MgO/CoFeB/Ta結構介面之垂直異向能探討

The Interface Effect on the Perpendicular Magnetic Anisotropy in MgO/CoFeB/Ta

申新煌(Hsin-Huang Shen)

指導教授 ： 陳恭

國立中正大學/理學院/物理學系/碩士(2012年)

摘要

磁性儲存記憶體(Magnetic Random Access Memory, MRAM)元件發展過程中，未來有採用垂直式記錄媒體之趨勢。垂直異向性(magnetic perpendicular anisotropy)的研究因此愈受到矚目。而MgO-CoFeB系統為目前研發MRAM重要的結構，其所具備的高異向能(Ku)、高磁阻率(MR%)以及高熱穩定性皆為MRAM不可或缺的特性。隨著元件尺寸日益縮小至奈米等級，介面效應也愈受到重視，然而在超薄鐵磁層介面結構中，垂直異向性的範圍以及利用退火關鍵步驟來調整垂直異向性的方式已經廣泛被研究，可是造成垂直異向性的詳細機制目前還尚未清楚，恐仍需更多的實驗結果。本論文延續本實驗室對此系統之研究，將透過熱退火處理對於覆蓋層影響垂直異向能之介面效應做深入的探討。
在這篇碩士論文中，我們利用濺鍍方式成長製作了一系列的MgO-CoFeB薄膜樣品，這些樣品是在矽基板(SiO2)上成長Ta(5)/ MgO(1)/Co20Fe60B20(x)/Ta(y)，分別經過不同溫度退火的程序對(A)固定覆蓋層y = 1.0 nm 或5.0 nm並改變鐵磁層CoFeB厚度x= 1.0~1.7 nm，以及(B)固定鐵磁層厚度x =1.2 nm和1.5 nm改變覆蓋層Ta厚度y = 0.5~5.0 nm，對這些樣品進行詳細的磁滯曲線分析與探討，以了解薄膜的物理特性。主要探討的部分包括：
覆蓋層厚度造成的死磁層
樣品經過熱退火處理之後，首先在(A)系列結構中，量測磁化量後利用m/A=M*(t-td) 鑒定死磁層(magnetic dead layer)的厚度，其中m為磁矩 (emu)，M為磁化強度 (emu/cm3)，A為基板面積(cm2)，t為濺鍍CoFeB厚度，td為死磁層厚度。實驗觀察出Ta為1.0 nm時，死磁層約為0.1 nm以及Ta為5.0 nm時，死磁層約為 0.5 nm，而M約為1190 emu/cm3。再透過另一個(B)系列的結構，來了解死磁層厚度在改變Ta護蓋層厚度時的變化趨勢，發現死磁層約為0.56 nm到達上限。此外發現改變退火溫度對於死磁層的影響並不顯著(約在1%之內)。
鐵磁層效應
我們選用(A)系列結構中校訂等效異向能常數，其方式是利用等效異向能模型Keff=Kv+Ki/teff，其中Keff=MsHk/2為等效異向能(erg/cm3)，Kv=-2πMs2為體積異向能(erg/cm3)，Ki為介面異向能(erg/cm2)，teff為CoFeB有效厚度，Ms為單位體積飽和磁化量，Hk為難軸飽和場。實驗結果顯示當覆蓋層Ta為1.0 nm時，其Kv約8.8 Merg/cm3，Ki理想值約1.5 erg/cm2，M約為1180 emu/cm3；當Ta為5.0 nm時，其Kv約8.2 Merg/cm3， Ki理想值約0.9 erg/cm2，M約為1150 emu/cm3。從這些結果也發現最佳退火溫度為315℃，並且當CoFeB為1.2 nm及Ta為1.0 nm時有最大的等效異向能約為2.5 Merg/cm3。此外，隨著鐵磁層厚度逐漸增加，較厚的鐵磁層具有更大的介面異向能。
3.覆蓋層效應
除了鐵磁層的厚度之外，覆蓋層Ta厚度對於垂直異向性的影響也十分明顯，我們以(B)系列的結構實驗顯示當Ta愈厚時，死磁層厚度也跟著增加，造成CoFeB有效厚度減少，因此飽和磁化量、矯頑場以及異向場皆隨著Ta厚度增加而逐漸降低。而覆蓋層Ta厚度為0.8 nm時，其等效異向能和介面異向能皆為最大值。此外，假設由MgO/CoFeB提供的介面異向能為定值，由實驗結果我們推測介面異向能分別為與CoFeB接觸之兩側介面所組成，Ki(MgO/CoFeB) 約為1.3 erg/cm2，然而隨著覆蓋層Ta逐漸加厚，造成MgO/CoFeB介面異向能的下降，推測原因是Ta擴散進入CoFeB的多寡也間接影響MgO層的結晶品質。

