CN111320487A - Silicon Nitride Ceramic Surface Modification Assisted Direct Diffusion Bonding Method - Google Patents

Abstract

The invention provides a silicon nitride ceramic surface modification auxiliary direct diffusion bonding method, which comprises the steps of firstly pretreating the surface of silicon nitride ceramic to remove surface oil stains and impurities, and then carrying out laser irradiation treatment on the surface of silicon nitride ceramic under the protection of vacuum or inert gas; assembling the treated materials according to the sequence of the silicon nitride ceramics and the metal materials, placing the materials in a vacuum diffusion connecting device for pressurizing, heating and preserving heat for a period of time, utilizing the modified silicon nitride ceramics and the metal materials to generate a local eutectic liquid phase through eutectic reaction, and then solidifying the eutectic liquid phase to realize the connection of the ceramics and the metal. The method solves the problems of higher cost and less selection range of brazing filler metal in the existing active metal brazing method. The invention can be widely applied to the high-quality connection of ceramics and metals.

Description

Silicon Nitride Ceramic Surface Modification Assisted Direct Diffusion Bonding Method

technical field

The invention relates to the technical field of ceramic and metal connection, in particular to a method for direct diffusion connection assisted by surface modification of silicon nitride ceramics.

Background technique

With the current rapid development of new energy vehicles, renewable energy, and electronic components, higher requirements are placed on the performance of power electronic devices. As the most important high-power device in the field of power electronics, insulated gate bipolar transistors (IGBTs) have been widely used in electric vehicles, smart grids, and rail transit. And the heat conduction channel is the key base material in the module. At present, the commonly used IGBT module substrate materials are ceramic substrates, mainly including alumina, aluminum nitride and silicon nitride ceramic substrates. Compared with the two ceramic materials of alumina and aluminum nitride, silicon nitride ceramics have better performance. It has the advantages of excellent mechanical properties, thermal shock resistance, high temperature oxidation resistance and chemical corrosion resistance, and also has the characteristics of resistance to thermal and cold shock. It will not break when heated to above 1000 ℃ in the air, and it will not be broken after rapid cooling and rapid heating. Therefore, the use of silicon nitride ceramics as substrate materials in power modules is of great significance for promoting the application of high-power devices in extreme conditions and complex mechanical environments.

In terms of module packaging technology, surface copper plating technology, direct copper cladding technology and active metal brazing technology are mainly used. At present, the connection method between silicon nitride ceramic substrate and copper is mainly active metal brazing method. This method is to use metal material with a lower melting point than the base metal as the brazing filler metal, and heat the welding temperature to a temperature higher than that of the brazing filler metal and lower than that of the mother metal. The temperature of the melting point of the material is a method of using the metallurgical reaction between the active elements in the liquid solder and the base metal to promote the liquid solder to wet the base metal, so as to realize the connection.

However, the existing active metal brazing method has the problems of high cost and limited selection of brazing filler metals, and the reaction between the liquid brazing filler metal and the base metal inevitably occurs during the brazing process to form an interface reaction layer. And the intermediate intermetallic compound seriously affects the thermal conductivity of the substrate, causing the ceramic-metal composite substrate to be prone to cracking during the thermal cycle after welding, which seriously affects the reliability of the power module. Therefore, the present invention provides a simple The efficient silicon nitride ceramic surface modification assisted direct diffusion connecting method solves the above technical problems existing in the existing connecting method of silicon nitride ceramic substrate and copper.

SUMMARY OF THE INVENTION

The present invention is to solve the problem that the existing active metal brazing method has higher cost, less selection range of brazing material, and the inevitable reaction between the liquid brazing material and the base metal during the brazing process to form an interface reaction layer. The interface reaction layer and the intermediate intermetallic compound seriously affect the thermal conductivity of the substrate, which leads to the easy cracking of the ceramic-metal composite substrate after welding during the thermal cycle, which seriously affects the technical problem of the reliability of the power module. The surface of the silicon nitride ceramic is irradiated by laser to carry out surface modification pretreatment, and the direct connection between the silicon nitride ceramic substrate and the copper metal is realized by changing the characteristics of the ceramic surface microstructure and stress distribution state. The ceramic-metal obtained by this process The composite substrate has the advantages of high strength, strong impact resistance, strong current carrying capacity, good thermal conductivity and high reliability.

