Abstract:
A polishing pad for use in polishing a surface of a substrate comprises a pad main body having a polishing surface and a plurality of electrode portions formed within the pad main body and mutually spaced apart in a plane direction of the pad main body. Each electrode portion is formed of a conductive portion and an insulating portion formed on the conductive portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-086382, filed Mar. 27, 2000, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a polishing pad, polishing apparatus and polishing method, more specifically, to a polishing pad, polishing apparatus and polishing method suitable for manufacturing a semiconductor device.  
           [0003]    Recently, with the tendency of integration and miniaturization of semiconductor devices in the field of manufacturing semiconductor devices, various micro-processing techniques have been developed. Of them, chemical mechanical polishing (CMP) is an essential technique for forming a planarized interlayer insulating film, plug, buried metal wiring and buried isolation.  
           [0004]    However, the CMP techniques have a problem. It is necessary to determine the end point of the CMP (at which point the CMP is stopped). Usually, the end point is detected by monitoring the friction force between the surface of a semiconductor wafer and a polishing pad during the polishing. The friction force is monitored by measuring the current of a motor rotating a polishing table or a polishing head. The end point is determined based on a change in measurement value of the current. However, in some cases, depending upon the polishing conditions or the structure of a surface to be polished (polish-receiving surface), the friction force does not change significantly. Particularly when the polishing is performed while a lightweight is applied, the friction force rarely changes.  
           [0005]    There is another end-point detection method, in which the end point is optically monitored. In this method, the thickness of the surface film of a semiconductor wafer under polishing is optically measured, and the change of the film type is detected based on a reflection rate of the surface of the wafer. However, this method entails technical difficulties. Light must be applied by some means to the wafer surface under polishing.  
           [0006]    To overcome the aforementioned problems, U.S. Pat. No. 4,793,895 proposes a method for monitoring current flowing on a polish-receiving surface, by a conductive electrode formed on a polishing pad. However, this method has problems. The conductive electrode exposed in the surface of the polishing pad is susceptible to corrosion with a polishing solution. In addition, the exposed surface of the conductive electrode may adversely affect the polishing characteristics of CMP. In short, the material for the electrode is limited by the use of the conductive electrode. It is actually difficult to select the material having no adverse effect upon the polishing characteristics.  
           [0007]    As described above, how to detect the end-point is one of the problems associated with the CMP technique. However, conventional end-point detection methods do not always solve this problem.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a polishing pad, a polishing apparatus, and a polishing method capable of easily and accurately determining an end point of polishing such as CMP.  
           [0009]    According to a first aspect of the present invention, there is provided a polishing pad for use in polishing a surface of a substrate; comprising: a pad main body having a polishing surface; and a plurality of electrode portions formed in the pad main body and mutually spaced apart in a plane direction of the pad main body, each of the electrode portions being formed of an conductive portion and an insulating portion formed on the conductive portion.  
           [0010]    According to a second aspect of the present invention, there is provide a polishing apparatus for polishing a surface of a substrate by attaching the polishing pad thereto, comprising: a polishing table to which the polishing pad is to be attached; a holding portion for holding the substrate such that the surface of the substrate is in contact with the polishing surface and a surface of the insulating portion of the polishing pad attached to the polishing table; a driving portion for moving the polishing table and the holding portion relative to each other; a polishing liquid supply portion for supplying a polishing liquid onto a surface of the polishing pad attached to the polishing table; a voltage application portion for applying an alternating voltage between the electrode portions of the polishing pad attached to the polishing table, whereby a current flows along the surface of the substrate held by the holding portion; and a detecting portion for detecting the current via the electrode portions of the polishing pad attached to the polishing table.  
