Combined smart chemFET/resistive sensor array J.A. Covinqton, S.L. Tan, A. Hamilton, T. Koickal T. Pearce J.W. Gardner zyxwvuts School of Engineering School of Engineering Department of Engineering University of Warwick University of Edinburgh University of Leicester Edinburgh, EH9,3JL, UK Coventry, CV4 7AL, UK zyxwvuts J.A.Covington@wanvick.ac.uk Leicester, LEI 7RH, UK zy Abstract zyxwvutsrq Here we describe a novel CMOS compatible gas sensor array based zyxwvutsrq on a combined resistive/chemFET sensor cell. We have fabricated an a r r q zyxwvuts of 70 sensors with integrated drive, gain and baseline removal circuitry using an AMS 0.6 pm CMOS process. The sensing materials are carbon b1acWpolymer composite zyxwvutsrqp (CB) thin films, which have been previously reported to have good vapour-sensing proper- ties. Different CBfilms have been deposited onto the sensor array and have been shown to respond differently to vola- tile organic compounds. This combined sensing element both reduces silicon area and, more importantly, measures diflerent physical properties of the same gas sensitive ma- terial improving discrimination and giving more insight into the sensing mechanism. Keywords Combined cbemFET/resistive Gas Sensor, Sensor Array, Offset Cancellation. INTRODUCTION There have been a number of attempts over the last decade to integrate chemical sensors within a standard CMOS process. Early work used a resistive chemical sensor with separate ASIC (Application Specific Integrated Circuit) that contained signal processing circuiby [I]. Later designs directly integrated the sensor with signal processing cir- cuitry, resulting in a number of sophisticated devices. For example Hagleitner et al. zyxwvutsrq [Z] have created a chip with four different integrated chemical sensors employing different sensing mechanisms. Work with FETs as chemical sensors has been investigated since its discovey in 1975 [3]. Such devices would seem ideal for CMOS integration due to the FET being the comerstone of modem electronics. Unfortu- nately, the high operating temperature (>ZOO "C) and non- standard gate material, in this case palladium, makes these devices difficult to integrate. Research into polymer gate FET devices bas removed some of these problems, such as the high operating temperature and,using polysilicon as the contact to the sensing material [4]. The main issue with this technology is the post-processing required lo expose the polysilicon gate and gate oxide. An alternative approach is to use a capacitive coupled design, where a capacitive plate is connected to the floating gate of a FET [5]. Alternative work has investigated large resistive chemical sensor arrays with an excess of 1000 sensing elements [6]. This array used a carbon black composite polymer as the sensing ma- terial and was fabricated using a standard CMOS process. Here we report on a combined chemFET/resistive sensor array fabricated using an AMs 0.6 pm CUP CMOS proc- ess, employing carbon black (CB) composite materials, with integrated signal processing circuitry. It differs from previous work in a number of ways. Firstly, it is the first resistive CMOS compatible sensor array integrating differ- ent CB's polymers into the same array. In addition, it is the largest FET sensor array using CB materials. Lastly, it is the first time both a chemoresistor and chemFET have been combined with integrated signal processing electronics. COMBINED SENSOR CONCEPT The resistive sensor component is simply formed by depos- iting CB polymer between two sensor electrodes. The FET section is based on the floating gate concept, were the sens- ing material is capacitive coupled to the floating gate of a FET. As the gate is floating, any potential created through the interaction of the sensing material with the target gas or vapour appears, due to capacitive coupling, on the gate of the FET. It bas previously been reported [SI that this is due to a change of workfunction within the sensing material. The floating gate will have an absolute potential subject to variations in the fabrication process. A biasing plate is added underneath the sensing plate and biased to ensure the transistor is tuned-on. The capacitive plate is placed in- between the two resistive electrode elements, hence the same sensing material is used for both resistive and FET measurements. This is shown in figure 1 below. operation. Here polymer composite materials are used as the sensing film. These combine an insulating polymer/rubber, usually used in gas chromatography with carbon black nanoparti- cles. The carbon endows electrical properties to the com- posite material. In the resistive configuration, exposure to a 0-7803-81 33-5/03/$17.00 02003 IEEE 1120