Patent Publication Number: US-11660722-B2

Title: Polishing system with capacitive shear sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 62/726,122, filed Aug. 31, 2018, the disclosure of which is incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to in-situ monitoring of friction during polishing of a substrate. 
     BACKGROUND 
     An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface, and planarizing the filler layer until the non-planar surface is exposed. For example, a conductive layer may be deposited onto a patterned dielectric layer. After planarization, the portions of the metal layer in trenches in the dielectric layer can provide conductive lines, vias, contact pads, and the like. In addition, planarization may be needed to provide a suitably flat substrate surface 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 head. The exposed surface of the substrate is placed against a polishing surface, such as a rotating polishing pad. The carrier head provides a controllable load of the substrate against the polishing pad. A polishing slurry, typically including abrasive particles, is supplied to the polishing surface. 
     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, when a desired amount of material has been removed, or when an underlying layer has been exposed. 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. 
     In-situ monitoring of the substrate has been performed, e.g., with optical or eddy current sensors, in order to detect the polishing endpoint. However, techniques relying on detection of a change in conductivity or reflectivity between two substrate layers deposited upon a substrate can be ineffective when the two layers have similar conductivity and reflectivity. 
     SUMMARY 
     In general, in one aspect, a chemical mechanical polishing system includes a platen to support a polishing pad, a carrier head to hold a substrate and bring a lower surface of the substrate into contact with the polishing pad, and an in-situ friction monitoring system including a friction sensor. The friction sensor includes a pad portion having a substrate contacting portion with an upper surface to contact the lower surface of the substrate, and a pair of capacitive sensors positioned below and on opposing sides of the substrate contacting portion. 
     Implementations may include one or more of the following features. 
     The in-situ friction monitoring system may be configured to determine a sequence of differences over time between a first signal from a first of the pair of capacitive sensors and a second signal from a second of the pair of capacitive sensors. The controller may be configured to determine at least one of a polishing endpoint or a change to a pressure applied by the carrier head based on the sequence of differences. 
     The friction sensor may include a lower body having a first pair of electrodes formed thereon, a polymer body having a second pair of electrodes formed thereon and aligned with the first pair of electrodes, and a pair of gaps between the first pair of electrodes and the second pair of electrodes, each stack of a first electrode, gap and second electrode providing one of the pair of capacitive sensors. The polymer body may include a main body and a plurality of projections extending from the main body to contact the lower body, and recesses between the projections may define the gaps. The polymer body may be a molded silicone. The lower body may be a printed circuit board. The pad portion may be supported on the polymer body. 
     The pad portion may include a lower portion, the substrate contacting portion may project upwardly from the lower portion, and the lower portion may extend laterally beyond all sides of the substrate contacting portion. 
     The system may include the polishing pad. The pad portion may be integrally joined to a remainder of a polishing layer of the polishing pad. The pad portion may include a lower portion, the substrate contacting portion may project upwardly from the lower portion, and the lower portion may extends laterally beyond all sides of the substrate contacting portion to be joined to the polishing pad. The friction sensor may be secured to the polishing pad. A bottom surface of the friction sensor may be coplanar with or recessed relative to a bottom surface of the polishing pad. The upper surface of the pad portion may be coplanar with a polishing surface of the polishing pad. The substrate contacting portion and a polishing layer of the polishing pad may be a same material. 
     The friction sensor may include two pairs of capacitive sensors, each pair of capacitive sensors positioned below and on opposing sides of the substrate contacting portion. The in-situ friction monitoring system may be configured to determine a total frictional force as a square root of a sum of the squares of a plurality of differences, the plurality of differences including first difference between signals from a first pair of the two pairs of capacitive sensors and a second difference between signals from a second pair of the two pairs of capacitive sensors. 
