A method and application of metal hot pressing bonding

Abstract

This invention discloses a method for metal hot-press bonding and its application. The method specifically includes: providing a first substrate and a second substrate, the first substrate having a first metal bonding surface and the second substrate having a second metal bonding surface; aligning the first and second substrates to bring the first and second metal bonding surfaces into contact; and performing hot-press bonding by controlling temperature and applying pressure. The hot-press bonding includes a heating and high-pressure stage, a heat-holding and low-pressure stage, and a cooling and pressure-releasing stage; the temperature of the heat-holding and low-pressure stage is the bonding temperature. This invention achieves hot-press bonding of metals in an air environment through coordinated temperature and pressure control, resulting in a well-bonded interface that is free of oxidation, dense, and pore-free.

Description

Technical Field

[0001] This invention belongs to the field of three-dimensional packaging and relates to a direct bonding method for interconnecting metal electrodes between chips or wafers, specifically a metal thermocompression bonding method and its application. Background Technology

[0002] 3D interconnect technology is a novel packaging technology that uses through-silicon vias (TSVs) and microbumps to stack multiple chips vertically, thereby increasing chip interconnect density, reducing package size, and improving electrical and thermal transfer efficiency. The size of interconnect bumps in 3D packaging has been reduced from over 100 micrometers in traditional flip chips to tens or even a few micrometers. However, as the size and spacing of interconnect microbumps decrease, reliability issues such as electromigration, thermal migration, intermetallic compound (IMC) embrittlement, and bridging of Sn-based solders become more pronounced, rendering 3D packaging less suitable.

[0003] Single-metal bump interconnects avoid the problems of alloy solder composition segregation and intermetallic compound growth mentioned above, making them the preferred choice for researchers. Among them, Cu (Cu) has become a focus of attention for next-generation bonding materials due to its low resistivity, high thermal conductivity, strong resistance to electromigration, and high cost-effectiveness. However, the Cu surface is extremely prone to oxidation and lacks stable and dense oxides, which greatly affects the bonding strength and the electrical and thermal conductivity of the solder joint after bonding. Reducing Cu surface oxidation is a key factor affecting the quality of Cu-Cu bonding. Researchers have proposed the following solutions: bonding in an oxygen-free environment to provide ultra-high vacuum conditions (Suga et al., JVAC SCI TECHNOLA, 2003, DOI: 10.1116 / 1.1537716); bonding in a reducing atmosphere to prevent oxidation and further reduce oxides (Masahisa Fujino, Jpn.J.Appl.Phys., 2017, DOI: 10.7567 / JJAP.56.04CC01); depositing an inert metal passivation layer, such as Au, Pt, Pd, etc., to isolate oxygen (Kuan-Neng Chen, Electron Device Lett, 2021, DOI: 10.1109 / LED.2021.3105434); depositing a bilayer of Cu with a highly reactive metal to sacrificial anode to protect the cathode (CN110476240A), etc. However, the above solutions inevitably have problems such as stringent requirements for the bonding environment and increased manufacturing costs. In addition, multilayer metal deposition also affects the bonding strength and electrothermal transfer performance.

[0004] To address the problems of existing CuCu bonding methods, there is an urgent need for an innovative CuCu bonding solution that can meet the oxidation resistance requirements of the Cu metal surface and ensure bonding strength and interconnect transport performance without introducing more hot-pressing bonding process steps and preparation costs. Summary of the Invention

[0005] To address the oxidation problem during CuCu bonding, the present invention aims to provide a method for metal hot pressing bonding and its application.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] The first aspect of this invention provides a method for metal hot-press bonding, comprising the following steps:

[0008] A first substrate and a second substrate are provided, the first substrate having a first metal bonding surface and the second substrate having a second metal bonding surface; the first substrate and the second substrate are aligned so that the first metal bonding surface and the second metal bonding surface are in contact, and thermo-press bonding is performed by controlling the temperature and applying pressure;

[0009] The hot-press bonding includes a heating and high-pressure stage, a heat-holding and low-pressure stage, and a cooling and pressure-releasing stage.

[0010] The temperature during the heat preservation and low-pressure stage is the bonding temperature.

[0011] Furthermore, the heating rate during the high-pressure heating stage is 3–10 °C / min, the temperature is raised to 200 °C–400 °C, and the pressure is 20–35 MPa.

