Method and device for processing microstructure with high longitudinal-width ratio through axial shaping laser

Abstract

The invention relates to the technical field of laser processing, in particular to a method and device for processing a high-longitudinal-width-ratio microstructure through axial shaping laser. According to the method, the complex coding hologram is loaded on the spatial light modulator, so that the axial intensity distribution of the Bessel beam can be accurately regulated and controlled, and the cylindrical voxel with the adjustable height can be prepared through single exposure. The voxel height can be continuously and gradually changed only by switching holograms generating Bessel beams with different focal lengths, and meanwhile, the maximum transverse size is kept unchanged. According to the method, frequent axial mechanical scanning or power adjustment is not needed in the machining process, and the machining efficiency and the operation stability can be improved; the method is suitable for rapid processing of microstructures with high longitudinal-width ratios, such as microcolumns, microtubes, cytoskeletons and binary diffraction devices.

Description

Technical Field

[0001] This invention relates to the field of laser processing technology, specifically to a method and apparatus for axially shaping laser processing of microstructures with high aspect ratio. Background Technology

[0002] High aspect ratio (HAR) microstructures have become a core research object and key technological challenge in the field of micro-nano fabrication due to their enormous application potential in microelectromechanical systems, micro-optics, biology, and metamaterials. A typical characteristic of these structures is that their longitudinal dimension is much larger than their lateral dimension, which places extremely high demands on the resolution, aspect ratio capability, efficiency, and three-dimensional morphology controllability of fabrication techniques.

[0003] Currently, the fabrication methods for HAR microstructures can be mainly divided into the following categories: Traditional micro / nano fabrication techniques: Represented by deep reactive ion etching (DRIE), these techniques can achieve structures with steep sidewalls and extremely high aspect ratios on silicon wafers, but are difficult to apply to non-silicon materials or to achieve true three-dimensional arbitrary shapes. Femtosecond laser direct writing and modification techniques: Femtosecond laser-induced assisted etching: Laser scanning modification is first performed inside a transparent material, and then wet etching is used to selectively remove the modified areas. This method can create microchannels in hard and brittle materials such as glass, but the sidewall roughness and morphology accuracy are difficult to control, and it is difficult to achieve arbitrary three-dimensional designs. Femtosecond laser direct writing: A high numerical aperture objective lens is used to focus a femtosecond laser, and the polymer photoresist is scanned point by point through a three-dimensional moving sample stage. Although this method can achieve complex three-dimensional molding with sub-micron resolution, the fabrication of a single high aspect ratio microstructure is extremely time-consuming.

[0004] By shaping femtosecond lasers into long-depth-of-focus beams using conical lenses or spatial light modulators, high aspect ratio micropillar structures can be fabricated efficiently. However, the depth of focus, lateral dimensions, and focal position of such beams are often coupled; adjusting one parameter (such as depth of focus) often leads to changes in other parameters (such as focal position), requiring frequent adjustments to the total exposure power or depth-direction movement of the sample. This makes it impossible to fabricate highly programmable, complex HAR microstructures within a fixed focal plane. Therefore, the current field of femtosecond laser fabrication of high aspect ratio microstructures urgently needs to develop a programmable, efficient, and easy-to-operate solution. Summary of the Invention

[0005] One of the objectives of this application is to provide a method for axial shaping laser processing of high aspect ratio microstructures that can avoid the need for parameter adjustment coupling linkage, compared with the prior art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for axially shaped laser processing of microstructures with high aspect ratio includes the following steps: Step S1: Generate an initial laser using a femtosecond laser and measure the intensity distribution of the initial laser light field; Step S2: Based on the intensity distribution of the initial laser field, determine the voxel parameters according to the high aspect ratio microstructure; Step S3: Calculate and generate a set of Bessel beam holograms based on voxel parameters. The holograms are used to load onto the spatial light modulator, which modulates the initial laser output beam into a Bessel beam with a controllable axial intensity distribution. Step S4: Adjust the processing energy of the Bessel beam by using an intensity attenuation device, and at the same time adjust the objective lens corresponding to the spatial light modulator so that the objective lens is aligned with the sample on the translation stage. Step S5: Control the translation stage to move while gradually replacing the hologram loaded in the spatial light modulator. Project Bessel beams of different lengths at different processing points of the sample. The Bessel beams are exposed at the processing points to generate voxels of corresponding heights. Step S6: After post-processing the voxels, a high aspect ratio microstructure with a preset height distribution is obtained.

[0007] The above technical solution produces the following technical effects: The technical solution described in this application allows for precise control of the axial intensity distribution of a Bessel beam by loading a complex-coded hologram onto a spatial light modulator, thereby enabling the fabrication of height-adjustable columnar voxels in a single exposure. Specifically, by simply switching between holograms generating Bessel beams with different focal lengths, the voxel height can be continuously and gradually varied while maintaining the maximum lateral dimension unchanged.

[0008] As a further improvement to the method of axially shaping laser processing of high aspect ratio microstructures of this application, the voxel parameters include: Voxel diameter is the width of the voxel. Voxel spacing is the distance between adjacent voxels; Voxel height refers to the height distribution of voxels.

[0009] As a further improvement to the method of axially shaped laser processing of high aspect ratio microstructures of this application, the step of calculating and obtaining the hologram in step S3 includes: Step S31: Calculate the initial surface phase distribution of the generated Bessel beam, in which the main lobe width of the Bessel beam matches the voxel diameter. Step S32: Calculate and generate the axial intensity distribution curve of the Bessel beam, and match the voxel height with the axial intensity distribution curve. Step S33: Based on the axial intensity distribution curve, calculate and obtain the initial surface complex amplitude distribution curve that realizes the axial intensity distribution curve in the Bessel beam; Step S34: Generate multiple holograms corresponding to the initial surface complex amplitude distribution curve through complex amplitude encoding.

