D-Band SIW Transitions in GaAs IPD Technology: Triple-Stack Vertical Flip-Chip and WR-06 Interconnects

This paper presents a D-band transition architecture based on a three-substrate GaAs integrated passive device (IPD) platform, combining a dielectric-loaded WR-06 waveguide interface with two flip-chip vertical SIW transitions. The multi-substrate SIW path enables compact 3D routing and provides a symmetric back-to-back (B2B) configuration for de-embedding. A quartz-loaded WR-06 aperture is employed to enhance TE10-to-SIW mode conversion, while Au-bump vertical posts ensure low-loss SIW continuity across substrates. Measurements using a 140–220 GHz setup with AFR de-embedding shows a 144–163 GHz bandwidth with S11 < –10 dB and a 4.9 dB B2B insertion loss at 153.5 GHz. After subtracting the 4 dB SIW propagation loss associated with the 18.68-mm SIW path, the intrinsic loss of all transitions is approximately 0.9 dB. These results demonstrate that the proposed multi-substrate GaAs IPD structure achieves low-loss and packaging-compatible D-band interconnects suitable for sub-THz system integration.