Wordline overdriving test: An effective predictive testing method for SRAMs against BTI aging

Negative bias temperature instability (NBTI) and positive bias temperature instability (PBTI) cause SRAM stability degradation during its lifetime. Hence, some of the SRAM cells that are functional initially may fail due to aging after usage. This causes field failures of the shipped chips. In this paper we focus on screening out these SRAM cells which may fail after aging during post-fabrication testing. We propose wordline overdriving test (WODT), a novel predictive testing method for SRAMs. During the test we overdrive the SRAM wordline to simulate the effect of aging, hence we are able to detect the cells that will fail after aging over a desired lifetime. We compare our proposed method with low-VDD test, and show that WODT outperforms low-VDD test with a large margin by achieving dramatically lower quality loss.