Patent Publication Number: US-10759020-B2

Title: Calibration method for eddy current sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of priority from Japanese Patent Application No. 2017-087080 filed on Apr. 26, 2017, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a calibration method for an eddy current sensor. 
     BACKGROUND ART 
     In connection with enhanced integration and increased density of semiconductor devices, wires of circuits have recently become finer and finer, and the number of layers of multilayer interconnections has also increased. In order to realize multilayer interconnection while micronizing a circuit, it is necessary to precisely perform planarization processing on the surface of a semiconductor device. 
     CMP (Chemical Mechanical Polishing) is known as a technique of planarizing the surface of a semiconductor device. A polishing apparatus for performing CMP includes a polishing table to which a polishing pad is stuck, and a top ring for holding a polishing target (for example, a substrate such as a semiconductor wafer, or each kind of film formed on the surface of a substrate). The polishing apparatus polishes the polishing target by pressing the polishing target held by the top ring against the polishing pad while rotating the polishing table. 
     The polishing apparatus includes a film thickness measuring device to detect an endpoint of a polishing step based on the film thickness of a polishing target. The film thickness measuring device has a film thickness sensor for detecting the film thickness of the polishing target. An eddy current sensor is known as a representative of the film thickness sensor. 
     The eddy current sensor is disposed in a hole formed in a polishing table, and detects the film thickness of the polishing target when it faces the polishing target while rotated along with the polishing table. The eddy current sensor causes the polishing target such as a conductive film to induce eddy current therein, and detects variation of the thickness of the polishing target from variation of magnetic field occurring due to the eddy current induced in the polishing target. In order to use the eddy current sensor as a film thickness sensor, a calibration for obtaining a correspondence relationship between the film thickness and the measurement value of the eddy current sensor is necessary before an actual measurement is started. 
     The conventional calibration has been performed as follows. When the eddy current sensor is mounted in the polishing apparatus, a polishing pad is interposed between the conductive film as the polishing target and the eddy current sensor. When the thickness of the polishing pad varies, the output of the sensor also varies. In a prior art described in JP2007-263981A, the calibration is performed by using plural polishing pads having different pad thicknesses and plural calibration wafers having different film thicknesses. Since different pad thicknesses and different film thicknesses are combined with one another, measurement is performed a number of times to obtain a correspondence relationship between the film thickness of the polishing target and the measurement value of the eddy current. 
     The prior art has also the following problems. First, since the plural polishing pads for calibration are disposed on the polishing table, it is necessary to peel off a polishing pad which has already been stuck to the polishing table. Accordingly, one polishing pad must be discarded for the calibration, resulting in increase of the cost. Secondly, since a calibration wafer is manually placed on the polishing pad, there is a problem that the positional precision is low. Additionally, an error may occur, since dust or the like is attached to the polishing pad or the calibration wafer due to manual operation. Thirdly, since the calibration wafer is repetitively used, oxidation or the like occurs, so that deterioration of the calibration wafer may cause an error. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP2007-263981A 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     A mode of the present invention has been implemented to dissolve the problem as described above, and has an object to provide a calibration method for an eddy current sensor that is capable of performing a calibration without peeling off a polishing pad. 
     Solution to Problem 
     In order to solve the foregoing problem, a first aspect adopts a configuration of a calibration method for an eddy current sensor for determining a correspondence relationship between a film thickness of a polishing target and a measurement value of the eddy current sensor to measure the film thickness of the polishing target by the eddy current sensor when the polishing target is polished while pressed against a polishing face, comprising: a first step of measuring an output of the eddy current sensor while the polishing target whose film thickness has been known is in contact with the polishing face, thereby obtaining a measurement value of the eddy current sensor which corresponds to the film thickness; and a second step of measuring an output of the eddy current sensor when the polishing target is polished while pressed against the polishing face, thereby obtaining a measurement value of the eddy current sensor that corresponds to a film thickness during polishing, wherein the correspondence relationship between the film thickness of the polishing target and the measurement value of the eddy current sensor is determined from the measurement value obtained in the first step and the measurement value obtained in the second step. 
     In this embodiment, no calibration wafer is used. A polishing target (for example, wafer) which is an actual product whose film thickness has been known can be used. Plural polishing pads having different pad thicknesses are unnecessary, and plural calibration wafers having different film thicknesses are unnecessary. It is not necessary to peel off a polishing pad which has already been struck to a rotating table. A calibration can be performed by using a polishing pad which has already been struck to the rotating table. 
     The film thickness of the polishing target means the film thickness of a film formed on the surface of the polishing target. When plural films are formed on the surface of the polishing target, the film thickness means the film thickness of a film which is located at the outermost portion and is to be polished. The measurement value of the eddy current sensor indicates a signal or data which is directly or indirectly obtained from the output of the eddy current sensor. 
     A second aspect adopts a configuration of a calibration method in which the second step is performed by using the polishing target used in the first step. 
     A third aspect adopts a configuration of a calibration method in which the polishing target used in the first step and the polishing target used in the second step are different from and independent of each other. 
     A fourth aspect adopts a configuration of a calibration method further comprising a step of measuring an output of the eddy current sensor while the eddy current sensor is moved from one end of the polishing target to another end of the polishing target on the polishing target, and a step of determining a rate of change of an obtained measurement value at each point on the polishing target, detecting positions of the one end and the other end of the polishing target from the rates of change, and determining a center position of the polishing target from the detected positions of the one end and the other end. 
     A fifth aspect adopts a configuration of a calibration method further comprising a third step of measuring an output of the eddy current sensor at least two points on a route from one end of the polishing target toward another end of the polishing target while the eddy current sensor is moved along the route, setting the measurement value at a predetermined position on the route as a reference value, and obtaining information on a first difference between the measurement value at each position on the route and the reference value, and a fourth step of determining, based on the information, a second difference between the measurement value obtained in the first and second steps and the difference at a position on the route which corresponds to each point at which the first measurement value is obtained. 
     A sixth aspect adopts a configuration of a calibration method further comprising a third step of measuring an output of the eddy current sensor at at least one point on a route from one end of the polishing target toward another end of the polishing target while the eddy current sensor is moved along the route when polishing of the polishing target is finished, and obtaining information on a measurement value at each position on the route when the polishing of the polishing target is finished, and a fourth step of determining, based on the information, a difference between the measurement value obtained in the first and second steps and the measurement value when the polishing is finished at a position on the route which corresponds to each point at which the measurement value is obtained. 
