Method for producing a donor substrate for transferring a piezoelectric layer onto a support substrate

Abstract

The invention relates to a method for producing a donor substrate (1) for transferring a piezoelectric layer onto a support substrate, which comprises the following successive steps: • (a) providing a piezoelectric substrate (5) and a manipulation substrate (2); • (b) depositing a photo-polymerisable adhesive layer (6) on a main face of the manipulation substrate (2) or the piezoelectric substrate (5); • (c) bonding the piezoelectric substrate (5) with the manipulation substrate (2) via the adhesive layer (6) to form a heterostructure (7); • (d) irradiating the heterostructure (7) with a luminous flux to polymerise the adhesive layer (6); • (e) thermally treating the irradiated heterostructure (7); and • (f) thinning the piezoelectric substrate (5) by its face opposite the manipulation substrate (2), so as to form the donor substrate (1).

Description

[0001] DESCRIPTION

[0002] Method of manufacturing a donor substrate for transferring a piezoelectric layer onto a support substrate

[0003] TECHNICAL FIELD

[0004] The invention relates to a method of manufacturing a donor substrate for transferring a piezoelectric layer onto a support substrate, a method of transferring a piezoelectric layer onto such a support substrate and a method of manufacturing a bulk acoustic wave device comprising such a transfer. The invention finally relates to a donor substrate for transferring a piezoelectric layer.

[0005] STATE OF THE ART

[0006] The transfer of an active layer, i.e. intended for the formation of components for electronic, optical or optoelectronic devices, onto a support substrate via an electrically insulating layer, is widely used in the microelectronics industry.

[0007] An example of a well-known active layer transfer process is the Smart Cut™ process. This process involves forming a weakened zone by implanting atomic species into a donor substrate to delimit a layer of interest to be transferred, bonding the donor substrate to the support substrate, and then detaching the donor substrate along the weakened zone to transfer the layer of interest to the support substrate.

[0008] In some situations, it is not possible to directly bond the donor substrate to the support substrate. More particularly, the bonding of the donor substrate to the support substrate can be achieved by means of oxide layers previously formed on the surface of each of the two substrates. To strengthen the oxide-oxide bonding between the piezoelectric substrate and the support substrate, it is known to carry out, after bonding, a consolidation annealing. Said consolidation annealing is typically carried out at a temperature between 100 °C and 300 °C.

[0009] The Smart Cut™ process may also require annealing to achieve detachment of the layer of interest to be transferred, in a temperature range of 100°C to 600°C.

[0010] When the donor substrate and the donor substrate have different coefficients of thermal expansion - which is the case for example between a donor substrate made of a piezoelectric material and a silicon support substrate - such annealing causes significant deformation of the assembly of the two substrates, which is detrimental to the transfer insofar as it can induce breakage of the substrates.

[0011] To minimize such deformation, it is possible to form an intermediate substrate called a pseudo-donor substrate, in which the donor substrate is assembled on a temporary handling substrate (“handle substrate” in English).

[0012] The manufacturing process of the pseudo-donor substrate generally comprises several steps. Thus, a layer of thick piezoelectric material is bonded to the handling substrate, for example a silicon substrate. Then, the layer of piezoelectric material is thinned and possibly trimmed. Finally, the free surface of the thinned layer of piezoelectric material is polished, for example in a chemical-mechanical polishing (CMP) process, and possibly covered with a thin layer of oxide so as to achieve the oxide-oxide bonding previously described with the support substrate.

[0013] The bonding of the piezoelectric substrate and the manipulation substrate to form the pseudo-donor substrate could be achieved by means of oxide layers previously formed on the surface of each of the two substrates. However, the deposition of an oxide layer on the piezoelectric substrate causes a significant curvature ("bow" according to English terminology) of said piezoelectric substrate, which is not very compatible with the subsequent steps of the process. Furthermore, the formation of oxide layers necessary for bonding is long and expensive. Finally, as mentioned above, the heterostructure cannot be subjected to consolidation annealing due to the differences in thermal expansion coefficients between the piezoelectric substrate and the manipulation substrate. However, in the absence of consolidation annealing, the bonding energy of the oxide layers of the two substrates remains very low, so that the mechanical strength of the pseudo-donor substrate is insufficient.Therefore, failure at the bonding interface may occur during the thinning step of the thick piezoelectric substrate.

[0014] An interesting alternative to oxide-oxide bonding for assembling the heterostructure constituting the donor pseudo-substrate is the use of a photopolymerizable adhesive layer: the photopolymerizable adhesive layer is deposited on one side of the handling substrate or the piezoelectric material layer, the handling substrate is bonded with the piezoelectric material layer via the adhesive layer and then the heterostructure thus formed is irradiated by a light flux so as to polymerize the adhesive layer.

