A method and apparatus for femtosecond laser drilling of silicon nitride ceramics

Abstract

The present application relates to the technical field of silicon nitride ceramic opening, and particularly relates to a femtosecond laser opening method and device for silicon nitride ceramic; the femtosecond laser opening method for silicon nitride ceramic comprises the following steps: pre-preparation; opening a fixed silicon nitride ceramic substrate based on a three-stage scanning strategy, and effectively limiting the depth of field by Rayleigh length; the three-stage scanning strategy is: S1. low-energy grooving; S2. high-energy multiple cutting; S3. low-energy edge trimming; in each stage of the three-stage processing process, the scanning is performed according to a preset two-dimensional hole vector path to execute annular feeding. The present application can realize high-quality and controllable micro-hole array processing on the silicon nitride ceramic substrate, and 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:

[0007] Preparatory work: Fix the silicon nitride ceramic substrate to be processed on the processing platform and lock it in place;

[0008] 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.

[0009] The three-stage scanning strategy is as follows:

[0010] 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.

[0011] S2. High-energy multiple cutting: keeping the laser wavelength unchanged as in step S1, increasing the single-pulse energy and the repetition frequency, and reducing the scanning speed, multiple scanning is performed on the slotted silicon nitride ceramic substrate to complete the removal of the main body area to be drilled;

[0012] S3. Low-energy contour trimming: after the hole has been drilled through, the laser is used to trim the hole wall;

[0013] 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 hole opening on the silicon nitride ceramic substrate, and lower than the ablation threshold of the silicon nitride ceramic substrate;

[0014] In each stage of the three-stage processing process, the scanning is performed according to the preset two-dimensional hole position vector path with circular feed.

[0015] Preferably, when step S1 is performed, it also includes expanding the processing area range on the silicon nitride ceramic substrate according to the actual size, and providing a buffer zone on the periphery of the processing area.

[0016] Preferably, when step S2 is performed, in order to avoid energy accumulation at the hole opening, a reserved lead-in section or a lead-out section is provided at the beginning and 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.

[0017] Preferably, when step S2 is performed, in order to suppress the effect of heat accumulation, the same path is scanned multiple times based on a preset time interval.

[0018] The preset time interval is greater than the plasma plume lifetime and the heat diffusion characteristic time in femtosecond laser processing.

[0019] Preferably, when step S2 is performed, in order to make the pulse distribution uniform and avoid the accumulation of one-way thermal gradient, the scanning mode uses an alternating clockwise and counterclockwise scanning mode.

[0020] A femtosecond laser drilling device for silicon nitride ceramic is used to realize a femtosecond laser drilling method for silicon nitride ceramic, comprising:

[0021] 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 on a three-axis motion platform connected in sequence by optical paths;

[0022] and an upper computer for outputting control signals and connected to the dual-wavelength femtosecond laser, the scanning galvanometer system, the three-axis motion platform, and the industrial camera signal;

[0023] The three-axis motion platform is a processing platform; the scanning galvanometer system performs circular scanning according to the preset two-dimensional hole position vector path.

[0024] The technical scheme of the present application has the following advantages:

[0025] The present application combines the ultrafast cooling processing characteristics of femtosecond laser with high-precision scanning control, breaks through the limitations of cracks, hole wall melting and low efficiency in traditional mechanical drilling and thermal laser drilling, and can realize high-quality and controllable micro-hole array processing on a silicon nitride ceramic substrate, having wide engineering application value. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the technical scheme in the specific embodiments or prior art of the present application, the drawings needed to be used in the specific embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings described below are some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0027] Figure 1 The scanning path schematic diagram of step S1 is shown in the figure;

[0028] Figure 2 The scanning path schematic diagram of step S2 is shown in the figure;

[0029] Figure 3 The scanning path schematic diagram of step S3 is shown in the figure. DETAILED DESCRIPTION

[0030] The technical scheme of the present application will be described in detail below with reference to the drawings. Obviously, the described embodiments are some embodiments of the present application, not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0031] In the description of the present application, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like 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 the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. In addition, the terms "first", "second", "third" are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0032] In the description of the present application, it should be noted that unless otherwise explicitly specified and limited, the terms "mounting", "connection", "linking" should be understood in a broad sense, for example, it can be fixed connection, or detachable connection, or integral connection; it can be mechanical connection, or electrical connection; it can be direct connection, or indirect connection through intermediate medium, or internal communication of two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0033] In addition, the technical features involved in the different embodiments of the application described below can be combined with each other as long as there is no conflict.

