U.S. patent application number 10/741908 was filed with the patent office on 2005-01-27 for reverse pulse plating composition and method.
This patent application is currently assigned to Shipley Company, L.L.C.. Invention is credited to Barstad, Leon R., Buckley, Thomas, Cruz, Raymond, Goodrich, Trevor, Hamm, Gary, Kapeckas, Mark J., Price, Katie, Reddington, Erik, Sonnenberg, Wade.
Application Number | 20050016858 10/741908 |
Document ID | / |
Family ID | 32990583 |
Filed Date | 2005-01-27 |
United States Patent
Application |
20050016858 |
Kind Code |
A1 |
Barstad, Leon R. ; et
al. |
January 27, 2005 |
Reverse pulse plating composition and method
Abstract
A composition and method for electroplating a metal on a
substrate. The composition has a chloride to brightener
concentration ratio of from 20:1 to 125:1. The method of
electroplating, which employs the composition, employs pulse
patterns that improve physical properties of metal surfaces.
Inventors: |
Barstad, Leon R.; (Raynham,
MA) ; Buckley, Thomas; (Rocky Hill, CT) ;
Cruz, Raymond; (Waltham, MA) ; Goodrich, Trevor;
(Windsor, MA) ; Hamm, Gary; (Medford, MA) ;
Kapeckas, Mark J.; (Marlborough, MA) ; Price,
Katie; (Stoneham, MA) ; Reddington, Erik;
(Ashland, MA) ; Sonnenberg, Wade; (Edgartown,
MA) |
Correspondence
Address: |
John J. Piskorski
c/o EDWARDS & ANGELL, LLP
P.O. Box 9169
Boston
MA
02209
US
|
Assignee: |
Shipley Company, L.L.C.
Marlborough
MA
|
Family ID: |
32990583 |
Appl. No.: |
10/741908 |
Filed: |
December 19, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60435976 |
Dec 20, 2002 |
|
|
|
Current U.S.
Class: |
205/104 |
Current CPC
Class: |
C25D 3/38 20130101; C25D
5/18 20130101; C25D 3/02 20130101 |
Class at
Publication: |
205/104 |
International
Class: |
C25D 005/18 |
Claims
What is claimed is:
1. A composition comprising chloride ion and a brightener having a
concentration ratio of chloride ion to brightener of from 20:1 to
125:1 at a brightener concentration of from 0.001 ppm to 1.0
ppm.
2. The composition of claim 1, wherein the chloride ion to
brightener concentration ratio is from 25:1 to 120:1.
3. The composition of claim 1, wherein a source of chloride ion is
sodium chloride, potassium chloride, hydrogen chloride, or mixtures
thereof.
4. The composition of claim 1 wherein the brightener comprises a
compound having a formula HS--R--SO.sub.3X,
XO.sub.3--S--R--S--S--R--SO.sub.3X or
XO.sub.3--S--Ar--S--S--Ar--SO.sub.3X where R is a substituted or
unsubstituted alkyl group, Ar is an aryl group, and X is a counter
ion.
5. The composition of claim 1, further comprising metal ions,
levelers, suppressors, buffers, surfactants, or mixtures
thereof.
6. The composition of claim 5, wherein the metal ions comprise
copper ions, nickel ions, tin ions, lead ions, chromium ions,
palladium ions, gold ions, silver ions, platinum ions, indium ions,
cadmium ions, bismuth ions, cobalt ions, rhodium ions, ruthenium
ions, or zinc ions.
7. The composition of claim 5, wherein the levelers comprise lactam
alkoxylates having a formula: 2where A is a hydrocarbon radical,
R.sup.1 is hydrogen or methyl, n is an integer of from 2 to 10, and
n' is an integer from 1 to 50.
8. The composition of claim 5, wherein the levelers comprise a
polyalkylene glycol ether of formula
[R.sub.2--O(CH.sub.2CH.sub.2O).sub.m-
(CH(CH.sub.3)--CH.sub.2O.sub.p--R.sup.3].sub.a where m is an
integer of from 8 to 800, p is an integer of from 0 to 50, R.sup.2
is a (C.sub.1-C.sub.4)alkyl, R.sup.3 is an aliphatic chain or an
aromatic group, and a is 1 or2.
9. The composition of claim 5, wherein the levelers comprise
compounds having a formula N--R.sup.4--S where R.sup.4 is a
substituted or unsubstituted alkyl group or a substituted or
unsubstituted aryl group.
10. The composition of claim 5, wherein the suppressors comprise
polyhydroxy compounds.
11. A method comprising: (a) generating an electromotive force
through a cathode, anode and a composition in electrical
communication to provide an electric field around the cathode, the
anode and the composition, the composition comprises metal ions,
brighteners and chloride ions, the chloride ions to brighteners
concentration ratio is from 20:1 to 125:1; (b) modifying the
electric field around the cathode, the anode and the composition to
provide a pulse pattern or a combination of pulse patterns
comprising (i) cathodic current followed by anodic current; (ii)
cathodic current followed by anodic current followed by cathodic DC
current; (iii) cathodic current followed by anodic current followed
by equilibration; or (iv) cathodic current followed by anodic
current followed by cathodic DC current then followed by
equilibration to electroplate a metal on the cathode.
