Low-noise infrared detector and preparation method thereof

Abstract

The invention discloses a low-noise infrared detector and a preparation method thereof, the low-noise infrared detector comprises a substrate, a metal electrode, an electron transport layer and a quantum dot layer, the ligand of quantum dots in the quantum dot layer is at least one of dithioglycol and tetrabutylammonium iodide, the electron transport layer is one of a graphene layer and a transition metal disulfide layer. The quantum dot layer replaced by the specific ligand is matched with the electron transport layer, contact between the quantum dot layer and the electron transport layer is improved, the defect mode of the quantum dot layer is reduced, the surface of the quantum dot is further passivated, the performance of the electron transport layer is more stable, noise in the infrared detector is reduced, and the performance of the infrared detector is improved. And the noise performance of the device is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of optoelectronic chips, and particularly relates to a low-noise infrared detector and a preparation method thereof. Background Art

[0002] Graphene is a material composed of carbon atoms and has excellent carrier transport characteristics, which enables the rapid transport of electrons in graphene. Under light illumination, the separation and conduction of carriers will be extremely rapid, which means that the photodetector made of graphene has lower noise and faster response speed.

[0003] When the quantum dot / graphene composite photodetector is illuminated, the quantum dot layer absorbs light energy to generate electron-hole pairs. Under the action of the built-in electric field, holes tend to be injected into the graphene layer, while electrons are captured by the quantum dot layer, thereby suppressing the rapid recombination of electrons and holes and increasing the device current. Combining the excellent transport performance of graphene and the narrow bandgap characteristics of quantum dots, a device with higher gain and faster response is obtained. However, although these systems can achieve higher gain, their noise impact is large.

[0004] The information disclosed in this background art section is only intended to enhance the overall understanding of the present invention and should not be regarded as an admission or any form of suggestion that this information constitutes prior art already known to those of ordinary skill in the art. Summary of the Invention

[0005] The purpose of the present invention is to provide a low-noise infrared detector and a preparation method thereof, and the low-noise infrared detector has low noise and excellent performance.

[0006] In order to achieve the above purpose, the technical solution provided by a specific embodiment of the present invention is as follows:

[0007] A low-noise infrared detector includes a substrate, a metal electrode, an electron transport layer, and a quantum dot layer. The ligand of the quantum dots in the quantum dot layer is at least one of ethanedithiol and tetrabutylammonium iodide, and the electron transport layer is one of a graphene layer and a transition metal disulfide layer.

[0008] In one or more embodiments of the present invention, the material of the transition metal disulfide layer is molybdenum disulfide or tungsten disulfide.

[0009] In one or more embodiments of the present invention, the quantum dots in the quantum dot layer are ligand-exchanged, and the quantum dots are selected from: lead sulfide quantum dots, lead selenide quantum dots, and mercury telluride quantum dots.

[0010] In one or more embodiments of the present invention, the particle size of the quantum dots in the quantum dot layer is 3-7 nm.

[0011] In one or more embodiments of the present invention, the thickness of the electron transport layer is 0.2 nm to 0.4 nm, and the thickness of the quantum dot layer is 10 nm to 30 nm.

[0012] The technical solution provided by another specific embodiment of the present invention is as follows:

[0013] A method for preparing a low-noise infrared detector includes the following steps:

[0014] Provide a substrate and form an electron transport layer on the surface of the substrate;

[0015] Form a metal electrode on the substrate;

[0016] Form a quantum dot layer at least on the electron transport layer with quantum dots after ligand replacement to obtain a low-noise infrared detector.

[0017] In one or more embodiments of the present invention, the quantum dot layer is obtained by coating with quantum dot ink. The quantum dot ink includes quantum dots and an organic solvent, and the mass concentration of the quantum dot ink is 280 g / L to 320 g / L.

[0018] In one or more embodiments of the present invention, the quantum dots are prepared as follows:

[0019] Disperse quantum dots with a long-chain ligand as the initial ligand in an organic solvent to make a quantum dot solution;

[0020] Under an inert atmosphere, the quantum dot solution and a replacement ligand form a mixed system for reaction. The replacement ligand is ethanedithiol or tetrabutylammonium iodide;

[0021] Add a reagent to precipitate the quantum dots, centrifuge, wash, and dry to obtain a quantum dot material.

