A method for purifying a gold source by pre-melting for electron beam evaporation

Abstract

The application discloses a gold source pre-melting purification method for electron beam evaporation, which comprises the following steps: dividing the gold source into N batches, wherein the volume of each batch of gold source accounts for 1 / 5-1 / 3 of the volume of the crucible; putting the first batch of gold source into the crucible, heating to complete melting under a vacuum of ≤8*10 ‑4 10%-30% of the rated power of the electron gun under a vacuum of Pa, and keeping for 1-3 minutes; taking out after cooling, wiping the surface of the gold ingot with a dust-free cloth dipped in an alcoholic solvent; putting the wiped gold ingot back into the crucible, adding the second batch of gold source, and repeating the above heating, cooling and wiping steps until all batches are processed; melting the finally obtained gold ingot and using it for electron beam evaporation coating. The application can effectively remove the black substances appearing on the surface of the gold source after melting, and significantly improve the quality of the gold film.

Description

Technical Field

[0001] This invention belongs to the field of vacuum coating technology, specifically relating to a method for pre-melting and purifying gold sources for electron beam evaporation. Background Technology

[0002] In the fields of semiconductor device manufacturing, optoelectronic device packaging, and microelectromechanical systems (MEMS) manufacturing, electron beam evaporation is a commonly used physical vapor deposition method for preparing gold (Au) electrodes and gold thin films. Gold, due to its excellent conductivity, chemical stability, oxidation resistance, and good solderability, is widely used in the preparation of ohmic contact electrodes, interconnects, pads, reflective layers, and various functional thin films. In actual industrial production, gold sources are typically purchased from specialized material suppliers in the form of gold wires, ingots, sheets, or particles. From a production process perspective, gold sources undergo relatively strict surface treatment before leaving the factory; therefore, before melting, their surfaces usually exhibit a clean and bright metallic luster, with no obvious contaminants, oxide layers, or discolored areas visible to the naked eye. However, long-term production practice and process research have revealed a common and thorny problem in the electron beam evaporation process of purchased gold wires or ingots: when the gold source is heated to complete melting by an electron beam in a vacuum environment, visible black floating matter often gradually appears on the originally clean melt surface, forming discontinuous patches or continuous thin film-like coatings. When heating stops and the melt cools and solidifies, these black substances remain on the surface of the gold ingot, forming obvious black patches or streaks. More seriously, during the subsequent continuous evaporation process, these black substances will partially sputter or deposit onto the surface of substrates such as chips and wafers along with the evaporation of the gold material, causing defects such as black spots, pinholes, decreased adhesion, and abnormal resistivity in the deposited gold film. In severe cases, this can even lead to the scrapping of an entire batch of products, seriously affecting product yield and long-term reliability.

[0003] To address this issue, existing technologies have proposed several purification methods, primarily including chemical cleaning, high-temperature annealing, and acid washing. Chemical cleaning uses organic solvents to ultrasonically clean the surface of the gold source to remove grease and organic contaminants. However, this method has limited effectiveness against stubborn residues embedded in surface microcracks, and the solvent itself may remain, introducing new sources of contamination. High-temperature annealing requires specialized equipment, has a long process flow, high energy consumption, and cannot be directly implemented in conventional evaporation processes. Acid washing is ineffective against organic and carbonaceous contaminants. Therefore, there is an urgent need for a simple, efficient, and targeted purification method to remove the black substances on the surface of the melted gold source, which can be easily implemented directly on existing electron beam evaporation equipment. Summary of the Invention

[0004] The technical problem to be solved by the present invention is that black substances appear on the surface of the gold source after melting, which leads to coating defects. To this end, the present invention proposes a gold source purification method that effectively removes the black substances that appear on the surface of the gold source after melting, and significantly improves the quality of the gold film.

[0005] To address the aforementioned technical problems, the present invention provides the following technical solution: A method for pre-melting and purifying gold sources for electron beam evaporation includes the following steps: S1. Divide the gold source with a total weight of W to be evaporated into N batches, where N≥2; S2. Place the i-th batch of gold source into the crucible, and place the crucible in the electron beam evaporation stage for vacuum treatment; S3. Heat the gold source in the crucible until the gold source is completely melted, and maintain it in the molten state for a preset time to allow impurities to float to the surface of the melt; S4. Stop heating and allow the melt to cool and solidify. Remove the crucible and use a physical wiping method to remove impurities from the surface of the solidified gold ingot. S5. Determine if there are any unprocessed batches. If yes, take the next batch of gold source and add it to the crucible, and repeat steps S3 to S5; otherwise, proceed to step S6. S6. Place the crucible containing the gold source, which has undergone all batch pre-melting and wiping treatments, back into the vacuum chamber, evacuate the vacuum, heat and melt it for evaporation coating.

