Multi-state resistance switching and variability analysis of HfOx based RRAM for ultra-high density memory applications

In this study, we have investigated the multi-level cell (MLC) characteristics and variability analysis of multiple resistance states of one of the most promising and extensively studied binary oxide (HfO_x) based nanometer scale RRAM stack by varying the switching current. The device size and thickness of stack layers were confirmed by transmission electron microscope (TEM) images. In the CMOS friendly stack with TiN/Ti/HfO_x/TiN structure, 3 distinct levels of low resistance states (LRS) with same high resistance state (HRS) were successfully obtained which can be used in 2-bit per cell storage. It was found that the switching variability was the strong function of the number of defects in the filament or the switching current (I_C) and it significantly improved at higher I_C. All the resistance levels show good inter switching ability and reliability characteristics such as read disturb immunity, read pulse endurance (>10⁸ times) and data retention.