A highly uniform copper microsphere for advanced electronic packaging microbumps and its preparation method

Abstract

This invention discloses a highly uniform copper microsphere for advanced electronic packaging microbumps and its preparation method. The method involves adding a copper precursor to an organic solvent system to form a reaction solution. Without pre-adding an alkaline regulator, the temperature is first raised to a first temperature range. While the reaction system is within the first temperature range, an alkaline regulator is added and held for a predetermined time to trigger mesophase nucleation. Subsequently, the temperature is raised to a second temperature range higher than the first temperature range and held for a predetermined time, further converting the mesophase into metallic copper microspheres. After the reaction, the microspheres are separated, washed, and dried to obtain the copper microspheres. This invention achieves separation and control of the nucleation stage and subsequent transformation stage through a process control method of "segmented temperature path + alkaline triggering in the first temperature range," which is beneficial for obtaining copper microspheres with narrow particle size distribution, regular morphology, and good dispersibility, and can be used in fields such as advanced electronic packaging microbumps.

Description

Technical Field

[0001] This invention belongs to the field of advanced electronic packaging and metal microsphere material preparation technology, specifically relating to a copper microsphere and its preparation method, and particularly to a preparation method for copper microspheres with uniform size and controllable morphology by synergistically controlling the temperature path and reaction triggering conditions in an organic solvent system for packaging microbump applications. Background Technology

[0002] With the development of advanced packaging technologies, the size and spacing of microbumps are constantly decreasing, placing higher demands on the dimensional accuracy, morphological uniformity, and dispersibility of the base particulate materials. Copper microspheres, due to their excellent electrical and thermal conductivity, low cost, and suitability for high-density interconnects, are considered important candidate raw materials for packaging microbumps and related conductive materials. Existing methods for preparing copper microspheres mainly include physical and wet chemical methods, with wet chemical methods offering advantages such as simple equipment, lower cost, and ease of scale-up. However, existing methods often employ isothermal reactions or single heating methods, relying primarily on final temperature, holding time, or alkali concentration to adjust product morphology. Nucleation, growth, and intermediate phase transformation processes are often coupled, easily leading to problems such as wide particle size distribution, insufficient sphericity uniformity, and agglomeration. Therefore, there is an urgent need to provide a preparation method that can achieve uniform copper microsphere size and controllable morphology to meet the application requirements of advanced electronic packaging microbumps.

[0003] Among existing wet chemical preparation routes for copper microspheres, a relatively typical scheme (Logutenko et al. A novel method to prepare copper microspheres via chemical reduction route. Journal of Materials Research and Technology, 2021, 13: 1254-1265) involves using a copper complex formed from divalent copper salt and 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEAA) in a benzyl alcohol system as a precursor. After adding sodium hydroxide, the temperature is increased in one step from room temperature to approximately 195°C, followed by reduction via a Cu2O mesophase to obtain metallic copper microspheres. This route is simple and easy to scale up, but it has the following drawbacks: Firstly, the nucleation and growth process is complex compared to the Cu2O→Cu process. 0Phase transformations occur overlapping on the same temperature platform, with Cu2O residue reaching over 80 wt% in the initial reaction stage. The processes are difficult to decouple, resulting in product particle size variation coefficients generally ranging from 25% to 30%. Secondly, the alkaline regulator is added all at once before heating, making the product particle size, morphology, and relative composition highly sensitive to alkalinity and reaction time, leading to a narrow process window and poor batch repeatability. Thirdly, natural cooling is often used after the reaction, allowing particles to continue growing and agglomerating during cooling, further exacerbating batch fluctuations. Therefore, it is difficult to meet the requirements of advanced electronic packaging microbumps for narrowly distributed, highly consistent copper microspheres. Summary of the Invention

