The present invention relates generally to chemical
mechanical polishing of substrates, and more particularly
to a carrier head for a chemical mechanical polishing
system. Reference should be made to the Applicants' US Patent Specification
No. 5 643 053.
Integrated circuits are typically formed on
substrates, particularly silicon wafers, by the
sequential deposition of conductive, semiconductive or
insulative layers. After each layer is deposited, the
layer is etched to create circuitry features. As a
series of layers are sequentially deposited and etched,
the outer or uppermost surface of the substrate, i.e.,
the exposed surface of the substrate, becomes
increasingly more non-planar. This non-planar outer
surface presents a problem for the integrated circuit
manufacturer. If the outer surface of the substrate is
non-planar, then a photoresist layer placed thereon is
also non-planar. A photoresist layer is typically
patterned by a photolithographic apparatus that focuses
a light image onto the photoresist. If the outer surface
of the substrate is sufficiently non-planar, then the
maximum height difference between the peaks and valleys
of the outer surface may exceed the depth of focus of the
imaging apparatus, and it will be impossible to properly
focus the light image onto the outer substrate surface.
It may be prohibitively expensive to design new
photolithographic devices having an improved depth of
focus. In addition, as the feature size used in
integrated circuits becomes smaller, shorter wavelengths
of light must be used, resulting in further reduction of
the available depth of focus. Therefore, there is a need
to periodically planarize the substrate surface to
provide a substantially planar layer surface.
Chemical mechanical polishing (CMP) is one accepted
method of planarization. This planarization method
typically requires that the substrate be mounted to a
carrier or polishing head. The exposed surface of the
substrate is then placed against a rotating polishing
pad: The carrier provides a controllable load, i.e.,
pressure, on the substrate to push it against the
polishing pad. In addition, the carrier may rotate to
provide additional motion between the substrate and
polishing pad. A polishing slurry, including an abrasive
and at least one chemically-reactive agent, is
distributed over the polishing pad to provide an abrasive
chemical solution at the interface between the pad and
substrate. A CMP process is fairly complex, and differs
from simple wet sanding. In a CMP process the reactive
agent in the slurry reacts with the outer surface of the
substrate to form reactive sites. The interaction of the
polishing pad and abrasive particles with the reactive
sites results in polishing.
An effective CMP process has a high polishing rate
and generates a substrate surface which is finished
(lacks small-scale roughness) and flat (lacks large-scale
topography). The polishing rate, finish and flatness are
determined by the pad and slurry combination, the
relative speed between the substrate and pad, and the
force pressing the substrate against the pad. Because
inadequate flatness and finish can create defective
substrates, the selection of a polishing pad and slurry
combination is usually dictated by the required finish
and flatness. Given these constraints, the polishing
rate sets the maximum throughput of the polishing
apparatus.
The polishing rate depends upon the force pressing
the substrate against the pad. Specifically, the greater
this force, the higher the polishing rate. If the
carrier head applies a non-uniform load, i.e., if the
carrier applies more force to one region of the substrate
than to another, then the high pressure regions will be
polished faster than the lower pressure regions.
Therefore, a non-uniform load may result in non-uniform
polishing of the substrate.
An additional consideration in the production of
integrated circuits is process and product stability. To
achieve a high yield, i.e., a low defect rate, each
successive substrate should be polished under
substantially similar conditions. Each substrate should
be polished by approximately the same amount so that each
integrated circuit is substantially identical.
In view of the foregoing, there is a need for a
chemical mechanical polishing apparatus which optimizes
polishing throughput, while providing the desired
flatness and finish. Specifically, the chemical
mechanical polishing apparatus should have a carrier head
which applies a substantially uniform load to the
substrate.
In one aspect, the invention is directed to a
carrier for positioning a substrate on a polishing
surface in a chemical mechanical polishing apparatus.
The carrier comprises a housing and a containment
assembly connected to the housing to hold a layer of
conformable material. The layer of conformable material
provides a mounting surface for a substrate.
Implementations of the invention may include the
following. The containment assembly may include a
flexible membrane defining an enclosed volume, with the
conformable material disposed within the enclosed volume.
The flexible membrane may be attached to a backing
member, and a flexible connector may connect the backing
member to the housing. The flexible connecter may form
a pressure chamber between the housing and the backing
member. The flexible membrane may include a first
flexible membrane portion defining a first enclosed
volume and a second flexible membrane portion defining a
second enclosed volume, and the conformable material may
include a first conformable material having a first
viscosity disposed in the first enclosed volume and a
second conformable material having a second viscosity
disposed in the second enclosed volume. The conformable
material may be a viscoelastic material such as silicone,
gelatin, or urethane. The conformable material may have
a durometer measurement, such as between about twenty-five
and thirty-five, selected to provide both elasticity
and normal strain in response to an applied load. The
containment assembly may include a base member with a
recess, and the layer of conformable material may be
disposed in the recess to provide the mounting surface.
The base member may be detachably connected to the
carrier head. A retaining ring having approximately the
same thickness as the substrate may be connected to the
mounting surface. A shield which is thinner than the
retaining ring may be connected to the base member and
project over a portion of the layer of conformable
material to surround the retaining ring. The shield may
be positioned to prevent the conformable material from
extruding when the substrate is pressed against the
polishing surface. The carrier may further comprise a
chucking mechanism to attach the substrate to the
mounting surface. The chucking mechanism may include a
passageway formed through the layer of conformable
material to the mounting surface, and a pump may be
connected to the passageway to suction the substrate to
the mounting surface. The passageway may have a diameter
such that the passageway does not collapse if the pump
applies suction to the passageway. The chucking
mechanism may include a pocket between the substrate and
the layer of conformable material to suction the
substrate to the mounting surface.
Advantages of the invention include the following.