[0010] As a further improvement to the method for axially shaping laser processing of high aspect ratio microstructures of this application, the calculation method for the initial surface complex amplitude distribution curve in step S33 includes the following steps: Step S331, setting the axial intensity distribution curve of the Bessel beam as... And the light field of the Bessel beam is on the central axis of the beam. The complex amplitude at point is expressed as: ; Where z is the propagation distance of the Bessel beam. Let r be the longitudinal component of the wave vector of the Bessel beam, r represent the radial coordinate, and represent the distance from a point in the optical field to the central axis of the beam. Step S332: Based on the axial strength distribution curve The spatial spectrum of the initial plane is solved using the following functional relationship. : ; Where k0 is the wave number, For longitudinal spatial frequency; Step S333: Obtain the spatial spectrum Substituting the angular spectrum propagation formula, calculate the complex amplitude distribution curve of the light field on the input plane. ; The expression for the angular spectrum propagation formula is: ; Where r is the radial coordinate. Radial spatial frequency, It is a zero-order Bessel function of the first kind; Step S334: Obtain the initial surface complex amplitude distribution curve based on the light field complex amplitude distribution curve. satisfy: ; The initial surface complex amplitude distribution curve contains amplitude information. With phase information x represents the x-coordinate of the initial surface, and y represents the y-coordinate of the initial surface.

[0011] As a further improvement to the method of axially shaped laser processing of high aspect ratio microstructures in this application, the method of generating a hologram of the initial surface complex amplitude distribution curve by complex amplitude encoding is as follows: Amplitude information is encoded using a complex encoding method. and phase information Encoded as pure phase distribution Pure phase distribution satisfy: ; Where (m,n) are the pixel indices of the spatial light modulator. To separate the phase of the blazed grating for the diffraction order, For the modulo operator, the parameter and parameters They respectively satisfy: ; ; Through pure phase distribution Obtain the corresponding hologram; satisfy: ; in, To measure the amplitude of the incident light on the spatial light modulator, For the shining angle, λ is the center wavelength of the incident light on the spatial light modulator.

[0012] As a further improvement to the method of axial shaping laser processing of microstructures with high aspect ratio in this application, in step S32, the axial intensity distribution curve satisfies the following: Bessel beams with different axial intensities have the same maximum on-axis intensity, and the maximum on-axis intensity is above the polymerization threshold of the sample in step S4.

[0013] As a further improvement to the method of axial shaping laser processing of microstructures with high aspect ratio in this application, the initial laser is collimated, expanded and polarized and then irradiated onto a spatial light modulator.

[0014] As a further improvement to the method of axial shaping laser processing of microstructures with high aspect ratio in this application, in step S1, the measurement method for measuring the intensity distribution of the initial laser light field is as follows: replace and align an image sensor device with the same spatial position as the spatial light modulator, adjust the power of the initial laser so that the image sensor is not saturated at the point of maximum initial laser intensity, and obtain the intensity distribution of the actual light field of the initial laser based on the corresponding pixels of the image sensor.

[0015] The second objective of this application is to provide an axially oriented laser processing apparatus for microstructures with high aspect ratios, used to realize the above-mentioned method of axially oriented laser processing for microstructures with high aspect ratios.

[0016] To achieve the above-mentioned objectives, this application implements the following technical solution: An apparatus for axially shaping laser processing of microstructures with high aspect ratios includes: Femtosecond lasers are used to emit initial laser light; Polarization state adjustment component, used to adjust the polarization state of the initial laser; Collimation and beam expanding assembly, used to collimate and expand the initial laser beam; A spatial light modulator is used to receive an initial laser beam that has been collimated, expanded, and polarized. The spatial light modulator is electrically connected to a computer. The computer loads a hologram obtained by performing axially shaped laser processing of high aspect ratio microstructures onto the spatial light modulator. The spatial light modulator outputs a Bessel beam with a controllable axial intensity distribution. An asymmetric 4-F system is used for spatial filtering of Bessel beams and focusing the filtered Bessel beams through an objective lens; The translation stage is used to receive the sample. The sample is exposed by the Bessel beam focused by the objective lens to produce voxels of corresponding height. The translation stage is electrically connected to the computer, and the computer controls the movement of the translation stage.

[0017] The above technical solution produces the following technical effects: The apparatus for axially shaping laser processing of high aspect ratio microstructures described in this application is highly compatible with existing femtosecond laser two-photon polymerization processing platforms. Furthermore, this apparatus can precisely control the axial intensity distribution of the Bessel beam, thereby achieving high-precision processing of high aspect ratio microstructures. In practical applications, the precise computer control of the translation stage allows for flexible adjustment of the sample position according to different processing requirements, enabling the Bessel beam to accurately expose the sample and generate voxels with specific heights and shapes, effectively improving the efficiency and quality of microstructure processing.

[0018] As a further improvement to the device for axial shaping laser processing of high aspect ratio microstructures in this application, the polarization state adjustment component includes a polarization state adjustment element and a polarization beam splitter, which modulates the initial laser to a polarization state that matches the polarization sensitivity of the spatial light modulator. The collimation and beam expanding assembly includes a plano-convex lens and a plano-concave lens arranged sequentially. After being adjusted by the polarization state adjustment component and the collimation and beam expansion component, the initial laser light is deflected by a reflector and incident on the liquid crystal surface of the spatial light modulator at an off-axis incident angle of 10°. The asymmetric 4-F system includes a telephoto lens and a variable aperture. The Bessel beam passes through the telephoto lens and the variable aperture in sequence and is transmitted to the objective lens. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are some embodiments of the present invention. For those skilled in the art, it is obvious that other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 This is a flowchart illustrating the axial shaping laser processing method for high aspect ratio microstructures according to the present invention.

