Formic acid vacuum sintering method for improving the quality of chip metallization layer bonding

Abstract

This invention relates to the field of semiconductor device packaging technology, specifically to a formic acid vacuum sintering method for improving the bonding quality of chip metallization layers. The method includes the following steps: assembling the chip, solder pad, and casing and placing them in a vacuum furnace; performing a first vacuum evacuation of the furnace chamber; filling the furnace chamber with nitrogen to one atmosphere; performing a second vacuum evacuation of the furnace chamber; introducing formic acid into the chamber under vacuum; raising the chamber temperature to a first temperature T1 and holding it at that temperature; performing a third vacuum evacuation of the chamber at the first temperature T1; introducing formic acid into the chamber again under vacuum; raising the chamber temperature to a second temperature T2 higher than the melting point of the solder pad and holding it at that temperature; performing a fourth vacuum evacuation of the furnace chamber; and finally cooling the product. This method, by combining a vacuum environment with a formic acid atmosphere at specific stages, effectively removes oxides and promotes atomic diffusion, thereby significantly enhancing the bonding strength of the metallization layer.

Description

Technical Field

[0001] This invention relates to the field of semiconductor device packaging technology, and more specifically to a formic acid vacuum sintering method for improving the bonding quality of chip metallization layers. Background Technology

[0002] In the packaging and manufacturing process of semiconductor devices, especially high-reliability power semiconductor devices used in aerospace and other fields, chips need to be fixed to the package through a sintering process. The metallization layer on the back of the chip, which is usually a multi-layered metal structure, is the key to achieving metallurgical bonding with the solder and the package. If the interlayer bonding quality of the metallization layer itself is poor, or if oxidation occurs on the surface of the metallization layer during sintering, the chip and the package will not bond firmly, and the metallization layer will fall off during subsequent testing or use, seriously affecting the reliability and lifespan of the device.

[0003] Currently used open or semi-open nitrogen-hydrogen protected eutectic sintering methods, while providing a certain reducing atmosphere, offer limited protection for chips with poor metallization bonding quality. They struggle to completely remove trace oxides from metallization gaps or surfaces, failing to effectively promote sufficient diffusion of metal atoms at the interface. Furthermore, due to the limited protective atmosphere, open or semi-open nitrogen-hydrogen protected eutectic sintering methods are prone to oxidation after prolonged high-temperature sintering, resulting in short sintering times and limited time for sufficient metal atom diffusion at the interface. After sintering using these methods, shear force tests frequently reveal delamination between metallization layers or between the metallization layer and the silicon substrate, representing a major technical defect in existing technologies. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a formic acid vacuum sintering method to improve the bonding quality of chip metallization layers. This method aims to solve the technical problem that existing sintering methods are unable to effectively improve the bonding quality of chip metallization layers and are prone to causing the metallization layers to detach after sintering.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A formic acid vacuum sintering method for improving the bonding quality of chip metallization layers includes the following steps: S1. Assemble the chip, solder pad, and casing, and place them in a vacuum furnace; S2. Perform the first vacuum evacuation on the vacuum furnace cavity; S3. Fill the furnace cavity with nitrogen to one atmosphere; S4. Perform a second vacuuming of the furnace cavity; S5. In a vacuum environment, formic acid is introduced into the cavity; S6. Raise the cavity temperature to the first temperature T1 and hold it at that temperature (raise the temperature in four gradients, each gradient controlling a different heating rate, raise it to the rapid decomposition temperature of formic acid, hold it for a period of time, and then raise it to temperature T1). The second temperature T1 is higher than the rapid decomposition temperature of formic acid. S7. At the first temperature T1, the cavity is evacuated for the third time; S8. In a vacuum environment, formic acid is introduced into the cavity again; S9. Raise the cavity temperature to a second temperature T2 and maintain the temperature, wherein the second temperature T2 is higher than the melting point of the solder sheet; S10. Perform the fourth vacuuming of the furnace cavity; S11. Cool the sintered product.

[0006] Furthermore, the target vacuum level for the first, second, third, and / or fourth vacuum evacuation is no greater than 100 Pa.

[0007] In one implementation, formic acid is introduced in step S5 before the cavity is heated; and / or, formic acid is introduced in step S8 before the cavity temperature reaches the second temperature T2.

[0008] In one embodiment, the first temperature T1 is a temperature above the rapid decomposition temperature of formic acid and below the liquidus temperature of the solder sheet; the second temperature T2 is 30°C to 70°C higher than the melting point of the solder sheet.

[0009] Furthermore, the second temperature T2 is 45°C to 55°C higher than the melting point of the solder sheet.

[0010] Furthermore, the target vacuum level is 45 Pa to 55 Pa.

[0011] In one specific embodiment, the first temperature T1 is 270°C to 287°C, and the holding time is 100s to 140s; the second temperature T2 is 330°C to 350°C, and the holding time is 40s to 60s.

