Low-Loss Microcoaxial Rat-Race Hybrid for Si-Based Microwave Integrated Circuits

This letter presents a microcoaxial rat-race hybrid for Si-based Ku-Band microwave applications. The device fabrication is conducted using a low-thermal-budget multilayer Cu Damescene process, which is fully compatible with CMOS back-end-of-line (BEOL) technology. The reported on-chip rat-race hybrid shows a best transmission loss (0.32 dB for sum ports and 0.25 dB for difference ports) of the reported on-chip rat-race hybrids. The demonstrated rat-race hybrid has isolation better than 20 dB and signal reflection of all ports less than -12 dB in the operation band, which is corresponding to a fractional bandwidth (FBW) of about 34% and 29% in the symmetrical and differential modes, respectively. As the lines of the hybrid are folded to closely routed meander structures, more than 75% of size reduction is achieved. With these advanced features, the microcoaxial rat-race can be used for the high performance Si-based microwave applications.