           [0011]    According to a third aspect of the present invention, there is provide a polishing apparatus for polishing a surface of a substrate by attaching a polishing pad; comprising: a polishing table to which the polishing pad having a plurality of holes is to be attached, the polishing table including a polishing-table main body having a main surface and a plurality of electrode portions formed in the polishing-table main body and mutually spaced apart in a plane direction of the polishing-table main body, each of the electrode portions being formed of a conductive portion and an insulating portion formed on the conductive portion, each of the electrode portions having a protruding portion protruding from a plane including the main surface, the protruding portion being capable of fitting into the corresponding hole formed in the polishing pad; a holding portion for holding the substrate such that the surface of the substrate is in contact with a polishing surface of the polishing pad attached to the polishing table and a surface of the insulating portion of the polishing table; a driving portion for moving the polishing table and the holding portion relative to each other; a polishing liquid supply portion for supplying a polishing liquid onto a surface of the polishing pad attached to the polishing table; a voltage application portion for applying an alternating voltage between the electrode portions, whereby a current flows along the surface of the substrate held by the holding portion; and a detecting portion for detecting the current via the electrode portions.  
           [0012]    According to a fourth aspect of the present invention, there is provide a polishing apparatus for polishing a surface of a substrate, comprising: a polishing table including a polishing-table main body having a polishing surface and a plurality of electrode portions formed in the polishing-table main body and mutually spaced apart in a plane direction of the polishing-table main body, each of the electrode portions being formed of a conductive portion and an insulating portion formed on the conductive portion, a holding portion for holding the substrate such that the surface of the substrate is in contact with the polishing surface and a surface of the insulating portion of the polishing table; a driving portion for moving the polishing table and the holding portion relative to each other; a polishing liquid supply portion for supplying a polishing liquid onto the polishing surface of the polishing table; a voltage application portion for applying an alternating voltage between the electrode portions, whereby a current flows along the surface of the substrate held by the holding portion; and a detecting portion for detecting the current via the electrode portions.  
           [0013]    According to the present invention, it is possible to effectively detect the end point of polishing in a polishing process of a conductive film (any film such as semiconductor films e.g., polysilicon film, other than metal films may be used as long as it has a conductivity) on the surface of a substrate. More specifically, the end point of polishing can be easily and accurately determined by detecting the conductivity of the substrate surface by means of a plurality of electrode portions. As a result, the polishing is neither insufficient nor excessive and appropriately performed.  
           [0014]    The conductive portion is protected by the insulating portion. Therefore, even if a polishing solution having a strong corrosion property is used, the conductive portion is not corroded. The current is detected by applying a high-frequency alternating voltage between the electrode portions and by functioning the insulating portion as a capacitor. Since the conductive portion is not directly exposed to the polishing solution and the high-frequency alternating voltage is applied, it is possible to prevent electrolysis of the polishing solution. Furthermore, the insulating portion may be formed of a material chosen from various insulating materials. Therefore, the insulating portion may be formed of the same or analogous material to that of the polishing pad. As a result, it is successfully prevent the electrode portion from adversely affecting the polishing properties.  
           [0015]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0016]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
         [0017]    [0017]FIGS. 1A and 1B are schematic views of a structure of a polishing apparatus according to a first embodiment of the present invention;  
         [0018]    [0018]FIGS. 2A and 2B are schematic views for explaining a polishing process by the polishing apparatus according to the present invention;  
         [0019]    [0019]FIG. 3 is an arrangement of an equivalent circuit of a polishing apparatus according to the present invention, during a polishing process;  
         [0020]    [0020]FIG. 4 is a graph showing a change of current with polishing time when polishing is performed by a polishing apparatus according to the present invention;  
         [0021]    [0021]FIGS. 5A and 5B are schematic views showing a structure of a polishing apparatus according to a second embodiment of the present invention;  
         [0022]    [0022]FIGS. 6A and 6B are schematic views showing a structure of a polishing apparatus according to a third embodiment of the present invention; and  
         [0023]    [0023]FIGS. 7A and 7B are schematic views showing a structure of a polishing apparatus according to a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Now, embodiments of the present invention will be explained with reference to the accompanying drawings.  
