FR3023207A1 - METHOD OF USING A MICRO-MACHINING SYSTEM TO FORM A PATTERN ON A MATERIAL - Google Patents

Abstract

The invention relates to a method of using a micro-machining system based on a pulsed light beam to form a pattern on a material, wherein a maximum number of Nk points is computed simultaneously with each pulse of the light beam by beam shaping of a laser shot of k pulses, and where a phase modulation is parameterized to form the light beam into a plurality N of points less than or equal to the maximum number of points Nk achievable at each pulse of the light beam.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to the field of micro-machining of materials, in particular allowing the marking of these materials, with a method and a method of micromachining system adapted to industrial rates, allowing for example to mark products for identification and / or authentication applications of such products. STATE OF THE ART At the present time, and in a context of rapidly expanding labeling, existing laser technologies have largely won through their ability to machine a large majority of materials, allowing both respond to current industrial issues while demonstrating a high potential for added value depending on operating parameters and processes. However, there are certain markets where laser technologies are reaching their limits, namely high-speed production, with, for example, the agri-food, pharmaceutical, fiduciary or electronics sectors, subject to generally speaking to the manufacture of products of small dimensions but in very large quantities. The most widely used marking technology today is based on the combination of a laser source of various radiation properties (power, rate, energy, wavelength, pulse duration, etc.) coupled to a deflection head . This head allows both the focusing of the laser beam, that is to say the spatial concentration thereof at a single point, and its controlled and automated movement in the space of the room to be marked by analogy with the tip of a pen. The deflection head is generally a galvanometric head composed of two mirrors rotating about two orthogonal axes. The motorization of these two mirrors generates an angular deflection of the beam in the desired direction, subsequently converted linearly into a distance on the plane of the sample by a focusing lens of the "f-theta" type. Other technologies still based on the mechanical movement of optics (mirrors, prisms, disks, polygons ...) have also been developed from specific needs but also to accelerate the marking speeds.

Nevertheless, the use of these systems on high production rates raises multiple problems concerning for example the speed of execution and stability of the mirrors in rotation, the synchronization of the movements on the rate of the laser beam, the correction of the displacement according to the conveying speed of the products to be marked. All these limiting points treated, only new systems, are faster and therefore more expensive, or more ingenious but always very specific, remain able to possibly be able to meet this demand. The Ichihara et al. No. 5,734,145 proposes for example a complex mechanical assembly combining two galvanometric mirrors, a rotating polygonal mirror and a liquid crystal mask in order to accelerate the laser machining of images on production lines. There are other developments tending to modify the light beam used for marking. First of all, there are the so-called "amplitude mask" technologies, characterized by the use of masks of identical shape to the shape to be machined and therefore having two major defects: the uniqueness of the mask as well as the loss of energy by blocking the beam in areas not to be marked. For example, reference can be made to patent documents published under US 4,128,752 and FR 2,909,922. Dynamic modulators are another direct beamforming technology. They are active optical elements for spatially modulating laser optical radiation and have the ability to reflect or transmit a large proportion of the incident laser energy at the expense of a more complex physics and therefore more difficult to exploit. In US Pat. No. 4,734,558 and US Pat. No. 4,818,835, for example, polarization shaping laser marking systems are presented. The modulators are addressed respectively optically (illuminated mask) and electrically to the image of the figure to be marked. After transmission or reflection on the latter, the marking laser beams are then spatially modulated into two polarizations, one of which is eliminated on passing through an analyzer. Only the portion of energy having the right polarization is finally transmitted to the focusing lens for marking the filtered form by imaging relation. Although these two methods both exploit dynamic modulators, the use of analyzers matches the deficiencies of the amplitude mask systems with the gross loss of energy that does not have the correct polarization. In the US patent application 2001 / 045,418, it is proposed to use micro-mirror matrices to divide a laser beam into several independently controlled sub-beams for the purpose of simultaneous multipoint marking. The low resolution of the micro-mirrors nonetheless generates a significant limit in terms of resolution and by extension in image generation flexibility. In addition, the modulator used is of the amplitude modulator type and the image generated on the surface of the modulator is directly reproduced on the material by imaging relation, again implying as a disadvantage a loss of energy partially absorbed or ejected. In patent application FR 2 884 743 is proposed a machining solution by phase shaping on laser beams of femtosecond pulse duration (fs).

A feedback loop between a beam analysis downstream of the shaping and the shaping itself also makes it possible to optimize this shaping for high speed applications with reduced thermal effects under these pulse regimes. The solution proposed in this patent application remains nevertheless exploitable for a limited number of industrial because requiring particular laser sources very specific, expensive and imposing a complex assembly and often expensive to implement. An object of the present invention is to provide a method and a micromachining system for forming a pattern on a material, for purposes of identification marking and / or authentication for example, which can be used industrially, in particular being simple to implement, with pre-existing devices and not requiring complex adjustments. In particular, an object of the present invention is to provide a method and a micromachining system for forming a pattern on a material to increase productivity over existing methods and systems.

Another object of the present invention is to provide a method and a micromachining system for forming a pattern on a material constituting a marking for identification and / or authentication purposes without it being necessary to changing the parameterization of said method or micro-machining system between two successive markings.

Yet another object of the present invention is to provide a method of using a micromachining system for optimizing the marking rate of different products depending on the material constituting said product and the micro-machining system. as such.

