A Low-Temperature, Low-Pressure Copper-Copper Direct Bonding Method Based on Heterogeneous Copper Structures

Abstract

This invention relates to the field of semiconductor packaging technology and discloses a low-temperature, low-pressure copper-copper direct bonding method based on a heterostructure copper structure. The method includes: forming electroplated metal underlayers on first and second silicon substrates, followed by degreasing, acid washing, ultrapure water cleaning, and drying; electroplating copper layers to form the first layer (111) textured nanotwinned copper layer and the second layer (110) textured nanotwinned copper layer; chemically and mechanically polishing the two copper layers, followed by rinsing and drying; cleaning and drying the polished copper layers and aligning them for bonding; completing the bonding using a bonding machine; and releasing the pressure and removing the layers after cooling to below 50°C. This invention significantly reduces the risk of thermal damage to heat-sensitive devices, greatly reduces equipment costs due to lower pressure and vacuum requirements, and simplifies the process flow.

Description

Technical Field

[0001] This invention relates to the field of semiconductor packaging technology, and more specifically, to a low-temperature, low-pressure copper-copper direct bonding method based on a heterogeneous copper structure. Background Technology

[0002] As integrated circuit manufacturing technology continues to advance towards higher performance, higher integration, and lower power consumption, 3D integration and advanced packaging have become key pathways to perpetuating Moore's Law. Among these technologies, copper-copper direct bonding has become one of the core technologies for achieving high-reliability 3D stacking due to its advantages such as low resistivity, high thermal conductivity, excellent electromigration resistance, and the ability to achieve high-density interconnects.

[0003] To achieve reliable copper-copper thermocompression bonding, the process typically requires temperatures of ≥400°C and pressures of ≥100MPa. The high-temperature process can easily cause damage to heat-sensitive devices, chip warping, and thermal stress accumulation, severely limiting its application in advanced packaging. Furthermore, to prevent copper surface oxidation, the process often needs to be carried out in an inert gas or high-vacuum environment, significantly increasing equipment complexity and manufacturing costs.

[0004] To address the aforementioned issues, low-temperature bonding can currently be achieved through pre-bonding surface treatment techniques and copper material structure control. Surface treatment techniques involve physical or chemical treatment of the copper surface before bonding to remove oxide layers, organic contaminants, and enhance surface activity, thereby reducing bonding temperature and pressure. For example, surface activated bonding (SAB) uses high-energy plasma (such as argon, hydrogen, nitrogen, or mixed gases) to bombard the material surface in an ultra-high vacuum environment to remove contaminants and oxide layers, thus cleaning the surface and enabling strong bonding without heat treatment. However, this method has extremely high requirements for equipment and environment, and poor mass production compatibility. Self-assembled monolayer (SAM) or metal passivation layer (such as Au, Ag, Co) treatment forms a temporary protective layer to prevent oxidation. During bonding, the SAM structure decomposes or Cu diffuses through the metal passivation layer, promoting interfacial connection. While these methods can reduce bonding temperature, they introduce additional materials and process steps, resulting in high costs and hindering large-scale applications.

[0005] Furthermore, during the bonding process, the diffusion paths of copper atoms are divided into surface diffusion and grain boundary diffusion. By controlling the crystal orientation, grain size and defect structure of copper materials, low-temperature bonding can be achieved by utilizing the diffusion characteristics of its intrinsic structure. For example, (111) nano-twinned copper (nt-Cu) has an extremely low surface energy and the highest surface diffusion rate on its (111) crystal plane, which can be bonded at 150-250℃ and low pressure. However, the bonding interface formed at low temperature is flat and the interface voids are difficult to eliminate. It requires subsequent high-temperature annealing (>250℃) to improve the bonding quality. Nano-fine-grained copper (nc-Cu) has a very high grain boundary density due to its small particle size (usually less than 100nm). The grain boundary diffusion rate is significantly improved, and rapid recrystallization and grain growth can occur at low temperature, which can achieve cross-interface connection. Nano-fine-grained copper has poor thermal stability and a significant self-annealing effect occurs at room temperature. The grains coarsen rapidly, the grain boundary density decreases, and the storage time (q-time) from electroplating to bonding is short, which cannot meet the process turnover and transportation requirements in actual production, thus restricting its mass production feasibility.

