U.S. patent application number 11/180468 was filed with the patent office on 2007-01-18 for plating method.
This patent application is currently assigned to Rohm and Haas Electronic Materials LLC. Invention is credited to Robert A. Binstead, Robert D. Mikkola, Chunyi Wu.
Application Number | 20070012576 11/180468 |
Document ID | / |
Family ID | 37660677 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070012576 |
Kind Code |
A1 |
Binstead; Robert A. ; et
al. |
January 18, 2007 |
Plating method
Abstract
Methods of depositing layers of copper that selectively
incorporate certain impurities are provided. Such copper layers
reduce stress-induced void formation in wide copper lines under
vias.
Inventors: |
Binstead; Robert A.;
(Marlborough, MA) ; Wu; Chunyi; (Shrewsbury,
MA) ; Mikkola; Robert D.; (Grafton, MA) |
Correspondence
Address: |
S. Matthew Cairns;Rohm and Haas Electronic Materials LLC
455 Forest Street
Marlborough
MA
01752
US
|
Assignee: |
Rohm and Haas Electronic Materials
LLC
Marlborough
MA
|
Family ID: |
37660677 |
Appl. No.: |
11/180468 |
Filed: |
July 13, 2005 |
Current U.S.
Class: |
205/291 ;
205/118 |
Current CPC
Class: |
H05K 3/423 20130101;
C25D 5/18 20130101; C25D 5/10 20130101; H05K 2203/1476 20130101;
C25D 3/38 20130101 |
Class at
Publication: |
205/291 ;
205/118 |
International
Class: |
C25D 3/38 20060101
C25D003/38; C25D 5/02 20060101 C25D005/02 |
Claims
1. A method of depositing copper comprising the steps of: a)
contacting an electronic device substrate having an aperture with a
copper electroplating bath comprising a source of copper ions, an
electrolyte, and a disulfide-containing accelerator; b) depositing
a layer of copper in the aperture using a duty cycle comprising 1)
applying a first current density for a first period to
electrochemically reduce the disulfide-containing accelerator to a
thiol compound at a copper surface; and 2) applying a second
current density for a second period; and c) repeating step b) until
a desired copper deposit is obtained; wherein the second current
density is less than the first current density.
2. The method of claim 1 wherein the copper deposit comprises from
1 to 500 ppm of average total impurities after annealing of the
copper.
3. The method of claim 2 wherein the impurities comprise one or
more of carbon, oxygen, nitrogen, sulfur and chloride.
4. The method of claim 1 wherein the first period is up to 5
seconds.
5. The method of claim 4 wherein the duty cycle has a frequency of
0.1 to 10 Hz.
6. The method of claim 1 wherein the first current density is in
the range of 10 to 100 mA/cm.sup.2.
7. The method of claim 1 wherein the second current density is in
the range of 1 to 20 mA/cm.sup.2.
8. The method of claim 1 wherein the duty cycle has a ratio of step
1) to step 2) of 1:1 to 10:1.
9. An electronic device comprising a first layer of metal and a
second layer of metal, wherein the first layer of metal comprises
average total impurities in the range of up to 10 ppm and the
second layer of metal comprises average total impurities in the
range of 10 to 100 ppm.
10. The electronic device of claim 10 wherein the first and second
metal layers are copper.
Description
[0001] The present invention relates generally to the field of
metal plating. In particular, the present invention relates to the
electrodeposition of copper.
[0002] Copper is used in the manufacture of many electronic
devices. For example, in the manufacture of integrated circuits
copper damascene processes (including dual damascene) involve the
formation of inlaid copper wiring patterns with the simultaneous
formation of via connections between metal layers. In such
processes, the copper is deposited electrolytically using direct
current.
[0003] The purity of the electrolytically deposited copper becomes
more important as the size of the electronic devices shrink. High
levels of impurities in small copper deposits will increase the
resistivity of the copper. Accordingly, the trend in the industry
is toward copper electroplating baths that provide purer copper
deposits in order to reduce the resistivity of the deposits.
[0004] Stress-induced voiding occurs in copper deposits in dual
damascene structures where voids are formed under vias that connect
to wide metal lines. Such voiding leads to failures in the device.
One theory attributes the formation of such voiding to vacancies
that develop in the copper deposits when the copper is not properly
annealed. See, for example, E. T. Ogawa et al., Stress-Induced
Voiding Under Vias Connected to Wide Cu Metal Lines, IEEE
International Reliability Physics Symposium Proceedings (2002),
40.sup.th, pp 312-321, which discusses the formation of voids under
vias due to stress. Regardless of how such voiding occurs, the use
of higher purity copper in the wide metal lines exacerbates the
formation of such voiding. There is a need in the industry for high
purity copper deposits that do not form stress-induced voids.
