Patent ID: 8535858

Claim:
A method for forming an overlay mark in a substrate, comprising steps of: applying a photoresist layer on a substrate; providing a photomask, the photomask comprising a plurality of patterns, at least one of the patterns comprising a plurality of ring areas and a plurality of inner areas enclosed by the ring areas, wherein the light transmittancy of the ring areas is different from that of the inner areas; performing an exposure and development process by using the photomask, so that the photoresist layer has a plurality of mark patterns, at least one of the mark patterns comprises a plurality of holes and a plurality of pillars within the holes, and a gap is between the sidewall of each hole and the sidewall of the pillar; forming a plurality of spacers on the sidewalls of the holes and the sidewalls of the pillars, wherein the bottoms of the spacers in the gaps join together to form a plurality of thick spacers; removing the photoresist layer, wherein the thick spacers remain; and etching the substrate to form an overlay mark corresponding to the thick spacers.