Manufacturing Guidelines for W-Band Full-Metal Waveguide Devices

Manufacturing Guidelines for W-Band Full-Metal Waveguide Devices: Selecting the most appropriate technologyPablo Sanchez-Olivares, Marta Ferreras, Eduardo Garcia-Marin, Lucas Polo-López, Adrián Tamayo-Domínguez, Juan Córcoles, José Manuel Fernández-González, José Luis Masa-Campos, José R. Montejo-Garai, Jesús M. Rebollar-Machain, Jorge A. Ruiz-Cruz, Manuel Sierra-Castañer, Manuel Sierra-Pérez, Mariano Barba, José Luis Besada, Jesús Grajal65map02-sanchez-olivares-opener-3143435IMAGE LICENSED BY INGRAM PUBLISHINGThis article analyzes some of the key aspects of manufacturing techniques for passive components and antennas in the millimeter band, with emphasis on the W band. Even experienced microwave designers might face great technological challenges when upscaling their designs to higher-frequency bands. Thus, several manufacturing technologies are widely analyzed in this work by designing, manufacturing, and measuring different components in the W band. Subtractive manufacturing techniques, such as computer numerical control (CNC) milling and electrical discharge machining (EDM), as well as additive manufacturing techniques, including stereolithography (SLA), direct metal laser sintering (DMLS), and diffusion bonding (DB) are explored. The advantages and disadvantages of these technologies are evaluated using features such as manufacturing tolerances, accuracy, surface roughness, and cost. The purpose is to offer some guidelines for the selection of the most appropriate manufacturing technology in the W band, bearing in mind both the performance of the different manufacturing methods and the particular circuital topologies of the component under development.IntroductionThe millimeter-wave (mm-wave) frequency band is increasingly interesting for low-cost and high-volume applications. These frequencies, including the W band, provide great benefits, such as broad allocated bandwidth and shorter wavelengths. These properties are currently in high demand in a wide range of different scenarios, including wireless communications, imaging, sensing, and life science [1]. In the field of wireless communications, the recent advent of high-speed and ubiquitous Internet access has given rise to increasing interest in the underused part of the spectrum corresponding to mm-wave bands, especially for short-range communications [2]. The need for large data throughputs in future 5G mobile links can be accomplished by the available bandwidth in the W band [3]. In addition to cellular networks, the physical layer for the new standards in Wi-Fi communications has been built in the mm-wave band [4]. On the other hand, mm-wave communication systems have been also proposed for satellite links [5] and military systems [6].mm-Wave and low-terahertz radiations have also been used for imaging in security applications [7] and material characterization [8]. Penetration through a large variety of materials and nonionizing characteristics are the two main properties that facilitate noninvasive inspection and enable the detection of concealed items. These properties of mm-wave radiation and the security threats from terrorism have driven the design and manufacture of scanners for personnel in transportation hubs [9]. The ability of mm-waves to penetrate diverse dielectrics is harnessed for material inspection and characterization and nondestructive testing (NDT) [8]. The techniques developed for locating superficial and hidden defects are the basis for quality control processes in different industries: the pharmaceutical sector [10], construction [11], and food production [12]. NDT and spectroscopy approaches can be combined for the extraction of specific material characteristics and, ultimately, material classification. A possible killer application for mm-wave technology could come from the automobile industry in its effort to develop autonomous driving solutions [13]. This industry requires sensors for precise localization and navigation, and mm-wave radars enable both a high resolution and a detection range of hundreds of meters. These instruments also offer new opportunities for anticollision systems, trajectory estimation, and terrain mapping sensing [14], [15].The aforementioned applications demand the development of all kinds of efficient, affordable, and scalable devices [16], [17]. In the case of passive components, such as filters, multiplexers, antennas, couplers, and so on, the manufacturing process plays a crucial role. Although, from a theoretical point of view, a microwave component can be directly scaled to a higher-frequency band without affecting its functioning, several practical problems arise when trying to build a design in the W band by using the same mainstream subtractive manufacturing techniques as for lower-frequency designs. The main issue to overcome is the appearance of power leakage due to poor electrical contact among the different parts of a split-block device. For this reason, monolithic manufacturing should be adopted whenever possible. Nevertheless, waveguide technology usually uses split-block designs together with certain strategies to prevent power leakage at the seams. A wide variety of alternative waveguide and packaging technologies can be found [17, Tab. 1]. The technology selected to manufacture high-volume structures must be repeatable and affordable.Another relevant issue regarding the manufacturing of prototypes in the W band is their small feature size, which may be challenging to achieve using traditional subtractive techniques. The advent of additive manufacturing in recent years provides new alternatives that can solve this issue. Moreover, alternative techniques, such as electroforming (EF) and additive manufacturing, may bring about the monolithic implementations of devices that were unfeasible using subtractive manufacturing. The authors have experience in the design and manufacturing of 3D printed prototypes [18], [19], [20]. Moreover, the DB technique can be a high-performance solution to build multilayer planar structures with intricate internal channels, which is impractical with conventional additive manufacturing processes [21]. Based on the expertise of the authors in this field, the goal of this work is to provide some useful guidelines to assist a microwave designer experienced in lower-frequency bands (e.g., X, Ku, and Ka bands) and who wants to undertake the design and manufacture of higher-frequency components, in particular, in the W band. To assess the main issues of mm-wave component manufacture, several passive antenna solutions have been designed, manufactured and experimentally evaluated in this band.Subtractive Manufacturing TechniquesWhile manufacturing accuracy is the main concern in the Ku and Ka bands, it becomes a critical factor for W-band prototyping, especially for resonant devices. Thus, high-precision manufacturing becomes essential. With dimensional tolerances of around 20 µm and a surface roughness of N6 (0.8 µm), CNC machining (i.e., milling, turning, boring, and so on) of waveguide blocks is the primary method for manufacturing these devices. CNC machines can preserve their accuracy across the total work volume, avoiding split-block misalignment. Furthermore, they are equipped with massive capacity tool charges, easing all machining operations to be finished in a single clamping. In general, conventional subtractive manufacturing techniques should be mainly preferred by the designer for their high level of technical and economic maturity as long as they are appropriate for the materials that are used and their accuracy and tolerances suffice for the intended application.Another technology suitable for the W band is EDM, also known as spark machining, a metal manufacturing method through which the desired shape is obtained by electrical discharges or sparks using different electrodes (rams or wires). Material is eliminated from the work piece by means of current discharges between two electrodes separated by a dielectric liquid and subjected to an electric voltage. Its tolerance of around 20 µm or better promotes its use in waveguide devices. When the required level of accuracy cannot be realized using CNC milling due to drill diameter restrictions, more accurate techniques such as EDM can be used, although this will increase the cost of the prototype.Ideally, a mm-wave device should be manufactured in a single piece to avoid power losses due to electrical contact issues among different parts. Nevertheless, for components of a certain complexity, this will not be possible to achieve. In these situations, the design should be divided into several parts that would be independently manufactured to be assembled later to make up the whole device. This division should result in the smallest number of pieces to minimize uncertainties and power losses at the joined surfaces, which must be carefully aligned before screw fastening. Moreover, certain strategies based on the knowledge of the electromagnetic fields involved in the operation of the device can be exploited to divide the design along planes that reduce power losses due to contact issues.This is exemplified by the power combiner proposed in [22], where the two components of the device (transducer and divider) are split into halves using an E-plane configuration to reduce energy losses (Figure 1). This device is designed to present an operation bandwidth of 12 GHz centered at 94 GHz. A mode transducer is used to generate the circular waveguide

