Lithography mask having overlay mark and related method

A method, comprising:

• forming an actual mask overlay mark in a mask based on a designed mask overlay mark, the actual mask overlay mark including a plurality of overlay patterns, no dimension of each of the plurality of overlay patterns exceeding about a first wavelength over twice a first numerical aperture, the first wavelength and the first numerical aperture being associated with a first light source of a lithography scanner;
• determining an overlay offset between the actual mask overlay mark and the designed mask overlay mark by a mask metrology apparatus that includes a second light source having a second wavelength and a second numerical aperture, wherein a pitch between adjacent two of the plurality of overlay patterns does not exceed the second wavelength divided by twice the second numerical aperture;
• forming patterned light by reflecting light of the first light source by the mask;
• forming a patterned layer of a wafer including exposing a layer to the patterned light without exposing the layer to a pattern of the actual mask overlay mark;
• and
• patterning a layer underneath the patterned layer through openings in the patterned layer.

A mask, comprising:

• a device feature pattern;
• and
• a mask overlay mark adjacent the device feature pattern, the mask overlay mark including a plurality of overlay patterns, wherein from a top view:
• each of the plurality of overlay patterns has no dimension that exceeds about 100 nanometers (nm);
• and
• a first pitch between an adjacent two of the plurality of overlay patterns is less than about 2×(λ/(4·NA)), λ being a wavelength of a measuring light of a mask metrology apparatus and NA being a numerical aperture of the measuring light.