U.S. patent application number 15/873660 was filed with the patent office on 2018-05-24 for method and apparatus for dynamic current distribution control during electroplating.
The applicant listed for this patent is Lam Research Corporation. Invention is credited to Zhian He.
Application Number | 20180142374 15/873660 |
Document ID | / |
Family ID | 50772312 |
Filed Date | 2018-05-24 |
United States Patent
Application |
20180142374 |
Kind Code |
A1 |
He; Zhian |
May 24, 2018 |
METHOD AND APPARATUS FOR DYNAMIC CURRENT DISTRIBUTION CONTROL
DURING ELECTROPLATING
Abstract
A method of electroplating a metal on a cathodically biased
wafer substrate employs an electroplating apparatus having a main
anode, an auxiliary electrode and an ionic current collimator,
where the ionic current collimator is configured to direct ionic
current that is generated by the main anode from a periphery of a
plating vessel to its center. During electroplating, in a first
electroplating stage (e.g., when terminal effect is pronounced),
the metal is plated onto the wafer while the auxiliary electrode is
cathodically biased; and in a second electroplating stage (e.g.,
when terminal effect subsides), the metal is plated onto the wafer
while the auxiliary electrode is anodically biased. In some
embodiments the auxiliary electrode is located between the ionic
current collimator and the wafer, and is configured to redistribute
ionic current that has passed through the collimator. The method is
useful for improving the uniformity of electroplating.
Inventors: |
He; Zhian; (Lake Oswego,
OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lam Research Corporation |
Fremont |
CA |
US |
|
|
Family ID: |
50772312 |
Appl. No.: |
15/873660 |
Filed: |
January 17, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14067616 |
Oct 30, 2013 |
9909228 |
|
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15873660 |
|
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61730285 |
Nov 27, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 5/022 20130101;
C25D 17/001 20130101; C25D 17/007 20130101; C25D 7/123 20130101;
C25D 17/002 20130101; C25D 17/10 20130101; C25D 17/008 20130101;
C25D 21/12 20130101 |
International
Class: |
C25D 17/00 20060101
C25D017/00; C25D 7/12 20060101 C25D007/12 |
Claims
1. A method of electroplating a metal on a wafer substrate, the
method comprising: (a) providing the wafer substrate to an
electroplating apparatus having a wafer holder, and a plating
vessel containing a main anode, an auxiliary electrode, and an
ionic current collimator, wherein the ionic current collimator is
configured to direct ionic current from the periphery to the center
of the plating vessel; (b) in a first electroplating stage,
electroplating metal onto the wafer substrate while cathodically
biasing the auxiliary electrode; and (c) in a second electroplating
stage, electroplating metal onto the wafer substrate while
anodically biasing the auxiliary electrode.
2. The method of claim 1, wherein the second electroplating stage
is performed after the first electroplating stage, and wherein
during the first electroplating stage a cathodic current initially
applied to the auxiliary electrode is at least about 300% of a
cathodic current applied to the wafer substrate at the same
time.
3. The method of claim 1, wherein in the second electroplating
stage, the auxiliary electrode is anodically biased to a greater
extent than the main anode at least during a portion of the second
electroplating stage.
4. The method of claim 1, further comprising cathodically biasing
the main anode during at least a portion of the second
electroplating stage.
5. The method of claim 1, further comprising moving the ionic
current collimator and the auxiliary electrode in a direction
perpendicular to a plating surface of the wafer substrate, during
the electroplating.
6. The method of claim 1, further comprising the steps of: applying
photoresist to the wafer substrate; exposing the photoresist to
light; patterning the photoresist and transferring the pattern to
the wafer substrate; and selectively removing the photoresist from
the wafer substrate.
7. The method of claim 1, wherein the ionic current collimator
comprises: (i) a central portion in the form of an open cylinder
extending in a direction that is perpendicular to a plating surface
of the wafer substrate, wherein the openings of the cylinder
provide a route for the ionic current; and (ii) a current
restricting portion connected to the central portion, the current
restricting portion extending in a direction that is parallel to
the plating surface of the wafer substrate.
8. The method of claim 7, wherein the current restricting portion
of the ionic current collimator extends to sidewalls of the plating
vessel, and is configured to block ionic current at the periphery
of the plating vessel.
9. The method of claim 8, wherein the current restricting portion
of the ionic current collimator is attached to the sidewalls of the
plating vessel.
10. The method of claim 1, wherein the ionic current collimator is
made of a dielectric material that is not permeable to electrolyte
and is selected from the group consisting of polycarbonate,
polyethylene, polypropylene, polyvinylidene difluoride (PVDF),
polytetrafluoroethylene, and polysulphone.
11. The method of claim 1, wherein the ionic current collimator
does not contact the main anode and is spaced from the main anode
by a distance of at least about 15% of the wafer substrate
radius.
12. The method of claim 1, wherein the auxiliary electrode resides
over the main anode and between the ionic current collimator and
the wafer substrate holder.
13. The method of claim 1, wherein the ionic current collimator
serves as a platform supporting the auxiliary electrode.
14. The method of claim 1, wherein the auxiliary electrode
comprises copper at least on the surface of the auxiliary
electrode.
15. The method of claim 1, wherein a footprint of the auxiliary
electrode onto the main anode is at least about 40% of a total
anode area.
16. The method of claim 1, wherein the auxiliary electrode has a
generally toroidal shape and a thickness of at least about 20
mm.
17. The method of claim 1, wherein the auxiliary electrode has a
working surface of at least about 600 cm.sup.2.
18. The method of claim 1, wherein the apparatus further includes a
thief cathode, which is cathodically biased during a portion of
electroplating.
19. The method of claim 1, wherein the apparatus further includes a
thief cathode, which is cathodically biased during the first
electroplating stage and during a portion of the second
electroplating stage.
20. The method of claim 1, wherein the auxiliary electrode changes
polarity from cathodic to anodic after a defined sheet resistance
of the electroplated layer on the wafer substrate is reached.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of and claims priority to
U.S. application Ser. No. 14/067,616 filed on Oct. 30, 2013, titled
"Method and Apparatus for Dynamic Current Distribution Control
during Electroplating" naming Zhian He as the inventor, which
claims benefit of prior U.S. Provisional Application No. 61/730,285
filed on Nov. 27, 2012, titled "Method and Apparatus for dynamic
Current Distribution Control during Electroplating" naming Zhian He
as the inventor, which are herein incorporated by reference in
their entirety and for all purposes.
FIELD OF THE INVENTION
[0002] The present invention pertains to methods and apparatuses
for electroplating. Specifically, the invention pertains to
electroplating tools used for electrodeposition of metals in
semiconductor processing.
BACKGROUND OF THE INVENTION
[0003] The transition from aluminum to copper in integrated circuit
(IC) fabrication required a change in process "architecture" (to
damascene and dual-damascene) as well as a whole new set of process
technologies. One process step used in producing copper damascene
circuits is the formation of a "seed-" or "strike-" layer, which is
then used as a base layer onto which copper is electroplated
(electrofill). The seed layer carries the electrical plating
current from the edge region of the wafer (where electrical contact
is made) to all trench and via structures located across the wafer
surface. The seed film is typically a thin conductive copper layer.
It is separated from the insulating silicon dioxide or other
dielectric by a barrier layer. The use of thin seed layers having
dual barrier-seed function (e.g. alloys of copper, or other metals,
such as ruthenium and tantalum), has also been investigated.
[0004] As semiconductor industry advances, technology nodes are
moving towards very thin and resistive seed regime for
electrochemical fill. It becomes a very challenging problem to
achieve uniform initial plating across the wafer with such
resistive seed layers. To effectively plate a large surface area,
the plating tool makes electrical contact to the conductive seed
only in the edge region of the wafer substrate. There is no direct
contact made to the central region of the substrate. Hence, for
highly resistive seed layers, the potential at the edge of the
layer is significantly greater than at the central region of the
layer. Without appropriate means of resistance and voltage
compensation, this large edge-to-center voltage drop could lead to
an extremely non-uniform plating thickness distribution, primarily
characterized by thicker plating at the wafer edge. This effect is
known as terminal effect.
[0005] The non-uniform plating thickness will be even more
pronounced as the industry transitions from 300 mm wafer to 450 mm
wafer.
