5.8‐inch QHD flexible AMOLED display with enhanced bendability of LTPS TFTs

Abstract

By applying the curve‐type thin film transistor (TFT) with longitudinal strain, TFT parameters do change little down to the 2R bending. The mobility variation range reduces down to 4% compared with 28% of the line‐type channel with transverse strain. The smaller variation is preferred for a high quality display. We clarified that majority carrier's effective mass and scattering rate are dominant factors influencing the bended TFT's performance, which can be controlled by the strain orientation and channel shape. This understanding and improvement was embedded in the 5.8″ flexible QHD active matrix organic light emitting diode panel with multi edge curvature of Galaxy S8. Through this achievement, we made our flexible premium active matrix organic light emitting diode panels more performable, reliable, and highly productive in small R bending circumstance.

P‐131: TCAD Modeling of Mechanical Stress for Simulation of Thin Film Transistor on Flexible Substrate

“…As we can see in Figure 10, field effect mobility increases with tensile strain or downward bending and threshold voltage decreases with tensile strain. This is very good agreement with published result [2] [3]. In compressive strain or upward bending, field effect mobility is saturated to 0.8 as opposed to tensile strain.…”

“…The corresponding maximum current and field effect mobility does not change as like the polyimide substrate case because of reduced stress value. The thickness variation of polysilicon thin film on polyimide substrate can be neglected, but the lateral thickness variation of length is 50 nm at 1.2 GPa stress which can be maximum process induced stress as explained in [3]. Interestingly, in case of bending stress, polyimide has less stress than glass substrate on two different substrate thickness as shown in Figure 8.…”

“…To see the stress effect on mobility enhancement in polysilicon TFT case, we used mobility enhancement model using c-Si strained mobility model as well as band gap change assuming poly-Si has preferred (100) orientation as described in the reference [3]. According to [3], polysilicon has residual intrinsic stress in the deposited film and this is 750-850 MPa biaxial tensile strain (ε = 0.47~0.53 % ).…”

“…Despite the fact that many papers [1][2] [3] described well the reason of device degradation by bending stress test, it does not tell us about the details of actual stress propagation in thin film device structures. Minsoo Bae, et al [2] reported bending stress effects on the electrical characteristics of polysilicon thin film transistor fabricated on polyimide substrate, and reported increasing field effect mobility and output current with tensile bending stress, while threshold voltage decrease with tensile strain.…”

“…They observed that a trilayer has less stress than a polyimide monolayer on the film surface where active devices are processed. Jaeseob, et al [3] reported the stress effect on three different transistor shape and explained curved shape is less sensitive to external bending stress, especially in the transverse direction to channel direction. They extracted biaxial stress in the polysilicon film from Raman spectra and explained why mobility and other electrical properties are changed by stress, assuming polysilicon has (100)-orientation preference, and concluded that biaxial as well as uniaxial bending stress effect the device performance.…”