Eddy current monitoring detects vibrations during grinding

Abstract

An object is brought into contact with a polishing pad of the polishing system, a polishing liquid is supplied to the polishing pad, relative motion is generated between the object and the polishing pad while the object is in contact with the polishing pad, a signal is generated from an in-situ eddy current monitoring system during the relative motion while the object is in contact with the polishing pad, and mechanical vibration of the polishing system is detected based on the signal from the in-situ eddy current monitoring system.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE This disclosure relates to chemical mechanical polishing, and more particularly, to eddy current monitoring during polishing. [Background technology]

[0002] Integrated circuits are typically formed on a substrate by sequentially depositing conductive, semiconductive, or insulating layers on a silicon wafer. Various manufacturing processes require planarization of layers on the substrate. For example, one manufacturing step involves depositing a filler layer over a non-planar surface and then planarizing the filler layer. In certain applications, the filler layer is planarized until the top surface of the patterned layer is exposed. For example, a metal layer may be deposited on a patterned insulating layer to fill the trenches and holes in the insulating layer. After planarization, the remaining metal in the trenches and holes of the patterned layer forms vias, plugs, and lines to provide conductive paths between thin film circuits on the substrate.

[0003] Chemical mechanical polishing (CMP) is an accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier 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, forcing it against the polishing pad. A polishing slurry containing abrasive particles is typically supplied to the surface of the polishing pad.

[0004] Some systems monitor the substrate in situ during polishing, for example through the polishing pad. One monitoring technique is to induce eddy currents in a conductive layer of the substrate and detect changes in the eddy currents as the conductive layer is removed. Summary of the Invention

[0005] In one aspect of the method, computer program product, or polishing system, a signal is generated from an in-situ eddy current monitoring system during relative motion in which the object is in contact with the polishing pad, and mechanical vibration of the polishing system is detected based on the signal from the in-situ eddy current monitoring system.

[0006] In another aspect, a method of chemical mechanical polishing includes contacting a substrate with a polishing pad, supplying a polishing fluid to the polishing pad, generating relative motion between the substrate and the polishing pad, sweeping a sensor of an in situ eddy current monitoring system in a path across the substrate while the substrate is being polished, selecting portions of signals from the in situ eddy current monitoring system that correspond to off-metal locations of the sensor, the off-metal locations excluding at least locations below the substrate, selecting portions of signals from the in situ eddy current monitoring system that correspond to off-metal locations of the sensor, measuring noise in the portions of the signals that correspond to the off-metal locations of the sensor, and comparing the measured noise to a threshold value to determine whether to issue an alert.

[0007] In another aspect, a computer program product tangibly encoded on a non-transitory computer readable medium has instructions to cause one or more computers to receive a signal from an in situ eddy current monitoring system including a sensor sweeping beneath a carrier head of a polishing system; select portions of the signal that correspond to off-metal locations of the sensor, the off-metal locations excluding at least locations of the sensor that are beneath the carrier head; measure noise in the portions of the signal that correspond to the off-metal locations of the sensor; and compare the measured noise to a threshold to determine whether to issue an alert.

[0008] In another aspect, a polishing system includes a rotatable platen that holds a polishing pad, a carrier head that holds a substrate in contact with the polishing pad, a motor that rotates the platen, an in situ eddy current monitoring system including sensor locations in the platen such that the sensor sweeps across the carrier head with each rotation of the platen, and a controller, the controller is configured to receive a signal from the in situ eddy current monitoring system, select portions of the signal that correspond to off-metal locations of the sensors, the off-metal locations excluding at least locations of the sensors that are below the carrier head, measure noise in the portions of the signal that correspond to the off-metal locations of the sensors, and compare the measured noise to a threshold value to determine whether to issue an alert.

[0009] Implementations may include one or more of the following features: The controller may be configured to provide a visual or audio alert to an operator The object may be a substrate for integrated circuit manufacturing, a carrier head retaining ring, or a conditioner disk.

[0010] Implementations may include one or more of the following advantages: The onset of vibration in the polishing system can be detected and an alert can be issued to stop polishing or take corrective action. Damage to the inner surface of the retaining ring, e.g., grooving of the inner diameter, can be reduced or avoided. Overpolishing of the edges can be reduced, improving yield. The polishing process and hardware can be screened to prevent this vibration from occurring. This detection can be performed with existing hardware, e.g., existing eddy current monitor systems, thus enabling a low-cost solution.

