Measurements of the bulk, C-axis electromechanical coupling constant as a function of AlN film quality

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/sup 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/spl deg/, K/sup 2/ saturates at approximately 6.5%, equivalent to the value for epitaxial AlN. For FWHM values >4/spl deg/, K/sup 2/ gradually decreases to approximately 2.5% at a FWHM of 7.5/spl deg/. 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/spl deg/ FWHM is required.