Method for transferring at least one layer of material from a first substrate to a second substrate

Abstract

The invention relates to a method for transferring at least one layer of material, comprising: producing first and second separating layers (108, 110), one against the other, on a first substrate (104); producing the layer to be transferred on the second separating layer (110); securing the layer to be transferred to a second substrate (106), forming a stack of different materials; and performing mechanical separation at the interface between the separating layers; in which the materials of the stack are such that the interface between the first and second separating layers has the weakest adhesion force, and the method comprises a step reducing an initial adhesion force of the interface between the first and second separating layers.

Claims

1. A method for transferring at least one layer of material from a first substrate to a second substrate, comprising: producing first and second separating layers disposed one against the other on a face of the first substrate, and such that the first separating layer is disposed between the first substrate and the second separating layer; producing said at least one layer of material to be transferred on the second separating layer; securing said at least one layer of material to be transferred to the second substrate, forming a stack of different materials; performing mechanical separation at an interface between the first and second separating layers, such that the first separating layer remains integral with the first substrate and that the second separating layer remains integral with said at least one layer of material to be transferred; and between the step of producing first and second separating layers and the step of securing the at least one layer of material, at least one step reducing an initial adhesion force of the interface between the first and second separating layers, wherein the materials of the stack are chosen such that the interface between the first and second separating layers corresponds to the interface, of all of the interfaces of the stack, having a weakest adhesion force, wherein one of the first and second separating layers includes at least one of SiO.sub.2 and silicon nitride and the other of the first and second separating layers includes at least one noble metal, and wherein said at least one step reducing the initial adhesion force of the interface between the first and second separating layers includes the implementation of at least one heat treatment at a temperature modifying a stress of the noble metal.

2. The method according to claim 1, wherein the noble metal includes platinum.

3. The method according to claim 1, wherein the step of securing is implemented: by means of an adhesive layer produced on said at least one layer of material to be transferred or on the second substrate, or by thermocompression between a first metal bonding layer, produced on said at least one layer of material to be transferred, and a second metal bonding layer produced on the second substrate, or by direct bonding between a first bonding layer, produced on said at least one layer of material to be transferred, and a second bonding layer produced on the second substrate.

4. The method according to claim 1, wherein said at least one layer of material to be transferred includes at least one layer of at least one material among: a pyroelectric material, a piezoelectric material, a ferroelectric material and a dielectric material.

5. The method according to claim 4, wherein: said at least one material includes lead; said one of the first and second separating layers includes SiO.sub.2; said at least one step reducing the initial adhesion force of the interface between the first and second separating layers includes the implementation of at least one heat treatment forming, at the interface between the first and second separating layers, an alloy of lead and SiO.sub.2 in liquid phase, then of a cooling forming cavities at the interface between the first and second separating layers.

6. The method according to claim 4, wherein said at least one layer of material to be transferred includes at least one first electrode layer such that said at least one layer of at least one material is disposed between the first electrically conductive layer and the second separating layer.

7. The method according to claim 4, wherein the second separating layer includes at least one noble metal.

8. The method according to claim 4, wherein said at least one layer of material to be transferred includes at least one second electrode layer disposed between said at least one layer of at least one material and the second separating layer.

9. The method according to claim 1, wherein said at least one layer of material to be transferred includes at least one first layer of at least one first material among: pyroelectric material, piezoelectric material, ferroelectric material, and dielectric material, and further comprising, before the step of securing, producing at least one second layer of at least one second material among: pyroelectric material, piezoelectric material, ferroelectric material and dielectric material, and of at least one third electrode layer disposed between the second substrate d said at least one second layer of at least one second material.

10. The method according to claim 4, further comprising at least one of the following steps: before the step of securing, producing cavities in the second substrate, through a face of the second substrate whereon said at least one layer of material to be transferred is secured; after the step of securing, producing cavities in the second substrate, through a face opposite to that whereon said at least one layer of material to be transferred is secured.

11. The method according to claim 1, wherein the second substrate includes at least one polymer material, and wherein the step of securing is implemented at a temperature less than or equal to approximately 150° C.

12. The method according to claim 1, further comprising, after the step of mechanical separation, a step of etching the second separating layer.

13. The method according to claim 1, further including, between the step of producing said at least one layer of material to be transferred and the step of securing said at least one layer of material to be transferred to the second substrate, a step of producing functional MEMS and/or NEMS and/or microelectronic type components from said at least one layer of material to be transferred.

14. The method according to claim 1, wherein, after the step of mechanical separation, the preceding steps of the transfer method are each repeated at least once in order to transfer at least one other layer of material from the first substrate to said at least one layer of material.

15. The method for producing components of at least one of the follow types: MEMS, NEMS and microelectronic, comprising the implementation of a transfer method according to claim 1, then producing components from the second substrate and from said at least one layer of material to be transferred.

16. The method according to claim 15, wherein the components produced are piezoelectric transducers and/or pyroelectric and/or ferroelectric components and/or include MIM and/or coplanar type capacitors, and wherein said at least one layer of material to be transferred includes at least one layer of at least one material among: a pyroelectric material, a piezoelectric material, a ferroelectric material and a dielectric material.

