Method for fabricating free-standing gallium nitride substrates

Abstract

The present disclosure discloses a method for fabricating a free-standing gallium nitride substrate, including: 1) providing a composite substrate comprising a sapphire substrate and a gallium nitride thin film; 2) forming a temporary bonding layer on the gallium nitride thin film; 3) bonding a transfer substrate to the composite substrate by the temporary bonding layer; 4) peeling off the sapphire substrate by a laser peeling process; 5) weakly bonding the receiving substrate and the gallium nitride thin film, and peeling off the transfer substrate from the gallium nitride thin film by losing the temporary bonding layer; and 6) epitaxially growing a gallium nitride epitaxial layer on the gallium nitride thin film, and realizing separation between the gallium nitride thin film and the receiving substrate by losing the weak bond due to lattice mismatch stress and / or thermal mismatch stress between the gallium nitride thin film, the gallium nitride epitaxial layer, and the receiving substrate. The present application effectively overcomes the drawback that the thickness of a hetero gallium nitride epitaxial layer is limited by lattice mismatch and thermal mismatch, thereby improving the quality of the free-standing gallium nitride substrate and reducing the manufacturing cost of the free-standing gallium nitride substrate.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE Embodiments of the present application relate to the field of semiconductor manufacturing, for example, methods for fabricating free-standing gallium nitride substrates. [Background technology]

[0002] Third-generation semiconductor materials, represented by gallium nitride (GaN) and its alloys, have been a new type of semiconductor material that has been highly valued internationally for the past dozen years. They have many excellent properties, such as a large band gap, high electron saturation drift velocity, small dielectric constant, good thermal conductivity, and structural stability, and have great application prospects in both the optoelectronic and microelectronic technology fields. In the optoelectronic field, the band gap of III-group nitrides can be continuously adjusted within the range of 0.7 to 6.2 eV, covering wavelengths from red light to ultraviolet light, making it possible to manufacture light-emitting devices and white light illumination in the green, blue, and ultraviolet wavelength bands. In addition, ultraviolet LEDs, which have recently emerged, have also shown special applications in screen printing, polymer curing, and environmental protection, greatly stimulating researchers' research enthusiasm. GaN lasers also play an important role in the field of information storage, and can also be applied in various fields such as medical diagnosis, undersea exploration, and communications.

[0003] Because it is difficult to produce bulk GaN single crystals and it is difficult to obtain large-sized, high-quality bulk single crystal GaN substrates, epitaxial growth of GaN is usually performed by heteroepitaxy. However, it has been shown by both theory and experiment that device performance can be significantly improved by homoepitaxy using GaN as the substrate. Therefore, the production of free-standing GaN substrates has attracted people's attention.

[0004] At present, large-area GaN freestanding substrates are generally obtained by vapor-growth of GaN thick film on hetero substrate and then separating the original hetero substrate. Among them, sapphire substrate is the most commonly used substrate. To obtain the freestanding substrate, the sapphire substrate needs to be removed. Sapphire is hard and has stable chemical properties, so it is difficult to remove by chemical etching or mechanical polishing. At present, laser peeling is often used to separate GaN from the sapphire substrate. However, the technical cost of laser peeling is high, and in the process of laser peeling, the high-pressure gas generated after high-temperature decomposition of GaN at the interface is likely to damage the GaN freestanding substrate to be prepared. If it is light, it will generate a large number of dislocations and microcracks in the GaN freestanding substrate, which will affect the quality of the subsequent device; if it is heavy, it will completely break up the GaN freestanding substrate, which will greatly reduce the yield.

[0005] In summary, when epitaxial growth is performed using hetero-material epitaxy, the thickness of the heteroepitaxial gallium nitride is limited due to lattice mismatch and thermal mismatch. At the same time, the process of dissociating the gallium nitride single crystal is very difficult, which is particularly evident in large-sized sapphire / gallium nitride thick film composite substrates (HVPE single-stage epitaxial wafers). Summary of the Invention [Problem to be solved by the invention]

[0006] The following is a general overview of the subject matter detailed in this document, which is not intended to limit the scope of protection of the claims.

