A 325F2 Physical Unclonable Function Based on Contact Failure Probability With Bit Error Rate < 0.43 ppm After Preselection With 0.0177% Discard Ratio

Preselection methods in which unstable bitcells are discarded are widely used to enhance the stability of the physical unclonable function (PUF). If the discard range of the preselection is too narrow, all unstable bitcells cannot be discarded, whereas, if the discard range is too wide, the PUF circuit area increases because the number of redundant bitcells should increase. Moreover, the discard range of most previous PUFs is not determined by theoretical analysis but rather by repeated measurements, which requires a long test time and cannot determine the accurate discard range to guarantee the PUF stability. This article demonstrates an efficient preselection method to determine the discard range through a theoretical analysis of the PUF entropy source for the PUF based on contact failure probability (contact PUF). After this preselection with the discard ratio of 0.0177%, no PUF responses changed among 2367040 bits from 1138 chips, i.e., bit error rate < 0.43 ppm, in seven harsh environmental tests regulated by the Joint Electron Device Engineering Council (JEDEC) standard. The proposed preselection method also enables the contact PUF to be implemented in a small area due to the low discard ratio. A small-area design methodology of contact PUF bitcells is also proposed. The PUF bitcell area per bit is
$325F^{2}$
, the smallest area compared to previous PUFs. The contact PUF with the proposed preselection applied is fabricated in 100-nm CMOS technology. It achieves 0.4999 of uniqueness and 0.9942 of Shannon entropy and passes all applicable randomness tests stipulated by the National Institute of Standards and Technology (NIST) SP800-22.