Bump structure and production method therefor

Abstract

In this bump structure and production method therefor, a UBM layer mainly composed of Ni is designed to have a P content of 2-5 wt%. Setting the P content of the UBM layer to 2 wt% or more makes it possible to form a bump layer on the electrode pad with the UBM layer therebetween. If the P content of the UBM layer is 6 wt% or more, a P-rich layer formed in the vicinity of the interface with the bump layer under high temperatures such as in the reflow process becomes excessively thick, and the bonding strength between the UBM layer and the bump layer is consequently reduced. Therefore, by setting the P content of the UBM layer mainly composed of Ni to 2-5 wt%, high bonding strength between the UBM layer and the bump layer can be achieved.

Description

Bump structure and method for manufacturing the same

[0001] The present disclosure relates to a bump structure and a method for manufacturing the same.

[0002] A technique is known in which solder bumps are provided on electrode pads for electrical connection to an external device in a semiconductor device. Patent Document 1 below discloses a technique in which a barrier metal layer mainly composed of Ni formed by electroless plating is interposed between an electrode pad and a solder bump. It is known that sodium hypophosphite is used as a reducing agent in electroless Ni plating, and P in the reducing agent precipitates together with Ni, so that the barrier metal layer contains P. When Ni in the barrier metal layer diffuses into the solder bump under high temperature such as reflow treatment of the solder bump, a layer containing a large amount of P (P-rich layer) is formed on the surface of the barrier metal layer. This P-rich layer causes a decrease in bonding strength between the barrier metal layer and the electrode pad. Patent Document 1 discloses a technique in which a barrier metal layer is composed of two layers having different P contents to make it difficult for a P-rich layer to occur.

[0003] Japanese Patent Application Laid-Open No. 2006-147952

[0004] Among the two-layered barrier metal layers according to the above-described conventional technology, the barrier metal layer having a higher P content is prone to generate a P-rich layer, and the barrier metal layer having a lower P content has higher hardness, so cracks and chipping are likely to occur. In addition, it is necessary to give consideration to adhesion between the two layers, and insufficient adhesion may result in a decrease in bonding strength.

[0005] One aspect of the present disclosure aims to provide a bump structure with improved bonding strength and a method for manufacturing the same.

[0006] A bump structure according to one aspect of the present disclosure includes a bump layer mainly composed of Sn formed on an electrode, and an under-bump layer of single-layer structure mainly composed of Ni, containing 2 wt% or more and 5 wt% or less of P, and interposed between the electrode and the bump layer.

[0007] A method for manufacturing a bump structure according to one aspect of this disclosure includes the steps of: preparing a substrate on which electrodes are formed on a main surface; covering the electrodes with a single-layer underbump layer mainly composed of Ni and containing 2 wt% to 5 wt% of P; and covering the underbump layer with a bump layer mainly composed of Sn.

[0008] In the above-described bump structure and its manufacturing method, an underbump layer mainly composed of Ni is interposed between the electrode and the bump layer, and since the underbump layer contains 2 wt% or more of P, a bump layer mainly composed of Sn can be formed on the electrode pad via the underbump layer. In addition, since the underbump layer contains 5 wt% or less of P, the thickness of the P-rich layer that forms between the underbump layer and the bump layer at high temperatures is suppressed, thereby achieving high bonding strength. Furthermore, by making the underbump layer a single-layer structure, poor adhesion between layers that occurs when the underbump layer has a multi-layer structure is avoided, and high bonding strength can be achieved.

[0009] According to various aspects of this disclosure, a bump structure with improved joint strength and a method for manufacturing the same are provided.

[0010] This is a schematic cross-sectional view showing a bump structure according to one embodiment. This is a flowchart showing the procedure for manufacturing the bump structure shown in Figure 1. This is a diagram showing the steps for manufacturing the bump structure shown in Figure 1. This is a table showing the experimental results. This is a graph showing the relationship between P content and average shear strength in the experimental results.

[0011] Embodiments of this disclosure will be described in detail below with reference to the attached drawings. In the description of the drawings, the same or equivalent elements will be denoted by the same reference numeral, and redundant descriptions will be omitted.

[0012] First, a bump structure 1 according to one embodiment will be described with reference to Figure 1. The bump structure 1 is composed of a bump layer 20 formed on an electrode pad 12 (electrode) formed on a flat main surface 10a of a substrate 10. The substrate 10 has insulating properties and may be, for example, a silicon substrate or an organic substrate.

