Study on an evaluation technique for millimeter-wave circuit substrate materials

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In design of millimeter-wave planar circuits, it is necessary to evaluate the copper-clad substrates. The relative permittivity of the insulator and the surface resistance of the copper should be measured at the millimeter-wave frequency band. Most of the insulator for the circuit substrate consist of the laminated glass-cloth sheets which are contained the low-loss plastic resin. Therefore, its relative permittivity has anisotropy generally for their dielectric properties. The circuit substrate suppliers, however, hardly show us data of the anisotropic permittivity in their catalog. The circuit designers frequently are troubled with the disagreement between the measured and pre-estimated results for the test-manufactured circuits or devices.On the other hand, for the conductor properties, the influence of the boundary situation between the insulator and the copper film on its effective conductivity should be evaluated to estimate the conductor loss of the designed circuits.In this study, we investigate a measurement method to evaluate the anisotropy of the relative permittivity of the dielectric laminate substrates using Whispering-Gallery mode dielectric resonator method, at first. Some dielectric substrate materials are measured in the 50 to 75 GHz band and the anisotropic relative permittivities are evaluated. The measured results of the relative permittivitiy in planar direction are compared with the measured results by the cut-off cylindrical wave guide method. Secondary, we investigate the measurement techniques to evaluate the millimeter-wave surface resistance of the conductor materials.