JP5542802B2 - Endpoint detection in chemical mechanical polishing using multiple spectra - Google Patents

Description

  The present invention relates generally to spectroscopic monitoring of a substrate during chemical mechanical polishing.

  Integrated circuits are typically formed on a substrate by sequential deposition of conductive, semiconductive, or insulating layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of the patterned layer is exposed. A conductive filler layer can be deposited on the patterned insulating layer, for example, to fill the grooves or holes in the insulating layer. After planarization, the portions of the conductive layer that remain between the raised patterns of the insulating layer form vias, plugs, and wiring that provide conductive paths between the thin film circuits on the substrate. For other applications such as oxide polishing, the filler layer is planarized until a predetermined thickness is left over the non-planar surface. In addition, planarization of the substrate surface is usually required for photolithography.

  Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be attached to a carrier or polishing head. The exposed surface of the substrate is typically placed in contact with the rotating abrasive disc pad or belt pad. The polishing pad can be a standard pad or a fixed abrasive pad. Standard pads have a durable rough surface, while fixed abrasive pads have abrasive particles held in a containment medium. The carrier head provides a controllable load on the substrate to press the substrate against the polishing pad. A polishing liquid, such as a slurry containing abrasive particles, is typically supplied to the surface of the polishing pad.

  One problem with CMP is determining whether the polishing process is complete, i.e., whether the substrate layer has been planarized to the desired flatness or thickness, and when the desired amount of material has been removed. is there. Overpolishing (removing too much) of the conductive layer or film leads to increased circuit resistance. On the other hand, insufficient polishing (too little removal) of the conductive layer leads to electrical shorts. Changes in the initial thickness of the substrate layer, slurry composition, polishing pad condition, relative speed between the polishing pad and the substrate, and loading on the substrate can cause changes in the material removal rate. These changes cause changes in the time required to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined simply as a function of polishing time.

  In one general aspect, a computer-implemented method includes obtaining at least one current spectrum using an in situ optical monitoring system, comparing the current spectrum to a plurality of different reference spectra, Determining whether a polishing endpoint has been reached for a substrate having an outermost layer undergoing polishing based on the comparing step. The current spectrum is that of light reflected from a substrate having an outermost layer being polished and at least one sublayer. The plurality of reference spectra represents the spectrum of light reflected from a substrate having an outermost layer having the same thickness and a lower layer having a different thickness.

  Embodiments can include one or more of the following. Determining whether the polishing endpoint has been reached may include calculating a difference between the current spectrum and a reference spectrum. Determining whether the polishing endpoint has been reached may include determining whether at least one of the differences has reached a threshold value. At least one of the differences may be a minimum difference. Determining whether the polishing endpoint has been reached may include activating an endpoint detection algorithm when at least one of the differences reaches a threshold value. Determining whether the polishing endpoint has been reached may include generating a difference trace that includes a plurality of points, where each of these points is a minimum difference calculated for platen rotation. Represents. The endpoint detection algorithm may include determining whether the difference trace has reached a minimum value. Determining whether the difference trace has reached a minimum value may include calculating a slope of the difference trace or determining whether the difference trace has risen to a threshold above the minimum value. . The reference spectrum can be generated experimentally or from theory.

  In another aspect, a computer program product encoded on a tangible program carrier is operable to cause a data processing apparatus to perform operations including the method steps above.

  As used herein, the term substrate can include, for example, a product substrate (eg, it includes multiple memories or processor dies), a test substrate, a bare substrate, and a gating substrate. The substrate can be at various stages of integrated circuit fabrication, for example, the substrate can be a bare wafer, or it can be one or more deposited layers and / or patterned layers. Can be included. The term substrate can encompass circular disks and rectangular sheets.

  Possible advantages of embodiments of the invention can include one or more of the following. The sensitivity of the endpoint detection system to changes between sublayers or patterns between the substrates can be reduced, and therefore the reliability of the endpoint system can be improved. The use of multiple reference spectra (as opposed to a single reference spectrum) determines the endpoint by providing a difference or endpoint trace that is generally smoother than the trace generated by using a single reference spectrum technique Improve the accuracy of

  The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the invention will be apparent from the description, drawings, and claims.

