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

Lead-free piezoelectric (K, Na)NbO
3
-CaTiO
3
(KNN-CT) films were prepared on Pt/Ti/SiO
2
/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 CaTiO
3
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
)NbO
3
-nCaTiO
3
(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 |e
31
*|=|d
31
|/s
11,p
>6.0C/m
²
(where s
11,p
is elastic compliance of the KNN-CT films), whereas n=0.000 |e
31
*|<5.0C/m
²
. In XRD analysis, (002) peak angle of these pseudocubic perovskite films increased with increase of n. From these results, we conclude that CaTiO
3
was successfully substituted as part of KNN and piezoelectric constant increased by effect of CaTiO
3
. In addition, the KNN-CT films with x=0.587–0.617 and n=0.027-0.053 exhibited higher |e
31
*| than any other x and n. Moreover, by 750°C post-annealing process, |e
31
*| of the KNN-CT film with x=0.587 and n=0.040 was enhanced and the highest |e
31
*|=11.7C/m
²
was confirmed. This result of our experiment shows one of highest piezoelectric coefficient for lead free piezoelectric films.