WO2001091034A1

WO2001091034A1 - Method for marking a material and method for reading said marking

Info

Publication number: WO2001091034A1
Application number: PCT/FR2001/001576
Authority: WO
Inventors: Bernard Cretin; Bruno Cavallier
Original assignee: Centre National De La Recherche Scientifique (Cnrs)
Priority date: 2000-05-23
Filing date: 2001-05-22
Publication date: 2001-11-29
Also published as: FR2809517B1; FR2809517A1; AU2001263997A1

Abstract

The invention concerns a method for reading structures or data written in a material, which consists in scanning the surface of the material, without modifying the structures or data, with an electromagnetic reading source. The invention is characterised in that it consists in collecting an image of the photothermal or photo-thermoelastic contrast of the scanned surface. The invention also concerns a method for writing in a material, which consists in exposing said material to a electromagnetic writing source to bring about a phase transition in a volume located in said material. The invention is characterised in that it consists in modifying, during said phase transition, the thermal, and optionally the mechanical, properties of the material, so as to modify the photothermal or photothermoplastic contrast of the material and in marking it for subsequent reading of the photothermal or photo-thermoelastic contrast of the marked surface.

Description

METHOD OF MARKING A MATERIAL AND METHOD OF READING THIS MARKING

The invention relates to the field of reading and writing markings on products. More specifically, according to one aspect, the invention relates to a method of reading a marking inscribed on a material, with an electromagnetic reading source. According to another aspect the invention relates to a marking method, in which a material is exposed to an electromagnetic source to cause a phase transition in a localized volume of this material. The invention also relates to the products marked by this marking process.

Optical reading methods are already known, in which the reflection coefficient of an electromagnetic source on a material is measured. This type of optical reading process is for example commonly used to read compact discs. In this case, the electromagnetic source is a laser.

An object of the invention is to provide a method for reading a new type of marking of a material.

This object is achieved by a process for reading structures or information written in a material, in which the surface of the material is scanned, without modifying the structures or the information, with an electromagnetic reading source, characterized in that the material is subjected to two different beams for reading: a first excitation beam, for heating the material and a second detection beam for detecting the variation in the thermal properties of the material. According to an advantageous characteristic, an image of the photo-thermal or photo-thermo-elastic contrast of the surface thus scanned is collected.

In the case of the invention, the reading is carried out by a spatial reading of the photo-thermal and / or photo-thermo-elastic properties of a material. To make this type of survey, a source of electromagnetic radiation is used. Advantageously, this is modulated in intensity. Typically it is a laser source. It modulates the temperature of the material. The interaction between the electromagnetic wave and the material produces, by photo-thermal conversion, a localized temperature rise. The resulting heating generates many physical phenomena in the material and its environment. The reading process according to the invention is based on the detection of the thermal and / or thermo-elastic properties of the materials.

The analysis of the thermal properties of the materials is carried out for example, by measurements of the reflectivity. Indeed, the reflectivity of a material depends on its temperature. The analysis of the thermo-elastic properties of the materials is carried out for example by an interferometric method, by measuring the local deformations of the materials generated by thermo-elastic conversion.

The reading method according to the invention advantageously comprises the following characteristics, taken independently or in combination: - the amplitude and the phase of the signal corresponding to the dynamic thermal reflectivity of the material or to its thermoelastic deformations are separated by synchronous detection or an equivalent method, for example by correlation;

- the photo-thermal or photo-thermo-elastic contrast image is collected without contact with the surface of the material;

- the parameters of the electromagnetic reading source are chosen so as to detect a writing structure underlying the surface; and

the parameters of the electromagnetic reading source are chosen so as to detect a photo-thermal or photo-thermo-elastic contrast due to a phase difference of the material.

Advantageously, according to the reading method according to the invention, an image of the photothermal contrast is collected, by measuring the infrared emission. This technique makes it possible to use only one receiver and to get rid of the need to have a source of excitation for the detection operation.

According to another aspect the invention is a method of writing a material in which this material is exposed to an electromagnetic writing source to cause a phase transition in a localized volume of this material, characterized in that one modifies, during this phase transition, the thermal, and possibly mechanical, properties of the material, so as to change the photo contrast -thermal or photo-thermo-elastic of the material and to mark it for a later reading of the photothermal or photo-thermo-elastic contrast.

