Method and system for low damage liftoff of gallium nitride substrates using dual pulse width laser

Abstract

This invention belongs to the field of third-generation semiconductor material processing technology. It proposes a method and system for low-damage removal of gallium nitride (GaN) substrates using dual-pulse lasers. First, a double-sided polished bulk GaN substrate is selected, and a common-path removal system using switchable femtosecond and picosecond lasers is constructed. The substrate is fixed on a two-dimensional motion platform. The picosecond laser is incident from the nitrogen surface to a predetermined depth inside the substrate, forming initial modification lines at a first line spacing. Then, a femtosecond laser is used to scan between adjacent initial modification lines at a second line spacing to generate supplementary modification lines, connecting the two types of modification lines to form a continuous composite removal layer. After applying adhesive to both sides of the substrate for fixation, tensile force is applied along the composite removal layer to complete the separation. Finally, an acidic or oxidizing cleaning solution combined with ultrasonic cleaning is used to remove residual gallium metal from the removal surface. After rinsing and drying, the finished product is obtained. This invention is suitable for removing bulk GaN substrates without natural interfaces and optimizes the quality of the removal interface.

Description

Technical Field

[0001] This invention relates to the field of third-generation semiconductor material processing technology, and in particular to a method and system for low-damage stripping of gallium nitride substrates using dual-pulse-width lasers. Background Technology

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Gallium nitride (GaN), as a core material of third-generation wide-bandgap semiconductors, possesses excellent electrical and thermal properties and is widely used in power electronics, radio frequency, and optoelectronic devices. Traditional substrate processing mainly relies on mechanical thinning and polishing. With the development of laser processing technology, laser lift-off is gradually being used for the separation of GaN-related structures. Existing laser lift-off schemes mostly target the inherent interface between the GaN epitaxial layer and the heterostructure substrate for simultaneous beam-combined processing. There are also process routes that use a single-pulse-width laser to modify the interior of the substrate. Various laser processing methods have gradually replaced some mechanical processing procedures and become the mainstream R&D direction for the fine separation of GaN substrates. The industry continues to explore low-damage and controllable lift-off processes adapted to bulk GaN substrates.

[0004] Existing laser lift-off processes struggle to stably construct a complete and continuous lift-off path within bulk gallium nitride substrates. The superposition of energy from synchronously combined laser beams can easily induce disordered expansion of defects within the substrate. Single-pulse laser processing cannot simultaneously address the continuity of the modified region and the need for thermal impact suppression. It is also impossible to form an orderly layered weakening structure at a predetermined depth in the substrate through step-by-step differentiated scanning. Furthermore, it is difficult to rely on the space reserved for gas release during step-by-step scanning to form a unified lift-off layer by connecting adjacent modified regions. This process is unsuitable for processing scenarios where bulk gallium nitride substrates lack natural separation interfaces. It is also difficult to achieve smooth lift-off along a predetermined plane while simultaneously controlling residual metal defects at the lift-off interface, which is detrimental to obtaining gallium nitride substrate wafers with lower damage levels and stable separation morphology. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, such as large mechanical damage and high material loss in GaN substrate processing, and the difficulty of directly applying existing laser lift-off techniques to low-damage separation of GaN substrates, a method and system for low-damage lift-off of gallium nitride substrates using dual-pulse lasers is provided. By using picosecond and femtosecond lasers in a stepwise synergistic effect, a continuous lift-off layer is constructed at a predetermined depth inside the GaN substrate. First, a picosecond laser is used to scan and process the substrate with a large line spacing to form an initial modified region inside the GaN substrate. Then, a femtosecond laser is used to perform supplementary scanning with a smaller line spacing to connect adjacent modified regions and form a continuous composite lift-off layer. Finally, combined with external force separation, low-damage and controllable lift-off of the GaN substrate along the predetermined lift-off layer is achieved.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for low-damage stripping of gallium nitride substrates using a dual-pulse-width laser.

[0007] A method for low-damage lift-off of gallium nitride substrates using dual-pulse lasers includes the following steps: Provide a bulk gallium nitride substrate with polished nitrogen and gallium surfaces, and build a dual-pulse laser lift-off system equipped with a switchable picosecond and femtosecond laser, a focusing optical system and a two-dimensional motion platform; A gallium nitride substrate is fixed on a two-dimensional motion platform. A picosecond laser is incident from the nitrogen surface and focused to a preset depth inside the gallium nitride substrate. Multiple initial modification lines are generated at intervals along the specified crystal orientation of the gallium nitride substrate with a first line spacing. A femtosecond laser is incident from the nitrogen surface and focused to a preset depth. It scans parallel to the adjacent initial modification lines with a second line spacing to generate a supplementary modification line. All initial modification lines and supplementary modification lines are connected to form a continuous composite exfoliation layer. After the nitrogen and gallium surfaces of the gallium nitride substrate are bonded together with adhesive, a tensile force is applied to separate them along the composite release layer. After stripping, the gallium nitride substrate is removed by immersion in an acidic or oxidizing cleaning solution at room temperature combined with ultrasonic cleaning. The remaining gallium metal on the stripped surface is then removed by rinsing and drying to complete the process.

[0008] In one implementation of the first aspect of the present invention, the gallium nitride substrate is a cuboid block structure, and the picosecond laser and femtosecond laser are incident perpendicularly on the crystal surface of the gallium nitride substrate. Both sides of the gallium nitride substrate are polished to reduce laser incident scattering loss. The preset depth is set based on the target peel thickness. The preset depth is the formation position of the composite peel layer inside the gallium nitride substrate.

[0009] In one implementation of the first aspect of the present invention, the first line spacing is the distance between the center lines of adjacent initial modification lines, the second line spacing is the distance between the supplementary modification line and the adjacent initial modification line, and the value of the second line spacing is half of the value of the first line spacing.

[0010] As a further limitation of the first aspect of the present invention, a range of values ​​for the first line spacing is defined. The lower limit of the first line spacing range is used to avoid void defects caused by thermal superposition of adjacent scans. The upper limit of the first line spacing range is used to ensure that the femtosecond laser can connect adjacent initial modification lines. The interval region formed by the first line spacing is used to retain gas release channels to weaken the superposition of heat accumulation.

[0011] As a further limitation of the first aspect of the present invention, a second line spacing range is defined. The lower limit of the second line spacing range is used to avoid damage to the peeling layer caused by local energy density, and the upper limit of the second line spacing range is used to ensure that the supplementary modification line can induce crack generation and connect the initial modification lines on both sides.

[0012] In one implementation of the first aspect of the present invention, the picosecond laser and the femtosecond laser are uniformly set with pulse repetition frequency and scanning speed within a fixed range, and the uniformity of energy deposition inside the gallium nitride substrate is controlled by matching the pulse repetition frequency and scanning speed; the picosecond laser scan travels along a specific crystal orientation of the gallium nitride substrate, and the femtosecond laser scanning direction is kept parallel to the picosecond laser scanning direction.

[0013] In one implementation of the first aspect of the present invention, a two-dimensional motion platform is used in conjunction with an electrically operated flip-up mirror to switch the optical path. The electrically operated flip-up mirror is located at the intersection of the light outputs of the picosecond laser and the femtosecond laser, and a single laser is time-divisionally guided into the focusing optical system to complete step-by-step scanning.

[0014] In one implementation of the first aspect of the present invention, ultrasonic cleaning relies on the cavitation effect to desorb the residual gallium metal on the stripping surface. After cleaning, the gallium nitride wafer is rinsed with deionized water, acetone or isopropanol in sequence, and then dried.

[0015] In one implementation of the first aspect of the present invention, after the substrate separation is completed, an organic solvent is used to remove the adhesive from the stripped gallium nitride wafer, and then it is sent to a cleaning process to remove the adhering material on the stripped surface. When the adhesive is applied and bonded, the two sides of the gallium nitride substrate are constrained as a whole. When a pulling force is applied, the separation is completed along a single plane of the composite stripping layer.

[0016] Secondly, the present invention provides a system for low-damage stripping of gallium nitride substrates using dual-pulse laser.

