US20140020830A1

US20140020830A1 - Carrier Head Sweep Motor Current for In-Situ Monitoring

Info

Publication number: US20140020830A1
Application number: US13/553,009
Authority: US
Inventors: Jagan Rangarajan; Alpay Yilmaz
Original assignee: Applied Materials Inc
Current assignee: Applied Materials Inc
Priority date: 2012-07-19
Filing date: 2012-07-19
Publication date: 2014-01-23

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.

Description

TECHNICAL FIELD

This disclosure relates to monitoring polishing using motor current.

BACKGROUND

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.
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.
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.
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

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.
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.
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.
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.
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

FIG. 1 illustrates a plan view of an example of a polishing apparatus.

FIG. 2 illustrates a schematic cross-sectional view of an example of a polishing apparatus.

FIG. 3 illustrates a carriage assembly for a carrier head.

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.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Each carrier head 126A, 126 b 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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_A.
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.
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_Bthat differs from the first time T_A.
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.
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.
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.
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.
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.
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.

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.