A femtosecond laser processing method for high-aspect-ratio transparent material micropores

Abstract

The application discloses a kind of high aspect ratio transparent material micro-hole femtosecond laser processing method, this method uses the burst mode of femtosecond laser to process transparent material, including: guide hole establishment stage, with first burst number and first energy density to transparent material processing, form the guide hole structure extending along its thickness direction;Deep hole processing stage, with second burst number and second energy density continue to process along the direction of guide hole structure extension, form high aspect ratio micro-hole;Wherein second energy density is less than first energy density, second burst number is greater than or equal to first burst number.The application is by stage regulation burst mode energy and burst number, utilize guide hole structure optimization laser propagation path, improve energy coupling efficiency, reduce heat accumulation effect, while guaranteeing processing efficiency, significantly improve hole wall quality, suitable for silicon carbide, sapphire, glass and other various transparent hard and brittle material high aspect ratio micro-hole processing.

Description

Technical Field

[0001] This invention relates to the field of ultrafast laser precision machining technology, and in particular to a femtosecond laser machining method for micropores in transparent materials with high aspect ratio. Background Technology

[0002] Silicon carbide (SiC), as a typical third-generation wide-bandgap semiconductor material, possesses comprehensive properties such as wide bandgap, high breakdown electric field, high electron mobility, high thermal conductivity, high hardness, and excellent high-temperature resistance and corrosion resistance. It has broad application prospects in fields such as new energy vehicles, power electronics, high-frequency communications, aerospace, optoelectronic devices, MEMS devices, and high-end equipment manufacturing. With the rapid development of the third-generation semiconductor industry, high-quality silicon carbide microstructure processing technology has become one of the key technologies restricting the improvement of device manufacturing and packaging performance. High aspect ratio microvias, as key structures in three-dimensional packaging interconnects, heat dissipation channels, microfluidic chips, MEMS devices, and optoelectronic devices, are widely used in vertical interconnects of power devices, micro-nozzle arrays, cooling microchannels, and high-density integrated packaging. As device integration continues to increase, higher requirements are placed on microvia structures, demanding not only large processing depth and high aspect ratio, but also smooth hole walls, high dimensional consistency, small heat-affected zone, minimal recast layer, and low oxidation level.

[0003] However, due to the characteristics of silicon carbide, such as high Mohs hardness, high brittleness, high melting point, strong chemical stability, and low fracture toughness, traditional machining methods such as mechanical drilling, electrical discharge machining, ultrasonic machining, and chemical etching generally suffer from low machining efficiency, severe tool wear, insufficient machining accuracy, and easy generation of cracks, chipping, and subsurface damage, making it difficult to meet the needs of modern high-end manufacturing for high-quality, high-precision micro-hole machining.

[0004] Femtosecond lasers, with pulse widths on the order of femtoseconds, are much shorter than the thermal diffusion time of the material's crystal lattice. Therefore, materials can be ablated ultrafastly through nonlinear absorption mechanisms such as multiphoton absorption and avalanche ionization. This results in advantages such as a small heat-affected zone, high processing precision, and applicability to various materials. Compared to continuous-wave lasers and nanosecond lasers, femtosecond lasers effectively reduce thermal diffusion effects, minimize melting and recasting, and improve processing precision, thus offering significant advantages in the field of silicon carbide micro-hole processing.

[0005] However, traditional single-pulse femtosecond lasers still have significant limitations in the fabrication of high aspect ratio micro-holes. As the hole depth increases, the laser needs to propagate within the narrow micro-hole, resulting in multiple reflections, scattering, and increasingly stronger plasma shielding effects, which continuously reduce the effective laser energy reaching the bottom of the hole. Simultaneously, the large amount of plasma, molten particles, and debris generated during ablation cannot be promptly removed from the micro-hole, further hindering effective coupling between the laser and the material, leading to a significant decrease in material removal efficiency. When a certain depth is reached, the ablation rate at the bottom of the hole gradually decreases, while the area at the hole opening continues to expand due to continuous laser irradiation, ultimately limiting the aspect ratio of the fabrication process and making further improvements difficult.

[0006] Therefore, there is an urgent need to propose a new femtosecond laser micro-hole processing method to achieve stable manufacturing of high aspect ratio and high quality micro-holes while ensuring processing efficiency, and to provide a new technical means for the precision manufacturing of third-generation semiconductor devices and other hard and brittle materials. Summary of the Invention

[0007] This invention aims to at least partially solve one of the technical problems in the above-mentioned technology, namely, the shortcomings of femtosecond laser processing of high aspect ratio micro-holes, such as the continuous decrease in effective laser energy reaching the bottom of the hole as the hole depth increases, the significant decrease in material removal efficiency, and the limitation on the processing aspect ratio. The invention provides a method for processing high aspect ratio transparent material micro-holes using a composite burst mode femtosecond laser based on guide hole assistance, so as to achieve high aspect ratio and high quality micro-hole processing while ensuring processing efficiency.

