P‐12: Improve the Reliability of a‐IGZO TFT through Optimizing the Threshold Voltage and Channel Thickness in AMOLED Hybrid Backplane

Abstract

In the fabrication of flexible a‐IGZO and LTPS thin film transistor (TFT) hybrid backplane, various threshold voltage (Vth) of the a‐IGZO TFT was achieved in this paper by regulating the O2/Ar ratio during the deposition of a‐IGZO film by sputtering. The results show that the Vth increases with the increase of the O2/Ar ratio, and the reliability tests indicate that in a certain range negative adjustment of Vth can significantly improve the positive bias temperature instability (PBTI) of IGZO TFT, and maintain good electrical uniformity. The increase of the O2/Ar ratio reduces the number of oxygen vacancy (Vo) in the a‐IGZO channel, and thereby Vth is positively shifted, which is confirmed by the TCAD simulation. The Vth value can indicate the Vo concentration in a‐IGZO TFT channel partly, and appropriately higher amount of Vo is beneficial to improve the PBTI, which is attributed to Fermi level (EF) shifting closer to conduction band. Raised EF means less unoccupied trap states, resulting in less carrier trapping under PBTI. Furthermore, the research also explores different a‐IGZO channel thicknesses and finds that when the Vth values are similar, thinner channel layers endow a‐IGZO TFT with improved PBTI. By optimizing the Vth and channel thickness, a‐IGZO TFTs with a channel thickness of 25nm and a Vth value of 0.2V have been prepared in a hybrid backplane. Under positive bias temperature stress for 5 hours, the Vth is positively shifted less than 0.5 V, which could lay the foundation for achieving high‐reliability flexible backplane for AMOLEDs.