PROCESS FOR THE PREPARATION AND TRANSFER OF A TWO-DIMENSIONAL MATERIAL

Abstract

The invention relates to a method for producing and transferring a two-dimensional material, comprising the following steps: forming on a growth substrate (10) a stack (20) comprising a layer of two-dimensional material (21), a stiffening layer (22) and a sacrificial layer (23) made of polymer material, the step of forming the stack (20) comprising the following sub-steps:growing the layer of two-dimensional material (21) on the growth substrate (10);depositing the stiffening layer (22) on the layer of two-dimensional material (21); andforming the sacrificial layer (23) on the stiffening layer (22); separating the growth substrate (10) and the stack (20) by immersing them in a liquid (30); transferring (S3) the stack (20) onto a target substrate (40), the two-dimensional material layer (21) being arranged between the stiffening layer (22) and the target substrate (40); removing the sacrificial layer (23);and remove the stiffening layer (22). Figure to be published with the abstract: Figure 1E;

Description

Title of the invention: METHOD FOR PRODUCING AND TRANSFERRING 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 and transferring a two-dimensional material. 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 transportation.

[0003] 2D materials are also considered promising in the field of micro / nanoelectronics, insofar as they make it possible to obtain crystalline layers of very low thickness, typically less than a nanometer. It is thus envisaged to produce electronic components (such as transistors or photodetectors) of nanometric dimensions on substrates of various natures, 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 one wishes to integrate this two-dimensional material, called the target substrate or final substrate. Indeed, this target substrate can already support components (or parts of components) which 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 the conservation of the intrinsic properties of the 2D material after transfer.

[0005] The known transfer methods can be divided into two categories, that involving delamination of the 2D material by means of a liquid, called the wet method, and that where the delamination is mechanical, called the dry method.

[0006] In a wet transfer process, a polymer is deposited on the surface of the 2D material, itself arranged on its growth substrate, then this growth substrate growth 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 then 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 a surface area of ​​a few cm2. However, they are not suitable for larger surface areas and the clean room environment of the microelectric 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 generally involves depositing a thin layer of strained metal onto the 2D material, followed by depositing a thick layer of polymer acting as a handle. By exerting force on 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 surfaces and cleanroom environments. They also allow the number of monolayers transferred to be controlled (by adjusting the stress level of the metal). However, they can generate defects such as cracks. Summary of the invention

[0010] There is therefore a need for a method for producing and transferring a two-dimensional material which does not generate residues or defects in the two-dimensional material.

[0011] According to the invention, this need is tended to be satisfied by providing a method for producing and transferring a two-dimensional (2D) material, comprising the following steps: • form on a growth substrate a stack comprising a layer of two-dimensional material, a stiffening layer and a sacrificial layer of polymer material, the stack formation step comprising the following sub-steps: • growing the two-dimensional material layer on the growth substrate; • deposit the stiffening layer on the layer of two-dimensional material; and • form the sacrificial layer on the stiffening layer; • separate the growth substrate and the stack by immersing them in a liquid; • transferring the stack onto a target substrate, the layer of two-dimensional material being arranged between the stiffening layer and the target substrate; • remove the sacrificial layer; and • remove the stiffening layer.

[0012] The separation in the liquid, combined with the combination of the sacrificial layer and the stiffening layer, allows a transfer of the 2D material layer (on the target substrate) without polymer residues and without defects such as folds or cracks. The sacrificial layer facilitates the separation of the stack and the growth substrate, while the stiffening layer allows a total removal of the polymer layer and prevents the 2D material layer from forming folds during the separation step.

[0013] In a first embodiment of the method, the stack is transferred onto the target substrate by recovering the stack in the liquid with the target substrate.

[0014] In a second embodiment, the step of forming the stack further comprises a sub-step of depositing a manipulation layer on the sacrificial layer, the method further comprising a step of removing the manipulation layer between the transfer step and the step of removing the sacrificial layer.

