U.S. patent number 3,635,758 [Application Number 04/847,421] was granted by the patent office on 1972-01-18 for electroless metal deposition.
This patent grant is currently assigned to Photocircuits Corporation. Invention is credited to John F. McCormack, Frederick W. Schneble, Jr., Rudolph J. Zeblisky.
United States Patent |
3,635,758 |
Schneble, Jr. , et
al. |
January 18, 1972 |
ELECTROLESS METAL DEPOSITION
Abstract
An improved method for enhancing the ductility of electroless
metal deposits is provided in which an electroless metal bath
contains an extraneous ion which has a preferential capacity for
being coulombically adsorbed at the outer layer of an electric
double layer present on an interface in contact with such bath on
which said metal is electrolessly depositing, such ion being
present in an amount sufficient to reduce the inclusion of hydrogen
in the electrolessly deposited metal.
Inventors: |
Schneble, Jr.; Frederick W.
(Oyster Bay, L.I., NY), McCormack; John F. (Roslyn Heights,
NY), Zeblisky; Rudolph J. (Hauppauge, NY) |
Assignee: |
Photocircuits Corporation (Glen
Cove, NY)
|
Family
ID: |
25300584 |
Appl.
No.: |
04/847,421 |
Filed: |
August 4, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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523902 |
Feb 1, 1966 |
|
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Current U.S.
Class: |
427/98.4;
106/1.26; 427/306; 427/98.5; 427/99.5 |
Current CPC
Class: |
C23C
18/40 (20130101) |
Current International
Class: |
C23C
18/40 (20060101); C23C 18/31 (20060101); B44d
001/18 (); C23c 003/02 () |
Field of
Search: |
;117/47R,130,13E,212
;106/1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hayes; Lorenzo B.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. application Ser.
No. 523,902, filed Feb. 1, 1966, and now abandoned.
Claims
What is claimed is:
1. In a method for electrolessly depositing copper which comprises
contacting a surface which is sensitive to the reception of
electroless copper, with an electroless copper deposition solution
containing an ion of copper, a complexing agent for said ion, a
reducing agent for said ion, and a pH regulator; the improvement
which comprises maintaining in the solution an inorganic extraneous
ion which has a preferential capacity for being coulombically
adsorbed at the outer layer of an electric double layer present at
the electroless metal deposition surface, in an amount sufficient
to reduce the inclusion of hydrogen in the electrolessly deposited
metal and less than 0.08 mole per liter, said inorganic extraneous
ion being derived from a compound which is a member selected from
the group consisting of orthophosphoric acid, pyrophosphoric acid,
metaphosphoric acid and polyphosphoric acid.
2. The method of claim 1, wherein said extraneous ion not
electrostatically adsorbed at the outer layer is chemisorbed at the
inner layer of the electric double layer.
3. The method of claim 1, wherein the extraneous ion is a
phosphorus containing ion.
4. The method of claim 1, wherein the electroless metal deposition
solution comprises as a stabilizing agent a member selected from
the group consisting of divalent sulfur compounds and sulfites.
5. The method of claim 1, wherein the electroless metal deposition
solution comprises as an agent for increasing the rate of
deposition of the electroless metal a member selected from the
group consisting of hexacyanoferrate (II) and hexacyanoferrate
(III).
6. The method of claim 5, wherein the electroless metal deposition
solution comprises as a stabilizing agent a member selected from
the group consisting of divalent sulfur compounds and sulfites.
7. The method of claim 1, wherein the surface contacted with the
electroless metal deposition solution is selectively sensitized in
designated areas, thereby forming a printed circuit.
8. The method of claim 1, wherein the electroless metal deposition
solution comprises less than 25 parts per million of an ion having
an oxidation potential greater than the oxidation potential of the
particular ion being electrolessly deposited.
9. The method of claim 1, wherein the extraneous ion is present in
an amount of from about 0.02 to 0.08 mole per liter.
Description
Generally stated, the subject matter of the present invention
relates to a process for enhancing the ductility of electroless
metal deposits. More particularly, the invention relates to a
process for enhancing the ductility of electroless metal deposits
which comprises employing a plating bath containing an extraneous
ion which has a preferential capacity for being coulombically
adsorbed at the outer layer of an electric double layer present on
an interface in contact with such bath on which said metal is
electrolessly depositing.
