MEASUREMENT OF PAD THICKNESS AND CONTROL OF CONDITIONING

Abstract

A conditioning process includes rotating a polishing pad about an axis of rotation, conditioning the polishing pad by sweeping an abrasive disk in a path across a surface of the polishing pad between an inner radial distance from the axis of rotation and an outer radial distance from the axis of rotation, sweeping a sensor across the polishing pad while conditioning the polishing pad, measuring a thickness of the polishing pad at a plurality of positions between the inner radial distance and the outer radial distance with the sensor, and adjusting at least one of a dwell time or a pressure of the abrasive disk against the polishing pad for a portion of the path based on measurements of the thickness by the sensor such that the polishing pad wears to a more uniform thickness than without such adjustment.

Description

TECHNICAL FIELD The present disclosure relates to control of conditioning during chemical mechanical polishing.

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 conductive 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. A polishing liquid, typically including an abrasive slurry, is supplied to the surface of the polishing pad.

Over time, the polishing process can heat and compress the polishing pad surface, creating “glazing” of the polishing pad which can reduce the polishing rate and adversely affect substrate uniformity. In addition, the polishing process can embed debris in the polishing pad, increasing defects. To counteract these problems, the surface of the polishing pad surface can be “conditioned,” a process which typically presses a rotating abrasive disk against the surface of the polishing pad. The conditioning process abrades the polishing pad surface, deglazing and removing debris from the polishing pad surface.

SUMMARY

Since conditioning abrades the polishing pad surface, it can gradually wear away the polishing pad. One problem is that wearing of the polishing pad from conditioning can be non-uniform, e.g., more pad material can removed from an annular region between the center and edge of the polishing pad. This non-uniform wear results in non-uniformity in the polishing pad thickness, which can lead to non-uniformity in the polishing rate. Another unrelated problem is that if a polishing pad is not properly installed on the platen, air bubbles can be trapped between the polishing pad and platen. These air bubbles can create bumps in the pad surface that can lead to polishing non-uniformity or defects.

The thickness profile of the polishing pad can be measured by a sensor, e.g., an eddy current sensor that is attached to the conditioning head or arm. The dwell time and/or downward pressure of the conditioning disk can be controlled based on the measured thickness profile so that the polishing pad is maintained with a uniform thickness. Alternatively or in addition, air bubbles between the polishing pad and the platen can be detected.

In one aspect, a method of controlling a conditioning process includes rotating a polishing pad about an axis of rotation, conditioning the polishing pad by sweeping an abrasive disk in a path across a surface of the polishing pad between an inner radial distance from the axis of rotation and an outer radial distance from the axis of rotation, sweeping a sensor across the polishing pad while conditioning the polishing pad, measuring a thickness of the polishing pad at a plurality of positions between the inner radial distance and the outer radial distance with the sensor, and adjusting at least one of a dwell time or a pressure of the abrasive disk against the polishing pad for a portion of the path based on measurements of the thickness by the sensor such that the polishing pad wears to a more uniform thickness than without such adjustment.

Implementations may include one or more of the following features. The dwell time of the abrasive disk for the portion of the path may be adjusted. The pressure of the abrasive disk against the polishing pad for the portion of the path may be adjusted. Sweeping the abrasive disk may include suspending the abrasive disk from an arm and pivoting the arm between a first angle and a second angle. A pressure profile for the conditioning disk may be stored. The pressure profile may identify a pressure to apply as a function of an angular position of the arm. Adjusting the pressure may include adjusting the pressure profile. A position profile for the conditioning disk may be stored. The position profile may identify an angular position for the arm as function of a time. Adjusting the dwell time may include adjusting the position profile. The sensor may be suspended from the arm. The sensor may be positioned adjacent the disk. Each measurement of the thickness by the sensor may be stored with an angular position of the arm at a time an associated measurement was made. The sensor may be an eddy current sensor.

In another aspect, a method of operating a polishing system includes installing a polishing pad on a rotatable platen, sweeping a sensor across the polishing pad while the polishing pad is on the platen, measuring a distance between the sensor and the platen at a plurality of positions between an inner radial distance and an outer radial distance with the sensor, and detecting an air bubble trapped between the polishing pad and the sensor based on measurements of the distance by the sensor.

Implementations may include one or more of the following features. The polishing pad may be removed prior to polishing a substrate with the polishing pad. The sensor may be suspended from an arm of a conditioner apparatus. Sweeping the sensor may include pivoting the arm. The sensor may be an eddy current sensor.

