Mobile Handheld FMCW Terahertz Multilayer Thickness Inspection

Schreiner, Nina S.; Weber, Stefan; Keil, Andreas; Friederich, Fabian

doi:10.1109/irmmw-thz.2019.8873942

Cited by 3 publications

(2 citation statements)

References 3 publications

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“…To investigate the potential for measurements of thin single layers, 13 individual calibration foils made of biaxially-oriented polyethylene terephthalate with thicknesses between 72.7 and 291 µm were inspected in a previous study [33]. The results showed a deviation of less than 5% down to a layer thickness of approximately 121 µm, which corresponds to roughly 6% of the Rayleigh resolution limit.…”

Section: Measurement Resultsmentioning

confidence: 99%

“…The inherent thickness resolution is limited by the measurement system’s bandwidth and its pre-evaluated refractive index but is overcome by a correlation-based signal processing approach, which compares the measured signal with the simulation result of an appropriate signal model. In this way, layer thicknesses down to roughly 6% of the Rayleigh resolution limit [33] and up to several centimeters of thickness can be determined. By analyzing the influence of multiple reflections on the accuracy, we found increasing deviations between the physical and measured thicknesses with increasing contrast of the refractive indices between adjacent layers.…”

Section: Discussionmentioning

confidence: 99%

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

Schreiner

Sauer-Greff

Urbansky

et al. 2019

Sensors

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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.

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Section: Measurement Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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

Schreiner

Sauer-Greff

Urbansky

et al. 2019

Sensors

View full text Add to dashboard Cite

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Optimization Algorithms for Accurate FMCW Millimeter Wave and Terahertz Thickness Measurements

Schreiner

Bortz

Sauer-Greff

et al. 2021

2020 50th European Microwave Conference (EuMC)

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Kilohertz Pixel-Rate Multilayer Terahertz Imaging of Subwavelength Coatings

et al. 2022

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The acquisition speed of terahertz time-domain spectroscopy systems has undergone a significant improvement in recent years. With the development of dual-laser-based sampling techniques such as ASOPS or ECOPS, waveform acquisition rates in the kilohertz range have become feasible. Here, we present measurements of sub-wavelength layers in multilayer systems at a rate of 1.6 kHz, where the individual layer thicknesses are analyzed in real time. For demonstration, we image layers on metallic and non-metallic disks with a 300 mm diameter, acquiring 240 000 pixels in only 2.5 min. By combining a rotation of the samples with a linear translation of the measurement head, we maximize the measurement yield. Owing to a “single-shot” (625 µs) dynamic range of 40 dB and a bandwidth above 3 THz of the ECOPS system, we achieve sub-micron repeatability for each layer.

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Mobile Handheld FMCW Terahertz Multilayer Thickness Inspection

Cited by 3 publications

References 3 publications

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

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

Optimization Algorithms for Accurate FMCW Millimeter Wave and Terahertz Thickness Measurements

Kilohertz Pixel-Rate Multilayer Terahertz Imaging of Subwavelength Coatings

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