The exposure state of the photoresist is described by
the concentration of the photoactive compound (PAC)
,
which constitutes the latent bulk image.
The bulk image is transferred into the photoresist by
light absorption. Thereby
the PAC is dissolved and the optical
properties, e.g., the refractive index
,
are changed.
As usually this exposure/bleaching reaction is modeled by
Dill's `ABC'-model
[13]
where
is the exposing light intensity.
Consequently, the EM field inside the nonlinear photoresist
has to be determined.
Because the bleaching rate is small as compared to the frequency of the
EM field, we apply a quasi-static approximation, i.e.,
where the initial PAC distribution is homogeneous,
.
Furthermore, we assume a steady-state field distribution within a time step
.
Therefore, the EM field is time-harmonic
and obeys the Maxwell equations in the form of
The complex permittivity
is related to the refractive index by Maxwell's law,
,
and the exposing light intensity is given by
[14]
The equation set
(2)
to
(4)
represents
the simulation model for the exposure/bleaching reaction,
whereby an efficient solution of
the inhomogeneous but linear Maxwell equations
(3)
is the crucial point for the applicability of the model.
The principal simulation flow of the exposure/bleaching
module is illustrated in
Fig. 7.