To this end, the present invention provides a silicon nitride ceramic surface modification assisted direct diffusion connection method, comprising the following steps:

(1) Put the silicon nitride ceramics into the acetone solution for ultrasonic cleaning for 10min-20min to remove the surface oil and impurities, and then use 600#, 1200#, 2000# sand discs to grind and polish the surface of the silicon nitride ceramics step by step. ;

(2) The silicon nitride ceramic obtained in step (1) is placed on a heating platform, heated under vacuum or inert gas protection state, and the heating temperature is controlled to be 100°C-300°C, and then the heated silicon nitride is heated. The ceramic is placed on the laser processing platform, and the surface of the silicon nitride ceramic is irradiated by laser under the protection of vacuum or inert gas, that is, the modification of the ceramic surface is realized;

(3) Machining the metal material to obtain a sample of the metal material to be connected, then put the metal material into the acetone solution for ultrasonic cleaning for 10min-20min, and then use 400#, 800#, 400#, 800#, 1200#, 2000# sandpaper is polished and polished step by step;

(4) Assembling the surface-modified silicon nitride ceramic obtained in step (2) and the metal material obtained in step (3) in the order of ceramic-metal material and placing it in a vacuum diffusion connection device, vacuuming, The connection structure is pressurized and heated for connection, and cooled to room temperature, that is, the direct diffusion connection between the silicon nitride ceramic and the metal material is realized.

Preferably, in step (2), the heating temperature is 100°C-250°C, and the heating process is carried out under the protection of vacuum or inert gas.

Preferably, the laser current in step (2) is 320A-350A, the laser pulse width is 1.8ms-2.5ms, the laser frequency is 3Hz-5Hz, and the laser scanning speed is 50mm/min-250mm/min.

Preferably, in step (2), the inert gas is any one of helium or argon as the protective gas.

Preferably, the metal material in step (3) is copper.

Preferably, in the connection process in step (4), the degree of vacuum in the diffusion connection device is 1.5×10 ^-3 Pa-6.5× ^{10 -3} Pa.

Preferably, the axial pressure applied by the ceramic and copper composite in step (4) is 2MPa-5MPa.

Preferably, in the step (4), the heating process is as follows: firstly, the temperature increase rate is controlled to be 10°C/min-20°C/min to be heated to 750°C and kept for 10 minutes, and then the temperature increase rate is controlled to be 5°C/min-10°C/min and heated to 750°C for 10 minutes. 850℃-1000℃, heat preservation for 30min-120min, and finally control the cooling rate of 5℃/min-10℃/min to cool down to 300℃ and then cool with the furnace.

Preferably, the heating process in step (4) is as follows: firstly, the temperature increase rate is controlled to be 10°C ± 1°C/min to be heated to 750°C ± 1°C for 10 minutes, and then the temperature increase rate is controlled to be 5°C ± 0.5°C/min to be heated to 900°C , hold for 60min, and finally control the cooling rate of 5℃±0.5℃/min to cool down to 300℃ and then cool with the furnace.

The beneficial effect of the present invention is that, due to the use of pulsed laser to modify the surface of silicon nitride ceramics, on the one hand, silicon nitride is decomposed to form silicon element after the ceramic surface is modified, and a local liquid phase is formed by silicon-copper eutectic reaction, so that the The direct connection between the ceramic and the metal is realized at a lower temperature, which reduces the reaction between the solder metal and the base metal in the existing silicon nitride ceramic and metal connection to form a large number of intermetallic compounds, which affects the performance of the joint; at the same time, due to the surface modification of the silicon nitride ceramic After that, the surface microstructure and state are changed, which effectively promotes the diffusion of atoms during the connection process and the combination of the two materials, and improves the connection strength between the ceramic and the metal.