SUMMARY
[0006] The difficulty in controlling the terminal effect is further
exacerbated by the fact that it is very pronounced in the beginning
of electroplating when the seed layer on the wafer is most
resistive, but is rapidly diminishing during the course of
electroplating. As electroplating proceeds, the plated layer
becomes thicker and more conductive, thereby reducing the terminal
effect. Therefore, during a single electroplating process, very
different ionic current environments should be created in the
plating apparatus in order to compensate for the terminal effect in
the beginning of the process, and to continue electroplating after
the terminal effect has subsided.
[0007] The needs for controllable electroplating on resistive seed
layers are addressed herein by providing an apparatus and a method
for electroplating that make use of a focus ring (also referred to
as an ionic current collimator) positioned in the proximity of an
anode, and an auxiliary electrode with flexibly adjustable
electrical characteristics. The ionic current collimator provides a
resistive correction for the terminal effect by restricting the
ionic current at the periphery, and by directing the ionic current
to the central portions of the wafer substrate. However, the use of
ionic current collimator alone would have resulted in unnecessarily
center-thick plating. An auxiliary electrode residing around the
ionic current collimator corrects this problem, by diverting the
ionic current provided by the collimator away from the center, and
modifying it to make it more uniform. In some embodiments, the
auxiliary electrode is configured to serve as an auxiliary cathode
in the beginning of the electroplating process, when the terminal
effect is most pronounced, and is also configured to serve as an
auxiliary anode (or even as a main anode) later in the process. The
auxiliary electrode, in some embodiments, is configured to be able
to accept a large amount of current (e.g., at least about 200% of
the current supplied to the wafer in the beginning of
electroplating, such as at least about 400% of the current supplied
to the wafer in the beginning of electroplating), and is
characterized by an unusual location in the electroplating
apparatus. The auxiliary electrode, in some embodiments at least
partially resides over the main anode, and has a non-zero footprint
on the anode. In some embodiments, the auxiliary electrode is also
characterized by a large surface area, which allows it to accept
large currents without building excessively high current densities.
For example, the auxiliary electrode in some embodiments is capable
of accepting currents of between about 10-75 A, such as between
about 20-50 A.
[0008] In one aspect, an apparatus for electroplating metal on a
wafer substrate is provided. The apparatus comprises: (a) a plating
vessel configured for holding an electroplating solution therein;
(b) a wafer holder configured for holding the wafer substrate in
position during electroplating, the wafer holder having one or more
electrical power contacts arranged to contact an edge of the
substrate and provide electrical current to the substrate during
electroplating, wherein the apparatus is configured for
cathodically biasing the wafer substrate during electroplating; (c)
an anode (also referred to as "main anode") residing in the plating
vessel and configured to be anodically biased during at least a
portion of electroplating; (d) an ionic current collimator
proximate the anode, wherein the ionic current collimator is a
non-conductive member configured to direct the ionic current from
the anode generally from the periphery to the center of the plating
vessel; and (e) an auxiliary electrode configured to be both
cathodically and anodically biased during electroplating.
[0009] In another aspect, a method for electroplating a layer of
metal on a wafer substrate is provided. In some embodiments, the
method involves (a) providing the wafer substrate to an
electroplating apparatus having a wafer holder, and a plating
vessel containing a main anode, an auxiliary electrode, and an
ionic current collimator, wherein the ionic current collimator is
configured to direct ionic current from the periphery to the center
of the plating vessel; (b) in a first electroplating stage,
electroplating metal onto the wafer substrate while cathodically
biasing the auxiliary electrode; and (c) in a second electroplating
stage, electroplating metal onto the wafer substrate while
anodically biasing the auxiliary electrode.
[0010] In another aspect, a non-transitory computer readable medium
comprising instructions for control of an electroplating apparatus
is provided. The program instructions will include code for
performing the methods provided herein, such as (a) in a first
electroplating stage, electroplating metal onto the wafer substrate
while cathodically biasing the auxiliary electrode; and (b) in a
second electroplating stage, electroplating metal onto the wafer
substrate while anodically biasing the auxiliary electrode.
[0011] These and other features and advantages of the present
invention will be described in more detail below with reference to
the associated drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1A is a schematic cross-sectional view of an
electroplating apparatus in accordance with an embodiment presented
herein.
[0013] FIG. 1B is a schematic cross-sectional view of an
electroplating apparatus in accordance with another embodiment
presented herein.
[0014] FIG. 1C is a schematic cross-sectional view of an
electroplating apparatus in accordance with another embodiment
presented herein.
[0015] FIG. 2A is a schematic cross-sectional view of an ionic
current collimator residing over an anode.
[0016] FIG. 2B is a schematic top view of an ionic current
collimator.
[0017] FIG. 3A is a schematic cross-sectional view of an auxiliary
electrode, an ionic current collimator and an anode in accordance
with an embodiment presented herein.
[0018] FIG. 3B is a schematic top view of an auxiliary electrode in
accordance with an embodiment presented herein.
[0019] FIG. 4 is a schematic diagram illustrating electrical
connectivity between the controller, power supply, and components
of the plating cell in accordance with an embodiment provided
herein.
[0020] FIG. 5A is a process flow diagram for an electroplating
method in accordance with an embodiment provided herein.
[0021] FIG. 5B is an example of an algorithm for determining and
using controller instructions in accordance with an embodiment
provided herein.
[0022] FIG. 6A shows a computational modeling result illustrating
ionic current distribution in the electroplating apparatus during
the first stage of plating, in accordance with an embodiment
provided herein.
[0023] FIG. 6B shows a computational modeling result illustrating
ionic current distribution in the electroplating apparatus during
the second stage of plating, in accordance with one embodiment
provided herein.
[0024] FIG. 7 is a plot illustrating preferred current levels for
different components of the electroplating cell, as a function of
seed layer sheet resistance, in accordance with one example
presented herein.
[0025] FIG. 8 is a plot illustrating preferred current levels for
an auxiliary electrode as a function of electroplating time.
[0026] FIG. 9 is an illustration of instantaneous current
distribution on the wafer substrate at various stages of the
plating.
DETAILED DESCRIPTION
[0027] The methods and apparatus provided herein are useful for
electroplating a variety of metals including but not limited to
copper and its alloys on semiconductor substrates having one or
more recessed features (e.g., trenches and vias). The methods and
apparatus are useful for electroplating on 300 mm and,
particularly, on 450 mm semiconductor wafers and on resistive seed
layers. For example, the apparatus and methods, can be used, in
some embodiments, for electroplating on seed layers having seed
sheet resistance of up to about 50 Ohm/sq. (inclusive of this
number), e.g. with sheet resistance of between about 10-50 Ohm/sq.,
such as between about 20-40 Ohm/sq. Examples of substrates that can
be processed by provided methods include, without limitation, a 300
mm wafer having a copper seed layer having a thickness of between
about 10-2000 A, or a 450 mm wafer having a copper seed layer
having a thickness of between about 20-2000 A. In some embodiments
the initial copper seed layer thickness is between about 10-100 A,
e.g., between about 10-50 A.
[0028] The methods and apparatus described herein can be used to
provide plated layers having excellent center-to-edge uniformity
due to their high capability for controlling ionic current
environment within the electroplating bath. While in many
implementations uniform plating is desired, in some embodiments,
when center-thick or edge-thick plating is required, the apparatus
can be configured to control ionic current distribution such as to
introduce the desired non-uniformity.
[0029] In one aspect, an apparatus for electroplating is provided.
The apparatus comprises: (a) a plating vessel configured for
holding an electroplating solution therein; (b) a wafer holder
configured for holding the wafer substrate in position during
electroplating, the wafer holder having one or more electrical
power contacts arranged to contact an edge of the substrate and
provide electrical current to the substrate during electroplating,
wherein the apparatus is configured for cathodically biasing the
wafer substrate during electroplating; (c) an anode residing in the
plating vessel (also known as the main anode); (d) an ionic current
collimator proximate the anode, wherein the ionic current
collimator is a non-conductive member configured to direct the
ionic current from the anode generally from the periphery to the
center of the plating vessel; and (e) an auxiliary electrode
configured to be both cathodically and anodically biased during
electroplating.