The carrier provides uniform loading of the backside of
the substrate to evenly polish the substrate. The
conformable material deforms and redistributes its mass
if the polishing pad is tilted, the substrate is warped,
or there are irregularities on the backside of the
substrate or the underside of the rigid surface. The
conformable layer is chemically inert vis-a-vis the
polishing process. The carrier head is also able to
vacuum chuck the substrate to lift the substrate off the
polishing pad.
Additional advantages of the invention will be set
forth in the description which follows, and in part will
be obvious from the description, or may be learned by
practice of the invention. The advantages of the
invention may be realized by means of the
instrumentalities and combinations particularly pointed
out in the claims.
The accompanying drawings, which are incorporated in
and constitute a part of the specification, schematically
illustrate the present invention, and together with the
general description given above and the detailed
description given below, serve to explain the principles
of the invention.
FIG. 1 is a schematic perspective view of a chemical
mechanical polishing apparatus.
FIG. 2 is a cross-sectional view of the support
assembly, carrier head and polishing pad of the chemical
mechanical apparatus of FIG. 1.
FIG. 3A is a schematic cross-sectional view of the
carrier head and polishing pad of the chemical mechanical
apparatus of FIG. 1.
FIG. 3B is a schematic cross-sectional view of an
alternate carrier head.
FIG. 4 is a schematic cross-sectional view of a
carrier head having multiple enclosed volumes filled with
a conformable material.
FIG. 5 is a schematic cross-sectional view of a
carrier head having a loading mechanism.
FIG. 6 is an exploded perspective view of a chemical
mechanical polishing apparatus.
FIG. 7 is a schematic top view of a carousel, with
the upper housing removed.
FIG. 8 is a cross-sectional view of the carousel of
FIG. 7 along line 8-8.
FIG. 9A is a schematic cross-sectional view of a
carrier head including bellows and a layer of conformable
material in accordance with the present invention.
FIG. 9B is a view of the carrier head of FIG. 9A in
which the bellows are replaced by a flexible membrane.
FIG. 10 is an exaggerated cross-sectional view of a
substrate in contact with the layer of conformable
material of the carrier head of FIG. 9A or FIG. 9B.
FIG. 11A is a schematic cross-sectional view of a
carrier head according to the present invention
illustrating vacuum chucking lines in the layer of
conformable material.
FIG. 11B is a view of the carrier head of FIG. llA
in which the vacuum chucking lines are closed by
application of a load to the carrier head.
FIG. 12A is a schematic cross-sectional view of a
carrier head according to the present invention
incorporating a vertically-movable cylinder for forming
a vacuum pocket.
FIG. 12B is view of the carrier head of FIG. 12A in
which the vertically-movable cylinder has been positioned
to form a vacuum pocket.
FIG. 13 is a schematic cross-section view of another
embodiment of a carrier head according to the present
invention.
Referring to FIGS. 1 and 2, a chemical mechanical
polishing (CMP) apparatus 30 generally includes a base 32
which supports a rotatable platen 40 and a polishing pad
42. The CMP apparatus 30 further includes a carrier or
carrier head 100 which receives a substrate 10 and
positions the substrate on the polishing pad. A support
assembly 60 connects carrier head 100 to base 32. The
carrier head is positioned against the surface of the
polishing pad by support assembly 60.
If substrate 10 is an eight-inch (200 mm) diameter
disk, then platen 40 and polishing pad 42 will be about
twenty inches in diameter. Platen 40 is preferably a
rotatable aluminum or stainless steel plate connected by
a drive shaft (not shown) to a drive mechanism (also not
shown). The drive shaft may also be stainless steel.
The drive mechanism, such as a motor and gear assembly,
is positioned inside the base to rotate the platen and
the polishing pad. The platen may be supported on the
base by bearings, or the drive mechanism may support the
platen. For most polishing processes, the drive
mechanism rotates platen 40 at thirty to two-hundred
revolutions per minute, although lower or higher
rotational speeds may be used.
Referring to FIG. 3A, polishing pad 42 may be a hard
composite material having a roughened polishing surface
44. The polishing pad 42 may be attached to platen 40 by
a pressure-sensitive adhesive layer 49. Polishing pad 42
may have a fifty mil thick hard upper layer 46 and a
fifty mil thick softer lower layer 48. Upper layer 46 is
preferably a material composed of polyurethane mixed with
other fillers. Lower layer 48 is preferably a material
composed of compressed felt fibers leached with urethane.
A common two-layer polishing pad, with the upper layer
composed of IC-1000 and the lower layer composed of SUBA-4,
is available from Rodel, Inc., Newark, Delaware (IC-1000
and SUBA-4 are product names of Rodel, Inc.).
Referring to FIG. 1, a slurry 50 containing a
reactive agent (e.g., deionized water for oxide
polishing), abrasive particles (e.g., silicon dioxide for
oxide polishing) and a chemically reactive catalyzer
(e.g., potassium hydroxide for oxide polishing) is
supplied to the surface of polishing pad 42. A slurry
supply tube or port 52 distributes or otherwise meters
the slurry onto the polishing pad. The slurry may also
be pumped through passages (not shown) in platen 40 and
polishing pad 42 to the underside of substrate 10.
To properly position the carrier head with respect
to the polishing pad, support assembly 60 includes a
crossbar 62 that extends over the polishing pad.
Crossbar 62 is positioned above the polishing pad by a
pair of opposed upright members 64a, 64b and 66, and a
biasing piston 68. One end of crossbar 62 is connected
to upright members 64a and 64b by means of a hinge 65.
The other end of crossbar 62 is connected to the biasing
piston 68. The biasing piston may lower and raise
crossbar 62 in order to control the vertical position of
the carrier head. The second upright member 66 is
positioned adjacent to the biasing piston 68 to provide
a vertical stop which limits the downward motion of the
crossbar.
To place a substrate on carrier head 100, the
crossbar is disconnected from the biasing piston, and the
crossbar is rotated about hinge 65 to lift carrier head
100 off the polishing pad. The substrate is then placed
in the carrier head, and the carrier head is lowered to
place substrate 10 against polishing surface 44 (see FIG.