Description of drawings

Fig. 1 is the schematic diagram of the surface modification process of silicon nitride ceramics of the present invention;

Fig. 2 is the macroscopic photograph and scanning electron microscope photograph before and after the surface modification of silicon nitride ceramics of the present invention;

FIG. 3 is a scanning electron microscope photograph of the connection interface between silicon nitride ceramics and metallic copper according to one embodiment of the present invention.

Detailed ways

Example 1

As shown in Figures 1 to 3, it is a photo of a sample obtained by the method of direct diffusion connection assisted by surface modification of silicon nitride ceramics in this embodiment, wherein Figure 1 is the surface modification process of silicon nitride ceramics in this embodiment Schematic diagram, FIG. 2 is a macroscopic photo and a scanning electron microscope photo before and after the surface modification of the silicon nitride ceramic in this embodiment, and FIG. 3 is a scanning electron microscope photo of the connection interface between the silicon nitride ceramic and metallic copper in this embodiment. In this embodiment, a silicon nitride ceramic surface modification assisted direct diffusion connection method includes the following steps:

(1) Put the silicon nitride ceramics into the acetone solution for ultrasonic cleaning for 10min-20min to remove the surface oil and impurities, and then use 600#, 1200#, 2000# sand discs to grind and polish the surface of the silicon nitride ceramics step by step. ;

(2) Place the silicon nitride ceramic obtained in step 1 on a heating platform, heat it under the protection of argon gas, and control the heating temperature to be 200°C, and then place the heated silicon nitride ceramic on the laser processing platform. , under the protection of argon gas, the surface of silicon nitride ceramics is irradiated by laser, the laser current is 340A, the laser pulse width is 1.8ms, the laser frequency is 4Hz, and the laser scanning speed is 100mm/min, that is, the improvement of the ceramic surface is realized. sex;

(3) The metal copper is mechanically processed to obtain a sample of the metal material to be connected, and then the metal copper is put into the acetone solution for ultrasonic cleaning for 10min-20min, and then the metal copper joint surfaces to be welded are respectively 400#, 800#, 1200#, 2000# sandpaper is polished and polished step by step;

(4) The surface-modified silicon nitride ceramic obtained in step (2) and the metallic copper obtained in step (3) are assembled in the order of ceramic-metal material and placed in a vacuum diffusion bonding device. The assembly is subjected to an axial pressure of 5MPa, and heated when the vacuum degree reaches 1.5×10 ^-3 Pa. First, the heating rate is controlled to be 10°C/min to 750°C for 10 minutes, and then the heating rate is controlled to be 5°C/min. To 900 ℃, keep the temperature for 60 minutes, and finally control the cooling rate of 5 ℃/min to cool down to 300 ℃ and then cool with the furnace, that is, to realize the direct diffusion connection between silicon nitride ceramics and metal copper.

In this embodiment, the laser modification technology is combined with the instantaneous liquid phase diffusion connection method, which effectively removes the inert layer on the ceramic surface and realizes the change of the chemical composition and surface state of the ceramic surface. The diffusion between the ceramics and the metal achieves a high-quality connection at a lower temperature.

In this embodiment, pulsed laser is used to modify the surface of silicon nitride ceramics. On the one hand, silicon nitride is decomposed to form silicon element after the ceramic surface is modified, and a local liquid phase is formed by silicon-copper eutectic reaction, so that at a lower temperature Realize the direct connection between ceramic and metal, and reduce the reaction between the solder metal and the base metal to form a large number of intermetallic compounds in the existing silicon nitride ceramic and metal connection, which affects the performance of the joint; at the same time, the surface microscopic surface is caused by the surface modification of the silicon nitride ceramic. The change of structure and state effectively promotes the diffusion of atoms and the combination of the two materials during the connection process, and improves the connection strength between the ceramic and the metal.

