Micro-Machined Terahertz Orthomode Transducer 69 views

Terahertz receivers used in radar, radiometry, and communications systems require the ability to simultaneously handle two orthogonally polarized signals over a common antenna aperture. This function is performed by the orthomode transducer (OMT), a passive three-port waveguide component that separates or combines two linearly polarized signals at a common port. As operating frequencies increase into the sub-terahertz range, the waveguide dimensions shrink to hundreds of micrometers, and OMT fabrication by conventional CNC machining becomes constrained by achievable dimensional tolerances. This dissertation presents the design, fabrication, and characterization of a silicon DRIE-micromachined WR-1.9 OMT intended for operation across the 400–600 GHz band. A Class I asymmetric T-junction OMT topology was designed in Ansys HFSS, with an 11-step impedance-matching profile optimized down to a six-step profile to reduce fabrication complexity. Integrated 90° port transitions were added to redirect the rectangular output ports into the plane of the silicon wafer, enabling co-planar connection to standard WR-2.2 and WR-1.5 vector network analyzer (VNA) extender heads. Full-wave simulation of the complete OMT with transitions demonstrates return loss greater than 10 dB, insertion loss below 1 dB, and port isolation and cross-polarization greater than 35 dB across the full 400–600 GHz band. Tolerance analysis confirms robustness to ±5 µm fabrication deviations, and the design scales successfully to the WR-1 band (750–1100 GHz). As a process qualification vehicle for the full OMT, a horizontal-polarization back-to-back 90° waveguide transition — which shares identical step depths with the OMT — was fabricated and characterized. The fabrication process employs an ICP-CVD SiO₂ hard mask deposited in multiple stages, each patterned by a polyimide/Al bilayer liftoff sequence. Bosch DRIE etching with SF₆/C₄F₈ cycling achieves a Si/SiO₂ selectivity of approximately 120:1 with thermal management through cycled cooling intervals to prevent step merging during extended etches. Metallization was performed by Cr/Ag sputter deposition (~700 nm Ag) after electroless gold plating proved incapable of achieving adequate film thickness. Three nominally identical devices on the same wafer were measured using a Rohde & Schwarz ZVA67 VNA with VDI WR-2.2 and WR-1.5 frequency extenders and a custom CNC-machined adapter block. Device-to-device insertion-loss variation was approximately 0.3 dB, confirming the uniformity of the DRIE process. The de-embedded back-to-back insertion loss ranged from approximately 2 dB at 400 GHz to 5 dB near 600 GHz, with the adapter block contribution remaining embedded across the band and the measured return-loss resonant structure in qualitative agreement with simulation. These results validate the silicon DRIE micromachining process for the subsequent fabrication of the full WR-1.9 OMT and its eventual scaling toward 1 THz.

PHD (Doctor of Philosophy)

Orthomode Transducers; Terahertz; Micromachining

English

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2026-05-23