Method for producing a layer by thinning and ion penetration

Abstract

A method for producing a layer of composition AA′BO.sub.3, wherein A consists of at least one element selected from the group consisting of: Li, Na, K, Ca, Mg, Ba, Sr, Pb, La, Bi, Y, Dy, Gd, Tb, Ce, Pr, Nd, Sm, Eu, Ho, Zr, Sc, Ag and Tl, and B consists of at least one element selected from the group consisting of: Nb, Ta, Sb, Ti, Zr, Sn, Ru, Fe, V, Sc, C, Ga, Al, Si, Mn, Zr and Tl, is described. The method includes providing a donor substrate of composition ABO.sub.3, forming a layer of composition ABO.sub.3 by thinning the donor substrate, and exposing the layer of composition ABO.sub.3 to a medium containing ions of an element A′ belonging to the same list of elements as A, A′ being different from A, such that the ions penetrate into the layer of composition ABO.sub.3 to form the layer of composition AA′BO.sub.3.

Claims

1. A method for producing a layer of composition AA′BO.sub.3 wherein an element A consists of at least one element selected from a group consisting of: Li, Na, K, Ca, Mg, Ba, Sr, Pb, La, Bi, Y, Dy, Gd, Tb, Ce, Pr, Nd, Sm, Eu, Ho, Zr, Sc, Ag, and Tl, and wherein an element B consists of at least one element selected from the group consisting of: Nb, Ta, Sb, Ti, Zr, Sn, Ru, Fe, V, Sc, C, Ga, Al, Si, Mn, and Tl, the method comprising: providing a donor substrate of composition ABO.sub.3; forming a layer of composition ABO.sub.3 by thinning the donor substrate; and before and/or after the thinning of the donor substrate, exposing the layer of composition ABO.sub.3 to a medium containing ions of an element A′ belonging to the same list of elements as the element A, the element A′ being different from the element A, such that the ions of the element A′ penetrate into the layer of composition ABO.sub.3 to convert the entirety of the layer of composition ABO.sub.3 into the layer of composition AA′BO.sub.3, the layer of composition AA′BO.sub.3 exhibiting a uniform composition.

2. The method of claim 1, wherein the ions of the element A′ penetrate into the layer of composition ABO.sub.3 by an ion exchange mechanism.

3. The method of claim 1, wherein the medium containing the ions of the element A′ is a liquid and the layer of composition ABO.sub.3 is immersed in a bath of the liquid.

4. The method of claim 1, wherein the layer of composition ABO.sub.3 is immersed in a bath comprising an acid solution of a salt comprising the element A′.

5. The method of claim 1, wherein the medium containing the ions of the element A′ is in gaseous phase and the layer of composition ABO.sub.3 is exposed to the medium.

6. The method of claim 1, wherein the layer of composition ABO.sub.3 is monocrystalline.

7. The method of claim 1, wherein the thickness of the layer of composition ABO.sub.3 is greater than 2 μm, the layer of composition ABO.sub.3 being self-supporting after the thinning of the donor substrate.

8. The method of claim 1, further comprising: producing a bulk acoustic wave device, the producing comprising forming electrodes on two opposite main faces of the layer of composition AA′BO.sub.3, which comprises a piezoelectric layer.

9. The method of claim 1, further comprising: producing a surface acoustic wave device, the producing comprising forming two interdigitated electrodes on a surface of the layer of composition AA′BO.sub.3, which comprises a piezoelectric layer.

10. The method of claim 1, wherein the medium containing the ions of the element A′ is in solid phase, a layer of the medium being deposited on the layer of composition ABO.sub.3.

11. The method of claim 10, further comprising at least one annealing step to cause diffusion of the element A′ from the medium to the layer of composition ABO.sub.3.

12. The method of claim 1, wherein the ions of the element A′ penetrate into the layer of composition ABO.sub.3 by implantation.

13. The method of claim 12, further comprising at least one heat treatment step for diffusing implanted ions into the layer of composition ABO.sub.3.