[0021] Figure 2 This is a curve showing the relationship between light intensity and axial distance in a light beam. Figure 3 This is a schematic diagram showing the relationship between voxel diameter, voxel height, and beam intensity. Figure 4 This is a schematic diagram of the device for axial shaping laser processing of microstructures with high aspect ratios, as described in this invention.

[0022] Figure 5 This is a hologram and a schematic diagram of the spatial distribution of light field intensity generated by the axial strength control method described in this invention; Figure 6 SEM image of a high aspect ratio microtube array prepared by a method for axially shaping laser processing of high aspect ratio microstructures according to an embodiment of the present invention; Figure 7 Height curve of a high aspect ratio microtube array prepared by the method of axial shaping laser processing of high aspect ratio microstructures according to an embodiment of the present invention.

[0023] 1-Femtosecond laser; 2-Intensity attenuation device; 3-Polarization state adjustment element; 4-Polarization beam splitter; 5-Plan-convex lens; 6-Plan-concave lens; 7-Mirror; 8-Spatial light modulator; 9-Long focal length lens; 10-Variable aperture; 11-Semi-transparent mirror; 12-Objective lens; 13-Translation stage; 14-Image sensor; 15-Computer. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] To facilitate a proper understanding of the solutions provided in the following embodiments of the present invention, the terms involved in the present invention will be explained as follows before describing the technical solutions provided by the present invention: The Spatial Light Modulator (SLM) is a device that modulates a light field by loading a computationally generated hologram. It can precisely control parameters such as the phase and amplitude of the light beam, thereby achieving flexible control over the spatial distribution of the light beam (such as the axial intensity distribution).

[0026] Complex-coded holograms are holograms that integrate the amplitude and phase information of a target light field into a pure phase distribution using complex amplitude coding techniques. Specifically, the target amplitude and phase are encoded into a pure phase distribution using a complex coding method. The coding formula needs to combine the incident light amplitude and the phase of the blazed grating used to separate the diffraction order. After solving the system of equations, a phase mask that can generate a Bessel beam with a preset axial intensity distribution is obtained.

[0027] A voxel is a three-dimensional structure formed by the polymerization of photoresist under specific conditions and irradiation by a beam of light. The photoresist is the initial photosensitive material (liquid or solid) and the raw material for forming voxels. Furthermore, when the intensity of the Bessel beam's axial intensity distribution exceeds the photoresist polymerization threshold, a polymerization reaction occurs in that region, resulting in columnar voxels.

[0028] A Bessel beam is a laser beam with a unique spatial intensity distribution. Its core characteristic is a long depth of focus (long focal depth characteristic) in the propagation direction (axial direction), and a Bessel function distribution in the transverse intensity distribution, with concentrated energy in the central main lobe surrounded by multiple side lobes. In this invention, the axial intensity distribution of the Bessel beam is precisely controlled by loading a complex-coded hologram onto the spatial light modulator 8.

[0029] Bessel function distribution: The transverse intensity of a Bessel beam exhibits a zero-order, first-kind Bessel function distribution, with concentrated energy in the central main lobe and oscillating attenuation of the side lobes with radial distance. The main lobe width is determined by the beam cone angle and wavelength, and the transverse dimension can be locked by fixing the radial spatial frequency parameter. Specifically, the zero-order, first-kind Bessel function distribution satisfies the following formula: ; In this application, the above formula The x parameter is .

[0030] The blaze angle refers to the tilt angle of the grooves in a blazed grating, used to control the propagation direction and energy distribution of diffracted light. Its core function is to concentrate laser energy to a specific diffraction order (such as the +1st order), achieving separation of the useful light field from other orders and avoiding stray light interference.

[0031] The hologram of the Bessel beam with controllable axial intensity distribution is a phase hologram generated by reverse engineering using a computer. Its core function is to precisely control the axial intensity distribution of the Bessel beam after loading it onto the spatial light modulator (SLM) to meet the processing requirements of microstructures with high aspect ratios.

[0032] The following detailed description is exemplary and intended to provide further detailed explanation of the invention. Unless otherwise specified, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this invention is for describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention.

[0033] Example 1 like Figure 1 As shown, the method for axially shaping laser processing of high aspect ratio microstructures according to the present invention includes the following steps: S1. An initial laser is generated by a femtosecond laser 1, and the spatial distribution of the intensity of the light field illuminating the spatial light modulator 8 after collimation, beam expansion, and polarization state adjustment is measured simultaneously. Alternatively, the spatial light modulator 8 can be removed and replaced with an image sensor 14. By aligning the image sensor 14 with the same spatial position as the spatial light modulator 8, the power of the incident laser is adjusted so that the image sensor 14 is not saturated at its maximum intensity. The actual intensity distribution of the light field can then be obtained based on the corresponding pixels of the image sensor 14.

[0034] S2. Define the voxel parameters according to the structure to be processed. The voxel parameters include: voxel diameter (width of the voxel); voxel spacing (distance between adjacent voxels); and voxel height (height distribution of the voxels). In practice, the width of the corresponding voxel is selected based on the width of the high aspect ratio microstructure to be processed. The height of the voxel is selected based on the required heights of the high aspect ratio microstructure to be processed. The voxel spacing parameter is selected based on the required spacing between the voxels of the high aspect ratio microstructure to be processed.

