Micro-gas-sensor with conducting polymers Q. Fang * , D.G. Chetwynd, J.A. Covington, C.-S. Toh, J.W. Gardner Electronic and Electrical Engineering, University College London, Torrington Place, London WCIE 7JE, UK Abstract In this paper design and fabrication of micro-gas-sensors, the deposition of four different poly(pyrrole) (PPY) thin films as chemoresistor in the micro-gas-sensors by electrochemical processing and chemical oxidation, and characterisation these films by FTIR, SEM, surface test instrument (WYKO NT2000) and optical microscopy, are reported. The effect of anions in PPY thin films, thickness and surface roughness of the PPY films on the response of micro-gas-sensors and the effect of ethanol vapour concentration on the response of polymer films have also been investigated. The experimental results show that a typical sensitivity to ethanol of ca. 5 mV/ppm was observed at 30 8C and the gas sensitivity of the integrated silicon planar micro-sensor depends on the counter ions in the PPY films, PPY film thickness and surface morphologies. The gas sensitive mechanism of the conducting PPY thin films is also discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Micro-gas-sensor; Conducting polymers; Poly(pyrrole) (PPY); Gas sensitivity 1. Introduction Gas sensors have been manufactured as a commercial product for more than three decades in many applications. Recent advances in micro-technology and micro-machining techniques have led to integrate a gas sensor system (includ- ing gas-sensitive thin film, micro-heater, electrodes and wires) onto a silicon chip [1,2]. The performance of gas sensitive materials including MOS, ionic conductor and conducting polymers can be improved by control of the composition, morphology and increasing operating tem- peratures. Poly(pyrrole) (PPY) is one of the most promising candidates for use as an active component in many techno- logical applications such as light emitting diodes (LEDs), solar cells, sensors and actuators. Currently there is a con- siderable interest in conducting polymer materials used in micro-gas-sensor [3,4], since they are expected to possess unique electronic functions, to operate around room tem- perature and devices based on molecules instead of bulk semiconductors. As a chemoresistor, however, there are three disadvantages to their use have been reported for some conducting polymers, such as PPY and polyaniline (PAN). Firstly, the specificity of an individual conducting polymer chemoresistor is poor, since these materials are sensitive to a wide range of vapours. Secondly, conducting polymer che- moresistors show a significant temperature coefficient of resistance of about 10 3 8C [5]. Third, a batch-to-batch variation in resistance of about 35% has been found [6]. To solve the poor selectivity problem, gas sensor array and pattern recognition analysis has been widely used for the identification of odours or gases [7,8]. To reduce the effect of variations in batch conditions and operating temperature on the device base-line signal, a micro-bridge structure with four arms has also been constructed and tested. This is owing to quite many factors that influence the gas-sensitive proper- ties of PPY-based polymers, much of the structure—prop- erty relationship still remains to be determined. In this work the authors describe the design and fabrica- tion of micro-gas-sensors used PPY as chemoresistor, the deposition and characterisation of PPY thin film in the micro-gas-sensors by electrochemical processing and che- mical oxidation and by FTIR, SEM, surface test instrument (WYKO NT2000) and optical microscopy, are reported. The effect of anions in PPY thin films, thickness and surface roughness of the PPY films on the response of micro-gas- sensors and the effect of ethanol vapour concentration on the response of polymer films have also been investigated. 2. Experimental details 2.1. Device design and fabrication Fig. 1 shows the cross-section view of the micro-gas- sensor structure. As shown in Fig. 1, the micro-gas-sensor is composed of PPY-sensing film, Pt micro-heater and Au electrodes on a stress free SiN x /SiO 2 /SiN x diaphragm. The micro-gas-sensor device has been designed using the Sensors and Actuators B 84 (2002) 66–71 * Corresponding author. Tel.: þ44-20-7679-3196; fax: þ44-20-7388-9325. E-mail address: q.fang@ee.ucl.ac.uk (Q. Fang). 0925-4005/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0925-4005(01)01076-0