Characteristics of Top-Gate Polysilicon Thin-Film Transistors Fabricated on Fluorine-Implanted and Crystallized Amorphous Silicon Films

This paper presents a comprehensive study on the characteristics of n- and p-channel polycrystalline—silicon (poly-silicon) thin-film transistors (TFTs) fabricated on fluorine-implanted-then-crystallized amorphous silicon films. Amorphous silicon films of two thicknesses were implanted with different energies and various dosages of fluorine, and studied using transmission electron microscopy (TEM) and secondary-ion mass spectrometry (SIMS). The electrical char-acteristics of TFTs fabricated on the films were correlated with the results of TEM and SIMS. It was found that field-effect mobilities of both n- and p-channel devices were improved by the fluorine implantation thanks to the enhanced grain size and the fluorine passivation effect. For the p-channel device, the fluorine implantation did not improve the sub-threshold swing and even degraded it after hydrogenation. This result was thought to be caused by the fluorine-induced negative charges in oxides. However, a thin active layer and a deep implantation reduced this degradation. In 1980 Kanrins et al.1 were the first to suggest that flu-orine be used to passivate grain boundaries of the polysil-icon TFT. The concept was not realized until 1991, howev-er, when fluorine ion implantation (FlI) was used to incorporate fluorine into the poly-Si TFT2 and studie