Femtosecond laser trepanning method and device for silicon nitride ceramic

Abstract

The invention relates to the technical field of silicon nitride ceramic trepanning, in particular to a femtosecond laser trepanning method and device for silicon nitride ceramic. The invention discloses a femtosecond laser trepanning method for silicon nitride ceramics. The method comprises the following steps: pre-preparation work; the fixed silicon nitride ceramic substrate is holed based on a three-stage scanning strategy, and the effective depth of field is constrained by the Rayleigh length; the three-stage scanning strategy is as follows: S1, low-energy grooving; s2, high-energy multiple cutting; s3, performing low-energy stroke trimming; annular feeding is executed according to a preset two-dimensional hole site vector path during scanning in each stage of the three-stage machining process. According to the method, high-quality and high-controllability micropore array processing can be realized on the silicon nitride ceramic substrate, and the method has wide engineering application value.

Description

Technical Field

[0001] This invention relates to the field of silicon nitride ceramic aperture technology, and specifically to a femtosecond laser aperture method and apparatus for silicon nitride ceramics. Background Technology

[0002] Existing hole-opening methods mainly rely on mechanical drilling and thermal laser drilling, with ultrafast lasers considered the optimal solution. Ultrafast laser processing technology, especially femtosecond lasers, due to their extremely short pulse widths and extremely high peak power, can achieve non-thermal vaporization ablation instantaneously before significant thermal diffusion occurs in the material. In particular, femtosecond laser processing of ceramic materials can significantly reduce the heat-affected zone, avoiding edge cracking and hole wall melting, making it especially suitable for the fabrication of high-precision micro-hole arrays. By rationally adjusting the energy density and repetitive scanning strategy of the femtosecond laser, high-sphericity and high-consistency through-hole processing can be achieved on silicon nitride ceramic substrates, thus meeting the stringent requirements of electronic packaging and high-power heat dissipation modules. This has led to an increasing number of researchers utilizing femtosecond lasers for hole processing in silicon nitride ceramics.

[0003] However, the current methods for drilling silicon nitride ceramics using femtosecond lasers, such as 1) the three-step processing method: high-power drilling, low-power drilling, and low-power finishing, are more suitable for processing relatively thick silicon nitride ceramic materials, such as 4mm; 2) the spiral layer-by-layer scanning processing method: the roundness can reach up to 95%.

[0004] Based on this, those skilled in the art urgently need to propose a novel processing flow for creating holes in silicon nitride ceramic sheets using femtosecond lasers, in order to improve the technical problems existing in the prior art and enhance the technical effect. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defects existing in the prior art, thereby providing a femtosecond laser drilling method and apparatus for silicon nitride ceramics.

[0006] A femtosecond laser aperture method for silicon nitride ceramics includes: Preparatory work: Fix the silicon nitride ceramic substrate to be processed on the processing platform and lock it in place; A three-stage scanning strategy is used to create openings in a fixed silicon nitride ceramic substrate, and the effective depth of field is constrained by Rayleigh length. The three-stage scanning strategy is as follows: S1. Low-energy grooving: A laser capable of stably removing the shallow layer of a silicon nitride ceramic substrate is used to scan the silicon nitride ceramic substrate multiple times to form a shallow groove. S2. High-energy multiple cutting: Keeping the laser wavelength unchanged from step S1, increasing the single pulse energy and repetition frequency, reducing the scanning speed, and performing multiple scans on the grooved silicon nitride ceramic substrate to complete the removal of the main area to be cut. S3. Low-energy outlining and finishing: After the hole has been penetrated, the hole wall is outlined and finished using a laser; The laser used in step S3 has a lower absorption rate than the laser used in step S2, and the corresponding energy density is higher than the removal threshold of surface debris and spatter at the opening on the silicon nitride ceramic substrate, but lower than the ablation threshold of the silicon nitride ceramic substrate. In each stage of the three-stage processing technology, scanning is performed in a circular feed based on a preset two-dimensional hole position vector path.

[0007] Preferably, when performing step S1, the processing area on the silicon nitride ceramic substrate is expanded according to the actual size, so that a buffer zone is provided around the processing area.

[0008] Preferably, in order to avoid energy accumulation at the orifice during step S2, a reserved entry or exit section is set at the beginning or end of the scanning path, the industrial camera shutter is only opened when entering the effective path, and a preset linear power ramp is used for gradual entry and exit.