SUMMARY OF THE INVENTION Thus, there is provided a method of using a micromachining system to form a pattern on a material, the system comprising a device for emitting a spatially and temporally coherent pulsed light beam, a dynamic optical modulation device of said light beam comprising a phase modulation for forming said light beam at a plurality of points and a device for focusing the light beam shaped on a surface of said material, the method being characterized in that it comprises the steps following: - Calculation of a threshold energy density F threshold (i) from which the material reacts as a function of the number of pulses (i) of said light beam, and determination of a threshold power Pseuil (i) associated; - Calculation of an available power Pdispo output of the modulation device from characteristic parameters of the transmitting device and the phase modulation device; - Setting a pulse train by choosing a number of pulses k of said light beam to react the material; - Calculation of a maximum number of points Nk achievable at each pulse of the light beam, assuming that the number of shaped points does not affect the threshold at which the material reacts, according to the relation: O Nk = Pdispo Pseuii (k) - Parameterization of the phase modulation for shaping the light beam into a plurality N of points less than or equal to the maximum number of points Nk achievable at each pulse of the light beam. Preferred but nonlimiting aspects of this method of use, taken alone or in combination, are the following: the phase modulation is parameterized so as to form the light beam into a plurality N of points less than or equal to half the maximum number Nk points achievable at each pulse of the light beam. the method comprises a complementary step of calculating a number of pulse trains necessary for the formation of the complete pattern by dividing the number of points forming the pattern by the plurality N of points chosen for the parameterization of the phase modulation. the parameterization of the pulse train consists in choosing the number of pulses k as a function of the calculation of the threshold energy density F sol (i), the number of pulses k being an integer selected from the number of pulses k200 corresponding to a threshold energy density equal to 200% of the minimum threshold energy density, and the number of k100 pulses corresponding to the lowest number of pulses for which the energy density is equal to the density minimum threshold energy. the calculation of the threshold energy density Fseuli (i) is carried out considering that the light beam has a Gaussian shape and that the material irradiated with the light beam reacts from a threshold energy density Fseuii given by the following formula : -D2 / with Fpeak 2Pmoy V7tCO2 co 22 Fcreated threshold where D is the physical impact diameter of the light beam on the material, Fpeak is the maximum energy density taken at the optical axis and expressed as a function of the average power Pmoy laser, v is the pulse rate and w is the radius of the light beam in the focusing plane of the focusing device. the calculation of the available power Pd, spo at the output of the modulation device is done according to the following formula: Pdispo = PlaserU% X% (C) (CNk d) v% - w'DAP - laser with: or% the transmission percentage of the dynamic optical modulation device; o v% the percentage available after dissymmetry effect of the pattern to be marked; o w% the percentage lost by the light beam at a central focus point not subjected to conformation of the dynamic optical modulation device; o x% (C) the percentage available after the effect of the application of a curvature C on a phase card for a modulation setpoint applied to the dynamic optical modulation device; o c and d number of impact coefficients reflecting the multiplicity of focus points of the phase map used for the modulation setpoint applied to the dynamic optical modulation device; the efficiency associated with Nk points being (cNk + d) where f is the focal length of the focusing device. - the threshold power Pseu (i) is given by the formula: Virw2Fseuii (i) Pseuil (i) = 2 - the maximum number of points Nk achievable at each pulse of the light beam is given by the formula: Z - dX Nk CX - Yk with: - X = Plaseru% x% (C) v%; - Yk Fseuil (k) vuw2. 2 - Z w% Piaser.

According to another aspect, there is provided a micromachining method for forming a pattern on a material, comprising the following steps: emission of a spatially and temporally coherent pulsed light beam; Dynamic conformation of said light beam by a dynamic optical modulation device comprising a phase modulation for forming said light beam according to a plurality of points forming the pattern; - Focusing of the light beam thus shaped on a surface of said material; wherein the formation of the pattern on the material is performed with a single pulse train comprising a number of pulses of said light beam strictly less than the number of points forming the pattern, and wherein the emission of the light beam is controlled so that each The pulse has a determined pulse duration of between 10 ps and 100 ns, preferably between 100 ps and 10 ns, and more preferably between 300 ps and 8 ns. Preferred but nonlimiting aspects of this micromachining process, taken alone or in combination, are the following: the process is used to form the same pattern according to the same micromachining parameters on several identical products, where each pattern is registered after being trained to allow individual product authentication. the pulse train comprises a number of pulses at least two times smaller than the number of points forming the pattern, preferably at least ten times smaller, and more preferably at least one hundred times smaller. the pulse train comprises less than one thousand pulses, preferably less than one hundred pulses, more preferably less than ten pulses, and more preferably the pulse train comprises a single pulse. the method further comprises a step of calculating a modulation setpoint from an input setpoint corresponding to the pattern, said modulation setpoint being imposed on the modulation device in order to effect the dynamic conformation of the light beam. the emission of the light beam is controlled so that each pulse has a determined energy of between 10 pJ and 30 mJ, preferably of between 100 pJ and 15 mJ, and more preferably between 1 mJ and 10 mJ. the emission of the light beam is controlled so that the pulses of the pulse train have a rate of between 10 Hz and 30 kHz, preferably between 20 Hz and 5 kHz, and more preferably between 250 Hz and 1 kHz; . the emission of the light beam is controlled so that the pulse train delivers an average power of between 50 pW and 20 W, preferably between 10 mVV and 5 W, and more preferably between 20 mVV and 2 W. the emission of the light beam is controlled in order to have a rectilinear polarization before the dynamic optical modulation. A micro-machining system is also proposed for forming a pattern on a material from an emission of a spatially and temporally coherent pulsed light beam, comprising: a light beam control device comprising means for limiting the transmitting said light beam into a pulse train comprising a defined number of pulses, and means for setting said light beam to a pulse duration of between 10 ps and 100 ns; - A dynamic optical modulation device comprising means for modulating the light beam parameterized by the control device according to at least one phase modulation from a modulation setpoint, in order to form the light beam according to a plurality of points forming the motive; A control device provided for imposing the modulation setpoint on the modulation device and comprising means for calculating the modulation setpoint from an input setpoint corresponding to the pattern; - A focusing device arranged to focus the light beam shaped by the modulation device on a surface of said material.

