gold layer. Although enclosed micro-cavity can also be fabricated by anodic bond- ing of wafers with micro-wells, the micro-cavities are inaccessible because of the non-permeable nature of the wafer [8].

THEORY The synthesis of zeolites on various substrates have been widely studied. The preparation involves two main steps, namely seeding and hydrothermal regrowth. In the seeding process, nano-sized zeolite crystals were coated on the silicon wafer using an organic linkers. The seeded wafer underwent hydrothermal synthesis in which the seeds acted as nuclea- tion sites for the growth of zeolite crystals. The zeolite crystals grew and overlapped to form a continuous film that had excellent adhesion to the wafer. Chau and Yeung [9] has successfully prepared zeolite enclosed microtunnels and micro- chunnels with the aid of organic templates. In this study, self-enclosed micro-cavities on wafer using zeolitic material were successfully fabricated. In the process, a zeolite film was grown on silicon wafer, followed by pore activation and etching step. Etchant molecules diffused through the zeolite pores and etched the silicon underneath. Figure 1 illustrates the fabrication process of the micro- cavities.

EXPERIMENTAL Zeolite-enclosed micro-cavities were fabricated using <100> silicon wafer by the method described in [10]. In brief, the wafer was pre-cleaned by a mixture of sulphuric acid and hydrogen peroxide (i.e. Piranha solution), followed by HF:H 2O(1:50) to remove organic contaminants and native oxide from the silicon surface. The wafer was coated with a layer of nano-zeolite seeds by the aid of an organic surfactant linker, (3-mercaptopropyl)trimethoxysilane (Aldrich, 98%). The silicalite-1 zeolite film was grown on the wafer from a synthesis solution containing 40 tetraethyl orthosilicate: 10 tetrapropylammonium hydroxide: 20,000 H 2O in molar basis. The chemicals were purchased from Aldrich. The seeded wafer was immersed vertically in the synthesis solution and underwent hydrothermal synthesis in a Tef- lon-lined autoclave at 130 oC for 24 hours. After the synthesis, the sample was thoroughly rinsed with distilled deionized water and dried in a 65 oC oven. A photoresist layer, AZ4620 (Clariant) was spin-coated on the Sil-1 film, followed by pattern transfer of 50um X 50um square arrays using conventional photolithography technique. The photoresist acted as a protective layer during the regrowth of thin layer of ZSM-5 zeolite, as shown in Figure 1. The synthesis solution for the growth of ZSM-5 consisted of 80 SiO 2: 10 Na 2O: 1 TPA 2O: 8 Al 2O3: 40000 H 2O in molar basis which was prepared from TEOS (Aldrich, 98%), alumina precursor, sodium hydroxide (BDH, 99%) TPAOH (1 M, Aldrich) and DDI water. The synthesis took place at 150 oC for 48 hours and 3 um ZSM-5 layer was grown.

978-0-9798064-3-8/µTAS 2010/$20©2010 CBMS 525 14th International Conference on Miniaturized Systems for Chemistry and Life Sciences 3 - 7 October 2010, Groningen, The Netherlands Silicon wafer Seeding Hydrothermal synthesis Controlled (ZSM-5) activation Hydrothermal synthesis (Sil-1) Wet etching

Photolithography

Figure 1: Process flow for the fabrication of zeolite self-enclosed microcavities

The pores of zeolite composite were then ozone treated to selectively remove the organic structure directing agent (SDA) molecules from the zeolite pores. The TPA + molecules were more readily removed from the Sil-1 compared to ZSM-5, thus the Sil-1 becomes porous while the ZSM-5 remained dense and nonporous after the treatment. The ozone concentration was 106 ppm at flow rate of 157 (cm 3/min) at 3 psi. During the etching step, the etchant diffuses through the open Sil-1 pores and etched the silicon beneath to form the self-enclosed microcavities. The area protected by the ZSM-5 layer was unetched. The zeolite membrane-enclosed mi- crocavities were obtained by placing the sample in 100ml of 25wt% TMAH solution at 60 oC (Moses Lake, 25wt%) for up to 4 hours. Sample without micro-cavities was prepared for comparison. Nitric acid and pyridine were loaded to the microcavities in order to demonstrate the possibility of loading chemical into the cavity under vacuum. The sample was rinsed with DI water and immersed in 50 ml deionized distilled water for the chemical releasing experiment. The released acid was monitored by a pH meter, while pyridine was measured with UV/visible spectrometer.

RESULTS AND DISCUSSION After the hydrothermal synthesis, the quality of zeolite film on the silicon surface was examined by scanning electron microscope. This is essential to check for imperfection in the zeolite film which can result in leakage of the stored chemi- cal. A typical fabricated microcavity structures is shown in Figure 2a. The silicalite-1 zeolite film is found to be uniform and free of crack and have a thickness of about 4 microns. Using a needle, the membrane enclosing a microcavity was poked open for closer inspection in Fig. 2b and 2c. The figures show that typical anisotropic etching of silicon beneath the thin, freestanding zeolite membrane layer.

a b c

Figure 2: Scanning electron microscopic images of the zeolite enclosed micro-cavities on silicon wafer: (a) Top view, (b) prospective view and (c) cross-section

Figure 3a shows the acid releasing from the nitric acid-loaded microcavities. The control sample resulted a drop of pH from 5.6 to 5.4. The slight decrease in pH is due to the desorption of hydrogen ions from the porous zeolite. On the other hand, acids stored within fully-etched microcavities induced a significant drop in the solution pH from pH 5.6 to 4.1. Based on the mass balance of hydrogen ions, the estimated volume of microcavities is 250 nanoliters, which is 20% larger than the theoretical value calculated from the geometry. It is probably due to the experimental error since the solu- tions were not stirred during the pH measurements. For the same reason, the proton transport property through the zeolite film can only be estimated to be in the range of 1 to 6 mmol m -2 s-1. Nevertheless, the experimental data showed that it is feasible to store and release the liquid chemicals in the zeolite-enclosed microcavities.

526 Figure 3b shows the cumulative amount of pyridine released from the micro-cavities. More than 90% of the stored py- ridine is released in 10 minutes. This experiment proves the feasibility of loading and releasing organic molecules through the zeolite-enclosed micro-cavities.

6.0 a 1.2 b

5.5 0.9 5.0

pH 0.6 4.5 without cavity with cavity 4.0 0.3

Without cavity (umol) Pyridinereleased With cavity 3.5 0 0 3 6 9 12 0 3 6 9 12 Time (min) Time (min)

Figure 3: Releasing of (a) 1.6M HNO3 and (b) 99% pyridine from micro-cavities

CONCLUSION In this work self-enclosed zeolite micro-cavities are successfully fabricated. Since the zeolitic membrane is chemically bonded on the wafer, no chemical leakage is possible in this device and it is particularly important for toxic reagent han- dling. Storage and releases of chemicals including inorganic nitric acid, and organic pyridine solution in the micro- cavities are demonstrated. As the sizes and shapes of the micro-cavities are based on photolithographic micro-pattern dur- ing the fabrication, various fluidic enclosed micro-structures can be fabricated by this proposed technique. It is potentially applicable for various micro-devices such as micro-reactors and micro-sensors.

ACKNOWLEDGEMENTS Funding from Hong Kong Research Grant Council (605009) is gratefully acknowledged.

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CONTACT *K.F. Lam, tel: +44-(0)-20 7679 3037; [email protected]

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