Method and test bench for adhesion of the interface between two materials

Abstract

Title: Method and Test Bench for the Adhesion of the Interface Between Two Materials The invention consists of a method for testing the adhesion of an interface (21) between at least a first (11) and a second (12) layer of a stack (10), the second layer defining a first interface (21) with the first layer, the stack further comprising an ablation layer (13), the ablation layer and the first layer being arranged to form a second interface (22), the method comprising applying at least one laser pulse (100a) to the ablation layer by a laser (100), observing the second layer with an observation device (200) to detect the formation of a void zone (300) between the first and second layers and deducing a failure threshold between the first and second layers. Figure for the abstract: Fig. 2B

Description

Title of the invention: Method and test bench for the adhesion of the interface between two materials technical field

[0001] The present invention relates to a method for testing the adhesion of two materials. Its particularly advantageous application is to perform an adhesion test of an interface between two layers of a stack, more particularly of a stack comprising several layers. STATE OF THE ART

[0002] When two materials, identical or of different nature, are assembled, it may be of interest to check the good mechanical performance of the assembly in the environment where it will be used, or even to determine the breaking point of the adhesion zone known as the interface.

[0003] Several techniques exist for testing the adhesion between two layers of materials in an assembly. Generally, the method relies on the use of a relatively intense laser pulse focused on one face of the assembly. This laser method is called LAS AT, for Laser Shock Adhesion Test.

[0004] The laser pulse focused on the target surface generates a dense, high-temperature plasma. This initiates a compression wave, followed by a relaxation wave, which propagates through the assembly. When the compression wave reaches the free rear face of the assembly, it is reflected as a tensile wave. The intersection of the tensile wave with the incident relaxation wave will result in maximum tension in a specific area. Knowing the dimensions and mechanical properties of the materials, and adjusting the laser parameters, makes it possible to locate this maximum tension in the vicinity of the tested interface. If the tensile strength amplitude exceeds the mechanical failure threshold of the interface, the two materials separate, thus testing adhesion. This technique is illustrated in [Fig. 1] and is described in patent FR2681427A1. Observation of the free rear face of the assembly allows observation of the adhesion failure.However, this solution relies on various complex adjustments of the laser pulse, therefore numerous trials are necessary.

[0005] This method can be optimized by using a second laser pulse. Similarly, each pulse will generate traction waves. However, this solution requires complex wave timing calculations in order to to control the location of maximum traction in the interface region. Furthermore, several parameters influence the results, which introduces uncertainty.

[0006] One object of the present invention is therefore to provide a solution for improving the execution of an adhesion test of the interface between two layers. Another object is to provide a solution for simplifying the evaluation of the failure threshold of the interface between two layers. SUMMARY

[0007] To achieve this objective, according to one embodiment, a method for testing the adhesion of a first interface between at least a first and a second layer of a stack is provided, the first layer having a front face and a rear face opposite the front face of the first layer, the second layer having a front face defining the first interface with the rear face of the first layer, the stack further comprising an additional layer called the ablation layer, preferably metallic, having a front face and a rear face opposite the front face of the ablation layer and disposed on the front face of the first layer so as to form a second interface, the method comprising the following steps: • apply at least one laser pulse through, and preferably onto, the front face of the laser ablation layer, • to perform an observation of a rear face of the second layer, opposite the front face of the second layer, using an observation device in order to detect the creation of a void zone between the first and second layers and deduce a failure threshold of the first interface, the material of the ablation layer being chosen so as to have an acoustic impedance Zn lower than the acoustic impedance Zn of the material of the first layer, the laser pulse, a Zn / Zn ratio and a thickness En of the ablation layer being configured so that the laser pulse applied to the front face of the ablation layer by a laser generates: • a first compression wave propagating from the front face of the ablation layer to the second interface within the ablation layer, • a first reflected compression wave propagating within the ablation layer from the second interface to the front face of the ablation layer, resulting from the reflection of the first compression wave on the second interface, • a first tensile wave propagating within the ablation layer from the front face of the ablation layer to the second interface, resulting from the reflection of the first compression wave reflected on the first face of the ablation layer, • a wave, called a traction daughter wave resulting from the transmission of the first traction wave, the traction daughter wave propagating within the first layer from the second interface to the first interface, Fonde traction daughter wave being configured so as to generate a tensile force at the first interface, creating the void zone between the first and second layers when the amplitude of the laser pulse exceeds a threshold called Sr100a rupture, • a transmitted daughter wave propagating within the second layer from the first interface to the back face of the second layer and resulting from the transmission of the traction daughter wave from the first interface, at least before the creation of the void zone.

[0008] Thus, by observing the rear face of the second layer, it is possible to detect the amplitude or energy density of the laser pulse at which the wave arriving at the rear face disappears or is strongly attenuated, indicating the creation of a void zone at the first interface. It is then easy to deduce whether or not the adhesion has broken. Furthermore, with knowledge of the material parameters, it is also possible to link the induced velocity of the rear face and therefore determine the failure threshold of this first interface.