[0009] Preferably, in order to suppress the heat accumulation effect, the same path is scanned multiple times based on a preset time interval when performing step S2; The preset time interval is greater than the plasma plume lifetime and thermal diffusion characteristic time in femtosecond laser processing.

[0010] Preferably, in order to ensure uniform pulse distribution and avoid unidirectional thermal gradient accumulation during step S2, the scanning mode adopts an alternating clockwise and counterclockwise scanning mode.

[0011] A femtosecond laser drilling device for silicon nitride ceramics, used to implement a femtosecond laser drilling method for silicon nitride ceramics, comprising: The optical path is connected in sequence to a dual-wavelength femtosecond laser, an optical path switching device, a beam expander, a scanning galvanometer system, a field lens, and a silicon nitride ceramic substrate to be processed located on a three-axis motion platform. And a host computer for outputting control signals, which are respectively connected to the dual-wavelength femtosecond laser, the scanning galvanometer system, the three-axis motion platform and the industrial camera signal; The three-axis motion platform serves as the machining platform; the scanning galvanometer system performs a circular scan based on a preset two-dimensional hole position vector path.

[0012] The technical solution of this invention has the following advantages: This invention combines the ultrafast cold processing characteristics of femtosecond lasers with high-precision scanning control, overcoming the limitations of traditional mechanical drilling and thermal laser drilling, which are prone to cracking, hole wall melting, and low efficiency. It can achieve high-quality and highly controllable micro-hole array processing on silicon nitride ceramic substrates, and has broad engineering application value. Attached Figure Description

[0013] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram of the scan path in step S1; Figure 2 This is a schematic diagram of the scan path in step S2; Figure 3 This is a schematic diagram of the scanning path for step S3. Detailed Implementation

[0015] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. 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.

[0016] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0017] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; 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; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0018] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0019] Example 1 A femtosecond laser aperture device for silicon nitride ceramics includes: The optical path is connected in sequence to a dual-wavelength femtosecond laser, an optical path switching device, a beam expander, a scanning galvanometer system, a field lens, and a silicon nitride ceramic substrate to be processed located on a three-axis motion platform. And a host computer for outputting control signals, which are respectively connected to the dual-wavelength femtosecond laser, the scanning galvanometer system, the three-axis motion platform and the industrial camera signal; The three-axis motion platform is the machining platform; the scanning galvanometer system performs a circular scan based on the preset two-dimensional hole position vector path; and the field lens is an F-θ focusing field lens.

[0020] The dual-wavelength femtosecond laser serves as the core light source, with an output pulse width of approximately 200 fs, a center wavelength of 1030 nm, and a frequency doubling wavelength of 515 nm. It features an adjustable repetition frequency of 100 kHz–1 MHz and a single pulse energy of 1–100 μJ, ensuring that the energy density exceeds the ablation threshold of the silicon nitride ceramic substrate.

[0021] The laser beam, after being adjusted by a beam expander, enters the scanning galvanometer system. High-speed deflection in the X and Y directions completes the scanning path, and the beam is focused onto the workpiece surface under the action of an F-θ focusing field lens, thus obtaining a stable and consistent focused spot throughout the entire processing area. The three-axis motion platform primarily performs workpiece positioning and area transposition during processing, working with an industrial camera to achieve reference point identification and automatic array position correction. The entire device is uniformly scheduled by a host computer, using hardware-synchronized pulse gating for laser and scanning to ensure strict consistency between pulse timing and path arc length, reducing local overburning at the start and end points, and ensuring strict synchronization between the laser pulse and the galvanometer scanning trajectory. During actual assembly, the size and structure of the device can be modified according to actual needs, but the optical path connection relationships of this embodiment must be met to ensure accurate scanning of the silicon nitride ceramic substrate to be processed on the three-axis motion platform. The device as a whole possesses programmable path scheduling, automatic multi-point array identification, and continuous processing capabilities, making it particularly suitable for high-precision drilling of small-hole matrix structures. Its control core consists of an industrial computer or FPGA, which can achieve strict synchronization between the laser pulse and the galvanometer scanning path, thereby ensuring clear aperture boundaries, rounded contours and good repeatability.