Preferred but non-limiting aspects of this micromachining system, taken alone or in combination, are as follows: the system further comprises a set of optical elements arranged so that the focused light beam is oriented at 90 ° to to the light beam at the input of the system. - The system has a footprint of less than 200x200x200 mm3. DESCRIPTION OF THE FIGURES Other features and advantages of the invention will become apparent from the description which follows, which is purely illustrative and nonlimiting and should be read with reference to the accompanying drawings, in which: FIG. 1 is a perspective representation the proposed micro-machining system. FIG. 2 is a schematic representation of all the elements involved in the optical path of the light beam when using the micromachining system of FIG. 1, from the generation of the beam to the marking of the sample. FIG. 3 illustrates the principle of operation of a phase modulation device that can be used in the micromachining system of FIG. 1. FIG. 4 shows two laser marking results on material with the micro-processing method. proposed machining to form a datamateur type pattern. - Figure 5 highlights the uncontrolled authenticating aspect maximized by the proposed micromachining process, according to different observation points. FIG. 6 is a graph illustrating the evolution of the reaction threshold of a material as a function of the number of pulses per laser shot. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the micromachining of materials, that is, the structural modification of materials on a small scale with respect to the dimensions of said material. A particular example of micromachining is the marking of material, that is to say the creation of specific patterns by structural modification of the material. The following description is made with regard to this particular micromachining example that is the marking but the associated invention is in no way limited to this particular example and covers the entire field of micromachining. The basic principle of the proposed invention consists in modifying a light beam making it possible to make the marking to create several marking points that can be focused at the same time on a surface of the material that is to be marked. In contrast to the techniques of the prior art where the marking is carried out by moving a single point focused on the material, the plurality of focusing points on the surface of the material to be marked is positioned here, making it possible to carry out a marking in the manner of 'a tampon. It is thus possible to mark a pattern in a number of pulses of the light beam which is smaller than the number of dots forming the pattern. This technique has an obvious advantage in an industrial environment since it makes it possible to carry out complex markings in reduced times, and thus to increase the production rates without complicating the marking system. Such a marking technique makes it possible to overcome the problems of beam displacement and therefore of synchronization with the light source, while not necessarily soliciting high pulse rates.

In a conventional marking scheme with a laser light beam, there is a laser source, a set of laser beam control optics (including, for example, mirrors, lenses, polarization optics, filters) and focusing optics. final.

A complementary block is provided here for spatially modulating the light beam dynamically, in particular to control the shape of this beam, that is to say the spatial distribution of optical energy, in order to create several light spots in the plane. focusing of the final optics, corresponding to the marking plane on the material.

The modulator is an active optical element for spatially modulating the laser optical radiation. This modulation may concern the amplitude and / or the phase and / or the polarization of the radiation, independently or otherwise. Preferably, a phase modulation will always be performed, and it may even be possible to have particular modes of marking where pure phase modulation is preferred.

This pure phase modulation can for example be carried out using one or more liquid crystal layers, for example parallel nematic liquid crystals. The application of a local electric field on this layer (for example by applying an electrical control, or an optical control converted into electrical control, on two electrodes located on either side of the layer) allows the rotation of the crystals on themselves, and thus the local modification of the optical index and thus by extension the optical path difference of the laser radiation. Such modulators are called SLM (acronym for "Spatial Light Modulator" meaning "Spatial Light Modulator"). The light beam, for example a laser beam, incident on the modulator is controlled to enable efficient marking, in particular from an industrial point of view, while being compatible with the modulation device used, in particular the optical resistance of the modulator. The incident light radiation is generally derived from a source making it possible to emit a pulsed light beam, spatially and temporally coherent, such as a laser beam. The wavelength of the radiation may be any. A preferred domain is established for the visible and near-IR wavelength range ("Infra-Red") of wavelength in the range [350nm-2pm], and ideally in the band corresponding to wavelength radiation in the range [1 pm-2pm]. The light beam is pulsed, that is to say composed of a succession of pulses. Furthermore, the transmission is controlled so that the beam is emitted in the form of pulse trains, which are also referred to as shots, where a pulse train is formed of a defined number of pulses of the light beam. . The present system is compatible with the various modes of fire existing at present without constraints in terms of pulse rate.

Thus, the beam can be emitted in the form of a "shooting on demand". In this case, a succession of pulses is generated by a trigger signal (often electrical and / or TTL, acronym for "Transistor-Transistor Logic") issued by a third control element (computer, PLC, etc.). ). For example, a laser pulse can be generated at each rising edge of a TTL signal, or at each press of a button. The beam can be emitted in the form of a "continuous shot on clock base". In this case, the laser source has a regular clock signal and delivers a series of pulses at the same rate as the clock. The beam can also be emitted in the form of a "clock-triggered firing", also known as a "burst" mode, which is preferred. In this case, the laser source has a regular clock signal and a trigger signal, often called "trigger" or "gate". The principle is similar to the continuous case presented above, except that firing is also limited by the state of the trigger signal. For example, a laser pulse is emitted only in the case of a rising edge of the clock signal and a triggering signal in logic state 1. The duration of the pulses is also controlled to be between 10 picoseconds ( ps) and 100 nanoseconds (ns), preferably between 100 picoseconds and 10 nanoseconds, and more preferably between 300 picoseconds and 8 nanoseconds.

Such pulse durations are particularly advantageous since they are compatible with most light sources, especially laser sources, which are widely used in an industrial environment. The method and the micro-machining system proposed are therefore easily transposable to current industrial conditions. In addition, these pulse durations are also compatible with large amounts of energy that may be useful for marking certain materials of a pattern having many points in a very small number of pulses. The light beam is preferably controlled so that the energy per pulse and the average power of the beam are sufficient for marking, especially taking into account the intermediate losses incurred in the system and in particular due to the modulator, while being less than threshold values beyond which the modulator could be damaged.

Thus preferably the emission of the light beam is controlled so that each pulse has a determined energy of between 10 pJ and 30 mJ, preferably between 100 pJ and 15 mJ, and more preferably between 1 mJ and 10 mJ.

More preferably, the emission of the light beam is controlled so that the pulse train delivers an average power of between 50 pW and 20 W, preferably between 10 mW and 5 W, and more preferably between 20 mW and 2 W. The rate at which each pulse of the pulse train is delivered also plays a role which should be taken into account with regard to the marking on the one hand, but also the possible damage of the modulator. The rate chosen is also strongly related to the desired industrial marking conditions. Thus preferably the emission of the light beam is controlled so that the pulses of the pulse train have a rate of between 10 Hz and 30 kHz, preferably between 20 Hz and 5 kHz, and more preferably between 250 Hz and 1 kHz. Preferably, for a sufficiently structured marking, a high peak power is used. However, to avoid degrading the modulation device, a moderate average power is preferred. In the end, the emission of the light beam is controlled to have sufficient pulsed energy while having a moderate rate. One of the solutions making it possible to limit the degradation of the modulation device is to make markings in which each pulse train necessary for marking a pattern comprises the lowest possible number of pulses.