     A seventh aspect adopts a configuration of a calibration method for an eddy current sensor for determining a center position of a polishing target to measure a film thickness of the polishing target by the eddy current sensor when the polishing target is polished while pressed against a polishing face, comprising: a step of measuring an output of the eddy current sensor when the eddy current sensor is moved from one end of the polishing target to another end thereof on the polishing target; and a step of determining a rate of change of an obtained measurement value at each point on the polishing target, detecting positions of the one end and the other end of the polishing target from the rates of change, and determining a center position of the polishing target from the detected positions of the one end and the other end. 
     An eighth aspect adopts a configuration of a calibration method for an eddy current sensor for determining a variation of a measurement value on a route from one end of a polishing target toward another end thereof, the variation occurring when an eddy current sensor is moved along the route, to measure a film thickness of the polishing target by the eddy current sensor when the polishing target is polished while pressed against a polishing face, comprising: measuring an output of the eddy current sensor at least two points on a route from one end of the polishing target toward another end of the polishing target while the eddy current sensor is moved along the route, setting the measurement value at a predetermined position on the route as a reference value, and obtaining information on a first difference between the measurement value at each position on the route and the reference value, and determining, based on the information, a second difference between a measurement value obtained by the eddy current sensor in an actual measurement after a calibration is finished, and the first difference at a position on the route which corresponds to each point at which the measurement value is obtained. 
     A ninth aspect adopts a configuration of a calibration method for an eddy current sensor that takes account of a measurement value of the eddy current sensor under a state that polishing of a polishing target is finished in order to measure a film thickness of the polishing target by the eddy current sensor when the polishing target is polished while pressed against the polishing face, comprising: measuring an output of the eddy current sensor at at least one point on a route from one end of the polishing target toward another end of the polishing target while the eddy current sensor is moved along the route when the polishing of the polishing target is finished, and obtaining information on the measurement value at each position on the route when the polishing of the polishing target is finished; and determining, based on the information, a difference between a measurement value obtained by the eddy current sensor in an actual measurement after a calibration is finished, and the measurement value when the polishing is finished at a position on the route which corresponds to each point at which the measurement value is obtained. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing the overall configuration of a polishing apparatus; 
         FIG. 2  is a block diagrams showing an exemplified configuration of an eddy current sensor for measuring impedance; 
         FIG. 3  is an equivalent circuit diagram of the block diagram of  FIG. 2 ; 
         FIG. 4  is a perspective view showing an exemplified configuration of a sensor coil of the eddy current sensor; 
         FIG. 5  is a circuit diagram showing a connection example of the sensor coil of  FIG. 4 ; 
         FIG. 6  is a block diagram showing a synchronous detection circuit of a sensor coil output; 
         FIG. 7  is a graph showing a circular locus of a resistance component (X) and a reactance component (Y) on an impedance coordinate plane with variation of the thickness of a conductive film; 
         FIG. 8  is a graph obtained by counterclockwise rotating the graph figure of  FIG. 7  by 90 degrees and then translating the graph figure; 
         FIG. 9  is a graph showing variation of an arc-shaped locus on the coordinates X, Y according to the distance corresponding to the thickness of a polishing pad being used; 
         FIG. 10  is a diagram showing that an angle α is identical irrespective of difference of the thickness of a polishing pad  108 ; 
         FIG. 11  is a diagram showing a proportional connection between 1/tan α (=Ta) and the film thickness t; 
         FIG. 12  is a measurement point under clear polishing; 
         FIG. 13A  shows the magnitude of a signal obtained by a measurement, and  FIG. 13B  shows the rate of change (differential or difference) of the signal; 
         FIG. 14  is a plan view showing a polishing table portion of  FIG. 1 ; 
         FIG. 15  is a diagram showing baseline processing and zero calibration processing; and 
         FIG. 16  is a flowchart showing the overall calibration method. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments according to the present invention will be described with reference to the drawings. In the following embodiments, the same or corresponding members are represented by the same signs, and duplicative descriptions thereof are omitted. 
       FIG. 1  is a schematic diagram showing the overall configuration of a polishing apparatus according to an embodiment of the present invention. As shown in  FIG. 1 , a polishing apparatus  100  includes a polishing unit  150  for polishing a polishing target (for example, a substrate such as a semiconductor wafer, or each kind of film formed on the surface of the substrate)  102 . The polishing unit  150  includes a polishing table  110  having an upper surface on which a polishing pad  108  for polishing the polishing target  102  can be fitted, a first electric motor  112  for rotating the polishing table  110 , a top ring  116  capable of holding the polishing target  102  and a second electric motor  118  for rotating the top ring  116 . 
     The polishing unit  150  includes a slurry line  120  for supplying abrasive liquid containing a polishing material onto the upper surface of the polishing pad  108 . The polishing apparatus  100  includes a polishing apparatus controller  140  for outputting various kinds of control signals related to the polishing unit  150 . 
     The polishing apparatus  100  includes an eddy current sensor  210  that is disposed in a hole formed in the polishing table  110  and detects the film thickness of the polishing target  102  along a polishing face with the rotation of the polishing table  110 . 
     When polishing the polishing target  102 , the polishing apparatus  100  supplies polishing slurry containing abrasive grains from the slurry line  120  to the upper surface of the polishing pad  108 , and rotates the polishing table  110  by the first electric motor  112 . Then, the polishing apparatus  100  presses the polishing target  102  held by the top ring  116  against the polishing pad  108  while the top ring  116  is rotated around a rotational axis which is eccentric from the rotational axis of the polishing table  110 . As a result, the polishing target  102  is polished by the polishing pad  108  which holds the polishing slurry, whereby the polishing target  102  is planarized. 
     A receiver  232  is connected to the eddy current sensor  210  via rotary joint connectors  160  and  170 . The receiver  232  receives a signal output from the eddy current sensor  210 , and outputs the signal as impedance. 
     As shown in  FIG. 1 , a film thickness measuring device  230  performs predetermined signal processing on the impedance output from the receiver  232 , and outputs the processed impedance signal to an endpoint detector  240 . 
     The endpoint detector  240  monitors the change of the film thickness of the polishing target  102  based on the signal output from the film thickness measuring device  230 . The endpoint detector  240  is connected to the polishing apparatus controller  140  for performing various kinds of control related to the polishing apparatus  100 . When detecting a polishing endpoint of the polishing target  102 , the endpoint detector  240  outputs a signal representing the detection of the polishing endpoint to the polishing apparatus controller  140 . When receiving the signal representing the polishing endpoint from the endpoint detector  240 , the polishing apparatus controller  140  causes the polishing apparatus  100  to finish the polishing. The polishing apparatus controller  140  controls the press force being applied to the polishing target  102  based on corrected film thickness data during polishing. 
     Here, a calibration in this embodiment will be briefly described. When the film thickness is measured by the eddy current sensor  210 , it is necessary to determine a correspondence relationship between data obtained from the output of the eddy current sensor  210  and the film thickness in advance. In this embodiment, an angle α is determined from the output of the eddy current sensor  210 . Definition of the angle α and how to determine the angle α will be described later. 
