Extreme ultraviolet interference lithography was carried out at the long undulator beamline in NewSUBARU. It was confirmed that the spatial coherence length is 1.1 mm using a 10-µm-wide slit in the Young's double slit experiment. A 25-nm half pitch (hp) resist pattern was successfully replicated by extreme ultraviolet interference lithography (EUV-IL) utilizing a two-window transmission grating pattern of a 50-nm line and space (L/S). For the replication of a 20-nm L/S resist pattern by EUV-IL, we contrived a fabrication process that is suitable for a transmission grating pattern of 40-nm L/S and smaller. Employing a hard-mask process with a silicon dioxide (SiO2) layer on a tantalum–nitride (TaN) layer in the fabrication of a two-window transmission grating, we successfully achieved five times larger dry-etch selectivity in comparison with a non-hard-mask process. As a result, we confirmed the ability to apply this process to a 40-nm hp grating.

EUV interference lithographic exposure tool was developed to evaluate hp 20 nm and below. The transmission diffraction grating with hp 30-nm absorber pattern was succeeded to fabricate. In the fabrication process of the transmission diffraction grating, SiO 2 hard mask process and the center stop process were applied to obtain high contrast of the interference fringes to replicate resist fine pattern. In addition, the vibration effect was succeeded to reduce. As results, hp 22.5 nm, hp 20 nm, hp 17.5 nm, and hp 15 nm resist patterns were succeeded to replicate by the two-luminous-flux interference exposure using the two-window transmission diffraction grating in EUV-IL. In addition, the hole pattern of hp 35 nm and hp 28-nm the four-luminous-flux interference exposure using the four-window transmission diffraction grating was succeeded to replicate.

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.

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