In any case, the pulse train comprises a number of pulses of said light beam strictly less than the number of points forming the pattern, which is particularly advantageous from an industrial point of view to increase production rates without having necessarily increase the operating rates of the micromachining system, including the light source, which is also in the direction of preserving the modulator. For example, the pulse train comprises a number of pulses at least two times smaller than the number of points forming the pattern, preferably at least ten times smaller, and more preferably at least one hundred times smaller.

According to a preferred embodiment, the pulse train comprises less than one thousand pulses, preferably less than one hundred pulses, more preferably less than ten pulses.

Optimally, the pulse train comprises a single pulse to form the marking in the material. The modulation applied by the modulator is calculated so as ultimately to obtain the desired focused beam shape.

In fact, a modulation setpoint is calculated from an input setpoint corresponding to the pattern, said modulation setpoint being imposed on the modulation device in order to effect the dynamic conformation of the light beam. In the case of a modulator comprising a phase modulation, this calculation can for example be performed by a phase map computation algorithm of the family of genetic algorithms, or iterative IFTA-type Fourier transform algorithms (corresponding to the acronym "Iterative Fourier Transform Algorithms"), or more generally by any optimization algorithm adapted to this problem. In addition, simple optical functions can be added, such as, but not limited to: - a transverse shift of all the points (tilt or phase ramp); an axial offset of all the points (phase curvature); known beam conformations (axicon, vortex). The target shape corresponding to the shape of the pattern can in principle be any. In the case where one seeks to use the marked pattern in the material to allow a certain traceability of the marked piece, we can privilege certain forms, such as for example: - Any forms composed of a set of focal points (forms multipoints); - Shapes representing a string of alphanumeric characters in a form "in the clear" (digits and letters) or encrypted (bar code, two-dimensional code - Datamatrix, QR-code, Aztec code, etc. -, etc.). The optical path before and after the modulation device is composed of a set of optical elements such as, but not limited to, mirrors, lenses, afocal systems, optical isolators, wave plates, separator elements and filters, shutters and security elements. These optical elements are chosen according to the desired application, in particular the marking envisaged, and the characteristics of the modulator. For example, before the modulation, we will provide a set of elements to: - Adapt the size of the beam to the various elements, in particular to the active surface of the modulator. - Adapt the energy and power level of the optical radiation to the constraints imposed by the modulator. After modulation, for example, a set of elements will be chosen to: - Adapt the size and the "virtual" position of the modulation modulator of the modulator. - Focus the laser radiation on the surface of the target to be marked. Since the laser beam is focused on the target, the spatial density of energy, otherwise known as fluence and expressed for example in J / cm 2, is increased. This high concentration of energy induces the modification of the target material. This modification can take various forms, and in particular: Morphological, for example, by the creation of microcavities, structures or textures, deposits, or the modification of the surface state. - Chemical in the form of oxidation, modification of the chemical structure for example. - Physics, with for example the modification of the optical properties (index, reflection, absorption), mechanical, or structural. The modifications of particular interest here are those that can be highlighted by vision tools, thus having an impact on the visual rendering (in the broad sense and not only of the human eye) of the target.

In Figures 1 and 2 is shown an example of a micro-machining system for marking materials in industrial conditions, and can be used in a compact and integrated environment. According to this example, the micromachining system - also called the marking head - is placed between a laser source 8 - which could be another light source - and a material to be marked 12 and preferably comprises: - An opening 2 for a an input laser beam with a diameter chosen to maximize the filling of the active surface of the optical modulator, without the need for the diameter to be greater than the active surface. For example, there will be a diameter less than or equal to 8 mm; - A dynamic optical modulator 3 allowing in particular the spatial modulation of the phase of this laser beam; A set of control optics 4 for the position of said beam, allowing for example the reorientation of the latter in a direction perpendicular to the input direction in accordance with the usual laser marking heads and / or the folding of the optical path to maintain the entire system in a space that is similar to or smaller than the usual laser marking heads (typically less than 200x200x200 mm3); - A focusing element 7 for concentrating the energy of the form generated by the modulator 3 on the material 12 - this element can be indifferently spherical lens type or aspheric, thin, doublet or achromatic triplet, f-theta and / or telecentric. The focusing may for example be perpendicular to the input face of the system, thanks to the set of optics; An opening 6 facing away from the focusing lens 7 which may or may not accommodate a display means of the marking area; A control electronics 5, whether on-board or not, including the control of the optical modulator 3 and / or the light source and / or the management of a database and / or the graphic interface for communication with the operator or the other components of the marking / micro-machining installation. An example of a marking method with a micromachining system as proposed is described with reference to FIGS. 2 and 3.

In the first place, a light source, such as a laser 8, is used to make the marking. This source is characterized by the emission of a pulsed light beam that is spatially and temporally coherent. Preferably, the emission is controlled so that the light beam has a given rectilinear polarization. This polarization can for example be imposed using a polarizer and / or a wave plate placed in the path of the light beam. As stated above, the light beam may have a wavelength ranging from near infrared to visible at typical values of 350 nm to 2 μm. As mentioned above, a beam with a specific pulse duration included in a preferential range between 10 ps and 100 ns is used.

The nominal power of the light beam is chosen according to the modulator 3 itself, sensitive to heating materialized by the average power of the laser. For example, a nominal power of less than or equal to 10W will be used. Indeed, while the viscosity of liquid crystals decreases with temperature making them faster to changes in state, a too high temperature induces a reversible melting of these crystals and the loss of modulation effects. The control 9 of this power, whether internal or external to the reference laser, is therefore important for the proper functioning of the micromachining system. It can be associated with this control that of the number of pulses generated (generally according to shots of "burst" type), also important in the context of very high speed applications where the window of optimization of the process in terms of quality- efficiency-speed can become very narrow.