[0017] A system for low-damage removal of gallium nitride substrates using dual-pulse-width lasers, utilizing the method for low-damage removal of gallium nitride substrates using dual-pulse-width lasers as described in the first aspect of the present invention, includes a picosecond laser source, a picosecond laser source moving device, a femtosecond laser source, a femtosecond laser source moving device, an electrically rotating mirror frame, a two-dimensional platform, and a removal device. The motorized flip-up mirror frame switches between two laser common-path outputs in a time-division manner, enabling step-by-step processing of large-pitch pre-scanning and small-pitch supplementary scanning. The picosecond laser source moving device and the femtosecond laser source moving device control the corresponding laser sources to move along preset trajectories. The two-dimensional platform supports and fixes the gallium nitride substrate, and the peeling device applies a separation force to the gallium nitride substrate after dual laser scanning.

[0018] Compared with the prior art, the beneficial effects of the present invention are: This invention utilizes picosecond and femtosecond laser pulse widths to perform stepwise substrate internal modification processing, enabling the construction of a continuous composite lift-off layer at a predetermined depth within a bulk gallium nitride substrate. This alleviates the difficulty of balancing the continuity of modification and thermal impact control in single-pulse laser processing. Picosecond lasers use a large line spacing to first scan and generate initially modified lines with spaced distribution. Channels for decomposition gas to escape are left between adjacent modified lines, reducing the disorderly expansion of internal defects. Femtosecond lasers supplement the scanning in the middle region of the initial modified lines, connecting the dispersed initial modified regions to form a complete and unified peeling path, which is suitable for processing scenarios where there is no natural separation interface on the bulk gallium nitride substrate. After processing, the substrate is separated by adhesive bonding and tensile force, which can maintain the integrity of the overall substrate structure. Subsequently, the peeling interface is treated with a suitable cleaning solution and ultrasonic cleaning, which reduces the amount of gallium metal deposits left on the peeling surface. The step-by-step scanning is completed by using two lasers with switchable optical paths to stably control the energy deposition state inside the substrate, so that the substrate can be smoothly delaminated along the preset plane, optimize the forming state of the peeling interface, reduce various internal and surface damages generated during the substrate separation process, and improve the interface quality of the finished gallium nitride substrate after peeling.

[0019] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0021] Figure 1 A schematic flowchart of a method for low-damage removal of gallium nitride substrate using dual-pulse laser, provided as an exemplary embodiment of the present invention; Figure 2 A schematic diagram of a gallium nitride substrate provided for an exemplary embodiment of the present invention; Figure 3 A schematic diagram of a stripping system provided for an exemplary embodiment of the present invention; Figure 4 A schematic diagram of picosecond laser large-pitch scanning provided as an exemplary embodiment of the present invention; Figure 5 A schematic diagram of a femtosecond laser small-pitch scanning forming a continuous peeling layer provided as an exemplary embodiment of the present invention; Figure 6 A schematic diagram of a stripping device provided for an exemplary embodiment of the present invention. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0024] This invention provides a method for low-damage lift-off of gallium nitride (GaN) substrates using dual-pulse lasers. The core concept of this method lies in leveraging the absence of natural separation interfaces within bulk GaN substrates. A step-by-step collaborative processing strategy using picosecond and femtosecond lasers is employed to artificially construct a continuous and controllable composite lift-off layer at a predetermined depth within the GaN substrate. Specifically, a picosecond laser is first used for pre-scanning with a large line spacing to form initially distributed modification lines within the GaN substrate. Picosecond lasers possess high processing efficiency and strong internal modification capabilities. Their pulse energy can induce localized nonlinear absorption and multiphoton ionization processes within the GaN substrate, causing structural weakening and microcrack initiation in the focused region of the substrate material.

[0025] Because picosecond lasers have relatively long pulse widths and significant thermal effects, dense scanning with small gaps can easily lead to heat accumulation between adjacent scanning areas. Simultaneously, gallium nitride (GaN) undergoes localized decomposition under laser irradiation, producing gaseous products such as nitrogen. If these gases cannot be released in time, they can form large-sized voids within the substrate, leading to localized bulging and cracking. Therefore, this embodiment of the invention employs a larger initial line spacing for picosecond laser pre-scanning, maintaining a certain interval between adjacent initial modification lines. This interval reduces the thermal overlap between adjacent scanning areas and provides a release channel for the gases generated by GaN decomposition, effectively reducing the generation of large-sized voids and localized cracking defects.

[0026] After completing the picosecond laser pre-scan, a femtosecond laser is used to perform a supplementary scan between adjacent initial modification lines with a smaller line spacing. Femtosecond lasers have shorter pulse widths and smaller heat-affected zones, enabling more precise local modification. The ultrashort pulse characteristics of femtosecond lasers give their energy deposition process within the material extremely high spatiotemporal locality, significantly suppressing thermal diffusion effects. This allows for the formation of supplementary modification regions between adjacent initial modification lines without introducing excessive thermal damage.

[0027] By supplementing the scan with a femtosecond laser, new microcracks can be induced between adjacent picosecond laser initial modification lines, promoting the propagation and interconnection of microcracks between adjacent modification regions. Ultimately, the initial modification lines formed by the picosecond laser, the supplementary modification lines formed by the femtosecond laser, and the crack-connected regions together constitute a continuous composite release layer. This release layer forms a macroscopically continuous and microscopically controllable weakening plane within the gallium nitride substrate, serving as a predetermined path for subsequent external force separation.

[0028] Compared to methods that use picosecond or femtosecond lasers alone, the dual-pulse laser stepwise collaborative processing strategy of this invention can simultaneously ensure the continuity of the lift-off layer, damage suppression, and processing efficiency. The initial modification line formed in the picosecond laser pre-scanning stage provides the basic weakening structure and gas release channel, while the femtosecond laser supplementary scanning stage realizes the fine connection between adjacent modification regions. The two work together to construct a continuous, low-damage composite lift-off layer inside the gallium nitride substrate, laying the foundation for subsequent controllable separation.

[0029] In this embodiment of the invention, the gallium nitride substrate is a bulk gallium nitride sample with nitrogen and gallium surfaces arranged opposite to each other. Gallium nitride crystals belong to the hexagonal wurtzite crystal system and have polarity along the c-axis, with the nitrogen and gallium surfaces corresponding to two opposite directions of the c-axis. During laser lift-off processing, the laser is typically incident from the nitrogen surface because the nitrogen surface has higher chemical stability and lower surface state density than the gallium surface, which is beneficial for stable laser energy transmission and focusing.

[0030] Both the nitrogen and gallium surfaces of the gallium nitride substrate are polished. Polishing significantly reduces the surface roughness of the substrate, minimizing surface scattering and energy fluctuations, thereby improving the focusing accuracy and energy deposition uniformity of the laser at a predetermined depth within the substrate. Simultaneously, polishing removes scratches, initial cracks, and residual damage layers introduced during surface processing, reducing the probability of disordered crack propagation and edge chipping during subsequent separation. Furthermore, the smooth surface after polishing facilitates uniform adhesive application and stable sample fixation during subsequent adhesive coating and fixation operations, and also facilitates the characterization and evaluation of the peeled surface quality.

[0031] like Figure 1 As shown in the figure, the method for low-damage removal of gallium nitride substrates using dual-pulse lasers provided in this invention includes the following steps: providing a gallium nitride substrate, establishing a dual-pulse laser removal system, pre-scanning with a large picosecond laser spacing, supplementary scanning with a small femtosecond laser spacing, fixing with adhesive and applying external force for separation, and cleaning to remove residual gallium metal. The entire process, from workpiece preparation, system setup, dual-laser step-by-step processing, substrate separation to cleaning, forms a complete processing chain.

[0032] Example 1 See Figures 1 to 6This embodiment provides a method for low-damage removal of gallium nitride substrates using a dual-pulse laser, the method comprising the following steps: S101: Provides a gallium nitride substrate.

