JP2016178217A - Method of manufacturing bump electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a bump electrode with high junction reliability by preventing a void from being generated caused by residual air bubbles near an interface of a base metal layer.SOLUTION: A base metal layer 6 is formed which includes a nickel metal layer 8 formed from nickel or a nickel alloy and a copper thin film layer 9 formed on a surface of the nickel metal layer 8 and having thickness equal to or greater than 0.1 μm and equal to or smaller than 0.5 μm and formed from copper or a copper alloy. After a solder plating layer 13 is formed on the copper thin film layer 9 of the base metal layer 6, the solder plating layer 13 is molten, thereby forming solder bump 10.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method of manufacturing a bump electrode used for connecting a semiconductor device to a substrate by flip chip mounting or the like.

In recent years, with the rapid progress of the network information society, flip chip mounting has become widespread as high-density mounting corresponding to high functionality and downsizing of semiconductor devices. In this flip chip mounting, bump electrodes provided on a substrate for connecting semiconductor devices are plated with solder on a bump forming base metal layer (Under Bump Metal) formed on the substrate in the case of plating. It is formed by reflow processing.
A problem with this type of bump electrode is that voids called voids may be formed inside the bumps after the reflow process. If such voids are generated, the bonding reliability is lowered.

Conventionally, the following techniques have been proposed to prevent the generation of voids.
In Patent Document 1, a convex portion is formed on the surface of an electrode (underlying metal layer), and bubbles are raised from the interface between the solder bump and the electrode by rising along the side surface of the convex portion by buoyancy in the reflow process. Is described as preventing the generation of voids.
In Patent Document 2, the solder bump is first heated and melted in a reduced-pressure atmosphere, then heated in a pressurized atmosphere pressurized more than the reduced-pressure atmosphere, and then cooled while substantially maintaining the pressurized atmosphere. Thus, it is disclosed that the solder bump is solidified. According to this, even if a void exists in the solder bump, the void volume shrinks to (pressure in a reduced pressure atmosphere) / (pressure in a pressurized atmosphere), so that the void is substantially eliminated. It is stated that you can.

Patent Document 3 discloses that a zinc diffusion prevention layer containing one of tin and nickel is formed on an electrode containing copper, and a solder bump made of a tin-zinc solder alloy is formed thereon. In addition, it is described that zinc diffusion is prevented by a zinc diffusion preventing layer to prevent formation of a brittle copper-zinc intermetallic compound layer and formation of voids.
Patent Document 4 discloses that when a void moves inside the molten solder and reaches the surface, the oxide film on the solder surface is removed and the void is released by setting the reflow atmosphere as a reducing atmosphere. In other words, it is described that the solder is prevented from solidifying while the void is included.

JP 2005-353915 A Japanese Patent Laid-Open No. 10-163213 JP 2001-298051 A JP 2009-81398 A

  Although the methods described in these patent documents are effective for removing bubbles inside the solder layer, there still remains a problem that bubbles slightly remain in the vicinity of the interface with the base metal layer. In particular, it is remarkable when the base metal layer is made of copper, and further improvement has been desired.

  The present invention has been made in view of such circumstances, and an object thereof is to produce a bump electrode with high bonding reliability by preventing generation of voids due to residual bubbles in the vicinity of the interface of the base metal layer. To do.

  The bump electrode manufacturing method of the present invention includes a nickel metal layer made of nickel or a nickel alloy, and a copper or copper alloy formed on the surface of the nickel metal layer with a thickness of 0.1 μm to 0.5 μm. A base metal layer having a thin film layer is formed, a solder plating layer is formed on the copper thin film layer of the base metal layer, and then the solder plating layer is melted to form solder bumps.

  Generally, copper having high electrical conductivity is used as the base metal layer. When a solder plating layer is formed on the copper and a reflow process is performed, a part of the copper diffuses into the molten solder. The copper and tin in the solder easily form an intermetallic compound. When the molten solder solidifies, the copper and tin are intermetallic compound (copper tin alloy) at the interface between the base metal layer and the solder plating layer. ) Layer. Since this intermetallic compound layer is formed in a fine concavo-convex shape, bubbles generated in the molten solder are attached so as to be trapped in the concavo-convexities of the intermetallic compound during solidification, which causes voids near the interface. Become.

