Abstract:
A system for processing a wafer is provided. The system includes a chemical mechanical planarization (CMP) tool. The CMP tool includes a wafer carrier defined within a housing. A carrier film is affixed to the bottom surface and supports a wafer. A sensor embedded in the wafer carrier. The sensor is configured to induce an eddy current in the wafer to determine a proximity and a thickness of the wafer. A cluster of sensors external to the CMP tool is included. The cluster of sensors is in communication with the sensor embedded in the wafer carrier and substantially eliminates a distance sensitivity. The cluster of sensors provides an initial thickness of the wafer to allow for a calibration to be performed on the sensor embedded in the wafer carrier. The calibration offsets variables causing inaccuracies in the determination of the thickness of the wafer during CMP operation. A method and an apparatus are also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is related to U.S. patent application Ser. No. 10/186,932, filed on the same day as the instant application and entitled “METHOD AND APPARATUS OF ARRAYED SENSORS FOR METROLOGICAL CONTROL.” The disclosure of this related application is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates generally to semiconductor fabrication and more specifically to in-line metrology for process control during wafer processing. 
   During semiconductor fabrication a there are multiple steps where an underlying substrate is subjected to the formation, modification and removal of various layers. The small feature sizes, tight surface planarity requirements, combined with the constant quest to increase throughput, makes it highly desirable to stop the process when the correct thickness has been achieved, i.e., when an endpoint has been obtained for the process step. 
   Eddy current sensors are used for displacement, proximity and film thickness measurements. The sensors rely on the induction of current in a sample by the fluctuating electromagnetic field of a test coil proximate to the object being measured. Fluctuating electromagnetic fields are created as a result of passing an alternating current through the coil. The fluctuating electromagnetic fields induce eddy currents which perturb the applied field and change the coils inductance. 
     FIG. 1  is a simplified schematic diagram of the principle upon which an eddy current sensor operates. An alternating current flows through coil  100  in close proximity to conducting object  102 . The electromagnetic field of the coil induces eddy currents  104  in conducting object  102 . The magnitude and the phase of the eddy currents in turn effect the loading on the coil. Thus, the impedance of the coil is impacted by the eddy currents. This impact is measured to sense the proximity of conducting object  102  as well as a thickness of the object. Distance  106  impacts the effect of eddy currents  104  on coil  100 , therefore, if object  1002  moves, the signal from the sensor monitoring the impact of eddy currents on coil  100  will also change. 
   Attempts to use eddy current sensors to measure thickness of a film has resulted in limited success. Since the signal from the eddy current sensor is sensitive to both the thickness of the film and distance of the substrate to the sensor, there are two unknowns that must be resolved.  FIG. 2  is a schematic diagram of a wafer carrier having an eddy current sensor for measuring the thickness of a wafer during a chemical mechanical planarization process (CMP). Wafer carrier  108  includes eddy current sensor  110 . During a CMP operation, wafer  114  supported by carrier film  112  of carrier  108  is pressed against pad  116  to planarize a surface of the wafer. Pad  116  is supported by stainless steel backing  118 . 
   One shortcoming of the configuration of  FIG. 2  comes from the variability of the carrier film, which, being only 0.020″ thick can undergo variations up to 0.006″ from sample to sample. Process conditions, in particular, film compression due to wafer load, affect the sensor-metal layer distance. Thus, the carrier film and variable process conditions cause a substantial variability in the distance between the wafer and the sensor. Accordingly, it becomes extremely difficult to calibrate for all the variables that effect the distance, which in turn impacts the thickness measurement of the sensor. Another shortcoming of this configuration is caused by the presence of another conducting material separate from the conducting material being measured and is commonly referred to as a third body effect. If the thickness of the conductive layer is less than the so-called skin depth, the electromagnetic field from the coil will not be completely absorbed and will partially pass through to stainless steel backing  118  of pad  116  of  FIG. 2 . The electromagnetic field will induce additional eddy currents within the stainless steel belt, thereby contributing to the total signal from the eddy current sensor. Furthermore, it should be appreciated that the pad wears or erodes over time, causing variation in the distance between the stainless steel backing and the eddy current sensor, which influences the appropriated contribution to the total eddy current sensor signal. Thus, a wear factor has to be considered as the wafers are continuously being processed. Consequently, due to the variability injected into the thickness measurement, the amount of error is unacceptably high and unpredictable. 
   Additionally, in line metrology used for semiconductor processing properties is incorporated in most semiconductor manufacturing equipment and particularly with CMP tools. However, obtaining the required data takes time, which in some cases is as long as the wafer processing time for a particular fabrication step. Furthermore, the current approaches add stations to the processing systems resulting in an increased footprint for the systems as well as an extra travel pathway for the wafer. 
   In view of the foregoing, there is a need to provide a method and system to capture incoming wafer characteristics, such as thickness of the wafer and films deposited on the wafer, on the fly so that the characteristics can be used to process the wafer more accurately and more efficiently. 
