A differential vertical hybrid coupler and low capacitance RF pads for millimeter-wave applications in 28 nm CMOS FDSOI

As millimeter-wave applications mostly rely on differential topologies, a differential design for quadrature hybrid coupler is also desired. The proposed Differential Vertical Hybrid Coupler (DVHC) concept is presented and dimensions are given for a 75 GHz operating frequency using 28 nm CMOS FDSOI technology. Simulated insertion loss is only 0.5 dB while return loss and isolation are close to 30 dB exhibiting very weak frequency dependence on all ports. Measurements on test structures confirm wideband operation and low insertion loss. Additionally, a flip-chip compatible RF pad with ultra-low parasitic capacitance is designed here and characterized (11 fF reported at 75 GHz).