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The Use of Gratings to Produce Oblique Beams for Focus Sensing in Cases of Little Working Distance

IP.com Disclosure Number: IPCOM000122090D
Original Publication Date: 1991-Oct-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 1 page(s) / 39K

Publishing Venue

IBM

Related People

Goodman, D: AUTHOR

Abstract

A well-known means of focus sensing on optical systems, e.g., lithographic projectors, depends on the displacement of oblique beams of light, as a function of object distance, Fig. 1. An axial displacement of the object or image-receiving plane gives rise to a lateral displacement of the reflected oblique beam. Sensing this beam determines focus.

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The Use of Gratings to Produce Oblique Beams for Focus Sensing in
Cases of Little Working Distance

      A well-known means of focus sensing on optical systems, e.g.,
lithographic projectors, depends on the displacement of oblique beams
of light, as a function of object distance, Fig. 1.  An axial
displacement of the object or image-receiving plane gives rise to a
lateral displacement of the reflected oblique beam.  Sensing this
beam determines focus.

      Some lenses, for instance, the high NA ones used in modern chip
lithography, have short working distances so that it is difficult to
accommodate the optical systems associated with the oblique beams.
Disclosed here is a method of surmounting this difficulty.

      Gratings are placed on two diametrically opposed regions of the
surface of the lens nearest to the surface being focused, Fig. 2.
These bend the light which may then be brought in at non-oblique
angles.  At a convenient interface elsewhere in the system, the beams
can be brought into and out of the lens.  The gratings can be
produced photographically or holographically.

      Disclosed anonymously.