Effects of Total Ionizing Dose on SRAM Physical Unclonable Functions

The effects of total ionizing dose (TID) on static random access memory (SRAM) physical unclonable functions (PUFs) are studied through X-ray and proton irradiation of commercially available SRAM. Negative shifts in the fractional Hamming weight (FHW) were measured with increasing TID, indicating a migration of bistable cells toward logic low. Additionally, positive shifts in the intra-die fractional Hamming distance (FHD) were measured and indicate changes to the virtual fingerprint of an SRAM PUF with TID, especially in devices that were dosed while holding data. Shifts in inter-die FHD were negligible, allowing individual SRAMs still to be easily identified based on the FHD between a known and unknown sample even after moderate amounts of TID. In some cases, SRAMs could still be identified by their PUFs after the devices had failed. In all cases, the irradiated SRAM devices retained their virtual fingerprint after recovery through annealing.