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

Abstract

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. #

Microfabrication of lead-free (K, Na)NbO<inf>3</inf> piezoelectric thin films by dry etching

“…The calculated values of d 31 are -110 pm/V, which is almost independent of applied voltage. This value is approximately consistent with previous report (d 31 = -96.3 ~ -138.2 pm/V)[7].…”

“…Although Ba or Bi-based materials (such as BaTiO 3 , (Bi,Na)TiO 3 , (Bi,K)TiO 3 and their solutions) have been widely investigated as an alternative materials [5,6], piezoelectric coefficients of their thin films are still far lower than those of PZT-based thin films. On the other hand, we deposited KNN thin films with a preferential <001>-orientation on Si substrates by RF magnetron sputtering, and found out that their piezoelectric coefficients d 31 were comparable to those of PZT thin films [7]. In order to realize a variety of lead-free piezoelectric MEMS applications, practical microfabrication method of KNN thin films should be established.…”

In Situ XRD Observation of Crystal Deformation of Piezoelectric (K,Na)NbO3 Thin Films

“…The detailed deposition conditions have been described elsewhere. 17 The film thickness was 2 μm. Before the in situ XRD measurements, we confirmed the crystal structures by conventional XRD using a fouraxis diffractometer with Cu Kα radiation source (Rigaku, SmartLab).…”

“…The films exhibited a polycrystalline and perovskite structure with a preferential (001) orientation in the out-of-plane direction (see Figure S1, Supporting Information), as reported previously. 15,17 The samples were cut into rectangular cantilever beams to determine the transverse piezoelectric coefficients (e 31,f ) by the cantilever method, which we used to evaluate both the direct and converse piezoelectric effects. The converse piezoelectric coefficient was determined by measuring the cantilever displacement induced by an applied electric field.…”

“…Numerous researchers have reported that KNN thin films exhibit strong piezoelectric properties, and piezoelectric energy harvesters with KNN thin films have an output power comparable to that of harvesters with PZT thin films. 17,18 However, the origin of the strong piezoelectric properties of KNN thin films remains unclear because crystallographic deformation by piezoelectric effects is not yet understood in detail. Recent progress with in situ X-ray diffraction (XRD) measurements has enabled us to observe directly how the crystal structures of ferroelectric thin films change under an applied electric field.…”