JP2001348671A - Electroless gold plating solution and electroless gold plating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare an electroless gold plating solution excellent in stability and capable of performing a plating without being added with an addition agent. SOLUTION: This electroless gold plating solution contains gold salt consisting of a bis (thiourea) gold (I) complex ([Au(NH2NHCS)2]X (wherein, X is a monovalent anion selected from the groups consisting of a halogen ion such as Cl- and a nitric ion) and a reducing agent. As the reducing agent, one kind selected from sodium hydpophosphite, thiourea and tartaric acid or their mixture is preferable. Further, in the electroless gold plating method using the gold plating solution, preferably, the plating is performed in such a manner that the pH of the gold plating solution is controlled to 2.0 to 5.0, and the temperature is controlled to room temperature to 80 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a gold plating solution used for an electroless gold plating method, and more particularly to a non-cyanide electroless gold plating solution. The present invention also relates to an electroless gold plating method using the gold plating solution.

[0002]

2. Description of the Related Art Gold plating has not only been used for decorations since ancient times, but also has been widely used for forming circuit patterns of electric and electronic members because of its excellent electrical properties, corrosion resistance, solderability, and the like.

Conventional gold plating methods include:
Most of the methods are based on an electrolytic plating method in which an object to be plated is immersed in a plating solution and energized to form a film. However, in the case of performing plating by electrolytic plating, if an attempt is made to form a complicated circuit pattern, a power supply line to an object to be plated becomes complicated. In recent years, the demand for higher density of electric and electronic components and the need for finer and more integrated circuit boards mounted on them have been increasing. In addition to increasing the cost and cost, it is practically impossible to form an ultrafine pattern.

Therefore, application of electroless plating is considered as a plating method that can cope with miniaturization and integration of circuit boards. This electroless plating is a substitution type electroless plating using a substitution reaction by a local battery action that occurs when an object to be plated is immersed, and a metal is mixed with a plating solution by using a reducing agent and a reducing agent. Although it is distinguished from reduction type electroless plating to be deposited, plating can be performed without supplying external power, so feed lines are unnecessary even when forming complex circuit patterns, and plating equipment In addition, the plating process does not become complicated.

[0005] By the way, as a plating solution used for gold plating, a cyan-based gold plating solution containing cyanide is generally used. However, since cyan-based gold salts are highly toxic, work safety and wastewater treatment are difficult. Not desirable from a viewpoint. Further, when a cyan-based gold plating solution is used, there is a problem that it is difficult to form a fine circuit pattern because excess cyanide peels and damages a resist pattern of a semiconductor component. Therefore, in electroless plating, it is considered that it is desirable to use a plating solution containing no cyanide. For example, gold sulfite (Na ₃ Au (SO ₃ ) ₂ ) is considered.
Application of a non-cyanide gold plating solution such as a solution is under study.

[0006]

However, any of the non-cyanide gold plating solutions known so far has a problem of low stability. For example, in the above-mentioned gold sulfite, the sulfite ions in the solution are oxidized by oxygen in the atmosphere and the concentration thereof is decreased, so that the stability of the gold (I) complex is reduced and the plating solution is decomposed. is there. Then, the use of the plating solution having low stability causes a gold sedimentation phenomenon in which gold of the plating solution precipitates and precipitates in the plating solution tank or the piping, which hinders the plating operation.

Therefore, in non-cyanide electroless plating, plating is performed by adding additives such as a stabilizer and a complexing agent to the plating solution to prevent decomposition of the plating solution. Such measures increase the cost of the stabilizer because the cost of the stabilizer and the plating solution manufacturing process become complicated. Therefore, at present, electroless plating using a non-cyanide-based gold plating solution has not been practically used.

The present invention has been made in view of the above background, and has as its object to provide an electroless gold plating solution having excellent stability and capable of performing a plating operation without adding an additive. I do. Another object of the present invention is to provide a gold plating method which is excellent in workability using the plating solution and can produce a film having good properties.