[0030] The ionic current collimator is made of dielectric material
(e.g., plastic) that is not permeable to electrolyte. Examples of
suitable materials include polycarbonate, polyethylene,
polypropylene, polyvinylidene difluoride (PVDF),
polytetrafluoroethylene, and polysulphone. In some embodiments the
ionic current collimator comprises two portions: (i) the central
portion which is generally in the form of an open cylinder
extending in the direction that is perpendicular to the plane of
the wafer substrate (referring to the plane of the plating surface
of the substrate), and, typically, co-centered with the center of
the wafer substrate and the center of the anode, where the openings
of the cylinder provide a route for the ionic current; and (ii) the
current restricting portion which is connected to the cylindrical
portion, e.g., at the end of the cylindrical portion that is
proximate the anode, and which is generally parallel to the plane
of the wafer substrate. The current restricting portion typically
extends to the sidewalls of the plating vessel and is secured
(e.g., attached) to these walls such that the ionic current
collimator is held in place, and such that the ionic current from
the anode would not be able to escape at the periphery of the
plating vessel. Thus, the ionic current collimator can direct
substantially all of the current from the main anode through its
central cylindrical opening generally in the direction of the
center of the wafer. The ionic current collimator, in some
embodiments, does not contact the anode and is spaced apart from
the anode by at least about 15% of the wafer radius (e.g., by at
least about 40 mm), for example by around 60 mm. The spacing from
the anode determines the amount of anode utilization, and also
impacts the thickness or current density profile. For example, if
the collimator is too close to the anode, the anode utilization may
be relatively small.
[0031] In some embodiments, the ionic current collimator is
stationary, and does not move during electroplating.
[0032] In other embodiments, the apparatus is configured to move
the ionic current collimator along an axis that is perpendicular to
the plane of the wafer substrate. For example, the collimator can
be moved closer to the wafer when more ionic current at the wafer
center is desired, and can be moved away from the wafer when less
center-focused current is needed. In some implementations, the
apparatus includes a mechanism configured for moving the ionic
current collimator. For example, the collimator can be moved with a
bellows-type mechanism.
[0033] The auxiliary electrode is located between the anode and the
wafer substrate or the wafer substrate holder (referring to a
position on an axis that is perpendicular to the wafer surface),
and further away from the anode than the current-restricting
portion of the collimator. In some embodiments, the
current-restricting portion of the collimator serves as a
convenient platform on which the auxiliary electrode resides in the
electroplating apparatus. The auxiliary electrode is electrically
connected to a power supply and can be biased negatively or
positively, as desired. When biased negatively, the auxiliary
electrode serves as an auxiliary cathode and is capable of
diverting ionic current toward itself, thereby reducing the current
experienced by the wafer substrate, and redistributing the
center-focused current exiting the cylindrical portion of the
collimator. When biased positively, the auxiliary electrode serves
as an anode and is capable of providing additional ionic current to
the wafer. The auxiliary electrode is typically made of the same
material that is being electroplated. For example, when copper is
electrodeposited on the wafer substrate, a copper auxiliary
electrode is used, and serves as a source of copper ions when the
auxiliary electrode serves as an anode. In some embodiments, the
auxiliary electrode has a core made of any suitable metal and has a
coating of the metal that is being plated (e.g., copper coating).
The coating may be pre-made, or can be generated during initial
stages of electroplating, when the auxiliary electrode serves as a
cathode (due to electrodeposition on the cathodically biased
auxiliary electrode).
[0034] Other important characteristics of the auxiliary electrode
are its unusual location and its size. In some embodiments, the
auxiliary electrode is located not beyond the circumference of the
anode, but relatively closer to the center of the plating bath, and
around the opening of the current collimator. In such stacked
configuration, utilization of the surfaces for both the main anode
and the auxiliary electrode would be greater compared to an
arrangement in which the anode and the auxiliary electrode reside
in the same plane. The ability to utilize greater surface areas,
allows the use of high currents for both the main anode and the
auxiliary electrode without building unnecessarily high current
densities on these electrodes. This is a significant advantage,
because plating on resistive seed layers often calls for the use of
very high currents.
[0035] In some embodiments, the footprint of the auxiliary
electrode projected onto the anode is at least about 40% of the
total anode area, e.g., between about 60 and 80% of the total anode
area. Such position of the auxiliary electrode allows for efficient
use of the auxiliary electrode both in an anode mode and for
redistribution of central current from the collimator. Further, in
some embodiments the auxiliary electrode has a large surface area.
When the surface area is large, the auxiliary electrode can accept
a very high current (as often needed to compensate for the terminal
effect caused by highly resistive seed layers), without building
undesirably high current densities. In some embodiments, the
working surface area of the auxiliary electrode (the area that is
in contact with electrolyte) is at least about 600 cm.sup.2, such
as between about 900 cm.sup.2 and 1200 cm.sup.2. In some
embodiments, the auxiliary electrode has a generally toroidal
shape, having a thickness (difference between outer and inner
radius) that is at least about 20 mm, such as between about 20 mm
and 80 mm. For example, the difference between the outer and inner
radius, in some embodiments is at least about 5% of the wafer
radius, such as between about 8-40% of the wafer radius.
[0036] In some embodiments the auxiliary electrode is stationary.
In other embodiments the auxiliary electrode is configured to be
movable along an axis that is perpendicular to the plane of the
wafer substrate. The auxiliary electrode can be moved together with
a movable ionic current collimator, or separately from the
collimator. For example, the auxiliary electrode in an anode mode
can be moved closer to the wafer, in order to provide more plating
current to the center of the wafer. In some embodiments, the
electroplating apparatus includes a mechanism configured for moving
the auxiliary electrode (e.g., a bellows-like mechanism).
[0037] In addition to the ionic current collimator and the
auxiliary electrode, the electroplating apparatus may further
include additional elements that are useful for mitigating the
terminal effect. In some embodiments the apparatus further includes
an ionically resistive element having electrolyte-permeable pores
or holes, where the element resides in close proximity of the wafer
substrate (e.g., within about 5 mm of platable surface of the
wafer). The ionically resistive ionically permeable element is
useful for improving plating uniformity on thin resistive seed
layers. The ionically resistive ionically permeable element
presents a uniform current density in the proximity of the wafer
cathode and therefore serves as a virtual anode. Accordingly, the
ionically resistive ionically permeable element is also referred to
as a high-resistance virtual anode (HRVA)
[0038] In certain embodiments, the HRVA is located in close
proximity to the wafer. In certain embodiments, the HRVA contains a
plurality of through-holes that are isolated from each other and do
not form interconnecting channels within the body of HRVA. Such
through-holes will be referred to as 1-D through-holes because they
extend in one dimension, typically, but not necessarily, normal to
the plated surface of the wafer. These through-holes are distinct
from three-dimensional porous networks, where the channels extend
in three dimensions and form interconnecting pore structures. An
example of a HRVA is a disk made of an ionically resistive
material, such as polycarbonate, polyethylene, polypropylene,
polyvinylidene difluoride (PVDF), polytetrafluoroethylene,
polysulphone and the like, having between about 6,000-12,000 1-D
through-holes. In other embodiments, the HRVA is a porous structure
in which at least some of the pores are interconnected and
therefore allow some two- or three-dimensional movement of
electrolyte therein. The disk, in many embodiments, is
substantially coextensive with the wafer (e.g., has a diameter of
about 300 mm when used with a 300 mm wafer) and resides in close
proximity of the wafer, e.g., just below the wafer in a
wafer-facing-down electroplating apparatus. In some embodiments,
the disk is relatively thin, for example between about 5 and 50 mm
thick. The plating electrolyte contained within the pores of the
HRVA allows ionic current to pass though the disk, but at a
significant voltage drop compared to the system as a whole. For
example, the voltage drop in the HRVA may be greater than about
50%, for example, between about 55 and 95%, of the total voltage
drop between the counter electrode (anode) and the wafer peripheral
edge. In certain embodiments, the plated surface of the wafer
resides within about 10 mm, and in some embodiments, within about 5
mm, of the closest HRVA surface.
[0039] Further, in some embodiments the apparatus includes a
secondary cathode, which is typically located at the periphery of
the wafer substrate (e.g., having no footprint projected to the
wafer). This secondary cathode, also referred to as a thieving
cathode is negatively biased during at least a portion of
electroplating and is configured to divert at least a portion of
ionic current from the wafer periphery, thereby reducing plated
thickness at the very edge of the wafer.