[0012] The insulation temperature during the low-pressure insulation stage is 200℃~400℃, the insulation time is 10~30min, and the pressure is ≤5Mpa.

[0013] The cooling and depressurization stage involves releasing pressure immediately after the heat preservation and low-pressure stage ends, and then cooling down to room temperature along with the furnace.

[0014] Furthermore, the first metal bonding surface is a metal micro-bump;

[0015] The second metal bonding surface is a metal microbump or a through-silicon via (TSV) lead-out metal pad.

[0016] Furthermore, the surface roughness Ra of the metal microbumps is ≤10nm in the range of 10×10μm;

[0017] The surface roughness of the metal micro-bumps is ≤5%;

[0018] The diameter of the metal micro-bumps is between 5 and 2100 μm, and the height is between 5 and 50 μm.

[0019] Preferably, the metal microbumps form an array structure, and the spacing between the metal microbumps is ≥10μm.

[0020] Furthermore, the material of the first substrate is Si, glass, or SiC semiconductor material, and the material of the second substrate is Si, glass, or SiC semiconductor material.

[0021] Furthermore, before hot-press bonding, the first substrate with the first metal bonding surface and the second substrate with the second metal bonding surface are sequentially cleaned with a formic acid-ethanol mixture solution and ultrasonically cleaned with deionized water.

[0022] Preferably, the formic acid content in the formic acid-ethanol solution is 40-65 vol%, and the soaking time using the formic acid-ethanol mixture is 30-60 seconds.

[0023] Preferably, the ultrasonic cleaning with deionized water has a frequency of 40-60 kHz, a temperature of 20-25°C, and a cleaning time of 1-3 minutes.

[0024] In the above technical solution of the present invention, the first metal is one of copper, silver and gold;

[0025] The second metal is one of copper, silver, and gold;

[0026] Preferably, the first metal is copper, and the second metal is copper.

[0027] Furthermore, when the first metal or the second metal is copper, the copper microbumps are (111)nt-Cu microbumps.

[0028] Further, the fabrication of the first or second substrate having (111)nt-Cu microbumps specifically includes: 1) depositing a SiO2 insulating layer on the first or second substrate; 2) applying photoresist and patterning it; 3) etching SiO2 and sputtering to deposit a Ti adhesion layer and a Cu seed layer; 4) applying photoresist to form an electroplated pattern; 5) electroplating to deposit metal Cu layer microbumps; 6) stripping the photoresist and etching the Ti / Cu seed layer;

[0029] Preferably, the electroplating in step 5) is DC electroplating.

[0030] A second aspect of the present invention provides the application of the above-described metal hot-press bonding method in interconnections between chips, between chips and wafers, or between wafers.

[0031] The present invention has the following advantages:

[0032] This invention provides a method for metal bonding in an air environment. Through coordinated temperature and pressure control, metal hot-press bonding is performed, resulting in a well-bonded interface that is oxidation-free, dense, and free of pores. Specifically, taking copper-copper hot-press bonding as an example, a higher pressure is used during the low-temperature heating stage to ensure tight contact between the Cu bumps and the bonding metal, preventing oxygen from entering and causing oxidation. During the high-temperature holding stage, the pressure is reduced to minimize thermal stress caused by the mismatch in the coefficients of thermal expansion of the chip, promoting diffusion and accelerating bonding with a lower pressure. During the cooling stage, the pressure is released, and the furnace cools down, reducing thermal stress. This process does not require creating ultra-high vacuum or reducing atmosphere conditions, nor does it require introducing additional protective layers or fabrication processes. By systematically regulating the bonding temperature and pressure curves, oxygen entry after Cu bump contact is controlled, preventing interface oxidation during bonding. Subsequently, temperature control ensures successful bonding. Furthermore, this invention uses (111)nt-Cu, which has high oxidation resistance, as the bonding metal, further improving the oxidation resistance of the bonding interface. The CuCu hot-press bonding method of the present invention is highly efficient, low-cost, and highly reliable, and the bonded structure has high strength and good electrical and thermal conductivity. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of temperature and pressure coordinated control.

[0034] Figure 2 This is a process flow diagram for bonding Cu bumps to Cu bumps in 3D packaging.

[0035] Figure 3 Flowchart for the preparation of Cuμ-bump.

[0036] Figure 4 This is a schematic diagram of the bonding structure.

[0037] Figure 5 The diagram illustrates the process parameters for the coordinated control of temperature and pressure in hot-press bonding in Example 1.