     In another aspect, a polishing pad includes an assembly surrounded by a lower portion of the polishing pad, and an upper portion including a pad portion disposed on the assembly and at least a portion of a polishing layer disposed on the lower portion. The assembly includes a lower body having a first pair of electrodes formed thereon, a polymer body having a second pair of electrodes formed thereon and aligned with the first pair of electrodes, and a pair of gaps between the first pair of electrodes and the second pair of electrodes. 
     In another aspect, a method of monitoring a frictional coefficient of a substrate during a polishing operation includes positioning a surface of a substrate in contact with a polishing surface and simultaneously in contact with a top surface of a substrate contacting member, causing relative motion between the substrate and the polishing surface, the relative motion applying a frictional force to the substrate contacting member which increases pressure on a first capacitive sensor and decreases pressure on a second capacitive sensor, and generating a signal indicating a shear on the substrate contacting member based on a difference between signals from the first and second capacitive sensors. 
     In another aspect, a method of fabricating a polishing pad includes providing an assembly surrounding by a lower portion of a polishing pad, and fabricating an upper portion of the polishing pad by an additive manufacturing process that includes droplet ejection of pad precursor material onto the assembly and the lower portion. The assembly includes a lower body having a first pair of electrodes formed thereon, a polymer body having a second pair of electrodes formed thereon and aligned with the first pair of electrodes, and a pair of gaps between the first pair of electrodes and the second pair of electrodes. 
     Implementations may have some, all, or none of the following advantages. Planarization of a layer being polished, or exposure of any underlying layer, may be detected more accurately and/or when the layer being polished and the layer to be exposed have similar optical or conductive properties. The friction sensor can be small, and complex mechanical parts can be avoided. The friction sensor can be integrated with the polishing pad, enabling ease of manufacture. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1 A  is a schematic side view, partially cross-sectional, of a chemical mechanical polishing station that includes an eddy current monitoring system. 
         FIG.  1 B  is a schematic top view of a chemical mechanical polishing station. 
         FIG.  2    is a schematic cross-sectional side view of a friction sensor in a portion of a polishing pad. 
         FIG.  3    is a schematic top view of the friction sensor and polishing pad of  FIG.  2   .  FIG.  2    is a cross-section along line  2 - 2  in  FIG.  3   . 
         FIG.  4    is a flow chart illustrating a method of monitoring during polishing. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Friction-based monitoring of chemical mechanical polishing has been proposed. For example, a sensor include a flexible plate, e.g., a leaf spring, on which a piece of polishing pad is mounted. The sensor can measure the strain on a flexible plate to generate a signal representative of the friction from the substrate. However, such sensor can be bulky. For example, the vertical length of the plate may present form factor problems given the space available in the platen. Moreover, installation of the sensor in the polishing pad can be cumbersome. However, a capacitive sensor can take up less space, can generate a signal representative from which a direction of friction can be determined, and/or can be integrated into a polishing pad for ease of installation. In addition, a capacitive sensor can provide increased precision and accuracy in friction measurements. Contacts for the sensor can be placed on the bottom of the polishing pad, such that electrical connection to other circuitry can be performed easily. 
       FIGS.  1 A and  1 B  illustrate an example of a polishing apparatus  100 . The polishing apparatus  100  includes a rotatable disk-shaped platen  120  on which a polishing pad  110  is situated. The platen is operable to rotate about an axis  125 . For example, a motor  121  can turn a drive shaft  124  to rotate the platen  120 . 
     The polishing pad  110  can be a two-layer polishing pad with an outer polishing layer  112  and a softer backing layer  114 . The polishing layer  112  can be formed to have a plurality of plateaus  116  separated by grooves  118  (see  FIG.  2   ). The grooves  118  in the polishing surface of the polishing layer  112  can serve to carry a polishing liquid. 
     The polishing apparatus  100  can include a port  130  to dispense the polishing liquid  132 , such as slurry, onto the polishing pad  110 . 