[0007] In view of the shortcomings of the related art described above, the embodiments of the present application provide a method for fabricating a free-standing gallium nitride substrate to solve the problem in the related art that the thickness of heteroepitaxial gallium nitride is limited due to the large lattice and thermal mismatch between the thick gallium nitride epitaxial layer and the sapphire substrate. [Means for solving the problem]

[0008] An embodiment of the present application includes the steps of: 1) providing a composite substrate including a sapphire substrate and a gallium nitride thin film formed on the sapphire substrate; 2) forming a temporary bonding layer on the gallium nitride thin film; 3) providing a transfer substrate and bonding the transfer substrate to the composite substrate by the temporary bonding layer; 4) peeling off the sapphire substrate by a laser peeling process to expose the gallium nitride thin film; and 5) providing a receiving substrate and weakly bonding the receiving substrate and the gallium nitride thin film, and then deactivating the temporary bonding layer to bond the transfer substrate to the gallium nitride thin film. and 6) epitaxially growing a gallium nitride epitaxial layer on the gallium nitride thin film, and after the gallium nitride epitaxial layer has grown to a certain thickness, causing a weak bonding state between the gallium nitride thin film and the receiving substrate to be lost due to lattice mismatch stress and thermal mismatch stress between the gallium nitride thin film and the gallium nitride epitaxial layer and the receiving substrate, thereby achieving separation between the gallium nitride thin film and the receiving substrate and obtaining a free-standing gallium nitride substrate.

[0009] Preferably, step 5) includes: forming a first metal grid on a surface of the receiving substrate, forming a second metal grid on a surface of the gallium nitride thin film, stacking the first metal grid and the second metal grid, and then interdiffusing the first metal grid and the second metal grid through a bonding process to weakly bond the receiving substrate and the gallium nitride thin film, and simultaneously deactivating the temporary bonding layer in the bonding process to peel off the transfer substrate from the gallium nitride thin film.

[0010] Preferably, the temperature of the bonding process is below the melting temperature of the first and second metal grids.

[0011] Preferably, the material of the first metal grid comprises one of Ti, Cr and Mo, and the material of the second metal grid comprises one of Ti, Cr and Mo.

[0012] Preferably, forming a first metal grid on the surface of the receiving substrate includes: forming a photoresist layer on the surface of the receiving substrate, and forming a grid-groove-shaped photolithography pattern on the receiving substrate after an exposure process and a development process; and forming a first metal grid on the surface of the receiving substrate by a deposition process and a metal stripping process; and forming a second metal grid on the surface of the gallium nitride thin film includes: forming a photoresist layer on the surface of the gallium nitride thin film, and forming a grid-groove-shaped photolithography pattern on the gallium nitride thin film after an exposure process and a development process; and forming a second metal grid on the surface of the gallium nitride thin film by a deposition process and a metal stripping process.

[0013] Preferably, the first metal grid and the second metal grid have the same shape and size, and the first metal grid and the second metal grid overlap each other when the receiving substrate and the gallium nitride thin film are bonded.

[0014] Preferably, step 2) forms a temporary bonding layer on the gallium nitride thin film by a spin-coating process, and the temporary bonding layer includes one of an epoxy resin and a high temperature wax.

[0015] Preferably, the material of the transfer substrate and the receiving substrate includes one of sapphire, silicon and quartz.

[0016] Preferably, step 4) further comprises the step of acid-washing the exposed surface of the gallium nitride thin film so as to remove metallic gallium remaining on the surface of the gallium nitride thin film.

[0017] Preferably, step 6) comprises epitaxially growing a gallium nitride epitaxial layer on the gallium nitride thin film by a hydride vapor phase epitaxy process. Effect of the Invention

[0018] As described above, the method for fabricating a free-standing gallium nitride substrate according to the embodiment of the present application has the following beneficial effects.

