Study of IC aging on ring oscillator physical unclonable functions

Silicon Physical Unclonable Functions (PUF) are novel circuits that exploit the random variations that exist in CMOS manufacturing process to generate chip-unique random bits. As their applications are security based, it is highly desired to have PUF responses that are very stable against the reliability issues that exist in Integrated Circuits (IC). One of the major sources of unreliability in the technology nodes 90nm and below is device aging. Aging is primarily due to phenomena like Bias Temperature Instability (BTI) and Hot Carrier Injection (HCI). In this work, we study the effect of device aging on the stability of Ring Oscillator PUFs for different PUF circuit-level choices and operating conditions. We observe that most of the PUF's aging instability happens early in its lifetime. Due to the typical differential nature of PUF structures, stability does not change significantly with age. Further, a high correlation has been observed between instability that is caused due to aging and instability that is caused due to temperature. In various RO-PUF setups and operating conditions, we observe that around 4% of the PUF bits are prone to instability due to aging.