Patent Publication Number: US-11045921-B2

Title: Polishing apparatus and polishing method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This document claims priority to Japanese Patent Application No. 2017-98254 filed May 17, 2017, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Manufacturing processes of semiconductor devices include a process of polishing a dielectric film, e.g., SiO 2 , and a process of polishing a metal film, e.g., copper or tungsten. Manufacturing processes of backside illumination CMOS sensor and through-silicon via (TSV) include a process of polishing a silicon layer (silicon wafer), in addition to the polishing processes of the dielectric film and the metal film. Polishing of a wafer is terminated when a thickness of a film (e.g., the dielectric film, the metal film, or the silicon layer), constituting a wafer surface, has reached a predetermined target value. 
     Polishing of a wafer is carried out using a polishing apparatus. In order to measure a film thickness of a non-metal film, such as a dielectric film or a silicon layer, the polishing apparatus generally includes an optical film-thickness measuring device. This optical film-thickness measuring device is configured to direct a light, which is emitted from a light source, to a surface of the wafer, and to analyze a spectrum of reflected light from the wafer to thereby detect the film thickness of the wafer. 
     A quantity of light emitted by the light source is gradually lowered with an operating time of the light source. Thus, when the quantity of light from the light source is lowered to a certain extent, it is necessary to calibrate the optical film-thickness measuring device. Further, before a service life of the light source is reached, the light source needs to be replaced with a new one. However, it takes a certain time to perform the calibration of the optical film-thickness measuring device. Beside, a tool for the calibration is needed. Moreover, the decrease in the quantity of light from the light source may be caused by factors other than the light source, and thus it is difficult to accurately determine the service life of the light source. 
     SUMMARY OF THE INVENTION 
     According to embodiments, there are provided a polishing apparatus and a polishing method capable of accurately determining a service life of a light source, and further capable of accurately measuring a film thickness of a substrate, such as a wafer, without calibrating an optical film-thickness measuring device. 
     Embodiments, which will be described below, relate to a polishing apparatus and a polishing method for polishing a wafer having a film forming a surface thereof, and more particularly to a polishing apparatus and a polishing method for polishing a wafer, while detecting a film thickness of the wafer by analyzing optical information contained in reflected light from the wafer. 
     In an embodiment, there is provided a polishing apparatus comprising: a polishing table for supporting a polishing pad; a polishing head configured to press a wafer against the polishing pad; a light source configured to emit light; an illuminating fiber having a distal end arranged at a predetermined position in the polishing table, the illuminating fiber being coupled to the light source; a spectrometer configured to decompose reflected light from the wafer in accordance with wavelength and measure an intensity of the reflected light at each of wavelengths; a light-receiving fiber having a distal end arranged at the predetermined position in the polishing table, the light-receiving fiber being coupled to the spectrometer; a processor configured to determine a film thickness of the wafer based on a spectral waveform indicating a relationship between the intensity of the reflected light and wavelength; an internal optical fiber coupled to the light source; and an optical-path selecting mechanism configured to selectively couple either the light-receiving fiber or the internal optical fiber to the spectrometer. 
     The processor stores therein, in advance, a correction formula for correcting the intensity of the reflected light, the correction formula being a function which includes, as variables, at least the intensity of the reflected light and an intensity of light transmitted to the spectrometer through the internal optical fiber. 
     The correction formula is represented by 
     
       
         
           
             
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     where E(λ) is an intensity of the reflected light at a wavelength λ, B(λ) is a reference intensity at the wavelength λ which is measured in advance, D 1 (λ) is a dark level at the wavelength λ obtained under a condition that light is cut off immediately before or immediately after the reference intensity B(λ) is measured, F(λ) is an intensity of the light at the wavelength λ transmitted to the spectrometer through the internal optical fiber immediately before or immediately after the reference intensity B(λ) is measured, D 2 (λ) is a dark level at the wavelength λ obtained under a condition that light is cut off immediately before or immediately after the intensity F(λ) is measured, G(λ) is an intensity of the light at the wavelength λ transmitted to the spectrometer through the internal optical fiber before the intensity E(λ) is measured, and D 3 (λ) is a dark level at the wavelength λ obtained under a condition that light is cut off before the intensity E(λ) is measured, and immediately before or immediately after the intensity G(λ) is measured. 
