Method for transferring complete two-dimensional material on steep side wall through wet method based on stress transfer principle

Abstract

The invention relates to the technical field of two-dimensional materials, in particular to a method for transferring a complete two-dimensional material on a steep side wall through a wet method based on the stress transfer principle. The method comprises the following steps: coating a polymer supporting layer on the surface of a two-dimensional material, continuously coating a photoresist layer on the surface of the two-dimensional material with the polymer supporting layer, and then patterning the photoresist layer to form a stress transfer region without photoresist and a stress protection region with photoresist; and finally, transferring the two-dimensional material with the composite sacrificial coating onto a target substrate. Wherein the target substrate is provided with a plane structure with steps. By regulating the pattern of the photoresist layer, the stress concentration degrees of areas with different thicknesses are different, so that the stress release of the two-dimensional material is realized; natural fracture occurs in an area without photoresist protection, and it is ensured that the two-dimensional material is completely transferred on the side wall; the limitation of the prior art is solved, and a new way is provided for preparing a novel two-dimensional device.

Description

Technical Field

[0001] The present invention relates to the technical field of two-dimensional materials, and in particular to a method for wet-transferring a complete two-dimensional material on a steep side wall based on the stress transfer principle. Background Art

[0002] Two-dimensional materials are an emerging type of nanomaterials. Due to their unique physical and chemical properties, they have broad application prospects in the fields of electronics, optoelectronics, energy, environment, and biosensors. In addition, two-dimensional materials have great potential to become the constituent materials of the new generation of transistors in integrated circuits below the 1nm node. However, the preparation and transfer process of two-dimensional materials faces many challenges.

[0003] Currently, two-dimensional materials are usually prepared by mechanical exfoliation or chemical vapor deposition, and then need to be transferred to the desired substrate. Traditional dry transfer methods have some limitations, such as difficulty in controlling the directional arrangement of materials, high risk of contamination, and strict requirements on the surface morphology of the substrate. When there are steep steps or other complex three-dimensional structures on the substrate surface, dry transfer often cannot achieve complete and uniform transfer of two-dimensional materials.

[0004] The wet transfer method transfers the two-dimensional material in a liquid medium, avoiding mechanical damage to the material under dry conditions, and has the advantages of simple operation and low cost. However, the existing wet transfer method is mainly for planar substrates. For substrates with three-dimensional structures, especially those with steep steps, it is still difficult to achieve complete transfer of two-dimensional materials. Summary of the invention

[0005] The present invention provides a method for wet transfer of two-dimensional materials, which is a method for wet transfer of complete two-dimensional materials on steep side walls based on the principle of stress transfer, to solve the above-mentioned defects in the prior art. This method can realize complete and high-quality transfer of two-dimensional materials on a substrate surface with complex three-dimensional structures such as steep high side walls, overcoming the limitation that two-dimensional materials can only be transferred on flat surfaces in the past, and greatly broadening the application scenarios of two-dimensional materials.

[0006] Based on this, the present invention has the following technical solutions: In a first aspect, the present invention provides a method for wet transfer of two-dimensional materials, comprising: coating a polymer support layer on the surface of the two-dimensional material, further coating a photoresist layer on the surface of the two-dimensional material with the polymer support layer, and then patterning the photoresist layer to form a stress transfer area without photoresist and a stress protection area with photoresist; finally, transferring the two-dimensional material with the composite sacrificial coating to a target substrate; Wherein, the target substrate has a planar structure with steps.

[0007] The present invention is based on the principle of stress transfer, and adopts patterned photoresist and polymer support layer as composite sacrificial coating during transfer. Different thicknesses of the composite sacrificial coating at different positions lead to different stress strains, thereby achieving different stress distributions in different areas of the two-dimensional material. Natural fracture after transfer occurs in the area without photoresist protection, ensuring the complete transfer of the two-dimensional material on the step side wall.

[0008] In the present invention, the photoresist is used as a stress protection layer, and after patterning, a stress transfer area without photoresist and a stress protection area with photoresist are formed, wherein the patterned area (i.e., the stress protection area with photoresist) is used to protect the two-dimensional material on the step, while other areas are used as stress transfer areas without photoresist, ensuring that the part of the two-dimensional material that is naturally torn will be in the place without the photoresist layer.

