Conductor loss in thin-film transmission lines

Abstract

By means of the surface impedance applicable to a finite film thickness, the conductor loss of a transmission line is investigated. It is found that there exists an optimum conductor thickness at which the conductor loss becomes a minimum. This thickness is predicted to be π/2 of the skin depth. Next, by using the thin-film microstrip line with varying conductor thickness and width, the measurement of transmission loss is carried out. It is confirmed that the optimum conductor thickness is near π/2 times the skin depth. In the process of investigation, the relationship with the surface current density in the incremental inductance method, often used as a simple computation method of conductor loss, is presented. The calculation of the conductor loss of a multilayered conductor is made possible. Previously, it has been considered that the conductor loss is sufficiently small if the conductor thickness is more than three times the skin depth. It is shown in the present study that the conductor loss becomes small with a conductor thickness about one-half as great. In addition to the increased performance of a transmission line, a contribution to reduction of both the consumption of valuable interconnect materials (such as gold) and of fabrication cost is expected. © 1999 Scripta Technica, Electron Comm Jpn Pt 2, 82(10): 83–91, 1999