     As described later, 1/tan α calculated from the angle α and the film thickness t are proportional to each other. That is, for 1/tan α=Ta, the relation of the film thickness t=A_th×Ta is satisfied. Here, A_th represents a factor of proportionality. In an actual measurement of the film thickness, Ta can be obtained from a measurement value of the eddy current sensor  210 . Accordingly, the factor of proportionality A_th in the correspondence relationship between the output of the eddy current sensor  210  and the film thickness which is represented by “film thickness t=A_th×Ta” may be determined in the calibration. When the factor of proportionality A_th is determined, the film thickness can be calculated by determining the angle α from the output of the eddy current sensor  210  in the actual measurement after the calibration. The measurement value of the eddy current sensor  210  which is obtained from the output of the eddy current sensor  210  means impedance (X,Y) described later, or the angle α described above, tan α, 1/tan α, Ta, etc. 
       FIG. 2  shows the eddy current sensor  210  equipped with the polishing apparatus  100 . In the eddy current sensor, the impedance when viewing a conductive film side from a sensor coil of the eddy current sensor  210  varies, and the film thickness is detected from the variation of the impedance. The eddy current sensor  210  is configured such that the sensor coil is arranged in the neighborhood of the polishing target  102  as a detection target, and an AC signal source  124  is connected to the sensor coil. Here, the polishing target  102  as the detection target is a copper-plated film (may be a deposition film of a metal material such as Au, Cr or W) which is formed, for example, on a semiconductor wafer W and has a thickness of about 0 to 2 μm. The sensor coil is arranged in the neighborhood of the conductive film as the detection target, for example, at a distance of about 0.5 to 5 mm from the conductive film as the detection target. A synchronous detection circuit  126  detects an impedance Z (whose components are X and Y) including the polishing target  102  as the detection target when viewed from the sensor coil side (described in detail later). 
     In an equivalent circuit shown in  FIG. 3 , the oscillation frequency of the AC signal source  124  is fixed, and when the film thickness of the polishing target  102  changes, the impedance Z when viewing the sensor coil side from the AC signal source  124  varies. That is, in the equivalent circuit shown in  FIG. 3 , an eddy current I 2  flowing in the polishing target  102  is determined by equivalent resistance R 2  and self-inductance L 2  of the polishing target  102 . When the film thickness changes, the eddy current I 2  varies, and it is captured as a variation of the impedance Z when viewed from the AC signal source  124  side through mutual inductance M with the sensor coil side. Here, L 1  represents a self-inductance component of the sensor coil, and R 1  represents a resistance component of the sensor coil. 
     The eddy current sensor will be specifically described hereunder. The AC signal source  124  is an oscillator having a fixed frequency of about 1 to 50 MHz, and for example, a crystal oscillator is used. Current I 1  flows in the sensor coil with an AC voltage supplied from the AC signal source  124 . The flow of current in the coil arranged in the neighborhood of the polishing target  102  makes a magnetic flux interlinked across the polishing target  102  to form mutual inductance M therebetween, so that the eddy current I 2  flows in the polishing target  102 . Here, R 1  represents equivalent resistance on the primary side containing the sensor coil, and L 1  likewise represents self-inductance on the primary side containing the sensor coil. On the polishing target  102  side, R 2  represents the equivalent resistance corresponding to an eddy current loss, and L 2  represents the self-inductance of the polishing target  102 . The impedance Z when viewing the sensor coil side from terminals  128 ,  130  of the AC signal source  124  varies depending on the magnitude of the eddy current loss formed in the polishing target  102 . 
       FIG. 4  shows an exemplified configuration of the sensor coil in the eddy current sensor according to this embodiment. The sensor coil is configured so that a coil for forming an eddy current in the conductive film and a coil for detecting the eddy current of the conductive film are separated from each other, and comprises three-layer coils wound around a bobbin  311 . Here, an exciting coil  312  at the center is an exciting coil connected to the AC signal source  124 . The exciting coil  312  forms an eddy current in the polishing target  102  on the semiconductor wafer W arranged in the neighborhood of the exciting coil  312  by magnetic field which is formed by a voltage supplied from the AC signal source  124 . A detection coil  313  is arranged on the upper side (the conductive film side) of the bobbin  311 , and detects a magnetic field generated by the eddy current formed in the conductive film. A balance coil  314  is arranged on the opposite side to the detection coil  313  of the exciting coil  312 . 
       FIG. 5  shows a connection example of each coil. The detection coil  313  and the balance coil  314  constitute an in-series circuit of opposite phases as described above, and both the ends thereof are connected to a resistance bridge circuit  317  containing a variable resistor  316 . The coil  312  is connected to an AC signal source  203 , and generates an alternating magnetic flux to form an eddy current in a conductive film  201 ′ arranged in the neighborhood of the coil  312 . By adjusting the resistance values of the variable resistors VR 1 , Vr 2 , the output voltage of the in-series circuit comprising the coils  313  and  314  is adjustable to be equal to zero when the conductive film is absent. 
       FIG. 6  shows an example of a measurement circuit for the impedance Z when viewing the sensor coil  202  side from the AC signal source  203  side. In the measurement circuit for the impedance Z shown in  FIG. 6 , an impedance plane coordinate value (X,Y) (that is, a resistance component (X), a reactance component (Y)), an impedance (Z=X+iY), and a phase output (θ=tan −1 R/X) which depend on the variation of the film thickness can be taken out. Accordingly, by using these signal outputs, it is possible to detect the progress status of more multifaceted processing, for example, measure the film thickness based on the magnitude of each kind of component of the impedance. 
     As described above, the signal source  203  for supplying an AC signal to the sensor coil arranged in the neighborhood of the semiconductor wafer W on which the polishing target  102  as the detection target is formed is an oscillator having a fixed frequency comprising a crystal oscillator. The AC signal source  203  supplies a voltage having a fixed frequency of 1 to 50 MHz for example. The AC voltage formed in the signal source  203  is supplied to the exciting coil  312  via a bandpass filter  302 . Signals detected at the terminals  128 ,  130  of the sensor coil are passed through a high-frequency amplifier  303  and a phase shift circuit  304 , and then input to a synchronous detector comprising a cos synchronous detection circuit  305  and a sin synchronous detection circuit  306 . A cos component (X component) and a sin component (Y component) of the detection signal are taken out by the synchronous detector. Here, two signals of an in-phase component (0°) and an orthogonal component (90°) of the signal source  203  are formed from an oscillation signal formed in the signal source  203  by the phase shift circuit  304 . These signals are introduced to the cos synchronous detection circuit  305  and the sin synchronous detection circuit  306  respectively to perform the synchronous detection described above. 
     Unnecessary components having high frequencies of not less than the frequency of a signal component, for example, high-frequency components of 5 KHz or more are removed from the synchronously detected signal by low pass filters  307  and  308 . The synchronously detected signal includes an X component output as a cos synchronous detection output and a Y component output as a sin synchronous detection output. The magnitude of the impedance Z, (X 2 +Y 2 ) 1/2  is obtained from the X component output and the Y component output by a vector calculation circuit  309 . Furthermore, a phase output (θ=tan −1 Y/X) is likewise obtained from the X component output and the Y component output by a vector calculation circuit (θ processing circuit)  310 . Here, these filters are provided to remove noise components of the sensor signal, and cut-off frequencies corresponding to various kinds of filters are set. 
     Next, points (coordinate values (X,Y)) on the impedance plane coordinate system which correspond to impedances obtained for different distances between the polishing target  102  and the eddy current sensor  210  form different circles. The respective centers of the different circles are located on the same line (second line). A first point is one common point to the different circles. These matters will be described. 
     The following expressions are satisfied in a sensor side circuit and a conductive film side circuit shown in  FIG. 3 , respectively:
 
