Layout-aware optimization of STT MRAMs

We present a layout-aware optimization methodology for spin-transfer torque (STT) MRAMs, considering the dependence of cell area on the access transistor width (W_FET), number of fingers in the access transistor and the metal pitch of bit- and source-lines. It is shown that for W_FET less than a critical value (∼7 times the minimum feature length), one-finger transistor yields minimum cell area. For large W_FET, minimum cell area is achieved with a two-finger transistor. We also show that for a range of W_FET, the cell area is limited by the metal pitch of bit- and source-lines. As a result, in the metal pitch limited (MPL) region, W _FET can be increased with no change in the cell area. We analyze the impact of increase in W_FET in the MPL region on the write margin and cell tunneling magneto-resistance (CTMR) of different genres of STT MRAMs. We consider conventional STT MRAM cells in the standard and reverse-connected configurations and STT MRAMs with tilted magnetic anisotropy for the analysis. By increasing W_FET from the minimum to the maximum value in the MPL region (at iso-cell area) and reducing read voltage to achieve iso-read disturb margin, 2X improvement in write margin and 27% improvement in CTMR is achieved for the reverse-connected STT MRAM. Similar trends are observed for other STT MRAM cells.