Defect inspection of EUV mask blank using confocal microscopy: simulation and experiment

A theoretical model of confocal microscopy of phase defect in EUV mask blank is developed using Fourier optics and rigorous coupled-wave analysis(RCWA). This model is verified by comparison with experimental data and then applied to calculating confocal microscopy signal of phase defect with conformal and flat surface for wavelength of 488 nm and 266 nm, respectively. From this simulation, it is shown that phase defect with flat surface is undetectable even by 266 nm-wavelength confocal microscope, while it is printable at 13.5 nm-wavelength. Subsequent simulation of energy flow rate through Mo/Si multilayer shows that this is because incident optical wave is absorbed within 4 pairs of Mo/Si multilayer, but 40 pair of Mo/Si multilayer is semi-transparent at 13.5 nm-wavelength. Based on this result, a deposition scheme of Mo/Si multilayer is suggested.