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Chapter 1 State of the Art of Integrated Inductor

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Chapter 1 State of the Art of Integrated Inductor TTHHÈÈSSEE En vue de l'obtention du DOCTORAT DE L’UNIVERSITÉ DE TOULOUSE Délivré par Université Toulouse 3 Paul Sabatier (UT3 Paul Sabatier) Discipline ou spécialité : Micro-nano systems Présentée et soutenue par NGUYEN Thi Yen Mai Le Mardi 9 Décembre 2014 Titre : Ferrite-based micro-inductors for Power Systems on Chip: from material elaboration to inductor optimisation Rapporteurs LABOURE Eric, Université Paris Sud ROY Saibal, Tyndall Institute Ecole doctorale : GEET Unité de recherche : LAAS, CNRS Directeur(s) de Thèse : BRUNET Magali, LAAS, CNRS LAUR Jean-Pierre, LAAS, CNRS Autre(s) membre(s) du Jury : VALDEZ-NAVA Zarel, LAPLACE, CNRS LEFRANC Pierre, G2E Lab PARRA Thierry, UT3 Paul Sabatier VOIRON Frédéric, IPDiA 1 2 Acknowledgement This PhD thesis was carried out within the group of Power Management System Integration (ISGE) at Laboratory for Analysis and Architecture of Systems (LAAS) in collaboration with Laboratory on Plasma and Conversion of Energy (LAPLACE) and with the group of Power Electronics at Grenoble Electrical Engineering Laboratory (G2E lab). Foremost, I thank the successive heads of the LAAS-CNRS, Raja Chatila, Jean Louis Sanchez and Jean Arlat who welcomed me during those three years of research. Then, I would like to thank Marise Bafleur and Frédéric Morancho, the successive heads of ISGE, for welcoming me in the group and enabling me to undertake my research. I would like to express the deepest appreciation to my supervisors, Magali Brunet and Jean- Pierre Laur (LAAS). I am grateful for their time, ideas and offered funding to establish and enrich my PhD experience. Without their persistent support this dissertation would not have been accomplished. I would like to thank Zarel Valdez-Nava, Vincent Bley and Céline Combettes (LAPLACE) for their cooperation in the magnetic materials. They gave me many advices and help for the fabrication of the ferrites and the ferrite pastes and also welcomed me at their laboratory for processing the magnetic materials. I thank Pascal Dufour and Marie-Claire Barthelemy (CIRIMAT) for the magnetic material fabrication and the TMA measurements. I would also like to thank Pierre Lefranc (G2E lab) for his guidance in simulation and optimization. He is always patient to explain the inductor modeling to me. Their contribution made my thesis. During my PhD, working in the clean room needed lots of aid from the technicians. I would like to thank especially David Bourrier for his work in metal electro-deposition, Samuel Charlot for the process of screen printing, wire bonding and flip chip testing, David Colin for the process of cutting and polishing wafers, Jean-Christophe Marrot for the process of sintering, Ludovic Salvagnac for the process of metal deposition, Pierre-Francois Calmon for his fabrication of masks, Franck Carcenac for the training of SEM and EDX, Adrian Laborde and Laurent Mazenq for the training of the photolithography process and thank Monique Benoit for the supply of the experimental instruments. I would like to thank Nicolas Mauran for his training and help in electrical measurements. I am grateful to his solutions for the problem in the equipment of measurement. 3 I also thank Claude Laffore, Ascension De Sousa Berdat, the successive secretaries of ISGE, Camille Cazeneuve, Claire Bardet (Personnel LAAS), Marie-Thérèse Funch (CNRS Delegation) and Marie Estruga (GEET) for helping me in administrative affairs. I would like to acknowledge the funding sources of my PhD, PRIIM project. Finally, I would like to thank my husband Dac-Kien Lai for his encouragement and his understanding of my research. I would like to express my gratitude to my Vietnamese friends for pleasant time we had together in Toulouse. Toulouse, 24th January 2015 Yen Mai Nguyen 4 “Stay hungry. Stay foolish” Steve Jobs, former CEO of Apple Computer and of Pixar Animation Studios June 12, 2005 To my friends, my husband and my family Toulouse 26th January 2015 Yen Mai Nguyen 5 6 Title: Ferrite-based micro-inductors for Power Systems on Chip: from material elaboration to inductor optimisation Abstract: On-chip inductors are key passive elements for future power supplies on chip (PwrSoC), which are expected to be compact and show enhanced performance: high efficiency and high power density. The objective of this thesis work is to study the material and technology to realize small size (<4 mm²) and low profile (< 250 µm) ferrite-based on-chip inductor. This component is dedicated to low power conversion (≈ 1 W) and should provide high inductance density and high quality factor at medium frequency range (5-10 MHz). Fully sintered NiZn ferrites are selected as soft magnetic materials for the inductor core because of their