電阻式記憶體之特性研究及電性探討

Abstract

隨著可攜式電子元件的蓬勃發展，如手機及數位相機，非揮發性記憶元件在半導體產業的需求也日益增加。現今主流的非揮發性記憶元件為快閃式記憶體，然而快閃式記憶體存在一些必須克服的問題，如操作電壓較高、操作速度較慢以及記憶力隨元件尺寸微縮而降低等。因此新興的非揮發性記憶體也逐漸受到重視。在這之中，以電阻式記憶體（RRAM）最被看好。電阻轉換記憶元件具有操作電壓低、功率消耗低、操作速度快、可微縮性高、記憶時間長、耐久力佳及尺寸小、易和半導體製程匹配等優勢。RRAM是透過外加電壓或電流將氧化層決定至高或低阻值，以用以作為分辨0或1不同阻態之記憶體應用。然而，至今主要的轉態機制又依材料特性而異，而且影響RRAM的電性表現的效應至今依然有著探討的空間。 在此論文中，分別以二氧化鉿、三氧化二釓和鋁酸鑭材料來作為氧化層，並探討其在不同效應下的電性表現行為。這三種高介電材料是近年來閘極氧化層應用的候選材料。因此若能有良好的電阻轉態特性，則有利於1T1R在製程上的整合製作。探討的參數變因包含了本質的調變和外質的調變。本質調變參數有：上電極材料（鉑、鈀、鎳、鈦、鉭、鋁、鎢、鋁、氮化鈦）、操作模式（電壓驅動和或電流驅動）、操作極性（單極操作和雙極操作）、電極製程條件（電子束蒸鍍和濺鍍沉積）、沉積氧分壓條件、沉積溫度條件。而外質調變參數有：氮環境熱處理、氮氧環境中熱處理、紫外光效應。無論是單極操作或雙極操作的電性轉態行為，我們均提出可能的物理模型架構來解釋轉態機制，並且我們也能在電性表現上有所提升和改善。 研究結果顯示，我們上述幾項研究變因參數皆會對轉態特性的好壞有所影響，而且氧化層和上電極之間的介面對於轉態特性的好壞扮演著極為重要的角色。選取適當的製程方式和製程參數，再搭配適合的上電極，我們能大幅提升單極操作的二氧化鉿的轉態特性。而當我們對介面進行處理，如照紫外光、以氟化氫溶液蝕刻表面、熱處理，皆得到相對較差的特性。我們的可逆實驗更加證實了介面的好壞可由我們人為的影響，來決定最後單極轉態電性的操作穩定性。在不同上電極材料的研究中，我們曉得二氧化鉿單極操作的特性是和上電極材料有關，更深入的研究，發現是上電極和氧化層之間所形成的中間層在主導著轉態電性的好壞。普遍接受的單極操作機制是，在ON操作時施加正偏壓將氧化層裂化，大量的氧離子被粹取至上電極介面。而OFF操作時，大量電流流過細小的細絲傳導路徑，產生高溫的焦耳熱，而因造成細絲路徑斷開，而回到高電阻態。因此，我們選取能提升二氧化鉿材料熱穩定性的材料鋁來作適當的添加，當二氧化鉿的熱穩定性被提升時，的確能有效的提升其在單極操作時的耐久度。在不同的熱處理和氧處理下，當介面的缺陷更加的被修復時，而造成細絲路徑的聚積，只是部份幾條細絲做傳導，因此在ON/OFF之間的切換更加的好控制，因此電壓散佈度也能有效的控制。而當氧化層表面受紫外光照射，或是電漿離子轟擊時，會造成氧化層表面的缺陷數量增加，如此會造成細絲路徑更加的散亂和任意的形成，因此會得到相對較差的轉態特性。 在雙極操作方面，我們也進行了研究。雙極特性的穩定與否，電極材料亦扮演著關鍵角色。正開負關或是負開正關的電阻轉態特性及其特性的穩定度，決定於介面層形成的好壞。我們以脈衝雷射沉積法(PLD)，來探討不同沉積溫度和氧分壓環境下所沉積的氧化層薄膜，其對於轉態特性的影響。研究成果顯示，在高溫下沉積的薄膜，其結晶性較好，表面的粗糙度變差，而在下電極和氧化層之間會有中介層形成。這層中介層也提供了氧的儲存槽的作用，以供氧離子能作來回的移動，因此會有較佳的電阻轉態的特性。施加負偏壓，將氧離子驅入至儲存槽中，因而路徑形成，轉態至ON態，施加正偏壓，將氧離子拉回氧化層中，因此路徑被修復斷開，轉態至OFF態。而在氧分壓高的環境中沉積，過多的氧原子會在沉積過程中反覆的和濺擊出來的沉積原子反應和碰撞，長成的薄膜結構較為緻密，可長成非晶態的鋁酸鑭氧化層。在氧分壓大中的環境沉積，能得到較粗糙的表面，薄膜和氧也鍵結愈強，表示氧空缺的量會大幅降低。而氧分壓大沉積過程易有碰撞，有些氧離子會被撞擊至試片表面，愈易造成中介層的生成。電性表現方面，在氧分壓大中沉積者有較大崩潰電壓、較低漏電流、較穩定的轉態特性及較佳的操作電壓散佈度。從電性傳導的角度，來回推適合穩定RRAM操作的電性傳導應該為何。值得提出的一點是，我們亦成功的製作出透明的電阻式記憶體，克服了ITO電極不適合作為電極操作的問題。並擴展電阻式記憶體的可利用範圍。

