Miniature 60 GHz slow-wave CPW branch-line coupler using 90 nm digital CMOS process

The performance of a miniature 60 GHz branch-line coupler (BLC) based on a slow-wave coplanar waveguide (SW-CPW), implemented in the back-end-of-the-line of a low-cost 90 nm digital CMOS technology, is reported. Design insights addressing the coupler structural dimensions, artificial dielectric, and loss are discussed. The BLC achieves a measured insertion loss of 2.8 dB, isolation of 25 dB at 60 GHz, and return losses (all four ports) of greater than 10 dB from 46 GHz to at least 67 GHz. The measured amplitude imbalance and phase difference are 0.8 dB and 90.3° at 60 GHz, respectively. By utilising the slow-wave technique, an area reduction of >75% compared to a conventional CPW design has been accomplished with a compact coupler core size of only 290×285 µm2 (0.083 mm2).