[0009] Thus, the invention proposes a solution to improve the method for performing adhesion tests on an interface between two layers. It further proposes a solution for evaluating the failure threshold of the interface between two layers. The presence of the ablation layer initially generates a compression wave, which is reflected as a new compression wave at the second interface. This new compression wave can then be totally reflected at the front face of the ablation layer. Total reflection results in the creation of a tensile wave that propagates through the ablation layer to the interface between the first and second layers, thereby creating a failure. The ablation layer can thus directly generate a high-amplitude tensile wave. The adhesion failure can then be observed at the rear of the stack.Additionally, the failure threshold can also be measured at the rear of the stack. This solution thus offers a simplification through the direct and controlled generation of a tensile wave. This avoids complex timing calculations, simplifying both the adhesion test and, separately and additionally, the method for evaluating the failure threshold. Furthermore, it limits... The various and complex adjustments of the laser pulse thus limit uncertainties. Furthermore, by separating the amplitude of the tensile wave used to generate the rupture from the amplitude of the compression waves, observation of the rupture is facilitated. Optionally, measurement of the rupture threshold is also simplified. In particular, the presence of noise during observation is limited, thereby improving accuracy.

[0010] According to another aspect, an adhesion test bench of a first interface configured to implement the test method and comprising, preferably along a propagation direction, at least one laser source configured to deliver at least one laser pulse, said stack comprising the ablation layer and said observation device.

[0011] Thus, the test bench makes it possible to observe and test in a simple and precise way the adhesion and by extension the breaking threshold between the first and second layers of the stack easily. BRIEF DESCRIPTION OF THE FIGURES

[0012] The aims, objects, features and advantages of the invention will become clearer from the detailed description of an embodiment thereof, which is illustrated by the following accompanying drawings in which:

[0013] [Fig.1] Fig.1 represents an adhesion test method according to a prior art method.

[0014] [Fig.2A] [Fig.2B] Figures 2A and 2B represent the present method for testing the adhesion of an interface between two layers of a stack.

[0015] [Fig.3] Fig.3 represents the wave amplitude profile over time and depending on their positions in the stacking as obtained by a numerical simulation.

[0016] The drawings are given by way of example and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate understanding of the invention and are not necessarily to scale with practical applications. For example, the directions of the waves are schematic and do not represent the exact propagation. DETAILED DESCRIPTION

[0017] Before proceeding with a detailed review of embodiments of the invention, optional features that may be used in combination or alternatively are listed below:

[0018] According to one example, the ablation layer and the first layer are in contact at the second interface. Thus, there is no material between the ablation layer and the first layer.

[0019] According to one example, the first layer and the second layer are in contact at the first interface. Thus, there is no material between the first and the second layer.

[0020] According to one example, the front face of the ablation layer is placed in contact with air or a material having an acoustic impedance Zm lower than the acoustic impedance Zn of the ablation layer material. This creates confinement, thereby generating a compression wave of greater amplitude, and subsequently, a tensile wave of greater amplitude.

[0021] According to one example, the ablation layer material is taken from aluminium, silver, copper or gold.

[0022] According to one example, the application of the laser pulse is configured so that the laser pulse applies an initial pressure on the front face of the ablation layer greater than 1 GPa. Thus, the laser pulse is sufficient to test the adhesion between the first layer and the second layer.

[0023] According to one example, the application of the laser pulse is configured so that the laser pulse has an energy density greater than or equal to 1 J / cm2, preferably greater than or equal to 5 J / cm2, preferably greater than or equal to 10 J / cm2. Thus, the pulse is sufficient to cause the creation of the vacuum zone.

[0024] According to one example, the laser pulse application is configured such that the laser pulse has a duration t, the ablation layer (13) has a speed of sound (c), and the thickness En of the ablation layer is configured such that the ratio Ei3 / (2*c*t) is strictly greater than 1. Preferably, En / (2*c*t) > 2. Preferably, En / (2*c*t) > 5. Preferably, En / (2*c*t) > 10, where c is the speed of sound in the ablation layer. The thickness of the ablation layer is thus sufficient to allow the first compression wave to be separated temporally from the first tension wave.

[0025] According to one example, the application duration of the laser pulse is greater than 1 nanosecond. This makes it possible to generate a wave which, on the one hand, will attenuate relatively little during its propagation and, on the other hand, will leave sufficient time for the interface to react to the stimulus and possibly break.

[0026] According to one example, the laser pulse has a wavelength greater than 250 nanometers, preferably greater than 500 nanometers, and preferably equal to 1000 nanometers. This allows the necessary absorption to be generated at the ablation layer. This, in turn, makes it possible to generate a wave with sufficient amplitude to test the adhesion between the first and second layers of the stack at the first interface.

[0027] According to one example, the ablation layer is deposited on the first layer by a method chosen from: • cathodic sputtering, • a plasma-assisted chemical vapor deposition.

[0028] This allows us to have a second interface thickness between the ablation layer and the first layer that can be neglected.

[0029] According to one example, the direction of propagation is substantially perpendicular to a plane formed by the second interface.

[0030] According to an example, the first interface and the second interface define parallel planes.

[0031] According to one example, the first interface and the front face of the ablation layer define parallel planes.

[0032] According to one example, the rear face of the second layer is observed using a velocimeter. This allows, by knowing the mechanical properties of the materials, a relationship between the speed of the rear face and the stress. Thus, the tensile stress dynamics at the rear face of the second layer can be observed; if these dynamics become zero, then there has been a rupture between the first and second layers.

