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An auxiliary optical system is described which is designed to provide dark-field illumination at the mask and wafer for alignment purposes for a photolithographic exposure tool which us laser light source.
English (United States)
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Dark-field Fiber-optic Mask Illuminator
An auxiliary optical system is described which is designed to provide dark-
field illumination at the mask and wafer for alignment purposes for a
photolithographic exposure tool which us laser light source.
The auxiliary light source comprises a visible, spectrally selected (green),
collimated light beam which is provided by a short-arc mercury lamp used with a
collimating lens and a band- pass filter. This light is combined collinearly with the
laser beam, which may comprise an excimer laser beam, for example, by using a
dichroic beamsplitter. For exposures, the laser beam reaches the mask through
a "fly's-eye" lens array and a fiber- optic coupler 1.
For dark-field illumination of the mask, a specially fabricated auxiliary flys'-
eye lens array 2 is brought into the proper position in the path of the auxiliary
beam. This lens array differs from the arrays used for exposure since each of its
lens elements 3 has a central obscuration region produced by a suitable opaque
coating 4. Thus, each lens element 3 produces a ring-field illumination cone 5
surrounding a central obscuration cone 6. The desired f numbers of these cones
are obtained by appropriate choice of the focal length of the lenses 3 and
diameter of the opaque coating 4.