Optical proximity correction challenges with highly elliptical contacts
2010
The steady march of Moore's law demands ever smaller feature sizes to be printed and
Optical Proximity Correction to correct to ever tighter dimensional tolerances. Recently
pitch doubling techniques has relieved the pressure on CD reduction, which instead of
being achieved lithographically are reduced by subsequent etching or chemical
interaction with spin-on layers. CD tolerance reductions, however, still need to match the
overall design rule shrinkage. The move to immersion lithography, where effective
Numerical Apertures now reach 1.35, has been accompanied by a significantly reduction
in depth of focus, especially on isolated contacts. To remedy this, RET techniques such
as assist feature placement, have been implemented. Certain local placements of assist
features and neighboring contacts are observed to result in highly elliptical contacts being
printed. In some layouts small changes in the aspect ratio of the contact on the mask leads
to strong changes in the aspect ratio of the printed contact, whereas in other layouts the
response is very weak. This effect can be described as an aspect ratio MEEF. The latter
type of contact can pose a significant challenge to the OPC recipe which is driven by the
need to place the printed contour within a small range of distance from target points
placed on the midpoint of edges of a nominally square contact. The OPC challenge
naturally will be compounded when the target layout is rectangular in the opposite sense
to the natural elliptical shape of the printed contact. Approaches to solving this can vary
from intervening at the assist feature placement stage, at the possible loss of depth of
focus, to accepting a certain degree of ellipticity in the final contour and making the OPC
recipe concentrate on minimizing any residual errors. This paper investigates which
contact layouts are most challenging, discusses the compromises associated with
achieving the correction target and results are shown from a few different approaches to
resolving these issues.
Keywords:
- Correction
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