Correction of the phase retardation caused by intrinsic birefringence in deep UV lithography

In the year 2001 it was reported that the birefringence induced by spatial dispersion (BISD), sometimes also called intrinsic birefringence, had been measured and calculated for fluorides CaF₂ and BaF₂ in the deep UV range. It was also shown that the magnitude of the BISD in these cubic crystals is sufficiently large to cause serious problems when using CaF₂ for lithographic objectives at 157 nm and possibly also in the case of high numerical aperture immersion objectives at 193 nm. Nevertheless the single-crystal fluorides such as CaF₂ are the only materials found with sufficient transmissivity at 157 nm and they are widely used at 193 nm for chromatic correction. The BISD-caused effects lead to the loss of the image contrast. In this work we discuss issues related to the design of optical systems considering the BISD effect. We focus on several approaches to the compensation of the BISD-related phase retardation and give examples of lithographic objectives with the compensated phase retardation.