It is a figure which shows a board | substrate. It is a figure which shows a chemical mechanical polishing apparatus. It is a bird's-eye view of a polishing pad, and shows a position where an in-situ measurement result is acquired. It is a flowchart of polishing end point determination. 3 illustrates a difference trace from a spectroscopic monitoring system. It is a flowchart of embodiment of grinding | polishing end point determination.

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

  As shown in FIG. 1, the substrate 10 is a wafer 12, an outermost layer 14 to be polished, and between the outermost layer 14 and the wafer 12, some of which are typically patterned. Alternatively, a plurality of lower layers 16 can be included. One potential problem associated with spectroscopic endpoint detection during chemical mechanical polishing is that the thickness (s) of the underlying layer (s) can vary from substrate to substrate. As a result, a substrate with the outermost layer having the same thickness may actually reflect different spectra depending on the underlying layer (s). As a result, the target spectrum used to trigger the polishing endpoint for some substrates may not function properly for other substrates, for example if the underlying layers have different thicknesses. However, it is possible to compensate for this effect by comparing the spectrum obtained during polishing against multiple spectra, where the multiple spectra represent changes in the lower layer (s).

  FIG. 2 shows a polishing apparatus 20 operable to polish the substrate 10. The polishing apparatus 20 includes a rotatable disk-shaped platen 24 on which a polishing pad 30 is located. The platen is operable to rotate about the axis 25. For example, the motor can rotate the drive shaft 22 to rotate the platen 24.

  Optical access 36 through the polishing pad is provided by including an opening (ie, a hole through the pad) or a solid window. The solid window can be secured to the polishing pad, but in some embodiments can be supported on the platen 24 and protrude into the opening of the polishing pad. The polishing pad 30 is typically placed on the platen 24 such that an opening or window lies on the optical head 53 located in the recess 26 of the platen 24. The optical head 53 consequently has optical access through an opening or window to the substrate being polished. The optical head will be further described later.

  The polishing apparatus 20 includes a composite slurry / rinse arm 39. During polishing, the arm 39 is operable to dispense a polishing liquid 38 such as a slurry. Alternatively, the polishing apparatus includes a slurry port operable to dispense slurry onto the polishing pad 30.

  The polishing apparatus 20 includes a carrier head 70 that is operable to contact the polishing pad 30 and hold the substrate 10. The carrier head 70 is suspended from a support structure 72, such as a carousel, and is connected to a carrier head rotation motor 76 by a carrier drive shaft 74 so that the carrier head can rotate about an axis 71. In addition, the carrier head 70 can vibrate laterally in radial slots formed in the support structure 72. In operation, the platen is rotated about its central axis 25 and the carrier head is rotated about its central axis 71 and moved laterally across the top surface of the polishing pad.

  The polishing apparatus also includes an optical monitoring system that can be used to determine the polishing endpoint as discussed below. The optical monitoring system includes a light source 51 and a photodetector 52. Light impinges from the light source 51 through the optical access 36 of the polishing pad 30, is reflected by the substrate 10, returns through the optical access 36, and is transmitted to the photodetector 52.

  The branch optical cable 54 can be used to transmit light from the light source 51 to the optical access 36, and conversely from the optical access 36 to the photodetector 52. The branch optical cable 54 can include a “trunk” 55 and two “branches” 56 and 58.

  As described above, the platen 24 includes the recess 26 in which the optical head 53 is positioned. The optical head 53 holds one end of the trunk 55 of the branch fiber cable 54, and the cable 54 is configured to transmit light to and from the substrate surface being polished. The optical head 53 can include one or more lenses or windows that overlie the end of the branch fiber cable 54. Alternatively, the optical head 53 can simply hold the end of the trunk 55 adjacent to the solid window of the polishing pad. The optical head 53 can hold the above-described nozzle of the flushing system. The optical head 53 may be removed from the recess 26 as necessary, for example, for prevention or improved maintenance.

