METHOD FOR MANUFACTURING A LAYER

Abstract

The invention relates to a method for manufacturing a layer of composition AA'BO3 where A consists of at least one element from: Li, Na, K, H, Ca, Mg, Ba, Sr, Pb, La, Bi, Y, Dy, Gd, Tb, Ce, Pr, Nd, Sm, Eu, Ho, Zr, Sc, Ag, Tl and B consists of at least one element from: Nb, Ta, Sb, Ti, Zr, Sn, Ru, Fe, V, Sc, C, Ga, Al, Si, Mn, Zr, Tl, characterized in that it comprises: - the provision of a donor substrate (100) of composition AA'BO3, - the formation of a layer (10) of composition AA'BO3 by thinning said donor substrate (100), - before and / or after the thinning step, the exposure of said layer (10) of composition AA'BO3 to a medium (M) containing ions of an element A' belonging to the same list of elements as A or being hydrogen, A' being different from A, so as to make said ions penetrate said layer (10) to form a layer of composition AA'BO3.

Description

According to one embodiment, the medium containing the ions of element A' is a liquid and the layer is immersed in a bath of said liquid. For example, the layer of composition ABO3 may be immersed in a bath comprising an acidic solution of a salt comprising element A'. According to another embodiment, the medium containing the ions of element A' is in the gas phase and the layer is exposed to said gas. According to another embodiment, the medium containing the ions of element A' is in solid phase, a layer of said medium being deposited on the layer of composition ABO3. Advantageously, the method comprises at least one annealing step to promote the diffusion of element A' from the medium towards the layer of composition ABO3. According to one embodiment of the invention, the layer of composition ABO3 is monocrystalline. According to one embodiment, thinning the donor substrate comprises applying a recipient substrate to the donor substrate, the ABO3 composition layer being at the interface, and transferring said layer to the recipient substrate. Applying the recipient substrate may include depositing said substrate onto the donor substrate. Alternatively, applying the recipient substrate comprises bonding said substrate to the donor substrate. The thickness of the layer transferred onto the receiving substrate is advantageously less than 20 μm. According to a particular embodiment, at least one electrically insulating layer and / or at least one electrically conductive layer is formed at the interface between the receiving substrate and the layer. According to one embodiment, the thickness of the layer of composition ABO3 is greater than 2 μm, preferably greater than 20 μm, said layer being self-supporting following thinning of the donor substrate. Thinning of the donor substrate may include the following steps: - the formation of a weakening zone in the donor substrate so as to delimit the ABO3 composition layer, - detachment of the donor substrate along the weakening zone. Said weakening zone is advantageously formed by ion implantation in the donor substrate. According to a particular embodiment, element A is lithium and element A' is sodium and / or potassium. Element B can then advantageously be niobium and / or tantalum. The invention also relates to a method of manufacturing a bulk acoustic wave device comprising forming electrodes on two main faces opposite sides of a piezoelectric layer, characterized in that it comprises the manufacture of said piezoelectric layer by a method as described above. The invention also relates to a method of manufacturing a surface acoustic wave device, comprising the formation of two interdigitated electrodes on the surface of a piezoelectric layer, characterized in that it comprises the manufacture of said piezoelectric layer by a method as described above. BRIEF DESCRIPTION OF THE DRAWINGS Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings in which: Figures 1A to 1E schematically illustrate steps of the method for manufacturing a monocrystalline layer of composition AA'BO3 according to one embodiment of the invention, Figure 2 is a sectional view of a surface acoustic wave filter, Figure 3 is a sectional view of a bulk acoustic wave filter. For reasons of readability of the figures, the elements illustrated are not necessarily represented to scale. Furthermore, elements designated by the same reference signs in different figures are identical. