Flexible substrate with a stack of support layers and adhesive layers

Abstract

The present description relates to a flexible substrate (100) comprising at least a first support layer (102) disposed between first and second adhesive layers (104, 106) and a second support layer (108) such that the second adhesive layer (106) is arranged between the first and second support layers (102, 108), and comprising a first adhesive face (107) formed by the first adhesive layer (104).

Description

DESCRIPTION TITLE: Flexible substrate with stack of support layers and adhesive layers The present application claims the benefit of priority from French patent application number 23 / 14405, filed on December 18, 2023, entitled "Flexible substrate with stack of support layers and adhesive layers", which is incorporated by reference to the fullest extent permitted by law. Technical field

[0001] The present description relates to a flexible substrate comprising a stack of support layers and adhesive layers. Such a flexible substrate is for example suitable for carrying out a transfer of a functional layer from a first substrate to a second substrate, for example for transferring a functional layer from an initial substrate to a final destination substrate. Prior art

[0002] A functional layer refers to a single layer or a stack of several layers. More particularly, the functional layer may have a characteristic allowing the operation of at least one active component (for example a laser, a diode, a transducer, an actuator or actuator, a memory cell, a battery, etc.) or passive component (for example an electrical capacitor, an optical waveguide, a lens, etc.). This characteristic may be linked to the composition and / or the structural organization of the functional layer from which emerge remarkable properties which may be mechanical, thermal, electrical, dielectric, piezoelectric, magnetic, optics, etc., and which are the basis of the component's operation.

[0003] There are many situations where it is necessary to perform the transfer of a functional layer from an initial substrate to a final destination substrate.

[0004] For example, the preparation of the functional layer may require particular processing conditions (e.g. in terms of temperature, pressure, chemistry, etc.) and / or particular substrates on which the composition and / or structural organization of the functional layer can be achieved. Thus, the optimal initial substrate used for the preparation of the functional layer may differ from the final substrate on which the functional layer is integrated. For example, it is possible that the final substrate does not support the preparation conditions of the functional layer, or the final substrate may include other components and / or elements that do not support these preparation conditions, or the final substrate may not allow the preparation of the functional layer.It is also possible that the initial substrate is not suitable for the use of the functional layer once the product is finalized, for example because it is too fragile, too thick, not flexible enough, etc.

[0005] According to another example, it may be necessary to transfer the functional layer from the initial substrate used for its preparation to the final substrate when a step of treating the face of the functional layer located against the initial substrate must be implemented.

[0006] For example, the functional layer may correspond to a layer of material comprising, for example, at least one of the following semiconductor materials: silicon, silicon carbide, germanium, III-V compound such as AsGa, InP or GaN, II-IV compound such as CdTe or ZnO. It can also correspond to a layer of material comprising for example at least one piezoelectric material such as LiNbO3, LiTaO3, PZT or PMN-PT. It can also correspond to a layer of magnetic material or functional oxide such as ZrO2, YSZ, yttrium-stabilized ZrO2, SrTiO3 or GaO2.

[0007] The material of the functional layer may be monocrystalline. It is also possible for the material of the functional layer to be polycrystalline, for example when the production conditions are optimized to obtain, for example, a particular density and size of crystal grains and / or a preferential crystal orientation.

[0008] According to another example, the functional layer may comprise at least one junction and include, for example, a doping variation of the pn type or of the npn or p-np type, or correspond to a power diode or a solar cell. The functional layer may also correspond to an optical stack that can be used to produce a laser, for example of the VCSEL type, or a light-emitting diode (including a junction and a multiple quantum well structure). The functional layer may also be used to produce an actuator and / or a piezoelectric sensor comprising a metal / piezoelectric material stack, or even a solid-state battery. According to another example, the functional layer may comprise electronic components produced by monolithic 3D integration.

[0009] Such a transfer can be achieved by implementing a prior transfer of the functional layer from the initial substrate to a temporary, or sacrificial, rigid substrate. For example, such a transfer may involve the creation of a fracture, or embrittlement, interface in the initial substrate, under the functional layer, then the implementation of direct bonding of the functional layer against the temporary substrate, then a separation of the initial substrate from the functional layer at the fracture interface. The functional layer is then transferred to the final substrate using the temporary substrate as a mechanical handle. The temporary substrate is then destroyed to release the functional layer.

[0010] The transfer method described above can be applied to carry out, for example, a full-plate transfer of the functional layer onto the final substrate. It is also possible for this method to be implemented to carry out a collective transfer of several elements or portions of material, for example in chip format, and together forming a functional layer.

