Luận án: Phát triển nanowires từ tính đa lớp cho cảm biến GMR - Thong Chi Le

DEVELOPMENT OF MULTILAYERED MAGNETIC NANOWIRES FOR GIANT MAGNETORESISTIVE SENSORS DEVELOPMENT OF MULTILAYERED MAGNETIC NANOWIRES FOR GIANT MAGNETORESISTIVE SENSORS A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering By Thong Chi Le HoChiMinh City University of Technology Bachelor of Science in Electronic Engineering, 1993 HoChiMinh City University of Natural Sciences Master of Science in Electronic Physics, 1998 December 2009 University of Arkansas ABSTRACT Switched reluctance motors (SRMs) are widely used for variable-speed applications due to their simplicity and ruggedness but their primary disadvantage is large torque ripple and they require complicated control techniques. One approach for reducing the torque pulsation is based on estimating the instantaneous torque in real-time and compensating with the motor drive power electronics. Flux density in the machine is directly measured by using embedded magnetic field sensors. The intent is to develop a fundamentally new approach to the design and operation of electric machines that will result in significantly improved efficiency and power density.

This dissertation describes the development of giant magnetoresistive (GMR) sensors with multilayered magnetic nanowires containing alternating ferromagnetic and nonmagnetic layers. Pulsed electrochemical deposition was used to control the periodic structure of multilayered nanoscale nanowires with preferential crystalline orientation. Co/Cu multilayered magnetic nanowires were grown through polymer nanopore membrane on substrates by lithography-assisted template bonding (LATB) method. Layer thicknesses and crystalline structure were varied and controlled to have an effect on the GMR results.

Measurements and results of embedded GMR sensors in a prototype motor are also presented. This dissertation is approved for recommendation to the Graduate Council Dissertation Director: Dr. Roy McCann Dissertation Committee: Dr. Vijay Varadan Dr.

Hargsoon Yoon Dr. Simon Ang Dr. Edward Pohl DISSERTATION DUPLICATION RELEASE I hereby authorize the University of Arkansas Libraries to duplicate this dissertation when needed for research and/or scholarship. Agreed Thong Chi Le Refused Thong Chi Le ACKNOWLEDGEMENTS First of all, I would like to thank Ministry of Education and Training of Vietnam offering a scholarship to pursue my Ph.

degree at University of Arkansas in the United States of America. I would like to acknowledge and extend my heartfelt gratitude to my advisor, Dr. McCann, who encouraged and challenged me through my academic program at University of Arkansas. He and the other faculty members, Dr.

Hargsoon Yoon and Dr. Varadan, guided me through the dissertation process. Under their support and guidance, my dissertation became a reality. Special appreciation goes to Dr.

Hargsoon Yoon and Dr. Varadan for their advice about the theory of nanotechnology. I wish to thank all other committee members in my Doctoral Dissertation Committee, Dr. Simon Ang and Dr.

Edward Pohl, for their vital encouragements and supports. I would like to thank Dr. Jining Xie, Dr. Linfeng Chen, Dr.

Taeksoo Ji and Dr. Mourad Benarama for their guidance and supports. I wish to thank Dr. Randy Brown and Dr.

Terry Martin for their teaching and guidance. A special thanks to HiDEC staff, Errol Porter, Mike Steger, Michael D. Glover, Jeffrey Mincy, Tom Cannon, and Alan Tolan, for their training and maintaining the fabrication equipment in HiDEC and INB Lab. I would like to thank the graduate students that I worked with at HiDEC, Devesh C.

Deshpande, Vasuda Ramachandran, Phillip T. Hankins, and Sechang Oh, for their assistance and supports. Most especially to my parents, Le Chi Thang and Ton Nu Tu Uyen, my brothers and sisters, my wife, Tran Thi Thu Hang, and my children, Le Chi Uyen Nhi and Le Chi Tu Van, for their love and encouragement. v TABLE OF CONTENTS  1.2 Size comparison of commercial magnetic sensors .3 Multilayered magnetic nanowires .5 Outline of the Dissertation . SWITCHED RELUCTANCE MOTOR .2 History of the Switched Reluctance Motor .3 Advantages and Disadvantages .3 Interlayer exchange coupling .3 Origin of the GMR effect .1 The discovery of GMR .3 Theory of perpendicular giant magnetoresistance in magnetic multilayers 43 3.4 Fabrication of multilayered magnetic multilayers .5 Applications of multilayered magnetic nanowires .2 Thin film deposition .1 Physical Vapor Deposition (PVD) .2 Chemical Vapor Deposition (CVD).2 Electrodeposition of Co/Cu multilayered thin films .2 Scanning Electron Microscopy (SEM) .3 Atomic Force Microscopy (AFM) . GMR SENSOR FABRICATION .2 Fabrication of Co/Cu multilayered nanowires .3 Single layer Co nanowire growth .4 Co/Cu multilayered nanowire growth .3 Development of multilayered magnetic nanowires for GMR sensors .5 Experimental results and discussion . MEASUREMENTS AND RESULTS . CONCLUSIONS AND FUTURE WORK.

