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DISSERTATION CONTRIBUTIONS OF GAS-PHASE PLASMA CHEMISTRY TO SURFACE MODIFICATIONS AND GAS-SURFACE INTERACTIONS: INVESTIGATIONS OF FLUOROCARBON RF PLASMAS Submitted by Michael F. Cuddy II Department of Chemistry In partial fulfillment of the requirements For the Degree of Doctor of Philosophy Colorado State University Fort Collins, Colorado Summer 2012 Doctoral Committee: Advisor: Ellen R. Fisher Amber Krummel Nancy E. Levinger Dawn Rickey Azer P. Yalin ABSTRACT CONTRIBUTIONS OF GAS-PHASE PLASMA CHEMISTRY TO SURFACE MODIFICATIONS AND GAS-SURFACE INTERACTIONS: INVESTIGATIONS OF FLUOROCARBON RF PLASMAS The fundamental aspects of inductively coupled fluorocarbon (FC) plasma chem- istry were examined, with special emphasis on the contributions of gas-phase species to surface modifications. Characterization of the gas-phase constituents of single-source CF4-, C2F6-, C3F8-, and C3F6-based plasmas was performed using spectroscopic and mass spectrometric techniques. The effects of varying plasma parameters, including applied rf power (P) and system pressure (p) were examined. Optical emission spectroscopy (OES) and laser-induced fluorescence (LIF) spectroscopy were employed to monitor the behav- ior of excited and ground CFx (x = 1,2) radicals, respectively. Mass spectrometric tech- niques, including ion energy analyses, elucidated behaviors of nascent ions in the FC plasmas. These gas-phase data were correlated with the net effect of substrate processing for Si and ZrO2 surfaces. Surface-specific analyses were performed for post-processed substrates via x-ray photoelectron spectroscopy (XPS) and contact angle goniometry. Generally, precursors with lower F/C ratios tended to deposit robust FC films of high sur- face energy. Precursors of higher F/C ratio, such as CF4, were associated with etching or removal of material from surfaces. Nonetheless, a net balance between deposition of FC moieties and etching of material exists for each plasma system. ii The imaging of radicals interacting with surfaces (IRIS) technique provided in- sight into the phenomena occurring at the interface of the plasma gas-phase and substrate of interest. IRIS results demonstrate that CFx radicals scatter copiously, with surface scatter coefficients, S, generally greater than unity under most experimental conditions. Such considerable S values imply surface-mediated production of the CFx radicals at FC- passivated sites. It is inferred that the primary route to surface production of CFx arises from energetic ion bombardment and ablation of surface FC films. Other factors which may influence the observed CFx scatter coefficient include the surface with which the radical interacts, the vibrational temperature (ΘV) of the radical in its gas-phase, and radi- cal interactions in the gas phase. The analyses of ΘV in particular were extended to dia- tomic radicals from other plasma sources, including nitric oxide and fluorosilane systems, to gauge the contributions of vibrational energy to surface reactivity. In general, a mono- tonic increase in S is observed for CF, NO, and SiF radicals with increasing ΘV. Preliminary results for mixed plasma precursor systems (i.e. FC/H2, FC/O2) indi- cate that the choice of feed gas additives has a profound effect on surface modification. Hydrogen additions tend to promote FC film deposition through scavenging of fluorine atoms, whereas oxygen consumes polymerizing species, thus favoring etching regimes. Time-resolved optical emission spectroscopy (TR-OES) studies of gas-phase species elu- cidate the mechanisms by which these processes occur. Ultimately, the work presented herein expands the fundamental chemical and physical understanding of fluorocarbon plasma systems. iii ACKNOWLEDGMENTS I am eternally grateful to all those who have guided, mentored, and supported me in pursuit of this educational milestone. There are countless individuals to whom I owe thanks. To my advisor, Prof. Ellen R. Fisher, whose guidance through the morass of sci- entific muddle helped hone my competence. Early in my graduate career, I was humbled by all that which I did not know; your counsel propelled me to approach those obstacles as a scientist. To my fellow graduate students in the Fisher group, whose solidarity and encouragement made the experience not only bearable, but enjoyable. To my advisors in Chemistry at Eastern Illinois University, Dr. Barbara Lawrence and Drs. Richard and El- len Keiter, who inspired an impetuous, young undergraduate researcher to pursue a PhD. To my family, especially my mother, Lora, my father, Michael, and my brothers, Jesse and Connor, who fostered my interests in science and mathematics from the start. To my wife, Kristin, whose tolerance for my idiosyncrasies defies my understanding. To the SMART Fellowship, that funded nearly my entire graduate career. To those I’ve forgot- ten, to those come and gone, to those friends and acquaintances, to those teachers and colleagues, to those who have influenced, motivated, inspired and educated, I am dearly indebted. Thank you, this dissertation is dedicated to you. iv TABLE OF CONTENTS ABSTRACT ....................................................................................................................... ii ACKNOWLEDGMENTS ............................................................................................... iv TABLE OF CONTENTS ..................................................................................................v LIST OF TABLES ........................................................................................................... ix LIST OF FIGURES ...........................................................................................................x CHAPTER 1. Introduction ................................................................................................1 1.1. Plasma Chemistry and Physics .........................................................................2 1.2. Motivations for Fluorocarbon Investigations ....................................................6 1.3. Overview of Research .....................................................................................10 1.4. References .......................................................................................................15 CHAPTER 2. Experimental Methods .............................................................................18 2.1. General Information ........................................................................................19 2.2. Gas-phase Plasma Diagnostics .......................................................................20 2.3. Gas-Surface Interactions .................................................................................28 2.4. Surface Analyses .............................................................................................29 2.5. References .......................................................................................................32 CHAPTER 3. Investigation of the Roles of Gas-Phase CF2 Molecules and F Atoms During Fluorocarbon Plasma Processing of Si and ZrO2 Substrates ..............................34 3.1. Introduction .....................................................................................................35 3.2. Results .............................................................................................................37 v 3.3. Discussion .......................................................................................................49 3.4. Summary .........................................................................................................60 3.5. Supplemental Information ..............................................................................61 3.6. References .......................................................................................................63 CHAPTER 4. Contributions of CF and CF2 Species to Fluorocarbon Film Composition and Properties for CxFy Plasma-Enhanced Chemical Vapor Deposition ........................66 4.1. Introduction .....................................................................................................67 4.2. Results .............................................................................................................69 4.3. Discussion .......................................................................................................78 4.4. Summary .........................................................................................................88 4.5. Supplemental Information ..............................................................................90 4.6. References .......................................................................................................91 CHAPTER 5. Energy Partitioning in Diatomic Radicals and Its Influence on Surface Scatter Coefficients for NO, SiF and CF in Inductively Coupled Plasmas ....................94 5.1. Introduction .....................................................................................................95 5.2. Results .............................................................................................................97 5.3. Discussion .....................................................................................................109 5.4. Summary .......................................................................................................121 5.5. References .....................................................................................................122 CHAPTER 6.
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