Effect of film thickness on piezoelectric vibrator using hydrothermally synthesized epitaxial (K_0.88Na_0.12)NbO₃ film

Two types of vibrators using hydrothermally synthesized ferroelectric (K, Na)NbO₃ films were fabricated on SrTiO₃ substrates. Cantilever piezoelectric vibrators with film thicknesses of 3.5-22.3 μm were used under low-frequency and no resonance drive. The obtained output displacement increased almost linearly as the film thickness increased under both unipolar and bipolar drives. The obtained transverse piezoelectric coefficient, e _31,f, was almost independent of film thickness, ≈−5 C m⁻². On the other hand, a longitudinal piezoelectric vibrator was used under the resonance of half-length longitudinal vibration. The electromechanical coupling factor, k _eff², and the mechanical quality factor, Q _m, were evaluated. k _eff² and Q _m respectively increased from 0.14% to 1.23% and decreased from 556 to 224 as the film thickness increased from 2.5 to 26 μm. By applying a large voltage of 50 V_0-p, a large vibration velocity of 1.5 m s⁻¹ was obtained for the vibrator with a film thickness of 26 μm.