Ultralow Energy SIMS

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A STUDY OF THE EFFECTS OF ULTRALOW-ENERGY SECONDARY ION MASS SPECTROMETRY (SIMS) ON SURFACE TRANSIENT AND DEPTH RESOLUTION AB RAZAK CHANBASHA (M.Sc. University of Strathclyde, UK) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE (2007) Acknowledgements i Acknowledgements This has been one of the most arduous projects that I have undertaken. This would not have been possible without the opportunity and the trust that has been given to me by my mentor and supervisor, Professor Andrew Wee Thye Shen. Prof. Wee has been most encouraging and had never failed to keep up with my progress throughout the course of this research. To him, I am indebted. I would also like to thank Mr. Ridzuan Wu Chia Chung, Managing Director at Omega Scientific Pte Ltd for the use of the ATOMIKA SIMS without which this research would not have taken place. I am also grateful to my colleague Mr. Amir Jantan for maintaining the SIMS instrument in excellent condition. I am also fortunate to have research colleagues Dr. Liu Rong and Dr. Md. Abdul Kader Zilani, formerly of Surface Science Laboratory (NUS), both of whom I have had regular discussions with throughout the course of my study. Thank you, Liu Rong and Zilani. Finally, my utmost gratitude goes to my family. To my wife, Aishah for her untiring support, constant encouragement and having to tolerate my spending family-time away from home. To my children, Irshaad, Muneerah, Shafeeq and Nazheef, I hope to have inspired them to continuously quest for knowledge. I am also grateful to my parents for instilling in me the value of education. Table of contents ii Table of contents Acknowledgments i Table of contents ii Summary vi Abbreviations and symbols viii Figure captions x List of Tables xii Publications xiii Chapter 1: Introduction 1 1.1 The need for SIMS in the semiconductor industry. 1 1.2 Challenges of ultralow-energy SIMS 3 1.2.1 Surface transient effects 3 Surface transient effects with ultralow-energy O2+ sputtering 4 Surface transient effects with ultralow-energy Cs+ sputtering 6 1.2.2 Sputter rate 9 1.2.3 Depth resolution 10 Factors affecting depth resolution 10 Methods used to improve depth resolution: O2+ bombardment 13 (a) Lower primary ion energy 13 (b) Varying the incident angle 13 (c) Oxygen flooding and sample rotation 15 Methods used to improve depth resolution: Cs+ bombardment 15 1.3 Outline of research project 17 References 19 Chapter 2: SIMS Principles 25 2.1 Introduction 25 2.2 Fundamentals of SIMS 26 2.2.1 Sputtering & collision theory 26 2.2.2 Sputtering yield 30 2.2.3 Sputter rate 31 2.2.4 Secondary ion emission 32 Secondary ion yield and ionisation probability 35 Ionisation mechanism with O2+ ion beam 35 Ionisation mechanism with Cs+ ion beam 36 Secondary ion species 38 2.3 Factors affecting depth profiling 38 Table of contents iii 2.3.1 Primary ion species 39 2.3.2 Primary ion energy 39 2.3.3 Primary ion angle of incidence 40 2.3.4 Crater edge effect 41 2.3.5 Sample charging effect 42 2.4 Quantification of depth profiles 43 2.4.1 Relative sensitivity factors and absolute standards 43 2.4.2 Depth calibration 45 References 47 Chapter 3: Experimental and instrumentation 51 3.1 SIMS instrumentation 51 3.1.1 Introduction 51 3.1.2 Vacuum system and sample handling 52 Vacuum system and monitors 52 Sample introduction 54 Sample manipulator 55 3.1.3 Primary ion gun 56 O2+ source 56 Cs+ source 57 FLIG 58 3.1.4 Secondary Ion Optics 60 3.1.5 Quadrupole mass spectrometry 60 3.1.6 Secondary ion detection 63 3.1.7 Data acquisition and electronics 63 3.2 Atomic force microscopy 64 3.2.1 Principles 64 3.2.2 Contact mode 66 3.2.3 Non-contact mode 66 3.2.4 Tapping mode 67 3.3 Experimental 68 3.3.1 Sample 68 3.3.2 Analysis parameters with O2+ SIMS 69 3.3.3 Analysis parameters with Cs+ SIMS 70 3.3.4 Sputter rate determination 71 References 71 Chapter 4: Effect of ultralow-energy O2+ SIMS on Si surface transient 73 4.1 Introduction 73 4.2 Results & discussion 74 4.2.1 Surface transients 74 Surface spikes and incomplete oxidation 75 Transient width 79 Table of contents iv 4.2.2 Sputter rates 82 4.3 Summary 88 References 89 Chapter 5: Effect of ultralow-energy O2+ SIMS on depth resolution 91 5.1 Introduction 91 5.2 Results & discussion 93 5.2.1 Depth resolution 93 Depth resolution in terms of FWHM 93 Depth resolution in terms of exponential decay 100 MRI model and evaluation 103 5.2.2 Dynamic range 107 5.3 Summary 108 References 110 Chapter 6: Effect of ultralow-energy Cs+ SIMS on Si surface transient 111 6.1 Introduction 111 6.2 Results & discussion 112 6.2.1 Surface transients 112 30Si- profiles 113 Transient width 117 Steady state intensity of 30Si- profiles 119 6.2.2 Sputter rates 120 6.3 Summary 125 References 126 Chapter 7: Effect of ultralow-energy Cs+ SIMS on depth resolution 128 7.1 Introduction 128 7.2 Results & discussion 129 7.2.1 Depth resolution 129 Depth resolution in terms of FWHM 131 Depth resolution in terms of exponential decay 137 Depth resolution evaluated with MRI model 141 7.2.2 Dynamic range 146 7.3 Summary 148 References 151 Table of contents v Chapter 8: Conclusion 152 8.1 Surface transient 152 8.2 Sputter rate 154 8.3 Depth resolution 155 8.4 Dynamic range 156 8.5 Optimum conditions for analysis 157 8.6 Proposed future work 158 Appendix A 159 Depth profiles with O2+ primary ion beam Appendix B 171 Depth profiles with Cs+ primary ion beam Summary vi Summary Ultralow-energy secondary ion mass spectrometry (SIMS) has been introduced to meet the increasing demand for depth profiling of ultrashallow junctions (< 20 nm) and ultrathin films following the developments in device miniaturisation in the semiconductor industry. This challenge for accurate profiling at the near surface (SIMS transient region) and for achieving high depth resolution is directly influenced by the probe energy and the incident angle of the primary ion used. However, issues such as surface roughening, atomic mixing, secondary ion yield, sputter rates and for ultralow-energy Cs+, a poorly focused beam are important considerations. The objective of this research is to understand the trends, characteristic behaviour and processes involved with the use of ultralow-energy SIMS. Obtaining such information will aid method development and the optimization of parameters for accurate depth profiling. In this research,