U.S. patent application number 09/782442 was filed with the patent office on 2001-08-16 for method and apparatus for planarization of a substrate.
This patent application is currently assigned to Micron Technology, Inc.. Invention is credited to Moore, Scott E..
Application Number | 20010014537 09/782442 |
Document ID | / |
Family ID | 22629874 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010014537 |
Kind Code |
A1 |
Moore, Scott E. |
August 16, 2001 |
Method and apparatus for planarization of a substrate
Abstract
Method and apparatus for a chemical-mechanical-polishing (CMP)
support pad structure are described. The support pad structure is
made with a housing to hold an open-cell material and a fluid. The
fluid is supplied in a gaseous and/or a liquid state to affect
rigidity of the pad. By controlling rigidity, material removal
rates may be directly affected during CMP processing. Moreover,
fluid flow through the support pad structure may be regulated to
adjust temperature during CMP processing.
Inventors: |
Moore, Scott E.; (Meridian,
ID) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH
P.O. BOX 2938
MINNEAPOLIS
MN
55402
US
|
Assignee: |
Micron Technology, Inc.
|
Family ID: |
22629874 |
Appl. No.: |
09/782442 |
Filed: |
February 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09782442 |
Feb 13, 2001 |
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09172950 |
Oct 14, 1998 |
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6187681 |
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Current U.S.
Class: |
438/690 ;
156/345.12; 257/E21.23; 438/691; 438/692 |
Current CPC
Class: |
B24B 37/24 20130101;
B24B 37/26 20130101; B24B 49/14 20130101; H01L 21/30625
20130101 |
Class at
Publication: |
438/690 ;
438/691; 438/692; 156/345 |
International
Class: |
C23F 001/02; H01L
021/461 |
Claims
What is claimed is:
1. In a chemical-mechanical-polishing system having a platen and a
polishing pad, an arrangement comprising: a support pad disposed
between said platen and said polishing pad, said support pad
comprising: a housing, said housing defining a chamber; a matrix
having voids therein, said matrix located within said chamber; a
fluid disposed within said chamber; and said matrix permeable with
respect to said fluid.
2. The arrangement of claim 1, wherein said chamber is pressurized
by said fluid, said fluid in at least one of a gaseous state, a
gelatinous state, and a liquid state.
3. The arrangement of claim 2, wherein pressure in said chamber is
in a range of approximately 1 to 100 p.s.i.
4. The arrangement of claim 1, wherein said matrix is coupled to
said housing to limit deformation of said chamber.
5. In a chemical-mechanical-polishing system having a platen and a
polishing pad, an arrangement comprising: a support pad disposed
between said platen and said polishing pad, said support pad
comprising: a housing, said housing defining a volume; a webbing
located within said volume and coupled to said housing to limit
deformation of said volume; and a fluid disposed within said
volume.
6. The arrangement of claim 5, wherein said housing is pressurized
by said fluid.
7. The arrangement of claim 6, wherein pressure in said volume is
in a range of approximately 1 to 100 p.s.i.
8. The arrangement of claim 5, wherein said webbing provides
baffling within said volume of said housing.
9. In a chemical-mechanical-polishing system having a platen and a
polishing pad, an arrangement comprising: a support pad disposed
between said platen and said polishing pad, said support pad
comprising: a pressurizable member having variable deformability,
said pressurizable member providing an enclosure; a matrix having
voids therein, said matrix located within said enclosure; a webbing
located within said enclosure and coupled to said pressurizable
member to limit deformation thereof; a fluid disposed within said
enclosure; and said matrix permeable with respect to said
fluid.
10. The arrangement of claim 9, wherein said matrix is coupled to
said pressurizable member to limit deformation of said
enclosure.
11. The arrangement of claim 9, further comprising at least one
tube operatively coupled to deliver said fluid to said
enclosure.
12. The arrangement of claim 9, further comprising: an inlet tube
operatively coupled to deliver said fluid to said enclosure; and an
outlet tube operatively coupled to remove said fluid from said
enclosure.
13. The arrangement of claim 12, wherein said webbing provides
baffling for regulating transmission of said fluid from said inlet
tube to said outlet tube.