[0033] like Figure 2 As shown, the gallium nitride substrate is a bulk gallium nitride sample with a cuboid shape. The gallium nitride substrate has nitrogen and gallium faces arranged opposite each other; the nitrogen face is labeled as follows: The gallium face is marked as the corresponding crystal face. The nitrogen and gallium surfaces of the gallium nitride substrate are both polished to form a polished surface.

[0034] Gallium nitride (GaN) crystals belong to the hexagonal wurtzite crystal system and exhibit significant anisotropy. In laser lift-off processing, crystal orientation has a crucial impact on crack propagation behavior and lift-off quality. For example... Figure 2 As shown, a three-dimensional crystal orientation coordinate system is established on the gallium nitride substrate, with the three coordinate axes corresponding to the crystal orientation indices. , , .

[0035] In this embodiment, the laser scanning direction is selected along... The laser scanning is performed along the crystal orientation. This crystal orientation corresponds to the m-axis of the gallium nitride crystal. When laser scanning is performed along this direction, the crack propagation path is relatively controllable, which is beneficial for forming straight and continuous modification lines. At the same time, scanning along this crystal orientation can reduce crack bifurcation and disordered propagation caused by crystal anisotropy, thereby improving the continuity of the release layer and the flatness of the release surface.

[0036] S102: Establish a dual-pulse laser ablation system.

[0037] like Figure 3 As shown, the dual-pulse laser ablation system adopts a common-path time-division switching structure design. The system mainly includes a picosecond laser, a femtosecond laser, an electrically operated tilting mirror frame, a half-wave plate, a beam expander, a reflector, a focusing objective lens, and a two-dimensional motion platform.

[0038] Picosecond and femtosecond lasers are positioned above and below each other at the intersection of the optical paths. A motorized tilting mirror is located at the intersection of the two laser output paths. By electrically controlling the tilting angle of the mirror, either the picosecond or femtosecond laser can be selectively guided into the subsequent common processing optical path. When picosecond laser scanning is required, the motorized tilting mirror rotates to the corresponding angle, allowing the picosecond laser to enter the common optical path through mirror reflection; when femtosecond laser scanning is required, the motorized tilting mirror rotates to the other angle, allowing the femtosecond laser to enter the common optical path through mirror reflection.

[0039] After entering the common optical path, the laser beam passes sequentially through a half-wave plate, a beam expander, and a reflector. The half-wave plate adjusts the laser's polarization direction to match the polarization requirements of subsequent optical components. The beam expander increases the laser beam diameter, reduces the laser power density, avoids damage to the optical component surfaces, and provides a suitable incident beam size for the subsequent focusing objective. The reflector adjusts the optical path direction, ensuring the laser beam is perpendicularly incident on the focusing objective.

[0040] The focusing objective lens focuses the laser beam to a predetermined depth within the gallium nitride substrate. The numerical aperture and focal length of the focusing objective lens determine the size and positional accuracy of the laser focus. In this embodiment, an objective lens with a suitable numerical aperture is selected to match the laser focus size with the target modification linewidth, while ensuring a sufficient working distance so that the laser can penetrate the surface of the gallium nitride substrate and be focused to the predetermined depth.

[0041] A two-dimensional motion platform is used to support and fix a gallium nitride (GaN) substrate and control its movement along a predetermined path in a horizontal plane. Through precise motion control of the two-dimensional motion platform, the relative scanning motion of the laser focus with respect to the GaN substrate can be achieved, thereby forming modified lines of a predetermined shape and distribution inside the substrate.

[0042] The system design of this common-path time-division switching has the following advantages: First, the two lasers share the same focusing optics system and motion platform, which simplifies the system structure and reduces equipment costs; second, the rapid switching between the two lasers is achieved through an electrically rotating mirror frame, which improves processing efficiency; finally, the common-path design ensures that the focal positions of the picosecond laser and the femtosecond laser are highly consistent, which is conducive to forming a continuous and aligned composite stripping layer.

[0043] S103: Picosecond laser large-pitch pre-scan.

[0044] A gallium nitride (GaN) substrate is fixed on a two-dimensional motion platform with the nitrogen-side of the substrate facing upwards. The motorized rotating mirror mount is adjusted to allow the picosecond laser to enter the common optical path. The picosecond laser is incident perpendicularly from the nitrogen-side of the GaN substrate, and the focal point of the picosecond laser is focused to a predetermined depth inside the GaN substrate by a focusing objective.

[0045] like Figure 4 As shown, the preset depth refers to the formation position of the composite release layer inside the gallium nitride substrate, and this depth is set according to the target release thickness. For example, if it is necessary to release a gallium nitride wafer of a certain thickness from the gallium nitride substrate, the preset depth is set to the position at the corresponding distance from the nitrogen surface of the gallium nitride substrate. By precisely controlling the position of the focusing objective, the laser focus can be accurately positioned at the preset depth.

[0046] Controlling the two-dimensional motion platform to move the gallium nitride substrate along The crystal orientation is scanned relative to the laser focal point. During the scan, the picosecond laser forms an initial modification line at a predetermined depth within the gallium nitride substrate. After scanning the first initial modification line, the two-dimensional motion platform moves a distance equal to the first line spacing in a direction perpendicular to the scanning direction, and then scans the second initial modification line. This process is repeated to form multiple parallel initial modification lines spaced apart within the gallium nitride substrate.

[0047] The first line spacing refers to the distance between the center lines of adjacent picosecond laser initial remodeling lines. The selection of the first line spacing needs to achieve a balance between the pre-weakening effect of the initial remodeling lines, thermal effect control, and gas release space.

[0048] The mechanisms by which picosecond lasers interact with gallium nitride (GaN) substrates primarily involve nonlinear absorption, multiphoton ionization, plasma formation, and thermal effects. When a picosecond laser pulse is focused into a GaN substrate, due to the large bandgap of GaN, the energy of a single photon is insufficient to directly excite electron transitions. Therefore, energy deposition is mainly achieved through a multiphoton absorption process. Under the influence of a high-intensity laser field, electrons in the GaN material gain sufficient energy by simultaneously absorbing multiple photons, transitioning from the valence band to the conduction band, forming free electrons and holes.

[0049] As the density of free electrons increases, electrons further absorb laser energy through inverse bremsstrahlung, leading to a sharp increase in electron temperature and the formation of high-temperature, high-density plasma. The formation and expansion of the plasma exert strong mechanical and thermal shocks on the surrounding materials, causing structural weakening, microcrack initiation, and local phase transitions in the focal region.

[0050] Because picosecond lasers have relatively long pulse widths, the heat generated during the laser process has time to diffuse to the surrounding area, forming a heat-affected zone (HAZ). If the initial modification lines of adjacent picosecond lasers are too close, the HAZs of adjacent scanning areas will overlap, leading to an enhanced cumulative thermal effect. Excessive heat accumulation can cause excessive thermal decomposition of the gallium nitride substrate near the modification zone, generating a large number of gaseous products, and may lead to localized melting, recrystallization, and stress concentration.

[0051] Meanwhile, gallium nitride undergoes a decomposition reaction at high temperatures, producing nitrogen gas and metallic gallium. As a gaseous product, nitrogen gas, if not released from the remodeling region in time, will accumulate inside the substrate, forming bubbles or voids. If adjacent initial remodeling lines are too close together, the gases generated in adjacent remodeling regions will overlap, increasing gas pressure, leading to larger voids, and even causing localized bulging and cracking.

[0052] Therefore, this embodiment employs a larger initial line spacing for picosecond laser pre-scanning, maintaining a certain interval between adjacent initial modification lines. This interval region can, on the one hand, reduce the superposition of thermal effects between adjacent scanning regions and reduce cumulative thermal effects; on the other hand, it can provide a release channel for the gas generated by gallium nitride decomposition, allowing the gas to diffuse and release along the interval region to the substrate surface or edge, thereby effectively reducing the generation of large-size voids and localized fracture defects.

