Effect of Lattice Strain and Improvement of the Piezoelectric Properties of (K,Na)NbO3 Lead-Free Film

(K,Na)NbO3 (KNN) films with very high transverse piezoelectric coefficient d(31), which attained values comparable to those of Pb(Zr,Ti)O-3 (PZT) films for the first time, were successfully deposited on Pt/Ti/SiO2/Si phi 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)(similar to-100 pm/V) that can be uniformized on phi 4-in. substrates and with a standard deviation that decreases to about 4 pm/V. (C) 2010 The Japan Society of Applied Physics