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Generally, a layer of composition ABO3 is formed by thinning a donor substrate of composition ABO3. Said thinning can be carried out by any appropriate technique, among which we can cite the Smart Cut™ process by which we form, by implantation of ionic species in the donor substrate, a weakening zone delimiting the layer of interest; any other technique for forming the weakening zone that can be used; or the implementation of one or more steps of etching the donor substrate so as to retain only the layer of interest. If the layer is sufficiently thick, it can be self-supporting, i.e. it does not need to be applied to a stiffening substrate to provide sufficient mechanical strength. Alternatively, particularly if the layer is thin, the thinning of the donor substrate is preceded by a step of applying a receiving substrate to the donor substrate. The application of said receiving substrate may be carried out in particular by bonding or by deposition on the donor substrate. From the layer of composition ABO3 thus obtained, a layer of more complex composition of type AA'BO3 is obtained, in which A' belongs to the same list of elements as A but is different from A or is hydrogen. Obtaining such a layer involves exposing the layer of composition ABO3 to a medium comprising ions of the element A', so as to cause at least some of said ions to penetrate into the layer. The conversion of the layer of initial composition ABO3 into a layer of composition AA'BO3 advantageously involves an ion exchange mechanism which is used in the field of glasses. For example, it is known to replace a portion of the Na+ ions present in certain glasses with Ag+ or K+ ions, by implementing a treatment consisting of immersing the glass in a bath of molten salts, for example AgNO3 or KNO3, respectively. Similarly, the invention can be implemented by exposing the layer to be converted into a layer of more complex composition to a medium containing ions of the element A' with which it is desired to enrich said layer, said medium possibly being liquid (for example a bath of an acid solution of a salt comprising the element A'), gaseous or solid). With reference to Figures 1A to 1D, a method of manufacturing a layer of composition AA'BO3 according to an embodiment implementing the Smart Cut™ method is considered, comprising the following steps: - the supply of a donor substrate of composition ABO3, - the formation of a weakening zone 101 by implantation of ionic species (for example hydrogen and / or helium) in the donor substrate 100 so as to delimit the layer 10 to be transferred (see figure 1A), - the application of a receiving substrate 110 on the donor substrate 100, the layer 10 to be transferred being at the interface (see figure 1B), - detaching the donor substrate 100 along the weakening zone 101 so as to transfer the layer 10 onto the receiving substrate 110 (see figure 1C). Alternatively (not shown) to the Smart Cut™ process, the transfer of the layer 10 onto the receiving substrate may be carried out by thinning the donor substrate 100 by its face opposite the bonding interface, down to the layer 10 to be transferred. This thinning may involve at least one etching, one chemical-mechanical polishing and / or any other suitable technique. The donor substrate may be a solid substrate of the material under consideration. Alternatively, the donor substrate may be a composite substrate, i.e. formed from a stack of at least two layers of different materials, one surface layer of which is made of the material under consideration. Among the piezoelectric materials of particular interest are perovskite and related materials, of ABO3 structure. However, the interest that can be shown in these materials is not limited to their piezoelectric character. In particular for other applications, for example related to integrated optics, we can also be interested in them where appropriate for their dielectric permittivity, for their refractive indices, or even for their pyroelectric, ferroelectric or even ferromagnetic properties for example and depending on the case. A few large families emerge. One of them derives in particular from binary materials such as LiNbO3, LiTaO3, KNbO3, KTaO3 to arrive at a general formula of type ABO3 where A is made up of one or more of the following elements: Li, Na, K, H and where B is made up of one or more of the following elements: Nb, Ta, Sb, V. Another large family derives from the materials SrTiO3, CaTiO3, SrTiO3, PbTiO3, PbZrO3 in particular to arrive at a general formula of type ABO3 where A is made up of one or more of the following elements: Ba, Ca, Sr, Mg, Pb, La, Y and where B is made up of one or more of the following elements: Ti, Zr, Sn. Other less widespread families can also be derived from BiFeO3, or LaMnO3, BaMnO3, SrMnO3, or LaAIO3, or LiAIO3, LiGaO3, or CaSiO3, FeSiO3, MgSiO3, or DyScO3, GdScO3 and TbScO3. In the end, we can summarize by considering that A is made up of one or more of the following elements among: Li, Na, K, H, Ca, Mg, Ba, Sr, Pb, La, Bi, Y, Dy, Gd, Tb, Ce, Pr, Nd, Sm, Eu, Ho, Zr, Sc, Ag, Tl and B is made up of one or more of the following elements among: Nb, Ta, Sb, Ti, Zr, Sn, Ru, Fe, V, Sc, C, Ga, Al, Si, Mn, Zr, Tl. Some of these materials are monocrystalline; others are not monocrystalline. The crystalline nature and composition of the donor substrate are chosen by the person skilled in the