[0004] This invention aims to provide a copper microsphere suitable for the preparation of microbumps in advanced electronic packaging and its preparation method. By constructing a preset thermal history in an organic solvent system and limiting the timing of adding an alkaline regulator to a specific temperature window, a precursor pre-organization environment is first established without the pre-addition of an alkaline regulator. Then, the nucleation of the intermediate phase is triggered in the first temperature range, and the metallization transformation is completed in the second temperature range. This achieves synergistic control of the particle size distribution, morphological consistency and dispersibility of the copper microspheres.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing copper microspheres includes the following steps: (1) Add the copper precursor to an organic solvent system containing a complexing agent or stabilizer to form a reaction solution; (2) Without adding an alkaline regulator, the reaction solution is heated to bring the reaction system into the first temperature range; (3) When the reaction system is in the first temperature range, an alkaline regulator is added and the system is kept in the first temperature range for a predetermined time to trigger and regulate the nucleation and growth of the copper-containing mesophase; (4) The temperature is then increased to allow the reaction system to enter a second temperature range higher than the first temperature range, and this is maintained for a predetermined time so that the copper-containing intermediate phase is further converted into metallic copper microspheres. (5) The obtained product is separated, washed and dried to obtain copper microspheres.

[0006] The alkaline regulator is not added at the initial stage of the reaction, but is added as a reaction triggering condition within the first temperature range; the first temperature range is used to regulate the formation of the copper-containing mesophase, the second temperature range is used to promote the conversion of the copper-containing mesophase into metallic copper, and the first temperature range is lower than the second temperature range.

[0007] The specific steps and conditions are as follows: (1) The copper precursor is added to an organic solvent system containing a complexing agent or a stabilizer, so that the copper precursor is fully dispersed or dissolved in the organic solvent system to form a reaction solution; the complexing agent or stabilizer is used to regulate the coordination state of the copper precursor, the formation process of the copper-containing mesophase or the surface stability of the copper microspheres, thereby providing a reaction environment for the subsequent controlled nucleation and growth of the copper-containing mesophase.

[0008] In the above scheme, the organic solvent system includes a high-boiling-point organic solvent with reducing effect. The high-boiling-point organic solvent is an alcohol organic solvent, which can be selected from benzyl alcohol, polyols or combinations thereof. Preferably, the high-boiling-point organic solvent is benzyl alcohol, which participates in the formation process of copper-containing mesophase and metallic copper microspheres as a reaction solvent and reducing agent.

[0009] In the above scheme, the copper precursor is a copper salt or copper complex capable of forming a copper-containing intermediate phase in the organic solvent system, and may be a divalent copper salt; preferably, the copper precursor is selected from one or more of copper nitrate, copper acetate, copper acetylacetonate or their hydrates; more preferably, the copper precursor is copper nitrate trihydrate.

[0010] In the above scheme, the complexing agent or stabilizer is an organic ligand capable of regulating the coordination state of the copper precursor, the formation of the copper-containing intermediate phase, or the surface stability of the copper microspheres. The organic ligand can be selected from carboxylic acid ligands, amine ligands, polyol ligands, or combinations thereof. Optionally, the complexing agent or stabilizer is a carboxylic acid ligand. More preferably, the complexing agent or stabilizer is 2-[2-(2-methoxyethoxy)ethoxy]acetic acid.

[0011] (2) Without adding an alkaline regulator, the reaction solution is heated according to a preset temperature path to bring the reaction system into the first temperature range; the first temperature range is used to control the formation process of the copper-containing mesophase, so that the copper-containing mesophase undergoes controlled nucleation and growth within a predetermined temperature window; the first temperature range is 140~160℃, preferably 145~155℃.

[0012] (3) When the reaction system is in the first temperature range, an alkaline regulator is added to trigger and regulate the nucleation and growth of the copper-containing intermediate phase. The alkaline regulator is not added at the initial stage of the reaction, but is added as a reaction triggering condition in the first temperature range, so as to avoid the alkaline regulator from intervening in the reaction process at the initial stage of heating.

[0013] In the above scheme, the alkaline regulator is an alkaline substance that can increase the alkalinity of the reaction system and trigger or promote the formation and transformation of the copper-containing mesophase, and can be selected as an inorganic base; preferably, the alkaline regulator is an aqueous solution of alkali metal hydroxide; more preferably, the alkaline regulator is an aqueous solution of sodium hydroxide.

[0014] (4) After adding the alkaline regulator, maintain the temperature in the first temperature range for a predetermined time to stabilize the nucleation and growth process of the copper-containing mesophase; after adding the alkaline regulator in the first temperature range, maintain the temperature for 5 to 15 minutes, preferably 5 to 10 minutes.

