Apparatus and method for acoustic monitoring of cmp retaining ring

Abstract

A chemical mechanical polishing (CMP) apparatus includes: a platform supporting a polishing pad; a carrier head holding a substrate surface against the polishing pad; an acoustic sensor supported on the platform; and a motor generating relative motion between the platform and the carrier head for polishing the substrate. The carrier head includes a retaining ring for holding the substrate, and the acoustic sensor travels in a path beneath 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.

Description

[Technical Field]

[0001] This disclosure relates to in-situ monitoring of chemical mechanical polishing, and more specifically, to acoustic monitoring. [Previous Technology]

[0002] Integrated circuits are typically formed on a substrate by sequentially depositing conductive layers, semiconductor layers, or insulating layers on a silicon wafer. One fabrication step involves depositing a fill layer on a non-planar surface and planarizing the fill layer. For some applications, such as using conductive filler material to fill trenches or holes in an insulating layer, the fill layer is planarized until the top surface of the patterned layer is exposed. For 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. Furthermore, photolithography typically requires planarization of the substrate surface.

[0003] Chemical mechanical polishing (CMP) is an accepted planarization method. This planarization method typically requires mounting a substrate 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 controlled load on the substrate to push it against the polishing pad. The carrier head typically includes a retaining ring to surround and hold the substrate below the carrier head. The lower surface of the retaining ring is compressible against the polishing pad and can wear down as polishing progresses. Therefore, 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 embodiment, a chemical mechanical polishing (CMP) apparatus includes: a platform supporting a polishing pad; a carrier head holding a surface of a substrate against the polishing pad; an acoustic sensor supported on the platform; and a motor generating relative motion between the platform and the carrier head for polishing the substrate. The carrier head includes a retaining ring for holding the substrate, and the acoustic sensor travels in 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 embodiment, the chemical mechanical polishing apparatus includes: a platform supporting a polishing pad; a carrier head holding a surface of a substrate against the polishing pad; an acoustic sensor supported on the platform; and a motor generating relative movement between the platform and the carrier head to polish the substrate. The carrier head includes a retaining ring for holding the substrate, and the acoustic sensor travels in a path below the carrier head and the retaining ring. A controller is configured to select a portion of the acoustic sensor corresponding to a signal from the acoustic sensor below the retaining ring, and is configured to generate an alarm or modify a polishing parameter based on the selected portion of the signal from the acoustic sensor.

[0007] One or more of the following advantages can be achieved. The retaining ring can be monitored instantly and in situ. In-situ monitoring can be used to determine if the retaining ring has been "broken in," thereby reducing the time required for polishing system qualification. The wear condition of the retaining ring can be monitored. The signal from the acoustic monitoring system, depending on the flatness or taper angle of the bottom surface of the retaining ring, can be used as a basis for adjusting other polishing parameters to improve the uniformity of inter-wafer (WTW) and intra-wafer (WIW) polishing.

[0008] Details of one or more embodiments are set forth in the accompanying drawings and the following description. Other features, characteristics, and advantages will be apparent from the specification and drawings, as well as from the claims.

Implementation Method

[0013] During polishing, the polishing pad abrades the bottom surface of the retaining ring. Typically, wear does not occur radially across the retaining ring at a uniform rate, resulting in a non-flat geometry on the bottom surface. However, the wear of the retaining ring eventually reaches equilibrium, causing the bottom surface of the retaining ring to maintain a geometry substantially the same as the ring wear until processing or polishing conditions change. Due to the continuous change in the geometry of the retaining ring, the polishing rate profile will drift until the retaining ring reaches equilibrium.

[0014] To reduce polishing variations between or across substrates, the retaining ring can be "destroyed" before the polishing process. One method of destroying the retaining ring is to simulate substrate polishing, for example, by pressing the retaining ring against a moving polishing pad, causing the ring to wear down until an equilibrium geometry is achieved. However, the disadvantage of "destruction" is that it is time-consuming and requires the use of polishing equipment. As a result, the destruction process results in downtime for the polishing equipment, during which polishing cannot be performed, increasing the cost of ownership.

[0015] By performing acoustic monitoring on the retaining ring, the time of retaining ring failure can be determined instantly and in situ. Therefore, the time required for failure can be reduced.

[0016] As another problem, the influence of the substrate on the inner surface of the retaining ring will create grooves on the inner surface. The grooves will affect the pressure applied to the edge of the substrate, thereby affecting the uniformity of polishing.