3A).
Support assembly 60 includes a transfer case 70
which is suspended from crossbar 62 to controllably orbit
and rotate the substrate about the polishing pad. The
transfer case 70 includes a drive shaft 72 and a housing
74. The housing 74 includes a fixed inner hub 76 and an
outer hub 78. The fixed inner hub 76 is rigidly secured
to the underside of crossbar 62, for example by a
plurality of bolts (not shown). The rotatable outer hub
78 is journalled to fixed inner hub 76 by upper and lower
tapered bearings 77. These bearings provide vertical
support to rotatable outer hub 78, while allowing it to
rotate with respect to the fixed inner hub. The drive
shaft 72 extends through fixed inner hub 76 and is also
vertically supported by tapered bearings 79 which allow
the drive shaft 72 to rotate with respect to the fixed
inner hub 76.
As discussed in aforementioned U.S. Patent
Application Serial No. 08/173,846, a first motor and gear
assembly 80 is connected to drive shaft 72 to control the
orbital motion of the carrier head, and a second motor
and gear assembly 84 is connected by means of a pulley 85
and drive belts 86 and 87 to rotatable outer hub 78 to
control the rotational motion of the carrier head. One
end of a horizontal cross arm 88 is connected to the
lower end of drive shaft 72. The other end of crossarm
88 is connected to the top of a secondary vertical drive
shaft 90. The bottom of secondary drive shaft 90 fits
into a cylindrical depression 112 in the carrier head.
Thus, when drive shaft 72 rotates, it sweeps secondary
drive shaft 90 and carrier head 100 in an orbital path.
Support assembly 60 also includes a rotational
compensation assembly to control the rotational speed of
carrier head 100. The compensation assembly includes a
ring gear 94 which is connected to the bottom of
rotatable outer hub 78 of housing 74, and a pinion gear
96 connected to secondary drive shaft 90 immediately
below cross arm 88. Ring gear 94 has an inner toothed
surface, and the pinion gear 96 includes an outer toothed
surface which engages the inner toothed surface of ring
gear 94. As cross arm 88 pivots, it sweeps pinion gear
96 around the inner periphery of ring gear 94. A pair of
dowel pins 98 extend from the pinion gear 96 into a pair
of mating dowel pin holes 114 in the carrier head to
rotationally fix the pinion gear with respect to the
carrier head. Thus, the rotational motion of rotatable
outer hub 78 is transferred to carrier head 100 through
ring gear 94, pinion gear 96, and pins 98.
The compensation assembly allows the user of CMP
apparatus 30 to vary both the rotational and orbital
components of motion of the carrier head, and thereby
control the rotation and orbit of substrate 10. By
rotating rotatable outer hub 78 while simultaneously
rotating drive shaft 72, the effective rotational motion
of carrier head 100 may be controlled. Carrier head 100
and substrate 10 may be caused to rotate, orbit, or
rotate and orbit. The carrier head rotates or orbits at
about thirty to two-hundred revolutions per minute (rpm).
As the substrate orbits, the polishing pad may be
rotated. Preferably, the orbital radius is no greater
than one inch, and the polishing pad rotates at a
relatively slow speed, e.g., less than ten rpm and more
preferably at less than five rpm. The orbit of the
substrate and the rotation of the polishing pad combine
to provide a nominal speed at the surface of the
substrate of 1800 to 4800 centimeters per minute.
A substrate is typically subjected to multiple
polishing steps including a main polishing step and a
final polishing step. For the main polishing step,
carrier head 100 applies a force of approximately four to
ten pounds per square inch (psi) to substrate 10,
although carrier head 100 may apply more or less force.
For a final polishing step, carrier head 100 may apply
about three psi.
Generally, carrier head 100 transfers torque from
the drive shaft to the substrate, uniformly loads the
substrate against the polishing surface and prevents the
substrate from slipping out from beneath the carrier head
during polishing operations.
As shown in more detail in FIG. 3A, carrier head 100
includes three major assemblies: a housing assembly 102,
a substrate loading assembly 104, and a retaining ring
assembly 106.
The housing assembly 102 is generally circular so as
to match the circular configuration of the substrate to
be polished. The housing assembly 102 may be machined
aluminum. The top surface of housing assembly 102
includes a cylindrical hub 110 having cylindrical recess
112 for receiving secondary drive shaft 90. At least one
passageway 116 connects recess 112 to the bottom of
housing assembly 102.
As shown in FIG. 2, drive shaft 72 includes one or
more channels 150 and secondary drive shaft 90 includes
one or more channels 152, to provide fluid or electrical
connections to the carrier head. A rotary coupling 154
at the top of drive shaft 72 couples channel(s) 150 to
one or more fluid or electrical lines 156. For instance,
one of lines 156 may be a conformable material supply
line as described below. Another rotary coupling (not
shown) in cross arm 88 connects channel(s) 150 in drive
shaft 72 to channel(s) 152 in secondary drive shaft 90.
As shown, passageway 116 passes through housing assembly
102 to connect to channel 152 to substrate loading
assembly 104.
As the polishing pad rotates, it tends to pull the
substrate out from beneath the carrier head. Therefore,
carrier head 100 includes a retaining ring assembly 106
which projects downwardly from housing assembly 102 and
extends circumferentially around the outer perimeter of
the substrate. The retaining ring assembly 106 may be
attached with a key-and-keyway assembly 120 to housing
assembly 102 so that the retaining ring assembly rests on
the polishing pad and is free to adjust to variations in
the height of the polishing surface 44. An inner edge
122 of retaining ring assembly 106 captures the substrate
so that the polishing pad cannot pull the substrate from
beneath the carrier head. Retaining ring assembly 106
may be made of a rigid plastic material.
