Procedia Engineering 25 (2011) 1669 – 1672

1877-7058 © 2011 Published by Elsevier Ltd.doi:10.1016/j.proeng.2011.12.413

Available online at www.sciencedirect.comAvailable online at www.sciencedirect.com

Procedia Engineering 00 (2011) 000–000

Procedia Engineering

www.elsevier.com/locate/procedia

Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece

ZnO Modified High Aspect Ratio Carbon Electrodes for Hydrogen Sensing Applications

Umesh Singha, Hyun Ae Leeb, Young-Chul Byunb, Amit Kumara,

Sudipta Seala, Hyoungsub Kimb, Hyoung J. Choa,b* aAdvanced Materials Processing Analysis Centre (AMPAC) and Department of Mechanical, Materials & Aerospace Engineering, University of

Central Florida, Orlando, USA b School of Advanced Materials Science and Engineering (WCU Program),

Sungkyunkwan University, Suwon, Korea

Abstract

Design, fabrication, and testing results of a hydrogen gas sensor based on a surface modified high density and high aspect ratio 3-D carbon post microarray are described in this paper. After the conversion of photoresist patterns into carbon electrodes, the increased surface was conformally coated with a metal oxide semiconductor film by atomic layer deposition (ALD). A maximum sensitivity as high as 100 (Rair/ Rhydrogen) has been observed in the fabricated sensor even in the presence of moisture at low temperature. A fast initial response (90% resistance drop in 30 sec at 3000 ppm hydrogen) of the sensor could be utilized for early leak detection. The methodology of surface modification as well as the test result shows good promise for various chemical sensing devices based on the proposed strategy. © 2011 Published by Elsevier Ltd. Keywords: Hydrogen; Gas Sensors; MEMS; Carbon; Atomic Layer Deposition; ZnO

1. Introduction

Due to the necessity and demand of alternative fuel source, hydrogen has obtained increased public interest in recent years. For safe handling of hydrogen fuel, it is imperative to develop sensors working at atmospheric temperature with fast response and good sensitivity. Among various types of sensors, conductometric gas sensors based on semiconductor metal oxides such as tin oxide (SnO2), zinc oxide (ZnO), indium oxide (In2O3), and tungsten oxide (WO3) are most widely studied [1].

* Corresponding author. Tel.: +1-407-823-5014; fax: +1-407-823-0208. E-mail address: hjcho@ucf.edu.

1670 Umesh Singh et al. / Procedia Engineering 25 (2011) 1669 – 1672 Author name / Procedia Engineering 00 (2011) 000–000

Although it has been reported that the strategy of increasing surface to volume ratio using nanoscale features is effective on improving the sensor performance, these nanostructures often require complicated fabrication steps and yet have a limited sensitivity at low temperatures [2,3]. One way to increase the surface with the limited floor area is the use of high aspect ratio structures. Glassy carbon posts that can be obtained by carbon microelectromechanical (C-MEMS) technology are conductive and significantly increase the surface to volume ratio with their three-dimensional architecture and compatible with batch microfabrication [4].

In this study we report the extension of C-MEMS technology for gas sensing applications with a conformal coating of semiconductor metal oxide film in which the carbon posts provide a structural support with an increased surface area while the metal oxide contributes to variation of electrical resistance in the presence of hydrogen.

2. Experimental

2.1. Fabrication

For the C-MEMS microsensor platform, first the bottom interdigitated electrodes were designed and fabricated. Then the carbon post arrays were fabricated on top of these interdigitated electrodes (IDE’s). Finally these carbon posts were conformally coated with ZnO using atomic layer deposition (ALD). The configuration of the device is shown in Fig 1(a).

(a) (b) Fig. 1. (a) Schematic illustration of ZnO modified carbon post arrays on top of Pt/Ti interdigitated electrodes; (b) Fabrication steps for ALD-ZnO / glassy carbon post arrays (top to bottom): SU-8 post arrays are defined by photolithography; carbon posts are obtained after pyrolysis; ZnO film is coated by ALD

The fabrication steps for the C-MEMS hydrogen sensor are shown in Fig 1 (b). Starting with an oxidized Si wafer,

the bottom electrodes were fabricated by a lift-off process of Pt (300nm)/ Ti (20nm) film. Then an epoxy based negative photoresist SU8-2050 (Microchem Corp.) was spin-coated on the Pt IDE’s to the thickness of 80 ~ 90 µm. The final post array patterns were prepared after photolithography.

The C-MEMS structures were obtained using a two step pyrolysis process in forming gas environment (5% H2 in N2) [4]. In the first step, the pyrolysis was done by heating the samples at 350 oC for 30 min and ramping up the temperature to 900 oC. In the final step, samples were kept for 1 hr and then cooled down to the room temperature. Fig 2(a) shows a tilted angle SEM image of glassy carbon posts where the individual post diameter is 50 µm and its aspect ratio is 1.8.

ZnO films were deposited on top of C-MEMS structure using ALD technique. Diethyl zinc [DEZn, ((C2H5)2Zn)] and H2O were used as metal and oxidant precursors, respectively. After the ALD process, individual chips were diced and sputter-coated with Pt for 2 sec. Finally, the diced chips were packaged and wire-bonded for testing. Fig 2(b) shows a packaged device. The inset figure shows ALD-ZnO/glassy carbon posts on the interdigitated electrodes. A reference sample without C-MEMS structures was prepared for comparative study.

