U.S. patent application number 12/962601 was filed with the patent office on 2011-03-31 for apparatus and method for improving uniformity in electroplating.
Invention is credited to HOOMAN HAFEZI, Aron Rosenfeld.
Application Number | 20110073483 12/962601 |
Document ID | / |
Family ID | 36928110 |
Filed Date | 2011-03-31 |
United States Patent
Application |
20110073483 |
Kind Code |
A1 |
HAFEZI; HOOMAN ; et
al. |
March 31, 2011 |
APPARATUS AND METHOD FOR IMPROVING UNIFORMITY IN ELECTROPLATING
Abstract
A method and apparatus for plating a metal onto a substrate. One
embodiment of the present invention provides an apparatus for
electroplating a substrate. The apparatus comprises a fluid basin,
an anode disposed near a bottom of the fluid basin, a restrictor
disposed above the anode, and a substrate support member configured
to move the substrate within the fluid basin among different
elevations relative to the restrictor. Plating profiles on the
substrate may be adjusted by changing the elevation of the
substrate during plating.
Inventors: |
HAFEZI; HOOMAN; (Redwood
City, CA) ; Rosenfeld; Aron; (Palo Alto, CA) |
Family ID: |
36928110 |
Appl. No.: |
12/962601 |
Filed: |
December 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11362433 |
Feb 24, 2006 |
7846306 |
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12962601 |
|
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60656161 |
Feb 25, 2005 |
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60687404 |
Jun 3, 2005 |
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Current U.S.
Class: |
205/137 |
Current CPC
Class: |
C25D 17/02 20130101;
C25D 5/04 20130101; C25D 7/123 20130101; C25D 5/18 20130101; C25D
17/007 20130101; C25D 17/002 20130101; C25D 17/001 20130101; C25D
17/10 20130101; H01L 21/2885 20130101 |
Class at
Publication: |
205/137 |
International
Class: |
C25D 5/00 20060101
C25D005/00 |
Claims
1. A method for plating a metal onto a substrate, comprising:
applying a plating bias to the substrate while moving the substrate
in a plating solution to adjust the plating current distribution
across the substrate.
2. The method of claim 1, further comprising: providing a plating
cell having in a fluid volume to retain the plating solution
therein, wherein the fluid volume has a restricted section having a
smaller sectional area than a plating surface on the substrate;
providing an anode on one side of the restrict section; and
positioning the substrate in a first position, wherein the
substrate and the anode are on opposite sides of the restricted
section; and applying a first waveform between the substrate and
the anode.
3. The method of claim 2, wherein the applying the plating bias to
the substrate while moving the substrate further comprising: moving
the substrate to a second position to adjust a distance between the
substrate and the restricted section; and applying a second
waveform between the substrate and the anode.
4. The method of claim 3, wherein the second position is further
away from the restricted section than the first position.
5. The method of claim 2, wherein the anode having an upper surface
smaller than the plating surface of the substrate, and the
restricted section is formed by the upper surface of the anode.
6. The method of claim 2, wherein the restricted section is formed
by a shield having an inner diameter smaller than the diameter of
the substrate.
7. The method of claim 2, further comprising: providing an
auxiliary anode disposed approximate the restricted section; and
applying an electric bias to the auxiliary anode.
8. The method of claim 7, further comprising: providing a
protective tube to encase the auxiliary anode, wherein the
protective tube comprising an ionic membrane; and supplying an
electrolyte to the protective tube.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of the
co-pending U.S. patent application Ser. No. 11/362,433, filed Feb.
24, 2006, which claims benefit of U.S. Provisional Patent
Application Ser. No. 60/656,161 filed Feb. 25, 2005 and U.S.
Provisional Patent Application Ser. No. 60/687,404 filed Jun. 3,
2005, which are herein incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] Embodiments of the invention generally relate to methods and
apparatus for electrochemical processing. One embodiment of the
invention relates to an electrochemical processing cell configured
to have an electric field with different profiles at different
locations.
[0004] 2. Description of the Related Art
[0005] Metallization of high aspect ratio 90 nm and smaller sized
features is a foundational technology for future generations of
integrated circuit manufacturing processes. Metallization of these
features is generally accomplished via an electrochemical plating
process. In a typical process scheme, a metal, such as copper, is
plated onto a substrate comprising an array of integrated circuit
devices with open vias and trenches. The plating process is carried
out in such a manner as to fill the vias and trenches on the
substrate surface with metal and to further deposit an additional
amount of metal known as the overburden on the substrate. The
overburden is required to enable subsequent polishing and
planarization of the deposit through a process step such as
chemical mechanical polishing. The total amount of metal deposited
in the electrochemical plating process is typically 0.2 to 1
.mu.m.
[0006] However, electrochemical plating (ECP) of these features
presents several challenges to conventional gap fill methods and
apparatuses. One such problem, for example, is that electrochemical
plating processes generally require a conductive seed layer to be
deposited onto the features to support the subsequent plating
process. Conventionally, these seed layers have had a thickness of
between about 1000 .ANG. and about 2500 .ANG.; however, as a result
of the high aspect ratios of 90 nm features, seed layer thicknesses
must be reduced to less than about 500 .ANG., or even below 100
.ANG.. This reduction in the seed layer thickness increases
resistivity of the substrate causing a "terminal effect," which is
an increase in the deposition thickness near the perimeter of the
substrate being plated.
[0007] The terminal effect is most severe at the beginning of an
electrochemical plating process, for example, within about the
first 10 seconds of the electrochemical plating process, when the
substrate resistivity is at the highest level. This stage is also
the critical stage when the features on the substrates are being
filled. The terminal effect results in a large difference in the
plating rate across the substrate, leading to variations in film
properties such as film composition and resistivity between the
center and edge of the substrate. More importantly, a highly
non-uniform plating rate across the substrate during the filling of
features forces the features at either the center or edge of the
wafer to fill under sub-optimal conditions, resulting in problems,
such as incomplete filling and trapped voids inside the
features.
[0008] Additionally, it is often desirable to modulate the plating
rate at the edge of the substrate after the features have been
filled and while the overburden is being deposited. For example,
processes that follow electrochemical plating, such as chemical
mechanical polishing, may yield better performance if the plated
film is thinner at the edge than at the center of the wafer. This
is because certain polishing processes are edge-slow (edge fast),
so that a slightly edge-thin (edge thick) deposit profile after
plating results in an optimally uniform profile after
polishing.
[0009] Therefore, control of the plating rate at the edge of the
substrate is desired to mitigate the terminal effect and adjust the
overall plating profile. Attempts have been made modulate the
plating profile at the edge of the substrate through various
apparatus and methods. For example, Conventional plating cells have
been modified to include resistive elements, multi-segment anodes,
or passive shield or flange members. These configurations were only
partly successful because of their lack of proximity to the
perimeter of the substrate or inability to dynamically adjust the
profile. For example, resistive elements or passive shield or
flange members were only sufficient to generate one type of plating
profile, such as uniform or edge-thin, whereas future generations
of plating processes may require both types of profiles to be
available during the plating process.
[0010] Therefore, there exists a need for an apparatus and method
for minimizing terminal effect and/or adjusting plating profile
dynamically.
SUMMARY OF THE INVENTION
[0011] Embodiments of the present invention provide apparatus and
method for minimizing terminal effect and adjusting plating
profiles.
[0012] One embodiment of the present invention provides an
apparatus for electroplating a substrate. The apparatus comprises a
fluid basin, an anode disposed near a bottom of the fluid basin, a
restrictor disposed above the anode, and a substrate support member
configured to move the substrate within the fluid basin among
different elevations relative to the restrictor.
[0013] Another embodiment of the present invention provides an
apparatus for electroplating a substrate. The apparatus comprises a
fluid basin configured to retain a plating solution in a fluid
volume therein, an anode disposed on a bottom of the fluid basin, a
substrate support member configured to position the substrate in
the fluid basin along a trajectory, wherein the fluid volume has a
restricted section with an inner diameter smaller than the diameter
of the substrate.
