Acoustic Monitoring of the CMP Holding Ring

Abstract

A chemical mechanical polishing apparatus includes a platen that supports a polishing pad, a carrier head for holding the surface of a substrate against the polishing pad, an acoustic sensor supported on the platen, and a motor for generating relative movement between the platen and the carrier head so as to polish the substrate. The carrier head includes a retaining ring for holding the substrate, and the acoustic sensor travels within a path below the carrier head and the retaining ring. A controller is configured to analyze signals from the acoustic sensor and determine characteristics of the retaining ring based on the signals.

Description

Technical Field

[0001] The present disclosure relates to in - situ monitoring of chemical mechanical polishing, and more particularly to acoustic monitoring.

Background Art

[0002] Integrated circuits are generally formed on a substrate by successive deposition of conductive, semiconductive, or insulating layers on a silicon wafer. One manufacturing step involves depositing a fill layer on a non - planar surface and planarizing the fill layer. In some applications, for example, to fill trenches or holes in an insulating layer with a conductive fill material, the fill layer is planarized until the upper surface of the patterned layer is exposed. In other applications, such as oxide polishing, the fill layer is planarized, for example, by polishing for a predetermined time period to leave a portion of the fill layer on the non - planar surface. Further, planarization of the substrate surface is typically required for photolithography.

[0003] Chemical mechanical polishing (CMP) is an accepted method of planarization. This planarization method generally requires that the substrate be mounted on a carrier head or polishing head. The exposed surface of the substrate is generally placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to press the substrate against the polishing pad. The carrier head generally surrounds the substrate and includes a retaining ring for holding the substrate beneath the carrier head. The lower surface of the retaining ring can be pressed against the polishing pad and may wear as polishing progresses. Thus, the retaining ring may need to be replaced periodically.

[0004] In some systems, the substrate is monitored in - situ during polishing. Acoustic monitoring of the substrate during polishing has been proposed.

Summary of the Invention

[0005] In one aspect, a chemical mechanical polishing apparatus includes a platen for supporting a polishing pad, a carrier head for holding the surface of a substrate against the polishing pad, an acoustic sensor supported on the platen, and a motor for generating relative movement between the platen and the carrier head to polish the substrate. The carrier head includes a retaining ring for holding the substrate, and the acoustic sensor travels within a path below the carrier head and the retaining ring. A controller is configured to analyze a signal from the acoustic sensor and determine a characteristic of the retaining ring based on the signal.

[0006] In another aspect, a chemical mechanical polishing apparatus includes a platen for supporting a polishing pad, a carrier head for holding the surface of a substrate against the polishing pad, an acoustic sensor supported on the platen, and a motor for generating relative movement between the platen and the carrier head to polish the substrate. The carrier head includes a retaining ring for holding the substrate, and the acoustic sensor travels within a path below the carrier head and the retaining ring. The controller is configured to select a portion of the signal from the acoustic sensor corresponding to when the acoustic sensor is below the retaining ring and, based on the selected portion of the signal from the acoustic sensor, generate an alert or modify a polishing parameter.

[0007] One or more of the following possible advantages may be realized. The retaining ring can be monitored in real-time and in an in-situ manner. Whether the retaining ring has been "broken in" can be determined by in-situ monitoring, thus reducing the time required for certification of the polishing system. The wear status of the retaining ring can be monitored. The signal from the acoustic monitoring system may depend on the flatness or taper angle of the bottom surface of the retaining ring, and this signal can be used as a basis for adjusting other polishing parameters to improve wafer-to-wafer (WTW) and within-wafer (WIW) polishing uniformity.

[0008] 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.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0010] In the figures, like reference numerals indicate like elements.

[0011] During polishing, the polishing pad wears the bottom surface of the holding ring. Generally, the wear does not occur at a uniform rate radially across the holding ring, and thus the bottom surface takes on non-flat shape dimensions. However, the wear of the holding ring eventually reaches an equilibrium state, and thus the bottom surface of the holding ring retains substantially the same shape dimensions when the ring has worn until the process or polishing conditions change. Due to the ongoing change in the holding ring shape dimensions, the polishing rate profile will drift until the holding ring reaches an equilibrium state.

