Impact of mask blank and mirror surface roughness on actinic EUVL mask blank inspection signal

Abstract

The impact of mask blank surface roughness and mirror surface roughness on the defect inspection signal is presented. The power spectrum density (PSD) of the roughness is assumed to be inversely proportional to the square of the spatial frequency. The PSD was estimated based on the mask blank surface roughness (rms), and then the background intensity was calculated using the PSD of the spatial frequencies between the inner and outer NA. The results indicate that the larger outer NA leads to an increase in the background intensity, and that a mask blank roughness of 0.15 nm generates a background intensity of 0.15 - 0.23 %. We also analyzed the effect of the mirror surface roughness on the background intensity and on the defect signal contrast. A point spread function (PSF) of scattered light from the mirror surface was calculated using the estimated PSD, and the defect images were simulated for the inspection optics by employing Fourier technique. The degradation of defect images caused by the mirror roughness was calculated by using the convolution of the PSF with the simulated images. Based on the results, it is concluded that the roughness has a large impact on the maximum intensity of the defect signal but has little effect on the background intensity. It was also learned that the degradation rate of the defect signal contrast is approximately proportional to the square of the mirror roughness.