Hollow metal composite substrate, method for manufacturing the same and use thereof

Abstract

This invention provides a hollow metal composite substrate, its fabrication method, and its applications. The fabrication method includes: depositing a metal thin film comprising an adhesion metal layer, a barrier layer, and a back reflective layer on the front side of a flip-chip epitaxial wafer; forming a patterned photoresist mask on the back reflective layer; depositing a thick metal film covering the photoresist mask and exposed areas; thinning and / or planarizing the metal layer; removing the photoresist mask to form multiple discrete metal support pillars and hollow isolation channels between adjacent support pillars; and depositing a stress-balanced thin film with compressive stress on the surfaces of the support pillars and hollow isolation channels. This invention effectively cuts off the in-plane transmission path of thermal stress through the hollow isolation channels and discrete support pillar array, achieving stress decoupling and significantly reducing the risk of substrate warpage and microcracks. Simultaneously, the hollow structure reduces the equivalent bending stiffness of the substrate, improves flexibility, and reduces weight, making it suitable for high-efficiency flexible thin-film solar cells in aerospace and other fields.

Description

Technical Field

[0001] This invention relates to the field of solar cell technology, and in particular to a hollow metal composite substrate for thin-film solar cells, its preparation method, and its application. Background Technology

[0002] With the development of space solar cell technology, high-efficiency multi-junction solar cells based on flexible substrates have gradually become a research hotspot due to their advantages such as light weight, rollability, portability, and high power-to-weight ratio. Flexible solar cells can be rolled up and stored, which can effectively reduce the size of solar panels, improve the carrying capacity of spacecraft, and reduce launch costs. At the same time, it can also meet the application needs of near-space spacecraft such as solar-powered drones and unmanned airships.

[0003] Currently, III-V compound multijunction solar cells generally use thick, rigid Ge or GaAs substrates (typically ≥140 μm thick), resulting in heavy and inflexible cells that limit their further application in the space sector. To address this issue, existing technologies have proposed various flexible substrate solutions.

[0004] 1. Chinese patent CN202310980698.7 discloses a Cu / Ni periodic composite substrate, which forms a periodic solid substrate by sequentially composited multiple Cu layers and Ni layers on a seed layer, and adjusts the overall thermal expansion coefficient by utilizing the difference in thermal expansion coefficients of Cu and Ni.

[0005] However, this plan has the following shortcomings: (1) Nickel has extremely high tensile stress and is very hard. When depositing the first 2μm Ni layer, the huge tensile stress may cause epitaxial cracking instantly. At the same time, multiple Cu / Ni cycles mean that multiple stress change sections need to be introduced. The stress concentration at the interface is a high-incidence area for material delamination. (2) Ni is very hard, and the bending stiffness of the Cu / Ni composite layer is much greater than that of pure copper. It cannot be bent at a small radius. The flexible battery made by this scheme is not a "flexible film" but a "high-strength rigid sheet". (3) Cu and Ni will form a galvanic cell in a humid environment. The Cu layer will act as the anode and accelerate corrosion. In high-temperature processes or long-term operation, Cu and Ni will diffuse into each other, which may lead to interface embrittlement.

[0006] 2. Chinese patent CN201810904936.5 discloses a pure copper electroplated flexible substrate, which forms a copper thin film on the back electrode as a flexible substrate.

[0007] This solution has the following shortcomings: (1) The coefficient of thermal expansion of copper is much higher than that of GaAs. After cooling in a high-temperature process, the epitaxial layer will be subjected to huge tensile stress, leading to microcracks or even rupture. (2) Copper is a “fast-diffusion” element in semiconductors such as silicon and GaAs. It is very easy to migrate into the active region of the battery to form a “recombination center”, which seriously reduces the photoelectric conversion efficiency. (3) Copper is easily oxidized in the air, generating non-conductive oxides, which greatly increases the series resistance of the battery and threatens its long-term reliability.

[0008] In summary, existing flexible substrate technology still has shortcomings in terms of thermal stress release, flexibility, and thermal expansion matching with the battery epitaxial layer, and a new solution is urgently needed. Summary of the Invention

[0009] The purpose of this invention is to provide a hollow metal composite substrate, its preparation method and application. By forming hollow isolation channels and discrete support pillar arrays in the metal substrate, and in conjunction with a stress-balancing thin film, thermal stress is effectively released, thereby improving the flexibility and mechanical stability of the substrate.

