Patent Publication Number: US-11638982-B2

Title: Core configuration for in-situ electromagnetic induction monitoring system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/726,148, filed Oct. 5, 2017, which claims priority to U.S. Provisional Application Ser. No. 62/411,407, filed on Oct. 21, 2016, and which claims priority to U.S. Provisional Application Ser. No. 62/415,641, filed on Nov. 1, 2016, each of which is incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to electromagnetic induction monitoring, e.g., eddy current monitoring, during processing of a substrate. 
     BACKGROUND 
     An integrated circuit is typically formed on a substrate (e.g. a semiconductor wafer) by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer, and by the subsequent processing of the layers. 
     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 filler layer can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. The filler layer is then polished until the raised pattern of the insulative layer is exposed. 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. In addition, planarization may be used to planarize the substrate surface for lithography. 
     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 rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing liquid, such as slurry with abrasive particles, is supplied to the surface of the polishing pad. 
     During semiconductor processing, it may be important to determine one or more characteristics of the substrate or layers on the substrate. For example, it may be important to know the thickness of a conductive layer during a CMP process, so that the process may be terminated at the correct time. A number of methods may be used to determine substrate characteristics. For example, optical sensors may be used for in-situ monitoring of a substrate during chemical mechanical polishing. Alternately (or in addition), an eddy current sensing system may be used to induce eddy currents in a conductive region on the substrate to determine parameters such as the local thickness of the conductive region. 
     SUMMARY 
     In one aspect, an apparatus for chemical mechanical polishing includes a support for a polishing pad having a polishing surface, and an electromagnetic induction monitoring system to generate a magnetic field to monitor a substrate being polished by the polishing pad. The electromagnetic induction monitoring system includes a core and a coil wound around a portion of the core. The core includes a back portion, a center post extending from the back portion in a first direction normal to the polishing surface, and an annular rim extending from the back portion in parallel with the center post and surrounding and spaced apart from the center post by a gap. The center post has a first width in a second direction parallel to the polishing surface, the annular rim has a second width in the second direction and the gap has a third width in the second direction. The third width is less than the first width, and a surface area of a top surface of the annular rim is at least two times greater than a surface area of a top surface of the center post. 
     Implementations may include one or more of the following features. 
     The second width may be greater than the first width. The second width may be 1.1 to 1.5 times greater than the first width. The third width may be 50% to 75% of the first width. The surface area of the top surface of the annular rim may be at least three times greater than the surface area of the top surface of the center post. A height of the center post may be equal to a height of the annular rim portion. The third width may be between about 30% and 70% of the second width. The coil and core may be configured to provide a resonant frequency of at least 12 MHz, e.g., between about 14 and 16 MHz. The core may be nickel zinc ferrite. 
     In another aspect, an apparatus for chemical mechanical polishing includes a support for a polishing pad having a polishing surface, and an electromagnetic induction monitoring system to generate a magnetic field to monitor a substrate being polished by the polishing pad. The electromagnetic induction monitoring system includes a core and a winding assembly. The core includes a back portion, a center post extending from the back portion in a first direction normal to the surface of the platen, and an annular rim extending from the back portion in parallel with the center post and surrounding and spaced apart from the center post by a gap. The center post has a first width in a second direction parallel to the surface of the platen, the annular rim has a second width in the second direction, and the gap has a third width in the second direction. The winding assembly is a cylindrical body fitting in the gap. The winding assembly includes a coil wound around the center post, and the winding assembly has a fourth width between an inner diameter and an outer diameter of the cylindrical body. The fourth width is at least 80% of the third width. 
     Implementations may include one or more of the following features. 
     The winding assembly may include a bobbin, the coil may be wound around the bobbin, and an inner surface of the bobbin may provide the inner diameter of the winding assembly. The inner surface of the bobbin may contact an outer surface of the center post. The winding assembly may include a tape contacting and surrounding the coil, and an outer surface of the tape may provide the outer diameter of the winding assembly. The outer surface of the tape may contact an inner surface of the annular rim. 
     The coil may have no more than two winding layers around the center post, e.g., the coil may have a single winding layer around the center post. The fourth width may be at least 90% of the third width. The third width may be about 1 to 2 mm. The third width may be less than the first width, and a surface area of a top surface of the annular rim may be at least two times greater than a surface area of a top surface of the center post. 
