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The Manufacture and Characterisation of Microscale Magnetic Components

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The Manufacture and Characterisation of Microscale Magnetic Components David Flynn A dissertation submitted for the degree of Doctor o f Philosophy Heriot-Watt University School of Engineering and Physical Sciences March 2007 This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that the copyright rests with its author and that no quotation from the thesis and no information derived from it may be published without the prior written consent of the author or of the University (as may be appropriate). l Abstract This thesis presents the work undertaken by the author within the Microsystems Engineering Centre in the School of Engineering and Physical Sciences at Heriot-Watt University, Edinburgh. This research has focused on the manufacture and characterisation of microscale magnetic components, namely microinductors and transformers for DC-DC converter applications. The development of these components has involved an investigation into manufacturing processes for microscale magnetic components, a study into suitable magnetic materials and methods of tailoring their electric and magnetic properties, the development of models within ANSYS to aid the understanding of current and flux density distribution within components, experimental characterisation of fabricated devices and the development of an optimal design process. Micromachined inductors and transformers with solenoid and pot-core geometries utilizing a variety of magnetic cores have been fabricated on glass using micromachined techniques borrowed from the LIGA process. Components were characterised over the l-10MHz frequency range with a low frequency impedance analyser. The performance of prototype components was then improved via an optimal design process. Analytical, experimental and simulated data was used to develop some of the most state-of-the-art components to date with efficiencies in excess of 90% and high power densities greater than 100 W/cc. Results have indicated that electrodeposited alloys out performed the commercial alloy for high frequency operation. With the cost effective production of thin film laminate core layers and magnetic material development fundamental to the success of future magnetic components. li Dedication To my family Your love and support gives me strength to endeavour against all that life tests me with and inspiration so that I may push myself to be all that I can be. In Loving Memory of Steven Flynn and Mark Flynn m Acknowledgements I would like to express my gratitude for the support, encouragement and guidance provided by my academic supervisor Professor Marc Desmulliez. I would also like to thank Dr Resham Dhariwal for his advice and guidance and for being the second reader of this thesis. Thanks also to Professor Philip Prewett for serving as the external examiner of this thesis. I would like to acknowledge the financial and technical support of Raytheon Systems Limited. In particular the time and effort dedicated to the project by Mr Anthony Toon and Mr Les Allen. I would also like to acknowledge the financial support of both the Engineering and Physical Sciences Research Council (EPSRC) the Scottish Manufacturing Institute (SMI). I would also like to thank the Institution of Engineering and Technology (IET) for their support in the form of the Robinson Scholarship and Leslie H Paddle Scholarship. Finally, I would also like to extend my thanks to my colleagues and friends within the Microsystems Engineering Centre and School of Engineering and Physical Sciences for their support, advice and assistance in the course of my research. IV Table of Contents Page Chapter 1 Motivation Behind the miniaturisation of microscale components and thesis outline 1 1.1 Introduction, main objectives and aim 1 1.2 Motivation of microscale magnetic component 2 1.2.1 Market trends of passive power components 4 1.2.2 Technology drivers of passive power components 5 1.3 The challenges of miniaturising magnetic components 8 1.4 Thesis layout 8 References Chapter 2 Basics of power components Fundamental design equations and material properties 13 2.1 Introduction 13 2.2 Fundamental design equation 13 2.3 Introduction to magnetic material 19 2.3.1 Magnetisation and magnetic materials 20 2.3.2 Eddy current phenomena within thin films 25 2.3.3 Origins of anisotropy 26 2.4 Magnetic materials for DC-DC converters operating in the MHz frequency range 30 2.5 Summary 32 References Chapter 3 Literature review of power magnetic components 37 3.1 Introduction 37 3.2 Basics of passive power inductive components 37 3.2.1 Inductors 37 3.2.2. Transformers 42 3.3 Introduction to switch mode power supplies 46 3.3.1 DC-DC converter configurations 48 v 3.3.1.1 Buck