Improvement of ON/OFF current ratio of n-channel offset-gate polysilicon thin film transistors by cesium ion implantation into passivation layer

Incorporation of positive charges into the passivation layer has been shown to improve the drive current of n-channel offset-gate polysilicon thin film transistors (TFTs). Previously we have shown that hydrogen plasma treatment can induce such positive charges. In this paper we propose another method, i.e. ion implantation, to achieve the same goal. Cesium was chosen as the implanting element because it is easy to form a single positive ion and has relatively high atomic mass that prevents the penetration of ions into the underlying semiconductor layer. MOS capacitors were used to monitor the charging effect of Cs⁺ in the dielectric layer which was thermally grown silicon dioxide of thickness in the range of 100 to 300 nm. Results indicate that the fixed oxide charge density is proportional to the ion dosage; however, for a Cs dose over 1×10¹⁵ cm^-2 the MOS capacitors could not be accumulated easily. Increasing the ion energy was found to enhance the charging effect in the oxide layer; however, if the energy penetrates into the Si-SiO₂ interface, then the MOS capacitor was damaged by creating defects in the interface. A 20 times increase of drive Si TFTs with single passivation layer after a 1×10¹⁴ cm^-2 and 40 keV Cs implantation