# CNT-PUFs: Highly Robust and Heat-Tolerant Carbon-Nanotube-Based Physical Unclonable Functions +
## 4 Fabrication and Implementation of CNT-PUFs  
### W wafer-Level Fabrication Process  
Our CNT-PUFs were fabricated under clean-room conditions utilising a nano-device platform technology for large FET arrays on 200 mm silicon wafers. This platform includes on-top CMOS-compatible surface micro-machining processes with standard projection lithography in line with scalable nanomaterial integration processes for the fabrication of more than 2000 CNT-PUFs per wafer in parallel and at an almost 100% yield. The fabricated sample configuration displays an intermediate processing step aiming for crossbar architectures. There, individual CNT-FETs will be interconnected by a \(12\times 12\) electrode matrix to form a 144-bit PUF. Figure 2 shows the simplified processing scheme on a schematic cross-section of PUF cells (a-c), a top-view light microscope image of a CNT-PUF section together with a photograph of a fully processed 200 mm wafer (d), as well as an Atomic Force Microscopy (AFM) image showing stochastically assembled CNTs over a buried gate electrode (e). First, buried electrodes for back gates and wirings were integrated using a copper single damascene technology on 2 \(\upmu\)m thermally grown silicon oxide layers. There, silicon dry etching was used to etch the trenches. After this process, a metal layer stack was deposited consisting of 20 nm of titanium and 20 nm of titanium nitride as barrier and adhesion layers followed by 70 nm of copper as a seed layer for the subsequent electrochemical deposition step of copper. Using chemical-mechanical polishing, the copper and the barrier were polished away, revealing 500 nm-wide gate electrodes with a dishing below 20 nm, as shown in Figure 2a. Next, the high-k gate-dielectric was deposited by plasma-enhanced Atomic Layer Deposition (ALD) of 20 nm of hafnium dioxide. Afterwards, highly-enriched semiconducting CNTs with a mean CNT length of 2 \(\upmu\)m and a diameter in between 1.2 nm and 1.4 nm (IsoSol-S100(R) [40] from Nanointegris Technologies Inc., which is located in Boisbriand, QC, Canada) were integrated by an automated printing-like method. The density of the stochas  
Figure 2: Schematic cross-section view of the fabrication process of passivated CNT-FET arrays (**a–c**), light microscope top-view image of a section of a CNT-PUF (**d**), and an AFM image of stochastically assembled CNTs (**e**). The inset in (**d**) shows a photograph of the finally processed 200 mm wafer. This figure was adapted and modified from our previous work [19], which this work extends. tically assembled CNTs was controlled by the CNT dispersion concentration and deposition process parameters such as the dispersion flow rate and the printing velocity. The CNT density was analysed by AFM and scanning electron microscopy. Detailed information on the CNT raw material, the CNT integration process, and the CNT-FET passivation is provided within the Supplementary Materials file. The wafer was subsequently immersed into toluene with a 1 vol.% of tetrafluoridic acid, into acetone, and into toluene (at 50 \({}^{\circ}\)C) for 30 min in order to remove residual polymers. The CNT assembly was then structured using lithography, inductively coupling oxygen plasma etching, and wet-resist removal in acetone with ultrasonic treatment. Afterwards, the contact electrodes were realised via Electron-Beam Lithography (EBL) with a subsequent ion-beam sputter deposition of 0.5 nm of chromium and 20 nm of platinum and an ultrasound-supported lift-off process in acetone. A channel length of 500 nm was realised, and the contact electrodes were precisely aligned over the gate electrode, as shown in Figure 2b. The channel width \(W_{\text{ch}}\) was varied to be 500 nm, 1000 nm, and 2000 nm, respectively. Thus, the average number of CNTs per device and, consequently, the number of devices with an electrical connection between the electrodes can be controlled. Afterwards, the wafer was annealed at 500 \({}^{\circ}\)C under an argon-hydrogen atmosphere at a pressure of 5 mbar for 30 min.