Thin-Film Oxide Transistor by Liquid Process (2): UV and Solvothermal Treatments for TFT Fabrication

Abstract

As previously noted, this chapter describes the effect of UV (ultraviolet) irradiation on the pyrolysis of oxide precursors. In the pyrolysis process of oxide materials, heat is the usual energy source to decompose starting materials to the final products (solids). However, when UV light is used in a pyrolysis process together with heat, a tremendous effect is expected. In this chapter, we present experimental results concerning the UV irradiation under an oxygen atmosphere (UV/O3 treatment) or nitrogen atmosphere to semiconductor (in Sect. 17.1) and insulator (in Sect. 17.3) precursor films. Using this technology, we demonstrated the second example of an all-liquid-processed TFTin Sect. 17.2. The UV irradiation is not only effective for enhancing the properties of oxide films but also can lower the process temperature and be used as a patterning tool. When the UV irradiation technology is used in combination with solvothermal synthesis of solution, which was already described in Chap. 13, it enables the low-temperature solidification of oxide materials at less than 200 °C, as described in Sect. 17.3. In Sect. 17.4, the solidification mechanism of oxide precursor including UV irradiation treatment is fully studied in detail taking an InO cluster gel as a specimen. The cluster gel is an assembly of hybrid clusters, which has In–O cores coordinated with organic ligand molecules. As the structure and composition of cluster gel is clearly understood, this specimen is an ideal material for investigating the mechanism of UV irradiation. It is found that the UV reactions generate new carbon bonds having higher binding energy. The combination of thermal and UV treatments makes possible the growth of fine In–O crystals with reduced (2%) carbon elements. The UV irradiation is not restricted to a tool assisting in the pyrolysis process, but it can be used as a patterning tool of precursor gel films. After confirming the ability of UV light to pattern various materials, we fabricate TFTs only using UV light as a patterning method and demonstrated the operation of the TFT, as described in Sect. 17.5.