Transmission electron microscopy study of hydrogen-induced degradation in strontium bismuth tantalate thin films

The fabrication of nonvolatile ferroelectric random access memories based o n SrBi2Ta2O9 (SBT) or other ferroelectric capacitors require exposure of th ese capacitors to processing gases mixtures including hydrogen. This result s in a strong degradation of the capacitor electrical properties, mainly du e to the interaction of hydrogen with the ferroelectric layer. Using surfac e analysis methods, we previously determined that the hydrogen-induced degr adation of SBT capacitors might be partially due to the degradation of the near surface region of the SBT layer. It was also demonstrated that oxygen annealing after the hydrogen exposure results in the recovery of the degrad ed SBT surface layer and the electrical properties of the capacitor. We hav e now performed detailed cross sectional transmission electron microscopy s tudies of virgin, hydrogen, and oxygen annealed SBT/Pt/TiO2/SiO2/Si heteros tructures. These studies combined microstructural imaging with a nanoscale compositional analysis of the SBT layer as a function of depth from the fre e surface of the film and across grain boundaries. High resolution energy d ispersive x-ray spectroscopy revealed that hydrogen annealed SBT layers hav e a deficiency of Bi up to a depth of similar to 30 nm underneath the free surface, and that there is accumulation of Bi at the grain boundaries of pr istine, hydrogen, and oxygen-recovery annealed SBT layers. This suggests th at the diffusion of Bi is largely controlled by grain boundaries and they p lay a critical role in controlling the electrical properties of SBT capacit ors. (C) 2000 American Institute of Physics. [S0021-8979(00)08604-7].