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

Abstract

The present application relates to the technical field of two-dimensional material, and particularly relates to a method for wetly transferring intact two-dimensional material on steep side wall based on stress transfer principle. The method comprises the following steps: coating a polymer support layer on the surface of two-dimensional material, coating a photoresist layer on the surface of two-dimensional material with the polymer support layer, then performing patterning on 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. By adjusting the pattern of the photoresist layer, the stress concentration degree of different thickness areas is different, and the stress release of the two-dimensional material is realized; the natural fracture occurs in the area without photoresist protection, and the intact transfer of the two-dimensional material on the side wall is ensured; the limitation of the prior art is solved, and a new way for preparing novel two-dimensional devices is provided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of two-dimensional materials, and particularly relates to a method for wetly transferring a complete two-dimensional material on a steep side wall based on a stress transfer principle. BACKGROUND

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

[0003] At present, 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 for the surface topography of the substrate. When the substrate surface has steep steps or other complex three-dimensional structures, dry transfer often cannot achieve complete and uniform transfer of two-dimensional materials.

[0004] Wet transfer methods achieve the transfer of two-dimensional materials in a liquid medium, avoiding mechanical damage to the materials under dry conditions, and have the advantages of simple operation and low cost. However, existing wet transfer methods mainly target flat substrates, and it is still difficult to achieve complete transfer of two-dimensional materials for substrates with three-dimensional structures, especially for substrates with steep steps. SUMMARY

[0005] The present application provides a method for wetly transferring two-dimensional materials, which is a method for wetly transferring a complete two-dimensional material on a steep side wall based on a stress transfer principle, to solve the above-mentioned defects in the prior art. The method can achieve complete and high-quality transfer of two-dimensional materials on the surface of a substrate 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, and greatly expanding the application scenarios of two-dimensional materials.

[0006] Based on this, the present application has the following technical solutions:

[0007] In a first aspect, the present application provides a method for wetly transferring two-dimensional materials, which comprises: coating a polymer support layer on the surface of two-dimensional materials, continuing to coat a photoresist layer on the surface of two-dimensional materials with the polymer support layer, and then patterning the photoresist layer to form a stress transfer zone without photoresist and a stress protection zone with photoresist; finally, transferring two-dimensional materials with a composite sacrificial coating to a target substrate.

[0008] The target substrate has a planar structure with steps.

[0009] The present application is based on the stress transfer principle, and uses the patterned photoresist and polymer support layer as a composite sacrificial coating during transfer, and uses the different thicknesses of the composite sacrificial coating at different positions to achieve different stress distributions in different regions of the two-dimensional material; wherein the natural fracture after transfer occurs in the region without photoresist protection, ensuring that the two-dimensional material is completely transferred on the step sidewall.

[0010] In the present application, the photoresist acts as a stress protection layer, and after patterning, a stress transfer zone without photoresist and a stress protection zone with photoresist are formed, wherein the patterned region (i.e. the stress protection zone with photoresist) serves as a protection for the two-dimensional material on the step, and the other region serves as a stress transfer zone without photoresist, ensuring that the naturally torn part of the two-dimensional material is in the place without photoresist layer.

[0011] In the present application, the target substrate has a planar structure with steps; specifically, it can be a step structure or a steep high sidewall with a high aspect ratio.

[0012] In the specific implementation process, those skilled in the art can use the existing and conventional methods in the art to pattern the coated photoresist, which is not specifically limited here.

[0013] In the present application, spin coating, spraying or dipping can be used to uniformly coat a layer of photoresist on the surface of the polymer support layer; spin coating is the most common method, which can be specifically implemented by rotating the substrate at high speed to make the photoresist uniformly distributed by centrifugal force. After coating, preliminary heat treatment (soft baking) is performed to remove the solvent in the photoresist and to preliminarily solidify the photoresist layer, thereby improving the adhesion. Then the two-dimensional material coated with 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 intended pattern position on the substrate. Through the exposure system of the photolithography machine, a specific wavelength of light (such as ultraviolet light) is used to irradiate the photoresist on the substrate, and the pattern is transmitted 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 the subsequent development and etching steps. Then a developing solution is used to treat, which dissolves the photoresist in the unexposed area (positive photoresist) or the photoresist in the exposed area (negative photoresist), forming the desired convex or concave pattern.

[0014] In the present application, 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 lithography light cannot pattern the PMMA layer, only the photoresist layer can be patterned, which leads to the preparation of the support layer and the photoresist layer.