[0040] The apparatus will further include one or more power
supplies and a controller in association with the power supplies
and with the elements of the apparatus, wherein the controller is
configured to perform the methods described herein. For example the
controller may include instructions (e.g., in the form of program
instructions or pre-built logic blocks) to specify electrical
characteristics (e.g., current, voltage, power, polarity) provided
to one or more components selected from the group consisting of the
wafer substrate, the auxiliary electrode, an anode, and the
secondary (thief electrode). The instructions may be provided in
some embodiments, by time-characteristic sequences (e.g.,
time-current sequences) for each of the elements.
[0041] In some embodiments, the main anode is located in a
separated anode chamber, while the wafer substrate is located in a
cathode chamber, wherein the two chambers are separated by an
ion-permeable membrane (e.g., a Nafion.RTM. membrane). The
compositions of electrolyte in the anode and cathode chambers may
be different. For example, catholyte in the cathode chamber may
contain organic plating additives, while the anolyte is free of
organic additives. In one configuration, the ionic current
collimator and the auxiliary electrode are located in the separated
anode chamber.
[0042] In some embodiments, the auxiliary electrode is also
separated from the anode by a cationic membrane such as a
Nafion.RTM. membrane, while remaining in ionic communication with
the electrolyte (e.g. anolyte). For example, the auxiliary
electrode may reside in a chamber defined by the walls of the
current collimator, the walls of the plating vessel, and the
cationic membrane. The membrane preferably does not allow
particulate material, which may form at the electrode due to
flaking, to travel across the membrane. The use of a membrane to
isolate the auxiliary electrode can lead to electroplating with
fewer defects.
[0043] In another aspect, a method for electroplating a layer of
metal on a wafer substrate is provided. In some embodiments, the
method involves (a) providing the wafer substrate to an
electroplating apparatus having a wafer holder, and a plating
vessel containing a main anode, an auxiliary electrode, and an
ionic current collimator, wherein the ionic current collimator is
configured to direct ionic current from the periphery to the center
of the plating vessel; (b) in a first electroplating stage,
electroplating metal onto the wafer substrate while cathodically
biasing the auxiliary electrode; and (c) in a second electroplating
stage, electroplating metal onto the wafer substrate while
anodically biasing the auxiliary electrode.
[0044] Electroplating is performed by making one or more electrical
contacts at the periphery of the wafer substrate, wherein the
contacts are made to the conductive seed layer residing on the
wafer substrate and by negatively biasing the wafer substrate, so
that it serves as a main cathode. In the beginning of the plating,
the seed layer on the substrate is highly resistive and initially a
relatively large current should be applied to a negatively biased
auxiliary electrode. Typically, the cathodic current initially
applied to the auxiliary electrode is at least about 200%, such as
at least about 300%, more preferably at least about 500%, such as
between about 400-600% of the cathodic current applied to the wafer
substrate. For example, when between about 10-15 A current is
applied to the wafer, between about 50-75 A is applied initially to
the auxiliary electrode (functioning as a cathode). In some
embodiments, the cathodic current initially applied to the
auxiliary electrode is between about 10-75 A, such a s between
about 20-50 A. As plating proceeds and the terminal effect
subsides, the cathodic current applied to the auxiliary electrode
is reduced, in some embodiments. Reduction of cathodic current to
the auxiliary electrode can follow a number of current vs. time
functions, e.g., over time the current can be reduced linearly,
exponentially, or follow a polynomial function. After the
reduction, the auxiliary electrode is biased positively and starts
serving as an auxiliary anode. In some embodiments the current
supplied to the auxiliary electrode (now in anode mode) is
increased over time. In some embodiments the auxiliary electrode at
least during a portion of electroplating time receives more anodic
current than the main anode, thereby essentially serving as the
main anode in the system. In some embodiments, when the auxiliary
electrode is anodically biased, the main anode (which was
anodically biased in the beginning of plating), is switched to
being biased cathodically and remains cathodically biased at least
for a portion of electroplating. In some embodiments, in order to
compensate for the very strong terminal effect in the beginning of
the plating, the plating tool may be configured to favor plating in
the center of the wafer at the beginning of electroplating. For
example, in a plating apparatus the electrical path from the anode
to the edge of the wafer is configured to be more resistive than
the electrical path to the center of the wafer. In some
embodiments, as plating proceeds, and terminal effect diminishes,
electroplating rate in the center of the wafer can become too fast
thereby having the potential to cause current density
non-uniformity across the wafer. In such cases, the current
provided by the auxiliary electrode (which acts as an anode in the
second stage of electroplating) will be increased with time and the
current provided by the main anode will be decreased with time. In
some cases, where the plating apparatus provides particularly large
amounts of plating current at the center of the wafer, the main
anode is switched to be biased cathodically in order to help
further reduce the center-thick plating and to keep uniform current
density distribution across the whole wafer. In such cases, the
main anode acts as a centrally based secondary cathode during a
portion of electroplating, e.g., during at least a portion of the
second stage of electroplating.
[0045] In some embodiments, the methods provided herein involve
moving the ionic current collimator along an axis that is
perpendicular to the plane of the wafer substrate during
electroplating. In some embodiments, the methods provided herein
involve moving the auxiliary electrode along an axis that is
perpendicular to the plane of the wafer substrate during
electroplating. The ionic current collimator and the auxiliary
electrode in some embodiments are moved together in one block. In
other embodiments, the auxiliary electrode is moved separately from
the collimator.
[0046] In some embodiments, in addition to the ionic current
collimator and the auxiliary electrode, the electroplating
apparatus further includes additional elements that are useful for
mitigating the terminal effect. In some embodiments, an ionically
resistive ionically permeable member, also known as HRVA resides
between the anode and the wafer substrate. The auxiliary electrode
preferably resides between the HRVA and the anode in the plating
chamber. In some embodiments, the apparatus further includes a
secondary cathode, which is typically located at the periphery of
the wafer substrate (e.g., having no footprint projected to the
wafer). This secondary cathode, also referred to as a thieving
cathode is negatively biased during at least a portion of
electroplating and is configured to divert at least a portion of
ionic current from the wafer periphery, thereby reducing plated
thickness at the very edge of the wafer. In some embodiments
provided methods comprise providing a cathodic current to the
secondary cathode, wherein the current is between about 100-400%,
more preferably is between about 200-300% of the current provided
to the wafer substrate at the beginning of electroplating. As
electroplating proceeds, the current supplied to the secondary
cathode may be reduced, e.g., to zero, or to a small constant
current.
[0047] The use of a flexible auxiliary electrode, where the
flexibility refers to its ability to function both as a cathode and
an anode, and to be able to follow a number of current-time
routines as desired by the user, allows for efficient control of
ionic current distribution during the entire course of
electroplating. Thereby, uniformly plated metal layers can be
obtained even when highly resistive seed layers are used or when
large wafers (e.g., 450 mm wafers) are used. The auxiliary
electrode and the ionic current collimator work in synergy to
mitigate terminal effect. The collimator directs the ionic current
from the periphery generally in the central direction, thereby
reducing the terminal effect. In the presence of the collimator a
relatively smaller current can be provided to the auxiliary
electrode (in a cathode mode) at the beginning of electroplating,
to achieve mitigation of terminal effect. In addition, in some
embodiments, the ionic current collimator provides an efficient
platform in the plating chamber for a large auxiliary
electrode.
[0048] The apparatus and process 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.
Typically, though not necessarily, such tools/processes will be
used or conducted together in a common fabrication facility.
Lithographic patterning of a film typically comprises some or all
of the following steps, each step enabled with a number of possible
tools: (1) application of photoresist on a workpiece, i.e.,
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 or 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 workpiece 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. In some embodiments, the
electroplating methods provided herein further include lithographic
steps of: applying photoresist to the workpiece; exposing the
photoresist to light; patterning the resist and transferring the
pattern to the workpiece; and selectively removing the photoresist
from the work piece. In some embodiments, a system is provided
which includes the electrodeposition apparatus described herein and
a stepper.
