Recently, trends for the development of flexible displays such as liquid crystal displays (LCDs), organic light-emitting diodes (OLEDs), thin-film transistors (TFTs), touch panel and e-paper have been necessitated of outstanding electrodes on highly transparent and flexible substrates. Indium tin oxide (ITO) is a well-known metal oxide degenerate semiconductor. It has a wide band gap (~3.5 eV), low resistivity (~10-4 Ω?cm) and high transmittance (~85 %) in the visible range. PET (polyethylene terephthalate) is a polymer which is widely employed in many industrial applications. It has a high flexibility and a good transparency (~92 %) in the visible range. Because of superior properties, ITO and PET have been mainly used as the flexible substrate and the transparent electrode, respectively. However, ITO requires relatively high process temperatures (>200℃) to improve the crystallinity. However, it is not possible these high process temperature to apply ITO films on PET substrates because of the low glass temperature (Tg) of the PET. ITO films on PET substrates generally show low crystallinity and low conductivity.
In this study, ITO thin films were deposited onto PET substrates with various deposition conditions by RF magnetron sputtering using an In-10 at% Sn alloy target. The distance between the target and substrates was about 8 cm. The films were deposited in the mixture of Ar and O2 and the total gas flow rate was maintained at 50 sccm.
The effects of RF power, O2 partial pressure and film thicknesses on the crystallinity, resistivity and transmittance of ITO film were systematically investigated. ITO thin films were deposited at room temperature without intentionally heating of substrates. The target was pre-sputtered for 10 mins in order to remove the surface oxide and the contaminant on the surface.
The ITO film thickness was measured by using an ellipsometry and the electrical properties were characterized by Hall measurements. Optical transmittance was measured by an UV-vis spectrophotometer. The crystallinity of ITO films was evaluated by using X-ray diffraction (XRD) and the surface morphology of the deposited ITO films was examined by scanning electron microscope (SEM).