[0008] To achieve the above objectives, this invention proposes a femtosecond laser processing method for micropores in high aspect ratio transparent materials. The method employs a femtosecond laser burst mode to process the transparent material, and the processing includes the following steps: The transparent material is processed using a first burst number and a first energy density to form a guide hole structure extending along the thickness direction of the transparent material. The transparent material is further processed by deep hole machining using a second burst number and a second energy density along the direction of the guide hole structure to form micropores with a high aspect ratio on the transparent material; Wherein, the second energy density is less than the first energy density, and the second number of bursts is greater than or equal to the first number of bursts.

[0009] According to the present invention, a femtosecond laser processing method for micropores in transparent materials with high aspect ratio is disclosed. This method utilizes the burst mode of a femtosecond laser to rapidly establish a guide hole structure with a high burst energy density and a low number of bursts in the initial stage of processing. This creates an initial processing channel with a certain depth and a small aperture on the material surface. The guide hole structure extends along the material thickness direction, serving as a stable channel for subsequent femtosecond laser propagation. After the guide hole is formed, deep hole processing continues along the guide hole with a lower burst energy density. Due to the presence of the guide hole, subsequent lasers can propagate directly along the guide hole axis to the bottom region of the hole, significantly reducing reflection and scattering losses at the hole opening, improving the propagation efficiency of the laser inside the deep hole, and allowing more laser energy to be deposited in the bottom region of the hole, thereby improving the longitudinal material removal capability. Simultaneously, reducing the burst energy effectively reduces the heat absorbed by the hole wall per unit time, significantly suppressing the heat accumulation effect, reducing the formation of molten material and resolidified layers, thereby improving the hole wall quality and suppressing oxidation and graphitization. In other words, this method allows the establishment of the guide hole and deep hole processing to correspond to different processing objectives, enabling dynamic adjustment of femtosecond laser processing parameters according to the processing stage. During the guide hole establishment stage, a high-energy burst mode is employed as a pre-processing technique. This rapidly removes material through high instantaneous peak power, establishing a guide hole structure of a certain depth in a very short time. Compared to traditional processing methods, this guide hole not only serves as the initial micropore structure formed by ablation but, more importantly, as a stable channel for subsequent femtosecond laser propagation. After the guide hole is formed, subsequent lasers do not need to consume a large amount of energy to process the material surface; instead, they can propagate directly along the guide hole axis to the bottom region. This significantly reduces laser reflection and scattering losses at the hole opening, improves laser propagation efficiency within the deep hole, and allows more laser energy to be deposited at the bottom region, thereby enhancing longitudinal material removal capabilities.

[0010] By utilizing a single femtosecond laser source and adjusting only two parameters—burst number and energy density—in stages, the propagation conditions of femtosecond lasers within deep holes are effectively improved. This increases laser energy utilization in the hole bottom region, reduces heat accumulation and material damage on the hole wall, and achieves synergistic optimization between high processing efficiency, high aspect ratio, high hole wall quality, and low thermal impact. This method is simple, reproducible, and widely applicable. It is not only suitable for silicon carbide materials but can also be extended to high-quality micro-hole processing of gallium nitride, sapphire, alumina ceramics, silicon nitride ceramics, and other high-hardness, high-brittle materials, demonstrating promising engineering applications and industrialization value.

[0011] In addition, the femtosecond laser processing method for micropores in high aspect ratio transparent materials proposed above according to the present invention may also have the following additional technical features: Optionally, the first energy density is 260~350 J / cm³. 2 The second energy density is 150~200 J / cm³.2 By controlling the energy density of the borehole stage at 260~350 J / cm², the material surface can be rapidly penetrated to form a borehole structure within a smaller number of bursts; by reducing the energy density of the deep hole stage to 150~200 J / cm², heat accumulation can be effectively suppressed while ensuring continuous ablation at the bottom of the hole.

[0012] Optionally, the second energy density is 50% to 80% of the first energy density. By controlling the energy density of the deep hole stage to 50% to 80% of that of the pilot hole stage, the deep hole machining efficiency after the pilot hole is established can be guaranteed, and thermal damage can be significantly reduced, thus achieving a balance between machining efficiency and machining quality.