[0049] Another aspect of the invention is an apparatus configured
to accomplish the methods described herein. A suitable apparatus
includes hardware for accomplishing the process operations and a
system controller having instructions for controlling process
operations in accordance with the present invention. The system
controller will typically include one or more memory devices and
one or more processors configured to execute the instructions so
that the apparatus will perform a method in accordance with the
present invention. Machine-readable media containing instructions
for controlling process operations in accordance with the present
invention may be coupled to the system controller. In some
embodiments, an apparatus is provided, wherein the apparatus
comprises a plating vessel, a wafer holder, an auxiliary electrode,
an ionic collimator and a controller comprising program
instructions and/or built in logic for (a) in a first
electroplating stage, electroplating metal onto the wafer substrate
while cathodically biasing the auxiliary electrode; and (b) in a
second electroplating stage, electroplating metal onto the wafer
substrate while anodically biasing the auxiliary electrode.
[0050] Advanced technologies call for the electroplating of metals
onto wafers with sheet resistances of 10 ohm per square and higher
(even 20 ohms per square or 40 ohms per square or higher). As seed
layers gets thinner, and as wafer sizes get bigger, the difference
in plating thickness between the center and edge (thus terminal
effect) becomes more pronounced. This requires ever more aggressive
techniques to compensate for the terminal effect. During plating,
the thickness of metal and the sheet resistance can drop several
orders of magnitude in a short time, and so methods and apparatus
capable of plating uniformly on the wafer throughout a process
where there may be a rapidly initially varying and later a
relatively constant sheet resistance are required. Embodiments of
the present invention address the challenges presented by such high
resistance seed layers, the rapid dynamic variance in the seed
electrical parameters, and the extreme terminal effect they
present.
[0051] Embodiments of the present invention pertain to methods and
apparatuses for electroplating a substantially uniform layer of
metal onto a work piece having a seed layer thereon. In certain
embodiments, a plating cell includes both an ionic current
collimator positioned in the proximity of an anode, and an
auxiliary electrode configured to function as an auxiliary cathode
in the beginning of electroplating and as an auxiliary anode at
later stages of electroplating. The described configuration,
particularly when used in combination with a HRVA and a secondary
cathode, can maintain uniform current distribution throughout
plating process. In some cases, however, it may be desirable to use
embodiments of the invention to create a non-uniform current
density that is experienced by the wafer. For example, it may be
desirable to create a non-uniform current density, resulting in
non-uniform metal plating, during overburden deposition to aid in
chemical mechanical polishing (CMP), wet chemical etching,
electropolishing, or electromechanical polishing.
[0052] A cross-sectional schematic view of an example of an
electroplating apparatus having an ionic current collimator and a
flexible auxiliary electrode is shown in FIG. 1A. Electrical
connectivity is not shown to preserve clarity
[0053] The apparatus depicted in FIG. 1A includes a plating vessel
101, which is configured to hold electrolyte 102 (e.g. an aqueous
solution of a copper salt, an acid, and electroplating additives)
in contact with the wafer substrate 103 during electroplating. The
wafer substrate 103 is held in a face-down orientation by the wafer
holder 104. The wafer holder includes electrical contacts that are
configured to contact the wafer 103 at its periphery (but not in
the center), which are electrically connected to a power supply
(not shown). In some embodiments, the wafer holder 104 is a
clamshell apparatus which makes contacts to the periphery of the
wafer through a number of contact fingers housed behind a typically
elastic "lip seal", which serves to seal the clamshell and keep the
edge contact region and wafer backside substantially free of
electrolyte, as well as to avoid any plating onto the contacts. A
general description of a clamshell-type plating apparatus having
aspects suitable for use with this invention is described in detail
in U.S. Pat. No. 6,156,167 issued to Patton et al., and U.S. Pat.
No. 6,800,187 issued to Reid et al, which are both incorporated
herein by reference for all purposes. The wafer holder is also
configured to rotate the wafer substrate during electroplating.
[0054] The apparatus is configured to negatively bias the wafer
substrate during electroplating such that it serves as a cathode.
The electroplating vessel 101 further includes an anode 105,
located at a distance below the wafer substrate. The anode is
electrically connected to a power supply and is configured to be
positively biased relative to the wafer substrate during plating
(e.g., it can be held at a ground potential). An active anode (e.g.
copper-containing anode) is typically used. For applications used
for deposition of other metals either an active anode or an inert
anode could be used. It is noted that in some embodiments, the main
anode 105 may also be configured to serve as a cathode during some
stages of electroplating (e.g., when the auxiliary electrode
assumes an anodic function). In these embodiments, the main anode
is connected with one or more power supplies that are capable of
biasing it both anodically and cathodically.
[0055] In the depicted embodiment, the apparatus includes an anode
chamber 107 and a cathode chamber 109, which are separated by an
ion-permeable membrane 111. A cationic membrane, such as Nafion may
be used as a membrane 111. The composition of electrolyte in the
anode chamber 107 and in the cathode chamber 111 may be the same or
different. In some embodiments, the electrolyte in the anode
chamber (anolyte) contains ions of plateable metal, but no organic
plating additives, while the electrolyte in the cathode chamber
(catholyte) contains both ions of plateable metal and organic
plating additives (e.g., one or more of accelerators, suppressors,
and levelers). The cationic membrane 111 allows ionic communication
between the separated anolyte chamber and the cathode chamber,
while preventing the particles generated at the anode from entering
the proximity of the wafer and contaminating it. The cationic
membrane is also useful in prohibiting non-ionic and anionic
species such as bath additives from passing though the membrane and
being degraded at the anode surface, and to a lesser extent in
redistributing current flow during the plating process and thereby
improving the plating uniformity. Detailed descriptions of suitable
ionic membranes are provided in U.S. Pat. Nos. 6,126,798 and
6,569,299 issued to Reid et al., both incorporated herein by
reference. A detailed description of suitable cationic membranes is
provided in U.S. patent application Ser. No. 12/337,147, entitled
Electroplating Apparatus with Vented Electrolyte Manifold, filed
Dec. 17, 2008, incorporated herein by reference. Further detailed
description of suitable cationic membranes is provided in U.S.
patent application Ser. No. 61/139,178, entitled Plating Method and
Apparatus with Multiple Internally Irrigated Chambers, filed Dec.
19, 2008, incorporated herein by reference.
[0056] An ionically resistive ionically permeable element 113 (also
known as HRVA) is located directly below the wafer 103 and resides
in the cathode chamber 109. HRVA, in some embodiments, is a plate
made of a dielectric material, which has a plurality of
non-communicating holes, which provide a resistive path for ionic
current in the proximity of the substrate. HRVA is described in
detail in the US Patent Pub. No. 2010/0116672 by Mayer et al.
titled Method and Apparatus for Electroplating, filed Jun. 9, 2009,
incorporated herein by reference.
[0057] A thieving secondary cathode 115 resides in this embodiment
in its own chamber filled with electrolyte and is in ionic
communication with electrolyte in the cathode chamber. The
secondary cathode 115 is electrically connected to a power supply
and is configured to be negatively biased at least during a portion
of electroplating. The opening of the chamber, which defines the
virtual thief cathode is located between the HRVA 113 and the wafer
103, referring to position on the vertical axis. This location
allows for efficient diversion of ionic current from the near edge
region of the wafer. A thief cathode (both physical and virtual) in
this location is described in detail in the U.S. patent application
Ser. No. 12/481,503, previously incorporated by reference. In some
embodiments the thief cathode is a metal ring peripherally located
relative to the wafer substrate.
[0058] Referring again to FIG. 1, the anode chamber 107 houses the
ionic current collimator 117 and an auxiliary electrode 119. The
ionic current collimator 117 is located above the anode 105. The
current collimator has a current restricting portion generally
parallel to the plane of the anode, and attached to the walls of
the plating chamber, and a cylindrical central portion which is
generally in the form of an open cylinder extending in the
direction that is perpendicular to the plane of the wafer
substrate, and, typically, co-centered with the center of the wafer
substrate and the center of the anode. The openings of the cylinder
provide a route for the ionic current to move from the anode upward
in the direction of the wafer substrate. The current collimator as
a whole directs the current from the anode in the direction of
central portions of the wafer, and thereby provides a resistive
compensation for the terminal effect. However, this compensation
alone is not sufficient for plating on highly resistive seed
layers. Accordingly, an auxiliary electrode configured to
redistribute the current from the opening of the collimator, away
from the center of the wafer is needed.