[0017] By acoustically monitoring the retaining ring, grooves formed on the inner surface of the retaining ring can be detected. This allows for quick replacement of the retaining ring, thereby reducing the possibility of uneven polishing.

[0018] More generally, acoustic monitoring of the retaining ring can be used to improve the polishing process. For example, signals from the acoustic monitoring system can indicate the geometry and / or smoothness of the lower surface of the retaining ring, which affects the polished profile of the substrate, particularly near the substrate edges. Therefore, such signals can be used as inputs to control various polishing parameters, such as the pressure applied to different areas of the substrate, to improve polishing performance.

[0019] Any one of these problems can be used to solve them independently of the other problems, or the system can solve multiple problems.

[0020] Figure 1 illustrates an example of a polishing apparatus 100. The polishing apparatus 100 includes a rotatable disc platform 120 on which a polishing pad 110 is located. The polishing pad 110 may be a two-layer polishing pad having an outer polishing layer 112 and a softer backing layer 114. The platform is operable to rotate about an axis 125. For example, a motor 121 (e.g., a DC induction motor) can rotate a drive shaft 124 to rotate the platform 120.

[0021] The polishing apparatus 100 may include a port 130 for dispensing polishing fluid 132 (e.g., abrasive slurry) onto the polishing pad 110. The polishing apparatus may also include a polishing pad adjuster for abrading the polishing pad 110 to maintain the polishing pad 110 in a consistent abrasive state.

[0022] The polishing apparatus 100 includes a carrier head 140. The carrier head 140 is operable to hold a substrate 10 against a polishing pad 110. The carrier head 140 includes a retaining ring 170 to hold the substrate 10 below a flexible membrane 144. The retaining ring 170 may include a lower portion 172 formed of an abrasion-resistant plastic (e.g., PPS) compatible with the polishing process, and an upper portion 174 formed of a more rigid material (e.g., metal). Alternatively, the entire retaining ring may be formed of plastic. The retaining ring 170 is typically secured to the carrier head 140 using screws, allowing the retaining ring 170 to be removed and replaced.

[0023] The carrier head 140 also includes one or more independently controllable pressurizable chambers defined by membranes, such as three chambers 146a to 146c, which can apply independently controllable pressure to associated areas on the flexible membrane 144, thereby applying pressure to the substrate 10 (see FIG1). Although only three chambers are illustrated in FIG1, for ease of illustration, there may be one or two chambers, or four or more chambers, such as five chambers.

[0024] The carrier head 140 is suspended from a support structure 150 (e.g., a turntable or track) and connected to a carrier head rotation motor 154 (e.g., a DC induction motor) via a drive shaft 152, allowing the carrier head to rotate about an axis 155. Optionally, each carrier head 140 can oscillate laterally, for example on a slider on the turntable 150, or by oscillating due to the rotation of the turntable itself, or by sliding along a track. In typical operation, the platform rotates about its central axis 125, and each carrier head rotates about its central axis 155 and translates laterally across the top surface of the polishing pad.

[0025] A controller 190 (e.g., a programmable computer) is connected to motors 121 and 154 to control the rotational rates of platform 120 and carrier head 140. For example, each motor may include an encoder to measure the rotational rate of the associated drive shaft.

[0026] 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 may be mounted at one or more locations on the platform 120. In particular, the in-situ acoustic monitoring system may be configured to detect acoustic signals from the retaining ring 170.

[0027] A position sensor (e.g., an optical interruptor or rotary encoder connected to the platform rim) can be used to sense the angular position of the platform 120. This only permits a portion of the signal measured when the sensor 162 is close to the retaining ring 170 (e.g., when the sensor 162 is below the retaining ring) to be used as an indication of the retaining ring state in subsequent signal processing.

[0028] In the implementations shown in Figures 1 and 3, the acoustic monitoring system 160 includes an acoustic sensor 162 supported and placed by a platform 120 to receive acoustic signals from the substrate 10 via a polishing pad 110. The acoustic sensor 162 may be partially or completely located in a recess 164 in the top surface of the platform 120. In some implementations, the top surface of the acoustic sensor 162 is coplanar with the top surface of the platform 120.