[0014] Yet another embodiment of the present invention provides a
method for plating a metal onto a substrate. The method comprises
moving the substrate in a plating solution to adjust a plating
current distribution across the substrate. The method further
comprises providing a plating cell having a plating solution
retained in a fluid volume, wherein the fluid volume has a
restricted section having a smaller sectional area than a plating
surface on the substrate, providing an anode on one side of the
restricted section, positioning the substrate in a first position,
wherein the substrate and the anode are on opposite sides of the
restricted section, and applying a first electric bias between the
substrate and the anode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] So that the manner in which the above recited features of
the present invention can be understood in detail, a more
particular description of the invention, briefly summarized above,
may be had by reference to embodiments, some of which are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments of
this invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments.
[0016] FIG. 1A illustrates a partial sectional perspective view of
an exemplary electrochemical plating cell of the invention.
[0017] FIG. 1B illustrates an enlarged partial perspective view of
the groove displayed in FIG. 1A in which the encased auxiliary
electrode is placed.
[0018] FIG. 1C illustrates an enlarged view of the wall of the
outer basin of the plating cell displayed in FIG. 1A showing how
the encased auxiliary electrode assembly enters the basin of the
plating cell.
[0019] FIG. 1D illustrates an alternate placement of the encased
auxiliary electrode assembly.
[0020] FIG. 2 illustrates a perspective view of an anode base plate
of the invention.
[0021] FIG. 3 illustrates a perspective view of an exemplary anode
base plate of the invention having an anode positioned therein.
[0022] FIG. 4 illustrates an exploded perspective view of an
exemplary membrane support member of the invention.
[0023] FIG. 5 illustrates a partial sectional view of an edge of
the plating cell of the invention.
[0024] FIG. 6 illustrates a plating thickness plot for a plating
cell with the encased auxiliary electrode assembly in fluid
communication with the catholyte compared to a plating cell without
the encased auxiliary electrode assembly.
[0025] FIG. 7 illustrates a flow diagram of a method for immersing
a substrate into a plating solution in accordance with one or more
of the embodiments of the invention.
[0026] FIGS. 8A-8C schematically illustrate an electrochemical
plating cell configured to provide an adjustable plating
profile.
[0027] FIG. 9A illustrates a sectional view of an electrochemical
plating cell having an anode smaller than the substrate.
[0028] FIG. 9B illustrates a sectional view of an electrochemical
plating cell having a diffuser plate with restricted area for fluid
to pass through.
[0029] FIG. 9C illustrates a section view of an electrochemical
plating cell having a shield and an auxiliary electrode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0030] The present invention generally provides an electrochemical
plating cell, with an encased auxiliary electrode assembly in fluid
communication with the cathode compartment, configured to uniformly
plate metal onto a substrate.
[0031] FIG. 1A illustrates a partial sectional perspective view of
an exemplary electrochemical plating cell 100 containing an encased
auxiliary electrode assembly 130 of the invention. The encased
auxiliary electrode assembly 130 is composed of an auxiliary
electrode 132 surrounded by electrolyte in a protective tube 134.
In another embodiment, the electrolyte is absent and the protective
tube is attached directly to the surface of the auxiliary
electrode. The encased auxiliary electrode assembly 130 is
generally configured to be used as a cathode when the substrate
contacts (not shown) are configured as cathodes. Also, when
configured as a cathode, the encased auxiliary electrode assembly
130 can be used (when the substrate contacts are configured as
anodes) as a cathode for deplating copper that accumulates on the
substrate contacts. The protective tube 134 of the encased
auxiliary electrode assembly 130 prevents catholyte solution
organics from entering the encased auxiliary electrode assembly 130
while also preventing copper sludge and plated copper formed at the
encased auxiliary electrode assembly 130 from exiting the
protective tube 134 and contaminating the catholyte solution.
[0032] The encased auxiliary electrode assembly 130 includes an
auxiliary electrode 132 is preferably platinum but may be
manufactured from copper or other metals known to be effective as
either a soluble or insoluble electrode in an electrochemical
plating cell. Additionally, the auxiliary electrode 132 may be
manufactured from a core material, such as copper, stainless steel,
titanium, or other cost effective core electrode material, and then
the outer surfaces, i.e., the upper surface of the auxiliary
electrode 132 that is in fluid contact with the electrolyte, may
then be plated with another metal, such as platinum, titanium, or
other electrode material. This configuration allows the cost of
auxiliary electrode 132 to be reduced, as a more cost effective and
electrically conductive material is used to manufacture the core of
the auxiliary electrode 132, while another more costly but
desirable electrode material, i.e., platinum, is used for the
exposed surfaces of the auxiliary electrode 132. The auxiliary
electrode 132 can be a wire, a ring, a torroid or of other
shape.
[0033] Due to the proximity of the auxiliary electrode 132 to the
primary cathode (the contact ring, pins and substrate) a large
amount of copper sludge is generated at the auxiliary electrode
132. The protective tube 134 prevents this copper sludge from
exiting the encased auxiliary electrode assembly 130 and
contaminating the catholyte solution. Further, the protective tube
134 prevents the auxiliary electrode 132 from consuming and
degrading the organic additives (levelers, suppressors and
activators) in the catholyte solution. Preferably, the protective
tube 134 can be composed of an ion exchange material such as
Nafion.RTM., CMX-SB or Vicor membrane. Also, the protective tube
134 can be composed of a porous membrane. One example of a
hydrophilic porous membrane is the Durapore Hydrophilic Membrane,
available from Millipore Corporation, located in Bedford, Mass.
Other examples of conventional membranes include porous glass,
porous ceramics, silica aerogels, organic aerogels, porous
polymeric materials, and filter membranes. Specific membranes
include carbon filter layers, Kynar polymer layers or polypropylene
membranes.
[0034] When in a plating configuration, the protective tube 134
allows copper ions to plate onto the auxiliary electrode 132 but
does not allow the copper sludge to exit the protective tube and
contaminate the catholyte solution, and as such, protective tube
134 is generally an ionic or ion exchange-type membrane. Ion
exchange membranes generally include fixed negatively charged
groups, such as SO.sub.3.sup.-, COO.sup.-, HPO.sub.2.sup.-,
SeO.sub.3.sup.-, PO.sub.3.sup.2-, or other negatively charged
groups amenable to plating processes. As such, the protective tube
134 is configured to allow a particular type of ion to travel
through the membrane, while preventing other types of ions from
traveling or passing through the membrane. More particularly, the
protective tube 134 may be a cationic membrane that is configured
to allow positively charged copper ions (Cu.sup.2+ and H.sup.+) to
pass therethrough, i.e., to allow copper ions to travel from the
catholyte solution through the protective tube 134 where the copper
ions may then be plated onto the auxiliary electrode 132. Further,
the cationic membrane may be configured to prevent passage of
negatively charged ions and electrically neutral species in the
solution, such as the ions that make up the plating solution and
catholyte additives. It is desirable to prevent these catholyte
additives from traveling through the protective tube 134 and
contacting the auxiliary electrode 132, as the additives are known
to breakdown upon contacting the auxiliary electrode 132. More
particularly, membranes with negatively charged ion groups like
SO.sub.3.sup.- etc. not only operate to facilitate Cu ion transport
from the anolyte to the catholyte, but also to prevent penetration
of accelerators to the auxiliary electrode. The accelerators are
generally negatively charged organic ions, such as, for example,
.sup.-SO.sub.3.sup.---C.sub.3H.sub.6--S--S--C.sub.3H.sub.6.sup.-SO.sub.3.-
sup.-, so they can't penetrate into or through the cation membrane
and contact the auxiliary electrode where they are consumed and/or
depleted.
[0035] Protective tube 134, for example, may be a Nafion.RTM.
membrane manufactured by Dupont Corporation. Nafion.RTM. membrane
is an example of a poly (tetrafluoroethylene) based ionomer.
Nafion.RTM. membrane has several desirable characteristics for
electrochemical plating applications, such as its thermal and
chemical resistance, ion-exchange properties, selectivity,
mechanical strength, and insolubility in water. Nafion.RTM.
membrane is also a cationic membrane based on a fluorized polymer
matrix. Because of its fluorized matrix, Nafion.RTM. membrane
exhibits excellent chemical stability, even in concentrated basic
solutions. More particularly, Nafion.RTM. membrane is a
perfluorinated polymer that contains small proportions of sulfonic
or carboxylic ionic functional groups, and has been shown to be
effective in transmitting metal ions (copper ions in the present
embodiment) therethrough, even at low plating current densities.