[0015] (5) After the reaction is triggered and maintained in the first temperature range, the temperature is increased to allow the reaction system to enter the second temperature range, which is higher than the first temperature range, and maintained for a predetermined time so that the copper-containing intermediate phase is further converted into metallic copper microspheres; the second temperature range is 185~200℃, preferably 190~195℃; and the second temperature range is maintained for 10~60 min, preferably 20~40 min.

[0016] In the above scheme, based on the amount of copper element, complexing agent or stabilizer and alkaline regulator, the molar ratio of the copper precursor, complexing agent or stabilizer to alkaline regulator is 1:(0.5~3):(0.5~4), preferably 1:(0.8~1.5):(1.5~2.5).

[0017] In the above scheme, the reaction system is stirred during the heating and reaction process, and the stirring speed is 300~700 rpm, which can be selected as 450~550 rpm.

[0018] In the above scheme, after step (4) is completed, the reaction system is cooled to below 50°C before separation, washing and drying.

[0019] In the above scheme, the copper microspheres prepared by the method have a particle size of 100 nm to 5 μm, a particle size distribution (PDI) of ≤20%, and high uniformity; wherein, PDI is defined as the particle size variation coefficient based on SEM statistics, i.e., PDI = σ / σ is the standard deviation of particle size. Hereinafter, CV refers to the number-average particle size.

[0020] In the above scheme, the copper microspheres can be used to prepare microbumps for advanced electronic packaging.

[0021] The first temperature range is used to regulate the formation of the copper-containing mesophase, and the second temperature range is used to promote the conversion of the copper-containing mesophase into metallic copper. This invention limits the addition of the alkaline regulator to the first temperature range, transforming the alkaline regulator from a component involved throughout the initial stage of the reaction into a reaction triggering condition coupled with the temperature window. Simultaneously, by segmenting the temperature paths of the first and second temperature ranges, the formation process of the copper-containing mesophase and the subsequent conversion into metallic copper are controlled in stages. Therefore, the nucleation, growth, and conversion into metallic copper microspheres of the copper-containing mesophase no longer completely overlap, which helps to shorten the nucleation window, reduce the differences in growth processes between different particles, and improve the particle size distribution, sphericity, and dispersibility of the final copper microspheres.

[0022] This invention also reveals a significant nonlinear synergistic relationship between the two process features: "segmented temperature path" and "alkali addition triggering in the first temperature range." The improvement in uniformity achievable by using either single variable independently is limited; only by using both simultaneously can the linear superposition expectation be overcome, resulting in highly uniform spherical copper microspheres. Furthermore, if the alkali addition timing deviates from the first temperature range window, the product morphology will undergo unpredictable deterioration.

[0023] Compared with the prior art, the present invention has at least the following beneficial effects: (1) This invention does not simply rely on final temperature, holding time or alkali dosage to adjust copper microsphere products, but rather uses a process control method of “alkali triggering in the first temperature range + conversion in the second temperature range” to regulate the formation of copper-containing intermediate phase and the generation of metallic copper microspheres in stages.

[0024] (2) In this invention, the temperature is first raised to the first temperature range without the addition of an alkaline regulator, so that the reaction system forms a relatively stable reaction environment before the alkaline regulator is introduced; then the alkaline regulator is added in the first temperature range and maintained for a predetermined time, which is conducive to the concentrated formation of copper-containing mesophase within the predetermined temperature window and reduces the widening of particle size distribution caused by excessive overlap of nucleation and growth processes during continuous heating.

[0025] (3) The present invention uses the first temperature range to regulate the formation of copper-containing mesophase and the second temperature range to promote the transformation of copper-containing mesophase into metallic copper, so that the temperature is changed from a single final state parameter to a path regulation parameter, which is beneficial to improving the uniformity of copper microspheres in terms of particle size, morphology consistency and dispersibility.

[0026] (4) The copper microspheres obtained by the present invention have a narrow particle size distribution and a regular morphology, making them suitable for applications such as advanced electronic packaging microbumps that require high uniformity of particle size and morphology. Attached Figure Description

[0027] Figure 1 This is a TEM image of the copper microspheres obtained in Example 1.

[0028] Figure 2(a) is a SEM image (10k magnification) of the copper microspheres obtained in Example 1.