[0029] In some implementations, the portion of the polishing pad directly above the acoustic sensor 162 may include an acoustic window 119, for example, a region having a lower acoustic impedance than the surrounding polishing material. The acoustic window 119 may extend through the polishing layer 112, or the backing layer 114, or both. However, if the acoustic transmission of the polishing pad is sufficiently high, the acoustic window 119 may not be necessary.

[0030] The acoustic sensor 162 is a contact acoustic sensor having a portion of a polishing pad, such as a polishing layer 112, a backing layer 114, or an acoustic window 119, connected to (e.g., in direct contact, or having only an adhesive layer for attachment, or only having an acoustic gel for transmitting acoustic signals therefrom). For example, the acoustic sensor 162 may be an electromagnetic acoustic transducer or a piezoelectric acoustic transducer. A piezoelectric sensor may include a rigid contact plate, such as stainless steel, positioned to contact the subject to be monitored, and a piezoelectric component on the back side of the contact plate, such as a piezoelectric layer sandwiched between two electrodes.

[0031] In some implementations, the acoustic sensor 162 is positioned within a recess 169 in the housing 163. An optional spring 165 may be arranged between the housing 163 and the support 167 to provide pressure against the housing 163. The pressure on the housing 163 compresses the acoustic sensor 162 to contact a portion of the polishing pad 110. Alternatively, the spring 165 may directly compress the acoustic sensor 162, for example, without the housing. In some implementations, the spring 165 is a long-travel spring 165 to supply pressure similar to that of a strong spring 165 over a greater compression range.

[0032] The acoustic sensor 162 can be connected to a power supply and / or other signal processing electronics 166 via a circuit system 168 through a rotary coupling (e.g., a mercury slip ring).

[0033] 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 the signal from the 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, to monitor the polishing of the interlayer dielectric (ILD) in shallow trench isolation (STI), a frequency range of 225 kHz to 350 kHz can be monitored.

[0034] 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). If desired, the signal from acoustic sensor 162 can then be further amplified and filtered, and digitized via an A / D port to a high-speed data acquisition board, such as in electronic device 166. Data from acoustic sensor 162 can be recorded in a range similar to or different from that of generator 163, such as a higher range, e.g., from 1 to 10 MHz, e.g., 1 to 3 MHz or 6 to 8 MHz. In implementations where acoustic sensor 162 is a passive acoustic sensor, a frequency range from 100 kHz to 2 MHz can be monitored, e.g., 500 kHz to 1 MHz (e.g., 750 kHz).

[0035] If positioned in platform 120, acoustic sensor 162 may be located at the center of platform 120, for example, at the rotation axis 125, at the edge of platform 120, or at the midpoint (e.g., 5 inches from the rotation axis for a platform with a diameter of 20 inches).

[0036] Referring to FIG. 2, due to the movement of platform 120 (as shown by arrow A), sensor 162 scans along a path (as shown by dashed arrow B) traveling beneath retaining ring 170 and substrate 10. Acoustic monitoring system 160 (e.g., controller 190) may be configured to sample signals from sensor 162 as the sensor passes beneath retaining ring 170. For example, controller 190 may determine the angular position of sensor 162 based on input from a motor encoder or platform position sensor and compare it to the positions of carrier head 140 and retaining ring 170 (e.g., based on scan information). The determination of the sensor's position relative to the substrate is discussed in U.S. Patent Nos. 6,159,073 and 6,296,548. This allows for the identification and selection of portions of the signal received when sensor 162 is beneath retaining ring 170. A portion of the signal corresponding to a sensor in another location (e.g., below substrate 10 or not at all below carrier head 140) may be used for other monitoring purposes, such as detecting defects on substrate 10, but need not be used for the retaining ring monitoring discussed below.

[0037] Referring back to Figures 1 and 3, the acoustic signal generated by the interface between the retaining ring 174 and the polished layer 112 of the pad 110 travels through the polished pad 110 and is received by the acoustic sensor 162. The acoustic sensor 162 transmits the received signal to the signal processing electronics 166 to perform a function on the received acoustic signal. The electronics 166 may include, for example, a filter, an amplifier, a spectrum analyzer, a data acquisition system (DAQ), or other components for processing the received acoustic signal. Typically, the electronics 166 may include a general-purpose programmable computer, a special-purpose circuit system, or a combination thereof. In some implementations, the electronics 166 is within a controller 190, for example, implemented by the controller 190.