Specifically, Nafion.RTM. membranes have shown to be effective at
transmitting between about 93% and about 97% of copper ions
therethrough at plating current densities of between about 5
mA/cm.sup.2 and about 20 mA/cm.sup.2. Additionally, at current
densities of between about 20 mA/cm.sup.2 and about 60 mA/cm.sup.2,
Nafion.RTM. membrane transmits between about 97% and about 93% of
copper ions therethrough. The above noted transmission percentages
were observed using a copper sulfate solution having a pH value of
about 3.4. Nafion's.RTM. general chemical structure (illustrated
below as Diagram 1), illustrates where X is either a sulfonic or
carboxylic functional group and M is either a metal cation in the
neutralized form or an H.sup.+ in the acid form.
##STR00001##
[0036] As a result of electrostatic interactions, the ionic groups
that form Nafion.RTM. membrane tend to aggregate to form tightly
packed regions referred to as clusters. The presence of these
electrostatic interactions between the ions and the ion pairs
enhance the intermolecular forces and thereby exert a significant
effect on the properties of the parent polymer, which makes
Nafion.RTM. membrane, or other membranes having similar physical
and/or operational characteristics, a desirable ionic membrane for
use in electrochemical plating cells having separated anolyte and
catholyte chambers.
[0037] Other membranes that may be used in embodiments of the
invention include various cationic and anionic membranes. For
example, ionic membranes manufactured by Tokuyama of Japan, i.e.,
CMX-SB ionic membranes that are based on a polydivinilbenzol
matrix, may be used to isolate a catholyte solution from an anolyte
solution in an electrochemical plating cell. CMX-SB membranes have
been shown to be effective in transmitting copper ions while
preventing organic plating additives from transmitting
therethrough.
[0038] Additionally, CMX-SB membranes have shown acceptable
resistance to transmission of positive hydrogen ions. More
particularly, CMX membranes have been shown to transmit above about
92% of copper ions at a current density of about 10 mA/cm.sup.2,
and above about 98% at a current density of about 60 mA/cm.sup.2.
Ionics CR-type membranes from Ionics Inc. have also shown to be
able to transmit above about 92% of copper ions at about 10
mA/cm.sup.2 and above about 88% of copper ions at about 60
mA/cm.sup.2.
[0039] With regard to other properties of the above noted membranes
(Ionics, CMX, and Nafion.RTM. membrane), each exhibit relatively
high conductivity, i.e., about 41.2, 35.3, and 24.2 ohm cm.sup.2 at
10 mA/cm.sup.2 for Ionics, Neosepta and Nafion.RTM. membrane,
respectively. Additionally, water moves through the membranes from
the anolyte into the catholyte compartment. This effect essentially
dilutes the catholyte and is undesirable. For example, between
about 0.5 and about 3 liters of water penetrates into the catholyte
per 24 hours (or per 200 wafers) depending on the membrane type and
electrolysis conditions. For example, CMX shows the minimal water
transport at about 1.5 ml/wafer, the Ionics membrane shows about 5
ml/wafer, and Nafion.RTM. membrane shows about 6.5 ml/wafer. The
transport properties of the CMX and Nafion.RTM. membranes result in
the CuSO.sub.4/H.sub.2SO.sub.4 concentration ratio remaining
relatively constant, even after about 200 substrates are plated.
This indicates that copper acid concentration changes will be lower
than 2%, if the penetrated water will be removed, e.g., by enforced
evaporation. As such, the use of CMX or Nafion.RTM. membrane
requires only a small device to accelerate the water evaporation to
4-6 liters/day. However, Ionics membranes require an additional
device that extracts the excess of H.sub.2SO.sub.4 coming from the
anolyte. Table 1 illustrates the respective properties of the above
noted membranes.
[0040] Vicor membranes may also be used to advantage in the plating
cell of the invention. Other membranes that may be used in the
plating cell of the invention include Neosepta.RTM. membranes
(ionic and non-ionic) manufactured by Tokuyama, Aciplex.RTM.
membranes, Selemlon.RTM. membranes, and Flemion membranes (all of
which are available as ionic and non-ionic) from Asahi Corporation,
Raipare.TM. membranes from Pall Gellman Sciences Corporation, and
C-class membranes from Solvay Corporation.
[0041] Referring to FIG. 1A, the auxiliary electrode 132 is in
electrical communication with a power supply 180 that is configured
to electrically bias the auxiliary electrode 132 anodically or
cathodically, i.e., the auxiliary electrode 132 may be biased
cathodically to assist in controlling the plating uniformity during
the plating process which essentially supplements or adds to the
fields generated by the primary cathode of the plating cell or to
deplate metal from the contact pins that conduct the plating bias
to the substrate in the plating cell when the primary cathode is
biased anodically. As such, the auxiliary electrode 132 may be used
to either withdraw some of the electrical field directed to the
primary cathode or to deplate the contact pins of the substrate
contact ring when the primary cathode is biased as an anode.
TABLE-US-00001 TABLE I Water Cu/Acid transfer, Resistance Ratio
Membrane Cu.sup.2+ transfer, % ml/Amphr ohm cm2 Deviation, % Ionics
90-95 8-11.5 53 4% Nafion 95-98 4-7.5 36 2% CMX 97-98 5.0-3.1 47
1%
[0042] In operation the auxiliary electrode 132 may be cathodically
biased in order to deplate copper that accumulates on substrate
contacts used to communicate a plating bias to a substrate during
plating operations. As is known in the art, copper tends to build
up on the electrical substrate contacts (as a result of the
contacts being in communication with the plating solution during
the plating process) and may cause varying resistances between the
respective contacts and the substrates being plated, which often
results in uniformity variations between plated substrates. As
such, it is desirable to periodically remove the accumulated copper
from the substrate contacts so that plating uniformity between
substrates may be maximized. The removal processes is generally
conducted at a time period when no substrates are being plated,
i.e., between plating substrates. At this time a substrate contact
ring (or other apparatus that includes the elements used to
electrically contact the substrate during the plating process) is
immersed in the plating solution such that the electrical contact
pins are in fluid communication with the plating solution. Once
immersed, a deplating bias is applied between the contact pins and
the auxiliary electrode 132. More particularly, the deplating bias
is configured such that the auxiliary electrode 132 is the cathode
electrode and the substrate contact pins are the anodic electrodes.
In this configuration the substrate contact pins supply the copper
ions to the reduction reaction, and as such, the copper that was
plated onto the contact pins during plating operations is removed
from the contact pins, transported through the plating solution,
and deposited on the auxiliary electrode 132 in the reduction
process resulting from the application of the deplating bias. The
deplating bias may be between about 3 volts and about 7 volts, for
example, and may have a duration of between about 10 seconds and
about 30 seconds. Additionally, the deplating time may be increased
to above 30 seconds if the time between deplating processes is
long, i.e., if the number of substrates plated has been excessive
and the copper buildup on the contact pins is more than can be
removed in 30 seconds. In this situation the deplating time or
duration may be calculated as 20 seconds multiplied by the number
of substrates plated since the last deplating process. Thus, for
example, if 20 substrates have been plated since the last contact
pin deplating process, then the duration of the deplating process
may be about 400 seconds to remove the excessive accumulation of
copper on the contact pins. Embodiments of the invention
contemplate that the contact pins may be deplated between every
substrate that is plated in order to maximize uniformity and
throughput. In this configuration the deplating process will likely
have a duration of less than about 20 seconds. However, the
inventors have found that the deplating process may be extended to
between every second, third, or fourth plated substrate without a
substantial degradation in the uniformity. In this configuration
the deplating time may be between about 20 seconds and about 80
seconds, for example.
[0043] The copper ions that deposit or accumulate on the auxiliary
electrode 132 during the deplating process generally do not have an
effect upon plating uniformity. However, copper deposits plated
onto the auxiliary electrode 132 as a result of the plating or
deplating process may later be reintroduced into the plating bath
via application of a forward plating bias to the auxiliary
electrode 132 (along with the plating anode 105) during a plating
process. This configuration essentially configures the auxiliary
electrode 132 as a secondary or auxiliary anode to the primary
plating anode 105, and as such, when the forward plating bias is
applied to the electrodes (the plating anode 105 and the auxiliary
electrode 132), the copper ions that have plated onto auxiliary
electrode 132 will be removed from the auxiliary electrode 132 via
the reduction reaction that supports the plating process and
reintroduced into the plating solution. More particularly, the
auxiliary electrode 132 may be electrically biased during plating
operations to the same polarity as the anode 105, and as such, the
auxiliary electrode 132 may contribute to the plating reaction,
i.e., supply copper ions to the plating solution, as well as
generating a magnetic and/or electric field in the plating cell.
