U.S. patent application number 09/782818 was filed with the patent office on 2001-07-12 for chemical mechanical polishing head assembly having floating wafer carrier and retaining ring.
This patent application is currently assigned to Mitsubishi Materials Corporation. Invention is credited to Chin, Scott, Dickey, Tanlin K., Dyson, William JR., Geraghty, John J., Moloney, Gerard S..
Application Number | 20010007810 09/782818 |
Document ID | / |
Family ID | 22992002 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007810 |
Kind Code |
A1 |
Moloney, Gerard S. ; et
al. |
July 12, 2001 |
Chemical mechanical polishing head assembly having floating wafer
carrier and retaining ring
Abstract
The invention provides structure and method for achieving a
uniformly polished or planarized substrate such as a semiconductor
wafer including achieving substantially uniform polishing between
the center of the semiconductor wafer and the edge of the wafer. In
one aspect the invention provides a polishing apparatus including a
housing, a carrier for mounting a substrate to be polished, a
retaining ring circumscribing the carrier for retaining the
substrate, a first coupling attaching the retaining ring to the
carrier such that the retaining ring may move relative to the
carrier, a second coupling attaching the carrier to the housing
such that the carrier may move relative to the housing, the housing
and the first coupling defining a first pressure chamber to exert a
pressure force against the retaining ring, and the housing and the
second coupling defining a second pressure chamber to exert a
pressure force against the subcarrier. In one embodiment, the
couplings are diaphragms. The invention also provides a retaining
ring having a special edge profile that assists in smoothing an
pre-compressing the polisihng pad to increase polisihng uniformity.
A method for polisihing and a semiconductor manufacture is also
provided.
Inventors: |
Moloney, Gerard S.;
(Milpitas, CA) ; Chin, Scott; (Palo Alto, CA)
; Geraghty, John J.; (Burlingame, CA) ; Dyson,
William JR.; (San Jose, CA) ; Dickey, Tanlin K.;
(Sunnyvale, CA) |
Correspondence
Address: |
FLEHR HOHBACH TEST
ALBRITTON & HERBERT LLP
Suite 3400
Four Embarcadero Center
San Francisco
CA
94111-4187
US
|
Assignee: |
Mitsubishi Materials
Corporation
Tokyo
JP
|
Family ID: |
22992002 |
Appl. No.: |
09/782818 |
Filed: |
February 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09782818 |
Feb 13, 2001 |
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09261112 |
Mar 3, 1999 |
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6231428 |
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Current U.S.
Class: |
451/72 ; 451/41;
451/56 |
Current CPC
Class: |
B24B 49/16 20130101;
B24B 37/30 20130101; B24B 37/32 20130101 |
Class at
Publication: |
451/72 ; 451/41;
451/56 |
International
Class: |
B24B 001/00; B24B
007/19; B24B 007/30 |
Claims
We claim:
1. A polishing apparatus comprising: a housing; a disc shaped
carrier for mounting a substrate to be polished; a retaining ring
substantially circumscribing said carrier for retaining said
substrate in a pocket formed by said retaining ring and a surface
of said carrier; a first flexible coupling attaching said retaining
ring to said carrier such that said retaining ring may translate in
at least one dimension and tilt about an axis relative to said
carrier; a second flexible coupling attaching said carrier to said
housing such that said carrier may translate in at least one
dimension and tilt about an axis relative to said housing; said
housing and said first flexible coupling defining a first chamber
in fluid communication with a first source of pressurized gas such
that when gas at a first pressure is communicated to said first
chamber a first force is exerted against said retaining ring; and
said housing and said second flexible coupling defining a second
chamber in fluid communication with a second source of pressurized
gas such that when gas at a second pressure is communicated to said
second chamber a second force is exerted against said
subcarrier.
2. The polishing apparatus in claim 1, wherein said translation and
tilt of said carrier is independent of said translation and tilt of
said retaining ring.
3. The polishing apparatus in claim 1, wherein said translation and
tilt of said carrier is coupled to a predetermined extent with said
translation and tilt of said retaining ring.
4. The polishing apparatus in claim 1, wherein said translation and
tilt of said carrier and said translation and tilt of said
retaining ring each have a component that is independent of the
other and a component that is dependent on the other.
5. The polishing apparatus in claim 4, wherein the extent to which
said translation and tilt components of said carrier and said ring
are coupled is dependent on material characteristics of said first
and second flexible couplings and geometry characteristics of said
attachments.
6. The polishing apparatus in claim 5, wherein said material
characteristics that affect the extent of coupling include
elasticity, stiffness, and spring constant; and said geometry
characteristics include distance between attachment locations
between said ring and said carrier and distance between attachment
locations between said carrier and said housing; the geometry of
the interface between said first and second diaphragms and adjacent
structures of said housing, said retaining ring, and said
carrier.
7. The polishing apparatus in claim 1, wherein said first pressure
and said second pressure are different pressures.
8. The polishing apparatus in claim 1, wherein said first pressure
and said second pressure are substantially equal pressures.
9. The polishing apparatus in claim 1, wherein said first pressure
and said second pressure are substantially equal pressures and the
force exerted on said retaining ring and on said carrier are
determined by the surface area of said retaining ring and said
carrier over which each said pressure is applied.
10. The polishing apparatus in claim 1, wherein said first pressure
and said second pressure may independently be positive pressure or
negative pressure (vacuum).
11. The polishing apparatus in claim 10, wherein the depth of a
pocket formed by a surface of said carrier and an inner cylindrical
surface of said retaining ring is established during a substrate
loading phase by said first pressure and said second pressure.
12. The polishing apparatus in claim 1, wherein said substrate
comprises a semiconductor wafer.
13. The polishing apparatus in claim 1, wherein said retaining ring
further comprises: a lower surface for contacting an external
polishing pad during polishing; an inner cylindrical surface
disposed adjacent to an outer circumferential surface of said
carrier and the periphery of a substrate mounting surface of said
carrier, said inner cylindrical surface and said carrier mounting
surface periphery forming a pocket for maintaining said substrate
during polishing; and a pad conditioning member disposed at the
lower outer radial portion of said retaining ring where said
retaining ring contacts said pad during polishing and defining a
shape profile transitioning between a first planar surface
substantially parallel to a plane of said polishing pad and a
second planar surface substantially perpendicular to said polishing
pad.
14. The polishing apparatus of claim 13, wherein said pad
conditioning member is characterized by presenting an angle
substantially between 15 degrees and substantially 25 degrees out
of parallel with respect to the nominal plane of said polishing
pad.
15. The polishing apparatus of claim 13, wherein said pad
conditioning member is characterized by presenting an angle
substantially between 18 degrees and substantially 22 degrees out
of parallel with respect to the nominal plane of said polishing
pad.
16. The polishing apparatus of claim 13, wherein said pad
conditioning member is characterized by presenting an angle
substantially 20 degrees out of parallel with respect to the
nominal plane of said polishing pad.
17. The polishing apparatus of claim 13, wherein said pad
conditioning member is characterized by presenting an angle
substantially 20 degrees out of parallel with respect to the
nominal plane of said polishing pad; and further characterized by
presenting a second angle of substantially 70 degrees out of
parallel with respect to the nominal plane of said polishing
pad.
18. The polishing apparatus of claim 13, further characterized in
that said portion presenting a substantially 20 degree angle
extends a distance of between 0.03 and 0.04 inches from said lower
planar retaining ring surface, and said portion presenting a
substantially 70 degree angle extends to at least a distance of
about 0.2 inches from said lower planar retaining ring surface.
19. The polishing apparatus of claim 13, wherein said pad
conditioning member is characterized by: presenting an angle
substantially between 15 degrees and substantially 25 degrees out
of parallel with respect to a nominal plane of said polishing pad;
and presenting a second substantially between 65 degrees and
substantially 75 degrees out of parallel with respect to said
nominal plane of said polishing pad.
20. The polishing apparatus in claim 1, wherein said first pressure
on said carrier is in the range between substantially 1.5 psi and
substantially 10 psi and the second pressure on said retaining ring
is in the range between substantially 1.5 psi and substantially 9.0
psi.
21. The polishing apparatus in claim 1, wherein said flexible
coupling comprises a diaphragm.
22. The polishing apparatus in claim 1, wherein said diaphragm is
formed from a material selected from the group consisting of:
metal, plastic, rubber, polymer, titanium, stainless-steel, carbon
fibre composite, and combinations thereof.
23. The polishing apparatus in claim 1, wherein said carrier is
formed from ceramic material.
24. A substrate retaining ring for a polishing machine, said
retaining ring comprising: a lower surface for contacting an
external polishing pad during polishing; an inner cylindrical
surface disposed adjacent to an outer circumferential surface of
said carrier and the periphery of a substrate mounting surface of
said carrier, said inner cylindrical surface and said carrier
mounting surface periphery forming a pocket for maintaining said
substrate during polishing; and a pad conditioning member disposed
at the lower outer radial portion of said retaining ring where said
retaining ring contacts said pad during polishing and defining a
shape profile transitioning between a first planar surface
substantially parallel to a plane of said polishing pad and a
second planar surface substantially perpendicular to said polishing
pad.
25. The substrate retaining ring of claim 24, wherein said pad
conditioning member is characterized by presenting an angle
substantially between 15 degrees and substantially 25 degrees out
of parallel with respect to the nominal plane of said polishing
pad.
26. The polishing apparatus of claim 24, wherein said pad
conditioning member is characterized by presenting an angle
substantially between 18 degrees and substantially 22 degrees out
of parallel with respect to the nominal plane of said polishing
pad.