[0033] It is specified that, within the framework of the present invention, the terms "on", "overcomes", "covers", "underlying", "opposite" and their equivalents do not necessarily mean "in contact with". Thus, for example, the depositing, transferring, gluing, assembling or applying a first layer on a second layer does not necessarily mean that the two layers are directly in contact with each other, but means that the first layer at least partially covers the second layer by being either directly in contact with it, or by being separated from it by at least one other layer or at least one other element.

[0034] Conversely, when it is indicated that a first layer is in contact with a second layer, this means that there is no other layer between this first and second layer, apart from a possible layer of adhesive. This adhesive layer preferably has a thickness of the same order of magnitude as the roughness of the first and / or the second layer. Preferably, this adhesive layer has a thickness less than or equal to 10 pm (micron-meters).

[0035] A layer may also be composed of several sub-layers of the same material or of different materials.

[0036] In the following description, unless otherwise indicated, when referring to absolute position qualifiers, such as the terms "front", "back", reference is made to the direction of propagation of the laser incident on the stack.

[0037] A layer usually has two opposite and parallel faces. Unless otherwise specified, the thickness of a layer is measured along a line perpendicular to its faces.

[0038] In the following description, the unit GPa (Giga-Pascal) corresponds to 109 Pa (Pascal), also corresponding to 109 kg*m1*s'2.

[0039] In the following description, the unit J (Joule) corresponds to kg*m2*s2.

[0040] In this description, the term "stress threshold" can be understood as a mechanical stress value beyond which two layers can completely or at least locally detach. The stress threshold value can be expressed in Pascals. A breaking stress can then be specified to reach this stress threshold.

[0041] In this description, the term "break threshold" can also be understood as the threshold of the laser pulse amplitude, implicitly associated with the mechanical break threshold of the interface. The value corresponding to the laser fluence would then be expressed in units of energy per unit area. The two definitions of break threshold can be related to each other.

[0042] In the present description, the wave amplitudes correspond to the transmitted stresses. Thus, a stress with a positive amplitude corresponds to a compressive wave and a stress with a negative amplitude corresponds to a tensile wave. The transmitted (T) and reflected (R) portions of a wave at the interface between two materials with impedances Zi and Z2 are evaluated mathematically based on hydrodynamic calculations: T = 2Z2 / (Zi+Z2) and R = (Z2-Z1) / (Z1+Z2).

[0043] In the present description, a comparison may be made between the amplitudes of the tension and compression waves and in terms of threshold, the values ​​of the amplitudes / thresholds shall be considered as being expressed in absolute values.

[0044] In the present description, applying a pulse through a face means that the laser reaches that face either directly or by passing through an additional layer disposed on that face.

[0045] In the present description, applying a pulse to a face means that the laser hits that face directly on that face.

[0046] The present invention relates to a method for testing the adhesion of a first interface 21 between at least a first layer 11 and a second layer 12 of a stack 10. The stack 10 further comprises an additional layer, referred to as the ablation layer 13. The method comprises at least one step of applying at least one laser pulse 100a to the ablation layer 13 using a laser 100. The laser pulse 100a can then create at least one compression wave followed by a tension wave in the stack 10. More specifically, said compression wave propagates through the ablation layer 13, and then a portion, referred to as the reflected portion, of this layer is reflected at a second interface 22. The second interface 22 being positioned between the ablation layer 13 and the first layer 11. Said reflected portion of said compression wave will then be reflected at a front face 13a of the ablation layer 13. The front face 13a of the ablation layer 13 being preferentially in contact with the air. The reflected portion of the compression wave is totally reflected into a first tension wave. This traction wave can then propagate through the stack 10 to the first interface 21. If the amplitude of the incident laser pulse exceeds a threshold known as the Sriooa breaking point, then the amplitude of the traction wave will be sufficient to create a void zone 300 at the first interface 21. The void zone 300 may extend at least partially over the first interface 21. Preferably, the void zone 300 extends over the entire first interface 21.The void zone 300 can be created invariantly in the transverse plane over the entire first interface 21. A break in adhesion can then be observed between the first 11 and the second layer 12. Also, the process also includes at least one step of observing the second layer 12 by an observation device 200. More precisely, from the observation of a back face 12b of the second layer 12, it is possible to detect from what amplitude or power of the Fonde laser 100 arriving at the back face 12b disappears or is strongly attenuated, indicating the creation of the void zone 300 at the first interface 21, thus indicating a break in adhesion.

[0047] Additionally, the failure threshold of this first interface 21 can also be easily deduced. Deducing the failure threshold of an interface between two layers from observation of the back face of the stack, for example with a velocimeter, is widely known in the prior art. Reference may be made, for example, to the following publication [S. Bardy et al, Optics & Laser Technology 124, 105983 (2020)]. More generally, the basic principle is as follows. The velocimeter allows measurement of the velocity of face 12b induced by the mechanical wave. This velocity is proportional to the velocity of the material at the face, with a factor close to two. The material velocity is estimated using the Rankine-Hugoniot relations or by performing numerical simulations with a hydrodynamic code.These estimates are based on knowledge of the mechanical parameters of the material in question, including the speed of sound and the shear modulus.

