Antenna on Chip (AoC) Design Using Metasurface and SIW Technologies for THz Wireless Applications

: This paper presents the design of a high-performance 0.45–0.50 THz antenna on chip (AoC) for fabrication on a 100-micron GaAs substrate. The antenna is based on metasurface and substrate-integrated waveguide (SIW) technologies. It is constituted from seven stacked layers consisting of copper patch–silicon oxide–feedline–silicon oxide–aluminium–GaAs–copper ground. The top layer consists of a 2 × 4 array of rectangular metallic patches with a row of subwavelength circular slots to transform the array into a metasurface. This essentially enlarges the effective aperture area of the antenna. The antenna is excited using a coplanar waveguide feedline that is sandwiched between the two silicon oxide layers below the patch layer. The proposed antenna structure reduces substrate loss and surface waves. The AoC has dimensions of 0.8 × 0.8 × 0.13 mm 3 . The results show that the proposed structure greatly enhances the antenna’s gain and radiation efﬁciency, and this is achieved without compromising its physical size. The antenna exhibits an average gain and efﬁciency of 6.5 dBi and 65%, respectively, which makes it a promising candidate for emerging terahertz applications.


Introduction
Smart wireless devices have grown in popularity at an exponential rate. This has caused an explosion of data traffic in the limited prescribed bandwidth resources [1]. It is estimated that the data rate in the next decade is going to be in the order of Tbps [2,3]. Although it has been shown that THz communication can easily provide a Gbps data rate, we are at a nascent stage of development for data rates in the Tbps [4,5]. THz waves benefit from being sandwiched between the millimeter and light waves in the electromagnetic spectrum. Compared with millimeter waves, THz waves have a wider usable frequency band and focused beam directivity that makes them highly unsusceptible to interference issues. Compared with light waves, THz waves have a stronger penetration power [6].
Antennas have a direct bearing on the performance of wireless systems in terms of operating bandwidth, radiation gain, and efficiency. These parameters have an impact on the system's data transmission rate, imaging resolution, and range. Compared with millimeter waves, the design of THz antennas is highly challenging as THz antennas operate at much higher frequencies and their size is significantly smaller. Moreover, the packaging of THz antennas is limited by materials and process technologies. Additionally, Electronics 2021, 10, 1120 2 of 8 free-space path loss is an inevitable characteristic of all wireless communication. However, THz waves are greatly absorbed by the atmosphere. Hence, increasing the gain of the THz antenna is an important requirement to compensate for the free-space path loss. Moreover, THz antennas suffer from relatively high loss, which makes them currently impractical for on-chip fabrication.
An antenna is the first element in a wireless receiver and the last element in a transmitter. To guarantee optimum power transfer between the antenna and the RF-integrated circuitry, it is necessary to include an impedance matching network at the junction of the antenna and the RF circuitry. Typically, antennas are matched to a 50 Ohm impedance system, which normally requires converting the complex impedance of the system to 50 Ohm [7]. This is achieved by employing conjugate matching, in which the imaginary section of two impedances has an identical magnitude but reverse signs. Additionally, bond wires are used to connect the THz antenna to the integrated circuit. As bond wires are not characterized well at THz, the matching is often suboptimal [6].
On-chip realizations of antennas overcome the abovementioned limitations as impedances of integrated circuit elements do not need to be matched with 50 Ohm [8][9][10]. Co-design of circuits and antennas guarantee that complex impedances are conjugately matched with no need for a matching circuit, thus saving several additional elements, area, expense, and numerous design attempts. Furthermore, the antenna on chip (AoC) omits any doubt with bond-wires as metal interlinks are used to interface the integrated circuit to the antenna feed point [11].
Antennas for on-chip applications need to be planar as they need to be fabricated using the same integrated circuit processing. A low-resistivity substrate (10-20 Ω cm) is a common choice for the fabrication of CMOS circuits, which is necessity to overcome the latch-up issue. This type of substrate, however, greatly limits the efficiency of planar antennas resulting from high ohmic and dielectric loss in the underlying silicon substrate. A dipole antenna fabricated on a 10 Ωcm silicon substrate typically has a limited efficiency of 10% [12,13].