The writing method according to the invention advantageously has the following characteristics taken independently or in combination:

- the electromagnetic writing source is a laser source; - the writing is adapted to be read by collecting an image of photo-thermal or photo-thermo-elastic contrast, of a surface scanned with an electromagnetic reading source; in this case, the reading is advantageously carried out using a power of the electromagnetic writing source greater than the power of the electromagnetic reading source, at least by a factor of 1.5, and preferably by a factor of between 2 and 5; and

- the writing is adapted to be read by collecting an image of optical contrast, of a surface scanned with an electromagnetic reading source. The methods in accordance with the invention find applications in the fields in which discrete (optically invisible, for example) and reliable markings may be required (heating above 300 ° C. is tolerable without deterioration for most metallic materials and time alone does not cause any degradation in the medium term, which allows markings / recordings in the very long term). Among the possible applications, we can cite:

- discreet marking of banknotes or other monetary securities (coins, ...), bank cards, marking of confidential documents or security documents; - the traceability of valuables and in particular jewelry (marking to control the origin and belonging); - the anti-theft marking on different types of vehicles (the discretion of the marking is here a guarantee of security; moreover, the reading process according to the invention detects in the volume of the marked object - often metallic; the marked object can therefore undergo all the usual climatic conditions for decades without the reading being altered;

- the storage of information in analog or digital form, unalterable over time; the density of information can be equivalent to that of a current compact disc (audio CD, CD - R, DVD-RAM for example);

- etc. The invention according to another aspect is a product marked by the writing method according to the invention.

Other objects, aspects and advantages of the invention will be better understood with the aid of the detailed description which follows. The invention will also be better understood with the aid of the drawings in which: FIG. 1 represents a diagram of an embodiment of a device by the implementation of the writing and reading methods in accordance with the present invention;

- Figures 2a to 2e represent a set of reading images obtained in the context of a first example of implementation of the reading method according to the invention, on the device shown in Figure 1;

FIGS. 3a to 3e represent a set of reading images obtained in the context of a second example of implementation of the reading method according to the invention, on the device represented in FIG. 1,

- Figure 4 schematically shows a marking structure capable of being read in the context of a third example of implementation of the reading method according to the invention, on the device illustrated in Figure 1;

- Figure 5 schematically illustrates, in section, a test sample for the implementation of a reading method according to the invention; and

FIG. 6 represents the evolution of the optical (solid line) and thermal (dotted line) signals as a function of the area of the sample comprising the marking structure shown in FIG. 4. The invention is described in detail below in the context of its implementation with a particular, but not limiting, example of a writing and reading device.

According to this example, represented in FIG. 1, the writing and reading device for implementing the methods according to the invention, comprises a first excitation source 2, an intensity modulator 4, a low frequency generator 6, a mirror 8, a lens 10, a second excitation source 12, a photo detector 14, a demodulator 16, first 18 and second 20 synchronous detectors, a microcomputer 22, a sample holder 24, and a device movement 26 of the sample holder 24.

The first excitation source 2 is a power laser, for example an argon laser. The intensity modulator 4 is an acousto-optical modulator which diffracts the light coming from the first excitation source 2, according to the level of electric power which is applied to it. The intensity modulator 4 is controlled by control electronics (not shown) and the low frequency generator 6.

The mirror 8 is a dichroic mirror. The dichroic mirror 8 protects the photo-detector 14 from the “parasitic” beam delivered by the first excitation source 2. The objective 10 is a microscope objective which focuses the beam coming from the intensity modulator 4, on the sample placed on the sample holder 24. The second excitation source 12 is a helium-neon laser. The operation of this device is as follows. The writing process consists in transforming the surface and to a certain extent the volume of the material of the sample by heating, irreversibly in normal conditions of conservation of the latter (it should be noted that in general, an erasure writing is possible by the same method). For example, an amorphous material will be transformed by recrystallization. In the case of metals, rough polishing of the surface makes it possible to disorganize the surface of the material. During the writing operation, the material will tend to regain the organized structure of the metal locally. For the implementation of the writing process, only the excitation part of the device is used. This excitation part is essentially composed of the first excitation source 2, of the intensity modulator 4 and its control electronics, as well as of the low frequency generator 6. The beam coming from the first excitation source 2, is modulated in intensity by the intensity modulator 4. This beam is firstly reflected by the dichroic mirror 8 then focused using the microscope objective 10. The intensity modulator 4 and the displacement device 26 are controlled by the microcomputer 22 so that the writing is carried out with the required power and in the appropriate places.

