U.S. patent application number 10/672416 was filed with the patent office on 2005-03-31 for copper bath for electroplating fine circuitry on semiconductor chips.
This patent application is currently assigned to Innovative Technology Licensing, LLC. Invention is credited to Tench, D. Morgan, White, John T..
Application Number | 20050067297 10/672416 |
Document ID | / |
Family ID | 34376355 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050067297 |
Kind Code |
A1 |
Tench, D. Morgan ; et
al. |
March 31, 2005 |
Copper bath for electroplating fine circuitry on semiconductor
chips
Abstract
Bottom-up filling of fine Damascene trenches and vias in
semiconductor chips is attained using a copper pyrophosphate
electroplating bath with a single accelerating additive species
present at low concentration (<5 .mu.M). This bath is much
easier to control than the acid copper sulfate bath, which employs
a complicated additive system involving a minimum of two organic
additives and chloride ion (as well as significant additive
breakdown products). Pyrophosphate copper deposits exhibit stable
properties without annealing and are typically twice as hard as
acid sulfate copper deposits, which facilitates chemical mechanical
planarization. The mechanical properties and texture of the
fine-grained pyrophosphate copper deposits are also much less
substrate dependent, which minimizes the effects of variations and
flaws in the barrier and seed layers. Attack of copper seed layers
is minimized for the copper pyrophosphate bath, which operates in
the pH 8 to 9 range. The resistivity of pyrophosphate and annealed
acid sulfate copper deposits are substantially equivalent.
Inventors: |
Tench, D. Morgan;
(Camarillo, CA) ; White, John T.; (Lancaster,
CA) |
Correspondence
Address: |
Rockwell Scientific Company
MCA15
P.O. Box 1085
Thousand Oaks
CA
91358-0085
US
|
Assignee: |
Innovative Technology Licensing,
LLC
|
Family ID: |
34376355 |
Appl. No.: |
10/672416 |
Filed: |
September 26, 2003 |
Current U.S.
Class: |
205/296 ;
205/297; 205/298; 257/E21.175; 257/E21.585 |
Current CPC
Class: |
H01L 21/2885 20130101;
C25D 3/58 20130101; H05K 3/423 20130101; C25D 3/38 20130101; H01L
21/76877 20130101 |
Class at
Publication: |
205/296 ;
205/297; 205/298 |
International
Class: |
C25D 003/38 |
Claims
We claim:
1. A copper electroplating bath, comprising: water as a solvent;
copper ions; anions that strongly complex said copper ions so as to
substantially increase the overpotential for copper
electrodeposition such that the copper deposition rate at a given
cathode potential is substantially suppressed; and an organic
additive compound that tends to accelerate the copper
electrodeposition rate.
2. The copper electroplating bath of claim 1, wherein said anions
are of a type selected from the group consisting of pyrophosphate,
cyanide, citrate, tartrate, phosphate, glycerolate,
ethylenediaminetetraacetic acid, carboxylic acids, triethanolamine,
amines, phosphonates, and mixtures thereof.
3. The copper electroplating bath of claim 1, further comprising:
cations other than copper ions added to the electroplating bath as
a salt of said anions, such that said anions are present in the
electroplating bath in stoichiometric excess relative to said
copper ions.
4. The copper electroplating bath of claim 3, wherein said cations
other than copper ions are not electroactive at the potential used
for copper electrodeposition, such that relatively pure copper
metal is deposited.
5. The copper electroplating bath of claim 4, wherein said cations
other than copper ions are selected from the group consisting of
K.sup.+, Na.sup.+, and NH.sub.4.sup.+ ions.
6. The copper electroplating bath of claim 1, father comprising: a
surfactant.
7. The copper electroplating bath of claim 1, further comprising:
ions of at least one electroactive metal selected from the group
consisting of silver, zinc, cadmium, iron, cobalt, nickel, tin,
lead, bismuth, antimony, gallium and indium, such that a copper
alloy deposit is obtained.
8. The copper electroplating bath of claim 1, wherein said organic
additive compound contains at least one chemical element selected
from the group consisting of sulfur, nitrogen and phosphorous.
9. The copper electroplating bath of claim 1, whereby copper metal
is electrodeposited in Damascene trenches and vias to form
circuitry on semiconductor chips.
10. A copper electroplating bath, comprising: water as a solvent;
copper ions; pyrophosphate anions; cations other than copper ions
added to the electroplating bath as a salt of said anions, such
that said anions are present in the electroplating bath in
stoichiometric excess relative to said copper ions; and an organic
additive compound that tends to accelerate the copper
electrodeposition rate.
11. The copper electroplating bath of claim 10, wherein said
cations other than copper ions are not electroactive at the
potential used for copper electrodeposition, such that relatively
pure copper metal is deposited.
12. The copper electroplating bath of claim 11, wherein said
cations other than copper ions are selected from the group
consisting of K.sup.+, Na.sup.+, and NH.sub.4.sup.+ ions.
13. The copper electroplating bath of claim 10, further comprising:
a surfactant.
14. The copper electroplating bath of claim 13, wherein said
surfactant is polyoxyethylene(10)isooctylphenylether.
15. The copper electroplating bath of claim 10, further comprising:
ions of at least one electroactive metal selected from the group
consisting of silver, zinc, cadmium, iron, cobalt, nickel, tin,
lead, bismuth, antimony, gallium and indium, such that a copper
alloy deposit is obtained.
16. The copper electroplating bath of claim 10, wherein said
organic additive compound is 2,5-dimercapto-1,3,4-thiadiazole at a
concentration in the range from 1 to 5 .mu.M.
17. The copper electroplating bath of claim 10, wherein the
temperature is maintained between 50.degree. C. and 60.degree.
C.
18. The copper electroplating bath of claim 10, wherein the pH is
maintained in the 8.0 to 8.8 range.
19. The copper electroplating bath of claim 10, further comprising;
ammonia or ammonium ion.
20. The copper electroplating bath of claim 10, further comprising:
nitrate ion.
21. The copper electroplating bath of claim 10, whereby copper
metal is electrodeposited in Damascene trenches and vias to form
circuitry on semiconductor chips.
22. A copper electroplating bath, comprising: water as a solvent;
copper ions; pyrophosphate anions; cations other than copper ions
added to the electroplating bath as a salt of said anions, such
that said anions are present in the electroplating bath in
stoichiometric excess relative to said copper ions; and
2,5-dimercapto-1,3,4-thiadiazole at a concentration in the range
from 1 to 5 .mu.M, whereby copper metal is electrodeposited in
Damascene trenches and vias to form circuitry on semiconductor
chips.
23. A copper electroplating bath, comprising: water as a solvent;
copper ions; pyrophosphate anions; cations other than copper ions
added to the electroplating bath as a salt of said anions, such
that said anions are present in the electroplating bath in
stoichiometric excess relative to said copper ions; an organic
additive compound that tends to accelerate the copper
electrodeposition rate; and a surfactant.
24. A copper electroplating bath, comprising: water as a solvent;
copper ions; pyrophosphate anions; cations other than copper ions
added to the electroplating bath as a salt of said anions, such
that said anions are present in the electroplating bath in
stoichiometric excess relative to said copper ions;
2,5-dimercapto-1,3,4-thiadiazole at a concentration in the range
from 1 to 5 .mu.M; and a surfactant, whereby copper metal is
electrodeposited in Damascene trenches and vias to form circuitry
on semiconductor chips.
