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Open AccessArticle

Multilayer Thickness Measurements below the Rayleigh Limit Using FMCW Millimeter and Terahertz Waves

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Center for Materials Characterization and Testing, Fraunhofer Institute for Industrial Mathematics ITWM, 67663 Kaiserslautern, Germany
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Institute of Communications Engineering, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany
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Department of Physics and Research Center OPTIMAS, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany
*
Authors to whom correspondence should be addressed.
Sensors 2019, 19(18), 3910; https://doi.org/10.3390/s19183910
Received: 31 May 2019 / Revised: 15 August 2019 / Accepted: 6 September 2019 / Published: 11 September 2019
(This article belongs to the Section Physical Sensors)
We present thickness measurements with millimeter and terahertz waves using frequency-modulated continuous-wave (FMCW) sensors. In contrast to terahertz time-domain spectroscopy (TDS), our FMCW systems provide a higher penetration depth and measurement rates of several kilohertz at frequency modulation bandwidths of up to 175 GHz. In order to resolve thicknesses below the Rayleigh resolution limit given by the modulation bandwidth, we employed a model-based signal processing technique. Within this contribution, we analyzed the influence of multiple reflections adapting a modified transfer matrix method. Based on a brute force optimization, we processed the models and compared them with the measured signal in parallel on a graphics processing unit, which allows fast calculations in less than 1 s. TDS measurements were used for the validation of our results on industrial samples. Finally, we present results obtained with reduced frequency modulation bandwidths, opening the window to future miniaturization based on monolithic microwave integrated circuit (MMIC) radar units. View Full-Text
Keywords: FMCW; thickness measurement; Rayleigh limit; millimeter-wave; terahertz; non-destructive testing; high accuracy; multiple reflections FMCW; thickness measurement; Rayleigh limit; millimeter-wave; terahertz; non-destructive testing; high accuracy; multiple reflections
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Schreiner, N.S.; Sauer-Greff, W.; Urbansky, R.; von Freymann, G.; Friederich, F. Multilayer Thickness Measurements below the Rayleigh Limit Using FMCW Millimeter and Terahertz Waves. Sensors 2019, 19, 3910.

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