     The reference intensity B(λ) is an intensity of the reflected light from a silicon wafer which is measured by the spectrometer when a silicon wafer with no film thereon is being water-polished in the presence of water on the polishing pad, or when a silicon wafer with no film thereon is placed on the polishing pad. 
     The reference intensity B(λ) is an average of multiple values of intensity of the reflected light from the silicon wafer which have been measured under the same condition. 
     The processor instructs the optical-path selecting mechanism to couple the internal optical fiber to the spectrometer before the wafer is polished. 
     The processor is configured to generate an alarm signal when the intensity of light transmitted to the spectrometer through the internal optical fiber is lower than a threshold value. 
     The illuminating fiber has a plurality of distal ends arranged at different locations in the polishing table, and the light-receiving fiber has a plurality of distal ends arranged at the different locations in the polishing table. 
     The illuminating fiber has a plurality of first illuminating strand optical fibers and a plurality of second illuminating strand optical fibers, and light-source-side ends of the plurality of first illuminating strand optical fibers and light-source-side ends of the plurality of second illuminating strand optical fibers are distributed evenly around a center of the light source. 
     An average of distances from the center of the light source to the light-source-side ends of the plurality of first illuminating strand optical fibers is equal to an average of distances from the center of the light source to the light-source-side ends of the plurality of second illuminating strand optical fibers. 
     A light-source-side end of the internal optical fiber is located at the center of the light source. 
     A part of the plurality of first illuminating strand optical fibers, a part of the plurality of second illuminating strand optical fibers, and a part of the internal optical fiber constitute a trunk fiber bound by a binder, and other part of the plurality of first illuminating strand optical fibers, other part of the plurality of second illuminating strand optical fibers, and other part of the internal optical fiber constitute branch fibers which branch off from the trunk fiber. 
     There is provided a polishing method comprising: directing light from a light source to a spectrometer through an internal optical fiber to measure an intensity of the light, the light source being coupled to the spectrometer through the internal optical fiber, pressing a wafer against a polishing pad on a polishing table to polish the wafer, during polishing of the wafer, directing light to the wafer and measuring an intensity of reflected light from the wafer; correcting the intensity of reflected light from the wafer based on the intensity of light transmitted to the spectrometer through the internal optical fiber; and determining a film thickness of the wafer based on a spectral waveform indicating a relationship between the corrected intensity and wavelength of light. 
     The intensity of reflected light from the wafer is corrected with use of a correction formula represented by 
     
       
         
           
             
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     where E(λ) is an intensity of the reflected light at a wavelength λ, B(λ) is a reference intensity at the wavelength λ which is measured in advance, D 1 (λ) is a dark level at the wavelength λ obtained under a condition that light is cut off immediately before or immediately after the reference intensity B(λ) is measured, F(λ) is an intensity of the light at the wavelength λ transmitted to the spectrometer through the internal optical fiber immediately before or immediately after the reference intensity B(λ) is measured, D 2 (λ) is a dark level at the wavelength λ obtained under a condition that light is cut off immediately before or immediately after the intensity F(λ) is measured, G(λ) is an intensity of the light at the wavelength λ transmitted to the spectrometer through the internal optical fiber before the intensity E(λ) is measured, and D 3 (λ) is a dark level at the wavelength λ obtained under a condition that light is cut off before the intensity E(λ) is measured, and immediately before or immediately after the intensity G(λ) is measured. 
     The reference intensity B(λ) is an intensity of the reflected light from a silicon wafer which is measured by the spectrometer when a silicon wafer with no film thereon is being water-polished in the presence of water on the polishing pad, or when a silicon wafer with no film thereon is placed on the polishing pad. 
