17 June 2009 Thin film interferometer using a light source with spectrally nonequidistantly distributed sampling points
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Numerous processes, e.g. in semiconductor and optics producing industries require film thickness observation. These measuring systems depend on different working principles, e.g. spectral reflectometry or ellipsometry. The spectral reflectometry interrogation method can be evaluated by various algorithms depending on resolution and measuring range demanded. All methods require a broad spectral distribution of the light source in order to sample the signal sufficiently for parameter extraction. Spectral sampling is often realized using a spectroscope, which produces equidistant sampling points in frequency space. In contrast to conventional spectrally broad light sources, the one employed here emits several spectral lines, which are non-equidistantly distributed. It also introduces problems like variations of intensity in the output spectrum and narrow wavelength bands, in which the reflected spectrum can be investigated. Non-equidistant sampling points additionally imply problems in conventional analysis algorithms, e.g. a FFT anticipates equidistant sampling points. Narrow wavelength bands imply little information to interrogate at the same spectral resolution of the interrogator. Strong variations of intensity lead to high noise levels at wavelengths with low intensities. Therewith, accuracy, resolution and measuring range are limited. An interrogator based on a Hg-Ar light source, a fiber coupler and a commercial spectroscope is described in this work. Both, accuracy and measuring range, are investigated by simulation and are experimentally proven on a glass on silicon demonstrator. Introducing an advanced algorithm, uncertainties invoked by the source's spectral and intensity distribution are minimized and resolution as well as measuring range are increased.
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Florian Hirth, Sven Dudeck, Martin Jakobi, Detlef Gerhard, Alexander W. Koch, "Thin film interferometer using a light source with spectrally nonequidistantly distributed sampling points", Proc. SPIE 7389, Optical Measurement Systems for Industrial Inspection VI, 73891K (17 June 2009); doi: 10.1117/12.827537; https://doi.org/10.1117/12.827537

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