[0015] According to a development of this second embodiment, the method further comprises a step of drying the stack between the separation step and the transfer step.

[0016] According to another development compatible with the previous one, the handling layer is formed from an adhesive film which can be removed by heat treatment or from an adhesive film which can be removed by exposure to ultraviolet radiation.

[0017] In addition to the characteristics which have just been mentioned in the preceding paragraphs, the preparation and transfer 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 stiffening layer is made of a material having a Young's modulus greater than 50 GPa, preferably greater than 70 GPa; • the stiffening layer is made of alumina (A12O3), aluminum nitride (AIN) or a metal, for example gold or nickel; • the stiffening layer has a thickness of between 5 nm and 100 nm, preferably between 5 nm and 15 nm; • the sacrificial layer is made of polystyrene or formed from a resin; • the liquid is deionized water or an ionic solution; and • the two-dimensional material is graphene, boron nitride with a structure hexagonal crystalline (h-BN) or a transition metal dichalcogenide. BRIEF DESCRIPTION OF THE FIGURES

[0018] 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 1A to 1G schematically represent a first mode of implementation of a method for producing and transferring 2D material according to the invention; and • Figures 2A to 21 schematically represent a second mode of implementation of the method for producing and transferring 2D material according to the invention.

[0019] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED DESCRIPTION

[0020] Figures 1A to 1G illustrate steps S1 to S5 of a method for producing and transferring a two-dimensional material according to a first embodiment of the invention.

[0021] A two-dimensional (2D) material refers to a material composed of a mono-atomic or mono-molecular sheet (also called a monolayer) or a stack of N identical mono-atomic or mono-molecular sheets (N being a natural number greater than or equal to 2). By identical sheets, we mean sheets having atoms or molecules of the same nature and ordered in the same way. The 2D material is called "mono-layer" when it comprises only one sheet and "multi-layer" when it comprises several sheets. Within each sheet, the atoms (or molecules) are linked (linked) to each other (them) by covalent bonds. The different sheets of a multi-layer 2D material are linked to each other by van der Waals forces.

[0022] Here, the material is considered to have “2D” properties when it comprises fewer than ten mono-atomic or mono-molecular layers (N<10). Beyond that, its properties are those of a solid material.

[0023] The method firstly comprises a step S1 of forming, on a growth substrate 10, a stack 20 comprising a layer of 2D material 21, a stiffening layer 22 and a sacrificial layer 23 made of polymer material. This first step S1 is here broken down into several sub-steps S1 to S13 represented by figures 1A to 1C.

[0024] Sub-step SI 1 of [Fig.lA] consists of growing the 2D material layer 21 from a surface 10s of the growth substrate 10.

[0025] The 2D material may be graphene, hexagonal boron nitride (h-BN), or a transition metal dichalcogenide, such as tungsten disulfide (WS2), molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), or tungsten diselenide (WSe2). The 2D material is bonded to the surface 10s of the growth substrate 10 by van der Waals forces.

[0026] The growth substrate 10 is a wafer serving as a support for the growth (or development) of the 2D material. The surface 10s of the growth substrate 10 corresponds to one of the main faces of this wafer.

[0027] As illustrated by [Fig. 1A], the growth substrate 10 may comprise a support layer 11 made of a first material and a surface layer 12 made of a second material distinct from the first material. For example, the first material is silicon (Si) and the second material is silicon dioxide (SiO2). The surface layer 12 is arranged on the support layer 11 and the 2D material layer 21 is grown on the surface layer 12. The surface 10s of the growth substrate 10 then belongs to the surface layer 12.

[0028] Alternatively, the growth substrate 10 is made of a single material, for example sapphire (A12O3).

[0029] The growth technique used to grow the 2D material on the surface 10s of the growth substrate 10 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.