[0005] It has been surprisingly found that impurities can be
selectively incorporated into copper metal lines during
electroplating of the copper. Such selective incorporation of
impurities in wide metal lines reduces the formation of
stress-induced voids under vias connected to such metal lines.
[0006] In one embodiment, the present invention provides a method
of depositing copper including the steps of: a) contacting an
electronic device substrate having apertures with a copper
electroplating bath including a source of copper ions, an
electrolyte, and a disulfide-containing accelerator; b) depositing
a layer of copper in the apertures using a duty cycle including 1)
applying a first current density for a first period to
electrochemically reduce the disulfide-containing accelerator to a
thiol compound at a copper surface; and 2) applying a second
current density for a second period; and c) repeating step b) until
a desired copper deposit is obtained; wherein the second current
density is less than the first current density. The present method
is useful for incorporating impurities at a desired level within
the copper deposit. In particular, the present invention is useful
in the manufacture of integrated circuits, and more specifically in
the deposition of wide metal lines in the manufacture of integrated
circuits.
[0007] FIG. 1 is a secondary ion mass spectrogram showing impurity
levels as a function of copper film depth for a prior art
process.
[0008] FIG. 2 is a secondary ion mass spectrogram showing impurity
levels as a function of copper film depth for a prior art
process.
[0009] FIG. 3 is a secondary ion mass spectrogram showing impurity
levels as a function of copper film depth for the process of the
invention.
[0010] As used throughout the specification, the following
abbreviations shall have the following meanings: nm=nanometers;
g/L=grams per liter; mA/cm.sup.2 =milliamperes per square
centimeter; .mu.m=micron=micrometer; ppm=parts per million,
mL/L=milliliter/liter;.degree. C.=degrees Centigrade; sec.=seconds;
msec.=milliseconds; g=grams; DC =direct current; Hz=Hertz; and
.ANG.=Angstroms.
[0011] As used throughout the specification, "feature" refers to
the geometries on a substrate. "Apertures" refer to recessed
features, such as vias and trenches. As used throughout this
specification, the term "plating" refers to copper electroplating,
unless the context clearly indicates otherwise. "Deposition" and
"plating" are used interchangeably throughout this specification.
"Defects" refer to surface defects of a copper layer, such as
protrusions and pits, as well as defects within a copper layer,
such as voids. "Wide metal lines" refers to metal lines having a
width of >1 .mu.m. The terms "layer" and "film" are used
interchangeably and refer to a metal deposit, particularly a copper
deposit, unless the context clearly indicates otherwise.
[0012] The term "alkyl" includes linear, branched and cyclic alkyl.
"Accelerator" refers to an organic additive that increases the
plating rate of a metal during electroplating. "Suppressors" (also
known as "carriers") refer to organic additives that suppress the
plating rate of a metal during electroplating. "Leveler" refers to
an organic additive that is capable of providing a substantially
planar metal layer. The terms "leveler" and "leveling agent" are
used interchangeably throughout this specification. The term
"halide" refers to fluoride, chloride, bromide and iodide. As used
herein, "duty cycle" means the relationship between the time period
of high current density and the time period of low current density.
A 75% duty cycle means that for a given time the ratio of time
periods of high to low current density is 3:1 (or that the high
current density is applied for 75% of the time and the low current
density is applied for 25% of the time).
[0013] The indefinite articles "a" and "an" are intended to include
both the singular and the plural. 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%.
[0014] A wide variety of electronic device substrates may be plated
with copper according to the present invention. Suitable substrates
include, without limitation: printed circuit board substrates,
integrated circuit substrates such as wafers used in the
manufacture of integrated circuits, electronic packages such as
lead frames and electronic interconnects such as wafer bumps; and
optoelectronic device substrates such as hermetic sealing
layers.
[0015] A wide variety of copper electroplating baths may be used
with the present invention. Copper electroplating baths typically
contain a source of copper ions, an electrolyte, a source of
chloride ions, and a disulfide-containing accelerator. More
typically, organic additives such as a suppressor are added to the
copper electroplating baths. The copper electroplating baths may
optionally contain a leveler.
