Abstract:
Methods and apparatus for feedback controlled polishing. A computer program product for generating feedback for chemical mechanical polishing. The product includes instructions operable to cause a processor to receive monitoring information during a current polishing cycle in which a first polishing process is performed on a substrate that includes a metal layer. The first polishing process clears the metal layer from the substrate during the current polishing cycle. The product includes instructions to calculate a representation of a clearing profile of the first polishing process. The calculation is based on the monitoring information received during the current polishing cycle. The product includes instructions to detect non-uniformity in the representation. The product includes instructions to generate, from the non-uniformity detected, feedback information for improving the uniformity of a clearing profile of the first polishing process for a subsequent polishing cycle.

Description:
This application claims priority to Provisional U.S. Application Ser. No. 60/366,271, filed on Mar. 22, 2002, which application is hereby incorporated by reference in its entirety. 

   BACKGROUND 
   The present invention relates to chemical mechanical polishing of substrates. 
   An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface, and planarizing the filler layer until the top surfaces of raised structures of the underlying layer are exposed. For example, a conductive filler layer can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. In this case, the portion of the layer that is raised above the bottom of the trenches or holes form the raised pattern. The filler layer is then polished until the raised pattern of the insulative layer is exposed. The process of planarizing the filler layer until the top surfaces of raised structures of the underlying layer are exposed is also referred to as clearing the filler layer. The time required to clear the filler layer will be referred to in this specification as time clearing time. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs, and lines that provide conductive paths between thin film circuits on the substrate. Copper damascene is one example of the above described fabrication step. 
   Chemical mechanical polishing (“CMP”) is one polishing technique that can be used for planarization. CMP typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing disk pad or belt pad. The polishing pad can be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad. 
   Planarization can generally include different polishing processes. For example, planarization can include a first polishing process, for removing the bulk of a filler layer, and a second polishing process, for removing the small amount of the filler layer remaining. The term polishing process refers to a combination of particular actions performed, in a particular sequence, using particular devices and particular materials to remove material. A polishing process can apply one or more polishing techniques. 
   A polishing process has parameters, exhibits characteristics, and yields results. Parameters of a polishing technique can include, for example, slurry flow rate, force on a abrasive surface being used to polish the substrate, and the radial speed at which the abrasive surface is being rotated. The characteristics of a polishing process can be the manner in which the process removes material, also referred to as the removal profile of the polishing process. A removal profile of a polishing process, which, as described, is a manner in which the polishing process removes material, is different from a thickness profile of a layer, which is the shape of a cross section of the layer. A removal profile is also different from a clearing profile of a substrate, which is the resulting shape of a cross section of a substrate after a filler layer has been cleared. Performing a polishing process on a substrate, that has an initial or pre-polish thickness profile, usually changes the pre-polish thickness profile to a resulting or post-polish thickness profile. 
   When they are used to manufacture integrated circuits, polishing processes are usually performed in cycles. For example, given a particular polishing process, the actions of this particular process are repeated for each substrate in an assembly line of substrates. A cycle can include one or more polishing processes. For example, given the first and second polishing processes described above, the actions of these processes are repeated for each substrate in the assembly line of substrates. 
   A clearing profile of a substrate is typically not uniform. When planarization includes a first and a second polishing process, such as, for example, the above described polishing processes, there are three possible causes of the non-uniformity. These are variations in the pre-polish thickness profile of the substrate and variations in the removal profiles of the first and second polishing processes being used to clear a filler layer from the substrate. 
   A polishing process such as, for example, one that applies CMP, is complete when a substrate layer has been planarized to a desired flatness or thickness, when a filler layer has been cleared, or when a desired amount of material has been removed. The completion or end of the polishing process is sometimes referred to as the polishing end point. In-situ monitoring of the substrate can been performed, for example, with optical or capacitance sensors, in order to detect a polishing endpoint. Other proposed endpoint detection techniques have involved measurements of friction, motor current, slurry chemistry, acoustics and conductivity. One detection technique that has been considered is to induce an eddy current in the metal layer and measure the change in the eddy current as the metal layer is removed. 
