A SERS substrate based on a grating-coupled MIM structure and its preparation method and application

Abstract

The present invention belongs to the technical field of surface enhanced Raman spectroscopy detection, and specifically discloses a SERS substrate based on a grating-coupled MIM structure, a preparation method thereof, and an application thereof. In the MIM structure, an Ag layer is selected as a SERS active layer, which solves the problem of non-repeatable substrate preparation caused by the random generation of "hot spots" when Ag particles are used in traditional manufacturing schemes; an SU8 layer is selected as an insulating layer in the MIM structure, which solves the problem of low interference field intensity caused by the low refractive index of the SiO2 layer when a SiO2 layer is used in traditional manufacturing schemes; and the grating structure is manufactured by laser interference lithography technology and displacement Talbot lithography technology, which solves the problems of high cost and low efficiency caused by the use of technologies such as electron beam lithography and focused ion beam lithography.

Description

Technical Field

[0001] The present invention relates to the technical field of surface enhanced Raman spectroscopy detection, in particular to a SERS substrate based on a grating coupled MIM structure and a preparation method and application thereof. Background Art

[0002] Surface-enhanced Raman scattering (SERS) is a powerful trace detection and analysis technique with a wide range of applications in food safety, healthcare, environment, biology, and other fields. Since its discovery in 1974, it has received tremendous research attention. The Raman signal of molecules adsorbed on SERS substrates is significantly enhanced due to electromagnetic enhancement and chemical enhancement mechanisms. Among them, the electromagnetic enhancement is proportional to the fourth power of the local electric field strength, and the chemical enhancement is related to the charge transfer effect between the substrate and the analyte. The typical enhancement factor of SERS is between 10^5 and 10^8, mainly provided by the electromagnetic enhancement on the rough metal surface. Generally, electromagnetic enhancement plays a more important role in SERS, so most research on SERS substrates focuses on maximizing the electromagnetic field strength.

[0003] Surface plasmons are the collective oscillations of free electrons on a metal surface under the influence of an external electromagnetic field. To produce these collective oscillations, the material's dielectric constant typically requires a large negative real part and a small imaginary part. Precious metals typically meet these requirements. The intensity of surface-enhanced Raman scattering (SERS) is highly dependent on the metal-enhanced near-field generated by SERS at the metal-dielectric interface.

[0004] Surface plasmon waves typically appear below the ultraviolet band, and visible light and near-infrared bands cannot directly couple with plasmons on smooth metal surfaces. Because the propagation constant of surface plasmon waves in free space is always greater than that of plane waves, i.e., there is a momentum mismatch, free-space waves incident on a smooth interface cannot directly excite surface plasmons. Therefore, in an optical state, providing a momentum-matched prism, a metal surface with a certain degree of roughness, or a grating is a necessary condition for coupling external radiation and exciting surface plasmons. In the present invention, a one-dimensional grating is used for coupling to achieve momentum matching.

[0005] In addition to the electromagnetic field enhancement, optical interference can also enhance Raman scattering, a process known as interference-enhanced Raman scattering (IERS). When molecules are placed on top of a transparent dielectric film with an opaque metallic reflective layer underneath, the multi-beam interference effect induced by the Fabry-Perot (FP) cavity results in a stronger Raman scattering intensity than without the reflective layer. The Raman scattering enhancement due to interference is only 20–60, far lower than the enhancement caused by the local electric field. Therefore, the impact of IERS has not attracted much attention from researchers. Recently, metal-insulator-metal (MIM) structures have been widely studied and applied to structural color, perfect absorbers, photovoltaics, and SERS. As SERS substrates, MIM structures can produce strong electromagnetic enhancement due to the mutual coupling of IERS FP modes and surface plasmon resonances. Therefore, grating-coupled MIM structures, as a new type of SERS-active substrate, are being used to study the role of interference in Raman enhancement, addressing the long-standing lack of understanding of the Raman enhancement mechanism.