Proposed in this paper is a technique to overcome the limitations on bandwidth, gain, and efficiency of THz antennas fabricated on-chip. This is achieved by employing two different technologies, namely a substrate-integrated waveguide (SIW) and a 2D metamaterial, which is commonly referred to as a metasurface [14]. The proposed technique reduces the loss due to the substrate and suppresses the adverse effects of surface waves. The results confirm this has notable improvement in the radiation characteristics of AoC.

Antenna on Chip Design
The proposed AoC is constituted from several stacked layers comprising Cu-SiO 2 -Cu-SiO 2 -Al-GaAs-Cu as shown in Figure 1. The top layer consists of a 2 × 4 array of rectangular copper patches. Etched on the patches is a row of subwavelength circular slots. The antenna is excited using proximity coupling at the patch edge, which is analogous to a gap coupling patch. This is achieved with a microstrip coplanar waveguide feedline, which is created by having both sides next to the feedline grounded. The feedline is sandwiched between the two silicon oxide layers, below which is a layer with aluminium patches that are separated from each other by a narrow gap. This layer acts like a partially reflective surface. The outer periphery of the aluminium patches is studded with an arrangement of densely lined metallic posts or vias connecting it to the ground-plane through the GaAs substrate to create a SIW electromagnetic resonant cavity. Incident electromagnetic waves that penetrate through the gaps in the aluminium layer are reflected at the groundplane. The thickness of the GaAs substrate is such that the phase shift of the ground-plane reflected waves is in phase with the waves reflected at the surface of the copper patches. With this arrangement, the aluminium surface acts like an artificial magnetic conductor (AMC) that fully reflects incident waves with a near zero degrees reflection phase [15]. This configuration results in significantly enhanced directivity. The circular slots across the rectangular patches act like miniature resonators that radiate energy at the THz band. at the surface of the copper patches. With this arrangement, the aluminium surface acts like an artificial magnetic conductor (AMC) that fully reflects incident waves with a near zero degrees reflection phase [15]. This configuration results in significantly enhanced directivity. The circular slots across the rectangular patches act like miniature resonators that radiate energy at the THz band. The antenna was implemented on a GaAs substrate of 100-micron thickness, a dielectric constant of 12.94, and a loss-tangent of 0.006. The proposed SIW structure reduces substrate loss, radiation leakage, and adverse effects of surface wave propagation. This is achieved by carefully choosing the diameter and spacing of the metallic vias [16]. The subwavelength circular slots transform the array into a metasurface that has an effect of amplifying the effective aperture area of the antenna, the consequence of which is an enhancement in the radiation gain and efficiency of the antenna. In addition, the slots also improve the impedance match and bandwidth of the antenna.
The optimized structural parameters of the proposed AoC are given in Table 1. Optimization was done using CST Microwave Studio, which is a 3D full-wave electromagnetic simulation tool. The optimization involved parameterizing the antenna structure to achieve the target goal of |S11| ≤ −10 dB across 450-500 GHz. The optimizer used in CST Microwave Studio was the Covariance Matrix Adaptation Evolutionary Strategy The proposed SIW structure reduces substrate loss, radiation leakage, and adverse effects of surface wave propagation. This is achieved by carefully choosing the diameter and spacing of the metallic vias [16]. The subwavelength circular slots transform the array into a metasurface that has an effect of amplifying the effective aperture area of the antenna, the consequence of which is an enhancement in the radiation gain and efficiency of the antenna. In addition, the slots also improve the impedance match and bandwidth of the antenna.
The optimized structural parameters of the proposed AoC are given in Table 1. Optimization was done using CST Microwave Studio, which is a 3D full-wave electromagnetic simulation tool. The optimization involved parameterizing the antenna structure to achieve the target goal of |S 11 | ≤ −10 dB across 450-500 GHz. The optimizer used in CST Microwave Studio was the Covariance Matrix Adaptation Evolutionary Strategy (CMA-ES). This optimizer was used because it has a relatively fast convergence, and it uses the history of the previous iterations to improve the performance of the algorithm while avoiding local optimums. The AoC has a form factor of 0.8 × 0.8 × 0.13 mm 3 . The effect of the circular slots and metallic vias on the performance parameters of the AoC are shown in It is evident that the metasurface and SIW significantly improve the antenna's impedance bandwidth to better than S 11 ≤ −15 dB, and the radiation gain and efficiency to over 0.45-0.5 THz. The average impedance match over 0.45-0.5 THz is approximately 30 dB, which an improvement of 15 dB without the slots and vias. The antenna has a fractional bandwidth of 10.52%. By incorporating the slots in the patches, the average radiation gain of the antenna increases to 6.5 dBi. This is an improvement of 2.7 dBi without the slots. The corresponding radiation efficiency increases to approximately 65%, which is an improvement of 12% without the slots. The average gain with the inclusion of the metallic vias is 6.5 dBi, which is an improvement of 4.4 dBi without the vias. The average efficiency with the metallic vias increases to about 65%, which is an improvement of 19% without the vias.
