Threshold voltage instability and polyimide charging effects of LTPS TFTs for flexible displays

In this paper, we investigate the V_th shift of p-type LTPS TFTs fabricated on a polyimide (PI) and glass substrate considering charging phenomena. The V_th of the LTPS TFTs with a PI substrate positively shift after a bias temperature stress test. However, the V_th with a glass substrate rarely changed even with increasing stress. Such a positive V_th shift results from the negative charging of fluorine stemmed from the PI under the gate bias. In fact, the C-V characterization on the metal-insulator-metal capacitor reveals that charging at the SiO₂/PI interface depends on the applied gate bias and the PI material, which agrees well with the TCAD simulation and SIMS analyses. As a result, the charging at the SiO₂/PI interface contributes to the V_th shift of the LTPS TFTs leading to image sticking.