PREDICTING MASS LOADING SENSITIVITY FOR ACOUSTIC-WAVE SENSORS OPERATING IN AIR

A model has been developed for calculating acoustic wave modes in mult i-layered piezoelectric devices which use interdigital transducers (ID Ts) as transmitter and receiver. The model is used to simulate the tra nsduction and reception of waves in acoustic wave sensors (e.g. biosen sors, gas sensors or chemical sensors) configured as delay-line oscill ators. First, all of the modes which may in principle propagate in the device are calculated for a specified frequency range. The insertion loss for each mode is then calculated approximately. Results from pert urbation theory are then used for those modes with small insertion los ses, to study the effect on the wave velocity of a solid mass-loading layer, calculating the inertial and elastic effects separately. The ef fect of a change in electrical conditions at the sensing surface is al so calculated using results from perturbation theory. The model is use d to identify wave modes in Rayleigh and SH devices manufactured from ST-cut quartz, by comparing measured and predicted insertion loss freq uency spectra. Higher frequency modes appearing in the Rayleigh device are identified as quasi-SH and Lamb modes. The principal and secondar y modes of the SH device are identified, with the model suggesting tha t it may be the SH1 mode, rather than the SH0 mode, which is responsib le for operation of the device. Calculations of the expected mass-load ing sensitivity of the modes in these devices are made and compared wi th experiment. We find that the elasticity of a loading layer can sign ificantly reduce the expected frequency changes, by up to around 15% f or Lamb-like modes, and around 7% for SH-like modes. Electrical effect s tend to negate this reduction. However, we conclude that the inertia l mass-loading effect is predominant in the experimental work referred to in this paper. (C) 1997 Elsevier Science S.A.