U.S. patent application number 16/154243 was filed with the patent office on 2019-05-16 for low copper electroplating solutions for fill and defect control.
The applicant listed for this patent is Novellus Systems, Inc.. Invention is credited to Jonathan David Reid, Jian Zhou.
Application Number | 20190145017 16/154243 |
Document ID | / |
Family ID | 51221749 |
Filed Date | 2019-05-16 |
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United States Patent
Application |
20190145017 |
Kind Code |
A1 |
Zhou; Jian ; et al. |
May 16, 2019 |
LOW COPPER ELECTROPLATING SOLUTIONS FOR FILL AND DEFECT CONTROL
Abstract
Certain embodiments herein relate to a method of electroplating
copper into damascene features using a low copper concentration
electrolyte having less than about 10 g/L copper ions and about
2-15 g/L acid. Using the low copper electrolyte produces a
relatively high overpotential on the plating substrate surface,
allowing for a slow plating process with few fill defects. The low
copper electrolyte may have a relatively high cloud point.
Inventors: |
Zhou; Jian; (West Linn,
OR) ; Reid; Jonathan David; (Sherwood, OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Novellus Systems, Inc. |
Fremont |
CA |
US |
|
|
Family ID: |
51221749 |
Appl. No.: |
16/154243 |
Filed: |
October 8, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13753333 |
Jan 29, 2013 |
10214826 |
|
|
16154243 |
|
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Current U.S.
Class: |
205/123 ;
205/118; 205/136 |
Current CPC
Class: |
C25D 7/123 20130101;
C25D 3/38 20130101; C25D 5/10 20130101; C25D 5/48 20130101; H01L
21/2885 20130101; C25D 21/12 20130101; C25D 17/001 20130101; H01L
21/76877 20130101 |
International
Class: |
C25D 7/12 20060101
C25D007/12; H01L 21/288 20060101 H01L021/288; C25D 5/48 20060101
C25D005/48; C25D 3/38 20060101 C25D003/38; H01L 21/768 20060101
H01L021/768; C25D 5/10 20060101 C25D005/10 |
Claims
1. A method of plating copper into damascene features, comprising:
receiving a substrate having a seed thickness of about 200
nanometers, on average, or thinner; electrically biasing the
substrate; immersing the substrate in an aqueous low copper
acid-containing electrolyte comprising less than about 10 grams per
liter copper ions and at least one suppressor compound, whereby the
low copper electrolyte induces a cathodic overpotential on the seed
sufficient to protect the seed from dissolution by acid in the
electrolyte during immersion; electroplating copper into the
features at a current density of about 3 mA/cm.sup.2 or less; and
removing the substrate from the electrolyte.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of and claims priority to
U.S. patent application Ser. No. 13/753,333 naming Zhou et al. as
inventors, titled "LOW COPPER ELECTROPLATING SOLUTIONS FOR FILL AND
DEFECT CONTROL" filed Jan. 29, 2013, which is incorporated herein
by reference in its entirety and for all purposes.
FIELD OF THE INVENTION
[0002] The present disclosure relates generally to copper
electroplating of damascene interconnects, and more specifically,
to a low-copper, low acid electrolyte and a method for using the
electrolyte under conditions that enhance suppression of copper
plating to promote void-free fill of submicron damascene
features.
BACKGROUND
[0003] Electrolytes used in electroplating copper into damascene
interconnects typically contain a copper salt, an acid, halide
ions, an accelerator, a suppressor and a leveler. The copper salt
is the copper source for the deposition. Acid is generally used to
control the conductivity of the plating bath. Halide ions may act
as bridges to assist the adsorption of certain organic additives
(e.g., accelerator, suppressor and/or leveler) onto a substrate
surface to encourage a bottom-up fill mechanism, described
herein.
[0004] Example copper salts include, but are not limited to, copper
sulfate, copper methanesulfonate, copper pyrophosphate, copper
propanesulfonate, etc. As used herein, the concentration of copper
ions reflects the concentration (mass per volume) of copper
cations, and does not include the mass of any anions associated
with the copper cations. Example acids include, but are not limited
to, sulfuric acid and methanesulfonic acid. As used herein, the
concentration of acid reflects the concentration (mass per volume)
of the entire acid molecule, not the mass of hydrogen cations
alone. Example halide ions include, but are not limited to,
chloride, bromide, iodide, and combinations thereof.
[0005] It may be desirable to strongly polarize the substrate in
the cathodic direction, particularly during the initial stages of
electroplating onto a seed layer. Such polarization may protect the
seed layer from dissolution. One way to achieve such polarization
is by providing a strong "suppressor" in the electrolyte.
[0006] FIG. 2 shows a copper wafer 201 plated in an electrolyte
containing a strong suppressor. It is a high molecular weight
suppressor with high ratio of hydrophobic propylene oxide versus
hydrophilic ethylene oxide The cloud point of the electrolyte is
27.degree. C., and plating occurred at 21.degree. C. The plated
wafer contains visible streaks caused by non-uniform suppressor
adsorption on the wafer due to suppressor agglomeration. The use of
some electrolytes may result in more subtle forms of such defects
that are not detectable by visual inspection, but which are
detectable through common defect metrology such as the AIT, SP1, or
SP2 series of tools from KLA-Tencor of San Jose, Calif.
[0007] Another technique for increasing the polarization of a
substrate is to increase halide ion concentrations or change the
halide ion composition. This technique is further described in U.S.
Pat. No. 8,268,155, incorporated by reference herein. The halide
may affect the suppressing effect of a suppressor or other
additive. However, the increase in overpotential that may be gained
by changing halide ion concentration or composition is limited, and
may not sufficient to provide the conditions needed for a uniform,
reproducible fill of small 10-20 nm features. Further, the
concentration of halide ions should be relatively low in the
electrolyte in order to avoid incorporation of the halides into the
plated films or the formation of center voids due to insufficient
bottom-up fill by over-suppression of copper deposition.
[0008] While conventional electroplating solutions have worked well
for previous generations of damascene interconnect fabrication
processes, new challenges are emerging as smaller features and
thinner seed layers are used. Thus, it has recently been found that
there exists a need for a method of electroplating damascene
interconnects that better protects the seed layer from dissolution
and allows the deposition to occur over a longer timeframe to
achieve a more reproducible, more uniform fill of features.
SUMMARY
[0009] Certain embodiments herein relate to a method of
electroplating copper into damascene features using a low copper
concentration electrolyte having less than about 10 g/L copper ions
and about 2-15 g/L acid, and plating at a current density of about
3 mA/cm.sup.2 or less. Using the low copper electrolyte produces a
relatively high overpotential on the plating substrate surface,
allowing for a slow plating process with few fill defects.
Suppressor polymers in the low copper electrolyte may have a
relatively high cloud point.
[0010] In one aspect, the embodiments herein provide a method of
electroplating copper into damascene features, including receiving
a substrate with a seed thickness of about 200 nm or less, on
average; electrically biasing the substrate; immersing the
substrate in an aqueous low copper acid-containing electrolyte
having at least one suppressor compound and less than about 10 g/L
copper ions, such that the low copper electrolyte induces a
cathodic overpotential on the seed sufficient to protect the seed
from dissolution by acid in the electrolyte during immersion;
electroplating copper into the features at a current density of
about 3 mA/cm.sup.2 or less; and removing the substrate from the
electrolyte. In certain embodiments, the electrolyte may include
between about 2-15 g/L acid, or between about 5-10 g/L acid. The pH
of the electrolyte may be between about 0.2-2 in some
implementations. The electrolyte may also include between about
10-500 milligrams per liter active organic additives. In some
implementations, the active organic additives may include one or
more accelerator compound. The concentration of accelerator may be
less than about 20 milligrams per liter, or less than about 10
milligrams per liter. In certain cases, the active organic
additives include one or more leveler compound. Certain embodiments
use a suppressor compound that is a polymeric compound. In some
implementations, the electrolyte includes less than about 5 g/L
copper ions. Further, the electrolyte may include between about
10-150 milligrams per liter halide ions. In some implementations,
the substrate has at least some features with openings smaller than
about 20 nm. The substrate may be a 450 mm wafer in certain cases.