12. The method of claim 11, wherein the cathodic current is from 40
ms to 1 second and the anodic current is from 0.25 ms to 15 ms for
pulse pattern (i).
13. The method of claim 11, wherein the cathodic current is from 40
ms to 1 second and the anodic current is from 0.25 ms to 15 ms and
the cathodic DC current is from 5 seconds to 90 seconds of pulse
pattern (ii).
14. The method of claim 11, wherein the cathodic current is from 40
ms to 1 second and the anodic current is from 0.25 ms to 15 ms and
equilibration is from 5 seconds to 90 seconds.
15. The method of claim 11, wherein the cathodic current is from 40
ms to 1 second and the anodic current is from 0.25 ms to 15 ms and
the cathodic DC current is from 5 seconds to 90 seconds and
equilibration is from 5 seconds to 90 seconds.
16. The method of claim 11, wherein a current density is from 5
mA/cm.sup.2 to 200 mA/cm
17. The method of claim 11, wherein the metal that is plated
comprises copper, tin, nickel, cobalt, chromium, cadmium, lead,
silver, gold, platinum, palladium, bismuth, indium, rhodium,
ruthenium, iridium, zinc or alloys thereof.
18. The method of claim 11, wherein the composition further
comprises levelers, suppressors, surfactants, buffers or mixtures
thereof.
19. The method of claim 11, wherein the cathode is a printed
circuit board or a silicon wafer.
20. The method of claim 11, wherein the composition comprises
brighteners in an amount of from 0.001 ppm to 1.0 ppm.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention is directed to a reverse pulse plating
composition and method. More specifically, the present invention is
directed to a reverse pulse plating composition and method that
reduces brightener decomposition and reduces defects of an
electroplated metal layer.
[0002] Numerous compositions and methods for electroplating
articles with metal layers or coatings are employed in many
industries. Such methods may involve passing a current between two
electrodes in a plating composition or solution where one of the
electrodes is an article to be metal plated. Using an acid copper
plating solution for purposes of illustration, a plating solution
may contain (1) dissolved copper (cupric ions), usually copper
sulfate, (2) an acid electrolyte such as sulfuric acid in an amount
sufficient to impart conductivity to the solution, and (3)
additives to improve efficiency of the plating reaction and the
quality of the metal deposit. Such additives include, for example,
surfactants, brighteners, levelers, suppressants, and corrosion
inhibitors.
[0003] Metals that may be electroplated include, for example,
copper, copper alloys, nickel, tin, lead, gold, silver, platinum,
palladium, cobalt, chromium, and zinc. Electrolytic metal plating
solutions are used for many industrial applications. For example,
they may be used in the automotive industry as base layers for
subsequently applied decorative and corrosion protective coatings.
They also may be used in the electronics industry, such as in the
fabrication of printed circuit or wiring boards, and for
semiconductor devices. For circuit fabrication in a printed circuit
board, a metal such as copper is electroplated over selected
portions of the surface of a printed circuit board and onto the
walls of through-holes passing between the surfaces of the circuit
board base material. The walls of the through-holes are metallized
to provide conductivity between circuit layers on each surface of
the board.
[0004] Early efforts to make printed circuit boards use
electrolytic metal plating solution developed for decorative
plating. However, as printed circuit boards became more complex and
as industry standards became more rigorous, solutions used for
decorative plating were found to be inadequate for printed circuit
board fabrication. One serious problem encountered using
electrolytic metal plating solutions involved coatings of uneven
thickness on the walls of the through-holes with metal deposits
thicker at the tope and bottom of the through-holes and thinner at
the center, a condition known in the art as "dog boning". The
thinner deposit at the center of the through-hole may lead to
circuit defects and board rejection.
[0005] Dog boning is believed to be caused by a voltage drop
between the top surface of the through-hole and the center of the
through-hole. The potential drop is a function of current density,
a ratio of the length of the through-hole to the through-hole
diameter (aspect ratio) and board thickness. As the aspect ratio
and the thickness of the board increase, dog boning becomes more
severe due to a voltage drop between the surface of the board and
the center of the through-hole. This voltage drop is believed to be
caused by a combination of factors including solution resistance, a
difference in surface to through-hole over potential due to mass
transfer, i.e., a difference in the flow of solution through the
through-hole compared to the movement of the solution over the
surface of the board, and a charge transfer difference as a
consequence of the concentration of solution additives in the
through-hole compared to the surface.
[0006] The printed circuit board industry continuously seeks
greater circuit densification. To increase density, the industry
has resorted to multi-layer circuits with through-holes or
interconnections passing through multiple layers. Multi-layer
circuit fabrication results in an overall increase in the thickness
of the board and a concomitant increase in the length of an
interconnection passing through the board. This means that
increased circuit densification results in increased aspect ratios
and through-hole length and an increase in the severity of the dog
boning problem. For high density boards, aspect ratios may exceed
ten to one.
