U.S. patent application number 13/553009 was filed with the patent office on 2014-01-23 for carrier head sweep motor current for in-situ monitoring.
This patent application is currently assigned to APPLIED MATERIALS, INC.. The applicant listed for this patent is Jagan Rangarajan, Alpay Yilmaz. Invention is credited to Jagan Rangarajan, Alpay Yilmaz.
Application Number | 20140020830 13/553009 |
Document ID | / |
Family ID | 49945554 |
Filed Date | 2014-01-23 |
United States Patent
Application |
20140020830 |
Kind Code |
A1 |
Rangarajan; Jagan ; et
al. |
January 23, 2014 |
Carrier Head Sweep Motor Current for In-Situ Monitoring
Abstract
A chemical mechanical polishing system includes a platen to
support a polishing pad, two carrier heads configured to hold two
substrates against the polishing pad at the same time, two
actuators to sweep the two carrier heads laterally across the
polishing pad, an in-situ polishing monitoring system including a
two current sensors to sense two currents supplied to the two
actuators and generate two signals, and a controller to receive the
two signals and independently detect a two endpoints for the two
substrates based on the two signals.
Inventors: |
Rangarajan; Jagan; (Fremont,
CA) ; Yilmaz; Alpay; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rangarajan; Jagan
Yilmaz; Alpay |
Fremont
San Jose |
CA
CA |
US
US |
|
|
Assignee: |
APPLIED MATERIALS, INC.
Santa Clara
CA
|
Family ID: |
49945554 |
Appl. No.: |
13/553009 |
Filed: |
July 19, 2012 |
Current U.S.
Class: |
156/345.13 |
Current CPC
Class: |
B24B 37/105 20130101;
B24B 37/013 20130101; B24B 49/10 20130101 |
Class at
Publication: |
156/345.13 |
International
Class: |
B24B 29/00 20060101
B24B029/00 |
Claims
1. A chemical mechanical polishing system, comprising: a platen to
support a polishing pad; a first rotatable carrier head configured
to hold a first substrate against the polishing pad; a second
rotatable carrier head configured to hold a second substrate
against the same polishing pad at the same time that the first
carrier head holds the first substrate against the polishing pad; a
first actuator to sweep the first carrier head laterally across the
polishing pad while the first substrate contacts the polishing pad;
a second actuator to sweep the second carrier head laterally across
the polishing pad while the second substrate contacts the polishing
pad; and an in-situ polishing monitoring system comprising a first
current sensor to sense a first current supplied to the first
actuator and generate a first signal, a second current sensor to
sense a second current supplied to the second actuator and generate
a second signal, and a controller to receive the first signal and
the second signal and independently detect a first endpoint and a
second endpoint for the first substrate and the second substrate,
respectively, based on the first signal and the second signal.
2. The system of claim 1, further comprising a track, a first
carriage supported by the track, and a second carriage supported by
the track, wherein the first carrier head is suspended from the
first carriage and the second carrier head is suspended from the
second carriage.
3. The system of claim 2, wherein the first actuator is configured
to move the first carriage along the track to sweep the first
carrier head and the second actuator is configured to move the
second carriage along the track to sweep the second carrier
head.
4. The system of claim 3, wherein the track comprises a magnetic
track, the first actuator comprises a first linear motor coil and
the second actuator comprises a second linear motor coil.
5. The system of claim 1, wherein the first signal comprises a
first sequence of values and the second signal comprises a second
sequence of values and the controller is configured to detect a
first change in slope in the first sequence and to detect a second
change in slope in the second sequence.
6. The system of claim 5, wherein the controller is configured to
detect a first endpoint by detecting a decrease in slope in the
first sequence and to detect a second endpoint by detecting a
decrease in slope in the second sequence.
7. The system of claim 6, comprising the first substrate and the
second substrate, wherein each of the first substrate and the
second substrate include an overlying layer and an underlying layer
having a lower coefficient of friction against the polishing pad
than the overlying layer.
8. The system of claim 5, wherein the controller is configured to
detect a first endpoint by detecting an increase in slope in the
first sequence and to detect a second endpoint by detecting a
increase in slope in the second sequence.
9. The system of claim 8, comprising the first substrate and the
second substrate, wherein each of the first substrate and the
second substrate include an overlying layer and an underlying layer
having a higher coefficient of friction against the polishing pad
than the overlying layer.
