Method for producing a two-dimensional material

Abstract

The invention relates to a method for producing a two-dimensional material (10), comprising the following steps: - growing the two-dimensional material (10) on a surface (20s) of a growth substrate (20) such that the two-dimensional material is bonded to said surface by van der Waals forces, said surface consisting of a first material having a first relative permittivity; - providing a target substrate (30) having a surface (30s) consisting at least in part of a second material having a second relative permittivity strictly greater than the first relative permittivity; - assembling (S2) the growth substrate (20) and the target substrate (30) by direct adhesive bonding between the two-dimensional material (10) and the second material; and - separating the growth substrate (20) and the target substrate (30), whereby at least one part of the two-dimensional material (10) detaches from the growth substrate and remains adhesively bonded to the target substrate.

Description

DESCRIPTION TITLE: PROCESS FOR PRODUCING A TWO-DIMENSIONAL MATERIAL TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the field of two-dimensional materials. The invention relates more particularly to a method for producing a two-dimensional material on a substrate of interest. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (MOS2, MoSe2, MoTe2, WS2, WSe2, etc.), are in the form of a monolayer (atomic or molecular) or a stack of monolayers linked together by van der Waals forces. These materials have excellent mechanical, electrical, optical and thermal properties, which make them materials of choice for many applications in fields as diverse as information technology, communication technology, health, energy and transport.

[0003] 2D materials are also considered promising in the field of micro / nanoelectronics, as they allow the production of very thin crystalline layers, typically less than a nanometer. It is therefore envisaged to produce electronic components (such as transistors or photodetectors) of nanometric dimensions on substrates of various types, using the two-dimensional material as a semiconductor material.

[0004] The growth of a 2D material is generally carried out at a very high temperature (800 °C - 1200 °C), which is often incompatible with the substrate on which we wish to integrate this two-dimensional material, called the target substrate or final substrate. Indeed, this target substrate may already support components (or parts of components) that would be degraded during the synthesis of the 2D material. To overcome this problem, the growth and integration steps are separated. The 2D material is grown on a suitable growth substrate and then the 2D material is transferred from its growth substrate to the substrate of interest. The main difficulty lies in preserving the intrinsic properties of the 2D material after transfer.

[0005] Known transfer processes can be divided into two categories: one involving delamination of the 2D material using a liquid, called the wet process, and one where the delamination is mechanical, called the dry process.

[0006] In a wet transfer process, a polymer is deposited on the surface of the 2D material, which is itself placed on its growth substrate, and then the growth substrate is immersed in a liquid (which can be water, an acidic solution, or a basic solution). The 2D material then detaches from the growth substrate and floats on the surface of the liquid. The 2D material must then be recovered with the target substrate, and the polymer removed. The document [“Large-Area Transfer of 2D TMDCs Assisted by a Water-Soluble Layer for Potential Device Applications,” Madan Sharma et al., ACS Omega 2022, 7, 11731-11741] describes an example of a wet transfer process.

[0007] Wet transfer processes are simple to implement and suitable for transferring 2D material layers with an area of ​​a few cm 2 . However, they are not suitable for larger surface areas and the cleanroom environment of the microelectronics industry. In addition, they can leave polymer residues on the 2D material and generate defects such as folds or holes.

[0008] A dry transfer process typically involves depositing a thin layer of strained metal onto the 2D material, followed by depositing a thick layer of polymer as a handle. By applying force to the polymer handle, the 2D material can be completely separated from the growth substrate (a so-called delamination or exfoliation step). The 2D material / metal / polymer stack is then transferred to the target substrate. Finally, the metal layer and the polymer handle are removed successively by immersing the stack in a solvent and then in an etching solution. The papers [“Layer-engineered atomic-scale spalling of 2D van der Waals crystals”, Ji-Yun Moon et al., Matter, 5, 3935-3946, 2022] and [“Controlled crack propagation for atomic precision handling of wafer-scale two-dimensional materials”, Jaewoo Shim et al., Science 362, 665-670, 2018] describe two examples of dry transfer processes.

