Abstract:
Methods and apparatus for providing metrology for chemical mechanical polishing. A chemical mechanical polishing system includes a first polishing station, a second polishing station, a transport device, and a first measuring station. The transport device is configured to hold a workpiece during polishing at the first and second polishing stations and to move the workpiece from the first polishing station to the second polishing station. The first measuring station is situated to measure a characteristic of the workpiece when the transport device is holding the workpiece and when the workpiece is not in contact with a polishing pad of any of the first polishing station and the second polishing station.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of prior U.S. provisional application 60/590,730, filed Jul. 22, 2004, which is incorporated by reference herein. 
    
    
     BACKGROUND 
     The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to metrology for process control, such as end point determination. 
     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. For certain applications, the filler layer is planarized until the non-planar surface is exposed. A conductive filler layer, for example, can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. 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. For other applications, such as oxide polishing, the filler layer is planarized until a predetermined thickness is left over the non-planar surface. In addition, planarization of the substrate surface is usually required for photolithography. 
     Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is typically 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. 
     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 when a desired amount of material has been removed. Overpolishing (removing too much) of a conductive layer or film leads to increased circuit resistance. On the other hand, underpolishing (removing too little) of a conductive layer leads to electrical shorting. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. 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. 
     One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. By way of example, the substrate can be transferred to a metrology station where the thickness of a substrate layer is measured with a profilometer or a resistivity measurement. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This reloading is a time-consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable. 
     More recently, in-situ monitoring of the substrate has been performed, for example, with optical or eddy current sensors, in order to detect the polishing endpoint. Other proposed endpoint detection techniques have involved measurements of friction, motor current, slurry chemistry, acoustics and conductivity. 
     SUMMARY 
     In one aspect, the invention is directed to a chemical mechanical polishing system that includes a first polishing station, a second polishing station, a transport device, and a first measuring station. The transport device is configured to hold a workpiece during polishing at the first and second polishing stations and to move the workpiece from the first polishing station to the second polishing station. The first measuring station is situated to measure a characteristic of the workpiece when the transport device is holding the workpiece and when the workpiece is not in contact with a polishing pad of any of the first polishing station and the second polishing station. 
     In the instant specification, the term workpiece refers generally to a piece being polished. A work piece can be any sort of substrates, for example, a product wafer that includes multiple units of memory, a test wafer, and a gating wafer. 
     In another aspect, the invention is directed to a chemical mechanical polishing method that includes polishing a workpiece with a first polishing pad at a first polishing station. The method includes transporting the workpiece from the first polishing station to a second polishing station that includes a second polishing pad, wherein transporting is done by a transport device that includes one or more carrier heads. The method includes measuring a characteristic of the workpiece when the workpiece is being held by the transport device and when the workpiece is not in contact with either the first or second polishing pads. 
     Possible advantages of implementations of the invention can include one or more of the following. Sufficient data can be collected to improve control of a CMP polishing process while reducing or minimizing adverse effects on throughput capacity. Measurements can be made simultaneously on more than one substrate in the CMP tool. Measurements can be made on a portion of the substrate that is substantially free of slurry and other polishing residue, and the signal to noise ratio of signals received by sensors is consequently improved. 
     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 schematic exploded perspective view of a chemical mechanical polishing apparatus. 
         FIGS. 2A and 2B  show paths along which a carousel of the apparatus can carry a substrate. 
         FIG. 3  shows an implementation of an inter-platen monitoring module. 
         FIGS. 4A–4C  show examples of paths over a substrate surface, along which measurements can be taken. 
         FIG. 5  one arrangement of inter-platen modules. 
         FIG. 6A  shows a second arrangement of inter-platen modules. 
         FIG. 6B  shows a method for processing a wafer in a CMP apparatus that has the second arrangement of inter-platen modules. 
         FIG. 7  shows an example of positions between stations where an inter-platen module can be placed. 
         FIGS. 8A–8D  show various implementations of a flushing system. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a CMP apparatus  20 , which can polish one or more substrates, for example, substrate  10 . 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 . The transfer station  23  transfers the substrates between a carrier head and a loading apparatus. 
     Each polishing station can include a rotatable platen  24  on which is placed a polishing pad  30 , which can be, for example, a two-layer polishing pad, a fixed-abrasive pad with embedded abrasive particles, or a relatively soft 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. 
