Modeling SRAM start-up behavior for Physical Unclonable Functions

One of the emerging technologies for cryptographic key storage is hardware intrinsic security based on Physical Unclonable Functions (PUFs); a PUF is a physical structure of a device that is hard to clone due to its inherent, device-unique and deep-submicron process variations. SRAM PUF is an example of such technology that is becoming popular. So far, only a little is published about modeling and analysis of their start-up values (SUVs). Reproducing the same start-up behavior every time the chip is powered-on is crucial to produce the same cryptographic key. This paper presents an analytical model for SUVs of an SRAM PUF based on Static Noise Margin (SNM), and reports some industrial measurements to validate the model. Simulation of the impact of different sensitivity parameters (such as variation in power supply, temperature, transistor geometry) has been performed. The results show that out of all sensitivity parameters, variation in threshold voltage is the one with the highest impact. Industrial measurements on real memory devices validate the simulation results.