[0010] To achieve the above objectives, the solution of the present invention is as follows: A method for preparing a hollow metal composite substrate includes the following steps: S1: A metal thin film is deposited on the front side of a flip-chip epitaxial wafer, the metal thin film comprising at least an adhesion metal layer, a barrier layer and a back reflection layer; S2: A patterned photoresist mask is formed on the back reflective layer, wherein the pattern of the photoresist mask is a plurality of geometric patterns arranged in an array; S3: Deposit a thick metal film on the patterned flip-chip epitaxial wafer, the thick metal film covering the photoresist mask and the exposed areas not covered by the photoresist, to form a continuous metal layer; S4: Thinning and / or planarizing the metal layer; S5: Remove the photoresist mask to form multiple discrete metal support pillars on the back reflective layer, with hollow isolation channels formed between adjacent metal support pillars; S6: A stress-balancing film is deposited on the surface of the metal support column and the hollow isolation channel. The stress-balancing film has compressive stress to counteract the tensile stress generated by the metal thick film during the cooling process.

[0011] In a preferred embodiment, in step S1, the adhesive metal layer is titanium, the blocking layer is molybdenum, and the back reflective layer is silver.

[0012] In a preferred embodiment, in step S2, the geometric shape is one of a regular hexagon, a square, or a circle.

[0013] In a preferred embodiment, the geometric shape is a regular hexagon, with the side spacing between adjacent regular hexagons being 30 μm and the opposite side spacing being 250 μm.

[0014] In a preferred embodiment, in step S3, the deposited metal film is a metal or a metal alloy.

[0015] In a preferred embodiment, the metal thick film is one or a combination of aluminum, copper, nickel, silver, gold, and platinum, and its deposition thickness is 10~30μm.

[0016] In a preferred embodiment, in step S6, the stress-balancing film is a titanium-tungsten alloy film with a thickness ranging from 100 to 300 nm.

[0017] In a preferred embodiment, in step S4, a grinding process is used to thin and planarize the metal layer, so that the surface of the metal layer is flat.

[0018] The present invention also provides a hollow metal composite substrate, which is prepared by any of the above-mentioned methods, comprising: A thin metal film located below the metal support column, the thin metal film comprising an adhesive metal layer, a barrier layer and a back reflective layer.

[0019] The present invention also provides a thin-film solar cell, comprising: As mentioned above, hollow metal composite substrates; A flip-chip epitaxial wafer is located below the hollow metal composite substrate, with the front side of the flip-chip epitaxial wafer facing the hollow metal composite substrate; A front electrode is located on the side surface of the flip-chip epitaxial wafer that is away from the hollow metal composite substrate; The back reflective layer of the hollow metal composite substrate is positioned opposite to the front surface of the flip-chip epitaxial wafer.

[0020] After adopting the above solution, the beneficial effects of the present invention are as follows: 1. By forming a hollow isolation channel and a discrete support column array, the present invention divides the continuous metal film into independent support units, effectively cutting off the in-plane transmission path of thermal stress, realizing stress decoupling, and significantly reducing the risk of warping and microcracks in the substrate after high-temperature processing.

[0021] 2. This invention reduces the equivalent bending stiffness of the substrate by forming hollow isolation channels and a discrete support pillar array, giving the substrate better flexibility and making it adaptable to bending applications with smaller radii. It also reduces the weight of the substrate, which is beneficial for improving the power-to-weight ratio of the battery.

[0022] 3. The metal cover plate and the discrete support columns of the present invention form a sandwich structure similar to an I-beam, which reduces weight while maintaining good mechanical support strength and improving bending stiffness. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure after depositing a metal thin film on a flip-chip epitaxial wafer of a GaAs growth substrate in an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure after forming a patterned photoresist mask in an embodiment of the present invention; Figure 3 This is a schematic diagram of the structure after depositing a thick metal film in an embodiment of the present invention; Figure 4 This is a schematic diagram of the structure after the metal layer has been thinned and planarized in an embodiment of the present invention; Figure 5 This is a schematic diagram of the structure formed by removing the photoresist mask to create a hollow isolation channel and a metal support pillar in an embodiment of the present invention; Figure 6 This is a schematic diagram of the structure after the deposition of a stress-balanced thin film in an embodiment of the present invention; Figure 7 This is a schematic diagram of the overall structure of the thin-film solar cell in an embodiment of the present invention; Figure 8 This is a top view of the honeycomb array frame formed by a regular hexagonal photoresist mask in an embodiment of the present invention.

