Effect of Island Configuration and Neutral Axis Location for Mechanical Bending Strain on a-IGZO Thin Film Transistors

In the electronic-device market, flexible displays using plastic substrates are considered to be one of the most promising alternatives for realizing the new functional needs of flat-panel displays. They are largely composed of a display element, thin film transistor (TFT) backplane, and a flexible substrate; the displays are an integrated system of these units. This means that the mechanical flexibility of each element to the physical stress while preserving the electrical stability is quite important because they reliably enable the overall flexibility of the integrated bending system.

In this research, we report the effect of electromechanical and mechanical strain on coplanar amorphous indium-gallium-zinc-oxide (a-IGZO) thin film transistor regarding to the structural design of the device and the neutral axis location in bending. Here we show the highly reliable bending feature of the island structured (IS) device and its backplane after being subjected to an extreme cyclic bending stress [Fig.1]. The coplanar IS TFTs fabricated on polyimide (PI) substrate exhibit excellent bendability for an cyclic bending of 100,000 cycles with a radious less than 2 mm without a remarkable change of electrical properties although the coplanar device with the conventional structure shows an significant electrical failure at the same mechanical strain [Fig.2].

Competitive study of the device stability characterized by the neutral axis also reveals the strong effect of neutral plane (N.P) as well as the location from N.P for bending stress, which is also greatly assigned to the device configuration. That is, when the device is positioned close to the neutral plane using the sandwiched structure, the electrical characteristic of TFTs is quite stable for the induced mechanical bending without relying on the device configuration [Fig.3]. As the device is positioned apart from the neutral plane, the electrical performance is obiously degraded in the position about 30 μm from N.P on the device with the conventional structure as a function of bending cycles [Fig.4]. The onset of crack strain is also closely corresponds to that of the electrical failure of the device. On the otherhands, figure 4 clearly reveals that the positioning margin in the neutral zone is remarkably enhanced after emolying IS structure on a-IGZO devices for the extreme bending stress. IS TFTs achieved the stable electrical characterisic such as the mobility (μ) change less than 10 % compared to that of inital value (μ_o) for an cyclic bending of 100,000 cycles even they are subjcted to the mechanical tensile strain over 2.5 % without the cracks, which is corresponding to the device position in 50 μm far from N.P with a radius of 2 mm [Fig.4]. As a result, IS TFTs enable to bend until the radius of 1 mm thanks to the bendability enhancement, preserving μ decrease less than 20 % from μ_o. To our knowledge, this is one of the highest bendable feature adopting the inorganic TFT and its backplane which is also comparable with that of flexible organic device.

Since the mechanical flexibility of the backplane component is one of key elements to achieve the integrated bending system as we discussed eariler, this result shows unique potential of IS TFT and its backplane to open new form factor flexible displays such as folding or rolling displays.