14. The method of claim 1, wherein the element A is lithium and the element A′ is sodium and/or potassium.

15. The method of claim 14, wherein the element B is niobium and/or tantalum.

16. The method of claim 1, wherein the thinning of the donor substrate comprises applying a receiver substrate on the donor substrate, the layer of composition ABO.sub.3 being at an interface between the receiver substrate and the donor substrate, and transferring the layer of composition ABO.sub.3 onto the receiver substrate.

17. The method of claim 16, wherein the application of the receiver substrate comprises deposition of the receiver substrate on the donor substrate.

18. The method of claim 16, wherein the application of the receiver substrate comprises bonding of the receiver substrate on the donor substrate.

19. The method of claim 16, wherein the thickness of the layer of composition ABO.sub.3 is less than 20 μm.

20. The method of claim 16, wherein at least one electrically insulating layer and/or at least one electrically conducting layer is formed at an interface between the receiver substrate and the layer of composition ABO.sub.3.

21. The method of claim 16, wherein the thinning of the donor substrate comprises: forming a weakened zone in the donor substrate so as to delineate the layer of composition ABO.sub.3; and detaching the layer of composition ABO.sub.3 from the donor substrate along the weakened zone.

22. The method of claim 21, wherein the weakened zone is formed by ion implantation in the donor substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the disclosure will become clear from the detailed description that follows, with reference to the accompanying drawings, in which:

(2) FIGS. 1A to IF schematically illustrate the steps of the method for producing a monocrystalline layer of composition AA′BO.sub.3 according to one embodiment of the disclosure,

(3) FIG. 2 is a cross-sectional principal view of a surface acoustic wave filter, and

(4) FIG. 3 is a cross-sectional principal view of a bulk acoustic wave filter.

(5) For reasons of legibility 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

(6) Generally speaking, a layer of composition ABO.sub.3 is formed by thinning a donor substrate of composition ABO.sub.3. Thinning may be carried out by any appropriate technique, among which may be cited the SMART CUT® method, thanks to which is formed, by implantation of ionic species in the donor substrate, a weakened zone delineating the layer of interest; any other technique of formation of the weakened zone being able to be employed; or instead the implementation of one or more steps of etching the donor substrate so as to conserve only the layer of interest.

(7) If the layer is sufficiently thick, it may be self-supporting, that is to say that it does not need to be applied on a stiffener substrate in order to have sufficient mechanical strength.

(8) Alternatively, notably if the layer is thin, the thinning of the donor substrate is preceded by a step of applying, on the donor substrate, a receiver substrate. The application of the receiver substrate may be done notably by bonding or by deposition on the donor substrate.

(9) From the layer of composition ABO.sub.3 thereby obtained, a layer of more complex composition of AA′BO.sub.3 type is obtained, in which A′ belongs to the same list of elements as A but is different from A. Obtaining such a layer assumes exposing the layer of composition ABO.sub.3 to a medium comprising ions of the element A′, so as to make at least one part of the ions penetrate into the layer.

(10) The conversion of the layer of initial composition ABO.sub.3 into a layer of composition AA′BO.sub.3 advantageously involves an ion exchange mechanism that is used in the glass field. For example, it is known to replace a part of the Na+ ions present in certain glasses by Ag+ or K+ ions, by implementing a treatment consisting in immersing the glass in a bath of molten salts, for example, AgNO.sub.3 or KNO.sub.3, respectively. Similarly, the disclosed method may be implemented by exposing the layer to convert into a layer of more complex composition to a medium containing ions of the element A′ with which it is wished to enrich the layer, the medium being able to be liquid (for example, a bath of an acid solution of a salt comprising the element A′), gaseous or solid.

