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 polycrystalline-silicon (poly-Si) thin film transistors (TFTs) performance and hot carrier stress (HCS) reliability. Corning Code 1737 glass, fused silica and oxidized Si were used as substrates on which 1000 Å of base poly-Si films were deposited by low-pressure chemical vapor deposition at 620°C. Top-gated TFTs of nominal channel lengths ∼15 μm were fabricated on these films and tested before and after the application of HCS. It is found out that both n- and p-TFTs on fused silica substrates perform better than their counterparts on bare 1737 glass and oxidized Si substrates. Also n- and p-TFTs on fused 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 of TFTs via grain geometries and impurity in poly-Si which are controlled by poly-Si/substrate interface properties as well as the substrate's impurity content. Variations in TFTs leakage current, threshold voltage, and subthreshold swing with substrate type and with HCS are interpreted in terms of a model incorporating generation/population of traps in the poly-Si channel, bulk gate-oxide, or poly-Si/oxide interface coupled with field-assisted emission of carriers from these traps.