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 inte rferometric characterization with the phase-shifting point diffraction inte rferometer. High accuracy system wave front characterization has led to the determination of the best Schwarzschild objective and subaperture configur ation. These investigations and the results of imaging experiments are disc ussed. Two pinhole null tests have provided an in situ method of demonstrat ing reference wavefront accuracy of similar to lambda(EUV)/300.