Research On Novel High-Density Nonvolatile Memories

With the rapid development of the consumer electronics products, the demand for the storage capacity and data density of the nonvolatile memories has always been growing. To improve the memory density, the device size keeps scaling down, the multi-bit(multilevel) technology has been widely used, and 3D stacked technology gradually develops. However, the scaling down of the devices will finally encounter the physical limitation; there exists many problems in the performance and reliability of multibit technology; and 3D technology is not qualified to applications due to its many challenges, e.g., material, device structure, fabrication process, etc. Therefore, a novel nonvolatile memory, which is highly compatible with CMOS process, is needed to realize higher data density and lower per bit cost. Resitive Random Access Memory(RAAM) and its 3D integrated structure is one of the most potential candidates because of its simple cell structure, compact device array, excellent memory performance and great CMOS compatibility. This work focuses on how to improve the density of nonvolatile memory, mainly including research on resitive materials, RRAM device structure and performance, multilevel switching and 3D integration.Many kinds of resistive materials have been investigated for the application in the high-density memories, e.g., metal-oxide, carbon material, organic polymer, etc. RRAM devices based on these materials have been fabricated, and the switching behavior, physical mechanism and reliability have been studied, respectively. A hybrid filament model is proposed to explain the total ionizing dose effects of γ-ray radiation on the RRAM devices. Among all the RRAM devices in this work, HfO2-based devices have shown great potential for the high-density memory applications because of the large memory window, low operation voltage and satisfied data reliability, etc.For multilevel RRAM, on one hand, a combined operation scheme by modulating both current and voltage is proposed to enlarge memory window and optimize data reliability. On the other hand, RRAM devices with stack-layered structure are studied since it shows intrinsic potential in the multilevel memory capacity. Moreover, a microscopic model of the conductive filaments is established and simulated to explain the physical nature of the multilevel storage behavior and guide the optimization of the RRAM devices.As for the 3D technology, crossbar structure and vertical gate-all-around(GAA) structure devices are studied. For crossbar devices, a novel high-performance PNPN switch device is proposed, as well as its RRAM memory array, operation scheme, fabrication method, etc. While for vertical GAA devices, a multilayer GAA RRAM device is fabricated and its resitive switching performance is examined. Furthermore, the device array, operation scheme and peripheral circuits are proposed and preliminarily verified for future applications.