Development of actinic full-field EUV mask blank inspection tool at MIRAI-Selete

Abstract

We have developed an actinic full-field EUV mask blank inspection tool that consists of an EUV light source, a 26X Schwarzschild optics for dark-field imaging, an EUV-sensitive backside-illuminated charge-coupled-device (BI-CCD) camera, and a mechanical mask stage with a stroke range of 160 mm. A critical illumination system is employed by setting ellipsoidal and plane mirrors to illuminate an area of mask blank that is to be inspected. Since in this setup a circular area on the mask blank with approximately 0.8 mm diameter is illuminated, a 0.5×0.5 mm² square image area can be addressed without moving the mask stage. The inspection tool can also be operated under time delay and integration (TDI) mode by scanning the mask stage with a constant velocity. In spite of comparatively large effective pixel size of 500 nm on the mask blank, small defect-to-pixel ratio such as 0.12 for phase defect of 60 nm in width and 1.5 nm in height, was established as a measured value of defect detection sensitivity by using both static imaging mode and time-delay and integration (TDI) operation mode.