Fourier search for thickness
The FFT option in the thickness dropdown is an alternative option to let FILMeasure choose an ini-
tial thickness for analysis. The Fourier Transform method analyzes the oscillations present in the
spectrum and determines the film thicknesses based on the periodicity of those oscillations. It is
somewhat less robust than the Grid method, but is better at finding the correct thickness in cases
where the shape of the initial theoretical spectrum is different than the measured data (i.e., the reflec-
tance spectrum is non-ideal in some way) or in cases where there is more than one film thickness is
being measured.
Grid search for thickness
There are a number of methods that FILMeasure can use to determine thickness. None is perfect –
each is a different trade-off between speed, accuracy, and robustness (i.e., the ability to find the
best solution among many that are nearly as good). Because thickness can vary over many orders
of magnitude and many near-solutions exist, it is often best to use a very robust method to get close
to the best solution, and then let a more accurate method take over. One very robust method is the
Grid method, which can be activated by selecting the Grid option in the thickness dropdown. The
Grid method searches the entire allowed thickness range (as defined by the initial guess and the
constraints) to find the best initial thickness. However, on some very complex multilayer spectra, it
is possible for the Grid method to become confused and give the wrong answer. In such cases it is
best to use the Fourier Transform method to determine initial thicknesses, or to provide them
manually.
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