Photomask and method for forming overlay mark using the same

A method for forming an overlay mark in a substrate, comprising:

• 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, 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.

A method for insuring an alignment precision in a double patterning process, comprising:

• applying a first 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, so that the first photoresist layer has a plurality of first mark patterns, at least one of the first 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 first photoresist layer, wherein the thick spacers remain;
• etching the substrate to form an overlay mark corresponding to the thick spacers;
• applying a second photoresist layer on the substrate;
• performing an exposure and development process, so that the second photoresist layer has a plurality of second mark patterns, wherein the second mark patterns are located over the overlay mark;
• and
• performing a metrology process to determine the alignment precision by referring to the second mark patterns and the overlay mark.