Acoustic characterization of thin polymer layers for Love mode surface acoustic waveguide

Beyond the great gravimetric sensitivity provided by the polymer guiding layer of shear surface wave on quartz resulting from a low acoustic velocity, the use of photo resists provide economical means of depositing guiding layers of optimal thicknesses compared to inorganic layer deposition processes (typically PECVD deposition of silicon dioxide lasting several hours). We here analyze the evolution of the guiding layer properties, i.e. the acoustic velocity and losses, as a function of time (solvent evaporation following photo resist spin coating) and temperature (typical baking steps). The polymer films is deposited on an AT-cut quartz substrate patterned with passivated (SiO₂) interdigital transducers (Al) for generating 40 mum-wavelength shear waves converted to a guided Love mode in a delay line configuration. We exploit experimental characterization results via a model of acoustic wave propagation to determine physical parameters of the layer. We then have determined materials coefficient such as viscosity, elastic and thermoelastic constants and density out of the velocity and insertion loss measurements.