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 consisting of 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; forming a plurality of data sets by dividing said plurality of candidate defect lists into a plurality of groups with each data set consisting of all the candidate defects of one corresponding group; performing first stage grouping and filtering for said plurality of data sets to generate a plurality of first stage defect lists with each data set being processed independently to generate a corresponding first stage defect list consisting of a portion of the candidate defects in the corresponding data set by grouping candidate defects of similar or identical features into a same defect group and filtering out candidate defects of nuisance type based on a first pre-set criteria; performing second stage grouping and filtering for all the candidate defects in said plurality of first stage defect lists collectively together to generate a final defect list for said systematic defects by grouping candidate defects having similar or identical features into a same systematic defect group and filtering out candidate defects of non-systematic type based on a second pre-set criteria; wherein the defect attributes of each candidate defect are used in said first stage grouping and filtering and said second stage grouping and filtering; forming a group of previous first stage defect lists from all the first stage defect lists used in generating a previous final defect list by excluding one of the first stage defect lists; collecting a new data set consisting of all the candidate defects of at least one new candidate defect list obtained from inspecting at least one new wafer of the same semiconductor device; performing first stage grouping and filtering for said new data set to generate a new first stage defect list consisting of a portion of the candidate defects in the new data set; and performing second stage grouping and filtering for all the candidate defects in said new first stage defect list and the group of previous first stage defect lists collectively together to generate a new final defect list for said systematic defects.