   SUMMARY 
   The invention provides methods and apparatus, including computer program products, for performing feedback-controlled polishing processes. 
   In general, in one aspect, the invention provides a computer program product for generating feedback for chemical mechanical polishing. The product includes instructions operable to cause a processor to receive monitoring information during a current polishing cycle in which a first polishing process is performed on a substrate that includes a metal layer. The first polishing process clears the metal layer from the substrate during the current polishing cycle. The product includes instructions to calculate a representation of a clearing profile of the first polishing process. The calculation is based on the monitoring information received during the current polishing cycle. The product includes instructions to detect non-uniformity in the representation. The product includes instructions to generate, from the non-uniformity detected, feedback information for improving the uniformity of a clearing profile of the first polishing process for a subsequent polishing cycle. The product is tangibly stored on machine-readable medium. 
   In general, in another aspect, the inventions provides a method for generating feedback for chemical mechanical polishing. The method includes receiving monitoring information during a first polishing cycle in which a first polishing process is performed on a substrate that includes a metal layer. The monitoring information includes information for two or more sampling regions that correspond to different annular regions of the substrate. The first polishing process clears the first metal layer from the substrate during the first polishing cycle. The method includes calculating, for each sampling region, a clearing time of the first polishing process, the calculation being based on the monitoring information received. The method includes determining a target profile for a subsequent polishing cycle, the calculating being based on the clearing times calculated. 
   Possible advantages of implementations of the invention can include one or more of the following. Monitoring information, obtained during one polishing cycle and from one polishing station where a first polishing process is performed, can be used to adjust either the first polishing process in a subsequent polishing cycle or a second polishing process in the subsequent cycle. The first and second polishing processes are both performed for each polishing cycle. The second polishing process is one that is performed before the first polishing process is performed. The clearing times for different regions of a substrate can be derived from the monitoring information and, furthermore, can be used to adjust the polishing processes. The use of clearing times advantageously accounts for non-uniform removal profiles of the polishing processes, without requiring calculations or measurement of removal rates. Feedback as described in this specification can adjust for the slow drift in the polishing processes that might result from the aging of consumables such as, for example, polishing pad, conditioning disk, slurry, retaining rings on a head. The feedback can be controlled by software so that adjustment of a polishing process is head-specific, which can thus account for slight process differences between different polishing heads. Non-uniformity of a clearing profile is convergently reduced after several cycles. 
   Other features and advantages of the invention will become apparent from the following description, including the drawings and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flowchart illustrating a method for feedback controlled polishing. 
       FIG. 2  is a schematic exploded perspective view of a chemical mechanical polishing apparatus. 
       FIG. 3A  is a schematic side view, partially cross-sectional, of a chemical mechanical polishing station that includes an eddy current monitoring system and an optical monitoring system. 
       FIG. 3B  is a schematic top view of a platen from the polishing station of  FIG. 3A . 
       FIG. 3C  shows an example eddy current monitoring system. 
       FIG. 4  is a graph illustrating an amplitude trace from the optical monitoring system. 
       FIG. 5  is a flowchart illustrating a method for feedback-controlled polishing of a metal layer. 
       FIG. 6  is a flowchart illustrating an alternative method for feedback-controlled polishing of a metal layer. 
   

   DETAILED DESCRIPTION 
   A polishing cycle includes a first polishing process and a second polishing process that is performed after the first process is performed. The first polishing process is performed at a first polishing station. The second polishing process is performed at a second polishing station. In this implementation of a polishing cycle, monitoring information obtained from the second polishing station during a current polishing cycle is used as feedback to adjust either the first or second polishing process of a subsequent polishing cycle. After several cycles, the adjusted process can clear a filler layer simultaneously everywhere on a substrate. 
   Monitoring can be done using optical monitors, eddy current monitors, or a combined optical/eddy current monitor. One example of an optical monitor is the Full Scan ISRM, available from Applied Materials, of Santa Clara, Calif. The Full Scan ISRM detects changes in the intensity of light reflected off the substrate work surface. The intensity is nearly constant until the filler layer begins to clear. The Full Scan ISRM typically includes a detector that is translated relative to a substrate being monitored. Usually, the detector is translated so that different areas of the substrate can be monitored. The Full Scan ISRM provides a scan, which can be a graph showing reflected intensity as a function of time or position of the detector. The x-axis of the graph can be divide into scan regions, each of which can correspond to particular areas of a substrate being monitored. A particular polishing process can have a particular removal profile. In some cases, the removal profile is not uniform. The described scan regions can be defined so that the removal rates within each scan region are relatively constant. 