[0012] To reduce polishing variations between or across substrates, the retention ring can be "broken in" before being used in the polishing process. One way to break in the retention ring is, for example, to simulate substrate polishing by pressing the ring against a moving polishing pad such that the ring wears until it reaches its equilibrium shape dimensions. However, a drawback of "breaking in" is that it is time consuming and requires the use of the polishing apparatus. As a result, the break-in process is a downtime of the polishing apparatus during which polishing cannot be performed, increasing the cost of ownership.

[0013] By acoustically monitoring the retention ring, the time at which the retention ring is broken in can be determined in real time and in situ. Accordingly, the time required for break-in can be reduced.

[0014] As another problem, the effect of the substrate on the inner surface of the retention ring can create grooving on the inner surface. This grooving can affect the pressure applied to the substrate edge and thus can act on the polishing uniformity.

[0015] By acoustically monitoring the retention ring, the formation of grooving on the inner surface of the retention ring can be detected. This allows the retention ring to be replaced promptly, thus reducing the likelihood of polishing non-uniformity.

[0016] More generally, acoustic monitoring of the retention ring can be used to improve the polishing process. For example, signals from an acoustic monitoring system can indicate the shape dimensions and / or smoothness of the lower surface of the retention ring that can affect the polishing profile of the substrate, particularly near the substrate edge. Accordingly, these signals can be used as inputs for the control of various polishing parameters, such as the pressure applied in various regions of the substrate, to improve polishing performance.

[0017] Any of these problems can be addressed and used independently of other problems, or the system can address multiple problems.

[0018] FIG. 1 shows an example of a polishing apparatus 100. The polishing apparatus 100 includes a rotatable disk-shaped platen 120 with a polishing pad 110 thereon. The polishing pad 110 can be a two-layer polishing pad having an outer polishing layer 112 and a softer backing layer 114. The platen is operable to rotate about an axis 125. For example, a motor 121, such as a DC induction motor, can rotate a drive shaft 124 to rotate the platen 120.

[0019] The polishing apparatus 100 can include a port 130 for dispensing a polishing liquid 132, such as an abrasive slurry, onto the polishing pad 110. The polishing apparatus can also include a polishing pad conditioner for polishing the polishing pad 110 to maintain the polishing pad 110 in a consistent abrasive state.

[0020] The polishing apparatus 100 includes a carrier head 140. The carrier head 140 is operable to hold a substrate 10 against the polishing pad 110. The carrier head 140 includes a holding ring 170 for holding the substrate 10 below a flexible membrane 144. The holding ring 170 can include a lower portion 172 formed of a wearable plastic, such as PPS, suitable for the polishing process, and an upper portion 174 formed of a more rigid material, such as metal. Alternatively, the entire holding ring can be formed of plastic. The holding ring 170 is generally fixed to the carrier head 140 with screws so that the holding ring 170 can be removed and replaced.

[0021] The carrier head 140 includes one or more independently controllable pressurizable chambers, defined by a membrane, that can apply independently controllable pressure to associated zones on the flexible membrane 144 and thus on the substrate 10, such as, for example, three chambers 146a - 146c (see FIG. 1). For ease of explanation, only three chambers are shown in FIG. 1, but there can be one or two chambers, or four or more chambers, such as, for example, five chambers.

[0022] The carrier head 140 is suspended from a support structure 150, such as a carousel or a track, and is connected by a drive shaft 152 to a carrier head rotation motor 154, such as a DC induction motor, so that the carrier head can rotate about an axis 155. Optionally, each carrier head 140 can vibrate laterally, for example, on a slider on the carousel 150, or by the rotational vibration of the carousel itself, or by sliding along a track. During normal operation, the platen is rotated about its central axis 125, each carrier head is rotated about its central axis 155, and is translated laterally across the upper surface of the polishing pad.

[0023] A controller 190, such as a programmable computer, is connected to the motors 121, 154 to control the rotation rates of the platen 120 and the carrier head 140. For example, each motor can include an encoder that measures the rotation rate of the associated drive shaft.

[0024] The polishing apparatus 100 includes at least one in - situ acoustic monitoring system 160. The in - situ acoustic monitoring system 160 includes one or more acoustic signal sensors 162. Each acoustic signal sensor can be installed at one or more locations on the platen 120. In particular, the in - situ acoustic monitoring system can be configured to detect acoustic signals from the retaining ring 170.

