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研究生:鄭智維
研究生(外文):Cheng Chih-Wei
論文名稱:CoFeB結構垂直磁異向性之縱深分析
論文名稱(外文):Depth profile analysis in perpendicular magnetic CoFeB structure
指導教授:陳恭陳恭引用關係
指導教授(外文):Chern Gung
口試委員:黃迪靖蘇炯武蔡崇智
口試委員(外文):Huang Di-JingSu Chiung-WuTSAI TSUNG-CHIH
口試日期:2011-06-15
學位類別:碩士
校院名稱:國立中正大學
系所名稱:物理學系暨研究所
學門:自然科學學門
學類:物理學類
論文種類:學術論文
論文出版年:2011
畢業學年度:99
語文別:中文
論文頁數:65
中文關鍵詞:垂直異向性鈷鐵硼縱深分析覆蓋層效應
外文關鍵詞:perpendicular anisotropyCoFeBDepth profile
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磁性隨機記憶體(Magnetic Random Access Memory, MRAM)元件的發展中,垂直異向性(Perpendicular Magnetic Anisotropy, PMA)的應用對於熱穩定性的改善是非常重要的關鍵。然而,絕大多數擁有垂直異向性現象的材料,如:L10-FePt合金、Co/Pt合金或多層膜以及過渡金屬-稀土化合物等等,無法同時滿足所有元件其他需求如:高磁阻率、高異向能常數和低翻轉電流等。在2010年7月日本東北大學H.Ohno等人發表了具有垂直異向性的 Ta/CoFeB/MgO/CoFeB/Ta 穿隧結構,並將準自旋閥結構分成上結構(Top structure)和下結構(Bottom structure)探討,他們發現將鐵磁層CoFeB的厚度下降到1.3nm時,經過升溫退火的過程之後,磁滯曲線的易軸會由水平導向轉到垂直導向。由於過去以較厚的(> 3 nm)鐵磁層的CoFeB/MgO/CoFeB 結構具備高磁阻率和低翻轉電流等特性,將此結構延伸到垂直異向性的實驗引此引起了廣泛的注意。
在這篇碩士論文中,我們用濺鍍方式製作了一系列的CoFeB-MgO的薄膜樣品並對這些樣品進行了詳細的磁性與結構組成的分析,主要探討的課題包括:
I. 製作樣品分成上下結構:
上結構:
Sub/Ta(5)/MgO(1)/CoFeB(x)/Ta(1、5)及 Sub/Ta(5)/MgO(1)/CoFeB(1.2)/Ta(y)
下結構:
Sub/Ta(10)/CoFeB(x)/MgO(1)及 Sub/Ta(10)/CoFeB(x)/MgO(1)/Ta(5) (以上單位均為nm)
II. 垂直異向性與鐵磁特性: 經由實驗發現,退火的過程成為垂直異向性表現優劣的關鍵,樣品經過退火之後,其飽和磁化量、矯頑場與方正性的表現都比退火前好上許多,且方正性(Mr/Ms)與矯頑場(Hc)會有隨著厚度增加而降低的現象,而退火之後垂直異向性出現的範圍約在 X=1.1~1.7 nm,比退火前x=1.1~1.5nm的範圍大一些。
III. 覆蓋層效應: 將改變覆蓋層厚度的樣品經過退火之後,其擁有垂直異向性的範圍約為 y=1~1.75 nm,顯示覆蓋層Ta的厚度對於垂直異向性的出現造成ㄧ定的影響,其飽和磁化量、方正性與矯頑場會隨著厚度增加而降低。
IV. xps 量測與界面分析: 由XPS縱深與元素分析顯示,覆蓋層Ta有明顯擴散進入鐵磁層的現象,但進入的型態有分別,覆蓋層Ta厚度為1nm的樣品在CoFeB的深度其滲透進入的Ta型態為氧化的TaOx。而Ta 2nm的樣品在CoFeB的深度中則為純金屬Ta佔大多數,因此在具有垂直異向性的樣品中觀察到的TaOx是否對於垂直異向性有直接性的影響還需再做更進一步的探討。

