THz-TDS Reflection Measurement of Coating Thicknesses at Non-Perpendicular Incidence: Experiment and Simulation
Time-domain spectroscopy (TDS) in the Terahertz (THz) frequency range is gaining 1 in importance in nondestructive testing of dielectric materials. One application is the layer 2 thickness measurement of a coating layer. To determine the thickness from the measurement data, 3 the refractive index of the coating layer must be known in the surveyed frequency range. For 4 perpendicular incidence of the radiation, methods exist to extract the refractive index from the 5 measurement data itself without prior knowledge. This paper extends these methods for non6 perpendicular incidence, where the polarization of the radiation becomes important. Furthermore, 7 modifications considering effects of surface roughness of the coating are introduced. The new 8 methods are verified using measurement data of a sample of Inconel steel coated with yttria9 stabilized zirconia (YSZ) and with COMSOL simulations of the measurement setup. To validate 10 the thickness measurements, scanning electron microscopy (SEM) images of the layer structure 11 are used. The results show good agreement with an average error of 1 % for the simulation data 12 and under 4 % for the experimental data compared to reference measurements. 13