U.S. patent application number 12/141876 was filed with the patent office on 2009-01-08 for method of removal profile modulation in cmp pads.
Invention is credited to Rajeev Bajaj, Alan Nolet.
Application Number | 20090011679 12/141876 |
Document ID | / |
Family ID | 40221825 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090011679 |
Kind Code |
A1 |
Bajaj; Rajeev ; et
al. |
January 8, 2009 |
METHOD OF REMOVAL PROFILE MODULATION IN CMP PADS
Abstract
A polishing pad includes a plurality of polishing surfaces, a
first group of the polishing surfaces made of a first material
having a first coefficient of friction and a second group of the
polishing surfaces made of a second material having a second
coefficient of friction. The first and second groups of polishing
surfaces may be arranged over the polishing pad so as to provide a
non-planar material removal profile. The polishing surface layout
may be designed by evaluating a material removal profile for an
existing polishing pad of known characteristics, observing how
variations in polishing surface densities and/or coefficients of
friction affect that material removal profile, and then mapping the
polishing surface coefficients of friction and density profiles to
the subject polishing pad layout.
Inventors: |
Bajaj; Rajeev; (Fremont,
CA) ; Nolet; Alan; (Atherton, CA) |
Correspondence
Address: |
SONNENSCHEIN NATH & ROSENTHAL LLP
P.O. BOX 061080, WACKER DRIVE STATION, SEARS TOWER
CHICAGO
IL
60606-1080
US
|
Family ID: |
40221825 |
Appl. No.: |
12/141876 |
Filed: |
June 18, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11697622 |
Apr 6, 2007 |
|
|
|
12141876 |
|
|
|
|
Current U.S.
Class: |
451/5 ; 451/527;
703/1 |
Current CPC
Class: |
B24B 37/26 20130101 |
Class at
Publication: |
451/5 ; 451/527;
703/1 |
International
Class: |
B24D 11/00 20060101
B24D011/00; B24B 1/00 20060101 B24B001/00 |
Claims
1. A polishing pad, comprising a plurality of polishing surfaces, a
first group of the polishing surfaces made of a first material
having a first coefficient of friction and a second group of the
polishing surfaces made of a second material having a second
coefficient of friction, the first and second groups of polishing
surfaces arranged over the polishing pad so as to provide a
non-planar material removal profile.
2. The polishing pad of claim 1, wherein the first and second
groups of polishing surfaces are arranged to provide an edge fast
material removal profile.
3. The polishing pad of claim 1, wherein the first and second
groups of polishing surfaces are arranged to provide an edge slow
removal profile.
4. The polishing pad of claim 1, wherein the first and second
groups of polishing surfaces are arranged to provide a center fast
removal profile.
5. The polishing pad of claim 1 wherein the first and second groups
of polishing surfaces are arranged to provide a center slow removal
profile.
6. The polishing pad of claim 1, wherein the polishing surfaces
comprise individual polishing elements, each polishing element
supported by an underlying compressible foam layer and oriented
with a long axis normal to a plane defined by the underlying
compressible foam layer.
7. The polishing pad of claim 6, wherein the polishing elements are
spaced apart from one another so that displacement of one polishing
element along its long axis in a direction normal to the plane
defined by the underlying foam layer does not materially affect
displacement of adjacent ones of the polishing elements.
8. The polishing pad of claim 1, wherein the polishing elements of
the first group are made of polyurethane and the polishing elements
of the second groups are made of polyoxymethylene.
9. The polishing pad of claim 1, wherein the first and second
groups of polishing surfaces are arranged in different densities
over the polishing pad.
10. The polishing pad of claim 9, wherein the different densities
vary by radial distance of the polishing surfaces from a center of
the polishing pad.
11. The polishing pad of claim 9, wherein the different densities
vary orthogonally across the polishing pad.
12. The polishing pad of claim 9, wherein polishing surface density
variations are implemented by varying groove pitch over the
polishing pad.
13. The polishing pad of claim 9, wherein polishing surface density
variations are implemented by varying groove size over the
polishing pad.
