Multilayer film multi-photon photoetching method based on photosensitive difference

Abstract

The invention belongs to the technical field of mask photoetching, and particularly relates to a multilayer film multi-photon photoetching method based on photosensitivity difference. According to the invention, the multi-photon absorption effect of femtosecond laser is utilized, and high-resolution photoetching is carried out on a double-layer or multi-layer film at a tightly focused laser focus; through combination of differences of photosensitive characteristics of photoresist materials, accurate control of a multi-layer micro-nano structure is realized in single scanning. Wherein photoetching is carried out by utilizing multiple photoetching schemes such as a two-photon mechanism, a three-photon mechanism and even a multi-photon mechanism, or a mixture of absorption mechanisms of different numbers of photons in each layer, and the like, so that the performance is improved. By utilizing the multi-photon effect of the femtosecond laser and the sensitivity difference of the multi-layer photoresist, the prepared structure has high precision (can be controlled at the magnitude of 10 nanometers to 100 nanometers), the line width, the material, the shape and the like of the structure have independent control capability, and the wide application prospect is shown.

Description

Technical Field

[0001] This invention belongs to the field of mask lithography technology, specifically relating to a multi-photon lithography method for multilayer thin films based on differences in photosensitivity. Background Technology

[0002] With the continuous development of modern micro-nano manufacturing technology, photolithography, as a core process in fields such as microelectronics, optical devices, and microelectromechanical systems (MEMS), is evolving towards higher resolution, more complex structures, and higher efficiency. Traditional photolithography methods, such as ultraviolet lithography and electron beam lithography, while widely used in the fabrication of large-scale integrated circuits and micro-nano structures, have limitations in achieving high-resolution nanostructures at the subwavelength scale due to optical diffraction limits and process complexity. Furthermore, the photoresist materials used in existing photolithography technologies typically possess only a single photosensitivity, making it difficult to precisely control the shape, linewidth, and material properties of each layer during the patterning of multilayer structures, thus failing to meet the increasingly complex demands of nanofabrication.

[0003] To overcome these technological bottlenecks, multiphoton lithography has gradually emerged as a highly promising micro- and nanofabrication technology in recent years due to its high resolution, flexibility, and unique nonlinear optical effects. Multiphoton lithography utilizes high-intensity femtosecond lasers focused to the nanoscale, inducing photochemical reactions in materials through multiphoton absorption effects, thereby achieving precise nanostructure fabrication. This technology can overcome the optical diffraction limit, constructing ultra-fine structures in three-dimensional space. However, in practical applications, existing multiphoton lithography processes still face many challenges. Especially when fabricating complex multilayer structures, the limited selection of photoresist materials often makes it difficult to achieve independent control of the characteristics of each layer (such as size, linewidth, material, and shape) without sacrificing resolution.

[0004] To address these issues, researchers have begun exploring the use of different types of photoresist materials to improve the flexibility and functionality of the photolithography process. However, existing multilayer photoresist technologies still suffer from limitations in material selection, complex processes, and poor stability when dealing with complex nanostructures. Summary of the Invention

[0005] The purpose of this invention is to provide a high-precision, multi-photon lithography method for multilayer thin films based on photosensitive differences, which allows for independent control of the size, linewidth, material, and shape of structural features.

[0006] This invention proposes a multiphoton lithography method for multilayer thin films based on photosensitive differences. Utilizing the multiphoton absorption effect of femtosecond lasers, it performs high-resolution lithography on bilayer or multilayer thin films at a tightly focused laser focal point. By combining the differences in the photosensitive properties of the photoresist materials, precise control of multilayer micro / nano structures is achieved in a single scan. Various lithography schemes can be employed, including two-photon, three-photon, and even multiphoton mechanisms, or a mixture of different numbers of photon absorption mechanisms in each layer, to improve performance. This invention combines the multiphoton effect of femtosecond lasers with the sensitivity differences of multilayer photoresists, resulting in structures that not only possess high precision but also exhibit independent control over linewidth, material properties, and shape, demonstrating broad application prospects. The final nanostructures can be controlled to a precision of 10 to 100 nanometers, with high repeatability and the ability to customize various structures according to requirements, exhibiting high flexibility, high precision, and wide adaptability.

[0007] The multiphoton lithography method for multilayer thin films based on photosensitive differences proposed in this invention comprises the following steps:

[0008] Step (1): Use oxygen plasma to bombard the substrate to perform surface treatment on the substrate.

