Interface between electrode and PZT memory device Original Research Article

PbZrTiO3 (PZT) is one of the most promising ferroelectric materials for ferroelectric random access memory (FeRAM). But, there is a problem that the PZT ferroelectric properties are degraded with platinum (Pt) electrodes after annealing in a hydrogenous atmosphere during LSI fabrication. This degradation has been attributed to the catalytic nature of Pt, which dissociates H2 into protons which then migrate into the PZT and reduce it at elevated temperature. In this theory, it is unclear how Pt and PZT physically interact at the interface. Also, the mechanism concerning the generation of an interfacial layer of oxygen vacancies has not been addressed, which creates some ambiguity in the model. We researched the interface using secondary ion mass spectroscopy (SIMS) and field-emission type transmission electron microscopy (FE-TEM) in order to more clearly understand its impact on the degradation mechanism. We have verified atomically that annealing a Pt/PZT/Pt capacitor results in Pb diffusion from the PZT into the Pt electrode and Pt diffusion from the electrode into the PZT. We next verified that the interdiffusion is not a usual thermal interdiffusion process, but appears rather to be generated by the reaction with hydrogen on both top and bottom Pt electrodes. Finally, we present a model of how the effects of hydrogen reduction combine with the Pt catalysis to form oxygen vacancies. Lead and Pt interdiffuse easily through these vacancies at both interfaces. The model presented here can predict the distribution of vacancies and demonstrates the limitations of the recovery anneal. It also supplements our understanding of the degradation process and provides additional credibility to the above theory.