[0035] Furthermore, the axial intensity distribution curve of the Bessel beam corresponding to the target with the desired voxel height is calculated; the selected axial intensity distribution curve satisfies the following condition: A set of Bessel beams with different axial intensities should have the same maximum on-axis intensity. Their maximum on-axis intensity is set to remain constant and above the polymerization threshold of the sample (preferably photoresist) for a predetermined transmission distance. Outside this distance, the intensity decays according to a predetermined function over the transmission distance until it falls below the photoresist polymerization threshold. Furthermore, the endpoints of the aforementioned intervals where the maximum on-axis intensity of the Bessel beams with different axial intensities remains constant should coincide.

[0036] For details, please refer to Figure 2As can be seen from the content, in the specific implementation of the technical solution of this application, whether the Bessel beam can form voxels depends on the polymerization threshold of the sample (especially the photoresist). Only when the light intensity meets or exceeds the polymerization threshold will the corresponding voxel be formed. That is, the photoresist polymerization reaction needs to reach a specific energy threshold (i.e., the "photoresist polymerization threshold"). In the axial intensity distribution of the Bessel beam, the maximum axial intensity is the peak value of the beam energy. Only when this peak intensity is higher than the polymerization threshold can the beam ensure that the photoresist polymerization is initiated in the range where the axial intensity is higher than the threshold. If the maximum axial intensity is insufficient or lower than the threshold, even if there is an axial distribution, the photoresist cannot be effectively polymerized, resulting in the voxel not being formed. The constant maximum axial intensity ensures that voxels of different heights have consistent energy during the polymerization process, avoiding changes in lateral dimensions (determined by the width of the main lobe of the Bessel beam) or degree of polymerization due to intensity fluctuations. If the maximum axial intensity changes, it may lead to uneven voxel diameter or decreased height control precision, destroying the core design goal of "decoupling height gradient and lateral dimension stability".

[0037] S3. A set of Bessel beams with controllable axial intensity distribution is generated by computer 15: the initial complex amplitude distribution of the beam that realizes the above distribution curve is calculated based on the axial target intensity distribution curve; one of them is selected and loaded onto the spatial light modulator 8 so that the outgoing beam after passing through the spatial light modulator 8 is a Bessel beam with controllable axial intensity distribution.

[0038] Furthermore, the steps for calculating and obtaining the hologram in step S3 include: Step S31: Calculate the initial surface phase distribution of the generated Bessel beam, in which the main lobe width of the Bessel beam matches the voxel diameter. In this application, the voxel diameter and voxel height are decoupled through the initial surface distribution of the Bessel beam. Specifically, the lateral intensity distribution of the Bessel beam follows a Bessel function distribution, and its central main lobe width (which determines the voxel diameter) is determined by the beam's cone angle and wavelength. In hologram design, by fixing the parameters of the initial surface phase distribution (such as the radial spatial frequency), it can be ensured that Bessel beams with different focal lengths have the same main lobe width, thereby keeping the maximum lateral dimension of the voxel constant.

[0039] like Figure 3As can be seen (for the specific principle, refer to the article "Long-scale multiphoton polymerization voxel growth investigation using engineered Bessel beams" in Optical Materials ExPREss), the voxel height generated by a single exposure of a Bessel beam can be changed by altering the overall intensity of the beam. However, when increasing or decreasing the voxel height in this way, the width of the beam exceeding the exposure threshold also increases, causing the diameter of the voxel to change with the height.

[0040] Specifically, as the intensity of the light beam increases during sample exposure, the height of the voxels formed by polymerization also increases (the corresponding relationship can also be referenced). Figure 2 (See the diagram). However, current technical solutions, in order to change the voxel height, often directly use an intensity attenuation device 2 and the femtosecond pulsed laser source itself to adjust the beam intensity, thereby changing the height of the voxels formed by exposure. However, in the above methods, as the light intensity changes, the lateral distribution of the voxels (i.e., the diameter of the voxels) also changes. Figure 3 As shown, the lateral distribution of voxels gradually increases along with their height. This is not suitable for the morphological requirements of some microstructures. However, by using the complex amplitude encoding method described in this application, the target amplitude (controlling the lateral size) and phase (controlling the axial distribution) are independently encoded into the hologram. When switching holograms, only the phase parameter related to the axial intensity distribution is changed, while the lateral amplitude parameter remains constant, thus achieving decoupled control of "gradual height change" and "stable lateral size".

[0041] Step S32: Calculate and generate the axial intensity distribution curve of the Bessel beam, and match the voxel height with the axial intensity distribution curve. Step S33: Based on the axial intensity distribution curve, calculate and obtain the initial surface complex amplitude distribution curve that realizes the axial intensity distribution curve in the Bessel beam; Specifically, the calculation method for the initial surface complex amplitude distribution curve includes the following steps: Step S331: Set the axial intensity distribution curve of the Bessel beam as follows: And the light field of the Bessel beam is on the central axis of the beam. The complex amplitude at point is expressed as: ; Where z is the propagation distance of the Bessel beam. Let r be the longitudinal component of the wave vector of the Bessel beam, r represent the radial coordinate, and represent the distance from a point in the optical field to the central axis of the beam. Step S332: Based on the axial strength distribution curve The spatial spectrum of the initial plane is solved using the following functional relationship. : ; Where k0 is the wave number, For longitudinal spatial frequency; Step S333: Obtain the spatial spectrum Substituting the angular spectrum propagation formula, calculate the complex amplitude distribution curve of the light field on the input plane. When r=0, it means that the point is located on the central axis of the beam (i.e., on-axis position). At this time, the intensity of the Bessel beam reaches its maximum value (corresponding to "maximum on-axis intensity"), which is the core region of the photoresist polymerization reaction. At the same time, the intensity at this position must be higher than the photoresist polymerization threshold.

