# Antenna on Chip (AoC) Design Using Metasurface and SIW Technologies for THz Wireless Applications
## 2 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 \(\times\) 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 ground-plane. 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. 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 \(\left.1\right|\mathrm{S}_{11}\)\(\mathrm{\SIUnitSymbolI}\)\Figures 2-4. It is evident that the metasurface and SIW significantly improve the antenna's impedance bandwidth to better than S\({}_{11}\leq-15\) 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\({}^{\circ}\) and that in the \(xz\)-plane is 17\({}^{\circ}\). 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.