Achromatic Deep Ultraviolet Lens Using Novel Optical Materials

The development of an achromatic lens in the deep ultraviolet (DUV) region is reported. The dispersion and Abbe numbers of lithium calcium aluminum fluoride (LiCaAlF6), magnesium fluoride (MgF2), lithium fluoride (LiF), calcium fluoride (CaF2), and fused silica (SiO2) are investigated in the wavelength range from 200 to 300 nm to determine appropriate material combinations for minimizing chromatic aberration in a doublet. Based on the results of these investigations, a prototype DUV objective lens is fabricated using a LiCaAlF6(convex lens)-SiO2(concave lens) doublet. The objective lens has a focal length of 10 mm, working distance of 2 mm, and numerical aperture (NA) of 0.2. The objective lens is evaluated to have a chromatic aberration of about 2.8% and resolution of at least 2 mu m. It is also evaluated to have a smaller spherical aberration compared with a reflective objective lens with a similar NA.