Characterization of Automotive Paint Permittivity for mmWave Radar Applications

A wideband dielectric permittivity measurement method to characterize thin-film automotive paints in millimeter-wave frequency bands is presented. The proposed measurement fixture is based on a coplanar waveguide (CPW) transmission line that is lithographically fabricated on a low-loss quartz wafer. The thin-film material sample is placed over the central Section CPW and wideband transmission and reflection coefficient measurements are collected with the aid of contact probes. On-wafer through-reflect-line standards are used to calibrate and de-embed the measurement fixture as well as the vector network analyzer (VNA) repeatable errors. Subsequently, we use the Nicolson–Ross–Weir (NRW) extraction procedure to determine the effective permittivity of a paint-loaded Section of the CPW fixture loaded with the thin film sample. The permittivity and loss tangent of the sample film are then calculated analytically, using conventional conformal-mapping-based formulation of the CPW geometry. The validity and the repeatability of the proposed measurement method are demonstrated using a representative, 122-
$\mu \text {m}$
-thick automotive paint film. We compare the performance of the proposed fixture with the free-space quasi-optical permittivity measurements. The proposed approach is particularly simple and effective in accurately characterizing the dielectric permittivity and loss tangents of thin and flexible paint films, enabling effective performance optimization of 60 and 77 GHz millimeter-wave automotive radars. The advantages of the proposed fixture for thin-film material evaluation, particularly for accurate dielectric loss characterization of thin automotive paints are summarized.