10. The system of claim 1, comprising a first motor to rotate the
first carrier head and a second motor to rotate the second carrier
head.
11. The system of claim 10, wherein the in-situ polishing
monitoring system includes a third current sensor to sense a third
current supplied to the first motor and generate a third signal,
and a fourth current sensor to sense a fourth current supplied to
the second motor and generate a fourth signal.
12. The system of claim 11, wherein the controller is configured to
receive the third signal and the fourth signal and independently
detect the first endpoint based on the first signal and third
signal, and detect the second endpoint based on the second signal
and the fourth signal.
13. The system of claim 12, wherein the controller is configured to
add the first signal and the third signal to generate a first
combined signal and to detect the first endpoint based on the first
combined signal, and to add the second signal and the fourth signal
to generate a second combined signal and to detect the second
endpoint based on the second combined signal.
14. The system of claim 12, wherein the controller is configured to
detect the first endpoint based on detecting an endpoint in either
the first signal or the third signal, and to detect the second
endpoint based on detecting an endpoint in either the second signal
or the fourth signal.
15. The system of claim 12, wherein the controller is configured to
detect the first endpoint based on detecting an endpoint in both
the first signal and the third signal, and to detect the second
endpoint based on detecting an endpoint in both the second signal
and the fourth signal.
16. A chemical mechanical polishing system, comprising: a platen to
support a polishing pad; a rotatable carrier head configured to
hold a substrate against the polishing pad; an actuator to sweep
the carrier head laterally across the polishing pad while the
substrate contacts the polishing pad; and an in-situ polishing
monitoring system comprising a current sensor to sense a current
supplied to the actuator and generate a signal, and a controller to
receive the signal and detect an endpoint for the substrate based
on the signal.
17. The system of claim 16, further comprising a track, a carriage
supported by the track, wherein the carrier head is suspended from
the carriage, and wherein the actuator is configured to move the
carriage along the track to sweep the carrier head.
18. The system of claim 16, wherein the signal comprises a sequence
of values and the controller is configured to detect a change in
slope in the sequence.
19. The system of claim 16, comprising a motor to rotate the
carrier.
20. The system of claim 19, wherein the in-situ polishing
monitoring system includes a second current sensor to sense a
second current supplied to the motor and generate a second signal,
and wherein the controller is configured to receive the second
signal and detect the endpoint based on the first signal and second
signal.
Description
TECHNICAL FIELD
[0001] This disclosure relates to monitoring polishing using motor
current.
BACKGROUND
[0002] An integrated circuit is typically formed on a substrate by
the sequential deposition of conductive, semiconductive, or
insulative layers on a silicon wafer. One fabrication step involves
depositing a filler layer over a non-planar surface and planarizing
the filler layer. For certain applications, the filler layer is
planarized until the top surface of a patterned layer is exposed. A
conductive filler layer, for example, can be deposited on a
patterned insulative layer to fill the trenches or holes in the
insulative layer. After planarization, the portions of the metallic
layer remaining between the raised pattern of the insulative layer
form vias, plugs, and lines that provide conductive paths between
thin film circuits on the substrate. For other applications, such
as oxide polishing, the filler layer is planarized until a
predetermined thickness is left over the non planar surface. In
addition, planarization of the substrate surface is usually
required for photolithography.
[0003] Chemical mechanical polishing (CMP) is one accepted method
of planarization. This planarization method typically requires that
the substrate be mounted on a carrier or polishing head. The
exposed surface of the substrate is typically placed against a
rotating polishing pad. The carrier head provides a controllable
load on the substrate to push it against the polishing pad. An
abrasive polishing slurry is typically supplied to the surface of
the polishing pad.
[0004] One problem in CMP is determining whether the polishing
process is complete, i.e., whether a substrate layer has been
planarized to a desired flatness or thickness, or when a desired
amount of material has been removed. Variations in the slurry
distribution, the polishing pad condition, the relative speed
between the polishing pad and the substrate, and the load on the
substrate can cause variations in the material removal rate. These
variations, as well as variations in the initial thickness of the
substrate layer, cause variations in the time needed to reach the
polishing endpoint. Therefore, determining the polishing endpoint
merely as a function of polishing time can lead to overpolishing or
underpolishing of the substrate.
[0005] In some systems, the substrate is monitored in-situ during
polishing, e.g., by monitoring the torque required by a motor to
rotate the platen or carrier head. However, existing monitoring
techniques may not satisfy increasing demands of semiconductor
device manufacturers.
