A method and device for gallium nitride stripping

Abstract

The present invention discloses a method and a device for gallium nitride stripping. The method includes: combining a first laser and a second laser and irradiating a gallium nitride epitaxial wafer, and making the focused spots of the first laser and the second laser coincide at the interface between the gallium nitride layer and the substrate of the gallium nitride epitaxial wafer, so as to separate the gallium nitride layer from the substrate. The present invention uses a femtosecond pulse to assist a nanosecond pulse to irradiate the gallium nitride epitaxial wafer, so that it has both the thermal processing effect of the nanosecond laser and the cold processing effect of the femtosecond laser, thereby effectively reducing the ablation of the gallium nitride material and enabling the gallium nitride layer and the substrate to be smoothly separated.

Description

Technical Field

[0001] The present invention belongs to the technical field of substrate peeling of semiconductor materials, and particularly relates to a gallium nitride peeling method and device. Background Art

[0002] Gallium nitride (GaN) is a semiconductor material with significant wide bandgap characteristics. Due to its good spectral selectivity, thermal stability, radiation resistance, and high response sensitivity, it is widely used in the fields of microwave power transistors and blue light-emitting devices. Heteroepitaxial technology is commonly used in scientific research and production practice to cultivate GaN crystals. Because the lattice mismatch between sapphire and GaN is low and the price is low, sapphire has always been regarded as the mainstream substrate for epitaxial growth of GaN materials. However, at the same time, the non-conductivity and low thermal conductivity of sapphire actually affect the light-emitting efficiency of LED devices. Therefore, peeling the sapphire substrate combined with GaN by technical means has become a necessary process in LED production. In this process, how to efficiently and non-destructively separate the GaN crystal from its growth substrate (such as sapphire) has become a key technical problem. These GaN-based flexible electronic devices usually need to transfer the original GaN functional layer to the final flexible substrate through mechanical, chemical, or laser peeling methods to achieve preparation. The efficient and high-quality peeling process directly determines the preparation cost and performance of flexible GaN-based electronic devices.

[0003] Laser lift-off technology (LLO) has become the mainstream method for large-scale industrial transfer of GaN functional layers and GaN-based devices due to its advantages such as small device damage, good equipment openness, and flexible application methods. Currently, LLO technology mainly uses excimer nanosecond pulsed lasers to achieve GaN device peeling by inducing rapid temperature rise and decomposition of materials near the interface between sapphire and GaN. Due to the limitation of the nanosecond pulse width, GaN devices are extremely vulnerable to thermal damage during the peeling process. And usually, the high peak power of ultrafast lasers often causes film breakdown, resulting in the direct failure of the peeled devices.

[0004] CN114220740B provides a gallium nitride substrate peeling method, which specifically includes: obtaining a gallium nitride substrate with a gallium nitride epitaxial structure directly grown on its upper surface; using a laser beam to scan and irradiate the inside of the gallium nitride substrate through the epitaxial structure to generate a decomposition layer in the gallium nitride substrate; the laser beam is a laser with a pulse width less than 10 15 s magnitude, and the distance between the decomposition layer and the upper surface of the gallium nitride substrate is less than the thickness of the gallium nitride substrate; separating the gallium nitride substrate at the decomposition layer to obtain a peeled gallium nitride substrate and a semiconductor device. This scheme directly replaces the nanosecond laser of the traditional LLO technology with a femtosecond laser and directly uses the femtosecond laser for peeling. Although it can reduce the diffusion of heat, its extremely high peak power may cause film breakdown and damage the semiconductor device.

[0005] CN116207044B provides a laser lift-off method, device and medium for gallium nitride materials. Specifically, a metal layer is deposited on the surface of the deposited gallium nitride heterostructure material to form a gallium nitride composite; in response to the completion of the deposition of the metal layer, the substrate on the gallium nitride composite is lifted off by laser, and the metal layer on the gallium nitride composite is washed away by a predetermined method to obtain gallium nitride materials. With the high ductility of the metal layer, during the laser lift-off process, the pressure generated by the thermal decomposition of GaN to generate N2 and the large stress generated by the internal stress relaxation of Fe-doped GaN materials can be effectively absorbed, greatly improving the yield rate of the formed gallium nitride materials and the quality of the gallium nitride materials. This solution may not be able to achieve uniform absorption of pressure and stress by using the added metal layer, and the production of the metal layer and the subsequent removal of the metal layer are relatively complex.

[0006] CN113770512B provides a method for rapidly preparing a flexible gallium nitride photodetector by laser. Specifically, a flexible substrate is attached to a gallium nitride epitaxial wafer, the focal plane position of the light beam is adjusted, and it is ensured that the light beam is incident from the side of the substrate of the gallium nitride epitaxial wafer, so that the light beam is scanned and irradiated along a specified route, and the original rigid transparent substrate of the epitaxial wafer is removed to obtain a Ga metal nanoparticle / gallium nitride thin film / flexible substrate structure, and interdigital electrodes are prepared on the surface of the Ga metal nanoparticles. This solution selects a laser beam with a pulse width of 10 picoseconds to strip gallium nitride. Compared with nanosecond lasers, picosecond lasers cannot achieve such a high thermal effect. Compared with femtosecond lasers, the degree of gasification expansion during the picosecond laser lift-off process is more intense and the thermal effect is more obvious. It is necessary to reasonably control the energy input while considering the lift-off efficiency.

