  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.
