LED thin film device and preparation method thereof

Abstract

According to the LED thin-film device and the preparation method thereof provided by the invention, the problem that a silicon support substrate is broken due to overlarge warping degree after evaporation is avoided in a manner of bonding and then performing back gold evaporation. By evaporating a plurality of stress-adjusting metal layers, the metal on the back surface of the silicon support substrate generates downward large pressure stress on the silicon support substrate so as to counteract the pressure stress back to the silicon support substrate generated after a substrate of an epitaxial wafer is removed, so that the warping degree of a thin film device is reduced, and large-scale production is realized. The preparation method comprises the following steps: providing an LED epitaxial wafer, wherein the LED epitaxial wafer comprises a substrate and an epitaxial layer; sequentially preparing a reflector layer and a first bonding layer on the epitaxial layer; providing a silicon support substrate, and preparing a second bonding layer on the front surface of the silicon support substrate; the thickness of the silicon support substrate is less than 180 [mu] m; the LED epitaxial wafer and the silicon supporting substrate are bonded; evaporating a metal layer for adjusting stress on the back surface of the silicon support substrate; the metal layer sequentially comprises a bonding layer, a stress adjusting layer and a contact layer; and removing the substrate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor manufacturing, in particular to an LED thin film device and a preparation method thereof. BACKGROUND

[0002] In the preparation process of the vertical structure LED, wafer bonding needs to be performed between the epitaxial wafer and the support substrate, and the substrate of the epitaxial wafer needs to be removed subsequently. However, after the substrate is removed, the device is prone to obvious warping due to excessive residual stress, which hinders the smooth production, and therefore, the industry conventional solution is to use a thick support substrate to suppress the warping. Specifically, a 2-inch wafer is matched with a support substrate with a thickness of about 430 μm, and a 4-inch wafer is matched with a support substrate with a thickness of about 625 μm. Meanwhile, in order to cope with the corrosion of various acid and alkali solutions in which the wafer needs to be soaked in the process, a corrosion-resistant metal layer is evaporated on the back of the support substrate, and the outermost layer is usually gold (Au).

[0003] In order to meet the requirement of the dicing thickness, the support substrate needs to undergo thinning and re-evaporation of the back gold, and the specific process is as follows: After the previous process is completed, the support substrate of the wafer-bonded thin film device is thinned until the thickness of the thin film device is less than 200 μm to meet the requirement of dicing. However, this thinning process has a chain reaction: when the support substrate is thinned, the gold (Au) metal layer originally plated on the back of the substrate for corrosion prevention is removed at the same time. Since the metal layer is a key structure for realizing the electrical connection between the device and the external circuit, in order to ensure the electrical connection, the back gold needs to be re-evaporated on the surface of the thinned support substrate.

[0004] The above process has the following problems: first, the process of thinning and second evaporation of the back gold of the support substrate increases the process complexity and production cost; second, the thinning needs to remove the back metal by grinding, and compared with the silicon material commonly used for grinding the support substrate, the metal grinding has higher requirements for the wear resistance and hardness of the thinning machine grinding wheel, which not only aggravates the equipment wear and tear but also may reduce the thinning accuracy; third, after the support substrate is thinned, the warping degree of the thin film device increases significantly, which easily causes the breakage of the thin film device during the second evaporation of the back gold, thereby increasing the difficulty of the operation of the subsequent process steps such as point measurement and dicing. SUMMARY

[0005] Based on the problems of the conventional vertical structure LED preparation process, such as the multiple processes, high equipment requirements, and easy breakage of the thin film device after the support substrate is thinned, the present application provides an LED thin film device and a preparation method thereof, which effectively control the warping degree of the thin film device, reduce the risk of breakage, and simplify the process flow, improve the production efficiency and product yield.