25. A copper electroplating bath, comprising: water as a solvent;
copper ions; pyrophosphate anions; cations other than copper ions
added to the electroplating bath as a salt of said anions, such
that said anions are present in the electroplating bath in
stoichiometric excess relative to said copper ions;
2,5-dimercapto-1,3,4-thiadiazole at a concentration in the range
from 1 to 5 .mu.M, polyoxyethylene(10)isooctylphenylether as a
surfactant; ammonia or ammonium ion; and nitrate ion, whereby
copper metal is electrodeposited in Damascene trenches and vias to
form circuitry on semiconductor chips.
26. A process for electrodepositing copper circuitry in trenches
and vias on semiconductor chips, comprising the steps of: providing
a semiconductor chip with trenches and vias to be filled with
copper; placing said chip in contact with an electroplating bath,
said bath comprising: water as a solvent, copper ions,
pyrophosphate anions, cations other than copper ions added to the
electroplating bath as a salt of said anions, such that said anions
are present in the electroplating bath in stoichiometric excess
relative to said copper ions, and 2,5-dimercapto-1,3,4-thiadiazole
at a concentration in the range from 1 to 5 .mu.M, and
electrodepositing copper in said trenches and vias.
27. The process of claim 26, wherein said cations other than copper
ions are selected from the group consisting of K.sup.+, Na.sup.+,
and NH.sub.4.sup.+ ions.
28. The process of claim 26, wherein the electroplating bath
further comprises a surfactant.
29. The process of claim 28, wherein said surfactant is
polyoxyethylene(10)isooctylphenylether.
30. The process of claim 26, wherein the temperature of the plating
bath is maintained at a temperature between 50.degree. C. and
60.degree. C.
31. The process of claim 26, wherein the pH of the electroplating
bath is maintained in the 8.0 to 8.8 range.
32. The process of claim 26, wherein the electroplating bath
further comprises ammonia or ammonium ion.
33. The process of claim 26, wherein the electroplating bath
further comprises nitrate ion.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention is concerned with fabrication of copper
integrated circuits on semiconductor chips, and in particular with
electrodeposition of copper circuitry.
[0003] 2. Description of the Related Art
[0004] The electronics industry is transitioning from aluminum to
copper as the basic metallization for semiconductor integrated
circuits (IC's) in order to increase device switching speed and
enhance electromigration resistance. The leading technology for
fabricating copper circuitry on semiconductor chips is the
"Damascene" process (P. C. Andricacos, Electrochem. Soc. Interface,
Spring 1999, p.32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S.
Pat. No. 5,209,817 to Ahmad et al.). In this process, vias are
etched in the chip's dielectric material, which is typically
silicon dioxide, although materials with lower dielectric constants
are under development. A barrier layer, e.g., titanium nitride
(TiN), tantalum nitride (TaN) or tungsten nitride (WN.sub.x), is
deposited on the sidewalls and bottoms of the trenches and vias,
typically by reactive sputtering, to prevent Cu migration into the
dielectric material, which would degrade the device performance.
Over the barrier layer, a thin copper seed layer is deposited,
typically by sputtering, to provide enhanced conductivity and good
adhesion. Copper is then electrodeposited (electroplated) into the
trenches and vias. Copper deposited on the outer surface, i.e.,
outside of the trenches and vias, is removed by chemical mechanical
planarization (CMP). A capping or cladding layer (e.g., TiN, TaN or
WN.sub.x) is applied to the exposed copper circuitry to suppress
oxidation and migration of the copper. Alternative barrier/capping
layers based on electrolessly deposited cobalt and nickel are under
investigation [e.g., A. Kohn, M. Eizenberg, Y. Shacham-Diamand and
Y. Sverdlov, Mater. Sci. Eng. A302, 18 (2001)]. The "Dual
Damascene" process involves deposition in both trenches and vias at
the same time. In this document, the term "Damascene" also
encompasses the "Dual Damascene" process.
[0005] Damascene copper electrodeposition is typically performed
from an acid copper sulfate electroplating bath, which requires a
minimum of two types of organic additives to provide good deposit
properties and complete filling of the trenches and vias. The
"suppressor" additive (also called the "polymer", "carrier", or
"wetter", depending on the bath supplier) is typically a polymeric
organic species, e.g., high-molecular-weight polyethylene or
polypropylene glycol, which adsorbs strongly on the copper cathode
surface, in the presence of chloride ion, to form a film that
sharply increases the overpotential for copper electrodeposition
[M. R. H. Hill and G. T. Rogers, J. Electroanal. Chem. 86, 179
(1978)], i.e., suppresses the rate of copper deposition at a given
potential. This greatly enhances the throwing power and leveling
characteristics of the bath, and inhibits uncontrolled copper
plating that would result in powdery or nodular deposits. The
"anti-suppressor" additive (also called the "brightener",
"accelerator" or simply the "additive", depending on the bath
supplier) also adsorbs strongly on the copper surface and counters
the suppressive effect of the suppressor so as to enhance the
copper electrodeposition rate [W. O. Fritag, C. Ogden, D. Tench and
J. White, Plating Surf. Fin. 70(10), 55 (1983)]. The
anti-suppressor acts primarily as a catalyst and is not consumed
rapidly in the electrodeposition process. As curvature develops in
the copper deposit at the bottom edges of the Damascene features,
the surface area is decreased, which increases the concentration of
adsorbed anti-suppressor and accelerates deposition at the feature
bottoms to produce "superconformal" deposition or "bottom-up"
filling [D. Josell, D. Wheeler and T. P. Moffat, Electrochem. &
Solid-State Letters 5(4), C49 (2002)].
[0006] For proper functioning of the acid copper sulfate additive
system, a delicate balance between the suppressor and
anti-suppressor additives must be maintained. In addition, it is
necessary to control the concentration of chloride ion, which is
known to be essential to the functioning of the suppressor and
anti-suppressor additives in acid copper sulfate baths [e.g., J. D.
Reid and A. P. David, Plating Surf. Fin. 74(1), 66 (1987); J. J.
Kelly, C. Tian and A. C. West, J. Electrochem. Soc. 146(7), 2540
(1999)]. An imbalance in the additive system typically results in
unacceptable voids or defect lines in the Damascene copper deposit,
formed as the copper deposits on opposing sidewalls of the feature
grow together (in the absence of bottom-up filling).
[0007] As the feature size for the Damascene process has shrunk
below 0.2 .mu.m, it has become necessary to utilize a third organic
additive in acid copper sulfate baths in order to suppress
overplating of the trenches and vias. Note that excess copper on
Damascene plated wafers (called the "overburden") is typically
removed by chemical mechanical planarization (CMP) but the copper
layer must be relatively uniform for efficient CMP removal. The
third additive is called the "leveler" (or "booster", depending on
the bath supplier) and is typically an organic compound containing
nitrogen or oxygen that also tends to decrease the copper plating
rate. The leveler is generally present at a relatively low
concentration so that it is most effective at the outer surface of
the wafer where it is effectively replenished by solution
agitation.
[0008] The concentrations of organic additives in copper
electroplating baths are typically determined from the effect that
they exert on the copper electrodeposition rate measured via cyclic
voltammetric stripping (CVS) analysis [D. Tench and C. Ogden, J.