[0015] As preferred, during the transferring, the stress protection zone is aligned with the step structure of the target substrate, and the place where the photoresist protects is ensured to fall on the step structure.

[0016] In the present application, when the height of the step is below 5 microns, the conventional transferring method of the prior art is prone to cause breakage on the sidewall, while the above-mentioned transferring method can complete high-quality tear-free transferring.

[0017] As a preferred embodiment of the present application, the method for wet transferring two-dimensional materials comprises:

[0018] S1: transferring the two-dimensional material to a sapphire substrate to obtain a two-dimensional material with a substrate;

[0019] 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 transferring zone without photoresist and a stress protection zone with photoresist;

[0020] S3: releasing the two-dimensional material with a composite sacrificial coating layer obtained in S2 to remove the sapphire substrate;

[0021] S4: transferring the two-dimensional material with a composite sacrificial coating layer obtained after removing the sapphire substrate in S3 to a target substrate.

[0022] In the present application, for some special two-dimensional materials, protection and patterning can be directly performed on their specific target substrates, and the two-dimensional material can be released.

[0023] As preferred, after the transferring, the polymer support layer and the photoresist layer are removed, and finally a complete two-dimensional material layer without tearing on the target substrate is obtained.

[0024] As preferred, the two-dimensional material comprises graphene, two-dimensional transition metal chalcogenide TMDs material, and various two-dimensional materials suitable for large-area transferring; the two-dimensional transition metal chalcogenide TMDs material comprises molybdenum disulfide, tungsten disulfide, tungsten selenide, or bismuth selenide.

[0025] For any two-dimensional material, since it can be on various substrates, it can be first transferred to water, then fished to a sapphire substrate, and then subsequent operations are performed, and finally released in a BOE solution, so as to complete the preparation of a thin film structure.

[0026] When the two-dimensional material is graphene, it can be pasted on a hard substrate, uniform glue photolithography and patterning are performed, then the structure is placed in an ammonium persulfate solution to dissolve the copper foil, thereby obtaining the two-dimensional material. Then it is fished to a sapphire substrate, and subsequent operations are performed, and finally released in the BOE solution, thereby completing the preparation of the thin film structure.

[0027] When the two-dimensional material is molybdenum disulfide, it can be directly coated with a polymer and a photoresist and subjected to photolithography and development, and then released by BOE.

[0028] 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.

[0029] In the present application, the photoresist can be a positive photoresist or a negative photoresist; preferably, the photoresist includes AZ601, AZ series photoresist or AR series photoresist.

[0030] Preferably, the polymer includes PMMA or PPC.

[0031] Preferably, the target substrate is a non-conductive smooth substrate.

[0032] Further preferably, a low-temperature etching method can be used to ensure the smooth surface.

[0033] Further preferably, the preparation method of the target substrate includes etching a silicon deep step structure; then performing a thermal oxidation treatment to generate a silicon dioxide layer on the step surface; then removing the silicon dioxide layer using a buffer oxidation etchant to obtain a smooth silicon surface; and then performing a 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.

[0034] The present application finds that by using the above process to construct a non-conductive smooth surface, the two-dimensional material can be well received, which is more conducive to achieving high-quality transfer, and the operation is simple, low-cost, and can realize high-quality wet transfer of two-dimensional materials on complex three-dimensional surfaces.

[0035] In the present application, during the wet transfer process, the pattern of the photoresist / PMMA / photoresist in the two-dimensional material and the pattern on the smooth step substrate are aligned to ensure that the photoresist protection area can fall on the step, and the water is evaporated by standing, thereby protecting the two-dimensional material on the step through different stress distribution of different areas.

[0036] In a second aspect, the present application provides a method for preparing a two-dimensional material electronic device, which includes the above-mentioned method for wet transfer of two-dimensional materials.

[0037] The method for wet transfer of two-dimensional materials provided by the application adopts a PMMA layer as a support layer and a photoresist layer as a stress protection layer to form a composite sacrificial coating structure; the stress concentration degree of different thickness regions is different by regulating the pattern of the photoresist layer, so that stress release of the two-dimensional material is realized; natural fracture occurs in the region 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 way is provided for preparing novel two-dimensional devices. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0039] Figure 1 The schematic diagram of the two-dimensional material and the test electrode structure transferred on the steep side wall based on the stress transfer principle provided by the application is shown. Among them, 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.

[0040] Figure 2 The schematic diagram of the smooth step substrate preparation method provided by the embodiment of the application is shown. Among them, 201 is an etched step, 202 is a material with a step surface of silicon dioxide, 203 is a smooth step surface, and 204 is a non-conductive smooth substrate.

