Effect of Interdigited in the Sensed of Carbon Monoxide Using ZnO Films

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Carbon monoxide (CO) gas is colorless, odorless and highly toxic. Natural sources account for 40% of the CO in the environment while 40% is the result of human activities. It is produced by the incomplete combustion of fuels and exhaust emission [1]. Zinc oxide (ZnO) is an n-type metal oxide semiconductor (MOS) that has been widely used for gas sensors along many years because of their chemical response to different adsorbed gases, high chemical stability, amenability to doping, non-toxicity, affordability, and relative fabrication simplicity. It has a wide range of electronic, chemical, and physical characteristics and has become a well-known material for commercial sensors because of the sensitivity of its properties under variations of its chemical environment [2,3]. In the present work we use ZnO films for sensing carbon monoxide with the resistive method. The ZnO films were deposited by a sputtering system using radiofrequency (RF), upon substrates of (001) p-type,1.5 x 1.5 cm2 sized silicon. The substrates were washed with a conventional RCA cleaning procedure. ZnO was deposited on the substrate for 45 minutes, obtaining a thickness of approximately 150 nm, under a pressure of 7.1 mTorr, a power of 125 W and an Ar flow of 10%, the blank of ZnO has purity of 99.99%, the deposit was made at room temperature. After the deposition, the metallic interdigitated electrodes were defined by using lithography. Designed electrodes have different distances between the fingers namely: 200, 400 and 800 microns. Also, different metals were used: 50 nm thick aluminum, 25 nm thick gold and 25 nm/50 nm thick gold /aluminum layers.The gas detection properties were evaluated by measuring the changes of the resistance of the sensing structure under air and CO gas. Results for ZnO structures using two different contact metals. The best response to different concentrations of CO was found with the use of Al as the metallic contact, the difference being greater at high concentrations of CO, and at a relatively low measuring temperature of 240 °C with respect to the other two metal contacts. As expected, the films with a distance between fingers of 800 micros in the interdigitate electrodes were those that presented the greatest response to the gas of interest, this because the surface exposed to gas was larger with respect to the other two designs.