An SRAM reliability test macro for fully automated statistical measurements of V_MIN degradation

Negative bias temperature instability (NBTI) has been considered as a main reliability issue in SRAMs since the threshold voltage degradation of PMOS transistors due to NBTI has raised minimum operating voltage (V_MIN) over time. This paper explains an SRAM reliability test macro designed in a 1.2 V, 65 nm CMOS process technology for statistical measurements of V_MIN degradation coming from NBTI. An automated test program efficiently collects statistical V_MIN data and reduces test time. The proposed test structure enables V_MIN degradation measurements for different SRAM failure modes such as the SNM-limited case and the access-time-limited case. The V_MIN dependency on initial device mismatch and stored data is also presented. The measured time to cell data flip affected by NBTI shows the similar trend of NBTI following a power-law dependency on stress time.