   The Full Scan ISRM can also provide one or more traces. A trace can be a graph showing reflected intensity, as detected by the detector at a particular position, as a function of time. A trace can, thus, show change in reflected intensity at a particular area of a substrate being monitored. 
   One example of a combined optical/eddy current monitor is the iScan ISRM, also available from Applied Materials. The iScan can provide information similar to those described with respect to the Full Scan ISRM. Monitoring information, for example, the scan and traces described, can be sent to a computer for processing and storage. The computer can use the monitoring information as feedback to change the first or second polishing processes in subsequent polishing cycles. Devices like the described monitoring systems are further described in commonly assigned U.S. Pat. No. 6,602,724, which is hereby incorporated by reference in its entirety. 
   Particular polishing processes can result in different post-polish thickness profiles. The post-polish thickness profile of a substrate subject to the second polishing process or, in cases where the second polishing process clears a filler layer, the clearing profile of the substrate, for example, can be determined by both the pre-polish thickness profile of the substrate and the removal profile of the second polishing process. 
   As discussed, the interval of time needed to clear a filler layer is the clearing time. The clearing time for each scan region can be derived from the monitoring information and used to adjust the polishing processes in the subsequent polishing cycles. Specifically, the clearing times can be used to construct either a target pre-polish thickness profile of a substrate incoming to the second station or a post-polish thickness profile of the substrate after being subject to the second polishing process at the second station. Non-uniformity in the clearing profile, i.e., the difference between or among clearing times, can be detected. This information can be used to adjust a polishing cycle. For example, if the clearing time for a particular scan region is less than clearing times of other scan regions, then this information can be used to increase the clearing time of the particular scan region in the next polishing cycle. Adjustment can include making the removal rate slower by, for example, twenty percent. Adjustment can also include making the pre-polish thickness of the incoming substrate greater by, for example, twenty percent at the area that corresponds to the particular region. 
   As shown in  FIG. 1 , a system performing method  100  monitors material removal at a second polishing station during a current polishing cycle (step  102 ). During each polishing cycle, a first and second polishing process are sequentially performed on a current substrate. The first polishing process is performed at a first polishing station and the second polishing process is performed at the second polishing station. The first polishing process can be, for example, a bulk removal process and the second polishing process can be, for example, a slow clearing process. Monitoring can be done by, for example, a Full Scan ISRM. 
   The system uses the monitoring information obtained during the second polishing process of the current polishing cycle to calculate clearing times of the second polishing process (step  104 ). The system can use the monitoring information, for example, traces obtained with the Full Scan ISRM, which can show intensity of reflected light over time for each scan region, to calculate the clearing time for each scan region. 
   The system detects non-uniformity between or among the clearing times (step  106 ). Detection can be done by comparing the deviation of each clearing time from an average clearing time. Various other statistical techniques can be applied to detect non-uniformity. 
   The system generates feedback information (step  108 ). The system can use the detected non-uniformity to generate feedback information. The system can, for example, calculate, for each scan region, a ratio of the average clearing time and the clearing time of the region. The ratio calculated is either less than 1, equal to 1, or greater than 1. Alternatively, the system can generate a target thickness profile, for use in the next polishing cycle, that is either a post-polish thickness profile that the first polishing process produces or a target clearing profile that the second polishing process produces. The system can calculate the target profile by calculating the target thickness for each region. Calculation, in this case, can include taking the above described ratio and multiplying it to the average of the thickness profile as it exists before the current substrate was subject to the second polishing process. The post-polish thickness profile can be used to adjust parameters of the first polishing process. The clearing profile can be used to adjust parameters of the second polishing process. 