[0025] To sense the angular position of the platen 120, a position sensor, such as an optical interrupter connected to the edge of the platen or a rotary encoder, can be used. This makes it possible for only the portion of the signal measured when the sensor 162 is close to the retaining ring 170, for example, when the sensor 162 is below the retaining ring, to be used as indicating the retaining ring condition in subsequent signal processing.

[0026] In the implementations shown in FIGS. 1 and 3, the acoustic monitoring system 160 includes an acoustic sensor 162 supported and positioned by a platen 120 for receiving acoustic signals from the substrate 10 through the polishing pad 110. The acoustic sensor 162 can be partially or fully within a recess 164 on the upper surface of the platen 120. In some implementations, the upper surface of the acoustic sensor 162 is in the same plane as the upper surface of the platen 120.

[0027] In some implementations, the portion of the polishing pad immediately above the acoustic sensor 162 can include an acoustic window 119, for example, a region having a lower acoustic impedance than the surrounding polishing material. The acoustic window 119 can extend through the polishing layer 112, or the backing layer 114, or both. However, if the acoustic transmission of the polishing pad is high enough, the acoustic window 119 may not be necessary.

[0028] The acoustic sensor 162 is a contact acoustic sensor having a surface connected to (e.g., in direct contact with, or having only an adhesive layer for adhesion to, or having only an acoustic gel for acoustic signal transmission from) a portion of the polishing pad, such as the polishing layer 112, or the backing layer 114, or the acoustic window 119. For example, the acoustic sensor 162 can be an electromagnetic acoustic transducer or a piezoelectric acoustic transducer. The piezoelectric sensor can include a rigid contact plate, such as stainless steel, placed in contact with the body to be monitored, and a piezoelectric assembly on the back side of the contact plate, such as a piezoelectric layer sandwiched between two electrodes.

[0029] In some implementations, the acoustic sensor 162 is located within a recess 169 in the housing 163. An optional spring 165 can be disposed between the housing 163 and the support 167 to provide pressure against the housing 163. The pressure on the housing 163 presses the acoustic sensor 162 into contact with a portion of the polishing pad 110. Alternatively, the spring 165 can press directly against the acoustic sensor 162, for example, when the housing is not used. In some implementations, the spring 165 is a long travel spring that supplies a pressure similar to that of a strong spring 165 over a larger compression range.

[0030] The acoustic sensor 162 can be connected by a circuit 168 to a power source and / or other signal processing electronics 166 through a rotary joint, such as a mercury slip ring.

[0031] In some implementations, the in-situ acoustic monitoring system 160 is a passive acoustic monitoring system. In this case, the signal is monitored by the acoustic sensor 162 without generating a signal from an acoustic signal generator (or the acoustic signal generator can be completely omitted from the system). The passive acoustic signal monitored by the acoustic sensor 162 can be in the range of 50 kHz to 1 MHz, for example, 200 to 400 kHz, or 200 kHz to 1 MHz. For example, in the monitoring of the polishing of the interlayer dielectric (ILD) in shallow trench isolation (STI), a frequency range of 225 kHz to 350 kHz can be monitored.

[0032] The signal from sensor 162 can be amplified by a built-in internal amplifier or by an external amplifier. In some implementations, the amplification gain is between 40 and 60 dB (e.g., 50 dB). The signal from acoustic sensor 162 is then further amplified and, if necessary, filtered and can be digitized, for example, in electronics 166 through an A / D port to a high-speed data acquisition board. The data from acoustic sensor 162 can be recorded in a range similar to that of generator 163 or in a different, for example, higher range, such as 1 - 10 MHz, for example, 1 - 3 MHz or 6 - 8 MHz. In implementations where acoustic sensor 162 is a passive acoustic sensor, a frequency range up to 100 kHz - 2 MHz, such as 500 kHz - 1 MHz (e.g., 750 kHz), can be monitored.

[0033] When located within platen 120, acoustic sensor 162 can be positioned at the center of platen 120, for example, on the rotation axis 125, at the edge of platen 120, or at an intermediate point (e.g., 5 inches from the rotation axis for a 20 - inch diameter platen).

