A Ferroelectric Memory Embedded In A RFID Transponder With 2.4V Operation And 10 Year Retention At 70/spl deg/C

Abstract

A FeRAM with l0year data retention at 70°C was embedded in a RFID tag. The use of SrBi2 Ta2 0 9 allowed a 2.4V operation with a long access distance of 50cm from a ReaderlWriter. Introduction A 288bit SBTO (SrBi2Ta209) based FeRAM (ferroelectric random access memory) embedded IC for contactless RFID (radio frequency identification) tags (transponders) has been developed. The emphasis is placed on its advantages in retention at high temperatures and low-voltage operation allowing contactless data communication at far distances. FeRAM Performance A material solution to fatigue in ferroelectric memories has been achieved by the use of SBTO with Pt electrodes:[l]. In addition, the FeRAM using SBTO exhibits low switching voltages and high-switching speeds: [2-31. Therefore SBTO memories are the best choice for nonvolatile memories for use in low power and high speed applications. The FeRAM is composed of two transistors and two SBTO capacitors per bit. The SBTO capacitors were fabricated using a MOD (metallo-organic decomposition) technique: [2]. Figure 1 shows the operating voltage range of the embedded FeRAM. The lower limit of the range at an access time of 1 . 2 , ~ s is 2.4V, low enough for write or read. Figure 2 shows the temperature dependence of MTBF (mean time between failure) for data retention of the FeRAM, programmed at 2.4V. The activation energy of MTBF in data retention is 1 .1 5eV. The data retention of FeRAM is estimated to be longer than 10 years at 70°C. The time dependence of remnant polarization of SBTO capacitors for different write voltages is shown in Figure 3. The remnant polarization written at 2.4V is found to be preserved with a sufficient margin for longer than 10 years at 70°C. RFID Tag System Figure 4 shows a block diagram of the RFID tag circuit incorporating a FeRAM. The power generator supplies DC voltage by rectifying the current induced by the electromagnetic field at a frequency of 125 kHz, provided by a Readerwriter. The receiver demodulates the signals from the Readerwriter by phase shift keying. Then the transmitter sends RF signals back to the Readerwriter, also by phase shift keying. The control logic circuit coordinates the cooperative operation of the FeRAM, transmitter and receiver. Figure 5 shows a photo of the IC. Performances and Characteristics of RFID Tag Figure 6 shows the generated DC voltage at the power generator versus access distance between a RFID tag and a Readerwriter. Since the FeRAM operates even at 2.4V, the RFID tag can communicate with the Readerwriter at a distance of up to 50cm. The followings are typical performances and characteristics of the RFID tag: Memory technology Memory size Data retention Endurance Transmit carrier frequency : Receive carrier frequency : Transmit modulation Receive modulation Chip size SBTO FeRAM 288bit 1 Oyears at 7 0°C 1 x 10"cycles 62.SkHz 125kHz BPSK@7812.Sbps BPSK@78 12.5bps 2.76x2.3 8mm2