Microscopic Analyses of Electrical Conductivity of Micromachined-Folded Waveguides Based on Surface Roughness Measurement for Terahertz Vacuum Electron Devices

Manufacturing slow-wave vacuum electron device (VED) components operating in a terahertz (THz) region required the use of proper techniques and detailed plans. Moreover, how to fabricate components and evaluate manufactured ones are essential in identifying defects, checking their performance, and finding what needs to be improved. This study introduces estimated conductivity based on the surface roughness and evaluations of fabricated folded waveguides (FWGs), which is one of the THz slow-wave VED components, using nanoscale precision computer numerical control (nano-CNC) machining. Produced circuits operating in the G-band (0.11-0.3 THz) and Y-band (0.325-0.5 THz) were diagnosed in terms of the RF transmission loss, achievable surface roughness, tolerance, and flatness. During testing, our devices exhibited the highest reported R
_α
(arithmetic mean surface roughness) at less than 11.17 nm in the Y-band. The conductivities based on the measured surface roughness were estimated in the G-band and the Y-band. Through a simulation and an experiment, we analyzed the reasons for the losses caused by the reduced electrical conductivity. The main contribution of this study is the demonstration of an achievable conductivity of the THz FWGs fabricated with nano-CNC machining and an improved surface finishing capability of the fabrication technique using lubricants.