Method for checking alignment accuracy using overlay mark

A method for checking alignment accuracy, which is applicable for checking the alignment accuracy in a photolithography process for defining an upper layer on a wafer with respect to a lower layer on the wafer, the method comprising:

• forming a plurality of overlay marks, comprising:
• forming a plurality of outer marks in the lower layer on the wafer when the lower layer on the wafer is defined, wherein each outer mark of the plurality of the outer marks comprises an interior profile and an exterior profile;
• forming a plurality of inner marks above the lower layer surrounded by the plurality of the outer marks during the photolithograph process, wherein each inner mark of the plurality of the inner marks comprises an interior profile and an exterior profile, and each inner mark of the plurality of the inner marks is formed correspondingly above each outer mark of the plurality of the outer marks, and the interior profile of the each outer mark of the plurality of the outer marks is a sidewall of the each outer mark of the plurality of the outer marks that is the closest to the each inner mark of the plurality of the inner marks formed correspondingly above the each outer mark of the plurality of the outer marks;
• performing a measurement process to obtain a first relation between only the interior profile of the each outer mark of the plurality of the outer marks and only the interior profile of another outer mark of the plurality of the outer marks and a second relation between the each correspondingly above inner mark of the plurality of the inner marks and another correspondingly above inner mark of the plurality of the inner marks or to obtain a third relation between only the interior profile of the each outer mark of the plurality of the outer marks and the each correspondingly above inner mark of the plurality of the inner marks;
• and
• calculating a X-directional alignment and a Y-directional alignment for the photolithography process with respect to the lower layer using the first and the second relations or the third relation.