For the implementation of the reading method, an excitation operation and a detection operation are carried out. The writing marking is read by an optical, thermal or thermo-elastic method using the device described above in relation to FIG. 1, sensitive to the changes in properties induced by phase transformation. The reading, which is carried out at a lower temperature than the marking, does not modify the marking carried out.

For the excitation operation, the excitation part described above is again used, but the average power of the first excitation source 2 is reduced to remain in reversible regime, that is to say for a reading. non-destructive. At the output of the objective 10, the optical energy contained in the focused beam is transformed into thermal waves by photo-thermal conversion thus generating local heating of the sample. Then by thermo-elastic conversion the rise in temperature generates local dilations of the sample. These dynamic thermoelastic deformations evolve at the modulation frequency of the low frequency generator 6 and are detected at the surface of the sample by the detection part.

For the detection operation, the second excitation source 12 emits a beam which is reflected perpendicular to the surface of the sample. Part of this beam is used to provide optical and thermal images respectively linked to the static and dynamic reflection coefficients, while the other part allows the detection of displacements. normal to the surface of the sample, ie thermoelastic deformations, by interferometry.

A separating plate, not shown, next to the dichroic mirror 8, reflects, on the photodetector 14, approximately 10% of the beam coming from the second excitation source 12 and reflected by the sample. The electronics associated with the photodetector 14 delivers two signals. The first signal is filtered by a low-pass device and corresponds to the static component of the reflectivity, ie the optical signal. The second signal is filtered in a high-pass filter and consequently corresponds to the dynamic component of the reflectivity, that is to say to the thermal signal which is sent, via the demodulator 16, to the first synchronous detector 18.

In addition, the rise in temperature generates local expansions of the material. These thermoelastic deformations, which evolve at the rate of the modulation frequency, are detected on the surface of the sample by the beam of a high resolution heterodyne interferometric probe 13 (typically 10 ^"2 pmΛ / Hz). optical pathways are converted by this interferometric probe 13, in the form of an electrical signal by a broadband photodetector (laser probe) and are demodulated by specific phase demodulation electronics. The signal thus obtained is sent to the second detector synchronous 20 in order to extract the amplitude (energetic part) and the phase (“propagative” part) of the thermoelastic signal.

The information obtained point by point is sent to the microcomputer 22 which manages the display of the results as well as the scanning provided by the displacement device 26. The latter comprises two XY cross translation units of high precision. An analog-digital converter, not shown in FIG. 1, makes it possible to acquire “in real time” the different signals from the first 18 and second 20 synchronous detectors as well as the optical signal. This microscope operates entirely without contact with an adjustable working frequency between a few hundred Hertz and a few hundred kiloHertz. This microscope is a microscope multi-acquisition, it allows, as indicated above, to obtain several types of images:

• an optical image of the surface, by an operation analogous to a scanning optical microscope, • two photoreflectivity images (amplitude and phase): these are photo-thermal images,

• two photo-thermo-elastic images (amplitude and phase).

The device described above is entirely suitable for industrial use for materials having a high optical reflection coefficient and a low roughness. This last point is important if one wishes to avoid diffraction of the probe beam by the surface of the sample (in practice, the diffraction effect is limited by the microscope objective).

It should be noted that optical detection can be useful for "visual" positioning, that is to say in relation to the optically visible elements of the sample.

Three examples of the implementation of the reading method according to the invention are described below.