17. A method for transferring at least one layer of material from a first substrate to a second substrate, comprising: producing first and second separating layers disposed one against the other on a face of the first substrate, and such that the first separating layer is disposed between the first substrate and the second separating layer; producing said at least one layer of material to be transferred on the second separating layer; securing said at least one layer of material to be transferred to the second substrate, forming a stack of different materials; performing mechanical separation at an interface between the first and second separating layers, such that the first separating layer remains integral with the first substrate and that the second separating layer remains integral with said at least one layer of material to be transferred; and between the step of producing first and second separating layers and the step of securing the at least one layer of material, at least one step reducing an initial adhesion force of the interface between the first and second separating layers, wherein the materials of the stack are chosen such that the interface between the first and second separating layers corresponds to the interface, of all of the interfaces of the stack, having a weakest adhesion force, wherein the at least one layer of material to be transferred includes at least one layer of at least one material among: a pyroelectric material, a piezoelectric material, a ferroelectric material and a dielectric material, and wherein the at least one material includes lead, the one of the first and second separating layers includes SiO.sub.2, and said at least one step reducing the initial adhesion force of the interface between the first and second separating layers includes the implementation of at least one heat treatment forming, at the interface between the first and second separating layers, an alloy of lead and SiO.sub.2 in liquid phase, then of a cooling forming cavities at the interface between the first and second separating layers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be better understood upon reading the description of examples of embodiment given, purely by way of indicative and non-limiting example, while referring to the appended drawings wherein:

(2) FIGS. 1A to 1D show the steps of a method for transferring at least one layer of material according to a first embodiment;

(3) FIGS. 2A to 2D show the steps of a method for transferring at least one layer of material according to a second embodiment;

(4) FIGS. 3A to 3D show the steps of a method for transferring at least one layer of material according to a third embodiment;

(5) FIG. 4 shows a stack of materials formed during a method for transferring at least one layer of material according to a fourth embodiment;

(6) FIG. 5 shows a stack of materials formed during a method for transferring at least one layer of material according to one variant of the fourth embodiment;

(7) FIGS. 6A to 6C show the steps of a method for transferring at least one layer of material according to a fifth embodiment;

(8) FIGS. 7A to 7C show the steps of a method for transferring at least one layer of material according to one variant of the fifth embodiment;

(9) FIGS. 8A to 8C show the steps of a method for transferring at least one layer of material according to a sixth embodiment;

(10) FIGS. 9A and 9B show devices produced from layers transferred by a method for transferring at least one layer of material according to the sixth embodiment;

(11) FIGS. 10A to 10F show the steps of a method for transferring at least one layer of material according to a seventh embodiment;

(12) FIG. 11 shows a structure obtained by implementing a method for transferring at least one layer of material according to an eighth embodiment;

(13) Identical, similar or equivalent portions of the various figures described hereafter bear the same numerical references so as to facilitate the passage from one Figure to the other.

(14) The various portions shown in the figures are not necessarily according to a uniform scale, in order to make the Figures more readable.

(15) The various possibilities (variants and embodiments) must be understood as not being exclusive from one another and may be mutually combined.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(16) Reference is first of all made to FIGS. 1A to 1D that show the steps of a method for transferring at least one layer of material according to a first embodiment.

(17) As shown in FIG. 1A, the reference 102 designates the layer or layers of materials intended to be transferred from a first substrate 104, also known as donor substrate and that corresponds to the substrate whereon the layer or layers 102 are initially produced, to a second substrate 106, also known as receiver substrate, shown on FIG. 1B.

(18) The or each of the layers of materials to be transferred 102 may be thin or thick, and has for example a thickness between approximately 1 nm and 1 mm. The nature of the material or materials of said layer or layers 102 depends on the components intended to be produced from said layer or layers 102. This method may apply to the transfer of any type of material.

(19) According to another example of embodiment, one or more microelectronic components such as transistors, light-emitting diodes, etc. may be produced, prior to the transfer of the layer or layers 102 to the second substrate 106, in the layer or layers 102.

(20) The first substrate 104 corresponds here to a semiconductor substrate comprising for example silicon, and whereof the thickness is for example between approximately 100 μm and 1 mm.

(21) The second substrate 106 includes at least one material corresponding for example to a semiconductor such as silicon, or glass, sapphire, metal or a polymer material. In the case of a second compliant/flexible substrate 106, it may be disposed on a rigid support during the transfer of the layer or layers thereon. The thickness of the second substrate 106 is for example equal to a plurality of hundreds of microns. When the second substrate 106 corresponds to a substrate made of polymer material, the thickness thereof is for example between approximately 1 μm and 1 mm.

(22) An interface of weak adhesion force is formed between the layer or layers 102 and the first substrate 104. In this first embodiment, said interface is formed thanks to a first separating layer 108 here comprising at least SiO.sub.2 and/or silicon nitride such as SiN and/or Si.sub.3N.sub.4, and a second separating layer 110 comprising at least one noble metal. The interface of weak adhesion force corresponds to the interface between said two separating layers 108, 110, and is obtained thanks to the nature of the materials of the separating layers 108, 110 that only adhere weakly to one another. The material of the first separating layer 108 may advantageously correspond to SiO.sub.2. The first separating layer 108 is for example carried out by thermal oxidation of the upper face of the first substrate 104. The thickness of the first separating layer 108 is for example between approximately 1 nm and 20 μm.

(23) The noble metal of the second separating layer 110 includes at least one of the following elements: platinum, gold, silver, rhodium, osmium, palladium, ruthenium, iridium. The thickness of the second separating layer 110 is for example between approximately 1 nm and 200 nm.

