Improving SRAM Vmin and yield by using variation-aware BTI stress

We propose a novel method that exploits BTI to partially offset variation and thus improve SRAM Vmin and yield. We show correlation between a bitcell's power-up state and its static noise margin. By applying stress with periodic re-power-up, device mismatch can be compensated by BTI induced changes. The proposed method has no extra design and area cost. It can be applied during burn-in test to offset manufacturing variation and/or used during the lifetime of the chip to offset variation from real-time aging and hence continue to improve the margins. Simulations in 45nm show that write, read, and hold Vmin at 6σ can be reduced by 128, 75, and 91 mV, respectively. Measurements from a 16Kb 45nm SRAM demonstrate the improvement of Vmin and yield.

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