ENGINEERING GAS SENSORS WITH AEROSOL NANOCRYSTALS Ganhua Lu 1 , Liying Zhu 2 , Stephen Hebert 2 , Edward Jen 3 , Leonidas Ocola 4 , and Junhong Chen 1 1 Department of Mechanical Engineering and Laboratory for Surface Studies, University of Wisconsin- Milwaukee, Milwaukee, WI 53211, e-mail: jhchen@uwm.edu 2 Department of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI 53211 3 Nicolet High School, 6701 North Jean Nicolet Road, Glendale, WI 53217 4 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439 INTRODUCTION Rutile tin oxide (SnO 2 ) is a wide band gap (3.6 eV at 300K [1]) n-type semiconductor material. It is widely used as sensing elements in gas sensors [2]. The sensing mechanism is generally attributed to the significant change in the electrical resistance of the material associated with the adsorption /desorption of oxygen on the semiconductor surface [3]. The formation of oxygen adsorbates (O 2 - or O - ) results in an electron-depletion surface layer due to the electron transfer from the oxide surface to oxygen [4]. Recent studies [5, 6] have shown that use of tin oxide nanocrystals significantly improves the dynamic response and the sensitivity of sensors since the electron depletion may occur in the whole crystallite. Here we report on the fabrication and characterization of a miniaturized gas sensor based on tin oxide nanocrystals. A simple, convenient and low-cost mini-arc plasma source is used to synthesize high-quality tin oxide nanoparticles in aerosol phase at atmospheric pressure. The nanoparticle sensor is then fabricated by electrostatic assembly of product tin oxide nanoparticles onto e-beam lithographically patterned interdigitated electrodes. The microfabricated nanoparticle sensor exhibits good sensitivity and dynamic response to low- concentration ethanol vapor and hydrogen gas diluted in air. EXPERIMENTAL METHOD The fabrication process of an interdigitated sensor substrate is illustrated in Fig. 1. The interdigitated electrode facilitates the electrical assembly of nanoparticles and the subsequent sensor characterization. First, a precursor solution (Flowable Oxides, FOx12, Dow Corning) was spin-coated onto a cleaned silicon wafer. The wafer was then baked to form a thin and uniform SiO 2 top layer and to evaporate the residual solvent. A photoresist solvent (ZEP: Au, 1:1) was spin-coated onto the SiO 2 layer and baked to form a hardened film and to remove remaining solvent. Using an electron-beam lithography facility (30 kV Raith 150 e-beam tool), a pre-designed pattern was latently transferred to the resist layer via direct e-beam “writing”. Photoresist exposed to e-beam was subsequently removed by rinsing it with an appropriate solvent. A metal layer (Au) was then deposited onto the surface of the substrate by sputtering. Finally, remaining resist and unwanted metal were dissolved by soaking the substrate in a solvent. Photoresist SiO 2 Si Spin coating Photoresist SiO2 Si Spin coating SiO 2 Si Lithography SiO2 Si Lithography SiO 2 Si Development SiO2 Si Si SiO 2 Au Au Au Metal evaporation Si SiO2 Au Au Metal evaporation Si SiO 2 Au Liftoff Si SiO2 Liftoff SiO 2 Si SiO2 Si Cleaning & Oxidation E-beam Fig. 1. Fabrication process of the nanoparticle sensor substrate. Aerosol tin oxide nanoparticles with an average size of 10- 20 nm were produced using a mini-arc plasma reactor shown in Fig. 2(a) [7]. The atmospheric mini-arc reactor consists of a tungsten cathode and a graphite anode. A commercial tungsten inert gas (TIG) arc welder was used to drive the dc arc. Purified argon was used as the plasma and carrier gas. The high temperature in the arc melts and vaporizes the solid tin placed in the graphite crucible. A pure and cold nitrogen flow was injected to quench the tin vapor and nucleate tin nanoparticles, which were then oxidized to form SnO 2 nanoparticles by exposing them to clean air at the reactor exit. A fraction of the nanoparticles from the mini-arc reactor are electrically charged by the plasma or the thermionic emission [7]. The charged tin oxide nanoparticles from the mini-arc nanoparticle generation system were assembled onto the prefabricated substrate using electrostatic force to build gas sensors, as shown in Fig. 2(b). The mechanism for sensor operation is that nanoparticles deposited between any two fingers close the electrical circuit to form a sensor, with the impedance of which changing in response to the exposed analyte molecules. 1 Copyright © 2007 by ASME and Argonne National Laboratories Proceedings of MNC2007 MicroNanoChina07 January 10-13, 2007, Sanya, Hainan, China MNC2007-21301