Laser Ablation of PZT Films for Use in Bimorph Actuator Structures
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LASER ABLATION OF PZT FILMS FOR USE IN BIMORPH ACTUATOR STRUCTURES
JOHANNES FRANTI'I, H. MOILANEN, S. LEPPAVUORI AND A. UUSIMAKI University of Oulu, Microelectronics and Material Physics Laboratories, SF-90570 Oulu, Finland
ABSTRACT Laser ablation has been used to produce piezoelectric films for use in a low voltage cantilever bimorph actuator structure. Nd doped lead zirconium titanate (PZT) films were deposited by a pulsed Q-switched Nd:YAG laser (wavelength 1064 nm) on to unheated silver-coated oxidised silicon substrates at a distance of 30 mm between substrate and target. Growth rates were typically 20 A/s. The as-deposited PZT films were amorphous but contained small microcrystalline pyrochlore and perovskite regions. Optimisation of the annealing conditions resulted in perovskite as the main phase (T=750 'C, t=2 h). A special fabrication technique (PbO covered PZT) was used to minimise film defects mainly caused by large particles. In order to improve the density of the film and deposition rates of the ablated PZT film, ablations using an XeCl excimer laser (wavelength 308 nm) under the influence of a static magnetic field were also studied. The decreased particle size in the deposited films showed that the surface quality of the films fabricated by the excimer laser was better than that of films deposited by the Nd:YAG laser. Films were studied using EDS, XRD and also micro-Raman spectrometry. Bimorph cantilever element structures were realised using Nd:YAG laser ablation. DC bending characteristics of these low voltage cantilever bimorphs were measured as a function of voltage using a Michelson interferometer. Displacements in the order of microns were obtained from a 15 mm x 4 mm x 80 ýtm element with a driving voltage level of 10 V.
INTRODUCTION In practice, two types of actuators are commonly used: ceramic plate stack and bimorph element. The bimorph element is composed of two active piezoelectric ceramic layers. Operation of the actuator is based on an unstiffened piezoelectric effect, where elongation of the ceramic material perpendicular to the poling axis is utilised. The displacements reached using this kind of component are high compared to those of a stack actuator, but on the other hand, maximum force and operation frequency may be two decades lower. Lead zirconium titanate (PZT) material is widely used in actuator applications'. 2. It has a perovskite structure formed by solid solution of the ferroelectric PbTiO 3 (PT) and antiferroelectric PbZrO 3 . Bimorph structures have the advantage of having the piezoelectric properties of poled PZT. Usually, the ratio of the titanium and zirconium atom % is 48:52, a composition which is near to the morphotropic phase boundary, where the piezoelectric effect is strong 3. In these experiments, the PZT was Nd doped, and the ratio of the titanium and zirconium atom % was 45:55. The atom % ratio of the lead and neodymium was 97:2. The structure of the PbTiO 3 is a tetragonal perovskite and that of the PbZrO 3 orthorhombic structure at room temperature, and
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