Broadband CPW multilayer directional couplers on GaAs for MMIC applications

This paper describes the design and characterisation of wide band multilayer directional couplers, fabricated on GaAs semi-insulating substrates with two conductive layers of gold separated by polyimide as a dielectric layer, operating at a centre frequency of 24 GHz. These components are very useful in the construction of multilayer MMICs and are compact and easy to design employing multilayer technology. High frequency simulations were carried out using Agilent´s Advanced Design System (ADS). In the design iteration, the thickness of the dielectric layer was varied to optimize the coupling and the isolation factors. The effects of metal overlap to improve the isolation factor of the directional couplers have been investigated in full. In this work, on-wafer RF measurements were carried out on the fabricated directional couplers using a HP85107A network analyser. Directional couplers with 90° phase angle, coupling factor of 5 dB and isolation of 10 dB were achieved over the 10 to 35 GHz frequency band. We further compare the results of simulations with those of measured data for final design optimisations.