Effects of F+ Implantation on the Characteristics of Poly-Si Films and Low-Temperature n-ch Poly-Si Thin-Film Transistors

Fluorine was implanted onto poly-Si films through an oxide layer and its passivation effect on poly-Si films and low-temperature n-ch TFTs was investigated. The implant energy of F+ ions was 40 keV, and the F+ dose was varied from 5×1012 to 4×1015 cm-2. The fluorine implanted inside poly-Si film preferentially diffused toward the poly- Si/SiO2 interface during annealing at 600° C. The residual implant damage was found to remain inside poly-Si films after annealing at 600° C when the F+ dose was ≥5×1014 cm-2. The resistivity and activation energy of the p-type poly-Si resistors were increased as the F+ dose increased, resulting from the increase of residual implant damage. However, the characteristics of the n-ch poly-Si TFT were clearly improved when the F+ dose was in the range of 5×1014 to 2×1015 cm-2, where the trap state density at the poly Si/SiO2 interface was found to be reduced because of the passivation effect by fluorine atoms. These results indicated that the fluorine passivation at the poly- Si/SiO2 interface was effective in an appropriate F+ dose range, while the passivation inside the poly-Si film (or grain boundary passivation) was not.