Current Status and Future Challenges of Resistive Switching Memories

Resistive switching memories (ReRAM), including transition metal oxide memory (TMO-RAM) and conducting bridge memory (CB-RAM), are some of the most promising new technologies that may scale beyond the charge-storage flash memories. Understanding the fundamental operation mechanism is one important challenge to control the key parameters and to choose the ReRAM material. The knowledge on ReRAM reliability is also insufficient to meet the future challenges. This paper will review the recent major approach of WOX ReRAM and Cu-based CB-RAM. Introduction Conventional charge-storage non-volatile memories are approaching their scaling limit [1]. A number of new non-volatile memories have been proposed [2-5] and among them the resistive switching memory is considered one of the most promising candidates [4-5]. To enable this new technology, researchers and engineers are working on four major topics: write power, scaling possibility, ReRAM materials, and cross point array architecture. The write power of ReRAM is lower than several other emerging memories such as PCM and MRAM [6-8]. However, the high forming voltage/current is still a concern, and even the regular write current affects the bandwidth. Although devices smaller than 10nm have been reported [7-8] recently, but array uniformity and reliability are still not proven. And finally, a proliferation of ReRAM materials has yet to converge to a few exhibiting all desired properties. ReRAM Materials Fig. 1 shows the ReRAM feature size evolution compared with charge-storage memory [7-19]. Already ReRAM has passed the scaling limit of charge-storage memories. Fig. 2 shows the relationship between switching current and speed [7-19]. The observation that only a few points are in the low-current/high-speed region indicates that there may exist a tradeoff between operating current and switching speed. The fact that most of the transition metal oxides can be switched not only in bipolar but also in unipolar modes actually triggers a critical issue on how to choose the best material and the best operation mode. Thus fundamental material studies including the switching mechanism, the conducting mechanism, and the material characteristics are the most important topics in ReRAM research. There seems consensus now that the bipolar switching behavior of TMO-RAM and CB-RAM is dominated by the movement of anions (Valency change effect) and cations (Electrochemical effect) (Fig. 3a and 3b) [9,18]. For unipolar opeation, it is believed that the switching mechanism is based on thermochemical effect (Fig. 3c) [20]. However, the detailed physical models still need further improvements and the knowledge for fine tuning the physical parameters to improve the device performances including speed, current, resistance window, data retention, and cycling endurance is still under development. Field Enhancement Structure for WOX ReRAM Since the forming process is an important issue and needs to be solved, a self-aligned field-enhancement device structure is proposed and its 2D simulation for 20 and 100 nm cells are shown in Fig. 4. By oxidizing the TiN liner into an insulating TiNOX the WOX is forced to protrude above the remaining TiN. Fig. 5a shows significantly higher electric field at the center of the WOX when the size of the W plug scales. Fig. 5b shows that the voltage required for the initial forming process falls rapidly when the cell size scales. Therefore, at 60nm or below, the initial forming process is practically eliminated. The 60nm WOX device not only shows forming-free property, but also good electrical performance. Fig. 6a shows the cycling endurance of the 60nm devices is > 10 times, and a 10X resistance window is well maintained by program-verify algorithms. Excellent thermal stability is Fig.1. Evolution for ReRAM and charge-storage memory. ReRAM shows promising scalability beyond 1X nm technology node. Fig.2. Switching current versus switching time for both TMO-RAM and