WO2012038842A1

WO2012038842A1 - Product embodying a physical unclonable function

Info

Publication number: WO2012038842A1
Application number: PCT/IB2011/053453
Authority: WO
Inventors: Tamas Visegrady; Heiko Wolf
Original assignee: International Business Machines Corporation
Priority date: 2010-09-22
Filing date: 2011-08-03
Publication date: 2012-03-29
Also published as: JP5782519B2; GB2497032B; DE112011103162T5; CN103124979A; JP2013539874A; CN103124979B; DE112011103162B4; GB2497032A; GB201303961D0

Abstract

The invention is directed to products embodying a PUF. A method for manufacturing such a product is disclosed which relies on a material having one surface with "deterministic" asperities. The method further uses particles dimensioned such as to be able to be trapped by the asperities of the surface. Generally, the method enables particles (20) to randomly deposit on and get trapped by asperities (14) of the material surface (12), such as to obtain a pattern that forms the PUF. The resulting PUF is made easier to read out since the general pattern and the location of the particles are known. Only the filling level (of a given type) of the particles is random.

Description

PRODUCT EMBODYING A PHYSICAL UNCLONABLE FUNCTION

FIELD OF THE INVENTION

The invention relates in general to the field physical unclonable functions and methods of manufacturing and/or challenging products embodying such functions.

BACKGROUND OF THE INVENTION

A Physical Unclonable Function (also called Physically Unclonable Function or PUF for short) is a function embodied in a physical structure, which is easy to evaluate but hard to fully characterize, see e.g., Wikipedia for a nice introduction (htt :// en . wikipedia . or g/wiki/Physie ally unclonable function) . The concept was already used in the early 90's, see e.g., EP0583709 (Al). Typically, the structure containing the PUF has random components introduced during the manufacture of the structure, which cannot be entirely controlled. Thus, when applying a physical stimulus (the challenge) to the structure embodying the PUF, a result is obtained (the response) which cannot be entirely predicted, owing to said random components. However, this result is (ideally infinitely) reproducible. Thus, a given challenge and its response together form a unique challenge-response pair. For example, a cracked or rough surface can be challenged by a ray of incident light producing a unique scattering pattern. Two PUFs that are manufactured with essentially the same process will possess distinct challenge -response behaviors. This results from a complex interaction of the challenge with the random components of the structure. Modeling this interaction exactly is assumed to be not possible in practice. PUFs are accordingly said to be unclonable.

Different sources of randomness can be relied on. In that respect, a distinction can be made between PUFs in which randomness is extrinsically introduced and PUFs using intrinsic randomness of a physical system.

For example, microelectronic semiconductor chips feature intrinsic randomness in their detailed electronic characteristics due to small variations in their fabrication process, e.g., silicon PUFs exploits intrinsic random variations in delays of wires and gates. Advantages are the downwards scaling and IC integration possibilities. More problematic are the typical long-term drift and reading reproducibility.

Examples of PUFs using extrinsic randomness are the optical PUFs and coating PUFs. An optical PUF can for instance be obtained from a transparent material doped with light scattering particles. A random and unique scattering pattern is produced when illuminating the material, see e.g., R. Pappu, B. Recht, J. Taylor, and N. Gershenfeld. Physical One-Way functions. Science, 297(5589):2026-2030, Sep 2002. http://dx.doi.org/10.1 126/science.1074376.

Coating PUFs are also known; they can be built e.g., in the top layer of an IC, by filling a space defined by the comb structure with an opaque material, randomly doped with dielectric particles, see e.g., B. Skoric, S. Maubach, T. Kevenaar, and P. Tuyls. Information-theoretic analysis of capacitive physical unclonable functions. J. Appl. Phys., 100(2): 024902, Jul 2006. http://dx.doi.Org/10.1063/l.2209532.

In general, extrinsic PUFs allow for high reproducibility and ease of reading, but the incorporation of markers is often difficult and the size- scalability is limited.

In each of the above cases, intrinsic or extrinsic randomness is exploited for creating PUFs with a respective, unique challenge -response pair. Such functions are for instance important for use in anti-counterfeiting and cryptography applications. Unfortunately, the randomness component of a PUF may drift or deteriorate with time.

More generally, reading out the PUF is made difficult due to deterioration or drift with time and/or some intrinsic difficulty.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a method for manufacturing a product embodying a physical unclonable function or PUF, comprising the steps of: providing a material with one surface having deterministic asperities and particles trappable at said asperities; and enabling particles to randomly deposit on the surface and get trapped at said asperities, such as to obtain the PUF..