     During a polishing step, a polishing liquid  38 , for example, an abrasive slurry or abrasive-free solution, can be supplied to the surface of the polishing pad  30  by a slurry supply port or combined slurry/rinse arm  39 . Different slurry solutions may be used at different polishing stations. 
     A rotatable multi-head carousel  60  supports four carrier heads  70   a – 70   d . The carousel  60  is rotated by a central post  62  about a carousel axis  64  by a carousel motor assembly (not shown) to orbit the carrier heads and the substrates attached thereto between the polishing stations  22   a – 22   c  and the transfer station  23 . The carrier heads can be spaced at substantially equal angular intervals around the carousel axis  64 . Similarly, the polishing stations and the transfer station can be arranged at substantially equal angular intervals around the carousel axis  64 . Three of the four carrier heads can receive and hold substrates, including polishing them by pressing them against the polishing pads, during which time, the fourth of the carrier heads can deliver a polished substrate to the transfer station  23  and receive an unpolished substrate from the transfer station  23 . Furthermore, the carrier heads can move their respective substrates in directions parallel to the carousel axis  64 . That is, the carrier heads can move the substrate toward and away from the platen surface. Descriptions of a suitable carrier head  70  can be found in U.S. Pat. Nos. 6,422,927 and 6,857,945, the entire disclosures of which are incorporated by reference. 
     Each carrier head 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 can independently and laterally move, including oscillate, in a radial slot  72  formed in a carousel support plate  66 . 
     During polishing, the carrier heads press their respective substrates against the corresponding polishing pad, the platens are rotated about their central axis, and the carrier heads are rotated about their respective central axes and translated laterally across the surface of the polishing pad. 
     Paths along which the carousel  60  and its carrier heads can carry a substrate are described in reference to  FIGS. 2A and 2B .  FIG. 2A  illustrates a circular path  63 , along portions of which the carousel  60  can move a substrate. The carousel axis  64  is the center of the circular path  63 . (Although the carousel  60  can carry multiple substrates, only one substrate is shown in  FIGS. 2A and 2B  for ease of exhibition.) The substrate is being held by a carrier head that is positioned at a particular position along the corresponding radial slot. To move the substrate  10  along the circular path  63 , the position of the carrier head along the radial slot is maintained while the carousel  60  rotates so that the substrate orbits around the carousel axis  64 . While the carousel  60  is moving the substrate  10  along the circular path  63 , the carrier head can rotate the substrate  10  about the axis of rotation of the carrier head. (The thick arrows indicate some of the possible degrees of freedom of movement available to the carousel and carrier head.) 
     The carousel  60  can move the substrate  10  along paths other than that shown in  FIG. 2A  by repositioning the carrier head holding the substrate in other positions along the radial slot  72 . The innermost and outermost circular paths through which the substrate can be moved are illustrated by phantom lines  63   a  and  63   b , respectively, corresponding to the innermost and outermost positions of the carrier head in the radial slot. Instead of a circular path, the carousel  60  can move the substrate along a spiral or elliptical path by translating the carrier head along its radial slot while rotating about the carousel axis  64 . As indicated above, the carousel  60  can hold four substrates and can move the substrates along different paths, either circular or elliptical. 
     To move substrates from station to station, the multi-head carousel  60  carries a substrate along a path that is a portion of one of the possible paths described above, for example, a circular arc or a segment of an elliptical or spiral path.  FIG. 2B  provides an example in which the carousel  60  moves the substrate  10  from the first polishing station  22   a  to the second polishing station  22   b . The path along which the carousel  60  moves the substrate  10  is an arc  65  of the circular path  63 . 
     The CMP apparatus  20  can include one or more in-situ monitoring modules that can determine a change in the thickness of a film on the substrate. The in-situ monitoring module is situated such that the determination can be made in real-time and while the film is being polished. Each polishing station can include an independent in-situ monitoring module. For example, an in-situ monitoring module can be situated in a recess that is formed in platen  24 . Each in-situ monitoring module can include one or more different types of monitoring systems such as an eddy current monitoring system, for example, as described in commonly owned U.S. patent application Ser. No. 10/633,276, an optical monitoring system, for example, as described in U.S. Pat. No. 6,159,073, or a friction-based monitoring system, for example, as described in U.S. patent application Ser. No. 10/977,479, each of which is incorporated by reference. The in-situ monitoring system can function as a polishing process control and endpoint detection system. A suitable in-situ monitoring module is further described in commonly owned U.S. patent application Ser. No. 10/124,507, filed on Apr. 16, 2002, and Ser. No. 10/123,917, also filed on Apr. 16, 2002, which are hereby incorporated by reference in their entireties. 