[0009] The substrate is surface-treated by ultrasonic vibration with 95% ethanol for 10-15 minutes to improve the adhesion and uniformity of the photoresist. The substrate is then placed in an oxygen plasma treatment device for surface activation, removing organic matter from the substrate surface and further improving the adhesion of the photoresist.

[0010] Step (2): Prepare a multilayer photoresist film on the substrate.

[0011] On a surface-treated substrate, photoresists with different photosensitivity are sequentially spin-coated using a spin coater. Specifically, a photoresist with higher photosensitivity (i.e., increased solubility after photosensitization, indicating poor solubility) is designated as resist A, and a photoresist with lower photosensitivity (i.e., decreased solubility after photosensitization, indicating good solubility) is designated as resist B, forming a multilayer photoresist film. The spin coater speed is set to 2000-5000 rpm, and the spin coating time is 30-60 seconds. After spin-coating the first layer of photoresist, the substrate is pre-baked on a hot plate at 80℃-100℃ for 8-10 minutes to ensure the uniformity and adhesion of the photoresist. The second layer of photoresist is then spin-coated using the same procedure. The thickness of the two photoresist layers can be varied by controlling the spin coating parameters.

[0012] Step (3): Photolithography structure design and processing.

[0013] The required microstructure patterns are designed using computer-aided design software and converted into spatial dot matrix files suitable for direct writing with femtosecond lasers. During the design of two-dimensional planar patterns and three-dimensional nanostructures, the spacing between each point is controlled to be 100-300nm, and different shape parameters (including squares, rectangles, lines, etc.) are designed to ensure the precise positioning of the laser in the photosensitive adhesive layer. Then, the designed microstructure patterns are converted into spatial dot matrix files to control the path of femtosecond laser scanning and the distribution of exposure points.

[0014] Step (4): Femtosecond laser Z-axis single-scan multiphoton effect lithography.

[0015] A femtosecond laser direct writing system with a wavelength of 800nm ​​is used, and the beam shape is adjusted to generate a multiphoton effect at a tightly focused spot. At the same time, the power, focusing factor, and exposure time of the femtosecond laser are adjusted according to the photosensitivity and thickness of the photoresist to ensure the accuracy and resolution of the exposure. A single scan exposure is performed on the double-layer photoresist, and the spot stays at each designated position for a period of time, so that the photoresist reacts at the spot and forms the preset microstructure.

[0016] Step (5): Development and post-processing.

[0017] After laser exposure is completed, a developing solution is used to develop the substrate, removing the unexposed photoresist and retaining the exposed areas to obtain the designed microstructure. Finally, the exposed substrate is immersed in the developing solution, and the unexposed areas are removed by soaking or stirring. After development, the substrate is rinsed with deionized water to remove residual developing solution and dried with nitrogen.

[0018] Furthermore:

[0019] In step (1), the oxygen plasma treatment equipment has an oxygen plasma treatment time of 2-5 minutes and a plasma energy of 30-100W.

[0020] In step (2), the spin-coated A and B photoresists can have different structural combinations, specifically using a combination of thin-layer dried photoresist B and thick-layer dried photoresist A, or a combination of thin-layer dried photoresist B and liquid photoresist A. The spin-coating thickness of thick-layer dried photoresist A is 100nm-5000nm, and the spin-coating thickness of thin-layer dried photoresist B is 80nm-1000nm. The spin-coating thickness of B is less than that of A.

[0021] In step (2), the spin-coated A and B adhesives can have different combinations of upper and lower spin coating. Specifically, the spin coating sequence can be one of the following: upper A and lower B, upper A and lower A, upper B and lower A, or upper B and lower B.

[0022] In step (2), the A-resin is selected from commercial photoresist AR-N4400. As a thick resist, it is sensitive to various light sources such as ultraviolet light, electron beam, and X-rays, and can be used for various processes such as LIGA, MEMS, and lift-off. The B-resin is selected from commercial photoresist AR-P3100. As a thin resist, it can be used for fine structure processing and mask fabrication.

[0023] In step (4), the adjustment to different beam shapes can be one of the following: uniform beam, Bessel beam, Gaussian beam, flat-top beam, or vortex beam.

[0024] In step (4), the spot diameter of the femtosecond laser is 1-2 μm, the pulse width is 100 fs, the power is 7-15.2 mW, the spot spacing is 100 nm-200 nm, the laser spot dwell time during the scanning process is 1000 μs-3000 μs, and the repetition frequency is 80 MHz.