[0007] Another problem encountered in metal electroplating are
defects such as intermittent surface roughness and non-uniform
surface appearance of the plated metal. Intermittent surface
roughness and non-uniform surface appearance are believed to be
caused by non-uniform current distribution across the surface of
the printed wiring board that is being plated. The non-uniform
current distribution results in an unequal or uneven deposit of
metal on the board surface resulting in the surface roughness and
non-uniformity of plated metal layers.
[0008] Another defect, which is often observed, is the formation of
dendrites or "whiskers". Whiskers are believed to be crystals of
the metal being plated and grow out of the plated surface. Whiskers
may range in diameter of from less than 1 micron to as large as
several millimeters. Although the cause of whisker growth has been
the subject of some debate, there is no question that whiskers are
undesirable for a variety of electrical, mechanical, and cosmetic
reason. For example, whiskers are readily detached and carried by
cooling air flows into electronic assemblies, both within and
external to electronic article housings, where they may cause
short-circuit failure.
[0009] Plating metal is a complex process that involves multiple
ingredients in a plating bath. In addition to metal salts that
provide a source of metal, pH adjusters and surfactants or wetting
agents, many plating baths contain chemical compounds that improve
various aspects of the plating process. Such chemical compounds or
additives are auxiliary bath components that are used to improve
the brightness of the metal plating, the physical properties of the
plated metal especially with respect to ductility and throwing
power of an electroplating solution or bath. Throwing power of the
solution defined as the ratio of current density flowing at the
center of the through-hole to the current density flowing at the
surface of the through-hole. Optimum throwing power is achieved
when the current density at the center of the through-hole is the
same as the current density flowing at the through-hole surface.
However, such a current density is difficult to achieve.
[0010] A main concern is additives that have an effect on the
bright finish, leveling and uniformity of metal deposition on
surfaces. Maintaining bath concentrations of such additives within
close tolerances is important to obtain high quality metal
deposits. The additives do breakdown during metal plating. The
additives breakdown due to oxidation at the anode, by reduction at
the cathode, and by chemical degradation.
[0011] When additives breakdown during plating, the breakdown
products may result in metal layer deposit characteristics that are
less than satisfactory for industry standards. Regular additions of
additives based upon empirical rules established by workers in the
industry to try and maintain optimum concentrations of the
additives have been employed. However, monitoring the
concentrations of the additives that improve metal plating is still
difficult because additives are present in plating baths in small
concentrations, i.e., parts per million of solution. Accordingly,
the level of the additives in the bath eventually changes such that
the additive concentrations are out of the acceptable range of
tolerance. If the additive concentration goes too far out of range
of tolerance, the quality of the metal deposit suffers and the
deposit may be dull in appearance and/or brittle or powdery in
structure. Other consequences include low throwing power and/or
plating folds with bad leveling. Electroplating of through-hole
interconnections in the manufacture of multi-layer printed circuit
boards is an example of where quality plating is required.
[0012] Many of the foregoing problems are found in reverse pulse
plating baths and methods. Reverse pulse plating is an
electroplating process where the electrical current is alternated
between a cathodic current (forward pulse) and an anodic current
(reverse pulse) during the electroplating process. Typical pulses
or waveforms are a reverse to forward voltage ratio of 3 to 1 and
times of 10 to 20 milliseconds for the forward waveform and 0.5 to
1 millisecond for the reverse. However, such waveforms often result
in undesirable intermittent surface roughness and non-uniform
surface appearance on plated metal layers, especially at current
densities of 100 amps/cm.sup.2.
[0013] Another problem with reverse pulse plating baths is their
short bath life, which may be in terms of a few days, i.e., two to
three days, of optimum performance. Preferably optimum bath
performance is continuous (from 6 months to at least a year). The
longer the duration of the optimum performance of a bath the more
economically efficient is the electroplating process. The short
life of a reverse pulse plating bath is due to additive breakdown,
especially due to the build-up of brightener by product. The rate
at which byproducts form is primarily governed by the brightener
concentration and secondarily by the idle time at which the
byproduct is formed on an anode surface. Reverse pulse plating
often uses high brightener concentrations, i.e., in excess of 1 ppm
(part per million), to help prevent or reduce poor performance in
leveling, throwing power and corner cracking. Poor throwing power
results in rough metal surfaces and non-uniform metal layers.
Corner cracking is a condition where the plated metal layer begins
to separate from the plated substrate. However, high brightener
concentrations may result in high concentrations of byproducts,
which may shorten the electroplating bath life. Accordingly, there
is a need for an improved reverse pulse plating composition or bath
and an improved reverse pulse plating method to address the
foregoing problems.
SUMMARY OF THE INVENTION
[0014] The present invention is directed to a composition including
chloride and a brightener having a concentration ratio of the
chloride to the brightener ranges from 20:1 to 125:1, and a
brightener concentration of 0.001 ppm to 1.0 ppm. The composition
may be employed as a metal plating solution or bath for
electrolytic deposition of metals on a substrate. In addition to
the chloride and brightener, the composition includes a source of
metal ions. The source of metal ions may be a salt of the metal
that is to be electroplated on the substrate.
