Project C09 - Ceramic Technologies for Passive Sub-mm Localization Infrastructure

Principal Investigators: Prof. Dr. Niels Benson, UDE / Dr. Alejandro Jimenez-Saez, TUDa

Achieved results and methods

Following the successful demonstration of additively manufactured ceramics applied to sub-THz RRTs in the 1st phase, projects C09 and C13 merged within the C09 project for the 2nd phase of MARIE, focusing on ‘THz RFID Tags & Components enabled by Additive Manufacturing’. For this purpose, different RRT concepts were evaluated as outlined in the following:

In the 1st phase, low-Q RRTs were based on encoding a CR with a Frequency Selective Surface (FSS) placed in its aperture [13], achieving operation up to 240 GHz with a cross-dipole based FSS [15, ch. 5, pp. 100-110]. However, the single resonance present in such structure limited the maximum number of RRTs that can be implemented in the WR-3 band. Up to 5 RRTs can be implemented without overlapping notches, while 10 are the minimum required for 3D indoor localization according to [16].

2nd Phase Solution: A 2-notch FSS based on Split Ring Resonators (SRRs) has been developed to operate in the WR-3 range. Two SRRs with different resonance frequencies are etched within a metallic layer placed over a RT 5880 dielectric substrate. The frequency signature is controlled by independently varying the radius of each SRR. Thus, two notches are achieved between 220 GHz to 330 GHz, which allow for the implementation of up to 15 non-overlapping signatures [1].

The advantage of these low-Q RRTs based on CRs is that they are simple to design and implement, although they are limited by the CR in terms of FoV. Further, the employed PTFE would not withstand the MARIE fire scenario, due to its melting point at T = 327°C.

2nd Phase Solution: Leveraging the potential of metallic 3D printing, a hemispherical 3D FSS was designed in brass and used to encode a spherical lens reflector enabling FoVs of 120° as shown in Fig. 1 [11, 17]. Quasi-isotropic encoding is possible by replacing the metallic backside with another hemispherical FSS. Since the FSS is printed in brass (melting point at 900°C) and is used in combination with a fused silica lens (melting point of 1700°C), this result constitutes the first proven W-band RRT with a broad 3D-field of view, suitable for ultra-high temperatures.

In the 2nd phase, the advantages of the high Q-factor concept has been demonstrated in cluttered environments [15, ch. 5, pp. 123-127]. In consequence, this concept, that in the 1st phase operated at 80 GHz, was redesigned and demonstrated at 240 GHz in close collaboration with C12, as their HR-Si processing via DRIE allowed for the implementation of very small structures, while employing a material with moderately high  and very low dielectric loss. This enabled a 2D PhC-based RRT with two resonators featuring Q-factors above 500. Further, the readout range and angular operation could be extended to approximately 50 cm (with our measurement setup) and ±70º with the help of a HR-Si Luneburg lens [3]. However, this two-piece structure is 2D, and therefore only works when the reader and tag are in-plane (i.e., at the same height). To be used as RRTs in a real indoor scenario, the tags require a 3D field of view so that they provide a backscattered response regardless of the interrogation position. Furthermore, the operation of silicon RRT is limited to operational temperatures below 100°C as demonstrated in the 1st phase [15, ch. 2, pp. 15-22].

Solution: Leveraging the potential of LCM technology, a QCTO  Luneburg lens was developed [4], which is able to operate up to approximately 98 GHz. To complete the RRT, the lens is integrated with a frequency-coded reflective layer, for which we developed a 3D PhC with two embedded resonators. Fig. 2a displays an image of the RRT, where the QCTO lens is placed on top of the PhC. Fig. 2b illustrates the time-frequency intensity map of the RRT, where the two implemented high-Q resonances are clearly detectable up to 10 ns, i.e., after the environmental clutter fades away. Finally, Fig. 2c displays the result of evaluating the backscattered power from the RRT as a function of the angle of incidence. Three high magnitude peaks are appreciable. Two more were expected at ±80°, but are not visible due to the large scanning loss of the lens.  