155 ix LIST OF FIGURES Figure 1-1: The Hall Effect is illustrated by a semiconductor slab [5]. 4 Figure 1-2: The change of resistance in the GMR sensors [5]. 5 Figure 1-3: Honeywell magnetic sensors [10]. 8 Figure 1-4: Asahi magnetic sensors HG106C, HG-0111, HG-302, HW101A, HW-108A, EW-410B, EW-610B [13].

9 Figure 1-5: ALPS magnetic sensors series HGAR, HGDE [14]. 10 Figure 2-1: The switched reluctance motor. 21 Figure 2-2: The 8/6 SRM [131]. 23 Figure 2-3: A phase torque of a SRM [40].

27 Figure 3-1: Giant Magnetoresistance. 31 Figure 3-2: Schematic 3d and 4s densities of states in transition metals. The position of the Fermi levels in Zn, Cu, Ni, Co, Fe and Mn are shown. 33 Figure 3-3: Schematic 3d and 4s up- and down-spin densities of states in a transition metal with exchange interaction included [53].

34 Figure 3-4: The density of states for a non-magnetic state (a) and a ferromagnetic state (b) [51]. 34 Figure 3-5: Electrical resistance [56]. 36 Figure 3-6: Schematic illustration of the behavior of the exchange coupling as a function of distance [51]. 39 Figure 3-7: Magnetoresistance measurements at 4.2K for the Fe/Cr multilayer system [28].

41 Figure 3-8: Magnetoresistance measurements at room temperature for the trilayer system Fe/Cr/Fe [29]. 41 Figure 3-9: Understanding GMR. 42 Figure 4-1: Typical system for e-beam evaporation of materials [69]. 55 Figure 4-2: The e-beam evaporator at INB lab, University of Arkansas.

55 Figure 4-3: Typical RF sputtering system [69]. 56 Figure 4-4: Typical hot-wall LPCVD reactor [69]. 58 Figure 4-5: Schematic diagram of PECVD system [130]. 58 Figure 4-6: The PECVD system at HiDEC, University of Arkansas.

59 x Figure 4-7: Schematics of an electrolytic cell for plating metal "M" from a solution of the metal salt "MA" [71]. 61 Figure 4-8: Two general lithographic processes [132]. 63 Figure 4-9: The mask alignment and UV expose tool Suss Microtec MA150 at HiDEC, University of Arkansas. 64 Figure 4-10: Comparison of RIE vs.

69 Figure 4-11: Typical parallel-plate reactive ion etching system [82]. 69 Figure 4-12: The RIE system at HiDEC, University of Arkansas. 70 Figure 4-13: Ion milling system [83]. 71 Figure 4-14: The ion milling system at INB lab, University of Arkansas.

72 Figure 4-15: Bragg’s law and diffraction [129]. 73 Figure 4-16: The XRD system at AAL lab, University of Arkansas. 74 Figure 4-17: The schematic diagram of the SEM [93]. 76 Figure 4-18: FESEM JEOL JSM-6335F at University of Arkansas.

77 Figure 4-19: Atomic Force Microscope (AFM) [94]. 78 Figure 4-20: The force between the tip and the sample [95]. 78 Figure 5-1: Fabrication procedure for single layer Co nanowires: (a) Cr/Au layer deposited on a substrate, (b) attachment of polycarbonate membrane, (c) lithography process, (d) cylindrical nanopores opened by UV exposure and developing, (e) nanowires growth, and (f) vertically aligned nanowires after removal of polycarbonate membrane. 82 Figure 5-2: Deposition current recorded at the deposition potential of -1.

86 Figure 5-3: SEM pictures of single layer Co nanowires (sample S-1). 87 Figure 5-4: X-ray diffraction patterns of single layer Co nanowires (sample S-1). 87 Figure 5-5: Magnetization loop (B-H loop) and the determination of coercivity HC and remanence BR. 88 Figure 5-6: (a) Magnetization loop and (b) coercivity and remanence of Co nanowires (sample S-1) when the applied field is perpendicular or parallel to the nanowire axis.

