Apparatus for projecting a reduced image of a photomask using a schwarzschild objective

摘要

An optical system for projecting an image of a photomask on a substrate, using a Schwarzschild objective includes an excimer laser, beam shaping optics for shaping a laser beam from the laser and a beam-dividing prism. The beam-dividing prism has a dividing-face including four facets inclined at an angle to each other. The four facets divide the shaped beam into four beam-portions propagating at an angle to the system axis. The beam-portions overlap at the photomask and mutually diverge into the entrance aperture in the concave mirror of the Schwarzschild objective such that all of the light in the beam portions is incident on the convex mirror of the objective in an annular zone outside of the central obscuration zone of the convex mirror. This essentially eliminates transfer losses normally caused by this obscuration zone.