Stabilized Electroless Plating Solutions

Abstract

An electroless copper plating solution is characterized by the addition of a small but effective amount of a source of mercury ions for increased bath stability. It is known in the art that solutions for electroless copper plating are unstable and tend to decompose with use. It is also know that decomposition can be retarded and the useful life of a copper solution increased by the addition of various additives, frequently catalytic poisons, in very small concentrations. In accordance with the present invention, it has been found that the stability of an electroless plating solution can be substantially increased by the addition of a source of mercury ions alone as a primary stabilizer, or preferably, by the addition of the mercury ions as a secondary stabilizer in combination with a prior art stabilizer as primary stabilizer. The combination of stabilizers provides a synergism with stability substantially improved over that obtainable with either component of the combination alone. One characteristic of the present invention is that the co-deposition of mercury in minute amounts with the plating metal.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application of U.S. Pat. Application Ser. No. 785,350, filed Dec. 19, 1968, now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a copper-depositing composition and more particularly, to an electroless copper plating solution having extended life and temperature tolerance and characterized by the addition of a stabilizer comprising a source of mercury ions alone or in combination with a prior art stabilizer.

2. Description of the Prior Art

Electroless metal deposition refers to the chemical plating of a metal over an active surface by chemical reduction in the absence of an external electric current. Processes and compositions useful therefor are known, are in substantial commercial use, and are described in numerous publications. For example, compositions for depositing electroless copper are described in U.S. Pat. Nos. 2,938,805; 3,011,920; and 3,383,224 included herein by reference.

Known electroless copper deposition solutions generally comprise at least four ingredients dissolved in solvent, usually water. They are (1) a source of the copper ions, (2) a reducing agent such as formaldehyde, (3) an acid or hydroxide pH adjuster to provide required pH, and (4) a complexing agent for copper ions sufficient to prevent their precipitation in solution. A large number of suitable complexing ions for electroless metal solutions are described in the above noted publications and also in U.S. Pat. Nos. 2,874,072; 3,075,586; and 3,075,855 also incorporated herein by reference.

Although electroless copper solutions have been used for many years, the commercially used formulations have not been fully satisfactory for several reasons. Among these are relatively slow deposition rates and bath instability. It has been shown that plating rate is dependent to some extent, upon the concentration of the reducing agent in the plating solution and that increased concentration will generally result in an increased rate of deposition. However, increased concentration of reducing agent also results in decreased bath stability. This is evidenced by a decrease in the time in which the plating solution will undergo uncontrollable decomposition (trigger).

It is known in the art that certain additives or inhibitors added to an electroless copper solution in properly controlled trace quantities act as stabilizers and retard the rate of bath decomposition. Generally speaking, these additives, or stabilizers as they are referred to in the art, are catalytic poisons. The concentration of the stabilizer in solution is usually critical. Trace quantities, typically in the range of a few parts per million, provide stability. An excess of stabilizer will partially or totally stop deposition of the electroless copper.

STATEMENT OF THE INVENTION

The present invention is predicated upon the discovery that the addition of a small but effective amount of a source of mercury ions to substantially any electroless copper solution improves stability without retarding the rate of deposition and in some solutions, provides a somewhat improved rate of deposition. Moreover, in accordance with the invention, it has been found that the addition of a combination of a mercury compound with a prior art stabilizer to an electroless copper solution provides a synergism resulting in a substantially increased solution stability. Accordingly, the present invention provides an electroless copper deposition solution comprising (1) a source of copper ions, (2) a reducing agent therefor such as formaldehyde, (3) a pH adjuster, (4) a complexing agent for the copper ions sufficient to prevent their precipitation in solution, and (5) a stabilizer for the solution which may be a source of mercury ions alone, as a primary stabilizer or in combination with a prior art stabilizer. Metal deposits from the electroless copper solutions of this invention are characterized by the co-deposition of small quantities of mercury.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As noted above, the mercury ion is believed to be responsible for increased bath stability. Both simple and complex mercury compounds soluble in the electroless metal plating solution are suitable sources of the mercury ion for purposes of the present invention. Typical examples of mercury compounds include mercuric acetate, mercuric benzoate, mercuric bromide, mercurous chloride, mercuric carbonate, mercuric chlorate, mercuric chloride, mercuric iodate, mercurous nitrate, mercuric nitrate, mercuric sulphate and mercuric ammonium chloride.