14. In a chemical-mechanical-polishing system having a platen and a
polishing pad, an arrangement comprising: a support pad disposed
between said platen and said polishing pad, said support pad
comprising: a housing, said housing defining a volume; an open-cell
material located within said volume; a fluid disposed within said
volume; an inlet tube operatively coupled to deliver said fluid to
said volume; a manifold operatively coupled to said inlet tube to
receive said fluid and to distribute said fluid in said volume; and
an outlet tube operatively coupled to said manifold to remove said
fluid from said volume.
15. The arrangement of claim 14, wherein said open-cell material is
configured to provide said manifold.
16. In a chemical-mechanical-polishing system having a platen and a
polishing pad, a support pad comprising: a housing, said housing
having a top layer and at least one side layer; a surface of said
platen, said top layer, and said side layer in combination defining
a space; a fluid located within said space; and a porous solid
member, permeable with respect to said fluid, located in said space
and coupled to said housing to limit deformation of said space.
17. In a chemical-mechanical-polishing system having a wafer
carrier, a support pad comprising: a flexible structural member,
said flexible structural member having a bottom layer and at least
one side layer; a surface of said wafer carrier, said bottom layer,
and said side layer in combination defining an enclosure; a fluid
located within said enclosure; and a porous solid member, permeable
with respect to said fluid, located within saud enclosure, said
porous solid member coupled to said flexible structural member and
to said wafer carrier to limit deformation of said enclosure.
18. In a chemical-mechanical-polishing system having a platen and a
polishing pad, a support pad comprising: a housing, said housing
having a top layer and at least one side layer; a surface of said
platen, said top layer, and said side layer in combination defining
a volume; webbing located within said volume and coupled to said
housing and said platen to limit deformation of said volume; and a
fluid located within said volume.
19. In a chemical-mechanical-polishing system having a wafer
carrier, a support pad comprising: a housing, said housing having a
bottom layer and at least one side layer; a surface of said wafer
carrier, said bottom layer, and said side layer in combination
defining a volume; webbing located within said volume and coupled
to said housing and said wafer carrier to limit deformation of said
volume; and a fluid located within said volume.
20. An apparatus for use in planarizing a semiconductor substrate
assembly, comprising: a chemical-mechanical-polishing pad; a
housing, said housing having a bottom layer and at least one side
layer; a surface of said chemical-mechanical-polishing pad, said
bottom layer, and said side layer in combination defining a volume;
a fluid located within said volume; and a porous solid member,
permeable with respect to said fluid, located within said volume
and coupled to said chemical-mechanical-polish- ing pad and said
housing to limit expansion of said volume.
21. An apparatus for use in planarizing a semiconductor substrate
assembly, comprising: a chemical-mechanical-polishing pad; a
housing, said housing having a bottom layer and at least one side
layer; a surface of said chemical-mechanical-polishing pad, said
bottom layer, and said side layer in combination defining a volume;
webbing located within said volume and coupled to said housing and
said chemical-mechanical-polishing pad to limit expansion of said
volume; and a fluid located within said volume.
22. In a chemical-mechanical-polishing system having a platen and a
polishing pad, an arrangement comprising: a support pad disposed
between said platen and said polishing pad, said support pad
comprising: a polymeric open-cell foam, said polymeric open-cell
foam having a skin, said skin defining an enclosed region; a fluid
disposed within said enclosed region.
23. In a chemical-mechanical-polishing system having a platen and a
polishing pad, an arrangement comprising: a support pad disposed
between said platen and said polishing pad, said support pad
comprising: elastomeric open-cell foam, said elastomeric open-cell
foam having a skin, said skin defining an enclosed region; and a
fluid disposed within said enclosed region.
24. In a chemical-mechanical-polishing system having a platen and a
polishing pad, a method for thermal control during substrate
assembly planarization, comprising: providing a pad structure, said
pad structure having a housing, said housing having an inlet tube
and an outlet tube, said pad structure having a sponge-like
material disposed within said housing; locating said pad structure
between said polishing pad and said platen; providing fluid to said
housing through said inlet tube; removing said fluid from said
housing through said outlet tube; monitoring temperature of said
fluid; and adjusting temperature of said fluid during said
planarization.
