Abstract:
A computer program product for process control in chemical mechanical polishing is described. The product includes instructions to cause a processor to receive a measurement of an initial pre-polishing thickness of a layer of a substrate from a metrology station, determine a value for a parameter of an endpoint algorithm from the initial thickness of the substrate, receive a monitoring signal generated from monitoring in-situ polishing of the substrate, process the monitoring signal to detect a signal feature indicating a final or intermediate endpoint and send instructions to stop polishing when an endpoint criterion is detected using the endpoint algorithm with the determined value for the parameter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation and claims the benefit of priority under 35 U.S.C. Section 120 of U.S. application Ser. No. 10/330,687, filed Dec. 27, 2002 now U.S. Pat. No. 6,939,198, which claims priority to U.S. Provisional Application Ser. No. 60/344,411, filed on Dec. 28, 2001, the entire contents of which are incorporated herein by reference. 

   BACKGROUND 
   The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting a polishing endpoint during a chemical mechanical polishing operation. 
   An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, planarization is often needed to remove a filler layer until an underlying stop layer is exposed, or to create a layer with a defined thickness. 
   Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. Conventionally, the exposed surface of the substrate is placed against a rotating polishing pad, although a linear belt or other polishing surface can be used. The polishing pad may 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 (also, some polishing processes use a “nonabrasive” process). 
   One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness or whether an underlying layer has been exposed. If an excessive amount of material is removed (overpolishing), the substrate is rendered unusable. If, on the other hand, if an insufficient amount of material is removed (underpolishing), the substrate must be reloaded into the CMP apparatus for further processing. This is a time-consuming procedure that reduces the throughput of the CMP apparatus. 
   The polishing rate is sensitive to the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate, as well as the initial substrate topography. In addition, there can be variations in the thickness of the layers in the substrate layers. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time. 
   Various methods are used to monitor and control the CMP planarity and layer thickness during polishing. For example, the substrate thickness can be monitored in-situ by an optical sensor, such as an interferometer. Alternatively, exposure of an underlying layer and the associated change in reflectivity of the substrate can be detected a reflectometer. In addition, various method are used to measure the layer thickness before or after thickness (e.g., in an in-line metrology station). For example, a spectrometer, such as the NovaScan 210, manufactured by the Nova Corporation of Israel, can be used an in-line metrology device. 
   Although these techniques are satisfactory, there is still room for improvement in the accuracy of the determination of the polishing endpoint. 
   SUMMARY 
   In one aspect, the invention is directed to a computer-implemented method for process control in chemical mechanical polishing. The method includes measuring an initial pre-polishing thickness of a substrate at a metrology station, determining a parameter of an endpoint algorithm from the initial thickness of the substrate, polishing a plurality of substrates at a polishing station, and stopping polishing when an endpoint criterion is detected using the endpoint algorithm. 
   Implementations of the invention may include one or more of the following features. Each of the plurality of substrates may be monitored in-situ during polishing and generating a monitoring signal. The endpoint algorithm may include processing the monitoring signal to detect a signal feature indicating a final or intermediate endpoint. The parameter may represent an initial delay time before the processing step begins, an overpolish time after detection of the signal feature, a total polishing time, or a number of interference cycles. A polishing rate may be determined from the monitoring signal, and the parameter may be adjusted based on the polishing rate. The adjusting step may occur after a predetermined amount of an expected polish time has elapsed. The first substrate may be one of the plurality of substrates. 
   In another aspect, the invention is directed to a computer-implemented method for process control in chemical mechanical polishing. The method includes measuring an initial pre-polishing thickness of a monitor substrate at a metrology station, determining a delay parameter of an endpoint algorithm from the initial thickness of the monitor substrate, polishing a product substrate at a polishing station, waiting for the delay parameter once polishing of the product substrate has commenced, monitoring the product substrate in-situ during polishing after the delay step to detect the endpoint criterion, and stopping polishing when the endpoint criterion is detected. 
   In another aspect, the invention is directed to a computer-implemented method for process control in chemical mechanical polishing. The method includes polishing a monitor substrate, monitoring the monitor substrate in-situ during the polishing step to determine a lot representative removal rate, measuring an initial substrate thickness of a product substrate at a metrology station, calculating an endpoint time from the initial substrate thickness and the lot representative removal rate, polishing the product substrate, and stopping polishing of the substrate at the endpoint time. 