[0011] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will become apparent from the description and drawings, and from the claims. [Brief description of the drawings]

[0012] [Figure 1] 1 is a schematic cross-sectional view of an example polishing station including an eddy current monitoring system. [Diagram 2] 1 is a schematic top view of an example chemical mechanical polishing station illustrating the scan path of a sensor across a substrate. [Diagram 3] 1 is a schematic cross-sectional view illustrating an example of a magnetic field generated by a sensor of an eddy current monitoring system. [Figure 4A] 1 is a schematic graph of a signal from an eddy current monitoring system. [Figure 4B] 4 is a schematic graph of a signal from an eddy current monitoring system when vibration occurs in the polishing system. [Diagram 5] FIG. 1 is a flow diagram of a method for monitoring the thickness of a conductive layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] One of the problems that can occur in chemical mechanical polishing is uneven polishing, e.g., over-polishing the edge of the substrate (usually considered the outer 5-10 mm of the substrate). Damage to the inner diameter surface of the retaining ring, e.g., the formation of scratches or grooves on the inner diameter surface, can correlate with some instances of uneven polishing. Without being limited by theory, it is hypothesized that certain polishing conditions result in high friction and stiction between the polishing pad and the substrate and / or retaining ring. It is theorized that this high friction results in a slip-stick motion of the substrate and / or retaining ring (rather than a relatively uniform motion) as it passes over the polishing pad. In particular, it is hypothesized that the slip-stick motion can cause the substrate to vibrate relative to the retaining ring, causing the edge of the substrate to bite into and damage the inner surface of the retaining ring, leaving scratches or grooves. The unevenness of the inner surface of the ring can result in uneven polishing, e.g., over-polishing at the edge of the substrate.

[0014] Without being limited to theory, this slip-stick effect may be more likely to occur in "aggressive" polishing processes, e.g., a combination of low slurry flow rate, high temperature, and high pressure. Such aggressive polishing may be required for polishing to planarize certain materials or to achieve high polishing rates. In some cases, aggressive polishing can be performed without associated retaining ring damage and uneven polishing. However, uneven polishing may progress quickly after normal polishing of multiple substrates. Again, hypothetically, in aggressive polishing processes, even small changes in temperature, slurry distribution, etc. may be enough to start to show the slip-stick effect. Unfortunately, this slip-stick effect is not immediately evident with many monitoring techniques, e.g., motor torque monitoring. Furthermore, due to the combination and contribution of other variables, e.g., pad roughness, slurry viscosity, etc., it may not be possible to designate a specific parameter, such as platen motor torque, carrier head torque, or pad temperature, as the boundary at which the slip-stick effect occurs.

[0015] However, the slip-stick effect results in vibration energy being transferred to the polishing system. In particular, an eddy current monitoring system can be used to detect mechanical vibrations in the polishing system, for example due to the slip-stick effect, which can provide a warning or change operating parameters to avoid damage to the retaining ring.

[0016] 1 and 2 show an example of a polishing station 20 of a chemical mechanical polishing system. The polishing station 20 includes a rotatable, disk-shaped platen 24 on which a polishing pad 30 is positioned. The platen 24 is operable to rotate about an axis 25. For example, a motor 22 may rotate a drive shaft 28 to rotate the platen 24. The polishing pad 30 may be a two-layer polishing pad having an outer polishing layer 34 and a soft backing layer 32.

[0017] The polishing station 20 may include a feed port or combined feed-rinse arm 39 for dispensing a polishing fluid 38, such as a polishing slurry, onto the polishing pad 30. The polishing station 20 may include a pad conditioner device, such as a conditioner head 40 having a conditioning disk 42, to maintain the surface roughness of the polishing pad.

[0018] Carrier head 70 is operable to hold substrate 10 against polishing pad 30. Carrier head 70 is suspended from a support structure 72, e.g., a carousel or track, and is connected by a drive shaft 74 to a carrier head rotation motor 76 such that the carrier head can rotate about axis 71. Optionally, carrier head 70 can be oscillated laterally, e.g., on a slider on the carousel, by movement along the track, or by rotational oscillation of the carousel itself.

[0019] The carrier head 70 may include a retaining ring 84 for holding the substrate. In some implementations, the retaining ring 84 may include a highly conductive portion, for example, the carrier ring may include a thin lower plastic portion 86 that contacts the polishing pad and a thicker upper conductive portion 88. In some implementations, the highly conductive portion is metal, for example the same metal as the layer being polished, for example copper.