[0011] When the functional layer is to be cut into several separate portions, the second substrate onto which the functional layer is transferred does not correspond to the final destination substrate, but to a second temporary substrate. A cutting of the functional layer as well as of the first temporary substrate is implemented, for example by sawing, after the transfer onto this second temporary substrate. The cut portions of the first temporary substrate and of the functional layer are then transferred individually (technique called "pick and place" in English) onto the final destination substrate. The first temporary substrate is then removed in order to retain only the portions of the functional layer on the final destination substrate.

[0012] Alternatively, it is possible to secure the functional layer to a temporary substrate, to cut the first substrate and the functional layer, to individually transfer the cut portions of the first substrate and the functional layer to the final substrate, and to remove the initial substrate to retain only the portions of the functional layer.

[0013] A common drawback of all the above transfer methods is their high cost due to the destruction of the temporary substrate or the initial substrate at the end of the process, these substrates being expensive due to the materials used. In addition, no temporary substrate is suitable for carrying out functional layer transfer on a curved or non-planar substrate.

[0014] Document US 2021 / 0260859 A1 describes another transfer method. Again, this method is not suitable for carrying out a functional layer transfer on a curved or non-planar substrate.

[0015] US 6,100,166 A1 describes a method for transferring a functional layer using a flexible substrate to separate the functional layer from the initial substrate. The flexible substrate used is however not suitable for carrying out manipulation of the resulting structure, nor for further processing or transfer of the functional layer. Summary of the invention

[0016] There is a need to provide a flexible substrate that does not exhibit at least some of the disadvantages set out above.

[0017] An embodiment overcomes all or part of the drawbacks of the known methods and proposes a flexible substrate comprising at least a first support layer. disposed between first and second adhesive layers and a second backing layer such that the second adhesive layer is disposed between the first and second backing layers, and comprising a first adhesive face formed by the first adhesive layer.

[0018] According to a particular embodiment, the flexible substrate comprises a thickness (which corresponds to the total thickness of the flexible substrate) of between 10 μm and 800 μm.

[0019] According to a particular embodiment, at least one of the first and second support layers comprises a polymer and / or a metallic material.

[0020] According to a particular embodiment, a material of at least one of the first and second adhesive layers comprises adhesive properties that can be modified by implementing a step of treating said at least one of the first and second adhesive layers.

[0021] According to a particular embodiment, one of the first and second adhesive layers comprises a material whose adhesive properties are reduced when this material is subjected to UV treatment, and the other of the first and second adhesive layers comprises a material whose adhesive properties are reduced when this material is subjected to heat treatment.

[0022] According to a particular embodiment, the material of said one of the first and second adhesive layers comprises photodegradable nitrogen-based compounds, and the material of said other of the first and second adhesive layers comprises acrylic.

[0023] According to a particular embodiment, the materials of the first and second adhesive layers are such that the adhesive strength of the second adhesive layer is lower than that of the first adhesive layer.

[0024] According to a particular embodiment, the flexible substrate further comprises at least one magnetic layer arranged between the first adhesive layer and the first support layer or between the first support layer and the second adhesive layer or between the second adhesive layer and the second support layer.

[0025] According to a particular embodiment, the magnetic layer corresponds to a ferromagnetic and / or diamagnetic and / or paramagnetic layer.

[0026] According to a particular embodiment, the flexible substrate further comprises at least one marking layer configured to be marked by laser and interposed between the first support layer and the second adhesive layer and / or between the first adhesive layer and the first support layer.

[0027] According to a particular embodiment, the marking layer comprises at least one metal and / or one polymer and / or one glass and / or one crystal.

[0028] According to a particular embodiment, the flexible substrate comprises dimensions adapted to the implementation of a method for transferring a functional layer from a first substrate to a second substrate. The dimensions of the flexible substrate may depend on several parameters, such as for example the dimensions of the functional layer, those of the second substrate, the application envisaged for the functional layer, the steps of the transfer method implemented, etc.

[0029] According to a particular embodiment, the flexible substrate further comprises a removable protective layer arranged against the first face. Brief description of the drawings

[0030] These and other features and advantages will be set forth in detail in the following description of particular embodiments given without limitation in relation to the attached figures, among which:

[0031] Figure 1 shows an example of a flexible substrate according to a particular embodiment;

[0032] Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 represent steps of a method of transferring a functional layer using a flexible substrate according to a particular embodiment;

[0033] Figure 10 represents a step of a method of transferring a functional layer according to an alternative embodiment;

[0034] Figure 11 and Figure 12 represent alternative embodiments of a flexible substrate according to particular embodiments. Description of the embodiments

[0035] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0036] For the sake of clarity, only the steps and elements useful for understanding the embodiments described have been represented and are detailed.

[0037] Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") together, this means that These two elements can be connected or linked through one or more other elements.

[0038] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", "on", "under", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures corresponding to a normal position of use.

[0039] In the figures, in order to facilitate their reading, the different elements and the different layers are not represented on the same scale in relation to each other.

