On-Chip Air-Gapped Cavity Resonators and Filters for mm-Wave IC Applications

This paper presents the detailed design, fabrication, and measurement of cavity resonators and filters for silicon (Si)-based millimeter-wave integrated circuits. The design of the cavity resonators and filters is conducted using Ansoft High Frequency Structure Simulator. The cavity resonators and filters are fabricated on a 4-in Si substrate using the air-gapped microcoax technology, which is based on a multilayer low-temperature copper Damascene process. The fabricated cavity resonators have measured unloaded quality (Qu) values of 666 and 864 for 36 and 60 GHz, respectively. Two-pole bandpass cavity filters have achieved an insertion loss close to 0 dB and a return loss better than 20 dB for both 36and 60-GHz passbands. The cavity resonator and filter have simple structures and avoided excessive global optimization for the design. Hence, the accuracy, time, and complexity of the design can be improved significantly. The fabrication technology demonstrates the advantages of ultralow loss and dispersion, perfect impedance matching, pure transverse electromagnetic propagation mode, and ultrahigh isolation and power-handling capability. Furthermore, the process has a low thermal budget and is fully compatible with CMOS back-end-of-line (BEOL) process, which can be integrated with other air-gapped microcoaxial components to realize a high-performance microwave system on chip.