[0012] Furthermore, the cooling process in step S11 includes the step of blowing inert protective gas into the cavity for forced cooling.

[0013] Furthermore, between steps S5 and S6, there is also a step of blowing air into the cavity to remove condensed moisture.

[0014] The present invention has at least the following beneficial effects: In this invention, oxygen in the cavity is eliminated by performing four vacuuming operations at a specific stage, creating an oxygen-free low-pressure environment. This fundamentally prevents oxidation of the metal surface during the heating process, and the low-pressure environment reduces the resistance to metal atom diffusion.

[0015] Secondly, formic acid is introduced twice before the critical heating point. During the heating process, the formic acid decomposes to produce hydrogen gas. The hydrogen gas can effectively reduce the trace metal oxides that already exist or may form on the surface of the chip's metallization layer, thereby removing the oxide layer barrier that hinders atomic-level bonding.

[0016] Finally, by precisely controlling the first temperature T1 and the second temperature T2, pretreatment and oxide reduction are carried out before the solder sheet is completely melted, and then the temperature is raised above the melting point of the solder sheet to achieve full melting and wetting. This promotes atomic interdiffusion and metallurgical bonding between the solder and the metallization layer, as well as between the layers of the metallization layer.

[0017] In summary, the present invention can significantly enhance the bonding force between chip metallization layers and the bonding strength with the solder interface, effectively solving the problem of peeling caused by poor original quality of chip metallization layers or oxidation during sintering, thereby greatly improving the packaging reliability and service life of semiconductor devices. Attached Figure Description

[0018] Figure 1 A process flow diagram of a sintering method for improving the bonding quality of chip metallization layers provided in an embodiment of the present invention. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be further described in detail below with reference to the embodiments of the present invention.

[0020] The sintering method provided by this invention is particularly suitable for the packaging and manufacturing processes of power semiconductor devices in fields such as aerospace, aviation, and automotive electronics, where reliability requirements are extremely high. This method aims to solve the problem of chip metallization layer detachment caused by poor bonding quality or oxidation in traditional sintering processes.

[0021] Embodiments of the present invention provide a sintering method for improving the bonding quality of chip metallization layers. The core of this method lies in introducing a reducing atmosphere (formic acid) at two key temperature nodes in an oxygen-free vacuum environment, and precisely controlling the heating and holding processes to synergistically enhance the bonding between the internal structure and interfaces of the chip metallization layer.

[0022] First, two vacuuming operations completely eliminate oxygen from the chamber, creating a low-oxygen or even oxygen-free environment. This effectively prevents oxidation of the metal surface at high temperatures and reduces the obstruction of atomic diffusion by gas molecules. Second, formic acid is introduced before heating to the first temperature T1 and the second temperature T2. During heating, the formic acid decomposes to produce hydrogen gas. This hydrogen gas can penetrate deep into the microscopic interface of the chip's metallization layer, reducing existing metal oxides and clearing obstacles for interatomic metallurgical bonding. Finally, the first temperature T1 is set above the rapid decomposition temperature of formic acid and below the liquidus temperature of the solder pad. At this temperature, the hydrogen gas from the decomposition of formic acid can fully reduce the surface oxide layer. Subsequently, the temperature is raised to the second temperature T2, which is higher than the melting point of the solder pad, allowing the solder to completely melt and fully wet the chip's metallization layer and the casing, promoting sufficient interdiffusion of metal atoms at the interface, thereby forming a strong metallurgical bond.

[0023] The following will combine Figure 1 The sintering method of this application is described in detail. Figure 1 As shown, the method mainly includes the following steps: S1. The chip with completed back metallization (such as Ti / Ni / Ag or Cr / Ni / Ag structure), the pre-shaped solder pad (such as Pb92.5Sn5Ag2.5 eutectic solder) and the shell (such as MoCu, CuW material) are assembled according to the design requirements and then placed into the cavity of the vacuum sintering furnace.

[0024] S2, the vacuum furnace cavity is evacuated for the first time to remove most of the air, especially oxygen, from the cavity. By controlling the target vacuum level at, for example, no more than 100 Pa, the oxygen partial pressure can be significantly reduced, creating a favorable initial oxygen-free environment for subsequent steps. In a preferred embodiment, the target vacuum level is further controlled in the range of 45 Pa to 55 Pa, for example, 50 Pa. This vacuum level ensures efficient oxygen removal while also considering equipment cost and process stability.

[0025] S3. Nitrogen gas is introduced into the furnace cavity to one atmosphere to completely replace the residual air inside the furnace cavity. A small amount of air (containing O2) may still remain in the furnace walls, dead corners, and powder pores. After introducing nitrogen gas, this residual air is "pushed out" by the nitrogen, significantly reducing the oxygen content.