     The CMP apparatus  20  includes one or more inter-platen monitoring modules configured to effect measurements of a substrate while the substrate is being held by the carrier head but is not in contact with a polishing pad. The inter-platen monitoring modules can include any combination of the above described eddy current, optical, and friction based monitoring systems. Additionally, the inter-platen monitoring module can include an optical monitoring system that uses white light. The white light system can use white light having wavelengths, for example, between 200–800 nanometers. Alternatively, white light having other wavelengths can be used. The white light optical system includes one or more light sensors to measure light intensity and a controller that applies principles of refractometry, spectometry, and/or elipsometry to calculate film thickness or changes in film thickness. 
       FIG. 3  shows an implementation  400  of an inter-platen monitoring module. The implementation  400  includes source  89 , sensor  91 , and controller  93  for the source and sensor. The source  89  can be a white light source, and the sensor  91  can be a white light sensor. The implementation  400  can include multiple sources and multiple sensors. 
     The source  89  and the sensor  91  can be configured and connected to exchange information with a controller  93 , which in turn can be configured and connected to exchange information with a computer  90 . The computer  90  can be a general purpose computer that is configured to control the CMP apparatus  20 . 
     The inter-platen monitoring module need not include a light source, a light sensor, or controller for the light source and the light sensor as these components can be located outside of the module. One or more optical fibers can convey light from the light source to the module, which directs the light to a substrate being measured. One or more optical fibers can convey light received at the module to the light sensor. The same optical fibers can be used for conveying light from the light source and for conveying light to the sensor. The optical fibers can be bifurcated. The inter-platen monitoring module can thus be reduced in size. Moreover, one controller can provide the requisite control for multiple inter-platen monitoring modules. 
     The inter-platen monitoring modules can be situated so that they can take measurements of the substrate without significant impact on the throughput capacity of the CMP apparatus  20 . In particular, an inter-platen monitoring module can be positioned so that (i) the time needed to transport a substrate from a first station (either polishing or transfer) to a position where the module can effect one or more measurements plus the time needed to transport the substrate from the position to a second station (either polishing or transfer) do not significantly vary from (ii) the time that would be needed, if no inter-platen monitoring were performed, to transport the substrate from the first station to the second station. A position where the inter-platen monitoring module can effect one or more measurements is generally one that is within in the working range of sensors and emitters of monitoring systems of the module. A position over a module, for example, is one where one or more measurements can be effected. Note that the first and second polishing stations can perform sequential polishing steps. In some implementations, transporting can include slowing down or stopping motion. That is, the carousel and carrier head can slow down or stop their moving of the substrate while transporting the substrate from one station to another. In other implementations, transporting is performed without slowing or stopping motion of the substrate. 
     As shown in  FIG. 4A , an inter-platen module  51  can be positioned along a path  65   a  traversed by the substrate  10 . As shown in  FIG. 4B , the inter-platen module  51  can perform a series of measurements along path  65   b  as the carousel  60  moves the substrate  10  across the module. In this case, measurements are taken while the carousel  60  rotates about the carousel axis  64 . Optionally, the carousel  60  can temporarily slow down its rotation about the carousel axis  64  while measurements are being taken. Alternatively, the carousel  60  can position the substrate over the inter-platen module  51  and then stop rotating about the carousel axis  64  and hold the substrate still until measurements are completed. In general, however, the carousel  60  will not reverse direction. In any case, the carrier head holding the substrate can move the substrate toward the module  51  so that the surface of the substrate is closer to the module  51 . In addition, the carrier head can rotate about its own axis of rotation and/or translate along the radial slot so that the sensors of the inter-platen module can take measurements at different points of the substrate surface.  FIG. 4C  provides an example in which measurements are taken along a spiral path  71 , resulting from rotation of the carrier head while the carrier head translates angularly over the one or more sensors of the inter-platen monitoring module. Note that the inter-platen module  51  can include a sensor or, alternatively, a mechanism for conveying signals to a sensor located outside of the module (for example, an optical fiber). 