[0006] However, the existing MIM structured SERS substrates have the following disadvantages:

[0007] (1) Previous studies have mostly focused on using particles to generate "hot spots" to increase the electromagnetic field intensity. However, since the particles are usually randomly arranged, the SERS substrates produced do not have a certain uniformity and repeatability;

[0008] (2) In the past, MIM structures often used SiO2 as the insulating layer material. However, due to the low refractive index of SiO2, the refractive power of the incident light is not strong when it enters the insulating layer, and the multi-beam interference effect in the insulating layer is not obvious.

[0009] (3) Currently, grating structures are mostly manufactured using electron beam lithography technology, but due to the need for a high vacuum environment and serial operation, the manufacturing cost is high and the efficiency is low. Summary of the Invention

[0010] In order to solve the above technical problems, the present invention provides a SERS substrate based on a grating-coupled MIM structure and a preparation method and application thereof.

[0011] To achieve the above object, the present invention is implemented according to the following technical solutions:

[0012] The first object of the present invention is to provide a SERS substrate based on a grating-coupled MIM structure, comprising a silicon substrate, a reflective layer located on the upper surface of the substrate, an insulating layer located on the upper surface of the reflective layer, a grating structure layer located on the upper surface of the insulating layer, and a SERS active layer located on the upper surface of the grating structure layer; the SERS active layer is an Ag layer, the insulating layer is an SU8 photoresist layer, and the reflective layer is an Au layer.

[0013] Furthermore, the thickness of the Ag layer is 40 nm, the grating height of the grating structure layer is 250 nm, the thickness of the SU8 photoresist layer is 20 nm, and the thickness of the Au layer is 50 nm.

[0014] The SERS active substrate of the present invention is prepared with the active layer made of Ag, because Ag is more sensitive to resonance and can provide higher detection sensitivity; the insulating layer is prepared with SU8, and previous studies mostly used SiO2 as the insulating layer material, but at the excitation wavelength (633nm) used in the present invention, the refractive index of SU8 is 1.598, which is higher than the refractive index of SiO2 1.444. Therefore, the use of a material with a higher refractive index will make the interference field intensity generated by the insulating layer greater; the reflective layer is prepared with Au, and commonly used materials for plating metal reflective films include Au, Ag, and Al. The reflectivity of Au in the visible light range (greater than 600nm) reaches more than 95%, the reflectivity of Ag in the visible light range (greater than 500nm) is about 95%, and the reflectivity of Al in the visible light range reaches more than 90%. Since the excitation wavelength selected in the present invention is 633nm, Au is selected as the reflective layer material.

[0015] A second object of the present invention is to provide a method for preparing a SERS substrate based on a grating-coupled MIM structure as claimed in claim 1 or 2, characterized in that it comprises the following steps:

[0016] S1. Clean the surface of the silicon substrate until there is no impurity;

[0017] S2. Using vacuum coating, a 50 nm thick Au layer is deposited on the surface of the silicon substrate as a reflective layer;

[0018] S3, spin-coating a 20 nm thick SU8 photoresist layer on the upper surface of the reflective layer, and performing pre-baking, exposure, post-baking, development, and hardening treatments on the SU8 photoresist layer to serve as an insulating layer;

[0019] S4. Spin-coat a positive photoresist on the upper surface of the insulating layer at a speed of 2500 rpm, with a pre-baking temperature of 100° C. for 60 seconds to ensure that the organic solvent in the positive photoresist can be fully volatilized and solidified. Expose the exposed area using laser interference lithography and displacement Talbot lithography, select a developer that matches the applied positive photoresist, and adjust the development time according to the required structural parameters. During the development process, slightly shake the sample up and down and left and right to allow the positive photoresist in the exposed area to fully react and dissolve with the developer to form a grating structure layer with a height of 250 nm.

[0020] S5. Using a vacuum coating method, a 40 nm thick Ag layer is deposited on the upper surface of the formed grating structure layer as a SERS active layer.

[0021] Furthermore, in step S1, the silicon substrate is ultrasonically cleaned in sequence using anhydrous ethanol and acetone, and deionized water is used to remove residues of the cleaning solution.