As indicated above, a set of alignment optics 10 as well as resizing the beam 11, placed upstream of the modulator 3, may be relevant in the context of optimal operation of the system. The phase modulator 3 may be of the LCOS SLM type or of the ITO optical valve type. Beam diameters are preferred for covering most of the active surface 16 of said modulator without the need for the diameter to be larger than the active area. For example, typical values of diameter less than or equal to 8 mm are used. This active surface is in the form of a matrix of electrically addressed liquid crystals in the case of an SLM, or optically addressed in the case of optical valves, in order to induce their local rotation and to create a variation of the path by variation. optical index and exploitation of the birefringence of these crystals, as for example in the case of so-called "parallel nematic" liquid crystals. The propagation of the laser beam and in particular its wavefront, initially assimilated as a plane or curve 14, is then modified. The modulation applied by the modulator is calculated so as to obtain the desired beamform only in FIG. the focussing zone by the lens 7. A set of positioning optics 4 is preferably used again in order to align the laser beam in this same lens but also in order to minimize the bulk of the system, while maintaining the necessary configurations (low incidence on the modulator in reflection for example). The target form 19 can in principle be arbitrary and dynamically change depending on the input commands 15. The refresh rate of the modulation boards is dependent on the characteristics of the modulator chosen, but is based on typical values of 60 Hz and below. In the case of a pure phase modulator 3, the computation of the modulation to be applied may for example be carried out by a phase-map calculation algorithm of the family of genetic algorithms, iterative Fourier transform I FTA algorithms or any other algorithm adapted to this problem. This calculation takes into account the optical configuration implemented, in particular on the following aspects: - Size and shape of the beam 13, and more broadly characteristic of the optical input path; - Characteristics of the radiation (for example its wavelength); - Characteristics of the final focusing lens 7 (in particular focal length); - Optical conjugation and physical distance of the modulator (or its image) by this same lens. It should be noted that the distance separating the last image of the modulator before focusing the final optics should ideally be close to the focal length of the latter, and generally less than 100 times this focal length. The calculation algorithm, which may or may not be integrated in the marking head 1, then generates a phase map in the form of a gray-scale image, each shade of which is associated with a percentage of phase shift and thus of rotation of the crystals. maximum amplitude being a function of the characteristics of the SLM-radiation pair but also of the operator's choices. In addition, the initial modulation may be supplemented by mathematical / optical functions such as transverse offset (tilt, prism or phase ramp), axial offset (phase curvature), convolution (sum of phase cards), etc. .

The figure to be marked 15 and the calculation algorithm are predefined in order to integrate in the final sample marking 12 an embodiment in multipoint form in order to promote the reaction of the material, whether it be of the morphological type (cavity, texturing, deposition, surface condition, etc.), chemical (oxidation, etc.), physical (optical (refractive index, absorption, reflection, transmission, etc.) or mechanical (residual stresses, etc.)). The number of points that can be achieved simultaneously depends on multiple conditions such as the characteristics of the laser (energy, power, polarization, wavelength, pulse duration, cadence, etc.) of the described micro-machining system (energy tolerance, percentage of transmission, focusing force ...) but also irradiated material. High laser energies are therefore preferentially recommended because they are distributed between the different marking points. Knowing the tolerances of the modulators used in the context of this invention in terms of average power, pulse rate limits are also taken into consideration. For example, it is established that pulsed radiation consisting of pulses of about 500 ps to about 100 ns, each carrying 2 mJ or less, and transmitted at rates of 1 kHz and below is particularly suitable. In addition, the following operating modes have revealed a good ability to generate multipoint patterns on multiple materials by means of a single pulse or a train of a few pulses (typically less than 100 pulses, preferably less than 10 pulses). ): - Pulse duration of 400 ps (plus or minus 5%), energy of 2 mJ or less, and rate of 1 kHz or less; - Pulse duration of 7 ns (plus or minus 5%), energy of 7 mJ or less, and rate of 20 Hz or less; - Pulse duration of 87 ns (plus or minus 5%), energy of 100 pJ or less, and rate of 25 kHz or less; We will now describe an exemplary embodiment demonstrating the feasibility of marking identification codes on materials intended for high speed through the system and micro-machining process described above. This example is described with reference to FIG. 4. The laser used in this example has the following characteristics: Wavelength: 1064 nm; - Output power: 2.2W; Pulse duration: 10 ns; - Rate: 1 kHz; - Polarization: linear; - Shots in burst mode involving pulse-to-pulse control. The configuration has been chosen to take advantage of the maximum energy deliverable by this system, namely for a minimum rate of 1 kHz. The output diameter of the beam is a function of the power demand, so it was set at its maximum value of 2.2 W, an available energy of about 2.2 mJ. Its variation is therefore controlled externally by the half-waveband / polarization splitter pair 9, which furthermore makes it possible to maintain the correct rectilinear polarization at the input of the marking head 1. The optical assembly 11 is similar to a telescope and consists of two focal lenses defined to increase the beam by a factor of 2, from a diameter of about 4 mm laser output to about 8 mm on the modulator 3.

The optical assembly 10 is composed of mirrors disposed downstream of the assembly 11 in order to optimize the beam entry 2 into the marking head 1 without losses. The phase modulator 3 is of the LCOS SLM type with a resolution of 1920x1080 and has an area of 15.3x8.6 mm2 (square pixels of 8 pm side / pitch). The focusing lens 7 comprises for its part a single thin lens treated anti-reflection in the infrared focal length 100 mm.

