Microfabrication of folded waveguide using UV-LIGA for 220 GHz traveling wave tube

Folded waveguide slow-wave structure as a traveling wave tubes main component is becoming smaller and smaller with vacuum electron devices operating frequency rises towards THz. And the fabrication tolerances are becoming increasingly critical. Traditional methods dont adapt to these micro-scale ranges. UV-LIGA is an attractive technique for fabricating this structure. This paper focuses on microfabrication techniques using SU8 photoresists to create this circuit at 220 GHz and a special mould made by PDMS and embedded polymer monofilament to assure the thickness and shape of the beam tunnel. By optimizing the process parameters, using gravity-assisted deposition technique, SU8 mold with coincident thickness and smooth surface is realized. After electroforming in a bath composed with sulfuric acid and copper sulfate and drawing out polymer monofilament, the microstructure is fabricated.