Flexible Device Applicable High Mobility InGaZnO/polyimide Thin Film Transistor at Low Temperature Using Interfacial Defect Control

Polyimide (PI) is currently used in various fields, such as substrate, passivation, gate insulator (GI), because PI can be fabricated in the condition of low temperature (~ Room temperature), low cost, large area deposition. Many research have reported PI which has high dielectric (≥ 2) constant and withstand high temperatures (~ 300 °C). In the case of a device using a soft insulating film such as PI, the fabrication method is limited according to the characteristics of PI. In the case of organic thin film transistor (OTFT), in which PI is mainly used as a gate insulator, the carrier mobility of organic semiconductor films is strongly influenced by the crystallinity, molecular packing structures of the organic thin films and charge traps at the gate dielectric/semiconductor interface. Even if certain molecular organic semiconductors can form various molecular packing structures by changing film formation process, the OTFT fabrication has very low reproducibility. It is difficult to analyze and control the composition and defect states in the semiconductor/GI interface of OTFT compared to inorganic device, because polymer materials are mainly used. The operation of the TFT device is affected by a channel formation in the active/GI interface. The analysis of the grain boundary, crystallinity, and defect states in the interface must be supported to adjust the device characteristics according to the purpose. In this study, we suggest a low-temperature (≤100 °C) fabrication method of high mobility (> 10 cm²/Vs) oxide thin film transistors using 6FDA-MDA as a gate insulator with Al₂O₃ buffer layer and analysis of the qualitative/quantitative defect sites in the interface between PI and IGZO layer. Spectroscopic ellipsometry (SE) and x-ray photoelectron spectroscopy (XPS) are used to the analyze qualitative physical properties of the interface, for example fermi level in the band gap, thickness, composition. Quantitative defect state analysis according to the defect activation energy of the IGZO channel region was conducted by photo induced current transient spectroscopy (PICTS). The improvement of device reliability, especially for PBS seems to be due to the decrease in the deep level defect states in the interface, which is related to the trend of change in composition in the interface due to the application of buffer layer. We applied the optimum fabrication condition to the flexible device with parylene/PET substrate and conducted a bending test to confirm the stability of the device over 1000 times.