[0006] Therefore, developing a copper-copper bonding method that combines low-temperature bonding capability, good thermal stability, high interface reliability, and compatibility with mass production processes has become a key challenge for advancing 3D integration technology. Summary of the Invention

[0007] The purpose of this invention is to provide a low-temperature, low-pressure direct copper-copper bonding method based on heterogeneous copper structures, aiming to solve the aforementioned problems. In this invention, "heterogeneous" refers to the two copper plating layers constituting the bonding interface having different crystallographic orientations or microstructural characteristics. Specifically, one is a nanotwinned copper with a (111) orientation, and the other is a nanotwinned copper or nanocrystalline copper with a (110) orientation. The differences in crystal orientation, microstructure, and diffusion characteristics between the two constitute the "heterogeneous" characteristic of this invention.

[0008] This invention provides a low-temperature, low-pressure copper-copper direct bonding method based on a heterostructure copper, comprising: S1: A metal underlayer for electroplating is formed on the first silicon substrate and the second silicon substrate respectively, and the first silicon substrate and the second silicon substrate are degreased, pickled, cleaned with ultrapure water and dried. S2: Copper plating layers are formed by electroplating on the first silicon substrate and the second silicon substrate respectively, wherein the copper plating layer on the first silicon substrate is a nanotwinned copper layer with (111) texture, and the copper plating layer on the second silicon substrate is a nanotwinned copper layer with (110) texture. S3: Perform chemical mechanical polishing on the nanotwinned copper layer with texture (111) and the nanotwinned copper layer with texture (110) respectively to obtain a bonding surface with a bonding surface roughness Rq≤5nm, and rinse and dry with ultrapure water. S4: After polishing, the nanotwinned copper layer with the (111) texture and the nanotwinned copper layer with the (110) texture are sequentially cleaned with acetone, cleaned with isopropanol, pickled with citric acid, rinsed with ultrapure water and dried, and then aligned vertically. S5: Place the aligned sample into the bonding machine, apply a vertical pressure of 1-5 MPa to the sample and evacuate to 100-500 Pa, heat to 150-250 °C at a heating rate of 10 °C / min and hold at the temperature and pressure for 30-90 min to complete the bonding. S6: After bonding is completed, stop heating and allow the sample to cool naturally to below 50°C while maintaining the vertical pressure. Then, release the pressure and remove the bonded sample.

[0009] Furthermore, the metal substrate is composed of a TiN adhesion layer and a Cu seed layer, and is formed on the first silicon substrate and the second silicon substrate by physical vapor deposition.

[0010] Furthermore, the thickness of the TiN adhesion layer is 20 nm, and the thickness of the Cu seed layer is 50 nm.

[0011] Furthermore, in step S2, the electroplating temperature is 15–25°C, the stirring rate is 400–800 rpm, the current density is 4–7 ASD, and the electroplating time is 20–60 min; the pickling in step S1 uses 20 wt% dilute sulfuric acid or citric acid.

[0012] Furthermore, the chemical mechanical polishing described in step S3 employs a rotary polisher, and the polishing pad is a porous polyurethane pad; the polishing slurry contains a silica suspension and ammonium persulfate, wherein the silica particle size is 50 nm, the silica concentration is 2–5 wt%, the ammonium persulfate concentration is 0.2–0.8 wt%, and the pH of the polishing slurry is 8.5 ± 0.5; the polishing pad rotation speed is 50–150 rpm, the applied downward pressure is 2–6 psi, and the polishing slurry flow rate is 30–50 mL / min.

[0013] Furthermore, in step S4, the acetone cleaning time is 2 minutes, the isopropanol cleaning time is 2 minutes, and the citric acid pickling time is 1 minute.

[0014] Furthermore, in step S5, the bonding temperature is 160-200℃, the vertical pressure is 2.5MPa, the vacuum degree is about 200Pa, and the heat and pressure holding time is 60min.

