WAFER SURFACE CLEANING FOR SILICON HOMOEPITAXY WITH AND WITHOUT ECR HYDROGEN PLASMA EXPOSURE by Hyoun-woo Kim B.S., Seoul National University (1986) M.S., Seoul National University (1988) Submitted to the Department of Materials Science and engineering in Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY at the MASSACHUSETFS INSMruTE OF TECHNOLOGY September 1994 O Massachusetts Institute of Technology 1994 JI / Signature of Author Department of Materials Science and Engineering August 5, 1994 ( .f Certified by I 1 Rafael Reif, Thesis Supervisor Professor, Department of Electrical Engineering and Computer Science Director, Microsystems Technology Laboratories Accepted by Carl V. Thompson Professor of Electronic Materials Chairman, Departmental Committee on Graduate Students MASSACHUSETTSINSTITUTE nr -T'rirmhAOnGY lA SEP 2 7 1994 LIBRAeES Science WAFER SURFACE CLEANING FOR SILICON HOMOEPITAXY WITH AND WITHOUT ECR HYDROGEN PLASMA EXPOSURE by Hyoun-woo Kim Submitted to the Department of Materials Science and engineering in Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY Abstract The objective of this thesis is to investigate the low temperature in-situ cleaning process using the ECR (Electron Cyclotron Resonance) hydrogen plasma technique and the ex-situ cleaning process using the HF dipping method, for low temperature silicon homoepitaxial growth. A Load Lock Chamber was installed on the MS-CVD (Multi-chamber Chemical Vapor Deposition) reactor, to reduce the possibility of introducing contaminants into the system. The ECR plasma system was selected because it could deliver a higher density of low energy ions in a well-regulated manner, as compared to a conventional RF (Radio Frequency) system. Hydrogen gas was selected because of its light mass and its ability to react chemically with surface contaminants. Epitaxial layers were deposited on top of the in-situ cleaned wafers and the structural qualities of the epitaxial layers and the epilayer/substrate interface were investigated by XTEM (Cross-sectional Transmission Electron Microscopy) and RBS (Rutherford Backscattering Spectroscopy) techniques. SIMS (Secondary Ion Mass Spectroscopy) was used to detect the oxygen and carbon contaminants at the interface. Process variables such as DC bias, cleaning time, microwave power, and gas pressure were set, and when the in-situ ECRhydrogen plasma process at 600°C was optimized, a defect-free epitaxial layer with an almost invisible interface was obtained. The effect of in-situ cleaning temperature on cleaning efficiency was then studied in the range of 25°C to 660°C and it was revealed that a defect-free epitaxial layer could be deposited with room temperature in- 1B situ cleaning. Ex-situ cleaning with HF dipping was performed and the effects of rinsing and drying techniques were investigated. Although rinsing encouraged the growth of natural surface oxide to grow, it improved the surface smoothness of the resulting epitaxial layer. The blow-drying technique was more efficient than the spin-drying technique. When in-situ cleaning was optimized at 600°C, it succeeded in removing natural surface oxide. When the in-situ cleaning was repeated at 660°C, it was even more efficient in removing the surface oxide from the wafer. Room temperature in- situ cleaning was efficient in removing carbon species from the wafer surface. The reaction mechanisms of oxygen and carbon removal were discussed. Thesis Supervisor: Dr. Rafael. Reif Title: Professor of Electrical Engineering and Computer Science Director, Microsystems Technology Laboratories 2 Acknowledgments I would like to thank my supervisor, Prof. Rafael Reif, for his patience and timely advises throughout my career at M.I.T. He provided me an opportunity for independent research. I would like to thank SRC/SEMATECHfor partially funding this project. I thank Prof. Harry Tuller and Prof. Lionel Kimmerling for reviewing this thesis and providing helpful suggestions. My gratitude goes to my colleagues, Julie Tsai, Ken Liao, Alex Cherkassky, Weize Chen, Andy Tang, Rajan Naik, Ben Tao and Dr. Takashi Noguchi for wonderful collaborations and friendships. I thank to Dr. Zhen-Hong Zhou in training me on the MS-CVDreactor and very helpful discussions, to Dr. Tri- Rung Yew, Dr. Euijoon Yoon, and Dr. Jaegab Lee for teaching me XTEM sample preparations, to Dr. Chang-Kyung Kim for helpful discussions on material characterizations. I thank to M. Frongillo for training me on TEM and ion milling, to J. Chervinski for training me on RBS. I am grateful to the staff in Microsystems Technology Laboratories: Octavio Hurtado, Paul Mcgrath, and Joe I)iMaria for their helps with facilities. A special thanks goes to Carolyn Zaccaria for great helps. I thank to Samsung company for providing me a scholarship and encouraged me throughout my PhD course. My father and mother have provided love and support throughout my life and I also appreciate encouragement from my wife' family. I specially thank my wife, Hye-gyu Lee, for her love, support and patience, without her this work was impossible. I also thank my lovely daughter, Juna for her happy smiles. Great thanks go to people, who have prayed for me throughout my life. Thank God and glorify Him with this small dissertation. 