[0006] In some embodiments of the present invention, in step S1, the weight w of each batch of gold source i Set the gold source weight to 1 / 5 to 1 / 3 of the effective capacity of the crucible used.

[0007] In some embodiments of the present invention, in step S1, the first batch of gold source is spread evenly to cover the bottom of the crucible; subsequent batches of gold source are added directly.

[0008] In some embodiments of the present invention, in step S3, the background vacuum level of the vacuum condition is ≤8×10⁻⁶. -4 Pa.

[0009] In some embodiments of the present invention, in step S3, the gold source is heated by turning on the electron gun, and the heating power is controlled at 10% to 30% of the rated power of the electron gun.

[0010] In some embodiments of the present invention, in step S3, the holding time of the molten state is 1 to 3 minutes.

[0011] In some embodiments of the present invention, in step S5, the physical wiping method involves wiping the surface of the solidified gold ingot with a clean, lint-free cloth; the wiping includes wiping with a lint-free cloth soaked in organic solvent.

[0012] In some embodiments of the present invention, the organic solvent is an alcohol solvent; the alcohol solvent includes anhydrous ethanol or isopropanol.

[0013] In some embodiments of the present invention, the gold source is one or a combination of gold wire, gold ingot, gold sheet or gold particle.

[0014] This invention also provides a method for pre-melting and purifying gold sources for electron beam evaporation, comprising the following steps: The gold source is divided into N batches, with each batch of gold source occupying 1 / 5 to 1 / 3 of the crucible volume. Place the first batch of gold sources into the crucible, within ≤8×10 -4 Under a vacuum of Pa, heat with 10%-30% of the rated power of the electron gun until completely melted, and maintain for 1-3 minutes; After cooling, remove the gold ingot and wipe its surface with a lint-free cloth dampened with an alcohol solvent. Place the wiped gold ingot back into the crucible, add the second batch of gold source, and repeat the heating, cooling, and wiping steps above until all batches are processed. The resulting gold ingots are melted and used for electron beam evaporation coating.

[0015] The technical solution of the present invention has the following technical effects compared with the prior art: The gold source pre-melting purification method for electron beam evaporation provided by this invention can remove the black substance that floats on the surface after the gold source is melted by pre-melting in batches and mechanical wiping. The black spot defect rate on the surface of the gold film after coating is significantly reduced and the film quality is significantly improved. No special equipment is required, and it can be implemented using the existing electron beam evaporation stage without increasing the additional cost. This purification method does not use chemical cleaning agents and has no risk of residue. Attached Figure Description

[0016] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, which will help to understand the purpose and advantages of the present invention, wherein: Figure 1 This is a flowchart of the gold source pre-melting and purification method for electron beam evaporation according to the present invention.

[0017] Figure 2 This is a diagram showing the surface state of Jinyuan after melting, cooling and solidification in the pre-melting purification method of the present invention. Figure 3 This is a diagram showing the state of Jinyuan after physical wiping using the pre-melting purification method of this invention. Detailed Implementation

[0018] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. 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.

[0019] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0020] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0021] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0022] The following is a specific embodiment of the gold source pre-melting and purification method for electron beam evaporation provided by the present invention. In this embodiment, the metal material to be evaporated is a gold ingot with a purity of 99.99% and a total weight of 216 grams. Research has found that trace carbonaceous impurities and organic residues inside the gold source spontaneously float to the surface after high-temperature melting because their density is lower than that of gold. Based on this mechanism, the present invention removes impurities through the following measures. This method strictly follows... Figure 1 The flowchart shown is executed, including the following steps: S1: Batch Preparation: The amount of gold added at one time is controlled to be 1 / 5 to 1 / 3 of the crucible capacity to ensure that the molten gold has a large surface area to depth ratio. A shallower molten pool allows impurities with lower density to float to the surface in a short time, avoiding the problem of impurities not effectively precipitating due to excessively deep molten pools. In this embodiment, the gold ingots with a total weight of 216 grams to be used for evaporation coating are divided into 4 batches (N=4), with each batch of gold ingots weighing 54 grams. The weight of each batch of gold source is approximately 1 / 4 of the capacity of the 40cc copper crucible used. The operator first checks the appearance of the gold ingots to confirm that the surface has a clean metallic luster and no obvious contaminants. Then, the gold ingots are picked up with clean tweezers and placed in a dust-free packaging bag that has been ultrasonically cleaned and dried with acetone and anhydrous ethanol for later use. The first batch of gold source should be spread evenly to cover the entire bottom of the crucible, forming a uniform thin layer, to avoid the electron beam directly bombarding the exposed bottom of the crucible, which could cause the crucible to be punctured or damaged by local overheating. Subsequent batches of gold source can be added directly, as there is already a molten or solidified metal layer protecting the bottom, so there is no need to spread it evenly again.

