Ocean Optics NanoCalc User Manual

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Ocean Optics Germany GmbH Thin Film Metrology

FFT
This method is very fast and applicable only for relatively thick layers, but not very precise !! It may be
refined by pressing the check box “use extended search after FFT or failure” in menu FitParameters.
You will see the fouriertransformed spectrum on the screen with different peaks. As your original signal is
NOT a sum of harmonic functions, there are peaks that do not correspond to a layer thickness (e.g. typically
there is a peak corresponding more or less to the sum of all thicknesses). All peaks that can be identified are
marked with a colored circle.
The scale in FFT mode is a scale of optical thickness (=product of geometrical thickness and refraction
index), not of geometrical thickness. If you SHIFT+Click on the values of refraction index (below the word
“SETUP” in main menu, the FFT scale will be recalculated for the value of the corresponding layer.

You have the choice between different options in menu EditStructure ("FixedLimits", "NarrowLimits",
"WideLimits" and "UserLimits")

"FixedLimitsMode"
In this mode the values of low and high limits of your estimate are made equal to the thickness. This
means that this layer is regarded as perfectly well-known.
Such a layer is excluded from data extraction algorithms.

"NarrowLimitsMode"
NanoCalc uses the value of your thickness estimation to calculate low and high limits which are quite
narrow (about ± 100 nm). A search is done ONLY within these limits. If your limits were to narrow this
search will fail and you have to try wider limits (like WideLimitsMode)

"WideLimitsMode"
The low limit and high limit is set according to the EditStructure setup menu:
1. relative wide limits: a symmetrical region is given, like ± 500 nm
2. absolute wide limits: the values of lower and upper wide limits are given as absolute values
In any case: a search is done ONLY within these limits. This means that the user does not need any
knowledge about the thickness. This option MAY be slow (but not necessarily !)

"UserLimitsMode"
NanoCalc fits the spectrum between low and high limits which were input by the user. Here the maximum
thickness is 300 micrometers.