  The platen includes a removable in-situ monitoring module 50. The in-situ monitoring module 50 can include a light source 51, a photodetector 52, and one or more of electrical circuits for transmitting and receiving signals to and from the light source 51 and the photodetector 52. For example, the output of the detector 52 may be a digital electronic signal that goes through a rotary coupling on the drive shaft 22, such as a slip ring, to a controller for an optical monitoring system. Similarly, the light source may be turned on or off in response to control commands in a digital electronic signal going from the controller through the rotary coupler to the module 50.

  The in-situ monitoring module can also hold the respective ends of the branch portions 56 and 58 of the branch optical fiber 54. The light source is operable to transmit light, which is transmitted through the branch 56 out of the end of the trunk 55 in the optical head 53 and hits the substrate being polished. The light reflected from the substrate is received at the end of the trunk 55 in the optical head 53 and transmitted through the branch 58 to the photodetector 52.

  In one embodiment, the branch fiber cable 54 is a bundle of optical fibers. The bundle includes a first group of optical fibers and a second group of optical fibers. The first group of optical fibers are connected to transmit light from the light source 51 to the polished substrate surface. The second group of optical fibers are connected to receive light reflected from the substrate surface being polished and to transmit the received light to a photodetector. The optical fibers can be arranged such that the second group of optical fibers forms an X-like shape centered on the longitudinal axis of the branch optical fiber 54 (as viewed in the cross section of the branch fiber cable 54). Alternatively, other arrangements can be implemented. For example, the second group of optical fibers can form a V-like shape that is a mirror image of each other. Suitable branch optical fibers can be found in Carrollton, Texas, Verity Instrument, Inc. Available from

  The light source 51 is operable to emit white light. In one embodiment, the emitted white light includes light having a wavelength of 200-800 nanometers. A suitable light source is a xenon lamp or a xenon mercury lamp.

  The photodetector 52 can be a spectrometer. A spectrometer is basically an optical instrument for measuring the intensity of light over a portion of the electromagnetic spectrum. A suitable spectrometer is a diffraction grating spectrometer. A typical output for a spectrometer is the intensity of light as a function of wavelength.

  The light source 51 and the photodetector 52 are connected to a computing device operable to control their operation and receive their signals. The computing device can include a microprocessor, such as a personal computer, located near the polishing apparatus. With respect to control, the computing device can, for example, synchronize the activation of light source 51 with the rotation of platen 24. As shown in FIG. 3, the computer can cause the light source 51 to emit a series of flashes that begin immediately before the substrate 10 passes over the in situ monitoring module and end immediately thereafter. (Each of the drawn points 301 to 311 represents the position where the light from the in-situ monitoring module has been hit and reflected.) Alternatively, the computer immediately before the substrate 10 passes over the in-situ monitoring module. Light that starts and ends immediately after can be continuously emitted to the light source 51. In either case, the signal from the detector can be integrated over the sampling period to produce a spectral measurement result at the sampling frequency. Although not shown, the alignment of the substrate 10 with the monitoring module for each passage on the monitoring module may be different from the previous passage. Over one rotation of the platen, the spectrum is obtained from different radii on the substrate. That is, some spectra are obtained from locations closer to the center of the substrate, and some are obtained from locations closer to the edge. In addition, a series of spectra may be acquired over time over multiple rotations of the platen.

  In operation, the computing device may receive a signal carrying information describing the spectrum of light received by the photodetector 52, eg, for a particular flash of the light source or detector time frame. Thus, this spectrum is a spectrum measured in situ during polishing.

  Without being limited to any particular theory, the spectrum of light reflected from the substrate 10 changes due to changes in the thickness of the outermost layer as polishing progresses, and hence a series of time-varying spectra. Cause it to occur. Moreover, a specific spectrum is presented by a stack of specific thickness.

  The computing device can process the signal to determine the end point of the polishing step. In particular, the computing device can execute logic to determine when the end point has been reached based on the measured spectrum.

  In short, the computing device can compare the measured spectrum with a plurality of reference spectra and use the result of the comparison to determine when the end point has been reached.

  As used herein, a reference spectrum is a predetermined spectrum that is generated prior to polishing of a substrate. The reference spectrum can have an association with a value of the substrate characteristic, such as the thickness of the outermost layer, that is defined prior to the polishing operation. The reference spectrum can be generated experimentally, for example by measuring a spectrum from a test substrate having a known layer thickness, or can be generated from theory.