[0038] The signal from the acoustic sensor 162 (e.g., after amplification, preliminary filtering, and digitization) may undergo data processing, for example, in the controller 190, for detecting various stages of the retaining ring (e.g., retaining ring failure or slotting on the inner surface of the retaining ring), or for feedback or feedforward control of polishing parameters. The controller 190 may generate an alarm indicating the type of event, for example, an alarm indicating which of the following has occurred: retaining ring failure or slotting (or other possible events, such as unclassified events).

[0039] In some implementations, the controller 190 is configured to monitor changes in the acoustic signal strength. For example, in an active acoustic monitoring system (where sensor 162 emits acoustic energy), the received signal strength is compared with the emitted signal strength to generate a normalized signal, and the normalization can be monitored over time to detect changes. As another example, in a passive acoustic monitoring system, the received signal strength is compared with a measured initial signal strength (e.g., the initial signal strength obtained when the retaining ring is newly installed on the carrier head) to generate a normalized signal, and the normalization can be monitored over time to detect changes. These changes can indicate that the retaining ring has been damaged.

[0040] In some practices, frequency analysis of the signal is performed. For example, frequency domain analysis can be used to determine the relative power changes of spectral frequencies and when a membrane transition occurs at a specific radius. Information about the timing of the radius-based transition can be used to trigger endpoints. As another example, a Fast Fourier Transform (FFT) can be performed on the signal to generate a frequency spectrum. Specific frequencies can be monitored, and if the intensity in the frequency band exceeds a critical value, this can indicate a change in the holding loop, such as the holding loop being damaged or a slot being formed on the inner surface of the holding loop. Alternatively, if the location (e.g., wavelength) or bandwidth of a local maximum or minimum value in a selected frequency range exceeds a critical value, this can indicate a change in the holding loop.

[0041] In some implementations, the spectrum of a signal (e.g., power, wavelength, or frequency spectrum) can be compared with a reference spectrum. If the difference (e.g., the sum of squares of the differences in power, wavelength, or frequency range) exceeds or falls below a critical value, this can indicate a change in the holding ring, such as the holding ring being damaged or a slot being formed on the inner surface of the holding ring.

[0042] The appropriate characteristics of the signal to be monitored and the appropriate criteria for triggering the indication of changes in the retaining ring can be determined empirically. For example, polishing can be performed using a retaining ring known to be in equilibrium of wear, and the spectrum of the signal from that retaining ring can be used as a reference spectrum. As another example, polishing can be performed using both a new retaining ring and a retaining ring known to be in equilibrium of wear. The spectrum of the signal can be compared to empirically determine the power, wavelength, or frequency band to be monitored, and whether the worn retaining ring has a higher or lower signal strength within that band. Criteria for generating an alarm indicating that the retaining ring has been damaged can be derived, and the controller 190 can be configured to test whether the signal meets the criteria.

[0043] As another example, polishing can be performed using a retaining ring known to have slots on its inner surface, and the spectrum of the signal from that retaining ring can be used as a reference spectrum. As another example, polishing can be performed using both a new retaining ring and a retaining ring known to have slots on its inner surface. The spectrum of the signal can be compared to empirically determine the power, wavelength, or frequency band for monitoring, and whether the retaining ring with slots has a higher or lower signal strength within the band. Criteria for generating an alarm signal indicating that the retaining ring has slots can be derived, and the controller 190 can be configured to test whether the signal meets the criteria.

[0044] During operation, acoustic signals are collected from the in-situ acoustic monitoring system 160. These signals are monitored to detect changes in the retaining ring. Detection of a change may trigger an alarm to the operator or automatically stop the polishing operation. If the retaining ring is damaged, it can still be used. The retaining ring can be removed from the polishing system for use in another polishing system, or the polishing system can be switched from polishing a virtual substrate to polishing a device substrate for actual integrated circuit production. If the retaining ring has slots on its inner surface, the operator can replace it.

[0045] In some implementations, controller 190 controls one or more components of device 100, such as a motor that controls the rotational rate of carrier head 140 or platform 120, or a pressure controller that controls the pressure within chambers 146a to 146c based on received acoustic signals.

[0046] The acoustic monitoring signals collected from the holding ring can be used for feedforward to control substrate processing in subsequent processing operations, such as polishing at a subsequent station, or for feedback to control subsequent substrate processing at the same polishing station. For example, the signal intensity in one or more power, wavelength, or frequency band can be correlated with the polishing rate in one or more regions (e.g., radial regions) of the substrate. The control algorithm can accept the acoustic monitoring signals and determine adjustments to one or more pressures applied to the polishing head to improve polishing uniformity.