[0034] Embodiment 1

[0035] A femtosecond laser drilling device for silicon nitride ceramic, comprising:

[0036] 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 on a three-axis motion platform are connected in sequence by optical paths.

[0037] And a host computer for outputting control signals and being connected with the dual-wavelength femtosecond laser, the scanning galvanometer system, the three-axis motion platform, and the industrial camera, respectively.

[0038] The three-axis motion platform is a processing platform; the scanning galvanometer system performs annular scanning according to a preset two-dimensional hole position vector path; and the field lens is an F-θ focusing field lens.

[0039] The dual-wavelength femtosecond laser as the core light source outputs a pulse width of about 200fs, a center wavelength of 1030nm, a frequency-doubled wavelength of 515nm, an adjustable repetition frequency of 100kHz-1MHz, and a single pulse energy of 1-100μJ, which can ensure that the energy density exceeds the ablation threshold of the silicon nitride ceramic substrate.

[0040] The laser beam enters the scanning galvanometer system after being adjusted by the beam expander, and the scanning path is completed by high-speed deflection in the X and Y directions, and is focused to the workpiece surface under the action of the F-θ focusing field mirror, so that a stable and consistent focused spot is obtained in the entire processing area. The three-axis motion platform mainly completes the positioning and area transposition of the workpiece during processing, and cooperates with the industrial camera to realize the reference point identification and automatic correction of the array position. The operation of the entire device is uniformly scheduled by the upper computer, and the laser and scanning are controlled by the hardware synchronous pulse, so that the pulse timing and path arc length are strictly consistent, the local overburning of the start and end sections is reduced, and the strict synchronization between the laser pulse and the galvanometer scanning trajectory is ensured. During the actual assembly of the overall device, the volume and structure can also be changed according to actual needs, but the light path connection relationship of the embodiment must be met to ensure that the silicon nitride ceramic substrate to be processed on the three-axis motion platform can be accurately scanned. The device as a whole has the capabilities of programmable path scheduling, multi-point array automatic identification and continuous processing, and is particularly suitable for high-precision hole opening of small hole matrix structures. The control core is composed of an industrial computer or an FPGA, which can realize the strict synchronization of the laser pulse and the galvanometer scanning path, so as to ensure that the hole diameter boundary is clear, the contour is round and the repeatability is good.

[0041] Embodiment 2

[0042] On the basis of embodiment 1, a femtosecond laser hole opening method for silicon nitride ceramic is specifically disclosed, comprising:

[0043] Preparation work: fix the silicon nitride ceramic substrate to be processed on the processing platform, and perform positioning and locking;

[0044] Based on the three-stage scanning strategy, the fixed silicon nitride ceramic substrate is opened, and the effective depth of field is constrained by the Rayleigh length;

[0045] The three-stage scanning strategy is:

[0046] S1. Low-energy grooving: using a laser capable of stably removing the shallow layer of the silicon nitride ceramic substrate, the silicon nitride ceramic substrate is scanned multiple times to form a shallow groove;

[0047] S2. High-energy multiple cutting: keeping the laser wavelength unchanged as in step S1, increasing the single-pulse energy and repetition frequency, and reducing the scanning speed, the silicon nitride ceramic substrate with the groove is scanned multiple times to complete the removal of the main body area to be opened;

[0048] S3. Low-energy edge trimming: after the hole is penetrated, the laser is used to trim the hole wall;

[0049] 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 the surface debris and spatter at the opening of the silicon nitride ceramic substrate, and lower than the ablation threshold of the silicon nitride ceramic substrate;

[0050] In each stage of the three-stage processing process, the scanning is performed according to the preset two-dimensional hole site vector path with annular feeding.