[0019] In the embodiment of the present application, the receiving substrate and the metal grid are used to change the strong chemical bond between the sapphire substrate and the gallium nitride epitaxial layer into a weak bonding state realized by the metal grid, thereby creating a weak connection state between the gallium nitride thin film and the receiving substrate, and the lattice constraint of the receiving substrate is weakened when the gallium nitride epitaxial layer is grown by the hydride vapor phase epitaxy process, so that the gallium nitride epitaxial layer can be grown to a thick state, and when a certain thickness is reached, the lattice mismatch stress and thermal mismatch stress can be used to break through the weak connection state between the gallium nitride thin film and the receiving substrate, and the gallium nitride epitaxial layer can be separated from the receiving substrate, so that a thick gallium nitride epitaxial layer can be obtained by only one epitaxial growth. The embodiments of the present application effectively overcome the drawback that the thickness of a hetero gallium nitride epitaxial layer is limited due to lattice mismatch and thermal mismatch, thereby improving the quality of the free-standing gallium nitride substrate and reducing the manufacturing cost of the free-standing gallium nitride substrate.

[0020] Other aspects may be understood after reading and understanding the drawings and detailed description. [Brief description of the drawings]

[0021] The drawings are intended to provide a further understanding of the technical aspects of the present application, are a part of the specification, and are used to interpret the technical aspects of the present application together with the examples of the present application, but are not intended to limit the technical aspects of the present application.

[0022] [Figure 1] 2A to 2C are schematic structural diagrams presented at each step of a method for fabricating a free-standing gallium nitride substrate according to an embodiment of the present application; [Diagram 2] 2A to 2C are schematic structural diagrams presented at each step of a method for fabricating a free-standing gallium nitride substrate according to an embodiment of the present application; [Diagram 3] 2A to 2C are schematic structural diagrams presented at each step of a method for fabricating a free-standing gallium nitride substrate according to an embodiment of the present application; [Figure 4] 2A to 2C are schematic structural diagrams presented at each step of a method for fabricating a free-standing gallium nitride substrate according to an embodiment of the present application; [Diagram 5] 2A to 2C are schematic structural diagrams presented at each step of a method for fabricating a free-standing gallium nitride substrate according to an embodiment of the present application; [Figure 6] 2A to 2C are schematic structural diagrams presented at each step of a method for fabricating a free-standing gallium nitride substrate according to an embodiment of the present application; [Figure 7] 2A to 2C are schematic structural diagrams presented at each step of a method for fabricating a free-standing gallium nitride substrate according to an embodiment of the present application; [Figure 8] 2A to 2C are schematic structural diagrams presented at each step of a method for fabricating a free-standing gallium nitride substrate according to an embodiment of the present application; [Figure 9] 2A to 2C are schematic structural diagrams presented at each step of a method for fabricating a free-standing gallium nitride substrate according to an embodiment of the present application; [Explanation of symbols]

[0023] 101···sapphire substrate, 102···gallium nitride thin film, 103···transfer substrate, 104···temporary bonding layer, 105···receiving substrate, 106···first metal grid, 107···second metal grid, and 108···gallium nitride epitaxial layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, the embodiments of the present application will be described by specific specific examples, and those skilled in the art can easily understand other advantages and effects of the present application from the contents disclosed in this specification. The present application may be implemented or applied by other different specific embodiments, and each detail in the specification may be modified or changed in various ways without departing from the spirit of the present application based on different perspectives and applications.

[0025] For example, when describing the embodiments of the present application in detail, in order to facilitate the description, the cross-sectional views showing the device structures are partially enlarged without regard to the general ratio, and the schematic diagrams are merely examples and should not limit the scope of protection of the present application here. In addition, in actual fabrication, the dimensions of the three-dimensional space of length, width and depth should be included.

[0026] For ease of description, spatial relationship terms such as "below," "belower," "lower than," "underside," "above," "on," and the like may be used herein to describe the relationship of one element or feature to another element or feature shown in the figures. It is understood that these spatial relationship terms are intended to include other orientations of the device in use or operation other than those depicted in the figures. Also, when a layer is said to be "between" two layers, it may be the only layer between the two layers, or there may be one or more intervening layers therebetween.

[0027] In the context of this application, a structure in which a first feature is described is "on" a second feature may include embodiments in which the first feature and the second feature are formed such that they are in direct contact, or may include embodiments in which other features are formed between the first feature and the second feature, such that the first feature and the second feature may not be in direct contact.

