A structure in semiconductor fabrication includes at least a first periodic asymmetric feature and a periodic asymmetric second feature. The first feature contains a plurality of periodically distributed first elements. The first feature has a first asymmetric profile such that the first feature no longer has the same first asymmetric profile when it is rotated by 180 degrees. The second feature contains a plurality of periodically distributed second elements. The second feature has a second asymmetric profile such that the second feature no longer has the same second asymmetric profile when it is rotated by 180 degrees. The second asymmetric profile is different from the first asymmetric profile.