Piezoelectric thin film AlN has great potential for on-chip devices such as thin-film resonator (TFR)-based bandpass filters. The AlN electromechanical coupling constant, K(2), is an important material parameter that determines the maximum possible bandwidth for bandpass filters. Using a previously published extraction technique, the bulk c-axis electromechanical coupling constant was measured as a function of the AlN X-ray diffraction rocking curve [full width at half maximum (FWHM)]. For FWHM values of less than approximately 4 degrees , K (2) saturates at approximately 6.5%, equivalent to the value for epitaxial AlN. For FWHM values >4 degrees , K(2) gradually decreases to approximately 2.5% at a FWHM of 7.5 degrees . These results indicate that the maximum possible bandwidth for TFR-based bandpass filters using polycrystalline AlN is approximately 80 MHz and that, for 60-MHz bandwidth PCS applications, an AlN film quality of >5.5 degrees FWHM is required.
This paper compares three different piston mode designs for temperature-compensated surface acoustic wave (TC-SAW) resonators using SiO2/LiNbO3 structure. It was shown that in rough approximation, phase shift given by extra elements for the piston mode operation is determined by their total mass. Thus, the hammer head design without additional metal layers does not work properly when the SiO2 layer is thick due to insufficient mass. On the other hand, piston mode designs using metal dots or stripes is effective to suppress the transverse mode resonances even when the SiO2 layer is thick. Although larger metal thickness is preferable for the wideband operation, it also makes the split of main resonance. Thus, the optimal metal thickness can be found from this trade off, and then the optimal metal width can be found to achieve good transverse mode suppression.
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