[0015] The photopolymerized adhesive layer provides the pseudo-donor substrate with good mechanical strength. It also eliminates the need for high-temperature process steps that can cause significant curvature of the substrate. Furthermore, the formation of such an adhesive layer is very simple to implement and inexpensive.

[0016] After bonding said pseudo-donor substrate and the support substrate, the piezoelectric substrate is held between the handling substrate and the support substrate. The choice of materials and thicknesses of the handling substrate and the support substrate ensures a certain symmetry of the thermal expansion coefficients, and thus minimizes the deformation of the assembly during the application of heat treatments.

[0017] However, when implementing such a method for manufacturing a piezoelectric-on-insulator structure, the applicants observe bonding imperfections between the donor pseudo-substrate and the support substrate, in the form of holes, called "edge bonding voids" in English, in which bonding does not occur at the periphery of the substrates.

[0018] BRIEF DESCRIPTION OF THE INVENTION

[0019] An aim of the invention is to design a donor pseudo-substrate for transferring a portion of a thinned piezoelectric layer onto a support substrate, the donor pseudo-substrate being formed by assembling the thinned piezoelectric layer and a handling substrate via a polymer adhesive layer, which makes it possible to improve the quality of the bonding between the thinned piezoelectric layer of the donor pseudo-substrate and the support substrate.

[0020] To this end, the invention proposes a method for manufacturing a donor substrate for transferring a piezoelectric layer onto a support substrate comprising the following successive steps:

[0021] (a) providing a piezoelectric substrate and a manipulation substrate,

[0022] (b) depositing a photopolymerizable adhesive layer on a main face of the handling substrate or the piezoelectric substrate,

[0023] (c) bonding the piezoelectric substrate with the manipulation substrate via the adhesive layer to form a heterostructure,

[0024] (d) irradiating said heterostructure with a light flux to polymerize the adhesive layer,

[0025] (e) heat treatment of the irradiated heterostructure,

[0026] (f) thinning the piezoelectric substrate by its face opposite the handling substrate, so as to form said donor substrate. The heat treatment of the heterostructure comprising the thick piezoelectric layer, the photopolymerized adhesive layer and the handling substrate makes it possible to increase the rigidity of said photopolymerized adhesive layer. The resulting donor pseudo-substrate has better mechanical strength and is less sensitive to the vibrations generated during the thinning step, which makes it possible to obtain a free surface of the thinned piezoelectric layer that is flatter, in particular at the periphery of the donor pseudo-substrate, so that the quality of the bonding to the support substrate is improved.

[0027] According to other optional features of the method of manufacturing a donor substrate for the transfer of a piezoelectric layer onto a support substrate taken alone or in combination when technically possible:

[0028] - the method for manufacturing a donor substrate further comprises a step (g) of chemical-mechanical polishing of the free surface of the thinned piezoelectric substrate;

[0029] - the heat treatment (e) is carried out so as to increase the degree of crosslinking of the polymer in the photopolymerized adhesive layer and / or the rigidity of said photopolymerized adhesive layer;

[0030] - the heat treatment (e) is carried out so that the Young's modulus of the adhesive layer after irradiation and heat treatment is between 0.05 GPa and 10 GPa, said Young's modulus being measured by nanoindentation;

[0031] - heat treatment (e) comprises the application of a temperature between 90°C and 110°C for a period between 1 hour and 12 hours, in a diazo atmosphere;

[0032] - the light flux is applied through the piezoelectric substrate;

[0033] - the luminous flux has a wavelength between 200 nm and 500 nm;

[0034] - the thickness of the photopolymerizable adhesive layer is between 1 pm and 50 pm and the irradiation energy received by said adhesive layer during the irradiation step is between 0.7 J / cm 2 and 10 J / cm 2 ;

[0035] - the deposition of the photo-polymerizable adhesive layer is carried out by centrifugal coating;

[0036] - the photopolymerizable adhesive layer comprises an isocyanurate, acrylate or epoxy glue crosslinkable by ultraviolet radiation with or without a hardening agent;

[0037] - the bonding step is carried out at a temperature between 10°C and 50°C and / or in which the irradiation step is carried out at a temperature between 10°C and 50°C. The invention also relates to a method for transferring a piezoelectric layer onto a support substrate comprising:

[0038] - the formation of a donor substrate by implementing the method of manufacturing a donor substrate as previously described,

[0039] - the formation of a weakening zone in the thinned piezoelectric substrate so as to delimit the piezoelectric layer to be transferred,

[0040] - the supply of the support substrate,

[0041] - bonding said donor substrate to the support substrate, the piezoelectric layer to be transferred being located at the bonding interface,

[0042] - detachment of the donor substrate along the weakening zone so as to transfer the piezoelectric layer to be transferred onto the support substrate.