   In another aspect, the invention is directed to a computer-implemented method for process control in chemical mechanical polishing. The method includes measuring an initial substrate thickness of a product substrate at a metrology station, polishing the product substrate, monitoring the product substrate in-situ during polishing to determine a polishing rate, calculating an endpoint time from the initial thickness and the polishing rate, and stopping polishing of the substrate when the endpoint time is detected. 
   In another aspect, the invention is directed to a computer-implemented method for process control in chemical mechanical polishing. The method includes measuring an initial thickness of a product substrate at a metrology station, determining a target amount to remove from the initial substrate thickness and a selected target thickness, determining an endpoint criterion based on the amount to remove, monitoring the substrate with an in-situ monitoring system during polishing to detect an actual thickness removed from the substrate, and stopping polishing when the target amount to remove is detected. 
   In another aspect, the invention is directed to a computer-implemented method for process control in chemical mechanical polishing of a substrate that includes an upper layer with an exposed surface covering a lower layer. The method includes measuring a thickness of the lower layer of a substrate at a metrology station, determining an overpolish time from the thickness of the under-layer wafer, polishing the exposed surface of the substrate, monitoring the substrate with an in-situ monitoring system to detect an intermediate endpoint criterion, and stopping polishing of the substrate when the overpolish time has elapsed after the intermediate endpoint criterion has been detected. 
   In another aspect, the invention is directed to a computer-implemented method for process control in chemical mechanical polishing. The method includes polishing a monitor substrate, monitoring the monitor substrate with an in-situ monitoring system to detect an intermediate endpoint criterion representing an underpolished state of the substrate, stopping polishing of the monitor substrate when the intermediate endpoint is detected, measuring a post-polishing thickness of the monitor substrate at a metrology station, calculating an overpolish time from a difference between the post-polishing thickness and a desired thickness, polishing a product substrate, monitoring the product substrate with an in-situ monitoring system to detect the intermediate endpoint criterion, and stopping polishing of the substrate when the overpolish time has elapsed after the intermediate endpoint criterion has been detected. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic plan view of a substrate processing system. 
       FIG. 2  is a schematic cross-sectional view of a polishing station in the substrate processing system. 
       FIG. 3  is a flowchart illustrating a method according to the invention. 
       FIG. 4  is a flowchart illustrating another method according to the invention. 
       FIG. 5  is a flowchart illustrating another method according to the invention. 
       FIG. 6  is a flowchart illustrating another method according to the invention. 
       FIG. 7  is a flowchart illustrating another method according to the invention. 
       FIG. 8  is a flowchart illustrating another method according to the invention. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a substrate processing system  20  includes a chemical mechanical polisher  22 , a wet robot  24 , a cleaner  26 , a factory interface module  28 , and an in-line metrology station  30 . Substrates  10  are transported to the substrate processing system  20  in cassettes  12 , and are extracted from the cassettes  12  by a robot  18  in the factory interface module  28  for transport to the polisher  22  or the cleaner  26 . The operations of the substrate processing system  20  are coordinated by controller  32 , which may include one or more programmable digital computers executing distributed control software. A description of a similar substrate processing system may be found in U.S. patent application Ser. No. 09/543,858, the entire disclosure of which is incorporated herein by reference. 
   The polisher  22  can be a Mirra® chemical mechanical polisher manufactured by Applied Materials, Inc. of Santa Clara, Calif. A description of a polisher may be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. An exemplary polisher includes three polishing stations  25   a ,  25   b  and  25   c,  and a transfer station  27 . At each polishing station, a substrate undergoes a polishing process defined by multiple polishing parameters, such as the rotation rate of the polishing pad, the rotation rate of the carrier head, the slurry composition and slurry flow rate, the pressure applied to the substrate, and the like. These polishing parameters can be controlled by the controller  32 . 
