具垂直異向性磁穿隧元件之自旋軌道矩磁化翻轉與製程優化

Abstract

磁阻式隨機存取記憶體在記憶體市場佔有一席之地，其中自旋軌道矩式磁阻式隨機存取記憶體具有很大的優勢，其具有非揮發的特性和較低的能耗，相較於自旋轉移力矩式軌道矩，自旋軌道矩式讀寫電流分離架構使其具有更高的可靠度，為了具有更好的熱穩定性及更高儲存密度，具垂直異向性的自旋軌道矩式記憶體也推動了磁性記憶體的發展邁向嶄新的一頁。在本文中探討自旋軌道矩式記憶體的核心結構，磁穿隧元件，利用不同鈷鐵硼鐵磁層及氧化鎂穿隧層的調整，並配合適當的熱處理製成具垂直異向性的試片，且為了防止離子束蝕刻本身造成的回鍍現象，本論文進行一系列的改善，包括訂定一套準則控制蝕刻時間，並以這個為基礎，進行不同角度、蝕刻深度、不同能量的離子束蝕刻及額外的小角度蝕刻的製程優化，最終防止回鍍現象，並且改善成品的良率至八成及提升穿隧磁阻至20%，良好的垂直異向性及較大矯完力的釘扎層也經由這些製程優化後得到。最後，在本文也實現以電流誘發自旋軌道矩翻轉其自由層的磁化方向，並且得到熱穩定性因子可達97的微米等級磁性穿隧元件。

Magnetoresistive Random Access Memory occupies a place in the RAM market. Among the variety of RAM, spin-orbit torque magnetoresistive random access memory (SOT-MRAM) has enormous advantages, including non-volatile and lower consumption. Compared with spin-transfer torque magnetoresistive random access memory (STT-MRAM), SOT-MRAM has better endurance due to the circuit architecture separating the read and write paths. To attain better thermal stability and high storage density, perpendicular magnetic anisotropy SOT-MRAM has been proposed, and it boosts developments for SOT-MRAM. In this thesis, I investigate the core part of SOT-MRAM, called magnetic tunnel junction (MTJ). I utilize different CoFeB and MgO tunneling barrier thicknesses and proper heat treatment to fabricate SOT-MTJ with perpendicular magnetic anisotropy. In addition, I conduct a series of adjustments for ion beam etching to prevent the redeposition effect. The method improvements include a standard for etching time, primary etching angle, the intensity of the etching beam, depth of channel layer, and additional small angle etching. After optimized fabrication via mentioned approaches, it can minimize and even prevent redeposition in MTJ. Moreover, the yield rate is up to 80%, and the TMR ratio reaches 20%. After optimization, well-defined PMA and relatively large fixed layer coercivity can also be obtained. Finally, current-induced SOT-driven magnetization switching can be realized, and the thermal stability factor with 97 can be derived from our micro-scale p-MTJs.