The extreme ultraviolet microscope (EUVM) has been developed for an actinic mask inspection of a EUV finished mask and a EUV blank mask. Using this microscope, amplitude defects on a finished mask and phase defects on a glass substrate are observed. However, it has a problem of low contrast, which originates from 1) thermal noise of a charge coupled device (CCD) camera, 2) wave aberrations of an optical component, and 3) a nonuniform illumination intensity. To resolve these issues, EUVM was improved. 1) To reduce a thermal noise, a cooled CCD camera is installed. 2) To remove wave aberrations of a back-end turning mirror, a Mo/Si multiplayer-coated thick glass substrate with a high surface accuracy is employed instead of a Si wafer substrate. Furthermore, in situ alignment was carried out to remove wavefront aberrations for a Schwarzschild imaging optics. In addition, 3) by installing a scanning system on the front-end turning mirror, a highly uniform illumination intensity was achieved. As a result, images of less than 100 nm without astigmatism were obtained.

We constructed an extreme ultraviolet microscopy (EUVM) system for actinic mask inspection that consists of Schwarzschild optics and an X-ray zooming tube. This system was used to inspect completed extreme ultraviolet lithography (EUVL) masks and Mo/Si coated substrates on ultralow expansion (ULE) glass. We also have fabricated programmed phase defects on the blanks used for inspection. The EUVM system was capable of resolving a programmed line-pit defect with a width of 40 nm and a depth of 10 nm and also that with a width of 70 nm and a depth of 2 nm. However, a 75-nm-wide, 1.5-nm-deep pit defect was not resolved. The EUVM system was also capable of resolving programmed hole-pit defects with widths ranging from 35 to 170 nm and depths ranging from 2.2 to 2.5 nm. However, 20-nmwide, 1.5-nm-deep hole-pit defects were not resolved. These results agree with the simulation results perfectly. Thus, in this study, critical dimensions of a pit defects on mask blanks were determined to be a width of 20 nm and a depth of 2 nm.

This paper is concerned with the observation of phase defects in an extreme ultraviolet lithography (EUVL) mask using an EUV microscope developed by the University of Hyogo. It is very important to determine the type and size of defects on a substrate that are printable after deposition of a multilayer film. Thus, some mask blanks with programmed hole-pit defects with different widths and depths were fabricated by a new process. In addition, critical dimensions of a pit defect were investigated using the EUV microscope. As a result, 4.0-nm-deep hole-pit defects with widths larger than 35 nm were resolved. However, 4.0-nm-deep hole-pit defects with widths smaller than 25 nm were not resolved. On the other hand, 3.0- and 2.0-nm-deep hole-pit defects with widths larger than 60 nm were resolved. However, hole-pit defects with widths smaller than 40 nm were not resolved. Furthermore, the EUVM system was capable of clearly resolving 1.0-nm-deep hole-pit defects with widths larger than 70 nm. However, hole-pit defects with widths smaller than 60 nm were not resolved. From these results, we have determined the size of phase defects that are printable or not by observing phase defects that have various widths and depths on mask blanks utilizing the EUV microscope.

We constructed an extreme ultraviolet microscope (EUVM) system for actinic mask inspection that consists of Schwarzschild optics and an X-ray zooming tube. This system was used to inspect finished extreme ultraviolet lithography (EUVL) masks and Mo/Si coated substrates of ULE glass. And we have fabricated programmed phase defects on the blanks used for inspection. The EUVM was able to resolve a programmed line-pit defect with a width of 40 nm and a depth of 10 nm, and also with a width of 70 nm and a depth of 2.0 nm. However, a 75-nm-wide 1.5-nm-deep pit defect was not resolved. Also, the EUVM was able to resolve a programmed hole-pit defects with widths ranging from 35 nm to 170 nm and depths ranging from 2.5 nm to 2.2 nm.However, 20-nm-wide 1.5-nm-deep hole-pit defects were not resolved. These results agree with the simulation results perfectly. Thus, in this study, one critical dimension of a pit defects was experimentaly estimated to be a width of 20 nm and a depth of 2.0 nm.