[0040] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%.

[0041] Each layer of material can correspond to a single layer of material or a stack of multiple stacked layers.

[0042] An exemplary embodiment of a flexible substrate 100, or flexible substrate, is described below in connection with FIG. 1.

[0043] The flexible substrate 100 comprises a first support layer 102 disposed between first and second adhesive layers 104, 106. The flexible substrate 100 further comprises a second support layer 108 such that the second adhesive layer 106 is disposed between the first and second support layers 102, 108.

[0044] According to an exemplary embodiment, the total thickness of the flexible substrate 100 is for example of the order of 200 μm, with first and second support layers 102, 108 each of thickness for example equal to 90 μm and first and second adhesive layers 104, 106 each of thickness for example equal to 10 μm. Alternatively, the thicknesses of the layers of the flexible substrate 100 may be different from these examples. In addition, as a variant, the thicknesses of the first and second support layers 102, 108 may be different from each other, and / or the thicknesses of the first and second adhesive layers 104, 106 may be different from each other. Generally, the thickness of the flexible substrate 100 may be between 10 μm and 800 μm.

[0045] According to an exemplary embodiment, the first and second support layers 102, 108 comprise a polymer, for example PET (polyethylene terephthalate), PE (polyethylene), PP (polypropylene), PVC (polyvinyl chloride), PO (polyolefin), etc., and / or comprise a metallic material. Alternatively, the materials of the first and second support layers 102, 108 may be different from one another.

[0046] According to an exemplary embodiment, a material of at least one of the first and second adhesive layers 104, 106 may comprise adhesive properties that can be modified by implementing a step of treating said at least one of the first and second adhesive layers 104, 106.

[0047] This modification of the adhesive properties may correspond to a reduction or even a suppression of the adhesive properties of the treated layer(s), for example to facilitate the detachment of one and / or the other of the first and second adhesive layers 104, 106 during the use of the flexible substrate 100. For example, such a modification of the adhesive properties of a material of one of the adhesive layers 104, 106 may be carried out by implementing an exposure of this material to a certain temperature and / or radiation at a certain wavelength.

[0048] Alternatively, this modification of the adhesive properties may correspond to an increase in the adhesive properties of the treated layer(s), to strengthen the adhesive strength of one or more of the adhesive layers 104, 106 when using the flexible substrate 100.

[0049] In a particular configuration, one of the first and second adhesive layers 104, 106 may comprise a material whose adhesive properties are reduced when this material is subjected to a UV treatment, and the other of the first and second adhesive layers 104, 106 may comprise a material whose adhesive properties are reduced when this material is subjected to a heat treatment. For example, the material of one of the first and second adhesive layers 104, 106 may comprise photodegradable nitrogen-based compounds (whose adhesive properties become very weak after exposure to UV radiation), and the material of the other of the first and second adhesive layers 104, 106 may comprise acrylic (whose adhesive properties become very weak after exposure to a certain temperature, for example, above about 190°C for acrylic based on methacrylate monomers).

[0050] For example, the adhesive properties of the first and second adhesive layers 104, 106 may be such that their adhesive strength is between 0.01 N / cm and 15 N / cm. The adhesive strength of each of the first and second adhesive layers 104, 106 may be measured in accordance with the measurement method known as "Peel Adhesion", or peel adhesion, as described in the ASTM D3330 or AFERA ​​4001 standard.

[0051] In the example of figure 1, the flexible substrate 100 comprises a first face 107 formed by one of the faces of the first adhesive layer 104, and a second face 109 formed by one of the faces of the second support layer 108. In the example described, unlike the second face 109, the first face 107 of the flexible substrate 100 has non-zero adhesive properties making it possible to stick this first face 107 of the flexible substrate 100 against an element to which the flexible substrate 100 is intended to be secured.

[0052] In addition to the above characteristics relating to the adhesive properties of the first and second adhesive layers 104, 106, the layers 102 to 108 of the flexible substrate 100 are such that it is flexible, or supple. Considering the flexural rigidity of the STE 100, this can be defined by the equation:

[0054] with D the flexural rigidity of the STE 100, in GPa.pm 3 ;

[0055] E the Young's modulus of STE 100, in GPa;

[0056] h the thickness of the STE 100, in pm;

[0057] v the Poisson's ratio of STE 100.

[0058] The flexural rigidity D of STE 100 can be between 1 GPa.pm 3 and 10 7 GPa.pm 3 .

[0059] An exemplary method of transferring a functional layer using the flexible substrate 100 is described below in connection with FIGS. 2-9. In the described method, the flexible substrate 100 serves as a temporary substrate with which a functional layer 110 is transferred from a first substrate to another substrate.