[0053] However, the initial line spacing cannot be too large. If the initial line spacing is too large, the distance between adjacent picosecond laser initial modification lines will be too far, significantly increasing the difficulty for induced cracks to effectively connect between adjacent modification lines during subsequent femtosecond laser supplementary scanning. This is because the crack propagation capability is limited by the material's fracture toughness and stress field distribution. If the distance between adjacent modification lines exceeds the effective crack propagation range, even if the femtosecond laser forms a supplementary modification zone in the middle region, it will be difficult to induce cracks to penetrate adjacent initial modification lines, resulting in discontinuous regions in the composite exfoliation layer, affecting the stability of subsequent separation and the quality of the exfoliated surface.

[0054] Therefore, the selection of the first line spacing needs to comprehensively consider thermal impact control, gas release space, and crack connectivity. By reasonably setting the first line spacing, favorable conditions can be created for subsequent femtosecond laser supplementary scanning while ensuring the initial pre-weakening effect of the modified lines, ultimately forming a continuous, low-damage composite exfoliation layer.

[0055] In picosecond laser pre-scanning, the selection of laser parameters has a significant impact on the modification effect. Laser power determines the energy deposited in the material per unit time; too low a power results in insignificant modification, while too high a power may lead to excessive ablation and thermal damage. Pulse repetition frequency determines the number of laser pulses per unit time; too low a repetition frequency results in low processing efficiency, while too high a repetition frequency enhances the thermal accumulation effect between adjacent pulses. Scanning speed determines the residence time of the laser focus in the material and the spatial interval between adjacent pulses; too slow a scanning speed results in excessive energy deposition per unit length, while too fast a scanning speed results in insufficient energy deposition.

[0056] By appropriately matching the laser power, pulse repetition frequency, and scanning speed, the energy deposition density and heat accumulation level inside the gallium nitride substrate can be controlled, thereby forming an effective modification line while keeping thermal damage and gas voids within acceptable limits. In this embodiment, the pulse repetition frequency and scanning speed of the picosecond laser are set within a specific range. By matching their values, the uniformity and controllability of energy deposition inside the gallium nitride substrate are achieved.

[0057] S104: Femtosecond laser small-pitch supplementary scanning.

[0058] After completing the picosecond laser large-pitch pre-scan, the motorized tilting mirror mount is adjusted to bring the femtosecond laser into the common optical path. The femtosecond laser is incident perpendicularly from the nitrogen surface of the gallium nitride substrate, and the focusing objective lens focuses the femtosecond laser to the same preset depth as the picosecond laser.

[0059] like Figure 5 As shown, the initial scanning path of the femtosecond laser is positioned midway between two adjacent picosecond laser initial modification lines in the plane. A two-dimensional motion platform is controlled to perform a line scan motion of the gallium nitride substrate relative to the laser focal point along a direction parallel to the initial modification lines. During the scan, the femtosecond laser forms a supplementary modification line between the adjacent picosecond laser initial modification lines.

[0060] The second line spacing refers to the distance between the femtosecond laser supplementary scan line and the adjacent picosecond laser initial remodeling line. Since the initial scan path of the femtosecond laser is located in the middle of two adjacent picosecond laser initial remodeling lines, the value of the second line spacing is half that of the first line spacing. This design allows the femtosecond laser supplementary remodeling line to equally divide the interval region between adjacent picosecond laser initial remodeling lines into two parts, thereby achieving uniform connection between adjacent remodeling regions.

[0061] The mechanism of femtosecond lasers inside gallium nitride substrates is similar to that of picosecond lasers, mainly including nonlinear absorption, multiphoton ionization, and plasma formation. However, due to the shorter pulse width of femtosecond lasers, their process has higher spatiotemporal locality and a smaller thermally affected zone.

[0062] The pulse width of femtosecond lasers is typically on the order of tens to hundreds of femtoseconds, much smaller than the thermal diffusion timescale of materials. During the femtosecond laser pulse, laser energy is primarily deposited in the electronic system, causing a sharp rise in electron temperature, while the lattice temperature remains almost constant. After the pulse ends, the high-temperature electrons transfer energy to the lattice through electron-phonon coupling. However, due to the extremely short pulse width, this energy transfer process is strictly limited in both time and space, significantly suppressing the thermal diffusion effect.

[0063] This ultrashort pulse characteristic enables femtosecond lasers to achieve highly localized energy deposition within an extremely small volume, inducing structural phase transitions, microcrack initiation, and stress concentration in materials, while having minimal thermal impact on the surrounding area. Therefore, femtosecond lasers are ideally suited for precision machining and low-damage material modification.

[0064] In this embodiment, the role of the femtosecond laser supplementary scanning is to form a supplementary remodeling region between adjacent picosecond laser initial remodeling lines, and to induce crack propagation and connectivity between adjacent remodeling regions. The remodeling region formed by the femtosecond laser on the supplementary scanning path serves as the starting point for crack initiation, while the strong stress field generated by its ultrashort pulse characteristics can promote crack propagation towards the picosecond laser initial remodeling lines on both sides.

[0065] Since the initial modification lines of the picosecond laser have already created a certain degree of structural weakening and microcracks within the gallium nitride substrate, the cracks generated by the femtosecond laser in the supplementary modification region are more likely to propagate along these pre-weakened areas, ultimately achieving connectivity between adjacent modification lines. In this way, the initial modification lines formed by the picosecond laser, the supplementary modification lines formed by the femtosecond laser, and the crack-connected regions together constitute a macroscopically continuous and microscopically controllable composite exfoliation layer.

[0066] The selection of the second line spacing has a significant impact on the effectiveness of femtosecond laser supplementary scanning. If the second line spacing is too small, the distance between the femtosecond laser supplementary scanning line and the picosecond laser initial remodeling line will be too close, easily causing excessively dense local energy input. Although the heat-affected zone of the femtosecond laser is small, if the distance between the supplementary remodeling line and the initial remodeling line is too close, the stress fields and microcrack regions of the two remodeling regions will overlap, which may lead to excessively strong local stress concentration, causing disordered crack propagation or local fracture. At the same time, the excessively close distance will also increase the accumulation of gas generated by gallium nitride decomposition in local areas, increasing the risk of void defects.

[0067] If the second line spacing is too large, the distance between the femtosecond laser-supplemented modification zone and the adjacent picosecond laser initial modification line will be too far, resulting in insufficient crack connectivity. Although the femtosecond laser can form a modification zone and induce crack initiation on the supplementary scanning path, the driving force for crack propagation to the adjacent initial modification line is insufficient due to the excessive distance, making it difficult to achieve effective connectivity and leading to discontinuous regions in the composite exfoliation layer.

[0068] Therefore, the selection of the second line spacing needs to strike a balance between local energy density control and crack connectivity. By setting the second line spacing to half of the first line spacing, the femtosecond laser supplementary remodeling line is positioned in the middle of the adjacent picosecond laser initial remodeling line. This ensures that there is sufficient distance between the supplementary remodeling region and the initial remodeling line to avoid excessive energy superposition, while also ensuring the effectiveness of crack propagation, thereby achieving reliable connectivity between adjacent remodeling regions.

[0069] Similar to picosecond lasers, the selection of laser parameters for femtosecond lasers also has a significant impact on the modification effect. In this embodiment, the pulse repetition frequency and scanning speed of the femtosecond laser are also set within a specific range and are consistent with or similar to the parameters of the picosecond laser to ensure that the energy deposition density and modification effect of the two lasers inside the gallium nitride substrate are matched, which is beneficial to the formation of a uniform and continuous composite lift-off layer.

[0070] Through a step-by-step collaborative processing method involving picosecond laser large-pitch pre-scanning and femtosecond laser small-pitch supplementary scanning, a continuous composite release layer was formed at a predetermined depth inside the gallium nitride substrate. This composite release layer consists of an initial modification line formed by picosecond laser, a supplementary modification line formed by femtosecond laser, and a crack-connecting region, forming a macroscopically continuous weakening plane inside the gallium nitride substrate, providing a predetermined separation path for subsequent external force separation.

[0071] S105: Apply adhesive to fix and then separate by applying external force.