[0047] In some implementations, a portion of the signal obtained from a sensor located beneath the substrate can be used. For example, a slot in the inner surface of the retaining ring can generate vibrations in the substrate, which can be sensed by an acoustic sensor. Similarly, a retaining ring known to have slots on its inner surface can be used to perform polishing, and the spectrum of the signal (including a portion of the signal corresponding to the sensor located beneath the substrate) can be used, either as a reference spectrum or as a criterion for generating an alarm. In operation, the portion of the signal corresponding to the sensor located beneath the substrate can be compared and analyzed to determine whether the criterion is met, or it can undergo a Fourier transform and be compared with a reference spectrum.

[0048] The implementations and all functional operations described herein can be implemented in digital electronic circuit systems or in computer software, firmware, or hardware, including the structural components disclosed herein and their structural equivalents, or combinations thereof. The implementations described herein can be implemented as one or more non-transitory computer program products, that is, tangibly contained in one or more computer programs in a machine-readable storage device for execution or control of their operation by a data processing device (e.g., a programmable processor, a computer, or multiple processors or computers).

[0049] A computer program (also known as a program, software, software application, or code) may be written in any form of programming language, including compiled or interpreted languages, and may be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unit suitable for a computing environment. A computer program does not necessarily correspond to a file. A program may be stored as part of a file containing other programs or data, in a single file dedicated to the program in question, or in multiple coordinated files (e.g., a file storing one or more modules, subroutines, or portions of code). Computer programs may be deployed to execute on one computer or multiple computers at one site, or distributed across multiple sites and interconnected via a communications network.

[0050] The processing and logic flows described in this specification can be executed by one or more programmable processors to execute one or more computer programs to perform functions by manipulating input data and generating output. These processing and logic flows can also be executed by a dedicated logic circuit system, and the device can also be implemented as a dedicated logic circuit system, such as an FPGA (Field Programmable Gate Array) or an ASIC (Application-Specific Integrated Circuit).

[0051] The term "data processing device" encompasses all devices, apparatuses, and machines used for processing data, including, for example, programmable processors, computers, or multiple processors or computers. In addition to hardware, the device may include code that creates an execution environment for the computer program in question, such as code constituting processor firmware, protocol stacks, database management systems, operating systems, or combinations thereof. Processors suitable for executing computer programs include, for example, both general-purpose and special-purpose microprocessors, and any one or more processors of any type of digital computer.

[0052] Computer-readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media, and memory devices, such as semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks, such as internal hard disks or removable disks; magneto-optical disks; and CD ROMs and DVD-ROMs. The processor and memory may be supplemented or integrated into a dedicated logic circuit system.

[0053] Although this specification contains numerous details, these should not be construed as limiting the scope of claims, but rather as descriptions of specific features of particular paradigms. Certain features described in this specification in the context of separate implementations may also be combined. Conversely, various features described in the context of a single implementation may also be implemented separately or in any suitable sub-combination in multiple embodiments.

[0054] Many embodiments have been described. However, it should be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims. [Simplified Explanation of the Diagram]

[0009] Figure 1 illustrates a schematic cross-sectional view of an example of a polishing system.

[0010] Figure 2 shows a top view of the polishing system.

[0011] Figure 3 illustrates a schematic cross-sectional view of the acoustic monitoring sensor below the retaining ring of the carrier head.

[0012] In the drawings, the same reference numerals indicate the same elements. [Biomaterial Storage]

[0056] Domestic storage information (please note in order of storage institution, date, and number): None. International storage information (please note in order of storage country, institution, date, and number): None.

Claims

1. A chemical mechanical polishing apparatus, comprising: A platform supporting a polishing pad; A carrier head that holds a surface of a substrate against a polishing pad, the carrier head including a retaining ring for holding the substrate; an acoustic sensor supported on a platform; a motor that generates relative movement between the platform and the carrier head to polish the substrate and causes the acoustic sensor to travel in a path below the carrier head and the retaining ring; and a controller configured to analyze a signal from the acoustic sensor and determine a characteristic of the retaining ring based on the signal, and configured to detect, based on the signal, that the retaining ring is broken in before polishing a device substrate for actual integrated circuit production.

