Micromachining system based on femtosecond laser single-pulse two-photon polymerization and high-aspect-ratio micropillar processing method

Abstract

The present invention discloses a micromachining system and a method for machining micropillars with high aspect ratios based on single-pulse two-photon polymerization of a femtosecond laser, belonging to the field of femtosecond laser micro-nanomachining technology. The system comprises a femtosecond laser optical system, a motion platform assembly, and a diode trigger control assembly thereof. The light field distribution of the incident laser is adjusted by double diffraction through a double aperture, and then a high-aspect-ratio ellipsoidal focused light spot is formed by focusing through a microscope objective lens. By coordinating the femtosecond laser pulse frequency and the motion platform movement speed, each point of the sample is exposed to only a single femtosecond laser pulse. Then, based on the femtosecond laser energy threshold effect, the polymerization area of ​​the ellipsoidal focused light spot in the photoresist is precisely controlled, achieving two-photon polymerization of micropillars of different heights and diameters. This method achieves high-throughput processing of three-dimensional micropillar arrays, greatly improving the efficiency of femtosecond laser microstructure processing.

Description

Technical Field

[0001] The present invention relates to the field of femtosecond laser micro-nanofabrication technology, specifically a microfabrication system and a method for fabricating high-aspect-ratio micropillars based on femtosecond laser single-pulse two-photon polymerization. This technology has broad potential applications in optoelectronics and biomedicine, such as integrated optical devices, flexible electronic devices, microrobotics, and three-dimensional cell growth control. Background Art

[0002] As a new type of processing tool, femtosecond laser has two significant characteristics: one is that the pulse width is extremely short, in the order of femtoseconds; the other is that the peak power is extremely high (10 12 ~10 15 W / cm 2 ). Such pulse width and power density have brought unprecedented high time resolution, high electric and magnetic field strength, high pressure and high temperature extreme physical conditions to scientific research. The emergence of femtosecond lasers has provided a more precise processing method for basic and applied fields such as physics, chemistry, biology, and medicine. Two-photon polymerization is another important application of femtosecond lasers. Unlike traditional photopolymerization, the nonlinear absorption effect of two photons can prepare three-dimensional micro / nanostructures with a resolution less than the diffraction limit inside the material. The unique ability of femtosecond lasers to process three-dimensional microstructures in transparent materials makes it show great potential in the field of three-dimensional high-precision micro-nano device manufacturing.

[0003] Currently, high-throughput microstructure fabrication based on femtosecond laser two-photon polymerization is primarily achieved through two approaches: femtosecond laser direct writing (Femtosecond laser direct writing) and femtosecond laser processing based on spatial light modulation (SLM). Incorporating a scanning galvanometer into a femtosecond laser direct writing system can significantly increase the speed of Femtosecond laser direct writing while maintaining processing resolution. However, when processing millimeter- or even centimeter-scale microstructures, laser direct writing often requires tens of hours or even days, severely impacting processing efficiency. Femtosecond laser processing based on spatial light field modulation (SLM) can achieve high-efficiency patterning. However, the SLM algorithm is complex, and the SLM components are expensive. Furthermore, when processing high-aspect-ratio microstructures, existing SLM-based femtosecond laser processing techniques can only be achieved through layer-by-layer scanning, resulting in low processing efficiency. These factors severely limit the application and commercialization of femtosecond laser processing systems. Therefore, a simple and efficient single-pulse Femtosecond laser two-photon polymerization method is urgently needed to achieve efficient processing of high-aspect-ratio microstructures. Summary of the Invention

[0004] In response to the shortcomings of the existing technology, the present invention provides a micromachining system based on femtosecond laser single-pulse two-photon polymerization and a high-aspect-ratio microcolumn processing method, which utilizes the energy threshold effect of femtosecond laser within the laser focal spot to achieve high-throughput additive manufacturing of three-dimensional microcolumn arrays.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A micromachining system based on femtosecond laser single-pulse two-photon polymerization, comprising a femtosecond laser processing optical system, a motion platform system, and a diode trigger control system; wherein:

[0007] Femtosecond laser processing optical system: including femtosecond laser, ultrafast reflector, double aperture optical adjustment component, light energy attenuation component and microscope objective lens, used to generate high aspect ratio focused spot of femtosecond laser;

[0008] Motion platform system: including piezoelectric ceramic motion platform and macro motion platform. Both the piezoelectric ceramic motion platform and macro motion platform are connected to the host computer control system. The relative position of the sample to be processed and the laser focal spot is controlled by the motion platform system.

