Amorphous silicon TFTs on steel-foil substrates

We describe the successful fabrication of device-quality a-Si:H thin-film transistors (TFTs) on stainless-steel foil substrates. These TFTs demonstrate that transistor circuits can be made on a flexible, non-breakable substrate. Such circuits could be used in reflective or emissive displays, and in other applications that require rugged macroelectronic circuits. Two inverted TFT structures have been made, using 200 mu m thick stainless steel foils with polished surfaces. In the first structure we used the substrate as the gate and utilized a homemade mask set with very large feature sizes: L = 45 mu m; W = 2.5 mm. The second, inverted staggered, structure used a 9500 Angstrom a-SiNx:H passivating/insulating layer deposited on the steel to enable the use of isolated gates. For this structure we used a mask set which is composed of TFTs with much smaller feature sizes. Both TFT structures exhibit transistor action. Current-voltage characterization of the TFTs with the inverted staggered structure shows typical on/off current ratios of 10(7), leakage currents on the order of 10(-12) A, good linear and saturation current behavior, and channel mobilities of 0.5 cm(2)/V . sec. These characteristics clearly identify the TFTs grown on stainless steel foil as being of device quality.