More particularly, since the auxiliary electrode 132 is positioned
radially outward of the substrate perimeter being plated, the
electric field from the auxiliary electrode 132 may be used to
provide an additional element of control over plating uniformity
across the surface of the substrate as a result of the field effect
of the auxiliary electrode 132. The minimal current supplied to the
auxiliary electrode 132 during the plating process may be
calculated to generate an electric field sufficient to interact
with the substrate and/or the minimal current may be calculated to
generate a shaping field, i.e., the field generated by auxiliary
electrode 132 (which has a small magnitude) may primarily be used
to shape the field generated by the anode 105 (which has a much
larger magnitude, as the majority of the plating current is
traveling through the anode 105). Alternatively, auxiliary
electrode 132 may be electrically biased to the same electrical
potential as the primary anode 105. In this configuration the anode
105 and the auxiliary electrode 132 essentially operate as a
unitary anode.
[0044] Additional embodiments of the invention contemplate that the
auxiliary electrode 132 may be active (have a forward or plating
bias applied thereto) for either the entire plating process time
(the time when the primary anode is active) or for only a portion
of the plating process. In embodiments where the auxiliary
electrode 132 is active for only a portion of the plating process
duration, the auxiliary electrode 132 may be activated for a time
period that is calculated to remove the copper deposits therefrom.
For example, if the auxiliary electrode 132 is activated for 20
seconds during the deplating process, then the auxiliary electrode
132 may be activated for another 20 seconds during the plating
process. The effect of this configuration would be to clean the
auxiliary electrode, i.e., to redeposit the copper that was plated
onto the auxiliary electrode during the deplating process into the
electrolyte solution, assuming that equal power is applied during
both the deplating and plating steps.
[0045] Referring to FIG. 1A, the plating cell 100 further includes
an outer basin 101 and an inner basin 102 positioned within outer
basin 101. The encased auxiliary electrode assembly 130 is
positioned in a groove 120 located in the lower portion of the
inner basin 102 of the plating cell. The inner basin 102 is
generally configured to contain a plating solution that is used to
plate a metal, e.g., copper, onto a substrate during an
electrochemical plating process. During the plating process, the
plating solution is generally continuously supplied to the inner
basin 102 (at about 1 gallon per minute for a plating cell having a
10 liter total capacity (capacity generally includes the cell
volume and the supply tank), for example), and therefore, the
plating solution continually overflows the uppermost point of the
inner basin 102 and runs into the outer basin 101. The overflow
plating solution is then collected by the outer basin 101 and
drained therefrom for recirculation into the inner basin 102. As
illustrated in FIG. 1, plating cell 100 is generally positioned at
a tilt angle, i.e., a frame portion 103 of the plating cell 100 is
generally elevated on one side such that the components of plating
cell 100 are tilted between about 3.degree. and about 30.degree..
Therefore, in order to contain an adequate depth of plating
solution within inner basin 102 during plating operations, the
uppermost portion of inner basin 102 may be extended upward on one
side of plating cell 100, such that the uppermost point of inner
basin 102 is generally horizontal and allows for contiguous
overflow of the plating solution supplied thereto around the
perimeter of inner basin 102.
[0046] FIG. 1B illustrates an enlarged partial perspective
sectional view of the lower portion of the inner basin 102
containing the groove 120 where the encased auxiliary electrode
assembly 130 is placed.
[0047] FIG. 1C illustrates an enlarged view of the outer basin 101
of the plating cell 100 displayed in FIG. 1A showing how the
encased auxiliary electrode assembly 130 enters the electrochemical
the plating cell 100 through a T-connection 140. After traversing
the outer basin 101 of the plating cell 100, the encased auxiliary
electrode assembly 130 circumnavigates the plating cell within the
groove 120 and exits at an outlet 170. Upon entering T-connection
140, the auxiliary electrode 132 is combined with electrolyte from
an electrolyte source 190 and enters the protective tube 134 to
form the encased auxiliary electrode assembly 130. The T-connection
140 is shown with an electrode inlet 136, an electrolyte inlet 138
and an electrode outlet 142 where the encased auxiliary electrode
assembly 130 traverses the wall of the outer basin 101 of the
plating cell. The electrode inlet 136, electrolyte inlet 138 and
electrode assembly outlet 142 are sealed in a well known manner
with seals 144a, 144b and 144c respectively.
[0048] The electrolyte, shown by an arrow 139 entering through the
electrolyte inlet 138, is generally an anolyte and is kept separate
from a catholyte in the catholyte compartment. The anolyte in the
protective tube 134 can either be flowing or stagnant. There are
several advantages to using anolyte in the encased auxiliary
electrode assembly 130. First, anolyte does not contain catholyte
organics (levelors, suppresors, and activators) which may be
quickly consumed or degraded by the auxiliary electrode 132.
Second, the use of anolyte is more cost effective than catholyte
but still provides the desired conduction. In another embodiment
(not shown) the protective membrane adheres directly to the upper
surface of the auxiliary electrode and functions without
electrolyte.
[0049] FIG. 1D illustrates an additional embodiment of the enlarged
partial perspective sectional view of the lower portion of the
inner basin 102 containing the encased auxiliary electrode assembly
130. The encased auxiliary electrode assembly 130 is positioned
between an upper surface 151 of the inner basin 102 and a support
member 150. The support member 150 is fastened to the upper surface
151 of the inner basin 102 thus securing the encased auxiliary
electrode assembly 130 to the upper surface 151 of the inner basin
102 while also allowing for expansion of the protective tube 134.
The support member 150 and upper surface 151 of the inner basin are
welded together, bolted together or fastened using other
conventional means.
[0050] FIG. 2 illustrates a perspective view of the anode base
member 104 without the anode 105. The upper surface of the anode
base member 104 generally includes an annular recessed portion 201
defined by a vertical wall 208 and configured to receive a disk
shaped anode 105 therein. Further, the bottom surface of annular
recessed portion 201 generally includes a plurality of anode base
channels 202 formed therein. Each of anode base channels 202 are
generally positioned in parallel orientation with each other,
extend across the lower portion of the annular recessed portion
201, and terminate at the periphery of the annular recessed portion
201. Additionally, the periphery of recessed region 201 includes an
annular drain channel 203 that extends around the perimeter of
recessed portion 201. Each of the plurality of parallel positioned
anode base channels 202 terminate at opposing ends into annular
drain channel 203. Therefore, anode base channels 202 may receive
dense fluids from anode slots 107 (further discussed herein) and
transmit the dense fluids to the annular drain channel 203 via
anode base channels 202. The vertical wall 208 that partially
defines recessed portion 201 generally includes a plurality of
anode base slots 204 formed into the vertical wall 208. The anode
base slots 204 are generally positioned in parallel orientation
with each other, and further, are generally positioned in parallel
orientation with the plurality of anode base channels 202 formed
into the bottom surface of recessed portion 201. The anode base
member 104 also includes at least one conduit 205 configured to
dispense a fluid into an anode region of the plating cell 100,
along with at least one plating solution supply conduit 206 that is
configured to dispense a plating solution into the cathode
compartment of plating cell 100. The respective conduits 205 and
206 are generally in fluid communication with at least one fluid
supply inlet 109 (illustrated in FIG. 1A) positioned on a lower
surface of the anode base member 104.
