Measurements of the Bulk, c-axis Electromechanical Coupling Constant as a Function of the AIN Film Quality

Piezoelectric thin-film AIN has great potential for on-chip devices such as Thin-Film Resonator (TER) based bandpass filters. The AIN electromechanical coupling constant, K sup 2, is an important material parameter which 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 AIN X-Ray Diffraction Rocking Curve (FWHM). For FWHM values of less than approximately 4 degrees, K sup 2 saturates at approximately 6.5%, equivalent to the value for epitaxial AIN> For FWHM values greater than 4 degrees, K sup 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 AIN is approximately 80MHz, and that for 60 MHz bandwidth PCS applications, an AIN film quality of better than 5.5 degrees FWHM is required.