U.S. patent application number 15/295163 was filed with the patent office on 2017-05-25 for method of electroplating low internal stress copper deposits on thin film substrates to inhibit warping.
The applicant listed for this patent is Rohm and Haas Electronic Materials LLC. Invention is credited to Luis Gomez, Yu Hua Kao, Mark Lefebvre, Lingyun Wei.
Application Number | 20170145577 15/295163 |
Document ID | / |
Family ID | 57256162 |
Filed Date | 2017-05-25 |
United States Patent
Application |
20170145577 |
Kind Code |
A1 |
Kao; Yu Hua ; et
al. |
May 25, 2017 |
METHOD OF ELECTROPLATING LOW INTERNAL STRESS COPPER DEPOSITS ON
THIN FILM SUBSTRATES TO INHIBIT WARPING
Abstract
Thin film substrates are electroplated with copper from low
internal stress, high ductility acid copper electroplating baths.
During the copper electroplating process the thin film substrates
can warp or bow. To address the problem of warping or bowing during
copper electroplating the thin film substrate is held by a securing
means which inhibits the thin film substrate from excessive
activity.
Inventors: |
Kao; Yu Hua; (Shrewsbury,
MA) ; Wei; Lingyun; (Shrewsbury, MA) ; Gomez;
Luis; (Holden, MA) ; Lefebvre; Mark; (Hudson,
NH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rohm and Haas Electronic Materials LLC |
Marlborough |
MA |
US |
|
|
Family ID: |
57256162 |
Appl. No.: |
15/295163 |
Filed: |
October 17, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62257262 |
Nov 19, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 3/38 20130101; C25D
17/06 20130101; C25D 17/001 20130101; C25D 17/00 20130101; C25D
5/04 20130101 |
International
Class: |
C25D 3/38 20060101
C25D003/38; C25D 17/06 20060101 C25D017/06; C25D 5/04 20060101
C25D005/04 |
Claims
1. A method of copper electroplating a thin film substrate
comprising: a) providing the thin film substrate; b) attaching the
thin film substrate to a securing means to inhibit substantially
all motion of the thin film substrate in relation to the securing
means; c) attaching one or more electrical contacts to the thin
film substrate; d) passing the thin film substrate attached to the
securing means and with the one or more electrical contacts through
a low stress, high ductility acid copper electroplating bath
wherein the thin film substrate and securing means with the one or
more electrical contacts remain substantially in a single plane
while passing through the copper electroplating bath; and e)
electroplating low stress, high ductility copper on the thin film
substrate with the low stress, high ductility copper electroplating
bath.
2. The method of claim 1, wherein the thin film substrate is 220
.mu.m thick or less but greater than 0.
3. The method of claim 1, wherein the thin film substrate attached
to the securing means is passed through the low stress, high
ductility copper electroplating bath by a conveyor.
4. The method of claim 3, wherein the securing means is a plating
jig joined to the conveyor.
5. The method of claim 1, wherein the securing means is a conveyor
comprising a grove for securing the thin film substrate during
electroplating.
6. The method of claim 1, wherein the securing means is a conveyor
comprising a plurality of pairs of rolling balls securing the thin
film substrate during electroplating.
7. The method of claim 1, wherein the low internal stress, high
ductility acid copper electroplating bath comprises one or more
sources of copper ions, an electrolyte, one or more branched
polyalkylenimines, one or more accelerators and one or more
suppressors.
8. The method of claim 1, wherein the low internal stress, high
ductile acid copper electroplating bath comprises one or more
sources of copper ions, an electrolyte, one or more
polyallylamines, one or more accelerators and one or more
suppressors.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to a method of
electroplating low internal stress copper deposits on thin film
substrates to inhibit warping of the thin film substrates. More
specifically, the present invention is directed to a method of
electroplating low internal stress copper deposits on thin film
substrates to inhibit warping of the thin film substrates by using
a securing means to inhibit excessive activity of the thin film
substrate during copper electroplating.
BACKGROUND OF THE INVENTION
[0002] Internal or intrinsic stress of electrodeposited metals is a
well known phenomenon caused by imperfections in the electroplated
crystal structure. After the electroplating operation such
imperfections seek to self correct and this induces a force on the
deposit to contract (tensile strength) or expand (compressive
stress). This stress and its relief can be problematic. For
example, when electroplating is predominantly on one side of a
substrate it can lead to curling, bowing and warping of the
substrate depending on the flexibility of the substrate and the
magnitude of the stress. Stress can lead to poor adhesion of the
deposit to the substrate resulting in blistering, peeling or
cracking. This is especially the case for difficult to adhere
substrates, such as semiconductor wafers or those with relatively
smooth surface topography. In general, the magnitude of stress is
proportional to deposit thickness thus it can be problematic where
thicker deposits are required or indeed may limit the achievable
deposit thickness.
