High-impedance multi-conductor transmission-lines for integrated applications at millimeter-wave frequency

This paper describes novel integrated transmission lines based on slotted-floating shield for ultra-wide band operation at millimeter-wave frequencies. The designed structures consist of two main conductors used for signal and ground, along with a floating conductor, which improves the performance of the line. The implemented designs increase the line-impedance and reduce the wavelength of the guided signal with respect to normal CPW and microstrip solutions, while retaining low propagation losses. Four different designs with different floating conductor geometries are realized and presented. All configurations aim at avoiding the penetration of the electromagnetic field into the process substrate and flow of return currents into the thinnest metals of the process. Both effects are known to be the main source of propagation losses for integrated transmission lines. Experimental demonstration is provided with the fabrication of test-structures in a 130 nm SiGe BiCMOS process. The simulation results have been validated against on-chip measurements, de-embedded with an Open-Short technique. Agreement between measurements and simulations is demonstrated in the frequency range of 1 GHz to 50 GHz. Simulations of the cross-section field-distribution are also shown, confirming the reduction of return currents into the lossy substrate. An increase in line impedance of 18% and 44% against conventional CPW and microstrip is measured for equal cross-section area and conductors width. At the same time, the propagation losses are lowered by 7% and 15%, and the slow-wave nature of the lines reduces the guided wavelength by 21% and 26% against the two conventional structures.