     Certain implementations can include one or more of the following advantages. Spatial resolution of the eddy current sensor can be improved. The eddy current sensor can be configured for monitoring of conductive features that have a high impedance, e.g., metal sheets formed of a low conductance metal such as titanium or cobalt, metal residue, or metal lines. 
     The details of one or more implementations 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. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic side view, partially cross-sectional, of a chemical mechanical polishing station that includes an electromagnetic induction monitoring system. 
         FIG.  2    is a schematic top view of the chemical mechanical polishing station of  FIG.  1   . 
         FIGS.  3 A- 3 C  are schematic cross-sectional side views illustrating a method of polishing a substrate. 
         FIG.  4    is a schematic circuit diagram of a drive system for an electromagnetic induction monitoring system. 
         FIGS.  5 A and  5 B  are schematic top and side views of the core of the electromagnetic induction monitoring system. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     A CMP system can use an eddy current monitoring systems to detect a thickness of a metal layer on a substrate during polishing. During polishing of the metal layer, the eddy current monitoring system can determine the thickness of the metal layer in different regions of the substrate. The thickness measurements can be used to detect the polishing endpoint or to adjust processing parameters of the polishing process in real time to reduce polishing non-uniformity. 
     One issue with eddy current monitoring is that the eddy current is induced in the conductive layer in a region whose size depends on the spread of the magnetic field; the greater the spread of the magnetic field, the lower the resolution of the eddy current monitoring system. With ever increasing demands of integrated circuit fabrication, there is a need for increased spatial resolution of the eddy current sensor, e.g., in order to provide improved control of the polishing parameters. Appropriate selection of the physical configuration of the magnetic core can reduce the spread of the magnetic field and provide improved resolution. 
       FIGS.  1  and  2    illustrate an example of a polishing station  20  of a chemical mechanical polishing apparatus. The polishing station  20  includes a rotatable disk-shaped platen  24  on which a polishing pad  30  is situated. The platen  24  is operable to rotate about an axis  25 . For example, a motor  22  can turn a drive shaft  28  to rotate the platen  24 . The polishing pad  30  can be a two-layer polishing pad with an outer polishing layer  34  and a softer backing layer  32 . 
     The polishing station  22  can include a supply port or a combined supply-rinse arm  39  to dispense a polishing liquid  38 , such as slurry, onto the polishing pad  30 . The polishing station  22  can include a pad conditioner apparatus with a conditioning disk to maintain the surface roughness of the polishing pad. 
     The carrier head  70  is operable to hold a substrate  10  against the polishing pad  30 . The carrier head  70  is suspended from a support structure  72 , e.g., a carousel or a track, and is connected by a drive shaft  74  to a carrier head rotation motor  76  so that the carrier head can rotate about an axis  71 . Optionally, the carrier head  70  can oscillate laterally, e.g., on sliders on the carousel or track  72 ; or by rotational oscillation of the carousel itself. 
     In operation, the platen is rotated about its central axis  25 , and the carrier head is rotated about its central axis  71  and translated laterally across the top surface of the polishing pad  30 . Where there are multiple carrier heads, each carrier head  70  can have independent control of its polishing parameters, for example each carrier head can independently control the pressure applied to each respective substrate. 
     The carrier head  70  can include a flexible membrane  80  having a substrate mounting surface to contact the back side of the substrate  10 , and a plurality of pressurizable chambers  82  to apply different pressures to different zones, e.g., different radial zones, on the substrate  10 . The carrier head can also include a retaining ring  84  to hold the substrate. 
     A recess  26  is formed in the platen  24 , and optionally a thin section  36  can be formed in the polishing pad  30  overlying the recess  26 . The recess  26  and thin pad section  36  can be positioned such that regardless of the translational position of the carrier head they pass beneath substrate  10  during a portion of the platen rotation. Assuming that the polishing pad  30  is a two-layer pad, the thin pad section  36  can be constructed by removing a portion of the backing layer  32 , and optionally by forming a recess in the bottom of the polishing layer  34 . The thin section can optionally be optically transmissive, e.g., if an in-situ optical monitoring system is integrated into the platen  24 . 
