The fabrication of large-area thin-film transistor (TFT) arrays on thin flexible plastic substrates requires deposition of thin film layers at relatively low temperatures since the upper working temperature of low-cost plastic films should not exceed ∼200°C. In this paper, we report a fabrication process of a-Si:H TFTs at 120°C on flexible polyimide substrates for large-area imaging applications.
Kapton HN (DuPont) films 50 and 125 μm thick and 3 inches in diameter, were used as substrates. Both sides of the polyimide substrate were first covered with 0.5 μm thick a-SiNx. The TFT structure includes: 120 nm thick room-temperature sputtered Al gate, 250 nm thick PECVD deposited a-SiNx for the gate dielectric, 50 nm thick a-Si:H deposited by PECVD from silane-hydrogen gas mixture, 50 nm thick n+ a-Si:H source- and drain contacts, and roomtemperature sputtered Al top contact metallization. We used dry etching for all layers except for the gate and top metal, which were patterned using wet etchants. For purpose of TFT performance comparison, Coming 7059 glass substrates were used.
The performance of the fabricated TFT and its improvement with use of optimized a-Si:H and a-SiNx quality will be presented along with a discussion of the intrinsic mechanical stress in the thin film layers will also be discussed.