[0043] According to other optional characteristics of the method of transferring a piezoelectric layer onto a support substrate taken alone or in combination when technically possible:

[0044] - the method comprises, before bonding, the formation of an oxide layer, or a nitride layer, or a layer comprising a combination of nitride and oxide, or a superposition of at least one oxide layer and one nitride layer on the support substrate;

[0045] - the formation of the weakening zone is carried out by implantation of atomic species in the piezoelectric substrate;

[0046] - the handling substrate and the support substrate are made of materials such that the difference in coefficient of thermal expansion between the material of the handling substrate and the support substrate is less than or equal to 5%, preferably approximately equal to 0%.

[0047] The invention also relates to a method for manufacturing a bulk acoustic wave device comprising the deposition of electrodes on two opposite faces of a piezoelectric layer, characterized in that it comprises the manufacturing of said piezoelectric layer by a method of transferring a piezoelectric layer onto a support substrate as previously described.

[0048] The invention finally relates to a donor substrate for the transfer of a piezoelectric layer, consisting of a heterostructure comprising a piezoelectric substrate bonded to a handling substrate, said substrate being characterized in that it comprises, at the interface between the piezoelectric substrate and the handling substrate, a polymerized adhesive layer whose Young's modulus is between 0.05 GPa and 10 Gpa.

[0049] According to other optional characteristics of the donor substrate for the transfer of a piezoelectric layer taken alone or in combination when technically possible:

[0050] - the thickness of the polymerized adhesive layer is between 1 pm and 50 pm;

[0051] - the polymerized adhesive layer comprises an isocyanurate, acrylate or epoxy glue crosslinkable by ultraviolet radiation with or without a hardening agent.

[0052] BRIEF DESCRIPTION OF THE FIGURES

[0053] Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which:

[0054] - Figure 1A represents a topology profile produced using a mechanical profilometer and showing the undulations present on the free surface of a donor substrate having been subjected to a heat treatment according to an embodiment of the invention, Figure 1B represents a topology profile produced using a mechanical profilometer and showing the undulations present on the free surface of a manufactured donor substrate not having been subjected to said heat treatment (said profiles are presented on the same scale);

[0055] - Figure 2 represents a sectional view of a donor substrate manufactured according to an embodiment of the method according to the invention comprising from its rear face to its front face: a handling substrate, a photopolymerized adhesive layer and a thinned piezoelectric layer;

[0056] - Figure 3 represents a sectional view of a handling substrate and a piezoelectric substrate provided for implementing the method of the invention;

[0057] - figure 4 represents a sectional view of a step of depositing a photopolymerizable adhesive layer on the handling substrate according to an embodiment of the method according to the invention;

[0058] - Figure 5 represents a sectional view of the multilayer structure obtained after a step of bonding the piezoelectric substrate and the handling substrate by means of a photopolymerizable adhesive layer according to an embodiment of the method according to the invention;

[0059] - Figure 6 represents a sectional view of a step of irradiation of the multilayer structure of Figure 5; - Figure 7 represents a sectional view of the multilayer structure obtained after irradiation then heat treatment of the structure represented in Figure 5 according to an embodiment of the method according to the invention;

[0060] - figure 8 represents a sectional view of a step of thinning the piezoelectric substrate in the multilayer structure of figure 7 according to an embodiment of the method according to the invention;

[0061] - figure 9 represents a sectional view of a support substrate for the transfer of a piezoelectric layer to be transferred;

[0062] - figure 10 represents a sectional view of a step of forming a weakening zone within the piezoelectric layer thinned by implantation of atomic species so as to delimit a piezoelectric layer to be transferred according to an embodiment of the method of transferring a piezoelectric layer onto a support substrate according to the invention;

[0063] - figure 11 represents a sectional view of a step of bonding the implanted support substrate of figure 10 with the support substrate of figure 9, the piezoelectric layer to be transferred being at the interface;

[0064] - Figure 12 represents a sectional view of the multilayer structure obtained after the transfer of the piezoelectric layer to be transferred onto the support substrate by detachment along the weakening zone.

[0065] - Figure 13 represents a sectional view of a bulk acoustic wave device.

[0066] For readability reasons, the drawings are not necessarily drawn to scale.

[0067] DETAILED DESCRIPTION OF EMBODIMENTS

[0068] The invention relates to a method for manufacturing a pseudo-donor substrate for transferring a piezoelectric layer onto a support substrate, said pseudo-donor substrate being formed by assembling a thick piezoelectric layer and a handling substrate via a photopolymerized adhesive layer and then thinning said thick piezoelectric layer, for example by grinding.

[0069] The multilayer structures resulting from the transfer of the piezoelectric layer from such a donor pseudo-substrate to a support substrate have numerous holes at their periphery, at their bonding interface, which reduce the quality of the bonding between the transferred active layer and the receiving substrate. Following the step of thinning the thick piezoelectric layer, the inventors observed that the free surface of the thinned piezoelectric layer of the donor pseudo-substrate has at its periphery, over a width of approximately 5 mm, a relief in the form of hollows and bumps ("wavyness" according to the Anglo-Saxon term visible in Figure 1A).