   SUMMARY OF THE INVENTION 
   Broadly speaking, the present invention fills these needs by providing a sensor cluster or sensor array incorporated into the front end of a processing tool to generate a thickness profile of a semiconductor substrate prior to its processing by the tool. The sensor array or sensor cluster is incorporated into the system by combining it with existing motion actuating devices so as not to alter the footprint of the tool or the flow path of the wafer. In the most of the cases the existing motion actuating devices, like robotics, center aligners, notch detectors etc., are not designed to keep the wafer in the same plane during the process cycle. Therefore, a distance insensitive solution is provided herein. 
   In accordance with one embodiment of the present invention, a wafer aligner is provided. The wafer aligner includes a rotating support configured to support a semiconductor substrate. A cluster of sensors configured to detect a signal indicating a metal film thickness of the semiconductor substrate is included. The cluster of sensors includes sensors located at opposing sides of the semiconductor substrate. The cluster of sensors is configured to be substantially insensitive to a distance of the semiconductor substrate to the sensors located above and below the semiconductor substrate. 
   In another embodiment, a device for providing in-line metrology capabilities as a semiconductor substrate is being transported is provided. The device includes a transport mechanism configured to support the semiconductor substrate. A sensor cluster configured to detect a signal indicating a metal film thickness of the semiconductor substrate while the semiconductor substrate is being transported is included. The sensor cluster generates a thickness profile of the semiconductor substrate. The thickness profile is stored electronically. The sensor cluster is further configured to be substantially insensitive to a change in distance between the semiconductor substrate and corresponding sensors of the sensor cluster. 
   In accordance with yet another embodiment of the present invention, a system for processing a semiconductor substrate is provided. The system includes a load port configured to hold a semiconductor substrate and a robot arm configured to transport the semiconductor substrate. An aligner configured to receive the semiconductor substrate from the robot arm is included. The aligner includes a sensor cluster configured to detect a signal indicating a metal film thickness of the semiconductor substrate to generate a thickness profile of the semiconductor substrate. The sensor cluster includes sensors located at opposing sides of the semiconductor substrate. The sensor cluster is configured to be substantially insensitive to a distance of the semiconductor substrate to the sensors located above and below the semiconductor substrate. A controller is in communication with the sensor cluster, where the controller is configured to receive the thickness profile is included. A process tool sensor in communication with the sensor cluster through the controller is included. The process tool sensor is configured to receive the thickness profile from the controller to calibrate the process tool sensor. A recipe for a processing operation performed by a process tool containing the process tool sensor is configured to be modified in response to the thickness profile being received by the process tool sensor. 
   In still yet another embodiment, a method for mapping a thickness profile of a semiconductor substrate to allow for customized processing operations based upon the thickness profile is provided. The method initiates with selecting a wafer. Then, the selected semiconductor substrate is transported to a processing tool. The transporting includes rotating the semiconductor substrate. Next, a thickness profile of the semiconductor substrate is detected while the semiconductor substrate is rotating. Then, the thickness profile of the semiconductor substrate is provided to a controller associated with a process tool. In response to the thickness profile to the controller, a recipe associated with the process tool is adjusted for the thickness profile of the semiconductor substrate. 
   In another embodiment, a method for determining an endpoint thickness for a chemical mechanical planarization (CMP) operation is provided. The method initiates with providing a first sensor external to the CMP module. Then, a thickness of a wafer is detected by the first sensor. Next, the thickness of the wafer is supplied to a second sensor in the CMP module. Then, the second sensor is calibrated according to the thickness of the wafer. Next, CMP operating parameters are adjusted based on a signal from the second sensor in order to optimize a CMP operation for the wafer. 
   It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the principles of the invention. 
       FIG. 1  is a simplified schematic diagram of the principle upon which an eddy current sensor operates. 
       FIG. 2  is a schematic diagram of a wafer carrier having an eddy current sensor for measuring the thickness of a wafer during a chemical mechanical planarization process (CMP). 
       FIG. 3  is a simplified schematic diagram of coupled sensors for measuring thickness of an incoming wafer in accordance with one embodiment of the invention. 
       FIG. 4  is a graph of the signal of coupled eddy current sensors configured as in  FIG. 3  in accordance with one embodiment of the invention. 
       FIG. 5  is a simplified schematic diagram of an alternative configuration of coupled sensors for measuring an incoming thickness of a wafer or film in accordance with one embodiment of the invention. 
       FIG. 6A  is a simplified schematic diagram of another alternative configuration of coupled sensors for measuring thickness of an incoming wafer in accordance with one embodiment of the invention. 
       FIG. 6B  is a graph illustrating the stability of an average signal when using an eddy current sensor cluster as configured to the side of the graph for detecting a film thickness in accordance with one embodiment of the invention. 
       FIG. 7A  is a simplified schematic diagram of an incoming thickness sensor coupled to a downstream CMP process thickness sensor in accordance with one embodiment of the invention. 
