A Physical Unclonable Function Based on Variations of Write Times in STT-MRAM due to Manufacturing Defects

TL;DR

Problem: authentication of hardware devices via unique, unclonable signatures. Approach: micromagnetic simulations of MTJs in STT-MRAM under thermal noise to map switching probability as a function of pulse width for six defect morphologies. Contributions: demonstrates defect-morphology–dependent switching profiles can realize PUFs, reports inter-Hamming distances near the ideal value of $0.5$, and proposes distribution-based extensions for stronger fingerprints. Significance: enables scalable, robust hardware authentication using manufacturable MTJs whose defect morphology acts as a biometric.

Abstract

A physical unclonable function (PUF) utilizes the unclonable random variations in a device's responses to a set of inputs to produce a unique "biometric" that can be used for authentication. The variations are caused by unpredictable, unclonable and random manufacturing defects. Here, we show that the switching time of a magnetic tunnel junction injected with a spin-polarized current generating spin transfer torque is sensitive to the nature of structural defects introduced during manufacturing and hence can be the basis of a PUF. We use micromagnetic simulations to study the switching times under a constant current excitation for six different (commonly encountered) defect morphologies in spin-transfer-torque magnetic random access memory (STT-MRAM) to establish the viability of a PUF.

Figures (3)

• Figure 1: Six different defect morphologies in an elliptical cobalt soft layer of major axis 100 nm, minor axis 90 nm, and average thickness 3 nm. $C_0$ is a defect-free pristine elliptical soft layer; $C_1$ has a 5-nm diameter hole in the center that is 2 nm deep; $C_{2-3}$ has one-half thicker than the other, with the thicker half 4 nm and the thinner half 2 nm for a step size of 2 nm; $C_4$ has a 10-nm wide rim that rises 1 nm above the surface; $C_5$ has a 5-nm diameter rivet that rises 1 nm above the surface; finally $C_6$ has through-hole with a diameter of 5 nm. Reproduced from [2] with a CC-BY 4.0 license.
• Figure 2: Switching probability for the six different defect morphologies as a function of current pulse width for a current amplitude of 3 mA at a temperature of 300 K. The dashed line shows the probabilities for a pulse width of 0.75 ns.
• Figure 3: Switching time distribution for the six different defect morphologies for a current pulse amplitude of 3 mA at a temperature of 300 K.