[0024] Label Explanation: 1. Flip-chip epitaxial wafer; 2. Metal thin film; 3. Adhesive metal layer; 4. Barrier layer; 5. Back reflector layer; 6. Photoresist mask; 7. Metal thick film; 8. Metal support pillar; 9. Hollow isolation channel; 10. Stress-balanced thin film; 11. Metal cover layer; 12. Front electrode; 13. Antireflection coating; 14. GaAs growth substrate. Detailed Implementation

[0025] The present invention will now be further described in conjunction with the accompanying drawings and specific embodiments.

[0026] Example 1: This embodiment provides a hollow metal composite substrate and its preparation method, as well as a thin-film solar cell containing the substrate.

[0027] Specifically, the preparation method of this hollow metal composite substrate includes: I. Providing flip-chip epitaxial wafers like Figure 1 As shown, a flip-chip epitaxial wafer 1 is provided. The flip-chip epitaxial wafer 1 is grown on a 4-inch GaAs growth substrate 14 using an MOCVD device to grow an epitaxial layer of a flip-chip multi-junction GaAs thin film battery.

[0028] II. Depositing Metal Thin Films Take the above-mentioned inverted epitaxial wafer 1, immerse it sequentially in acetone and isopropanol, ultrasonically clean each for 10 minutes, rinse with water and then spin dry. For example... Figure 1 As shown, an adhesion metal layer 3, a barrier layer 4, and a back reflective layer 5 are sequentially deposited on the front side of the flip-chip epitaxial wafer 1, together forming a metal thin film 2. In this embodiment, the adhesion metal layer 3 is made of titanium (Ti) with a thickness of 50 angstroms (Å); the barrier layer 43 is made of molybdenum (Mo) with a thickness of 1000 angstroms (Å); and the back reflective layer 5 is made of silver (Ag) with a thickness of 30000 angstroms (Å). The deposition method is electron beam evaporation. Ti serves as the adhesion layer, Mo is the barrier layer 4, which has high hardness and also acts as a stress buffer, and Ag serves as the back reflector and can also be used as a bonding layer.

[0029] III. Forming a patterned photoresist mask like Figure 2 and Figure 8 As shown, photoresist with a thickness of 10 μm is coated on the back reflective layer 5. After homogenization, exposure, and development, a patterned photoresist mask 6 is formed, exposing areas not covered by photoresist. The pattern of the photoresist mask 6 is an array of multiple geometric shapes, such as squares, circles, and regular hexagons. In this embodiment, regular hexagons are used to form a honeycomb-like array framework. The side spacing between adjacent regular hexagons is 30 μm, and the side-to-side distance between regular hexagons is 250 μm. The total area of ​​the regular hexagon array accounts for approximately 75% of the wafer area. The thickness of the photoresist should be selected based on the subsequent deposition thickness of the metal film 7 to ensure smooth stripping during photoresist removal.

[0030] IV. Depositing Thick Metal Films like Figure 3 As shown, a thick metal film 7 is deposited on a patterned flip-chip epitaxial wafer 1. The thick metal film 7 covers the photoresist mask 6 and the exposed areas not covered by the photoresist, forming a continuous metal layer. In this embodiment, aluminum (Al) is deposited as the thick metal film 7 by electron beam evaporation, with a deposition thickness of 20 μm. In other embodiments, magnetron sputtering or electroplating can also be used for deposition, and the metal material can be one or a combination of copper (Cu), nickel (Ni), silver (Ag), gold (Au), and platinum (Pt), with a deposition thickness ranging from 10 to 30 μm.

[0031] V. Thinning and Planarization Treatment like Figure 4As shown, the epitaxial wafer after coating (thick metal film 7) is placed with a blue film attached to the substrate side, and the coated side facing upwards. It is then placed on a grinding machine for thinning and / or planarization. In this embodiment, a grinding process is used to thin and planarize the metal layer surface, eliminating uneven steps and making the metal layer surface smooth and of uniform thickness. The surface roughness of the ground metal layer should meet the requirements of subsequent processes.

[0032] VI. Glue Removal and Cleaning The thinned flip-chip epitaxial wafer 1 was sequentially immersed in three stripping solutions, each for 10 minutes, followed by ultrasonic immersion in isopropanol solution for 10 minutes to remove the photoresist inside the metal and residual photoresist after the blue film was stripped from the substrate. Figure 5 As shown, after resist removal, the metal layer forms multiple discrete metal support pillars 8 on the back reflective layer 5, with hollow isolation channels 9 formed between adjacent metal support pillars 8. The cross-sectional shape of the metal support pillars 8 is consistent with the pattern of the photoresist mask 6, and in this embodiment, it is a regular hexagon. The width of the hollow isolation channel 9 is the side spacing of the adjacent regular hexagons, 30 μm. After resist removal, the coated metal forms a structure with metal cover plates on the top and bottom, and a columnar metal array supported by regular hexagonal bases in the middle, wherein the thickness of the columnar metal is 10 μm, and the thickness of the upper metal cover plate is 5 μm.