[0009] Dry transfer processes are suitable for large areas and cleanroom environments. They also allow for control of the number of transferred monolayers (by playing on the level of stress of the metal). On the other hand, they can leave polymer residues and / or metal residues on the 2D material and generate defects, such as cracks. SUMMARY OF THE INVENTION

[0010] There is therefore a need for a process for producing a 2D material on a given target substrate, which is suitable for large areas, compatible with the microelectronics industry and which does not generate residues or defects in the 2D material.

[0011] According to the invention, this need is tended to be satisfied by providing a method for producing a two-dimensional material on a substrate called a target substrate, comprising the following steps: growing the two-dimensional material on a surface of a growth substrate so that the two-dimensional material is bonded to the surface of the growth substrate by van der Waals forces, the surface of the growth substrate consisting of a first material having a first relative permittivity; providing the target substrate, the target substrate having a surface consisting at least in part of a second material having a second relative permittivity strictly greater than the first relative permittivity; assembling the growth substrate and the target substrate by direct bonding between the two-dimensional material and the second material;and separating the growth substrate and the target substrate, whereby at least a portion of the two-dimensional material detaches from the growth substrate and remains adhered to the target substrate.;

[0012] Providing on the surface of the target substrate a second material having a relative permittivity greater than that of the first material, and therefore a polarizability greater than that of the first material, makes it possible to obtain, after bonding, a (van der Waals) bond between the 2D material and the target substrate that is stronger than the (van der Waals) bond between the 2D material and the growth substrate. At least part of the two-dimensional material detaches from the growth substrate and remains bonded to the target substrate, because the latter is more strongly bound to the two-dimensional material than the growth substrate.

[0013] Thanks to direct bonding, the production method according to the invention is simple to implement, compatible with the microelectronics industry and applicable to large-surface 2D material layers. In addition, it does not form any polymer or metal residue, nor does it cause deterioration, because no material is deposited on the 2D material, unlike the transfer methods of the prior art.

[0014] In addition to the characteristics which have just been mentioned in the preceding paragraphs, the preparation method according to the invention may have one or more additional characteristics among the following, considered individually or according to all technically possible combinations: the two-dimensional material and the surface of the target substrate each have a surface roughness of less than 0.5 nm in quadratic mean value; the separation of the growth substrate and the target substrate is initiated by the insertion of a blade between the growth substrate and the target substrate; the growth substrate and the target substrate are immersed in deionized water or an ionic solution during the separation step; the two-dimensional material is composed of a mono-atomic or mono-molecular sheet and the mono-atomic or mono-molecular sheet is transferred onto the target substrate;the two-dimensional material is composed of a stack of monoatomic or mono-molecular sheets and one or more monoatomic or mono-molecular sheets are transferred onto the target substrate; the two-dimensional material is graphene, boron nitride with a hexagonal crystal structure or a transition metal dichalcogenide; the first material is chosen from silicon dioxide (SiC) and sapphire (AI2O3); the second material is chosen from silicon (Si), titanium oxides and advantageously titanium dioxide (TiC), hafnium oxides and; advantageously hafnium dioxide (HfC), and nickel oxides (NiOx); and the two-dimensional material is molybdenum disulfide (M0S2), the first material is silicon dioxide (SiC) and the second material is silicon (Si).

[0015] In an alternative embodiment, the production method comprises the following steps: growing the two-dimensional material on a surface of a growth substrate such that the two-dimensional material is bonded to the surface of the growth substrate by van der Waals forces and composed of a stack of mono-atomic or mono-molecular sheets, the surface of the growth substrate consisting of a first material having a first relative permittivity; providing the target substrate, the target substrate having a surface consisting at least in part of a second material having a second relative permittivity less than or equal to the first relative permittivity; assembling the growth substrate and the target substrate by direct bonding between the two-dimensional material and the second material; and separating the growth substrate and the target substrate;the two-dimensional material having an out-of-plane relative permittivity strictly lower than the first relative permittivity and the second relative permittivity, from which it results that only a part of the mono-atomic or mono-molecular sheets detaches from the growth substrate and remains stuck to the target substrate during the separation step.; BRIEF DESCRIPTION OF THE FIGURES

[0016] Other characteristics and advantages of the invention will emerge clearly from the description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which: Figures 1 A to 1 C schematically represent steps of a method for producing 2D material according to the invention; Figure 2 represents a first mode of implementation of the production method in the case of a multi-layer 2D material; Figure 3 represents a second mode of implementation of the production method in the case of a multi-layer 2D material; and Figure 4 represents a target substrate configured to be partially covered with 2D material using the production method according to the invention.

