Bump structure and method of manufacturing the same

Abstract

The embodiment of the present disclosure provides a bump structure and a preparation method thereof. The method comprises the following steps: providing a wafer; forming a UBM layer electrically connected with a pad on the surface of the wafer; electroplating a palladium bump layer electrically connected with the pad on the UBM layer, and electroplating a copper bump layer on the palladium bump layer to form a palladium-copper bump; performing chemical plating on the surface of the palladium-copper bump to form a first protective layer wrapping the palladium-copper bump; and performing chemical plating on the surface of the first protective layer to form a second protective layer wrapping the first protective layer. The palladium bump layer and the copper bump layer are formed in sequence to form the palladium-copper bump, the palladium bump layer increases the overall bump bonding force, provides reliability and improves the overall appearance; and the first protective layer and the second protective layer realize all-around coverage of the palladium-copper bump, thereby fundamentally eliminating oxidation and significantly improving product reliability.

Description

Technical Field

[0001] This disclosure pertains to the field of semiconductor integrated circuit technology, specifically relating to a bump structure and its fabrication method. Background Technology

[0002] In the packaging of display driver chips (DDICs), bumps are a key structure for achieving electrical interconnection and mechanical connection between the chip and the glass substrate (such as COG technology) or flexible circuit board (such as COF technology). As display technology develops towards higher resolution and higher refresh rates, the size of bumps continues to shrink and the density continues to increase, placing higher demands on their electrical performance, mechanical reliability, and process stability.

[0003] Traditional bump fabrication methods often employ copper or gold plating, but these methods have the following drawbacks: 1) Insufficient bonding: The bonding between copper and the underlying seed layer is weak, and it is prone to delamination under thermal or mechanical stress.

[0004] 2) Poor morphology control: Lateral growth is prone to occur during the copper electroplating process, which leads to tilting of the bump sidewalls and affects the subsequent bonding accuracy.

[0005] 3) Poor corrosion resistance: Copper is easily oxidized, and long-term use may lead to failure due to electromigration.

[0006] 4) Excessive gold layer leads to high cost: If a thick gold layer is used directly, the material cost will increase significantly.

[0007] To address the aforementioned problems, it is necessary to propose a reasonably designed and effective bump structure and its fabrication method. Summary of the Invention

[0008] The present disclosure aims to at least solve one of the technical problems existing in the prior art, and to provide a bump structure and a method for preparing the same.

[0009] One aspect of this disclosure provides a method for fabricating a bump structure, the method comprising: A wafer is provided, the wafer having a passivation layer and pads; A UBM layer electrically connected to the pads is formed on the surface of the wafer; A palladium bump layer electrically connected to the pads is formed by electroplating on the UBM layer, and a copper bump layer is formed by electroplating on the palladium bump layer to form palladium-copper bumps; Chemical plating is performed on the surface of the palladium-copper bump to form a first protective layer that encapsulates the palladium-copper bump; Chemical plating is performed on the surface of the first protective layer to form a second protective layer that encapsulates the first protective layer.

[0010] Optionally, the thickness of the formed copper bump layer is greater than the thickness of the palladium bump layer.

[0011] Optionally, a palladium bump layer electrically connected to the pads is formed by electroplating on the UBM layer, including: A photoresist layer is formed on the UBM layer; Pattern the photoresist layer to form openings on the photoresist layer; The palladium bump layer is formed by electroplating within the opening.

[0012] Optionally, after forming the palladium-copper bumps, the method further includes: Remove any remaining photoresist layer; Remove excess UBM layers so that the UBM layers are electrically connected only to the palladium-copper bumps.

[0013] Optionally, chemical plating is performed on the surface of the palladium-copper bump to form a first protective layer encapsulating the palladium-copper bump, comprising: The top and side walls of the palladium-copper bumps are cleaned and activated; A uniform and dense layer of metallic nickel is deposited on the top and sidewalls of the palladium-copper bump by an autocatalytic reaction to form the first protective layer; The surface of the first protective layer is cleaned and dried.

[0014] Optionally, a chemical plating process is performed on the surface of the first protective layer to form a second protective layer encapsulating the first protective layer, comprising: Remove the passivation oxide film from the surface of the first protective layer; A uniform and dense metallic gold layer is deposited on the surface of the first protective layer through an autocatalytic reaction to form the second protective layer; The surface of the second protective layer is cleaned and dried.

[0015] Optionally, after forming the second protective layer, the method further includes: The entire bump structure is annealed to adjust the hardness of the conductive bump structure.

