U.S. patent number 3,607,317 [Application Number 04/796,542] was granted by the patent office on 1971-09-21 for ductility promoter and stabilizer for electroless copper plating baths.
This patent grant is currently assigned to Photocircuits Corporation. Invention is credited to Fredrick W. Schneble, Jr..
United States Patent |
3,607,317 |
Schneble, Jr. |
September 21, 1971 |
DUCTILITY PROMOTER AND STABILIZER FOR ELECTROLESS COPPER PLATING
BATHS
Abstract
The present invention provides a means for controlling the
stability of electroless copper plating baths and enhancing the
ductility properties of the electroless metal deposits produced
therefrom.
Inventors: |
Schneble, Jr.; Fredrick W.
(Oyster Bay, NY) |
Assignee: |
Photocircuits Corporation (Glen
Cove, NY)
|
Family
ID: |
25168439 |
Appl.
No.: |
04/796,542 |
Filed: |
February 4, 1969 |
Current U.S.
Class: |
427/443.1;
427/437; 106/1.26 |
Current CPC
Class: |
C23C
18/40 (20130101) |
Current International
Class: |
C23C
18/31 (20060101); C23C 18/40 (20060101); C23c
003/02 () |
Field of
Search: |
;106/1
;117/130,13E,160 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hayes; Lorenzo B.
Claims
I claim:
1. In an electroless copper deposition solution which comprises
water, a complexing agent for copper ion to be deposited, a
reducing agent for the copper ion to be deposited, a stabilizing
agent for the copper ion to be deposited, and an agent capable of
adjusting pH, the improvement which comprises maintaining in the
solution a small effective stabilizing amount of at least about 0.1
milligram per liter of a polyalkylene oxide, having up to about 7
carbon atoms per alkylene moiety and a molecular weight of at least
about 6000.
2. The solution of claim 1, wherein the content of said
polyalkylene oxide is between about 0.4 and 80.0 milligrams per
liter, said amount being insufficient to prevent electroless
deposition of copper but sufficient to enhance the stability of the
solution at said conditions.
3. The solution of claim 1, wherein the polyalkylene oxide content
is between about 1 and 10 milligrams per liter.
4. The solution of claim 1, wherein the polyalkylene oxide has a
molecular weight ranging from 6,000 to greater than 5,000,000.
5. The solution of claim 1, wherein the polyalkylene oxide is a
member selected from the group consisting of polyethylene oxide,
polypropylene oxide and polybutylene oxide.
6. The solution of claim 1, wherein the pH adjuster maintains the
pH in the alkaline range.
7. The solution of claim 1, wherein the stabilizing agent is sodium
cyanide.
8. The solution of Claim 1, wherein the reducing agent for the
copper ion is formaldehyde.
9. In a process for depositing copper on a catalytic surface
comprising contacting said surface with an electroless copper
solution comprising a water soluble copper salt, a complexing agent
for a copper ion to be deposited, a reducing agent for the copper
ion to be deposited, a stabilizing agent for the copper ion to be
deposited, and an agent for adjusting pH, the improvement which
comprises maintaining in the solution a small effective stabilizing
amount of at least about 0.1 milligrams per liter of a polyalkylene
oxide, having up to about seven carbon atoms per alkylene moiety
and a molecular weight of at least about 6000.
10. The process of claim 9, wherein the polyalkylene oxide is
present in an amount of between about 0.4 and 80.0 milligrams per
liter, said amount being insufficient to prevent electroless
deposition of copper but sufficient to enhance the stability of the
solution at said conditions.
11. The process of claim 9, wherein the polyalkylene oxide is a
member selected from the group consisting of polyethylene oxide,
polypropylene oxide and polybutylene oxide.
12. The process of claim 9, wherein the polyalkylene oxide is
present in an amount of between about 1.0 and 10.0 milligrams per
liter.
13. The process of claim 9, wherein the polyalkylene oxide has a
molecular weight ranging from 6,000 to greater than 5,000,000.