${\text{TE}}_{01}$

mode from the rectangular waveguide

${\text{TE}}_{10}$

mode. Then, the signal entering this circular waveguide is divided among five rectangular ports, exciting the

${\text{TE}}_{10}$

mode at each of them at an equal magnitude (± 0.4 dB) and phase (± 3.5 °). A figure of merit labeled as the power-combining efficiency (as defined in [23]) can be calculated for this device by adding the complex transmission coefficient to each of the five ports. As evident in Figure 2, a simulation considering a finite conductivity for the metallic walls

${(}{\sigma}_{\text{Brass}} = {15.9}\,{\cdot}\,{10}^{6}{\text{S / m}}{)}$

yields a power-combining efficiency of 92%; however, the measured value is slightly lower due to tolerances associated to the manufacturing of the device. Nevertheless, given the difficulties of manufacturing a device of this kind in the W band, the obtained 87% efficiency can be considered a remarkable achievement.sanchez-olivares01-3143435Figure 1. A representation of the power combiner. (a) The two components of the device (transducer and divider) are split at the E-plane for manufacturing. (b) The assembled device. (c) The prototype manufactured via EDM. (Source: Adapted from [22].)sanchez-olivares02-3143435Figure 2. The efficiency of the manufactured power combiner.Another successful prototype based on classic subtractive manufacturing is the monopulse feed illustrated in Figure 3. The proposed geometry compacts all passive elements of a monopulse radar front end into a single component [24], offering circular-polarization-based duplexing and a 2D angular discrimination of targets when hybrid phase–amplitude monopulse comparison is applied [25]. Figure 3(a) shows the manufactured prototype, while the different blocks that make up the monopulse antenna are presented in Figure 3(b). This device generates opposite circular polarizations using an orthomode transducer (OMT) + horn arrangement. Furthermore, this design also includes a mode transducer and magic-T junction acting as a modal filter to extract the difference pattern.sanchez-olivares03-3143435Figure 3. The monopulse waveguide duplexer at 94 GHz. (a) The prototype manufactured through EDM and CNC. (b) The mechanical design of all the metal blocks, including the waveguide structures for electromagnetic analysis (left) and their metal enclosures (right). Tx: transmitter; Rx: receiver. (Source: Adapted from [24].)As a tradeoff between the ease of manufacture and low insertion loss, the structure was divided into four aluminum blocks, as depicted in Figure 3(b). Blocks 1, 3, and 4 were manufactured via a combination of ram–EDM, wire–EDM, and turning, with a tolerance of ±20 µm. Note that this was possible because the most-internal waveguide cavities were designed to be smaller than the most-external ones. The ridged steps of the septum were machined within metal block 1 through the sinker–EDM, thus avoiding misalignment errors. The H-plane structure in block 2 was manufactured using classic CNC with an end mill. All affected corners were appropriately rounded during design optimization to consider the 0.3-mm milling tool radius. Finally, all the pieces were assembled from the bottom (block 1) to the top (block 4) using precision screws and alignment pins placed as close as possible to the waveguides to minimize the insertion loss level. The metal post of the magic-T junction is a grub screw that was thinned on one edge through turning. It was introduced from the top of block 3 until it stuck out at the correct length from the other side.The measured prototype resulted in isolation and return losses better than 22 and 23 dB, respectively, at the desired operation frequency of 94 GHz. Figure 4 conveys the measured monopulse patterns, showing that angular tracking is possible for targets located up to ±8.5° from the broadside direction. The maximum values of the directivity, realized gain, and axial ratio are detailed in Figure 5. Very good agreement between simulations and measurements is achieved for all cases. Figure 5 demonstrates that both sum patterns are circularly polarized, featuring a 23.9-dB gain and less than a 1-dB axial ratio at 94 GHz. There is a difference between the gain and directivity of about 0.7–0.9 dB, which is equivalent to an approximate radiation efficiency of between 80 and 85%.sanchez-olivares04-3143435Figure 4. The monopulse waveguide duplexer: measured and simulated monopulse amplitude patterns at 94 GHz in planes

${\phi}$

= 0, 15, 30, 45, 60, 75, and 90°. CP: copolar; XP: cross-polar.sanchez-olivares05-3143435Figure 5. The monopulse waveguide duplexer: the measured and simulated directivity, realized gain, and axial ratio of transmit and receive sum patterns. dBi: decibels per isotropic.The presented subtractive manufacturing techniques have several advantages in terms of accuracy, high conductivity, and low cost. Nevertheless, if a component cannot be manufactured in a single unit, it is necessary to separate it into parts in an appropriate way. It is also interesting to observe in the previous prototypes that the holes for the assembling screws are significantly bigger than the channels forming the waveguide sections. Hence, when manufacturing device parts, it best to build the holes first and then, after mechanical tensions have dissapeared from the material, build the waveguide channels. If done otherwise, the tensions caused by manufacturing the screw holes could give rise to deformations of the waveguide sections that would alter the performance of the device.EFEF is a well-established technology that uses a metal forming process with parts that are manufactured by means of electrodeposition on a previously mechanized model called a mandrel. The process needs a direct current through an electrolyte containing salts of the metal being deposited until the required electroform thickness is achieved. Finally, the mandrel is eliminated by chemical dissolution or an equivalent procedure. The EF process is summarized in Figure 6. In the case of mm-wave components, the key advantage of EF with respect to other classic manufacturing processes is that a device is not split into two or more parts. Therefore, alignment issues between blocks are removed, and the deterioration of the insertion loss due to possible air gaps and faulty contacts is avoided. In addition, tolerances below 5–10 µm can be obtained as well as a surface roughness lower than 0.8 µm (N6).sanchez-olivares06-3143435Figure 6. The main EF steps.Another interesting point is related to the possibility of implementing very complex geometries with this technology, such as flared-type mode transducers [26]. EF is also used for implanting different wideband waveguide components, including horns [27], OMTs [28], and polarizers [29], with successful results. In this contribution, a 10th-order wideband bandpass filter working in the W band (75–110 GHz) is presented. Its high performance, based on waveguide direct coupled cavities, has significant application in radio astronomy receivers [30]. In this configuration, the thickness of the iris between cavities is a key design parameter. By means of EF, thicknesses as low as 0.2 mm are obtained; using classic machining techniques, this value is normally limited to 0.5 mm to guarantee good electrical contact [31]. These facts definitely contribute to the measured insertion loss level below 0.4 dB within the bandpass, which is a critical parameter for low-noise receivers. Figure 7 compares measurements and simulations, considering an effective conductivity of