When immersing the substrate in certain embodiments, the substrate
may be immersed at an angle relative to the surface of the
electrolyte and then oriented horizontally. The electroplating
operation may include electroplating copper during a first plating
phase to fill the substrate features with copper at a first
deposition rate; and electroplating copper during a second plating
phase to deposit an overburden layer of copper on the substrate at
a second deposition rate that is higher than the first deposition
rate. In some embodiments, the first deposition rate is between
about 0.5-5 mA/cm.sup.2 (e.g., about 5 mA/cm.sup.2 or less, or
about 3 mA/cm.sup.2 or less). In certain cases the first deposition
rate is higher (e.g., about 10 mA/cm.sup.2 or less). The second
deposition rate may, in certain embodiments, be between about 10-15
mA/cm.sup.2. The method of electroplating may also include
performing a post-plating treatment on the substrate. In certain
implementations, the post-plating treatment includes rinsing and/or
planarizing the substrate.
[0011] In another aspect of the embodiments herein, a method is
provided for electroplating copper into damascene features,
including receiving a substrate with a seed thickness of about 200
nm or less, on average; electrically biasing the substrate;
immersing the substrate in an aqueous low copper acid-containing
electrolyte including at least one suppressor compound, at least
one accelerator compound, and less than about 10 g/L copper ions;
during immersion, electroplating copper into the features by a
bottom-up fill mechanism, where the low copper electrolyte permits
plating using a sufficiently low current density such that the time
of plating is adequate to allow the suppressor and accelerator to
adsorb onto the seed and thereby enable bottom-up fill during
immersion; and removing the substrate from the electrolyte. In
certain implementations, the current density during electroplating
is less than about 5 mA/cm.sup.2, or less than about 3 mA/cm.sup.2.
In some embodiments, electroplating copper into the features
includes electroplating copper during a first plating phase to fill
the features with copper at a first deposition rate, and
electroplating copper during a second plating phase to deposit an
overburden layer of copper on the substrate at a second deposition
rate that is higher than the first deposition rate. The electrolyte
may include between about 2-15 g/L acid. In certain embodiments,
the electrolyte includes less than about 5 g/L copper ions.
[0012] Another aspect of the disclosed implementations is an
electrolyte including between about 1-10 g/L copper cations;
between about 2-15 g/L acid; halide ions; one or more suppressor
compound; one or more accelerator compound; where the electrolyte
has a cloud point above 50.degree. C. In certain embodiments, the
acid is sulfuric acid. In other embodiments the acid may be
methanesulfonic acid. The halide ions may be chloride ions. In some
implementations, the copper cations are provided in a compound that
dissociates into cations and anions, and the anions associated with
the copper cations are the same species as anions formed from the
acid. In certain embodiments, the concentration of accelerator
compound in electrolyte is less than about 20 milligrams per liter,
or less than about 10 milligrams per liter.
[0013] These and other features will be described below with
reference to the associated drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows 100 nm trenches plated at constant currents
between 1 and 4 Amps, for a fixed amount of charge passed.
[0015] FIG. 2 shows a copper wafer that was plated at 21.degree. C.
in a plating bath containing a strong suppressor with a low cloud
point of 27.degree. C. Polymer agglomeration defects are
visible.
[0016] FIG. 3 shows galvanic polarization results collected on
copper coated platinum rotating disk electrodes in three copper
sulfate solutions, both with and without organic additives. The
graphs illustrate increases in polarization resulting from
decreases in copper ion concentration and acid concentration.
[0017] FIG. 4 is a graph demonstrating increased uniformity of fill
fraction (and therefore fill rate) between upstream and downstream
features on a substrate at low copper ion concentrations as opposed
to high copper ion concentrations in electrolyte.
[0018] FIG. 5 shows a correlation between AFM RMS roughness and SP2
haze signals of electroplated copper films.
[0019] FIG. 6 shows haze maps (left) and histograms (right) of 50
.ANG. films plated on 400 .ANG. seed layers in high copper (top)
and low copper (bottom) electrolytes.
[0020] FIG. 7 shows a flowchart of a method of electroplating
copper according to the embodiments herein.
[0021] FIG. 8 shows the fill of 48 nm trenches plated in three
electrolytes. The left panel corresponds to a film plated in a high
copper, high acid electrolyte, the middle panel corresponds to a
film plated in a low copper, high acid electrolyte, and the right
panel corresponds to a film plated in a low copper, low acid
electrolyte.
[0022] FIG. 9 shows a schematic of a top view of an example
electrodeposition apparatus.
[0023] FIG. 10 shows a schematic of a top view of an alternative
example electrodeposition apparatus.
DETAILED DESCRIPTION
[0024] In this application, the terms "semiconductor wafer,"
"wafer," "substrate," "wafer substrate," and "partially fabricated
integrated circuit" are used interchangeably. One of ordinary skill
in the art would understand that the term "partially fabricated
integrated circuit" can refer to a silicon wafer during any of many
stages of integrated circuit fabrication thereon. Further, the
terms "electrolyte," "plating bath," "bath," and "plating solution"
are used interchangeably. The term "low copper electrolyte" is
understood to mean "electrolyte having a low concentration of
copper ions." A low concentration of copper ions is understood to
mean a concentration of less than about 10 g/L copper ions. The
following detailed description assumes the invention is implemented
on a wafer. However, the invention is not so limited. The work
piece may be of various shapes, sizes, and materials. In addition
to semiconductor wafers, other work pieces that may take advantage
of this invention include various articles such as printed circuit
boards and the like.
[0025] In the following description, numerous specific details are
set forth in order to provide a thorough understanding of the
presented embodiments. The disclosed embodiments may be practiced
without some or all of these specific details. In other instances,
well-known process operations have not been described in detail to
not unnecessarily obscure the disclosed embodiments. While the
disclosed embodiments will be described in conjunction with the
specific embodiments, it will be understood that it is not intended
to limit the disclosed embodiments.
[0026] The embodiments herein utilize a low copper concentration
electrolyte to achieve a high overpotential plating environment.
For example, the copper concentration may be reduced by about 75%
to 90% of the nominal mass/volume concentration used in
conventional low acid copper plating baths, as compared to the
conventional baths. In certain applications, the copper ion
concentration is less than about 10 g/L or less than about 5 g/L.
Some embodiments also utilize a low acid concentration electrolyte
to further increase the overpotential. For example, the electrolyte
may be between about 2-15 g/L, between about 5-10 g/L, less than
about 10 g/L, or less than about 5 g/L acid. The increased
overpotential provides various advantages such as allowing plating
to occur at a slower rate, thereby making the deposition easier to
control and resulting in a more uniform, more easily reproducible
fill. By increasing the total amount of plating time, the influence
of initial plating non-uniformities that occur during immersion
become less important, and a more uniform fill results.
Typically, the copper electroplating solution includes organic bath
additives to permit controlled high quality electrofill of recesses
in a damascene substrate. Such additives typically include a
suppressor and an accelerator and possibly a leveler. One role of
the suppressor is to suppress electroplating and increase the
surface polarization of the plating substrate. Before further
describing the use of low copper concentration electroplating
solutions, a discussion of plating additives is presented.
[0027] Suppressors
[0028] While not wishing to be bound to any theory or mechanism of
action, it is believed that suppressors (either alone or in
combination with other bath additives) are surface-kinetic
polarizing compounds that lead to a significant increase in the
voltage drop across the substrate-electrolyte interface, especially
when present in combination with a surface chemisorbing halide
(e.g., chloride or bromide). The halide may act as a bridge between
the suppressor molecules and the wafer surface. The suppressor both
(1) increases the local polarization of the substrate surface at
regions where the suppressor is present relative to regions where
the suppressor is absent, and (2) increases the polarization of the
substrate surface generally. The increased polarization (local
and/or general) corresponds to increased resistivity/impedance and
therefore slower plating at a particular applied potential.
[0029] It is believed that suppressors are not incorporated into
the deposited film, though they may slowly degrade over time.
Suppressors are often relatively large molecules, and in many
instances they are polymeric in nature (e.g., polyethylene oxide,
polypropylene oxide, polyethylene glycol, polypropylene glycol,
etc). Other examples of suppressors include polyethylene and
polypropylene oxides with S- and/or N-containing functional groups,
block polymers of polyethylene oxide and polypropylene oxides, etc.
The suppressors can have linear chain structures or branch
structures. It is common that suppressor molecules with various
molecular weights co-exist in a commercial suppressor solution. Due
in part to suppressors' large size, the diffusion of these
compounds into a recessed feature is relatively slow.