[0003] Most metals including copper deposited from an acid
electroplating process exhibits internal stress. Commercial copper
acid electroplating processes utilize various organic additives
which beneficially modify the electroplating process and deposit
characteristics. It is also known that deposits from such
electroplating baths may undergo room temperature self annealing.
Transformation of the grain structure during such self annealing
concurrently results in a change in the deposit stress, often
increasing it. Not only is internal stress problematic in itself
but is typically subject to change on aging as the deposit self
anneals with time resulting in unpredictability.
[0004] The fundamental mechanism of alleviating intrinsic stress in
copper electroplating is not well understood. Parameters, such as
reducing deposit thickness, lowering current density, i.e., plating
speed, substrate type, seed layer or under plate selection,
electroplating bath composition, such as anion type, additives,
impurities and contaminants are known to affect deposit stress.
Such empirical means of reducing stress have been employed though
typically are not consistent or compromise the efficiency of the
electroplating process.
[0005] Recent work directed to developing electroplating baths to
address the problem of internal stress has been somewhat
successful; however, as the plating industry moves to thinner
substrates warping has become an increasing concern. Even many of
the improved copper electroplating baths directed to addressing the
problems of internal stress have not been able to resolve the
warping problem when plating on thin substrate. Accordingly, there
is a need for a method of reducing or eliminating the problem of
metal plated thin film substrate warping.
SUMMARY OF THE INVENTION
[0006] A method of copper electroplating a thin film substrate
comprising: providing the thin film substrate; attaching the thin
film substrate to a securing means to inhibit substantially all
motion of the thin film substrate in relation to the securing
means; attaching one or more electrical contacts to the thin film
substrate; passing the thin film substrate attached to the securing
means and with the one or more electrical contacts through a low
stress, high ductility copper electroplating bath wherein the thin
film substrate and securing means with the one or more electrical
contacts remain substantially in a single plane while passing
through the copper electroplating bath; and electroplating low
stress, high ductility copper on the thin film substrate with the
low stress, high ductility copper electroplating bath.
[0007] The method enables the thin film substrate to pass through a
low internal stress, high ductility copper electroplating bath such
that the thin film substrate does not warp or bow after copper
electroplating. The thin film substrate remains substantially flat
with the copper deposit plated on it.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIGS. 1A-F are illustrations of copper grain growth for two
different copper electroplating arrangements.
[0009] FIGS. 2A-B are illustrations of a copper thin film joined to
a plating jig with electrical and non-electrical contacts.
[0010] FIGS. 3A-C are illustrations of a copper thin film with
electrical contacts secured to a conveyor and two alternative
conveyor designs for securing the copper thin film.
[0011] FIGS. 4A-B are illustrations of copper thin films connected
to three electrical connectors on one side or at the top of the
film.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The following abbreviations have the following meanings
unless the context clearly indicates otherwise: .degree. C.=degrees
Celsius; g=grams; mL=milliliter; L=liter; ppm=parts per
million=mg/L; A=amperes=Amps; DC=direct current; m=meters;
dm=decimeter; mm=millimeter; .mu.m=micrometers; nm=nanometers;
Mw=weight average molecular weight; ASD=A/dm.sup.2; v=volts; 2.54
cm=1 inch; lbf=pound-force=4.44822162 N; N newtons; psi=pounds per
square inch=0.06805 atmospheres; 1
atmosphere=1.01325.times.10.sup.6 dynes/square centimeter; and
RFID=radio frequency identification.
[0013] As used throughout this specification, the term
"depositing", "plating" and "electroplating" are used
interchangeably. The term "lateral" means from the side or sides.
The term "axis" refers to an imaginary line about which a body
rotates. The term "moiety" means a part or a functional group of a
molecule. The moiety
##STR00001##
Indefinite articles "a" and "an" include both the singular and the
plural. The term "ductility" means a solid material's ability to
deform under tensile stress. The term "tensile stress" means the
maximum stress a material withstands before failing. The term
"plane" means a flat surface on which a straight line joining any
two points on it lies wholly in it.
[0014] All percentages and ratios are by weight unless otherwise
indicated. All ranges are inclusive and combinable in any order
except where it is clear that such numerical ranges are constrained
to add up to 100%.
