Method and system for spatiotemporal focusing femtosecond laser welding of dissimilar hard and brittle materials

Abstract

The application discloses a spatiotemporal synchronous focusing femtosecond laser welding heterogeneous hard and brittle material method and system, and belongs to the technical field of femtosecond laser welding application. The system comprises five subsystems of spatial dispersion, focusing welding, top imaging monitoring, side plasma signal collection and computer control. The femtosecond laser is used for welding of heterogeneous materials after spatial dispersion and focusing. The laser is focused on the optical contact surface of the double-layer material, the welding result is observed from the top CCD imaging during the welding process, and the plasma morphology at the weld is observed from the side high-speed camera. The relative position relationship between the focal point and the contact surface of the sample is adjusted in time, that is, the femtosecond laser welding process is observed through multi-angle observation, so that the focal point is prevented from splitting and damaging the material inside due to self-focusing; meanwhile, whether fine particles are ejected at the weld is observed, so that the laser energy loss and the weld strength reduction caused by poor optical contact of the material are reduced. The welding result is reliable, the repeatability is high, the heat affected zone is small, and the packaging requirements of precise microelectronic devices are met.

Description

Technical Field

[0001] This invention relates to a method and system for spatiotemporally synchronized focusing femtosecond laser welding of heterogeneous hard and brittle materials, belonging to the field of femtosecond laser welding application technology. Background Technology

[0002] Laser welding is a crucial technology in laser processing. Utilizing a high-energy-density laser beam as a heat source, it offers advantages over other methods of joining dissimilar materials (such as adhesive bonding and anodic bonding), including high speed, minimal deformation, lack of material limitations, and the absence of a vacuum environment. Femtosecond laser welding, a significant branch of laser welding, is used to join dissimilar hard and brittle materials. Its advantage lies in the fact that the ultrashort pulse duration of the femtosecond laser can connect hard and brittle materials under instantaneous high temperature and pressure, resulting in high-quality welds and a small heat-affected zone.

[0003] Therefore, femtosecond laser welding has become a highly attractive high-quality packaging method. However, in practical applications, due to the nonlinear absorption and self-focusing phenomenon of femtosecond lasers within the material, the laser energy attenuates sharply when it passes through the hard and brittle upper material to the interface of the heterogeneous material. The focal point shifts forward to the interior of the upper material, which is not conducive to weld formation and can easily cause damage to the interior of the upper material.

[0004] By employing spatiotemporal focusing technology to improve femtosecond laser focusing, the laser is strongly focused at the focal point, where the pulse width is minimized. The farther the distance on the optical axis from the geometric focal point, the wider the pulse width. This increased pulse width reduces the self-focusing phenomenon of the filament within the material, resulting in a significant reduction in the focal length. This characteristic effectively reduces the nonlinear absorption of the laser by hard and brittle materials during propagation, improves energy utilization, and reduces internal damage to the upper hard and brittle materials. Summary of the Invention

[0005] This invention aims to provide a method and system for spatiotemporally focused femtosecond laser welding of heterogeneous hard and brittle materials. By observing the focal point position in real time during processing, it avoids self-focusing from affecting weld strength and damaging the interior of the hard and brittle materials. This invention has the advantages of high efficiency and simplicity, enabling high-throughput, low-damage welding of heterogeneous hard and brittle materials, and has significant application value in fields such as microelectronic component packaging and aerospace.

[0006] The objective of this invention is achieved through the following technical solution.

[0007] This invention discloses a method for spatiotemporally focused femtosecond laser welding of heterogeneous hard and brittle materials. The femtosecond laser, after spatial dispersion and focusing, is used for welding heterogeneous materials. The welding process is monitored in real-time from multiple angles through top imaging, side plasma signal collection, and computer control to avoid self-focusing causing focal point splitting and reducing weld strength. Simultaneously, the weld seam is observed for the ejection of fine particles to prevent welding failure. The combination of multi-angle observation and femtosecond laser welding results in reliable welding results with high repeatability and a small heat-affected zone, meeting the packaging requirements of precision microelectronic devices.