The E-plane and H-plane radiation patterns of the proposed AoC at 0.45 THz are shown in Figure 5. Clearly, the radiation patterns exhibit relatively high directivity in both planes. The half-power beam width in the xy-plane is 26 • and that in the xz-plane is 17 • . In addition, the side-lobe and back-lobe levels are all below −17.5 dBi in the xz-plane and below −22 dBi in the xy-plane. (CMA-ES). This optimizer was used because it has a relatively fast convergence, and it uses the history of the previous iterations to improve the performance of the algorithm while avoiding local optimums. The AoC has a form factor of 0.8 × 0.8 × 0.13 mm 3 . The effect of the circular slots and metallic vias on the performance parameters of the AoC are shown in Figures 2-4. It is evident that the metasurface and SIW significantly improve the antenna's impedance bandwidth to better than S11 ≤ −15dB, and the radiation gain and efficiency to over 0.45-0.5 THz. The average impedance match over 0.45-0.5 THz is approximately 30 dB, which an improvement of 15 dB without the slots and vias. The antenna has a fractional bandwidth of 10.52%. By incorporating the slots in the patches, the average radiation gain of the antenna increases to 6.5 dBi. This is an improvement of 2.7 dBi without the slots. The corresponding radiation efficiency increases to approximately 65%, which is an improvement of 12% without the slots. The average gain with the inclusion of the metallic vias is 6.5 dBi, which is an improvement of 4.4 dBi without the vias. The average efficiency with the metallic vias increases to about 65%, which is an improvement of 19% without the vias.    The E-plane and H-plane radiation patterns of the proposed AoC at 0.45 THz are shown in Figure 5. Clearly, the radiation patterns exhibit relatively high directivity in both planes. The half-power beam width in the xy-plane is 26° and that in the xz-plane is 17°. In addition, the side-lobe and back-lobe levels are all below −17.5 dBi in the xz-plane and below −22 dBi in the xy-plane.    The E-plane and H-plane radiation patterns of the proposed AoC at 0.45 THz are shown in Figure 5. Clearly, the radiation patterns exhibit relatively high directivity in both planes. The half-power beam width in the xy-plane is 26° and that in the xz-plane is 17°. In addition, the side-lobe and back-lobe levels are all below −17.5 dBi in the xz-plane and below −22 dBi in the xy-plane.    The E-plane and H-plane radiation patterns of the proposed AoC at 0.45 THz are shown in Figure 5. Clearly, the radiation patterns exhibit relatively high directivity in both planes. The half-power beam width in the xy-plane is 26° and that in the xz-plane is 17°. In addition, the side-lobe and back-lobe levels are all below −17.5 dBi in the xz-plane and below −22 dBi in the xy-plane.   Table 2 compares the salient features of the proposed on-chip antenna with AoC reported in the literature. The maximum gain of the proposed antenna of 7.4 dBi is lower than [17,18] but it is comparable to references [11,19,20]. Additionally, it has a maximum Electronics 2021, 10, 1120 6 of 8 efficiency of 70%, which is lower than [17,19,20] but it is comparable to [11]. However, compared with the cited work in Table 2 the proposed antenna operates over a much  higher frequency (between 450 and 500 GHz). Moreover, other than the antenna in [21] that operates over a much lower frequency range (50-70 GHz), the proposed antenna is much less thick with respect to the operating wavelength than other on-chip antennas reported to date. This is attributed to combining metasurface and SIW technologies in the implementation of the on-chip antenna. The thinner antenna structure is important to prevent the generation of surface or substrate modes that can adversely affect the antenna's performance, especially at THz frequencies [22].