[0015] Thin film substrates of the present invention are secured by
a securing means such that the thin film substrates remain in one
plane during the electroplating process. The securing means
prevents the thin film substrate from any activity or change in
planar position throughout the electroplating process. It restrains
undesired motion of the thin film substrate while the substrate
passes through a plating bath. In contrast, conventional processes
for electroplating thin film substrates typically involve passing
the thin films through a copper electroplating bath where the thin
film substrate is allowed significant activity or motion throughout
the electroplating process. Such thin film substrates are typically
joined to a conveyor system at one point such that the thin film
substrates rotate about the axis where they are joined to the
conveyor and readily move from side to side outside a single plane.
Such activity often results in a warped thin film substrate. Such
warping is often more pronounced after the plated substrates are
allowed to age at room temperature. Copper plated on the substrates
typically cracks or peels. While not being bound by theory, copper
grain growth is different in the two plating processes. When the
thin film substrate is secured such that it remains in a single
plane during electroplating, more copper grains/nuclei fit in voids
between an initial layer of copper nuclei. This is more favorable
to inhibit substrate warping. In conventional processes, the
substrate has flexibility to move during plating. Once the copper
nucleation starts, there is less copper nuclei being able to fit in
the voids before the initial copper nuclei evolve/anneal to larger
grain size. Therefore the volume contraction during annealing is
larger than the volume contraction of thin films plated according
to the method of the present invention.
[0016] Thin films of the present invention have thickness ranges of
220 .mu.m or less but greater than 0. Preferably the thin films
have a thickness range from 50 .mu.m to 150 .mu.m, more preferably
from 50 .mu.m to 100 .mu.m. Such thin film substrates are patterned
by copper seeds which have thicknesses of a few nanometers. The
patterns can be a plurality of asymmetric lines which can vary in
width and are orientated in one direction on one side of the
substrate. Warping typically occurs along the lines.
[0017] FIGS. 1A-F compares the two processes. FIGS. 1A and 1B
illustrate the initial plating deposit. FIG. 1A is the deposit
plated according to the method of the present invention where the
thin film substrate is held in one plane during plating by a
securing means. In contrast, FIG. 1B is the deposit plated on the
thin film substrate using a conventional process where the thin
film is allowed substantial freedom of movement during plating.
FIG. 1C illustrates that small copper nuclei or grains are allowed
to be deposited within the voids amongst the initial deposit layer
due to limited motion of the substrate, while in conventional
design shown in FIG. 1D the copper nuclei or small grains from the
initial copper deposit layer have already started to anneal or grow
and therefore the thin film substrate bends in order to accommodate
the volume shrinkage. In other words, more copper grains are fitted
in the design of the present invention and thus less volume
contraction than in the conventional approach. The present
invention provides a physical force to counteract the stress caused
by the annealing. FIG. 1E illustrates the copper grain size of the
thin film substrate plated according to the method of the present
invention after aging at room temperature. The copper grain size
has increased to the point of being equivalent to the grain sizes
of the copper of FIG. 1F using a conventional process; however, no
warping is present.
[0018] Securing means for thin film substrates can be a plating jig
such as shown in FIG. 2A where the thin film substrate is joined to
the jig by three electrical contacts and three non-electrical
contacts. Alternatively, the thin film substrate can be secured by
six electrical contacts. The thin film is secured to the plating
jig such that it does not move but remains stationary in relation
to the jig. The jig is then secured to a conveyor belt such that it
remains stationary with respect to its position on the conveyor.
The jig can be secured to the conveyor at any one of its points
provided it remains stationary to its position on the conveyor.
Typically the jig is secured to the conveyor belt at one or more
points along its top edge and at one or more points along its
bottom edge. Nuts and bolts or clamps or other securing means can
be used to join the plating jig to the conveyor belt. The conveyor
belt can include a plurality of plating jigs such that a plurality
of thin films can be joined to the conveyor at the same time and
copper electroplated on the thin films in sequence. The conveyor is
activated by a motor which passes the thin films to a tank
containing a low internal stress, high ductility copper
electroplating bath. The tank includes one or more counter
electrodes. The electrodes can be soluble or insoluble electrodes.
On entering the plating tank the electrical contacts joined to the
thin film are placed under a potential from an electric rail
through gliding contacts joined to the electrical contacts at the
ends opposite to where they make contact with the thin film. The
potential applied to the electrical contacts is from 10 v to 60 v
provided by a rectifier. As each thin film substrate passes through
the plating tank each remains in the same plane such that they do
not move outside the plane during electroplating. The only motion
experienced by each thin film is the motion of moving in a
continuous plane from the entrance of the plating tank until it
passes out of the tank at an opposite end. The thin films do not
revolve about an axis or move laterally to the plane in which the
thin films pass through the plating tank during the electroplating
process. In general, the rate at which the thin films pass through
the electroplating tank is from 0.2 m/minute to 5 m/minute. Such
speeds have minimal to no impact on warping of the thin films.