[0060] In the example of figure 2, the flexible substrate 100 is secured, at its first face 107, to the layer functional layer 110 intended to be transferred from a first substrate 112 to a second substrate 114 (visible in FIG. 5). In the exemplary embodiment described, before implementing this joining, a weakening region 116 is formed between the functional layer 110 and the first substrate 112. Different techniques can be implemented to produce the weakening region 116, the choice of the technique used being a function in particular of the material(s) of the functional layer 110 and the first substrate 112. According to a first example, the weakening region 116 can be formed by carrying out an ion implantation in the first substrate 112, followed by annealing resulting in the formation of bubbles allowing the subsequent separation of the functional layer 110 from the first substrate 112.According to a second example, the weakening region 116 may be formed by porosification in the first substrate 112, the subsequent separation of the functional layer 110 from the first substrate 112 being able to be carried out by etching. According to a third example, the weakening region 116 may be formed by laser lift-off. Other techniques may be implemented to form the weakening region 116.

[0061] Alternatively, it is possible that the weakening region 116 is replaced by an interface of low adhesion force, such as for example that described in document FR3082997A1.

[0062] According to an exemplary embodiment, the functional layer 110 may correspond to a thin upper part of the first substrate 112. In this case, it is the production of the weakening region 116 which defines the functional layer 110 whose material (or at least one of the materials) corresponds to that of the first substrate 112. For example, the first substrate 112 may correspond to a substrate of piezoelectric material and the functional layer 110 may correspond to a thin upper part originating from the first substrate 112. According to another exemplary embodiment, the functional layer 110 may correspond to a layer of material produced by epitaxy on the first substrate 112. More generally, the material of the functional layer 110 may correspond to a monocrystalline material which cannot be obtained directly, without transfer, on the final substrate.

[0063] The thickness of the functional layer 110 may be less than 20 μm, or even less than or equal to 1 μm. Furthermore, the thickness of the functional layer 110 may depend on its nature, i.e. whether it corresponds to a single layer of materials or to a stack of several layers of materials, and / or whether it comprises electrical and / or electronic elements and / or components.

[0064] The technique(s) implemented to secure the flexible substrate 100 to the functional layer 110 may depend in particular on the materials of the flexible substrate 100 and the functional layer 110. In the exemplary embodiment described here, the flexible substrate 100 is applied for example by lamination onto the functional layer 110. Other techniques may be implemented to achieve this securing. The securing between the flexible substrate 100 and the functional layer 110 is ensured by the adhesive properties of the first adhesive layer 104, at the first face 107 of the flexible substrate 100.

[0065] In the described embodiment, as shown in FIG. 3, a separation of the functional layer 110 and the first substrate 112 is then carried out. During this separation, the adhesion forces between the different layers of the flexible substrate 100 and between the first adhesive layer 104 and the functional layer 110 are greater than an adhesion force between the functional layer 110 and the first substrate 112. According to an exemplary embodiment, this separation may correspond to a tearing carried out by securing a rigid peripheral element (not visible in the figures) to the flexible substrate 100 and using this element as a mechanical handle. Other separation methods are possible, whether mechanical, thermal, by lift-off, etc.

[0066] In the embodiment described, as shown in FIG. 4, a cutting of the functional layer 110 into several distinct portions mechanically held together by at least one of the layers of the flexible substrate 100 is then implemented. This cutting step can be used to define the shape of elements, or portions, intended to be subsequently transferred to different destination substrates. For example, when the functional layer 110 corresponds to a piezoelectric layer, this cutting step can be used to cut the shape of piezoelectric portions intended to serve as sensitive elements of different piezoelectric sensors.

[0067] The cutting (represented by cutting lines designated by the reference 118) can be carried out through at least the functional layer 110, the first adhesive layer 104 and the first support layer 102. In the example of FIG. 4, the cutting is also carried out through a portion of the second adhesive layer 106. As a variant, depending in particular on the thickness of the second support layer 108, this cutting can also be carried out through the entire thickness of the second adhesive layer 106, or even through a portion of the thickness of the second support layer 108.

[0068] According to an exemplary embodiment, the cutting is carried out by laser. Other techniques for singulating the portions of the functional layer 110, for example mechanical, can be implemented to carry out this cutting.

[0069] As shown in FIG. 5, the functional layer 110 is transferred onto a second substrate 120 using the flexible substrate 100 as a transport element for the functional layer 110. In the exemplary embodiment described, given the cutting of the functional layer 110 previously carried out, this transfer corresponds to a collective joining of the portions of the functional layer 110 onto the second substrate 120.

[0070] In the example of Figure 5, the second substrate 120 corresponds to a rigid substrate on which an adhesive layer 122, comprising for example a silicone gel, is arranged to ensure the bonding between the functional layer 110 and the second substrate 120. As a variant, it is possible for the second substrate 120 to correspond to a flexible substrate, for example similar to or different from the first support layer 102 and / or the second support layer 108. Other variants are also conceivable, such as for example a second substrate 120 corresponding to a ceramic plate with which the adhesion of the functional layer 110 is ensured by suction, by electrostatic forces, etc.