         [0025]    (First embodiment)  
         [0026]    [0026]FIGS. 1A and 1B are schematic views of a structure of a polishing apparatus according to the present invention. FIG. 1A is a schematic side-view of the apparatus. FIG. 1B is a schematic top-view of the gist portion of the apparatus.  
         [0027]    As shown in FIGS. 1A and 1B, a polishing table  101  rotates horizontally around a rotation shaft  102  in the direction of the arrow. A polishing pad  103  is attached to the surface of the polishing table  101 .  
         [0028]    A holding portion  104  for holding a semiconductor wafer  106  is arranged above and off the center of the polishing table  101 . The holding portion  104  rotates around the rotation shaft  105 , in the same direction as that of the polishing table  101 . The holding portion  104  holds a semiconductor wafer  106  to be polished by a vacuum chuck. The semiconductor wafer  106  thus held is pressed at a predetermined pressure by a driving mechanism such as a cylinder, such that the polishing-receiving surface of the semiconductor wafer  106  comes into contact with the polishing surface of the polishing pad  103 .  
         [0029]    A slurry supply pipe  107  is disposed above the center portion of the polishing table  101 . A slurry  108  is supplied on the polishing pad  103  from the slurry supply pipe  107 .  
         [0030]    Two electrode portions  109  are buried in the main body of the polishing pad  103  formed of an insulating material. The electrode portions are concentrically arranged at a distance from each other. Each of the two electrode portions  109  has a laminate structure formed of a conductive portion  109   a  and an insulating portion  109   b  (insulating film or insulating plate). The insulating portion  109   b  is stacked on the conductive portion  109   a.  The distance between two electrode portions  109  is set at such a value that the surface of the semiconductor wafer  106  comes into contact with two electrode portions  109  (insulating portion  109   b ).  
         [0031]    Each of the two electrode portions  109  is designed to be in contact with a terminal  110 . A high frequency voltage is applied to the electrode portions  109  via the terminals  110  by a voltage generating portion  111   a.  The current flowing between the two electrode portions  109  is detected by a current detector  111   b.  The CMP operation is controlled based on the detection results.  
         [0032]    [0032]FIG. 2A is a cross-sectional view of the structure of the semiconductor wafer  106  to be subjected to CMP by the polishing apparatus shown in FIGS. 1A and 1B. A silicon oxide film  11  is formed on the main surface side of the semiconductor wafer. A metal film  12  is formed so as to fill wiring grooves formed in the silicon oxide film  11 . In the first embodiment, damascene wiring is formed by CMP on the surface of the semiconductor wafer having the aforementioned structure, as shown in FIG. 2B.  
         [0033]    The holding portion  104  (rotation speed: 50 rpm) holding the semiconductor wafer  106  is pressed against the polishing table  101  (rotation speed: 50 rpm) with the polishing pad  103  attached, at a pressure of 200 g/cm 2 . Furthermore, the slurry  108  is supplied dropwise on the polishing pad  103  from the slurry supply pipe  107  at a rate of 200 ml/min. In the case where the metal film  12  is tungsten, ferric nitrate (about 8 wt %) dissolved in an alumina dispersion solution is used as the slurry  108 . The slurry  108  is supplied between the semiconductor wafer  106  and the polishing pad  103  and then CMP is applied to the surface of the semiconductor wafer  106 .  
         [0034]    When the metal film  12  is formed over the entire surface of the semiconductor wafer  106 , a large amount of current flows between the two conductive portions  109 . However, as the thickness of the metal film  12  reduces, resistance increases. As a result, the current flowing between the two conductive portions  109  gradually decreases. On and after the metal film  12  is completely removed except within the grooves, the current value becomes constant at a low level. Therefore, the end point of the polishing process can be determined based on the change in current.  
