1. A method of identifying systematic defects in wafer inspection using hierarchical grouping and filtering, comprising the steps of: collecting a plurality of candidate defect lists obtained from inspecting a plurality of wafers of a same semiconductor device, each of said candidate defect lists comprising a plurality of candidate defects obtained from inspecting one of said plurality of wafers, each of said candidate defects being represented by a plurality of defect attributes including a defect location; dividing said plurality of candidate defect lists into a plurality of sets; performing first stage grouping and filtering for the candidate defects in the candidate defect lists of each set to generate a first stage defect list respectively for each set; and performing second stage grouping and filtering for the candidate defects in all the first stage defect lists together to generate a final defect list for said systematic defects; wherein the defect attributes of each candidate defect are used in said first stage grouping and filtering and said second stage grouping and filtering.