Processing - structure - property interrelationships of ferroelectric thin films with emphasis on formation kinetics

Lead zirconate titanate (PZT) is a ferroelectric material which has many interesting properties. Recently, PZT thin films have been considered as one of the most promising materials for the application of nonvolatile electronic memories. In this study, a sol-gel process for PZT film preparation was adopted and greatly modified. PZT films with very desirable electrical properties have been successfully prepared by this modified sol-gel process.

One of the problems of incorporating PZT films into the DRAM devices is the need of high post-deposition annealing temperatures which complicates their integration into the existing semiconductor manufacturing process. In this work, formation kinetics of PZT films were studied and the nucleation was found to be the rate-limiting step in the formation of the perovskite phase. Based on this finding, a seeding process was invented to encourage the nucleation of the perovskite phase. As a result of this seeding process, the transformation temperature has been lowered by as much as 100°C. The seeded PZT films also have good ferroelectric properties.

The ferroelectric domain structures, and the metastable pyrochlore phase including its transformation to the perovskite phase have been investigated by transmission electron microscopy (TEM). The domain structures of the PZT films had the {lID} <110> orientation and most of them were 90° domains. The TEM study of the pyrochlore to perovskite transformation provides valuable insight on the formation of the perovskite phase.

Among all the processing steps, the drying process of the sol-gel films created the highest growth stress. In addition, the thin film stress study was also used to determine the transformation stress and Curie temperature.

The effects of composition, thermal processing conditions, and film thickness on electrical properties have been studied. Some of the notable results are as follow: (1) PZT films with a Zr/Ti ratio of 53/47, the morphotropic boundary (MPB) composition, have the highest remanent polarization and the lowest coercive field. (2) The optimum annealing temperatures for most of the PZT compositions are found to be about 50°C higher than the completion temperature of the perovskite formation (T_c^per) of the same composition. (3) PZT films with film thicknesses greater than or equal to 170 nm have electrical properties very close to those of the thicker films and are not susceptible to dielectric breakdown at an applied voltage of 5 V.