Watching a thin film crystallize: Microstructure Evolution in HfO2 Based Memristive Devices
Metal-oxide based resistive random access memory (RRAM) devices with hafnia as the insulator material are promising next generation non-volatile memory devices with back end of-line processes compatibility in the current semiconductor fabrication process and good chemical and thermal stability. Here, we show that by grain boundary engineering of HfO2 thin films, the most prominent constraining properties, i.e. high forming voltage and high device-to-device variability, can be controlled/tackled. Predefined grain boundaries, interconnecting top and bottom electrode, can serve as predefined pathways, locally constricting the formation of the conductive filament.
In this study, a TiN/HfO2/Pt stack grown on c-cut sapphire was investigated by monitoring local structural changes such as crystallization and grain growth within the dielectric layer with Scanning Precession Electron Diffraction (SPED) during in situ heating in the transmission electron microscope. By using reactive molecular beam epitaxy (RMBE) a 50 nm epitaxial TiN (111) was deposited followed by a 12 nm thick amorphous hafnia layer. The stack combination was completed by ex situ sputter deposition of 100 nm Pt. By using Focused Ion Beam (FIB), a cross sectional lamella was prepared and transferred onto a Micro Electrical Mechanical System (MEMS) based heating chip. Laterally resolved structural information was acquired by SPED (4D-STEM) using a MerlinEM Medipix3 (Quantum Detectors) direct electron detector in order to track the structural changes in the dielectric hafnia layer during in situ heating. In addition, structural changes have been compared to an ex situ annealed stack.
Within this work, the formation of grain boundaries in hafnium oxide was investigated in situ by monitoring grain evolution during heating inside a transmission electron microscope yielding a unique insight into the grain growth mechanism. Further ex situ annealing samples, which resulted in a phase change from amorphous to polycrystalline hafnia, showed improved forming voltages and device-to-device variability. The results of this study serve as a basis for a direct structure-property correlation in this system.