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Optimum Dark Field Illumination for Coordinate Measurements On Wafers

IP.com Disclosure Number: IPCOM000099512D
Original Publication Date: 1990-Jan-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 152K

Publishing Venue

IBM

Related People

Wagner, D: AUTHOR

Abstract

The production of future semiconductor elements is limited by the tolerances for mask and wafer alignment and the decreasing overlay to be observed for ever increasing chip dimensions. This necessitates highly precise X-Y coordinate measurements on wafers, which in the interest of economical production can only be made by high-precision devices operating at wafer level.

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Optimum Dark Field Illumination for Coordinate Measurements On Wafers

       The production of future semiconductor elements is limited
by the tolerances for mask and wafer alignment and the decreasing
overlay to be observed for ever increasing chip dimensions.  This
necessitates highly precise X-Y coordinate measurements on wafers,
which in the interest of economical production can only be made by
high-precision devices operating at wafer level.

      The lines on the wafer consist, in addition to metallic
conductors (great contrast, amplitude object), of resist and oxide
structures (the contrast and phase object of which depend on the
layer thickness).  With such phase objects with different intensity
profiles (Fig. 1), it is difficult, even for intelligent evaluator
means, to detect the edge location.  A usual way out is the use of
dark field illumination where the edges of lines of different
thickness have the same profile, and the maximum indicates the edge
location (Fig. 2).

      The quality of optical imaging depends on the contrast
intensity which is generally limited by the amount of stray light.

      For bright field imaging, attempts have been made to limit the
stray light by stops (aperture and field stops). In the dark field
arrangements currently available, a single field stop is totally
ineffective.  The optical characteristics of the mirror system
(surrounding the object) are so poor that fields are illuminated in
the millimeter range.  For alignment and measuring jobs, however,
fields of about 50 mm are fully sufficient.  This means that the
object is illuminated at a power a thousand times higher than that
actually required.  This has two serious disadvantages:

      1.   The amount of stray light increases with the factor of
overillumination.

      2.   The unnessarily high radiation power excessively heats the
object.  This may lead to measurable deformation of the only 2 mm
thick membrane, particularly in the case of X-ray masks.  In the
image, dark field illumination reaches an intensity which corresponds
to only N 1/20 of the bright field intensity.  Therefore, dark field
illumination mostly employs mercury high-pressure lamps because of
their high radiation density.

      The new method is described first with respect to a bright
field application.  It is assumed that the visual field...