BACKGROUND OF THE INVENTION
Electroless or autocatalytic metal deposition solutions are
characterized by a capacity to deposit metal on a wide variety of
conducting, nonconducting, or insulating surfaces without the
assistance of an external supply of electrons. Typically, such
solutions comprise a solvent, a supply of ions of a metal to be
deposited, an agent capable of reducing the ions of the metal to be
deposited, a complexing agent for the ions of the metal to be
deposited, and a pH regulator.
Such solutions are particularly suitable for metallizing insulating
substrata on surfaces which have been suitably treated to make them
sensitive to the reception of electroless metal deposition. Such
sensitization techniques include the well-known treatment with an
acidic aqueous solution of stannous chloride (SnCl.sub.2), followed
by treatment with a dilute aqueous acidic solution of palladium
chloride (PdCl.sub.2). Alternatively, sensitization may be achieved
by treating the insulating substrate 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.
Alternate ways of achieving sensitization of insulating substrata
to the reception of electroless copper are disclosed in copending
application Ser. No. 249,063, filed Jan. 2, 1963, and U.S. Pat. No.
3,146,125.
Most electroless metal solutions of the type described produce
metal deposits which contain a substantial amount of hydrogen, for
reasons which will be hereinafter clarified. Such deposits are
brittle, break under vibration and bending, and otherwise exhibit
poor ductility. For this reason, such solutions are not generally
satisfactory for use where rugged, tenacious electroless metal
deposits capable of withstanding rough mechanical handling and
bending forces are desired. More particularly, the inherent
tendency for electroless metal solutions to produce brittle metal
deposits has severely retarded the widespread use of such
solutions, for example, in the manufacture of printed circuits and
the metallization of plastics generally.
The present invention has for an object the production of
electroless metal having a reduced hydrogen content.
Another object is the production of ductile electroless metal
deposits which are capable of withstanding bending forces.
A further object of the present invention is the provision of novel
and improved processes for providing insulating substrates
including plastics with adherent, ductile electroless metal
deposits.
Still another object is the provision of novel and improved
processes for the production of reliable, accurate and mechanically
rugged printed circuits.
Other objects of the invention will in part be obvious from the
following description and will in part be made clear
hereinafter.
THE INVENTION
To achieve the foregoing objects, and in accordance with its
purpose, as embodied and broadly described, the present invention
provides an improved process for electrolessly depositing metal in
which the ductility of the electroless metal deposits is enhanced.
The process comprises maintaining in the electroless metal
deposition solution an extraneous ion which is electrostatically
adsorbed at the outer layer of the electric double layer present at
the surface on which deposition is occurring. The effect of outer
layer adsorption of the extraneous ion is to reduce or minimize
inner layer adsorption at the deposition surface of hydrogen ions,
thereby reducing the opportunity for hydrogen to be included in the
electroless deposit.
By extraneous ion is meant an ion which does not interfere with and
is not required for the main electroless deposition reaction to
occur.
The invention will be particularly described with reference to
electroless copper deposition, which is a preferred embodiment.
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, e.g., derived from a water-soluble copper salt, a reducing
agent for cupric ions, a complexing agent for cupric 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 (mono-, di-, tri-, and tetra-sodium)
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-.gamma.-lactone and modified
ethylenediamineacetates are also useful, and in certain instances
give even better results than the pure sodium
ethylenediaminetetraacetates. One such material is
N-hydroxyethylethylenediaminetriacetate. Other materials suitable
for use as cupric complexing agents are disclosed in U.S. Pat. Nos.
2,996,408; 3,075,856; 3,075,855; and 2,938,805.
Preferred reducing agents for cupric ion for use in alkaline
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 as alkali
metal borohydrides, e.g., sodium and potassium borohydride, as well
as substituted borohydrides, e.g., sodium trimethoxy borohydride.
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 reducing agents for cupric ion in acid electroless
copper solutions are hypophosphites, such as sodium and potassium
hypophosphite, and the like.
Reducing agents of the type described release hydrogen when
oxidized.
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 the sulfuric acid.
In operation of the bath, the copper salt serves as a source of
cupric ions, and the reducing agent reduces the cupric ions to
metallic form. When reducing agents of the type described above are
oxidized to provide electrons for the reduction of the cupric ions,
hydrogen is released at the site of deposition. The complexing
agent serves to complex the cupric ion, and at the same time makes
the cupric ion 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.
It should be understood, however, that every constituent 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 effect 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 continually
being consumed, so that the bath is in a constant state of
change.
