Integration of an optical height sensor in mask inspection tools

A method, comprising:

• positioning a photomask on a stage in a vacuum chamber;
• measuring heights of patterned areas on a surface of the photomask using an optical height sensor that is at least partially disposed in the vacuum chamber, wherein the optical height sensor uses light that is not extreme ultraviolet (EUV) light and that has wavelengths longer than EUV light;
• producing a focal map for the photomask using the measured heights on the surface of the photomask, comprising:
• adjusting the measured heights of the patterned areas based on fill factors for the patterned areas, and
• applying a height offset to the adjusted heights to account for a height difference between the adjusted heights and an EUV focal position, in accordance with a calibration height-offset map;
• and
• inspecting the photomask for defects using an EUV objective disposed in the vacuum chamber, comprising:
• translating the stage horizontally and vertically in accordance with the focal map;
• and
• collecting EUV light from the photomask using the EUV objective in accordance with the focal map.

A photomask-inspection system, comprising:

• a vacuum chamber comprising a plate that divides the vacuum chamber into a first sub-chamber and a second sub-chamber;
• a stage, disposed in the second sub-chamber of the vacuum chamber, to support a photomask and to translate the photomask horizontally and vertically;
• an optical height sensor, at least partially disposed in the vacuum chamber, to measure heights on a surface of the photomask, wherein:
• the optical height sensor comprises a broadband light interferometer,
• the broadband light interferometer comprises an interferometric objective disposed in the first-sub-chamber of the vacuum chamber, and
• the optical height sensor is to use light that is not extreme ultraviolet (EUV) light and that has wavelengths longer than EUV light;
• a stage controller to translate the stage horizontally and vertically in accordance with a focal map for the photomask produced using the measured heights on the surface of the photomask;
• and
• an EUV objective, disposed in the first-sub-chamber of the vacuum chamber, to collect EUV light from the photomask in accordance with the focal map, in order to inspect the photomask for defects,
• wherein the plate comprises:
• a window to pass light between the interferometric objective and the photomask, and
• a hole to pass the EUV light between the photomask and the EUV objective.