[0017] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED DESCRIPTION

[0018] Figures 1A to 1C illustrate steps S1 to S3 of a method for producing a two-dimensional material 10 on a substrate 30 hereinafter called the target substrate.

[0019] A two-dimensional (2D) material refers to a material composed of a single-atomic or single-molecular sheet (also called a monolayer) or a stack of N identical single-atomic or single-molecular sheets (N being a natural number greater than or equal to 2). Identical sheets are sheets with atoms or molecules of the same nature and ordered in the same way. A 2D material is called a "single-layer" when it comprises only one sheet and a "multi-layer" when it comprises several sheets. Within each sheet, the atoms (or molecules) are linked together by covalent bonds. The different sheets of a 2D multi-layer material are linked together by van der Waals forces.

[0020] Here, the material is considered to have "2D" properties when it comprises fewer than ten mono-atomic or mono-molecular layers (N<10). Beyond this, its properties are those of a massive material.

[0021] Step S1 shown in Figure 1 A includes providing a growth substrate 20, growing the 2D material 10 from a surface 20s of the growth substrate 20, and providing the target substrate 30.

[0022] The 2D material 10 is thus bound to the surface 20s of the growth substrate 20 by van der Waals forces. The potential energy of the van der Waals forces, also called van der Waals binding (or interaction) energy, between the 2D material 10 and the surface 20s of the growth substrate 20 is denoted “Ei” in the figures.

[0023] The 2D material 10 can be graphene, hexagonal boron nitride (h-BN), or a transition metal dichalcogenide, such as tungsten disulfide (WS2), molybdenum disulfide (M0S2), molybdenum diselenide (MoSe2), or tungsten diselenide (WSe2).

[0024] The growth substrate 20 is a wafer serving as a support for the growth of the 2D material 10. The surface 20s of the growth substrate 20 is made of a first material having a first relative permittivity Sri (also called dielectric constant). This first material is for example silicon dioxide (SiC) or sapphire (AI2O3). The growth substrate 20 may be formed entirely of the first material, as shown in FIG. 1A. Alternatively, it may comprise a support layer made of another material, for example silicon, and a so-called surface layer made of the first material (typically SiC) and arranged on the support layer.

[0025] The growth technique used to grow the 2D material 10 on the surface 20s of the growth substrate 20 may be atomic layer deposition (ALD), vapor phase epitaxy (VPE), metal organic chemical vapor deposition (MOCVD), plasma enhanced chemical vapor deposition (PECVD), or molecular beam epitaxy (MBE). It depends on the 2D material to be grown. The technique used generates van der Waals forces between the 2D material 10 and the surface 20s of the growth substrate 20.

[0026] The target substrate 30 has a surface 30s consisting at least in part of a second material having a second relative permittivity s r2 strictly greater than the first relative permittivity e ri (first material / surface 20s of the growth substrate 20). The target substrate 30 may comprise a support layer 31 made of another material (for example silicon) and a surface layer 32 made (at least in part) of the second material and arranged on the support layer 31 (see Fig. 1A). The surface 30s of the target substrate 30 then belongs to the surface layer 32. Alternatively, the target substrate 30 may be formed entirely of the second material.

[0027] The provision of the target substrate 30 can thus comprise an operation of depositing the surface layer 32 of second material on the support layer 31.

[0028] The target substrate 30 is also in the form of a wafer. It may be intended for the manufacture of integrated circuits and comprise electronic components or parts of electronic components, typically in the support layer 31.

[0029] Preferably, the surface 20s of the growth substrate 20 corresponds to one of its main faces and the surface 30s of the target substrate 30 corresponds to one of its main faces.

[0030] In step S2 of Figure 1 B, the growth substrate 20 (covered with the 2D material 10) and the target substrate 30 are assembled by direct bonding (in other words, without adding adhesive or metallic material) between the 2D material 10 and the second material forming the surface 30s of the target substrate 30. The free surface of the 2D material 10 is thus brought into (direct) contact with the surface 30s of the target substrate 30.