[0015] On another aspect, the present invention protects a copper-copper bonding structure prepared by the above-mentioned heterogeneous low-temperature low-pressure copper-copper bonding method, comprising a first silicon substrate and a second silicon substrate, a nanotwin copper layer with (111) texture located on the first silicon substrate, and a nanotwin copper layer with (110) texture located on the second silicon substrate, wherein the nanotwin copper layer with (111) texture and the nanotwin copper layer with (110) texture are bonded to each other to form a copper-copper bonding interface.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: Existing mainstream cryogenic solutions (such as surface activation, ultra-high vacuum, and metal passivation layers) all require expensive and complex specialized equipment, resulting in high maintenance costs and limited production capacity. This invention can achieve bonding in a normal low vacuum (100-500Pa) atmosphere, is compatible with existing thermoforming bonding machines in mainstream packaging production lines, has low equipment modification costs, and is easy to replicate and integrate for production capacity.

[0017] Traditional nanocrystalline copper, due to its "self-annealing" effect, requires strict time control (q-time) from electroplating completion to bonding, which is almost impossible in multi-process, cross-plant production. The heterogeneous material system constructed in this invention effectively solves the stability problem of single materials by combining copper materials with low-temperature grain growth capabilities (such as (110) nanotwinned copper, nanocrystalline copper, etc.) with (111) nanotwinned copper with high surface diffusion activity. Specifically, the (110) nanotwinned copper, as the preferred embodiment of this invention, can maintain structural stability at room temperature for a long time, greatly relaxing the time constraints for production scheduling and material flow, meeting the stringent requirements of mass production for process window tolerance. Even for nanocrystalline copper, which has a self-annealing tendency, bonding with (111) nanotwinned copper can reduce its sensitivity to process windows.

[0018] This invention achieves bonding at 200℃ and 2.5MPa, which not only protects temperature-sensitive advanced logic chips, memory chips (such as DRAM), and microelectromechanical systems (MEMS) devices, but also significantly reduces the risk of chip warpage and breakage caused by high voltage, improving overall packaging yield. The shear strength can stably reach over 34MPa, and the fracture is located in the region between the TiN adhesion layer and the Si wafer, rather than the copper-copper bonding interface, indicating higher bonding strength. Regarding the bonding interface quality, the spontaneous filling and elimination of interface voids is achieved through a synergistic mechanism of surface diffusion and grain growth and creep, resulting in an interface without continuous voids, laying the foundation for thermal management and anti-electromigration performance.

[0019] Both types of copper materials in this invention can be achieved by adjusting existing copper electroplating processes (adjusting additives and current density), without the need to develop entirely new material deposition equipment, making it easy to implement in foundries and packaging plants. The process flow is simple, requiring no additional core equipment. Electroplating, CMP, cleaning, and alignment bonding are all standard processes in semiconductor manufacturing and packaging. The core bonding step is completed on a thermoforming bonding machine, eliminating the need for additional surface activation or ultra-high vacuum bonding equipment. This invention is applicable to mainstream three-dimensional integrated advanced packaging fields, including chip-to-wafer (C2W) bonding, wafer-to-wafer (W2W) bonding, and chiplet heterogeneous integration, and is a key interconnect technology for high-bandwidth memory (HBM) and system-in-package (SiP). Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 The SEM and XRD patterns of the cross-section of the nanotwinned copper coating provided in Example 1 of this invention; Figure 2 The SEM and EBSD images of the bonding interface provided in Embodiment 1 of the present invention; Figure 3 The bonding interface SEM and EBSD diagrams provided in Embodiment 2 of the present invention; Figure 4 This is a SEM image of the bonding interface provided in Embodiment 3 of the present invention.