[0009] In the present invention, the target substrate has a planar structure with steps; specifically, it may be a step structure or a steep high side wall with a high aspect ratio.

[0010] In the specific implementation process, those skilled in the art can pattern the coated photoresist using existing and conventional methods in the art, which are not specifically limited here.

[0011] In the present invention, a layer of photoresist can be uniformly coated on the surface of the polymer support layer by spin coating, spray coating or dip coating; spin coating is the most common method, and the photoresist can be uniformly distributed by rotating the substrate at high speed and using centrifugal force. After coating, a preliminary heat treatment (soft baking) is performed to remove the solvent in the photoresist, preliminarily solidify the photoresist layer, and improve adhesion. Then, the two-dimensional material coated with the photoresist is placed in a photolithography machine, and an alignment system is used to ensure that the pattern on the mask is aligned with the expected pattern position on the substrate. Through the exposure system of the photolithography machine, light of a specific wavelength (such as ultraviolet light) is used to irradiate the photoresist on the substrate, and the pattern is transferred through the mask. During the exposure process, the photosensitive components of the photoresist undergo a chemical reaction. After exposure, post-baking can be performed to further solidify the photoresist and enhance its stability in subsequent development and etching steps. Then, a developer is used to dissolve the photoresist in the unexposed area (positive photoresist) or the photoresist in the exposed area (negative photoresist) to form the desired convex or concave pattern.

[0012] In the present invention, the photoresist layer is different from the PMMA support layer and can be well attached to the PMMA layer without gaps. The PMMA layer is an electron beam photoresist, so ordinary photolithography light cannot pattern the PMMA layer, and can only photolithography the photoresist layer, which leads to the preparation of the support layer and the photolithography layer.

[0013] Preferably, during the transfer, the stress protection area is adjusted to be aligned with the step structure of the target substrate to ensure that the area protected by the photoresist falls on the step structure.

[0014] In the present invention, when the height of the step is less than 5 μm, the conventional transfer method in the prior art is very likely to cause cracks on the side wall, while the above transfer method can achieve high-quality tear-free transfer.

[0015] As a preferred embodiment of the present invention, the method for wet transfer of two-dimensional materials comprises: S1: transferring the two-dimensional material onto a sapphire substrate to obtain a two-dimensional material with a substrate; S2: sequentially coating a polymer support layer and a photoresist layer on the two-dimensional material with a substrate, and then patterning the photoresist layer to form a stress transfer area without photoresist and a stress protection area with photoresist; S3: releasing the two-dimensional material with the composite sacrificial coating obtained in S2 and removing the sapphire substrate; S4: The two-dimensional material with the composite sacrificial coating obtained in S3 after removing the sapphire substrate is transferred to a target substrate.

[0016] In the present invention, for some special two-dimensional materials, protection and patterning as well as release of the two-dimensional materials can be performed directly from their specific target substrates.

[0017] Preferably, after the transfer, the polymer support layer and the photoresist layer are removed, so as to finally obtain a complete two-dimensional material layer without tearing on the target substrate.

[0018] Preferably, the two-dimensional material includes graphene, two-dimensional transition metal chalcogenide TMDs material and various two-dimensional materials suitable for large-area transfer; the two-dimensional transition metal chalcogenide TMDs material includes molybdenum disulfide, tungsten disulfide, tungsten selenide or bismuth selenide.

[0019] For any two-dimensional material, since it may be on a variety of substrates, it can be first transferred into water, then scooped onto a sapphire substrate, and then subjected to subsequent operations, and finally released into a BOE solution, thereby completing the preparation of the thin film structure.

[0020] When the two-dimensional material is graphene, since it grows on copper foil, it can be applied to a hard substrate first, subjected to photolithography and patterning, and then placed in an ammonium persulfate solution to dissolve the copper foil, thereby obtaining the two-dimensional material. It is then scooped onto a sapphire substrate, subjected to subsequent operations, and finally released in a BOE solution, thereby completing the preparation of the thin film structure.

[0021] When the two-dimensional material is molybdenum disulfide, since it is originally on a hard substrate such as sapphire, it can be directly coated with polymer and photoresist and subjected to photolithography and development, and then the film can be released through BOE.

