Ultra-stable surface-embedded physically unclonable functions

Abstract

The invention relates to ultra-stable surface-embedded physically unclonable functions used to provide security in the field of data security, where counterfeit products and identity breaches pose an increasing threat to the world economy, national security and human health.

Description

[0001] ULTRA-STABLE SURFACE-EMBEDDED PHYSICALLY UNCLONABLE

[0002] FUNCTIONS

[0003] TECHNICAL FIELD

[0004] The invention relates to ultra-stable surface-embedded physically unclonable functions used to provide security in the field of data security, where counterfeit products and identity breaches pose an increasing threat to the world economy, national security and human health.

[0005] PRIOR ART

[0006] Counterfeit goods and identity breaches pose an increasing threat to the world economy, national security and human health. With the advancement of technology, it is becoming increasingly difficult to provide security for personal data, critical documents, information and objects. This situation has increased the need for next-generation encryption approaches that are difficult to clone, while increasing the interest in research on anti -counterfeiting and authentication applications.

[0007] Encryption systems are generally generated by deterministic and stochastic processes. The production of deterministic security labels, also known as the traditional method, does not involve any randomness. Since they are designed by the manufacturer and always produce the same output, they can be repeated in principle. Therefore, security labels produced by deterministic processes can be copied even if they are produced in a complex manner in different forms such as dimensional, spatial or geometric shapes and can inherently be imitated. For example, a security system is created with holograms or watermarks created in physically different forms such as shapes, numbers or symbols on globally used paper money. Even if they provide a certain level of protection, security labels created with deterministic production processes are insufficient to prevent counterfeiting with today's developing technology. In recent years, there has been increasing research on the use of physical systems instead of mathematical functions for cryptographic applications. This approach, known as "Physically Uncl enable Functions" (PUFs), is based on the use of a physical system based on stochastic processes. PUFs are generated in such a way that they give a different output each time and are one-way functions that require many years to decipher. Therefore, it is a physical encryption method based on randomness in its production.

[0008] PUFs have a wide range of applications that offer unique opportunities for secure and quality life in today's world. In addition to secure communication and data storage in various industries such as automotive, aerospace, finance, medical and military, PUFs are used in Internet of Things (loT) devices and privacy protection, DRAM process variations to generate cryptographic keys, and device authentication. The randomness and authentication of PUF- based security labels can be confirmed by NIST (National Institute of Standards and Technology) through a number of multi-stage procedures, using specific standards and authentication process image algorithms that are globally recognised.

[0009] When the current PUF-based studies in the literature related to the subject of the invention are analysed;

[0010] In our study, low-cost, scalable and ambient production of PUFs consisting of micrometre and nanometre scale patterns by electrospraying of bulk polymeric materials is presented. By utilising the electrohydrodynamic instabilities in the substructure of the device, PUF-based security labels with random positions and features on the surfaces were produced. (Esidir, Abidin, et al. "Unclonable Features via Electrospraying of Bulk Polymers." ACS Applied Polymer Materials 4.8 (2022): 5952-5964.)

[0011] Lee, Sangsun, et al. show that graphene-based PUF systems with dual source of randomness can be fabricated by using dewetted poly(2-vinylpyridine) (P2VP) islands as etching masks for CVD graphene with abundant microscopic layers that can be well resolved using micro-Raman spectroscopy. (Lee, Sangsun, et al. "Graphene-Based Physically Unclonable Functions with Dual Source of Randomness." ACS applied materials & interfaces (2023).) In the study by Torun, Neslihan et al. dewetting instabilities in nanoscopic (thickness <100 nm) polymer films are presented as a form of PUF. The inherent randomness involved in the dewetting process provided a highly suitable platform for producing unclonable surfaces. Thermal annealing-induced dewetting of P2VP on polystyrene grafted substrates enables the generation of randomly located functional features separated at the microscopic length scale, a requirement set by optical authentication systems. (Torun, Neslihan, et al. "Physically unclonable surfaces via dewetting of polymer thin films." ACS applied materials & interfaces 13.9 (2021): 11247-11259.)

