The mechanism of hydrogen plasma passivation for poly-crystalline silicon （poly-Si） thin films is investigated by optical emission spectroscopy （OES） combined with Hall mobility, Raman spectra, absorption coefficient spectra, and so on. It is found that different kinds of hydrogen plasma radicals are responsible for passivating different defects in polySi. The Ha with lower energy is mainly responsible for passivating the solid phase crystallization （SPC） poly-Si whose crystallization precursor is deposited by plasma-enhanced chemical vapor deposition （PECVD）. The H＊ with higher energy may passivate the defects related to teh Ni impurity around the grain boundaries more effectively. In addition, Hβ and H7 with the highest energy are required to passivate intra-grain defects in the poly-Si crystallized by SPC but whose precursor is deposited bv low pressure chemical vapor deposition（LPCVD）
A technique to improve and accelerate aluminum induced crystallization（AIC） by using hydrogen plasma is proposed.Raman spectroscopy and secondary ion mass spectrometry of crystallized poly-Si thin films show that hydrogen plasma radicals reduce the crystallization time of AIC.This technique shortens the annealing time from 10 to 4 h and increases the Hall mobility from 22.1 to 42.5 cm^2/（V·s）.The possible mechanism of AIC assisted by hydrogen radicals is also discussed.