(11) With reference to FIGS. 1A to 1D, a method is considered for producing a layer of composition AA′BO.sub.3 according to one embodiment implementing the SMART CUT® method, comprising the following steps: providing a donor substrate of composition ABO.sub.3, the formation of a weakened zone 101 by implantation of ionic species (for example, hydrogen and/or helium) in the donor substrate 100 so as to delineate the layer 10 to transfer (cf. FIG. 1A), the application of a receiver substrate 110 on the donor substrate 100, the layer 10 to transfer being at the interface (cf. FIG. 1B), the detachment from the donor substrate 100 along the weakened zone 101 so as to transfer the layer 10 onto the receiver substrate 110 (cf. FIG. 1C).

(12) In an alternative (not illustrated) to the SMART CUT® method, the transfer of the layer 10 onto the receiver substrate may be carried out by thinning the donor substrate 100 by its face opposite to the bonding interface, down to the layer 10 to transfer. This thinning may involve at least one etching, chemical mechanical polishing and/or another appropriate technique.

(13) The donor substrate may be a bulk substrate of the considered material. Alternatively, the donor substrate may be a composite substrate, that is to say formed of a stack of at least two layers of different materials, of which a superficial layer consists of the considered material.

(14) Among piezoelectric materials of particular interest are perovskites and comparable materials, of ABO.sub.3 structure. However, the interest that can be placed in these materials is not limited to their piezoelectric character. Notably for other applications, for example, linked to integrated optics, interest could also be taken in them if need be for their dielectric permittivity, for their refractive indices, or instead for their pyroelectric, ferroelectric or instead ferromagnetic properties, for example, and depending on the case. Several large families stand out. One of them derives notably from binary materials such as LiNbO.sub.3, LiTaO.sub.3, KNbO.sub.3, KTaO.sub.3 to end up with a generic formula of ABO.sub.3 type where A consists of one or more of the following elements: Li, Na, K and where B consists of one or more of the following elements: Nb, Ta, Sb, V. Another large family derives from SrTiO.sub.3, CaTiO.sub.3, SrTiO.sub.3, PbTiO.sub.3, PbZrO.sub.3 materials notably to end up with a generic formula of ABO.sub.3 type where A consists of one or more of the following elements: Ba, Ca, Sr, Mg, Pb, La, Y and where B consists of one or more of the following elements: Ti, Zr, Sn. Other less widespread families may also be derived from BiFeO.sub.3, or instead LaMnO.sub.3, BaMnO.sub.3, SrMnO.sub.3, or instead LaAlO.sub.3, or instead, LiAlO.sub.3, LiGaO.sub.3, or instead CaSiO.sub.3, FeSiO.sub.3, MgSiO.sub.3, or instead DyScO.sub.3, GdScO.sub.3 and TbScO.sub.3.

(15) In the end, it could be summarized by considering that A consists of one or more of the following elements selected from: Li, Na, K, 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 one or more of the following elements selected from: Nb, Ta, Sb, Ti, Zr, Sn, Ru, Fe, V, Sc, C, Ga, Al, Si, Mn, Zr, Tl.

(16) Some of these materials are monocrystalline; others are not monocrystalline.

(17) The crystalline nature and the composition of the donor substrate are selected by those skilled in the art depending on the destination of the layer to transfer.

(18) The receiver substrate has a function of mechanical support of the transferred layer. It may be of any nature and, advantageously but not imperatively, adapted to the targeted application, the transferred layer optionally being able to be transferred later onto another substrate. The receiver substrate may be bulk or composite.

(19) According to one embodiment, the application of the receiver substrate on the donor substrate is carried out by bonding.

(20) Alternatively, the application of the receiver substrate on the donor substrate is carried out by a deposition of the receiver substrate on the donor substrate. Any suitable deposition technique, such as, for example, but in a non-limiting manner, an evaporation, a cathodic sputtering, an aerosol sputtering, a chemical phase deposition, an electrodeposition, a spread coating, a spin coating, a varnishing, a screen printing, an immersion, may be used. Such a solution is particularly advantageous to compensate for poor adhesion of the donor substrate vis-à-vis the receiver substrate.

(21) Optionally, the method comprises forming at least one electrically insulating layer and/or at least one electrically conducting layer (not represented) at the interface between the receiver substrate 110 and the layer 10 to transfer.