     The reference intensity B(λ) is an average of multiple values of intensity of the reflected light from the silicon wafer which have been measured under the same condition. 
     The process of directing the light from the light source to the spectrometer through the internal optical fiber to measure the intensity of light is performed before polishing of the wafer. 
     The polishing method further comprises generating an alarm signal when the intensity of light transmitted to the spectrometer through the internal optical fiber is lower than a threshold value. 
     If the intensity of the light is lower than the threshold value, the wafer is returned to a substrate cassette without performing polishing of the wafer. 
     According to the above-described embodiments, the light emitted by the light source is transmitted to the spectrometer through the internal optical fiber. Because the light is directly transmitted to the spectrometer without being directed to the wafer, the processor can accurately determine the service life of the light source based on the intensity of light measured by the spectrometer. Further, the processor corrects the intensity of the reflected light from the wafer during polishing of the wafer with use of the intensity of light transmitted to the spectrometer through the internal optical fiber, i.e., an internal monitoring intensity. Since the corrected intensity of the reflected light contains correct optical information of the wafer, the processor can determine an accurate film thickness of the wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing an embodiment of a polishing apparatus; 
         FIG. 2  is a plan view showing a polishing pad and a polishing table; 
         FIG. 3  is an enlarged view showing an optical film-thickness measuring device (film-thickness measuring apparatus); 
         FIG. 4  is a schematic view illustrating the principle of the optical film-thickness measuring device; 
         FIG. 5  is a graph showing an example of a spectral waveform; 
         FIG. 6  is a graph showing a frequency spectrum obtained by performing Fourier transform process on the spectral waveform shown in  FIG. 5 ; and 
         FIG. 7  is a schematic view showing an arrangement of light-source-side ends of first illuminating strand optical fibers and light-source-side ends of second illuminating strand optical fibers. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments will be described below with reference to the drawings.  FIG. 1  is a view showing an embodiment of a polishing apparatus. As shown in  FIG. 1 , the polishing apparatus includes a polishing table  3  supporting a polishing pad  1 , a polishing head  5  for holding a wafer W and pressing the wafer W against the polishing pad  1  on the polishing table  3 , a polishing-liquid supply nozzle  10  for supplying a polishing liquid (e.g., slurry) onto the polishing pad  1 , and a polishing controller  12  for controlling polishing of the wafer W. 
     The polishing table  3  is coupled to a table motor  19  through a table shaft  3   a , so that the polishing table  3  is rotated by the table motor  19  in a direction indicated by arrow. The table motor  19  is located below the polishing table  3 . The polishing pad  1  is attached to an upper surface of the polishing table  3 . The polishing pad  1  has an upper surface, which provides a polishing surface  1   a  for polishing the wafer W. The polishing head  5  is secured to a lower end of a polishing head shaft  16 . The polishing head  5  is configured to be able to hold the wafer W on its lower surface by vacuum suction. The polishing head shaft  16  can be elevated and lowered by an elevating mechanism (not shown in the drawing). 
     Polishing of the wafer W is performed as follows. The polishing head  5  and the polishing table  3  are rotated in directions indicated by arrows, while the polishing liquid (or slurry) is supplied from the polishing-liquid supply nozzle  10  onto the polishing pad  1 . In this state, the polishing head  5  presses the wafer W against the polishing surface  1   a  of the polishing pad  1 . The surface of the wafer W is polished by a chemical action of the polishing liquid and a mechanical action of abrasive grains contained in the polishing liquid. 
