Platinum (100) hillock growth in Pt/Ti electrode stack for SrBi2Ta2O9 ferroelectric random access memory

Abstract

The ferroelectric material SrBi2Ta2O9 (SBT) has been extensively investigated in connection with integrating nonvolatile ferroelectric random-access memory (FeRAM). The SBT layer must be annealed in an oxygen atmosphere after deposition to crystallize the ferroelectric oxide film, which induces Pt hillock formation in a Pt/Ti electrode stack. The Pt hillock in a Pt/Ti electrode stack has been the main concern in SBT FeRAM due to reliability problems, such as capacitor shorts. Reportedly, the compressive stress generated in thin film is widely accepted as being responsible for the occurrence of hillocks in thin film and the main mass transport mechanism for hillock formation is the grain boundary diffusion for thin film with a columnar structure. In this study, three factors are considered in the total compressive stress generated during both deposition and post-annealing in Pt/Ti electrode stack: intrinsic stress, thermal stress, and extrinsic stress. Moreover, we found that an orientation relationship of Pt (100)(hillock)//Pt (111)(thin film) existed between the Pt hillock and the thin film. The Pt hillock was a single crystal, having facets with polyatomic steps. From these results, we suggest that the Pt hillock growth mechanism is the layer growth of flat faces, which shapes the hillock into a tetrahedron single crystal.