25. In a chemical-mechanical-polishing system having a platen and a
polishing pad, a method for adjusting responsiveness of said
polishing pad during planarization of a substrate assembly,
comprising: providing a pad structure, said pad structure having a
housing, said housing having an inlet tube and an outlet tube, said
pad structure having a sponge-like material disposed within said
housing; locating said pad structure between said polishing pad and
said platen; providing fluid to said housing through said inlet
tube; removing said fluid from said housing through said outlet
tube; monitoring pressure of said fluid; and adjusting pressure of
said fluid during said planarization.
26. The method of claim 25, wherein said pressure is maintained in
a range of approximately 1 to 100 p.s.i.
27. The method of claim 25, wherein said pressure is maintained in
a range of approximately 2 to 20 p.s.i.
28. The method of claim 25, wherein said pressure is at or above a
polishing force pressure.
29. The method of claim 28, wherein said polishing force pressure
is in a range of approximately 1 to 10 p.s.i.
30. In a chemical-mechanical-polishing system having a wafer
carrier, a method for thermal control during polishing, comprising:
providing a pad structure, said pad structure having a housing,
said housing having an inlet tube and an outlet tube, said pad
structure having an sponge-like material disposed within said
housing; locating said pad structure in operative proximity to said
wafer carrier; providing fluid to said housing through said inlet
tube; removing said fluid from said housing through said outlet
tube; monitoring temperature of said fluid; and adjusting
temperature of said fluid during said polishing.
31. In a chemical-mechanical-polishing system having a wafer
carrier, a method for adjusting responsiveness of a polishing pad
during planarization of a substrate assembly, comprising: providing
a pad structure, said pad structure having a housing, said housing
having an inlet tube and an outlet tube, said pad structure having
a sponge-like material disposed within said housing; locating said
pad structure in operative proximity to said wafer carrier;
providing fluid to said housing through said inlet tube; removing
said fluid from said housing through said outlet tube; monitoring
pressure of said fluid; and adjusting pressure of said fluid during
said planarization.
32. The method of claim 31, wherein said pressure is in a range of
approximately 1 to 100 p.s.i.
33. The method of claim 31, wherein said pressure is in a range of
approximately 2 to 20 p.s.i.
34. The method of claim 31, wherein said pressure is at or above a
polishing force pressure.
35. The method of claim 34, wherein said polishing force pressure
is in a range of approximately 1 to 10 p.s.i.
36. In a chemical-mechanical-polishing system having a platen and a
polishing pad, a method for thermal control during polishing of a
substrate assembly, comprising: providing a pad structure having a
housing; locating said pad structure in near proximity to said
substrate assembly for thermal conductivity thereto; providing
fluid to and from said housing; and varying temperature of said
fluid during said polishing.
37. In a chemical-mechanical-polishing (CMP) system having a platen
and a polishing pad, a method for adjusting responsiveness of said
CMP system during polishing of a substrate assembly, comprising:
providing a pad structure having a housing; providing fluid to and
from said housing; and varying pressure of said fluid to said
housing during said polishing.
38. In a chemical-mechanical-polishing system having a platen and a
polishing pad, a method for thermal influence on a substrate
assembly, comprising: providing a pad structure having a housing;
locating said pad structure in near proximity to said substrate
assembly for thermal conductivity thereto; providing fluid to and
from said housing; and varying temperature of said fluid.
39. In a chemical-mechanical-polishing (CMP) system, a method for
adjusting responsiveness of said CMP system for polishing a
substrate assembly, comprising: providing a pad structure having a
housing; providing fluid to and from said housing; and varying
pressure of said fluid to said housing.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates generally to mechanical
polishing of a semiconductor substrate, and more particularly to a
support pad for use in chemical-mechanical-polishing to planarize a
substrate assembly.
BACKGROUND OF THE INVENTION
[0002] In the semiconductor industry, a technology known as
chemical-mechanical-polishing ("CMP") has been developed. In "CMP,"
a semiconductor wafer is place between a polishing pad and a wafer
carrier. The wafer carrier is a mechanical device used to provide
rotation of and downward pressure on the semiconductor wafer. In
this manner, the semiconductor wafer is forcibly held and rotated
against the polishing pad. As a result, material is abraded from
the surface of the semiconductor wafer in contact with the
polishing pad.
[0003] Slurry, a liquid chemical composition generally containing
particulate, may be dispensed on the polishing pad. The slurry
conventionally is chemically active to remove unwanted material
from the semiconductor wafer. This chemical activity may be changed
during CMP processing. The slurry particulate facilitates abrasion.