     As discussed above, the inter-platen modules are situated at a point along one of the above described paths used to transfer substrates between stations (which can be consecutive stations) of the CMP apparatus  20 . The inter-platen module can take measurements as the substrate is being transferred from station to station so that substantially no additional time is needed beyond that required to make the transfer. For example, the carousel  60  and the carrier head holding the substrate can move as though no measurements are being taken. Thus, re-programming of the movements for the carousel and carrier heads is not necessary if the inter-platen monitoring module is installed as a retro fit in the CMP apparatus. Retrofitting can but does not necessarily include, for example, replacing a platen and polishing pad assembly that does not include inter-platen modules with one that does. 
     The number of inter-platen monitoring modules required generally depends on the polishing recipe. In general, there should be a sufficient number of modules so that measurements can be taken as needed to provide proper control of the polishing process. By way of example, consider a polishing process that includes three polishing steps performed sequentially at the first, second, and third polishing stations situated as shown in  FIG. 5 . The substrate is first polished in a first polishing step at the first polishing station, then moved to and polished in a second polishing step at the second polishing station, and then moved to and polished in a third polishing step at the third polishing station. The first, second, and third polishing steps can have different removal rates and remove 90%, 9%, and 0.9%, respectively, of the film. Alternatively, the polishing steps can have substantially the same polishing rates. 
     In this case, assume that proper control requires measurements to be taken before each polishing step. Consequently, three inter-platen modules are needed. Module  77  measures thickness before the first polishing step. Module  79  measures thickness before the second polishing step. Module  81  measures thickness before the third polishing step. 
     Note that, in general, an n number of substrates can be measured simultaneously when there are n inter-platen monitoring modules and at least n number of carrier heads. The implementation of  FIG. 5  for example, where there are three inter-platen modules and four of carrier heads, three substrates can be measured simultaneously. Sensors of the three inter-platen modules are positioned so that when one substrate being held by a carrier head is positioned over the sensors of one module, two other substrates being held by other carrier heads of the carousel are also positioned over the sensors of the other two modules. Three substrates can thus be measured at one time and the effect of measurement on throughput can be minimized or reduced. 
       FIGS. 6A and 6B  show a case where polishing consists of three polishing steps, performed sequentially at the first, second, and third polishing stations. In this case, assume proper control requires measurements be taken before and after each polishing step. Consequently, four inter-platen monitoring modules are needed. 
       FIG. 6A  shows an over head view of: (i) the first, second, and third platens of polishing stations  22   a ,  22   b , and  22   c , respectively, (ii) transfer station  23 , and (iii) four inter-platen monitoring modules  77 ,  79 ,  81 , and  83 . As can be seen, the four inter-platen monitoring modules are situated between the four stations. 
       FIG. 6B  shows a method  600  for processing a wafer (a type of substrate). At the transfer station  23 , a current wafer is loaded onto a carrier head (step  602 ). 
     The carrier head transports the current wafer from transfer station  23  to the first platen, including pausing at or moving through one or more positions where inter-platen module  77  can effect measurements (step  604 ). When the current wafer is at a position where inter-platen monitoring module  77  can effect measurements, inter-platen monitoring module  77  takes one or more measurements, including a current film thickness of the current wafer. 
     Because the carrier head can pause or move through more than one position where measurements can be effected by an inter-platen monitoring module, more than one location on the current wafer can be measured by the inter-platen monitoring module. Thus, a inter-platen monitoring module can effect measurements at multiple locations on the current wafer as the wafer is being transported from one platen to another platen, from a platen to transfer station  23 , or from transfer station  23  to a platen. 
     At the first platen, the first polishing step is performed (step  606 ). During the first polishing step, the in-situ monitoring module or modules of polishing station  22   a  perform in-situ measurements. 
     After completion of the first polishing step, the carrier head transports the current wafer from the first platen to the second platen, including pausing at or moving through one or more positions where inter-platen module  79  can effect measurements (step  608 ). When the current wafer is at a position where inter-platen monitoring module  79  can effect measurements, inter-platen monitoring module  79  takes one or more measurements, including a current film thickness of the current wafer. 