 R   1   I   1   +L   1   dI   1   /dt+MdI   2   /dt=E   (1)
 
 R   2   I   2   +L   2   dI   2   /dt+MdI   1   /dt= 0  (2)
 
     Here, M represents the mutual inductance, R 1  represents the equivalent resistance of the sensor side circuit, and L 1  represents the self-inductance of the sensor side circuit. R 2  represents the equivalent resistance of the conductive film in which an eddy current is induced, and L 2  represents the self-inductance of the conductive film in which the eddy current flows. 
     Here, when I n =A n e jωt  (sine wave) is set, the foregoing expressions (1) and (2) are represented as follows:
 
( R   1   +jωL   1 ) I   1   +jωMI   2   =E   (3)
 
( R   2   +jωL   2 ) I   2   +jωMI   1 =0  (4)
 
     From these expressions (3) and (4), the following expression (5) is derived. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     Accordingly, the impedance Z of the sensor side circuit is represented by the following expression (6):
 
 Z=E/I   1   ={R   1 +ω 2   M   2   R   2 /( R   2   2 +ω 2   L   2   2 )}+ jω{L   1 −ω 2   L   2   M   2 /( R   2   2 +ω 2   L   2   2 )}  (6)
 
     Here, when the real part of Z (resistance component) and the imaginary part of Z (inductive reactance component) are represented by X and Y, the foregoing expression (6) is represented as follows:
 
 Z=X+jωY   (7)
 
     Here, when Rx=ω 2 M 2 /(R 2   2 +ω 2 L 2   2 ) is set, the expression (7) is represented as follows:
 
 X+jωY =[ R   1   +R   2   R   x ]+ j ω[ L   1   −L   2   R   x] 
 
     Accordingly, X=R 1 +R 2 R x , and Y=ω[L 1 −L 2 R x ] are obtained. 
     By solving these expressions for R 2  and L 2 ,
 
 R   2 =ω 2 ( X−R   1 ) M   2 /((ω L   1   −Y ) 2 +( X−R   1 ) 2 )  (8)
 
 L   2 =ω(ω L   1   −Y ) M   2 /((ω L   1   −Y ) 2 +( X−R   1 ) 2 )  (9)
 
     A symbol k shown in  FIG. 7  represents a coupling coefficient, and the following relational expression (10) is satisfied:
 
 M=k ( L   1   L   2 ) 1/2   (10)
 
     By applying this expression to (9),
 
( X−R   1 ) 2 +( Y −ω(1−( k   2 /2)) L   1 ) 2 =(ω L   1   k   2 /2) 2   (11)
 
     This expression represents an equation of a circle, and it represents that X and Y form a circle, that is, the impedance Z forms a circle. 
     The eddy current sensor  210  outputs the resistance component X and the inductive reactance component Y of the impedance of the electrical circuit containing the coils of the eddy current sensor  210 . The resistance component X and the inductive reactance component Y are film thickness signals reflecting the film thickness, and vary depending on the thickness of the conductive film on the substrate. 
       FIG. 7  is a diagram showing a graph which is drawn by plotting X and Y varying together with the film thickness of the conductive film on the XY coordinate system. The coordinates of a point T∞ represent X and Y when the film thickness is infinite, that is, R 2  is equal to zero. The coordinate of a point T 0  (first point) represents X and Y when the film thickness is equal to zero, that is, R 2  is infinite if the conductivity of the substrate is non-negligible. A point Tn (second point) to be positioned from the values of X and Y advances to the point T 0  while drawing an arc locus as the thickness of the conductive film decreases. 
       FIG. 8  is a diagram showing a graph obtained by counterclockwise rotating the graph figure of  FIG. 7  by 90 degrees and further translating the graph figure. As shown in  FIG. 8 , the point Tn to be positioned from the values of X and Y advances to the point T 0  while drawing an arc locus as the film thickness decreases. The coupling coefficient k represents a rate at which magnetic field generated by one coil is transmitted to another coil. The maximum value of k is equal to 1, and when the distance between the coils increases, that is, the thickness of the polishing pad  108  increases, k decreases. 
     A distance G between the coil of the eddy current sensor  210  and a substrate W changes depending on the thickness of the polishing pad  108  interposed between the coil and the substrate W. As a result, the arc locus of the coordinates X, Y change depending on the distance G (G 1  to G 3 ) corresponding to the thickness of the polishing pad  108  to be used as shown in  FIG. 9 . As is apparent from  FIG. 9 , the coordinates X, Y for the same film thickness for the distances G 1  to G 3  are connected to one another by a line (hereinafter referred to as an equal film thickness line (first lines)), and in this case, equal film thickness lines each of which passes through the coordinates X, Y for the same film thickness intersect to one another at an intersection point P irrespective of the distance G between the coil and the polishing target  102 . The point P is the first point T 0 . This equal film thickness line m (n: 1, 2, 3, . . . ) inclines at an angle α corresponding to the thickness of the conductive film (polishing target  102 ) with respect to the diameter (second line)  12  passing through the first point in  FIG. 9 . The diameters (second lines) of circles passing through the first point are identical to one another irrespective of the distance G. 
     The angle α is an intersecting angle at which the first line for connecting the first point (T 0 ) corresponding to the impedance for the film thickness of zero and the second point (Tn) corresponding to the impedance for the non-zero film thickness and the diameter of the circle passing through the first point (T 0 ) intersect to each other. When the thickness of the conductive film is equal, the angle α is equal irrespective of the difference in thickness of the polishing pad  108 . This point will be described with reference to  FIG. 10 . 
     The coordinate (X,Y) of the point Tn is represented by using the angle α shown in  FIG. 10 . From  FIG. 10 ,
 
 X=R   1 +ω( k   2 /2) L   1  sin α  (12)
 
 Y =ω(1−( k   2 /2) L 1−ω( k   2 /2) L   1  cos α  (13)
 
     From (8) and (9) described above,
 
 R   2   /L   2 =ω( X−R   1 )/(ω L   1   −Y )
 
     By substituting (12) and (13) into the above expression,
 
 R   2   /L   2 =ω sin 2/α(1+cos 2α)=ω tan α  (14)
 