         [0035]    [0035]FIG. 3 shows an equivalent circuit of the system of the present invention. In this circuit, the resistance of the conductive portion  109   a  is represented by r, the capacitance of the insulating portion  109   b  is C, the surface resistance of the semiconductor wafer  106  between the electrode portions  109  is r w , and the resistance associated with the ionic conductance of the slurry on the polishing pad  103  between the electrode portions  109  is r i . The resistances r w  and r i  are arranged in parallel. The resistance r and the capacitance C are connected in series to the parallel arrangement portion.  
         [0036]    The value of the resistance r w  increases as the thickness of the metal film is reduced by CMP. The resistance value varies from 1 Ω up to infinite. The resistance r is about 10 Ω. The resistance r i , although varies depending upon the type and concentration of the slurry, is usually about 100 Ω. If the frequency f of the high frequency voltage to be applied to the electrode portion  109  is increased, and the capacitance C of the insulating portion  109   b  is increased, the effect of the capacitance C can be ignored to some extent (r&gt;&gt;½πfC). To increase the capacitance C, it is necessary that the insulating portion  109   b  is formed of a material having a large dielectric constant, the film thickness of the insulating portion  109   b  is reduced and the area of the terminal  110  in contact with the insulating portion  109   b  is increased.  
         [0037]    [0037]FIG. 4 shows a graph showing current (actual value) versus polishing time. The graph is drawn on the basis of the calculation which is made under the assumption that the thickness of the metal film reduces at a constant rate, in other words, the metal film is polished at a constant polishing rate.  
         [0038]    The current reduces up to a point of tc, and thereafter it becomes constant. Therefore, the point of tc is the time point at which the metal film is just removed. This calculation is made under the assumption that the metal film is removed uniformly from the wafer at the time point tc. The thin film effect of the metal film (the increase of resistance due to electrons scattered at the interface) is ignored in the calculation. In practice, the current does not change exactly in the same manner as that of the calculation, in the vicinity of the time point t c . Taking this point into consideration, the polishing is preferably finished at the time point which is about 10% longer than the end point of at which the polishing is considered to finish.  
         [0039]    To avoid an adverse effect upon the semiconductor device, it is desirable that the voltage to be applied between the electrode portions  109  be as low as possible within the detectable range. The voltage on the order of mV or less is suitable. The high frequency voltage to be applied between the electrode portions  109  is preferably 1 kHz or more, more preferably, 10 kHz or more.  
         [0040]    In the present invention, a high frequency voltage is applied via the insulating portion  109   b.  Therefore, the insulating portion  109   b  is formed thin by using a material having a high dielectric constant. Furthermore, to avoid an adverse effect upon the polishing characteristics, it is preferable that the material having identical or analogous elasticity and viscosity to those of the polishing pad  103  be used as the material for the insulating portion  109   b.  Moreover, to avoid formation of a step between the insulating portion  109   b  and the peripheral polishing pad  103 , the insulating portion  109   b  and the polishing pad  103  are formed so as to be polished at an equal rate during the polishing process or dressing process. In addition, it is necessary to select a material which does not elude a contaminant to the wafer, after the CMP.  
         [0041]    In the present invention, not the conductive portion  109   a  but the insulating portion  109   b  is exposed. Since the material of the insulating portion  109   b  can be selected from a wide variety of materials compared to the conductive portion  109   a,  it is possible that the insulating portion  109   b  is formed of the material having the aforementioned properties. The insulating portion  109   b  is preferably formed of the same material as that of the polishing surface of the polishing pad  103 . For example, a polymer resin such as polyurethane is preferably used as a material of the insulating portion  109   b  and the polishing pad  103 .  
         [0042]    In the present invention, since the conductive portion  109   a  is covered with the insulating portion  109   b,  the conductive portion  109   a  can be successfully prevented from being corroded. Therefore, various materials also can be used in the conductive portion  109   a.    
         [0043]    (Second embodiment)  
         [0044]    [0044]FIGS. 5A and 5B are schematic views showing a structure of a polishing apparatus according to a second embodiment of the present invention. FIG. 5A is a schematic side-view of the apparatus. FIG. 5B is a schematic top-view of the gist portion of the apparatus.  
