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During the sputtering process, the bombardment effect of negative oxygen ions (NOIs) on the bias stability of amorphous indium gallium zinc oxide based thin film transistors (a-IGZO TFTs) was investigated. To control the bombardment energy of NOIs, a-IGZO active layer was prepared by using a superimposed rf/dc magnetron sputter. The electrical characteristics of a-IGZO TFTs under a negative bias illumination stress (NBIS) shows the different trend with that in the dark. We propose that the heavy defects such as neutral oxygen vacancy defects (Vo) as bulk defects are closely related with the bombardment energy of the negative oxygen ions.