Detecting and characterizing mask blank defects using angular distribution of scattered light

A method comprising:

• limiting an illumination angle of light emitted from an illumination source to a dark field optical unit by a first iris diaphragm located at the illumination source;
• limiting scattering angle of the light scattered after reflecting from a mask blank by a scattering limit unit located at exit of the dark field optical unit;
• and
• generating an angular distribution of the scattered light, the angular distribution being used to characterize criticality of a defect found on the mask blank;
• wherein limiting scattering angle comprises:
• limiting a minimum scattering angle of the scattered light by a second iris diaphragm.

A system comprising:

• a mask stage to move a mask blank;
• an illumination source to produce a light for inspecting the mask blank;
• a dark field optical unit optically coupled to the illumination source to provide scattered light from the light deflected onto the mask blank;
• and
• a defect inspection tool coupled to the illumination source and the dark field optical unit comprising:
• a first iris diaphragm located at the illumination source to limit an illumination angle of the light emitted from the illumination source to the dark field optical unit,
• a scattering limit unit located at exit of the dark field optical unit to limit scattering angle of the scattered light after reflecting from the mask blank, and
• a defect analyzer optically coupled to the dark field optical unit to generate an angular distribution of the scattered light, the angular distribution being used to characterize criticality of a defect found on the mask blank;
• wherein the scattering limit unit comprises:
• a second iris diaphragm to limit a minimum scattering angle of the scattered light.