(24) In the first embodiment described here, an adhesive layer 112 is disposed on the second substrate 106. According to one particular example of embodiment, it is possible that the functional elements are present on the second substrate 106, for example electronic components. In this case, the adhesive layer 112 covers said functional elements. By way of example, the adhesive layer 112 may correspond to a layer of polymer adhesive or an adhesive dry film, whereof the thickness is for example between approximately 100 nm and 1 mm.

(25) The assembly formed of the first substrate 104, of the separating layers 108, 110 and of the layer or layers 102 is secured on the second substrate 106 such that the layer or layers 102 are disposed against the adhesive layer 112, that is to say between the two substrates 104, 106. Within the stack thus obtained, the interface of weak adhesion force formed between the separating layers 108, 110 corresponds, of all of the interfaces of the stack, to the interface that has the weakest adhesion force between two successive layers of the stack.

(26) Mechanical separation is subsequently performed at said interface of weak adhesion force between the separating layers 108, 110. Said mechanical separation is for example performed by introducing a blade between the two wafers. At the end of said mechanical separation, the stack remaining kept includes the second substrate 106 whereon the layer or layers 102 are secured by means of the adhesive layer 112. The second separating layer 110, which may be kept or removed after the step of separation, is disposed on the layer or layers 102 (see FIG. 1C).

(27) FIG. 1D shows the assembly formed of the first substrate 104 and of the first separating layer 108, separated from the other layers. Said first substrate 104 may be used to perform the transfer of one or more other layers 102 to another second substrate 106 or to the same substrate 106 already integrating one or more layers previously transferred.

(28) In the first embodiment described in relation to FIGS. 1A to 1D, the first separating layer 108 includes SiO.sub.2 and/or silicon nitride and the second separating layer 110 includes the noble metal. This may be advantageous when the material of the second separating layer 110 is subsequently used for producing components from the layers transferred to the second substrate 106. According to an alternative embodiment, it is however possible that the first separating layer 108 includes the noble metal and that the second separating layer 110 includes SiO.sub.2 and/or silicon nitride.

(29) In the first embodiment described above, the securing between the layer or layers 102 and the second substrate 106 is ensured by the adhesive layer 112. Alternatively, it is possible that said securing is carried out in another way. For example, it is possible to perform a direct bonding, or molecular bonding, between two bonding layers. Each of said two bonding layers has properties adapted to such a direct bonding: very low roughness, adapted material, etc. Said two bonding layers correspond for example to layers of SiO.sub.2, of Si or also of SiN, and each include for example a thickness between 1 nm and 1 mm.

(30) According to another example of embodiment, the securing between the layer or layers 102 and the second substrate 106 may be ensured by a thermocompression bonding. In this case, two metal bonding layers are produced, one on the layer or layers 102 and the other on the second substrate 106. Said metal bonding layers correspond for example to layers comprising copper or gold, and whereof the thickness is for example between 1 nm and 100 μm.

(31) Any other bonding means may also be envisaged for carrying out the securing between the layer or layers 102 and the second substrate 106, from the moment that said securing does not form an interface having a weaker adhesion force than that formed between the separating layers 108, 110.

(32) FIGS. 2A to 2D show the steps of a method for transferring at least one layer of material according to a second embodiment.

(33) In said second embodiment, the element to be transferred from the first substrate 104 to the second substrate 106 corresponds to a stack of a plurality of layers intended to be used to produce piezoelectric and/or pyroelectric and/or ferroelectric and/or dielectric devices. Said stack of a plurality of layers to be transferred is formed on the first separating layer 108, and includes: the second separating layer 110, which comprises for example platinum and whereof the thickness is for example equal to approximately 100 nm; a layer 114 of piezoelectric and/or pyroelectric and/or ferroelectric and/or dielectric material, comprising for example PZT and/or AlN and/or KNN ((K.sub.xNa.sub.1-x) NbO.sub.3), and whereof the thickness is for example between approximately 1 nm and 100 μm and here equal to approximately 500 nm. an optional grip layer 116, which comprises for example ruthenium and/or tungsten and/or chromium and/or titanium and/or TiO.sub.2, and whereof the thickness is for example between approximately 0 nm and 10 nm; an electrode layer 118, which includes an electrically conductive material, for example the same metal as that of the second separating layer 110, here platinum and/or ruthenium and/or iridium, and whereof the thickness is for example between approximately 1 nm and 500 nm and here equal to that of the second separating layer 110, that is to say approximately 100 nm.

(34) The presence or not of the grip layer 116 in particular depends on the adhesion of the electrode layer 118 on the layer 114.

(35) In said stack, the second separating layer 110 is configured to form, in the devices that will be produced from the stack of layers transferred to the second substrate 106, one of the electrodes that is located on one side of the portions from the layer 114 of each of the devices. The electrode layer 118, which is here metal, is intended to form the other of the two electrodes of said devices.

(36) An optional layer 120 forming a diffusion barrier, comprising for example RuO.sub.2 and/or Ni and/or TiN and/or WN and whereof the thickness is for example equal to approximately 60 nm or more generally between approximately 0 nm and 100 nm, is disposed on the electrode layer 118. The presence or not of the layer 120 depends on the thermal budget to which the stack is subjected after the transfer.

