Flexible preparation method of femtosecond laser-induced multi-form three-dimensional metal micro-cone structure

Abstract

This invention provides a method for the flexible fabrication of multi-morphological three-dimensional metal microcone structures induced by femtosecond lasers, comprising the following steps: Step 1: fabricating a metal film layer; Step 2: setting the single-pulse energy of the femtosecond laser beam according to the morphology and size of the target multi-morphological three-dimensional microcone structure, and pre-setting the target spot morphology and spatial energy distribution of the focused processing beam; Step 3: activating the femtosecond laser source, the femtosecond laser beam being shaped by a beam modulation and focusing module, and focusing on irradiating the upper surface of the metal film layer to induce the generation of the target multi-morphological three-dimensional microcone structure; wherein, the laser single-pulse energy density is lower than the metal film layer ablation threshold. This invention uses single-pulse energy and spot morphology as control means, the single-pulse energy determines the height and overall size of the microcone, and the spot energy distribution determines the shape of the microcone. It achieves customized and controllable fabrication of various multi-morphological three-dimensional microcone structures on the metal film surface through direct writing, with high freedom of morphology control, adapting to the structural requirements of different functional devices.

Description

Technical Field

[0001] This invention relates to the field of laser micro / nano manufacturing technology, specifically to a method for flexible fabrication of multimorphic three-dimensional metal microcone structures induced by femtosecond laser. Background Technology

[0002] Three-dimensional metallic microconical structures can achieve functions such as local electromagnetic field enhancement and optical response modulation through morphology control. Their morphology, size and arrangement directly determine the core performance of the device. They are core functional materials for devices such as biosensor chips, optical metasurfaces and micro-optical elements. Their core functions include providing surface-enhanced Raman scattering and electromagnetic wave frequency selective absorption.

[0003] Currently, the mainstream fabrication methods for three-dimensional metallic microconical structures include photolithography, dry etching, and electrochemical deposition. Among them, photolithography usually uses photoresist as an intermediary, requires customized masks, has a complex process flow and long preparation cycle, and is prone to structural collapse and dimensional instability, making it difficult to achieve controllable fabrication of high-precision microstructures. Dry etching has a narrow etching atmosphere and process window, and is prone to problems such as side etching, uneven etching rate, and rough sidewalls, resulting in high equipment investment and operating costs. Electrochemical deposition has problems such as poor process compatibility and easy introduction of impurities, and has limited ability to precisely control the morphology and size of structures, making it unsuitable for the batch fabrication needs of large-area, arrayed surface microstructures.

[0004] Femtosecond lasers, characterized by ultrashort pulse widths and ultra-high peak power, inject laser energy into materials and rapidly diffuse it within an extremely short time, significantly reducing the heat-affected zone and enabling high-precision, low-damage processing. This provides a new pathway for the low-damage fabrication of three-dimensional microconical structures on metal film surfaces. However, most existing femtosecond laser processing techniques for metal films rely on laser ablation to achieve structural shaping, which easily causes damage to the metal film and substrate cracking. A few non-ablation processing techniques can only achieve three-dimensional microconical structures with a single morphology through single energy adjustment, and cannot achieve diversified and high-precision control of the morphology and size of microconical structures by separately adjusting the spot morphology and energy.

[0005] In summary, existing technologies struggle to balance structural precision, morphological flexibility, and large-area fabrication efficiency, thus limiting the application of surface microstructures. Therefore, there is an urgent need for a femtosecond laser processing method that is ablation-free, mask-free, has a simple process flow, and allows for the independent control of multiple parameters to fabricate diverse three-dimensional microconical structures on metal film surfaces. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the purpose of this invention is to provide a flexible fabrication method for femtosecond laser-induced multimorphic three-dimensional metal microcone structures.

