An Al-doped assisted Cu-Cu low-temperature and low-pressure bonding method and its application

Abstract

This invention relates to the field of integrated circuit manufacturing technology, and discloses an Al-doped assisted Cu-Cu low-temperature, low-pressure bonding method and its application. The method includes: firstly, performing alkaline degreasing and acid washing to remove the oxide layer on a substrate with a copper seed layer; then, preparing a fine-grained copper underlayer by DC electroplating; next, using this as the working electrode, performing pulse electroplating to achieve copper-aluminum co-deposition to prepare a Cu-Al alloy bonding layer; finally, bonding the two components to be bonded together with their plating layers facing each other, and maintaining a constant temperature and pressure of 1-15 MPa and 160-300°C in a vacuum environment to complete the Cu-Cu metallurgical bonding. This invention significantly reduces the temperature and pressure required for Cu-Cu bonding.

Description

Technical Field

[0001] This invention relates to the field of integrated circuit manufacturing technology, and more specifically, to an Al-doped-assisted Cu-Cu low-temperature and low-pressure bonding method and its application. Background Technology

[0002] As integrated circuit manufacturing technology continues to advance towards high performance, high integration, and low power consumption, 3D integration and advanced packaging have become key paths to perpetuating Moore's Law. Among these, copper-copper direct bonding technology has become a core technology due to its advantages such as low resistivity, high thermal conductivity, and the ability to achieve high-density interconnects. However, achieving reliable pure copper thermocompression bonding typically requires high temperatures of ≥400℃ and high pressures of ≥100MPa. These high temperatures and pressures easily damage heat-sensitive devices, cause chip warping, and accumulate thermal stress, severely limiting its application in advanced packaging. Simultaneously, copper surfaces readily form a natural oxide layer in the air, which severely hinders atomic diffusion at the bonding interface. To address this issue, existing technologies typically rely on ultra-high vacuum environments, complex plasma surface activation (SAB), or the introduction of additional metal passivation layers (such as Au, Ag, etc.), which significantly increases equipment complexity and manufacturing costs, and also results in poor mass production compatibility.

[0003] Therefore, developing a new copper-copper bonding method that can both improve atomic diffusion rate at the intrinsic material level to reduce bonding temperature / pressure and overcome surface oxide obstacles in situ and simplify the process has become a key challenge that the industry urgently needs to solve. Summary of the Invention

[0004] The purpose of this invention is to provide an Al-doped Cu-Cu low-temperature and low-pressure bonding method and its application, aiming to solve the above-mentioned problems.

[0005] This invention provides an Al-doped assisted Cu-Cu low-temperature, low-pressure bonding method, comprising the following steps: S1. Substrate pretreatment and preparation of fine-grained copper underlayer: After the substrate with copper seed layer is subjected to alkaline degreasing and acid pickling to remove oxide layer pretreatment, fine-grained copper underlayer is prepared by electrodeposition on the surface of copper seed layer of substrate using DC electroplating process. S2. Preparation of Al-doped Cu-Al alloy bonding layer: Using the substrate with fine-grained copper underlayer obtained in step S1 as the working electrode, copper-aluminum co-deposition is performed by pulse electroplating process to prepare an Al-doped Cu-Al alloy bonding layer on the surface of the fine-grained copper underlayer. The percentage of Al atoms in the Cu-Al alloy bonding layer is 2-10%. S3. Low-temperature and low-pressure vacuum hot-press bonding: Two pieces of workpieces with the Cu-Al alloy bonding layer prepared on their surfaces are bonded together with the bonding layers facing each other. They are placed in a vacuum hot-press bonding device, and a contact pressure of 1-15 MPa is applied in a vacuum environment. The temperature is raised to 160-300℃ and held at a constant temperature and pressure for 30-90 minutes to complete the Cu-Cu metallurgical bonding.

[0006] Further, in step S1, the plating solution used for DC electroplating contains 50-100 g / L copper sulfate pentahydrate, 3-10 mL / L sulfuric acid, and 30-80 ppm sodium chloride; the process parameters for DC electroplating are: current density 3-8 A / dm², electroplating temperature 20-35℃, stirring speed 500-1000 rpm, and electroplating time 5-20 min.

[0007] Further, in step S2, the plating solution used for pulse electroplating contains 0.2-5 M / L aluminum salt and 100-2000 ppm copper salt; The aluminum salt is one or more of aluminum trifluoromethanesulfonate, aluminum bis(trifluoromethanesulfonyl)imide, aluminum bis(fluorosulfonyl)imide, and aluminum trifluoroacetate. The copper salt is copper sulfate, copper methanesulfonate, copper pyrophosphate, copper fluoroborate, copper aminosulfonate, and copper acetate, or their respective hydrates.