Substrate loading assembly 104 is located beneath
housing assembly 102 in the recess formed by retaining
ring assembly 106. Substrate loading assembly 104 may
include a removable carrier plate 124, a membrane 134
which defines an enclosed volume 126, and a removable
carrier film 128. Enclosed volume 126 may be located in
the cylindrical recess surrounded by retaining ring
assembly 106.
The removable carrier plate 124 may be a circular
stainless-steel disk of approximately the same diameter
as the substrate. The lower surface of the carrier
plate, or the lower surface of the housing if the carrier
plate is not present, provides a face 130 to which
membrane 134 may be adhesively attached.
The enclosed volume 126 is filled with a conformable
material 132. The conformable material 132 is a non-gaseous
material which undergoes viscous, elastic, or
viscoelastic deformation under pressure. Preferably,
conformable material 132 is a viscoelastic material, such
as a silicon, a gelatin, or another substantially
resilient yet viscous substance which will redistribute
its mass under pressure. The pressure applied during
polishing is substantially uniformly distributed across
substrate 10 by means of the conformable material in
enclosed volume 126.
As shown in FIG. 3A, membrane 134 defines enclosed
volume 126. The membrane is comprised of a flexible,
stretchable and compressible material such as rubber.
Membrane 134 may entirely encapsulate conformable
material 132. An upper surface 136 of membrane 134 is
placed against face 130. Alternately, as shown in FIG.
3B, the enclosed volume may be formed by extending the
membrane across the recess beneath face 130 and filling
the enclosed volume with conformable material 132.
Carrier film 128 may be attached to a lower surface
138 of membrane 134. Carrier film 128 is formed of a
thin circular layer of a porous material such as
urethane. Carrier film 128, if used, is sufficiently
thin and flexible that it substantially conforms to the
surface of substrate 10. Carrier film 128 provides a
mounting surface 142 to which substrate 10 is releasably
adhered by surface tension. Alternately, if the carrier
film is not used, the lower surface of membrane 134 may
be porous to accomplish the same thing (see FIG. 5).
Carrier film 128 is sufficiently thin and flexible so
that it substantially conforms to the surface of
substrate 10.
The space defined by retaining ring assembly 106 and
mounting surface 142 provides a substrate receiving
recess 140. The substrate is placed against mounting
surface 142, causing conformable material 132 and carrier
film 128, if present, to deform to contact the substrate
across its entire backside. Carrier head 100 is then
lowered to bring the substrate into contact with
polishing surface 44. The load applied to the substrate
is transferred through conformable material 132.
The polishing surface 44 may be non-planar; e.g., it
may have sloping contours. Carrier plate 124 and the
underside or surface 141 of housing assembly 102 may also
be non-planar. The polishing pad may be tilted relative
to the carrier head. In addition, the backside of
substrate 10 may have surface irregularities. The
substrate could also be warped. The conformable material
132 ensures a uniform distribution of the carrier load on
the substrate for both large scale effects (e.g., a
tilted polishing pad) and small scale effects (e.g.,
surface irregularities on the backside of the substrate).
Conformable material 132 conforms to the substrate
surface as well as to face 130. That is, the conformable
material inside membrane 134 redistributes its mass to
conform to surface irregularities on the backside of the
substrate and face 130. Because the conformable material
contacts substrate across its entire back surface, and
because the conformable material has a uniform density,
it ensures a uniform load across the backside of the
substrate. In addition, conformable material 132 may
flow and deform. This permits the substrate to tilt with
respect to housing assembly 102 to follow the contours of
the polishing pad. In summary, the conformable material
ensures that carrier head 100 uniformly loads the
substrate against the polishing surface 44.
When carrier head 100 rotates at high speeds,
centrifugal force will tend to push the conformable
material in the enclosed volume outwardly toward the edge
of the carrier head. This tends to increase the density
of the conformable material near the perimeter of
enclosed volume. Consequently, the conformable material
near the edge of the enclosed volume will tend to become
less compressible than the center, and a non-uniform load
may be applied to the substrate.
To prevent this non-uniform load, enclosed volume
126 is connected by passageway 116, channels 150 and 152,
and conformable material supply line 156 to a supply 158.
Supply 158 can provide conformable material at a constant
pressure to enclosed volume 126. Consequently, when
carrier head 100 rotates and conformable material 132 is
forced toward the edge of the enclosed volume, supply 158
provides additional conformable material to the center of
the enclosed volume and maintains the conformable
material at a substantially uniform distribution
throughout enclosed volume 126. This uniform
distribution of conformable material ensures uniform
polishing at the center and edges of the substrate.
Supply 158 may also be used to control the viscosity
of conformable material 132. By increasing the pressure
on the conformable material, the density of conformable
material 132 can be increased. If the density of
conformable material 132 increases, its viscosity will
decrease.
The minimum pressure from supply 158 must overcome
the load applied by the carrier head to the substrate;
otherwise, this load will force the conformable material
back through passageway 116. When the carrier head stops
rotating, the conformable material is uniformly redistributed
throughout membrane 134. The excess
conformable material then flows back through passageways
116, 150 and 152 to supply 158.
In another implementation, conformable material 132
may be a material, such as rubber, which is sufficiently
rigid that it does not flow under the influence of
centrifugal forces. In this implementation, the
distribution of conformable material 132 does not change
significantly when carrier head 100 rotates. Thus,
conformable material supply 158 is not required.
As shown in FIG. 4, substrate loading assembly 104
may include multiple compartments or enclosed volumes 160
and 162. The enclosed volumes 160 and 162 are defined by
two or more membrane portions. The membrane portions may
be separate, discrete membranes, or they may be different
portions of a single membrane. Enclosed volume 160 may
be a circular disk, located above the center of mounting
surface 142, and enclosed volume 162 may be an annular
ring surrounding enclosed volume 160. The enclosed
volumes 160 and 162 contain conformable materials 164 and
166, respectively. Conformable materials 164 and 166
have different viscosities. By selecting the relative
viscosities of conformable materials 164 and 166, over-polishing
of the substrate edge may be avoided and more
uniform polishing of the substrate may be achieved. Each
enclosed volume may be connected by a passageway 168 to
a supply (not shown).