[0030] Then, during sub-step S12 of [Fig. 1B], the stiffening layer 22 is deposited on the 2D material layer 21. The stiffening layer 22 is advantageously formed from a material other than a polymer material.

[0031] The stiffening layer 22 is preferably made of a material having a Young's modulus greater than 50 GPa, more preferably greater than 70 GPa. It can in particular be made of alumina (A12O3), aluminum nitride (AIN) or a metal, for example gold (Au) or nickel (Ni).

[0032] The stiffening layer 22 preferably has a thickness of between 5 nm and 100 nm, advantageously between 5 nm and 15 nm in the case of a stiffening layer 22 made of metal such as nickel. This low thickness limits the mechanical stress in the stiffening layer 22.

[0033] The technique used to deposit the stiffening layer 22 is preferably chosen from evaporation techniques (in particular vacuum evaporation), atomic layer deposition (or ALD), assisted or not by a plasma (PEALD, for “plasma-enhanced ALD”), pulsed laser ablation (or PLD, for “pulsed laser deposition”) and ion beam deposition (or IBD, for “ion beam deposition”). The deposition technique is chosen so as not to damage the 2D material.

[0034] Sub-step S13 of [Fig.1C] consists of forming the sacrificial layer 23 of polymer material on the stiffening layer 22. The formation of the sacrificial layer 23 may comprise several successive operations: the dissolution of the polymer material in a solvent to obtain a polymer solution, the deposition of a film of the solution on the stiffening layer 22, for example by spin coating, and the evaporation of the solvent, for example by heating. In the case of a very volatile solvent such as toluene, the evaporation takes place during the centrifugation. The sacrificial layer 23 may in particular be formed of polystyrene (PS).

[0035] The sacrificial layer 23 may also be formed from a resin like those used for photolithography steps in the microelectronics industry. The formation of the sacrificial layer 23 then comprises an operation of spreading the resin, for example by centrifugal coating. It may also comprise a step of crosslinking the resin, for example by heat treatment (typically from 100°C to 150°C).

[0036] The thickness of the sacrificial layer 23 is for example between 400 nm and 1 pm.

[0037] With reference to [Fig. 1D], the method then comprises a step S2 called wet delamination, during which the substrate 10 and the stack 20 are separated by immersing them in a liquid 30. The liquid 30 is such as to break the bonds between the growth substrate 10 and the 2D material layer 21, in particular the van der Waals bonds. This liquid 30 may be deionized water or an ionic solution.

[0038] Wet delamination is a gentle process that generates little stress in the 2D material layer, unlike dry delamination using a handle layer. It does not fracture the 2D material layer 21.

[0039] The stiffening layer 22 provides rigidity to the 2D material layer 21 to prevent it from forming folds during this wet delamination step S2.

[0040] In step S3 of [Fig. 1E], the stack 20 is transferred onto a target substrate 40, also called the substrate of interest or final substrate, so as to arrange the 2D material layer 21 between the stiffening layer 22 and the target substrate 40. Preferably, the 2D material layer 21 is brought into contact with a surface 40s of the target substrate 40.

[0041] Like the growth substrate 10, the target substrate 40 may comprise a support layer made of a material (for example silicon) and a surface layer composed of one or more other materials and disposed on the support layer.

[0042] The target substrate 40 is also in the form of a wafer. It may be intended for the manufacture of integrated circuits and may comprise electronic components or parts of electronic components (typically in the support layer), in which it is desired to integrate at least part of the 2D material layer 21. The surface 40s of the target substrate 40 corresponds to one of the main faces of this wafer.

[0043] In this first embodiment, the stack 20 is transferred onto the target substrate 40 by recovering the stack 20 in the liquid 30 with the target substrate 40. The transfer is therefore accomplished wet, by immersing the target substrate 40 in the liquid 30.

[0044] After its development on the growth substrate 10 (see [Fig. 1A]), the 2D material layer 21 was therefore transferred from the growth substrate 10 to the target substrate 40.

