US20040231995A1

US20040231995A1 - Printed circuit boards and the methods of their production

Info

Publication number: US20040231995A1
Application number: US10/849,820
Authority: US
Inventors: Kenji Murao
Original assignee: Hitachi Ltd; Nitto Denko Corp
Current assignee: Hitachi Ltd; Nitto Denko Corp
Priority date: 2003-05-23
Filing date: 2004-05-21
Publication date: 2004-11-25
Also published as: JP2004346381A; CN1575106A; KR20040103770A; TW200507721A; TWI235630B

Abstract

It is an object of the present invention to provide a copper electroplating method for a printed circuit board having via-holes, which can reproducibly secure good plated film quality and via-hole filling capacity even when the board includes a resist or the like. The electroplating bath for electroplating of a printed circuit board, containing at least one compound selected from the group consisting of pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety.

Description

FIELD OF THE INVENTION

The present invention relates to a printed circuit board in which a circuit board is subjected to a copper electroplating to form a desired electrical circuit, more particularly to a printed circuit board in which via-holes of a printed circuit board having blind via-holes are filled with a copper electroplating, and a method for producing the same. Further, the present invention relates to a copper electroplating method for electroplating via-holes of a printed circuit board having blind via-holes with copper, and a copper electroplating bath for electroplating a varying board with copper.

BACKGROUND OF THE INVENTION

Recently, electronic devices are increasingly required to be compacter and more functional. The printed circuit board on which a varying electronic device is mounted is increasingly required to have a denser wiring, higher degree of integration and higher contact reliability, in order to cope with the above trends. In order to satisfy these requirements, the so-called build-up process, in which conductive layer and insulating layer are alternately laminated on a base substrate after undergoing a full-additive process or semi-additive process to form a fine and three-dimensional circuit, has been widely employed.

One example of the methods widely used for forming these fine wirings comprises deposition of a thin copper film on a surface of an insulating resin layer by sputtering or the like, forming a desired resist pattern on the thin film and copper electroplating the laminate. It is a common practice in this method to secure an electrical contact between a conductive layer present below an insulating layer and a conductive layer provided above it by means of connecting holes by the name of via-holes, where each via-hole is plated with a conductive material on the wall or totally filled with a conductive material.

More specifically, an insulating layer is irradiated with carbon dioxide gas or bored by a mechanical means (e.g., drill) on the connecting or conducting passage area, and each hole is plated with a conductive material on the wall or totally filled with a conductive material. This procedure can secure electrical contact between the conducting layers with the insulating layer in-between, and is widely employed by the industry concerned.

SUMMARY OF THE INVENTION

However, it is more preferable to fill each via-hole totally with a conductive material by electroplating than to plate the via-hole wall only with a conductive material, because of much improved reliability of the electrical contact.

Filling via-holes by electroplating carries another advantage when 3 or more conductive layers are laminated via an insulating layer, because the via-holes between the first and second conductive layers can be arranged immediately above those between the second and third conductive layers.

In result, this structure reduces a space for the via-holes as a whole, thereby allowing the devices to be mounted at a higher density. Via-holes generally have a diameter of several tens microns in consideration of the trends to finer wirings.

When via-holes are filled with a conductive material by electroplating, it is necessary to form a thin film of conductive material beforehand by electroless copper plating or the like on the surface to be electroplated. At the same time, special considerations are required for the subsequent electroplating by which via-holes are filled with copper.

More specifically, the surface, on which a conductive circuit is formed, has a flat surface other than via-holes and a concave portion of via-hole, with the result that it may be excessively coated with a plated conductive film when the via-holes are completely filled with a conductive material by electroplating. Conversely, when the surface is to be coated with a conductive layer of adequate thickness by electroplating, the via-holes may be filled insufficiently.

In order to avoid these difficulties, copper electroplating baths incorporated with several types of additives have been widely used to fill the via-holes with a conductive material by electroplating and, at the same time, to coat the surface with a conductive layer of adequate thickness by copper electroplating.

The copper electroplating bath prepared to adequately fill via-holes is incorporated with three types of additives (polymer component, leveler component and brightener component). The procedure for filling via-holes with the above bath has been widely recognized.

