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Technology Update
A high-resolution 193 nm
interferometer is based on a
variation of Young's classic dual slit interference
experiment. In the example shown at the left an
aperture plate with two 15 micron pinholes spaced 300 microns
apart is back-illuminated by a 193 nm laser and the
pinholes are relay imaged at 100X de-magnification onto
the surface of an alternating aperture phase shift photomask. These pinholes
essentilly form two optical probes, each 150 nm in
diameter and spaced 3 microns apart, which are then used to
determine the phase shift of extremely small
features etched onto the mask such as trenches and
contact holes. The images depicted in the panel above
are taken in reflection: the
left is at-focus, the middle defocused by +2 microns and
the right defocused by +5 microns. The fringes that
appear in the two right images are the
near field patterns formed by the interference the two
spots just above the mask surface. One of the issues we are continuing to study
in this R&D project is the effect that the relatively
broad near field radiation patterns have on the phase metrology
in areas of dense mask features.
The interferometer is part of a high resolution microscope
that features a powerful Corning Tropel 0.75 NA Panther
catadioptric objective lens designed for operation at
193 nm. The photomask is illuminated in transmission
from below the stage by an Actinix Model 3193
laser source. A Sony DUV CCD camera is positioned on top
of the scope to image the mask at a magnification of
100X (the images shown in the panel above are captured
with this camera in reflection from the mask). The condenser lens below the mask has an NA of 0.32,
giving an illumination partial coherence of 0.43. The blue camera at the bottom next to
the stage is a cooled astronomical camera with a CCD that
is extremely sensitive to 193 nm light. This camera images the
transmitted localized fringes that result from the light
interfering from the
two 150 nm diameter probes, thus we acquire interferograms of photomask features
at geometrical scales below the
wavelength of the illumination light.
The diagram at the left shows how the
phase shift of a feature is determined. The etch
depths in these masks are ideally made to produce phase
shifts of about 180
degrees. The instrument needs to perform two measurements:
reference and test. The
reference measurement is made by positioning both spots
on a flat open area on the mask surface (the mesa) in
close proximity to the feature under test. The fringes
are formed below the mask as light propagates away from
the imaged probe spots. The
condenser lens serves to relay this fringe field to the
cooled CCD. At the test location the mask is positioned
such that one spot remains on the flat mesa and the second spot is positioned within the feature
area. Again the fringes are integrated on the CCD and if
there is a phase shift, the fringe field should be
displaced by an amount equal to that shift (we assume a
priori the etch depth is approximately one wavelength so
we don't have to unwrap the phase.)
An actual phase measurement on a
chromeless phase lithography (CPL) mask is shown at the left.
The mask layout diagram shows the mesa (0
degrees) as red areas and the etched trenches as gold. We
make the reference measurement on the mesa as diagrammed
by the two white probes positioned in the red area and then we make the test
measurement nearby with one spot on the gold border area. Since
the border feature is isolated we don't expect to see
any crosstalk in the interferogram from near field
artifacts. As shown we obtain relatively clean interferograms for
both the reference and test measurements. The large hole in
the center of the fringes is caused by the central
obscuration inherent in the catadioptric objective lens,
so we sample the interferogram on the edge of the
pattern. The plot
shows that indeed we can detect the phase shift of
isolated features at sub-wavelength scales with a good signal-to-noise ratio. Each measurement is made in about
one second. Further work is underway to examine the tool
capabilities with dense
features.
This technology is related to US
patent 7,042,577. |
This project was funded by NSF grant 0450620.