     The polishing apparatus can also include a polishing pad conditioner  170  to abrade the polishing pad  110  to maintain the polishing pad  110  in a consistent abrasive state. In addition, conditioning improves consistency of friction between the substrate and the polishing pad. The polishing pad conditioner  170  can include a conditioner head  172  that permits the conditioner head  172  to sweep radially over the polishing pad  110  as the platen  120  rotates. The conditioner head  172  can hold a conditioner disk  176 , e.g., a metal disk having abrasives, e.g., diamond grit, on the lower surface. The conditioning process tends to wear away the polishing pad  110  over time, until the polishing pad  110  needs to be replaced. 
     The polishing apparatus  100  includes at least one carrier head  140 . The carrier head  140  is operable to hold a substrate  10  against the polishing pad  110 . The carrier head  140  can have independent control of the polishing parameters, for example pressure, associated with each respective substrate. 
     In particular, the carrier head  140  can include a retaining ring  142  to retain the substrate  10  below a flexible membrane  144 . The carrier head  140  also includes a plurality of independently controllable pressurizable chambers  146  defined by the membrane, and which can apply independently controllable pressures to associated zones on the flexible membrane  144  and thus on the substrate  10 . Although only three chambers  146  are illustrated in  FIG.  1    for ease of illustration, there could be one or two chambers, or four or more chambers, e.g., five chambers. 
     The carrier head  140  is suspended from a support structure  150 , e.g., a carousel or a track, and is connected by a drive shaft  152  to a carrier head rotation motor  154  so that the carrier head can rotate about an axis  155 . Optionally the carrier head  140  can oscillate laterally, e.g., on sliders on the carousel  150  or track; or by rotational oscillation of the carousel itself. In operation, the platen is rotated about its central axis  125 , and the carrier head is rotated about its central axis  155  and translated laterally across the top surface of the polishing pad. 
     While only one carrier head  140  is shown, more carrier heads can be provided to hold additional substrates so that the surface area of polishing pad  110  may be used efficiently. 
     The polishing apparatus  100  also includes an in-situ monitoring system  200 . In particular, the in-situ monitoring system  200  generates a time-varying sequence of values that depend on the friction of the surface of layer on the substrate  10  that is being polished. The in-situ monitoring system  200  includes a sensor  202  which generates a signal that depends on the frictional coefficient of a localized, discrete area of the substrate  10 . Due to relative motion between the substrate  10  and the sensor  202 , measurements can be taken at different locations on the substrate  10 . 
     The CMP apparatus  100  can also include a position sensor  180 , such as an optical interrupter, to sense when the sensor  202  is beneath the substrate  10 . For example, the optical interrupter  180  could be mounted at a fixed point opposite the carrier head  140 . A flag  182  is attached to the periphery of the platen. The point of attachment and length of flag  182  is selected so that it interrupts the optical signal of sensor  180  while the sensor  202  sweeps beneath substrate  10 . Alternatively or in addition, the CMP apparatus  100  can include an encoder to determine the angular position of platen. 
     If needed, sense circuitry  250  can be used to receive an analog signal, e.g., a voltage or current level, from the sensor  202 , e.g., on wires  252 . The sense circuitry  250  can be located in a recess in the platen  120 , or could be located outside the platen  120  and be coupled to sensor  202  through a rotary electrical union  129 . In some implementations, the drive and sense circuitry receives multiple analog signals from the sensor  202 , and converts those analog signals into a serial digital signal. 
     A controller  190 , such as a general purpose programmable digital computer, receives the signal from the sensing circuitry  250  or directly from the sensor  202 . The controller  190  can include a processor, memory, and I/O devices, as well as an output device e.g., a monitor, and an input device, e.g., a keyboard. The signals can pass from the sensor  202  to the controller  190  through the rotary electrical union  129 . Alternatively, the sense circuitry  250  could communicate with the controller  190  by a wireless signal. 