[0051] Specifically:

[0052] This embodiment takes a 0.5mm silicon nitride ceramic substrate as the element to be processed, and details the processing of a hole with a diameter of about 420μm at the upper opening and a diameter of about 400μm at the lower opening, and a true circularity of more than 98%. Figures 1-3 The detailed processing flow example method processes a hole with a diameter of about 420μm at the upper opening and a diameter of about 400μm at the lower opening, and a true circularity of more than 98%.

[0053] Preparation work:

[0054] Unlike other hole opening methods, since the application requires the use of two wavelengths of laser, a dual-wavelength femtosecond laser is required in this embodiment, and an optical path switching device is provided for this purpose, as well as optical elements such as a beam expander mirror containing two wavebands of 515nm and 1030nm, a scanning galvanometer system, etc.

[0055] Before processing, the 0.5mm-thick silicon nitride ceramic substrate is first moved to the set position by the three-axis motion platform, and high-precision hole array reference alignment is achieved through the industrial camera recognition function, and positioning is completed. It should be noted that after the three-axis motion platform is locked, the galvanometer scanning system performs hole site trajectory scanning according to the preset G code path or vector graphics file, such as DXF.

[0056] Based on the three-stage scanning strategy, the fixed silicon nitride ceramic substrate is drilled, and the effective depth of field is constrained by the Rayleigh length.

[0057] It should be noted that constraining the effective depth of field by the Rayleigh length allows the three-stage scanning strategy to be performed at approximately the same energy density in each stage.

[0058] The three-stage scanning strategy is:

[0059] Step S1:

[0060] Low-energy slotting: a laser capable of stably removing the shallow layer of the silicon nitride ceramic substrate is used to scan the silicon nitride ceramic substrate along the scanning path multiple times to form a shallow slot.

[0061] Objective: To establish a stable incident channel on the dense layer on the surface of the silicon nitride ceramic substrate; specifically, to form a shallow slot.

[0062] A 515nm laser with a pulse width of 300fs, a repetition frequency of 250kHz, a single pulse energy of 3.2μJ, and a galvanometer spot diameter of 20μm is used, and the scanning speed is 1.2m / s.

[0063] When the pulse width is 300fs, the peak power density formula is: ; wherein, represents peak power density; represents single pulse energy; represents pulse width; represents galvanometer spot diameter.

[0064] The calculated peak power density is , which is higher than the multi-photon ablation threshold of silicon nitride, and can ensure stable removal of the shallow layer. For example, Figure 1 is a schematic diagram of the path planned when step S1 is performed. During the execution of step S1 processing Figure 1 The red part represents the actual scanning path of the laser.

[0065] When femtosecond laser interacts with silicon nitride ceramic substrate, local resolidification phenomenon may occur after ablation removal of the material. The particles vaporized or melted will be re-deposited near the hole wall due to the rapid cooling of the plasma plume, forming a strip of debris. Due to the certain adhesion and fusion characteristics of these debris particles, it is difficult to completely remove them in time by relying solely on suction or air flow dust removal devices, which may further lead to decreased efficiency of subsequent pulse action, uneven energy deposition, and even affect the stability of the slot depth and edge profile.

[0066] To suppress this phenomenon, when step S1 is performed, the processing area range on the silicon nitride ceramic substrate is also expanded according to the actual size, and a buffer zone is provided on the periphery of the processing area, thereby providing a more sufficient channel for the escape and air flow carrying of debris. This strategy effectively reduces the accumulation of debris at the hole opening and hole wall, and improves the cleanliness and energy utilization rate of the processing process. The specific design of the width of the buffer zone can be set according to the actual selected femtosecond laser and the specific material of the material.

[0067] And through experimental verification, it is found that when the scanning mode of step S1 is used, after five repeated scans, the slot depth can be stably controlled at 12-15 μm, the edge profile is smooth and clear, and there is no obvious burr or resolidification adhesion, and the slot quality remains stable. The results show that expanding the scanning area not only improves the debris discharge efficiency, but also significantly improves the hole opening edge quality and processing consistency, providing an ideal initial condition for subsequent main removal and finishing processes.

