U.S. patent application number 15/261244 was filed with the patent office on 2016-12-29 for control of electrolyte hydrodynamics for efficient mass transfer during electroplating.
The applicant listed for this patent is Novellus Systems, Inc.. Invention is credited to Steven T. Mayer, David W. Porter.
Application Number | 20160376722 15/261244 |
Document ID | / |
Family ID | 45398858 |
Filed Date | 2016-12-29 |
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United States Patent
Application |
20160376722 |
Kind Code |
A1 |
Mayer; Steven T. ; et
al. |
December 29, 2016 |
CONTROL OF ELECTROLYTE HYDRODYNAMICS FOR EFFICIENT MASS TRANSFER
DURING ELECTROPLATING
Abstract
Described are apparatus and methods for electroplating one or
more metals onto a substrate. Embodiments include electroplating
apparatus configured for plating highly uniform metal layers. In
specific embodiments, the apparatus includes a flow-shaping element
made of an ionically resistive material and having a plurality of
channels made through the flow shaping element. The channels allow
for transport of the electrolyte through the flow shaping element
during electroplating. The channel openings are arranged in a
spiral-like pattern on the substrate-facing surface of the flow
shaping element such that the center of the spiral-like pattern is
offset from the center of the flow shaping element.
Inventors: |
Mayer; Steven T.; (Aurora,
OR) ; Porter; David W.; (Sherwood, OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Novellus Systems, Inc. |
Fremont |
CA |
US |
|
|
Family ID: |
45398858 |
Appl. No.: |
15/261244 |
Filed: |
September 9, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14309723 |
Jun 19, 2014 |
9464361 |
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15261244 |
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13172642 |
Jun 29, 2011 |
8795480 |
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14309723 |
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61361333 |
Jul 2, 2010 |
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61374911 |
Aug 18, 2010 |
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61405608 |
Oct 21, 2010 |
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Current U.S.
Class: |
204/261 |
Current CPC
Class: |
C25D 21/10 20130101;
C25D 17/001 20130101; C25D 5/08 20130101; C25D 17/002 20130101;
C25D 17/02 20130101; C25D 17/008 20130101; C25D 5/04 20130101 |
International
Class: |
C25D 17/00 20060101
C25D017/00; C25D 5/04 20060101 C25D005/04; C25D 21/10 20060101
C25D021/10; C25D 5/08 20060101 C25D005/08; C25D 17/02 20060101
C25D017/02 |
Claims
1. (canceled)
2. A method of electroplating on a substrate comprising features
having a width and/or depth of at least about 2 micrometers, the
method comprising: (a) providing the substrate to a plating
chamber, wherein the plating chamber is configured to contain an
electrolyte and an anode during electroplating of metal onto the
substrate, wherein the plating chamber comprises: (i) a substrate
holder holding the substrate such that a plating face of the
substrate is separated from the anode during electroplating, and
(ii) a flow shaping element shaped and configured to be positioned
between the substrate and the anode during electroplating, the flow
shaping element having a flat surface that is substantially
parallel to and separated from the plating face of the substrate by
a distance of about 10 millimeters or less during electroplating,
wherein the flow shaping element has a plurality of holes; and (b)
electroplating a metal onto the substrate plating surface while
rotating the substrate and while flowing the electrolyte such that
a transverse flow rate of the electrolyte across the center point
of the plating face of the substrate is at least about 3 cm/second,
wherein flowing the electrolyte comprises diverting an electrolyte
flow exiting the holes of the flow shaping element to a transverse
electrolyte flow that is substantially parallel to the plating face
of the substrate.
3. The method of claim 2, wherein the electroplated metal is
selected from the group consisting of copper, tin, a tin-lead
composition, a tin-silver composition, nickel, a tin-copper
composition, a tin-silver-copper composition, gold, and alloys
thereof.
4. The method of claim 2, wherein the average flow velocity of the
electrolyte exiting the holes of the flow shaping element is at
least about 10 cm/second.
5. The method of claim 2 comprising rotating the substrate at a
rate of at least 30 rpm during electroplating.
6. The method of claim 2, wherein the holes of the flow shaping
element are non-communicating channels.
7. The method of claim 2, wherein the electrolyte flows across the
plating face of the substrate at a center point of the substrate at
a transverse flow rate of about 5 cm/second or greater during
electroplating.
8. The method of claim 2, wherein the flow shaping element
comprises an ionically resistive material selected from the group
consisting of polyethylene, polypropylene, polyvinylidene
diflouride (PVDF), polytetrafluoroethylene, polysulphone, and
polycarbonate.
9. The method of claim 2, wherein the flow shaping element is a
disk having between about 6,000-12,000 holes.
10. The method of claim 2, wherein the flow shaping element has a
non-uniform density of holes, with a greater density of holes being
present in a region of the flow shaping element that faces a
rotational axis of the substrate plating face.
11. The method of claim 2, wherein the flow shaping element is
between about 5 mm and about 10 mm thick.
12. The method of claim 2, further comprising reversing a direction
of rotation of the substrate with respect to the flow shaping
element during electroplating.
13. The method of claim 2, wherein the features on the substrate
are wafer level packaging features.
14. The method of claim 2, wherein the method comprises
electroplating metal in the features at a rate of at least 5
micrometers per minute.
15. The method of claim 2, wherein the plating chamber comprises at
least one electrolyte flow port, configured to increase transverse
flow of electrolyte.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/309,723, filed Jun. 19, 2014, which is a
division of U.S. patent application Ser. No. 13/172,642, filed Jun.
29, 2011 (issued as U.S. Pat. No. 8,795,480), which claims benefit
under 35 U.S.C. .sctn.119(e) of U.S. Provisional Patent Application
No. 61/361,333, filed Jul. 2, 2010, U.S. Provisional Patent
Application No. 61/374,911, filed Aug. 18, 2010, and U.S.
Provisional Patent Application No. 61/405,608, filed Oct. 21, 2010,
each of which is incorporated herein by reference in its
entirety.
FIELD
[0002] The invention relates to methods and apparatus for
controlling electrolyte hydrodynamics during electroplating. More
particularly, methods and apparatus described herein are
particularly useful for plating metals onto semiconductor wafer
substrates.
BACKGROUND
[0003] Electrochemical deposition processes are well-established in
modern integrated circuit fabrication. The movement from aluminum
to copper metal lines in the early years of the twenty-first
century drove a need for increasingly more sophisticated
electrodeposition processes and plating tools. Much of the
sophistication evolved in response to the need for ever smaller
current carrying lines in device metallization layers. These copper
lines are formed by electroplating the metal into very thin,
high-aspect ratio trenches and vias in a methodology commonly
referred to as "damascene" processing.
[0004] Electrochemical deposition is now poised to fill a
commercial need for sophisticated packaging and multichip
interconnection technologies known generally as wafer level
packaging (WLP) and through silicon via (TSV) electrical connection
technology. These technologies present their own very significant
challenges.
[0005] The technologies require electroplating on a significantly
larger size scale than damascene applications. Depending on the
type and application of the packaging features (e.g. through chip
connecting TSV, interconnection redistribution wiring, or chip to
board or chip bonding, such as flip-chip pillars), plated features
are usually, in current technology, greater than about 2
micrometers and typically 5-100 micrometers (for example, pillars
may be about 50 micrometers). For some on-chip structures such as
power busses, the feature to be plated may be larger than 100
micrometers. The aspect ratios of the WLP features are typically
about 1:1 (height to width) or lower, while TSV structures can have
very high aspect ratios (e.g., in the neighborhood of about
20:1).
[0006] Given the relatively large amount of material to be
deposited, not only feature size, but also plating speed
differentiates WLP and TSV applications from damascene
applications. For many WLP applications, plating must fill features
at a rate of at least about 2 micrometers/minute, and typically at
least about 4 micrometers/minute, and for some applications at
least about 7 micrometers/minute. At these higher plating rage
regimes, efficient mass transfer of metal ions in the electrolyte
to the plating surface is important.
[0007] Higher plating rates present challenges with respect to
uniformity of the electrodeposited layer, that is, plating must be
conducted in a highly uniform manner. For various WLP applications,
plating must exhibit at most about 5% half range variation radially
along the wafer surface (referred to as a within wafer
non-uniformity, measured as a single feature type in a die at
multiple locations across the wafer's diameter). A similar equally
challenging requirement is the uniform deposition (thickness and
shape) of various features of either different sizes (e.g. feature
diameters) or feature density (e.g. an isolated or imbedded feature
in the middle of an array). This performance specification is
generally referred to as the within die non-uniformity. Within die
non-uniformity is measured as the local variability (e.g. <5%
half range) of the various features types as described above versus
the average feature height or shape within a given wafer die at
that particular die location on the wafer (e.g. at the mid radius,
center or edge).
[0008] A final challenging requirement is the general control of
the within feature shape. A line or pillar can be sloped in either
a convex, flat or concave fashion, with a flat profile generally,
though not always, preferred. While meeting these challenges, WLP
applications must compete with conventional, inexpensive pick and
place routing operations. Still further, electrochemical deposition
for WLP applications may involve plating various non-copper metals
such as lead, tin, silver, nickel, gold, and various alloys of
these, some of which include copper.
SUMMARY
[0009] Described herein are apparatus and methods for
electroplating one or more metals onto a substrate. Embodiments are
described generally where the substrate is a semiconductor wafer;
however the invention is not so limited. Embodiments include
electroplating apparatus configured for, and methods including,
control of electrolyte hydrodynamics for efficient mass transfer
during plating so that highly uniform plating layers are obtained.
In specific embodiments, the mass transfer is achieved using a
combination of impinging flow and shear flow at the wafer
surface.
[0010] One embodiment is an electroplating apparatus including: (a)
a plating chamber configured to contain an electrolyte and an anode
while electroplating metal onto a substantially planar substrate;
(b) a substrate holder configured to hold the substantially planar
substrate such that a plating face of the substrate is separated
from the anode during electroplating; (c) a flow shaping element
including a substrate-facing surface that is substantially parallel
to and separated from a plating face of the substrate during
electroplating, the flow shaping element including an ionically
resistive material with a plurality of non-communicating channels
made through the flow shaping element, where the non-communicating
channels allow for transport of the electrolyte through the flow
shaping element during electroplating; and (d) a flow diverter on
the substrate-facing surface of the flow shaping element, the flow
diverter including a wall structure partially following the
circumference of the flow shaping element, and having one or more
gaps, and defining a partial or "pseudo" chamber between the flow
shaping element and the substantially planar substrate during
electroplating.
[0011] In one embodiment, the flow shaping element is disk-shaped
and the flow diverter includes a slotted annular spacer attached
to, or integrated onto, the flow shaping element. In one
embodiment, the wall structure of the flow diverter has a single
gap and the single gap occupies an arc of between about 40 degrees
and about 90 degrees. The wall structure of the flow diverter may
be between about 1 mm and about 5 mm high. In certain embodiments,
the flow diverter is configured such that a top surface of the wall
structure is between about 0.1 and 0.5 mm from a bottom surface of
the substrate holder during electroplating and the top surface of
the flow shaping element is between about 1 and 5 mm from the
bottom surface of the substrate holder during electroplating. The
number and configuration of the through holes in the flow shaping
element are discussed in more detail below. The holes may be in
uniform and/or non-uniform patterns on the flow shaping element. In
certain embodiments, a flow shaping element is termed a "flow
shaping plate."
[0012] In certain embodiments, the apparatus is configured to flow
electrolyte in the direction of the substrate plating face and
under conditions that produce an average flow velocity of at least
about 10 cm/s exiting the holes of the flow shaping element during
electroplating. In certain embodiments, the apparatus is configured
to operate under conditions that produce a transverse electrolyte
velocity of about 3 cm/sec or greater across the center point of
the plating face of the substrate.
[0013] In certain embodiments, the wall structure has an outer
portion that is higher than an inner portion. Embodiments include
features that restrict the flow of electrolyte out of the pseudo
chamber except for the one or more gaps which form a vent region in
the pseudo chamber.
[0014] One embodiment is an apparatus for electroplating metal onto
a substrate, the apparatus including: (a) a plating chamber
configured to contain an electrolyte and an anode while
electroplating metal onto the substrate; (b) a substrate holder
configured to hold the substrate such that a plating face of the
substrate is separated from the anode during electroplating, the
substrate 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; (c) a flow shaping
element shaped and configured to be positioned between the
substrate and the anode during electroplating, the flow shaping
element having a flat surface that is substantially parallel to and
separated from the plating face of the substrate by a gap of about
10 millimeters or less during electroplating, and the flow shaping
element also having a plurality of holes to permit flow of the
electrolyte toward the plating face of the substrate; (d) a
mechanism for rotating the substrate and/or the flow shaping
element while flowing electrolyte in the electroplating cell in the
direction of the substrate plating face; and (e) a mechanism for
applying a shearing force to the electrolyte flowing at the plating
face of the substrate; where the apparatus is configured for
flowing electrolyte in the direction of the substrate plating face
under conditions that produce an average flow velocity of at least
about 10 cm/s exiting the holes of the flow shaping element during
electroplating and for flowing electrolyte in a direction parallel
to the plating face of the substrate at an electrolyte velocity of
at least about 3 cm/sec across the center point of the plating face
of the substrate. Various shearing force mechanisms are described
in more detail below.
[0015] One embodiment is a method of electroplating on a substrate
including features having a width and/or depth of at least about 2
micrometers, the method including: (a) providing the substrate to a
plating chamber configured to contain an electrolyte and an anode
while electroplating metal onto the substrate, where the plating
chamber includes: (i) a substrate holder holding the substrate such
that a plating face of the substrate is separated from the anode
during electroplating, and (ii) a flow shaping element shaped and
configured to be positioned between the substrate and the anode
during electroplating, the flow shaping element having a flat
surface that is substantially parallel to and separated from the
plating face of the substrate by a gap of about 10 millimeters or
less during electroplating, where the flow shaping element has a
plurality of holes; (b) electroplating a metal onto the substrate
plating surface while rotating the substrate and/or the flow
shaping element and while flowing the electrolyte in the
electroplating cell in the direction of the substrate plating face
and under conditions that produce an average flow velocity of at
least about 10 cm/s exiting the holes of the flow shaping
element.
