Guidelines for mitigating NBTI degradation in on-chip memories

Negative Bias Temperature Instability (NBTI) is one of the dominant factors determining a device lifetime. NBTI causes a threshold voltage shift on a PMOS transistor. Modern LSI often has large on-chip SRAMs such as cache memories. NBTI affects the SRAM cell as degradation in Static Noise Margin (SNM), which is a measure of the read stability of the cell. Hence, a special technique for mitigating NBTI on on-chip SRAMs is required. We investigate features of NBTI via detailed simulations and find that a stress probability and a stress-recovery cycle are important parameters for mitigating it. These parameters are dependent upon the values stored in the cell and the value is dependent upon the on-chip memory configurations and applications. This paper presents the relationship among NBTI degradation, memory configurations, and target applications by focusing on the values stored in SRAM cells. Furthermore, these observations lead us to discuss guidelines for mitigating NBTI degradation of on-chip SRAMs.