14.7 In-situ techniques for in-field sensing of NBTI degradation in an SRAM register file

SRAM register files have sensitive circuitry and often operate with high switching activity and at high temperature. This makes them particularly vulnerable to aging by negative-bias temperature instability (NBTI) degradation of their PMOS devices. We propose a technique to sense this aging degradation; it is an in-situ technique sensing the threshold voltage (V_t) of PMOSs directly in bitcells, and can operate in-field, thanks to the ability to sense V, robustly across temperature and voltage variations. This technique can be foundational for several dynamic reliability management (DRM) approaches, including: 1) sensing V, values periodically (e.g., every several months) for evaluating the amount and the rate of NBTI degradation; 2) sensing V, differences between two PMOSs in a bitcell to determine their strength skew and to estimate the minimum functional voltage (V_MIN) degradation; and, 3) using the skew information across bitcells to create recovery vectors, which can be used to recover the aged PMOSs and thereby rebalance the skews. Existing in-situ techniques using ring oscillators or current sensors to sense bitcell reliability and performance cannot support in-field operation, which is a critical issue for DRM since it is impractical to control environmental parameters, particularly temperature, during sensing.