In this work, the surface cleaning of amorphous silicon mandrel at Fin-FET technology node is demonstrated. The unclean mandrel surface can cause abnormal growth of the following ALDSIN spacer deposition which may transfer to protrusion or bridge defect at the final structures. We present several mandrel cleaning methods to study the quality of deposited ALDSIN and then put forward a model of abnormal growth of ALDSIN. Finally, it is demostrated that the protrusion or bridge defect can be obviously improved and avoid the yield loss by the optimization of the mandrel cleaning process.