[0007] Therefore, how to reduce the damage in the LLO process to improve the lift-off reliability is an urgent problem to be solved at present. Summary of the Invention

[0008] The main object of the present invention is to provide a lift-off method and device for high-quality and rapid lift-off of gallium nitride epitaxial wafers.

[0009] To achieve the foregoing invention object, the technical solution adopted by the present invention includes: a gallium nitride lift-off method, including:

[0010] Providing a gallium nitride epitaxial wafer, the gallium nitride epitaxial wafer including a substrate and a gallium nitride layer grown on the substrate;

[0011] Combining and irradiating the first laser and the second laser on the gallium nitride epitaxial wafer, and making the focused spots of the first laser and the second laser coincide at the interface between the gallium nitride layer and the substrate, so as to separate the gallium nitride layer from the substrate;

[0012] Wherein, both the first laser and the second laser are pulsed lasers, and the first laser can undergo single-photon absorption at the interface between the gallium nitride layer and the substrate, and the second laser can undergo two-photon absorption at the interface between the gallium nitride layer and the substrate.

[0013] In a preferred embodiment, the first laser is a nanosecond laser and the photon energy is higher than the bandgap of gallium nitride; the second laser is a femtosecond laser and the photon energy is lower than the bandgap of gallium nitride.

[0014] In a preferred embodiment, the second laser is also used to generate plasma at the interface between the gallium nitride layer and the substrate, and the plasma can at least reflect the first laser.

[0015] In a preferred embodiment, the pulse frequency of the first laser is 30KHz to 80KHz, the pulse width is 5ns to 20ns, the pulse fluence is 150 to 250mJ / cm 2 ², and the pulse has a Gaussian distribution; the pulse frequency of the second laser is 300KHz to 600KHz, the pulse width is 20fs to 100fs, and the pulse fluence is 1.5 to 2.5J / cm 2 ².

[0016] In a preferred embodiment, the combined laser formed by combining the first laser and the second laser is focused on the interface between the gallium nitride layer and the substrate, and the focused spot of the combined laser is scanned from the edge to the inside of the interface between the gallium nitride layer and the substrate, so as to separate the gallium nitride layer from the substrate.

[0017] In a preferred embodiment, after the combined laser formed by combining the first laser and the second laser is focused on the interface between the gallium nitride layer and the substrate, the focused spot of the combined laser is scanned from the edge to the inside of the interface between the gallium nitride layer and the substrate by using the optimal power combination and the optimal spot size of the two lasers, so as to separate the gallium nitride layer from the substrate.

[0018] In a preferred embodiment, the obtaining of the optimal power combination of the two lasers includes: linearly changing the powers of the two lasers after the combined laser is focused on the interface, scanning a line of laser on the interface of the gallium nitride sample each time after the change, and selecting the power combination with the smoothest decomposition at the interface as the optimal power combination.

[0019] In a preferred embodiment, the obtaining of the optimal spot size includes: changing the spot sizes of the two lasers after obtaining the optimal power combination, scanning a line of laser on the interface each time after the change, and selecting the one with the best peeling efficiency according to the decomposition situation at the interface as the optimal spot size.

[0020] In a preferred embodiment, it further includes: performing a secondary scan on the interface, where the laser power used in the secondary scan is less than that in the primary scan, and the scanning speed is faster than that in the primary scan.

[0021] On the other hand, the technical solution adopted by the present invention includes: a gallium nitride stripping device, which is used to strip a gallium nitride epitaxial wafer. The gallium nitride epitaxial wafer includes a substrate and a gallium nitride layer grown on the substrate. The device includes:

[0022] A first laser for emitting a first laser;

[0023] A second laser for emitting a second laser, and the first laser and the second laser are combined and enter a scanning galvanometer;

[0024] The scanning galvanometer is used to make the focused spots of the first laser and the second laser coincide at the interface between the gallium nitride layer and the substrate, so as to separate the gallium nitride layer from the substrate;

[0025] Wherein, both the first laser and the second laser are pulsed lasers, and the first laser can undergo single-photon absorption at the interface between the gallium nitride layer and the substrate, and the second laser can undergo two-photon absorption at the interface between the gallium nitride layer and the substrate.

[0026] In a preferred embodiment, the first laser uses a diode-pumped solid-state laser with a wavelength of 355 nm, and the pulse fluence of the emitted first laser is 150 - 250 mJ / cm 2 , the pulse frequency is 30 KHz - 80 KHz, and the pulse width is 5 ns - 20 ns; and / or, the second laser uses a femtosecond laser with a wavelength of 532 nm, and the pulse fluence of the emitted second laser is 1.5 - 2.5 J / cm 2 , the pulse frequency is 300 KHz - 600 KHz, and the pulse width is 20 fs - 100 fs.

