# Carbon Nanotube Based Waveguide and Application to Digital Communication in mmW Band

Rong Tao Jiang, Chong Wei Tan, Xing Hai Zhao, Stéphane Bila, Dominique

Baillargeat, Philippe Coquet, Beng Kang Tay, Pascal Szriftgiser, Guillaume

Ducournau

► To cite this version:

Rong Tao Jiang, Chong Wei Tan, Xing Hai Zhao, Stéphane Bila, Dominique Baillargeat, et al. Carbon Nanotube Based Waveguide and Application to Digital Communication in mmW Band. 2024 54th European Microwave Conference (EuMC), Sep 2024, Paris, France. pp.1074-1077, 10.23919/EuMC61614.2024.10732139. hal-04764993

HAL Id: hal-04764993

https://hal.science/hal-04764993v1

Submitted on 26 Feb 2025

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# Carbon Nanotube Based Waveguide and Application to Digital Communication in mmW Band

Rong Tao Jiang<fnref n="1" />,<fnref n="2" />, Chong Wei Tan<fnref n="2" />, Xing Hai Zhao<fnref n="2" />, Stephane Bila<fnref n="6" />, Dominique Baillargeat<fnref n="5" />, Philippe Coquet<fnref n="1" />,<fnref n="4" />,
Beng Kang Tay<fnref n="1" />,<fnref n="2" />, P. Szriftgiser<fnref n="3" />, G. Ducournau<fnref n="4" />

Co-corresponding Authors: guillaume.ducournau@univ-lille.fr, EBKTAY@ntu.edu.sg

## INTRODUCTION

Millimeter-wave (mmW) band, corresponding to frequencies spanning from 30 to 300 GHz, are integral across diverse modern applications, including radio astronomy, remote sensing, automotive radars, imaging, security screening, and the rapidly evolving landscapes of 5G and 6G telecommunications. Particularly, the high-frequency bands within this range, such as the W-band (75-110 GHz) and D-band (110-170 GHz), are attracting more attention due to their potential to deliver higher data transmission rates and broader bandwidths. This is particularly significant given the congestion of bandwidths at lower frequencies. As the exploration of higher frequencies extends into the mmW band and beyond, the hollow rectangular waveguide (HRWG) emerges as an increasingly appealing interconnect option, owing to its advantageous features of low-loss air medium, isolated propagation environment, and high-power handling capability. Conventionally, waveguide components have been manufactured using metal blocks through CNC milling, but these methods often incur high costs. Additionally, as frequencies increase, the need for smaller dimensions and tighter tolerances poses challenges for traditional fabrication approaches. In this context, there is a need to develop alternative guided propagation structures. In this work, we proposed a novel waveguide technology based on vertically aligned carbon nanotubes (VACNTs). VACNTs, renowned for their exceptional mechanical and electrical properties, have attract significant attention for electronic applications. However, the high CNT growth temperature (>650°C) has limited their deployment in various applications. The recently developed low temperature CNTs transfer technology [1] overcomes this limitation and has been used here to fabricate rectangular waveguide in W band.

The VACNT waveguide measurement results obtained demonstrate good waveguide performances with a flat bandwidth over almost an octave, 60-110 GHz, with losses very close to simulated curves. Promising performances of these structures are extrapolated up to the D-band from

simulations. Moreover, an experimental validation of carrying wideband digital modulated channels in the 60GHz with this waveguide is also presented, enabled thanks to the flat response of the VACNT.

## I. VACNT BASED WAVEGUIDE DESIGN AND FABRICATION

As shown in Fig. 1, VACNTs are used to replace the metallic side walls of the HRWG, utilizing their inherent conductivity primarily in the vertical direction. This property enables the waveguide to support the transmission of the transverse electric (TE) mode.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Schematic of the VACNT-based waveguide structure [2]</figcaption></figure>

Coplanar waveguides (CPW) that are terminated with coupling probes, also made of VACNTs, are used to feed in and feed out the waveguide. The VACNT probes are injected from the top substrate to ensure magnetic coupling within the waveguide. Through substrate vias (TSV) are carefully deployed around the CPW to mitigate substrate mode. 3D EM simulation software is utilized to design and optimize the waveguide performance, such as fine-tuning impedance matching to ensure minimal insertion loss.

The challenges that have obstructed the integration of VACNTs into conventional circuit have been the high VACNT growth temperature (>650°C), and the poor adhesion of VACNTs on the substrate. The low temperature VACNTs transfer technology enables VACNTs to be transferred from grown substrate to various types of substrates at monolithic

<footnote n="1">¹CINTRA IRL 3288, CNRS NTU THALES, Nanyang Technological University, Research Techno Plaza, Singapore 637553</footnote>

<footnote n="2">²Centre for Micro- and Nano-Electronics (CMNE), School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798</footnote>

<footnote n="3">³PhLAM Laboratoire de Physique des Lasers, Atomes et Molécules, 59650 Villeneuve d'Ascq, France</footnote>

<footnote n="4">⁴Univ. Lille, CNRS, Institut d'Electronique, Microélectronique et Nanotechnologie, IEMN, UMR 8520, F-59000 Lille, France</footnote>

<footnote n="5">⁵CNRS@CREATE Ltd, 1 create way, create tower, #08-01, Singapore, 138602</footnote>

<footnote n="6">⁶Univ. Limoges, CNRS, XLIM, UMR 7252, F-87060 Limoges, France</footnote>
microwave integrated circuit (MMIC) compatible temperature (below 300 °C) [3].

