非揮發性二氧化鉿電阻式記憶體之研究

Abstract

在現今日新月異的科技時代，從一開始的個人電腦記憶體市場逐漸地轉移到消費性電子產品時，因為可攜式電子產品像是行動電話、數位相機、平板電腦等大量興起，使得非揮發性記憶體需求量自1990年代後期起有著爆炸性的成長。為了追求高密度、高存取速度，低功率消耗和便宜的資料儲存記憶體，造就了非揮發性記憶體相關科技的蓬勃發展。在未來，為了提供相較於現今主流的快閃記憶體更高的記憶體密度，更快的操作速度，更低的功耗，以及更低的成本，新世代非揮發性記憶體的技術開發是相當重要的研究課題。由於爲了克服傳統記憶體的限制，有許多人都在研究下一世代的非揮發性記憶體。在許多新穎的記憶體技術中，考量未來的微縮潛力，電阻式記憶體被認為是最可行的選擇之一，因為其擁有低功率消耗、高速操作和高密度整合等特性，因此深具潛力。 此論文使用相容於傳統半導體製程的二元氧化物為基礎的材料製作非揮發性電阻式記憶體元件，使用數種氧化物材料並搭配不同的金屬上電極製備出類似於傳統電容結構的金屬/氧化層/半導體電阻式記憶體。我們將藉由電性量測和物性分析的技術來探討記憶體元件的切換特性及記憶表現。另一方面，透過元件結構的變化進一步釐清電阻切換機制並提出一合理的物理模型來解釋本論文中所觀察到電阻式記憶體元件的切換行為。 首先，我們利用二氧化鉿材料作為電阻切換層製作出具有不同金屬電極的氧化鉿電阻式記憶體，指出不同金屬上電極對切換模式有很大的影響。發現當使用氮化鈦/鈦、鉑和鉭金屬作為上電極時，氧化鉿電阻式記憶體呈現出雙極切換行為；反之，當採用銅和鎳金屬時，則具有單極切換的特性。此結果可用存在於二氧化鉿電阻層中不同形式的絲狀傳導路徑來解釋。氧缺主導的傳導絲狀需藉由外加電場方向的改變驅使氧陰離子的遷移進而達成傳導路徑的連接與斷裂造成雙極切換，然而，具有電化學反應的銅和鎳金屬則可藉由擴散方式進入二氧化鉿電阻層中形成金屬性傳導絲狀，再藉著重置電流產生的焦耳熱就能夠使之斷裂達到單極切換。進一步使用鎳金屬結合其他二元氧化物材料包括二氧化矽、三氧化二鋁和二氧化鋯研究發現鎳燈絲造成的單極切換行為與作為阻值切換層的氧化物材料有密切關係。接著，我們設計幾種不同雙層電阻結構的電阻式記憶體元件，其目的是用來探討絲狀傳導路徑連接與斷裂發生的位置並提出一包含傳導絲狀的組成與形狀的電阻切換模型來說明阻值切換的現象。 最後，在比較各種不同形式的電阻式記憶體之後，我們提出一個可整合於現今主流的互補式金氧半場效電晶體技術之鎳/二氧化鉿/矽電阻式記憶體。此記憶體元件展現了高元件良率、低操作電流 (小於 10 微安培)、高開關電阻值比 (大於 102) 、低變異性 (包含重覆覆寫過程與不同元件間)、長保存時間(大於十年)、高可靠度，且更重要的是其具有單極切換的能力非常適合與二極體整合應用在未來高密度的記憶體陣列上。

As the PC-driven memory market gradually shifted to a consumer device-driven market, battery-powered portable electronics, such as mobile phones, digital cameras, and tablet devices, have significantly increased the demand for nonvolatile memory (NVM) from the late 1990s. NVM technology has been extensively developed to achieve high density and speed, low power consumption, and affordable data storage. Next-generation NVM has even better specifications and is of significant technological importance. Studies have examined several next-generation NVMs to overcome the limitations of conventional memory. Resistive-switching random access memory (RRAM) has superior scalability, low power consumption, high-speed operation, and high-density integration; thus, it is considered a promising candidate. This study involved successfully fabricating various RRAMs with CMOS-compatible binary oxide materials combined with different metal electrodes. The device characteristics were achieved through a capacitor-like metal/insulator/Si (MIS) structure. The resistive-switching (RS) properties and memory performance were characterized by electrical measurement and physical analysis techniques. Furthermore, the RS mechanism was achieved by examining the bilayer-structured RRAM. This study suggests a reasonable physical model for explaining the RS phenomenon in the RRAM. An HfO2-based RRAM with TiN/Ti, Ta, Pt, Cu, and Ni metal electrodes was fabricated. The devices with TiN/Ti, Ta, and Pt top electrodes (TE) show bipolar RS, but those with Cu and Ni TEs exhibit nonpolar RS, suggesting that the behavior of RS is strongly dependent on the metal electrodes. Bipolar RS occurs because a bulk RS of HfO2 is present from the migration of oxygen anions, and the nonpolar RS with electrochemically active electrodes such as Ni and Cu are the result of the migration of metal cations, electrochemical metallization, and local Joule heating effect. Conversely, RRAMs with SiO2, Al2O3, HfO2, and ZrO2 films using Ni TEs were also fabricated to investigate the effects of the crystallinity of binary oxides on Ni metal filaments. Moreover, various bilayer-structured RRAMs were designed to examine the RS location where the connection and rupture of the conducting filaments occur. Thereafter, this study proposes the RS model involving both the filamentary composition and shape. Finally, a fully CMOS-compatible Ni/HfO2/Si RRAM cell with promising NVM characteristics is demonstrated. This RRAM cell possesses a high device yield, stable switching endurance (>2000 cycles), low operational current (<10 μA), excellent thermal stability, long data retention (10 years), good immunity to read disturbance, and unipolar RS characteristics suitably integrated with a rectifying diode for future high-density memory array applications.