關鍵字

垂直異向性 ； 介面異向能 ； 死磁層

並列摘要

In develop of magnetic random access memory (MRAM) device, using the perpendicular recording media has become a trend in the future. In hence, the study magnetic perpendicular anisotropy has become popular. Recently, it is found that MgO-CoFeB based MRAM structure is important because these systems have high effective anisotropy (Ku), high magnetoresistive ratio (MR%) and high thermal stabilization these advantage. A surface/interface effect, which is usually ignored in a bulk material, becomes crucial as the size reduces to nanometer scale. However, the mechanism of PMA is not yet clear and more experimental results will be needed. In this thesis, we fabricated a series of top CoFeB-MgO based structures by sputtering system and did a detailed study on the interface structure and magnetic characterizations. Two sets of samples are prepared: Si/SiO2/Ta(5)/MgO(1)/ Co20Fe60B20(x)/Ta(y) (units: nm.) and these samples are post-annealed at different annealing temperatures. Set A consists of fixed y =1.0 or 5.0 and changed x=1.0 - 1.7. Set B consists of fixed x=1.2 or 1.5 and changed y=0.5 - 5.0. In order to realize magnetic characterizations of films, we did detailed measurements of the magnetic hysteresis (M-H) curve analysis and the results are divided into three parts:
1. The magnetic dead layer (MDL) induced by depends upon the thickness of Ta capping layer:
For the samples in set A, we used m/A=M(t-td) formula to calibrate the MDL thickness, where m is magnetic moment (emu), M is magnetization (emu/cm3), A is substrate area (cm2), t is deposited thickness of CoFeB and td is the thickness of MDL. We observed that the MDL is 0.1 nm when Ta is 1.0 nm while the MDL is 0.5 nm when Ta is 5.0 nm, and M is approximately 1190 emu/cm3 in both cases. For set B for the dependence of MDL on thickness of Ta capping layer is calibrated. We found that the thickness of MDL linearly drops from ~ 0.56 nm to 1 nm Ta decreases from 5 nm to 1 nm. In addition, the effect of annealing temperature on MDL is not significant.
2. The effect of thickness of magnetic layer on the effective anisotropy:
In set A, the effective anisotropy of ferromagnetic films is calibrated. An effective anisotropy constant may be defined as Keff =Kv+Ki/teff, where Keff =MsHk/2 is the effective anisotropy (erg/cm3), Kv=-2πMs2 is the bulk anisotropy (erg/cm3), Ki is the interface anisotropy (erg/cm2), teff is the effective thickness of CoFeB, Ms is magnetization and Hk is saturation field of hard axis. In our experiments, we found
that the Kv is 8.8 Merg/cm3, optimal Ki is 1.5 erg/cm2 and M is 1180 emu/cm3 when capping layer Ta is 1.0 nm while the Kv is 8.2 Merg/cm3, optimal Ki is 0.9 erg/cm2 and M is 1150 emu/cm3 when Ta is 5.0 nm. In addition, the optimal annealing temperature of 315℃ with CoFeB 1.2 nm, the effective anisotropy reaches 2.5 Merg/cm3.
3. The effect of thickness of cap layer on the effective anisotropy :
The impact of the thickness of capping layer Ta on the perpendicular anisotropy is investigated. In set B, we show that increasing the thickness of the cap layer Ta also leads to the increase of the MDL. As a result, the Ms, Hc and Hk all become smaller. The effective anisotropy and interface anisotropy constant reach maxima when Ta is 0.8 nm. In addition, we found the Ki(MgO/CoFeB) is approximately 1.3 erg/cm2. The Ki(MgO/CoFeB) degrades as the capping layer Ta thickness increasing. This may be due to the Ta diffusion into CoFeB layer and indirectly introduce defects and affect the crystalline quality of MgO layer.

並列關鍵字

perpendicular anisotropy ； interface anisotropy ； magnetic dead layer

參考文獻

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