Complementary metal-oxide-semiconductor thin-film transistor circuits from a high-temperature polycrystalline silicon process on steel foil substrates

We fabricated CMOS circuits from polycrystalline silicon films on steel foil substrates at process temperatures up to 950°C. The substrates were 0.2-mm thick steel foil coated with 0.5-μm thick SiO₂. We employed silicon crystallization times ranging from 6 h (600°C) to 20 s (950°C). Thin-film transistors (TFTs) were made in either self-aligned or nonself-aligned geometries. The gate dielectric was SiO₂ made by thermal oxidation or from deposited SiO₂. The field-effect mobilities reach 64 cm²/Vs for electrons and 22 cm²/Vs for holes. Complementary metal-oxide-silicon (CMOS) circuits were fabricated with self-aligned TFT geometries, and exhibit ring oscillator frequencies of 1 MHz. These results lay the groundwork for polycrystalline silicon circuitry on flexible substrates for large-area electronic backplanes.