     The polishing apparatus includes an optical film-thickness measuring device (i.e., a film thickness measuring apparatus)  25  for measuring a film thickness of the wafer W. This optical film-thickness measuring device  25  includes a light source  30  for emitting light, an illuminating fiber  34  having distal ends  34   a ,  34   b  arranged at different locations in the polishing table  3 , a light-receiving fiber  50  having distal ends  50   a ,  50   b  arranged at the different locations in the polishing table  3 , a spectrometer  26  for decomposing reflected light from the wafer W in accordance with wavelength and measuring an intensity of the reflected light at each of wavelengths, and a processor  27  for producing a spectral waveform indicating a relationship between the intensity and the wavelength of the reflected light. The processor  27  is coupled to the polishing controller  12 . 
     The illuminating fiber  34  is coupled to the light source  30  and is arranged so as to direct the light, emitted by the light source  30 , to the surface of the wafer W. The light-receiving fiber  50  is coupled to an optical-path selecting mechanism  70 . One end of an internal optical fiber  72  is coupled to the light source  30 , while the other end of the internal optical fiber  72  is coupled to the optical-path selecting mechanism  70 . Further, the optical-path selecting mechanism  70  is coupled to the spectrometer  26  through a connecting optical fiber  74 . 
     The optical-path selecting mechanism  70  is configured to optically couple either the light-receiving fiber  50  or the internal optical fiber  72  to the spectrometer  26  through the connecting optical fiber  74 . More specifically, when the optical-path selecting mechanism  70  is activated to optically couple the light-receiving fiber  50  to the spectrometer  26 , the reflected light from the wafer W is transmitted to the spectrometer  26  through the light-receiving fiber  50 , the optical-path selecting mechanism  70 , and the connecting optical fiber  74 . When the optical-path selecting mechanism  70  is activated to optically couple the internal optical fiber  72  to the spectrometer  26 , the light emitted by the light source  30  is transmitted to the spectrometer  26  through the internal optical fiber  72 , the optical-path selecting mechanism  70 , and the connecting optical fiber  74 . Operations of the optical-path selecting mechanism  70  are controlled by the processor  27 . 
     Examples of the optical-path selecting mechanism  70  include an optical switch. The optical switch may be of a type which has an actuator for moving a first optical path to selectively couple the first optical path to at least one of second optical paths, or may be a type which has a shutter for blocking at least one of second optical paths coupled to first optical paths, respectively. 
     The distal end  34   a  of the illuminating fiber  34  and the distal end  50   a  of the light-receiving fiber  50  are adjacent to each other. These distal ends  34   a ,  50   a  constitute a first optical sensor  61 . The other distal end  34   b  of the illuminating fiber  34  and the other distal end  50   b  of the light-receiving fiber  50  are adjacent to each other. These distal ends  34   b ,  50   b  constitute a second optical sensor  62 . The polishing pad  1  has through-holes  1   b ,  1   c  located above the first optical sensor  61  and the second optical sensor  62 , respectively. The first optical sensor  61  and the second optical sensor  62  can transmit the light to the wafer W on the polishing pad  1  through the through-holes  1   b ,  1   c  and can receive the reflected light from the wafer W through the through-holes  1   b ,  1   c.    
     In one embodiment, the illuminating fiber  34  may have only one distal end arranged at a predetermined position in the polishing table  3 , and the light-receiving fiber  50  may also have only one distal end arranged at the predetermined position in the polishing table  3 . In this case also, the distal end of the illuminating fiber  34  and the distal end of the light-receiving fiber  50  are adjacent to each other. The distal end of the illuminating fiber  34  and the distal end of the light-receiving fiber  50  constitute an optical sensor for transmitting the light to the wafer W on the polishing pad  1 , and receiving the reflected light from the wafer W. 
       FIG. 2  is a plan view showing the polishing pad  1  and the polishing table  3 . The first optical sensor  61  and the second optical sensor  62  are located at different distances from a center of the polishing table  3 , and are arranged away from each other in the circumferential direction of the polishing pad  3 . In the embodiment shown in  FIG. 2 , the second optical sensor  62  is located across the center of the polishing table  3  from the first optical sensor  61 . The first optical sensor  61  and the second optical sensor  62  move across the wafer W alternately in different paths each time the polishing table  3  makes one revolution. More specifically, the first optical sensor  61  sweeps across the center of the wafer W, while the second optical sensor  62  sweeps across only the edge portion of the wafer W. The first optical sensor  61  and the second optical sensor  62  direct the light to the wafer W alternately, and receive the reflected light from the wafer W alternately. 
