TaOx/TiO2 Bilayer Resistive-switching Random Access Memory for Flexible Array and 3D Vertical Structure Applications

• Main Authors: Lai, Wei-Li, 賴韋利
• Other Authors: Hou, Tuo-Hung
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/j2234n

碩士 === 國立交通大學 === 電子工程學系 電子研究所 === 104 === Since Moore’s Law has proposed by Gordon E. Moore in 1965, the semiconductor devices have become smaller, denser, cheaper, and faster. His prediction has driven technology for a half century. However, recent miniaturization has become difficult when reaching the physical limits—the atomic scale. Hence, in order to prolong Moore’s Law, the concept of More Moore and More than Moore are proposed. For memory devices, one of the representative examples of More Moore is the BiCS 3D NAND memory demonstrated by Toshiba, which provides higher bit density to continue the Moore's law by using vertical stacking. For the More than Moore approach, flexible memories create new application spaces and have gained increasing attention recently. For the future More Moore and More than Moore memory development, resistive-switching random access memory (RRAM) is regarded as one of the most promising candidates for next-generation memory because of its process compatibility with standard VLSI processes, simple cell structure, low process temperature, small footprint of 4F2, etc. In this thesis, we focus on a promising Ta/TaOx/TiO2/Ti bilayer RRAM structure. We successfully fabricated RRAM array on a flexible substrate, and further optimized the process parameter for 3D vertical RRAM application. For the flexible RRAM, the process temperature is the main concern. We investigated the process temperature effect on the device switching characteristics. The result shows that increasing the TiO2deposition temperature improved the device switching characteristics. Therefore, an optimized deposition temperature for the flexible substrate was used to demonstrate a 8×8flexible crossbar array. The reliability test on the flexible array was also conducted. No significant performance degradation was found under bending. As for the 3D vertical RRAM structure, the Ti horizontal electrode located between the SiO2insulatinglayers is easily oxidized because of the high reactivity of Ti, which limits the minimal Ti thickness one can use and thus vertical scaling capability. Therefore, we explored the possibility of using TiN to replace the Ti electrode. However, the RRAM device completely lost its switching characteristics by using the TiN electrode. The switching characteristics can only be recovered by inserting a TiOx layer between TiO2and TiN. We found that the Schottky barrier at this interface is crucial for device performance. Our device with the TiN electrode shows superior characteristics, including: (1) forming free, (2) self-compliance,(3) small variability. (4) self-rectifying ratio higher than four orders of magnitudes at +/-2V. Finally, we attempted to deposit our film stack by using atomic layer deposition (ALD) instead of sputtering to achieve better film quality and step coverage. The preliminary results suggest this is a promising direction to pursue in the future.