[0008] Further, in step S2, the process parameters for pulse electroplating are: average current density 1-6.25A / dm², pulse width 0.2-1s, stirring speed 1000-2000rpm, and total electroplating time 3-10min.

[0009] Furthermore, in step S3, the vacuum level of the vacuum environment ranges from 500 Pa to 10⁻³ mbar.

[0010] Furthermore, in step S3, the temperature for constant temperature and pressure holding is 180-200℃, the contact pressure is 10MPa, and the constant temperature and pressure holding time is 60min.

[0011] Furthermore, in step S3, the temperature for constant temperature and pressure holding is 250-300℃, the contact pressure is 10MPa, and the constant temperature and pressure holding time is 60min.

[0012] On the other hand, the present invention also protects the application of the above-mentioned Al-doped Cu-Cu low-temperature and low-pressure bonding method in three-dimensional integration and advanced packaging of integrated circuits.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Significantly reduced thermal budget and stress: Bonding temperature as low as 180°C, and contact pressure required only about 10 MPa. This effectively protects advanced logic chips, memory chips, and heat-sensitive devices from thermal damage, and significantly reduces the risk of chip warpage and breakage due to high pressure, thereby improving overall packaging yield.

[0014] 2. Low equipment modification cost: Thanks to the passivation and consumption of oxygen by Al, the bonding process only needs to be carried out in a normal low vacuum environment, which is compatible with the existing thermoforming bonding machines of mainstream packaging production lines.

[0015] 3. The Al-doped Cu thin film of this invention can be directly prepared using conventional electroplating processes, perfectly compatible with existing semiconductor electroplating production lines. No additional expensive physical vapor deposition equipment is required, making it easy to achieve large-scale mass production in foundries and packaging plants. Attached Figure Description

[0016] 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.

[0017] Figure 1 TEM cross-sectional characterization of the 180°C bonding interface provided in an embodiment of the present invention; Figure 2 SEM image and EDS element distribution map of the 180°C bonding interface provided in this embodiment of the invention; Figure 3 These are magnified SEM images of the interface of copper-copper bonding at different temperatures provided in embodiments of the present invention. Figure 4 The interface diffusion MD calculation fitting curve provided for this invention.

[0018] Figure 1 In the figure, (a) is the STEM-HAADF image; (b) is the HRTEM image of the marked area; (c) is the selected electron diffraction spot for the yellow box in (b); and (d) is the selected electron diffraction spot for the red box in (b). Figure 2 (a) shows the elemental distribution of the surface scan of the STEM-HAADF image; (b) shows the Cu element; (c) shows the Al element; and (d) shows the O element. Figure 3 In the image, (ac) is the SEM image at 250℃, and (df) is the SEM image at 300℃. Detailed Implementation

[0019] 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.

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] The following examples all use the same substrate and pretreatment process: a silicon wafer with a (111) oriented copper seed layer is used as the cathode substrate. Insulating adhesive is used to control the electroplating area to 2cm × 2cm. The cathode substrate is placed in an alkaline degreasing solution and degreased at a constant voltage of 12V for 15s. After rinsing with pure water, it is acid-washed in a 20% sulfuric acid solution for 15s to remove the oxide layer, and then placed in an electrolyte for electroplating.

[0022] Preparation of fine-grained copper substrate: In a plating bath containing 62.5 g / L CuSO4·5H2O, 5.55 mL / L H2SO4 and 50 ppm NaCl, a current density of 6 A / dm² was applied, and electrodeposition was performed at 25 °C with magnetic stirring (800 rpm) for 10 min.

[0023] Example 1: Hot-press bonding at 250℃

[0024] Copper-aluminum co-deposition (Al doping): Using the pre-plated sample as the working electrode, 40 mL of a copper-aluminum mixed plating solution was added to a 50 mL electrolytic cell. By adjusting the concentration ratio of copper and aluminum salts in the mixed plating solution and controlling the average current density and duty cycle parameters of the pulse electrodeposition, pulse electrodeposition was performed at a magnetic stirring speed of 1000 rpm to obtain a micron-sized Cu-Al alloy film with an Al atomic percentage of 10%.

[0025] Bonding process: Two samples are bonded together with their coatings facing each other and placed into the chamber of a WH-2000 vacuum hot press bonding machine. Background vacuum is evacuated to 10... -3 The load is on the order of mbar. A mechanical load of 100 kgf (corresponding to a contact pressure of approximately 10 MPa) is applied. The temperature is then increased to 250°C at a rate of 10°C / min and held at this temperature and pressure for 1 hour. After bonding is complete, the pressure is released, and the product is cooled in the furnace to below 40°C before being removed.