high resistivity and moderate permeability stable in the frequencies range of interest. Two techniques are developed for the ferrite cores: screen printing of in-house made ferrite powder and cutting of commercial ferrite films, followed in each case by sintering and pick-and place assembling to form the rectangular toroid inductor. Test inductors were realized first so that the characterization could be carried out to study the magnetic properties of the ferrite core and the volumetric core losses. The core losses were fit from the measured curve with Steinmetz equation to obtain analytical expressions of losses versus frequency and induction. The second phase of the thesis is the design optimization for the on-chip ferrite based inductor, taking into account the expected losses. Genetic algorithm is employed to optimize the inductor design with the objective function as minimum losses and satisfying the specification on the inductance values under weak current-bias condition. Finite element method for magnetics FEMM is used as a tool to calculate inductance and losses. The second run of prototypes was done to validate the optimization method. In perspective, processes of thick-photoresist photolithography and electroplating are being developed to realize the completed thick copper windings surrounding ferrite cores. Résumé: Les composants passifs intégrés sont des éléments clés pour les futures alimentations sur puce, compactes et présentant des performances améliorées: haut rendement et forte densité de puissance. L'objectif de ce travail de thèse est d'étudier les matériaux et la technologie pour réaliser de bobines à base de ferrite, intégrées sur silicium, avec des faibles empreintes (<4 mm ²) et de faible épaisseur (<250 µm). Ces bobines, dédiées à la conversion de puissance (≈ 1 W) doivent présenter une forte inductance spécifique et un facteur de qualité élevé dans la gamme de fréquence visée (5-10 MHz). Des ferrites de NiZn ont été sélectionnées comme matériaux magnétiques pour le noyau des bobines 7 en raison de leur forte résistivité et de leur perméabilité stable dans la gamme de fréquence visée. Deux techniques sont développées pour les noyaux de ferrite: la sérigraphie d’une poudre synthétisée au laboratoire et la découpe automatique de films de ferrite commerciaux, suivi dans chaque cas du frittage et le placement sur les conducteurs pour former une bobine rectangulaire. Des bobines tests ont été réalisées dans un premier temps afin que la caractérisation puisse être effectuée : les propriétés magnétiques du noyau de ferrite notamment les pertes volumiques dans le noyau sont ainsi extraites. L’équation de Steinmetz a permis de corréler les courbes de pertes mesurées avec des expressions analytiques en fonction de la fréquence et de l'induction. La deuxième phase de la thèse est l'optimisation de la conception de la micro-bobine à base de ferrite, en tenant compte des pertes attendues. L’algorithme générique est utilisé pour optimiser les dimensions de la bobine avec pour objectif ; la minimisation des pertes et l’obtention de la valeur d'inductance spécifique souhaitée, sous faible polarisation en courant. La méthode des éléments finis pour le magnétisme FEMM est utilisée pour modéliser le comportement électromagnétique du composant. La deuxième série de prototypes a été réalisée afin de valider la méthode d'optimisation. En perspective, les procédés de photolithographie de résine épaisse et le dépôt électrolytique sont en cours de développement pour réaliser les enroulements de cuivre épais autour des noyaux de ferrite optimisés et ainsi former le composant complet. 8 Table of contents General introduction ........................................................................................................................... 13 Chapter 1 State of the art of integrated inductor ............................................................................. 19 1. Basic formulas for micro-inductors with magnetic core ................................................................... 19 1.1. Magnetic material ................................................................................................................... 19 1.2. Micro-inductor ........................................................................................................................ 19 1.3. Micro-inductor in Buck converter .......................................................................................... 22 2. Literature review on integrated micro-inductors ............................................................................... 23 2.1. Micro-inductor structures ....................................................................................................... 23 2.2. Magnetic materials and processing technology for cores ......................................................
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