  In the present invention, at least the surface portion of the base metal layer has a two-layer structure consisting of a nickel metal layer made of nickel or a nickel alloy and a copper thin film layer formed thinly thereon, thereby melting the solder plating layer. In this case, even if the copper in the copper thin film layer diffuses into the molten solder and an intermetallic compound is formed between copper and tin, the intermetallic compound is grounded because the copper thin film layer is kept as a thin layer. Without being deposited in layers on the surface of the metal layer, it is easy to peel off from the underlying metal layer. Therefore, even if bubbles are generated, the molten solder can be levitated and discharged without being stuck to the interface.

In this case, when the thickness of the copper thin film layer is less than 0.1 μm, the effect cannot be sufficiently exerted, and conversely, all diffuses during the reflow process, and bubbles are exposed in the exposed nickel metal layer. May be adsorbed. On the other hand, if the thickness of the copper thin film layer exceeds 0.5 μm, the solidified intermetallic compound layer becomes thicker and the unevenness increases, so that bubbles remain on the underlying metal layer even after reflow treatment. Easy to do.
Note that after the formation of the solder bumps, the copper thin film layer almost disappears, but it may remain with a very small thickness.
Further, as the base metal layer, a copper metal layer made of copper or a copper alloy similar to the conventional copper metal layer may be formed under the nickel metal layer, and at least the surface portion of the base metal layer has a nickel metal layer and a copper thin film. What is necessary is just to be formed with the layer.

  The present invention also provides an intermediate for forming a bump electrode in which a solder plating layer is formed on a base metal layer, the base metal layer comprising a nickel metal layer made of nickel or a nickel alloy, and the nickel metal A copper thin film layer made of copper or a copper alloy formed with a thickness of 0.1 μm or more and 0.5 μm or less on the surface of the layer, and the solder plating layer is formed on the copper thin film layer.

  By reflowing the bump electrode forming intermediate, a good bump electrode without voids can be formed on the base metal layer.

  According to the present invention, it is possible to prevent the generation of voids due to the remaining of bubbles in the vicinity of the interface of the base metal layer, and to manufacture a bump electrode with high bonding reliability.

It is sectional drawing which shows the bump electrode to which the method of this invention is applied. It is sectional drawing which showed the process of forming the bump electrode of FIG. 1 in order of (a)-(d).

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows a bump electrode to which the method of the present invention is applied. A bump electrode 3 is formed on an electrode pad 2 of a substrate 1.
The substrate 1 has a circuit layer, an insulating layer and the like formed on the surface of a silicon wafer 4. In FIG. 1, an electrode pad 2 is laminated on the surface of the silicon wafer 4, except for the central portion of the pad 2, An insulating layer 5 is formed on the surface of the silicon wafer 4.

The central part of the electrode pad 2 is not covered with the insulating layer 5, and a solder bump 10 is formed on the central part of the electrode pad 2 through a base metal layer (Under Bump Metal) 6 in a solder shape. Thus, the bump electrode 3 is configured. In the illustrated example, the base metal layer 6 is formed in a dish shape in which the central portion is recessed downward, but the shape is not limited to a dish shape, and may be a flat plate shape or the like. In the example shown in FIG. 2, the base metal layer 6 includes a copper metal layer 7 made of copper or a copper alloy, and nickel metal made of nickel or a nickel alloy (pure nickel is preferable) formed on the copper metal layer 7. The layer 8 has a three-layer structure including a copper thin film layer 9 made of copper or a copper alloy (pure copper is preferable) formed on the nickel metal layer 8 with a slight thickness. It will be described later.
Moreover, as a solder material used as the solder bump 10, Sn-Ag alloy, Pb-Sn alloy, Sn-Bi alloy, Sn-Zn alloy, Sn-Sb alloy, Sn-Cu alloy, Sn-Ag-Cu alloy, etc. An Sn-based alloy composed of Sn and an additive component is suitable.

