Method for transferring a thin layer to a supporting substrate having a different thermal expansion coefficient - Patents.com

Abstract

To provide a novel method for transferring a thin layer made of a first material onto a support substrate made of a second material.SOLUTION: Provided is a method for transferring a thin layer (3) made of a first material to a support substrate (7) made of a second material, the first material and the second material having different thermal expansion coefficients. The method includes the steps of: preparing a donor substrate (1) composed of an assembly of a thick layer (1a) formed of the first material and a handling substrate (1b), the thermal expansion coefficient of the handling substrate (1b) being similar to that of the support substrate (7), and the donor substrate (1) having a main face (4) on the side of the thick layer (1a); introducing light species into the thick layer (1a) to define the thin layer (3) between an embrittlement plane (2) and the main face (4); assembling the main face (4) of the donor substrate with one face (6) of the support substrate (7); and detaching the thin layer (3) from the embrittlement plane (2), the step including the application of heat treatment.SELECTED DRAWING: Figure 1C

Description

[Technical Field]

[0001] The present invention relates to the field of heterogeneous structures combining two substrates with different thermal expansion coefficients. More particularly, the present invention relates to a method for transferring a thin layer onto a supporting substrate. This manufacturing method is used, for example, in the fields of microelectronics, micromechanics, photonics, etc. [Background technology]

[0002] Various methods are known from the prior art for forming thin layers on a support substrate, such methods may be, for example, molecular beam epitaxy, plasma sputtering, plasma deposition (laser pulsed deposition) or application of the Smart Cut™ technique, which obtains thin layers from a bulk substrate by cracking at weak zones (or embrittled planes) formed in the bulk substrate by implantation of light species.

[0003] The invention relates more particularly to the formation of thin layers made of ferroelectric material obtained by applying the method as taught in patent application FR 2 914 492.

[0004] The application of the Smart Cut method is particularly suitable when the thermal expansion coefficient of the material of the thin layer to be transferred is similar to that of the supporting substrate to which the transfer is to be made.

[0005] Conversely, the temperature to which the assembly formed from the support substrate and the donor substrate can be subjected is limited. Thus, FR 2 856 192 A1 recalls that heat treatments above a temperature determined by the value of the thermal expansion coefficient of the materials can lead to uncontrolled cracking of one of the substrates and / or delamination of the donor substrate or thin layers. This is problematic in the Smart Cut method, since it may be necessary to carry out at least one heat treatment at a sufficient temperature, for example, to strengthen the adhesion of the donor substrate to the support substrate or to promote cracking of the donor substrate at embrittlement planes. Summary of the Invention [Problem to be solved by the invention]

[0006] One goal of the present invention is to propose a method for transferring a thin layer made of a first material onto a support substrate made of a second material, the first and second materials having different thermal expansion coefficients, which at least partially solves the above-mentioned problems. In particular, this method finds application in transferring a thin layer made of a ferroelectric material onto a support substrate having an expansion coefficient different from that of the material of which the thin layer is made, for example made of silicon. [Means for solving the problem]

[0007] To achieve one of these goals, the subject matter of the present invention proposes a method for transferring a thin layer made of a first material to a support substrate made of a second material, the first and second materials having different thermal expansion coefficients. According to the invention, the method for transferring a thin layer comprises the steps of: providing a donor substrate consisting of an assembly of a thick layer made of a first material and a handling substrate, the handling substrate having a thermal expansion coefficient similar to that of the support substrate, the donor substrate having a main surface facing the thick layer; introducing a light species into the thick layer to create an embrittlement plane in the thick layer and defining a thin layer between the embrittlement plane and the main surface of the donor substrate; assembling the main surface of the donor substrate with one surface of the support substrate; and separating the thin layer from the embrittlement plane, the separating step comprising applying a heat treatment.

[0008] The assembly formed from the donor substrate and the support can be subjected to temperatures significantly higher than those applied in the context of the "direct" approach according to the prior art, so that the donor substrate does not include any handling substrate, without risking uncontrolled cracking of one of the substrates or delamination of the donor substrate or the thin layers. The balanced structure in terms of the thermal expansion coefficients of this assembly makes it possible in particular to facilitate the step of separating the thin layers by subjecting the assembly to relatively high temperatures.