[0006] In a first aspect, the present application provides a preparation method of an LED thin film device, comprising the following steps: S1, providing an LED epitaxial wafer, the LED epitaxial wafer comprising a substrate and an epitaxial layer; a mirror layer and a first bonding layer are sequentially prepared on the epitaxial layer; the thermal expansion coefficient of the substrate is less than the thermal expansion coefficient of the epitaxial layer; S2, providing a silicon support substrate, a second bonding layer is prepared on the front surface of the silicon support substrate; the thickness of the silicon support substrate is less than 180 μm; S3, bonding the LED epitaxial wafer and the silicon support substrate; S4, evaporating a stress-adjusting metal layer on the back surface of the silicon support substrate; the metal layer comprises a bonding layer, a stress-adjusting layer and a contact layer in sequence; S5, removing the substrate, and completing the preparation of the LED thin film device.

[0007] The preparation method of the LED thin film device provided in the application selects a thin silicon support substrate with a thickness meeting the wafering requirement, and thus subsequent thinning is not needed, and the process steps are reduced. Only one metal layer needs to be evaporated on the back surface of the silicon support substrate, and thus secondary evaporation is not needed, time and cost are saved. Bonding first and then evaporating the metal layer can solve the problem of broken pieces after evaporating the back metal layer of the silicon support substrate. In addition, after the substrate is removed, the front surface of the silicon support substrate will be subjected to compressive stress generated by the epitaxial layer, the mirror and the bonding layer (since the thermal expansion coefficients of the epitaxial layer, the mirror and the bonding layer are all greater than that of the silicon support substrate, after cooling, the shrinkage degree is greater than that of the silicon support substrate, and thus the silicon support substrate is subjected to compressive stress), and the thickness of the silicon support substrate is relatively thin, and a certain warping will be generated. When the metal layer on the back surface is evaporated, heating is needed, and a certain thermal stress is obtained on the back surface of the silicon support substrate, which can reduce the compressive stress on the front surface of the silicon support substrate, and thus the warping degree of the LED thin film device is reduced.

[0008] As an optional solution of the preparation method, the silicon support substrate is a 2-inch or 4-inch wafer, and the thickness of the silicon support substrate is 170 μm to 175 μm.

[0009] As an optional solution of the preparation method, the temperature for evaporating the metal layer in step S4 is 100 ℃ to 200 ℃.

[0010] As an optional solution of the preparation method, the bonding layer is Ti, and the thickness of the bonding layer is 500 Å to 1000 Å.

[0011] As an optional solution of the preparation method, the stress-adjusting layer is Cu, and the thickness of the stress-adjusting layer is 120000 Å.

[0012] As an alternative of the preparation method of the present application, the stress adjustment layer is composed of a first stress adjustment layer, a second stress adjustment layer and a third stress adjustment layer; the first stress adjustment layer is Cu, the thickness of the first stress adjustment layer is 20000-45000 angstroms; the second stress adjustment layer is In, the thickness of the second stress adjustment layer is 1000-1200 angstroms; the third stress adjustment layer is Cu, the thickness of the third stress adjustment layer is 45000 angstroms.

[0013] As an alternative of the preparation method of the present application, the contact layer is Au, the thickness of the contact layer is 2000-2500 angstroms.

[0014] As an alternative of the preparation method of the present application, the contact layer is composed of a first contact layer, a second contact layer and a third contact layer; the first contact layer is Au, the thickness of the first contact layer is 400-600 angstroms; the second contact layer is Pt, the thickness of the second contact layer is 400-600 angstroms; the third contact layer is Au, the thickness of the third contact layer is 1000-2000 angstroms.

[0015] As an alternative of the preparation method of the present application, the substrate is a silicon substrate, and the epitaxial layer is a gallium nitride-based epitaxial layer.

[0016] In a second aspect, the present application also provides an LED thin film device prepared by the above method.

[0017] Additional aspects and advantages of the present application will be better understood from the following descriptions in conjunction with the accompanying drawings. Embodiments of the present application will be illustrated and described, and their objectives, as well as advantages, will become apparent. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The flowchart of the embodiment of the present application is shown.