(22) In the case where the layer 10 is sufficiently thick to confer thereon a certain mechanical strength, notably during the operation of detachment along the weakened zone or during its later use, the step of application of the receiver substrate may be omitted. The layer 10 is then called 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 energy of implantation of the ionic species is greater than 1 MeV.

(23) Whether the layer 10 of initial composition ABO.sub.3 is self-supporting or transferred onto the receiver substrate 110, it is next converted into a layer of composition AA′BO.sub.3 where A′ is an element belonging to the same list as A but is different than element A.

(24) This modification of the composition of the transferred layer 10 is carried out by exposing the layer (and optionally the whole of the receiver substrate that supports it) to a medium M comprising ions of the element A′ (cf. FIG. 1D).

(25) This exposure has the effect of making ions of the element A′ migrate from the medium M to the transferred layer 10, thereby enriching the transferred layer with element A′.

(26) Optionally, atoms of the element A situated in the layer 10 can migrate to the medium M.

(27) Similarly, if obtaining the layer 10 requires the implementation of an implantation of H+ ions, of hydrogen atoms present in the transferred layer at the location of atoms of the element A, the hydrogen atoms can migrate to the medium M. A reverse proton exchange type mechanism is brought into play in this migration. Reverse proton exchange is described, in relation with an entirely other application than that addressed by the present disclosure, in the article of Yu. N. Korkishko et al. entitled “Reverse proton exchange for buried waveguides in LiNbO.sub.3,” J. Opt. Soc. Am. A, Vol. 15, No. 7, July 1998.

(28) According to one embodiment, the layer 10 is exposed to the medium M before the step of thinning the donor substrate and thus has a composition AA′BO.sub.3 before being transferred onto the receiver substrate or be formed as a self-supporting layer, if need be.

(29) Optionally, the exposure to the medium M may be implemented both before and after the thinning of the donor substrate.

(30) Furthermore, different elements A′ may be inserted into the layer 10, by successively exposing the layer to different media each comprising ions of the element A′. It is thus possible to obtain a layer 10 having a complex composition with several elements of the list to which the element A belongs.

(31) An ion exchange type mechanism is brought into play in the migration of ions of the element A′ into the layer of initial composition ABO.sub.3.

(32) The medium M may be a liquid, in which case, the transferred layer is immersed in a bath of the liquid.

(33) Alternatively, the medium M may be gaseous, in which case, the transferred layer is placed in an enclosure containing the gas.

(34) Those skilled in the art are able to define the operating conditions of this exposure, notably the composition of the medium, the duration and the temperature of exposure, as a function of the desired composition of the layer.

(35) According to one particular embodiment, illustrated in FIG. 1E, the exposure to the medium may be carried out by implantation of ions of the element A′ into the transferred layer (this implantation is shown schematically by arrows). Since the implantation results in a peak of ions implanted at a determined depth of the layer, it must be followed by a heat treatment intended to cause a diffusion of the ions into the whole of the thickness of the layer, in order to homogenize the composition thereof. Such an annealing also makes it possible to repair crystalline defects created by the implantation. This annealing is generally carried out immediately after the implantation. In certain cases, it starts during the implantation step itself, notably when the substrate subjected to the implantation heats up under the exposure of the corresponding energy flux. But the annealing can also take place later in the sequencing of steps of methods. It is thereby possible to intercalate other steps between the implantation step and the annealing step, such as, for example, the lift-off of a superficial passivation layer, or instead a polishing step, an etching step or instead a layer transfer operation. The annealing may moreover be carried out in several stages, which can be spread out over different steps of the overall method. The implantation may be of conventional type, that is to say, in which the ions are transported in the form of a beam of ions accelerated to a certain energy (typically several tens to several hundreds of keV). In an alternative, the implantation may be carried out by plasma immersion, which makes it possible to implant rapidly a high dose of A′ ions in the vicinity of the surface of the transferred layer. These techniques are known to those skilled in the art.