[0038] Figure 6A This is a SEM image of the overall bonding structure in Example 1.

[0039] Figure 6B This is a magnified view of the bonding interface in Example 1.

[0040] Figure 7A This is a TEM image of the CuCu bonding interface in Example 1.

[0041] Figure 7B Example 1: EDS line scan analysis of the elemental distribution at the Cu-Cu bonding interface showed good interfacial bonding and no enrichment of oxygen elements.

[0042] Figure 8 This is a comparative SEM image of the CuCu bonding interface. Detailed Implementation

[0043] To better understand the present invention, it is now further described with reference to the following embodiments and accompanying drawings. The embodiments are for illustrative purposes only and do not limit the invention in any way. In the embodiments, all original reagents and materials are commercially available, and experimental methods not specifically specified are conventional methods and conditions well known in the art, or according to the conditions recommended by the instrument manufacturer.

[0044] The temperature and pressure coordinated control method for thermocompression bonding of this invention is applicable to heterogeneous or homogeneous bonding between metals (such as Cu, Ag, Au, etc.). Taking Cu-Cu thermocompression bonding as an example, the process method of this invention can be applied to bonding between Cu bumps in three-dimensional packaging and bonding between Cu pads and Cu bumps at the leads of through-silicon vias (TSVs).

[0045] Taking the CuCu bonding process applied to the bonding of Cu bumps to Cu bumps in 3D packaging as an example, the materials of the first and second substrates are both Si, and the copper microbumps are (111)nt-Cuμ-bumps. The process flow diagram is as follows. Figure 2 As shown, the specific steps are described below:

[0046] Step 1: Fabricate 111nt-Cu microbump structures on the substrate. The fabrication process is as follows: Figure 3 As shown. Specifically, it includes: 1) thermally oxidizing and depositing a SiO2 insulating layer on a Si wafer; 2) applying photoresist and patterning it; 3) etching SiO2 and sputtering to deposit a Ti adhesion layer and a Cu seed layer; 4) applying photoresist to form an electroplating pattern; 5) electroplating to deposit microbumps (μ-bumps) of a Cu layer to form a Cu μ-bump array structure; 6) stripping the photoresist and etching the Ti / Cu seed layer. Among them, the Ti adhesion layer can improve the (111) selectivity of the Cu seed layer, thereby giving the electroplated Cu layer better (111) selectivity and reducing the electroplating transition layer.

[0047] In a preferred embodiment, the electroplated Cu layer is achieved by using DC electroplating with commercial active additives, resulting in a coating with a (111)-preferred oriented nanotwinned Cu structure ((111)nt-Cu), which has better oxidation resistance. DC electroplated Cu has better thermal stability compared to pulse electroplating.

[0048] In a preferred embodiment, the Cuμ-bump diameter is 5–2100 μm, the μ-bump height is 5–50 μm, and the pad spacing is ≥10 μm.

[0049] Step 2: Reduce the surface roughness of the prepared Cuμ-bump to achieve surface planarization. Specifically, mechanical polishing or chemical mechanical polishing (CMP) can be used to reduce surface roughness and achieve surface planarization.

[0050] In a preferred embodiment, the surface roughness Ra of the polished Cuμ-bump is ≤10nm (within the range of 10×10μm); the surface unevenness is ≤5%.

[0051] Step 3: Cut chip dies from the entire wafer.

[0052] Step 4: The prepared die containing (111)nt-Cuμ-bump was ultrasonically cleaned sequentially with a mixture of formic acid and ethanol and deionized water to remove surface organic impurities and oxide layer. After cleaning, it was quickly dried with N2.

[0053] In a preferred embodiment, the formic acid ethanol solution contains 40–65 vol% formic acid and the soaking time is 30–60 s.

[0054] In a preferred embodiment, the ultrasonic cleaning frequency of deionized water is 40-60KHz, the temperature is 20-25℃, the cleaning time is 1-3min, and after removal, it is quickly dried with nitrogen gas.