[0013] The electrode pad 12 is in the form of a film and has a thickness of, for example, 0.1 to 30 μm (1 μm as an example). The electrode pad 12 can be made of a metal film or an alloy film, for example, Cu, Cu alloy, Al, or Al alloy. The electrode pad 12 according to this embodiment is made of Cu. The electrode pad 12 according to this embodiment has a circular outer shape with a diameter of 2 μm or more when viewed in plan (i.e., when viewed from a direction perpendicular to the main surface 10a of the base material 10). The outer shape of the electrode pad 12 may be an elliptical or polygonal shape with a width of 2 μm or more when viewed in plan.

[0014] On the main surface 10a of the substrate 10, the electrode pad 12 is surrounded by a frame 14. The frame 14 is insulating and can be made of a resist material containing, for example, an insulating resin. The frame 14 has an opening 14a through which the surface of the electrode pad 12 is exposed. In this embodiment, the frame 14 is circular in shape with a diameter of 1 to 50 μm (5 μm as an example) in plan view. Since the diameter of the opening 14a of the frame 14 is smaller than the diameter of the electrode pad 12, only a part of the surface of the electrode pad 12 is exposed through the opening 14a, not the entire surface. The frame 14 may cover the entire outer edge of the electrode pad 12 or it may cover it partially. The opening 14a of the frame 14 may be circular, elliptical, or polygonal in plan view. If the opening 14a is a polygon in plan view, the average value of the distances between substantially opposite sides in the polygon can be considered as the diameter of the opening 14a.

[0015] The bump layer 20 integrally covers the electrode pad 12 and the frame 14. The bump layer 20 according to this embodiment is composed of an embedded portion 21 located within the opening 14a of the frame 14 and a main body portion 22 located above the upper surface 14b of the frame 14.

[0016] The embedded portion 21 is provided to fill the opening 14a of the frame 14 and has the same planar shape as the opening 14a of the frame 14. In other words, in this embodiment, the embedded portion 21 of the bump layer 20 has a circular planar shape, and more specifically, it is cylindrical.

[0017] The main body portion 22 is substantially dome-shaped, with its upper surface curved in a convex manner. In this embodiment, the main body portion 22 is a perfect circle with a diameter of 15 μm or less (8 μm as an example) in plan view. The main body portion 22 may be elliptical or polygonal in plan view. If the main body portion 22 is polygonal in plan view, the average value of the distances between substantially opposite sides in the polygon can be considered as the diameter of the main body portion 22. Since the diameter of the main body portion 22 is larger than the diameter of the opening 14a and the diameter of the embedded portion 21 of the frame 14, the main body portion 22 covers the upper surface 14b in the peripheral region of the opening 14a. The main body portion 22 may cover the entire peripheral region of the opening 14a or it may cover it partially. By designing the main body portion 22 of the bump layer 20 to have a diameter of 15 μm or less, the bump layer 20 can be made extremely small in diameter. This allows for miniaturization of the entire bump structure 1, thereby enabling a narrower pitch and higher density arrangement by bringing the bump structures 1 closer together.

[0018] The bump layer 20 is made of a material mainly composed of Sn. The bump layer 20 according to this embodiment is substantially made of pure Sn. The bump layer 20 made of pure Sn can be dissolved by reflow soldering to remove whiskers that form on the surface of the bump layer 20. The bump layer 20 has a thickness of, for example, 2 μm or more (8 μm as an example). By making the bump layer 20 bulky with a thickness of 2 μm or more, sufficient clearance can be ensured during bonding.

[0019] The bump layer 20 is covered by a coating layer 24. Specifically, the coating layer 24 covers the surface (i.e., the convex curved surface) of the main body portion 22 of the bump layer 20. The coating layer 24 can function as an antioxidant film that suppresses oxidation of at least the surface of the main body portion 22 of the bump layer 20. The coating layer 24 has a thickness of, for example, 0.01 μm or more and 0.5 μm or less (0.1 μm as an example). The coating layer 24 according to this embodiment is made of a material mainly composed of Ag. The coating layer 24 may be made of a material mainly composed of Au or Pd in ​​addition to Ag.