Example 2

A method for direct diffusion connection assisted by surface modification of silicon nitride ceramics in this embodiment includes the following steps:

(1) Put the silicon nitride ceramics into the acetone solution for ultrasonic cleaning for 10min-20min to remove the surface oil and impurities, and then use 600#, 1200#, 2000# sand discs to grind and polish the surface of the silicon nitride ceramics step by step. ;

(2) The silicon nitride ceramic obtained in step (1) is placed on a heating platform, heated under the protection of argon gas, and the heating temperature is controlled to be 200°C, and then the heated silicon nitride ceramic is placed on the laser processing On the platform, the surface of silicon nitride ceramics is irradiated by laser under the protection of argon gas, the laser current is 335A, the laser pulse width is 1.8ms, the laser frequency is 4Hz, and the laser scanning speed is 80mm/min, that is, the ceramic surface is realized. modification;

(3) The metal copper is mechanically processed to obtain a sample of the metal material to be connected, and then the metal copper is put into the acetone solution for ultrasonic cleaning for 10min-20min, and then the metal copper joint surfaces to be welded are respectively 400#, 800#, 1200#, 2000# sandpaper is polished and polished step by step;

(4) The surface-modified silicon nitride ceramic obtained in step (2) and the metallic copper obtained in step (3) are assembled in the order of ceramic-metal material and placed in a vacuum diffusion bonding device. The assembly is subjected to an axial pressure of 4MPa, and heated when the degree of vacuum reaches 1.5×10 ^-3 Pa. First, the heating rate is controlled to be 10°C/min to 750°C for 10 minutes, and then the heating rate is controlled to be 5°C/min. To 850 ℃, keep the temperature for 90 minutes, and finally control the cooling rate of 5 ℃/min to cool down to 300 ℃ and then cool with the furnace, that is, to realize the direct diffusion connection between silicon nitride ceramics and metal copper.

After experiments, some process parameters in this example have been adjusted relative to those in Example 1, and the expected goal can also be achieved.

Example 3

A method for direct diffusion connection assisted by surface modification of silicon nitride ceramics in this embodiment includes the following steps:

(1) Put the silicon nitride ceramics into the acetone solution for ultrasonic cleaning for 10min-20min to remove the surface oil and impurities, and then use 600#, 1200#, 2000# sand discs to grind and polish the surface of the silicon nitride ceramics step by step. ;

(2) Place the silicon nitride ceramic obtained in step 1 on a heating platform, heat it under the protection of argon gas, and control the heating temperature to be 200°C, and then place the heated silicon nitride ceramic on the laser processing platform. , Under the protection of argon gas, the surface of silicon nitride ceramics is irradiated by laser, the laser current is 320A, the laser pulse width is 1.8ms, the laser frequency is 3Hz, and the laser scanning speed is 200mm/min, that is, the improvement of the ceramic surface is realized. sex;

(3) The metal copper is mechanically processed to obtain a sample of the metal material to be connected, and then the metal copper is put into the acetone solution for ultrasonic cleaning for 10min-20min, and then the metal copper joint surfaces to be welded are respectively 400#, 800#, 1200#, 2000# sandpaper is polished and polished step by step;

(4) Assemble the surface-modified silicon nitride ceramic obtained in step 2 and the metallic copper obtained in step 3 in the order of ceramic-metal material and place them in a vacuum diffusion bonding device, and apply The axial pressure of 2MPa is heated when the vacuum degree reaches 5×10 ^-3 Pa. First, the heating rate is controlled to be 10°C/min to 750°C for 10 minutes, and then the temperature rise rate is controlled to be 5°C/min to be heated to 900°C. Hold for 60 minutes, and finally control the cooling rate of 5 °C/min to cool down to 300 °C and then cool with the furnace, that is, to realize the direct diffusion connection between silicon nitride ceramics and metal copper.

After experiments, some process parameters in this example have been adjusted relative to those in Example 1, and the expected goal can also be achieved.

