Method and apparatus for inspecting a mask substrate for defects, method of manufacturing a photomask, and method of manufacturing a semiconductor device
View Patent ↗According to one embodiment, a method of inspecting a mask substrate for defects, includes acquiring a defocus image of a partial region of a mask substrate using a dark-field optical system, acquiring a just-focus image of the partial region using the dark-field optical system, generating a set composed of first signals obtained from the defocus image and having signal intensities equal to or higher than a first threshold value, excluding, from the set, the first signals pertaining to parts in which signal intensities of signals obtained from the just-focus image are equal to or higher than a second threshold value, determining an inspection threshold value for signal intensities, on the basis of the first signals not excluded from, and remaining in, the sea.
1. An apparatus for inspecting a mask substrate for defects, the apparatus comprising:
a stage configured to place a mask substrate;
a dark-field optical system configured to acquire an image of the mask substrate placed on the stage;
a defocus-image acquiring unit configured to acquire a defocus image of a partial region of a mask substrate using the dark-field optical system;
a just-focus image acquiring unit configured to acquire a just-focus image of the partial region of the mask substrate using the dark-field optical system;
a set generating unit configured to generate a set composed of first signals obtained from the defocus image and having signal intensities equal to or higher than a first threshold value;
a signal excluding unit configured to exclude, from the set, the first signals pertaining to parts in which signal intensities of signals obtained from the just-focus image are equal to or higher than a second threshold value;
an inspection threshold-value determining unit configured to determine an inspection threshold value for signal intensities, on the basis of the first signals not excluded from, and remaining in, the set;
an inspection-region image acquiring unit configured to acquire an image of an inspection region of the mask substrate using the dark-field optical system; and
a defect determining unit configured to determine, based on the image of the inspection region, that any part having a signal intensity equal to or higher than the inspection threshold value has a defect,
2. The apparatus according to claim 1 , wherein determining the inspection threshold value by the inspection threshold-value determining unit includes:
acquiring the number of parts having a signal intensity equal to or higher than a certain value, based on the first signals not excluded from, and remaining in, the set; and
determining, as the inspection threshold value, a signal intensity in a case where the number of such parts is equal to or smaller than a prescribed number.
3. The apparatus according to claim 1 , wherein the mask substrate is a mask blank.
4. The apparatus according to claim 3 , wherein the mask blank is a reflection-type mask blank for EUV exposure.
5. The apparatus according to claim 1 , wherein at least one of the signal intensity of the signal acquired from the defocus image and the signal intensity of the signal acquired from the just-focus image corresponds to a signal intensity obtained by subtracting an average signal intensity measured in a region outside a region of interest from a signal intensity measured in the region of interest.
6. The apparatus according to claim 1 , wherein the defocus image and the just-focus image are acquired by a TDI camera.