Detection signal analysis of actinic inspection of EUV mask blanks using dark-field imaging

MIRAI Project has developed a novel actinic (at-wavelength) inspection tool for detecting critical multilayer defects on EUV mask blanks using a dark-field imaging and a laser-produced plasma (LPP) light source. Characterization of this experimental actinic inspection tool is ongoing to define the detailed specification of a proto-type tool. One of the important factors which improve the sensitivity of the inspection tool is the suppression of background noise and the optimization of detective conditions to get a high intensity signal. In this paper, characterization results of background noise and through focus imaging are presented. The multi-coated layer roughness-induced scattering noise which is a main factor of background noise is in proportion to the square of high and mid intermediate range roughness. The background level is expected to be suppressed to about two-thirds of an ordinary level, by improvement of multi-coated layer blank making. To inspect various defects with high sensitivity, through focus characteristics on various programmed defects with dot, hole, line, groove shapes is examined. Best focus in which a maximum defect signal is obtained is different between pattern types, especially hole and dot, and a common focus level through various small patterns can not be secured. Signal-to-background ratio (SBR) we proposed is a good parameter for defect detection because it has a wide focus latitude and it is possible to detect both small hole and dot defects with a common focus level.