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研究生:趙建都
研究生(外文):Chao, Chien-Tu
論文名稱:奈米磁穿隧結元件之翻轉特性研究
論文名稱(外文):Switching characteristics of nano-scaled magnetic tunnel junction
指導教授:吳仲卿
指導教授(外文):Wu, Jong-Ching
學位類別:博士
校院名稱:國立彰化師範大學
系所名稱:物理學系
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:2013
畢業學年度:101
語文別:英文
論文頁數:111
中文關鍵詞:磁穿隧結奈米製程磁學電流驅動翻轉
外文關鍵詞:magnetic tunnel junctionnano-fabricationmagnetismcurrent-induced magnetization switching
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本研究目的主要是發展奈米製程及探討奈米級磁性穿隧結元件的翻轉特性,分別經由施加磁場以及極化電流來操控磁化方向的翻轉並探討其特性。研究之磁性穿隧結薄膜由磁控濺鍍的方式製備,隨後採用典型的由上至下的製程方式,將磁性薄膜利用光微影術、電子束微影術及離子蝕刻技術等製程方式,製作出奈米級的元件。
首先利用量測不同溫度下的磁滯曲線,來檢視人工反鐵磁的固定層的熱穩定性,並且為了探究其尺寸效應,製作尺度分別為200奈米 × 300奈米、500奈米 × 750奈米和2微米 × 3微米的元件陣列,並量測未圖型化的薄膜作為比較。根據一系列的磁滯曲線顯示,交換偏壓耦合及RKKY耦合在不同尺寸的樣品中,顯現出相似的溫度相關性,透過數值擬合,得到交換偏壓耦合的阻隔溫度在427 K以上,此數值高於實際應用上的操作溫度。另外,由RKKY耦合強度隨溫度的變化趨勢,可得出間隔層釕的費米速度約為105 米/秒。
接著採用標準的四點量測方式來量測磁阻的變化並了解自由層磁化方向翻轉的情形,藉由在不同溫度下量測到的磁阻曲線,可以看出自由層的矯頑場隨溫度下降而平緩上升的趨勢,當溫度降低至特定溫度以下後,矯頑場則呈現陡升的現象。此外,改變不同的磁場掃描速率所量得的磁阻曲線,可用以推斷自由層的熱穩定因子以及異向場的數值大小。
電流驅動磁化方向翻轉的量測是以為時50奈秒的脈衝電流來翻轉自由層的磁化方向,利用高頻示波器以時域穿透式的量測法來擷取穿透的訊號,並以此訊號的振幅監測元件磁電阻值的變化。因此,可以計算出成功翻轉的機率以及解析出翻轉時間的分布,藉此擬合出臨界翻轉電流密度及熱穩定因子。

The research interests of this dissertation are to develop nanofabrication processes and analyze the switching configurations of nano-scale magnetic tunnel junctions (MTJs). The stacked MTJ films are prepared and sequentially patterned into nano-scaled devices using a top-down process that combines photo-lithography, electron beam lithography, and ion beam etching. The magnetization switching properties are investigated by applying the external magnetic field and/or spin-polarized current.
The thermal stability of the synthetic antiferromagnetic (SAF) pinned layer is first identified through temperature dependent magnetometric measurement using a vibrating sample magnetometer (VSM). In order to characterize size effects, MTJ arrays with regular dimensions of 200 nm × 300 nm, 500 nm × 750 nm, and 2 μm × 3 μm are manufactured on different chips, and the hysteresis loops of the extended film are measured for comparison. According to sequences of hysteresis loops, these four samples reveal similar temperature dependencies on exchange bias and the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling effect. The blocking temperature of this exchange bias system is fitted to be above 427 K, which is higher than the operational temperature in real products. Also, the Fermi velocity of the Ru layer is obtained to be about 105 m/s through the strength of temperature dependent RKKY coupling.
Subsequently, magnetoresistance (MR) measurement is adopted to investigate the switching characteristics of the free layer. The overall configuration of the coercive field of the free layer is that it increases gradually with decreasing temperature in high temperature regions but rapidly increases below a critical temperature. In addition, the thermal stability factor and the anisotropic field of the free layer are obtained using MR measurements under various field sweep rates.
The current-induced magnetization switching (CIMS) measurement is executed at room temperature. A pulse current with a duration of 50 ns is utilized to drive the nano-scale MTJ device. The transmitted pulse profile is used to monitor the resistance variation by using a real-time oscilloscope based on time-domain transmission (TDT) measurements. This process makes it easy to obtain the switching probability and resolve the distribution of the switching time. Therefore, the intrinsic current density and thermal stability are also derived.