[0016] Typical sources of copper ions are any copper compounds that
are soluble in the electroplating bath. Suitable copper compounds
include, but are not limited to, copper salts such as copper
sulfate, copper persulfate, copper halide, copper chlorate, copper
perchlorate, copper alkanesulfonate such as copper
methanesulfonate, copper alkanol sulfonate, copper arylsulfonate,
copper fluoroborate, cupric nitrate, copper acetate, and copper
citrate. Copper sulfate is preferred. Mixtures of copper compounds
may be used. Such sources of copper ions are generally commercially
available.
[0017] The source of copper ions may be used in the present
electroplating baths in a relatively wide concentration range.
Typically, the copper ion source is present in an amount sufficient
to provide an amount of copper ion of 10 to 80 g/L in the plating
bath. More typically, the amount of copper source provides 15 to 65
g/L of copper ions in the plating bath. The copper plating bath may
also contain amounts of other alloying elements, such as, but not
limited to, tin, zinc, indium, antimony, and the like. Such
alloying elements are added to the electroplating baths in the form
of any suitable bath-solution salt. Thus, the copper electroplating
baths useful in the present invention may deposit copper or copper
alloy.
[0018] The electrolyte may be alkaline or acidic and is typically
acidic. Any acid which is compatible with the copper compound may
be used in the present invention. Suitable acids include, but are
not limited to: sulfuric acid, acetic acid, fluoroboric acid,
nitric acid, sulfamic acid, phosphoric acid, hydrogen halide acids
such as hydrochloric acid, alkanesulfonic acids and arylsulfonic
acids such as methanesulfonic acid, toluenesulfonic acid,
phenolsulfonic acid and benzenesulfonic acid, and halogenated acids
such as trifluoromethylsulfonic acid and haloacetic acid. Typically
the acid is sulfuric acid, alkanesulfonic acid or arylsulfonic
acid. Mixtures of acids may be used. In general, the acid is
present in an amount to impart conductivity to a bath containing
the acidic electrolyte composition. Typically, the pH of the acidic
electrolyte of the present invention has a value of less than 7,
and more typically less than 2. Exemplary alkaline electroplating
baths use pyrophosphate as the electrolyte, although other
electrolytes may be employed. It will be appreciated by those
skilled in the art that the pH of the electrolyte may be adjusted
by any known methods, if necessary.
[0019] The total amount of acid electrolyte used in the present
electroplating baths may be from 0 to 100 g/L, and typically from 0
to 60 g/L, although higher amounts of acid may be used for certain
applications, such as up to 225 g/L or even 300 g/L. It will be
appreciated by those skilled in the art that by using copper
sulfate, a copper alkanesulfonate or a copper arylsulfonate as the
copper ion source, an acidic electrolyte can be obtained without
any added acid.
[0020] A wide variety of disulfide-containing accelerators may be
employed in the present copper electroplating baths. Such
accelerators may be used alone or as a mixture of two or more. In
general, the disulfide-containing accelerators have a molecular
weight of 5000 or less and more typically 1000 or less.
Disulfide-containing accelerators that also have sulfonic acid
groups are generally preferred, particularly compounds that include
a group of the formula R'--S--S--R--SO.sub.3X, where R is an
optionally substituted alkyl (which include cycloalkyl), optionally
substituted heteroalkyl, optionally substituted aryl group, or
optionally substituted heteroalicyclic; X is hydrogen or a counter
ion such as sodium or potassium; and R'is hydrogen or an organic
residue, such as a group of the formula --R--SO.sub.3X or a
substituent of a larger compound. Typically alkyl groups will have
from 1 to 16 carbons, more typically 1 to 8 or 12 carbons.
Heteroalkyl groups will have one or more hetero (N, O or S) atoms
in the chain, and typically have from 1 to 16 carbons, more
typically 1 to 8 or 12 carbons. Carbocyclic aryl groups are typical
aryl groups, such as phenyl and naphthyl. Heteroaromatic groups
also will be suitable aryl groups, and typically contain 1 to 3 of
one or more of N, O and S atoms and 1 to 3 separate or fused rings
and include, e.g., coumarinyl, quinolinyl, pyridyl, pyrazinyl,
pyrimidyl, furyl, pyrrolyl, thienyl, thiazolyl, oxazolyl,
oxidizolyl, triazole, imidazolyl, indolyl, benzofuranyl, and
benzothiazol. Heteroalicyclic groups typically will have 1 to 3 of
one or more of N, O and S atoms and from 1 to 3 separate or fused
rings and include, e.g., tetrahydrofuranyl, thienyl,
tetrahydropyranyl, piperdinyl, morpholino, and pyrrolindinyl.