[0016] In one embodiment, the electrolyte flows across the plating
face of the substrate at a center point of the substrate at a rate
of about 3 cm/second or greater and shearing force is applied to
the electrolyte flowing at the plating face of the substrate. In
one embodiment, the metal is electroplated in the features at a
rate of at least about 5 micrometers/minute. In one embodiment, the
thickness of the metal electroplated on the plating surface of the
substrate has a uniformity of about 10% or better when plated to a
thickness of at least 1 micrometer.
[0017] Methods described herein are particularly useful for
electroplating Damascene features, TSV features and wafer level
packaging (WLP) features, such as a redistribution layer, a bump
for connecting to an external wire and an under-bump metallization
feature.
[0018] Particular aspects of embodiments described herein are
included below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1A is a perspective of a semiconductor wafer holder and
positioning mechanism used for electroplating onto the wafer.
[0020] FIG. 1B is a cross-section of the wafer holder described in
relation to FIG. 1A.
[0021] FIG. 1C is a cross-section of a wafer plating apparatus
showing aspects of a flow shaping plate having multiple through
holes for electrolyte flow.
[0022] FIG. 1D is a graph showing a reduced deposition rate near
the center of a wafer as compared to outer regions when using the
flow shaping plate as described in relation to FIG. 1C at high
deposition rate plating regimes.
[0023] FIG. 2A is a perspective of an exemplary flow diverter and
flow shaping plate assembly.
[0024] FIG. 2B is a cross-section of the flow diverter as described
in relation to FIG. 2A in relation to a wafer holder.
[0025] FIGS. 2C-D are top views of flow dynamics on the top of the
flow shaping plate when a flow diverter as described in relation to
FIG. 2A is used.
[0026] FIGS. 2E-I depict various aspects of the assembly as
described in relation to FIG. 2A along with wafer holder and
electrolyte chamber hardware.
[0027] FIG. 3A shows a top view and cross-section of a flow
diverter/flow shaping plate assembly where the flow diverter has a
vertical surface element for aiding in transverse fluid flow across
a wafer during plating.
[0028] FIG. 3B is a cross-section showing the relationship between
the flow diverter as described in relation to FIG. 3A and a wafer
holder assembly.
[0029] FIG. 3C is a graph showing plating uniformity results
obtained using a flow diverter/flow shaping plate assembly as
described in relation to FIGS. 3A and 3B.
[0030] FIG. 3D shows cross-sections of a number of flow diverters
having vertical surface elements.
[0031] FIG. 3E shows flow patterns resulting from using flow
diverters as described herein with flow shaping plate's having
square pattern through hole placement.
[0032] FIGS. 4A-B show top view of flow shaping plate's with spiral
through hole patterns, where the origin of the spiral pattern is in
different locations on the flow shaping plate.
[0033] FIG. 4C shows a top view and perspective of a flow shaping
plate with a spiral through hole pattern, where the spiral pattern
is offset from the center of the flow shaping plate face such that
what would be the origin of the spiral pattern is not included in
the through hole pattern.
[0034] FIG. 5A shows flow patterns resulting from using a flow
diverter as described in relation to FIG. 3A is used in conjunction
with a flow shaping plate as described in relation to FIG. 4C
during plating.
[0035] FIG. 5B shows plating uniformity results when using the flow
diverter/flow shaping plate combination as described in relation to
FIG. 5A.
[0036] FIG. 6 is a cross-section of a flow shaping plate having
variable flow through properties in order to compensate for lower
plating rate near the center of the wafer as observed when using
conventional flow shaping plate through holes.
[0037] FIG. 7A is a top view of flow dynamics on the top of the
flow shaping plate when a flow port transverse flow enhancement is
used.
[0038] FIGS. 7B-G depict various apparatus for enhancing transverse
flow across a work piece plating surface.
[0039] FIG. 8A is a cross-section of a flow shaping plate having
angled through holes in order to compensate for lower plating rate
near the center of the wafer as observed when using conventional
flow shaping plate through holes.
[0040] FIGS. 8B-C are graphs of plating uniformity obtained when
using angled flow shaping plates.
[0041] FIGS. 9A-B are cross-section and perspective, respectively,
of a paddle wheel type assembly for creating turbulent transverse
flow across a wafer surface during electroplating.
[0042] FIG. 10 is a perspective of a wafer holder showing
directional vectors and rotation for orbital motion of the wafer
holder.
[0043] FIGS. 11A-B are perspective and perspective cross-section of
a flow shaping plate having an embedded rotational element for
creating transverse flow at the center of a wafer during
plating.
[0044] FIG. 12 is a flow diagram outlining aspects of a method
described herein.
[0045] FIG. 13 is a graph showing plating uniformity obtained when
transverse flow is used during plating.
DETAILED DESCRIPTION
A. General Apparatus Context
[0046] The following description of FIGS. 1A and 1B provides some
general non-limiting context for the apparatus and methods
described herein. Various features presented in the following
discussion are also presented in one or more of the figures
described above. The discussion of such features in the following
is intended only to supplement description of embodiments included
herein. Particular focus in later figures is toward a wafer holder
assembly in relation to various flow shaping plates and flow
diverters and thus an exemplary positioning mechanism, rotating
mechanism and wafer holder is described.
[0047] FIG. 1A provides a perspective view of a wafer holding and
positioning apparatus 100 for electrochemically treating
semiconductor wafers. Apparatus 100 has various features shown and
described in subsequent Figures. For example, it includes wafer
engaging components (sometimes referred to herein as "clamshell"
components). The actual clamshell includes a cup, 102, and a cone,
103 that clamps a wafer securely in the cup.
[0048] Cup 102 is supported by struts 104, which are connected to a
top plate 105. This assembly (102-105), collectively assembly 101,
is driven by a motor 107, via a spindle 106. Motor 107 is attached
to a mounting bracket 109. Spindle 106 transmits torque to a wafer
(not shown in this figure) to allow rotation during plating. An air
cylinder (not shown) within spindle 106 also provides vertical
force to clamp the wafer between the cup and cone 103. For the
purposes of this discussion, the assembly including components
102-109 is collectively referred to as a wafer holder 111. Note
however, that the concept of a "wafer holder" extends generally to
various combinations and sub-combinations of components that engage
a wafer and allow its movement and positioning.
[0049] A tilting assembly including a first plate, 115, that is
slidably connected to a second plate, 117, is connected to mounting
bracket 109. A drive cylinder 113 is connected both to plate 115
and plate 117 at pivot joints 119 and 121, respectively. Thus,
drive cylinder 113 provides force for sliding plate 115 (and thus
wafer holder 111) across plate 117. The distal end of wafer holder
111 (i.e. mounting bracket 109) is moved along an arced path (not
shown) which defines the contact region between plates 115 and 117,
and thus the proximal end of wafer holder 111 (i.e. cup and cone
assembly) is tilted upon a virtual pivot. This allows for angled
entry of a wafer into a plating bath.
[0050] The entire apparatus 100 is lifted vertically either up or
down to immerse the proximal end of wafer holder 111 into a plating
solution via another actuator (not shown). Thus, a two-component
positioning mechanism provides both vertical movement along a
trajectory perpendicular to an electrolyte and a tilting movement
allowing deviation from a horizontal orientation (parallel to
electrolyte surface) for the wafer (angled-wafer immersion
capability). A more detailed description of the movement
capabilities and associated hardware of apparatus 100 is described
in U.S. Pat. No. 6,551,487 filed May 31, 2001 and issued Apr. 22,
2003, which is herein incorporated by reference in its
entirety.
[0051] Note that apparatus 100 is typically used with a particular
plating cell having a plating chamber which houses an anode (e.g.,
a copper anode) and electrolyte. The plating cell may also include
plumbing or plumbing connections for circulating electrolyte
through the plating cell--and against the work piece being plated.
It may also include membranes or other separators designed to
maintain different electrolyte chemistries in an anode compartment
and a cathode compartment. In one embodiment, one membrane is
employed to define an anode chamber, which contains electrolyte
that is substantially free of suppressors, accelerators, or other
organic plating additives.
[0052] The following description provides more detail of the cup
and cone assembly of the clamshell. FIG. 1B depicts a portion, 101,
of assembly 100, including cone 103 and cup 102 in cross-section
format. Note that this figure is not meant to be an accurate
depiction of the cup and cone assembly, but rather a stylized
depiction for discussion purposes. Cup 102 is supported by top
plate 105 via struts 104, which are attached via screws 108.
Generally, cup 102 provides a support upon which wafer 145 rests.
It includes an opening through which electrolyte from a plating
cell can contact the wafer. Note that wafer 145 has a front side
142, which is where plating occurs. So, the periphery of wafer 145
rests on the cup. The cone 103 presses down on the back side of the
wafer to hold it in place during plating.
[0053] To load a wafer into 101, cone 103 is lifted from its
depicted position via spindle 106 until cone 103 touches top plate
105. From this position, a gap is created between the cup and the
cone into which wafer 145 can be inserted, and thus loaded into the
cup. Then cone 103 is lowered to engage the wafer against the
periphery of cup 102 as depicted.
[0054] Spindle 106 transmits both vertical force for causing cone
103 to engage a wafer 145 and torque for rotating assembly 101.
These transmitted forces are indicated by the arrows in FIG. 1B.
Note that wafer plating typically occurs while the wafer is
rotating (as indicated by the dashed arrows at the top of FIG.
1B).
[0055] Cup 102 has a compressible lip seal 143, which forms a
fluid-tight seal when cone 103 engages wafer 145. The vertical
force from the cone and wafer compresses lip seal 143 to form the
fluid tight seal. The lip seal prevents electrolyte from contacting
the backside of wafer 145 (where it could introduce contaminating
atoms such copper directly into silicon) and from contacting
sensitive components of apparatus 101. There may also be seals
located between the interface of the cup and the wafer which form
fluid-tight seals to further protect the backside of wafer 145 (not
shown).
[0056] Cone 103 also includes a seal 149. As shown, seal 149 is
located near the edge of cone 103 and an upper region of the cup
when engaged. This also protects the backside of wafer 145 from any
electrolyte that might enter the clamshell from above the cup. Seal
149 may be affixed to the cone or the cup, and may be a single seal
or a multi-component seal.
[0057] Upon initiation of plating, wafer 145 is introduced to
assembly 102 when cone 103 is raised above cup 102. When the wafer
is initially introduced into cup 102--typically by a robot arm--its
front side, 142, rests lightly on lip seal 143. During plating the
assembly 101 rotates in order to aid in achieving uniform plating.
In subsequent figures, assembly 101 is depicted in a more
simplistic format and in relation to components for controlling the
hydrodynamics of electrolyte at the wafer plating surface 142
during plating. Thus, an overview of mass transfer and fluid shear
at the work piece follows.
B. Mass Transfer and Fluid Shear at the Work Piece Plating
Surface
[0058] As indicated various WLP and TSV structures are relatively
large and therefore require rapid, yet highly uniform, plating
across the wafer surface. Although various methods and apparatus
described hereinafter are suitable for these purposes, the
invention is not limited in this way.
[0059] Certain embodiments described herein employ a rotating work
piece, which in certain operating regimes approximates a classical
rotating disk electrode. The rotation of the electrode results in
flow of electrolyte upwards towards the wafer. The flow at the
surface of the wafer may be laminar (as generally employed in a
classical rotating disk electrode) or turbulent. As mentioned,
electroplating cells employing horizontally oriented rotating
wafers are conventionally employed in electroplating apparatus such
as the Sabre.RTM. line of plating systems available from Novellus
Systems, Inc. of San Jose, Calif.
[0060] In various embodiments, a flat flow shaping plate, having
multiple through holes in a generally vertical orientation, is
deployed within the electroplating apparatus a short distance from
the plating surface, e.g., the flat surface of the flow shaping
plate is about 1-10 mm from the plating surface. Examples of
electroplating apparatus containing flow shaping elements are
described in U.S. patent application Ser. No. 12/291,356, filed
Nov. 7, 2008 (issued as U.S. Pat. No. 8,308,931 on Nov. 13, 2012),
which is incorporated herein by reference in its entirety. As
depicted in FIG. 1C, plating apparatus 150 includes a plating cell,
155, which houses anode 160. In this example, electrolyte 175 is
flowed into cell 155 through anode 160 and the electrolyte passes
through a flow shaping element 170 having vertically oriented
(non-intersecting) through holes through which electrolyte flows
and then impinges on wafer 145 which is held in, positioned and
moved by, wafer holder 101. Flow shaping elements such as 170
provide uniform impinging flow upon the wafer plating surface;
however, it has been found (and as described in more detail below)
that when plating in WLP and TSV plating rate regimes, where larger
features are being filled at higher plating rates (for example
relative to plating rates for certain Damascene processing), lower
plating rates are observed in the central region of the wafer as
compared to the outer regions. This result is typified in FIG. 1D
which shows plating uniformity as a function of deposition rate vs.
radial position on a 300 mm wafer. In accordance with certain
embodiments described herein, apparatus utilizing such flow shaping
elements are configured and/or operated in a manner that
facilitates high rate and very uniform plating across the face of
wafer including plating under high rate deposition regimes such as
for WLP and TSV applications. Any or all of the various embodiments
described can be implemented in the context of Damascene as well as
TSV and WLP applications.
[0061] Assuming the rotating work piece is horizontally oriented,
at a plane some distance below the wafer surface the bulk
electrolyte flow is primarily in the vertical direction. When it
approaches and contacts the wafer surface, the presence of the
wafer (and its rotation) redirects and forces the fluid to flow
outward toward the wafer periphery. This flow is normally laminar.
In the ideal case, the current density at the electrode surface is
described by the Levich equation, which indicates that the limiting
current density is proportional to the square root of electrode's
angular velocity. This limiting current density is uniform over the
radial extent of the rotating electrode, primarily because the
boundary layer thickness is of constant thickness and is
independent of both the radial or azimuthal location.
[0062] In various embodiments, the apparatus provides very high
rate vertical flow rates through the pores in a flow shaping plate.
In various embodiments, those pores are holes in the flow shaping
plate that are all independent (i.e., non-interconnecting--there is
no fluidic communication between individual holes) and are oriented
in a primarily vertical orientation to direct flow upwards at the
wafer surface a short distance above the pore exit. Typically,
there are many such pores in the flow shaping plate, often at least
about 1000 such pores or at least about 5000 such pores.