Table of Contents

Chapter 1 Introduction 1
1.1 Development of Magnetic Tunnel Junction 1
1.2 Prospective Application of Magnetic Tunnel Junction 4
1.3 Motivation of this Dissertation 6
1.4 Overview of this Dissertation 7

Chapter 2 Theories and Literature Review 9
2.1 Micromagnetism 9
2.2 Magnetic Tunnel Junction 15
2.3 Interlayer Coupling Effects in MTJ 20
2.3.1 Néel Coupling 20
2.3.2 RKKY Coupling 22
2.3.3 Exchange Bias 23
2.4 Field Switching 29
2.5 Current-Induced Magnetization Switching 34

Chapter 3 Experimental Procedure 41
3.1 Lithography 41
3.1.1 Photo-lithography 43
3.1.2 Electron Beam Lithography 46
3.2 Etching Technique 50
3.3 Fabrication Procedure 55
3.4 Characterization Method 61
3.4.1 Vibrating Sample Magnetometer 61
3.4.2 Physical Property Measurement System 62
3.4.3 Magnetoresistance Measurement 62

Chapter 4 Results and Discussion 66
4.1 Characterization of the Stacked Magnetic Tunnel Junction Films 66
4.2 Field Switching Characterization 70
4.2.1 Temperature Dependence 70
4.2.2 Angular Dependence 80
4.2.3 Magnetic Field-Sweeping Dependence 84
4.3 Current Switching Characterization 86
4.3.1 Characterization Method 88
4.3.2 Switching Probability 91
4.3.3 Switching Time Distribution 98

Chapter 5 Conclusions and Prospective Works 106
5.1 Conclusions 106
5.2 Prospective Works 108

Appendix 110


List of Figures

1.1.1 Schematic illustrations of electron tunneling through (a) an amorphous AlOx barrier and (b) a crystalline MgO (001) barrier. After Ref. [1.9]. 3

1.2.1 Schematic of magnetoresistive head for hard-disk recording. After Ref. [1.16]. 5

1.2.2 Principle of magnetoresistive random access memory in the basic cross-point architecture. After Ref. [1.16]. 5

2.1.1 The schematic of the precessional motion of a magnetic moment with a damping term. After Ref. [2.3]. 14

2.2.1 Schematic of a tunneling barrier in an insulating layer sandwiched between two metallic electrodes. After Ref. [2.7]. 15

2.2.2 Schematic illustration of electron tunneling in a magnetic tunnel junction. (a) is the parallel configuration and (b) is the anti-parallel configuration. After Ref. [2.11]. 17

2.2.3 The TDOS for Fe(001)|8MgO|Fe(001) in parallel configuration. (a) Majority band and (b) minority band with different symmetries. After Ref. [2.14]. 18

2.2.4 Illustration of the MTJ schemes. The top panels indicate the parallel configuration while the bottom panels imply the anti-parallel configuration. (a) Simple energy barrier: tunnel electrons experience the same decay rate inside the barrier. (b) Symmetry filtering barrier: in epitaxial/textured systems, tunnel electrons with different Bloch state symmetry decay at very different rates inside the barrier as a result of symmetry matching between the barrier and electrodes. After Ref. [2.19]. 19

2.3.1.1 Schematic of the Néel coupling effect. After Ref. [2.3]. 21

2.3.1.2 Schematic of Néel coupling with finite thickness. After Ref. [2.22]. 21

2.3.3.1 (a) Hysteresis loops of Co nano-particles with native oxide on the surface measured at 77 K. The dashed lines show the hysteresis loop when the Co nano-particles are cooled without a magnetic field. The solid lines show the hysteresis loop when Co nano-particles are cooled in a saturating magnetic field. After Ref. [35]. (b) The intuitive picture of exchange bias between FM/AFM layers with a field cooling process. After Ref. [2.37]. 23