Substituents of substituted alkyl, heteroalkyl, aryl or
heteroalicyclic groups include, e.g., C.sub.1-8 alkoxy; C.sub.1-8
alkyl, halogen such as F, C1 and Br; cyano; and nitro.
[0021] More specifically, useful disulfide-containing accelerators
include those of the following formulae;
XO.sub.3S--R--S--S--R--SO.sub.3 X and
XO.sub.3S--Ar--S--S--Ar--SO.sub.3X, wherein R in the above formulae
is an optionally substituted alkyl group, and typically is an alkyl
group having from 1 to 6 carbon atoms, more typically is an alkyl
group having from 1 to 4 carbon atoms; Ar is an optionally
substituted aryl group such as optionally substituted phenyl or
naphthyl; and X is hydrogen or a suitable counter ion such as
sodium or potassium. Exemplary disulfide-containing accelerators
include, without limitation, bis-sulfopropyl disulfide and
bis-sodium-sulfopropyl disulfide.
[0022] Optionally, an additional accelerator that does not contain
a disulfide group may be used in combination with the present
disulfide-containing accelerator. Typical additional accelerators
are sulfur-containing and contain one or more sulfur atoms and may
be, without limitation, thiols, mercaptans, sulfides, disulfides
and organic sulfonic acids. In one embodiment, such additional
accelerator compound has the formula XO.sub.3S--R--SH, wherein R is
an optionally substituted alkyl group, and typically is an alkyl
group having from 1 to 6 carbon atoms, more typically is an alkyl
group having from 1 to 4 carbon atoms and X is hydrogen or a
suitable counter ion such as sodium or potassium.
[0023] Exemplary additional accelerators include, without
limitation, N,N-dimethyl-dithiocarbamic acid-(3-sulfopropyl)ester;
3-mercapto-propylsulfonic acid-(3-sulfopropyl)ester;
3-mercapto-propylsulfonic acid (sodium salt); carbonic
acid-dithio-o-ethylester-s-ester with 3-mercapto-1 -propane
sulfonic acid (potassium salt); 3-(benzthiazolyl-s-thio)propyl
sulfonic acid (sodium salt); pyridinium propyl sulfobetaine;
1-sodium-3-mercaptopropane-1-sulfonate; sulfoalkyl sulfide
compounds disclosed in U.S. Pat. No. 3,778,357; the peroxide
oxidation product of a dialkyl
amino-thiox-methyl-thioalkanesulfonic acid; and combinations of the
above. Additional suitable accelerators are also described in U.S.
Pat. Nos. 3,770,598; 4,374,709; 4,376,685; 4,555,315; and
4,673,469.
[0024] The amount of the disulfide-containing accelerators present
in a freshly prepared copper electroplating bath is typically from
0.1 to 1000 ppm. More typically, the disulfide-containing
accelerator compounds are present in an amount of from 0.5 to 300
ppm, still more typically from 1 to 100 ppm, and even more
typically from 2 to 50 ppm. Any additional accelerators present in
the copper electroplating bath is used the amounts described for
the disulfide-containing accelerators.
[0025] In general, the copper electroplating baths also contain
water. The water may be present in a wide range of amounts. Any
type of water may be used, such as distilled, deionized or tap.
[0026] It will be appreciated by those skilled in the art that one
or more other components may be added to the copper electroplating
baths of the present invention, such as, e.g., suppressors,
levelers, halide ions, and other alloying materials.
[0027] Any suppressor may optionally be used in the present
electroplating baths. Suppressors, as used throughout this
specification, refer to any compounds that suppress the plating
rate of copper as compared to baths without such suppressors.
Suitable suppressors include polymeric materials, preferably having
heteroatom substitution, particularly oxygen linkages. In general,
suppressors are typically polyethers, such as, but not limited to,
those of the formula R--O--(CXYCX'Y'O).sub.nH wherein R is an aryl,
alkyl or alkenyl group containing from 2 to 20 carbons; X, Y, X',
and Y' are each independently hydrogen, alkyl, such as methyl,
ethyl or propyl, aryl such as phenyl, and aralkyl such as benzyl;
and n is an integer from 5 to 100,000. It is preferred that one or
more of X, Y, X' and Y' is hydrogen. More than one suppressor may
be used.