[0025] In step (5) of this invention, A glue uses conventional developer and the development time is 1-2 minutes; B glue requires a longer development time, usually 2-3 minutes.

[0026] The main technical features and advantages of this invention are as follows:

[0027] This invention presents a high-resolution photolithography method based on multilayer photoresist films with varying multiphoton photosensitivity, enabling the fabrication of microstructures with resolutions up to the tens of nanometers level, overcoming the diffraction limit of traditional photolithography techniques. The invention involves sequentially and uniformly spin-coating two photoresist layers with different photosensitivity or photoresist properties onto a substrate, employing different thicknesses and orientations to create various thin-film photoresist material systems. This invention allows for the flexible construction of high-resolution, complex, multi-layered nanostructures. Furthermore, by utilizing the multiphoton effect at the femtosecond laser focal point, single-scan exposure is achieved, significantly simplifying the process and improving processing efficiency. This invention not only allows for independent control of the feature size, linewidth, material, and shape of the structure but also demonstrates broad adaptability and multifunctionality in nanopatterning applications, exhibiting significant innovative value. This invention has enormous application potential in integrated circuit manufacturing, optical device fabrication, micro / nano sensors, and microelectromechanical systems (MEMS). Attached Figure Description

[0028] Figure 1 This is a flowchart of the high-resolution photolithography method for multiphoton photosensitivity difference in Embodiment 1 of the present invention.

[0029] Figure 2 This is a two-photon scanning diagram of a photosensitive differential multilayer thin film in Embodiment 1 of the present invention, created using multiphoton lithography.

[0030] Figure 3This is a SEM image after laser exposure processing in Embodiment 1 of the present invention. The dashed line represents the area reserved for metal layer deposition.

[0031] Figure 4 This is Example 1 of the present invention. Figure 3 SEM image of the area marked by dashed lines after metal layer deposition.

[0032] Figure 5 This is a flowchart and effect diagram of Embodiment 1 of the present invention.

[0033] Figure 6 This is a multilayer photolithography illustration of two different beam exposure combinations in this invention.

[0034] Figure 7 This is a schematic diagram of multilayer photolithography using photoresists of different shapes and thicknesses in Embodiment 3 of the present invention.

[0035] Figure 8 This is a SEM image after laser exposure processing in Embodiment 3 of the present invention. The dashed line represents the area reserved for metal layer deposition.

[0036] Figure 9 This is Example 3 of the present invention. Figure 8 SEM image of the area marked by dashed lines after metal layer deposition.

[0037] Figure 10 This is a multilayer photolithography illustration of different light beam exposures and step-by-step development in Embodiment 4 of the present invention. Detailed Implementation

[0038] The present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0039] Example 1

[0040] (1) The substrate was ultrasonically vibrated with 95% ethanol for 15 minutes to perform surface treatment. Then the substrate was placed in an oxygen plasma treatment device for surface activation. The oxygen plasma treatment time was 3 minutes and the plasma energy was 50W.

[0041] (2) On the surface-treated substrate, a spin coater with a rotation speed of 2000 rpm and a spin coating time of 30 seconds is used to spin coat a 150 nm thick layer of A-resin (photoresist AR-N4400). The substrate is then pre-baked on a hot plate at 80℃-100℃ for 10 minutes. Then, the spin coater with a rotation speed of 5000 rpm and a spin coating time of 60 seconds is used to spin coat a 100 nm thick layer of B-resin (photoresist AR-P3100). This forms a multilayer photoresist film combination with different photosensitivity, consisting of A on top and B on the bottom.

[0042] (3) The required microstructure pattern is designed using computer-aided design software, with a spacing of 100 nm between each point and a rectangular shape. The designed microstructure pattern is then converted into a spatial dot matrix file containing specific coordinate information, which is used to control the path of the femtosecond laser scanning and the distribution of exposure points.

[0043] (4) A femtosecond laser direct writing system was used to perform photolithography according to the designed dot matrix file. Under the multiphoton effect, the molecules in the photoresist undergo nonlinear absorption reactions, thereby forming micro- and nano-structures. The laser moves along the path set in the dot matrix file through a computer control system. A single scan exposure was performed in the upper layer photoresist (A-resist) using parameters such as a femtosecond laser spot diameter of 1 μm, a pulse width of 100 fs, a power of 7 mW, a dot pitch of 100 nm, a laser dot dwell time of 1000 μs scanning speed, and a Gaussian beam parameter of 80 MHz repetition frequency. The laser dot stayed at each designated position for a period of time, allowing the photoresist to react at the dot and form the preset microstructure. Due to its high photosensitivity, the A-resist could undergo photochemical reactions at a lower laser power, while the B-resist required a higher power to produce the multiphoton effect. Then, the power was adjusted to 15.2 mW and a single scan exposure was performed in the lower layer photoresist (B-resist).