[0030] Accelerators
[0031] While not wishing to be bound by any theory or mechanism of
action, it is believed that accelerators (either alone or in
combination with other bath additives) tend to locally reduce the
polarization effect associated with the presence of suppressors,
and thereby locally increase the electrodeposition rate. The
reduced polarization effect is most pronounced in regions where the
adsorbed accelerator is most concentrated (i.e., the polarization
is reduced as a function of the local surface concentration of
adsorbed accelerator). Example accelerators include, but are not
limited to, dimercaptopropane sulfonic acid, dimercaptoethane
sulfonic acid, mercaptopropane sulfonic acid, mercaptoethane
sulfonic acid, bis-(3-sulfopropyl) disulfide (SPS), and their
derivatives. Although the accelerator may become strongly adsorbed
to the substrate surface and generally laterally-surface immobile
as a result of the plating reactions, the accelerator is generally
not incorporated into the film. Thus, the accelerator remains on
the surface as metal is deposited. As a recess is filled, the local
accelerator concentration increases on the surface within the
recess. Accelerators tend to be smaller molecules and exhibit
faster diffusion into recessed features, as compared to
suppressors.
[0032] Levelers
[0033] While not wishing to be bound by any theory or mechanism of
action, it is believed that levelers (either alone or in
combination with other bath additives) act as suppressing agents to
counteract the depolarization effect associated with accelerators,
especially in the field region and at the side walls of a feature.
The leveler may locally increase the polarization/surface
resistance of the substrate, thereby slowing the local
electrodeposition reaction in regions where the leveler is present.
The local concentration of levelers is determined to some degree by
mass transport. Therefore levelers act principally on surface
structures having geometries that protrude away from the surface.
This action "smooths" the surface of the electrodeposited layer. It
is believed that leveler reacts or is consumed at the substrate
surface at a rate that is at or near a diffusion limited rate, and
therefore, a continuous supply of leveler is often beneficial in
maintaining uniform plating conditions over time.
[0034] Leveler compounds are generally classified as levelers based
on their electrochemical function and impact and do not require
specific chemical structure or formulation. However, levelers often
contain one or more nitrogen, amine, imide or imidazole, and may
also contain sulfur functional groups. Certain levelers include one
or more five and six member rings and/or conjugated organic
compound derivatives. Nitrogen groups may form part of the ring
structure. In amine-containing levelers, the amines may be primary,
secondary or tertiary alkyl amines. Furthermore, the amine may be
an aryl amine or a heterocyclic amine. Example amines include, but
are not limited to, dialkylamines, trialkylamines, arylalkylamines,
triazoles, imidazole, triazole, tetrazole, benzimidazole,
benzotriazole, piperidine, morpholines, piperazine, pyridine,
oxazole, benzoxazole, pyrimidine, quonoline, and isoquinoline.
Imidazole and pyridine may be especially useful. Leveler compounds
may also include ethoxide groups. For example, the leveler may
include a general backbone similar to that found in polyethylene
glycol or polyethyelene oxide, with fragments of amine functionally
inserted over the chain (e.g., Janus Green B). Example epoxides
include, but are not limited to, epihalohydrins such as
epichlorohydrin and epibromohydrin, and polyepoxide compounds.
Polyepoxide compounds having two or more epoxide moieties joined
together by an ether-containing linkage may be especially useful.
Some leveler compounds are polymeric, while others are not. Example
polymeric leveler compounds include, but are not limited to,
polyethylenimine, polyamidoamines, and reaction products of an
amine with various oxygen epoxides or sulfides. One example of a
non-polymeric leveler is 6-mercapto-hexanol. Another example
leveler is polyvinylpyrrolidone (PVP).
[0035] Bottom-Up Fill
[0036] In the bottom-up fill mechanism, a recessed feature on a
plating surface tends to be plated with metal from the bottom to
the top of the feature, and inward from the side walls towards the
center of the feature. It is important to control the deposition
rate within the feature and in the field region in order to achieve
uniform filling and avoid incorporating voids into the features.
The three types of additives described above are beneficial in
accomplishing bottom-up fill, each working to selectively increase
or decrease the polarization at the substrate surface.
[0037] After the substrate is immersed in electrolyte, the
suppressor adsorbs onto the surface of the substrate, especially in
exposed regions such as the field region. At the initial plating
stages, there is a substantial differential in suppressor
concentration between the top and bottom of a recessed feature.
This differential is present due to the relatively large size of
the suppressor molecule and its correspondingly slow transport
properties. Over this same initial plating time, it is believed
that accelerator accumulates at a low, substantially uniform
concentration over the entire plating surface, including the bottom
and side walls of the feature. Because the accelerator diffuses
into features more rapidly than the suppressor, the initial ratio
of accelerator:suppressor within the feature (especially at the
feature bottom) is relatively high. The relatively high initial
accelerator:suppressor ratio within the feature promotes rapid
plating from the bottom of the feature upwards and from the
sidewalls inwards. Meanwhile, the initial plating rate in the field
region is relatively low due to the lower ratio of
accelerator:suppressor. Thus, in the initial plating stages,
plating occurs relatively faster within the feature and relatively
slower in the field region.
[0038] As plating continues, the feature fills with metal and the
surface area within the feature is reduced. Because of the
decreasing surface area and the accelerator substantially remaining
on the surface, the local surface concentration of accelerator
within the feature increases as plating continues. This increased
accelerator concentration within the feature helps maintain the
differential plating rate beneficial for bottom-up fill.
[0039] In the later stages of plating, particularly as overburden
deposits, the accelerator may build up in certain regions (e.g.,
above filled features) undesirably, resulting in local
faster-than-desired plating. Leveler may be used to counteract this
effect. The surface concentration of leveler is greatest at exposed
regions of a surface (i.e., not within recessed features) and where
convection is greatest. It is believed that the leveler displaces
accelerator, increases the local polarization and decreases the
local plating rate at regions of the surface that would otherwise
be plating at a rate greater than at other locations on the
deposit. In other words, the leveler tends, at least in part, to
reduce or remove the influence of an accelerating compound at the
exposed regions of a surface, particularly at protruding
structures. Without leveler, a feature may tend to overfill and
produce a bump. Therefore, in the later stages of bottom-up fill
plating, levelers are beneficial in producing a relatively flat
deposit.
[0040] The use of suppressor, accelerator and leveler, in
combination, may allow a feature to be filled without voids from
the bottom-up and from the sidewalls-inward, while producing a
relatively flat deposited surface. The exact identity/composition
of the additive compounds are typically maintained as trade secrets
by the additive suppliers, thus, information about the exact nature
of these compounds is not publicly available.
[0041] Plating Techniques and Process Considerations
[0042] Typical electroplating baths for filling damascene
interconnects generally contain relatively high concentrations of
copper cations (e.g., 40 g/L). High copper concentrations were
understood to be beneficial because higher copper concentrations
result in higher limiting currents that may be used during plating.
Higher currents were seen as beneficial because they increased the
rate of electrodeposition and thereby decreased processing time.
Moreover, if the concentration of copper is too low, the
electrolyte may experience copper depletion, resulting in
significant fill defects. When the mass transfer of copper to the
plating surface is too low at a given current (e.g., when the
concentration of copper is too low or when the electrolyte is
insufficiently turbulent), there is insufficient copper at the
plating surface to sustain the reduction reaction. Instead, a
parasitic reaction must occur to sustain the current delivered to
the substrate. For example, the electrolyte itself may begin to
decompose and generate gasses at the plating interface, resulting
in significantly non-uniform plating and nodular growths on the
substrate.
[0043] When plating with these high copper electrolytes, small
features tend to fill fairly quickly (e.g., within 1 to 2 seconds).
Because the timeframe for filling the small features is so short,
the resulting fills often contain defects or are otherwise
non-uniform. For example, because of the nature of the physical
vapor deposition (PVD) processes used to deposit a copper seed
layer, the seed thickness is not uniformly distributed, especially
in areas along the sidewall of a feature. In some areas the seed
layer may be so thin that the seed becomes discontinuous, which may
lead to the formation of sidewall voids during subsequent rapid
deposition.
