Enhanced write margin of perpendicular MRAM cells using thick MgO cap layer

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Implementation of spin transfer torque magneto-resistive random access memory (STT-MRAM) in memory chips requires that the write margin of the MRAM cell, defined as the difference between breakdown voltage and write voltage for the specified endurance, write error rate, and write speed, is sufficiently large in order to accommodate resistance variations arising from external chip circuitry. We show that by increasing only the thickness of the MgO cap layer to make its resistance-area product close to that of the main MgO barrier, the write margin can be increased substantially without affecting the thermal stability of the cell.