# Antenna-on-Chip for Millimeter Wave Applications Using CMOS Process Technology
## 1 Introduction  
In the future, communication frequency band, millimeter wave, and sub-terahertz (Sub-THz) technologies can meet the requirements of the fifth-generation (5G) communication system. Therefore, to provide a wider bandwidth and faster transmission speed in the communication frequency band at this stage, the communication frequency band is developing towards the millimeter wave frequency band [1; 2; 3]. However, in some countries such as China and South Korea that use the 6 GHz frequency band, 5G mobile communication has been listed as a commercial frequency band. Hence, many transmission requirements increase, and the frequency bands below 6 GHz can no longer satisfy the transmission requirements of existing users. The millimeter wave is one of the most efficient solutions in improving the transmission efficiency of fifth-generation mobile communication and meeting users' needs. Through the characteristics of a millimeter wave, it can achieve a low delay, high-speed transmission, and other characteristics to meet the transmission needs of communication users [4; 5]. The millimeter wave frequency band has various applications, such as fifth-generation (5G) communication systems, satellite communications, synthetic aperture radars, wideband high-speed point-to-point communications, automotive radars, and other technologies. However, millimeter wave frequency bands have shorter wavelengths than other application frequency bands, so the assembly components are smaller, making wireless systems more compact [6; 7]. With the advancement of chip-integrated circuit manufacturing in recent years, component miniaturization can be easily achieved. Additionally, because of the advancement of radio frequency integrated circuits (RFIC), society is gradually paying attention to the concept of the antenna-on-chip. The wavelength of the millimeter wave frequency band is shorter than that of other frequency bands, which is conducive to small-size assembly. Therefore, millimeter wave antennas can be easily fabricated on wafers [6,8,9]. The method of the antenna-on-chip is suitable for making millimeter wave antennas. The antenna-on-chip can avoid mutual influence in the circuit through circuit design such as integrated circuits (IC) [10]. In a transceiver, the antenna is often used as the first and last element of the transmitter and receiver. However, the antenna and transceiver circuit must match the 50 ohm impedance system impedance. The requirement of 50 ohm impedance matching is overcome in the antenna-on-chip. This is because the RF front-end and antenna can be co-designed in the chip, saving the external design area and components [11,12]. There are many challenges in antenna-on-chips. First of all, the Si substrate of the CMOS process is not suitable for the structural design of the antenna because the typical six-layer metal layer CMOS process is the reason that affects the radiation performance and efficiency of the antenna. Second, the distance between the metal layers in the manufacturing process is only 10 um, the thickness of the Si substrate is 500 um, and the low resistivity is 10 \(\Omega\) cm. The above reasons cause most of the electromagnetic waves of the antenna to propagate to the substrate, which greatly reduces the gain of the antenna-on-chip. The high dielectric constant of the Si substrate (\(\epsilon_{r}=11.9\)) is also a reason for the reduction in the radiation efficiency of the antenna-on-chip. The electromagnetic waves generated by the antenna will be absorbed by the Si substrate instead of propagating into the air, so the radiation efficiency and gain of the antenna-on-chip cannot be compared with traditional typical antennas [13, 14, 15, 16]. Many problems and challenges are often faced in the design of antenna-on-chips. Due to the high dielectric constant and low resistivity of the Si substrate made of wafer CMOS, the gain and radiation efficiency of the antenna-on-chip decrease, resulting in a more significant transmission loss for the antenna-on-chip [14,15].