In this paper, N-2 plasma surface treatment on high performance low-temperature poly-Si thin-film transistors (LTPS-TFTs) with HfO2 gate dielectric is demonstrated. A significant performance improvement by N-2 plasma surface treatment is observed, including the threshold voltage V-TH reduction similar to-0.94 V, subthreshold swing S.S. improvement from 0.227 V/dec. to 0.188 V/dec., field effect mobility mu(FE) enhancement similar to+61% and driving current I-drv enhancement similar to+95%. The individual impacts of interfacial layer growth effect and trap passivation effect of poly-Si channel film are investigated by the plasma induced interfacial layer (PIL) removal process. The results show that the PIL growth effect has more contribution to the improvement of V-TH reduction and I-dry enhancement than the trap passivation effect of poly-Si channel film. Consequently, the interfacial layer engineering would be very important for the development of high performance LTPS-TFTs. (C) 2014 Elsevier Ltd. All rights reserved.