Improvement of Piezoelectric Properties of (K,Na)NbO3 Films Deposited by Sputtering

(K,Na)NbO₃ (KNN) films with high transverse piezoelectric coefficients were successfully deposited onto Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structures with preferential <001 > orientation. To improve their piezoelectric properties, we investigated the effects of annealing after the deposition and the Na/(K+ Na) ratio of the films. Annealing in air at 750 °C led to a decrease in the residual strain in the KNN crystal and the disappearance of openings at the grain boundary, thereby improving the transverse piezoelectric coefficient and leakage current properties. We also investigated the transverse piezoelectric coefficient and dielectric constant as a function of the Na/(K+ Na) ratio; both had maximum values at a ratio of approximately 0.55. For the KNN films, e₃₁^* ranged between -10.0 and -14.4 C/m²; thus, it was superior to previously reported values for lead-free piezoelectric films and was comparable to the best commercially available Pb(Zr,Ti)O₃ films.