Investigations of thin film structures of WO3 and WO3 with Pd for hydrogen detection in a surface acoustic wave sensor system

A single thin film sensor structure Of WO3 (similar to 50 nm) and bilayer sensor structure Of WO3 (similar to 50 nm) with a very thin film of palladium (Pd similar to 18 nm) on the top, have been studied for hydrogen gas-sensing application at similar to 30 degrees C and similar to 50 degrees C. The structures were obtained by vacuum deposition (first the WO3 and than the Pd film) onto a LiNbO3 Y-cut Z-propagating substrate making use of the surface acoustic wave method and additionally (in this same technological processes) onto a glass substrate with a planar microelectrode array for simultaneously monitoring of the planar resistance of the structure. In the case of a bilayer structure a very good correlation has been observed between these two methods frequency changes in SAW method correlate very well with decreases of the bilayer structure resistance. These frequency changes are on the level of 2.4 kHz to 4% of hydrogen concentration in dry air, whereas in the case of a single WO3 structure almost no frequency shift is observed. (C) 2007 Elsevier B.V. All rights reserved.