JPH11214421A - Method for forming electrode of semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide electroless nickel plating and gold plating methods on an aluminum electrode for forming a highly reliable electrode. SOLUTION: Zinc is deposited 14 on the aluminum electrode 12 with zincate treating liquid containing sodium hydroxide and zinc oxide. Then, sodium hypophosphite being a reducing agent is dissolved in pure water. Pure water is added while it is adjusted to pH 9.0-12.0 with sodium hydroxide solution and it is immersed into solution 15 which is set to be 1000 mL in total. Then, nickel plating is executed with the condition of pH 4.0-6.8 at 80-90 deg.C by using oxidation-reduction reaction electroless nickel plating liquid containing sulfur compound with the aluminum electrode 12 of the semiconductor element as reaction accelerator. Thus, a nickel film 16 containing phosphorus and gold plating films 17 and 18 are obtained in all the aluminum electrodes 12 of the semiconductor element by executing substitution reaction electroless gold plating and oxidation reduction-type electroless plating.

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 electrodes of a semiconductor device which is necessary when a semiconductor device is mounted by using a wireless bonding method such as a flip chip method or a film carrier method.

[0002]

2. Description of the Related Art In recent years, with the miniaturization of electronic equipment, ICs,
2. Description of the Related Art High density and high integration of semiconductor elements such as LSIs are being promoted. Also, from the viewpoint of the mounting surface of the semiconductor element, there is a tendency that the pitch between the electrodes is reduced and the number of input / output electrodes is increased. Furthermore, thinner devices such as calculators, notebook computers, and mobile phones are required.

[0003] To meet these demands, wireless bonding methods such as the flip chip method and the TAB method are matched from the viewpoints of reliability resulting from collective joining and alignment accuracy, thinner mounting, higher density, and the like. It is expected to become one of the major pillars in the mounting technology of semiconductor devices in the future, and much research and development has been made.

In the wireless bonding method, generally, a metal projection called a bump electrode or a bump is formed on an aluminum electrode of a semiconductor element.

As a method of forming a protruding electrode on an aluminum electrode of such a semiconductor device, many methods have been proposed and some of them have been commercialized. Among these methods, a method of forming a bump by electroless plating, which is expected to be low in cost, has been studied. For example, JP-A-63-3055
32, JP-A-64-81344,
An example thereof is a palladium nucleation method disclosed in JP-A-69524.

This method will be described with reference to FIG.
As shown in FIG. 4A, the surface of the aluminum electrode 3 of the semiconductor element 2 made of the silicon substrate 1 is pretreated with a dilute solution of nitric acid (or phosphoric acid) and then washed. 4 is a passivation film. Subsequently, as shown in FIG. 4 (b), the semiconductor element 2 was placed in a palladium activator consisting of 1 g of palladium chloride, 100 cc of hydrochloric acid and 9.54 L of water.
The precipitate 5 of palladium is adhered to the surface of the aluminum electrode 3 exposed by immersion for 60 seconds.

Next, as shown in FIG. 4C, palladium 5 deposited on the surface of the aluminum electrode 3 of the semiconductor device
After the semiconductor element 2 has been washed to such an extent that
Is immersed in an electroless nickel plating solution having a temperature of 4 to 6 and a temperature of 80 to 90 ° C., and subjected to electroless plating for about 1 hour to form a nickel bump 6 having a thickness of 20 μm around the exposed aluminum electrode 3. I do. Subsequently, after cleaning the semiconductor element 2, the semiconductor element 2 is immersed in an electroless gold plating solution to perform electroless gold plating, and a 1 μm
m of the gold layer 7 is formed.

[0008]

However, in the above conventional example, palladium is deposited on the surface of a semiconductor element other than aluminum (non-conductive portion: for example, a passivation film) by catalyzing treatment with a palladium activator, When nickel plating is performed, there is a possibility that the plating reaction starts with the precipitated palladium as a nucleus,
If the selectivity of the reaction is lacking and the electrical insulation on the surface of the nonconductor is deteriorated or the electrode pitch is narrowed, the plating film is bridged between the electrodes, resulting in a short circuit between the electrodes. In applications other than semiconductors, in the electroless nickel plating specification by zincate treatment, which is generally performed as a plating method on aluminum metal plates, nickel plating on the aluminum surface used as the electrode of the semiconductor element is an aluminum electrode Of aluminum is, for example, Al
In the case of an alloy such as -1% Si, the surface unevenness of the plating film is extremely large and the adhesion strength is weak, and thus it is insufficient as an electrode for connecting a semiconductor element and a circuit board. When electroless nickel plating was performed on the actual working semiconductor element, an electrode (specific electrode: for example, GND or _Vcc ) to which no metal adhered was observed. This tendency is a phenomenon that occurs on many types of electrodes on semiconductor elements, and that the electrodes at the same position are not plated.