[0015] The composition of the present invention also may include
other additives such as levelers, suppressors, carriers,
surfactants, buffers as well as other components that may be used
in electroplating baths. The compositions of the present invention
may have an aqueous or organic solvent.
[0016] Another embodiment of the present invention is directed to a
method which includes (a) generating an electromotive force through
a cathode, anode and a composition in electrical communication to
provide an electric field around the cathode, the anode and the
composition, the composition comprises metal ions, brighteners and
chloride ions, the chloride ions to brighteners are at a
concentration ratio of from 20:1 to 125:1; (b) modifying the
electric field around the cathode, the anode and the composition to
provide a pulse pattern or a combination of pulse patterns
comprising (i) cathodic current followed by anodic current; (ii)
cathodic current followed by anodic current followed by cathodic DC
current; (iii) cathodic current followed by anodic current followed
by equilibration; or (iv) cathodic current followed by anodic
current followed by cathodic DC current then followed by
equilibration to electroplate a metal on the cathode.
[0017] Advantageously, the compositions and methods prevent or at
least reduce dendrite or whisker formation on metal plated
substrates, reduce dog boning as well as intermittent surface
roughness, and provide a uniform metal layer on the substrates.
Other advantages include improved leveling performance, improved
throwing power and reduced corner cracking. Also additive
decomposition is reduced to provide electroplating baths having a
longer operating life.
[0018] A primary objective of the present invention is to provide a
composition with reduced additive breakdown.
[0019] Another objective is to provide a composition that has an
improved electroplating life.
[0020] A further objective of the present invention is to provide a
method of metal plating a substrate that reduces metal plating
defects.
[0021] Still yet another objective is to provide a method of
plating a metal that has an improved throwing power.
[0022] Other objectives and advantages of the methods and
compositions may be ascertained by a person of skill in the art
after reading the disclosure of the invention and the appended
claims.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Compositions include chloride ion and brighteners in
concentration ratios of from 20:1 to 125:1, and a brightener
concentration of 0.001 ppm to 1.0 ppm. The compositions also may
include other additives depending on the particular function of the
composition. The compositions may be employed as electroplating
solutions for plating a metal on a substrate. When the compositions
are employed as electroplating baths, metal ions of the metal to be
plated are included in the composition as well as other additives
to assist in optimizing performance of the electroplating bath.
[0024] The compositions are suitable for electroplating by reverse
pulse plating. Accordingly, another embodiment of the present
invention is a reverse pulse plating method to electroplate a metal
on a substrate. An electromotive force (emf) is generated from a
suitable electrical source to provide an electric field around an
electroplating apparatus including an anode, cathode and a
composition including chloride ions and brighteners in a
concentration ratio of from 20:1 to 125:1 and metal ions. The
anode, cathode and composition are in electrical communication with
each other to provide a complete electrical circuit with the source
of the electromotive force. Typically the cathode is the substrate
on which the metal is plated.
[0025] During electroplating of a metal, the electric field around
the electroplating apparatus may be modified to provide (i) a
cathodic current (forward pulse or waveform) followed by an anodic
current (reverse pulse or waveform); (ii) a cathodic current
followed by an anodic current (reverse pulse or waveform) followed
by cathodic DC current (direct current); (iii) a cathodic current
followed by an anodic current (reverse pulse or waveform) followed
by equilibration (open circuit); (iv) a cathodic current followed
by an anodic circuit (reverse pulse or waveform) followed by
cathodic DC current (direct current) then followed by equilibration
(open circuit); or combinations of pulse patterns (i), (ii), (iii),
or (iv) provided that the net result of the pulse electroplating
process results in a metal layer formed on the substrate to be
metal plated. Net current for each pattern or combination of
patterns is in the cathodic or plating direction. During cathodic
current (AC or alternating current) a metal is being plated on the
cathode, while during anodic current metal is being removed or
stripped from the cathode. During cathodic DC current metal is
again being plated on the cathode, and during equilibration there
is no metal being deposited on the cathode or stripped from the
cathode. There is no plating or stripping during equilibration
because the electrical circuit is open and there is no emf to plate
or strip. In other words, workers choose a particular pulse pattern
or combination of pulse patterns such that the net result provides
a metal layer or coat on the substrate, which typically is the
cathode of the plating apparatus. The particular order of each
pulse pattern and the time duration during an electroplating
process of each pulse pattern and their respective waveforms, DC
currents and equilibrations may vary depending on the dimensions of
the substrate and the desired thickness of the metal layer(s).
Reverse to forward voltage ratios range from 1.5 to 5.5, preferably
from 2.5 to 3.5. The pulse patterns provide for reduced
intermittent surface roughness and improved uniform metal layer(s)
in contrast to many conventional pulse plating patterns. The pulse
plating patterns also have improved throwing power in contrast to
many conventional pulse plating patterns.