[0027] In a preferred embodiment, the device further includes an optical path structure, which is at least used to combine the first laser and the second laser into a combined laser with coincident spots and supply it to the scanning galvanometer.

[0028]

[0029] In a preferred embodiment, the optical path structure includes a nanosecond optical path structure located between the first laser and the scanning galvanometer and a femtosecond optical path structure located between the second laser and the scanning galvanometer. The nanosecond optical path structure and the femtosecond optical path structure are combined to adjust the two laser beams to have the same spot size and combine them into a combined laser with coincident spots and input it to the scanning galvanometer.

[0030] In a preferred embodiment, the nanosecond optical path structure includes a first reflector, a first beam expander, a first half-wave plate, a first linear polarizer, and a dichroic mirror. The first reflector is configured to reflect the first laser to the first beam expander. The first beam expander is configured to collimate and expand the first laser and supply it to the first half-wave plate. The first half-wave plate is configured to rotate the polarization direction of the first laser and supply it to the first linear polarizer. The first linear polarizer is configured to cooperate with the first half-wave plate to adjust the power of the first laser and then supply it to the dichroic mirror. The dichroic mirror is configured to combine the processed first laser and the second laser into a combined laser beam and output it to the scanning galvanometer.

[0031] In a preferred embodiment, the femtosecond optical path structure includes a second reflector, a second beam expander, a second half-wave plate, a second linear polarizer, and a third reflector. The second reflector is configured to reflect the second laser emitted by the second laser source to the second beam expander. The second beam expander is configured to collimate and expand the second laser and supply it to the second half-wave plate. The second half-wave plate is configured to rotate the polarization direction of the laser and supply it to the second linear polarizer. The second linear polarizer is configured to cooperate with the second half-wave plate to adjust the power of the second laser and then supply it to the third reflector. The third reflector is configured to output the processed second laser to the scanning galvanometer.

[0032] In a preferred embodiment, the scanning galvanometer further includes an achromatic field-flattening focusing lens for focusing the two laser beams.

[0033] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0034] The present invention uses a dual-beam to strip gallium nitride, with the femtosecond pulse assisting the nanosecond pulse, simultaneously possessing the thermal working principle of the nanosecond laser and the cold processing effect of the femtosecond laser, greatly suppressing heat diffusion, thereby effectively reducing the ablation of the material, and enabling the smooth separation of the gallium nitride epitaxy and the sapphire substrate.

[0035] The present invention can flexibly change the spot size of the dual-beam through the combination of two beam expanders. Through testing, the spot size most conducive to improving the stripping efficiency can be found. Moreover, the combination of the half-wave plate and the linear polarizer can linearly change the power of the two laser beams, without the need to replace components, with flexible operation, and can precisely control the laser power. Through testing, the power ratio most conducive to improving the stripping efficiency can be found. Then, laser scanning is performed along the specified path so that the gas generated during the decomposition process can escape from the separated area, thereby greatly reducing the air pressure on the GaN film.

[0036] After one scan, the present invention uses a low-energy pulse to scan again along the original path, making the decomposition at the interface between the sapphire substrate and the gallium nitride epitaxy smoother and more complete. Description of the Drawings

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0038] Figure 1 It is a schematic flow chart of a gallium nitride stripping method in an embodiment of the present invention;

[0039] Figure 2 It is a schematic structural diagram of a gallium nitride epitaxial wafer in an embodiment of the present invention;

[0040] Figure 3 It is a schematic scanning path diagram of a scanning galvanometer in an embodiment of the present invention;

[0041] Figure 4 It is a schematic structural diagram of a gallium nitride stripping device in an embodiment of the present invention.

[0042] Explanation of reference numerals:

[0043] 1. First laser; 2. Second laser; 3. Scanning galvanometer; 31. Apochromatic flat-field focusing lens; 4. Controller; 5. Gallium nitride epitaxial wafer; 51. Substrate; 52. Gallium nitride layer; 53. PDMS thermal release glue; 54. Silicon wafer; 6. Z-axis displacement stage; 7. Nanosecond optical path structure; 71. First reflector; 72. First beam expander; 73. First half-wave plate; 74. First linear polarizer; 75. Dichroic mirror; 8. Femtosecond optical path structure; 81. Second reflector; 82. Second beam expander; 83. Second half-wave plate; 84. Second linear polarizer; 85. Third reflector. Detailed embodiments

[0044] The present invention will be more fully understood by the following detailed embodiments to be read in conjunction with the accompanying drawings. The detailed embodiments of the present invention are disclosed herein; however, it should be understood that the disclosed embodiments are only exemplary of the present invention, and the present invention can be embodied in various forms. Therefore, the specific functional details disclosed herein should not be construed as limiting, but only as a basis for the claims and as a representative basis for teaching those skilled in the art to employ the present invention in any appropriate detailed embodiment in different ways.