[0070] The inventors hypothesize that it is this peripheral relief that generates the holes in the final multi-layer structure.

[0071] Furthermore, the inventors assume that this relief in the form of hollows and bumps is formed due to the entry into resonance of said thick piezoelectric layer under the effect of the vibrations generated by the thinning process, and note that this relief is all the more marked as the mechanical strength of the piezoelectric layer - handling substrate assembly is weak.

[0072] For this purpose, the invention relates to a method for manufacturing a donor substrate for transferring a piezoelectric layer onto a support substrate. An example of a donor substrate 1 according to the invention is shown in Figure 2. The donor substrate 1 has a multilayer structure comprising, from its rear face to its front face:

[0073] - a handling substrate 2,

[0074] - a photopolymerized adhesive layer 3,

[0075] - a thinned piezoelectric substrate 4.

[0076] For example, the thinned piezoelectric substrate 4 is made of a material such as lithium tantalate (LiTaOs), lithium niobate (LiNbOs), barium titanate (BaTiOs) and / or lead zirconate titanoate (PZT). The piezoelectric layer 4 has a thickness of between 50 nm and 20 pm, preferably a thickness of between 100 nm and 10 pm.

[0077] The piezoelectric material of the piezoelectric substrate 4 and the material of the support substrate have very different coefficients of thermal expansion. Depositing a layer of piezoelectric material without a handling substrate on the support substrate would expose the resulting multilayer structure to significant deformations during the implementation of thermal annealing, for example to reinforce the bonding interface between the layer of piezoelectric material and the support substrate. The handling substrate 2 is therefore preferably manufactured from a material whose coefficient of thermal expansion is close to that of the material of the support substrate on which the thinned piezoelectric substrate 4 is intended to be bonded. By close, we mean a difference in coefficient of thermal expansion between the material of the handling substrate 2 and the material of the support substrate less than or equal to 5%, and preferably equal to or close to 0%.Suitable materials are, for example, silicon, sapphire, polycrystalline aluminum nitride, or gallium arsenide. Preferably, the handling substrate 2 is made of the same material as the support substrate. In the present invention, the coefficient of thermal expansion in a plane parallel to the main surface of the substrates is of interest. The handling substrate 2 has a thickness of between 100 μm and 2 mm, preferably a thickness of between 200 μm and 1 mm. Preferably, the handling substrate 2 has a thickness close to that of the support substrate, so that the structure obtained after bonding the donor substrate to the support substrate is as symmetrical and balanced as possible in terms of mechanical and thermal behavior.A coefficient of thermal expansion and a thickness of the handling substrate 2 close respectively to the coefficient of thermal expansion and the thickness of the support substrate make it possible to minimize the stresses on the multilayer structure and its deformation under the effect of temperature variations.

[0078] Referring to Figure 3, the method of manufacturing the donor substrate comprises providing a piezoelectric substrate 5 and the manipulation substrate 2.

[0079] The piezoelectric substrate 5 has a thickness of between 100 pm and 2 mm, preferably a thickness of between 200 pm and 1 mm. The piezoelectric substrate 5 is formed from the piezoelectric material which constitutes the thinned piezoelectric layer 4 in the final support substrate 10. The piezoelectric substrate 5 may therefore comprise LiTaCh, LiNbOs, BaTiCh and / or PZT.

[0080] The method for manufacturing the donor substrate according to the invention further comprises the deposition of a photopolymerizable adhesive layer 6 on a main face of the handling substrate 2 or of the piezoelectric substrate 5. By way of example, Figure 4 represents a particular embodiment in which the photopolymerizable adhesive layer 6 is deposited on the handling substrate 2. The deposition of the photopolymerizable adhesive layer is advantageously carried out by centrifugal coating, or "spin coating" according to the English terminology.

[0081] This technique consists of rotating the substrate on which the photopolymerizable layer is intended to be deposited on itself at a substantially constant and relatively high speed, in order to spread said photopolymerizable layer uniformly over the entire surface of the substrate by centrifugal force. For this purpose, the substrate is typically placed and held by vacuum on a rotating plate. Those skilled in the art are able to determine the operating conditions, such as the volume of adhesive deposited on the surface of the substrate, the rotation speed of the substrate, and the minimum deposition time depending on the desired thickness of the adhesive layer. Deposition by spin coating advantageously makes it possible to control the uniformity of the thickness of the deposited photopolymerizable adhesive layer. The thickness of the deposited photopolymerizable adhesive layer is typically between 1 and 50 μm.A thickness of less than 1 pm is not achievable with a photopolymerizable adhesive layer that is too viscous. In addition, below a thickness of less than 1 pm, there is a risk that in some areas the thickness of the photopolymerizable adhesive layer will be almost zero. A thickness greater than 50 pm is not achievable with a photopolymerizable adhesive layer that is too liquid. In addition, since the photopolymerized polymer is softer than the other layers, a thickness greater than 50 pm risks affecting the good mechanical strength of the donor substrate.