     The controller  190  can be configured to convert the signals from the sensor  202  into a series of values indicative of the coefficient of friction of the substrate  10 . As such, some functionality of the controller  190  can be considered part of the in-situ monitoring system  200 . 
     Since the sensor  202  sweeps beneath the substrate with each rotation of the platen, information on the friction is accumulated in-situ and on a continuous real-time basis (once per platen rotation). The controller  190  can be programmed to sample measurements when the substrate generally overlies the sensor  202  (as determined by the position sensor  180 ). As polishing progresses, the coefficient of friction of the surface of the substrate changes, and the sampled signals can vary with time. The time varying sampled signals may be referred to as traces. The measurements from the monitoring systems can be displayed on the output device during polishing to permit the operator of the device to visually monitor the progress of the polishing operation. 
     In operation, the CMP apparatus  100  can use the in-situ monitoring system  200  to determine when the bulk of the filler layer has been removed and/or to determine when the underlying stop layer has been substantially exposed. In particular, when an underlying layer is exposed, there should be a sudden change in the coefficient of friction. This change can be detected, e.g., by detecting changes in slope of the trace, or by detecting that the amplitude or slope of the trace passes a threshold value. Detection of exposure of the underlying layer can trigger the polishing endpoint and halt polishing. 
     The controller  190  may also be connected to the pressure mechanisms that control the pressure applied by carrier head  140 , to carrier head rotation motor  174  to control the carrier head rotation rate, to the platen rotation motor  121  to control the platen rotation rate, or to slurry distribution system  130  to control the slurry composition supplied to the polishing pad. In addition, the computer  190  can be programmed to divide the measurements from the sensor  202  from each sweep beneath the substrate into a plurality of sampling zones  194 , to calculate the radial position of each sampling zone, and to sort the amplitude measurements into radial ranges. After sorting the measurements into radial ranges, information on the film thickness can be fed in real-time into a closed-loop controller to periodically or continuously modify the polishing pressure profile applied by a carrier head in order to provide improved polishing uniformity. 
     Now referring to  FIGS.  2  and  3   , the sensor  202  can includes a pad portion  210  having a top surface  212  configured to contact the substrate, and at least one pair of capacitive sensors  220  positioned below and on opposite sides of the pad portion  210 . The sensor  202  can include a lower body  240 , which can be a printed circuit board, and a polymer body  230 . Gaps between the lower body  240  and the polymer body  230  define the spaces between opposite electrodes of the capacitive sensors  220 . 
     The pad portion  210  includes a substrate contacting portion  214 , the upper surface of which provides the top surface  212  to contact the polishing pad. The substrate contacting portion  214  can have a lateral cross-section (see  FIG.  3   ) which is square, circular, or some other suitable shape. The substrate contacting portion  214  can have a width W of about 0.2-0.5 mm, and a height H of about 0.2-1 mm. The height H of the upper portion  214  can be greater than the width W of the substrate contacting portion  214 . 
     The pad portion  210  can optionally also include a lower portion  216  that extends laterally outward on all sides of the substrate contacting portion  214 ; the lower portion  216  that has a lateral dimension that is greater than the lateral dimension of the substrate contacting portion  214 . The lower portion  216  can extend entirely across the capacitive  5  sensors  220 , and can extend entirely across the polymer portion  240 . The lower portion  216 , if present, can have a height less than the height of the upper portion, e.g., about 0.1-0.5 mm. 
     In some implementations, the lower portion  216  extends to and contacts the remainder of the polishing pad  110 . The lower portion  216  can be secured to the polishing layer  112 , e.g., with an adhesive. Alternatively, the lower portion  216  can be integrally joined to the remainder of the polishing pad  110 , i.e., without an adhesive, seam or similar discontinuity. 
     In some implementations, there is a gap between the side edges of the lower portion  216  and the polishing pad  110 . 