[0072] After the dual-laser step-by-step processing is completed, a continuous composite release layer has formed inside the gallium nitride substrate, but the substrate as a whole remains intact. In order to achieve controllable separation along the composite release layer, the gallium nitride substrate needs to be fixed with adhesive and external force needs to be applied.

[0073] like Figure 6 As shown, the purpose of adhesive bonding is to adhere the nitrogen and gallium surfaces of the gallium nitride (GaN) substrate to the support substrate or clamp, respectively. This allows the substrate to separate along a single plane of the composite release layer when external force is applied, without disordered breakage or edge chipping. In practice, adhesive is applied to both the nitrogen and gallium surfaces of the GaN substrate, and then the nitrogen and gallium surfaces are respectively attached to the upper and lower clamps. After the adhesive cures, the GaN substrate is firmly fixed between the upper and lower clamps.

[0074] Subsequently, a vertically opposite pulling force is applied to the upper and lower clamps using a separation device. Under the action of the pulling force, the stress concentration at the composite release layer inside the gallium nitride substrate is enhanced, and the cracks rapidly propagate along the composite release layer and penetrate the entire substrate cross section, ultimately achieving complete separation of the gallium nitride substrate along the composite release layer, forming two independent gallium nitride wafers.

[0075] The method of using adhesive coating for fixation combined with external force separation has the following advantages: First, adhesive coating fixation restricts the overall position of the gallium nitride substrate on both sides, avoiding local movement or warping of the substrate during separation and ensuring the stability and controllability of the separation process; second, separation is achieved by applying tension, which, compared with methods such as mechanical cutting or prying, allows for better control of the separation path and reduces edge chipping and disordered fracture; finally, the adhesive coating can absorb the impact and vibration generated during separation to a certain extent, further protecting the gallium nitride wafer from additional damage.

[0076] After separation, an organic solvent is used to detach the two gallium nitride (GaN) wafers from the upper and lower jigs, and a de-adhesive treatment is performed to remove any residual adhesive from the surface of the GaN wafers. Commonly used organic solvents include acetone and isopropanol, which can effectively dissolve the adhesive without corroding or damaging the gallium nitride material itself.

[0077] S106: Cleaning to remove residual gallium metal.

[0078] After the gallium nitride substrate is separated along the composite release layer, a certain amount of metallic gallium usually remains on the release surface. This metallic gallium mainly originates from the decomposition reaction of gallium nitride under laser irradiation. Under laser-induced high temperature and high pressure conditions, gallium nitride decomposes, producing nitrogen gas and metallic gallium. Nitrogen gas, as a gaseous product, can diffuse and be released through the gas release channel, while metallic gallium, as a liquid or solid product, may adhere to the release surface.

[0079] Gallium residue can affect the surface quality of the gallium nitride wafer's stripped surface and its subsequent applications. Therefore, the stripped surface needs to be cleaned to remove gallium residue and other deposits.

[0080] The cleaning process includes two steps: chemical cleaning and ultrasonic cleaning. First, the stripped gallium nitride wafer is immersed in a cleaning solution and treated at room temperature for a period of time. The cleaning solution is preferably an acidic or oxidizing solution capable of removing residual gallium. Acidic cleaning solutions can chemically react with gallium, converting it into soluble salts, thus achieving removal; oxidizing cleaning solutions can oxidize gallium to gallium oxide, which is more easily removed in subsequent cleaning steps than gallium.

[0081] Building upon chemical cleaning, ultrasonic cleaning technology is further employed to remove residual gallium metal from the surface of the stripped area. Ultrasonic cleaning utilizes the cavitation effect generated by ultrasound in a liquid. When ultrasound propagates in a liquid, the periodic changes in sound pressure cause a large number of tiny bubbles to form. These bubbles rapidly grow and collapse under the influence of sound pressure, generating strong localized shock waves and microjets.

[0082] These shock waves and microjets act on the gallium nitride wafer stripping surface, effectively stripping and dispersing the residual gallium metal adhering to the surface, causing it to detach and suspend in the cleaning solution, thereby achieving removal. Compared with methods such as mechanical wiping, ultrasonic cleaning is non-contact, uniform, and efficient, achieving thorough cleaning of the stripping surface without introducing additional mechanical damage.

[0083] After cleaning, the gallium nitride wafer is rinsed with deionized water, acetone, or isopropanol to remove cleaning solution residue and suspended impurities. This rinsing step is typically repeated multiple times to ensure the cleanliness of the gallium nitride wafer surface. Finally, the gallium nitride wafer is dried to remove surface moisture, completing the entire cleaning process.

[0084] The above cleaning process can effectively remove residual gallium metal and other deposits from the surface of gallium nitride wafers, resulting in clean, high-quality gallium nitride wafers, laying the foundation for subsequent device fabrication or other applications.

[0085] This embodiment provides a method for low-damage lift-off of gallium nitride (GaN) substrates using dual-pulse lasers. This method employs a step-by-step collaborative process of picosecond laser pre-scanning with large spacing and femtosecond laser supplementary scanning with small spacing to construct a continuous, low-damage composite lift-off layer within the GaN substrate. Combined with adhesive coating and external force separation, this achieves controllable lift-off of the GaN substrate. This method effectively reduces surface cracks, subsurface damage, and edge chipping caused by traditional machining, improving the lift-off quality and yield of GaN wafers.

[0086] Example 2 Based on Example 1, this embodiment further defines the range of values ​​for the first line spacing and the second line spacing to optimize the continuity of the composite release layer and the quality of the release surface.

[0087] In this embodiment, the first line spacing is within a specific range. The lower limit of this range is used to avoid the problem of void defects caused by thermal superposition during the initial remodeling line scanning of adjacent picosecond lasers, while the upper limit is used to ensure that adjacent initial remodeling lines can be effectively connected during subsequent femtosecond laser supplementary scanning.

[0088] When the initial line spacing is less than the lower limit, the distance between adjacent picosecond laser initial modification lines is too close, and the heat-affected zone generated by the laser and the gas effects generated by the local decomposition of gallium nitride are prone to superimpose. The superposition of the heat-affected zone leads to an enhanced cumulative thermal effect, causing excessive thermal decomposition of the gallium nitride substrate near the modification zone, generating a large amount of gaseous products. The superposition of gas effects increases the gas pressure, causing gas to accumulate in local areas and form large-sized voids. These voids not only affect the continuity of the composite release layer, but may also become the starting point for disordered crack propagation during subsequent separation, leading to local bulging and fracture defects.

[0089] When the initial line spacing exceeds the upper limit, the distance between adjacent picosecond laser initial modification lines becomes too large. During subsequent femtosecond laser supplementary scanning, even if a supplementary modification line is formed in the middle, the induced crack cannot effectively propagate to the adjacent initial modification lines and achieve connectivity. This is because the crack propagation capability is limited by the material's fracture toughness and stress field distribution. When the distance between adjacent modification lines exceeds the effective crack propagation range, the driving force for crack propagation is insufficient, leading to discontinuous regions in the composite exfoliation layer and affecting the stability of subsequent separation.

[0090] By limiting the first line spacing to a specific range, the pre-weakening effect of the initial remodeling line, thermal effect control, and gas release space can be taken into account during the picosecond laser pre-scanning stage. At the same time, it creates favorable conditions for subsequent femtosecond laser supplementary scanning, ensuring that adjacent remodeling zones can be effectively connected to form a continuous composite exfoliation layer.

[0091] Meanwhile, the gaps formed by the first line spacing act as gas release channels during the picosecond laser pre-scanning process. Nitrogen gas generated by the decomposition of gallium nitride under laser irradiation can diffuse and release along these gaps towards the substrate surface or edge, preventing excessive gas accumulation within the modified region. This gas release mechanism effectively mitigates the cumulative effect of heat accumulation, reduces the generation of large-size voids and localized fracture defects, and improves the quality of the composite release layer.

[0092] In this embodiment, the range of values ​​for the second line spacing is also limited to a specific range. The lower limit of this range is used to avoid damage to the exfoliation layer caused by local energy density, while the upper limit is used to ensure that the femtosecond laser supplementary remodeling line can induce crack generation and connect the picosecond laser initial remodeling lines on both sides.

