Harmonic frequency analysis of SAW resonator chemical sensors: application to the detection of carbon dioxide and humidity

We have investigated the possibility of increasing surface acoustic wave (SAW) gas sensor sensitivity and detection limit by operating a SAW device at its fundamental frequency (250 MHz for the devices used in this paper), while monitoring frequency changes at a higher harmonic. In particular we have compared frequency changes at the third harmonic with that of the fundamental mode, using carbon dioxide (CO2) and water vapor as our test gases. The results showed that sensitivity is increased by a factor of three and the detection limit is improved by a factor of two. Three different polymers were used in our investigations. Of the three polymer coatings studied-BMBT, PEI and Versamid 900-PEI gave the greatest frequency change per unit change in humidity, followed by Versamid 900 and BMBT, in that order. The frequency change yielded by PEI (5.7 kHz per percent change in humidity) was about a factor of ten increase compared with the other two polymers. PEI was also found to be relatively more sensitive to CO2 compared with the other polymers. A frequency shift of about 1 kHz was measured for 240 ppm of CO2 in nitrogen. This was comparable to the frequency shift obtained for Versamid 900. The corresponding frequency shift for BMBT was about 700 Hz. (C) 1998 Elsevier Science S.A. All rights reserved.