After the thin films pass out of the plating tank with their copper
plated surfaces, they are retrieved at the end of the plating line
or plating circuit. The securing means for the thin film substrates
can also be a conveyor which has a grove for securing one end of
the thin film or it can be a plurality of pairs of rolling balls as
shown in FIGS. 3A-C. The bottom end of the thin film substrate may
rest secured in the grove of the conveyor with a plurality of
electrical connectors joined to the opposite end of the thin film
as shown in FIGS. 3A and 3B. The grove is sufficiently wide enough
to accept the bottom end of the thin film and at the same time
secure it such that is does not move laterally from its position in
the grove. The electrical connectors can also be joined to the thin
film as one or both of the two sides instead of at the top.
Alternatively, the thin film may be secured by the conveyor with
the grove at its top with the electrical connectors at the bottom
or sides (not shown). As described above, the conveyor drives the
thin films to a plating tank and when the thin films enter the
plating tank the connectors make contact at their opposite ends to
an electric rail by means of a gliding contact. The thin films then
pass through the plating tank in one plane where they are copper
plated and then pass out at the opposite end where they are
retrieved.
[0019] Alternatively, the conveyor has a plurality of pairs of
rolling balls as shown in FIG. 3C. The thin film is secured between
the rolling balls in a similar fashion as with the conveyor with
the groove except the balls are motorized such that they rotate and
drive the thin film forward toward and into the plating tank while
maintaining the thin film in one pale such that it does not move
laterally during electroplating.
[0020] Copper metal is electroplated from low stress, high
ductility aqueous acid copper electroplating baths. Preferably such
aqueous acid copper metal electroplating baths include one or more
sources of copper ions, an electrolyte, one or more branched
polyalkylenimines, one or more accelerators and one or more
suppressors such that the copper deposits have low internal stress
and high ductility, preferably minimal change in stress as the
copper deposit ages and high ductility. The low internal stress
copper deposits may have a matt appearance with a relatively large
as deposited grain size, typically of 2 microns or more. The acid,
low stress, high ductility copper baths also may include one or
more sources of chloride ions and one or more conventional
additives typically included in acid copper electroplating baths.
Preferably, one or more sources of chloride ions are included in
the acid copper electroplating baths.
[0021] The one or more branched polyalkylenimines include, but are
not limited to compounds having a general formula:
##STR00002##
where R.sub.1, R.sub.2, R.sub.3, R.sub.4 and R.sub.5 may be
hydrogen or a moiety having a general formula:
##STR00003##
[0022] where R.sub.6 and R.sub.7 are the same or different and are
hydrogen or a moiety having a general formula:
##STR00004##
with the proviso that at least one of R.sub.1, R.sub.2, R.sub.3,
R.sub.4 and R.sub.5 is the moiety having formula (II) and n, p, q,
r, s, t and u are the same or different and are integers of 2 to 6
and m is an integer of 2 or greater. Preferably, at least two of
R.sub.1, R.sub.2, R.sub.3, R.sub.4 and R.sub.5 are the moiety
having formula (II), more preferably three of R.sub.1, R.sub.2,
R.sub.3, R.sub.4 and R.sub.5 are the moiety having formula (II).
Preferably, at least one of R.sub.6 and R.sub.7 is the moiety
having formula (III) with the remainder being hydrogen. Preferably,
the variables n, p, q, r, s, t and u are the same or different and
are 2 to 3, more preferably, n, p, q, r, s, t and u are 2.
[0023] Examples of preferred branched polyalkylenimines are the
following polyethylenimines:
##STR00005##
where the variable m is as defined above.
[0024] An example of another branched polyalkylenimine is the
dendrimer having the following structure:
##STR00006##
[0025] The branched polyalkylenimines are included in the acid
copper electroplating baths in amounts of 0.1 to 10 ppm, preferably
from 0.1 to 5 ppm, more preferably from 0.1 to 2 ppm and most
preferably 0.1 to 1 ppm. The preferred and the most preferred
branched polyalkylenimines may be included in the low stress, high
ductility acid copper electroplating baths in amounts of 0.2 ppm to
0.8 ppm.
[0026] In general Mw may range from 1000 and greater. Typically the
Mw may range from 4000 to 60,000, more typically from 10,000 to
30,000.
