具交叉結構二氧化鉿電阻式記憶體之可靠度與非線性特性研究

Abstract

近年來，由於快閃記憶體面臨到微縮物理極限與操作電壓過大等問題，因此，具簡單結構、操作速度快與低功率消耗的電阻式記憶體元件，極有可能取代傳統的快閃記憶體，成為下個世代非揮發性記憶體的主流。二氧化鉿為現今先進互補式金氧半場效電晶體的主流材料，因此以二氧化鉿薄膜為電阻轉態層的電阻式記憶體元件被大量的研究與探討。且於過去研究文獻顯示，二氧化鉿薄膜電阻式記憶體具有低電壓操作、低功率消耗、高密度堆疊結構等極佳的記憶體操作特性。 本篇論文分為兩個部分，第一部分探討二氧化鉿電阻式記憶體的熱穩定性。由於二氧化鉿的熱穩定性不佳，經由後續400 oC退火處理後，會產生氧離子逸失等問題，嚴重影響二氧化鉿電阻式記憶體的進一步開發與應用。因此，為了抑制上述現象，本論文利用內嵌一層具低自由能特性的氧化鋁薄膜於二氧化鉿電阻轉態層與上電極之間，藉此抑制退火處理後的二氧化鉿薄膜中的氧離子逸失現象。實驗結果顯示，此結構確實可有效抑制二氧化鉿薄膜中的氧離子逸失現象，並於高溫加速測試下，能提升資料保存能力特性，並且在30奈秒的快速脈衝電壓操作下，高低阻態電阻比值可維持大於10倍與高達一百萬次電阻轉換操作。 第二部分改將寬能隙薄膜嵌入於電阻轉態層與底電極之間，低阻態的電流-電壓特性將會從線性轉換成非線性。此寬能隙材料於底電極處成為穿隧能障，所以在小電壓時電子將直接穿隧此能障，而在大電壓時進行FN穿隧，使得電壓大到一定程度時電流突增。此非線性電阻式記憶體元件無須串聯非線性被動元件即可改善陣列式記憶體讀取錯誤的問題。

Recently, the challenges of flash memory device are scaling limits and high operation voltage, etc. Hence, resistive switching RAM has received great attention as the next generation nonvolatile memory device due to its advantages of simple structure, fast operation speed and low power consumption. Hafnium oxide has been employed as a high-k material for gate oxide for CMOS process and it also a superior material for RRAM. The HfO2-based RRAM shows good characteristics on low operation voltage, low power consumption and ultra-high density integration The organization of the thesis can separated into two parts. In the first part, the thermal stability of the HfO2-based RRAM is discussed. This result shows that oxygen will release from HfO2 film during 400 oC post metal annealing process. It will affect the characteristics of the HfO2-based RRAM. Therefore, inserting a thin film with low Gibbs free energy between the resistive switching layer and bottom electrode can suppress the oxygen releasing problem which is due to high temperature annealing. Moreover, the data retention test under high temperature was improved. Under the high speed pulse operation of 30 ns, the device switched up to 106 cycles and the resistive ratio was larger than 10. In the second part, the large band gap Al2O3 layer was inserted between resistive switching layer and the bottom electrode and the I-V characteristic in low resistance state will change from linear to nonlinear. The Al2O3 and HfO2 layer plays a role as tunnel barrier and transport layer, respectively. At low voltage region, the electrons transport through Al2O3 by direct tunneling. However, the electrons transport through Al2O3 by Fowler-Nordheim tunneling and the current increased abruptly at high voltage region. This nonlinear RRAM improves the problems of misreadng in crossbar array without a nonlinear passive device in series.