Electrolyte flowing out of these holes may produce a set of
individual "microjets" of high velocity fluid that directly impinge
on the wafer surface. In some cases, the flow at the work piece
plating surface is not laminar, i.e., the local flow is turbulent
or transitional between turbulent and laminar. In some cases, the
local flow at the hydrodynamic boundary layer of the wafer surface
is defined by a Reynolds number of about 10.sup.5 or greater at the
wafer surface. In other cases, the flow at the work piece plating
surface is laminar and/or characterized by a Reynolds number of
about 2300 or lower. In accordance with specific embodiments
described herein, the flow rate of fluid emanating from an
individual hole or pore in the flow plate in the vertical
direction, to the wafer surface (and through the through holes in
the flow shaping plate), is on the order of about 10 cm/second or
greater, more typically about 15 cm/second or greater. In some
cases, it is about 20 cm/second or greater.
[0063] Additionally, the electroplating apparatus may be operated
in a manner so that local shearing of the electrolyte between the
flow shaping plate and the electrode occurs. Shearing of the fluid,
particularly the combination of impinging and shearing flow may
maximize convection within the reactor for features whose sizes are
on the length scale of the typical boundary layer thickness. In
many embodiments, this length scale is on the order of micrometers
or even 10 s of micrometers. Flow shearing can be established in at
least two manners. In the first case, it is accomplished by the
relative proximity of a generally stationary flow shaping plate to
a high speed relative-moving wafer surface located a few
millimeters away. This arrangement establishes relative motion, and
consequently shearing flow, by linear, rotational and/or orbital
motion. Taking a non-moving flow shaping plate as a point of
reference, the fluid local shear will be given by the local point
on the wafer's velocity divided by the plate-to-wafer gap (units
(cm/sec)/(cm)=sec.sup.-1), while the required shear stress to keep
the wafer moving is simply this value times fluid's viscosity.
Generally (for a Newtonian fluid) in this first mode of shearing,
the velocity profile generally increases linearly between the two
planar surfaces. A second approach to establishing local shearing
involves introducing conditions within the flow plate/wafer gap
that create or induce lateral fluid motion in the gap between the
two flat surfaces (either in the absence of or in additional to any
relative motion of the plate). A pressure difference and or
entrance and exit port for fluid into and out of the gap moves
fluid substantially parallel to the two surfaces, including across
the center of rotation of the wafer. Assuming a stationary wafer,
the maximum velocity associated with imposed flow is observed in
the middle of the flow-plate/wafer gap, and the local shear is
proportional to the local fluid flow density or average velocity
(cm.sup.3/sec/cm or cm/sec) divided by the wafer-to-flow-plate gap,
with a maximum velocity at the center of the gap. While the first
mode of shearing of a classical rotating disk/wafer does not create
any shearing of fluid at the wafer center, the second mode, which
may be implemented in various embodiments, does create fluid
shearing at the wafer center. Therefore, in certain embodiments,
the electroplating apparatus is operated under conditions that
produce a transverse relative electrolyte velocity of about 3
cm/sec or greater (or about 5 cm/sec or greater) within a few mm
from the wafer surface across the center point of the plating face
of the substrate.
[0064] When operating at such high vertical flow rates through a
flow shaping plate, high plating rates can be attained, typically
on the order of about 5 micrometers/minutes or higher, particularly
in feature being formed in a through resist layer of photoresist
with a 1:1 aspect ratio 50 um deep. Further, while not wishing to
be held to any particularly principle or theory, when operating
under shearing conditions as described herein, advantageous
convective patterns and associated enhanced transport of material
within the recessed fluid-containing-portion of the structure being
plated enhances both the deposition rate and uniformity, leading to
very uniform shaped features both within individual dies and over
the entire face of the plating work piece, frequently varying by no
more than about 5% over the plating surface. Regardless of the
mechanism of action, the recited operation leads to remarkably
uniform and rapid plating.
[0065] As mentioned above, it is interesting to note that in the
absence of an appropriate combination of both a flow impinging and
shearing condition created by apparatus herein, such as high
vertical impinging flow rates on the work piece surface, or flow
shearing alone, will not easily yield highly uniform plating both
within and over the wafer surface of large, WLP size features.
[0066] Consider first the situation of plating a substantially flat
surface. Here, the term substantially flat means a surface whose
feature or roughness are less than the calculated or measured mass
transfer boundary layer thickness (generally a few tens of
micrometers). Any surface having recessed features smaller than
about 5 micrometers, such as 1 micrometer or less, such as
typically used in copper damascene plating, are therefore
substantially flat for this purpose. When using classical
convection, an example being a rotating disk or fountain plating
system, the plating is theoretically and practically very uniform
across the work piece face. Because the depths of features are
small compared to the mass transfer boundary thickness, the
internal feature mass transfer resistance (associated with
diffusion inside the feature) is small. Importantly, shearing the
fluid, for example, by using a flow shearing plate, theoretically
will not alter the mass transport to a flat surface, because the
shearing velocities and associated convection are all in the
direction normal to the surface. To aid mass transfer to the
surface, convection must have a component of velocity toward the
surface. In contrast, a high velocity fluid moving in the direction
of the surface, such as that resulting from fluid passing through
an anisotropic porous plated (e.g., a flow shaping plate as
described herein), can create a large impinging flow with a
component of velocity towards the surface, and therefore
substantially decrease the mass transport boundary layer.
Therefore, again for a substantially flat surface, impinging flow
will improve transport, but shearing (as long as turbulence is not
created) will not improve transport. In the presence of turbulence
(chaotic motion of fluid), such as that created in the gap between
the wafer and a shearing plate in close proximity to a rotating
work piece, one can considerably reduce the mass transfer
resistance and enhance uniform convective condition, creating
condition for very thin boundary layer thicknesses, because some of
the chaotic motion is directing fluid to the surface. The flow to
the substantially flat surface may or may not be turbulent over the
entire radial extent of the work piece, but can generally results
in very uniform within feature and within wafer deposition.
[0067] It is important to understand the limitation of the concept
of a boundary layer thickness, a highly simplified, conceptual
region of space that lumps mass transfer resistance into an
equivalent surface film. It is functionally limited to representing
the distance over which reactants' concentration change as they
diffuse to a generally flat surface, loosing some significance when
applied to "rougher" surfaces. It is true that thin boundary layers
are generally associated with high rates of transport. But it is
also true that some conditions that do not lead to improved
convection to a flat surface, can improve convection to a rough
one. It is believed that for WLP scale "rough" surface, there is an
added, hitherto unappreciated, characteristic of fluid shearing
that can be used, in combination with impinging flow, to enhance
convection to such rougher surface, such as patterned surfaces with
features larger than the mass transfer boundary layer thickness.
The perceived reason for this difference between substantially flat
and substantially rough surface behavior is associated with an
enhanced material replenishment that can be created to stir the
matter held in the cavity as it passes over the mouth of the
feature, mixing and transporting of fluid to and away from the
relatively large recessed features. The creation of the
intra-feature circulation condition is instrumental in achieving
very high rate, global and microscopically uniform deposition in
WLP type structures.
[0068] With large and relatively deep (1:0.5 width to depth or
greater aspect ratio) features, using impinging flow alone may be
only partially effective, because impinging fluid must diverge
radially outwards from the feature cavity opening as it approaches
the open pore. Fluid contained within the cavity is not effectively
stirred or moved and may remain essentially stagnant, leaving
transport with the feature to be primarily by diffusion alone.
Therefore, it is believed that when plating WLP scale features
under operating conditions of either primarily impinging or
shearing flow alone, convection is inferior to that using the
combination of the two. And the mass transfer boundary layer that
is associated with an equivalent convection conditions to a flat
surface (flat on the order of the boundary layer) will naturally be
generally uniform, but in the situation encountered in WLP scale
feature plating, the boundary layer thickness, generally comparable
to the size of the features being plating and on the order of a few
tens of micrometers, requires, for uniform plating, conditions
which are quite different.
[0069] Finally, a combination and crossing a laminar impinging flow
with a laminar shearing flow is believed to be able to create
micro-flow vortices. These micro-vortices, which alone may be
laminar in nature, can potentially become turbulent in nature, and
in line with the discussion above, be useful in enhancing
convection to both flat and rough surface plating. It should be
appreciated that the above explanation is submitted only to aid in
understanding the physical underpinnings of mass transfer and
convection in wafers having WLP or WLP-like features. It is not a
limiting explanation of the mechanisms of action or necessary
plating conditions for the beneficial methods and apparatus
described herein.
[0070] It has been observed by the inventors that when rotating a
patterned substrates--particularly those having features of similar
size to the mass transfer boundary layer (e.g., recesses or
protrusions on the order of micrometers or tens of micrometers such
as commonly encountered on TSV and WLP substrates)--can produce a
"singularity" or plating aberration at the center of the rotating
substrate (see FIG. 1D). This plating non-uniformity occurs at the
axis of rotation of the flat plating surface where the angular
velocity is at or near zero. It has also been observed in some of
the apparatus employing a flow shaping plate as described above, in
the absence of some other center-aberration-mediating mechanisms.
In such cases without these mechanisms, the plating rate is
remarkably uniform and rapid with generally flat features across
the patterned work piece surface everywhere, except at the center
of the work piece, where the rate is significantly lower and the
feature shapes are generally non-uniform (for example concave near
the center). This is particularly interesting, given that plating
under similar conditions on an unpatterned substrate produces an
entirely uniform plating profile or sometimes even an inverse
plating profile (i.e., the plating rate is remarkably uniform
across the work piece surface everywhere except at the center where
it is significantly higher, resulting a domed center region). In
other tests, where the total impinging flow volume, and/or velocity
is increased at the center, it is found that the rate of deposition
can be increased there, but the general shape of the feature at the
center remain largely unchanged (domed and irregular rather than
flat).
[0071] This center non-uniformity may be mitigated or eliminated by
providing a lateral moving fluid that will create a shearing force
at the substrate center to the electrolyte flowing across the
plating face of the substrate. This shearing force may be applied
by any of a number of mechanisms, some of which will be described
herein. Briefly, the mechanisms include (1) a flow shaping plate
having variation from uniformity in number, orientation and
distribution of holes at or near the center of the rotating
substrate, such as a flow shaping plate in which at least some of
the holes proximate to the center of the rotating work piece have
an angle deviating from vertical (more generally, an angle that is
not perpendicular to the plating face of the rotating substrate),
(2) a lateral component of relative motion between the work piece
surface and the flow shaping plate (e.g., a relative linear or
orbital motion such as is sometimes applied in chemical mechanical
polishing apparatus), (3) one or more reciprocating or rotating
paddles (e.g., a paddlewheel or impeller) provided in the plating
cell, (4) a rotating assembly attached to or proximate to the flow
shaping plate and offset from the axis of rotation of the work
piece, (5) an azimuthally non-uniform flow restrictor (sometimes
termed a "flow diverter") attached to or proximate the
circumference of the flow shaping plate and extending toward the
rotating work piece, and (6) other mechanisms of introducing
lateral flow across the general wafer surface including the
center.
[0072] Each of these mechanisms will be described and exemplified
in more detail below. Regarding the first listed mechanism, the
non-uniformity in distribution of plate holes may be (a) an
increased density of holes in the center region of the plate and/or
(b) a randomness in the distribution of holes in the center region.
Regarding, the fifth of the listed mechanisms, the flow diverter
effectively provides a nearly closed chamber between the rotating
substrate and the flow shaping plate. In some cases, as more fully
described below, the flow diverter and associated hardware provides
or enables creation of a very small gap (e.g., about 0.1 to 0.5 mm)
over the majority of the region between a substrate holder
periphery and the top of the edge element. In the remaining
periphery region, there is a gap in the edge element that provides
a larger gap with a relatively low resistance path for electrolyte
to flow out of the nearly closed chamber. See e.g., FIGS. 2A-C.
C. Design and Operating Parameters
[0073] Various relevant parameters will be discussed in this
section. These parameters are often interrelated. Nevertheless,
they will be described separately to provide examples of a general
operating space and a general apparatus design space. Those of
skill in the art will fully appreciate that appropriate
combinations of these parameters can be chosen, when considering
the teachings of this disclosure, to effect particular results such
as desired plating rates or uniform deposition profiles.
Additionally, some of the parameters presented herein may scale
with the size of the substrate and features being plated and/or the
electroplating cell in which they apply. Unless otherwise
specified, the recited parameters are appropriate for plating 300
mm wafers using an electroplating cell having an electrolyte
chamber volume, below the flow shaping plate of greater than about
1 liter.
Electrolyte Flow Rate Exiting the Holes of Flow Shaping Plate and
Impinging on Wafer
[0074] As indicated, the flow rate through holes in the flow
shaping plate may be relevant to the operation of the plating cell.
Typically, it is desired to have a high rate of impinging flow
passing through the flow shaping plate. In certain embodiments,
this exiting flow rate from individual holes in the plate is at
least about 10 cm/second and often as great as about 15 cm/second
or even about 20 cm/second or greater. The distance from the plate
hole and the wafer surface is generally less than 5 mm, thereby
minimizing any potential dissipation of the above stated fluid
velocity before striking the wafer surface. Essentially, each of
the apertures of each through hole provides a microjet of impinging
flow.
[0075] In flow shaping plates having relatively small openings
(e.g., on the order of 0.03 inches in diameter or less), viscous
wall forces typically dominate inertial hydrodynamic forces inside
the openings. In such cases, the Reynolds number will be well below
the turbulent value threshold (>2000) for flow in a pipe. Thus,
the flow inside the holes themselves typically will be laminar.
Nevertheless, the flow hits the plating surface hard and directly
(e.g., at a right angle), after traveling at, e.g., 10-20 cm/sec.
It is believed that this impinging flow is at least partially
responsible for the observed beneficial results. For example,
measurements of the limiting current plating rates of copper to a
flat wafer were used to determine the boundary layer thickness with
and without the use of high velocity impinging fluid microjets. The
flow shaping plate was a 1/2 in thick plate with 6500 drilled 0.026
inch holes, evenly arranged over about a 300 mm diameter region.