[0016] Another aspect of this disclosure provides a bump structure, formed using the bump structure fabrication method described above, wherein the bump structure includes: A wafer, the surface of which is provided with a passivation layer and pads; A UBM layer is disposed on the surface of the wafer and electrically connected to the pads; Palladium-copper bumps are disposed on the UBM layer and electrically connected to the pads, wherein the palladium-copper bumps comprise palladium bump layers and copper bump layers stacked sequentially; A first protective layer is applied to the surface of the palladium-copper bump. The second protective layer is wrapped around the surface of the first protective layer.

[0017] Optionally, the thickness of the copper bump layer is greater than the thickness of the palladium bump layer.

[0018] Optionally, the first protective layer is made of nickel, and the second protective layer is made of gold.

[0019] The bump structure and its preparation method provided in this disclosure involve first electroplating a palladium bump layer on a UBM layer, and then electroplating a copper bump layer on the palladium bump layer to form a palladium-copper bump. This allows the palladium bump layer and the copper and titanium in the UBM layer to form a solid solution, achieving metallurgical bonding and significantly improving the bonding strength. The palladium bump layer itself has better morphological perpendicularity and a lattice constant close to that of copper, enabling the copper bump layer to grow epitaxially along the vertical direction, resulting in an overall palladium-copper bump morphology close to perpendicularity. By first electroplating to form the palladium-copper bump, and then forming a first and second protective layer, the palladium-copper bump is fully covered, fundamentally preventing oxidation and significantly improving product reliability. Attached Figure Description

[0020] Figure 1 This is a schematic flowchart of a method for preparing a bump structure according to one embodiment of the present disclosure; Figures 2 to 9 This is a process flow diagram of a bump structure fabrication method according to another embodiment of this disclosure; Figure 10 This is a schematic diagram of a bump structure in another embodiment of the present disclosure. Detailed Implementation

[0021] To enable those skilled in the art to better understand the technical solutions of the embodiments of this disclosure, the embodiments of this disclosure will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0022] like Figure 1 As shown, one aspect of this disclosure provides a method S100 for fabricating a bump structure, the method comprising: S110. Provide a wafer, the wafer having a passivation layer and pads.

[0023] like Figure 2 As shown, qualified wafers 110 are selected to avoid failures in subsequent processes. The surface of wafer 110 is provided with a passivation layer 111 and pads 112, wherein the passivation layer 111 has openings to expose the pads 112.

[0024] S120, A UBM layer electrically connected to the pads is formed on the surface of the wafer.

[0025] like Figure 3 As shown, a UBM layer 120 electrically connected to the pads 112 is formed on the entire surface of wafer 110 by physical vapor deposition. The UBM layer 120 includes a Ti layer and a Cu layer. The Ti layer covers the pads 112 and the passivation layer 111, preventing the Cu layer and pads 112 from diffusing into each other. The Cu layer provides conductivity for plating. The thickness of the Ti layer is 0.1 μm to 1 μm, and the thickness of the Cu layer is 0.05 μm to 1 μm.

[0026] S130. A palladium bump layer electrically connected to the pad is formed by electroplating on the UBM layer, and a copper bump layer is formed by electroplating on the palladium bump layer to form a palladium-copper bump.

[0027] like Figure 4 As shown, a photoresist layer 121 is formed on the UBM layer 120; the photoresist layer 121 is exposed and developed using a photomask to pattern it, thereby forming a shape on the photoresist layer 121 as shown. Figure 5 The opening 122 is shown.

[0028] like Figure 5 As shown, a palladium bump layer 131 is formed in the opening 122 by electroplating, and the thickness of the palladium bump layer 131 is 0.1μm~10μm.

[0029] like Figure 6 As shown, a copper bump layer 132 is formed on the palladium bump layer 131 by electroplating. The thickness of the copper bump layer 132 is 3μm~20μm. The palladium bump layer 131 and the copper bump layer 132 together form a palladium-copper bump 130.

[0030] In this embodiment, the palladium bump layer 131 increases the overall adhesion of the palladium-copper bump 130, provides reliability, and improves the overall morphology. In conventional processes, copper is directly electroplated onto the UBM layer, and the adhesion relies on physical adsorption. However, palladium can form solid solutions with both copper and titanium in the UBM layer, achieving metallurgical bonding and significantly improving the bonding strength. When the palladium bump layer 131 is electroplated onto the UBM layer 120 first, followed by the copper bump layer 132, copper atoms do not simply pile up on the palladium bump layer 131, but continue to grow directly on the palladium lattice. At the interface, atoms occupy each other's positions, forming a mixed region with gradually transitioning composition, which is the essence of metallurgical bonding. In addition, local microalloying reactions still occur at the atomic contact interface between palladium and titanium. Highly reactive titanium can form intermetallic compounds with palladium. Although this compound layer is extremely thin, it can achieve chemical bonding, providing strong adhesion. The palladium bump layer 131 itself has better morphological verticality and its lattice constant is close to that of copper, which makes the copper bump layer 132 grow epitaxially in the vertical direction, so that the overall palladium-copper bump 130 formed has a morphological verticality close to that of copper.

