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Gas-Phase Chemistry of Methyl-Substituted Silanes in a Hot-Wire Chemical Vapour Deposition Process

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Gas-Phase Chemistry of Methyl-Substituted Silanes in a Hot-Wire Chemical Vapour Deposition Process University of Calgary PRISM: University of Calgary's Digital Repository Graduate Studies The Vault: Electronic Theses and Dissertations 2013-08-27 Gas-phase Chemistry of Methyl-Substituted Silanes in a Hot-wire Chemical Vapour Deposition Process Toukabri, Rim Toukabri, R. (2013). Gas-phase Chemistry of Methyl-Substituted Silanes in a Hot-wire Chemical Vapour Deposition Process (Unpublished doctoral thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/26257 http://hdl.handle.net/11023/891 doctoral thesis University of Calgary graduate students retain copyright ownership and moral rights for their thesis. You may use this material in any way that is permitted by the Copyright Act or through licensing that has been assigned to the document. For uses that are not allowable under copyright legislation or licensing, you are required to seek permission. Downloaded from PRISM: https://prism.ucalgary.ca UNIVERSITY OF CALGARY Gas-phase Chemistry of Methyl-Substituted Silanes in a Hot-wire Chemical Vapour Deposition Process by Rim Toukabri A THESIS SUBMITTED TO THE FACULTY OF GRADUATE STUDIES IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY CALGARY, ALBERTA August, 2013 © Rim Toukabri 2013 Abstract The primary decomposition and secondary gas-phase reactions of methyl- substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS) and tetramethylsilane (TMS), in hot-wire chemical vapour deposition (HWCVD) processes have been studied using laser ionization methods in combination with time of flight mass spectrometry (TOF-MS). For all four molecules, methyl radical formation and hydrogen molecule formation have been found to be the common decomposition steps on both tungsten (W) and tantalum (Ta) filaments. The apparent activation energy ranges from 51.1 to 84.7 kJ · mol-1 for the methyl radical -1 formation and 55.4 to 70.7 kJ · mol for the formation of H2. Both activation energy values increase with the number of methyl substitutions in the precursor molecules on W and Ta filaments. The formation of these two species is initiated by Si-H bond cleavage. This cleavage is then followed by Si-CH3 bond breakage producing methyl radical, whereas two H adsorbates on the surface of the filament recombine releasing H2 into the gas phase, following Langmuir-Hinshelwood mechanism. The secondary gas-phase reactions of MMS and DMS in a HWCVD reactor have also been investigated. For DMS as a precursor gas, a competition between silene/ silylene chemistry occurring at low temperature and radical chain mechanism present at high temperature is observed. For MMS, its gas-phase chemistry involves exclusively silylene species, characterized by its insertion and dimerization reactions. It is concluded that both free-radical and silenes/silylenes intermediates play important roles in the gas- phase chemistry of methyl-substituted silanes. A comparison of the secondary gas-phase ii reactions of TMS, TriMS, DMS, and MMS revealed a switch in dominance from free- radical chemistry to silene/silylene chemistry as the number of methyl substitution on the precursor molecule is decreased. A study of the effect of deposition parameters, including precursor pressure and filament material, has shown that these parameters influenced significantly the gas-phase chemistry of TriMS and DMS. This is due to the competition between free-radical reactions and silylene/silene reactions. However, these parameters do not affect the gas-phase chemistry of MMS since it involves only one type of intermediate. iii Acknowledgements I would like to take this opportunity to present my appreciation to the people that provided me the possibility to accomplish my PhD study. Foremost I would like to express my sincere gratitude to my supervisor, Dr. Yujun Shi, for giving me the opportunity to complete my graduate study in her group and for her guidance throughout these years. Beside my supervisor, I would like to forward my gratitude to my committee members Dr. Hans Osthoff and Dr. Peter Kusalik for their encouragement, and insightful discussions. It has been a great pleasure to work along each of Dr. Shi’s group members, past and present. I would like to thank Brett and Ling for teaching me about the instrument and their help when I started with the lab work. I am also pleased to know Martin, Watheq, Chris, Jimmy, Ismail and Ebenezer. Your company and friendship kept enriched my life with laughter and joy. I specially thank Ismail for his help and contribution in the theoretical calculation carried out in this work. iv I would like to thank my friends Yiota, Angela and Samar, who were always great support in all my struggles and frustrations, as well as my achievements through these years. Yiota, even though you are far, your friendship and support is greatly appreciated. Ibro, I could never thank you enough, you were always there for me. My sincere thanks go to Bonnie King for her countless hours of support and motivation. I would like to thank also Andy Read and Todd Willis from the science workshop for their tremendous help in fixing and adjusting my instrument, as well as Edward Cairns and Mike Siewert for providing me with assistance with all sorts of electronics. Last but not least, I would like to thank my parents, Hatem Toukabri and Samira Kedadi for always believing in me. Their love and encouragement provided me with inspiration and driving force to complete this important stage of my life. I also would like to thank my siblings, Ala and Mariem Toukabri, and their families for their understanding and support throughout these last years. v Dedication To my parents Hatem Toukabri and Samira Kedadi vi Table of Contents Abstract ............................................................................................................................... ii Acknowledgements ............................................................................................................ iv Dedication .......................................................................................................................... vi Table of Contents .............................................................................................................. vii List of Tables .......................................................................................................................x List of Figures and Illustrations ......................................................................................... xi List of Schemes .............................................................................................................. xviii List of Symbols, Abbreviations and Nomenclature ......................................................... xix Epigraph ........................................................................................................................... xxi CHAPTER ONE: INTRODUCTION AND BACKGROUND ...........................................1 1.1 Thin Film Deposition Processes ................................................................................1 1.1.1 Physical Vapour Deposition ..............................................................................1 1.1.2 Chemical Vapour Deposition ............................................................................3 1.2 Hot-Wire Chemical Vapour Deposition (HWCVD) .................................................4 1.3 HWCVD of Silicon Carbide Thin Films .................................................................10 1.3.1 Silicon Carbide (SiC) Thin Films ....................................................................10 1.3.2 Growth of SiC Thin Films Using HWCVD Processes ....................................12 1.3.3 HWCVD of SiC Thin Films Using Methyl-Substituted Silanes .....................14 1.4 Gas-Phase Chemistry in HWCVD System ..............................................................15 1.4.1 Detecting the Gas-Phase Reaction Products in a HWCVD Process ...............15 1.4.2 Gas-Phase Chemistry in HWCVD Using Methyl-Substituted Silanes ...........19 1.4.3 Chemical Trapping of the Active Intermediates .............................................21 1.5 Objectives of this Thesis ..........................................................................................25 CHAPTER TWO: EXPERIMENTAL DETAILS .............................................................27 2.1 HWCVD Sources .....................................................................................................28 2.1.1 A Collision-Free Setup to Detect Decomposition Species on the Filament ....28 2.1.2 A Reactor Setup to Detect Species from Secondary Gas-phase Reactions .....30 2.2 Ionization Sources ....................................................................................................31 2.2.1 The Single Photon Ionization (SPI) Source .....................................................31 2.2.2 The Dual Single Photon Ionization and Laser Induced Electron Ionization (SPI/LIEI) Source ............................................................................................32 2.3 TOF Mass Spectra Collection ..................................................................................34 2.4 Synthesis of MMS-d3, DMS-d2 and TriMS-d1 ........................................................36 2.5 Sample Preparation and Introduction ......................................................................39 2.6 Computational Methods ...........................................................................................41
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