[0012] Kayaci, Nilgun et al. used fluorescence organic semiconductor (f-OSC) materials for the production of PUF surfaces by utilising their easy processing and superior physicochemical / optoelectronic / spectroscopic properties. Randomly positioned unclonable surfaces (OLE-PUF) were obtained by annealing the surfaces coated with green fluorescent oligo( p-phenylenethynylene) molecule under a moderate temperature of 170 °C for a short period of 5 minutes. Thanks to the multiple properties of the material, it has enabled the production of security labels with multiple coding such as fluorescence profile, excited state decay dynamics, Raman mapping / spectrum and infrared spectrum and stable against water, mechanical abrasion, UV-light. (Kayaci, Nilgun, et al. "Organic light-emitting physically unclonable functions." Advanced Functional Materials 32.14 (2022): 2108675.) )

[0013] Sun, Ningfei et al. proposed a universal, fractal -guided film annealing strategy for realising random Au network-based PUFs that can be designed on-demand in complexity, ensuring the intrinsic uniqueness and stability of labels. They presented a dynamic, deep learning-based authentication system with an extensible database to identify and track PUFs. (Sun, Ningfei, et al. "Random fractal-enabled physical unclonable functions with dynamic Al authentication." Nature Communications 14.1 (2023): 30 2185.)

[0014] Zhang, Tongtong et al. fabricated multimodal, dynamic and non-copyable anti-counterfeiting PUF-based labels based on diamond microparticles containing silicon vacancy centres. These chaotic microparticles are heterogeneously grown on a silicon substrate by chemical vapour deposition, facilitating low-cost scalable production. Highly stable photoluminescence signals from silicon vacancy centres and scattered light from diamond microparticles enabled high- capacity optical encoding. (Zhang, Tongtong, et al. "Multimodal dynamic and unclonable anticounterfeiting using robust diamond microparticles on heterogeneous substrate." Nature Communications 14.1 (2023): 2507.))

[0015] When the existing PUF-based inventions in the literature are analysed;

[0016] Invention WO2021133305 relates to the production of functional patterns having random positions on surfaces by exploiting the instability of polymer thin films. Invention CN110569948 A discloses a physically unclonable structural coloured anti -counterfeiting label identified by artificial intelligence. Invention CN111695658 discloses an anti -counterfeiting method based on a physical unclonable function (PUF). The method comprises providing a model formed by randomly distributing surface-enhanced Raman nanoparticles, receiving data from the model, and performing digital processing to generate anti -counterfeiting coding information. The invention CN114842735 discloses an anti -counterfeiting PUF label production method based on a precious metal fractal pattern. Patent WO2021076217 discloses the generation of a cryptographic key based on the addition of edible PUF in a random distribution on a pharmaceutical. In the invention numbered WO2021111349, a new approach is presented in the prevention of counterfeiting by preparing microspherical structures in a unique optical fingerprint-type texture with at least one minutiae based on PUF. In our patent application with application number 2023 / 001813 and titled "PUF Based Security Label Production Method from Foodstuffs", edible PUF-based security label production was carried out by randomly distributing erythrosin b adsorbed corn starch particles on the gelatin surface. In the patent application numbered WO2007128751, SU-8 photoresist film is coated on the silicon surface fortissue imaging in mass spectroscopy and the formation of hollow patterns on the film with UV light penetration and surface transfer processes with various etching methods are described.

[0017] Again in the literature, the invention numbered CN113293448 A is about the production of fibres with continuous structure by electrospinning method and the formation of a PUF by taking advantage of the random position and orientation of the fibres. Our invention application is based on the production of discrete microscopic and nanoscopic structures by electrospraying. Although both methods are based on electrohydrodynamic instabilities, there are major differences in the morphology and dimensions of the obtained structures and the process parameters and mechanisms affecting them. Using the technical infrastructure in our application numbered 2021 / 022173, it is possible to produce PUF patterns embedded in the silicon surface we have developed. With its unique morphology, it allows the production of discrete nanoscopic and microscopic sizes and provides convenience in authentication analyses. On the other hand, the unique elevations provided by the polymeric structures electrosprayed on the SU-8 photoresist film act as a barrier for UV penetration and act as a light mask. Preventing UV penetration is critical for the formation of cavities on the film and thus the transfer of randomly located PUF patterns to the surface and constitutes one of the main novelty of the study. In this context, considering that the complex and overlapping morphological structures in the fibre form in the application CN113293448 A will create a limiting effect on the contact line on the SU-8 film, it will not be possible to produce surface-embedded PUF patterns.

[0018] In our patent application with application number 2021 / 022173 and titled "A Physically Unclonable Function Generation System and Method", the PUF-based security label system and method based on the production of bulk polymers at random positions on surfaces by utilising the electrohydrodynamic instabilities underlying the electrospraying technique is presented.

