Abnormal behavior of threshold voltage shift in bias-stressed a-Si:H thin film transistor under extremely high intensity illumination.

We report on the unusual behavior of threshold voltage turnaround in a hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) when biased under extremely high intensity illumination. The threshold voltage shift changes from negative to positive gate bias direction after ∼30 min of bias stress even when the negative gate bias stress is applied under high intensity illumination (>400 000 Cd/cm(2)), which has not been observed in low intensity (∼6000 Cd/cm(2)). This behavior is more pronounced in a low work function gate metal structure (Al: 4.1-4.3 eV), compared to the high work function of Cu (4.5-5.1 eV). Also this is mainly observed in shorter wavelength of high photon energy illumination. However, this behavior is effectively prohibited by embedding the high energy band gap (∼8.6 eV) of SiOx in the gate insulator layer. These imply that this behavior could be originated from the injection of electrons from gate electrode, transported and trapped in the electron trap sites of the SiNx/a-Si:H interface, which causes the shift of threshold voltage toward positive gate bias direction. The results reported here can be applicable to the large-sized outdoor displays which are usually exposed to the extremely high intensity illumination.

Overlay accuracy on a flexible web with a roll printing process based on a roll-to-roll system.

For high-quality flexible devices from printing processes based on Roll-to-Roll (R2R) systems, overlay alignment during the patterning of each functional layer poses a major challenge. The reason is because flexible substrates have a relatively low stiffness compared with rigid substrates, and they are easily deformed during web handling in the R2R system. To achieve a high overlay accuracy for a flexible substrate, it is important not only to develop web handling modules (such as web guiding, tension control, winding, and unwinding) and a precise printing tool but also to control the synchronization of each unit in the total system. A R2R web handling system and reverse offset printing process were developed in this work, and an overlay between the 1st and 2nd layers of ±5µm on a 500 mm-wide film was achieved at a σ level of 2.4 and 2.8 (x and y directions, respectively) in a continuous R2R printing process. This paper presents the components and mechanisms used in reverse offset printing based on a R2R system and the printing results including positioning accuracy and overlay alignment accuracy.