Evaporated polycrystalline-silicon thin-film transistors on glass

Polycrystalline-silicon thin-film transistors (Polysilicon TFTs) have been fabricated on glass substrate for application in flat-panel displays. In order to make the process temperature as low as possible, and make it possible to use ordinary glass substrate, a coplanar structure was chosen. The polysilicon film was deposited by ultrahigh vacuum evaporation, the gate insulator by RF magnetron sputtering, and instead of standard doping processes like ion implantation, or thermal diffusion, Al vacuum deposition to control the substrate temperature was used for the source-drain electrodes formation. As a result, the highest temperature in the process was 550°C. TFTs which use aluminum oxides in their gate insulators usually operate in the P-channel enhancement mode, and have an OFF resistance of 10⁸Ω and an OFF/ON resistance ratio of 10⁴. If treated with oxygen plasma after formation of the source-drain electrodes, the stability of these devices improves remarkably.