Effect of sol infiltrations on electrical properties of PZT

Thin PZT films with a thickness of approximately 1 μm can readily be fabricated using a layered sol-gel deposition technique. The maximum thickness obtainable is limited by the time taken and the tendency of the films to crack and spall when many layers are deposited. Thicker layers may be obtained by depositing a powder-sol slurry whereby a PZT powder is mixed with a sol of approximately the same composition Using this layered slurry deposition technique it is possible to obtain films with a thickness in excess of 20 μm. The resulting films, however, are often porous, leading to poor properties and making subsequent patterning difficult. A technique for increasing the density of such films through the use of controlled heat treatments and sol infiltration is presented. It is shown that with increased levels of sol infiltration the density and dielectric constant of the films are maximised. Measurements of piezoelectric properties indicate that sol infiltrations have no significant effect on d₃₃. A sample with approximately 10% closed porosity was obtained following four infiltration steps per layer. This resulted in a maximum dielectric constant of approximately 700 and a d₃₃ of 70 pC N^-1 (poling conditions: 8 V μm^-1 for 5 min at 200°C). Examination of cross-sections of the films produced shows that for intermediate levels of sol infiltration (typically between 1 and 3) a density gradient is obtained with higher densities observed nearer the base of the film. It is postulated that the observed density gradient is a result of continued infiltration from the upper layers when further layers of slurry are deposited. The effects of changing process variables such as the number of infiltration steps and firing temperature on film structure and properties are discussed.