[0044] As indicated, the immersion time is frequently a significant
fraction of the total feature fill time in high copper
electrolytes. Therefore, non-uniformities may be introduced as a
consequence of the time difference between when the leading edge of
a wafer contacts the electrolyte and when the trailing edge of the
wafer contacts the electrolyte. Wafers are frequently immersed at
an angle that deviates from horizontal in order to minimize bubble
creation. In order to reduce non-uniformities introduced by the
time it takes for the wafer to be fully immersed, it would be
desirable to reduce the plating rate in recessed features,
particularly during immersion. Use of a low copper electrolyte
increases the polarization of the seed layer, thereby protecting it
from dissolution without requiring a strong electrical bias. As a
consequence, the low copper electrolyte permits seed layer
protection without rapid electroplating. Non-uniformities are
reduced.
[0045] Further, short feature filling timeframes required with high
copper electrolytes may result in poor fill performance because the
short timeframes are incompatible with the bottom-up fill mechanism
during immersion, which requires some time for the organic
additives to reach and adsorb onto the substrate surface at the
relevant locations described above. As a particular example, when
the accelerator compound(s) do not have time to reach and adsorb
onto the bottom surface of a feature before substantial deposition
begins, the bottom-up fill mechanism will be impaired and the
feature fill will likely be poor. One method of addressing this
concern is to use a high concentration of accelerator in the
electrolyte. However, high accelerator concentrations may be
undesirable in particular applications, and it is generally
advantageous to use low concentrations of these additives in order
to lower cost. The embodiments herein address the bottom-up fill
timeframe issue by plating at a low copper ion concentration and
consequently at a low current density, achieving a slower filling
timeframe, especially for small features. The longer filling
timeframe permits the organic additives sufficient time to diffuse
or otherwise travel to their proper locations, adsorb onto the
substrate surface, and promote bottom-up fill during immersion and
before substantial deposition occurs. In certain implementations,
the concentration of accelerator is less than about 20 milligrams
per liter, or less than 10 milligrams per liter.
[0046] In order to increase the timeframe over which plating
occurs, a lower current density is desired during plating. Lower
current densities may also exhibit higher bottom-up fill
efficiency. FIG. 1 shows the partial fill of 100 nm trenches with a
400 .ANG. seed layer plated at constant currents between 1 and 4
Amps, for a fixed amount of charge passed. The trenches plated at
the lower current are more filled because of increased bottom-up
fill efficiency. However, lower current densities result in faster
dissolution of the copper seed layer, leading to sidewall voids.
The minimum current density required for a void-free fill in
conventional electrolytes is still too high to allow good process
control (i.e., plating occurs too quickly to achieve a
reproducible, defect-free fill).
[0047] Typical electroplating baths and methods often cause the
copper seed layer to dissolve/corrode to a certain degree when the
substrate is first immersed in electrolyte. This dissolution may
result in voids or other defects/non-uniformities in the deposit.
The seed dissolution issue is especially problematic for thin seed
layers, as thin layers are more easily dissolved than thicker
layers. To achieve higher performance integrated circuits, many of
the features of the integrated circuits are being fabricated with
smaller feature sizes and higher densities of components. As the
industry moves towards smaller features, the seed layers must
become thinner and thinner to avoid "pinching off" the top of the
feature before the bottom of the feature can be properly filled. In
some damascene processing, for example, copper seed layers on 2X-nm
node features may be as thin as or thinner than 50 .ANG.. Technical
challenges arise with smaller feature sizes in producing metal seed
layers and metal interconnects substantially free of voids or
defects.
[0048] One technique for protecting a copper seed layer is to
increase the overpotential of the substrate-electrolyte interface.
The overpotential may be increased in a variety of ways.
[0049] For example, using a stronger suppressor or a higher
concentration of suppressor will result in a higher overpotential.
Unfortunately, suppressors tend to form visible agglomerations
above a threshold temperature, referred to as the cloud point.
These agglomerations result in significant deposition defects
because they can adhere to the surface of the substrate. Although
the cloud point is the temperature at which the agglomerations
become visible, it is believed that agglomerations begin forming at
temperatures lower than the cloud point. Thus, the deposition
process should be run at a temperature that is well below (e.g.,
20.degree. C. or more below) the cloud point of a particular
electrolyte. Stronger suppressors tend to have very low cloud
points (e.g., 27.degree. C. for a proprietary suppressor that has
more than 20 mV higher overpotential as compared to existing
"moderate strength" commercial suppressors). Moreover, higher
concentrations of suppressor result in lower cloud points (i.e.,
the more suppressor present in solution, the easier it is to form
agglomerations at lower temperatures). Thus, although electrolytes
with strong suppressors or high concentrations of weak to moderate
suppressors may exhibit good fill performance, they cannot
generally be used in conventional electroplating methods for small
10-20 nm features because they reduce the electrolyte's cloud level
to an unacceptably low temperature.
[0050] While suppressors may be referred to as "weak" or "strong,"
it should be understood that these terms are relative and may vary
over time. As the industry develops, stronger suppressors are
created, and suppressors that used to be seen as "strong" may now
be considered "moderate" or even "weak." Currently, the Excel
suppressor available from Enthone may be considered a strong
suppressor, while the Extreme Plus suppressor, also available from
Enthone, may be considered a moderate suppressor.
[0051] FIG. 3 shows the overpotential of three electrolytes and
demonstrates that the overpotential may be increased by using
electrolytes with low copper and low acid concentrations. In the
upper panel, the electrolytes do not include organic additives,
while in the lower panel the organic additives (6 mL/L Excel
accelerator and 4 ml/L Excel suppressor) are present. The
overpotential is measured by galvanostatic polarization between a
copper coated platinum rotating disk electrode (RDE) and an
Hg/HgSO.sub.4 reference electrode. A thin layer of 0.66 .mu.m
copper is plated on a platinum electrode in an organic
additive-free electrolyte. The electrode is then immersed in the
bath to be tested with a copper sheet as the counter electrode. The
voltage between the copper coated platinum RDE and the reference
electrode is monitored while the electrode is rotated at 100 RPM at
a current density of 10 mA/cm.sup.2. The overpotential is expressed
relative to the open-circuit voltage, which is the voltage between
the copper coated platinum RDE and the Hg/HgSO.sub.4 reference
electrode when no current is passed.
[0052] The three tested electrolytes are all copper sulfate
solutions. The first solution is a baseline solution of 40 g/L
Cu.sup.2+, 10 g/L acid, and 50 ppm Cl.sup.-. The second solution is
a low copper/high acid solution of 10 g/L Cu.sup.2+, 10 g/L acid,
and 50 ppm Cl.sup.-. The third solution is a low copper/low acid
solution of 10 g/L Cu.sup.2+, 5 g/L acid, and 50 ppm Cl.sup.-. As
used herein, "low" and "high" concentrations are compared relative
to one another. In other words, although 10 g/L acid is referred to
as a "high acid" solution, it should be understood that 10 g/L may
not be a high acid solution in absolute terms, but rather, is high
compared to the 5 g/L acid solution.
[0053] Where there were no organic additives present (FIG. 3, upper
panel), the use of the low copper/high acid solution increased the
overpotential by about 30 mV, and the use of the low copper/low
acid solution further increased the overpotential by about another
20 mV. Where the organic additives were present (FIG. 3, lower
panel), the use of the low copper/high acid solution increased the
overpotential by about 50 mV, and the use of the low copper/low
acid solution further increased the overpotential by about another
50 mV. Thus, the use of a low copper, low acid electrolyte may
increase the overpotential by about 100 mV when additives are
present, as compared to conventional electroplating solutions. This
increase in overpotential is larger than the increase typically
achieved by using stronger suppressor formulations or changing the
suppressor concentration in a given electrolyte (e.g., a typical
increase of about 10 to 20 mV). Advantageously, the low copper
electrolyte does not increase film impurities that typically occur
when strong halide ions (e.g., bromide ions) or high concentrations
of weaker halide ions are used.
[0054] Because the low copper concentration electrolytes exhibit
higher overpotential (stronger polarization) at a given current
density, the copper seed dissolution is also slower at a given
current density. Due to the lower seed dissolution rate, lower
current densities may be used without resulting in sidewall voids.
The lower current densities allow the plating to occur more slowly,
resulting in better fills with fewer defects in a more reproducible
process.