[0019] Figure 2 The cross-sectional view of the embodiment of the present application after step S3 is shown.

[0020] Figure 3 The cross-sectional view of the embodiment of the present application after step S4 is shown.

[0021] Figure 4 The cross-sectional view of the embodiment of the present application after step S5 is shown.

[0022] Figure 5 The distribution of the warpage of the embodiment of the present application after step S5 is shown.

[0023] Figure 6 The cross-sectional view of the embodiment of the present application after step S4 is shown.

[0024] Figure 7A cross-sectional view of the wafer after step S5 of Example 2 of the present application.

[0025] Figure 8 A warpage distribution diagram of the wafer after step S5 of Example 2 of the present application.

[0026] Illustration: 1 - substrate; 2 - epitaxial layer; 3 - mirror layer; 4 - bonding layer; 5 - silicon support substrate; 6 - adhesive layer; 7 - stress adjustment layer; 7-1 - first stress adjustment layer; 7-2 - second stress adjustment layer; 7-3 - third stress adjustment layer; 8 - contact layer; 8-1 - first contact layer; 8-2 - second contact layer; 8-3 - third contact layer. DETAILED DESCRIPTION

[0027] The present application is further described below in conjunction with the accompanying drawings and examples.

[0028] It should be noted that the following detailed description is exemplary in nature and is intended to provide further description of the application. Unless otherwise defined, 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 application belongs.

[0029] It is also important to note that the terms used herein are not intended to limit the exemplary embodiments to the specific embodiments which are described herein. Rather, it is contemplated that the exemplary embodiments are intended to cover all possible combinations and sub-combinations of the various features, steps, reagents, devices, components and / or methods described herein.

[0030] In the present application, the terms such as "upper", "lower", "horizontal", "intermediate" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, which is a relationship word determined only for the purpose of describing the structural relationship of the components or elements of the present application, and is not intended to specify any component or element in the present application, and cannot be understood as a limitation on the present application.

[0031] The example of the present application provides a preparation method of an LED thin film device, comprising the following steps: S1, provide an LED epitaxial wafer, the LED epitaxial wafer comprising a substrate 1 and an epitaxial layer 2; a mirror layer 3 and a first bonding layer are prepared on the epitaxial layer 2 in sequence; the thermal expansion coefficient of the substrate 1 is less than the thermal expansion coefficient of the epitaxial layer 2; it should be noted that after epitaxial growth and cooling to room temperature, the epitaxial layer 2 shrinks more and the substrate 1 shrinks less due to the thermal expansion coefficient of the substrate 1 being less than the epitaxial layer 2, so the substrate 1 exerts a tensile stress on the epitaxial layer 2, that is, there is a stretching force on the epitaxial layer 2, when the substrate 1 is completely removed later, the tensile stress on the epitaxial layer 2 is suddenly released, and the epitaxial layer 2 that was originally stretched wants to return to a free state, which will generate a large compressive stress on the front surface of the silicon support substrate 5.

[0032] S2, provide a silicon support substrate 5, and a second bonding layer is prepared on the front surface of the silicon support substrate 5; the thickness of the silicon support substrate 5 is less than 180 μm; it should be noted that a thin silicon support substrate with a thickness meeting the scribing requirement is directly selected, and subsequent thinning is not required, thereby reducing the process steps and saving time and cost.

[0033] S3, bond the LED epitaxial wafer and the silicon support substrate 5.

