With progresses in wireless communication and semiconductor technologies, radio frequency identification (RFID) technology has been used in more and more fields to replace some traditional technologies. An RFID system generally includes an RFID tag and an RFID read/write device (i.e., an RFID reader). The RFID tag also includes an antenna to communicate with the RFID reader. Because the cost for the RFID tags is still higher than the traditional bar code tags, large-scale applications of the RFID tags in certain areas, such as in retail business, are still limited. With respect to a total cost of the RFID tag, more than half of the total cost is associated with the antenna of the RFID tag.
The antenna in a traditional RFID tag often is external to the RFID chip, so called the off-chip antenna. Such off-chip antenna may have several disadvantages. For example, such off-chip antenna may be incompatible with current CMOS technology, which may be the main reason why the cost for making the antenna remains high. Also, such off-chip antenna may have a significantly large surface area, which may limit the applications of the RFID tags (such as RFID tags embedded in animals for tracing). Further, such off-chip antenna may increase the uncertainty and instability of an RFID tag system, because the RFID chip needs to be coupled to the antenna using coupling wires. In high-frequency applications, coupling wires may incur parasitic inductance, and the amount of parasitic inductance changes according to the length and angle of the coupling wires. Thus, it may be difficult to precisely control the conditions on the coupling wires during manufacturing process, which may result in undesired reliability.
To solve these problems, on-chip antenna is developed to reduce cost and size of the RFID tags. However, one of the major issues for making the on-chip antenna of a RFID tag is the antenna integration, i.e., creating a high-efficient antenna within a small area of the RFID chip. One of the main limitations of the antenna integration is energy loss on the silicon substrate.
The on-chip antenna is fabricated on the silicon substrate. When the RFID reader reads from or writes to an RFID tag, a portion of the electromagnetic waves from the RFID reader enter into the RFID tag circuitry in the form of energy, and a large portion of that energy enters into the silicon substrate. This may impact operations of the RFID tag circuitry and lower the efficiency of the RFID tag. Such difficulties integrating antenna onto a substrate may be caused by low resistivity, high dielectric constant and low thermal conductivity of an ordinary silicon substrate.
Certain approaches have been reported to try to solve the efficiency issues. For example, Nano Devices and Systems R&D Center in Japan tried proton implantation in the substrate to increase the substrate resistance (A B M H Rashid, S. Watanabe, T. Kikkawa, “High Transmission Gain Integrated Antenna on Extremely High Resistivity Si for ULSI Wireless Interconnect”, IEEE Electron Device Letters, December 2002, vol. 23, no. 12, pp. 731-733.); Delft University in Netherlands tried to use a substrate epitaxial layer of high resistivity polysilicon, and to fabricate integrated inductors or antenna on top of the polysilicon (P M Mendes, S. Sinaga, A. Polyakov, “Wafer-Level Integration of On-Chip Antennas and RF Passives Using High-Resistivity Polysilicon Substrate Technology”, 2004 Electronic Components and Technology Conference, 2004, pp. 1879-1884.); and also other laboratories tried to deposit a 10 μm thick layer of resin between the ordinary silicon substrate and the antenna to isolate the induced current in the substrate (Hiroshi Abe, Masakazu Sato, Kazuhisa Itoi, “Microwave Operation of On-Chip Antenna Embedded in WL-CSP”, IWAT 2005, March 2005, pp. 147-150.). Although these methods can reduce the energy loss of the substrate and increase the RFID tag efficiency to a certain degree, these methods often are incompatible with standard CMOS technology and associated with high cost and complex manufacturing processes. Thus, it may be impractical to use such methods in large-scale industrial productions.
The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.