[0045] The transfer step S3 may be followed by a drying step (not shown in the figures), for example for 24 hours in ambient air, to reinforce the adhesion of the stack 20 (and therefore the adhesion of the 2D material layer 21) on the target substrate 40.

[0046] The low stress in the stiffening layer 22 also reduces the risk of detachment (or lifting) of the 2D material layer 21.

[0047] At the end of the transfer step S3, a temporary structure is obtained comprising successively the target substrate 40, the 2D material layer 21, the stiffening layer 22 and the sacrificial layer 23 (see [Fig. 1E]). The following steps S4 and S5 of the method relate to the “disassembly” of this temporary structure to keep only the 2D material layer 21 on the target substrate 40.

[0048] Step S4 of [Fig. 1F] consists of removing the sacrificial layer 23, preferably by wet method, typically using a solvent. This solvent may be the same as that used to form the sacrificial layer 23.

[0049] Any polymer residues left by such shrinkage on the stiffening layer 22 can be removed by means of plasma etching, for example by means of an oxygen (O2) plasma in the case of a sacrificial layer 23 made of polystyrene.

[0050] Thus, the sacrificial layer 23 is removed entirely.

[0051] Finally, the stiffening layer 22 is also removed entirely during a step S5 represented by [Fig. 1G]. It is preferably removed by wet etching, for example using a sulfuric acid solution in the case of a nickel stiffening layer 22.

[0052] The method of Figures 1A-1G does not cause any contamination of the 2D material, nor does it cause defects such as folds or cracks in the 2D material layer 21. The sacrificial layer 23 of polymer material, combined with the stiffening layer 22, facilitates the separation step S2. The sacrificial layer 23 is separated from the 2D material layer 21 by the stiffening layer 22 and can therefore be removed entirely without risk of damaging the 2D material layer 21.

[0053] The method is furthermore simple to implement, compatible with clean room environments in the microelectronics industry and allows the transfer of large-area 2D material layers. In particular, it can be implemented with wafers of 300 mm diameter.

[0054] Figures 2A to 21 illustrate a second mode of implementation of the preparation and transfer method, which differs from the first essentially in that a handling layer 24 (also called a handle layer) is used to facilitate the transfer of the stack 20.

[0055] [Fig.2A], [Fig.2B] and [Fig.2C] represent the sub-steps S11, S12 and S13 of the step S11 of forming the stack 20. They are accomplished in the manner described previously in relation to FIGS. 1A-1C.

[0056] The manipulation layer 24 is deposited on the sacrificial layer 23 during a sub-step S14 of the step S1 of forming the stack 20. The sub-step S14 of depositing the manipulation layer 24 is represented by [Fig.2D] and subsequent to the sub-steps S11 to S13.

[0057] Thus, in this second embodiment, the stack 20 further comprises the handling layer 24.

[0058] The handling layer 24 is preferably formed from an adhesive film that can be peeled off by thermal treatment (“thermal release tape” in English) or from an adhesive film that can be peeled off by exposure to ultraviolet radiation (“ultraviolet release tape” in English). More generally, it can be formed by any adhesive having an adhesion energy after treatment lower than the Van der Waals energy between the 2D material layer 21 and the growth substrate 10.

[0059] After the wet delamination step S2 (see [Fig.2E]), the stack 20 is positioned on the target substrate 40 (the 2D material layer 21 between the stiffening layer 22 and the target substrate 40) during the transfer step S3 of [Fig.2F].

[0060] The handling layer 24 makes it possible to easily extract the other layers of the stack 20 (which are much thinner) from the liquid 30 without damaging them and avoids having to dip the target substrate 40 in the liquid 30 to recover the stack 20.

[0061] The handling layer 24 allows dry transfer of the stack 20. The stack 20 is therefore advantageously dried between the separation step S2 and the transfer step S3, for example using a blower.

