鋯酸鍶基電阻式記憶元件特性與機制之研究

Abstract

近年來，手機、數位相機和MP3隨身聽等消費性電子產品逐漸流行，使得非揮發性記憶體需求量大增，非揮發性記憶體必須具有於無電源供應時仍能維持其記憶狀態和操作時必須低耗能等特性，理想的非揮發性記憶體具備操作電壓低、低耗能、操作速度快、耐久力佳、記憶時間長、非破壞性讀取、製程簡單、可微縮及低成本等條件，目前並無任何非揮發性記憶體元件完全達到上述條件。目前市場主流的非揮發性記憶體仍以快閃式記憶體為主流，但其操作電壓大、高耗能和操作速度慢等缺點，將限制快閃式記憶體未來的發展。此外在元件微縮製程趨勢下，過薄的穿透閘極氧化層將導致資料存取性不佳，亦是當前嚴重的問題。 目前已有許多非揮發性記憶體材料和元件正被積極研發中，包括鐵電記憶體、磁記憶體、相變化記憶體和電阻式記憶體等。其中電阻式非揮發性記憶體具有操作電壓低、功率消耗低、寫入抹除時間短、可微縮、耐久力長、記憶時間長、記憶時間長、非破壞性讀取、製程簡單及製作成本低等優點，因此電阻式非揮發性記憶體是目前新興非揮發性記憶體元件中的研究重點。 本論文第一章根據已發表之論文，把現今電阻式記憶體研究之重點、現況與理論做一整理、歸納與比較。第二章為實驗步驟，介紹元件製作、材料分析儀器與量測方法。第三章探討釩摻雜對於鋯酸鍶薄膜物理與電阻轉態特性之影響，釩摻雜之鋯酸鍶薄膜有著穩定元件耐久力特性，但其操作電壓卻遠高於未摻雜鋯酸鍶薄膜，無法達到低電壓操作之需求。為了改善此問題，第四章提出氧氣快速熱退火製程提升薄膜品質，雖改善元件兩記憶狀態在連續操作下不穩定之問題，但氧氣退火鋯酸鍶薄膜其操作電壓相較於未退火處理薄膜高出1V左右。第五章利用2奈米厚鉑金屬之內嵌製程，經快速熱退火處理，於鋯酸鍶薄膜內形成鉑奈米團簇，可縮短電阻轉態區域，進而降低操作電壓，但鉑金屬並不適用於傳統互補式金氧半場效電晶體製程。第六章提出利用氧氣流量製程技術製作多氧層與少氧層之堆疊結構，限制元件電阻轉態特性於多氧之鋯酸鍶薄膜內。大幅降低鋯酸鍶薄膜之形成電壓與寫入電壓並提升鋯酸鍶薄膜的電阻轉態穩定度。本論文係利用最簡單之方式，在不增加任何製程前提下，成功地限制導電路徑導通與破裂之區域，進而減少兩記憶狀態於操作過程中所發生之變異與寫入失敗之危險。可大幅減少成本、有效減少變異並適合於大量生產。鋯酸鍶薄膜電阻式記憶體具有良好的電阻轉態特性，可應用於下世代非揮發性記憶體。 最後為本論文總結，並對未來研究工作提出具體建議。

With the popularity of the portable equipment such as mobile phone, digital camera, and MP3 player, the demand of nonvolatile memory has greatly increased in recent years. For ideal NVM devices, it is expected to possess the advantages of low operation voltage, low power consumption, high operation speed, high endurance, long retention time, nondestructive readout, simple structure, and low cost. However, there is no NVM device completely including the above properties up to now. Flash memory, the mainstream of NVM devices nowadays, suffers some severe issues including high operation voltage, high power consumption, and low operation speed. In addition, as the continuous device scaling down, it will meet the physical scaling limitation in the near future, further leading to poor retention time and coupling interference effect. Consequently, several high-potential candidates for the next-generation NVM application, including ferroelectric random access memory, magnetroresistive random access memory, phase change random access memory, and resistive random access memory (RRAM) have been proposed to replace flash memory. Among them, RRAM possesses the excellent advantages, including low operation voltage, low operation power, high operation speed, high scalability, good endurance, long retention time, nondestructive readout, simple structure, and low cost. As a result, RRAM has been investigated for the commercial NVM application. Chapter 1 introduces various types of novel memory devices and reviews current status of the resistive switching memory based on the material categories. Some important effects and the possible resistive switching mechanisms are organized and discussed in detail. The detailed experimental procedures of fabrication of SrZrO3 (SZO)-based memory devices, the principles of material analyses, and the related electrical measurements are all presented in Chapter 2 of this dissertation. In Chapter 3, the effects of vanadium doping on resistive switching characteristics and mechanism of RF-sputtered SZO-based thin films are investigated. The physical and electrical properties of SZO-based thin films, such as the forming voltage, turn-on voltage, HRS resistance, dielectric constant, and , are modulated by vanadium doping due to the suppression of oxygen vacancy formation. Although SZO thin films exhibited lower forming voltage and lower turn-on voltage than those of the vanadium-doped SZO thin films, the large dispersion of resistive switching parameters and the low device yield in SZO thin films limit their development in realizing the practical NVM application. In Chapter 4, the O2-600 oC RTA process is used to give a narrower distribution in the SZO bulk thin films, further reducing the large dispersion of resistive switching parameters and increasing the device yield in SZO thin films. O2-annealing process is reported to improve and stabilize resistive switching behavior of SZO-based memory devices, but it could increase forming voltage and turn-on voltage. In Chapter 5, embedding Pt metal layer into SZO thin films can significantly reduce forming voltage and turn-on voltage to -3.5 V and |2.3| V, respectively, due to the formation of Pt clusters. However, this E-Pt process is more complicated than the conventional process of silicon-based devices. In Chapter 6, the resistive switching region can be effectively reduced and localized within the oxygen-rich layer of SZO devices by the oxygen flow control process, leading to the low operation voltage and small dispersions of resistive switching parameters. Finally, Chapter 7 summarizes the finding and contributions of this dissertation and the future works are suggested for the further RRAM research.