It should be noted that many mercury compounds are not fully soluble in aqueous solutions and many are considered insoluble. However, the mercury compound, for purposes of the present invention, is required in amounts capable of dissociating to yield mercury ions in concentrations of only parts per million. Consequently, mercury compounds considered insoluble in aqueous solution may be soluble to the extent that they yield mercury ions in concentrations sufficient for purposes of the present invention. Mercury compounds soluble in aqueous solution are preferred.

Since the mercury ions are not catalytic poisons, their concentration in solution is not critical. Frequently, trace quantities are suitable. A preferred range comprises from about 1 to 100 parts per million. However, amounts up to saturation in solution may be used or amounts in excess of saturation may be used, the excess serving as a source of mercury ions for replenishment.

The particular mercury compound selected for purposes of the present invention is not critical provided it yields sufficient mercury ions and provided that the anion of the compound is not harmful to the plating solution.

In a preferred embodiment of this invention, the mercury compound is used as a stabilizer in combination with a prior art stabilizer. Materials known to the art as catalytic poisons to the deposition of electroless copper are frequently used in controlled amounts as stabilizers for electroless copper plating solutions. Perhaps the most widely used group of compounds of this nature are the divalent sulphur-containing compounds, many of which are disclosed in U.S. Pat. No. 3,361,540, incorporated herein by reference. Representative examples of such sulphur compounds are the inorganic sulfides such as sodium sulfide, potassium sulfide, sodium polysulfide, and potassium polysulfide; organic and inorganic thio compounds such as sodium thiocyanate, potassium thiocyanate, potassium dithionate, sodium thiosulfate, and potassium thiosulfate; and organic sulphur containing compounds such as thiourea, 2-mercaptobenzothiazole, 1,2-ethanedithiol, 1,2-benziosothiazane, methionine, 2,2'-thiodiethanol, dithioglycol, and thioglycollic acid.

The amount of the sulfur compound used in combination with the mercury compound is small and will vary depending upon the particular compound used. Typically, the sulfur compound is present in an amount less than that which will not stop deposition of the plating metal. Generally, the amount may vary from a trace to about 300 parts per million dependent upon the sulfur compound used.

There are many stabilizers in the prior art in addition to divalent sulfur compounds. One other class of stabilizers comprises the water soluble cyanide compounds defined broadly to include nitriles and dinitriles set forth in U.S. Pat. No. 3,310,430. Typical of such compounds are alkali metal cyanides such as sodium and potassium cyanide; nitriles such as alpha-hydroxynitrile, e.g., glyconitrile and lactonitrile, and dinitriles such as iminodiacetonitriles and 3,3'-iminodipropionitrile. The cyanide compound is used in an amount about equal to that of the divalent sulfur compound.

An additional class of stabilizers for electroless copper solutions is disclosed in copending U.S. Pat. No. 3,457,089, filed in the names of Charles Shipley and Michael Gulla. These stabilizers are acetylinic compounds corresponding to one of the following generic formulas:

R -- C .tbd. CH or R' -- C .tbd. C - R"

where each R is individually selected from the class of lower monovalent hydroxyalkyl, cyclohydroxyalkyl or hydroxyalkyl ether. Examples include ethynyl cyclohexanol, methyl butynol, methyl pentynol, dimethyl hexynol, 2-butyne-1, 4-diol, dimethyl hexynediol, propargyl alcohol, hexynol and ethyl octynol.

It has been found that the above class of compounds can be broadened to include substantially more members when used in combination with a mercury compound. For example, when used in combination with a mercury compound, each of R,R' and R" can be aryl or aliphatic including cycloaliphatic substituted with a water solubilizing group such as hydroxyl and carboxyl. For purposes of the present invention, the acetylinic compounds contemplated will be referred to by the term "solution soluble acetylinic compounds."

Additional examples of metals useful as stabilizers for electroless copper solutions are disclosed in the aforementioned U.S. Pat. No. 3,310,430.

An electroless copper plating solution stabilized with a mercury compound in accordance with this invention is used to deposit metal in the same manner as any prior art electroless copper solution. The surface of the part to be plated should be free of grease and contaminating material. Where a non-metallic surface is to be plated, the surface area to receive the deposit must first be sensitized to render it catalytic to the reception of the electroless metal as by the well-known treatment with an acidic aqueous solution of stannous chloride followed by treatment with dilute aqueous acidic solution of palladium chloride. Alternatively, extremely good sensitization of non-metallic surfaces is achieved by contact with a colloid of a precious metal having a protective stannic acid colloid and formed by the admixture of stannous chloride and a precious metal chloride, preferably palladium chloride, the stannous chloride being present in stoichmetric excess based upon the amount of precious metal chloride.

