Preparation And Properties Of PZT-based Double-layer Piezoelectric Films

With the development of miniaturization in electronic information systems(EIS), integration of circuit is put forward higher and higher requirements, but also promote the rapid development of integration in electronic thin films. Recently, the multilayer piezoelectric material is becoming the key point in the electronic material studies, due to the application in the high frequency of SAW devices, tunable capacitors and micro sensors and actuators. However, in order to fully realize the multi-functional of the complex piezoelectric films, the appropriate piezoelectric material is not only to be selected and integrated, the preparation process of the composite piezoelectric film is also necessary to be optimized.This dissertation mainly focuses on the preparation of bimorph structure based PZT layer, mainly the thin-?lm ZnO/PZT and AlN/PZT bimorph structure, and their fabrication, microstructures and properties were systematically studied. Simultaneously, the effects of thickness and oriention of PZT film on the structure and properties of the complex piezoelectric films were investigated. In addition, the influence of ZnO buffer layer on the structure and properties of AlN films was dicussed.1. The PZT films with compact structure, highly(111) preferred orientation, and excellent electrical properties were prepared by magnetron sputtering combined with rapid thermal annealing(RTA). First, the PZT films were prepared at substrate temperature of 200 °C, sputtering power of 170 W in argon and oxygen atmosphere with the gas flow ratio of Ar to O2 was 45/0.65 sccm; Then the PZT films were annealed at temperature of 650 °C and short annealing duration of 40 s in an infrared heated rapid heating system. With the annealing temperature increasing and annealing duration lengthening, the pyrochlore phase in PZT films could gradually translate to perovskite structure with(111) preferred orientation. However, when the sputtering power and substrate temperature increased, the PZT films showed(100) preferred orientation and high surface roughness. Meanwhile, the ferroelectric properties of the PZT films dropped significantly.2. ZnO films with highly c-axis orientation and excellent electric properties were successfully manufactured on(111)-orientated PZT layer by radio frequency reactive magnetron sputtering method. The effects of preparation and annealing parameters on structure and properties of ZnO films were discussed. The ZnO films showed a perfect(002) preferred orientation and dense, uniform microstructure, when prepared at substrate temperature of 600 °C, sputtering power of 110 W in argon and oxygen atmosphere with the gas flow ratio of Ar to O2 was 10/4 sccm. When the ZnO/PZT ?lms were annealed by RTA technologe, the ZnO films showed more dense microstructure and more excellent electric properties. The leakage current density and the piezoelectric coefficient were 10-8 A/cm2 and 8 pm/V, respectively. In addition, with the annealing temperature increasing, the residual stress in ZnO films could be relaxed and there was a transforming point of stress from press stress to strain stress during the post-annealing.3. AlN films with highly c-axis orientation and excellent electric properties were successfully manufactured on(111)-orientated PZT layer by DC reactive magnetron sputtering method. The leakage current density and the piezoelectric coefficient of the AlN/PZT films were 10-9 A/cm2 and 5 pm/V, respectively. The effects of preparation parameters on structure and properties of AlN films were investigated in detail. Results showed that the e sputtering power, gas flow ratio of Ar to N2, and sputtering pressure had significant effects on the structure composition and properties of AlN films. However, for the substrate tempreture, there is no significant impact on the orientation of AlN films, but the film crystal quality and residual stress. With the substrate tempreture increasing, the AlN films’ crystal quality was improved while the leakage current density dropped sharply. Simultaneously, the residual stress in AlN films first decreased then increased as increasing the substrate tempreture, there was a transforming point of stress from press stress to strain stress. This was also beneficial to relaxing the residual stress in AlN films.4. The dependence of structure and electric properties of ZnO、AlN films on PZT film thicknesses were investigated, the results showed that the PZT film thicknesses had a major impact on the crystal quality and electrical properties of the films. With the PZT film thicknesses increasing, the position of ZnO(002) and AlN(002) diffraction peak shifted observably to other 2θ angle. The ZnO films showed highest degree of(002) preferred orientation and crystal quality when the PZT film thickness was as thick as 330 nm, thus the leakage current density could maintain at a small value of 10-8 A/cm2. At present, the dielectric constant of the sample increased to 11.7, and the piezoelectric coefficient of the films was 8.4 pm/V. The AlN films had a perfect c-axis preferred orientation and better crystal structure when the PZT film thickness was 600 nm. The leakage current density of the AlN/PZT films decreased to10-9 A/cm2 and the value of dielectric constant was 16.3.5. The effects of PZT films with different orientation on the structure, micro morphology and electric properties of ZnO, AlN films were studied. The results indicted that only the ZnO, AlN films prepared on(111)-orientated PZT films had good quality and higher degree of(002) preferred orientation, though the PZT films with(100) and(110) orientation also had perfect crystal quality and surface topography. The values of FWHM of(002) rocking curve for the ZnO, AlN films were measured by X-ray ? scan was 2.8° and 3.7°, respectively. The values had a great decrease compared to the ZnO, AlN films that prepared on the PZT films with(100) and(110) orientation. Meanwhile, the best surface morphology appeared on the ZnO, AlN films, which had a low surface roughness and compact microstructure when prepared on the(111)-orientated PZT films. The ZnO, AlN films also exhibited excellent insulation performance when deposited on(111)-orientated PZT films, which had lowest leakage current density.6. Highly c-axis oriented AlN thin film was manufactured on Si(111) substrate using ZnO buffer layer by direct current(DC) magnetron sputtering method. The effects of the ZnO buffer layer on the structure and electric properties of AlN films were studied. The reasults showed that the AlN thin ?lms had a perfect c-axis preferred orientation and better crystal structure when using ZnO buffer layer. Morever, the electric properties of AlN films were improved remarkably. The leakage current density at 5 V bias voltages was 10-9 Acm-2. The value of the AlN films with ZnO buffer layer was 3 orders of magnitude lower than that without ZnO buffer layer. The piezoelectric coefficient of the AlN/ZnO films was 6 pm/V.