[0026] Examples of pulse patterns that may be used to electroplate
a substrate include pulse pattern (i) by itself during the entire
electroplating process; a combination of pulse patterns (i) and
(ii); a combination of pulse patterns (i), (ii) and (iii); a
combination of pulse patterns (i), (ii), (iii), and (iv); or a
combination of pulse patterns (i), (iii) and (iv). The particular
order of each pulse pattern and the time duration of each including
their respective waveforms, DC currents and equilibrations may vary
depending on the dimensions of the substrate and the desired
thickness of the metal layer(s). Some minor experimentation may be
employed to determine which combination of pulse patterns and
duration of the pulse patterns optimize the electroplating process
for a given substrate. Such minor experimentation is common in the
electroplating art to optimize electroplating processes. A
preferred pulse pattern is (i) a cathodic current (forward pulse or
waveform) followed by an anodic current (reverse pulse or
waveform).
[0027] Current densities may range from 5 milliamps(mA)/cm.sup.2 to
200 mA/cm.sup.2, preferably from 5 mA/cm.sup.2 to 125 mA/cm.sup.2,
more preferably from 5 mA/cm.sup.2 to 50 mA/cm.sup.2. Forward
pulses range in time from 40 milliseconds (ms) to 1 second,
preferably from 40 ms to 800 ms, and reverse pulses may range from
0.25 ms to 15 ms, preferably from 1 ms to 3 ms for pulse pattern
(i). Forward pulses range from 40 ms to 1 second, preferably from
40 ms to 800 ms and reverse pulses range from 0.25 ms to 15 ms,
preferably from 1 minute to 10 ms, and the DC current ranges from 5
seconds to 90 seconds, preferably from 10 seconds to 60 seconds for
pulse pattern (ii). Forward pulses range from 40 ms to 1 second,
preferably from 40 ms to 800 ms and reverse pulses range from 0.25
ms to 15 ms, preferably from 1 minute to 10 ms, and the
equilibration ranges from 5 seconds to 90 seconds, preferably from
10 seconds to 60 seconds in pulse pattern (iii). Forward pulses
range from 40 ms to 1 second, preferably from 40 ms to 800 ms,
reverse pulses range from 0.25 ms to 15 ms, preferably from 1
minute to 10 ms, DC current ranges from 5 seconds to 90 seconds,
preferably from 10 seconds to 60 seconds, and equilibration ranges
from 5 seconds to 90 seconds, preferably from 10 seconds to 60
seconds for pulse pattern (iv).
[0028] Pulse times, pulse patterns and applied voltages of the
cathodic and anodic waveforms may be adjusted to provide that the
overall process is cathodic, i.e., there is a net deposition of
metal on a substrate. Workers may adapt the pulse time waveforms
and their frequencies to a particular application based on the
teachings of the process of the invention.
[0029] The electroplating compositions may be employed to plate any
metal that may be electroplated on a substrate. Examples of such
metals include copper, tin, nickel, cobalt, chromium, cadmium,
lead, silver, gold, platinum, palladium, bismuth, indium, rhodium,
ruthenium, iridium, zinc, or alloys thereof. The electroplating
compositions are especially suitable for electroplating copper and
copper alloy to a substrate. Metals are included in the
compositions as soluble salts. Any suitable metal salt may be
employed to practice the present invention provided that the metal
salt is soluble in the composition solvent. Examples of suitable
copper compounds include copper halides, copper sulfates, copper
alkane sulfonate, copper alkanol sulfonate, or mixtures thereof.
Such copper compounds are water-soluble.
[0030] A sufficient amount of a metal salt is included in the
electroplating composition such that the concentration of the
respective metal ion is from 0.010 grams/liter to 200 grams/liter,
preferably from 0.5 grams/liter to 100 grams/liter. When copper is
the metal, a sufficient amount of copper salt is employed such that
the copper ion concentration preferably ranges from 0.01 to 100
grams/liter, more preferably from 0.10 grams/liter to 50
grams/liter. Solvents of the electroplating composition may be
water or an organic solvent such as alcohol or other suitable
organic solvent employed in electroplating. Mixtures of solvents
also may be employed.
[0031] Sources of chloride ion include any suitable chloride salt
or other source of chloride that is soluble in the electroplating
compositions solvent. Examples of such chloride ion sources are
sodium chloride, potassium chloride, hydrogen chloride (HCl), or
mixtures thereof. A sufficient amount of chloride ion source is
included in a composition such that the chloride ion concentration
ranges from 0.02 ppm to 125 ppm, preferably from 0.25 ppm to 60
ppm, more preferably from 5 ppm to 35 ppm.
[0032] Brighteners that may be employed in the compositions and
methods of the present invention include any brightener that is
suitable for the metal that is to be electroplated. Brighteners may
be specific for the metal that is plated. Workers in the art are
familiar with the particular brightener that may be employed with a
particular metal. Brighteners are included in the electroplating
compositions at a range of from 0.001 ppm to 1.0 ppm, preferably
from 0.01 ppm to 0.5 ppm, more preferably from 0.1 ppm to 0.5 ppm.