27. The substrate retaining ring of claim 24, wherein said pad
conditioning member is characterized by presenting an angle
substantially 20 degrees out of parallel with respect to the
nominal plane of said polishing pad.
28. The substrate retaining ring of claim 24, wherein said pad
conditioning member is characterized by presenting an angle
substantially 20 degrees out of parallel with respect to the
nominal plane of said polishing pad; and further characterized by
presenting a second angle of substantially 70 degrees out of
parallel with respect to the nominal plane of said polishing
pad.
29. The substrate retaining ring of claim 24, further characterized
in that said portion presenting a substantially 20 degree angle
extends a distance of between 0.03 and 0.04 inches from said lower
planar retaining ring surface, and said portion presenting a
substantially 70 degree angle extends to at least a distance of
about 0.2 inches from said lower planar retaining ring surface.
30. The substrate retaining ring of claim 24, wherein said pad
conditioning member is characterized by: presenting an angle
substantially between 15 degrees and substantially 25 degrees out
of parallel with respect to a nominal plane of said polishing pad;
and presenting a second substantially between 65 degrees and
substantially 75 degrees out of parallel with respect to said
nominal plane of said polishing pad.
31. A method of planarizing a semiconductor wafer, said method
including: supporting a back-side surface of said wafer with a
wafer support subcarrier; applying a polishing force against said
support subcarrier to press a front surface of said wafer against a
polishing pad; restraining movement of said wafer from said support
subcarrier during polishing with a retaining ring circumferentially
disposed around a portion of said subcarrier and said wafer; and
applying a pad conditioning force against said retaining ring to
press a front surface of said retaining ring against said polishing
pad.
32. The method in claim 31, wherein said pad conditioning force is
applied independently of said polishing force.
33. The method in claim 31, wherein said pad conditioning force is
coupled to said polishing force.
34. The method in claim 31, wherein said pad conditioning force is
applied to a first area of said pad in a direction orthogonal to a
plane defined by said pad surface, to a second area of said pad in
a direction having a first fractional component orthogonal to said
plane and having a second fractional component parallel to said
plane.
35. An article of manufacture comprising a semiconductor wafer
polished according to the method of claim 31.
36. An article of manufacture comprising a semiconductor wafer
planarized according to the method of claim 34.
Description
FIELD OF THE INVENTION
[0001] The invention relates to chemical mechanical planarization
and polishing of substrates including silicon surfaces, metal
films, oxide films, and other types of films on a surface, more
particularly to a polishing head including a substrate carrier
assembly with substrate retaining ring, and most particularly to a
two-pressure chamber polishing head and method for silicon or glass
substrate polishing and chemical mechanical planarization of
various oxides, metals, or other deposited materials on the surface
of such substrates wherein the substrate carrier and substrate
retaining ring are separately controllable.
BACKGROUND
[0002] Sub-micron integrated circuits (ICs) require that the device
surfaces be planarized at their metal inter-connect steps. Chemical
mechanical polishing (CMP) is the technology of choice for
planarizing semiconductor wafer surfaces. The IC transistor packing
density has been doubled about every 18 months for some number of
years and there has been consistent effort to maintain this
trend.
[0003] There are at least two methods by which to increase the
packing density of transistors on a chip. The first method is to
increase the device or die size. This is not always the best
method, however, because as the die size increases, the die yield
per wafer may typically decrease. Since the defect density per unit
area is the constraint factor, the amount of defect-free dies per
area decreases as the die size increases. Not only will the yield
be lower, but the number of dies that can be stepped (printed) on
the wafer will also decrease. The second method is to shrink the
size of the transistor feature. Smaller transistors mean a higher
switching speed, which is an added benefit. By decreasing the
transistor size, more transistors and more logic functions or
memory bits can be packed into the same device area without
increasing die size.
[0004] Sub-half micron technology has been rapidly evolved into
sub-quarter micron technology in the past few years alone. The
number of transistors being fabricated on each chip has increased
enormously--from hundreds of thousands transistors per chip three
years ago to several million transistors per chip today. This
density is expected to increase even further in the near future.
The current solution to the challenge is to build layers upon
layers of inter-connect wiring with insulating (dielectric) thin
films in between. The wiring is also connectable vertically through
vias; to achieve all electrical paths as required by the integrated
circuit functions.
[0005] Inlaid metal line structure, using inlaid metal lines
embedded in insulating dielectric layers, allows for metal wiring
connections to be made on the same plane as well as on an up and
down direction through plasma etched trenches and vies in the
dielectric layer. Theoretically, these connection planes can be
built with as many layers on top of each other as desired, as long
as each layer is well planarized with CMP process. The ultimate
limit of the interconnect is formed by the connection resistance
(R) and the proximity capacitance (C). The so-called RC constant
limits the signal-to-noise ratio and causes the power consumption
to increase, rendering the chip non-functional. According to
industry projections, the number of transistors to be integrated on
a chip will be as many as one billion, and the number of layers of
interconnect will increase to up to nine layers or more.
[0006] To meet the predicted inter-connect requirements, the CMP
process and CMP tool performance would advantageously be improved
to achieve reduce the wafer edge exclusion due to over- and
under-polishing from 6 mm to less than 3 mm so that the physical
area from which large dies may be formed, and reduce polishing
non-uniformity by providing a polishing head that is able to apply
uniform and appropriate force across the entire surface of the
wafer during polishing. Current variations in film uniformities
after CMP, at the wafer edge (2-15 mm from the edge) result in lost
die yield in the outer edges of the wafer. This edge non-uniformity
is due to either over or under polishing near the wafer edge. By
providing a CMP polishing head with the ability to adjust the
amount of edge polishing to compensate for over or under polishing,
significant yield improvements can be achieved.
[0007] Integrated circuits are conventionally formed on substrates,
particularly silicon wafers, by the sequential deposition of one or
more layers, which layers may be conductive, insulative, or
semiconductive. These structures are sometimes referred to as the
multi-layer metal structures (MIM's) and are important relative to
achieving close-packing of circuit elements on the chip with the
ever decreasing design rules.
[0008] Flat panel displays such as those used in notebook
computers, personal data assistants (PDAs), cellular telephones,
and other electronic devices, may typically deposit one or more
layers on a glass or other transparent substrate to form the
display elements such as active or passive LCD circuitry. After
each layer is deposited, the layer is etched to remove material
from selected regions to create circuitry features. As a series of
layers are deposited and etched, the outer or topmost surface of
the substrate becomes successively less planar because the distance
between the outer surface and the underlying substrate is greatest
in regions of the substrate where the least etching has occurred,
and the distance between the outer surface and the underlying
substrate is least in regions where the greatest etching has
occurred. Even for a single layer, the non-planar surface takes on
an uneven profile of peaks and valleys. With a plurality of
patterned layers, the difference in the height between the peaks
and valleys becomes much more severe, and may typically vary by
several microns.
[0009] A non-planar upper surface is problematic respective of
surface photolithography used to pattern the surface, and
respective of layers that may fracture if deposited on a surface
having excessive height variation. Therefore, there is a need to
planarize the substrate surface periodically to provide a planar
layer surface. Planarization removes the non-planar outer surface
to form a relatively flat, smooth surface and involves polishing
away the conductive, semiconductive, or insulative material.
Following planarization, additional layers may be deposited on the
exposed outer surface to form additional structures including
interconnect lines between structures, or the upper layer may be
etched to form vias to structures beneath the exposed surface.
Polishing generally and chemical mechanical polishing (CMP) more
particularly are known methods for surface planarization.
[0010] The polishing process is designed to achieve a particular
surface finish (roughness or smoothness) and a flatness (freedom
from large scale topography). Failure to provide minimum finish and
flatness may result in defective substrates, which in turn may
result in defective integrated circuits.
[0011] During CMP, a substrate such as a semiconductor wafer, is
typically mounted with the surface to be polished exposed, on a
wafer carrier which is part of or attached to a polishing head. The
mounted substrate is then placed against a rotating polishing pad
disposed on a base portion of the polishing machine. The polishing
pad is typically oriented such that it's flat polishing surface is
horizontal to provide for even distribution of polishing slurry and
interaction with the substrate face in parallel opposition to the
pad. Horizontal orientation of the pad surface (the pad surface
normal is vertical) is also desirable as it permits the wafer to
contact the pad at least partially under the influence of gravity,
and at the very least interact in such manner that the
gravitational force is not unevenly applied between the wafer and
the polishing pad. In addition to the pad rotation, the carrier
head may rotate to provide additional motion between the substrate
and polishing pad surface. The polishing slurry, typically
including an abrasive suspended in a liquid and for CMP at least
one chemically-reactive agent, may be applied to the polishing pad
to provide an abrasive polishing mixture, and for CMP an abrasive
and chemically reactive mixture at the pad substrate interface.
Various polishing pads, polishing slurries, and reactive mixtures
are known in the art, and which is combination allow particular
finish and flatness characteristics to be achieved. Relative speed
between the polishing pad and the substrate, total polishing time,
and the pressure applied during polishing, in addition to other
factors influence the surface flatness and finish, as well as the
uniformity. It is also desirable that the polishing of successive
substrates, or where a multiple head polisher is used, all
substrates polished during any particular polishing operation are
polished to the same extent, including removal of substantially the
same amount of material and providing the same flatness and finish.
CMP and wafer polishing generally are well known in the art and not
described in further detail here.