[0029] Figure 2(b) is a SEM image (5k magnification) of the copper microspheres obtained in Example 1.

[0030] Figure 2(c) is a statistical chart of the particle size of the copper microspheres obtained in Example 1.

[0031] Figure 3 The image shows the XRD pattern of the copper microspheres obtained in Example 1.

[0032] Figure 4(a) is a SEM image (10k magnification) of the copper microspheres obtained in Comparative Example 1.

[0033] Figure 4(b) is a SEM image (5k magnification) of the copper microspheres obtained in Comparative Example 1.

[0034] Figure 4(c) is a statistical chart of the particle size of the copper microspheres obtained in Comparative Example 1.

[0035] Figure 5(a) is a SEM image (10k magnification) of the copper microspheres obtained in Comparative Example 2.

[0036] Figure 5(b) is a statistical chart of the particle size of the copper microspheres obtained in Comparative Example 2.

[0037] Figure 6(a) is a SEM image (10k magnification) of the copper microspheres obtained in Comparative Example 3.

[0038] Figure 6(b) is a statistical chart of the particle size of the copper microspheres obtained in Comparative Example 3.

[0039] Figure 7 This is a SEM image of the copper microspheres obtained in Comparative Example 4. Detailed Implementation

[0040] The present invention will be described below with reference to specific embodiments. Those skilled in the art will understand that these embodiments are for illustrative purposes only and do not limit the scope of the invention in any way.

[0041] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, all reagents and materials used in the following examples are commercially available products.

[0042] Example 1: Weigh 256 mg of copper nitrate trihydrate and add it to 25 mL of benzyl alcohol, followed by 225 μL of 2-[2-(2-methoxyethoxy)ethoxy]acetic acid. Stir and mix thoroughly to form a reaction solution. Place the reaction solution in a reaction vessel and heat it from room temperature to 145–155 °C under stirring at approximately 500 rpm. When the reaction system reaches 145–155 °C, add 110 μL of 50% sodium hydroxide aqueous solution and maintain this temperature range for 5–10 min. Then, continue heating to 190–195 °C and maintain this temperature for 30 min. After the reaction is complete, centrifuge and wash with isopropanol and ethanol 2–3 times each, then dry to obtain the copper microsphere product.

[0043] The SEM results in Figures 2(a) and 2(b) show that the obtained copper microspheres are generally spherical with good dispersion and a relatively concentrated particle size distribution. Based on the SEM images, particle size statistics were performed on more than 300 particles (Figure 2(c)). The median particle size D of the obtained copper microspheres was... 50 The particle size is approximately 700 nm, the coefficient of variation (CV) is approximately 9.55%, and the particle roundness is approximately 0.987, indicating that the particles are at a high level in terms of size uniformity and sphericity. Figure 1 The image shows a TEM image of the copper microspheres obtained in Example 1. It can be seen that the obtained particles are regular spheres with complete outlines and clear edges, with a particle size of about 1 μm. The internal thickness of the particles is uniform, and no obvious pores or internal defects are observed, indicating that the overall structure of the particles is dense. Figure 3 The XRD pattern of the copper microspheres obtained in Example 1 shows three distinct diffraction peaks at 2θ≈43.3°, 50.4°, and 74.2°, which correspond to the (111), (200), and (220) crystal planes of face-centered cubic (FCC) metallic copper (standard card PDF 04-0836), indicating that the main phase of the obtained product is metallic copper.

[0044] Comparative Example 1: Weigh 256 mg of copper nitrate trihydrate and add it to 25 mL of benzyl alcohol. Then add 225 μL of 2-[2-(2-methoxyethoxy)ethoxy]acetic acid and 110 μL of 50% sodium hydroxide aqueous solution. After stirring and mixing evenly, directly heat to 190-195 °C and maintain for 30 min. After the reaction is completed, post-treatment is carried out in the same manner as in Example 1.

[0045] The SEM results in Figures 4(a) and 4(b) show that the obtained product is mainly composed of spherical particles. The particle size distribution in Figure 4(c) shows that the median particle size D of the obtained microspheres is... 50 The particle size is approximately 1130 nm, with a coefficient of variation (CV) of approximately 29.5%. The particle size range is large, with a clear coexistence of large and small particles, and poor size uniformity.