[0048] The stack 10 and its properties will now be described with reference to Figures 2A and 2B. Each layer of the stack 10 will also be described along a propagation direction P. The propagation direction P can, for example, correspond to the incident direction of the laser pulse 100a on the stack 10. Thus, the propagation direction P can, according to an example, correspond to the direction going from a laser 100 to an observation device 200.

[0049] The stack 10 thus comprises, along the propagation direction P, the ablation layer 13, the first layer 11, and the second layer 12. The assembly comprising the first layer 11 and the second layer 12 can then be defined as a composite. The composite can be considered as the element for which the adhesion test is to be performed.

[0050] The ablation layer 13 has a front face 13a. The front face 13a can be configured to receive the laser pulse 100a. The ablation layer 13 also has a rear face 13b. The rear face 13b of the ablation layer 13 is opposite the front face 13a of the ablation layer 13. The ablation layer 13 has a thickness En. The thickness En of the ablation layer 13 can correspond to the distance between the front face 13a and the rear face 13b of the ablation layer 13. Preferably, the front face 13a and rear face 13b of the ablation layer 13 are parallel. The thickness Eu is measured perpendicular to these faces.

[0051] The stack 10 then comprises, along the propagation direction P, the first layer 11. The first layer 11 has a front face 1la. The rear face 13b of the ablation layer 13 is disposed on the front face 1la of the first layer 11, preferably in contact with the latter. Thus, an interface, called the second interface 22, is formed by the rear face 13b of the ablation layer 13 and the front face 1la of the first layer 11.

[0052] In one example, the ablation layer 13 and the first layer 11 are in contact at the second interface 22. Thus, there is no material between the ablation layer 13 and the first layer 11. To achieve this, and in one example, the ablation layer 13 can be deposited on the first layer 11 by a method such as sputtering or plasma-enhanced chemical vapor deposition. Therefore, the thickness of the second interface 22 between the ablation layer 13 and the first layer 11 can be neglected.

[0053] The first layer 11 also has a rear face 11b. The rear face 11b of the first layer 11 is opposite the front face 1la of the first layer 11. Thus, the first layer 11 can have a thickness En. The thickness En of the first layer 11 can then extend between the front face 1la and the rear face 11b of the first layer 11. The thickness Eu of the ablation layer 13 can be less than the thickness En of the first layer 11.

[0054] The stack then comprises, along the propagation direction P, the second layer 12. The second layer 12 has a front face 12a. The rear face 11b of the first layer 11 being disposed on the front face 12a of the second layer 12. Thus an interface, called first interface 21, is formed by the back face 11b of the first layer 11 and the front face 12a of the second layer 12.

[0055] In one example, the first layer 11 and the second layer 12 are in contact at the first interface 21. Thus, there is no material between the first layer 11 and the second layer 12. A possible adhesive layer may, in one example, be positioned between the first layer 11 and the second layer 12. This adhesive layer preferably has a thickness of the same order of magnitude as the roughness of the first 11 and / or the second 12 layer. Thus, it may be, for example, less than or equal to 10 µm.

[0056] The second layer 12 also has a rear face 12b. The rear face 12b of the second layer 12 is opposite the front face 12a of the second layer 12. The rear face 12b is opposite the front face 13a of the ablation layer 13.

[0057] The ablation layer 13 is configured such that the laser pulse 100a applied to the front face 13a of the ablation layer 13 by a laser 100 generates at least one traction wave. The ablation layer 13 can be configured to allow the laser pulse 100a to retrace its steps in order to create a traction wave. The traction wave is then configured to create the void zone 300.

[0058] To achieve this, the material of the ablation layer 13 is chosen so as to have an acoustic impedance Zn lower than the acoustic impedance Zn of the material of the first layer 11. Thus, the initial compression wave induced by the laser pulse 100a will be reflected off the rear face 13b as a compression wave in the ablation layer 13. More precisely, this allows the compression wave to be maintained with sufficient amplitude to generate a tensile wave (after reflection off the front face 13a) which in turn has sufficient amplitude to cause rupture at the first interface 21. Sufficient amplitude is understood here to mean an amplitude (in absolute value) greater than the rupture threshold. Preferably, the parameters satisfy the equation below, with R being the reflection coefficient for the second interface 22 and a the initial maximum amplitude of the wave.

[0059] [Math.l] ----^-a^R

[0060] According to one example, the ablation layer 13 is configured to absorb the energy of the laser pulse 100a. The ablation layer 13 can be made of an absorbing material. The ablation layer 13 is preferably metallic. This allows for the efficient absorption of the laser pulse and the generation of an initial compression wave created by the laser pulse 100a in the stack 10. According to one example, the material of the ablation layer is taken from aluminum, silver, and... copper or gold. Thus, the absorption efficiency of the ablation layer 13 can be increased.

[0061] The method includes a step of applying at least one laser pulse 100a to the front face 13a of the ablation layer 13 by a laser 100. The application step may correspond to an irradiation step.

