Low-Characteristic-Impedance Transmission Line of a Benzocyclobutene-Based 3-Dimensional Structure at Millimeter-Wave Frequencies

A variety of three-dimensional (3-D) transmission line structures were fabricated using low-dielectric-constant benzocyclobutene (BCB) interlayers to achieve low characteristic impedances (Z ₀) at millimeter wave frequencies. The effects of structural variations on the Z ₀ of the transmission line structures were examined at a frequency of 50 GHz. Two different 3-D coplanar waveguide (CPW) structures, the bottom-ground and the double-ground structures, showed very low Z ₀ values of ∼7.3 and ∼9.4 Ω, respectively, at a signal linewidth of 100 μm and a bottom-ground spacing of 20 μm. When the ratio of the spacing of bottom-ground width to the signal linewidth became greater than 2.5, the Z ₀ was nearly saturated, showing a minimum Z ₀. The thin-film microstrip lines fabricated using the BCB inter layers showed a higher minimum Z ₀ of ∼25.5 Ω at a ground plane width of 55 μm and a signal linewidth of 35 μm than the 3-D CPW structures did. Among the 3-D transmission structures, the bottom-ground CPW was shown to be the most efficient structure to achieve a low Z ₀ by maximizing the capacitance between the signal and the ground lines at 50 GHz.