고집적 메모리 응용을 위한 원자 기반 문턱 전압 (선택) 소자 및 메모리 소자에 대한 연구

Abstract

Charge based memory devices such as dynamic random access memory (DRAM) and NAND has been continuously scaled down according to Moore's law, leading to high density and performance improvement of devices. As a result, we have been able to store and process huge amounts of information quickly and with high density, which has enabled the semiconductor industry to grow rapidly. However, due to scaling according to Moore's law, short channel effects such as drain-induce-barrier-lowering (DIBL) and gate-induce-drain-leakage (GIDL) occurred in conventional transistors. In addition, in the case of DRAM having 1 transistor (T) – 1 capacitor (C) unit cell, the capacitance of capacitor was reduced due to the reduction of the area of the capacitor by scaling, and a vertical capacitor was implemented to maximize capacitance, but it is having difficulties due to the mechanically unstable structure and various process issues (etching etc). It is very difficult to meet the requirements of sub-nm scaling, extremely low power and low leakage current using conventional electron-based memory devices. Therefore, two-terminal- and atomic-based memory and threshold (TS) devices with cross-point array architecture are necessary to overcome scaling limits of conventional memory devices for future memory technology. Firstly, among the various TS devices, the atomic-based TS device having metal-insulator-metal structure has an extremely low off current (leakage current) characteristic, so it can effectively prevent the sneak path current in a cross-point array. However, the atomic-based TS device has a slow turn-off speed and non-uniform switching characteristics due to its operating mechanism. To improve turn-off speed, volatility of metal filament was evaluated according to the density and stoichiometry of oxide-based solid electrolyte using plasma-enhanced-atomic-layer-deposition (PEALD), ALD, and sputter deposition processes. The trade-off relationship between turn-on and turn-off speed according to oxide density and stoichiometry was solved by sequentially depositing dense and porous oxides extremely thinly, and fast turn-off speed and uniformity were improved, which results from localized atomic switching in a sub-nm oxide layer. In addition, in order to improve the uniformity and volatility characteristics of atomic-based TS devices, the switching characteristics of an atomic-based TS device with a limited amount of metal source were evaluated through the diffusion of Ag into the chalcogen electrolyte induced by UV light exposure. The TS and uniformity characteristics vary according to the amount of Ag injected into the chalcogenide electrolyte, which varies according to the UV light exposure time. The TS device, in which Ag limited amount of Ag is present in the chalcogenide electrolyte by UV light, has improved characteristics that can uniformly control both the uniformity of the threshold voltage and the hold voltage, that is, the properties of filament formation and decomposition. When integrating the selector and memory in implementing an atom-based memory, it has a complex layered structure due to several intermediate metal layers. Accordingly, a hybrid memory device was implemented in which two layers of oxides having different properties were sequentially deposited between a pair of metal layers rather than dividing the selector device and the memory device based on the intermediate metal layer. Although it is the same oxide material, the density and stoichiometry vary according to different deposition methods, that is, PEALD and sputter processes, and the volatile and non-volatile characteristics of metal filaments are distinguished. Therefore, the characteristics of a hybrid memory device having an extremely low leakage current and a large window margin were evaluated through an electrolyte with the same oxide material but different physical properties. Conductive bridge random access memory (CBRAM) is of interest for emerging storage class memories as a next-generation high-density non-volatile memory with simple metal-insulator-metal structures in preparation for information society. In order to implement a extremely high-density storage memory, it is essential to store multi-level data storage in a single cell as well as selector devices with low leakage current for cross-point array structures. The challenge for multi-level data storage is to realize distinct and various quantized conductance states by precisely controlling the atomic-transport constituting the atomic-scale CF in the insulating layer by applying electrical energy. The transition energy of atoms in the insulating layer varies according to device stack, temperature and environment, and the relation between controllability of quantized conduction and the transition energy must be uncovered for elaborate control of multi-level data storage. Therefore, the relationship between transition energy and controllability of quantized conduction was evaluated by comparing oxide-based electrolyte and vacuum-based electrolyte, and it was found that sophisticated quantized conduction with low energy can be implemented through vacuum-based electrolyte implementation.