[0059] The auxiliary electrode 119 in the depicted embodiment
resides on top of the current-restricting portion of the ionic
current collimator 117. The auxiliary electrode 119 is electrically
connected to a power supply and is configured to be biased both
cathodically and anodically during the course of electroplating of
a single substrate. In some embodiments, the auxiliary electrode is
negatively biased in the beginning of electroplating, when the
terminal effect is pronounced, and later is anodically biased.
Because the auxiliary electrode serves as an anode, in some
embodiments it is made of a material that is plated on the wafer
substrate, e.g., copper during copper plating. In other
embodiments, it has a coating of a metal that is being plated on
the substrate and a core made of a different metal. Yet in other
embodiments the auxiliary electrode may be made of a material that
is different from the one being plated, but it is sufficiently
coated with the plated metal (e.g., copper) during the period when
it serves as a cathode. This deposited material is then redissolved
when the auxiliary electrode serves as an anode.
[0060] A cross-sectional schematic view of another example of an
electroplating apparatus having an ionic current collimator and an
auxiliary electrode is shown in FIG. 1B. In this apparatus, in
addition to the cationic membrane 111 residing below the HRVA, a
second cationic membrane 111a is added, such that the membrane 111a
resides directly above the auxiliary electrode 119, so that a
chamber 120 is formed. The auxiliary electrode chamber 120 is
defined by the sidewall of the apparatus on one side, by the ionic
current collimator 117 on the bottom and on the other side and by
the cationic membrane 111a on top. The cationic membrane 111a
separates the auxiliary electrode from the anode, such that any
particles generated at the auxiliary electrode, would not be able
to cross the membrane. The membrane, however, allows for ionic
communication between the auxiliary electrode chamber and anolyte,
as the cationic membrane allows for transfer of cations. The
cationic membrane 111a is attached to the sidewalls of the plating
vessel, and is typically also attached to the opening of the
cylinder of the ionic current collimator 117 (e.g., by an o-ring).
The auxiliary electrode may be prone to flaking, due to plating and
deplating cycles, and therefore its isolation via a cationic
membrane is preferred in some embodiments.
[0061] A cross-sectional schematic view of yet another example of
an electroplating apparatus having an ionic current collimator and
a flexible auxiliary electrode is shown in FIG. 1C. In this
implementation, the cationic membrane 111a isolating the auxiliary
electrode 119 in a chamber 120 is present, but the cationic
membrane 111 in the proximity of the HRVA is absent.
[0062] The relative positions of the ionic current collimator and
of the auxiliary electrode are significant characteristics of the
described apparatus. The ionic current collimator and the auxiliary
electrode work in synergy to provide a flexible solution for
modulating ionic current distribution and, consequently, for
plating uniformity during the course of electroplating under
changing conditions.
[0063] The current collimator is described in more detail with
reference to FIGS. 2A and 2B. FIG. 2A shows a diagrammatical
cross-sectional view of the ionic current collimator 217 residing
over the anode 205. The central portion 221 of the ionic current
collimator 217 is a hollow cylinder having a diameter d1 and height
d2. In some embodiments the diameter d1 is between about 30-70% of
the wafer radius, more preferably between about 40-60% of the wafer
radius, such as between about 90-135 mm. The height d2 can be
between about 30-60% of d1, in some embodiments. The central
portion of the ionic current collimator is attached to the current
restricting portion 223, where the current restricting portion 223
is parallel to the anode. The current restricting portion extends
to the walls of the plating vessel and has a length d3 of between
about 30-70% of the wafer radius. The ionic current collimator is
positioned such that it restricts the ionic current from escaping
at the periphery and directs substantially all current from the
anode to the opening of the cylinder in the central portion 221.
The top surface of the collimator and the HRVA bottom surface are
maintained at a non-zero distance to allow for current
redistribution to the auxiliary electrode. In some embodiment this
distance is less than half of the wafer radius.
[0064] In some embodiments the ionic current collimator does not
have two distinct portions described above, but simply has an ion
restricting portion with an opening in the center. The ionic
current collimator in these embodiments may have a toroidal shape
with a uniform thickness (along an axis perpendicular to the wafer
surface). However, an embodiment of collimator with a central
cylindrical portion extending upward towards the wafer has a number
of advantages over a simpler doughnut-shaped collimator. For
example, the collimator having a central cylindrical portion
extending upward is typically more effective in delivering the
ionic current to the central portions of the wafer, and also serves
as a more convenient platform for the auxiliary electrode.
[0065] FIG. 2B illustrates a top view of the ionic current
collimator 217, showing an extended disk of the current restricting
portion 223 around the opening of the central portion 221.
[0066] FIG. 3A illustrates a cross-sectional schematic view of a
portion of the electroplating apparatus which includes the anode
305, the current collimator 317, and the auxiliary electrode 319
which resides on the current restricting portion of the current
collimator. The auxiliary electrode 319 has a toroidal shape and is
located at least partially over the anode (has a non-zero footprint
when projected onto the anode). The footprint projected onto the
anode is shown as d5. In some embodiments, its footprint projected
onto the anode is at least about 40% of total anode area, e.g.,
between about 40 and 80% of the total anode area. In some
embodiments, the footprint of the current restricting portion of
the ionic current collimator onto the anode is very similar to the
footprint of the auxiliary anode, e.g., at least about 40% of total
anode area, e.g., between about 40 and 80%.
[0067] Further, in some embodiments the auxiliary electrode has a
large surface area. When the surface area is large, the auxiliary
electrode can accept a very high current (as often needed to
compensate for the terminal effect caused by highly resistive seed
layers), without building undesirably high current densities. In
some embodiments, the surface area of the auxiliary electrode is at
least about 600 cm.sup.2, such as between about 900 cm.sup.2 and
1200 cm.sup.2. FIG. 3B illustrates a top view of the toroidal
auxiliary electrode 319. In some embodiments, the electrode has a
radial thickness d6 (difference between outer and inner radius)
that is at least 60 mm, such as between about 60 mm and 150 mm.
[0068] FIG. 4 is a schematic illustration of electrical
connectivity between the elements of the electroplating apparatus
in accordance with one embodiment. The wafer substrate 403, the
anode 405, the secondary thief cathode 415 and the auxiliary
electrode 419 are connected to one or more power supplies 431
(shown as one block), which are configured to bias the substrate
403 negatively during electroplating, while concurrently biasing
the anode 405 positively relative to the substrate, and while
biasing the thief cathode 415 negatively at least during a portion
of electroplating of one substrate, and while biasing the auxiliary
electrode both negatively and positively during the course of
electroplating of one substrate. Typically, the auxiliary electrode
is negatively biased in the beginning of electroplating, and is
positively biased later in the plating process. In some
embodiments, the one or more power supplies 431 are also configured
to bias the main anode cathodically, when the auxiliary electrode
is biased anodically.
[0069] In certain embodiments, one or more power supplies are
provided for providing power to the wafer substrate, the auxiliary
electrode, and the secondary cathode. In some cases, a separate
power supply is provided for each of the auxiliary electrode, the
secondary cathode and the work piece; this allows flexible and
independent control over delivery of power to each cathode.
Alternatively, one power supply with multiple independently
controllable electrical outlets can be used to provide different
levels of current to the wafer, to the auxiliary electrode, and to
the secondary cathode. In the embodiment depicted in FIG. 4, the
anode is positively biased with respect to the wafer, the auxiliary
electrode, and the secondary cathode, and is sometimes
grounded.
[0070] In some embodiments the power supply 413 is a multi-channel
power supply. One channel of the power supply 431 negatively biases
the wafer 403 with respect to the anode; another channel of the
power supply negatively biases the secondary cathode 415 with
respect to the anode; and another channel of the power supply 431
negatively biases the auxiliary electrode in the beginning of
plating with respect to the anode. The power supply 431 can be
connected to a controller 433, which allows for independent control
of current and potential provided to the wafer, the secondary
cathode and auxiliary electrode of the electroplating apparatus.
