[0001] The electroless plating of copper onto a substrate is well-known in the prior art.
For instance, an electroless or autocatalytic copper plating bath usually contains
a cupric ion source, a reducing agent for the cupric ion, a chelating or complexing
agent, and a pH adjustor. In addition, if the surface being plated is not already
catalytic for the deposition of the desired metal, a suitable catalyst is deposited
on the surface prior to contact with the plating bath. Among the more widely employed
procedures for catalyzing a substrate is the use of stannous chloride sensitizing
solution and a palladium chloride activator to form a layer of metallic palladium
particles.
[0002] Although the technology relative to electroless copper plating is continually being
improved, there still remains room for additional improvement. Certain problems are
especially pronounced when preparing articles of very high quality such as those to
be employed in printed circuit applications (e.g., printed circuit boards which contain
high-density circuitry and large numbers of holes such as through-holes and blind
holes).
[0003] A major reason for yield loss in electroless copper plating is the formation of what
is known as extraneous copper or nodules. The formation of nodules in unwanted areas
on a substrate can result in short-circuiting by forming contact between circuit lines
on the substrate. In addition, such processes as providing protective coatings, providing
solder, and pin insertion are adversely affected by the presence of nodules on the
surface.
[0004] The problem of nodule formation can be avoided, however this has to be traded off
by the judicious selection of the bath and the conditions of plating by providing
a less-active bath.
[0005] It is the object of the invention to provide an electroless copper plating bath having
improved stability and a method for electroless copper plating using this bath where
in applying this method the formation of nodules is reduced if not entirely avoided
and at the same time the rate of plating can be increased.
[0006] This object is achieved by a plating bath as disclosed in claim 1 and by a method
as claimed in claim 9.
[0007] The plating bath of the present invention provides high-quality deposited copper
of improved ductility. Moreover, the longevity of the baths of the present invention
is relatively long (e.g., a bath can be used for about one week).
[0008] Advantageous embodiments of the inventive plating bath and the inventive method are
disclosed in the subclaims.
[0009] The invention will become more apparent from the following detailed description.
[0010] According to the present invention, it has been found that electroless copper plating
bath of improved stability and capable of providing for increased plating rates can
be achieved by providing about 1 part per billion to about 1,000 parts per billion
and preferably about 1 part per billion to about 500 parts per billion of a cationic
polymer from acrylamide and/or from methacrylamide.
[0011] It is believed that the cationic polymer, in the concentrations employed, helps in
the oxidation of Cu
+, thereby preventing bulk precipitation of CU20 which, in turn, enhances the stability
of the bath and helps in reducing nodule formation. Moreover,, it is believed, in
accordance with the present invention, that the cationic polymer acts as a complexing
or chelating agent for the cupric ion. Moreover, it is believed that the presence
of the cationic polymer in the plating bath acts as a bridging ligand between the
metal ions and the surface to be coated, thereby enhancing the rate of the electrochemical
reaction providing increased plating rate.
[0012] The preferred cationic polymers employed are available under the trade designation
"Reten".
[0013] The polymer from acrylamide and/or methacrylamide is a multifunctional cationic material
in that it must contain at least two active or available cationic moieties. The polymers
are at least water-miscible and are preferably water-soluble or at least soluble in
the water compositions employed in the present invention. The preferred cationic moieties
are quaternary phosphonium and quaternary ammonium groups. Polymers containing at
least two cationic moieties are commercially available and need not be described herein
in any great detail. Examples of commercially available multifunctional cationic polymers
are Reten 210, Reten 220, and Reten 300, marketed by Hercules, description of which
can be found in "Water-Soluble Polymers", Bulletin VC-482A, Hercules Incorporated,
Wilmington, Delaware 19899, disclosure of which is incorporated herein by reference.
[0014] Reten 210 is in powder form and is a copolymer of acrylamide and betamethacryloxyethyltrimethvlammonium
methyl sulfate having a Brookfield viscosity of a 1 % solution of 600-1000 cps.