[0045] An embodiment of the present invention proposes a solution for high-quality and rapid stripping of gallium nitride on a sapphire substrate. By selecting nanosecond lasers and femtosecond lasers with appropriate wavelengths corresponding to the single-photon absorption and two-photon absorption of gallium nitride respectively, the thermal working principle of the nanosecond laser is combined with the cold processing advantage of the femtosecond laser. The high reflectivity of the plasma generated by the femtosecond pulse is used to prevent the entry of the nanosecond pulse, thereby reducing laser ablation and thermal diffusion. Adjust the double-pulse beam combination to make the light spots coincide, and linearly adjust the power ratio of the two beams of light through the combination of a half-wave plate and a linear polarizer. Adjust the focusing light spot size by adjusting the beam expander, find the parameters with the highest stripping quality and scan along the specified path to quickly and efficiently strip gallium nitride from the sapphire substrate.

[0046] As Figure 1 shown, a method for gallium nitride stripping disclosed in an embodiment of the present invention specifically includes the following steps:

[0047] S1. Provide a gallium nitride epitaxial wafer 5.

[0048] In this embodiment, as Figure 2 shown, the gallium nitride epitaxial wafer 5 specifically includes a substrate 51, a gallium nitride layer 52, a PDMS thermal release glue 53, and a silicon wafer 54 arranged in sequence from top to bottom. Among them, the substrate 51 is specifically a sapphire substrate, the thickness of the substrate 51 is about 500 microns thick, and the epitaxially grown gallium nitride layer 52 is about 6 microns thick. Preferably, the PDMS thermal release glue 53 is used to adhere the epitaxial growth side of the gallium nitride layer to the silicon wafer 54 serving as a support substrate. The PDMS thermal release glue 53 has viscosity at a certain temperature and can play a positioning role. After processing, only need to heat to the set temperature, and the viscosity disappears after about ten minutes. Utilize the thermal release property of PDMS to transfer the gallium nitride epitaxial wafer 5 to the target position to achieve simple stripping. The PDMS thermal release glue 53 can effectively eliminate the strain generated by air pressure, and the silicon wafer 54 can provide support for the gallium nitride layer 52 during the separation process, thereby maintaining the integrity of the gallium nitride layer 52. During stripping, place the sample to be stripped (i.e., the gallium nitride epitaxial wafer 5) on the Z-axis displacement stage 6, and at the same time ensure that the light beam is incident from one side of the substrate 51.

[0049] S2. Combine the first laser and the second laser and irradiate the gallium nitride epitaxial wafer, and make the focusing light spots of the first laser and the second laser coincide at the interface between the gallium nitride layer and the substrate, so as to separate the gallium nitride layer from the substrate.

[0050] Specifically, first select appropriate lasers and optical elements to build an optical path. Combining Figure 4As shown, in this embodiment, a first laser is used to emit first laser light. Specifically, a diode-pumped solid-state laser with a wavelength of 355 nm (such as a DPSS laser, DX-355-15, Photonics Industries) is selected as the nanosecond laser pulse source for the first laser. The first laser light emitted is nanosecond laser light with a wavelength of 355 nm. The photon energy of the ultraviolet laser with a wavelength of 355 nm (specifically 3.49 eV) is higher than the bandgap of GaN (specifically 3.4 eV). A second laser is used to emit second laser light. Specifically, a femtosecond laser with a wavelength of 532 nm is selected as the femtosecond laser pulse source for the second laser. The second laser light emitted is femtosecond laser light with a wavelength of 532 nm. The photon energy of the laser with a wavelength of 532 nm is 2.33 eV, which is lower than the bandgap of GaN.

[0051] Optical elements are selected to build an optical path structure between the laser and the scanning galvanometer. Specifically, a nanosecond optical path structure 7 is built between the first laser 1 and the scanning galvanometer 3, and a femtosecond optical path structure 8 is built between the second laser 2 and the scanning galvanometer 3. In a specific embodiment, the nanosecond optical path structure 7 specifically includes a first reflector 71, a first beam expander 72, a first half-wave plate 73, a first linear polarizer 74, and a dichroic mirror 75. Among them, the first reflector 71 is used to reflect the first laser light emitted by the first laser 1 to the first beam expander 72; the first beam expander 72 is used to collimate and expand the first laser light to the first half-wave plate 73. The first beam expander 72 can specifically be formed by a combination of a positive and a negative lens. By adjusting the distance between the two lenses, the expansion of a circular collimated light spot can be achieved, and the diameter of the focused light spot can be adjusted; the first laser light passes through the combination of the first half-wave plate 73 and the first linear polarizer 74 after being collimated and expanded by the first beam expander 72. The first half-wave plate 73 can rotate the polarization direction of the light, and the first linear polarizer 74 can select the polarization direction to be transmitted. When the first half-wave plate 73 rotates, the transmitted light intensity will change. The combination of the two can achieve continuous linear adjustment of the light energy, and thus achieve laser power adjustment. This method does not require replacing components, is flexible in operation, and can precisely control the laser power; the dichroic mirror 75 is used to combine the processed first laser light and the second laser light into combined laser light and output it to the scanning galvanometer 3.