[0093] When the second line spacing is less than the lower limit, the distance between the femtosecond laser supplementary scanning line and the picosecond laser initial remodeling line is too close, which can easily lead to excessively dense local energy input. Although the heat-affected zone of the femtosecond laser is small, if the supplementary remodeling line and the initial remodeling line are too close, the stress fields and microcrack regions of the two remodeling regions will overlap, which may lead to excessive local stress concentration, causing disordered crack propagation or local fracture. At the same time, the excessively close distance will also increase the accumulation of gas generated by gallium nitride decomposition in local areas, increasing the risk of void defects and damage to the release layer.

[0094] When the second line spacing exceeds the upper limit, the distance between the femtosecond laser-supplemented remodeling zone and the adjacent picosecond laser initial remodeling line is too large, resulting in insufficient crack connectivity. Although the femtosecond laser can form a remodeling zone and induce crack initiation on the supplementary scanning path, the driving force for crack propagation to the adjacent initial remodeling line is insufficient due to the excessive distance, making effective connectivity difficult and leading to discontinuous regions in the composite exfoliation layer.

[0095] By limiting the second line spacing to a specific range, the femtosecond laser supplementary scanning can form an effective supplementary remodeling zone between adjacent picosecond laser initial remodeling lines, and induce crack propagation and connectivity between adjacent remodeling lines, while avoiding damage to the exfoliation layer caused by excessive local energy, thereby forming a continuous, low-damage composite exfoliation layer.

[0096] In some preferred embodiments, the first line spacing is a value near the middle of the interval, and the second line spacing is half of the first line spacing. This combination is located in the middle region of their respective value intervals, which can achieve a better balance between the connectivity of adjacent modified regions, crack penetration capability, thermal impact control, and gas release space. It can avoid the thermal impact and gas void superposition caused by excessively small line spacing, and avoid insufficient crack connection caused by excessively large line spacing. This is conducive to the formation of a continuous composite release layer with fewer defects, thereby improving the release stability and release surface quality of gallium nitride substrates.

[0097] Example 3 Based on Example 1, this embodiment further illustrates the matching relationship between the pulse repetition frequency and scanning speed of picosecond and femtosecond lasers, as well as the relationship between the laser scanning direction and the orientation of gallium nitride crystals.

[0098] In this embodiment, both picosecond and femtosecond lasers are configured with a fixed pulse repetition frequency and scanning speed within a fixed range. The pulse repetition frequency determines the number of laser pulses per unit time, while the scanning speed determines the residence time of the laser focus in the material and the spatial interval between adjacent pulses. By matching the pulse repetition frequency and scanning speed, the uniformity of energy deposition within the gallium nitride substrate can be controlled.

[0099] Specifically, the ratio of pulse repetition frequency to scanning speed determines the spacing between the interaction sites of adjacent laser pulses in the material; this spacing is called the pulse overlap ratio. When the pulse overlap ratio is small, the interaction regions between adjacent pulses are large, which may lead to discontinuities in the modification lines; when the pulse overlap ratio is large, the interaction regions between adjacent pulses overlap more, which may lead to excessively dense local energy deposition and enhanced heat accumulation.

[0100] By setting the pulse repetition frequency and scanning speed within a specific range and appropriately matching their values, the interaction positions of adjacent laser pulses in the material can maintain an appropriate spacing and overlap, thereby achieving uniformity and controllability of energy deposition within the gallium nitride substrate. Uniform energy deposition is beneficial for forming continuous and stable modified lines, avoiding breakage or excessive local damage to the modified lines caused by uneven energy distribution.

[0101] In this embodiment, a picosecond laser scan travels along a specific crystal orientation of the gallium nitride substrate. As previously mentioned, gallium nitride crystals belong to the hexagonal wurtzite structure and exhibit significant anisotropy. In laser lift-off processing, crystal orientation has a significant impact on crack propagation behavior and lift-off quality.

[0102] When laser scanning is performed along a specific crystal orientation, the crack propagation path is relatively controllable, which is beneficial for forming straight and continuous modification lines. This crystal orientation is the a-axis direction of gallium nitride crystals. When laser scanning is performed along this direction, cracks tend to propagate along the crystal plane rather than through the crystal, thereby reducing the risk of crack bifurcation and disordered propagation, and improving the continuity of the release layer and the flatness of the release surface.

[0103] The femtosecond laser scanning direction is parallel to the picosecond laser scanning direction. This design ensures that the femtosecond laser supplementary modification line and the picosecond laser initial modification line are consistent in crystal orientation, which is beneficial for crack propagation and connection along the same crystal direction, thereby forming a composite exfoliation layer with consistent orientation and good continuity.

[0104] Meanwhile, the parallel scanning design simplifies the programming and operation of the motion control system, improving processing efficiency. In actual processing, parallel scanning of picosecond and femtosecond lasers can be achieved simply by controlling the two-dimensional motion platform to move along two mutually perpendicular directions, without the need for complex angle adjustments or path planning.

[0105] Example 4 Based on Example 1, this embodiment further illustrates the working principle and time-division switching mechanism of the electrically reversible frame in the dual-pulse laser ablation system.

[0106] As previously mentioned, the dual-pulse laser ablation system employs a common-path time-division switching design. The motorized flip-up mirror frame is a key component for achieving time-division switching, and it is located at the intersection of the picosecond and femtosecond laser outputs.

[0107] A motorized reversible mirror frame typically consists of a rotatable mirror and a motorized drive. The mirror has a high reflectivity, effectively reflecting picosecond and femtosecond lasers. The motorized drive precisely controls the mirror's rotation angle, thereby changing the mirror's orientation.

[0108] When picosecond laser scanning is required, the electric drive unit controls the reflector to rotate to a specific angle, so that the laser beam emitted from the picosecond laser is incident on the mirror surface and enters the subsequent common processing optical path through mirror reflection. At this time, the laser beam emitted from the femtosecond laser cannot enter the common processing optical path due to the obstruction of the reflector or the mismatch of the angle, thus realizing the independent conduction of the picosecond laser.

[0109] When femtosecond laser scanning is required, the electric drive unit controls the reflector to rotate to a specific angle, so that the laser beam emitted from the femtosecond laser is incident on the mirror surface and enters the subsequent common processing optical path through mirror reflection. At this time, the laser beam emitted from the picosecond laser cannot enter the common processing optical path, thus realizing the independent conduction of the femtosecond laser.

[0110] By switching the motorized flip-up frame in time, picosecond and femtosecond lasers can be processed in steps on the same focusing optical system and motion platform. This simplifies the system structure, reduces equipment costs, and ensures that the focal positions of the two lasers are highly consistent, which is beneficial for forming a continuous and aligned composite stripping layer.

[0111] The switching speed of the electrically operated flip-up mirror frame has a certain impact on processing efficiency. In practical applications, electric drive devices with fast response speed and high positioning accuracy are usually selected to achieve rapid switching between picosecond lasers and femtosecond lasers, reduce waiting time during the switching process, and improve overall processing efficiency.

[0112] Example 5 Based on Example 1, this embodiment further illustrates the mechanism of cavitation effect during ultrasonic cleaning, as well as the selection principles for cleaning and rinsing solutions.

[0113] As mentioned earlier, after the gallium nitride substrate is separated along the composite release layer, a certain amount of metallic gallium will remain on the release surface. To remove these metallic gallium residues, a combination of chemical cleaning and ultrasonic cleaning methods is required.

[0114] Ultrasonic cleaning utilizes the cavitation effect generated by ultrasound waves in liquids to achieve its cleaning function. Cavitation refers to the phenomenon where, as ultrasound waves propagate in a liquid, the periodic changes in sound pressure cause the formation of numerous tiny bubbles. In the negative pressure phase of the sound wave, the local pressure in the liquid drops below its saturated vapor pressure, causing gases originally dissolved in the liquid to precipitate and form bubbles. In the positive pressure phase, the bubbles are compressed, their volume rapidly decreasing. When the bubble size shrinks to a certain extent, the bubbles violently collapse.