[0027] Preferred low internal stress, high ductility copper
electroplating baths can also include one or more polyallylamines
which have a general formula:
##STR00007##
where variable "y" is a number such that the Mw is 1000 g/mole or
greater. Preferably the Mw of the polyallylamines of the present
invention ranges from 4000 g/mole to 60,000 g/mole, more preferably
from 10,000 g/mole to 30,000 g/mole.
[0028] Polyallylamines are included in the aqueous acid copper
electroplating baths in amounts of 1 to 10 ppm, preferably from 1
to 5 ppm, more preferably from 1 to 2 ppm.
[0029] One or more accelerators are included in the low stress and
high ductility acid copper electroplating baths. Accelerators are
preferably compounds which in combination with one or more
suppressors may lead to an increase in plating rate at given
plating potentials. The accelerators are preferably sulfur
containing organic compounds. Preferably the accelerators are
3-mercapto-1-propane sulfonic acid, ethylenedithiodipropyl sulfonic
acid, bis-(w-sulfobutyl)-disulfide,
methyl-(w-sulfopropyl)-disulfide, N,N-dimethyldithiocarbamic acid
(3-sulfopropyl) ester,
(O-ethyldithiocarbonato)-S-(3-sulfopropyl)-ester,
3-[(amino-iminomethyl)-thiol]-1-propanesulfonic acid,
3-(2-benzylthiazolylthio)-1-propanesulfonic acid,
bis-(sulfopropyl)-disulfide and alkali metal salts thereof. More
preferably, the accelerator is chosen from 3-mercapto-1-propane
sulfonic acid and its alkali metal salts and
(O-ethyldithiocarbonato)-S-(3-sulfopropyl)-ester and its alkali
metal salts. Most preferably, the accelerator is chosen from
3-mercapto-1-propane sulfonic acid, sodium salt and
(O-ethyldithiocarbonato)-S-(3-sulfopropyl)-ester, potassium salt.
While not being bound by theory, it is believed that the one or
more accelerators in combination with the one or more branched
polyalkylenimines or polyallylamines enable the combination of a
low internal stress and high ductility copper metal film
deposit.
[0030] In general, such accelerators may be included in amounts of
1 ppm and greater. Preferably such accelerators may be included in
the acid copper electroplating baths in amounts of 2 ppm to 500
ppm, more preferably from 2 ppm to 250 ppm, most preferably the
accelerators are included in amounts of 3 ppm to 200 ppm. When the
accelerators are chosen from 3-mercapto-1-propane sulfonic acid and
its alkali metal salts they are most preferably included in amounts
of 3 ppm to 8 ppm, and
(O-ethyldithiocarbonato)-S-(3-sulfopropyl)-ester and its alkali
metal salts are most preferably included in amounts of 100 ppm to
200 ppm.
[0031] Suppressors included in the low stress, high ductility acid
copper electroplating baths include, but are not limited to,
polyoxyalkylene glycols, carboxymethylcellulose,
nonylphenolpolyglycol ether, octandiolbis-(polyalkylene
glycolether), octanolpolyalkylene glycol ethers, oleic
acidpolyglycol ester, polyethylenepropylene glycol, polyethylene
glycol, polyethylene glycoldimethylether, polyoxypropylene glycol,
polypropylene glycol, polyvinylalcohol, stearic acid polyglycol
ester and stearyl alcohol polyglycol ether. Such suppressors are
included in amounts of 0.1 g/L to 10 g/L, preferably from 0.1 g/L
to 5 g/L, more preferably from 0.1 g/L to 2 g/L and most preferably
from 0.1 g/L to 1.5 g/L.
[0032] Suitable copper ion sources are copper salts and include
without limitation: copper sulfate; copper halides such as copper
chloride; copper acetate; copper nitrate; copper tetrafluoroborate;
copper alkylsulfonates; copper aryl sulfonates; copper sulfamate;
copper perchlorate and copper gluconate. Exemplary copper alkane
sulfonates include copper (C.sub.1-C.sub.6)alkane sulfonate and
more preferably copper (C.sub.1-C.sub.3)alkane sulfonate. Preferred
copper alkane sulfonates are copper methanesulfonate, copper
ethanesulfonate and copper propanesulfonate. Exemplary copper
arylsulfonates include, without limitation, copper benzenesulfonate
and copper p-toluenesulfonate. Mixtures of copper ion sources may
be used. One or more salts of metal ions other than copper ions may
be added to the acid copper electroplating baths. Typically, the
copper salt is present in an amount sufficient to provide an amount
of copper ions of 10 to 400 g/L of plating solution. The
electroplating baths do not include any alloying metals. The
electroplating baths are directed to thin film copper deposits, not
copper alloy deposits or any other metal or metal alloy.