Despite the fact that the holes' area occupy only about 3% of the
total area below the wafer plating surface, and the rotating wafer
is directly above a hole for an equally small fraction of time, the
limiting current was found to increase as much as 100% percent when
changing the hole flow velocity from 3 cm/sec to 18.2 cm/sec while
the rotation of the wafer remained at 30 RPM.
Volumetric Flow Rate Through Flow Shaping Plate
[0076] The overall volumetric flow passing through the flow shaping
plate is directly tied to the linear flow rate from the individual
holes in the plate. For a typical flow shaping plate as described
herein (e.g., one of about 300 mm diameter having a large number of
equal diameter) a volumetric flow through the plate holes may be
greater than about 5 liters/minute, or greater than about 10
liter/minute, or sometime as great as 40 liters/minute or higher.
As an example, a volumetric flow rate of 24 liters/minute produces
a linear flow velocity at the exit of each hole of a typical plate
of about 18.2 cm/sec.
Flow Rate Laterally Across Center Axis of Rotation of Substrate
Work Surface
[0077] The flow immediately parallel to the surface of the rotating
substrate should generally be non-zero at the axis of rotation for
the substrate. This parallel flow is measured just outside the
hydrodynamic boundary layer on the substrate surface. In some
embodiments, the flow across the substrate center is greater than
about 3 cm/sec, or more specifically greater than about 5 cm/sec.
It is believed that such flows mitigate or eliminate the observed
decrease in plating rate at the rotation axis of patterned
wafers.
Pressure Drop of Electrolyte Flowing Through Flow Shaping Plate
[0078] In certain embodiments, the pressure drop of electrolyte
flowing through the holes of the flow shaping element is modest,
e.g., about 0.5 to 3 torr (0.03 psi or 1.5 torr in a specific
embodiment). In some designs such as those employing a flow
diverter structure described with respect to, for example, FIGS.
2A-I, the pressure drop across the plate should be significantly
larger than the pressure drop to the open gap in the shield or edge
element to ensure that the impinging flow on the substrate surface
is at least relatively uniform across the substrate surface.
Distance Between Wafer and Flow Shaping Plate
[0079] In certain embodiments, a wafer holder and associated
positioning mechanism hold a rotating wafer very close to the
parallel upper surface of the flow shaping element. In typical
cases, the separation distance is about 1-10 millimeters, or about
2-8 millimeters. This small plate to wafer distance can create a
plating pattern on the wafer associated with proximity "imaging" of
individual holes of the pattern, particularly near the center of
wafer rotation. To avoid this phenomenon, in some embodiments, the
individual holes (particularly at and near the wafer center) should
be constructed to have a small size, for example less than about
115.sup.th the plate to wafer gap. When coupled with wafer
rotation, the small pore size allows for time averaging of the flow
velocity of impinging fluid coming up as a jet from the plate and
reduces or avoids small scale non-uniformities (e.g., those on the
order of micrometers). Despite the above precaution, and depending
on the properties of the plating bath used (e.g. particular metal
deposited, conductivities, and bath additives employed), in some
cases deposition may be prone to occur in a micro-non-uniform
pattern as the time average exposure and proximity-imaging-pattern
of varying thickness (for example, in the shape of a "bulls eye"
around the wafer center) and corresponding to the individual hole
pattern used. This can occur if the finite hole pattern creates an
impinging flow pattern that is non-uniform and influences the
deposition. In this case, introducing lateral flow across the wafer
center has been found to largely eliminate any
micro-non-uniformities otherwise found there.
Porosity of Flow Shaping Plate
[0080] In various embodiments, the flow shaping plate has a
sufficiently low porosity and pore size to provide a viscous
backpressure and high vertical impinging flow rates at normal
operating volumetric flow rate. In some cases, about 1-10% of the
flow shaping plate is open area allowing fluid to reach the wafer
surface. In particular embodiments, about 2-5% the plate is open
area. In a specific example, the open area of the plate is about
3.2% and the effective total open cross sectional area is about 23
cm.sup.2.
Hole Size of Flow Shaping Plate
[0081] The porosity of the flow shaping plate can be implemented in
many different ways. In various embodiments, it is implemented with
many vertical holes of small diameter. In some cases the plate does
not consist of individual "drilled" holes, but is created by a
sintered plate of continuously porous material. Examples of such
sintered plates are described in U.S. Pat. No. 6,964,792, which is
herein incorporated by reference in its entirety. In some
embodiments, drilled non-communicating holes have a diameter of
about 0.01 to 0.05 inches. In some cases, the holes have a diameter
or about 0.02 to 0.03 inches. As mentioned above, in various
embodiments the holes have a diameter that is at most about 0.2
times the gap distance between the flow shaping plate and the
wafer. The holes are generally circular in cross section, but need
not be. Further, to ease construction, all holes in the plate may
have the same diameter. However this need not be the case, and so
both the individual size and local density of holes may vary over
the plate surface as specific requirements may dictate.
[0082] As an example, a solid plate made of a suitable ceramic or
plastic (generally a dielectric insulating and mechanically robust
material), having a large number of small holes provided therein,
e.g. 6465 holes of 0.026 inches diameter has been found useful. The
porosity of the plate is typically less than about 5 percent so
that the total flow rate necessary to create a high impinging
velocity is not too great. Using smaller holes helps to create a
large pressure drop across the plate than larger holes, aiding in
creating a more uniform upward velocity through the plate.
[0083] Generally, the distribution of holes over the flow shaping
plate is of uniform density and non-random. In some cases, however,
the density of holes may vary, particularly in the radial
direction. In a specific embodiment, as described more fully below,
there is a greater density and/or diameter of holes in the region
of the plate that directs flow toward the center of the rotating
substrate. Further, in some embodiments, the holes directing
electrolyte at or near the center of the rotating wafer may induce
flow at a non-right angle with respect to the wafer surface.
Further the holes in this region may have a random or partially
random distribution non-uniform plating "rings" due to any
interaction between the limited number of hole and the wafer
rotation. In some embodiments, the hole density proximate an open
segment of a flow diverter is lower than on regions of the flow
shaping plate that are further from the open segment of the
attached flow diverter.
Rotation Rate of Substrate
[0084] The wafer's rate of rotation can vary substantially. In the
absence of impinging flow and a flow shaping plate a small distance
below the wafer, rotation rates above 90 rpm should be avoided
because of turbulence generally forming at the wafer's outer edge
(and laminar flow remaining further in), resulting in radial
non-uniform convection conditions. However, in most of the
embodiment disclosed herein, such as those with imposed flow
turbulence and/or with the impinging flow shaping plate, much
larger ranges of rotation rates, for example from 20 to 200 rpm or
more, can be used. Higher rotation rates greatly increase the
shearing of most of the wafer surface with the exception of the
wafer center. Nevertheless, high rotation rates also tend to
amplify, focus or otherwise modify the relative scale of the center
singularity/aberration, so it is believed that introducing lateral
flow across the center is sometimes necessary to eliminate the
same, particularly when operating at higher rotation rates.
Rotation Direction of Substrate
[0085] In some embodiments, the wafer direction is changed
periodically during the electroplating process. One benefit of this
approach is that in an array of features or a portion of an
individual feature that previously was at the leading edge of the
fluid flow (in the angular direction) can become the features at
the trailing edges of the flow when the rotational direction
reverses. Of course, the opposite is also true. This reversal in
angular fluid flow tends to even out the deposition rate over the
features on the face of the work piece. In certain embodiments, the
rotation reversal takes place a number of times of approximately
equal durations throughout the entire plating process so that
convection versus feature depth convolutions are minimized. In some
cases, the rotation is reversed at least about 4 times during the
course of plating a wafer. For example, a series of oscillating 5
clockwise and 5 counterclockwise plating rotation steps can be
used. Generally, changing the direction of rotation can moderate
upstream/downstream non-uniformities in the azimuthal direction,
but have limited impact on radial non-uniformities unless
superimposed with other randomizing influences, such as impinging
flow and wafer cross flow.
Electrodeposition Uniformity Over Substrate Surface--Surface to
Edge
[0086] As indicated, it is generally desirable to plate all
features to a uniform thickness over the plating face of a wafer.
In certain embodiments, the plating rate and therefore the
thickness of the plated features has a within wafer half range (WIW
R/2%) non uniformity of 10% or less. The WIW-R/2 is defined as the
total thickness range of a particular feature type (i.e. a chosen
feature of a given size and having the same relative location with
each die on the wafer) collected at multiple die across the wafer
radius, divided by twice that feature's average thickness over the
entire wafer. In some cases, the plating process has a WIW-R/2
uniformity of about 5% or better. Apparatus and methods described
in this invention are capable of achieving or exceeding this level
of uniformity at high rates of deposition (e.g., 5
micrometers/minute or higher).
Electrodeposition Rate
[0087] Many WLP, TSV and other applications require a very high
rate of electrofill. In some cases, an electroplating process as
described herein fills micron scale features at a rate of at least
about 1 micrometers/minute. In some cases, it fills such features
at a rate of at least about 5 micrometers/minute (sometimes at
least about 10 micrometers/minute). Embodiments described herein
create efficient mass transfer so that such higher plating rates
can be used while maintaining high plating uniformity.
Additional Characteristics of Flow Shaping Plate
[0088] As indicated, the flow shaping plate can have many different
configurations. In some embodiments, it provides the following
general (qualitative) characteristics: 1) a no slip boundary
residing close to the rotating work piece to produce local shearing
force of the electrolyte at the work piece surface, 2) a
significant ionic resistance which may provide a more uniform
potential and current distribution over the work piece radius when
electroplating onto relatively thin metallized or otherwise highly
resistive surfaces, and 3) a large number of fluid microjets that
deliver very high velocity fluid directly onto the wafer surface.
The significant ionic resistance is important, because in both WLP
and TSV plating, there can be little or no metal deposition on the
wafer as a whole, the cross wafer resistance and resistance from
the wafer periphery to its center may remain high throughout the
entire process. Having a significant ionic resistance throughout
the entire plating process allow a useful means of maintaining a
uniform plating process and enables the use of thinner seed layers
than would be otherwise possible. This addresses the "terminal
effect" as described in U.S. patent application Ser. No.
12/291,356, previously incorporated by reference.
[0089] In many embodiments, the pores or holes of the flow shaping
element are not interconnected, but rather are non-communicating,
i.e., they are isolated from each other and do not form
interconnecting channels with the body of flow shaping element.
Such a hole may be referred to as a 1-D through-hole because it
extends in one dimension, in one example, normal to the plating
surface of the wafer. That is, the channels are oriented at an
angle of about 90.degree. with respect to the substrate-facing
surface of the flow shaping element. In one embodiment, the
channels of the flow shaping element are oriented at an angle of
about 20.degree. to about 60.degree. with respect to the
substrate-facing surface of the flow shaping element, in another
embodiment, about 30.degree. to about 50.degree. with respect to
the substrate-facing surface of the flow shaping element. In one
embodiment, the flow shaping element includes through-channels
oriented at different angles. The hole pattern on the flow shaping
element can include uniform, non-uniform, symmetric and asymmetric
elements, i.e. the density and pattern of holes may vary across the
flow shaping element. In certain embodiments, the channels are
arranged to avoid long range linear paths parallel to the
substrate-facing surface that do not encounter one of the channels.
In one embodiment, the channels are arranged to avoid long range
linear paths of about 10 mm or greater that are parallel to the
substrate-facing surface that do not encounter one of the
channels.
[0090] The flow shaping element may be constructed of an ionically
resistive material including at least one of polyethylene,
polypropylene, polyvinylidene difluoride (PVDF),
polytetrafluoroethylene, polysulphone, and polycarbonate. In one
embodiment, the flow shaping element is between about 5 mm and
about 10 mm thick.
[0091] In certain embodiments, the plurality of channels are
substantially parallel to one another, in another embodiment, at
least some of the plurality of channels are not parallel to one
another. In certain embodiments, the flow shaping element is a disk
having between about 6,000-12,000 holes. In one embodiment, the
flow shaping element has a non-uniform density of holes, with a
greater density of holes being present in a region of the flow
shaping element that faces a rotational axis of the substrate
plating face. In one embodiment, the plurality of holes in the flow
shaping element do not form communicating channels within the flow
shaping element and substantially all of the plurality of holes
have a principal dimension or a diameter of the opening on the
surface of the element facing the surface of the substrate of no
greater than about 5 millimeters.
[0092] It should be noted, that a flow shaping plate employed with
this invention may have certain characteristics that deviate from
those recited in U.S. patent application Ser. No. 12/291,356,
previously incorporated by reference. These include (1) a lower
ionic resistance (such as a resistance significantly smaller than
that of the seeded wafer), (2) a larger number of holes, and (3) a
thinner construction (e.g. the plate could be about one-quarter
inch or less in thickness).
[0093] With the above-described parameters in mind, apparatus and
methods are described in more detail below in conjunction with the
Figures.
D. Apparatus for Addressing Center Plating Non-Uniformity
[0094] While some aspects of the invention described herein may be
employed in various types of plating apparatus, for simplicity and
clarity, most of the examples will concern wafer-face-down
"fountain" plating apparatus. In such apparatus, the work piece to
plated (typically a semiconductor wafer in the examples presented
herein) that generally has a substantially horizontal orientation
(which may in some cases vary by a few degrees from true
horizontal) and rotates during plating with generally vertically
upward electrolyte convection. One example of a member of the
fountain plating class of cells/apparatus is the Sabre.RTM.
Electroplating System produced by and available from Novellus
Systems, Inc. of San Jose, Calif. Additionally, fountain
electroplating systems are described in, e.g., U.S. Pat. No.
6,800,187 and US Patent Application Publication US 2010-0032310A1
filed Feb. 11, 2010, which are incorporated herein by reference in
their entireties.
