Reliable cache design with detection of gate oxide breakdown using BIST

Scaling of device sizes has reduced gate oxide thickness to a few atomic layers, increasing the vulnerability of the gate oxide to breakdown. During breakdown, devices go through a gradual wearout process after an initial gate leakage increase leading to device failure. It is proposed that if wearout can be monitored, cache arrays with failing cells can be reliably operated after reconfiguration given available memory redundancy. Using experimentally verified gate oxide breakdown models, a detailed analysis of the effect of progressive gate oxide breakdown on the performance of a conventional 6T SRAM cell is presented for 45 nm predictive technology. The DC margin trends (Read, Write and Retention) and access times (Read and Write) during wearout are analyzed, and a cell breakdown point due to degradation in each of these parameters is defined. A combination of these results is used to formulate a practical definition for the hard-breakdown point of a cell. Using an on-chip PVT (process, voltage, and temperature) tolerant monitoring scheme, it has been shown that gradual wearout in SRAM cells, due to gate oxide breakdown, is detectible, and cell failure can be predicted before its occurrence.