[0007] The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to the present invention includes the following steps: Step 1: Prepare a metal film layer on the substrate and fix the substrate to the sample carrier module; Step 2: Based on the morphology and size of the target multimorphic three-dimensional microcone structure, set the single pulse energy of the femtosecond laser beam, and preset the target spot morphology and its spatial energy distribution of the focused processing beam; Step 3: Start the femtosecond laser source and emit a femtosecond laser beam. After the femtosecond laser beam is shaped by the beam control and focusing module, it forms a focused processing beam, which focuses and irradiates the upper surface of the metal film layer, inducing the generation of the target multi-morphological three-dimensional microcone structure on the surface of the metal film layer. The laser single-pulse energy density is controlled within a range below the metal film ablation threshold throughout the process.

[0008] Preferably, the sample carrying module includes a moving sample stage and a sample clamp; In step 1, after the metal film layer is prepared, the substrate is fixed onto the sample fixture; In step 3, the moving sample stage moves along a preset scanning path, causing the femtosecond laser to sequentially focus and irradiate different target positions on the upper surface of the metal film.

[0009] Preferably, the beam control and focusing module includes: The beam control and shaping optical path, along the transmission direction of the femtosecond laser beam, includes an attenuator, a beam expander, and an optical field shaping element, which are used to independently control the single-pulse energy, spot morphology, and spatial energy distribution of the femtosecond laser beam. The focusing processing module includes a flat achromatic focusing objective lens that matches the wavelength of the femtosecond laser, used to vertically focus the shaped femtosecond laser beam onto the upper surface of the metal film layer, forming a focused processing beam with controllable energy and morphology.

[0010] Preferably, the substrate comprises quartz glass, borosilicate glass, or K9 optical glass.

[0011] Preferably, the metal film is formed by electron beam evaporation.

[0012] Preferably, the metal type of the metal film includes gold, silver, copper, platinum, or titanium.

[0013] Preferably, the light field shaping element includes a liquid crystal spatial light modulator, a digital micromirror device, and / or a diffractive optical element, used to shape the incident light beam into a target light spot shape.

[0014] Preferably, both the femtosecond laser source and the beam modulation and focusing module are electrically connected to the control system; The control system is used to set the single-pulse energy of the femtosecond laser beam emitted by the femtosecond laser source, and to set the target spot morphology and its spatial energy distribution after shaping by the beam modulation and focusing module.

[0015] Preferably, the moving sample stage is electrically connected to the control system, which is used to control the scanning path of the moving sample stage.

[0016] Preferably, the target multimorphic three-dimensional microcone structure includes a single-peak microcone structure, an elliptical microcone structure, a strip-shaped microcone structure, and a bi-peak microcone structure.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses single-pulse energy and customized spot morphology as control methods. The single-pulse energy determines the height and overall size of the microcone, and the spot energy distribution determines the shape of the microcone. Customized and controllable fabrication of various three-dimensional microcone structures, including single-peak microcone structure, elliptical microcone structure, long strip microcone structure, and double-peak microcone structure, can be achieved on the surface of a metal film by direct writing. The morphology control has a high degree of freedom and can adapt to the structural requirements of different functional devices.

[0018] 2. The present invention uses laser energy below the metal film ablation threshold throughout the process, without ablation or material removal. It achieves the formation of multi-morphological three-dimensional microconical structures on the surface entirely through local melting jet forming and rapid cooling recrystallization of the metal film. The processing accuracy can reach the sub-micron level, and the structural integrity and consistency are far superior to ablation-type processing technology.

[0019] 3. This invention requires no mask, has a simple process flow, and reduces processing costs; through arrayed spot shaping, it can achieve parallel processing of single-pulse multi-structures, with high preparation efficiency, and is compatible with the needs of small-batch customization and large-scale production.