[0012] In U.S. Pat. No. 5,205,082 there is described a flexible
diaphragm mounting of the sub-carrier having numerous advantages
over earlier structures and methods, and U.S. Pat. No. 5,584,751
provides for some control of the down force on the retaining ring
through the use of a flexible bladder; however, neither these
patents describe structure for direct independent control of the
pressure exerted at the interface of the wafer and retaining ring,
or any sort of differential pressure to modify the edge polishing
or planarization effects.
[0013] In view of the foregoing, there is a need for a chemical
mechanical polishing apparatus which optimizes polishing
throughput, flatness uniformity, and finish, while minimizing the
risk of contamination or destruction of any substrate.
[0014] In view of the above, there remains a need for a polishing
head that provides a substantially uniform pressure across the
substrate surface being polished, that maintains the substrate
substantially parallel to the polishing pad during the polishing
operation, and that maintains the substrate within the carrier
portion of the polishing head without inducing undesirable
polishing anomalies at the periphery of the substrate.
SUMMARY
[0015] The invention provides structure and method for achieving a
uniformly polished or planarized substrate such as a semiconductor
wafer including achieving substantially uniform polishing between
the center of the semiconductor wafer and the edge of the wafer. In
one aspect the invention provides a polishing apparatus including a
housing, a carrier for mounting a substrate to be polished, a
retaining ring circumscribing the carrier for retaining the
substrate, a first coupling attaching the retaining ring to the
carrier such that the retaining ring may move relative to the
carrier, a second coupling attaching the carrier to the housing
such that the carrier may move relative to the housing, the housing
and the first coupling defining a first pressure chamber to exert a
pressure force against the retaining ring, and the housing and the
second coupling defining a second pressure chamber to exert a
pressure force against the subcarrier. In one embodiment, the
couplings are diaphragms.
[0016] In another aspect, the invention provides structure and
method for a substrate (semiconductor wafer) retaining ring for a
polishing or planarization machine wherein the retaining ring
includes a lower surface for contacting a polishing pad during
polishing, an inner surface disposed adjacent to an outer surface
of the carrier and the periphery of a substrate mounting surface of
the carrier, the inner surface and the carrier mounting surface
periphery forming a pocket for maintaining the substrate during
polishing, and a pad conditioning member disposed at the lower
outer radial portion of the retaining ring where the retaining ring
contacts the pad during polishing and defining a shape profile
transitioning between a first planar surface substantially parallel
to a plane of the polishing pad and a second planar surface
substantially perpendicular to the polishing pad. In one embodiment
of the invention, the substrate retaining ring is characterized by
presenting an angle between about 15 degrees and about 25 degrees
out of parallel with respect to the nominal plane of said polishing
pad. In a different embodiment, the substrate retaining ring is
characterized by presenting an angle substantially 20 degrees out
of parallel with respect to the nominal plane of said polishing
pad.
[0017] In another aspect, the invention provides a method of
planarizing a semiconductor wafer including: supporting a back-side
surface of the wafer with a wafer support subcarrier, applying a
polishing force against the support subcarrier to press a front
surface of the wafer against a polishing pad, restraining movement
of the wafer from the support subcarrier during polishing with a
retaining ring circumferentially disposed around a portion of the
subcarrier and the wafer, and applying a pad conditioning force
against the retaining ring to press a front surface of the
retaining ring against the polishing pad. In one embodiment of the
inventive method, the pad conditioning force is applied
independently of said polishing force, while in a different
embodiment, the pad conditioning force is somewhat coupled to the
polishing force. In another alternative embodiment, the pad
conditioning force is applied to a first area of the pad in a
direction orthogonal to a plane defined by the pad surface, to a
second area of said pad in a direction having a first fractional
component orthogonal to the plane and having a second fractional
component parallel to the plane using a retaining ring having a
chamfered edge profile.
[0018] In another aspect, the invention provides a semiconductor
wafer polished or planarized according to the inventive method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagrammatic illustration showing an embodiment
of a multi-head polishing/planarization apparatus.
[0020] FIG. 2 is a diagrammatic illustration showing a simple
embodiment of the inventive two-chambered polishing head.
[0021] FIG. 3 is a diagrammatic illustration showing a simple
embodiment of the inventive two-chambered polishing head in FIG. 3
further illustrating at exaggerated scale the manner in which
linking elements (diaphragms) permit movement of the wafer
subcarrier and wafer retaining ring.
[0022] FIG. 4 is a diagrammatic illustration showing a sectional
assembly drawing of embodiments of portions of the carousel, head
mounting assembly, rotary unions, and wafer carrier assembly.
[0023] FIG. 5 is a diagrammatic illustration showing a more
detailed sectional view of an embodiment of the inventive wafer
carrier assembly.
[0024] FIG. 6 is a diagrammatic illustration showing an exploded
assembly drawing illustrating elements of the embodiment of the
wafer carrier assembly shown in FIG. 5.
[0025] FIG. 7 is a diagrammatic illustration showing a detailed
sectional view of a portion of the embodiment of the wafer carrier
assembly of FIG. 5.
[0026] FIG. 8 is a diagrammatic illustration showing a detailed
sectional view of a different portion of the embodiment of the
wafer carrier assembly of FIG. 5.
[0027] FIG. 9 is a diagrammatic illustration showing a plan view of
an embodiment of the inventive retaining ring.
[0028] FIG. 10 is a diagrammatic illustration showing a sectional
view of the embodiment of the inventive retaining ring in FIG.
9.
[0029] FIG. 11 is a diagrammatic illustration showing a detail of
the embodiment of the inventive retaining ring in FIG. 9.
[0030] FIG. 12 is a diagrammatic illustration showing a perspective
view of the embodiment of the inventive retaining ring in FIG.
9.
[0031] FIG. 13 is a diagrammatic illustration showing a sectional
view through a portion of the retaining ring in FIG. 9,
particularly showing the chamfered transition region at the outer
radial periphery of the ring.
[0032] FIG. 14 is a diagrammatic illustration showing an embodiment
of the inventive retaining ring adapter used in the polishing head
of FIG. 5.
[0033] FIG. 15 is a diagrammatic illustration showing an
alternative view of the retaining ring adapter in FIG. 14.
[0034] FIG. 16 is a diagrammatic illustration showing a sectional
view of the retaining ring adapter in FIG. 14.
[0035] FIG. 17 is a diagrammatic illustration showing a detail of
the manner of attaching the retaining ring to the retaining ring
adapter in sectional view.
[0036] FIG. 18 is a diagrammatic illustration showing a detail of
the flushing channels and orifices for clearing polishing slurry
from the ring area.
[0037] FIG. 19 is a diagrammatic illustration of a hypothesized
retaining ring polishing pad interaction for a retaining ring
having a square corner at the ring-pad interface.
[0038] FIG. 20 is a diagrammatic illustration of a hypothesized
retaining ring polishing pad interaction for a retaining ring
having the inventive multi-planar chamfered transition region at
the ring-pad interface.
[0039] FIG. 21 is a diagrammatic flow-chart illustration of an
embodiment of a wafer loading procedure.
[0040] FIG. 22 is a diagrammatic flow-chart illustration of an
embodiment of a wafer polishing procedure.
[0041] FIG. 23 is a diagrammatic flow-chart illustration of an
embodiment of a wafer unloading procedure.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0042] In FIG. 1, there is shown a chemical mechanical polishing or
planarization (CMP) tool 101, that includes a carousel 102 carrying
a plurality of polishing head assemblies 103 comprised of a head
mounting assembly 104 and the substrate (wafer) carrier assembly
106 (See FIG. 3). We use the term "polishing" here to mean either
polishing of a substrate 113 generally including semiconductor
wafer substrates, and also to planarization when the substrate is a
semiconductor wafer onto which electronic circuit elements have
been deposited. Semiconductor wafers are typically thin and
somewhat brittle disks having diameters nominally between 100 mm
and 300 mm. Currently 200 mm semiconductor wafers are used
extensively, but the use of 300 mm wafers is under development. The
inventive design is applicable to semiconductor wafers and other
substrates at least up to 300 mm diameter, and advantageously
confines any significant wafer surface polishing nonuniformities to
no more than about the so-called 2 mm =exclusion zone at the radial
periphery of the semiconductor disc, and frequently to an annular
region less than about 2 mm from the edge of the wafer.
[0043] A base 105 provides support for the other components
including a bridge 107 which supports and permits raising and
lowering of the carousel with attached head assemblies. Each head
mounting assembly 104 is installed on carousel 102, and each of the
polishing head assemblies 103 are mounted to head mounting assembly
104 for rotation, the carousel is mounted for rotation about a
central carousel axis 108 and each polishing head assembly 103 axis
of rotation 111 is substantially parallel to, but separated from,
the carousel axes of rotation 108. CMP tool 101 also includes the
motor driven platen 109 mounted for rotation about a platen drive
axes 110. Platen 109 holds a polishing pad 135 and is driven to
rotate by a platen motor (not shown). This particular embodiment of
a CMP tool is a multi-head design, meaning that there are a
plurality of polishing heads for each carousel; however, single
head CMP tools are known, and inventive head assembly 103, retainer
ring 166, and method for polishing may be used with either a
multi-head or single-head type polishing apparatus.
[0044] Furthermore, in this particular CMP design, each of the
plurality of heads are driven by a single head motor which drives a
chain (not shown), which in turn drives each of the polishing heads
103 via a chain and sprocket mechanism; however, the invention may
be used in embodiments in which each head 103 is rotated with a
separate motor. The inventive CMP tool also incorporates a rotary
union 116 providing five different gas/fluid channels to
communicate pressurized fluids such as air, water, vacuum, or the
like between stationary sources external to the head and locations
on or within the wafer carrier assembly 106.