     R 2 /L 2  is dependent only on the film thickness, and also is not dependent on the coupling coefficient k. Therefore, it is not dependent on the distance between the eddy current sensor  210  and the polishing target  102 , that is, the thickness of the polishing pad  108 . R 2 /L 2  is dependent only on the film thickness, and thus the angle α is also dependent only on the film thickness. A film thickness calculator calculates the tangent of the angle α, and determines the film thickness from the tangent by using the relationship of (14). 
     A method of calculating the angle α and a method of calculating the film thickness will be described. When an eddy current formable in the polishing target  102  is detected as impedance by the eddy current sensor  210  to measure the film thickness of the polishing target, the film thickness measuring device  230  of  FIG. 1  receives the impedance from the receiver  232 , and determines the film thickness from the received impedance. The film thickness measuring device  230  includes an angle calculator  234  and a film thickness calculator  238 . 
     The angle calculator  234  calculates the angle α at which the first line for connecting the first point T 0  corresponding to the impedance for the film thickness of zero and the second point Tn corresponding to the impedance for the non-zero film thickness and a diameter  12  of the circle passing through the first pint T 0  intersect to each other. The film thickness calculator  238  calculates the tangent of the angle α, and determines the film thickness from the tangent. 
     Next, the film thickness calculator  238  for determining the film thickness from the tangent will be described. In this embodiment, the relationship between the reciprocal of the tangent and the film thickness is utilized. First, the relationship between the reciprocal of the tangent and the film thickness will be described. 
     The relationship of the foregoing (14), that is, the following expression is known between the tangent and the resistance value of the metal film (conductive film).
 
 R   2   /L   2 =ω tan α  (14)
 
Here, R 2  represents the resistance value of the metal film. Accordingly, R 2  and tan α are proportional to each other. Furthermore, R 2  has the following relationship with the film thickness.
 
 R   2   =ρL/tW   (15)
 
     Here, ρ: resistivity, L, W: the length and width of the metal film, and t: film thickness. 
     From (14) and (15), it is apparent that the film thickness t and the angle α have the following relationship:
 