  The reference spectrum can be a target spectrum, which can be an endpoint process compensated target spectrum or an uncompensated target spectrum. The uncompensated target spectrum is the spectrum presented by the substrate when the outermost layer has a target thickness. As an example, the target thickness can be 1 to 3 microns. Alternatively, the target thickness can be zero, for example when the target film is removed so that the underlying layer is exposed. However, there may be a time lag between when the system receives a spectrum representing the target thickness and when polishing stops (it is an endpoint detection algorithm, command that requires spectra from multiple platen rotations) May be due to the time that is transmitted from the controller to the processing system and the time required to stop the rotation of the platen). Therefore, the polishing end point can be set to a time before the target thickness is achieved. The end point process compensation target spectrum has substantially the target thickness when used to provide a polishing end point trigger under a specific end point algorithm and polishing control system, for example, no time offset compensation is made. It is a spectrum that results in a substrate with a thickness that is significantly closer to the target thickness.

  As noted above, there are multiple reference spectra for a particular target thickness of the outermost layer. That is because different thicknesses of the lower layer (s) for different substrates can result in different spectra even though the outermost layer has the same thickness. In addition, substrates for different integrated chip products will have different patterning of layers, which may also result in different spectra even though the outermost layer has the same thickness. Thus, there may be multiple spectra for a particular thickness of the outermost layer, and the multiple spectra are sublayers (because the substrate is intended to provide different products) Different spectra can be included for different thickness (es) or different patterns.

  The reference spectrum is collected prior to the polishing operation and the association of each reference spectrum with its associated substrate characteristics is preserved. The reference spectrum can be determined experimentally.

  For example, to determine a target spectrum, the properties of a “set” substrate that has the same pattern as the product substrate can be measured before polishing at a metrology station. The substrate characteristic can be the thickness of the outermost layer. The setting substrate is then polished while the spectrum is collected. The setting substrate can be periodically removed from the polishing system, and its properties are measured at a measurement station. The substrate may be overpolished so that a spectrum of light reflected from the substrate can be obtained when the target thickness is reached, i.e., polished beyond the desired thickness.

  The measured thickness and the acquired spectrum are used to select one or more spectra from the acquired spectrum that are determined to be presented by the substrate when the substrate has the thickness of interest. In particular, linear interpolation is performed using the measured pre-polishing film thickness and post-polishing substrate thickness to determine the time presented when the target thickness is achieved and the corresponding spectrum. be able to. The spectrum or multispectrum determined to be presented when the target thickness is achieved is designated to be the target spectrum or multitarget spectrum.

  These steps are then repeated for one or more additional configuration substrates that have the same pattern as the product substrate, but have different thickness (es) of the lower layer (s) to generate additional reference spectra. be able to. Thus, the resulting collection of reference spectra includes different target spectra for different target layer (s) thicknesses, but for the same target thickness.

  Alternatively or additionally, these steps can then be repeated for one or more additional setting substrates that have a different pattern than the product substrate to generate additional reference spectra. Thus, the resulting collection of reference spectra includes different target spectra for different patterns but for the same target thickness.

  Optionally, the collected spectrum is processed to increase accuracy and / or precision. The spectrum can be processed, for example, to normalize the spectrum to a common reference, to average the spectrum, and / or to filter noise from the spectrum.

  In addition, some or all of the reference spectra can be calculated from theory using, for example, an optical model of the substrate layer.

  FIG. 4 shows a method 200 for using spectrum-based endpoint determination logic to determine the endpoint of a polishing step. The product substrate is polished using the polishing apparatus described above (step 402). The following steps occur at each rotation of the platen.

  At least one spectrum of light reflected from the surface of the substrate being polished is measured (step 404). Optionally, multiple spectra can be measured, for example, spectra measured at different radii on the substrate can be obtained from a single rotation of the platen, for example at points 301 to 311 (FIG. 3). If multiple spectra are measured, one or more subsets of the spectra can be selected for use in the endpoint detection algorithm. For example, a spectrum measured at a sample location close to the center of the substrate (eg, at points 305, 306, and 307 shown in FIG. 3) may be selected. The spectrum measured during the current platen rotation is optionally processed to increase accuracy and / or precision.