[0028] Suitable suppressors include, but are not limited to: amines
such as ethoxylated amines; polyoxyalkylene amines and alkanol
amines; amides; poly-glycol type wetting agents such as
polyethylene glycols, polyalkylene glycols and polyoxyalkylene
glycols; high molecular weight polyethers; polyethylene oxides such
as those having a molecular weight in the range of 1,000 to
100,000; polyoxyalkylene block copolymers; alkylpolyether
sulfonates; complexing suppressors such as alkoxylated diamines;
and complexing agents for cupric or cuprous ions such as citric
acid, edetic acid, tartaric acid, potassium sodium tartrate,
acetonitrile, cupreine and pyridine.
[0029] Particularly useful suppressors include, but are not limited
to: ethyleneoxide/propyleneoxide ("EO/PO") block or random
copolymers; ethoxylated polystyrenated phenol having 12 moles of
ethyleneoxide ("EO"), ethoxylated butanol having 5 moles of EO,
ethoxylated butanol having 16 moles of EO, ethoxylated butanol
having 8 moles of EO, ethoxylated octanol having 12 moles of EO,
ethoxylated beta-naphthol having 13 moles of EO, ethoxylated
bisphenol A having 10 moles of EO, ethoxylated sulfated bisphenol A
having 30 moles of EO and ethoxylated bisphenol A having 8 moles of
EO.
[0030] In general, the suppressor may be added in any amount that
provides sufficient lateral growth of the copper layer. Typically,
the amount of suppressor is in the range of 0.001 to 10 g/L, and
more typically 0.1 to 2.0 g/L.
[0031] Levelers may optionally be added to the present
electroplating baths. In one embodiment, a leveler compound is used
in the present electroplating baths. Such levelers may be used in a
wide range of amounts, such as from 0.01 to 50 ppm or greater.
Examples of suitable leveling agents are described and set forth in
U.S. Pat. Nos. 3,770,598; 4,374,709; 4,376,685; 4,555,315;
4,673,459; and 6,610,192; and U.S. pat. application Ser. No.
2004/0249177. In general, useful leveling agents include those that
contain a substituted amino group such as compounds having
R--N--R', where each R and R'is independently a substituted or
unsubstituted alkyl group or a substituted or unsubstituted aryl
group. Typically the alkyl groups have from 1 to 6 carbon atoms,
more typically from 1 to 4 carbon atoms. Suitable aryl groups
include substituted or unsubstituted phenyl or naphthyl. The
substituents of the substituted alkyl and aryl groups may be, for
example, alkyl, halo and alkoxy. Sulfur-containing leveling agents
may also be used.
[0032] More specifically, suitable leveling agents include, but are
not limited to, 1-(2-hydroxyethyl)-2-imidazolidinethione;
4-mercaptopyridine; 2-mercaptothiazoline; ethylene thiourea;
thiourea; alkylated polyalkyleneimine; phenazonium compounds
disclosed in U.S. Pat. No. 3,956,084; N-heteroaromatic rings
containing polymers; quatemized, acrylic, polymeric amines;
polyvinyl carbamates; pyrrolidone; and imidazole. An exemplary
leveler is 1-(2-hydroxyethyl)-2-imidazolidinethione, although other
suitable levelers may be employed.
[0033] Other suitable levelers are reaction products of an amine
with an epihalohydrin, and preferably epichlorohydrin. Suitable
amines include, but are not limited to, primary, secondary or
tertiary amines, cyclic amines, aromatic amines and the like.
Exemplary amines include, without limitation, dialkylamines,
trialkylamines, arylalylamines, diarylamines, imidazole, triazole,
tetrazole, benzimidazole, benzotriazole, piperidine, morpholine,
piperazine, pyridine, oxazole, benzoxazole, pyrimidine, quinoline,
and isoquinoline. Imidazole is the preferred amine. Such amines may
be substituted or unsubstituted. By "substituted", it is meant that
one or more of the hydrogens on the amine are replaced by one or
more substituent groups, such as alkyl, aryl, alkoxy, halo, and
alkenyl. Other suitable reaction products of amines with
epichlorohydrin are those disclosed in U.S. Pat. No. 4,038,161
(Eckles et al.). Such reaction products are generally commercially
available, such as from Raschig, or may be prepared by methods
known in the art.
[0034] When present, the leveling agents are typically used in an
amount of 0.5 to 1000 ppm. More typically, the leveling agents are
used in an amount of 0.5 to 500 ppm, still more typically from 1 to
250 ppm, and even more typically from 1 to 50 ppm.
[0035] The present copper electroplating baths may optionally
contain a halide ion, and preferably do contain a halide ion.