[0008] This invention discloses a spatiotemporal focused femtosecond laser welding and weld seam real-time signal acquisition system, used to realize the method of welding heterogeneous hard and brittle materials based on spatiotemporal focused femtosecond laser. The system includes five subsystems: spatial dispersion, focused welding, top imaging monitoring, side plasma signal collection, and computer control.

[0009] The focused welding subsystem mainly consists of a laser (and a magnification stage), a half-wave plate, a polarizing beam splitter prism, an electronically controlled shutter, a focusing objective at the end of the optical path, and a three-dimensional translation stage for placing the sample. These components enable the laser to be focused onto the interface between the two materials, and the translation stage controls the displacement of the sample to achieve the welding of dissimilar materials.

[0010] The spatial dispersion subsystem mainly consists of a convex lens, a concave lens, and a pair of gratings. The laser beam diameter decreases after passing through the convex and concave lenses from the amplification stage; the beam reduction ratio is determined by the focal length and spacing of the lenses. The gratings are placed parallel and opposite to each other. After the beam-reduced laser passes through the grating pair, the time chirp is compensated, generating spatial chirp. The spatial dispersion of the laser is adjusted by changing the laser incident angle of the first grating and the distance between the grating pairs.

[0011] The top imaging monitoring subsystem mainly consists of a dichroic mirror, an objective lens for focusing the light spot at the end of the optical path, a beam splitter, a white illumination light source, and a charge-coupled device (CCD). During the welding process, the welding status of the double-layer materials is monitored through top imaging: when the laser sweeps directly across the contact plane, no fine particles are ejected around the weld, indicating that the hard and brittle dissimilar materials are welded well.

[0012] The side plasma signal collection subsystem mainly consists of a high-speed camera and a microscope tube. Before signal collection, the position of the high-speed camera and the magnification of the microscope tube need to be adjusted so that the focal point is slightly below the center of the imaging plane. Then, the plasma signal at the weld seam is collected in real time during the welding process, which facilitates timely adjustment of the relative position of the focal point and the sample contact interface.

[0013] The computer control subsystem is connected to the electronic shutter, CCD, high-speed camera, and 3D panning stage. It is used to control the shutter opening and closing, the panning stage movement, and to output the signals collected by the CCD and high-speed camera in real time.

[0014] This invention discloses a method for welding heterogeneous hard and brittle materials using a spatiotemporally focused femtosecond laser. The method is based on a spatiotemporally focused femtosecond laser welding and weld seam real-time signal acquisition system for welding and monitoring heterogeneous hard and brittle materials. The method includes the following steps:

[0015] Step 1: Polish the materials to be welded. The upper hard and brittle sample (glass, sapphire, ceramic, etc.) needs to be polished on three sides: the upper surface, the lower surface, and the side. The lower sample (metal, ceramic, semiconductor, etc.) needs to be polished on one side. Place the lower material on a three-dimensional translation stage, place the hard and brittle sample on it, and fit the two samples together to achieve optical contact.

[0016] Step 2: Collimate the optical path and adjust the laser incident angle and spacing of the grating pairs;

[0017] Step 3: Focus the laser light through the upper material onto the space between the samples;

[0018] Step 4: Adjust the top CCD so that it images onto the contact surface between the samples. Adjust the relative position of the high-speed camera and the samples so that the side surface of the sample is aligned with the lens barrel of the high-speed camera and the high-speed camera images onto the vertical plane where the focal point is located.

[0019] Step 5: Determine the moving speed and path of the translation stage during the welding process;

[0020] Step six involves opening the electronically controlled shutter to weld the double-layer sample. The top CCD imager is used to observe whether the weld has formed. Signals collected by a high-speed side camera are used to adjust the focus and relative position of the sample contact surface in real time, enabling multi-angle monitoring of the welding process. This prevents self-focusing from causing focus splitting and reducing weld strength. Simultaneously, the weld is observed for any microparticles ejected to prevent welding failure. The combination of multi-angle observation and femtosecond laser welding ensures reliable welding results, high repeatability, and a small heat-affected zone, meeting the packaging requirements of precision microelectronic devices.