SUMMARY
[0006] In some CMP systems, multiple substrates are polished
simultaneously on a single polishing pad. Consequently, the current
draw of the motor used to rotate the platen that supports the
polishing pad depends on the degree of polishing of both
substrates. This can make detection of exposure of an underlying
layer based on the platen motor current difficult. However, a
technique to counteract this problem is to independently monitor
the motor current of the individual motors that cause the
individual carrier heads to sweep laterally across the polishing
pad.
[0007] In one aspect, a chemical mechanical polishing system,
comprising a platen to support a polishing pad, a first rotatable
carrier head configured to hold a first substrate against the
polishing pad, a second rotatable carrier head configured to hold a
second substrate against the same polishing pad at the same time
that the first carrier head holds the first substrate against the
polishing pad, a first actuator to sweep the first carrier head
laterally across the polishing pad while the first substrate
contacts the polishing pad, a second actuator to sweep the second
carrier head laterally across the polishing pad while the second
substrate contacts the polishing pad, and an in-situ polishing
monitoring system including a first current sensor to sense a first
current supplied to the first actuator and generate a first signal,
a second current sensor to sense a second current supplied to the
second actuator and generate a second signal, and a controller to
receive the first signal and the second signal and independently
detect a first endpoint and a second endpoint for the first
substrate and the second substrate, respectively, based on the
first signal and the second signal.
[0008] Implementations can include one or more of the following
features. The system may include a track, a first carriage
supported by the track, and a second carriage supported by the
track. The first carrier head may be suspended from the first
carriage and the second carrier head may be suspended from the
second carriage. The first actuator may be configured to move the
first carriage along the track to sweep the first carrier head and
the second actuator may be configured to move the second carriage
along the track to sweep the second carrier head. The track may be
a magnetic track, the first actuator may include a first linear
motor coil and the second actuator may include a second linear
motor coil. The first signal may include a first sequence of
values, the second signal may include a second sequence of values,
and the controller may be configured to detect a first change in
slope in the first sequence and to detect a second change in slope
in the second sequence. The controller may be configured to detect
a first endpoint by detecting a decrease in slope in the first
sequence and to detect a second endpoint by detecting a decrease in
slope in the second sequence. Each of the first substrate and the
second substrate may include an overlying layer and an underlying
layer having a lower coefficient of friction against the polishing
pad than the overlying layer. The controller may be configured to
detect a first endpoint by detecting an increase in slope in the
first sequence and to detect a second endpoint by detecting a
increase in slope in the second sequence. Each of the first
substrate and the second substrate include an overlying layer and
an underlying layer having a higher coefficient of friction against
the polishing pad than the overlying layer. A first motor may
rotate the first carrier head and a second motor may rotate the
second carrier head. The in-situ polishing monitoring system may
include a third current sensor to sense a third current supplied to
the first motor and generate a third signal, and a fourth current
sensor to sense a fourth current supplied to the second motor and
generate a fourth signal. The controller may be configured to
receive the third signal and the fourth signal and independently
detect the first endpoint based on the first signal and third
signal, and detect the second endpoint based on the second signal
and the fourth signal. The controller may be configured to add the
first signal and the third signal to generate a first combined
signal and to detect the first endpoint based on the first combined
signal, and to add the second signal and the fourth signal to
generate a second combined signal and to detect the second endpoint
based on the second combined signal. The controller may be
configured to detect the first endpoint based on detecting an
endpoint in either the first signal or the third signal, and to
detect the second endpoint based on detecting an endpoint in either
the second signal or the fourth signal. The controller may be
configured to detect the first endpoint based on detecting an
endpoint in both the first signal and the third signal, and to
detect the second endpoint based on detecting an endpoint in both
the second signal and the fourth signal.
[0009] Implementations can include one or more of the following
potential advantages. Exposure of an underlying layer can be sensed
independently and more reliably for multiple substrates being
polished on a single polishing pad. Polishing for each substrate
can be halted more reliably at a target thickness.
[0010] The details of one or more embodiments are set forth in the
accompanying drawings and the description below. Other aspects,
features and advantages will be apparent from the description and
drawings, and from the claims.
DESCRIPTION OF DRAWINGS
[0011] FIG. 1 illustrates a plan view of an example of a polishing
apparatus.
[0012] FIG. 2 illustrates a schematic cross-sectional view of an
example of a polishing apparatus.
