TFCalc has two methods of determining the refractive index of a thin film layer:
- using a variable material. In this case, it is assumed that the index is
constant over a wavelength range. One (or more) layers in the coating design
can be designated as a variable material -- a material whose index can be changed
during optimization. The measurements (reflectance, transmission, ellipsometric)
are entered as optimization targets. Then optimization is used to find the
index and thickness of the variable index layers.
- fitting a dispersion formula. In this case, the index is assumed to follow
a formula containing a few parameters. It is assumed that measurements are
of a single layer coated on a substrate. Up to three files of measurements (R and/or T)
can be used. TFCalc solves a nonlinear least-squares problem to determine the
parameters in the dispersion formula. The user can (1) select which parameters
should be optimized and (2) constrain the values of the parameters. Here are the
dispersion formulas that are available for N:
Here are the dispersion formulas for K:
Except for the Drude formulas, any pair of dispersion formulas for N and K
can be combined.
TFCalc can now be used to determine the
index of a substrate from reflectance and transmittance measurements. The thickness
of the substrate must be known. Previously, TFCalc could determine the index of a
thin film layer on a known substrate.