[0022] Example 2 Based on Example 1, a femtosecond laser drilling method for silicon nitride ceramics is specifically disclosed, including: Preparatory work: Fix the silicon nitride ceramic substrate to be processed on the processing platform and lock it in place; A three-stage scanning strategy is used to create openings in a fixed silicon nitride ceramic substrate, and the effective depth of field is constrained by Rayleigh length. The three-stage scanning strategy is as follows: S1. Low-energy grooving: A laser capable of stably removing the shallow layer of a silicon nitride ceramic substrate is used to scan the silicon nitride ceramic substrate multiple times to form a shallow groove. S2. High-energy multiple cutting: Keeping the laser wavelength unchanged from step S1, increasing the single pulse energy and repetition frequency, reducing the scanning speed, and performing multiple scans on the grooved silicon nitride ceramic substrate to complete the removal of the main area to be cut. S3. Low-energy outlining and finishing: After the hole has been penetrated, the hole wall is outlined and finished using a laser; The laser used in step S3 has a lower absorption rate than the laser used in step S2, and the corresponding energy density is higher than the removal threshold of surface debris and spatter at the opening on the silicon nitride ceramic substrate, but lower than the ablation threshold of the silicon nitride ceramic substrate. In each stage of the three-stage processing technology, scanning is performed in a circular feed based on a preset two-dimensional hole position vector path.

[0023] Specifically: This embodiment uses a 0.5mm silicon nitride ceramic substrate as the component to be processed, as detailed below. Figure 1-3 A detailed example of a machining process for holes with an upper diameter of approximately 420 μm and a lower diameter of approximately 400 μm or more, and a roundness greater than 98%.

[0024] Preparatory work: Unlike other aperture opening methods, this application requires the use of two wavelengths of laser. Therefore, in this embodiment, a dual-wavelength femtosecond laser needs to be prepared in advance, and an optical path switching device is required for this purpose, as well as optical components such as a beam expander and a scanning galvanometer system that use two wavelengths including 515nm and 1030nm.

[0025] Before processing, the 0.5mm thick silicon nitride ceramic substrate is first moved to a set position by a three-axis motion platform, and the hole array is aligned with a reference using an industrial camera for high precision, thus completing the positioning and locking. It should be noted that after the three-axis motion platform is locked, the galvanometer scanning system performs hole position trajectory scanning according to a preset G-code path or vector graphics file, such as DXF.

[0026] A three-stage scanning strategy is used to create openings in a fixed silicon nitride ceramic substrate, and the effective depth of field is constrained by Rayleigh length. It should be noted that by constraining the effective depth of field with Rayleigh length, each stage of the three-stage scanning strategy can be performed at approximately the energy density.

[0027] The three-stage scanning strategy is as follows: Step S1: Low-energy grooving: A laser capable of stably removing the shallow layer of a silicon nitride ceramic substrate is used to scan the silicon nitride ceramic substrate multiple times along the scanning path to form a shallow groove.

[0028] Objective: To establish a stable incident channel in a dense layer on the upper surface of a silicon nitride ceramic substrate; specifically, to form a shallow groove.

[0029] The laser used was a 515nm laser with a pulse width of 300fs, a repetition frequency of 250kHz, a single pulse energy of 3.2μJ, a galvanometer spot diameter of 20μm, and a scanning speed of 1.2m / s.

[0030] When the pulse width is 300 fs, the peak power density formula is used: In the formula, Indicates peak power density; Indicates single pulse energy; Indicates the pulse width; This indicates the diameter of the galvanometer spot.

[0031] The calculated peak power density is as follows: Its value is higher than the multiphoton ablation threshold of silicon nitride, ensuring stable shallow layer removal. For example... Figure 1 This is a schematic diagram of the path planned in step S1; during the processing of step S1... Figure 1 The red portion represents the actual scanning path of the laser.

[0032] When a femtosecond laser interacts with a silicon nitride ceramic substrate, localized resolidification may occur after the material is ablated. Vaporized or molten particles may redeposit near the hole walls due to the rapid cooling of the plasma plume, forming ribbon-like debris. Because these debris particles have certain adhesive and agglomeration characteristics, they are difficult to completely remove in a timely manner by simply relying on suction or airflow dust removal devices. This may lead to a decrease in the efficiency of subsequent pulses, uneven energy deposition, and even affect the stability of the groove depth and edge morphology.

[0033] To suppress this phenomenon, step S1 further includes expanding the processing area on the silicon nitride ceramic substrate according to the actual dimensions, creating a buffer zone around the processing area to provide a more adequate channel for debris escape and airflow. This strategy effectively reduces debris accumulation at the orifice and orifice walls, improving the cleanliness and energy efficiency of the processing. The specific width of the buffer zone can be determined based on the selected femtosecond laser and the specific material composition.

