二元金屬氧化物應用於電阻式記憶體之界面特性研究

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、數位相機以及筆記型電腦等扮演著重要的角色。快閃記憶體為現今非揮發性記憶體的主流，其原理乃是利用電子儲存於懸浮閘極中，藉由起始電壓的偏移來判別記憶與否。傳統快閃記憶體仍具有著許多缺點，其中包含高的操作電壓、低的操作速度以及較差的耐久力，再者由於近年來元件製程不斷地微縮之下，快閃記憶體面臨了許多急欲克服之難題，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，造成儲存資料的喪失；此外，經由反覆長時間的操作之下，其易於穿遂氧化層內產生缺陷路徑以導致懸浮閘極全面性的漏電問題，如此瓶頸，加速了對於新世代非揮發性記憶體的研究腳步。有鑑於此，相關新世代非揮發性記憶體的發展，如鐵電記憶體、磁記憶體、相變化記憶體與電阻式記憶體等，已積極地展開一系列地研究工作，而其中電阻式記憶體乃是利用元件內部不同之電阻值作為其相對應之記憶狀態，並用電壓或電流脈衝在極短時間下改變電阻值，進而達到資料寫入與抹除之動作。 電阻式記憶體為－金屬/氧化層/金屬之電容結構，具有著與互補式金氧半電晶體製程相容、非破壞性讀取、多重記憶狀態、結構簡單以及所需面積小等優點，可用於嵌入式記憶體並極可能成為新世代之非揮發性記憶體。至今為止，相關的電阻轉態現象已被報導於各種不同材料中，諸如有機高分子材料、固態電解質、鈣鈦礦材料，以及過渡金屬氧化物等；然而，就目前電阻式記憶體的研究與實際應用上，操作切換速度、操作電壓、記憶保持時間與可靠度性能等方面，仍具有極大的改善空間。 本論文提出藉由電場驅動氧離子移動所造成之氧化還原為電阻轉換基礎，以活性金屬鈦作為上電極，會在二元金屬氧化物－氧化鋯、氧化銅等電阻層上自動對準(self-aligned)產生一界面層，在不需額外增加製程步驟下，藉由活性鈦電極捕捉氧離子的能力來調變氧化物中氧缺陷的密度位置，進而利用不同外加電場的極性來驅動鈦電極與氧化物界面間的氧離子移動，以侷限電阻轉換發生之區域，如此不僅大幅地減少了電阻轉換過程中出現的變異，更具有達到高可靠度的操作次數；其為一創新、簡單及有效的方法去抑制電阻轉換過程中所產生之變異。 於文中同時也建構出電阻式記憶元件其內部電阻轉換機制，研究中發現，不同厚度之金屬鈦電極將導致形成不同厚度之界面層，此一現象不僅改變了元件本身電阻轉換特性，於元件的可靠度與操作模式上，影響更為顯著，相關的電阻轉換機制與界面層形成厚度之相依性，已一併提出討論，再者，對於記憶狀態之導電機制分析與記憶元件之可靠度量測，如：記憶保持時間量測、操作次數耐久度測試、非破壞性讀取測試，以及多重儲存原理，於文中均有詳細的研究與探討。 此外，為改善電阻式記憶元件之電阻轉換特性，本論文針對元件結構上的改良，提出進一步有效地控制氧化鋯層中氧缺陷的濃度含量之方法；製程上改變其原本結構，在氧化鋯層內嵌入一極薄之鉬金屬層，隨後經由快速熱退火處理使鉬擴散至氧化鋯層內部，藉由調變氧化鋯層中氧缺陷的濃度含量，以大幅地提升電阻式記憶元件之電阻轉換特性，此一技術不僅可免除原本所需一高電場形成過程(forming process)之步驟，相關的研究成果顯現出此技術在非揮發性記憶體應用上的潛力。 最後對全文作一總結，並對未來可行的研究工作提出一些基本構想，希望以添加人造異質缺陷等方式，進而去侷限並控制電阻轉換發生之區域，來提升非揮發性電阻式記憶體高可靠度之效能。

Due to the popularity of consumer electronic products, such as mobile phone, laptop, and USB storage devices, the demand of nonvolatile memory (NVM) increases significantly with the years in the semiconductor industry. The mainstream of NVM nowadays is flash memory. The primary structure of flash memory is a MOSFET-like transistor with an inner floating gate. Based on the basic principle in flash memory, the logic high or low is determined by charges stored in the floating gate or not, which further alters the threshold voltage of the MOSFET-like transistor. The flash memory, however, has some drawbacks such as 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 novel NVMs such as ferroelectric random access memory (FeRAM), magnetroresistive random access memory (MRAM), phase change random access memory (PCRAM), and resistive random access memory (RRAM). RRAM primarily utilizes reversible resistive switching (RS) between the different resistance states to write/erase the user’s information, where the RS can be easily performed by applying voltage or current pulses. The RRAM device with a simple metal-insulator-metal structure has the merits of nondestructive readout, multi-level storage, high-density integration, and compatibility of the complementary metal oxide semiconductor (CMOS) process. So far, the RS phenomena have been observed in many materials, such as organic molecular, solid state electrolytes, perovskite materials, transition metal oxides, etc. However, several obstacles still need to be overcome in RRAM before realizing commercial applications, including the operation speed/voltage, the data retention time, and the related reliability issues. In this dissertation, the research purpose focuses on the interface engineering in the ZrO2- and CuxO-based RRAM devices. Due to the oxygen-gettering ability of the Ti top electrode, the self-aligned interface between Ti and oxide film serves as a series resistance and an oxygen sink. While applying the bipolar voltage sweeps, the interfacial oxygen migration would cause the redox reaction within the conducting filaments, leading to the formation and rupture of the conducting filaments. The effective RS region is suggested to be confined near the interface layer so that the RRAM device possesses lower variations of RS parameters. Moreover, we proposed a simple method to produce the various interface thicknesses within Ti/ZrO2 by changing the thickness of the Ti top electrode. The forming voltages and bias-polarity relation of the RS behaviors are found to be affected by the interface thicknesses between Ti and ZrO2 films. During the successive RS cycles, we also deduced that the evolution of the interface thickness has a significant influence on the device reliability. Some measurements related to the conduction mechanisms and reliability tests, such as data retention time, multi-level storage, nondestructive readout, and endurance cycling properties, are investigated in this thesis as well. To further improve the RS characteristics in RRAM, a simple and effective method was also proposed to control the oxygen vacancies in the ZrO2-based film by introducing an embedded a Mo metal layer. It shows that the forming process can be removed by inserting an embedded Mo metal within ZrO2 via a post-annealing process. Based on the experimental results, the embedded Mo serves as an oxygen getter, causing more oxygen vacancies to form within the ZrO2 layer. These controllable oxygen vacancies not only facilitate the formation of conducting filaments but also improve the memory performance in the ZrO2-based devices. Accordingly, our research indicates promise for the realization of RRAM by efficiently controlling the oxygen vacancies inside the oxide layer. At the end of this dissertation, the conclusion and the suggested future work are presented. We expect to confine the RS region by introducing the artificial extrinsic defects in RRAM devices and to further enhance their performance and reliability for next-generation NVM applications.