US Patent 9823585 EUV focus monitoring systems and methods

Systems and methods for monitoring the focus of an EUV lithography system are disclosed. Another aspect includes a method having operations of measuring a first shift value for a first patterned set of sub-structures of a focus test structure on a wafer and measuring a second shift value for a second patterned set of sub-structures of the test structure on the wafer. The test structure may be formed on the wafer using asymmetric illumination, with the first patterned set of sub-structures having a first pitch and the second patterned set of sub-structures having a second pitch that is different from the first pitch. The method may further include determining a focus shift compensation for an illumination system based on a difference between the first shift value and the second shift value.