         [0045]    As shown in FIGS. 5A and 5B, a polishing table  201  is designed to rotate horizontally around a rotation shaft  202  in the direction of the arrow. A polishing pad  203  is attached to the surface of the polishing table  201 .  
         [0046]    A holding portion  204  for holding a semiconductor wafer  206  is arranged above and off the center of the polishing table  201 . The holding portion  204  rotates around the rotation shaft  205  in the same direction as that of the polishing table  201 . The holding portion  204  holds a semiconductor wafer  206  by a vacuum chuck. The semiconductor wafer  206  is pressed at a predetermined pressure by a driving mechanism e.g., a cylinder such that the polishing-receiving surface of the semiconductor wafer  206  thus held is in contact with the polishing surface of the polishing pad  203 .  
         [0047]    A slurry supply pipe  207  is disposed above the center portion of the polishing table  201 . A slurry  208  is supplied onto the polishing pad  203  from the slurry supply pipe  207 .  
         [0048]    Two cylindrical electrode portions  209  are buried at a distance from each other in the main body of the polishing pad  203  formed of an insulating material. Each of the two electrode portions  209  has a laminate structure formed of a conductive portion  209   a  and an insulating portion  209   b  (insulating film or insulating plate), which is stacked on the conductive portion  209   a.  The distance between two conductive portions  209  is set in such a manner that the surface of the semiconductor wafer  206  is in contact with the two electrode portions  209  (insulating portion  209   b ) during polishing.  
         [0049]    The two electrode portions  209  are electrically connected to two conducting poles  212 , respectively. Each of the conducting poles  212  is connected to a conducting line  214  at the rear portion of the polishing table  201 . The conducting line  214  is electrically connected to a rotation terminal  215  formed around the rotation shaft  202  of the polishing table. A terminal  210  is further in contact with the rotation terminal  215 . With this structure, a high frequency voltage is applied between two electrode portions  209  from a voltage generating portion  211   a.  The current flowing between the two electrode portions  209  is detected by a current detecting portion  211   b.  The CMP operation is controlled based on the detection results.  
         [0050]    In the case where the polishing table  201  is made of a conductive material, an insulating tube  213  must be formed around the conducting pole  212 . In the case where the rotation shaft  202  of the polishing table is made of a conductive material, an insulating material must be provided between the rotation terminal  215  and the rotation shaft  202 .  
         [0051]    The same voltage as that of the first embodiment is applied between the electrode portions  209 . The electrode portions  209  are formed of the same material as in the first embodiment. The equivalent circuit used herein is the same as in the first embodiment.  
         [0052]    The substrate shown in FIG. 2A is subjected to CMP under the same conditions and in the same manner as in the first embodiment. As a result, damascene wiring (shown in FIG. 2B) is obtained.  
         [0053]    When the polishing table  201  is rotated, time zone A and time zone B are produced. In the time zone A, the semiconductor wafer  206  is in contact with the two electrode portions  209  (insulating portion  209   b ). In the time zone B, they are not in contact with each other. In the time zone A, both a surface conductivity of the semiconductor wafer  206  and the ion conductivity of the slurry interposed between the semiconductor wafer  206  and the polishing pad  203  contribute to the current, whereas, in the time zone B, only the ionic conductivity of the slurry placed on the polishing pad  203  contributes to the current.  
         [0054]    When the metal film  12  is present over the entire surface of the wafer in the time zone A, a large amount of current flows between the two electrode portions  209 . Since the resistance increases as the metal film  12  is reduced in thickness, the current flowing between the two electrode portions  209  gradually decreases. On and after the time the metal film  12  is completely removed except within a groove, the current value becomes constant at a low level. This is because only the ionic conductivity of the slurry contributes to the current. On the other hand, in the time zone B, the current value becomes constant at a low level no matter how thick the metal film  12  is. This is because only the ionic conductivity of the slurry contributes to the current.  