Electrochem. Soc. 125, 194 (1978)]. In the CVS method, the
potential of a platinum rotating disk electrode is cycled in the
plating bath between fixed potential limits so that metal is
alternately plated on and stripped from the electrode surface. The
copper electrodeposition rate is generally determined from the
voltammetric stripping peak area for the rotating electrode
(A.sub.r), which corresponds to the charge required to anodically
strip the copper deposited during a given cycle. To improve
measurement precision, A.sub.r is typically normalized via division
by the stripping peak area for the stationary electrode in the same
solution (A.sub.s), or via division by A.sub.r(0) measured for a
background electrolyte without organic additives. The CVS method
was first applied to control copper pyrophosphate baths (U.S. Pat.
No. 4,132,605 to Tench and Ogden) but has since been adapted for
control of the various additive components in acid copper sulfate
baths. For example, the acid copper suppressor concentration can be
determined by CVS response curve or dilution titration analysis [W.
O. Freitag, C. Ogden, D. Tench and J. White, Plating Surf. Fin.
70(10), 55 (1983)], and the anti-suppressor concentration can be
determined by the linear approximation technique (LAT) or modified
linear approximation technique (MLAT) described by R. Gluzman
[Proc. 70.sup.th Am. Electroplaters Soc. Tech. Conf., Sur/Fin,
Indianapolis, Ind. (June 1983)].
[0009] A major disadvantage of the acid copper sulfate system for
Damascene plating is the complicated additive system, whose
components must be closely controlled to obtain acceptable
deposits. Close control is difficult to attain since the various
additive species are generally present at very low concentrations
and exert synergistic effects. Plating bath suppliers typically
provide organic additives in the form of solutions that may contain
additives of more than one type (as well as inorganic species),
which exacerbates the difficulty of controlling the additive
system. In addition, additive components may be comprised of more
than one chemical species, and the suppressor additive generally
involves a range of molecular weights.
[0010] Furthermore, additive breakdown products tend to accumulate
in the plating bath and interfere with functioning of the additive
system. Such breakdown products, which include
lower-molecular-weight suppressor species, must be monitored and
periodically removed. Additive breakdown products are typically
removed by bleeding off part of the plating bath and replacing it
with fresh plating solution (bleed-and-feed), which is costly and
environmentally undesirable.
[0011] Another disadvantage of the acid copper sulfate system for
Damascene plating is that key properties of the deposit (grain
size, hardness and electrical conductivity) undergo slow changes at
ambient temperatures. To provide stable properties and maximum
electrical conductivity within a short time, deposits are annealed
(typically at about 200.degree. C.), which adds an extra processing
step and increases costs.
[0012] Still another disadvantage of the acid copper sulfate system
for Damascene plating is that the copper deposit tends to be
relatively soft (large-grained). During CMP processing, soft copper
tends to be removed faster than the surrounding dielectric material
and, especially for wide trenches and bond pads, may "dish" and
lose the planarity needed to facilitate bonding and minimize
circuit electrical resistance. Soft copper also tends to exacerbate
CMP erosion of copper and dielectric material in clusters of
closely-spaced narrow trenches.
[0013] Still another disadvantage of the acid copper sulfate system
for Damascene plating is that the deposit mechanical properties are
strongly dependent on the substrate [R. Haak, C. Ogden, and D.
Tench, Plating Surf. Fin. 68(10), p. 59 (1981); K. Abe, Y. Harada,
and H. Onoda, IEEE 98CH36173 Ann. Int. Rel. Phys. Symp., p. 342
(1998)]. Consequently, inconsistencies or changes in the barrier
and seed layers may significantly affect the properties of the
Damascene copper.
[0014] Still another disadvantage of the acid copper sulfate system
for Damascene plating is that the plating bath is strongly acidic
(typically, 10% sulfuric acid by volume). The strong acid tends to
attack the copper seed layer, which is a particular problem for
very narrow and/or deep Damascene features since the seed layer in
this case is necessarily thin and may not be uniform. To avoid
unacceptable thinning of the copper seed layer via acid attack, it
is often necessary to introduce the semiconductor wafer into the
plating bath with electrical power applied, i.e., "hot", which
constrains the plating cell design and may not be totally
effective.
[0015] One possibility for avoiding the complicated additive
systems employed in acid copper sulfate baths is to utilize a
copper plating bath based on an anion (pyrophosphate and cyanide,
for example) that forms strong complexes with copper ions. In this
case, the strongly complexing anion performs the function of
suppressing the copper electrodeposition rate (raising the copper
deposition overpotential) so that the polymeric suppressor additive
employed in acid copper sulfate baths is not needed. A copper
plating bath based on a strongly complexing anion (or anions) is
termed a "complexed bath" or "complexed copper bath" herein.
Complexed copper baths generally provide high throwing power
(uniform deposit thickness over irregular-shaped substrates) and
deposits with good mechanical properties, even without organic
additives. Organic brightening and leveling additives are typically
used in such baths to provide finer-grained deposits with enhanced
brightness (luster) and improved mechanical properties, and to
improve the uniformity and smoothness of deposits on irregular or
rough substrates. The copper pyrophosphate bath has been widely
used for electrodepositing uniform copper layers within
through-holes (and blind vias) in printed wiring boards
(PWB's).
[0016] Brightening and leveling additives in the complexed copper
baths of the prior art literature generally function by adsorbing
on the copper surface and blocking growth sites so that the copper
electrodeposition rate at a given electrode potential is suppressed
further (beyond the suppression provided by strongly complexing
anions). Within recessed areas of the deposit, the adsorbed
additive species, which is typically present at low concentration
in the bulk solution, becomes depleted as it is consumed in the
copper electrodeposition process so that the rate of copper
electrodeposition within the recessed areas increases. The additive
species is less depleted on flat or protruded areas of the deposit,
where it is more effectively replenished by solution flow produced
by bath agitation, so that the copper electrodeposition rate
remains substantially suppressed in these areas. More rapid copper
deposition within deposit recesses tends to level and brighten the
deposit. The increased overpotential provided by
leveling/brightening additives also improves the bath throwing
power (ability to provide uniform deposit thickness over
irregular-shaped substrates).
[0017] Complexed copper baths employing the brightening/leveling
additives of the prior art, which function by the
suppression-depletion mechanism described above, have very limited
utility for Damascene plating. Bottom-up filling by this mechanism
requires that copper deposition be accelerated at the feature
bottom by additive depletion during copper electrodeposition, and
be suppressed on the feature sidewalls by additive replenishment
via solution flow. Bath agitation produces negligible solution flow
within the very narrow Damascene features of principal interest to
the electronics industry (0.2 .mu.m or less). Under the
substantially stagnant conditions existing within narrow features,
the additive is uniformly depleted over the surface during copper
deposition and a conformal deposit is obtained. In this case,
opposing feature sidewalls grow together, producing a void or
defect line. The additive concentration gradient needed for
bottom-up filling by the suppression-depletion mechanism can only
be attained within relatively wide Damascene features, within which
bath agitation produces substantial solution flow. In addition, for
prior art additives used in strongly complexed baths, the feature
aspect ratio (depth to width ratio) cannot be so large that
substantial additive depletion occurs on the lower sidewall areas,
causing opposing sidewalls to grow together to create a void or
defect line.
[0018] A copper pyrophosphate system has been evaluated for plating
copper on silicon for potential micro-electro-mechanical systems
(MEMS) applications [M. Cerisier, K. Attenborough, J. Fransaer, C.
