Material Characterization by Millimeter-Wave Techniques

Abstract: This master thesis investigates material characterization by reflection and transmission of electromagnetic waves in the 40-60 GHz band (millimeter-wave spectrum) for different materials. The free-space measurement method is a fast, efficient and non-destructive way of examining a material and is being researched by both academics and industries. The theory of how electromagnetic waves interact with different materials such as dielectrics and conductors is reviewed as well as how the reflection and transmission from such materials can be computed theoretically. This theory is partially derived from Maxwell’s equations. From this theory, simulations are performed to get signal levels of reflection and transmission for different materials and varying material parameters. From the simulations it is shown that certain materials are better examined in either transmission or reflection. Measurements were performed in time domain (with a wavelet generator and an oscilloscope) and in frequency domain (with a network analyzer). Both reflection and transmission were measured for all samples. Four samples were investigated thoroughly: two PMMA (Poly(Methyl MethAcrylate)) samples, one silicon sample and a thin gold film sample. Before the measured data can be compared to the simulated, it is necessary to apply signal processing to both the measured and the simulated data. This is done to make sure the comparison of the two data sets works and it consists of removing multiple reflections and other unwanted noise from the signal. The material characterization could then be performed, by extracting a specific material parameter, such as permittivity or conductivity. This is done by comparing simulated data iteratively to measured data. The best fit should then, in theory, correspond to the actual material parameter. The material characterization worked, although sometimes differences in time and frequency domain were found. Permittivity values were extracted for the PMMA samples and conductivity values for the silicon and thin gold film samples. The values extracted compared well with reference values for the PMMA samples and the thin gold film sample.