[0095] As mentioned, it has been observed that on patterned wafers
the electroplating rate at the center of the wafer and over a small
radial region near it is relatively slower and the plating feature
shape inferior compared to that at the remainder of the wafer,
where the rate is substantially uniform. FIG. 1D depicts results
from an electroplating run of copper onto a 300 mm wafer when a
conventional fountain-type plating configuration is employed. These
results were obtained for a wafer plated with copper and having 50
micrometer wide features defined in 50 micrometer thick photoresist
plated at 3.5 micrometers/min. Plating was conducted while the
wafer was rotating at 90 rpm, with a flow plate as described above
and a total system flow rate of 20 lpm, but without means of
correcting for specifically introducing cross-center wafer flow
shear. When plating at high deposition rates, for example in rates
near of exceeding the upper limits of present WLP plating
capabilities regimes, conventional diffusers and wafer rotation
conditions are insufficient to prevent non-uniform deposition in a
region at the center of the wafer. This is believed due to slower
rotation, minimal impinging flow, and insufficient shearing of
fluid at the center region of the wafer. At the actual central axis
of rotation on the wafer surface, there is a "singularity"
associated with zero angular velocity.
[0096] Having efficient mass transfer capabilities, the singularity
can be compensated for and thus high rate uniform plating is
achieved; thus apparatus described herein are configured to
electroplate, for example, wafer level packaging features, TSV's
and the like. Various metals can be plated using apparatus
described herein, including metals that are traditionally difficult
to plate due to mass transfer issues. In one embodiment, apparatus
described herein are configured to electroplate one or more metals
selected from the group consisting of copper, tin, a tin-lead
composition, a tin silver composition, nickel, a tin-copper
composition, a tin-silver-copper composition, gold, and alloys
thereof.
[0097] Various mechanisms for addressing the observed
non-uniformity were identified above. In certain embodiments, these
mechanisms introduce fluid shearing at the surface of the rotating
work piece. Each of the embodiments are described more fully
below.
[0098] One embodiment is an electroplating apparatus including: (a)
a plating chamber configured to contain an electrolyte and an anode
while electroplating metal onto a substantially planar substrate;
(b) a substrate holder configured to hold the substantially planar
substrate such that a plating face of the substrate is separated
from the anode during electroplating; (c) a flow shaping element
including a substrate-facing surface that is substantially parallel
to and separated from a plating face of the substrate during
electroplating, the flow shaping element including an ionically
resistive material with a plurality of non-communicating channels
made through the flow shaping element, where the non-communicating
channels allow for transport of the electrolyte through the flow
shaping element during electroplating; and (d) a flow diverter on
the substrate-facing surface of the flow shaping element, the flow
diverter including a wall structure partially following the
circumference of the flow shaping element, and having one or more
gaps, and defining a partial or "pseudo" chamber between the flow
shaping element and the substantially planar substrate during
electroplating.
[0099] In one embodiment, the flow shaping element is disk-shaped
and the flow diverter includes a slotted annular spacer attached
to, or integrated onto, the flow shaping element. In one
embodiment, the wall structure of the flow diverter has a single
gap and the single gap occupies an arc of between about 40 and
about 90 degrees. The wall structure of the flow diverter may be
between about 1 mm and about 5 mm high. In certain embodiments, the
flow diverter is configured such that a top surface of the wall
structure is between about 0.1 and 0.5 mm from a bottom surface of
the substrate holder during electroplating and the top surface of
the flow shaping element is between about 1 and 5 mm from the
bottom surface of the substrate holder during electroplating.
[0100] In certain embodiments, the apparatus is configured to flow
electrolyte in the direction of the substrate plating face and
under conditions that produce an average flow velocity of at least
about 10 cm/s exiting the holes of the flow shaping element during
electroplating. In certain embodiments, the apparatus is configured
to operate under conditions that produce a transverse electrolyte
velocity of about 3 cm/sec or greater across the center point of
the plating face of the substrate.
[0101] In certain embodiments, the wall structure has an outer
portion that is higher than an inner portion. Embodiments include
features that restrict the flow of electrolyte out of the pseudo
chamber except for the one or more gaps which form a vent region in
the pseudo chamber.
[0102] One embodiment is an apparatus for electroplating metal onto
a substrate, the apparatus including: (a) a plating chamber
configured to contain an electrolyte and an anode while
electroplating metal onto the substrate; (b) a substrate holder
configured to hold the substrate such that a plating face of the
substrate is separated from the anode during electroplating, the
substrate 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; (c) a flow shaping
element shaped and configured to be positioned between the
substrate and the anode during electroplating, the flow shaping
element having a flat surface that is substantially parallel to and
separated from the plating face of the substrate by a gap of about
10 millimeters or less during electroplating, and the flow shaping
element also having a plurality of holes to permit flow of the
electrolyte toward the plating face of the substrate; (d) a
mechanism for rotating the substrate and/or the flow shaping
element while flowing electrolyte in the electroplating cell in the
direction of the substrate plating face; and (e) a mechanism for
applying a shearing force to the electrolyte flowing at the plating
face of the substrate; where the apparatus is configured for
flowing electrolyte in the direction of the substrate plating face
under conditions that produce an average flow velocity of at least
about 10 cm/s exiting the holes of the flow shaping element during
electroplating and for flowing electrolyte in a direction parallel
to the plating face of the substrate at an electrolyte velocity of
at least about 3 cm/sec across the center point of the plating face
of the substrate. Various shearing force mechanisms are described
in more detail below.
Flow Diverter
[0103] Certain embodiments impart lateral shearing at the wafer's
plating face, and particularly at the central axis of rotation on
the plating face. This shearing is believed to reduce or eliminate
the non-uniformity in deposition rate observed at the center of the
wafer. In this section, the shearing is imparted by using an
azimuthally non-uniform flow diverter attached to or proximate the
circumference of the flow shaping plate and extending toward the
rotating work piece. Generally a flow diverter will have a wall
structure that at least partially restricts the flow of electrolyte
from the pseudo chamber except at the vent portion of the pseudo
chamber. The wall structure will have a top surface, which in some
embodiments is flat and in others has vertical elements, slopes
and/or curved portions. In some embodiments described herein, the
top surface of an edge portion of the flow diverter provides a very
small gap (e.g., about 0.1 to 0.5 mm) between the bottom of the
wafer holder and flow diverter over the majority of the region
between a substrate holder periphery and the top of the edge
portion. Outside this region (between about 30 to 120 degrees arc),
there is a gap in the flow diverter body (for example a segment
removed from an annular body) that provides a relatively low
resistance path for electrolyte to flow out of the nearly closed
chamber formed between the wafer plating face, certain surfaces of
the wafer holder, the flow shaping plate and the interior surfaces
of the flow diverter.
[0104] In one embodiment, the electroplating apparatus' mechanism
for applying the shearing force includes a slotted spacer located
on or proximate to the circumference of the flow shaping element
and projecting toward the substrate holder to define a partial
chamber between the flow shaping element and the substrate holder,
where the slotted spacer includes a slot over an angular section to
provide a low resistance path for electrolyte flow out of the
partial chamber. FIGS. 2A-D and associated CAD FIGS. 2E-I depict an
implementation where a slotted spacer, 200, is used in combination
with a flow shaping plate 202 (5 in FIGS. 2E-K), in order to create
a diverter assembly, 204, which when positioned in close proximity
to a rotatable drive assembly 101 and when sufficient flow is
provided through the through holes of plate 202, provides
substantially uniform plating in high rate deposition regimes. FIG.
2A depicts how slotted spacer 200 (also referred to as an
azimuthally asymmetric flow diverter) combines with flow shaping
plate 202 to form assembly 204. Slotted spacer 200 can be attached,
for example, using screws and the like (not shown). One of ordinary
skill in the art would appreciate that, although embodiments are
described as individual flow shaping plates and flow diverters
combined in an assembly (e.g. slotted spacer 200 and plate 202,
together assembly 204), rather than such assemblies, a unitary body
milled from, for example, a block of material can serve the same
purpose. Thus, one embodiment is a flow shaping element having a
unitary body which is configured to serve the purpose of a flow
diverter/flow shaping plate assembly described herein.
[0105] Assembly 204 is positioned in close proximity to the
substrate to be plated. For example, the closest part of assembly
101 (the base of cup 102 as described in relation to FIGS. 1A and
1B) is within less than about one millimeter from the top of
azimuthally slotted spacer 200. In this way, a confined space or
pseudo chamber is formed between the wafer and the flow shaping
plate where the majority of the electrolyte impinging on the wafer
surface exits through the slotted portion of 200. Dimension A,
which may be defined as an angle or a linear dimension for a ring
of defined radius, can be varied to allow more or less flow through
the slot and dimension B can be varied to create a larger or
smaller volume in the aforementioned pseudo chamber. FIG. 2B is a
cross sectional depiction of assembly 206 positioned in close
proximity to assembly 101. In certain embodiments, a dimension C,
which is a gap between the top of spacer 200 and the bottom of
assembly 101, is on the order of about 0.1 to 0.5 mm, in another
embodiment about 0.2 to 0.4 mm.
[0106] FIG. 2C depicts the electrolyte flow pattern within the
pseudo chamber between the wafer and plate 202 when the wafer is
not rotating. More specifically, the figure depicts representative
vectors of a flow pattern immediately proximate the plating face of
the wafer. The electrolyte impinges on the wafer normal to the
plating surface, but then is deflected and flows parallel to the
plating surface and out of the slot of 200. This flow pattern is
produced by virtue of the resistance to flow through the narrow gap
C (see FIG. 2B) relative to the region where the segment is removed
from flow diverter 200 where a "vent" or larger opening in the
pseudo chamber resides. Note that the magnitude of the flow vectors
increases across the flow shaping plate from the area in the pseudo
chamber furthest from the vent region and toward the vent region.
This can be rationalized by considering the pressure differential,
for example, from the area furthest from the gap (higher pressure)
and the area proximate the gap (lower pressure). Also, the
electrolyte flowing in the area of the pseudo chamber furthest from
the vent does not enjoy the additive speed and momentum of combined
flow from the additional micro jets in the shaping plate as is true
in the region near the vent. In certain embodiments, described in
more detail below, these flow vector magnitudes are made more
uniform in order to further increase plating uniformity.
[0107] FIG. 2D depicts representative vectors of a flow pattern at
the wafer face when the wafer is rotating in one direction. Note
that the electrolyte flows laterally across the center (marked with
a bold "X") or axis of rotation of the rotating wafer. Thus shear
flow is established across the center of the wafer, mitigating or
eliminating the center slow plating (e.g. as described in relation
to FIG. 1D) observed when insufficient shearing flow exists.
[0108] In some embodiments, a substantially flow resistive but
ionically conducting film, such as a layer of flow resistive
micro-porous filter material or cationic conducting membrane (e.g.,
Nafion.TM.--a sulfonated tetrafluoroethylene based
fluoropolymer-copolymer available from E.I. du Pont de Nemours and
Company) is placed just below the flow plate in region of the plate
proximate the open flow slot of the flow diverter. In one
embodiment, the portion is about one-half of the area of the plate.
In another embodiment, the portion is about one-third the area of
the plate, in another embodiment, about one-quarter and in yet
another embodiment, the portion is less than one-quarter of the
area of the plate. This construction allows ionic current to pass
largely uninhibited through the holes there, but prevent flow
immerging upwards in that region, increasing the cross flow across
the wafer center for the same total flow rate, while normalizing
the flow vectors across the wafer plating surface. For example,
when the portion is half of the area of the plate, this results in
doubling the flow velocity in the holes located at the opposite
side of the slot and eliminating flow through holes on the half of
the plate proximate the slot. Skilled artisans would appreciate
that the shape and placement of the membrane can be optimized to
normalize the transverse flow vectors, depending on the particular
plating apparatus' configuration including the flow diverter/flow
shaping plate configuration. In lieu of such a membrane, the
through hole pattern of the flow shaping plate can be adjusted so
that the density of holes is lower proximate the gap in the flow
diverter; analogously the pattern of the holes proximate the gap
will depend on the particular system's configuration and operating
parameters. A more flexible approach is to use a flow shaping plate
with some fixed hole pattern and use the aforementioned membrane
and/or hole blocking to create the desired transverse flow
characteristics across the wafer plating surface. Further
discussion of improving transverse flow characteristics is included
in the discussion of subsequent Figures. For example methods and
apparatus for normalizing the transverse flow vectors across the
wafer plating surface are further described in relation to FIGS.
7A-C.
[0109] In FIGS. 2E-I, which are derived from CAD drawings of actual
plating apparatus components, show additional features of the
apparatus and particularly the diverter assembly. Where possible,
the numbering of some components in FIGS. 2E-I matches that for
previous figures, for example, wafer 145, flow diverter 200 and
flow shaping plate 202. Other features in FIGS. 2E-I are identified
by the following reference numbers. FIG. 2E shows assembly 204 in
perspective attached to a plating cell assembly, and wafer holder
101 as a cross-section. Reference number 206 identifies a "top
plate" for connecting to a "cup" 212 and allowing the cup to move
up and down to hold the wafer in position against a "cone" 210.
Struts 208 connect cup 212 to top plate 206. Mounted to cone 210 is
a housing 205 that holds various connections such as pneumatic and
electrical connections. The cone also includes a cut out 207 to
produce a flexible cantilever structure in the cone, and a sealing
O-ring 230. The cup 212 includes a main cup body or structure 222,
electrical contacts 224 for connecting with wafer 145, a bus plate
226 for delivering electricity to the contacts 224, and a cup
bottom 228, which defines a lower surface of the assembly 101
(FIGS. 2A-D, also note that FIGS. 1A and 1B and associated
description provide context on an exemplary wafer holding and
positioning assembly, 100, and a cross section of assembly
101.)
[0110] Slotted spacer 200 (also see FIGS. 2A-D) contacts flow
shaping plate 202 (also see FIGS. 2A-D). A cutout or slot 201
exists in the slotted spacer and, as explained, provides a low
resistance path for electrolyte to escape during electroplating. In
this example, mounting screws connect slotted spacer 200 to flow
shaping plate 202. Fixing members 220 connect plate 202 to a main
cell body 216. A circular wall 214 defines the outer region of a
cathode chamber which holds catholyte separated from an anode
chamber that holds anolyte.
[0111] A gap, 232 (see also dimension C of FIG. 2B), between the
plating surface of wafer 145 and the upper surface of flow shaping
plate 202. This gap may be about 2-4 millimeters in the interior
regions of the flow diverter. However, at the circumferential
points where the slotted spacer resides there is a gap 234 that is
only about 0.1 to 0.5 millimeters in some embodiments. This smaller
gap 234 is characterized by the distance between the upper surface
of slotted spacer 200 and the lower surface of cup bottom 228. Of
course, this small gap 234 does not exist at the opening 201 in the
spacer 200. At this opening, the gap between cup bottom and plate
202 is the same as gap 232. In certain embodiments, the difference
gap size between gaps 232 and 234 is approximately a multiple of
10.