[0022] In one or more embodiments of the present invention, the molar ratio of the replacement ligand to the quantum dots, calculated based on the ratio of the replacement ligand to the surface sites of the quantum dots, is (9 to 11):1.

[0023] In one or more embodiments of the present invention, the quantum dot layer is obtained by coating with quantum dot ink. Specifically, the quantum dot ink is spin-coated on the surface of the electron transport layer at a rotation speed of 2500 r / min to 3000 r / min for a spin-coating time of 30 s to 60 s. After spin-coating, annealing is performed at 60 °C to 100 °C for 8 min to 12 min.

[0024] Compared with the prior art, the present invention selects specific ligand-exchanged quantum dots, reduces the defect states of the quantum dot layer, passivates the surface of the quantum dot layer, improves the photon absorption ability of the quantum dot layer. At the same time, the quantum dot layer covers the surface of the electron transport layer, reduces the defect states of the electron transport layer, increases the stability of the electron transport layer, optimizes the carrier transport process in the quantum dot layer and the electron transport layer, and improves the noise performance of the infrared detector. Description of the Drawings

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0026] Figure 1 Schematic structural diagram of a low-noise infrared detector in an embodiment of the present invention;

[0027] Figures 2-5 Flow chart for preparing a low-noise infrared detector in an embodiment of the present invention;

[0028] Figure 6 Graph of the noise detection results of the low-noise infrared detector in Example 1;

[0029] Figure 7 Graph of the noise detection results of the infrared detector in Comparative Example 1.

[0030] Main reference numeral descriptions:

[0031] 1, substrate; 2, electron transport layer; 3, metal electrode; 4, quantum dot layer. Detailed Embodiments

[0032] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0033] Near-infrared detectors need to have characteristics such as high responsivity, low noise, and fast response, which pose relatively high requirements for the materials of the detectors. As a semiconductor material, lead sulfide has a large adjustable bandgap range (0.7 eV - 2.1 eV). At the same time, it has a high absorption coefficient for near-infrared light and a wide response range, making it an excellent choice for fabricating infrared detectors. Currently, although lead sulfide quantum dot materials perform well in the field of near-infrared detection, the ligands formed during the preparation process of lead sulfide quantum dot materials are long-chain ligands. Although they help disperse lead sulfide in non-polar solvents, they inhibit charge transport and affect the optoelectronic properties.

[0034] The traditional solid-state ligand exchange method deposits the quantum dot layer on the device by layer-by-layer spin coating (LBL). Although this method has advantages in terms of preparation cost and environmental friendliness, it is prone to generating cracks in the thin film, and the preparation efficiency of this method needs to be improved. In contrast, solution-phase ligand exchange uses metal halide salts to replace oleic acid ligands. It can not only better passivate lead sulfide quantum dots and reduce their surface defect states, but also more easily control the film thickness through a single-step spin coating process to achieve uniform deposition of the quantum dot film, and at the same time significantly simplifies the device preparation process.

[0035] In this invention, a graphene / quantum dot heterojunction photodetector is constructed by combining quantum dot materials with specific ligand substitution with an electron transport layer such as graphene. Thanks to the superiority of the ligand substitution method, the defect states in the quantum dot layer of the device are reduced, the film surface is further passivated, the noise in the detector is reduced, and thus the noise performance of the device is improved.

[0036] A specific embodiment of this invention provides a low-noise infrared detector, as Figure 1 shown. It includes a substrate 1, a metal electrode 3, an electron transport layer 2, and a quantum dot layer 4. The ligands of the quantum dots in the quantum dot layer 4 are at least one of ethanedithiol and tetrabutylammonium iodide, and the electron transport layer 2 is one of a graphene layer and a transition metal disulfide layer.