[0013] FIG. 3 illustrates a carriage assembly for a carrier
head.
[0014] FIGS. 4A and 4B are graphs of two motor current signals
generated by two sensors measuring current supplied to two motors
that cause two carrier heads to sweep laterally across the
polishing pad.
[0015] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0016] In some semiconductor chip fabrication processes, an
overlying layer, e.g., copper, silicon oxide or polysilicon, is
polished until an underlying layer of a different material, e.g., a
dielectric, such as silicon oxide, silicon nitride or a high-K
dielectric, is exposed. For some applications, the underlying layer
has a different coefficient of friction against the polishing layer
than the overlying layer. As a result, when the underlying layer is
exposed, the torque required by the motor to cause the platen or
carrier head to rotate at a specified rotation rate changes.
[0017] However, as noted above, if multiple substrates are being
polished simultaneously on the same polishing pad, the torque
required by the motor for the platen depends on the degree of
polishing of both substrates. This can make detection of exposure
of an underlying layer based on the platen motor current difficult.
However, by independently monitoring the current used by motors
that cause the carrier heads to sweep laterally across the
polishing surface, and independently detecting a change in motor
torque at these motors, the polishing endpoints can be determined
independently for the substrates.
[0018] FIG. 1 is a plan view of a chemical mechanical polishing
apparatus 100 for processing one or more substrates. The polishing
apparatus 100 includes a polishing platform 106 that at least
partially supports and houses one or more polishing stations 124.
The polishing apparatus 100 also includes a multiplicity of carrier
heads 126, each of which is configured to carry a substrate. The
multiplicity carrier heads 126 includes at least two carrier heads
126A, 126B. Each polishing station 124 is adapted to polish a
substrate that is retained in a carrier head 126.
[0019] The polishing apparatus 100 can also include one or more
load cups 122 adapted to facilitate transfer of a substrate between
the carrier heads 126 and a factory interface (not shown) or other
device (not shown) by a transfer robot 110. The load cups 122
generally facilitate transfer between the robot 110 and each of the
carrier heads 126.
[0020] Each polishing station 124 includes a polishing pad 130
supported on a platen 200 (see FIG. 2). The polishing pad 110 can
be a two-layer polishing pad with an outer polishing layer and a
softer backing layer.
[0021] At least one of the polishing stations 124 is sized such
that a plurality of carrier heads 126A, 126B can be positioned
simultaneously over the polishing pad 130 so that polishing of a
plurality of substrates can occur at the same time in the polishing
station 124. For example, each of the plurality of carrier heads
126A, 126B can hold a single substrate, and each carrier head 126A,
126B that is within the same polishing station can lower its
substrate into contact with the same polishing pad 130. Thus, a
plurality of substrates, e.g., one per carrier head, can be
polished simultaneously with the same polishing pad.
[0022] In some implementations, the polishing station 124 can
accommodate two carrier heads 126A, 126B. Thus, two substrates can
be polished simultaneously on the same polishing pad 130. However,
in some implementations the polishing station could be constructed
to accommodate three or more carrier heads.
[0023] The carrier heads 126 are coupled to a carriage 108 that is
mounted to an overhead track 128. The overhead track 128 allows
each carriage 108 to be selectively positioned around the polishing
platform 106. The configuration of the overhead track 128 and
carriages 108 facilitates positioning of the carrier heads 126
selectively over the polishing stations 124 and the load cups 122.
In the embodiment depicted in FIG. 1, the overhead track 128 has a
circular configuration (shown in phantom) which allows the
carriages 108 retaining the carrier heads 126 to be selectively
rotated over and/or clear of the load cups 122 and the polishing
stations 124. It is contemplated that the overhead track 128 may
have other configurations including elliptical, oval, linear or
other suitable orientation.
[0024] Each polishing station 124 of the polishing apparatus 100
can include a port 218 at the end of an arm 134 to dispense
polishing liquid 220 (see FIG. 2), such as abrasive slurry, onto
the polishing pad 130. Each polishing station 124 of the polishing
apparatus 100 can also include pad conditioning apparatus 132 to
abrade the polishing pad 130 to maintain the polishing pad 130 in a
consistent abrasive state.
[0025] FIG. 2 is a partial cross-sectional view of a polishing
station 124 of FIG. 1. The polishing station 124 includes the
platen 200 on which the polishing pad 130 can be mounted. The
platen 200 is coupled by a shaft 202 to a motor 204, e.g., a DC
induction motor, to rotate the platen 200 and the polishing pad 130
about a rotational axis.