Example 4

A method for direct diffusion connection assisted by surface modification of silicon nitride ceramics in this embodiment includes the following steps:

(1) Put the silicon nitride ceramics into the acetone solution for ultrasonic cleaning for 10min-20min to remove the surface oil and impurities, and then use 600#, 1200#, 2000# sand discs to grind and polish the surface of the silicon nitride ceramics step by step. ;

(2) The silicon nitride ceramic obtained in step (1) is placed on a heating platform, heated under the protection of argon gas, and the heating temperature is controlled to be 200°C, and then the heated silicon nitride ceramic is placed on the laser processing On the platform, the surface of silicon nitride ceramics is irradiated by laser under the protection of argon gas, where the laser current is 340A, the laser pulse width is 4ms, the laser frequency is 4Hz, and the laser scanning speed is 100mm/min, that is, the ceramic surface is irradiated. modified;

(3) The metal copper is mechanically processed to obtain a sample of the metal material to be connected, and then the metal copper is put into the acetone solution for ultrasonic cleaning for 10min-20min, and then the metal copper joint surfaces to be welded are respectively 400#, 800#, 1200#, 2000# sandpaper is polished and polished step by step;

(4) The surface-modified silicon nitride ceramic obtained in step (2) and the metallic copper obtained in step (3) are assembled in the order of ceramic-metal material and placed in a vacuum diffusion bonding device. The assembly is subjected to an axial pressure of 5MPa, and heated when the degree of vacuum reaches 5×10 ^-3 Pa. First, the heating rate is controlled to be 10°C/min to 750°C for 10 minutes, and then the heating rate is controlled to be 5°C/min. To 950 ℃, keep the temperature for 30 minutes, and finally control the cooling rate of 5 ℃/min to cool down to 300 ℃ and then cool with the furnace, that is, to realize the direct diffusion connection between silicon nitride ceramics and metal copper.

After experiments, some process parameters in this example have been adjusted relative to those in Example 1, and the expected goal can also be achieved.

Example 5

A method for direct diffusion connection assisted by surface modification of silicon nitride ceramics in this embodiment includes the following steps:

(1) Put the silicon nitride ceramics into the acetone solution for ultrasonic cleaning for 10min-20min to remove the surface oil and impurities, and then use 600#, 1200#, 2000# sand discs to grind and polish the surface of the silicon nitride ceramics step by step. ;

(2) Place the silicon nitride ceramic obtained in step 1 on a heating platform, heat it under the protection of argon gas, and control the heating temperature to be 200°C, and then place the heated silicon nitride ceramic on the laser processing platform. , Under the protection of argon gas, the surface of silicon nitride ceramics is irradiated by laser, the laser current is 340A, the laser pulse width is 1.8ms, the laser frequency is 4Hz, and the laser scanning speed is 200mm/min, that is, the improvement of the ceramic surface is realized. sex;

(3) The metal copper is mechanically processed to obtain a sample of the metal material to be connected, and then the metal copper is put into the acetone solution for ultrasonic cleaning for 10min-20min, and then the metal copper joint surfaces to be welded are respectively 400#, 800#, 1200#, 2000# sandpaper is polished and polished step by step;

(4) The surface-modified silicon nitride ceramic obtained in step (2) and the metallic copper obtained in step (3) are assembled in the order of ceramic-metal material and placed in a vacuum diffusion bonding device. The assembly is subjected to an axial pressure of 5MPa, and heated when the degree of vacuum reaches 5×10 ^-3 Pa. First, the heating rate is controlled to be 10°C/min to 750°C for 10 minutes, and then the heating rate is controlled to be 5°C/min. To 840 ℃, hold the temperature for 120 minutes, and finally control the cooling rate of 5 ℃/min to cool down to 300 ℃ and then cool with the furnace, that is, to realize the direct diffusion connection between silicon nitride ceramics and metal copper.