   Referring to  FIG. 2 , a polishing station in the polisher includes a rotatable platen  34  that supports a polishing pad  35 , e.g., a standard or a fixed-abrasive or polishing pad. A support structure  38  supports a carrier heads  14  that holds a substrate  10  against the polishing pad  35 . An aperture  37  is formed in the platen  34 , and a transparent window  36  is formed in a portion of the polishing pad  35  overlying the aperture. An optical monitoring system  40 , which can function as a reflectometer or interferometer, is secured to the platen  27  beneath the aperture  37  and rotates with the platen  34 . The optical monitoring system  40  includes a light source  44 , such as a laser, and a detector  46 , such as a photodiode. The light source generates a light beam  42  which propagates through transparent window  36  to impinge upon the exposed surface of the substrate  10 . The intensity of a reflected beam  48  from the substrate  10  is measured by the detector  46 . 
   In operation, the polisher  22  uses the optical monitoring system  40  to determine the amount of material removed from or remaining on the surface of the substrate during polishing, to determine when the surface has become planarized, or to determine when an underlying layer has been exposed. The light source  44  and the detector  46  are coupled to the controller  32 . The controller  32  may be a general purpose digital computer programmed to activate the light source  44  when the substrate generally overlies the window, to store intensity measurements from the detector  46 , to display the intensity measurements on an output device  49 , to sort the intensity measurements into radial ranges, and to apply the endpoint detection logic to the measured signals to detect the polishing endpoint. 
   Although the controller for the polishing system, the in-line metrology device and the in-situ endpoint detection system are shown as the same computer, these systems can be implemented as a distributed system, or as a set of independent systems that communicate using a preselected protocol. Moreover, various steps in the methods set forth below need not be performed on the same controller system. For example, the measurement of the initial layer thickness could be forwarded from the controller for the in-line metrology device to the controller for the polishing system. The controller for the polishing system could calculate values for the variables used by the endpoint detection algorithm, and then forward the calculated values to the controller for the in-situ endpoint detection system. 
   Although an optical monitoring system has been illustrated, the present process can work with other in-situ monitoring systems, such as eddy current, capacitive or vibration sensing systems. 
   Returning to  FIG. 1 , the in-line metrology station  30  includes a metrology device  60 . The in-line metrology station is located at a position separate from the polishing stations  25   a – 25   c,  where the substrate can be placed before or after polishing. For example, the in-line metrology station  30  can be positioned off the factory interface module  28  and accessible by the interface robot  18  as shown, or it can be positioned between the polisher  22  and the factor interface module and be accessible to the wet robot  24 , or it can be positioned in the input stage  62  or the output stage of the cleaner  26 . In general, the metrology device  60  is capable of measuring the thickness of one or more layers in the substrate, including underlying layers. The metrology device can be a spectrometer, such as the NovaScan 210 manufactured by Nova Corporation of Israel. Other suitable spectrometers that can measure the thickness of layers on the substrate  10  include spectrometers manufactured by KLA-Tencor Corporation and Thermawave Corporation. Since the metrology device  60  operates in-line rather than in-situ, it may be capable of higher accuracy measurements than the optical monitoring system  40 . 
   The overall approach of this invention to obtaining an accurate polishing endpoint is to premeasure the initial thickness of the substrate, or a layer in the substrate, by the in-line metrology device  60  prior to the substrate being polished, and use this data to adjust the endpoint polishing routine. 
     FIG. 3  illustrates a method  100  according to one implementation of the invention. First, the initial thickness of a layer on the substrate  10  is measured by the in-line metrology device  30  (step  110 ). The measurement of the layer thickness can be aligned with the critical areas on the substrate  10 . The initial thickness is stored in a memory of the controller  32 . The controller  32  then calculates an initial dead time (IDT) based on the initial layer thickness (step  120 ). The initial dead time can be calculated by dividing the difference between the an ideal starting thickness for the endpoint algorithm and the measured initial thickness by an estimated polishing rate. 
   Next, the controller  32  modifies the endpoint algorithm using the initial dead time. Specifically, the controller adjusts the algorithm so that data collection using the in-situ endpoint detection system  40  begins after expiration of the initial dead time from the start of polishing of the substrate. During polishing, the substrate  10  is placed in contact with the polishing pad  35  and monitored in-situ by the monitoring system  40  (step  140 ). As noted however, data collection from the monitoring system does not being until after the initial dead time has expired. Once the endpoint detection system begins operating, the controller applies endpoint detection logic to the trace from the monitoring system to detect the endpoint of the polishing process (step  150 ). Once the endpoint criteria are detected, polishing halts (step  160 ). 
