Bump structure, bump structure production method, and joint structure

Abstract

In this bump structure, a UBM layer mainly composed of Ni is interposed between an electrode pad and a bump layer, and the UBM layer is designed to have a B content of 0.1-3 wt%. Therefore, even when the bump structure is subjected to high temperatures such as in the reflow process, a P-rich layer is not generated between the UBM layer and the bump layer, or even if a P-rich layer were generated, the thickness of the P-rich layer can be suppressed. As a result, a decrease in bonding strength due to the P-rich layer is suppressed.

Description

Bump structure, method for manufacturing bump structure, and bonding structure

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

[0002] In semiconductor devices, a technique of providing solder bumps on electrode pads to achieve electrical bonding with an external device is known. 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.

[0003] Japanese Unexamined Patent Publication No. 2006-147952

[0004] In the above-mentioned conventional bump structure, 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 solder bump and the barrier metal layer.

[0005] An object of one aspect of the present disclosure is to provide a bump structure with improved bonding strength, a method for manufacturing the bump structure, and a bonding structure.

[0006] A bump structure according to one aspect of the present disclosure includes: a bump layer mainly containing Sn and formed on an electrode; and an under-bump layer of single-layer structure mainly containing Ni, containing 0.1 wt% or more and 3 wt% or less of B, and interposed between the electrode and the bump layer.

[0007] A method for manufacturing a bump structure according to one aspect of the present disclosure includes the steps of: preparing a base material having an electrode formed on a main surface thereof; covering the electrode with an under-bump layer of single-layer structure mainly containing Ni and containing 0.1 wt% or more and 3 wt% or less of B; and covering the under-bump layer with a bump layer mainly containing Sn.

[0008] A bonding structure according to one aspect of the present disclosure includes the bump structure described above.

[0009] In the above-described bump structure, method for manufacturing the bump structure, and bonding structure, an underbump layer mainly composed of Ni is interposed between the electrode and the bump layer, and since the underbump layer contains 0.1 wt% to 3 wt% of B, 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.

[0010] According to various aspects of this disclosure, a bump structure with improved joint strength, a method for manufacturing the bump structure, and a joint structure are provided.

[0011] 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 of the manufacturing method for the bump structure shown in Figure 1. This is a table showing the experimental results. This is a schematic cross-sectional view showing a joint structure equipped with the bump structure of Figure 1.

[0012] 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.

[0013] 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.

[0014] The electrode pad 12 is in the form of a film and has a thickness of, for example, 0.1 to 1 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, Al alloy, Au, or Au 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.

[0015] 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 opening 14a of the frame 14 is a perfect circle with a diameter of about 1 μm or more in plan view. The diameter of the opening 14a of the frame 14 is designed to be smaller than the diameter of the electrode pad 12, so that only a part of the surface of the electrode pad 12 is exposed through the opening 14a, rather than 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 an elliptical or polygonal shape with a width of about 1 μm or more 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.

[0016] The bump layer 20 integrally covers the electrode pad 12 and the frame 14. The bump layer 20 is substantially dome-shaped, with its upper surface curved in a convex manner. In this embodiment, the bump layer 20 is a perfect circle with a diameter of 15 μm or less (8 μm as an example) in plan view. The bump layer 20 may be elliptical or polygonal in plan view. If the bump layer 20 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 bump layer 20. Since the diameter of the bump layer 20 is larger than the diameter of the opening 14a of the frame 14, the bump layer 20 covers the upper surface 14b in the peripheral region of the opening 14a. The bump layer 20 may cover the entire peripheral region of the opening 14a or it may cover only a portion of it. By designing the bump layer 20 to have a diameter of 15 μm or less, the bump structure 20 as a whole can be miniaturized, thereby enabling a narrower pitch and higher density arrangement by bringing adjacent bump structures 1 closer together on the substrate 10.

[0017] 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 joining. The bump layer 20 can be formed by electroless plating, and in this case, compared to when it is formed by electrolytic plating, components in the plating solution are less likely to be mixed into the bump layer 20 as impurities, and high crystallinity can be achieved.

[0018] The bump layer 20 is covered by a coating layer 24. Specifically, the coating layer 24 covers the surface of the bump layer 20 (i.e., the convex curved surface). The coating layer 24 is made of a conductive material (metal or alloy), and may be made of a material mainly composed of Ag, Au, or Pd. In this embodiment, the coating layer 24 is made of Ag. 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 may be made of a single layer, or it may be made of multiple layers with different materials and compositions. The coating layer 24 can also function as an antioxidant film that suppresses oxidation of the surface of the bump layer 20.