[0034] S4, evaporate a stress-adjusting metal layer on the back surface of the silicon support substrate 5; the metal layer comprises a bonding layer 6, a stress-adjusting layer 7 and a contact layer 8 in sequence; it should be noted that after the substrate is removed, the front surface of the silicon support substrate will be subjected to a compressive stress generated by the epitaxial layer 2, the mirror layer 3 and the bonding layer 4, and the silicon support substrate is relatively thin, which will generate a certain amount of warping. When the metal layer on the back surface is evaporated, heating is required to allow the back surface of the silicon support substrate to obtain a certain amount of thermal stress, which can reduce the compressive stress on the front surface of the silicon support substrate, thereby reducing the warping degree of the LED thin film device. And the epitaxial wafer and the silicon support substrate wafer are bonded first and then the metal layer on the back surface of the silicon support substrate is evaporated, which can solve the problem of broken pieces after the silicon support substrate is evaporated.

[0035] S5, remove the substrate 1, and the preparation of the LED thin film device is completed.

[0036] In some embodiments of the present application, the silicon support substrate is a 2-inch or 4-inch wafer, and the thickness of the support substrate is 170 μm to 175 μm. It should be noted that the thickness of the silicon support substrate in this range can meet the scribing requirement and control the warping degree of the prepared LED thin film device within a certain range, and the epitaxial wafer substrate is not easy to break after being removed.

[0037] In some embodiments of the present application, the temperature for evaporating the metal layer in step S4 is 100℃ to 200℃. It should be noted that controlling the temperature for evaporating the metal layer within this range can allow the silicon support substrate to obtain a suitable amount of thermal stress, which can match the compressive stress on the front surface of the silicon support substrate.

[0038] In some embodiments of the present application, the adhesive layer is Ti, and the thickness of the adhesive layer is 500-1000 A. It should be noted that Ti has good adhesion with the silicon support substrate, and avoids the stress adjustment layer from falling off in subsequent preparation.

[0039] In some embodiments of the present application, the stress adjustment layer is Cu, and the thickness of the stress adjustment layer is 120000 A. It should be noted that the thermal expansion coefficient of Cu is 16.5x10 −6 K −1 , the thermal expansion coefficient of the silicon support substrate is 2.6x10 −6 K −1 , and Cu is much larger than silicon, and Cu shrinks more severely when cooled. Thermal stress is determined by the coefficient of thermal expansion (CTE) mismatch and the stiffness of the metal thin film (Young's modulus x thickness). Therefore, a thicker Cu layer is needed to apply a larger compressive stress to the back of the silicon support substrate, to offset the compressive stress on the front of the silicon support substrate from the epitaxial layer 2, the mirror layer 3 and the bonding layer 4, so as to obtain a thin film device with low warpage. In addition, Cu also has good ductility, which can avoid brittle cracking of thick metal films.

[0040] In some embodiments of the present application, the stress adjustment layer is composed of a first stress adjustment layer, a second stress adjustment layer and a third stress adjustment layer; the first stress adjustment layer is Cu, and the thickness of the first stress adjustment layer is 20000-45000 A; the second stress adjustment layer is In, and the thickness of the second stress adjustment layer is 1000-1200 A; and the third stress adjustment layer is Cu, and the thickness of the third stress adjustment layer is 45000 A. It should be noted that the second stress adjustment layer uses In to reduce the total thickness of the stress adjustment layer. The thermal expansion coefficient of Cu is 16.5x10 −6 K −1 , the thermal expansion coefficient of In is 32.1x10 −6 K −1 , and the thermal expansion coefficient of In is higher than that of Cu, and its thermal mismatch effect is more obvious, and its required thickness is lower. The specific principle is that when cooled (from 100°C to 25°C), the In thin film is constrained by the adjacent Cu layer, and is subjected to tensile stress by the first layer of Cu, and since the melting point of In is only 157°C, low-temperature plastic deformation is easy to occur, the compressive stress of In is released due to plastic deformation, the "pull back" effect of In layer on Cu layer is weakened, resulting in that Cu layer shrinks more freely, and the net tensile stress of Cu to the silicon support substrate is increased. If the thickness of In is too large, the compressive stress on the silicon support substrate is too large, and the thin film device is prone to have too large reverse warpage (i.e. convex state), and still cannot be successfully produced. In addition, since the melting point of In is low, In does not have the stability of Cu, and the ductility of In is not as good as that of Cu, so In can only be used in combination with Cu to adjust the stress, and In cannot be used as the stress adjustment layer.

