Methods for defect inspection, sorting, and manufacturing photomask blank

A method for inspecting a defect on a photomask blank, comprising the steps of:

• (A1) preparing a photomask blank having at least one thin film on an optically transparent substrate, the photomask blank bearing a defect on its surface,
• (A2) moving the photomask blank to move the defect on the photomask blank surface to a position observable by an inspection optical system, irradiating inspection light to a defect-bearing surface region, and collecting the reflected light from the irradiated region as a magnified image of the region via the inspection optical system,
• (A3) extracting a feature parameter from the magnified image, and
• (A4) identifying the shape of the defect on the basis of the feature parameter combined with the structure of the photomask blank thin film, wherein in step (A4), the shape of the defect is identified on the basis of data including the feature parameter of the magnified image and the structure of the photomask blank thin film, and with reference to a table which is previously formed based on optical simulation or empirical data for enabling a choice between pinhole defect and bump defect, and wherein
• i) provided that the outermost surface of the photomask blank under inspection is a thin film which is transparent to the inspection light, when a feature parameter indicating that the magnified image of the defect is a bright part-predominant image is extracted in step (A3), the defect detected is identified to be a pinhole defect, or
• ii) provided that the photomask blank under inspection has a film structure that a thin film at the outermost surface has a higher reflectivity of inspection light than an underlying layer, when a feature parameter indicating that the magnified image of the defect is a dark part-predominant image is extracted in step (A3), the defect detected is identified to be a pinhole defect.