[0034] Referring to FIG. 2, as the platen 120 moves (as indicated by arrow A), the sensor 162 sweeps in a path that travels beneath the retention ring 170 and the substrate 10 (as indicated by phantom arrow B). The acoustic monitoring system 160, e.g., the controller 190, can be configured to sample the signal from the sensor 162 as the sensor 162 passes beneath the retention ring 170. For example, the controller 190 can determine the angular position of the sensor 162 based on an input from a motor encoder or a platen position sensor and can compare this, for example, to the positions of the carrier head 140 and the retention ring 170 based on sweep information. Determination of the position of the sensor relative to the substrate is described in U.S. Patent No. 6,159,073 and U.S. Patent No. 6,296,548. This enables the portion of the signal received when the sensor 162 is beneath the retention ring 170 to be identified and selected. Portions of the signal corresponding to the sensor being in other positions, e.g., beneath the substrate 10 or not even beneath the carrier head 140 in the first place, can be used for other monitoring purposes, such as detection of defects on the substrate 10, but are not required for the retention ring monitoring described below.

[0035] Returning to FIGS. 1 and 3, an acoustic signal generated by the interface between the retention ring 174 and the polishing layer 112 of the pad 110 travels through the polishing pad 110 and is received by the acoustic sensor 162. The acoustic sensor 162 transmits what is received to the signal processing electronics 166, and the signal processing electronics 166 performs functions on the received acoustic signal. The electronics 166 can include, for example, a filter, an amplifier, a spectrum analyzer, a data acquisition system (DAQ), or other components for processing the received acoustic signal. Generally, the electronics 166 can include a general-purpose programmable computer, a dedicated circuit, or a combination thereof. In some implementations, the electronics 166 are within the controller 190 and are implemented, for example, by the controller 190.

[0036] For example, after amplification, preliminary filtering, and digitization, the signal from acoustic sensor 162 can undergo data processing, for example, in controller 190, either for detection of various stages of the retention ring, such as detection of when the retention ring has been broken in or when a grooving has been formed on the inner surface of the retention ring, or for feedback or feedforward control of the polishing parameters. Controller 190 can generate an alert indicating the type of event, for example, the alert can indicate which of a retention ring break-in, or grooving formation (or other possible events such as an unclassified event) has occurred.

[0037] In some implementations, controller 190 is configured to monitor changes in acoustic signal intensity. For example, in an active acoustic monitoring system (where sensor 162 emits acoustic energy), the intensity of the received signal is compared to the intensity of the emitted signal to generate a normalized signal, and the normalized signal can be monitored over time to detect changes. As another example, in a passive acoustic monitoring system, the intensity of the received signal is compared to a measured initial signal intensity, for example, obtained when the retention ring was newly installed on the carrier head, to generate a normalized signal, and the normalized signal can be monitored over time to detect changes. Such changes can indicate that the retention ring has been broken in.

[0038] In some implementations, frequency analysis of a signal is performed. For example, frequency domain analysis can be used to determine the change in relative power of spectral frequencies and to determine when a film transition occurs at a particular radius. Information regarding the time of the transition by radius can be used to trigger an end point. As another example, a fast Fourier transform (FFT) can be performed on the signal to generate a frequency spectrum. A particular frequency band can be monitored, and if the intensity in the frequency band exceeds a threshold, this can indicate a change in the retention ring, such as the retention ring has been broken in or that a grooving has been formed on the inner surface of the retention ring. Alternatively, if the location (e.g., wavelength) or bandwidth of a local maximum or minimum within a selected frequency range exceeds a threshold, this can indicate a change in the retention ring.

[0039] In some implementations, the spectrum of a signal (e.g., power, wavelength, or frequency spectrum) can be compared to a reference spectrum. If the difference, e.g., the sum of the squares of the differences over a power, wavelength, or frequency range, exceeds or falls below a threshold, this can indicate a change in the retention ring, such as the retention ring has been broken in or that a grooving has been formed on the inner surface of the retention ring.

[0040] The determination of the appropriate characteristics of the signal to be monitored and the appropriate criteria for triggering an indication of a change in the retaining ring can be made empirically. For example, polishing can be performed with a worn retaining ring known to be in an equilibrium state, and the spectrum of the signal from this retaining ring can be used as a reference spectrum. As another example, polishing can be performed with both a new retaining ring and a worn retaining ring known to be in an equilibrium state. The spectra of the signals are compared to empirically determine the power, wavelength, or frequency band for monitoring and whether the worn retaining ring has a higher or lower signal intensity within that band. A criterion for the signal can be derived to generate an alert indicating that the retaining ring has been broken in, and the controller 190 can be configured to test whether the signal meets that criterion.