[0022] The third object of the present invention is to provide a use of the SERS substrate based on the grating-coupled MIM structure as claimed in claim 1 in the preparation of an ultra-sensitive and repeatable trace substance detection device.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. The SERS substrate based on the grating-coupled MIM structure of the present invention exhibits high SERS detection sensitivity due to the combination of surface plasmon resonance in the SERS active layer and the insulating layer and interference-enhanced Raman scattering. Furthermore, due to the highly uniform structure of each component, the preparation of the novel substrate is also highly reproducible.

[0025] 2. The Ag layer is selected as the SERS active layer in the MIM structure, which solves the problem of non-repeatable substrate preparation caused by the random generation of "hot spots" when using Ag particles in traditional manufacturing schemes; the SU8 layer is selected as the insulating layer in the MIM structure, which solves the problem of low interference field intensity caused by the low refractive index of the SiO2 layer in traditional manufacturing schemes; the grating structure is manufactured by laser interference lithography technology and displacement Talbot lithography technology, which solves the high cost and low efficiency problems brought about by the use of technologies such as electron beam lithography and focused ion beam lithography.

[0026] 3. The SERS substrate based on the grating-coupled MIM structure of the present invention adopts laser interference lithography technology and displacement Talbot lithography technology to manufacture the grating structure, which can realize the preparation of SERS active substrate with high resolution, large area, low cost and high efficiency. The SERS substrate based on the grating-coupled MIM structure of the present invention can realize the manufacture of high-performance trace substance detection devices, bringing new application prospects for trace substance detection in the fields of biochemical sensing, disease diagnosis and environmental monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Flowchart for preparing a SERS substrate based on a grating-coupled MIM structure according to the present invention, with the preparation process from left to right and from top to bottom;

[0028] Figure 2 The setting relationship between the substrate and the mask;

[0029] Figure 3 The Raman spectra of the examples and some comparative examples are shown in FIG.

[0030] Figure 4 Raman spectrum measurement results of different grating structure parameters;

[0031] Figure 5 It is the SEM picture of embodiment;

[0032] Figure 6 This is an AFM three-dimensional image of the embodiment. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. The specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0034] Example

[0035] See also Figure 1 , the preparation process of SERS substrate based on grating-coupled MIM structure:

[0036] a. Ultrasonic cleaning of the silicon wafer was performed using anhydrous ethanol and acetone in sequence, and deionized water was used to remove the residual cleaning solution to ensure that there were no impurities on the silicon wafer surface;

[0037] b. Using vacuum coating, a 50 nm thick Au layer was deposited on the surface of the silicon wafer as the reflective layer of the SERS substrate;

[0038] c. Spin-coat SU8 at a certain speed to a thickness of 20 nm;

[0039] d. The SU8 layer is pre-baked, exposed, post-baked, developed, and hardened to serve as the insulating layer of the SERS substrate. It should be noted that a single-beam full-area exposure is used here to meet the exposure dose required for a 20 nm thick SU8 layer; no structural exposure is required.

[0040] e. Spin-coat a positive photoresist at a speed of 2500 rpm;

[0041] f. The pre-baking temperature is 100°C for 60 seconds to ensure that the organic solvent in the positive photoresist can be fully volatilized and solidified;

[0042] g. Select the appropriate exposure dose to ensure that the exposed area of the positive photoresist can be completely dissolved in the developer. It should be noted that during the exposure process using the displacement Talbot lithography technology, the distance between the substrate and the mask needs to be adjusted to obtain the ideal exposure structure. Figure 2 ;

[0043] h. Select a developer compatible with the coated positive photoresist, and the development time is 60s. During the development process, the sample is slightly shaken up and down so that the positive photoresist and the developer can fully react to form a grating structure layer with a height of 250nm.

[0044] i. Using vacuum coating, 40 nm thick Ag was deposited on the surface of the formed grating structure as a SERS active layer to obtain a SERS substrate based on a grating-coupled MIM structure.

[0045] Among them, the SEM image of the SERS substrate based on the grating coupled MIM structure is as follows: Figure 5 As shown, the AFM three-dimensional image is as follows Figure 6 As shown. Figure 5 、 Figure 6 It can be seen that the period of the grating structure in the embodiment is 400 nm, the line width is 150 nm, and the height is 250 nm.