     Polishing time, polishing rate, and/or a wafer thickness profile can be calculated based on the inter-platen measurements obtained in the current step. The calculations can be used for polishing the current wafer at the second platen (i.e., feed forward) and/or for polishing a next wafer at the first platen (i.e., feedback). The calculation can be performed by, for example, the above described computer  90 . Polishing time, polishing rate, and thickness profile calculation are further described below. 
     At the second platen, the second polishing step is performed (step  610 ). During the second polishing step, the in-situ monitoring module or modules of polishing station  22   b  perform in-situ measurements. 
     After completion of the second polishing step, the carrier head transports the current wafer from the second platen the third platen, including pausing at or moving through one or more positions where inter-platen module  81  can effect measurements (step  612 ). When the current wafer is at a position where inter-platen monitoring module  81  can effect measurements, inter-platen monitoring module  81  takes one or more measurements, including a current film thickness of the current wafer. Polishing time, polishing rate, and/or a wafer thickness profile can be calculated based on the inter-platen measurements obtained in the current step. The calculations can be used for polishing the current wafer at platen  3  and/or for polishing the next wafer at platen  2 . 
     At the third platen, the first polishing step is performed (step  614 ). During the third polishing step, the in-situ monitoring module or modules of polishing station  22   c  perform in-situ measurements. 
     After completion of the third polishing step, the carrier head transports the current wafer from the third platen to transfer station  23 , including pausing at or moving through one or more positions where inter-platen module  83  can effect measurements (step  616 ). When the current wafer is at a position where inter-platen monitoring module  83  can effect measurements, inter-platen monitoring module  83  takes one or more measurements, including a current film thickness of the current wafer. Polishing time, polishing rate, and/or a wafer thickness profile can be calculated based on the inter-platen measurements obtained in the current step. The calculations can be used for checking the post-polish thickness (or thickness profile) of the current wafer and/or polishing the next wafer at platen  3 . 
     To effect the above-described transport of the current wafer from the third platen to the transfer station  23 , the carrier head rotates counter clockwise (as indicated by arrow  61 ,  FIG. 6A ). However, in alternative implementations, the carrier head can effect the transport from the third platen to transfer station  23  by rotating clockwise (as indicated by arrow  63 ,  FIG. 6A ). In the latter case, one of inter-platen monitoring modules  77 ,  79 , and  81  is used instead of inter-platen monitoring module  83 . 
     At the transfer station, the wafer is unloaded from the carrier head (step  618 ). Another wafer can then be loaded onto the carrier head. 
     Typically, one or more gating wafers of a batch are processed before product wafers of the batch are processed. For a gating wafer, a removal rate for a platen can be calculated based on the pre-polished thickness and the post-polish thickness measured. A polish time can be calculated from the removal rate and be pre-set for the platen. For a product wafer, a post-polish thickness can be used to adjust the polish time of the platen. If post platen  1  thickness of a current wafer is thicker than expected, the polish time of platen  1  can be extended as appropriate for a subsequent wafer and/or the polish time of platen  2  can be extended for the current wafer. Moreover, removal rates can be calculated for product wafers, and the polish times of the platens can be adjusted to increase or maximize throughput. Note that adjustments and calculations can be effected based on averages of measurements for multiple wafers, for example, an average of measurements taken for five wafers. 
     In addition or as an alternative to using the measurements of the inter-platen monitoring modules to adjust polish times to achieve a target thickness, the measurements can be used to adjust polishing parameters to achieve a desired removal rate. In implementations where the carrier head can exert different pressures at difference portions of the wafer, the measurement can also be used to adjust the pressures of each zone to achieve a desired post-polish profile for the wafer. An example of a carrier that can exert different pressures at different portions of a wafer is provided in the above referenced U.S. Pat. Nos. 6,422,927 and 6,857,945. 
     In the above described implementations, an inter-platen monitoring module is positioned so that transporting a substrate from a first station to a position over the module and from the position over the module to a subsequent station does not require the carousel  60  to reverse its rotation about the carousel axis  64 . That is, during the transport of the substrate from one station to another station, the carousel rotates about the carousel axis in only one direction (either clockwise or counter clockwise). The carousel may pause or slow down but does not reverse its rotation during the transfer. Note that, the carrier head may need to make movements not performed during a strictly station-to-station transfer (i.e., one performed without taking measurements). Such movement can be, for example, a translation of the carrier head along its radial slot. 
