1. A method for designing a two-dimensional array overlay target for a wafer, comprising the steps of: selecting a plurality of two-dimensional array overlay targets having different overlay errors; calculating deviations of simulated diffraction spectra for the two-dimensional array overlay targets; selecting an error-independent overlay target by taking the generated deviations of the simulated diffraction spectra into consideration; and designing using a computer a two-dimensional array overlay target based on structural parameters of the selected error-independent overlay target.