Spin-on Nanoparticle Tin Oxide for Microhotplate Gas Sensors R.E. Cavicchi§, R. M. Walton§, M. Aquino-Class§, J. D. Allen§, B. Panchapakesan ‡, M. Whitmire¶ §National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ‡Department of Chemical Engineering, Univ. of Maryland, College Park, Maryland 20742 ¶Hitachi Instruments, Gaithersburg, MD Abstract A colloidal suspension of tin oxide nanoparticles is used to prepare a highly sensitive gas sensing film on a microhotplate. Fabrication consists of spin-coating the solution over an array of micromachined hotplates and annealing. A thermolithographic process using a nitrocellulose coating is demonstrated. Scanning electron microscopy images show the films to be smooth and consisting of 10 nm particles. Response to methanol shows a resistance change of a factor of two between zero-grade dry air and a concentration of 10 nanomoles/mole (10 ppB) at an operating temperature of 350 °C. Sensitivity is observed at operating temperatures as low as 150 °C, with generally lower recovery times. Tests comparing the response in saturated humid air and dessicated dry air show that recovery to wet air exposures is greatly accelerated at temperatures above 300 °C. Stable response with no evident poisoning of the sensor was observed in a run lasting 120 hours consisting of methanol and ethanol exposures. Temperature programmed sensing was used to create different response patterns in air, methanol, and toluene, useful for identification of a detected gas. The results suggest colloidal suspensions may be used in combination with the microhotplate platform to produce a low-power, highly sensitive, device that is fabricated using planar technology. Introduction It was shown by Xu et al. [1] that nanoparticle tin oxide produced enhanced gas- sensitivity in thick film devices. Yoo et al. [2-4] have fabricated nanoparticle tin oxide sensing films from a sol by spin-coating. In the field of photovoltaics, similar films are finding use in combination with light-absorbing dyes [5]. Ferrere et al.[6] used a commercially available colloidal suspension of tin oxide nanoparticles in the fabrication of dye-sensitized solar cells. This report presents results of nanoparticle films produced by spin-coating a colloidal suspension of Sb-doped tin oxide onto a micromachined sensing platform, the microhotplate. Related work on undoped spin-on tin oxide was presented at the 8 th International Meeting on Chemical Sensors[7,8]. The CMOS (complementary metal-oxide semiconductor)-compatible microhotplate consists of a suspended microbridge with an integrated heater, heat- distribution/thermometer plate, and sensing contacts, each separated from the others by a layer of SiO 2 [9]. The platform has the benefits of low-power operation and has been fabricated in arrays of as many as 48 sensing elements[10]. Because of the small thermal mass, rapid temperature changes on the millisecond time-scale may be used to enhance the ability of a sensor to identify the gas being detected [11]. This operating mode is referred to as “temperature programmed sensing” (TPS).