[0015] Reten 220 is in powder form and is a copolymer of acrylamide and betamethacryloxyethyltrimethylammonium
methyl sulfate having a Brookfield viscosity of a 1 % solution of 800-1200 cps.
[0016] Reten 300 is a liquid and is a homopolymer of betamethacryloxyethyltrimethylammonium
methyl sulfate having a Brookfield viscosity of a 1 % solution of 300-700 cps.
[0017] The molecular weight of the Reten polymers is usually relatively high and varies
from about 50,000 to about 1,000,000 or more. These high molecular weight polymers
are solid products and their main chemical backbone structure is polyacrylamide. The
cationic Reten (positive charge) is obtained by attaching to the polyacrylamide various
tetraalkyl ammonium compounds. These quaternary ammonium groups provide the number
of positive charges of the polymer. The preferred copper electroless plating baths
to which the cationic polymer from acrylamide and/or methacrylamide is added in accordance
with the present invention and their methods of application are disclosed in U.S.
Patents 3,844,799 and 4,152,467 disclosures of which are incorporated herein by reference.
[0018] Such copper electroless plating baths generally are aqueous compositions which include
a source of cupric ion, a reducing agent, a complexing agent for the cupric ion, and
a pH adjustor. The plating baths also preferably include a cyanide ion source and
an anionic surface-active agent. The cupric ion source generally used is a cupric
sulfate or a cupric salt of the complexing agent to be employed.
[0019] The cupric ion source is generally employed in amounts from about 3 to about 15 grams
per liter and preferably about 8 to about 12 grams per liter calculated as cupric
sulfate.
[0020] The most common reducing agent employed is formaldehyde which in the preferred aspects
of the present invention are used in amounts from about 0.7 to about 7 grams per liter
and most preferably from about 0.7 to about 2.2 grams per liter.
[0021] Examples of other reducing agents include formaldehyde derivatives or precursors
such as paraformaldehyde, trioxane, dimethylhydantoin, and gl
yoxal; borohydrides such as alkali metal alkali borohydrides (sodium and potassium
borohydride) and substituted borohydrides such as sodium trimethox
y borohydride; boranes such as amine borane (isopropyl amine borane and morpholine
borane).
[0022] Examples of some suitable complexing agents include Rochelle Salts, ethylene diamine
tetraacetic acid, the sodium (mono-, di-, tri-, and tetra-sodium) salts of ethylene
diamine tetraacetic acid, nitrilotriacetic acid and its alkali salts, gluconic acid,
gluconates, triethanol amine, glucono(gamma)-lactone, modified ethylene diamine acetates
such as N-hydroxy ethyl, ethylene diamine triacetate. In addition, a number of other
suitable cupric complexing agents are suggested in U.S. Patents 2,996,408; 3,075,856;
3,075,855; and 2,938,805 disclosures of which are incorporated herein by reference.
[0023] The amount of complexing agent is dependent upon the amount of cupric ions present
in the solution as generally from about 20 to about 50 grams per liter or in a 3-4
fold molar excess.
[0024] The plating bath also preferably contains an anionic surface active agent which assists
in wetting the surface to be coated. A satisfactory anionic surface active agent is,
for instance, an organic phosphate ester available under the trade designation "Gafac
RE-610". Generally, the anionic surface active agent is present in amounts from about
0.02 to about 0.3 grams per liter.
[0025] In addition, the pH of the bath is usually generally controlled, for instance, by
the addition of a basic compound such as sodium hydroxide or potassium hydroxide in
the necessary amount to achieve the desired pH. The preferred pH of the electroless
plating bath employed in accordance with the present invention is between 11.8 and
12.5.
[0026] Also, preferably, the plating bath contains a cyanide ion and most preferably contains
about 10 to about 25 milligrams per liter to provide a cyanide ion concentration in
the bath within the range of 0.0002 to 0.0004 molar. Examples of some cyanides which
can be employed according to the present invention are the alkali metal, alkaline
earth metal, and ammonium cyanides. In addition, the plating bath can include other
minor additives as known in the art.