Example 6

A method for direct diffusion connection assisted by surface modification of silicon nitride ceramics in this embodiment includes the following steps:

(1) Put the silicon nitride ceramics into the acetone solution for ultrasonic cleaning for 10min-20min to remove the surface oil and impurities, and then use 600#, 1200#, 2000# sand discs to grind and polish the surface of the silicon nitride ceramics step by step. ;

(2) The silicon nitride ceramic obtained in step (1) is placed on a heating platform, heated under the protection of argon gas, and the heating temperature is controlled to be 200°C, and then the heated silicon nitride ceramic is placed on the laser processing On the platform, the surface of silicon nitride ceramics is irradiated by laser under the protection of argon gas, where the laser current is 340A, the laser pulse width is 4ms, the laser frequency is 4Hz, and the laser scanning speed is 100mm/min, that is, the ceramic surface is irradiated. modified;

(3) The metal copper is mechanically processed to obtain a sample of the metal material to be connected, and then the metal copper is put into the acetone solution for ultrasonic cleaning for 10min-20min, and then the metal copper joint surfaces to be welded are respectively 400#, 800#, 1200#, 2000# sandpaper is polished and polished step by step;

(4) The surface-modified silicon nitride ceramic obtained in step (2) and the metallic copper obtained in step (3) are assembled in the order of ceramic-metal material and placed in a vacuum diffusion bonding device. The assembly is subjected to an axial pressure of 2MPa, and heated when the vacuum degree reaches 5×10 ^-3 Pa. First, control the heating rate to be 10°C/min to 750°C for 10 minutes, and then control the heating rate to 5°C/min. To 1000 ℃, hold for 60 minutes, and finally control the cooling rate of 5 ℃/min to cool down to 300 ℃ and then cool with the furnace, that is, to realize the direct diffusion connection between silicon nitride ceramics and metal copper.

Example 7

A method for direct diffusion connection assisted by surface modification of silicon nitride ceramics in this embodiment includes the following steps:

(1) Put the silicon nitride ceramics into the acetone solution for ultrasonic cleaning for 10min-20min to remove the surface oil and impurities, and then use 600#, 1200#, 2000# sand discs to grind and polish the surface of the silicon nitride ceramics step by step. ;

(2) placing the silicon nitride ceramic obtained in step (1) on a heating platform, heating in a vacuum state, and controlling the heating temperature to be 150° C., and then placing the heated silicon nitride ceramic on a laser processing platform, The surface of the silicon nitride ceramics is irradiated with laser light in a vacuum state, the laser current is 345A, the laser pulse width is 1.8ms, the laser frequency is 4Hz, and the laser scanning speed is 100mm/min, that is, the modification of the ceramic surface is realized;

(3) The metal copper is mechanically processed to obtain a sample of the metal material to be connected, and then the metal copper is put into the acetone solution for ultrasonic cleaning for 10min-20min, and then the metal copper joint surfaces to be welded are respectively 400#, 800#, 1200#, 2000# sandpaper is polished and polished step by step;

(4) The surface-modified silicon nitride ceramic obtained in step (2) and the metallic copper obtained in step (3) are assembled in the order of ceramic-metal material and placed in a vacuum diffusion bonding device. The assembly is subjected to an axial pressure of 5MPa, and heated when the vacuum degree reaches 1.5×10 ^-3 Pa. First, the heating rate is controlled to be 10°C/min to 750°C for 10 minutes, and then the heating rate is controlled to be 5°C/min. To 880 ℃, hold the temperature for 60 minutes, and finally control the cooling rate of 5 ℃/min to cool down to 300 ℃ and then cool with the furnace, that is, to realize the direct diffusion connection between silicon nitride ceramics and metal copper.

The technical solution of the present invention is not limited to the specific embodiments exemplified above, but also includes any combination of specific embodiments. The above embodiments use pulsed lasers to modify the surface of silicon nitride ceramics, on the one hand, use the ceramic surface After modification, silicon nitride is decomposed to form silicon element, and the silicon-copper eutectic reaction is used to form a local liquid phase, so as to realize the direct connection between ceramic and metal at a lower temperature, and reduce the solder metal and metal in the existing silicon nitride ceramic and metal connection. The base metal reacts to form a large number of intermetallic compounds, which affect the performance of the joint; at the same time, due to the change of the surface microstructure and state after the surface modification of the silicon nitride ceramic, it effectively promotes the diffusion of atoms and the combination of the two materials during the connection process. The strength of the connection between ceramic and metal.