[0051] FIG. 3 illustrates a perspective view of the anode base
member 104 having the disk shaped anode 105 positioned therein. The
anode 105, which is generally a disk shaped copper member, i.e., a
soluble-type copper anode generally used to support copper
electrochemical plating operations, generally includes a plurality
of parallel positioned anode slots 107 formed therein. The anode
slots 107 generally extend through the interior of anode 105 and
are in fluid communication with both the upper surface and lower
surface of anode 105, as illustrated in the cross section of anode
105 in FIG. 1. As such, anode slots 107 allow fluids to travel
through the interior of anode 105 from the upper surface to the
lower surface of the anode 105. However, when anode 105 is
positioned within annular recess 201 of the anode base member 104,
the parallel anode slots 107 of the anode 105 are generally
positioned orthogonal to both anode base slots 204 and anode base
channels 202 of the anode base member 104, as illustrated
cooperatively by FIGS. 2 and 3. Further, with regard to
positioning, the anode slots 107 are generally positioned such that
the tilt angle of the cell positions the slots orthogonal to fluid
flow as a result of the tilt, i.e., the anode slots 107 are
positioned such that fluid flowing across the surface of the anode
105 as a result of the tilt angle of the cell will intersect the
anode slots 107 and be received therein. Although the inventors
have illustrated the anode slots 107 being positioned orthogonally
to the fluid flow, other fluid intersection angles, such as angles
between about 5.degree. and about 89.degree., are contemplated
within the scope of the invention. Additionally, anode slots 107
generally do not continuously extend across the upper surface of
anode 105. Rather, anode slots 107 are broken into a longer segment
303 and a shorter segment 304, with a conductive spacer 305 between
the two segments, which operates to generate a longer current path
through anode 105 from one side to the other (when the current path
is measured orthogonal to the anode slots 107). Further, adjacently
positioned anode slots 107 have the conductive spacer 305
positioned on opposite sides of the anode upper surface for each
alternating anode slot 107. As such, the current path from the
lower side of the anode 105 to the upper side of the anode 105
(orthogonal to the direction of the anode slots 107) generally
includes a back and forth type path between the respective channels
107 through the spacer 305. Further, the positioning of spacers 305
and the anode slots 107 provides for improved concentrated
Newtonian fluid removal from the surface of the anode 105, as the
positioning of the anode slots 107 provides a shortest possible
distance of travel for the dense fluids to be received in the anode
slots 107. This feature is important, as dense fluids generally
travel slowly, and therefore, it is desirable.
[0052] In another embodiment of the invention anode 105 is
manufactured from a bipolar insoluble electrode material. In this
embodiment, the auxiliary electrode 132 may also be manufactured
from a bipolar insoluble electrode material. In this embodiment,
the anode 105 and auxiliary electrode 132 may be manufactured from
platinum or other metal that is inert and operable as an anode
material in an electrochemical plating solution. In this embodiment
of the invention, a copper dosing system, such as a copper
hydroxide dosing system, for example, may be used to replenish
copper into the plating solution, i.e., the anolyte and catholyte
of the plating cell, in place of the copper anode that supplies
copper to conventional soluble anode plating cells. The auxiliary
electrode 132 may be either consumable or a permanent part of the
cell.
[0053] Referring to FIG. 1A, the plating cell 100 further includes
a membrane support assembly 106 configured to support the membrane
108. The membrane support assembly 106 is generally secured at an
outer periphery thereof to the anode base member 104, and includes
an interior region that is configured to allow fluids to pass
therethrough via a sequence of oppositely positioned slots, bores,
or other fluid apertures (not shown). The membrane support assembly
106 may include an o-ring type seal (not shown) positioned near a
perimeter of the membrane 108, wherein the seal is configured to
prevent fluids from traveling from one side of the membrane 108
secured on the membrane support assembly 106 to the other side of
the membrane 108 without passing through the membrane 108
itself.
[0054] The implementation of the membrane between the anode and the
substrate being plated generates substantially different behavior
in the plating cell as compared to conventional plating cells, both
without membranes and those with the membranes discussed in this
application. Specifically, the behavior of a copper anode in an
acid-free CuSO.sub.4 solution is different from conventional anode
behavior. First, the sludge formation rate is lower at current
densities of up to about 60 mA/cm.sup.2 than that in
CuSO.sub.4/H.sub.2SO.sub.4 electrolyte, especially at
concentrations of less than about 0.5M. In more concentrated
CuSO.sub.4 solutions both the amount of sludge and the probability
of anode passiviation increases, especially at low flow rates
through the anode compartment. Further, although Cu.sup.+ generally
forms on the anode in both conventional tools and the tool of the
invention, in the configuration of the present invention it
accumulates only into the anolyte, mainly at current densities of
greater than about 30 mA/cm.sup.2, when the oxygen dissolved in
electrolyte has no time to convert Cu.sup.+ into Cu.sup.2+ again.
Further still, the stability of the anolyte and catholyte
compositions decreases dramatically because of the small volumes of
tanks.
[0055] FIG. 4 illustrates an exploded perspective view of an
exemplary membrane support assembly 106 of the invention. Membrane
support assembly 106 generally includes an upper ring shaped
support member 401, an intermediate membrane support member 400,
and a lower support member 402. Upper and lower support members 401
and 402 are generally configured to provide structural support to
intermediate membrane support member 400, i.e., upper support
member 401 operates to secure intermediate membrane support member
400 to lower support member 402, while lower support member 402
receives intermediate membrane support member 400. Intermediate
membrane support member 400 generally includes a substantially
planar upper surface having a plurality of bores (not shown)
partially formed therethrough. A lower surface of intermediate
membrane support member 400 generally includes a tapered outer
portion 403 and a substantially planar inner membrane engaging
surface 404. An upper surface of lower support member 402 may
include a corresponding tapered portion configured to receive the
tapered outer portion 403 of intermediate membrane support member
400 thereon. The membrane engaging surface 404 generally includes a
plurality of parallel positioned/orientated channels (not shown).
Each of the channels formed into the lower surface of intermediate
membrane support member 400 are in fluid communication with at
least one of the plurality of bores partially formed through the
planar upper surface. The channels operate to allow a membrane
positioned in the membrane support assembly to deform slightly
upward in the region of the channels, which provides a flow path
for air bubbles and less dense fluids in the cathode chamber to
travel to the perimeter of the membrane and be evacuated from the
anode chamber.
[0056] In operation, the plating cell 100 of the invention provides
a small volume (electrolyte volume) processing cell that may be
used for copper electrochemical plating processes, for example.
Plating cell 100 may be horizontally positioned or positioned in a
tilted orientation, i.e., where one side of the cell is elevated
vertically higher than the opposing side of the cell, as
illustrated in FIG. 1. If plating cell 100 is implemented in a
tilted configuration, then a tilted head assembly and substrate
support member may be utilized to immerse the substrate at a
constant immersion angle, i.e., immerse the substrate such that the
angle between the substrate and the upper surface of the
electrolyte does not change during the immersion process, or
alternatively, at an angle that varies during the immersion
process. Further, the immersion process may include a varying
immersion velocity, i.e., an increasing velocity as the substrate
becomes immersed in the electrolyte solution and rotation of the
substrate during the immersion process. The combination of the
constant immersion angle, rotation, and the varying immersion
velocity operates to eliminate air bubbles on the substrate
surface.
[0057] Assuming a tilted implementation is utilized, a substrate is
first immersed into a plating solution contained within inner basin
102. The immersion process generally includes positioning the
substrate onto a cathode substrate support member or substrate
contact ring. The substrate contact ring is generally configured to
both support the substrate for electrochemical processing, as well
as electrically contact the substrate to facilitate the
electrolytic plating reaction. The electrical contact between the
contact ring and the substrate is generally made via a plurality of
electrically conductive contact pins positioned and configured to
electrically engage a perimeter portion of the substrate and supply
a plating bias to the substrate sufficient to support plating
operations. Exemplary contact rings may be found in commonly
assigned U.S. Pat. No. 6,136,163, filed on Mar. 5, 1999 and
entitled Apparatus for Electrochemical Deposition with Thermal
Anneal, commonly assigned U.S. Pat. No. 6,251,236, filed on Nov.
30, 1998 entitled Cathode Contact Ring for Electrochemical
Deposition, and commonly assigned U.S. patent application Ser. No.
10/355,479, filed on Jan. 31, 2003 entitled Contact Ring with
Embedded Flexible Contacts. All of the above noted cases
illustrating contact rings are incorporated by reference herein in
their entirety.
