# Design, Fabrication and Testing of a Multifrequency Microstrip RFID Tag Antenna on Si
## 1 Introduction  
RFID (Radio Frequency Identification) systems are used in a variety of fields, including logistics, item-level inventory tracking, access control, race timing, tool tracing, and materials management [1, 2], with new applications currently appearing, e.g., health-related ones [3]. RFID tags may be classified into active, semi-passive, and passive ones. An active RFID tag uses a power supply and a transmitter for communication; a semi-passive RFID tag has a power source but no transmitter. A passive RFID tag functions using the electromagnetic wave power delivered by the reader, so it is the cheapest option. The typical frequency bands for RFID applications include 125/135 KHz (LF), 13.56 MHz (HF), 433 MHz, and 840-960 MHz (UHF), as well as 2.45 and 5.8 GHz in the microwave region [4].  
Several studies [5, 6] have explored Si-based designs for OCAs (On-Chip Antennas) with a view to improving gain and efficiency and reducing parasitic interconnections. Such antennas generally tend to have a narrow bandwidth (less than 10%) and a limited radiation efficiency and range; however, what distinguishes them is their small size, allowing integration into a variety of electronic devices [7]. Recent developments in the implementation of high-performance on-chip antennas for millimeter-wave and terahertz-integrated circuit applications are discussed in [8, 9]. On-chip antennas show a significant potential to accommodate applications in fields such as 5G, Internet of Things (IoT), autonomous vehicles, high-data-rate point-to-point communications, GPS, Wi-Fi, WLAN, Bluetooth, and wireless sensors. In recent work (see [9] and references therein), the use of novel substrate layers (such as silicon, graphene, polyimide, and GaAs) to facilitate IC integration has beenstudied, as well as a new excitation mechanism that appears to suppress the propagation of surface waves and reduce substrate loss.  
The rapid advances in IC technologies in recent decades involve various aspects, such as increased complexity, reduced size, higher frequency regions, etc. In this trend, the use of on-chip integrated antennas through, e.g., complementary metal oxide semiconductor (CMOS) technology, is becoming an increasingly attractive option. The Antennas on Si substrate are of recent interest [10; 11; 12; 13], offering advantages such as compact construction and size reduction compared to a common material like FR-4; hence, they are promising candidates for integration into current and future microwave and mm wave ICs. On the other hand, such RFID antennas have received relatively limited interest in the literature so far.  
In this work, we focus on the use of Si and Polyimide for the stack-up section. Our motivation lies in developing novel RFID tag antenna structures on Si, in anticipation of a significant growth in applications for this type of antenna in the near future. An RFID design on a Si-based structure is studied, with a view to improving the impedance matching, bandwidth, and gain performance. The main motivation is to achieve a configurable design of an RFID tag antenna on Si. With the parameterized approach, one can easily change the total area of the design, and consequently the frequency region, by changing only one dimension, and subsequently modifying it as specific needs arise. The coupling between the design lines can be controlled by varying the distance between the lines. We focus on the design of an RFID antenna at 2.4 GHz and 5.8 GHz. These frequencies have been chosen because of the substantial current demand for multi-frequency components in a variety of applications (Wi-Fi, Bluetooth, IoT, Automotive, Smart Home, etc.) that may be expected to further increase in the near future.  
Table 1 presents a comparison between the proposed antenna and some recent studies using FR-4, RT-Duroid 5880, jeans, and SiPh as substrates. The present design, based on a Si substrate and polyimide PI2525, appears capable of achieving a combination of relatively high gain and dual-band behavior.