[0013] Optionally, the number of the first bursts does not exceed 50. Controlling the number of bursts in the guide hole stage to within 50 allows the guide hole structure to be established in a very short time, avoiding the enlargement of the inlet aperture and the accumulation of thermal damage caused by prolonged high-energy processing.

[0014] Optionally, each burst sequence in the burst mode contains 2 to 8 sub-pulses, with a time interval of 22 ns between adjacent sub-pulses. The nanosecond-level time interval between sub-pulses ensures that the material modification induced by the previous sub-pulse has not yet fully relaxed before the subsequent sub-pulse arrives, forming an energy accumulation effect and significantly improving the material removal efficiency.

[0015] Optionally, the femtosecond laser has a pulse width of 290 fs, a repetition rate of 150 kHz, and a center wavelength of 1035 nm. Femtosecond lasers within this parameter range can achieve stable burst mode output, meeting the requirements for high-quality micro-hole processing of transparent, hard, and brittle materials.

[0016] Optionally, the transparent material is one of silicon carbide, gallium nitride, sapphire, glass, alumina ceramic, silicon nitride ceramic, or diamond. These materials are all typical transparent, hard, and brittle materials, with wide applications in semiconductors, optoelectronics, aerospace, and other fields.

[0017] Optionally, in both the pilot hole machining and the deep hole machining, the number of sub-pulses in each burst train is the same. By keeping the number of sub-pulses constant in both machining stages and only changing the energy density and number of bursts in the burst train, the control of process parameters is simplified, while ensuring the continuity and stability of the machining process.

[0018] Optionally, the first burst number is 15, and the first energy density is 287 J / cm³. 2 The second energy density is 172 J / cm³. 2This parameter combination achieved significant technical results on 4H-SiC single-crystal silicon carbide wafers: the processing depth increased from 275 μm in the traditional burst mode to 350 μm, an improvement of approximately 27.3%; the aspect ratio increased from 16.3 to 22.6, an improvement of approximately 38.6%; the hole wall roughness decreased from Ra 1.41 μm to 0.96 μm, a reduction of approximately 31.9%; and the oxidation and graphitization of the hole walls were effectively suppressed.

[0019] Furthermore, each burst train in the burst mode comprises 8 sub-pulses, with a time interval of 22 ns between adjacent sub-pulses and a time interval of 6.67 μs between adjacent burst trains. This sub-pulse timing parameter matches the 150 kHz repetition frequency of the femtosecond laser, enabling stable burst mode output and good energy accumulation effect. Attached Figure Description

[0020] Figure 1 A schematic diagram of the experimental setup for a femtosecond laser processing method for micropores in transparent materials with high aspect ratio; wherein, (a) is a schematic diagram of the femtosecond laser drilling experimental setup, (b) is a time pulse sequence diagram in single pulse mode, (c) is the corresponding temperature evolution curve in single pulse mode, (d) is a time pulse sequence diagram in burst mode, and (e) is the corresponding temperature evolution curve in burst mode. Figure 2 The images show a comparison of the cross-sectional SEM morphology of micropores processed using single burst mode and combined burst mode at different burst numbers, as well as curves showing the variation of borehole depth and aspect ratio. Among them, (a1)-(a6) are the cross-sectional SEM morphology of single burst mode at burst numbers of 15, 150, 1500, 15k, 150k, and 750k, (b1)-(b6) are the cross-sectional SEM morphology of combined burst mode at burst numbers of 15, 150, 1500, 15k, 150k, and 750k, (c) is the curve showing the variation of borehole depth with burst number, and (d) is the curve showing the variation of aspect ratio with burst number. Figure 3 Comparison of SEM morphology and EDS elemental analysis of micropore walls obtained by drilling under single-burst mode and combined-burst mode; where (a1, a2) are the SEM cross-sectional morphology and magnified SEM morphology of the sidewall of the borehole under burst mode, (b1, b2, b3) are the EDS elemental distribution map of the borehole under burst mode, (c) is the point analysis of the sidewall edge of the borehole under burst mode, (d1, d2) are the SEM morphology and magnified SEM morphology of the sidewall of the borehole under combined-burst mode, (e) is the EDS elemental distribution map of the borehole under combined-burst mode, and (f) is the point analysis of the sidewall edge of the micropore of the borehole under combined-burst mode. Figure 4 SEM images of the micro-orifice inlet morphology during drilling of composite burst modes under different burst numbers; Figure 5 SEM images of the micro-orifice exit morphology during drilling of composite burst modes under different burst numbers; Figure 6 Comparison of the sidewall contours of microholes drilled using single burst mode and combined burst mode; Figure 7 for Figure 3 A comparison of Raman spectra obtained at position D in single-burst mode and combined-burst mode boreholes; where (a) is the Raman spectrum of the single-burst mode borehole and (b) is the Raman spectrum of the combined-burst mode borehole. Figure 8 SEM images of the pilot hole in the composite burst mode drilling are shown; (a) is the pilot hole inlet SEM image under the first step of 15 bursts in the composite burst mode drilling, and (b) is the pilot hole cross section SEM image under the first step of 15 bursts in the composite burst mode drilling. Figure 9 The cross-sectional SEM morphology of boreholes drilled in single-pulse mode at different pulse numbers is shown. Among them, (a1)-(a6) are the cross-sectional SEM morphologies of boreholes with burst numbers of 15, 150, 1500, 15k, 150k and 750k. Figure 10 The inlet SEM morphology of boreholes drilled using single-pulse mode under different pulse conditions is shown. Among them, (a1)-(a6) are the cross-sectional SEM morphology when the burst number is 15, 150, 1500, 15k, 150k and 750k. Detailed Implementation