Van Haesendonck and J.-P. Celis, J. Electrochem. Soc. 146, 2156
(1999)]. In this case, however, copper was plated directly on the
silicon surface without the barrier and seed layers needed to
fabricate integrated circuits in the Damascene process. Deposition
was also performed on a planar surface from a copper pyrophosphate
bath at room temperature without organic additives, which would not
be suitable for plating trenches and vias in the Damascene process.
These workers thoroughly characterized the morphology of the
deposits obtained, but did not investigate the capability of the
bath for leveling or filling IC features with copper.
[0019] For bottom-up filling of narrow Damascene features with
relatively high aspect ratios from a complexed copper bath, an
additive that substantially accelerates the copper
electrodeposition rate is needed. In this case, accelerated
deposition at the feature bottom would be provided by the increase
in additive concentration resulting from curvature in the copper
deposit.
SUMMARY OF THE INVENTION
[0020] This invention provides an electroplating bath based on a
strongly complexing anion and an accelerating additive for
electrodepositing copper circuitry in Damascene trenches and vias
on semiconductor chips. This bath avoids the disadvantages of the
acid copper sulfate baths employed for Damascene plating in the
prior art. Use of a strongly complexing anion (pyrophosphate or
cyanide, for example) to suppress runaway copper electrodeposition
and improve the deposit properties eliminates the need for the
polymeric suppressor additive used in acid copper sulfate baths,
and avoids interference from breakdown products thereof. Bottom-up
filling of Damascene features is provided by a single accelerating
additive species. This additive is present at low concentrations
and is not rapidly consumed in the copper electrodeposition process
so that organic breakdown products are minimal. The low
accelerating additive concentration used and the inherently good
throwing power of strongly complexed baths tend to provide a
uniform "overburden" layer, which is more easily removed by
chemical mechanical planarization (CMP) and obviates the need for
the "leveler" additive employed in acid copper sulfate baths. The
strongly complexing anion is present in high concentration and is
not rapidly consumed in the electrodeposition process, which
facilitates control of the plating system.
[0021] In addition, the strongly complexed copper electroplating
baths of the present invention provide relatively fine-grained
deposits that have stable properties, whereas acid copper sulfate
deposits are generally large-grained and must be annealed to
stabilize key deposit properties (e.g., hardness and electrical
conductivity). The fine-grained deposits from the
strongly-complexed bath of the invention also tend to be harder and
more resistant to dishing during CMP processing. The properties of
fine-grained deposits also tend to be much less dependent on the
substrate properties so that variations and flaws in the seed and
barrier layers used in the Damascene process tend to be less
important. Furthermore, the strongly complexing anion used in the
bath of the present invention stabilizes the copper ions so that
the bath can operate at an alkaline pH (without precipitation of
copper hydroxide), which minimizes copper seed layer attack.
[0022] A preferred copper plating bath according to the present
invention is copper pyrophosphate and a preferred accelerating
additive is the 2,5-dimercapto-1,3,4-thiadiazole monomer at a
concentration of less than 5 .mu.M. The
2,5-dimercapto-1,3,4-thiadiazole dimer has previously been employed
as a brightening/leveling additive in copper pyrophosphate plating
baths used for printed wiring board (PWB) applications. For PWB
plating, the 2,5-dimercapto-1,3,4-thiadiazole (DMTD) monomer is
typically added to the plating bath at a sufficiently high
concentration (typically about 15 .mu.M) to form a substantial
concentration of the DMTD dimer, which may be detected via the
substantial decrease in the copper electrodeposition rate produced.
For the preferred copper pyrophosphate bath according to the
present invention, the concentration of DMTD monomer is maintained
below the concentration required to produce a substantial
concentration of the DMTD dimer. In this case, the effect of the
monomer on the copper electrodeposition rate is dominant, as
indicated by an increase in the copper electrodeposition rate upon
addition of a small amount of the DMTD monomer. Bottom-up filling
of narrow Damascene features (0.2 .mu.m) without voids or defect
lines was demonstrated using a copper pyrophosphate bath containing
the accelerating DMTD additive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 shows the effect of additions of DMTD monomer on the
CVS copper electrodeposition rate parameter (A.sub.r/A.sub.s)
measured after 16 hours equilibration at 55.degree. C. for a copper
pyrophosphate bath (pH 8.3) containing 22.5 g/L Cu.sup.2+ ion, 175
g/L P.sub.2O.sub.7.sup.4- ion, and 2.25 g/L NH.sub.3 (added as
NH.sub.40H solution). The CVS measurements were made at a Pt
rotating disk electrode (4 mm diameter, 2500 rpm) cycled at 50 mV/s
between -0.700 and +1.000 V vs. saturated calomel electrode
(SCE).
[0024] FIG. 2 depicts the equilibria involved in copper complexing
and dimerization of the 2,5-dimercapto-1,3,4-thiadiazole
additive.
[0025] FIG. 3 depicts the Damascene copper electrodeposition
process via cross-sections showing: (A) feature (trench or via)
without electrodeposited copper; (B) initial stage of copper
deposition with curvature developed at feature bottom; (C)
partially-filled feature; and (D) fully-filled feature with copper
overburden.
[0026] FIG. 4 depicts a cross-section of a Damascene copper deposit
containing a void resulting from conformal deposition rather than
bottom-up filling.
[0027] FIG. 5 shows the effect of additions of DMTD monomer on the
CVS copper electrodeposition rate parameter (A.sub.r/A.sub.s)
measured at two electrode rotation rates (1000 and 2500 rpm) for
the solution of FIG. 1 with 0.50 g/L Triton.RTM.-X surfactant
added. Other measurement conditions were the same as for the FIG. 1
data.
[0028] FIG. 6 shows an electron micrograph of a cross-section of
Damascene vias (0.13 .mu.m wide) that were completely filled with
copper from a copper pyrophosphate bath (55.degree. C.) containing
2.0 .mu.M (0.3 ppm) DMTD additive.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Technical terms used in this document are generally known to
those skilled in the art. The terms "electroplating", "plating" and
"electrodeposition" are equivalent. The term "electrode potential",
or simply "potential", refers to the voltage occurring across a
single electrode-electrolyte interface, whereas a "cell voltage" is
the total voltage applied between two electrodes. In practice, the
electrode potential often includes an appreciable resistive voltage
drop in the electrolyte, which typically remains constant and does
not affect voltammetric results. A "cyclic voltammogram" is a plot
of current or current density (on the y-axis) versus the working
electrode potential (on the x-axis) typically obtained by cycling
the working electrode potential with time between fixed negative
and positive limits. A "potentiostat" is an electronic device for
controlling the potential of a working electrode by passing current
between the working electrode and a counter electrode so as to
drive the working electrode to a desired potential relative to a
reference electrode.
[0030] The term "bath" denotes an electrolytic solution used for
electroplating. A copper plating bath based on strongly complexing
anions is termed a "complexed bath" or "complexed copper bath"
herein. The plural term "anions" encompasses both anions of
different types (e.g., pyrophosphate or cyanide), as well as the
plurality of anions of the same type that are generally present in
a solution of a given anion. The singular term "anion" denotes an
anion of a particular type. The term "copper ions" encompasses both
Cu.sup.2+ and Cu.sup.+ species. The dominant species in a complexed
copper bath may be either Cu.sup.2+ or Cu.sup.+ but both species
may be present in various complexes with anions, and Cu.sup.+ is
generally formed as an intermediate during copper electrodeposition
from the Cu.sup.2+ species. The term "overpotential" refers herein
to the increase in negative electrode potential required to produce
substantial copper electrodeposition for a complexed copper bath
compared to that required for an acid copper sulfate bath (in the
absence of organic additives). An "accelerating additive"
substantially enhances the copper electrodeposition rate at a given
cathode potential. Damascene "features" include both trenches and
vias. The symbol "M" means molar concentration, and ".mu.M" means
micromolar concentration.
