A 1 Mb Nonvolatile Embedded Memory Using 4T2MTJ Cell With 32 b Fine-Grained Power

Abstract

A 1 Mb nonvolatile embedded memory using a fou r transistor and two spin-transf er-torque (STT) magnetic tunnel junction (MTJ) cell is designed and fabricated to demonstrate its zero standby power and high performance. Th e power supply voltages of 32 cells along a word line (WL) are controlled simul- taneously by a power line (PL) driver to eliminate the standby power without impact on the access time. Thi s fine-grained power gating scheme also optimizes th e trade-off between macro size and operation power. The butter fly curve for the cell is measured to be asymmetric as predicted, enhanc ing the cell’s static noise margin (SNM) for data retention. The scaling of 1 Mb macro size is compared with that of the 6T SRAM counterpart, indicating that the former will become smaller than the latter at 45 nm technology node and beyond by m oderately thinning its tunnel dielectrics (MgO) in accordance with the shrink of the MTJ’s cross sectional area. The operat ion current of the macro is also shown to be almost unchanged over generations, while that of the 6T SRAM increases exponenti ally due to the degradation of MOSFET off-current as the d evice scales.