The Research Of Fatigue Mechanism Of PZT Ferroelectric Thin Film Used In Memory

Ferroelectric random access memories(FRAM) are currently one of the leading candidates for the next generation memory devices resulting from its favorable characteristics. But the degradation properties including fatigue, retention loss and imprint are important reliability issues which seriously impede the development and large-scale commercial use of FRAM. The three issues have inner link of microstructure. The mainstream view is that these failure mechanisms are greatly related with the oxygen vacancy.By observing the phenomenon during fatigue process of the PZT ferroelectric thin films and to do researches and analyze about the film's failure mechanism of fatigue, then try to propose a feasible way to improve the ability of anti-fatigue.(1) An extended sin2? method to analyze the residual stresses in PZT thin films with the standard XRD spectrum is proposed on the basis of the traditional method. The innovation lies in that it provides a feasible way to determine the residual stress from the sin2? method based on the normal X-ray incidence measurement, and it doesn't need the measurement of the spacing of a selected set of planes under different tilting angles inclined X-ray incidence ?. The reliability of the new method is confirmed by the comparison with the conventional sin2? method. The remnant polarization and coercive voltage were found to present regular changes in the initial 10 s, independent on the asymmetry or frequency of switching waves. We attributed the change to the relaxation of stress in the film and identified a coercive voltage V0 of 0.6V for the stress-free film.(2) By the analysis of leakage current under different range of voltage, it is found the conduction mechanism under small voltage is ohmic conduction mechanism which is caused by the drifting of carriers, the increase of leakage current under small voltage as a function of fatigue cycles, and it verifies the increase of oxygen vacancy concentration during fatigue process.(3) The variation of remnant polarization and coercive voltage dependent on the asymmetric square waves with different degrees and symmetric square wave with different frequencies. And by analyzing the behavior of the remnant polarization and coercive field, the asymmetric square wave with higher degree and symmetric square wave with lower frequency will accelerate the fatigue speed of film, owing to the high speed of oxygen vacancy generation. While applying asymmetric square wave to the film as fatigue stress, the oxygen vacancies will drift towards the interface under the applied voltage and this will result in different accumulation of oxygen vacancy concentration, which leads to a difference between the retention ability of positive and negative polarization. The increase of oxygen vacancy concentration will lead to a growing depolarization field, and then result in a worse polarization retention ability.(4) By analyzing the thermal effect caused by fatigue stress, we propose a feasible way to improve the ability of anti-fatigue by enhancing radiating.