14. The polishing pad of claim 1, wherein overall densities of
polishing surfaces per unit area of the polishing pad is uniform
for the polishing pad.
15. The polishing pad of claim 1, wherein the first and second
groups of polishing surfaces are laid out in uniform radial
arrangement over the entire polishing pad.
16. The polishing pad of claim 1, wherein each polishing surface of
the first and second groups has a common size.
17. The polishing pad of claim 1, wherein each polishing surface of
the first and second groups has a common shape.
18. The polishing pad of claim 1, wherein the polishing surfaces
comprise individual polishing elements laid out in an orthogonal
arrangement where inter-polishing element spacing is defined by a
pitch in each of two dimensions.
19. The polishing pad of claim 18, wherein the inter-polishing
element pitch in at least one of the two dimensions varies across
the polishing pad.
20. The polishing pad of claim 18, wherein the inter-polishing
element pitch in both of the two dimensions varies across the
polishing pad.
21. The polishing pad of claim 1, wherein the polishing surfaces
comprise individual polishing elements laid out in a radial pattern
such that polishing elements at different radial distances from a
center of the polishing pad have different inter-polishing element
spacing.
22. A method, comprising polishing a workpiece using a polishing
pad defined by any one of the preceding claims.
23. A method, comprising designing a layout of polishing surfaces
for a polishing pad to achieve a desired material removal profile
for the polishing pad by (a) determining a first material removal
profile of a first polishing pad having a known polishing surface
layout, (b) applying polishing surface coefficient of friction
variations and/or polishing surface density variations
corresponding to regions of the first polishing pad where changes
in the first material removal profile are desired, (c) determining
what fraction of a total polishing time is spent by an area of a
wafer of interest in the regions of the first pad, (d) determining,
based on these fractional times, corresponding variations in the
first material removal profile at pad-wafer locations of interest,
and (e) determining the new pad layout to achieve the desired
material removal profile by mapping new polishing surface
coefficients of friction and density profiles to the subject
polishing pad layout.
Description
RELATED APPLICATIONS
[0001] This is a Continuation-in-Part of U.S. patent application
Ser. No. 11/697,622, filed 6 Apr. 2007, which application is
assigned to the assignee of the present invention and incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of chemical
mechanical planarization (CMP) and relates specifically to a CMP
polishing pad having a non-planar material removal profiles and
methods for using such pads.
BACKGROUND
[0003] In modern integrated circuit (IC) fabrication, layers of
material are applied to embedded structures previously formed on
semiconductor wafers. Chemical mechanical planarization (CMP) is an
abrasive process used to remove excess material from these layers
and polish the resulting surface to achieve a desired structure and
material profile. CMP may be performed on both oxides and metals
and generally involves the use of chemical slurries applied in
conjunction with a polishing pad that is put in motion relative to
the wafer (e.g., rotational motion relative to the wafer). The
resulting smooth flat surface is necessary to maintain proper
photolithographic depth of focus for subsequent wafer processing
steps and to ensure that metal interconnects are not deformed over
underlying features on the wafer. Damascene processing requires
metal, such as tungsten or copper, to be removed from a top surface
of a dielectric to define interconnect structures, using CMP.
[0004] Polishing pads used in CMP processes are typically made of
urethanes, either in cast form and filled with micro-porous
elements, or from non-woven felt coated with polyurethanes. The
polishing surface of a polishing pad is typically a single,
continuous sheet of material, which may be grooved or perforated to
facilitate slurry distribution across the surface. During polishing
operations the polishing pad is rotated while contacting the wafer,
which is also rotated, with the slurry layer disposed between the
pad and the wafer, thus affecting polishing.
[0005] One consequence of the use of conventional polishing pads in
polishing processes is that as pad moves relative to the wafer,
sudden changes in pad compression will be experienced. These
variations will give rise to "edge effects" on the wafer, causing
material removal rates at the edge of the wafer to be different
from those experienced at other points across the diameter of the
wafer.
