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Generalized Basic Iterative Methods for Absolute Measurement of Optical Flats

Higher level of optical testing is required with the rapid development of modern optical manufacture, especially in the lithographic objective lens system. It requires the surface figure to nanometer RMS even sub-nanometer RMS. The requirement of the surface in EUV lithography objective reaches 0.1 nm RMS, which is a great challenge for optical metrology.

One of the key technologies is the absolute test method, which involves multiple measurements using a plurality of probes, artefacts, and/or set-ups. Interferometric measurement of the surface form or figure of optical surfaces generally involves a comparison of the surface under test with a reference surface. For high-accuracy measurements, it is necessary to separate reference errors from test surface errors, especially when the reference errors have the same order magnitude as the test surface errors.

The researchers from Institute of Optics and Electronics developed a generalized method to reconstruct the surface figure based on reverse iterative optimization scheme. The proposed basic iterative methods are generalized; can correctly reconstruct absolute figures with pixel-level spatial resolution; are easy to understand and implement; and computationally efficient. It has been proved by both simulation results and experimental results.

Comparison of convergence rate for different iterative methods (Experimental results) (Plotted by QUAN Haiyang)

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