       FIG. 7B  is a simplified schematic diagram of an alternative embodiment of an incoming thickness sensor coupled to a downstream CMP process thickness sensor. 
       FIGS. 8A and 8B  are graphs illustrating the correlation between the signal from an eddy current sensor for a film thickness and a signal from a standard resistivity film thickness measurement device in accordance with one embodiment of the invention 
       FIG. 9  is a graph illustrating a calibration curve for an eddy current sensor for measuring a copper film thickness in accordance with one embodiment of the invention. 
       FIG. 10  is a graph illustrating two eddy current sensor output signals during a CMP operation of a copper thin film on a substrate in accordance with one embodiment of the invention. 
       FIG. 11A  is a graph of an Infrared (IR) sensor signal measuring the temperature of a polishing belt over time in accordance with one embodiment of the invention. 
       FIG. 11B  is a graph of an Infrared signal of the wafer temperature over time in accordance with one embodiment of the invention. 
       FIG. 12  is a schematic diagram of 30 second time sequences T 1 –T 9  of a CMP process illustrating the removal of a copper film as measured by an eddy current sensor in the wafer carrier in accordance with one embodiment of the invention. 
       FIG. 13  is a high level schematic of a CMP system in accordance with one embodiment of the invention. 
       FIG. 14  is a cross sectional view of a simplified schematic of a wafer aligner with an integrated sensor cluster in accordance with one embodiment of the invention. 
       FIG. 15  is a more detailed side view of the sensor array containing a plurality of sensor clusters in accordance with one embodiment of the invention. 
       FIG. 16  is a top view of the sensor array containing a plurality of clustered sensors of  FIG. 15  in accordance with one embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Several exemplary embodiments of the invention will now be described in detail with reference to the accompanying drawings.  FIGS. 1 and 2  are discussed above in the “Background of the Invention” section. 
   Eddy current sensors (ECS) allow for measuring a metal film thickness of a moving wafer. It has been determined that ECS are capable of providing a fast enough response for a wafer moving under typical loading robotics velocity. Therefore, it is possible to perform the thickness measurements “on the fly” without impacting process throughput. Moreover, the movement of the wafer can be taken advantage of to produce a thickness profile from a limited number of sensors in a cluster configuration. For example, wafer aligners provide movement in a rotational direction and a linear radial direction. Accordingly, a cluster of sensors can capture a thickness profile of a wafer while the wafer is undergoing common automated wafer handling schemes. In one embodiment, a thickness profile can be generated for each wafer so that a recipe of a downstream process can be optimized for the thickness profile. 
   The Figures below initially provide configurations for an incoming sensor or sensor cluster that determines the thickness of a wafer under non-process conditions, i.e., non-disturbing conditions. This thickness can then be stored and/or communicated to downstream fabrication processes dealing with thin metal films. That is, the incoming thickness can be supplied to a similar thickness measuring device for the downstream fabrication process. The sensor associated with the downstream fabrication process can then be calibrated with the incoming thickness to make the unknowns or variables resulting from the processing conditions irrelevant. It should be appreciated that a cluster of sensors refers to the combination of two or three sensors to define a new metrological property. In one embodiment, the sensors are joined into clusters to remove sensitivity to distance. Sample results of data gathered with the sensor configurations are also provided herein. An array of sensors as used herein includes two or more clusters of sensors joined to cover, i.e., map, a larger area of a wafer. Additionally, three sensors located on the same side of a wafer can be considered an array. 
   When integrating the clustered sensor with a pre-existing station the thickness of the wafer can be mapped and stored for a downstream processing operation. In addition, when the wafers are removed from the processing chamber the thickness of the processed wafer can also be scanned to provide feedback as to the results of the processing operation. Accordingly, adjustments can be made to the recipe of the operation based on the feedback. Of course, the post processing results can be provided for further processing operations also. 
     FIG. 3  is a simplified schematic diagram of coupled sensors for measuring thickness of an incoming wafer in accordance with one embodiment of the invention. Top sensor  130  and bottom sensor  132  are configured to provide a signal indicating a thickness of wafer  138 . In one embodiment, sensors  130  and  132  are eddy current sensors. Wafer  138  includes substrate  142  and metal layer  140 . Axis  134  of top sensor is offset from axis  136  of bottom sensor  132 . One skilled in the art will appreciate that by offsetting top sensor  130  and bottom sensor  132 , where both sensors are eddy current sensors, the electromagnetic field produced by sensors  130  and  132  will not interfere, i.e., suppress each other. Where the frequencies are the same and axis  134  is aligned with axis  136  then suppression of the signals can occur in certain situations, however, as will be explained below adjustments can be made to avoid suppression of the signals. Controller  144  is in communication with sensors  132  and  134 . In one embodiment, controller  144  is configured to receive signals from sensors  132  and  134  and determine a thickness of wafer  138 . It should be appreciated that controller  144  averages the signals from sensor  130  and  132  to arrive at a signal indicating a thickness of the wafer. Furthermore, by providing top sensor  130  and bottom sensor  132  a change in distance  146  between the top sensor and a top surface of wafer  138  or a change in distance  148  between the bottom sensor and a bottom surface of wafer  138  are cancelled out. That is, a change in either distance when both the sensors are stationary is offset by the averaging of the readings so that the signal is now a function of thickness rather than thickness and proximity. In another embodiment, controller  144  communicates the calculated thickness to a downstream tool, such as a CMP tool, so that the downstream process can optimize the process settings, such as pressure of the down force applied and belt speed, based upon the incoming thickness of the wafer. 
