Preparation of (K, Na)NbO3-CaTiO3 film by RF magnetron sputtering

Lead-free piezoelectric (K, Na)NbO₃-CaTiO₃ (KNN-CT) films were prepared on Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering. Co-sputtering using a number of KNN-CT targets which vary in compositions of Na/(K+Na) ratio and amount of CaTiO₃ provided variations in compositions of KNN-CT films. The obtained KNN-CT films had compositions corresponding to the average compositions of the used targets represented by chemical formula of (1-n)(K_1-x, Na_x)NbO₃-nCaTiO₃ (x=0.460-0.617, n=0.000-0.065) and the compositional dependencies of their piezoelectric properties were studied. Most KNN-CT films with n>0.000 exhibited relatively high transverse piezoelectric coefficients |e₃₁ *|=|d₃₁|/s_11,p>6.0C/m² (where s_11,p is elastic compliance of the KNN-CT films), whereas n=0.000 |e₃₁ *|<;5.0C/m². In XRD analysis, (002) peak angle of these pseudocubic perovskite films increased with increase of n. From these results, we conclude that CaTiO₃ was successfully substituted as part of KNN and piezoelectric constant increased by effect of CaTiO₃. In addition, the KNN-CT films with x=0.587-0.617 and n=0.027-0.053 exhibited higher |e₃₁ *| than any other x and n. Moreover, by 750°C post-annealing process, |e₃₁ *| of the KNN-CT film with x=0.587 and n=0.040 was enhanced and the highest |e₃₁ *|=11.7C/m² was confirmed. This result of our experiment shows one of highest piezoelectric coefficient for lead free piezoelectric films.