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Integration of SnO2 sol-gel processes to gas sensor microfabrication: H2 and CO sensitivity evaluation

Sol-gel organic synthesis of SnO2 thin films from tin ethoxide precursor is reported here as a promising and cheap alternative of the 'classical' chemical and physical preparation methods of the SnO2 thin films, for gas sensing applications. A simple, integrated circuit compatible test structure, for rapid evaluation of the sensing properties of the SnO2 sol-gel derived thin films is described. The main features of our microstructure consists of a a heating resistor integrated on chip, made of highly boron doped silicon and a metallization system from Au/W deposited on a planarized chemically vapor deposited SiO2 layer. The SnO2 films have shown the well-known increase-maximum-decrease dependence of chemoresistance as a function of temperature, with a maximum at about 380 degrees C, when they are measured in clean, dry air. The sensitivity of SnO2 films to high concentration of H2 in air was studied within a quartz furnace, externally heated in the temperature range from 200 to 450 degrees C. The relative sensitivity is equal to 100 percent at temperatures as low as 200 degrees C, while its maximum value is anticipated to be above 450 degrees C. The CO sensing properties of SnO2 layers were evaluated as a function of input power applied on the integrated heating resistor. We have obtained relative sensitivities of 30 percent for 500 ppm CO concentration in dry air and an input power of 209 mW.