[0033] It should be noted that although the thick metal film 7 completely covers the photoresist mask 6, because the thick metal film 7 is deposited by electron beam evaporation in this embodiment, the metal atoms reach the substrate surface in a straight line. The metal deposited on the sidewalls of the photoresist mask 6 is extremely thin, forming natural weak areas. The resist remover can penetrate from the weak areas on the sidewalls and the sample edges, causing the photoresist mask 6 to swell and dissolve, thereby peeling off the metal layer covering it. This is the mature lift-off process in semiconductor manufacturing.

[0034] VII. Deposition of stress-balanced thin films 10 like Figure 6As shown, the flip-chip epitaxial wafer 1 was immersed in a mixed solution of phosphoric acid and water (volume ratio 1:16) for 2 minutes to remove the oxide layer on the Al surface. It was then rinsed with water, ultrasonically soaked in isopropanol for 5 minutes, and baked in a vacuum oven at 100°C for 10 minutes. A stress-balancing film 10 was then deposited on the surface of the metal support pillar 8 and the hollow isolation channel 9. In this embodiment, the stress-balancing film 10 is a titanium-tungsten (TiW) alloy film, deposited by magnetron sputtering, with a thickness of 200 nm. The TiW film has compressive stress characteristics, which can effectively counteract the tensile stress generated by the thick metal film 7 (Al film) during deposition and cooling, thereby suppressing substrate warping. In the composite structure of metal (Al / Ag) and GaAs, the metal shrinkage rate is much greater than that of GaAs during subsequent cooling processes, resulting in huge tensile stress at the interface, leading to wafer warping or delamination. Here, the compressive stress of the TiW thin film is used to offset the tensile stress of the metal layer, while its high hardness is used to protect the fragile hollow hexagonal array, playing a dual role as a "stress balancer" and a "structural reinforcement".

[0035] 8. Temporary bonding At this point, the hollow metal composite substrate is complete, and temporary bonding can then be performed. Specifically, temporary bonding adhesive is spin-coated onto the metal surface of the stress-balancing film 10 side of the flip-chip epitaxial wafer 1, temporarily bonding it to the rigid substrate to facilitate subsequent processes. The rigid substrate can be glass, sapphire, silicon, etc.; in this embodiment, it is sapphire.

[0036] IX. Subsequent Processes Of course, after the hollow metal composite substrate is prepared, the subsequent processes are the conventional processes for thin-film solar cells, including etching to remove the GaAs growth substrate 14, fabricating the front electrode 12 and antireflection film 13, debonding, annealing, dicing, end-face etching, etc., finally producing a thin-film battery chip with a composite metal flexible substrate having a metal support pillar 8 and a hollow isolation channel 9 structure, such as... Figure 7 As shown.

[0037] Example 2: This embodiment provides a hollow metal composite substrate, fabricated using any of the preparation methods described above, with reference to... Figure 6 ,include: A metal cover layer 11 located at the top, the metal cover layer 11 being composed of the stress-balancing thin film 10 and the underlying thick metal film 7; Multiple discrete metal support pillars 8 located below the metal cover layer 11, the metal support pillars 8 being formed by patterning the deposited metal thick film 7; Multiple hollow isolation channels 9 are located between adjacent metal support columns 8; A metal film 2 located below the metal support column 8, the metal film 2 including an adhesive metal layer 3, a barrier layer 4 and a back reflective layer 5.

[0038] Example 3: This embodiment provides a thin-film solar cell, referenced... Figure 7 ,include: As mentioned above, hollow metal composite substrates; A flip-chip epitaxial wafer 1 is located below the hollow metal composite substrate, and the front side of the flip-chip epitaxial wafer 1 faces the hollow metal composite substrate; A front electrode 12 is located on the side surface of the flip-chip epitaxial wafer 1 that is away from the hollow metal composite substrate; The back reflective layer 5 of the hollow metal composite substrate is positioned opposite to the front surface of the flip-chip epitaxial wafer 1.