[0071] The transfer method then comprises implementing a removal of the second support layer 108 and the second adhesive layer 106 (see FIG. 6). According to an exemplary embodiment, this removal corresponds to a peeling carried out by applying a mechanical force pulling on the second support layer 108. Alternatively, other techniques can be implemented to carry out this removal.

[0072] In the described embodiment, when removing the second support layer 108 and the second adhesive layer 106, the adhesive force between the second adhesive layer 106 and the first support layer 102 is lower than those between the first support layer 102 and the first adhesive layer 104, between the first adhesive layer 104 and the functional layer 110 and between the functional layer 110 and the second substrate 120 (between the functional layer 110 and the adhesive layer 122 in the example shown in FIG. 6).

[0073] According to an exemplary embodiment, the low adhesive force between the second adhesive layer 106 and the first support layer 102 can be obtained by subjecting the second adhesive layer 106 to a treatment reducing the initial adhesive force of the second adhesive layer 106. The nature of the treatment implemented can depend in particular on the nature of the material(s) of the second adhesive layer 106. For example, when the material of the second adhesive layer 106 comprises photodegradable nitrogen-based compounds, this treatment can correspond to an exposure of the material of the second adhesive layer 106 (for example through the second support layer 108) to UV radiation. In this example, the UV radiation used degrades the adhesive properties of the second adhesive layer 106 until they are very weak, or even zero.It is then possible to remove the second support layer 108 and the second adhesive layer 106 without removing other layers from the stack produced at this stage of the transfer process. Furthermore, the material of the first adhesive layer 104 can be chosen such that its adhesive properties are not or very little impacted by this treatment, as is for example the case by choosing a first adhesive layer 104 comprising acrylic whose adhesive properties are not modified by a UV treatment.

[0074] After removing the second support layer 108 and the second adhesive layer 106, the mechanical cohesion between the portions of the functional layer 110 is ensured by the second substrate 120 (and the adhesive layer 122 in the example described).

[0075] A second transfer of one or more of the portions of the functional layer 110 from the second substrate 120 onto at least one third substrate 124 is then implemented (see FIGS. 7 and 8). In the exemplary embodiment described, this second transfer is implemented individually for each of the portions of the functional layer 110, for example by placement equipment 126, i.e. of the “pick and place” type. Alternatively, other transfer techniques are possible.

[0076] In the exemplary embodiment described, during this second transfer, the adhesion force between the portions of the functional layer 110 and the second substrate 120 is less than those between the first support layer 102 and the first adhesive layer 104 and between the first adhesive layer 104 and the functional layer 110. In the exemplary embodiment described, this results in a force opposing the adhesion force between the second substrate 120 and the functional layer 110, applied to the second support layer 108 by the equipment 126, and which is greater than the adhesion force between the portion of the functional layer 110 and the second substrate 120. The force allowing this detachment of the portions of the functional layer 110 from the second substrate 120 corresponds for example to a suction force applied by the equipment 126 on each portion of the first support layer 102.

[0077] In the described embodiment, the third substrate 124 on which the portions of the layer functional layer 110 are transferred corresponds to the final destination substrate of these portions. This third substrate 124 may correspond to a rigid substrate, a flexible substrate, a PCB, etc. In a particular configuration different from that shown in FIGS. 7 and 8, the third substrate 124 may comprise a curved surface onto which one or more of the portions of the functional layer 100 are transferred.

[0078] In the embodiment described, the holding of the portions of the functional layer 110 on the third substrate 124 is ensured by a third adhesive layer 128 arranged between the third substrate 124 and the portion(s) of the functional layer 110. For example, the third adhesive layer 128 may comprise a solder paste or an electrically conductive film (deposited anisotropically or isotropically) making it possible to recover electrical contact with the portion(s) of the functional layer 110. If it is not necessary to recover electrical contact with the portion(s) of the functional layer 110, the third adhesive layer 128 may comprise, for example, an epoxy and / or acrylic resin.

[0079] As a variant of the previously described embodiment, after having implemented the steps previously described in connection with FIGS. 1 to 6, the portions of the functional layer 110 can be transferred not individually onto the third substrate 124, but collectively, for example using equipment provided with a roller against which the first support film 102 is secured and making it possible to collectively transfer the portions of the functional layer 110 onto the third substrate 124.

[0080] The first support layer 102 and the first adhesive layer 104 are then removed and separated from the layer functional layer 110 (see figure 9). According to an exemplary embodiment, this removal may correspond to a peeling carried out by applying a mechanical force pulling on the first support layer 102 and opposing the adhesion force between the first adhesive layer 104 and the functional layer 110. Alternatively, other techniques may be applied to carry out this removal of the first support layer 102 and the first adhesive layer 104.