[0046] Comparative Example 2: The remaining conditions were the same as in Example 1, except that 110 μL of a 50% sodium hydroxide aqueous solution was added to the reaction solution during preparation (i.e., at the same time as the addition of alkali in Comparative Example 1). The temperature was then raised to 145–155 °C and held, following the same temperature path as in Example 1, and then raised to 190–195 °C and held for 30 min. After the reaction, the post-treatment was performed in the same manner as in Example 1.

[0047] The SEM results in Figure 5(a) show that the obtained copper microspheres are still mainly spherical particles. The particle size statistics in Figure 5(b) show that the median particle size D of the obtained microspheres is... 50 The particle size is approximately 660 nm, and the coefficient of variation (CV) is approximately 19.9%. Although the median particle size is similar to that of Example 1, the particle size distribution is significantly wider than that of Example 1, showing a more obvious phenomenon of coexistence of large and small particles.

[0048] Comparative Example 3: The remaining conditions were the same as in Example 1, except that the reaction solution was not kept in the 145-155°C range. Instead, 110 μL of 50% sodium hydroxide aqueous solution was added at once when the temperature passed through this range during the heating process. After adding the alkali, the temperature was not kept in this range, but the temperature was directly increased to 190-195°C and held for 30 min. After the reaction was completed, the same post-treatment as in Example 1 was performed.

[0049] The SEM results in Figure 6(a) show that the obtained product still exhibits an overall morphology of spherical particle aggregation, but there are significant differences compared to Example 1. On the one hand, the aggregation and adhesion between particles are obvious, with a large number of particles tightly attached and even undergoing local melting, making it difficult to form the well-dispersed independent spherical particles observed in Example 1; on the other hand, some particles do not fully develop into complete spheres, but are formed by the aggregation of several secondary small particles, with a rough surface and an incompletely formed state. The particle size statistics in Figure 6(b) show that the median particle size D 50 The particle size is approximately 390 nm, with a particle size variation coefficient (CV) of approximately 28.8%. The particle size distribution is significantly broadened, which is close to that of Comparative Example 1 (CV approximately 29.5%). The coexistence of large and small particles is obvious, and the particle size uniformity is poor.

[0050] Comparative Example 4: The remaining conditions were the same as in Example 1, except that no alkaline regulator was added during the 145-155°C stage. Instead, 110 μL of a 50% sodium hydroxide aqueous solution was added after the temperature was raised to 190-195°C and maintained for 30 min. After the reaction was completed, the reaction was post-treated in the same manner as in Example 1.

[0051] Figure 7SEM results showed that the obtained products exhibited obvious morphological heterogeneity. After morphological statistics of more than 200 particles, about 41% of the obtained products were cubic or polyhedral particles with angular surfaces, and the remaining about 59% were nearly spherical (flower-shaped) particles with rough surfaces. There were also a small number of blocky and irregularly stacked particles. The particle morphology was not very consistent. Due to the significant differences in particle morphology, particle size statistics were no longer applicable.

[0052] Results analysis: Table 1

[0053] Based on the results in Table 1, Comparative Example 1, as the baseline path without introducing segmented temperature paths and post-addition alkali triggering, yielded copper microspheres D... 50 The particle size is approximately 1130 nm, with a CV of approximately 29.5%, a wide particle size distribution, and poor size uniformity. Based on Comparative Example 1, only a segmented temperature path was introduced while maintaining the initial one-pot alkali addition (Comparative Example 2). D 50 Narrowing the temperature to approximately 660 nm reduced the CV to approximately 19.9%, and improved uniformity by approximately 10 percentage points, indicating that the segmented temperature path contributes to the decoupling of nucleation and growth processes, but its improvement is limited when used alone. Based on Comparative Example 1, only the timing of alkali addition was moved to the first temperature range and no heat preservation was performed after alkali addition (Comparative Example 3), resulting in product D... 50 Although the particle size was reduced to about 390 nm, the CV was still at about 28.8%, which was close to that of Comparative Example 1, and the uniformity was improved by less than 1 percentage point. At the same time, the particle morphology showed typical characteristics of "agglomeration and adhesion, and incomplete development", indicating that without the first temperature range of heat preservation, simply changing the timing of alkali addition could not improve the particle size uniformity independently. Instead, it would cause new morphology deterioration problems due to the lack of size homogenization window for the primary intermediate phase.