[0026] Final product verification: Magnified SEM images of the interface show a dense, void-free interface achieved at 250℃. EDS elemental surface scanning confirms that Al and O elements exhibit significant co-segregation at the bonding interface, demonstrating good self-cleaning properties.

[0027] Example 2: Bonding verification at 300℃

[0028] Copper-aluminum co-deposition: The operation steps and solution formulation are exactly the same as in Example 1.

[0029] Bonding process: Sample loading and vacuuming (10 -3 The operation of applying pressure (mbar) and pressurizing (10MPa) is the same as above. The temperature is increased to the target bonding temperature of 300℃ at a rate of 10℃ / min, and then held at this temperature and pressure for 1 hour.

[0030] Final product verification: Cross-sectional SEM images confirmed that as the temperature increased, the interfacial diffusion and grain boundary migration of atoms became more complete, and a perfectly stitched interface with extremely high metallurgical bonding quality was obtained at 300℃.

[0031] Example 3: Extremely low temperature hot pressing bonding at 180℃

[0032] Copper-aluminum co-deposition: The operation steps are the same as in Example 1, and a Cu-Al alloy coating enriched with highly active doped Al atoms is obtained by electrodeposition.

[0033] Bonding process: Sample loading and vacuuming (10⁻³ mbar) and pressurization (10 MPa) were performed as described above. To verify the feasibility of bonding at extremely low temperatures, the target heating temperature was set to 180°C and held at constant temperature and pressure for 1 hour.

[0034] Final product validation: See Figure 1 As shown, wafer-level / chip-level interconnection was still achieved under an extremely low thermal budget of 180°C. (See also...) Figure 2 As shown, the TEM cross-section (STEM-HAADF image and HRTEM image) and selected area electron diffraction (SAED) spots clearly confirm that the Al atoms segregated to the interface in situ capture oxygen from the native oxide layer, generating discontinuous nanoscale Al2O3 particles. This not only opens up the diffusion channels for copper atoms but also plays a role in nano-dispersion enhancement.

[0035] See Figure 3 As shown, these are magnified SEM images of the interface after copper-copper bonding at different temperatures. (ac) represents 250°C, corresponding to Example 1; (df) represents 300°C, corresponding to Example 2.

[0036] Example 4

[0037] The difference between this embodiment and Embodiment 1 is that, in the copper-aluminum co-deposition (Al doping) step, the concentrations of copper salt and aluminum salt in the copper-aluminum mixed plating solution are adjusted so that the Al content of the obtained micron-sized Cu-Al alloy film is 5%.

[0038] Example 5

[0039] The difference between this embodiment and Embodiment 1 is that, in the copper-aluminum co-deposition (Al doping) step, the concentrations of copper salt and aluminum salt in the copper-aluminum mixed plating solution are adjusted so that the Al content of the obtained micron-sized Cu-Al alloy film is 2%.

[0040] Comparative Example 1

[0041] The difference between this embodiment and Embodiment 1 is that, in the copper-aluminum co-deposition (Al doping) step, only a single copper salt plating solution is used to prepare the top film (i.e., Al atomic percentage is 0%).

[0042] Conclusion Analysis

[0043] 1. Pure copper has a high solid-state diffusion activation energy and low atomic mobility at low temperatures, making it difficult to close interfacial voids and requiring high temperature and pressure for bonding. This application proposes an Al-doped Cu-Al alloy interconnect material as a bonding layer, utilizing the Al doping effect to significantly reduce the diffusion barrier between copper atoms at the interface and grain boundaries. For details, please refer to... Figure 4 As shown, the interface diffusion MD calculation fitting curves of Examples 1, 4, 5 and Comparative Example 1 reveal that Al doping effectively reduces the solid-state diffusion activation energy (Ea) of the system. The Ea of the pure copper system is 1.44 eV, while with 10% Al doping, Ea can be significantly reduced to 0.50 eV, demonstrating a significant barrier reduction. The introduction of Al atoms greatly enhances the activity of interface atoms. Therefore, the bonding temperature, which originally required above 400℃, can be significantly reduced to a low temperature of 180-200℃, allowing for rapid interface diffusion and rearrangement of atoms, achieving a dense bonded interface without obvious continuous voids.

[0044] 2. Copper surfaces are highly susceptible to oxidation. Conventional processes require ultra-high vacuum or complex plasma activation to remove interfacial oxides. This application utilizes the high oxygen evolution activity of Al, making it a sacrificial electrode during bonding to achieve in-situ self-cleaning of the interface. Compared to Cu, Al has a higher affinity for oxygen. During hot-press bonding, Al atoms spontaneously segregate towards the bonding interface containing trace amounts of oxygen, reducing the copper oxide at the interface in situ and generating nanoscale Al2O3 particles. This process disrupts the continuous copper oxide film, exposing a fresh metal surface. Therefore, an ultra-high vacuum environment or expensive surface plasma activation treatment is unnecessary. Excellent metallurgical bonding can be achieved under conventional low vacuum levels, significantly reducing equipment investment and process costs.

