Foiling falsification with micromarks

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Simone Mazzucato, technology research specialist at Sisma, discusses how lasers can be used to create hidden microfeatures in materials for anti-counterfeit purposes

Nowadays there is an ever-growing need for security and protection against falsification and counterfeit processes. Illicit fake products flourish in every market sector, with intellectual property rights crimes being valued at more than $450 billion annually worldwide1.

Thousands of solutions have been proposed and are available to address this phenomenon, spanning from simple external tags to sophisticated atomic checks. 

Valuable items are greatly exposed to falsification. For them, a higher level of security is needed, often achieved through a combination of different anti-counterfeit strategies applied at the same time. 

One appealing strategy is to add microfeatures to the surface of the item being protected. These features can be human-readable, for example in the form of alphanumeric characters, or machine readable in the form of simple micro-2D codes (data matrices or QR codes) or more sophisticated encrypted microcodes. The size of these structures has to be on the scale of micrometres or smaller. 

In this work we consider features that can be visualised through cheap optical devices such as small microscopes or powerful eyeglasses. Codes of smaller dimensions (in the nanoscale range) would need very expensive ‘printing’ and detection techniques to realise.

In order to reach the micro-level of miniaturisation, laser technology comes into play. Controlling the laser spot size and position is key to laser micromarking. This can be done in different ways, but each method comes with its pros and cons. 

The use of small focal lenses and motorised x-y stages offers more accurate control of the spot, but can be expensive and difficult to control, especially when marking bulky items or treating big areas.

Figure 1: Left: QR code marked with a UV nanosecond pulsed laser on a polished stainless steel. Right: Magnified view of the hidden microfeatures inside the QR code. (Credit: Sisma)

Programmable spatial light modulators modify the laser beam to the shape of the microcode, giving great and extremely fast results. However, this way the size of the feature is limited, the laser power is proportional to the complexity of the code and the machine has to be dedicated for this application. 

A third way, implemented by Sisma, exploits classical scan heads and galvanometric mirrors of commonly sold laser marking systems, but controls the laser beam to supply on-demand pulses, whose spatial combination on the final object creates the microfeature. This solution, based on single laser shot control, is simple, relatively cheap and enables microtexts or microcodes to be marked directly on the object’s surface, hidden within existing or new textures or paths. 

Theoretically, this solution can be implemented using almost any pulsed laser, regardless of the wavelength, average power or pulse duration. Special care should be taken to equalise the intensity of all laser shots, particularly the initial ones. This depends upon the laser source and its manufacturer control. The final result will instead depend upon these parameters, upon the machine configuration and the material to be treated.

Spot size is key

As taught by the optical beam propagation theory, the spot size after a converging lens (the theta-lens in our systems) depends upon the hardware configuration, according to the following rule:

Where f is the f-theta focal distance, λ the laser wavelength, and D the beam diameter before the lens. It is clear that UV lasers achieve smaller spot sizes compared to near-infrared fibre lasers. The spot size also depends on the material to be processed, according to its structure, composition and absorption coefficient. The duration of the interaction between the laser and the material also plays an important role: by using short or ultrashort laser pulses the heat affecting zone (HAZ), which contributes enormously to the final spot size on the sample, decreases, becoming almost negligible with ultrashort pulses. Finally, the visual resolution of the spots depends upon the surface quality: defects or imperfections in the surface can spoil the visibility of the spots, especially if they have a similar size. In such a case, however, changing the polarisation of the illuminating light can make the pattern distinguishable from the rest. 

A typical spot size, which can be observed by magnifying glasses, is in the order of 10-20µm. This size is easily achievable with f160 f-theta lenses, which are suitable for laser treating 100mm x 100mm wide areas. Again, the final spot size depends upon the factors listed above.

The smaller the spot, the higher the amount of information that can be stored. For metal surfaces the spot dimension can be even smaller than 10µm (especially in highly reflective metals such as gold and silver where only the peak of the Gaussian pulse interacts with the surface), whereas on transparent materials, such as plastics and glass, the spot is usually bigger.

Figure 2: Left: A photo marked with a near-infrared nanosecond pulsed fibre laser on a brass substrate. Right: Enlarging photo reveals hidden microtext in the whitish random spot background. (Credit: Sisma)

Two clarifying examples of this application are shown in figures 1 and 2. On polished stainless steel, a standard QR code (see figure 1) has been marked with an UV laser. The unicity of the 2D code is strengthened by replacing some pixels, or part of them, with microtexts or other microDM codes. Whereas the QR code is easily readable by a mobile camera, all hidden information requires some means of magnification to be spotted and decrypted. The greyish areas in the sample hide micropatterns but can also host other advanced anticounterfeit featuresSISMA has developed, as presented in2,3.

Microfeatures can be hidden within any standard marking or engraving patterns such as photos, textures or vector files. The second example (see figure 2) is a photo marked with a near-infrared nanosecond laser on a brass plate. Zooming on the sunglasses’ lenses the text ‘SISMA’ and ‘MICROTEXT’ can be spotted, camouflaged among the random laser spots. The content of the text or 2Dcodes can be personalised dynamically using information directly from a firm’s internal or external database.

At Sisma we have succeeded in fine-tuning this practical and technically flawless solution. It is capable of maximising the efficiency of the visual outcome while minimising component and management complexity. This makes the approach the most suitable for users who are shrewd and attentive, and who seek impeccability in process execution and repeatability.

References

[1] Euipo Europol: Situation Report on Counterfeiting and Piracy in the European Union, (2017) European Union

[2] Laser metal marking application: hidden anti-counterfeiting features - SISMA S.p.A. https://youtu.be/yw1-bc4O3O8 

[3] K. L. Wlodarczyk et al., Journal of Materials Processing Technology, 222, pp. 206-218 (2015)

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