Evaluation of homogeneity of single-crystal calcium fluoride (CaF₂) optical windows using DUV interferometry

In the semiconductor industry, the requirement for optical systems of high resolution with minimal aberration is becoming more stringent as the wavelength employed in the lithography tool is shortened. Index homogeneity, which is a key property for realizing aberration-free optics, has been conventionally characterized using a Fizeau interferometer with a visible light source. However, this method may not be technically feasible for the assessment of optical materials intended to operate in the ultraviolet (UV) wavelength region due to the steeper dispersion slope in that region. Here, we performed an optical testing for the index homogeneity of a single-crystal CaF₂ optical window, which was grown via Czochralski method, by conducting interferometric measurements in the DUV wavelength region. A home-built DUV Fizeau interferometer was developed and self-calibrated to examine the CaF₂ optical window for at-wavelength measurements. Furthermore, sub-aperture stitching interferometry was implemented to investigate the full aperture index distribution of the CaF₂ optical window. The results were compared with the inhomogeneity measured by a commercial visible-wavelength Fizeau interferometer, which showed a value almost twice as high as that of the visible interferometer.