Chloride and bromide are preferred halide ions, with chloride being
more preferred. Mixtures of halide ions may be used. A wide range
of halide ion concentrations (if a halide ion is employed) may be
suitably utilized, e.g. from 0 (where no halide ion employed) to
100 ppm of halide ion in the plating bath, more preferably from 25
to 75 ppm. Such halides may be added as the corresponding hydrogen
halide acid or as any suitable salt.
[0036] The electroplating baths may be prepared by combining the
source of copper ions, the electrolyte, the disulfide-containing
accelerator and any optional components in any order. Typically,
the plating baths of the present invention may be used at any
temperature from 10.degree. to 65.degree. C. or higher. It is
preferred that the temperature of the plating baths is from
10.degree. to 35.degree. C. and more preferably from 15.degree. to
30.degree. C.
[0037] The present plating baths are typically agitated during use.
Any suitable agitation method may be used with the present
invention and such methods are well-known in the art. Suitable
agitation methods include, but are not limited to, air sparging,
work piece agitation, impingement, rotation and the like. Such
methods are known to those skilled in the art.
[0038] When the present invention is used to plate an integrated
circuit substrate, such as a wafer, the wafer may be rotated such
as from 1 to 150 RPM and the plating solution contacts the rotating
wafer, such as by pumping or spraying. In the alternative, the
wafer need not be rotated where the flow of the plating bath is
sufficient to provide the desired metal deposit.
[0039] In general, the substrate to be copper plated is contacted
with the copper electroplating bath by a suitable means, such as by
immersion or by pumping or spraying. The substrate typically
functions as the cathode. An anode is added to the copper plating
bath and a potential is applied.
[0040] In one embodiment, the present invention provides a method
of depositing copper including the steps of: a) contacting an
electronic device substrate having apertures with a copper
electroplating bath including a source of copper ions, an
electrolyte, and a sulfur-containing compound; b) depositing a
layer of copper in the apertures using a duty cycle including 1)
applying a first current density for a first period to
electrochemically reduce the disulfide-containing accelerator to a
thiol compound at a copper surface; and 2) applying a second
current density for a second period; and c) repeating step b) until
a desired copper deposit is obtained; wherein the second current
density is less than the first current density.
[0041] The duty cycle may be repeated at a variety of different
frequencies. For example, the duty cycle may be repeated up to
multiple times per second or may take multiple seconds to perform
one duty cycle. The particular duty cycle chosen will depend upon
the size of the aperture to be copper plated, the particular copper
electroplating bath used and the level of impurities desired.
Suitable duty cycle frequencies are from 0.05 to 10 Hz (or cycles
per second) or even higher frequencies may be used, such as up to
100 Hz. In the manufacture of integrated circuits having wide metal
lines, a suitable duty cycle has a frequency of 0.1 to 10 Hz, more
typically from 0.1 to 5 Hz and still more typically from 0.1 to 2
Hz, although higher or lower frequencies may suitably be used.
[0042] A wide variety of current densities may be used for the
first current density. Suitable first current densities are from 1
to 100 mA/cm.sup.2 although higher or lower current densities may
be used. More typically, the first current density is from 5 to 100
mA/cm.sup.2, and still more typically from 15 to 90 mA/cm.sup.2. A
particularly suitable range of first current densities is from 40
to 85 mA/cm.sup.2 . A wide variety of current densities may be used
for the second current density, provided that the second current
density is less than the first current density. Exemplary second
current densities are from 1 to 50 mA/cm.sup.2, although higher or
lower current densities may be used. More typically, the second
current density is from 1 to 35 mA/cm.sup.2, still more typically
from 2 to 25 mA/cm.sup.2, and even more typically from 5 to 10
mA/cm.sup.2.
[0043] While not intending to be bound by theory, it is believed
that the first period of high current density reduces the
disulfide-containing accelerator to one or more thiol compounds.
Such thiol compounds may contain one or more thiol groups. It is
believed that the disulfide-containing accelerator is
electrochemically reduced at the freshly growing copper surface to
form the thiol compound. Such thiol compounds are believed to
adsorb on the copper surface during the relatively high current
density first period. In one embodiment, the first period is
performed for a time of 0.1 msec. to 10 sec., more typically from
0.1 msec. to 5 sec., and still more typically from 0.1 msec. to 1
sec. Further without wishing to be bound by theory, it is believed
that the longer the period of relatively low current density, the
greater the amount of total impurities incorporated into the copper
deposit. In one theory, but not the only theory, such period of
relatively low current density allows the copper surface to
recrystallize to incorporate any organic material on the copper
surface. Thus, the amount of total impurities incorporated into the
copper deposited can be controlled by the choice of second current
density and by the time period the substrate is subjected to this
current density.