Electroless copper solutions having the basic constituents
described produce copper deposits which contain a substantial
amount of included hydrogen. Such deposits are brittle, break under
vibration and bending, and otherwise exhibit poor ductility. Such
deposits also exhibit a dull surface, of poor color, which may best
be described as "smutty."
The electroless copper of this invention, as distinguished from
that of the prior art, has a greatly reduced hydrogen content, is
bright, and possesses enhanced ductility, as compared to the
electroless copper of the prior art.
The precise mechanism by which electroless metal, e.g., copper,
deposition occurs is complex and difficult of precise definition.
Without wishing to be limited thereto, the ensuing theory affords a
rational explanation for this phenomenon.
In electroless copper deposition from a solution containing a
solvent, a source of cupric ions, a reducing agent for cupric ions,
a complexing agent for cupric ions, and a pH regulator, the
reducing agent is oxidized at deposition surface areas to release
electrons to such areas. Cupric ions in contact with such areas
pick up the free electrons and are reduced to copper atoms, which
deposit on the adjacent surface areas and aggregate to form a
copper metal deposit.
The potential at which the process takes place is a compromise, or
mixed potential, between the two potentials which characterize the
oxidization of the reducing agent and the reduction of the cupric
ions.
To illustrate the mechanism, in one embodiment of a particularly
useable electroless copper deposition solution, the reducing agent
is formaldehyde, and the source of the cupric ions is a copper
salt, such as cupric sulfate. Using the mixed potential theory
heretofore described, the formaldehyde is oxidized to hydrogen and
formate ions at the surface at which deposition is occurring, and
an electron is given up to the surface for each molecule of
formaldehyde oxidized. The released electron is in turn taken up by
cupric ions at or in contact with the surface, to cause local
deposition of copper.
The surface on which electroless metal deposition occurs may be
considered a charged interface having the structure of an electric
double layer. For a description of the electric double layer
structure of such a charged interface, see: Proc. Roy. Soc. Series
A, 1963, pp. 55-79; and Bockris, Modern Aspects of
Electrochemistry, No. 1, 1954, pp. 103-173, which are herein
incorporated by reference as part of the instant specification.
According to the present invention, the outer layer of the charged
interface or surface at which deposition occurs, has
electrostatically or coulombically adsorbed thereon extraneous ions
which screen the surface at least in part from inner layer
adsorption of hydrogen released by the reducing agent. To
accomplish this result, there are maintained in the electroless
copper solution, particularly one having a high plating potential,
extraneous ions which are capable of being electrostatically or
coulombically adsorbed at the outer layer of the deposition
surface. The adsorbed, extraneous ions must be potentially
dependent, i.e., they must have the capacity to be adsorbed at the
potential of operation of the bath. They must also have the
capacity to be preferentially adsorbed, compared to the ions
normally present and required for successful operation of the bath.
The potential of operation of the bath, the mixed potential, is
between the potential which characterizes the reduction of the
cupric ions on the absence of the reducing agent and the potential
which characterizes the oxidation of the reducing agent in the
absence of the cupric ions. The former is between -0.2 and -0.5 and
the latter between -0.9 and -1.3 volts versus the saturated calomel
electrode depending on temperature and composition of the bath.
Thus, the adsorbed species must have the property to be adsorbed at
the potentials between -0.2 and -1.3 volts versus the saturated
calomel electrode.
The extraneous ions suitable for use must also, as stated
previously, have the capacity of being electrostatically adsorbed
at the outer layer of the electric double layer present at the
surface on which deposition is occurring. The outer layer of the
electric double layer may be considered as being separated from the
charged interface by a distance of about 1 to 2 molecular
diameters, or about 3 angstroms. The capacity of the extraneous
ions suitable for use herein to be electrostatically adsorbed at
the outer layer of the surface on which deposition is occurring
distinguishes such ions from those which may be specifically
adsorbed, i.e., chemisorbed, at the inner layer of the electric
double layer.
The adsorbed extraneous ions of this invention screen the
deposition surface from hydrogen ions which are released at the
surface by the main deposition reaction. Accordingly, the greater
the number of extraneous ions electrostatically adsorbed at the
outer layer, the less the tendency for hydrogen adsorption. A
decrease in the number of hydrogen ions adsorbed in the inner layer
leads directly to a decrease in hydrogen inclusion in the
electroless metal and a consequent increase in the ductility of the
deposited metal.
Extraneous ions having a preferential capacity for the
electrostatic or coulombic adsorption at the outer layer of the
deposition interface and therefore suitable for use herein are ions
which contain phosphorus and are derived, for example, from
inorganic phosphorus containing salts, acids, and bases; including
the phosphates, pyrophosphates, metaphosphates and polyphosphates.