[0031] The 2D material 10 is then bound by van der Waals forces to the surface 30s of the target substrate 30, constituted by the second material (see Fig. 1 B). This bond has an energy E2 strictly greater than the bond energy E1 between the 2D material 10 and the growth substrate 20, due to the fact that the second material has a relative permittivity Er2 greater than that (EM ) of the first material (surface 20s of the growth substrate 20).

[0032] Indeed, there is a relationship between the van der Waals binding energy and the polarizability of the interacting materials. Now the polarizability of a material depends directly on its relative permittivity E r (according to the Clausius-Mossotti relationship). The higher the relative permittivity Er of a material, the higher its polarizability and the stronger the van der Waals bond it is likely to create with another material.

[0033] Thus, a second material having a relative permittivity Er2 greater than that of the first material (EM) is chosen to form the surface 30s of the target substrate 30. so that the polarizability of the second material is greater than that of the first material and therefore the binding energy E2 between the 2D material 10 and the target substrate 30 is greater than the binding energy E1 between the 2D material 10 and the growth substrate 20.

[0034] Table 1 below gives the values ​​of the relative permittivity s r (or dielectric constant) for a selection of materials, including materials commonly used in the microelectronics industry.

[0035] [Table 1]

[0036] The second material is advantageously chosen from silicon (Si), titanium oxides and more particularly titanium dioxide (TiC), hafnium oxides and more particularly hafnium dioxide (HfC) and nickel oxides (NiOx), which have relative permittivity values ​​s r high, especially TiO2 and HfO2.

[0037] Preferably, the relative permittivity s r 2 of the second material is greater than or equal to 110% of the relative permittivity e ri of the first material (Sr2 > 1,1. Erl).

[0038] The bonding surfaces, namely the free surface of the 2D material 10 and the 30s surface of the target substrate 30, advantageously have a surface roughness of less than 0.5 nm. This roughness value, as well as all those given subsequently, are expressed as a root mean square value. The average roughness quadratic (denoted Rq) is determined by a statistical analysis of an atomic force microscope image, taking as a sample a surface of 1x1 pm 2

[0039] 2D material growth techniques make it possible to obtain a surface roughness of less than 0.5 nm. On the other hand, the target substrate 30 may have undergone, between the step S1 of providing the target substrate 30 and the step S2 of assembling by bonding, a polishing step (for example by chemical-mechanical planarization or CMP) of its bonding surface 30s so as to obtain a surface roughness value of less than 0.5 nm.

[0040] Finally, step S3 of FIG. 1 C consists of separating the growth substrate 20 and the target substrate 30. This results in at least a portion of the 2D material 10 detaching from the growth substrate 20. Indeed, the 2D material 10 remains bonded to the substrate with which it has the highest binding energy, here the target substrate 30 (E2>EI).

[0041] The 2D material 10 is thus transferred from the growth substrate 20 to the target substrate 30.

[0042] Separation of the growth substrate 20 and the target substrate 30 may be accomplished in various ways. For example, it may first be initiated by inserting a blade between the growth substrate 20 and the target substrate 30, either manually or by means of a machine. Then, a force is exerted on at least one of the substrates (or wafers) to separate them completely.

[0043] The separation of the growth substrate 20 and the target substrate 30 can further be facilitated by immersing the assembly of the two substrates in deionized water (DW) or an ionic solution during the separation step. After initiating the separation (e.g., by inserting a blade), the deionized water or the ionic solution progresses toward the center of the platelets and helps separate the two substrates. Deionized water is used when the first material (surface 20s) has a hydrophilic character. The ionic solution is used to break bonds of a type other than van der Waals bonds.

[0044] Thus, the production method is based on direct bonding between the 2D material 10, bonded to its growth substrate 20 with a first bonding energy Ei, and a second so-called bonding material, capable of bonding to the 2D material 10 with a second bonding energy E2 greater than the first bonding energy Ei.

[0045] As a result, the process of Figures 1 A-1 C is particularly simple to implement, compatible with clean room environments in the microelectronics industry and allows the transfer of large-area 2D material. In particular, it can be implemented with wafers of 300 mm diameter.