   The measurement of the substrate layer thickness and calculation of the IDT can be performed for just the first substrate in a cassette. Subsequently substrates from the same cassette can be polished using the same IDT and adjusted endpoint algorithm. 
   A potential advantage of this system is that it can prevents the endpoint detection system from triggering too early or too late. Specifically, when a layer having a large thickness, such as oxide layer having a thickness of more than 4000 Angstroms, is to be removed, the method prevents “order skipping”, i.e., mistakes by the endpoint detection system when the number of interference cycles is larger than expected. Since the endpoint detection algorithm does not begin until a point near where the expected number of interference cycles remain, “order skipping” can be avoided. 
     FIG. 4  illustrates a method  200  according to another implementation of the invention. In method  200 , the polishing endpoint for the product substrate  10  is determined based on the pre-determined removal rate for a monitor substrate and an initial thickness of the product substrate  10 . 
   Initially, the cassette  12  is delivered to the polisher  22  by the substrate processing system  20 . The cassette  12  includes a monitor substrate  15  in addition to the set of regular device substrates  10 . The monitor substrate  15  can be a blank oxide-coated wafer. The monitor substrate is polished (step  210 ) while being in-situ monitored by the system  40  (step  220 ). The controller  32  determines the polishing rate of the monitor substrate (step  222 ) based on the signals received from the monitoring system  40 . For each device substrate, the initial layer thickness is measured using the in-line metrology system  30  (step  224 ). The controller calculates the individual polishing time for each individual device substrate  10  (step  230 ), e.g., by dividing the measured thickness by the polishing rate of the monitor substrate. When the device substrate is being polished (step  240 ), the controller merely halts polishing (step  260 ) when the actual polishing time of the device substrate becomes equal to the expected polishing time previously determined by the controller  32  (step  250 ). 
     FIG. 5  illustrates a method  300  according to another implementation of the invention. In method  300 , both the in-situ removal rate and the in-line measurement of the initial thickness are used to determine the polishing time for the product substrate. 
   The initial thickness of a layer in a product substrate  10  is measured by the in-line metrology device  30  (step  310 ). The controller  32  determines an estimated polishing time for the product substrate  10  (step  320 ) from the initial thickness of the substrate and from a previously stored polishing rate, such as an empirically measured polishing rate or a polishing rate of a previously polished substrate as determine by the optical monitoring system  40 . The product substrate  10  is transferred to the polishing station  22  and polished while being monitored by the optical monitoring system  40  (step  330 ). After a preset amount of the estimated polishing time has occurred, e.g., about 80–90% of the estimated polishing time, the controller calculates the actual removal rate of the polisher  22  from the intensity measurements generated by the in-situ optical monitoring system  40 . Then the controller  32  re-calculates the polishing endpoint time based on the actual removal rate and the initial thickness of the product substrate (step  340 ). When the polishing time reaches recalculated endpoint time, the controller halts polishing of the product substrate. 
     FIG. 6  illustrates a method  400  according to another implementation of the invention. In method  400 , the initial thickness is used to calculate the number of interference peaks expected during polishing. The endpoint detection system counts this number of interference peaks before polishing is halted. 
   The initial thickness of a device substrate  10  is pre-measured by the in-line metrology device  30  (step  410 ). The controller  32  calculates an amount to be removed from the upper surface of the substrate based on the initial thickness and the target thickness of the layer on the substrate (step  420 ). The target thickness is a pre-determined value and is stored in the memory of the controller prior to the beginning of the polishing process. An expected number of peaks in the interference signal is then calculated from the amount to be removed (step  425 ). The product substrate is transferred to the polisher and polished while being in-situ monitored by the optical monitoring system  40  (step  440 ). The series of intensity values from the reflected beam can be stored in the controller, are analyzed, and the number of peaks in the sinusoidal signal is counted (step  430 ). When the desired number of peaks has been counted (step  440 ), polishing is halted. The number of peaks can be a fraction, in which case the endpoint detection system can continue polishing for a partial interference cycle, e.g., by monitoring the slope or other features of the interference signal, or by calculating the average duration of the previous interference cycles and polishing past an interference peak for a time equal to the fraction of the average duration. Alternatively, the desired thickness to be removed can be stored, and the controller can calculate the amount of thickness removed from the number of peaks in the intensity signal. 