2.3.3.2 Schematic of angles in an exchange bias system. After Ref. [2.37]. 24

2.3.3.3 Schematic of an ideal interface model. After Ref. [2.39]. 25

2.3.3.4 Schematic of the random field model. After Ref. [2.39]. 27

2.3.3.5 Schematic of the antiferromagnetic domain wall model. After Ref. [2.41]. 28

2.4.1 The relative configuration of a simplified single domain element under a magnetic field. 29

2.4.2 Hysteresis loops with various field angles ψ. After Ref. [2.1]. 31

2.4.3 Stoner–Wohlfarth switching Asteroid. After Ref. [2.3]. 32

2.5.1 Schematics of current-induced magnetization switching. After Ref. [2.52]. 35

2.5.2 Current-induced magnetization switching phase diagram. The three switching modes are thermal activation (solid line), dynamic reversal (dotted line) and precessional switching (thick solid line). The parameters are α = 0.02, HK = 500 Oe, and 4πMS = 18 kOe. After Ref. [2.53]. 35

2.5.3 (a) Field and (b) current driven magnetization switching with pulse duration of 30 ms. After Ref. [2.54]. 37

3.1.1 Schematics of two different tones of resist exposed by radiation source and developed in solution. 42

3.1.1.1 Schematics of photo-lithography followed by subtractive and additive process. (a) sample preparation (b) photoresist coating (c) exposure (d) development (e) ion beam etching (e’) metal deposition (f) and (f’) photoresist removal. 45

3.1.2.1 The e-beam writing system is the combination of commercial scanning electron microscope Hitachi S-4300SE and the nanometer pattern generation system (NPGS). 48

3.1.2.2 Schematic of e-beam lithography process. (a) Preparation of stacked MTJ film. (b) Spin-coating the photoresist ma-N2405. (c) Exposed by electron beam lithography. (d) Developed by solution. (e) Ion beam etching. 49

3.2.1 Schematics of facet effect taking place during the process of ion beam etching. After Ref. [3.1]. 52

3.2.2 The formation of re-deposition problem. After Ref. [3.1]. 52

3.2.3 Ion-beam etching system Elionix EIS-200ER. 53

3.2.4 Schematic illustrations of ion beam etching with a rotating stage and adjustable tilt roller, which manually manipulates the incident angle of ion beam. 53

3.2.5 Schematic illustrations of two-step ion beam etching technique with different incident angles. The gray part indicates the barrier layer of MTJ cell. (a) The desired etching depth was carried out by using ion beam etching with small incident angle. However, the re-deposition problem occurs on the sidewall. (b) The etching with high incident angle was used to clean the re-deposited metal because the high milling rate on sidewall. 54

3.3.1 Schematic of MTJ pillar array. 56

3.3.2 The SEM micrographs of MTJ pillar arrays with dimensions of (a) 200 nm × 300 nm, (b) 500 nm × 750 nm, and (c) 2 μm × 3 μm. 57

3.3.3 The complete fabrication process nano-scale MTJ pillar cells with self-aligned technique. (a) A nano-scale pattern is made using electron beam lithography. (b) The pattern is transferred to the stacked MTJ film using ion beam etching. (c) The photoresist and MTJ cell are fully covered by insulating layer SiO2. (d) The photoresist is removed with a solution in an ultrasonic bath, and then the top of the pillar cell is spontaneously exposed to the atmosphere. (e) The top contact is made using photo-lithography. 59