[0115] The collapse of a bubble releases enormous energy, creating extremely high temperatures and pressures near the collapse point, while simultaneously generating powerful shock waves and microjets. These shock waves and microjets act on solid surfaces, effectively stripping and dispersing contaminants adhering to the surface.

[0116] For gallium residue on the surface of gallium nitride wafers, the cavitation effect generated during ultrasonic cleaning can detach the gallium from the surface, causing it to detach and suspend in the cleaning solution. Compared to contact cleaning methods such as mechanical wiping, ultrasonic cleaning is non-contact, uniform, and efficient, achieving thorough cleaning of the detached surface without introducing additional mechanical damage.

[0117] The choice of cleaning solution has a significant impact on the cleaning effect. The preferred cleaning solution is an acidic or oxidizing solution capable of removing gallium residue. Acidic cleaning solutions can react chemically with gallium, converting it into soluble salts, thereby achieving removal. Commonly used acidic cleaning solutions include dilute solutions of inorganic acids such as hydrochloric acid, nitric acid, and sulfuric acid, as well as solutions of organic acids such as citric acid and oxalic acid.

[0118] Oxidizing cleaning solutions can oxidize metallic gallium into gallium oxide. Gallium oxide has higher chemical stability than metallic gallium, but its adhesion to gallium nitride substrates is weaker, making it easier to remove by subsequent ultrasonic cleaning or mechanical rinsing. Commonly used oxidizing cleaning solutions include hydrogen peroxide solution and potassium permanganate solution.

[0119] When selecting a cleaning solution, it is necessary to comprehensively consider the solution's gallium removal efficiency, its corrosiveness to gallium nitride, and its safety and environmental friendliness. Generally, a cleaning solution with high gallium removal efficiency, low corrosiveness to gallium nitride, and safety and environmental friendliness is chosen.

[0120] After cleaning, the gallium nitride wafer needs to be rinsed with deionized water, acetone, or isopropanol to remove cleaning solution residue and suspended impurity particles. Deionized water is the most commonly used rinsing solution, characterized by high purity and absence of impurity ions, effectively removing cleaning solution residue. Acetone and isopropanol are commonly used organic solvents with good solubility and volatility, capable of removing organic contaminants from the surface and rapidly evaporating during the drying process, leaving no residue.

[0121] The rinsing process typically needs to be repeated multiple times, with a fresh rinsing solution after each rinse to ensure the required cleanliness of the gallium nitride wafer surface. Finally, the gallium nitride wafer is dried to remove surface moisture, completing the entire cleaning process.

[0122] Example 6 Based on Example 1, this embodiment further illustrates the specific implementation method of the adhesive bonding and fixing operation, as well as the protective effect of the adhesive layer on the separation process.

[0123] As previously stated, after completing the dual-laser step-by-step processing, the gallium nitride substrate needs to be coated with adhesive to fix it, and external force is applied to achieve separation. The purpose of the adhesive bonding and fixing operation is to bond the nitrogen and gallium surfaces of the gallium nitride substrate to the support substrate or jig respectively, so that the substrate can be separated along a single plane of the composite release layer when external force is applied.

[0124] In practice, adhesive is first applied to both the nitrogen and gallium sides of the gallium nitride substrate. The adhesive should meet the following requirements: it should have good bonding strength to firmly fix the gallium nitride substrate to the fixture; it should have a certain degree of flexibility to absorb impact and vibration during separation; after curing, it should be able to withstand a certain amount of tensile force and not fall off prematurely during separation; and after curing, it should be soluble in organic solvents for easy removal of the adhesive.

[0125] Commonly used adhesives include epoxy resin adhesives, acrylic adhesives, and hot melt adhesives. When applying the adhesive, it is necessary to ensure that the adhesive is evenly distributed on the surface of the gallium nitride substrate, avoiding areas that are too thick or too thin. After the adhesive is applied, the nitrogen and magnesium sides of the gallium nitride substrate are respectively attached to the upper and lower clamps, and a certain pressure is applied to ensure that the adhesive makes full contact with the surfaces of the gallium nitride substrate and the clamps.

[0126] The adhesive is then cured. The curing method depends on the type of adhesive; common methods include room temperature curing, heat curing, and ultraviolet curing. After curing, the gallium nitride substrate is firmly fixed between the upper and lower clamps, forming a single integrated structure.

[0127] The adhesive bonding process confines the gallium nitride substrate on both sides, preventing it from shifting or warping under external force. This ensures the separation process proceeds along a single plane of the composite release layer. Without adhesive bonding, the gallium nitride substrate may shift, warp, or rotate under external force, causing the separation path to deviate from the composite release layer and resulting in disordered fractures or edge chipping.

[0128] Meanwhile, the adhesive layer also acts as a buffer and protector during the separation process. When tensile force is applied, stress concentration intensifies at the composite release layer inside the gallium nitride substrate, causing cracks to propagate rapidly and penetrate the entire substrate cross-section. At the moment the crack penetrates, a certain amount of impact and vibration is generated. Due to its flexibility, the adhesive layer can absorb these impacts and vibrations, reducing their impact on the gallium nitride wafer and thus protecting it from further damage.

[0129] After separation, the stripped gallium nitride (GaN) wafers are treated with organic solvents to remove the adhesive. Commonly used organic solvents include acetone, isopropanol, and xylene. During the removal process, the GaN wafer is immersed in the organic solvent, and the adhesive gradually dissolves under the action of the solvent, allowing the GaN wafer to detach from the fixture. After removal, the GaN wafer is cleaned and dried to remove residual solvent and adhesive from the surface, completing the entire coating, fixation, and separation process.

[0130] Example 7 This embodiment provides a system for low-damage removal of gallium nitride substrates using a dual-pulse-width laser. This system is used to implement the method for low-damage removal of gallium nitride substrates using a dual-pulse-width laser as described in any one of Embodiments 1 to 6.

[0131] like Figure 3 As shown, the system includes a picosecond laser, a femtosecond laser, a motorized tilting mirror mount, a half-wave plate, a beam expander, a reflector, a focusing objective (focusing optics system), and a two-dimensional motion platform. The picosecond and femtosecond lasers are respectively arranged on the upper and lower sides of the optical path intersection. The motorized tilting mirror mount is located at the intersection of the two laser output paths. By motorically controlling the tilting angle of the mirror mount, either the picosecond laser or the femtosecond laser can be selectively introduced into the subsequent common processing optical path.

[0132] Picosecond lasers are used to generate picosecond lasers. They are typically solid-state lasers or fiber lasers, capable of outputting laser pulses with pulse widths on the order of picoseconds. The wavelength of the picosecond laser is selected based on the absorption characteristics of gallium nitride (GaN) materials, usually choosing ultraviolet or near-ultraviolet wavelengths to improve the absorption efficiency of GaN materials.

[0133] A picosecond laser moving device is used to fix the picosecond laser and control its movement along a set path. In some embodiments, the picosecond laser is stationary, and the relative movement of the laser focus with respect to the gallium nitride substrate is achieved by moving a two-dimensional platform. In other embodiments, the picosecond laser is mounted on the moving device, and the moving device controls the picosecond laser to move along a preset trajectory, thereby achieving the relative movement of the laser focus with respect to the gallium nitride substrate.

[0134] Femtosecond lasers are used to generate femtosecond lasers. These lasers are typically Ti:sapphire lasers or fiber lasers, capable of outputting laser pulses with pulse widths on the order of femtoseconds. The wavelength of the femtosecond laser is also selected based on the absorption characteristics of gallium nitride (GaN) materials, usually in the ultraviolet or near-ultraviolet band.

[0135] The femtosecond laser moving device is used to fix the femtosecond laser and control its movement along a set path. Similar to the picosecond laser moving device, the femtosecond laser can be fixed or mounted on the moving device.