[0020] In the process, the platen rotates about its central axis 25 and the carrier head rotates about its central axis 71 and translates laterally across the top surface of the polishing pad 30. The carrier head 70 may include a flexible membrane 80 having a substrate mounting surface that contacts the backside of the substrate 10, and a number of pressurizable chambers 82 for applying different pressures to different zones on the substrate 10, such as different radial zones. The carrier head may also include a retaining ring 84 for holding the substrate.

[0021] The polishing system also includes an eddy current monitoring system 100 that may be coupled to or may be considered to include the controller 90. A rotary coupler 29 may be used to electrically connect components of the rotatable platen 24, such as sensors of an in situ monitoring system, to components external to the platen, such as drive and sensing circuitry or the controller 90.

[0022] The in-situ eddy current monitor system 100 is configured to generate a signal that depends on the depth of a layer of a conductive material, e.g., a metal such as copper, on a substrate. In a process, the polishing system can use the in-situ monitor system 100 to determine when the conductive layer reaches a target thickness, e.g., a target depth of the metal in a trench or a target thickness of a metal layer over a dielectric layer, and then stop polishing. Alternatively or additionally, the polishing system can use the in-situ monitor system 100 to determine the difference in thickness of the conductive material 16 across the substrate 10 and use this information to adjust the pressure in one or more chambers 82 of the carrier head 80 during polishing to reduce polishing non-uniformity.

[0023] A recess 26 may be formed in the platen 24, and optionally a thinned portion 36 may be formed in the polishing pad 30 above the recess 26. The recess 26 and thinned portion 36 may be positioned to pass under the substrate 10 during a portion of the platen rotation, regardless of the translational position of the carrier head. Assuming the polishing pad 30 is a two-layer pad, the thinned portion 36 may be constructed by removing a portion of the backing layer 32 and optionally forming a recess in the bottom of the polishing layer 34. The thinned portion may optionally be optically transparent, for example, if an in-situ optical monitoring system is integrated into the platen 24.

[0024] The in situ monitoring system 100 may include a sensor 102 disposed in the recess 26. The sensor 102 may include a magnetic core 104 positioned at least partially within the recess 26 and at least one coil 106 wound around a portion of the core 104. A drive and sense circuit 108 is electrically connected to the coil 106. The drive and sense circuit 108 generates a signal that may be sent to the controller 90. Although illustrated external to the platen 24, some or all of the drive and sense circuit 108 may be disposed within the platen 24.

[0025] 1 and 3, the drive and sense circuitry 108 applies an alternating current to the coil 106, generating a magnetic field 150 between two poles 152a and 152b of the core 104. In the process, as the substrate 10 is intermittently brought above the sensor 102, a portion of the magnetic field 150 extends into the substrate 10.

[0026] The circuit 108 may include a capacitor connected in parallel with the coil 106. The coil 106 and the capacitor together may form an LC resonant tank.

[0027] If it is desired to monitor the thickness of a conductive layer on a substrate, when the magnetic field 150 reaches the conductive layer, it can pass through the layer on the substrate and generate eddy currents, which change the effective impedance of the LC circuit.

[0028] The drive and sense circuitry 108 may include a marginal oscillator coupled to the combined drive and sense coil 106, and the output signal may be the current required to maintain a constant peak-to-peak amplitude of the sinusoidal oscillation, as described, for example, in U.S. Pat. No. 7,112,960. Other configurations of the drive and sense circuitry 108 are possible. For example, separate drive and sense coils may be wound on a core. The drive and sense circuitry 108 may run a current at a fixed frequency, and the signal from the drive and sense circuitry 108 may be the phase shift of the current in the sense coil relative to the drive coil, or the amplitude of the sensed current, as described, for example, in U.S. Pat. No. 6,975,107.

[0029] 2, as the platen 24 rotates, the sensor 102 sweeps under the substrate 10. By sampling the signal from the circuitry 108 at a particular frequency, the circuitry 108 generates measurements at a series of sampling zones 94 across the substrate 10. In each sweep, measurements at one or more of the sampling zones 94 may be selected or combined. In this manner, the selected or combined measurements over multiple sweeps provide a time-varying series of values.

[0030] The polishing station 20 may also include a position sensor 96, such as an optical interrupter, to sense when the sensor 102 is below the substrate 10 and when the sensor 102 is off the substrate. For example, the position sensor 96 may be mounted at a fixed location opposite the carrier head 70. A flag 98 may be mounted on the periphery of the platen 24. The mounting point and length of the flag 98 are selected such that the flag 98 can send a signal to the position sensor 96 when the sensor 102 has swept under the substrate 10. Alternatively or additionally, the polishing station 20 may include an encoder for determining the angular position of the platen 24.