[0022] Preferably, the thickness of the polymer support layer is 100 nm to 1 μm; and / or the thickness of the photoresist layer is 200 nm to 2 μm.

[0023] In the present invention, the photoresist may be a positive photoresist or a negative photoresist; preferably, the photoresist includes AZ601, AZ series photoresist or AR series photoresist.

[0024] Preferably, the polymer comprises PMMA or PPC.

[0025] Preferably, the target substrate is a non-conductive smooth substrate.

[0026] Further preferably, a low temperature etching method may be used to ensure a smooth surface.

[0027] Further preferably, the method for preparing the target substrate includes: deeply etching silicon to form a step structure; then performing thermal oxidation treatment to generate a silicon dioxide layer on the step surface; then using a buffered oxide etchant to remove the silicon dioxide layer to obtain a smooth silicon surface; then performing thermal oxidation treatment again or directly ICP-PECVD growing a smooth silicon dioxide medium to generate an insulating silicon dioxide layer on the step surface.

[0028] The present invention finds that by adopting the above-mentioned process to construct a non-conductive smooth surface, two-dimensional materials can be well received, which is more conducive to achieving high-quality transfer. In addition, the operation is simple and low-cost, and high-quality wet transfer of two-dimensional materials on complex three-dimensional surfaces can be achieved.

[0029] In the present invention, during the wet transfer process, the pattern of the photoresist in the photoresist / PMMA / two-dimensional material needs to be aligned with the pattern on the substrate with smooth steps to ensure that the area protected by the photoresist can fall on the steps, and by standing still to wait for water to evaporate, the two-dimensional material on the steps can be protected by the different stress distributions in different areas.

[0030] In a second aspect, the present invention provides a method for preparing a two-dimensional material electronic device, which includes the above-mentioned method for wet transfer of two-dimensional materials.

[0031] The method for wet transfer of two-dimensional materials provided by the present invention adopts a PMMA layer as a support layer and a photoresist layer as a stress protection layer to form a composite sacrificial coating structure; by adjusting the pattern of the photoresist layer, the stress concentration degrees in areas with different thicknesses are different, thereby achieving stress release of the two-dimensional material; natural fracture occurs in areas without photoresist protection, ensuring that the two-dimensional material is completely transferred on the side wall; the limitations of the prior art are solved, and a new approach is provided for preparing new two-dimensional devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0033] Figure 1 Schematic diagram of the wet transfer of two-dimensional materials and test electrode structures on steep side walls based on the stress transfer principle provided by the present invention. 101 is a smooth step substrate, 102 is a two-dimensional material transferred to the step and patterned, 103 is an electrode on the step, and 104 is an electrode under the step.

[0034] Figure 2 Schematic diagram of the method for preparing a smooth step substrate according to an embodiment of the present invention, wherein 201 is an etched step, 202 is a material whose step surface is silicon dioxide, 203 is a smooth step surface, and 204 is a non-conductive smooth substrate.

[0035] Figure 3 Schematic diagram of the method for preparing photoresist / PMMA / two-dimensional material structure of various two-dimensional materials according to the embodiment of the present invention. 301 represents the transfer flow chart when the two-dimensional material is graphene, 302 represents the transfer flow chart when the two-dimensional material is molybdenum disulfide, and 301 represents the transfer flow chart of any two-dimensional material.

[0036] Figure 4 401 is a photoresist of a stress protection layer, 402 is a supporting layer of PMMA, 403 is a two-dimensional material, and 404 is a rupture.

[0037] Figure 5 The figure is a scanning electron microscope picture of the smooth substrate of the embodiment provided by the present invention.

[0038] Figure 6 This is an optical microscope photo of the graphene transfer and test electrode preparation of the embodiment provided by the present invention, wherein 601 is the electrode on the step, 602 is the two-dimensional material transferred to the step, and 603 is the electrode under the step.

[0039] Figure 7 It is an electrical test curve diagram of the embodiment provided by the present invention. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] Unless otherwise specified, the various raw materials used in the examples and comparative examples are commercially available conventional raw materials, and the technical means used are conventional means well known to those skilled in the art.