[0023] S2: First Batch Loading. Take the first batch (i=1) of 54 grams of gold ingots and place them into a clean, dry 40cc copper crucible. Place the crucible on the crucible holder of the electron beam evaporation stage, ready for vacuum pre-melting. Close the vacuum chamber and start the mechanical pump and condenser pump units to evacuate the vacuum. After approximately 20 minutes of evacuation, the vacuum level inside the chamber reaches 4.0 × 10⁻⁶. -4 Pa, satisfying a background vacuum degree ≤ 8 × 10⁻⁶ -4 The required vacuum level (Pa) is as follows: The higher the vacuum level, the fewer residual gas molecules there are. This prevents gold from reacting with residual oxygen at high temperatures to form oxides. Furthermore, the high vacuum environment reduces the obstruction of gas molecules to the upward movement of impurities and prevents contamination of the melt surface by residual gas, ensuring the pure precipitation of the black substance.

[0024] S3: After reaching the preset vacuum level, the electron gun system is activated to heat the gold ingot in the crucible. The operator manually adjusts the filament current and beam current to focus the electron beam spot onto the gold ingot in the crucible. The initial heating power is set to 10% of the electron gun's rated power of 8kW, i.e., 0.8kW, and controlled within the range of 10% to 30% of the electron gun's rated power. This power is below the power threshold that would cause the gold to evaporate violently, avoiding violent tumbling or even splashing of the gold melt. Low-power heating allows impurities sufficient time to float to the surface. If the power is below 10%, the heat input is insufficient, the gold source is not fully melted, the melt has poor fluidity, and impurities cannot float effectively. If it is above 30%, the surface of the gold melt evaporates violently, generating convection and tumbling, which easily re-entrains the floated impurities into the melt, reducing the purification effect. During continuous heating, the gold ingot is observed to gradually turn red, soften, and eventually completely melt, with the entire process taking approximately 2 minutes. After the gold ingot is completely melted, the current power is maintained to keep the melt in a molten state for 3 minutes to provide sufficient diffusion time. During this period, through the observation window of the vacuum chamber, it could be clearly seen that tiny black floating objects gradually appeared on the originally bright surface of the gold melt, and slowly gathered into discontinuous black patches, which... Figure 2 The state indicated by the middle arrow is consistent.

[0025] S4: Cool and wipe away. After maintaining the molten state for 3 minutes, turn off the electron gun high voltage and stop heating. Allow the melt to cool naturally to room temperature in a vacuum, and it will completely solidify after about 15 minutes. Turn off the vacuum system and slowly fill the chamber with dry nitrogen gas to atmospheric pressure (breaking the vacuum). Open the chamber and carefully remove the gold ingot using clean stainless steel tweezers. Maintain the vacuum state while cooling until the temperature drops below room temperature to avoid opening the chamber at high temperatures, which could expose the gold ingot surface to air and cause oxidation (although gold is relatively stable, trace impurities may be oxidized) or allow it to absorb moisture and dust from the air. Breaking the vacuum after complete cooling effectively locks in the clean state before wiping.

[0026] The gold ingot was placed on a clean workbench and allowed to cool completely to room temperature. Then, the solidified surface of the ingot was gently wiped with a polyester fiber lint-free cloth. During wiping, it was observed that the black substance was easily removed, leaving noticeable black marks on the cloth. After wiping, the surface of the gold ingot regained its clean metallic luster. To further enhance the cleaning effect, the operator dampened the lint-free cloth with a small amount of anhydrous ethanol and wiped it again. The surface of the gold ingot immediately displayed a very bright metallic luster, with no visible black residue. Figure 3The conditions shown are consistent. After wiping, carefully inspect the surface of the gold ingot to ensure there are no visible black spots, then place the gold ingot in a desiccator for later use. Since the black substance may contain incompletely carbonized organic residues, alcoholic organic solvents can dissolve it, soften stubborn deposits, and reduce the difficulty of wiping. Anhydrous ethanol and isopropanol have low surface tension, spread easily, and have high saturated vapor pressure at room temperature, making them highly volatile. Even if there are trace amounts of residue after wiping, they will quickly evaporate before being placed in a vacuum chamber or during the vacuuming process.

