Challenges in the dimensional Calibration of submicrometer Structures by Help of optical Microscopy

A basic task in dimensional metrology is edge localisation. The distance of two neighboring edges in an object structure, for instance, can be determined by the evaluation of the intensity distribution by use of threshold or extreme-value criteria. However, the distributions in the images begin to overlap for structures with dimensions below λ/NA where λ is the wavelength and NA is the numerical aperture of the imaging lens. That’s why the distances of the extreme values or the thresholds become strongly dependent from the width of the structures and for still smaller structures the extrema usually merge into one extremum.