[0009] Diode trigger control system: includes a photodiode, which is used to detect femtosecond laser pulses and trigger the macro-motion platform to start moving through the host computer control system. By matching the femtosecond laser pulse frequency with the macro-motion platform speed, single-pulse two-photon polymerization processing is achieved.

[0010] The double-aperture optical adjustment component consists of two continuously adjustable irises, namely adjustable iris I and adjustable iris II. The incident femtosecond laser is diffracted twice by adjustable iris I and adjustable iris II in sequence, thereby modulating the spatial distribution of the light field; the light field modulated by the double-aperture light field modulation component is then focused by the microscope objective lens to form an ellipsoidal focused light spot with a high aspect ratio.

[0011] The aperture of the adjustable iris I is 1-10 mm, and the aperture of the adjustable iris II is 1-10 mm; the diameter of light passing through the adjustable iris I is 3 mm, and the diameter of light passing through the adjustable iris II is 2 mm.

[0012] The optical energy attenuation component includes a half-wave plate and a polarization beam splitter, which are used to continuously tune the incident laser energy; wherein: the incident laser reaches the polarization beam splitter after passing through the half-wave plate, and a portion of the laser reaching the polarization beam splitter is reflected to the photodiode, and the other portion passes through the polarization beam splitter to reach the ultrafast reflector, and is reflected by the ultrafast reflector to the double-aperture optical modulation component; the ultrafast reflector is suitable for the wavelength of femtosecond laser and is used to refract the propagation direction of the femtosecond laser.

[0013] The micromachining system also includes a sample stage, which is placed on a piezoelectric ceramic motion platform. The sample to be processed is fixed on the sample stage, and the laser is focused onto the sample to be processed by a femtosecond laser optical system. The sample to be processed includes a cover glass and a photoresist coated on the lower surface of the cover glass. The cover glass serves as a photoresist substrate, and the photoresist is used for two-photon polymerization material. Before two-photon polymerization processing, the photoresist needs to be heated in an environment of 100°C for 70 minutes.

[0014] The micromachining system also includes a real-time optical imaging system for real-time observation of the processing position and processing results; the real-time optical imaging system includes an LED lighting source, a dichroic mirror and a CMOS camera. The illumination light irradiated onto the sample by the LED lighting source passes through the microscope objective lens and the dichroic mirror to be imaged by the CMOS camera.

[0015] The method for processing high-aspect-ratio microcolumns based on single-pulse two-photon polymerization of femtosecond lasers comprises the following steps: adjusting the light field distribution of the incident laser by a double-aperture optical adjustment component through a double-diffraction method, and then focusing the light to form an ellipsoidal focused spot with a high aspect ratio through a microscope objective lens; cooperatively controlling the femtosecond laser pulse frequency and the motion speed of the macro-motion platform so that each point of the sample to be processed is exposed to only a single femtosecond laser pulse; then, based on the femtosecond laser energy threshold effect, precisely controlling the polymerization area of ​​the ellipsoidal focused spot in the photoresist, thereby achieving two-photon polymerization of microcolumns of different heights and diameters; and achieving single-pulse polymerization of high-aspect-ratio microcolumns through high-aspect-ratio focused spot modulation.

[0016] In this processing method, a single high-aspect-ratio microcolumn is obtained by two-photon polymerization of a single femtosecond laser pulse; after the two-photon polymerization is completed, the microstructure sample needs to be developed to remove excess photoresist to obtain the expected microcolumn structure.

[0017] In this processing method, the height of the microcolumn can be adjusted by adjusting the distance between the focused light spot and the photoresist-cover glass interface; the diameter of the microcolumn is determined by the cross-section between the focused light spot and the photoresist-cover glass interface, and will increase with the increase of the microcolumn height; the height and diameter of the microcolumn will increase with the increase of processing energy.

[0018] The microcolumns prepared by this processing method can reach a height of 78μm, a width range of 1.9 to 6μm, and a maximum aspect ratio of up to 19.

[0019] The present invention has the following beneficial effects and advantages:

[0020] 1. This invention uses a dual-aperture light field modulation method to perform dual-diffraction shaping on the incident femtosecond laser beam with a Gaussian distribution, forming a focused spot with a high aspect ratio after being focused by the objective lens. This light field modulation method is low-cost and easy to operate, greatly simplifying the femtosecond laser light field modulation process.

[0021] 2. The present invention is based on the principle of femtosecond laser single-pulse polymerization. Through femtosecond laser single-pulse exposure, micropillars with ultra-high aspect ratios can be processed. For example, the processing time of a micropillar with a height of tens of microns is only 220 fs, which greatly improves the processing efficiency of micropillars.