 R   2 ∝(½)∝ω tan α
 
     That is, 1/tan α∝t, and thus 1/tan α and the film thickness t are proportional to each other. The method of calculating the film thickness as described above will be described next. 
     First, the resistance component (X) and the reactance component (Y) on the impedance coordinate plane are obtained by the eddy current sensor  210  and the receiver  232 . Next, tan α is calculated by the foregoing method in the angle calculator  234 . 1/tan α and the film thickness t are proportional to each other. The film thickness t is determined from 1/tan α based on the proportional connection described later. 
     Next, a calibration to be performed in advance before the forgoing actual measurement will be described. According to this embodiment, in the calibration of the eddy current sensor  210 , when the polishing target  102  is polished while pressed against the surface (polishing face  104 ) of the polishing pad  108 , a correspondence relationship between the film thickness of the polishing target  102  and the measurement value of the eddy current sensor  210  is determined in order to measure the film thickness of the polishing target  102  by the eddy current sensor  210 . Here, the correspondence relationship means the proportional connection between 1/tan α and the film thickness t described above. 
     In the calibration, the output of the eddy current sensor  210  is measured while the polishing target  102  whose film thickness has been known is pressed against the polishing face  104 , thereby obtaining the measurement value of the eddy current sensor  210  which corresponds to the film thickness. The film thickness is measured in advance on the outside of the polishing apparatus  100 . The measured film thickness is input to the polishing apparatus  100  by user&#39;s operation of a terminal, and stored in the film thickness calculator  238 . 
     A wafer whose film thickness has been measured in advance is polished with water while the polishing table  110  is rotated. This will be hereunder referred to as “water polishing”. Since water is used in the “water polishing”, no polishing occurs actually. The reason why “water polishing” is performed resides in that the purpose of the water polishing is to obtain an output of the eddy current sensor  210  when a polishing target  102  whose film thickness has been known is used, and thus it is undesirable to polish the polishing target  102 . 
     The table rotational number of the polishing table  110  may be arbitrary, but it is desirable to be equal to the rotational number under actual polishing. With respect to the measurement value of the eddy current sensor  210 , an average value within a range of 20 mm in diameter from the center of the polishing target  102  is stored in the film thickness calculator  238 . The foregoing angle α is obtained from the measurement value of the eddy current sensor  210 . A measurement result is shown in  FIG. 11 .  FIG. 11  shows the proportional connection between 1/tan α (=Ta) and the film thickness t. The abscissa axis represents the measurement value 1/tan α of the eddy current sensor  210 , and the ordinate axis represents the film thickness t.  FIG. 11  shows a measurement value  58  of the eddy current sensor  210  for a known film thickness  56 . 
     Straight lines representing proportional connections shown in  FIG. 11  contain a straight line  50  obtained in a calibration stage based on some measurements of the eddy current sensor  210 , and a straight line  52  and a straight line  54  which are actually measured by changing the distance between the eddy current sensor  210  and the polishing target  102  in two ways for comparison with the result of the calibration. All of these straight lines satisfy the proportional connection of “film thickness=A_th×Ta”. A_th represents the gradient of the straight lines. The purpose of the calibration is to determine the straight line  50 . The straight line  50  is a straight line which would be settled when the gradient A_th is known. When the measurement value  58  is known, the gradient A_th is determined because the film thickness  56  has already been known. A method of obtaining the measurement value  58  will be described later. 
     Furthermore, the film thickness  56  of a wafer used for calibration may have some degree of variation. There is no problem in this embodiment insofar as the film thickness  56  is correctly measured. This is because even when the film thickness  56  is different, a different measurement value  58  is obtained depending on the different film thickness, and the gradient A_th in the relational expression of “film thickness=A_th×Ta” obtained from the film thickness  56  and the measurement value  58  is invariable. In the prior art, since a calibration wafer whose film thickness has been determined is used, the film thickness as a target is fixed in the calibration. 
     Next, it will be described with reference to  FIG. 12  that when the polishing target  102  is polished while pressed against the polishing face  104 , the output of the eddy current sensor  210  is measured to obtain measurement values of the eddy current sensor  210  which correspond to the film thickness during polishing. In this step, a straight line representing the diameter of a circle shown in  FIG. 10  is determined from the measurement values of the eddy current sensor  210  which correspond to the film thickness during polishing. The angle α corresponding to the measurement value  58  is determined from the obtained straight line and the measurement value of the eddy current sensor  210  measured with reference to  FIG. 11 , and the measurement value  58  is finally obtained. 
     In another expression, the following operation is performed in this step. When an eddy current formable in the polishing target is detected as an impedance by the eddy current sensor to measure the film thickness of the polishing target  102 , the impedance is input, and the film thickness is determined from the input impedance. When the resistance component and the reactance component of the impedance are associated with the axes of the coordinate system having two orthogonal coordinate axes respectively, points on the coordinate system which correspond to the impedance form at least a part of a circle of FIG.  10 . The film thickness measuring device calculates, in the angle calculator, an intersection angle α at which the first straight line  10  connecting the first point T 0  corresponding to the impedance when the film thickness is equal to zero and the second point Tn corresponding to the impedance when the film thickness is not equal to zero, and the diameter  12  of the circle passing through the first point intersect to each other, or the tangent tan α of the angle α. 
     In this step, measurement values of the eddy current sensor  210  are obtained while the polishing target  102  to which the metal film is stuck is polished. A predetermined film of the polishing target  102  is polished until a polishing endpoint. This overall polishing is referred to as “clear polishing”.  FIG. 12  shows measurement points in the clear polishing.  FIG. 12  is similar to  FIG. 10 . An arc curve  60  represents an area where measurement points used to determine an arc center coordinate  64  exist out of an area where measurement points in the clear polishing exist. In this embodiment, a measurement point  68  represents a measurement value at a polishing start point. The measurement point  68  corresponds to the measurement value  58  shown in  FIG. 11 . A measurement point  62  represents a measurement value at the end of the polishing. The polishing and measurement for the calibration are performed from the measurement point  68  to the measurement point  62 . The arc center coordinate  64  is determined from the arc curve  60  as described above. An arc center straight line  66  is obtained from the measurement point  62  and the arc center coordinate  64 . 
     In this embodiment, as described above, unlike the prior art, the calibration is performed by actually polishing one polishing target  102 . In the prior art, no polishing is performed, plural calibration wafers are used and a polishing pad is peeled off 
     From the arc center straight line  66  and the output (existing on the circle) of the eddy current sensor  210  measured with reference to  FIG. 11 , the angle α corresponding to the output is determined. When the angle α is determined, 1/tan α (=Ta) is determined, that is, the measurement value  58  is determined. The arc center straight line  66  can be represented as Y=A_Imp×X. Here, A_Imp represents the gradient of the arc center straight line  66 . 
     The step shown in  FIG. 12  is performed by using the polishing target used in the step shown in  FIG. 11 . That is, the calibration is performed by polishing one polishing target  102 . However, the polishing target used in the step shown in  FIG. 11  and the polishing target used in the step shown in  FIG. 12  may be different from and independent of each other. That is, the angle α may be calculated by using the output of the eddy current sensor  210  obtained in the step shown in  FIG. 11  for a polishing target whose film thickness has been known, and the arc center straight line  66  obtained in the step shown in  FIG. 12  for a polishing target which is different from and independent of the former polishing target, and then the measurement value  58  may be calculated. 
     Next, it will be described with reference with  FIGS. 13 and 14  that when the polishing target  102  has a circular shape, the center position of the circular shape is detected. The detection of the center position of the polishing target  102  is not directly related to the determination of the correspondence relationship between the film thickness of the polishing target and the measurement value of the eddy current sensor. However, there is a case where the polishing time, the polishing pressure, etc. are controlled during polishing by mainly detecting the film thickness at the center position of the polishing target  102 . Accordingly, it is important to accurately detect the center position of the polishing target  102  in the calibration stage. This detection is performed in the receiver  232 . 
     In this embodiment, the output of the eddy current sensor  210  is measured while the eddy current sensor  210  is moved from one end  76  of the polishing target  102  to another end  78  thereof on the polishing target  102  used in  FIGS. 11 and 12 . 
     In the rotation of the polishing table  110 , a dog  351  fitted to the outer peripheral surface of the polishing table  110  is detected by a dog sensor  350  as shown in  FIG. 14 . Signal processing of the polishing target  102  held by the top ring  116  is started based on a detection signal from the dog sensor  350 . That is, a sensor locus  352  traverses the polishing target  102  with the rotation of the polishing table  110 . 
     The polishing apparatus first receives a signal from the dog sensor  350  while the polishing table  110  makes one revolution. At this time, the polishing target  102  has not yet come onto the eddy current sensor  210 , so that the eddy current sensor  210  receives a weak signal outside the polishing target  102 . Thereafter, when the eddy current sensor  210  is located below the polishing target  102 , the eddy current sensor  210  receives a sensor signal whose level corresponds to an eddy current occurring in the conductive film or the like. After the polishing target  102  has passed over the eddy current sensor  210 , the eddy current sensor  210  receives a weak sensor signal outside the polishing target  102  under a state where no eddy current occurs. 
     The rate of change (differential or difference) of a measurement value obtained at each point on the polishing target  102  at the point on the polishing target  102  is determined.  FIG. 13A  shows the magnitude of a signal obtained by the measurement, and  FIG. 13B  shows the rate of change (differential or difference) of the signal. The abscissa axis represents the time, and the ordinate axis represents the absolute value of the impedance as the output of the eddy current sensor  210  in  FIG. 13A , and the time derivative of the absolute value of the impedance in  FIG. 13B . The positions of one end  76  (polishing start point  70 ) and the other end  78  (polishing endpoint  72 ) of the polishing target  102  can be detected from the rate of change. This is because the rate of change has a plus peak at the polishing start point  70  and has a minus peak at the polishing endpoint  72 . From the positions of the one end and the other end at which the peaks of the rate of change are detected, the center position of the polishing target  102  is determined as a middle point  74  between these positions. 
     The center position of the polishing target  102  can be stored as time information or distance information. In the case of the time information, the center position can be stored as a time from detection of the dog  351 . In the case of the distance information, the center position can be stored as a distance from the one end  76  on a route  80 . 
     This step can be performed on any stage in the calibration, but it is preferable that it is performed first in the calibration. With respect to the detection of the center position of the polishing target  102 , conventionally, a user views measurement data and visually determines where the center position is located. In this embodiment, the polishing apparatus automatically determines where the center position is located. The table rotational number of the polishing table  110  is arbitrary, but it is preferable that it is set to be equal to that under actual polishing. When this step is performed first in the calibration, the polishing target  102  to which the metal film is stuck is subjected to the water polishing. This is because when the polishing target  102  is polished, it is impossible to perform the measurement shown in  FIG. 11 . 
     With respect to the measurement values shown in  FIG. 13 , the following three conditions are monitored, and an error is determined when at least one of them is not satisfied. 
     1. In the case of the time value of the plus peak position&lt; the time value of the minus peak position, the polishing target  102  is normal. This is because when this condition is not satisfied, a measurement error may be considered. 
     2. The distance between the peaks is not more than (the wafer diameter +40 mm) and not less than (the wafer diameter −40 mm). When this condition is not satisfied, it may be considered that a measurement curve or a peak position obtained from the measurement curve is abnormal.
 
3. There is no peak within 10 mm from the position of the dog  351  which indicates the rotation position of the polishing table  110  provided to the polishing apparatus, that is, the polishing start position. This is because when this condition is not satisfied, it is considered that the position detection of the dog  351  is abnormal, so that abnormality or a measurement error of the dog  351  is considered.
 
