Electroplating method for forming AuAg alloy bump with component gradient structure

Abstract

The invention discloses an electroplating method for forming an AuAg alloy bump with a component gradient structure, which comprises the following steps: electroplating a base material in an Au / Ag alloy plating solution containing gold salt and silver salt according to the concentration ratio of the gold salt to the silver salt of 3: 7; the electroplating process comprises two stages, and different current densities are set; in the first stage, alloy electro-deposition is carried out by adopting the first current density; in the second stage, electroplating is continued by adopting second current density in the later period of alloy layer formation; the current densities of the two stages are different, so that component gradient distribution from bottom to top is formed in the alloy layer.

Description

Technical Field

[0001] The present invention is an electroplating method for forming an AuAg alloy bump with a composition gradient structure. Background Art

[0002] With the continuous development of integrated circuit packaging technology towards high density, miniaturization, and high reliability, metal bumps, as the key interconnection structure between chips and packaging substrates, have received extensive attention for their material properties and composition design. Especially in the field of high-end microelectronic packaging, bumps not only need to have good electrical conductivity and weldability but also excellent oxidation resistance, thermal stability, and reliable mechanical adhesion. Au / Ag (gold-silver) alloy has become an ideal candidate to replace pure gold electroplating materials due to its combination of the stability of gold and the high electrical conductivity of silver, and is widely used in the construction of electronic interconnection structures.

[0003] In the prior art, to reduce costs, AuAg alloy electroplating solutions generally adopt low gold content ratios, such as combinations with a gold salt to silver salt concentration ratio of 1:9 or 2:8. Such low gold ratio systems have certain advantages in controlling material costs, but there are still significant limitations in terms of functional performance and structure regulation. For example, in the 1:9 ratio, due to the too low gold content, it is difficult to achieve effective enrichment of gold elements during electro-deposition, resulting in insufficient gold content at the top of the electroplated layer and making it difficult to meet the process requirements of more than 80% gold purity, thus affecting its oxidation resistance and bonding reliability. Although the 2:8 ratio has some improvement, there is still room for improvement in terms of composition gradient regulation, stability of top gold enrichment, and process adaptability.

[0004] In addition, traditional electroplating processes mostly adopt a single current density, resulting in a uniform composition of the alloy layer and making it difficult to achieve a composition gradient structure from bottom to top, and unable to fully take into account the bonding strength of the bottom layer and the high functional requirements of the top layer. In recent years, the two-stage current density control strategy, as a new electro-deposition method, has shown broad prospects in constructing an alloy gradient structure, but it poses higher requirements for the reasonable setting of the gold-silver ratio in the electroplating solution.

[0005] Therefore, there is an urgent need to develop an AuAg alloy electroplating solution system based on a moderate gold content ratio, which can cooperate with the two-stage current process to form a composition gradient structure while meeting the requirements of high gold purity at the top and overall structural stability. Summary of the Invention

[0006] The purpose of the present invention is to solve the above deficiencies of the prior art and provide an electroplating method for forming an AuAg alloy bump with a composition gradient structure.

[0007] An electroplating method for forming an AuAg alloy bump with a composition gradient structure includes the following steps:

[0008] Electroplating treatment is carried out on a substrate in an Au / Ag alloy plating solution containing a gold salt and a silver salt, and the concentration ratio of the gold salt to the silver salt in the plating solution is 3:7;

[0009] The electroplating process includes two stages, and different current densities are set respectively:

[0010] In the first stage, alloy electro-deposition is carried out using the first current density;

[0011] In the second stage, electroplating is continued using the second current density in the later stage of the formation of the alloy layer;

[0012] The current densities of the two stages are different to form a compositional gradient distribution from the bottom to the top in the alloy layer.

[0013] Further, the first current density is 0.2 - 0.8 A / dm 2 , and the second current density is 1 - 5 A / dm 2 .

