Accuracy evaluation of point diffraction interferometer for EUVL mirror

The point diffraction interferometer (PDI) is expected to be a powerful measurement tool for EUVL projection optics. ASET have developed the PDI in corporation with Nikon. Half of transmitted wavefront generated by the pinhole is reflected by the test mirror and interfered with the other half. In PDI, the artificial reference is not used, then the very high accuracy measurement is enabled. Recently, the accuracy of PDI has been evaluated experimentally. Here, two types of accuracy are discussed. The term "repeatability" is the difference of two continuous measured data and the term "absolute accuracy" is the difference of measured figure from the real value.