   The system uses the feedback information to adjust, for a subsequent cycle, one of the first polishing process and the second polishing process (step  110 ). When the feedback information is the described ratio, adjustments depends on whether the ratio is less than 1, equal to 1, or greater than 1. If, for example, the ratio is greater than 1, then adjustment can be made to decrease the removal rate at this region during the next cycle. Alternatively, the target thickness profile or both the ratios and the target thickness profile can be similarly used as feed back information. In the case when the target thickness profile is used, adjustment depends on whether the target thickness for a region is less than, the same as, or greater than the calculated average thickness. As cycles occur, the clearing times converges and the typical non-uniformity is minimized. 
   If the removal profile for the second polishing process is available, then a ratio of the average removal rate and the removal rate for each region can be calculated and applied to calculate the target thickness for each region. The application of such a ration can increase the convergence rate. That is, the application of this ration of the average removal rate and the removal rate for each region can reduce the number of cycles needed to have the clearing times converge. 
   How removal rate or a removal profile is changed can be determined by a computer program product that models the polishing process being adjusted. The model can be based, for example, on empirical information. A product and model that can be used for feedback as described in this specification are further described in commonly assigned U.S. patent application Ser. No. 10/393,531, filed on Mar. 21, 2003, the entire specification is hereby incorporated by reference. 
     FIG. 2  shows a CMP apparatus  20 , in which one or more substrates  10  can be polished. A description of a similar polishing apparatus  20  can be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. Polishing apparatus  20  includes a series of polishing stations  22   a ,  22   b  and  22   c , and a transfer station  23 . Transfer station  23  transfers the substrates between the carrier heads and a loading apparatus. 
   Each polishing station includes a rotatable platen  24  on which is placed a polishing pad  30 . The first and second stations  22   a  and  22   b  can include a two-layer polishing pad with a hard durable outer surface or a fixed-abrasive pad with embedded abrasive particles. The final polishing station  22   c  can include a relatively soft pad or a two-layer pad. Each polishing station can also include a pad conditioner apparatus  28  to maintain the condition of the polishing pad so that it will effectively polish substrates. 
   As shown in  FIG. 3A , a two-layer polishing pad  30  typically has a backing layer  32  which abuts the surface of platen  24  and a covering layer  34  which is used to polish substrate  10 . Covering layer  34  is typically harder than backing layer  32 . However, some pads have only a covering layer and no backing layer. Covering layer  34  can be composed of foamed or cast polyurethane, possibly with fillers, e.g., hollow microspheres, and/or a grooved surface. Backing layer  32  can be composed of compressed felt fibers leached with urethane. A two-layer polishing pad, with the covering layer composed of IC-1000 and the backing layer composed of SUBA-4, is available from Rodel, Inc., of Newark, Del. (IC-1000 and SUBA-4 are product names of Rodel, Inc.). 
   During a polishing step, a slurry  38  ( FIG. 2 ) containing a liquid (e.g., deionized water for oxide polishing) and a pH adjuster (e.g., potassium hydroxide for oxide polishing) can be supplied to the surface of polishing pad  30  by a slurry supply port or combined slurry/rinse arm  39  ( FIG. 2 ). If polishing pad  30  is a standard pad, slurry  38  can also include abrasive particles (e.g., silicon dioxide for oxide polishing). 
   A rotatable multi-head carousel  60  supports four carrier heads  70 . (See  FIG. 2 .) The carousel is rotated by a central post  62  about a carousel axis  64  by a carousel motor assembly (not shown) to orbit the carrier head systems and the substrates attached thereto between polishing stations  22  and transfer station  23 . Three of the carrier head systems receive and hold substrates, and polish them by pressing them against the polishing pads. Meanwhile, one of the carrier head systems receives a substrate from and delivers a substrate to transfer station  23 . 
   Each carrier head  70  is connected by a carrier drive shaft  74  to a carrier head rotation motor  76  (shown by the removal of one quarter of cover  68 ) so that each carrier head can independently rotate about it own axis. In addition, each carrier head  70  independently laterally oscillates in a radial slot  72  formed in carousel support plate  66 . A description of a suitable carrier head  70  can be found in U.S. Pat. Nos. 6,422,927 and 6,450,868, the entire disclosures of which are incorporated by reference. In operation, the platen is rotated about its central axis  25 , and the carrier head is rotated about its central axis  71  and translated laterally across the surface of the polishing pad. Devices similar to the carrier head are described in the above mentioned U.S. Pat. No. 6,602,724. 