[0009]

Means for Solving the Problems The present inventors have made intensive studies to develop a non-cyanide gold plating solution applicable to the electroless gold plating method. Here, as a characteristic required for the non-cyanide-based gold plating solution applicable to the electroless gold plating method, as described above, it is required to have stability, that is, to perform plating under actual plating conditions (solution temperature). First, the fact that the compounds constituting the liquid do not decompose is
In addition to this, in addition to that, it can be manufactured at low cost without requiring complicated steps in its manufacture, and easily deposits gold when mixed with a reducing agent used in the reduction type electroless plating method That is, two characteristics are required, that is, the reduction potential of the gold salt constituting the plating solution is sufficiently noble and a sufficient redox potential difference occurs between the gold salt and the reducing agent. . The present inventors have considered this point,
Investigation was made to find the most suitable non-cyanide electroless gold plating solution satisfying the above conditions, and came to the present invention.

The first aspect of the present invention is an electroless gold plating solution containing a gold salt comprising a bis (thiourea) gold (I) complex represented by the following formula and a reducing agent.

[0011]

Embedded image (In the formula, X represents a monovalent anion selected from the group consisting of halogen ions such as Cl ⁻ and nitrate ions.)

In the present invention, the bis (thiourea) gold (I) complex, which is a gold salt as a main component thereof, is a non-cyanide compound containing no cyanide (CN), as is apparent from its constituent elements. It is. Therefore, there is no problem in terms of work safety and wastewater treatment, and the material has the characteristics as a non-cyanide-based gold plating solution as it is.

[0013] The stability of the plating solution, which is the first object of the present invention, also relates to the bis (thiourea).
The gold (I) complex is a compound that is stable to heat and light in the atmosphere and can exist stably even when dissolved in a solvent such as water. Therefore, according to the present invention, a plating operation can be performed in a stable state without the addition of an additive such as a stabilizer, and a plating solution can be manufactured without using an additive, thereby reducing the cost. can do.

The bis (thiourea) gold (I) complex is obtained by mixing thiourea with a halide of gold (a chloride such as chloroauric acid) or a nitrate of gold and utilizing the reduction action of thiourea. The reaction can be carried out. Therefore, it can be manufactured without going through complicated steps. Further, since the production amount relative to the amount of gold chloride as a raw material, that is, the yield is relatively high, the gold plating solution according to the present invention is suitable for industrial production and can be mass-produced at low cost.

Further, regarding the suitability of a reduction type electroless plating solution, the bis (thiourea) gold (I) complex has a relatively noble reduction potential as described later, and is generally used. Has a sufficient redox potential with the oxidation potential of the reducing agent to be used. Therefore, it is considered that the gold plating solution according to the present invention can be used as a plating solution for reduction-type electroless plating, and a film having a large film thickness can be efficiently produced.

The gold plating solution according to the present invention is based on the premise that a reducing agent is added. That is, the present invention is applied to a reduction type electroless plating method. This is because the film produced by the substitutional electroless plating without adding a reducing agent is thin, and in order to produce a film having a film thickness required for a circuit pattern of an electric / electronic member, the reduction type electroless plating is not necessary. This is because the electroplating method is efficient.

As the reducing agent, those having a reducing action and generally called a reducing agent can be used. Therefore, gold can be precipitated even by using a reducing agent having a strong reducing action such as L-ascorbic acid and sodium tetrahydroborate. Occurs, and the plating solution is colored, so that a film of good quality cannot be produced in some cases.

Therefore, as the reducing agent applied to the gold plating solution according to the present invention, sodium hypophosphite (NaPH ₂ O ₂ ), tartaric acid (C ₄ H)
_It is preferable to use at least one of ₆ O ₆ ) and thiourea (CS (NH ₂ ) ₂ ). According to the study of the present inventors, when these reducing agents are used, formation of a precipitate,
This is because coloring of the plating solution does not occur. And tartaric acid is preferable as a particularly preferable one of these three reducing agents. This is because when a plating solution using tartaric acid as a reducing agent is used, a good film having the most excellent smoothness can be produced.

Further, the concentration of each component of the gold plating solution according to the present invention is as follows: the bis (thiourea) gold (I) complex is 5.0 × 10 ^{−4 to} 1.0 ×. 10 ^-1 M,
It is preferable to use a reducing agent of 1.0 × 10 ^{−3 to} 1.0M. The reason why the concentration of the bis (thiourea) gold (I) complex, which is a gold salt, is set to the above range is that if the concentration is 5.0 × 10 ⁻⁴ M or less, gold does not precipitate, and 1.0 × 10 ⁻⁴ M or less. 10
This is because addition of ^-1 M or more does not affect the deposition rate and the amount of gold deposition. In consideration of the cost of the plating solution, the concentration of the bis (thiourea) gold (I) complex is 5.0 ×
It is particularly preferred to be in the range of 10 ^{−3 to} 1.0 × 10 ⁻² M. On the other hand, the reason why the concentration of the reducing agent is in the above range is that when the concentration is 1.0 × 10 ⁻³ M or less, the reducing action is poor and gold is not deposited. This is because there is no difference in the action and addition of 1.0 M or more wastes the reducing agent. The concentration of the reducing agent is particularly preferably 5.0 × 10 ⁻² M.

