磁阻式元件之磁阻抗及氧化鎢電阻式記憶體的研究

Abstract

本論文主要研究磁阻式元件之磁阻抗行為及氧化鎢記憶體特性，以求對於電阻式記憶體的物理特性能有更深入地了解。 磁阻式元件是磁性隨機存取記憶體中扮演相當重要的角色，其中包含類自旋閥元件及穿隧式磁阻元件。磁阻式元件的直流電性行為已經被廣泛的研究，然而關於交流電性的行為則是很少被研究。因此在本篇論文的前半部分我們主要探討磁阻式元件的交流特性。 我們提供了一個藉由類自旋閥的交流特性，來對奈米氧化層在類自旋閥的貢獻做非破壞性的分析。此外我們以等效電路模型來解釋穿隧式磁阻元件的交流特性，並且在穿隧式磁阻元件Ru (5nm) /Cu (10nm) /Ru(5nm) /IrMn(10nm) /CoFeB(4nm) /Al(1.2nm)-oxide /CoFeB (4nm) /Ru (5nm) 發現在很高的交流頻率17.7百萬赫茲，此元件的虛部磁阻抗，有非常劇烈的變化可達17000%，這個發現使的磁穿隧元件有潛力應用於高頻的感應式元件。 本論文的後半部分，研究有關於氧化鎢電阻式隨機存取記憶體。由於電阻式隨機存取記憶體，具有極佳的微縮能力及具備做成三維元件的可能性，所以一直被看好能取代電子式儲存的記憶體；然而其基礎理論並不完備，且元件的表現還有待更進步。因此我們提出了一個具新穎性的氧化鎢電阻式隨機存取記憶體，其製作方式只需在現有半導體製程多一道曝光程序，並且完全符合CMOS製程。我們以快速氧化技術所製備的氧化鎢電阻式隨機存取記憶體，可表現出極佳的元件特性，例如低電流操作、極短時間操作、好的記憶維持能力及可達百萬次的操作次數。 對於氧化鎢電阻式隨機存取記憶體，我們進一步發現頂部電極的功函數大小可以改變傳導機制，在低功函數頂部電極其電流傳導機制符合空間電荷限制電流的理論，而在高功函數頂部電極其電流傳導機制符合熱離子發射理論。此研究提供了藉著改變頂部電極，有機會改善元件的特性表現。所以根據這個發現，我們設著去研究利用鎳當頂部電極的氧化鎢電阻式隨機存取記憶體。此新元件表現出非常好的特性，如低於200微安培的操作電流、大於100倍的阻值變化及在攝氏85度的環境下，大於300年的記憶維持能力。

This dissertation is devoted to study the magneto impedance effect of magneto resistive elements, and tungsten oxide resistive memory extensively for providing in-depth their physical understanding. The magneto resistive elements such as pseudo spin valve (PSV) and magneto tunneling junction (MTJ) play a role for magneto random access memory (MRAM). The DC behaviors for magneto resistive elements have been widely studied, but the AC behaviors are still rare. Thus, in the first half of this dissertation we focus on the AC behaviors of magneto resistive elements. By using AC characteristics, we provide a new non-destructive analysis method to study the nano-oxide layer effects of PSV. Besides, the equivalent circuit model is used to describe the AC behaviors for MTJ. For Ru (5nm) /Cu (10nm) /Ru(5nm) /IrMn(10nm) /CoFeB(4nm) /Al(1.2nm)-oxide /CoFeB (4nm) /Ru (5nm) system, a huge imaginary part of magneto impedance ratio more than 17000% is observed at high frequency 17.7MHz. Magneto impedance effect of the MTJ is potentially a sensitive sensor for high frequencies. In the second half of this dissertation, tungsten oxide resistive random access memory is studied extensively. Although resistive random access memory (RRAM) is forecasted to be the promising solutions to replace the electron storage memory due to its excellent scalability and 3D possibility, up to now, the fundamental understanding is still very limited and device performance is required to improve. Thus, we proposed a novel WOX for the RRAM system, because it requires only one extra mask under standard CMOS process. By using rapid thermal oxidation (RTO) for the WOX RRAM, we have demonstrated its good performance such as low switching current, fast programming speed, good thermal stability, and high endurance > 1M times for nonvolatile memory. Under this study, we further find the work function (WF) of the electrode is a key element determining the conduction mechanism for WOX RRAM. SCLC (space charge limit current) mechanism and thermionic emission mechanism are identified for low WF and high WF electrodes, respectively. These studies imply that the device performance may be significantly improved by selecting the proper electrode. With this guideline, Ni/WOX/W device is investigated. The new Ni/WOX/W device shows very good performance that operates at <200uA switching current, a 100X resistance ratio window, and extremely good data retention of > 300 years at 85 C.