(Invited) Short Channel Coplanar Amorphous-Indium-Gallium-Zinc-Oxide Thin-Film Transistors on Polyimide Substrate for High Resolution Displays

Abstract

High-resolution active-matrix organic light-emitting diode (AMOLED) thin-film transistor (TFT) backplane with lower power consumption is needed for augmented reality (AR) applications. The near-to-eye format of AR terminals needs facing several technical challenges, such as higher optical efficiency, wider field of view, lighter weight, and smaller size to accelerate widespread applications. For these reasons, downscaling of TFTs with amorphous-indium-gallium-zinc-oxide (a-IGZO) or LTPS is required. The channel length should be less than 1 μm and the flexible substrate is better than glass because of light weight and very thin structure. TFT technologies on polyimide (PI) substrate can support the applications for foldable and rollable displays. For flexible AMOLED, PI layer of 10 to 20 um is widely used for foldable displays. The PI layer is coated by slot coating on the top of a carrier glass with a buffer layer of carbon nanotubes (CNTs) and graphene oxide (GO) mixture coated by spray. This layer can reduce the adhesion force between PI and glass and thus mechanical detachment can be possible. The CNTs are conducting so that it can release the electrostatic discharge (ESD) damage because CNTs/GO layer exists on the bottom side of the PI substrate. The PI substrate is covered by SiO2 and ZrAlOx layers (five layers in total starting with SiO2) by plasma enhanced chemical vapor deposition and spray pyrolysis, respectively, which acts as a gas barrier. The amorphous ZrAlOx film by spray coating can be used as a high-k gate insulator and also a gas barrier. The threshold voltage of a-IGZO TFTs tends to shift toward negative gate voltage with decreasing the channel length. In this work, a ZrAlOx passivation layer is introduced for the short channel, coplanar a-IGZO TFT. A very thin ZrAlOx layer is deposited by spray pyrolysis on the top of a-IGZO TFT at the substrate temperature of 350 ℃ before interlayer deposition. The a-IGZO TFT with a very thin ZrAlOx layer exhibits the stable threshold voltage dependence with decreasing the channel length. The 0.8 μm channel length a-IGZO TFT with ZrAlOx layer by spray pyrolysis at the substrate temperature of 350 ℃ exhibits the field-effect mobility of 7.09 cm2 V-1 s-1, the threshold voltage of -0.6 V, and on/off current ratio of 3.3 × 107. The formation of Zr-O bonds on the a-IGZO surface region reduces the carrier concentration in the a-IGZO TFT offset region, between source/drain contact and the channel, and blocks the carrier diffusion into the channel. This is the first reported self-aligned coplanar a-IGZO TFT under 1 μm channel length on a flexible substrate. Figure 1