The gold plating solution according to the present invention is produced by dissolving a bis (thiourea) gold (I) complex as a gold salt and a reducing agent in a solvent such as water. In this case, a reducing agent solution in which a reducing agent is dissolved in a solvent, a gold salt solution in which a gold salt is dissolved in a solvent are separately manufactured, and the gold salt solution is added to the reducing agent solution. Is preferred. It can be produced by adding a solid state gold salt to a reducing agent solution. However, depending on the type of the reducing agent, the solubility of the bis (thiourea) gold (I) complex decreases, so that bis (thiourea) gold ( This is because I) the complex may not be sufficiently dissolved.

As described above, the gold plating solution according to the present invention comprises:
It has sufficient characteristics required for a non-cyanide electroless plating solution. As a plating method using the gold plating solution according to the present invention, a normal electroless plating method, that is, immersion in a non-plating material in a gold plating solution, and holding for a predetermined time until a required film thickness is obtained. By doing.
Here, in the plating solution used in the reduction type electroless plating method as in the present invention, it is considered that the film formation rate and the properties of the film are affected by the temperature and pH of the plating solution.
The present inventors have studied the film formation rate and the film under these plating conditions, and have found an optimal plating method when the plating according to the present invention is used.

First, the pH of the plating solution is 2.0 to
It is preferable to carry out in the range of 5.0. The pH of the bis (thiourea) gold (I) complex solution which is a gold salt used in the present invention is about 4.0, and the bis (thiourea) gold (I) complex is stable in a low pH range. Yes, unstable at high pH,
In particular, if the pH is 5.0 or more, it may decompose and precipitate.

The temperature of the plating solution during the plating operation is preferably in the range of room temperature to 80 ° C. In the plating method using the bis (thiourea) gold (I) complex according to the present invention, the plating rate can be increased by increasing the plating solution temperature, but if the temperature exceeds 80 ° C., the plating rate is too high. This is because the properties of the film deteriorate. At 80 ° C. or higher, the bis (thiourea) gold (I) complex may be colored even if it is not decomposed.

In the electroless plating method according to the present invention, it is preferable to perform the plating treatment in a stationary state without stirring the plating solution. This is because, by stirring the plating solution, the supply of gold ions to the substrate surface is promoted and the plating rate is increased, but in this case, excessive gold precipitation occurs and the properties of the film deteriorate.

[0025]

Preferred embodiments of the present invention will be described below with reference to the drawings.

First Embodiment : In the present embodiment, bis (thiourea) gold (I) chloride is produced as a bis (thiourea) gold (I) complex, and the properties of the solution are evaluated. A plating solution mixed with various reducing agents was put into a bath, and the plating speed and film properties of each plating solution were examined.

Production of bis (thiourea) gold (I) chloride
And characteristic evaluation : gold chloride hydrate (H [AuCl ₄ ] .4H ₂
O) an aqueous solution in which 30 g is dissolved in water, and thiourea 22.2
g in water and mixed with water and stirred for 1 hour. Then, the mixed solution was filtered, the filtrate was concentrated at a temperature of 50 ° C. using an evaporator until crystals were precipitated, and further cooled with ice at 0 ° C. or lower for 30 minutes to grow crystals. The purified crystals were collected by a filter, washed with acetone and diethyl ether several times, and dried in a drier for 2 hours under vacuum. As a result, a white powder of bis (thiourea) gold chloride was obtained.

