RRAM Devices with Plasma Treated HfO2 with Ru as Top Electrode for In-Memory Computing Hardware

Abstract

Recently, many non-volatile memory devices such as resistive random-access memory (RRAM) have shown promise as candidates for analog in-memory computing applications related to inference and training in artificial intelligence. A RRAM cell has an MIM (metal insulator metal) structure that exhibits reversible resistive switching on application of positive or negative voltage. HfO2-based dielectrics with controlled distribution of defects or oxygen vacancies can potentially enable low power switching and multi-resistance levels. However, detailed studies of the power consumption, repeatability and retention during multi-level operation have not been undertaken previously. Transition metal oxide-based RRAM, using HfO2, executes change in resistance (switching behavior) via electrochemical migration of oxygen vacancies. Lack of enough oxygen vacancies in stoichiometric HfO2 limits the low power operation. Therefore, HfOx (with x < 2) has been studied for many RRAM devices (1). A uniform distribution of defects in HfO2 may help in switching but may not help in reducing the power. The distribution of these defects throughout the bulk of HfOx is, therefore, not well-understood. It was recently reported that high oxygen vacancy concentration close to top electrode reduces switching power (2).This work investigates the role of extra oxygen vacancies, introduced by plasma for a 6 nm thick HfO2 during deposition to reduce the power consumption. Initially TiN, which is a commonly used metal in CMOS technology, was used as the top electrode for treated HfO2 using a Si/PVD Ti/PVD TiN/HfO2/PVD TiN structure. A range of compliance currents (CC) from 1 nA to 500 nA were used to evaluate the switching characteristics. The TiN devices switch at and above a CC of 50 nA and were tested for sustainability for hundreds of cycles. In addition to oxygen vacancy rich treated-HfO2 near the top electrode, TaN and Ru electrodes were explored to enhance the switching behavior and power consumption. The role of both TaN and Ru as bottom metal was also evaluated. With Ru as top metal the Si/PVD Ti/PVD TiN/HfO2/Ru/PVD TiN structure, switched at 1 nA CC and at higher CC. Whereas when Ru was used as a bottom electrode, a Si/PVD Ti/PVD TiN/Ru/HfO2/ PVD TiN structure, devices were unable to switch below a CC of 50 mA. For TaN as top metal electrode, devices switched at and above 1 mA CC whereas with TaN as bottom metal the initial switching was at CC of 2 mA. It was concluded that use of Ru as a top metal significantly reduced the switching energy of the treated HfO2 RRAM device but was ineffective when used as a bottom metal. Potential mechanism for this observation will be discussed. References Chen, W. Lu, B. Long, Y. Li, D. Gilmer, G. Bersuker, S. Bhunia, and R. Jha, Semiconductor Science and Technology 30, 075002 (2015).Misra, P. Zhao, D. H. Triyoso, V. Kaushik, K. Tapily, R. D. Clark, S. Consiglio,2, T. Hakamata, C. S. Wajda, and G. J. Leusink, ECS J. Solid State Sci. Technol. 9, 05300, (2020).