     In general, the substrate contacting member portion  214  is formed of a material that does not adversely impact the polishing process, e.g., it should be chemically compatible with the polishing environment and sufficiently soft as to avoid scratching or damaging the substrate. The pad portion  210  can be the same material as the polishing layer  32  of the polishing pad  30 , e.g., a polyurethane. Alternatively, the pad portion  210  can be a different material than the polishing layer  32 , e.g., an acrylate. 
     The pad portion  210  can be supported on the polymer body  230 . The bottom surface of the pad portion  210  can be secured to the top surface of the polymer body  230 , e.g., by an adhesive or by fabricating the pad portion  210  directly on the polymer body  230 . 
     A plurality of projections  232  extend from the bottom of a main body  234  of the polymer body  230  to contact the lower body  240 , e.g., the printed circuit board. Recesses between the projections  232  define gaps  236  between the polymer body  230  and the lower body  240 . The polymer body  230  can be secured to the lower body  240 , e.g., by adhesive. The gaps  236  can partially underlie the substrate contacting portion  214 . For example, the width of the projection  232  can be less than the width W of the substrate contacting portion  214 . Alternatively, the gaps  236  can be laterally spaced so that they do not directly underlie the substrate contacting portion  214 . For example, the width of the projection  232  can be greater than the width W of the substrate contacting portion  214 . 
     The polymer body  230  can be silicone material, e.g., polydimethylsiloxane (PDMS). The polymer body  230  can be formed by a molding process, e.g., injection molding into the form having the projections  232  extending from a main body  234 . 
     The interior horizontal surfaces of the recesses can be coated with a conductive material to form electrodes  238 . The sidewall surfaces of the recesses (i.e., the sides of the projections) need not be coated. 
     As noted above, the lower body  240  can be a printed circuit board. Electrodes  242  are formed on a top surface of the lower body  240 , and conductive contacts  244  can be formed on the bottom surface of the lower body  240 . In addition, the lower body  240  can include conductive lead lines  246 , e.g., extending through the thickness of the lower body, to electrically connect each electrode  242  with a corresponding conductive contact  244 . 
     In some implementations, electrical contacts  254  can be formed on the top surface of the platen  120  (see  FIG.  1 A ). These electrical contacts  254  are connected by the wires  252  to the sense circuitry  250  and/or controller  190 . Thus, when the polishing pad  110  is installed on the platen  120 , each conductive contact  244  makes an electrical connection to a corresponding electrical contact  254 . This permits the electrical connection of the sensor  202  to other components, e.g., the sense circuitry  250  and/or controller  190 , to be made quickly and easily. 
     When the polymer body  230  is secured to the lower body  240 , each electrode  238  on the polymer body  230  is aligned with a corresponding electrode  242  on the lower body  240  with a gap  236  between. A set of two electrodes  238 ,  242  with a gap  236  therebetween provides a capacitive pressure sensor  220 . In brief, if the space between the electrodes  238 ,  242  changes, this will result in a change in capacitance and thus a change in a signal sensed by circuitry coupled to the sensor  220 , e.g., through the conductive contact  244 . 
     In a rest state, e.g., when not being compressed by pressure from a substrate, the gap  236  can have a height of 10 to 50 microns. The electrodes  238 ,  242  can have a lateral dimension of 0.5 to 1 mm. The electrodes  238 ,  242  and the gap  236  can be square, circular, or another suitable lateral cross-sectional shape. 
     A pair of capacitive pressure sensors  220   a ,  220   b  positioned on opposing sides of a midline of the substrate contacting portion  214  can provide a shear sensor. In particular, frictional drag on the substrate contacting portion  214  from the substrate will tend to apply a torque on the pad portion  210 . This will cause a differential in pressure on the two sensors  220   a ,  220   b . For example, if the substrate  10  is moving rightward across the polishing pad  110 , friction on the pad portion  210  will tend to increase pressure on the right-hand capacitive pressure sensor  220   a , and reduce pressure on the left-hand capacitive pressure sensor  220   b . Conversely, if the substrate  10  is moving leftward across the polishing pad  110 , friction on the pad portion  210  will tend to reduce pressure on the right-hand capacitive pressure sensor  220   a , and increase pressure on the left-hand capacitive pressure sensor  220   b.    
