Anodic Oxidation of Nitrogen-Added Al-Based Alloys for Thin-Film Transistors
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susceptible to stress migrations such as hillocks and whiskers under thermal stress, and therefore, the surface of the aluminum is often anodized. This anodized film functions as a protective layer and effectively suppresses the stress migrations. The electrical properties of the anodized film are important because it also functions as a gate insulator. The anodic oxidation of aluminum has been investigated, and the effects of the electrolyte, current density, and cathode position have been reported [3-5]. These oxidation conditions improved the electrical properties of the anodized aluminum film, but its leakage current was 1-3 orders of magnitude larger than that of films formed by chemical vapor deposition (CVD). We previously clarified the mechanism of the current leakage and reported that annealing can decrease the current leakage [6]. However, annealing did not adequately suppress the current leakage under a negative bias. In the work described in this paper, we added nitrogen to the aluminum oxide to decrease the current leakage, and used Al-based alloys for anodic oxidation, to increase the thermal resistance. Rare-earth metals such as gadolinium (Gd) and neodymium (Nd) have been studied as additional elements to increase the thermal resistance of aluminum [7-8]. However, the thermal resistances of these alloys are not sufficient for the fabrication of large, highresolution AMLCDs, and therefore, we used anodic oxidation for these alloys. The film thickness, surface morphology, nanostructure, and electrical properties of anodized nitrogen added Al-based alloys are described, and the effects of nitrogen addition are investigated. We plan to publish a separate paper on nitrogen-added Al-based alloys. 119
Mat. Res. Soc. Symp. Proc. Vol. 500 © 1998 Materials Research Society
EXPERIMENT Aluminum alloys were deposited on 5-inch-square LCD-grade glass substrates by means of a dc magnetron sputtering apparatus at a substrate temperature of 150°C. Gadolinium (Gd) and neodymium (Nd) were used as alloy components to increase the thermal resistance of the aluminum. Al-Gd alloy with a composition of I at.% was sputtered from a composite target, while Al-Nd alloy with a composition of 2 at.% was sputtered from an alloy target. Argon and nitrogen were used as the sputtering gases, and the pressure was controlled at 0.4 Pa. The flow ratios of the nitrogen were from 0% to 20% in the Al-Gd alloy, and from 0% to 15% in the AlNd alloy. The nitrogen contents were determined by electron spectroscopy for chemical analysis (ESCA: SSI-M-probe ESCA). The thicknesses of the sputtered aluminum alloys were measured by a surface profiler (Tencor-P1) at 290-390 nm in the Al-Gd alloy, and 170-300 nm in the AlNd alloy.
These alloys were anodized in an electrolyte consisting of a mixture of 3 wt% tartaric acid solution and ethylene glycol, whose pH was kept at 6.5-7.5 by addition of an ammonium solution. The cathode was positioned on the rear of the anode [3]. A positive bias, applied to the anode with a current density of 1.5 mA/cm 2, w
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