   One problem in CMP endpoint detection is the influence of the refractive index variation and thickness of the underlying layers on the traces measured by the in-situ monitoring system  40 . Specifically, when the thickness or the refractive index of the underlying layers vary, the correlation between the signal from the monitoring system  40  and the thickness of the substrate becomes distorted. For example, a large variation in the thickness of an underlying nitride layer may result in a large post-CMP variation of the oxide layer thickness. Although the thickness of the nitride layer can be consistent within a given lot, there can be large lot-to lot variations. 
     FIG. 7  illustrates a method  500  according to another implementation of the invention. In method  500 , the thickness of an underlying layer is used to adjust the endpoint algorithm. 
   The in-line metrology device  30  is used to measure the thickness of the underlying nitride layer of the first substrate from the cassette (step  510 ). The thickness of the underlying layer is used by the controller to calculate an overpolish time (step  520 ), and this overpolish time is used to adjust the endpoint control timing (step  530 ). The substrate  10  is polished while being in-situ monitored by the monitoring system  40  (step  540 ). The polishing endpoint is detected as normal (step  550 ), but polishing is not halted immediately. Instead, the controller  32  causes the polisher to continue to polish the substrate for the previously calculated overpolish time (step  555 ). After the overpolish time has expired, polishing is terminated (step  560 ). 
   In effect, the overpolish time corrects for the distortion in the trace caused by variation in the underlying layer thickness that would cause the endpoint to be triggered too early. The relationship between the underlying layer thickness and the necessary overpolish time can be determined empirically or theoretically, and can be stored as a look-up-table or the like. 
   Still another problem in CMP polishing is measuring the thickness of a substrate that includes a thin layer, for example, an anti-reflective coating (ARC) layer covering the upper oxide layer. Typically, the thickness of the ARC is approximately about 150–300 Angstroms of TiN. Although the thickness of the ARC layer is usually fairly consistent within one cassette, the ARC layers can vary in thickness significantly from one cassette to another. Specifically, in some lots, cassette-to cassette thickness variations of the ARC layer can exceed 100%. The TiN reflectivity and, consequently, the phase of the ISRM reflected beam  46  vary significantly with the variations in the RCA layer thickness. This can cause large differences in the amplitude of the signal from the in-situ monitoring system  40 . On the other hand, a measurement of the ARC layer with the in-line metrology device  35  alone may not be possible because of the small thickness of the layer. 
     FIG. 8  illustrates a method  600  according to another implementation of the invention. In method  600 , an optical endpoint detection criteria is selected that ensures that the ARC layer will be removed but that resulting substrate will be somewhat underpolished (step  605 ). A first device  17  substrate from the cassette  12  is polished using this optical endpoint detection criteria (step  610 ). Since the ARC layer has been removed, the in-line metrology device  30  can be used, as described above, to measure the thickness of the layer remaining on the underpolished substrate  17  at the metrology station  30  (step  615 ). This thickness is used to calculate an overpolish time based on the post-CMP thickness of the underpolished substrate  17  and a desired target thickness (step  620 ). The target thickness is pre-determined data which is stored in the controller prior to the beginning of the polishing process. The overpolish time can be calculated from a polish rate measured during by the in-situ monitoring system during polishing and the desired thickness to be removed (calculated from the thickness remaining and the target thickness). The timing portion of the endpoint algorithm is adjusted to use the overpolish time (step  630 ). 
   Subsequent device substrates  10  (i.e., after the first substrate  17 ) are polished while being monitored by the optical monitoring system  40  (step  640 ), the endpoint detection algorithm is used with the polishing criteria previously calculated to provide an underpolished substrate (step  650 ), and after the optical endpoint criteria are detected, the polisher continues to polish for the overpolish time (step  660 ). 
   A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.