[0045] In operation, the polishing platen 109 with adhered
polishing pad 135 rotates, the carousel 102 rotates, and each of
the heads 103 rotates about their own axis. In one embodiment of
the inventive CMP tool, the carousel axis of rotation is off-set
from the platen axis of rotation by about one inch. The speed at
which each component rotates is selected such that each portion on
the wafer travels substantially the same distance at the same
average speed as every other point on a wafer so as to provide for
uniform polishing or planarization of the substrate. As the
polishing pad is typically somewhat compressible, the velocity and
manner of the interaction between the pad and the wafer where the
wafer first contacts the pad is a significant determinant of the
amount of material removed from the edge of the wafer, and of the
uniformity of the polished wafer surface.
[0046] A polishing tool having a plurality of carousel mounted head
assemblies is described in U.S. Pat. No. 4,918,870 entitled
Floating Subcarriers for Wafer Polishing Apparatus; a polishing
tool having a floating head and floating retainer ring is described
in U.S. Pat. No. 5,205,082 Wafer Polisher head Having Floating
Retainer Ring; and a rotary union for use in a polisher head is
described in U.S. Pat. No. 5,443,416 and entitled Rotary Union for
Coupling Fluids in a Wafer Polishing Apparatus; each of which are
hereby incorporated by reference.
[0047] The inventive structure and method provide a two-chambered
head having a disc shaped subcarrier for mounting a substrate (i.e.
semiconductor wafer) 113 and an annular shaped retaining ring 166
disposed coaxially with, and fitting around both, the lower portion
of the subcarrier 160 and around the edge of the wafer substrate
113 to maintain the substrate directly underneath and in contact
with the subcarrier 160 and a polishing pad surface 135 which
itself is adhered to the platen 109. Maintaining the wafer directly
underneath the subcarrier is important for uniformity as the
subcarrier imposes a downward polishing force onto the back side of
the wafer to force the front side of the wafer against the pad. One
of the chambers (P2) 132 is in fluid communication with carrier 160
and exerts a downward polishing pressure (or force) during
polishing on the subcarrier 160 and indirectly of the substrate 113
against the polishing pad 135 (referred to as "subcarrier force" or
"wafer force"). The second chamber (P1) 131 is in fluid
communication with the retaining ring 166 via a retaining ring
adapter 168 and exerts a downward pressure during polishing of the
retaining ring 166 against the polishing pad 135 (referred to as
"ring force"). The two chambers 131, 132 and their associated
pressure/vacuum sources 114, 115 permit control of the pressure (or
force) exerted by the wafer 113 and separately by the retaining
ring 166 against the polishing pad surface 135.
[0048] While in one embodiment of the invention the subcarrier
force and ring force are selected independently, the structure can
be adapted to provide greater and lesser degrees of coupling
between the ring force and subcarrier force. By making appropriate
choices as the properties of a linkage between a head housing
supporting structure 120 and the subcarrier 160, and between the
subcarrier 160 and the ring 166, degrees of independence in the
range from independent movement of the subcarrier and ring to
strong coupling between the subcarrier and ring can be achieved. In
one embodiment of the invention, the material and geometrical
characteristics of linking elements formed in the manner of
diaphragms 145, 162 provide optimal linking to achieve uniform
polishing (or planarization) over the surface of a semiconductor
wafer, even at the edges of the substrate.
[0049] In another embodiment, the size and shape of the retaining
ring 166 is modified compared to conventional retaining ring
structures in order to pre-compress and/or condition the polishing
pad 135 in a region near the outer peripheral edge of the substrate
113 so that deleterious affects associated with the movement of
substrate 113 across pad 135 from one area of the pad to another
are not manifested as non-linearities on the polished substrate
surface. The inventive retaining ring 166 acts to flatten out the
pad 135 at the leading and training edges of motion so that before
the advancing substrate contacts a new area of the pad, the pad is
essentially flat and coplanar with the substrate surface; and, as
contact between the substrate and the pad is about to end, the pad
is kept flat and coplanar with the polished surface of the
substrate. In this way, the substrate always experiences a flat,
precompressed, and substantially uniform polishing pad surface.
[0050] The retaining ring pre-compresses the polishing pad before
it travels across the wafer surface. This results in the whole
wafer surface seeing a polishing pad with the same amount of
pre-compression which results in a move uniform removal of material
across the wafer surface. With independent control oc the retaining
ring pressure it is possible to modulate the amount of polishing
pad pre-compression, thus influencing the amount of material
removed from the wafer edge. Computer control, with or without
feedback, such as using end point detection means, can assist in
achieving the desired uniformity.
[0051] We first turn our attention to a simple first embodiment of
the inventive two-chambered polishing head 100 shown in FIG. 2 to
illustrate the manner in which selected aspects of the invention
operate. In particular we show and describe the manner in which
pressure to the retaining ring assembly (including retaining ring
adapter 168 and retaining ring 166) and the carrier 160 are
effectuated and controlled. We will then describe other aspects of
the invention relative to somewhat more elaborate alternative
embodiments that include additional optional, but advantageous
features.
[0052] Turret mounting adapter 121 and pins 122, 123 or other
attachment means facilitate alignment and attachment or mounting of
housing 120 to a spindle 119 mounted for rotation relative to
carousel 102, or in single head embodiments, to other supporting
structure, such as an arm that moves the head across the surface of
the pad while the head and pad are rotating. Housing 120 provides a
supporting structure for other head components. Secondary diaphragm
145 is mounted to housing 120 by spacer ring 131 to separate
secondary diaphragm from housing 120 to allow a range of vertical
and angular motion of the diaphragm and structures attached thereto
(including carrier 160) relative to a nominal secondary diaphragm
plane 125. (The primary and secondary diaphragms also permit some
small horizontal movement as a result of the angular tilt alone or
in conjunction with vertical translation that is provided to
accommodate angular variations at the interface between the
carrier-pad and retaining ring-pad interfaces, but this horizontal
movement is typically small compared to the vertical movement.)
[0053] Spacer ring 131 may be formed integrally with housing 120 in
this embodiment and provide the same function; however, as will be
described in an alternative embodiment (See for example, FIG. 5)
spacer ring 131 is advantageously formed from a separate piece and
attached to the housing with fasteners (such as screws) and
concentric O-ring gaskets to assure the attachment is air- and
pressure-tight.
[0054] Carrier 160 and retaining ring assembly 165 (including
retaining ring adapter 168 and retaining ring 166) are similarly
attached to primary diaphragm 162 which itself is attached to a
lower portion of housing 162. Carrier 160 and retaining ring 166
are thus able to translate vertically and tilt to accommodate
irregularities in the surface of the pad and to assist in
flattening the polishing pad where the pad first encounters
retaining ring 166 proximate the edge of the wafer 113.
Generically, this type of diaphragm facilitated movement has been
referred to as "floating," the carrier and retaining ring as
"floating carrier" and "floating retaining ring", and a head
incorporating these elements has been referred to as a "floating
head" design. While the inventive head utilizes "floating"
elements, the structure and method of operation are different than
that known in the art heretofore.
[0055] Flange ring 146 connects secondary diaphragm 145 to an upper
surface of carrier 160 which itself is attached to primary
diaphragm 162. Flange ring 146 and subcarrier 160 are effectively
clamped together and move as a unit, but retaining ring assembly
167 is mounted only to the primary diaphragm and is free to move
subject only to constraints on movement imposed by the primary and
secondary diaphragms. Flange ring 146 links primary diaphragm 162
and secondary diaphragm 145. Frictional forces between the
diaphragm and the flange ring and subcarrier assist in holding the
diaphragm in place and in maintaining a tension across the
diaphragm. The manner in which primary and secondary diaphragms
permit translational and angular movement of the carrier and
retaining ring is further shown by the diagrammatic illustration in
FIG. 3, which shows a greatly exaggerated condition in which the
nominal planar conformation of each diaphragm 145, 162 is altered
to permit the translational and angular degrees of freedom. This
exaggerated degree of diaphragm flexation illustrated in the
figure, especially in angular orientation, would not be expected to
be encountered during polishing, and the vertical translation would
typically be experienced only during wafer loading and unloading
operations. In particular, secondary diaphragm 145 experiences some
flexing or distortion in first and second flexation regions 172,
173 in the span between attachment to seal ring 131 and flange ring
146; and primary diaphragm experiences different flexing or
distortion at third, fourth, fifth, and sixth flexation regions
174, 175, 178, 179 where it spans its attachments to housing 120
and carrier 160.
[0056] In this description, the terms "upper" and "lower"
conveniently refer to relative orientations of structures when the
structure being described is used in its normal operating state,
typically as shown in the drawings. In the same manner, the terms
"vertical" and "horizontal" also refer to orientations or movements
when the invention or an embodiment or element of an embodiment is
used in its intended orientation. This is appropriate for a
polishing machine, as wafer polishing machines of the type known by
the inventors provide for a horizontal polishing pad surface which
fixes the orientations of other polisher components.
