Study on the Resistive Switching Characteristics of HfOx-Based Resistive Random Access Memory (RRAM) Devices

• Main Authors: Zhang, He-Xin, 張鶴馨
• Other Authors: Cheng, Huang-Chung
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/b4eywy

碩士 === 國立交通大學 === 電子研究所 === 107 === Non-volatile, high switching speed, low operation voltages and simple structure were the advantages of resistive random access memory (RRAM) for high competitiveness in the field of memory technology. Recently, RRAM had attracted great attention and be supposed to replace the conventional non-volatile memories (NVM), such as the flash memory technology, owing to the simple fabrication process. Nevertheless, there were still lots of drawbacks required to be solved for the RRAM devices, such as the bad uniformity and the uncertain switching mechanism. In this thesis, elevated-film-stack-structured HfOx-based RRAM with post-metal annealing and engineering H2O pressure of ALD sub-HfOx dielectric films were used to improve the switching characteristics. First of all, this thesis compared the effect of the post metal annealing on the resistive switch characteristics of the RRAM devices with the planar structure and ones with the elevated-film-stack structure. The result showed that the operation voltages after post-metal-annealing process were decreased significantly for both structures. According to the XPS analysis, these oxygen ions tended to accumulate in the Ti interfacial layer after post-metal annealing process. Therefore, the amount of oxygen vacancies in HfOx layer significantly increased. The forming voltage (Vform), set voltage (Vset), and the reset voltage (Vreset) effectively decreased because the filament easily formed and ruptured. For the operation voltage decrease, the perform of the elevated-film-stack-structured RRAM was better than the one of the planar-structured RRAM. In the TCAD simulation, the elevated-film-stack structure had the sharp corner which induces the local high electrical field and heat accumulation effect. They made the oxygen ion form significantly near the corner, so it improved the uniformity and decreased the operation voltages. In the second part, the RRAM cells of different film qualities were fabricated by engineering the H2O pressures in ALD process of depositing HfOx layer. The result showed that the H2O pressure decrease caused incomplete relation of the precursor TDMAH and H2O; thereby, the Hf-O binding energy became weak. Therefore, the oxygen scavenging ability of Ti would improve and then the interfacial layer TiOx would form between Ti and HfOx layers. The interfacial layer, which avoid device form hard breakdown, would become dense during SET process. The oxygen vacancies increase, forming voltage decrease, and form of the interfacial layer made filaments short and dense. The size of filaments determined the failure type. The RRAM cell with small amount of the initial oxygen vacancy tended to form the thick filament. After some operations, it couldn’t change from low resistance state to high resistance state (HRS degrade) because the thick filaments were hard to break. However, The RRAM cell with small amount of the initial oxygen vacancy tended to form the thin filament and thick interfacial layer. After some operations, it couldn’t change from high resistance state to low resistance state (LRS degrade) because the dense interfacial layer limited the current increase. Engineering the H2O pressure in ALD process made devices to achieve the suitable film quality and then improve the endurance. It was found that if the compliance current of the forming process increased, the switching resistance would become gradually smaller with increasing operation number in the reset process. The behavior of the gradual resistance change was achieved with increasing operation number. The compliance current increase made filament size become large and thickness of the interfacial layer increase which made oxygen ion diffusion slow so the filament could not rupture immediately in only one reset process. It must repeat operation to completely rupture the filaments. In this thesis, the TiN/Ti/HfOx/TiN stack RRAM devices under the PMA processes and with the elevated-film-stack structures improved the uniformity and decreased the operation voltage. The engineering oxygen vacancies of ALD sub-HfOx dielectric films created the self-compliance and analog reset characteristics. Such devices had the potential applications in the future non-volatile circuits.