[0022] Figure 1 In the image, (a) is a cross-sectional SEM image of the (110) nano-twinned copper coating, (b) is a cross-sectional SEM image of the (111) nano-twinned copper coating, (c) is an XRD pattern of the (110) nano-twinned copper coating, and (d) is an XRD pattern of the (111) nano-twinned copper coating. Figure 2 and Figure 3 In the image, (a) is the SEM image and (b) is the EBSD image; Figure 4 In the image, (a) is a SEM image of the (111) nano-twinned copper bonding interface, and (b) is a SEM image of the (110) nano-twinned copper bonding interface. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] As mentioned in the background section, traditional bonding requires high temperature and pressure, resulting in high costs. The high-temperature process can easily cause damage to heat-sensitive devices, chip warping, and thermal stress accumulation. Therefore, the inventors discovered that using a heterogeneous material system of "high surface diffusion copper" + "low-temperature grain growth copper" for copper-copper bonding can reduce the traditional temperature ≥400℃ and pressure ≥100MPa to 160-200℃ and 1-5MPa. This significantly reduces the risk of thermal damage to heat-sensitive devices, and greatly reduces equipment investment due to the reduced pressure and vacuum requirements, while also simplifying the process flow. Specifically, "high surface diffusion copper," such as (111) nanotwinned copper, has the highest surface diffusion rate among all crystal planes on its (111) crystal plane, which can drive copper atoms to migrate rapidly along the bonding interface at low temperatures, effectively promoting the initial closure of the interface and the filling of voids. "Low-temperature grain growth capability copper," such as (110) nanotwinned copper / nanofine-grained copper, has a low Young's modulus and hardness on its surface (e.g., the (110) surface), allowing it to undergo plastic deformation under low pressure and promoting initial tight contact at the interface. During low-temperature annealing, this type of material can undergo rapid recrystallization and grain growth. The migration and consumption of grain boundaries during grain growth release a large number of copper atoms, providing a sufficient atomic diffusion source for the entire bonding interface (especially adjacent high surface diffusion regions), thus achieving high-quality bonding.

[0025] In addition, both single-structure nt-Cu (i.e., nanotwinned Cu) and random nc-Cu have their own defects, making it impossible to balance performance and processability: (111) nt-Cu has a fast surface diffusion rate, but the number of surface atoms is limited. Copper atoms at the lattice cannot diffuse to the surface quickly, resulting in the inability to further eliminate interface voids. High-temperature annealing (>250℃) is required to improve the bonding quality. (110) Although nt-Cu has grain growth capability, its surface diffusion rate is much lower than that of (111) surface. It is difficult to achieve complete densification of the interface by grain growth alone. Nano-fine copper has poor thermal stability and undergoes a significant self-annealing effect at room temperature. The grains coarsen rapidly, and the grain boundary density decreases, which cannot meet the process turnover and transportation requirements in actual production, thus restricting its mass production feasibility. The inventors used the complementary properties of the two materials to synergistically solve the inherent problems of single materials. The bonding interface achieves a low-void bonding effect and gives the copper material good room temperature storage stability, thereby obtaining high reliability and mass production process compatibility. Specifically, at low temperatures, the plastic deformation and grain growth of (110) twinned copper / nanocrystalline copper provide more atomic migration flux, allowing a large number of copper atoms to diffuse to the bonding interface, creating a better diffusion environment for the adjacent (111) region. The (110) nanotwinned copper preferred in this invention is essentially a thermodynamically stable nanostructure. Its twin boundaries can effectively pin grain boundaries, suppressing grain coarsening at room temperature and solving the problem of extremely narrow processing windows due to self-annealing of traditional nanocrystalline copper.

[0026] Finally, existing technologies often require complex and expensive pretreatment or special environments to achieve low-temperature bonding. In response, the inventors have proposed a simplified and efficient pretreatment and process window adapted to the heterogeneous materials of this invention. The entire process requires low-cost equipment, involves simple steps, and is economical. In contrast to the "expensive" and "complex" nature of conventional low-temperature bonding technologies, this facilitates large-scale adoption in industry (especially in wafer-level packaging, chiplet integration, etc.), demonstrating significant economic efficiency and ease of use. Specifically, for the surface treatment of (111)nt-Cu and (110)nt-Cu, only standard CMP polishing to a roughness ≤5 nm is required, combined with conventional wet cleaning (acetone, acid pickling, ultrapure water rinsing, nitrogen drying) to remove contaminants and oxides, without relying on ultra-high vacuum, plasma activation, or costly metal passivation layers. The high surface diffusion and oxidation resistance of (111)nt-Cu, and the low-temperature grain growth capability of (110)nt-Cu, make the bonding process only need to be carried out in "ordinary low vacuum" (100-500 Pa), avoiding dependence on ultra-high vacuum equipment.

[0027] The heterobonding method provided by the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0028] Example 1: Bonding of (111) nanotwinned copper with (110) nanotwinned copper

[0029] (1) Step 1: Electroplating of (111) nanotwinned copper and (110) nanotwinned copper: A standard 8-inch silicon wafer was used as the substrate, on which a 20 nm thick TiN adhesion layer and a 50 nm thick Cu seed layer were pre-prepared by physical vapor deposition (PVD). The wafer was then cut into 10 mm × 10 mm samples.

