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Plasma Treatment Method for Ohmic Contacts on Zinc Oxide Thin Film Transistors

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Plasma Treatment Method for Ohmic Contacts on Zinc Oxide Thin Film Transistors Air Force Institute of Technology AFIT Scholar Theses and Dissertations Student Graduate Works 3-22-2019 Plasma Treatment Method for Ohmic Contacts on Zinc Oxide Thin iF lm Transistors Blaine Z. Underwood Follow this and additional works at: https://scholar.afit.edu/etd Part of the Power and Energy Commons Recommended Citation Underwood, Blaine Z., "Plasma Treatment Method for Ohmic Contacts on Zinc Oxide Thin iF lm Transistors" (2019). Theses and Dissertations. 2287. https://scholar.afit.edu/etd/2287 This Thesis is brought to you for free and open access by the Student Graduate Works at AFIT Scholar. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of AFIT Scholar. For more information, please contact [email protected]. Plasma Treatment Method for Ohmic Contacts on Zinc Oxide Thin Film Transistors THESIS Blaine Z. Underwood, Second Lieutenant, USAF AFIT-ENG-MS-19-M-062 DEPARTMENT OF THE AIR FORCE AIR UNIVERSITY AIR FORCE INSTITUTE OF TECHNOLOGY Wright-Patterson Air Force Base, Ohio DISTRIBUTION STATEMENT A APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED The views expressed in this document are those of the author and do not reflect the official policy or position of the United States Air Force, the United States Department of Defense or the United States Government. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. AFIT-ENG-MS-19-M-062 PLASMA TREATMENT METHOD FOR OHMIC CONTACTS ON ZINC OXIDE THIN FILM TRANSISTORS THESIS Presented to the Faculty Department of Electrical and Computer Engineering Graduate School of Engineering and Management Air Force Institute of Technology Air University Air Education and Training Command in Partial Fulfillment of the Requirements for the Degree of Master of Science Blaine Z. Underwood, BSEE Second Lieutenant, USAF March 2019 DISTRIBUTION STATEMENT A APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED AFIT-ENG-MS-19-M-062 PLASMA TREATMENT METHOD FOR OHMIC CONTACTS ON ZINC OXIDE THIN FILM TRANSISTORS THESIS Blaine Z. Underwood, BSEE Second Lieutenant, USAF Committee Membership: Maj. Tod V. Laurvick, PhD Chair Hengky Chandrahalim, PhD Member Michael L. Schuette, PhD Member AFIT-ENG-MS-19-M-062 Abstract This research utilizes plasma treatments as a method to decrease the contact resistance on zinc oxide (ZnO) thin film transistors (TFT). In recent years, researchers have achieved gigahertz RF switch cutoff frequency with ZnO TFTs. To further increase the cutoff frequency, the total resistance of this device must be minimized. Modern ZnO TFTs are fabricated with submicron channel lengths, which contains minimal resistance due to the TFT channel, making contact resistance significant to the total resistance of the device. This research developed a method to integrate plasma treatments into the fabrication of ZnO TFT to decrease the contact resistance, thus increasing the cutoff frequency of the device. Three plasma treatment methods were used, including Ar plasma generated in an inductively coupled reactive-ion etch (ICP-RIE) chamber, Ar plasma applied in-situ in a metal-sputter chamber before sputtering tungsten (W) contacts, and by remotely generated hydrogen plasma. The plasma treatments were implemented after the deposition of ZnO and prior to the deposition of sputtered tungsten (W) contacts. The plasma treatments increased the conductivity of the ZnO, which allowed for the formation of ohmic contacts on the ZnO TFT. The contact resistance of the untreated ZnO TFT sample was 1.9 ohm-mm, while the ICP-RIE Ar plasma