[0006] Further, conventional polishing pads have no inherent
ability to modulate their material removal profile across the
diameter of a wafer. Yet, in advanced wafer processing operations
there are multiple films being deposited, each of which have
specific deposition profiles. For example, electroplated copper
films tend to be thickest at the edge of a wafer, while some
dielectric films tend to have a smooth "M" or "W"-shaped profile
across a diameter of a wafer. In cases of critical process modules,
such as copper and STI polishing, the use of conventional polishing
pads can lead to over-polishing of some areas in order to ensure
that other areas of the wafer receive adequate polishing. In
advanced technology nodes, the margin for such over-polishing
(i.e., the ability to tolerate such conditions and still have an
acceptable wafer result from the process) is shrinking rapidly and
in some cases allows for less than 5% polish time. This leads to
loss of performance or, worse, loss of yield for some parts. There
is therefore a need to provide means for tuning the removal profile
of a polishing pad in polishing processes to minimize the need for
over-polishing across the diameter of a wafer.
SUMMARY OF THE INVENTION
[0007] In one embodiment, a polishing pad includes a plurality of
polishing surfaces, a first group of the polishing surfaces made of
a first material having a first coefficient of friction and a
second group of the polishing surfaces made of a second material
having a second coefficient of friction. The first and second
groups of polishing surfaces may be arranged over the polishing pad
so as to provide a non-planar material removal profile. The first
and second groups of polishing surfaces may be arranged to provide
(1) an edge fast material removal profile; (2) an edge slow removal
profile; (3) a center fast removal profile; or (4) a center slow
removal profile.
[0008] The polishing surfaces of the polishing pad may, in some
cases, be individual polishing elements, each polishing element
supported by an underlying compressible foam layer and oriented
with a long axis normal to a plane defined by the underlying
compressible foam layer. The polishing elements may be spaced apart
from one another so that displacement of one polishing element
along its long axis in a direction normal to the plane defined by
the underlying foam layer does not materially affect displacement
of adjacent ones of the polishing elements. In some cases, the
polishing elements of the first group may be made of polyurethane
and the polishing elements of the second groups may be made of
polyoxymethylene.
[0009] In varying embodiments of the invention, the first and
second groups of polishing surfaces may be arranged in different
densities over the polishing pad. In some cases, the different
densities vary by radial distance of the polishing surfaces from a
center of the polishing pad. In other cases, the different
densities vary orthogonally across the polishing pad. In still
other cases, the polishing surface density variations are
implemented by varying groove pitch and/or groove size over the
polishing pad. In still further cases, however, overall densities
of polishing surfaces per unit area of the polishing pad is uniform
for the polishing pad.
[0010] In some instances, the first and second groups of polishing
surfaces may be laid out in uniform radial arrangement over the
entire polishing pad. Each polishing surface of the first and
second groups may have a common size and/or common shape or
different sizes/shapes. Where the polishing surfaces are individual
polishing elements, they may be laid out in an orthogonal
arrangement where inter-polishing element spacing is defined by a
pitch in each of two dimensions. This inter-polishing element pitch
may vary in either or both of these dimensions. Further, where the
polishing surfaces are individual polishing elements they may be
laid out in a radial pattern such that polishing elements at
different radial distances from a center of the polishing pad have
different inter-polishing element spacing.
[0011] Further embodiments of the invention provide for polishing a
workpiece using a polishing pad configured in any of the
above-described fashions.
[0012] Moreover, designing a layout of polishing surfaces for a
polishing pad to achieve a desired material removal profile for the
polishing pad may be done by (a) determining a first material
removal profile of a first polishing pad having a known polishing
surface layout, (b) applying polishing surface coefficient of
friction variations and/or polishing surface density variations
corresponding to regions of the first polishing pad where changes
in the first material removal profile are desired, (c) determining
what fraction of a total polishing time is spent by an area of a
wafer of interest in the regions of the first pad, (d) determining,
based on these fractional times, corresponding variations in the
first material removal profile at pad-wafer locations of interest,
and (e) determining the new pad layout to achieve the desired
material removal profile by mapping new polishing surface
coefficients of friction and density profiles to the subject
polishing pad layout.