[0020] 4. The process of this invention has good compatibility and can be adapted to metal film samples of different thicknesses and different metal materials (gold, silver, copper, platinum, titanium, etc.). It is suitable for the fabrication of structures in multiple fields such as Raman enhanced detection of trace substances, selective absorbing metasurfaces, plasmonic sensing, and microfluidic chips. Attached Figure Description

[0021] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram illustrating the overall preparation principle of the present invention; Figure 2 This is a schematic diagram of the beam control and shaping optical path in this invention; Figure 3 The images shown are characterization diagrams of the microstructures induced on the surface of the metal film under different spot morphologies and laser energies in this invention. From top to bottom, they are: focused spot morphology diagram, energy distribution curve of spot cross section, and SEM characterization diagram of three-dimensional microcone structure on the surface of metal film.

[0022] The diagram shows: Detailed Implementation

[0023] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0024] This invention discloses a flexible fabrication method for femtosecond laser-induced multimorphic three-dimensional metal microcone structures. This method utilizes spatial light modulation to achieve programmable control of the femtosecond laser spot morphology. There is no ablation or material removal. The multimorphic three-dimensional microcone structure with the same shape as the modulated spot is formed on the surface of the metal film by direct writing through femtosecond laser-induced local melting of the metal film, jet forming, and rapid cooling and recrystallization after the pulse ends.

[0025] The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to the present invention employs, as follows: Figure 1 The femtosecond laser-induced surface microstructure fabrication device shown includes a femtosecond laser source 1, which outputs a femtosecond laser beam 2. After being shaped and focused by the beam shaping and focusing module 3, it forms a focused processing beam 4, which is incident vertically from the front of the metal film layer 5 and focuses on irradiating the upper surface of the metal film layer. The laser single pulse energy density is controlled to be lower than the ablation threshold of the metal film throughout the process, and a three-dimensional metal microcone structure 7 is generated on the metal surface. Figure 1 The right side simultaneously shows the continuous and controllable evolution of the height and bottom size of the multimorphic three-dimensional microcone structure 7 on the metal film surface under Gaussian spot irradiation as the laser single pulse energy increases.

[0026] like Figure 2 The diagram shows the beam control and shaping optical path of the present invention. An attenuator 31, a beam expander 33, an optical field shaping element 34, and an objective lens 35 are arranged sequentially along the laser transmission direction. After the femtosecond laser beam 2 is precisely controlled by the attenuator 31 to adjust the single pulse energy, it is deflected by the reflector 32 to the beam expander 33. After the laser beam is expanded and collimated, it is guided by another reflector 32 to the optical field shaping element 34 to complete the customized control of the spot shape. Finally, after being focused by the flat achromatic focusing objective lens 35, a focused processing beam 4 with matching energy and shape is formed, which is perpendicularly incident on the upper surface of the metal film layer 5.

[0027] The present invention provides a method for the flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metal microcone structures. This method utilizes spatial light modulation to achieve programmable control of the femtosecond laser spot morphology, eliminating ablation and material removal. It achieves multimorphic three-dimensional microcone structures on the metal film surface through femtosecond laser-induced local melting of the metal film, jet forming, and rapid cooling and recrystallization after the pulse ends, forming structures with the same shape as the modulated laser spot on the metal film surface via direct writing. The method includes the following steps: Step 1: A high-purity metal film 5 is prepared on the surface of the glass substrate 6 using a thin film deposition process. After routine cleaning and drying, the metal film sample is fixed on the sample stage fixture. Step 2: Based on the morphology and size of the target multi-morphological three-dimensional microcone structure 7, the single-pulse energy of the femtosecond laser beam 2 is set by the control system. At the same time, the target spot morphology and spatial energy distribution of the focusing processing beam 4 are preset by the optical field shaping element 34. The single-pulse energy determines the volume of the molten region and the magnitude of the thermoelastic stress, which in turn determines the jet height and overall size of the microcone structure. The spatial energy distribution of the spot determines the geometric shape and surface tension distribution of the molten region, which in turn determines the morphology of the microcone structure. Furthermore, it can be adjusted as needed during the processing to achieve flexible control of the morphology of the microcone structure in different regions. Step 3: Start the femtosecond laser source 1 and the high-precision motion sample stage 8. The control system drives the motion sample stage 8 to move along the preset scanning path, so that the femtosecond laser is focused to irradiate the upper surface of the metal film layer 5. Throughout the process, the laser single pulse energy density is controlled within a range lower than the metal film ablation threshold, and the target multi-morphological three-dimensional microcone structure 7 is induced to be generated on the surface of the metal film. The jet height and overall size of the microcone structure are precisely controlled by adjusting the single pulse energy, and the morphology of the microcone structure is precisely controlled by adjusting the spatial energy distribution of the light spot.