Alternatively, the semiconductor wafer is polished without slurry
by using a "fixed-abrasive polishing pad." A "fixed-abrasive
polishing pad" has protrusions formed on it to facilitate abrasion.
The semiconductor wafer is thus held and rotated against the
protrusions on the fixed-abrasive polishing pad.
[0004] Aside from removing unwanted material, CMP may be used to
planarize a surface of a semiconductor wafer. A uniform planar
surface is important for subsequent lithographic processing of the
semiconductor wafer, mainly due to depth of focus limitations of
lithographic equipment. Accordingly, to achieve a substantially
uniformly planar surface, it is important to apply pressure
uniformly. A problem has been a lack of uniform pressure resulting
in a non-uniform planar surface (e.g. a wedge-like shape).
[0005] One approach to achieve greater pressure uniformity has been
to place an underpad below the polishing pad. The underpad has more
give than the polishing pad, and thus it distributes downward force
from the wafer carrier more uniformly. However, conditions, such as
temperature, applied force, and friction, change during CMP, and an
underpad is not able to compensate for these changes.
[0006] Another approach to achieve a more uniform downward force
has been to use a wafer carrier having multiple bellows located
between a wafer carrier housing and a wafer carrier base, as is
described in U.S. Pat. No. 5,681,215 to Sherwood et al. As the
semiconductor wafer is held between the polishing pad and the wafer
carrier base, by pressurizing bellows against the wafer carrier
housing, force is directed onto the wafer carrier base, which in
turn pushes down on the semiconductor wafer. As each bellows may be
separately pressurized, downward force is supposedly adjustable
from location to location to improve uniformity; however, Sherwood
et al is a mechanically complicated approach.
[0007] Therefore, it would be desirable to have a CMP apparatus
that is less mechanically complicated than Sherwood et al, but
allows for rigidity adjustment unlike present day underpads.
Furthermore, it would be desirable to have a CMP apparatus that is
a disposable item like an underpad.
SUMMARY OF THE INVENTION
[0008] The present invention provides method and apparatus for CMP
with a support pad to provide a more uniform application of force
to polish a semiconductor substrate. Such a support pad is formed
with a housing. The housing defines a volume in which an open-cell
material, disposed in the volume, is coupled to the housing to
limit deformation of the volume. A fluid is disposed in the volume
to affect rigidity of the support pad. The fluid may be in a
gaseous and/or a liquid state and may be pressurized.
[0009] The housing may be attached to or in contact with a CMP
wafer carrier, a CMP platen, and/or a CMP polishing pad. Such a CMP
wafer carrier, platen, and/or pad may be used in combination with
the housing in defining a volume in which an open-cell material and
fluid is contained. Moreover, inlet and outlet tubes may be
employed for providing the fluid in and out of the volume. Such
ingress and egress of fluid may be used for thermal control.
Baffling or webbing may be located within the volume to regulate
fluid flow, to maintain pressurization, and/or to limit volume
deformation.
[0010] A support pad in accordance with the present invention may
be a sealed unit to better cope with a harsh CMP environment; such
a support pad may easily be removed and replaced. Moreover, a
sealed pad may be pressurized to a desired level for a CMP
operation. Alternatively, one or more tubes may be employed for in
situ maintenance and/or adjustment of support pad rigidity.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0011] Features and advantages of the present invention will become
more apparent from the following detailed description of the
preferred embodiment(s) described below in detail with reference to
the accompanying drawings where:
[0012] FIG. 1 is a cross-sectional view of an exemplary portion of
a CMP systems in accordance with the present invention.
[0013] FIG. 2 is a cross-sectional view of an exemplary portion of
an embodiment of a support pad structure in accordance with the
present invention.
[0014] FIG. 3 is a cross-sectional view of an exemplary portion of
an alternate embodiment of support pad structure in accordance with
the present invention.
[0015] FIG. 4 is a cross-sectional view of an exemplary portion of
a wafer carrier assembly in accordance with the present
invention.
[0016] FIG. 5 is a block diagram of an exemplary portion of an
embodiment of a fluid control system in accordance with the present
invention.