[0013] These and further embodiments of the invention are discussed
in detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The present invention is illustrated by way of example, and
not limitation, in the figures of the accompanying drawings, in
which:
[0015] FIG. 1 illustrates an example of a polishing pad having
individual polishing elements, densities of which across the pad
may be varied according to the present invention;
[0016] FIG. 2A shows a polishing pad having polishing elements laid
out in varying densities along circumferences of different radii in
accordance with an embodiment of the present invention.
[0017] FIG. 2B shows a polishing pad in which polishing elements
are laid out in an orthogonal arrangement and the inter-polishing
element spacing is defined by a pitch in two dimensions.
[0018] FIG. 2C illustrates a polishing pad similar to that shown in
FIG. 2B, with variations in polishing pad pitches in each
dimension.
[0019] FIG. 3 shows a polishing pad with a wafer overlaid thereon
(e.g., as may be the case during wafer polishing operations) and
containing polishing elements laid out in a fixed radial density
profile and in which polishing elements of two different types are
present.
DETAILED DESCRIPTION
[0020] Described below is a polishing pad suitable for use in CMP
and other applications, which pad is configured to provide a
tunable, material removal profile across a workpiece being
polished. Often, for example with CMP processes, the workpiece will
be a semiconductor wafer (either a bare wafer or one that has
already undergone one or more processing steps in a fabrication
process), but this is not necessarily so. Other workpieces that are
subject to polishing operations may also benefit from the use of
the polishing pads configured in accordance with the present
invention. Therefore, although the present polishing pad will be
discussed in the context of semiconductor polishing operations (in
particular CMP), it should be recognized that the present polishing
pad may find use in many other applications.
[0021] In various embodiments, the present polishing pad consists
of polishing elements which can be arranged over the surface of the
pad in different configurations. For example the polishing elements
may be arranged in a fixed area density configuration or a fixed
radial density configuration. These different layouts offer
specific advantages in their ability to deliver uniform material
removal profiles, depending on the motion of the pad relative to
the wafer. A fixed area density profile may be more advantageous
for linear or orbital relative motion, while a fixed radial density
profile maybe more suitable when relative rotary motion is
employed. Of course, other polishing element layouts may also be
used.
[0022] The above-cited U.S. patent application Ser. No. 11/697,622
described a polishing pad that includes a number of individual
polishing elements. Such a pad 100 is illustrated in cross-section
in FIG. 1. Each polishing element 102 is supported by an underlying
compressible foam layer 104 and extends vertically through holes in
a guide plate 106 affixed to that underlying foam layer. The
polishing elements are maintained so that they are approximately
normal with respect to a plane defined by the underlying foam layer
and are spaced apart from one another so that displacement of one
polishing element along its long axis in a direction normal to the
plane defined by the underlying foam layer does not materially
affect displacement of adjacent ones of the polishing elements.
[0023] The polishing pad described in the '622 application enabled
very uniform pressure to be exerted onto a wafer surface during
polishing operations. This translates to a very uniform material
removal profile in which edge effects typically associated with
conventional polishing pads are significantly reduced (or in some
cases eliminated altogether). Nevertheless, in some instances it is
desirable to tune the polishing profile of the polishing pad in
order to effect different material removal profiles. Hence, in
embodiments of the present invention, the individual polishing
elements may be organized in many different arrangements (e.g.,
different density profiles that have more or fewer pads per unit
area across the face of the pad) and/or may be made from different
materials (e.g., having different coefficients of friction for the
material being polished) in order to effect such varying material
removal profiles.
[0024] In one embodiment, a polishing pad configured to effect
non-planar material removal across a wafer during polishing is made
up of a number of individual polishing elements, each supported by
an underlying compressible foam layer and maintained in vertical
orientation with respect to said foam underlayer by a guide plate
having holes through which the individual polishing elements
protrude. The nominal diameter of an individual polishing element
may be approximately 0.25 inches and the nominal height of an
individual polishing element may be approximately 0.160 inches. The
compressible foam underlayer may be nominally 0.060 inches
thick.