[0057] We next turn our attention to the alternative and somewhat
more sophisticated embodiment of the inventive polishing head
assembly 103 illustrated in FIG. 4. Particular emphasis is directed
toward wafer carrier assembly 106; however, the rotary union 116
and head mounting assembly 104 components of the polishing head
assembly 103 are also described. We note that although some
structures in the first embodiment of the invention (See FIG. 2)
have somewhat different structures from those illustrated for this
alternative embodiment (See FIG. 4) identical reference numbers
have been retained so that the similar functions provided by the
elements in the two embodiments is made clear.
[0058] Polishing head assembly 103 generally includes a spindle 119
defining a spindle axis of rotation 111, a rotary union 116, and
spindle support means 209 including bearings that provide means for
attaching spindle 119 into a spindle support which is attached to
the bridge 107 in a manner that permits rotation of the spindle.
These spindle support structures are known in the mechanical arts
and not described here in any detail. Structure within the spindle
is illustrated and described as that structure pertains to the
structure and operation of rotary union 116.
[0059] Rotary union 116 provides means for coupling pressurized and
non-pressurized fluids (gases, liquids, vacuum, and the like)
between a fluid source, such as vacuum source, which is stationary
and non-rotating and the rotatable polishing head wafer carrier
assembly 106. The rotary union is adapted to mount to the
non-rotatable portion of the polishing head and provides means for
confining and continually coupling a pressurized or non-pressurized
fluid between a non-rotatable fluid source and a region of space
adjacent to an exterior surface of the rotatable spindle shaft 119.
While a rotary union is specifically illustrated in the embodiment
of FIG. 4, it will be understood that rotary unions are applicable
to the other embodiments of the invention.
[0060] One or more fluid sources are coupled to rotary union 116
via tubing and control valve (not shown). Rotary union 116 has a
recessed area on an interior surface portion which defines a
typically cylindrical reservoir 212, 213, 214 between interior
surface portion 216 of rotary union 116 and the exterior surface
217 of spindle shaft 119. Seals 218 are provided between the
rotatable shaft 119 and the nonrotatable portion of the rotary
union to prevent leakage between the reservoirs and regions
exterior to the reservoirs. Conventional seals as are known in the
mechanical arts may be used. A bore or port 201 is also provided
down the center of the spindle shaft to communicate a fluid via a
rotatable coupling.
[0061] Spindle shaft 119 has five passageways extending from the
exterior shaft surface and the top of the shaft to a hollow bores
within the spindle shaft. Due to the particular sectional view in
FIG. 4, only three of the five passageways are visible in the
drawing. From each bore the vacuum or other pressurized or
non-pressurized fluids are communicated via couplings and or tubing
within the wafer carrier assembly 106 to the location at which the
fluid is required. The precise location or existence of the
couplings are an implementation detail and not important to the
inventive concept except as described hereinafter. These recited
structures provide means for confining and continually coupling one
or more pressurized fluids between the region adjacent to the
exterior surface of the rotatable shaft and the enclosed chamber,
but other means may be used. A rotary union that provides fewer
channels than that in this particular embodiment of the invention
is described in U.S. Pat. No. 5,443,416 and entitled Rotary Union
for Coupling Fluids in a Wafer Polishing Apparatus, incorporated
herein by reference.
[0062] We now describe wafer carrier assembly 106 with respect to
FIG. 5 showing a sectional view through "Section A-A" of wafer
carrier assembly 106, and FIG. 6 showing an exploded assembly
diagram of wafer carrier assembly 106. It is clear from FIG. 6 that
wafer carrier assembly 106 has a high degree of symmetry about a
central axis; however, it will be observed that not all elements
are symmetrical with respect to the locations of holes, orifices,
fitting, notches, and the like detailed features. Rather than
describing wafer carrier assembly 106 with respect to any single
diagram, we refer to the combination of FIG. 5 (side-view through
Section A-A), FIG. 6 (exploded assembly drawing), FIG. 7 (enlarged
sectional view of right-hand side of FIG. 5), and FIG. 8 (enlarged
sectional view of left-hand side of FIG. 5) which show the
constituent elements from somewhat different perspectives and make
clearer the structure and operation of each element.
[0063] Chemical mechanical polishing as well as the characteristics
of polishing pads, slurry, and wafer composition, are well known
and not described with any degree of specificity except as is
necessary to understand the invention.
[0064] Functionally, wafer carrier assembly 106 provides all of the
structure needed to mount and hold a substrate 130 such as a
semiconductor wafer during the polishing operation. (Note that this
invention is applicable to polishing substrates other than
semiconductor wafers.) Carrier assembly 106 provides vacuum at one
surface of a wafer subcarrier through holes or apertures 147 for
holding the wafer during a period time between loading the wafer
and initiation polishing. It also provides a downward polishing
pressure on the wafer through the wafer subcarrier and a separate
downward pressure on a retaining ring for maintaining the wafer
within a pocket and for interacting with the polishing pad to
reduce or eliminate polishing nonuniformity near the edge over the
wafer. Wafer carrier assembly 106 also provides sources of fluids
such as the deionized water (DI water), pressurized air, and vacuum
at several chambers, orifices, and surfaces is described in greater
detail hereinafter. The wafer carrier assembly is particularly
important in that it provides a diaphragm mounted subcarrier and
retaining ring assembly which itself includes a retaining ring
adapter and a retaining ring. The diaphragm mounted components and
their structural and functional relationships with other elements
and chambers provide several of the advantageous features of the
invention.
[0065] The upper housing 120 is mounted to mounting adapter 121 via
four socket head screws, which in turn is mounted to the lower
portion of head mounting assembly 104 via screws and positioned by
first and second pins 122,123. Upper housing 120 provides a stable
member to which other elements of the wafer carrier assembly may be
mounted as described herein. Housing seal ring 129 is a generally
circular element which acts to separate the first pressure chamber
(P1) 131 from a second pressure chamber (P2) 132. The pair of
O-rings 137,139 are disposed within separate channels machined into
an upper surface of housing seal ring 131 which when attached to an
interior surface of upper housing 120 provides a leak-proof fluid
and pressure seal between housing seal ring 131 and upper housing
120. The pressure in first pressure chamber 131 is operative to
influence the downward acting pressure on retaining ring assembly
134 and its interaction with polishing pad 135. Pressure in second
pressure chamber 132 is operative to influence the downward acting
pressure on subcarrier 136 which in turn provides the polishing
pressure exerted between the lower surface of wafer 138 and
polishing pad 135. Optionally, a polymeric or other insert 161 may
be used between lower surface of subcarrier 106 in the upper, or
backside, surface of wafer 138. Internal structure within wafer
carrier assembly 106 provides both a degree of independence between
the pressure and/or movement of retaining ring assembly 134 and
subcarrier 136.
[0066] We note that one or more fittings 141 are provided to
communicate pressurized air from a location or source 114 external
to first pressure chamber 131 into the chamber, and one or more
fittings 142 are provided to communicate pressurized air from a
second external source or location 115 to second pressure chamber
132 in like manner. These fittings 141,142 are connected via
appropriate tubing to channels within head mounting assembly 104
and rotary union 116, and appropriate control circuitry to provide
the desired pressure levels. The manner and sequence in which
pressures, vacuum, and/or fluids are communicated are described
hereinafter.
[0067] The locking ring 144 is mounted to the lower surface of
housing seal ring 131 via eighteen screws and attaches secondary
diaphragm 145 between housing seal ring 131 and locking ring 144 by
virtue of sandwiching or clamping secondary diaphragm between the
two structures. Both housing seal ring 131 and locking ring 144 as
well as the portion of secondary diaphragm 145 clamped between
housing seal ring 131 and locking ring 144 are maintained in fixed
position relative to upper housing 120. The portion of secondary
diaphragm 145 lying radially interior to an inner radius of housing
seal ring 131 is clamped on a lower surface by an upper surface of
inner flanged ring 146 and on an upper surface by a lower surface
of inner stop ring 148. The inner flanged ring and inner stop ring
are attached by fastening means such as socket head cap screws
149.
[0068] Although housing seal ring 131, locking ring 144, and the
portion of secondary diaphragm 145 clamped between these two
structures maintain a fixed location relative to the surface of
upper housing 120, both inner flanged ring 146 and inner stop ring
148 being suspended from secondary diaphragm 145 are at least
somewhat free to move upward and downward relative to polishing pad
135 and upper housing 120, and to some degree, to change angular
orientation or tilt relative to polishing pad 135 and upper housing
120. The ability of this structure to move vertically upward in
downward and to tilt to alter its angular orientation permits
structures attached to it such as subcarrier 136, wafer 138, and
retaining ring assembly 134 to float on the surface of polishing
pad 134.
[0069] The nature of the material from which secondary diaphragm
145 is fabricated, as well as secondary diaphragm thickness (Td),
the distance between the clamped portion of secondary diaphragm 145
between the housing seal ring and the locking ring with respect to
the clamped portion of secondary diaphragm 145 between inner
flanged ring 146 and inner stop ring 148, as well as the physical
gap were separation between first vertical edges 151 of inner
flanged ring 146 and second vertical surfaces 152 of locking ring
144 adjacent to the first vertical edges 151 influence the amount
of vertical movement and the amount of tilt or angular motion.
These properties provide an effective spring constant of the
diaphragm. Although the primary and secondary diaphragms in
embodiments of the invention described here are formed from the
same material, in general, different materials may be used.