[0019] 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, 0.1 to 2.5 μm (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 of the UBM layer 30 (the surface in contact with the electrode pad 12) to the point furthest from the main surface 10a of the substrate 10 in a direction perpendicular to the main surface 10a of the substrate 10.

[0020] The UBM layer 30 is made of a material mainly composed of Ni and contains a small amount of B (boron). The B content of the UBM layer 30 is between 0.1 wt% and 3 wt%. The UBM layer 30 may contain no P at all, or it may contain a small amount of P. 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 also in direct contact with the bump layer 20. The UBM layer 30 can be formed by electroless plating. In this case, compared to when it is formed by electrolytic plating, components in the plating solution are less likely to be mixed into the UBM layer 30 as impurities, and high crystallinity can be achieved.

[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 reducing agent containing boron, and a complexing agent is used. As a result, as shown in Figure 3(b), the surface of the electrode pad 12 within the opening 14a is directly covered by the UBM layer 30.

[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, for example, a high-temperature treatment in a nitrogen atmosphere, 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. At this time, as shown in Figure 3(c), the opening 14a is completely filled by the UBM layer 30 and the upper surface 14b in the peripheral region of the opening 14a is covered.

[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, a UBM layer 30 mainly composed of Ni is interposed between the electrode pad 12 and the bump layer 20, and the B content of the UBM layer 30 is designed to be between 0.1 wt% and 3 wt%. Therefore, even when the bump structure 1 is subjected to high temperatures such as reflow processing, a P-rich layer does not form between the UBM layer 30 and the bump layer 20, or if it does form, the thickness of the P-rich layer can be suppressed. Accordingly, the bump structure 1 and its manufacturing method described above suppress the decrease in bonding strength caused by the P-rich layer, and high bonding strength can be achieved.

[0028] If the boron content of the UBM layer 30 is less than 0.1 wt%, microvoids may form at the interface between the UBM layer 30 and the bump layer 20. If the boron content of the UBM layer 30 exceeds 3 wt%, Sn will not precipitate in the bump layer 20, or will precipitate less easily. Therefore, by setting the boron content of the Ni-based UBM layer 30 to 0.1 wt% or more and 3 wt% or less, the bump layer 20 can be formed on the UBM layer 30, achieving high bonding strength between the two layers. 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, further increasing the bonding strength.

[0029] 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 0.1 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 0.1 μm or more. If the thickness of the UBM layer 30 exceeds 2.5 μm, the bump layer 20 becomes larger (larger in diameter), hindering the miniaturization of the bump structure. Therefore, the thickness of the UBM layer 30 is designed to be 2.5 μm or less.

[0030] 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.

[0031] The inventors conducted the following experiments regarding the thickness of the underbump layer described above. For the experiments, several bump structures having the above-described configuration but with different underbump layer thicknesses were prepared. Specifically, sample 1 had an underbump layer thickness of 0.13 μm, sample 2 had an underbump layer thickness of 0.35 μm, sample 3 had an underbump layer thickness of 0.66 μm, and sample 4 had an underbump layer thickness of 1.79 μm. The underbump layers of samples 1 to 4 were all made of a material mainly composed of Ni and containing B (B content 0.3 wt%). For comparison, sample 5 was also prepared with an underbump layer thickness of 2.33 μm. The underbump layer of sample 5 was made of a material mainly composed of Ni and containing P (P content 9 wt%). Reflow treatment was then performed on each of the bump structures of samples 1 to 5, and the shear strength after reflow treatment was measured. The reflow process was performed under conditions of an oxygen concentration of 500 ppm, a temperature of 250°C, and a holding time of 30 seconds.

[0032] 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.

[0033] For the electroless plating forming the underbump layer of samples 1 to 4, BEL-24KM (60 mL / L), BEL-24KR (60 mL / L), BEL-24KA (60 mL / L), and BEL-24KE (10 mL / L) from Uemura Kogyo Co., Ltd. were used as plating solutions containing B as a reducing agent, with the pH adjusted to 6.5 and the plating carried out under conditions of a bath temperature of 65°C. For the electroless plating forming the underbump layer of sample 5, KSB-33M (200 mL / L), KSB-33A (50 mL / L), KSB-33B- (25 mL / L), and KSB-33D (5 mL / L) from Uemura Kogyo Co., Ltd. were used as plating solutions containing P as a reducing agent, with the pH adjusted to 4.7 and the plating carried out under conditions of a bath temperature of 70°C.