Next, a method of manufacturing the bump electrode 3 configured as described above on the substrate 1 will be described in the order of steps shown in FIG.
(Copper metal layer formation process)
The substrate 1 having the electrode pad 2 and the insulating layer 5 formed on the surface of the silicon wafer 4 is formed in advance, and a copper metal layer 7a made of copper or a copper alloy is formed so as to cover the surface. In this case, the insulating layer 5 avoids the central portion of the electrode pad 2 and covers only the peripheral portion, and the copper metal layer 7 a formed thereon is formed in a bonded state with the central portion of the electrode pad 2. The copper metal layer 7a is generally formed by sputtering.

(Resist layer formation process)
Next, as shown in FIG. 2A, a resist layer 11 is formed on the copper metal layer 7 a of the substrate 1, and the resist layer 11 is exposed and developed so that the resist layer 11 is exposed above the electrode pad 2. An opening (pattern part) 12 is formed in a state where the upper surface of the copper metal layer 7a is exposed. The inner diameter of the opening 12 of the resist layer 11 is set corresponding to the outer diameter of the obtained bump electrode 3.

(Nickel metal layer, copper thin film layer formation process)
The substrate 1 is immersed in a nickel plating tank (not shown), the copper metal layer 7a is energized, and the nickel metal layer 8a is formed on the copper metal layer 7a exposed from the opening 12 of the resist layer 11 by electrolytic plating. Form. Next, a thin copper film layer 9a is formed on the nickel metal layer 8a so as to have a thickness of 0.1 μm to 0.5 μm (see FIG. 2B). When the copper thin film layer 9a is formed by electrolytic plating, for example, a copper sulfate bath mainly composed of copper sulfate (CuSO ₄ ) and sulfuric acid (H ₂ SO ₄ ) can be used. The temperature of the plating bath is 20 ° C. or more and 50 ° C. or less, and the current density is 1 A / dm ² or more and 20 A / dm ² or less.
Further, instead of electrolytic plating, the copper thin film layer 9a may be formed by a method such as sputtering. In the case of sputtering, a copper thin film layer is also formed on the resist layer.

(Solder plating process)
Next, the substrate 1 is immersed in a solder plating tank (not shown), and the copper metal layer 7a is energized to perform solder plating on the copper thin film layer 9a on the electrode pad 2 by electrolytic plating. As shown in 2 (c), the inside of the opening 12 of the resist layer 11 is filled with the solder plating layer 13.
The plating solution used for forming the solder plating layer 13 is, for example, the following composition in the Sn-Ag alloy plating solution.
Free acid (eg alkylsulfonic acid); 80-350 g / L
Sn ²⁺ ; 40-95 g / L
Ag ⁺ ; 0.1-3.0 g / L
Complexing agent; 140 to 300 g / L
Additive; 30-80ml / L

As electrolytic plating conditions, the bath temperature of the plating tank is set to 30 ° C., for example, and the current density is 3 A / dm ^{2 and} the amount of electrolysis is about 120 A · min.

(Resist layer removal and etching process)
Next, the resist layer 11 is dissolved and removed with a resist layer stripping solution to expose the solder plating layer 13, and then unnecessary portions of the copper metal layer 7a covered with the resist layer 11 are removed by etching, and solder plating is performed. Under the layer 13, a base metal layer 6 having a three-layer structure of a copper metal layer 7, a nickel metal layer 8, and a copper thin film layer 9 is formed. By this step, as shown in FIG. 2D, a bump electrode forming intermediate 20 provided in a state where the base metal layer 6 and the solder plating layer 13 are laminated on the electrode pad 2 is obtained. It is done.

(Reflow process)
Next, a reflow process for heating and melting the solder plating layer 13 is performed. As the reflow treatment, heating is performed at 230 ° C. to 250 ° C. for several tens of seconds in a nitrogen atmosphere, a low oxygen atmosphere, or a reducing atmosphere.

By this reflow process, the solder plating layer 13 is melted, and the copper of the copper thin film layer 9 is diffused into the molten solder. Also, the molten solder is rounded into a ball shape due to surface tension, and then cooled to solidify in a ball shape. At this time, an intermetallic compound (mainly Cu ₆ Sn ₅ ) is formed between copper diffused from the copper thin film layer 9 and solder tin, and this intermetallic compound is in the vicinity of the interface with the remaining copper thin film layer 9. As the copper in the copper thin film layer 9 diffuses, the thin copper thin film layer 9 disappears from the surface portion of the base metal layer 6, so that the intermetallic compound layer is near the interface. Therefore, even if bubbles are generated in the molten solder, they are floated and discharged from the surface without staying at the interface.