Description
The present invention relates to electroless plating of metals, and
more particularly to controlling the stability of electroless metal
plating baths and enhancing the physical properties of the
electroless metal deposits produced therefrom.
The present invention has for an object to improve the stability of
electroless copper baths without adversely affecting the deposition
rate or the physical properties of the electroless copper produced
therefrom.
Another object of this invention is to provide means for monitoring
electroless copper solutions so as to maintain them in a state of
dynamic equilibrium.
A further object of the invention is to provide electroless copper
deposition solutions which are capable of producing electroless
copper having enhanced physical properties including improved
ductility, brightness, and the like.
Still a further object of this invention is to provide new and
useful addition agents for controlling the stability of electroless
copper solutions.
The present invention and the agents described herein although
generally applicable to electroless metal plating solutions, are
particularly useful with electroless copper solutions.
Electroless copper solutions are capable of depositing copper
without the assistance of an external supply of electrons.
Typically, such solutions comprise water, a small amount of copper
ions, a complexing agent for copper ions, and a pH regulator.
The selection of the water soluble copper salt for such baths is
chiefly a matter of economics. Copper sulfate is preferred for
economic reasons, but the halides, nitrates, acetates and other
organic and inorganic acid salts of copper may also be used.
Rochelle salts, the sodium salts (mono-, di-, tri-, and
tetrasodium), salts of ethylenediaminetetraacetic acid,
nitrilotriacetic acid and its alkali salts, gluconic acid,
gluconates, and triethanolamine are preferred as copper ion
complexing agents, but commercially available glucono-9 -loctone
and modified ethlenediamineacetates are also useful, and in certain
instances, give even better results than the pure sodium
ethylenediamine-tetraacetates. Such materials are
N-hydroxyethylethylenediamine-triacetate and N, N, N', N'-tetrakis
(2-hydroxypropyl ethylenediamine). Other materials suitable for use
as cupric complexing agents are disclosed in U.S. Pat. Nos.
2,938,805, 2,996,408, 3,075,855 and 3,075,856.
Copper reducing agents which have been used in electroless metal
baths include formaldehyde, and formaldehyde precursors or
derivatives, such as paraformaldehyde, trioxane, dimethyl
hydantoin, glyoxal, and the like. Also suitable as reducing agents
in alkaline baths are borohydrides, such alkali metal borohydrides,
e.g., sodium and potassium borohydride, as well as substituted
borohydrides, e.g., sodium trimethoxyborohydride. As reducing
agents in such baths may also be used boranes, such as amine
borane, e.g., isopropylamine borane, morpholine borane, and the
like.
Typical of the copper reducing agents for use in acid electroless
copper solutions are hypophosphites, such as sodium and potassium
hypophosphite, and the like.
The pH adjuster or regulator may consist of any acid or base, and
here again the selection will depend primarily on economics. For
this reason, the pH adjuster on the alkaline side will ordinarily
be sodium hydroxide. On the acid side, pH will usually be adjusted
with an acid having a common anion with the copper salt. Since the
preferred copper salt is the sulfate, the preferred pH adjuster on
the acid side is sulfuric acid.
In addition to the materials placed in the copper bath solution, as
disclosed above, there may be materials placed in the bath to
further enhance the properties of the solution. These may include
cyanide compounds such as sodium and potassium cyanide and nitriles
such as acrylonitrile; 5 membered heterocyclics such as thiazoles
and iso-thiazoles, e.g., 2-mercaptobenzolthiazole and the like; and
sulfurated potash. Other materials suitable for stabilizing
electroless copper bath solutions are disclosed in U.S. Pat. Nos.
3,095,309, 3,257,215, and 3,361,580.