converter 5 0 3.3.1.2 Boost converter 50 3.3.1.3 Buck-Boost converter 51 3.3.1.4 Flyback converter 52 3.3.2 Frequency effect on passive components 53 3.4 Fabrication of power inductive components 55 3.4.1 Review of conventional fabrication 55 3.4.1.1 Review of conventional core materials 60 3.4.2 Review of micromachined fabrication approach 63 3.5 Summary of deposition techniques 67 3.6 Review of state of the art power magnetic components 68 References Chapter 4 Electrodeposition of Nickel-Iron 76 4.1 Introduction to Electrodeposition 76 4.1.1 Basics of Electrodeposition 76 4.2 Electrolyte Systems for Nickel-Iron deposition 79 4.2.1 Sulphate and chloride electrolytes 79 4.2.2 Sulfamate electrolytes 81 4.2.3 Periodic deposition of Nickel-Iron Alloys 82 4.3 Anomalous Deposition 83 4.4 Magnetic field annealing during electrodeposition 86 4.5 Experimental results for DC Nickel-Iron electrodeposition 88 4.5.1 DC Plating Cell 88 4.5.2 Experimental analysis of DC Ni-Fe 90 4.5.2.1 White light interferometer analysis 91 4.5.2.2 SEM-EDX Analysis 93 4.5.2.3 AFM and XDA Analysis 95 4.5.2.4 Electrical and magnetic characterisation 99 4.5.2.5 Magnetic field Annealing 105 4.6 Summary 109 References Chapter 5 Design of Experiments for electrodeposition of nickel-iron 113 5.1 Introduction 113 vi 5.2 Rationale for using DOE for nickel-iron electrodeposition 113 5.3 Experimental Set-up & Waveforms Generated by the DOE 115 5.4 Structural properties of DOE samples 119 5.5 Electrical Properties of DOE samples 127 5.6 Magnetic Properties of DOE Samples 129 5.7 Summary 133 References Chapter 6 Design of Microscale Magnetic Components 136 6.1 Introduction 136 6.2 Microfabricated Component Design 136 6.2.1 Basic geometry 140 6.3 Winding design 143 6.3.1 Skin and proximity effect 148 6.4 Core loss in micro-inductors and transformers 151 6.5 Component design: the solenoid approach 155 6.5.1 Solenoid anisotropic core 159 6.5.2 Solenoid air-gap 160 6.6 Thermal dissipation & electromigration 162 6.7 Pot core micro-inductors and transformers 162 6.8 Review of circuit parameters on component design 164 6.8 Influence of magnetic component on circuit operation 168 6.9 Summary 170 References Chapter 7 Microfabrication of Power Inductors and transformers 173 7.1 Introduction 173 7.2 Mask Design 173 7.2.1 Transferring acetate mask to titanium on glass 175 7.3 Micro-fabrication & assembly of solenoid microinductor 176 7.3.1 Preparation of the substrate 176 7.3.2 Deposition of sacrificial layer 177 7.3.3 Deposition of a titanium seed layer 179 7.3.4 Photoresist deposition and UV photolithography 182 7.3.5 Electrodeposition of copper and nickel 186 vii 7.3.6 Electrodeposition of gold 188 7.3.7 AZ stripping and seed layer etching 190 7.3.8 Flip-chip assembly of the micro-inductor 195 7.4 Fabrication of other microscale magnetic components 200 7.5 Other magnetic alloys investigated 204 7.6 Summary 205 References Chapter 8 Characterisation of Micro-Inductors and Transformers 207 8.1 Introduction 207 8.2 Low frequency impedance measurement 207 8.3 Measurement results of the microinductors 208 8.3.1 Solenoid micro-inductor: winding & dielectric assessment 208 8.3.1.1 Solenoid microinductor: resistance test 210 8.3.1.2 Solenoid micro-inductor: inductance & Q-factor test 214 8.3.1.3 Solenoid microinductor: anisotropic test 218 8.3.1.4 Solenoid micro-inductor: air-gap & core area tests 220 8.3.1.5 Solenoid micro-inductor: DC bias test 222 8.3.1.6 Solenoid micro-inductor: efficiency & power density 224 8.4 Pot-core micro-inductor 229 8.5 Pot-core micro-transformer 232 8.6 Summary 239 References Chapter 9 Modeling-based component optimization 242 9.1 Introduction 242 9.2 Modeling of the solenoid micro-inductor 242 9.3 Computer modeling of a one-turn pot-core micro-inductor 248 9.4 Optimization & Characterization of the Solenoid Micro-Inductor 258 9.5 Conclusions 262 References Chapter 10 Conclusions and future work 265 10.1 Conclusion 265 10.2 Future Work 271 viii Appendix A AC Inductance and AC Core Resistance Derivation Appendix B Fabrication of Solenoid Micro-Inductor List of Tables Table 2.1 The fundamental equations governing inductor and transformer operation: Faraday’s Law, Ampere’s Law and Lenz’s Law 18 Table 2.2: Relationship between magnetic parameters in cgs and S.I. units 19 Table 2.3 Core losses for various commercial ferrite materials 31 Table 2.4 Properties of Permalloy and a Typical High Performance MnZn Power Ferrite 32 Table 3.1 Advantages of planar magnetic components 5 8 Table 3.2 A comparison of deposition methods for micromachined power magnetic components 68 Table 4.1 Electrolyte composition of Nickel-Iron 89 Table 4.2 Magnetic and Electrical Properties of DC NiS0Fe20 104 Table 4.3 Anisotropic and Isotropic Nii0Fe20 film properties 108 Table 5.1 Pulse Reverse waveform parameters
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