JP2003096573A - Method for deposition of electroless plating film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for depositing an electroless plating film of uniform film thickness onto pad electrodes of a semiconductor substrate without applying plating resist to the back and sides of the semiconductor substrate. SOLUTION: A zinc immersion film 23 is deposited by using a zinc immersion process onto the pad electrode 21. After washed with pure water, the semiconductor substrate 15 as a substrate to be plated is immersed in an oxidation- reduction type electroless nickel plating solution using a nickel-plated stainless- steel bar as a counter electrode 12 and using this counter electrode 12 as a negative electrode while applying a minute positive potential to the semiconductor substrate 15. By this method, a bump 16 composed of an electroless nickel plating film of uniform film thickness can be formed on a plurality of pieces of the pad electrode 21 while obviating the necessity of the application of plating-resist to the back and sides of the semiconductor substrate 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an electroless plating film for selectively and uniformly depositing a plating metal on a pad electrode of a semiconductor substrate.

[0002]

2. Description of the Related Art When a barrier metal or a bump electrode is to be formed by electroless plating on a pad electrode composed mainly of aluminum formed on the active element surface of a semiconductor substrate, the pad electrode is selectively formed on the pad electrode. In addition, it is required that the electroless plating reaction be uniformly started and continued for a plurality of pad electrodes on the semiconductor substrate.

Therefore, in order to selectively and uniformly form an electroless plating film on the pad electrode, a method of activating zinc plating treatment or applying a catalyst such as palladium only on the pad electrode is used. However, a method of forming a protruding electrode using the zinc substitution method will be described below with reference to FIG.

3A to 3E are process sectional views showing a conventional method for forming electrodes on a semiconductor substrate. Although this process diagram shows a method of manufacturing one electrode on a semiconductor substrate, it should be understood that in this process, many electrodes can be formed at one time. First, as shown in FIG. 3A, a pad electrode 21 containing aluminum as a main component of the semiconductor substrate 15 and a passivation film 22 having an opening on the pad electrode are formed. Since a natural oxide film is formed in the opening immediately after being exposed to the atmosphere, the natural oxide film is removed by etching by immersing the natural oxide film in an acid solution or an alkaline solution. Next, as shown in FIG. 3B, a plating resist 31 is applied to the back surface and side surfaces of the semiconductor substrate 15 by spin coating. This is to prevent the plating reaction on the back and side surfaces of the semiconductor substrate.

Next, the semiconductor substrate 15 coated with the plating resist is dipped in an alkaline or weakly acidic zincate aqueous solution to remove the natural oxide film, as shown in FIG. 3 (c). 21 Zinc substitution film 2 on the surface
3 is deposited by substitution. If necessary, the zinc substitution film 23 may be immersed in dilute nitric acid or the like to remove the deposited zinc substitution film 23 by etching, and then again immersed in the zincate aqueous solution to form a second zinc substitution film. This is a method for obtaining a more uniform and dense zinc substitution coating 23, and the zinc substitution coating etching step and the zinc substitution step may be further repeated if necessary.

Next, the semiconductor substrate 15 is dipped in an oxidation-reduction type electroless plating solution to form a barrier metal or a bump electrode with a plating metal. Here, the case of using electroless nickel plating will be described. To do. As shown in FIG. 3D, in the case of the zinc substitution method, when the semiconductor substrate 15 is immersed in an electroless nickel plating solution, the zinc substitution film 23 is dissolved and substitution precipitation of nickel occurs near the surface of the pad electrode 21. After that, the electroless plating reaction is continued by the autocatalytic action of nickel, and the protruding electrodes 16 can be formed as shown in FIG.

Finally, the semiconductor substrate 15 is dipped in a resist stripping solution, and as shown in FIG.
The plating resist 31 applied to the back surface and the side surface of 5 is peeled off.

[0008]

SUMMARY OF THE INVENTION In the conventional method,
When the semiconductor substrate 15 is immersed in the electroless plating solution without applying the plating resist to the back surface and the side surfaces of the semiconductor substrate 15, the back surface and the back surface of the semiconductor substrate 15 made of silicon or the like which is not activated by the electroless plating solution and On the side surface, the electroless plating reaction should not occur originally, but the residue of zinc and the suspended matter in the electroless plating solution are the core when the washing with water after the zinc substitution treatment is insufficient, Electroless plating reaction may occur on the back and side surfaces of the semiconductor substrate 15. However, since the back surface and the side surface of the semiconductor substrate 15 are not activated, a uniform plating film cannot be formed, and a portion where plating is deposited and a portion where plating is not adhered occur.

