Fine Grained Wearout Sensing using Metastability Resolution Time

Vikram Suresh and Wayne Burleson
University of Massachusetts, Amherst


Reliability concerns due to Negative Bias Temperature Instability (NBTI) and Positive Bias Temperature Instability (PBTI) are increasing in nanometer CMOS memory and digital logic. Conventional chip lifetime enhancement techniques rely on run-time wearout sensors to detect device threshold voltage (Vth) shift. In this work, we present a novel wearout sensing technique using metastability resolution time. The circuit consists of a reference inverter and age-tracking inverter cross-coupled to form a metastable cell. The resolution time of this cell is impacted by temporal Vth shift in the tracking devices. The tracking devices are sized relatively large compared to reference inverter to minimize impact of process induced Vth offset. The resolution time measurement is performed in two stages using a ring oscillator based Time to Digital Converter (TDC). The first stage of measurement establishes a reference precision during run time. In the second stage, the actual resolution time is measured relative to the reference. This alleviates impact of process variation and measurement temperature. Implementation and simulation in 32nm Predictive Technology Model indicate a lightweight sensor with estimated area of ~105μm2. The resolution time of the circuit tracks Vth shift due to both NBTI and PBTI. A worst case measurement error at +/-3σ Vth offset is 9.3% of ΔVth. A nominal measurement time of ~1ns provides accurate estimate of Vth shift without allowing recovery time for tracking devices.