(37) A first metal bonding layer 122, adapted for the implementation of a thermocompression bonding, is disposed on the barrier layer 120. The barrier layer 120 is configured to form a diffusion barrier between the electrode layer 118 and the first metal bonding layer 122 and prevent a possible diffusion (depending on the thermal budget applied) of the metal from the first metal bonding layer 122 in the electrode layer 118 during the securing with the second substrate 106.

(38) As in the first embodiment, the first separating layer 108 includes SiO.sub.2 and/or silicon nitride such as SiN and/or Si.sub.3N.sub.4, and has for example a thickness equal to approximately 500 nm. The interface of weak adhesion force is formed here, as in the first embodiment, between the first separating layer 108 and the second separating layer 110 due to the weak adhesion force obtained between the SiO.sub.2 and/or the silicon nitride of the first separating layer 108 and the noble metal of the second separating layer 110.

(39) The stack shown in FIG. 2A is for example obtained by implementing the steps of: thermal oxidation of the first substrate 104, forming the first separating layer 108 at a front face of the first substrate 104; deoxidation (optional) of the rear face of the first substrate 104 (removing the oxide formed on the rear face of the first substrate 104 during the previous oxidation step); deposition, for example PVD (physical vapor deposition), of the noble metal of the second separating layer 110 on the first separating layer 108; deposition, for example by a sol-gel process, of the pyroelectric and/or piezoelectric and/or ferroelectric and/or dielectric material of the layer 114; successive depositions of the metals of the layers 116, 118, 120 and 122.

(40) The stack obtained at the end of these steps is shown in FIG. 2A.

(41) At the same time as the various layers previously described are produced on the first substrate 104, the second substrate 106 is also prepared for the transfer of the stack of layers 110, 114, 116, 118, 120, 122.

(42) As shown in FIG. 2B, the second substrate 106 is covered, on the side of the front and rear faces thereof, with oxide layers 124, 126 for example each one similar to the first separating layer 108. In the example of embodiment described here, each of the oxide layers 124, 126 includes SiO.sub.2 and a thickness for example equal to approximately 500 nm.

(43) The oxide layer 124, which corresponds to that which is located on the side where the first substrate 104 is intended to be transferred, forms an electrically insulated layer between second substrate 106 and the other layers that are located on the layer 124. The insulating layer 124 is covered with a grip layer 128 including for example a metal such as titanium, chromium, tungsten or ruthenium, and whereof the thickness is for example equal to approximately 20 nm or more generally between approximately 1 nm and 100 nm. Another layer 130 is disposed on the layer 128 and includes for example nitride, here TiN and/or WN, and has a thickness equal to approximately 60 nm or more generally between approximately 1 nm and 100 nm.

(44) Alternatively, it is possible that the oxide layer 124 is not present between the second substrate 106 and the grip layer 128. Indeed, it may be advantageous not to have SiO.sub.2 in the stack forming a piezoelectric transducer in order to facilitate the management of stresses, because the compressive stress of SiO.sub.2 may make the overall management of stresses in the piezoelectric structures more complex.

(45) A second metal bonding layer 132 is disposed on the layer 130. Each of the first and second metal bonding layers 122, 132 includes a material, here a metal, adapted for the implementation of a thermocompression between said two layers. In the particular example of embodiment described here, the first and second metal bonding layers 122, 132 include copper and each have a thickness equal to approximately 500 nm. According to another example of embodiment, the first and second metal bonding layers 122, 132 may include gold, and may each have a thickness equal to approximately 50 nm.

(46) The layer 130 forms here a diffusion barrier for preventing a diffusion of the metal of the second metal bonding layer 132 in the layers 128, 124 and in the second substrate 106. However, said layer 130 is optional.

(47) In the example of embodiment described here, the rear face of the second substrate 106 is covered by the oxide layer 126. Alternatively, it is possible to eliminate said oxide layer 126 by carrying out a deoxidation of the rear face of the second substrate 106, as for the first substrate 104.

(48) As shown in FIG. 2C, the first substrate 104 and the layers formed thereon are transferred to the second substrate 106 by performing a thermocompression bonding between the two metal bonding layers 122, 132. Said thermocompression bonding is for example implemented at a temperature between approximately 20° C. and 400° C., by applying a compressive force between approximately 1 kN and 50 kN, and during a duration between approximately 1 min and 30 min. The force and the duration with which the thermocompression bonding is implemented in particular depend on the thicknesses of the metal layers to be bonded. The parameters for implementing said thermocompression are adapted in particular depending on the properties of the metal bonding layers 122, 132.

(49) Mechanical separation of two substrates 104, 106 is subsequently performed at the interface of weak adhesion force between the separating layers 108, 110, as in the first embodiment previously described. At the end of said mechanical separation, the stack remaining kept includes the second substrate 106, and the layers 124, 128, 130, 132, 122, 120, 118, 116, 114 and 110 disposed on the second substrate 106 (see FIG. 2D).

(50) The first substrate 104 and the first separating layer 108 are thus separated from the other layers. Said first substrate 104 may be reused to carry out the transfer of other layers to another second substrate 106.

(51) The various examples and variants previously described in relation to the first embodiment may apply to said second embodiment.

(52) FIGS. 3A to 3D show the steps of a method for transferring at least one layer of material according to a third embodiment.

(53) FIG. 3A shows the second substrate 106 whereon a plurality of layers will be transferred. The substrate 106 includes the front face thereof whereon is disposed the insulating layer 124. The second metal bonding layer 132, which corresponds here to a layer of gold of thickness equal to approximately 50 nm, is disposed on the insulating layer 124.

