Julien Gratier

Transverse modes suppression and loss reduction for buried electrodes SAW devices

High coupling substrates using overcoat layers of silicon oxide for temperature compensation exhibit a strong affinity toward multi-mode behavior. Apodization, which is the standard technique to suppress the transverse modes, suffers from drawbacks such as reduction of the transduction efficiency and additional losses. By changing the transducer layout, the guiding can be improved and a “piston mode” can be obtained. This approach is demonstrated on resonators and DMS test devices manufactured on lithium niobate with an orientation close to YX128 deg.

A nonlinear P matrix model to simulate intermodulation products in SAW devices

Nonlinear intermodulation distortion inside duplexers is a limitation for the sensitivity of mobile devices receivers. A perturbation approach for non linear harmonic acoustic generation along the propagation of surface acoustic wave is derived. It is used to extend the P matrix model to non linear generation of harmonics and mixing products. Interface of the new model to harmonic balance simulators is discussed. Results are presented showing the validity of the approach.

Acoustic wave device employing reflective elements for confining elastic energy

An acoustic wave device includes electrodes carried on a surface of a piezoelectric material and an array of reflective obstacles such that elastic energy resulting from a piezoelectric effect is preferentially directed along a primary wave propagation path. The array of reflective obstacles are positioned generally parallel to the surface of the piezoelectric material and redirect acoustic waves typically reflected in other than a desirable direction to along a desired direction generally along the primary propagation path. The obstacles improve performance for SAW and BAW devices by effecting reflected energy and suppressing spurious modes.

Measurement and FEM/BEM simulation of transverse effects in SAW resonators in lithium tantalate

It is well known that transverse effects contribute significantly to the loss of SAW resonators on lithium tantalate. In particular, for frequencies above resonance, the surface wave is not guided inside the transducer and radiates into the busbars. In addition, because bulk modes can also be excited, scalar models are not sufficient to accurately predict transverse effects. It is also known that the layout of a SAW resonator (electrode gaps and dummy electrodes) has a strong impact on the transverse effects. In this paper, a periodic FEM/BEM model is presented and is used to simulate the transverse effects for various SAW resonator layouts. Test devices matching those simulated are fabricated and measured; the measured results are compared with the simulated results and show good agreement. By analyzing the dispersion curves produced from the FEM/BEM model in the different regions of the device, several frequency bands corresponding to different transverse behaviors are identified. These results are consistent with the elastic displacements, also computed by the FEM/BEM model. It is further shown that guided conditions in the gap between the transducer and the busbar occur for a frequency range above resonance. This result is in agreement with measurements showing that resonators with smaller gaps exhibit smaller spurious responses in their admittance.

FEM/BEM analysis of infinite periodic grating covered with an SiO 2 overlay

In order to improve the temperature stability of RF SAW devices, it is worth adding a layer of SiO2 on the surface of the piezoelectric substrate. This paper presents an original method, which allows the computation of the Harmonic Admittance of an infinite periodic grating covered with an SiO2 overlay. The origins of this method can be found in a previous paper. By comparison with S. Ballandras and K. Hashimoto works, this methods allows to take into account the very efficient Chebyshev-like charge distribution, together with a classical FEM basis interpolation for the stresses. FEM is used to model mechanical behavior of complex shape electrodes and BEM using a periodic Greens function Kernel is used to take into account the semi-infinite piezoelectric substrate. Validations are presented by comparison with K. Hashimotos software.