   Referring to  FIGS. 3A and 3B , a recess  26  is formed in platen  24 , and a transparent section  36  is formed in polishing pad  30  overlying recess  26 . Aperture  26  and transparent section  36  are positioned such that they pass beneath substrate  10  during a portion of the platen&#39;s rotation, regardless of the translational position of the carrier head. Assuming that polishing pad  32  is a two-layer pad, thin pad section  36  can be constructed by removing a portion of backing layer  32  and inserting a transparent plug  36  into the cover layer  34 . The plug  36  can be a relatively pure polymer or polyurethane, e.g., formed without fillers. In general, the material of transparent section  36  should be non-magnetic and non-conductive. 
   Referring to  FIGS. 2 ,  3 A, and  3 C, the first polishing station  22   a  ( FIG. 2 ) includes an in-situ eddy current monitoring system  40  ( FIG. 3C ) and an optical monitoring system  140  ( FIG. 3A ). The eddy current monitoring system  40  and optical monitoring system  140  can function as a polishing process control and endpoint detection system. The second polishing station  22   b  and the final polishing station  22   c  can both include just an optical monitoring system, although either may additionally include an eddy current monitoring system. 
   The optical monitoring system  140 , which can function as a reflectometer or interferometer, can be secured to platen  24  in recess  26  adjacent the eddy current monitoring system  40 . Thus, the optical monitoring system  140  can measure the reflectivity of substantially the same location on the substrate as is being monitored by the eddy current monitoring system  40 . Specifically, the optical monitoring system  140  can be positioned to measure a portion of the substrate at the same radial distance from the axis of rotation of the platen  24  as the eddy current monitoring system  40 . Thus, the optical monitoring system  140  can sweep across the substrate in the same path as the eddy current monitoring system  40 . 
   The optical monitoring system  140  includes a light source  144  and a detector  146 . The light source generates a light beam  142  which propagates through transparent window section  36  and slurry to impinge upon the exposed surface of the substrate  10 . For example, the light source  144  may be a laser and the light beam  142  may be a collimated laser beam. The light laser beam  142  can be projected from the laser  144  at an angle a from an axis normal to the surface of the substrate  10 . In addition, if the hole  26  and the window  36  are elongated, a beam expander (not illustrated) may be positioned in the path of the light beam to expand the light beam along the elongated axis of the window. In general, the optical monitoring system functions as described in U.S. Pat. Nos. 6,159,073 and 6,280,289, the entire disclosures of which are incorporated herein by references. 
     FIG. 4  shows an example of a trace  250  generated by an optical monitoring system. The overall shape of intensity trace  250  may be explained as follows. Initially, the metal layer has some initial topography because of the topology of the underlying patterned layer. Due to this topography, the light beam scatters when it impinges the metal layer. As the polishing operation progresses in section  252  of the trace, the metal layer becomes more planar and the reflectivity of the polished metal layer increases. As the bulk of the metal layer is removed in section  254  of the trace, the intensity remains relatively stable. Once the oxide layer begins to be exposed in the trace, the overall signal strength drops quickly in section  256  of the trace. Once the oxide layer is entire exposed in the trace, the intensity stabilizes again in section  258  of the trace, although it may undergo small oscillations due to interferometric effects as the oxide layer is removed. 
   Returning to  FIGS. 3A and 3B , the CMP apparatus  20  can also include a position sensor  80 , such as an optical interrupter, to sense when core  42  and light source  44  are beneath substrate  10 . For example, the optical interrupter could be mounted at a fixed point opposite carrier head  70 . A flag  82  is attached to the periphery of the platen. The point of attachment and length of flag  82  is selected so that it interrupts the optical signal of sensor  80  while transparent section  36  sweeps beneath substrate  10 . Alternately, the CMP apparatus can include an encoder to determine the angular position of platen. 