[0062] The laser pulse 100a, the Zi3 / Zn ratio, and the thickness En of the ablation layer 13 are configured such that the laser pulse 100a, focused onto the front face 13a of the ablation layer 13 by a laser 100, generates mechanical waves that will be described in the remainder of this description with reference to Figures 2A to 3. Figures 2A and 2B illustrate the stack 10 for which the adhesion of two materials, or of two layers 11, 12 of an assembly, is to be tested, and illustrate the dynamics of the mechanical compression and tension waves initially induced by the laser pulse 100a. Figures 2A and 2B also include a time dimension t. The time axis t can illustrate the time dynamics of the mechanical waves initially induced by the laser pulse 100a.

[0063] In the following description, reference will be made to zones. These zones are understood to be areas of a face having a surface in the plane transverse to the direction of propagation. This surface may be equal to the entire surface of the corresponding face.

[0064] According to one example, the laser pulse 100a will be focused on the front face 13a of the ablation layer at a source zone 1. The surface area of ​​the source zone 1 can be equal to the surface area of ​​the face 13a. The laser pulse 100a can then be parameterized so as to cause the creation of the void zone 300 and thus a break between the first layer 11 and the second layer 12.

[0065] In one example, the application of the laser pulse 100a is configured to apply an initial pressure to the front face 13a of the ablation layer 13. The ablation layer 13 can absorb the laser pulse 100a. This absorption can then lead to a significant increase in temperature in the vicinity of the front face 13a of the ablation layer 13. Thus, a pressure gradient can be generated and lead to mechanical stresses. Theoretical and numerical knowledge of the pressure profile induced by the laser pulse 100a will allow for better quantification of the results. In one example, a numerical simulation code can provide an estimate of the ablation pressure of certain materials.

[0066] More specifically, the laser pulse 100a can induce the application of a pressure greater than 1 GPa (Gigapascal). Thus, the laser pulse 100a is sufficient to test the adhesion between the first layer 11 and the second layer 12. Therefore, the laser pulse 100a is configured to apply a stress to the front face 13a of the ablation layer 13 and to exhibit an amplitude greater than 1 GPa. The profile of the laser pulse 100a at the source area 1 is illustrated in [Fig.3].

[0067] According to one example, the laser pulse 100a can have a wavelength X. Preferably, the wavelength X of the laser pulse 100a can then be greater than 250 nanometers. Preferably, the wavelength X of the laser pulse 100a can then be greater than 500 nanometers. Preferably, the wavelength X of the laser pulse 100a can then be equal to 1000 nanometers. This makes it possible to generate the necessary absorption at the ablation layer 13. Thus, this makes it possible to generate a wave with sufficient amplitude to test the adhesion between the first 11 and the second layer 12 of the stack 10.

[0068] According to one example, the application of the laser pulse 100a is configured so as to present an energy density greater than or equal to 1 J / cm2, preferably greater than or equal to 5 J / cm2, preferably greater than or equal to 10 J / cm2. Thus, the laser pulse 100a is sufficient to cause the creation of the void zone 300.

[0069] According to one example, the duration t of application of the laser pulse 100a is greater than 1 nanosecond. Thus, this makes it possible to generate a wave which, on the one hand, will attenuate relatively little during its propagation and, on the other hand, will leave sufficient time for the first interface 21 to react to the stimulus and possibly break.

[0070] Any gradient of mechanical stress propagates through the material; this dynamic is described by Euler's equations, to which the mechanical behavior is added. The mechanical stresses generated by the laser pulse 100a at the source zone 1 constitute the source of all the mechanical waves described in the following sections.

[0071] The laser pulse 100a will then generate a first compression wave 102. This first compression wave 102 will propagate from the front face 13a of the ablation layer 13 to the second interface 22, for example along the propagation direction P, within the ablation layer 13. Thus, the first compression wave 102 propagates between the source area 1 and an area 2 located on the second interface 22. The profile of the first compression wave 102 at the level of the second interface 22 and therefore at the level of area 2 is illustrated by [Fig.3].

[0072] Since the ablation layer 13 and the first layer 11 have different acoustic impedances, the first compression wave 102 can be partially transmitted 205 and reflected 203. As explained previously, the acoustic impedance Zn of the ablation layer 13 is lower than the acoustic impedance Zn of the material of the first layer 11. Therefore, the transmitted portion 205 of the first compression wave 102 can be a compression wave with an amplitude less than or equal to the amplitude of the first compression wave 102. The reflected portion 203 can also have an amplitude less than or equal to the amplitude of the first compression wave 102. The reflection and transmission coefficients depend on the material used. These coefficients are governed by known physical concepts.

[0073] The first reflected compression wave 203 corresponds to the reflected portion of the first compression wave 102. Thus, the first reflected compression wave 203 results from the reflection of the first compression wave 102 on the second interface 22. The first reflected compression wave can propagate within the ablation layer 13 in a direction opposite to the propagation direction P.

[0074] Thus, the first reflected compression wave 203 propagates between zone 2 and a zone 3. Zone 3 is positioned on the front face 13a of the ablation layer 13. By way of example, the front face 13a of the ablation layer 13 is said to be free. Under the action of a wave, this front face 13a of the ablation layer 13 can then move freely.