The integration of the lens and the 3D PhC is not straightforward, as even if both components are manufactured in , the front side of the 3D PhC has large gaps which complicate the placement of the lens to achieve a monolithic structure. Therefore, a matching layer is required to gradually couple the lens and the PhC structures, both electromagnetically (2nd phase) and mechanically for monolithic integration (3rd phase). However, the addition of the matching layer impacts the EM wave steering capabilities achieved by the lens, defocusing the coupling into the cavities embedded in the 3D PhC.

Solution: 1D PhCs are a potential coding structure able to address this issue: by designing them with the same unit cell size as the lens, a seamless integration between both the lens and the coding structure is possible, which is an objective for the 3rd phase of MARIE. As a first step, C09 has developed a reflective 1D PhC, which reduces sensitivity to the environment and achieves backscattering of the resonators’ response. Fig. 3a presents a sketch of the structure, while Fig. 3b showcases the readout of its resonance frequency.

To validate the performance of ceramic RRTs in harsh environments, high temperature measurements up to 1200°C have been performed. This maximum temperature was chosen to be in line with the MARIE indoor fire scenario, as well as with the German Fire Protection Association, which sets this limit for the design of fires employed for the verification of fire protection methods [18]. Furthermore, this is also the maximum temperature measured in a recent study burning a real house [19]. Three of the RRTs outlined above are capable of withstanding temperatures above 300°C, namely the frequency coded lens reflector (Brass), the reflective 1D PhC () and the combination of flattened lens and 3D PhC (). Their measured time-gated  in this high-temperature experiment is shown in Fig. 4. The frequency coded brass spherical lens reflector presents a coded response up to the melting point of the material (900°C). A wireless readout of the signature from the two -based tags is still possible at 1200°C, although with decreasing magnitude / Radar Cross Section (RCS), and lower Q-factor due to the increasing dielectric losses with temperature. Further, the variation of permittivity with an increase in temperature causes a frequency shift of 67 ppm/K in the peaks, giving the RRTs sensor functionality and thus enabling sensor fusion. To the best of our knowledge, these are the first mm-Wave frequency-coded RRTs tested and successfully operated at such high-temperatures.

Currently, ceramic LCM can realize lenses with operational frequencies up to 98 GHz, with the QCTO functionality made possible by permittivity gradients introduced through mixing Air- unit cells. Pushing this frequency to 300 GHz and beyond will be one of the objectives in the 3rd phase. To address this, the use of multi-materials during the AM process is required, in order to enable higher structural precision as outlined in the work packages below. The multi-material strategy does not necessarily require different molecules, as was demonstrated in [5] with the tuning of the permittivity of  by inducing a change in the material’s porosity. In the 2nd phase this was enabled by using different sintering temperatures ranging from 1250°C to 1650°C as outlined in Fig. 5. The dielectric parameters were extracted by using  PhC samples of identical size, which demonstrated a shift in resonance frequency up to 35 GHz, in dependence of their porosity [5]. The shift in bulk relative permittivity () and low dielectric loss () for the different sintering temperatures is demonstrated in Figs. 5a and 5b. These results were employed for the successful demonstration of a 3D-QCTO lens operating above 100 GHz [20]. In order to use this concept for dielectric Gradient Index (GRIN)-based lenses in the 3rd phase, the porosity will be influenced using porogens, which are volatile during temperature processing, instead of using a variation in sintering temperatures. This technique can potentially achieve the integration of multi- within the same structure, while enabling decreased dielectric losses as demonstrated in Fig. 5b as the consequence of higher sintering temperatures.

Complementing the 2nd phase MARIE efforts, the independent research group of Alejandro Jiménez-Sáez at TUDa has achieved state-of-the-art results on RIS structures based on the use of LC. Currently, most RIS designs are based on lumped semiconductor components and are therefore limited in their frequency scalability in terms of maximum wafer area, cost, and EM-efficiency. We have demonstrated the realization of a 60 GHz LC-based RIS, which, when compared to competing LC-based RIS structures, simultaneously provides fast response times (sub-100 ms), low insertion loss (< 7 dB), wide bandwidth (> 10%), and each phase shifter is able to provide a full (360°) phase shift [10]. Motivated by these new state of the art results and their alignment with the MARIE vision, further research on such structures beyond 200 GHz will be pursued within the 3rd phase.

