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, K2, 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°, K2 saturates at approximately 6.5%, equivalent to the value for epitaxial AlN. For FWHM values >4°, K2 gradually decreases to approximately 2.5% at a FWHM of 7.5°. 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° FWHM is required.
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