(36) According to another alternative, the medium M may be in solid phase, and the transferred layer 10 is exposed to the medium by deposition of a layer of the medium on the layer 10 (cf. FIG. 1F). “On” is here taken to mean either directly in contact with the layer 10, or through one or more layers formed of different materials, in so far as the intermediate layers do not block the migration of the element A from the layer constituting the medium M to the layer 10. A 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, optionally between two successive annealing steps.

(37) In the remainder of the text a layer of initial composition LiXO.sub.3, where X is niobium and/or tantalum, is taken as example. In other words, in this non-limiting example, the element A is lithium and the element B is niobium and/or tantalum, it being understood that those skilled in the art are able to define suitable conditions for the other materials cited above.

(38) To transform the layer of initial composition LiXO.sub.3 into a layer of composition LiKXO.sub.3 or LiNaXO.sub.3, the layer is exposed to an acid solution of a potassium or sodium salt.

(39) Those skilled in the art are able to define the operating conditions of this exposure, notably the composition of the medium, the duration and the temperature of exposure, as a function of the composition targeted for the transferred layer.

(40) Optionally, before the step of enrichment with element A′, a part of the thickness of the layer transferred onto the receiver substrate is removed. This removal may be carried out by chemical mechanical polishing, by etching or by any other appropriate technique.

(41) Two applications of the layer 10 of composition modified according to the disclosed method are described hereafter.

(42) FIG. 2 is a principal view of a surface acoustic wave filter.

(43) The filter comprises a piezoelectric layer 10 and two electrodes 12, 13 in the form of two interdigitated metal combs deposited on the surface of the piezoelectric layer. On the side opposite to the electrodes 12, 13, the piezoelectric layer rests on a support substrate 11. The piezoelectric layer 10 is monocrystalline, an excellent crystalline quality indeed being necessary so as not to cause attenuation of the surface wave.

(44) FIG. 3 is a principal view of a bulk acoustic wave resonator.

(45) The resonator comprises a thin piezoelectric layer (that is to say of thickness generally less than 1 μm, preferably less than 0.2 μm) and two electrodes 12, 13 arranged on either side of the piezoelectric layer 10. The piezoelectric layer 10 rests on a support substrate 11. To insulate the resonator from the substrate and thereby avoid 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 insulation could be achieved by arranging a cavity between the substrate and the piezoelectric layer. These different arrangements are known to those skilled in the art and thus will not be described in detail in the present text.

(46) In certain cases, the receiver substrate may not be optimal for the final application. It may then be advantageous to transfer the layer 10 onto a final substrate (not represented) of which the properties are selected as a function of the targeted application, by bonding it on the final substrate and by removing the receiver substrate by any suitable technique.

(47) In the case where it is wished to produce a surface acoustic wave device, metal electrodes 12, 13 in the form of two interdigitated combs are deposited on the surface of the layer 10 opposite to the receiver substrate 110 or, if need be, opposite to the final substrate (whether it is the receiver substrate 110 or the final substrate, the substrate forms the support substrate noted 11 in FIG. 2).

(48) In the case where it is wished to produce a bulk acoustic wave device, an adaptation of the method described above has to be made. On the one hand, before the bonding step illustrated in FIG. 1B, a first electrode is deposited on the free surface of the layer 10 to transfer of the donor substrate, this first electrode (reference number 13 in FIG. 3) being buried in the final stack. After the transfer step illustrated in FIG. 1C, a second electrode (reference number 12 in FIG. 3) is deposited on the free surface of the layer 10, opposite to 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 the transfer. On the other hand, to avoid the propagation of acoustic waves in the receiver substrate 110, it is possible to integrate therein an insulation means, for example, a Bragg mirror (as illustrated in FIG. 3) or a cavity etched beforehand in the receiver substrate or in the final substrate if need be.

(49) Finally, it goes without saying that the examples that have been given are only particular illustrations that are in no way limiting with regard to the application fields of the disclosed method.