Atomic Layer Etching of Metal Oxides and Atomic Layer Deposition of Metal Fluorides

Atomic Layer Etching of Metal Oxides and Atomic Layer Deposition of Metal Fluorides by Younghee Lee B.S., Seoul National University, l999 M.S., Seoul National University, 2001 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Chemistry and Biochemistry 2015 This thesis entitled: Atomic Layer Etching of Metal Oxides and Atomic Layer Deposition of Metal Fluorides written by Younghee Lee has been approved for the Department of Chemistry and Biochemistry Professor Steven M. George Professor Carl A. Koval Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Lee, Younghee (Ph.D., Chemistry) Atomic Layer Etching of Metal Oxides and Atomic Layer Deposition of Metal Fluorides Thesis directed by Professor Steven M. George Atomic control of thin film growth and removal is essential for semiconductor processing. Atomic layer deposition (ALD) is a thin film deposition technique that is based on self-limiting surface reactions that deposit thin conformal films with atomic scale precision. The ALD of metal fluorides has been limited by the difficulty of handling the hydrogen fluoride (HF) precursor that is necessary for metal fluoride film growth. The use of HF-pyridine as an HF reservoir has allowed the development of metal fluoride ALD processes such as AlF3, LiF, lithium ion conducting (AlF3)(LiF)x alloy, ZrF4, HfF4, MnF2, MgF2, and ZnF2. AlF3 ALD was studied using trimethylaluminum (TMA) and HF at temperatures from 150-300°C by in situ techniques such as quartz crystal microbalance (QCM), quadrupole mass spectrometry (QMS) and Fourier transform infrared (FTIR) spectroscopy. Ex situ characterization of the films was conducted using X-ray reflectivity (XRR) and spectroscopic ellipsometry (SE), grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). These high quality metal fluoride ALD films will be useful for many applications such as optical coatings and Lewis acid catalysts. ”Reverse ALD”, i.e. atomic layer etching (ALE), is a film removal technique that is able to remove thin films with atomic level control. Current ALE processes based on halogen adsorption and ion-enhancement are inherently directional processes that lead to an anisotropic removal of material. New thermal approaches for Al2O3 and HfO2 ALE were demonstrated using Sn(acac)2 and HF as the reactants. In situ and ex situ studies showed that thermal ALE of Al2O3 and HfO2 is possible at temperatures from 150-250°C. In the proposed reaction iv mechanism, the HF reactant fluorinates the metal oxide surfaces to form the corresponding metal fluoride and H2O as reaction products. The Sn(acac)2 reactant induces a ligand exchange reaction to produce volatile SnF(acac) and Al(acac)3 or Hf(acac)4. This etching approach based on fluorination and ligand exchange is general and can be applied to other metal oxides, as well as metal nitrides, metal phosphides, metal arsenides and elemental metals. Thermally driven ALE processes will provide an important tool for isotropically etching materials at the atomic scale. v “We discovered something today.” John Bardeen (1908-1991) vi ACKNOWLEDGEMENTS I would like to thank my advisor, Professor Steven George, for his support throughout my PhD study. Steve has always provided me with wonderful advice and encouraged me to pursue projects that I truly enjoy. I have enjoyed being a part of a great team. Andrew Cavanagh taught me everything I needed to know around the lab as well as preformed the XPS experiments for these studies. Virginia Anderson taught me about XRR measurements and supported me. Jaime DuMont conducted all of the FTIR studies in this study. Matthias Young guided me through XRD analysis. Daniel Higgs assisted me in studying the mechanical properties of our materials. Huaxing Sun also performed XPS experiments. Tim Porcelli helped conduct the AFM and SEM measurements. I am also grateful to the rest of the former and current George group members who have always been there to help me. I would like to thank our excellent collaborators, especially Dani PiPer and Professor Sehee Lee for the measurements of ion conductivity. I am very grateful to my friends, parents, and family. Ingrid and Rick made me love Boulder even more. My parents have always been supportive. I would like to thank my wonderful wife, Dayoung for being with me. This work was funded by the National Renewable Energy Laboratory, the Department of Energy, and the National Science Foundation. vii CONTENTS CHAPTER 1 Introduction 1 1.1 Atomic Layer Deposition 1.2 Atomic Layer Etching 