Interferometric measurement of phase in EUV masks

Abstract

In this paper we present a technique for measuring the phase of EUV phase shift masks. In this technique we image the mask with a specialized objective lens that generates a superposition of two laterally separated images that interfere onto a detector. The resulting interference contains information about both the amplitude and the phase of the mask. By changing the incident illumination angle, we can control the bulk phase of the interference fringes, which allows us to reconstruct the phase difference between two adjacent points on the mask using standard phase retrieval techniques. While this method only reconstructs phase differences, it can be used to fully characterize the amplitude and phase of features provided that they are adjacent to a flat reference area on the mask.