[0026] Each of the carrier heads 126A, 126B is coupled to a rotary
motor 214A, 214B by a drive shaft 208A, 208B. Thus, each motor
214A, 214B can independently rotate the respective carrier head
126A, 126B about a rotational axis relative to the polishing pad
130 and platen 200.
[0027] The polishing system 100 is configured to sweep the carrier
heads 126A, 126B laterally across the polishing surface 212 of
polishing pad 130. The lateral sweep is in a direction parallel to
the polishing surface 212. The lateral sweep can be a linear or
arcuate motion. In particular, each motor 214A, 214B, drive shaft
208A, 208B and carrier head 126A, 126B can be suspended by a
carriage 108A, 108B that is supported by the track 128. Each
carriage 108A, 108B, can be independently driven along the track by
an associated actuator 106A, 106B. Each actuator 106A, 106B can be
a DC motor.
[0028] Each carrier head 126A, 126b is operable to hold a substrate
216A, 216B against the polishing pad 130. Each carrier head 126A,
126B can have independent control of the polishing parameters, for
example pressure, associated with each respective substrate.
[0029] Each carrier head 126A, 126B can include a retaining ring
224 to retain a substrate 216A, 216B below a flexible membrane 230.
Each carrier head 126A, 126B includes one or more independently
controllable pressurizable chambers 228 defined by the membrane
230, which can apply independently controllable pressurizes to
associated zones on the flexible membrane 230 and thus on the
substrate 216A, 216B. Although only one chamber per carrier head is
illustrated in FIG. 2 for ease of illustration, there could be two
chambers or more chambers, e.g., three or five chambers.
[0030] Optionally, each carrier head 126A, 126B can be coupled by
the shaft 208A, 208B to a linear actuator to independently lift or
lower the respective carrier head 126A, 126B in the Z direction
relative to a polishing surface 212 of the polishing pad 130.
Alternatively, the Z direction actuation can be provided by an
actuator, e.g., a pressurizable chamber, within the carrier head
126A, 126B.
[0031] FIG. 3 illustrates an exemplary implementation of the
connection of a single carrier head 126 to the track 128 in more
detail. The track 128 includes a plate 250 from which are suspended
two concentric rails 252. A magnetic track 254 is also suspended
from the plate 300, e.g., between the two concentric rails 302. The
carriage 108 is suspended from two carriage adapter assemblies 256,
each of which is suspended from and slidably engages an associated
rail 252. The carriage 108 also supports the actuator 106, e.g., a
linear motor coil, which fits between the opposing magnets in the
magnetic track 254. Other components that are supported by the
carriage 108, e.g., the motor 214A, can be located inside a
carriage housing 260.
[0032] Since the drive shaft 208 is held rigidly against lateral
motion by the carriage assembly 108, the resistance against lateral
motion that must be overcome by the motor coil 106 provides a
reasonably reliable measure of the friction of the substrate held
by the associated carrier head 126 against the polishing pad
130.
[0033] Although FIGS. 1-3 illustrate the lateral motion of the
carrier head as provided by motion of the carriage 108 along the
track 128, in another implementation the lateral motion is provided
by a sub-carriage suspended from the carriage 108. Such a
sub-carriage is movable, e.g., along rails, relative to the
carriage 108 and includes a motor to drive the sub-carriage
relative to the carriage. The carriage 108 could remain stationary
during the polishing operation, with the lateral motion provided
solely by the sub-carriage. Current used by the motor for the
sub-carriage would be provided to the controller 190 and used for
endpoint detection. The sub-carriage could be movable in a
direction different from, e.g., orthogonal to, the track 128.
[0034] In operation, when an even number of substrates, such as two
semiconductor substrates 216A, 216B, are provided to the load cups
122 of the polishing module 100 (FIG. 1), each carrier head 126A,
126B is positioned over the load cups 122 to facilitate transfer of
the two substrates 216A, 216B from the load cups 122 to the carrier
heads 126A, 126B. Once the substrates 216A, 216B are retained in
the carrier heads 126A, 126B, the carrier heads 126A, 126B are
positioned over the polishing pad 130 by action of the carriages
108. Each carrier head 126A, 126B is urged toward the polishing
surface 212 of the polishing pad 130. The motor 204 rotates the
platen 200, the rotary actuators 214A, 214B rotate the carrier
heads 126A, 126B, and the actuators 106A, 106B cause the carrier
heads 126A, 126B to sweep across the polishing surface 212.