According to the first example of implementation, by way of illustration, the letters “LPMO” were marked on a sample of polycrystalline nickel (this sample having been roughly mechanically polished beforehand). Each letter has a height of 5 micrometers. Focusing has been optimized using a 10x type x100 lens which provides an optical resolution of around 0.5 μm. However, another objective 10 can be used depending on the size of the marked pattern and the depth of the marking. The thermal marking represented in FIG. 2 was carried out with a laser power of 180 mW and with the 10 × 100 objective, waiting for 100 ms at each minimum step (100 nm) of the displacement device 26.

FIGS. 2a to 2e correspond to the reading of the letters “LPMO” by the multi-acquisition photothermoelastic microscope described above. The size of the images presented is 28x13 microns. The reading is carried out as indicated previously. The modulation frequency is 8 kHz (non-critical value), the excitation power is 36 mW for reading (5 times less important than that used for marking). The optical images 2a of the photothermal amplitude 2b, of the thermoelastic amplitude 2d and of the thermoelastic phase 2e, do not exhibit the contrast of the marking of the letters "LPMO", by phase transformation, by recrystallization of nickel. Only the image of the photothermal phase 2f makes it possible to read the marking of letters "LPMO".

According to the second example of implementation of the reading method according to the invention, the material is optically marked, for example, by thermal marking, that is to say without removal of material. When a certain power threshold is exceeded depending on the material of the sample chosen and the surface condition, the area is damaged (melting of the material on the surface) leaving the information to appear optically. Many materials can be so marked (Silicon, steel, copper, nickel, NiB, etc. Two examples illustrated by Figures 3a and 3e and 4a to 4e correspond to applications of optical etching respectively on Silicon and on steel and its repercussions on thermal and thermoelastic images. The size of the letters thus marked can be less than 10 micrometers, while the human eye perceives objects of a size greater than 50 micrometers. According to the third example of implementation of the reading method according to the invention, information is detected which is optically masked by a metallic film on the surface. It suffices for this that the topographic effects are sufficiently weak not to allow visual detection. The results presented below show clearly that this process can be applied to reading information written beneath the surface of the object.

FIG. 5 schematically illustrates, in section, a test sample for this third example of implementation of the reading method according to the invention. This sample consists of a quartz substrate 30 on which is deposited a pattern 32 of aluminum, 500 Å thick, intended to be read, the whole being covered with a layer 34 of titanium, also 500A thick. The pattern 32 of aluminum and the layer 34 of titanium are optically opaque.

The experimental conditions associated with this third example of implementation of the reading method according to the invention are: x20 focusing objective, and modulation frequency of 8 kHz.

Several tests have been carried out in order to see if the pattern 32 subsurface is detectable. It should be noted that the detection beam (HeNe probe beam) is coincident with the excitation beam (Argon beam). The results are presented in the two tables below.

Table 1 represents the amplitude | Uz | and the phase Pha (Uz) of the thermo-elastic coefficient Uz in the direction normal to the surface of the sample, on the area of the sample with the subsurface pattern 32 (front) and on the area of the sample without the subsurface motif 32 (after).

Tables 1 and 2 also show the variations of this value as well as the dynamic reflectivity when passing from an area with the subsurface pattern 32 to an area without it.

Tables 1 and 2 group these different values, for different values of the magnification of objective 10 and the modulation frequency.

TABLE 1

Table 2 presents the amplitude | T | and the phase Pha (T) of the dynamic thermal reflectivity, on the area of the sample with the subsurface pattern 32 (before) and in the area of the sample without the subsurface pattern 32 (after).

TABLE 2

The results grouped in the previous tables show that to obtain the best results, that is to say the best contrasts, it is preferable to work at maximum focus (* 100) and with the highest possible modulation frequency (here 100 kHz ) for the detection of the photothermal signal which has a variation of 64% in amplitude.

The more the modulation frequency is increased the more parameters close to the surface are probed.

FIG. 6 shows the evolution of the optical (solid line) and thermal (dotted line) signals, as a function of the distance traveled X, during a scan passing from an area without the pattern 32 to an area with the pattern 32. The variations in these signals are reflected by variations in the gray level of the images. The right part of the two curves corresponding to the area without the pattern 32 and the left part of the two curves corresponding to the area with the pattern 32. For the acquisition of these curves, the time constant τ is 300 ms and the waiting time is 3 τ.