[0022] 3. This invention, based on the femtosecond laser energy threshold effect, is universally applicable to two-photon polymerization materials and is independent of femtosecond lasers and processing systems. It could significantly enhance the application of femtosecond laser two-photon polymerization in optoelectronic devices and biomedicine. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the optical system structure of the present invention;

[0024] Figure 2 Schematic diagram of the principle of single-pulse two-photon polymerization; among them: (a) is a schematic diagram of a microcolumn with zero height, (b) is a schematic diagram of two-photon polymerization of microcolumns with a specific height, and (c) is a schematic diagram of a high aspect ratio microcolumn obtained after development.

[0025] Figure 3 1 is an experimental effect diagram of the present invention; wherein: h represents the height of the microcolumn, and d represents the bottom diameter of the microcolumn.

[0026] In the figure: 1 is a femtosecond laser, 2 is a half-wave plate, 3 is a polarization beam splitter, 4 is an ultrafast mirror, 5 is a variable aperture I, 6 is a variable aperture II, 7 is a shutter, 8 is a photodiode, 9 is a dichroic mirror, 10 is a microscope objective, 11 is a sample stage, 12 is a piezoelectric ceramic motion platform, 13 is a macro motion platform, 14 is a host computer control system, 15 is a CMOS camera, 16 is a cover glass, 17 is a focused light spot, 18 is a photoresist, and 19 is a microcolumn. DETAILED DESCRIPTION

[0027] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0028] The present invention provides a micromachining system based on femtosecond laser single-pulse two-photon polymerization and a high-aspect-ratio microcolumn machining method. Figure 1 The figure shows a schematic diagram of the optical system of the present invention. The femtosecond laser processing system includes a femtosecond laser processing optical system, a motion platform system, a photodiode trigger control system, and a real-time optical imaging system. The specific structure and connection relationship of each part are as follows:

[0029] The femtosecond laser processing optical system is used to generate a high-aspect-ratio focused spot of a femtosecond laser. It includes a femtosecond laser 1, an ultrafast reflector 4, a dual-aperture optical adjustment component, a light energy attenuation component, and a microscope objective 10. The dual-aperture optical adjustment component consists of two continuously adjustable apertures (adjustable aperture I5 ​​and adjustable aperture II6). The incident femtosecond laser is diffracted twice by adjustable apertures I5 and II6, thereby modulating the spatial distribution of the light field. The light energy attenuation component includes a half-wave plate 2 and a polarization beam splitter 3 for continuously tuning the incident laser energy.

[0030] The motion platform system includes a piezoelectric ceramic motion platform 12 and a macro motion platform 13, both of which are connected to a host computer control system 14 for controlling the relative position of the processed sample and the laser focal spot.

[0031] The diode trigger control system includes a photodiode 8, which is connected to a host computer control system 14. The photodiode detects femtosecond laser pulses and triggers the macro-motion platform 13 to start moving through the host computer control system. By matching the femtosecond laser pulse frequency with the macro-motion platform speed, single-pulse two-photon polymerization processing is achieved.

[0032] The micromachining system also includes a real-time optical imaging system for real-time observation of the processing position and processing results; the real-time optical imaging system includes an LED lighting source, a dichroic mirror 9 and a CMOS camera 15. The illumination light irradiated onto the sample by the LED lighting source passes through the microscope objective lens 10 and the dichroic mirror 9 to be imaged by the CMOS camera 15.

[0033] The light emitted by the femtosecond laser 1 is rotated in polarization direction by the half-wave plate 2 and then attenuated by the polarization beam splitter 3. The light passing through the polarization beam splitter 3 is divided into two beams. One beam is incident on the photodiode 8 and triggers the macro-motion platform 13 to move horizontally through the host computer control system 14. The other beam is deflected by the ultrafast reflector 4. The reflected light first undergoes first-order diffraction through the continuously adjustable aperture I5 ​​and then undergoes second-order diffraction through the continuously adjustable aperture II6. After the host computer control system 14 controls the mechanical shutter 7 to open, the laser is reflected by the dichroic mirror 9 and enters the microscope objective 10 for focusing. The focused light spot is focused on the sample placed on the sample stage 11 for two-photon polymerization processing. The axial height of the sample is precisely controlled by the piezoelectric ceramic motion platform 12, and the coarse adjustment of the horizontal and axial position of the sample is achieved through the macro-motion platform 13.