[0046] It does not generate any polymer or metal residue, nor does it cause deterioration, because no material is deposited on the 2D material, unlike the transfer methods of the prior art.

[0047] The method of Figures 1A-1C is applicable to both a single-layer 2D material and a multi-layer 2D material.

[0048] When the 2D material 10 is composed of a single mono-atomic or mono-molecular sheet, this sheet detaches from the growth substrate 30 during the separation step.

[0049] Figures 2 and 3 illustrate two modes of implementation of the production process, when the 2D material 10 is composed of a stack of N mono-atomic or mono-molecular sheets 11 (for example, N=2 in the figures).

[0050] With reference to Figure 2, at least one sheet 11 (among the N) will remain stuck to the growth substrate 20 and at least one sheet 11 will be transferred to the target substrate 30 if the binding energy Ei between the 2D material 10 and the growth substrate 20 is strictly greater than the binding energy Es between the sheets 11. In other words, the separation will take place between two sheets 11.

[0051] With reference to Figure 3, if on the contrary the binding energy Es between the sheets 11 is strictly greater than the binding energy Ei between the growth substrate 20 and the 2D material 10, all of the sheets 11 (i.e. the N sheets) will be transferred onto the target substrate 30.

[0052] Thus, the transfer of the 2D material 10 can be partial or total, taking into account its thickness.

[0053] In an alternative implementation of the production method, applicable to a 2D material 10 composed of a stack of N mono-atomic or mono-molecular sheets 11, the 2D (multi-layer) material has an out-of-plane relative permittivity s r 3 strictly less than the first relative permittivity e ri (first material / surface 20s of the growth substrate 20) and at the second permittivity relative Er2 (second material I surface 30s of the target substrate 30), hence E3 < E1 and E3 < E2. It follows that the separation of the substrates (step S3 following the direct bonding step S2) will take place between two sheets 11. Thus, at least one sheet 11 (among the N) will remain bonded to the growth substrate 20 and at least one sheet 11 will be transferred to the target substrate 30. In this variant, the second relative permittivity 8r2 is less than or equal to the first relative permittivity e ri .

[0054] The 2D material 10 can be transferred so as to completely cover the surface 30s of the target substrate 30 or to cover only a part of the surface 30s of the target substrate 30. Indeed, when the second material constitutes only a part of the surface 30s of the target substrate 30, being located in one or more predefined zones 33 as represented by FIG. 4, the 2D material 10 is transferred only onto the zone(s) 33 of the target substrate 30. Nevertheless, if the zones 30 are sufficiently close to each other, the 2D material 10 can be transferred even between these zones 30, or even onto the entire surface 30s of the target substrate 30.

[0055] Thus, the transfer of the 2D material 10 can be partial or total, taking into account its surface area.

[0056] An example of the implementation of the development process will now be described.

[0057] Step S1

[0058] A stack of three monolayers of M0S2 was obtained on a growth substrate 20 of 200 mm diameter, according to the method described in the document [“Elaboration of monolayers of dichalcogenides of group (VI) transition metals by surface organometallic chemistry”, S. Cadot, Materials. University of Lyon, 2016. French. NNT: 2016LYSE1075. tel-01530918]. This method includes a low-temperature ALD deposition step (about 100 °C) and a high-temperature (900 °C) material crystallization step to obtain the typical lamellar structure of 2D materials. The growth substrate 20 comprises a silicon support layer (or base layer) and a SiC>2 surface layer, obtained for example by thermal oxidation of the silicon support layer. The 2D material 10 is therefore M0S2 and the surface 20s of the growth substrate 20 is composed (here entirely) of SiO2 (EM = 3.9). The target substrate 30 provided is entirely made of silicon (s r 2 = 11.68). Thus, the first material is SiO2 and the second material is silicon. The surface roughness of the 2D material 10 is less than 0.3 nm, that of the SiO2 surface layer is less than 0.2 nm and that of the silicon target substrate 30 is less than 0.1 nm.

[0059] Step S2

[0060] The growth substrate 20 and the target substrate 30 are then assembled, by direct bonding between the 2D material 10 and the silicon of the target substrate 30, preferably at room temperature and in ambient air.

