Hitoshi Hokari

Novel top‐gate zinc oxide thin‐film transistors (ZnO TFTs) for AMLCDs

— High-performance top-gate thin-film transistors (TFTs) with a transparent zinc oxide (ZnO) channel have been developed. ZnO thin films used as active channels were deposited by rf magnetron sputtering. The electrical properties and thermal stability of the ZnO films are controlled by the deposition conditions. A gate insulator made of silicon nitride (SiNx) was deposited on the ZnO films by conventional P-CVD. A novel ZnO-TFT process based on photolithography is proposed for AMLCDs. AMLCDs having an aperture ratio and pixel density comparable to those of a-Si:H TFT-LCDs are driven by ZnO TFTs using the same driving scheme of conventional AMLCDs.

Bottom-Gate Zinc Oxide Thin-Film Transistors (ZnO TFTs) for AM-LCDs

In this paper, high-performance bottom-gate thin-film transistors (TFTs) with transparent zinc oxide (ZnO) channels have been developed. The ZnO film for active channels was deposited by RF magnetron sputtering. The crystallinity of the ZnO film drastically improved when it was deposited on a doublelayer SiOx/SiNx gate insulator. In order to achieve a ZnO TFT back-plane for liquid-crystal display (LCD) with the required pattern accuracy, dry etching of the ZnO film in an Ar and CH4 chemistry has been developed. The etching rate and tapered profile of the ZnO film could be controlled by the Ar content in the etching gases of Ar and CH4. The saturation mobility (musat) of the ZnO TFT strongly depended on a gate voltage. A musat of 5.2 & cm2 .(V .s)-1 at VGS = 40 V and VDS = 10 V, and an on/off-current ratio of 2.7 x 107 were obtained. A drain-current uniformity of plusmn7% was achieved within a radius of 20 mm from the substrate center. A 1.46 -in diagonal LCD with 61 600 pixels has been driven by the ZnO-TFT back-plane. A moving picture image was available on fabricated LCD driven by the ZnO TFTs.

4.1: Distinguished Paper: High Mobility Top-Gate Zinc Oxide Thin-Film Transistors (ZnO-TFTs) for Active-Matrix Liquid Crystal Displays

High-performance top-gate ZnO thin-film transistors (TFTs) for AM-LCDs have been developed. Sputtered ZnO was used as an active channel and silicon nitride (SiNx) deposited by plasma enhanced chemical vapor deposition (P-CVD) was used as a gate insulator. Field effect mobility and threshold voltage of the ZnO-TFT are 50.3 cm2/V⋅sec and 1.1 V, respectively. We first demonstrated a 1.46″ diagonal AM-LCD driven by ZnO-TFTs.

Heat Resistance of Ga-Doped ZnO Thin Films for Application as Transparent Electrodes in Liquid Crystal Displays

The electrical characteristics and residual stresses in three types of Ga-doped ZnO (GZO) films formed using a reactive plasma deposition system (type 1 film) and a magnetron sputtering system utilizing dc power (type 2 film) or dc + radio-frequency (rf) power (type 3 film) were compared from the viewpoint of heat resistance as a function of the thermal process for liquid crystal display (LCD) fabrication. The resistivity of the as-formed GZO films prepared at 180°C can be ordered as (2.75 μΩ m) < type 3 film (3.88 μΩ m) < type 2 film (6.00 μΩ m). However, the heat resistance of the films was ordered as type 3 (200-250°C) < type 2 (250-300°C) < type 1 (350-A00°C). Thus, the GZO films prepared by magnetron sputtering utilizing rf + dc power did not satisfy the condition for transparent electrodes for LCDs in terms of heat resistance against the ca. 250°C processing step. LCD panels (3 in.) using the GZO transparent electrodes on the red-green-blue color filters were fabricated for demonstrating the feasibility of using this material as an electrode.

P-7: Ga-Doped ZnO Transparent Conductive Film as Substitution for ITO Common Electrode in TFT-LCDs

Electrical properties and thermal stability of Ga-doped ZnO (GZO) films were examined in light of the application to transparent common electrodes in thin film transistor liquid crystal displays (TFT-LCDs). Durable thermal stability of GZO films prepared by magnetron sputtering or ion plating was confirmed for 30 minutes at post annealing temperatures of 230 °C. Feasibility of the GZO transparent common electrodes has been examined thorough practical fabrication and field test of TFT-LCD utilizing the GZO films prepared by magnetron sputtering.