High-NA in-line projector for EUV lithography

We propose 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 canceled within each stage and between them, in a manner similar to that of the double-Gauss lens. The light rays pass through the central aperture in each mirror with acceptable obscuration. The numerical aperture is 0.5 and 0.55. It has a circular exposure field with a diameter of 20 and 30 mm, respectively. The residual radial distortion is rather high at a few microns at the field rim, and the scan motion causes image blurring in the scanner. Thus, we need to revert to the stepper design, and the field becomes smaller, i.e., 14 mm×14 mm square and 21×21 mm. However, we can remove the scanning mechanism from the photomask side; thus, it makes the system simple. It is important to note that both the wafer and the photomask remain stationary during the EUV exposure. 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 bypassing the central obscurations.