[0031] The thickness of the copper bump layer 132 is greater than the thickness of the palladium bump layer 131. In other words, the copper bump layer 132, as the main structure of the palladium-copper bump 130, has a much lower cost than gold bumps, thus saving costs.

[0032] Among them, such as Figure 7 As shown, after forming the palladium-copper bump 130, the remaining photoresist layer 121 and the excess UBM layer 120 are removed, so that the UBM layer 120 is only electrically connected to the palladium-copper bump 130 to prevent short circuits.

[0033] S140. Perform chemical plating on the surface of the palladium-copper bump to form a first protective layer that encapsulates the palladium-copper bump.

[0034] like Figure 8 As shown, firstly, the top and sidewalls of the palladium-copper bump 130 are cleaned and activated to ensure adhesion to the first protective layer 140. Secondly, a uniform and dense nickel layer is deposited on the top and sidewalls of the palladium-copper bump 130 via an autocatalytic reaction to form the first protective layer 140. Finally, the surface of the first protective layer 140 is cleaned and dried to prevent residual nickel salt crystals. In this embodiment, the first protective layer 140 is a nickel layer with a thickness of 0.1 μm to 0.5 μm.

[0035] The first protective layer is formed by chemical plating. This first protective layer can uniformly cover the top wall and all side walls of the copper bump, achieving true full-coverage protection, providing a functional barrier, effectively preventing copper oxidation, and improving corrosion resistance and mechanical support.

[0036] S150. Perform chemical plating on the surface of the first protective layer to form a second protective layer that encapsulates the first protective layer.

[0037] like Figure 9 As shown, firstly, the passivation oxide film on the surface of the first protective layer 140 is removed to prevent contamination of the plating solution for the second protective layer 150; secondly, a uniform and dense gold layer is deposited on the surface of the first protective layer 140 through an autocatalytic reaction to form the second protective layer 150; finally, the surface of the second protective layer 150 is cleaned and dried to form a bump structure. In this embodiment, the second protective layer 150 is a gold layer with a thickness of 0.5 μm to 2 μm.

[0038] By forming a dense and uniform second protective layer through chemical plating, excellent solderability and final protection can be provided. At the same time, compared with electroplated gold layers, the amount of gold used can be significantly reduced, reducing material costs. Furthermore, there is no edge effect, and the surface is conducive to subsequent bonding.

[0039] After forming the second protective layer, the method further includes: The entire bump structure is annealed to adjust the hardness of the conductive bump structure and improve the bonding force.

[0040] It should be noted that in this embodiment, the first protective layer 140 uses a nickel layer for the following reasons: Copper and gold will directly diffuse into each other under high temperature or long-term use, forming brittle intermetallic compounds, leading to decreased bonding strength and increased resistance. As the first protective layer, the nickel layer acts as a diffusion barrier, effectively preventing copper atoms from migrating to the gold layer and preventing gold atoms from penetrating into the copper, thereby maintaining the long-term stability of the bump composition and performance. Before electroless gold plating, the nickel layer completely encapsulates the active palladium-copper bumps, isolating them from external moisture and impurities, preventing oxidation or corrosion. Nickel is generally harder than gold, and as a bottom support, it provides a solid base for the softer gold layer, preventing the gold layer from being crushed or the bumps from undergoing excessive deformation during subsequent thermo-press bonding. There is a difference in the coefficient of thermal expansion between the palladium-copper bumps and the gold layer. The nickel layer has a certain degree of hardness and toughness, and can act as a stress buffer layer, alleviating the thermomechanical stress caused by temperature changes during encapsulation and preventing interface delamination.

[0041] It should be further explained that, in this embodiment, the second protective layer, made of gold, serves the following purposes: Gold possesses excellent solderability and contact properties, enabling it to form low-resistance, highly reliable mechanical and electrical connections with the packaging material. Gold is chemically very stable, does not readily oxidize in air, and resists the erosion of most corrosive gases and moisture. As the outermost layer, it provides the final corrosion-resistant barrier for the conductive bump structure, protecting the entire conductive bump structure from damage in harsh environments, which is crucial for the long-term reliability of consumer electronics products.