(54) FIG. 3B shows the first substrate 104 that includes the layers to be transferred. The first separating layer 108, corresponding for example to a layer of platinum, is disposed on the front face of the first substrate 104. The second separating layer 110 is disposed on the first separating layer 108, and corresponds here to a layer of SiO.sub.2.

(55) The layers to be transferred to the second substrate 106 are disposed on the second separating layer 110. Said layers correspond to: a grip layer 134, comprising for example TiO.sub.2, ensuring a good adhesion between the second separating layer 110 and an electrode layer 136 that here includes platinum and is intended to form a first electrode of the devices that will be produced from the layers transferred to the second substrate 106; the layer 114 of piezoelectric and/or pyroelectric and/or ferroelectric and/or dielectric material, which includes for example PZT the optional grip layer 116 and the electrode layer 118; the first metal bonding layer 122.

(56) As in the second embodiment previously described, a thermocompression bonding is subsequently performed between the first and second metal bonding layers 122, 132.

(57) Mechanical separation is subsequently performed at the interface of weak adhesion force formed between the separating layers 108, 110 as in the previous embodiments. FIG. 3C shows the second substrate 106 whereon the various layers have been transferred, and FIG. 3D shows the first substrate 104 remaining whereon the first separating layer 108 is kept. The second separating layer 110 that is located at the top of the layers transferred to the second substrate 106 may be eliminated.

(58) The various examples and alternative embodiments previously described in relation to the preceding embodiments may apply to said third embodiment.

(59) In the three embodiments previously described, the interface of weak adhesion force is formed thanks to the intrinsic properties of the materials of the separating layers 108, 110, due to the fact that SiO.sub.2 and/or silicon nitride and a noble metal adhere little to one another.

(60) Alternatively, it is possible that the weak adhesion force of the interface between the donor substrate (first substrate 104) and the layer or layers to be transferred to the receiver substrate (second substrate 106) is obtained thanks to at least one specific step intentionally degrading the adhesion properties of said interface and reducing the adhesion force at said interface between the materials of the first and second separating layers 108, 110. By way of example, during the deposition of the layer 114 on the second separating layer 110, it is possible to implement a heat treatment diffusing lead atoms from the PZT of the layer 114 towards the SiO.sub.2 of the first separating layer 108, through the second separating layer 110 that includes for example platinum. With such a heat treatment, for example implemented at a temperature higher than or equal to approximately 700° C., a SiO.sub.2+Pb mixture in liquid phase forms at the interface between the first and second separating layers 108, 110. When the temperature lowers, cavities are then created at said interface, in the layer formed of the SiO.sub.2 and lead mixture.

(61) FIG. 4 shows a structure obtained during the implementation of the method for transferring at least one layer according to a fourth embodiment. Said structure includes the first substrate 104 whereon are disposed the layers 108, 110, 114 and 118, and whereof the interface formed between the separating layers 108, 110 is degraded by cavities 138 created following the diffusion of lead from the PZT of the layer 114 in the SiO.sub.2 of the first separating layer 108. Said cavities 138 reduce the adhesion between the separating layers 108, 110.

(62) Said degradation, or reduction, of the adhesion force of the interface between the separating layers 108, 110 may therefore be obtained when the intrinsic properties of the materials used for said separating layers 108, 110 do not generate, without supplementary intervention, the formation of an interface of weak adhesion force.

(63) FIG. 5 shows such a variant wherein the second separating layer 110 corresponds here to a layer of TiO.sub.2. The thickness of said second separating layer 110 is for example equal to 20 nm. The reference 136 designates an electrode layer intended to form an electrode of components that will subsequently be produced, and that includes for example platinum. In this configuration, the adhesion force between the TiO.sub.2 of the second separating layer 110 and the platinum of the electrode layer 136 as well as that between the SiO.sub.2 of the first separating layer 108 and the TiO.sub.2 of the second separating layer 110 are not weak in relation to that of the other interfaces of said stack. Thanks to the heat treatment implemented after the deposition of the layer 114, the diffusion of lead from the PZT of the layer 114 up to the interface between the separating layers 108, 110 and the formation of cavities 138 at said interface make it possible to give it a weak adhesion force. Alternatively, the second separating layer may include ZrO.sub.2 or also ZnO or Ta.sub.2O.sub.5.

(64) Said intentional degradation of the separating interface between the first and second substrates is implemented for all of the embodiments previously described.

(65) Another possibility for degrading or reducing the adhesion force of the interface between the separating layers 108, 110 consists of implementing a heat treatment at a temperature modifying a stress of the noble metal of one of the two separating layers 108, 110. By considering the examples previously described where the second separating layer 110 includes a noble metal, and in particular platinum, which includes a compressive stress generated during the deposition thereof, the implementation of a heat treatment at a temperature for example higher than or equal to 250° C., or even higher than or equal to 300° C., modifies said stress that becomes a tensile stress. Said change in stress enables easier detachment during the implementation of the step of separating at the interface between the first and second separating layers 108, 110.

(66) In the second and third embodiments previously described, the layer 114 advantageously includes a pyroelectric and/or piezoelectric and/or ferroelectric and/or dielectric material and is disposed between two layers of metal electrodes so that said stack of layers is subsequently etched in order to form piezoelectric and/or pyroelectric and/or ferroelectric and/or dielectric functional components produced from the layers transferred to the second substrate 106.