It has been realized recently that a perpendicular magnetic anisotropy (PMA), which may enhance thermal stability, is crucial for the development of high density magnetic random access memory (MRAM) devices. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth / transition metal alloys, L10-ordered (Co or Fe)–Pt alloys, and Co/(Pd , Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time.
A recent observation of CoFeB-MgO-based magnetic tunnel junctions has shown perpendicular anisotropy and possible features to match the new generation MRAM applications. We studied annealing effects on perpendicular anisotropy in CoFeB-MgO magnetic tunnel junctions. The results show that annealing is an effective method to improve the perpendicular anisotropy of a CoFeB-MgO system. In addition, the physical origin of the PMA in the CoFeB-MgO based structure is not quite clear. Although it has been suggested that the PMA is mainly stabilized by the Fe 3d and O 2p hybridization at the CoFeB/MgO interface, the CoFeB/Ta interface has also been found to be crucial for the PMA in MgO/CoFeB/Ta structures. These all indicate that the PMA in CoFeB-MgO-based structures may depend upon various factors, and thus a detailed examination is needed.
In this thesis, we have fabricated a series of top and bottom CoFeB-MgO based structures and did a detailed study of the interface structure and magnetic characterizations. In addition, the capping effects on the CoFeB are also studied.
We found the perpendicular magnetic anisotropy only exists in a limited region of CoFeB thickness. In the top structure, the effective CoFeB thickness is between 1.1 – 1.7 nm while in the bottom structure the effective CoFeB thickness is 1.0 – 1.7 nm. The magnetic dead layer of the top structure is within 0.1 nm and no MDL is observed in the bottom structure. However, the coercivity of the bottom structure is below 30 Oe while the coercivity of the top structure may reach a much higher value of 180 Oe. The discrepancy of the coercivity between the bottom and bottom structure is attributed to the by the interface structure, which will be discussed in detail in this thesis. Moreover, the effective anisotropy constants, which are the main parameter for a perpendicular material, are also analyzed. The highest anisotropy energy of the present film is ~ which may correspond to a thermal factor of >40 kT of a 40 nm cell size. In priciciple, this value may be further enhanced if the interface of a thinner CoFeB film is smoother.
X-ray photoelectron spectroscopy (XPS) depth profile studies have been carried out to understand the element diffusion and chemical bonding at the interface of CoFeB. The boron, after 300 C anneal for an hour, boron clearly diffuses out and approaches to the Ta side. It is well known that Ta behaves as a good getter and this is why boron intends to diffuse to the Ta/CoFeB interface. However, the present study also reveals that it is TaOx which is dominated at the Ta/CoFeB interface. This observation may be useful for the understanding of the physical origin of the stabilization of the perpendicular anisotropy.

第一章.序論..............................................................15
第二章.理論背景...........................................................17
2.1 CoFeB-MgO-CoFeB Tunnel Junction......................................17
2.2 垂直異向性(Perpendicular magnetic anisotropy, PMA)....................17
2.3 Oxide Interface......................................................20
2.4 晶體結構與擴散作用.....................................................22
2.5 磁性穿隧接面(Magnetic Tunnel Junction , MTJ)..........................24
第三章.實驗流程...........................................................25
3.1 實驗步驟.............................................................25
3.2 X-射線光電子能譜儀 (X-ray Photoelectron Spectroscopy,XPS).............26
3.3 退火 ................................................................28
第四章.實驗結果與討論......................................................30
4.1 上結構MgO/Co20Fe60B20/Ta與下結構Ta/ Co20Fe60B20/MgO之垂直異向性探討.....30
Magnetic Dead Layer.....................................................30
等效異向性常數 (Effective Anisotropy Constant (Keff)).....................40
4.2 覆蓋層的效應 ........................................................43
4.3 擴散效應與深度分析....................................................50
第五章. 結論.............................................................62
參考文獻.................................................................65


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