# Antenna on Chip (AoC) Design Using Metasurface and SIW Technologies for THz Wireless Applications
## 1 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,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 \(\Omega\) 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 \(\Omega\)cm 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.