Compact Silicon-Micromachined Wideband 220–330-GHz Turnstile Orthomode Transducer

This paper reports on a turnstile-junction orthomode transducer (OMT) implemented by silicon micromachining in the 220-330-GHz band. Turnstile OMTs are very wideband and allow for co-planar ports but require accurate and complex geometries, which makes their fabrication challenging at higher frequencies. The compact 10 mm × 10 mm × 0.9 mm OMT-chip presented in this paper is the first micromachined full-band OMT in any frequency range and only the second turnstile OMT implemented above 110 GHz. The measured insertion loss (0.3 dB average, 0.6 dB worst case) and the cross polarization (60 dB average, 30 dB worst case) over the whole waveguide band represent the best performance of any wideband OMT, regardless of design or fabrication technology, in the 220-330-GHz band. The return loss with 22 dB average (16 dB worst case) is comparable with or better than previous works. This paper discusses design considerations and compromises of this complex 9-layer silicon micromachined device, including the influence of side-wall slopes, underetching, and postbonding misalignment between the chips. It is shown that for a device that is very sensitive to geometrical variations, such as a turnstile OMT, it is necessary to anticipate and compensate for any fabrication imperfections in the design to achieve high RF performance.