Proposal of Plane-Parallel Resonator Configuration for High-NA EUV Lithography

TL;DR

This paper proposes a novel plane-parallel resonator configuration for high-NA EUV lithography that enhances image contrast and depth-of-focus by using off-axis illumination and a single-path cavity design, reducing sensitivity to mirror vibrations.

Contribution

It introduces a new resonator setup with off-axis illumination and a Fourier filter, enabling high-contrast, high-DOF imaging in EUV lithography without resonant effects.

Findings

Long depth-of-focus achieved

High image contrast demonstrated

Feasible design with Wolter telescope

Abstract

Plane-parallel resonator configuration is proposed for high-NA EUV lithography, where the lithography mask and the wafer are parallelly arranged through two focusing mirrors. EUV light is injected through an off-axis rotating mirror at the back focal plane and provides off-axis illumination (precession beam) to the mask and bounces back twice (at the mask and the wafer), finally goes out from the resonator through the rotating mirror. This is a single path cavity, there is no resonant effect. The orbital error or vibration of the rotating mirror do not affect on the imaging quality. The off-axis illumination is essential for high-NA optics, which recovers the high spatial frequency, and improves the edge contrast. The matched annular-aperture is located at the back-focal plane of the projector mirror, which acts as Fourier filter passing only the horizontally scattered waves reflected by the density modulations F(kx,ky,0) : 0-th z-order Fourier component of the mask. During single precession of the beam, this system creates 2D image of the normally projected density map of the mask pattern onto the wafer, where the longitudinal variation (3D effect) disappears, and thus the mask-shadowing problem is moderated. The depth-of-focus (DOF) is long, and also the image contrast becomes very high. As the objective mirror, Schwarzschild objective or Wolter telescope will be a suitable candidate. Wolter telescope is axisymmetric and lighter than conventional solid concave mirror, therefore, a larger diameter can be fabricated in high precession. By optimizing Wolter telescope, it may be possible to cover the wafer field size 26 mm x 33 mm of single patterning. Example parameter design was performed, seems technically feasible.