金屬氧化物電阻式記憶體之軟崩潰機制研究與元件特性改善

Abstract

電阻式記憶體在穩定操作前都需要一個大電場去軟崩潰絕緣層[1, 2]，但大崩潰電場會劣化其他整合的元件，使元件在微縮上受到限制，所以降低崩潰電場是非常重要的課題，也是業界像是台積電想解決的問題。 從文獻得知RRAM的崩潰電場和絕緣層的鍵能有很大的關係，本篇文章著重於材料的化學特性去作探討---電負度，觀察絕緣層原子和原子之間電負度差是如何影響金屬氧化物電阻式記憶體[3, 4]，以氧離子對接不同的金屬離子作為RRAM絕緣層，分別是Bi2O3、SnO2、Ga2O3、ZnO、Al2O3、ZrO2、HfO2 [4]。實驗上得出金屬和氧的電負度差值越小，阻絲生成的電場就越小，然而也發現造成電阻式記憶體在阻絲生成過程中會有漏電流的主要原因和金屬氧化物的自身金屬功函數以及電子親和力有相當大的關係，漏電流所產生的熱，也會使阻絲形成時的鏈鎖反應更激烈，進而使崩潰電場下降。 根據以上實驗，以 Pt/Bi2O3/HfO2/TiN雙層新結構，可使氧化鉿電阻式記憶體的軟崩潰電場下降，然而藉由高斯電場定律可得知不同介電常數的薄膜疊層在一起時，給予一電場時，這兩層薄膜所獲得的實際電場會有所不同，後來選用k值相對於HfO2還小的氧化鋅ZnO做為疊層，再次使崩潰電場大幅下降，並搭配COMSOL電場分布模擬來驗證此概念。 最後再以疊層結構優化RRAM元件特性，將氧化鉍Bi2O3沉積在切換層的區域(Pt/HfO2/Bi2O3/TiN)，因為切換層中的氧和金屬鍵結容易斷鍵，使此元件擁有小的Set電壓以及更大的On/Off 比例。元件的可靠度也達一定的水準，也利用電性參數迴歸分析，更了解元件的切換機制。

RRAM device need a large voltage to form conduction path[1, 2]. Such large voltage will degrade these component which integrated with RRAM. Therefore, reduce the forming breakdown electric field is a very important subject waiting to be solved. From many literatures it can be learned that there is a relation between soft breakdown mechanism and bond energy of insulator. This article focuses on how the difference of electronegativity between the atoms of the insulating layer affect the metal oxide resistive memory [3, 4]. We chose seven kinds of metal oxide which have different electronegativity between oxygen and metal, such as Bi2O3, SnO2, Ga2O3, ZnO, Al2O3, ZrO2 and HfO2, to be insulating layer.[4] The experimental results show that the soft breakdown electric field will decline as difference of electronegativity between the metal and the oxygen decrease. Furthermore we found that the leakage current during forming process has a considerable relation with metal work function and electron affinity of metal oxide. The leakage current will generate heat and make chain effect in forming process more intense, so that the breakdown electric field decline. Based on the above experiments, Pt/Bi2O3/HfO2/ TiN Bilayer structure was manufactured. It can reduce soft breakdown electric field of hafnium oxide RRAM by inserting a Bi2O3 between Pt and HfO2. However, from Gauss’s law we can know that the electric field of Bi2O3 and HfO2 are different while a voltage is applied at electrode, because their dielectric constants are different. And then we chose ZnO to replace Bi2O3 because it has smaller dielectric constant than HfO2. This layer makes the breakdown electric field of RRAM device decrease significantly. COMSOL electric field distribution simulation was used to verify this concept. Finally we use other double layer structure to improve RRAM characteristic by depositing a thin film Bi2O3 at switching layer (Pt/HfO2/Bi2O3/TiN). Because the bond between oxide and metal is weak in switching region, this RRAM’s set voltage is smaller and the on/off ratio is larger than device without Bi2O3. The reliabilities of device are all reaching certain level. And we know more about the switching mechanism of (Pt/HfO2/Bi2O3/TiN) by the I-V parameter fitting.