[0028] It should be noted that the illustrations in this embodiment are merely for explaining the basic concept of the present application in a schematic manner, and therefore the drawings are not made according to the number, shape and size of the components in the actual implementation, but only show the components related to the present application, and the shape, number and proportion of each component in the actual implementation may be changed freely to some extent, and the arrangement of the components may be more complex.

[0029] As shown in FIGS. 1 to 9, this embodiment provides a method for fabricating a free-standing gallium nitride substrate, the method including:

[0030] As shown in FIG. 1, step 1) is performed by first providing a composite substrate including a sapphire substrate 101 and a gallium nitride thin film 102 formed on the sapphire substrate 101, and in one embodiment, the thickness of the gallium nitride thin film 102 is 4.5 μm.

[0031] As shown in FIG. 2, step 2) of forming a temporary bonding layer 104 on the gallium nitride thin film 102 and step 3) of providing a transfer substrate 103 and bonding the transfer substrate 103 to the composite substrate via the temporary bonding layer 104 are then performed.

[0032] In one embodiment, step 2) forms a temporary bonding layer 104 on the gallium nitride thin film 102 by a spin-coating process, and the temporary bonding layer 104 includes one of epoxy resin and high-temperature wax. For example, the high-temperature wax may have a melting temperature higher than 100° C., and is in a viscous solid state below 100° C., and when the temperature is higher than the melting temperature, it melts partially or completely, or decomposes and carbonizes to lose viscosity.

[0033] In one embodiment, the material of the transfer substrate 103 includes one of sapphire, silicon and quartz. For example, in this embodiment, the material of the transfer substrate 103 may be sapphire. The shape and size of the transfer substrate 103 are completely the same as those of the sapphire substrate 101.

[0034] As shown in FIG. 3, thereafter, step 4) is performed in which the sapphire substrate 101 is peeled off by a laser peeling process to expose the gallium nitride thin film 102, and the surface exposed to the outside of the gallium nitride thin film 102 is the N-face.

[0035] In one embodiment, the sapphire substrate 101 is peeled off from the gallium nitride thin film 102 by irradiating one side of the sapphire substrate 101 with a laser.

[0036] In one embodiment, step 4) further includes the step of pickling the exposed surface of the gallium nitride thin film 102 with an acid so as to remove metallic gallium remaining on the surface of the gallium nitride thin film 102 .

[0037] As shown in Figures 4 to 6, step 5) is then performed in which a receiving substrate 105 is provided, the receiving substrate 105 and the gallium nitride thin film 102 are weakly bonded together, and the temporary bonding layer 104 is deactivated to peel off the transfer substrate 103 from the gallium nitride thin film 102.

[0038] In one embodiment, step 5) includes:

[0039] As shown in FIG. 4, first, step 5-1) of forming a first metal grid 106 on the surface of the receiving substrate 105 is performed.

[0040] Specifically, forming the first metal grid 106 on the surface of the receiving substrate 105 includes: firstly forming a photoresist layer on the surface of the receiving substrate 105 by a spin-coating process; then forming a grid-groove-shaped photolithography pattern on the receiving substrate 105 after an exposure process and a development process; then forming a first metal coating layer on the surface of the receiving substrate 105 and the photolithography pattern by a deposition process; and then forming a protruding first metal grid 106 by a metal stripping process, i.e., simultaneously removing the photolithography pattern and the metal coating layer thereon.

[0041] In one embodiment, the temperature of the bonding process is lower than the melting temperature of the first metal grid 106. For example, the material of the first metal grid 106 includes one of Ti, Cr, and Mo.

[0042] As shown in FIG. 5, thereafter, step 5-2) of forming a second metal grid 107 on the surface of the gallium nitride thin film 102 is performed.

[0043] Specifically, forming a second metal grid 107 on the surface of the gallium nitride thin film 102 includes: forming a photoresist layer on the surface of the gallium nitride thin film 102 by a spin-coating process, and forming a grid-groove photolithography pattern on the gallium nitride thin film 102 after an exposure process and a development process; forming a second metal coating layer on the surface of the gallium nitride thin film 102 and the photolithography pattern by a deposition process, and forming a protruding second metal grid 107 by a metal stripping process, i.e., simultaneously removing the photolithography pattern and the metal coating layer thereon.