The values used for the determination of these conditions are examples, and other values may be applicable.
 
     Next, it will be described with reference to  FIGS. 14 and 15  that the output of the eddy current sensor  210  when the film thickness of the polishing target  102  is equal to 0 angstrom is adjusted to zero. There may be two types of adjustments for the above adjustment. A first type is an adjustment for a case where heat is generated by polishing the polishing target  102  and thus the temperature increases along the route  80  from one end  76  of the polishing target to the other end  78  of the polishing target. The output of the eddy current sensor  210  increases or decreases along the route  80  from the one end  76  to the other end  78 . A second type is an adjustment for a case where the output of the eddy current sensor  210  when the film thickness is equal to 0 angstrom is not necessarily equal to zero. The reason why the output of the eddy current sensor is not equal to zero is based on the characteristic of the signal processing circuit of the measuring device for measuring the output of the eddy current sensor  210 . 
     The first adjustment will be described with reference to  FIGS. 15A and 15B . This adjustment is performed in the receiver  232 .  FIG. 15A  shows the output of the eddy current sensor  210  when the film thickness of the polishing target  102  is equal to 0 angstrom. In  FIGS. 15A, 15B and 15C , the abscissa axis represents the time, and the ordinate axis represents the X or Y component of the impedance. 
     The eddy current sensor  210  is moved along the route  80  from the one end  76  of the polishing target  102  toward the other end  78  of the polishing target  102  to measure the output of the eddy current sensor at least two points on the route  80 . The case where the output of the eddy current sensor is measured at two points is, for example, a case where increase of the temperature is proportional to the distance from the one end  76 . At this time, a temperature variation component on the overall route  80  can be determined from the measurement at the two points. 
     In this embodiment, the measurement is continuously performed on the route  80 . A measurement value at a predetermined position on the route  80  is set as a reference value, and information representing the difference between a measurement value at each position on the route  80  and the reference value is obtained. In the case of  FIG. 15 , the predetermined position is set to the one end  76 . The predetermined position may be set to any position on the route  80 . The measurement value at the predetermined position on the route  80  and the measurement value at each position on the route  80  are the X or Y component of the impedance in the case of  FIG. 15A . The measurement value is not limited to the X or Y component of the impedance, but may be another value described above. The information representing the difference is the difference between X components of the impedance or the difference between Y components of the impedance. 
     A measurement value  82  represents the level of the measurement value at the one end  76 . The difference  88  between a measurement value  86  at some position  84  on the route  80  and the measurement value  82  is shown as an example of the information representing the difference. The difference  88  is considered as a temperature variation component. In this embodiment, the information representing the difference is obtained as the difference  88  itself at each point. When the difference  88  varies linearly, information on the equation of the straight line may be set as the information representing the difference. For example, the gradient of the straight line and the value at some position are set. 
     The difference between each measurement value obtained in the steps of  FIGS. 11 and 12  and the difference  88  at a position on the route  80  which corresponds to each point at which the above measurement value is obtained is calculated based on the information. As a result, the temperature variation component can be removed.  FIG. 15B  shows a result obtained by removing the temperature variation component from the measurement values of  FIG. 15A . In the following description, the processing of removing the temperature variation component will be referred to as “baseline processing”. The baseline processing is automatically performed every time the polishing table  110  makes one revolution. Since the purpose of the baseline processing is to detect and remove the temperature variation component, measurement values on the polishing target  102  whose film thickness is not equal to zero may be used. 
     The second adjustment will be described with reference to  FIGS. 15A and 15C . This adjustment is performed in the receiver  232 . When the polishing of the polishing target  102  is finished, that is, when the film thickness is equal to zero, the output of the eddy current sensor at at least one point on the route  80  is measured while the eddy current sensor  210  is moved along the route  80  from the one end  76  of the polishing target  102  toward the other end  78  of the polishing target  102 . A case where the output of the eddy current sensor  210  at only one point is sufficient is considered to be a case where the same measurement value is obtained at all points on the route  80 . 
     In this embodiment, the measurement is continuously performed on the route  80 . Information on the measurement value at each position on the route  80  at the time when the polishing of the polishing target  102  is finished is obtained. This information is shown in  FIG. 15A , and corresponds to the X component or Y component of the impedance. 
     The difference between a measurement value obtained in the steps of  FIGS. 11 and 12  and a measurement value (shown in  FIG. 15A ) when the polishing is finished at a position on the route  80  which corresponds to each point where the measurement value is obtained is determined based on the above information, whereby the components when the film thickness is equal to zero can be removed.  FIG. 15C  shows a result obtained by removing the above component from the measurement values of  FIG. 15A . In the following description, the processing for removing the component when the film thickness is equal to zero will be referred to as “zero calibration processing”. The zero calibration processing is automatically performed every time the polishing table  110  makes one revolution. When the measurement values of  FIG. 15A  are obtained for the baseline processing and the zero calibration processing, the table rotational number is arbitrary, but it is preferable that the table rotational number is set to be equal to that under actual polishing. In the zero calibration processing, a polishing target  102  to which no metal film is stuck is polished by using slurry. Furthermore, it is preferable that the processing is performed under the same pressure as the actual polishing. 
     Measurement values used in the processing shown in  FIGS. 13 to 15  can be measured by using the same one polishing target  102  as the polishing target  102  used in the processing shown in  FIGS. 11 and 12 . However, without being limited to this style, the measurement values used in the processing shown in  FIGS. 13 to 15  may be measured by using a different polishing target  102  from the polishing target  102  used in the processing shown in  FIGS. 11 and 12 , or measurement values which have already been measured may also be diverted. This is because the measurement values used in the processing shown in  FIGS. 13 to 15  are measurement values which are little dependent on the characteristic of the film. 
     Next, the processing flow for performing the processing shown in  FIGS. 11 to 15  by using one polishing target  102  will be described with reference to  FIG. 16 .  FIG. 16  is an entire flow of the calibration. 
     The flow of the processing of the entire flow is roughly as follows. In the first half of the flow, data necessary for the processing shown in  FIGS. 13, 11, 12 and 15  are obtained in this order. After all the data necessary for the processing are measured, the processing shown in  FIGS. 12 and 11  is performed in this order in the last half of the flow. At this time, the baseline processing and the zero calibration processing are performed by using the data obtained in the processing (step  164 ) shown in  FIG. 15 . 
     The details of the flow of the processing of the entire flow is as follows. When the calibration is started (step  152 ), the film thickness of the polishing target  102  used in the calibration is first measured by a film thickness measuring unit provided outside the polishing apparatus. The measured film thickness is input to the controller of the polishing apparatus  100  (step  154 ). 
     Next, data necessary for the processing shown in  FIG. 13  is obtained (step  156 ). That is, the water polishing is performed on the polishing target  102 , and the center position of the polishing target  10  is detected based on the peaks of the derivatives of a waveform. The data on the position of the polishing target  102  in the controller are adjusted based on the detection result. This step may be omitted. At that time, the center position which was obtained by a past measurement and has already been stored in the polishing apparatus is used. This is because the center position is little changed according to an individual polishing target  102 . 
     Next, data necessary for the processing shown in  FIG. 11  are obtained (step  158 ). That is, the water polishing is performed, and the output value of the eddy current sensor  210  at the center position of the polishing target  102  is obtained and stored. The thus-obtained data is represented by (X_th_raw, Y_th_raw). 
     Next, data necessary for the processing shown in  FIG. 12  is obtained (step  162 ). That is, polishing is performed with slurry until the film thickness is equal to zero. A data array at the center position of the polishing target  102  is obtained. The thus-obtained data array is represented by (X_clr[n], Y_clr[n]). Here, n represents an identification number of plural data. 
     Next, data necessary for the processing shown in  FIG. 15  is obtained (step  164 ). That is, polishing for the film thickness of zero is performed. Data at the center position of the polishing target  102  having no film is obtained. The thus-obtained data is represented by (X_zero, Y_zero). This step may be omitted. At that time, data which was obtained by a past measurement and has already been stored in the polishing apparatus is used. This is because the data varies little according to an individual polishing target  102 . 
     The necessary data is obtained as described above, and these data is processed as follows. First, in step  166 , (X_zero, Y_zero) is subtracted from (X_clr[n], Y_clr[n]), and then the arc center point and the radius are calculated according to the foregoing method. 
     Next, in step  168 , (X_zero, Y_zero) is subtracted from (X_th_raw, Y_th_raw), and then the measurement value Ta of the eddy current sensor  210  which corresponds to the known film thickness is calculated by using the arc radius. 
     Next, in step  172 , the gradient A_th is calculated from the input known film thickness and Ta obtained in step  168 , and the gradient A_imp is calculated from the arc center point. The foregoing processing completes the calibration. 
     According to this embodiment, the following problems associated with the prior art can be solved. That is, the prior art has the following problems: 
     1. it is necessary to peel off the polishing pad  108  and perform calibration by using a calibration wafer whose film thickness is known; 
     2. an error occurs due to a way of manually placing the calibration wafer, contamination of the calibration wafer or the like; and 
     3. a time-dependent error occurs due to deterioration of the characteristic due to oxidation or the like of the calibration wafer. 
     According to this embodiment, the problems associated with the prior art can be solved as follows. 
     1. The calibration can be performed without peeling off the polishing pad. 
     2. Since the apparatus can be automatically operated, neither positional deviation caused by a manual operation nor an error caused by contamination occurs. 
     3. Since the calibration can be performed with any polishing target  102  (wafer) whose film thickness has been measured, the calibration is not affected by deterioration of a calibration wafer. Any value is possible as the film thickness. In the prior art, the film thickness is limited to film thicknesses which calibration wafers have. However, according to this embodiment, any value may be used as the film thickness.
 