[0026] S4. A second vacuum is performed on the furnace cavity to reduce the total pressure inside the furnace and create a suitable low-pressure environment for formic acid atmosphere sintering. By controlling the target vacuum level at, for example, no more than 100 Pa, the total pressure inside the furnace can be significantly reduced, creating a good oxygen-free low-pressure environment for subsequent steps. In a preferred embodiment, the target vacuum level is further controlled in the range of 45 Pa to 55 Pa, for example, 50 Pa. This vacuum level can ensure efficient oxygen removal and maintain a low-pressure environment while taking into account equipment cost and process stability.

[0027] S5, while maintaining the aforementioned vacuum environment, for example, not exceeding 100 Pa or 45-55 Pa, introduce a certain amount of gaseous formic acid into the chamber. This step is preferably performed before the chamber temperature reaches the subsequent first temperature T1, for example, it can be done at room temperature or at the beginning of the heating process. The introduced formic acid gas will begin to decompose during the subsequent heating process.

[0028] As a further optimization, after step S5 and before step S6, a dry inert protective gas, such as high-purity nitrogen or argon, can be briefly blown into the cavity. The purpose is to use the airflow to remove moisture that may be generated due to formic acid condensation or ambient humidity, so as to prevent moisture from causing metal oxidation in subsequent high-temperature steps.

[0029] S6. The cavity temperature is increased from room temperature to a first temperature T1 at a certain heating rate, for example, 20℃ / min to 60℃ / min, and held at this temperature. The first temperature T1 is set above the rapid decomposition temperature of formic acid and below the liquidus temperature of the solder pad. At this temperature, the hydrogen gas from the decomposition of formic acid can fully reduce the surface oxide layer. For commonly used Pb92.5Sn5Ag2.5 solder pads, its solidus is approximately 287℃, and the first temperature T1 can be selected from 270℃ to 287℃. Holding at this temperature range, below the solidus temperature of the solder pad (287℃), keeps the solder pad solid, intact in shape, and in position, and is within the high activity range of formic acid, which can efficiently reduce the Pb / Sn / Ag surface oxide film. The holding time can be 100 seconds to 140 seconds, for example, 120 seconds. During this period, the formic acid introduced in step S5 continues to decompose and produce hydrogen gas, which can perform preliminary reduction and cleaning of the chip metallization layer surface at a relatively low temperature.

[0030] S7. At the end of or after the holding period at the first temperature T1, a third vacuum is performed on the cavity. The target vacuum level for this vacuuming can be the same as the first, for example, no more than 100 Pa or 45 Pa to 55 Pa. The main purpose of this step is to promptly remove the gaseous products generated after the decomposition of formic acid introduced in step S5, such as CO and H2O, as well as volatile substances that may be released from the material, purifying the cavity environment and preparing for subsequent steps at higher temperatures.

[0031] S8. Maintaining the vacuum achieved after the second vacuuming, formic acid is introduced into the chamber again. Similar to step S5, this introduction of formic acid is preferably carried out before the chamber temperature reaches the second temperature T2. The formic acid introduced this time will provide raw materials for the deep reduction reaction at a higher temperature.

[0032] S9, the cavity temperature is further increased from the first temperature T1 to the second temperature T2, and held at T2. The second temperature T2 is higher than the melting point of the solder used. Specifically, the second temperature T2 can be 30°C to 70°C higher than the melting point of the solder. For example, for a Pb92.5Sn5Ag2.5 solder with a melting point of 287°C, T2 can be 317°C to 357°C.

[0033] In a more preferred range, the second temperature T2 is 45°C to 55°C higher than the melting point of the solder pad, i.e., for the aforementioned solder pad, T2 can be 332°C to 342°C. The holding time can be 40 seconds to 60 seconds, for example, 50 seconds. During this stage, the solder pad completely melts into a liquid state, fully wetting the soldering area of ​​the metallization layer on the back of the chip and the casing. Simultaneously, the formic acid introduced in step S8 decomposes rapidly at a higher temperature, producing more reactive hydrogen gas that can completely reduce residual oxides at the interface and promote strong atomic interdiffusion between the liquid solder and the metallization layer, forming a strong and reliable metallurgical bonding interface and promoting the bonding between the layers of the metallization layer itself.

[0034] S10, at the end of or after the heat preservation stage at the first temperature T2, a fourth vacuum is performed on the cavity. The target vacuum level for this vacuuming can be the same as the first, for example, no greater than 100 Pa or 45 Pa to 55 Pa. The main purpose of this step is to promptly remove the gaseous products generated after the decomposition of formic acid introduced in step S8, such as CO and H2O, as well as volatile substances that may be released from the material, thus purifying the cavity environment.