[0082] The photopolymerizable adhesive layer 6 preferably comprises an isocyanurate, acrylate or epoxy adhesive crosslinkable by ultraviolet with a curing agent or not. For example, the photopolymerizable adhesive layer 6 comprises two isocyanurate monomers and a thiol derivative. Such an adhesive has the advantage of being polymerizable at wavelengths at which the piezoelectric material is at least partially transparent. In addition, it has good temperature resistance up to 250°C.

[0083] Bonding with a photopolymerizable adhesive layer has the advantage of involving fewer manufacturing steps than molecular bonding. It is therefore simple to implement and inexpensive. In addition, the photopolymerized adhesive layer provides the pseudo-donor substrate with good mechanical strength without requiring high-temperature thermal annealing, which could cause significant curvature of the substrate, which is not the case with molecular bonding. Finally, the polymer, initially liquid, will fill in the flatness defects and partially compensate for the edge drop due to the chamfering of the substrates. Bonding with a photopolymerizable adhesive layer therefore makes it possible to bond the substrates closer to their edge.

[0084] With reference to Figure 5, after the deposition of the photopolymerizable adhesive layer on a main face of the handling substrate 2 or the piezoelectric substrate 5, the piezoelectric substrate 5 is bonded to the handling substrate 2 via the photopolymerizable adhesive layer 6 to form a heterostructure 7. The bonding is preferably carried out at room temperature, i.e. approximately 20°C. It is however possible to carry out the bonding at a temperature between 10°C and 50°C, more preferably between 20°C and 30°C, preferably still at a temperature close to room temperature. Indeed, the further the bonding temperature is from room temperature, the more the heterostructure 7 will deform when it returns to its equilibrium temperature.In addition, the bonding step is advantageously carried out under vacuum, which makes it possible to desorb water from the surfaces forming the bonding interface, i.e. the surface of the adhesive layer and the surface of the handling substrate or the piezoelectric substrate.

[0085] Then, with reference to Figure 6, said heterostructure 7 is irradiated with a light flux to polymerize the adhesive layer.

[0086] For example, the light flux can be applied through the piezoelectric substrate 5 if said piezoelectric substrate 5 is at least partially transparent in the wavelength range making it possible to initiate the polymerization of the photopolymerizable adhesive layer 6. For example again, the light flux is applied through the handling substrate 2 if said handling substrate 2 is at least partially transparent in the wavelength range making it possible to initiate the polymerization of the photopolymerizable adhesive layer 6.

[0087] Irradiation is preferably carried out at a temperature between 10°C and 50°C, preferably still at a temperature close to room temperature. Indeed, the further the temperature is from room temperature, the more the heterostructure 7 will deform when it returns to its equilibrium temperature.

[0088] The irradiation energy received by the photopolymerizable adhesive layer 6 is advantageously between 0.7 J / cm 2 and 10 J / cm 2 . In the case where the irradiation of the photopolymerizable adhesive layer 6 is carried out through the handling substrate 2 or through the piezoelectric substrate 5, the absorption coefficient at the irradiation wavelength and the thickness of the material crossed should be taken into account to determine the irradiation power actually received by the photopolymerizable adhesive layer 6.

[0089] The wavelength of the incident light flux is chosen according to the nature of the photopolymerizable adhesive layer 6 and the material crossed by said incident light flux (handling substrate 2 or piezoelectric substrate 5). Indeed, the photopolymerizable adhesive layer 6 must absorb at the wavelength and said absorption must initiate polymerization. In addition, the material crossed by the incident light flux must not be completely absorbed by the material crossed, so that it would not be possible to irradiate the photopolymerizable adhesive layer 6 with the necessary power. Preferably, the light flux has a wavelength between 200 nm and 500 nm. Indeed, it is in this wavelength range that the piezoelectric substrate is advantageously the most transparent.

[0090] Irradiation of the heterostructure 7 comprising the thick piezoelectric layer 5, the photopolymerizable adhesive layer 6 and the handling substrate 2 allows the chain polymerization reaction of the monomer to be initiated. The inventors assume that the polymer chains grow until the steric hindrance is greater than the thermal agitation, and prevents the active sites from coming together, thus blocking the polymerization reaction. The steric hindrance thus limits the degree of crosslinking of the polymer chains, and therefore the rigidity of the photopolymerized adhesive layer, achievable following simple irradiation of the heterostructure.

[0091] After irradiation of the heterostructure by a light flux, the method according to the invention comprises an additional step of heat treatment of the irradiated heterostructure, so as to obtain the photopolymerized adhesive layer 3 as shown in Figure 7.