[0041] In some embodiments of the present application, the contact layer is Au, and the thickness of the contact layer is 2000-2500 A. It should be noted that Au has good thermal conductivity and corrosion resistance, and the presence of Au can protect the silicon support substrate, the adhesion layer and the stress adjustment layer from the corrosion of acid and alkali thermal solution and the influence of dry etching in subsequent processes.

[0042] In some embodiments of the present application, the contact layer is composed of a first contact layer, a second contact layer and a third contact layer; the first contact layer is Au, and the thickness of the first contact layer is 400-600 A; the second contact layer is Pt, and the thickness of the second contact layer is 400-600 A; the third contact layer is Au, and the thickness of the third contact layer is 1000-2000 A. It should be noted that Pt is used as the second contact layer because Pt is cheaper than Au, and because Pt generates greater thermal stress than Au under the same temperature difference and thickness, which can help the stress adjustment layer to adjust stress and save the amount of Au used. In addition, Pt also has good corrosion resistance.

[0043] In some embodiments of the present application, the substrate 1 is a silicon substrate, and the epitaxial layer 2 is a gallium nitride-based epitaxial layer.

[0044] In some embodiments of the present application, the materials of the first bonding layer and the second bonding layer are binary alloys, one of which is a low-melting-point metal, and the other of which is a high-melting-point metal, including but not limited to CuIn, AuIn, AuSn and NiSn.

[0045] The embodiments of the present application also provide an LED thin film device prepared by the above method. Embodiment 1

[0046] The present embodiment provides a preparation method of an LED thin film device. Taking an LED thin film device with a size of 4 inches as an example, the flowchart of the present embodiment is shown in Figure 1 It should be noted that since the thickness of some metal layers is too thin, it cannot be displayed in proportion, and therefore Figure 2 、 3 , 4, 6 and 7 are not drawn in actual proportion.

[0047] The preparation process of the present embodiment includes the following steps: Step S1, providing an LED epitaxial wafer, the LED epitaxial wafer including a substrate 1 and an epitaxial layer 2; a mirror layer 3 and a first bonding layer are sequentially prepared on the epitaxial layer 2. The substrate 1 is a silicon substrate with a size of 4 inches, and the specific radius is 50.8 mm. The material of the epitaxial layer 2 is GaN-based material, the mirror layer 3 is an Ag mirror, and the material of the first bonding layer is CuIn.

[0048] Step S2: Provide a silicon support substrate 5, and fabricate a second bonding layer on the front side of the silicon support substrate 5; the thickness of the silicon support substrate 5 is less than 180 μm. The silicon support substrate 5 is a 4-inch wafer with a thickness of 175 μm and a specific radius of 50.8 mm. The material of the second bonding layer is CuIn.

[0049] Step S3: Bond the LED epitaxial wafer and the silicon support substrate 5 together. The structure after bonding is as follows: Figure 2 As shown. The first bonding layer and the second bonding layer are bonded together to form bonding layer 4.

[0050] Step S4: Evaporate the adhesive layer 6, the first stress-adjusting layer 7-1, the second stress-adjusting layer 7-2, the third stress-adjusting layer 7-3, and the contact layer 8 on the back side of the silicon support substrate 5, as follows: Figure 3 As shown.

[0051] The adhesive layer 6 is made of metallic Ti with a thickness of 500 Å, which allows it to adhere well to the silicon support substrate 5. If the first stress-adjusting layer 7-1 (Cu) is directly applied to the substrate, Cu is prone to detachment.

[0052] The first stress-adjusting layer 7-1 is made of metallic Cu with a thickness of 45,000 Å; the second stress-adjusting layer 7-2 is made of metallic In with a thickness of 1,200 Å; and the third stress-adjusting layer 7-3 is made of metallic Cu with a thickness of 45,000 Å.

