THE EFFECTS OF SUBSTRATES ON THE CHARACTERISTICS OF POLYCRYSTALLINE SILICON THIN-FILM TRANSISTORS

We report on the impact of substrates on n-channel and p-channel polyc rystalline-silicon (poly-Si) thin film transistors (TFTs) performance and hot carrier stress (HCS) reliability. Coming Code 1737 glass, fuse d silica and oxidized Si were used as substrates on which 1000 Angstro m of base poly-Si films were deposited by low-pressure chemical vapor deposition at 620 degrees C. Top-gated TFTs of nominal channel lengths similar to 15 mu m were fabricated on these films and tested before a nd after the application of HCS. It is found out that both n- and p-TF Ts on fused silica substrates perform better than their counterparts o n bare 1737 glass and oxidized Si substrates. Also n- and p-TFTs on fu sed silica, unlike those on 1737 glass and oxidized silicon substrates , are observed to be more resistant to HCS. The implications are that the substrate influences the characteristics and the HCS reliability o f TFTs via grain geometries and impurity in poly-Si which are controll ed by poly-Si/substrate interface properties as well as the substrate' s impurity content. Variations in TFTs leakage current, threshold volt age, and subthreshold swing with substrate type and with HCS are inter preted in terms of a model incorporating generation/population of trap s in the poly-Si channel, bulk gate-oxide, or poly-Si/oxide interface coupled with field-assisted emission of carriers from these traps. (C) 1998 Elsevier Science Ltd. Ail rights reserved.