Metal additive micro-manufacturing to achieve enhanced air-bridge geometry for coplanar waveguide mm-wave GaN-on-SiC integrated circuits

This paper presents a novel cavity coplanar waveguide (CCPW) structure based on GaN-on-SiC technology for high-power microwave applications. The CCPW structure was fabricated using an emerging monolithic microwave integrated circuit (MMIC)-compatible localised electrodeposition metal additive micro-manufacturing (μAM) process, achieving an air-bridge height of 50 μm. Electromagnetic (EM) simulations revealed that introducing a cavity above the CPW improves impedance matching at mm-wave frequencies while providing a robust ground-return path. S-parameter measurements show that the CCPW provides a 6.5 dB improvement in reflection coefficient at 110 GHz compared to a standard coplanar waveguide (CPW) structure. Furthermore, both simulations and measurements indicate a broadband reflection coefficient trough suggesting the potential for broadband impedance matching in MMIC applications. To further analyse RF parasitics, a high-frequency equivalent circuit model was developed, demonstrating significant performance improvements of the CCPW compared to a printed air-bridge.