Of these three types of additives, the leveler component is normally of an organic dye. The mechanisms by which these three additive components work for filling via-holes by copper electroplating are not fully understood. However, it is widely accepted that the leveler component as one of the three additive components works to control copper separation rate both inside and outside of each via-hole. This is described below.

That is, the molecules constituting the leveler component are adsorbed on the boar surface to be electroplated to retard copper electroplating, but are depleted on the electrode (board surface to be electroplated) by being electrochemically decomposed or included in the plated film.

This depletion is accompanied by the leveler component molecules diffusing towards the board surfaces from the bulk bath, whose concentration remains essentially constant. Diffusion rate of the leveler component molecules is one of the factors that determine plating rate. The substance used as the leveler component generally has a relatively high molecular weight, and produces a gradient of its concentration in the passage (diffusion layer) between the bulk plating bath to the board surface at a plating rate beyond a certain level. On the other hand, the concave portion of the via-hole requires a longer diffusion path than the flat portion free of the via-holes to have a smaller quantity of the diffused leveler component at the bottom than the upper flat surface free of the via-holes. Therefore, the leveler component is present always at a lower concentration on the via-hole bottom surface than on the flat surface, with the result that the plated copper film grows faster on the via-hole bottom surface than on the flat surface. It is understood that the via-hole is filled with copper by electroplating by the above effect.

The leveler component having the above characteristics may cause undesirable effect of inhomogeneity of plated film thickness, when it is unevenly distributed on the board surface.

The uneven concentration distribution of the leveler component is more noted in the areas where it diffuses unevenly, e.g., in the vicinity of the resist wall, or the plating bath tends to flow unevenly, with the above-described via-hole insides and outsides set aside.

Therefore, electroplating to fill via-holes needs a plating bath completely free of leveler component or containing it at a low concentration. It is an object of the present invention to provide a plating method which fills via-holes while substantially removing the problems caused by a leveler component and, at the same time, secures a good plated film of uniform thickness on the other portion.

An organic dye used as the leveler is generally expensive, and use of a plating bath completely free of leveler component or containing it at a low concentration brings another advantage of reducing cost of the copper electroplating to fill the via-holes.

The problems resulting from use of a leveler are described taking a printed circuit board as an example, the similar problems should occur with a semiconductor board which are provided with finer via-holes.

Therefore, it is another object of the present invention to provide a printed circuit board electroplated with copper on the circuit board to form a desired electrical circuit thereon, wherein the via-holes in the printed circuit board are filled with copper by electroplating. It is still another object of the present invention to provide a copper electroplating method for a printed circuit board with blind via-holes to electroplate the via-holes. It is still another object of the present invention to provide a copper electroplating bath for electroplating a varying board with copper.

The inventors of the present invention have concluded, after having extensively studied to solve the above problems, that it is necessary to find out a method for filling via-holes by electroplating without depending on a leveler component.

They have also extensively studied functions of each of the above-described additives in searching for plating baths completely free of leveler component or containing it in trace quantities, to find that a leveler component has two functions.

One of the functions is to form assemblies of the molecules in the interface by stacking them, a property which many organic dye exhibit. Formation of these assemblies tends to occur in defects, e.g., dislocation or steps, in which addition of the copper atoms progresses most notably. As a result, it retards progress of plating in the cathodic interface.

The other function, which a leveler component exhibits, is to break an adsorption film of polymer covering the electrode surface, which is coated and blocked with the polymer component, e.g., polyethylene glycol, thereby facilitating access of the copper ion or brightener component to the board surface. It is a property of organic, ionic compounds on which leveler component charges are highly delocalized.

In other words, a leveler component represented by an organic dye is found to have two functions contradictory to each other, one is retarding the plating reaction and the other accelerating the reaction. For the latter function, a leveler is not necessarily of an associative dye. At the same time, it is meant that even an organic, ionic compound of relatively low molecular weight can exhibit the function, when filling of via-holes is achieved mainly by a brightener component function.