[0027] S5: Determine if there are any unprocessed batches of gold source. The first batch has been processed, but the second, third, and fourth batches remain unprocessed. Therefore, let i = i + 1 (i = 2), and add the 54-gram gold ingots from the second batch directly to the crucible that has undergone the first pre-melting and wiping (at this point, the crucible already contains the purified first batch of gold ingots, with a total weight of approximately 108 grams). Repeat steps S3 to S5. That is: place the gold source into the crucible and evacuate to 4.0 × 10⁻⁶. -4 At step Pa, the electron gun is turned on and heated at 0.8 kW to melt the second batch of gold ingots, which are then combined with the first batch of purified gold. The mixture is kept molten for 3 minutes. Black floating matter is observed to reappear on the surface of the melt, but in slightly fewer quantities than the first time. After cooling, the gold ingots are removed, and their surfaces are wiped again with a lint-free cloth soaked in anhydrous ethanol, restoring their clean luster. The third and fourth batches of gold ingots are processed sequentially using the same method. After the fourth batch is processed, it is determined whether there are any unprocessed batches. At this point, all batches have been processed (N=4, i=4 have been processed). The process proceeds to step S7, where batch processing and sequential wiping prevent the accumulation of impurities.

[0028] S6: Final melting and coating. The gold ingot containing all the gold sources that have undergone four batches of pre-melting and four wiping processes (now combined into a single ingot of approximately 216 grams) is placed back into the vacuum chamber of the electron beam evaporation stage. The chamber is closed, and the vacuum system is restarted to evacuate to 4.0 × 10⁻⁶. -4 Pa. After reaching the predetermined vacuum level, the electron gun is activated, and the gold ingot is completely melted at a power threshold of 3kW, which is required for the vigorous evaporation of gold. Careful observation through the viewing window reveals that the surface of the molten gold is pure and bright, without any visible black floating matter. The molten state is maintained for 2 minutes to ensure uniform and stable molten pool temperature. The crystal oscillator film thickness monitor is then activated, and after the evaporation rate stabilizes at 0.2 nm / s, the substrate baffle is opened, and gold film deposition begins on the pre-prepared 3-inch silicon wafer, with a deposition rate set at 0.2 nm / s and a target thickness of 60 nm. The evaporation rate is continuously monitored during deposition, and it is found that the rate fluctuates stably within ±3% of the set value, indicating a very stable molten pool. After deposition is complete, the baffle and electron gun are closed, and the wafer is removed after the chamber has cooled.

[0029] Control experiment As a control, 216 grams of untreated gold nuggets from the same batch were placed directly into another clean 40cc copper crucible and evaporated in the same electron beam evaporation stage under the same process parameters (baseline vacuum 4.0 × 10⁻⁶). -4 (Pa, heating power 3kW, deposition rate 0.2nm / s, target thickness 60nm) to evaporate and deposit a gold film onto another wafer. After deposition, the gold ingot was removed and observed to have obvious black floating matter on the surface of the control group gold ingot, and the evaporation rate fluctuated greatly during the coating process, varying within ±15%.

[0030] Effect verification After coating, the two groups of samples were tested and analyzed. First, a visual inspection was performed. The gold film on the silicon wafers in the cleaned group was a uniform golden yellow, bright and flawless. Scattered black spots were clearly visible on the surface of the gold film on the silicon wafers in the control group. Using a metallographic microscope (200x), 10 fields of view were randomly selected from both groups of samples for observation, and the number of black spots was counted. The results showed that 47 black spots were counted in the 10 fields of view of the control group, with an average black spot density of approximately 23.5 spots / cm², indicating a high defect rate. Only a very small number of black spots were counted in the 10 fields of view of the cleaned group, indicating a significantly better surface quality than the control group. A cross-cut adhesion test was conducted using 3M tape (model 600). After the tape was applied and removed from both groups of samples, no film peeling was observed, and the adhesion rating was 5B (the highest level), indicating that the cleaned treatment did not negatively affect the adhesion of the gold film. A four-probe resistance meter was used to measure the sheet resistance of the gold film. The sheet resistance of the gold film in the cleaned group was lower than that in the control group, indicating that the gold film in the cleaned group had a lower resistivity.

[0031] After the above purification treatment, the coating verification was carried out. The results showed that the gold source pre-melted and purified by the method of the present invention did not have obvious black floating matter on the surface of the melt during the electron beam evaporation process. The surface of the deposited gold film was smooth, and macroscopic defects such as black spots and pinholes were significantly reduced. The film quality was significantly improved, and the process stability and product yield were effectively improved.