The invention will be better understood by reference to the following examples where stability of solution was measured by the time it takes for a bath to spontaneously decompose (trigger) when plating a catalyzed cloth at one-half square foot per gallon. In all examples, catalyzed cloth was prepared by treating a cotton fabric according to the following sequence of steps:

1. Rinse cloth in a 20 percent (by weight) ammonium hydroxide solution maintained at room temperature for 5 minutes.

2. Rinse for 5 minutes in 20 percent acetic acid solution maintained at room temperature. Rinse in cold water.

3. Immerse for from 20 to 40 seconds in a sensitizing solution of a palladium colloid having a protective stannic acid colloid (Catalyst 6F) maintained at room temperature. Rinse in cold water.

4. Immerse for 1 to 3 minutes in a dilute hydrochloric acid solution maintained at room temperature. Rinse in cold water.

5. Dry cloth and cut to size.

In the following examples where rate of metal deposition from solution is recorded, rate was determined by plating over a phenolic substrate using the following procedure:

1. Cut phenolic board to a size measuring 2 inches by 2 inches.

2. Scrub clean with an abrasive cleaner. Rinse in cold water.

3. Treat for from 1 to 3 minutes with a non-ionic surfactant conditioner maintained at room temperature. Rinse in cold water.

4. Immerse for from 3 to 4 minutes in a sensitizing solution of colloidal palladium having a stannic acid protective colloid (Catalyst 6F) maintained at room temperature. Rinse in cold water.

5. Immerse for from 3 to 6 minutes in dilute hydrochloric acid solution maintained at room temperature. Rinse in cold water.

6. Deposit electroless metal for a period of 10 minutes.

7. Rinse, dry parts and measure thickness of deposit. In the examples, Example number and Bath number are used interchangeably. --------------------------------------------------------------------------- Examples 1 to 10

Cupric sulfate pentahydrate 8 g. Paraformaldehyde 7.5 g. Tetrahydroxypropyl- ethylenediamine 12 g. Triisopropanolamine 2 g. Sodium hydroxide (28 wt. %) 50 ml. Water to 1 liter Temperature 120.degree. .+-. 5.degree. F. __________________________________________________________________________

catalyzed cloth was plated with the above formulation with stabilizing additives added in amounts and with results as set forth in the following table: --------------------------------------------------------------------------- Stabilizing Agent (s) (p.p.m.)

Bath Time to Trigger No. Mercury Other (min.) __________________________________________________________________________ 1 -- -- 1/2- 2 2 mercuric acetate (20) -- >60 3 -- NaCN(5) 20-25 4 mercuric acetate NaCN(5) >60 (20) 5 -- methyl butynol 1/2-2 (50) 6 mercuric acetate methyl butynol >60 (20) (50) 7 -- thiomalic acid 20-25 (15) 8 mercuric acetate thiomalic acid >60 (20) (15) 9 mercuric chloride -- >60 (20) 10 mercuric chloride methyl butynol >60 (20) (50) 11 mercurous nitrate -- >60 12 mercurous nitrate methyl butynol >60 (20) (50) __________________________________________________________________________

From the above table, it is apparent that the addition of the mercury compound in small quantities to the copper deposition solution substantially extends the time for solution decomposition and provides a solution more stable than that possible with other prior art stabilizers for copper solutions. In addition, the combination of a mercury compound with the prior art stabilizers provides a synergism with solution stability extended to a time in excess of that using either stabilizer alone. --------------------------------------------------------------------------- Examples 13-16

Cupric sulfate pentahydrate 8 g. Paraformaldehyde 7.5 g. Rochelle salts.sup.(1) 40 g. NaOH (28 wt. %) 50 g. Water to 1 liter Temperature 120.degree. .+-. 5.degree. F. __________________________________________________________________________ (1) A sodium-potassium tartrate double salt.

Catalyzed cloth is plated by immersion in the above solution containing stabilizers in amounts and with results as set forth in the following table: --------------------------------------------------------------------------- Stabilizing Agent (p.p.m.)

