Lithography apparatuses apply a dose of radiation to pattern a pattern in substrate (or wafer). The dose is applied on portion of the substrate known as die. Radiation sources of lithography apparatuses are not fully stable, and the applied dose is not constant. Therefore, some dies may be overexposed, and other dies may be underexposed, The radiation may be deep ultraviolet (DUV) or extreme ultraviolet (EUV). Several calibration processes are known to mitigate this problem.
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