${\sigma} = {4}\,{\cdot}\,{10}^{7}{(}{\text{S / m}}{)}$

. It should be highlighted that this conductivity value is very close to the nominal conductivity of gold, an excellent result thanks to the EF process. Figure 8 displays the filter connected to the measurement workbench. The manufactured prototype is pictured in the inset. The superior experimental result opens the possibility of going to higher frequencies via EF technology.sanchez-olivares07-3143435Figure 7. The measurement and simulation comparison considering an effective conductivity of

${\sigma} = {4}\,{\cdot}\,{10}^{7}{(}{\text{S / m}}{)}$

. Inset: the 10th-order filter layout.sanchez-olivares08-3143435Figure 8. The filter connected to the measurement workbench. Inset: the manufactured prototype.Additive Manufacturing TechniquesGap waveguide technology has proved to be a solution when the geometry of a component prevents strategic cuts that minimize field leakage [32], [33]. The gap waveguide is based on the use of periodic metal pins facing a metal plate, separated by less than a quarter wavelength. These pins generate a propagation blocking condition, thus enabling the fields to be confined within a path similar to a conventional waveguide. In this way, high-frequency and high-performance devices can be produced without the need for good electrical contact between parts. However, the dimensions of these pins become smaller as the frequency increases. In the W band, the CNC machining of a part of a certain complexity in gap waveguide technology presents several problems: the manufacturing cost of these pins increases drastically, the pins may break during the process, and the required small diameter drills can be damaged.An alternative that avoids these issues is additive manufacturing. 3D printing is booming in all kinds of engineering sectors, as it enables complex devices to be manufactured quickly in a single piece. This technology applied to gap waveguide devices is very advantageous since it means a significant reduction in cost and minimizes pin manufacturing problems. There are currently different technologies in plastic and metallic materials, some of which are represented in Figure 9. The most basic is fused deposition modeling, which consists of depositing a molten plastic filament layer by layer. Other technologies based on ultraviolet curable resins, such as SLA and digital light processing, improve performance by solidifying the resin on a surface layer by layer. Further additive manufacturing techniques, such as material jetting, binder jetting, and nanoparticle jetting, are based on the sputtering of the material, which is progressively stacked on the part. Finally, selective LS and DMLS technologies apply a laser beam to plastic and metal particles so that they are welded together.sanchez-olivares09-3143435Figure 9. The 3D printing processes. (Source: www.3dhubs.com; edited with permission.)Two of these technologies are evaluated in this work: SLA and DMLS. They were selected because their performance enables the correct manufacturing of small elements, such as pins, and because they have low tolerances (for well-designed parts, tolerances in the x/y dimension are ±50 μm). Parts manufactured using SLA require an additional copper plating process. This can be critical at high frequencies, due to problems with the adhesion of the metal to the plastic, areas of poor metallization, and a bad finishing of the metal surfaces. However, if the metallization processes are well controlled, reduced thickness layers (around 5 – 10 μm) with a very smooth finish can be achieved.A monopulse radial line slot array (RLSA) antenna was proposed to experimentally evaluate SLA and DMLS manufacturing techniques. The antenna consists of two pieces: a Butler matrix based on a grooved gap waveguide (GGW) with two input ports to generate the sum and difference patterns and the RLSA antenna [34] in which the fields from the Butler matrix are coupled to produce the corresponding sum and difference patterns. Since the RLSA is embedded in a dielectric substrate, this part is manufactured using conventional printed circuit board (PCB) technology. However, the complexity of the Butler matrix is ideal for evaluating the two selected additive manufacturing processes.The monopulse operating principle of the RLSA antenna consists of the excitation of a rotating mode and radially symmetrical mode inside the dielectric substrate between parallel plates. Each of these modes produces, respectively, a sum radiation pattern and conical difference pattern. The manufactured prototypes are in Figure 10. The Butler matrix built using SLA and subsequent copper plating corresponds to the prototype in Figure 10(a), and the one produced with DMLS is in Figure 10(b). The sheets with the RLSAs have been implemented on a substrate with a dielectric constant of

