Aberration calibration in high-NA spherical surfaces measurement on point diffraction interferometry

Abstract

Systematic error in the point diffraction interferometer can significantly influence the measurement accuracy in the case that a high-numerical-aperture spherical surface is tested, in which wavefront aberrations could be observed and cannot be removed with the traditional calibration method. A calibration method, called symmetrical position aberration cancellation (SPAC) method, is proposed to remove the systematic error. With the SPAC method, the calibration needs two separate measurements with interferograms of opposite tilts and the systematic error can be reduced by averaging the test wavefronts of these two measurements. A geometrical optics model is presented for facilitating the calculation of the geometric aberrations that are introduced by the lateral deviation between the curvature center of the test surface and the center of the pinhole. Then a simulation model is built to analyze the influence of the lateral deviation and the numerical aperture of the surface under test on the systematic error. Both the computer simulation and experiments with a point diffraction interferometer (PDI) have been carried out to validate the proposed calibration method. In the final comparative experiment with a ZYGO interferometer, the PDI exhibits high precision in high-numerical-aperture spherical surfaces measurement based on the proposed calibration method.