Magnetic properties of co ta thin films and their applications in magnetic tunnel junctions

MAGNETIC PROPERTIES OF CO-TA THIN FILMS AND THEIR APPLICATIONS IN MAGNETIC TUNNEL JUNCTIONS FONG KIEN HOONG NATIONAL UNIVERSITY OF SINGAPORE 2003 MAGNETIC PROPERTIES OF CO-TA THIN FILMS AND THEIR APPLICATIONS IN MAGNETIC TUNNEL JUNCTIONS FONG KIEN HOONG (B Eng (Hons), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2003 Acknowledgment ACKNOWLEDGMENT The author would like to express his heartfelt gratitude to his mentors, Dr Vivian Ng and Dr Adeyeye Adekunle for their guidance, encouragement and advice throughout the course of the project The author would like to thank Miss Loh Fong Leong, Miss Liu Ling, Mr Walter Lim, Mr Lim Boon Chow, Miss Chan Soon Yeng, Miss Pang Siew It, Miss Zhao Yan and Mdm Ah Lian Kiat for their assistance and advice Special thanks to the author’s collaborators, Mr Hu Jiangfeng and Mr Chen Fang Hao for their invaluable contributions, assistance and advice In addition, the author would like to express his gratitude to his colleagues, Kyaw Min Tun, Darren, Alvin Wee, Zhao Qiang, May Thu Win, Wang Xiao Qiang, Chen Chao, Guo Jie and Chan Hui Ling for their companionship Last but not least, the author would like to thank all those who have contributed to this project in one way or another i Table of contents TABLE OF CONTENTS ACKNOWLEDGEMENT i TABLE OF CONTENTS ii SUMMARY vi SYMBOLS AND ABBREVIATIONS viii LIST OF FIGURES xi LIST OF TABLES xiv CHAPTER 1: INTRODUCTION 1.1 Background 1.2 Objectives 1.3 Organization CHAPTER 2: THEORY AND REVIEW 2.1 Introduction 2.2 Anisotropic Magnetoresistive Effects 2.3 Giant Magnetoresistive Effects 10 2.4 Tunneling Magnetoresistive Effects 14 2.5 Related Works on Materials Used in MTJ 19 2.6 Related Works on MTJ 21 2.7 Motivation 24 2.8 Summary 25 ii Table of contents CHAPTER 3: EXPERIMENTAL METHODOLOGIES 28 3.1 Introduction 28 3.2 Fabrication Techniques and Equipment 30 3.2.1: Types of Substrates 30 3.2.2: Cleaning 31 3.2.3: Lithography 31 3.2.4: Deposition 33 3.2.5: Lift-off 38 Characterization Techniques and Equipment 39 3.3.1: Thickness Characterization 39 3.3.2: Magnetic Properties Characterization 39 3.3.3: Imaging of Samples 42 3.3.4: Measurement Setups 47 Summary 50 3.3 3.4 CHAPTER 4: CHARACTERIZATION OF THIN FILMS 51 4.1 Introduction 51 4.2 Seed Layer 53 4.2.1: Characterization of Ni80Fe20 53 4.2.2: Characterization of Ta 55 4.3 Bottom Electrode 58 4.4 Insulating Barrier 59 4.5 Top Electrode 63 4.5.1: Co as Top Electrode 63 iii Table of contents 4.6 4.7 Co-Ta Alloy 65 4.6.1: Calibration of Co and Ta 65 4.6.2: Coercivity of Co-Ta Films 66 4.6.3: TEM of Co-Ta Grains 68 4.6.4: MFM Images of Co-Ta Films 69 4.6.5: TEM Images of Co-Ta Films 71 4.6.6: Crystal Structure of Co-Ta Films 72 4.6.7: Surface Roughness of Co-Ta (15%) 75 4.6.8: Summary of Studies on Co-Ta Films 76 Summary 77 CHAPTER 5: MAGNETIC TUNNEL JUNCTIONS 78 5.1 Introduction 78 5.2 Shadow Mask Design I 81 5.3 Magnetic Tunnel Junctions Using Shadow Mask I 85 5.3.1: Deposition Conditions 85 5.3.2: VSM Measurement 86 5.3.3: I-V Measurements 87 5.3.4: MR Measurements 87 5.4 Shadow Mask Design II 96 5.5 Shadow Mask II Magnetic Tunnel Junctions with Co Electrode 99 5.5.1: Deposition Conditions 99 5.5.2: VSM Measurement 100 5.5.3: I-V Measurements 100 iv Table of contents 5.6 5.7 5.5.4: MR