${\varepsilon}_{r} = {2.17}$

and loss tangent

${\tan}\,{\delta} = {0.0008}$

(at 10 GHz), according to the manufacturer. Figure 10 includes a detail of the pins and surfaces made through SLA and DMLS. In this design, the GGW pins have a height of 0.9 mm, diameter of 0.5 mm, and periodicity of 1.2 mm. As evident, there is a large difference in the roughness of the DMLS surfaces with respect to SLA.sanchez-olivares10-3143435Figure 10. Manufactured prototypes of the monopulse RLSA antenna. (a) The SLA Butler matrix and (b) DMLS Butler matrix. The RLSA is based on PCB technology in both cases. (Source: Adapted from [34].)The SLA prototype clearly shows the sum and difference beam for the monopulse function [Figure 11(a) and (b), respectively]. An appreciable amplitude imbalance is found in the measured sum and difference patterns with respect to the simulated ones (ideal). The sum pattern displays a widening of the main lobe and a null filling, while the difference pattern displays similar widening as well as an asymmetry of the two main lobes. A tolerance study was carried out in CST Microwave Studio (broadly explained in [34]). It was found that these degradations are mainly due to the combination of tolerance effects. First, there is a deviation of the dielectric constant of the substrate from 2.17 to 2.32. Second, there is a misalignment of only 0.15 mm between the RLSA antenna and Butler matrix.sanchez-olivares11-3143435Figure 11. The SLA monopulse RLSA antenna from Figure 10(a). The (a) simulated and measured sum pattern, (b) simulated and measured difference pattern, and (c) simulated (with tolerances) and measured directivity, realized gain, and axial ratio.This point is demonstrated by a comparison of measured and simulated patterns, including tolerances. In particular, the main beam broadening and null filling of the sum pattern are mainly due to the shift of the substrate permittivity, while the pattern asymmetries as well as the null deviation of the difference pattern mainly result from the misalignment between the upper and lower parts of the antenna. This analysis demonstrates that both the substrate permittivity and alignment of the different antenna parts are absolutely critical in these frequency bands. However, these tolerance errors are not derived from the SLA manufacturing process. Considering the close similarity between the measurements and simulations with tolerances, it can be concluded that the SLA-based printing process provides good performance and is suitable for manufacturing this antenna topology in the W band.The directivity, realized gain, and axial ratio values in the main beam direction are provided in Figure 11(c). Very close agreement between simulations (with tolerances) and measurements has been achieved, which again demonstrates the good performance of the SLA process to manufacture this antenna topology in the W band. The axial ratio of the SLA antenna is below 0.5 dB for circular polarization in most of the band of interest. Finally, the gain measurements made for the SLA prototype are detailed with the measured directivity. There is a difference of about 1.5 dB, which is equivalent to a radiation efficiency of around 70%. On the other hand, the patterns of the DMLS prototype do not show a monopulse behavior at all (Figure 12). This is mainly due to the surface imperfections in Figure 10(b) resulting from the DMLS manufacturing technology. In addition, the roughness reduces the effective conductivity of the material to such a degree that losses are so high and the resonant behavior of the circular coupling cavity is not observed.sanchez-olivares12-3143435Figure 12. The DMLS monopulse RLSA antenna from Figure 10(b). The (a) simulated and measured sum pattern and (b) simulated and measured difference pattern.These results show that SLA is feasible for the W band, with a very close similarity to the outcomes obtained in simulation. Some radiating properties, such as the maximum directivity and radiation pattern, have been degraded by tolerance errors, including misalignment among parts and a deviation of the substrate permittivity provided by the supplier. However, these effects have been characterized in simulation, and it has been demonstrated that they are not related to SLA manufacturing, so it is concluded that this method is suitable for the fabrication of this type of W-band designs. It should also be noted that an effective surface roughness root mean square of 0.3 μm has been estimated for the copper layer, providing a low loss level. Further information on the response of this antenna can be found in [34]. On the other hand, the measurement results clearly discard the DMLS technology, due to roughness issues in this frequency band. This assertion is confirmed by studies dedicated to DMLS, such as [35], in which several waveguide paths are manufactured and widely analyzed from 50 to 110 GHz, and [36], where a multilayer antenna is designed and manufactured via a powder metal laser process that provides approximate efficiencies between 30 and 60%.Diffusion BondingConventional machining and additive manufacturing have been widely validated for prototyping radio frequency (RF) components. Nevertheless, both technologies have limitations, especially when challenging devices are required: for instance, antenna arrays are complex and intricate structures that demand strict accuracy in frequencies higher than 60 GHz [35], [36]. Traditional machining has proved to be capable of attaining sufficient precision even at hundreds of gigahertz. However, to machine complex structures, a split-block strategy is unavoidable. This introduces uncertainty regarding the electrical contact among the multiple parts that need to be joined. Poor contacts are critical and may lead to significant leakage and component malfunction. On the other hand, additive manufacturing can produce complex devices as a single block, although the accuracy of the technology at the present time is limited for mm-wave, multilayer designs. Particles of resin and metal materials may be enclosed in the internal channels of the multilayer structure. In addition, postmetallization in multilayer structures is very challenging since the access of the metallizing fluid to the internal areas of the part cannot be ensured.In this context, researchers from the Tokyo Institute of Technology introduced DB to the RF community [37]. This technology was originally conceived for industrial applications, such as the production of heat exchangers. The DB technique provides the combination of the accuracy of conventional manufacturing techniques and the flexibility of layer-oriented processes. It is especially well suited for planar metallic structures, including antenna arrays [38], [39] and passive waveguide devices, such as filters and diplexers [40]. The primary phases of the DB process are detailed in Figure 13. The main idea is to assemble the RF component by stacking thin metallic layers. The layers are manufactured by chemical etching, which is a well-known process that provides very high tolerance on the order of 20 μm [41]. For DB purposes, a minimal thickness of the thin layers increases accuracy. The reason is that thick pieces are susceptible to a nonuniform etching width since chemical etching will remove more material from the most-exposed parts, namely, the outer areas of the walls. However, less layer thickness implies more layers to be etched, which will increase the manufacturing cost. For components up to 100 GHz, a layer thickness of between 0.1 and 0.3 mm is a good tradeoff. The layer thickness does not necessarily correspond to that of the waveguides and cavities present in the RF design. Thus, several thin layers with the same etching pattern may be stacked to attain the desired height.sanchez-olivares13-3143435Figure 13. The DB manufacturing process. The (a) etching, (b) heat and pressure, and (c) diffusion phases.In the next step, the thin metallic layers need to be joined together. This is when the DB process itself is applied. First, the layers should be stacked and carefully aligned, and environment cleanliness must be ensured, as impurities in the interface between layers might give rise to poor bonding. For this reason, a vacuum furnace is often used to carry out DB. Afterward, the main part of the process starts when a part is exposed to a proper combination of high temperatures and mechanical pressure [42]. Exposure time is also key to attaining high-quality bonding. The required temperatures never reach the melting point of the materials, usually remaining below 70% of that value. For this reason, deformation due to high temperatures is nonexistent. Furthermore, it must be highlighted that as the temperatures involved are lower than in other bonding and welding processes, the heating time is generally longer. Besides, the need for mechanical pressure must be taken into account since fragile parts in the design might deform and even break. Elements such as cantilevers and thin and tall walls might be a source of problems and should be inspected by the manufacturer to determine whether changes in the design are required.The proper combination of heat and pressure triggers an atomic process called diffusion. In this, the atoms in the interface between layers intertwine, filling the voids and creating an atomic-level bond. DB can occur between dissimilar metals and even ceramics, providing more flexibility in the choice of materials. In any case, the main attraction of DB compared to other technologies is that no additional joining elements (tightening screws, welding metal, and sticking epoxy) are required. This ensures the continuity of the material throughout the stacked structure, improving electrical contacts, preventing tensions due to different thermal expansion coefficients, and preserving the designed layer thicknesses [42]. Moreover, the union created in diffusion-bonded parts is considerably more robust than the alternative of using tightening screws. The use of screws is unattractive in mm-wave frequencies since their electrical size is big and, consequently, few screws can be placed in a part, which may lead to poor electrical contact. This is especially worrying in intricate components, such antenna arrays, where the screws cannot be placed in most areas to avoid interfering with internal channels. In summary, after DB, a multilayer stack is converted into a single-block structure with strong interlayer bonds.To experimentally validate DB technology for W-band components, a 94-GHz circularly polarized antenna array in full waveguide technology was proposed [43]. The manufacturing process was carried out at a Japanese company, Welcon [44]. Although many companies have the infrastructure to carry out DB, very few are able to operate with the accuracy required for mm-wave RF components. Two identical units were ordered to determine the repeatability of the DB manufacturing process (Figure 14). In Figure 14, the layered assembly can be seen, with a layer thickness of 0.2 and 0.3 mm. The material is copper to maximize the electrical conductivity and thus the total antenna efficiency, although in terms of weight, an aluminum solution would be preferred. Experimental results showed that the total radiation efficiency was between 50 and 80%, which is somewhat lower than expected for a waveguide antenna. In addition, a 5% shift was obtained in the frequency response, due to a proportional overetching of the copper layers.sanchez-olivares14-3143435Figure 14. The DB monopulse planar array. (a) Two units of the same antenna. (b) A detail of the circularly polarized radiating cavities. (c) A detail of a side view where the thin layers can be observed. (Sources: Adapted from [43] and [45].)These observations were transmitted to the manufacturer to compensate for the errors in the following iteration of the project. The proposed antenna design was more complex since a monopulse combining network and an amplitude taper were added. The antenna relies on four waveguide inputs to generate a monopulse radiation pattern on two planes. A monopulse beamforming network with waveguide couplers and phase shifters is assembled. A two-tier corporate feeding network provides an amplitude taper illumination to a 16 × 16 circularly polarized radiating array (Figure 14).Again, two identical prototypes were ordered from the same company to check the manufacturing repeatability. Experimental results demonstrated that the feedback provided to the manufacturer served to refine the manufacturing process [45]. The measured broadside gain for the sum monopulse pattern is presented in Figure 15. The results of the two manufactured units are almost identical, suggesting that DB can realize high repeatability. In addition, the total radiation efficiency is between 70 and 85%, which is a remarkable result for a W-band complex antenna. Furthermore, the axial ratio in Figure 15 displays good agreement in both prototypes, remaining under 2.3 dB to provide good polarization purity in the entire bandwidth. The radiation patterns have also been measured, and the results for one of the prototypes are plotted in Figure 16. The measured curves are in good agreement with the simulation predictions. Consequently, DB technology enables the production of high-efficiency planar components such as high-gain arrays in the W band.sanchez-olivares15-3143435Figure 15. The monopulse planar array manufactured through DB.sanchez-olivares16-3143435Figure 16. The monopulse planar array manufactured via DB: simulation and measurements of RHCP radiation patterns at 94 GHz in planes (a)