[0031] This invention provides an electroplating bath based on a
strongly complexing anion and an accelerating additive for
electrodepositing copper circuitry in trenches and vias on
semiconductor chips as part of the Damascene process. Complexing
the copper ions via an intrinsic component of the bath provides an
inherently high overpotential for copper deposition, which
eliminates the need for the suppressor additive and the associated
complicated additive system used in acid copper sulfate baths. For
example, the onset potential for copper deposition from a copper
pyrophosphate bath is typically about -0.5 V greater than that for
deposition from an acid copper sulfate bath (without organic
additives). In addition, complexed copper systems provide
fine-grained deposits which are typically much harder than the
large-grained acid sulfate copper deposits, and which exhibit
stable mechanical properties that do not change with time. The
mechanical properties and texture of the fine-grained deposits
produced by complexed baths are also much less substrate dependent
than those produced by acid copper sulfate baths, which minimizes
the effects of variations and flaws in the barrier and seed
layers.
[0032] Although complexed copper baths offer significant advantages
for Damascene plating, the prior art does not identify a suitable
organic additive providing good deposit properties and the
accelerated copper deposition needed for bottom-up filling of fine
features. Only additives that further suppress (decelerate) the
copper electrodeposition rate are reported for such baths in the
prior art literature. Suppressor additives can provide bottom-up
filling only if a difference in solution mass transport can be
established within the feature (between the feature top and
bottom), by bath agitation or other solution mass transport means.
For fine features (0.2 .mu.m or less in width) or those with
relatively high aspect ratios (>5:1), the plating solution is
practically stagnant within at least a substantial portion of the
feature so that bottom-up filling cannot be attained with the
decelerating additives used for complexed copper baths in the prior
art.
[0033] At a minimum, the plating bath of the present invention must
include water as a solvent, copper ions (in either the +1 or +2
oxidation state, or a mixture of the two states), anions that
strongly complex the copper ions so as to substantially increase
the overpotential for copper electrodeposition such that the copper
deposition rate at a given cathode potential is substantially
suppressed, and an organic additive compound that tends to
accelerate the copper electrodeposition rate. The plating bath may
also include one or more cations other than copper ions, auxiliary
complexing agents, non-complexing anions, additives to increase the
maximum usable current density, and surfactants as wetting agents
to aid in wetting the substrate and/or copper surface.
[0034] In complexed copper plating baths, a stoichiometric excess
of complexing anions to copper ions is typically present to enhance
the stability of the copper complex so as to adequately suppress
the copper electrodeposition rate and enhance the overall bath
stability. This stoichiometric excess of anions is usually provided
by addition of a salt of a metal other than copper that readily
dissociates in the bath (does not readily form a complex with the
anion). The metal cations other than copper ions derived from
addition of such a salt (K.sup.+ and Na.sup.+ ions, for example)
are typically not electroactive at the potential used for copper
electrodeposition, such that relatively pure copper metal is
deposited. Alternatively, the salt used to provide excess
complexing anions may contain non-metallic cations, e.g., ammonium
ions (NH.sub.4.sup.+), to avoid the presence of metallic ions other
than copper in the bath, which might degrade device performance if
not completely removed after the plating operation. Ions of an
electrodepositable metal other than copper can also be added to the
plating bath, such that an alloy deposit is obtained. Possible
metals for this purpose include silver, zinc, cadmium, iron,
cobalt, nickel, tin, lead, bismuth, antimony, gallium and indium.
In this case, the metal other than copper may be selected, for
example, to increase the resistance of the alloy to
electromigration.
[0035] Complexing anions that may be used to practice the present
invention include pyrophosphate, cyanide, citrate, tartrate,
phosphate, glycerolate, ethylenediaminetetraacetic acid (EDTA),
carboxylic acids, amines (e.g., triethanolamine), phosphonates, and
mixtures thereof. The efficacy of these complexing agents is
expected to vary and to depend on the pH of the plating solution.
The pyrophosphate and cyanide anions are preferred since they have
been widely used for copper plating. The copper pyrophosphate
system is most preferred since cyanide baths are undesirable for
environmental and safety reasons. Copper pyrophosphate baths with
decelerating organic additives were previously used extensively for
circuit board plating, but were largely replaced by acid copper
sulfate baths, primarily because the alkaline copper pyrophosphate
system is more susceptible to contamination by water soluble
organic coatings, which were adopted for environmental reasons.
Such water soluble coatings are typically not present during chip
plating by the Damascene process.
[0036] A preferred electroplating bath for practicing the present
invention is copper pyrophosphate containing 1 to 5 .mu.M
2,5-dimercapto-1,3,4-thiadiazole (DMTD) as an accelerating
additive. According to the prior art literature [C. Ogden and D.
Tench, J. Electrochem. Soc. 128, 539 (1981)], DMTD reacts in copper
pyrophosphate baths at the operating temperature (50-60.degree. C.)
to form the DMTD dimer, which is a decelerating additive species
that acts as a brightening and leveling agent. The prior art also
indicates that the accelerated copper deposition produced by the
DMTD monomer species is detrimental to the deposit properties and
uniformity, causing protrusions in the deposit to grow faster than
recessed areas [D. Tench and C. Ogden, J. Electrochem. Soc. 125,
1218 (1978)]. The printed wiring board plating problem known as
"foldback" has been shown to result from accelerated copper
deposition at protrusions within through-holes when the DMTD
concentration is too low for formation of enough dimer to suppress
the copper deposition rate and level the deposit. Thus, the prior
art teaches away from use of the DMTD monomer as an additive in
copper pyrophosphate baths. We have discovered, however, that the
DMTD monomer at an appropriately low concentration provides
bottom-up filling of fine Damascene trenches and vias.
[0037] FIG. 1 shows the effect of additions of DMTD monomer on the
CVS copper electrodeposition rate parameter (A.sub.r/A.sub.s)
measured in a copper pyrophosphate bath at 55.degree. C. The
accelerating effect of the DMTD monomer is evident from the initial
increase in A.sub.r/A.sub.s with increasing DMTD concentration. The
peak and decrease in A.sub.r/A.sub.s at higher concentrations
result from formation of the DMTD dimer, which decelerates the
copper electrodepositon rate. In prior art copper pyrophosphate
baths, the DMTD concentration has always been maintained at a
relatively high value, typically about 15 .mu.M (2 ppm) [C. Ogden
and D. Tench, Plating Surf. Fin. 66(9), 30 (1979)], at which the
dominant additive species is the dimer and leveling/brightening is
provided by the suppression-depletion mechanism. In the copper
pyrophosphate bath of the present invention, the DMTD concentration
is maintained at a value below 5 .mu.M for which the dominant
additive species is the monomer so that bottom-up filling of
Damascene features is attained. The DMTD was added as the
protonated DMTD species for which 1.5 mg/L corresponds to 10 .mu.M
concentration. The DMTD may also be added as a salt of another
cation, sodium or potassium ion, for example.
