Hydrogenation effects on polysilicon thin-film transistor structures

The authors have investigated the hydrogenation of various polysilicon TFT (thin-film transistors) structures in a parallel-plate reactor and studied the effect of plasma frequency (30 kHz or 13.56 MHz) on the device characteristics. The device structures included non-self-aligned metal gate, self-aligned polysilicon gate, and offset polysilicon gate. Both polysilicon- and metal-gate structures improved after the hydrogenation at 30 kHz. The subthreshold slope of polysilicon gate TFTs was lower by a factor of two than that of metal gate, showing that the polysilicon gate results in a more efficient hydrogenation of the device structure. The hydrogenation at 13.56 MHz either marginally improves the devices or under certain conditions actually degrades them. Electrical stress of n-channel TFTs (L=10 mu m, W=50 mu m) from the same substrate that were hydrogenated at different plasma frequencies was measured at room temperature under bias stress conditions of V/sub gs/=20 V and V/sub ds/=20 V. The threshold voltage (V/sub T/) was 0.46 V after the 30-kHz hydrogenation and became 0.5 V after stressing for 3*10/sup 4/ s. This decrease in the threshold voltage is indicative of the movement of positive charges in the gate dielectric.