[0020] An underbump layer 30 (hereinafter also referred to as the UBM layer) is interposed between the electrode pad 12 and the bump layer 20. The UBM layer 30 covers the entire area of ​​the electrode pad 12 exposed from the opening 14a within the opening 14a of the frame 14. In this embodiment, the UBM layer 30 directly covers the surface of the electrode pad 12. The UBM layer 30 is in the form of a film and has a thickness of, for example, 1 μm or more (1.5 μm as an example). The thickness of the UBM layer 30 can be defined, for example, as the height from the highest point to the bottom, and in this embodiment, it can be defined as the length from the bottom surface (the surface in contact with the electrode pad 12) of the UBM layer 30 to the point furthest from it in a direction perpendicular to the main surface 10a of the substrate 10. The UBM layer 30 is made of a material mainly composed of Ni and contains a small amount of P. The P content of the UBM layer 30 is 2 wt% or more and 5 wt% or less. The UBM layer 30 has a single-layer structure and is not composed of multiple layers. In other words, there are no other layers mainly composed of Ni, other than the UBM layer 30, between the electrode pad 12 and the bump layer 20. More specifically, the UBM layer 30 is in direct contact with the electrode pad 12 and is also in direct contact with the bump layer 20.

[0021] Next, the manufacturing method of the bump structure 1 described above will be explained with reference to Figures 2 and 3.

[0022] When fabricating the bump structure 1, first, a base material 10 is prepared (step S1 in Figure 2). In this step, the electrode pads 12 and frame 14 may be placed on the main surface 10a of the base material 10, or a base material 10 may be brought in with the electrode pads 12 and frame 14 already placed on the main surface 10a. In this state, as shown in Figure 3(a), a portion of the surface of the electrode pads 12 is exposed through the opening 14a of the frame 14.

[0023] Before forming the UBM layer 30 on the substrate 10, known pretreatments may be performed as needed. Examples of pretreatments include surface modification by excimer irradiation or plasma irradiation, degreasing, and acid cleaning.

[0024] Next, a UBM layer 30 is formed on the surface of the electrode pad 12 in the area exposed from the opening 14a of the frame 14 (step S2 in Figure 2). In this step, the UBM layer 30 is formed by reductive electroless plating. For this electroless plating, a plating solution containing a nickel compound, a complexing agent, and a reducing agent is used. From the viewpoint of improving the workability of electroless plating (bath stability, deposition rate), a plating solution containing hypophosphorous acid as a reducing agent can be used. As a result, as shown in Figure 3(b), the surface of the electrode pad 12 is directly covered by the UBM layer 30 within the opening 14a.

[0025] Furthermore, a bump layer 20 is formed on the electrode pad 12 via the UBM layer 30 (step S3 in Figure 2). In this step, the bump layer 20 is formed by reductive electroless plating. For this electroless plating, a plating solution containing a tin compound, a complexing agent, and a reducing agent is used, and buffering agents, antioxidants, surfactants, brighteners, etc., can be included as needed. After electroless plating, a reflow treatment is performed. The reflow treatment is a high-temperature treatment in a nitrogen atmosphere, for example, in which the bump layer 20 is melted and then rapidly cooled and solidified to obtain a dome-shaped bump layer 20. There are no particular restrictions on the reflow conditions, but for example, the atmosphere is an oxygen concentration of 1000 ppm or less, the temperature is 235 to 300°C, and the holding time is 5 to 120 seconds. As a result, as shown in Figure 3(c), the embedded portion 21 of the bump layer 20 directly covers the surface of the UBM layer 30 and completely fills the opening 14a, while the main body portion 22 of the bump layer 20 covers the upper surface 14b in the peripheral region of the opening 14a.

[0026] Subsequently, a coating layer 24 is formed on the bump layer 20 (step S4 in Figure 2), completing the bump structure 1 shown in Figure 1. The coating layer 24 is formed by reducing electroless plating. For this electroless plating, a plating solution containing a silver compound, a complexing agent, and a reducing agent is used.

[0027] In the bump structure 1 and its manufacturing method described above, the P content of the Ni-based UBM layer 30 is designed to be between 2 wt% and 5 wt%. If the P content of the UBM layer 30 is less than 2 wt%, the bump layer 20 cannot be formed on the electrode pad 12 via the UBM layer 30. However, by setting the P content to 2 wt% or more, the bump layer 20 can be formed on the electrode pad 12 via the UBM layer 30. If the P content of the UBM layer 30 is 6 wt% or more, the P-rich layer formed near the interface with the bump layer 20 under high temperatures such as the reflow process described above becomes too thick, reducing the bonding strength (shear strength) between the UBM layer 30 and the bump layer 20. Therefore, by setting the P content of the Ni-based UBM layer 30 to 2 wt% and 5 wt%, high bonding strength between the UBM layer 30 and the bump layer 20 can be achieved. Furthermore, by making the UBM layer 30 a single-layer structure, poor adhesion between layers that occurs when the underbump layer has a multi-layer structure is avoided, and the bonding strength can be further increased.

