K,Na)NbO 3 (KNN) films with high transverse piezoelectric coefficients were successfully deposited onto Pt/Ti/SiO 2 /Si substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structures with preferential h001i orientation. To improve their piezoelectric properties, we investigated the effects of annealing after the deposition and the Na=ðK þ NaÞ 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=ðK þ NaÞ ratio; both had maximum values at a ratio of approximately 0.55. For the KNN films, e à 31 ranged between À10:0 and À14:4 C/m 2 ; thus, it was superior to previously reported values for lead-free piezoelectric films and was comparable to the best commercially available Pb(Zr,Ti)O 3 films. #

We investigated fundamental parameters such as the Curie temperature, biaxial elastic modulus, and thermal expansion coefficient of (K,Na)NbO 3 (KNN) films on Pt/Ti/SiO 2 /Si substrates. From the temperature dependence of the dielectric constant of the KNN films, their Curie temperature was approximately 360 C; this value was confirmed from critical changes in the lattice parameters of the films and the stress induced in the films at approximately this temperature. By using the optical lever method, the biaxial elastic modulus and thermal expansion coefficient of the KNN films were found to be 92 GPa and 8:0 Â 10 À6 (1/ C), respectively. The fundamental properties of the KNN films were similar to those of widely used Pb(Zr,Ti)O 3 films, indicating that KNN films are potential candidates for lead-free piezoelectric thin films in micro-electro-mechanical system (MEMS) applications.

We fabricated piezoelectric MEMS energy harvesters (EHs) of lead-free (K,Na)NbO3 (KNN) thin films on microfabricated stainless steel cantilevers. The use of metal substrates makes it possible to fabricate thin cantilevers owing to a large fracture toughness compared with Si substrates. KNN films were directly deposited onto Pt-coated stainless steel cantilevers by rf-magnetron sputtering, thereby simplifying the fabrication process of the EHs. From XRD measurement, we confirmed that the KNN films on Pt-coated stainless steel cantilevers had a perovskite structure with a preferential (001) orientation. The transverse piezoelectric coefficient e 31f and relative dielectric constant εr were measured to be -3.8 C/m2 and 409, respectively. From the evaluation of the power generation performance of a KNN thin-film EH (length: 7.5 mm, width: 5.0 mm, weight of tip mass: 25 mg), we obtained a large average output power of 1.6 µW under vibration at 393 Hz and 10 m/s2.

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(K,Na)NbO3 (KNN) films with very high transverse piezoelectric coefficient d 31, which attained values comparable to those of Pb(Zr,Ti)O3 (PZT) films for the first time, were successfully deposited on Pt/Ti/SiO2/Si φ 4-in. substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structure with a <001> preferred orientation. Furthermore, we focused on the effect of lattice strain on d 31 of KNN to clarify the relationship between the piezoelectric properties and structural parameters apart from orientation. We found that -d 31 increases with decreasing lattice strain c/a ratio of KNN when the in-plane lattice parameter a increases and the out-of-plane lattice parameter c decreases. By controlling the lattice strain c/a ratio strictly and maintaining a homogeneous strain, we achieved a high d 31 (∼-100 pm/V) that can be uniformized on φ 4-in. substrates and with a standard deviation that decreases to about 4 pm/V.