Abstract
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.
Highlights
Millimeter and terahertz wave technology provides a non-destructive and contact-free method for accurate thickness measurements of dielectric multilayer structures
90 GHz center frequency with a modulation bandwidth of 39.5 GHz
The radars corresponding optical resolution limit vary throughout the samples, we compared our results with terahertz time-domain spectroscopy (TDS)
Summary
Millimeter and terahertz wave technology provides a non-destructive and contact-free method for accurate thickness measurements of dielectric multilayer structures. The measurement setups in [11,12,13,14] require both reflection and transmission measurements using a vector network analyzer (VNA) These approaches have the potential to determine sub-micrometer material thicknesses of a single layer and can be applied to measure single-layer thicknesses of multilayer structures [14]. The authors in [15] present a method, which can be applied for layer thickness measurements of multilayer structures (solely) in reflection geometry and with single-side access and a single sensor unit This is achieved by obtaining measurements over different angles of incidence or over a certain frequency range. We addressed measurements with reduced modulation bandwidths for the potential use of monolithic microwave integrated circuit (MMIC)-based integrated FMCW radars [20,21,22]
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