In another aspect, a conditioner system includes a base secured to a frame of a chemical mechanical polishing system, an arm connected to the frame and pivotally movable by the base over a polishing pad of the chemical mechanical polishing system, a conditioning head suspended from the arm to hold an abrasive disk against the polishing pad, and an eddy current sensor suspended from the arm and positioned to measure a thickness of the polishing pad.

Implementations may include one or more of the following features. The eddy current sensor may be suspended from the conditioning head. The eddy current sensor may be configured to contact the polishing pad. The eddy current sensor may be separated from the polishing pad by a gap. The conditioning head and arm may be configured such that pivoting the arm sweeps the abrasive disk in a path across a surface of the polishing pad between an inner radial distance from an axis of rotation of a platen and an outer radial distance from the axis of rotation, and a controller may be configured to adjust at least one of a dwell time or a pressure of the abrasive disk against the polishing pad for a portion of the path based on measurements of the thickness by the sensor such that the polishing pad wears to a more uniform thickness than without such adjustment.

Certain implementations can include one or more of the following advantages. The thickness of the polishing pad can be measured without halting of the polishing operation, thus improving throughput. The polishing pad can be maintained with a uniform thickness, thus improving polishing uniformity and reducing within-wafer non-uniformity (WIWNU). Air bubbles between the polishing pad and the platen can be detected, and the polishing pad can be replaced as a precautionary measure.

The details of one or more implementations 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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a chemical mechanical polishing apparatus.

FIG. 2 is a schematic top view of a chemical mechanical polishing apparatus.

FIG. 3 is a graph of sensor signal (in volts) as a function of thickness of a shim between the sensor and platen.

FIG. 4 is a flow chart of a method of operating a polishing system.

FIG. 5 is a flow chart of a method of controlling a conditioning process.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of a polishing apparatus 100. The polishing apparatus 100 includes a rotatable disk-shaped platen 120 on which a polishing pad 110 is situated. The platen is operable to rotate about an axis 125. For example, a motor 121 can turn a drive shaft 124 to rotate the platen 120. The polishing pad 110 can be detachably secured to the platen 120, for example, by a layer of adhesive. The polishing pad 110 can be a two-layer polishing pad with an outer polishing layer 112 and a softer backing layer 114.

The polishing apparatus 100 can include a combined slurry/rinse arm 130. During polishing, the arm 130 is operable to dispense a polishing liquid 132, such as a slurry, onto the polishing pad 110. While only one slurry/rinse arm 130 is shown, additional nozzles, such as one or more dedicated slurry arms per carrier head, can be used.

The polishing apparatus 100 can further include a carrier head 140. The carrier head 140 may be operable to hold a substrate 10 against the polishing pad 110. While only one carrier head 140 is shown, additional carrier heads can be used and may be preferable in some implementations. In such embodiments, each carrier head 140 can have independent or joint control of the polishing parameters, for example pressure, associated with each respective substrate.

The carrier head 140 can be suspended from a support structure 146, e.g., a carousel, and is connected by a drive shaft 142 to a carrier head rotation motor 144 so that the carrier head can rotate about an axis 148. Optionally, carrier head 140 can oscillate laterally, e.g., on sliders on the carousel 146; or by rotational oscillation of the carousel itself. In operation, the platen is rotated about its central axis 125 (see arrow A in FIG. 2), and the carrier head is rotated about its central axis 148 and translated laterally across the top surface of the polishing pad 110.

Referring to FIGS. 1 and 2, the polishing apparatus 100 also includes a polishing pad conditioner 150 to abrade the polishing pad 110 to maintain the polishing pad 110 in a consistent abrasive state. The pad conditioner 150 can be mounted on an immobile frame 126, e.g., the frame that supports the drive shaft 124 for the platen, of the polishing apparatus 100. The pad conditioner 150 includes a conditioner head 152, a base 156, and an arm 154. The arm 154 can have a first end connected to the conditioner head 152, e.g., the conditioner head 152 is supported from the first end of the arm 152. A second end of the arm 154 is connected to the base 154.

The conditioner head 152 includes an abrasive disk 160, e.g., a disk coated with diamond particle or the like, and an actuator 162 to apply a downward pressure to abrasive disk 160, e.g., through a drive shaft 164, in order to press the abrasive disk 160 against the polishing pad 110. The abrasive disk 160 can be rotatable. The actuator 162 can be located in the conditioner head 152 as illustrated in FIG. 1 with only the drive shaft 164 undergoing vertical potion, or the actuator 162 could be located in the base 156 so that both the arm 152 and entire conditioner head 152 undergo vertical motion. A motor, located in the conditioner head 152 or the base 156, can rotate the drive shaft 164 to rotate the abrasive disk 160.