Thus, chloride to brightener concentrations of the compositions
range from 20:1 to 125:1, preferably 25:1 to 120:1, more preferably
from 50:1 to 70:1. Such ranges of chloride ion to brightener are
suitable for reducing or preventing whisker formation, corner
cracking and brightener byproduct formation during electroplating,
especially for electroplating copper or copper alloys. Such
chloride to brightener ratios also improves leveling, and throwing
power of an electroplating bath, especially in copper or copper
alloy electroplating.
[0033] Examples of suitable brighteners include sulfur containing
compounds that have a general formula S--R--SO.sub.3, where R is
substituted or unsubstituted alkyl or substituted or unsubstituted
aryl group. More specifically, examples of suitable brighteners
include compounds having structural formulas HS--R--SO.sub.3X,
XO.sub.3--S--R--S--S--R--SO.sub.3X or
XO.sub.3--S--Ar--S--S--Ar--SO.sub.3- X where R is a substituted or
unsubstituted alkyl group, and preferably is an alkyl group having
from 1 to 6 carbon atoms, more preferably is an alkyl group having
from 1 to 4 carbon atoms; Ar is an aryl group such as phenyl or
naphthyl; and X is a suitable counter ion such as sodium or
potassium. Specific examples of such compounds include
n,n-dimethyl-dithiocarbamic acid-(3-sulfopropyl)ester, carbonic
acid-dithio-o-ethylester-s-ester with 3-mercapto-1-propane sulfonic
acid (potassium salt), bissulfopropyl disulfide (BSDS),
3-(benzthiazolyl-s-thio)propyl sulfonic acid (sodium salt),
pyridinium propyl sulfonic sulfobetaine, or mixtures thereof. Other
suitable brighteners are described in U.S. Pat. Nos. 3,770,598,
4,374,709, 4,376,685, 4,555,315, and 4,673,469. Also aromatic and
aliphatic quaternary amines may be added to the compositions to
improve metal brightness.
[0034] Examples of other suitable brighteners include
3-(benzthiazoyl-2-thio)-propylsulfonic acid sodium salt,
3-mercaptopropane-1-sulfonic acid sodium salt,
ethylenedithiodipropylsulf- onic acid sodium salt,
bis-(p-sulfophenyl)-disulfide disodium salt,
bis(.omega.-sulfobutyl)-disulfide disodium salt,
bis-(.omega.-sulfohydrox- ypropyl)-disulfide disodium salt,
bis-(.omega.-sulfopropyl)-disulfide disodium salt,
bis-(.omega.-sulfopropyl)-sulfide disodium salt,
methyl-(.omega.-sulfopropyl)-disulfide sodium salt,
methyl-(.omega.-sulfopropyl)-trisulfide disodium salt,
o-ethyl-dithiocarbonic acid-S-(.omega.-sulfopropyl)-ester potassium
salt, thioglycolic acid, thiosphosphoric
acid-o-ethyl-bis-(o-sulfopropyl)-ester disodium salt,
thiophosphoric acid-tris(.omega.-sulfopropyl)-ester trisodium salt,
N,N-dimethyldithiocarbamic acid (3-sulfopropyl) ester sodium salt
(DPS), (o-ethyldithiocarbonato)-S-(3-sulfopropyl)-ester potassium
salt (OPX), 3-[(amino-iminomethyl)-thio]-1-propanesulfonic acid
(UPS), 3-(2-benthiazolylthio)-1-propanesulfonic acid sodium salt
(ZPS), thiol of bissulfopropyl disulfide (MPS), or mixtures
thereof.
[0035] In addition to soluble metal compounds, chloride ions and
brighteners, the compositions of the present invention also may
include levelers, suppressors (carriers), surfactants, buffering
agents and other compounds used in conventional electroplating
baths.
[0036] Examples of suitable levelers include lactam alkoxylates
having a formula: 1
[0037] where A represents a hydrocarbon radical such as
--CH.sub.2--, R.sup.1 is hydrogen or methyl, n is an integer from 2
to 10, preferably from 2 to 5, and n' is an integer from 1 to 50.
Examples of such compounds include .beta.-propiolactam ethoxylate,
.gamma.-butyrolactam-he- xa-ethoxylate,
.delta.-valerolactam-octa-ethoxylate,
.delta.-valerolactam-penta-propoxylate,
.epsilon.-caprolactam-hexa-ethoxy- late, or
.epsilon.-caprolactam-dodeca-ethoxylate. Such leveling agents are
included in electroplating compositions in amounts of from 0.002 to
3 grams/liter, preferably from 0.005 to 0.2 grams/liter.
[0038] Another example of suitable levelers include polyalkylene
glycol ethers of formula:
[R.sup.2--O(CH.sub.2CH.sub.2O).sub.m(CH(CH.sub.3)--CH.sub.2O).sub.p--R.sup-
.3].sub.a
[0039] where m is an integer of from 8 to 800, preferably from 14
to 90, p is an integer of from 0 to 50, preferably from 0 to 20,
R.sup.2 is a (C.sub.1-C.sub.4)alkyl, R3 is an aliphatic chain or an
aromatic group and a is either 1 or 2.