On the other hand, flip chip mounting or T
In AB mounting, the height accuracy of the protruding electrodes may be extremely high depending on the mounting method. In this case, a method called leveling is used, in which the protruding electrodes are pressed from above to make the heights uniform. ing. However, the above-mentioned nickel projection electrode has a problem that it is not crushed due to high hardness of nickel, and a gold plating film having low hardness is required.

[0010] In order to solve this problem, studies have been made on an oxidation-reduction reaction type gold plating solution. However, conventional potassium borohydride or dimethylamine borane (D
MAB) or the like as a reducing agent, and an electroless gold plating solution using a metal cyanide such as potassium dicyanoaurate (I) as a metal salt, the solution is strongly alkaline (pH 13 to 13.6) and the solution temperature is low. Since the temperature is relatively high at 60 to 80 ° C., the bump electrode is formed by electroless plating a metal having low hardness, such as gold, because the passivation film of the semiconductor is eroded and there is no resist material resistant to gold plating. I couldn't. Further, since this gold plating solution contains a cyanide compound, there is also a problem in working environment and waste liquid treatment.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems and provides a highly reliable electrode or protruding electrode on an aluminum electrode surface of a semiconductor element at a low cost without adversely affecting a substrate, an aluminum electrode or a passivation film of the semiconductor element. It is an object of the present invention to provide a method for forming the same.

[0012]

In order to solve the above-mentioned problems, a method for forming an electrode of a semiconductor device according to the present invention provides a method for forming a metal on a surface of an aluminum electrode of a semiconductor device by applying nickel plating as a barrier metal or a projection electrode. After performing an etching treatment with a liquid or an alkaline liquid, a desmutting treatment is performed with a smut removing liquid, and then a zincate treatment for replacing aluminum and zinc is performed simultaneously with removing an aluminum oxide film from the aluminum electrode surface with an alkaline metal salt solution. ~ 3 times, to precipitate Zn fine particles on the aluminum electrode surface, and then activate the aluminum electrode surface by immersing it in an alkaline solution in which a reducing agent is dissolved, and further attach the reducing agent solution to the aluminum electrode. Oxidation-Reduction Reaction Considering the Reaction Accelerator in the State Perform nickel plating by immersing in an electroless nickel plating solution, and performs flash gold plated with an electroless gold plating solution of the substitution reaction type thereon. Depending on the mounting method, a thicker gold plating method is adopted by further immersing in a neutral, non-cyanide-based oxidation-reduction type electroless gold plating solution.

According to this method, a nickel plating film having good electrical conduction without corroding the passivation film and the aluminum electrode by a very simple facility and method, and a thick gold plating film can be used for all of the semiconductor elements. It can be formed on an aluminum electrode.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The nickel plating method on an aluminum electrode according to the present invention is based on the following. First, a semiconductor electrode having an aluminum electrode is etched with an acidic solution or an alkaline solution and then desmutted with a smut removing solution. Then, based on a solution obtained by dissolving zinc oxide in a sodium hydroxide solution, a zincate treatment was performed using an alkaline zincate solution in which a metal additive and a complexing agent were added, and a zincate treatment was performed on the surface of the aluminum electrode. Zinc is precipitated by a substitution reaction with aluminum, and then a reducing agent such as sodium hypophosphite used in a redox reaction type electroless nickel plating solution in the next step is dissolved, and the pH is further adjusted to 9.0.
The material is immersed in an alkaline solution adjusted to ~ 12.0, and then the above-mentioned material is attached to the surface of the alkaline solution containing the reducing agent. After immersion in a nickel plating solution, flash gold plating is performed with a substitution reaction type electroless gold plating solution. Thereafter, depending on the mounting method, the substrate is immersed in an oxidation-reduction type electroless gold plating solution composed of ascorbic acid or a salt thereof and gold sulfite or a salt thereof or chloroauric acid or a salt thereof.

In general, even if aluminum is etched with an acid or an alkali to remove a surface oxide film, an oxide film is easily formed again before the subsequent water washing treatment or the next plating treatment, and plating may not be easily attached. . Further, as a feature of the semiconductor element, each terminal such as _Vcc , GND, I / O, etc., which is an electrode of the semiconductor element, is connected to a functional element such as a transistor, a resistor, a capacitor, a PN junction, or _Vcc or GND depending on the semiconductor element.
Is short-circuited to the silicon substrate, which is the base of the semiconductor element, so that a potential difference occurs between the electrodes. For this reason, in a usual plating method, a local battery is formed between the electrodes, and a phenomenon occurs in which plating is not applied depending on the method of forming the local battery.