The base 154 can be secured to frame 126, and can pivotally connect the arm 154 to the frame 126. A motor 166 in or coupled to the base 154, e.g., between the base 154 and the frame 126, can cause the arm 154 to horizontally swing over the platen 120 (and the polishing pad 110 if it is installed).

In addition, in some implementations, the arm 154 can pivot vertically relative to the frame 16. In such implementations, the actuator 162 can located in the base 156, and can cause the arm 154 to swing vertically (see arrow D in FIG. 1). Thus, the abrasive disk 160 can be lowered into contact with the polishing pad 110, and the pressure of the abrasive disk 160 against the polishing pad 110, can be controlled.

In operation, the motor 166 causes the arm 154 to swing back and forth, thus causing the conditioner head 152 to sweep back and forth across the surface of a polishing pad 110 (see arrow B in FIG. 2). In conjunction, the actuator 162 presses the abrasive disk 160 down on the polishing pad 110 while the abrasive disk 160 rotates, thus conditioning the polishing pad.

The polishing apparatus 100 can also optionally include a cup 158, e.g., supported on the frame 126. The cup 158 can contain a fluid for rinsing the abrasive disk 160, and can include nozzles to spray the abrasive disk 160 and/or the underside of the conditioner head 152. Between conditioning operations, the arm 154 can pivot to locate the conditioning head 152 over the cup 158.

In some implementations, the polishing pad 110 is conditioned by the pad conditioner 150 while the polishing pad 110 polishes a substrate 10 which is mounted on the carrier head 140. The conditioner head 152 can sweep across the polishing pad 110 with a motion that is synchronized with the motion of the carrier head 140 to avoid collision.

For a conditioning operation, the base 156 can rotate the arm 154 between a first angle and a second angle. This action sweeps the abrasive disk in a path across a surface of the polishing pad 110 between an inner radial distance from the axis of rotation 125 and an outer radial distance from the axis of rotation 125.

A sensor 170 is suspended from the arm 154. For example, the sensor 170 connected directly to, e.g., suspended from, the conditioner head 152, or the sensor 170 can be connected directly to the arm 154 near the conditioner head 152. The sensor 170 can be connected to the actuator 162 such that the sensor 170 moves up and down with the abrasive disk 160, but does not rotate with the abrasive disk 160. The sensor 170 can be located at the bottom of a support 172 that is connected to the arm 154 or conditioner head 172.

In some implementations, the support 172 is configured such that the sensor 170 remains separated from the polishing pad 110 by a gap. For example, assuming that the arm 154 swings vertically (e.g., the abrasive disk 160 is lowered onto the polishing pad 110 by a change in the angle of inclination of the arm 154), the projection of the support 172 and sensor 170 below the conditioner head 152 can be less than the projection of the abrasive disk 160 below the conditioner head 152. Thus, when the actuator 162 lowers the arm 154 and the abrasive disk 160, the abrasive disk 160 contacts the polishing pad 110 before the sensor 170. This stops motion of the arm 154, and thus stops further downward motion of the sensor 170 so that the sensor remains separated from the polishing pad 110. In addition, since the arm 154 is stopped when the abrasive disk 160 contacts the polishing pad, the vertical position depends the thickness of the polishing pad, and consequently the distance of the sensor 170 from the platen 120 also depends on the thickness of the polishing pad 110.

In some implementations, the support 172 is configured such that the sensor 170 contacts the polishing pad 110. For example, the support 172 can include a spring to press the sensor 170 against the polishing pad 110, or the sensor 170 can simply rest on the polishing pad 110. In these implementations, since the sensor 170 is contacting the polishing pad 110, the distance of the sensor 170 from the platen 120 depends on the thickness of the polishing pad 110.

The sensor 170 is configured to measure a height of the top of the polishing pad 110 above the platen. In some implementations, the sensor 170 is an eddy current sensor. For example, an eddy current sensor can measure the distance between the sensor 170 and the platen 120. Since the vertical position of the sensor 170 depends on the thickness of the polishing pad 110, the signal provides either a measure of the height of the top of the polishing pad or a thickness of the polishing pad.

A controller 190, e.g., one or more programmable computers, can be connected to and receive signals from the sensor 170. Referring to FIG. 3, the sensor 170, e.g., the eddy current sensor, can be calibrated. For example, shims of known thickness between the sensor 170 and the platen 120, and a signal strength can be measured for each of these known thicknesses. A function, e.g., a line 200, can be fit to the data (i.e., the pairs of thickness and signal strength values). Returning to FIGS. 1 and 2, this function can be stored in the controller 190, which can then calculate the distance of the sensor 170 from the platen 120 for a particular signal strength from the sensor 170.

