60 GHz optimised nickel-free gold-plated enclosed coplanar waveguide liquid crystal phase shifter

Abstract

A novel nickel-free gold-plated enclosed coplanar waveguide is experimentally demonstrated for a liquid crystal tunable delay line phase shifter at 60 GHz. By numerical investigations, we find that conventional surface impedance model (without meshing inside metals) fails to accurately characterise the effect of gold-plating thickness on conductor loss, especially when the plating thickness is smaller than 4 times of the gold's skin depth. To address this, adaptive meshing inside metals is performed for accurate modelling of the device. The gold-plating thickness is then experimentally optimised and tailored for the liquid crystal-based device-making procedures, i.e. taking into account the accelerated copper migration into gold problem during the alignment agent baking process at 200°C. Measurement results of assembled phase shifters at 60 GHz verify that a direct gold plating of 2μm on Cu can reduce the insertion loss by 1.79 dB and 2.85 dB as compared with that with a thinner plating of 50 nm, and no plating, respectively. The maximum insertion loss of -2.996 dB demonstrated at 60 GHz for the 0-π phase shifter delivers a figure-of-merit of 60°/dB, an improvement of 42°/dB as compared with up-to-date literature.