[0040] Amounts of polyalkylene glycol ether that may be included in
the compositions ranges from 0.005 to 30 grams/liter, preferably
from 0.02 to 8.0 grams/liter. Relative molecular mass may be from
500 to 3500 grams/mole, preferably from 800 to 4000 grams/mole.
[0041] Such polyalkylene glycol ethers are known in the art or may
be produced according to processes which are known in the art by
converting polyalkylene glycols with an alkylating agent such as
dimethyl sulfate or tertiary butene.
[0042] Examples of such polyalkylene glycol ethers include dimethyl
polyethylene glycol ether, dimethyl polypropylene glycol ether,
di-tertiary butyl polyethylene glycol ether, stearyl monomethyl
polyethylene glycol ether, nonylphenol monomethyl polyethylene
glycol ether, polyethylene polypropylene dimethyl ether (mixed or
block polymer), octyl monomethyl polyalkylene ether (mixed or block
polymer), dimethyl-bis(polyalkylene glycol)octylene ether (mixed or
block polymer), and .beta.-naphthol monomehtyl polyethylene
glycol.
[0043] Additional levelers that may be employed to practice the
present invention include nitrogen and sulfur containing levelers
with a formula N--R.sup.4--S, where R.sup.4 is a substituted or
unsubstituted alkyl group or a substituted or unsubstituted aryl
group. The alkyl groups may have from 1 to 6 carbons, typically
from 1 to 4 carbons. Suitable aryl groups may include substituted
or unsubstituted phenyl or naphthyl. Substituents of the alkyl and
aryl groups may be, for example, alkyl, halo, or alkoxy. Examples
of specific levelers include
1-(2-hydroxyethyl)-2-imidazolidinethione, 4-mercaptopyridine,
2-mercaptothiazoline, ethylene thiourea, thiourea, and alkylated
polyalkyleneimine. Such levelers are included in amounts of 500 ppb
(parts per billion) or less, preferably from 100 to 500 ppb. Other
suitable leveling agents are described in U.S. Pat. Nos. 3,770,598,
4,374,709, 4,376,685, 455,315 and 4,673,459.
[0044] Any suppressor (carrier) that is employed in metal plating
may be employed in the practice of the present invention. While the
concentrations of suppressors may vary from one electroplating bath
to another, suppressors typically range from 100 ppm or greater.
Examples of such suppressors are polyhydroxy compounds such as
polyglycols, e.g., poly(ethylene glycol), poly(propylene glycol)
and copolymers thereof. An example of a preferred suppressor is
poly(ethylene glycol). A suitable concentration range for
poly(ethylene glycol) is from 200 ppm to 2000 ppm. The
poly(ethylene glycol) may range in molecular weight from 1000 to
12000, preferably from 2500 to 5000.
[0045] Any suitable buffer or pH adjuster may be employed in the
present invention. Such pH adjusters may include, for example,
inorganic acids such as sulfuric acid, hydrochloric acid, nitric
acid, phosphoric acid, or mixtures thereof Sufficient acid is added
to the compositions such that the pH ranges from 0 to 14,
preferably from 0 to 8.
[0046] During electroplating the compositions or electroplating
baths may range in temperature of from 20.degree. C. to 110.degree.
C. Temperature ranges for specific metals may vary and such
temperature ranges are well known in the art. Copper electroplating
baths may be maintained at a temperature range of from 20.degree.
C. to 80.degree. C. with acid copper baths (pH from 0 to 4) at
temperatures of from 20.degree. C. to 50.degree. C. Metal plating
is continued for a time sufficient to form a deposit of desired
thickness. Plating time for a printed wiring board may range from
45 minutes to 8 hours. For circuit board manufacture, a desired
thickness may range from 62 mils to 400 mils (0.001 mils/inch and
2.54 cm/inch).
[0047] The composition and method of the present invention is
suitable for metal plating through-holes of multi-layer circuit
boards with aspect ratios of at least 10:1 and through-hole
interconnections of at least 0.16 cm, and blind vias of 0.063 cm.
The composition and method or the present invention, in addition to
the other advantages, reduces or eliminates dog-boning in contrast
to many conventional electroplating methods.
[0048] Both vertical and horizontal plating processes may be
employed. In vertical processes the substrate, such as a printed
wiring board, is sunk in a vertical position into a container
containing a plating bath composition of the present invention. The
substrate, which functions as a cathode, is situated in the
vertical position opposite to at least one soluble or insoluble
anode. The substrate and the anode are connected to a current
source and an electrical current or electric field is generated the
substrate, anode and plating composition. Any suitable source for
emf may be employed. Various apparatus for generating an emf are
well known in the art. Plating composition is directed continuously
through a container with the cathode, anode and plating composition
by means of transporting equipment such as a pump. Any suitable
pump employed in electroplating processes may be employed to
practice the present invention. Such pumps are well known in the
electroplating industry and are readily available.