In other embodiments, the method may comprise one or more of the following features:

providing comprises providing the particles as a colloidal suspension in a liquid, the liquid being preferably water; and enabling particles to randomly deposit comprises applying the liquid to the surface having asperities;

the surface asperities of the material, the particles and the liquid provided are chosen such that particles are subject to capillary forces at the surface, the characteristic dimensions of both the particles and asperities being preferably on the order of micrometers;

enabling particles to randomly deposit further comprises: applying the liquid to the surface by maintaining a layer of the liquid against the surface having asperities with a lid, whereby a meniscus of the liquid is defined between the surface and one edge of the lid; and moving the lid or the surface according to an evaporation rate of the liquid at the level of the meniscus defined, the liquid being preferably heated; the liquid provided further comprises surfactants impacting a contact angle formed by the meniscus at the surface;

providing comprises providing particles of different types, and particles of different types preferably have different, respective colors;

- providing comprises providing a material having pre-processed surface asperities;

the deterministic surface asperities provided form a 2D array; the deterministic surface asperities provided form an array of open corners;

- the particles provided are beads, each comprising a fluorescent dye, and wherein the particles preferably are polystyrene beads;

the method of the invention further comprises previous steps of molding a prepolymer in a mold and polymerize the prepolymer to form the material with the surface asperities, the material being preferably PDMS;

- the method of the invention further comprises a step of fixing the deposited particles; and

the surface of the material provided has a set of asperities designed such that an asperity of the set with a particle trapped thereat produces a scattering pattern upon illumination which substantially differs from a scattering pattern produced by an asperity of the set without a particle trapped thereat. The invention is further directed, according to another aspect, to a product embodying a PUF, the product obtained according to embodiments of the method according to the invention.

According to a final aspect, the invention is also directed to a method of performing a challenge-response evaluation of a product according to the invention, comprising the steps of: providing the product according to the invention; stimulating the surface of the product with particles deposited in the deterministic asperities of the surface to obtain a response; and reading the response in accordance with the deterministic asperities.

Products and methods embodying the present invention will now be described, by way of non-limiting examples, and in reference to the accompanying drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 schematically illustrates a method for manufacturing a product embodying a PUF, in an embodiment of the present invention;

- FIG. 2 is a view of an experimental set-up for implementing both the method of e.g., FIG. 1, and a challenge-response evaluation of a PUF accordingly obtained, in an embodiment; FIG. 3 is a microscope view of ~1 μιη polystyrene particles suspended in water, and being deposited on a PDMS surface, according to an embodiment;

FIG. 4 is an optical bright field micrograph image showing fluorescent polystyrene spheres deposited and trapped in a corner array, according to an embodiment;

FIG. 5 is a negative of a grayscale version of a fluorescence microscope image (channel overlay) obtained for the deposited polystyrene spheres of FIG. 4;

FIG. 6 schematically depicts a PUF as obtained in embodiments, being challenged by a ray of incident light, and producing a unique scattering pattern; and - FIG. 7 illustrates steps of molding a prepolymer and polymerizing it to form a material with a suitable surface for implementing the method of FIG. 1 or 2.

DETAILED DESCRIPTION OF THE INVENTION

As an introduction to the following description, it is first pointed at general aspects of the invention, directed to products embodying a PUF. A method for manufacturing such a product relies first and foremost on a material having one surface with "deterministic" asperities, i.e., causally determined by preceding events or natural laws. The method further uses particles configured such that they can be trapped at asperities of the surface. Generally, said method enables particles to randomly deposit on and get trapped at asperities of the material surface, such as to obtain a patterned material surface, which forms the PUF. As it can be realized, the resulting PUF is made easier to read out owing to the (partial) knowledge one has of the surface, that is, the deterministic aspects thereof. For example, the general pattern and the location of the particles may be known in advance and only the filling level (of a given type) of particles is random.

An example of such a method is illustrated in FIG. 1. First, the method comprises providing a material 10 such as polydimethylsiloxane (PDMS), with one surface 12 having deterministic asperities 14. Particles 20 such as polystyrene (PS) beads are further provided, e.g., as a colloidal suspension. Such particles are chosen such that they can be trapped at said asperities. Typically, their characteristic dimensions are less than or on the order of the asperities', as illustrated in the enlarged view in FIG. 1. Particles are then allowed to randomly deposit S40 on and get trapped S50 at some of the asperities 14 of the surface 12. The pattern of deposited particles accordingly obtained forms the PUF. As noted, this pattern is partly deterministic, while the occupation probability by particles keeps a random component.

