氧化鎂基磁性穿隧接面之顯微結構與磁阻研究

Abstract

本研究以直流與射頻磁控濺鍍的方式在矽基板上鍍製磁性穿隧接面，利用超高真空的濺鍍機，配合電子束微影及氬離子蝕刻來製作微米尺度的自旋依賴穿隧接面。本實驗所鍍製的磁性穿隧接面是使用準自旋閥的結構，主要是利用兩層鐵磁性薄膜頑磁力不同，當施加一外加磁場時，隨著磁場的變化，形成兩鐵磁性層磁化方向平行與反平行兩種狀態，並利用此兩種狀態來得到低電阻和高電阻的差異。我們將針對MgO 薄膜成長於不同的鐵磁性層時（Co、CoFe、CoFeB、CoFeC)，探討其微結構和穿隧磁阻的變化，同時藉由改變工作壓力和熱處理溫度來探討MgO 薄膜微結構的改變。 經由穿透式電子顯微鏡的分析顯示，使用Co 或CoFe 底層將使MgO 薄膜形成奈米晶粒和非晶質混合的結構，且MgO 薄膜的成長不具特定結晶方向，呈現散亂的晶格排列方式。但是當MgO 薄膜成長於非晶質的CoFeB、CoFeC 或SiO2 底層時，MgO 薄膜在初鍍狀態即可具有(001)優選方向。從X 光繞射的分析顯示，當MgO 薄膜經過400 ℃、30 分鐘的熱處理後，其(001)織構未有顯著的提升，但有助於MgO 薄膜應力的釋放。 根據穿透式電子顯微鏡以及穿隧磁阻的研究結果顯示，當Os (2 nm)/CoFeB (6 nm)/MgO (2.5 nm)/CoFe (5 nm)/Ta (5 nm)樣品經過300 oC、30 分鐘的熱處理之後，可以得到約25 %的穿隧磁阻值且CoFeB薄膜會利用MgO 的表面形成MgO (001)[100]//CoFeB (001) [110]的磊晶關係。此外，由於CoFe 薄膜成長於MgO 表面時不具有特定的結晶方向，呈現散亂的晶格排列方式，當薄膜中局部區域有較多的MgO (001)//CoFe (001)磊晶關係存在時，將可得到較大的穿隧磁阻值（~90 %）。當我們於MgO 薄膜和CoFe 薄膜間引入CoFeB 中間層時，CoFeB/CoFe 上磁性層將於初鍍狀態形成（111）從優取向，此有助於提升初鍍薄膜的穿隧磁阻值，將會由Os (2 nm)/CoFeB (6 nm)/MgO (2.5 nm)/CoFe (5 nm)/Ta (5 nm)薄膜的1 %提高至Os (2 nm)/CoFeB (6 nm)/MgO (2.5 nm)/CoFeB (1.2 nm)/CoFe (3 nm)/Ta (5 nm)薄膜的12 %。但是當後者經過300 ℃、30 分鐘的熱處理後，仍可得到約25 %的穿隧磁阻值，即鐵磁性中間層的引入對於最佳穿隧磁阻值的提高並無顯著效果。此外，根據CoFeC/MgO/CoFe 磁性穿隧接面的研究結果顯示，當初鍍Os (2 nm)/CoFeC (6 nm)/MgO (2.5 nm)/CoFe (3 nm)/Ta(5 nm)樣品經過150 ℃的熱處理後，穿隧磁阻值可以由9 %提高至12 %，但是隨著熱處理溫度繼續提高，穿隧磁阻值和RA 值則逐漸降低。根據電流-電壓曲線的量測結果顯示Os/CoFeC/MgO/CoFe/Ta 樣品的絕緣層特性在經過350 ℃、30 分鐘熱處理後已被明顯的破壞。

Magnetic tunnel junctions are deposited on Si substrate by dc and rf magnetron sputtering in an ultra-high vacuum chamber. The micro-sized spin-dependent tunnel junction is patterned by e-beam lithography and ion-milling. The pseudo-spin valve magnetic tunnel junction is fabricated in this work. Due to the difference in the coercivity between two ferromagnetic layers, the parallel and antiparallel spin states of the two ferromagnetic layers can be obtained as the magnetic field is varied. The effects of underlayer materials, working pressure and annealing temperatures on the microstructure and magnetoresistance of the MgO film are investigated. TEM analysis shows that the as-deposited MgO film with Co or CoFe underlayer has nanocrystalline structure or mixing phases of nanocrystalline and amorphous. Besides, the MgO grains are random-oriented. The as-deposited MgO thin film with (001) preferred orientation is obtained as the MgO film is deposited on CoFeB、CoFeC or SiO2 underlayer. X-ray diffraction analysis shows that the (001) preferred orientation of the MgO thin film does not enhance significantly after post-annealing at 400 ℃ for 30 minutes. However, that is beneficial for relaxing the stress in the MgO thin film. Magnetoresistance measurements indicate that the tunneling magnetoresistance ratio of about 25 % is obtained by annealing the Os (2 nm)/CoFeB (5 nm)/MgO (2.5 nm)/CoFe (5 nm)/Ta (5 nm) thin film at 300 ℃ for 30 minutes. The MgO (001)[100]//CoFeB (001) [110] orientation relationship can be obtained according to the TEM analysis. Because the CoFe grains are random-oriented as it is deposited on the MgO layer, the larger tunneling magnetoresistance ratio of about 90 % can be obtained for the magnetic tunnel junction with better epitaxial growth of (001) CoFe grains on (001) MgO grains. The as-deposited CoFe thin film with (111) preferred orientation can be obtained by inserting the CoFeB thin layer between the MgO and CoFe thin layers. Therefore, the tunnel magnetoresistance ratio for the as-deposited magnetic tunnel junction is increased from 1 % for the Os (2 nm)/CoFeB (6 nm)/MgO (2.5 nm)/CoFe (5 nm)/Ta (5 nm) thin film to 12 % for the Os (2 nm)/CoFeB (6 nm)/MgO (2.5 nm)/CoFeB (1.2 nm)/CoFe (3 nm)/Ta (5 nm). It is observed that the insertion of the ferromagnetic spacer does not improve the magnetoresistance ratio after 300 ℃ annealing. The tunnel magnetoresistance ratio for the Os (2 nm)/CoFeB (6 nm)/MgO (2.5 nm)/CoFeB (1.2 nm)/CoFe (5 nm)/Ta (5 nm) thin film is about 25 % after annealing at 300 ℃ for 30 minutes. The tunnel magnetoresistance for the as-deposited Os (2 nm)/CoFeC (6 nm)/MgO (2.5 nm)/CoFe (3 nm)/Ta (5 nm) thin film is increased from 9 % to 12 % after annealing at 150 ℃. However, the tunnel magnetoresistance and the resistance-area product are decreased as the annealing temperature is increased above 150 ℃. From the I-V curve measurement of the Os/CoFeC/MgO/CoFe/Ta thin film, it can be see that the barrier properties is deteriorated after annealing at 350 ℃ for 30 minutes.