Referring to FIG. 5, carrier head 100 may be held in
a vertically-fixed position by support assembly 60 (see
FIG. 3A), and a force may be applied to substrate 10 by
the carrier head. In this embodiment, the loading
assembly 104 includes a flexible connector, such as a
bellows 170. The bellows 170 connects a substrate
backing member 174 to a bottom surface 173 of housing
assembly 102. The bellows 170 is expandable so that
substrate backing member 174 can move vertically relative
to housing assembly 102. The interior of bellows 170
forms a pressure chamber 176. Pressure chamber 176 can
be pressurized negatively or positively by a pressure or
vacuum source (not shown) which is connected to pressure
chamber 176 by a conduit 178. Membrane 134 is attached
to the bottom face of substrate backing member 174. By
pressurizing chamber 176, a force is exerted on
conformable material 132 to press the substrate against
the polishing pad. Thus, flexible connector 170 acts as
a loading mechanism, and replaces the biasing piston 68.
Enclosed volume 126 may be connected to a supply as
shown in the embodiment of FIG. 2. A flexible conduit
182, which may be a plastic tubing, connects a passageway
180 in substrate backing member 174 to passageway 116 in
housing assembly 102 for this purpose. The points at
which flexible conduit 182 is connected to passageways
180 and 116 may be sealed by appropriate fittings to
prevent conformable material 132 from leaking into
pressure chamber 176.
Referring to FIG. 6, in another embodiment, one or
more substrates 10 are polished by a chemical mechanical
polishing (CMP) apparatus 220. A complete description of
CMP apparatus 220 may be found in EP-A-0774323, the entire disclosure of
which is hereby incorporated by reference.
The CMP apparatus 220 includes a lower machine base
222 with a table top 223 mounted thereon and removable
upper outer cover (not shown). Table top 223 supports a
series of polishing stations 225a, 225b and 225c, and a
transfer station 227. Transfer station 227 forms a
generally square arrangement with the three polishing
stations 225a, 225b and 225c. Transfer station 227
serves multiple functions of receiving individual
substrates 10 from a loading apparatus (not shown),
washing the substrates, loading the substrates into
carrier heads (to be described below), receiving the
substrates from the carrier heads, washing the substrates
again, and finally transferring the substrates back to
the loading apparatus.
Each polishing station 225a-225c includes a
rotatable platen 230 on which is placed a polishing pad
232. If substrate 10 is an eight-inch (200 mm) diameter
disk, then platen 230 and polishing pad 232 will be about
twenty inches in diameter. Platen 230 is preferably a
rotatable aluminum or stainless steel plate connected by
stainless steel platen drive shaft (not shown) to a
platen drive motor (not shown). For most polishing
processes, the drive motor rotates platen 230 at thirty
to two-hundred revolutions per minute, although lower or
higher rotational speeds may be used.
Referring to FIG. 10, polishing pad 232 is a
composite material with a roughened polishing surface
234. Polishing pad 232 may be attached to platen 230 by
a pressure-sensitive adhesive layer 239. Polishing pad
232 may have a fifty mil thick hard upper layer 236 and
a fifty mil thick softer lower layer 238. Upper layer
236 is preferably a material composed of polyurethane
mixed with other fillers. Lower layer 238 is preferably
a material composed of compressed felt fibers leached
with urethane. A common two-layer polishing pad, with
the upper layer composed of IC-1000 and the lower layer
composed of SUBA-4, is available from Rodel, Inc.,
located in Newark, Delaware (IC-1000 and SUBA-4 are
product names of Rodel, Inc.).
Returning to FIG. 6, each polishing station 225a-225c
may further include an associated pad conditioner
apparatus 240. Each pad conditioner apparatus 240 has a
rotatable arm 242 holding an independently rotating
conditioner head 244 and an associated washing basin 246.
The conditioner apparatus maintains the condition of the
polishing pad so it will effectively polish any substrate
pressed against it while it is rotating.
A slurry 250 containing a reactive agent (e.g.,
deionized water for oxide polishing), abrasive particles
(e.g., silicon dioxide for oxide polishing) and a
chemically reactive catalyzer (e.g., potassium hydroxide
for oxide polishing), is supplied to the surface of
polishing pad 232 by a slurry supply tube 252.
Sufficient slurry is provided to cover and wet the entire
polishing pad 232. Two or more intermediate washing
stations 255a and 255b are positioned between neighboring
polishing stations 225a, 225b and 225c. The washing
stations rinse the substrates as they pass from one
polishing station to another.
A rotatable multi-head carousel 260 is positioned
above lower machine base 222. Carousel 260 is supported
by a center post 262 and rotated thereon about a carousel
axis 264 by a carousel motor assembly located within base
222. Center post 262 supports a carousel support plate
266 and a cover 268. Multi-head carousel 260 includes
four carrier head systems 270a, 270b, 270c, and 270d.
Three of the carrier head systems receive and hold
substrates, and polish them by pressing them against the
polishing pad 232 on platen 230 of polishing stations
225a-225c. One of the carrier head systems receives a
substrate from and delivers the substrate to transfer
station 227.
The four carrier head systems 270a-270d are mounted
on carousel support plate 266 at equal angular intervals
about carousel axis 264. Center post 262 allows the
carousel motor to rotate the carousel support plate 266
and to orbit the carrier head systems 270a-270d, and the
substrates attached thereto, about carousel axis 264.