4. Since plural calibrations shown in  FIGS. 11 to 15  are performed with one polishing target  102 , a labor can be saved.
 
     In this embodiment, the plural calibrations shown in  FIGS. 11 to 15  are performed with one polishing target  102 . However, the processing shown in each of  FIGS. 13 and 15  can be performed independently as described above. Data obtained by performing each processing independently can be shared among calibrations on plural different polishing targets  102 . 
     In this case, the processing is performed as follows. In the case of  FIG. 13 , when the polishing target is polished while pressed against the polishing face, the calibration of the eddy current sensor for determining the center position of the polishing target is executed to measure the film thickness of the polishing target by the eddy current sensor. This method includes the step of measuring the output of the eddy current sensor while the eddy current sensor is moved from one end of the polishing target to another end of the polishing target on the polishing target, and the step of determining the rate of change of the obtained measurement value at each point on the polishing target, detecting the positions of the one end and the other end of the polishing target from the rate of changes, and determining the center position of the polishing target from the detected positions of the one end and the other end. 
     In the case of the baseline processing shown in  FIG. 15B , when the polishing target is polished while pressed against the polishing face, the calibration of the eddy current sensor for determining the variation of the measurement value on a route along which the eddy current sensor is moved from one end of the polishing target to another end of the polishing target is executed to measure the film thickness of the polishing target by the eddy current sensor. This method measures the output of the eddy current sensor at at least two points on a route from one end of the polishing target toward the other end of the polishing target while the polishing target is moved along the route, setting the measurement value at a predetermined position on the route as a reference value, and obtaining information representing the difference between the measurement value and the reference value. The difference between a measurement value obtained by the eddy current sensor in an actual measurement after the above calibration is finished, and a difference at the position on the route which corresponds to each point at which the above measurement is obtained is determined based on the above information. 
     In the case of the zero calibration processing of  FIG. 15C , when the polishing target is polished while pressed against the polishing face, the calibration of the eddy current sensor which takes account of the measurement values of the eddy current sensor under the state that the polishing of the polishing target is finished is executed to measure the film thickness of the polishing target by the eddy current sensor. When the polishing of the polishing target is finished, this method measures the output of the eddy current sensor at at least one point on a route from one end of the polishing target toward another end of the polishing target while the eddy current sensor is moved along the route, and obtains information on the measurement value at each position on the route when the polishing of the polishing target is finished. The difference between a measurement value obtained by the eddy current sensor in an actual measurement after the above calibration is finished, and a measurement value when the polishing of the polishing target is finished, at the position on the route which corresponds to each point at which the above measurement value is obtained is determined based on the above information. 
     Exemplified embodiments according to the present invention have been described above. However, the foregoing embodiments of the present invention are presented to facilitate understanding of the present invention, and do not limit the present invention. The present invention may be modified or improved without departing from the subject matter of the present invention, and also contains equivalents thereof. The respective constituent elements described in claims and the specification may be arbitrarily combined or omitted to the extent that at least a part of the foregoing problem can be solved or at least a part of the effect can be obtained. 
     This application claims priority under the Paris Convention to Japanese Patent Application No. 2017-87080 filed on Apr. 26, 2017. The entire disclosure of Japanese Patent Laid-Open No. 2007-263981 including specification, claims, drawings and summary is incorporated herein by reference in its entirety. 
     REFERENCE SIGNS LIST 
     
         
         
           
               70  polishing start point 
               72  polishing endpoint 
               74  intermediate point 
               76  one end 
               78  another end 
               80  route 
               100  polishing apparatus 
               102  polishing target 
               104  polishing face 
               108  polishing pad 
               110  polishing table 
               140  polishing apparatus controller 
               150  polishing unit 
               210  eddy current sensor 
               230  film thickness measuring device 
               232  receiver 
               234  angle calculator 
               238  film thickness calculator 
               240  endpoint detector