     To detect the amount of shear, and thus measure the friction between the substrate and the substrate contacting portion  214 , a differential between the signals from the two sensors  220   a ,  220   b  can be calculated. For example, the signal from the right-hand capacitive pressure sensor  220   a  can be subtracted from the signal from the left-hand capacitive pressure sensor  220   b.    
     As shown in  FIG.  3   , in some implementations, the sensor  202  includes two pairs of capacitive pressure sensors  220  (i.e., four capacitive pressure sensors). The two sensors of each pair are positioned on opposing sides of a midline of the upper portion  214 . In addition, the two pairs can be arranged to measure shear along perpendicular axes. With this configuration, the in-situ monitoring system  200  can generate a measurement indicative of a total frictional force, e.g., as a square root of the sum of the squares of the shear measured in the two perpendicular directions. This calculation can be performed by the controller  190 . In some implementations, the sensor  202  includes three or more pairs of capacitive pressure sensors  220 , with each pair of capacitive pressure sensors  220  including two capacitive pressure sensors on opposite sides of the pad portion  210 . For example, although  FIG.  3    illustrates empty spots diagonally above and below the pad portion  210 , these spots could be occupied by additional capacitive pressure sensors. 
     Different substrate layers have different coefficients of friction between the deposited layers and the substrate contacting portion  214 . This difference in coefficients of friction means that different deposited layers will generate different amounts of frictional force, and thus different amounts of shear on the sensor  202 . If the coefficient of friction increases, the shear will increase. Similarly, if the coefficient of friction decreases, the shear will decrease. When deposited layer  16  has been polished down to expose the patterned layer  14 , the shear will change to reflect the different coefficient of friction between the material of the deposited layer  14  and the polishing pad  110 . Consequently, a computing device, such as the controller  190 , can be used to determine the polishing endpoint by monitoring the changes in shear, and thus friction, detected by the in-situ monitoring system. 
     Referring to  FIG.  4   , the controller can be used to control the polishing system  100 . An implementation of a computer program for chemical mechanical polishing begins with the initiation of a chemical mechanical polishing process on the substrate  10  ( 410 ). During the polishing process, the computer  90  receives input from the sensors  202  ( 420 ). Input from the individual capacitive sensors  220  can be received simultaneously or serially, and can be received continuously or periodically. The controller  190  (or the circuitry  250 ) receives the signals from the capacitive sensors  220  and determines the shear experienced by the sensor  202  ( 430 ). The controller  190  monitors the signal for changes in shear. When a change in shear indicates a desired polishing endpoint, the controller  190  ends the polishing process ( 440 ). 
     In some implementation, the controller  190  detects changes in the slope of the shear data to determine a polishing endpoint. The controller  190  can also monitor for shear signal smoothing to determine a polishing endpoint. Alternatively, the controller  190  consults a database containing pre-determined endpoint shear values based on the deposited layers used in order to determine the occurrence of an endpoint. 
     As noted above, the controller  190  can sort the measurements from the sensor  202  into radial ranges. The polishing parameters can then be adjusted based on the measurements, e.g., to provide improved uniformity. When the measurements indicate that an underlying layer has become exposed in a particular range, the polishing parameters can be adjusted to reduce the polishing rate in that range. Machine parameters that are independently controllable for the different radial ranges of the substrate, can then be controlled based on the measurements for the respective radial ranges. 
