Focus measurement

摘要

Using focus marks on wafers allows the use of diffraction based metrology to determine focus quality associated with a lithographic process. Typically a signal (for example a difference in intensity between a first and minus first order of the light diffracted by the mark on the wafer) is used to deduce a certain focus parameter at the location of the mark. The signal is compared to a calibration curve which relates a signal value to a focus value.