3.3.4 The SEM micrographs of nano-scale MTJ cell after ion beam etching. 60

3.3.5 The SEM micrographs of nano-scale MTJ cell after e-beam resist lift-off. 60

3.4.1.1 The vibrating sample magnetometer. 61

3.4.3.1 The probe station system with DC and RF probes. 63

3.4.3.2 Schematic of single-shot pulse measurement. 64

3.4.3.3 The equivalent circuit of the pulse measurement. 64

4.1.1 The layer structures of an MgO-based MTJ stacked film. 67

4.1.2 The hysteresis loop of an MTJ film displays three significant sub-loops (1)-(3) which represent the reversal of the pinned, free, and reference layers, respectively. The black, red, and blue arrows show the corresponding magnetization direction of the free, reference, and pinned layers, respectively. 69

4.2.1.1 The temperature dependence of hysteresis loops of (a) Sample A and (b) Sample D. 72

4.2.1.2 The exchange bias field as a function of temperature. (a) Sample A. (b) Sample B. (c) Sample C. (d) Sample D. 73

4.2.1.3 The saturated field of the reference layer as a function of temperature. (a) Sample A. (b) Sample B. (c) Sample C. (d) Sample D. 75

4.2.1.4 The temperature dependence of MR minor loops. 77

4.2.1.5 The effective dipolar field of the free layer as a function of temperature. 79

4.2.1.6 The coercive field of the free layer as a function of temperature. 79

4.2.2.1 Schematic illustration of measurement setup. 80

4.2.2.2 The minor M-H hysteresis loops of Sample A, B, C, and D at the low field ranges with field angle of 0°. 81

4.2.2.3 Fig. 4.2.2.3 (a) The angular dependence of hysteresis loops of (a) Sample A and (b) Sample D. 82

4.2.2.4 The coercive field as a function of field angle. 83

4.2.3.1 MR curves measured under various field sweeping rates. The inset shows the detailed switching events from AP to P state under different field sweep rates. 84

4.2.3.2 The coercive field as a function of field dwell time. 85

4.3.1 (a) The simulated impedance of coplanar waveguide with different lateral dimensions. (b) The schematic illustration of coplanar waveguide. 87

4.3.2 The transmitted pulse profile with different durations through a coplanar waveguide on ceramic substrate. 87

4.3.1.1 The transmitted pulse profiles through an MTJ device at (a) a high resistance state and (b) a low resistance state. 89

4.3.1.2 The transmitted pulse profiles through an MTJ device at initial high resistance state switches to low resistance state after an incubation time. 90

4.3.1.3 The normalized transmitted waveform is plotted by subtracting the non-switching background. 90

4.3.2.1 (a) The SEM micrograph of rectangular MTJ device with lateral dimensions of 120 nm by 250 nm. (b) The corresponding MR minor loop. 92

4.3.2.2 The normalized transmitted pulse profiles with 10 switching events. Each switching events is distinguished by different colors. t = 0 indicates the pulse onset. 92

4.3.2.3 The switching probability as a function of external magnetic field. Each curve indicates different writing current densities. The successful switching probability of (a), AP to P state and (b), P to AP state, is shown. 94

4.3.2.4 The cumulative probability as a function of the incubation time under constant current density and different magnetic fields. (a) shows the switch from P to AP state. (b) shows the switch from AP to P state. 96

4.3.2.5 The cumulative probability as a function of the incubation time under constant magnetic field and different current densities. (a) shows the switch from low to high state. (b) shows the switch from high to low state. 97

4.3.3.1 The histograms represent the switching time distribution from P to AP state under different external magnetic fields. 99

4.3.3.2 The histograms represent the switching time distribution from P to AP state under different current densities. 100

4.3.3.3 The mean switching time τ as a function of current density. Each color indicates a series of measurements under various magnetic fields. 102

4.3.3.4 The effective thermal stability factor (ΔI) as a function of current density, where ΔI ≡ E0(1-J/Jc0)/kBT. 103


List of Tables

4.2.1.1 The fitting parameters of exchange bias as a function of temperature. 74

4.2.1.2 The fitting parameters of saturated field as a function of temperature. 76

4.2.3.1 The fitting parameters of coercive field as a function of field dwell time. 85

4.3.3.1 The fitting parameters of switching time as a function of magnetic field. 103

4.3.3.2 The fitting parameters of effective thermal stability factor as a function of current density. 104

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Chapter 5
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