[0045] 3. The presence of continuous oxides at the bonding interface leads to a decrease in the mechanical strength and an increase in the resistivity of the joint. This application controls Al doping by adjusting electroplating parameters, resulting in a nano-dispersed distribution of Al2O3 rather than a continuous barrier layer. After the Al segregates to the interface and removes oxygen, it forms a discontinuous nano-scale precipitate at the interface. These nanoparticles not only do not completely block electron transport, but also play a role similar to "dispersion strengthening". Thus, the mechanical strength and reliability of the bonding joint are improved while ensuring conductivity.

[0046] 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 by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. An Al-doped assisted Cu-Cu low-temperature, low-pressure bonding method, characterized in that, Includes the following steps: S1. Substrate pretreatment and preparation of fine-grained copper underlayer: After the substrate with copper seed layer is subjected to alkaline degreasing and acid pickling to remove oxide layer pretreatment, fine-grained copper underlayer is prepared by electrodeposition on the surface of copper seed layer of substrate using DC electroplating process. S2. Preparation of Al-doped Cu-Al alloy bonding layer: Using the substrate with fine-grained copper underlayer obtained in step S1 as the working electrode, copper-aluminum co-deposition is performed by pulse electroplating process to prepare an Al-doped Cu-Al alloy bonding layer on the surface of the fine-grained copper underlayer. The percentage of Al atoms in the Cu-Al alloy bonding layer is 2-10%. S3. Low-temperature and low-pressure vacuum hot-press bonding: Two pieces of workpieces with the Cu-Al alloy bonding layer prepared on their surfaces are bonded together with the bonding layers facing each other. They are placed in a vacuum hot-press bonding device, and a contact pressure of 1-15 MPa is applied in a vacuum environment. The temperature is raised to 160-300℃ and held at a constant temperature and pressure for 30-90 minutes to complete the Cu-Cu metallurgical bonding.

2. The Al-doped assisted Cu-Cu low-temperature and low-pressure bonding method according to claim 1, characterized in that, In step S1, the plating solution used for DC electroplating contains 50-100 g / L copper sulfate pentahydrate, 3-10 mL / L sulfuric acid, and 30-80 ppm sodium chloride; the process parameters for DC electroplating are: current density 3-8 A / dm², electroplating temperature 20-35℃, stirring speed 500-1000 rpm, and electroplating time 5-20 min.

3. The Al-doped assisted Cu-Cu low-temperature and low-pressure bonding method according to claim 1, characterized in that, In step S2, the plating solution used for pulse electroplating contains 0.2-5 M / L aluminum salt and 100-2000 ppm copper salt; The aluminum salt is one or more of aluminum trifluoromethanesulfonate, aluminum bis(trifluoromethanesulfonyl)imide, aluminum bis(fluorosulfonyl)imide, and aluminum trifluoroacetate. The copper salt is copper sulfate, copper methanesulfonate, copper pyrophosphate, copper fluoroborate, copper aminosulfonate, and copper acetate, or their respective hydrates.

4. The Al-doped assisted Cu-Cu low-temperature and low-pressure bonding method according to claim 1, characterized in that, In step S2, the process parameters for pulse electroplating are: average current density 1-6.25 A / dm², pulse width 0.2-1 s, stirring speed 1000-2000 rpm, and total electroplating time 3-10 min.

5. The Al-doped assisted Cu-Cu low-temperature and low-pressure bonding method according to claim 1, characterized in that, In step S3, the vacuum level of the vacuum environment ranges from 500 Pa to 10 Pa. -3 mbar.

6. The Al-doped assisted Cu-Cu low-temperature and low-pressure bonding method according to claim 1, characterized in that, In step S3, the temperature for constant temperature and pressure holding is 180-200℃, the contact pressure is 10MPa, and the constant temperature and pressure holding time is 60min.

7. The Al-doped assisted Cu-Cu low-temperature and low-pressure bonding method according to claim 1, characterized in that, In step S3, the temperature for constant temperature and pressure holding is 250-300℃, the contact pressure is 10MPa, and the constant temperature and pressure holding time is 60min.

8. The application of the Al-doped Cu-Cu low-temperature and low-pressure bonding method according to any one of claims 1-7 in three-dimensional integration and advanced packaging of integrated circuits.

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