[0070] In one embodiment of invention adapted to mounting 200
millimeter (mm) semiconductor wafers, the diaphragm is made from
0.05 inch thick BUNA N with Nylon material made by INTERTEX. This
material has internal fibers that provide strength and stiffness
while also providing the desired degree of elasticity. Those
workers having ordinary skill in the art will appreciate in light
of description provided here, that different dimensions and
materials may be used to accomplish the same were similar
operation. For example, a thin metallic sheet or membrane may be
used for secondary diaphragm 145 so long as the thin metallic
membrane provides sufficient elasticity so that it can be deflected
vertically to respond to pressured applied to it and sufficient
angular movement so that it can maintain contact with the pad
during a polishing operation. In some instances, a flat sheet of
material may not in and of itself possess sufficient elasticity;
however, by forming the sheet in an appropriate manner such as with
corrugated annular grooves, bellows, or the like, a metal linking
element may provide alternative structures for the diaphragms
described here. Composite materials may also be used to provide the
desired properties. The relationship between the clamped and
un-clamped portion of secondary diaphragm 145 and the separation
between locking 144 and inner flanged ring 146 are shown in greater
detail in FIGS. 7 and 8.
[0071] Inner stop ring 148, in addition to clamping inner flanged
ring 146 to secondary diaphragm 145 provides a movement limit stop
function to prevent excessive upward movement of inner stop ring
148, diaphragm 145, inner flanged ring 146, and structures attached
thereto, from moving excessively upward into recess 152 within
upper housing 120. In one embodiment of the invention, inner stop
ring 148 and attached structures are able to move about 0.125
inches upward from a nominal position in which diaphragm 145 is
planar before a stop contact surface 153 of inner stop ring 148
contacts an opposing contact surface 154 of housing seal ring 131,
and about 0.10 inches downward from the nominal position, for a
total travel distance of about 0.25 inches. Only a portion of this
upward and downward (vertical) range of motion is needed during
actual polishing; the remainder being used to extend the carrier
beyond the bottom edge of the retaining ring during wafer
(substrate) loading and unloading operations. The ability to
project the edge of the subcarrier 160 beyond the lower edge of the
retaining ring is advantageous and facilitates the loading and
unloading operations.
[0072] The vertical range of motion is limited by mechanical stops
rather than by the diaphragm material. The use of stops prevents
unnecessary forces on the diaphragm when the carrier/wafer is not
in contact with the pad, such as during loading and unloading
operations, and during maintenance, or when powered-off that could
in the long-term stretch or distort the diaphragm. The inventive
structure also provides a carrier head assembly having an
automatically self-adjusting wafer mounting pocket depth.
[0073] Subcarrier 160 is mounted to a lower surface 156 of inner
flanged ring 146 by attachment means such as socket head cap screws
157 thereby effectively hanging subcarrier 160 from secondary
diaphragm 145 (supported by mechanical stops on the stop rings when
at the lower limit of its vertical range of motion, and prevented
from moving excessively upward by a second set of mechanical stops)
and providing the subcarrier with be vertical and angular motion
already described. Primary diaphragm 162 is clamped between a
circumferential ring of inner flanged ring 146 and attached to
upper surface 163 of subcarrier 160 by socket head cap screws 157
near the edge of the subcarrier. Subcarrier 160 being formed other
a nonporous ceramic material in at least one embodiment, is fitted
with stainless-steel inserts to receive the threaded portions of
screws 157.
[0074] We now describe aspects of retaining ring assembly 134
before describing important aspects of the interaction among
retaining ring 134, subcarrier 136, and primary diaphragm 162.
Retaining ring assembly 167 includes a retaining ring 166 and a
retaining ring adapter 168. In one embodiment, retaining ring 166
is formed from Techtron.TM. PPS (Polyphenylene Sulfide). Retaining
ring adapter 168 mounts to a lower surface 170 of outer stop ring
171 with primary diaphragm 162 clamped their between. Retaining
ring 166 is formed of TECHTRON material and is attached to
retaining ring adapter 168 via socket head screws through the
primary diaphragm and outer stop ring. A chamfered portion 180 of
retainer ring 166 at its outer radius advantageously reduces edge
polishing non-linear areas which are typically encountered using
conventional polishing tools. Outer stop ring 169 is co-axially
mounted with respect to inner flanged ring 146 but at a larger
radial distance from the center of the wafer carrier assembly 106,
but is neither mounted to inner flanged ring 146 nor to any other
elements except retaining ring adapter 168 and primary diaphragm
162, except that both outer stop ring 169 a and retaining ring
assembly 134 are coupled together by primary diaphragm 162. The
nature of this coupling is important to providing mechanical
properties that contribute to the polishing benefits provided by
this invention. Structures contributing to this coupling are
illustrated in a larger scale and greater detail in FIGS. 7 and
8.
[0075] We now describe the structure and overall operation of
primary diaphragm 162 and a manner in which it is attached to
subcarrier 160 and retaining ring assembly 134. We also describe
details of the wafer carrier assembly that contribute to its
ability to reduce non-linear areas, often referred to as "ringing",
at the edges of the polished wafer. First, it should be understood
that primary diaphragm 162 should have stiffness with elasticity so
that the coupling between pressure applied to subcarrier 160 and
the separate pressure applied to retaining ring 166, and the
movement of the subcarrier and retaining ring as a result of these
pressures and the counter-acting upward force of polishing pad 135
falls within the appropriate range. By this we mean essentially
that the movement of the retaining ring and of the subcarrier
should be independent within some range of motion, but at the same
time in some embodiments providing some coupling between the
motions all of the retaining ring and the subcarrier.
[0076] The desired degree of coupling is affected by several
factors, including: (i) controlling the span of primary diaphragm
162 between third clamped region 182 (between subcarrier 160 and
inner flanged ring 146) and fourth clamped region 183 (between
retaining ring adapter 168 and outer stop ring 169); (ii)
controlling the thickness and material properties of primary
diaphragm 162; (iii) controlling the geometry of the surfaces that
interact with the diaphragm 162 in the span region; (iv)
controlling the distance between opposing vertical surfaces 185 of
subcarrier 160, vertical surface 186 of retaining ring adapter 168,
and vertical surface 187 of retaining ring 166; and (v) controlling
the distance or clearance between surface 188 of retaining ring
adapter 168 and a vertical surface of 190 of lower housing 122, and
between a vertical surface 189 of retaining ring 166 and that same
vertical surface 190 of lower housing 122. By controlling these
factors both vertical motion and angular motion are allowed to
occur, but without excessive movement that might cause binding of
the retaining ring either against subcarrier 160 or lower housing
122.
[0077] In one embodiment of the invention, the distance d1 between
the subcarrier and the retaining ring adapter is 0.050 inches, the
distance d2 between the subcarrier and the retaining ring is 0.010
inches, the distance d3 between the retaining ring adapter and a
lower housing is about 0.5 inches, and the distance d4 between the
retaining ring and lower housing is 0.015 inches. These
relationships are illustrated in FIG. 7. Of course those workers
having ordinary scale in the art will appreciate that these
dimensions are exemplary and that other dimensions and
relationships may be provided to accomplish the same functionality.
In particular, one might expect that each of these dimensions might
be modified by up to about 30 percent or more and still provide
comparable operation, even if not optimal operation. Greater
variations of dimensional tolerances would likely provide an
operational but suboptimal apparatus.
[0078] We also note in the embodiment illustrated in FIGS. 7 and 8,
that outer radial portion of subcarrier 160 adjacent to spanning
portion of primary diaphragm 162 forms a substantially right angle
with vertical surface 185; however, the opposing vertical surface
of the retaining ring adapter has a beveled portion at the opposing
corner 194. Maintaining a corner having about a square (90 degree)
corner has been found to be beneficial for preventing subcarrier
binding with the retaining ring or the retaining ring adapter.
Furthermore, providing a slight bevel or chamfer 194 on the
adjacent surface of retaining ring adapter 168 has been found to
beneficial for retaining ring mobility without binding, but it has
been observed that if the bevel is too great, then some undesired
binding may occur. While this combination has been found to have
certain advantages, those workers having ordinary skill the art
will appreciate that other variations which facilitate smooth
motion control without binding of the adjacent components.
[0079] Further advantages of the invention have been realized by
providing a particular shape profile at the outer or radial surface
195 of retaining ring 166 in what will be referred to as a
transition region 206. Conventionally, retaining rings if provided
at all, have been formed with a substantially vertical outer wall
surface either because it provided a favorable surface profile to
slide against a mating surface such as the equivalent of inner
radial wall surface of lower housing 122, or because no thought was
given to the importance of the profile of the edge and a default
vertical profile was used. In one embodiment of the invention, the
retaining ring 166 has shape profile illustrated in FIGS. 9-13
which show various aspects of the retaining ring at different
levels of detail. FIG. 10 is a shows a sectional view of the
embodiment of the retaining ring in FIG. 9, while FIG. 11 shows an
detail, and FIG. 12 provides a perspective view of the retaining
ring. FIG. 13 is a diagrammatic illustration showing a sectional
view through a portion of the retaining ring particularly showing
the chamfered transition region at the outer radial periphery of
the ring.
[0080] For this embodiment of the retaining ring, a lower surface
201 which during polishing contacts polishing pad 135, transitions
through two beveled surfaces 202, 203 to a substantially vertical
surface 204 which in operation opposes a substantially parallel
vertical surface 189 on lower housing 122, though a clearance gap
is provided so as to eliminate binding. Surface 204 is
substantially orthogonal to upper retaining rig surface 205, and
upper surface 205 is substantially parallel to lower surface 201.
Desirably, during manufacture of the wafer carrier assembly, an
assembly fixture is used to maintain alignment of the constituent
parts, and shims are used to set the clearance gap and other
spacings between the ring 166 and the subcarrier 160 and housing
120, 122.
