Bump structure and production method therefor

Abstract

In this bump structure and production method therefor, a UBM layer is interposed between an electrode pad and a bump layer. The bump layer is mainly composed of Sn, which easily diffuses to the electrode pad. However, the UBM layer functions as a diffusion prevention layer so as to suppress such Sn diffusion. As a result, a bump structure having high dimensional accuracy 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] In a semiconductor device, a technique of providing a bump on an electrode pad for achieving electrical connection with an external device is known. Patent Document 1 below discloses a configuration in which a bump formed by electroless plating using Ni and Au is provided on an Al electrode pad.

[0003] Japanese Unexamined Patent Publication No. 2010-67711

[0004] The inventors have conducted repeated research on Sn bumps, and obtained the finding that it is difficult to form Sn bumps of desired dimensions on electrode pads because Sn easily diffuses into electrode pads. The inventors have further conducted research and newly found a technique capable of forming Sn bumps of desired dimensions on electrode pads.

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

[0006] A bump structure according to one aspect of the present disclosure comprises: a bump layer containing Sn as a main component and formed on an electrode; and an under-bump layer containing Pd as a main component 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 containing Pd as a main component; and covering the under-bump layer with a bump layer containing Sn as a main component.

[0008] In the above bump structure and the method for manufacturing the same, since the under-bump layer containing Pd as a main component is interposed between the electrode and the bump layer, diffusion of Sn, which is the main component of the bump layer, into the electrode is suppressed, whereby a bump structure having high dimensional accuracy can be obtained.

[0009] According to various aspects of the present disclosure, there are provided a bump structure with improved dimensional accuracy and a method for manufacturing the same.

[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 of the manufacturing method for the bump structure shown in Figure 1. This is an SEM image showing the experimental results. This is an SEM image showing 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 3 μm or less. The UBM layer 30 is composed of a material mainly composed of Pd. In this embodiment, the UBM layer 30 is substantially composed of pure Pd. The UBM layer 30 may contain a trace amount of P of about 3 wt%.

[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, 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 palladium 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 UBM layer 30 is interposed between the electrode pad 12 and the bump layer 20. Although Sn, the main component of the bump layer 20, tends to diffuse into the electrode pad 12 under high temperatures such as those in the reflow process described above, the UBM layer 30 functions as a diffusion prevention layer, suppressing such Sn diffusion. As a result, erosion of the electrode pad 12 due to Sn diffusion is less likely to occur, and a bump structure 1 with high dimensional accuracy can be realized.

[0028] The UBM layer 30 is not limited to being composed of pure Pd; any material containing 90 wt% or more of Pd as the main component can function as a diffusion barrier layer. In this case, the UBM layer 30 may contain trace amounts of P as additives (sub-components). A thickness of 0.1 μm or more of the UBM layer 30 can further suppress Sn diffusion, and a thickness of 1.0 μm or more can almost completely suppress Sn diffusion. Since increasing the thickness of the UBM layer 30 leads to increased costs, the thickness of the UBM layer 30 can be kept to 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 diffusion of Sn. As examples, several bump structures having the above-described configuration but with different underbump layer thicknesses were prepared. Specifically, four types of bump structures (samples 1 to 4) were prepared: Sample 1 with an underbump layer thickness of 0.1 μm, Sample 2 with an underbump layer thickness of 0.25 μm, Sample 3 with an underbump layer thickness of 0.5 μm, and Sample 4 with an underbump layer thickness of 1.0 μm. In addition, a bump structure differing only from the above-described configuration in that it lacked an underbump layer was prepared as a comparative example. Then, reflow processing was performed on each of the examples (samples 1 to 4) and the comparative example, and it was confirmed whether the Sn in the bump layer had diffused into the electrode pad after the reflow processing.

[0031] 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. In the experiment, before forming the underbump layer, surface modification by excimer irradiation, degreasing, and acid cleaning were performed as pretreatments. For the 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 the degreasing, a 100 mL / L concentration degreasing agent was used under conditions of pH < 1 and 5 minutes. For the acid cleaning, an acidic solvent with a concentration of 10 mL / L (18 g / L concentration) was used under conditions of pH < 1 and 1 minute.