[0042] Furthermore, the expression for the angular spectrum propagation formula is: ; Where r is the radial coordinate. Radial spatial frequency, It is a zero-order Bessel function of the first kind; Step S334: Obtain the initial surface complex amplitude distribution curve based on the light field complex amplitude distribution curve. satisfy: ; The initial surface complex amplitude distribution curve contains amplitude information. With phase information x represents the x-coordinate of the initial surface, and y represents the y-coordinate of the initial surface.

[0043] Step S34: Generate multiple holograms corresponding to the initial surface complex amplitude distribution curve through complex amplitude encoding.

[0044] Furthermore, the method for generating a hologram of the initial surface complex amplitude distribution curve through complex amplitude encoding is as follows: Amplitude information is encoded using a complex encoding method. and phase information Encoded as pure phase distribution Pure phase distribution satisfy: ; Where (m,n) is the pixel index of the spatial light modulator 8. To separate the phase of the blazed grating for the diffraction order, For the modulo operator, the parameter and parameters They respectively satisfy: ; ; Through pure phase distribution Obtain the corresponding hologram; satisfy: ; in, To measure the amplitude of the incident light on the spatial light modulator 8, For the shining angle, λ is the center wavelength of the incident light on the spatial light modulator 8.

[0045] S4. The pre-processed photoresist sample is controlled by an optical switch to open and close the optical path. The laser processing energy is adjusted by the light intensity attenuation device 2. The objective lens 12 is moved along the optical axis so that the generated Bessel beam simultaneously contacts the substrate and the photoresist sample, which can trigger a polymerization reaction. Specifically, the objective lens 12 is moved to allow the beam to enter the substrate to adjust the focus position. When the brightness of the ring-shaped light spot reflected from the substrate is observed to increase from weak to strong and reach its strongest point, this is when the axial intensity plateau of the beam just enters the substrate.

[0046] Furthermore, this step is a crucial preparatory stage before laser processing, primarily ensuring that the laser energy and focusing position meet the processing requirements. It specifically includes three core operations: Controlling the optical path: The laser can be precisely started and stopped through an optical switch device to avoid invalid exposure during non-processing periods.

[0047] Adjusting processing energy: The laser power is adjusted using a light intensity attenuation device to ensure that the energy reaches the threshold required for photoresist polymerization.

[0048] Positioning the processing location: Adjust the objective lens to move along the optical axis so that the Bessel beam focus is precisely aligned with the sample surface, ensuring that the voxel is formed in the preset position.

[0049] S5, programmable control of translation stage 13 movement and hologram switching, switching different holograms at different processing points, projecting Bessel beams of different lengths, and exposing the beams at preset positions in the sample to generate voxels of corresponding heights; S6. After post-processing the photoresist sample, a high aspect ratio structure with a preset height distribution is obtained.

[0050] In summary, the axial shaping laser processing method for high aspect ratio microstructures described in this invention reverse-engineers and generates Bessel beams with specific axial intensity distributions. By precisely controlling the length of the "high-intensity plateau" in the axial intensity curve while maintaining the plateau intensity and the core of the beam's lateral field distribution unchanged, a series of highly continuously varying voxels with a substantially constant maximum lateral dimension can be fabricated within a single exposure simply by switching holograms. This solves the problem of mutual constraints between morphological parameters in existing technologies, providing greater freedom for the design of complex three-dimensional microstructures.

[0051] The method for axially shaped laser processing of high aspect ratio microstructures described in this invention involves reverse designing and generating a Bessel beam with a specific axial intensity distribution. In this invention, the control of voxel height relies entirely on the electronic switching of different computational holograms loaded onto the spatial light modulator 8. This is a purely digital, non-contact control mode, reducing error sources and speed limitations caused by mechanical motion, and significantly improving processing repeatability, accuracy, and long-term stability.

[0052] The axial shaping laser processing method for high aspect ratio microstructures described in this invention inherits the high efficiency of "single-exposure forming," with extremely short manufacturing time for each high aspect ratio voxel. By programming and controlling the hologram and the two-dimensional translation stage 13 via computer 15, voxels of different heights can be rapidly processed at different positions on the sample, enabling rapid manufacturing of microstructures with complex height distributions. This method combines high efficiency and high flexibility, making it suitable for the fabrication of customized, functionally graded microstructure arrays, such as gradient microneedles and zoom microlens arrays.

[0053] Example 2 like Figure 4 As shown, unlike Example 1, this example provides an apparatus for axially shaping laser processing of microstructures with high aspect ratios, specifically including: Femtosecond laser 1 is used to emit the initial laser beam; Polarization state adjustment component, used to adjust the polarization state of the initial laser; Collimation and beam expanding assembly, used to collimate and expand the initial laser beam; Spatial light modulator 8 is used to receive the initial laser after collimation, beam expansion and polarization state adjustment. The spatial light modulator is electrically connected to computer 15. The computer 15 loads the hologram obtained by performing axial shaping laser processing of high aspect ratio microstructures onto the spatial light modulator 8. The spatial light modulator 8 outputs a Bessel beam with a controllable axial intensity distribution. An asymmetric 4-F system is used to spatially filter the Bessel beam and focus the filtered Bessel beam through objective lens 12. The translation stage 13 is used to receive the sample. The sample is exposed by the Bessel beam focused by the objective lens 12 to produce voxels of corresponding height. The translation stage 13 is electrically connected to the computer 15, and the computer 15 controls the movement of the translation stage 13.