[0046] Comparative Example 1

[0047] SERS substrate preparation process of grating + active layer:

[0048] a. Ultrasonic cleaning of the silicon wafer was performed using anhydrous ethanol and acetone in sequence, and deionized water was used to remove the residual cleaning solution to ensure that there were no impurities on the silicon wafer surface;

[0049] b. Spin-coat a positive photoresist at a speed of 2500 rpm;

[0050] c. The pre-baking temperature is 100°C and the time is 60s to ensure that the organic solvent in the positive photoresist can be fully volatilized and solidified;

[0051] d. Select an appropriate exposure dose to ensure that the exposed areas of the positive photoresist are completely dissolved in the developer. It should be noted that during the exposure process using shifted Talbot lithography, the distance between the substrate and the mask needs to be adjusted to obtain the ideal exposure structure;

[0052] e. Select a developer compatible with the coated positive photoresist, and the development time is 60s. During the development process, slightly shake the sample up and down so that the positive photoresist and the developer can fully react to form a grating structure layer with a height of 250nm.

[0053] f. Using vacuum coating, 40 nm thick Ag was deposited on the surface of the formed grating structure as the SERS active layer.

[0054] Comparative Example 2

[0055] SERS substrate preparation process of base + active layer:

[0056] a. Ultrasonic cleaning of the silicon wafer was performed using anhydrous ethanol and acetone in sequence, and deionized water was used to remove the residual cleaning solution to ensure that there were no impurities on the silicon wafer surface;

[0057] b. Using vacuum coating, 40nm thick Ag was deposited on the surface of the silicon wafer as the SERS active layer.

[0058] Comparative Example 3

[0059] Preparation process of SERS substrate with increased active layer thickness:

[0060] a. Ultrasonic cleaning of the silicon wafer was performed using anhydrous ethanol and acetone in sequence, and deionized water was used to remove the residual cleaning solution to ensure that there were no impurities on the silicon wafer surface;

[0061] b. Using vacuum coating, a 50 nm thick Au layer was deposited on the surface of the silicon wafer as the reflective layer of the SERS substrate;

[0062] c. Spin-coat SU8 at a certain speed to a thickness of 20 nm;

[0063] d. The SU8 layer is pre-baked, exposed, post-baked, developed, and hardened to serve as the insulating layer of the SERS substrate. It should be noted that a single-beam full-area exposure is used here to meet the exposure dose required for a 20 nm thick SU8 layer; no structural exposure is required.

[0064] e. Spin-coat a positive photoresist at a speed of 2500 rpm;

[0065] f. The pre-baking temperature is 100°C for 60 seconds to ensure that the organic solvent in the positive photoresist can be fully volatilized and solidified;

[0066] g. Select an appropriate exposure dose to ensure that the exposed area of the positive photoresist is completely dissolved in the developer. It should be noted that during the exposure process using shifted Talbot lithography, the distance between the substrate and the mask needs to be adjusted to obtain the ideal exposure structure;

[0067] h. Select a developer compatible with the coated positive photoresist, and the development time is 60s. During the development process, the sample is slightly shaken up and down so that the positive photoresist and the developer can fully react to form a grating structure layer with a height of 250nm.

[0068] i. Using vacuum coating, a 60 nm thick Ag layer was deposited on the surface of the formed grating structure as the SERS active layer.

[0069] Comparative Example 4

[0070] Preparation process of SERS substrate without insulating layer:

[0071] a. Ultrasonic cleaning of the silicon wafer was performed using anhydrous ethanol and acetone in sequence, and deionized water was used to remove the residual cleaning solution to ensure that there were no impurities on the silicon wafer surface;

[0072] b. Using vacuum coating, a 50 nm thick Au layer was deposited on the surface of the silicon wafer as the reflective layer of the SERS substrate;

[0073] c. Spin-coat positive photoresist at a speed of 2500 rpm;

[0074] d. The pre-baking temperature is 100°C and the time is 60s to ensure that the organic solvent in the positive photoresist can be fully volatilized and solidified;

[0075] e. Selecting an appropriate exposure dose to ensure that the exposed areas of the positive photoresist are completely dissolved in the developer. It should be noted that during the exposure process using shifted Talbot lithography, the distance between the substrate and the mask must be adjusted to achieve the ideal exposure structure.

