Revealing features of different optical shaping technologies by a point diffraction interferometer

Almost hidden residual defects of a test surface can be revealed using high precision instrument such as a point diffraction interferometer (PDI). In general, PDI is engaged to display the figure of a surface or wavefront with sub-nanometer accuracy paying attention to low-frequency configurations. Such technique is suited to test EUV or X-ray optics. The tool described in the paper is able to map absolute profile deviations of several angstroms and therefore it provides a new vision of a surface under test of various quality, e.g. detects specific characteristics which immediately disclose either lapping or diamond turning has been used to form the substrate. Such inspection may help optimize the processes in early stage of shape forming before final configuring.