式中、ａおよびｂは、それぞれスペクトルの波長の範囲の下限および上限であり、Ｉ_{ｃｕｒｒｅｎｔ}（λ）およびＩ_{ｒｅｆｅｒｅｎｃｅ}（λ）は、それぞれ所与の波長に対する現在のスペクトルの強度および目標スペクトルの強度である。別法として、差は、平均二乗誤差として計算でき、すなわち、

である。 The difference between each of the selected measured spectra and each of the reference spectra is calculated (step 406). The reference spectrum can be a target spectrum. In one embodiment, the difference is the sum of the intensity differences over a range of wavelengths. That is,

Where a and b are the lower and upper limits of the spectral wavelength range, respectively, and I _current (λ) and I _reference (λ) are the current spectral intensity and target spectral intensity for a given wavelength, respectively. It is. Alternatively, the difference can be calculated as a mean square error, i.e.

It is.

  One way to calculate the difference between each of the current spectra and each of the reference spectra is to select each of the current spectra. For each current spectrum selected, the difference is calculated against each of the reference spectra. Given the current spectra e, f, and g and the reference spectra E, F, and G, for example, the difference is the next combination of the current spectrum and the reference spectrum: e and E, e and F, e And G, f and E, f and F, f and G, g and E, g and F, and g and G.

  The calculated minimum difference is added to the difference trace (step 408). The difference trace is typically updated once per platen revolution. The difference trace is generally a single plot of the calculated difference (in this case, the minimum difference calculated for the current platen rotation). As an alternative to the minimum difference, another difference can be added to the trace, such as the median of the differences or the next of the minimum differences.

  Optionally, the difference trace can be processed, for example, to smooth out the difference trace by filtering out calculated differences that deviate beyond the threshold from one or more preceding calculated differences. can do.

  It is determined whether the difference trace is below a threshold (step 410). Once the difference trace falls below the threshold, endpoint logic is initiated and can be applied to detect the endpoint condition, eg, the lowest value of the difference trace (step 412). For example, the endpoint can be called when the difference trace starts to rise past a certain threshold of the lowest value, or when the slope of the difference trace falls below a threshold close to zero, or other window logic Applicable. Once the endpoint logic detects the endpoint condition (step 414), polishing is stopped (step 416).

  In some embodiments, once the difference trace falls below the threshold, the closest reference, eg, the particular reference spectrum that provided the smallest difference from the measured spectrum, is then the only one for the rest of the endpoint determination process. Used as a reference spectrum. This ensures that the endpoint is based on a target spectrum that represents a substrate similar to the substrate whose sublayer is being polished.

  By using multiple reference spectra representing substrates having sublayers of different thicknesses, the endpoint detection system may be less sensitive to sublayer changes, and thus the reliability of the endpoint system may be improved. Similarly, by using multiple reference spectra representing substrates having different patterns, the endpoint detection system may be less sensitive to pattern changes and thus improve the reliability of the endpoint system.

  If it is not determined that the difference trace has reached the minimum threshold range, polishing is allowed to continue and steps 404, 406, and 408 are repeated as necessary.

  FIG. 5 is an example graph of a difference trace as a function of time, illustrating the threshold. Trace 502 is a difference trace and may already be filtered and smoothed. End point detection 508 is triggered when the smoothed difference trace 502 reaches a threshold 504 above a minimum value 506.

  FIG. 6 shows a method 600 for determining the end point of the polishing step. Prior to the polishing operation, a reference spectrum is generated and collected experimentally, for example, by polishing a set substrate and measuring the spectrum, or calculated from theory using, for example, an optical model of the substrate layer. The The spectrum is stored in a library. However, unlike the process of FIG. 4, where only the target spectrum representing the target thickness is used, the reference spectrum of the library represents substrates having various different thicknesses in the outer layer. The measured spectrum is then compared to the library spectrum and one of the library spectra is selected as the match.