[0055] The low copper, higher overpotential electrolyte presents
several significant plating benefits explored in more detail
herein. First, the higher overpotential provides better seed
protection at a given current density. Better seed protection is
especially beneficial for small features (e.g., 10-20 nm nodes)
with thin seeds (e.g., thinner than about 100 .ANG.) which may
otherwise dissolve during plating, resulting in fill defects.
[0056] Next, higher overpotential promotes higher nucleation
density on the copper seed. The high nucleation density is
especially beneficial for thin seeds, especially those that may
have marginal/discontinuous seed coverage. As mentioned above, thin
seeds may have discontinuous coverage on a substrate surface due to
the nature of the PVD process used to deposit the seed layer. The
discontinuities in seed coverage tend to occur on the sidewalls of
features. However, the combination of improved seed protection and
high nucleation density enables plating to occur in small features
with thin seed without the formation of sidewall voids, or at a
minimum, with fewer/smaller voids than occur with conventional
methods. Moreover, the high overpotential at low copper
concentrations promotes high nucleation density at lower current
densities than what is typically used when plating in conventional
electrolytes. The high nucleation density promotes the formation of
a continuous copper film instead of a discontinuous copper film
having islands of copper.
[0057] Another advantage to using a low copper electrolyte is that
it can promote uniform fill across a pattern of dense features.
When many features are positioned near one another on a wafer,
conventional electrolytes often result in non-uniform fills between
the features. As electrolyte flows over the surface of a substrate,
organic additives present in the electrolyte adsorb onto the
surface, both within the features and in the field region. Features
which are relatively upstream in the electrolyte flow path may
therefore experience higher concentrations of additives, while
features which are relatively downstream may experience lower
concentrations of such additives.
[0058] The activity of the suppressor maxes out at a certain
threshold suppressor concentration, referred to herein as the
saturation point or suppressor saturation point. At concentrations
above the saturation point, additional suppressor has very little
or no effect on the overpotential at a fixed current density. It is
desirable for the electrolyte to be near or above the suppressor
saturation point during the initial stage of plating at all
locations on the substrate surface. Thus, the suppressor
concentration differences caused by flow direction (upstream vs.
downstream) and/or pattern density have little or no impact on
overpotential or copper deposition rate. If one location on a
substrate (e.g., an upstream location) experiences a suppressor
concentration above the saturation point, and another location on
the substrate (e.g., a downstream location) experiences a
suppressor concentration below the saturation point (the lower
suppressor concentration being a result of adsorbing suppressor
onto upstream locations more rapidly on the substrate surface), the
difference in additive concentrations/adsorption rates may result
in non-uniform fill between the upstream and downstream features.
This same phenomenon also results in fill differences between
features which are located in areas of dense features vs. features
which are more isolated. The suppressor saturation point increases
with increasing copper ion concentration in the electrolyte. Thus,
in solutions with lower copper ion concentrations, the suppressor
saturation point is lower, and it is easier for the suppressor
concentration to remain above the saturation point at all locations
during the initial stage of plating.
[0059] FIG. 4 shows the fill fraction of 20 nm technology nodes
(i.e., 2X-nm features) at three locations on a wafer (upstream edge
(LL, 402), center (C, 404), and downstream edge (UR, 406)) when
plated at three different copper concentrations (20, 30 and 40 g/L
copper ions). The other components of the electrolyte and the
additive concentrations were the same for all three electrolytes.
The fill fraction represents the fraction of the feature that was
filled with copper during deposition. The fill fraction may be
correlated to the fill rate. The features plated in electrolyte
having 40 g/L copper ions show the widest variation in fill
fraction. The uniformity of fill fraction increases slightly for
the features plated in electrolyte having 30 g/L copper ions, and
the uniformity is best for the features plated in electrolyte
having the lowest copper ion concentration at 20 g/L. At the low
copper condition, the fill fraction at the upstream edge was 85% of
that seen at the downstream edge, which is significantly more
uniform than at the 40 g/L copper ion condition where the fill
fraction at the upstream edge was only about 50% of that seen at
the downstream edge. For each set of features, there is a general
trend towards higher fill fraction at the downstream edge (406) as
compared to the upstream edge (402). This may be a result of
different surface concentrations of adsorbed additives.
[0060] Furthermore, low copper electrolytes are beneficial because
they increase the cloud point of the suppressor in the electrolyte.
As discussed above, suppressors form agglomerations above certain
temperatures. The temperature at which the agglomerations become
visible is referred to as the cloud point, though it is believed
that such agglomerations begin to form at temperatures lower than
the cloud point. The agglomerations result in significant plating
defects, and therefore, plating should occur at a temperature well
below (e.g., 20.degree. C. or more below) the cloud point of the
electrolyte. The increase in cloud point resulting from the use of
a low copper electrolyte is substantial. For example, the cloud
point of a 2% (vol/vol) suppressor solution may be increased by
about 25.degree. C. when the copper concentration is reduced from
40 to 5 g/L in copper sulfate plating baths. Therefore,
agglomeration defects can be significantly reduced or eliminated
through the use of a low copper electrolyte. The increased cloud
point also makes it possible to use strong suppressors (or higher
suppressor concentrations) that could not be used in higher copper
electrolytes due to their low cloud point. Furthermore, by
increasing the cloud point of the suppressor/electrolyte, a wider
range of process temperatures may be used for plating.
Specifically, plating may occur at higher temperatures than was
otherwise acceptable in conventional plating solutions.
[0061] Table 1 lists the cloud point of solutions of 2% (vol/vol)
commercial suppressor (Extreme Plus from Enthone) in two
electrolytes. The cloud point of the conventional high copper, high
acid electrolyte is 35.degree. C. The cloud point of the low
copper, low acid electrolyte is 25.degree. C. higher at 60.degree.
C. It is believed that all suppressors will exhibit similarly
increased cloud points in electrolytes with low copper ion
concentrations.
TABLE-US-00001 TABLE 1 Electrolyte Composition Cloud Point
(.degree. C.) 40 g/L Cu.sup.2+, 10 g/L acid, 50 ppm Cl.sup.- 35 5
g/L Cu.sup.2+, 5 g/L acid, 50 ppm Cl.sup.- 60
[0062] An additional benefit to using low copper, low acid
electrolytes is that the impact from the terminal effect is
reduced. When plating begins, there is a significant potential drop
between the edges of a wafer (where the wafer is connected to the
power supply) and its center. The difference in potential results
in faster plating at the edges and slower plating at the center of
the substrate during the initial plating stage. The resulting film
is generally edge-thick and center-thin, meaning that there may be
significant plating non-uniformities between the different areas on
a wafer. However, the impact of the terminal effect is reduced when
a low copper, low acid electrolyte is used due to the higher
impedance/lower conductivity of the electrolyte. Because of the
lower electrolyte conductivity (due at least in part to the lower
acid concentration), the voltage drop between the wafer edge and
center due to resistive seed becomes less significant. The terminal
effect is an important consideration when designing electroplating
systems, and will be even more important as the industry
transitions from 300 to 450 mm wafers. Further, the terminal effect
is increasingly important as thinner seed layers are used to
accommodate smaller features because the thinner seed layers
exhibit higher sheet resistance.
[0063] Method of Electroplating with Low Copper Electrolyte
[0064] The electrolyte used in embodiments herein may contain
copper ions, acid, water, halide ions, and organic additives such
as suppressors, accelerators and levelers. The composition of the
electrolyte is described in more detail below. The temperature of
the electrolyte during deposition may be between about
25-40.degree. C., for example 30.degree. C., to achieve a
reasonable deposition rate and temperature control.
[0065] In some embodiments, before a wafer enters the electrolyte,
a constant potential is applied to the wafer in order to prevent
seed dissolution or corrosion when the wafer first enters the
electrolyte. The constant potential entry is further described in
U.S. Pat. No. 6,551,483, issued Apr. 22, 2003, and incorporated by
reference herein. In other embodiments, the potential is dynamic
during the initial immersion period. Generally, a high potential is
desired during entry in order to provide better seed protection,
especially where thin copper seed layers are used. During the entry
phase, the copper plating is conformal (i.e., the copper deposition
rate is the same at all locations such as the field, sidewalls and
bottom of features). In conventional methods, where the entry
potential is too high, or where the potential is applied for too
long, excessive conformal plating inside the features may lead to
seam voids. However, low copper electrolytes allow for high
potential entry without causing excessive conformal plating inside
the substrate's features because a lower current density may be
used. The minimum current density required to support plating in a
particular electrolyte at a given mass transfer rate is
proportional to the copper concentration of bulk solution. The high
constant potential entry voltage followed by plating at a low
current density in low copper electrolyte provides better seed
protection, fill efficiency and uniformity, and process
consistency.