[0028] To implement the above method, the following methods are adopted: Figure 1 , Figure 2 The device shown is based on a conventional femtosecond laser micro / nano fabrication platform and includes a femtosecond laser output module, a beam control and shaping optical path, a focusing and processing module, a sample carrying module, and a control system.

[0029] The femtosecond laser output module includes a femtosecond laser source 1 for outputting an ultrashort pulse femtosecond laser beam 2 for processing. The beam control and shaping optical path, arranged sequentially along the laser transmission direction, includes an attenuator 31, a beam expander 33, and a light field shaping element 34. Its core function is to precisely and independently control the single-pulse energy and spot morphology of the femtosecond laser, i.e., the spatial energy distribution. The focusing processing module employs a flat-field achromatic focusing objective 35 matched to the femtosecond laser wavelength to vertically focus the shaped femtosecond laser beam onto the upper surface of the metal film, forming a focused processing beam 4 with controllable energy and morphology. The sample carrying module includes a high-precision three-dimensional electric displacement stage, i.e., a high-precision moving sample stage 8, and a sample clamp for fixing the glass substrate metal film sample to be processed, and enabling three-dimensional scanning motion with nanometer-level precision. The control system is electrically connected to the femtosecond laser output module, the beam control and shaping optical path, the focusing processing module, and the sample carrying module, respectively, for synchronously controlling the laser energy parameters, beam shaping state, and sample stage scanning path, achieving independent control of energy and morphology.

[0030] In a preferred embodiment, in step 1, the glass substrate 6 is one of quartz glass, borosilicate glass, or K9 optical glass; the metal film layer 5 is a metal thin film formed by electron beam evaporation, and the metal includes, but is not limited to, gold, silver, copper, platinum, or titanium.

[0031] In a preferred embodiment, in step 2, the spatial optical field shaping of the pulsed femtosecond laser is achieved through optical field shaping elements, including but not limited to liquid crystal spatial light modulators (SLMs), digital micromirror devices (DMDs), and diffractive optical elements (DOEs). These elements are used to shape the incident Gaussian beam into different spot morphologies, including but not limited to line-focused beams, elliptical flat-top beams, and ring beams. By changing the spot morphology and laser pulse energy, the jet forming process of the molten metal film is controlled. The locally stronger energy within the spot induces significant bulging of the metal film at that location. The intensity gradient of the spot determines the steepness of the bulging, thereby enabling the controllable fabrication of different types of multi-morphological three-dimensional microcone structures, including but not limited to single-peak microcone structures, elliptical microcone structures, elongated microcone structures, and bi-peak microcone structures. Figure 3 As shown, multi-morphological three-dimensional metal microcone structures with different morphologies are induced on the surface of the metal film. From top to bottom, the images are: the morphology of the focused spot, the energy distribution curve of the spot cross section, and the SEM characterization image of the multi-morphological three-dimensional microcone structure on the surface of the metal film. The formation mechanism is completely matched by the non-ablation melting and recrystallization forming mechanism.