  The spectra are indexed such that each of the collection of spectra representing a substrate having a particular sublayer thickness has a unique index value (a spectrum representing a substrate having a different sublayer thickness is the same index May be associated with a value). Indexing is performed such that the index values are arranged in the order in which the spectra were measured during polishing or in the order in which they are expected to be measured. The index value can be selected to increase monotonically as polishing progresses, for example, the index value may be proportional to the platen speed, for example linearly proportional. Thus, each index can be an integer, and the index can represent the expected platen rotation that the associated spectrum will appear in. The library can be implemented in the memory of the computer device of the polishing apparatus.

  One substrate in the batch of substrates is polished (step 602) and the next step is performed for each platen rotation. One or more spectra are measured to obtain a current spectrum for the current platen rotation (step 604). Of the spectra stored in the library, the spectrum closest to the current spectrum is determined (step 606). The index of the library spectrum closest to the current spectrum is determined from the library (step 608) and added to the endpoint index trace (step 610). As discussed above, the index can be determined prior to the polishing operation, and can be stored for later access, for example as a database relating the spectrum to the index. The endpoint is called when the endpoint trace reaches the target spectrum index (step 612).

  In some embodiments, the indices that match each acquired spectrum are plotted according to time or platen rotation. The line is fitted to the index plotted using robust line fitting. Where the line meets the target indicator defines the end point time or rotation.

  As discussed above, by using multiple reference spectra representing substrates with different thickness sublayers, the endpoint detection system is less sensitive to sublayer changes, and therefore the endpoint system's reliability. May improve sex.

  A method that can be applied during the endpoint process is to limit the portion of the library that is searched for matching spectra. Libraries typically include a wider range of spectra than can be obtained while polishing a substrate. A broad range means that the spectrum is obtained from a thicker starting outermost layer and after overpolishing. During substrate polishing, the library search is limited to a predetermined range of the library spectrum. In some embodiments, the current rotation index N of the substrate being polished is determined. N can be determined by searching the entire library spectrum. For spectra obtained during subsequent rotations, the library is searched within N degrees of freedom. That is, if an index is found to be N during one rotation, the range to be searched for is Y (Y + X) − during subsequent rotations after X rotations, assuming that the degree of freedom is Y. Y to (N + X) + Y. For example, in the first polishing rotation of the substrate, if the matching index is found to be 8 and the degree of freedom is selected to be 5, for the spectrum obtained during the second rotation, the exponent 9 ± Only the spectrum corresponding to 5 is examined looking for matches.

  All of the embodiments of the invention and the functional operations described in this specification are based on digital electronic circuits, or in computer software, firmware, or structural means disclosed herein and their structural equivalents. It can be implemented with hardware including, or a combination thereof. Embodiments of the present invention may be used as one or more computer program products, i.e., for execution by a data processing device, e.g., a programmable processor, computer, or multiple processors or computers, or to control their operations Can be implemented as one or more computer programs tangibly embodied in an information carrier, eg, in a machine-readable storage device or in a propagated signal. A computer program (also known as a program, software, software application, or code) can be written in any form of programming language, including a compiled or interpreted language, either as a stand-alone program or as a module It can be deployed in any form including components, subroutines, or other units suitable for use in a computer environment. A computer program does not necessarily correspond to a file. A program stores a part of a file that holds other programs or data, a single file dedicated to the program in question, or multiple collaborative files (eg, one or more modules, subprograms, or portions of code) File). A computer program can be developed to be executed on one computer or on multiple computers interconnected by a communication network, or distributed on multiple sites.

  The process and logic flows described herein are performed by one or more programmable processors that operate on input data and execute one or more computer programs to perform functions by generating output. Can do. Processes and logic flows can also be performed by special purpose logic circuits, such as FPGAs (Field Programmable Gate Arrays) or ASICs (Application Specific Integrated Circuits), and the devices can also be implemented as such.

