1. A method for designing a two-dimensional array overlay target set, comprising the steps of: selecting a plurality of two-dimensional array overlay target sets having different overlay errors; calculating a deviation of simulated diffraction spectra for each two-dimensional array overlay target set; selecting a sensitive overlay target set by taking the deviations of the simulated diffraction spectra into consideration; and designing a two-dimensional array overlay target set based on structural parameters of the sensitive overlay target set.