[0044] In one embodiment, the temperature of the bonding process is lower than the melting temperature of the second metal grid 107. For example, the material of the second metal grid 107 includes one of Ti, Cr, and Mo.

[0045] The materials of the metal grid 106 and the metal grid 107 may be the same or different.

[0046] As shown in FIG. 6, finally, step 5-3) is performed in which the first metal grid 106 and the second metal grid 107 are laminated, and then the first metal grid 106 and the second metal grid 107 are mutually diffused by a bonding process to weakly bond the receiving substrate 105 and the gallium nitride thin film 102, and at the same time, the temporary bonding layer 104 is deactivated by the bonding process to peel off the transfer substrate 103 from the gallium nitride thin film 102.

[0047] For example, in one embodiment, the temporary bonding layer 104 may be a high-temperature wax that is in a viscous solid state below 100° C., and when it is higher than the melting temperature, it will melt partially or completely, decompose, carbonize, and lose its viscosity. The bonding temperature is, on the one hand, lower than the melting temperature of the first metal grid 106 and the second metal grid 107, thereby avoiding the compatibility between the first metal grid 106 and the second metal grid 107, which causes the bonding strength to be too high, and the bonding temperature only makes them interdiffuse in the solid state to realize a weak connection, and, on the other hand, higher than the melting temperature of the high-temperature wax, thereby making it automatically lose its effectiveness, e.g., carbonize, and realize the separation between the transfer substrate 103 and the gallium nitride thin film 102. This embodiment can eliminate the extra peeling step of the transfer substrate 103, and greatly reduce the process time and process cost.

[0048] As shown in Figures 7 to 9, finally, step 6) is performed in which a gallium nitride epitaxial layer 108 is epitaxially grown on the gallium nitride thin film 102, and the gallium nitride epitaxial layer 108 is grown to a certain thickness. After that, the weak bonding state between the gallium nitride thin film 102 and the receiving substrate 105 is lost due to lattice mismatch stress and thermal mismatch stress between the gallium nitride thin film 102 and the gallium nitride epitaxial layer 108 and the receiving substrate 105, thereby achieving separation between the gallium nitride thin film 102 and the receiving substrate 105, and obtaining a free-standing gallium nitride substrate.

[0049] In one embodiment, the first metal grid 106 and the second metal grid 107 have the same shape and size, and when the receiving substrate 105 and the gallium nitride thin film 102 are bonded, the first metal grid 106 and the second metal grid 107 overlap each other. In the bonding process, the diffusion connection between only the metal located on the top surface of the photolithography pattern can be ensured, so as to weaken the connection strength between the receiving substrate 105 and the gallium nitride thin film 102; at the same time, since the metal grid has a certain height and may form a cavity between the metal grid after bonding, the lattice constraint of the receiving substrate 105 is weakened when the gallium nitride epitaxial layer 108 is grown in a hydride vapor phase epitaxy process, so that the gallium nitride epitaxial layer 108 is grown in a thick state, and when it reaches a certain thickness, the lattice mismatch stress and thermal mismatch stress can be used to break through the weak connection state between the gallium nitride thin film 102 and the receiving substrate 105, and the separation of the gallium nitride thin film 102 and the receiving substrate 105 can be realized. This embodiment can automatically separate the gallium nitride thin film 102 and the receiving substrate 105, greatly reducing the process cost.

[0050] In one embodiment, step 6) epitaxially grows a gallium nitride epitaxial layer 108 on the gallium nitride thin film 102 by a hydride vapor phase epitaxy process.

[0051] In one embodiment, the method further comprises polishing and / or cleaning the gallium nitride epitaxial layer 108 so as to obtain a gallium nitride epitaxial layer 108 with good surface quality and to obtain a high quality free-standing gallium nitride substrate.

[0052] As described above, the method for fabricating a free-standing gallium nitride substrate according to the embodiment of the present application has the following beneficial effects.

