Fabrication and microwave characterisation of multilayer circuits for MMIC applications

A technique to fabricate novel multilayer monolithic microwave integrated circuit (MMIC) structures using polyimide as insulating dielectric layer is described. The polyimide layer formation, curing and dry etching processes are investigated in an attempt to obtain high quality dielectric layers suitable for MMIC applications. By employing this technique, MMIC design and circuit routings become much easier and conventional microstrip transmission lines and lumped passive components can be realised with effective use of the substrate material. In the paper it is shown that a V-shaped MMIC coplanar waveguide (CPW) transmission lines with low characteristic impedance and edge offset broadside directional couplers can easily be designed using the multilayer technique. This implementation can avoid the well known current crowding effects on the conductor edges. Within the Ku and K band frequency range, circuits have been characterised and show good performances. By using the same technique, miniaturisation of a lumped-distributed CPW ring-resonator bandpass filter can be achieved with built-in shunt capacitors. Two transmission poles and two transmission zeros could be implemented by introducing two discontinuities. The measured results show a 3 dB bandwidth of 3.7 GHz at 13.7 GHz