As shown in FIG. 1, a bump electrode 10 in which a solder bump 3 is formed on the surface of the base metal layer 6 is formed on the electrode pad 2 of the substrate 1. In this case, by setting the film thickness of the copper thin film layer 9 appropriately, it diffuses and disappears completely during the reflow process. However, as shown in FIG. The copper thin film layer 9 may be formed.
In this reflow treatment step, the temperature rise to the reflow treatment temperature (230 ° C. to 250 ° C.) may be heated so as to have a temperature profile of two or more stages, and until the solder melting temperature is reached, Including those with pre-heat treatment that is held for a predetermined time at a temperature lower than the solder melting temperature.

In the bump electrode 3 formed in this way, solder bumps 10 are formed in a spherical shape on the base metal layer 6.
As described above, in the reflow processing step, since the remaining of bubbles near the interface of the base metal layer 6 is suppressed, the solder bumps 10 free from voids are formed, and high bonding reliability is obtained in component mounting. it can.

A tin-silver (Sn-Ag) alloy is used as a solder alloy, a resist layer formed of an organic film is formed on the copper metal layer, and nickel is formed on the copper metal layer in the opening (pattern part) of the resist layer. Bump electrodes having a height of 40 μm were formed on the base metal layer having a diameter of 110 μm by performing electrolytic plating of pure nickel and pure copper as the metal layer and the copper thin film layer and further performing solder plating. At that time, the thicknesses of the nickel metal layer and the copper thin film layer were changed as shown in the examples to form bump electrodes. Next, after stripping the resist, unnecessary portions of the copper metal layer that do not serve as a base were removed by etching. The reflow process was performed at 240 ° C. for 60 seconds in a nitrogen atmosphere.
In addition, the laminated structure of the nickel metal layer and the copper thin film layer was reversed, and a nickel metal layer formed on the copper thin film layer was also produced. For reference, a copper thin film layer was not formed, and only a nickel metal layer was formed on the copper metal layer.

About the obtained sample, the average void area ratio was measured.
The void area ratio was calculated as the ratio of the sum of the cross-sectional areas of the voids to the cross-sectional area of the solder bumps in the observation field of view, by observing the cross-section of the solder bumps. The void area ratio is an average value of 100 bump electrodes.
The results are shown in Table 1.

As shown in Table 1, as the surface portion of the base metal layer, a copper thin film layer formed on a nickel metal layer with a thickness of 0.1 μm to 0.5 μm has a small average void area ratio and reduced void generation. It turns out that there is an effect.
As described above, by forming the copper thin film layer with an appropriate thickness on the nickel metal layer, generation of voids can be reduced.

  In addition, this invention is not limited to the said embodiment, A various change can be added in the range which does not deviate from the meaning of this invention.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Electrode pad 3 Bump electrode 4 Silicon wafer 5 Insulating layer 6 Base metal layer 7 Copper metal layer 8 Nickel metal layer 9 Copper thin film layer 10 Solder bump 11 Resist layer 12 Opening part 13 Solder plating layer 20 Intermediate body for bump electrode formation

Claims (2)

  A base metal layer having a nickel metal layer made of nickel or a nickel alloy and a copper thin film layer made of copper or a copper alloy formed on the surface of the nickel metal layer with a thickness of 0.1 μm to 0.5 μm is formed. A method for manufacturing a bump electrode, comprising: forming a solder plating layer on the copper thin film layer of the base metal layer, and then melting the solder plating layer to form a solder bump. A bump electrode forming intermediate in which a solder plating layer is formed on a base metal layer, wherein the base metal layer has a nickel metal layer made of nickel or a nickel alloy and a thickness on the surface of the nickel metal layer. And a copper thin film layer made of copper or a copper alloy formed with a thickness of 0.1 μm or more and 0.5 μm or less, and the solder plating layer is formed on the copper thin film layer Forming intermediate.

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