The phosphate, tetrapyrophosphate and hexametaphosphate salts of
alkali and alkaline earth metals, and ammonia are preferred for
use. Preferred for use are sodium, potassium and ammonium
phosphates and polyphosphates.
The foregoing compounds are merely typical of those which are
capable of providing phosphorus containing ions which are suitable
for use herein.
The electrostatically absorbable ions described herein should be
maintained in the electroless metal solutions averaging between
about 0.02 and 0.1 mole per liter of solution, preferably between
about 0.03 and 0.08 mole per liter.
It should be emphasized however that the amount of
electrostatically absorbable ions will vary with the nature and
activity of the ion used, and with makeup of the solution and the
conditions, e.g., the temperature, concentration and pH, under
which it is used. The upper limit of such ions is an amount which
will prevent deposition of electroless metal under conditions of
use. The lower limit is the least amount of ion which will be
effective in manifesting the results described herein, again under
the particular conditions of use.
A typical electroless metal deposition bath made according to the
present invention will comprise:
Electroless metal salt 0.002 to 1.0 mole Reducing agent 0.03 to 4
moles Electroless metal complexing agent 0.7 to 40 times the moles
of metal salt Extraneous ion 0.02 to 0.1 mole pH adjuster
sufficient to give desired pH Water sufficient to make 1 liter.
Embodiments of a high plating potential electroless copper solution
comprise:
Copper salt 0.002 to 1 mole Reducing agent 0.03 to 4 moles Cupric
ion com- plexing agent 0.7 to 40 times the moles of copper
Extraneous ion 0.02 to 0.1 mole pH adjuster sufficient to give
desired pH Water sufficient to make 1 liter.
Preferred embodiments of highly active electroless copper solutions
comprise:
A soluble cupric salt, preferably cupric sulfate 0.002 to 0.60
moles Alkali metal hydroxide, preferably sodium hydroxide, to give
pH of 10-14 Reducing agent, preferably formaldehyde 0.03 to 1.3
moles Extraneous ion 0.03 to 0.08 mole Cupric ion complexing agent
0.7 to 15 times the moles of copper Water sufficient to make 1
liter.
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 extraneous ions, and to adjust them to their optimum value as
the bath is used.
Here it should be noted that the use of the electrostatically
absorbable ions described are present in easily detectable amounts,
thereby facilitating control of the operation.
For best results, surfactants in an amount of less than about 5
grams per liter may be added to the baths. Typical of suitable
surfactants are organic phosphate esters and oxyethylated sodium
salts.
Particularly, results are achieved when the extraneous ions of this
invention are added to or maintained in solutions which contain
small effective amounts, e.g., between 5 micrograms and 500
milligrams per liter, of water soluble cyanide compounds. Typical
of such cyanide compounds may be mentioned alkali cyanides, such as
sodium and potassium cyanide, nitriles, such as chloroacetonitrile,
alpha-hydroxy nitriles, e.g., glyconitrile and lactonitrile, and
dinitriles, e.g., succinonitrile, iminodiactonitrile, and 3,3'
iminodipropionitrile.
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, it is usual to find that the rate of plating is
increased, but the temperature is not highly critical, and, within
the usual operating range, excellent bright, ductile deposits of
electroless copper of reduced hydrogen content are obtained.
Performance data for baths made in accordance with the teachings
contained herein are given in table I. ##SPC1##
In table I, the solutions contained about 1 ml./l. of an organic
phosphate ester as a surfactant.
In table I, 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.
As shown in table I, the presence of phosphorus enhances the
ductility of the copper deposits to a truly remarkable degree.
Use of the phosphorus compounds described also improved stability
of the electroless copper solution to a marked degree.
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 brought out
hereinabove.
Where 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 autocatalytic metal baths and permitted
to remain in the bath until a metal deposit of the desired
thickness has built up.
As has been brought out heretofore, the solutions described herein
are advantageous for use in production of printed circuits. For
example, portions of the surface of an insulating substrate in the
form of a desired circuit pattern may be sensitized for the
reception of electroless metal. Following sensitization, the
substrate is immersed in the electroless metal solution of the type
described and permitted to remain therein until a metal deposit of
the desired thickness has been built up. The circuit may be formed
on one or more surfaces of the substrate. If desired,
interconnections between the surfaces may be provided by drilling
or punching holes and sensitizing the lateral walls thereof prior
to exposure of the substrate to the electroless metal solution. In
this embodiment, electroless metal builds up on the circuit
pattern, as well as on the walls surrounding the holes to form
interconnections.
