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 SiO(2)/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 SiO(2)/PI interface contributes to the V(th) shift of the LTPS TFTs leading to image sticking.