Models for characterizing the printability of buried EUV defects

Models for the printability of buried 3D EUV defect are analyzed and extended using rigorous electromagnetic simulation by TEMEPST. Parallel simulation on a network of workstation was used to examine the classical assumptions of coherent illumination, uniformly filling the entrance pupil and vertical propagation in the theoretical model of Gullikson for Gaussian defects. Results show that the limitation of the model is the lack of uniformity of filling the pupil for defects with diameter (2*sigma) larger than 0.20 lambda /NA at the wafer plane. Beyond this diameter the dip in the clear field intensity no longer follows the quadratic decrease with size and height of the model. Rather the dip quickly goes through its worst-case minimum intensity near 70 nm and then rises as the size further lowers the local surface slope. The worst-case image decrease from the clear field value for any sized Gaussian defect is roughly, 18% per mn of height. Thus isolated Gaussian defects with height less than 2 nm will never reduce the field intensity to less than 60% of the clear field value.