[0058] Once the substrate is immersed in the plating solution,
which generally contains copper sulfate, a chlorine ion source, and
one or more of a plurality of organic plating additives (levelers,
suppressors, accelerators, etc.) configured to control plating
parameters, an electrical plating bias is applied between a seed
layer on the substrate and the anode 105 positioned in a lower
portion of plating cell 100. The electrical plating bias generally
operates to cause metal ions in the plating solution to deposit on
the cathodic substrate surface. The plating solution supplied to
inner basin 102 is continually circulated through inner basin 102
via fluid inlet 109 and conduits 206. More particularly, the
plating solution may be introduced in plating cell 100 via a fluid
inlet 109. The solution may travel across the lower surface of the
anode base member 104 and upward through one of fluid conduits 206.
The plating solution may then be introduced into the cathode
chamber via a channel formed into plating cell 100 that
communicates with the cathode chamber at a point above membrane
support assembly 106 and in fluid communication with conduits 206.
Similarly, the plating solution may be removed from the cathode
chamber via a corresponding conduit 206. For example, as discussed
above with respect to FIG. 2, the anode base member 104 may include
first and second conduits 206 positioned on opposite sides of the
anode base member 204. The oppositely positioned conduits 206 may
operate to individually introduce and drain the plating solution
from the cathode chamber in a predetermined direction, which also
allows for flow direction control.
[0059] Once the plating solution is introduced into the cathode
chamber, the plating solution travels upward through a diffusion
plate 110. Diffusion plate 110, which is generally a ceramic or
other porous disk shaped member, generally operates as a fluid flow
restrictor to even out the flow pattern across the surface of the
substrate. Further, the diffusion plate 110 operates to resistively
damp electrical variations in the electrochemically active area of
the anode or cation membrane surface, which has been shown to
reduce plating uniformities. Additionally, embodiments of the
invention contemplate that the ceramic diffusion plate 110 may be
replaced by a hydrophilic plastic member, i.e., a treated
polyethylene (PE) member, a polyvinylidene (PVDF) member, a
polypropylene (PP) member, or other material that is known to be
porous and provide the electrically resistive damping
characteristics provided by ceramics. However, the plating solution
introduced into the cathode chamber, which is generally a plating
catholyte solution, i.e., a plating solution with additives, is not
permitted to travel downward through the membrane (not shown)
positioned on the lower surface of membrane support assembly 106
into the anode chamber, as the anode chamber is fluidly isolated
from the cathode chamber by the membrane. The anode chamber
includes separate individual fluid supply and drain sources
configured to supply an anolyte solution to the anode chamber. The
solution supplied to the anode chamber, which may generally be
copper sulfate in a copper electrochemical plating system,
circulates exclusively through the anode chamber and does not
diffuse or otherwise travel into the cathode chamber, as the
membrane 108 positioned on membrane support assembly 106 is not
fluid permeable in either direction.
[0060] Additionally, the flow of the fluid solution (anolyte, i.e.,
a plating solution without additives, which may be referred to as a
virgin makeup solution) into the anode chamber is also
directionally controlled in order to maximize plating parameters.
For example, anolyte may be communicated to the anode chamber via
an individual fluid inlet 109. Fluid inlet 109 is in fluid
communication with a conduit 205 formed into a lower portion of the
anode base member. A seal positioned radially outward of conduits
205, in conjunction with the surrounding structure, directs the
anolyte flowing out of conduits 205 upward and into anode base
slots 204. Thereafter, the anolyte generally travels across the
upper surface of the anode 105 towards the opposing side of the
anode base member 104, which in a tilted configuration, is
generally the lower side of plating cell 100. The anolyte travels
across the surface of the anode below the membrane positioned
immediately above. Once the anolyte reaches the opposing side of
anode 105, it is received into a corresponding conduit 205 and
drained from plating cell 100 for recirculation thereafter.
[0061] During plating operations, the application of the electrical
plating bias between the anode and the cathode generally causes a
breakdown of the anolyte solution contained within the anode
chamber. More particularly, the application of the plating bias
operates to generate multiple hydrodynamic or Newtonian layers of
the copper sulfate solution within the anode chamber. The
hydrodynamic layers generally include a layer of concentrated
copper sulfate positioned proximate the anode, an intermediate
layer of normal copper sulfate, and a top layer of lighter and
depleted copper sulfate proximate the membrane. The depleted layer
is generally a less dense and lighter layer of copper sulfate than
the copper sulfate originally supplied to the anode compartment,
while the concentrated layer is generally a heavier and denser
layer of copper sulfate having a very viscous consistency. The
dense consistency of the concentrated layer proximate the anode
causes electrical conductivity problems (known as anode
passiviation) in anodes formed without anode slots 107. However,
anode slots 107, in conjunction with the tilted orientation of
plating cell 100, operate to receive the concentrated viscous layer
of copper sulfate and remove the layer from the surface of the
anode, which eliminates conductivity variances. Further, as noted
above, plating cell 100 generally includes one side that is tilted
upward or vertically positioned above the other side, and
therefore, the upper surface of anode 105 is generally a plane that
is also tilted. This tilt causes the layer of concentrated copper
sulfate generated at the surface of the anode to generally flow
downhill as a result of the gravitational force acting thereon. As
the concentrated copper sulfate layer flows downward, it is
received within one of anode slots 107 and removed from the surface
of the anode 105. As discussed above, anode slots 107 are generally
parallel to each other and are orthogonal to anode base slots 204.
As such, each of anode slots 107 intersect several of anode base
channels 202 at the lower surface of the anode 105. This
configuration allows the concentrated copper sulfate received
within anode slots 107 to be communicated to one or more of anode
base channels 202. Thereafter, the concentrated copper sulfate may
be communicated via anode base channels 202 to the annular drain
channel 203 positioned within recessed portion 201. The annular
drain channel 203 in communication with anode base channels 202 may
generally be communicated through the anode base member 104 and
back to a central anolyte supply tank, where the concentrated
copper sulfate removed from the anode surface may be recombined
with a volume of stored copper sulfate used for the anolyte
solution. Similarly, the upper portion of anode chamber generates a
diluted layer of copper sulfate proximate the membrane. The diluted
layer of copper sulfate may be removed from the anode chamber via
an air vent/drain 501, as illustrated in FIG. 5.
[0062] FIG. 5 shows a partial sectional view of an edge of the
plating cell 100 showing a groove 120 where the encased auxiliary
electrode assembly 130 (FIG. 1B) is generally positioned. The
groove 120 is located in the lower portion of the inner basin 102.
Air vent/drain 501, which may include multiple ports, is generally
positioned on the upper side of electrochemical plating cell 100,
and therefore, is positioned to receive both bubbles trapped within
the anode chamber, as well as the diluted copper sulfate generated
at the membrane surface. Air vents 501 are generally in fluid
communication with the anolyte tank discussed above, and therefore,
conducts the diluted copper sulfate received therein back to the
anolyte tank, where the diluted copper sulfate may combine with the
concentrated copper sulfate removed via anode slots 107 to form the
desired concentration of copper sulfate within the anolyte tank.
Any bubbles trapped by air vent 501 may also be removed from the
cathode chamber vented to the atmosphere or simply maintained
within the anolyte tank and not recirculated into the cathode
chamber.
[0063] The catholyte solution (the solution used to contact and
plate metal/copper onto the substrate) generally includes several
constituents. The constituents generally include a virgin makeup
plating solution or VMS (a plating solution that does not contain
any plating additives, such as levelers, suppressors, or
accelerators, such as that provided by Shipley Ronal of
Marlborough, Mass. or Enthone, a division of Cookson Electronics
PWB Materials & Chemistry of London), water (generally included
as part of the VMS, but is may also be added), and a plurality of
plating solution additives configured to provide control over
various parameters of the plating process. The catholyte is
generally a low acid-type of plating solution, i.e., the catholyte
generally has between about 5 g/l of acid and about 50 g/l of acid,
or more particularly, between about 5 g/l and about 10 g/l. The
acid may be sulfuric acid, sulfonic acid (including alkane sulfonic
acids), pyrophosphoric acid, citric acid, and other acids known to
support electrochemical plating processes. The desired copper
concentration in the catholyte is generally between about 25 g/l
and about 70 g/l, preferably between about 30 g/l and about 50 g/l
of copper. The copper is generally provided to the solution via
copper sulfate, and/or through the electrolytic reaction of the
plating process wherein copper ions are provided to the solution
via the anolyte from a soluble copper anode positioned in the
catholyte solution. More particularly, copper sulfate pentahydrate
(CuSO.sub.4.5H.sub.2O) may be diluted to obtain a copper
concentration of about 40 g/l, for example. A common acid and
copper source combination is sulfuric acid and copper sulfate, for
example. The catholyte also has chlorine ions, which may be
supplied by hydrochloric acid or copper chloride, for example, and
the concentration of the chlorine may be between about 30 ppm and
about 60 ppm.