[0019] PUFs, which are non-reproducible by nature, are critical in terms of their stability properties as well as their unclonability, considering their functionality in various industrial application areas. In order for PUFs to fulfil their functions, they must be resistant to environmental risks. In the literature, PUF productions are generally based on material deposition on the substrate surface. Therefore, the preservation of PUF properties is related to the durability of the deposited material. Instead of depositing material on the substrate surface, producing structures embedded in the surface can be considered as a beneficial approach in the production of PUFs. PUFs with ultra-stable thermal and mechanical performance without the limitations of the deposited material are remarkable in that they can be developed with an innovative surface fabrication process.

[0020] In addition to the fact that security label systems can be produced with functional materials and unique techniques and can meet today's anti -counterfeiting technology needs, the security label produced must be able to respond to environmental conditions, extreme situations that may be encountered voluntarily or involuntarily in order to serve its purpose. Even when exposed to high thermal and mechanical effects, it is of critical importance that PUF-based security labels maintain their structure and can be verified in authentication systems, i.e. their functionality. In this context, considering the current literature applications based on PUF, which have been studied in different fields, as can be seen in Table 1, the majority of the thermal stability studies have been carried out at temperatures lower than 150°C. When we look at the studies where mechanical stability tests were performed, it is seen that only one of the tests such as abrasion, tape test, flexure test etc. was applied under minimal conditions. It is obvious that there is a deficiency in the literature in terms of examining the functionality of PUF-based safety labels. In this sense, it is essential for quality and safe life to develop innovative surface production methods by combining known practical applicable techniques and to increase the service life of PUF-based security labels by improving their stability. Table 1 shows the thermal and mechanical stability of the security labels produced by the deposition of PUF-based materials studied in the literature in recent years.

[0021]

[0022] DESCRIPTION OF THE INVENTION

[0023] In addition to presenting an innovative and unique surface production method in PUF -based security label production processes by adding on top of the known techniques in the current technique, a system that exhibits ultra-stability over the PUF -based security labels already available in the literature thermally and mechanically is presented.

[0024] When PUF -based security tag systems are analysed, recent studies have focused on material - specific, deposition-based encrypted surfaces. In the work of the invention , a new generation encryption system is presented by integrating known techniques overcoming the limiting effects of material deposition on stability. It is an important requirement for encryption methods to be resistant to environmental effects as well as being unclonable. A new generation encryption method is presented that is unique in both thermal and mechanical extreme conditions and in PUF-based encryption applications.

[0025] To summarise the advantages of this invention;

[0026] 1. A new generation of unclonable thermally and mechanically ultra-stable PUF-based encryption concept is presented for the first time in the literature with the production of random patterns in the form of cavities directly embedded in the substrate surface based on the use of structures obtained by electrospray deposition of light crosslinkable resin as a light mask, without the need for complex infrastructure such as lithography and the use of a different material on the surface.

[0027] 2. By filling these cavities with functional nanomaterials, the fabrication of hidden PUFs was also realised.

[0028] 3. Ultra-resistant PUFs were produced with the performance of the produced PUFs in terms of mechanical and thermal stability, which is far superior to the existing examples in the literature. 4. By selectively decorating the PUF patterns in the form of surface-embedded cavities with nanomaterials and thin films, both functionality and an additional security layer that can radiate at specific wavelengths or give characteristic signals are integrated with the PUF system and the opportunity to be characterised by different methods is provided.

[0029] Description of Drawings

[0030] Figure 1. Fabrication of surface-embedded nanostructures

[0031] Figure 2. AFM and SEM images taken during the fabrication process of ultra-resistant surface- embedded PUFs. a) AFM and SEM images of polymeric structures electrosprayed on SU-8 film. b) AFM and SEM images of cavities opened on the SU-8 surface. c) AFM and SEM images taken after the SU-8 cavities were etched and transferred to the silicon surface. d) AFM and SEM images of ultra-resistant PUFs embedded in silicon surface after burning of cross-linked SU-8 film at 500°C in a vacuum furnace.

