in semiconductor technology, a plane defined by one of the following: three points at specified locations on the front surface of a wafer; the least squares fit to the front surface of a wafer using all points within the fixed quality area (FQA); the least squares fit to the front surface of a wafer using all points within a site; or an ideal back surface (equivalent to the ideally flat chuck surface that contacts the wafer). [SEMI M1-94]
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