3D PRINTING

The advantages of 3D printing have allowed it to replace traditional fabrication methods for THz components. Rapid prototyping and the ability to fabricate design prototypes using a broad range of dielectric and metallic materials are the key advantages of the technique. Low material loss, little hazardous waste generation, the ability to precisely manufacture devices with microscale features, the ease of repeatability and the possibility of fabricating a wide range of components on planar and nonplanar structures using a single tabletop system are other advantages of additive manufacturing. Various 3D-printed antennas used for 5G applications were presented by Rui Xu et al.17 The impacts and future advancements in antenna fabrication is also investigated.

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3D-Printed Active and Passive THz Components

In the paper by Shu-Yan Zhu et al.,18 imprint and dry etching technologies are used to design a Gaussian beam antenna. The paper describes a 100 μm thick silicon substrate etched with a 50 μm × 190 μm slot pattern. The higher aspect ratio demonstrates the advantage of using dry etching technologies. The DRIE process is used to form pillar-shaped photoresist squares. The feed was fabricated by dry etching and optical lithography in a fabrication process shown in Figure 5.

Figure 5

Figure 5 Fabrication technology for silicon THz antenna feed.18

Figure 6

Figure 6 (a) Metal film deposition and laser surface patterning.21 (b) Optical image of metamaterial surface.21

A two-step 3D printing is used to print a D-Band antenna in the paper from Chao Gu et al.19 In this example, an antenna prototype is developed that uses metal printing followed by dielectric printing. The structure described in the paper gets 14.2 percent bandwidth and 15.5 dBi gain at 135 GHz, with the gain value resulting from gain enhancement techniques. Broadband propagation characteristics are achieved by using a dielectric ridge waveguide. 3D printing and gold sputtering are combined in the fabrication of topological waveguides in the paper from Muhammad Talal Ali Khan et al.20 The fabricated metallic waveguide with air channels described in this paper can be used in high speed interconnects.

LASER-BASED TECHNIQUES

Developments in laser-based manufacturing methods enable the fabrication of ultrathin structures. Transparent conducting surfaces are fabricated on dielectric substrates using laser-based techniques as described by Qinghua Wang et al.21 Two-step laser-based fabrication is used to achieve ultrathin THz bandpass filtering. The dielectric is coated with a 10 nm thick metal layer and patterns are formed by laser ablation. A nano-pulse laser is used to pattern the metal deposition. Surface resistance and visible transmittance of the fabricated structures are measured using THz time-domain spectroscopy. Figure 6a shows the thin metal film deposition and laser surface patterning used in this article. Figure 6b shows an optical image of the laser-based metamaterial surface patterned on an 8 nm thick copper film.

The direct laser writing method is used to fabricate a terahertz metamaterial absorber based on the fractal structure for wideband applications in an article from Hou-Bing Liu et al.22 From this paper, a negative glue covers the dielectric layer and the patterns are formed on a photoresist using direct laser writing, followed by metal deposition with magnetron sputtering. A laser is used in various processes such as engraving, melting, drilling, cutting, ablation and patterning of THz component fabrication. Direct laser patterning (DLP) is adapted as a suitable maskless method, in the fabrication of various micro apertures and surface patterns in different materials.23 Changes in design can be easily adapted in femtosecond laser DLP techniques since it uses a maskless printing method.

CONCLUSION

This article has presented an overview of recent developments in advanced fabrication techniques suitable for the THz regime. THz frequencies hold much promise and they are attracting substantial attention and investment. The availability of well-understood manufacturing techniques opens the opportunity to explore the promise of the THz frequency band for next-generation communication systems.

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