A Millimeter-Wave Miniature Branch-Line Coupler in 22-nm CMOS Technology

A 60-GHz innovative quadrature branch-line coupler is proposed in this letter. The device was implemented in a 22-nm CMOS process, from global foundries. The proposed design merges two miniaturization techniques. On the one hand, slow-wave microstrip (SW-MS) lines based on bed of nails are demonstrated in CMOS technology for the first time. On the other hand, the two quarter-wavelength MS line sections required for the branch-line coupler implementation are realized using the SW-MS transmission line configurations with different form factors. As a result, the achieved miniaturization versus electrical performance is very good as compared to the state-of-the-art. Design insights addressing the coupler dimensions and losses are discussed. The experimental assessment shows an adequate frequency response with amplitude and phase imbalance of 0.8 dB and 1.8 degrees at 60 GHz, respectively. The measured insertion loss is 2.35 dB at 60 GHz and the return loss is better than 15 dB at any port (from 54 to 66 GHz). The isolation is better than 15 dB (from 51 to 68 GHz). The overall size of the coupler is 0.3 mm x 0.22 mm, which is equivalent to 0.06.lambda(0) x 0.044.lambda(0), at 60 GHz.