Patent ID: 7244533

Claim:
A mask set used for determining layer to layer overlay error when forming a second layer pattern in a photoresist layer on a substrate having a first layer pattern and for removing unwanted portions of the second layer pattern in the photoresist layer, comprising: (a) a first layer mask having an inner device pattern area and a plurality of first reference marks, a chrome forbidden area surrounding the inner device pattern area, and an outer region surrounding the chrome forbidden area wherein each of said first reference marks has a center point with (x,y) coordinates; (b) a second layer mask having an inner device pattern area and a plurality of second reference marks, a chrome forbidden area surrounding the inner device pattern area, and an outer region surrounding the chrome forbidden area region wherein each second reference mark is matched with a first reference mark on the first layer mask and has a center point with the same (x,y) coordinates as a matching first reference mark on the first layer mask; and (c) a third mask with clear regions having the same (x,y) coordinates as the second reference marks on the second layer mask.