[0052] In a more specific embodiment, the femtosecond optical path structure 8 specifically includes a second reflector 81, a second beam expander 82, a second half-wave plate 83, a second linear polarizer 84, and a third reflector 85. Among them, the second reflector 81 is used to reflect the second laser emitted by the second laser 2 to the second beam expander 82; the second beam expander 82 is used to collimate and expand the second laser to the second half-wave plate 83. The second beam expander 82 is specifically formed by a combination of a positive lens and a negative lens. By adjusting the distance between the two lenses, the expansion of a circular collimated light spot can be achieved, and the diameter of the focused light spot can be adjusted; after the second laser is collimated and expanded by the second beam expander 82, it passes through the combination of the second half-wave plate 83 and the second linear polarizer 84. The second half-wave plate 83 can rotate the polarization direction of light, while the second linear polarizer 84 can select the transmitted polarization direction. When the second half-wave plate 83 rotates, the transmitted light intensity will change. The combination of the two can achieve continuous linear adjustment of light energy, and thus achieve laser power adjustment; the third reflector 85 is used to output the processed second laser to the dichroic mirror 75 mentioned above. The dichroic mirror 75 combines the first laser and the second laser into a combined laser and outputs it to the scanning galvanometer 3.

[0053] The combination of the nanosecond optical path structure 7 and the femtosecond optical path structure 8 adjusts the two laser beams into a combined laser with the same spot size and the ability to merge into a coincident spot and inputs it to the scanning galvanometer 3. Specifically, first, by adjusting the two beam expanders, that is, the first beam expander 72 and the second beam expander 82 (by adjusting the distance between the two beam expanders), the sizes of the two light spots are made consistent, which is convenient for making the two light spots coincide later. Then, the dichroic mirror 75 is used to combine the two processed pulsed lights (that is, the first laser and the second laser), and by adjusting the angles of the above three reflectors, the two light spots are controlled to completely coincide into a combined laser, and it is ensured that the combined laser is collimated and enters the scanning galvanometer 3.

[0054] The scanning galvanometer 3 is located above the gallium nitride epitaxial wafer 5 to be peeled, and is connected to the first laser 1 and the second laser 2 through the above optical path structure, so that the focused light spots of the first laser and the second laser coincide at the interface between the gallium nitride layer and the substrate, thereby separating the gallium nitride layer from the substrate. In this embodiment, the scanning galvanometer 3 specifically includes an achromatic flat-field focusing lens (F-theta lens) 31 for focusing the combined laser. The combined laser is focused to the interface between the substrate 51 and the gallium nitride layer 52 after passing through the F-theta lens of the scanning galvanometer 5. The use of the achromatic F-theta lens can ensure that the focal points of the two light beams are on the same plane. During implementation, the F-Theta lens usually consists of 3 to 5 spherical or aspherical lenses, which are used to correct the field area and distortion of the light beams in different fields of view on the image plane. In a specific embodiment, a telecentric achromatic F-Theta focusing field lens with an effective focal length of 56 mm and a scanning range of 20×20 mm is selected, and the initial focused light spot diameter is 8 microns.

[0055] After setting up the optical path, set the laser parameters of the first laser 1 and the second laser 2 and the scanning parameters of the scanning galvanometer 3 as described above. Specifically, it can be set through a controller, and the controller is connected to the first laser 1, the second laser 2 and the scanning galvanometer 3. Among them, the laser parameters include the pulse fluence, pulse frequency and pulse width of the laser, and the scanning parameters include the scanning speed and scanning path. In a specific embodiment, specifically set the pulse frequency of the first laser to be 30KHz to 80KHz, preferably 50kHz, the pulse width to be 5ns to 20ns, preferably 8ns, and the pulse fluence to be 150 to 250mJ / cm 2 , preferably 200mJ / cm 2 , and the pulse has a Gaussian distribution; and specifically set the pulse frequency of the second laser to be 300KHz to 600KHz, preferably 500KHz, the pulse width to be 20fs to 100fs, preferably 50fs, and the pulse fluence to be 1.5 to 2.5J / cm 2 , preferably 2J / cm 2 . And set the scanning speed to 2m / s, and the scanning path is to scan from the edge of the interface between the gallium nitride layer and the substrate to the inside, and the distance between each scanned circle is 20um. The scanning path in this embodiment is a rectangular scanning path, and the interval between the outer circle and the inner circle is set to 20 microns, and the scanning galvanometer 3 scans from its boundary from the outside to the inside according to the rectangular scanning path. It can make the gas generated during the decomposition process escape from the separated area, thereby greatly reducing the air pressure on the GaN film. Of course, the above laser parameters and scanning parameters can be adjusted as needed until the parameters with the highest peeling efficiency are found.

