Assessment and verification of thickness homogeneity in mask blank by utilizing main harmonics of triple-surface interferometry

To assess the thickness homogeneity of a mask blank using wavelength-modulating interferometry, novel methods for determining and verifying the thickness homogeneity using triple-surface interferometry are proposed. First, based on the optical path difference in triple-surface interferometry, the phases of main harmonics were defined, and the determination method of the back thickness variation was derived. Next, a verification method for thickness assessment was derived using the theoretical formula of the triple-surface interferometry. And then, the thickness assessment of the mask blank was conducted by wavelength-modulating Fizeau interferometer. The thickness homogeneity of the mask blank was determined by the phases of main harmonics extracted using harmonic interferometric-iterative analysis. Finally, the applicability of the verification method was evaluated by comparing the theoretical errors and standard deviations of the experimental results.