[0054] When both process controls mentioned above are used simultaneously, the copper microspheres obtained in Example 1 show a CV reduction of approximately 9.55%, a sphericity of approximately 0.987, good dispersibility, and a smooth surface. If the univariate improvement rates of Comparative Examples 2 and 3 relative to Comparative Example 1 are linearly superimposed, the expected upper limit of CV improvement would not exceed approximately 11 percentage points, meaning the CV is expected to decrease to approximately 19%. However, the measured improvement rate of Example 1 is approximately 20 percentage points, further reducing the CV to 9.55%, far exceeding the expected linear superposition. This indicates a clear nonlinear synergistic effect between the segmented temperature path and the alkali-triggered addition in the first temperature range; both are indispensable. Comparative Example 4 further illustrates that delaying the alkali addition timing is not necessarily better the later it is done. When the alkali addition timing is delayed to the second temperature range (190~195℃), about 41% of the resulting products are cubic / polyhedral particles with angular surfaces, and about 59% are flower-shaped particles with rough surfaces. Particle size statistics are no longer applicable. This morphological change indicates that the process window defined by the present invention, "alkali addition occurs within the first temperature range," has a significant boundary effect. Uniform spherical copper microspheres cannot be obtained outside the window.

[0055] In summary, the above comparison shows that there is a clear nonlinear synergistic relationship between the two process features defined in this invention: the improvement in uniformity that can be achieved by using either single variable independently is relatively limited. Only by using both simultaneously can the linear superposition expectation be crossed, and high-uniformity spherical copper microspheres with a CV of about 9.55% and a roundness of about 0.987 be obtained. Moreover, once the timing of alkali addition exceeds the first temperature range window, the morphology of the product may deteriorate unpredictably.

Claims

1. A method for preparing copper microspheres, characterized in that, Includes the following steps: (1) Add the copper precursor to an organic solvent system containing a complexing agent or stabilizer to form a reaction solution; (2) Without adding an alkaline regulator, the reaction solution is heated to bring the reaction system into the first temperature range; (3) When the reaction system is in the first temperature range, an alkaline regulator is added and the system is kept in the first temperature range for a predetermined time to trigger and regulate the nucleation and growth of the copper-containing mesophase; (4) The temperature is then increased to allow the reaction system to enter a second temperature range higher than the first temperature range, and this is maintained for a predetermined time so that the copper-containing intermediate phase is further converted into metallic copper microspheres. (5) The obtained product is separated, washed and dried to obtain copper microspheres.

2. The production method according to claim 1, characterized by, The first temperature range is 140~160℃; after adding an alkaline regulator within the first temperature range, maintain for 5~15 min; the second temperature range is 185~200℃; maintain for 10~60 min within the second temperature range.

3. The preparation method according to claim 1, characterized in that, The organic solvent system is composed of alcohol-based organic solvents.

4. The method of claim 1, wherein, The copper precursor is a divalent copper salt.

5. The preparation method according to claim 1, characterized in that, The reaction solution also contains a complexing agent or a stabilizer, wherein the complexing agent or stabilizer is 2-[2-(2-methoxyethoxy)ethoxy]acetic acid.

6. The method of claim 1, wherein, The alkalinity regulator is an inorganic or organic base.

7. The preparation method according to claim 1, characterized in that, The molar ratio of the copper precursor, complexing agent or stabilizer to the alkaline regulator is 1:(0.5~3):(0.5~4) based on the effective amounts of copper element, complexing agent or stabilizer and alkaline regulator; the reaction system is stirred during heating and reaction, and the stirring speed is 300~700 rpm.

8. The preparation method according to claim 1, characterized in that, After step (4) is completed, the reaction system is cooled to below 50°C before separation, washing and drying.

9. The copper microspheres prepared by the method according to any one of claims 1 to 8, characterized in that, Its particle size is 100 nm to 5 μm, and its PDI is ≤20%.

10. The application of the copper microspheres according to claim 9 in the fabrication of microbumps for advanced electronic packaging.

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