[0034] The clear apertures of the continuously adjustable apertures I5 and II6 need to be smaller than the diameter of the femtosecond laser beam. By adjusting the clear apertures of the two apertures (3 mm and 2 mm respectively), light field modulation is achieved, thereby forming a focused light spot with a high aspect ratio at the focusing point behind the objective lens.

[0035] Photodiode 8 detects femtosecond laser pulses. The photoelectric signal detected by the photodiode triggers macro-motion platform 12 to begin moving along a preset trajectory. By matching the femtosecond laser pulse frequency with the speed of motion platform 12, each processing location on the sample receives a single femtosecond laser exposure.

[0036] Figure 2 Schematic diagram of the principle of achieving high aspect ratio micro-pillar processing by femtosecond laser single-pulse two-photon polymerization. Figure 2 In (a), the lower surface of the coverslip substrate 16 is coated with photoresist 18. The femtosecond laser is focused by the microscope objective lens to form a high-aspect-ratio ellipsoidal focused spot 17. The lower end of the ellipsoidal focused spot is located exactly at the interface between the coverslip 16 and the photoresist 18. This position of the sample is defined as the initial position, and the corresponding micropillar height is zero. Figure 2 (b) is a schematic diagram of the sample stage after it has been moved a certain height relative to its initial position. At this point, part of the ellipsoidal focused spot is located inside the photoresist, and the other part is located inside the cover glass. Under this condition, turning on the femtosecond laser can cause the photoresist located in the focal spot area to undergo two-photon polymerization. By developing the photoresist, the following can be obtained: Figure 2 (c) shows the micropillar 19, where h represents the height of the micropillar and d represents the diameter of the bottom end of the micropillar.

[0037] The height h of the microcolumn can be adjusted by controlling the relative position of the cover glass 16 and the focused light spot 17. When the height h of the microcolumn increases, the diameter d of the bottom end of the microcolumn increases accordingly.

[0038] Considering the energy threshold effect of femtosecond laser two-photon polymerization, when the energy of the incident femtosecond laser increases, the volume of the ellipsoidal focused spot 17 will increase. Therefore, the height h and diameter d of the polymerized microcolumn will increase accordingly.

[0039] On the constructed micromachining system based on femtosecond laser single-pulse two-photon polymerization, high-throughput processing of patterned microcolumn arrays is achieved by controlling the macro-motion platform 13 to move along a specific trajectory. Figure 3 Based on Figure 1 Scanning electron microscope (SEM) scanning imaging results of the micropillar array obtained by two-photon polymerization using the constructed femtosecond laser processing system. The micropillar array processing results verify the effectiveness of the single-pulse two-photon polymerization described in the present invention. The femtosecond laser wavelength used in this experiment was 1030nm, the pulse width was 217fs, the processing pulse frequency was 50Hz, the movement speed of the macro-motion platform 12 was 50mm / s, and the photoresist used was SZ2080. When the power of the incident femtosecond laser was 0.4-0.8W, the diameter of the single-pulse polymerized micropillars varied from 1.9 to 6μm, the height of the micropillars varied from 0-70μm, and the maximum aspect ratio of the micropillars could be as high as 19.

[0040] This method modulates the incident laser light field distribution through double diffraction using a double aperture, and then focuses the light through a microscope objective to form a high-aspect-ratio ellipsoidal focused spot. Based on the femtosecond laser energy threshold effect, the polymerization area of ​​the focused spot in the photoresist is strictly controlled, enabling the processing of micropillars of varying heights and diameters. The high-aspect-ratio focused spot enables single-pulse polymerization of high-aspect-ratio micropillars. This effectively overcomes the low efficiency of existing femtosecond laser two-photon polymerization, enabling high-throughput processing of three-dimensional micropillar arrays and greatly improving the efficiency of femtosecond laser microstructure processing. This provides a reliable technical foundation for the commercial application of femtosecond lasers and helps expand their application areas.

