Measurement of absolute optical thickness distribution of a mask-glass by wavelength tuning interferometry

The surface flatness and the uniformity in thickness and refractive index of a mask-blank glass have been requested in semiconductor industry. The absolute optical thickness of a mask-blank glass of seven-inch square and 3mm thickness was measured by three-surface interferometry in a wavelength tuning Fizeau interferometer. Wavelength-tuning interferometry can separate in frequency space the three interference signals of the surface shape and the optical thickness. 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 transform. 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 632 nm and 642 nm. The optical thickness defined by the group refractive index at the central wavelength of 337 nm can be measured by this technique. Experimental results show that the cross talk in multiple-surface interferometry caused a systematic error of 2.0 microns in the measured optical thickness.