[0042] It should still be noted that, in this embodiment, the advantages of using chemical plating to form the first protective layer and the second protective layer are as follows: 1) Chemical plating does not require an external power source. It relies on a reducing agent in the solution to continuously reduce nickel ions on the catalytic surface (copper), and the deposited nickel layer itself can catalyze the continuous reaction. Electroplating, on the other hand, is driven by an external current, and nickel ions are reduced by gaining electrons on the cathode (wafer).

[0043] 2) Electroless plating produces a more robust protective layer: During electroless plating, the plating solution penetrates any wettable surface through diffusion and convection, including the entire sidewall, bottom edge, and even tiny recesses of the palladium-copper bump, achieving uniform thickness deposition in all areas. Electroplating, however, is limited by current distribution, with current density concentrated at the bump tips and edges, resulting in excessively thick plating in these areas. In contrast, the lower sidewalls and recesses, where lower current areas result in thin plating or even missed areas. Even the undercut formed at the bottom after UBM etching of the bump cannot be effectively covered.

[0044] 3) Uniform thickness of the protective layer: The protective layer formed by electroless plating has a uniform height, and the plating thickness is independent of the shape of the bump, with a tolerance of up to ±0.3μm. Electroless plating ensures that the set 0.25μm nickel layer is evenly distributed on all surfaces of the bump, achieving true full-coverage sealing. In contrast, electroplating on the same bump can result in thickness differences of more than 30% at different locations (such as the tip and the groove), affecting the reliability of the bump.

[0045] The bump structure preparation method provided in this disclosure first electroplats a palladium bump layer on the UBM layer, and then electroplats a copper bump layer on the palladium bump layer to form a palladium-copper bump. In this way, the palladium bump layer and the copper and titanium in the UBM layer can form a solid solution, achieving metallurgical bonding and significantly improving the bonding strength. The palladium bump layer itself has better morphological perpendicularity and its lattice constant is close to that of copper, allowing the copper bump layer to grow epitaxially in the vertical direction, making the overall palladium-copper bump morphology close to perpendicularity. By first electroplating to form the palladium-copper bump and then forming the first and second protective layers, the palladium-copper bump is fully covered, thereby fundamentally preventing oxidation and significantly improving product reliability.

[0046] like Figure 10 As shown, another aspect of the present disclosure provides a bump structure 100, which is formed by the bump structure preparation method S100 described above. The specific process of the bump structure preparation method S100 has been described in detail above and will not be repeated here.

[0047] The bump structure 100 includes a wafer 110, a UBM layer 120, a palladium-copper bump 130, a first protective layer 140, and a second protective layer 150.

[0048] A passivation layer 111 and a pad 112 are provided on the surface of wafer 110.

[0049] A UBM layer 120 is disposed on the surface of the wafer 110 and electrically connected to the pads 112. The UBM layer 120 can provide a conductive substrate for electroplating.

[0050] A palladium-copper bump 130 is disposed on the UBM layer 120 and electrically connected to the pad 112. The palladium-copper bump 130 comprises a palladium bump layer 131 and a copper bump layer 132 stacked sequentially. In this embodiment, the thickness of the palladium bump layer 131 is 0.1 μm to 10 μm, and the thickness of the copper bump layer 132 is 3 μm to 20 μm.

[0051] The first protective layer 140 is wrapped around the surface of the palladium-copper bump 130 to protect the entire palladium-copper bump 130.

[0052] The second protective layer 150 is disposed on the surface of the first protective layer 140 to protect the first protective layer 140.

[0053] The first protective layer 140 and the second protective layer 150 are formed by chemical plating process, and the specific process can be referred to the previous description.

[0054] For example, the thickness of the copper bump layer 132 is greater than the thickness of the palladium bump layer 131. That is, the copper bump layer 132, as the main structure of the palladium-copper bump 130, has a much lower cost than gold bumps, thus saving costs.

[0055] Preferably, in this embodiment, the first protective layer 140 is made of nickel, and the second protective layer 150 is made of gold. The first protective layer 140 is a nickel layer with a thickness of 0.1 μm to 0.5 μm, and the second protective layer 150 is a gold layer with a thickness of 0.5 μm to 2 μm.

[0056] In this embodiment, the nickel layer provides excellent barrier and support functions, while the gold layer provides good solderability and oxidation resistance. The combination of the two forms an efficient and low-cost dual-layer protection structure, ensuring the long-term reliability of the bumps during packaging and use.