[0041] Figure 3 The schematic diagram of the photoresist / PMMA / two-dimensional material structure preparation method of various two-dimensional materials of the embodiment provided by the application is shown. Among them, 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 303 represents the transfer flow chart of any two-dimensional material.

[0042] Figure 4 The difference comparison schematic diagram between the transfer method and the conventional transfer method in the embodiment provided by the application is shown. Among them, 401 is the photoresist of the stress protection layer, 402 is the support layer PMMA, 403 is the two-dimensional material, and 404 is the fracture.

[0043] Figure 5 The scanning electron microscope real object diagram of the smooth substrate of the embodiment provided by the application is shown.

[0044] Figure 6It is an optical microscope actual picture of graphene transfer and test electrode preparation of the embodiment provided by the present application. Wherein, 601 is an electrode on the step, 602 is a two-dimensional material transferred to the step, and 603 is an electrode under the step.

[0045] Figure 7 It is an electrical test curve graph of the embodiment provided by the present application. DETAILED DESCRIPTION

[0046] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the present application will be described clearly and completely below in combination with the drawings in the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0047] Unless otherwise specified, the various raw materials used in the embodiments and comparative examples are all commercially available conventional raw materials, and the technical means used is the conventional means well known to those skilled in the art.

[0048] The actual structure schematic diagram after transfer obtained by the following embodiments is shown in Figure 1 , which includes 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 proved only by measuring the conductivity of the electrodes 103 and 104.

[0049] Embodiment 1

[0050] The present embodiment provides a method for wet transfer of two-dimensional materials, which specifically comprises the following steps:

[0051] 1. Preparation of a target substrate, the preparation process is shown in Figure 2 , and the preparation method comprises the following steps:

[0052] A step 201 is etched by using a silicon deep etching device, a material 202 with a step surface of silicon dioxide is obtained by using a thermal oxidation method, and then the surface silicon oxide is removed by using BOE, so that a smooth step surface 203 can be obtained, and then a non-conductive smooth substrate 204 is obtained by using a thermal oxidation scheme.

[0053] 2. Preparation of a photoresist / PMMA / two-dimensional material structure, the preparation process is shown in Figure 3 , and the preparation method comprises the following steps:

[0054] For the photoresist / PMMA / graphene structure, the preparation process is shown in

[0055] Firstly, the copper foil is attached to the silicon wafer, and then the PMMA layer and AZ601 photoresist layer are spin-coated in sequence. Then, the photoresist is patterned by photolithography. Finally, the two-dimensional material is released in the ammonium persulfate solution.

[0056] For the photoresist / PMMA / molybdenum disulfide structure, the preparation process is shown in FIG. 302:

[0057] Since the molybdenum disulfide is on the sapphire substrate itself, the PMMA layer and AZ601 photoresist layer are spin-coated in sequence, and then the photoresist is patterned by photolithography. Finally, the two-dimensional material is released in the BOE solution.

[0058] For other arbitrary two-dimensional materials, the preparation process is shown in FIG. 303; since it can be on various substrates, it can be first transferred to water, then picked up on a sapphire substrate, and then the subsequent operation is performed. Finally, it is released in the BOE solution to complete the preparation of the thin film structure.

[0059] 3. Alignment and wet transfer are performed, and the photoresist pattern on the patterned photoresist / PMMA / two-dimensional material layer is aligned with the pattern on the step of the target substrate by angle slipping; the transfer process is shown in FIG. Figure 4 .

[0060] Figure 4 The differences between the transfer method of the embodiment and the conventional transfer method are compared in FIGS. 401 and 402, respectively. 401 is the photoresist of the stress protection layer, 402 is the PMMA support layer, 403 is the two-dimensional material, and 404 is the breaking point. Due to the presence of the stress protection layer, the crack of the two-dimensional material after being transferred to the step can be controlled not to be on the steep side wall. The conventional transfer method will cause the two-dimensional material to be more prone to break on the side wall. It can be seen that the transfer method of the embodiment can ensure that the two-dimensional material is not broken on the steep side wall.

[0061] Then, it is dried under the action of natural air drying; after heating, it is dissolved in a solvent to remove the photoresist / PMMA support layer, and a complete two-dimensional material layer without tearing on the side wall is obtained. The solvent can be acetone, DSMP, NMP, anisole, etc.

[0062] 4. The two-dimensional material is patterned and the electrode is deposited to complete the preparation of the entire structure.