[0041] As another example, polishing can be performed with a retaining ring known to have grooving on its inner surface, and the spectrum of the signal from this retaining ring can be used as a reference spectrum. As another example, polishing can be performed with both a new retaining ring and a retaining ring known to have grooving on its inner surface. The spectra of the signals are compared to empirically determine the power, wavelength, or frequency band for monitoring and whether the retaining ring with grooving has a higher or lower signal intensity within that band. A criterion for the signal can be derived to generate an alert indicating that the retaining ring has grooving, and the controller 190 can be configured to test whether the signal meets that criterion.

[0042] During operation, acoustic signals are collected from the in-situ acoustic monitoring system 160. The signals are monitored to detect changes in the retention ring. Detection of a change can trigger an alert to the operator or can automatically stop the polishing operation. In the case of a broken-in retention ring, the retention ring can be used. The retention ring can be removed from the polishing system for use in another polishing system, or the polishing system can be switched from polishing a dummy substrate to polishing a device substrate for actual integrated circuit production. In the case of a retention ring having grooving on its inner surface, the operator can replace the retention ring.

[0043] In some implementations, the controller 190 controls one or more components of the apparatus 100, a motor such as to control the rotation rate of the carrier head 140 or the platen 120, or a pressure controller that controls the pressure within the chambers 146a-c based on the received acoustic signals.

[0044] The acoustic monitoring signals collected from the retention ring can be used in feed-forward to control the processing of the substrate in subsequent processing operations, such as polishing in a subsequent station, or can be used in feedback to control the processing of subsequent substrates in the same polishing station. For example, the signal strength in one or more power, wavelength, or frequency bands can be correlated with the polishing rate in one or more regions of the substrate, such as a radial zone. A control algorithm can receive the acoustic monitoring signals and determine an adjustment of one or more pressures applied by the polishing head to improve polishing uniformity.

[0045] In some implementations, portions of signals obtained from sensors below the substrate may be used. For example, grooving on the inner surface of a retaining ring can generate vibrations in the substrate that can be sensed by an acoustic sensor. Again, the polishing can be performed with a retaining ring known to have grooving on its inner surface, and the spectrum of the signal (including the portion of the signal corresponding to the sensor being below the substrate) can be used either as a reference spectrum or to determine a reference for generating an alert. During operation, the portion of the signal corresponding to the sensor being below the substrate can be compared, analyzed, to determine whether its reference is met, or can undergo a Fourier transform and be compared to a reference spectrum.

[0046] All of the implementations and functional operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, or combinations thereof, including the structural means disclosed in this specification and their structural equivalents. The implementations described in this specification can be implemented as one or more non-transitory computer program products, i.e., one or more computer programs tangibly embodied in a machine-readable storage device, for execution by, or to control the operation of, a data processing apparatus, e.g., a programmable processor, a computer, or multiple processors or computers.

[0047] A computer program (also known as a program, software, software application, or code) can be written in any form of programming language, including compiler-type or interpreter-type languages. The computer program can be deployed 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. The program can be stored in a part of a file that holds other programs or data, in a single file dedicated to the program, or in multiple cooperating files (e.g., files that store one or more modules, subprograms, or portions of code). The computer program can be deployed to be executed on one computer or on multiple computers distributed at one site or across multiple sites and interconnected by a communication network.

[0048] The processes and logical flows described herein can be implemented 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 logical flows can also be implemented by, for example, dedicated logic circuitry, such as an FPGA (Field Programmable Gate Array) or ASIC (Application Specific Integrated Circuit), and the apparatus can also be implemented as dedicated logic circuitry.

[0049] The term "data processing apparatus" encompasses, by way of example, all apparatus, devices, and machines for processing data, including programmable processors, computers, or a plurality of processors or computers. The apparatus can include, in addition to hardware, code for creating an execution environment for the computer program, such as code forming processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. Processors suitable for the execution of a computer program include, by way of example, both general-purpose and special-purpose microprocessors, as well as any one or more processors of any kind of digital computer.

[0050] Computer-readable media suitable for storing computer program instructions and data include, by way of example, non-volatile memory, media, and memory devices, such as semiconductor memory devices, for example, EPROM, EEPROM, and flash memory devices, magnetic disks, such as internal hard disks or removable disks, magneto-optical disks, and all forms of CD ROM disks and DVD-ROM disks. Processors and memories can be supplemented by, or incorporated in, dedicated logic circuitry.

