Modeling of dielectric breakdown-induced time-dependent STT-MRAM performance degradation

In recent years, spin-transfer torque magnetoresistive random access memory (STT-MRAM) has gained a lot of interest as a promising memory candidate for future embedded applications. STT-MRAM possesses desirable memory attributes such as excellent readability, writability, stability, non-volatility, and unlimited endurance. Moreover, ITRS reports that STT-MRAM can endure 10¹⁵ cycle operations before breakdown thus meeting 10 yrs life-time. As shown in Fig. 1, STT-MRAM bitcell consists of one access transistor and one magnetic tunnel junction (MTJ) (1T-1R). One of the primary reliability concerns in STT-MRAM is the dielectric breakdown of the tunnel junction MgO in the MTJ known as time-dependent dielectric breakdown (TDDB). The thickness of MgO is on the order of 1nm and the voltage across the MTJ during write operation is approximately 0.7V resulting in electric field of ~10MV/cm across it which can induce TDDB. Thus, such high stress conditions can lead to lower breakdown time (T_BD) which can go even lower with further MgO thickness scaling. In addition to the hard breakdown (HBD) in MTJ which results in very low MTJ impedance and inability to function as memory, experimental results show that soft breakdowns (SBD) also exists. SBDs cause minor degradation in the MTJ resistance and they have shorter average time to appear compared to HBDs. In this paper, we explore in detail the physical mechanism behind both HBD and SBD, and using percolation model we estimate the time dependent degradation in the MTJ performance parameters such as tunneling magneto-resistance (TMR), write current (J_C), write-time (T_WR) and lifetime (T_LIFE).