Claims (9)

1. a silicon nitride ceramic surface modification assistant direct diffusion connecting method is characterized in that comprising the following steps: (1) Put the silicon nitride ceramics into the acetone solution for ultrasonic cleaning for 10min-20min to remove the surface oil and impurities, and then use 600#, 1200#, 2000# sand discs to grind and polish the surface of the silicon nitride ceramics step by step. ; (2) The silicon nitride ceramic obtained in step (1) is placed on a heating platform, heated under vacuum or inert gas protection state, and the heating temperature is controlled to be 100°C-300°C, and then the heated silicon nitride is heated. The ceramic is placed on the laser processing platform, and the surface of the silicon nitride ceramic is irradiated by laser under the protection of vacuum or inert gas, that is, the modification of the ceramic surface is realized; (3) Machining the metal material to obtain a sample of the metal material to be connected, then put the metal material into the acetone solution for ultrasonic cleaning for 10min-20min, and then use 400#, 800#, 400#, 800#, 1200#, 2000# sandpaper is polished and polished step by step; (4) Assembling the surface-modified silicon nitride ceramic obtained in step (2) and the metal material obtained in step (3) in the order of ceramic-metal material and placing it in a vacuum diffusion connection device, vacuuming, The connection structure is pressurized and heated for connection, and cooled to room temperature, that is, the direct diffusion connection between the silicon nitride ceramic and the metal material is realized. 2. The silicon nitride ceramic surface modification-assisted direct diffusion bonding method according to claim 1, characterized in that: in the step (2), the heating temperature is 100°C-250°C, and the heating process is protected by vacuum or inert gas. state. 3. The silicon nitride ceramic surface modification-assisted direct diffusion connection method according to claim 1, wherein the laser current in the step (2) is 320A-350A, and the laser pulse width is 1.8ms-2.5 ms, the laser frequency is 3Hz-5Hz, and the laser scanning speed is 50mm/min-250mm/min. 4 . The method for surface modification assisted direct diffusion connection of silicon nitride ceramics according to claim 1 , wherein in the step (2), the inert gas is any one of helium or argon as the protective gas. 5 . 5 . The method of claim 1 , wherein the metal material in the step (3) is copper. 6 . 6 . The method for assisted direct diffusion bonding by surface modification of silicon nitride ceramics according to claim 1 , wherein the vacuum degree in the diffusion bonding device during the connecting process in the step (4) is 1.5×10 ^-3 Pa- 6.5×10 ^-3 Pa. 7 . The method for assisted direct diffusion bonding of silicon nitride ceramic surface modification according to claim 1 , wherein in the step (4), the axial pressure exerted by the ceramic and copper assembly is 2MPa-5MPa. 8 . 8. The silicon nitride ceramic surface modification-assisted direct diffusion bonding method according to claim 1, characterized in that: in the step (4), the heating process is, firstly, the heating rate is controlled to be 10°C/min-20°C/ Min heating up to 750°C and holding for 10min, then control the heating rate to 5°C/min-10°C/min, heat up to 850°C-1000°C, hold for 30min-120min, and finally control the cooling at 5°C/min-10°C/min The rate is cooled to 300 ℃ and then cooled with the furnace. 9. The silicon nitride ceramic surface modification-assisted direct diffusion bonding method according to claim 1, characterized in that: in the step (4), the heating process is, firstly, the temperature increase rate is controlled to be 10°C ± 1°C/min. Keep the temperature at 750℃±1℃ for 10min, then control the heating rate to be 5℃±0.5℃/min, heat up to 900℃, keep the temperature for 60min, and finally control the cooling rate of 5℃±0.5℃/min to cool down to 300℃ and then cool with the furnace .

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