Barium Titanate and Barium Strontium Titanate Thin Films Were Deposited on Base Metal Foils Via Chemical Solution Deposition and Radio Frequency Magnetron Sputtering
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ABSTRACT AYGÜN, SEYMEN MURAT. Processing Science of Barium Titanate. (Under the direction of Jon-Paul Maria.) Barium titanate and barium strontium titanate thin films were deposited on base metal foils via chemical solution deposition and radio frequency magnetron sputtering. The films were processed at elevated temperatures for densification and crystallization. Two unifying research goals underpin all experiments: 1) To improve our fundamental understanding of complex oxide processing science, and 2) to translate those improvements into materials with superior structural and electrical properties. The relationships linking dielectric response, grain size, and thermal budget for sputtered barium strontium titanate were illustrated. (Ba0.6Sr0.4)TiO3 films were sputtered on nickel foils at temperatures ranging between 100-400 °C. After the top electrode deposition, the films were co-fired at 900 °C for densification and crystallization. The dielectric properties were observed to improve with increasing sputter temperature reaching a permittivity of 1800, a tunability of 10:1, and a loss tangent of less than 0.015 for the sample sputtered at 400 °C. The data can be understood using a brick wall model incorporating a high permittivity grain interior with low permittivity grain boundary. However, this high permittivity value was achieved at a grain size of 80 nm, which is typically associated with strong suppression of the dielectric response. These results clearly show that conventional models that parameterize permittivity with crystal diameter or film thickness alone are insufficiently sophisticated. Better models are needed that incorporate the influence of microstructure and crystal structure. This thesis next explores the ability to tune microstructure and properties of chemically solution deposited BaTiO3 thin films by modulation of heat treatment thermal profiles and firing atmosphere composition. Barium titanate films were deposited on copper foils using hybrid-chelate chemistries. An in-situ gas analysis process was developed to probe the organic removal and the barium titanate phase formation. The exhaust gases emitted during the firing of barium titanate films were monitored using a residual gas analyzer (RGA) to investigate the effects of ramp rate and oxygen partial pressure. The dielectric properties including capacitor yield were correlated to the RGA data and microstructure. This information was used to tailor a thermal profile to obtain the optimum -13 dielectric response. A ramp rate of 20 °C/min and a pO2 of 10 atm resulted in a permittivity of 1500, a loss tangent of 0.035 and a 90 % capacitor yield in 0.5 mm dot capacitors. Yield values above 90% represent a significant advantage over preexisting reports and can be attributed to an improved ability to control final porosity. Finally, the dramatic enhancement in film density was demonstrated by understanding the processing science relationships between organic removal, crystallization, and densification in chemical solution deposition. The in situ gas analysis was used to develop an each-layer-fired approach that provides for effective organic removal, thus pore elimination, larger grain sizes, and superior densification. The combination of large grain size and high density enabled reproducing bulk-like dielectric properties in a thin film. A room temperature permittivity of 3000, a 5 μF/cm2 capacitance density, and a dielectric tunability of 15:1 were achieved. By combining the data sets generated in this thesis with those of comparable literature reports, we were able to broadly rationalize scaling effects in polycrystalline thin films. We show that the same models successfully applied to bulk ceramic systems are appropriate for thin films, and that models involving parasitic interfacial layers are not needed. Developing better models for scaling effects were made possible solely by advancing our ability to synthesize materials thus eliminating artifacts and extrinsic effects. Processing Science of Barium Titanate by Seymen Murat Aygun A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Materials Science and Engineering Raleigh, North Carolina 2009 APPROVED BY: _______________________________ ______________________________ Jon-Paul Maria Zlatko Sitar Associate Professor Professor Materials Science and Engineering Materials Science and Engineering Committee Chair ________________________________ ________________________________ Gregory Parsons Yuntian Zhu Professor Associate Professor Chemical Engineering Materials Science and Engineering DEDICATION To my family ii BIOGRAPHY Seymen Aygün was born on August 11, 1980 to parents Baykut and Sezer Aygün in Ankara, Turkey. He graduated from METU (Middle East Technical University) High School and enrolled at the Metallurgical and Materials Science Department of METU. He received his B.S. in 2002. After that, he decided to continue his journey in the US and he worked with Dr. David Cann at Iowa State University of Science and Technology where he received his master’s degree in 2005. After spending a year and a half as a postmaster researcher under supervision of Dr. Jon-Paul Maria at North Carolina State University, he decided to pursue Ph.D. and received his degree in 2009. iii ACKNOWLEDGEMENTS First I would like to thank my mentor and friend Professor Jon-Paul Maria for the opportunities, guidance, and convincing me to pursue Ph.D. I am grateful for the four years I have spent at NCSU. I would also like to thank my friends at MSE; Jon Ihlefeld, Brian Laughlin, Mark Losego, Dipankar Ghosh, Spalding Craft, Patrick Daniels, Peter Gaifun Lam, Jimster, Erin Gross, Michelle Casper, Elizabeth Paisley, Jesse Jur, James Steel, and Tony Rice. I would like to acknowledge the help of Dr. Bill Borland with my research. I would like to thank Dick Parham and Edna Deas for their help and patience with my administrative problems. Also to my friends Metin, Eren, Erdem, Berna, and Arun. To Utkan for her love and support. Finally, and most importantly, I would like to express my gratitude to my family. Thank you for your constant love, support and patience. I could not have accomplished this without you. iv TABLE OF CONTENTS LIST OF FIGURES....................................................................................................... viii LIST OF TABLES ........................................................................................................ xiii CHAPTER 1. INTRODUCTION .......................................................................................1 CHAPTER 2. LITERATURE REVIEW............................................................................3 2.1 Dielectrics ...................................................................................................................3 2.2 Classification in Terms of Crystal Symmetry.............................................................11 2.3 Ferroelectricity ..........................................................................................................13 2.3.2 Theory of Ferroelectricity ...................................................................................14 2.3.3 Dielectric Properties of Ferroelectrics ..............................................................21 2.4 Perovskite Structure and The Archetypical Ferroelectric BaTiO3 ...............................31 2.4.1 Crystal Structure and Phase Transitions............................................................31 2.4.2 Dielectric Properties.........................................................................................35 2.4.3 Compositional Modification with SrTiO3 .........................................................38 2.4.4 Stress-Strain Effects.........................................................................................40 2.5 Scaling Effects in BaTiO3 ..........................................................................................43 2.5.1 Scaling Effects in Bulk BaTiO3 ........................................................................44 2.5.2 Scaling Effects in Thin Film BaTiO3 ................................................................49 2.6 Processing of BaTiO3 – (Ba,Sr)TiO3 Thin Films ........................................................57 2.6.1 Magnetron Sputtering......................................................................................58 2.6.2 Chemical Solution Deposition.........................................................................63 v 2.6.3 Firing of Thin Films on Base Metal Substrates.................................................81 References.......................................................................................................................85 CHAPTER 3. EXPERIMENTAL PROCEDURES .......................................................104 3.1 Barium Strontium Titanate Sputtering on Base Metal Foils......................................104 3.2 Chemical Solution Deposition of Barium Titanate ...................................................107 3.3 Low pO2 Processing of Thin Films ..........................................................................109 3.3.1 Firing of Sputtered BST Thin Films ...............................................................109 3.3.2 Firing and Probing the Phase Evolution of CSD Barium Titanate Thin Films.112 3.3.3 Reoxidation Anneals ......................................................................................116