[0014] Further, the duration of the electroplating in the second stage is 30 - 120 seconds.

[0015] Further, the duration of the electroplating in the first stage is 1000 - 2400 seconds.

[0016] Further, the thickness of the alloy bump is 5 - 11 μm, and the gold content purity within the thickness range of 1 - 2 μm at the top is more than 80%.

[0017] Further, the flow rate of the plating solution is maintained at 10 - 15 L / min and the shaking frequency is 10 - 30 Hz throughout the electroplating process.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0019] An AuAg alloy plating solution with a concentration ratio of gold salt to silver salt of 3:7 is used, combined with a two-stage current density control strategy, for preparing metal bumps with a compositional gradient structure. This design not only optimizes the electroplating behavior, but also improves the structure regulation ability and the final performance of the alloy layer. Compared with the traditional 1:9 ratio and the previously applied 2:8 ratio, this ratio shows more excellent comprehensive advantages in multiple key aspects.

[0020] First, from the working principle of electrodeposition, gold and silver have different electrode reduction orders during the electroplating process. Stable alloy deposition is achieved through low current density in the first stage, and high current density in the second stage strengthens the enrichment of gold elements at the top, constructing a composition gradient structure from bottom to top. Under this mechanism, the increase in the gold salt concentration in the electroplating solution helps to enhance the deposition ability of the top layer of gold, thus playing a decisive role in the final composition distribution. In contrast, the 1:9 ratio has extremely low gold content, and even through current control, it is difficult to form a top layer with a gold content of more than 80%. Although the 2:8 ratio is improved, it still has the problem of limited gradient control range.

[0021] Secondly, the 3:7 ratio significantly improves the reliability of top gold content control. In micro-bump structures, the top region typically requires high gold purity to ensure good conductivity and oxidation resistance. This ratio not only makes it easier to achieve the target gold purity of over 80%, but also provides a larger process window. Even with certain fluctuations in current density or plating time, the film quality and performance consistency can be maintained, effectively improving mass production stability.

[0022] Furthermore, the 3:7 ratio offers greater control over the composition gradient, enabling the alloy bump to achieve layer-by-layer enrichment of gold from bottom to top, resulting in a more natural transition and a more stable structure. This layered structure not only enhances mechanical adhesion in microelectronic packaging but also improves interfacial electrical properties and thermal stability. Compared to a 1:9 ratio, the gradient transition is more pronounced, and the division of structural and functional areas is more rational. Compared to a 2:8 ratio, the gold-rich areas are more extensive, their distribution more controllable, and their application adaptability is broader.

[0023] Finally, in terms of balancing performance and cost, the 3:7 ratio, while having a slightly higher gold content, still offers significant cost advantages compared to higher-gold plating solutions. Furthermore, it offers significant technical advantages in achieving high-reliability connections, enhancing electrical performance, and maintaining long-term stability, making it particularly suitable for the manufacturing of precision electronic devices requiring high functional gradients and packaging compatibility.

[0024] In summary, the Au / Ag alloy ratio of 3:7 is superior to the ratios of 1:9 and 2:8 in terms of process controllability, structural functionality and application adaptability, and is an ideal electroplating solution with both performance and economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is a flow chart of an electroplating method for forming an AuAg alloy bump with a composition gradient structure. DETAILED DESCRIPTION

[0026] In order to deepen the understanding of the present invention, the present invention will be further described in detail below with reference to embodiments and drawings. The embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0027] An electroplating method for forming an AuAg alloy bump with a compositional gradient structure, comprising the following steps: electroplating a substrate in an Au / Ag alloy plating solution containing a gold salt and a silver salt, where the concentration ratio of the gold salt to the silver salt in the plating solution is 3:7; the electroplating process includes two stages with different current densities set respectively: in the first stage, alloy electrodeposition is carried out using a first current density; in the second stage, electroplating is continued using a second current density in the later stage of the formation of the alloy layer; the current densities of the two stages are different to form a compositional gradient distribution from the bottom to the top in the alloy layer.