Therefore, considering the driving force of the reaction of electroless nickel plating, the reduction potential of Ni ²⁺ is pH on the acidic side.
It is almost constant up to around 4, but fluctuates considerably on the alkali side due to the stability of Ni complex ions. On the other hand, the redox potential of the reducing agent changes linearly over the entire pH range. Therefore, the value of the driving force of the plating reaction, that is, the reduction reaction is pH
Was also changed by

For example, in electroless nickel plating, Ni ²⁺ + 2e ⁻ = Ni at pH 4... E _o = −0.250 V H ₂ PO ₂ ⁻ + H ₂ O = H ₂ PO ₃ ⁻ + 2H ⁺ + 2e ⁻ . _{...... E o '= -0.750V E o} ' -E o = -0.5V pH12 in the ^{Ni-cit + 2e - = Ni} ......... E o = -0.4V H 2 PO 2 - + H 2 O = H 2 _{^{PO 3 - + 2H + + 2e}} - a _{......... E o '= -1.25V E o} ' -E o = -0.85V, it can be seen that the driving force of the reduction reaction higher pH increases.

Therefore, by utilizing the driving force of this reaction, the semiconductor
Generates reducing power that exceeds the potential difference between the electrodes of the body element
This is very important. In other words, the surface of each aluminum electrode
To e ^-(H_TwoPO_Two ^-+ H_TwoO = H_TwoPO_Three ^-+ 2H⁺+ 2e^-)
Is rich and Ni²⁺+ 2e^-= Promote Ni reaction
To proceed, raise the pH of the surface of each aluminum electrode.
(Alkali side) and hypophosphorous acid as a reducing agent
It is important to keep the amount of sodium rich
is there.

According to the present invention, a reducing agent (for example, such as sodium hydroxide solution) contained in the nickel plating solution is added to the sodium hydroxide solution which has a function of dissolving the aluminum oxide and does not hinder the plating operation even when brought into the next nickel plating. If redox electroless nickel plating is performed immediately after dissolving sodium hypophosphite) and directly adhering to the aluminum electrode of the semiconductor element, a large reducing power can be obtained without forming an oxide film on the electrode surface. Then, a nickel film is deposited around zinc particles previously deposited on the aluminum electrode. By performing the above-described treatment, the initial nickel deposition is facilitated when the semiconductor element having the aluminum electrode enters the plating solution. Also,
It was also confirmed that if this nickel plating could be performed, plating treatment such as electroless gold plating could be performed on this nickel film without any problem.

As the electroless plating solution, for example, an electroless Ni-P plating solution or an electroless Ni-B plating solution can be used.

As the electroless nickel plating solution, not only an alkaline plating solution using sodium hypophosphite as a reducing agent but also an acidic plating solution can be used. In the present invention, it has been experimentally found that the pH of the acidic plating solution is important to uniformly form a nickel plating film on each aluminum electrode.
Exceeds 7, the nickel hydroxide precipitates and becomes cloudy. However, the pH of the alkaline solution containing the reducing agent of the present invention is adjusted to 9.0 to 12.0. Can be plated very well without exceeding 7. Further, as the plating progresses and the nickel ions are consumed, the pH drops. However, even with this drop in pH, the pH was suppressed to some extent by bringing in the alkaline solution by the above treatment, and good results were obtained for plating adhesion. Further, if the pH of the alkaline solution containing the reducing agent is set to 12.0 or more, a bad effect such as attacking the passivation film of the semiconductor element occurs.
When plating thick, a bump having a stable shape can be formed by sufficiently swinging the semiconductor element in a plating solution.

Further, in the electroless nickel plating process, nickel itself deposited according to the mounting form to be used may be thickly plated (10 to 15 μm) to form a bump body or a part of a bump.

That is, when soldering to an electrode lead of a counterpart circuit board, electroless nickel plating is applied to the aluminum electrode of the semiconductor element by 2 to 5 μm, and the surface of the aluminum electrode is replaced with an electroless nickel plating of about 0.05 μm. Gold plating may be performed, and solder bumps may be formed thereon by wire bonding using, for example, a solder wire.