[0021] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0022] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present invention and to fully convey the scope of the invention to those skilled in the art.

[0023] This application is based on the inventor's considerations regarding the following: In order to improve the efficiency of traditional single-pulse femtosecond laser processing, burst-mode femtosecond laser processing technology has been proposed in recent years. Burst mode splits a high-energy laser pulse into multiple sub-pulses with extremely short time intervals. These sub-pulses form a burst pulse train, with each sub-pulse typically having a nanosecond-level time interval, while adjacent burst pulses maintain a microsecond-level repetition period. Because multiple sub-pulses can continue to act on the material before the previous sub-pulse has completely cooled, a significant energy accumulation effect is formed inside the material. Therefore, burst mode can significantly improve material removal efficiency and enhance deep hole processing capabilities.

[0024] However, burst-mode femtosecond laser processing in related technologies typically employs fixed processing parameters throughout the entire process. This means that from the material surface to the final formation of the through-hole, the same energy, the same number of sub-pulses, and the same processing method are used consistently. While this approach improves processing efficiency, as the processing depth increases, the continuous high-energy laser acts on the hole walls and bottom region, causing a large amount of heat to accumulate within a confined space. This leads to a continuous increase in the local temperature of the hole walls, resulting in phenomena such as melting, resolidification, and thermal oxidation of the material. The large amount of molten material that resolidifies on the hole walls forms a thick recast layer, increasing the hole wall roughness and affecting the dimensional accuracy and structural integrity of the micropores. Simultaneously, under high-temperature conditions, silicon in SiC materials is prone to oxidation, forming an oxygen-rich layer; some carbon elements undergo graphitization, forming a graphitized layer, further degrading material properties. Furthermore, the continuous accumulation of thermal stress on the hole walls due to continuous high-energy processing can induce surface microcracks, localized spalling, and hole wall damage, thereby reducing processing quality.

[0025] Furthermore, as the hole depth increases, the laser propagation environment inside the microhole changes significantly. In the initial stage of processing, the laser can directly act on the material surface; however, as the guide hole gradually forms, the processing transforms into deep hole processing in a confined space, and the laser propagation path, energy coupling mode, plasma confinement state, molten material discharge behavior, and heat accumulation mechanism all undergo significant changes. Therefore, different processing stages correspond to different material removal mechanisms. If a single fixed processing parameter is still used, it is impossible to simultaneously meet the rapid material removal capabilities required in the shallow hole processing stage and the low thermal damage processing requirements in the deep hole processing stage, resulting in a persistent and unresolved contradiction between processing efficiency and processing quality.

[0026] To address this, the present invention proposes a novel femtosecond laser micro-hole fabrication method. This method rapidly establishes a guide hole structure conducive to subsequent laser propagation in the initial processing stage. Furthermore, it modulates the burst mode laser energy based on changes in material removal characteristics at different processing stages, thereby improving the energy coupling efficiency of the laser within the deep hole, reducing heat accumulation effects, and minimizing the formation of recast layers, oxide layers, and graphitization layers on the hole wall. This ensures stable fabrication of high aspect ratio, high-quality silicon carbide micro-holes while maintaining processing efficiency, providing a new processing method for the precision manufacturing of third-generation semiconductor devices and other hard and brittle materials.

[0027] According to one embodiment of the present invention, please refer to Figure 1 a, Figure 1 Figure a shows an experimental setup diagram of a femtosecond laser processing method for micropores in transparent materials with high aspect ratio according to the present invention; the femtosecond laser processing system includes: a femtosecond laser, an optical path system, a focusing objective lens, a displacement platform, and a control system.