[0075] According to one example, the front face 13a of the ablation layer 13 is in contact with air. Indeed, the stack 10 is in air, the density of which is significantly lower than that of the materials considered. Similarly, the acoustic impedance Za of the air is negligible compared to the acoustic impedance Zn of the ablation layer 13. Under these conditions, the first reflected compression wave 203 arriving at zone 3 is preferably totally reflected into a wave retaining the same profile but with an opposite amplitude. Thus, the first reflected compression wave 203 becomes a first tensile wave 304.

[0076] According to one example, the front face 13a of the ablation layer 13 may also be in contact with a material. The material may then have an acoustic impedance Zm lower than the acoustic impedance Zn of the ablation layer material 13.

[0077] The first tensile wave 304 generated at zone 3 results from the reflection of the first compression wave 203 reflected from the first face 13a of the ablation layer 13. The first tensile wave 304 propagates from the first face 13a of the ablation layer 13 to the first interface 21. The first tensile wave 304 can propagate within the ablation layer 13 along the propagation direction P. The first tensile wave 304 propagates from zone 3 to a zone 4 positioned on the second interface 22. The profile of the first tensile wave 304 at zone 4 of the second interface 22 can be observed in [Fig. 3]. The first tensile wave 304 at the second interface 22 can then exhibit a so-called negative amplitude. The first tensile wave 304 can then present a profile similar to the first compression wave 102 but of opposite sign.The first traction wave 304 can have an amplitude in value. absolute less than the absolute value amplitude of the first compression wave 102.

[0078] As illustrated in [Fig. 3], at the second interface 22 (zones 2 and 4), two waves can be observed. The first compression wave 102 and the first tension wave 304 can be observed at the second interface 22. Observation of these two waves is possible because they arrive at the second interface 22 at two distinct times. To achieve this, and by way of example, the thickness En of the ablation layer 13 can be parameterized according to the laser pulse 100a. Thus, the application of the laser pulse 100a can have a duration t. The ablation layer 13 has a speed of sound c. The thickness En of the ablation layer 13 is chosen to be sufficient to allow the temporal separation of the first compression wave and the first tension wave. The thickness En of the ablation layer 13 can be configured so that the ratio Ei3 / (2*c*t) is strictly greater than 1. Preferably, En / (2*c*t) > 2.Preferably, En / (2*c*t) > 5. Preferably, En / (2*c*t) > 10. The temporal separation avoids a superposition of the two waves and therefore a decrease and / or a suppression of the amplitude of the first traction wave 304. The suppression of the amplitude of the first traction wave 304 at the level of the second interface 22 could limit a rupture at the level of the first interface 21.

[0079] In zone 4, the first traction wave 304 is partly transmitted and partly reflected. The reflected portion of the first traction wave 304 continues to travel back and forth within the ablation layer 13, its amplitude decreasing regularly and not considered in the present invention. The transmitted portion of the first traction wave 304 may then exhibit the same profile as the first traction wave 304. The transmitted portion of the first traction wave 304 is hereinafter referred to as the daughter traction wave 405.

[0080] A daughter traction wave 405 then propagates within the first layer 11. More precisely, the daughter traction wave 405 propagates between the second interface 22 and the first interface 21. The daughter traction wave 405 results from the transmission of the first traction wave 304. The daughter traction wave 405 propagates, according to one example, to a zone 5 of the first interface 21.

[0081] As illustrated in [Fig. 3], at zone 5 and therefore at the first interface 21, a wave with two principal components can be observed. The first component may be in compression and originates from the first transmission 205 at zone 2, which itself originates from the first compression wave 102. The second component may be in tension. This second component may correspond to the daughter tension wave 405 and originates from the second transmission at zone 4 originating from the first traction wave 304. Observation of these two waves is possible because they arrive at two distinct times at the first interface 21. As explained previously, the temporal separation, due to the thickness En of the ablation layer 13, prevents the two waves from overlapping. The overlap of the amplitude of the transmitted portion 205 of the first compression wave 102 and the daughter traction wave 405 would result in limited traction at the first interface 21, thus limiting the probability of the formation of a void zone 300.

[0082] The traction daughter wave 405 is therefore configured so as to create the void zone 300 between the first 11 and the second 12 layers if its amplitude is sufficient. According to an example, the amplitude of the traction daughter wave 405 will be sufficient when the amplitude of the laser pulse 100a exceeds a so-called breaking threshold Sriooa-

[0083] As an example, the traction daughter wave 405 may be partly reflected 506 and partly transmitted 508. The reflected portion 506, as well as the resulting waves 607, 707a, 707b, 707c, may not be observed if the void zone 300 is properly created. They will be described later in this description. The following paragraphs of this description focus on the transmitted portion 508 of the traction daughter wave 405.

[0084] A transmitted daughter wave 508 propagates within the second layer 12. The transmitted daughter wave 508 propagates from the first interface 21 (zone 5) to the rear face 12b of the second layer 12 (zone 8), preferably along the propagation direction P. The transmitted daughter wave 508 results from the transmission of the traction daughter wave 405 from the first interface 21. At the very least, the transmitted daughter wave 508 corresponds to the transmitted portion of the traction daughter wave 405 before the creation of the void zone 300.