[0038] FIG. 2 depicts the chemical equilibria involved in copper
complexing and dimerization of the DMTD additive. The key to the
strong accelerating effect of the DMTD species is its ability to
complex copper ions via two mercapto-groups, which assist in
deposition of two contiguous copper atoms that form a new copper
nucleus [C. Ogden and D. Tench, J. Electrochem. Soc. 128, 539
(1981)]. The compound (2-amino-5-mercapto-1,3,4-thiadiazole)
resulting from substitution of a copper-complexing amino-group for
one of the DMTD mercapto-groups also accelerates the copper
deposition rate in copper pyrophosphate baths, whereas the compound
(2-methyl-5-mercapto-1,3,4-thiadiazole) resulting from substitution
of a non-complexing methyl-group does not act as an accelerating
additive. Relatively reversible adsorption via the sulfide group
(--S--) allows the same DMTD molecule to assist in formation of
numerous copper nuclei, which strongly accelerates copper
deposition. The dimer species can also complex copper via two
mercapto-groups but, in this case, the complexed copper ions are
far apart and irreversible adsorption via the disulfide group
(--S--S--) serves to block growth sites and decelerate copper
deposition. Since an equilibrium is involved, both the monomer and
dimer are always present but at sufficiently low DMTD
concentrations (<5 .mu.M) the equilibrium strongly favors the
monomer, which exerts the dominant effect, accelerating the copper
electrodeposition rate.
[0039] A standard curve of a CVS rate parameter vs. additive
concentration, such as the one shown in FIG. 1 for the DMTD
additive, can be used to identify suitable accelerating additive
species and concentration ranges for practicing the present
invention. Use of a plot of A.sub.s vs. additive concentration
might seem to be more appropriate (to simulate stagnant conditions
within fine Damascene features) but in this case the effects of
species such as the DMTD dimer, which is rapidly consumed during
copper deposition, would be masked. For the DMTD additive, A.sub.s
continues to increase at higher DMDT concentrations for which
electrode rotation (solution agitation) would produce a decrease in
the copper deposition rate via DMTD dimer replenishment at the
cathode surface. Consequently, plots of A.sub.r/A.sub.s (or
A.sub.r) vs. additive concentration are preferred for identifying
suitable accelerating additives and additive concentration ranges
for the present invention.
[0040] Suitable accelerating additives produce an increase in the
CVS rate parameters (A.sub.r, A.sub.s and A.sub.r/A.sub.s), or
another parameter reflecting the copper electrodeposition rate,
with increasing additive concentration. Excessively high copper
deposition rates, produced by higher additive concentrations, can
result in unacceptable deposit properties [C. Ogden, D. Tench and
J. White, J. Appl. Electrochem. 12, 619 (1982)] and should be
avoided. In identifying a suitable additive concentration, the
effects of differences in current density and solution mass
transport between the copper deposition rate measurements and the
bath operating conditions should be considered. The positive slope,
and any peak and negative slope associated with dimerization or
another additive reaction, for plots of measured copper deposition
rate vs. additive concentration generally depend on current density
and solution mass transport. A preferred procedure is to
empirically determine the optimum additive concentration by plating
actual test samples at additive concentrations in the range for
which the copper deposition rate increases with increased additive
concentration. Numerous organic compounds containing sulfur,
nitrogen and/or phosphorus atoms may provide the accelerated copper
deposition in complexed copper baths needed for bottom-up filling
of fine Damascene features.
[0041] By analogy with DMTD, compounds having two mercapto-groups
in close proximity on a hetero-ring structure (2,6-dithiopurine,
for example) are likely to function as accelerating additives in
complexed copper baths. Some di-mercapto-compounds that do not
involve a hetero-ring structure (2,3-dimercapto-1-propanol, for
example) may also accelerate copper electrodeposition. By analogy
with 2-amino-5-mercapto-1,3,4-thiadiazole, compounds for which
amino-groups are substituted for one or both of the mercapto-groups
in dimercapto-compounds may also be useful as accelerating
additives.
[0042] FIG. 3 depicts cross-sections illustrating bottom-up filling
by an accelerating additive in the Damascene process. As shown in
FIG. 3(A), Damascene feature 101 (with sidewalls 102 and bottom
103) in semiconductor dielectric material 104 initially has a
barrier layer 105 and a copper seed layer 106, and the bottom edges
of feature 101 (intersection of sidewalls 102 and bottom 103) are
relatively sharp. The accelerating additive species may adsorb
uniformly over copper seed layer 106, or adsorb preferentially at
the sharp bottom edges of feature 101. As shown in FIG. 3(B),
curved surfaces 108 develop along the bottom edges of copper
deposit 109 during the early stages of copper electrodeposition due
to simultaneous deposition on the sidewalls 102 and bottom 103, and
possibly preferential adsorption of the accelerating additive along
the sharp bottom edges of feature 101. Curved surfaces 108 on
copper deposit 109 have reduced surface areas compared to the
corresponding regions of the sharp bottom edge of feature 101 so
that the concentration of adsorbed accelerating additive species,
which is not rapidly consumed in the copper deposition process, is
increased. This accelerates the copper deposition rate near the
bottom 103 of feature 101, which initiates bottom-up filling. As
shown in FIG. 3(C), curvature in copper deposit 109 eventually
extends across the bottom 110 of copper deposit 109, which further
accelerates bottom-up filling. As shown in FIG. 3(D), the Damascene
feature is completely filled with copper deposit 109, without a
void or defect line, and a layer of copper overburden 110 is
deposited. Overburden 110 is subsequently removed via CMP
processing.
[0043] FIG. 4 depicts a cross-section of the Damascene copper
deposit obtained by conformal copper electrodeposition rather than
bottom-up filling. In this case, copper deposit 301 has a void 302
and/or a defect line 303, which are typically formed when the
copper deposits on opposing sidewalls of the feature grow
together.
[0044] The present invention can be effectively practiced using
standard copper pyrophosphate bath formulations [J. W. Dini, Modern
Electroplating, 4.sup.th Edition, John Wiley & Sons, Ed. M.
Schlesinger and M. Paunovic, Chap. 2, Part D (2000)] with the
addition of an accelerating additive (which is not used in prior
art bath formulations). Copper pyrophosphate plating baths
typically contain 22-38 g/L copper ions (Cu.sup.2+), 150-250 g/L
pyrophosphate ions [(P.sub.2O.sub.7).sup.4-- ], and 1-3 g/L ammonia
(NH.sub.3). The optimum ratio of (P.sub.2O.sub.7).sup.4- to
Cu.sup.2+ is in the 7:1 to 8:1 range, which provides maximum bath
stability and good deposit properties. Ammonia serves as an
auxiliary complexing agent, which aids in copper anode dissolution.
Addition of 5-10 g/L nitrate ions (NO.sub.3.sup.-) may be
beneficial for improving the maximum usable current density.
Typically, copper ions are added as the copper pyrophosphate salt
(Cu.sub.2P.sub.2O.sub.7.3H.sub.2O), and nitrate and excess
pyrophosphate are added as the potassium, sodium or ammonium salts.
Ammonia may be added as the gas (NH.sub.3) or as ammonium hydroxide
(NH.sub.4OH). The bath pH is adjusted by addition of phosphoric
acid or the hydroxide of potassium, sodium or ammonia to maintain a
target value in the pH 8.0-8.8 range (typically around pH 8.3).
Best results are obtained by operating the bath at 50-60.degree. C.