${\phi} = {0}^{\circ}$

and (b)

${\phi} = {90}^{\circ}$

.DiscussionComparison of Manufacturing TechniquesFollowing the analysis of the designs presented in previous sections and based on the experience acquired, a performance comparison of the different manufacturing techniques presented as candidates for working in the W band is discussed in the following. The most important properties of the approaches evaluated in this work are summarized in Table 1. One of the first points to be analyzed is the ability to manufacture a device in a single block. In this sense, the classic manufacturing techniques, such as CNC and EDM, are more limited since they are subtractive methods. Other approaches, such as EF and additive manufacturing, have the ability to form complex and intricate devices in a single part. However, they present a limitation, with multilayer structures made up of many internal channels since, in this case, it will be necessary to produce each of the layers separately and join them all together, taking special care in the alignment of the parts. If a highly intricate multilayer device is to be manufactured in one piece, it is recommended to use DB because it is based on the separate crafting of individual layers that are fused, resulting in a single piece.Table 1. A comparison of manufacturing methods for passive devices at the W band.sanchez-olivares_t1-3143435The best surface finishes and roughness are found in advanced manufacturing techniques, such as EDM (around 20–50 µm) and EF (below 5 µm), while additive methods lack this precision. In particular, DMLS technology has significant surface roughness that completely rules out its use in the manufacture of W-band devices that will perform well. In terms of weight, a crucial point in many applications, manufacturing techniques based on direct metal treatment result in heavier devices. In general, subtractive manufacturing techniques and DB are rather heavy, especially compared to techniques such as EF and SLA.However, if the highest possible effective conductivity is desired, it is recommended to use classic manufacturing approaches, such as CNC and EDM (which directly provide the conductivity levels of the metal that are used), although it has also been found that more advanced techniques, including EF and DB, can provide high effective levels of conductivity. However, additive manufacturing approaches provide lower effective levels of conductivity, mainly due to their surface finish and roughness.Finally, costs (for small-scale production) and accessibility must be discussed. One of the great advantages of additive manufacturing techniques is their lower expense. On the other hand, those techniques that, in general, offer better performance (such as EF and DB) tend to be more expensive and less accessible. In other words, based on the authors’ experience, there are few companies that develop these manufacturing techniques with good results in a frequency band as high as the W band.Manufacturing GuidelinesAs a conclusion and based on all the studies that were carried out, this article aims to provide a set of guidelines to an expert designer who intends to manufacture devices in the W band. As explained in previous sections, the most appropriate manufacturing method will strongly depend on the topology of the device to be crafted. As a summary, Figure 17 presents a flowchart detailing the recommended manufacturing approaches. Basically, we must distinguish whether the device is a multilayer structure or not. In the case of it not being multilayer, consider the following:sanchez-olivares17-3143435Figure 17. The recommended manufacturing methods at the W band.

If the device can be manufactured in a single piece, it is recommended to use any of the subtractive techniques (such as CNC and EDM) and EF. In general, subtractive techniques offer a higher level of effective conductivity, but the surface finishes of the EF parts are significantly higher, especially if the device has a complex topology.

If the device cannot be manufactured in a single piece, then it is recommended to split it into several parts. At this point, it is recommended that the designer perform the appropriate tests to find those parts of the structures that are susceptible to prevent field leakage, as in the following:

If the device can be split in such a way to prevent leakage, then subtractive manufacturing and EF techniques are recommended because they offer the best electrical performance.
If the device cannot be divided into parts so that field leakage is prevented, then additive manufacturing techniques using gap waveguide technology are advised.

On the other hand, if the designed device is a multilayer structure with many intricate channels, apply the following:

Use additive techniques to manufacture the different layers, including gap waveguide technology that becomes essential in these frequency bands to prevent field leakage between layers. Among the techniques evaluated, SLA is recommended, while DMLS is totally discarded. Currently, DMLS is not considered a sufficiently mature technology for the manufacture of W-band devices that will have good performance.
If a multilayer structure is desired in a single block, a more elaborate manufacturing technology, such as DB, is recommended. The device is manufactured in multiple thin layers, and then, after the appropriate combination of pressure and temperature, the layers are fused together resulting in a single part. Despite the advantages of this technology, the drawbacks are its high cost compared to other technologies and limited accessibility.

As an additional key recommendation derived from this study, when working on the design of W-band passive devices, a crucial point is the mandatory interaction between the electrical engineer and mechanical technologist. In addition to reducing manufacturing time, a large number of problems can be avoided in advance by knowing the limitations/restrictions of each technology. Foreknowledge of the accuracy of the selected process can prevent a major issue before the experimental results.

Conclusions

Manufacturing accuracy is a critical factor for W-band prototyping, especially in the case of narrow-band devices. The manufacturing processes suitable for passive devices in this frequency band differ substantially from the production considerations and technologies typically used in the microwave and lower mm-wave bands. This work provided an experimental analysis of the most suitable manufacturing techniques for the implementation of passive prototypes in W-band waveguide technology. Several designs based on different monopulse antenna topologies have been manufactured and measured to show the performance of the proposed production techniques in the W band and provide details to carry out the processes in the most appropriate way.