[0056] After setting the parameters, move the above z-axis displacement stage 6 to find the focus so that the focusing plane of the combined laser is exactly at the junction of the substrate 51 and the gallium nitride layer 52. At this time, the combined laser is focused at the interface, causing the gallium nitride layer 52 to absorb heat and decompose, thereby separating the gallium nitride layer 52 from the substrate. The thermal decomposition only occurs in the junction area where the light spot is focused, which can effectively avoid affecting other areas due to incidental thermal damage.

[0057] In the above embodiments of the present invention, femtosecond laser-assisted nanosecond laser is used for laser lift-off, which can reduce ablation while obtaining the energy for absorption and decomposition at the interface between the substrate 51 and the gallium nitride layer 52, compensating for the problems of rapid temperature rise and thermal diffusion in traditional nanosecond laser lift-off of gallium nitride. Specifically, since the first laser is a nanosecond laser and its photon energy (3.49 eV) is higher than the bandgap of GaN (3.4 eV), single-photon absorption occurs at the interface between the gallium nitride layer 52 and the substrate 51 for the first laser. The first laser irradiates the interface between the substrate 51 and the epitaxially grown gallium nitride layer 52, and the gallium nitride layer 52 absorbs a large amount of photon energy and decomposes to form liquid Ga and nitrogen gas. The second laser is a femtosecond laser with a photon energy of 2.33 eV, which is lower than the bandgap of GaN (3.4 eV), so two-photon absorption occurs at the interface between the gallium nitride layer 52 and the substrate 51 for the second laser, and the second laser can also generate plasma at the interface. Due to the cumulative effect of the pulses, the thermal effect of the ultrafast laser action still exists, but it is almost cold processing compared with the nanosecond laser, and an appropriate thermal effect can improve the lift-off efficiency, promote the gasification of the ablated material, and accelerate the formation of high-density plasma. The absorption coefficient of the high-density plasma for laser is much larger than that of the medium, which can prevent redundant absorption of laser energy by gallium nitride, and the high reflectivity of the plasma can prevent the entry of nanosecond pulses, thereby reducing the laser ablation effect.

[0058] Preferably, after the combined laser beam is focused on the interface between the gallium nitride layer 52 and the substrate 51, the optimal power combination and the optimal spot size of the two laser beams can be adjusted to obtain the combined laser beam with these two parameters of the optimal power combination and the optimal spot size to perform laser scanning on the interface between the gallium nitride layer 52 and the substrate 51, so as to achieve the highest stripping efficiency. Among them, the combination of the above-mentioned half-wave plate and linear polarizer can be used to adjust and obtain the optimal power combination of the two laser beams. The specific operation in this embodiment is to first keep the power of the first laser unchanged, change the power of the second laser from small to large, and control the combined laser beam to scan one row after each change, with a 20-micron interval between each row. After removing the sample, observe the ablation situation of each row of the sample under a microscope, then finely adjust the power of the first laser, and also control the combined laser beam to scan one row after each change, with a 20-micron interval between each row, and select the power combination with the smoothest decomposition at the interface as the optimal power combination for processing. After setting the optimal power combination, the distance between the two lenses can be adjusted by adjusting the two beam expanders to change the size of the focused spot, so as to obtain the optimal spot size of the combined laser beam. Note that the sizes of the two spots should always overlap. The specific operation is to adjust the beam expander to slowly change the spot size of the combined laser beam from 8 microns to 100 microns, and control the combined laser beam to scan one row at the interface after each change in the spot size, with a 20-micron interval between each row. After removing the gallium nitride epitaxial wafer 5 sample, observe the ablation situation of each row of the sample under a microscope, and select the spot size with the best stripping efficiency according to the decomposition situation at the interface as the optimal spot size.

[0059] After determining the optimal spot size, scan the interface from the outside to the inside along the specified path, starting from the boundary of the interface and scanning the entire sample. In this way, the gas generated during the decomposition process can escape from the separated area, thereby greatly reducing the air pressure on the GaN film.

[0060] Preferably, after scanning once along the above scanning path, since there may still be slight unevenness at the interface, it is preferably scanned again along the original path with low-energy pulses to make the decomposition at the interface flat. The laser power used for the secondary scanning is preferably less than that of the primary scanning, and the scanning speed is faster than that of the primary scanning. The combination of the two scans can greatly improve the morphology of the gallium nitride interface. In a specific embodiment, during the secondary scanning, the combination of the half-wave plate and the linear polarizer is used to adjust the power of the two pulsed lasers to half of the previous value, and the galvanometer scanning speed is adjusted to 3 m / s. The interface between the substrate and the gallium nitride layer after the secondary scanning is very smooth. Experiments show that the present invention can achieve good non-destructive stripping of gallium nitride epitaxial wafers, the stripping success rate is greater than 97%, and the surface roughness of the stripped gallium nitride can be reduced to below 10 nm. By controlling the energy of the laser well, the surface roughness can even reach 5 nm.

