Impact of organic inter-layer dielectric for improvement in mechanical flexibility of self-aligned coplanar in-Ga-Zn-O thin-film transistor

A novel device structure has been suggested to introduce organic inter-layer dielectric (ILD) for improving mechanical flexibility of self-aligned (SA) coplanar In-Ga-Zn-O (IGZO) thin-film transistor (TFT). The SA coplanar TFTs fabricated on poly(ethylene naphthalate) (PEN) substrate showed a high saturation mobility of 11.4 cm2/V∙s, low threshold voltage of 0.26 V, steep subthreshold swing of 115 mV/dec, and high on/off current ratio (~109) at flat state. In the proposed structure, all-oxide gate stacks were defined into an island configuration embedded in the organic ILD, and hence, the mechanical stress applied to the active region could be effectively relieved. To investigate the feasibility of a new device concept, the mechanical properties of the fabricated TFTs were characterized. When the PEN thickness was varied 25 and 50 μm, the critical radius of curvature (RC) values were estimated to be 1 and 3 mm, respectively, which were superior to those for conventional oxide devices. In addition, the mechanical durability during the cyclic bending test at RC of 5 mm was significantly improved from 20,000 to 70,000 cycles when the PEN film thickness was reduced from 50 to 25 μm. The effects of PEN thickness on the mechanical stability of the SA coplanar TFTs were quantitatively discussed from a view point of surface strain. As a result, robust mechanical bendability of proposed device was confirmed even under harsh deformation conditions.