Suspended Slow-Wave transmission lines for mm-wave applications

This paper presents and validate the concept of a suspended, slow-wave microstrip transmission line that uses air as a substrate. The lines are fabricated on a low-cost interposer technology, the metallic nanowire membrane (MnM), that allows selective growth of copper nanowires, enabling transmission lines with a wide range of Zc. Lines with widths of $35 \mu \mathrm{m}, 25\mu \mathrm{m}$ and $15\mu \mathrm{m}$ were designed and fabricated with 1, 2 or 4 suspended segments of $250\mu \mathrm{m}$ or $500\mu \mathrm{m}$ of length. Parametric extraction from the measured S-parameters showed $\varepsilon_{eff}$ ranging from 5 to 7.5, $\alpha$ smaller than 0.8 dB/mm at 70 GHz and Q as high as 55. Z c varied between $65 \Omega$ and $90 \Omega$.