[0044] (5) After laser exposure, develop the substrate with photoresist B for 3 minutes to remove unexposed photoresist B; then develop it with photoresist A for 2 minutes to remove unexposed photoresist A. Retain the exposed areas to obtain the designed microstructure. Finally, immerse the exposed substrate in the developer and remove the unexposed areas by soaking or stirring. After development, rinse the substrate with deionized water to remove residual developer and dry it with nitrogen gas to form a complete rectangular cross-section structure.

[0045] The overall process steps of this embodiment are as follows: Figure 1 As shown; a schematic diagram of photolithography after spin coating is shown below. Figure 2 As shown, using a layer of A-type adhesive on top of B-type adhesive ensures that during two-photon scanning, the contact area between adhesive B and the light spot is smaller than that between adhesive A and the light spot. This results in a cross-linked structure where the opening of the upper film is smaller than that of the lower film, which is beneficial for preserving the structure after development. After achieving a rectangular structure through two-photon scanning, the processed structure was imaged using a scanning electron microscope to obtain the SEM image, as shown below. Figure 3 As shown in the figure, the central rectangular area and the bright white rectangle represent the untreated area of ​​the photoresist, while the grayish rectangular frame outside the bright white rectangle represents the structure treated with two-photon photons. Figure 3 Taking the structure within the dashed box as an example, the rectangular cross-sectional structure retained after development and post-processing is shown below. Figure 4 As shown. The overall photolithography process and results are as follows. Figure 5As shown, the final lithographic structure effect is obtained by sequentially performing multiphoton lithography, metal layer deposition, layer-by-layer development, and post-processing.

[0046] Example 2

[0047] Steps (1), (2), (3), and (5) are the same as in Example 1, except that step (4) uses a different Gaussian beam exposure processing method than in Example 1. The laser exposure scanning diagram of this example is shown below. Figure 6 As shown, a uniform laser beam is first used to scan through photoresist A, and then the beam is adjusted to a Bessel beam to scan through photoresist B to achieve multiphoton high-resolution lithography.

[0048] Example 3

[0049] Steps (1), (4), and (5) are the same as in Example 2. However, in step (2), the combination of AB photoresist of the same shape and thickness as in Example 2 differs. In this example, a thick dry photoresist B with a thickness of 3 μm is spin-coated on the upper layer, and photoresist A is spin-coated on the lower layer for laser direct writing lithography. In step (3), the lithography structure is designed as a linear structure, unlike the rectangular structure designed in Example 2. A schematic diagram of the different combinations of photoresist AB structures in this example, after spin-coating, is shown below. Figure 7 As shown; after realizing the rectangular structure using two-photon scanning, the processed structure was imaged using a scanning electron microscope to obtain the SEM image, as shown. Figure 8 As shown in the figure, the two black linear structures represent the photoresist areas processed to achieve the linear structure; with Figure 7 Taking the structure within the dashed box as an example, the linear cross-sectional structure diagram retained after development and post-processing is as follows: Figure 9 As shown.

[0050] Example 4

[0051] Steps (1), (2), and (3) are the same as in Example 3. However, in step (4), unlike Example 3 which used multiple beams sequentially, this example uses a Gaussian beam to perform a single scan on photoresists A and B of different thicknesses. In step (5), the same development time of 2 minutes is used for development. Since the exposed structure of photoresist A is retained after exposure, while the exposed structure of photoresist B is removed, the development time is controlled to ensure that photoresist A is not completely developed, thus supporting the retained structure of photoresist B. On the other hand, since photoresist A is covered by the upper layer of photoresist B, it is inferred that the developer will develop the upper layer of photoresist B first, flowing into the slit of photoresist B to develop photoresist A, allowing photoresist A to begin contacting the developer and forming a structure. A schematic diagram of the laser exposure scanning and development process in this example is shown below. Figure 10As shown, a Gaussian beam is used to perform a single scan on photoresists A and B of different thicknesses. Both photoresists A and B are developed using the same development time. By controlling the different development effects of photoresists A and B, photoresist B is fully developed while photoresist A is partially developed, in order to manufacture a structure after processing and developing photoresists A and B.