[0053] Contact layer 8 is made of Au with a thickness of 2500 Å. Au, as a contact layer, has good corrosion resistance and thermal conductivity. After removing the epitaxial substrate, the thin film device still needs to undergo other processes, including acid-base hot solution etching and dry etching. Au can protect the silicon support substrate 5, adhesive layer 6, and stress adjustment layer 7 from corrosion.

[0054] S5. Remove substrate 1 to complete the fabrication of the LED thin-film device, such as... Figure 4 As shown. When removing substrate 1, the silicon substrate is first thinned using a thinning machine to a remaining thickness of approximately 100 μm. The remaining silicon substrate is then removed using a mixed aqueous solution of nitric acid and glacial acetic acid.

[0055] This embodiment also provides an LED thin-film device, the structure of which is as follows: Figure 4 As shown. The LED thin-film device provided in this embodiment can ensure a warpage of less than 100 μm, and its warpage is as follows. Figure 5 As shown. The specific reason is as follows: After epitaxial growth and cooling to room temperature, because the coefficient of thermal expansion of the substrate (silicon) is smaller than that of the epitaxial layer 2 (GaN), GaN shrinks more and Si shrinks less. Therefore, Si exerts tensile stress on GaN (i.e., a force in the stretching direction on GaN). When Si is completely removed, the tensile stress on GaN is suddenly released, and the previously stretched epitaxial layer needs to return to a free state, which manifests as an upward stress, i.e., as shown.Figure 4 The front surface of the silicon support substrate 5 is subjected to a large upward compressive stress. However, the back surface of the silicon support substrate 5 has the stress adjustment layer 7, which generates a downward compressive stress on the back surface of the silicon support substrate 5, and the two tend to balance, so that a low level of warping can be maintained, thus enabling mass production of the chips. Embodiment 2

[0056] This embodiment provides a preparation method of an LED thin film device. Taking an LED thin film device with a size of 4 inches as an example, the preparation process of this embodiment includes the following steps: Step S1, providing an LED epitaxial wafer, the LED epitaxial wafer including a substrate 1 and an epitaxial layer 2; a mirror layer 3 and a first bonding layer are sequentially prepared on the epitaxial layer 2. The substrate 1 is a silicon substrate with a size of 4 inches, and the specific radius is 50.8 mm. The material of the epitaxial layer 2 is GaN-based material, the mirror layer 3 is Ag mirror, and the material of the first bonding layer is CuIn.

[0057] Step S2, providing a silicon support substrate 5, and preparing a second bonding layer on the front surface of the silicon support substrate 5; the thickness of the silicon support substrate 5 is less than 200 μm. The silicon support substrate 5 is a 4-inch wafer with a thickness of 175 μm, and the specific radius is 50.8 mm. The material of the second bonding layer is CuIn.

[0058] Step S3, bonding the LED epitaxial wafer and the silicon support substrate 5, and the structure after bonding is as shown in Figure 2 The first bonding layer and the second bonding layer form a bonding layer 4 after bonding.

[0059] Step S4, evaporating a bonding layer 6, a stress adjustment layer 7, a first contact layer 8-1, a second contact layer 8-2 and a third contact layer 8-3 on the back surface of the silicon support substrate 5, as shown in Figure 6

[0060] The bonding layer 6 is metal Ti with a thickness of 500 Å, which can be well adhered to the silicon support substrate 5. If the stress adjustment layer 7 is directly contacted with the substrate, Cu is prone to fall off.

[0061] The thickness of the stress adjustment layer is 120000 Å.

[0062] The first contact layer is Au with a thickness of 500 Å; the second contact layer is Pt with a thickness of 500 Å; and the third contact layer is Au with a thickness of 1500 Å.

