Bump structure

Abstract

In this bump structure and method for producing the same, whiskers generated on the surface of a bump layer can be suppressed by a coating layer covering the surface. Since the bump layer is formed by electroless plating, the bump layer itself contributes to the suppression of whisker generation. This is considered to be a result of the internal stress of the bump layer formed by electroless plating being ±0 kg / mm2 or being compressive stress.

Description

Bump structure

[0001] The present disclosure relates to a bump structure.

[0002] In a semiconductor device, a technique of providing a bump on an electrode pad for electrical connection with an external device is known. The following Patent Document 1 discloses a configuration in which a Sn-containing bump is provided on a conductive layer by electrolytic plating.

[0003] Japanese National Publication of International Patent Application No. 2007-506284

[0004] In the bump structure according to the above-described related art, whiskers may occur in Sn-containing bumps. The inventors have repeatedly conducted research on Sn-containing bumps and newly found a technique capable of suppressing whiskers.

[0005] An object of one aspect of the present disclosure is to provide a bump structure in which whiskers are suppressed.

[0006] A bump structure according to one aspect of the present disclosure includes a bump layer containing Sn as a main component and formed on an electrode, and a conductive coating layer covering a surface of the bump layer. In such a bump structure, whiskers generated on the surface of the bump layer are suppressed by the coating layer covering the surface.

[0007] According to various aspects of the present disclosure, a bump structure in which whiskers are suppressed is provided.

[0008] BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic sectional drawing which showed the bump structure which concerns on one Embodiment. It is the flowchart which showed the procedure which manufactures the bump structure shown in FIG. 1. It is the figure which showed the process of the manufacturing method of the bump structure shown in FIG. 1. It is a SEM photograph which showed the result of an experiment.

[0009] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and duplicate descriptions are omitted.

[0010] 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.

[0011] The electrode pad 12 is in the form of a film and has a thickness of, for example, 0.1 to 2 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.

[0012] 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 a perfect circle 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 elliptical or polygonal in plan view. If the opening 14a 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 opening 14a.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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 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 of different materials.

[0018] 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 can be made of a material mainly composed of Ni. In this case, the UBM layer 30 may contain a trace amount of P of about 5 wt%. The UBM layer 30 can also be made of a material mainly composed of Pd.

[0019] Next, the manufacturing method of the bump structure 1 described above will be explained with reference to Figures 2 and 3.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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. After electroless plating, a reflow process is performed. 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. During the reflow process, the Ag in the coating layer 24 can diffuse into the bump layer 20. In this case, the Ag in the coating layer 24 and the Sn in the bump layer 20 are alloyed.

[0025] Here, we will explain the whiskers that form in the bump layer 20.

[0026] Because the bump layer 20 is composed of pure Sn, whiskers are prone to forming on its surface. When forming the bump layer 20, the whiskers that form on the surface of the bump layer 20 can be temporarily removed by dissolving the coating by reflow. However, whiskers may still form thereafter depending on conditions related to the internal stress of the bump layer 20, conditions related to the external pressure applied to the bump layer 20 from the outside, and conditions related to temperature. Therefore, in the bump structure 1 and its manufacturing method described above, a coating layer 24 is provided to prevent whisker formation. That is, the coating layer 24 has at least the function of preventing whisker formation. The coating layer 24 can also function as an antioxidant film that suppresses oxidation of the surface of the main body portion 22 of the bump layer 20.

[0027] The inventors conducted the following experiment regarding whisker suppression by a coating layer. For the experiment, twelve bump structures with the above-described configuration, including a coating layer, were prepared as examples, and twelve bump structures differed from the above-described configuration only in that they lacked a coating layer, as comparative examples. For each of the examples and comparative examples, the time elapsed until whiskers appeared was measured under a temperature condition of 85°C. As a result, in the comparative example bump structure without a coating layer, whiskers appeared in at least one of the twelve bump structures before 250 hours had elapsed. Figure 4 shows an SEM image of the whiskers that appeared in the comparative example bump structure. On the other hand, in the example bump structure with a coating layer, no whiskers appeared even after 250 hours, and still no whiskers appeared even after more than 500 hours. From these results, it was confirmed that the coating layer is effective in suppressing whiskers.

[0028] Therefore, in the bump structure 1 and its manufacturing method, whiskers that form on the surface of the bump layer 20 can be suppressed by the coating layer 24 that covers the surface.

[0029] Furthermore, since the bump layer 20 is formed by electroless plating, the bump layer 20 itself contributes to suppressing whisker formation. This is because the bump layer 20 formed by electroless plating has an internal stress of ±0 kg / mm². ２ Alternatively, this is thought to be due to compressive stress. The inventors actually measured the internal stress of the bump layer 20 multiple times using a strip-type electrodeposited stress tester manufactured by Fuji Chemical Co., Ltd., and found that the internal stress was ±0 kg / mm ２ Alternatively, it was confirmed that the stress becomes compressive (for example, 1.5 MPa).

[0030] Furthermore, 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] Furthermore, in bump structure 1, the UBM layer 30 is interposed between the electrode pad 12 and the bump layer 20. 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, but 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.

[0032] 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.

[0033] 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, with Sn as the main component, and a coating layer having conductivity and covering the surface of the bump layer. [Appendix 2] The bump structure according to Appendix 1, wherein the coating layer has Ag, Au, or Pd as the main component. [Appendix 3] The bump structure according to Appendix 1 or 2, wherein the coating layer is composed of multiple layers. [Appendix 4] The bump structure according to any one of Appendix 1 to 3, wherein the thickness of the coating layer is 0.05 μm or more and 0.2 μm or less. [Appendix 5] The bump structure according to any one of Appendix 1 to 4, wherein the thickness of the bump layer is 2 μm or more.

[0034] 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, with Sn as the main component, and a coating layer that is conductive and covers the surface of the bump layer.

2. The bump structure according to claim 1, wherein the coating layer is mainly composed of Ag, Au, or Pd.

3. The bump structure according to claim 1, wherein the coating layer is composed of multiple layers.

4. The bump structure according to claim 1, wherein the thickness of the coating layer is 0.05 μm or more and 0.2 μm or less.

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

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