[0061] Step S3

[0062] The separation of the growth substrate 20 and the target substrate 30 is accomplished by immersion in deionized water (DW). It is initiated by the introduction of a blade. The hydrophilicity of SiO2 allows an advance of the deionized water to the SiO2 / MoS2 interface and a separation of the substrates at this interface.

[0063] The 2D material 10 is thus transferred onto the target substrate 30 made of silicon (with which the van der Waals bonds are the strongest). All the monolayers of M0S2 are transferred here, because the out-of-plane relative permittivity of M0S2 (Sr3 = 11,111) is greater than the relative permittivity e ri of the first material (and therefore E3 > E1; see Fig.3).

Claims

CLAIMS

1. A method of producing a two-dimensional material (10) on a substrate (30) called the target substrate, comprising the following steps: - growing (S1) the two-dimensional material (10) on a surface (20s) of a growth substrate (20) such that the two-dimensional material (10) is bonded to the surface (20s) of the growth substrate (20) by van der Waals forces, the surface (20s) of the growth substrate (20) being made of a first material having a first relative permittivity (EM); - providing (S1) the target substrate (30), the target substrate (30) having a surface (30s) consisting at least in part of a second material having a second relative permittivity (Er2) strictly greater than the first relative permittivity (EM); - assembling (S2) the growth substrate (20) and the target substrate (30) by direct bonding between the two-dimensional material (10) and the second material; and - separating (S3) the growth substrate (20) and the target substrate (30), whereby at least a portion of the two-dimensional material (10) detaches from the growth substrate (20) and remains adhered to the target substrate (30).

2. The method of claim 1, wherein the separation of the growth substrate (20) and the target substrate (30) is initiated by inserting a blade between the growth substrate and the target substrate.

3. Method according to one of claims 1 and 2, wherein the growth substrate (20) and the target substrate (30) are immersed in deionized water or an ionic solution during the separation step (S3).

4. A method according to any one of claims 1 to 3, wherein the two-dimensional material (10) is composed of a monoatomic or monomolecular sheet and wherein the monoatomic or monomolecular sheet is transferred onto the target substrate (30).

5. A method according to any one of claims 1 to 3, wherein the two-dimensional material (10) is composed of a stack of mono-atomic or mono-molecular sheets (11) and wherein one or more mono-atomic or mono-molecular sheets (11) are transferred onto the target substrate (30).

6. A method according to any one of claims 1 to 5, wherein the two-dimensional material (10) is graphene, hexagonal boron nitride (h-BN) or a transition metal dichalcogenide.

7. A method according to any one of claims 1 to 6, wherein the first material is selected from silicon dioxide (SiC) and sapphire (AI2O3).

8. A method according to any one of claims 1 to 7, wherein the second material is selected from silicon (Si), titanium oxides, hafnium oxides and nickel oxides (NiOx).

9. A method according to any one of claims 1 to 8, wherein the two-dimensional material (10) is molybdenum disulfide (M0S2), the first material is silicon dioxide (SiC) and the second material is silicon (Si).

10. A method of producing a two-dimensional material (10) on a substrate (30) called the target substrate, comprising the following steps: - growing (S1) the two-dimensional material (10) on a surface (20s) of a growth substrate (20) such that the two-dimensional material (10) is bonded to the surface (20s) of the growth substrate (20) by van der Waals forces and composed of a stack of monoatomic or mono-molecular sheets (11), the surface (20s) of the growth substrate (20) being made of a first material having a first relative permittivity (e ri ) ; - providing (S1) the target substrate (30), the target substrate (30) having a surface (30s) consisting at least in part of a second material having a second relative permittivity (s r 2) less than or equal to the first relative permittivity (e ri ) ; - assembling (S2) the growth substrate (20) and the target substrate (30) by direct bonding between the two-dimensional material (10) and the second material; and - separating (S3) the growth substrate (20) and the target substrate (30); method in which the two-dimensional material (10) has an out-of-plane relative permittivity (Er3) strictly lower than the first relative permittivity (EM) and the second relative permittivity (Er2), from which it results that only a part of the mono-atomic or mono-molecular sheets (11) detaches from the growth substrate (20) and remains stuck to the target substrate (30) during the separation step.

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