[0053] In the embodiment of the present application, the receiving substrate 105 and the metal grid are used to change the strong chemical bond between the sapphire substrate 101 and the gallium nitride epitaxial layer 108 into a weak bonding state realized by the metal grid, thereby creating a weak connection state between the gallium nitride thin film 102 and the receiving substrate 105, and the lattice constraint of the receiving substrate 105 is weakened when the gallium nitride epitaxial layer 108 is grown by the hydride vapor phase epitaxy process, so that the gallium nitride epitaxial layer 108 can be grown to a thick state. When the gallium nitride epitaxial layer 108 reaches a certain thickness, the lattice mismatch stress and thermal mismatch stress can be used to break through the weak connection state between the gallium nitride thin film 102 and the receiving substrate 105, and the gallium nitride epitaxial layer 108 can be separated from the receiving substrate 105, so that a thick gallium nitride epitaxial layer 108 can be obtained by only one epitaxial growth.

[0054] Therefore, the embodiments of the present application effectively overcome various shortcomings in the related art and have high industrial applicability.

[0055] Example 1 As shown in FIGS. 1 to 9, this embodiment provides a method for fabricating a free-standing gallium nitride substrate, the method including:

[0056] As shown in FIG. 1, step 1) is first performed by providing a composite substrate including a sapphire substrate 101 and a gallium nitride thin film 102 formed on the sapphire substrate 101, and the gallium nitride thin film 102 has a thickness of 4.5 μm.

[0057] As shown in FIG. 2, step 2) of forming a temporary bonding layer 104 on the gallium nitride thin film 102 and step 3) of providing a transfer substrate 103 and bonding the transfer substrate 103 to the composite substrate via the temporary bonding layer 104 are then performed.

[0058] Step 2) forms a temporary bonding layer 104 on the gallium nitride thin film 102 by a spin-coating process, the temporary bonding layer 104 being a high-temperature wax, which has a melting temperature greater than 100°C and is in a viscous solid state below 100°C, and when the temperature is higher than the melting temperature, it melts partially or completely, or decomposes and carbonizes, losing its viscosity.

[0059] The material of the transfer substrate 103 is sapphire, and its shape and size are completely the same as those of the sapphire substrate 101.

[0060] As shown in FIG. 3, thereafter, step 4) is performed in which the sapphire substrate 101 is peeled off by a laser peeling process to expose the gallium nitride thin film 102, and the surface exposed to the outside of the gallium nitride thin film 102 is the N-face.

[0061] The sapphire substrate 101 is peeled off from the gallium nitride thin film 102 by irradiating one surface of the sapphire substrate 101 with a laser.

[0062] Step 4) further includes a step of pickling the exposed surface of the gallium nitride thin film 102 by immersing it in a mixture of HCl:H2O=1:1 for 30 seconds, thereby removing metallic gallium remaining on the surface of the gallium nitride thin film 102.

[0063] As shown in Figures 4 to 6, step 5) is then performed in which a receiving substrate 105 is provided, the receiving substrate 105 and the gallium nitride thin film 102 are weakly bonded together, and the temporary bonding layer 104 is deactivated to peel off the transfer substrate 103 from the gallium nitride thin film 102.

[0064] Step 5) includes the following steps: As shown in FIG.

[0065] Specifically, forming the first metal grid 106 on the surface of the receiving substrate 105 includes: firstly forming a photoresist layer on the surface of the receiving substrate 105 by a spin-coating process; then forming a grid-groove-shaped photolithography pattern on the receiving substrate 105 after an exposure process and a development process; then forming a first metal coating layer on the surface of the receiving substrate 105 and the photolithography pattern by a deposition process; and then forming a protruding first metal grid 106 by a metal stripping process, i.e., simultaneously removing the photolithography pattern and the metal coating layer thereon.

[0066] The temperature of the bonding process is 400° C., which is lower than the melting temperature of the first metal grid 106. The material of the first metal grid 106 is Ti.

[0067] As shown in FIG. 5, thereafter, step 5-2) of forming a second metal grid 107 on the surface of the gallium nitride thin film 102 is performed.