[0032] Figure 3. Investigation of the effect of solvent and SU-8 film thickness on the production of surface-embedded PUFs and determination of the appropriate etching time. a) Optical microscope images of polymeric structures electrosprayed on SU-8 thin film after exposure to UV light and immersion in solvents, showing the solvent effect on the opening of voids. The solvent immersion was carried out under the same conditions in different solvents (Scale bars: 50 pm). b) The effects of different coating thicknesses on the opening of voids were investigated using SU-8 thin films with thicknesses of 1, 1.5, 2 and 5 micrometres. Solvent immersion was carried out in chlorobenzene under the same conditions. The positions of the voids and their opening percentages are given in the bottom row (Scale bars: 50 pm). c) Determination of the appropriate processing time by etching the voids on SU-8 film for different times. The depths of the voids for etching times of 150, 210, 270, 330, 450 and 570 s are given graphically. (Scale bars: 100 pm)

[0033] Figure 4. a) SEM and EDX mapping images of surface-embedded PUFs taken at each production stage are given. b) FT-IR spectrum analysis of SU-8 thin film before and after exposure to UV light.

[0034] Figure 5. Key generation and extraction of PUF parameters. a) Representative optical microscopy image of the sample used to analyse surface -embedded PUFs. Binarisation and size reduction of the images for the generation of a security key (bits 0 and 1 represent black and white colour bits respectively). b) Homogeneity of bits obtained from 30 different PUF keys of an PUF chip. c) Intra Hamming distance (HDINTRA) and inter Hamming distance (HDINTER) distributions. HDINTRA and HDINTER distributions are shown on the left and right, respectively. d) Pairwise comparison map of HDINTRA values of 30 different FCFs.

[0035] Figure 6. Investigation of the stability of surface -embedded PUFs. The key generation was produced from optical microscope images of surface-embedded microstructures etched for 150, 210, 270 and 330 seconds. After the stability tests, the security keys were regenerated and their similarity with the security key generated before the test was compared and given as %. a) 150 s, b) 210 s c) 270 s, d) 330 s abrasion, water impact, sand impact and high temperature durability tests were analysed. (Given in columns from top to bottom)

[0036] Figure 7. Filling of nanoscopic depth PUFs embedded in the surface with functional materials. a) Quantum dot, b) Fluorescence microscope image of gold nanoparticle filled PUFs and binary code. DETAILED DESCRIPTION OF THE INVENTION

[0037] In this detailed description, the inventive ultra-stable surface-embedded PUFs are described only for the purpose of a better understanding of the subject matter and in a non-limiting manner.

[0038] In this approach, firstly, SU-8, a light crosslinkable resin, was coated as a thin film on the substrate silicon surface. Polymer structures were deposited on the photoresist coated surface by electrospraying process. The electrohydrodynamic instabilities underlying the electrospraying process result in the polymeric structures have random sizes, positions and morphologies. Thus, a "Physically Unclonable Function" (PUF) based encryption based on randomness was made possible. In the next step, the surface with polymeric structures deposited on SU-8 film was exposed to UV light to crosslink the photoresist. During the light treatment, the polymeric structures deposited by electrospraying acted as a light mask and prevented light from penetrating the photoresist film. Thus, cross-linking of the photoresist in the regions under the polymeric structures was prevented. In the next step, the substrate was washed in a solvent. In this washing process, uncrosslinked polymeric structures and photoresist areas in the lower region were removed and cavities were opened on the photoresist film. At the end of this process, a surface containing cavities at random positions and sizes on the photoresist film was obtained. In order to achieve the targeted high thermal and mechanical stability, these cavities were transferred to the substrate. For this transfer process, the substrate was subjected to dry etching by inductively coupled reactive ion etching (ICP-RIE) Technique. The cavities produced at random positions with nanoscopic depths embedded in the surface constitute an optical microscope verifiable PUF. The photoresist was removed by incineration. In order to further improve the security level of verifiable and difficult to forge encryption systems with different mechanisms, the functionalisation of these cavities with functional materials has also been investigated in this context. For this purpose, materials that can be deposited as quantum dots and thin films were used. The properties of these materials such as fluorescence radiation and structural colour generation were used in encryption and authentication.

[0039] Exemplary embodiment 1

[0040] Formation of random cavities in the light-crosslinkable resin layer The processes of obtaining cavities by forming random structures by electrospraying on SU-8 photoresist thin film, a light crosslinkable resin, have been studied. The main reason for the electrospraying process is the electrohydrodynamic instabilities underlying the process. Due to these instabilities, the structures to be formed contain a real randomness both in size and position. In addition to being a stochastic process, electrospray is a simple, practical and cost- effective method. Electrospray requires a high voltage source, a conductive nozzle with a small aperture (e.g. syringe needle) and a conductive collector. The metal needle is connected to a syringe containing a solution. Using a syringe pump, the solution can be pumped into the metal needle at a controlled speed. With the application of a high voltage (1-30 kV), the solution at the needle tip becomes charged and under the influence of electrical forces the liquid droplet takes a conical shape, also known as a Taylor cone. When the electrical force exceeds a certain limit, it overcomes the surface tension of the solution and exhibits spraying behaviour from the needle tip towards the collector in the form of a liquid droplet. As shown in Figure 2, while the solution droplet moves between the oppositely charged needle tip and the conductive collector, if the distance between the collector and the tip is large enough, the solvent in the solution is completely vaporised due to the air friction acting on it, and both the separated and solidified particles are deposited on the collector in a completely random manner without any extra treatment.