[0021] Beneficial effects:

[0022] 1. This invention discloses a method and system for spatiotemporally focused femtosecond laser welding of heterogeneous hard and brittle materials. During the welding process, the welding results are observed from a top CCD imager, and the plasma morphology at the weld seam is observed from a side high-speed camera. These two perspectives jointly monitor the welding process, allowing for timely adjustment of the relative position of the focal point and the sample contact surface. In other words, by combining multi-angle observation with femtosecond laser welding, the controllability of laser welding is improved based on spatiotemporal focusing. This invention is applicable to welding of almost all hard and brittle materials and non-hard / brittle / hard materials (ceramics and alloys, ceramics and sapphire, etc.). The welding results are stable, reliable, and highly repeatable, with high experimental efficiency.

[0023] 2. The spatiotemporal focusing femtosecond laser welding method and system for heterogeneous hard and brittle materials disclosed in this invention, on the basis of achieving the beneficial effect 1, realizes real-time multi-angle monitoring of welding, avoids self-focusing causing focal point splitting and reducing weld strength, and observes whether there are micro-particles ejected at the weld. It can effectively avoid nonlinear absorption of laser and laser self-focusing phenomenon in hard and brittle materials, and helps to reduce the loss of laser energy and the damage caused by focal point splitting to the upper hard and brittle material. Attached Figure Description

[0024] Figure 1 A flowchart of a method for spatiotemporally focused femtosecond laser welding of heterogeneous hard and brittle materials.

[0025] Figure 2 This is a structural diagram of a femtosecond laser welding and monitoring system based on spatiotemporal focusing.

[0026] Figure 3 Comparative electron micrographs of the contact surfaces of samples made of the same material that were not welded and the weld seams of samples that were welded and then disassembled. The left image shows the surface morphology of the unwelded sample, and the right image shows the weld seam morphology of the sample that was welded and then disassembled.

[0027] Among them, 1-femtosecond laser, 2-half-wave plate, 3-polarizing beam splitter prism, 4-electronic shutter, 5-convex lens, 6-concave lens, 7-first grating, 8-second grating, 9-dichroic mirror, 10-focusing objective lens, 11-double-layer sample with optical contact, 12-three-dimensional translation stage, 13-beam splitter, 14-illumination source, 15-top CCD, 16-high-speed camera, 17-lens tube, 18-computer control system.

[0028] Figure 4 The image shows the plasma luminescence morphology at the weld joint during the welding process. The left image shows low weld strength due to laser self-focusing causing focal point splitting; the right image shows high weld strength achieved by adjusting the laser focal point position to avoid focal point splitting. Detailed Implementation

[0029] To better understand the method of the present invention, the technical solution of the present invention will be further described in detail below with reference to specific embodiments.

[0030] like Figure 1 As shown in this embodiment, a method for welding heterogeneous hard and brittle materials using spatiotemporal focusing femtosecond laser is disclosed. The original femtosecond laser pulse is dispersed by a grating and then refocused, resulting in a reduced focal length and increased light intensity at the focal point, which enables high-quality welding of heterogeneous hard and brittle materials.

[0031] like Figure 2 As shown, the spatiotemporally focused femtosecond laser welding and monitoring system includes five subsystems: focused welding, spatial dispersion, top imaging monitoring, side plasma signal collection, and computer control.

[0032] The focused welding subsystem consists of a femtosecond laser 1, a half-wave plate 2, a polarizing beam splitter prism 3, an electronically controlled shutter 4, a focusing objective lens 10 at the end of the optical path, and a three-dimensional translation stage 12 for placing the sample 11. These components allow the laser to be focused onto the interface between the two materials, and the translation stage controls the displacement of the sample to achieve heterogeneous material welding. The laser energy can be continuously adjusted through the coordinated action of the half-wave plate 2 and the polarizing beam splitter prism 3.

