View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by ScholarBank@NUS INVESTIGATION OF HIGH-К DIELECTRIC FILMS INCORPORATED WITH LANTHANUM AND THEIR APPLICATION IN FLASH MEMORY DEVICES HE WEI NATIONAL UNIVERSITY OF SINGAPORE 2009 INVESTIGATION OF HIGH-К DIELECTRIC FILMS INCORPORATED WITH LANTHANUM AND THEIR APPLICATION IN FLASH MEMORY DEVICES He Wei (B. Eng., Harbin Institute of Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 Abstract Abstract The SONOS-type of flash memory is one of the promising candidates to replace the conventional floating-gate type flash memory to enable continued memory device down-sizing. For a typical SONOS-type flash memory, there is a tradeoff between data retention and operation speeds. In order to break this tradeoff, Al2O3 as a blocking oxide has been proposed and exhibits promising results. However, the permittivity of Al2O3 is not high (only around 9) and even higher permittivity dielectrics, such as LaAlOx and HfLaOx, were investigated here to further improve the memory performance. The lanthanum-incorporated dielectrics (LaAlOx and HfLaOx) were deposited using the atomic layer deposition (ALD) method at 300ºC. The enhancement of deposition rate of La2O3 was observed when co-introducing the Hf or Al precursor into the process chamber to form HfLaOx or LaAlOx, respectively. Overall, the ALD processes of both HfLaOx and LaAlOx showed good self-limiting behavior, good film uniformity, low carbon impurity and a strictly linear relationship between the film thickness and deposition cycles, indicating good ALD characteristics. Both ALD LaAlOx and HfLaOx layers exhibited wide energy band gap and relatively large conduction band offset. LaAlOx films with a higher La percentage showed higher permittivity. The permittivity of amorphous LaAlOx films with 46% La was around 18 and these films could stand annealing temperature up to 850ºC. In I Abstract contrast, at low temperatures (~ 500ºC), HfLaOx with 8 at.% La would crystallize to a cubic-like phase and this phase has a much greater permittivity (~ 38) than that of amorphous or monoclinic phases (~ 20). This increase in permittivity resulted in a significant improvement in the overall leakage current. Moreover, the permittivities and leakage currents are dependant on annealing temperatures. Both LaAlOx and HfLaOx were used as blocking oxides in nitride-based SONOS-type flash memories. In comparison with memory cells using Al2O3 blocking oxide, those with LaAlOx or HfLaOx blocking oxide exhibited improved memory characteristics, such as faster operation speeds, wider saturation window, and better time-to-breakdown (tBD) reliability. Furthermore, memory cells with LaAlOx blocking oxide showed improved retention performance below 120ºC, which is similar to normal memory working temperature. On the other hand, memory cells using HfLaOx blocking oxide showed degraded retention performance due to their relatively low conduction band offset. By compiling above data retention results, it is found that the conduction band offset of blocking oxide over Si is required to be greater than 2.4 eV. However, few dielectrics can satisfy such requirement. In addition, by using the modified Yang’s model, the trapping energy depth in nitride films was calculated to be ~ 0.7 eV below the nitride conduction band edge. Overall, LaAlOx is a promising candidate as a blocking oxide to further boost the memory performances. And HfLaOx, due to its improved permittivity under cubic phase, may be useful in other applications, e.g. in DRAMs. II Acknowledgements Acknowledgements Many colleagues and individuals who have directly or indirectly assisted in the preparation of this manuscript are much appreciated. First of all, I would like to express my sincere gratitude to my thesis advisors, namely, Prof. Daniel Chan and Prof. Byung-jin Cho for their invaluable guidance, wisdom, and kindness in instructing and encouraging me during my postgraduate study at NUS. I will definitely benefit from the experience and knowledge I have gained from them throughout my life. I am especially grateful to Prof. Cho who provided me an opportunity to join Silicon Nano Device Lab and start the postgraduate study; secondly for his patience and painstaking efforts devoted to my research as well as his kindness and understanding which accompanied me over the last four years. Besides, I am also grateful to Prof. Chan’s, who guided me through the Ph.D study, offered me a position to continue the study and spent a lot of time in correcting the thesis. Hence, my best wishes will go to Prof. Cho and Prof. Chan as I am deeply appreciated for their generous help. Besides, I have had a great pleasure of collaborating with numerous talented