[0051] Although this specification contains many details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular examples. Also, some of the features described herein in the context of separate implementations can be combined. Conversely, various features described in the context of a single implementation can be implemented separately in multiple implementations, or in any suitable sub-combination.

[0052] Some implementations have been described. Nevertheless, it will be understood that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, other implementations are within the scope of the following claims.

Claims

1. A chemical mechanical polishing apparatus, comprising: A platen for supporting a polishing pad; A carrier head for holding the surface of a substrate against the polishing pad, the carrier head including a retaining ring for holding the substrate; An acoustic sensor supported on the platen; A motor for generating relative movement between the platen and the carrier head so as to polish the substrate and so that the acoustic sensor travels in a path below the carrier head and the retaining ring; A controller configured to analyze a signal from the acoustic sensor and determine a characteristic of the retaining ring based on the signal. The chemical mechanical polishing apparatus.

2. The apparatus according to claim 1, wherein the controller is configured to detect that the retaining ring has been broken in based on the signal.

3. The apparatus according to claim 2, wherein the controller is configured to generate an alert in response to detecting that the retaining ring has been broken in.

4. The apparatus according to claim 1, wherein the controller is configured to detect that a grooving has been formed on an inner surface of the retaining ring based on the signal.

5. The apparatus according to claim 4, wherein the controller is configured to generate an alert in response to detecting that a grooving has been formed on the inner surface of the retaining ring.

6. The apparatus according to claim 1, wherein the controller is configured to generate a measured spectrum of the signal.

7. The apparatus according to claim 6, wherein the controller is configured to compare the measured spectrum with a reference spectrum.

8. The apparatus according to claim 6, wherein the controller is configured to detect a signal intensity in a band in the measured spectrum and compare the signal intensity with a threshold value.

9. The apparatus according to claim 1, wherein the controller is configured to select a portion of the signal corresponding to the acoustic sensor being positioned below the retaining ring.

10. A chemical mechanical polishing apparatus, comprising: A platen for supporting a polishing pad; A carrier head for holding the surface of a substrate against the polishing pad, the carrier head including a retaining ring for holding the substrate; An acoustic sensor supported on the platen; A motor for generating relative movement between the platen and the carrier head such that the substrate is polished and such that the acoustic sensor travels within a path below the carrier head and the retaining ring, A controller configured to select a portion of a signal from the acoustic sensor corresponding to the acoustic sensor being below the retaining ring and to generate an alert or modify a polishing parameter based on the selected portion of the signal from the acoustic sensor A chemical mechanical polishing apparatus comprising the same. **Claim 11** The apparatus of claim 10, wherein the controller is configured to detect that the retaining ring has been broken in based on the selected portion of the signal. **Claim 12** The apparatus of claim 10, wherein the controller is configured to detect that a grooving has been formed on an inner surface of the retaining ring based on the selected portion of the signal. **Claim 13** The apparatus of claim 10, wherein the controller is configured to modify the polishing parameter based on the selected portion of the signal. **Claim 14** A method of a chemical mechanical polishing apparatus, comprising: Contacting a surface of a substrate with a polishing pad; Generating relative movement between the substrate and the polishing pad such that the substrate is polished and using a carrier head for holding the substrate against lateral movement; Sweeping an acoustic sensor in a path below the carrier head and the retaining ring during polishing; Analyzing a signal from the acoustic sensor and determining a characteristic of the retaining ring based on the signal A method of a chemical mechanical polishing apparatus including the same. **Claim 15** The method of claim 14, comprising detecting that the retaining ring has been broken in based on the signal. **Claim 16** The method of claim 15, comprising generating an alert in response to detecting that the retaining ring has been broken in. **Claim 17** The method of claim 14, comprising detecting that a grooving has been formed on an inner surface of the retaining ring based on the signal. **Claim 18** The method of claim 17, comprising generating an alert in response to detecting that a grooving has been formed on the inner surface of the retaining ring. **Claim 19** The method according to claim 14, comprising modifying the polishing parameters based on the signal. **Claim 20** The method according to claim 14, comprising selecting a portion of the signal corresponding to the acoustic sensor being located below the holding ring.

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