[0054] Furthermore, the polarization state adjustment component includes a polarization state adjustment element 3 and a polarization beam splitter 4, which modulate the initial laser to a polarization state that matches the polarization sensitivity of the spatial light modulator 8. The collimation and beam expanding assembly includes a plano-convex lens 5 and a plano-concave lens 6 arranged sequentially. After being adjusted by the polarization state adjustment component and the collimation and beam expansion component, the initial laser light is deflected by the reflector 7 and incident on the liquid crystal surface of the spatial light modulator 8 at an off-axis incident angle of 10°. The asymmetric 4-F system includes a telephoto lens 9 and a variable aperture 10. The Bessel beam passes through the telephoto lens 9 and the variable aperture 10 in sequence and is transmitted to the objective lens 12.

[0055] In the specific implementation process, the beam is generated by the femtosecond laser 1 and first incident on the light intensity attenuation device 2, such as a neutral density attenuator or an acousto-optic modulator. There, the energy is attenuated to a suitable power level according to a preset processing energy, ensuring that the subsequent two-photon absorption process is near the convergence threshold. The attenuated beam from the spatial light modulator 8 is then passed through the polarization state adjustment element 3 (a quarter-wave plate) and the polarization beam splitter 4, and is modulated to a polarization state matching the polarization sensitivity of the spatial light modulator 8.

[0056] Furthermore, the beam from the spatial light modulator 8 then enters the beam expanding and collimating system, passing sequentially through a plano-convex lens 5 and a plano-concave lens 6, or using other combinations of beam expanders. The beam waist radius of the spatial light modulator 8 beam is enlarged to ensure that the size of the light spot subsequently incident on the spatial light modulator 8 fills the effective aperture of its liquid crystal panel. The expanded beam from the spatial light modulator 8 is then deflected by a reflector 7 and incident on the liquid crystal surface of the reflective spatial light modulator 8 at an off-axis incident angle of 10°.

[0057] In this process, computer 15 calculates a complex coded hologram based on the height parameters of the microstructure to be processed using a reverse design algorithm and loads it into spatial light modulator 8, where it is encoded into a Bessel beam with a controllable axial intensity distribution. The spatial light modulator 8 beam, after modulation, becomes a diffracted beam carrying axial shaping information and enters an asymmetric 4-F system composed of a telephoto lens 9 and a variable aperture 10. The telephoto lens 9, acting as the first Fourier lens of the 4-F system, propagates the spatial light modulator 8 beam from the plane of the spatial light modulator 8 to its Fourier plane, i.e., the back focal plane of the telephoto lens 9. A variable aperture 10 is positioned at the Fourier plane of the spatial light modulator 8, filtering out zero-order diffracted light and higher-order stray light as the spatial light modulator 8 beam passes through it.

[0058] Furthermore, the spatially filtered beam from the spatial light modulator 8 is focused by the microscope objective 12, which serves as the second lens in the 4-F system, with its front focal plane coinciding with the rear focal plane of the telephoto lens 9. Due to the imaging characteristics of the 4-F system, the beam from the spatial light modulator 8 reproduces the light field distribution modulated by the spatial light modulator 8 at the rear focal plane of the objective 12, i.e., the processing plane, and is focused onto the photoresist sample supported on the two-dimensional high-precision translation stage 13. The beam from the spatial light modulator 8 induces localized aggregation only in the plateau region where the axial intensity is higher than the aggregation threshold, forming a high aspect ratio columnar voxel whose height corresponds to the length of the high-intensity plateau of the spatial light modulator 8 beam.

[0059] Furthermore, it is worth noting that the semi-transparent mirror 11 introduces illumination light into the optical path while allowing backscattered light from the spatial light modulator 8 beam to pass through. This backscattered light is then received by the image sensor 14 via the imaging system, enabling real-time monitoring and processing quality control of the focusing state of the spatial light modulator 8 beam. By programming and controlling the two-dimensional movement of the translation stage 13 and the synchronous switching of the hologram on the spatial light modulator 8 through the computer 15, the spatial light modulator 8 beam is assigned different axial lengths at different processing positions, thereby fabricating a high aspect ratio microstructure array with stepped or continuously varying heights in a single scan.

[0060] Example 3 To further demonstrate the technical effect of the present application's technical solution, specifically, in this embodiment, a femtosecond pulsed laser with a center wavelength of 532nm, a repetition frequency of 100 kHz, and a pulse width of approximately 270 fs is used. The laser emitted from the laser is attenuated by a neutral density attenuator, collimated, expanded, and polarized before illuminating the spatial light modulator 8. The spatial distribution of the light field intensity is measured by removing the spatial light modulator 8 and replacing it with an image sensor 14. By aligning the incident laser with the same spatial position as the spatial light modulator 8, the power of the incident laser is adjusted so that the maximum light intensity does not saturate the image sensor 14. The intensity distribution of the incident light field can be obtained based on the corresponding pixels of the image sensor 14. A Gaussian beam is used to fit the measured intensity. Since the light intensity in the optical path can be adjusted by the attenuator, only the normalized intensity needs to be examined. satisfy w = 3 mm.

[0061] In this embodiment, the structure to be processed is a high aspect ratio microtube array with stepped height, comprising nine microtubes, increasing in spacing from 15 μm to 25 μm. A Bessel beam cone angle of 0.86 mrad was selected, and nine different Bessel beam axial intensity distribution curves were chosen. (j=1-9) is shown in the following formula. Note that the light intensity here is a normalized expression and does not refer to the actual intensity.

