Ellipsometry and spectral reflectance, on the other hand, are both non-contact optical techniques that permit to determinate precisely layer thicknesses. The Arcspectro THINFILM NIR spectrometer uses the spectral reflectance of the sample to determine the thickness of the different layers that compose the sample.
Due to the wave-like behavior of the light, the phenomenon of light interferences appears into a layer thinner than the half coherence length of the light. The light reflected at the first interface can interfere with the light reflected at the second interface and their interaction is dependant on the wavelength of the light and the refractive index of the layer. This effect leads to oscillations into the reflectance spectrum measured by this instrument. The frequency of this oscillation gives information about the thickness of the layer and its refractive index

As depicted in the schematic above, the first part of the incoming light is reflected at the interface n1/n2 (point A). The amount of light reflected depends on the incidence angle, n1 and n2. Some light pass through the first interface and is reflected at the second interface (n2/substrate at point B). The two “rays” of light that are reflected back by the sample have a phase difference linked to the optical path difference (OPD) given by ABC-AD.
If this difference is equal to a multiple of 2n, there is a constructive interference and it will lead to a maximum in the reflectance spectrum. If this difference worth 2n+1 , there is a destructive interference and it will result in a minimum in the reflectance spectrum. Finally, some wavelengths will lead to constructive interferences and some to destructive ones. Consequently, modulation with a certain contrast (depending on the phase step between the layers) of the measured spectrum is observed. By applying a frequency analysis (by FFT) of the measured spectrum (picture right) we can deduce the thickness of the layer present on the substrate.
The precision of the measurement will essentially be given by the performance of the spectrometer. The Arcspectro- NIR spectrometer has an excellent SNR and wavelength precision which cannot be achieved by usual grating spectrometers. Also, its high resolution permits to characterize relatively thick films (up to 800um) over a large spectral range.

Other parameters such as the precision of the refractive index of the material, the flatness and alignment of the sample, the divergence of the light will influence the absolute precision of the measurement.