[0025] The polishing pad may include polishing elements made of two
(or more) different materials, each having a different coefficient
of friction for the film or material being polished. The polishing
elements may be arranged such that the polishing elements with
lower coefficient of friction, such as Delrin.TM.
(polyoxymethylene), alternate in a desired fashion or density with
polishing elements having higher coefficients of friction (e.g.,
polyurethane polishing elements). The ratio of one variety of
polishing element to another may be varied to achieve different
material removal rate performance.
[0026] The nominal coefficient of friction for polyurethane against
an SiO.sub.2 surface is 0.45, whereas the coefficient of friction
for Delrin under similar conditions is 0.15. Therefore, the present
polishing pad is expected to have a much lower material removal
contribution from polishing elements made of Delrin than for other
polishing elements, thus leading to a lower overall material
removal rate than for a pad having only non-Delrin polishing
elements. During polishing, an area of a wafer in contact with a
section of the pad that contains both Delrin and non-Delrin (e.g.,
polyurethane) polishing elements will have lower rate of material
removal than an area of the wafer in contact with only non-Delrin
(e.g., polyurethane) polishing elements.
[0027] In addition, embodiments of the present polishing pad may
have polishing elements arranged to provide a fixed area density of
polishing elements or a fixed radial density of polishing elements
(either in total or per polishing element material). These
different layouts offer specific advantages in their ability to
deliver differing material removal rates profiles, depending on the
relative motion of the polishing pad to the wafer. A fixed area
density profile may be more advantageous for linear or orbital
relative motion, while a fixed radial density profile maybe more
suitable when relative rotary motion is employed. FIG. 2A shows a
polishing pad 200 having an arrangement of polishing elements 202
in which the polishing elements 202 are laid along circumferences
of different radii (marked as R1, R2, R3 and R4). FIG. 2B shows a
polishing pad 204 in which the polishing elements 202 are laid out
in an orthogonal arrangement where the inter-polishing element
spacing is defined by a pitch in both X and Y directions. As shown
in FIG. 2C, either or both pitches (X, Y) may be varied across a
diameter of a wafer 206 to provide two or more pitches (X1, X2; Y1,
Y2) in each dimension.
[0028] FIG. 3 shows a polishing pad 300 with a wafer 302 overlaid
on the pad (e.g., as may be the case during wafer polishing
operations). The pad 300 contains polishing elements 304A, 304B
laid out in a fixed radial density profile in which polishing
elements of two different types are present. That is, polishing
elements 304A are made of a different material than polishing
elements 304B. For example, polishing elements 304A may be made of
polyurethane while polishing elements 304B may be made of material
having a different coefficient of friction with respect to a
surface of wafer 302 (or a film on wafer 302) presently being
polished. Note that the polishing elements of different materials
may be used with any of the configurations shown in FIGS.
2A-2C.
[0029] In an embodiment of the invention, a determination as to
which polishing element material combination and/or layout to use
in order to achieve a desired film removal profile may be
accomplished. For example, the material removal rate for a given
substrate or film will be proportional to the area of the polishing
pad in contact with that substrate or film. This will, in turn, be
affected by the density of polishing elements per unit area of the
pad. The effective polishing element density of a pad may be varied
by changing the pitch and/or size of polishing elements in any
given area thereof. Therefore, assuming that as the wafer rotates
relative to the pad, and the wafer traverses the entire radius of
the pad during a polishing operation, then by varying the layout
density of polishing elements (hence the effective contact area of
the pad) and calculating the time spent by the wafer in areas of
different polishing element densities across the entire wafer, a
removal profile for the pad/substrate (or pad/film) combination can
be predicted.
[0030] Conversely, by knowing the removal profile for a given
polishing element material/density layout, a polishing pad can be
constructed to achieve a desired layout/material removal profile.
Of course, the same processes can be applied to polishing pads
using polishing elements of different coefficients of friction (or
other polishing element characteristics that affect material
removal rates) in lieu of or in addition to varying polishing
element densities. Stated differently, the above-described
processes can be used to develop custom tailored pads for given
substrates or films and selected polishing element materials to
provide desired material removal rates.
