Improved thin film model for overlay metrology

Meeting the stricter overlay measurement error requirements of next-generation lithography is a challenge to conventional optical metrology solution associated with bright-field microscopy. A modified thin film model was developed to simulate the optical image intensity profile from novel overlay targets with design rule features. The image is calculated based on diffraction theory, which is simpler than the rigorous application of Maxwell's equations in three dimensions. The model is matched to the image by adding the contributions from all of the patterned regions in the target, and multiplying by a complex reflectance transfer matrix, which embodies all of the material characteristics. The overlay error in the target and the optical configuration parameters are modified to find the best fit between the image and the model. Although this method makes several assumptions about the formation of an image, very close agreement between the model and the image is obtained.