   A general purpose programmable digital computer  90  receives the intensity signals and phase shift signals from the eddy current sensing system  40 , and the intensity signals from the optical monitoring system  140 . Since the monitoring systems sweep beneath the substrate with each rotation of the platen, information on the metal layer thickness and exposure of the underlying layer is accumulated in-situ and on a continuous real-time basis (once per platen rotation). The computer  90  can be programmed to sample measurements from the monitoring system when the substrate generally overlies the transparent section  36  (as determined by the position sensor). As polishing progresses, the reflectivity or thickness of the metal layer changes, and the sampled signals vary with time. As discussed, the time varying sampled signals may be referred to as traces. The measurements from the monitoring systems can be displayed on an output device  92  during polishing to permit the operator of the device to visually monitor the progress of the polishing operation. The computer  90  can use traces to control the polishing process and determine the end-point of the metal layer polishing operation. The computer  90  can also process the traces, e.g., calculate clearing times as described above, and use the information obtained from processing the traces as feedback to change the polishing processes. 
   In operation, CMP apparatus  20  uses eddy current monitoring system  40  and optical monitoring system  140  to determine when the bulk of the filler layer has been removed and to determine when the underlying stop layer has been substantially exposed. The computer  90  applies process control and endpoint detection logic to the sampled signals to determine when to change process parameter and to detect the polishing endpoint. Possible process control and endpoint criteria for the detector logic include local minima or maxima, changes in slope, threshold values in amplitude or slope, or combinations thereof. The CMP apparatus  20  can also use the monitoring systems, as described above, to provide feedback control to adjust parameters at each station. Pressure, for example, of the described chambers in the carrier head can be adjusted. The rotation speed of a carrier head, for example, can also be adjusted. 
   In addition, the computer  90  can be programmed to divide the measurements from both the eddy current monitoring system  40  and the optical monitoring system  140  from each sweep beneath the substrate into a plurality of sampling zones  96 , to calculate the radial position of each sampling zone, to sort the amplitude measurements into radial ranges, to determine minimum, maximum and average measurements for each sampling zone, and to use multiple radial ranges to determine the polishing endpoint, as discussed in U.S. Pat. No. 6,399,501, the entirety of which is incorporated herein by reference. 
   Computer  90  may also be connected to the pressure mechanisms that control the pressure applied by carrier head  70 , to carrier head rotation motor  76  to control the carrier head rotation rate, to the platen rotation motor (not shown) to control the platen rotation rate, or to slurry distribution system  39  to control the slurry composition supplied to the polishing pad. Specifically, after sorting the measurements into radial ranges, information on the metal film thickness can be fed in real-time into a closed-loop controller to periodically or continuously modify the polishing pressure profile applied by a carrier head, as discussed in U.S. Pat. No. 6,776,692, the entirety of which is incorporated herein by reference. For example, the computer could determine that the endpoint criteria have been satisfied for the outer radial ranges but not for the inner radial ranges. This would indicate that the underlying layer has been exposed in an annular outer area but not in an inner area of the substrate. In this case, the computer could reduce the diameter of the area in which pressure is applied so that pressure is applied only to the inner area of the substrate, thereby reducing dishing and erosion on the outer area of the substrate. 
   When the computer  90  is programmed to generate feedback as described in  FIG. 1 , the programming can be implemented as a computer program product that interacts with programs that controls the CMP apparatus  20 . The computer  90  can include a computer program product that takes as input the feedback described above in reference to  FIG. 1 , determines which of the described parameters of the CMP apparatus  20  to adjust, and outputs control signals for adjusting the CMP apparatus  20 . Product similar to those described are described in the above referenced U.S. patent application Ser. No. 10/393,531. 