[0028] Femtosecond lasers are used to provide pulsed laser beams, including single-pulse mode and burst mode, such as... Figure 1 b and Figure 1 As shown in c, in single-pulse mode, the pulse period is 6.67 μs and the frequency is 150 kHz. After each pulse, the temperature rises rapidly and then gradually decays; in burst mode, as... Figure 1 d and Figure 1 As shown in e, the sub-pulse interval within each burst is 22 ns, the frequency is 45 MHz, each burst contains 2 to 8 sub-pulses, and the group interval remains 6.67 μs. Multiple sub-pulses within each group cause the temperature to rise in a stepwise manner and then decay.

[0029] The optical path system is set in the output optical path of the femtosecond laser and includes a mirror, shutter, beam expander, and dichroic mirror, used for transmitting and shaping the laser beam. After the laser beam is output, it passes through the mirror, shutter, and beam expander in sequence, and is then reflected by the dichroic mirror before entering the focusing objective lens.

[0030] The focusing objective is a 10× microscope objective with a numerical aperture of 0.3, used to focus the laser beam onto the surface of a transparent material sample, with a focused spot diameter of approximately 4.3 μm.

[0031] The displacement platform is a high-precision XYZ three-dimensional motion platform used to support and move transparent material samples, ensuring sample position stability during processing and avoiding processing errors caused by vibration or displacement. The displacement platform utilizes existing drive devices capable of XYZ three-dimensional motion, such as ball screw slides driven by servo motors, air-bearing platforms driven by linear motors, or nano-displacement stages driven by piezoelectric ceramics. Its motion accuracy and stroke range are selected according to processing requirements.

[0032] The control system is electrically connected to the femtosecond laser and the displacement platform. It controls the laser's operating mode and parameters, and the movement trajectory of the displacement platform, achieving precise matching between the laser focus and the workpiece position. The control system is as follows: Figure 1 As shown in Figure a, the computer-controlled processing path and motion platform enable precise matching between the laser focus and the workpiece position.

[0033] In addition, the processing system also includes a CCD vision system, which is coupled to the optical path system through a beam splitter to monitor the laser focus position and processing status of the processing area in real time.

[0034] According to one embodiment of the present invention, the processing method includes the following steps: sample pretreatment, femtosecond laser processing system setup, guide hole establishment, deep hole processing, and processing quality inspection. Unlike traditional burst-mode femtosecond lasers that always use fixed processing parameters to complete the entire drilling process, the present invention uses different burst energy levels for staged processing based on the changing characteristics of material removal behavior and laser propagation conditions at different processing stages, significantly improving the quality of micro-hole processing while ensuring processing efficiency.

[0035] Example 1: Silicon carbide micropore fabrication In this embodiment, under the same processing equipment, the same focusing conditions, and the same scanning parameters, a comparative experiment was conducted using a single burst mode and a compound burst mode, respectively. The experimental parameters for the two processing methods are shown in Table 1.

[0036] Table 1. Laser processing parameters used in each method

[0037] 1. Sample pretreatment The processing material used in this embodiment was a commercial 4H-SiC single-crystal silicon carbide wafer with a sample thickness of 350 μm. All wafer surfaces were double-sided polished to ensure laser focusing quality and experimental repeatability. Before processing, the silicon carbide sample was first ultrasonically cleaned sequentially in anhydrous ethanol and deionized water for 10 minutes in each solution to thoroughly remove oil, dust, and other impurities from the sample surface. After cleaning, the sample was placed in a clean environment to air dry and then fixed. Figure 1 A high-precision XYZ three-dimensional motion platform is used to ensure the stability of the sample position during processing and to avoid processing errors caused by vibration or displacement.

[0038] 2. Femtosecond laser processing system setup like Figure 1As shown in Figure a, this embodiment uses a Femto2-R60-DNG-0100 femtosecond laser manufactured by Huari Laser Technology Co., Ltd. The laser's center wavelength is 1035 nm, pulse width is 290 fs, and repetition frequency is 150 kHz. In burst mode, each burst contains 8 sub-pulses, i.e., the number of pulses per burst (NPPB) is 8. The time interval between adjacent sub-pulses is approximately 22 ns, and the time interval between adjacent bursts is approximately 6.67 μs. After the laser beam is output, it passes through the optical path system and is focused onto the surface of the silicon carbide sample by a 10× microscope objective with a numerical aperture of 0.3. The focused spot diameter is approximately 4.3 μm. The entire processing process is monitored in real-time by a CCD vision system, and the processing path and motion platform are controlled by a computer to achieve precise matching between the laser focus and the workpiece position. In this embodiment, the burst laser energy density (F... burst = Number of pulses within a burst (N) PPB ) × Single pulse laser energy (F sub-pulse ).

