電阻式隨機存取記憶元件之製作、改良與特性研究

Abstract

隨著數位行動生活的到來，非揮發性記憶體在可攜式電子產品，如：手機、數位相機跟筆記型電腦扮演著重要的角色。快閃記憶體是現今非揮發性記憶體的主流，但是它有著許多缺點，包含：高的操作電壓、低的操作速度與較差的耐久力。近年來因傳統快閃式記憶體在不斷微縮下，面臨了許多急欲克服之難題，例如儲存在懸浮閘極中之電荷，因穿遂氧化層過薄而隨時間漸漸流失，造成資料流失；此外，在長時間操作之下，易在穿遂氧化層內產生缺陷以及超作電壓過高…等，如此瓶頸，加快了下世代非揮發性記憶體之研究腳步。下世代非揮發性記憶體有：鐵電記憶體、磁記憶體與電阻式記憶體等，其正如火如荼地發展。而其中電阻式記憶體是利用元件內部不同之電阻值當作其相對應之記憶狀態，並用電壓或電流脈衝在及短時間下(~10ns)改變電阻值，進而達到資料寫入之動作。 夏普公司於2002年國際電子元件會議中首先發表錳酸鐠鈣此材料於金屬-絕緣層-金屬結構中，其存在電阻轉換現象並可利用其元件內部電阻值之不同而應用於非揮發性記憶體，即所謂電阻式記憶體。電阻式記憶體具有著與互補式金氧半電晶體製程相容、結構簡單、可微縮化、操作速度快與低耗電等優點，可用於嵌入式記憶體並極可能成為下世代之非揮發性記憶體。韓國三星公司更於2004年國際電子元件會議中，發表利用二元過渡金屬氧化物搭配0.18μm製程，成功製作出電阻式記憶體；隔年，並發表利用栓塞式底電極以減少兩個記憶狀態之電阻值於連續操作過程中所發生之變異。現今，電阻轉態現象已於許多材料中被觀察到了，如有機高分子材料、固態電解質、鈣鈦礦材料像，鋯酸鍶、鈦酸鍶與錳酸鐠鈣；過渡金屬氧化物，像氧化鎳、氧化鈦、氧化鋯與氧化銅等。 本論文首先根據已發表之論文，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。而在將把電阻式記憶體於商業化應用之前，還是有著許多重要、尚未瞭解的問題，包含：電阻轉態機制的基本原理和可靠度之爭議。本論文以鋯酸鍶與一些二元氧化物為材料，針對這些議題提供一些較深入的看法並進一步地解決。在本論文前部分，為利用凝膠法製備鋯酸鍶；首先提出利用一層5nm之鉻金屬內嵌於鋯酸鍶薄膜內，搭配快速熱退火處理，解決其在連續的操作過程中出現不穩定的電阻轉換之問題。 相對於鈣鈦礦材料，二元金屬氧化物有著較少組成成分跟與傳統互補式金氧半電晶體製程相容之優勢。因此，後半部論文主要研究一些二元氧化物，如：氧化鋅、氧化鈰、氧化鉻、氧化鐵、氧化鑭、氧化鋯、氧化鋁；分析其電阻轉態特性，發現氧化鋯與氧化鋁具有優異的特性。利用鈦金屬強烈吸氧之特性，成功製作不需高電壓形成過程(forming process)之記憶元件；不需要額外的高電壓，進而減少在電路設計上的複雜度。針對氧化鋯作分析，利用界面層解釋以鈦金屬為上電極時，記憶元件有著極高的良率、穩定的特性、極性相關與自我限流的轉態行為跟載子傳輸機制的改變等。對這尚未明瞭的電阻轉態機制，提供不同的觀點。最後對全文作一總結，並對未來可行的研究工作做一建議。

With the arrival of Digital Age, nonvolatile memory (NVM) plays an important role for portable electronic products, such as the mobile phone, digital camera, and notebook computer. Flash memory is the mainstream among the nonvolatile memory devices nowadays, but it has many drawbacks, including high operation voltage, low operation speed, and poor endurance. In addition, when the device dimensions are continuously scaled down, the flash memory faces the challenge of thin tunneling oxide that causes an unsatisfactory retention time. Consequently, there are many proposals for new nonvolatile memories such as the ferroelectric random access memory (FeRAM), the magnetic random access memory (MRAM), and the resistive random access memory (RRAM). As for RRAM, the digital data can be stored in two memory states with high and low resistivities, ON-state and OFF-state, respectively. The two memory states can be easily switched by voltage biases or pulses, which enhance the possibility of the application in circuit level. In the International Electron Devices Meeting (IEDM) in 2002, Sharp company reported that RRAM had advantages over both dynamic random access memory (DRAM) and flash memory due to its nonvolatility, simple device structure, and high operation speed. Thereafter, much attention devoted to the development of RRAM devices. Resistive switching phenomena have been observed in many materials, which can be roughly categorized into four groups (1) organic molecular materials, (2) solid state electrolytes (or called programmable metallization cell), (3) perovskite structures such as SrZrO3 (SZO), SrTiOx and Pr0.7Ca0.3MnO3, and (4) transition metal oxides (TMOs) such as NiO, TiO2, ZrO2, Cu2O, and etc. There are still many important unresolved problems, including the original resistive switching mechanisms and the reliability issues (such as endurance test, reset failure, and variations of resistive switching parameters), which are all needed to be identified before realizing commercial applications. The major goals of the dissertation are to give more insights into these issues and find solutions to them. The purpose of this research focuses on the fundamentals of the SZO- and ZrO2-based resistive switching memory devices. Based on the basic understandings, the SZO- and ZrO2-based memory device is modified to achieve the improvements in the device yield, reliability, and manufacturability for the next-generation nonvolatile memory application. We have proposed novel, simple, and effective manners by embedding a thin Cr layer and using Ti top electrode, which causes an inner space charge region and a self-aligned interface. No extra manufacture cost is needed to improve the device performance. Moreover, the self-aligned interface serves as a series resistance and an oxygen sink, which is demonstrated to be important for the enhancement of the device characteristics. In the final part of this dissertation, the conclusions and the suggested future works are presented.