[0076] f. Select the developer supporting the coated positive photoresist, the development time is 60s. During the development process, the sample is slightly shaken up and down so that the positive photoresist and the developer can fully react to form a grating structure layer with a height of 250nm.

[0077] g. Using vacuum coating, 40 nm thick Ag was deposited on the surface of the formed grating structure as the SERS active layer.

[0078] Comparative Example 5

[0079] Preparation process of SERS substrate without reflection layer:

[0080] a. Ultrasonic cleaning of the silicon wafer was performed using anhydrous ethanol and acetone in sequence, and deionized water was used to remove the residual cleaning solution to ensure that there were no impurities on the silicon wafer surface;

[0081] b. Spin-coat SU8 at a certain speed to a thickness of 20 nm;

[0082] c. The SU8 layer is pre-baked, exposed, post-baked, developed, and hardened to serve as the insulating layer of the SERS substrate. It should be noted that a single-beam full-area exposure is used here to meet the exposure dose required for a 20 nm thick SU8 layer; no structural exposure is required.

[0083] d. Spin-coat a positive photoresist at a speed of 2500 rpm;

[0084] e. The pre-baking temperature is 100°C and the time is 60s to ensure that the organic solvent in the positive photoresist can be fully volatilized and solidified;

[0085] f. Selecting an appropriate exposure dose to ensure that the exposed area of the positive photoresist is completely dissolved in the developer. It should be noted that during the exposure process using shifted Talbot lithography, the distance between the substrate and the mask needs to be adjusted to obtain the ideal exposure structure;

[0086] g. Select the developer supporting the coated positive photoresist, the development time is 60s. During the development process, the sample is slightly shaken up and down so that the positive photoresist and the developer can fully react to form a grating structure layer with a height of 250nm.

[0087] h. Using vacuum coating, 40 nm thick Ag was deposited on the surface of the formed grating structure as the SERS active layer.

[0088] Comparative Example 6

[0089] Preparation process of SERS substrate with varying grating height:

[0090] a. Ultrasonic cleaning of the silicon wafer was performed using anhydrous ethanol and acetone in sequence, and deionized water was used to remove the residual cleaning solution to ensure that there were no impurities on the silicon wafer surface;

[0091] b. Using vacuum coating, a 50 nm thick Au layer was deposited on the surface of the silicon wafer as the reflective layer of the SERS substrate;

[0092] c. Spin-coat SU8 at a certain speed to a thickness of 20 nm;

[0093] d. The SU8 layer is pre-baked, exposed, post-baked, developed, and hardened to serve as the insulating layer of the SERS substrate. It should be noted that a single-beam full-area exposure is used here to meet the exposure dose required for a 20 nm thick SU8 layer; no structural exposure is required.

[0094] e. Spin-coat a positive photoresist at a speed of 2500 rpm;

[0095] f. The pre-baking temperature is 100°C for 60 seconds to ensure that the organic solvent in the positive photoresist can be fully volatilized and solidified;

[0096] g. Select an appropriate exposure dose to ensure that the exposed area of the positive photoresist is completely dissolved in the developer. It should be noted that during the exposure process using shifted Talbot lithography, the distance between the substrate and the mask needs to be adjusted to obtain the ideal exposure structure;

[0097] h. Select a developer compatible with the coated positive photoresist, and adjust the development time according to the desired structural parameters. During the development process, slightly shake the sample up and down to allow the positive photoresist and developer to fully react, forming grating structure layers with heights of 220nm and 380nm, respectively.

[0098] i. Using vacuum coating, 40 nm thick Ag was deposited on the surface of the formed grating structure as the SERS active layer.