[0041] Unclonable features were produced using a commercial electrospinner (Holmarc HO-NFES- 040). Electrospraying was performed by homogeneously dissolving 30 wt% P2VP polymer in a solvent mixture of chlorobenzene / DMF (dimethylformamide) (9 / 1 v / v). The fabrication of PUF patterns at random positions and sizes using P2VP solution was carried out on SU-8 film, a light crosslinkable resin photoresist. Before film coating, the silicon substrate was sonicated in ethanol for 15 minutes for cleaning and then treated in a UV-ozone device for 30 minutes. After the surface cleaning of the silicon substrate, thin film production was carried out on the surface by rotational coating method. SU-8 thin film was produced by spin-coating technique at a speed of 55 s and 4000 rpm. With the spin-coating technique, coatings in the form of thin films are made on the surface of the materials by taking advantage of the centrifugal forces acting on the substrate surface. With the effect of rotational speed, the solvent is removed from the coating material in liquid form and a film layer in solid form is obtained on the surface. There are critical parameters in the formation of the film layer. These parameters are the appropriate viscosity of the liquid solution and the rotational speed of the device. While there is a linear relationship between solution viscosity and film thickness, there is an inverse proportional situation between rotational speed and film thickness. In order to see the effect of film thickness, which is considered to be critical in the study of the invention, rotational speed (for example 1000-4000 rpm) and two different viscosities (SU-8 2 and SU-8 10) were studied. As a result of the studies, "SU-8 2" and 4000 rpm rotational speed were continued. Electrosprayed polymeric structures formed randomly on the photoresist film were exposed to 400 W UV light and crosslinking process was performed. Here, the structures produced at random positions and sizes by electrospraying were used as light masks as mentioned above. The areas under the electrosprayed polymeric structures prevented UV light penetration and prevented the photoresist from crosslinking. In this way, it was possible to remove the noncrosslinked electrosprayed polymeric patterns and the photoresist in the lower region by washing in chlorobenzene solvent for 1 minute. Within the scope of the invention, the effects of different organic solvents (chlorobenzene, acetone, chloroform, toluene) on the opening of cavities on the film were investigated in detail. In this context, it was decided to use chlorobenzene solvent in the production of surface-embedded PUFs. Finally, randomly opened cavities were obtained on SU-8 film.

[0042] Exemplary embodiment 2

[0043] Transfer of cavities in the photoresist layer to the substrate

[0044] Research has been carried out on the dry etching of the cavities drilled on the photoresist film on the silicon surface by reactive ion etching method, the etching process times and the development of surface-embedded PUFs. In this invention, the dry etching of randomly located and sized cavities on the SU-8 film coated on silicon substrate was carried out by ICP-RIE method. ICP-RIE is a selective dry etching method used in the manufacturing technology of various semiconductor devices. The etching is used to form non-planar microstructures, pits, mesa structures and sidewalls inclined at a controlled angle. Combining high finishing precision and reproducibility, the ICP-RIE method is suitable for etching hard materials such as SiC, GaN, diamond, SiO2. The ICP-RIE method allows deep anisotropic, isotropic or directional etching of patterns. A balance between etching rate and anisotropy can be achieved by varying the etching conditions, e.g. by changing the composition of the working gas, selecting suitable process parameters or choosing a mask material. The dry etching process, which constitutes the second aspect of this invention, is of great importance in the formation of surface-embedded PUFs. Etching process was performed at different times with reactive ion etching device. In this way, surface -embedded PUF-based security label patterns with different nanoscopic depths could be produced. There are basically two reasons for producing embedded features at different depths. In the first of these, the patterns to be transferred to the surface by the etching process should have clear traces in the optical microscope. It has been confirmed that by increasing the etching time, sharper and clearer boundaries will be formed and accordingly, images can be taken better with optical systems. Since the authentication of surface -embedded PUFs will be carried out with the data obtained from the images with an optical microscope or a compact camera, the etching times were scanned to determine the appropriate time and images. The second reason is that mechanical stability can be increased with depth.