[0068] Specifically, forming a second metal grid 107 on the surface of the gallium nitride thin film 102 includes: forming a photoresist layer on the surface of the gallium nitride thin film 102 by a spin-coating process, and forming a grid-groove photolithography pattern on the gallium nitride thin film 102 after an exposure process and a development process; forming a second metal coating layer on the surface of the gallium nitride thin film 102 and the photolithography pattern by a deposition process, and forming a protruding second metal grid 107 by a metal stripping process, i.e., simultaneously removing the photolithography pattern and the metal coating layer thereon.

[0069] The temperature of the bonding process is 400° C., which is lower than the melting temperature of the second metal grid 107. The material of the second metal grid 107 is Ti.

[0070] As shown in FIG. 6, finally, step 5-3) is performed in which the first metal grid 106 and the second metal grid 107 are laminated, and then the first metal grid 106 and the second metal grid 107 are mutually diffused by a bonding process to weakly bond the receiving substrate 105 and the gallium nitride thin film 102, and at the same time, the temporary bonding layer 104 is deactivated by the bonding process to peel off the transfer substrate 103 from the gallium nitride thin film 102.

[0071] The temporary bonding layer 104 is a high-temperature wax that is in a solid state with viscosity below 100°C, and when it is higher than the melting temperature, it melts partially or completely, or decomposes and carbonizes to lose its viscosity. The bonding temperature is 400°C, which is lower than the melting temperature of the first metal grid 106 and the second metal grid 107 on the one hand, thereby avoiding the compatibility between the first metal grid 106 and the second metal grid 107 to cause too high bonding strength, and the bonding temperature only makes them interdiffuse in a solid state to realize a weak connection, and on the other hand, the bonding temperature is 400°C, which is higher than the melting temperature of the high-temperature wax, thereby making it automatically lose its function and realize the separation between the transfer substrate 103 and the gallium nitride thin film 102. This embodiment can eliminate the extra peeling step of the transfer substrate 103, and greatly reduce the process time and process cost.

[0072] As shown in Figures 7 to 9, finally, a gallium nitride epitaxial layer 108 is epitaxially grown on the gallium nitride thin film 102, and the gallium nitride epitaxial layer 108 is grown to a certain thickness. Then, due to the lattice mismatch stress and thermal mismatch stress between the gallium nitride thin film 102 and the gallium nitride epitaxial layer 108 and the receiving substrate 105, the weak bonding state between the gallium nitride thin film 102 and the receiving substrate 105 is lost, and separation between the gallium nitride thin film 102 and the receiving substrate 105 is realized, and a free-standing gallium nitride substrate is obtained in step 6). After measurement, the thickness of the free-standing gallium nitride substrate finally obtained is about 500 μm.

[0073] The first metal grid 106 and the second metal grid 107 have the same shape and size, and when the receiving substrate 105 and the gallium nitride thin film 102 are bonded, the first metal grid 106 and the second metal grid 107 overlap each other. In the bonding process, the diffusion connection between only the metal located on the top surface of the photolithography pattern can be ensured, so as to weaken the connection strength between the receiving substrate 105 and the gallium nitride thin film 102; at the same time, since the metal grid has a certain height and may form a cavity between the metal grid after bonding, the lattice constraint of the receiving substrate 105 is weakened when the gallium nitride epitaxial layer 108 is grown in a hydride vapor phase epitaxy process, so that the gallium nitride epitaxial layer 108 is grown in a thick state, and when it reaches a certain thickness, the lattice mismatch stress and thermal mismatch stress can be used to break through the weak connection state between the gallium nitride thin film 102 and the receiving substrate 105, and the gallium nitride thin film 102 and the receiving substrate 105 can be separated. This embodiment can automatically peel off the gallium nitride thin film 102 and the receiving substrate 105, greatly reducing the process cost.

[0074] Step 6) epitaxially grows a gallium nitride epitaxial layer 108 on the gallium nitride thin film 102 by a hydride vapor phase epitaxy process.