[0033] Suitable electrolytes include, but are not limited to,
sulfuric acid, acetic acid, fluoroboric acid, alkanesulfonic acids
such as methanesulfonic acid, ethanesulfonic acid, propanesulfonic
acid and trifluoromethane sulfonic acid, aryl sulfonic acids such
as benzenesulfonic acid, p-toluenesulfonic acid, sulfamic acid,
hydrochloric acid, hydrobromic acid, perchloric acid, nitric acid,
chromic acid and phosphoric acid. Mixtures of acids may be used in
the present metal plating baths. Preferred acids include sulfuric
acid, methanesulfonic acid, ethanesulfonic acid, propanesulfonic
acid, hydrochloric acid and mixtures thereof. The acids may be
present in an amount in the range of 1 to 400 g/L. Electrolytes are
generally commercially available from a variety of sources and may
be used without further purification.
[0034] One or more optional additives may also be included in the
electroplating composition. Such additives include, but are not
limited to, levelers, surfactants, buffering agents, pH adjustors,
sources of halide ions, organic acids, chelating agents and
complexing agents. Such additives are well known in the art and may
be used in conventional amounts.
[0035] Levelers may be included in the acid copper electroplating
bath. Such levelers include, but are not limited to, organic sulfo
sulfonates such as 1-(2-hydroxyethyl)-2-imidazolidinethione (HIT),
4-mercaptopyridine, 2-mercaptothiazoline, ethylene thiourea,
thiourea, those disclosed in U.S. Pat. No. 6,610,192 to Step et
al., U.S. Pat. No. 7,128,822 to Wang et al., U.S. Pat. No.
7,374,652 to Hayashi et al. and 6,800,188 to Hagiwara et al. Such
levelers may be included in conventional amounts. Typically they
are included in amounts of 1 ppb to 1 g/L.
[0036] Conventional nonionic, anionic, cationic and amphoteric
surfactants may be included in the acid copper electroplating
baths. Such surfactants are well known in the art and many are
commercially available. Typically the surfactants are nonionic. In
general, surfactants are included in conventional amounts.
Typically they may be included in the electroplating baths in
amounts of 0.05 g/l to 15 g/L.
[0037] Halogen ions include chloride, fluoride, and bromide. Such
halides are typically added into the bath as a water soluble salt
or acid. Preferably, the copper electroplating baths include
chloride. Chloride is preferably introduced into the bath as
hydrochloric acid or as sodium chloride or potassium chloride.
Preferably chloride is added to the bath as hydrochloric acid.
Halogens may be included in the baths in amounts of 20 ppm to 500
ppm, preferably 20 ppm to 100 ppm.
[0038] The low stress, high ductility acid copper electroplating
baths have a pH range less than 1 to less than 7, preferably from
less than 1 to 5, more preferably from less than 1 to 2, most
preferably the pH is less than 1 to 1.
[0039] Electroplating may be by DC plating, pulse plating, pulse
reverse plating, light induced plating (LIP) or light assisted
plating. Preferably the low stress, high ductility copper films are
plated by DC, LIP or light assisted plating. In general, current
density ranges from 0.5-50 ASD depending on the application.
Typically, the current density ranges from 1-20 ASD or such as
15-20 ASD. Electroplating is done at temperature ranges from
15.degree. C. to 80.degree. C. or such as from room temperature to
60.degree. C. or such as from 20.degree. C. to 40.degree. C. or
such as from 20.degree. C. to 25.degree. C.
[0040] The internal stress and ductility of the copper films may be
determined using conventional methods. Typically low internal
stress is measured using a deposit stress analyzer, such as is
available from Specialty Testing and Development Co., Jacobus, Pa.
The low internal stress may be determined by the equation
S=U/3T.times.K, where S is stress in psi, U is number of increments
of deflection on a calibrated scale, T is deposit thickness in
inches and K is the test strip calibration constant. The constant
may vary and is provided with the deposit stress analyzer. Low
internal stress is measured immediately after plating and then
after ageing for a few days, preferably two days after the copper
film is deposited on a substrate, such as a conventional
copper/beryllium alloy test strip. Internal stress measurements
immediately after electroplating and after ageing are made at room
temperature. While room temperature may vary, for purposes of
measuring internal stress, room temperature typically ranges from
18.degree. C. to 25.degree. C., preferably from 20.degree. C. to
25.degree. C. Preferably, a copper film of 1-10 .mu.m is plated on
the test strip, more preferably 1-5 .mu.m. Initial internal stress
which is measured immediately after plating copper on the substrate
may range from 0 psi to 950 psi, preferably from 0 psi to 520 psi,
more preferably from 0 psi to 505 psi at room temperature. After
ageing, such as for two days, the internal stress may range from
300 psi to 900 psi, preferably from 300 psi to 850 psi, more
preferably from 300 psi to 800 psi at room temperature. While
internal stress may vary slightly from the two days ageing time
period, the measurement of the internal stress of a copper film
typically does not significantly change at room temperature after
the two day ageing period.