[0039] 3. Pilot hole establishment stage After the laser system is set up, the first step is to establish the pilot hole. This invention differs from methods that use a fixed energy level for drilling in a single burst mode; instead, it uses a higher burst energy series to rapidly establish the pilot hole in the initial processing stage. In this embodiment, the femtosecond laser is switched to burst mode, maintaining N... PPB Keeping the value 8 constant, the laser energy density corresponding to the total energy of the burst series is adjusted to reach 287 J / cm² by adjusting the average laser power. 2 That is, the energy of a single-pulse laser is 35.9 J / cm². 2 Only 15 bursts (i.e., the first burst count is 15) are needed to form a guide hole structure of a certain depth on the silicon carbide surface. The high burst energy can quickly complete material breakdown and initial ablation, forming a deep and narrow guide hole on the material surface, while maintaining a small entrance aperture, thus establishing a stable laser propagation channel for subsequent deep hole processing.

[0040] 4. Deep Hole Machining Stage After the first-stage guide hole is formed, the deep hole machining stage begins, which involves increasing the aspect ratio of the micro-hole. This stage still employs burst mode machining, but the energy density of the burst series is reduced, thus changing the machining method to low-thermal-damage machining. In this embodiment, by reducing the average laser power, the laser energy density corresponding to the total energy of the burst series is reduced to 172 J / cm². 2 That is, the energy of a single-pulse laser is 21.5 J / cm². 2All other processing parameters remained unchanged. Drilling experiments were then conducted using bursts of 150, 1500, 15000, 150000, and 750000 bursts to investigate drilling behavior at different processing stages. Since the pilot hole was already formed, subsequent laser beams could propagate stably along it to the bottom of the hole. Therefore, even with reduced burst energy, the bottom of the hole still received sufficient energy for continuous ablation. Simultaneously, reducing the burst energy effectively reduced the heat absorbed by the hole wall per unit time, significantly suppressing the heat accumulation effect, reducing the formation and resolidification of molten material, and thus improving the hole wall quality.

[0041] As the machining process continues, the guide hole continuously acts as a laser propagation channel, concentrating the laser propagation direction more towards the bottom of the hole, reducing repeated ablation at the hole opening, and improving longitudinal material removal efficiency. Simultaneously, the guide hole provides a smoother path for the plasma and ablation debris generated during machining, reducing plasma shielding and improving the energy coupling efficiency between the laser and the material at the bottom of the hole. Therefore, the guide hole-assisted composite burst mode machining strategy can maintain high machining stability throughout the drilling process, allowing the drilling depth to continuously increase without the significant decrease in machining efficiency that occurs with increasing hole depth, as seen in traditional single burst modes.

[0042] 5. Post-processing and testing After processing, the samples were sequentially ultrasonically cleaned in anhydrous ethanol and deionized water for 10 min each to remove residual debris and loose recast layer from the pores. Subsequently, the micropore inlet, outlet, and pore wall morphology were observed using a Zeiss SUPRA 55 field emission scanning electron microscope (SEM) to analyze the thickness of the recast layer and the distribution of molten particles. The micropore depth, inlet diameter, outlet diameter, and pore wall surface roughness were measured using a KEYENCE VK-X1000 laser confocal microscope. Energy dispersive spectroscopy (EDS) was used to detect changes in oxygen content in the pore walls to evaluate the degree of material oxidation during processing. Raman spectroscopy was used to analyze changes in the pore wall material structure to evaluate the degree of graphitization of the SiC material.

[0043] 6. Results Analysis like Figure 2 As shown, Figure 2 The micropore morphologies of silicon carbide materials processed under different burst conditions (a) and (b) are presented. Figure 2 (a1)-(a6) are the cross-sectional SEM morphologies of a single burst mode at burst numbers of 15, 150, 1500, 15k, 150k, and 750k, respectively. Figure 2Figures (b1)-(b6) show the cross-sectional SEM morphology of the composite burst mode at burst numbers of 15, 150, 1500, 15k, 150k, and 750k, respectively. Experimental results show that, under the same processing conditions, the maximum processing depth obtained by the single burst mode is approximately 275 μm, while the processing depth of the composite burst mode reaches 350 μm, an improvement of approximately 27.3%. The maximum aspect ratio obtained by the single burst mode is approximately 16.3, while the maximum aspect ratio of the composite burst mode reaches 22.6, an improvement of approximately 38.6%. Figure 2 (c) and (d) respectively compare the changes in processing depth and aspect ratio with the number of bursts under the two processing strategies. It can be seen that the femtosecond laser processing method based on guide hole-assisted composite burst mode in this embodiment exhibits higher drilling efficiency and better deep hole processing capability throughout the entire processing process.