[0081] In the exemplary embodiment described, during this removal, the adhesive force between the portion(s) of the functional layer 110 and the first adhesive layer 104 is lower than that between the functional layer 110 and the third substrate 124 (between the functional layer 110 and the third adhesive layer 128 in the example of FIG. 9). According to an exemplary embodiment, the low adhesive force between the first adhesive layer 104 and the functional layer 110 can be obtained by subjecting the first adhesive layer 104 to a treatment reducing the initial adhesive force of the first adhesive layer 104. The nature of the treatment implemented can depend in particular on the nature of the material(s) of the first adhesive layer 104.For example, when the material of the first adhesive layer 104 comprises acrylic, this treatment may correspond to a heat treatment, for example carried out at a temperature of at least 190°C. In this example, the temperatures to which the first adhesive layer 104 is exposed degrade the adhesive properties of the first adhesive layer 104 to the point where they are very low, or even zero. It is then possible to remove the first support layer 102 and the first adhesive layer 104 without separating the portion(s) of the functional layer 110 from the third substrate 124. Furthermore, the material of the third adhesive layer 128 may be chosen such that its adhesive properties are not or are very little impacted by. this heat treatment, as is for example the case by choosing a third adhesive layer 128 comprising a material comprising photodegradable nitrogen-based compounds whose adhesive properties are not modified by heat treatment.

[0082] Following this removal of the first support layer 102 and the first adhesive layer 104, the stack(s) obtained from the portions of the functional layer 110 and the third substrate 124 can be used to produce a device. For example, the portions of the functional layer 110 can correspond to thin portions of piezoelectric material used for producing piezoelectric sensors. Many other applications are possible.

[0083] In the example transfer method described above, treatment steps are described for reducing the adhesive forces of the first and second adhesive layers 104, 106 just before implementing the steps of removing these layers. As an alternative or in combination with these steps of reducing the initial adhesive forces of the adhesive layers, it is possible to implement, during the transfer method, steps of treating the adhesive layers strengthening their adhesive force. For example, before the first transfer of the piezoelectric layer 110 onto the second substrate 120, it is possible to implement one or more treatment steps increasing the adhesive forces of the first and second adhesive layers 104, 106.According to another example, before the removal of the second support layer 108 and the second adhesive layer 106, it is possible to implement one or more treatment steps increasing the adhesive strength of the first adhesive layer 104 and / or the adhesive layer 122. According to another example, before the. second transfer of the portions of the functional layer 110 onto the third substrate 124, it is possible to implement one or more treatment steps increasing the adhesive strength of the first adhesive layer 104. By way of example, such a step of strengthening the adhesive strength of an adhesive layer may correspond to crosslinking by UV treatment in the case of an adhesive layer comprising an acrylic glue, or temperature curing in the case of an adhesive layer comprising an epoxy glue.

[0084] In the previously described method example, the material of the first adhesive layer 104 may comprise acrylic whose adhesive properties become very weak after exposure to a certain temperature, and the material of the second adhesive layer 106 may comprise photodegradable nitrogen-based compounds so that its adhesive properties become very weak after exposure to UV radiation. Alternatively, it is possible for the materials of the first and second adhesive layers 104, 106 to be such that their adhesive properties become very weak when exposed to different temperatures or different wavelengths.

[0085] Furthermore, during the removals mentioned above and implemented during the method, the cohesive force of the functional layer 110 is greater than the adhesive force of the removed layers.

[0086] As a variant of the above method in which the adhesive strengths of the adhesive layers 104 and 106 are modified, the materials of the first and second adhesive layers 104, 106 may have by default adhesive properties, or adhesion properties, such that the adhesive strength of the second adhesive layer 106 is lower than that of the adhesive layer 122, itself lower than that of the first adhesive layer 104, itself lower than that of the third adhesive layer 128.

[0087] As a variant of the example of the transfer method described above, it is possible to implement, for example before the securing of the flexible substrate 100 to the functional layer 110, and / or between the separation of the functional layer 110 and the first substrate 112 and the securing to the second substrate 120, and / or between the separation of the functional layer 110 and the second substrate 112 and the securing to the third substrate 124, one or more steps of treatment of the functional layer 110. Such treatment steps correspond for example to steps of cleaning, etching (for example by laser, plasma, etc.), deposition, activation, etc. Such treatment steps can be implemented for one and / or the other of the faces of the functional layer 110 given that each of the faces of the functional layer 110 is accessible during the transfer method thanks to the use of the flexible substrate 100.