[0085] The arrival of transmitted daughter Fonde 508 on zone 8 can then cause a displacement of the rear face 12b of the second layer 12. The rear face 12b of the second layer 12 is configured to deform under the effect of the wave propagating in the stack 10. The rear face 12b of the second layer 12 is configured so as to allow observation by the observation device 200 of the evolution of this deformation. The creation of the void zone 300 at the level of the first interface 21 will attenuate or interrupt this deformation, thus making it possible to identify that there has been a rupture and subsequently to determine the rupture threshold of the first interface 21.

[0086] Therefore, if the amplitude of the laser pulse 100a was insufficient, then the maximum tensile amplitude of the daughter wave 405 arriving at the first interface 21 is less than the mechanical breaking threshold of the first interface 21, and the daughter wave 405 is fully transmitted. transmitted 508 observed at the rear face 12b of the second layer 12 may present substantially the same profile as that of the first traction wave 304 observed at the second interface 22. According to this example, there is no observed rupture and therefore there is no creation of a void zone 300.

[0087] If the maximum tensile amplitude of the daughter wave 405 arriving at the first interface 21 exceeds the mechanical failure threshold of the first interface 21, the daughter wave 405 will not be fully transmitted. The transmitted daughter wave 508 observed at the rear face 12b of the second layer 12 will have a zero amplitude at the time the failure threshold is reached. This example is illustrated in [Fig. 3]. The amplitude of the daughter wave 405 is then sufficient to cause a rupture of the first interface 21 and thus the creation of a void zone 300. The last transmitted stress value, the amplitude of the tensile wave, can correspond to the mechanical failure threshold, which can be related to the failure threshold SrioOaissu of the laser pulse 100a.

[0088] Figure 3 illustrates an example in which a rupture is observed at the first interface 21. Figure 3 is a numerical simulation illustrating three wave profiles visible at the front face 13a of the ablation layer 13, at the second interface 22, and at the first interface 21. The different observable amplitudes correspond to the amplitude of the initial compression wave induced by the laser pulse 100a at zone 1, the first compression wave 102 at zone 2, the tensile wave 304 at zone 4, the transmitted portion 205 of the first compression wave 102, and the daughter tensile wave 405. These curves are obtained when the ablation layer 13 is aluminum with a thickness En of 16 pm (micrometers). The first layer 11 and the second layer 13 have a thickness of 100 pm.The laser pulse 100a is parameterized to have an intensity of 10 GW / cm2, a duration t of 1.6 ns (nanoseconds) and a wavelength X of 1 pm. A failure threshold value is set at 0.5 GPa (Gigapascal), the daughter tensile wave 405 is then truncated at this same value following the formation of the void zone 300 at the first interface 21 signifying the loss of adhesion of the assembly.

[0089] Other waves observable depending on the case, with reference to Figures 2A and 2B, are described below. The dynamics of these waves do not affect the general principle described above.

[0090] According to an example and as explained previously, the reflected portion 506 and the resulting waves 607, 707a, 707b, 707c may not be observed if the void zone 300 is indeed created. In fact, if the void zone 300 is created, the rear face 11b of the first layer 11 can become a free surface. The impact of a wave on a free surface which may involve total internal reflection the reflected part 506 as well as the resulting waves cannot be observed.

[0091] According to one example, the transmitted daughter wave 508 arrives at the rear face 12b of the second layer 12 in a zone 8 and is then totally reflected 809. The daughter wave 508 is totally reflected because the rear face 12b of the second layer 12 is a free surface. This reflected wave 809 can then propagate within the second layer 12 towards the first interface 21, towards a zone 9. The following observations are not relevant in the present case.

[0092] Furthermore, if there has been no break in the first interface 21, all the secondary waves 607, 707a, 707b, 707c, and 909a originating from zones 6 and 9 can also be observed by the observation device 200. The secondary waves 607, 707a, 707b, 707c, and 909a originate from reflections and / or transmissions at the second interface 22, the first interface 21, or the rear face 12b of the second layer 12. The observation of these secondary waves implies that there has been no loss of adhesion. In this case, the interface break threshold is not reached, and the laser pulse 100a is therefore insufficient.

[0093] According to one example, the adhesion test method for the first interface 21 is carried out using a test bench. The test bench may then include, preferably along the propagation direction P, at least one laser source 100. This laser source 100 enables the laser pulse 100a to be generated. The laser 100 may, for example, enable the use of the LAS AT technique. The laser 100 may be an Nd:YAG laser.

[0094] The test bench can also include, depending on the propagation direction P, the stack 10. The stack 10 includes the ablation layer 13 and can then present the front face 13a of the ablation layer 13 positioned with regard to the laser 100.

[0095] According to one example, the test bench finally includes the observation device 200. The observation device 200 can be a device using an interferometric optical technique that takes advantage of the Doppler effect.

[0096] According to one example, the rear face 12b of the second layer 12 is observed using a velocimeter. This allows, by knowing the mechanical properties of the materials, a relationship between speed and stress. Thus, the dynamics of Fonde fille 405 can be observed at the rear face 12b of the second layer 12. Consequently, if this dynamic becomes zero or decreases rapidly and sharply, then there has been a rupture between the first layer 11 and the second layer 12.

[0097] In light of the preceding description, it is clear that the described method and test bench make it possible to considerably simplify the determination of a failure at an interface. They optionally allow for the measurement of a failure threshold between layers. Thus, the adhesion test can be a measurement of a breaking point between two layers. Similarly, the adhesion test can be a test with a breaking point known in advance and fixed; the 100a laser pulse is then parameterized to reach this breaking point.

