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The uniformity of the surface flatness, optical thickness distribution, and refractive index have been requested in semi-conductor industry. The absolute optical thickness of a mask-blank glass of seven-inch square and 3.1 mm thickness was measured by three-surface interferometry in a wavelength tuning Fizeau interferometer. Wavelength-tuning interferometry can separate three interference signals of the front surface shape, the rear surface shape and the optical thickness in frequency space. The wavelength of a tunable laser diode source was scanned linearly from 632 nm to 642 nm and a CCD detector recorded two thousand interference images. The number of phase variation of the interference fringes during the wavelength scanning was counted by a temporal discrete Fourier transformation. The initial and final phases of the interferograms before and after the scanning were measured by a phase shifting technique with fine tunings of the wavelengths at 632nm and 642 nm. The optical thickness defined by the group refractive index at the central wavelength of 637 nm can be measured by this technique. Experimental results show that the cross talk in multiple-surface interferometry caused a systematic error of 0.6 microns in the measured optical thickness.

Measurement of the Absolute Optical Thickness Distribution of a Mask-blank Glass by a Phase-shifting, Wavelength Tuning Interferometry