Thin-film transistors (TFTs) prepared using oxide semiconductors have attracted considerable attention for use in driving devices owing to their excellent electrical properties (1). Alternative facile fabrication methods based on solution processing are recognized as advanced oxide TFT fabrication methods (2, 3). Solution-process fabrication is promising technology owing to advantages such as lower processing costs, easier large-area fabrication, and higher throughput compared to conventional vacuum processing. However, it has several limitations that must be mitigated so that the method can be used in practical applications. One such limitation is the processing temperature for the fabrication of solution-processed oxide TFTs. In general, solution-processed oxide TFTs need relatively high-temperature annealing for metal–oxide–metal bond formation and decomposition of the solvents and ligands of metal salts. High-temperature annealing over 400 °C cannot incorporate onto the flexible substrate.　Another limitation is that the fabrication process is complex. Although solution processing can be performed using basic equipment such as a coater and annealing equipment, it requires a complicated photolithography process for device integration. A simple fabrication process based on direct patterning procedures is useful for maximizing the advantages of solution processing.

In this presentation, we introduce our recent studies on techniques for fabricating low-temperature aqueous solution-processed oxide TFTs, as well as simpler fabrication based on the direct patterning process. We demonstrate the improvement in the characteristics of low-temperature oxide TFTs by applying the hydrogen injection and oxidation (HIO) process (4). From the technical standpoint, decomposing residual species efficiently is of primary importance. We initiated the reduction reaction by hydrogen injection for decomposing the residual spices. The characteristics of the resulting In-Ga-Zn oxide (IGZO)-based TFTs were evaluated by applying the HIO method at low temperatures. The fabrication of IGZO TFTs on film substrate were also demonstrated. For process simplification, we report a direct photoreactive patterning method for fabricating solution-processed IGZO TFTs using a simple aqueous IGZO precursor without any the need for organic-based additives. This is accomplished by photo-oxidation of water molecules based on free radical reactions induced by short-wavelength UV light. In addition, a soft etching step using a dilute organic acid is employed. By combining these processes, a photoreactive patterning system capable of generating uniform island shapes for fabricating solution-processed IGZO TFTs is demonstrated. We demonstrate the advantages of the TFTs fabricated using the direct patterning process by comparing their performance with that of the TFTs fabricated using the conventional photolithography process.

1. K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature, 432, 488 (2004).
2. E. Fortunato, P. Barquinha, and R. Martins, Adv Mater, 24, 2945 (2012).
3. S. J. Kim, S. Yoon, and H. J. Kim, Japn J Appl Phys, 53 (2014).
4. M. Miyakawa, M. Nakata, H. Tsuji, Y. Fujisaki, and T. Yamamoto, AIP Advances, 6 (2016).