[0066] Potentiostatic wafer entry may lead to non-uniform film
roughness across the wafer when used with conventional
electrolytes. When a wafer enters the plating solution at a fixed
potential, the leading edge of the wafer often experiences a high
initial current density compared to the rest of the wafer.
Moreover, the leading edge is "wetted" first, meaning that it
adsorbs the organic additives before the rest of the wafer. This
differential in current density and/or additive adsorption time may
cause non-uniform film roughness across the wafer. Film roughness
may be analyzed through correlation to haze signals (e.g., haze
signals collected on a KLA-Tencor metrology tool such as SP2),
further discussed in U.S. Pat. No. 7,286,218, issued Oct. 23, 2007,
and incorporated by reference herein.
[0067] FIG. 5 shows a correlation curve between atomic force
microscopy (AFM) root mean square (RMS) roughness and haze signals
of electroplated films on 400 .ANG. seed layer. The plated film
thicknesses range from 5 to 120 nm. The plating baths include
different additive packages from two different chemical vendors.
The electrolyte used in plating had a copper ion concentration of
40 g/L, and it is believed that the copper ion concentration does
not affect the correlation between AFM RMS roughness and haze
signals. As used in this analysis, higher haze values correspond to
rougher films.
[0068] FIG. 6 shows the haze maps and histograms of two 50 .ANG.
films plated on 400 .ANG. copper seed layers deposited through PVD.
The upper panels of FIG. 5 correspond to a film plated in a
conventional high copper electrolyte comprising 40 g/L Cu.sup.2+,
10 g/L acid, 50 ppm Cl.sup.-, 12 ml/L accelerator, 4 ml/L
suppressor and 4 ml/L leveler. The lower panels of FIG. 5
correspond to a film plated in a low copper electrolyte comprising
5 g/L Cu.sup.2+, 10 g/L acid, 50 ppm Cl.sup.-, 3 ml/L accelerator,
2 ml/L suppressor and 1 ml/L leveler. It should be noted that the
different additive concentrations used herein may have some impact
on haze values or uniformity. The additive concentrations are
generally optimized for a particular desired fill, and such
optimized additive concentrations are different for different
electrolytes. Thus, the comparison shown here may include effects
from the interaction among copper cations and electrolyte additives
as well. The entry speed was 200 mm/s (in the z-direction, normal
to the surface of the wafer), at a rotation speed of 150 RPM. The
potentiostatic entry voltage was 0.5 V, and after a trigger delay
time of 0.1 s, a current of 1 A was applied. Notably, the film
plated in the low copper electrolyte has a tighter distribution of
haze signals, and the haze signals max out at a lower value as
compared to the film plated in the high copper electrolyte. This
means that the low copper electrolyte resulted in a smoother, more
uniform film. Based on the correlation curve in FIG. 5, the film
roughness of the films plated in the low copper and high copper
electrolytes are about 2-3 nm and 2-5 nm, respectively. While not
wishing to be bound by any theory, it is believed that the
improvement in film roughness uniformity may be caused by lower
current density or deposition rate at a given potential during
initial wafer entry.
[0069] In many embodiments herein, after the initial immersion
period, a substrate is plated at a relatively low constant current
or current density as the features are filled. For example, the
substrate may be plated at a current density between about 0.5-5
mA/cm.sup.2 during this fill stage. In some cases the current
density during this fill stage is about 5 mA/cm.sup.2 or less, or
about 3 mA/cm.sup.2 or less. In certain implementations the current
density during the fill stage is higher, for example, about 10
mA/cm.sup.2 or less. A high mass transfer rate is beneficial during
the fill stage in order to avoid copper depletion inside the
features, especially at relatively higher current densities. Higher
mass transfer rates may be achieved by increasing the flow rate of
the electrolyte, as well as by increasing the rotation speed of the
electrode, which increases convection to the substrate surface. The
mass transfer rate should generally be sufficiently high such that
the electrodeposition reaction is kinetically controlled, not mass
transfer controlled. In certain embodiments, the current or current
density may be dynamic over time. Many suitable electroplating
processes and systems may be used to implement the embodiments
described herein. Examples of such processes and systems are
described in the following U.S. patents and U.S. patent
applications, each incorporated herein by reference in its
entirety: U.S. Pat. Nos. 6,333,275 and 8,308,931, and U.S. Patent
Application No. 61/315,679, filed Mar. 19, 2010 and titled
"Electrolyte Loop with Pressure Regulation for Separated Anode
Chamber of Electroplating System."
[0070] After plating is complete, the substrate may be removed from
the electrolyte, rinsed, dried and processed for further use.
[0071] FIG. 7 shows an example of a method of electroplating copper
700 according to the embodiments described herein. The plating
process may be divided into the following basic stages: immersion,
fill, overburden and removal. In the immersion stage beginning at
block 701, a negative bias is applied to the wafer. At block 703,
the biased wafer is immersed into a low copper concentration
electrolyte at an angled orientation. The wafer enters the plating
bath at a high constant potential between about 1-2V, depending on
seed thickness. A higher potential may be applied when the seed is
thinner or when the sheet resistance of the seed is higher. The
wafer enters the plating bath at an angle (e.g., 2-4 degrees) with
respect to the surface of the plating solution in order to avoid
trapping air bubbles on the wafer surface. The entry angle may be
fixed, or it may be dynamic as the wafer becomes more fully
immersed. The vertical speed of the wafer during immersion may be
between about 5-300 mm/s, between about 5-200 mm/sec, or between
about 100-300 mm/sec in certain implementations. The vertical speed
should be relatively fast in order to achieve uniform initial
plating across the wafer, but should not be so fast as to cause
defect issues due to splashing. The rotation speed of the electrode
may be optimized such that there is (1) a low limiting current at
high entry voltage during potentiostatic entry, and (2) a uniform
initial plating rate across the wafer. A low rotation speed is
generally beneficial where there is a low limiting current, while a
high rotation speed is generally beneficial to promote uniform
initial plating across the wafer. Thus, the entry rotation speed of
the electrode should be set at an intermediate level, for example,
between about 30-120 RPM. The flow rate of plating solution during
the initial immersion stage may be relatively low (e.g., between
about 3-6 LPM). After the wafer is fully immersed in the plating
bath, the wafer is set to a tilt angle of 0.
[0072] During the fill stage of plating at block 705, the features
on the substrate undergo bottom-up fill as described herein. The
current density during the fill stage may be relatively low (e.g.,
between about 0.5-5 mA/cm.sup.2). Further, the mass transfer rate
during the fill stage may be relatively low in order to avoid
diffusion of leveler into the features. For example, the flow rate
of electrolyte may be about 6 LPM during this stage. The electrode
rotation speed may be relatively low during the fill stage (e.g.,
between about 12-30 RPM).
[0073] After the features are filled, an overburden stage occurs at
block 707. During the overburden stage, a higher current and/or
current density is applied (e.g., between about 10-15 mA/cm.sup.2),
and a higher rate of mass transfer is used (e.g., an electrolyte
flow rate between about 12-20 LPM, and an electrode rotation speed
between about 60-120 RPM). Next, at block 709 the wafer may be
removed from the electrolyte. At block 711 the wafer may be
optionally rinsed, dried and processed for further use. Next, at
block 713 the wafer is planarized to remove overburden. Operations
711 and 713 may occur in either order.