[0098] In particular, it is not necessary to perform timing calculations. Furthermore, the adjustment of the laser pulse duration is considerably simplified.

[0099] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention.

Claims

1. Demands Method for testing the adhesion of a first interface (21) between at least a first (11) and a second (12) layers of a stack (10), the first layer (11) having a front face (1la) and a rear face (11b) opposite the front face (1la) of the first layer (11), the second layer (12) having a front face (12a) defining with the rear face (11b) of the first layer (11) the first interface (21), the stack further comprising an additional layer called the ablation layer (13), preferably metallic, having a front face (13a) and a rear face (13b) opposite the front face (13a) of the ablation layer (13), the rear face (13b) of the ablation layer (13) being disposed on the front face (1la) of the first layer (11) so as to form a second interface (22), the method comprising the following steps: • apply at least one laser pulse (100a) by, and preferably on, the front face (13a) of the ablation layer (13) by a laser (100), • to carry out an observation of a rear face (12b) of the second layer (12), opposite the front face (12a) of the second layer (12), by an observation device (200) so as to detect the creation of a void zone (300) between the first layer (11) and the second layer (12) and deduce a rupture threshold of the first interface (21), the material of the ablation layer (13) being chosen so as to have an acoustic impedance Zn lower than an acoustic impedance Zn of the material of the first layer (11), the laser pulse (100a), a Zi3 / Zn ratio and a thickness En of the ablation layer (13) being configured such that the laser pulse (100a) applied to the front face (13a) of the ablation layer (13) by a laser (100) generates: • a first compression wave (102) propagating from the front face (13a) of the ablation layer (13) to the second interface (22) within the ablation layer (13),

2.

3.

4. • a first reflected compression wave (203) propagating within the ablation layer (13) from the second interface (22) to the front face (13a) of the ablation layer (13) and resulting from the reflection of the first compression wave (102) on the second interface (22), • a first traction wave (304) propagating within the ablation layer (13) from the front face of the ablation layer to the second interface (22) and resulting from the reflection of the first reflected compression wave (203) on the first face (13a) of the ablation layer (13), • a wave, called the daughter wave (405) of traction, resulting from the transmission of the first traction wave (304), propagating within the first layer (11) from the second interface (22) to the first interface (21), the daughter wave (405) of traction being configured so as to generate a tensile force at the first interface (21), creating the void zone (300) between the first (11) and the second (12) layer when the amplitude of the laser pulse (100a) exceeds a threshold called the breaking threshold Sri00a, • a transmitted daughter wave (508) propagating within the second layer (12) from the first interface (21) to the rear face (12b) of the second layer (12) and resulting from the transmission of the traction daughter wave (405) from the first interface (21), at least before the creation of the void zone (300). Method according to the preceding claim in which the ablation layer (13) and the first layer (11) are in contact at the level of the second interface (22). Method according to any one of the preceding claims wherein the first layer (11) and the second layer (12) are in contact at the first interface (21). A method according to any one of the preceding claims, wherein the front face (13a) of the ablation layer (13) is disposed in contact with air or a material having an impedance acoustic Zm lower than the acoustic impedance Zn of the ablation layer material (13).

5. A method according to any one of the preceding claims in which the ablation layer material (13) is taken from aluminium, silver, copper or gold.

6. A method according to any one of the preceding claims wherein the application of the laser pulse (100a) is configured so that the laser pulse applies an initial pressure on the front face (13a) of the ablation layer (13) greater than 1 GPa.

7. A method according to any one of the preceding claims wherein the application of the laser pulse (100a) is configured so that the laser pulse has an energy density greater than or equal to 1 J / cm2, preferably greater than or equal to 5 J / cm2, and more preferably greater than or equal to 10 J / cm2.

8. A method according to any one of the preceding claims wherein the application of the laser pulse (100a) is configured such that the laser pulse has a duration (t), the ablation layer (13) has a speed of sound (c), and wherein the thickness En of the ablation layer (13) is configured such that the ratio En / (2*c*t) is strictly greater than 1, preferably En / (2*c*t) > 2, preferably Ei3 / (2*c*t) > 5, preferably En / (2*c*t) > 10.

9. Method according to the preceding claim wherein, the duration (t) of application of the laser pulse (100a) is greater than 1 nanosecond.

10. A method according to any one of the preceding claims wherein the laser pulse (100a) has a wavelength (X) greater than 250 nanometers, preferably greater than 500 nanometers, preferably equal to 1000 nanometers.

11. A method according to any one of the preceding claims wherein the ablation layer (13) is deposited on the first layer (11) by a method taken from: • sputtering, • plasma-assisted chemical vapor deposition.

12. A method according to any one of the preceding claims in which the observation of the rear face (12b) of the second layer (12) is carried out using a velocimeter.

13. Adhesion test bench of a first interface (21) configured to implement the test method according to any one of the preceding claims and comprising, preferably along a propagation direction (P), at least one laser source (100) configured to deliver at least one laser pulse (100a), said stack (10) comprising the ablation layer (13) and said observation device (200).

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