art according to the destination of the layer to be transferred. The receiving substrate has a mechanical support function for the transferred layer. It can be of any nature and, advantageously but not necessarily, adapted to the intended application, the transferred layer possibly being transferred subsequently to another substrate. The receiving substrate can be solid or composite. According to one embodiment, the application of the receiving substrate to the donor substrate is carried out by bonding. Alternatively, the application of the receiver substrate to the donor substrate is carried out by depositing the receiver substrate on the donor substrate. Any suitable deposition technique, such as, for example but not limited to, evaporation, sputtering, aerosol spraying, chemical phase deposition, electrodeposition, coating, spin deposition, varnishing, screen printing, immersion, can be used. Such a solution is particularly advantageous for overcoming poor adhesion of the donor substrate to the receiver substrate. Optionally, the method comprises the formation of at least one electrically insulating layer and / or at least one electrically conductive layer (not shown) at the interface between the receiving substrate 110 and the layer 10 to be transferred. In the case where the layer 10 is sufficiently thick to give it a certain mechanical strength, in particular during the detachment operation along the weakening zone or during its subsequent use, the step of applying the receiving substrate can be omitted. The layer 10 is then said to be self-supporting after its detachment from the rest of the donor substrate. In this case, the thickness of the layer 10 is typically greater than 2 μm, preferably greater than 20 μm, and the implantation energy of the ionic species is greater than 1 MeV. Whether the layer 10 of initial composition ABO3 is self-supporting or transferred onto the receiving substrate 110, it is then converted into a layer of composition AA'BO3 where A' is an element belonging to the same list as A but which is different from the element A or is hydrogen. This modification of the composition of the transferred layer 10 is carried out by exposing said layer (and possibly the entire receiving substrate which supports it) to a medium M comprising ions of the element A' (see figure 1D). This exposure has the effect of causing ions of element A' to migrate from the medium M to the transferred layer 10, thus enriching the transferred layer in element A'. Eventually, atoms of element A located in layer 10 can migrate to medium M. Similarly, if obtaining layer 10 required the implementation of an implantation of H+ ions, hydrogen atoms present in the layer transferred to the location of atoms of element A, said hydrogen atoms can migrate towards the medium M. A mechanism of the reverse proton exchange type is involved in this migration. Reverse proton exchange is described, in connection with a completely different application than that addressed by the present invention, in the article by Yu. N. Korkishko et al entitled “Reverse proton exchange for buried waveguides in LiNbO3”, J. Opt. Soc. Am. A, Vol. 15, No. 7, July 1998. According to one embodiment, layer 10 is exposed to medium M before the step of thinning the donor substrate and thus has a composition AA'BO3 before being transferred to the recipient substrate or being formed as a self-supporting layer, if applicable. Optionally, exposure to M medium can be implemented both before and after thinning of the donor substrate. Furthermore, different elements A' can be inserted into the layer 10, by successively exposing said layer to different media each comprising ions of element A'. It is thus possible to obtain a layer 10 having a complex composition with several elements from the list to which element A and / or hydrogen belongs. An ion exchange type mechanism is involved in the migration of ions from element A' to the layer of initial composition ABO3. The medium M may be a liquid, in which case the transferred layer is immersed in a bath of said liquid. Alternatively, the medium M may be gaseous, in which case the transferred layer is placed in an enclosure containing said gas. A person skilled in the art is able to define the operating conditions of this exposure, in particular the composition of the medium, the duration and the temperature of exposure, depending on the composition of the desired layer. According to another alternative, the medium M may be in solid phase, and the transferred layer 10 is exposed to said medium by depositing a layer of said medium on the layer 10 (see FIG. 1E). By "on" is meant here either directly in contact with said layer 10, or via one or more layers formed from different materials, insofar as said intermediate layers do not block the migration of the element A' from the layer constituting the medium M to the layer 10. Better