[0009] Other advantageous and non-limiting features of the present invention, obtained alone or in any technically feasible combination, are: the thermal expansion coefficient of the first material constituting the thick layer differs from the thermal expansion coefficient of the second material constituting the support substrate by at least 10% at room temperature; the difference between the thermal expansion coefficients of the components of the handling substrate and the components of the support is smaller in absolute value than the difference between the thermal expansion of the thick layer and the thermal expansion of the support substrate; the implanted light species are hydrogen ions and / or helium ions; the first material is a ferroelectric material such as LiTaO3, LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3 or KTaO3; The material of the support substrate is silicon, The handling substrate is of the same nature as the support substrate; the handling substrate has a thickness equal to the thickness of the support substrate; the thick layer has a thickness between 10 and 400 micrometers to allow sampling of one or more thin layers; a donor substrate is obtained by bonding a source substrate and a handling substrate; The bonding is achieved by molecular adhesion, thinning the source substrate to form a thick layer; The thinning step is carried out by milling and / or mechanical-chemical polishing and / or etching.

[0010] Other features and advantages of the present invention will become apparent from the following detailed description of the invention, which proceeds with reference to the accompanying drawings. [Brief explanation of the drawings]

[0011] [Figure 1A] 1 is a diagram of an embodiment of the method according to the present invention; [Figure 1B] 1 is a diagram of an embodiment of the method according to the present invention; [Figure 1C] 1 is a diagram of an embodiment of the method according to the present invention; [Figure 1D] 1 is a diagram of an embodiment of the method according to the present invention; [Figure 2] 1 is a schematic diagram of a method for forming a donor substrate according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0012] To simplify the following description, the same reference numerals are used in the prior art or in various embodiments of the presented method for identical elements or elements that perform the same function.

[0013] The drawings are shown diagrammatically and not to scale for clarity, in particular the thicknesses of layers are not to scale relative to the lateral dimensions of those layers.

[0014] In other parts of this description, the term "coefficient of thermal expansion" used in relation to a layer or substrate refers to the coefficient of expansion in a defined direction in a major surface defining this layer or this substrate. If the material is anisotropic, the coefficient value that is retained is the value of maximum amplitude. The coefficient value is measured at room temperature.

[0015] The present invention relates to a method for transferring a thin layer 3 of a first material onto a support substrate 7 of a second material, the first and second materials having different thermal expansion coefficients, the term "different" meaning that these coefficients differ by at least 10%.

[0016] In the context of this description, by way of example, the thin layer 3 is made of a ferroelectric material and the support 7 is made of silicon (with a thermal expansion coefficient of 2.6×10 -6 K -1 We will consider that it is made from (presumably

[0017] It is recalled that a ferroelectric material is a material that naturally possesses an electric polarization, and that this polarization can be reversed upon the application of an external electric field. A ferroelectric domain denotes each continuous region of the material in which the polarization is uniform (all dipole moments are aligned parallel to one another in a given direction). Thus, a ferroelectric material may be characterized as "monodomain," if the material consists of a single region in which the polarization is uniform, or as "multidomain," if the ferroelectric material contains multiple regions with potentially different polarities.

[0018] Generally, it is desired to have the ferroelectric thin layer entirely monodomain in nature.

[0019] Referring to FIG. 1A, the donor substrate 1 is, for example, LiTaO3 (2×10 -6 K -1 (z), 16×10 -6 K -1 The donor substrate 1 comprises a thick layer 1a of ferroelectric material, which may be LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3, or KTaO3 (x, y), and a handling substrate 1b. The donor substrate 1 can be in the form of a standard-sized circular wafer, e.g., 150 mm or 200 mm in diameter. However, the invention is not limited to these dimensions or this shape. The thick layer 1a of material can be sampled from an ingot of ferroelectric material, and the sampling is performed so that the thick layer 1a has a predetermined crystal orientation. This orientation is chosen depending on the intended application. Thus, if it is desired to utilize the properties of the thin layer to form a SAW filter, it is common practice to choose a 42°RY orientation. However, the invention is not limited to any particular crystal orientation.

[0020] Advantageously, the handling substrate 1b consists of one or more materials that provide a thermal expansion coefficient close to that of the material constituting the support substrate 7. The term "close" means that, in absolute value, the difference between the thermal expansion coefficient of the handling substrate 1b and that of the support 7 is smaller than the difference between the thermal expansion coefficient of the thick layer 1a and that of the support substrate 7.

[0021] It is preferred that the handling substrate 1b and the support substrate have the same thermal expansion coefficient. During assembly of the donor substrate and the support, a structure is formed that can withstand heat treatment at relatively high temperatures. For ease of implementation, this can be achieved by selecting the handling substrate 1b to be made of the same material as that of the support substrate 7.