Measurements 101 5.5.5: Comparison of Batch N with Batch G 104 Magnetic Tunnel Junctions with Co-Ta Electrode 105 5.6.1: Deposition Conditions 105 5.6.2: VSM Measurement 106 5.6.3: I-V Measurements 106 5.6.4: MR Measurements 107 5.6.5: Comparison of Batch N with Batch G 109 5.6.6: Review of Magnetic Tunnel Junctions Fabricated 111 Summary 113 CHAPTER 6: CONCLUSION AND FUTURE RECOMMENDATIONS 115 6.1 Conclusion 115 6.2 Problems Encountered 117 6.3 Future Recommendations 119 Appendix A Shadow Mask Design I 121 B Shadow Mask Design II 125 C MR Measurements of Batch G at 0° 129 D MR Measurements of Batch G at 90° 131 List of Publications 133 v Summary SUMMARY Magnetic tunnel junctions (MTJ) show great promise to become the next candidate for magnetic data storage devices like the hard disk and magnetic random access memory This is due to their sensitivity to low magnetic fields, non-volatility as well as radiation hardness The common structure of a MTJ consists of two ferromagnetic layers sandwiching a thin insulating layer The common ferromagnetic materials used include Ni80Fe20 and Co In this project, the metal Ta was examined to determine its suitability as the base contact of the MTJ It was found that Ni80Fe20 gives a smoother film than Ta, thus discounting the use of Ta as the base contact The alloy Co-Ta was also examined to determine its suitability to be part of the tri-layer structure It was found that the Co-Ta film exhibits vastly different magnetic properties when doped with different concentrations of Ta The coercivity of Co-Ta films initially increases with the increase in Ta ratio, but once the concentration reaches 15%, the coercivity decreases rapidly until around 10 Oe It was found that the changes in magnetic properties of the Co-Ta films are due to many contributing factors The factors are namely grain size changes, presence of different crystal orientations, different degrees of crystallinity as well as different magnetic domain configurations The variation in coercivity of the Co-Ta material suggests a possibility of using it to replace Co as the top electrode to complement the bottom electrode Ni80Fe20 vi Summary MTJ devices of Ni80Fe20/Al2O3/Co tri-layer were fabricated using shadow masks The devices show a large dependence on the shape of the electrodes It was found that the orientation of the top electrode is critical in giving a better magnetoresistive response Subsequently, another batch of devices was fabricated using a different set of shadow masks in order to reduce the shape anisotropic effects This has succeeded to a certain extent in that the switching fields of the electrodes are much lower compared to the previous batch A third batch of devices using Co-Ta as the top electrode was fabricated It was found that MTJs using this material still exhibit tunneling characteristics, but however, the magnetoresistive ratio was lower than when only Co was used This was attributed to the presence of the non-magnetic Ta, which reduced the spin polarization of Co vii Symbols and Abbreviations SYMBOLS AND ABBREVIATIONS Å Angstroms (10-10 m) Al Aluminium Al2O3 Aluminium oxide AFM Atomic Force Microscope AMR Anisotropic Magentoresistive Ar Argon B Magnetic flux density Co Cobalt Co50Fe50 Cobalt Iron alloy with 50% Cobalt and 50% Iron composition Cr Chromium D(EF) Density of electron