[0092] The heat treatment according to the invention of the irradiated heterostructure prior to the thinning of the thick piezoelectric layer makes it possible to increase the thermal agitation within the adhesive layer and to re-align active sites on the polymer chains so as to continue the crosslinking of the polymer initiated by the irradiation. The heat treatment therefore makes it possible to achieve a degree of crosslinking of the polymer greater than that achievable by simple irradiation, whatever the irradiation dose used, and therefore a greater rigidity of the photopolymerized adhesive layer. The pseudo-donor substrate thus has better mechanical strength and less sensitivity to vibrations from the thinning process: the free surface of the pseudo-donor substrate is flatter at the end of the thinning step.

[0093] The heat treatment of the previously irradiated heterostructure is preferably carried out so that the Young's modulus of the adhesive layer after irradiation and heat treatment is between 0.05 GPa and 10 GPa, preferably between 3.5 GPa and 10 GPa.

[0094] The Young's modulus is measured by nano-indentation: a tip of calibrated shape and size made of a hard material, for example a diamond tip, is applied with a force of a given intensity to the surface of the material whose Young's modulus is to be evaluated. The Young's modulus is then evaluated as a function of the size of the imprint left in the material.

[0095] The nano-indentation for measuring the Young's modulus of the photopolymerized adhesive layer 3 is carried out on a multilayer structure comprising a test photopolymerized adhesive layer deposited on a handling substrate without a thick piezoelectric layer (or conversely on a thick piezoelectric substrate without a handling substrate), said test photopolymerized adhesive layer having been prepared under the same conditions as the photopolymerized adhesive layer 6. More specifically, the preparation of the test photopolymerized adhesive layer comprises the deposition of a test photopolymerizable adhesive layer and the irradiation and then the heat treatment of the deposited test photopolymerizable adhesive layer, the deposition, irradiation and heat treatment conditions applied to the test photopolymerizable adhesive layer being identical to those applied to the photopolymerizable adhesive layer 6.By identical irradiation conditions is meant that the irradiation power received by the test photopolymerizable adhesive layer is identical to the irradiation power received by the photopolymerizable adhesive layer 6. Since the irradiation of the photopolymerizable adhesive layer 6 is carried out through the handling substrate 2 or through the piezoelectric substrate 5, the absorption coefficient at the irradiation wavelength and the thickness of the material crossed should be taken into account to determine the irradiation power to be applied to the test photopolymerizable adhesive layer. A Young's modulus of the photopolymerized adhesive layer of between 0.05 Ga and 10 GPa advantageously makes it possible to reduce the number of holes or "edge bonding voids" at the bonding interface between the donor substrate comprising such a photopolymerized adhesive layer 3 and a support substrate.For example, a Young's modulus of the photopolymerized adhesive layer 3 greater than 3.5 GPa makes it possible to divide by five the number of holes or "edge bonding voids" relative to a bonding interface between a support substrate and a donor substrate comprising a photopolymerized adhesive layer which has not been subjected to the heat treatment according to the invention.

[0096] A simple irradiation of the heterostructure 7 does not make it possible to achieve a Young's modulus greater than 3.5 GPa, even by increasing the irradiation power received up to 400 mW. In addition, too great an increase in the irradiation power, due to the fact that the piezoelectric substrate 5 absorbs part of the insolation, creates a significant risk of deformation of said piezoelectric substrate 5.

[0097] According to a particularly advantageous embodiment of the heat treatment of the irradiated heterostructure, said heat treatment comprises the application to said heterostructure of a temperature of between 90°C and 110°C for a duration of between 1 hour and 12 hours, preferably between 1 hour and 5 hours, more preferably between 1 hour and 3 hours in a nitrogen atmosphere. Such a temperature range advantageously makes it possible to obtain a Young's modulus of the photopolymerized adhesive layer greater than 3 GPa after 3 hours, and a maximum Young's modulus of the order of 10 GPa after 5 hours. Such durations advantageously make it possible to maintain a reasonable process time for the production of plates in volume.

[0098] A temperature below 110°C advantageously makes it possible to avoid the risk of deformation of the structure due to the very different thermal expansion coefficients between the piezoelectric substrate 5 and the handling substrate 2.

[0099] After the heat treatment of the heterostructure 7, the piezoelectric substrate 5 is thinned by its face opposite the handling substrate, as shown in FIG. 8, so that the thinned piezoelectric substrate 4 has a thickness of between 1 μm and 100 μm, preferably a thickness of between 5 μm and 50 μm. The thinning of the handling substrate 5 is for example carried out by coarse grinding, which makes it possible to quickly reduce the thickness of the heterostructure. Then, finer grinding can be implemented to continue to reduce the thickness of the heterostructure, but by reducing the roughness of the surface of the final donor substrate 1.

[0100] Finally, chemical mechanical polishing (CMP) can be carried out to smooth the free surface of the thinned piezoelectric substrate 4 opposite the handling substrate 2, so as to achieve the desired roughness for bonding the donor substrate 1 to the support substrate 12 and thus improve the bonding quality.