[0068] Step S2:

[0069] High-energy multiple cutting: keeping the laser wavelength unchanged as step S1, increasing the single pulse energy and repetition frequency, and reducing the scanning speed, multiple scanning is performed on the slotted silicon nitride ceramic substrate to complete the removal of the main area to be drilled;

[0070] The 515 nm laser is used, the single pulse energy is 20 μJ, the galvanometer spot diameter is 20 μm, the repetition frequency is 500 kHz, and the scanning speed is 0.6 mm / s. The laser energy density corresponding to the parameters is much higher than the ablation threshold of the material. In order to describe the depth accumulation effect of repeated processing, a logarithmic ablation model is used in this embodiment: ;

[0071] wherein, is the ablation coefficient of the material, which can be obtained by calibration, = 0.95 μm; is the real-time cumulative depth of repeated processing; is the peak energy density of the laser used in step S2; is the ablation threshold of the silicon nitride material.

[0072] It should be noted that the logarithmic ablation model essentially only describes the lateral information: the logarithmic relationship between the ablation pit diameter and the laser energy density, which is used to extract the ablation threshold and the galvanometer spot radius, and does not include time or longitudinal ablation dynamics. Therefore, according to the conventional theory, the processing depth cannot be directly given, and the cumulative depth and other related effects cannot be reflected. However, the processing depth of the technical solution of the present embodiment is linear in implementation, which is an empirical fitting, and therefore does not belong to the derivation of the logarithmic ablation model itself, but is an extrapolation of the lateral formula to the depth direction. Therefore, the skilled person in the art can derive it by experiment after step S2 of the present embodiment, and therefore it will not be described in detail.

[0073] Under the above conditions, the average removal depth of a single cycle scanning is about 20 μm, and about 24 times of repeated processing is needed to complete the penetration for a silicon nitride ceramic substrate with a thickness of 0.5 mm.

[0074] In addition, in order to avoid energy accumulation at the aperture during the execution of step S2, a reserved lead-in section or a lead-out section is arranged at the beginning and end of the scanning path, the industrial camera shutter is only opened when entering the effective path, and a pre-set linear power ramp is used for gradual entry and exit. Specifically, the length of the reserved lead-in section or the lead-out section is 50 μm; the pre-set linear power is 0.3 ms.

[0075] In order to suppress the thermal accumulation effect during the execution of step S2, the same path is scanned multiple times based on a pre-set time interval;

[0076] The pre-set time interval is greater than the plasma plume lifetime and the thermal diffusion characteristic time in femtosecond laser processing, which ensures cold processing from the time domain. Specifically, the pre-set time interval is Δt = 10 ms;

[0077] In order to make the pulse distribution uniform and avoid the accumulation of one-way thermal gradient, the scanning mode uses a clockwise and counterclockwise alternating scanning mode.

[0078] Experiments show that about 23-25 times of repeated scanning is needed to complete the through hole of 500 μm thick ceramic under the process of step S2, the obtained hole wall edge is straight, the roundness is better than 98%, and the heat affected zone width is less than 2 μm, which lays a quality foundation for the final edge modification.

[0079] Step S3:

[0080] Low-energy edge modification: after the hole is penetrated, the laser is used to modify the edge of the hole wall;

[0081] Objective: to eliminate the splashes and micro-cracks on the hole edge after the hole is penetrated.

[0082] The femtosecond laser with a wavelength of 1030 nm is used,

[0083] The absorption rate of 1030 nm laser in silicon nitride ceramic substrate is lower than that of 515 nm laser, which can only remove the debris and resolidified layer on the hole wall and hole edge without further removing the silicon nitride ceramic substrate.