As the arm 154 pivots, the sensor 170 will sweep across radial positions on the polishing pad between the inner radial distance and the outer radial distance travelled by the abrasive disk 160 (see arrow C in FIG. 2). Thus, the sensor 170 generates a plurality of measurements of the thickness of the polishing pad 110, with at least some of the measurements made at positions between the inner radial distance and the outer radial distance.

The controller 190 can also be connected to the motor 166 and/or the actuator 162. The controller 190 can be configured to control the motor 166 to control the radial sweep of the abrasive disk 120 across the polishing pad and/or control the actuator 162 to control the downward pressure of the abrasive disk 120 against the polishing pad. For example, the controller 190 can store an position profile for the conditioning disk. The position profile can identify an angular position for the arm as function of a time. As another example, the controller 190 can store a pressure profile for the conditioning disk. This pressure profile can identify a pressure to apply as a function of an angular position of the arm.

Each measurement of the thickness by the sensor 170 can be stored, e.g., by the controller 190, with an angular position of the arm 154 at a time an associated measurement was made. This angular position of the arm 154 can be obtained from the position profile, or from a rotary encoder the measures the position of the arm 154.

FIG. 4 illustrates a method 220 of operating the polishing system 100 that can take advantage of the measurements from the sensor 170. Initially, the polishing pad 110 is installed on the platen 120 (step 222). The sensor 170 is swept across the polishing pad 110 while the polishing pad 110 is on the platen 120 (step 224). This can use the sensor 170 attached to the arm 154 of the conditioner system 150 as discussed above, or the sensor 170 could be attached to another movable part. A distance between the sensor and the platen is measured at a plurality of positions (step 226). These positions can be between an inner radial distance and an outer radial distance of the axis of rotation of the platen. Based on the measured distances, an air bubble trapped between the polishing pad and the sensor can be detected (step 228). For example, a non-uniformity in the thickness measurement that exceeds a threshold value for a “fresh” polishing pad (i.e., a polishing pad that has not previously been used for polishing) can indicate the presence of an air bubble, since the fresh polishing pad should have good thickness uniformity. The polishing pad 110 can then be removed, preferably prior to polishing a substrate with the polishing pad. The controller 190 can be configured to automatically generate an alert if the thicknesses exhibit a non-uniformity that exceeds the threshold, or a user could visually inspect the data, e.g., on a display with a graphical user interface, and make the determination.

In some implementations, the controller is configured to adjusting at least one of a dwell time or a pressure of the abrasive disk against the polishing pad for a portion of the path based on measurements of the thickness by the sensor such that the polishing pad wears to a more uniform thickness than without such adjustment.

FIG. 5 illustrates a method 240 of controlling conditioning that can take advantage of the measurements from the sensor 170. The rotating polishing pad 110 is conditioned by the abrasive disk 160 (step 242) For example, the abrasive disk 160 can be swept in a path across a surface of the polishing pad 110 between an inner radial distance from the axis of rotation 125 and an outer radial distance from the axis of rotation 125. The sensor 170 is swept across the polishing pad 110 while the polishing pad 110 is conditioned (step 244), and a thickness of the polishing pad 110 is measured at a plurality of positions (step 246). The positions can be between the inner radial distance and the outer radial distance. The controller adjusting at least one of a dwell time or a pressure of the abrasive disk against the polishing pad for a portion of the path based on measurements of the thickness by the sensor such that the polishing pad wears to a more uniform thickness than without such adjustment (step 248). In general, in regions, e.g., radial regions, where the polishing pad is thinner, the pressure of the abrasive disk and/or the dwell time can be reduced, thus reducing the wear on those regions. Alternatively or in addition, in regions, e.g., radial regions, where the polishing pad is thicker, the pressure of the abrasive disk and/or the dwell time can be increased, thus increasing the wear on those regions.

To adjust the dwell time, the position profile can be adjusted. In some implementations, only the dwell time is adjusted. In some implementations, only the pressure is adjusted. In some implementations, both the dwell time and the pressure are adjusted. To adjust the pressure, the stored pressure profile can be adjusted. With the sensor 170 located on the arm 154, since the thickness measurements can be stored with the associated angular position of the arm 154, and since the pressure profile and position profile also use the angular position of the arm 154, it may be possible for the controller 190 to determine the adjustments without having to convert measurements made in one frame of reference to another frame of reference, thereby reducing the complexity of the control algorithm.