[0055] Step 5: Align the two cleaned dies to make contact between the μ-bumps. Apply pressure and heat to the two dies to begin thermocompression bonding. Thermocompression bonding includes a high-pressure heating stage, a low-pressure holding stage, and a cooling and pressure release stage. Specific temperature and pressure coordination control is as follows... Figure 1 As shown, the temperature control stage is divided into three parts: heating stage, holding stage, and cooling stage. Correspondingly, the pressure-assisted stages are the high-pressure zone, low-pressure zone, and no-pressure zone (pressure release zone). The bonding temperature is 200℃~400℃, the heating rate is 3~10℃ / min, the holding time is 10~30min, and the cooling stage involves furnace cooling (approximately 1℃ / min). The pressure range in the high-pressure zone is 20~35MPa; the pressure range in the low-pressure zone is ≤5MPa.

[0056] Example 1

[0057] This embodiment provides bonding between Cu bumps, as detailed below:

[0058] 1. Fabrication of a 111nt-Cuμ-bump bonded chip structure. Polycrystalline Cu was used as the substrate material, and the Cu seed layer structure sputtered on a Si substrate was simulated. A whole (111)nt-Cu layer was deposited by DC electroplating with commercial additives, with a coating thickness of 50 μm. The (111) orientation preference was 85%, and the vertical columnar crystals containing parallel twin structures satisfied the bonding structure conditions.

[0059] 2. The surface of 111nt-Cu was smoothed and its roughness was reduced by mechanical polishing. The surface roughness Ra = 7nm (10×10μm) and the surface unevenness ≤5%.

[0060] 3. Cut out two chip dies of different sizes, with the upper die measuring 3×3mm and the lower die measuring 5×5mm.

[0061] 4. The prepared chip containing (111)nt-Cuμ-bump was sequentially cleaned with a 50 vol% formic acid ethanol solution for 30 s to remove surface organic impurities and oxide layer. Then, it was cleaned with deionized water to remove organic residues. The deionized water ultrasonic cleaning frequency was 55 kHz, the temperature was 25 ℃, and the cleaning time was 1 min. After cleaning, it was quickly dried with nitrogen gas.

[0062] 5. Align the two cleaned chips so that the Cu metal between the upper and lower chips is in contact, with the bonding structure as follows: Figure 4 As shown, two chips are thermo-bonded in air, with temperature and pressure jointly controlling the process parameters as follows. Figure 5 As shown in the figure. The bonding temperature is 300℃, the heating rate is 5℃ / min, the holding time is 20min, and then the cooling rate is 1℃ / min. During the heating stage, a pressure of 30MPa is applied in the high-pressure zone, and during the holding stage, a pressure of 5MPa is applied in the low-pressure zone. The pressure is immediately released upon entering the cooling stage, entering the pressure-free zone.

[0063] 6. After mounting the bonded sample with epoxy resin and polishing it sequentially with sandpaper and diamond polishing compound, SEM images were taken. Figure 6 shows the SEM image of the bonding interface. As can be seen from the figure, there is no obvious bonding interface between the upper and lower chips, indicating high bonding quality.

[0064] 7. TEM was used to further magnify and observe the CuCu bonding interface. TEM samples of the CuCu bonding interface were prepared using FIB. Figure 7A This is a TEM image of the CuCu bonding interface sample, where the arrow indicates... Figure 7B EDS component analysis line scan range. (Through) Figure 7B As can be seen, there are no prominent oxygen peaks at the bonding interface, indicating the absence of oxygen segregation or enrichment, meaning there is no obvious oxide layer at the interface. Combined with TEM images, this shows that using this bonding process, the bonding interface is free of oxide layer and pores.

[0065] 8. The bonded sample was subjected to shear strength testing, and the bond strength reached 86 MPa.

[0066] Comparative Example 1

[0067] This comparative example demonstrates the bonding between Cu bumps, as detailed below:

[0068] 1. Fabrication of a 111nt-Cuμ-bump bonded chip structure. Polycrystalline Cu was used as the substrate material, and the Cu seed layer structure sputtered on a Si substrate was simulated. A whole (111)nt-Cu layer was deposited by DC electroplating with commercial additives, with a coating thickness of 50 μm. The (111) orientation preference was 85%, and it contained vertical columnar crystals with parallel twin structures, which met the bonding structure requirements.

[0069] 2. Surface roughness is reduced through mechanical polishing, with a surface roughness Ra = 10⁶ nm (10 × 10 μm).

[0070] 3. Cut two chip dies of different sizes.

[0071] 4. The prepared chip containing (111)nt-Cuμ-bump was sequentially cleaned with a 50 vol% formic acid ethanol solution for 30 s, followed by ultrasonic cleaning with deionized water at a frequency of 55 kHz and a temperature of 25 ℃ for 1 min. After cleaning, the chip was quickly dried with nitrogen gas.