Power supply 431 causes an electrical current to flow from anode
405 to wafer 403, plating metal onto the wafer. The channel
connected to secondary cathode 415 of power supply 431 causes the
electrical current to flow from anode 405 to the secondary cathode
415, thereby partially or substantially diverting current that
flows from anode 405 to wafer 403. Depending on the relative
potential applied on the auxiliary electrode 419, auxiliary
electrode 419 may either pull current from anode 405, or provide
current to wafer 403. The electrical circuit described above may
also include one or several diodes (not shown) that will prevent
reversal of the current flow, when such reversal is not
desired.
[0071] With separate power supplies or power supply channels for
the auxiliary electrode, the secondary cathode, and the wafer, the
current applied to each of the electrodes may be dynamically
controlled. As the wafer is electroplated with metal, the sheet
resistance decreases and the current distribution non-uniformity
may be reduced, making the secondary cathode unnecessary after a
certain thickness of metal is achieved. The current supplied to the
secondary cathode and the auxiliary electrode may be dynamically
controlled to account for a reduction of the wafer's sheet
resistance and for the associated more uniform current distribution
that normally results without the activation of the auxiliary
electrode. In some embodiments, no current is supplied to the
secondary cathode after the sheet resistance of the wafer drops to
a defined level such as about 1 ohm per square and less. In some
embodiments, the current that is supplied to the auxiliary
electrode (in cathode mode) changes polarity after the sheet
resistance of the wafer drops to a defined level such as about 7.5
ohm per square or lower, after which point the auxiliary cathode
becomes an anode and starts to supply current to wafer 403. In
other words, in one process sequence, in the very beginning of
plating, both the secondary cathode (thief) and the auxiliary
electrode (in cathode mode) are powered, and typically each
receives more current than the wafer. Next, after a certain amount
of plating, the polarity of the auxiliary electrode is switched
from negative to positive, and then, after a further amount of
plating, power to the thief is turned off, while the wafer is
negatively biased all throughout the process receiving constant or
changing current.
[0072] A controller 433 is electrically connected with the one or
more power supplies 431 and is configured to control the electrical
parameters applied to the apparatus components. For example the
controller can control one or more of power, current and voltage
applied to the components of the apparatus, and is capable of
dynamically changing these parameters during the course of
electroplating. In some embodiments, the controller includes
program instructions, or logic for performing the methods provided
herein.
[0073] An electroplating method, in accordance with one embodiment,
is illustrated by the process diagram shown in FIG. 5A. The process
starts in 501 by placing a wafer having a continuous seed or
barrier-seed layer in a wafer holder of an electroplating apparatus
having an ionic current collimator configured to direct ionic
current from the anode to the central portions of the wafer. Next,
in 503, the wafer (in contact with electrolyte) is biased
negatively to plate metal on the seed or barrier-seed layer, while
concurrently a thief cathode located at the periphery of the wafer
is negatively biased to divert ionic current from the near-edge
region of the wafer. Concurrently, an auxiliary electrode located
between the anode and the wafer is negatively biased. Next, in 505,
the current supplied to the auxiliary electrode is reduced, and
next, the auxiliary electrode is biased positively. Further, the
current supplied to the thief cathode is reduced to a small amount,
or the power to the thief cathode is turned off. Further, the
current supplied to the main anode is reduced during the course of
plating or even reversed to cathodic current, in some embodiments.
The metal is plated in 507 until a desired thickness is
reached.
[0074] In some embodiments the cathodic current provided to the
auxiliary electrode in the beginning of electroplating is at least
200% of the current provided to the wafer substrate, such as at
least 400% of the current provided to the wafer substrate. In some
embodiments, this current is rapidly reduced, e.g., in the first 5
seconds of electroplating time, and is then switched to anodic
current, which may be increased during the course of
electroplating. In some embodiments, the current is decreased
following an exponential function. In other embodiments, the
current is decreased following a polynomial function. The time of
the switch to the anodic mode can be determined, by calculating the
projected sheet resistance of the wafer based on a number of input
parameters (e.g., type of metal plated, size of wafer, and plating
speed). Alternatively the projected sheet resistance may be
calculated by measuring the number of coulombs that pass through
the system, and knowing the type of metal being plated, and the
size of the wafer. The switch time may occur after a certain
projected sheet resistance has been reached. For example, for a
deposition of a known metal on a wafer of a known size, it can be
calculated at what time the sheet resistance decreases below a
predetermined number. Therefore, the auxiliary electrode can be
switched to an anode mode when the sheet resistance drops below a
predetermined number (e.g., below 7.5 ohm/sq.)
[0075] FIG. 5B illustrates an exemplary algorithm for configuring a
system controller to perform the methods provided herein. In
operation 511 a plurality of input parameters are provided. These
input parameters may include the size of the wafer, the type of
metal, and the plating speed. Next, in 513, current-time profiles
for one or more of the wafer, the secondary thief cathode, and the
auxiliary electrode are calculated based on the input parameters.
In 517, the controller having these current-time instructions is
interfaced with the one or more power supplies and with the
components of the electroplating system. Next, in 519, the metal is
plated following the instructions provided by the system
controller.
[0076] Controller 433 in conjunction with power supplies 431 allows
for independent control of current and potential provided to the
wafer, the auxiliary cathode, and the second auxiliary cathode of
the electroplating apparatus, as well as control over other plating
components. Thus, controller 433 is capable of controlling power
supplies 431 to generate the current profiles described above. The
controller, however, generally is not capable of independently
determining if one of the conditions described above (e.g., sheet
resistance reaching a level of 1 ohm per square or lower) has been
met, though an estimate of the sheet resistance can be made based
on a known total cumulative amount of charge passed to the wafer at
any given time. Thus, the controller may be used in conjunction
with sensors that may determine whether a condition has been met.
Alternatively, the controller may be programmed with a separate
current versus time profile for each of the wafer, auxiliary
electrode, and the secondary cathode. The controller may also
measure the charge (coulombs=integral of amperage*time) supplied to
the wafer, auxiliary electrode, and secondary cathode, and base the
current-time profile on these data.
[0077] Controller 433 may be configured to control electrical power
delivered to the auxiliary electrode in a manner that produces a
more uniform current distribution from the anode after
electroplating a defined amount of metal onto the substrate or
after electroplating for a defined period of time. Controller 433
may also be configured to control electrical power delivered to a
secondary cathode adapted for diverting a portion of ionic current
from an edge region of the substrate. Furthermore, controller 433
may be configured to ramp down electrical power delivered to the
auxiliary electrode 419 and the secondary cathode 415, each at
different rates, as metal is deposited on the substrate.
Additionally, controller 433 may be configured to supply anodic
current after supplying cathodic current to the auxiliary electrode
after the sheet resistance of the substrate surface reaches a first
threshold level, and to supply no current or substantially no
current to the secondary cathode after the sheet resistance of the
substrate surface reaches a second threshold level.
[0078] Controller 433 may be further designed or configured to
control the position of the wafer relative to the anode position,
the rotation of the wafer in the wafer holder, etc. In the case
where the auxiliary electrode and/or the collimator is movable,
controller 433 can also control the movement parameters of the
collimator and/or of the auxiliary cathode, such as the speed of
movement and timing of starting and stopping the movement. The
positions of the collimator and of the auxiliary cathode may be
controlled based on a number of factors including, but not limited
to, the sheet resistance of the substrate surface, time (i.e. how
long the electrodeposition process has been going), and the amount
of metal deposited onto the substrate surface. These factors allow
for dynamic control of the collimator position, resulting in more
uniform deposition across the wafer. For example, the controller
may be configured and may comprise program instructions to initiate
movement of one or both of the collimator and of the auxiliary
electrode after a threshold sheet resistance is reached, or after a
predetermined amount of time, or after a predetermined amount of
plating has occurred.
[0079] FIG. 6A illustrates results of computational modeling
illustrating ionic current distribution in the beginning of
electroplating, when the auxiliary electrode serves as an auxiliary
cathode. In the illustrated system, the ionic current is directed
from the main anode 605 (at a ground potential) to the central
opening of the ionic current collimator 617, from where it proceeds
to the central portions of the wafer 603 (negatively biased), and
is partially diverted towards the negatively biased auxiliary
electrode 619. The excess of ionic current reaching the edges of
the wafer 603 is diverted to a negatively biased thief cathode 615
which resides in a chamber around the periphery of the wafer and is
connected to the main plating bath via a narrow channel.