At the site where the electroless plating reaction occurs on the back surface and the side surface of the semiconductor substrate 15, a partial reduction in potential occurs due to the redox reaction of the electroless plating, and as a result, the variation in potential across the semiconductor substrate 15 occurs. Will be caused. This variation in potential also affects the plurality of pad electrodes 21 of the semiconductor substrate 15, resulting in non-uniform thickness of the electroless plating film on the plurality of pad electrodes 21.
A defect such as non-adhesion of plating to some of the pad electrodes 21 occurred, which was a factor of impairing the selectivity to the pad electrodes 21.

Even if the semiconductor substrate 15 is coated with plating resist on the back and side surfaces of the semiconductor substrate 15, internal wiring (not shown) formed under the passivation film 22 on the active element surface of the semiconductor substrate 15 is not shown. ), The electrons generated in the pad electrode 21 may move and cause a potential difference with other pad electrodes.
Due to this effect, the pad electrode 21 is negatively charged, but since metal plating is not formed on this pad electrode, this also causes non-uniformity of the film thickness of the electroless plating film on the pad electrode 21. This causes a defect such as non-adhesion of plating to the pad electrode 21 of the portion.

Further, the electroless plating film has a semiconductor substrate 1
When grown on the back surface of the semiconductor substrate 5, the plating reaction also propagates from the back surface of the semiconductor substrate 15 to the passivation film 22 on the active element surface of the semiconductor substrate 15 via the side surface. Was a factor that undermines.

[0012] For the above-mentioned reason, in the conventional method, it is indispensable to apply the plating resist 31 to the back surface and the side surface of the semiconductor substrate 15, and furthermore, the plating resist 31 requires a peeling step after the plating treatment.

Therefore, an object of the present invention is to improve the above-mentioned problems of the prior art, and to form a uniform electroless plating film on a plurality of pad electrodes of a semiconductor substrate without applying a plating resist on the back and side surfaces of the semiconductor substrate. Is provided.

[0014]

In order to solve the above problems, in the present invention, the following electroless plating film forming method is adopted.

That is, the first means for solving the above problems in the present invention is a method of forming an electroless plating film on a pad electrode of a semiconductor substrate, and the pad electrode is selectively activated. And a step of immersing the semiconductor substrate in an electroless plating solution and simultaneously depositing an electroless plating film while applying a potential to the semiconductor substrate by an external power source. Further, it is desirable that the activation step in the first means is either a catalyst application method or a zinc substitution method. In the first or second means, in the step of depositing the electroless plating film while applying a potential to the semiconductor substrate, the applied potential is used for the redox potential of the plating metal ion and the electroless plating solution. It is desirable that the positive potential is less than the difference from the redox potential of the reducing agent.

(Function) In the method of forming an electroless plating film of the present invention, a positive potential is applied from an external power source to a semiconductor substrate whose plating reaction is to be partially suppressed, so that the electroless plating film is partially formed. It is a method of selectively forming an electroless plating film only on a necessary portion while suppressing the formation.

As an example, redox electroless nickel plating using hypophosphite as a reducing agent will be described. This electroless nickel plating has a redox potential of -0.80 V (pH 5.0, 25 ° C, NHE standard) of hypophosphite ion and a -0.25 V (pH 5) of nickel ion (Ni2 +). 0.0, 25 ° C, NHE standard) becomes the electromotive force, and the electroless plating reaction starts and continues. If it is a minute potential within the range that does not impair this electromotive force,
Even if the semiconductor substrate is used as a positive electrode and a potential opposite to the plating reaction is applied from an external power source, the electroless plating reaction can be continued.

Further, the internal wiring formed inside the semiconductor substrate may cause a potential difference on the surface of a plurality of pad electrodes. However, by applying a minute potential from an external power source to the entire semiconductor substrate, a plurality of potential differences can be produced. The potential of the pad electrode surface can be made uniform, and an electroless plating film having a uniform film thickness can be obtained.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION A method for forming an electroless plating film according to the present invention is a method in which a semiconductor substrate and an insoluble electrode serving as a counter electrode thereof are immersed in an electroless plating solution to form an external layer between the semiconductor substrate and the insoluble electrode. This is a method of forming an electroless plating film while applying a constant potential difference from a power source for a fixed period of time.

A method for forming electrodes on a semiconductor substrate by the method for forming an electroless plating film according to the present invention will be described below with reference to the drawings.

As shown in FIG. 2A, a pad electrode 21 containing aluminum as a main component and a passivation film 22 having an opening on the pad electrode 21 are formed on the semiconductor substrate 15 as in the conventional manufacturing method. Form. Further, the natural oxide film formed on the pad electrode 21 is removed by etching with an acid solution or an alkaline solution. As the etching solution, a dilute solution of sodium hydroxide or dilute sulfuric acid can be used. It is preferable to use nitric acid in order to remove the oxide film and form a thin passive film.