     Referring to  FIG.  3 A , the polishing system  20  can be used to polish a substrate  10  that includes a conductive material overlying and/or inlaid in a patterned dielectric layer. For example, the substrate  10  can include a layer of conductive material  16 , e.g., a metal, e.g., copper, aluminum, cobalt or titanium, that overlies and fills trenches in a dielectric layer  14 , e.g., silicon oxide or a high-k dielectric. Optionally a barrier layer  18 , e.g., tantalum or tantalum nitride, can line the trenches and separate the conductive material  16  from the dielectric layer  14 . The conductive material  16  in the trenches can provide vias, pads and/or interconnects in a completed integrated circuit. Although the dielectric layer  14  is illustrated as deposited directly on a semiconductor wafer  12 , one or more other layers can be interposed between the dielectric layer  14  and the wafer  12 . 
     Initially, the conductive material  16  overlies the entire dielectric layer  14 . As polishing progresses, the bulk of the conductive material  16  is removed, exposing the barrier layer  18  (see  FIG.  3 B ). Continued polishing then exposes the patterned top surface of the dielectric layer  14  (see  FIG.  3 C ). Additional polishing can then be used to control the depth of the trenches that contain the conductive material  16 . 
     Returning to  FIG.  1   , the polishing system  20  includes an in-situ electromagnetic induction monitoring system  100  which can be coupled to or be considered to include a controller  90 . A rotary coupler  29  can be used to electrically connect components in the rotatable platen  24 , e.g., the sensors of the in-situ monitoring systems, to components outside the platen, e.g., drive and sense circuitry or the controller  90 . 
     The in-situ electromagnetic induction monitoring system  100  is configured to generate a signal that depends on a depth of the conductive material  16 , e.g., the metal. The electromagnetic induction monitoring system can operate either by generation of eddy-currents in the conductive material, which can be either the sheet of conductive material that overlies the dielectric layer or the conductive material remaining in trenches after the dielectric layer is exposed, or generation of current in a conductive loop formed in a trench in the dielectric layer on the substrate. 
     In operation, the polishing system  20  can use the in-situ monitoring system  100  to determine when the conductive layer has reached a target thickness, e.g., a target depth for metal in a trench or a target thickness for a metal layer overlying the dielectric layer, and then halts polishing. Alternatively or in addition, the polishing system  20  can use the in-situ monitoring system  100  to determine differences in thickness of the conductive material  16  across the substrate  10 , and uses this information to adjust the pressure in one or more chambers  82  in the carrier head  80  during polishing in order to reduce polishing non-uniformity. 
     The in-situ monitoring system  100  can include a sensor  102  installed in a recess  26  in the platen  24 . The sensor  102  can include a magnetic core  104  positioned at least partially in the recess  26 , and at least one coil  106  wound around a portion of the core  104 . Drive and sense circuitry  108  is electrically connected to the coil  106 . The drive and sense circuitry  108  generates a signal that can be sent to the controller  90 . Although illustrated as outside the platen  24 , some or all of the drive and sense circuitry  108  can be installed in the platen  24 . 
     Referring to  FIG.  2   , as the platen  24  rotates, the sensor  102  sweeps below the substrate  10 . By sampling the signal from the circuitry  108  at a particular frequency, the circuitry  108  generates measurements at a sequence of sampling zones  94  across the substrate  10 . For each sweep, measurements at one or more of the sampling zones  94  can be selected or combined. Thus, over multiple sweeps, the selected or combined measurements provide the time-varying sequence of values. 
     The polishing station  20  can also include a position sensor  96 , such as an optical interrupter, to sense when the sensor  102  is underneath the substrate  10  and when the sensor  102  is off the substrate. For example, the position sensor  96  can be mounted at a fixed location opposite the carrier head  70 . A flag  98  can be attached to the periphery of the platen  24 . The point of attachment and length of the flag  98  is selected so that it can signal the position sensor  96  when the sensor  102  sweeps underneath the substrate  10 . 
     Alternately or in addition, the polishing station  20  can include an encoder to determine the angular position of the platen  24 . 
     Returning to  FIG.  1   , a controller  90 , e.g., a general purpose programmable digital computer, receives the signals from the in-situ monitoring system  100 . Since the sensor  102  sweeps beneath the substrate  10  with each rotation of the platen  24 , information on the depth of the conductive layer, e.g., the bulk layer or conductive material in the trenches, is accumulated in-situ (once per platen rotation). The controller  90  can be programmed to sample measurements from the in-situ monitoring system  100  when the substrate  10  generally overlies the sensor  102 . 
