Amorphous silicon crystallization and polysilicon thin film transistors on SiO2 passivated steel foil substrates

We formed polycrystalline silicon films on SiO2 passivated steel substrates by the furnace annealing of hydrogenated amorphous silicon (a-Si:H) at temperatures of 600–950°C. With the high temperature tolerance of steel substrates, the shortest crystallization was done at 950°C for 20 s. Thin film transistors were fabricated using these materials in two ways: (1) deposited source/drain, non-self-aligned process, (2) ion-implanted source/drain, self-aligned process. In both cases, the electron linear mobility was larger than 10 cm2/V s, the OFF current was ∼1 nA, and the ON current to OFF current ratio was >105. Furthermore, we introduced integrated circuit fabrication technology by using thermal oxidation for making the gate dielectric. The thermal oxidation was done at 950°C in dry oxygen for 40 min. The transistors with thermal gate oxide showed no significant difference from the transistors with deposited oxide. The successful introduction of an important IC process brings us closer to a high-performance transistor technology on unbreakable, flexible, steel substrates.