A 1Mb 28nm STT-MRAM with 2.8ns read access time at 1.2V VDD using single-cap offset-cancelled sense amplifier and in-situ self-write-termination

1T1R spin-transfer-torque (STT) MRAM is a promising candidate for next-generation high-density embedded non-volatile memory [1-2]. However, 1T1R STT-MRAM suffers from limited sensing margin and high write power. As shown in Fig. 30.2.1(a), sense amplifier design is challenging due to the small difference (only 2x) between the high-resistance state (R AP ) and the low-resistance state (R P ), as well as R AP degradation with increasing temperature. Moreover, R P and R AP resistance distributions shift with process variation, requiring a read reference (V ref ) that tracks process. To improve the sensing margin, several offset-cancellation methods have been reported to reduce sense amplifier mismatch [3]. However, these methods use multiple capacitors and hence incur significant area overheads. To address this issue, we propose an offset-cancelled sense amplifier that uses only a single capacitor to significantly improve the sensing margin by more than 60%. A second design challenge for STT-MRAM stems from the high current needed to flip a cell during a write operation. For non-volatile memory applications with a 10-year retention time requirement, the write current can be as high as several hundred μA. However, as shown in Fig. 30.2.1(b), the required write time varies with the state change required (0→1 or 1→0), process variation, and temperature. As a result, a fixed write time that ensures successful write for all conditions wastes a significant energy for typical or average conditions. We propose an in situ write-self-termination method to reduce write energy in most scenarios. The sense amplifier is reconfigured to continuously monitor the write operation and automatically shuts off the write drivers when the state transition is detected, without an area or timing penalty. In addition, dual dummy columns are added in each array to provide read V ref tracking of row-wise PVT variation. A 1Mb STT-MRAM was fabricated in 28nm technology, and achieves a 2.8ns read-access time at 25°C and 3.6ns at 120°C, respectively. With in-situ self-write-termination the write power is reduced by 47% with a 20ns write-access time at 25°C and by 60% at 120°C.