[0063] S5, removing the substrate 1 to complete the preparation of the LED thin film device, and the structure is as shown in Figure 7 ​​

[0064] The embodiment also provides an LED thin film device, which has a structure as shown in Figure 7 The LED thin film device provided by the embodiment can ensure that the warping degree is below 200 μm, as shown in Figure 8 The specific reasons are as follows: after epitaxial growth and cooling to room temperature, because the thermal expansion coefficient of the substrate (silicon) is less than that of the epitaxial layer 2 (GaN), the GaN shrinks more and the Si shrinks less, thus the Si exerts a tensile stress on the GaN (i.e. a force in the tensile direction of the GaN), when the Si is completely removed, the tensile stress on the GaN is suddenly released, and the epitaxial layer which was originally stretched wants to restore to a free state, showing an upward stress, i.e. a large upward compressive stress on the front surface of the silicon support substrate 5, as shown in Figure 7 However, the back surface of the silicon support substrate 5 has the stress adjusting layer 7, which generates a downward compressive stress on the back surface of the silicon support substrate 5, and the two tend to be balanced, so as to maintain a low level of warping, thus enabling the chips to be mass-produced.

[0065] In the description of the specification, the description of "some embodiments" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0066] The above-described embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the patent scope of the present application. It should be noted that, for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are all within the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method of fabricating an LED thin film device, characterized by, The method comprises the following steps: S1, providing an LED epitaxial wafer, the LED epitaxial wafer comprising a substrate and an epitaxial layer; a mirror layer and a first bonding layer are prepared on the epitaxial layer in sequence; the thermal expansion coefficient of the substrate is less than the thermal expansion coefficient of the epitaxial layer; S2, providing a silicon support substrate, a second bonding layer is prepared on the front surface of the silicon support substrate; the thickness of the silicon support substrate is less than 180 μm; S3, bonding the LED epitaxial wafer and the silicon support substrate; S4, evaporating a stress-adjusting metal layer on the back surface of the silicon support substrate; the metal layer comprises a bonding layer, a stress-adjusting layer and a contact layer in sequence; S5, removing the substrate, and completing the preparation of the LED thin film device.

2. The method of claim 1, wherein: The silicon support substrate is a 2-inch or 4-inch wafer, and the thickness of the silicon support substrate is 170 μm to 175 μm.

3. The method of claim 1, wherein: The temperature for evaporating the metal layer in step S4 is 100 ℃ to 200 ℃.

4. The method of claim 1, wherein: The bonding layer is Ti, and the thickness of the bonding layer is 500 Å to 1000 Å.

5. The method of claim 1, wherein: The stress-adjusting layer is Cu, and the thickness of the stress-adjusting layer is 120000 Å.

6. The method of claim 1, wherein: The stress-adjusting layer is composed of a first stress-adjusting layer, a second stress-adjusting layer and a third stress-adjusting layer; the first stress-adjusting layer is Cu, and the thickness of the first stress-adjusting layer is 20000 Å to 45000 Å; the second stress-adjusting layer is In, and the thickness of the second stress-adjusting layer is 1000 Å to 1200 Å; the third stress-adjusting layer is Cu, and the thickness of the third stress-adjusting layer is 45000 Å.

7. The method of claim 1, wherein: The contact layer is Au, and the thickness of the contact layer is 2000 Å to 2500 Å.

8. The method of claim 1, wherein: The contact layer is composed of a first contact layer, a second contact layer and a third contact layer; the first contact layer is Au, and the thickness of the first contact layer is 400 Å to 600 Å; the second contact layer is Pt, and the thickness of the second contact layer is 400 Å to 600 Å; the third contact layer is Au, and the thickness of the third contact layer is 1000 Å to 2000 Å.

9. The method of claim 1, wherein: The substrate is a silicon substrate, and the epitaxial layer is a gallium nitride-based epitaxial layer.

10. An LED thin film device, characterized by, The LED thin film device is prepared by the method in any one of claims 1 to 9.

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