When a brightener approaches the electrode surface, the surface area in the via-hole is reduced as the plating reaction proceeds, with the result that the brightener component is concentrated on the surface, and the reaction proceeds faster in the via-hole inside than in the outside. The via-hole filling process by the brightener concentration on the surface is based on its property of being depleted at a lower rate in the plating reaction process.

Based on the above concept, the inventors of the present invention have searched for alternative via-hole filling additives having a lower molecular weight than the conventional leveler component, not associative at least at around a plating potential, soluble in water and active in the interface.

The associative property of the conventional leveler component is closely related to the fact that it is of an organic dye. Therefore, it is an important condition for the additive component that replaces the conventional leveler component not to substantially exhibit strong light absorption in the visible region.

The new leveler component will exhibit a high via-hole filling capacity when used in combination with the brightener component, as discussed above. It is found that the new additive for the present invention can fill the via-holes and, at the same time, give the plated film of high homogeneity on the portion other than the via-holes.

It is also found, in the case of a board with a plated resist pattern, that the new additive can give a highly flat and excellent plated film even in the vicinity of the wall. It is well known that the conventional leveler frequently deteriorates plated film flatness significantly in the vicinity of the plated resist wall.

The good results demonstrated by the method of the present invention results from its independence on a compound of very low diffusibility used for the conventional leveler for via-hole filling.

In the method of the present invention, which uses no component of low diffusibility, problems resulting from diffusion of the copper ion itself, which are concealed in the presence of the leveler component of strong functions, tend to be more noted.

These problems sometimes observed in the vicinity of the plated resist wall. They result from retarded diffusion of the copper ion across the resist wall. The effect of retarded symmetry of the copper ion diffusion is much smaller than that of a leveler, but exactly opposite in direction.

In other words, the copper ion is more used for plating as it diffuses more, unlike the case with a leveler. It is possible to improve homogeneity of the plated film thickness to a still higher extent by utilizing the functions of the cooper ion and leveler component working in the opposite direction under the same geometric conditions which limit diffusion of the solute.

The above can be achieved by only incorporating a trace quantity of a leveler component in the electroplating bath, in addition to the above-described polymer component, brightener component and organic ionic compound. A leveler is incorporated only in a quantity sufficient for canceling out small fluctuations of film thickness resulting from asymmetry of the copper ion diffusion.

The inventors of the present invention have noted a specific quaternary, nitrogen-containing heterocyclic compound as the copper plating promoter which satisfies the above requirements, achieving the present invention.

Thus, the first aspect of the present invention relates to a printed circuit board, the second aspect relates to a method for producing the same, the third aspect relates to an electroplating method, and the fourth aspect relates to a copper electroplating bath. The copper electroplating bath is incorporated with at least one compound selected from the group consisting of pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety as a copper plating promoter.

In the present invention, the pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl or N-aryl moiety etc. mean an organic pyridinium cation represented by the general formula:

(wherein, R ₁is an organic moiety selected from the group consisting of alkyl, phenyl and aralkyl moiety), an organic bipyridinium cation represented by the general formula:

(wherein, R ₂and R₃are each an organic moiety selected from the group consisting of alkyl, phenyl and aralkyl moiety),

an organic phenanthrolinium cation represented by the general formula:

(wherein, R ₄is an organic moiety selected from the group consisting of alkylene and aralkylene moiety), an organic quinolinium cation represented by the general formula:

(wherein, R ₅is an organic moiety selected from the group consisting of alkyl, phenyl and aralkyl moiety), and

an organic phenazinium cation represented by the general formula:

(wherein, R ₆is an organic moiety selected from the group consisting of alkyl, phenyl and aralkyl moiety),

Of these five types of the cationic salts, the organic bipyridinium salt is particularly preferable for its excellent via-hole filling capacity and availability in the markets.

The N-alkyl, N-phenyl and N-aralkyl moieties etc. represented by one of R ₁to R₅may be substituted with a varying substituent or unsubstituted. Sulfonic acid moiety is a preferable substituent, because these moieties substituted therewith can be more compatible with the sulfonic acid anion normally present in the copper electroplating bath.

The pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl or N-aryl moiety etc. are clearly distinguished from an organic dye which has been traditionally used as the leveler component. These cationic salts are characterized by a low molar extinction coefficient of only 5000 M ⁻¹cm⁻¹(M: moles/litter) in a wavelength region of 400 to 700 nm.

Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the printed circuit board electroplated with copper, prepared in EXAMPLE 1, for the portion containing via-holes, drawn based on the microgram. [0050]
FIG. 2 is a cross-sectional view of the printed circuit board electroplated with copper, prepared in COMPARATIVE EXAMPLE 1, for the portion containing via-holes, drawn based on the microgram. [0051]
FIG. 3 is a cross-sectional view of the printed circuit board electroplated with copper, prepared in COMPARATIVE EXAMPLE 5, for the portion containing via-holes, drawn based on the microgram.[0052]

DESCRIPTION OF THE REFERENCE NUMERALS AND SIGNS

[0053] 1: Base substrate
[0054] 2: Copper foil layer
[0055] 3: Resist
[0056] 4: Epoxy resin layer
[0057] 5: Copper-plated film

DETAILED DESCRIPTION OF THE INVENTION

The via-holes for the present invention are generally 10 to 600 μm deep and 5 to 800 μm in diameter for printed circuit boards. These via-holes can be produced by boring an insulating resin layer put between two conductive layers with laser beams. It is needless to say that the via-holes are finer when the copper plating of the present invention is applied to a semi-conducting board. [0058]
Each via-hole can be electroplated, when its bottom and sides are made conductive beforehand by electroless plating or sputtering. It is also possible to make a desired portion outside of the via-hole conductive by a similar means. [0059]
The representative composition of a copper electroplating bath for filling via-holes by the copper electroplating method of the present invention is copper sulfate pentahydrate: 55 to 240 g/L, concentrated sulfuric acid: 60 to 260 g/L and chlorine ion content: 60 to 100 ppm. It should be noted that implementation of the present invention does not become immediately difficult when the composition is out of the above range. [0060]
The base composition prepared above for the copper electroplating bath is incorporated with polyethylene glycol as a polymer component at 20 to 300 mg/L and sodium salt of bis-(3-sulfopropyl) disulfide (hereinafter referred to as SPS), represented by (NaO[0061] ₃S—CH₂—CH₂—CH₂—S—) ₂, as a brightener component at 1 to 30 mg/L. The above solution is further incorporated with at least one selected from the group consisting of the above-described five types of cationic salts, i.e., pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl or N-aryl moiety etc. at 2 to 2000 μmols/L for the electroplating.
It is preferable, depending on circumstances, to incorporate a trace quantity of an organic dye or the like as a leveler in the electroplating bath composition prepared above to still improve flatness of the plated film. The anions which can be suitably used as the counter ion of the above-described organic cation include a halide ion, e.g., chloride ion, and sulfonate ion. The ionic compound is incorporated as the counter ion at a much lower content than any other component of the base plating bath, and cannot greatly affect the ionic concentration of the bath. No trouble is anticipated when the chloride or sulfonate ion originally present in the base composition of the plating bath is used as the counter ion, unless its concentration is out of the originally intended suitable ion concentration range. [0062]
For energizing the cathode and anode for electroplating, either a potentiostatic or galvanostatic method can satisfy the object of the present invention. Galvanostatic copper electroplating is preferably carried out at a current density applied of 2.5A dm[0063] ⁻², but the effect of the present invention can be realized at 0.5 to 6.0A dm⁻².

EXAMPLES

The present invention is described by EXAMPLES. [0064]

Examples 1

A base substrate having a copper foil coated on a surface thereof was coated with a 15 μm thick insulating layer of known epoxy resin normally used for the build-up process, and then the surface was irradiated with carbon dioxide gas laser beams to form via-holes, 60 μm in diameter at the opening, 50 μm in diameter at the bottom and 15 μm deep. The coated base substrate was immersed in a permanganic acid solution for desmearing, and then provided with a catalyst by a known treatment method, to form a 0.6 μm thick electroless, copper-plated film. The treated base substrate was cut into 60 mm by 60 mm specimens in such a way that each had via-holes. Each specimen was electroplated at a current density of 2.5A dm[0065] ⁻²for 26 minutes in a plating bath of the composition given in Table 1 while it was kept upright. During the plating, the bath was stirred by airing in the electroplating tank having inner dimensions of width: 250 mm, depth: 200 mm and height: 200 mm.

