Toshiaki Arai

Mo-capped Al-Nd alloy for both gate and data bus lines of liquid crystal displays

Abstract Aluminum–neodymium (Al–Nd) alloy and molybdenum (Mo)-capped structures were applied to both gate and data bus lines of liquid crystal displays (LCD). We investigated the hillock resistance and electrical properties of Al–Nd alloys. Their structures were studied by cross-sectional transmission electron microscopy (TEM) and SEM. Adding 2 at.% Nd to Al effectively prevented the Al film from forming hillock and whisker. The electrical resistivity varied with the annealing temperature after deposition: the resistivity was 4.3 μΩ cm with annealing at 350°C and 10.0 μΩ cm with annealing at 250°C. Mo is used not only to suppress hillocks, but also for taper etching of lines and as a contact layer with other materials. We investigated the effects of adding Nd to Al, and found that the Nd addition improved the step coverage, thermal resistance, patternability, and mechanical strength of the lines. By applying a common structure and metals for both gate and data bus lines, we could increase the productivity of TFT-LCDs.

Aluminum-based gate structure for active-matrix liquid crystal displays

This paper describes the development of an Al-based thin-film gate structure for use in large, high-resolution active-matrix liquid crystal displays (AMLCDs). Aluminum films are suitable for forming the data lines of such displays, but they are not suitable for forming the gate lines because of the hillock-induced shorts that can occur to overlying metal lines during the heating necessary for insulator deposition. Alloying with yttrium, gadolinium, and neodymium was examined with the aim of reducing hillock and whisker formation during such heating. Although Al films alloyed with 2 at.% of those metals exhibited low hillock densities (10-100 mm-2), the densities were not low enough for the fabrication of SXGA (1280 × 1024 pixels) panels. After investigation of several means to further reduce the formation of hillocks and whiskers, the most effective approach was found to be anodization of the Al-alloy gate lines, suitably patterned for anodization, followed by photoresist application and laser-cutting steps. Illustratively, by use of an anodized Al-Nd (2 at.%) thin-film gate structure, the short-circuit defect rate and contact defect rate for an 11.3-in.-diagonal experimental SVGA (800 × 600 pixels) display could be effectively reduced to zero.

Pinholes in Al thin films: their effects on TFT characteristics and a taguchi method analysis of their origins

Abstract The effects of pinholes in the aluminum gate electrode on the characteristics of thin-film transistors (TFTs) were investigated. It was found that the TFT characteristics are degraded by pinholes in the aluminum thin film in the case of an etching-stopper-type inverted staggered TFT fabricated by using a back-side exposure process. By means of Taguchi methods, it was found that the generation of pinholes in the aluminum thin film is strongly related to the sputtering parameters, and that the pinhole density is related to the surface roughness of the Al thin film.

Al-based sputter-deposited films for large liquid-crystal-display

Abstract The relationship between the properties of Al thin film sputter-deposited on large glass substrate (550×650 mm) and the sputtering process parameters were clearly identified by means of Taguchi methods. The number of magnet passes, the substrate temperature, and the Ar pressure have significant effects on the microstructure of Al thin film. Taguchi methods contribute to the optimization of the sputtering process by reducing the number of test trials from 324 to 18. The results indicate good agreement between the properties of the simulated and actual sputter-deposited Al film.

Annealing effects of anodized Al-based alloy for thin-film transistors

Abstract The annealing effects of anodized aluminum-based alloys were investigated. Gadolinium and neodymium were employed as respective alloy components and the surface roughness, nanostructure, leakage current and breakdown electric field of these anodized films were investigated as functions of the annealing time. The leakage current was decreased by annealing for 0.5 h at 350°C, but was increased by annealing for 1 h and was decreased again by further annealing. The breakdown electric field and surface roughness showed the same tendency as the leakage current with respect to the annealing time. In the case of the anodized aluminum–gadolinium alloy, annealing for 2 h led to a lower leakage current of 7.4 E–13 A/V, a higher breakdown electric field of 9.9 MV/cm and a lower average roughness of 4.9 nm. The leakage current was studied further by the triangular voltage sweep method. Under a negative bias, the leakage current behaved quite differently from that under a positive bias. In the latter case, it was 1 order of magnitude smaller and could be greatly reduced by annealing for 0.5 h at 350°C. In the former case, however, it was large enough after annealing for 0.5 h and remained after further annealing.

Nitrogen-added Al rare-earth alloys for thin film transistors

Abstract The effects of nitrogen addition to aluminum-rare-earth alloys were investigated. Yttrium and gadolinium were employed as respective alloy components. The electrical properties and hillock densities of alloy films were investigated, and their nanostructures were studied by cross-sectional transmission electron microscopy. Nitrogen effectively decreases the grain size, and causes the columnar structure, that is generally present in aluminum-based alloys, to disappear. Nitric aluminum-rare-earth alloys have a strong resistance to hillock formation, and formed no micro-voids even after annealing at 450°C. An N 2 flow rate of 1.3–10% in the sputtering gas gives a low hillock density of 2.0 E +1–7.7 E −1 pcs/mm 2 after annealing at 350°C in both nitric Al-rare-earth alloys. In the case of patterned nitric aluminum-yttrium alloys, an N 2 flow rate of less than 1.3% causes large side-hillocks after annealing at over 350°C. As an optimum value, an N 2 flow rate of 2.5% results in a low hillock density of 1.9 E +1 pcs/mm 2 and a low resistivity of 8.6 μΩ cm after annealing at 350°C.