     In particular, the measurements may then be used for real-time closed loop control of the pressure applied by the carrier head  140 . For example, if the controller  190  detects that the friction is changing in one radial zone, e.g., at the edge of the substrate, this can indicate that the underlying layer is being exposed, e.g., the underlying layer is being exposed first at the edge of the substrate. In response, the controller  190  can cause the carrier head  140  to reduce the pressure applied at the edges of the substrate. In contrast, if the controller  190  has not detected a change in friction in another radial range, e.g., a center portion of the substrate, this can indicate that the underlying layer is not yet exposed. The controller  190  can cause the carrier head  140  to maintain the pressure applied at the center of the substrate. 
     Referring to  FIG.  1 B , the in-situ monitoring system can include multiple sensors  202 . For example, the in-situ monitoring system can include multiple sensors  202  placed at substantially the same distance from but at equal angular intervals around the axis of rotation of the platen. As another example, there can be sensors  202  positioned at different radial positions on the polishing pad  110 . For example, the sensors  202  can be arranged in 3×3 grid. Increasing the number of sensors permits an increase in the sampling rate from the substrate  10 . 
     To fabricate the sensor  202 , the lower body  240  can be fabricated, e.g., as a printed circuit board having the electrodes  242 . The polymer body  230  can be fabricated by injection molding. The electrodes  238  can be deposited in the recesses between the projections  232 , e.g., by a sputtering process. The polymer body  230  is aligned and secured to the lower body  240  to form the capacitive sensors  220 . 
     The assembly of the polymer body  230  and lower body  240  can then be placed into an aperture in the backing layer  114 . Then the polishing layer  112  can be fabricated on top of the assembly and the backing layer. For example, the polishing layer  112  can be fabricated by a 3D printing process, e.g., by ejection and curing of droplets of pad precursor material. This permits the pad portion  210  and the remainder of the polishing layer  112  to be fabricated together as one continuous piece, i.e., without an adhesive, seam or similar discontinuity. 
     Alternatively, the polishing layer  112  could be fabricated separately, and then placed on the assembly and the backing layer  114 , and secured, e.g., by adhesive. 
     Alternatively, the pad portion  210  can be secured to the assembly of the polymer body  230  and lower body  240  separately. Thereafter, the sensor  202  can be installed in the polishing pad  110 , e.g., by being inserted into an aperture in the polishing pad  110  and secured, e.g., by adhesive. 
     The monitoring system 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 standard (e.g., polyurethane with or without fillers) rough pad, a soft pad, or a fixed-abrasive pad. 
     The functional operations described in this specification, e.g., for the controller and/or sense circuitry, can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structural means disclosed in this specification and structural equivalents thereof, or in combinations of them. Embodiments can be implemented as one or more computer program products, i.e., one or more computer programs tangibly embodied in an information carrier, e.g., in a non-transitory machine readable storage medium or in a propagated signal, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple processors or computers. A computer program (also known as a program, software, software application, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file. A program can be stored in a portion of a file that holds other programs or data, in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. 
     The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of this disclosure. For example:
         The top surface of the polymer body need not be coplanar with the top surface of the backing layer.   Although  FIG.  2    illustrates the polishing pad as having two layers, the polishing pad could be a single layer pad. A recess could be formed in the back surface of the polishing pad and the sensor inserted into the recess.   The polishing pad could be built up around the sensor by a 3D printing process. For example, the assembly of the polymer body and lower body could be placed on a print stage, and the lower portion of the polishing pad could be fabricated around the assembly, e.g., by selectively ejecting droplets of precursor material into areas around but not on the assembly. This can build layers until the top of the pad material is coplanar with the top of the assembly. After this point, droplets of precursor material could be ejected across both the previously formed layers and the assembly, thus forming the pad portion and the upper portion of the remainder of the polishing pad.   The technique of fabrication of the polishing pad by 3D printing around the assembly can be used for a single-layer pad, in which case the same material can be used throughout the pad, or for a multi-layer pad, in which case a different precursor or different curing technique can be used to form the lower portion (and thus the backing layer) of the polishing pad around the assembly.       

     Accordingly, other embodiments are within the scope of the following claims.