[0037] Specifically, the quantum dot layer 4 absorbs photons to generate electron-hole pairs. Among them, the electrons will be transferred to the electron transport layer 2 under the action of the built-in electric field, while the holes remain in the quantum dot layer 4. By forming a built-in potential, it promotes the transfer of photo-generated charges from the quantum dots to the electron transport layer 2 channel, thereby improving the performance.

[0038] The quantum dot layer 4 is disposed on the surface of the electron transport layer 2. First, the quantum dot layer 4 covers the surface of the electron transport layer 2, physically blocking the adsorption of polar molecules and avoiding the interference of polar molecules (such as H2O, O2) in the air on the carrier concentration. Second, the quantum dot layer 4 can cover the defects on the surface of the electron transport layer 2, reducing the influence of defect states. Taking the graphene layer as an example, the quantum dot layer 4 can cover the grain boundaries and adsorption defects on the surface of graphene, reducing the influence of graphene defect states. Moreover, using ethanedithiol (EDT) and tetrabutylammonium iodide as ligands of the quantum dots, there are intermolecular interactions between the ligands and the electron transport layer 2, such as van der Waals forces and electrostatic interactions, which can further stabilize the surface of the electron transport layer 2, reduce charge scattering centers, and improve the carrier mobility. In addition, the quantum dot layer 4 also causes N-type doping, such as adjusting the Fermi level of graphene to be closer to the Dirac point, reducing the carrier concentration fluctuation. Through the above effects, the contact between the quantum dot layer and the electron transport layer is improved, the carrier transport process in the quantum dot layer 4 and the electron transport layer 2 is optimized, and the noise performance of the infrared detector is improved.

[0039] Further, the material of the transition metal dichalcogenide layer is molybdenum disulfide or tungsten disulfide.

[0040] Specifically, the layered structures of molybdenum disulfide and tungsten disulfide are beneficial to the rapid transport of electrons. Using them as the electron transport layer 2 can better improve the performance of the infrared detector.

[0041] Further, the quantum dots in the quantum dot layer 4 are ligand-exchanged, and the quantum dots are selected from: lead sulfide quantum dots, lead selenide quantum dots, mercury telluride quantum dots, and the thickness is 10nm - 30nm.

[0042] Specifically, strong coordination bonds can be formed between lead sulfide, lead selenide, mercury telluride and the ligands (ethanedithiol, tetrabutylammonium iodide). The ligands can effectively passivate the quantum dots, reduce the influence of their surface defect states, enable them to effectively absorb photons to generate electron-hole pairs, and then promote the transfer of photo-generated charges to the electron transport layer 2 through the built-in potential, thereby improving the performance of the infrared detector.

[0043] Further, the thickness of the electron transport layer 2 is 0.2nm - 0.4nm.

[0044] Further, the substrate 1 includes a silicon substrate, and a silicon dioxide dielectric layer with a thickness of 250nm - 300nm is provided on the surface of the silicon substrate. The metal electrode 3 is composed of two layers of metals. The bottom layer is an adhesion layer disposed on the surface of the substrate. The material of the adhesion layer is chromium metal, titanium metal or niobium metal, and the thickness is 15nm - 20nm. The upper layer is an inert metal layer, and its material is gold, and the thickness is 60nm - 80nm. The thicknesses of the adhesion layer and the inert metal layer make the thickness of the electrode 75nm - 100nm.

[0045] Another specific embodiment of the present invention provides a method for fabricating a low-noise infrared detector, including steps 1-3.

[0046] Step 1: Provide a substrate 1 and form an electron transport layer 2 on the surface of the substrate 1.

[0047] Specifically, the substrate 1 includes a silicon substrate, and a silicon dioxide dielectric layer is provided on the surface of the silicon substrate. The thickness of the silicon dioxide dielectric layer is 250 nm to 300 nm. The electron transport layer 2 is a graphene layer or a transition metal dichalcogenide layer, with a thickness of 0.2 nm to 0.4 nm. The transition metal dichalcogenide layer is made of molybdenum disulfide or tungsten disulfide.

