Nanoscale surface roughness effects on THz vacuum electron device performance

Vacuum electron devices are the most promising solution to generate power at Watt level at millimeter waves and terahertz frequencies. The three dimensional nature of metal structures required to provide an effective interaction between an electron beam and THz signal poses relevant fabrication challenges. At the increase of the frequency, losses are a relevant detrimental effect on performance. In particular, the skin depth, in the order of one hundred nanometers or less, constrains the maximum surface roughness of the metal surfaces below those values. Microfabrication techniques were proved in principle to achieve values of surface roughness at nanoscale level, but the use of different metals and affordable microfabrication techniques requires a further investigation for a repeatable quality of the metal surfaces. This paper will discuss on the nanoscale issues of metal waveguides for a 0.346 THz backward wave tube oscillator and a 0.22 THz traveling wave tube.