[0084] In addition, in step S3, the specific parameters are set as: single pulse energy =2.0 μJ, spot diameter =25 μm, scanning speed =1.8 m / s, pulse frequency =200 kHz, and repetition number 2 circles; the laser energy density corresponding to the parameters of step S3 is slightly higher than the removal threshold of surface debris and splashes in actual application, but is lower than the ablation threshold of ceramic matrix, so that only cleaning is realized without engraving. After step S3, the hole wall surface is smooth, the cracks are suppressed, the edge residues are removed, the true roundness of the final hole is maintained at more than 98%, the heat affected zone width is less than 2 μm, and the processing quality meets the requirements of high reliability application.

[0085] Further, after the processing is completed, the scanning electron microscope observation shows that the hole edge is clear without resolidified debris, the matrix structure arrangement is accurate, and the hole spacing is maintained within the design value range of 70 μm. The processing efficiency and hole quality meet the expectations, which proves that the scheme of the embodiment can realize stable batch hole opening while ensuring high precision and high consistency, and has the feasibility and reliability of practical engineering application. Summary: in actual array structure processing, the galvanometer system completes hole array scanning point by point according to the matrix path, and each hole is about 10 times of repeated processing to form a stable through structure. By optimizing the pulse energy, scanning speed and pulse overlap rate, the energy distribution is uniform during processing, which ensures the hole diameter consistency and edge integrity. The embodiment can stably realize a 10x10 matrix structure, the hole spacing is maintained at 70 μm, the processing efficiency is one hole per 10 seconds, which can meet the actual production demand under the requirements of high precision and high consistency.

[0086] Obviously, the above embodiments are merely example for clearly illustrating but not limitation to the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, all the embodiments need not and can not be enumerated. The obvious changes or variations derived from the above description are still within the protection scope of the present application.

Claims

1. A method of femtosecond laser drilling of silicon nitride ceramic, characterized by, The method comprises the following steps: Preparation: fix the silicon nitride ceramic substrate to be processed on the processing platform, and perform positioning and locking; Based on the three-stage scanning strategy, the fixed silicon nitride ceramic substrate is drilled, and the effective depth of field is limited by the Rayleigh length; The three-stage scanning strategy is as follows: S1. Low-energy slotting: using a laser capable of stably removing the shallow layer of the silicon nitride ceramic substrate, the silicon nitride ceramic substrate is scanned multiple times to form a shallow slot; S2. High-energy multiple cutting: keeping the laser wavelength unchanged as in step S1, increasing the single-pulse energy and repetition frequency, and reducing the scanning speed, the silicon nitride ceramic substrate with the slot is scanned multiple times to complete the removal of the main body area to be drilled; S3. Low-energy edge trimming: after the hole is penetrated, the laser is used to trim the hole wall; 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 the surface debris and spatter at the drilling site on the silicon nitride ceramic substrate, and lower than the ablation threshold of the silicon nitride ceramic substrate; In each stage of the three-stage processing process, the scanning is performed according to the preset two-dimensional hole vector path with circular feed.

2. The method of claim 1, wherein the method is for a silicon nitride ceramic. When performing step S1, the processing area range on the silicon nitride ceramic substrate is expanded according to the actual size, and a buffer zone is provided on the periphery of the processing area.

3. The method of claim 1, wherein the method is for a silicon nitride ceramic. When performing step S2, in order to avoid energy accumulation at the hole, a reserved lead-in section or a lead-out section is arranged at the beginning and 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.

4. The method of claim 1, wherein the method is for a silicon nitride ceramic. When performing step S2, in order to suppress the effect of heat accumulation, 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 the heat diffusion characteristic time in femtosecond laser processing.

5. The method of claim 1, wherein the method is for a silicon nitride ceramic. When performing step S2, in order to make the pulse distribution uniform and avoid the accumulation of one-way thermal gradient, the scanning mode adopts the clockwise and counterclockwise alternating scanning mode.

6. A device for femtosecond laser drilling of silicon nitride ceramic, characterized by A femtosecond laser drilling method for silicon nitride ceramic according to any one of claims 1-5, comprising: 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 on a three-axis motion platform connected in sequence; and an upper computer for outputting control signals and connected with 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 a processing platform; the scanning galvanometer system performs circular scanning according to the preset two-dimensional hole vector path.

Images