(67) In the embodiments and examples of embodiment described, the second substrate 106 whereon at least one layer of material is transferred corresponds to an unstructured layer of material. Alternatively, it is possible that cavities are produced through at least one portion of the thickness of the second substrate 106, from the rear face of said second substrate 106, so that the layer or layers transferred can be used to form membranes of the devices produced from the layer or layers transferred.

(68) In addition, it is possible that the layer of material transferred is structured and forms part of one or more functional components already produced before the transfer to the second substrate 106.

(69) According to another variant, it is possible that the layers supplementary to those described in the preceding embodiments are present on the second substrate 106 before the securing thereof with the first substrate 104. For example, when the layer or layers transferred to the second substrate 106 are intended to be used for producing RF capacity type components, it is possible that one or more metal layers having a low resistivity (comprising for example gold) are present on the second substrate 106. This variant is particularly advantageous when the layers transferred to the second substrate 106 include a layer comprising a “high-k” dielectric material (or with high permittivity, for example higher than approximately 3.4, and because such a dielectric material is very complex to produce on a metal layer of low resistivity.

(70) The transfer method described here may advantageously be used to produce components forming electromagnetic transducers integrating materials that will make it possible to produce actuator and/or sensor functions. The sensor function is obtained by direct piezoelectric effect (a mechanical stress generates the appearance of electric charges in the material) whereas the actuator function is obtained by inverse piezoelectric effect (in this case an electric polarization external to the material generates a mechanical deformation of said latter).

(71) In order to produce the actuator function, a piezoelectric material having a transversal piezoelectric coefficient of the highest possible coefficient d31 is generally chosen. This is the case of PZT (or doped PZT (La, Mn, Nb), KNN, BNT-BT, or also PMN-PT).

(72) In order to produce the sensor function, a material having the best compromise between a high transversal piezoelectric coefficient e31 and a low dielectric permittivity ε.sub.r, that is to say with the largest Figure of Merit FOM=e31/ε.sub.r, is generally chosen. This is typically the case of AlN (or also Sc-doped AlN).

(73) The layer transfer method described here may advantageously apply to produce such an electromechanical transduction structure. A fifth embodiment of the transfer method is described below in relation to FIGS. 6A to 6C, for example for producing a PMUT or an energy recovery system.

(74) The second substrate 106 is first of all prepared to receive the layer or layers to be transferred from the first substrate 104. In the example described here, the first and second substrates 104, 106 are intended to be secured to one another by direct bonding, or molecular bonding. Thus, a first direct bonding layer 142 adapted for such a direct bonding is formed on the face of the second substrate 106 that is located on the side where the layer or layers will be transferred. Cavities 144 are subsequently produced through the first direct bonding layer 142 and a portion of the thickness of the second substrate 106 (see FIG. 6A). Said cavities 144 are intended to be located under the membranes of the electromechanical transduction structures that will subsequently be produced from the layers that will be transferred to the second substrate 106.

(75) The layers to be transferred are produced on the first substrate 104. As in the preceding embodiments, the interface of weak adhesion force is formed by the separating layers 108, 110 whereon are located the layer 114 that includes here a piezoelectric and/or pyroelectric and/or ferroelectric and/or dielectric material, the grip layer 116 comprising for example TiO.sub.2, the electrode layer 118 that includes for example platinum and that is intended to form one of the electrodes of the electromagnetic transduction elements (the other electrode being intended to be formed by the second separating layer 110), and a second direct bonding layer 146 adapted for the implementation of a direct bonding with the other direct bonding layer 142.

(76) The direct bonding layers 142, 146 include for example SiO2, or SiN, or Si, and have for example a thickness between approximately 1 nm and 100 μm.

(77) The two substrates 104, 106 are subsequently secured to one another by direct bonding between the direct bonding layers 142, 146, then mechanical separation is performed at the interface of weak adhesion force formed between the separating layers 108, 110. The two separate substrates 104, 106, with the various layers transferred to the second substrate 106, are shown in FIG. 6C. The layers 142, 146 bonded to one another are subsequently used to form the membranes whereon the electromagnetic transduction elements are produced from the layers 114, 110 and 136.

(78) According to one variant of this fifth embodiment, the securing between the substrates 104, 106 may be obtained by thermocompression by using metal bonding layers 122, 132 between the substrates 104, 106, as previously described in relation to FIGS. 2A-2B and 3A-3C. In this case, it is possible to not produce the electrode layer 118, and the metal bonding layer obtained by the implementation of the thermocompression may be intended to form one of the electrodes of the electromechanical transduction elements. Such a variant is described below in relation to FIGS. 7A to 7C.

(79) The second substrate 106 is first of all prepared to receive the layer or layers to be transferred from the first substrate 104. A metal bonding layer 132 is formed on the face of the second substrate 106 that is located on the side where the layer or layers will be transferred. In the example of embodiment described here, the metal bonding layer 132 includes gold or copper. Cavities 144 are subsequently produced through the first metal bonding layer 132 and a portion of the thickness of the second substrate 106 (see FIG. 7A).