[0034] Furthermore, experimental verification revealed that when using the scanning mode in step S1, after five repeated scans, the groove depth could be stably controlled at 12–15 μm, with a smooth and clear edge contour, no obvious burrs or re-solidification adhesion, and stable groove quality. These results indicate that expanding the scanning area not only improves debris removal efficiency but also significantly improves the edge quality and processing consistency of the orifice, providing ideal initial conditions for subsequent body removal and finishing processes.

[0035] Step S2: High-energy multiple cutting: Keeping the laser wavelength unchanged from step S1, increasing the single pulse energy and repetition frequency, reducing the scanning speed, and performing multiple scans on the grooved silicon nitride ceramic substrate to complete the removal of the main area to be cut. A 515nm laser with a single-pulse energy of 20μJ, a galvanometer spot diameter of 20μm, a repetition frequency of 500kHz, and a scanning speed of 0.6mm / s was used. The laser energy density corresponding to these parameters is much higher than the ablation threshold of the material. To describe the cumulative effect of repeated processing depth, a logarithmic ablation model is used in this embodiment. ; in, The ablation coefficient of the material can be obtained through calibration. =0.95μm; The real-time cumulative depth for repeated processing; The peak energy density of the laser used in step S2; This represents the ablation threshold of silicon nitride materials.

[0036] It should be noted that the logarithmic ablation model essentially only describes lateral information: the logarithmic relationship between the ablation pit diameter and the laser energy density, used to extract the ablation threshold and the galvanometer spot radius. It does not include temporal or longitudinal ablation dynamics, therefore, according to conventional theory, it cannot directly provide the processing depth, nor can it reflect cumulative depth and related effects. However, the technical solution in this embodiment exhibits a linear processing depth during implementation, which is an empirical fit. Therefore, it is not a derivation of the logarithmic ablation model itself, but rather an extrapolation of the lateral formula to the depth direction layer by layer. Those skilled in the art can derive this principle through experiments after step S2 of this embodiment, so it will not be elaborated further.

[0037] Under the above conditions, the average removal depth of a single cycle scan is about 20 μm, and for a silicon nitride ceramic substrate with a thickness of 0.5 mm, it takes about 24 repeated processing cycles to complete the penetration.

[0038] In addition, to avoid energy buildup at the orifice during step S2, reserved entry or exit sections are set at the beginning and end of the scanning path. The industrial camera shutter only opens when entering the effective path, and a preset linear power ramp is used for gradual entry and exit. Specifically, the length of the reserved entry or exit section is 50μm; the preset linear power is 0.3ms.

[0039] Furthermore, in order to suppress the heat accumulation effect during step S2, the same path is scanned multiple times based on a preset time interval; The preset time interval is longer than the plasma plume lifetime and thermal diffusion characteristic time in femtosecond laser processing, ensuring cold processing in the time domain. Specifically, the preset time interval is Δt = 10 ms; In order to ensure uniform pulse distribution and avoid unidirectional thermal gradient accumulation during step S2, the scanning mode adopts an alternating clockwise and counterclockwise scanning mode.

[0040] Experiments show that, under the process in step S2, approximately 23-25 ​​repeated scans are required to penetrate a 500μm thick ceramic. The resulting hole wall edges are straight, the roundness is better than 98%, and the width of the heat-affected zone is less than 2μm, laying a quality foundation for the final tracing and finishing.

[0041] Step S3: Low-energy outlining and finishing: After the hole has been penetrated, the hole wall is outlined and finished using a laser; Objective: To eliminate spatter and microcracks at the edge of the hole after it has been penetrated.

[0042] It uses a femtosecond laser with a wavelength of 1030nm. The absorption rate of 1030nm laser in silicon nitride ceramic substrate is lower than that of 515nm laser. It can remove only the debris and resolidified layer at the hole wall and hole opening without removing the silicon nitride ceramic substrate again.

[0043] In addition, the specific parameter set in step S3 is: single pulse energy. =2.0 μJ, spot diameter =25 μm, scanning speed =1.8 m / s, pulse frequency =200 kHz, repeated 2 times; the laser energy density corresponding to the parameters in step S3 is slightly higher than the removal threshold of surface debris and spatter in practical applications, but lower than the ablation threshold of the ceramic substrate, thus achieving cleaning without etching. After the treatment in step S3, the hole wall surface is smooth, cracks are suppressed, edge residues are removed, and the final roundness of the hole is maintained above 98%, and the width of the heat-affected zone is less than 2μm, ensuring that the processing quality meets the requirements of high reliability applications.

