Failure analysis of 6T SRAM on low-voltage and high-frequency operation

Careful analysis of SRAM bit failure at high-frequency operation has been described. Using the nanoprober technique, MOS characteristics of failure bit in actual memory cells had been measured directly. It was confirmed that the drain current of a PMOS was about one order in magnitude smaller and the threshold voltage was about 1 V higher than that for normal bits. A newly developed, unique selective etching technique using hydrazine mixture showed these degradations were caused by local gate depletion, and TEM observation showed the PMOS gate poly-Si of the failure bit had a huge grain. Minimizing grain size of the gate poly-Si is found to be quite effective for improving drain current degradation and suppressing this failure mode.

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