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Concept
Thin-film bulk acoustic resonator
Summary
A thin-film bulk acoustic resonator (FBAR or TFBAR) is a device consisting of a piezoelectric material manufactured by thin film methods between two conductive – typically metallic – electrodes and acoustically isolated from the surrounding medium. The operation is based on the piezoelectricity of the piezolayer between the electrodes.
FBAR devices using piezoelectric films with thicknesses ranging from several micrometres down to tenths of micrometres resonate in the frequency range of 100 MHz to 20 GHz. FBAR or TFBAR resonators fall in the category of bulk acoustic resonators (BAW) and piezoelectric resonators and they are used in applications where high frequency, small size and weight is needed.
The crystallographic orientation of a thin film depends on the piezomaterial selected and many other items like the surface on which the film is grown and various manufacturing - thin film growth - conditions (temperatures selected, pressure, gases used, vacuum conditions etc.).
Any material like lead zirconate titanate (PZT) or barium strontium titanate (BST) from the list of piezoelectric materials could act as an active material in an FBAR. However two compound materials aluminium nitride (AlN) and zinc oxide (ZnO) are the two most studied piezoelectric materials manufactured for high frequency FBAR realisations. This is due to the fact that the properties like stoichiometry of two compound materials can be easier to control compared to three compound materials manufactured by thin film methods. For example it is known that thin film ZnO with C axis of the crystal structure (crystalline Z axis) normal to the substrate surface excites longitudinal (L) waves. Shear (transverse) (S) waves are excited if C axis of the film crystal structure is 41o tilted. It is also possible – depending on the crystal structure of the film – that both waves (L & S) are excited. Therefore the understanding and control of the crystal structure of the manufactured piezoelectric film is crucial for the operation of the FBAR.
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