Ta/TaOx/TiO2/Ti雙氧化層電阻式記憶體於交錯式陣列與軟性電子之應用

Abstract

電阻式記憶體(RRAM)擁有雙端點金屬-絕緣層-金屬(MIM)結構之記憶胞(memory unit cell)，適合應用於交錯式被動陣列(crossbar passive array)，具有最小單元面積4F2優秀的微縮潛力，然而在被動式陣列中記憶胞之間的漏電流路徑(sneak current path)干擾造成了讀取邊際(read margin)的退化，而漏電流路徑與陣列的尺寸有相當程度的相依性，如何抑制漏電流將是實現高密度記憶體陣列必須克服的關鍵問題。許多研究團隊採用額外串接一非線性元件的方式來阻斷漏電流路徑，但是額外串接的非線性元件增加了製程的複雜度，或是使用到貴重金屬電極，導致製作成本提高，因此開發具有自我整流(self-rectifying)特性的電阻式記憶體會是一更有效抑制漏電流的方式，具有自我整流特性的電阻式記憶體不僅同樣可以達到非線性的需求，並且不需要額外非線性元件設計。 Ta/TaOx/TiO2/Ti雙氧化層電阻式記憶體具有有良好的自我整流特性(>103)、穩定的記憶窗口(memory window)與可室溫製作的特點，不僅適合製作交錯式陣列(crossbar array)，也提供了製作於軟性基板上可撓式記憶體的新選擇。然而雙氧化層電阻式記憶體的操作機制目前仍不甚清楚，因此在這篇論文中我們有系統地改變雙氧化層電阻式記憶體的製程條件與材料組成，釐清各層與阻態切換特性與整流特性之相依性，我們發現TaOx層對整流特性有很大的影響，而TiO2層則直接影響到阻態切換的特性，最後我們並提出一個可以合理解釋實驗結果的物理模型。 在雙氧化層電阻式記憶體的應用上，我們成功將雙氧化層電阻式記憶體製作在軟性基板上，發現在彎曲測試中仍保有良好的記憶窗口、整流特性與可靠度。我們也成功將雙氧化層電阻式記憶體製作成交錯式陣列，在2×2交錯式陣列測試中記憶胞確實可以有效抑制漏電流，且記憶胞所以儲存的阻態不受周圍記憶胞寫入操作的干擾，最後在6×6交錯陣列中亦可清楚辨別每一記憶胞的高阻態與低阻態。

Resistive-switching random access memory (RRAM) is suitable for the crossbar passive-array configuration because of its two-terminal metal-insulator-metal (MIM) memory unit cell. Although crossbar RRAM with a 4F2 unit cell size is attractive, the sneak current path issue limits the array size because of the degradation of read margin. Therefore, how to suppress sneak current flowing through unselected memory cells is a critical concern. Many researchers adopted additional nonlinear devices in series connection with RRAM to improve the selectivity. However, additional nonlinear devices complicated the process flow and some of them even used noble metal electrodes, such as Pt, that are not compatible with the semiconductor process. Therefore, developing a self-rectifying RRAM device is a more attractive alternative because self-rectifying RRAM provides necessary nonlinearity without additional nonlinear devices. Ta/TaOx/TiO2/Ti bilayer RRAM contains numerous desired characteristics, including: (1) large rectifying ratio (>103), (2) stable memory window (~10x), and (3) room temperature process. Therefore, it can be implemented in crossbar arrays and on flexible substrates. However, the operating mechanism of the bilayer device is still not completely understood. In this study, we varied the process conditions and film stacks in bilayer RRAM to verify the roles of two different oxide layers. We found that the TaOx layer significantly affects the self-rectifying characteristics, while the TiO2 layer is responsible for resistive switching. Finally, we propose a physical-based model to explain the experimental results. For the applications of bilayer RRAM, we successfully fabricated the bilayer RRAM device on flexible substrates. The flexible bilayer RRAM device maintains excellent electrical characteristics at both bending and flat states. The memory window remains stable at different bending radius and after 103 bending cycles. We also successfully fabricated crossbar arrays by using the bilayer RRAM device. In a 2×2 crossbar array, the selected memory cell demonstrates stable switching characteristics while the rest unselected cells are at low resistance state (LRS). In addition, the stored states are not disturbed by the write operations of neighboring cells. At last, we demonstrate that successful SET, RESET, and read operations in every functional memory cell in a 6×6 crossbar array, highlighting the excellent ability of suppressing sneak current.