[0063] The present application relates to a method for wet transfer of complete two-dimensional material on the side wall of steep high step based on stress release principle. Specifically, first, select the required two-dimensional material originally on its intrinsic substrate; coat polymethyl methacrylate (PMMA) on the surface of the two-dimensional material as a support layer; spin coating photoresist on the PMMA layer and photoetching as a stress protection layer. Then, use the specific method of step 2 to separate the patterned photoresist / PMMA / two-dimensional material layer from its specific intrinsic substrate; wash the separated patterned photoresist / PMMA / two-dimensional material layer in deionized water; wet transfer the patterned photoresist / PMMA / two-dimensional material layer on the substrate surface of the smooth side wall constructed in step 1; align the photoresist pattern on the patterned photoresist / PMMA / two-dimensional material layer with the pattern on the step of the substrate through angular slip; dry under the action of natural air drying; after heating, dissolve and remove the photoresist / PMMA support layer in the solvent to obtain a complete two-dimensional material layer without tearing on the side wall.

[0064] The key of the present application is to ensure the integrity and high-quality transfer of two-dimensional material on complex surface structure through the pre-designed stress release structure, which is particularly suitable for steep high side wall and other complex structure surfaces, and solves the problem of easy tearing of two-dimensional material during transfer to the side wall structure in the prior art.

[0065] Test Example 1

[0066] In order to verify the effect of the embodiment of the present application, the structure of the device is characterized, and the electrical performance of the structure is tested. Referring to Figure 5 is the preparation of a smooth substrate in the embodiment of the present application, and it can be seen that the steep side wall is very smooth. This can make the two-dimensional material adhere well without damage. Figure 6 is the optical microscope image of the complete 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 is the test of the conduction between the electrode on the step and the electrode under the step. Since graphene has good conductivity, graphene is used in this embodiment to make a complete structure, and the electrical curve is measured. 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 scheme.

[0067] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method of wet transfer of two-dimensional material, characterized by, The method comprises the following steps: 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. The target substrate has a planar structure with steps.

2. The method of wet transfer of two-dimensional material according to claim 1, wherein, During the transferring, the stress protection area is aligned with the step structure of the target substrate to ensure that the photoresist protection area can fall on the step structure.

3. The method of wet transfer of two-dimensional material according to claim 1, wherein, The height of the step is less than 5 μm.

4. The method of wet transfer of two-dimensional material according to any one of claims 1 to 3, wherein, The method comprises the following steps: S1: transferring the two-dimensional material to a sapphire substrate to obtain a two-dimensional material with a substrate; S2: coating a polymer support layer and a photoresist layer on the two-dimensional material with the substrate in sequence, and then patterning the photoresist layer to form a stress transfer area without photoresist and a stress protection area with photoresist; S3: releasing the thin film of the two-dimensional material with the composite sacrificial coating obtained in S2 to remove the sapphire substrate; S4: transferring the two-dimensional material with the composite sacrificial coating obtained after removing the sapphire substrate in S3 to a target substrate.

5. The method of wet transfer of two-dimensional material according to any one of claims 1 to 3, wherein, 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 of wet transfer of two-dimensional material according to any one of claims 1 to 3, wherein, The photoresist comprises an AZ series photoresist or an AR series photoresist; the AZ series photoresist comprises AZ601.

7. The method of wet transfer of two-dimensional material according to any one of claims 1 to 3, wherein, The polymer comprises PMMA or PPC.

8. The method for wet transfer of two-dimensional materials according to any one of claims 1 to 3, characterized in that: The two-dimensional material is a two-dimensional material suitable for large-area transfer; the two-dimensional material suitable for large-area transfer comprises graphene or a two-dimensional transition metal dichalcogenide (TMD) material; the two-dimensional transition metal dichalcogenide (TMD) material comprises molybdenum disulfide, tungsten disulfide, tungsten selenide or bismuth selenide.

9. The method of wet transfer of two-dimensional material according to any one of claims 1 to 3, wherein, The target substrate is a non-conductive smooth substrate; the preparation method comprises the following steps: etching a silicon to form a step structure; then performing a thermal oxidation treatment to generate a silicon dioxide layer on the step surface; then removing the silicon dioxide layer by using a buffer oxidation etchant to obtain a smooth silicon surface; then performing a 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 of fabricating a two-dimensional material electronic device, comprising: The method comprises the steps of any one of claims 1 to 9. The method comprises the steps of any one of claims 1 to 9.

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