Abstract
Phase-shifting Point Diffraction Interferometer (PSPDI) utilizing nearly perfect spherical wavefront diffracted by a pinhole as reference wavefront, which diminishes the influence of reference optics used in traditional interferometers, has been developed with high accuracy, repeatability and reproducibility. Accuracy of PSPDI is mainly limited by the quality of diffracted reference wavefront. We analyze the quality of diffracted reference wavefront by using of Rayleigh- Sommerfeld diffraction theory and performed FDTD numerical simulation. Based on analysis, we have developed a phase-shifting point diffraction interferometer. Ultra-precise pinhole alignment technical, high stable mount, high stable testing environment and error source insensitive data processing algorithm was used to achieve high stability and accuracy. Via accuracy evaluation, a deep sub-nanometer system error of developed PSPDI is obtained. A cross comparison of PSPDI measurement and measurement of another kind of interferometer was done, and the difference was 0.16nmRMS. The developed PSPDI has been applied in spherical mirror testing and EUV projection objective testing.
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