X-ray LIGA Microfabricated Circuits for a Sub-THz Wave Folded Waveguide Traveling-Wave-Tube Amplifier

We report experimental results on the microfabrication of three types of folded waveguide circuits with a high aspect ratio by using a high-energy X-ray LIGA technique. A W-band folded waveguide with a total of 80 periods was designed for a traveling-wave-tube (TWT) amplifier operating at 90 GHz–100 GHz. The measured return loss and insertion loss of the W-band fabricated circuit were–10 dB and 0.42 dB/cm, respectively. A G-band circuit showed a higher insertion loss of 1.57 dB/cm. An additional circuit fabrication for a 850-GHz folded waveguide TWT was also successfully carried out. The broad-wall angle measured with respect to the narrow wall of the rectangular waveguide was very close to 90◦, indicating an excellent resolution compared to that of other microfabrication techniques, such as deep reactive ion etching (DRIE) and ultraviolet (UV) lithography, electroplating, and polymer replication (LIGA).