Gravimetric sensitivity of transverse waves trapped by metal gratings on thin quartz plates

It has been theoretically and experimentally demonstrated that Surface Transverse Wave (STW) resonators fabricated on thin quartz plates (AT and Z cuts) exhibit attractive properties with respect to classical shear plate mode wave delay lines. Many modes can be generated and trapped using thin metal strip gratings deposited on top side of the plate. Insertion losses lower than -10 dB and and factor in excess of 25000 were measured close to 130 MHz. The possibility to use the bottom side for sensing purpose appears very interesting. A general model is proposed based on a rigorous calculation of mechanical and electrical boundary conditions, giving access to the gravimetric sensitivity of each mode excited in the plate. Experiments have been performed to check the validity of the proposed theoretical analysis. Aluminum and chromium layers have been deposited by thermal evaporation on Z and AT cut plate devices to evaluate the gravimetric sensitivity of our devices. Measured properties are related to the difference of phase velocities of the waves in the quartz plate and the layer used to load the device. Comparison with already published data shows that the proposed theoretical approach provides better results than other analyses based on perturbation methods. Discussions are eventually proposed to explain the residual discrepancies between our model and experimental values.