Water-related instability in TFTs formed using deposited gate oxides

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Abstract
Polycrystalline silicon thin film transistors formed with undensified gate oxides have shown instability phenomena not observed for TFTS formed with oxides annealed at 600 degrees C or above. These instabilities are related to ionized water in the oxide, with positive (H+) and negative (OH-) ions drifting on the application of gate bias to produce both negative and positive threshold voltage shifts. The mobility for the OH- ions is found to be lower than that for the H+ ions, as might be expected. In addition, kinks in the subthreshold characteristic are sometimes seen, and these are shown to be due to a sidewall effect. The water diffuses in from the atmosphere laterally under the gate electrode, and this gives rise to a gate length dependence of the effects. From this dependence a diffusion coefficient approximately 3*10-12 cm2 s-1 is deduced at room temperature. This is some eight orders of magnitude higher than that for fused silica, and confirms the porosity of the material.