Each carrier head system 270a-270d includes a
polishing or carrier head 300. Each carrier head 300
independently rotates about its own axis, and
independently laterally oscillates in a radial slot 272
formed in carousel support plate 266. A carrier drive
shaft 274 connects a carrier head rotation motor 276 to
carrier head 300 (shown by the removal of one-quarter of
cover 268). There is one carrier drive shaft and motor
for each head.
Referring to FIG. 7, in which cover 268 of carousel
260 has been removed, carousel support plate 266 supports
the four carrier head systems 270a-270d. Carousel
support plate includes four radial slots 272, generally
extending radially and oriented 90° apart. Radial slots
272 may either be close-ended (as shown) or open-ended.
The top of support plate supports four slotted carrier
head support slides 280. Each slide 280 aligns along one
of the radial slots 272 and moves freely along a radial
path with respect to carousel support plate 266. Two
linear bearing assemblies bracket each radial slot 272 to
support each slide 280.
As shown in FIGS. 7 and 8, each linear bearing
assembly includes a rail 282 fixed to carousel support
plate 266, and two hands 283 (only one of which is
illustrated in FIG. 8) fixed to slide 280 to grasp the
rail. Two bearings 284 separate each hand 283 from rail
282 to provide free and smooth movement therebetween.
Thus, the linear bearing assemblies permit the slides 280
to move freely along radial slots 272.
A bearing stop 285 anchored to the outer end one of
the rails 282 prevents slide 280 from accidentally coming
off the end of the rails. One of the arms of each slide
280 contains an unillustrated threaded receiving cavity
or nut fixed to the slide near its distal end. The
threaded cavity or nut receives a worm-gear lead screw
286 driven by a slide radial oscillator motor 287 mounted
on carousel support plate 266. When motor 287 turns lead
screw 286, slide 280 moves radially. The four motors 287
are independently operable to independently move the four
slides along the radial slots 272 in carousel support
plate 266.
A carrier head assembly or system, each including a
carrier head 300, a carrier drive shaft 274, a carrier
motor 276, and a surrounding non-rotating shaft housing
278, is fixed to each of the four slides. Drive shaft
housing 278 holds drive shaft 274 by paired sets of lower
ring bearings 288 and a set of upper ring bearings 289.
Each carrier head assembly can be assembled away from
polishing apparatus 220, slid in its untightened state
into radial slot 272 in carousel support plate 266 and
between the arms of slide 280, and there tightened to
grasp the slide.
A rotary coupling 290 at the top of drive motor 286
couples two or more fluid or electrical lines 292 into
two or more channels 294 in drive shaft 274. Channels
294 are used, as described in more detail below, to
pneumatically power carrier head 300, to vacuum-chuck the
substrate to the bottom of the carrier head and to
actuate a retaining ring against the polishing pad.
During actual polishing, three of the carrier heads,
e.g., those of carrier head systems 270a-70c, are
positioned at and above respective polishing stations
225a-225c. Carrier head 300 lowers a substrate into
contact with polishing pad 232, and slurry 250 acts as
the media for chemical mechanical polishing of the
substrate or wafer. The carrier head 300 uniformly loads
the substrate against the polishing pad.
The substrate is typically subjected to multiple
polishing steps, including a main polishing step and a
final polishing step. For the main polishing step,
usually performed at station 225a, carrier head 300
applies a force of approximately four to ten pounds per
square inch (psi) to substrate 10. At subsequent
stations, carried head 300 may apply more or less force.
For example, for a final polishing step, usually
performed at station 225c, carrier head 300 may apply a
force of about three psi. Carrier motor 76 rotates
carrier head 300 at about thirty to two-hundred
revolutions per minute. Platen 230 and carrier head 300
may rotate at substantially the same rate.
Generally, carrier head 300 holds the substrate
against the polishing pad and evenly distributes a
downward pressure across the back surface of the
substrate. The carrier head also transfers torque from
the drive shaft to the substrate and ensures that the
substrate does not slip from beneath the carrier head
during polishing.
Referring to FIG. 9A, carrier head 300 includes a
housing assembly 302, a loading mechanism 304 and a base
assembly 306. The drive shaft 274 is connected to
housing assembly 302. Loading mechanism 304 connects
housing assembly 302 to base assembly 306. The loading
mechanism applied a load, i.e., a downward pressure, to
base assembly 306. The base assembly 306 transfers the
downward pressure from loading mechanism 304 to substrate
10 to push the substrate against the polishing pad. Base
assembly 306 includes a conformable layer 308 to evenly
distribute the downward pressure across the back surface
of the substrate. Each of these components will be
described in greater detail below.
Housing assembly 302 may be formed of aluminum or
stainless steel. The housing assembly is generally
circular in shape to correspond the circular
configuration of the substrate to be polished. The top
surface of the housing assembly may include a cylindrical
hub 320 having a threaded neck 322. To connect drive
shaft 274 to carrier head 300, two dowel pins 324 may be
inserted into matching dowel pin holes in hub 320 and a
flange 296. Then, a threaded perimeter nut 298 is
screwed onto threaded neck 322 to firmly attach carrier
head 300 to drive shaft 274. When drive shaft 274
rotates, dowel pins 324 transfer torque to housing
assembly 302 to rotate the carrier head about the same
axis as the drive shaft.
At least two conduits 326 and 328 extend through hub
320. There may be one conduit for each channel 294 in
drive shaft 274. When carrier head 300 is attached to
drive shaft 274, the dowel pins align the carrier head so
that conduits 326 and 328 connect to channels 294. O-rings
(not shown) may be positioned in hub 320
surrounding each conduit 326 and 328 to form a fluid-tight
seal between the conduits to the channels.
Loading mechanism 304 forms a vertically-movable
seal between housing assembly 302 and base assembly 306
and defines a pressure chamber 330. A gas, such as air,
is pumped into and out of pressure chamber 330 through
conduit 326 to control the load applied to base assembly
306. When air is pumped into pressure chamber 330, base
assembly 306 is forced downwardly to bring substrate 10
into contact with polishing pad 232. When air is pumped
out of pressure chamber 330, base assembly is lifted
upwardly to remove the substrate from polishing pad 232.