Subtractive manufacturing techniques present multiple advantages in terms of accuracy, high conductivity, and low cost. Traditional techniques, such as CNC, and more accurate techniques, such as EDM, have been evaluated. However, the devices can be excessively heavy, and leakage effects can be destructive unless there is a possibility of splitting a whole block into parts in an appropriate way. Leakage losses can also be very high in multilayer structures.

Other techniques based on electrodeposition, including EF, have many advantages over subtractive methods. EF provides high levels of effective conductivity, low weight, and higher-precision surface finishes than subtractive techniques. Furthermore, it is possible to manufacture complex parts in a single block since they are based on metal deposition on a previously mechanized model. In short, it can be a suitable manufacturing approach for the W band. However, it suffers from the same limitations as subtractive techniques when it comes to manufacturing complex multilayer structures.

Alternatively, additive techniques enable complex structures to be manufactured in few parts, thus preventing the harmful effects and high costs of subtractive approaches. Techniques such as SLA that require a subsequent metallization process have proved to be appropriate for the high-quality manufacturing of W-band devices. In contrast, other additive techniques that do not require device postmetallization, such as DMLS, are ruled out in this frequency band because their tolerance, at around 100 µm, is above the required limit.

Finally, additive manufacturing technologies have significant constructive limitations, especially for challenging devices made up of multiple layers and intricate internal channels. In this context, innovative manufacturing techniques, such as DB, enable the production of high-efficiency planar components in frequencies at least as high as 100 GHz. This technology, which is based on the milling and stacking of multiple metal layers that are fused together to form a single piece, makes it possible to manufacture an arbitrarily shaped, single-block device with high performance. However the costs associated with this technique are currently high.

Acknowledgment

This work is partially supported by the Madrid Regional Government, under project S2013/ICE3000 (SPADERADAR-CM).

Author Information

Pablo Sanchez-Olivares (pablo.sanchezo@upm.es) is an assistant professor at Universidad Politécnica de Madrid, Madrid, 28040, Spain. He received his Ph.D. degree from Universidad Autonoma de Madrid in 2018. His research interests include planar array antennas, phased array antennas, and waveguide antennas.

Marta Ferreras (marta.ferreras@upm.es) is a doctoral student in the Department of Signals, Systems, and Radiocommunications, Universidad Politécnica de Madrid, Madrid, 28040, Spain. Her research interests include on-chip terahertz electronics and millimeter-wave antennas and systems for active imaging and radar applications. She is a Student Member of IEEE.

Eduardo Garcia-Marin (eduardo.garciam@uam.es) is with the Radio-Frequency, Circuits, Antennas, and Systems Group, Department of Electronic and Communication Technologies, Autonoma University of Madrid, Madrid, 28049, Spain. He received his Ph.D. degree in telecommunication engineering from Autonoma University of Madrid in 2021. His research interests include passive and active array antennas and antenna manufacturing techniques.

Lucas Polo-López (lucas.polo-lopez@insa-rennes.fr) is with the Radio-Frequency, Circuits, Antennas, and Systems Group, Department of Electronic and Communication Technologies, Autonoma University of Madrid, Madrid, 28049, Spain, where he received his Ph.D. degree in telecommunication engineering in 2020. His research interests include the computer-aided design of horn antennas and passive waveguide devices.

Adrián Tamayo-Domínguez (a.tamayo@upm.es) is an assistant professor at Universidad Politécnica de Madrid, Madrid, 28040, Spain, where he received his Ph.D. degree in 2020. His research interests include radio-frequency circuits, gap waveguides, and higher symmetries applied to planar antennas. He is a Student Member of IEEE.

Juan Córcoles (juan.corcoles@uam.es) is an associate professor at Autonoma University of Madrid, Madrid, 28049, Spain. He received his Ph.D. degree in electrical engineering from Universidad Politécnica de Madrid in 2009. His research interests include microwave circuits and antennas. He is a Senior Member of IEEE.

José Manuel Fernández-González (josemanuel.fernandez.gonzalez@upm.es) is an associate professor at Universidad Politécnica de Madrid, Madrid, 28040, Spain, where he received his Ph.D. degree in 2009. His research interests include phased array antennas, radio-frequency circuits, and metamaterial structures. He is a Senior Member of IEEE.

José Luis Masa-Campos (joseluis.masa@uam.es) is an assistant professor in the Radio-Frequency, Circuits, Antennas, and Systems Group, Department of Electronic and Communication Technologies, Autonoma University of Madrid, Madrid, 28049, Spain. He received his Ph.D. degree in telecommunication engineering from Technical University of Madrid in 2006. His research interests include active and passive planar array antennas.

José R. Montejo-Garai (joseramon.montejo@upm.es) is a full professor at Universidad Politécnica de Madrid, Madrid, 28040, Spain, where he received his Ph.D. degree in 1994. His research interests include waveguide structures, advanced synthesis theory, and computer-aided design for microwave and millimeter-wave passive devices.

Jesús M. Rebollar-Machain (jesusmaria.rebollar@upm.es) is a professor of electromagnetic theory at Universidad Politécnica de Madrid, Madrid, 28040, Spain, where he received his Ph.D. degree in 1980. His research interests include electromagnetic wave propagation and interactions of electromagnetic fields. He is a Member of IEEE.

Jorge A. Ruiz-Cruz (jorge.ruizcruz@uam.es) is a full professor at Autonoma University of Madrid, Madrid, 28049, Spain. He received his Ph.D. degree from Universidad Politécnica de Madrid in 2005. His research interests include the computer-aided design of microwave passive devices and circuits. He is a Senior Member of IEEE.

Manuel Sierra-Castañer (manuel.sierra@upm.es) is a senior professor at Universidad Politécnica de Madrid, Madrid, 28040, Spain, where he received his Ph.D. degree in 2000. His research interests include planar antennas and antenna measurement systems. He is a Senior Member of IEEE.

Manuel Sierra-Pérez (manuel.sierra.perez@upm.es) is an emeritus professor at Universidad Politécnica de Madrid, Madrid, 28040, Spain, where he received his Ph.D. degree in 1980. His research interests include passive and active array antennas. He is a Life Senior Member of IEEE.

Mariano Barba (mariano.barba@upm.es) was an associate professor in the Department of Electromagnetism and Circuit Theory, Universidad Politécnica de Madrid, Madrid, 28040, Spain, where he received his Ph.D. degree in 1996. He was a Member of IEEE.

José Luis Besada (besada@gr.ssr.upm.es) is a full professor in the Department of Signals, Systems, and Radiocommunications, Universidad Politécnica de Madrid, Madrid, 28040, Spain, where he received his Ph.D. degree in 1979. His research interests include the design of reflector antennas and antenna measurement systems.