On the other hand, when the Au-Sn eutectic is formed and joined to the Sn-plated electrode lead of the mating circuit board as in TAB mounting, nickel plating is 10 to 15 μm on the aluminum electrode of the semiconductor element. Then, the surface may be subjected to substitutional electroless gold plating of about 0.05 μm, and further a redox electroless gold plating film of about 2 μm may be formed thereon to form bumps.

When the electrode leads of the counterpart circuit board are bonded to the bumps by anisotropic conductive rubber or conductive adhesive, electroless nickel plating of 10 to 15 μm is applied on the aluminum electrodes of the semiconductor element. Then, the surface may be subjected to substitution type electroless gold plating of about 0.05 μm to be used as a bump.

According to this method, a nickel plating film and a gold plating film having good electrical continuity without corrosion of the passivation film and the aluminum electrode are formed on all the aluminum electrodes of the semiconductor element by extremely simple equipment and method. It becomes possible.

Hereinafter, specific embodiments of the present invention will be described with reference to FIGS. 1 (a) to 1 (d), 2 and 3.

First, as shown in FIG. 1A, an aluminum (Al-1% Si) electrode 1 is formed on a silicon substrate 11 on which various transistors and wirings are formed by a conventional method.
After the formation of No. ₂ , a passivation film 13 made of Si ₃ N ₄ was formed on the entire surface, and the passivation film 13 was selectively removed by etching to prepare a semiconductor device in which most of the aluminum electrode 12 was exposed.

Next, as shown in FIG. 1B, the surface of the aluminum electrode 12 of the semiconductor element was soft-etched with an acidic solution containing a fluorinated compound and then washed with pure water. Next, desmut treatment is performed with a smut removing liquid,
Subsequently, the semiconductor element was immersed in a zincate treatment solution based on a solution in which zinc oxide was dissolved in a sodium hydroxide solution containing Fe as a metal additive for 20 seconds, and the surface of the aluminum electrode 12 exposed by the substitution reaction was removed. After depositing zinc deposit 14, the precipitate was washed with pure water.

Next, the deposited zinc is immersed in a 50% nitric acid solution for 30 seconds to remove the deposited zinc, washed with water, subjected to a second zincate treatment and peeling treatment, further subjected to a third zincate treatment, and then washed with pure water. did.

Next, as shown in FIG. 1C, 25 g of sodium hypophosphite as a reducing agent was dissolved in 750 mL of water, and then pH 9.0 to 12.1 was dissolved with a sodium hydroxide solution.
After immersing for 10 seconds in a solution 15 made up to a total of 1000 mL by adding pure water while adjusting the concentration to 0, or immediately or immersed in pure water, the semiconductor element is made of the following composition, and further containing a sulfur compound (for example, thiol A few ppm as a reaction accelerator, adjust the pH to 4.0 to 6.8, and immerse in an oxidation-reduction type electroless nickel plating solution at a temperature of 80 to 90 ° C. for about 10 minutes. Thus, a nickel film 16 containing phosphorus of 4 to 5 μm was formed on the aluminum electrode 12.

The aluminum electrode is made of Al-1% Si
By performing the zincate treatment three times, a plating film excellent in surface shape and physical properties could be obtained. However, when the aluminum electrode contains Cu, for example, Al-1% S
In the case of i-2% Cu, the nickel plating film had a surface roughness of 1 μm or less and a shear strength of 20 gf / 25 × 45 μm or more which had no problem in mounting even if the zincate treatment was performed once.

Further, the adhesion was improved by adding Fe to the zincate treatment liquid. Further, by adding Fe, the granular precipitates on the surface (zinc-iron-based alloy film) become
There was a tendency to be dense.

In addition, an excellent plating film shape was obtained by adding a sulfur compound (eg, thiodiglycolic acid) as a reaction accelerator to the oxidation-reduction reaction nickel plating solution at a concentration of several ppm. It was also found that the amount of the reaction accelerator composed of the sulfur compound had a large effect on the shape of the plating film.

Further, as shown in FIG. 1 (d), the semiconductor element is washed with pure water, and a substitution reaction type electroless gold plating solution (for example, OPC Muden Gold manufactured by Okuno Pharmaceutical Co., Ltd., pH 5.8) , Liquid temperature 90 ° C) for about 10 minutes,
A flash gold plating film 17 having a thickness of 0.05 μm was formed on the surface of the nickel film 16.

Here, the composition of the oxidation-reduction reaction type electroless nickel plating solution used in this experimental example is shown below.

