High-NA In-Line Projector for EUV Lithography

This paper proposes a simple, four-mirror, in-line projector for high-NA EUV lithography that eliminates the most troublesome mask 3D effect. The design consists of a two-stage concave-convex pair, where optical aberrations are cancelled within each stage and between them, in a manner similar to that of a double-Gauss lens. The light rays pass through the central aperture in each mirror with acceptable obscuration. The numerical aperture (NA) is 0.5 and 0.7 for Hyper-NA. It has a circular exposure field with a diameter of 26 mm. The residual radial distortion is rather high at a few microns at the field rim, and the scan motion causes image blurring. Thus, we need to revert to the stepper design, and the field becomes smaller, i.e. 18 mm x 18 mm square. However, this brings an important benefit: we can remove the scanning mechanism from the photomask side. It is important to note that both the wafer and the photomask remain stationary during the EUV exposure. This guarantees superior overlay control and results in enhanced productivity. This approach serves to simplify the system and reduce electrical consumption also. Illumination will be provided through two rectangular scan-mirrors located in front of the mask, providing dual line scan field, which matches with off-axis illumination enhancing the resolution and bypasses the central obscurations.