[0045] After the formation of the surface-embedded hollow-shaped PUF label patterns, incineration was carried out for the removal of the cross-linked SU-8 photoresist, taking advantage of the article by Dentinger et al. in the literature where removal processes specifically for cross-linked SU-8 photoresist were studied. Incineration was carried out in a muffle furnace at 500°C for ~1 hour. In this way, the cross-linked SU-8 photoresist could be removed from the surface. Only silicon substrate and randomly formed PUF patterns in the form of cavities embedded in this surface were successfully produced in this way.

[0046] Exemplary embodiment 3

[0047] Functionalisation of cavities with nanomaterials

[0048] As another aspect of the invention, surface-embedded PUF systems have been decorated with functional materials and studies have been carried out on the generation of hidden and high- stability ciphers. On the silicon substrate, the substrate was transferred to the surface by dry etching process, and two approaches were studied in order to fill the nanometre deep cavities embedded in the surface and integrate them with functional materials. In the first of these, after the SU-8 film was removed by burning, spin coating was used to selectively fill the nanometresized cavities embedded in the surface of the quantum dot by capillary interactions under the effect of centrifugal forces. In the second approach, materials were coated into the cavities by physical vapour deposition (PVD) method just before the SU-8 film was burned. The functional materials that could not adhere to the SU-8 photoresist were removed by sonication in organic solvent for 30 s, while the materials filled into the cavities retained their structure. In the incineration process of SU-8 photoresist, gold with high melting temperature inside the cavities preserved its structure while evaporating away at 500°C.

[0049] Exemplary embodiment 4

[0050] Authentication of surface-embedded PUFs.

[0051] The electrospraying method has been used as a light mask for the production of discrete and random PUF patterns of cavities on the silicon substrate surface. Thanks to the electrohydrodynamic instabilities that provide randomness, a database was created from optical microscope images taken at 10x magnification for each security tag produced during the authentication process of the security labels after the nanometre deep cavities opened on the silicon surface. The optical microscope images were converted into binary form in 16x 16 dimensions. A security key consisting of 256 bit length Os and Is was generated to be used in randomness, homogeneity, uniqueness and reliability analyses. As a result of these analyses, important contributions have been made in the encryption and generation of security keys by confirming that the PUF -based security tag patterns to be generated can indeed be generated randomly.

[0052] The PUF analyses were performed by calculating the following parameters.

[0053] Uniformity

[0054] The uniformity for each key was calculated with the following formula. Uniformity n : Number of bits ri,i= rlth binary bit of the n-bit response (0 or 1)

[0055] Uniqueness The uniqueness was calculated by the following formula with the keys obtained from the image. s= Number of keys from different chips, s=10 images.

[0056] HD (Ri, Rj) : Hamming distance between chips i and j

[0057] Normalised Hamming Distance: Hamming distance divided by the number of bits,

[0058] » normalised Hamming distance sum in uniqueness formula

[0059] Reliability

[0060] Reliability gives information about the reproduction of the PUF bits. Calculations were made according to the formulae given below. m: Number of samples studied under different conditions

[0061] Reliability = 1.00-HDINTRA

[0062] Frequency Test

[0063] Frequency test was used for the randomness of the keys. erfc: Complementary error function ir n: Bit count

[0064] Xrfc-l n= Binary bit (0 or 1) of the n-bit response from a switch

Claims

CLAIMS1. The invention relates to an ultra-stable surface-embedded physically unclonable function (PUF), characterized by:- coating of negative-tone SU-8 photoresist, a light-crosslinkable resin, onto a silicon substrate, preferably in the form of a thin film, preferably 1 micrometre thick, by spin coating,- deposition of polymer structures by electrospraying on photoresist thin film coated surface,- the electrohydrodynamic instabilities underlying the electrospraying process, the random sizes, positions and morphologies of polymeric structures, and the formation of a PUF based encryption based on randomness,- cross-linking of photoresist by exposing the surface with polymeric structures deposited on SU-8 film to UV light,- formation of cavities at random positions in the light-crosslinkable resin film,- the use of polymer structures produced by electrospraying as a light mask and the formation of cavities as a result of light exposure / washing,- transfer of randomly positioned cavities in the light crosslinkable resin to the substrate by dry or wet etching,- realisation of ultra-stable surface-embedded PUFs depending on the depth of the cavities at random positions.- increasing the security level by functionalising the cavities with nanomaterials operations.

Images