[0022] To form the donor substrate 1, a bulk block of ferroelectric material is pre-assembled with a handling substrate 1b, for example by molecular adhesive bonding techniques. A layer of ferroelectric material 1a is then formed by thinning, for example by milling and / or mechanical-chemical polishing and / or etching. This method is shown diagrammatically in FIG. 2. Prior to assembly, it may be envisaged to form an adhesion layer on one and / or the other of the contacting surfaces (for example by deposition of silicon oxide and / or silicon nitride). The assembly may involve the application of a low-temperature heat treatment (for example, between 50 and 300°C, typically 100°C), which makes the bonding energy strong enough to enable the subsequent thinning step.

[0023] The handling substrate 1b can be chosen to have a thickness substantially equal to that of the support substrate 7. The thinning step is performed so that the thick layer 1a has a thickness small enough that stresses generated during heat treatments applied elsewhere in the method reduce the strength of the thick layer 1a. At the same time, this thickness is large enough to allow the thin layer 3 or multiple such layers to be sampled from that thickness. This thickness can be, for example, between 5 and 400 microns.

[0024] The method comprises introducing at least one light species into the donor substrate 1. This introduction can correspond to implantation, i.e. ion bombardment, of light species such as hydrogen and / or helium ions into the main surface 4 of the donor substrate 1.

[0025] In a manner known per se, as shown in FIG. 1B, the implanted ions serve to form an embrittlement plane 2 that demarcates the thin layer 3 of ferroelectric material to be transferred, located on the main surface 4, and another portion 5 that constitutes the rest of the substrate.

[0026] In practice, the dose and implantation energy of the implanted species are chosen depending on the thickness of the layer to which transfer is desired and on the physicochemical properties of the donor substrate. Thus, in the case of a donor substrate 1 made of LiTaO3, an implantation with an energy of 30-300 keV is used to define a thin layer 3 of the order of 200-2000 nm. E 16~5 E 17at / cm 2 It may be chosen to inject a dose of hydrogen.

[0027] In the next step, shown in FIG. 1C, the main surface 4 of the donor substrate 1 is assembled with one surface 6 of a support substrate 7. The support substrate 7 can have the same dimensions and shape as the donor substrate. For reasons of availability and cost, the support substrate 7 is a monocrystalline or polycrystalline silicon wafer. However, more generally, the support substrate 7 can be made of any material, for example silicon, sapphire, or glass, and can have any shape.

[0028] Before this step, it is possible to envisage preparing the surface of the substrate to be assembled by a cleaning, brushing, drying, polishing or plasma activation step.

[0029] The assembling step can correspond to placing the donor substrate 1 in close contact with the support substrate 7 by molecular adhesion and / or electrostatic bonding. Optionally, in order to facilitate the assembly of the two substrates 1, 7, particularly when the two substrates 1, 7 are assembled by direct bonding, at least one intermediate layer can be formed on the main surface 4 of the donor substrate 1 or on the flat surface 6 to be assembled of the support substrate 7, or on both, before assembly. This intermediate layer can be made of, for example, silicon oxide, silicon nitride, or polycrystalline silicon and have a thickness between a few nanometers and a few microns. The intermediate layer can be produced according to various techniques known in the art, such as thermal oxidation, thermal nitridation, or chemical deposition (PECVD, LPCVD, etc.).

[0030] At the end of this assembly step, the assembly shown in FIG. 1C is obtained, comprising two bonded substrates with the planar surface 6 of the support substrate 7 adhered to the major surface 4 of the donor substrate 1.

[0031] The assembly is then treated to separate the thin layer 3 of ferroelectric material from the donor substrate 1, for example by cleaving at the embrittlement plane 2.

[0032] This separation step can therefore involve applying a heat treatment to the assembly in a temperature range of about 80°C to 500°C, allowing the transfer of the thin layer 3 to the support substrate 7. In addition to the heat treatment, this step can involve the application of a blade or a jet of a gaseous or liquid fluid to the embrittlement surface 2. In the case of ferroelectric materials, care is taken not to exceed the Curie temperature of the ferroelectric material, so as not to deteriorate the properties of the thin layer.

[0033] After this separation step, the structure 9 shown in Figure 1D is obtained, which comprises a thin layer 3 of ferroelectric material having a first free surface 8 and a main surface 4 arranged relative to a support substrate 7.

[0034] The assembly formed from the donor substrate 1 and the support 7 can be subjected to temperatures significantly higher than those applied in the context of the "direct" approach according to the prior art, so that the donor substrate does not include any handling substrate, without risking uncontrolled cracking of one of the substrates or delamination of the donor substrate 1 or thin layer 3. The balanced structure in terms of the thermal expansion coefficients of this assembly therefore makes it possible to facilitate the step of separating thin layer 3 by subjecting the assembly to relatively high temperatures, for example between 80°C and 500°C.