[0032] Example 1 This embodiment discloses a multi-morphological three-dimensional metal microcone structure array formed by direct writing of different spot morphologies. Step 1. Select a quartz glass substrate 6 with a thickness of 1 mm and prepare a pure metal film layer 5 (e.g., gold film) with a thickness of about 50 nm on its surface by electron beam evaporation thin film deposition process. After the prepared sample is cleaned and dried by conventional process, it is fixed on a high-precision moving sample stage 8 by sample stage clamp.

[0033] Step 2. Clarify the quantitative correspondence of core process parameters: Single-pulse energy is linearly positively correlated with thermoelastic stress, thus determining the height and overall size of the microcone jet, and energy adjustment does not change the basic morphology of the microcone; the spatial energy distribution of the light spot determines the boundary of the molten region and the surface tension field, which corresponds to the morphology of the microcone. Based on this principle, precise parameter customization can be achieved. By matching the corresponding light spot according to the target morphology and then deducing the required single-pulse energy, the fabrication can be completed.

[0034] Step 3. The femtosecond laser source 1 adjusts the single pulse energy to 0.1μJ–50μJ through attenuator 31, which is continuously adjustable, and then enters the optical field shaping element 34 after passing through beam expander 33.

[0035] Step 4. Load different phase masks onto the light field shaping element 34 to generate the following spot shapes: standard Gaussian beam, elliptical flat-top beam, line-focused beam, and double-peak beam. Record the intensity distribution of each spot at the focal plane using a CCD.

[0036] Step 5. Divide the processing area into several array regions, and select different spot morphologies from the above description for processing in different sub-arrays. In this embodiment, for a standard Gaussian spot array, six sets of gradient single-pulse energies of 0.15μJ, 0.25μJ, 0.35μJ, 0.45μJ, 0.55μJ, and 0.65μJ were further set up for comparative experiments, covering the entire range from the start of the jet to the maximum achievable height.

[0037] Step 6. After processing, the different subarray regions were characterized using scanning electron microscopy (SEM). The standard Gaussian beam-induced region used 0.15 μJ as the jet initiation threshold; below this energy, there was no obvious jet structure. As the single-pulse energy increased, the microcone structure maintained its standard single-peak morphology without ablation marks. The diameter of the microcone base also increased linearly with energy. Within the energy range of 0.15 μJ to 0.65 μJ, the microcone jet height increased linearly from 200 nm to 1.2 μm, with a linear fit R² > 0.98. The elliptical flat-top beam-induced region formed an elliptical microcone structure, with the major axis dimension significantly larger than the minor axis dimension. This morphology originated from the elliptical energy distribution of the flat-top beam spot. The molten region was elliptical, and the thermoelastic stress was uniformly distributed along the elliptical contour. The elliptical morphology remained constant within the energy range, with only the major and minor axis dimensions increasing linearly with energy. The line-focused beam-induced region... The structure forms an elongated microcone extending along the line direction, with multiple shorter jets visible on the ridge. This morphology originates from the one-dimensional energy distribution of the line spot. The molten region is elongated, and the thermoelastic stress is continuously distributed along the line, driving the molten metal to form a continuous elongated jet. The tiny jets on the ridge correspond to the minute undulations in the energy distribution of the line spot. Energy changes only alter the overall height of the jet and the width of the ridge, while the basic elongated morphology remains unchanged. The bimodal beam-induced region forms a bimodal microcone structure with two adjacent jets whose tips bridge each other. This morphology originates from the dual energy peak distribution of the bimodal beam. The two energy peaks induce the formation of two independent molten regions and generate upward jets. The liquid metal at the tips of the two adjacent jets attracts, fuses, and bridges each other under the action of surface tension, eventually solidifying to form a symmetrical bimodal bridging structure. The energy concentration region within the beam spot corresponds to the bulging position of the metal film, and the intensity gradient of the beam spot directly determines the steepness of the sidewalls of the microcone structure, perfectly matching the forming mechanism of this invention.