treatment, in-situ Ar plasma treatment, and H plasma treatment demonstrated minimum contact resistances of 1.2, 1.0, and 1.5 ohm-mm respectively. The in-situ Ar plasma treatment demonstrated the best results, with a decrease in contact resistance of 47%. iv AFIT-ENG-MS-19-M-062 This work is dedicated to my lovely wife and three daughters. They are my greatest supporters and source of motivation. I could not have completed this work without my wife's constant devotion to our family. v Acknowledgements To my brother and colleague, George; we were great teammates back in the days on the Track and Field team in high school, Electrical Engineering in undergraduate school, and now in the Master's program at AFIT. Twenty-five years of working together will now come to an end, and I cannot thank you enough for being there every step of the way. I'd like to thank the faculty and staff of AFIT, especially Maj Laurvick and Dr. Chandrahalim; for their guidance and support throughout this process. I appreciate the extra hours you put in to make sure I had the resources needed for my research. To my colleagues in the Microelectronics program, Enoc Flores, Tory Robinson, Mike Dela Cruz, Jon Smith, and Paul Michaud; I am thankful for the help each of you gave me in our classes together and throughout my research. To the staff at AFRL, particularly Dr. Schuette, Dr. Leedy, Dr. Green, Kyle Liddy, Jason Hickey, and Andrew Browning; Your expertise and assistance were invaluable to the completion of this research. Blaine Z. Underwood vi Table of Contents Page Abstract . iv Acknowledgements . vi List of Figures . ix List of Tables . xii I. Introduction . .1 1.1 Background . .1 1.2 Problem Statement . .1 1.3 Limitations . .2 1.4 Approach . .3 1.4.1 Plasma Surface Treatment . .3 1.4.2 Channel Recovery . .3 1.5 Summary . .4 II. Literature Review . .5 2.1 History of TFTs. .5 2.2 Metal Oxide TFTs . .5 2.3 ZnO Semiconductor Properties . .8 2.3.1 n-Type Conductivity . .8 2.4 ZnO Native Defects . .8 2.4.1 Oxygen Vacancy . .9 2.4.2 Zn Interstitials . .9 2.5 ZnO Growth Process . 10 2.5.1 Pulsed Laser Deposition of ZnO . 11 2.6 ZnO TFTs . 12 2.7 ZnO Ohmic Contacts . 12 2.8 Selective Etch Contact Method . 15 2.9 Plasma Treatment Solution . 16 2.9.1 Hydrogen Plasma Treatment . 16 2.9.2 Argon Plasma Treatment . 17 2.10 Treatments to Decrease Conductivity . 18 2.10.1 Oxygen Plasma Treatment . 18 2.10.2 Oxygen Anneal . 19 2.11 Contact Resistance Measurements . 21 2.11.1 Transfer Length Method . 21 2.11.2 Cross-Bridge Kelvin Resistor . 25 vii Page III. Methodology . 27 3.1 Introduction . 27 3.2 Implementation of Plasma Surface Treatments . 32 3.3 Factor Selection for Experiments . 33 3.3.1 Plasma-Enhanced Atomic Layer Deposition Tool . 33 3.3.2 ICP-RIE Tool . 34 3.3.3 RF Sputtering Tool . 34 3.3.4 Factor Selection Conclusion . 35 3.4 Plasma Treatment Experimental Design . 35 3.4.1 Hydrogen Plasma Treatment . 36 3.4.2 ICP-RIE Chamber Argon Plasma Treatment . 36 3.4.3 In-situ Argon Plasma Treatment. 37 3.5 Plasma Etching Experiment . 38 3.6 Contact Resistance Measurements . 39 3.7 Channel Recovery Experiment . 42 3.7.1 Oxygen Plasma Channel Recovery Experiment . 42 3.7.2 Oxygen Anneal Channel Recovery Experiment . 43 IV. Results and Analysis . 45 4.1 Chapter Overview . 45 4.2 Surface Etching Experiment . 45 4.2.1 Surface Etching Experiment Summary . 48 4.3 Contact Resistance Results . 49 4.3.1 Contact Resistance Summary . 53 4.4 Contact Resistance vs. Etch Depth . 54 4.4.1 Contact Resistance vs. Etch Depth Analysis . 56 4.5 Channel Recovery . 56 4.5.1 Channel Recovery Analysis . 58 V. Conclusion . 60 5.1 Conclusions of Research . 60 5.2 Recommendations for Future Research . 60 5.2.1 Ga-doped ZnO Ohmic Layer . 62 Bibliography . ..
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