[0028] This embodiment is based on the principle of the reduction of metal ions on the electrode surface during the electrodeposition process. By regulating the ratio of the gold salt to the silver salt in the electroplating solution and setting the current density in stages, the deposition rate and ratio of gold and silver in the Au / Ag alloy can be controlled. Using a lower current density in the first stage helps to promote the uniform initial deposition of the alloy components; increasing the current density in the second stage speeds up the deposition rate and enhances the priority of the reduction of gold ions, thus obtaining a structure with a higher gold content at the top of the alloy layer. This staged control strategy causes an obvious compositional gradient to form from the bottom to the top of the entire alloy layer, that is, a higher silver content at the bottom and a higher gold content at the top, thereby optimizing the physical and electrical properties of the alloy layer.

[0029] This method realizes the gradient regulation of the composition of the Au / Ag alloy layer through simple two-stage current control, effectively improving the functionality of the electroplated layer structure. For example, in microelectronic packaging, the high-gold-content region at the top is beneficial to subsequent welding and bonding performance, while the region with a relatively high silver content at the bottom takes into account both cost control and electrical conductivity. In addition, since no complex process control links are introduced, this method has good industrial feasibility and replicability.

[0030] In a possible embodiment, the first current density is 0.2 - 0.8 A / dm 2 , and the second current density is 1 - 5 A / dm 2 .

[0031] By setting different current densities in the two stages, the composition regulation during the alloy electrodeposition process is achieved. Using a lower current density (0.2 - 0.8 A / dm 2 ) in the first stage can reduce the reduction rate of gold ions, making the silver content relatively high in the initial stage of the alloy layer. In the second stage, a higher current density (1 - 5 A / dm 2 ) is used. Since the standard electrode potential of gold ions is relatively high, they are more easily reduced at a high current density, thus promoting an increase in the gold content in the later stage of deposition and forming the required compositional gradient distribution.

[0032] This current density configuration enables the electroplating process to have better controllability and repeatability, ensuring the formation of a gradient structure with an increasing gold content from bottom to top, effectively improving the welding adaptability and surface corrosion resistance of the alloy layer while maintaining the rationality of the overall cost.

[0033] In a possible implementation, the duration of the second-stage electroplating is 30 - 120 seconds.

[0034] Controlling the duration of the second stage helps to regulate the thickness of the gold content at the top of the alloy. In the time range of 30 - 120 seconds, the high current density enables the rapid reduction of gold ions, causing gold to be mainly deposited on the surface layer, thereby forming a high-purity gold coating in the top region of the finally formed alloy layer and effectively achieving the required composition gradient.

[0035] By reasonably setting the electroplating time of the second stage, it is possible to ensure the formation of a gold-rich layer with a moderate thickness, enabling the top region to avoid material waste while maintaining good electrical conductivity and oxidation resistance, and improving the overall structural performance and process economy.

[0036] In a possible implementation, the duration of the first-stage electroplating is 1000 - 2400 seconds.

[0037] The first stage is the main deposition stage of the alloy layer. By controlling the electroplating time between 1000 and 2400 seconds, it is possible to ensure the formation of a base layer structure with an appropriate thickness and a relatively high silver content. The low current density deposition within this time range is conducive to uniform deposition and avoids excessive internal stress, thereby improving the stability of the overall structure.

[0038] This duration setting achieves good electroplating uniformity and structural integrity, provides a solid foundation for the subsequent gold enrichment in the second stage, reduces the oxidation risk of silver, and helps to improve the electrical conductivity of the bottom layer and the overall corrosion resistance.

[0039] In a possible implementation, the thickness of the alloy bump is 5 - 11 μm, and the gold content purity within the thickness range of 1 - 2 μm at the top is above 80%.

