Effect of Zr/Ti Ratio on the Reliability Characteristics Behavior of Sol-Gel Derived PZT Films on Pt/IrO~2 Electrode

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 FRAME. 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 bound 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, H-2 degradation of PZT capacitor with Pt/IrO2 was studied. It was found that H-2 annealing degraded the PZT capacitors even at temperatures as low as 150 degrees C. Thus, imprint and H-2 degradation are serious problems for PZT capacitors and these will be a key issue in the reliability of FRAM.