penetration of the ions of the element A into the layer 10 is made possible by one or more annealing steps. The deposited layer containing the element A may be removed at the end of the operation, possibly between two successive annealing steps. In the remainder of the text, we take as an example a layer of initial composition LiXO3, where X is niobium and / or tantalum. In other words, in this non-limiting example, element A is lithium and element B is niobium and / or tantalum, it being understood that a person skilled in the art is able to define the appropriate conditions for the other materials mentioned above. To transform the layer of initial composition LiXO3 into a layer of composition LiKXO3 or LiNaXO3, said layer is exposed to an acid solution of a potassium or sodium salt. A person skilled in the art is able to define the operating conditions of this exposure, in particular the composition of the medium, the duration and the temperature of exposure, depending on the composition targeted for the transferred layer. Optionally, before the step of enriching in element A', part of the thickness of the layer transferred to the receiving substrate is removed. This removal can be carried out by chemical-mechanical polishing, by etching or by any other appropriate technique. Two applications of the layer 10 of modified composition according to the invention are described below. Figure 2 is a schematic view of a surface acoustic wave filter. Said filter comprises a piezoelectric layer 10 and two electrodes 12, 13 in the form of two interdigitated metal combs deposited on the surface of said piezoelectric layer. On the side opposite the electrodes 12, 13, the piezoelectric layer rests on a support substrate 11. The piezoelectric layer 10 is monocrystalline, an excellent crystalline quality being in fact necessary to not cause attenuation of the surface wave. Figure 3 is a schematic view of a bulk acoustic wave resonator. The resonator comprises a thin piezoelectric layer (i.e. generally less than 1 μm thick, preferably less than 0.2 μm) and two electrodes 12, 13 arranged on either side of said piezoelectric layer 10. The piezoelectric layer 10 rests on a support substrate 11. To isolate the resonator from the substrate and thus prevent the propagation of waves in the substrate, a Bragg mirror 14 is interposed between the electrode 13 and the substrate 11. Alternatively (not illustrated), this isolation could be achieved by providing a cavity between the substrate and the piezoelectric layer. These different arrangements are known to those skilled in the art and will therefore not be described in detail in this text. In some cases, the receiving substrate may not be optimal for the final application. It may then be advantageous to transfer the layer 10 onto a final substrate (not shown) whose properties are chosen according to the intended application, by adhering it to said final substrate and removing the receiving substrate by any suitable technique. In the case where it is desired to manufacture a surface acoustic wave device, metal electrodes 12, 13 are deposited on the surface of the layer 10 opposite the receiving substrate 110 or, where appropriate, the final substrate (whether it is the receiving substrate 110 or the final substrate 111, said substrate forms the support substrate denoted 11 in FIG. 2), in the form of two interdigitated combs. In the case where it is desired to manufacture a bulk acoustic wave device, an adaptation of the method described above must be carried out. On the one hand, before the bonding step illustrated in Figure 1B, a first electrode is deposited on the free surface of the layer 10 to be transferred from the donor substrate, this first electrode (referenced 13 in Figure 3) being buried in the final stack. After the transfer step illustrated in Figure 1C, a second electrode (referenced 12 in Figure 3) is deposited on the free surface of the layer 10, opposite the first electrode. Another option is to transfer the layer 10 onto a final substrate as mentioned above and to form the electrodes before and after said transfer. On the other hand, to avoid the propagation of acoustic waves in the receiving substrate 110, it is possible to integrate therein an isolation means which may be, for example, a Bragg mirror (such as shown in Figure 3) or a cavity previously etched in the receiving substrate or in the final substrate where applicable. Finally, it goes without saying that the examples which have just been given are only specific illustrations and are in no way limiting as to the fields of application of the invention. REFERENCES W. Steichen and S. Ballandras, Acoustic components used for filtering - Review of different technologies, Engineering Techniques, E2000, 2008 10 Yu. N. Korkishko et al, Reverse proton exchange for buried waveguides in LiNbO3, J.Opt. Soc. A, Vol. 15, No. 7, July 1998