[0028] Furthermore, under high temperatures such as those encountered during the reflow process described above, Sn from the bump layer 20 tends to diffuse into the electrode pad 12. However, the UBM layer 30 functions as a diffusion-preventing layer, suppressing such Sn diffusion. If the thickness of the UBM layer 30 is less than 1.0 μm, it does not function as a sufficiently effective diffusion-preventing layer for practical purposes, and the bump layer 20 will not melt during the reflow process due to diffusion. Therefore, the thickness of the UBM layer 30 is designed to be 1.0 μm or more. If the thickness of the UBM layer 30 exceeds 2.0 μm, the bump layer 20 becomes larger (increases in diameter), hindering the miniaturization of the bump structure. Therefore, the thickness of the UBM layer 30 is designed to be 2.0 μm or less.

[0029] The bump layer 20 and UBM layer 30 according to this embodiment are formed by electroless plating. In the fine bump structure 1 described above, it is difficult to form the bump layer 20 and UBM layer 30 by electroplating. Electroplating requires a seed layer, but it is difficult to pattern the seed layer with high positional accuracy in the narrow area on the surface of the electrode pad 12 surrounded by the frame 14. Even if it were possible to provide it, the resist used for patterning may become a residue, leading to a decrease in yield and an increase in cost. In this embodiment, the bump layer 20 and UBM layer 30 are formed by electroless plating, eliminating the need for a seed layer. This allows the bump layer 20 and UBM layer 30 to be formed in the fine bump structure 1 described above with high yield and low cost. When the bump layer 20 is formed by electroless plating, compared to when it is formed by electroplating, components in the plating solution are less likely to be mixed into the bump layer 20 as impurities, and high crystallinity can be achieved.

[0030] The inventors conducted the following experiments regarding the P-rich layer. For the experiments, several bump structures having the above-described configuration were prepared, each with a different P content in the underbump layer. Specifically, three types of bump structures (samples 1 to 3) were prepared: Sample 1 with an underbump layer P content of 3 wt%, Sample 2 with an underbump layer P content of 5 wt%, and Sample 3 with an underbump layer P content of 7 wt%. Reflow treatment was performed on each of samples 1 to 3, and the shear strength after reflow treatment was measured. The reflow treatment was performed under conditions of an oxygen concentration of 500 ppm, a temperature of 250°C, and a holding time of 30 seconds.

[0031] In the experiment, before forming the underbump layer, surface modification by excimer irradiation, degreasing, and acid cleaning were performed as pretreatment. For surface modification, excimer irradiation was performed 10 times at a distance of 3 mm from the substrate and at a speed of 15 mm / second. For degreasing, a 100 mL / L concentration degreasing agent was used under conditions of pH < 1 and 5 minutes. For acid cleaning, an acidic solvent (97 wt% sulfuric acid) with a concentration of 10 mL / L (18 g / L concentration) was used under conditions of pH < 1 and 1 minute.

[0032] In the electroless plating process for forming the underbump layer of sample 1, a plating solution containing Epitas NLL-1-M (250 mL / L), Epitas NLL-1-1B (27 mL / L), and Epitas NLL-1-A (45 mL / L) manufactured by Uemura Kogyo Co., Ltd. was used. The pH was adjusted to 6.4 with 10% sulfuric acid and 10% NaOH, and an underbump layer with a thickness of 2.5 μm was formed at a bath temperature of 78°C and a deposition rate of 8 μm / hour. For the electroless plating that formed the underbump layer of sample 2, a plating solution containing Melplate NI-6505LF1 (70 mL / L), Melplate NI-6505LF2 (140 mL / L), and Melplate NI-6505LF3 (10 mL / L) manufactured by Meltex Co., Ltd. was used. The pH was adjusted to 4.6 with 10% sulfuric acid and ammonia, and an underbump layer with a thickness of 2.5 μm was formed at a bath temperature of 78°C and a deposition rate of 12 μm / hour. For the electroless plating that formed the underbump layer of sample 3, a plating solution containing Melplate NI-865TM1 ​​(60 mL / L) and Melplate NI-865TM2 (120 mL / L) manufactured by Meltex Co., Ltd. was used. The pH was adjusted to 4.6 with sulfuric acid and ammonia, and an underbump layer with a thickness of 2.5 μm was formed at a bath temperature of 78°C and a deposition rate of 9 μm / hour.

[0033] Each bump layer of samples 1 to 3 was formed to a thickness of 4 μm under the conditions of pH 8.5, 70°C, and 60 minutes, using a prepared bath concentration of 0.08 mol / L stannous chloride dihydrate, 0.35 mol / L trisodium citrate diammonium, 0.10 mol / L trisodium nitrilotriacetate, and 0.06 mol / L 20% titanium trichloride, as an example.