Jesús Grajal (jesus.grajal@upm.es) is a full professor in the Department of Signals, Systems, and Radiocommunications, Universidad Politécnica de Madrid, Madrid, 28040, Spain, where he received his Ph.D. degree in 1998. His research interests include semiconductor device modeling and high-frequency circuit and system design. He is a Member of IEEE.

References

[1] D. M. Mittleman, “Perspective: Terahertz science and technology,” J. Appl. Phys., vol. 122, no. 23, p. 230,901, Dec. 2017, doi: 10.1063/1.5007683.

[2] R. Lombardi, “Microwave and millimetre-wave for 5G transport,” ETSI, Sophia Antipolis, France, White Paper, Feb. 2018.

[3] T. S. Rappaport et al., “Millimeter wave mobile communications for 5G cellular: It will work!” IEEE Access, vol. 1, pp. 335–349, May 2013, doi: 10.1109/ACCESS.2013.2260813.

[4] T. Nitsche, C. Cordeiro, A. B. Flores, E. W. Knightly, E. Perahia, and J. C. Widmer, “IEEE 802.11 ad: Directional 60 GHz communication for multi-Gigabit-per-second Wi-Fi,” IEEE Commun. Mag., vol. 52, no. 12, pp. 132–141, Dec. 2014, doi: 10.1109/MCOM.2014.6979964.

[5] J. N. Pelton, “New millimeter, terahertz, and light-wave frequencies for satellite communications,” in Handbook of Satellite Applications, Cham, Switzerland: Springer, 2017, pp. 413–429.

[6] S. L. Cotton, W. G. Scanion, and B. K. Madahar, “Millimeter-wave soldier-to-soldier communications for covert battlefield operations,” IEEE Commun. Mag., vol. 47, no. 10, pp. 72–81, Oct. 2009, doi: 10.1109/MCOM.2009.5273811.

[7] R. Appleby and H. B. Wallace, “Standoff detection of weapons and contraband in the 100 GHz to 1 THz region,” IEEE Trans. Antennas Propag., vol. 55, no. 11, pp. 2944–2295, Nov. 2007, doi: 10.1109/TAP.2007.908543.

[8] P. J. Shull, Nondestructive Evaluation: Theory, Techniques, and Applications. Boca Raton, FL, USA: CRC Press, 2016.

[9] D. M. Sheen, D. L. McMakin, and T. E. Hall, “Three-dimensional millimeter-wave imaging for concealed weapon detection,” IEEE Trans. Microw. Theory Techn., vol. 49, no. 9, pp. 1581–1592, Sep. 2001, doi: 10.1109/22.942570.

[10] K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Exp., vol. 11, no. 20, pp. 2549–2554, Oct. 2003, doi: 10.1364/OE.11.002549.

[11] A. Abina, U. Puc, A. Jeglič, and A. Zidanšek, “Applications of terahertz spectroscopy in the field of construction and building materials,” Appl. Spectrosc. Rev., vol. 50, no. 4, pp. 279–303, Apr. 2015, doi: 10.1080/05704928.2014.965825.

[12] Z. Yan, Y. Ying, H. Zhang, and H. Yu, “Research progress of terahertz wave technology in food inspection,” THz Phys., Devices, Syst., vol. 6373, p. 63730R, Oct. 2006, doi: 10.1117/12.686840.

[13] I. Skog and P. Handel, “In-car positioning and navigation technologies: A survey,” IEEE Trans. Intell. Transp. Syst., vol. 10, no. 1, pp. 4–21, Mar. 2009, doi: 10.1109/TITS.2008.2011712.

[14] T.-Y. Lee, V. Skvortsov, M. Kim, S. Han, and M. Ka, “Application of W-band FMCW radar for road curvature estimation in poor visibility conditions,” IEEE Sensors J., vol. 18, no. 13, pp. 5300–5312, Jul. 2018, doi: 10.1109/JSEN.2018.2837875.

[15] D. Jasteh, E. G. Hoare, M. Cherniakov, and M. Gashinova, “Experimental low-terahertz radar image analysis for automotive terrain sensing,” IEEE Geosci. Remote Sens. Lett., vol. 13, no. 4, pp. 490–494, Apr. 2016, doi: 10.1109/LGRS.2016.2518579.

[16] K. K. Samanta, “3D/multilayer heterogeneous integration and packaging for next generation applications in millimeter-wave and beyond,” in Proc. 2017 IEEE MTT-S Int. Microw. RF Conf. (IMaRC), Ahmedabad, pp. 294–297, doi: 10.1109/IMaRC.2017.8611009.

[17] J. Campion et al., “Toward industrial exploitation of THz frequencies: Integration of SiGe MMICs in silicon-micromachined waveguide systems,” IEEE Trans. THz Sci. Technol., vol. 9, no. 6, pp. 624–636, Nov. 2019, doi: 10.1109/TTHZ.2019.2943572.

[18] J. R. Montejo-Garai, J. A. Ruiz-Cruz, and J. M. Rebollar, “Evaluation of additive manufacturing techniques applied to a waveguide mode transducer,” IEEE Trans. Compon. Packag. Manuf. Technol., vol. 10, no. 5, pp. 887–894, May 2020, doi: 10.1109/TCPMT.2020.2982735.

[19] E. García-Marín, J. L. Masa-Campos, P. Sánchez-Olivares, and J. A. Ruiz-Cruz, “Evaluation of additive manufacturing techniques applied to ku-band multilayer Corporate waveguide antennas,” IEEE Antennas Wireless Propag. Lett., vol. 17, no. 11, pp. 2114–2118, Nov. 2018, doi: 10.1109/LAWP.2018.2866631.

[20] A. Tamayo-Dominguez, J. Fernandez-Gonzalez, and M. Sierra-Castañer, “3-D-printed modified butler matrix based on gap waveguide at W-band for monopulse radar,” IEEE Trans. Microw. Theory Techn., vol. 68, no. 3, pp. 926–938, Mar. 2020, doi: 10.1109/TMTT.2019.2953164.

[21] E. Garcia-Marin, J. L. Masa-Campos, and P. Sanchez-Olivares, “Diffusion bonding manufacturing of high gain W-band antennas for 5G applications,” IEEE Commun. Mag., vol. 56, no. 7, pp. 21–27, Jul. 2018, doi: 10.1109/MCOM.2018.1700986.

[22] J. R. Montejo-Garai, J. A. Ruiz-Cruz, and J. M. Rebollar, “5-way radial power combiner at W-band by stacked waveguide micromachining,” Nuclear Instrum. Methods Phys. Res. A, Accel., Spectrometers, Detect. Associated Equip., vol. 905, pp. 91–95, Oct. 2018, doi: 10.1016/j.nima.2018.07.031.