[0042] The actual structure diagram after transfer obtained in the following examples is shown in Figure 1 , including a smooth step substrate 101, a two-dimensional material 102 transferred to the step and patterned, an electrode 103 on the step, and an electrode 104 under the step. The effectiveness of the method can be demonstrated by simply measuring the conductivity of electrodes 103 and 104.

[0043] Example 1

[0044] This embodiment provides a method for wet transfer of two-dimensional materials, which specifically includes: 1. Prepare the target substrate. The preparation process is as follows: Figure 2 As shown, the preparation method comprises the following steps: A step 201 is etched out using silicon deep etching equipment, and a material 202 having a silicon dioxide surface is obtained by thermal oxidation. BOE is then used to remove the surface silicon oxide, thereby obtaining a smooth step surface 203. A non-conductive smooth substrate 204 is then obtained by thermal oxidation.

[0045] 2. Prepare photoresist / PMMA / 2D material structure, the preparation process is as follows Figure 3 As shown, the preparation method comprises the following steps: For the photoresist / PMMA / graphene structure, the preparation process is shown in Figure 301: First, the copper foil is bonded to the silicon wafer, and then the PMMA layer and the AZ601 photoresist layer are spin-coated in sequence. Then, photolithography is performed for patterning. Finally, the two-dimensional material is released in an ammonium persulfate solution.

[0046] For the photoresist / PMMA / molybdenum disulfide structure, the preparation process is shown in Figure 302: Since molybdenum disulfide is already on the sapphire substrate, the PMMA layer and the AZ601 photoresist layer are directly spin-coated in sequence, followed by photolithography patterning, and finally the two-dimensional material is released in the BOE solution.

[0047] For any other two-dimensional material, the preparation process is shown in Figure 303; since it may be on various substrates, it can be transferred to water first, then scooped onto a sapphire substrate, and then subjected to subsequent operations, and finally released into a BOE solution to complete the preparation of the thin film structure.

[0048] 3. Perform alignment and wet transfer, and align the photoresist pattern on the patterned photoresist / PMMA / two-dimensional material layer with the pattern on the step of the target substrate by angle sliding; the transfer process is as follows Figure 4 shown.

[0049] Figure 4 Figures a and b in the figure respectively compare the differences between the transfer method of this embodiment and the traditional transfer method. 401 is the photoresist of the stress protection layer, 402 is the supporting layer PMMA, 403 is the two-dimensional material, and 404 is the crack. Due to the presence of the stress protection layer, the cracks of the two-dimensional material can be controlled not on the steep sidewall after being transferred to the step. While the traditional transfer method will make it easier to crack on the sidewall, it can be clearly seen that the transfer method in this embodiment can ensure that the two-dimensional material does not crack on the steep sidewall.

[0050] Then, the material is dried in air under natural air drying; after heating, the material is placed in a solvent to dissolve and remove the photoresist / PMMA support layer, thereby obtaining a complete two-dimensional material layer without tearing on the side wall. The solvent may be acetone, DSMO, NMP, anisole, etc.

[0051] 4. Perform two-dimensional material patterning and electrode deposition to complete the preparation of the entire structure.

[0052] The present invention relates to a method for wet-transferring a complete two-dimensional material on a side wall of a steep high step based on the principle of stress release. Specifically, the required two-dimensional material originally on its intrinsic substrate is first selected; polymethyl methacrylate (PMMA) is coated on the surface of the two-dimensional material as a support layer; photoresist is spin-coated on the PMMA layer and photolithography is performed as a stress protection layer. Then, the patterned photoresist / PMMA / two-dimensional material layer is peeled off from its specific intrinsic substrate using the specific method of the above step 2; the peeled patterned photoresist / PMMA / two-dimensional material layer is washed in deionized water; the patterned photoresist / PMMA / two-dimensional material layer is wet-transferred on the surface of the substrate with a smooth side wall constructed in step 1; the photoresist pattern on the patterned photoresist / PMMA / two-dimensional material layer is aligned with the pattern on the step of the substrate by angle sliding; the layer is dried under the action of natural air drying; after heating, the layer is placed in a solvent to dissolve and remove the photoresist / PMMA support layer, so as to obtain a complete two-dimensional material layer without tearing on the side wall.

