Polycrystalline silicon thin-film transistors on flexible steel foil substrates for complementary-metal-oxide-silicon technology

We made complementary metal-oxide-silicon circuits from polycrystalline silicon thin film transistors on steel foil substrates. As-rolled steel foils can be planarized and electrically insulated with a combination of spin-on and plasma-deposited SiO2, which also functions as the barrier against contaminant diffusion. The processes at temperatures of up to 950degreesC include the furnace crystallization of amorphous silicon precursor films, thermal annealing of ion implants in self-aligned geometries, and thermal oxidation of the polycrystalline silicon film. Individual thin film transistors have reached electron and hole mobilities of 60 cm(2)V(-1)s(-1) and 15 cm(2)V(-1)s(-1), respectively. The propagation delay in ring oscillators is 1 mus per gate, and is determined by the channel resistance and the coupling capacitance between thin film transistor and substrate. Our work introduces polycrystalline silicon circuits on steel foil as a robust technology for flexible backplanes.