Theoretical analysis and simulation of micromachined THz waveguide embedded in LTCC multi-layer packaging substrate for high throughput data exchange backbone and vacuum electronic devices applications

Micromachined waveguides embedded in LTCC (Low Temperature Co-fired Ceramic) multi-layer packaging substrate are excellent candidates for vacuum microelectronic devices in submillimeter wave or terahertz band, and data exchange backbone structures with high data throughput capabilities in highly integrated system-in-package (SIP). In this paper, theoretical and numerical simulation on the fundamental characteristics of a folded waveguide (FWG) and a directional coupler of ridge waveguide in terahertz band are discussed. The dispersion and interaction impedance, which are the two key parameters of cold-test characteristic of a folded waveguide, are derived from the equivalent circuit of the structure and numerical simulation. With frequency rising, interaction impedance gradually decreases, which means attenuation increases in the high frequency band. This can be improved by including more structure cycles to get a larger phase shift. Then the coupling coefficient and directivity of the directional coupler are analyzed by small aperture theory and simulated by HFSS. At the frequency of 134GHz, the coupling is designed to be 50dB and the directivity is above 20dB at a passband of 31%. Finally, the difference between theoretical solutions and simulation results is interpreted.