[0064] As noted above, the plating solution (catholyte) generally
contains one or more plating additives configured to provide a
level of control over the plating process. The additives may
include suppressors at a concentration of between about 1.5 ml/l
and about 4 ml/l, preferably between about 2 ml/l and 3.0 ml/l.
Exemplary suppressors include ethylene oxide and propylene oxide
copolymers. Additives may also include accelerators at a
concentration of between about 3 ml/l and about 10 ml/l, preferably
within the range of between about 4.5 ml/l and 8.5 ml/l. Exemplary
accelerators are based on sulfopropyl-disulfide or
mercapto-propane-sulphonate and their derivatives. Additionally,
another additive that may optionally be added to the catholyte
solution is a leveler at a concentration of between about 1 ml/l
and about 12 ml/l, or more particularly, in the range of between
about 1.5 ml/l and 4 ml/l.
[0065] The anolyte solution, as noted above, is generally contained
in the volume below the membrane and above the anode. The anolyte
solution may be simply the catholyte solution without the plating
additives, i.e., levelers, suppressors, and/or accelerators.
However, the inventors have found that specific anolyte solutions,
other than just stripped catholyte solutions, provide a substantial
improvement in plating parameters. Specifically, copper transfer
through the membrane and prevention of copper sulfate and hydroxide
precipitation, i.e., when the Cu ions transport through membrane,
copper sulfate accumulates in the anolyte and starts to precipitate
on the anode provoking its passiviation, are improved. When pH of
the anolyte is maintained above about 4.5 to about 4.8, copper
hydroxide starts to deposit from Cu salt solutions, i.e.,
Cu.sup.2++2H.sub.2O=Cu(OH).sub.2 (deposit)+2H.sup.+. More
particularly, the inventors have found that if the anolyte can be
configured to supply between about 90% and about 100% of the copper
to the catholyte, then the membrane essentially operates as a clean
copper anode, i.e., the membrane provides copper to the catholyte
without the disadvantages associated with the electrochemical
reaction that takes place at the surface of the anode (sludge
formation, additive consumption, planarity variations due to
erosion, etc.). The anolyte of the invention generally includes a
soluble copper II salt (copper ions are not complexed with ligands
like NH.sub.3, or EDTA or phyrophoshoric acid anions, as Cu
transports through the membrane together with this ligand, like
Cu(NH.sub.3).sub.4.sup.2+ will transport together with NH.sub.3,
such as copper sulfate, copper sulfonate, copper chloride, copper
bromide, copper nitrate, or a blend of any combination of these
salts in an amount sufficient to provide a concentration of copper
ions in the catholyte of between about 0.1M and about 2.5M, or more
particularly, between about 0.25M and about 2M.
[0066] Additionally, the pH of the anolyte solution will generally
be between about 1.5 and about 6, or more particularly, between
about 2 and 4.8, for example. The pH is maintained in this range,
as increasing the pH above this range in conventional plating
configurations has been shown to cause copper hydroxide
precipitation. Additionally, when the pH is below 2, and
particularly if the pH is below 1.5, then the solution supports a
substantial increase in the hydrogen ion (H.sup.+) transport
through the membrane from the anolyte to the catholyte. In this
situation, the bulk of the plating current is carried by the
H.sup.+ ions and the copper ion transport is reduced. As such, the
copper ion concentration in the catholyte decreases, potentially to
a critical level that will not support plating, while
simultaneously the sulfuric acid concentration in the catholyte
increases. The anolyte can generally use any soluble Cu.sup.2+
salt, such as CuSO.sub.4 (solubility 300 g/L), CuBr.sub.2
(solubility more that 2 kg/L), CuCl.sub.2 (solubility 700 g/L),
CuF.sub.2 (47 g/L), Cu(NO.sub.3).sub.2 (1300 g/L) etc. The
selection of anions depends on their impact to prevent or minimize
Cu(I) formation and anode passiviation, on penetration through the
membrane etc. For instance, the anolyte can be CuSO.sub.4 (0.5 M)
with small additions of Cu(NO.sub.3) to activate anode surface and
minimize Cu(I) formation. To minimize Cu(I) formation, small
additions of Cu(ClO.sub.3).sub.2 (solubility 2 kg/L) or
Cu(lO.sub.3).sub.2--solubility 1 g/L may be used. In similar
fashion to the catholyte, the source of copper in the anolyte
(aside from the anode) may be copper sulfate pentahydrate
(CuSO.sub.4.5H.sub.2O) at between about 51 g/L and 70 g/L, or at
between about 0.75 M and about 0.95 M. Alternatively, in a
preferred embodiment, the copper source may be between about 51 g/L
and about 60 g/L, preferably about 54 g/L, and at a molarity of
between about 0.8 M and about 0.9 M, preferably about 0.85 M.
[0067] FIG. 6 illustrates a plating thickness plot for a plating
cell with the encased auxiliary electrode assembly in fluid
communication with the catholyte compared to a plating cell without
the encased auxiliary electrode assembly. The substrate plated had
a diameter of 300 mm. This specific example is for 200 .ANG. plated
on a 300 .ANG. Cu seed layer. The auxiliary electrode current for
the "with auxiliary electrode" data is 0.4 A. The auxiliary
electrode will work the same way for other seed layers, current
densities, and plating thicknesses, but the auxiliary electrode
current will vary.
[0068] The plating thickness plot of FIG. 6 results from the
auxiliary electrode absorbing electric flux near the perimeter of
the substrate. This essentially results in the anode electrically
seeing a substrate that has a larger surface area than the
substrate being plated and as such, the terminal effect is shifted
to the encased auxiliary electrode assembly 130 from the perimeter
of the substrate, i.e., the increased thickness near the perimeter
is shifted to the auxiliary electrode and off or away from the
perimeter of the substrate. Further, since the auxiliary electrode
may be controlled, the deposition thickness near the perimeter of
the substrate may also be controlled.
[0069] FIG. 7 illustrates a method for immersing a substrate into a
plating solution in accordance with one or more of the embodiments
of the invention. This method 700 involves modulating the current
density applied to the substrate/wafer in a manner that is
synchronized with the wafer's motion and exerting strict control of
the current density uniformity across the wafer surface. This
approach targets the first few seconds of plating (or the first 50
to about 100 .ANG. of deposition) when features are filled.
[0070] The initial steps 710 and 720 involve development of an
optimized waveform that is applied to the wafer during the initial
seconds of plating including the immersion sequence. Details of the
immersion sequence and waveform development are described in U.S.
patent application Ser. No. 11/052,443, entitled "Immersion Process
For Electroplating Applications," assigned to Applied Materials,
Inc., filed Feb. 7, 2005 and herein incorporated by reference to
the extent not inconsistent with the invention. The waveform
consists of several steps whose durations and amplitudes are tuned
to protect the seed-layer induce bottom-up growth, and achieve
void-free fill. The steps are closely synchronized with the
trajectory of the wafer as it is immersed and moved into its final
plating position.
[0071] The next step 730 involves dynamic control of the
instantaneous current distribution on the wafer. This has been
achieved by model-guided design of the plating cell 100
configuration, which includes a diffusion plate 110 and an encased
auxiliary electrode 132 to compensate for the terminal effect and
produce uniform distribution of the instantaneous current density
during the stages of plating. Specifically, the auxiliary electrode
132 current is dynamically varied in tandem with the waveform
applied to the wafer and the growing thickness of the deposit.
[0072] As discussed earlier, "terminal effect" causes difference in
deposition rate between different points across a substrate. The
severity of the "terminal effect" are generally determined by size
of the substrate and thickness of a seed layer, or the deposited
metal layer. Because the thickness of the deposited metal layer
increases as a plating process goes on, the severity of "terminal
effect" changes continuously, which may require the compensation of
"terminal effect" to be dynamic or adjustable. For example, when
the substrate's resistivity is high, it may be desirable to plate
under "edge-thin" conditions to compensate for the severe terminal
effect during the initial stages of plating. As the terminal effect
disappears, it may become desirable to plate under uniform plating
conditions. As the plating process is continues and the overburden
is deposited, it may be desirable to plate under an edge-thin,
edge-thick, or uniform condition or with a specific plating profile
that is optimum for subsequent processes such as CMP.
