KNN lead-free piezoelectric films grown by sputtering

Abstract

Potassium sodium niobate [(K,Na)NbO3, KNN] films are promising lead-free piezoelectric materials for microelectromechanical systems (MEMS) devices. We previously developed technologies for forming high-quality KNN films by sputtering, which showed excellent piezoelectric properties comparable to lead zirconate titanate (PZT) films. In the present study, we addressed several challenges with the aim of introducing KNN films into commercialized MEMS devices. First, we optimized the dielectric and piezoelectric properties to realize a suitable performance for actuator and sensor devices. The sensor-type KNN films had a low dielectric constant (248) and a self-poling function, which are greatly beneficial for sensor device performance and the fabrication process. The actuator-type KNN films had a very high e31 value of −13.5 C/m2, which is almost comparable to the top level of PZT films. Next, we found that the DC stress lifetime of KNN films strongly depended on the material used for the adhesion layer of the top electrode. When we used a Pt/RuO2 top electrode, the actuator-type KNN films had a long lifetime (>130 000 s, 300 kV/cm and 200 °C), which is long enough to be used for commercialized MEMS devices. Furthermore, we realized 8-in. KNN wafers with good thickness uniformity across the wafer (±3.5%) by introducing a mass-production-ready sputtering tool: the EB-2500 produced by Canon Anelva. In the near future, these promising results will open the way to replace PZT with KNN in the piezoMEMS industry.