89 Figure 5-7: Differential Pressure Laminator Optek DPL-24 at University of Arkansas. 91 Figure 5-8: Fabrication procedure of Co/Cu multilayered nanowires: (a) Ti/Au layer deposited on a substrate, (b) attachment of polycarbonate membrane, (c) lithography xi process, (d) cylindrical nanopores opened by UV exposure and developing, (e) top electrode deposited on the top of membrane, and (f) nanowires growth. 93 Figure 5-9: Microscope image of the bottom and top electrodes. 94 Figure 5-10: Schematic diagram for nanowire growth and contact formation.

94 Figure 5-11: (a) Co/Cu deposition for sample S-2 by pulse sequence; (b) Co/Cu deposition for sample S-5 by pulse sequence with open circuit potential (OCP) control 97 Figure 5-12: Anodic deposition current when switching from the Co potential (-1.0 V) to the Cu potential (-0.5 V) in the pulse sequence (a), and in the pulse sequence with OCP (b). 98 Figure 5-13: SEM images of Co/Cu multilayered nanowires: (a) sample S-2, (b) sample S-3, (c) sample S-4, and (d) sample S-5. 100 Figure 5-14: X-ray diffraction patterns of: (a) single layer Co nanowires (S-1), (b) multilayered Cu/Co nanowires (S-2), (c) multilayered Cu/Co nanowires (S-3), and (d) multilayered Cu/Co nanowires with OCP control (S-5). 101 Figure 5-15: (a) Magnetization loop and (b) coercivity and remanence of Co nanowires (sample S-2) when the applied field is perpendicular or parallel to the nanowire axis.

103 Figure 5-16: The deposition current increase significantly when an electrical contact between nanowires and the top electrode is made. 104 Figure 5-17: The GMR effect is measured by applying a variable external magnetic field using electromagnet (a) and magnet controller (b). 105 Figure 5-18: The GMR results of 100-nm diameter Co/Cu multilayered nanowires when applied magnetic field is parallel and perpendicular to the nanowire axis. 106 Figure 5-19: The inherent connection between nanowires in nanowire arrays.

108 Figure 5-21: A depiction of GMR sensors: (a) cross-sectional view, (b) 45o angle view, (c) top view. 109 Figure 5-22: The level 1 photomask used in fabrication to define the area covered by grown nanowires, which varies from 20 μm by 1700 μm to 500 μm by 1700 μm. 111 Figure 5-23: The level 2 photomask. This created the key marks used for the alignment of the level 3 mask after the sample was flipped.

111 Figure 5-24: The level 3 mask used to form the electrodes. 112 Figure 5-25: The level 4 photomask. This is used to etch the gold layer between two electrodes. 112 xii Figure 5-26: A cross-section diagram of fabrication procedure for GMR sensors: (a) deposit Au layer on Ti foil, (b) attach membrane and selectively open nanopore, (c) grow Co/Cu multilayerd nanowires, and (d) remove membrane.

114 Figure 5-27: Photographs of samples after attaching membrane (a), growing nanowires (b), parylene coating (c), creating electrodes (d), and bonding process (e). 117 Figure 5-28: Photograph of final GMR sensors. 119 Figure 5-29: Recorded potential (a) and current (b,c) during pulsed deposition. Co and Cu are deposited for 0.5 s at -1 V and for 20 s at -0.

Co deposition current is -3.5 mA and Cu deposition current is -18 μA. 121 Figure 5-30: (a) SEM image for 100 nm Co/Cu nanowires electrodeposited with deposition potentials of -1.5 V and deposition time of 0.5 s/20 s, and (b) its magnified image. 122 Figure 5-31: (a) SEM image for 100 nm Co/Cu nanowires electrodeposited with deposition potentials of -1.5 V and deposition time of 0.5 s/15 s, and (b) its magnified image. 123 Figure 5-32: X-ray diffraction patterns of Co/Cu nanowires.

Co was deposited at -1.5 s and Cu was deposited at -0. 125 Figure 5-33: Magnetic hysteresis loop of Co/Cu multilayered nanowires. Co was deposited at -1.5 s and Cu was deposited at-0. 125 Figure 5-34: Schematic showing the GMR results.

The resistance changes when applying the field perpendicular and parallel to the nanowire axis are 7% and 5%, respectively. 126 Figure 6-1: Diagram of circuit converting resistance to voltage. 130 Figure 6-2: The current source mirror built from two N-channel MOSFETs and two P- channel MOSFETs .