[0048] Taking graphene as an example, first prepare a graphene film by chemical vapor deposition method, and then transfer the graphene film to the surface of the substrate. The transfer method adopts conventional methods, such as wet transfer, dry transfer or mechanical exfoliation. Graphene is preferably monolayer graphene.

[0049] After transferring the graphene to the substrate, spin-coat a layer of photoresist on the graphene surface, leave the required photoresist structure through exposure and development, etch the graphene by oxygen plasma, and then remove the photoresist with acetone solution to obtain the required electron transport layer 2.

[0050] Step 2: Form a metal electrode 3 on the substrate 1.

[0051] Specifically, spin-coat a photoresist on the surface of the substrate 1, leave the photoresist structure through exposure and development, and then deposit metal. The metal electrode 3 consists of two layers of metal. The bottom layer is an adhesion layer, and the adhesion layer is made of chromium metal, titanium metal or niobium metal. The upper layer is an inert metal layer, and its material is gold. Then use acetone solution to remove the photoresist and strip the metal film on the photoresist surface together to finally form the metal electrode 3.

[0052] Step 3: Form a quantum dot layer 4 at least on the electron transport layer 2 with ligand-exchanged quantum dots to obtain a low-noise infrared detector.

[0053] Specifically, use a spin coater to spin-coat a quantum dot ink with a mass concentration of 280 g / L to 320 g / L on the electron transport layer 2, with a rotation speed of 2500 r / min to 3000 r / min and a spin-coating time of 30 s to 60 s. After spin-coating, anneal at 60 °C to 100 °C for 8 min to 12 min.

[0054] The quantum dot ink uses hexane, toluene or n-octane as an organic solvent. The quantum dots are lead sulfide, lead selenide or mercury telluride. The ligand of the quantum dots is ethanedithiol or tetrabutylammonium iodide. The quantum dots are dispersed in the organic solvent to make the quantum dot ink.

[0055] Further, the quantum dots are prepared as follows: Quantum dots with a long-chain ligand (such as oleic acid) as the initial ligand are dispersed in an organic solvent to form a quantum dot solution with a concentration of 4 mg / L to 6 mg / L; under an inert atmosphere (nitrogen or argon), the quantum dot solution and a replacement ligand (ethanedithiol or tetrabutylammonium iodide) form a mixed system for reaction, and the molar ratio of the replacement ligand to the quantum dots, calculated based on the ratio of the replacement ligand to the surface sites of the quantum dots, is (9 to 11):1; after the reaction, a reagent (acetone) is added to precipitate the quantum dots, and they are centrifuged, washed, and dried to obtain the quantum dot material.

[0056] Specifically, the reaction temperature is room temperature (25 °C) or slightly heated, and the reaction is stirred for 30 min to 2 h. The terminator is acetone. After the quantum dots are precipitated, they are centrifuged at 4000 rpm to 6000 rpm for 4 min to 6 min, and the solid is collected. Then the solid is washed alternately with acetone and isopropanol multiple times to remove the unreacted replacement ligand and by-products. The washed solid is dried in a vacuum drying oven at 35 °C to 45 °C for 1.5 h to 2.5 h.

[0057] The present invention will be further described in detail below with reference to specific embodiments.

[0058] Example 1

[0059] Preparation of quantum dots:

[0060] Lead sulfide quantum dots with oleic acid as the ligand (purchasing manufacturer: Suzhou Xingshuo Nano Technology Co., Ltd., model: lead sulfide quantum dots; wavelength: 1550 ± 50 nm) are dispersed in toluene with a concentration of 10 mg / mL and ultrasonically treated for 10 min.

[0061] Methanol is added to the solution, and the volume ratio of toluene to methanol is 1:3. It is centrifuged at a speed of 5000 rpm for 5 min to precipitate the quantum dots. The supernatant is discarded, and the precipitate is washed repeatedly with the toluene-methanol mixture 3 times to remove the free original ligand.