[0017] Reference numbers refer to the same or equivalent parts of
embodiment(s) of the present invention throughout the several
figures of the drawing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0018] Referring to FIG. 1, there is shown a cross-sectional view
of an exemplary portion of a CMP system 10. CMP system 10 comprises
motors 24 and 25, support pads 11, drive shafts 14 and 15,
polishing pad 12, platen 13, wafer carrier housing 16, and
retaining ring 17. Motors 24 and 25 may be employed to rotatively
drive shafts 15 and 14, respectively. Drive shafts 14 and 15 may be
hollow, such that cavities, as indicated by dashed lines 80 and 81,
may house tubing to transport fluids to and from support pads 11,
as will become apparent.
[0019] A support pad 11, in accordance with the present invention,
may be located below polishing pad 12, and/or between wafer carrier
16 and substrate assembly 23. By "substrate assembly," it should be
understood to include a bare substrate or wafer, as well as a
substrate having one or more layers and/or features formed thereon
and/or therein.
[0020] Several embodiments of support pads 11 are explained in more
detail below, where it will become apparent that by employing a
support pad of the present invention, a substantially uniformly
distributed pressure may be obtained.
[0021] Referring now to FIG. 2, there is shown a cross-sectional
view of an exemplary portion of an embodiment of a support pad 40
in accordance with the present invention. Support pad 40 comprises
a non-rigid housing 33, filler 39, inlet tube 31 and outlet tube
32.
[0022] In defining volume 100, housing 33 comprises upper layer 37,
lower layer 36, and at least one outer or side layer 38. Layers 36,
37, and 38 may be made from polycarbonate, polyurethane, or other
suitable material. Upper layer 37 includes inner top surface 98,
and bottom layer 36 includes inner bottom surface 99. Layers 36 and
37 may be bonded, clamped, fused, glued, epoxied, and in like
manner attached or coupled to layer 38, filler 39, and/or webbing
68 (shown in FIG. 3) to limit ballooning of volume 100 and/or
housing 33. Alternatively, layers 36, 37, and 38 may be integrally
formed as one unitized housing 33. Filler 39 may be formed after at
least partial formation of housing 33 in order to substantially
fill volume 100. Alternatively, filler 39 may be prefabricated for
fitting into volume 100. Filler 39 may be formed of an open-cell
material. By open-cell material, it is meant a matrix having
adjacent, suspended voids ("open-cells"). In accordance with an
embodiment of the present invention, matrix with embedded voids is
gas or liquid permeable, and more particularly is capable of
providing at least one conduit for transmitting a fluid from a
proximal surface to a distal surface. Materials used in forming
open-cell structures include polymers, monomers, elastomers,
carbon, silicon. Some materials for forming open-cell material
include polyurethane, polypropylene, polyethylene, polyolefin,
polychloroprene, rubber, synthetic-rubber, among others. Each
substance may be used in forming a sponge-like material as is well
known. An open-cell foam with sufficient rigidity to withstand
downward polish force may be employed.
[0023] Inlet tube 31 and/or outlet tube 32 may be removed or
omitted where support pad 40 has a sealed volume, as indicated by
dash lines 28 and 29. For example, inlet tube 31 may be used for
dispensing a fluid into volume 100 defined by housing 33. After
such dispensing is completed, inlet tube 31 may be sealed (fused,
glued, epoxies, bonded, and the like) to form a sealed volume. A
sealed volume support pad may be more beneficial for
implementations where it is rotated during CMP.
[0024] With reference to FIG. 1, hollow drive shafts 14, 15 may
meet holes in platen 13 and/or wafer carrier 16, as indicated by
dashed lines 80 and 81. Such configurations may be employed at
least in part to house one or more tubes 31 and/or 32.
[0025] Inlet tube 31 and outlet tube 32 may comprise one or more
tubes for transporting fluid to and from volume 100. Furthermore,
inlet tube 31 and outlet tube 32 in combination may form a manifold
having one or more tubing branches or alternatively open-cell
material may be configured to provide a manifold, as illustratively
indicated by dashed lines 188.