[0039] Explanation of the key process principles of this invention: The core design concept of this invention is a structure of "metal support pillar 8 + hollow isolation channel 9". The structure's flexibility is used to offset the negative impact of material rigidity. This is an effective solution to the huge thermal expansion coefficient mismatch between GaAs and metals (Cu, Ni, Al, Ag, etc.). Especially in medium-sized wafers such as 4-inch wafers, it can significantly improve the yield and reliability of devices.

[0040] Compared with the planar solid-layer copper or Cu / Ni periodic composite substrates in the prior art, the hollow metal composite substrate of the present invention has the following advantages: 1. First, it significantly improves thermal stress matching. The hollow isolation channel 9 structure blocks the stress accumulation path. In a planar structure, stress accumulates linearly from the center to the edge (with the highest stress at the edge). The hollow isolation channel 9 cuts off the stress transmission path. The deformation of each metal support column 8 is limited to a local range, avoiding long-distance stress superposition. At the same time, the hollow isolation channel 9 cuts the large-area continuous metal film into independent "islands" or "frames." When the temperature rises, each "island" can expand freely outward without transferring the huge accumulated stress to the entire wafer, forming a decoupling effect. From a macroscopic mechanical perspective, this porous or mesh-like structure greatly reduces the equivalent Young's modulus of the metal layer in the in-plane direction (XY axis). According to the bimetallic strip effect formula, stress is proportional to modulus. As the modulus decreases, the generated thermal stress naturally decreases.

[0041] Specifically, in a bimetallic structure, if the two metal layers are completely rigidly constrained (i.e., cannot expand or contract freely), the resulting internal thermal stress... The basic calculation formula is:

[0042] in, For elastic modulus, is the coefficient of thermal expansion of the metal. This represents the temperature change. The formula clearly shows that thermal stress... With elastic modulus They exhibit a strictly proportional relationship. If the elastic modulus of the material... If the temperature is lowered, the resulting thermal stress will decrease proportionally under the same temperature change.

[0043] 2. Secondly, it enhances mechanical strength and resistance to deformation. The metal cover plate on the hollow structure provides planar support for the graphic metal, and the planar design facilitates subsequent processes. In sheet metal mechanics, a reasonable cavity design can enhance mechanical properties. The hollow structure in this invention increases the interfacial moment of inertia, similar to the principle of "I-beams" in construction, increasing the thickness of the structure and improving bending stiffness. This means that the upper metal cover plate is less likely to collapse or bend in the vertical direction. In addition, it can prevent overall instability. Compared with simple thin metal foil, this "sandwich structure" with metal support columns 8 can better maintain its shape and provide sufficient mechanical support.

[0044] 3. Furthermore, optimizing thermal performance provides insulation and buffering. The thermal conductivity of a hollow cavity (air or vacuum) is much lower than that of metal. This structure can form a thermal barrier layer, slowing down the direct transfer of heat from the metal layer to the GaAs epitaxial layer, or protecting internal components during drastic external temperature fluctuations. It can also withstand thermal shock. During rapid temperature increases and decreases (thermal shock), the "buffer space" provided by the hollow structure allows the metal to expand and contract, reducing the risk of delamination or cracking caused by instantaneous thermal expansion and contraction, forming an energy-absorbing effect similar to a "lattice" or "honeycomb." This structure aligns with the design concept of zero-thermal-expansion lattice composite materials. Although the metal itself expands significantly, by designing a hollow structure, the metal skeleton can undergo slight bending or torsional deformation (i.e., structural compliance) when heated, thus "absorbing" some of the strain energy caused by the mismatch in thermal expansion coefficients, rather than converting it all into destructive interfacial shear stress.

[0045] This invention has advantages in terms of process and integration. On the one hand, it is adaptable to heterogeneous integration, allowing other functions to be integrated inside the metal cover to achieve vertical electrical interconnection or fluid channels. On the other hand, it reduces weight by removing some metal materials to form cavities, which directly reduces the weight of the overall structure and enhances the ultra-lightweight characteristics of flexible thin-film solar cells, which is beneficial for applications in the aerospace field.

[0046] Furthermore, the hollow structure can extend the sound propagation path, providing shock absorption and sound insulation, which can make flexible solar cells more resistant to mechanical vibration and reduce the damage of vibration to the GaAs lattice.