     In general, an inter-platen monitoring module can be positioned so that transporting a substrate from a first station (for performing a first polishing step) to a position over the module and from the position over the module to a second station (for performing a next polishing step) does not require the device transporting the substrate to reverse direction about or along any of the degrees of freedom of movement which use is required to effect the transport. A degree of freedom of movement can be, for example, rotation about an axis and translation along a path, either radial or otherwise. Motion about or along a degree of freedom of movement without a reverse in direction is said to be monotonic. For example, the rotation about axis  64  to move a substrate from the first station to the subsequent station described in the preceding paragraph is monotonic. 
       FIG. 7  provides an example in which inter-platen modules are positioned in between polishing stations. Any point in annular section  95  is considered to be in between the polishing stations. Annular section  95  is defined by rays  85   a  and  85   b  and circles  87   a  and  87   b . Rays  85   a  and  85   b  start at the carousel axis  64  and are tangent to the circumferences of the polishing stations  22   a  and  22   b  at the points  97  and  99 , respectively. Circles  87   a  and  87   b  (only portions of which are shown) have centers that are at the carousel axis  64 . Circle  87   a  is tangent to the circumference of the polishing stations  22   a  and  22   b  at points  101  and  103 , respectively. Circle  87   b  is tangent to the circumference of the polishing stations  22   a  and  22   b  at points  105  and  107 , respectively. When the platens do not have equal circumferences, the circles  87   a  and  87   b  are tangent to the circle having the greater circumference at points similar to those indicated so as to maximize the area of the annular section  95 . 
     Optionally, the CMP apparatus  20  includes one or more flushing systems. A flushing system can, for example, use one or more fluids to provide consistently a homogeneous medium through which light can travel to and from the surface of the film that is to be or that has been polished. The fluid can flush away polishing residue, for example, slurry, from the surface of the film being measured. 
       FIG. 8A  shows an implementation of a flushing system for an inter-platen monitoring module. In this implementation, an optical fiber  67  is situated inside a tube  69 , through which de-ionized water can be pumped. When the substrate is positioned over one end of the optical fiber, the optical fiber  67  directs light from a source  89  to the film. Light reflected from the film travels back through the optical fiber  67  to a sensor  91 . The ends of the optical fiber  67  and tube  69  are situated so that a jet of de-ionized water washes away polishing residue and provides a homogeneous medium for light to travel to and from the film surface. The optical fiber can be a bifurcated fiber, or simply two adjacent optical fibers. 
       FIG. 8B  shows another implementation of the flushing system, which includes a flush head  53  that includes an input port  54 , an opening  55  that is adjacent to a substrate being polished, and a quartz window  56 . There can be more than one input port. De-ionized water can be supplied to the flush head  53  through the port  54  and can exit the flush head  53  through the opening  55  that is adjacent to the substrate. A light head  57  operable to transmit and receive light is situated adjacent to the quartz window  56  so that light transmitted from and received by the light head  57  (i.e., incident light and reflected light, respectively) pass through the flush head  53  as illustrated. Light is provided to and received from the light head  57  through an optical fiber  58 . Alternatively, the light source and sensor can be included in the light head, in which case the cable depicted can be an electrical cable rather than an optical fiber. 
     During in-situ monitoring, de-ionized water flows into the input port  54 , through the flush head  53 , out the opening  55 , and impacts against a substrate surface where the incident light impinges (i.e., the substrate surface being measured). A constant flow of de-ionized water can thus wash away slurry as well as other optical impediments to incident and reflected light. Furthermore, a constant flow of slurry can provide a homogenous medium, i.e., the de-ionized water, through which incident and reflected light can pass. 
       FIG. 8C  shows another implementation of the flushing system, which is similar to the one described above in reference to  FIG. 8B  but includes a cover  59 A for the flush head  53 . The cover  59 A is situated to cover the flush head  53  when measurements are not being taken during polishing, thus, protecting the flush head  53  and the light head  57  from contamination with slurry. During in-situ monitoring, the cover  59 A is situated to expose the flush head  53  and permit monitoring and flushing. 