     In addition, the controller  90  can be programmed to calculate the radial position of each measurement, and to sort the measurements into radial ranges. 
       FIG.  4    illustrates an example of the drive and sense circuitry  108 . The circuitry  108  applies an AC current to the coil  106 , which generates a magnetic field  150  between two poles  152   a  and  152   b  of the core  104 . In operation, when the substrate  10  intermittently overlies the sensor  104 , a portion of the magnetic field  150  extends into the substrate  10 . 
     The circuitry  108  can include a capacitor  160  connected in parallel with the coil  106 . Together the coil  106  and the capacitor  160  can form an LC resonant tank. In operation, a current generator  162  (e.g., a current generator based on a marginal oscillator circuit) drives the system at the resonant frequency of the LC tank circuit formed by the coil  106  (with inductance L) and the capacitor  160  (with capacitance C). The current generator  162  can be designed to maintain the peak to peak amplitude of the sinusoidal oscillation at a constant value. A time-dependent voltage with amplitude VO is rectified using a rectifier  164  and provided to a feedback circuit  166 . The feedback circuit  166  determines a drive current for current generator  162  to keep the amplitude of the voltage VO constant. Marginal oscillator circuits and feedback circuits are further described in U.S. Pat. Nos. 4,000,458, and 7,112,960. 
     As an eddy current monitoring system, the electromagnetic induction monitoring system  100  can be used to monitor the thickness of a conductive layer by inducing eddy currents in the conductive sheet, or to monitor the depth of a conductive material in a trench by inducing eddy currents in the conductive material. Alternatively, as an inductive monitoring system, the electromagnetic induction monitoring system can operate by inductively generating a current in a conductive loop formed in the dielectric layer  14  of the substrate  10  for the purpose of monitoring, e.g., as described in U.S. Patent Publication No. 2015-0371907. 
     If monitoring of the thickness of a conductive layer on the substrate is desired, then when the magnetic field  150  reaches the conductive layer, the magnetic field  150  can pass through and generate a current (if the target is a loop) or create an eddy-current (if the target is a sheet). This creates an effective impedance, thus increasing the drive current required for the current generator  162  to keep the amplitude of the voltage VO constant. The magnitude of the effective impedance depends on the thickness of the conductive layer. Thus, the drive current generated by the current generator  162  provides a measurement of the thickness of the conductive layer being polished. 
     Other configurations are possible for the drive and sense circuitry  108 . For example, separate drive and sense coils could be wound around the core, the drive coil could be driven at a constant frequency, and the amplitude or phase (relative to the driving oscillator) of the current from the sense coil could be used for the signal. 
       FIGS.  5 A and  51 B  illustrate an example of a core  104  for the in-situ monitoring system  100 . The core  104  has a body formed of a non-conductive material with a relatively high magnetic permeability (e.g., μ of about 2500 or more). Specifically, the core  104  can be nickel-zinc ferrite or magnesium-zinc ferrite. 
     In some implementations, the core  104  is coated with a protective layer. For example, the core  104  can be coated with a material such as parylene to prevent water from entering pores in the core  104 , and to prevent coil shorting. 
     The core  104  can be round core, also known as a pot core. The core  104  includes a back portion  120 , a center post  122  that extends from the back portion  120 , and an annular rim  124  surrounding and spaced apart from the center post  122  by a gap  126  and also extending from the back portion  120 . The annular rim  124  can be spaced apart from the center post  122  by a uniform distance around the perimeter of the center post  122 . The annular rim  124  can completely enclose the center post  122  (as seen in the top view of  FIG.  4 B ). 
     A winding assembly  130  fits into the gap  126 . The winding assembly can a cylindrical body. The winding assembly has a width (W 4 ), which can be the distance between an inner diameter and an outer diameter of the cylindrical body. 
     The winding assembly  130  includes at least the coil  106 , which is wound around the center post  122  of the core  104 , e.g., only around the center post  122 . In order to reduce the required width of the gap  126 , the coil  106  can have just one or two layers of windings. 
     The winding assembly  130  can also include a bobbin  132 . The bobbin  132  fits around the center post  122 , and the coil  106  is wound around the bobbin  132 . The bobbin  132  can also include a cap  136  that rests against the top surface of the post  124  to set the vertical position of the coil portion. This permits easier assembly of the sensor  102 . The bobbin can be a dielectric material, e.g., a plastic. The inner surface of the bobbin  132  can provide the outer diameter of the winding assembly. 