[0044] like Figure 3 As shown, Figure 3 SEM morphology and EDS elemental analysis results of the micropore walls obtained by two processing strategies are presented. Figure 3 (a1, a2) represent the SEM cross-sectional morphology and magnified SEM morphology of the sidewall of the borehole in the single burst mode. Figure 3 (b1, b2, b3) is the EDS element distribution map corresponding to a single burst mode borehole; Figure 3 (c) represents the point analysis of the borehole sidewall edge in a single burst mode; Figure 3 (d1, d2) represent the SEM morphology of the borehole in the composite burst mode and the magnified SEM morphology of the sidewall. Figure 3 (e) is the EDS element distribution map corresponding to the composite burst mode borehole; Figure 3 (f) represents the point analysis of the micro-hole sidewall edge in the composite burst mode drilling. The results show that the hole wall processed by the guide hole-assisted composite burst mode femtosecond laser processing method in this embodiment is smoother and more continuous, with a significant reduction in molten particles and resolidified layers. At the same time, the oxygen content of the hole wall is significantly lower than that of the single burst mode, indicating that guide hole assistance and reduction of burst series energy can effectively suppress high-temperature oxidation reaction and reduce thermal damage to the hole wall.

[0045] like Figure 4 and Figure 5 As shown, Figure 4 and Figure 5 The entry and exit morphology under the compound burst mode is shown. Figure 4SEM characterization of the micro-orifice inlet morphology during drilling with a composite burst mode at different burst numbers; at a burst number of 15 (corresponding to the pilot hole establishment stage), the micro-orifice inlet is a regular circle with smooth edges and almost no processing damage; after entering the deep hole processing stage, a small amount of edge chipping appears at the inlet edge when the burst number is 150, but as the burst number further increases, the inlet morphology gradually returns to regularity, indicating that continuous processing after reducing energy density is beneficial to improving the inlet quality; however, when the burst number reaches 750k, the inlet size increases significantly, and melting and erosion residues appear at the edge, indicating that there is a reasonable upper limit to the burst number in the deep hole processing stage; Figure 5 SEM characterization of the micropore exit morphology during drilling under a combined burst mode with different burst numbers. It can be seen that as the burst number increases, the exit morphology gradually becomes more regular. When the burst number reaches 750k, the exit exhibits a regular perfect circle, indicating that a sufficient number of bursts allows the micropore to be completely penetrated and form a good exit morphology. Overall, the micropore edges are clear and complete, the orifice contour is regular, the number of molten particles is significantly reduced, and almost no large-area recast layer adhesion is observed around the micropore.

[0046] like Figure 6 As shown, Figure 6 The images show the sidewall profiles of microholes drilled using single-burst and combined-burst modes. Further measurements of the hole wall roughness were performed using a laser confocal microscope. The average surface roughness of the hole wall obtained by the combined-burst mode was approximately 0.96 μm, a reduction of about 31.9% compared to the 1.41 μm obtained by the single-burst mode. This indicates that staged burst energy control can significantly improve the surface quality of the hole wall and enhance the precision of microhole machining.

[0047] like Figure 7 As shown, Figure 7 for Figure 3 Raman spectra obtained at position D in (a) single-burst mode and (b) combined-burst mode drilling. The Raman spectroscopy analysis results further show that the intensity of the D peak and G peak of the hole wall obtained by the combined-burst mode is significantly reduced, while the characteristic peaks of SiC crystal remain at a high intensity, indicating that the graphitization phenomenon caused by continuous high-energy processing is effectively suppressed and the crystal structure of the material is more intact.

[0048] like Figure 8 The image shows the machining result of the first step of the pilot hole in the compound burst mode. Figure 8 a represents the surface morphology of the entrance. Figure 8 b represents the corresponding cross-sectional morphology. This result indicates that the first step of the composite burst mode processing mainly achieves the formation of the guide hole in the material, preparing for the second step of deepening. The low-energy processing in this step can reduce the formation of subsequent hole wall recasting layer, oxide layer and graphitization layer.