[0027] The preferred plating baths employed have a specific gravity within the range of
1.060 to 1.080. Moreover, the temperature of the bath is preferably maintained between
70°C and 80°C and most preferably between 70°C and 75°C. For a discussion of the preferred
plating temperature coupled with the preferred,cyanide ion concentrations, see U.S.
Patent 3,844,799.
[0028] In addition, it is preferred to maintain the 0
2 of the bath between 2 ppm and 4 ppm and preferably about 2.5 to about 3.5 ppm, as
discussed in U.S. Patent 4,152,467. The 0
2 content can be controlled by injecting oxygen and an inert gas into the bath.
[0029] The overall flow rate of the gases into the bath is generally from about 28.32 to
about 566.4 1 per minute per 3785 1 (about 1 to about 20 standard cubic feet per minute
per thousand gallons) of bath and preferably from about 141.6 to about 283.2 1 per
minute per 3785 1 (about 5 to about 10 standard cubic feet per minute per thousand
gallons) of bath.
[0030] The preferred plating rates employed in accordance with the present invention are
about 5.08 to about 7.62 µm (about 0.2 to about 0.3 mils) of plated copper thickness
per hour.
[0031] The following non-limiting example is presented to illustrate the present invention.
EXAMPLE 1
[0032] A plating bath containing about 9 grams per liter of cupric sulfate, about 2.0 grams
per liter of formaldehyde, about 36 grams per liter of ethylene diamine tetraacetic
acid, about 28 milligrams per liter of sodium cyanide, about 1.2 parts per billion
of Reten 210, and about 0.05 grams per liter of Gafac is preferred. The bath has a
pH of about 12. The bath is fed through a plating tank at a temperature of about 73°C.
The plating tank contains substrates having a thin layer of copper on the surface
thereof. The oxygen content of the bath during plating is about 3 ppm. The rate of
plating is about 5.08 µm per hour. The nodule rating of the substrate is 1 (nodule
rating refers to nodules per 6.4516 cm
2 (1 square inch) with 1 being the best and 5 being the worst}. Similar results are
obtained with dielectric substrates catalyzed for plating copper electroless plating.
1. Electroless copper plating bath which comprises:
A. cupric ion source in an amount from about 3 to about 15 grams per liter and preferably
from about 8 to about 12 grams per liter calculated as cupric sulfate;
B. a reducing agent for the cupric ion source in an amount from about 0.7 to about
7 grams per liter and preferably from about 0.7 to about 2.2 grams per liter calculated
as formaldehyde;
C. a complexing agent, like ethylene diamine tetraacetic acid or salt thereof, for
the cupric ion in an amount of about 20 to 50 grams per liter; and
D. about 1 part per billion to about 1,000 parts per billion of a cationic polymer
from acrylamide or methacrylamide, or both.
2. Plating bath according to claim 1 which contains about 1 part per billion to about
500 parts per billion of said cationic polymer.
3. Plating bath according to claim 1 or 2 which contains an anionic surface-active
agent, generally in an amount from about 0.02 to about 0.3 grams per liter.
4. Plating bath according to any one of claims 1 to 3 having a pH of about 11.8 to
about 12.5.
5. Plating bath according to any one of claims 1 to 4 which also contains about 10
to about 25 milligrams per liter of a cyanide ion.
6. Plating bath according to any one of claims 1 to 5 wherein said cationic polymer
is a multifunctional cationic polymer.
7. Plating bath according to any one of claims 1 to 6 wherein said cationic polymer
is a copolymer of acrylamide and ammonium quaternary compound.
8. Plating bath according to any one of claims 1 to 7 which also contains between
about 2 and about 4 ppm O2.
9. Method for coating a substrate which comprises contacting the substrate with an
electroless copper plating bath according to claim 1.
10. Method according to claim 9 wherein said electroless copper plating bath is maintained
at a temperature of about 70°C to about 80°C.