2D Array Overlay Target Design for Semiconductor Metrology

The patent replaces image-based overlay metrology with diffraction-based metrology. Instead of directly imaging the overlay structures, the system uses diffraction patterns formed by illuminated gratings to determine overlay errors. This substitution eliminates the limitations of diffraction and proximity effects that plague image-based methods, achieving higher measurement precision through a different physical mechanism.

The patent changes the measurement parameter from direct imaging to diffraction spectrum analysis. By measuring the diffraction patterns and analyzing spectral characteristics rather than direct images, the system achieves superior overlay measurement precision. The empirical model-based method further changes parameters by using regression lines fitted to diffraction data from multiple grating configurations.

The patent applies preliminary action by pre-calculating diffraction spectra for various overlay error conditions and storing them in a database. During actual measurement, the system only needs to compare the measured diffraction pattern against this pre-computed database using empirical regression models, eliminating the need for time-consuming fabrication and measurement of multiple test gratings while maintaining high measurement accuracy.

The patent uses copying by creating a database of simulated diffraction spectra that represents various overlay error conditions. Instead of physically fabricating multiple grating structures to characterize the system, the method creates virtual copies of diffraction patterns through simulation, which can then be used for rapid comparison and measurement during actual overlay error detection.