UC Berkeley

In recent years, resistive random access memory (RRAM) has gained significant attention as one of the promising candidates for next generation memory applications. This is due to its anticipated advantages versus Flash technology with respect to high density, low power and fast read and write speed. The main operation mechanism of these devices is a resistance change induced by filament formation through metal-cations or oxygen vacancies.

In the first part of this work, a Kinetic Monte Carlo (KMC) simulator is built to study the filament formation process in an electrochemical metallization (ECM) RRAM. This simulator takes into account most important physical and chemical processes such as oxidation, reduction, metal crystallization, adsorption, desorption and ionic transportation. The characteristics of the forming voltage, forming time and switching speed are investigated. In addition, studies on filament overgrowth and on-state resistance distribution are presented. Further, filament topography, which strongly influences device properties, is studied under different device operation conditions. The simulator also predicts that depending on the strength of lateral electric field, the conductive filament can break at various locations during the RESET process. The simulation results are verified by experiments conducted on Ag/Ag2S/W and Cu/H2O/Cu systems.

In the second part of this work, RRAM memory devices based on amorphous Yttria stabilized Zirconia (YSZ) are systematically studied. The effects of different top electrodes of Au, Cu, Ni, Al and Ti are investigated. And the characteristics and the mechanisms of Ti/YSZ and Cu/YSZ are studied in details. It is found that Ti/YSZ has much better endurance, retention and reliability than Cu/YSZ. The underlining physics driving this behavior is investigated. In addition, it is found that Ti/YSZ has very smooth transition in the RESET stage and the off state resistance exponentially increases with an increase of erase voltage. Based on those properties, a multi-level programming (MLP) cell is realized that shows good endurance. The underlying physics that makes the MLP possible for Ti/YSZ is investigated. Finally, it is shown that an incremental step pulse programming (ISPP) technique can significantly increase the device endurance and reliability. Furthermore, it can optimize the tradeoff between resistance programming window and device lifetime.