[0088] According to an alternative embodiment illustrated by FIG. 10, it is possible that, prior to the securing of the flexible substrate 100 to the functional layer 110, a magnetic layer 130, that is to say a layer of material having magnetic properties, is produced on the functional layer 110. This magnetic layer 130 may correspond to a ferromagnetic layer and may comprise, for example, iron, nickel or cobalt. It is also possible that the magnetic layer 130 corresponds to a diamagnetic layer comprising, for example, carbon, copper or silver. It is also possible that the magnetic layer 130 corresponds to a paramagnetic layer comprising, for example, tungsten, aluminium or lithium. It is also possible that the magnetic layer 130 comprises several magnetic materials, combining or not these different magnetic properties. The magnetic layer 130 can be produced by implementing a deposition on the functional layer 110, for example by evaporation, PECVD deposition (plasma-enhanced chemical vapor deposition), electrochemical deposition, etc. After the production of the magnetic layer 130 on the functional layer 110, the flexible substrate 100 can be secured to the magnetic layer 130, as shown in FIG. 10. The other steps of the method can be similar to those previously described. At the end of the transfer process, the magnetic material remaining on the functional layer 110 can be removed or preserved.

[0089] According to another exemplary embodiment illustrated in FIG. 11, it is possible that the magnetic layer 130 is not produced on the functional layer 110 prior to the securing of the flexible substrate 100, but that it is integrated into the flexible substrate 100, for example between the first support layer 102 and the first adhesive layer 104. In this case, the magnetic layer 130 can be removed when removing the first support layer 102 and the first adhesive layer 104. Alternatively, the magnetic layer 130 could be arranged between the first support layer 102 and the second adhesive layer 106, or between the second support layer 108 and the second adhesive layer 106.

[0090] According to another variant, it is possible to envisage having at least a first magnetic layer interposed between the functional layer 110 and the flexible substrate 100, and at least a second magnetic layer arranged in the substrate 100. Such a configuration can make it possible to increase the overall magnetic power formed by these layers compared to a single layer of magnetic material, or else make it possible to combine the magnetic properties of two magnetic layers having different properties. It is also possible to have several magnetic layers present in the flexible substrate 100.

[0091] The presence of the magnetic layer 130 between the functional layer 110 and the flexible substrate 100 and / or in the flexible substrate 100 makes it possible to improve the management and processing of the assembly produced and handled during the various steps of the method. The presence of the magnetic layer 130 can make it possible to ensure the flatness of the assembly produced during the various steps, for example by using a magnetic flat tool to which the assembly is held thanks to the presence of the magnetic layer 130. The presence of the magnetic layer 130 can also make it possible to apply, thanks to the magnetic forces generated, other objects to this assembly, for example for the implementation of certain processing steps of the functional layer 110 (for example a mask for the implementation of a deposit on the functional layer 110).The presence of the magnetic layer 130 can also help to maintain the cohesion of the different parts of the assembly obtained after the cutting step. According to a particular embodiment, it is also possible that the maintenance of the functional layer 110 on the second substrate 120 is ensured by magnetic forces exerted between the magnetic layer 130 and another magnetic material present on or in the second substrate 120.

[0092] According to an alternative embodiment illustrated by FIG. 12, it is possible for the flexible substrate 100 to comprise a marking layer 132 configured to be able to be marked by laser engraving. The marking layer 132 is not transparent to the wavelength of the laser used to carry out the desired marking. The marking layer 132 can be interposed between the first support layer 102 and the second adhesive layer 106 or between the first adhesive layer 104 and the first support layer 102. In the example of FIG. 12, the marking is carried out through the second support layer 108 and the second adhesive layer 106 which are, in this example, transparent to the wavelength of the laser used. For example, the marking layer 132 may comprise a metal such as aluminum, copper, nickel, gold, platinum, and / or a polymer such as PET, PVC, polyimide, PP, polycarbonate, etc., and / or glass for example of the borosilicate, soda-lime, quartz glass type, and / or a crystal such as silicon, quartz, sapphire. Preferably, the material of the marking layer 132 may be inorganic, and / or may be metallic to allow thermal dissipation of the heat generated by the marking.In addition, the material of the marking layer 132 may be opaque with respect to wavelengths in the visible range, which makes it possible to simplify the alignment of the marked elements and / or the recognition of the marking carried out subsequently.

[0093] The marking of the layer 132 may be carried out in one or more steps implemented before the second transfer from the second substrate 120 to the third substrate 124. According to an exemplary embodiment, this marking may correspond to an identifier used to identify the different portions of the functional layer 110 after the implementation of the second transfer, and / or may correspond to alignment marks. It is for example possible, by this marking, to indicate whether the portions of the functional layer 110 are intended or not to be used subsequently.

[0094] According to a variant, the marking layer 132 can also serve as a magnetic layer capable of fulfilling the same functions as those previously described for the The magnetic layer 130 if the material(s) are suitable for this, as is the case for example for a marking layer 132 comprising nickel.