     The winding assembly  130  can also include a tape  134  that covers the outer surface of the coil  106 , e.g., to protect the coil  106 . The outer surface of the tape  134  can provide the inner diameter of the winding assembly. 
     The back portion  120  of the core  106  can be a generally planar body and can have a top face parallel to the top surface of the platen, e.g., parallel to the substrate and the polishing pad during the polishing operation. The back portion  120  can have a height (H) that is measured normal to the top surface of the platen. The center post  122  and the annular rim  124  extend from the back portion  120  in a direction normal to a top surface of the back portion  120  and extend in parallel with each other. The center post  122  and the annular rim  124  can have the same height. 
     In some implementations, the core  104  is generally circular. For example, the back portion  120  can be disk-shaped, the center post  122  can be circular, and the annular rim  124  can similarly be ring-shaped. However, other configurations are possible that maintain an annular configuration for the rim  124 , e.g., the center post  122  could be a square and the rim  124  could similarly trace the perimeter of a square. 
     The center post  122  has a width (W 1 ) and the annular rim  124  has a width (W 2 ), each of which can be measured along a direction parallel to the top surface of the platen, e.g., parallel to the faces of the substrate and polishing pad during the polishing operation, and are substantially linear and extend in parallel to each other. The width W 1  of the center post  122  can be substantially the minimum possible while providing the necessary magnetic flux for a clear signal. 
     The annular rim  124  is separated from the center post  122  by a gap having a width (W 3 ). The width of the gap  126  can be substantially the minimum possible while providing room for the winding assembly  130  to fit in the gap  126 . For example, the width (W 4 ) of the winding can be at least 80%, e.g., about 90%, of the width of the gap  126 . This maintains the magnetic field in a region close to the center post  122 , and increases spatial resolution. In some implementations, the outer surface of the winding assembly  130  contacts the inner surface of the annular rim  124 . 
     The widths W 1 , W 2  and W 3  can be selected such that the surface area of the annular rim  124  is larger, e.g., at least two times larger, e.g., at least three times larger, e.g., at least four times larger, than the surface area of the center post  122 . This permits more flux lines to be collected and pushed toward the inner diameter of the annular rim  124 , thereby further improving the spatial resolution. 
     For center post having a larger width, e.g., where W 1  is 3 mm or larger, the surface area of the annular rim  124  can be at least two times, e.g., two to three times, larger than the surface area of the center post  122 . For this case, the width W 3  can be up to 1 mm. For center post having a larger width, e.g., where W 1  is less than 3 mm, the surface area of the annular rim  124  can be at least four times, e.g., four to six times, larger than the surface area of the center post  122 . For this case, the width W 3  can be up to 2 mm. 
     The annular rim  124  can have a width W 2  that is larger than the half the width W 1  (e.g., larger than the radius) of the center post  122 . In some implementations, the annular rim  124  has a width W 2  that is larger than the width W 1  (e.g., larger than the diameter) of the center post  122 , e.g., at least 10% larger. For example, the center post  122  can have a width of 1.5 mm, the gap  126  can have a width of about 1 mm, and the annular rim  124  can have a width of about 1.75 mm. 
     The center post  122  and the annular rim  124  have a height Hp, which is the distance that the they extend from the back portion  120  of the core  104 . The height Hp can be greater than the widths W 1  and W 2 . In some implementations, the height Hp is the same as the distances W 3  separating the prongs  504   a - c.    
     In general, the in-situ eddy current monitoring system  400  is constructed with a resonant frequency of about 50 kHz to 50 MHz. For example, for the eddy current monitoring system  400  shown in  FIG.  4 A , the coil  422  can have an inductance of about 0.1 to 50 microH, e.g., 0.75 uH, and the capacitor  424  can have a capacitance of about 40 pF to about 0.022 uF, e.g., 150 pF. 
     The electromagnetic induction 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 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 can be a fluid bearing between the platen and polishing pad during polishing. The polishing pad can be a standard (e.g., polyurethane with or without fillers) rough pad, a soft pad, or a fixed-abrasive pad. 
     In addition, although the description above has focused on polishing, the core design can be applicable to in-situ monitoring during other substrate processing tools and steps that modify the thickness of the layer on the substrate, e.g., etching or deposition, and to in-line or stand-alone system measurements. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.