[0101] Supplementing the irradiation of the photopolymerizable adhesive layer 6 with a heat treatment according to the method of the invention advantageously makes it possible to increase the mechanical strength of the donor substrate. The better mechanical strength of the donor substrate provides said donor substrate with better resistance to the vibrations generated by the grinding and polishing steps, so that the relief observed at the periphery of the thinned piezoelectric layer made of hollows and bumps is much less marked. Thus, the free surface of the thinned piezoelectric layer is flatter at the edge of the plate, which allows better quality bonding of the donor substrate to the support substrate. Figure 1B represents the topology profile produced using a mechanical profilometer, on the free surface of a donor substrate having undergone the heat treatment according to an embodiment of the invention.The amplitude of the undulations is therefore reduced in comparison with the profile of Figure 1A representing the topology profile on the free surface of a donor substrate which has not undergone said heat treatment.

[0102] The invention also relates to a method of transferring a piezoelectric layer onto a support substrate.

[0103] A donor substrate comprising the piezoelectric layer to be transferred is initially provided. The donor substrate is preferably obtained by the manufacturing method previously described according to the first subject of the invention.

[0104] With reference to Figure 9, a support substrate 8 is also provided capable of receiving the piezoelectric layer to be transferred. Preferably, the handling substrate 2 and the support substrate 8 are made of materials such that the difference in thermal expansion coefficient between the material of the handling substrate 2 and the support substrate 8 is less than or equal to 5%, preferably approximately equal to 0%.

[0105] A weakening zone is formed in the thinned piezoelectric substrate 4 so as to delimit a piezoelectric layer to be transferred 9. The depth of the weakening zone relative to the exposed surface of the thinned piezoelectric substrate 4 determines the thickness of the piezoelectric layer to be transferred 9.

[0106] According to a preferred embodiment shown in Figure 10, the weakening zone is formed by implantation of atomic species in the thinned piezoelectric substrate, the implantation being represented in Figure 10 by the black arrows. The atomic species are implanted at a determined depth of the thinned piezoelectric substrate 4 which determines the thickness of the piezoelectric layer to be transferred 9.

[0107] When the embrittlement zone is formed by implantation of atomic species, the implanted atomic species are preferably hydrogen ions and / or helium ions.

[0108] According to one embodiment, an oxide layer, or a nitride layer, or a layer comprising a combination of nitride and oxide, or a superposition of at least one oxide layer and one nitride layer on the support substrate (not shown) is then formed. Such an oxide or nitride layer or one comprising a combination of nitride and oxide advantageously makes it possible to improve the bonding energy between the two substrates.

[0109] With reference to Figure 11, the donor substrate is then bonded to the support substrate 8, the piezoelectric layer to be transferred 9 and the possible dielectric layer being located at the bonding interface.

[0110] The formed multilayer structure then successively comprises, from a rear face to a front face, the support substrate 8, the possible dielectric layer, the thinned piezoelectric substrate 4, the photopolymerized layer 3 and the handling substrate 2.

[0111] The formed multilayer structure has few holes on the periphery of the structure at the bonding interface between the donor substrate and the support substrate, ("edge bonding voids" in English), the detection of said holes being carried out by laser detection. Such an effect is advantageously obtained because the donor substrate according to the invention has a very low relief on the free surface of the piezoelectric layer thinned prior to the bonding of said donor substrate with the support substrate.

[0112] With reference to Figure 12, the donor substrate is then detached along the weakening zone so as to transfer the piezoelectric layer to be transferred 9 onto the support substrate 8.

[0113] Detachment along the embrittlement zone may be triggered by mechanical action and / or thermal energy input. The thermal energy input may include annealing in a furnace at a temperature between 150°C and 300°C, preferably between 150°C and 220°C to prevent degradation of the polymer.

[0114] Finally, due to a low number of holes at the bonding interface between the donor substrate and the piezoelectric layer, a better transfer of the piezoelectric layer onto the support substrate at the edge of the plate is obtained.

[0115] The invention extends to a method of manufacturing a bulk acoustic wave device comprising the deposition of electrodes on two opposite faces of a piezoelectric layer, characterized in that it comprises the manufacturing of said piezoelectric layer by a method of transferring the piezoelectric layer onto a support substrate according to any one of the embodiments previously described.

[0116] More precisely, before bonding the piezoelectric layer to be transferred 9 from the donor substrate to the support substrate 8, a first electrode is deposited on the free surface of said layer 9, this first electrode (referenced 10 in FIG. 13) being buried in the final stack. After the step of transferring the piezoelectric layer 9 from the donor substrate to the support substrate 8, a second electrode (referenced 11 in FIG. 13) is deposited on the free surface of the layer 9, opposite the first electrode. To prevent the propagation of acoustic waves in the support substrate 8, it is possible to integrate therein an isolation means which may be, for example, a Bragg mirror 12 (as illustrated in FIG. 13) or a cavity previously etched in the support substrate 8.