[0041] Ductility is measured using conventional elongation tests
and apparatus. Preferably, elongation testing is done using
industrial standard IPC-TM-650 methods with an apparatus such as an
Instron pull tester 33R4464. Copper is electroplated on a substrate
such as a stainless steel panel. Typically copper is electroplated
as a thin film on the substrate to a thickness of 50-100 .mu.m,
preferably from 60-80 .mu.m. The copper is peeled from the
substrate and annealed for 1-5 hours, preferably from 2-5 hours.
Annealing is done at temperatures of 100-150.degree. C., preferably
from 110-130.degree. C., and then the copper is allowed to come to
room temperature. Elongation or load of maximum tensile stress is
typically not a pre-set parameter. The greater the load of maximum
tensile stress a material can withstand before failing or cracking,
the higher or the better the ductility. Typically elongation is
done at loads of maximum tensile stress of 50 lbf or greater.
Preferably, elongation is done at 60 lbf or greater. More
preferably elongation is done at loads of maximum tensile stress of
70 lbf to 90 lbf. Elongation ranges from greater than or equal to
8%, preferably from 9% to 15%.
[0042] The methods of the present invention are used to plate
copper on thin film substrates such as semiconductor wafers or
metal thin films, or on sides of substrates where bowing, curling
or warping are problems. The methods can also be used to plate
copper on difficult to adhere to substrates where blistering,
peeling or cracking of the deposit are common. For example, the
methods may be used in the manufacture of printed circuit and
wiring boards, such as flexible circuit boards, flexible circuit
antennas, RFID tags, electrolytic foil, semiconductor wafers for
photovoltaic devices and solar cells, including interdigitated rear
contact solar cells, heterojunction with intrinsic thin layer (HIT)
cells and fully plated front contact cells. The methods are used to
preferably plate copper at thickness ranges of 15 .mu.m to 5 mm,
more preferably from 20 .mu.m to 1 mm. When copper is used as the
principle conductor in the formation of contacts for solar cells,
the copper is preferably plated to thickness ranges of 20 .mu.m to
60 .mu.m, more preferably from 30 .mu.m to 50 .mu.m.
[0043] The following examples are provided to illustrate the
invention, but are not intended to limit its scope.
Example 1
[0044] The following aqueous acid copper electroplating baths are
prepared at room temperature.
TABLE-US-00001 TABLE 1 Component Bath 1 Bath 2 Copper sulfate 160
g/L 160 g/L Sulfuric acid 150 g/L 150 g/L Chloride (as hydrochloric
acid) 60 ppm 60 ppm Bis-sodium sulfopropyl disulfide 4 ppm 0
3-mercapto-1-propane sulfonic 6 ppm 0 acid, sodium salt
O-ethyldithiocarbonato)-S- 0 150 ppm (3-sulfopropyl)-ester,
potassium salt Polyoxy-alkylene glycol 0.15 g/L 0.9 g/L
Polyethylene glycol 0.18 g/L 1.1 g/L Branched polyethylenimine 0.75
ppm 0.75 ppm (Mw = 25000)
[0045] The components of the copper electroplating baths are made
up using conventional laboratory procedures where organics are
added to water followed by adding the inorganic components.
Stirring or agitation with heat application at temperatures of
below 30.degree. C. is done to be certain that all of the
components are solubilized in the water. The baths are allowed to
come to room temperature prior to copper electroplating. The pH of
the acid copper electroplating baths ranges from less than 1 to 1
at room temperature and during copper electroplating.