[0081] It has been determined empirically, that providing that this
transition region 206 substantially improves the qualities of the
edges of the polished wafer by eliminating nonlinearities in the
polishing. These nonlinearities typically appear as troughs and
peaks (waves or rings) within about three to five millimeters or
more from the outer edge of the wafer. Without benefit of theory,
the nature of this transition region 206 is thought to be important
because the retaining ring in addition to holding the wafer in a
pocket against the subcarrier during polishing operation also acts
to press or flatten the polishing pad just prior to that portion of
the pad contacting the wafer when the retaining ring is at the
leading edge of motion and to expand of the region over which the
pad is flat when that portion all of the retaining ring is a
trailing edge portion of the wafer. A fact, the retaining ring
maintains surface coplanarity with and around the wafer so that any
conditions that cause the polishing pad 135 to buckle or distort,
the accumulation of polishing slurry at the leading edge, or other
non-linear or non-coplanar effects, occur outside of or under the
retaining ring and not under or adjacent to the edge of the
wafer.
[0082] Is also been determined that the particular retaining ring
geometry in the transition region 206, that is the optimal angles
for the transition region of .alpha.1=20 degrees, .alpha.2=20
degrees, and .alpha.3=90 degrees, is optimal for a multi-head
polishing apparatus and for a particular combination of polishing
pad 135, a polishing pad rotational speed of about 30 revolutions
per minute (RPM), a wafer carrier assembly rotational speed of
about 26 RPM, 200 mm diameter silicon wafers, a polishing pressure
of for example, about five pounds per square inch (5 psi), and a
TECHTRON material retaining ring. In this multi-head carousel based
polisher, the effective linear speed of the ring across the surface
of the pad is about 80-200 feet/min. Polishing pressures may be
varied over a greater range to achieve the desired polishing
effect. For example, the pressure on the subcarrier is typically in
the range between about 1.5 psi and about 10 psi and the pressure
on the retaining ring is typically in the range between about 1.5
psi and about 9.0 psi, though the pressure on the retaining ring
can be the same as the pressure on the subcarrier. While the
invention is not limited to any particular polishing pad types, one
polishing pad useful for chemical mechanical polishing or
planarization with the inventive head is the Rodel.RTM. CR
IC1400-A4 (Rodel Part No. P05695, Product Type IC1400, K-GRV, PSA).
This particular pad 135 has a nominal 35.75 inch diameter,
thickness range between about 2.5 mm and about 2.8 mm, deflection
of between about 0.02 mm and about 0.18 mm, compressibility of
between about 0.7 and about 6.6 percent, and rebound of about 46
percent (all measured with the RM-10-27-95 test method. Another
alternative is the Rodel CR IC1000-A4, P/V/SUBA type pads (Rodel
Part No. P06342).
[0083] Retaining ring has a thickness of about 0.25 inches and the
20 degree bevel portion 202 at the lower surface of the ring
extends upward about 0.034 inches and the vertical portion 204
extends about 0.060 inches before meeting the second beveled
segment 203. These exemplary dimensions are illustrated in the
drawing. For this particular combination of variables, it has been
determined empirically that these angles are somewhat sensitive to
about plus or minus two degrees for optimal performance; however,
it is expected that somewhat greater range, for example from at
least about plus or minus four degrees about the angles given
provides useful results. However; it is noted that while the
principal of providing a transition region for the retaining ring
is a significant determining factor in achieving uniform polishing
particularly at the edges of the wafer, the actual shape of this
transition region may require tuning to particular physical
parameters associated with the polishing operation. For example,
use of different polishing pad's (particularly if they are of a
different thickness, compensability, resiliency, or friction
coefficient), different platen rotational speed, different carousel
rotational speed, different wafer carrier assembly rotational
speed, and even different polishing slurry may suggest an
alternative transition region geometry for optimal results.
Fortunately, once a CMP polishing tool is set up, these parameters
normally do not change, or can be adjusted in accordance with
standard quality control procedures performed during CMP toll
setup.
[0084] For single head polishers (including for example, polishers
of the type wherein the polishing pad rotates, the head rotates,
and the head is driven to oscillate back and forth on a linear
reciprocating motion) the same parameters are expected to pertain
but the effective linear speed of the leading edge of the retaining
ring across the pad will be a pertinent parameter rather than the
combination of polishing pad speed, carousel speed, and head
speed.
[0085] In one embodiment of the invention pertaining to the
inventive retaining ring structure, the 20 degree transition angle
on retaining ring provides substantial advantages over conventional
square cornered retaining ring edge designs. The transition region
is able to pre-compress and smooth the pad before the wafer gets
into the area, thereby eliminating the "ringing marks" on the edge
of the wafer.
[0086] Therefore, while the particular 20-degree angle chamfer
combination for structure illustrated in FIG. 13 has shown
excellent results for the system described, other modified
transition region structures that transition between the parallel
and the perpendicular may be optimal for other CMP polisher
configurations, including, for example a radially shaped transition
confirmation, elliptically shaped conformations, linear transition
region having only single chamfer between surfaces 201 and 209, and
confirmations which provide different angles and/or more surfaces
in the transition region.
[0087] We now briefly describe additional details for retaining
ring adapter 168 relative to FIGS. 14-18. FIG. 14 is a diagrammatic
illustration showing an embodiment of the inventive retaining ring
adapter used in the polishing head of FIG. 5, and FIG. 15 shows an
alternative view of the same ring. FIG. 16 is a diagrammatic
illustration showing a sectional view of the retaining ring adapter
in FIG. 14, and FIG. 17 shows a sectional view detail of the manner
of attaching the retaining ring to the retaining ring adapter. FIG.
18 shows some additional detail of the flushing channels and
orifices for clearing polishing slurry from the ring area.
[0088] With reference to these figures, retaining ring adapter 168
is typically formed of metal to provide appropriate strength,
dimensional stability, and the like properties of a structure
within the head. On the other hand, the retaining ring continuously
floats on the surface of the polishing pad during a polishing
operation and must be compatible with that environment, and in
addition should not deposit material onto the pad that may be
harmful to the polishing operation. Such material is typically as
softer material, such as the TECHTRON material used in one
embodiment of the invention. The retaining ring is also a wear
item. Therefore, it is advantageous to provide separate retaining
ring adapter and replaceable retaining rings, though in theory an
integral structure providing both functions can be used, albeit not
with optimum characteristics.
[0089] Retaining ring adapter 168, in addition to providing means
for attaching retaining ring 166 to primary diaphragm 162, includes
a plurality of "T"-shaped channels or orifices for cleaning slurry
that may gather: (i) between the subcarrier 160 and retaining ring
166 (and retaining ring adapter 168), or (ii) between retaining
ring 166 (and retaining ring adapter 168) and lower housing 122. In
the embodiment of the invention illustrated in FIGS. 14-18, five
such T-shaped (or inverted T-shaped) channels are provided,
disposed at substantially equal intervals around the periphery of
the retaining ring adapter 168. The first vertically downwardly
extending (approximately 0.115 inch diameter) hole 177 extends
downward from an upper surface of retaining ring adapter 168 about
0.125 inches to intersect a second horizontally extending bore 176
(approximately 0.1 inch diameter) that extends between surface 186
adjacent subcarrier surface 185 and surface 196 which opens onto a
space continuous with a region between the inner surface of lower
housing 122 and the outer radio portions of retaining ring adapter
168.
[0090] By forcing deionized water through the first orifice the
space between subcarrier and retaining ring is cleared of any
slurry, and by forcing water through the second orifice, the region
between retaining ring and lower housing is kept clear of slurry.
Separate channels and orifices may alternatively be provided
extending separately to the ring-housing area and to the
ring-subcarrier area, but no particular advantage is provided by
such structure. The discharge pressure and volume should be
adjusted to produce adequate clearing action. Detail of these
orifices is also illustrated in FIG. 18. Means to communicate fluid
from an exterior source through rotary union 116 and to the fitting
197 are implementation details and are not shown.
[0091] In one embodiment of the invention, five 0.100 inch
"T"-shaped holes or channels are provided for head flushing.
High-pressure deionized water is forced through the these holes to
dislodge and clear any accumulated slurry. A 0.45 inch wide by 0.20
inch step on the top surface of the retaining ring adapter 168
provides sufficient physical space for cleaning water flow to clear
slurry deposits and as a result to maintain unrestricted motion of
the retaining ring relative to both the carrier and the housing.
Free movement of the subcarrier and retaining ring are important
for maintaining uniform polishing at the edge of the wafer. The
square edge of the subcarrier allows the retaining ring to move
separately from the subcarrier and keep certain distance in a
vertical direction.
[0092] Subcarrier 160 also has additional properties. In one
embodiment, subcarrier 160 is a solid round non-porous ceramic disk
having a diameter of about eight inches (7.885 inches in one
particular embodiment) for the version of the polishing tool
applicable to 200 mm wafers. The subcarrier has a square edge on
its upper and lower surfaces, and is lower surface is lapped for
flatness and smoothness. Six vacuum holes (0.040 in. diameter) are
provided in the subcarrier opening onto the lower surface of the
subcarrier where the subcarrier mounts the backside of the wafer.
These holes are in fluid communication with the single bore 184 at
the top center of the subcarrier. The fitting, a male thread 10-32
NPT one touch connector, is provided on the upper surface of
subcarrier for connection to tubing through the rotary union and to
an exterior source of vacuum, pressurized air, or water.
