# CNT-PUFs: Highly Robust and Heat-Tolerant Carbon-Nanotube-Based Physical Unclonable Functions +
## 2 Related Work
This work demonstrated good results regarding the uniqueness and robustness of the fabricated PUFs, reporting intra-device Hamming distances of 6.3% after two weeks and 8.3% after two months. In comparison, our work allows for an average intra-device Hamming distance value of 0.1% at normal temperature (and 2% at differing temperatures) even after an extended time period. In 2021, [PERSON] and [PERSON] [32] proposed a ternary-cycle-operator-based PUF utilising CNT-FETs. However, this work provided simulation results only regarding the uniformity of the relevant PUF responses and did not examine their robustness. Most recently, in 2022, [PERSON] et al. [33] described the fabrication of identical pairs of ternary PUFs based on transferred Chemical-Vapour-Deposition (CVD)-grown CNTs ondual-line structures, which they referred to as \"twin PUFs\". The individual PUFs were examined at room temperature and at 100 \({}^{\circ}\)C and exhibited high uniformity and uniqueness, as well as high consistency to their pairs, over a six-month period. The authors noted an intra-device Hamming distance value close to zero over six months, which appears to be an ideal result. However, we need to note that this result concerned a PUF whose characteristics are reproducible and, thus, predetermined during the manufacturing process (other examples of such PUFs include the ones described in [34] and in [35] and are also individually extremely robust), and not a PUF whose characteristics are not predetermined, but are only based on the slight intrinsic variations of the manufacturing process, leading to highly unpredictable responses, like the CNT-PUF that our work examined. Moreover, the intra-device Hamming distance was based on a comparably weak database of only two measurements. Ageing or fatigue arising from electrical loading upon cyclic readout was not considered. Considering parallel fabrication on large substrates such a CNT transfer process provokes biased and, thus, predictable PUFs. This is because CVD-grown horizontal CNTs lead to highly aligned unidirectional assemblies up to several millimetres along the CNT axis and, thus, cause repeating bit sequences in neighbouring PUFs. We generally note an increased interest in CNT-based PUFs as demonstrated by the relevant literature. At the same time, however, we also observed that the relevant literature either fails to report on results regarding the stability of the examined CNT-based PUFs or the relevant average intra-device Hamming distance values are significantly higher than the 0.06% value that our work allows for, under normal conditions. Moreover, most of these works were based on simulation results, and the few of them that dealt with fabricated structures required either additional lithography steps for the definition of CNT deposition sites, e.g., [26], or sophisticated transfer processes, e.g., [33]. Furthermore, we noticed that the CNT transfer processes as described by [PERSON] et al. were hardly 200 mm wafer scalable, and the dielectrophoretic CNT integration from [PERSON] et al. requires an additional expensive lithography and etching step. In contrast, our work presents a large-scale fabrication of 144-bit CNT-PUFs by highly scalable and automated printing-like processes on 200 mm wafers for stochastic CNT assemblies with a minimal amount of dispersion (<200 uL/wafer), featuring high uniformity and uniqueness. In addition, for the first time, the robustness of the CNT-PUFs was characterised in detail through repeated measurements utilising a cyclic thermal load between 23 \({}^{\circ}\)C and 120 \({}^{\circ}\)C. Moreover, to the best of our knowledge, this work is the first to examine passivated CNT-PUFs. Passivation of p-type CNT-FETs is typically connected to reduced maximum currents and a shift of the transfer curve [36] due to the absence of the electrostatic p-doping from physisorbed oxygen molecules [37; 38; 39].