Claims

CLAIMS 1. Method for manufacturing a donor substrate (1) for transferring a piezoelectric layer onto a support substrate comprising the following successive steps: (a) providing a piezoelectric substrate (5) and a manipulation substrate (2), (b) depositing a photopolymerizable adhesive layer (6) on a main face of the handling substrate (2) or the piezoelectric substrate (5), (c) bonding the piezoelectric substrate (5) with the handling substrate (2) via the adhesive layer (6) to form a heterostructure (7), (d) irradiating said heterostructure (7) with a light flux to polymerize the adhesive layer (6), (e) heat treatment of the irradiated heterostructure (7), (f) thinning the piezoelectric substrate (5) by its face opposite the handling substrate (2), so as to form said donor substrate (1).

2. Method according to the preceding claim, further comprising a step (g) of chemical-mechanical polishing of the free surface of the thinned piezoelectric substrate (4).

3. Method according to one of the preceding claims, in which the heat treatment (e) is carried out so as to increase the degree of crosslinking of the polymer in the photopolymerized adhesive layer (3) and / or the rigidity of said photopolymerized adhesive layer (3).

4. Method according to one of the preceding claims, in which the heat treatment (e) is carried out so that the Young's modulus of the adhesive layer (3) after irradiation and heat treatment is between 3.5 GPa and 10 GPa, said Young's modulus being measured by nanoindentation.

5. Method according to one of the preceding claims, in which the heat treatment (e) comprises the application of a temperature between 90°C and 110°C for a duration between 1 hour and 12 hours, in a nitrogen atmosphere.

6. Method according to one of the preceding claims, in which the light flux is applied through the piezoelectric substrate (5).

7. Method according to one of the preceding claims, in which the luminous flux has a wavelength between 200 nm and 500 nm.

8. Method according to one of the preceding claims, in which the thickness of the photopolymerizable adhesive layer (6) is between 1 pm and 50 pm and the irradiation energy received by said adhesive layer (6) during the irradiation step is between 0.7 J / cm 2 and 10 J / cm 2 .

9. Method according to one of the preceding claims, in which the deposition of the photopolymerizable adhesive layer (6) is carried out by centrifugal coating.

10. Method according to one of the preceding claims, in which the photopolymerizable adhesive layer (6) comprises an isocyanurate, acrylate or epoxy glue crosslinkable by ultraviolet radiation with or without a hardening agent.

11. Method according to one of the preceding claims, in which the bonding step is carried out at a temperature between 10°C and 50°C and / or in which the irradiation step is carried out at a temperature between 10°C and 50°C.

12. Method for transferring a piezoelectric layer onto a support substrate comprising: - the formation of a donor substrate (1) by implementing the method according to any one of the preceding claims, - the formation of a weakening zone in the thinned piezoelectric substrate (4) so ​​as to delimit the piezoelectric layer to be transferred (9), - the supply of the support substrate (8), - bonding said donor substrate (1) to the support substrate (8), the piezoelectric layer to be transferred (9) being located at the bonding interface, - detaching the donor substrate (1) along the weakening zone so as to transfer the piezoelectric layer to be transferred (9) onto the support substrate (8).

13. Method according to the preceding claim, comprising, before bonding, the formation of an oxide layer, or a nitride layer, or a layer comprising a combination of nitride and oxide, or a superposition of at least one oxide layer and one nitride layer on the support substrate (8).

14. Method according to one of claims 12 or 13, in which the formation of the weakening zone is carried out by implantation of atomic species in the thinned piezoelectric substrate (4).

15. Manufacturing method according to one of claims 12 to 14, in which the handling substrate (2) and the support substrate (8) are manufactured from materials such that the difference in coefficient of thermal expansion between the material of the handling substrate (2) and the support substrate (8) is less than or equal to 5%, preferably approximately equal to 0%.

16. Method for manufacturing a bulk acoustic wave device comprising the deposition of electrodes on two opposite faces of a piezoelectric layer, characterized in that it comprises the manufacturing of said piezoelectric layer by a method according to one of claims 12 to 15.

17. Donor substrate for the transfer of a piezoelectric layer, consisting of a heterostructure comprising a piezoelectric substrate bonded to a handling substrate (2), said substrate being characterized in that it comprises, at the interface between the piezoelectric substrate and the handling substrate, a polymerized adhesive layer (3) whose Young's modulus is between 3.5 GPa and 10 Gpa.

18. Substrate according to the preceding claim, in which the thickness of the polymerized adhesive layer (3) is between 1 μm and 50 μm.

19. Substrate according to one of claims 17 or 18, in which the polymerized adhesive layer (3) comprises an isocyanurate, acrylate or epoxy glue crosslinkable by ultraviolet radiation with or without a hardening agent.

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