Patent ID: 7359577

Claim:
A method for aligning two selected layers of two or more layers of a semiconductor wafer, a first layer of said two selected layers defining a first mark of at least one mark of said first layer, said first mark having a pitch grating d 1 , a second layer of said two selected layers defining a second mark of at least one mark of said second layer, said second mark having a pitch grating d 2 , said first mark and said second mark overlapping each other, the method comprising steps of: directing light from at least one light source to at least one first portion and at least one second portion of each of said first and second marks; receiving at least one first signal from said light directed at said at least one first portion of each of said first and second marks; receiving at least one second signal from said light directed at said at least one second portion of said first and second marks; comparing said at least one first signal and said at least one second signal to determine an overlaid image distribution shift Δx; determining an actual lay shift Δs =Δx(d 2 −d 1 )/d 1 ; and adjusting said two selected layers of said semiconductor wafer by said Δs relative to each other to thereby align said two selected layers of said semiconductor wafer with each other.