[0034] Each bump layer of samples 1 to 5 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.

[0035] Cross-sectional images of each of the samples 1 to 5 taken with a scanning electron microscope (SEM) are shown in Figure 4. When the presence or absence of a P-rich layer was observed from the cross-sectional images at 12,000x magnification, a P-rich layer was observed only in sample 5, while no P-rich layer was observed in samples 1 to 4.

[0036] Furthermore, the shear strength of samples 1 to 5 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 9.73 g, the average shear strength of sample 2 was 10.04 g, the average shear strength of sample 3 was 10.37 g, the average shear strength of sample 4 was 10.40 g, and the average shear strength of sample 5 was 4.36 g.

[0037] From the above results, it was confirmed that samples 1 to 4, in which the underbump layer was composed of a material mainly composed of Ni and containing B, and in which no P-rich layer was observed, obtained sufficiently high shear strength. Furthermore, it was confirmed that sufficiently high shear strength could be obtained when the thickness of the underbump layer was within the range of 0.1 to 2.5 μm. On the other hand, it was confirmed that sample 5, in which the underbump layer was composed of a material mainly composed of Ni and containing P, and in which a P-rich layer was observed, exhibited a relatively significantly lower shear strength.

[0038] Figure 5 shows a bump joint structure 100 comprising a pair of bump structures 1A and 1B similar to the bump structure 1 described above. In the bump joint structure 100, the first bump structure 1A provided on the first substrate 10A and the second bump structure 1B provided on the second substrate 10B are joined together and electrically connected to each other. The first substrate 10A may be, for example, an electronic component or a mounting substrate, and similarly, the second substrate 10B may also be an electronic component or a mounting substrate.

[0039] The first bump structure 1A and the second bump structure 1B can be joined by solid-phase diffusion bonding while facing each other. Solid-phase diffusion bonding is performed, for example, by pressing the first bump structure 1A and the second bump structure 1B against each other at a temperature below the melting point in a vacuum or an inert gas atmosphere. At this time, each bump layer 20 of the first bump structure 1A and the second bump structure 1B does not melt substantially, and their shape is substantially maintained before and after bonding. During solid-phase diffusion bonding, the coating layer 24 covering the surface of each bump layer 20 of the first bump structure 1A and the second bump structure 1B is fed out from the bonding interface 5, and at the bonding interface 5, the bump layer 20 of the first bump structure 1A and the bump layer 20 of the second bump structure 1B are in direct contact. Therefore, in the bump bonded structure 100, the bump layers 20 facing each other are joined at the bonding interface 5. The surface of the bump layer 20 other than the bonding interface 5 is covered by the coating layer 24 even after the first bump structure 1A and the second bump structure 1B are bonded together.

[0040] The joined first bump structure 1A and second bump structure 1B are integrally covered by a coating resin 102. The coating resin 102 has insulating properties and is composed of, for example, an epoxy resin or a polyurethane resin. If one of the base materials 10A and 10B is an electronic component and the other is a mounting substrate, the coating resin 102 may be an underfill material filled between the electronic component and the mounting substrate.

[0041] 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 0.1 wt% to 3 wt% of B. [Appendix 2] The bump structure according to Appendix 1, wherein the thickness of the underbump layer is in the range of 0.1 to 2.5 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 0.1 wt% to 3 wt% of B; 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. [Note 9] A bonding structure comprising the bump structure according to any one of Notes 1 to 4.

[0042] 1, 1A, 1B... Bump structure, 10, 10A, 10B... Substrate, 12... Electrode pad, 14... Frame, 20... Bump layer, 24... Coating layer, 30... Underbump layer (UBM layer), 100... Bump joint structure, S1 to 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 0.1 wt% to 3 wt% of B.

2. The bump structure according to claim 1, wherein the thickness of the underbump layer is in the range of 0.1 to 2.5 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 0.1 wt% to 3 wt% of B; 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.

9. A joint structure comprising the bump structure described in claim 1.

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