# Waveguide Manufacturing Technologies for Next-Generation Millimeter-Wave Antennas
## 4 Stereolithography (SLA) and Post-Metallization Process  
An alternative that avoids the main drawback of DMLS (i.e., a rough finish of some internal surfaces) is provided by other technologies based on ultraviolet curable resins, such as stereolithography (SLA), which are based on the solidification of the resin layer-by \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & [36] & [37] & [38] & [39] \\ \hline N\({}^{\circ}\) of elements & 4 \(\times\) 1 & 4 \(\times\) 4 & 4 \(\times\) 1 & 20 \(\times\) 1 \\ \(f_{0}\) (GHz) & 17.0 & 28.0 & 5.5 & 9.4 \\ Scanning range ( \(\circ\) ) & 50 & 40 & 90 & 14 \\ Power losses (dB) & 0.46 & 8.50 & 1.50 & 0.73 \\ Measured gain (dB) & 12.0 & 10.7 & 11.0 & 19.1 \\ BW @ 12 dB & 23.5 & 9.0 & 12.8 & 4.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of phased arrays with beam-steering capabilities. Figure 9: H-plane cut (\(\phi=0\)) of the radiation pattern produced by the waveguide array for different scanning directions \(\theta_{0}\). The solid line corresponds to the measurements and the dotted line corresponds to the simulations. layer [26]. This technology allows the correct manufacturing of small elements and has low tolerances of approximately \(\pm\) 50 \(\upmu\)m. However, the prototypes manufactured with SLA require an additional plating process, which can be critical at high frequencies due to metal-to-plastic adhesion problems, an unacceptable metal surface roughness, or areas of poor metallization. Nevertheless, if metallization processes are well-controlled, reduced layer thicknesses (5-10 \(\upmu\)m) with a smooth finish can be achieved. Likewise, metal coating on plastics presents some mechanical deficiencies that cannot be obviated. This is a well-known technology, especially in the jewelry and souvenirs industries. Nevertheless, the metallic coatings used in these industries are poor metal alloys that cannot be used in microwave devices or antennas due to the conductivity required for efficient performance. On the other hand, the metal coating added to jewelry or souvenir pieces can present interesting mechanical properties that could be sufficient for some uses in the assembly process of RF devices. The key point is the preparation stage (prior to metallization) applied to the polymerized resin used in SLA plastic 3D printing, which can ensure a strong, uniform, and durable adhesion of the metal to the plastic. If this preparation stage of the polymerized resin is not properly executed, the external copper coating becomes fragile, crumbling and peeling like an eggshell. This effect can be seen in Figure 10, where two photos taken in our laboratory in a 3D printer show some initial prototypes manufactured with another companies' external copper coating. Swissto12 [40] patented a copper metal plastic chemical coating that has been successfully used up to millimeter wave band [41; 42; 43; 44; 45; 46]. This chemical process allows a homogeneous metallic coating to be obtained with a very low roughness (much lower than the typical roughness of DMLS technology). Thus, the combination of a high-precision stereolithography process with a subsequent low roughness metallization process allows the fabrication of passive devices and RF antennas at very high frequencies. For example, some designs of Ka-band antenna elements or microwave devices, such as a monolithic V-band OMT, were presented in [41]. All of them showed a high-quality response, as well as high agreement between the simulations and measurements. The transmission of W-band rectangular waveguides fabricated by SLA and metallized by the Swissto12 chemical process was shown in [42]. An effective surface roughness of a few hundred nanometers and an effective copper conductivity of 2 \(\times\) 10\({}^{7}\) S/m were estimated. Similarly, a Ka-band monolithic 2 \(\times\) 2 horn array with a very complex topology was also presented with very satisfactory results.