The desired VACNT patterns are first grown on the silicon wafer using conventional CVD method, Fig. 2.a). Next, a layer of titanium/gold (Ti/Au) is deposited on top of the VACNTs, to be used as adhesion layer, Fig. 2.b). Subsequently, the VACNT is aligned and transferred to the waveguide top cover using thermocompression bonding as shown in Fig. 2.c). In Fig. 2.d), vacuum suction aids in the detachment of VANT from the original silicon wafer. Another Ti/Au layer is deposited on the transferred VACNT, followed by bonding the VACNT to the waveguide bottom substrate which incorporate the CPW feedlines. A complete HRWG structure with VACNT sandwiched between substrates, substituting the metallic side walls, is then obtained as shown in Fig. 2.f).

<figure><img src="image_3.png" /><figcaption>Fig 2. CNT transfer technology process. a) VACNT grown on silicon wafer b) Ti/Au layer deposited on the VACNT c) align silicon wafer to the silicon top cover and initiate the transfer d) VACNT transferred to the silicon top cover e) Ti/Au layer deposited on the transferred VACNT f) Completed HRWG structure with alumina bottom substrate bonded to the transferred VACNT</figcaption></figure>

Thanks to this transfer technology, arbitrary, complex, high resolution (µm range), and high aspect ratio 3D VACNT geometric structures can be transferred with low temperature at the desired location, with precision positioning (accuracy of 0.01mm) and strong interfacial adhesion. The technology is also an add-on technique to the traditional CMOS technology. An example of the VACNT structure to be used in W-band waveguide is shown in Fig. 3. The designed waveguide features a cross-section measuring 2.6 mm in width and 0.1 mm in height, with the distance between the VACNT coupling probes to be 4.2 mm.

<figure><img src="image_4.png" /><figcaption>Fig 3. SEM view of VACNT transferred on waveguide top cover.</figcaption></figure>

## II. S PARAMETER MEASUREMENT AND DATACOM VALIDATION THROUGH THE CNT WAVEGUIDE

### A. S-parameters up to 110 GHz

As shown in Fig. 4, the measured S parameters closely align with the simulated S parameters, with the simulated $S_{21}$ transmission coefficient showing a range of approximately -1.4dB across frequencies from about 75GHz to 94GHz. The measured $S_{21}$ presented a lower value of around -3dB within the same frequency range. The latest measurement results have been achieved by optimizing the fabrication process, such as precise alignment of VACNT coupling probe.

<figure><img src="image_5.png" /><figcaption>Fig 4. S parameter of VACNT waveguide. Measured $S_{21}$ transmission coefficient are represented by square markers, whereas the simulated parameters are not marked.</figcaption></figure>

### B. Experimental testbed

In order to showcase the possibility to use the VACNT as interconnects for wideband modulated signals, the waveguide structures have been testbed within a telecommunication testbed at 62 GHz. In this setup, photomixing approach is first used to generate I/Q modulated signals with a tunable carrier frequency. An arbitrary waveform generator (AWG) is generating I/Q baseband data, to optically modulate a 1.55 µm laser. A second optical tone is then mixed with the first one, before an optical amplification stage (erbium-doped fiber amplifier, EDFA), then passed through a polarization controller, voltage-controlled attenuator, and THz waves generated using a fast InP photodiode. The VACNT waveguide is then probed using a set of infinity type probe from formfactor [4].
After propagation inside the VACNT waveguide, the modulated 62 GHz signal is detected with a 70 GHz real-time oscilloscope (RTO), without any amplification stage. The signal is then analyzed using the VSA tool [5] for error vector magnitude (EVM) measurement. Fig. 5. summarizes the experimental testbed, while Fig. 6 shows the probing of the VACNT during experiments. 2 modulation formats were tested in this work: Quadrature Phase Shift Keying (QPSK) and quadrature amplitude modulation with 16 symbols (QAM16), using 5GB. So far the proof-of-concept was done only at 62 GHz, from available component at the time of the tests.

expected to be close to RF losses in the device. From the performance obtained, a 8 dB power penalty is observed, corresponding to the overall losses of the VACNT waveguide and probes, and in line with the measured performance at 62 GHz presented on the Fig. 4. Leveraging the wide bandwidth of the VACNT, the structure could be used in the E-band (71-76 and 81-86 GHz) but also up to 110 GHz and even in the D-band. This last point is presently under investigation, especially for the beyond 110 GHz testbed.