Claims

1. A multiphoton lithography method for multilayer thin films based on photosensitive differences, characterized in that, The specific steps are as follows: Step (1) involves bombarding the substrate with oxygen plasma to perform surface treatment; including: The substrate surface is pretreated by ultrasonic vibration with 95% ethanol for 10-15 minutes to improve the adhesion and uniformity of the photoresist; then the substrate is placed in an oxygen plasma treatment device for surface activation to remove organic matter from the substrate surface and improve the adhesion of the photoresist. Step (2) involves fabricating a multilayer photoresist film on a substrate; including: Using a spin coater, photoresists with different photosensitivity are sequentially spin-coated onto a surface-treated substrate. Specifically, a photoresist with higher photosensitivity is designated as A, and a photoresist with lower photosensitivity is designated as B, forming a multilayer photoresist film. The spin coater speed is set to 2000-5000 rpm, and the spin coating time is 30-60 seconds. After spin-coating the first layer of photoresist, the substrate is pre-baked on a hot plate at 80℃-100℃ for 8-10 minutes. The second layer of photoresist is then spin-coated using the same operation. The thickness of the two photoresist layers is changed by controlling the spin coating parameters. Step (3), photolithography structure design and processing; The required microstructure patterns are designed using computer-aided design software and converted into spatial dot matrix files suitable for femtosecond laser direct writing. In the design process of two-dimensional planar patterns and three-dimensional nanostructures, the spacing between each point is controlled to be 100-300nm, and different shape parameters are designed to ensure the precise positioning of the laser in the photosensitive adhesive layer. Then, the designed microstructure patterns are converted into spatial dot matrix files to control the path of femtosecond laser scanning and the distribution of exposure points. Step (4): Femtosecond laser Z-axis single-scan multiphoton effect lithography; A femtosecond laser direct writing system with a wavelength of 800nm ​​is used, and the beam shape is adjusted to generate a multiphoton effect at a tightly focused spot. At the same time, the power, focusing factor, and exposure time of the femtosecond laser are adjusted according to the photosensitivity and thickness of the photoresist to ensure the accuracy and resolution of the exposure. A single scan exposure is performed on the double-layer photoresist, and the spot stays at each designated position for a period of time, so that the photoresist reacts at the spot and forms the preset microstructure. Step (5), development and post-processing; After laser exposure is completed, a developing solution is used to remove the unexposed photoresist and retain the exposed areas to obtain the designed microstructure.

2. The multilayer thin film multiphoton lithography method according to claim 1, characterized in that, In step (1), the oxygen plasma treatment equipment has an oxygen plasma treatment time of 2-5 minutes and a plasma energy of 30-100W.

3. The multilayer thin film multiphoton lithography method according to claim 2, characterized in that, In step (2), the spin-coated A and B photoresists have different structural combinations, specifically using a combination of thin-layer dry photoresist B and thick-layer dry photoresist A, or a combination of thin-layer dry photoresist B and liquid photoresist A; the spin-coating thickness of thick-layer dry photoresist A is 100nm-5000nm, the spin-coating thickness of thin-layer dry photoresist B is 80nm-1000nm, and the spin-coating thickness of B is less than the spin-coating thickness of A.

4. The multilayer thin film multiphoton lithography method according to claim 3, characterized in that, In step (2), the spin-coated A and B adhesives can be combined in different ways, including one of the following: A on top and B on the bottom, A on top and A on the bottom, B on top and A on the bottom, and B on top and B on the bottom.

5. The multilayer thin film multiphoton lithography method according to claim 4, characterized in that, In step (2), the A-resin is photoresist AR-N4400; the B-resin is photoresist AR-P3100.

6. The multilayer thin film multiphoton lithography method according to claim 1, characterized in that, In step (4), the adjustment to different beam shapes is specifically one of uniform beam, Bessel beam, Gaussian beam, flat-top beam, and vortex beam.

7. The multilayer thin film multiphoton lithography method according to claim 1, characterized in that, In step (4), the spot diameter of the femtosecond laser is 1-2 μm, the pulse width is 100 fs, the power is 7-15.2 mW, the spot spacing is 100 nm-200 nm, the laser spot dwell time during the scanning process is 1000 μs-3000 μs, and the repetition frequency is 80 MHz.

8. The multilayer thin film multiphoton lithography method according to claim 1, characterized in that, In step (5), the development time for A glue is 1-2 minutes; the development time for B glue is 2-3 minutes.

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