[0044] In the present process, the range of amounts of impurities
incorporated in the copper layer as deposited, that is before
annealing) may be from 1 to 500 ppm for each impurity, such as
chloride, sulfur, carbon, oxygen and nitrogen. The total amount of
impurities before annealing may be up to a couple of thousand ppm.
. Typically, such total impurities are in the range of from 1 to
500 ppm, more typically from 1 to 300 ppm. The impurity levels are
determined by Secondary Ion Mass Spectrometry ("SIMS"), which
provides a value of ion concentration per unit area, as compared to
an ion implanted standard. The average impurity values are obtained
by summing the ppm values from the SIMS analysis for each data
point for each impurity and then dividing by the total number of
data points for the depth (in nm) of the copper layer evaluated.
The average impurity levels throughout the depth of the copper
deposit are much lower than the individual values. For example, an
impurity level of chloride ion by SIMS analysis may show a maximum
value of 200 ppm for a given unit area, where the average chloride
ion impurity level may only be 5 ppm for the entire copper deposit.
In one embodiment, the range of average total impurity level is
from 1 to 500 ppm, and more typically from 1 to 300 ppm.
[0045] In integrated circuit manufacture, copper layers are
typically annealed. During such annealing step, certain impurities,
such as sulfur and oxygen, are typically reduced. Copper layers
deposited according to the present invention, following annealing,
typically have average total impurities in the range of 1 to 500
ppm, more typically 1 to 300 ppm, and still more typically from 1
to 250 ppm. In one embodiment, the average total impurity level
following annealing is from 1 to 100 ppm.
[0046] After the desired copper deposit is obtained, an optional
further plating step may be employed to smooth the surface of the
deposit. Such optional plating step includes applying a current
density for a third period. A further optional resting step may be
included. No current is applied during the resting portion of the
step. In one embodiment, the third current density is in the range
of 20 to 90 mA/cm.sup.2 and typically 30 to 60 mA/cm.sup.2. In
another embodiment, additional plating steps are performed, such
additional steps may include cycling the plating on and off to
smooth the surface of the copper deposit.
[0047] The present invention is useful for depositing copper as
well as copper alloys such as, but not limited to, copper-silver,
copper-tin and tin-copper-silver. The present invention is expected
to be beneficial in the deposition of metals other than copper,
such as silver and tin.
[0048] An advantage of the present invention is that it provides
for the tailoring of doping (impurity) levels in a metal layer,
particularly a copper layer, to balance electromigration
performance and void stress migration performance. High purity
levels (i.e. low doping levels) are advantageous from
electromigration performance. However, the incorporation of certain
levels of impurities may be beneficial for void stress migration
control where small vias land on a wide line.
[0049] A further advantage of the present invention is that a
single metal plating bath may be used to provide an electronic
device having a first metal layer having a first purity and a
second metal layer having a second purity, where the purities of
the two metal layers are different. In this way, a metal layer can
be deposited having a desired level of total impurities needed for
a specific purpose, such as for control of void stress migration.
Accordingly, the present invention provides an electronic device
including a first layer of metal and a second layer of metal,
wherein the first layer of metal includes total impurities in the
range of up to 10 ppm and the second layer of metal includes total
impurities in the range of 10 to 100 ppm. In one embodiment, the
first and second metal layers are copper. For example, in an
integrated circuit, the first layer of metal may be a via layer, a
small line (i.e., a line having a width of .ltoreq.1 .mu.m), or a
mixture of these and the second layer of metal may be a wide
line.
EXAMPLE 1-10
[0050] A copper plating bath was prepared by combining copper
sulfate (40 g/L of copper ion), sulfuric acid (10 g/L),
hydrochloric acid (50 mg/L of chloride ion), a disulfide-containing
sulfonic acid accelerator (10 mL/L), an EO/PO copolymer suppressor
(5 mL/L), a leveler (3 mL/L) that is a reaction product of an
epoxide and imidazole and water.