[0053] The key to the present invention is to ensure the integrity and high-quality transfer of two-dimensional materials on complex surface structures through a pre-designed stress release structure. It is particularly suitable for complex structural surfaces such as steep high side walls, and solves the problem in the prior art that two-dimensional materials are easily torn during the transfer process to the side wall structure.

[0054] Test Example 1

[0055] In order to verify the effect of the embodiment of the present invention, the structure of the device was characterized and the electrical performance of the structure was tested. Figure 5 , is the preparation of a smooth substrate in an embodiment of the present invention, and it can be seen that the steep sidewall is very smooth. The two-dimensional material can be well attached without being damaged. Figure 6 This is an optical microscope image of a fully fabricated device. 601 is the electrode on the step, 602 is the two-dimensional material transferred to the step, and 603 is the electrode under the step. Figure 7 To test the conductivity between the upper electrode and the lower electrode of the step, since graphene has good conductivity, the embodiment here uses graphene to make a complete structure and measures the electrical curve. It can be seen that graphene has very good conductivity, which proves that graphene still has good continuity on the side wall, proving the effectiveness of the solution.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for wet transfer of two-dimensional materials, characterized in that: include: A polymer support layer is coated on the surface of the two-dimensional material, a photoresist layer is further coated on the surface of the two-dimensional material with the polymer support layer, and then the photoresist layer is patterned to form a stress transfer area without photoresist and a stress protection area with photoresist; finally, the two-dimensional material with the composite sacrificial coating is transferred to a target substrate; Wherein, the target substrate has a planar structure with steps.

2. The method for wet transfer of two-dimensional materials according to claim 1, characterized in that: During the transfer, the stress protection area is adjusted to be aligned with the step structure of the target substrate to ensure that the area protected by the photoresist can fall on the step structure.

3. The method for wet transfer of two-dimensional materials according to claim 1 or 2, characterized in that: The height of the step is 5 μm or less.

4. The method for wet transfer of two-dimensional materials according to any one of claims 1 to 3, characterized in that: include: S1: transferring the two-dimensional material onto a sapphire substrate to obtain a two-dimensional material with a substrate; S2: sequentially coating a polymer support layer and a photoresist layer on the two-dimensional material with a substrate, and then patterning the photoresist layer to form a stress transfer area without photoresist and a stress protection area with photoresist; S3: releasing the two-dimensional material with the composite sacrificial coating obtained in S2 and removing the sapphire substrate; S4: The two-dimensional material with the composite sacrificial coating obtained in S3 after removing the sapphire substrate is transferred to a target substrate.

5. The method for wet transfer of two-dimensional materials according to any one of claims 1 to 4, characterized in that: The thickness of the polymer support layer is 100 nm to 1 μm; and / or the thickness of the photoresist layer is 200 nm to 2 μm.

6. The method for wet transfer of two-dimensional materials according to any one of claims 1 to 5, characterized in that: The photoresist includes AZ601, AZ series photoresist or AR series photoresist.

7. The method for wet transfer of two-dimensional materials according to any one of claims 1 to 6, characterized in that: The polymer includes PMMA or PPC.

8. The method for wet transfer of two-dimensional materials according to any one of claims 1 to 7, characterized in that: The two-dimensional materials include graphene, two-dimensional transition metal chalcogenide TMDs materials and various two-dimensional materials suitable for large-area transfer; the two-dimensional transition metal chalcogenide TMDs materials include molybdenum disulfide, tungsten disulfide, tungsten selenide or bismuth selenide.

9. The method for wet transfer of two-dimensional materials according to any one of claims 1 to 8, characterized in that: The target substrate is a non-conductive smooth substrate; its preparation method includes: deeply etching silicon to form a step structure; then performing thermal oxidation treatment to generate a silicon dioxide layer on the step surface; then using a buffered oxidation etchant to remove the silicon dioxide layer to obtain a smooth silicon surface; then performing thermal oxidation treatment again or directly growing a smooth silicon dioxide medium by ICP-PECVD to generate an insulating silicon dioxide layer on the step surface.

10. A method for preparing a two-dimensional material electronic device, characterized in that: A method for wet transfer of two-dimensional materials comprising the steps of any one of claims 1 to 9.

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