Next, as shown in FIG. 2B, a zinc substitution film 23 is deposited by substitution on the pad electrode 21 from which the oxide film has been removed by the zinc substitution method. In this zinc substitution method, zinc is deposited only on the site where aluminum is eluted due to the ionization tendency, so that the zinc substitution film can be selectively formed on the pad electrode 21 containing aluminum as a main component. Specifically, it is immersed at room temperature for 30 to 60 seconds using a commercially available zincate treatment liquid. Then, after washing with pure water, if necessary, further dipping in dilute nitric acid or the like to remove the zinc substitution film 23 once deposited by etching, and then dipping in the zincate treatment liquid again to deposit a second zinc substitution film. May be.

Then, after washing with pure water, as shown in FIG. 1, the semiconductor substrate 15 is immersed in the redox electroless nickel plating solution 14 while applying a DC potential from the external power source 11 to the back side thereof. However, in the present embodiment, the counter electrode 12 is made of a stainless steel rod plated with nickel, and the counter electrode 12 is used as a negative electrode to apply a positive potential of +0.1 to 0.2 V to the semiconductor substrate 15, which is the substrate to be plated. By immersing in electroless nickel plating solution 14 while applying
The protruding electrodes 16 were formed by electroless nickel plating as shown in FIG. 2 (c).

Although the plating film formation rate is slower than that of the conventional method, the method of the present invention can form a uniform plating film on a plurality of pad electrodes.
Although nickel plating is deposited on the counter electrode 12, the reaction does not affect the electroless nickel plating on the pad electrode 21.

In this example, a commercially available redox electroless nickel-phosphorus alloy plating solution using hypophosphite as a reducing agent was used, and the pH was 5.0 ± 0.1 at 90 ° C. for 30 minutes. It was possible to form uniform nickel protrusion electrodes of 5 to 6 μm on all the pad electrodes 21 of the semiconductor substrate 15 without depositing a plating film on the back surface and the side surfaces of the semiconductor substrate 15. According to the method of the present invention, the projection electrode 16 having a thicker plating film can be formed by further extending the time for immersing the semiconductor substrate 15 in the electroless plating solution 14.

Further, in this embodiment, the positive electrode is applied to the counter electrode 12 to form the protruding electrode 16 on the semiconductor substrate 15 which is the substrate to be plated by electroless plating, but like the conventional method. Although it is necessary to apply a plating resist to the back surface and the side surface of the semiconductor substrate 15, the semiconductor substrate 15 to be plated is provided only for the purpose of equalizing the potential of the surface of the pad electrodes 21 of the semiconductor substrate 15. It is also possible to apply a small negative potential.

According to the means of the present invention, a potential difference less than the difference between the redox potential of the reducing agent of the electroless plating solution and the redox potential of the plating metal ions is applied to the semiconductor substrate 15 from the external power source. By selecting the potential difference, it is possible to similarly form a plating film of copper, cobalt, or an alloy thereof on the pad electrode 21.

Further, even if a method of substituting an active metal other than zinc or an activation method of applying a catalyst such as palladium is used as the activation treatment, the plating on the pad electrode 21 of the semiconductor substrate 15 is similarly performed. It goes without saying that a film can be formed.

[0029]

As described above, according to the present invention, the plating resist is applied to the back surface and the side surface of the semiconductor substrate, and the resist stripping step in the subsequent step is unnecessary, and the plating film is deposited on the back surface and the side surface of the semiconductor substrate. It is possible to form a barrier metal or a bump electrode having a uniform film thickness on a plurality of pad electrodes of a semiconductor substrate without any operation.

[Brief description of drawings]

FIG. 1 is a schematic view showing a method for forming an electroless plating film of the present invention.

FIG. 2 is a process sectional view showing an electrode forming method on a semiconductor substrate of the present invention.

FIG. 3 is a process cross-sectional view showing a conventional method for forming an electrode on a semiconductor substrate.

[Explanation of symbols]

11 External power supply 12 opposite poles 13 plating tank 14 Electroless plating solution 15 Semiconductor substrate 16 protruding electrode 21 Pad electrode 22 Passivation film 23 Zinc substitution film 31 Plating resist

Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H01L 21/92 604M

Claims (3)

[Claims] 1. A method of forming an electroless plating film on a pad electrode of a semiconductor substrate, comprising the step of selectively activating the pad electrode, and immersing the semiconductor substrate in an electroless plating solution, And a step of depositing an electroless plating film while applying a potential to the semiconductor substrate by an external power source. 2. The method for forming an electroless plating film according to claim 1, wherein the activating step is either a catalyst application method or a zinc substitution method. 3. The step of depositing an electroless plating film while applying a potential to the semiconductor substrate, wherein the applied potential is the oxidation-reduction potential of the plating metal ion and the oxidation of the reducing agent used in the electroless plating solution. The electroless plating film forming method according to claim 1 or 2, wherein the positive potential is less than the difference from the reduction potential.