states near the Fermi level DI De-ionized DC Direct Current EF Fermi energy Fe Iron FM Ferromagentic GMR Giant Magnetoresistive H Magnetic field strength HC Coercivity I Current IPA Iso-propanol k Fermi wave vactors viii Chapter 6.3 Conclusion and Future Recommendations Future Recommendations There exist many avenues for future research in this area Below are some of the recommendations Fabrication of small MTJs The existing MTJs made using the shadow masks are big and are affected by the designs of the mask In order to eliminate the effects due to the shape of the bottom electode, it is necessary to use photolithographical methods to fabricate the MTJs The use of photolithography allows MTJs of specific sizes to be fabricated, thus eliminating the present problems due to the shapes of the bottom electrode After photolithography, Electron Beam lithography can be employed to define sub-micron size patterns to fabricate nanoscale devices Insulating Barrier Recent reports of half-metals like Fe3O4[1,2] suggest a possibility of using this material as the barrier of the MTJ However, it is known that the deposition of this material to obtain the required properties is very challenging Deposition conditions The setup of the evaporator and sputtering machine only allows room temperature and high temperature deposition Low temperature deposition methods may be able to produce higher quality films This can be done by having a cooling chamber next to the 119 Chapter Conclusion and Future Recommendations deposition machines such that the samples can be cooled and then films can be immediately deposited Another addition to the deposition condition is the application of a magnetic field The presence of a magnetic field during deposition allows the inducement of an easy axis in the samples This is especially useful if exchange biasing is to be employed References: [1] C M Fang, G A de Wijs and R A de Groot, J Appl Phys., 91, 8340, (2002) [2] S V Molnar and D Read, Proc IEEE, 91, 715, (2003) 120 Appendix APPENDIX A: SHADOW MASK DESIGN I Shadow mask I contains four masks All dimensions listed are in micrometer 800 200 500 400 100 1200 7800 10000 10000 Figure A1: Bottom electrode 121 Appendix 6000 6000 Figure A2: Insulator layer 122 Appendix 7800 200 500 400 100 Figure A3: Top electrode 123 Appendix 1200 1200 Figure A4: Contact pads 124 Appendix APPENDIX B: SHADOW MASK DESIGN II Shadow mask II contains four masks All dimensions listed are in micrometer 200 x 200 700 700 400 450 2000 1200 450 10000 300 350 10000 Figure B1: Bottom electrode 125 Appendix 1000 1000 Figure B2: Insulator layer 126 Appendix 2000 350 350 400 1200 400 Figure B3: Top electrode 127 Appendix 1000 2000 Figure B4: Contact pads 128 Appendix APPENDIX C: MR MEASUREMENTS OF BATCH G AT 0º A1 A2 844 Magnetoresistance(Ω) Magnetoresistance(Ω) 1118 1114 1110 1106 -150 -50 50 150 -150 Applied Field (Oe) 836 -50Applied Field (Oe) 50 A3 A4 940 756 Magnetoresistance(Ω) Magnetoresistance(Ω) 840 937 934 754 752 750 748 931 -150 -50 50 Applied Field (Oe) -150 150 -50 B2 Magnetoresistance(Ω) Magnetoresistance(Ω) 560 558 556 522 519 554 516 50 150 -150 B4 1218 684 Applied Field (Oe) 129 50 150 1206 -150 150 150 1212 681 50 50 Applied Field (Oe) 690 687 -50 -50 B3 Magnetoresistance(Ω) Magnetoresistance(Ω) Applied Field (Oe) -150 150 525 562 -50 50 Applied