Due to the popularity of portable equipment, such as mobile phone and MP3 player, the requirements of nonvolatile memory (NVM) increase significantly in the semiconductor industry. Flash memory is the mainstream among the nonvolatile memory devices nowadays, but several bottlenecks emerge, such as high operation voltage, low operation speed, poor endurance, and the scaling problem. The device dimensions are continuously scaled down, the flash memory faces the challenge of thin tunneling oxide that causes an unsatisfactory retention time. Therefore, the novel NVMs attract much attention in recent years. One of the promising candidates of next-generation NVMs is the resistive random access memory (RRAM) owing to its low operation voltage and power, high operation speed, high scalability, good endurance, small size, etc. However, the RRAM devices still exist many issues needed to be improved before the commercial application. In this thesis, we investigated the electrical characteristics by using HfOx, Gd2O3, and LaAlO3 as the insulator layer. The three kinds of high-k dielectrics are the candidates that have been identified by the semiconductor industry as a potential replacement for SiO2 in gate dielectric layers. Once these materials can exhibit reliable resistive switching properties, it can be a good choice for 1T1R integration application. The varying parameters we focus on including the intrinsic modulation (about metal-insulator-metal itself) and extrinsic modulation (post treatment). The intrinsic modulated parameters, such as top electrode material (Pt, Pd, Ni, Ti, Ta, Al, W, Cu, and TiN were used, respectively), operation mode (voltage bias operation or current bias operation), operation polarity (unipolar switching or bipolar switching), electrode fabrication process (E-Gun deposition or sputtering deposition), oxygen partial pressure condition and deposition temperature and the extrinsic modulated parameters, such as thermal treatment in N2, thermal treatment in N2/O2, UV light exposure effect, etc., were all discussed. For the unipolar and bipolar resistive switching characteristics, we both proposed the possible model to explain the switching mechanism. In addition, we can improve the electrical properties by modulating the above treatment. The results reveal that the switching characteristics can be influenced by the modulated parameters listed above. In the investigation on electrical properties of HfOx films by using different metal materials as top electrode, we observed that the unipolar switching characteristics are highly correlated to the top electrode materials. We further found that the interfacial layer formed between electrode and insulator film during deposition greatly dominates the switching properties. Based on the proposed unipolar switching mechanism, the general accepted model is the formation and rupture of the conducting filamentary paths, which is driven by the field for SET (OFF to ON) operation and thermal effect for RESET (ON to OFF) operation. We tried to improve the thermal instability of HfOx by adding Al thin layer at the top insulator surface. Once the thermal stability of HfOx was improved, higher thermal stability of HfAlO layer can effectively prevent the thermal noise against the unintentional stimulus, so the switching characteristics will be much better than that of other samples. In the study on post treatment, when samples are treated by oxygen gas at high temperature, defects at the surface will be oxidized and reduced. This can effectively confine the conducting filamentary paths to some local weak point, thus the random distribution of the conducting filamentary paths inside the insulator films can be avoided. The high and low resistance values can be greatly improved under the certain conditions of oxygen gas treatment. When samples surface were exposed under UV light (wavelength and laser intensity is about 365 nm and 80 mW/cm2) or plasma ions bombardment, defect density at the insulator surface will be increased. Once the defect density increases, the filamentary paths tend to form in a random connection under the applied bias, so a much poor switching properties would be expected. We can modulate the defect density at the insulator surface by the depicted experiments above to control the resistive switching characteristics. In bipolar operation, the electrode material also plays an important role to the bipolar switching properties. We also observed that the operation polarity depends on the interfacial layer. We investigated the process condition effect to the switching properties based on by pulsed laser deposition. Different process condition, such as deposition temperature and oxygen partial pressure, were experimented and discussed. The results reveal that when thin films deposited at higher temperature, it has better crystallinity, larger surface roughness, and a thicker interfacial layer formed at the bottom electrode and insulator interface. The higher crystallinity can provide more grain boundaries inside the insulator films, which facilitate the migration of the movable oxygen ions. The interfacial layer can serve as a good oxygen reservoir for the oxygen ions rested under the negative bias. So, the state can be switched to ON state. When the positive bias was applied, the oxygen ions back to oxidize with the metallic defects inside insulator films, so the state was switched to OFF state. We also discuss the amorphous insulator films as the application for RRAM devices on LaAlO3 thin films grown under different oxygen partial pressure. We clearly explain how different oxygen partial pressure influences the surface roughness, the formation of the interfacial layer, the leakage current density, the forming voltage and the transparent resistive switching characteristics of the LaAlO3 thin films. The micro-structure and oxygen concentration different inside LAO thin films may be the main reason for the distinction of the electrical characteristics, as well as the resistive switching properties. LAO films grown at higher oxygen partial pressure is beneficial for a more reliable resistive switching performance, because the formation of the interfacial layer and lower oxygen vacancy concentration exist in the LAO thin film. Migration of the oxygen ions between the interfacial layer and the LAO films under applied bias may be the possible switching mechanism. We successfully fabricated the transparent RRAM (T-RRAM) based on Gd2O3 and LaAlO3 thin films on glass substrate and extended the application of RRAM to the transparent electronics filed.