         [0055]    As described above, there are two time zones A and B, so that the current (actual value) changes in a pulse fashion. Since the rotation number of the polishing table  201  is 50 rpm, the pulse has a period of 1.2 sec. The height of the pulse gradually reduces. Although the current changes in a pulse fashion, the current changes basically in the same manner as that of FIG. 4. Therefore, the end point of polishing can be determined in the same manner as in the first embodiment.  
         [0056]    (Third embodiment)  
         [0057]    [0057]FIGS. 6A and 6B are schematic views of a structure of a polishing apparatus according to a third embodiment of the present invention. FIG. 6A is a schematic side-view of the apparatus. FIG. 6B is a schematic top-view of the gist portion of the apparatus. The basic structure is analogous to that of the second embodiment. The like reference numerals are used to designate like structural elements of FIGS. 6A and 6B corresponding to those like in FIGS. 5A and 5B.  
         [0058]    In this embodiment, the top portion of each of the conducting poles  212  is allowed to protrude from the upper surface of the polishing table  201 , as shown in FIGS. 6A and 6B. The protruding portion is designated by a reference numeral  222 . The polishing pad  203  has holes corresponding to the protruding portions  222 . The polishing pad  203  is attached to the polishing table  201  by inserting the protruding portions  222  through the holes. The upper surface of the conducting pole  212  is covered with an insulating portion  221  (insulating film or insulating plate). The insulating portion  221  may be provided in the polishing pad  203 . The insulating portion  221  may be formed of the same material as employed in the previous embodiments. The upper surface of the protruding portion  222 , in other words, the upper surface of the insulating portion  221 , is on the same level with the upper surface of the polishing pad  203 .  
         [0059]    The equivalent circuit and the method of determining the end point of the polishing are the same as in previous embodiments.  
         [0060]    According to this embodiment, the upper portions of the conducting poles  212  and the insulating portions  221  are allowed to protrude from the upper surface of the polishing table  201  to form the protruding portions  222 . All things to do is to form holes for the protruding portions  222  in the polishing pad  203 . Therefore, the polishing pad  203  can be formed at low cost. In this respect, this embodiment is advantageous.  
         [0061]    (Fourth embodiment)  
         [0062]    [0062]FIGS. 7A and 7B are schematic views of a structure of a polishing apparatus according to a fourth embodiment of the present invention. FIG. 7A is a schematic side-view of the apparatus. FIG. 7B is a schematic top-view of the gist portion of the apparatus. The basic structure of the apparatus is analogous to that of the second embodiment. The like reference numerals are used to designate like structural elements of FIGS. 7A and 7B corresponding to those like in FIGS. 5A and 5B.  
         [0063]    In the aforementioned embodiments, the polishing pad is detachably provided to the polishing table. However, in this embodiment, a substrate is polished by a polishing surface integrated into the polishing table.  
         [0064]    The surface region  232  of the polishing table  231  is used as a polishing surface, as shown in FIGS. 7A and 7B. The conducting pole  212  is formed in the polishing table in the same manner as in FIGS. 6A and 6B. The upper surface of the conductive pole  212  is covered with an insulating portion  233 . The insulating portion  233  is formed of the same material as used in the previous embodiments. More specifically, the insulating portion  233  is formed of the same or analogous material as that of the surface region  232 . The insulating portion  233  is formed at the same level as that of the surface region  232 .  
         [0065]    In this embodiment, the equivalent circuit and the method of determining the end point of the polishing are the same as those of the previous embodiments. The same effects can be obtained.  
         [0066]    In the embodiments mentioned above, two electrode portions are formed. The number of the electrodes may be increased. A plurality of electrodes may be brought into contact with the surface of the semiconductor wafer. Where a plurality of electrode portions is used, the degrees of polishing at the center and peripheral portion of the wafer can be monitored. In other words, it is possible to monitor the in-plane uniformity of a semiconductor wafer.  
         [0067]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.