In operation of the bath, the copper salt serves as a source of
copper ions, and the reducing agent reduces the copper ions to
metallic form. The reducing agent is itself oxidized to provide
electrons for the reduction of the copper ions. The complexing
agent serves to complex the copper ion so that it will not be
precipitated, e.g., by hydroxyl ions and the like, and at the same
time makes the copper available as needed to the reducing action of
the reducing agent. The pH adjuster serves chiefly to regulate the
internal plating potential of the bath.
The performance of the electroless copper baths is improved by
addition thereto of certain surfactants in an amount of less than
about 5 grams per liter. Such surfactants include organic phosphate
esters and oxyethylated sodium salts, and mixtures thereof.
Preferred surfactants are alkylphenoxy polyethoxy phosphate esters.
Such surfactants may be obtained under the trade name of Gafec RE
610
It should be understood, however, that every constitutent in the
electroless copper bath has an effect on plating potential, and
therefore must be regulated in concentration to maintain the most
desirable plating potential for the particular ingredients and
conditions of operation. Other factors which affect internal
plating voltage, deposition quality and rate include temperature
and degree of agitation, in addition to type and concentration of
the basic ingredients mentioned.
In electroless plating baths, the bath constituents are
continuously being consumed, so that the bath is in a constant
state of change. Control of such baths, so as to maintain a
relatively high plating rate over relatively long periods of time
is exceedingly difficult. As a result, such baths, and particularly
those having a high plating potential, i.e., highly active baths,
tend to become unstable and to spontaneously decompose with use.
Heretofore, spontaneous decomposition of high plating potential
baths has been an important factor in limiting the commercial
acceptance of electroless copper solutions as a substitute for or a
competitor of electroplating baths.
According to the present invention, it has been discovered that
certain agents, when added to electroless copper plating solutions,
serve to maintain the baths in a dynamic state of equilibrium for
long periods of time and to prevent spontaneous decomposition.
The addition agents of this invention render electroless copper
solutions less sensitive to changes of temperature and
concentration, and therefore permit greater variation in operating
conditions, ingredient concentration, temperature, and types of
ingredients than have heretofore been considered possible.
The stabilizing agents of this invention are simple or complex
compounds of polyalkylene oxides such as polyethylene
polypropylene, and polybutylene. The polyalkylene oxides containing
less than five to seven carbon atoms are preferably included as the
stabilizing agents. Although all these polyalkylene oxides are
applicable in the present invention, the polyethylene oxides are
preferable and have shown advantageous results when used in various
electroless plating solutions.
The polyethylene oxides according to this invention are nonionic
water soluble homopolymer resins having high molecular weights
ranging from several hundred thousands to five million and above.
They are thermoplastics which are completely soluble in water and
compatible in moderate concentrations of electrolytes.
The polyethylene oxides suitable for use in the present invention
are sold by the Union Carbide Corporation under the trade name of
POLYOX resins. The water soluble resins are manufactured in
different grades of varying molecular weights. The preferred grade
being that produced under the trade name of POLYOX coagulant,
having a molecular weight of greater than 5,000,000.
The amount of polyethylene oxide maintained in the baths will be a
small effective amount. Ordinarily, its concentration will average
between about 0.4 and 80.0 milligrams per liter of solution,
preferably about 1.0 milligrams per liter.
It should be emphasized however that the small effective amount of
polyethylene oxide will vary with the nature of the particular
compound used, and with makeup of the solution and the conditions
e.g. temperature, under which it is used.
It is noted that the stabilizing agents of the present invention,
e.g., polyethylene oxides, may be added to electroless plating
solutions containing known stabilizers as sodium cyanide and
2-mercapto-benzathiazole, to greatly enhance the stability of such
plating solutions. With a small amount of a polyethylene oxide
added to the plating solutions containing these known stabilizers,
the stability of the plating solutions is greatly increased.
In addition to stabilizing the bath, the polyethylene oxides
enhance the physical properties of the electroless copper deposits,
particularly the ductility. This is a completely surprising result
and contributes materially to the value of the baths utilizing the
instant invention.
