In this work, we report an efficient way to improve characteristics of polysilicon thin film transistors (TFTs). A TFT treated with H2O microwave plasma shows an excellent subthreshold swing of 130 mV DEC-1 and a threshold voltage of 2.2 V while TFT treated with conventional H-2 radio frequency plasma shows an subthreshold swing of 200 mV DEC-1,md a threshold voltage of 3.2 V. We also find that a TFT treated with H2O plasma also shows better interface strength under electric stress than a TFT treated with H-2 plasma. The oxide integrity of a TFT with H2O plasma under constant current stress is also better than a TFT with H-2 plasma.