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Subtractive Plasma-Assisted-Etch Process for Developing High Performance Nanocrystalline Zinc-Oxide Thin-Film-Transistors
AIR FORCE INSTITUTE OF TECHNOLOGY WRIGHT-PATTERSON AFB OH GRADUATE SCHOOL OF ENGINEERING AND MANAGEMENT
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Thin-Film-Transistors TFTs employing undoped zinc-oxide ZnO thin-films are currently being investigated by the Air Force for microwave switching applications. Since the on-resistance Ron of the device scales with channel length LC, ZnO TFT optimization should be focused on reducing LC, therefore minimizing the associated insertion losses. In this research, deep sub-micron scaling of ZnO TFTs was undertaken using a subtractive reactive-ion-etch RIE process. Under optimum processing conditions, ZnO TFTs with LC as small as 155 nm were successfully demonstrated. The active ZnO channels of the TFTs were patterned by selective SF6-RIE of a tungsten ohmic film through electron-beam defined openings in a polymethyl-methacrylate PMMA based resist. Through electrical testing, the width normalized Ron of ZnO TFTs with 155 nm channels was extracted as 3.6 omega dot mm and the devices were found to operate at drain current densities and transconductance values of 830 mAmm and 121 mSmm, respectively. Additionally, a total width-normalized source and drain parasitic resistance of 2.1 omega dot mm was observed using a gated transfer length method TLM, indicating the tungsten-ZnO interface is low resistance. This demonstration of high performance and low Ron suggests the potential for ZnO TFTs in switching and microwave power applications.
APPROVED FOR PUBLIC RELEASE