2. The device as claimed in claim 1, wherein the controller is configured to generate an alarm in response to detection that the holding loop is engaged.

3. The device as claimed in claim 1, wherein the controller is configured to generate a measured spectrum of the signal.

4. The device as claimed in claim 3, wherein the controller is configured to compare the measured spectrum with a reference spectrum.

5. The device as claimed in claim 3, wherein the controller is configured to detect a signal strength in a band of the measured spectrum and compare the signal strength with a threshold value.

6. The device as claimed in claim 1, wherein the controller is configured to select the portion of the signal corresponding to the acoustic sensor located below the retaining ring.

7. A chemical mechanical polishing apparatus, comprising: A platform supporting a polishing pad; A carrier head that holds a surface of a substrate against a polishing pad, the carrier head including a retaining ring for holding the substrate; an acoustic sensor supported on a platform; a motor that generates relative movement between the platform and the carrier head to polish the substrate and causes the acoustic sensor to travel in a path below the carrier head and the retaining ring; and a controller configured to analyze a signal from the acoustic sensor and determine a characteristic of the retaining ring based on the signal, and configured to detect the formation of a slot on an inner surface of the retaining ring based on the signal.

8. The device as claimed in claim 7, wherein the controller is configured to generate an alarm in response to the detection of a slot being formed on the inner surface of the retaining ring.

9. A chemical mechanical polishing apparatus, comprising: A platform supporting a polishing pad; A carrier head that holds a surface of a substrate against a polishing pad, the carrier head including a retaining ring for holding the substrate; an acoustic sensor supported on a platform; a motor that generates relative movement between the platform and the carrier head to polish the substrate, and causes the acoustic sensor to travel in a path below the carrier head and the retaining ring; and a controller configured to select a portion of the acoustic sensor from the acoustic sensor corresponding to the portion of the acoustic sensor below the retaining ring, and configured to generate an alarm or modify a polishing parameter based on the selected portion of the signal from the acoustic sensor, and configured to detect, based on the selected portion of the signal, that the retaining ring is engaged before polishing a device substrate for actual integrated circuit production.

10. The device as claimed in claim 9, wherein the controller is configured to modify the polishing parameters based on the selected portions of the signal.

11. A chemical mechanical polishing apparatus, comprising: A platform supporting a polishing pad; A carrier head that holds a surface of a substrate against a polishing pad, the carrier head including a retaining ring for holding the substrate; an acoustic sensor supported on a platform; a motor that generates relative movement between the platform and the carrier head to polish the substrate, and causes the acoustic sensor to travel in a path below the carrier head and the retaining ring; and a controller configured to select a portion of a signal from the acoustic sensor corresponding to the portion of the acoustic sensor below the retaining ring, and configured to generate an alarm or modify a polishing parameter based on the selected portion of the signal from the acoustic sensor, and configured to detect the formation of a slot on an inner surface of the retaining ring based on the selected portion of the signal.

12. A method of a chemical mechanical polishing apparatus, comprising the steps of: contacting a surface of a substrate with a polishing pad; generating relative motion between the substrate and the polishing pad to polish the substrate, and using a carrier head to maintain the substrate against lateral motion; during polishing, scanning an acoustic sensor in a path below the carrier head and a retaining ring; analyzing a signal from the acoustic sensor and determining a characteristic of the retaining ring based on the signal; and detecting, based on the signal, that the retaining ring is engaged before polishing a device substrate for actual integrated circuit production.

13. The method as described in claim 12 includes the following steps: generating an alarm in response to the detection that the holding ring is engaged.

14. The method as described in claim 12, comprising the step of: modifying a polishing parameter based on the signal.

15. The method as described in claim 12, comprising the step of: selecting the signal to correspond to a portion of the acoustic sensor placed below the retaining ring.

16. A method of a chemical mechanical polishing apparatus, comprising the steps of: contacting a surface of a substrate with a polishing pad; generating relative motion between the substrate and the polishing pad to polish the substrate, and using a carrier head to maintain the substrate against lateral motion; during polishing, scanning an acoustic sensor in a path below the carrier head and a retaining ring; analyzing a signal from the acoustic sensor and determining a characteristic of the retaining ring based on the signal; and detecting the formation of a groove on an inner surface of the retaining ring based on the signal.

17. The method as described in claim 16, comprising the step of: generating an alarm in response to the detection of a slot formed on the inner surface of the retaining ring.

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