[0074] FIG. 8 shows the fill of 48 nm trenches (i.e., features)
plated in two-component plating solutions in three different
electrolyte compositions. "Two-component plating solution" means
that the electrolyte included both accelerator and suppressor. The
left panel of FIG. 8 corresponds to trenches filled in a high
copper/high acid electrolyte having 40 g/L Cu.sup.2+, 10 g/L acid,
50 ppm Cl.sup.-, 6 ml/L accelerator, and 4 mL/L suppressor. The
middle panel corresponds to trenches filled in a low copper/high
acid electrolyte having 5 g/L Cu.sup.2+, 10 g/L acid, 50 ppm
Cl.sup.-, 3 ml/L accelerator, and 2 mL/L suppressor. The right
panel corresponds to trenches filled in a low copper/low acid
electrolyte having 5 g/L Cu.sup.2+, 5 g/L acid, 50 ppm Cl.sup.-, 3
ml/L accelerator, and 2 mL/L suppressor. Each set of trenches was
plated for 0.15 seconds in a beaker with a potentiostatic entry at
1V as compared to an Hg/HgSO.sub.4 reference electrode, followed by
plating at a constant current density of 3 mA/cm.sup.2. The
rotation speed of the electrode during plating was 100 RPM. The
trenches filled in the high copper/high acid solution (FIG. 8, left
panel) show conformal plating with significant seam void defects,
while the trenches plated in the low copper solutions (FIG. 8,
center and right panels) show no such defects. The use of the low
copper electrolyte substantially reduced the number and severity of
plating defects seen in the features and appears to have eliminated
such defects altogether.
[0075] Composition of Electrolyte
[0076] Electrolytes used in the embodiments disclosed herein may
contain copper ions, acid, water, halide ions, and organic
additives such as suppressors, accelerators and levelers. The
concentration of copper ions (Cu.sup.2+) in the electrolyte may be
between about 1-10 g/L. In certain implementations the
concentration of copper ions is about 20 g/L or less, about 10 g/L
or less, or about 5 g/L or less. In some cases, the concentration
of copper ions is between about 4-10 g/L. The copper ions are
typically provided in the form of a copper salt. Example copper
salts include, but are not limited to, copper sulfate, copper
methanesulfonate, copper pyrophosphate, copper propanesulfonate,
etc. The copper concentrations cited herein include only the mass
of the copper cations and do not include the mass of any associated
anions. The concentration of acid in the electrolyte may be less
than about 10 g/L acid. In some cases, the acid concentration is
between about 2-15 g/L acid, though in certain implementations the
concentration is limited to between about 5-10 g/L. Other
implementations of the invention may utilize acid concentrations
above 15 g/L. Example acids include, but are not limited to,
sulfuric acid and methanesulfonic acid. The acid will dissociate
into cations and anions, and the anions are typically the same
species as anions formed from the copper salt. The acid
concentrations cited herein include the mass of both the hydrogen
cation and the associated anion in solution. The acid concentration
may be varied to control the conductivity of the plating bath. In
addition to increasing the polarization of the electrolyte as
described above, low acid electrolytes may be further beneficial
because they use less acid (reducing cost), are safer to use, and
are easier on the plating equipment (causing less equipment damage
over time).
[0077] The electrolytes used herein are typically aqueous and
generally contain halide ions. Example halide ions include, but are
not limited to, chloride ions, bromide ions, iodide ions, and
combinations thereof. It is believed that halide ions act as
bridges to assist the adsorption of organic additives on the copper
surface to achieve bottom-up fill of the features. The
concentration of halide ions may range between about 10-100 ppm,
e.g., about 50 ppm. In certain embodiments, the oxygen level of the
electrolyte is less than about 1-2 ppm in order to minimize seed
oxidation/corrosion in the electrolyte.
[0078] Next, the electrolyte may contain organic additives such as
suppressors, accelerators and/or levelers. A detailed description
of the function, interaction and identity of these additives is
included above. The concentration of organic additives in the
electrolyte may range between about 10-500 mg/L. This concentration
corresponds to the mass of active components in the additives and
does not include the mass of non-active components. The use of a
low copper electrolyte allows plating to occur at relatively low
suppressor concentrations as compared to conventional
electroplating electrolytes. In certain embodiments, the
concentration of suppressor is between about 50-200 ppm, between
about 50-300 ppm, or below about 200 ppm. Because the concentration
of suppressor is relatively low, the electrolyte will tend to have
a cloud point that is relatively high. In some embodiments, the
cloud point of the electrolyte is between about 40-100.degree. C.,
between about 50-100.degree. C., or between about 60-100.degree.
C.
[0079] Three example electrolyte compositions according to the
embodiments herein are shown in Table 2. These compositions are
included for exemplary purposes only, and should not be construed
as limiting.
TABLE-US-00002 TABLE 2 Electrolyte 1 Electrolyte 2 Electrolyte 3
Copper ion 4-20 4-20 4-20 concentration (g/L) Sulfuric acid 5-10
5-10 5-10 concentration (g/L) Halide ion 10-100 10-100 mg/L
Cl.sup.-; 10-100 concentration (mg/L) mg/L Cl- 0.5-25 mg/L Br.sup.-
mg/L Br.sup.-; Organic additive 10-500 10-500 10-500 concentration
(mg/L)
[0080] Apparatus for Plating
[0081] Many apparatus configurations may be used in accordance with
the embodiments described herein. One example apparatus includes a
clamshell fixture that seals a wafer's backside away from the
plating solution while allowing plating to proceed on the wafer's
face. The clamshell fixture may support the wafer, for example, via
a seal placed over the bevel of the wafer, or by means such as a
vacuum applied to the back of a wafer in conjunction with seals
applied near the bevel.
[0082] The clamshell fixture should enter the bath in a way that
allows good wetting of the wafer's plating surface. The quality of
substrate wetting is affected by multiple variables including, but
not limited to, clamshell rotation speed, vertical entry speed, and
the angle of the clamshell relative to the surface of the plating
bath. These variables and their effects are further discussed in
U.S. Pat. No. 6,551,487, incorporated by reference herein. In
certain implementations, the electrode rotation rate is between
about 5-125 RPM, the vertical entry speed is between about 5-300
mm/s, and the angle of the clamshell relative to the surface of the
plating bath is between about 1-10 degrees. One of the goals in
optimizing these variables for a particular application is to
achieve good wetting by fully displacing air from the wafer
surface.
[0083] The electrodeposition methods disclosed herein can be
described in reference to, and may be employed in the context of,
various electroplating tool apparatuses. One example of a plating
apparatus that may be used according to the embodiments herein is
the Lam Research Sabre tool. Electrodeposition, including substrate
immersion, and other methods disclosed herein can be performed in
components that form a larger electrodeposition apparatus. FIG. 9
shows a schematic of a top view of an example electrodeposition
apparatus. The electrodeposition apparatus 900 can include three
separate electroplating modules 902, 904, and 906. The
electrodeposition apparatus 900 can also include three separate
modules 912, 914, and 916 configured for various process
operations. For example, in some embodiments, one or more of
modules 912, 914, and 916 may be a spin rinse drying (SRD) module.
In other embodiments, one or more of the modules 912, 914, and 916
may be post-electrofill modules (PEMs), each configured to perform
a function, such as edge bevel removal, backside etching, and acid
cleaning of substrates after they have been processed by one of the
electroplating modules 902, 904, and 906.
[0084] The electrodeposition apparatus 900 includes a central
electrodeposition chamber 924. The central electrodeposition
chamber 924 is a chamber that holds the chemical solution used as
the electroplating solution in the electroplating modules 902, 904,
and 906. The electrodeposition apparatus 900 also includes a dosing
system 926 that may store and deliver additives for the
electroplating solution. A chemical dilution module 922 may store
and mix chemicals to be used as an etchant. A filtration and
pumping unit 928 may filter the electroplating solution for the
central electrodeposition chamber 924 and pump it to the
electroplating modules.
[0085] A system controller 930 provides electronic and interface
controls required to operate the electrodeposition apparatus 900.
The system controller 930 (which may include one or more physical
or logical controllers) controls some or all of the properties of
the electroplating apparatus 900. The system controller 930
typically includes one or more memory devices and one or more
processors. The processor may include a central processing unit
(CPU) or computer, analog and/or digital input/output connections,
stepper motor controller boards, and other like components.
Instructions for implementing appropriate control operations as
described herein may be executed on the processor. These
instructions may be stored on the memory devices associated with
the system controller 930 or they may be provided over a network.
In certain embodiments, the system controller 930 executes system
control software.
