Structure engineering for hillock-free pure aluminum sputter deposition for gate and source line fabrication in active-matrix liquid crystal displays

In this article we present the results of a study aimed at developing hillock-free, pure-Al thin-film material suitable for the fabrication of gate and source lines in thin film transistor active-matrix liquid crystal displays. Strong Al(111) texture was shown to be a key attribute for achieving good resistance to hillock formation. To obtain this strong Al(111) texture, we explored the incorporation of a thin metal layer, under the Al film, and we showed that Ti could be an appropriate candidate for further optimization. Key variables affecting the quality of Al were found to be the roughness of the glass substrate, the thickness of the titanium, the background vacuum quality, and the sputtering temperature. By optimizing the deposition process for both Al and Ti layers, we showed that aluminum films with very strong (111) texture (rocking curve full width at half maximum 0.86°), very smooth surface (root mean square surface roughness 1.8 nm), and a uniform, columnar grain size (0.3–0.5 μm) could be deposited by dc magnetron sputtering. In addition to improving the aluminum microstructure, optimization of the gate dielectric process was also performed with the aim of reducing hillock formation. Combined optimization of the sputtering and gate dielectric deposition processes yielded an overall reduction in hillock density of 3.5–4 orders of magnitude, without the need for anodic oxidation or other means of capping the aluminum film. By combining Al/Ti dc sputtering, dry-etching technology for one-step patterning of the composite Al/Ti film, and gate insulator deposition at 300 °C we demonstrated, for the first time, gate metallization technology based on pure Al with excellent step coverage and ultralow hillock density $(<103 cm−2).$