[0034] The scanning electron microscope (SEM) images of samples 1 to 3 are shown in Figure 4. The P-rich layers were observed at 35,000x magnification, and their thicknesses were measured. The thickness of the P-rich layer in sample 1 was 0.078 μm, in sample 2 it was 0.082 μm, and in sample 3 it was 0.13 μm.

[0035] The shear strength of samples 1 to 3 was measured as follows. Specifically, using a Bond Tester (4000Plus) manufactured by Nordson Corporation and a shear tool with a shear plane of 10 μm, the shear strength was measured five times for each sample at a shear height of approximately 1.5 to 2.0 μm at a speed of 150 μm / second, and the average was calculated. As a result, the average shear strength of sample 1 was 9.518 g, the average shear strength of sample 2 was 9.762 g, and the average shear strength of sample 3 was 4.388 g. In other words, as shown in the graph in Figure 5, sufficiently high shear strength was obtained for samples 1 and 2, while the shear strength of sample 3 was relatively remarkably low. Therefore, it was confirmed that a sufficiently high shear strength for practical use can be obtained by keeping the P content of the underbump layer to 5 wt% or less.

[0036] This disclosure is not limited to the embodiments described above and can be modified in various ways. For example, the bump layer may not include an embedded portion and may consist only of a main body portion located above the upper surface of the frame. In this case, the upper surface of the underbump layer may be flush with the upper surface of the frame, or the underbump layer may protrude from the opening of the frame. Furthermore, the bump structure may not include a frame.

[0037] As can be understood from the above description, this specification discloses the following: [Appendix 1] A bump structure comprising a bump layer formed on an electrode, mainly composed of Sn, and a single-layer underbump layer interposed between the electrode and the bump layer, mainly composed of Ni and containing 2 wt% to 5 wt% of P. [Appendix 2] The bump structure according to Appendix 1, wherein the thickness of the underbump layer is in the range of 1.0 to 2.0 μm. [Appendix 3] The bump structure according to Appendix 1 or 2, wherein the thickness of the bump layer is 2 μm or more. [Appendix 4] The bump structure according to any one of Appendix 1 to 3, further comprising a coating layer covering the surface of the bump layer, mainly composed of Ag, Au, or Pd. [Note 5] A method for manufacturing a bump structure, comprising the steps of: preparing a substrate on which electrodes are formed on a main surface; covering the electrodes with a single-layer underbump layer mainly composed of Ni and containing 2 wt% to 5 wt% of P; and covering the underbump layer with a bump layer mainly composed of Sn. [Note 6] The method for manufacturing a bump structure according to Note 5, wherein in the step of covering with the underbump layer, the underbump layer is formed by electroless plating. [Note 7] The method for manufacturing a bump structure according to Note 5 or 6, wherein in the step of covering with the bump layer, the bump layer is formed by electroless plating. [Note 8] The method for manufacturing a bump structure according to any one of Notes 5 to 7, further comprising the step of covering the bump layer with a coating layer mainly composed of Ag, Au, or Pd.

[0038] 1...Bump structure, 10...Substrate, 12...Electrode pad, 14...Frame, 20...Bump layer, 24...Coating layer, 30...Underbump layer (UBM layer), S1-S4...Process.

Claims

1. A bump structure comprising a bump layer formed on an electrode, mainly composed of Sn, and a single-layer underbump layer interposed between the electrode and the bump layer, mainly composed of Ni and containing 2 wt% to 5 wt% of P.

2. The bump structure according to claim 1, wherein the thickness of the underbump layer is in the range of 1.0 to 2.0 μm.

3. The bump structure according to claim 1, wherein the thickness of the bump layer is 2 μm or more.

4. The bump structure according to claim 1, further comprising a coating layer mainly composed of Ag, Au, or Pd that covers the surface of the bump layer.

5. A method for manufacturing a bump structure, comprising the steps of: preparing a substrate on which electrodes are formed on a main surface; covering the electrodes with a single-layer underbump layer mainly composed of Ni and containing 2 wt% to 5 wt% of P; and covering the underbump layer with a bump layer mainly composed of Sn.

6. The method for manufacturing a bump structure according to claim 5, wherein in the step of covering with the underbump layer, the underbump layer is formed by electroless plating.

7. The method for manufacturing a bump structure according to claim 5, wherein in the step of covering with the bump layer, the bump layer is formed by electroless plating.

8. The method for manufacturing a bump structure according to claim 5, further comprising the step of covering the bump layer with a coating layer mainly composed of Ag, Au, or Pd.

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