As evoked above too, particles are preferably provided as a colloidal suspension in a liquid 30, e.g., water. Thus, particles are easily enabled to randomly deposit on the surface by applying S20 the liquid thereto. The random distribution of particles in the liquid ensures a random filling in fine. Less practical variants may consist of mechanically dispensing, e.g., sputtering particles to the surface asperities.

Preferably, the particles and the liquid are chosen such that particles are subject to capillary forces (as denoted by F_c in FIG. 1) at the surface, during the deposition. In that respect, characteristic dimensions of particles and asperities typically are on the order of micrometers. Accordingly, trapping the particles at the asperities can be partly assisted by capillary forces. In variants, particles get trapped by way of their momenta. In other variants, the respective conformations of asperities vs. particles allow the particles to get trapped at the asperities. Particles can in fact be slightly larger than the asperities at issues, e.g., holes in the surface, provided they are deformable and that they have sufficient momentum when reaching an asperity, or that sufficient force is applied thereto, e.g., capillary forces. The latter scenario is preferred and is assumed in the following.

In that respect, one way to achieve a capillarity-assisted deposition process is to apply the liquid 30 to the surface 12, by maintaining a layer 30 of the liquid against the surface with a lid 40. A meniscus 32 will accordingly form between the surface 12 and one edge 41 of the lid 40. The meniscus is an air-liquid interface at which liquid likely evaporates (step S30) at a rate determined by the geometry and thermodynamic conditions of the experiment, causing in turn the meniscus to retract.

As illustrated in the enlarged view in FIG. 1 , this meniscus 32 further exerts pressure on particles close to the interface upon retracting (see e.g., Malaquin et al., Langmuir

2007, 23, 11513), thereby causing the particles to get trapped at asperities.

Interestingly, both the capillary forces and the confinement forces that result once particles are trapped are strong and short-ranged. Accordingly, they result in a highly accurate pattern of deposited particles, at least in a context such as that of FIG. 1. Capillary forces act during a short period of time, i.e., corresponding to the capillary breakup of the meniscus when retracting, as depicted in the enlarged view of FIG. 1.

Better performances can further be achieved by suitably tuning the contact angle of the meniscus at the surface, e.g., by choosing a convenient liquid and/or surfactants. A surfactant is a surface-active molecule. At low concentrations, surfactant molecules likely reside at the air-water interface, where they reduce the surface tension. Reaching the CMC (critical micelle concentration), they additionally start forming micelles. A surfactant molecule usually has a hydrophilic headgroup and a hydrophobic tailgroup (a long alkyl chain). The hydrophilic headgroup can be cationic, anionic, or non-ionic. Whether a ionic (and which charge) or a non-ionic surfactant is to be used or not may depend on the colloidal system. Surfactants should preferably be selected not to cause agglomeration and precipitation of the colloidal particles. Sometimes mixtures of surfactants are advantageous. Useful concentrations are mostly in the mM range but may vary significantly. Using surfactants, the contact angle can be tuned towards smaller values, such that the meniscus projection onto the surface is as large as possible, and that a corresponding force has a vertical, downwardly directed component, as illustrated.

The relative dimensions of particles and asperities, the concentration of particles in the liquid, the nature of particles and liquid can be adjusted by a trial and error process. Suitable examples are given below.

Following this principle, the entire surface 12 can be patterned by moving the lid (step S20) or the surface according to the evaporation rate of the liquid at the level of the meniscus 32. In that respect, FIG. 3 shows a microscope view of particles being deposited on a patterned PDMS surface. Here the particles are ~1 μιη PS particles suspended in water. Particles 20 already deposited on the array 16 are visible on the left side of the meniscus 32. On the right side, particles are still suspended in water 30.

Note that the process can be assisted by heating the liquid during the operation, in order to speed up the process.

Advantageously, the particles used are of different types, allowing different axes of randomness which can later on be exploited during a challenge-step. For example, the particles may have different colors. Thus, particles of a given color fills asperities at random (asperities are not color-selective).

FIG. 2 depicts a capillarity-assisted particle assembly tool, which allows for implementing methods such as described above. This tool includes:

A stepper motor 61, for driving a motorized translation stage 62;

On top of stage 62 is a heat exchanger 63, with fluid input 63' and output 63 " , for heat-assisting the process ;

A Peltier element 64 is further provided, i.e., a solid-state heat pump which transfers heat from one side of the device to the other;

The material 10 to be patterned is put on the Peltier element, with a colloidal suspension 20, 30 on top, applied and maintained by a lid 40 (i.e., a mere confinement slide), as described in reference to FIG. 1. A guided slide holder (not shown) allows for moving the lid.