[0035] S11 involves cooling the sintered product to a temperature low enough for safe removal. To further improve efficiency and prevent oxidation during the initial cooling phase, this cooling process may include a forced cooling step of blowing an inert protective gas, such as high-purity nitrogen, into the chamber. Rapid and uniform cooling can be achieved by controlling the flow rate and pressure of the inert gas.

[0036] Through the synergistic effect of the above steps, this method can significantly improve the bonding strength of the metallization layer of the chip after sintering, and reduce or even eliminate the failure of delamination caused by interface oxidation or poor bonding of the metallization layer itself.

[0037] Table 1 below shows the experimental results comparing the method of this application with two conventional methods. The experiment used the same chip with a Ti / Ni / Ag metallization layer structure and Pb92.5Sn5Ag2.5 solder, and the casing was made of MoCu material.

[0038] Table 1 Comparison of chip shear force test results under different sintering methods The data above shows that when the method of this application is adopted, that is, to create and maintain a low-oxygen environment by four vacuuming cycles, and to introduce formic acid twice before the first temperature T1 (270°C to 287°C) and the second temperature T2 (337°C to 357°C), and to combine it with specific holding times (100s to 140s, 40s to 60s), the chip failure mode changes from "metallization layer peeling" to "solder layer or silicon wafer cohesive fracture", and the peeling rate is significantly improved.

[0039] This indicates that the bonding strength between the chip metallization layer and the solder, as well as the metallization layer itself, is higher than the cohesive force of the solder or silicon material itself. This effectively solves the metallization layer detachment problem described in the background art, significantly improving the packaging reliability of semiconductor devices. The fundamental reason lies in the fact that the process design of this method synergistically achieves deep deoxidation, staged efficient reduction, and optimized temperature history, thereby greatly improving the quality of the interfacial metallurgical bonding.

[0040] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A formic acid vacuum sintering method for improving the bonding quality of chip metallization layers, characterized in that: Includes the following steps: S1. Assemble the chip, solder pad, and casing, and place them in a vacuum furnace; S2. Perform the first vacuum evacuation on the vacuum furnace cavity; S3. Fill the furnace cavity with nitrogen to one atmosphere; S4. Perform a second vacuuming of the furnace cavity; S5. In a vacuum environment, formic acid is introduced into the cavity; S6. Raise the cavity temperature to the first temperature T1 and hold it at that temperature (raise the temperature in four gradients, each gradient controlling a different heating rate, raise it to the rapid decomposition temperature of formic acid and hold it for a period of time before raising it to temperature T1). The second temperature T1 is higher than the rapid decomposition temperature of formic acid. S7. At the first temperature T1, the cavity is evacuated for the third time; S8. In a vacuum environment, formic acid is introduced into the cavity again; S9. Raise the cavity temperature to a second temperature T2 and maintain the temperature, wherein the second temperature T2 is higher than the melting point of the solder sheet; S10. Perform the fourth vacuuming of the furnace cavity; S11. Cool the sintered product.

2. The formic acid vacuum sintering method for improving the bonding quality of chip metallization layers as described in claim 1, characterized in that: The target vacuum level for the first, second, third, and / or fourth vacuuming is no greater than 100 Pa.

3. The formic acid vacuum sintering method for improving the bonding quality of chip metallization layers as described in claim 1, characterized in that: Formic acid is introduced in step S5 before the cavity temperature rises; and / or, formic acid is introduced in step S8 before the cavity temperature reaches the second temperature T2.

4. The formic acid vacuum sintering method for improving the bonding quality of chip metallization layers as described in any one of claims 1 to 3, characterized in that: The first temperature T1 is above the rapid decomposition temperature of formic acid and below the liquidus temperature of the solder sheet; the second temperature T2 is 45°C to 55°C higher than the melting point of the solder sheet.

5. The formic acid vacuum sintering method for improving the bonding quality of chip metallization layers as described in claim 4, characterized in that: The second temperature T2 is 45°C to 55°C higher than the melting point of the solder sheet.

6. The formic acid vacuum sintering method for improving the bonding quality of chip metallization layers as described in claim 2, characterized in that: The target vacuum level is 45 Pa to 55 Pa.

7. The formic acid vacuum sintering method for improving the bonding quality of chip metallization layers as described in claim 4, characterized in that: The first temperature T1 is 270℃ to 287℃, and the holding time is 100s to 140s; the second temperature T2 is 330℃ to 350℃, and the holding time is 40s to 60s.

8. The formic acid vacuum sintering method for improving the bonding quality of chip metallization layers as described in any one of claims 1 to 7, characterized in that: The cooling process in step S11 includes the step of blowing inert protective gas into the cavity for forced cooling.

9. The formic acid vacuum sintering method for improving the bonding quality of chip metallization layers as described in claim 8, characterized in that: Between steps S5 and S6, there is also a step of blowing air into the cavity to remove condensed moisture.

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