Impact of the interface capacity on failure mechanisms and size effects in ferroelectric thin films Von der Fakultät für Elektrotechnik und Informationstechnik der Rheinisch-Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Ingenieurwissenschaften genehmigte Dissertation Von Diplom Ingenier Ulrich Ellerkmann aus Aachen Berichter: Univ.-Prof. Dr. Rainer Waser Univ.-Prof. Dr. rer. Nat. Matthias Wuttig Tag der mündlichen Prüfung: 12.07.2005
Berichte aus der Halbleitertechnik Ulrich Ellerkmann Impact of the interface capacity on failure mechanisms and size effects in ferroelectric thin films D 82 (Diss. RWTH Aachen) Shaker Verlag Aachen 2006
Bibliografische Information der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.ddb.de abrufbar. Zugl.: Aachen, Techn. Hochsch., Diss., 2005 Copyright Shaker Verlag 2006 Alle Rechte, auch das des auszugsweisen Nachdruckes, der auszugsweisen oder vollständigen Wiedergabe, der Speicherung in Datenverarbeitungsanlagen und der Übersetzung, vorbehalten. Printed in Germany. ISBN-10: 3-8322-5053-0 ISBN-13: 978-3-8322-5053-9 ISSN 0945-0785 Shaker Verlag GmbH Postfach 101818 52018 Aachen Telefon: 02407 / 95 96-0 Telefax: 02407 / 95 96-9 Internet: www.shaker.de email: info@shaker.de
III Acknowledgement This dissertation was written during my Ph.D.-studies at the Institut für Werkstoffe der Elektrotechnik of the Rheinisch-Westfälische Technische Hochschule Aachen, Germany (RWTH Aachen). I would like to express my gratitude to Prof. Rainer Waser for giving me the opportunity to do research at the Institut für Werkstoffe der Elektrotechnik in the field of ferroelectrics and for providing an excellent working and learning environment. I highly appreciate his advice and support. I am also indebted to Prof. M. Wuttig who kindly agreed to be the co-examiner in the jury. Additionally, I would like to thank P. Schorn, P. Gerber and especially Dr. U. Böttger from the FeRAM team for many fruitful and stimulating discussions. I would like to express my gratitude to our projekt partner Infineon Technologies AG, especially Dr. R. Bruchhaus, Dr. N. Nagel, Dr. G. Beitel and C. Scheibe and the members from Toshiba Cooperations of the FRAM Developement Alliance (FDA) between Infineon and Toshiba, especially K. Yamakawa and I. Kunishima, which also provided some of the PZT thin films investigated in this work, for many stimulating discussions and outstanding cooperation with RWTH Aachen in the FeRAM project. I am grateful to Dr. T. Schneller and R. Gerhardt (RWTH Aachen) for preparing the PZT precursor solutions; A. Roelofs, S. Clemens and G. Darlinski for being great office mates; T. Kever and C. Nauenheim for contributing to this thesis by their assistance as student workers; D. Erdoglija for preparing numerous top electrodes on PZT thin films, P. Roegels for his expertise and assistance with vacuum systems and G. Wasse for taking SEM pictures; S. Tiedke and T. Schmitz (aixacct Systems) for their support in electrical characterization; my colleagues at the Institut für Werkstoffe der Elektrotechnik and at the Forschungszentrum Jülich for their valuable support and many suggestions.
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V Contents 1 Introduction 1 1.1 Motivation............................ 1 1.2 Objectives............................ 3 2 Ferroelectric Materials and their Application in Ferroelectric Memories 5 2.1 Ferroelectric Materials and their Properties.......... 5 2.1.1 Definition and Basic Properties of a Ferroelectric... 5 2.1.2 Material Systems.................... 7 2.2 Ferroelectric Random Access Memories............ 12 2.2.1 Operation Principle................... 12 2.2.2 Cell Structure...................... 13 3 Failure Mechanisms and Size-Effect 17 3.1 Failure Mechanisms....................... 17 3.1.1 Fatigue......................... 17 3.1.2 Imprint......................... 20 3.1.3 Retention........................ 21 3.2 Size-Effects........................... 22 3.2.1 Decrease of Remanent Polarization.......... 23 3.2.2 Increase of Coercive Field............... 25 3.2.3 Decrease of the Permittivity.............. 28 3.2.4 Change of Domain Structure.............. 29 3.2.5 Decrease of the Piezoelectric Coefficients....... 30 3.2.6 Variation of the Phase Transition............ 30 3.3 Interface Capacity........................ 31
VI 4 Experimental Methods 40 4.1 Sample Preparation....................... 40 4.2 Physical Characterization Methods............... 44 4.3 Electrical Characterization Methods.............. 48 4.3.1 Hysteresis Measurement................ 49 4.3.2 Imprint Measurements................. 52 4.3.3 Fatigue Measurement................. 53 4.3.4 Small Signal Measurements.............. 54 5 Reducing the Film Thickness 58 5.1 State of the Art......................... 58 5.2 Variations from the Standard Route............... 59 5.2.1 Different Dilutions of the Precursor Solution..... 60 5.2.2 Crystallizing under different Atmospheres: O 2 vs. N 2 65 6 Results and Discussion 69 6.1 Suppression of the Tunability.................. 69 6.1.1 Bias dependence of the Interface Capacity...... 70 6.2 Fatigue............................. 88 6.3 Imprint............................. 98 6.4 Scalability of the Coercive Field................ 102 7 Summary and Outlook 109 7.1 Summary............................ 109 7.2 Outlook............................. 111 References 112