Browse Prior Art Database

Pupil Filling for Optical Measurement Systems

IP.com Disclosure Number: IPCOM000107919D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 1 page(s) / 62K

Publishing Venue

IBM

Related People

Progler, CJ: AUTHOR [+2]

Abstract

Proposed is a modification of the illuminator used in optical alignment or overlay measurement systems in which the spatial frequency content of the illuminating light is maximized for improved system performance. This is achieved by completely filling or overfilling the projection lens pupil with the effective source image in order to achieve a greater symmetry of the image and, hence, accuracy of centerline estimates.

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Pupil Filling for Optical Measurement Systems

      Proposed is a modification of the illuminator used in optical
alignment or overlay measurement systems in which the spatial
frequency content of the illuminating light is maximized for improved
system performance.  This is achieved by completely filling or
overfilling the projection lens pupil with the effective source image
in order to achieve a greater symmetry of the image and, hence,
accuracy of centerline estimates.

      Selection of the filling ratio on current state of the art
semiconductor metrology optics is not optimal for accurate estimation
of centerline.  Typically, the filling ratio for alignment/overlay
equipment is tuned for CD measurement or carried directly over from
brightfield microscopy yielding ratios from approximately 0 to .7.
The systems are then used for centerline estimation applications with
the expectation that the same pupil filling ratio will be optimal.
Image symmetry and process insensitivity, two key requirements for
accurate, robust overlay and alignment measurement, are neglected in
this CD-based filling optimization.

      It is proposed to increase the filling ratio to 1.0 or higher
over the conventional filling ratios of 0 to .7. Although some
reduction in image contrast may result at a filling ratio of 1.0, the
final image will be a more accurate representation of the true target
position and less susceptible to illuminator and sample asymmetry.
In addition, thin fil...