Design Space Analysis for Cross-Point 1S1MTJ MRAM: Selector–MTJ Cooptimization

To increase the density of magnetoresistive random access memory (MRAM) beyond the 1T1MTJ MRAM cell in use today, the design space for 1S1MTJ MRAM array is analyzed by cooptimizing both selectors and MTJs. Current low-resistance MTJs for 1T1MTJ MRAM are not suitable for 1S1MTJ MRAM. Threshold-type selectors would induce a strong read disturb on the MTJ due to the snapback voltage ( ${V}_{\mathrm {TH}}$ – ${V}_{\mathrm {HOLD}}$ ) when the selector is turned on. Also, exponential-type selectors would degrade the read margin (RM) due to its large ON-state resistance. When using existing selectors to achieve a 1-M-bit 1S1MTJ array, it is necessary to adjust the product of resistance and area (RA) and the diameter of the MTJ. An MTJ with the RA $= 15\,\,\Omega \cdot \mu \text{m}^{2}$ and the diameter = 50 nm can meet the criterion of RM > 10% for both exponential-type selectors (exponential slope = 300–500 mV/decade with the current density ~ 1 MA/cm ² ) and threshold-type selectors ( ${V}_{\mathrm {TH}}$ – ${V}_{\mathrm {HOLD}} \sim ~250$ mV). A design space accommodating a selector variation of around 1% can be found for MTJs with tunnel magnetoresistance ratio (TMR) < 250%. With an increased TMR of 250%–350% of the MTJ, the tolerance of variations for exponential-type selectors and threshold-type selectors can be improved to 2% and 4%, respectively. This provides a chance for the 1S1MTJ MRAM with existing selectors.