The bis (thiourea) gold (I) chloride was evaluated for reduction potential. This evaluation was performed by measuring a cyclic voltammetry curve (hereinafter, referred to as a CV curve) of the bis (thiourea) gold (I) chloride aqueous solution. The CV curve is obtained by dissolving bis (thiourea) gold (I) chloride produced by the above method in pure water,
The measurement was carried out by electrolyzing an electrolytic solution prepared by preparing an aqueous solution of 1 mM or 1.0 mM and adding potassium nitrate as an indicator electrolyte to the aqueous solution to a concentration of 0.5 M. For the measurement, a gold plate (dimension 1 × 1 cm, surface area 2 cm ² ) was used for the working electrode and the counter electrode, and a silver / silver chloride electrode (Ag / AgCl) was used as a reference electrode.
Was used. The CV curve was measured with a potentiostat / galvanostat, and the potential value was swept at 1 speed.
00 mV / sec.

FIG. 1 shows CV curves of aqueous solutions of 0.5 mM and 1.0 mM bis (thiourea) gold (I) chloride.
FIG. 1 also shows a CV curve of a potassium nitrate aqueous solution as an indicator electrolyte as a reference curve. As can be seen from FIG. 1, in the CV curve of the bis (thiourea) gold (I) chloride aqueous solution, a reduction peak not found in the CV curve of potassium nitrate was +0.2 V (vs. Ag / AgC
1) and -0.7 v (vs. Ag / AgCl), but the peak of +0.2 V (vs. Ag / AgCl) becomes sharper as the concentration of bis (thiourea) gold (I) chloride is increased. Has become. Therefore, the reduction potential of bis (thiourea) gold (I) chloride is +0.2 V (vs. Ag / A
gCl). This indicates that the bis (thiourea) gold (I) chloride aqueous solution can be used together with most of the commonly used reducing agents as a reduction type electroless plating solution.

Bathing of gold plating solution : A bath of plating solution was prepared using bis (thiourea) gold (I) chloride produced by the above method. Sodium hypophosphite, thiourea, and tartaric acid are dissolved as reducing agents in 20 mL of pure water, and a solution of bis (thiourea) gold (I) chloride in 20 mL of pure water is slowly added to this solution, and both are added. The plating solution was mixed to form a bath. At this time, the concentrations of bis (thiourea) gold (I) chloride and each reducing agent were respectively based on the total amount of the plating solution.
The concentration was adjusted to 0.005M and 0.05M, and three types of plating solutions were produced for each reducing agent. In the present embodiment, the reduction is performed by mixing the reducing agent solution and the bis (thiourea) gold (I) chloride solution. However, as described above, the bis (thiourea) gold is added to the reducing agent solution. (I) The chloride may be added in a solid state.

Plating treatment : Using this plating solution, a Ni plating substrate was subjected to gold plating treatment. The substrate used was N
Using an iFe plate (20 × 20 × 0.3 mm) obtained by subjecting a 0.5 μm-thick Ni plating to a cut of 0.8 × 12 × 0.3 mm, electrolytic degreasing is performed, and a 50% hydrochloric acid solution is used. Immediately after the oxide film was removed by immersion for 30 sec and etching to remove the oxide film, it was subjected to plating treatment.

Then, 30 mL of the three kinds of plating solutions produced by the above method were sampled and placed in a beaker, and the substrate was immersed in the plating solution to perform plating. The plating process at this time is performed without stirring the plating solution. As plating conditions at this time, the pH of the plating solution was adjusted to 4.0, and the temperature of the plating solution was set to room temperature.

As a result, it was confirmed that any of the plating solutions manufactured in the present embodiment deposited a film which was considered to be gold. FIG. 2 shows the measurement results of the time change of the thickness of the film deposited in each plating solution. From FIG. 2, it can be seen that, among the three plating solutions manufactured in the present embodiment, the plating solution using sodium hypophosphite and tartaric acid as a reducing agent has a substantially linear relationship between the film thickness and the plating time. It was found that the plating rate was constant. On the other hand, in the plating solution using thiourea as a reducing agent, it was found that the plating rate was not constant, although the film thickness increased with the plating time with the plating time. In addition, when compared with a plating solution using sodium hypophosphite and tartaric acid as a reducing agent, the plating rate of which is almost constant, the plating rate using a sodium hypophosphite as a reducing agent is slightly higher. all right.

Here, in order to confirm that the film deposited by each plating solution was gold, XRD (X-ray diffraction analysis) was performed on the substrate surface after the film deposition, and all the films were gold. It was confirmed that. FIG. 3 shows an X-ray diffraction profile of a film deposited with a plating solution using tartaric acid as a reducing agent.