     FIG. 4  is a graph of the signal from coupled eddy current sensors configured as in  FIG. 3  in accordance with one embodiment of the invention. Graph  150  plots an eddy current sensor output in volts versus time. Line  152  represents the signal from a sensor located below a wafer such as bottom sensor  132  of  FIG. 3 . Line  154  represents the signal from a sensor located above the wafer such as top sensor  130  of  FIG. 3 . Bold line  156  represents the average of lines  152  and  154 . It will be apparent to one skilled in the art that line  156  provides a substantially constant signal. That is, by combining a signal from the top sensor with a signal from a bottom sensor, the dependence of the signal on the distance of the sensor is eliminated. For example, as wafer  138  of  FIG. 3  moves closer to top sensor  130 , the signal increases. Region  158  exemplifies the movement of the wafer closer to the top sensor. Thus, the signal intensity from the top sensor increases accordingly as illustrated by line  152  in region  158 . At the same time, the wafer is moving away from the bottom sensor. Therefore, the signal intensity from the bottom sensor decreases similar to the increase of the signal intensity from the top sensor, as illustrated by line  154  in region  158 . Consequently, an average of the top and bottom signal intensity stays constant. Since the signal intensity is linearly related to the distance of the sensor to the object, a change in signal intensity caused by movement of the object toward a first stationary sensor is offset by an opposite change in intensity caused by movement of the object away from a second stationary sensor. In turn, the sensitivity of the signal intensity to distance is eliminated. 
   Thus, by clustering a sensor or clusters of sensors with a sensor operating under working conditions, such as a sensor embedded in a wafer carrier of a CMP tool, the sensor operating under working conditions can be calibrated to more accurately provide information regarding the removal rate and process endpoint. That is, an accurate measurement of an incoming film thickness or wafer thickness is provided so that inaccuracies caused under processing conditions can be compensated through a calibration setting. Furthermore, the sensor cluster can be utilized as a run-to-run process control where a wafer has a thickness profile that is mapped by a first sensor or cluster of sensors and the profile is downloaded into a controller of the process tool to customize the process, such as a CMP process, to remove the correct amount of film thickness. 
     FIG. 5  is a simplified schematic diagram of an alternative configuration of coupled sensors for measuring an incoming thickness of a wafer or film in accordance with one embodiment of the invention. Here, top sensor  130  and bottom sensor  132  share the same vertical axis  160 . In order to eliminate any interference or suppression of the signals between the top and bottom sensors a different frequency can be applied for each of the respective sensors. Additionally, a phase shift can be applied so that the two sensors are out of phase. That is, one sensor overturns a wave of the signal by 180 degrees to eliminate suppression of the signal. Since distance is eliminated as a variable here as described above, the signal intensity is a function of thickness. Stated as a mathematical equation: S=k(THK), where S is the signal intensity, k is the sensitivity coefficient and THK is the thickness. Where the signal intensity and the sensitivity coefficient are known for the above equation, the thickness can be determined through a calibration curve, in one embodiment. This determined thickness can be supplied to a downstream process tool dealing with thin metal films in a semiconductor fabrication process, such as a CMP tool as discussed with reference to  FIGS. 8 and 9 . 
     FIG. 6A  is a simplified schematic diagram of another alternative configuration of coupled sensors for measuring thickness of an incoming wafer in accordance with one embodiment of the invention. In this embodiment, the sensor cluster is configured to determine the thickness of wafer  138  along axis  162  of top sensor  130 . Bottom sensors  132   a  and  132   b  are positioned such that a distance from each sensor to axis  162  is the same. Thus, by averaging the signals from sensors  132   a  and  132   b , the signal, and consequently the thickness, along axis  162  is determined. Here, interference or suppression of the signals between the top and bottom sensors is not a concern as bottom sensors  132   a  and  132   b  are offset from an axis of top sensor  130 . Regions  164 ,  166 ,  168 ,  170  and  172  denote movement of wafer  138  as the wafer passes through a space defined between top sensor  130  and bottom sensors  132   a  and  132   b . The significance of these regions are described in greater detail with reference to  FIG. 6B . 