Claims

CLAIMS 1. Method for manufacturing a layer of composition AA'BO3 where A consists of at least one element from among: Li, Na, K, Ca, Mg, Ba, Sr, Pb, La, Bi, Y, Dy, Gd, 5 Tb, Ce, Pr, Nd, Sm, Eu, Ho, Zr, Sc, Ag, Tl and B consists of at least one element from among: Nb, Ta, Sb, Ti, Zr, Sn, Ru, Fe, V, Sc, C, Ga, Al, Si, Mn, Zr, Tl, characterized in that it comprises: - the provision of a donor substrate (100) of composition ABO3, - the formation of a layer (10) of composition ABO3 by thinning said donor substrate (100), - before and / or after the thinning step, exposing said layer (10) of composition ABO3 to a medium (M) containing ions of an element A' forming part of the same list of elements as A, A' being different from A, so as to cause said ions to penetrate into said layer (10) to form a layer of composition AA'BO3. 15 2. Method according to claim 1, in which the ions of element A' penetrate into the layer (10) by an ion exchange mechanism.

3. Method according to one of claims 1 or 2, in which the medium (M) 20 containing the ions of the element A' is a liquid and the layer (10) is immersed in a bath of said liquid.

4. Method according to one of claims 1 to 3, in which the layer (10) of composition ABO3 is immersed in a bath comprising an acid solution of a salt comprising element A'.

5. Method according to one of claims 1 or 2, in which the medium (M) containing the ions of element A' is in gas phase and the layer (10) is exposed to said gas. 30 6. Method according to one of claims 1 or 2, in which the medium (M) containing the ions of element A' is in solid phase, a layer of said medium (M) being deposited on the layer (10). 35 7, Method according to claim 6, comprising at least one annealing step to promote the diffusion of element A' from the medium (M) to the layer (10) 8. Method according to one of claims 1 to 7, in which the layer (10) is monocrystalline.

9. Method according to one of claims 1 to 8, in which the thinning of the donor substrate (100) comprises the application of a receiver substrate (110) on the donor substrate (10), the layer (10) being at the interface, and the transfer of the layer (10) on the receiver substrate (110).

10. The method of claim 9, wherein applying the recipient substrate comprises depositing said substrate (110) onto the donor substrate (100).

11. The method of claim 9, wherein applying the recipient substrate comprises bonding said substrate (110) to the donor substrate (100). 15 12. Method according to one of claims 9 to 11, in which the thickness of the layer (10) is less than 20 pm.

13. Method according to one of claims 9 to 12, in which at least one electrically insulating layer and / or at least one electrically conductive layer is formed at the interface between the receiving substrate (110) and the layer (10), 14. Method according to one of claims 1 to 8, in which the thickness of the layer (10) is greater than 2 μm, preferably greater than 20 μm, said layer being self-supporting following thinning of the donor substrate (100). 25 15. The method of claim 9, wherein thinning the donor substrate (10) comprises the following steps: - the formation of a weakening zone (101) in the donor substrate (100) so as to delimit the layer (10) of composition ABOg, 30 - detachment of the donor substrate (100) along the weakening zone (101).

16. The method of claim 15, wherein the weakening zone (101) is formed by ion implantation in the donor substrate (100). 35 17. Method according to one of claims 1 to 16, in which A is lithium and element A' is sodium and / or potassium.

18. The method of claim 17, wherein element B is niobium and / or tantalum.

19. A method of manufacturing a bulk acoustic wave device, 5 comprising the formation of electrodes (12, 13) on two opposite main faces of a piezoelectric layer (10), characterized in that it comprises the manufacture of said piezoelectric layer (10) by a method according to one of claims 1 to 18.

20. Method of manufacturing a surface acoustic wave device, 10 comprising the formation of two interdigitated electrodes (12, 13) on the surface of a piezoelectric layer (10), characterized in that it comprises the manufacture of said piezoelectric layer by a method according to one of claims 1 to 18.

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