Flexible Oxide Thin-Film Transistors Fabricated at Room Temperature By Introducing Polymer Dielectric and Microwave Annealing
Authors: Jang SeongCheol , Kim Jaehyun , Lee Kyung Jin , Kim Hyun-Suk 
Published: December 2020

Amorphous oxide semiconductors (AOSs) have attracted considerable attention as an active layer material for thin-film transistors (TFTs), due to their many advantages such as high field effect mobility and large area uniformity compared to amorphous Si counterparts. In particular, amorphous indium gallium zinc oxide (a-IGZO) have widely studied for high performance TFTs which require high mobility, high on/off ratio, low leakage current and room temperature fabrication. Even if, a-IGZO TFTs can be deposited at room temperature, additional thermal treatment is still required in order to obtain satisfactory electrical characteristic and stability.

Organic materials can be used for dielectrics of flexible TFTs because of their flexible mechanical properties. Generally, organic dielectric films are fabricated by solution process like spin-coating. Spin coating process can be easy and cheap method, however, the quality of films is not as good as the one with vacuum process because of the residual impurities and thermal damage to the substrate during post-coating bake. Poly(chloro para-xylylene) (Parylene C) can be fabricated via pyrolysis-CVD process at room temperature, which lead to unique characteristics of pinhole-free nature. Also, electrical properties of parylene C films are quite enough to utilize as dielectric layer of TFTs. Moreover, parylene C can act as passivation layer due to their remarkable chemical resistance.

In this work, microwave annealing and parylene C are adopted to IGZO TFTs to reduce overall process temperature. Microwave can selectively heat target material by conversion of electromagnetic energy to thermal energy inside of the target material. Finally, flexible IGZO TFTs were realized on PET substrate owing to low process temperature of microwave annealing and parylene C deposition.

Transfer and output characteristics were measured in order to investigate the possibility of parylene C as an insulator on oxide TFTs. The device exhibit typical output characteristics with a clear pinch-off, current saturation and no current crowding. When the applied gate voltage bias was 20 V, the saturation drain current was around 230 μA at drain voltage of 20 V. From transfer curve, fully room temperature fabricated IGZO TFTs exhibit mobility, Vth, S.S and hysteresis of 12.9 cm2/Vs, 0.796 V, 0.651 V/dec and 0.62 V.

Flexible IGZO TFTs were fabricated using PET substrate which shows low glass transition temperature. The device was well operated which indicate microwave annealing and parylene C deposition process is suitable for low glass transition temperature substrate. The electrical stability of the IGZO TFTs with parylene C gate dielectric under bending stress was evaluated. After bending 15,000 times, switching characteristics was still remained. Mobility and S.S were hardly deteriorated, however, Vth shows small negative shift of near -1 V.

In this work, IGZO TFTs were fabricated at room temperature by introducing microwave annealing and room temperature deposited gate dielectric, parylene C. By combining microwave annealing and room temperature fabricated parylene C, the process temperature was decreased enough to utilize flexible substrate with low glass transition temperature. Fabricated flexible IGZO TFTs was shown to withstand harsh 15,000 bending test.

Figure 1