1671Umesh Singh et al. / Procedia Engineering 25 (2011) 1669 – 1672 Author name / Procedia Engineering 00 (20111) 000–000 3

(a) (b)

Fig. 2. (a) Tilted SEM image showing carbon post arrays after pyrolysis; (b) A packaged sensor (inset: ALD-ZnO/glassy carbon posts on interdigitated electrodes)

2.2. Test

The testing of all C-MEMS and reference samples were done at room temperature (22 oC) with relative humidity of

35 % under dynamic condition. Turbo pumps were used to maintain the air pressure within the test chamber to 50 Torr. Mass flow controllers were used to regulate the volume of hydrogen. In this dynamic test condition, a desired amount of hydrogen was continuously flowed into the chamber and simultaneously pumped out through the outlet for the duration of test. The hydrogen was supplied for 30 minutes with a 50% duty cycle. Both the devices were tested at a fixed concentration of 3000 ppm hydrogen which is less than 10 % of lower explosive limit (LEL) of hydrogen in air. The change of resistance was recorded each second by a computer running LABVIEW (National Instruments Corporation). Sensitivity (S) was defined as the ratio of resistance in the air (Rair) to the minimum resistance (Rgas) in the presence of hydrogen.

3. Results and Discussion

Fig 3(a) shows the cyclic response of reference and C-MEMS sensors at 3000 ppm hydrogen level at room temperature. Tested sensors had a 20 nm thick ZnO film as a sensing element and Pt clusters as a catalyst for hydrogen dissociation. Initially both sensors showed high resistance in the air, Rair, which was eventually dropped to minimum resistance, Rgas, in the presence of hydrogen. The working principle based on the surface adsorbed oxygen species (O2- or O-) and the variation of the resulting space charge layer upon exposure to hydrogen was described elsewhere [1].

The ZnO C-MEMS sensor showed a tenfold increase of the sensitivity (S= Rair / Rgas = 100) as compared to the control sample (S=10) at 3000 ppm H2 level. The increase in the sensitivity of C-MEMS sensors can be attributed to the 3D electrode structure which increases the total surface area; charge carriers increases as the number of available reaction sites increases.

Fig 3(b) shows the initial time response of the resistance which may be used as an indicator of the sensor response speed. The ZnO C-MEMS exhibited a rate of 4.5 KΩ/sec while the control sample showed 1.2 KΩ/sec; the initial resistance drop is four times faster in C-MEMS. Based on a Schottky barrier present at electrode/sensing material interface, the response speed depends how fast the hydrogen atoms could diffuse to the interface to lower the Schottky barrier height [5]. It seems that the porous structure of carbon contributes to the percolation of ZnO as ALD progresses and thus greatly increases the interfacial area [6]. Furthermore due to their small size, diffusion of hydrogen atoms through the interface does not appear to be limited. Therefore, increase of the interfacial area between sensing materials and electrodes using C-MEMS/ALD process is found effective in improving hydrogen sensor performances in both sensitivity and response time.

1672 Umesh Singh et al. / Procedia Engineering 25 (2011) 1669 – 1672 Author name / Procedia Engineering 00 (2011) 000–000

0 20 40 60 80 100 120 140 160 180 200 220 240

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Fig. 3. (a) Cyclic and; (b) initial response of sensors at room temperature under dynamic testing condition (22 oC, 3000 ppm H2)

4. Conclusion

ZnO modified high aspect ratio carbon electrodes were fabricated and characterized for hydrogen gas sensing applications. A maximum sensitivity as high as 100 was observed even in the presence of moisture (35% RH) at low temperature (22 oC) at 3000 ppm hydrogen concentration. The fast response speed (dropping rate of resistance = 4.5 KΩ/sec) promises the use of sensors in early leak detection.

Acknowledgements

This work was supported by National Science Foundation, USA (ECCS-0801744) and World Class (WCU) University program through the National Research Foundation of Korea (R32-2009-000-10124-0)

References

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[2] Tonezzer M, Lacerda RG. Zinc oxide nanowires on carbon microfiber as flexible gas sensor. Physica E: Low-dimensional Systems and Nanostructures 2010;doi:10.1016/j.physe.2010.11.029.

[3] Ra HW, Choi KS, Kim JH, Hahn YB, Im YH. Fabrication of ZnO Nanowires Using Nanoscale Spacer Lithography for Gas Sensors. Small 2008;4:8- 1105.

[4] Wang C, Taherabadi L, Jia G, Madou M, Yeh Y, Dunn B. C-MEMS for the Manufacture of 3D Microbatteries. Electrochemical and Solid-State Letters 2004; 7:11- A435.

[5] Peng Z, Vincent A, Kumar A, Seal S, Cho HJ. A Low-Energy Room-Temperature Hydrogen Nanosensor: Utilizing the Schottky Barriers at the Electrode/Sensing-Material Interfaces. IEEE Electron Device Letters 2010; 31:7-770.

[6] Lee HA, Byun YC, Singh U, Cho HJ, Kim, H. Surface Modification of Carbon Post Arrays by Atomic Layer Deposition of ZnO Film. Journal of Nanoscience and Nanotechnology In print.