Further, the film deposited with a plating solution using tartaric acid as a reducing agent was subjected to SEM (scanning electron microscope).
And the surface morphology was observed. FIG. 4 shows an SEM image at that time, and it was found that the gold film deposited with the plating solution using tartaric acid as a reducing agent was relatively smooth.

Second Embodiment : In this embodiment, a plating solution using tartaric acid as a reducing agent is used.
The plating treatment was carried out while changing the pH, and the effect of pH on the plating rate was examined.

The plating solution manufactured by the same method as in the first embodiment was subjected to the same pretreatment as in the first embodiment.
The iFe substrate was immersed and plated. At this time, the pH of the plating solution is adjusted by a plating solution (pH
= 2.3) and the plating solution (pH = 4.
0) were used. FIG. 5 is a diagram showing a change in the film thickness at this time. From FIG. 5, it was found that the plating rate of the gold plating solution according to the present invention was increased by increasing the pH value. This is considered to be because the oxidation potential of tartaric acid as a reducing agent shifts to a lower direction as the pH of the plating solution increases, and the redox potential difference with the gold (I) complex increases.

Third Embodiment In the present embodiment, a plating treatment was performed on a plating solution using tartaric acid as a reducing agent while changing the temperature of the plating solution, and the effect of the temperature on the plating rate was examined.

The plating solution here is the same as the plating solution manufactured in the first embodiment. Further, a substrate on which the same treatment as in the first embodiment was performed was used as a substrate on which plating was performed. Then, the plating treatment was performed at three temperatures: room temperature, 40 ° C., and 60 ° C. FIG. 6 shows a change in the thickness of the film when plating is performed at each plating solution temperature. From FIG. 6, it was confirmed that the plating rate of the gold plating solution according to the present invention increased with increasing solution temperature, and it was found that the plating rate could be adjusted by the solution temperature.

[0040]

As described above, the gold plating solution according to the present invention is excellent in stability and can be plated without being decomposed without adding an additive during plating. Also,
In the plating solution according to the present invention, the plating rate can be controlled by adjusting the plating solution pH and the solution temperature, and a film having good properties can be stably manufactured by an operation within an appropriate range.

[Brief description of the drawings]

FIG. 1: 0.5 mM and 1.0 mM bis (thiourea)
The figure which shows the CV curve of gold (I) chloride aqueous solution.

Fig. 2 Measurement result of time change of film thickness deposited in each plating solution

FIG. 3 is a view showing an X-ray diffraction profile of a film deposited with a plating solution using tartaric acid as a reducing agent.

FIG. 4 is a view showing an SEM image of a film deposited with a plating solution using tartaric acid as a reducing agent.

FIG. 5 shows two types of plating solutions (pH = 2.3, pH =
The figure which shows the change of the film thickness when plating processing is performed in 4.0).

FIG. 6 is a diagram showing a change in film thickness when plating is performed at each plating solution temperature.

Continued on the front page F term (reference) 4K022 AA02 AA42 AA47 BA03 DA01 DB02 DB04 DB07 DB08 DB24 5E343 BB23 CC78 DD33 DD36 GG06

Claims (5)

[Claims] 1. An electroless gold plating solution comprising a gold salt comprising a bis (thiourea) gold (I) complex represented by the following formula, and a reducing agent. Embedded image (In the formula, X represents a monovalent anion selected from the group consisting of halogen ions such as Cl ⁻ and nitrate ions.) 2. The bis (thiourea) gold (I) complex of the formula (1) is used in an amount of 5.0 × 10 ^{−4 to} 1.0 × 10 ⁻¹ M, and the reducing agent is used in an amount of 1.10 × 10 ⁻¹ M.
The electroless gold plating solution according to claim 1, wherein the electroless gold plating solution is contained at a concentration of 0 × 10 ^{−3 to} 1.0 M. 3. The reducing agent is one of sodium hypophosphite, thiourea, tartaric acid or a mixture thereof.
Or the electroless gold plating solution according to claim 2. 4. An electroless gold plating method for plating an object to be plated by immersing the object in the gold plating solution according to claim 1, wherein the pH of the gold plating solution is 2. An electroless gold plating method of plating with 0 to 5.0. 5. The electroless gold plating method according to claim 4, wherein the plating is performed by setting the temperature of the gold plating solution at room temperature to 80 ° C.