Field (Oe) B1 -150 150 1200 -50 Applied Field (Oe) Appendix C1 C2 452 Magnetoresistance(Ω) Magnetoresistance(Ω) 780 776 772 449 446 768 -150 -50 Applied Field (Oe) 50 -150 150 -50 C3 Magnetoresistance(Ω) Magnetoresistance(Ω) 1224 504 1220 1216 1212 500 -50 50 -150 150 D3 1246 Magnetoresistance(Ω) Magnetoresistance(Ω) D2 1270 -50 50 150 1242 1240 1238 -150 Applied Field (Oe) D4 1114 50 Applied Field (Oe) 1236 -50 50 Applied Field (Oe) 1118 1110 -50 150 1244 1266 Magnetoresistance(Ω) 50 1278 1274 -150 -50 Applied Field (Oe) Applied Field (Oe) -150 150 D1 508 -150 50 Applied Field (Oe) 150 130 150 Appendix A1 A2 1195 855 MagnetoresistanceΩ() Magnetoresistance(Ω) APPENDIX D: MR MEASUREMENTS OF BATCH G AT 90º 1190 1185 1180 852 849 1175 -100 -50 846 50 100 -100 Applied Field (Oe) -50 A3 Magnetoresistance(Ω) Magnetoresistance(Ω) 946 -100 100 -50 B2 596 532 50 100 50 100 524 520 Applied Field (Oe) 50 -100 100 -50 Applied Field (Oe) B3 B4 632 1230 Magnetoresistance(Ω) Magnetoresistance(Ω) 528 584 628 624 620 1220 1210 1200 616 -50 100 Applied Field (Oe) B1 Magnetoresistance(Ω) Magnetoresistance(Ω) Applied Field (Oe) 50 588 -100 50 780 776 592 -50 784 942 -100 100 788 950 -50 50 A4 954 -100 Applied Field (Oe) Applied Field (Oe) 50 -100 100 131 -50 Applied Field (Oe) C1 C2 828 490 Magnetoresistance(Ω) Magnetoresistance(Ω) Appendix 824 820 816 812 488 486 484 482 808 -100 -50 Applied Field (Oe) 50 -100 100 -50 Magnetoresistance(Ω) Magnetoresistance(Ω) 100 50 100 50 100 1230 514 511 508 1224 1218 1212 1206 505 -50 50 D1 C3 -100 Applied Field (Oe) 50 -100 100 -50 Applied Field (Oe) Applied Field (Oe) D2 D3 1234 Magentoresistance(Ω) Magnetoresistance(Ω) 1386 1380 1374 1228 1222 1368 -100 -50 1216 50 100 -100 Applied Field (Oe) D4 Magnetoresistance(Ω) 1100 1096 1092 1088 -100 -50 Applied Field (Oe) 50 100 132 -50 Applied Field (Oe) List of Publications LIST OF PUBLICATIONS 1) “Magnetic properties of Co-Ta alloy”, K H Fong and V Ng (prepare to submit for publication) 133 [...]... of 10% Ta ratio, (ii) Co- Ta of 15% Ta ratio, (iii) Co- Ta of 20% Ta ratio and (iv) Co- Ta of 25% Ta ratio 68 Fig 4.6.4 MFM images of (i) Co, (ii) Co- Ta (5%), (iii) Co- Ta (10%), (iv) Co- Ta (15%), (v) Co- Ta (20%) and (vi) Co- Ta (25%) 69 Fig 4.6.5 TEM images showing the crystal structures of (i) Co- Ta of 10% Ta ratio, (ii) Co- Ta of 15% Ta ratio, (iii) Co- Ta of 20% Ta ratio and (iv) Co- Ta of 25% Ta ratio... decades since the discovery of TMR effects in FM/I/FM structures by Julliére in 1975[6] before interests in this field were revived This is due primarily to the difficulty of fabricating thin films of high quality to obtain sufficiently high MR values, resulting in little interests in this feild However, with the advancement in deposition techniques, and its potential in MRAMs, interests in MTJ have... the conductance of Fe/Ge/Pb and Fe/Ge /Co structures with the semiconductor layer oxidized[6] He studied the conductances of the structures when the two ferromagnetic films are parallel and antiparallel, and found that the conductance is higher at the parallel state than the anti-parallel state He attributed the phenomenon to the tunneling of electrons through the oxidized semiconductor barrier, and. .. determines the sensitivity of the device 3 Chapter 1 1.2 Introduction OBJECTIVES The objectives of this project centers around the use of tantalum in the fabrication of MTJ devices: 1) Design shadow and photo masks for the fabrication of MTJ devices 2) Explore the use of tantalum as part of the base contact of the device 3) Explore the possibility of using a new ferromagnetic layer for the fabrication of. .. 