Unveiling the Vulnerability of Oxide-Breakdown-Based PUF

This work reports a potential vulnerability of an oxide-breakdown-based Physical Unclonable Function (PUF). This generates a unique chip key based on the stochastic competition between the formation of oxide breakdown in pairs of identical transistors. Depending on which transistor breaks within each pair, a bit value of ‘0’ or ‘1’ is assigned to it. Combining the bits corresponding to several transistor pairs, the key is generated. This type of PUF had been considered secure until now. However, we show that, using Voltage Contrast Scanning Electron Microscopy (VC-SEM), it is possible to determine which transistor has oxide breakdown within each pair, and thus extract the PUF key with an accuracy larger than 99.9%. For this, the diffusion regions of the inspected transistors must have contacts. Furthermore, at least two contacts per cell (e.g., one for each of the two identical transistors) are needed due to the differential nature of the analysis.