[0112] As an alternative set of embodiments, liquid flow is used as
a barrier to create the shear flow as described herein. In these
embodiments, the edge gap is not necessarily quite as small as
described above, for example 2 mm, but the effect of creating cross
flow still results. In one example, where the cell is generally as
described as in relation to FIGS. 2A-I, in the region slotted
spacer 200 would typically occupy, there is a mechanism (e.g., one
or more fluid jets) for creating an upward flowing stream of fluid
directed substantially upwards towards the wafer holder, thereby
creating a liquid "wall" in the region where fluid would otherwise
try "leak" through the gap. In another embodiment, the spacer
extends outwards beyond the periphery of the wafer holder and then
laterally upward in the direction of the wafer itself a distances
of from about 1 to 10 cm, thereby creating a "leaky" cup in which
the wafer and its holder fits. Like the flow diverter, the leaky
cup has a section of its wall missing, through which the liquid
that enters the flow plate exits the gap between it and wafer must
exit. While the above embodiments may reduce the need for an
extremely small gap between the wafer and the insert, the total
cross flow across the wafer center is in part determined by the
flow shaping plate to wafer distances, and this parameter typically
remains largely the same as described above.
[0113] FIG. 2H shows a more complete depiction (as a cross section)
of the electroplating cell. As shown, the electroplating cell
includes an upper or cathode chamber 215 defined in part by
circular wall 214. The upper catholyte chamber and lower anode
chamber of the cell are separated by an ionic transfer membrane 240
(e.g., Nafion.TM.) and an inverted conically shaped support
structure 238. The number 248 indicates the flow path lines of the
electrolyte up to and through flow shaping plate 202. The anode
chamber includes a copper anode 242 and a charge plate 243 for
delivering power to the anode. It also includes an inlet manifold
247 and a series of flutes 246 for delivering electrolyte to the
anode surface in a manner that irrigates the top surface of the
anode. Passing through the center of the anode 242 and the anode
chamber is a catholyte flow inlet 244. This structure delivers
catholyte to upper chamber 215 along streamlines 248 as shown by
the radial/vertical arrows in FIG. 2H. FIG. 2I depicts flow
streamlines 248 for electrolyte flowing through holes in shaping
plate 202 and into gap 232, adjacent the plating surface of the
wafer.
[0114] Some of the cell features shown in FIGS. 2E-I are also shown
in FIGS. 1A, 1B and 3B described below. The apparatus will include
one or more controllers for controlling, inter alia, the
positioning of the wafer in the cup and cone, the positioning of
the wafer with respect to the flow shaping plate, the rotation of
the wafer and the delivery of current to the anode and wafer.
[0115] Some general but non-limiting features of flow diverter
embodiment are set forth below in Roman numerals I-XII below.
[0116] I. A structure for creating a small gap region and nearly
closed wafer to flow shaping plate "chamber."
[0117] II. In more specific embodiments, the nearly closed wafer to
flow shaping plate chamber is created by forming a very small gap
(e.g., about 0.1 to 0.5 mm) between the majority of the space
between a wafer holder periphery and a peripheral edge element
(slotted spacer), located either on, or as part of, the flow
shaping plate.
[0118] III. The apparatus rotates the wafer at a relatively high
angular velocities (e.g., at least about 30 rpm) above the flow
shaping plate, thereby creating a high degrees of fluidic-shearing.
This fluidic shearing is caused by the large velocity difference
between the moving wafer and the (stationary) upper surface of the
shaping plate which is in close proximity to the wafer.
[0119] IV. A region of the cell that acts as a fluidic outlet
"vent." This vent is an opening, or in some cases, an outlet gap
(e.g., the gap in the slotted spacer described above). It creates
an opening in the "chamber" between flow shaping plate and the
rotating wafer. The vent directs fluid that moves upward through
the flow shaping plate to change directions by 90 degrees and move
at a high velocity parallel to the wafer surface, at an angle
toward the vent location. This outlet vent or gap encompasses an
angular portion of the outer circumference of the "chamber" (outer
edge of the wafer/cup and/or the flow shaping plate) to introduce
azimuthal asymmetry in the chamber. In some cases, the angle
subtended by the vent or gap is about 20 to 120 degrees or about 40
to 90 degrees. It is through this gap that the vast majority of the
fluid that enters the cell chamber and subsequently passes through
the holes in the shaping plate, eventually exits the cell (and is
recaptured for recirculation to the bath).
[0120] V. The (fluid) flow shaping plate typically has a low
porosity and pore size that introduces a substantial viscous
backpressure at operating flow rates. As an example, a solid plate
having a large number of very small holes provided therein, e.g.,
6465.times.0.026 inches in diameter, has been shown useful. The
porosity of the plate is typically less than about 5 percent.
[0121] VII. In certain embodiments employing a flow shaping plate
of about 300 mm diameter (and having a large number of holes), a
volumetric flow of about 5 liters/minute or greater is employed. In
some cases, the volumetric flow is at least about 10 liter/minute,
and sometime as great as 40 liters/minute.
[0122] VIII. In various embodiments, the magnitude of the pressure
drop across the flow shaping plate is approximately equal to or
larger than, the pressure drop between the outlet gap and a
position within the "chamber" opposite the outlet gap and below the
wafer, and therefore acts as a flow manifold.
[0123] IX. The flow shaping plate delivers a substantially uniform
flow directly at and largely upwards toward the wafer. This avoids
the situation where the majority of the flow might otherwise enter
the chamber from the flow shaping plate but be preferentially
routed (short circuited) by a path that is primarily outward near
and through the outlet gap.
[0124] X. Unlike the case with a large gap (greater than a
millimeter) between the edge of wafer and the shaping plate and
without a flow diverter, as flow accumulates in the region below
the wafer, the path of least resistance is altered from that of a
radially outwards trajectory to one that must now primarily pass
parallel to the wafer and in the direction of the outlet gap.
Therefore, fluid is directed to traverse in a lateral direction
parallel to the wafer surface, and of particular note, traverses
and passes across the center of wafer (or axis of wafer rotation).
It is no longer directed radially outwards in all directions from
the center.
[0125] XI. The velocity of the transverse flow at the center and
other locations depends on a number of design and operating
parameters, including the size of the various gaps (flow shaping
plate to wafer, outlet gap, slotted spacer to wafer holder
peripheral bottom), the total flow, wafer rotation rate. However,
in various embodiments, the flow across the wafer center is at
least about 3 cm/sec, or at least about 5 cm/sec.
[0126] XII. A mechanism to tilt the wafer and holder to allow for
"angled entry" may be used. The tilt may be toward the gap or vent
in the upper chamber.
[0127] Other embodiments include flow diverters that include a
vertical surface that further inhibits flow out of the pseudo
chamber except for at the vent or gap. The vertical surface can be
described as a FIG. 3A depicts a flow diverter/flow shaping plate
assembly, 304, that includes flow shaping plate 202 (as described
previously) and a flow diverter 300. Flow diverter 300 is much like
flow diverter 200 as described in relation to FIG. 2A, as it has a
generally annular shape with a segment removed; however, flow
diverter 300 is shaped and configured to have a vertical element.
The bottom portion of FIG. 3A shows a cross-section of flow
diverter 300. Rather than a flat top surface that is below the
lowest surface of the wafer holder, as in flow diverter 200, the
top surface of flow diverter 300 is shaped to have, starting from
the inner circumference and moving radially outward, an upwardly
sloping surface that eventually becomes a vertical surface,
terminating at a top (in this example flat) surface that is above
the lowest surface of the wafer holder. Thus, in this example, the
wall structure has an outer portion that is higher than an inner
portion. In certain embodiments, the outer portion is between about
5 mm and about 20 mm in height and the inner portion is between
about 1 mm and about 5 mm in height.
[0128] In the example of FIG. 3A, the flow diverter has a vertical
interior surface, 301. The surface need not be perfectly vertical,
as for example, a sloped surface will suffice. The important
feature in this embodiment is that the narrow gap between the top
surface of the flow diverter and the bottom surface of the wafer
holder, distance C in FIG. 2B, is extended to include some sloped
and/or vertical component of the wafer holder surface. In theory,
this "narrow gap extension" need not include any sloped or vertical
surface, rather it could include expanding the area where the upper
surface of the flow diverter and the lower surface of the wafer
holder are registered in order to create the narrow gap and/or
narrowing further the narrow gap to inhibit fluid escape from the
pseudo chamber. However, with the import of reducing an apparatus'
overall footprint, it is oftentimes more desirable to simply extend
the narrow gap to sloped and/or vertical surfaces to obtain the
same result of less fluid loss through the narrow gap.
[0129] Referring to FIG. 3B, which depicts a partial cross-section
of assembly 304 registered with wafer holder 101, a vertical
surface, 301, in this example along with a vertical portion of the
wafer holder 101, extends the aforementioned narrow gap (for
example refer to "C" in FIG. 2B) between the flow diverter top
surface and the wafer holder. Typically, but not necessarily, as
depicted in FIG. 3B, the distance, as indicated by 302, between
these vertical and/or sloped surfaces is smaller than the distance
C between the horizontal surface of the flow diverter and the wafer
holder. In this rendition, the portion, 202a, of flow shaping plate
202 having no through holes and the portion, 202b, having the
through holes, are depicted. In one embodiment, the flow diverter
is configured such that an inner surface of the wall structure is
between about 0.1 and about 2 mm from an outer surface of the
substrate holder during electroplating. In this example, gap 302
represents this distance. This further narrowing of the gap creates
more fluid pressure in the pseudo chamber and increases shear flow
across the wafer plating surface and out the vent (where segmented
portion of flow diverter 300 opposes wafer holder 101. FIG. 3C is a
graph showing uniformity of plated copper on a 300 mm wafer as a
function of varying the described vertical gap. As indicated, at
various gap distances, highly uniform plating can be achieved.
[0130] FIG. 3D depicts a number of variations of the cross-section
of flow diverters, 305-330, having a vertical element. As depicted,
the vertical surface need not be precisely normal to the plating
surface and there need not be a sloping portion of the top surface
of the flow diverter (see e.g. cross-section 315). As depicted in
cross-section 320, the inner surface of the flow diverter may be
entirely a curved surface. Cross-section 310 shows that there may
only be a slanted surface that extends the gap. One of ordinary
skill in the art would appreciate that the shape of the flow
diverter may depend on the wafer holder to which it registers in
order to create the gap extension. In one embodiment, the surface
that deviates from the horizontal (as compared to, for example, the
top surface of the flow shaping plate) has at least one portion
that deviates from between about 30 degrees to about 90 degrees
(normal to) from horizontal.
[0131] The flow diverters as described in relation to FIGS. 3A-D
aid in creating more uniform transverse flow between the wafer
plating surface and the flow shaping plate. FIG. 3E shows top view
Surf Image Haze Maps of the transverse flow patterns created when a
flow diverter as described in relation to FIGS. 2A-I (left portion
of FIG. 3E) is used as compared to when a flow diverter as
described in relation to FIGS. 3A-D is used (right portion of FIG.
3E). These haze maps are a result of flowing plating solution
onto/across the wafer having a seed layer without applying a
plating current. The sulfuric acid in the plating solution etches
the seeded wafer surface and thus creates a pattern that reflects
the flow pattern, when analyzed with a laser-based particle/defect
detector. In each test, a flow shaping plate such as 202 was used,
where the hole patterns were a regular and uniform square pattern
of holes across the entire area of the plate inside the flow
diverter inner circumference (and where the segment removed from
the diverter would reside were it not removed). The drawing in the
upper middle of FIG. 3E indicates the orientation of the flow
diverter and flow direction from the upper left to the lower right
and out the gap. The darker portions of the haze maps indicate
vertical impinging flow, while the lighter areas indicate
transverse flow. As seen in the left hand map, there are many
branches of the dark areas indicating confluence of vertical flows
across the wafer. That is, presumably due to the regular
distribution of the through holes on the flow shaping plate
surface, there are long-range pathways for fluid where the
transverse component of flow is less than the impinging component
of flow. These long-range pathways can negatively affect plating
uniformity across the wafer plating surface, and it is desirable to
minimize the long-range pathways. As indicated by the haze map on
the right side of FIG. 3E, when using a flow diverter as described
in relation to FIGS. 3A-D (having a gap extending element), for
example a vertical interior surface, there is an increased amount
and more uniform transverse flow across the wafer.
Non-Uniform Hole Distribution on Flow Shaping Plate
[0132] In certain embodiments, flow shaping plates have non-uniform
distribution of through holes in order to, alone or in combination
with flow diverters, create increased and/or more highly uniform
transverse flow across the wafer surface during plating.
[0133] In some embodiments the non-uniform hole distribution is a
spiral pattern. FIG. 4A shows a top view of one such flow shaping
plate, 400. Note that the center of the spiral pattern of through
holes is offset from the center of the circular area of the holes
at a distance D. FIG. 4B shows a similar flow shaping plate, 405,
where the offset is greater, a distance E. FIG. 4C depicts another
similar flow shaping plate, 410, (top and perspective views,
respectively) where the center of the spiral pattern of holes is
not included in the circular area occupied by the holes, rather the
offset is such that what would be the center of the spiral pattern
of holes is not included in the circular area that includes the
through holes. Using such offset spiral patterns provides for
improved transverse flow across the wafer surface during plating.
Such flow shaping plates are described in more detail in U.S.
provisional patent application, Ser. No. 61/405,608, as
incorporated by reference above.
[0134] FIG. 5A depicts a haze map showing flow patterns resulting
from using a flow diverter as described in relation to FIG. 3A is
used in conjunction with a flow shaping plate as described in
relation to FIG. 4C (without wafer rotation). The haze map
indicates, due to the non-uniform through hole patterns, in this
example a spiral pattern, there is nearly complete transverse flow,
with minimal if any long-range pathways for fluid flow where
impinging components of flow dominate. FIG. 5B shows plating
uniformity results when using the flow diverter/flow shaping plate
combination as described in relation to FIG. 5A at a specified gap
(3 mm) between the diverter and wafer holder. The plating
uniformity on a 300 mm wafer is quite high.