     FIG. 6B  is a graph illustrating the stability of an average signal when using an eddy current sensor cluster as configured to the side of the graph for detecting a film thickness in accordance with one embodiment of the invention. In this configuration, eddy current sensor cluster includes a top sensor  130  and two bottom sensors  132   a  and  132   b . Wafer  138  travels between top sensor  130  and bottom sensors  132   a  and  132   b . Graph  171  plots the ECS readings in volts versus the time in milliseconds. Line  169  represents the reading from sensor  132   a , while line  173  represents the reading from sensor  132   b . Line  175  represents the reading from top sensor  130 . In one embodiment, the signals from sensors  132   a  and  132   b  are averaged. This result is then averaged with the signal from sensor  130 . The final averaged signal is represented by line  177 . Graph  171  illustrates the various positions of wafer  138  as it passes through the sensor cluster. For example, wafer  138  comes into the sensor cluster and is represented on the graph by region  164 . It should be appreciated that the ECS reading of 11 volts represents a starting point corresponding to a thickness of 0 as the wafer is not being measured here. The signals stabilize as the wafer edge passes through the sensor cluster. Then, the wafer continues along this midpoint travel path as represented in region  166 . As can be seen, the average reading represented by line  177  stays relatively steady. 
   Next, the wafer is moved up by 0.020 inches. While the signal intensity, i.e., a difference between the measured signal and the reference signal at 0 thickness, from top sensor  130  becomes stronger, the signal intensity from bottom sensors  132   a  and  132   b  becomes weaker. However, the average represented by line  177  remains relatively constant. The wafer is then moved down 0.020 inches from the midpoint path. Accordingly, the signal intensity from top sensor  130  becomes weaker, while the signal intensity from bottom sensors  132   a  and  132   b  becomes stronger. As above, the average of the top sensor signal and the bottom sensor signal remains relatively constant. Thus, as mentioned above, the sensor cluster supplies a stable signal where the distance of the wafer from the sensor is irrelevant as the average of a top and bottom signal offsets variability from the wafer moving or even variation of the signal from wafer warping. Wafer  138  then moves out of the sensor cluster as depicted in region  172  where the signals vary as the sensors see the edge of the wafer upon exit. It should be appreciated that regions  164 – 172  of  FIG. 6A  represent a similar pattern of movement as described with respect to  FIG. 6B . A graph of the movement of  FIG. 6A  would yield a substantially constant average signal as in  FIG. 6B . One skilled in the art will appreciate that the sensors can be configured with one bottom sensor and one top sensor, one bottom sensor and two bottom sensors, or any other suitable configuration to offset the movement of the wafer so that a stable reading is maintained. 
     FIG. 7A  is a simplified schematic diagram of an incoming thickness sensor coupled to a downstream CMP process thickness sensor in accordance with one embodiment of the invention. Here, incoming thickness sensor  130   a  determines a thickness of the wafer and/or thin film  140  over substrate  142  of wafer  138 . The signal indicating the determined thickness is communicated to controller  144 . In turn, controller  144  communicates the signal to sensor  130   b , which is embedded in wafer carrier  174  for a CMP process. In one embodiment sensors  130   a  and  130   b  are eddy current sensors. In another embodiment, sensors  130   a  and  130   b  are infrared sensors. It should be appreciated that by providing sensor  130   b  with the thickness of incoming wafer  138 , a calibration can be performed to substantially eliminate the sensitivity to the distance between the sensor and the wafer. The variability in the distance between sensor  130   b  and wafer  138  can be caused by carrier film  176  compressing during working conditions or just due to the inherent variation of the thickness of the carrier film, which can be as great as +/−3 millimeters. Additionally, the distance between the top of polishing pad  178  and stainless steel backing  180  impacts the signal from sensor  130   b . Again, the signal indicating the thickness of incoming wafer  138  can be used to calibrate sensor  130   b  to substantially eliminate variability caused by polishing pad tolerances and pad erosion that impact the distance between the top of polishing pad  178  and stainless steel backing  180 . 
     FIG. 7B  is a simplified schematic diagram of an alternative embodiment of an incoming thickness sensor coupled to a downstream CMP process thickness sensor. In  FIG. 7B , a sensor cluster consisting of top sensor  130   a  and bottom sensors  132   a  and  132   b , is in communication with controller  144 . Here, a suitable sensor cluster, such as the sensor cluster of  FIGS. 6A and 6B , is provided to determine the incoming thickness of wafer  138  or of thin film  140  of the wafer. One skilled in the art will appreciate that the sensor cluster with reference to  FIGS. 3 and 5  are suitable sensor clusters that can also be used to determine the thickness of wafer  138 . In one embodiment, controller  144  averages the signal from bottom sensors  132   a  and  132   b  in order to determine a thickness signal for the thickness of wafer  138  along an axis of top sensor  130   a  through the wafer. The averaged signal of the bottom sensors is then averaged with the signal from top sensor  130   a  to determine a thickness of wafer  138  or thin film  140 . This thickness is then communicated to embedded sensor  130   b . As mentioned above with reference to  FIG. 7A , an auto calibration can be performed for sensor  130   b  where the sensitivity to the distance between the sensor and wafer  138  and the sensitivity to the distance between the top of polishing pad  178  and stainless steel backing  180  are substantially eliminated. That is, the auto calibration can be performed in real-time to adjust the ECS readout for variation in sensor proximity due to pad-wear or other mechanical drift issues with the CMP carrier-to-plates mechanical displacements. 