   A method of polishing a metal layer, such as a copper layer, is shown in flowchart form in  FIG. 5 . A current substrate is polished at the first polishing station  22   a  to remove the bulk of the metal layer (step  502 ). The polishing process is monitored by the eddy current monitoring system  40  ( FIG. 3A ). When a predetermined thickness, e.g., 2000 Angstroms, of the copper layer remains over an underlying barrier layer, the polishing process is halted and the current substrate is transferred to the second polishing station  22   b  (step  504 ). This first polishing endpoint can be triggered when the phase shift signal exceeds an experimentally determined threshold value. Example polishing parameters for the first polishing station include a platen rotation rate of 93 rpm, a carrier head pressure of about 3 psi, and an IC-1010 polishing pad. As polishing progresses at the first polishing station, the radial thickness information from the eddy current monitoring system  40  can be fed into a closed-loop feedback system to control the pressure and/or the loading area of the carrier head  200  on the substrate. The pressure of the retaining ring on the polishing pad may also be adjusted to adjust the polishing rate. This permits the carrier head to compensate for the non-uniformity in the polishing rate or for non-uniformity in the thickness of the metal layer of the incoming substrate. As a result, after polishing at the first polishing station, most of the metal layer has been removed and the surface of the metal layer remaining on the substrate is substantially planarized. 
   At the second polishing station  22   b , the current substrate is polished at a lower polishing rate than at the first polishing station. For example, the polishing rate is reduced by about a factor of 2 to 4, i.e., by about 50% to 75%. To reduce the polishing rate, the carrier head pressure can be reduced, the carrier head rotation rate can be reduced, the composition of the slurry can be changed to introduce a slower polishing slurry, and/or the platen rotation rate could be reduced. For example, the pressure on the substrate from the carrier head may be reduced by about 33% to 50%, and the platen rotation rate and carrier head rotation rate may both be reduced by about 50%. Example polishing parameters for the second polishing station  22   b  include a platen rotation rate of 43 rpm, a carrier head pressure of about 2 psi, and an IC-1010 polishing pad. 
   Optionally, when the polishing begins at the second polishing station, the current substrate may be briefly polished, e.g., for about 10 seconds, at a somewhat higher pressure, e.g., 3 psi, and rotation rate, e.g., 93 rpm (step  506 ). This initial polishing, which can be termed an “initiation” step, may be needed to remove native oxides formed on the metal layer or to compensate for ramp-up of the platen rotation rate and carrier head pressure so as to maintain the expected throughput. 
   The polishing process is monitored at the second polishing station  22   b  by the optical monitoring system  140  ( FIG. 3A ). Polishing proceeds at the second polishing station  22   b  until the metal layer is removed and the underlying barrier layer is exposed (step  508 ). Of course, small portions of the metal layer can remain on the substrate, but the metal layer is substantially entirely removed. The optical monitoring system is useful for determining this endpoint, since it can detect the change in reflectivity as the barrier layer is exposed. Specifically, the endpoint for the second polishing station can be triggered when the amplitude or slope of the optical monitoring signal falls below an experimentally determined threshold value across all the radial ranges monitored by the computer. This indicates that the barrier metal layer has been removed across substantially all of the substrate. Of course, as polishing progresses at the second polishing station  22   b , the reflectivity information from the optical monitoring system  140  can be fed into a closed-loop feedback system to control the pressure and/or the loading area of the carrier head  200  on the substrate to prevent the regions of the barrier layer that are exposed earliest from becoming overpolished. 
   The reflectivity information obtained from the optical monitoring system  140  can also be used, as described in  FIG. 1 , to provide feedback, for a next polishing cycle when a next substrate is being polished, to change the polishing process at either the first polishing station  22   a  or the second polishing station  22   b  (step  510 ). The reflectivity information, e.g., the described scan and traces, can be used to calculate clearing times in each of the sampling zones  96  ( FIG. 3B ). The computer  90  can calculate the clearing times of each sampling zone. The sampling zones can correspond to different annular regions of the current substrate. Calculations include measuring the interval between the time when polishing starts and when and end point is reached as determine by, for example, when the intensity trace for the sampling zone drops (as shown, e.g., in  FIG. 4 , when the trace drops from intensity level  254  to intensity level  258 ). Calculations can also include calculating an average clearing time and clearing time ratios as described in reference to  FIG. 1 . 
   By reducing the polishing rate before the barrier layer is exposed, dishing and erosion effects can be reduced. In addition, the relative reaction time of the polishing machine is improved, enabling the polishing machine to halt polishing and transfer to the third polishing station with less material removed after the final endpoint criterion is detected. Moreover, more intensity measurements can be collected near the expected polishing end time, thereby potentially improving the accuracy of the polishing endpoint calculation. However, by maintaining a high polishing rate throughout most of the polishing operation at the first polishing station, high throughput is achieved. Preferably, at least 75%, e.g., 80–90%, of the bulk polishing of the metal layer is completed before the carrier head pressure is reduced or other polishing parameters are changed. 