The electroplated base substrate having via-holes was cut and ground, to observe the via-hole cross-section. FIG. 1 presents a microscopic cross-sectional view which schematically shows the base substrate electroplated with copper, prepared in EXAMPLE 1. As shown, the via-hole was filled with copper by electroplating. The surface was good in flatness, with irregularities of only around ±2 μm even in the vicinity of the resist.

TABLE 1


Composition of the plating bath used in EXAMPLE 1

	Plating bath components	Concentration

	Copper sulfate pentahydrate	200 g/L
	Concentrated sulfuric acid	60 g/L
	Hydrochloric acid (0.1 mol/L)	17 mL/L
	Polyethylene glycol 4000	100 mg/L
	SPS
	5 mg/L
	Benzyl biologen chloride	25 μmol/L

Comparative Example 1

A base substrate was electroplated in the same manner as in EXAMPLE 1, except that the electroplating bath composition given in Table 1 contained no benzyl biologen chloride. The electroplated base substrate having via-holes was cut and ground to observe the via-hole cross-section. FIG. 2 presents a microscopic cross-sectional view which schematically shows the base substrate electroplated with copper, prepared in COMPARATIVE EXAMPLE 1. As shown, the via-hole was not filled sufficiently with copper by electroplating. The plated film surface was found to have irregularities of ±5 μm or more. [0067]

Example 2

A base substrate was electroplated in the same manner as in EXAMPLE 1, except that the plating bath composition was replaced by the one given in Table 2. The electroplated base substrate having via-holes was cut and ground to microscopically observe the via-hole cross-section. It was observed that the via-hole was filled with copper by electroplating, and the copper-plated film surface was good in flatness in the vicinity of the resist.

TABLE 2


Composition of the plating bath used in EXAMPLE 2

	Plating bath components	Concentration

	Copper sulfate pentahydrate	200 g/L
	Concentrated sulfuric acid	80 g/L
	Hydrochloric acid (0.1 mol/L)	17 mL/L
	Polyethylene glycol 4000	200 mg/L
	SPS	8 mg/L
	Hexylpyridinium chloride	35 μmol/L

Compapative Example 2

A base substrate was electroplated in the same manner as in EXAMPLE 2, except that the electroplating bath composition given in Table 2 contained no hexylpyridinium chloride. The electroplated base substrate having via-holes was cut and ground to microscopically observe the via-hole cross-section. It was observed that the via-hole was not filled sufficiently with copper by electroplating, and the plated film surface had irregularities of ±5 μm or more. [0069]

Example 3

A base substrate was electroplated in the same manner as in EXAMPLE 1, except that the plating bath composition was replaced by the one given in Table 3. The electroplated base substrate having via-holes was cut and ground to microscopically observe the via-hole cross-section. It was observed that the via-hole was filled with copper by electroplating, and the copper-plated film surface was high in flatness showing no phenomenon of growth of film thickness in the vicinity of the resist.

TABLE 3


Composition of the plating bath used in EXAMPLE 3

	Plating bath components	Concentration

	Copper sulfate pentahydrate	200 g/L
	Concentrated sulfuric acid	100 g/L
	Hydrochloric acid (0.1 mol/L)	17 mL/L
	Polyethylene glycol 4000	100 mg/L
	SPS	6 mg/L
	Hexylquinolinium chloride	30 μmol/L

Compapative Example 3

A base substrate was electroplated in the same manner as in EXAMPLE 3, except that the electroplating bath composition given in Table 3 contained no hexylquinolinium chloride. The electroplated base substrate having via-holes was cut and ground to microscopically observe the via-hole cross-section. It was observed that the via-hole was not filled sufficiently with copper by electroplating, and the plated film surface had irregularities of ±5 μm or more. [0071]

Example 4

A base substrate was electroplated in the same manner as in EXAMPLE 1, except that the plating bath composition was replaced by the one given in Table 4. The electroplated base substrate having via-holes was cut and ground to microscopically observe the via-hole cross-section. It was observed that the via-hole was filled with copper by electroplating, and the copper-plated film surface was high in flatness showing no phenomenon of growth of film thickness in the vicinity of the resist.