3 Contents 1 Introduction and Background ................................................... 11 1.1 Background and Motivation .................................................... 11 1.2 Organization of Thesis .................................................... 16 1.3 Literature Review .................................................... 17 1.3.1 Applications of Silicon Epitaxial Layers ........................................ 17 1.3.2 Fundamentals of ECRPlasma .................................................... 23 2 Reactor Description ..................................................... 34 2.1 Introduction .................................................... 34 2.2 Chambers .................................................... .. 35 2.3 Plasma System .................................................... 37 2.4 Heating System .................................................... 39 2.5 Wafer Delivery System ..................................................... 40 2.6 Gas Handling System ...................................................................................... 41 2.7 Pumping System ............................................................................................. 43 2.8 Quadrupole Mass Spectrometer (QMS)......................................................... 44 3 Experimental Procedures .......................................................... 46 3.1 Surface Cleaning and Deposition .................................................... 46 3.1.1 Ex-situ Cleaning ........................................ ............ 46 3.1.2 In-situ Plasma Cleaning ................................................................... 47 3.1.3 Deposition Procedures ...................................................................... 48 3.2 Materials Characterization Techniques ........... ......... 48 3.2.1 Haze inspection and Nomarski Optical Microscopy ..................... 48 3.2.2 Plain-view Transmission Electron Microscopy ........................... 50 3.2.3 Cross-sectional Transmission Electron Microscopy .................... 50 4 3.2.4 Secondary Ion Mass Spectroscopy ................................................... 52 3.2.5 Ion Channeling / Rutherford Backscattering Spectroscopy ....................................................................................... 52 3.2.6 FTIR Measurements - In-situ Monitoring Technique ................. 53 4 In-situ cleaning: part 1 ............................................................ 56 4.1 Introduction .................................................................................................... 56 4.2 Experimental ..................................................... 58 4.3 Results ...................................................... 59 4.3.1 Standard Condition ...................................................... 59 4.3.2 Effect of DC Bias...................................................... 61 4.3.3 Effect of Cleaning Time ...................................................... 66 4.3.4 Effect of Microwave Power ..................................................... 66 4.3.5 Effect of Cleaning Gas (H2) Pressure ............................................... 68 4.4 Discussions...................................................... 72 5 In-sItu cleaning: part 2 ........................................ 75 5.1 Introduction ........................................ 75 5.2 Experimental ................................................................................................... 77 5.3 Results and Discussion ........................................ 79 5.4 Conclusion and Summary........................................ 94 6 Ex-situ cleaning with HF dipping ............................................. 96 6.1 Introduction .................................................................................................... 96 6.2 Experimental ................................................................................................... 98 6.3 Results ....................................... 99 6.4 Discussions....................................................................................................... 110 7 Surface Contamination : Oxygen and Carbon ............................. 115 7.1 Introduction ...................................................................................................