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Microwave circuits are popularly formed on multi-layered organic or non-organic substrates. In the microwave circuits, the attenuation of planar transmission lines such as striplines, microstrip lines, and coplanar lines are determined by their conductor loss, dielectric loss and radiation loss. Among them, the conductor loss is a major factor in the attenuation of the planar transmission lines. A new measurement method is needed to evaluate the conductivity of transmission line on or in the substrates such as the organic, ceramic and LTCC (low temperature co-fired ceramics) substrates.

The IEC 61338-1-3 described the measurement method for the surface resistance R_s and effective conductivity σ on the surface of the conductor. The term σ is designated as σ_s in this PAS, and is called surface conductivity (Figure 1). This PAS describes a measurement method for resistance and effective conductivity at the interface between conductor layer and dielectric substrate designated as R_i and σ_i respectively, and are called interface resistance and interface conductivity.

For the transmission line in the substrates, the electric current is concentrated at the interface between conductor layer and dielectric substrate, because the skin depth δ in the conductor is the order of μm in thickness at the microwave frequencies. In microstrip lines, the current is concentrated at the interface, rather than at the open face of the conductor. Furthermore, in copper-clad organic substrates, the interface side of the copper foil has rugged structure to hold the strong adhesive strength. In LTCC substrates, the interface between the conductor and ceramics has a rough structure, depending on the co-firing process and the material compositions. The interface conditions increase the conductor loss. Therefore, the evaluation of R_i and σ_i is important to design microwave circuit and to improve the conductor fabrication process.

This measurement method has the following characteristics:

- the interface resistance Ri is obtained by measuring the resonant frequency f₀ and unloaded quality factor Q_u of a TE_01δ mode dielectric rod resonator shown in Figure 2;

- the interface conductivity σ_i and the relative interface conductivity σ _ri =σ _i / σ ₀ are calculated from the measured R_i value, where σ ₀ = 5,8x10⁷ S/m is the conductivity of standard copper;

- the measurement uncertainty of σ _ri ( Δσ _ri ) is less than 5%.