graduate students and colleagues over the past four years. Firstly, I would like to give my thanks to my colleagues in Prof. Cho’s group, including Dr. Kim Sun-Jung, Dr. Hwang Wansik, Dr. Shen Chen, Ms. Zhang Lu, and Ms. Pu Ji for their useful discussions and kind assistances. Secondly, many thanks also go to Mr. Zhang Gang, III Acknowledgements Mr. Sun Zhiqiang, Mr. Ma Fajun and other technical staffs (Mr. Yong YuFoo, Mr. Lau BoonTeck, Mr. Tang Patrick, and O Yan Wai Linn) for their knowledge and experiences which had benefited me, as well as the long lasting friendship. I would also like to extend my appreciation to all NUS members including professors (such as Prof. Samudra Ganesh, Dr. Zhu Chunxiang, Dr. Yeo Yee-Chia, Dr. Yuan Zeliang, etc.), non-academic staffs and graduate students for the terrific academic environment, as well as the technical staffs in IMRE who have assisted me in material analysis, such as Dr. Zhang Zheng, Lim Poh Chong, Doreen Lai Mei-Ying, etc. Last but not least, my deepest love and gratitude will go to my family, especially to my wife and my parents for their self-giving love, patience and support throughout my life. He Wei Nov 2009 IV Table of Contents Table of Contents Abstract......................................................................................................................... I Acknowledgements ....................................................................................................III Table of Contents ........................................................................................................V List of Tables ...........................................................................................................VIII List of Figures.............................................................................................................IX List of Symbols and Abbreviations .......................................................................XVI Chapter 1 Introduction of Flash Memory and High-К Dielectrics.........................1 1.1 Overview..............................................................................................................1 1.2 Nitride-based SONOS-type Flash Memory.........................................................4 1.2.1 Overview of Non-volatile Memory Candidates............................................4 1.2.2 A Brief Review of SONOS-type Flash Memory ..........................................8 1.2.3 Using High-κ Dielectric to Improve Memory Performance.......................11 1.3 Lanthanum-incorporated High-κ Dielectrics .....................................................13 1.3.1 Dielectric Parameters for the Selection of High-κ Thin Film.....................13 1.3.2 Lanthanum-incorporated High-κ Dielectrics: LaAlOx and HfLaOx ...........17 1.3.3 Current Developments of LaAlOx and HfLaOx Dielectrics........................19 1.3.4 Using ALD LaAlOx and HfLaOx Dielectrics in Memory Application.......23 1.4 Outline of the Thesis..........................................................................................25 References................................................................................................................26 Chapter 2 High-К Dielectrics Engineering by Atomic Layer Deposition (ALD).35 2.1 Introduction........................................................................................................35 2.1.2 Introduction of ALD Principles ..................................................................36 V Table of Contents 2.1.2 A Brief Review of ALD Precursors............................................................39 2.3 Experiment Conditions ......................................................................................41 2.4 ALD Process of Lanthanum-incorporated Dielectrics.......................................41 2.5 Characterization of Deposited Film...................................................................53 2.6 Summary............................................................................................................57 References................................................................................................................57 Chapter 3 Characteristics of Lanthanum-incorporated Dielectrics .....................61 3.1 Introduction........................................................................................................61 3.2 Experiments .......................................................................................................62 3.3 Physical Characteristics of HfLaOx and LaAlOx Films.....................................66 3.4 Electrical Characteristics of HfLaOx Films .......................................................75