Design and characterization of a 180-GHz on-chip integrated broadband balun

This paper presents the design and characterization of a passive planar balun centered at 180 GHz for monolithic integration. A novel approach for on-wafer characterization is presented and analyzed, which relies on via-hole structures and their metal pads to achieve on-chip termination with high return loss. The balun characterization shows a mean insertion loss of 1 dB and a return loss better than 12 dB between 140 and 220 GHz. The phase difference is 177.2° at 180 GHz and varies almost linearly around 180° over the desired frequency range. The phase balance is within ±5° over a 15-GHz bandwidth between 174 and 189 GHz; the chip area is below 0.05 mm2.