[0062] The washed quantum dots are dispersed in chloroform with a concentration of 5 mg / mL. Under a nitrogen atmosphere, according to the molar ratio of ethanedithiol to the surface sites of the quantum dots being 10:1, that is, 20 μL of ethanedithiol is added based on 1 mL of the quantum dot solution, and the reaction is stirred at room temperature for 1 h. An excessive amount of acetone is added to precipitate the quantum dots, and they are centrifuged at a speed of 5000 rpm for 5 min to collect the precipitate. The precipitate is washed alternately with acetone and isopropanol 3 times to remove the unreacted ethanedithiol and by-products. Then it is dried in a vacuum drying oven at 40 °C for 2 h to obtain the quantum dots, namely, ethanedithiol-modified lead sulfide quantum dots.

[0063] Example 2

[0064] As Figure 2 and Figure 3, take a substrate, the substrate includes a silicon substrate, and the surface of the silicon substrate has a silicon dioxide dielectric layer with a thickness of 285 nm. Transfer the graphene film grown by chemical vapor deposition to the surface of the substrate. The graphene film is monolayer graphene with a thickness of 0.4 nm. As Figure 4 , spin-coat a layer of photoresist on the surface of the graphene film, leave the required photoresist structure through exposure and development, then etch the graphene by oxygen plasma, and then remove the photoresist with acetone solution to obtain the required electron transport layer.

[0065] As Figure 5 , spin-coat photoresist on the substrate, leave the photoresist structure through exposure and development, and then deposit metal to form a metal electrode. The metal electrode consists of two layers of metal. The bottom layer is an adhesion layer, the material is chromium metal with a thickness of 15 nm, and the upper layer is an inert metal, the material is gold with a thickness of 65 nm. Then remove the photoresist with acetone solution, and at this time, peel off the metal film on the surface of the photoresist together to finally form a metal electrode.

[0066] Spin-coat quantum dot ink with a mass concentration of 300 g / L on the graphene film. The quantum dot ink is prepared by dispersing the quantum dots in Example 1 in toluene. During spin-coating, the rotation speed is 2500 r / min, the time is 60 s, the spin-coating thickness is 30 nm, and after spin-coating, anneal at 90 °C for 10 min to form a quantum dot layer, and prepare a low-noise infrared detector with the structure as Figure 1 shown.

[0067] Comparative Example 1

[0068] , take a substrate, the substrate includes a silicon substrate, and the surface of the silicon substrate has a silicon dioxide dielectric layer with a thickness of 285 nm. Transfer the graphene film grown by chemical vapor deposition to the surface of the substrate. The graphene film is monolayer graphene with a thickness of 0.4 nm. Spin-coat a layer of photoresist on the surface of the graphene film, leave the required photoresist structure through exposure and development, then etch the graphene by oxygen plasma, and then remove the photoresist with acetone solution to obtain the required electron transport layer.

[0069] , spin-coat photoresist on the substrate, leave the photoresist structure through exposure and development, and then deposit metal. The metal electrode consists of two layers of metal. The bottom layer is an adhesion layer, the material is chromium metal with a thickness of 15 nm, and the upper layer is an inert metal, the material is gold with a thickness of 65 nm. Then remove the photoresist with acetone solution, and at this time, peel off the metal film on the surface of the photoresist together to finally form a metal electrode.

[0070] A quantum dot ink with a mass concentration of 300 g / L was spin-coated on the graphene film. The quantum dot ink was prepared by dispersing a quantum dot material (purchased from Suzhou Xingshuo Nanotechnology Co., Ltd., model: lead sulfide quantum dots; wavelength: 1550 ± 50 nm) in toluene. During spin coating, the rotation speed was 2500 r / min, the time was 60 s, and the spin coating thickness was 30 nm. After spin coating, the film was annealed at 90°C for 10 min to form a quantum dot layer, thereby preparing an infrared detector.