[0032] This embodiment also investigated the effect of different wiping methods on the purification effect. In another set of experiments, the same batch-by-batch pre-melting process was used on the same batch of gold ingots, but the wiping method was dry wiping (wiping directly with a lint-free cloth without applying ethanol). The results showed that the dry wiping method could also remove most of the black substances, and the surface of the gold ingots was basically clean, but the wet wiping method (applying ethanol) showed a better cleaning effect, and the surface of the gold ingots was shinier.

[0033] In summary, the gold source pre-melting and purification method for electron beam evaporation provided in this embodiment effectively removes the black substance that floats on the surface after the gold source melts by organically combining batch pre-melting and mechanical wiping, thus significantly improving the surface quality of the gold film. This method is simple to operate, requires no additional equipment investment, and produces no chemical pollution. It can significantly improve the quality and process stability of electron beam evaporated gold films, and has broad application prospects in semiconductor manufacturing, optoelectronic device packaging, and other fields.

[0034] 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 for pre-melting and purifying gold sources for electron beam evaporation, characterized in that, Includes the following steps: S1. Divide the gold source with a total weight of W to be evaporated into N batches, where N≥2; S2. Place the i-th batch of gold source into the crucible, and place the crucible in the electron beam evaporation stage for vacuum treatment; S3. Heat the gold source in the crucible until the gold source is completely melted, and maintain it in the molten state for a preset time to allow impurities to float to the surface of the melt; S4. Stop heating and allow the melt to cool and solidify. Remove the crucible and use a physical wiping method to remove impurities from the surface of the solidified gold ingot. S5. Determine if there are any unprocessed batches. If yes, take the next batch of gold source and add it to the crucible, and repeat steps S3 to S5; otherwise, proceed to step S6. S6. Place the crucible containing the gold source, which has undergone all batch pre-melting and wiping treatments, back into the vacuum chamber, evacuate the vacuum, heat and melt it for evaporation coating.

2. The method for pre-melting and purifying gold source for electron beam evaporation according to claim 1, characterized in that, In step S1, the weight w of each batch of gold resources i Set the gold source weight to 1 / 5 to 1 / 3 of the effective capacity of the crucible used.

3. The method for pre-melting and purifying gold source for electron beam evaporation according to claim 1 or 2, characterized in that, In step S1, the first batch of gold source is spread evenly to cover the bottom of the crucible; subsequent batches of gold source are added directly.

4. The gold source pre-melting and purification method for electron beam evaporation according to claim 1, characterized in that, In step S3, the background vacuum level of the vacuum condition is ≤8×10⁻⁶. -4 Pa.

5. The gold source pre-melting and purification method for electron beam evaporation according to claim 1, characterized in that, In step S3, the gold source is heated by turning on the electron gun, and the heating power is controlled at 10% to 30% of the rated power of the electron gun.

6. The gold source pre-melting and purification method for electron beam evaporation according to claim 1, characterized in that, In step S3, the molten state is maintained for 1 to 3 minutes.

7. The method for pre-melting and purifying gold source for electron beam evaporation according to claim 1, characterized in that, In step S5, the physical wiping method involves wiping the surface of the solidified gold ingot with a clean, lint-free cloth; the wiping includes wiping with a lint-free cloth soaked in organic solvent.

8. The gold source pre-melting and purification method for electron beam evaporation according to claim 7, characterized in that, The organic solvent is an alcohol solvent; the alcohol solvent includes anhydrous ethanol or isopropanol.

9. The method for pre-melting and purifying gold source for electron beam evaporation according to claim 1, characterized in that, The gold source is one or a combination of gold wire, gold ingot, gold sheet or gold particle.

10. A method for pre-melting and purifying gold sources for electron beam evaporation, characterized in that, Includes the following steps: The gold source is divided into N batches, with each batch of gold source occupying 1 / 5 to 1 / 3 of the crucible volume. Place the first batch of gold sources into the crucible, within ≤8×10 -4 Under a vacuum of Pa, heat with 10%-30% of the rated power of the electron gun until completely melted, and maintain for 1-3 minutes; After cooling, remove the gold ingot and wipe its surface with a lint-free cloth dampened with an alcohol solvent. Place the wiped gold ingot back into the crucible, add the second batch of gold source, and repeat the heating, cooling, and wiping steps above until all batches are processed. The resulting gold ingots are melted and used for electron beam evaporation coating.

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