Overall, the higher molecular weight polyethylene oxides provide
greater stability of the plating baths and enhance the ductility of
the copper deposited from the plating solutions of this invention.
For example, a bath having a polyethylene oxide of a molecular
weight of 100,000 has greater stability and provides a higher
ductility of copper deposits than a comparable bath having a
polyethylene oxide of a molecular weight of 6,000. The optimum
range of molecular weight of the polyethylene oxides for enhanced
ductility is between 100,000 and 6,000,000.
It has also been found that the rate of deposition of the metal
from the plating baths using polyethylene oxides is increased with
the use of compounds having high molecular weights ranging over
1,000,000.
Typical electroless copper deposition baths made according to the
present invention are as shown below. A polyethylene oxide having a
molecular weight of greater than 5,000,000 is used in each bath.
##SPC1##
In considering the general and specific working formulas set forth
herein, it should be understood that as the baths are used up in
plating, the ingredients will be replenished from time to time.
Also, it is advisable to monitor the pH, and the concentration of
the additive element described, and to adjust them to their optimum
value as the bath is used.
The baths may be used at widely varying temperatures, e.g., between
15.degree. and 100.degree. C., although they will usually be used
between about 20.degree. and 80.degree. C. As the temperature is
increased, but the temperature is not highly critical and, within
the usual operating range, excellent bright, ductile deposits of
copper are obtained.
Bath 3 varies from the other two baths in that it contains one
stabilizing agent, polyethylene oxide, the agent provided according
to the present invention. Bath 3 was very stable and the copper
deposited from the bath had a high ductility.
Performance data for the Baths 1 and 2 made in accordance with the
teachings contained herein are given in tables I and II,
respectively. The tables show the results with respect to the rate
of copper deposition, ductility of deposition and stability of
baths prepared according to formulas set forth herein. ##SPC2##
##SPC3##
In each of the baths, both a printed circuit plate and a ductility
sample were placed for a period of about 5 hours.
In the tables, the ductility is measured by bending the copper
deposit through 180.degree., in one direction, creasing, then
returning it to its original position, with pressing along the
crease to flatten it, this cycle constituting one bend.
Use of the polyethylene oxide in copper solutions improves and
maintains the stability as is brought out in tables I and II. As
indicated in the tables, when the polyethylene oxide was added to
the baths the plating solutions became stable. A plating solution
is stable when no powder is formed on the plated specimen. Another
showing of stability of a plating solution is when there are no
deposits of metal on the walls or bottom of the bath container in
24 hours. Conversely, if there are deposits of metal on the walls
and at the bottom of the plating bath container in less than 24
hours, the solution is not stable.
As also shown by the tables I and II, the presence of polyethylene
oxide also enhances the ductility of the copper deposits.
In using the electroless copper solutions to plate metal, the
surface to be plated must be free of grease and other contaminating
material.
Where a nonmetallic surface is to be plated, the surface area to
receive the deposit must first be sensitized to render it catalytic
to the reception of electroless copper, as by the well known
treatment with an acidic aqueous solution of stannous chloride (SN
C1.sub.2) followed by treatment with a dilute aqueous acidic
solution of palladium chloride (Pd C1.sub.2).
Alternatively, extremely good sensitization of nonmetallic surfaces
is achieved by contact with an acidic solution containing a mixture
of stannous chloride and precious metal chloride, such as palladium
chloride, the stannous chloride being present in stoichiometric
excess, based on the amount of precious metal chloride.
Where a metal surface is to be plated, it should be degreased, and
then treated with an acid, such as hydrochloric or phosphoric acid,
to free the surface of oxides.
Following pretreatment and/or sensitization, the surface to be
plated is immersed in the electroless copper baths, and permitted
to remain in the bath until a copper deposit of the desired
thickness has been built up.
The invention in its broader aspects is not limited to the specific
steps, processes and compositions shown and described, but
departures may be made therefrom within the scope of the
accompanying claims without sacrificing its chief advantages.
* * * * *