[0071] Perform noise detection on the infrared detectors in the embodiments and comparative examples: Place the sample to be tested on the platform and select a suitable gain amplifier. Observe through a 5x optical microscope and insert the test probe into the sample electrode. After confirming that the test probe is in good contact with the sample electrode, cover the area with an iron shielding box to form an equipotential body with the conductive base to isolate it from external environmental interference. Select different gain multiples in the test program. When an overload occurs, the previous multiple of the gain multiple is the correct gain multiple (generally the gain multiple is 10). 7 or 10 8 ). At this gain multiple, the low-frequency noise of the sample is tested. The test program will perform 100 mean square calculations to obtain the low-frequency noise data and power density spectrum of the device. In device testing, bias is generally not applied. If abnormal data points appear in the test results (usually caused by environmental interference), the abnormal data can be selectively deleted. The test results are as follows: Figure 6 and Figure 7 shown.

[0072] Figure 6 and Figure 7 In the figure, the horizontal axis is the frequency of the noise signal, and the vertical axis is the noise power density. It can be seen from the figure that when the noise signal frequency is 1 Hz, the noise power density of the infrared detector in Comparative Example 1 is 2.88×10 -16 A ^2 / Hz, the noise power density of the infrared detector in Example 1 is 3.22×10 -18 A ^2 / Hz, compared with comparative example 1, the noise of the infrared detector in Example 1 is significantly reduced, showing better performance, indicating that the present invention selects ethanedithiol and tetrabutylammonium iodide as quantum dot ligands, which can reduce the noise of the infrared detector and improve the performance of the infrared detector by cooperating with the electron transport layer.

[0073] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above-described exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, in any aspect, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

[0074] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment only contains an independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A low-noise infrared detector, comprising a substrate, a metal electrode, an electron transport layer and a quantum dot layer, characterized in that: The ligand of the quantum dots in the quantum dot layer is at least one of ethanedithiol and tetrabutylammonium iodide, and the electron transport layer is one of a graphene layer and a transition metal disulfide layer.

2. The low-noise infrared detector according to claim 1, characterized in that: The material of the transition metal disulfide layer is molybdenum disulfide or tungsten disulfide.

3. The low-noise infrared detector according to claim 1, characterized in that: The quantum dots in the quantum dot layer are ligand-substituted, and the quantum dots are selected from: lead sulfide quantum dots, lead selenide quantum dots, and mercury telluride quantum dots.

4. The low-noise infrared detector according to claim 1, characterized in that: The particle size of the quantum dots in the quantum dot layer is 3-7 nm.

5. The low-noise infrared detector according to claim 1, characterized in that: The thickness of the electron transport layer is 0.2nm-0.4nm, and the thickness of the quantum dot layer is 10nm-30nm.

6. A method for preparing a low-noise infrared detector according to any one of claims 1 to 5, characterized in that: The steps include: Providing a substrate, and forming an electron transport layer on the surface of the substrate; forming a metal electrode on a substrate; At least on the electron transport layer, quantum dots replaced with ligands form a quantum dot layer to obtain a low-noise infrared detector.

7. The method for preparing a low-noise infrared detector according to claim 6, characterized in that: The quantum dot layer is obtained by coating with quantum dot ink, wherein the quantum dot ink comprises quantum dots and an organic solvent, and the mass concentration of the quantum dot ink is 280 g / L to 320 g / L.

8. The method for preparing a low-noise infrared detector according to claim 6, characterized in that: The preparation of the quantum dots is as follows: Dispersing quantum dots with long-chain ligands as initial ligands in an organic solvent to prepare a quantum dot solution; Under an inert atmosphere, the quantum dot solution and the replacement ligand form a mixed system for reaction, wherein the replacement ligand is ethanedithiol or tetrabutylammonium iodide; Reagents are added to precipitate quantum dots, and the quantum dots are centrifuged, washed, and dried to obtain quantum dot materials.

9. The method for preparing a low-noise infrared detector according to claim 8, characterized in that: The molar ratio of the replacement ligand to the quantum dot is (9-11):1, calculated as the ratio of the replacement ligand to the surface sites of the quantum dot.

10. The method for preparing a low-noise infrared detector according to claim 6, characterized in that: The quantum dot layer is obtained by coating the quantum dot ink, specifically: the quantum dot ink is spin-coated on the surface of the electron transport layer at a rotation speed of 2500r / min to 3000r / min for 30s to 60s, and after spin-coating, annealed at 60°C to 100°C for 8min to 12min.

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