[0080] FIG. 6B illustrates results of computational modeling
illustrating ionic current distribution at a later stage in
electroplating. In this case power to the secondary thief cathode
615 is turned off, and the auxiliary electrode 619 is positively
biased and serves as an anode. The main anode 605 remains at a
ground potential. It can be seen that the ionic current is now
supplied both by the anode 605 and by the auxiliary electrode 619,
where the ionic current from 619 is directed primarily to
non-central regions of the wafer substrate 603. Thus, in a first
stage of electroplating the system contains one anode, and three
cathodes, while in the later stage of electroplating the system
contains two anodes and one cathode (wafer).
[0081] FIG. 7 illustrates a plot of optimal instantaneous currents
on the components of the plating cell for seed layers having
different sheet resistances, in accordance with one implementation.
The range of sheet resistances is from about 0.05 ohm/sq
(corresponding to about 4000 .ANG. copper layer) to about 50
ohm/sq. The curve (a) shows that the cathodic current provided to
the wafer substrate is constant at 10 A. Curve (b) illustrates the
cathodic current provided to the secondary cathode during plating.
As the sheet resistances decrease, the amount of cathodic current
supplied to the secondary cathode also decreases. Curve (c)
illustrates the current supplied to the auxiliary electrode. For
highly resistive seed layers (7.5-50 Ohm/sq) a cathodic current is
supplied to this electrode. The cathodic current decreases as the
sheet resistance decreases from 50 Ohm/sq. to 7.5 Ohm/sq. At 7.5
Ohm/square no current is supplied to this electrode, and at lower
sheet resistances, the polarity of the electrode is switched, and
it starts accepting positive current and starts serving as an
auxiliary anode. The anodic current is increased, as the sheet
resistance is further decreased. Curve (d) shows the current at the
main anode. The current at the anode decreases as the sheet
resistances decrease (anodic current for the main anode is defined
as positive in this plot).
[0082] FIG. 8 illustrates one exemplary scenario of current vs.
time profile that may be employed for the auxiliary electrode when
plating on resistive seeds. The process starts by applying a
cathodic current of about 50 A to the auxiliary cathode, rapidly
decreasing the cathodic current over less than 5 seconds, then
transitioning to anodic current, then increasing the anodic
current, and then plating at a relatively constant anodic current
at the auxiliary electrode for at least 30 seconds. The current to
the wafer follows the following waveform: 10A for the first 5
seconds, followed by 15 A for the next 30 seconds, followed by 90 A
for the remainder of the plating time. For the purposes of this
calculation the current is normalized to a constant wafer current
of 10 A. The current to the main anode decreased over time, such
that after about 30 seconds of plating, the anodic current of the
auxiliary electrode was higher than at the "main" anode. Therefore,
the anodes switched roles in this scenario, with the auxiliary
anode serving as the "main" anode during the later portion of
plating.
[0083] FIG. 9 illustrates computational modeling results showing
distribution of current at the surface of the wafer during one
exemplary electroplating sequence. The X-axis refers to a radial
position on the wafer, starting at a center (0 m) and extending to
the edge (0.225 m). The Y-axis shows the level of current in
Amp/m.sup.2. The electroplating process begins in (a), where the
sheet resistance of the seed layer is 50 ohm.sq., the wafer
receives a cathodic current of 10 A, the secondary thief cathode
receives a cathodic current of 27.7 A, and the auxiliary electrode
receives a cathodic current of 49.7 A. Next, after 0.45 seconds in
(b) the sheet resistance drops to 30 Ohm/sq., the wafer receives a
cathodic current of 10 A, the secondary thief cathode receives a
smaller cathodic current of 16.8 A, and the auxiliary electrode
receives a smaller cathodic current of 24.6 A. Next, after 2.1
seconds after beginning of plating, in (c) the sheet resistance
drops further to 10 Ohm/sq., the wafer receives a cathodic current
of 10 A, the secondary thief cathode receives a smaller cathodic
current of 5.6 A, and the auxiliary electrode receives a smaller
cathodic current of 2.7 A. Next, after 11 seconds after beginning
of plating, in (d) the sheet resistance drops further to 1 Ohm/sq.,
the wafer receives a cathodic current of 10 A, the secondary thief
cathode receives a smaller cathodic current of 0.1 A, and the
auxiliary electrode receives switches polarity and receives an
anodic current of -5 A. Finally, after 45 seconds after beginning
of plating, in (e) the sheet resistance drops further to 0.05
Ohm/sq., the wafer receives a cathodic current of 10 A, the
secondary thief cathode is turned off and receives zero current,
and the auxiliary electrode receives a higher anodic current of
-6.9 A. It can be seen that a highly uniform distribution of
current across the wafer surface, and, consequently, uniform
plating can be achieved using provided electroplating
sequences.
[0084] The programming of the apparatus controller with a desired
current-time characteristic can be performed, in some embodiments,
following the exemplary guidelines presented below.
[0085] As shown in FIG. 7, the optimal current on the auxiliary
electrode and on the secondary cathode is linearly correlated with
the instantaneous sheet resistance of the seed wafer substrate.
Thus in a plating process, due to constant plating occurring on the
wafer substrate, the preferred process involves dynamic changing
and smart control of current on the auxiliary electrode and on the
secondary cathode.
[0086] Control of the current level on the auxiliary electrode (as
well as on the secondary cathode) is designed in some embodiments
based on the linear correlation between the optimal current level
and the instantaneous substrate sheet resistance. The following
three aspects can be merged into a single mathematical model which
can be implemented through waveform control (current-time function
for each of the components of the apparatus). This function can be
included in the controller in the form of program instructions,
e.g., via inclusion into the power supply firmware control unit.
According to a first aspect, there is a linear correlation between
the sheet resistance of the wafer substrate and the optimal current
on the auxiliary cathode. According to a second aspect, in a
plating process, the metal growth rate (thus the "seed" thickness
increase rate) follows a step function. Thus, over a certain time
period the growth rate is constant. Therefore, the thickness of the
metal on the wafer substrate is linearly correlated with the time
over each time period. According to a third aspect, the correlation
between the sheet resistance of the wafer substrate and the
thickness of the "seed" layer is not linear. Instead it could be
mathematically illustrated as a polynominal function or an
exponential function. The presented three correlations, when merged
together, lead to a polynominal and/or exponential correlation
between the optimal auxiliary electrode current and plating time,
as illustrated in FIG. 8. Accordingly, in some embodiments, an
instantaneous measurement of the substrate sheet resistance is not
necessary.
[0087] In some embodiments, the control of the system is
implemented using the following steps without using the sensors. In
a first step, computational modeling is performed to determine the
optimal current on the auxiliary electrode versus wafer sheet
resistance for a given hardware design, as illustrated in FIG. 7.
Next, confirming experiments are performed to finalize the
determination of correlation between auxiliary electrode current
and time for different metal layer growth rates. The obtained
correlation will be suitable for providing control for a selected
hardware design with certain hardware dimensions and arrangement.
The obtained correlation can then be used to configure power supply
firmware and software and to pre-define the constants of a
polynominal function and/or an exponential function used by the
controller, and thus define the current-time function. Input
parameters provided by the apparatus user to configure the
controller may include: initial seed layer thickness on the
substrate, initial current on the auxiliary electrode, information
that defines starting point of polynominal and/or exponential
function, and initial plating current on the wafer substrate, which
is determined by factors other than the auxiliary electrode
parameters. Factors that determine the plating current on the wafer
substrate include but are not limited to: types of structures on
the wafer substrate, plating bath chemistry used for plating on the
wafer substrate, integration requirements, etc.
CONCLUSION
[0088] It is understood that the examples and embodiments described
herein are for illustrative purposes only and that various
modifications or changes in light thereof will be suggested to
persons skilled in the art. Although various details have been
omitted for clarity's sake, various design alternatives may be
implemented. Therefore, the present examples are to be considered
as illustrative and not restrictive, and the invention is not to be
limited to the details given herein, but may be modified within the
scope of the appended claims. Further it is understood that many
features presented in this application can be practiced separately
as well as in any suitable combination with each other, as will be
understood by one of skill in the art.
* * * * *