Oxygen Plasma Treatment Induced Mobile Carriers Generation in Amorphous Metal Oxide Semiconductors for Thin-Film Transistors and Their Integration with Active-Matrix Digital Microfluidics

Abstract

This thesis explores a mobile carrier generation method in commonly used amorphous metal oxide semiconductor (AMOS) indium-gallium-zinc oxide (IGZO) and indium-tin-zinc oxide (ITZO). The research demonstrates that oxygen (O₂) plasma treatment significantly improves the conductivity of these materials, with resistivity influenced by various factors such as deposition precursors and thickness of silicon oxide (SiO_x) cover, annealing time, plasma excitation conditions, and the thickness of AMOS. The lowest resistivities achieved were 1.2 mΩ∙cm for ITZO and 2 mΩ∙cm for IGZO. The donors generated by O2 plasma can maintain even at elevated temperatures of 150 ℃.

The O₂ plasma treatment was effectively utilized to form the source/drain (S/D) regions of self-aligned (SA), top-gate (TG) ITZO and IGZO TFTs. The fabrication process for optimal performance included specific annealing steps in O₂ and precursors of SiO_x as gate insulator (GI) and passivation (PV), leading to enhanced device characteristics. The lateral dopant profile analysis revealed a rapid decrease in carrier concentration near the S/D regions, contributing to well-defined transfer characteristics across varying channel lengths.

The thesis further investigates the mechanism of O₂ plasma-induced generation of mobile charge carriers in AMOS through various analytical techniques. The findings suggest that electrons accumulating on the SiO_x surface during plasma treatment attract holes from photon-excited electron-hole pairs, stabilizing electrons in AMOS assisted by the heating from ion bombardment. This mechanism underscores the necessity of SiO_x deposition with carrier gases of SiH₄ for effective donor generation through O₂ plasma.

Additionally, an active-matrix pixel circuit supporting continuous current for electro-dewetting digital microfluidics was designed and fabricated using the developed IGZO and ITZO TG SA TFTs. Successful demonstrations of basic droplet operations, such as transport, splitting, and merging, highlight the practical applications of the research findings.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Man Hoi WONG (Supervisor)