[0086] The system control software in the electrodeposition
apparatus 900 may include instructions for controlling the timing,
mixture of electrolyte components (including the concentration of
one or more electrolyte components), inlet pressure, plating cell
pressure, plating cell temperature, substrate temperature, current
and potential applied to the substrate and any other electrodes,
substrate position, substrate rotation, and other parameters of a
particular process performed by the electrodeposition apparatus
900. The system control logic may also include instructions for
electroplating under conditions that are tailored to be appropriate
for a low copper concentration electrolyte and the high
overpotential associated therewith. For example, the system control
logic may be configured to provide a relatively low current density
during the bottom-up fill stage and/or a higher current density
during the overburden stage. The control logic may also be
configured to provide certain levels of mass transfer to the wafer
surface during plating. For example, the control logic may be
configured to control the flow of electrolyte to ensure sufficient
mass transfer to the wafer during plating such that the substrate
does not encounter depleted copper conditions. In certain
embodiments the control logic may operate to provide different
levels of mass transfer at different stages of the plating process
(e.g., higher mass transfer during the bottom-up fill stage than
during the overburden stage, or lower mass transfer during the
bottom-up fill stage than during the overburden stage). Further,
the system control logic may be configured to maintain the
concentration of one or more electrolyte components within any of
the ranges disclosed herein. As a particular example, the system
control logic may be designed or configured to maintain the
concentration of copper cations between about 1-10 g/L. System
control logic may be configured in any suitable way. For example,
various process tool component sub-routines or control objects may
be written to control operation of the process tool components
necessary to carry out various process tool processes. System
control software may be coded in any suitable computer readable
programming language. The logic may also be implemented as hardware
in a programmable logic device (e.g., an FPGA), an ASIC, or other
appropriate vehicle.
[0087] In some embodiments, system control logic includes
input/output control (IOC) sequencing instructions for controlling
the various parameters described above. For example, each phase of
an electroplating process may include one or more instructions for
execution by the system controller 930. The instructions for
setting process conditions for an immersion process phase may be
included in a corresponding immersion recipe phase. In some
embodiments, the electroplating recipe phases may be sequentially
arranged, so that all instructions for an electroplating process
phase are executed concurrently with that process phase.
[0088] The control logic may be divided into various components
such as programs or sections of programs in some embodiments.
Examples of logic components for this purpose include a substrate
positioning component, an electrolyte composition control
component, a pressure control component, a heater control
component, and a potential/current power supply control
component.
[0089] In some embodiments, there may be a user interface
associated with the system controller 930. The user interface may
include a display screen, graphical software displays of the
apparatus and/or process conditions, and user input devices such as
pointing devices, keyboards, touch screens, microphones, etc.
[0090] In some embodiments, parameters adjusted by the system
controller 930 may relate to process conditions. Non-limiting
examples include bath conditions (temperature, composition, and
flow rate), substrate position (rotation rate, linear (vertical)
speed, angle from horizontal) at various stages, etc. These
parameters may be provided to the user in the form of a recipe,
which may be entered utilizing the user interface.
[0091] Signals for monitoring the process may be provided by analog
and/or digital input connections of the system controller 930 from
various process tool sensors. The signals for controlling the
process may be output on the analog and digital output connections
of the process tool. Non-limiting examples of process tool sensors
that may be monitored include mass flow controllers, pressure
sensors (such as manometers), thermocouples, optical position
sensors, etc. Appropriately programmed feedback and control
algorithms may be used with data from these sensors to maintain
process conditions.
[0092] In one embodiment, the instructions can include inserting
the substrate in a wafer holder, tilting the substrate, biasing the
substrate during immersion, and electrodepositing a copper
containing structure on a substrate.
[0093] A hand-off tool 940 may select a substrate from a substrate
cassette such as the cassette 942 or the cassette 944. The
cassettes 942 or 944 may be front opening unified pods (FOUPs). A
FOUP is an enclosure designed to hold substrates securely and
safely in a controlled environment and to allow the substrates to
be removed for processing or measurement by tools equipped with
appropriate load ports and robotic handling systems. The hand-off
tool 940 may hold the substrate using a vacuum attachment or some
other attaching mechanism.
[0094] The hand-off tool 940 may interface with a wafer handling
station 932, the cassettes 942 or 944, a transfer station 950, or
an aligner 948. From the transfer station 950, a hand-off tool 946
may gain access to the substrate. The transfer station 950 may be a
slot or a position from and to which hand-off tools 940 and 946 may
pass substrates without going through the aligner 948. In some
embodiments, however, to ensure that a substrate is properly
aligned on the hand-off tool 946 for precision delivery to an
electroplating module, the hand-off tool 946 may align the
substrate with an aligner 948. The hand-off tool 946 may also
deliver a substrate to one of the electroplating modules 902, 904,
or 906 or to one of the three separate modules 912, 914, and 916
configured for various process operations.
[0095] An example of a process operation according to the methods
described above may proceed as follows: (1) electrodeposit copper
onto a substrate to form a copper containing structure in the
electroplating module 904; (2) rinse and dry the substrate in SRD
in module 912; and, (3) perform edge bevel removal in module
914.
[0096] An apparatus configured to allow efficient cycling of
substrates through sequential plating, rinsing, drying, and PEM
process operations may be useful for implementations for use in a
manufacturing environment. To accomplish this, the module 912 can
be configured as a spin rinse dryer and an edge bevel removal
chamber. With such a module 912, the substrate would only need to
be transported between the electroplating module 904 and the module
912 for the copper plating and EBR operations.
[0097] An alternative embodiment of an electrodeposition apparatus
1000 is schematically illustrated in FIG. 10. In this embodiment,
the electrodeposition apparatus 1000 has a set of electroplating
cells 1007, each containing an electroplating bath, in a paired or
multiple "duet" configuration. In addition to electroplating per
se, the electrodeposition apparatus 1000 may perform a variety of
other electroplating related processes and sub-steps, such as
spin-rinsing, spin-drying, metal and silicon wet etching,
electroless deposition, pre-wetting and pre-chemical treating,
reducing, annealing, photoresist stripping, and surface
pre-activation, for example. The electrodeposition apparatus 1000
is shown schematically looking top down in FIG. 10, and only a
single level or "floor" is revealed in the figure, but it is to be
readily understood by one having ordinary skill in the art that
such an apparatus, e.g. the Novellus Sabre.TM. 3D tool, can have
two or more levels "stacked" on top of each other, each potentially
having identical or different types of processing stations.
[0098] Referring once again to FIG. 10, the substrates 1006 that
are to be electroplated are generally fed to the electrodeposition
apparatus 1000 through a front end loading FOUP 1001 and, in this
example, are brought from the FOUP to the main substrate processing
area of the electrodeposition apparatus 1000 via a front-end robot
1002 that can retract and move a substrate 1006 driven by a spindle
1003 in multiple dimensions from one station to another of the
accessible stations--two front-end accessible stations 1004 and
also two front-end accessible stations 1008 are shown in this
example. The front-end accessible stations 1004 and 1008 may
include, for example, pre-treatment stations, and spin rinse drying
(SRD) stations. Lateral movement from side-to-side of the front-end
robot 1002 is accomplished utilizing robot track 1002a. Each of the
substrates 1006 may be held by a cup/cone assembly (not shown)
driven by a spindle 1003 connected to a motor (not shown), and the
motor may be attached to a mounting bracket 1009. Also shown in
this example are the four "duets" of electroplating cells 1007, for
a total of eight electroplating cells 1007. The electroplating
cells 1007 may be used for electroplating copper for the copper
containing structure and electroplating solder material for the
solder structure. A system controller (not shown) may be coupled to
the electrodeposition apparatus 1000 to control some or all of the
properties of the electrodeposition apparatus 1000. The system
controller may be programmed or otherwise configured to execute
instructions according to processes described earlier herein.
[0099] The electroplating apparatus/methods described hereinabove
may be used in conjunction with lithographic patterning tools or
processes, for example, for the fabrication or manufacture of
semiconductor devices, displays, LEDs, photovoltaic panels and the
like. Generally, though not necessarily, such tools/processes will
be used or conducted together in a common fabrication facility.
Lithographic patterning of a film generally comprises some or all
of the following steps, each step enabled with a number of possible
tools: (1) application of photoresist on a work piece, i.e., a
substrate, using a spin-on or spray-on tool; (2) curing of
photoresist using a hot plate or furnace or UV curing tool; (3)
exposing the photoresist to visible, UV, or x-ray light with a tool
such as a wafer stepper; (4) developing the resist so as to
selectively remove resist and thereby pattern it using a tool such
as a wet bench; (5) transferring the resist pattern into an
underlying film or work piece by using a dry or plasma-assisted
etching tool; and (6) removing the resist using a tool such as an
RF or microwave plasma resist stripper.
* * * * *