[0062]

[0063] in , , At this time, the beam intensity plateau region preset length Considering the beam elongation caused by refraction when the beam is incident on the photoresist layer from air, the preset value for the structural height generated by the above beam exposure can be calculated as follows: .

[0064] The required phase hologram is calculated through programming, and the hologram is divided into 1920 sections according to the number and size of pixels in the spatial light modulator 8. The system comprises 1080 sub-regions, numbered from top to bottom and left to right, with each pixel designated as (m, n). The width of each sub-region is determined by its width. The coordinates of the center point of each pixel are calculated as ( According to the target axial strength distribution curve respectively; The spatial spectrum of the initial plane can be solved using the following relationship: ; Where k0 is the wave number, For longitudinal spatial frequency; Substituting the obtained spatial spectrum S into the angular spectrum propagation formula, according to: ; Therefore, the complex amplitude distribution of the target light field on the input plane is calculated. Substituting the obtained spatial spectrum S into the angular spectrum propagation formula, due to the aperture limitation of the system, the maximum radial spatial wave vector that can pass through is measured to be 0.08. Based on the modulation integral interval, to conform to the actual system as follows: The complex amplitude distribution of the target light field on the input plane was calculated. Where r is the radial coordinate, Radial spatial frequency, It is a zero-order Bessel function of the first kind.

[0065] get That is, to achieve the initial surface amplitude distribution of the above distribution curve, it is necessary to include amplitude information. With phase information ; A phase hologram realizing the above-mentioned initial surface complex amplitude distribution is generated by complex amplitude encoding, and the target amplitude is then encoded using a complex encoding method. and phase Encoded as pure phase distribution The encoding formula is as follows:

[0066] Where (m,n) is the SLM pixel index. The amplitude of the incident light. This is the phase of the blazed grating used to separate the diffraction orders. Its phase is represented as... The blazing grating angle is set to This is used to coordinate the optical path and separate the diffraction orders. By solving the above system of equations, a set of curves that can generate axial intensity distribution is obtained. Corresponding phase mask of Bessel beam .

[0067] Loading holograms on SLM The axial intensity distribution of the generated light field is obtained by image sensor 14.

[0068] refer to Figure 5 , Figure 5 This embodiment provides a set of holograms and corresponding schematic diagrams of the axial distribution of light field intensity generated by the axial intensity control method described in this invention.

[0069] Adjusting the attenuator to set the generated beam power to 0.03mW, the processed photoresist sample is placed on translation stage 13. The sample is a flat SU8-2015 photoresist layer covering a glass substrate with a thickness of 40 mm. Adjust the position of the 50x microscope objective 12 relative to the sample until the end of the beam is just in contact with the substrate.

[0070] Furthermore, the switching of the hologram and the movement of the translation stage 13 are controlled by programming: the sample is moved to the processing area 1, and the hologram loaded by the SLM is switched to... Move the sample along a circular trajectory, with intervals of 0.2 on the trajectory. A processing point is set up. After the sample is moved to the processing point and brought to a standstill, the optical path is controlled by a galvanometer to perform point-by-point exposure. The exposure time for each point is 0.1 s. Then, the sample is moved to the next processing point for exposure, and so on, until all processing points in the processing area have been traversed. The above processing process is repeated for other processing areas until all structures to be processed are completed.

[0071] The processed sample was placed on a hot plate, heated to 95 degrees Celsius for 6 minutes, then immersed in the developing solution for 4 minutes. Afterward, the sample was rinsed with acetone to remove any remaining developing solution. The processing result was observed under a scanning electron microscope. Figure 6 .

[0072] like Figure 7 As shown, the microtube height from 1 to 9 achieves a range from 15... By 25 The linear gradient, the measured height and the design value The root mean square error is less than 0.8. .

[0073] This embodiment demonstrates that by synchronizing programmable voxels with scanning motion in real time, the present invention can efficiently and flexibly manufacture functional microstructures with complex three-dimensional morphological evolution, showcasing its powerful capabilities in the fields of microfluidics, biomimetic scaffolds, and customized micro-optical components manufacturing.

[0074] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0075] It is noteworthy that those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0076] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, as well as combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0077] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0078] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

[0080] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0081] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for axially shaped laser processing of microstructures with high aspect ratio, characterized in that, Includes the following steps: Step S1: Generate an initial laser using a femtosecond laser and measure the intensity distribution of the initial laser light field; Step S2: Based on the intensity distribution of the initial laser field, determine the voxel parameters according to the high aspect ratio microstructure; Step S3: Calculate and generate a set of Bessel beam holograms based on the voxel parameters. The holograms are used to load onto a spatial light modulator, which modulates the emitted beam of the initial laser into the Bessel beam with a controllable axial intensity distribution. Step S4: Adjust the processing energy of the Bessel beam by means of an intensity attenuation device, and at the same time adjust the objective lens corresponding to the spatial light modulator so that the objective lens is aligned with the sample on the translation stage; Step S5: Control the translation stage to move, and at the same time gradually replace the hologram loaded in the spatial light modulator. Project the Bessel beams of different lengths at different processing points of the sample, and expose the Bessel beams at the processing points to generate voxels of corresponding heights. Step S6: After post-processing the voxels, the high aspect ratio microstructure with a preset height distribution is obtained.

2. The method for axially shaped laser processing of microstructures with high aspect ratio according to claim 1, characterized in that, The voxel parameters include: Voxel diameter is the width of the voxel. Voxel spacing is the distance between adjacent voxels; Voxel height refers to the height distribution of the voxels.

