High-accuracy interferometry of extreme ultraviolet lithographic optical systems

Recent improvements in extreme ultraviolet (EUV) lithographic imaging with Schwarzschild objectives have come as a direct result of at-wavelength interferometric characterization with the phase-shifting point diffraction interferometer. High accuracy system wave front characterization has led to the determination of the best Schwarzschild objective and subaperture configuration. These investigations and the results of imaging experiments are discussed. Two pinhole null tests have provided an in situ method of demonstrating reference wavefront accuracy of ∼λEUV/300.