As used in the instant specification, the term substrate can include, for example, a product substrate (e.g., which includes multiple memory or processor dies), a test substrate, a bare substrate, and a gating substrate. The substrate can be at various stages of integrated circuit fabrication, e.g., the substrate can be a bare wafer, or it can include one or more deposited and/or patterned layers. The term substrate can include circular disks and rectangular sheets.

Embodiments of the invention and all of the functional operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structural means disclosed in this specification and structural equivalents thereof, or in combinations of them. Embodiments of the invention can be implemented as one or more computer program products, i.e., one or more computer programs tangibly embodied in an information carrier, e.g., in a machine-readable non-transitory storage device or in a propagated signal, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple processors or computers. A computer program (also known as a program, software, software application, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file. A program can be stored in a portion of a file that holds other programs or data, in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network.

The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit).

The above described polishing apparatus and methods can be applied in a variety of polishing systems. Either the polishing pad, or the carrier head, or both can move to provide relative motion between the polishing surface and the substrate. For example, the platen may orbit rather than rotate. 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 orientation during operation.

Particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results.

Claims

1. A method of controlling a conditioning process, comprising:

rotating a polishing pad about an axis of rotation;

conditioning the polishing pad by sweeping an abrasive disk in a path across a surface of the polishing pad between an inner radial distance from the axis of rotation and an outer radial distance from the axis of rotation;

sweeping a sensor across the polishing pad while conditioning the polishing pad;

measuring a thickness of the polishing pad at a plurality of positions between the inner radial distance and the outer radial distance with the sensor; and

adjusting at least one of a dwell time or a pressure of the abrasive disk against the polishing pad for a portion of the path based on measurements of the thickness by the sensor such that the polishing pad wears to a more uniform thickness than without such adjustment.

2. The method of claim 1, comprising adjusting the dwell time of the abrasive for the portion of the path.

3. The method of claim 1, comprising adjusting the pressure of the abrasive disk against the polishing pad for the portion of the path.

4. The method of claim 1, wherein sweeping the abrasive disk includes suspending the abrasive disk from an arm and pivoting the arm between a first angle and a second angle.

5. The method of claim 4, comprising storing a pressure profile for the conditioning disk, the pressure profile identifying a pressure to apply as a function of an angular position of the arm, and wherein adjusting the pressure comprises adjusting the pressure profile.

6. The method of claim 4, comprising storing an position profile for the conditioning disk, the position profile being an angular position for the arm as function of a time, and wherein adjusting the dwell time comprises adjusting the position profile.

7. The method of claim 4, comprising suspending the sensor from the arm.

8. The method of claim 7, wherein the sensor is positioned adjacent the disk.

9. The method of claim 7, further comprising storing each measurement of the thickness by the sensor with an angular position of the arm at a time an associated measurement was made.

10. The method of claim 1, wherein the sensor comprises an eddy current sensor.

11. A method of operating a polishing system, comprising:

installing a polishing pad on a rotatable platen;

sweeping a sensor across the polishing pad while the polishing pad is on the platen;

measuring a distance between the sensor and the platen at a plurality of positions between an inner radial distance and an outer radial distance with the sensor; and

detecting an air bubble trapped between the polishing pad and the sensor based on measurements of the distance by the sensor.

12. The method of claim 11, further comprising removing the polishing pad prior to polishing a substrate with the polishing pad.

13. The method of claim 11, further comprising suspending the sensor from an arm of a conditioner apparatus.

14. The method of claim 13, wherein sweeping the sensor includes pivoting the arm.

15. The method of claim 1, wherein the sensor comprises an eddy current sensor.

16. A conditioner system, comprising:

a base secured to a frame of a chemical mechanical polishing system;

an arm connected to the frame and pivotally movable by the base over a polishing pad of the chemical mechanical polishing system;

a conditioning head suspended from the arm to hold an abrasive disk against the polishing pad; and

an eddy current sensor suspended from the arm and positioned to measure a thickness of the polishing pad.

17. The conditioner system of claim 16, wherein the eddy current sensor is suspended from the conditioning head.

18. The conditioner system of claim 16, wherein the eddy current sensor is configured to contact the polishing pad.

19. The conditioner system of claim 16, wherein the eddy current sensor is separated from the polishing pad by a gap.

20. The conditioner system of claim 16, wherein the conditioning head and arm are configured such that pivoting the arm sweeps the abrasive disk in a path across a surface of the polishing pad between an inner radial distance from an axis of rotation of a platen and an outer radial distance from the axis of rotation, and comprising a controller configured to adjust at least one of a dwell time or a pressure of the abrasive disk against the polishing pad for a portion of the path based on measurements of the thickness by the sensor such that the polishing pad wears to a more uniform thickness than without such adjustment.