TABLE 4


Composition of the plating bath used in EXAMPLE 4

	Plating bath components	Concentration

	Copper sulfate pentahydrate	200 g/L
	Concentrated sulfuric acid	70 g/L
	Hydrochloric acid (0.1 mol/L)	17 mL/L
	Polyethylene glycol 4000	150 mg/L
	SPS	7 mg/L
	Ethylphenazinium chloride	30 μmol/L

Comparative Example 4

A base substrate was electroplated in the same manner as in EXAMPLE 4, except that the electroplating bath composition given in Table 4 contained no ethylphenazinium chloride. The electroplated base substrate having via-holes was cut and ground to microscopically observe the via-hole cross-section and the via-hole periphery in contact with the resist wall. It was observed that the via-hole was not filled sufficiently with copper by electroplating, and the plated film surface had irregularities of ±5 μm or more. [0073]

Comparative Example 5

A base substrate was electroplated in the same manner as in EXAMPLE 1, except that the electroplating bath composition contained Janus Green B as a leveler in place of benzyl biologen chloride for the present invention (Table 5). The electroplated base substrate having via-holes was cut and ground to microscopically observe the via-hole cross-section and the via-hole periphery in contact with the resist wall. FIG. 3 presents a microscopic cross-sectional view which schematically shows the base substrate electroplated with copper, prepared in COMPARATIVE EXAMPLE 5. As shown, the via-hole was filled with copper to some extent by the electroplating, but thickness of the plated film increased to 5 μm or more in the area in contact with the resist wall. It was therefore found that flatness of the plated film as one of the objects of the present invention could not be secured.

TABLE 5


Composition of the plating bath used in
COMPARATIVE EXAMPLE 5

	Plating bath components	Concentration

	Copper sulfate pentahydrate	210 g/L
	Concentrated sulfuric acid	80 g/L
	Hydrochloric acid (0.1 mol/L)	17 mL/L
	Polyethylene glycol 4000	120 mg/L
	SPS
	5 mg/L
	Janus Green B	20 μmol/L

It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims. [0075]

Effects of the Present Invention

As described above, the method of the present invention for electroplating a board with via-holes can well fill the holes with copper and realize high flatness of the plated film. [0076]

Claims

What is claimed is:

1. A printed circuit board having via-holes plated by a copper electroplating, wherein the via-holes are substantially filled with a copper electroplating using a copper electroplating bath containing at least one compound selected from the group consisting of pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety.

2. A method for producing a printed circuit board having via-holes plated by a copper electroplating, wherein an electroplating bath used in the copper electroplating contains at least one compound selected from the group consisting of pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety.

3. An electroplating method which conducts a copper electroplating using an electroplating bath containing at least one compound selected from the group consisting of pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety.

4. The electroplating method according to claim 3, wherein the N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety contains sulfonic acid moiety.

5. The electroplating method according to claim 3, wherein the salts have a low molar extinction coefficient of 5000 M⁻¹cm⁻¹(M: moles/litter) in a wavelength region of 400 to 700 nm.

6. A copper electroplating bath used in a copper electroplating which contains at least one compound selected from the group consisting of pyridinium, bipyridinium, phenanthrolinium, quinolinium and phenazinium salts in the form of onium with an N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety.

7. The copper electroplating bath according to claim 6, wherein the N-alkyl, N-aralkyl, N-aryl, N-alkylene or N-aralkylene moiety contains sulfonic acid moiety.

8. The copper electroplating bath according to claim 6, wherein the salts have a low molar extinction coefficient of 5000 M⁻¹cm⁻¹(M: moles/litter) in a wavelength region of 400 to 700 nm.