An optical microscope 65 may further be provided for monitoring the process, as well as a camera 66 (for measuring the contact angle).

In a specific embodiment, the PUF is fabricated by capillary deposition of ~1 μιη fluorescent beads into an array 16 of asperities which are corners 17 open on top, as represented in FIG. 4. Other types of arrays/asperities might be used for capillary deposition, similar to crystal lattices. The corner array has given spacing and pattern, e.g., a square lattice with translation vector a, with lal ~ 10 μιη. lal can yet be tuned to optimize the subsequent scattering pattern (e.g., minimize crosstalk at reading). The colloid used for capillary assembly contains a mixture of beads of different (fluorescent) colors. The deposition of beads into asperities may be designed such as to obtain a high yield. Yet, it is not selective for the color of beads. Thus, the deposition results in a random placement of differently colored beads on the template, as depicted in FIG. 5. The resulting bead array is unclonable because beads are too small to be placed "manually" in a large number and in affordable time.

The challenge-step is carried out by e.g., UV/V illumination of the bead array, where each bead responds by emission of its corresponding fluorescence color, as illustrated in FIG. 6. Such a characterization technique is known per se: basically, the patterned surface is illuminated (step SI 00) to obtain a scattering pattern, the latter collected by any suitable camera (step SI 10). The obtained pattern of colored spots is essentially unclonable. This PUF is easy to read out (steps S120) since the pattern and the location of the beads is known. Only the color of the bead is random in that case (assuming a filling probability of ~1). Now, since the bead positions are determined through the template, fixed features can conveniently be inserted within the challenge-step, such as a coordinating grid or alignment features to help automatic recognition, step 120. In other words, the said "fixed features" reflect the deterministic nature of the asperities. More generally, the patterned surface of the product shall be stimulated such as to obtain a unique response, which can in turn be read, according to the challenge-response principle evoked earlier. In the present case, reading out the response can be facilitated owing to the partial knowledge one has of the surface, i.e., due to the deterministic aspects thereof. As said, fixed features can advantageously be exploited, such as a coordinating grid or alignment features to help automatic recognition, thereby facilitating the interpretation of the response. For instance, features such as alignment features as known from standard lithography applications might be used. Also, miniaturized logos and codes (e.g. field numbers, coordinates,...) are possible. This would for instance not be possible with glass-based PUFs which rely on surface cracks. A challenge -response evaluation process based on e.g., light- scattering is else generally known per se.

In a variant, PUFs could also be produced by enabling random deposition of beads onto a surface, whose asperities are not or less deterministic than in the above examples, based on predefined patterns such as arrays. For example, features of the surface (e.g., defaults) can be determined in some extent, e.g., statistically. However, the challenge/response is likely more problematic in such cases. Namely, at low bead density/concentration, beads would produce a low number of responses (which needs larger area and/or longer read out time), whereas at high concentrations they may deposit to close from each other, making the read out more difficult or even impossible (crosstalk). Thus, one understands that a convenient surface should have reasonably predefined asperities. A practical solution is to rely on a pre-processed surface or on a surface offering preferred sites, which results in a predetermined pattern of deposited particle (i.e., ordering parameters are provided which go beyond the mere random location of trap sites).

In carrying out embodiments of the present invention, the following advantages are generally sought:

- Optical reading and scanning is essentially deterministic contrary to semiconductor-related PUFs which are statistical. - Deterioration with time is supposed to be low, there is essentially no drift as in semiconductor-related PUFs.

- Incidentally, reading may be augmented by error-correction code compressing to less, but reliably reconstructable core bits.

As an additional security feature, the pattern of the assembly sites can be designed to create a characteristic diffraction pattern upon illumination with a light source, e.g., a laser. Assembly site may for instance be designed in such a way that empty sites and filled sites produce a different pattern. This can in turn be used to verify whether beads have been assembled and not just fluorescent ink is producing the control image. Furthermore, the particles can be modified with quantum dots emitting a unique (fingerprint) spectrum.

Preferably, fluorescent (red, green, and blue) polystyrene (PS) beads of 1 μιη diameter are assembled into corners of 2 μιη length, 1 μιη width, and ~1 μιη height by capillary assembly, as evoked earlier. The corner pattern determines the position of the beads, but the assembly process is not selective for the colors of the beads. Thus, a random color pattern can be produced.