   Sensor  130   b  of  FIG. 7B  is disposed over spacer  175 . Spacer  175  is aligned with the bottom surface of wafer carrier  174 . Spacer  175  is composed of any suitable material that is non-conductive. In one embodiment, spacer  175  is a polymer. In another embodiment, spacer  175  is between about 1 millimeter (mm) and about 1.5 mm thick. It should be appreciated that spacer  175  provides a window for sensor  130   b  to transmit and receive signals indicating the thickness and proximity of a wafer or film on a wafer. 
   While the embodiments of  FIGS. 7A and 7B  illustrate a sensor or sensor cluster for pre-CMP processing, a sensor or sensor cluster can also be located post CMP processing to provided information configured to improve run-to-run process control. While the incoming thickness allows for specific recipes to be downloaded to the process tool station to compensate for any incoming film thickness, the post CMP thickness allows for the correction of any detected process variation determined in the post-CMP thickness uniformity measurement. That is, the post-CMP thickness uniformity measurement is provided as feedback to sensor  130   b , in order for sensor  130   b  to further fine tune the calibration settings to obtain an accurate endpoint. In one embodiment, controller  144  provides the feedback to sensor  130   b  from a post-CMP sensor cluster. In another embodiment, the eddy current sensors are commonly available eddy current sensors, such as GP-A series analog displacement sensors available from SUNX Limited. In another embodiment, multiple sensors can be placed in the wafer carrier of  FIGS. 7A and 7B . The multiple sensors can be linked together to detect both wafer proximity and metal film-thickness. For example, a capacitance sensor can be included in the wafer carrier to determine a distance between the wafer and the ECS sensor. As the capacitance sensor is linked to the ECS sensor, the distance can be provided to the ECS sensor. 
     FIGS. 8A and 8B  are graphs illustrating the correlation between the signal from an eddy current sensor for a film thickness and a signal from a standard resistivity film thickness measurement device in accordance with one embodiment of the invention. Line  190  of  FIG. 8A  represents the signal from a typical resistance sensor approach. Line  192  represents the signal from an eddy current sensor in the presence of a third metal body, such as the stainless steel backing of a polishing pad or belt, a wafer carrier, an air bearing platen, etc, of a CMP tool, at different points from the center of the wafer. Line  194   a  represents the signal from an eddy current sensor without the presence of a third metal body. Thus, the eddy current sensor closely correlates with the signals from a standard four point approach of the resistance measurement. Additionally, the percent deviation is within +/−5%, as illustrated by triangles  196 , where each of the triangles corresponds to a difference between respective points on the lines above each of the triangles. 
     FIG. 8B  similarly illustrates eddy current sensor measurements correlated to signals from a typical resistance sensor approach. Here, line  190   b  represents the signal from a typical resistance sensor approach, while  194   b  represents the signal from an eddy current sensor without the presence of a third metal body. It should be appreciated that the wafer measured in  FIG. 8A  is different than the wafer measure in  FIG. 8B . Again the ECS signal  194   b  closely correlates with the RS75 signal. That is, the percent deviation between the signals is generally within +/−5% as illustrated by triangles  196 . One skilled in the art will appreciate that the points at the ends of the lines correspond to the edge of the film, i.e., edge of the wafer, and are not considered as relevant. 
     FIG. 9  is a graph illustrating a calibration curve for an eddy current sensor for measuring a copper film thickness in accordance with one embodiment of the invention. Line  198  represents a film thickness of copper and the associated ECS voltage reading for that thickness without the presence of a third metal body. Line  200  represents a film thickness of copper and the associated ECS voltage reading for that thickness in the presence of a third metal body. One skilled in the art will appreciate that the calibration curves can be applied to the sensors described above with reference to  FIGS. 3 ,  5 ,  6 A,  6 B,  7 A and  7 B. Additionally, calibration curves can be generated for any conductive film layer, as copper is described here for illustrative purposes only and not meant to be limiting. 
     FIG. 10  is a graph illustrating two eddy current sensor output signals during a CMP operation of a copper thin film on a substrate in accordance with one embodiment of the invention. Line  210  represents the ECS signal over time of the leading edge of the wafer undergoing the CMP operation. Line  212  represents the ECS signal over time of the trailing edge of the wafer undergoing the CMP operation. It should be appreciated that the region defined between line  214  and  216  removes the topography of the wafer, while the region after line  216  removes the copper overburden from the wafer. Point  210 - 1  represents the endpoint, i.e., clearing of the copper overburden, of the leading edge of the wafer. Point  212 - 1  represents the endpoint of the trailing edge of the wafer. One skilled in the art will appreciate that the information gathered from the ECS embedded in the wafer carrier will yield continuous data for determining a removal rate. Additionally, the removal rate variation between the leading edge and the trailing edge can be observed. Where the sensor is embedded in the wafer carrier, the sensor provides continuous real time data as to the thickness of the wafer or a film on the wafer being measured. That is, there is not a window where the sensor takes a snapshot once per revolution of a polishing belt or pad which provides discrete measurements. The embodiments described herein provide continuous measurement and thickness monitoring. 