[0061] After the scanning is completed, the substrate 51 is removed to obtain a complete and smooth gallium nitride thin film. The gallium metal remaining on the sample is removed with hydrochloric acid solution, and then the sample is washed with deionized water.

[0062]

[0063] As Figure 4 shown is a gallium nitride stripping device disclosed in an embodiment of the present invention. The device is used for stripping a gallium nitride epitaxial wafer, and mainly includes a first laser 1, a second laser 2, a scanning galvanometer 3 and a controller 4. Among them, the first laser 1 and the second laser 2 respectively emit lasers and are combined into a beam to the scanning galvanometer 3 under the control of the controller 4, and the combined laser irradiates and strips the gallium nitride epitaxial wafer 5 through the scanning galvanometer 3. Among them, the specific structures and working principles of the first laser 1, the second laser 2, the scanning galvanometer 3 and the controller 4 can refer to the descriptions in the above device, and will not be elaborated here.

[0064] Based on the content of the above embodiments, it can be seen that the present invention has at least the following advantages: 1. The present invention strips the gallium nitride epitaxial wafer by selecting a double beam, and uses femtosecond pulses to assist nanosecond pulses to irradiate the gallium nitride epitaxial wafer, simultaneously having the thermal working principle of nanosecond laser and the cold processing effect of femtosecond laser, greatly suppressing heat diffusion, thereby effectively reducing the ablation of the material and enabling the smooth separation of the gallium nitride layer and the substrate. 2. The present invention can flexibly change the spot size of the double beam through the combination of two beam expanders, find the spot size most conducive to improving the stripping efficiency, and the combination of the half-wave plate and the linear polarizer can linearly change the power of the two beams of light, without replacing components, with flexible operation, and can accurately control the laser power, find the power ratio most conducive to improving the stripping efficiency, and then perform laser scanning along the specified path so that the gas generated during the decomposition process can escape from the separated area, thereby greatly reducing the air pressure on the GaN film. 3. After one scan in the present invention, a low-energy pulse is used to scan again along the original path, making the decomposition at the interface between the sapphire substrate and the gallium nitride epitaxial layer smoother and more complete. 4. The present invention uses an achromatic F-theta field lens, which can ensure that the focal points of the two beams of light are on the same plane. 5. The present invention does not need to grow other buffer layers and absorption layers, and the heat generated by laser stripping can be effectively controlled, greatly simplifying the process.

[0065] All aspects, embodiments, features and examples of the present invention should be considered illustrative in all respects and are not intended to limit the present invention. The scope of the present invention is only defined by the claims. Without departing from the spirit and scope of the claimed invention, those skilled in the art will appreciate other embodiments, modifications and uses.

[0066] In the present invention, the use of titles and chapters does not mean limiting the present invention; each chapter can be applied to any aspect, embodiment or feature of the present invention.

Claims

1. A gallium nitride stripping method, characterized in that, Including: Providing a gallium nitride epitaxial wafer, the gallium nitride epitaxial wafer including a substrate and a gallium nitride layer grown on the substrate; Combining a first laser and a second laser and irradiating the gallium nitride epitaxial wafer, and making the focused spots of the first laser and the second laser coincide at the interface between the gallium nitride layer and the substrate, so as to separate the gallium nitride layer from the substrate; Wherein, both the first laser and the second laser are pulsed lasers, and the first laser can undergo single-photon absorption at the interface between the gallium nitride layer and the substrate, and the second laser can undergo two-photon absorption at the interface between the gallium nitride layer and the substrate; It further includes focusing the combined laser formed by combining the first laser and the second laser on the interface between the gallium nitride layer and the substrate, and using the optimal power combination and the optimal spot size of the two lasers to scan the focused spot of the combined laser from the edge of the interface between the gallium nitride layer and the substrate towards the inside, so as to separate the gallium nitride layer from the substrate; The obtaining of the optimal power combination of the two lasers includes: linearly changing the powers of the two lasers after the combined laser is focused on the interface, and laser-scanning one row on the interface of the gallium nitride sample each time after the change, and selecting the power combination with the smoothest decomposition at the interface as the optimal power combination; the obtaining of the optimal spot size includes: changing the spot sizes of the two lasers after obtaining the optimal power combination, and laser-scanning one row on the interface each time after the change, and selecting the one with the best peeling efficiency according to the decomposition situation at the interface as the optimal spot size.

2. The gallium nitride stripping method according to claim 1, characterized in that: The first laser is a nanosecond laser, and the photon energy is higher than the bandgap of gallium nitride; the second laser is a femtosecond laser, and the photon energy is lower than the bandgap of gallium nitride; and / or, the second laser is also used to generate plasma at the interface between the gallium nitride layer and the substrate, and the plasma can at least reflect the first laser.

