Kazufumi Suenaga

Curie Temperature, Biaxial Elastic Modulus, and Thermal Expansion Coefficient of (K,Na)NbO3 Piezoelectric Thin Films

We investigated fundamental parameters such as the Curie temperature, biaxial elastic modulus, and thermal expansion coefficient of (K,Na)NbO3 (KNN) films on Pt/Ti/SiO2/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 8.0×10-6 (1/°C), respectively. The fundamental properties of the KNN films were similar to those of widely used Pb(Zr,Ti)O3 films, indicating that KNN films are potential candidates for lead-free piezoelectric thin films in micro-electro-mechanical system (MEMS) applications.

Lead-Free Piezoelectric MEMS Energy Harvesters of (K,Na)NbO3 Thin Films on Stainless Steel Cantilevers

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 e31f and relative dielectric constant er 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.

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

(K,Na)NbO3 (KNN) films with high transverse piezoelectric coefficients were successfully deposited onto Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structures with preferential orientation. To improve their piezoelectric properties, we investigated the effects of annealing after the deposition and the Na/(K+ Na) 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/(K+ Na) ratio; both had maximum values at a ratio of approximately 0.55. For the KNN films, e31* ranged between -10.0 and -14.4 C/m2; thus, it was superior to previously reported values for lead-free piezoelectric films and was comparable to the best commercially available Pb(Zr,Ti)O3 films.

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 d31, 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 4-in. substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structure with a preferred orientation. Furthermore, we focused on the effect of lattice strain on d31 of KNN to clarify the relationship between the piezoelectric properties and structural parameters apart from orientation. We found that -d31 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 d31 (~-100 pm/V) that can be uniformized on 4-in. substrates and with a standard deviation that decreases to about 4 pm/V.

Effect of grain size on degradation of Pt/PLZT/Pt capacitor

Abstract The effect of grain size on electric properties has been investigated. Various (111) oriented PLZT films were prepared using the RF magnetron sputtering method on Pt/Ti/SiO2/Si substrates. Grain size and surface roughness were observed by AFM. The results showed that; 1) surface roughness depends on average grain size and 2) decreasing grain size decreases process degradation and fatigue. The grain boundary was thought to be responsible for process damage, such as reduction by hydrogen, at electrode interface tops. Electric field strength at grain boundaries also increased with increases in surface roughness.

Evaluation of Crystal Orientation for (K,Na)NbO3 Films Using X-ray Diffraction Reciprocal Space Map and Relationship between Crystal Orientation and Piezoelectric Coefficient

We have found an effective method for the evaluation of the crystal orientation of (K,Na)NbO3 (KNN) films in the (K,Na)NbO3/Pt/Ti/SiO2/Si structure using X-ray diffraction (XRD) reciprocal space maps. Previously, the crystal structure and orientation of such (K,Na)NbO3 films were evaluated using 2θ/θ XRD, and were considered to be the pseudocubic perovskite structure with preferential (001) orientation and no (111) orientation. Here, we applied the new method using XRD reciprocal space maps, and discovered that the (K,Na)NbO3 films had some degree of KNN(111) orientation. We calculated the KNN(001)- and KNN(111)-orientation volume fractions for the (K,Na)NbO3 films from the (101) diffraction peaks originating from the KNN(001)- and KNN(111)-orientation elements in the XRD reciprocal space maps, considering the calibration factors obtained from pole-figure simulations, and examined the relationship between the crystal orientation and d31 piezoelectric coefficient in the (K,Na)NbO3 films. The results indicated that the d31 piezoelectric coefficient increases with increasing (001)-orientation volume fraction.

Degradation of Pt/PLZT/Pt capacitors caused by hydrogen in interlayer dielectrics

Abstract A correlation between ferroelectric properties of the PLZT capacitors and amount of H2 and H2O gasses desorbed from interlayer dielectrics were investigated quantitatively. H2 and H2O gasses desorbed from interlayer dioxides were analyzed using thermal desorption spectroscopy. Polarization charges and its aging characteristics of memory array capacitors with area of 2 μm squares did not depend on amount of desorbed H2 explicitly, but strongly on H2O desorption. It is considered that silanol and hydroxyalkyl groups worked as hydrogen donor with catalytic activities of Pt top electrodes. As a result, hydrogen atoms should work as a major degradation agent for the imprint phenomena of memory array capacitors. Precise control of hydroxyl groups contained in the interlayer dielectrics and passivation films resulted in very small retention degradation caused by imprint phenomena, and realized high reliability and high density FRAM technologies.