[0030] The silicon substrate was subjected to a series of treatments: 7 seconds of degreasing, 30 seconds of citric acid pickling, 30 seconds of ultrapure water rinsing, and drying. The cleaned silicon substrate was then electroplated in two different twin copper plating solutions to obtain (111) nano-twinned copper layers and (110) nano-twinned copper layers (temperature 25℃ / speed 600rpm / time 40min / current density 6ASD). The cross-sectional morphology and XRD patterns of the electroplated (111) nano-twinned copper and (110) nano-twinned copper are shown below. Figure 1 As shown, (110) nanotwinned copper has a large number of grain boundaries perpendicular to the substrate, forming a columnar crystal structure. (111) nanotwinned copper also has a columnar crystal structure, but there are a large number of twin boundaries parallel to the substrate inside the grains. The two coatings have obvious (110) and (111) textures, respectively.

[0031] (2) Step 2: Polishing the surface of the twinned copper plating

[0032] Chemical mechanical polishing (CMP) was used to polish the surfaces of (111) and (110) twinned copper using a rotary polisher to obtain atomically smooth bonded surfaces. A porous polyurethane pad was used as the polishing pad. The polishing solution consisted of 3 wt% silica suspension (50 nm particle size), 0.4 wt% ammonium persulfate, and pH 8.5. The polishing pad rotation speed was 100 rpm, and the polishing solution flow rate was 40 mL / min. A profilometer showed that the polishing removal of the copper plating was 1.2 μm. The surface roughness of (111) nanotwinned copper and (110) nanotwinned copper were 3.5 and 3.7 nm, respectively. After polishing, the copper plating was rinsed with ultrapure water and dried.

[0033] (3) Step 3: Pre-bonding treatment

[0034] After polishing, the (111) and (110) nanotwinned copper coatings were cleaned with acetone for 2 min and isopropanol for 2 min to remove surface organic contaminants. Then, the coatings were acid-washed with citric acid for 1 min, rinsed with ultrapure water, and dried. Finally, the silicon substrates of the (111) and (110) nanotwinned copper coatings were aligned vertically.

[0035] (4) Step 4: Bonding

[0036] The aligned bonding sample is quickly placed into the bonding stage, a vertical pressure of 2.5 MPa is applied, and a vacuum is drawn (approximately 200 Pa). The upper and lower stages are simultaneously heated to the target temperature of 200°C (heating rate 10°C / min), and held at this temperature and pressure for 60 minutes to complete the bonding. After the bonding time is completed, heating is stopped, and the sample is allowed to cool naturally to below 50°C while maintaining pressure. The pressure is then released, and the bonding sample is removed.

[0037] Bonding effect as Figure 2 As shown, (110) nanotwinned copper grows into large-sized grains with random grain orientation, while (111) nanotwinned copper exhibits higher structural stability, maintaining a high (111) orientation, and some bonding interfaces show cross-interface migration of grain boundaries. The shear strength of the bonded joint was tested using a push-pull force tester. To meet the test requirements, a copper plating layer (4×4mm) was applied to a small silicon substrate. 2 ) and a large copper plating layer (10×10mm) on a silicon substrate 2 Copper-copper bonding was performed to form a bump-like structure. Three parallel samples were tested, and the average shear strength was calculated to be 34.16 MPa. The fracture area was located between the TiN adhesion layer and the Si substrate, indicating that the actual bonding strength is higher than the existing value.

[0038] Example 2: Temperature Parameter Boundary Verification

[0039] This embodiment aims to verify the feasibility of the technology of the present invention at a lower temperature (160°C).

[0040] (1) Step 1: Electroplating of (111) nanotwinned copper and (110) nanotwinned copper: This step is exactly the same as step one in Example 1, and (111)nt-Cu coatings and (110)nt-Cu coatings with the same microstructure characteristics are prepared.

[0041] (2) Step 2: Polishing the surface of the twinned copper plating

[0042] This step is exactly the same as step two in Example 1, ensuring that the surface roughness Rq of the bonding surfaces of the two samples is less than 5nm, wherein the surface roughness of (111) nanotwinned copper and (110) nanotwinned copper are 3.6 and 3.4nm respectively, and the coating removal thickness is 1.3μm.

