Optical thickness measurement of mask blank glass plate by the excess fraction method using a wavelength-tuning interferometer

●The optical thickness was measured by wavelength tuning and phase-shifting.●The correlation integral was used to reduce the nonlinearity of wavelength tuning.●Two kinds of the optical thicknesses were synthesized to reduce the crosstalk noise.●Optical thickness and interference order were calculated by the excess fraction method.

The absolute optical thickness of a 140-mm2 mask blank glass plate 3.1 mm thickness was measured by three-surface interferometry using a wavelength-tuning Fizeau interferometer. The interference order was determined by the excess fraction method. The wavelength of a tunable laser diode was scanned linearly from 632 to 642 nm, and a CCD detector recorded 2000 interference images. Two kinds of optical thicknesses measured by discrete Fourier analysis and phase-shifting were synthesized to obtain the optical thickness with respect to the ordinary refractive index. The optical thickness defined by the group refractive index at the 637 nm central wavelength was measured by wavelength scanning. The optical thickness deviation defined by the ordinary refractive index was measured using tunable phase-shifting. The systematic errors caused by nonlinearity in the wavelength tuning were corrected through correlation analysis between the theoretical and observed interference fringes.