Articles Anal. Chem. 1994,66, 2201-2219
Dynamics and Response of Polymer-Coated Surface Acoustic Wave Devices: Effect of Viscoelastic Properties and Film Resonance Stephen J. Martin' and Gregory C. Frye Microsensor Research and Development Department, Sandia National Laboratories, Albuquerque, New Mexico 87185 Stephen D. Senturla Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technobgy, Cambridge, Massachusetts 02 139
The response of polymer-coatedsurfaceacoustic wave (SAW) devices to temperature changes and polymer vapor absorption is examined. A perturbationalapproachis used to relate velocity and attenuationresponsesto film translationaland strain modes generated by the SAW. Two distinct regimes of fdm behavior arise, causing different SAW responses. For glassy films, displacementis nearly uniform across the film thickness, varying only in the direction of propagation. A model developed to predict velocity and attenuation in this regime (model l),reduces to the familiar Tiersten (Wohltjen) equation for purely elastic films. For elastomeric (rubbery) fdms, inertial effects cause a phase lag to occur across the film for shear displacements. A model to account for these cross-filmdisplacement gradients (model 2) predicts a characteristic resonant response when the film phase shift reaches m/2, where n is an odd integer. These model predictionsare compared with measured responses from polyisobutylenecoated SAW devices as temperature is varied and during exposure to high vapor concentrations. Surface acoustic wave (SAW) devices use interdigital transducersphotolithographically patterned on the surface of a piezoelectric crystal (Figure 1) to excite and detect surface waves.' These waves are excited in a frequency band centered at fo = u / d , where u is the SAW propagation velocity and d is the transducer period. The SAW wavelength A generated atfo matches the transducer period d. SAW devices typically operate in the 30-300-MHzrange and use ST-cut2 quartz substrates for high-temperaturestability near room tempera( I ) White, R. M.; Volltmer, F. W. Appl. Phys. Lett. 1965, 7, 314. (2) The Euler angles for ST-cut quartz are Oo, 132.7S0, Oo. SAW velocity in the A'-propagation direction is 3.158 X lo-' cm/s.
0003-2700/94/03852201$04.50/0 0 1994 American Chemlcal Society
ture. In the delay-line device configuration? input and output transducers are typically separated by 100-200 wavelengths. The extreme sensitivity of SAW devices to thin-film properties has been used both (1) to characterize thin film elastic and viscoelastic properties: and (2) to construct gas and vapor sensors.5 In these applications,a thin film (thickness h > 1 and rubbery if UT