[0031] 1, a controller 90, e.g., a general purpose programmable digital computer, receives signals from a sensor 102 of an in situ monitoring system 100. As the sensor 102 sweeps under the substrate 10 with each rotation of the platen 24, information is accumulated in situ (once per platen rotation) regarding the depth of the conductive layer, e.g., the bulk layer or conductive material within a trench. The controller 90 can be programmed to sample measurements from the in situ monitoring system 100 when the substrate 10 is generally over the sensor 102.

[0032] Additionally, the controller 90 can be programmed to calculate the radial location of each measurement and sort the measurements into radial ranges. By arranging the measurements into radial ranges, data regarding the conductive film thickness in each radial range can be provided to a controller (e.g., the controller 90) to adjust the polishing pressure profile applied by the carrier head. The controller 90 can also be programmed to apply endpoint detection logic to the series of measurements generated by the in situ monitor system 100 signals to detect the polishing endpoint.

[0033] As the sensor 102 sweeps under the substrate 10 with each rotation of the platen 24, information regarding the thickness of the conductive layer is accumulated in situ on a continuous, real-time basis. During polishing, measurements from the sensor 102 may be displayed on an output device to allow an operator of the polishing station to visually monitor the progress of the polishing process, although this is not required.

[0034] 4A is a graph showing a signal 200 output by eddy current monitoring system 100 as a function of time during a "normal" process with no unexpected mechanical vibrations. Peaks 202 in signal 200 correspond to measurements taken as the sensor passes under substrate 10, increasing in signal strength due to magnetic field interaction with metal layers on the substrate and / or metal portions of the carrier head. Peaks 202 have some "noise" as the sensor passes over various areas with different feature densities, different metal depths, etc., resulting in fluctuating signal strength.

[0035] In contrast, valley 204 of signal 200 corresponds to measurements taken when the sensor is "off the substrate," i.e., not below the substrate. As shown in zoomed-in portion 210 of the graph, the signal is generally flat and the noise is low at valley 204. The signal is generally at a minimum because there are no metal layers on the substrate and / or metal parts of the carrier head with which the magnetic field interacts.

[0036] A "step" 206 at the base of the peak 202 may correspond to the measurement taken when the sensor passes under the retaining ring.

[0037] FIG. 4B is a graph showing a signal 200′ output by the eddy current monitor system 100 as a function of time during a polishing process where vibrations occur due to, for example, stick-slip effects. Again, the peaks 202 of the signal 200′ correspond to measurements taken as the sensor passes under the substrate 10. In some processes, vibrations may not occur immediately. Thus, the initial valleys 204a may be generally flat and low noise. However, when vibrations occur in the polishing system, they may appear as noise in several subsequent valleys 204b. This vibration may be caused by changes in the polishing environment, for example, heat buildup on the polishing pad or changes in the distribution of slurry over time. The appearance of noise in the valleys 204b is unexpected, since there is virtually no metal above the sensor during the valleys 204b. Again, without being limited to a particular theory, it is hypothesized that vibration energy is transferred to the sensor 102, causing the sensor 120 to vibrate relative to the platen 24 (see FIG. 1), causing the assembly to deviate from its calibration condition, resulting in a fluctuating signal.

[0038] 1, the controller 90 may be configured to detect an increase in noise or the presence of noise above a threshold in a portion of the signal corresponding to the sensor 102 being located where a signal from metal above the polishing pad is not expected, i.e., not under the carrier head 70 (including the substrate 10 and the retaining ring 84) or other metal components of the polishing system adjacent and above the polishing pad that may induce a signal, such as not under the metal conditioning head 40 or the conditioning disk 42. The positions of the sensor 102 where a signal from metal is not expected may be referred to as "off-metal" positions.

[0039] The controller 90 can select the portion of the signal that corresponds to the off-metal position of the sensor 102 based on the data from the position sensor 96. For example, the portion of the signal that corresponds to the flag 98 can be excluded. There may be additional flags for the conditioner head 40 and / or other metal components positioned above the polishing pad, and the portions of the signal that correspond to those flags can also be excluded so that the remaining portions of the signal correspond to "off-metal" positions. Alternatively or additionally, the controller can make the selection based on angular position data from the encoder, for example, by comparing the angular positions from the encoder to a set of thresholds, for example, from a look-up table, that indicate which angular positions should be included or excluded. Alternatively or additionally, the controller can make the selection based on signal processing of the signal 200 to detect peaks 202 (see FIGS. 4A-4B) that are then excluded.