Loading mechanism 304 may include a cylindrical
bellows 332 which is bolted or fixed to housing assembly
302 and base assembly 306 to form pressure chamber 330.
Bellows 332 may be a stainless steel cylinder which
expands or contracts depending upon whether a gas is
supplied to or removed from pressure chamber 330.
Bellows 332 may include upper and lower support plates
334 and 336 which are bolted or otherwise secured to
housing assembly 302 and a base assembly 306,
respectively. A cylindrical seal 338 may fit into a
circumferential groove 312 on rim 310 of housing 302 and
in a circumferential groove 339 in an upwardly-extending
wall portion 318 of, base assembly 306. The seal 338
surrounds and protects bellows 332 from the corrosive
effects of slurry 250. When housing assembly 302 is
rotated, bellows 332 transfers torque from the housing
assembly to the base assembly, causing it to also rotate.
However, because the bellows are flexible, base assembly
306 can pivot with respect to the housing assembly about
an axis parallel to the surface of the polishing pad to
remain substantially parallel to the polishing pad
surface.
Base assembly 306 includes a rigid backing fixture
or plate 350 and a detachable module 352 which is
attached to the underside of backing plate 350. Backing
plate 350 may be generally disk-shaped to match the
configuration of substrate 30, and may be formed of a
metal such as aluminum or stainless steel. Module 352
includes a rigid support fixture or cup 354, conformable
layer 308, an annular shield ring 360, and an annular
retaining ring 362. Each of these elements will be
discussed in detail below.
Module 352 may be removably attached to backing
plate 350 by various attachment mechanisms, such as
bolts, screws, key and key slot combination, vacuum
chucking, or magnets. As such, module 352 can be
detached and replaced if it is damaged or worn out. In
addition, it may be replaced to change the polishing
parameters. For example, different modules may
incorporate conformable layers with different durometer
measurements. The different modules may also have
different retaining ring widths or retaining ring
heights. The height and width of the retaining ring
affects the polishing rate near the edge of the
substrate. These module features can be selected to
provide an optimal polishing performance.
Cup 354 may be formed of aluminum or stainless steel
and may have an outer lip or rim 356 which projects
downwardly to surround a recess. The conformable layer
308 is disposed within the recess so that the bottom
surface of the conformable layer is substantially flush
with the bottom surface of rim 356. The recess may be
approximately one-eighth to one-quarter inch deep.
The conformable layer 308 is made of a visco-elastic
material that has a substantially homogeneous density.
Conformable layer 308 is elastic; i.e., it will return to
its original shape when an applied load is removed.
Conformable layer 308 is slightly compressible. In
addition, conformable layer 308 undergoes normal strain;
i.e., it will redistribute its mass in directions normal
to an applied load. The durometer measurement of the
conformable layer must be carefully selected. If the
durometer measurement is too low, the material will lack
elasticity. On the other hand, if the durometer
measurement is too high, the material will not undergo
normal strain. Conformable layer 308 may have a
durometer measurement of between approximately fifteen to
twenty-five on the Shore scale. Preferably, conformable
layer 308 has a durometer measurement of about twenty-one
on the Shore scale. The conformable material may have an
adhesive surface so that it adheres to the walls of cup
354. In addition, it should be resistant to heat and be
chemically inert vis-a-vis the polishing process. An
appropriate conformable material is a urethane material
available from Pittsburgh Plastics of Zelienopal,
Pennsylvania. Module 352 may be manufactured by pouring
liquid urethane into cup 354 and curing it to form layer
309.
Referring to FIG. 10, conformable layer 308 permits
substrate 10 to shift or pivot to accommodate changes in
the surface of the polishing pad. Conformable layer 308
deforms to match the back side of substrate 10 and evenly
distribute the load from loading mechanism 304 to the
substrate. For example, if substrate 10 is warped,
conformable layer 308 will, in effect, conform to the
contours of the warped substrate.
A thin sheet 358 of a low-friction material may be
laminated to the outer surface of conformable layer 308
to provide a low-friction substrate mounting surface 364.
The sheet 358 may be a seven mil thick film of urethane
having a durometer measurement of approximately eighty-three
on the Shore scale. Sheet 358 permits the
conformable material layer 308 to closely conform to the
back side of substrate 10 but prevents the substrate from
adhering to the conformable material. Sheet 358 is
sufficiently thin that substrate 10 may be considered to
be in direct contact with conformable layer 308.
Referring to FIG. 9A, module 352, as previously
noted, also includes shield ring 360 and retaining ring
362. Shield ring 360 is formed of a rigid material such
as aluminum or stainless steel and is positioned below
comformable layer 308 to be substantially flush with the
bottom surface of rim 356 and the conformable layer.
Shield ring 360 holds conformable layer 308 with the
recess of cup 354 when a load is applied to substrate 10.
Shield ring 360 may be appropriately secured to rim 356
such as by screws or bolts (not shown).
Retaining ring 362 is an annular rigid ring,
positioned within the circumference of shield ring 360.
Retaining ring 362 may be adhesively attached directly to
conformable layer 308. Retaining ring 362 may be formed
of a hard plastic or ceramic material. Retaining ring
362 is separated from shield ring 360 by a small gap "r"
so that the retaining ring may shift or pivot to
accommodate variations in the vertical height of the
surface of polishing pad 232. In operation, substrate 10
fits into a circular recess defined by retaining ring 362
and abuts mounting surface 364 of the conformable layer.
Retaining ring 362 and substrate 10 have substantially
the same thickness, so that retaining ring 362 also
contacts polishing pad 232. The shear force created by
the relative velocity between substrate 10 and polishing
pad 232 tends to push the substrate from beneath carrier
head 300. Retaining ring 362 prevents substrate 10 from
moving from beneath base assembly 306.