[0057] The bump structure provided in this embodiment is prepared using the bump structure preparation method described above. The palladium-copper bump includes a palladium bump layer and a copper bump layer stacked sequentially. The palladium bump layer and the copper and titanium in the UBM layer can form a solid solution to achieve metallurgical bonding, which significantly improves the bonding strength. The palladium bump layer itself has better morphological verticality and its lattice constant is close to that of copper, which allows the copper bump layer to grow epitaxially in the vertical direction, making the overall palladium-copper bump morphology close to verticality. The palladium-copper bump is first formed by electroplating, and then the first protective layer and the second protective layer are formed to achieve all-round coverage of the palladium-copper bump, thereby fundamentally preventing oxidation and significantly improving product reliability.

[0058] The bump structure provided in this disclosure can be used in the display driver chip packaging structure. The display driver packaging can be divided into two types: COG (Chip On Glass): An IC and its leads are added to an LCD display. The IC is directly bonded to the LCD screen using ACF (an anisotropic conductive adhesive) via thermoforming. The hardness and height of the bump structure directly affect the COG package. The hardness of the bump structure can be controlled within the range of 75HV to 105HV, and the height of the bump structure can be controlled within the range of 6μm to 15μm.

[0059] COF (Chip on Film): An IC packaging technology that uses thermoforming to bond bump structures on a chip to internal pins on a flexible substrate circuit. The hardness and height of the bump structure directly affect COF packaging. The hardness of the bump structure can be controlled within the range of 50 HV to 65 HV, and the height of the bump structure can be controlled within the range of 6 μm to 18 μm.

[0060] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the embodiments of this disclosure, and the embodiments of this disclosure are not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the embodiments of this disclosure, and these modifications and improvements are also considered to be within the protection scope of the embodiments of this disclosure.

Claims

1. A method for fabricating a bump structure, characterized in that, The method includes: A wafer is provided, the wafer having a passivation layer and pads; A UBM layer electrically connected to the pads is formed on the surface of the wafer; A palladium bump layer electrically connected to the pads is formed by electroplating on the UBM layer, and a copper bump layer is formed by electroplating on the palladium bump layer to form palladium-copper bumps; Chemical plating is performed on the surface of the palladium-copper bump to form a first protective layer that encapsulates the palladium-copper bump; Chemical plating is performed on the surface of the first protective layer to form a second protective layer that encapsulates the first protective layer.

2. The preparation method according to claim 1, characterized in that, The thickness of the formed copper bump layer is greater than the thickness of the palladium bump layer.

3. The preparation method according to claim 2, characterized in that, A palladium bump layer electrically connected to the pads is formed by electroplating on the UBM layer, including: A photoresist layer is formed on the UBM layer; Pattern the photoresist layer to form openings on the photoresist layer; The palladium bump layer is formed by electroplating within the opening.

4. The preparation method according to claim 3, characterized in that, After forming the palladium-copper bumps, the method further includes: Remove any remaining photoresist layer; Remove excess UBM layers so that the UBM layers are electrically connected only to the palladium-copper bumps.

5. The preparation method according to claim 1, characterized in that, A chemical plating process is performed on the surface of the palladium-copper bump to form a first protective layer encapsulating the palladium-copper bump, comprising: The top and side walls of the palladium-copper bumps are cleaned and activated; A uniform and dense layer of metallic nickel is deposited on the top and sidewalls of the palladium-copper bump by an autocatalytic reaction to form the first protective layer; The surface of the first protective layer is cleaned and dried.

6. The preparation method according to claim 1, characterized in that, A second protective layer is formed by chemical plating on the surface of the first protective layer, encapsulating the first protective layer, including: Remove the passivation oxide film from the surface of the first protective layer; A uniform and dense metallic gold layer is deposited on the surface of the first protective layer through an autocatalytic reaction to form the second protective layer; The surface of the second protective layer is cleaned and dried.

7. The preparation method according to claim 1, characterized in that, After forming the second protective layer, the method further includes: The entire bump structure is annealed to adjust the hardness of the conductive bump structure.

8. A bump structure, characterized in that, The bump structure is prepared by the method of any one of claims 1 to 7, wherein the bump structure comprises: A wafer, the surface of which is provided with a passivation layer and pads; A UBM layer is disposed on the surface of the wafer and electrically connected to the pads; Palladium-copper bumps are disposed on the UBM layer and electrically connected to the pads, wherein the palladium-copper bumps comprise palladium bump layers and copper bump layers stacked sequentially; A first protective layer is applied to the surface of the palladium-copper bump. The second protective layer is wrapped around the surface of the first protective layer.

9. The bump structure according to claim 8, characterized in that, The thickness of the copper bump layer is greater than the thickness of the palladium bump layer.

10. The bump structure according to claim 8, characterized in that, The first protective layer is made of nickel, and the second protective layer is made of gold.

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