[0035] A controller 190, such as a programmable computer, is
connected to each motor 204, 214A, 214B to independently control
the rotation rate of the platen 120 and the carrier heads 126A,
126B. For example, each motor can include an encoder that measures
the rotation rate of the associated drive shaft. Similarly, the
controller 190 is connected to each actuator 106A, 106B to
independently control the lateral motion of each carrier head 126A,
126B. For example, each actuator can include a linear encoder that
measures the position of the carriage along the track 128.
[0036] The controller 190 can include a central processing unit
(CPU) 192, a memory 194, and support circuits 196, e.g.,
input/output circuitry, power supplies, clock circuits, cache, and
the like. The memory is connected to the CPU 192. The memory is a
non-transitory computable readable medium, and can be one or more
readily available memory such as random access memory (RAM), read
only memory (ROM), floppy disk, hard disk, or other form of digital
storage. In addition, although illustrated as a single computer,
the controller 190 could be a distributed system, e.g., including
multiple independently operating processors and memories.
[0037] For each actuator 106A, 106B, a feedback control circuit,
which could be in the actuator itself, part of the controller, or a
separate circuit, receives the lateral sweep rate or position from
the encoder and adjusts the current supplied to the actuator to
ensure that the sweep matches at a desired sweep profile received
from the controller.
[0038] The polishing apparatus also includes an in-situ monitoring
system, e.g., a motor current or motor torque monitoring system,
which can be used to determine a polishing endpoint. The in-situ
monitoring system includes a sensor to measure a motor torque used
and/or a current supplied to actuators 106A and 106B.
[0039] For example, a current sensor 170A can monitor the current
supplied to the actuator 106A and a current sensor 170B can monitor
the current supplied to the actuator 106B. The output signal of the
current sensors 170A, 170B is directed to the controller 190.
Although the current sensors 170A, 170B are illustrated as part of
the actuators 106A, 106B, the current sensors could be part of the
controller (if the controller itself outputs the drive current for
the actuators) or separate circuits.
[0040] The output of each sensor 170A, 170B can be a digital
electronic signal (if the output of the sensor is an analog signal
then it can be converted to a digital signal by an ADC in the
sensor or the controller). Each digital signal is composed of a
sequence of signal values, with the time period between signal
values depending on the sampling frequency of the sensor. This
sequence of signal values can be referred to as a
signal-versus-time curve. Various filtering algorithms can be
applied to the "raw" signal from each sensors 170A, 170B to
generate the signal-versus-time curve.
[0041] In addition to receiving signals from the in-situ monitoring
system (and any other endpoint detection systems), the controller
190 can be connected to the polishing apparatus 100 to control the
polishing parameters, e.g., the various rotational rates of the
platen(s) and carrier head(s) and pressure(s) applied by the
carrier head, by connection to the respective motors or
actuators.
[0042] In conjunction, the combination of the sensors 170A, 170B
and the controller 190 can provide an in-situ monitoring system,
which can be used to independently detect polishing endpoints for
the substrates 216A, 216B and/or determine whether to adjust a
polishing parameter for the carrier heads 126A, 126B. In
particular, the in-situ monitoring system uses the motor current
drawn by the actuators 106A, 106B to determine the polishing
endpoints for the substrates 216A, 216B and/or determine whether to
adjust the polishing parameters for the carrier heads 126A,
126B.
[0043] FIG. 4A illustrates a signal-versus-time curve 300 for the
sensor 170A associated with the carrier head 126A. The controller
190 is configured to detect the polishing endpoint for the carrier
head 126A based on the signal-versus-time curve 300.
[0044] Initially, while the overlying layer is being polished, the
coefficient of friction between the substrate and the polishing pad
remains relatively constant, resulting in a relatively flat portion
302 of the curve 300. Eventually, a portion of the underlying layer
is exposed and the coefficient of friction starts to change. As a
result, there is a sudden increase in the slope of the curve 300,
providing a sloped portion 304. Once the underlying layer is
completely exposed, e.g., coefficient of friction stabilizes and
again becomes relatively constant, resulting in a second relatively
flat portion 306 of the curve 300.
[0045] In general, it is desirable to halt polishing once the
underlying layer is completely exposed, but without overpolishing.