Referring to FIG. 9B, in another embodiment, in
which similar parts are referred to with primed numbers,
loading mechanism 304' may include a flexible membrane
340 instead of a bellows. Flexible membrane 340 may be
an annular sheet of silicone approximately sixty mils
thick, with inner and outer edges 342 and 344. The inner
edge 342 is clamped between an inner clamp ring 346 and
base assembly 306', whereas outer edge 344 is clamped
between an outer clamp ring 348 and housing assembly
302'. The clamp rings attach the flexible membrane to
the housing assembly and the base assembly to form
pressure chamber 330'. Flexible membrane 340 acts as a
diaphragm which rolls or unrolls, depending upon the
vertical distance across pressure chamber 330'.
Housing assembly 302' includes two opposing flanges
314 which project downwardly from rim 310. Each flange
314 may have a rectangular slot 315. A torque pin 316
extends through each rectangular slot 315 and is secured
in a receiving recess 317 in upward-extending wall
portion 318' of backing plate 350 of base assembly 306'.
The width of rectangular slot 315 is comparable to the
width of torque pin 316 so that the pin cannot move
horizontally in the slot. When drive shaft 274 rotates
housing assembly 302', torque pins 316 transfer torque
from the housing assembly to the base assembly. The
height of rectangular slot 315 is greater than the height
of torque pin 316 so that the pin can move vertically in
the slot. Thus, base assembly 306' must rotate with
housing assembly 302', but it is free to move vertically
with respect to the housing assembly.
As discussed above, carrier head 300 may lift
substrate 10 away from polishing pad 232 in order to move
the substrate from one polishing station to another. In
addition, the substrate may be ejected from carrier head
300 to return the substrate to transfer station 227 (see
FIG. 6). Specifically, carrier head 300 may vacuum-chuck
or pressure-eject the substrate to or from mounting
surface 364, as explained in more detail below.
The carrier head includes several fluid lines which
permit a gas, such as air, to flow into and out of base
assembly 306 to vacuum-chuck or pressure-eject the
substrate. Because base assembly 306 and housing
assembly 302 can move vertically relative to each other,
flexible fluid conduits are used to link conduit 328 to
a passageway 370 in backing plate 350. As shown in FIG.
9A, the flexible fluid conduit may be a metal bellows
372. The metal bellows can expand and contract to match
the distance across chamber 330. Alternately, as shown
in FIG. 9B, the flexible fluid conduit may be a plastic
tubing 374 positioned within chamber 330'. The plastic
tubing may, for example, be wrapped in a half, a three-quarter,
a full turn. When base assembly 306' moves
relative to the housing assembly, the tubing coils or
uncoils to match the distance across chamber 330'.
Referring to FIG. 11A, in one implementation,
passageway 370 is connected to one or more passages 376
of cup 354. In addition, vacuum-chucking passages 380
extend through conformable layer 308 from passages 376 in
cup 354 to mounting surface 364. Each vacuum chucking
passage 380 is simply a hole in the conformable layer.
The-hole is large enough so that it does not collapse
when a vacuum is applied but small enough so that it does
collapse when a load is applied to the substrate.
A pump 382 is connected via fluid line 292, channel
294, conduit 328, conduit 372, passageway 370, passages
376, and vacuum-chucking passages 380 to mounting surface
364. If a vacuum is applied to passages 380 by pump 382,
substrate 10 will be vacuum-chucked to mounting surface
364. If air is forced into passages 380 by pump 382,
substrate 10 will be ejected from mounting surface 364.
Referring to FIG. 11B, when substrate 10 is
positioned against polishing pad 232 and a load is
applied, conformable layer 308 will be compressed and
vacuum-chucking passages 380 will collapse. Thus, the
passages do not significantly affect the distribution of
the load across the backside of the substrate. When the
load is removed, conformable layer 308 will return to its
normal state and vacuum-chucking passages 380 will
reopen. Each vacuum-chucking passage 380 should be
between approximately one-eighth and one-quarter of an
inch in diameter.
Referring to FIGS. 12A and 12B, in another
implementation, substrate 10 is vacuum-chucked to carrier
head 300 by the formation of a vacuum pocket. As shown
in FIG. 12A, module 352 may include a vertically-movable
disk 390. Conformable layer 308 may be adhesively
attached to disk 390. Disk 390 has a diameter less than
that of the substrate, and it may be connected to the
activating mechanism of an air cylinder 392. Air
cylinder 392 may be positioned in cup portion 354, and it
392 may be powered by a pump 382. The pump is connected
to the air cylinder by the flexible conduit, passageway
370, and passages 376. The actuating mechanism of air
cylinder 392 may move disk 390 between a first position
in which the disk is flush with a bottom surface 394 of
base 378 of cup 354 (see FIG. 12A) and a second position
in which the disk has been drawn upwardly away from the
substrate. In the second position, the portion of
conformable layer 308 beneath the disk will be pulled
upwardly. Since the edges of conformable layer 308
remain in contact with substrate 10, whereas the center
of conformable layer 308 is drawn away from the center of
substrate 10, a vacuum pocket 398 is formed between the
substrate and the conformable layer. This vacuum pocket
vacuum-chucks the substrate to the carrier head.
A conformable layer in accordance with the present
invention may be incorporated into various other carrier
head designs, such as the one described in JP-A-322071, the entire disclosure of
which is hereby incorporated by reference.
Referring specifically to FIG. 13, such a carrier
head 400 includes a housing assembly 402, a base assembly
404 and a retaining ring assembly 406. A conformable
layer 408, similar in composition and structure to the
conformable layer described above, may be adhered or
attached to a surface 418 of base assembly 404 to provide
a substrate mounting surface 410.
The present invention has been described in terms of
a preferred embodiment. The invention however, is not
limited to the embodiment depicted and described.
Rather, the scope of the invention is defined by the
appended claims.