Methods for monitoring source symmetry of photolithography systems

A method for monitoring a source symmetry of a photolithography system, comprising:

• providing a first reticle having a plurality of first mark patterns and a plurality of second mark patterns;
• providing a second reticle also having a plurality of the first mark patterns and a plurality of the second mark patterns;
• forming first bottom overlay alignment marks on a first wafer using the first reticle;
• forming first top overlay alignment marks on the first bottom overlay alignment marks using the second reticle;
• forming second bottom overlay alignment marks on a second wafer using the second reticle;
• forming second top overlay alignment marks on the second bottom overlay alignment marks using the first reticle;
• measuring a first overlay shift of the first top overlay alignment marks and the first bottom overlay alignment marks;
• measuring a second overlay shift of the second top overlay alignment marks and the second bottom overlay alignment marks;
• and
• obtaining an overlay shift caused by the source asymmetry based on the first overlay shift and the second overlay shift.