Influence of AlN quality on the transverse and longitudinal coupling coefficients of acoustic devices

The transverse and longitudinal piezoelectric response of AlN polycrystalline thin films is analyzed by means of measurements in surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices fabricated on films of identical characteristics. The crystal quality of the films is correlated with the measured coupling factors; it is found that the presence of grains with the c-axis tilted with respect to the substrate normal is detrimental to the piezoelectric response of AlN. However, the presence of tilted crystal affects differently the longitudinal and transverse response of the devices, degrading more severely the latter as the amount of tilted grains increases. The presence of such defects leads to the destruction of the mechanical coherence in the transverse deformation when the electric field parallel to the surface is applied.