(Invited) Oxide TFT Fabrication with Various Low Temperature Techniques

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Oxide thin film transistors (TFTs) have attracted much attention for next-generation electronics because of their superior performance compared with conventional a–Si TFTs such as high mobility and optical transmittance in the visible light.1 Some display manufacturing companies have already manufactured oxide TFTs based products, but there are some issues remained for oxide TFTs to be used for flexible displays. One of the issues is the high-temperature annealing process (> 300°C). Due to low thermal durability, flexible substrates cannot be processed in high temperatures.2-3 In this study, we propose various researches to lower the processing temperatures in oxide TFTs: photocatalytic reactions of titanium dioxide (PRT) and rotating magnetic field (RMF) treatment. In addition, we suggest ethyl cyanoacrylate (ECA) based instant glue (IG) as a new material for the passivation layers of oxide TFTs with low processing temperature. Photocatalytic Reaction of Titanium Dioxide (PRT) In this study, we fabricate indium–zinc oxide (IZO) TFTs using PRT. The hydroxyl radicals are generated by reaction between titanium dioxide (TiO2) and water molecules during UV irradiation. These hydroxyl radicals decompose the 2-methoxyethanol (2ME; one of the representative solvents) into smaller organic molecules. The decomposed 2ME has low boiling points and molar masses compared with non-PRT treated 2ME. Therefore, PRT treatment not only provides superior electronic devices caused by less defect sites in the oxide channel layers, but also allows low temperature process at 230°C. Additionally, since TiO2 particles can generate hydroxyl radicals permanently under UV irradiation, the PRT treated IZO TFTs show improved electrical characteristics even after four times recycling of TiO2. Rotating magnetic field (RMF) treatment In this work, we fabricate indium–gallium–zinc oxide (IGZO) TFTs by RMF treatment at a low temperature of 150°C. For RMF treatment, IGZO films are exposed to periodically rotating magnetic field and thermal energy of 150°C. The magnetic field and thermal energy simultaneously facilitate the interaction and attraction of metal and oxygen atoms leading to formation of M–O bonds in IGZO films. As a result, RMF treated IGZO TFTs exhibited improved electrical performances and positive bias stress (PBS) characteristics compared with those of IGZO TFTs treated by only thermal annealing process at 300°C. Instant Glue (IG) Passivation We introduce IG as a novel material for the passivation layer of IGZO TFTs. The ECA-based IG is a moisture-curing adhesive, which is liquid at room temperature and cures rapidly by reacting with moisture in atmosphere. As it is solidified rapidly at room temperature, it has shorter process time than that of conventional organic materials; the IG passivation layer is prepared successfully just for 1 min at 150°C. The IG passivation layer can modify the chemical stoichiometry of IGZO films and prevent any interaction between the back surface of the IGZO TFT and ambient gas. Therefore, the IGZO TFTs with an IG passivation layer show improved electrical characteristics and reliability compared to those without passivation layer. References Jae Won Na, You Seung Rim, Hee Jun Kim, Jin Hyeok Lee, Seonghwan Hong, and Hyun Jae Kim, ACS Appl. Mater. Interfaces 9, 35, 29849 (2017).Hua Xu, Dongxiang Luo, Min Li, Miao Xu, Jianhua Zou, Hong Tao, Linfeng Lan, Lei Wang, Junbiao Peng, and Yang Cao, J. Mater. Chem. C 2, 7, 1255 (2014).Seonghwan Hong, Sung Pyo Park, Yeong-gyu Kim, Byung Ha Kang, Jae Won Na, and Hyun Jae Kim, Scientific Reports 7, 1, 16265 (2017). Figure. Schematics of various techniques to lower the processing temperatures for oxide TFTs, (a) photocatalytic reaction of titanium dioxide, (b) rotating magnetic field treatment, (c) instant glue passivation. Figure 1