[0044] Furthermore, scanning electron microscopy (SEM) observation after processing showed that the hole edges were clear, with no re-solidified debris residue, precise matrix structure arrangement, and hole spacing maintained within the design range of 70 μm. Both processing efficiency and hole quality met expectations, proving that this embodiment can achieve stable batch drilling while ensuring high precision and high consistency, demonstrating feasibility and reliability for practical engineering applications. In summary, in actual array structure processing, the galvanometer system completes the hole array scanning point-by-point according to the matrix path, with each hole undergoing approximately 10 repeated processing steps to form a stable through-hole structure. By optimizing pulse energy, scanning speed, and pulse overlap rate, uniform energy distribution is achieved during processing, ensuring hole diameter consistency and edge integrity. This embodiment can stably achieve a 10×10 matrix structure with a hole spacing maintained at 70 μm, and a processing efficiency of one hole every 10 seconds, meeting the actual production needs under high precision and high consistency requirements.

[0045] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A femtosecond laser aperture method for silicon nitride ceramics, characterized in that, include: Preparatory work: Fix the silicon nitride ceramic substrate to be processed on the processing platform and lock it in place; A three-stage scanning strategy is used to create openings in a fixed silicon nitride ceramic substrate, and the effective depth of field is constrained by Rayleigh length. The three-stage scanning strategy is as follows: S1. Low-energy grooving: A laser capable of stably removing the shallow layer of a silicon nitride ceramic substrate is used to scan the silicon nitride ceramic substrate multiple times to form a shallow groove. S2. High-energy multiple cutting: Keeping the laser wavelength unchanged from step S1, increasing the single pulse energy and repetition frequency, reducing the scanning speed, and performing multiple scans on the grooved silicon nitride ceramic substrate to complete the removal of the main area to be cut. S3. Low-energy outlining and finishing: After the hole has been penetrated, the hole wall is outlined and finished using a laser; The laser used in step S3 has a lower absorption rate than the laser used in step S2, and the corresponding energy density is higher than the removal threshold of surface debris and spatter at the opening on the silicon nitride ceramic substrate, but lower than the ablation threshold of the silicon nitride ceramic substrate. In each stage of the three-stage processing technology, scanning is performed in a circular feed based on a preset two-dimensional hole position vector path.

2. The femtosecond laser drilling method for silicon nitride ceramics according to claim 1, characterized in that, When performing step S1, the processing area on the silicon nitride ceramic substrate is expanded according to the actual size, so that a buffer zone is provided around the processing area.

3. The femtosecond laser drilling method for silicon nitride ceramics according to claim 1, characterized in that, To avoid energy buildup at the orifice during step S2, a reserved entry or exit section is set at the beginning or end of the scanning path. The industrial camera shutter only opens when entering the effective path and a preset linear power ramp is used for gradual entry and exit.

4. The femtosecond laser drilling method for silicon nitride ceramics according to claim 1, characterized in that, To suppress the heat accumulation effect during step S2, the same path is scanned multiple times based on a preset time interval. The preset time interval is greater than the plasma plume lifetime and thermal diffusion characteristic time in femtosecond laser processing.

5. The femtosecond laser drilling method for silicon nitride ceramics according to claim 1, characterized in that, In order to ensure uniform pulse distribution and avoid unidirectional thermal gradient accumulation during step S2, the scanning mode adopts an alternating clockwise and counterclockwise scanning mode.

6. A femtosecond laser aperture device for silicon nitride ceramics, characterized in that, A method for implementing a femtosecond laser aperture opening method for silicon nitride ceramics as described in any one of claims 1-5 includes: The optical path is connected in sequence to a dual-wavelength femtosecond laser, an optical path switching device, a beam expander, a scanning galvanometer system, a field lens, and a silicon nitride ceramic substrate to be processed located on a three-axis motion platform. And a host computer for outputting control signals, which are respectively connected to the dual-wavelength femtosecond laser, the scanning galvanometer system, the three-axis motion platform and the industrial camera signal; The three-axis motion platform serves as the machining platform; the scanning galvanometer system performs a circular scan based on a preset two-dimensional hole position vector path.

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