[0135] Non-uniform through hole patterns can include other than
spiral forms. And in certain embodiments, flow diverters are not
used in combination with flow shaping plates with hole
non-uniformity. For example, FIG. 6 depicts an assembly, 600,
illustrating one configuration to address the center slow plating
issue. Plating apparatus 600 has a plating bath, 155, which has an
anode, 160, and an electrolyte inlet, 165. In this example, a flow
shaping plate, 605, produces non-uniform impinging flow across the
wafer. Specifically as shown, there is greater flow at the center
of the wafer than in the outer regions due to a non-uniform
distribution of holes in the flow shaping plate, e.g., a variance
in the radial distribution of holes size and density. As indicated
by the heavy dotted arrows, in this example, greater flow is
created near the center of the wafer to compensate for the
insufficient mass transfer and resultant lower plating rates seen
at the center of the wafer (for example refer to FIG. 1D).
[0136] While not wishing to be bound by theory, it is believed that
insufficient fluid shearing and hence non-uniform mass transfer
across the surface of the wafer in conventional plating regimes as
described above. By increasing the flow rate at the center of the
wafer relative to the other areas of the wafer (as depicted by the
higher density of dashed arrows near the center of the cathode
chamber versus the outer regions), lower plating rates nearer the
center of the wafer can be avoided. This result can be achieved by,
for example, increasing the number of holes in, and/or the angle of
orientation with respect to the wafer, for example, a flow shaping
plate in order to increase the number of impinging flow jets and
the amount of resultant shearing in the center region.
[0137] In general, the hole density, size, and/or distribution
(e.g., uniform or random) is changed near the center of the flow
shaping plate. In some embodiments, the hole density increases near
the center. Alternatively or in addition, the holes assume a
somewhat random distribution in their pattern near the center,
which the hole distribution may be provided in a regular or
periodic arrangement elsewhere on the flow shaping. In some
embodiments, partial coverings may be provided to cover some holes
in certain regions of a flow shaping plate. In certain embodiments,
these coverings include an ionically conductive flow inhibitive
member. This will allow the end user to customize the hole density
and/or distribution to meet particular electroplating needs.
Flow Port Transverse Flow Enhancement
[0138] In some embodiments, electrolyte flow ports are configured
to aid transverse flow, alone or in combination with a flow shaping
plate and a flow diverter as described herein. Various embodiments
are described below in relation to a combination with a flow
shaping plate and a flow diverter, but the invention is not so
limited. Note, as described in relation to FIG. 2C, in certain
embodiments it is believed that the magnitude of the electrolyte
flow vectors across the wafer surface are larger proximate the vent
or gap and progressively smaller across the wafer surface, being
smallest at the interior of the pseudo chamber furthest from the
vent or gap. As depicted in FIG. 7A, by using appropriately
configured electrolyte flow ports, the magnitude of these
transverse flow vectors is more uniform across the wafer
surface.
[0139] FIG. 7B depicts a simplified cross-section of a plating
cell, 700, having a wafer holder, 101, which is partially immersed
in an electrolyte, 175, in plating bath 155. Plating cell 700
includes a flow shaping plate, 705, such as those described herein.
An anode, 160, resides below plate 705. On top of plate 705 is a
flow diverter, 315, such as described in relation to FIGS. 3A and
3D. In this figure, the vent or gap in the flow diverter is on the
right side of the diagram and thus imparts transverse flow from
left to right as indicated by the largest dotted arrow. A series of
smaller vertical arrows indicate flow through the vertically
oriented through holes in plate 705. Also below plate 705 are a
series of electrolyte inlet flow ports, 710, that introduce
electrolyte into the chamber below plate 705. In this figure, there
is no membrane separating an anolyte and catholyte chamber, but
this can also be included in such plating cells without departing
from the scope of this description.
[0140] In this example, flow ports 710 are distributed radially
about the interior wall of cell 155. In certain embodiments, in
order to enhance the transverse flow across the wafer plating
surface, one or more of these flow ports is blocked, for example,
flow ports on the right hand side (as drawn), proximate the vent or
gap in the pseudo chamber formed between the wafer, plate 705 and
flow diverter 315. In this way, although impinging flow is
permitted through all the through holes in plate 705, the pressure
at the left side, distal of the gap or vent in the pseudo chamber,
is higher and thus the transverse flow across the wafer surface (in
this example shown as left to right flow) is enhanced. In certain
embodiments, the blocked flow ports are positioned about an azimuth
that is at least equal to the azimuth of the segmented portion of
the flow diverter. In a specific embodiment, the electrolyte flow
ports on a 90.degree. azimuthal section of the circumference of the
electrolyte chamber below the flow shaping plate are blocked. In
one embodiment, this 90.degree. azimuthal section is registered
with the open segment of the flow diverter annulus.
[0141] In other embodiments, the electrolyte inlet flow port or
ports are configured to favor higher pressure in the area below the
portion of the flow diverter distal of the vent or gap (indicated
by Y in FIG. 7B). In some instances, simply physically blocking
(e.g., via one or more shut off valves) selected inlet ports is
more convenient and flexible than designing a cell with
particularly configured electrolyte inlet ports. This is true
because the configuration of the flow shaping plate and the
associated flow diverter can change with different desired plating
results and thus it is more flexible to be able to vary the
electrolyte inlet configuration on a single plating cell.
[0142] In other embodiments, with or without blocking one or more
electrolyte inlet ports, a dam, baffle or other physical structure
is configured to favor higher pressure in the area below the
portion of the flow diverter distal of the vent or gap. For
example, referring to FIG. 7C, a baffle, 720, is configured to
favor higher pressure in the area below the portion of the flow
diverter distal of the vent or gap (indicated by Y in FIG. 7C).
FIG. 7D is a top view of plating cell 155, without wafer holder
101, flow diverter 315 or flow shaping plate 705, showing that
baffle 720 promotes electrolyte flow emanating from ports 720 to
confluence at area Y and thus increase pressure in that area
(supra). One of ordinary skill in the art would appreciate that a
physical structure may be oriented in a number of different ways,
e.g. having horizontal, vertical, sloped or other elements in order
to channel flow of the electrolyte in order to create a higher
pressure region as described and thus promote transverse flow
across the wafer surface in the pseudo chamber where the shear flow
vectors are substantially uniform.
[0143] Some embodiments do include electrolyte inlet flow ports
configured for transverse flow enhancement in conjunction with flow
shaping plate and flow diverter assemblies. FIG. 7E depicts a
cross-section of components of a plating apparatus, 725, for
plating copper onto a wafer, 145, which is held, positioned and
rotated by wafer holder 101. Apparatus 725 includes a plating cell,
155, which is dual chamber cell, having an anode chamber with a
copper anode, 160, and anolyte. The anode chamber and cathode
chamber are separated by a cationic membrane 740 which is supported
by a support member 735. Plating apparatus 725 includes a flow
shaping plate, 410, as described herein. A flow diverter, 325, is
on top of flow shaping plate 410, and aides in creating transverse
shear flow as described herein. Catholyte is introduced into the
cathode chamber (above membrane 740) via flow ports 710. From flow
ports 710, catholyte passes through flow plate 410 as described
herein and produces impinging flow onto the plating surface of
wafer 145. In addition to catholyte flow ports 710, an additional
flow port, 710a, introduces catholyte at its exit at a position
distal to the vent or gap of flow diverter 325. In this example,
flow port 710a's exit is formed as a channel in flow shaping plate
410. The functional result is that catholyte flow is introduced
directly into the pseudo chamber formed between the flow plate and
the wafer plating surface in order to enhance transverse flow
across the wafer surface and thereby normalize the flow vectors
across the wafer (and flow plate 410).
[0144] FIG. 7F depicts a flow diagram similar to that in FIG. 2C,
however, in this figure, the flow port 710a (from FIG. 7E) is
depicted. As seen in FIG. 7F, flow port 710a's exit spans 90
degrees of the inner circumference of flow diverter 325. One of
ordinary skill in the art would appreciate that the dimensions,
configuration and location of port 710a may vary without escaping
the scope of the invention. One of skill in the art would also
appreciate that equivalent configurations would include having the
catholyte exit from a port or channel in flow diverter 325 and/or
in combination with a channel such as depicted in FIG. 7E (in flow
plate 410). Other embodiments include one or more ports in the
(lower) side wall of a flow diverter, i.e. that side wall nearest
the flow shaping plate top surface, where the one or more ports are
located in a portion of the flow diverter opposite the vent or gap.
FIG. 7G depicts a flow diverter, 750, assembled with a flow shaping
plate 410, where flow diverter 750 has catholyte flow ports, 710b,
that supply electrolyte from the flow diverter opposite the gap of
the flow diverter. Flow ports such as 710a and 710b may supply
electrolyte at any angle relative to the wafer plating surface or
the flow shaping plate top surface. The one or more flow ports can
deliver impinging flow to the wafer surface and/or transverse
(shear) flow.
[0145] In one embodiment, for example as described in relation to
FIGS. 7E-G, a flow shaping plate as described herein is used in
conjunction with a flow diverter such as described in relation to
FIGS. 3A-3D, where a flow port configured for enhanced transverse
flow (as described herein) is also used with the flow plate/flow
diverter assembly. In one embodiment the flow shaping plate has
non-uniform hole distribution, in one embodiment, a spiral hole
pattern.
Angled Holes in Flow Shaping Plate
[0146] Another way to increase transverse flow and thereby achieve
more uniform plating in high-rate plating regimes is to employ an
angled-hole orientation in the flow shaping plate. That is, a flow
shaping plate having through-holes that are non-communicating (as
described above) and with the hole dimension angled relative to top
and bottom parallel surfaces through which the hole extends. This
is illustrated in FIG. 8A, which depicts an assembly, 800. The
through holes in the flow shaping plate, 805, are angled and thus
the electrolyte flow impinging on the surface of wafer 145 strike
at a non-normal angle and thus impart shearing at the center of the
rotating wafer. Further details of flow shaping plates having such
angled orientation are provided in provisional U.S. Patent
Application No. 61/361,333 filed on Jul. 2, 2010, which is
incorporated herein by reference.
[0147] FIG. 8B is a graph showing plating thickness variation with
respect to radial position on a 300 mm wafer plated with copper
when using a flow shaping plate with six thousand or nine thousand
angled through holes, optimizing flow rates and each with 90 rpm
wafer rotation. As seen from the data, at 24 lpm using a flow plate
with six thousand holes, the plating is not as uniform as, for
example, when the plate has nine thousand holes at a flow rate
through the plate of 6 lpm. Thus, the number of holes, flow rate,
etc. can be optimized when using flow shaping plates with angled
through holes to obtain sufficient shearing flow to obtain uniform
plating across the wafer surface. FIG. 8C is a graph showing
deposition rate vs. radial position on a 200 mm wafer when plated
with copper using a flow shaping plate with angled through holes.
At 6 lpm, the uniformity is greater than at 12 lpm. This
demonstrates that by using flow shaping plates with angled through
holes, mass transfer across the wafer can be adjusted to compensate
for low plating rates at the wafer center. Angled through hole flow
shaping plates result in remarkably uniform plating conditions over
a wide range of boundary layer conditions.
Paddle Shearing Cell Embodiments
[0148] FIG. 9A depicts another embodiment, where rotating paddles,
900, are used to increase convection and create shearing in the
electrolyte flow at the wafer surface just below a rotating wafer,
thus provide improved mass transfer under high-rate plating
conditions. In this embodiment, paddle wheels 900 are provided as
spindles with interweaving paddles (see FIG. 9B). In this
embodiment, the paddle wheels 900 are mounted on a base 905, which
integrates into a plating chamber where the paddle wheels are in
close proximity to the plating surface of wafer 145 during plating.
This creates an increased convection, and in some cases both
substantial shearing and turbulence, at the wafer surface and thus
sufficient mass transfer in high-rate plating regimes. Base 905 has
a number of holes, 910, for allowing electrolyte to flow through.
On the lower right of base 905 is a drive mechanism for driving the
spindles having paddle wheels 900. The paddle assembly includes
counter rotating impellers mounted as an assembly on a base. The
base with the paddle assembly is a modular unit that fits between,
for example, the wafer and a cationic membrane used to delineate a
cathode chamber from an anode chamber. Thus the paddle assembly is
positioned in close proximity to the wafer plating surface, in the
catholyte, to create shearing flow in the electrolyte at the wafer
surface.
Orbital or Translational Motion of Substrate with Respect to Flow
Shaping Plate
[0149] FIG. 10 depicts an embodiment where orbital motion is
employed to affect improved shear flow at the center axis of the
wafer surface. In this example, a plating chamber is employed where
the plating chamber has sufficient diameter to accommodate wafer
holder 101 when assembly 101 is orbiting in the electrolyte. That
is, assembly 101, which holds the wafer during plating, not only
rotates clockwise and counter clockwise along the Z axis (as
depicted) but also has a translational motion along the X axis
and/or the Y axis. In this way the center of the wafer does not
experience a region of lesser shearing over the flow plate or
turbulence relative to the rest of the wafer surface. In one
embodiment, the electroplating apparatus' mechanism for applying
the shearing force includes a mechanism for moving flow shaping
element and/or the substrate in a direction that moves a rotational
axis of the substrate plating face to a new position with respect
to the flow shaping element.
[0150] As those of skill in the art will appreciate, orbital
motions can be implemented in numerous ways. Chemical mechanical
polishing apparatus provide a good analogy and many orbital systems
employed for CMP can be employed with good effect in the present
invention.