Claims

1. A micromachining system based on femtosecond laser single-pulse two-photon polymerization, characterized by: The micromachining system includes a femtosecond laser processing optical system, a motion platform system and a diode trigger control system; wherein: Femtosecond laser processing optical system: includes a femtosecond laser, an ultrafast reflector, a double-aperture optical adjustment component, a light energy attenuation component, and a microscope objective lens, and is used to generate a high-aspect-ratio focused light spot of the femtosecond laser. The double-aperture optical adjustment component is composed of two continuously adjustable apertures, namely adjustable aperture I and adjustable aperture II. The incident femtosecond laser is diffracted twice by adjustable aperture I and adjustable aperture II in sequence, thereby modulating the spatial distribution of the light field. The light field modulated by the double-aperture optical adjustment component is then focused by the microscope objective lens to form a high-aspect-ratio ellipsoidal focused light spot. Motion platform system: including piezoelectric ceramic motion platform and macro motion platform. Both the piezoelectric ceramic motion platform and macro motion platform are connected to the host computer control system. The relative position of the sample to be processed and the laser focal spot is controlled by the motion platform system. Diode trigger control system: includes a photodiode, which is used to detect femtosecond laser pulses and trigger the macro-motion platform to start moving through the host computer control system. By matching the femtosecond laser pulse frequency with the macro-motion platform speed, single-pulse two-photon polymerization processing is achieved; The high aspect ratio micropillars prepared based on the micromachining system have a micropillar height of 78 μm, a micropillar width ranging from 1.9 to 6 μm, and a maximum aspect ratio of 19.

2. The micromachining system based on femtosecond laser single-pulse two-photon polymerization according to claim 1, characterized in that: The aperture of the adjustable iris I is 1-10 mm, and the aperture of the adjustable iris II is 1-10 mm; the diameter of light passing through the adjustable iris I is 3 mm, and the diameter of light passing through the adjustable iris II is 2 mm.

3. The micromachining system based on femtosecond laser single-pulse two-photon polymerization according to claim 1, characterized in that: The optical energy attenuation component includes a half-wave plate and a polarization beam splitter, which are used to continuously tune the incident laser energy; wherein: the incident laser reaches the polarization beam splitter after passing through the half-wave plate, and a portion of the laser reaching the polarization beam splitter is reflected to the photodiode, and the other portion passes through the polarization beam splitter to reach the ultrafast reflector, and is reflected by the ultrafast reflector to the double-aperture optical adjustment component; the ultrafast reflector is suitable for the wavelength of femtosecond laser and is used to refract the propagation direction of the femtosecond laser.

4. The micromachining system based on femtosecond laser single-pulse two-photon polymerization according to claim 1, characterized in that: The micromachining system also includes a sample stage, which is placed on a piezoelectric ceramic motion platform. The sample to be processed is fixed on the sample stage, and the laser is focused on the sample to be processed by a femtosecond laser processing optical system. The sample to be processed includes a cover glass and a photoresist coated on the lower surface of the cover glass. The cover glass serves as a photoresist substrate, and the photoresist is used for two-photon polymerization. Before two-photon polymerization processing, the photoresist needs to be heated in an environment of 100°C for 70 minutes.

5. The micromachining system based on femtosecond laser single-pulse two-photon polymerization according to claim 1, characterized in that: The micromachining system also includes a real-time optical imaging system for real-time observation of the processing position and processing results; the real-time optical imaging system includes an LED lighting source, a dichroic mirror and a CMOS camera. The illumination light irradiated onto the sample by the LED lighting source passes through the microscope objective lens and the dichroic mirror to be imaged by the CMOS camera.

6. A method for processing high aspect ratio micropillars using the micromachining system based on femtosecond laser single-pulse two-photon polymerization according to claim 1, characterized in that: This method adjusts the light field distribution of the incident laser through double diffraction by a double-aperture optical adjustment component, and then focuses the light through a microscope objective to form an ellipsoidal focused spot with a high aspect ratio. By coordinating the femtosecond laser pulse frequency and the motion speed of the macro-motion platform, each point of the sample to be processed is exposed to only a single femtosecond laser pulse. Then, based on the energy threshold effect of femtosecond laser, the polymerization area of ​​the ellipsoidal focused spot in the photoresist was precisely controlled to achieve two-photon polymerization of microcolumns of different heights and diameters; through high-aspect-ratio focused spot modulation, single-pulse polymerization of high-aspect-ratio microcolumns was achieved.

7. The method for fabricating a high aspect ratio microcolumn according to claim 6, wherein: A single high aspect ratio microcolumn is obtained by two-photon polymerization of a single femtosecond laser pulse; after the two-photon polymerization is completed, the sample to be processed needs to be developed to remove excess photoresist to obtain the expected microcolumn structure.

8. The method for fabricating a high aspect ratio microcolumn according to claim 6, wherein: The height of the micropillar is adjusted by adjusting the distance between the focused light spot and the photoresist-cover glass interface; the diameter of the micropillar is determined by the cross-section between the focused light spot and the photoresist-cover glass interface, and will increase with the increase of the micropillar height; the height and diameter of the micropillar will increase with the increase of processing energy.

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