[0136] The motorized flip-up mirror mount is positioned at the intersection of the output paths of the femtosecond and picosecond lasers. The mount controls the rotation angle of the mirror via a motorized drive, selecting either the femtosecond or picosecond laser to enter the subsequent shared processing optical path as needed. After entering the common optical path, the laser sequentially passes through a half-wave plate, a beam expander, and a reflector. The half-wave plate adjusts the laser's polarization direction, the beam expander increases the laser beam diameter, and the reflector adjusts the optical path direction, ensuring the laser beam is perpendicularly incident on the focusing objective. This motorized flip-up mirror mount enables step-by-step processing of large-pitch pre-scanning and small-pitch supplementary scanning, allowing both lasers to operate in a time-sharing manner within the same optical system. This ensures processing accuracy while improving system flexibility and economy.

[0137] Two-dimensional platforms are used to support and fix gallium nitride (GaN) substrates. A two-dimensional platform typically includes two mutually perpendicular linear motion axes, enabling precise movement of the GaN substrate in any direction within the horizontal plane. The motion accuracy and repeatability of the two-dimensional platform have a significant impact on the quality of laser processing, typically requiring motion accuracy at the micrometer or even sub-micrometer level.

[0138] The stripping device is used to apply a separation force to a gallium nitride substrate after dual-laser scanning. For example... Figure 6 As shown, the stripping device typically includes an upper clamp, a lower clamp, and a tension application mechanism. The upper and lower clamps are used to fix the nitrogen and gallium surfaces of the gallium nitride substrate, respectively, and the tension application mechanism is used to apply opposing tensions in the vertical direction to the upper and lower clamps, thereby achieving the separation of the gallium nitride substrate along the composite stripping layer.

[0139] The dual-pulse laser low-damage lift-off system for gallium nitride substrates provided in this embodiment achieves low-damage, controllable lift-off of gallium nitride substrates through the coordinated operation of a picosecond laser, a femtosecond laser, a motorized flip-up mirror frame, a two-dimensional platform, and a lift-off device. This system is compact, flexible in operation, and offers high processing precision, meeting the practical requirements for gallium nitride substrate lift-off processing.

[0140] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0141] This invention employs a step-by-step collaborative processing strategy of picosecond laser large-pitch pre-scanning and femtosecond laser small-pitch supplementary scanning to construct a continuous, low-damage composite release layer inside a gallium nitride substrate. By combining adhesive coating fixation and external force separation, controllable release of the gallium nitride substrate is achieved. This effectively reduces surface cracks, subsurface damage, and edge chipping caused by traditional mechanical processing, improves the release quality and yield of gallium nitride wafers, and provides a new technical approach for efficient and low-damage processing of gallium nitride substrates.

[0142] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. 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 low-damage lift-off of gallium nitride substrates using dual-pulse laser, characterized in that, Includes the following processes: Provide a bulk gallium nitride substrate with polished nitrogen and gallium surfaces, and build a dual-pulse laser lift-off system equipped with a switchable picosecond and femtosecond laser, a focusing optical system and a two-dimensional motion platform; A gallium nitride substrate is fixed on a two-dimensional motion platform. A picosecond laser is incident from the nitrogen surface and focused to a preset depth inside the gallium nitride substrate. Multiple initial modification lines are generated at intervals along the specified crystal orientation of the gallium nitride substrate with a first line spacing. A femtosecond laser is incident from the nitrogen surface and focused to a preset depth. It scans parallel to the adjacent initial modification lines with a second line spacing to generate a supplementary modification line. All initial modification lines and supplementary modification lines are connected to form a continuous composite exfoliation layer. After the nitrogen and gallium surfaces of the gallium nitride substrate are bonded together with adhesive, a tensile force is applied to separate them along the composite release layer. After stripping, the gallium nitride substrate is removed by immersion in an acidic or oxidizing cleaning solution at room temperature combined with ultrasonic cleaning. The remaining gallium metal on the stripped surface is then removed by rinsing and drying to complete the process.

2. The method for low-damage lift-off of gallium nitride substrate using dual-pulse laser as described in claim 1, characterized in that, The gallium nitride substrate has a rectangular block structure. Picosecond lasers and femtosecond lasers are incident perpendicularly on the crystal plane of the gallium nitride substrate. Both sides of the gallium nitride substrate are polished to reduce laser incident scattering loss. The preset depth is set based on the target peel thickness. The preset depth is the formation position of the composite peel layer inside the gallium nitride substrate.

3. The method for low-damage lift-off of gallium nitride substrates using dual-pulse laser as described in claim 1, characterized in that, The first line spacing is the distance between the center lines of adjacent initial reforming lines, and the second line spacing is the distance between the supplementary reforming line and the adjacent initial reforming line. The value of the second line spacing is half of the value of the first line spacing.

4. The method for low-damage lift-off of gallium nitride substrate using dual-pulse laser as described in claim 3, characterized in that, The range of values ​​for the first line spacing is defined. The lower limit of the first line spacing range is used to avoid void defects caused by thermal superposition of adjacent scans. The upper limit of the first line spacing range is used to ensure that the femtosecond laser can connect adjacent initial modification lines. The interval region formed by the first line spacing is used to retain gas release channels to weaken the superposition of thermal accumulation.

5. The method for low-damage lift-off of gallium nitride substrate using a dual-pulse laser as described in claim 3, characterized in that, The range of values ​​for the second line spacing is defined. The lower limit of the second line spacing range is used to avoid damage to the exfoliation layer caused by local energy concentration, and the upper limit of the second line spacing range is used to ensure that the supplementary modification line can induce crack generation and connect the initial modification lines on both sides.

6. The method for low-damage lift-off of gallium nitride substrate using dual-pulse laser as described in claim 1, characterized in that, Picosecond and femtosecond lasers are uniformly set with pulse repetition frequency and scanning speed within a fixed range. The uniformity of energy deposition inside the gallium nitride substrate is controlled by matching the pulse repetition frequency and scanning speed. The picosecond laser scans along a specific crystal orientation of the gallium nitride substrate, while the femtosecond laser scans in the same direction as the picosecond laser.

7. The method for low-damage lift-off of gallium nitride substrate using dual-pulse laser as described in claim 1, characterized in that, A two-dimensional motion platform, in conjunction with an electrically operated flip-up mirror mount, switches the optical path. This electrically operated flip-up mirror mount is located at the intersection of the picosecond laser and the femtosecond laser beams, allowing a single laser beam to enter the focusing optical system in a time-division manner to complete step-by-step scanning.

8. The method for low-damage lift-off of gallium nitride substrate using dual-pulse laser as described in claim 1, characterized in that, Ultrasonic cleaning relies on the cavitation effect to desorb residual gallium metal from the stripping surface. After cleaning, the gallium nitride wafer is rinsed with deionized water, acetone or isopropanol in sequence, and then dried.

9. The method for low-damage lift-off of gallium nitride substrate using dual-pulse laser as described in claim 1, characterized in that, After the substrate separation is completed, the adhesive is removed from the stripped gallium nitride wafer using an organic solvent, and then it is sent to the cleaning process to remove the adhering substances on the stripped surface. When the adhesive is applied and bonded, the two sides of the gallium nitride substrate are restricted as a whole. When the tension is applied, the separation is completed along a single plane of the composite stripping layer.

10. A system for low-damage lift-off of gallium nitride substrates using dual-pulse laser, characterized in that, The method for low-damage removal of gallium nitride substrate using dual-pulse laser as described in any one of claims 1-9 includes a picosecond laser source, a picosecond laser source moving device, a femtosecond laser source, a femtosecond laser source moving device, an electrically operated flip-up mirror frame, a two-dimensional platform, and a removal device. The motorized flip-up mirror frame switches between two laser common-path outputs in a time-division manner, enabling step-by-step processing of large-pitch pre-scanning and small-pitch supplementary scanning. The picosecond laser source moving device and the femtosecond laser source moving device control the corresponding laser sources to move along preset trajectories. The two-dimensional platform supports and fixes the gallium nitride substrate, and the peeling device applies a separation force to the gallium nitride substrate after dual laser scanning.

Images