[0040] Once the portions of the signal corresponding to the off-metal locations are selected, e.g., valleys 204a, 204b, the "noise" of each portion may be measured. Typically, one noise measurement may be generated per valley 204. Various techniques are possible for measuring the noise in the off-metal portions of the signal, such as standard deviation, maximum-minimum difference, or total trace length. Total trace length is a simple calculation, is sensitive to noise, and is generally not affected by the substrate signal. Yet another possibility is to Fourier transform the signal 200 into a frequency spectrum (equivalent to a wavelength or wavenumber spectrum) and measure the power in a preselected portion of the spectrum, e.g., 1-4 kHz. Any of these techniques will produce a measurement indicative of the noise in the off-metal portions of the signal.

[0041] The controller 90 may then compare the measurements to a stored threshold value. If the noise exceeds the threshold value, the controller 90 may generate a warning signal. This may be a visual or audible signal to the operator so that the operator may decide to stop polishing. Alternatively, the warning signal may cause the controller 90 to automatically stop the polishing process. In either case, the operator may then take corrective action, such as adjusting polishing control parameters, such as slurry flow rate, carrier head pressure, or delivery of heat or coolant, or replacing a part, such as replacing a retaining ring, to prevent uneven polishing of subsequent substrates.

[0042] Although the above description has focused on detecting mechanical vibrations during the polishing process, it is also possible to use this technique for process and hardware screening, for example, during the "qualification" of a polishing system or polishing recipe. For example, the polishing system can be operated without a substrate present on the carrier head, or using a "blank" substrate (not a device substrate for integrated circuit manufacturing). If mechanical vibrations are detected, the polishing system or polishing recipe is not qualified.

[0043] FIG. 5 is a flow diagram of a method 500 for monitoring mechanical vibrations. Assuming that polishing of a substrate is desired (as opposed to a screening process without a substrate), the substrate is placed on the carrier head and brought into contact with the polishing surface of the polishing pad (502). Without the substrate, the retaining ring of the carrier head contacts the polishing pad. A polishing fluid (e.g., a slurry) is applied to the polishing pad (504) and relative motion is generated between the carrier head and the polishing pad, e.g., the platen is rotated (506). As this occurs, the system is monitored with an in situ eddy current monitoring system (508) and a signal, e.g., a series of signal values, is generated. During the polishing process, the portion of the signal corresponding to the sensor being below the substrate can be used to detect the thickness of the metal layer on the substrate and detect the polishing endpoint. Separately, portions of the signal corresponding to the sensor being in an "off-metal" position are selected (510). The noise in these "off-metal" portions of the signal is measured and compared to a threshold (512). If the measured noise exceeds a stored threshold, an alert can be generated (514).

[0044] The polishing apparatus and method described above can be applied to various polishing systems. As long as there is a period when the sensor is in the "off-metal" position, either the polishing pad or the carrier head, or both, can move to generate relative motion between the polishing surface and the substrate. For example, the platen can move in an orbit rather than rotate. The polishing pad can be a circular (or other shaped) pad fixed to the platen. The polishing layer can be a standard (e.g., polyurethane with or without fillers) abrasive material, a soft material, or a fixed abrasive material. The term relative positioning is used to refer to the relative positioning within the system, not with respect to gravity. It should be understood that during the polishing process, the polishing surface and the substrate can be held in a vertical orientation or any other orientation.

[0045] Although the above description has focused on the analysis of the "off-metal" portion of the signal, it is also possible to use the "on-metal" portion of the signal to detect mechanical vibrations. In general, the signal variations due to mechanical vibrations occur at different frequencies than the variations due to the substrate, e.g., the patterned metal. Thus, the signal can be filtered, e.g., with a high-pass or band-pass filter, and the filtered signal can be analyzed to determine whether or not to issue an alarm. The particular frequency range of the filter can be determined empirically. The noise of the filtered signal can then be compared to a threshold value to determine whether or not to issue an alarm.

[0046] The functional operations of the controller 90 may be implemented using one or more computer program products, i.e., one or more computer programs tangibly embodied in a non-transitory computer-readable storage medium for execution by or to control the operation of a data processing device, e.g., a programmable processor, a computer, or multiple processors or computers.

[0047] A number of embodiments of the invention 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 embodiments are within the scope of the following claims.