[0043] (3) Step 3: Pre-bonding treatment

[0044] This step is exactly the same as step three in Example 1.

[0045] (4) Step 4: Bonding

[0046] The bonding process is the same as step four of Example 1, except that the bonding temperature is changed to 160℃. All other parameters remain unchanged: vacuum degree 200Pa, pressure 2.5MPa, bonding time 60 minutes, and heating rate 10℃ / min. The bonding effect is as follows: Figure 3 As shown, a few grains at the bonding interface still show a tendency to grow across the interface. The (110) nanotwinned copper grows into large-sized grains, while the (111) nanotwinned copper has higher structural stability and still maintains a high (111) orientation. At the same time, some interfaces show grain growth across the interface, and there are no obvious voids at the interface, indicating that high bonding quality can still be achieved at a low temperature of 160℃. Shear tests show that the average shear strength is 30.5 MPa. The strength is lower than that of Example 1, but still much higher than that of the completely unbonded state.

[0047] This embodiment demonstrates that 160°C is close to the effective bonding temperature limit of the method of the present invention under the current parameters. It verifies that even at lower temperatures, the synergistic mechanism of heterogeneous copper-copper bonding, namely the combination of grain boundary migration and recrystallization of (110) oriented copper and the high surface diffusion of (111) oriented copper, can still achieve a certain quality of bonding. It proves that the present invention does not depend on a specific high temperature point, but can work within a temperature window.

[0048] Example 3: Comparison of (111) nanotwinned copper or (110) nanotwinned copper bonding in single materials

[0049] To demonstrate the advantages and necessity of "heterogeneous material synergy" in this invention, two control experiments were set up in this embodiment, using single (111)nt-Cu and single (110)nt-Cu respectively, and bonding was carried out under the same process conditions as in Example 1 of this invention, for comparison.

[0050] (1) Step 1: Electroplating of (111) nanotwinned copper and (110) nanotwinned copper: This step is exactly the same as step one in Example 1, and two (111)nt-Cu coatings with the same microstructure characteristics and two (110)nt-Cu coatings are prepared.

[0051] (2) Step 2: Polishing the surface of the twinned copper plating

[0052] This step is exactly the same as step two in Example 1, ensuring that the roughness Rq of the bonding surface is less than 5nm, wherein the surface roughness of (111) nanotwinned copper and (110) nanotwinned copper are 3.3 and 3.5nm respectively, and the coating removal thickness is 1.2μm.

[0053] (3) Step 3: Pre-bonding treatment

[0054] This step is exactly the same as step three in Example 1.

[0055] (4) Step 4: Bonding

[0056] The bonding process is the same as step four of Example 1, and alignment and hot-press bonding are performed under exactly the same conditions as in Example 1 (bonding at 200°C): vacuum degree 200 Pa, temperature raised to 200°C, pressure applied at 2.5 MPa, and maintained for 60 minutes. The bonding effect is as follows. Figure 4 As shown, the (111)nt-Cu bonding interface has discontinuous voids of varying sizes, indicating that although the extremely high surface diffusion rate of the (111) facet promotes atomic migration in the local contact area during the bonding process, there is a lack of copper atoms diffusing along the grain boundaries, which cannot fill the large voids caused by the micro-undulations of the surface. The (110)nt-Cu bonding interface has some tortuosity, and the voids observed at the interface are generally smaller (nanometer-scale), but more numerous than those at the (111)nt-Cu bonding interface. This indicates that the (110)nt-Cu undergoes certain plastic deformation and grain boundary migration at low temperatures, which can better adapt to the surface morphology. Even in areas with large surface undulations, the voids can be reduced through the good plastic deformation and creep of the (110) facet. However, the diffusion rate of the (110) facet is lower than that of the (111) facet, and there are still small voids at the interface.

[0057] In summary, the key to achieving high-quality bonding at low temperatures lies in simultaneously providing high surface diffusion channels at the interface and eliminating interfacial porosity through grain boundary migration / recrystallization. Single-structure copper materials struggle to achieve both. Therefore, this invention combines two copper materials with specific structures that each enhance one of these capabilities, achieving interfacial void closure at low temperatures through their synergistic function.

[0058] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the present invention.