[0072] 5. Align the two cleaned chips so that the Cu metal between the upper and lower chips makes contact.

[0073] 6. Perform thermocompression bonding on the two chips. The bonding temperature is 300℃, the heating rate is 10℃ / min, the holding time is 30min, followed by a cooling rate of 1℃ / min. No pressure is applied during the heating phase, a pressure of 20MPa is applied during the holding phase, and the pressure is stopped at the start of the cooling phase.

[0074] 7. During the pressureless bonding phase at the heating stage, the high surface roughness of the bonding interface allows oxygen to enter, leading to severe oxidation. A comparative SEM image of the bonding interface is shown below. Figure 8 As shown, a thick oxide layer exists at the bonding interface.

[0075] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method of metal thermocompression bonding, characterized by, Includes the following steps: A first substrate and a second substrate are provided, the first substrate having a first metal bonding surface and the second substrate having a second metal bonding surface; the first substrate and the second substrate are aligned so that the first metal bonding surface and the second metal bonding surface are in contact, and thermocompression bonding is performed by controlling the temperature and applying pressure, wherein the thermocompression bonding method is direct thermocompression bonding of copper metals; The hot-press bonding includes a heating and high-pressure stage, a heat-holding and low-pressure stage, and a cooling and pressure-releasing stage; the temperature of the heat-holding and low-pressure stage is the bonding temperature. The heating rate during the high-pressure heating stage is 3~10 ℃ / min, heating to 200 ℃~400 ℃, with a pressure of 20~35 MPa; the holding temperature during the low-pressure holding stage is 200 ℃~400 ℃, the holding time is 10~30 min, and the pressure is ≤5 MPa; the cooling and pressure relief stage involves releasing the pressure immediately after the low-pressure holding stage ends, and cooling down to room temperature along with the furnace. The first metal bonding surface is a metal microbump, which is a copper bump; the second metal bonding surface is a metal microbump or a through-silicon via (TSV) lead-out metal pad, which is a Cu pad.

2. The method of claim 1, wherein, The surface roughness Ra of the metal microbumps is ≤10nm in the range of 10×10μm; The surface roughness of the metal micro-bumps is ≤5%; The diameter of the metal micro-bumps is between 5 and 2100 μm, and the height is between 5 and 50 μm.

3. The method of claim 2, wherein, The metal microbumps form an array structure, and the spacing between the metal microbumps is ≥10μm.

4. The method of claim 1, wherein, The first substrate is made of Si, glass, or SiC semiconductor material, and the second substrate is made of Si, glass, or SiC semiconductor material.

5. The method according to claim 1, characterized in that, Before hot-press bonding, the first substrate with the first metal bonding surface and the second substrate with the second metal bonding surface are sequentially cleaned with a formic acid-ethanol mixture solution and ultrasonically cleaned with deionized water.

6. The method according to claim 5, characterized in that, The formic acid in the formic acid-ethanol solution has a formic acid content of 40-65 vol%, and the soaking time using the formic acid-ethanol mixture is 30-60 s.

7. The method according to claim 5, characterized in that, The ultrasonic cleaning with deionized water is performed at a frequency of 40-60 kHz, a temperature of 20-25 ℃, and a cleaning time of 1-3 min.

8. The method according to any one of claims 1-7, characterized in that, The first metal is one of copper, silver, and gold; The second metal is one of copper, silver, and gold.

9. The method according to claim 8, characterized in that, The first metal is copper, and the second metal is copper.

10. The method according to claim 8, characterized in that, When the first metal or the second metal is copper, the copper bump is a (111)nt-Cu microbump.

11. The method according to claim 8, characterized in that, The preparation of a first substrate or a second substrate having (111)nt-Cu microbumps specifically includes: 1) depositing a SiO2 insulating layer on the first substrate or the second substrate; 2) applying photoresist and patterning it; 3) etching SiO2 and sputtering to deposit a Ti adhesion layer and a Cu seed layer; 4) applying photoresist to form an electroplated pattern; 5) electroplating to deposit metal Cu layer microbumps; 6) stripping the photoresist and etching the Ti / Cu seed layer.

12. The method according to claim 11, characterized in that, The electroplating described in step 5) is DC electroplating.

13. The application of the method according to any one of claims 1-12 in interconnection between chips, between chips and wafers, or between wafers.

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