# Additive Manufactured Waveguide for E-Band Using Ceramic Materials
## 1 Introduction  
In passive systems for higher frequency ranges above 20 GHz, waveguide technology offers several major advantages over printed circuit boards (PCBs) which are limited to a planar structure by their nature. While small cross-section microstrip lines exhibit relatively high conductor loss scaling in a \(\propto\sqrt{f}\) fashion with frequency, inevitable dielectric loss resulting from the PCB substrate comes on top scaling even in a direct proportionality with frequency [1]. Conductor surface roughness (\(R_{\text{q}}\)) further increases conductor loss effects while at the same time also generating an impact on the phase behavior [2; 3; 4]. Both loss mechanisms are addressed by hollow waveguides. The larger cross-section allows for a reduction in conductor losses, while the absence of lossy laminate material makes dielectric losses negligible. Passive components suhc as waveguide circuits [5], compact couplers [6], horn antennae [7; 8] or slotted waveguide array (SWA) antennae [9; 10] can be realized in waveguide technology. However, those systems are typically rather bulky and weighty due to their full metal nature and conventional manufacturing of such includes expensive micro machining, highprecision milling and assembly. In addition, the complexity of manufacturing increases disproportionately with increasing system complexity. The latter drawbacks could be successfully addressed and overcome by novel additive manufacturing (AM) methods including polymer-based 3D printing, such as digital light processing (DLP) [11] or stereolithography (SLA) [12], and a subsequent metallization process. By making use of the slotted waveguide approach, where non-radiating slots are introduced into the narrow walls of the waveguide, printing and metallization qualities can be enhanced, enabling the fabrication of even relatively complex structures for millimeter-wave (mmWave) frequency range, such as waveguides, or waveguide paths for E- [13; 14] and D-band [15], hybrid couplers for both bands [16; 17; 18], W-band SWAs [19], helix antennae for 77 GHz [20] that can be used in imaging applications [21], wavemode transitions [22; 23] and even entire six-ports from one monolithic model [24]. All those components are made of photopolymer epoxy resins, which offer advantages such as low weight, low surface roughness (typ. \(R_{\mathrm{q}}<1\) um) and a well established plating process. However, those materials have low thermal stability up to 80-300 \({}^{\circ}\)C and poor chemical resistance. Moreover, their suitability for vacuum environments, such as space applications, has not yet been proven. Hence, there is a potential demand for materials that overcome those challenges while at the same time being printable, lightweight and, therefore, meeting the requirements imposed by passive mmWave systems. Ceramic materials offer excellent high-temperature properties, are chemically stable and can be used in vacuum environments [25; 26]. Therefore, they are well suited for the desired applications. In current research, various AM approaches for ceramic materials have been proposed. For example, in [27; 28; 29], fused deposition modeling (FDM) was applied by using polymer-based composite materials. By mixing ceramic fillers into a resin, various ceramic materials can be produced by SLA and DLP [30; 31; 32; 33; 34]. Recent progress in robocasting as described, e.g., in [35], allows for direct writing of ceramic materials. Compared to FDM, SLA and robocasting, DLP enables an almost free design customization, a printing of detailed structures and a short curing time [31] and is, therefore, especially attractive for fabrication of complex structures, such as waveguide geometries. However, AM of ceramic materials has, so far, not been applied to the fabrication of waveguides and systems for mmWave applications. Therefore, this contribution introduces an AM concept and design guidelines for producing ceramic waveguides and waveguide systems. The contribution is organized as follows: The next section describes the novel AM process.