3. The method for axially shaped laser processing of high aspect ratio microstructures according to claim 2, characterized in that, The step of calculating and obtaining the hologram in step S3 includes: Step S31: Calculate and generate the initial surface phase distribution of the Bessel beam, wherein the main lobe width of the Bessel beam in the initial surface phase distribution matches the voxel diameter; Step S32: Calculate and generate the axial intensity distribution curve of the Bessel beam, wherein the axial intensity distribution curve matches the voxel height; Step S33: Based on the axial intensity distribution curve, calculate and obtain the initial surface complex amplitude distribution curve that realizes the axial intensity distribution curve in the Bessel beam; Step S34: Generate multiple holograms corresponding to the initial surface complex amplitude distribution curve through complex amplitude encoding.

4. The method for axially shaped laser processing of high aspect ratio microstructures according to claim 3, characterized in that, In step S33, the method for calculating the initial surface complex amplitude distribution curve includes the following steps: Step S331, setting the axial intensity distribution curve of the Bessel beam as... And the light field of the Bessel beam is on the central axis of the beam. The complex amplitude at point is expressed as: ； Where z is the propagation distance of the Bessel beam. Let r be the longitudinal component of the wave vector of the Bessel beam, r represent the radial coordinate, and represent the distance from a point in the optical field to the central axis of the beam. Step S332: Based on the axial strength distribution curve The spatial spectrum of the initial plane is solved using the following functional relationship. : ； Where k0 is the wave number, For longitudinal spatial frequency; Step S333: Obtain the spatial spectrum Substituting into the angular spectrum propagation formula, calculate the complex amplitude distribution curve of the light field on the input plane. ; The expression for the angular spectrum propagation formula is: ； Where r is the radial coordinate. Radial spatial frequency, It is a zero-order Bessel function of the first kind; Step S334: Obtain the initial surface complex amplitude distribution curve based on the light field complex amplitude distribution curve. satisfy: ； The initial surface complex amplitude distribution curve contains amplitude information. With phase information x represents the horizontal coordinate of the initial surface, and y represents the vertical coordinate of the initial surface.

5. The method for axially shaped laser processing of high aspect ratio microstructures according to claim 4, characterized in that, The method for generating the hologram that realizes the initial surface complex amplitude distribution curve through complex amplitude encoding is as follows: The amplitude information is encoded using a complex encoding method. and the phase information Encoded as pure phase distribution The pure phase distribution satisfy: ； Where (m,n) are the pixel indices of the spatial light modulator. To separate the phase of the blazed grating for the diffraction order, For the modulo operator, the parameter and parameters They respectively satisfy: ； ； Through the pure phase distribution Obtain the corresponding hologram; The satisfy: ； in, To measure the amplitude of the incident light on the spatial light modulator, For the shining angle, The center wavelength of the incident light on the spatial light modulator.

6. The method for axially shaped laser processing of high aspect ratio microstructures according to claim 3, characterized in that, In step S32, the axial intensity distribution curve satisfies the following: the Bessel beams with different axial intensities have the same maximum on-axis intensity, which is above the polymerization threshold of the sample in step S4.

7. The method for axially shaped laser processing of high aspect ratio microstructures according to claim 1, characterized in that, The initial laser beam is collimated, expanded, and polarized before being applied to the spatial light modulator.

8. The method for axially shaped laser processing of high aspect ratio microstructures according to claim 1, characterized in that, In step S1, the measurement method for measuring the intensity distribution of the initial laser light field is as follows: replace and align an image sensor with the same spatial position as the spatial light modulator, adjust the power of the initial laser so that the image sensor is not saturated at the point of maximum intensity of the initial laser, and obtain the intensity distribution of the actual light field of the initial laser based on the corresponding pixels of the image sensor.

9. A device for axially shaping laser processing of microstructures with high aspect ratio, characterized in that, include: Femtosecond lasers are used to emit initial laser light; A polarization state adjustment component is used to adjust the polarization state of the initial laser. A collimation and beam expanding assembly is used to collimate and expand the initial laser beam. A spatial light modulator is used to receive an initial laser beam that has been collimated, expanded, and polarized. The spatial light modulator is electrically connected to a computer, which loads a hologram obtained by executing a method for axially shaped laser processing of high aspect ratio microstructures as described in any one of claims 1-8 onto the spatial light modulator. The spatial light modulator outputs a Bessel beam with a controllable axial intensity distribution. An asymmetric 4-F system is used to spatially filter the Bessel beam and focus the filtered Bessel beam through an objective lens; A translation stage is used to receive the sample, which is exposed by the Bessel beam focused by the objective lens to produce voxels of corresponding height. The translation stage is electrically connected to the computer, which controls the movement of the translation stage.

10. The apparatus for axially shaping laser processing of microstructures with high aspect ratio according to claim 9, characterized in that, The polarization state adjustment component includes a polarization state adjustment element and a polarization beam splitter, wherein the polarization state adjustment element and the polarization beam splitter modulate the initial laser to a polarization state that matches the polarization sensitivity of the spatial light modulator; The collimation and beam expanding assembly includes a plano-convex lens and a plano-concave lens arranged sequentially. The initial laser, after being adjusted by the polarization state adjustment component and the collimation and beam expansion component, is deflected by a reflector and incident on the liquid crystal surface of the spatial light modulator at an off-axis incident angle of 10°. The asymmetric 4-F system includes a telephoto lens and a variable aperture, and the Bessel beam passes sequentially through the telephoto lens and the variable aperture and is transmitted to the objective lens.

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