US Patent 9316925 Methods for monitoring source symmetry of photolithography systems

A method for monitoring the source symmetry of a photolithography system is provided. The method includes providing a first reticle; and providing a second reticle. The method also includes forming first bottom overlay alignment marks on a first wafer using the first reticle; and forming first top overlay alignment marks on the first bottom overlay alignment marks using the second reticle. Further, the method includes forming second bottom overlay alignment marks on a second wafer using the first reticle; and forming second top overlay alignment marks on the second bottom overlay alignment marks using the second reticle. Further, the method also include measuring a first overlay shift; measuring a second overlay shift; and obtaining an overlay shift caused by the source asymmetry based on the first overlay shift and the second overlay shift.