Bath Time to Trigger NO. Mercury Other (min.) __________________________________________________________________________ 13 -- -- 1/2-2 14 mercuric acetate -- 3-4 (20) 15 -- NaCN(5) 3-4 16 mercuric acetate NaCN(5) 10-12 (20) __________________________________________________________________________

the addition of the mercuric acetate to the above formulation provided some improvement in stability, about equivalent to that obtainable with sodium cyanide. However, the combination of the mercuric acetate with the sodium cyanide provided substantially greater stability. It should be noted that though time to trigger is short for the above formulation, it is somewhat accelerated by the elevated temperature at which the bath is used, and the time measurement reported is not typical of a formulation used in a commercial facility. --------------------------------------------------------------------------- Examples 17-20

Cupric sulfate pentahydrate 8 g. Paraformaldehyde 7.5 g. Pentahydroxypropyldiethylene- triamine 20 g. Sodium hydroxide (28 wt. %) 50 ml. Water to 1 liter Temperature 120.degree. .+-. 5.degree. F. __________________________________________________________________________

catalyzed cloth was plated by immersion in the above formulation with stabilizers added in amounts and with results as set forth in the following table: --------------------------------------------------------------------------- Stabilizing Agent (s) (p.p.m.)

Bath Time to Trigger No. Mercury Other (min.) 17 -- -- 1/2- 2 18 mercuric acetate -- 2-3 (40) 19 -- methyl butynol 1/2-2 (50) 20 mercuric acetate methyl butynol >60 (40) (50) __________________________________________________________________________

The synergism between the mercury compound and the prior art stabilizers is readily shown by reference to the above table where using either stabilizer alone provided a plating solution that triggered within a few minutes. Combining the stabilizers (Bath No. 20) provided a bath that did not decompose within 60 minutes.

EXAMPLES 21-26

Using the bath formulation of Examples 1-8 at a temperature of 75.degree. .+-. 5.degree. F., the effect of the addition of stabilizers on plating rate was determined by plating a phenolic substrate in the manner noted above. The bath formulations used and plating rate are set forth in the following table:

Plating Rate Bath No. (in./10 min.) __________________________________________________________________________ 1 86.times.10.sup.-.sup.6 2 80.times.10.sup.-.sup.6 5 74.times.10.sup.-.sup.6 6 72.times.10.sup.-.sup.6 7 36.times.10.sup.-.sup.6 8 42.times.10.sup.-.sup.6 __________________________________________________________________________

The addition of mercuric acetate alone to the plating solution (Bath 2) results in some slight decrease in plating rate. The addition of methyl butynol (Bath 5) causes a somewhat greater decrease in rate. However, the addition of thiomalic acid to the plating solution (Bath 7) results in a substantial decrease in rate. Combination of mercuric acetate with the thiomalic acid (Bath 8) does not further decrease the rate, but surprisingly provides a somewhat increased rate of copper deposition.

EXAMPLES 27-32

It is known in the art that contaminants dragged into an electroless plating solution results in a substantial decrease in bath stability. To determine the effect of contamination of an electroless solution containing a mercury compound, various contaminants known to accelerate triggering of an electroless solution were added to baths Nos. 7 and 8 in amounts and with results as set forth in the following table:

Bath Time to Trigger No. Contaminant (Min.) __________________________________________________________________________ 7 -- > 60 8 -- >60 7 PVP/VA 535.sup. (2) 20-25 8 PVP/VA 535.sup.(2) >60 7 dextrose.sup.(3) 10-12 8 dextrose.sup.(3) 30-35 __________________________________________________________________________

(2) Polyvinylpyrrolidone-vinyl acetate copolymer added in an amount of 50 ppm. (3) Added in an amount of 1/2 gram.

EXAMPLES 33-36

It is known in the art that increased bath temperature results in a more rapid decomposition of an electroless plating solution. To determine the effect of temperature on the plating solutions of this invention containing mercury ions alone or in combination with other stabilizers, Baths Nos. 1, 2, 7 and 8 were used to plate catalyzed cloth at varying temperatures with results as set forth in the following table:

Bath Time to Trigger (Min.) No. 70.degree.-80.degree. F. 120.degree.-130.degree. F. 150.degree - .degree. F. __________________________________________________________________________ 1 20-22 1-2 0 2 >60 >60 2-3 7 >60 10-12 0-1/2 8 >60 >60 >60 __________________________________________________________________________

bath No. 8 illustrates the substantial improvement and the synergism resulting from the combination of a mercury compound with a prior art stabilizing compound.