(Composition of Redox Reaction Type Electroless Nickel Plating Solution) Nickel sulfate 7.4 g / L sodium hypophosphite 13 g / L Rossel salt 14 g / L malic acid 10 g / L sodium acetate 6 g / L thioglycolic acid A 300 mg / L aqueous sodium hydroxide solution was added to adjust the pH to 4.0 to 6.0.
Adjusted to 8 By the above plating process, Ni (4 to 5 μm) -Au (0.
(05 μm). This plating film 18
Was extremely firmly adhered to the aluminum electrode 12. As shown in FIG. 2, a solder wire is formed on the plating film 18 by a wire bonding method.
Was formed, and this semiconductor element was mounted on an Ag-Pd electrode wired on a ceramic substrate by a face-down method, and good bonding was confirmed.

In the above embodiment, Ni;
The application of a plating film of 0.05 μm in thickness of Au and 0.05 μm was shown. However, as shown in FIG. 3, nickel plating was performed for 30 minutes by the same electroless plating as in the above embodiment, and the nickel film 16 (12 to 14 μm) was formed. )-After forming a plating film of the substitution-reaction type electroless gold plating film 17 (0.05 μm), it is further composed of the following composition, and has a pH of 7.0 and a temperature of 60 ° C. A gold plating film 20 having a thickness of about 2 μm is formed by immersion in a liquid for 120 minutes, and a bump 21 having a height of about 15 μm is formed to form an inner lead of a tin (Sn) -plated TAB tape carrier and Au. TAB mounting in which -Sn eutectic was formed and joined was also possible.

(Composition of redox reaction type electroless gold plating solution) Sodium chloroaurate 10 g / L sodium sulfite 25 g / L sodium thiosulfate 50 g / L sodium ascorbate 80 g / L ammonium chloride 5 g / L

[0040]

As described above, according to the present invention, the G of a semiconductor element, which has been difficult to perform by conventional plating, with an extremely simple operation.
A nickel film can be stably formed at low cost on an aluminum electrode including specific pads such as ND and _Vcc . Therefore,
Based on the nickel film formation according to the present invention, it is possible to form a highly reliable protruding electrode with no short-circuit between electrodes at a high yield on the film by using a technique such as gold plating or solder wire bonding. The industrial value is great, such as realizing a simple face-down mounting method.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a semiconductor element in each plating process on an aluminum electrode according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a semiconductor element when a solder wire is wire-bonded on a plating film on which a substitution reaction type electroless gold plating has been performed after a nickel film is formed in the same embodiment.

FIG. 3 is a cross-sectional view of the semiconductor device when a nickel film formation-substitution reaction type electroless gold plating is formed and then a redox reaction type gold plating is performed to form a TAB mounting bump in the embodiment.

FIG. 4 is a cross-sectional view of a semiconductor element in each conventional plating process on an aluminum electrode.

[Explanation of symbols]

 12 Aluminum electrode 14 Zinc precipitate 15 Alkaline solution containing reducing agent 16 Nickel film 17 Flash gold plating film 19 Solder wire bump 20 Thick gold plating film

Claims (7)

[Claims] When a nickel plating is applied to a surface of an aluminum electrode of a semiconductor element as a barrier metal or a projection electrode, a light etching step of etching the above material with an acidic solution or an alkaline solution, and a zincate using an alkaline zincate solution. A zincate treatment step of performing a treatment, an activation step of activating the aluminum electrode surface by immersing the aluminum electrode in an alkaline solution in which a reducing agent is dissolved, and the aluminum electrode in a state where the reducing agent solution is adhered to the aluminum electrode. An electroless nickel plating step of immersing in an oxidation-reduction type electroless nickel plating solution. 2. The electrode formation of a semiconductor device according to claim 1, wherein the alkaline solution in which the reducing agent is dissolved comprises sodium hypophosphite and sodium hydroxide and has a pH of 9.0 to 12.0. Method. 3. The method according to claim 1, wherein the zincate treatment is performed a plurality of times. 4. The method according to claim 1, wherein the zincate treatment liquid comprises zinc oxide and sodium hydroxide, and contains Fe as a metal additive. 5. The method according to claim 1, wherein a reaction accelerator comprising a sulfur compound is added to the electroless nickel plating solution.
The method for forming an electrode of a semiconductor device according to the above. 6. The method according to claim 5, wherein the sulfur compound is thiodiglycolic acid. 7. After the step of electroless nickel plating, after flash gold plating with a substitution reaction type electroless gold plating solution, an oxidation-reduction reaction type electroless gold plating solution is mixed with ascorbic acid or a salt thereof, gold sulfite or 2. The method for forming an electrode of a semiconductor device according to claim 1, wherein the electrode is immersed in a gold plating solution comprising the salt or chloroauric acid or a salt thereof to perform thick gold plating.