[0038] The above results demonstrate that the final microcone structure corresponds perfectly to the modulated spot morphology. Within the effective processing window, the single-pulse energy and the height of the microcone jet exhibit a precise linear relationship, and energy adjustment only changes the dimensional parameters without affecting the basic morphology of the microcone. Based on the independent control mechanism of thermoelastic stress driving size and surface tension regulating morphology, this invention can achieve precise parameterized customized fabrication. According to the specified morphology and height requirements of the target microcone structure, by adjusting the spot morphology and single-pulse energy respectively, flexible direct writing of multi-morphological three-dimensional metal microcone structures consistent with the modulated spot shape can be achieved.

[0039] In the description of this application, it should be understood that the terms "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0040] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A method for flexible fabrication of multi-morphological three-dimensional metallic microcone structures induced by femtosecond laser, characterized in that, Includes the following steps: Step 1: Prepare a metal film layer (5) on the substrate (6) and fix the substrate (6) on the sample carrier module; Step 2: Based on the morphology and size of the target multimorphic three-dimensional microcone structure (7), set the single pulse energy of the femtosecond laser beam (2), and preset the target spot morphology and its spatial energy distribution of the focused processing beam (4); Step 3: Start the femtosecond laser source (1) and emit a femtosecond laser beam (2). After the femtosecond laser beam (2) is shaped by the beam control and focusing module (3), it forms a focused processing beam (4) and focuses on irradiating the upper surface of the metal film layer (5) to induce the generation of the target multimorphic three-dimensional microcone structure (7) on the surface of the metal film layer (5). The laser single-pulse energy density is controlled within a range below the ablation threshold of the metal film layer (5) throughout the process.

2. The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to claim 1, characterized in that, The sample carrying module includes a moving sample stage (8) and a sample clamp; In step 1, after the metal film layer (5) is prepared, the substrate (6) is fixed on the sample fixture; In step 3, the moving sample stage (8) moves along a preset scanning path, so that the femtosecond laser sequentially focuses on different target positions on the upper surface of the metal film layer (5).

3. The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to claim 1, characterized in that, The beam control and focusing module (3) includes: The beam control and shaping optical path includes an attenuator (31), a beam expander (33), and an optical field shaping element (34) in sequence along the transmission direction of the femtosecond laser beam (2), which are used to independently control the single pulse energy, spot morphology, and spatial energy distribution of the femtosecond laser beam (2). The focusing processing module includes a flat achromatic focusing objective (35) that matches the wavelength of the femtosecond laser, used to vertically focus the shaped femtosecond laser beam (2) onto the upper surface of the metal film layer (5) to form a focused processing beam (4) with controllable energy and morphology.

4. The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to claim 1, characterized in that, The substrate (6) includes quartz glass, borosilicate glass or K9 optical glass.

5. The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to claim 1, characterized in that, The metal film (5) is formed by electron beam evaporation.

6. The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to claim 1, characterized in that, The metal type of the metal film (5) includes gold, silver, copper, platinum or titanium.

7. The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to claim 3, characterized in that, The light field shaping element (34) includes a liquid crystal spatial light modulator, a digital micromirror device and / or a diffractive optical element, used to shape the incident light beam into a target light spot shape.

8. The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to claim 1, characterized in that, The femtosecond laser source (1) and the beam modulation and focusing module (3) are both electrically connected to the control system; The control system is used to set the single pulse energy of the femtosecond laser beam (2) emitted by the femtosecond laser source (1), and to set the target spot morphology and its spatial energy distribution after being shaped by the beam control and focusing module (3).

9. The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to claim 3, characterized in that, The moving sample stage (8) is electrically connected to the control system, which is used to control the scanning path of the moving sample stage (8).

10. The method for flexible fabrication of femtosecond laser-induced multimorphic three-dimensional metallic microcone structures according to claim 1, characterized in that, The target multimorphic three-dimensional microcone structure (7) includes a single-peak microcone structure, an elliptical microcone structure, a strip-shaped microcone structure, and a bi-peak microcone structure.

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