To enhance the stability of the electroless metal solutions
described herein, divalent sulfur compounds, as well as sulfites,
may be maintained therein during operation.
Among the organic sulfur compounds may be mentioned the following:
aliphatic sulfur-nitrogen compounds, such as thiocarbamates, e.g.,
thiourea; 5-membered heterocyclics containing S-N in the 5
-membered ring, such as thiazoles and isothiazoles, e.g.,
2-mercapto benzol thiazole and the like; dithiols, e.g.,
1,2-ethanedithiol and the like; 6-membered heterocyclics containing
S-N in the ring, such as thiazines, e.g., 1,2-benzisothiazine,
benzothiazine, and the like; thioamino acids, such as methinonine,
cysteine, cystine, and the like; thio derivatives of alkyl glycols,
such as 2,2' thiodiethanol, dithiodiglycol, and thioglycollic acid;
and the like. Among the inorganic sulfur compounds may be
mentioned: alkali metal sulfides, e.g., sodium sulfide, potassium
sulfide, sodium polysulfide, potassium polysulfide; alkali metal
thiocyanates, such as sodium and potassium thiocyanates; and alkali
metal dithionates, such as sodium and potassium dithionate.
The foregoing sulfur compounds are merely typical of sulfur
compounds which are capable of stabilizing autocatalytic copper
baths as taught herein.
The amount of sulfur compound required is a small effective amount
and will vary, depending upon the particular compound used, from a
trace to about 300 parts per million (p.p.m.) or more.
Solutions containing small effective amounts of sulfur component
are especially useful in the preparation of printed circuits. In
the absence of sulfur, there is a tendency with electroless
solutions of the type described for nonsensitized areas of the
insulating substrates, following prolonged immersion in the
solution, to become sensitized and to receive scattered spot
deposits of metal. As will be readily appreciated, deposition of
electroless metal in areas of the substratum where metal is not
desired would raise havoc with control techniques in the
preparation of printed circuits.
Additionally, when autocatalytic metal baths are used commercially,
deposition of extraneous metal occurs on insulating walls of
containers housing the electroless metal solution.
When sulfur is present, the tendency for such scattered spot
deposits of metal to form is substantially reduced if not
completely eliminated.
The extraneous ions described herein are especially suitable for
use in electroless copper solutions which are to be run
substantially free of ferric and ferrous ions.
According to copending application Ser. No. 523,863, of Frederick
W. Schneble, Jr., John F. McCormack and Rudolph J. Zeblisky, filed
Feb. 1, 1966, the reduction or substantial elimination from
electroless metal deposition solutions of ions which have an
oxidation potential greater than the oxidation potential of the ion
of the metal sought to be electrolessly deposited also alleviates
the problem of spontaneous decomposition ordinarily associated with
such solutions. More specifically, the copending application
suggests maintaining the concentration of such ions below about 25
parts per million.
The expression "oxidation potential" used herein should be
understood as having the definition set forth in Latimer, Oxidation
Potentials, 2nd ed., Prentice Hall, 1952.
In electroless copper solutions designed to be run for long periods
of time, ions of iron, i.e., ferrous and ferric ions, are
particularly troublesome in this respect, and the concentration
should be maintained below about 25 parts per million.
The extraneous ions, e.g., phosphates, of this invention do not
react with ferric and ferrous ions and therefore do not interfere
with removal of ferrous and ferric ions from electroless copper
solutions. Rather, with the extraneous ions of this invention, the
electroless copper solutions may be continuously or periodically
treated to remove the deleterious ions described immediately above,
as by raising the pH of the solution to precipitate iron as the
hydroxide.
It has also been discovered that the addition of complex
cyanometallic compounds such as ferricyanides or ferrocyanides to
electroless copper solutions of the type described herein results
in an increase in the rate of deposition. Accordingly, the present
invention contemplates the addition to or maintenance in the
electroless metal solutions described of small effective amounts of
water soluble ferrocyanide or ferricyanide salts, e.g., water
soluble alkali metal, alkaline earth metal, and ammonium salts of
hexacyanoferrate (II) and hexacyanoferrate (III). In general, the
amount of such salts will range from about 1 to about 320 parts per
million, calculated as iron (Fe).
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 departing from the principles of the
invention and without sacrificing its chief advantages.
* * * * *