[0047] Comparative Example 1: Using the same steps S1 and S2 as in Example 1, a 2Ti / Mo / Ag metal thin film was deposited on the flip-chip epitaxial wafer 1, followed by the direct deposition of an Al thick film. After temporary bonding and etching to remove the substrate, a front electrode 12 and an anti-reflection film 13 were fabricated. After debonding, the entire wafer curled directly into a cylindrical shape towards the Al metal surface and the epitaxial layer was torn, indicating that the Al thick film had extremely high tensile stress before undergoing the high-temperature process, making it impossible to perform subsequent high-temperature annealing and cutting processes.

[0048] Comparative Example 2: Using the same steps as in Example 1, but without depositing a TiW stress balance film, after temporary bonding and etching to remove the substrate, the front electrode 12 and antireflection film 13 are fabricated. After debonding, the wafer warps toward the Al metal surface, with an edge warping of 5mm-10mm, while the edge warping in Example 1 is 1mm-2mm. It can be seen that the TiW alloy effectively improves the wafer warping, making the battery chip flatter.

[0049] The directional terms used in this specification are defined relative to the structures shown in the accompanying drawings. They are relative concepts and may therefore vary depending on their location and usage. Therefore, these or other directional terms should not be interpreted as restrictive.

[0050] The above description is only a preferred embodiment of the present invention and is not intended to limit the design of this case. All equivalent changes made based on the key design features of this case shall fall within the protection scope of this case.

Claims

1. A method for preparing a hollow metal composite substrate, characterized in that: Includes the following steps: S1: A metal thin film is deposited on the front side of a flip-chip epitaxial wafer, the metal thin film comprising at least an adhesion metal layer, a barrier layer and a back reflection layer; S2: A patterned photoresist mask is formed on the back reflective layer, wherein the pattern of the photoresist mask is a plurality of geometric patterns arranged in an array; S3: Deposit a thick metal film on the patterned flip-chip epitaxial wafer, the thick metal film covering the photoresist mask and the exposed areas not covered by the photoresist, to form a continuous metal layer; S4: Thinning and / or planarizing the metal layer; S5: Remove the photoresist mask to form multiple discrete metal support pillars on the back reflective layer, with hollow isolation channels formed between adjacent metal support pillars; S6: A stress-balancing film is deposited on the surface of the metal support column and the hollow isolation channel. The stress-balancing film has compressive stress to counteract the tensile stress generated by the metal thick film during the cooling process.

2. The method for preparing a hollow metal composite substrate as described in claim 1, characterized in that: In step S1, the adhesive metal layer is titanium, the barrier layer is molybdenum, and the back reflective layer is silver.

3. The method for preparing a hollow metal composite substrate as described in claim 1, characterized in that: In step S2, the geometric shape is one of a regular hexagon, a square, or a circle.

4. The method for preparing a hollow metal composite substrate as described in claim 3, characterized in that: The geometric shape is a regular hexagon, with a side spacing of 30 μm between adjacent regular hexagons and a side spacing of 250 μm between opposite regular hexagons.

5. The method for preparing a hollow metal composite substrate as described in claim 1, characterized in that: In step S3, the deposited metal film is a metal or a metal alloy.

6. The method for preparing a hollow metal composite substrate as described in claim 5, characterized in that: The metal thick film is one or a combination of aluminum, copper, nickel, silver, gold, and platinum, and its deposition thickness is 10~30μm.

7. The method for preparing a hollow metal composite substrate as described in claim 1, characterized in that: In step S6, the stress-balanced film is a titanium-tungsten alloy film with a thickness ranging from 100 to 300 nm.

8. The method for preparing a hollow metal composite substrate as described in claim 1, characterized in that: In step S4, a grinding process is used to thin and planarize the metal layer, making the surface of the metal layer smooth.

9. A hollow metal composite substrate, characterized in that: Prepared by the preparation method according to any one of claims 1 to 8, comprising: A metal cover layer at the top, the metal cover layer being composed of the stress-balanced thin film and the underlying thick metal film; Multiple discrete metal support pillars located beneath the metal cover layer, the metal support pillars being formed by patterning the deposited metal thick film; Multiple hollow isolation channels located between adjacent metal support columns; A thin metal film located below the metal support column, the thin metal film comprising an adhesive metal layer, a barrier layer and a back reflective layer.

10. A thin-film solar cell, characterized in that: include: A hollow metal composite substrate as described in claim 9; A flip-chip epitaxial wafer is located below the hollow metal composite substrate, with the front side of the flip-chip epitaxial wafer facing the hollow metal composite substrate; A front electrode is located on the side surface of the flip-chip epitaxial wafer that is away from the hollow metal composite substrate; The back reflective layer of the hollow metal composite substrate is positioned opposite to the front surface of the flip-chip epitaxial wafer.

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