[0073] In this invention, the plating profile on the substrate is
adjusted over the course of the plating process by plating in an
apparatus where the electric field varies significantly as a
function of the position of the substrate. This allows the
different plating profiles to be achieved by moving the substrate.
In the examples below, the electric field is made to vary within
the plating cell as a function of the substrate's height above a
restrictor element, and the wafer is moved to different heights
during the plating process to achieve the different desired
profiles. It should be noted however that the dynamic adjustment of
the substrate position during the plating process to access
different plating profiles is a general concept applicable to any
plating apparatus where the plating profile on the substrate can be
made to vary substantially with the substrate's position.
[0074] FIGS. 8A-8C schematically illustrate an electrochemical
plating cell 800 configured to provide an adjustable plating
profile. A substrate 801 is retained in a substrate support 802
configured to transfer the substrate 801 vertically. A restrictor
803 is disposed parallel to the substrate 801. The restrictor 803
has a diameter smaller than the diameter of the substrate 801.
During process, the substrate 801 and the restrictor 803 are both
immersed in a plating solution, and an electric field is applied to
the substrate 801 via the restrictor 803. Curves 804 indicates the
current density of the electric field.
[0075] FIGS. 8A-8C illustrate the substrate 801 has a vertical
elevation Z1, Z2 and Z3 respectively relative to the restrictor
803, wherein elevation Z1 is the smallest and elevation Z3 is the
largest. At elevation Z1, shown in FIG. 8A, the current density 804
on the surface of the substrate 801 has an edge thin pattern for
the substrate 801 is close to the restrictor 803. At elevation Z3,
shown in FIG. 8C, the current density 804 on the surface of the
substrate 801 has an edge thick pattern for the substrate 801 is
far away from the restrictor 803. At elevation Z2, shown in FIG.
8B, the current density 804 is relatively uniformly distributed on
the surface of the substrate 801. Therefore, an edge thin plating
profile may be achieved by positioning the substrate 801 at a low
elevation, e.g., elevation Z1, an edge thick plating profile may be
achieved by positioning the substrate 801 at a high elevation,
e.g., elevation Z3, a uniform plating profile may be achieved by
positioning the substrate 801 at a proper elevation, such as
elevation Z2.
[0076] In one embodiment, the substrate 801 may be moved among
different elevations during the plating process while the plating
bias is being applied to the substrate 801 through the restrictor
803 having a smaller area than the substrate 801.
[0077] It should be noted that the restrictor 803 may be any means
that functions as a means to restrict or "retain" the plating
current within an area smaller than the plating surface on the
substrate. Exemplary restrictor may be an anode with a smaller
surface area than the substrate, or a diffuser plate with a smaller
open area then the substrate, or part of a fluid basin having a
smaller diameter than the rest of the fluid basin, or any means to
restrict the path of plating electric field between the anode and
the substrate.
[0078] FIG. 9A illustrates an electrochemical plating cell 900A
configured to have an adjustable electric field across a substrate
during a plating process. The plating cell 900A comprises a fluid
basin 901 defining a fluid volume 909 configured to contain at
least one electrolyte. The plating cell 900A further comprises a
substrate support member 902 configured to support a substrate 903
therein. The substrate support member 902 contacts the substrate
903 via a plurality of conductive contact pins 910. The substrate
support member 902 is further configured to move the substrate 903
and to spin the substrate 903 during process. An anode 908 is
disposed in the fluid volume 909. The anode 908 may be a disk
having a smaller diameter than the substrate 903. The substrate 903
may be moved vertically in the fluid volume 909, for example, into
position 903A or 903B, during plating to achieve an edge thin, an
edge thick or a uniform plating profile.
[0079] During different stages of a plating process, the substrate
903 may be moved to different elevations and/or biased by different
waveforms to achieve a desired plating profile. For example, a
first waveform may be applied to the substrate 903 from the
substrate 903 first immersed into the plating solution until the
substrate 903 reaches a first plating position. In one embodiment,
the first waveform may be configured to prevent a seed layer on the
substrate 903 from being deplated. Then a second waveform may be
applied to the substrate 903 when the substrate 903 is in the first
plating position. In one embodiment, the first plating position may
have a low elevation relative to the anode 908, for example,
position 903B, to generate an edge thin profile and the second
waveform may be configured accordingly. The substrate 903 can then
be moved to a second plating position and a third waveform may be
applied to the substrate 903. In one embodiment, moving the
substrate 903 from the first plating position to the second plating
position may be conducted when the plated layer becomes thicker and
the terminal effect becomes less severe. The second plating
position may have a relative higher elevation compared to the first
plating position, such as position 903A, therefore, providing a
relatively uniform plating profile.
[0080] FIG. 9B illustrates an electrochemical plating cell 900B
configured to have an adjustable electric field across a substrate
during a plating process. An anode 911 is disposed near the bottom
of the fluid volume 909 of the fluid basin 901. A diffuser plate
913 is placed above the anode 911. The diffuser plate 913 may be
secured in place by a ring shaped support 912 extended from the
fluid basin 901. The diffuser plate 913 may have a circular area to
allow the plating solution inside the fluid volume 909 to pass
through. The circular area may include a plurality of vertical
channels to guide the flow of the plating solution in the fluid
volume 909. The circular area is smaller than the surface area of
the substrate 903. In one aspect, the diffuser plate 913 is
configured to control the fluid flow in the fluid volume 909. In
another aspect, the diffuser plate 913 also restricts the plating
current within the circular area smaller than the substrate 903,
which allows the plating profile to be adjusted by moving the
substrate 903 vertically during the plating process. For example,
the substrate 903 may be positioned at position 903D for an edge
thin plating profile and be moved to position 903C for a more even
plating profile compared to at position 903D.
[0081] FIG. 9C illustrates an electrochemical plating cell 900C
configured to have an adjustable electric field across a substrate
during a plating process. The substrate support member 902 contacts
the substrate 903 via a plurality of conductive contact pins 910.
An anode 904 is disposed near the bottom of the fluid basin 901. A
power supply 906 is configured to apply an electric bias between
the conductive contact pins 910 and the anode 904. An auxiliary
electrode 905 is disposed in the fluid basin 901. The auxiliary
electrode--905 is coupled to a power supply 916 which applies a
bias to the auxiliary electrode 905 to adjust the electric field
between the substrate 903 and the anode 904. The electrochemical
plating cell 900 further comprises a shield 907 configured to
shield part of the fluid volume 909 from being affected by the
auxiliary electrode 905. The shield 907 may be a ring having an
inner diameter smaller than the outer diameter of the substrate
903.
[0082] In one embodiment, the auxiliary electrode 905 may be an
encased electrode as described above. The auxiliary electrode 905
may be biased independently from the anode 904 and the substrate
903. The effect of the auxiliary electrode 905 may be adjusted by
controlling the bias applied to the auxiliary electrode 905 and/or
adjusting the distance between the substrate 903 and the auxiliary
electrode 905.
[0083] The shield 907 further serves as a restrictor to retain the
plating current between the anode 904 and the substrate 903.
Therefore, the plating profile on the substrate 903 can be adjusted
by moving the substrate 903 to change the elevation of the
substrate 903 relative to the shield 907.
[0084] In this configuration, the plating profile may be changed by
applying an appropriate bias to the auxiliary electrode in
combination with positioning the substrate 903 at different
elevations above the shield 907. As a result, a set of different
plating profiles, which are not otherwise available, can be
generated on the substrate 903, including edge-thin, edge thick or
uniform, and other desirable plating profiles.
[0085] In another embodiment, an auxiliary electrode may be used in
combination of a diffuser plate with a restricted area for fluid to
pass through. For example, referring to FIG. 9B, an auxiliary
electrode 917 may be disposed over the ring shaped support 912.
[0086] While the foregoing is directed to embodiments of the
present invention, other and further embodiments of the invention
may be devised without departing from the basic scope thereof, and
the scope thereof is determined by the claims that follow.
* * * * *