[0026] Referring to FIG. 3, there is shown a cross-sectional view
of an exemplary portion of an alternate embodiment of a support pad
50 in accordance with the present invention. Support pad 50 is
similar to support pad 40 of FIG. 2, except bottom layer 36 has
been omitted, webbing 68 has been added to form baffles, and tubes
31 and 32 have been reoriented. Bottom layer 36 may be omitted in
place of top surface 89 of platen 13 and/or bottom surface 88 of
wafer carrier 16 (shown in FIG. 1). Outer layer 38 is attached to
platen layer 13 and/or wafer carrier 16 depending on configuration
to be employed. Webbing 68, which may have passages 86, may be used
to limit deformation of volume 100, to regulate or direct fluid
flow, and/or to maintain pressurization of filler 39. Tubes 31 and
32 may be at least partially housed in either drive shaft 14 or 15
(shown in FIG. 1). Seals 83 may be employed to limit or prevent
fluid leakage.
[0027] Referring to FIG. 4, there is shown a cross-sectional view
of an exemplary portion of an embodiment of a wafer carrier
assembly 70 in accordance with the present invention. Wafer carrier
assembly 70 includes an alternate embodiment of a sealed support
pad 90 of the present invention. Open-cell foam 71 may have a skin
forming at least a portion of housing 94 of sealed support pad 90.
Alternatively, open-cell foam 71 may be laminated at least in part
to form at least a portion of housing 94. Also, corrugated webbing
72 may be used to form baffles to limit deformation of housing 94.
Webbing 72 may be attached to interior surfaces of housing 94 at
one or more contact points 73.
[0028] Referring now to FIG. 5, there is shown a block diagram of
an exemplary portion of an embodiment of fluid control system 60
for use with support pad 40 in accordance with the present
invention. Fluid is supplied from fluid supply 57 to pump 58. Fluid
is pumped from pump 58 along in-feed tubing 62 to support pad 40,
e.g., through inlet tube 31 (shown in FIG. 2).
[0029] Exiting fluid flows from support pad 40, through out-feed
tubing 61 to controller-conditioner 59. Controller-conditioner 59
may be used to monitor fluid pressure and/or temperature. If
temperature is too high or too low, controller-conditioner 59 may
divert fluid to temperature pump 56 in order to increase or
decrease temperature of fluid 54. In this manner, support pad 40
may be employed at least in part for temperature control during
CMP. Controller-conditioner 59 in electrical communication with
regulated pump 58 via path 63 may regulate pressure of fluid
supplied to support pad 40. Pressure may then be referenced to
absolute pressure, atmospheric pressure, or polish downward force.
In one embodiment, pressure ranges from approximately 1 to 100
p.s.i. However, while not wishing to be bound by any particular
theory, it is believed that the pressure should be selected to be
greater than the polish downward force being applied. As CMP
downward force is typically in a range of approximately 1 to 10
p.s.i., a pressure in a range of approximately 2 to 20 p.s.i. may
be employed therefor. Tubing 64 allows fluid 54 transfer between
controller/conditioner 59 and pump 58.
[0030] Notably, the present invention may also be employed in a
single-ended system. In which system, in-feed tubing 62 is used for
ingress and egress of fluid 54 to support pad 40, and out-feed
tubing 61 is omitted.
[0031] Support pad 40 rigidity may be controlled at least in part
by pressurization of volume 100 (shown in FIG. 2) by fluid 54.
Rigidity of support pad 40 may also be effected by the type of
fluid selected. Fluid 54 may be in a liquid state, a colloidal
(gelatinous) state, and/or a gaseous state. Moreover, gas may be
introduced and at least partially dissolved in a liquid to adjust
compressibility. Also, a support pad 11 (shown in FIG. 1) may be
pressurized to volumetrically expand to maintain substrate assembly
23 in contact with CMP pad 12 for polishing.
[0032] In accordance with the present invention, during CMP,
support pad rigidity may be controlled to affect material removal
rate. Generally, a more rigid support pad will aid removal of more
material than a less rigid support pad. Also, by controlling fluid
flow through a support pad, temperature may be adjusted during CMP
processing, including adjustment. Also, a support pad may be
pressurized to compensate for changes in a downwardly directed
force used in polishing a substrate assembly.
[0033] The present invention has been particularly shown and
described with respect to certain preferred embodiment(s). It
should be readily apparent to those of ordinary skill in the art
that various changes and modifications in form and detail may be
made without departing from the spirit and scope of the present
invention as set forth in the appended claims.
* * * * *