Measuring technology for wavefront aberration of EUVL objective system

In order to measure the wavefront aberration of the EUVL objective system with the super high accuracy, the double-fiber phase-shifting point diffraction interferometer was introduced, the measuring program was optimized, the troon structure was introduced to phase-shifting system and OPD-adjusting system, and the endface of fiber was specially treated. The error sources which can affect the accuracy and wavefront repeatability were introduced to the interferograms to analyze, the accuracy and wavefront repeatability of the interferometer were analyzed, in order to verify the feasibility of double-fiber phase-shifting point diffraction interferometer and correctness of theoretical analysis, the principle experimental device was built, the same tested optical system was measured by 512 times, the measured results were devided into 8 groups, at last, the wavefront repeatability of 8 groups of data is 0.13 nm, it is better than λ/4000.