[0049] In the horizontal plating process, the substrate or cathode
is transported through a conveyorized unit in a horizontal position
with a horizontal direction of movement. Electroplating composition
is injected continuously from below and/or above and onto the
substrate by means of splash nozzles or flood pipes. Anodes are
arranged at a spacing relative to the substrate and are brought
into contact with the electroplating composition by means of a
suitable device. The substrate is transported by means of rollers
or plates. Such horizontal apparatus are well known in the art.
[0050] The compositions and method of the present invention
eliminate or reduce dog-boning, increase throwing power, reduce or
prevent corner cracking as well as whisker formation, and provide
an improved metal layer surface and leveling performance.
Additionally, the compositions of the present invention are more
stable than many conventional plating compositions. Accordingly,
the present invention is an improvement in the metal plating
art.
[0051] While the present invention is described with an emphasis on
electroplating in the printed wiring board industry, the present
invention may be employed in any suitable plating process. The
composition and method may be employed in metal plating in the
manufacture of electrical devices such as printed circuit and
wiring boards, integrated circuit, electrical contact surfaces and
connectors, electrolytic foil, silicon wafers for microchip
applications, semi-conductors and semi-conductor packaging, lead
frames, optoelectronics, and optoelectronic packaging, and solder
bumps, such as on wafers.
[0052] All numerical ranges in the present application are
inclusive and combinable.
[0053] The following example is provided to better describe the
present invention, and is not intended to limit the scope of the
invention.
EXAMPLE 1
Compositions to Reduce or Eliminate Whiskers
[0054] Eight copper metal electroplating baths were prepared to
verify the ability of chloride to prevent or reduce whisker
(dendrite) formation on copper metal surfaces during electroplating
of copper on a substrate. Each electroplating composition or bath
was an aqueous bath that contained 80 grams/liter of copper sulfate
pentahydrate as the metal ion source, 225 grams/liter of sulfuric
acid to maintain the pH of the baths at 4.0. Chloride ion
concentration in each of the baths was 25 ppm. The chloride ion
source was HCl. In addition to the foregoing components, each bath
also contained a carrier component at a concentration of either
0.25 ppm or 1 ppm, and a brightener (BSDS) in an amount of either
0.1 ppm or 0.2 ppm to provide a chloride to brightener ratio of
either 125:1 or 250:1. Carriers that were employed in each solution
are disclosed in the table below. All of the carriers listed in the
table below are block copolymers.
[0055] Each bath was placed in a separate standard 1.5 liter
Gornell cell and a 9.5 cm.times.8.25 cm copper clad panel (cathode)
was placed in each cell with air circulation and mechanical
agitation during the electroplating process. A copper anode was
employed as the auxiliary electrode. Current density during the
electroplating process was maintained at 32 mAmps/cm.sup.2. Each
panel was electroplated for 60 minutes using a forward to reverse
waveform of 10 ms to 0.2 ms. The source of the emf was a Technu
pulse rectifier.
1TABLE Carrier Level Sample in parts Chloride/Brightener Whisker
Number Carrier per million Ratio Count 1 Ingepal CA877 1 250 6 2
Ingepal CA877 1 125 1 3 Pluronic .RTM. F68 1 250 >5 4 Pluronic
.RTM. F68 1 125 1 5 Nape 14-90 0.25 250 >5 6 Nape 14-90 0.25 125
1 7 Tetronic .RTM. 304 1 250 2 8 Tetronic .RTM. 304 1 125 0
[0056] After each panel was plated with a copper layer, the panels
were removed from the Gornell cell and examined for whiskers.
Examination was done with the naked eye and by touching the
surfaces of each panel and counting the whiskers.
[0057] Panels that were plated in baths with a chloride to
brightener ratio of 125 had whisker counts of 1 or 0 (samples 2, 4,
6, and 8). Panels that had a chloride to brightener ratio of 250
had whisker counts of 6, >5 or 2 samples 1, 3, 5, and 7).
Accordingly, the compositions that had a chloride to brightener
ratio of 125 eliminated or reduced whisker count.
EXAMPLE 2
Whisker Reduction
[0058] Four electroplating baths were prepared to verify the
function of the pulse waveform with respect to the formation for
whiskers (dendrites). All four baths contained the same
concentration of chemical components, and all substrates were
plated using the same anodes, and tank assembly. The anodes were
freshly etched prior to each plating experiment. The concentration
of inorganic components in each bath was 82 g/L
CuSO.sub.4.5H.sub.2O, 216.5 g/L H.sub.2SO.sub.4, and Cl-/brightener
ratio was 44. The concentration of suppressor in each bath was 15
ml/l. In a 1.5 liter Haring plating cell, a 15 cm.times.6.3 cm
copper clad panel was electroplated at 10.7 mA/cm2 in each plating
bath using a different pulse waveform as shown in the Table. After
plating, the boards were physically scanned for whiskers, see
Table. As shown in the Table, as the forward wave was made longer,
the number of whiskers was reduced significantly. This effect was
particularly marked as the forward wave reaches 50 ms and
above.
2TABLE Forward time, ms Reverse time, ms Number of Whiskers 10 0.5
69 20 1 37 50 2.6 27 100 5 21
* * * * *