  The above-described polishing apparatus and method can be applied to various polishing systems. The polishing pad, the carrier head, or both can be moved to provide relative movement between the polishing surface and the substrate. For example, the platen may orbit rather than rotate. The polishing pad can be a circular (or some other shape) pad that is secured to the platen. Some aspects of the endpoint detection system may be applicable when the linear polishing system is, for example, a continuous or reel-to-reel belt in which the polishing pad moves linearly. The abrasive layer can be a standard (eg, polyurethane with or without filler) abrasive material, a soft material, or a fixed abrasive material. It should be understood that relative positioning conditions are used and the polishing surface and substrate can be held in a vertical orientation or some other orientation.

  A particular embodiment of the present invention has been described. Other embodiments are within the scope of the claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results.

Claims (15)

A step of obtaining at least one spectrum using situ optical monitoring system, resulting spectrum is reflected from each of the outermost layer and at least one respective substrate having a lower layer undergoing polishing light spectrum der of is, the steps spectrum of the light reflected from the substrate is that determined by the thickness of the thickness and the lower layer of the outermost layer,
Comparing the obtained spectrum with a plurality of different reference spectra , each of the plurality of reference spectra representing a spectrum of light reflected from a respective one of a plurality of reference substrates, each reference substrate; Each outermost layer and at least one respective sublayer, and each outermost layer of a plurality of reference substrates share a common thickness, while the at least one respective sublayer is among the plurality of reference substrates, a step that differ in terms of at least the thickness or patterning,
Determining whether a polishing endpoint has been reached for the substrate having the outermost layer undergoing polishing based on the comparing step. The method of claim 1, wherein determining whether the polishing endpoint has been reached comprises calculating a difference between the obtained spectrum and the reference spectrum.   The method of claim 2, wherein determining whether the polishing endpoint has been reached comprises determining whether at least one of the differences has reached a threshold value.   The method of claim 3, wherein the at least one of the differences is a minimum of the differences or a median of the differences.   The method of claim 2, wherein determining whether the polishing endpoint has been reached comprises invoking an endpoint detection algorithm when at least one of the differences reaches a threshold value.   6. The method of claim 5, wherein determining whether the polishing endpoint has been reached comprises generating a difference trace that includes a plurality of points, each representing a minimum of the difference calculated for platen rotation. the method of.   The method of claim 6, wherein the endpoint detection algorithm includes determining whether the difference trace has reached a minimum value.   The method of claim 7, wherein determining whether the difference trace has reached a minimum comprises calculating a slope of the difference trace.   The method of claim 6, wherein the endpoint detection algorithm includes determining whether the difference trace has risen to a threshold value that is greater than the minimum value. The method of claim 1, further comprising obtaining the plurality of spectra at different times. The method of claim 10, wherein a plurality of acquired spectra comprises a series of acquired spectra based on a plurality of sweeps of the in situ optical monitoring system across the substrate. Spectrum obtained multiple is include a plurality of resulting spectrum one based on the same sweep of the in situ optical monitoring system across the substrate, The method of claim 10. To generate a plurality of differences between the plurality of spectra obtained with the plurality of reference spectra further comprises a plurality of spectra obtained, wherein the based on the same sweep step for comparing said plurality of reference spectra The method according to claim 12.   The method of claim 13, further comprising determining a minimum of the plurality of differences and determining whether a polishing endpoint has been reached using the minimum of the plurality of differences. A step of obtaining at least one spectrum using situ optical monitoring system, resulting spectrum is reflected from each of the outermost layer and at least one respective substrate having a lower layer undergoing polishing light spectrum der of is, the steps spectrum of the light reflected from the substrate is that determined by the thickness of the thickness and the lower layer of the outermost layer,
Comparing the obtained spectrum with a plurality of different reference spectra , each of the plurality of reference spectra representing a spectrum of light reflected from a respective one of a plurality of reference substrates, each reference substrate; Each outermost layer and at least one respective sublayer, and each outermost layer of a plurality of reference substrates share a common thickness, while the at least one respective sublayer is among the plurality of reference substrates, a step that differ in terms of at least the thickness or patterning,
Based on the comparing step, the data processing device is operable to cause the data processing device to perform an operation including determining whether the polishing end point has been reached for the substrate having the outermost layer undergoing polishing. A computer program product encoded on a tangible program carrier.

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