1.3 Introduction ORGANISATION OF THESIS The thesis is divided into six main chapters, as detailed in the Table of Contents Chapter 2 discusses the theory on magnetoresistance, including anisotropic, giant and tunneling magnetoresistance In addition, there will be a review on some of the works done on MTJ by other researchers This section includes work that are investigated on the materials used in the... fraction of the mean free path of an electron in the multi-layer system The first condition is met by inserting a non -magnetic spacer between the ferromagnetic layers to help them achieve independent rotation of their magentizations The second condition requires precise deposition techniques in order to ensure a thin film of less than 2 nm GMR effects arise because of spin-dependent scattering either... Fe/Ge/Fe and Fe/Ge /Co junction in 1975 Since then, the interests in MTJs subsided due to the technical difficulties in making high quality barriers It was not until some 20 years later that this obstacle was cleared due to advancement in deposition techniques As of now, a MR ratio of 58.8% was reported[2] by a group of Japanese researchers using a Ta/ Cu /Ta/ NiFe/Cu/MnIr/CoFe/AlO/CoFe/NiFe/Cu /Ta structure... effective spin polarization An MTJ employing TMR effects has potential in magnetic random access memory (MRAM)[8], as shown in figure 2.4.2 The MTJ devices are placed in a matrix with conducting wires sandwiching them In order to read from a MRAM, a current is passed to the Word line and voltage measurements are made from the corresponding Bit line In the example shown, the red coloured MTJ is being read... curves of (i) PA1_0, (ii) PB1_0, (iii) PA2_0 and (iv) PB2_0 107 Fig 5.6.4 MR curves of (i) PA1_90, (ii) PB1_90, (iii) PA2_90 and (iv) PB2_90 xiii 92 108 List of Tables LIST OF TABLES Table 3.3.1 Comparisons between the various modes of AFM 44 Table 4.2.1 Surface roughness of Ni80Fe20 films deposited at various conditions 55 Table 4.2.2 Surface roughness of Ta films deposited at various conditions 57 Table... Ta and Ni80Fe20 are thus used in this project as seed layers for the device There are only a small number of ferromagnetic materials available, and as such, had been extensively investigated In addition, many alloys of two ferromagnetic materials have also been studied The common configurations favoured by researchers include CoFe/NiFe[12,13,14], Co/ Fe[15,16,], Co/ NiFe[16,17,18] and Co/ CoFe[19,21] Co ... Ta ratio, (ii) Co- Ta of 15% Ta ratio, (iii) Co- Ta of 20% Ta ratio and (iv) Co- Ta of 25% Ta ratio 68 Fig 4.6.4 MFM images of (i) Co, (ii) Co- Ta (5%), (iii) Co- Ta (10%), (iv) Co- Ta (15%), (v) Co- Ta. .. Co- Ta (20%) and (vi) Co- Ta (25%) 69 Fig 4.6.5 TEM images showing the crystal structures of (i) Co- Ta of 10% Ta ratio, (ii) Co- Ta of 15% Ta ratio, (iii) Co- Ta of 20% Ta ratio and (iv) Co- Ta of. .. Co as Top Electrode 63 iii Table of contents 4.6 4.7 Co- Ta Alloy 65 4.6.1: Calibration of Co and Ta 65 4.6.2: Coercivity of Co- Ta Films 66 4.6.3: TEM of Co- Ta Grains 68 4.6.4: MFM Images of Co- Ta