Metallic 3D Printed Rectangular Waveguides (WR3) for Rapid Prototyping of THz Packages

In this work, laser beam melting (LBM) is used for direct 3D printing of hollow-metallic rectangular waveguides (WRs) within the WR3-band (230-320 GHz). By using laser beam melting of stainless steel (SS) 316L powder, a WR3 waveguide is fabricated in a single 3D printing process. This eliminates the need for costly mechanical and chemical post-printing processes required in traditional micromachining and non-metallic 3D printing technologies. In addition, the metallic LBM waveguides outperform the non-metallic ones in mechanical robustness. On the other hand, the surface roughness for LBM SS316L is typically higher as compared to non-metallic printed structures. For rectangular waveguides, this leads to slightly higher transmission losses. According to these performances, 3D metallic printing can be considered as a suitable technology for rapid prototyping, e.g. for high-frequency photodiode (PD) packages, where complex PD packages are printed in a single fabrication step. Since the length of the rectangular waveguide in the terahertz photodiode package is only a few millimeter long, slightly higher transmission losses can be accommodated. To investigate the use of LBM technology for THz PD packages, the impact of the surface roughness on the electrical performance of the fabricated waveguides is numerically and experimentally analyzed. The measured average inner surface roughness is about 50 μm, whereas it is about 47.7 μm at the WR3 flange surface, resulting in an RF insertion loss of about 0.7 dB/mm. By smoothing the WR3 flange surface to values as low as 2 μm via a simple milling process, RF losses are reduced down to 0.3 dB/mm.