1. Field of the Invention
The invention relates to the field of circuits and microsystems having one or more piezoelectric and/or acoustic resonators.
2. Description of the Related Art
Such circuits may be used as filters in mobile telephony applications. A noise-contaminated signal is applied to the upper electrode. A filtered signal centered on the resonance frequency of the resonator is retrieved on the lower electrode. This frequency is determined directly by the thickness of a piezoelectric film included between the two electrodes. A combination of resonators can be used to form a filter function. The resonator is used as a high-Q filter element.
The resonators are securely attached to the integrated circuit while being acoustically or mechanically isolated with respect to the latter. To this end, a support capable of providing such isolation may be envisaged. The support may comprise, alternately, high acoustic impedance layers and low acoustic impedance layers. The support may comprise a suspended membrane.
The actual resonator comprises an active element formed of piezoelectric material placed between two electrodes. The resonance frequency of the resonator depends essentially on the thickness of the piezoelectric material, on its mechanical properties and on the mechanical properties of the electrodes. Resonators may be interconnected in various configurations to produce filters. The center frequency of such combinations depends on the resonance frequency of each of the resonators forming them.
The U.S. Document 2002/0089395 describes resonators mounted on one and the same substrate and having different resonance frequencies due to different thicknesses of the piezoelectric material of the active elements.
A need is apparent for resonators mounted on the same substrate and exhibiting resonance frequencies that are markedly different, for example 900 MHz and 1800 MHz.
Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above.