Enhancing the Reliability of SC PUF Through Optimal Capacitor Configuration

Switched-capacitor (SC) PUF has been used in the protection of security chips from invasive attacks due to their capacitive sensitivity. However, non-volatile memory cells are often used to bypass the unstable SC PUF units, which then become insensitive to outside attacks, creating a vulnerability that can be exploited by probing or Focus Ion Beam (FIB) attacks, thereby posing a potential security risk if these units are used to cover sensitive areas of the security chip. This paper proposes a method to increase stability while maintaining anti-invasive-attack capabilities by utilizing the configurability of the four capacitors in each SC PUF unit, and selecting the configuration that provides a larger mismatch of capacitor ratios, resulting in a larger output voltage mismatch. By doing so, the variation of offset for the latch-styled sense amplifier (LSSA) is suppressed, and the stability of the SC PUF is significantly increased. Additionally, changing the polarity of the LSSA’s inputs to match its offset also increases the effective input voltage of the LSSA. The proposed SC PUF using optimal capacitor configuration is fabricated using a 12 nm FinFET CMOS process. Test results demonstrate that the bit error rate of the SC PUF is reduced from about 17% to about 1.5%, and most of the SC PUF units can serve as anti-invasive-attack shields of the security chip.