Filter Design Method Based On Thin Film Bulk Acoustic Resonator

Nowadays, more and more wireless applications are coming to our everyday lives.However, the spectrum resources are limited, and the most favorable bands have beenassigned to the mobile communications and government projects. The remaining ofspectrum bands are neither easy to use nor benefit to exploit. Under this challenge, theprimary difficulty we are confronted with is how to improve the isolation between theadjacent bands, which is the key to business success. The RF filter plays a crucial rolein the wireless communications with a main function of selecting the target band aswell as reducing the interference from the others. The frequency selectivecharacteristic of filter determines efficiency of the usage of band. In other words, thisfeature refers to whether the roll-off is good or not. In the actual design, there aremany complicated aspects, such as the material loss and the temperature sensitivity,should be taken into account carefully.Although the surface acoustic wave (SAW) filters based on the interdigitalstructures have been well developed to provide the RF signal filtering, at highoperation frequencies, for example, above2GHz, it becomes challenging to producesub-micro patterns of electrodes utilizing photolithography in SAW filters, and theproperties of the filter deteriorates substantially. Before FBAR devices appear, theengineers use dielectric filters, waveguide filters and LC filters as a foundation forsolutions of applications at2GHz. But there are various problems in these solutions,for instance, the ohm effect, the skin effect and the eddy current effects of metals inthe LC filter will contribute to a low quality factor, less than50, thus it cannot providea good frequency selective feature. With the Commercially available FBARpiezoelectric resonators have been successful in realizing RF filters, the excellentperformance of FBAR filters are able to fulfill the most of communication standards.This thesis starts from the filter networks, and analyzes the classic filter designmethod, then contrives to apply it into FBAR filter design. The basic building elementfor the filters is resonator. Accordingly, the following part of this paper derives asimple one-dimension electrical model and a complex one-dimension physical modelbased on the piezoelectric constitutive relations in order to simulate and predict theelectrical characteristics of the filter in the EDA software. At last, this thesis presents some significant FBAR filter topologies, and combines it with a series of experiments,following this process: spec analysis, picking up a suited topology, optimizing theparameters of resonators, and measuring and tuning the final products.