All of these elements involve a marking distance between impacts ranging from a few microns to a few tens of microns, depending on the resolution of the phase map applied to the SLM. The command input image 15 is a datamatrix of 14x14 modules, each of which consists of a single pixel. In order to avoid an unsightly overlap on the material knowing impact sizes generally greater than 26 μm, empty (white) pixels artificially space the future marking points, as in FIG. computation for the generation of the associated phase map 17 is IFTA iterative type, the number of iterations being here defined almost arbitrarily, but nevertheless greater than ten, so that the optimization induced by this calculation is substantially stabilized and therefore fluctuates very little from one iteration to the next. The samples of labeled materials 21 and 22 are of the polymer type covered with a metallization of respective colors silver and gold a few micrometers thick. The image acquisition was obtained by means of a x40 magnification microscope with transmission illumination. Pulse trains comprising 25 pulses were used to ablate the varnish, ie 25 ms @ 1kHz for marking a datamatrix of about 720 pm side and having 108 points. Under similar conditions but with a standard galvanometer head deflection head employing all available power at a single focusing point, the laser required for marking such a datamatrix should provide a minimum rate of approximately 4 kHz to be competitive with the same time of 25 ms. At this rate, this assumes a single pulse per marking point and a virtual absence of time lost during the various mechanical repositioning. Compared to a conventional marking solution, the technology developed here assumes greater flexibility in the context of high-speed and on-the-fly marking applications and therefore shorter and more optimizable completion times. The second embodiment aims to highlight the authentication aspect maximized by the proposed micro-machining system through an example of simple marking. This example is described with reference to FIG. 5. The laser used in this example has the following characteristics: Wavelength: 1064 nm; - Output power: 6VV; Pulse duration: 80 ns; - Rate: 25 kHz; - Polarization: random; - Pulse control modulation: 5 kHz.

The configuration of the various optical elements is similar to the previous case. Nevertheless, this laser whose specifications are more common has much more power for a rate even higher. Also, it does not integrate pulse-to-pulse control and an external modulation (of the "trigger" or "gate" type as mentioned above) of lower rate is necessary in order to maintain a constant output power output while having sufficient control over the number of pulses irradiating the sample. Therefore, for a maximum frequency modulation of 5 kHz at a rate of 25 kHz, the minimum number of pulses per shot allowed by the set is 5. Also, the polarization of this system is random. The passage in a polarizer cube ensures entry into the marking head in the correct polarization despite a loss of energy of half. 100 μJ of energy (ie 2.5 VV average power at 25 kHz) finally reach the phase modulator 3. The control input image 15 is a matrix 5x5 impacts, or 25 points distributed uniformly and regularly on a surface of 230x230 pm2.

The calculation algorithm for the generation of the associated phase map 17 is again IFTA iterative type. In the context of this example, the control of the number of iterations is essential here. The material to be marked is PE polymer type with aluminum coating several hundred nanometers.

The series of images 24 of FIG. 5 highlights several dot matrices made exactly in the same illumination and modulation configurations. Only the positioning on the sample varies. There may be slight fluctuations from one marking to another (typically 24a and 24b), representative of the fluctuations in the properties of the irradiated materials, whether by beam shaping or by galvanometric head. The series of images 25 and 26 are nevertheless specific to the use of the formatting head described in this document. The series of images 25 highlights several markings from the same source image but whose phase maps are different. This uncontrolled process is intrinsic to the algorithm used which does not converge to a single phase map solution. This fluctuation, associated with that of the incident light source and the local properties of the material is added to the uniqueness of the marking and therefore to this authenticating aspect specific to the laser material interaction. Finally, the series of images 26 shows the effect of the number of iterations for the same card being computed, where the eight of images 26a to 26h correspond to tests with 1, 2, 3, 4, 5, 10, 20, and 50 iterations respectively. The deviation from the source image is all the more important as the number of iterations is small because of a partially converged phase map solution. It may be noted the absence of certain marking points 26a and the appearance of other parasitic points 26b highlighting the ability to create a marking with a unique and uncontrolled signature. This exemplary embodiment clearly demonstrates the importance of such an innovation, particularly in the market for traceability and the fight against counterfeiting where the needs in terms of marking speed and authenticating prospects are proven. Thus, the same pattern can be formed according to the same micromachining parameters on several identical products, where each pattern is recorded after having been formed so as to allow individual authentication of said products. The micromachining system and process presented allow a significant productivity gain. Indeed, in the first approach, the use of a shaped laser beam allows the increase in the area (or the number of points) marked (e) (s) in a single laser shot. The time required for unit marking is therefore reduced by a factor equal to the number of points simultaneously marked (or the ratio of the surfaces). In addition the use of current optical modulators allows to consider a dynamic modulation (shape change over time) with a frequency of 10Hz-20Hz or even up to 60Hz. In the case of a need to change the shape between two markings (for example in the case of a unit numbering of products), the process has a potential rate of 36,000 to 72,000 pieces per hour. These rates are a priori five to ten times higher than current rates. In addition, the system and micro-machining process presented allow to mark complex shapes in few shots. Current solutions being based on the displacement of a focused beam, the realized pattern inherently has rounded edges on the scale of the beam size (often of the order of ten to hundred micrometers). The use of a shaped beam makes it possible to envisage obtaining a form that until now was very complex on a small scale, in particular having straight or sharp angles. Moreover, in the particular context of laser marking of encrypted information in the form of two-dimensional codes, the datamatrix format is now preferred to the QR-code format because it is faster to mark with equivalent content (since it presents displacements between two successive modules less numerous). This second type has the advantage of easier reading. The use of the installation described here makes it possible to overcome this limitation by presenting an identical realization time for the two types of coding, with equivalent content, because of the simultaneous marking of all the points constituting the code. within the limit where this number of points remains lower than the maximum number of points that can be made simultaneously. Finally, the system and the micro-machining process presented are of particular interest with regard to the authentication of products, in traceability or anti-counterfeiting applications, for example. They make it possible to simply create a unique marking. During the interaction of the light radiation with the target, the result obtained is highly dependent on the optical properties of the beam and the physical properties of the target. By combining a control of the spatial distribution of the optical energy, and an intrinsic non-uniformity of the local properties of the material, a significant variation of the marking result can be obtained from one shot to the other. The registration of a signature of these non-repeatable aspects allows a posteriori the authentication of the support. In order to use the micro-machining system and method presented in an optimized manner, a method for calculating the number of simultaneous impacts achievable on a given material is also proposed. For this purpose, we identify all the influence parameters that can play on the characteristics of the laser-material interaction at the origin of the multipoint tagging feasibility described above. These parameters can be grouped into three categories: - Laser parameters: these are the characteristics specific to the laser source, or the source of the light radiation in general; - Parameters of the micromachining system and process: these are the characteristics specific to the micromachining system, its parameterization and its operation, making it possible to treat the beam from the laser to the sample; Material parameters: these are the characteristics specific to the final study material, the laser-material interaction being dependent on the properties of each material such as absorption-transmission properties, melting-vaporization temperatures, etc. After identification of these different parameters, a simulation as close as possible to the multipoint marking reality can be realized in order to optimize the use of the proposed micro-machining system. This use can also be based on actual marking tests. First, list the parameters that characterize the light source, usually a laser source. When using the micro-machining system with a pre-existing industrial laser, these parameters are often found in the documentation associated with said laser. In particular: - Wavelength λ - Pulse rate v - Pulse duration T - Polarization p - Average power Shave - Exit output diameter - Divergence a For maximized operation of the proposed marking head more high, the wavelength λ of the laser is preferably adapted to the various process optical processes, the polarization p is rectilinear in an orientation defined by the marking head, the outgoing beam output diameter is adapted to the optical zone active modulator and the divergence of the minimized beam (so-called collimated beam). In the latter two cases, a judiciously chosen lens set makes it possible both to adapt the size of the beam to the entrance of the marking head but also to reduce the divergence until a quasi-collimated beam is obtained. In the parameters of the system and the micro-machining process, there is a distinction between all the optical elements enabling the characteristics of the laser output beam to be readjusted (the notions of divergence and polarization have been mentioned), the specific characteristics of the desired marking. (focal), and the properties of the shaping marking head which has been described in detail above. It is considered here that the laser beam corresponds to the input data of the marking head. The latter is characterized by: - Percentage of transmission u%: the SLM being a pixellated optic, a significant percentage of lost energy is to be considered in estimating the amount of energy available for formatting; this parameter also incorporates the imperfections of the optics processing component path before and after the modulator. - Percentage available after dissymmetry effect v%: a loss of energy due to the pattern to be marked between symmetrical shaping with respect to the optical axis and unsymmetrical shaping, symbolized by an estimated constant to v%; - Percent lost by central spot w%: due to the non-perfection of optical and laser properties as well as calculation approximations, a quantity of energy, again estimated by a constant w%, is found in a focus point central, not subject to the shaping applied on the beam by the proposed system. - Percentage available by adding a curvature x% (C): in the image of a symmetrical shaping or not, the realization of a phase map including a value of curvature influences on the quantity of available energy for shaping. C is the curvature value applied as part of a layout; - Coefficients of number of impacts c &d; percentage available by the number of points (cNk + d): in the image of a symmetrical or non-symmetrical shaping, the realization of a phase map generating on the material a different number of points of focus influences the quantity available energy for shaping the same number of points. - Focal distance f: working distance as a function of the converging focussing lens defined. This parameter makes it possible to define, among other things, the size of the laser beam at the marking plane and therefore the energy density in this plane.