     FIG. 11A  is a graph of an infrared (IR) sensor signal measuring the temperature of a polishing belt over time in accordance with one embodiment of the invention. One skilled in the art will appreciate that a silicon substrate is transparent to the infrared signal, therefore, the infrared signal can detect the temperature of the thin film of the wafer being planarized by the surface of the polishing surface. The lines of the graph of  FIG. 11A  represent the monitoring of the infrared signal at various locations of the belt, such as the center front and back of the belt relative to an operator. 
     FIG. 11B  is a graph of an infrared signal of the wafer temperature over time in accordance with one embodiment of the invention. Here, the wafer temperature is being monitored to monitor temperature variation during CMP process flow. For each of the embodiments of  FIGS. 11A and 11B , line  218  and  220 , respectively, intersect the response lines of the graphs at the endpoint of the associated processes. That is, the belt temperature and wafer temperature begin to decrease at the endpoint, which is at an inflection point of the curves. In one embodiment, the wafer temperature changes by about 20 degrees Celsius and the belt temperature changes by about 10 degrees Celsius during the processing. 
     FIG. 12  is a schematic diagram of 30 second time sequences T 1 –T 9  of a CMP process illustrating the removal of a copper film as measured by an eddy current sensor in the wafer carrier in accordance with one embodiment of the invention. Each of time sequences T 1 –T 9  show the ECS signal on the y axis in volts and the time interval in seconds on the x axis. Time sequence T 1  illustrates the initiation of the process, while time sequence T 2  illustrates the beginning of the removal of the copper film. That is, 479 Å of material is removed in time sequence T 2 . Time sequences T 3 –T 8  illustrate approximate 30 second periods and the associated amount of material removed during the time sequence. Time sequence T 9  illustrates the occurrence of the endpoint condition. 
     FIG. 13  is a high level schematic of a CMP system in accordance with one embodiment of the invention. As will be described below, the sensor cluster or array of sensor clusters can be incorporated into the front end of the CMP processing system to provided a thickness profile of the wafers as they are delivered to the processing module or received from the processing module. Load port modules  230   a ,  230   b , and  230   c  contain wafers to be processed and/or wafers that have been processed. Robot  240  is configured to access load port modules  230   a–c  to transport the wafers to another station, such as dry buffer  208  or aligner  234 . Robot  236  transports the wafer to and from the CMP processing module. Region  232  represents a spin, rinse and dry (SRD) module. It should be appreciated that a suitable sensor cluster, such as the sensor clusters discussed with reference to  FIGS. 7A and 7B , can be integrated within the front end system of  FIG. 13  in one embodiment. For example, the sensor cluster or even an array of sensor clusters can be integrated with aligner  234 . Accordingly, as aligner  234  rotates the wafer, the thickness profile can be detected by the sensor cluster. Thus, no additional space is required, i.e., the footprint of the system is not affected, and the pathway of the wafer is not altered to obtain the information. While the sensor cluster is discussed with respect to a CMP tool, it should be appreciated that the sensor clusters can be incorporated with any tools dealing with thin metal films and using wafer aligners. Furthermore, a single sensor cluster can be incorporated into the embodiments described herein, or alternatively, an array of sensor clusters can be incorporated to cover a larger area for a more complete thickness profile of a wafer. 
     FIG. 14  is a cross sectional view of a simplified schematic of a wafer aligner with an integrated sensor cluster in accordance with one embodiment of the invention. Wafer aligner  234  includes spindle  244  that is configured to rotate wafer  138 . In one embodiment, wafer  138  is held to spindle  244  through suction applied to a bottom surface of the wafer. Top sensor array  242   a  and bottom sensor array  242   b  each include a plurality of clustered sensors in each array. The clustered sensors of each array are configured to detect a signal indicating the thickness of a metal film on wafer  138 . One suitable sensor cluster is an eddy current sensor cluster as described above with reference to  FIGS. 6B and 7B . Top sensor array  242   a  is affixed to a top portion of aligner  234 , while bottom sensor array is affixed to a bottom portion of the aligner. It should be appreciated that while top sensor array  242   a  and bottom sensor array  242   b  are illustrated as being offset from a center of the wafer, however, the sensor array and the sensor clusters making up the sensor array, can be positioned in any number of suitable locations over the wafer. In one embodiment, the sensor clusters included in top sensor array  242   a  and bottom sensor array  242   b  are offset between top and bottom sensors of a cluster such as with reference to  FIGS. 3 ,  6 A and  6 B. In another embodiment, an axis of a top sensor of a sensor cluster in top array  242   a  is the same as an axis of a corresponding bottom sensor as discussed with reference to  FIG. 5 . 
