Direct Bonding Technique for Integration and Vertical Interconnection of Metal-Air Coaxial Lines

This article introduces a novel application of thermocompression bonding (TCB) for the 3-D integration of metal-air coaxial lines (MACLs). The approach leverages a prestressed protrusion of the inner conductor to ensure reliable metal-to-metal contact, enabling multilayer interconnection without solder or interposers. Double-layer MACL prototypes were fabricated and evaluated through electromagnetic and thermomechanical simulations, fabrication tolerance studies, and reliability testing. The bonded interconnects achieved insertion loss (IL) below 0.45 dB, isolation exceeding 58 dB across the 1-43-GHz frequency range, and shear strength of up to 8 MPa. The proposed method combines broadband, low-loss transmission with robust mechanical integrity while mitigating issues associated with thermal expansion mismatch. To illustrate its applicability, a compact switch-based true time delay unit (TDU) was demonstrated, further underscoring the potential of TCB-enabled MACL for ultrawideband, highly integrated radio frequency (RF) components and microsystems.