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D. Tsankov et al.: FTIR Ellipsometry as a Tool for Studying Organic Layers
refractive index (n ) and the geometrical thickness was accomplished by fitting the
1
computed to the experimental D spectra in a spectral range free of absorption. The
curve fitting procedure was done iteratively by minimizing simultaneously the square
deviations for two measurements taken at incidence angles 60ꢀ and 65ꢀ. The found
values n ¼ 1:36 and d ¼ 25 nm were further used for data refining by oscillator fit
1
calculations. As can be seen from Fig. 7 the fit for D is very good, partly certain devia-
tion remains in the tan Y spectrum, which in principle is more sensitive to data incon-
sistencies and experimental shortcomings.
5. Conclusion
Infrared spectroscopic ellipsometry was used to explore optical constants and thickness
of various organic layers deposited on metallic, semiconducting and polymeric sub-
strates. Large scale of layer thicknesses ranging from a few nanometers to micrometers
can be examined. The thickness evaluations of technical polymer layer coatings tend to
be restricted to some hundred nanometers only. Most often they contain different addi-
tives aiming to improve their mechanical toughness as well as their chemical and ther-
mal resistance. Depending on the particular polymer and its application, type and
amount of the additive may vary but in any case it influences the layer homogeneity
and its background absorption. This problem may become severe in case of thicker
technical coatings. If the transmittance is too poor, the polymer coating cannot be
recognized as a surface film by the probing radiation due to lack of reflection from the
layer/substrate interface. Such layers behave like bulk samples. The examples presented
here prove the method as a reliable analytical tool for probing layers in low nanometer
scale. It is capable of performing a non-destructive analysis even of technically rough
surfaces that would have been difficult if not impossible, by standard FTIR techniques.
The calculation algorithm based on the rigorous electromagnetic theory can also supply
reliable data for anisotropic optical constants.
Acknowledgements We are grateful to R. Dietel, Institute of Thin Film Technology
and Microsensorics, Teltow, Germany, for making available the LB-films and Mrs I.
Fischer (ISAS) for technical assistance. The financial support by the Senatsverwaltung
fu¨r Wissenschaft, Forschung und Kultur des Landes Berlin and by the Bundesminister-
ium fu¨r Bildung und Forschung is gratefully acknowledged.
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