Example 2
[0046] A plurality of thin films of copper having thicknesses of
100 .mu.m are joined to non-conducting rectangular flat panel
plating jigs as illustrated in FIG. 2A. Each thin film is secured
to a flat panel plating jig by six electrical contacts, three on
each side of the thin film or in the alternative by three
electrical contacts and three non-electrical contacts as shown in
FIGS. 2A and 2B. The non-conducting flat panel plating jigs with
the thin films are secured to a conveyor system where the plating
jig is joined to the conveyor such that it does not move outside a
single plane and which transports the thin films through plating
tanks which contain Bath 1 or Bath 2 as in Example 1. The thin
films remain substantially in one plane during the electroplating
process. When the thin copper films joined to the flat panel jigs
pass into one of the two copper plating baths, the electrical
contacts are placed under a potential from an electric rail which
makes electrical contact with the contacts joining the thin films
of copper. Each panel is electroplated with a layer of copper 30
.mu.m thick. Copper electroplating is initiated at 1.5 ASD to 3 ASD
to plate an initial 0.1-4 .mu.m of copper then the current density
is increased to 20ASD to complete the plating. Plating is DC and at
room temperature. After the thin films are plated with copper they
pass out of the copper plating tanks, they are rinsed with DI water
and examined for any warping. None of the thin films show
observable signs of warping immediately after plating. The samples
are then allowed to age at room temperature for about 24 hours.
After 24 hours the samples are observed for warping. There are no
observable signs of substrate or copper layer warping.
Example 3
[0047] The following aqueous acid copper electroplating baths are
prepared at room temperature.
TABLE-US-00002 TABLE 2 Component Bath 3 Copper sulfate 160 g/L
Sulfuric acid 150 g/L Chloride (as hydrochloric acid) 60 ppm
(O-ethyldithiocarbonato)-S- 175 ppm (3-sulfopropyl)-ester,
potassium salt Polyoxy-alkylene glycol 0.15 g/L (PolyMax .TM. PA-
66/LC solution) Polyethylene glycol 0.18 g/L (PEG 12000)
Polyallylamine 1.25 ppm (Mw = 15000)
[0048] The components or the copper electroplating bath are made up
using conventional laboratory procedures where organics are added
to water followed by adding the inorganic components. Stirring or
agitation with heat application at temperatures of below 30.degree.
C. is done to be certain that all of the components are solubilized
in the water. The bath is allowed to come to room temperature prior
to copper electroplating. The pH of the acid copper electroplating
bath is less than 1 at room temperature and during copper
electroplating.
Example 4
[0049] A plurality of thin films of copper having thicknesses of
150 .mu.m are connected to three electrical contacts and inserted
in a conveyor with groves or alternatively a conveyor with rollers
as illustrated in FIGS. 3A-C. The groove and rollers secure the
thin films such that they remain in one plane during transport to
and during passage through low internal stress, high ductility
copper electroplating baths. Two plating tanks include the low
internal stress, high ductility copper electroplating baths as
shown in Example 3. When the thin copper films pass into one of the
two copper plating baths, the electrical contacts are placed under
a potential from an electric rail which makes electrical contact
with the contacts joining the thin films of copper. Each panel is
electroplated with a layer of copper 30 .mu.m thick. Electroplating
is done at room temperature. The current density is at 20 ASD using
DC. After the thin films are plated with copper they pass out of
the copper plating tanks, they are rinsed with tap water and
examined for any warping. None of the thin films show observable
signs of warping. The samples are then allowed to age at room
temperature for 24 hours. After 24 hours, the samples are observed
for warping. No warping was observed for any of the samples.
Example 5 (Comparative)
[0050] A plurality of thin films of copper having thicknesses of
120 .mu.m are connected to three electrical connectors on one side
or at the top as shown in FIGS. 4A and 4B. The connectors are
joined to a conveyor which transports the plurality of thin films
to a copper plating tank which includes one of the low internal
stress, high ductility copper electroplating baths shown in tables
1 and 2 above. The thin films are secured to the conveyor only by
means of the electrical connectors. When the thin copper films pass
into one of the three copper plating baths, the electrical contacts
are placed under a potential from an electric rail which makes
electrical contact with the contacts joining the thin films of
copper. Each panel is electroplated with a layer of copper 30 .mu.m
thick at a current density of 20ASD with DC. Electroplating is done
at room temperature. During passage through the electroplating
baths the thin films change their planar orientation due to contact
with the aqueous plating baths as well as due to the continuous
stirring of the baths during electroplating. The thin films are not
copper plated in a single continuous plane as in Examples 2 and 4.
After the thin films are plated with copper they pass out of the
copper plating tanks, they are rinsed with tap water and examined
for any warping. All of the samples had some observable signs of
warping. The samples are then aged at room temperature for 24
hours. After 24 hours, the warping of all of the samples is more
pronounced. The plated copper layers for most of the samples show
signs of peeling from the thin film substrate. All of the
substrates have bowing.
* * * * *