(80) The layers to be transferred are produced on the first substrate 104. As in the preceding embodiments, the interface of weak adhesion force is formed by the separating layers 108, 110. On the example of embodiment shown in FIG. 7B, the first separating layer 108 includes platinum and the second separating layer 110 includes SiO.sub.2. In this variant, the second separating layer 110 is not intended to be used as an electrode and has a thickness for example between approximately 1 nm and 20 μm and is intended to form an elastic membrane of the devices produced subsequently from the stack of layers transferred to the second substrate 106. Alternatively, if the devices to be produced are not intended to have elastic membranes, the second separating layer 110 may be etched after the transfer to the second substrate 106.

(81) A grip layer 134 comprising for example TiO.sub.2 is produced on the second separating layer 110. An electrode layer 136 comprising for example platinum and that is intended to form an electrode of the electromechanical transduction elements subsequently produced is formed on the grip layer 134. The layer 114 of piezoelectric and/or pyroelectric and/or ferroelectric and/or dielectric material, another grip layer 148 (of thickness for example between approximately 5 nm and 10 nm and comprising W and/or TiN and/or Cr and/or Ru) and the other metal bonding layer 122, comprising for example gold or copper, are formed on the electrode layer 136 (FIG. 7B).

(82) The two substrates 104, 106 are subsequently secured to one another by thermocompression between the metal bonding layers 122, 132, then a separation is performed at the interface of the separating layers 108, 110. The two separate substrates 104, 106, with the various layers transferred to the second substrate 106, are shown in FIG. 7C. The electromechanical transduction elements are subsequently produced from the layers transferred to the second substrate 106. In the structure obtained, the metal bonding layers 122, 132 together form the lower electrode of each of the electromechanical transduction elements.

(83) According to one variant of the example previously described, the second separating layer 110 intended to form part of the membranes of the electromechanical transduction elements may be replaced by a stack comprising the second separating layer 108 (of thickness thinner than in the preceding example), a semiconductor layer, for example silicon, and a semiconductor oxide layer, for example SiO.sub.2. Said layers may in particular form a SiO.sub.2—Si—SiO.sub.2 stack intended to form part of the membranes of the electromechanical transduction elements, and whereof the thicknesses of said layers are chosen to adjust the overall stress of said membranes.

(84) The various examples and variants previously described may apply to the examples of embodiment described above wherein the cavities 144 are produced in the second substrate 106.

(85) The layer transfer method described here may advantageously apply to produce MIM-type capacitors or capacities. Such a transfer method according to a sixth embodiment is described below in relation to FIGS. 8A to 8C.

(86) The layers to be transferred are produced on the first substrate 104, for example made of silicon. As in the preceding embodiments, the interface of weak adhesion force is formed by the separating layers 108, 110. The first separating layer 108 comprising the SiO.sub.2 is disposed between the second separating layer 110 comprising the noble metal, here platinum, and the first substrate 104, so that the second separating layer 110 can be used to subsequently form one of the electrodes of each of the capacities or capacitors that will be produced from the layers transferred to the second substrate 106. The layer 114, that includes here a dielectric material, is produced on the second separating layer 110, then the electrode layer 118 is produced on the layer 114. A wide range of materials is available to form the electrode layer 118, such as for example Pt, Ru, RuO.sub.2, Ir, Au, Cu, etc. The layer 114 may include for example a dielectric material or else a piezoelectric and/or ferroelectric and/or pyroelectric material, according to the device intended to be produced (capacitor, transducer, etc.).

(87) The first substrate 104 and the layers produced thereon are transferred to the second substrate 106 which corresponds here to a substrate including at least one polymer material by means of the adhesive layer 112 (FIG. 8B). A rigid support may be disposed under the second substrate 106 in order to facilitate the implementation of said bonding.

(88) Separation is subsequently performed at the interface of the separating layers 108, 110. The second substrate 106 with the various layers transferred thereon is shown in FIG. 8C.

(89) In this sixth embodiment, the second transfer layer 110 is intended to form the upper electrode of the capacities or capacitors that will be produced from the layers transferred to the second substrate 106. Alternatively, it is possible to etch the second transfer layer 110 in order to replace it with another electrode material deposited on the layer 114.

(90) Such a sixth embodiment may be used to produce capacitors, for example for disconnection, transducers (actuator and/or sensor), vibration energy recovery devices, or also ferroelectric memories.

(91) According to one variant, it is possible to not produce the electrode layer 118. In this case, the layer 114 is disposed directly on the adhesive layer 112. Such a variant is for example implemented in order to produce coplanar capacities, obtained by subsequently etching the upper electrode layer (layer 110) in order to form the electrodes of said capacities that are located on a single face (upper face in FIG. 8C) of the layer 114.

(92) According to one variant, the securing between the first and second substrates 104, 106 may be performed by thermocompression bonding by forming the metal bonding layers 122, 132. In this case, said metal bonding layers 122, 132 may form together one of the electrodes of the capacities produced from the layers transferred to the second substrate 106. A grip layer may be formed on the layer 114 prior to the deposition of the first metal bonding layer 122. Similarly, another grip layer may be produced on the second substrate 106 prior to the deposition of the metal bonding layer 132. Said grip layers may in this case form part of the electrode that will be formed by the metal bonding layers 122, 132.

(93) According to another variant, direct bonding may be implemented between the direct bonding layers 142, 146 as previously described.

