Measurement Of The Degree Of Coherence In Conventional Microscopes

Abstract

The spatial degree of coherence affects the quality of imaging systems. Especially in microlithography and microscopy the image quality is optimized by choosing the "right" degree of coherence. Hence it is necessary to know how to adjust the desired degree of coherence. The illumination system of a microscope allows one to vary several parameters : field stop, aperture stop and Kohler/critical illumination. The purpose of this paper is to determine the influence of these parameters on the degree of coherence in the object plane of a usual illumination system. The degree ofcoherence is measured as variation of modulation of interference fringes, produced by a shear interferometer, with the shear distance. For any kind of illuminating aperture (circular, i.e. bright-field, annular, i. e. dark-field, rectangular and double slit) and for both Kohler and critical illumination we found a very good agreement with the concept of Hopkins' effective source, predicting a Fourier relationship between the intensity distribution in the pupil of the condenser and the degree of coherence in the object plane.