   Each of the sensor clusters of the arrays of  FIG. 14  are in communication with controller  144 . As mentioned above, controller  144  is configured to average the signals from the sensor cluster to determine a thickness of the wafer. In one embodiment, a plurality of sensors are included in each top sensor array  242   a  and bottom sensor array  242   b . Thus, as wafer  138  is spinning for the alignment process, the sensor arrays can map the thickness profile of the wafer through the sensor clusters included in the sensor arrays. In one embodiment, controller  144  is a computer which controls the process operation which wafer  138  is being delivered to. Here, the computer can store the thickness profile and adjust a recipe for the processing operation when wafer  138  is undergoing the processing operation. That is, the sensor clusters of the sensor array, when placed in the front end of the processing system allows for the customization for each wafer as it is processed. For example, the process variables for a CMP recipe, such as pressure, belt speed, etc., can be adjusted for a particular wafer. In other words, each process operation can be customized for a particular wafer. One skilled in the art will appreciate that controller  144  can be in communication with another controller or computer controlling the process operation and provide the thickness profile to the other controller or computer. 
     FIG. 15  is a more detailed side view of the sensor array containing a plurality of sensor clusters in accordance with one embodiment of the invention. Here, top sensor array  242   a  includes top sensors  242   a - 1 ,  242   a - 2  and  242   a - 3 . Bottom sensor array  242   b  includes bottom sensors  242   b - 1 ,  242   b - 2 , and  242   b - 3 . In one embodiment each of the sensors are eddy current sensors. In the embodiment of  FIG. 15 , top eddy current sensors are offset from corresponding bottom eddy current sensors of a clustered set of sensors. However, as mentioned above, the top and bottom eddy current sensors can share a common axis by applying a phase shift between the top and the bottom sensors or using a different frequency. 
     FIG. 16  is a top view of the sensor array containing a plurality of clustered sensors of  FIG. 15  in accordance with one embodiment of the invention. Here it can be seen that top sensor array  242   a  includes top sensor  242   a - 1 – 242   a - 3 . Bottom sensor array  242   b  includes bottom sensors  242   b - 1 – 242   b - 3  and  242   c - 1 – 242   c - 3 . Thus, the sensor clusters of the sensor array are configured as described with reference to  FIGS. 6B and 7B . As wafer  138  rotates in the aligner in the direction of arrow  250 , the sensors generate a thickness profile of wafer  138  by detecting signals indicative of the thickness. Additionally, the sensors can monitor the thickness of the wafers as the wafer is moving linearly, in the direction of arrow  252 , as the wafer is brought into the aligner. It should be appreciated that the sensor array can have any number of sensor clusters in any suitable pattern to map the thickness profile. Additionally, the sensors do not have to be evenly placed over the wafer. In one embodiment, the non-symmetrical placement of the corresponding top and bottom sensors, i.e., sensor clusters may provide a more detailed map of the thickness profile of wafer  138 . 
   In summary, the present invention provides for the determination of an endpoint of a semiconductor fabrication process, such as a CMP process, through a clustered sensor configuration. The clustered sensors allow for the determination of the endpoint and associated removal or deposition rates by initially determining a thickness of a film on the wafer under non-process conditions. In one embodiment, the thickness of the film being measured is between about 0 microns and 2 microns. The determined thickness is provided to a second sensor associated with the process operation in order to calibrate the sensor so that variables from processing conditions that cause error in the thickness measurement are substantially eliminated. It should be appreciated that while the embodiments have been described in terms of a CMP process, the clustered sensors or arrays containing clustered sensors are not limited to a CMP process. For example, the sensors can be used within any semiconductor process that removes or deposits a layer or film on a substrate, such as etch, deposition and photoresist stripping processes. 
   In addition, the present invention allows for mapping a thickness profile of a wafer so that a process operation can use the information to optimize the process variables for the incoming wafer thickness. One skilled in the art will appreciate that the thickness of the wafer can also be monitored upon the completion of the processing operation. For example, after the illustrative CMP operations used above, the thickness of the wafer can be monitored as the wafer is brought back to a load module. The post-process monitoring can be used to provide feedback to further optimize the processing parameters. Alternatively, the post-process monitoring can be used for the next process the wafer is exposed to where the thickness profile of the wafer is helpful information. 
   Moreover, the eddy current sensor clusters were described as being integrated with aligners in one embodiment of the invention. It should be appreciated that any tool generating full or partial wafer rotation can be effective for generating a thickness profile in conjunction with the sensor clusters. In addition, the sensor clusters can be employed along the wafer path, such that the robot passes the wafer between the sensor cluster or array of sensor clusters. In turn, the thickness profile is detected from the linear radial motion of the wafer between the sensor clusters. Thus, the measurements are performed on the fly at typical robotics loading/unloading velocity. That is, system throughput is not impacted. 
   The invention has been described herein in terms of several exemplary embodiments. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. The embodiments and preferred features described above should be considered exemplary, with the invention being defined by the appended claims.