   Once the metal layer has been removed at the second polishing station  22   b , the substrate is transferred to the third polishing station  22   c  (step  512 ) for removal of the barrier layer. Example polishing parameters for the second polishing station include a platen rotation rate of 103 rpm, a carrier head pressure of about 3 psi, and an IC-1010 polishing pad. Optionally, the substrate may be briefly polished with an initiation step, e.g., for about 5 seconds, at a somewhat higher pressure, e.g., 3 psi, and platen rotation rate, e.g., 103 rpm (step  514 ). The polishing process is monitored at the third polishing station  22   c  by an optical monitoring system, and proceeds until the barrier layer is substantially removed and the underlying dielectric layer is substantially exposed (step  516 ). The same slurry solution may be used at the first and second polishing stations, whereas another slurry solution may be used at the third polishing station. 
   An alternative method of polishing a metal layer, such as a copper layer, is shown in flowchart form in  FIG. 6 . This method is similar to the method shown in  FIG. 5 . However, both the fast polishing step and the slow polishing step are performed at the first polishing station  22   a . Removal of the barrier layer is performed at the second polishing station  22   b , and a buffing step is performed at the final polishing station  22   c.    
   Various aspects of the invention, including the method steps described, can be implemented as a computer program product, i.e., a computer program tangibly embodied in an information carrier, e.g., in a machine-readable storage device, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, multiple computers, or a test system. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. 
   Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. 
   Information carriers suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in special purpose logic circuitry. 
   The invention has been described in terms of particular implementations. Other implementations are possible. For example, the steps of the invention can be performed in a different order and still achieve desirable results. 
   The feedback method described can be incorporated in a variety of polishing systems. The described method is useful not only for CMP process but any polishing process. Adjustment to a polishing process is not limited to changing the example parameters described. Rather, adjustment can include any combination of the following: changing a slurry rate, changing membrane pressure, changing platen radial speed, changing a combination of membrane pressure and platen radial speed, and changing the different pressures applied to the back of a substrate. 
   The eddy current and optical monitoring systems can be used in a variety of polishing systems. Either the polishing pad, or the carrier head, or both can move to provide relative motion between the polishing surface and the substrate. The polishing pad can be a circular (or some other shape) pad secured to the platen, a tape extending between supply and take-up rollers, or a continuous belt. The polishing pad can be affixed on a platen, incrementally advanced over a platen between polishing operations, or driven continuously over the platen during polishing. The pad can be secured to the platen during polishing, or there could be a fluid bearing between the platen and polishing pad during polishing. The polishing pad can be a standard (e.g., polyurethane with or without fillers) rough pad, a soft pad, or a fixed-abrasive pad. Rather than tuning when the substrate is absent, the drive frequency of the oscillator can be tuned to a resonant frequency with a polished or unpolished substrate present (with or without the carrier head), or to some other reference. 
   Although illustrated as positioned in the same hole, the optical monitoring system  140  could be positioned at a different location on the platen than the eddy current monitoring system  40 . For example, the optical monitoring system  140  and eddy current monitoring system  40  could be positioned on opposite sides of the platen, so that they alternately scan the substrate surface. 
   Various aspects of the invention, such as placement of the coil on a side of the polishing surface opposite the substrate or the measurement of a phase difference, still apply if the eddy current sensor uses a single coil. In a single coil system, both the oscillator and the sense capacitor (and other sensor circuitry) are connected to the same coil. 
   The feedback calculations described can be performed by one or more computer program products. These products can interface with the computer program products that control the polishing processes. The products for feedback and for control can reside on a same or on different computers. Alternatively, the feedback calculations described and the polishing process control can be performed by the same computer program product. 
   The substrate described can be any type of substrate. For example, the substrate can be a wafer. 
   The invention has been described in terms of particular embodiments. Other embodiments are within the scope of the following claims. For example, the steps of the invention can be performed in a different order and still achieve desirable results.