All these parameters being characterized, the power available for beam shaping is then calculated by the following formula: Pdispo = Plaser11% X% (C) (CNk C) V% - wUP - laser Each material having its specific absorption properties, it is characterized by thresholds in terms of energy and energy density and from which it begins to react and change appearance. The determination of these thresholds is important to validate or not the feasibility of marking with a given laser. To determine the reaction threshold of the material, Liu's method is preferably used, which assumes the perfect transverse shape of a laser beam (Gaussian form) of the following formula: (-2r2 / 2Pmoy F (r) = Fcretee ' coz) with Fpeak = Or 'V1TW 2 - F is the density of energy (in J / cm2, also commonly called fluence), - r is the distance to the optical axis, Fpeak - F, is the maximum fluence taken at the optical axis and expressed as a function of the average laser power Pmoy, - v is the pulse rate, and w is the radius of the beam at the focusing plane (also commonly called "waist") and is directly dependent on the focal length f of the lens of the system. Liu's method considers in particular that the irradiated material reacts from a certain energetic density, Fseuil, thus associated with a physical impact diameter D, that is: -D2 Fseuil = Fcrêtee 2W2 Hence after remanipulation of this equation: D2 = 2w2 in (Fpeak) 2 (° 21n (Fseuii) Finally, it appears that the impact diameter on the machined material D increases linearly as a function of In (Fpeak) and thus indirectly as a function of the power The threshold Fseuil finally considers itself as the ordinate at the origin of this line.It should be noted that this right given in fluence is also valid in energy and intensity by simple relations of proportionality between these different values. VTCW 2 It is also noted that the pulse duration affects this marking threshold Fseuii in the sense that a shorter pulse duration decreases it.The number of impacts that can be simultaneously achieved will theoretically ically increase as the pulse duration decreases.

Liu's method presented above corresponds to a simulation performed on the basis of gaussian-shaped laser fire with only one pulse. The threshold thus defined corresponds to the monopulse laser threshold of the test material. Preferably, this method is transposed to laser shots composed of several pulses by repeating the calculations several times for several numbers of specific pulses. Consequently, a phenomenon, known as an incubation phenomenon, appears on the majority of real materials, characterized by the decrease of the threshold of a material with the increase of the number of laser pulses constituting the laser shot. By noting i the number of pulses comprising a laser shot, this phenomenon can therefore be represented by replacing the threshold P preceding threshold by the function Pseuil (i).

FIG. 6 is a typical diagrammatic representation illustrating the evolution of the threshold as a function of the number of pulses per pulse train. We see that there is a saturation effect of the incubation illustrated on the graph by the horizontal asymptote.

