Fabrication techniques for tunable surface acoustic wave resonators on giant ΔE layers

Cellular multiband phones require a multitude of different filter components. For higher integration and a reduction of the number of components a preferable alternative to standard SAW (surface acoustic wave) filters would be one single tunable filter for Rx and Tx band, respectively. Up to now only tunable SAW delay lines changing the delay time or phase have been reported. However, the center frequency of these devices remained unchanged. We demonstrate the tuning of the center frequency of a one-port resonator as a basic filter element, by changing the propagation velocity underneath the IDT. This is achieved by using a multi layer structure exhibiting ΔE effect. By an external magnetic field the elastic stiffness is varied and hence the propagation velocity. Both, a layer by layer deposition and a sacrificial layer technique are used to realize first samples. The basic setup is formed by a piezoelectric ZnO-layer on an amorphous magnetostrictive layer of FeCoSiB. By using the conventional layer by layer deposition technique a crystallization of the amorphous layer occurs, as demonstrated by several analytic methods. The increase in crystallinity of the magnetostrictive layer causes a decrease in the magnitude of giant ΔE effect and thus a reduction of tunability. This effect is avoided using the sacrificial layer technique, which additionally leads to a higher c-axis orientation of the ZnO. Although samples realized with conventional technique yield a tuning range already better than values reported so far, samples realized with the sacrificial layer technique exceed the first method by a factor of five in the tuning range.