It is well known that PZT material properties are strongly dependent on the Zr/Ti ratio. The reliability characteristics, such as the retention and imprint properties, of PZT thin film correlate with the reliability of FRAM(R). PZT films with various Zr/Ti ratios, 60/40, 52/48, 45/55, 40/60 and 30/70 were prepared on Pt/IrO2 and Pt/Ti electrodes by a sol-gel process. With lower Zr/Ti ratio, the grain size becomes smaller and the film was highly oriented to (111) crystallographic plane. But, pyrochlore phase in the PZT films on Pt/IrO2 electrode was detected by SEM and XRD. Hysteresis and pulse responses were measured on the capacitors. With lower Zr/Ti ratio, Pr and Vc become larger. It was found that the preferential (111) orientation played an important role in determining Pr. Voltage shifts which are related to imprint are dependent on the Zr/Ti compositional ratio. Increasing the Ti concentration causes the voltage shift to increase due to more oxygen vacancies. But, this tendency was not in accordance with the results of Qos which were measured by capacitor test simulation of imprint properties for FRAM operation, because the results of Qos did not change monotonically with Ti concentration. However, the capacitor for 45/55 film grown on Pt/IrO2 had good imprint property, similar to the capacitor for sputtered PLZT. In addition, H2 degradation of PZT capacitor with Pt/IrO2 was studied. It was found that H2 annealing degraded the PZT capacitors even at temperatures as low as 150. Thus, imprint and H2 degradation are serious problems for PZT capacitors and these will be a key issue in the reliability of FRAM.

A behavioral model for ferroelectric capacitors is developed. There are two requirements for the circuit simulation model; one is to reproduce the hysteretic behavior of the polarization under arbitrary voltage history, and the other is to describe the time dependence of polarization change. A parallel element model has been proposed to meet the first requirement. This model reproduces the minor loops of the hysteresis by assuming that the ferroelectric capacitor consists of the parallel capacitors of different polarization and coercive voltages. In order to add the function to describe the time dependence of the polarization change, we propose a method of measuring the switching response for individual parallel elements and the model which describes the response. In the measurement, the voltage applied to the capacitor is raised in two steps. After the first step, the voltage is kept at an intermediate level for a period of time, then raised again to the final level and the polarization change was recorded as a function of time. Because the capacitor elements with the coercive voltage lower than the intermediate level complete switching during the first step, the polarization change of the whole capacitor during the second step is attributed to the capacitor elements with the coercive voltage higher than the intermediate level. This procedure is repeated with changing the intermediate level, and the switching response of each capacitor element is obtained by taking the finite differences between the adjacent sets of data. The measurement on a sol-gel derived SrBi2Ta2O9 capacitor revealed that the switching time depended only on the difference between the applied voltage and the coercive voltage of each capacitor element. The time dependence of the polarization change is implemented to the model by inserting a nonlinear resistor in series with each capacitor, which reproduces the polarization switching under arbitrary voltage change without any fitting parameters.