[0049] To demonstrate the comparison effect, as shown in Table 1, the femtosecond laser was switched to single-pulse mode for processing, i.e., single-pulse mode was used to drill holes in silicon carbide. The cross-sectional morphology of the holes drilled in single-pulse mode at different pulse numbers is shown in Table 1. Figure 9 As shown, the entrance shape is as follows Figure 10 As shown, by Figure 9 It is evident that the machining depth of the micropore increases with the increase of the number of pulses; however, in single-pulse mode, there are obvious longitudinal microcracks on the sidewall of the hole, resulting in poor hole wall integrity. As the number of pulses increases, the cracks continue to extend and expand along the hole wall into the material, while local chipping and debris adhesion occur inside the hole, gradually exacerbating machining damage. Even with increased machining depth, single-pulse drilling still struggles to achieve high-quality micropore structures with smooth inner walls and no obvious crack defects.

[0050] Therefore, this embodiment presents a method for processing high aspect ratio micro-holes in silicon carbide using a composite burst mode femtosecond laser with a guide hole-assisted process. By rapidly establishing a guide hole in the early stages of processing and implementing a low-energy burst sequence in the subsequent deep hole processing stage, the propagation conditions of the femtosecond laser inside the deep hole are effectively improved, the laser energy utilization rate in the bottom region of the hole is increased, and the heat accumulation and material damage on the hole wall are reduced. This achieves a synergistic optimization between high processing efficiency, high aspect ratio, high hole wall quality, and low thermal impact.

[0051] In summary, the method for processing micro-holes in high aspect ratio transparent materials using a composite burst mode femtosecond laser based on guide hole assistance proposed in this invention effectively improves the propagation conditions of the femtosecond laser inside the deep hole by rapidly establishing the guide hole in the early stage of processing and reducing the burst energy in the subsequent deep hole processing stage. This increases the laser energy utilization rate in the bottom region of the hole, reduces heat accumulation and material damage on the hole wall, and achieves synergistic optimization between high processing efficiency, high aspect ratio, high hole wall quality, and low thermal impact. This method is simple, reproducible, and widely applicable. It is not only suitable for silicon carbide materials but can also be extended to high-quality micro-hole processing of gallium nitride, sapphire, glass, diamond, and other high-hardness, high-brittle materials. It has promising engineering application prospects and industrialization value in fields such as third-generation semiconductor device manufacturing, advanced packaging, aerospace, and microelectromechanical systems (MEMS).

[0052] In the description of this invention, it should be understood that the terms "upper surface", "lower surface", "left and right sides", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing this invention 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, and therefore should not be construed as a limitation of this invention.

[0053] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0054] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0055] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0056] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. The illustrative expressions of the above terms in this specification should not be construed as necessarily referring to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0057] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A femtosecond laser processing method for micropores in high aspect ratio transparent materials, characterized in that, Transparent materials are processed using a femtosecond laser burst mode, and the processing includes the following steps: The transparent material is processed using a first burst number and a first energy density to form a via structure extending along the thickness direction of the transparent material. The transparent material is further processed by deep hole machining using a second burst number and a second energy density along the direction of the guide hole structure to form micropores with a high aspect ratio on the transparent material; Wherein, the second energy density is less than the first energy density, and the second number of bursts is greater than or equal to the first number of bursts; The transparent material is one of silicon carbide, gallium nitride, sapphire, glass, alumina ceramic, silicon nitride ceramic, or diamond.

2. The femtosecond laser processing method as described in claim 1, characterized in that, The first energy density is 260~350 J / cm³ 2 The second energy density is 150~200 J / cm³. 2 .

3. The femtosecond laser processing method as described in claim 1, characterized in that, The second energy density is 50% to 80% of the first energy density.

4. The femtosecond laser processing method as described in claim 1, characterized in that, The first outbreak number does not exceed 50.

5. The femtosecond laser processing method as described in claim 1, characterized in that, Each burst sequence in the burst mode contains 2 to 8 sub-pulses, and the time interval between adjacent sub-pulses is 22 ns.

6. The femtosecond laser processing method as described in claim 1, characterized in that, The femtosecond laser has a pulse width of 290 fs, a repetition rate of 150 kHz, and a center wavelength of 1035 nm.

7. The femtosecond laser processing method as described in claim 5, characterized in that, In the machining of the guide hole and the machining of the deep hole, the number of sub-pulses in each burst is the same.

8. The femtosecond laser processing method as described in claim 1, characterized in that, The first burst number is 15, and the first energy density is 287 J / cm³. 2 The second energy density is 172 J / cm³. 2 .

9. The femtosecond laser processing method as described in claim 7, characterized in that, In the burst mode, each burst sequence contains 8 sub-pulses, the time interval between adjacent sub-pulses is 22 ns, and the time interval between adjacent burst sequences is 6.67 μs.

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