Selected project-related publications

[1]

J. Sánchez-Pastor, A. Kamel, M. Schüßler, R. Jakoby and A. Jiménez-Sáez, “Double-Notch Frequency-Coded Corner Reflectors for sub-THz chipless RFID Tags”, IEEE Antenna and Wireless Propagation Letters (2024). Accepted (in press).

[2]

J. Sánchez-Pastor, U. S. K. P. Miriya Thanthrige, F. Ilgac, A. Jiménez-Sáez, P. Jung, A. Sezgin and R. Jakoby, “Clutter Suppression for Indoor Self-Localization Systems by Iteratively Reweighted Low-Rank Plus Sparse Recovery”, Sensors (2021). [DOI: 10.3390/s21206842]

[3]

P. Kadera, J. Sánchez-Pastor, L. Schmitt, M. Schüßler, R. Jakoby, M. Hoffmann, A. Jiménez-Sáez and J. Lacik, “Sub-THz Luneburg lens enabled wide-angle frequency-coded identification tag for passive indoor self-localization”, International Journal of Microwave and Wireless Technologies (2022). [DOI: 10.1017/S175907872200054X]

[4]

P. Kaděra, J. Sánchez-Pastor, H. Eskandari, T. Tyc, M. Sakaki, M. Schüßler, R. Jakoby, N. Benson, A. Jiménez-Sáez, and J. Láčík, “Wide-Angle Ceramic Retroreflective Luneburg Lens Based on Quasi-Conformal Transformation Optics for mm-Wave Indoor Localization”, IEEE Access (2022). [DOI: 10.1109/ACCESS.2022.3166509]

[5]

K.-D. Jenkel, J. Sánchez-Pastor, M.M. Baloochian, R. Jakoby, M. Sakaki, A. Jiménez-Sáez and N. Benson, “Effect of sintering temperature on the dielectric properties of 3D-printed alumina (Al2O3) in the W-band”, Journal of the American Ceramic Society (2023). [DOI: 10.1111/jace.19597]

[6]

J. Ornik, M. Sakaki, M. Koch, J. C. Balzer and N. Benson, “3D Printed Al2O3 for Terahertz Technology”, IEEE Access (2020). [DOI: 10.1109/ACCESS.2020.3047514]

[7]

K.-D. Jenkel, B. Sievert, A. Rennings, M. Sakaki, D. Erni, and N. Benson, “Radar Cross-Section of Ceramic Corner Reflectors in the W-Band Fabricated with the LCM-Method”, IEEE Journal of Radio Frequency Identification (2023). [DOI: 10.1109/JRFID.2023.3265079]

[8]

H. Guerboukha, M. Sakaki, R. Shrestha, J. Li, J. Kölbel, N. Benson and D. M. Mittleman, “3D-Printed Photonic Crystal Sub-Terahertz Leaky-Wave Antenna”, Advanced Materials Technologies (2024), [DOI: 10.1002/admt.202300698]

[9]

Batra, A.A. Abbas, J. Sánchez-Pastor, M. El-Absi, A. Jiménez-Sáez, M. Khaliel, J. Barowski, M. Wiemeler, D. Göhringer, I. Rolfes, R. Jakoby, and T. Kaiser, “Millimeter Wave Indoor SAR Sensing Assisted With Chipless Tags-Based Self-Localization System: Experimental Evaluation”, IEEE Sensors Journal (2024). [DOI: 10.1109/JSEN.2023.3332431]

[10]

R. Neuder, M. Späth, M. Schüßler and A. Jiménez-Sáez, “Architecture for sub-100 ms Liquid Crystal Reconfigurable Intelligent Surface Based on Defected Delay Lines”, Communications Engineering (2024), Accepted (in press), [Preprint DOI: 10.21203/rs.3.rs-3296270/v1]