[0075] In this embodiment, the method further includes polishing and / or cleaning the gallium nitride epitaxial layer 108, so as to obtain a gallium nitride epitaxial layer 108 with good surface quality and obtain a high-quality free-standing gallium nitride substrate.

[0076] The above embodiments are not intended to limit the present application, but merely to exemplify the principles and effects of the present application. Those skilled in the art may modify or change the above embodiments without violating the spirit and scope of the present application. Therefore, any equivalent modifications or changes made by those with ordinary knowledge in the art without departing from the spirit and technical ideas set forth in the present application should still be covered by the claims of the present application.

Claims

1. 1) providing a composite substrate comprising a sapphire substrate and a gallium nitride thin film formed on the sapphire substrate; 2) forming a temporary bonding layer on the gallium nitride thin film; 3) providing a transfer substrate and bonding the transfer substrate to the composite substrate by the temporary bonding layer; 4) peeling off the sapphire substrate by a laser peeling process to expose the gallium nitride thin film; 5) providing a receiving substrate, forming a first metal grid on a surface of the receiving substrate, forming a second metal grid on a surface of the gallium nitride thin film, laminating the first metal grid and the second metal grid, and then using a bonding process to mutually diffuse the first metal grid and the second metal grid to weakly bond the receiving substrate and the gallium nitride thin film, and at the same time, using the bonding process to deactivate the temporary bonding layer to peel off the transfer substrate from the gallium nitride thin film; 6) epitaxially growing a gallium nitride epitaxial layer on the gallium nitride thin film, and growing the gallium nitride epitaxial layer to a certain thickness, and then causing a lattice mismatch stress and a thermal mismatch stress between the gallium nitride thin film and the gallium nitride epitaxial layer and the receiving substrate to break the weak bonding state between the gallium nitride thin film and the receiving substrate, thereby realizing separation between the gallium nitride thin film and the receiving substrate and obtaining a free-standing gallium nitride substrate. A method for fabricating a free-standing gallium nitride substrate.

2. the temperature of the bonding process is lower than the melting temperature of the first metal grid and the second metal grid; A method for fabricating the free-standing gallium nitride substrate of claim 1.

3. the material of the first metal grid includes one of Ti, Cr, and Mo, and the material of the second metal grid includes one of Ti, Cr, and Mo; A method for fabricating the free-standing gallium nitride substrate of claim 1.

4. The method of forming a first metallic grid on a surface of a receiving substrate includes the steps of: forming a photoresist layer on the surface of the receiving substrate, and forming a grid groove-shaped photolithography pattern on the receiving substrate after an exposure process and a development process; forming a first metal grid on a surface of the receiving substrate by a deposition process and a metal release process; The method for forming a second metal grid on a surface of the gallium nitride thin film includes the steps of: forming a photoresist layer on the surface of the gallium nitride thin film, and forming a grid groove-shaped photolithography pattern on the gallium nitride thin film after an exposure process and a development process; forming a second metal grid on the surface of the gallium nitride thin film by a deposition process and a metal release process; A method for fabricating the free-standing gallium nitride substrate of claim 1.

5. the first metal grid and the second metal grid have the same shape and size, and when the receiving substrate and the gallium nitride thin film are bonded, the first metal grid and the second metal grid are overlapped with each other; A method for fabricating the free-standing gallium nitride substrate of claim 1.

6. Step 2) forms a temporary bonding layer on the gallium nitride thin film by a spin-coating process, the temporary bonding layer comprising one of an epoxy resin and a high-temperature wax; A method for fabricating the free-standing gallium nitride substrate of claim 1.

7. The material of the transfer substrate and the receiving substrate includes one of sapphire, silicon, and quartz; A method for fabricating the free-standing gallium nitride substrate of claim 1.

8. Step 4) further includes a step of pickling the exposed surface of the gallium nitride thin film so as to remove metallic gallium remaining on the surface of the gallium nitride thin film. A method for fabricating the free-standing gallium nitride substrate of claim 1.

9. Step 6) epitaxially growing a gallium nitride epitaxial layer on the gallium nitride thin film by a hydride vapor phase epitaxy process; A method for fabricating the free-standing gallium nitride substrate of claim 1.

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