The corner pattern can be molded from a structured silicon master in polydimethylsiloxane (PDMS) or in a polymer resist (by nanoimprint lithography), as illustrated in FIG. 7. For example, a PDMS prepolymer 10' can be molded (step S10 - S12) in a mold 5, prior to polymerization (S12) and extraction thereof (S14). Such a molding technique is known per se. The polymerized form provides a suitable material for manufacturing a PUF as described above. A corner pattern can else be formed using any material (polymer, glass, semiconductor, metal, metal oxide) that is prone to any kind of lithography or molding method. A surface treatment of such a material might then be applied to achieve the necessary wetting properties for capillary assembly.

After capillary deposition, a single PS bead ideally resides in each of the corners, FIG. 4. Fluorescent colors 20', 20", 20" 'of the beads can be observed under a fluorescent microscope (as illustrated in FIG. 5, where a negative of a grayscale image is rendered). Incidentally, using three colors (RGB fluorescent) and only 100 assembly sites already results in 5.15378 10⁴⁷ alternative assemblies (assuming one particle per site). Relying instead on a single-type particle but tuning the deposition process to reach a filling probability of ½ yields 1.26765 10³⁰ alternative configurations.

The steps of particle deposition may be followed by any convenient steps for fixing the particles (e.g., applying an additional layer, optically inactive).

Finally, the present invention is directed to a product embodying a PUF, wherein the product is obtained according to any of the methods discussed above. While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. For example, the process can be dynamically (or spatially) modulated, by changing the composition over time or its concentration or by modulating deterministic aspects of the surface 12 along the material. For example, some isolated asperities may be provided or the array can change along the surface (one part being square, another rectangle, etc.). This can be useful to provide specific tags or identifiers. For example, specific asperities may be pre-processed which are the signature of a given company, a given class of products thereof, etc.

Claims

1. A method (S20-S40) for manufacturing a product (15) embodying a physical unclonable function or PUF, comprising the steps of:

- providing a material (10) with one surface (12) having deterministic asperities (14) and particles (20) trappable at said asperities; and

- enabling particles to randomly deposit (S40) on the surface and get trapped (S50) at said asperities (14), such as to obtain the PUF.

2. The method of claim 1, wherein:

- providing comprises providing the particles as a colloidal suspension in a liquid (30), the liquid being preferably water; and

- enabling particles to randomly deposit comprises applying (S20) the liquid to the surface having asperities.

3. The method of claim 2, wherein the surface asperities of the material, the particles and the liquid provided are chosen such that particles are subject to capillary forces (F_c) at the surface, the characteristic dimensions of both the particles and asperities being preferably on the order of micrometers.

4. The method of claim 3, wherein enabling particles to randomly deposit further comprises:

- applying the liquid to the surface (12) by maintaining a layer (30) of the liquid against the surface having asperities with a lid (40), whereby a meniscus (32) of the liquid is defined between the surface (12) and one edge (41) of the lid; and

- moving (S20) the lid or the surface according to an evaporation rate (S30) of the liquid at the level of the meniscus defined, the liquid being preferably heated.

5. The method of claim 4, wherein the liquid provided further comprises surfactants impacting a contact angle formed by the meniscus at the surface.

6. The method of any one of claims 1 to 5, wherein providing comprises providing particles of different types, and wherein particles of different types preferably have different, respective colors.

7. The method of any one of claims 1 to 6, wherein providing comprises providing (S10 - S 14) a material (10) having pre-processed surface asperities.

8. The method according to claim 7, wherein the deterministic surface asperities provided form a 2D array (16).

9. The method of claim 8, wherein the deterministic surface asperities provided form an array (16) of open corners (17).

10. The method of claim 1 to 9, wherein the particles provided are beads (17), each comprising a fluorescent dye, and wherein the particles preferably are polystyrene beads.

11. The method of any one of claims 1 to 10, further comprising previous steps of molding (S10 - S14) a prepolymer (10') in a mold (5) and polymerize (S12) the prepolymer to form the material (10) with the surface asperities, the material being preferably PDMS.

12. The method of any one of claims 1 to 11, further comprising a step of fixing the deposited particles.

13. The method of any one of claims 1 to 12, wherein the surface of the material provided has a set of asperities designed such that an asperity of the set with a particle trapped thereat produces a scattering pattern upon illumination which substantially differs from a scattering pattern produced by an asperity of the set without a particle trapped thereat.

14. A product embodying a PUF, the product obtained according to the method of any one of claims 1 to 13.

15. A method of performing a challenge-response evaluation of a product according to claim 14, comprising the steps of:

- providing the product according to claim 14;

- stimulating (SI 00) the surface of the product with particles deposited in the deterministic asperities of the surface to obtain (SI 10) a response; and

- reading (S120) the response in accordance with the deterministic asperities.