[0062] Another, longer drying may be performed after the transfer of the stack 20 to strengthen its adhesion with the target substrate 40, as previously described.

[0063] In this second embodiment of the method, the dismantling of the temporary structure obtained at the end of the transfer (and further comprising the handling layer 24) further comprises a step S3' of removing the handling layer 24. This step S3' of removing the handling layer 24, illustrated by [Fig.2G], is accomplished before step S4 of removing the sacrificial layer 23.

[0064] For example, in the case of a handling layer 24 formed of an adhesive film removable by heat treatment, the temporary structure is heated, typically to a temperature between 100°C and 130°C.

[0065] Step S4 of removing the sacrificial layer 23 and step S5 of removing the stiffening layer 22, illustrated respectively by [Fig.2H] and [Fig.21], are then carried out in the manner described previously in relation to FIGS. 1F and 1G.

[0066] The sacrificial layer 23 is all the more advantageous in this second embodiment since it prevents residues of the handling layer 24 from being deposited on the 2D material layer 21. Indeed, if the handling layer 24 were directly deposited on the stiffening layer 22, adhesive residues would be scattered in the etching solution of the stiffening layer 22 and would be redeposited on the 2D material layer 21. Here, the adhesive residues are eliminated when the sacrificial layer 23 is removed.

[0067] The production and transfer method according to the invention is applicable both to a single-layer 2D material and to a multi-layer 2D material.

Claims

Claims

1. Method for producing and transferring a two-dimensional material, comprising the following steps: - forming (SI) on a growth substrate (10) a stack (20) comprising a layer of two-dimensional material (21), a stiffening layer (22) and a sacrificial layer (23) made of polymer material, the step of forming the stack (20) comprising the following sub-steps: • growing (SI 1) the layer of two-dimensional material (21) on the growth substrate (10); • depositing (S 12) the stiffening layer (22) on the layer of two-dimensional material (21); and • forming (S 13) the sacrificial layer (23) on the stiffening layer (22); - separating (S2) the growth substrate (10) and the stack (20) by immersing them in a liquid (30);- transferring (S3) the stack (20) onto a target substrate (40), the two-dimensional material layer (21) being arranged between the stiffening layer (22) and the target substrate (40); - removing (S4) the sacrificial layer (23); and - removing (S5) the stiffening layer (22).;

2. The method of claim 1, wherein the step (S1) of forming the stack (20) further comprises a sub-step (S14) of depositing a handling layer (24) on the sacrificial layer (23), the method further comprising a step (S3') of removing the handling layer (24) between the transfer step (S3) and the step (S4) of removing the sacrificial layer (23).

3. The method of claim 2, further comprising a step of drying the stack (20) between the separation step (S2) and the transfer step (S3).

4. Method according to one of claims 2 and 3, in which the handling layer (24) is formed of an adhesive film removable by heat treatment or an adhesive film removable by exposure to ultraviolet radiation.

5. The method of claim 1, wherein the stack (20) is

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11. transferred onto the target substrate (40) by recovering the stack (20) in the liquid (30) with the target substrate (40). Method according to any one of claims 1 to 5, in which the stiffening layer (22) is made of a material having a Young's modulus greater than 50 GPa, preferably greater than 70 GPa. Method according to any one of claims 1 to 6, in which the stiffening layer (22) is made of alumina (Al2O3), aluminum nitride (AIN) or a metal, for example gold or nickel. Method according to any one of claims 1 to 7, in which the stiffening layer (22) has a thickness of between 5 nm and 100 nm, preferably between 5 nm and 15 nm. A method according to any one of claims 1 to 8, wherein the sacrificial layer (23) is made of polystyrene or formed of a resin. A method according to any one of claims 1 to 9, wherein the liquid (30) is deionized water or an ionic solution. The method of any one of claims 1 to 10, wherein the two-dimensional material is graphene, hexagonal boron nitride (h-BN) or a transition metal dichalcogenide.

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