[0031] In various embodiments of the invention then, the polishing
element density of a polishing pad may be varied over areas from 10
mm.sup.2 to 1000 mm.sup.2, and preferably from 30 mm.sup.2 to 250
mm.sup.2. Such polishing element densities may be varied radially
across the pad surface or orthogonally across the pad surface. The
polishing element density range across the pad may be between
20%-80% of the entire pad surface, and preferably between 35-60% of
the pad surface. The polishing element density variation may be
achieved by varying polishing element pitch and, optionally, the
size of the discrete polishing element surfaces which contact the
substrate or film to be polished. Alternatively, polishing element
density variation may achieved by varying groove pitch and/or
groove size, as in the case of continuous polishing surfaces for
continuous layer polishing pads.
[0032] In a further embodiment of the invention, desired material
removal rates for a polishing pad may be obtained by varying
polishing surface coefficients of friction over an area of the
polishing pad. Such area may vary between 10 mm.sup.2-1000
mm.sup.2, and preferably between 30 mm.sup.2-250 mm.sup.2.
Coefficients of friction of the polishing surfaces may be varied
across the pad in the range of 0.1-0.8, and preferably 0.3-0.6.
This variation in the coefficients of friction may be achieved by
varying the material of which the subject polishing surface is
made. In some cases, the polishing surface may be made up of
multiple individual polishing element surfaces, while in other
cases a continuous polishing surface may be used.
[0033] A polishing pad configured in accordance with the above
practices may therefore contain a plurality of polishing surface
materials, each having a different coefficient of friction, and
arranged so as to achieve non-planar removal profile. For example,
the materials may be arranged to achieve edge fast removal profile,
an edge slow removal profile, a center fast removal profile, or a
center slow removal profile. In one instance, a polishing pad
contains a plurality of polishing surface materials each having
different coefficient of friction and the materials of different
coefficients of friction are arranged according to different layout
densities over the surface of the pad to achieve the non planar
removal profile.
[0034] For example, some of the plurality of polishing surface
materials (which may be individual polishing element surfaces or
areas of a continuous polishing surface) may be made from
polyurethane and others of the plurality of polishing surfaces are
made of Delrin. Each of the respective polishing surfaces may be
arranged in a radial manner across the pad such that Delrin
polishing surfaces make up 5-50% density in locations of the total
polishing surface of the pad corresponding to areas requiring low
material removal rates. The pad may be used in polishing operations
such that it contacts a wafer or other substrate (or a film
thereon), optionally in the presence of slurry, such that the wafer
areas requiring low material removal rates are in contact with pad
areas containing Delrin polishing surfaces in the desired density.
In some instances, the overall density of polishing surfaces may be
uniform per unit area of the polishing pad. Alternatively, or in
addition, the polishing surfaces may be laid out in a uniform
radial arrangement. Optionally, both the Delrin and polyurethane
polishing surfaces may have a common shape and size, but in other
cases the respective polishing surfaces may have different sizes
and shapes.
[0035] A polishing pad polishing surface layout designed to achieve
a desired material removal profile may be achieved by (a)
determining a removal profile of a known polishing pad polishing
surface layout, (b) applying polishing surface coefficient of
friction variations and/or density variations corresponding to
regions of the pad where changes in material removal rates are
desired, (c) determining what fraction of the total polishing time
is spent by an area of the wafer of interest in the region of
modified polishing surface coefficients of friction/densities of
the pad, (d) determining, based on these fractional times,
corresponding variations in material removal rates at a pad-wafer
locations of interest, and (e) determining the new pad layout to
achieve the desired material removal profile by mapping new
polishing surface coefficients of friction and density profiles to
the subject polishing pad layout.
[0036] Thus, a polishing pad suitable for use in CMP and other
applications, which pad is configured to provide a tunable,
material removal profile across a workpiece has been described. In
the above description, a number of specifically illustrated
embodiments were discussed in order to better explain the present
invention, however, these examples should not be read to limit the
scope of the invention. Instead, the invention should be measured
only in terms of the following claims.
* * * * *