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Classical methods for modeling electromagnetic scattering from the topography of lithographic reticles must place
a high premium on fast computation, and toward that end they apply pre-stored perturbations (e.g. the so-called
boundary layers) to feature edges in order to approximate the impact of finite-thickness mask films. Though
approximate, these methods involve E&M calculations with vector fields, and so employ edge-field corrections
that are different for edges oriented parallel or perpendicular to the vector field. As a result these methods entail a
requirement for two separate aerial image simulations using orthogonal source polarizations in order to represent
unpolarized illumination. This imposes a minimum 2X runtime penalty relative to baseline thin-mask (TMA)
simulations, since the known method for combining the effect of both polarizations into one single set of imaging
TCCs applies only to thin-mask calculations. More severe performance penalties are common in so-called sparse
imaging methodologies when topographic effects are included, since the separated treatment of feature edges and
the internal area of the features can increase the number of memory lookups required.
In this paper an isotropic field perturbation approach is evaluated, in which an isotropic edge field correction,
common to all edge orientations, mimics the effect of the true parallel and perpendicular edge field perturbations
when the mask is illuminated with unpolarized light, as well as in certain cases of polarized illumination. The
isofield is not an ad hoc empirical correction but rather an accurate approximation in the limit of modest departures
from scalar TMA. More specifically, we show that the isofield model accounts for vector imaging effects with full
accuracy in the TMA terms, and in an approximate way in the electromagnetic edge-field terms that becomes
accurate when the polarization dependence of the TMA terms is small. We will show with comparison to more
rigorous electromagnetic models and simulations, as well as against wafer measurements that the accuracy loss
relative to classic polarized EMF correction approach is within a small percentage on mask blanks where the
electromagnetic edge field perturbation terms are small relative to the TMA term. Methodology to extend these
models into the subwavelength diffraction regime will be discussed.
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