[0033] The spatial dispersion subsystem consists of a convex lens 5, a concave lens 6, and a pair of gratings 7 and 8. The laser beam from the amplification stage passes sequentially through the convex lens 5 and the concave lens 6, resulting in a reduced beam diameter. The beam reduction ratio is determined by the focal length and spacing of the lenses. The gratings 7 and 8 are placed parallel and opposite to each other. After the beam-reduced laser passes through the grating pair, the time chirp is compensated, generating spatial chirp. The spatial dispersion of the laser is adjusted by changing the laser incident angle of the first grating and the distance between the grating pairs.

[0034] The top imaging monitoring subsystem consists of a dichroic mirror 9, a focusing objective lens 10 for focusing the light spot at the end of the optical path, a beam splitter 13, an illumination source 14, and a top CCD 15. During the welding process, the welding status of the double-layer materials can be monitored through top imaging: when the laser sweeps directly across the contact plane, no obvious fine particles are ejected around the weld, indicating that the hard and brittle dissimilar materials are welded well.

[0035] The side plasma signal collection subsystem consists of a high-speed camera 16 and a lens barrel 17. Before signal collection, the position of the high-speed camera and the magnification of the lens barrel need to be adjusted so that the imaging plane is at the focal point. Then, the plasma signal at the weld seam is collected in real time during the welding process, which facilitates timely adjustment of the relative position of the focal point and the sample contact interface.

[0036] The computer control system 18 is connected to the electronic shutter 4, the top CCD 15, the high-speed camera 16, and the three-dimensional translation stage 12. It is used to control the shutter opening and closing, the movement of the translation stage, and to output the signals collected by the CCD and the high-speed camera in real time.

[0037] The operation and fine-tuning methods of the spatiotemporally focused femtosecond laser welding and monitoring system disclosed in this example are as follows:

[0038] Step 1: Polish the materials to be welded. Polish the upper hard and brittle sample on three sides (upper and lower surfaces and side surfaces), and polish the lower sample (metal, ceramic, semiconductor, etc.) on one side. Place the lower material on a three-dimensional translation stage, place the hard and brittle sample on it, and fit the double-layer sample 11 to achieve optical contact (judgment method: Newton's rings visible to the naked eye are generated on the contact surface).

[0039] Step 2: Collimate the optical path. Adjust the laser incident angle and spacing of the gratings to the first grating 7 and the second grating 8 so that the output laser enters the focusing objective 10 vertically and does not produce visible divergence in front of the focusing objective 10.

[0040] Step 3: Focus the laser light through the upper material onto the optical contact surface between the samples.

[0041] Step 4: Adjust the top CCD 15 so that it images onto the optical contact surface between the samples. Adjust the relative position of the high-speed camera 16 and the sample 11 so that the side surface of the sample is aligned with the lens barrel 17 of the high-speed camera and the high-speed camera 16 images onto the vertical plane where the focal point is located. Adjust the focal point so that it is slightly below the center of the imaging plane.

[0042] Step 5: The computer control system 18 determines the moving speed and path of the translation stage 12 during the welding process.

[0043] Step 6: Open the electronic shutter 4 to weld the double-layer sample 11, and observe whether it is welded through the top CCD 15. A welded sample has smooth edges and no microparticles protruding, while an unwelded sample has rough edges and microparticles protruding around it. The electron microscope image is shown below. Figure 3 As shown. The reason for the failure to weld is that the gap between the bonded samples is too large in some areas, failing to meet the optical contact requirements. This phenomenon should be avoided as much as possible during the welding process. Using signals collected by a high-speed side camera, the relative position of the focus and the sample contact surface should be adjusted in a timely manner: When the welding area is large, slight differences in the horizontal position of the sample may cause the focus to be higher than the sample contact surface. This phenomenon will reduce the shear strength of the weld and cause internal modification and fracture of the upper hard and brittle material. When this phenomenon is detected by the high-speed camera, the translation stage should be adjusted in time to make the focus act on the optical contact surface of the sample. For example: Figure 3 The plasma splitting phenomenon shown in the left figure can be mitigated by adjusting the height of the 12-dimensional translation stage until the requirements of femtosecond laser welding are met, thus achieving high-quality welding of heterogeneous hard and brittle materials.