[0051] Wafers were plated by immersing them individually in the
copper plating bath with rotation to cause net mass transport to
the wafer surface. Different first and second current densities
were used for each wafer. In each case, a rectangular pulsed
waveform having a 75% duty cycle was used. Copper was deposited to
approximately 1 .mu.m. After deposition, the wafers were removed
from the plating bath, rinsed and dried. The copper deposits were
then analyzed by Secondary Ion Mass Spectrometry ("SIMS") for total
impurity levels and found to contain oxygen, nitrogen, chlorine,
sulfur and carbon as impurities. The approximate total amount of
impurities (C, N, O, S, Cl) in each deposit is reported in the
following table. TABLE-US-00001 High Current Low Current Average
Total Density Density Frequency Impurity Level Example
(mA/cm.sup.2) (mA/cm.sup.2) (Hz) (ppm) 1 40 5 1 11.0 2 40 5 0.5
14.2 3 40 5 0.25 13.6 4 55 5 1 26.4 5 55 5 0.5 21.0 6 55 5 0.25
20.7 7 65 5 0.25 26.0 8 65 5 0.1 24.4 9 85 5 0.25 38.3 10 85 5 0.1
35.5
EXAMPLE 11--Comparative
[0052] The plating bath of Examples 1-10 was used to deposit copper
on a wafer using a DC waveform. The average (C, N, O, S, Cl)
impurity level by SIMS analysis was <5 ppm.
EXAMPLE 12--Comparative
[0053] The plating bath of Examples 1-10 was used to deposit a
copper film on a wafer using a with a current density of 7
mA/cm.sup.2 for the first 100 nm of copper deposited, followed by
40 mA/cm.sup.2 for approximately the next 900 nm of copper
deposited. The non-annealed copper deposit (approximately 1000 nm
thick) was then analyzed for impurity levels using SIMS. The
results are shown in FIG. 1 and illustrate the principal
impurities: carbon, sulfur, chlorine, nitrogen, and oxygen. The
plotted concentration values are not average values but are instead
actual data points. The increasing levels of impurities observed
near 0 nm of copper deposit depth arose from surface contamination
from the additives in the plating bath. The high level oxygen
impurity at depths of >800 nm arose from the TaO liner used to
fabricate the silicon test wafer. These data show, for example, a
maximum concentration of approximately 6 ppm of chloride ion per
unit area in the region of 400-700 nm depth. The average total
impurity level of this copper deposit was quite low, i.e. , 5 ppm.
The average total impurity level was not controlled using this
process.
EXAMPLE 13 --Comparative
[0054] The plating bath of Examples 1-10 was used to deposit a
copper film on a wafer using a constant current density of 7
mA/cm.sup.2 for the entire depth of the copper deposit
(approximately 1000 nm). The non-annealed copper deposit was then
analyzed for impurity levels using SIMS. The results are shown in
FIG. 2 and illustrate the principal impurities: carbon, sulfur,
chlorine, nitrogen, and oxygen. The plotted concentration values
are not average values but are instead actual data points. The
increasing level of oxygen at depths of >850 nm arose from the
TaO liner used to fabricate the silicon test wafer. The very high
levels of impurities observed between 300 and 450 nm depth arose
from a natural surface recrystallization phenomenon that results in
the incorporation of surface adsorbates from the plating bath and
exposure of a fresh surface of copper atoms. This natural cycle of
accumulation of surface adsorbates followed by recrystallization of
the surface layers can be repeated indefinitely if plating is
continued at low current density. However, when such
recrystallization occurs is not predictable. Accordingly, the
average total impurity level cannot be controlled using such
natural recrystallization process.
EXAMPLE 14
[0055] The plating bath of Examples 1-10 was used to deposit a
copper film on a wafer using alternating current densities of 5
mA/cm.sup.2 (for 100 nm of copper deposit) and 60 mA/cm.sup.2 (for
22.5 nm) repeated four times. The final 410 nm of copper deposit
was plated at 5 mA/cm.sup.2. The non-annealed copper deposit was
then analyzed for impurity levels using SIMS. The results are shown
in FIG. 3 and illustrate the principal impurities: carbon, sulfur,
chlorine, nitrogen, and oxygen. The plotted concentration values
are not average values but are instead actual data points. The high
level of oxygen at depths of >950 nm arose from the TaO liner
used to fabricate the silicon test wafer. The very high levels of
other impurities observed between 300 and 950 nm depth arose from
induced surface recrystallization caused by enrichment with surface
adsorbed thiols that are produced during brief applications for a
high current density plating pulse (1 sec, 60 mA/cm.sup.2). Pulsed
waveforms of the present invention can be used, therefore, to
incorporate very large levels of impurities compared with a
conventional DC plating process.
* * * * *