[0040] Combining multi-stage current density control and deposition time adjustment, during the entire electroplating process, the alloy growing layer by layer forms a structure with an overall thickness of 5 - 11 μm, where a high-purity region with a gold content ≥ 80% is formed at the top under the conditions of high current density and rapid deposition in a short period, effectively meeting the requirements of high-performance package connection.

[0041] This structure realizes the top function optimization on the basis of ensuring overall cost control, enhances the welding reliability and the stability of electrical connection. The high-purity gold surface layer also effectively improves the oxidation and corrosion resistance, making the alloy bump more suitable for the high-density interconnect environment.

[0042] In a possible implementation, the electroplating solution maintains a flow rate of 10 - 15 L / min and a shaking frequency of 10 - 30 Hz throughout the electroplating process.

[0043] Maintaining a stable flow rate and a moderate shaking frequency helps to evenly distribute metal ions in the plating solution, avoid ion concentration gradients and polarization phenomena, and improve the stability of the electrodeposition process. A flow rate of 10 - 15 L / min can effectively carry away reaction products and replenish fresh ions, while a shaking frequency of 10 - 30 Hz enhances solution perturbation, which is beneficial to improving the deposition rate and uniformity.

[0044] This dynamic control method of the electroplating solution significantly improves the quality of the alloy layer, reduces surface defects and compositional non-uniformity, optimizes electroplating efficiency and equipment utilization rate, and enhances the repeatability and industrial adaptability of the process.

[0045] In summary, this process method involves electroplating a substrate in an Au / Ag alloy plating solution containing gold salt and silver salt, where the concentration ratio of gold salt to silver salt in the plating solution is 3:7. The electroplating process includes two stages, with different current densities set respectively to form a compositional gradient distribution from the bottom to the top, specifically including:

[0046] In the first stage, a first current density of 0.2 - 0.8 A / dm 2 is used for alloy electrodeposition, and the electroplating duration is 1000 - 2400 seconds;

[0047] In the second stage, in the later stage of the alloy layer formation, a second current density of 1 - 5 A / dm 2 is used to continue electroplating, and the electroplating duration is 30 - 120 seconds;

[0048] The overall thickness of the alloy bumps formed under the action of the above two stages is 5 - 11 μm, and the gold content purity within the thickness range of 1 - 2 μm at the top is above 80%;

[0049] During the electroplating process, the electroplating solution maintains a flow rate of 10 - 15 L / min and is shaken at a frequency of 10 - 30 Hz to enhance the uniformity of ion distribution and the electrodeposition effect.

[0050] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. An electroplating method for forming an AuAg alloy bump with a compositional gradient structure, characterized in that, The steps include: Electroplating a substrate in an Au / Ag alloy plating solution containing a gold salt and a silver salt, where the concentration ratio of the gold salt to the silver salt in the plating solution is 3:7; The electroplating process includes two stages, with different current densities set respectively: In the first stage, alloy electrodeposition is carried out using a first current density; In the second stage, electroplating is continued using a second current density in the later stage of the formation of the alloy layer; The current densities of the two stages are different to form a compositional gradient distribution from the bottom to the top in the alloy layer.

2. The electroplating method according to claim 1, wherein The first current density is 0.2 - 0.8 A / dm 2 , and the second current density is 1 - 5 A / dm 2 .

3. The electroplating method according to claim 2, characterized in that, The duration of the electroplating in the second stage is 30 - 120 seconds.

4. The electroplating method according to claim 3, wherein The duration of the electroplating in the first stage is 1000 - 2400 seconds.

5. The electroplating method according to claim 4, wherein The thickness of the alloy bump is 5 - 11 μm, and the gold content purity within the thickness range of 1 - 2 μm at the top is above 80%.

6. The electroplating method according to claim 5, characterized in that, The flow rate of the plating solution is maintained at 10 - 15 L / min and the shaking frequency is 10 - 30 Hz throughout the electroplating process.

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