The Argon laser can provide adjustable power from one milliWatt to about two watts. However, the implementation of the writing method requires, according to the invention, little power due to the focusing of the beam by the microscope objective 10. A power of 180 mW for example can be used. On the other hand, the available power can prove to be very useful in the case of an unfocused excitation beam for parallel imaging with a CDD camera for example. The power density in the non-focused case can thus reach 7.35.10 ⁵ W / m ² , and in the focused case 1.65.10 ¹² W / m ² (with an x100 objective and knowing that the minimum diameter of the beam is 1 , 5 μr).

As far as beam intensity modulation is concerned, the acousto-optical modulator is conventionally positioned in Bragg incidence, thus allowing the beam to be divided into two parts (order 0 and order +1 or -1) whose intensities are related to the amplitude of the applied voltage. The acousto-optic modulator 4 control electronics deliver a 40 MHz frequency signal which generates, by piezoelectric conversion, a diffraction grating in the modulator. This signal at 40MHz is amplitude modulated by the low frequency generator thus leading to beam intensity modulation. Also, it is preferable that the low frequency generator, which is the source of the system, has a great stability in phase because it delivers a voltage intended to modulate the intensity of the beam of the laser of power, and consequently conditions the quality of excitation. It should however be noted that the acousto-optical modulator is not useful if the laser 2 can be directly modulated.

Claims

1. Method for reading structures or information inscribed in a material, in which the surface of the material is scanned, without modifying the structures or the information, with an electromagnetic reading source (2), characterized in that the the material is subjected to two different beams for reading: a first excitation beam, for heating the material and a second detection beam for detecting the variation in the thermal properties of the material.

2. Method according to claim 1, characterized in that an image of the photo-thermal or photo-thermo-elastic contrast of the surface thus scanned is collected.

3. Method according to claim 1 or 2, characterized in that an image of the photo-thermal contrast is collected, by measuring the dynamic thermal reflectivity of the material or the infrared emission.

4. Method according to claim 1 or 2, characterized in that an image of the photo-thermo-elastic contrast is collected, by detecting the thermo-elastic deformations by interferometry.

5. Method according to one of claims 3 and 4, characterized in that one separates the amplitude and the phase of the signal corresponding to the dynamic thermal reflectivity of the material or to its thermoelastic deformations, by synchronous detection or correlation.

6. Method according to one of the preceding claims, characterized in that the image of the photo-thermal or photo-thermo-elastic contrast is collected without contact with the surface of the material.

7. Method according to one of the preceding claims, characterized in that the electromagnetic reading sources used for excitation and for detection, are laser sources (2).

8. Method according to one of the preceding claims, characterized in that the parameters of the electromagnetic reading source (2) are chosen, so as to detect a writing structure (32) underlying the surface .

9. Method according to one of the preceding claims, characterized in that the parameters of the electromagnetic reading source (2) are chosen, so as to detect a photothermal or photo-thermo-elastic contrast due to a phase difference of the material.

10. A method of writing a material, in which this material is exposed to an electromagnetic writing source (2) to cause a phase transition in a localized volume of this material, characterized in that one modifies , during this phase transition, the thermal, and possibly mechanical, properties of the material, so as to change the photo-thermal or photo-thermo-elastic contrast of the material and to mark it for a later reading of the photo-thermal contrast or photo-thermo-elastic of the marked surface.

11. Method according to claim 10, characterized in that the electromagnetic writing source (2) is a laser source.

12. Method according to one of claims 10 and 11, characterized in that the writing thus produced is adapted to be read by collecting an image of photo-thermal or photo-thermo-elastic contrast, of a surface scanned with an electromagnetic reading source (2).

13. Method according to claim 12, characterized in that the writing is carried out using a power of the electromagnetic writing source (2) greater than the power of the electromagnetic reading source (2), at least a factor of 1.5, and preferably a factor of between two and five.

14. Method according to one of claims 10 and 11, characterized in that the writing thus produced is adapted to be read by collecting an image of optical contrast, of a surface scanned with an electromagnetic reading source (2) .

15. Product marked by a process according to one of claims