3. The gallium nitride stripping method according to claim 2, characterized in that: The pulse frequency of the first laser is 30KHz to 80KHz, the pulse width is 5ns to 20ns, and the pulse fluence is 150 to 250mJ / cm 2 , and the pulse has a Gaussian distribution; the pulse frequency of the second laser is 300KHz to 600KHz, the pulse width is 20fs to 100fs, and the pulse fluence is 1.5 to 2.5J / cm 2 .

4. The gallium nitride stripping method according to claim 1, wherein, Specifically including: Focusing the combined laser formed by combining the first laser and the second laser on the interface between the gallium nitride layer and the substrate, and making the focused spot of the combined laser scan from the edge of the interface between the gallium nitride layer and the substrate towards the inside, so as to separate the gallium nitride layer from the substrate.

5. The gallium nitride stripping method according to claim 4, wherein It further includes: Performing a secondary scan on the interface, the laser power used in the secondary scan is less than that in the primary scan, and the scan speed is faster than that in the primary scan.

6. A gallium nitride stripping device, characterized in that, The device is used for peeling a gallium nitride epitaxial wafer, the gallium nitride epitaxial wafer including a substrate and a gallium nitride layer grown on the substrate, and the device includes: A first laser for emitting a first laser; A second laser for emitting a second laser, the first laser and the second laser are combined and enter a scanning galvanometer; The scanning galvanometer is used to make the focused spots of the first laser and the second laser coincide at the interface between the gallium nitride layer and the substrate, so as to separate the gallium nitride layer from the substrate; Wherein, both the first laser and the second laser are pulsed lasers, and the first laser can undergo single-photon absorption at the interface between the gallium nitride layer and the substrate, and the second laser can undergo two-photon absorption at the interface between the gallium nitride layer and the substrate; It further includes focusing the combined laser formed by combining the first laser and the second laser on the interface between the gallium nitride layer and the substrate, and using the optimal power combination and the optimal spot size of the two lasers to scan the focused spot of the combined laser from the edge to the inside of the interface between the gallium nitride layer and the substrate, so as to separate the gallium nitride layer from the substrate; The acquisition of the optimal power combination of the two lasers includes: linearly changing the powers of the two lasers after the combined laser is focused on the interface, scanning a line of laser on the interface of the gallium nitride sample each time after the change, and selecting the power combination with the smoothest decomposition at the interface as the optimal power combination; the acquisition of the optimal spot size includes: changing the spot sizes of the two lasers after obtaining the optimal power combination, scanning a line of laser on the interface each time after the change, and selecting the one with the best peeling efficiency according to the decomposition situation at the interface as the optimal spot size.

7. The gallium nitride peeling device according to claim 6, characterized in that, The first laser is a diode-pumped solid-state laser with a wavelength of 355 nm, and the pulse fluence of the first laser emitted is 150 - 250 mJ / cm 2 , the pulse frequency is 30 KHz - 80 KHz, and the pulse width is 5 ns - 20 ns; and / or, the second laser is a femtosecond laser with a wavelength of 532 nm, and the pulse fluence of the second laser emitted is 1.5 - 2.5 J / cm 2 , the pulse frequency is 300 KHz - 600 KHz, and the pulse width is 20 fs - 100 fs.

8. The gallium nitride stripping device according to claim 6, characterized in that, The device further includes an optical path structure, which is at least used to combine the first laser and the second laser into a combined laser with overlapping spots for the scanning galvanometer, and / or, the optical path structure includes a nanosecond optical path structure between the first laser and the scanning galvanometer and a femtosecond optical path structure between the second laser and the scanning galvanometer. The nanosecond optical path structure and the femtosecond optical path structure are combined to adjust the two lasers into a combined laser with the same spot size and overlapping spots and input it to the scanning galvanometer, and / or, the nanosecond optical path structure includes a first reflector, a first beam expander, a first half-wave plate, a first linear polarizer and a dichroic mirror. The first reflector is used to reflect the first laser to the first beam expander, the first beam expander is used to collimate and expand the first laser to the first half-wave plate, the first half-wave plate is used to rotate the polarization direction of the first laser to the first linear polarizer, the first linear polarizer is used to combine with the first half-wave plate to adjust the power of the first laser and then give it to the dichroic mirror, and the dichroic mirror is used to combine the processed first laser and the second laser into a combined laser and then output it to the scanning galvanometer; and / or, the femtosecond optical path structure includes a second reflector, a second beam expander, a second half-wave plate, a second linear polarizer and a third reflector. The second reflector is used to reflect the second laser emitted by the second laser to the second beam expander, the second beam expander is used to collimate and expand the second laser to the second half-wave plate, the second half-wave plate is used to rotate the polarization direction of the laser to the second linear polarizer, the second linear polarizer is used to combine with the second half-wave plate to adjust the power of the second laser and then give it to the third reflector, and the third reflector is used to output the processed second laser to the scanning galvanometer; and / or, the scanning galvanometer further includes an achromatic flat-field focusing lens for focusing the two lasers.

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