[0032] The underbump layer was formed using a plating solution containing palladium at a concentration of 1 g / L under conditions of pH 5.8, 55°C, and 20 minutes. More specifically, the underbump layer was formed using Altarea TPD-23 electroless palladium plating solution manufactured by Uemura Kogyo Co., Ltd., with a mixture of TPD-23M 100 mL / L, TPD-23-B 80 mL / L, TPD-23-C 15 mL / L, 2 mol / L hydrochloric acid 70 mL / L, and palladium at 1 g / L, under conditions of pH 5.8, 55°C, and 20 minutes. The thickness of the underbump layer was adjusted by the plating time. As an example, the electroless Sn plating that forms the bump layer was formed to a thickness of 4 μm under the conditions of pH 8.5, 70°C, and 60 minutes, using a bath composition 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.

[0033] The experimental results were observed using a scanning electron microscope (SEM) and are shown in the perspective view (10,000x magnification) and cross-sectional view (12,000x magnification) in Figure 4. When Sn from the bump layer diffuses into the electrode pad, an intermetallic compound (IMC) phase is formed near the upper surface of the electrode pad. The IMC is composed of Sn and Cu (or Sn, Cu, and Pd). Since the formation of IMC was suppressed to some extent in all of the samples 1 to 4 of the example, it was recognized that the diffusion of Sn from the bump layer into the electrode pad was suppressed. More specifically, it was confirmed that the suppression of IMC increased in the order of samples 1 to 4, that is, as the thickness of the underbump layer increased. In particular, in sample 4, where the thickness of the underbump layer was 1.0 μm, the Sn from the bump layer did not diffuse into the electrode pad at all. In this case, no IMC was observed in the cross-sectional SEM image, and a straight, clear boundary appeared between the electrode pad and the underbump layer. On the other hand, in the comparative example without an underbump layer, as shown in the SEM cross-section in Figure 5, the Sn in the bump layer diffused into the electrode pad, and the formation of IMC near the upper surface of the electrode pad was confirmed.

[0034] 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.

[0035] 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 an underbump layer interposed between the electrode and the bump layer, mainly composed of Pd. [Appendix 2] The bump structure according to Appendix 1, wherein the thickness of the underbump layer is 1 μm or more. [Appendix 3] The bump structure according to Appendix 1 or 2, wherein the underbump layer contains 90 wt% or more of Pd and contains P as an additive. [Appendix 4] The bump structure according to any one of Appendixes 1 to 3, wherein the thickness of the bump layer is 2 μm or more. [Appendix 5] The bump structure according to any one of Appendixes 1 to 4, wherein the width of the bump layer is 10 μm or less. [Appendix 6] The bump structure according to any one of Appendixes 1 to 5, further comprising a coating layer covering the surface of the bump layer, mainly composed of Ag or Au. [Note 7] 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 an underbump layer mainly composed of Pd; and covering the underbump layer with a bump layer mainly composed of Sn. [Note 8] The method for manufacturing a bump structure according to Note 7, wherein in the step of covering with the underbump layer, the underbump layer is formed by electroless plating. [Note 9] The method for manufacturing a bump structure according to Note 7 or 8, wherein in the step of covering with the bump layer, the bump layer is formed by electroless plating. [Note 10] The method for manufacturing a bump structure according to any one of Notes 7 to 9, further comprising the step of covering the bump layer with a coating layer mainly composed of Ag, Au, or Pd.

[0036] 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 an underbump layer interposed between the electrode and the bump layer, mainly composed of Pd.

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

3. The bump structure according to claim 1, wherein the underbump layer contains 90 wt% or more of Pd and contains P as an additive.

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

5. The bump structure according to claim 1, wherein the width of the bump layer is 10 μm or less.

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

7. 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 an underbump layer mainly composed of Pd; and covering the underbump layer with a bump layer mainly composed of Sn.

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

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

10. The method for manufacturing a bump structure according to claim 7, further comprising the step of covering the bump layer with a coating layer mainly composed of Ag, Au, or Pd.

Images