[0035] A finishing step can then be carried out on the thin layer 3 and in particular on the free surface 8 of the thin film in order to restore and / or improve the desired properties of this layer. As is known per se, this finishing can involve polishing, etching, sacrificial oxidation or annealing under a neutral, reducing or oxidizing atmosphere.

[0036] In the context of the example described so far, in which the thin layer 3 is made of a ferroelectric material, this finishing step can correspond to a heat treatment of the thin layer 3 followed by polishing, which sequence can restore the initial monodomain nature of the thick layer 1a, for example so that the transferred thin layer 3 retains the 42° RY orientation. However, the invention is not limited to any particular finishing sequence.

[0037] The heat treatment makes it possible to correct any crystalline defects present in the thin layer 3. In addition, the heat treatment contributes to the consolidation of the bond between the thin layer 3 and the support 7. The heat treatment brings the structure to a temperature between 300°C and the Curie temperature of the ferroelectric material for a period of time ranging from 10 seconds to 10 hours. This heat treatment is preferentially carried out by exposing the free surface of the thin layer 3 to an oxidizing or neutral gas atmosphere.

[0038] The preparation method further comprises thinning the thin layer 3 after the heat treatment. This thinning can correspond to polishing the first free surface 8 of the thin layer 3, for example by mechanical, mechanochemical and / or chemical etching thinning techniques. This makes it possible to prepare the free surface 8 such that its roughness is insignificant, for example less than 0.5 nm RMS for a 5×5 μm area by atomic force measurement (AFM), and to remove surface parts of the first free surface 8 of the thin layer 3 that are prone to residual defects.

[0039] Naturally, the invention is not limited to the examples described and variations of the embodiments may be introduced into the invention without departing from the scope of the invention as defined by the claims.

[0040] Furthermore, the invention applies to any "foreign" structure in which there is a difference in the thermal expansion coefficient between the thin layer 3 and the supporting substrate 7, which is for example the case in silicon-on-quartz or silicon-on-sapphire structures.

[0041] 1...donor substrate, 1a...thick layer, 1b...handling substrate, 3...thin layer, 4...main surface of donor substrate, 6...one surface of support substrate, 7...support substrate.

Claims

1. 1. A method for transferring a thin layer (3) of a first material onto a support substrate (7) of a second material, said first material and said second material having different thermal expansion coefficients, comprising: a step of preparing a donor substrate (1) consisting of an assembly of a thick layer (1a) made of the first material and a handling substrate (1b), wherein the difference in thermal expansion coefficient between the components of the handling substrate (1b) and the components of the support substrate (7) is smaller in absolute value than the difference in thermal expansion coefficient between the thick layer (1a) and the support substrate (7), and the donor substrate (1) has a main surface (4) on the side of the thick layer (1a); b) introducing light species into said thick layer (1 a) to create an embrittlement plane (2) in said thick layer (1 a) and to define said thin layer (3) between said embrittlement plane (2) and said main surface (4) of said donor substrate (1), said implanted light species being hydrogen ions and / or helium ions; c. assembling the main surface (4) of the donor substrate and one surface (6) of the support substrate (7); d. separating said thin layer (3) from said embrittlement surface (2), comprising applying a heat treatment; Including, the thermal expansion coefficient of the first material constituting the thick layer (1a) and the thermal expansion coefficient of the second material constituting the support substrate (7) differ by at least 10% at room temperature; the first material is a ferroelectric material; The step a Bonding a bulk block of ferroelectric material to said handling substrate (1b); thinning a bulk block of said ferroelectric material to form said thick layer (1a); A method comprising:

2. 2. The method of claim 1, wherein the first material is a ferroelectric material such as LiTaO3, LiNbO3, LiAlO3, BaTiO3, PbZrTiO3, KNbO3, BaZrO3, CaTiO3, PbTiO3, or KTaO3.

3. 3. The method according to claim 1 or 2, wherein the material of the support substrate (7) is silicon.

4. The method according to any one of claims 1 to 3, wherein the handling substrate (1b) has a thickness equal to the thickness of the support substrate (7).

5. A method according to any one of claims 1 to 4, wherein said thick layer (1a) has a thickness between 10 and 400 micrometers to allow sampling of one or more thin layers.

6. The method of claim 1 , wherein the bonding is achieved by molecular adhesion.

7. The method of claim 1 , wherein the thinning is performed by milling and / or mechanical-chemical polishing and / or etching.

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