Reliability improvement of SRAM PUFs based on a detailed experimental study into the stochastic effects of aging

Physical Unclonable Functions (PUFs) have gained attention as a lightweight hardware security primitive. In particular, the SRAM-based PUF uses the unpredictable power-up value of the cells within an SRAM. Although these values should ideally be always the same within each SRAM to accomplish a correct PUF operation, this is often not the case, especially when factors like circuit aging are considered. While certain studies explore the effects of aging on SRAM PUFs, they often simplify the analysis. For instance, some studies assume that only Bias Temperature Instability (BTI) contributes to circuit degradation while others evaluate the overall degradation without accounting for the stochastic effects of aging on each individual cell. In this work, we first perform a detailed characterization of the nature of aging in SRAM PUFs, demonstrating that the impact of Non-Conductive Hot-Carrier Injection cannot be neglected. We also show that different cells degrade differently, highlighting the importance of accounting for the stochasticity of aging. After that, a method based on the Data Retention Voltage metric to select the cells with the most stable power-up response is introduced. Using these cells to generate the PUF identifier will result in a more stable response, and thus a better PUF performance.