Therefore the controller 190 can be configured to detect the change
from the sloped portion 304 to the flat portion 306 of the
signal-versus time curve 300. For example, the controller 190 can
be configured to calculate the slope of the curve 300, and to
compare the slope to a threshold value. Thus, the controller 190
can detect the polishing endpoint for the substrate 216A in the
carrier head at a first time T.sub.A.
[0046] FIG. 4B illustrates a signal-versus-time curve 310 for the
sensor 170B associated with the carrier head 126B. The
signal-versus-time curve 310 is similar to the signal-versus-time
curve 300, with a relatively flat portion 312, a sloped portion
314, and a second relatively flat portion 316. However, if the
polishing rates at the carrier heads 126A, 126B differ, the
underlying layer may be exposed at different times, resulting in
the sloped portion 314 occurring at a different time than the
sloped portion 304.
[0047] The controller 190 is configured to independently detect the
polishing endpoint for the carrier head 126B based on the
signal-versus-time curve 310. For example, the controller 190 can
be configured to detect the change from the sloped portion 314 to
the flat portion 316 of the signal-versus time curve 310. For
example, the controller 190 can be configured to calculate the
slope of the curve 310, and to compare the slope to a threshold
value. Thus, the controller 190 can independently detect the
polishing endpoint for the substrate 216B in the carrier head at a
second time T.sub.B that differs from the first time T.sub.A.
[0048] In the example illustrated in FIGS. 4A and 4B, the
underlying layer has a higher coefficient of friction than the
overlying layer, so that the motor current draw increases when the
underlying layer is exposed. However, the underlying layer could
have a lower coefficient of friction than the overlying layer, so
that the motor current draw decreases when the underlying layer is
exposed, and the detection algorithm could be adjusted
appropriately. In addition, the signal-versus time curves 300, 310
are relatively simple; more complex curves are possible if multiple
layers are being exposed or the substrate starts with significant
topology, in which case more complex algorithms may be needed to
trigger the endpoint detection.
[0049] Optionally, for each motor 214A, 214B, a feedback control
circuit, which could be in the motor itself, part of the
controller, or a separate circuit, receives the measured rotation
rate from the encoder and adjusts the current supplied to the motor
to ensure that the rotation rate of the drive shaft matches at a
rotation rate received from the controller.
[0050] In addition, current sensors can monitor the current
supplied to the motors 214A, 214B, the output signal of the current
sensors can be directed to the controller 190 to generate a
sequence of signal values for each motor 214A, 214B. Like sensors
170A, 170B, the current sensors for the motors 214A, 214B, could be
part of the motors, part of the controller (if the controller
itself outputs the drive current for the motors) or separate
circuits.
[0051] In some implementations, the sequence of signal values for
the motor current for motor 214A can be combined with the sequence
of signal values for the actuator 106A. Similarly, in some
implementations, the sequence of signal values for the motor
current for motor 214B can be combined with the sequence of signal
values for the actuator 106B. For example, in some implementations,
the signals values associated with the same carrier head could
simply be added together. As another example, in some
implementations, the controller 190 can be configured to indicate
an endpoint for a substrate when an endpoint is detected in both
the signal values for the actuator and the signal values for the
motor. As another example, in some implementations, the controller
190 can be configured to indicate an endpoint for a substrate when
an endpoint is detected in either the signal values for the
actuator or the signal values for the motor.
[0052] The above described polishing apparatus and methods can be
applied in a variety of polishing systems. For example, rather than
be suspended from a track, multiple carrier heads can be suspended
from a carousel, and lateral motion of the carrier heads can be
provided by a carriage that is suspend from and can move relative
to the carousel. The platen may orbit rather than rotate. Although
a plurality of polishing stations are illustrated in FIG. 1, there
could be just a single polishing station. The polishing pad can be
a circular (or some other shape) pad secured to the platen. Some
aspects of the endpoint detection system may be applicable to
linear polishing systems (e.g., where the polishing pad is a
continuous or a reel-to-reel belt that moves linearly). The
polishing layer can be a standard (for example, polyurethane with
or without fillers) polishing material, a soft material, or a
fixed-abrasive material. Terms of relative positioning are used; it
should be understood that the polishing surface and substrate can
be held in a vertical orientation or some other orientations.
[0053] While the foregoing is directed to embodiments of the
present invention, other and further embodiments of the invention
may be devised without departing from the basic scope thereof, and
the scope thereof is determined by the claims that follow.
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