[0095] In all the exemplary embodiments, the substrates used in the transfer method may correspond to cylindrical wafers such as those used in the field of microelectronics.

[0096] In all the embodiments, before the flexible substrate 100 is used, it is possible for its first face 107 to be protected by a removable protective layer, not shown in the figures. This removable protective layer can serve to preserve the adhesive properties of the first face 107. This removable protective layer is in this case removed before the flexible substrate 100 is used.

[0097] Thus, in an exemplary embodiment of this transfer method, from an initial substrate, a functional layer can be detached by being fixed to the flexible substrate 100 having a structure allowing a posteriori a singularization of the functional layer into smaller elements and their selective or collective transfer onto a final flat or curved substrate.

[0098] For all the embodiment examples described, the flexible substrate 100 can allow:

[0099] - a check of the flatness of the assembly including the flexible substrate 100 during the different stages;

[0100] - protection of the functional layer 110;

[0101] - easier handling of the functional layer 110;

[0102] - a singularization of the functional layer 110 into several elements facilitated by maintaining the assemblies;

[0103] - a collective or individual transfer of portions of the functional layer 110;

[0104] - access to both sides of the functional layer 110 during the transfer process;

[0105] - a transfer onto a final flat or curved substrate thanks to the flexible nature of the substrate 100;

[0106] - a dissociation of the first and second transfers implemented, one being able to be of the pick and place type of the portions of the functional layer 110 and the other being able to correspond to a collective transfer of the functional layer 110 or of the portions of the functional layer 110, which makes it possible for example to transfer portions of the functional layer 110 with a spacing relative to each other ("pitch" in English), different before and after the transfer.

[0107] Additionally, compared to using two separate adhesive strips, using the flexible substrate 100 has the following advantages:

[0108] - time saving (a single step of securing the flexible substrate 100 to the functional layer 110, unlike the use of two separate adhesive strips requiring the implementation of two separate steps to secure them to the functional layer);

[0109] - reduction of defectivity (in particular the appearance of bubbles, particles, etc.);

[0110] - release of constraints (for example in terms of temperature, pressure, contamination, environment) of compatibility of the first substrate 112 with the steps linked to the securing of the flexible substrate 100 to the functional layer 110 (the securing of the substrate 100 to the functional layer 110 can be carried out without having to heat or apply pressure, unlike the successive joining of two adhesive strips which may require a particular temperature and / or pressure to avoid the presence of bubbles between them).

[0111] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art.

[0112] Finally, the practical implementation of the embodiments and variants described is within the reach of the person skilled in the art from the functional indications given above.

Claims

CLAIMS 1. Flexible substrate (100) comprising at least a first support layer (102) disposed between first and second adhesive layers (104, 106) and a second support layer (108) such that the second adhesive layer (106) is disposed between the first and second support layers (102, 108), and comprising a first adhesive face (107) formed by the first adhesive layer (104), and further comprising at least one ferromagnetic layer (130) disposed between the first adhesive layer (104) and the first support layer (102) or between the first support layer (102) and the second adhesive layer (106) or between the second adhesive layer (106) and the second support layer (108).

2. Flexible substrate (100) according to claim 1, comprising a total thickness of the flexible substrate (100) of between 10 μm and 800 μm.

3. Flexible substrate (100) according to one of the preceding claims, wherein at least one of the first and second support layers (102, 108) comprises a polymer and / or a metallic material.

4. Flexible substrate (100) according to one of the preceding claims, wherein one of the first and second adhesive layers (104, 106) comprises a material whose adhesive properties are reduced when this material is subjected to UV treatment, and the other of the first and second adhesive layers (104, 106) comprises a material whose adhesive properties are reduced when this material is subjected to heat treatment.

5. Flexible substrate (100) according to claim 4, wherein the material of said one of the first and second adhesive layers (104, 106) comprises compounds nitrogen-based photodegradable, and the material of said other of the first and second adhesive layers (104, 106) comprises acrylic.

6. Flexible substrate (100) according to one of the preceding claims, wherein the materials of the first and second adhesive layers (104, 106) are such that the adhesive strength of the second adhesive layer (106) is lower than that of the first adhesive layer (104).

7. Flexible substrate (100) according to one of the preceding claims, further comprising at least one marking layer (132) configured to be laser marked and interposed between the first support layer (102) and the second adhesive layer (106) and / or between the first adhesive layer (104) and the first support layer (102).

8. Flexible substrate (100) according to claim 7, wherein the marking layer (132) comprises at least one metal and / or one polymer and / or one glass and / or one crystal.

9. Flexible substrate (100) according to one of the preceding claims, further comprising a removable protective layer disposed against the first face (107).

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