Design, Fabrication, and Characterization of an Air Dielectric Semi-Coaxial Line

This paper describes a semi-coaxial transmission line (SCTL) which utilizes air as the working dielectric and is fabricated on a non-conducting substrate using a simple surface micromachining process. The transmission line design is suitable for both on and off-chip transmission lines and shows good performance till 100 GHz. The semi-coax design shows low loss and performance approaching that of a full-air dielectric rectacoax. The center conductor comprises an approximately 20 μm tall SU-8 core support, a thick electroplated semi-cylindrical outer shield, and an enclosed air cavity. The three-sided coated SU-8 conductor has a very low electric field in the SU-8, so the design reduces dielectric loss even in a poor dielectric like SU-8. This obviates the need for suspended lines, which have reliability issues. A semi-coax line with a characteristic impedance of 37-41 Ω has been fabricated on a Borofloat glass substrate with copper conductors. The line characteristics have been measured from 1 to 100 GHz and showed the attenuation constant of 0.13 dB/mm at 30 GHz and 0.42 dB/mm at 97.5 GHz for a semi-coax transmission line (SCTL). The effective relative permittivity is measured to be 1.88. The measurements align closely with HFSS simulations, affirming the reliability of this semi-coaxial transmission line. The design is particularly well-suited for applications within the V and W band frequency ranges, demonstrating its potential for advanced high-frequency applications.