EXAMPLES 27-39

Copper deposited from Baths Nos. 2, 10 and 13 were analyzed for mercury content. It was found that the mercury concentration in the copper deposit from each bath was 0.038%, 0.0025% and 0.105% respectively. Copper deposited from equivalent plating solutions but free of a mercury compound contained no detectable quantities of mercury.

From the above, it should be readily apparent that the following advantages are gained from the addition of a source of mercury ions to an electroless metal solution.

1. Mercury ions used alone are primary stabilizers for electroless copper solutions.

2. Mercury ions in combination with prior art stabilizers for electroless copper solutions exhibit a synergism that improves stability of solution to an extent greater than that obtainable with either the mercury ions or the prior art stabilizer used alone.

3. The addition of a mercury ion to an electroless copper plating solution does not significantly affect the rate of deposition, as with other stabilizing agents known as catalytic poisons.

4. The addition of mercury ions to an electroless copper plating solution allows for a greater operational temperature tolerance in that higher temperatures are permissible due to the stabilizing effect of the mercury ions.

5. The addition of mercury ions to an electroless copper plating solution, to some extent, lessens the detrimental effect of contaminants dragged into the plating bath.

6. Substantially any source of mercury ions is usable to stabilize electroless copper plating solution provided it is not associated with an anion detrimental to electroless plating of the metal (such as the sulphur or cyanide anion in larger than tolerable amounts) and provided further that the mercury source is soluble to the extent that it yields at least a few parts per million of mercury ions in solution.

7. Mercury co-deposits with the electroless copper being plated. From 0.05 to a maximum of 1.0% co-deposited mercury is desirable, and preferably from 0.1% to 0.5%.

The reasons for improved bath stability resulting from the addition of a source of mercury ions to the electroless copper solution are not fully understood, but are believed to be associated with the evolution of hydrogen gas as the metal deposits. An unusual property of mercury is its high overvoltage for hydrogen and it is believed that this property is associated with the stabilization mechanism.

It should of course be understood that changes may be made in the specific embodiments described herein without departing from the scope of the invention as defined by the following claims.

Claims

We claim:

1. In an aqueous electroless copper plating solution including a source of cupric ions, a complexing agent sufficient to render said cupric ions soluble in solution and a reducing agent for said cupric ions;

the improvement comprising mercury ions in solution in a small but effective amount capable of providing increased bath stability.

2. In a basic aqueous electroless copper plating solution including a source of cupric ions, a complexing agent sufficient to render said cupric ions soluble in solution, free hydroxide and formaldehyde as a reducing agent for said cupric ions;

the improvement comprising the addition of mercury ions in an amount of at least one part per million parts of solution to provide increased bath stability.

3. The composition of claim 2 where the mercury ions are derived from a mercury salt having an anionic portion non-interfering with said electroless plating solution.

4. The composition of claim 2 where the mercury ions are in solution in an amount from one part per million parts of solution to saturation.

5. The composition of claim 2 where the mercury ions are in solution in an amount varying from one to 100 parts per million parts of solution.

6. The composition of claim 2 containing a second stabilizing agent selected from the group consisting of divalent sulphur compounds, cyanide compounds including nitriles and dinitriles and acetylinic compounds.

7. The composition of claim 6 where the acetylinic compounds corresponds to one of the following generic formulas:

R - C .tbd. CH or R' - C .tbd. C - R"

where each of R, R', and R" are radicals selected from the class of monovalent hydroxyalkyl, cyclohydroxyalkyl and hydroxyalkyl ether.

8. The composition of claim 6 where the second stabilizer is an alkali metal cyanide.

9. The composition of claim 6 where the second stabilizer is a thio compound.

10. A method for increasing the stability of an electroless copper plating solution and retarding the time in which the electroless metal plating solution spontaneously decomposes, said electroless copper plating solution including a source of cupric ions, a complexing agent sufficient to render said cupric ions soluble in solution and a reducing agent for said cupric ions, said method comprising the step of adding mercury compound to said electroless copper plating solution in a concentration capable of providing a small but effective amount of mercury ions in solution.

11. An article of manufacture comprising a substrate coated with electroless copper deposited from the solution of claim 1 and characterized by the co-deposition of mercury.