<figure><img src="image_6.png" /><figcaption>Fig. 5. Experimental testbed. AWG: Arbitrary Waveform Generator, MZM: Mach-Zehdner Modulator, EDFA: Erbium Doped Fiber Amplifier, VOA: Variable Optical Attenuator, PD: Photodiode, RTO: Real-time Oscilloscope. The VACNT waveguide is inserted between the two probes.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. View of the experimental testbed where the VACNT waveguide is fed by a Datacom signal at 62 GHz. Right probe output is directly connected to the RTO.</figcaption></figure>

### C. Back-to-back and VACNT waveguide transmitted EVM performances

To assess the capability to use the optimized VACNT waveguide as interconnect for communication applications, the comparison between back-to-back (direct connection between photo-diode and RTO) as well as transmitted over the VACNT waveguide was done.

The Fig. 7 and 8 show the EVM performances measured as function of the relative power of the transmitter. At the PIN-PD stage, the optical input power is varied to adjust the 62 GHz injected power in the VACNT waveguide. The EVM obtained curves are following a similar trend, where the EVM is reduced when an increased transmit power is used.

The EVM values (B2B and transmitted via the VACNT waveguide) are compared at low power regime, where no saturation of the EVM is obtained. The power difference observed to reach a fixed EVM for B2B and VACNT waveguide, respectively, is defined as the power penalty. As the VACNT response is quite flat, the power penalty is

<figure><img src="image_8.png" /><figcaption>Fig. 7. EVM performances curves for a QPSK-5GB signal (10 Gbit/s), in back-to-back and transmitted over the VACNT waveguide.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. EVM performances curves for a QAM16-5GB signal (20 Gbit/s), in back-to-back and transmitted over the VACNT waveguide.</figcaption></figure>
CONCLUSION

In this work, we have shown that VACNT can be used to
develop waveguide structures that can be an alternative to
conventional metallic waveguides.

The VACNT waveguide has been measured and it has shown a very good transmission and flatness. While the bandwidth has been measured so far up to 110 GHz, the design of the waveguide can address frequencies up to 145 GHz, which is of interest for interconnects in future backhauling applications in the D-band. Wideband digital modulated channels transmission in the 60GHz with this waveguide has also been validated. Future works will consider the characterization of such structures up to 170 GHz, and also waveguide design scaling as well as propagation losses studies up to the 300 GHz bands.

REFERENCES

ACKNOWLEDGMENT

This work is supported by the France 2030 programs, PEPR (Programmes et Equipements Prioritaires pour la Recherche), CPER Wavetech, and University of Lille, and Ultra-High Data-rate (UHD) IEMN flagship. The PEPR is operated by the Agence Nationale de la Recherche (ANR), under the grants ANR-22-PEEL-0006 (FUNTERA, PEPR 'Electronics') and ANR-22-PEFT-0006 (NF-SYSTERA, PEPR 5G and beyond - Future Networks). The Contrat de Plan Etat-Region (CPER) WaveTech is supported by the Ministry of Higher Education and Research, the Hauts-de-France Regional council, the Lille European Metropolis (MEL), the Institute of Physics of the French National Centre for Scientific Research (CNRS) and the European Regional Development Fund (ERDF).

This work is also supported by the National Research
Foundation, Singapore and Infocomm Media Development
Authority under its Future Communications Research &
Development Programme (FCP-CNRS-RG-2022-
022, REQ0396559), and Singapore MOE Tier 2, MOE2018-
T2-2-105 and partially funded by MOE grant AcRF Tier 1-
2021-T1-001-064 (RG 55/21).

<footnote>[1] 'Wang, J.; Coquet, P.; Tay, B.K. Nanostructure Transfer Method. Singapore Patent 10201807403V, 29 August 2018.'</footnote>

<footnote>[2] J. M. de Saxcé *et al.*, 'E-Band Vertically Aligned Carbon Nanotubes-Based Air-Filled Waveguide', *IEEE Microwave and Wireless Technology Letters*, vol. 33, no. 4, pp. 395-398, Apr. 2023, doi: 10.1109/LMWT.2022.3227946.</footnote>

<footnote>[3] C. F. Siah et al., 'Development of a CMOS-Compatible Carbon Nanotube Array Transfer Method', *Micromachines*, vol. 12, p. 95, Jan. 2021, doi: 10.3390/mi12010095.</footnote>

<footnote>[4] Form factor Infinity probes. Available on-line Accessed March 9th,
2024. https://www.formfactor.com/product/probes/infinity/infinity-probe/</footnote>

<footnote>[5] VSA/Pathwave software. Available on-line, accessed March 9th, 2024
https://www.keysight.com/zz/en/lib/resources/software-releases/89600-vsa-2023.html</footnote>