U.S. patent number 3,663,242 [Application Number 05/075,737] was granted by the patent office on 1972-05-16 for stabilized electroless plating solutions.
This patent grant is currently assigned to Shipley Company, Inc.. Invention is credited to Oleh B. Dutkewych, Michael Gulla.
United States Patent |
3,663,242 |
Gulla , et al. |
May 16, 1972 |
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.
Inventors: |
Gulla; Michael (Newton, MA),
Dutkewych; Oleh B. (Medfield, MA) |
Assignee: |
Shipley Company, Inc. (Newton,
MA)
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Family
ID: |
22127682 |
Appl.
No.: |
05/075,737 |
Filed: |
September 25, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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785350 |
Dec 19, 1968 |
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Current U.S.
Class: |
106/1.23;
106/1.26; 427/437; 106/1.22; 427/306 |
Current CPC
Class: |
C23C
18/405 (20130101) |
Current International
Class: |
C23C
18/40 (20060101); C23C 18/31 (20060101); C23c
003/02 () |
Field of
Search: |
;106/1
;117/130,13E,47R,160 |
References Cited
[Referenced By]
U.S. Patent Documents
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3403035 |
September 1968 |
Schneble et al. |
3457089 |
July 1969 |
Shipley et al. |
|
Foreign Patent Documents
Primary Examiner: Hayes; Lorenzo B.
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.
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.
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.
* * * * *