(94) According to another variant, the second substrate 106 may be used as flexible membranes for the devices produced from the layers transferred. In this case, the second substrate 106 is disposed on a mechanical support 150 that may be etched from the rear face thereof in order to release the membrane via the creation of a cavity 152 under the membrane. FIG. 9A shows a MIM capacity of a piezoelectric transducer device wherein the second substrate 106 forms a compliant/flexible membrane of said capacity. FIG. 9B shows a coplanar capacity of such a device wherein the second separating layer 110 is etched such that the remaining portions of said layer form the electrodes of the capacity (in FIG. 9B, no lower electrode is present between the layer 114 and the second substrate 106).

(95) In the embodiments and examples of embodiment described above, a single layer of piezoelectric material 114 is used. Alternatively, the transfer method may be implemented to form on the second substrate 106 the stack of layers including two superimposed piezoelectric layers, one of said two layers advantageously having a piezoelectric material adapted to provide a piezoelectric actuation, that is to say having a transversal piezoelectric coefficient d31 the highest possible, such as for example PZT, or La-doped PZT, Mn or Nb, or KNN, or BNT-BT, or PMN-PT, and the other of said two layers having a piezoelectric material adapted to provide the sensor function, that is to say having the best compromise between a high transversal piezoelectric coefficient e31 and a low dielectric permittivity, such as for example AlN or Sc-doped AlN. For this, each of said piezoelectric layers is produced on one of the two substrates 104, 106.

(96) Indeed, some piezoelectric MEMS/NEMS devices operate both in actuator mode and in sensor mode. This is the case of PMUT (Piezoelectric Micromachined Ultrasonic Transducers). They operate as actuators in order to transmit sound waves (transmission) and as sensors in order to receive the sound waves in return (reception). The PMUT devices integrate the same piezoelectric material that will make possible both the transmission of ultrasonic (US) waves and the reception of US waves in return. The electromagnetic transducers very often include in the form of type of membrane a drumhead, that is to say a membrane (elastic layer+so-called “motor” layer (piezoelectric or other)) suspended above a cavity.

(97) The transfer method proposed here is advantageous because such a stack of two different piezoelectric materials is difficult to obtain with the conventional methods of the prior art, due to technological constraints such as the thermal budget, the diffusion of elements during annealing that require the addition of barrier layers, a delicate crystal growth, the management of mechanical stresses, etc. When two layers of piezoelectric materials are produced, it is not necessary to form a Si, SiO.sub.2 or SiN based elastic or mechanical membrane because during actuator operation, the piezoelectric material that is used as a sensor plays the role of elastic membrane and during sensor operation, the piezoelectric material that is used as an actuator plays the role of elastic membrane.

(98) Such a transfer method implemented to provide a stack comprising two layers of piezoelectric materials is described below and in relation to FIGS. 10A to 10F.

(99) As shown in FIG. 10A, a portion of the stack, corresponding to the actuator portion, is formed on the second substrate 106. Said portion of the stack includes the insulating layer 124, the grip layer 134, the electrode layer 136, the second piezoelectric layer 114.2 that includes for example PZT, and the second metal bonding layer 132.

(100) The other portion of the stack, corresponding to the sensor portion, is produced on the first substrate 104. Said other portion of the stack includes the first and second separating layers 108, 110, the first piezoelectric layer 114.1 that includes for example AlN, and the other metal bonding layer 122 (FIG. 10B).

(101) The first and second substrates 104, 106 are secured to one another by a thermocompression bonding implemented between the metal bonding layers 122, 132.

(102) Mechanical separation is subsequently performed between the separating layers 108, 110. The second separating layer 110 remaining on the first piezoelectric layer 114.1 is eliminated, then a new electrode layer 154, comprising for example Mo is formed on the first piezoelectric layer 114.1 (FIG. 10C). In the structure thus obtained, the layers 154 and 136 as well as the metal bonding layers 122, 132 form the electrodes (the metal bonding layers form a common electrode used both by the sensor portion and by the actuator portion of the devices).

(103) Grip layers and/or layers forming diffusion barriers (for blocking for example the diffusion of gold coming from the metal bonding layers) may be disposed between the two metal bonding layers 122, 132 and the piezoelectric material layers 114.1, 114.2.

(104) The second substrate 106 and the layer 124 are subsequently etched from the rear face of the second substrate 102, forming the cavities 144 (a single cavity 144 is shown in FIG. 10D) under the portions of the stack intended to form the membranes of the devices. The other layers of the stack are subsequently etched according to the desired configuration for the devices (see FIG. 10E).

(105) Alternatively, it is possible that the cavities 144 are not formed through the layer 124, as is the case on the example in FIG. 10F. The layer 124 then forms an etching stop layer during the producing of cavities 144 and may also be used as elastic layer of the device(s) subsequently produced.

(106) According to another variant, before the securing of the two substrates 104, 106, the portion of the stack corresponding to the sensor portion (including the piezoelectric layer 114.2) may be produced on the second substrate 106 and the portion of the stack corresponding to the actuator portion (including the piezoelectric layer 114.2) may be produced on the first substrate 104. In this case, the electrode formed between the second substrate 106 and the piezoelectric layer 114.1 may include the molybdenum formed on a growth layer present on the insulating layer 124.

(107) According to another variant, the two portions of the stack produced on the two substrates 104, 106 may be secured to one another by means of direct bonding. The stack obtained at the end of such a securing is shown for example in FIG. 11. The layer 142+146 from the direct bonding may be used as elastic layer of the device(s) subsequently produced.

(108) The various examples of embodiment and the various alternative embodiments described may apply for each of the embodiments.