Advantages of a new sub-nanometer aspheric profiling technique with respect to the unique requirements of EUV lithography mirrors

We have developed a new noncontacting approach for obtaining the full aperture, absolute aspheric profile of optical surfaces. The approach has many advantages for a wide range of optics, including self-referencing and motion-insensitive operation; extremely high accuracy; compact size; and the ability to test concave, flat, and convex optics over a wide range of spatial frequencies. In a separate paper, we describe the underlying theory of operation, a prototype instrument, preliminary measurement results, and projected accuracies. In this paper, we discuss the specific advantages that are especially relevant for Extreme Ultra Violet (EUV) lithography components. These mirrors are responsible for the fabulously accurate imaging of the reticle onto the wafer. They therefore have typical surface accuracy requirements of a fraction of a nanometer, with the need to characterize the errors over an enormous range of spatial frequencies. The need to provide diffraction-limited imaging at EUV wavelengths over large Fields Of View (FOV's) and Numerical Apertures (NA's) puts a premium on freedom in the optical design. Specifically, the use of aspheres and convex mirrors can be of great help. Therefore, performance, FOV, and NA all benefit from the most accurate and flexible metrology. All of these factors make this new profiling technique well suited to the unique requirements of EUV lithography mirrors.