     The cover can include a reference reflector  59 B, which can be a mirror or a piece of blank silicon wafer. The reflector  59 B can be used to calibrate the one or more monitoring systems of an inter-platen monitoring-module before each measurement performed by the inter-platen monitoring module. Calibration can account for long term and short term spectrum drifts of the light source and, thus, facilitate accurate measurements. 
       FIG. 8D  shows another implementation of the flushing system, which includes a flush jet source  73  and a dry jet source  75 . The flush jet source produces a jet of liquid, which can be de-ionized water or IPA. The dry jet source produces a jet of gas, which can be N 2  or CDA. The sources are aimed at the substrate surface being measured. 
     During in-situ monitoring, the substrate surface to be measured is first washed with a jet of liquid and then dried with a jet of gas. Measurements can then be effected. 
     Optionally, the CMP apparatus  20  can include one or more position sensors situated and configured to detect and determine when an inter-platen module are beneath a substrate. Examples of position sensors include an optical signal interrupter and an encoder configured to determine angular positions of the carousel and the inter-platen sensor. In general, the optical interrupter operates by having a flag, which can be made of a material that blocks light, that is strategically positioned to block a light signal when the workpiece is positioned over the sensor. An interruption in the light signal, thus, indicates that the workpiece is positioned over the sensor. Measurements of the workpiece can thus be taken during the interruption. 
     Alternatively or in addition to the above-described flag mechanism, a cross rotation motor having an encoder can be implemented to detect and determine when the inter-platen module are beneath a workpiece. In general, the encoder senses the rotational position of the carousel  60  and can indicate when a workpiece being held by one of the carousel&#39;s carrier heads overlies an inter-platen monitoring module. 
     A general purpose programmable digital computer, for example, the above described computer  90 , can be configured to receive signals from the inter-platen modules. The computer can be programmed to sample measurements from the monitoring system when the substrate generally overlies the sensors of the inter-platen modules (as determined, for example, by the above described position sensor). The computer can perform the above described calculations and adjustments. The computer can, for example, calculate film thickness and adjust the polishing time of: (i) the previous polishing step, i.e., the polishing time for a subsequent substrate at the polishing station that the substrate being measured just left, (ii) the subsequent polishing step, i.e., the polishing time at the polishing station to which the substrate being measured will be transferred, or (iii) both of items (i) and (ii). Adjustment can be based on the equation:
 
Change in thickness=Polishing Rate×Polishing Time  (Eq. 1)
 
The polishing rate can usually be empirically derived and typically changes as consumables, for example, the polishing pad, wear with use and age. The computer can use the polishing time to determine when an end point has been achieved. An end point is reached, for example, when the polishing time of the last polishing step has expired.
 
     Alternatively or additionally, the computer can adjust a polishing rate. Such adjustments can be effected by changing controllable polishing parameters, for example, carrier head rotation speed, slurry flow, conditioning, and the amount of force used to press a substrate against a polishing pad. 
     The computer can be programmed to determine a radial position on the substrate for each measurement made by the inter-platen monitoring module and/or to sort the measurements into radial ranges. The computer can calculate a wafer thickness profile by calculating a layer thickness for each radial range. 
     The inter-platen modules 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 (for example, polyurethane with or without fillers) rough pad, a soft pad, or a fixed-abrasive pad. 
     The inter-platen modules may collect sufficient information without requiring the use of in-situ monitoring modules to effectively monitor and control a polishing process. 
     The inter-platen monitoring modules can be implemented for various polishing processes, including but not limited to those for shallow trench isolation, polishing inter-layer dielectric, inter-metal dielectric, pre-metal dielectric, silicon on insulator, and poly materials. 
     Although described in the context of polishing of an oxide layer, some aspects of the invention would be applicable to polishing of other dielectric layers and metals layers, for example, a layer of tungsten or copper. The polished layer can be a barrier layer. 
     The above described inter-platen module can use other types of lights other than those described. The module can use, for example, ultraviolet light or infrared light. 
     Computer programs to carry out the invention may be tangibly embodied in computer-readable medium, for example, disks or memory of the above described computer. 
     The present invention has been described in terms of a preferred embodiment. The invention, however, is not limited to the embodiment depicted and described. Rather, the scope of the invention is defined by the appended claims.