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5.3 Field Calculation over Nonplanar Topography

The growing complexity of the devices is mainly realized by decreasing the
minimal feature size and simultaneously increasing the nonplanarity of the
topography. Both trends contradict most of the stringent assumptions required
for the simple planar imaging models. Second-order effects such as light
scattering due to the inhomogeneity of the resist and especially the notching
phenomenon due to a nonplanar reflective substrate have to be caught by the
simulation. Hence rigorous EM methods are being developed that directly
solve the Maxwell equations in either the *spatial-*, *time-* or
*frequency-domain*. A survey over representative examples belonging to
these basic categories is given below.
However, as the chemical reaction of the resist is only understood to a
certain depth and as the extreme computational requirements prohibit a quick
everyday application of rigorous field solvers, approximate EM simulation
techniques suited
for nonplanar topography have been proposed recently. For the sake of
completeness one promising approach based on the *geometrical theory of
diffraction* will be described at the end of this chapter.

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**Up:** 5. Photoresist Exposure/Bleaching Simulation
** Previous:** 5.2.4 Beam Propagation Method
*Heinrich Kirchauer, Institute for Microelectronics, TU Vienna *

1998-04-17