[0044] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for welding heterogeneous hard and brittle materials using spatiotemporal focused femtosecond laser, based on a spatiotemporal focused femtosecond laser welding and real-time signal acquisition system at the weld seam, characterized in that: The system comprises five subsystems: spatial dispersion, focused welding, top imaging monitoring, side plasma signal collection, and computer control. The focused welding subsystem mainly consists of a laser, a half-wave plate, a polarizing beam splitter prism, an electronically controlled shutter, a focusing objective at the end of the optical path, and a three-dimensional translation stage for placing the sample. Through these components, the laser can be focused onto the interface between the two materials, and the translation stage controls the displacement of the sample to achieve welding of heterogeneous hard and brittle materials. The spatial dispersion subsystem mainly consists of a convex lens, a concave lens, and a pair of gratings. The laser beam diameter decreases after passing through the concave and convex lenses, and the beam reduction ratio is determined by the focal length and spacing of the concave and convex lenses. The gratings are placed parallel and opposite to each other. After the laser beam is reduced, the time chirp is compensated after passing through the grating pair, resulting in spatial chirp. The spatial dispersion of the laser is adjusted by changing the laser incident angle of the first grating and the distance between the grating pairs. The top imaging monitoring subsystem mainly consists of a dichroic mirror, an objective lens for focusing the light spot at the end of the optical path, a beam splitter, a white light for illumination, and a charge-coupled device (CCD). During the welding process, the welding status of the double-layer material is monitored by top imaging: when the laser is scanning the contact plane directly, no fine particles are ejected around the weld, indicating that the dissimilar hard and brittle materials are welded well. The side plasma signal collection subsystem mainly consists of a high-speed camera and a lens barrel. Before signal collection, the position of the high-speed camera and the magnification of the lens barrel need to be adjusted so that the focus is located slightly below the center of the imaging plane. Then, the plasma signal at the weld seam is collected in real time during the welding process, which facilitates timely adjustment of the relative position of the focus and the sample contact interface. The computer control subsystem is connected to the electronic shutter, CCD, high-speed camera, and 3D translation stage, and is used to control the shutter opening and closing, the movement of the translation stage, and to output the signals collected by the CCD and high-speed camera in real time. Femtosecond lasers are used for welding heterogeneous hard and brittle materials after spatial dispersion and focusing. The welding process is monitored in real time from multiple angles through top imaging monitoring, side plasma signal collection, and computer control to avoid self-focusing that causes focal point splitting and reduces weld strength. At the same time, it is observed whether there are microparticles ejected from the weld to avoid welding failure. The combination of multi-angle observation and femtosecond laser welding makes the welding results reliable, highly repeatable, and with a small heat-affected zone, which meets the packaging requirements of precision microelectronic devices.

2. The method for spatiotemporally focused femtosecond laser welding of heterogeneous hard and brittle materials as described in claim 1, characterized in that: Includes the following steps, Step 1: Polish the materials to be welded. The upper hard and brittle sample needs to be polished on three sides: the upper surface, the lower surface, and the side. The lower sample needs to be polished on one side. Place the lower material on a three-dimensional translation stage, place the hard and brittle sample on it, and fit the two samples together to achieve optical contact. Step 2: Collimate the optical path and adjust the laser incident angle and spacing of the grating pairs; Step 3: Focus the laser light through the upper material onto the space between the samples; Step 4: Adjust the top CCD so that it images onto the contact surface between the samples. Adjust the relative position of the high-speed camera and the samples so that the side surface of the sample is aligned with the lens barrel of the high-speed camera and the high-speed camera images onto the vertical plane where the focal point is located. Step 5: Determine the moving speed and path of the translation stage during the welding process; Step six: Open the electronically controlled shutter to weld the double-layer sample. Observe whether the weld is successful through top CCD imaging. Adjust the focus and the relative position of the sample contact surface in a timely manner through the signal collected by the side high-speed camera, and monitor the welding from multiple angles in real time.

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