[0099] Application Examples

[0100] The SERS substrate using the grating-coupled MIM structure of the embodiment can be used to prepare ultra-sensitive and repeatable trace substance detection devices. Prepared according to conventional methods and utilizing the sandwich structure design concept, a micro-nano sensor with SERS functionality is obtained. Using rhodamine 6G (R6G) as a probe molecule, Raman spectroscopy detection is performed, and it can be used to identify substances such as malachite green, melamine, and cytosine, ultimately bringing certain application prospects for trace detection in the field of biochemical sensing. For comparison, the substrates of Comparative Examples 1 to Comparative Example 6 were also prepared into micro-nano sensors with SERS functionality, and Raman spectroscopy detection was performed using rhodamine 6G (R6G) as a probe molecule.

[0101] Among them, using Example 4 and Comparative Example 5 as the main comparison objects, the measurement results are shown in Figure 3 ,Depend on Figure 3 It can be seen that the Au+SU8 structure of the embodiment has the best Raman enhancement effect compared with the Au and SU8 structures in the comparative example, so the Au+SU8 structure is used as the SERS substrate in the present invention.

[0102] Based on the Au+SU8 structure as the SERS substrate in the present invention, grating structures with different line widths and heights were prepared and Raman spectroscopy was performed to determine the optimal grating structure on the SERS substrate. The measurement results are shown in Figure 4 Among them, curve A represents the grating structure with a line width of 150nm and a height of 250nm; curve B represents the grating structure with a line width of 170nm and a height of 380nm; curve C represents the grating structure with a line width of 120nm and a height of 220nm. Figure 4 It can be seen that when the Au+SU8 structure is used as the SERS substrate in the present invention, the Raman enhancement effect of the SERS substrate is best when the line width of the grating structure is 150nm and the height is 250nm. Therefore, the structural parameters of the line width of 150nm and the height of 250nm are used as the ideal grating structure of the SERS substrate in the present invention.

[0103] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. Any technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. A SERS substrate based on a grating-coupled MIM structure, comprising a silicon substrate, a reflective layer located on the upper surface of the substrate, an insulating layer located on the upper surface of the reflective layer, a grating structure layer located on the upper surface of the insulating layer, and a SERS active layer located on the upper surface of the grating structure layer; characterized in that: The SERS active layer is an Ag layer, the insulating layer is an SU8 photoresist layer, and the reflective layer is an Au layer; The thickness of the Ag layer is 40 nm, the grating height of the grating structure layer is 250 nm, the thickness of the SU8 photoresist layer is 20 nm, and the thickness of the Au layer is 50 nm.

2. A method for preparing a SERS substrate based on a grating-coupled MIM structure according to claim 1, characterized in that: The following steps are involved: S1. Clean the surface of the silicon substrate until there is no impurity; S2. Using vacuum coating, a 50 nm thick Au layer is deposited on the surface of the silicon substrate as a reflective layer; S3, spin-coating a 20 nm thick SU8 photoresist layer on the upper surface of the reflective layer, and performing pre-baking, exposure, post-baking, development, and hardening treatments on the SU8 photoresist layer to serve as an insulating layer; S4. Spin-coat a positive photoresist on the upper surface of the insulating layer at a speed of 2500 rpm, with a pre-baking temperature of 100°C for 60 seconds to ensure that the organic solvent in the positive photoresist can be fully volatilized and solidified. Expose the exposed area using laser interference lithography and displacement Talbot lithography. Select a developer that matches the applied positive photoresist. Adjust the development time according to the required structural parameters. During the development process, slightly shake the sample up and down and left and right to allow the positive photoresist in the exposed area to fully react and dissolve with the developer to form a grating structure layer with a height of 250 nm. S5. Using a vacuum coating method, a 40 nm thick Ag layer is deposited on the upper surface of the formed grating structure layer as a SERS active layer.

3. The method for preparing a SERS substrate based on a grating-coupled MIM structure according to claim 2, wherein: In step S1, anhydrous ethanol and acetone are used to ultrasonically clean the silicon substrate in sequence, and deionized water is used to remove the residue of the cleaning solution.

4. A use of the SERS substrate based on the grating-coupled MIM structure as claimed in claim 1 in the preparation of an ultrasensitive and repeatable trace substance detection device.

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