The first step of the simulation procedure for estimating the number of simultaneous points theoretically achievable on a given material by a given laser is to recover or calculate the various parameters mentioned. Secondly, knowing the marking threshold of the material as well as the laser power available for shaping, it is assumed that the number of shaped points does not affect the marking threshold of the material, assuming that these impacts are dissociated. Therefore, it is estimated that the available power is equivalent to the reaction threshold power of the material by the maximum number of points theoretically achievable by shaping, and this for a number k of pulses per laser shot, ie: Pdispo = NkPseuil (k) Applying this relation to the equation of the reaction threshold of the material, for a number k of pulses per pulse train, we have: F (k) threshold - 2P threshold (k) is Pcuspo Nk co2Fseuii (k) virco2 2 Finally, by associating this equation with that explained in the description of the parameters of the system and micro-machining method, an equation with one unknown in Nk is obtained, the other parameters being known, namely: % X% (C) (CNk d) 12% - W% P Fseuil (k) NkinrW2 - - - laser = 2 And where finally the estimate of the maximum number of points Nk simultaneous by shaping the head of marking with a pulse train comprising k pulses: Z-dX Nk cX-Y k with: - X = Piaseru% x% (C) V % Y Fseuu (k) lnrco2 - 1k - This procedure finally allows using the various parameters of the complete system to estimate in a high degree the number of maximum points Nk achievable simultaneously by beam shaping of a laser shot of k pulses, knowing the variations of the laser source used and the material to be marked. 2 - Z P / aser By extension N.0 we will note the limit value of the series (which converges a priori because of the saturation of the incubation), which represents the maximum number of points that can be inscribed simultaneously in the absolute. From this maximum number of points Nk that can be produced simultaneously by beam shaping of a laser shot of k pulses, the micro-machining system can be adjusted by parameterizing, in particular, the modulation device in order to shape the light beam. a plurality N of points less than or equal to the maximum number of points Nk achievable at each pulse of the light beam. To be certain that each point is well marked on the material, it is possible to set this number N of conformation points smaller than the maximum number of points Nk that can be produced simultaneously by beam shaping of a laser firing of k pulses. which makes it possible to have in fact an energy available for the higher marking. Preferably, the phase modulation is parameterized to form the light beam into a plurality N of points less than or equal to half the maximum number of points Nk that can be produced at each pulse of the light beam. It is also possible to deduce the number of pulse trains necessary for the formation of the complete pattern, in particular for the case of very complex patterns, by dividing the number of points forming the pattern by the number N of conformation points chosen for the pattern. parameterization of the phase modulation. BIBLIOGRAPHIC REFERENCES - US 5,734,145 - US 4,128,752 - FR 2,909,922 - US 4,734,558 - US 4,818,835 - US 2001 / 045,418 - FR 2 884 743 30

Claims (8)

REVENDICATIONS1. A method of using a micro-machining system to form a pattern on a material, the system comprising a device for emitting a spatially and temporally coherent pulsed light beam, a dynamic optical modulation device of said light beam comprising a phase modulation for shaping said light beam at a plurality of points and a focusing device of the light beam shaped on a surface of said material, the method being characterized in that it comprises the following steps: - Calculation of a density of threshold energy Fseuil (i) from which the material reacts as a function of the number of pulses (i) of said light beam, and determination of a threshold power Pseuil (i) associated; - Calculation of an available power Pdispo output of the modulation device from characteristic parameters of the transmitting device and the phase modulation device; - Setting a pulse train by choosing a number of pulses k of said light beam to react the material; - Calculation of a maximum number of points Nk achievable at each pulse of the light beam, assuming that the number of shaped points does not affect the threshold at which the material reacts, according to the relation: 0 Nk = Pdispoi Pseuii (k) - Parameterization of the phase modulation for shaping the light beam into a plurality N of points less than or equal to the maximum number of points Nk achievable at each pulse of the light beam. 2. Method according to claim 1, wherein the phase modulation is configured to form the light beam into a plurality N of points less than or equal to half the maximum number of points Nk achievable at each pulse of the light beam. 3. Method according to any one of claims 1 or 2, comprising a complementary step of calculating a number of trains of pulses necessary for the formation of the complete pattern by dividing the number of points forming the pattern by the plurality N of points chosen for the parameterization of the phase modulation. 4. Method according to any one of claims 1 to 3, wherein the parameterization of the pulse train consists in choosing the number of pulses k according to the calculation of the threshold energy density Fseuii (i), the number of pulses k being an integer selected from the number of pulses k200 corresponding to a threshold energy density equal to 200% of the minimum threshold energy density, and the number of pulses k100 corresponding to the number of pulses the weaker for which the energy density is equal to the minimum threshold energy density. 5. Method according to any one of claims 1 to 4, wherein the calculation of the threshold energy density Fs', i (i) is performed considering that the light beam has a Gaussian shape and that the material irradiated with the light beam reacts from a threshold energy density Fthreshold given by the following formula: -D2 / 2co 2 2Pm "with Fpeak = Fpole Fcretee virco2 where D is the physical impact diameter of the light beam on the material, Fpeak is the maximum energy density taken at the optical axis and expressed as a function of the average laser power Pmoy, v is the pulse rate and w is the radius of the light beam in the focusing plane of the focusing device. 6. Method according to claim 5, wherein the calculation of the available power Pdispo output of the modulation device is according to the following formula: Pdispo = PlaserU% X% (C) (CNk d) v% - w'DAP - - laser with: - u% the transmission percentage of the dynamic optical modulation device; - v% the percentage available after dissymmetry effect of the pattern to be marked; w% the percentage lost by the light beam at a central focusing point not subject to conformation of the dynamic optical modulation device; - x% (C) the percentage available after the effect of the application of a curvature C on a phase map for a modulation setpoint applied to the dynamic optical modulation device; - c and d number of impact coefficients reflecting the multiplicity of focus points of the phase map used for the modulation setpoint applied to the dynamic optical modulation device; the efficiency associated with Nk points being (cNk + d) where f is the focal length of the focusing device. 7. The method of claim 6, wherein the threshold power Pseu, i (i) is given by the formula: Vir (02 F threshold (i) Pseuil (i) = 2 8. The method of claim 7, wherein the maximum number of points Nk achievable at each pulse of the light beam is given by the formula: Z - dX Nk CX - Yk with: - X = PlaserU% X% (C) V% Yk = Fseuil (k) vru,) 2. 2 - Z = w% Piaser.5