Claims

1. 1. A method of chemical mechanical polishing comprising the steps of: contacting the substrate with a polishing pad; supplying a polishing liquid to the polishing pad; generating relative motion between the substrate and the polishing pad; sweeping a sensor of an in-situ eddy current monitoring system in a path across the substrate while the substrate is being polished; selecting portions of signals from the in-situ eddy current monitoring system that correspond to off-metal locations of the sensor, the off-metal locations excluding at least locations below the substrate; and measuring noise in a portion of the signal corresponding to the off-metal location of the selected sensor; comparing the measured noise to a threshold value to determine whether to issue an alert; A method comprising:

2. The method of claim 1 , comprising issuing an alert in response to the measured noise exceeding the threshold.

3. The method of claim 2 , wherein the warning comprises a visual or audio warning to an operator.

4. The method of claim 2 , further comprising stopping polishing in response to said warning.

5. A computer program product tangibly encoded on a non-transitory computer readable medium, the computer program product comprising: receiving a signal from an in situ eddy current monitoring system including a sensor that sweeps beneath a carrier head of the polishing system; selecting portions of the signal that correspond to off-metal locations of the sensor, the off-metal locations excluding at least locations of the sensor that are below the carrier head; and measuring noise in a portion of the signal corresponding to the off-metal location of the selected sensor; comparing the measured noise to a threshold value to determine whether to issue an alert; A computer program product comprising instructions to cause a

6. 6. The computer program product of claim 5, further comprising instructions for issuing an alert in response to the measured noise exceeding the threshold.

7. The computer program product of claim 6 , further comprising instructions for stopping polishing in response to said warning.

8. 6. The computer program product of claim 5, wherein the instructions to measure noise in the selected portion of the signal include instructions to calculate one or more of a standard deviation, a maximum-minimum difference, or a total trace length of the signal.

9. 6. The computer program product of claim 5, wherein the instructions for measuring the noise of the selected portion of the signal include instructions for performing a Fourier transform on the selected portion of the signal and calculating the power of the signal in a range of frequencies.

10. 6. The computer program product of claim 5, wherein the instructions for selecting the portion of the signal include instructions for receiving platen position data from a sensor and comparing the platen position data to a range of stored platen position values.

11. 6. The computer program product of claim 5, wherein the instructions to select a portion of the signal include instructions to perform signal processing of the signal to detect peaks in the signal corresponding to the sensor passing under a substrate and to exclude the peaks.

12. 6. The computer program product of claim 5, wherein the instructions to measure the noise include instructions to detect each sweep of the sensor below a substrate and generate a noise value for each sweep.

13. 1. A polishing system comprising: a rotatable platen for holding a polishing pad; a carrier head that holds a substrate in contact with the polishing pad; A motor that rotates the platen; an in situ eddy current monitoring system including a sensor location within the platen such that a sensor sweeps across the carrier head with each rotation of the platen; A controller, receiving a signal from the in situ eddy current monitor system; selecting portions of the signal that correspond to off-metal locations of the sensor, the off-metal locations excluding at least locations of the sensor that are below the carrier head; and measuring noise in portions of the signal corresponding to selected off-metal locations of the sensor; comparing the measured noise to a threshold value to determine whether to issue an alert; a controller configured to A system comprising:

14. The system of claim 13 , wherein the controller is configured to issue an alarm in response to the measured noise exceeding the threshold.

15. The system of claim 14 , wherein the controller is configured to stop polishing in response to the alert.

16. 1. A method comprising: contacting the object with a polishing pad of a polishing system; supplying a polishing liquid to the polishing pad; generating relative motion between the object and the polishing pad while the object is in contact with the polishing pad; generating signals from an in-situ eddy current monitoring system during relative motion while the object is in contact with the polishing pad; detecting mechanical vibrations of the polishing system based on signals from the in-situ eddy current monitoring system; A method comprising:

17. 17. The method of claim 16, wherein detecting mechanical vibrations includes applying a filter to at least a portion of the signal corresponding to a sensor of the eddy current monitoring system being below the object to produce a filtered signal in which fluctuations in the signal due to the object are removed.

18. The method of claim 17 , wherein detecting mechanical vibrations comprises measuring noise in the filtered signal.

19. 17. The method of claim 16, comprising sweeping a sensor of an in-situ eddy current monitoring system in a path across the object, and detecting mechanical vibrations comprises selecting portions of the signal from the in-situ eddy current monitoring system that correspond to off-metal locations of the sensor, the off-metal locations excluding at least locations below the object.

20. 20. The method of claim 19, wherein detecting the mechanical vibration comprises measuring noise in a portion of the signal that corresponds to the off-metal location of a selected one of the sensors.

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