[0093] The holes are formed by boring a first hole 184 into the top
surface of the subcarrier 160, then boring six holes radially
inward from the cylindrical edge of the subcarrier to the center
bore hole 184. Six holes are then bored from the lower surface of
the subcarrier upward from the lower subcarrier surface until they
intersect the six radially extending holes or bores 191 to complete
the connection to the central bore hole 184. The portion of the
radially extending holes between the six vertically extending holes
and the cylindrical edge over the subcarrier are then filled with a
stainless steel plugs 181 or other means to prevent leakage of air,
vacuum, pressure, or water. These holes and channels are used to
supply vacuum to the backside of the wafer in order to hold the
wafer to the subcarrier, and to provide pressurized air or water or
a combination of the two to urge the wafer away from the subcarrier
during wafer unload operations.
[0094] We now address one hypothesized explanation for the reason
the inventive retaining ring perform so well in conditioning the
pad 135. FIG. 19 is a diagrammatic illustration of a hypothesized
retaining ring polishing pad interaction for a retaining ring
having a square corner at the ring-pad interface. In this example,
the square edge of the pad causes the pad to compress and buckle
upward as the edge of the ring presses forward and downward against
it. The pad experiences the impact of the ring and oscillations
develop in the pad that extend to an area beneath the wafer. On the
other hand, with the inventive retaining ring illustrated in it is
hypothesized that the retaining ring to polishing pad interaction
for a retaining ring having the inventive multi-planar chamfered
transition region at the ring-pad interface causes fewer
oscillations in the pad, or lower magnitude oscillations that die
out before reaching the wafer surface. The beneficial effects are
also achieved in part by applying only a fractional component of
the retaining ring downward pressure at the outer radial edge of
the ring, and gradually increasing the pressure as at smaller
radii. In effect, the transition region guides the pad under the
ring and increases pressure as the pad passes thereby reducing the
impact of the ring on the pad and causing a more gradual
application of force.
[0095] We now describe three embodiments of head wafer load/unload
and polishing procedures associated with the inventive structure
and method. FIG. 21 illustrates a diagrammatic flowchart of the
head wafer load procedure 501. It should be understood that this
procedure includes several steps which are performed in a preferred
embodiment of the invention; however, it should be understood that
not all of the steps described are essential steps, rather the
several optimal but provide for optimal one-year optimal results in
the overall procedure.
[0096] Robotic wafer handling equipment is commonly used in the
semiconductor industry, particularly where processes are carried
out in clean room environments. In this context, a Head Load Module
(HLM) and a Head Unload Module (HULM) are provided to present
wafers to the CMP tool for polishing and to receive wafers from the
CMP tool when polishing is completed. Even where the HLM and HULM
may be identical robots, two separate machines may be used, one to
present clean dry wafers and the second to receive wet wafers
coated with polishing slurry. Typically the HLM and HULM include a
stationary portion and an articulated arm portion that moves a
robotic hand, paddle, or other wafer grasping means in three
dimensions, including the ability to rotate. The hand is moved
under computer control to move the wafer from a storage location to
the CMIP tool and back to water or another storage location after
polishing or planarization has been completed. The following
procedures refer to the manner in which the HLM or HULM interacts
with the CMP tool and more specifically with components of the
wafer carrier assembly.
[0097] First, the loading of a wafer to the head is initiated (Step
502). This includes the controlled movement of the HIM robotic arm
from a "home" position to "head" position (Step 503). Home position
for the HLM is a position wherein the robot loading arm is outside
of the carousel and away from the head. Head position is a position
of the robotic arm where the robotic arm is extending beneath the
carousel under the polishing head and presenting the wafer to the
head for mounting. In Step 504, head subcarrier extends out
(downward) under the influence of pressure into chamber P2 132 so
that the carrier face extends below the lower edge of the retaining
ring; the robotic arm then extends upward to urge the wafer against
the carrier face. Springs are provided so that hard contact that
might damage the wafer is avoided. Next, HLM nozzle optionally
sprays DI water onto the head, and the head flush valve is turned
on so that the valve is open for DI water to pass through the valve
(Step 505). The HLM then goes back to the "home" position and loads
to wafer (Step 506). Then, the HLM goes to "head" position (Step
507). Next, the computer checks the head vacuum switched to verify
that is working (Step 508). The working head vacuum switch is
important because it ensures that the vacuum is working so that the
head is able to pick up the wafer from the extended arm of the
robot. If the head vacuum switch is not working the head cleaning
cycle is repeated starting at Step 502 until a working head vacuum
switch is verified, Manning the head subcarrier vacuum is turned on
so as to be ready to receive a wafer (Step 509).
[0098] The HLM goes up to the head wafer loading position (Step
510), and head subcarrier picks up the wafer from the HLM (Step
511). Next we determined if he wafer has action been picked up a by
the subcarrier applying the vacuum at the back side of the wafer,
and if the wafer is on the subcarrier, the head subcarrier
retraction with the wafer attached (Step 512) and wafer polishing
procedures then began (Step 513). On the other hand, if the wafer
is not on the subcarrier, the HL and goes down and then backup in
an attempt to reload the wafer onto the head (Step 514) and repeats
Steps 510 through 511 until it is verified that the wafer is on the
subcarrier.
[0099] The wafer polishing procedures now described relative to
FIG. 22 which shows a diagrammatic flowchart of the polishing
procedure (Step 521). Wafer polishing begins after the wafer has
been loaded onto the subcarrier as previous described (Step 522).
The polishing head attached to the turret and carousel assemblies
is moved downward to the polish position so that the wafer is
placed in contact with the polishing pad adhered to the platen, and
the head wafer backside vacuum which had been on to assisting
adhering the wafer to the subcarrier is turned off (Step 523). The
vacuum valve then closes and remains closed until just prior to
polishing. Then it is opened, uncovered and checked to verify wafer
presence prior to polish and then closed again (Step 524). At this
stage of the process the vacuum switch should normally be off, and
if the vacuum switch is on, alarm is triggered in the form of an
audible, and visual, or other indicator (Step 525). After vacuum
switch is off, the process proceeds by applying air pressure to
each of the two chambers in the head chamber P1 and chamber P2
(Steps 526, 527). The air or other fluid pressure applied to
chamber P1 controls the pressure or force on the subcarrier and as
a result in the polishing pressure exerted on the front surface of
the wafer against the opposing surface of the polishing pad (Step
526) the air or fluid pressure applied to chamber P2 controls be
pressure exerted against the retaining ring, which pressure serves
both to maintain the wafer within a pocket defined by the retaining
ring and to place the polishing pad in the immediate vicinity all
of the edge of the wafer into a condition optimal for polishing the
wafer and eliminating non-linear polishing effects at the edge of
the wafer (Step 527).
[0100] Once appropriate pressures in the two chambers has been
established the platen motor is energized (Step 528), and the
carousel motors and head motors are energized (Step 529) to cause
rotation all the platen carousel and head motors in a predetermined
manner and thereby initiate polishing of the wafer's (Step 530).
After the wafers have been polished, the heads and carousel
(attached to a bridge assembly) are raised away from the polishing
pad (Step 531), and head subcarrier is retracted from the lowest
position to the highest position inside the head so that the wafer
can be easily separated from the pad (Step 532). The polishing
having completed wafer unloading procedures are initiated (Step
530).
[0101] Wafer unload procedures (Step 541) are now described
relative to the diagrammatic flowchart in FIG. 23. Wafer unload
begins (Step 542) by extending the head subcarrier towards the Head
UnLoad Module (HULM) (Step 543). Next, the HULM is moved to a
"head" position (Step 544). Next a head flush operation is
initiated to clean spaces between the subcarrier and retaining ring
(Step 545), and between portions of retaining ring and the lower
housing (Step 546). The head flush switch "ON" operation clauses
the deionized (DI) water to be sent under pressure from an external
source to the rotary union 116 (including spindle 119) and into the
head through mounting adapter 121 and communicated via tubing and
fittings to carrier-ring flush orifices and to ring-housing flush
orifices. A purge operation (Step 545) is also performed by
applying deionized water to be backside of the wafer through a
central bore 184 at the upper surface of the subcarrier and via
channels 191 and holes 147 extending from the central bore to the
subcarrier-wafer mounting surface. When an optional insert is
provided between the subcarrier-wafer mounting surface and the
backside of the wafer, holes are also provided through the insert
so that deionized water, pressurized air, or vacuum may be applied
through the insert. The purge operation also includes application
of high-pressure clean dry air (CDA) the through the subcarrier
holes to push off the wafer onto to the HULM ring which has been
brought into proximity to receive the wafer as is pushed off the
subcarrier (Step 546). If after this first purge operation the
wafer has been urged off of the subcarrier and onto the HULMH, then
the HULM is moved back to its "home" position (Step 547).
Unfortunately, the single purge cycle may not always be sufficient
to urge the wafer from the subcarrier, and in such instance the
HULM is moved downward. The procedures are repeated beginning at
Step 545 with additional purge cycle's until the wafer has been
removed from subcarrier and is captured by the HULM.
[0102] Although the foregoing invention has been described in some
detail by way of illustration and example for purposes of clarity
of understanding, it will be readily apparent to those of ordinary
skill in the art in light of the teachings of this invention that
certain changes and modifications may be made thereto without
departing from the spirit or scope of the appended claims. All
publications and patent applications cited in this specification
are herein incorporated by reference as if each individual
publication or patent application were specifically and
individually indicated to be incorporated by reference.
* * * * *