Reliability Of Flexible Low Temperature Poly-Si Thin-Film Transistors Under Dynamic Bending Stress

In recent years,low-temperature polycrystalline Si(LTPS)thin-film transistors(TFTs)are widely used in flexible displays and sensors due to their high carrier mobility and CMOS compatibility.In the practical condition,flexible TFTs will be subject to sustained mechanical stress(such as twisting,stretching,bending,etc.).Among various stress modes,the most typical one is bending stress.The degradation of TFT characteristics will affect the performance of the overall electronic device.Therefore,the study on the reliability of flexible LTPS TFT devices under dynamic bending stress is of great significance.After investigating other research groups work on flexible TFT reliability,we independently designed and manufactured the flexible thin-film material bending test apparatus for our work.Subsequently,the location of neutral plane of the TFT was calculated by theoretical formula and the strain distribution in TFT structure was simulated by finite element software-ANSYS.By monitoring the evolution of characteristics of the TFT after subject to given bending cycles,the degradation features of the flexible LTPS TFT under the dynamic bending stress was investigated.Furthermore,the dependency of the degradation rate of the TFT parameters on the bending cycles and curvature is studied,respectively.This paper further studies the abnormal continuous degradation phenomenon in TFT characteristics after stress removal.Experiment results show that for flexible p-channel LTPS TFT,spontaneous degradation will occur,no matter TFT subject to either tensile or compressive bending stress,along either channel width or length direction.The amount of spontaneous degradation of the device is related to the number of bending cycles,and it is most significant after a moderate number of bending cycles.In the end,this paper rectified and improved the degradation model of LTPS TFT under dynamic bending stress in the references.Degradation and spontaneous degradation behavior of LTPS TFTs under dynamic bending stress are caused by electron trapping into defects in the gate insulator.Experiments show that the electron trapping behavior occurs during dynamic bending process and idle time after stress removal,and has no relationship with the TFT characteristic measurement process.As for the source of the electron,we can reasonably infer two possibilities.One is due to interaction between TFT and the environment.The other is due to the internal charge transfer within the TFT.