(typically 55.degree. C.). Temperatures higher than 60.degree. C.
tend to cause rapid decomposition of pyrophosphate to
orthophosphate. The concentration of orthophosphate
[(HPO.sub.4).sup.2-] should be maintained at less than 110 g/L by
diluting or periodically dumping the bath.
[0045] Another feature of the present invention is the use of a
surfactant to improve wetting within small Damascene features.
Surfactants are usually employed in acidic plating baths for
depositing less noble metals (nickel, for example) to help dislodge
hydrogen gas bubbles formed as a side reaction during the metal
deposition process. Complexed copper plating baths typically
operate with substantially 100% current efficiency so that hydrogen
bubble formation is not an issue. Complexed copper baths of the
prior art have generally not employed surfactants or wetting
agents. For Damascene plating, however, a surfactant may be useful
to help dislodge air bubbles entrapped within small features when
the semiconductor wafer is introduced into the plating solution,
particularly in the face-down configuration. A significant concern
is that the surfactant might substantially interfere with the
functioning of the accelerating additive. We have discovered,
however, that a surfactant may be used in the electroplating bath
of the present invention, which employs an accelerating
additive.
[0046] FIG. 5 shows the effect of additions of DMTD monomer on the
CVS copper electrodeposition rate parameter (A.sub.r/A.sub.s)
measured at 1000 and 2500 rpm in a copper pyrophosphate bath
(55.degree. C.) containing 0.5 mg/L
polyoxyethylene(10)isooctylphenylether (sold commercially as
Triton.RTM. X-100) as a surfactant. As seen by comparing FIGS. 1
and 5, the behavior with and without the surfactant added is
substantially the same in the low additive concentration range
corresponding to increased copper deposition rate with increased
DMTD concentration (note scale difference for x-axis). This
indicates that a surfactant may be used in the complexed copper
bath of the present invention to improve wetting within Damascene
features. At higher DMTD concentrations, the surfactant produces a
relatively precipitous decrease in copper deposition rate,
apparently by augmenting the effect of the DMTD dimer suppressor
species. In the additive concentration range useful for the present
invention (<5 .mu.M), the electrode rotation rate (1000 or 2500
rpm) used for CVS measurements has little effect on A.sub.r/A.sub.s
vs. DMTD concentration plots.
[0047] A wide variety of surfactants could be used within the scope
of the present invention to improve wetting in small Damascene
features. Preferably, the surfactant is substantially stable
against cathodic decomposition, so that good deposit properties are
obtained and excessive amounts of detrimental contaminants are not
formed. The Triton.RTM. surfactants have been widely used as a
polarographic maximum suppressors and are known to be stable at
substantial cathodic potentials. Surfactants typically employed in
baths for electrodepositing nickel (and other less-noble metals)
that could be used in complexed copper baths include sodium lauryl
sulfate and sodium dodecyl sulfate.
[0048] The teachings of the prior art counsel against the approach
of the present invention. The fine-grained deposits provided by
complexed copper baths are reported to exhibit poor resistance to
electromigration [e.g., C. Ryu, K. Kwon, A. L. S. Loke, J. M.
Dubin, R. A. Kavari, G. W. Ray, and S. S. Wong, Symp. on VLSI Tech.
(Jun. 8-11, 1998)]. Early work [e.g., T. Nitta, J. Electrochem.
Soc. 140, 1131 (1993)] showed that acid sulfate copper deposits
have a higher activation energy for electromigration after
annealing, which increases the grain size. Acid sulfate copper
deposits are routinely annealed for Damascene applications, to
enhance electromigration resistance and to stabilize the properties
for more consistent CMP results. Other studies compared
large-grained acid sulfate copper deposits with vacuum-deposited or
chemical-vapor-deposited (CVD) materials, which are of only
moderate grain size, and found that the large-grained deposits have
higher resistance to electromigration.
[0049] Contrary to these teachings, the inventors believe that the
fine-grained deposits obtained from complexed copper baths will
prove to have good electromigration resistance. The poor
electromigration resistance observed for CVD copper materials of
moderate grain size may be attributed to micro-voids (indicated by
less than theoretical density). On the other hand, deposits from
strongly complexed baths (e.g., copper pyrophosphate) containing
organic additives typically approach theoretical density
(indicating minimum void volume), and are extremely fine-grained.
Furthermore, the high density of the "necks" which connect the
crystallites together in such deposits should actually inhibit
copper atom motion and impede electromigration. Our preliminary
tests indicate that the electromigration resistance of deposits
from copper pyrophosphate baths can be at least as high as deposits
from acid copper sulfate baths.
[0050] The complexed copper plating bath of the present invention
offers significant advantages in terms of both simplicity and
performance compared to the acid copper sulfate bath. A major
advantage is that the single accelerating additive species employed
in the complexed bath can readily be controlled to provide
bottom-up filling, whereas the acid copper sulfate system used for
Damascene plating in the prior art requires a complicated additive
system that is difficult to control. Additive-additive interactions
and the dynamic balance required between consumption and mass
transport limit the efficacy of acid copper sulfate baths for
Damascene plating of narrow features with high aspect ratios. The
effective concentration of the DMTD additive in the copper
pyrophosphate bath can be determined by CVS via a simple
A.sub.r/A.sub.s measurement [D. Tench and C. Ogden, J. Electrochem.
Soc. 125 (2), 194 (1978)]. This additive is present at low
concentrations and is not rapidly consumed in the copper
electrodeposition process so that organic breakdown products are
minimal, whereas breakdown products of the suppressor and
anti-suppressor additives must be frequently diluted in acid copper
sulfate baths to avoid degradation of the deposit quality. Another
advantage of complexed copper baths is that the additive can be a
known compound (e.g., DMTD) rather than a proprietary formulation,
which allows the user to exercise better control of the plating
process. Furthermore, attack of copper seed layers, which is a
problem for acid copper sulfate baths, is minimized for alkaline
complexed copper baths.
[0051] In addition, the low accelerating additive concentration
used and the inherently good throwing power of complexed copper
baths tend to provide a uniform "overburden" layer, which is more
easily removed by chemical mechanical planarization (CMP) and
obviates the need for the "leveler" additive employed in acid
copper sulfate baths. In addition, the fine-grained deposits
provided by complexed copper baths tend to have stable properties
that are relatively insensitive to the substrate, whereas annealing
is required to stabilize the properties of acid copper deposits,
which tend to depend strongly on the substrate. The fine-grained
deposits from complexed copper baths also tend to be about twice as
hard as those from acid copper sulfate baths so that dishing during
CMP processing is less of an issue. The resistivity of
pyrophosphate and annealed acid sulfate copper deposits are
equivalent.
[0052] The simple accelerating additives used in the complexed
copper bath of the present invention also tend to be less sensitive
to degradation by ac voltages and are therefore more amenable for
use with pulse plating or periodic reverse pulse plating to enhance
the deposit properties or overall plating rate. In contrast,
organic additives of the types typically employed in acid copper
sulfate baths tend to be consumed at a much faster rate under
alternating current (ac) conditions, making control of additive
concentrations and bath purity more difficult. This restricts the
options in terms of additive types and operating conditions for
pulse plating from acid sulfate systems.
