On process simplification and reliability of elevated-metal metal-oxide thin-film transistor

Abstract

With their relatively lower process temperature, higher field-effect mobility, lower leakage current and higher transparency, metal-oxide semiconductors (MOs) such as zinc oxide and its variants are being pursued as promising alternatives to silicon-based materials for the construction of thin-film transistors (TFTs) in next-generation flat-panel displays. However, architectural limitations and reliability issues of MO TFTs currently hinder wider application of the technology to the construction of displays. A three-mask architecture has been proposed for the realization of an elevated-metal metal-oxide (EMMO) TFT with a (mask-less) self-aligned definition of the active island. The process-simplified device, dubbed 3-EMMO TFT, merges all benefits of the two conventional bottom-gate transistor architectures: a lower manufacture cost, a smaller device footprint, reduced overlap capacitances and better device characteristics. Superior to the conventional stretched-exponential equation for interpreting the time-dependence of the shift in the turn-on voltage of a MO TFT under stress, a more physically based model incorporating the photo-generation, transport and trapping of holes is presently formulated. It is theoretically deduced and experimentally verified that the degradation kinetics is either generation- or transport-limited, depending on the magnitude and direction of the local electric field inside the channel but normal to the channel/interface during the stress. The correlation of the origin of TFT degradation and oxygen-vacancy (Vo) related defects is clarified in the model and studied by conducting a comparison of the effects of post-metallization oxidizing and non-oxidizing annealing. Accordingly, an oxidation-last annealing is suggested to reduce the population of V_O in MOs to improve TFT reliability. Finally, a 1.22-inch, in-plane switching liquid-crystal watch panel based on EMMO TFT technology is designed, fabricated and tested. The successful demonstration of the watch panel verifies the feasibility of applying EMMO TFT technology to the construction of flat-panel displays

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology