21 June 2015 Problems in thin film thickness measurement resolved: improvements of the fast Fourier transform analysis and consideration of the numerical aperture of microscope headers and collimators
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Abstract
Thin film thickness determination with a reflectometer suffers from two problems. One problem is the leakage in the Fast Fourier Transform caused by the fact that the two variables wavenumber 1/λ and optical thickness n⋅d are not really independent, since the refractive index n of the film material itself depends upon the wavenumber. This causes uncertainties in the thickness determination in the order of up to 5% for highly refractive materials like semiconductors. We present a simple but effective improvement of this contribution of the leakage that reduces the uncertainty to less than 2% for highly refractive materials.

Another problem that mainly affects thin films below about 2 μm arises if one uses measuring heads collimators or even microscope headers to obtain high lateral resolutions in the thickness determination. The use of a header introduces angles of incidence different from the default angle α = 0° in reflectometry. Then, the measured reflectance becomes polarization-dependent and the angle must be explicitly considered in the evaluation algorithm. For a microscope header however, all angles between 0° and the angle of aperture must be considered. We will present a solution that allows to reduce the work for each header on taking into account the polarization of the reflected light and a corresponding effective angle αeff.
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M. Quinten, M. Quinten, F. Houta, F. Houta, T. Fries, T. Fries, } "Problems in thin film thickness measurement resolved: improvements of the fast Fourier transform analysis and consideration of the numerical aperture of microscope headers and collimators", Proc. SPIE 9526, Modeling Aspects in Optical Metrology V, 95260R (21 June 2015); doi: 10.1117/12.2184684; https://doi.org/10.1117/12.2184684
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