New In-Situ Process of Top Gate Nanocrystalline Silicon Thin Film Transistors Fabricated at 180 °C for the Suppression of Leakage Current

We have successfully fabricated top gate nanocrystalline silicon (nc-Si) thin film transistor (TFT) using an inductively coupled plasma chemical vapor deposition (ICP-CVD) at substrate temperature of 180°C with various nc-Si thicknesses. The field effect mobilities of conventional top gate TFTs with the nc-Si thicknesses of 60, 90 and 130 nm are 26, 77 and 119 cm²/Vsec due to the increase of nc-Si grain size, respectively. However, the leakage current measured at V_GS=-4.4 V increased with increase of nc-Si thickness because channel resistance increased. We fabricated a new in-situ process in order to suppress leakage current. The new in-situ process for the protection of the interface from the native oxide and impurities in the air is the continuous deposition of nc-Si and SiO₂ layers without any vacuum break. The leakage current of the nc-Si TFT was successfully suppressed from 5.2x10^-9 to 3.6x10^-10. A by employing the new in-situ process. The subthreshold swing was reduced from 274 to 230 mV/dec and the field effect mobility was improved from 77 to 99 cm²/Vsec, respectively.