Off-Axis Rotating Element as Part of Flow Shaping Plate
[0151] In one embodiment, the electroplating apparatus' mechanism
for applying the shearing force includes mechanism for rotating the
substrate and/or the flow shaping element is configured to reverse
a direction of rotation of the substrate with respect to the flow
shaping element. In certain embodiments however, the electroplating
apparatus' mechanism for applying the shearing force includes a
mechanism for rotating an off-axis shearing plate located between
the flow shaping element and the plating face of the substrate to
produce a flow of electrolyte across a rotational axis of the
substrate plating face. FIG. 11A depicts an embodiment, where an
assembly, 1100, includes, for example, a flow shaping plate, 1105,
with a rotatable disk, 1110, embedded in or attached to it. Disk
1110 can freely rotate upon a central axis, and is driven, in this
example, by the angularly-rotating and moving fluid created in the
gap between the flow plate and a wafer (not shown) that is rotating
a few millimeters above flow plate 1105 and rotatable disk 1110. In
some embodiments the rotatable disk moves (rotates) simply by
coupling to the shearing of the fluid in the gap and over a
rotatable disk flat surface. In other embodiments there are a set
of electrolyte flow coupling fins, which in this example are
situated in depressions 1115 in disk 1110 (but can also be above
the plate of the flow plate) and aid in inducing the rotational
motion. Hence, in this embodiment, an external mechanism of
powering the rotation of the disk, other than from the rotation of
the wafer above the plate and disk itself, is not required. This
embodiment can be combined with that of the flow diverter, to
create a larger flow shearing condition both at the wafer center
and other locations, as well as minimize any upstream-downstream
flow induced plating non-uniformities caused by, for example, wafer
rotation alone.
[0152] In the depicted embodiment, disk 1110 is configured so that
at least a portion of its surface area lies below the center region
of wafer 145. Since disk 1110 rotates during plating, lateral flow
is created in the region near the center of the wafer and thus
improved mass transfer is achieved for uniform plating in high-rate
plating regimes. While shearing at the wafer surface (other than at
the wafer center) is typically created, in the absence of the
rotatable disk 1110, by the motion of the rotating wafer above the
flow plate 1105, in embodiments employing a disk, shearing of fluid
is created at the wafer center by the relative motion of a
rotatable disk or similar element with respect to a substantially
locally-non-moving wafer. In this example with rotatable disk 1110,
the through holes in both the flow plate and rotatable disk are
normal (or substantially normal) to the plating surface of the
wafer and of the same size and density, but this is not limiting.
In the region of the rotating disk, in certain embodiments, the sum
of the individual flow holes in the plate and in the rotating disk
are equal in length to those of the holes in the plate outside of
the region were the rotating disk resides. This construct ensures
that the ionic resistance to current flow in these two region of
the flow plate/rotating disk member are substantially equal. There
is typically a small vertical separation or gap between the bottom
surface of the rotatable disk and the flow plate to accommodate the
presence of a small barring and/or to ensure that the rotating disk
moves freely and does not rub on the flow plate surfaces.
Furthermore, in some embodiments, the top surface of these two
elements nearest the wafer are arranged to be substantially at the
same general height or distance from the wafer. To meet these two
conditions, there may be a section of additional material in the
flow shaping plate that protrudes below the flow plate's lower
surface.
[0153] In another embodiment, angled through holes, such as those
described in relation to FIG. 4, are employed, alone or in
combination with normally oriented through holes.
[0154] In one embodiment, disk 1110 is driven mechanically, for
example, analogous to the paddles described in relation to FIGS.
9A-B. The disk also may be driven by applying a time varying
magnetic or electrical field to magnets contained within or on the
disk, or can be magnetically coupled to via an internal element
contained in the rotating wafer holder and the rotating disk. In
the later case, as a specific example, a set of equally spaced
magnets in the periphery of the wafer holding and rotating
clamshell create a coupling for to a corresponding set of magnets
embedded in the rotating disk 1110. As the magnets in the wafer
holder move/rotate about the center of the wafer and cell, they
drive the disk to move in the same direction and the wafer/holder.
The individual magnets eventually move further away from the
individual magnet in the disk that they are most strongly couple
to, but another magnetic pair in the disk and wafer holder than
approach each other as they both rotate with the wafer holder/disk
rotation. Also, the motion of the rotating disk can be achieved by
coupling its motion to the fluid flow entering the cell, thereby
eliminating the need for a separate motor or electrical components
or extra moving part in a corrosive electrolyte. FIG. 11B is a
cross-section of assembly 1100.
[0155] Other similar apparatus and driving mechanism that create
central shearing have been envisioned and are considered within the
scope of this invention, as they are readily adopted minor
modification of the principles presented herein. As one further
example, rather than a rotating disk, one can employ a rotating
impeller or moving propeller, again either driven by the induced
flow of a moving wafer, by the flow of fluid through the flow plate
holes, or by other coupling external means, but also arranged to
rotating in a reciprocating off-center of the axis of rotation of
the wafer and cell, can be employed.
E. Plating Methods for Addressing Center Plating Non-Uniformity
[0156] FIG. 12 depicts a process flow, 1200, according to a method
of electroplating described herein. A wafer is positioned in a
wafer holder, see 1205. The wafer and holder are optionally tilted
for angled immersion in the plating cell electrolyte, see 1210. The
wafer is then immersed in the electrolyte, see 1215. Then
electroplating is commenced under shearing hydrodynamic conditions
and with microjets of electrolyte impinging on the wafer plating
surface, see 1220. Then the method is complete.
[0157] As described above, in one embodiment, a flow diverter has
described herein is used and the wafer and holder are tilted so
that the leading edge of the wafer and holder (the low side of the
tilted assembly) is registered with the gap in the flow diverter
(e.g. having a slotted annular structure, the slot is forms a
portion of the vent or gap). In this way, the wafer holder wafer
can be brought as close as possible to the final desired gap
distance during immersion and thus save having to immerse at a
greater distance from the flow diverter and then position closer,
at the desired gap distances described herein.
[0158] FIG. 13 shows a result of plating using methods and
apparatus described herein, where transverse shear flow is used for
efficient mass transfer during plating. The two curves show results
with and without shear flow as described herein. Without shear flow
at the center of the wafer, the singularity or aberration and lack
of sufficient shear flow produces a profile as described in
relation to FIG. 1. But with shear flow as described herein, in
this example using a slotted spacer type flow diverter as described
for example in relation to FIG. 2A, plating deposition rates are
substantially uniform across the plating surface of the wafer
[0159] One embodiment is a method of electroplating on a substrate
including features having a width and/or depth of at least about 2
micrometers, the method including: (a) providing the substrate to a
plating chamber configured to contain an electrolyte and an anode
while electroplating metal onto the substrate, where the plating
chamber includes: (i) a substrate holder holding the substrate such
that a plating face of the substrate is separated from the anode
during electroplating, and (ii) a flow shaping element shaped and
configured to be positioned between the substrate and the anode
during electroplating, the flow shaping element having a flat
surface that is substantially parallel to and separated from the
plating face of the substrate by a gap of about 10 millimeters or
less during electroplating, where the flow shaping element has a
plurality of holes; (b) electroplating a metal onto the substrate
plating surface while rotating the substrate and/or the flow
shaping element and while flowing the electrolyte in the
electroplating cell in the direction of the substrate plating face
and under conditions that produce an average flow velocity of at
least about 10 cm/s exiting the holes of the flow shaping element.
In one embodiment, the electrolyte flows across the plating face of
the substrate at a center point of the substrate at a rate of about
3 cm/second or greater and shearing force is applied to the
electrolyte flowing at the plating face of the substrate. In one
embodiment, the metal is electroplated in the features at a rate of
at least about 5 micrometers/minute. In one embodiment, the
thickness of the metal electroplated on the plating surface of the
substrate has a uniformity of about 10% or better when plated to a
thickness of at least 1 micrometer. In one embodiment, applying the
shearing force includes moving the flow shaping element and/or the
substrate in a direction that causes a rotational axis of the
substrate plating face to move to a new position with respect to
the flow shaping element. In one embodiment, applying the shearing
force includes rotating an off-axis shearing plate located between
the flow shaping element and the plating face of the substrate to
produce a flow of electrolyte across a rotational axis of the
substrate plating face. In another embodiment, applying the
shearing force includes causing the electrolyte to flow laterally
across the face of the substrate toward a gap in a ring structure
provided around the periphery of the flow shaping element. In one
embodiment, the direction of rotation of the substrate with respect
to the flow shaping element is alternated during plating.
[0160] In one embodiment, the holes in the flow shaping element do
not form communicating channels within the body, and where
substantially all holes have a principal dimension or a diameter of
the opening on the surface of the element facing the surface of the
substrate of no greater than about 5 millimeters. In one
embodiment, the flow shaping element is a disk having between about
6,000-12,000 holes. In one embodiment, the flow shaping element has
a non-uniform density of holes, with a greater density of holes
being present in a region of the flow shaping element that faces a
rotational axis of the substrate plating face.
[0161] Methods described herein can be used for electroplating
Damascene features, TSV features and wafer level packaging (WLP)
features, such as a redistribution layer, a bump for connecting to
an external wire and an under-bump metallization feature. Further
discussion of WLP plating as it relates to embodiments described
herein is included below.
F. WLP Plating
[0162] Embodiments described herein can be used for WLP
applications. Given the relatively large amount of material to be
deposited in WLP regimes, plating speed differentiates WLP and TSV
applications from damascene applications, and thus efficient mass
transfer of plating ions to the plating surface is important. Still
further, electrochemical deposition of WLP features may involve
plating various combinations of metals such as combinations or
alloys of lead, tin, silver, nickel, gold, and copper as described
above. Related apparatus and methods for WLP applications are
described in U.S. Provisional application, Ser. No. 61/418,781,
filed Dec. 1, 2010, which is incorporated by reference herein in
its entirety.
[0163] Electrochemical deposition procedures may be employed at
various points in the integrated circuit fabrication and packaging
process. At the IC chip level, damascene features are created by
electrodepositing copper within vias and trenches to form multiple
interconnected metallization layers. As indicated,
electrodeposition processes for this purpose are widely deployed in
current integrated fabrication processes.
[0164] Above the multiple interconnection metallization layers, the
"packaging" of the chip begins. Various WLP schemes and structures
may be employed and a few of them are described here. In some
designs, the first is a redistribution layers (also referred to
"RDL"), which redistributes upper level contacts from bond pads to
various under bump metallization or solder bump or ball locations.
In some cases, the RDL lines help to match a convention die
contacts to pin out arrays of standard packages. Such arrays may be
associated with one or more defined standard formats. RDLs may also
be used to balance the signal delivery times across the different
lines in the package, which lines may have different
resistance/capacitance/inductance (RCL) delays. Note that the RDL
may be provided directly on top of damascene metallization layers
or on a passivation layer formed over the top metallization layer.
Various embodiments of the present invention may be employed to
electroplate RDL features.
[0165] Above the RDL, the package may employ the "under bump
metallization" (or UBM) structures or features. The UBM is the
metal layer feature that forms base for a solder bump. The UBM may
include one or more of the following: an adhesion layer, a
diffusion barrier layer, and an oxidation barrier layer. Aluminum
is frequently used as an adhesion layer because it provides a good
glass-metal bond. In some cases, an interlayer diffusion barrier is
provided between the RDL and UBM to block, e.g., copper diffusion.
One interlayer material, which may be electroplated in accordance
with the principles disclosed herein, is nickel for example.
[0166] Bumps are used for soldering or otherwise attaching external
wires to the package. Bumps are used in flip chip designs to
produce smaller chip assemblies than employed in wire bonding
technology. A bump may require an underlying interlayer material to
prevent diffusion of, e.g., tin from the bump from reaching copper
in an underlying pad. The interlayer material may be plated in
accordance with the principles of this invention.
[0167] Additionally and more recently, copper pillars may be
electroplated in accordance with the methods and apparatus herein
to create flip chip structures and/or to make contact between the
UBM and/or the bump of another chip or device. In some cases,
copper pillars are used to reduce the amount of solder material
(such as reduce the amount of total lead solder in the chip), and
to enable much tighter pitch control that can be achieved when
using solder bumps.
[0168] Additionally, the various metals of the bumps themselves may
be electroplated, with or without first forming copper pillars.
Bumps may be formed from high melting point lead-tin compositions
including lower melting lead-tin eutectics, and from lead-free
compositions such as tin-silver alloys. Components of the under
bump metallization may include films of gold or nickel-gold alloys,
nickel, and palladium.
[0169] Thus, it should be apparent that WLP features or layers that
may be plated using the inventions described herein are a
heterogeneous group, both in terms of geometry and materials. Some
examples of materials that may be electroplated in accordance with
the methods and apparatus described herein to form WLP features are
listed below.
[0170] 1. Copper: As explained copper may be employed to form
pillars, which may be used under the solder joint. Copper is also
used as the RDL material.
[0171] 2. Tin solder materials: Lead-tin--Various composition of
this combination of elements currently includes about 90% of the
market soldering in IC applications. The eutectic material
typically includes about 60% atomic lead and about 40% atomic tin.
It is relatively easy to plate because the potential of deposition,
E.sub.0s, of the two elements is nearly identical (differ by about
10 mV). Tin-silver--Typically this material is contains less than
about 3% atomic silver. A challenge is to plate tin and silver
together and maintain the proper concentration. Tin and silver have
very different E.sub.0s (differing by almost 1 V) with silver being
more noble and plating in preference to tin. Hence even in
solutions having very low concentration of silver, the silver
preferentially plates and can be quickly depleted from the
solution. This challenge suggests that it would be desirable to
plate 100% tin. However, elemental tin has a hexagonal close packed
lattice, which results in formation of crystal grains having
different CTEs in different crystal directions. This can gives rise
to mechanical failures during normal use. Tin also is known to form
"tin whiskers", a phenomenon known to be able to create shorts
between adjacent features.
[0172] 3. Nickel: As mentioned, this element is used in UBM
applications, primarily as a copper diffusion barrier.
[0173] 4. Gold
[0174] In one embodiment, the aforementioned electroplated feature
is a wafer level packaging feature. In one embodiment, the wafer
level packaging feature is a redistribution layer, a bump for
connecting to an external wire, or an under-bump metallization
feature. In one embodiment, the electroplated metal is selected
from the group consisting of copper, tin, a tin-lead composition, a
tin-silver composition, nickel, a tin-copper composition, a
tin-silver-copper composition, gold, and alloys thereof.
[0175] Although the foregoing invention has been described in some
detail for purposes of clarity of understanding, it will be
apparent that certain changes and modifications may be practiced
within the scope of the appended claims. Therefore, the present
embodiments 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 and equivalents
of the claims.
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