[23] M. S. Gupta, “Power combining efficiency and its optimisation,” IEE Proc. H - Microw., Antennas Propag., vol. 139, no. 3, pp. 233–238, Jun. 1992, doi: 10.1049/ip-h-2.1992.0043.

[24] M. Ramírez-Torres et al., “Technological developments for a space-borne orbital debris radar at 94 GHz,” in Proc. IEEE Radar Conf. (RadarConf 2018), Oklahoma City, OK, USA, Apr. 2018, pp. 564–569, doi: 10.1109/RADAR.2018.8378621.

[25] S. M. Sherman and D. K. Barton, Monopulse Principles and Techniques, 2nd ed. Norwood, MA, USA: Artech House, 2011.

[26] L. Epp, P. Khan, and A. Silva, “Ka-band wide-band gap solid-state power amplifier: Hardware validation,” Jet Propulsion Lab., Pasadena, CA, USA, Interplanetary Network Progress Rep. 42-163, Nov. 15, 2005.

[27] R. Banham et al., “Electroformed front-end at 100 GHz for radio-astronomical applications,” Microw. J., vol. 48, pp. 112–122, Aug. 2005.

[28] G. Pisano et al., “A broadband WR10 turnstile junction orthomode transducer,” IEEE Microw. Wireless Compon. Lett., vol. 17, no. 4, pp. 286–288, 2007, doi: 10.1109/LMWC.2007.892976.

[29] G. Pisano, R. Nesti, M. W. Ng, A. Orfei, D. Panella, and P. Wilkinson, “A novel Q-band polarizer with very flat phase response,” J. Electromagn. Waves Appl., vol. 26, nos. 5–6, pp. 707–715, 2012, doi: 10.1080/09205071.2012.710795.

[30] C. A. Leal-Sevillano et al., “Development of low loss waveguide filters for radio-astronomy applications,” Infrared Phys. Technol., vol. 61, pp. 224–229, Nov. 2013, doi: 10.1016/j.infrared.2013.08.012.

[31] C. A. Leal-Sevillano, J. R. Montejo-Garai, J. A. Ruiz-Cruz, and J. M. Rebollar, “Low-loss elliptical response filter at 100 GHz,” IEEE Microw. Wireless Compon. Lett., vol. 22, no. 9, pp. 459–461, 2012, doi: 10.1109/LMWC.2012.2212237.

[32] D. Zarifi, A. Farahbakhsh, and A. U. Zaman, “A gap waveguide-fed wideband patch antenna array for 60-GHz applications,” IEEE Trans. Antennas Propag., vol. 65, no. 9, pp. 4875–4879, Sep. 2017, doi: 10.1109/TAP.2017.2722866.

[33] J. L. Vazquez-Roy, A. Tamayo-Domínguez, E. Rajo-Iglesias, and M. Sierra-Castañer, “Radial line slot antenna design with groove gap waveguide feed for monopulse radar systems,” IEEE Trans. Antennas Propag., vol. 67, no. 10, pp. 6317–6324, Oct. 2019, doi: 10.1109/TAP.2019.2922742.

[34] A. Tamayo-Domínguez, J. M. Fernández-González, and M. Sierra-Castañer, “Monopulse radial line slot array antenna fed by a 3D-printed cavity-ended modified butler matrix based on gap waveguide at 94 GHz,” IEEE Trans. Antennas Propag., early Access, 2021, doi: 10.1109/TAP.2021.3060045.

[35] S. Verploegh, M. Coffey, E. Grossman, and Z. Popovič, “Properties of 50–110-GHz waveguide components fabricated by metal additive manufacturing,” IEEE Trans. Microw. Theory Techn., vol. 65, no. 12, pp. 5144–5153, Dec. 2017, doi: 10.1109/TMTT.2017.2771446.

[36] J. Hirokawa, S. Ito, S. Suetsugu, M. Zhang, and M. Ando, “Fabrication of a double-layer corporate-feed waveguide slot array by powder metal laser process,” in Proc. Int. Conf. Numerical Electromagn. Model. Optimiz. RF, Microw., THz Appl. (NEMO), Pavia, 2014, pp. 1–4, doi: 10.1109/NEMO.2014.6995662.

[37] J. Hirokawa, M. Zhang and M. Ando, “94GHz fabrication of a slotted waveguide array antenna by diffusion bonding of laminated thin plates,” in Proc. IEEE SENSORS, Christchurch, 2009, pp. 907–911, doi: 10.1109/ICSENS.2009.5398159.

[38] M. Sano, J. Hirokawa, and M. Ando, “A hollow rectangular coaxial line for slot array applications fabricated by diffusion bonding of laminated thin metal plates,” IEEE Trans. Antennas Propag., vol. 61, no. 4, pp. 1810–1815, Apr. 2013, doi: 10.1109/TAP.2012.2232268.

[39] E. Garcia-Marin, J. L. Masa-Campos, and P. Sanchez-Olivares, “Implementation of millimeter wave antenna arrays by diffusion bonding,” in Proc. 11th Global Symp. Millimeter Waves (GSMM), Boulder, CO, USA, 2018, pp. 1–4, doi: 10.1109/GSMM.2018.8439652.

[40] X. Xu, M. Zhang, J. Hirokawa, and M. Ando, “E-band plate-laminated waveguide filters and their integration into a corporate-feed slot array antenna with diffusion bonding technology,” IEEE Trans. Microw. Theory Techn., vol. 64, no. 11, pp. 3592–3603, Nov. 2016, doi: 10.1109/TMTT.2016.2602859.

[41] M. Sano, J. Hirokawa, and M. Ando, “Single-layer corporate-feed slot array in the 60-GHz band using hollow rectangular coaxial lines,” IEEE Trans. Antennas Propag., vol. 62, no. 10, pp. 5068–5076, Oct. 2014, doi: 10.1109/TAP.2014.2346213.

[42] R. Rusnaldy, “Diffusion bonding: An advanced of material process,” ROTASI, vol. 3, no. 1, pp. 23–27, 2012.

[43] E. Garcia-Marin, J. L. Masa-Campos, and P. Sanchez-Olivares, “Implementation of millimeter wave antenna arrays by diffusion bonding,” in Proc. 2018 11th Global Symp. Millimeter Waves (GSMM), Boulder, CO, USA, pp. 1–4, doi: 10.1109/GSMM.2018.8439652.

[44] “Thermal one-stop WELCON,” WELCON. https://www.welcon.co.jp/

[45] E. Garcia-Marin, J.-L. Masa-Campos, and P. Sanchez-Olivares, “Diffusion-bonded W-band monopulse array antenna for space debris radar,” AEU - Int. J. Electron. Commun., vol. 116, p. 153,061, Mar. 2020, doi: 10.1016/j.aeue.2019.153061.

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