1.3 Outline of Remaining Chapters 1.4 References 2 ALD of AlF3 Using Trimethylaluminum and Hydrogen Fluoride 23 2.1 Introduction 2.2 Experimental 2.2A Viscous Flow Reactor with in situ QCM and QMS 2.2B FTIR Spectroscopy 2.2C Ex situ Film Characterization using XRR, XRD and SE 2.2D XPS and RBS 2.3 Results and Discussion 2.3A Growth of AlF3 Films 2.3B Nucleation and Reaction Mechanism for AlF3 ALD 2.3C. Ex situ AlF3 Film Characterization 2.4 Conclusions 2.5 Acknowledgements 2.6 References 3 ALD of LiF and Lithium Ion Conducting (AlF3)(LiF)x Alloys Using Trimethylaluminum, Lithium Hexamethyldisilazide and Hydrogen Fluoride 60 3.1 Introduction 3.2 Experimental viii 3.2A Viscous Flow Reactor with in situ QCM and QMS 3.2B XRR, GIXRD, and SE 3.2C XPS 3.2D ICP-MS and IC 3.2E EIS 3.3 Results and Discussion 3.3A Growth of LiF Films 3.3B Growth of (AlF3)(LiF)x Alloy Films 3.3C Ex situ analysis of LiF and (AlF3)(LiF)x Alloy Films 3.3D Measurement of Ionic Conductivity of (AlF3)(LiF)x Alloy Films by EIS 3.4 Conclusions 3.5 Acknowledgements 3.6 References 4 ALD of Metal Fluorides Using Various Metal Precursors and Hydrogen Fluoride 95 4.1 Introduction 4.2 Experimental 4.2A Viscous Flow Reactor with in situ QCM 4.2B Ex situ Film Characterization Using XRR, XRD, SE, and XPS 4.3 Results and Discussion 4.3A. Growth of ZrF4 Films Using Tetrakis(ethylmethylamido)zirconium and HF 4.3B. Growth of ZrF4 Films Using or Zirconium tert-butoxide and HF 4.3C Growth of HfF4 Films Using tetrakis(dimethylamido)hafnium and HF 4.3D Growth of MnF2 Films Using Bis(ethylcyclopentadienyl)manganese and HF 4.3E Growth of MgF2 Films Using Bis(ethylcyclopentadienyl)magnesium and HF 4.3F Growth of ZnF2 Films Diethylzinc and HF 4.3G Reaction Mechanism and Stoichiometry 4.3H Ex situ Film Characterization Using XRR, XRD, SE, and XPS 4.4 Conclusions ix 4.5 Acknowledgements 4.6 References 5 Atomic Layer Etching of Al2O3 Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)2 and HF 132 5.1 Introduction 5.2 Experimental 5.2A Viscous Flow Reactor Equipped for in situ QCM Measurements 5.2B Sample Preparation and ex situ Film Analysis 5.3 Results and Discussion 5.4 Generality and Advantages of Thermal ALE Approach 5.5 Conclusions 5.6 Acknowledgements 5.7 References 6.. Mechanism of Thermal Al2O3 Atomic Layer Etching Using Sequential Reactions with Sn(acac)2 and HF 167 6.1 Introduction 6.2 Experimental 6.2A Viscous Flow Reactor Equipped for in situ QCM Measurements 6.2B FTIR Spectroscopy Measurements 6.3 Results and Discussion 6.3A QCM Measurements versus Temperature 6.3B FTIR Studies of Al2O3 ALE 6.3C Studies of Al2O3 ALE Nucleation 6.3D Proposed Al2O3 ALE Reaction Mechanism 6.4 Conclusions 6.5 Acknowledgements 6.6 References 7.. Atomic Layer Etching of HfO2 Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)2 and HF 207 x 7.1 Introduction 7.2 Experimental 7.2A Viscous Flow Reactor Equipped for in situ QCM Measurements 7.2B Si Wafers, XRR, XPS and SE 7.2C FTIR Spectroscopy Measurements 7.3 Results and Discussion 7.3A QCM Measurements 7.3B XRR, XPS and SE Measurements 7.3C FTIR Spectroscopy Measurements 7.3D Nucleation Behavior and Proposed HfF4 Formation 7.3E Proposed HfO2 ALE Reactions 7.4 Conclusions 7.5 Acknowledgements 7.6 References 8 Bibliography 253 xi TABLES Table 1-1. Various metal precursors used for metal fluorides ALD. Table 2-1. ∆MTMA, ∆MHF, MGPC, ∆MTMA/MGPC, and x for AlF3 ALD at different 2 temperatures. ∆MTMA, ∆MHF, and MGPC are expressed in units of ng/(cm cycle). Growth rate is expressed in unit of Å /cycle Table 4-1. Metal fluorides, metal precursors, acronym of metal precursors, and the bubbler temperature. Table 4-2. Metal fluorides, Metal fluorides, acronym of metal precursors, MGPC, and number of HF on metal fluoride. Table 4-3. Metal fluorides, acronym of metal precursors, growth rate at 150°C, density, and the refractive index. Table 5-1. ∆MSn, ∆MHF and MCPC for Al2O3 ALE at different temperatures. Table 5-2. Atomic Layer Etching Reactions for Various Materials Table 6-1. ∆MSn, ∆MHF, MCPC, ∆MSn/MCPC, x, and x(MCPC) for Al2O3 ALE at different 2 temperatures. ∆MSn, ∆MHF, MCPC, and x(MCPC) are expressed in units of ng/(cm cycle). Table 7-1. ∆MSn, ∆MHF, MCPC, ∆MSn/MCPC, x, and x(MCPC) for HfO2 ALE at different 2 temperatures. ∆MSn, ∆MHF, MCPC and x(MCPC) are expressed in units of ng/(cm cycle). xii FIGURES Figure 1-1. Schematic of the general ALD process based on sequential, self-limiting surface reactions. Figure 1-2. Schematic of surface chemistry for Al2O3 ALD using TMA and H2O as the reactants.

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