Claims

1. A low-temperature, low-pressure copper-copper direct bonding method based on a heterogeneous copper structure, characterized in that, include: S1: A metal underlayer for electroplating is formed on the first silicon substrate and the second silicon substrate respectively, and the first silicon substrate and the second silicon substrate are degreased, pickled, cleaned with ultrapure water and dried. S2: Copper plating layers are formed by electroplating on the first silicon substrate and the second silicon substrate respectively, wherein the copper plating layer on the first silicon substrate is a nanotwinned copper layer with (111) texture, and the copper plating layer on the second silicon substrate is a nanotwinned copper layer with (110) texture. S3: Perform chemical mechanical polishing on the nanotwinned copper layer with texture (111) and the nanotwinned copper layer with texture (110) respectively to obtain a bonding surface with a bonding surface roughness Rq≤5nm, and rinse and dry with ultrapure water. S4: After polishing, the nanotwinned copper layer with the (111) texture and the nanotwinned copper layer with the (110) texture are sequentially cleaned with acetone, cleaned with isopropanol, pickled with citric acid, rinsed with ultrapure water and dried, and then aligned vertically. S5: Place the aligned sample into the bonding machine, apply a vertical pressure of 1-5 MPa to the sample and evacuate to 100-500 Pa, heat to 150-250 °C at a heating rate of 10 °C / min and hold at the temperature and pressure for 30-90 min to complete the bonding. S6: After bonding is completed, stop heating and allow the sample to cool naturally to below 50°C while maintaining the vertical pressure. Then, release the pressure and remove the bonded sample.

2. The low-temperature, low-pressure copper-copper direct bonding method based on heterogeneous copper structures according to claim 1, characterized in that, The metal substrate consists of a TiN adhesion layer and a Cu seed layer, and is formed on the first silicon substrate and the second silicon substrate by physical vapor deposition.

3. The low-temperature, low-pressure copper-copper direct bonding method based on heterogeneous copper structures according to claim 2, characterized in that, The thickness of the TiN adhesion layer is 20 nm, and the thickness of the Cu seed layer is 50 nm.

4. The low-temperature, low-pressure copper-copper direct bonding method based on heterogeneous copper structures according to claim 1, characterized in that, In step S2, the electroplating temperature is 15-25℃, the stirring rate is 400-800rpm, the current density is 4-7ASD, and the electroplating time is 20-60min; in step S1, the pickling uses 20wt% dilute sulfuric acid or citric acid.

5. The low-temperature, low-pressure copper-copper direct bonding method based on heterogeneous copper structures according to claim 1, characterized in that, The chemical mechanical polishing described in step S3 uses a rotary polisher, and the polishing pad is a porous polyurethane pad. The polishing solution contains a silica suspension and ammonium persulfate, wherein the silica particle size is 50 nm, the silica concentration is 2-5 wt%, the ammonium persulfate concentration is 0.2-0.8 wt%, and the pH of the polishing solution is 8.5±0.

5. The polishing pad rotation speed is 50-150 rpm, the applied downward pressure is 2-6 psi, and the polishing solution flow rate is 30-50 mL / min.

6. The low-temperature, low-pressure copper-copper direct bonding method based on heterogeneous copper structures according to claim 1, characterized in that, In step S4, the acetone cleaning time is 2 minutes, the isopropanol cleaning time is 2 minutes, and the citric acid pickling time is 1 minute.

7. The low-temperature, low-pressure copper-copper direct bonding method based on heterogeneous copper structures according to claim 1, characterized in that, In step S5, the bonding temperature is 160-200℃, the vertical pressure is 2.5MPa, the vacuum degree is about 200Pa, and the heat and pressure holding time is 60min.

8. A copper-copper bonding structure, characterized in that, The structure includes a first silicon substrate and a second silicon substrate, a nanotwinned copper layer with a (111) texture on the first silicon substrate, and a nanotwinned copper layer with a (110) texture on the second silicon substrate, wherein the nanotwinned copper layer with a (111) texture and the nanotwinned copper layer with a (110) texture are bonded to each other to form a copper-copper bonding interface; the copper-copper bonding structure is prepared by the low-temperature, low-pressure copper-copper direct bonding method based on heterostructure copper structure according to any one of claims 1-7.

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