[0053] Although the plating bath of the present invention is
discussed herein with respect to the Damascene process for making
copper integrated circuits on semiconductor chips, this bath should
also be useful for a wide variety of applications. For example, the
bath could be applied to fabrication of micro-electro-mechanical
systems (MEMS) devices, and especially those involving integration
with electronic devices. This bath is particularly useful for
bottom-up filling of small "blind" features. Any use of the bath of
this invention is explicitly claimed.
DESCRIPTION OF A PREFERRED EMBODIMENT
[0054] Damascene copper plating with the bath of the present
invention can be performed in a laboratory apparatus, or with a
commercial plating tool (may need to be modified for operation at
elevated temperature). Good results are obtained by rotating the
wafer during plating and/or providing a uniform laminar flow of
solution over the wafer surface via pumping the solution through a
nozzle system. The wafer holder should have a seal designed to
avoid solution contact with the electrical contact made to the
copper seed layer around the periphery of the wafer side to be
plated. The holder should be designed to have a low profile and/or
other means for avoiding disruption of laminar flow over the wafer
surface. Pumped solution from a nozzle is typically directed toward
the center of the rotating wafer, but could be directed toward
other wafer locations. Plating may also be performed from a
stagnant plating bath.
[0055] The preferred electroplating bath for practicing the
invention is a copper pyrophosphate bath containing 22-38 g/L
copper ions (predominantly Cu.sup.2+), 150-250 g/L pyrophosphate
ions [(P.sub.2O.sub.7).sup.4-], 1-3 g/L ammonia (NH.sub.3), 1-5
.mu.M 2,5-dimercapto-1,3,4-thiadiazole (DMTD), and optionally 5-10
g/L nitrate ions (NO.sub.3.sup.-). Copper ions are preferably added
as the copper pyrophosphate salt
(Cu.sub.2P.sub.2O.sub.7.3H.sub.2O), and nitrate and excess
pyrophosphate are preferably added as the potassium, sodium or
ammonium salts. Addition as ammonium pyrophosphate is preferred to
avoid the possibility of integrated circuit degradation by sodium
or potassium ions. Ammonia may be added as the gas (NH.sub.3) or as
ammonium hydroxide (NH.sub.4OH). The bath pH is preferably
maintained at about pH 8.3 by addition of phosphoric acid or the
hydroxide of potassium, sodium or ammonia. The bath temperature
should be maintained in the 50-60.degree. C. range, preferably near
55.degree. C. The concentration of orthophosphate
[(HPO.sub.4).sup.2-] should be maintained at less than 110 g/L by
diluting or periodically dumping the bath.
[0056] The efficacy of the invention for Damascene plating was
demonstrated by copper plating coupons (typically 2 cm square) cut
from an eight-inch-diameter silicon wafer coated with a metal
nitride barrier layer and a copper seed layer. The wafer contained
vias 0.13 .mu.m wide (5.4:1 aspect ratio), vias 0.2 .mu.m wide (4:1
aspect ratio), trenches 0.33 .mu.m wide (2:1 aspect ratio), and
trenches 0.38 .mu.m wide (0.7:1 aspect ratio). Copper was plated at
35 mA/cm.sup.2 to an average thickness of 1.0 .mu.m in a 600-mL
beaker containing a copper anode (2 cm square) and 500 mL of
plating solution maintained at 55.degree. C. on a hot plate. The
copper pyrophosphate plating solution contained 23.5 g/L copper ion
(Cu.sup.2+), 176.3 g/L pyrophosphate ion
[(P.sub.2O.sub.7).sup.4.sup.-], 2.1 g/L ammonia (NH.sub.3), 7.4 g/L
nitrate ion (NO.sub.3.sup.-), and various concentrations of
2,5-dimercapto-1,3,4-thiadiazole (DMTD). The solution pH was
adjusted to 8.2 by addition of phosphoric acid. In some cases, 0.50
g/L Triton.RTM.-X was added as a surfactant. During plating, the
coupon and the anode were held vertically near the sides of the
beaker (8 cm apart) and the solution was stirred with a
Teflon-coated magnetic stir bar (200 rpm).
[0057] Coupons were mechanically cleaved and cross-sections were
examined by scanning electron microscopy (SEM) in the as-cleaved
state. For each plating condition, 10 to 20 Damascene features of
each type were examined for bottom-up filling, except when the
cleave lines did not intersect a sufficient number of features of a
given type. In some cases, defects in the copper deposit resulting
from extraneous causes were observed. In particular, lack of
plating on a portion of some feature side walls apparently resulted
from poor wetting and/or high electrical resistance of the copper
seed layer due to oxidation. This was evident since a prior
electrochemical reduction in borate buffer solution (pH 8.3) was
shown to reduce the incidence of this type of defect. Such
oxidation is likely to have occurred since the wafers available at
the time had been stored in air for several months. Other sporadic
defects observed for copper deposits, especially within fine
features, most probably resulted from the poorly controlled beaker
plating process and the crude cross-sectioning procedure used.
Mechanical cleaving is known in the art to sometimes introduce
voids associated with "pull away" of a portion of the deposit.
Because of the extraneous factors affecting the quality of the
Damascene copper deposit, good bottom-up filling was judged to have
been attained when the majority of the features were filled with
copper exhibiting no voids or defect lines. Plating conditions
shown to provide good bottom-up filling should yield low defect
rates for a properly controlled production wafer plating process
involving fresh copper seed layers.
EXAMPLE 1
[0058] Coupons were plated from copper pyrophosphate solutions
(55.degree. C.) containing 0.0, 0.33, 0.67, 1.0, 1.3, 1.7, 2.0,
2.3, 3.3, 5.0 or 8.3 .mu.M of the DMTD additive (without surfactant
added). Good bottom-up filling of the smallest features (0.13 .mu.m
vias) was attained with the 2.0 .mu.M solution, as indicated by the
micrograph in FIG. 6. Good bottom-up filling of all of the larger
features (0.20, 0.33 and 0.38 .mu.m wide) was attained with the
1.3, 1.7 and 2.0 .mu.M solutions. Incomplete filling was observed
for all features with all of the other solutions, except that good
bottom-up filling was observed for the two larger features (0.33
and 0.37 .mu.m wide) with the 3.3 .mu.M solution. These data show
that the DMTD monomer additive of the present invention provides
good bottom-up filling of Damascene features as fine as 0.13 .mu.m
wide (5.4:1 aspect ratio). The optimum DMTD concentration was about
2 .mu.M for the conditions used but would be expected to depend on
the plating parameters, such as current density, solution
composition, and solution mass transport.
EXAMPLE 2
[0059] Coupons were plated from copper pyrophosphate solutions
(55.degree. C.) containing 0.50 g/L Triton.RTM.-X surfactant and
0.0, 0.33, 0.67, 1.0, 1.3, 1.7, 2.0, or 5.7 .mu.M of the DMTD
additive. As was the case without surfactant added, a DMTD
concentration of about 2 .mu.M provided good bottom up filling of
even the smallest Damascene features (0.13 .mu.m vias) in the
presence of the Triton.RTM.-X surfactant.
[0060] The preferred embodiments of this invention have been
illustrated and described above. Modifications and additional
embodiments, however, will undoubtedly be apparent to those skilled
in the art. Furthermore, equivalent elements may be substituted for
those illustrated and described herein, parts or connections might
be reversed or otherwise interchanged, and certain features of the
invention may be utilized independently of other features.
Consequently, the exemplary embodiments should be considered
illustrative, rather than inclusive, while the appended claims are
more indicative of the full scope of the invention.
* * * * *