       FIG. 3  is an enlarged view showing the optical film-thickness measuring device (i.e., the film-thickness measuring apparatus)  25 . The illuminating fiber  34  has a plurality of first illuminating strand optical fibers  36  and a plurality of second illuminating strand optical fibers  37 . Distal ends of the first illuminating strand optical fibers  36  and distal ends of the second illuminating strand optical fibers  37  are bound by binders  32 ,  33 , respectively. These distal ends constitute the distal ends  34   a ,  34   b  of the illuminating fiber  34 . 
     Light-source-side ends of the first illuminating strand optical fibers  36 , light-source-side ends of the second illuminating strand optical fibers  37 , and a light-source-side end of the internal optical fiber  72  are coupled to the light source  30 . A part of the first illuminating strand optical fibers  36 , a part of the second illuminating strand optical fibers  37 , and a part of the internal optical fiber  72  constitute a trunk fiber  35  bound by a binder  31 . The trunk fiber  35  is coupled to the light source  30 . The other part of the first illuminating strand optical fibers  36 , the other part of the second illuminating strand optical fibers  37 , and the other part of the internal optical fiber  72  constitute branch fibers which branch off from the trunk fiber  35 . 
     In the embodiment shown in  FIG. 3 , three branch fibers branch off from one trunk fiber  35 . Four or more branch fibers can branch off by adding strand optical fibers. Further, a diameter of the fiber can be easily increased by adding strand optical fibers. Such a fiber constituted by the plurality of strand optical fibers has advantages that it can be easily bent and is not easily broken. 
     The light-receiving fiber  50  includes a plurality of first light-receiving strand optical fibers  56  bound by a binder  51 , and a plurality of second light-receiving strand optical fibers  57  bound by a binder  52 . The distal ends  50   a ,  50   b  of the light-receiving fiber  50  is constituted by distal ends of the first light-receiving strand optical fibers  56  and distal ends of the second light-receiving strand optical fibers  57 , respectively. The distal end  34   a  of the first illuminating strand optical fibers  36  and the distal end  50   a  of the first light-receiving strand optical fibers  56  constitute the first optical sensor  61 . The distal end  34   b  of the second illuminating strand optical fibers  37  and the distal end  50   b  of the second light-receiving strand optical fibers  57  constitute the second optical sensor  62 . Opposite ends of the first light-receiving strand optical fibers  56  and the second light-receiving strand optical fibers  57  are coupled to the optical-path selecting mechanism  70 . 
     The optical-path selecting mechanism  70  and the spectrometer  26  are electrically coupled to the processor  27 . The optical-path selecting mechanism  70  is operated by the processor  27 . When the wafer W is to be polished, the processor  27  operates the optical-path selecting mechanism  70  to optically couple the light-receiving fiber  50  to the spectrometer  26 . More specifically, each time the polishing table  3  makes one revolution, the processor  27  operates the optical-path selecting mechanism  70  to couple the first light-receiving strand optical fibers  56  and the second light-receiving strand optical fibers  57  alternately to the spectrometer  26 . The first light-receiving strand-optical fiber  56  are coupled to the spectrometer  26  while the distal end  50   a  of the first light-receiving branch fibers  56  are present under the wafer W, and the second light-receiving strand optical fibers  57  are coupled to the spectrometer  26  while the distal end  50   b  of the second light-receiving branch fiber  57  are present under the wafer W. 
     In the present embodiment, the optical-path selecting mechanism  70  is configured to optically couple any one of the first light-receiving strand optical fibers  56 , the second light-receiving strand optical fibers  57 , and the internal optical fiber  72  to the spectrometer  26 . This structure makes it possible to transmit only the reflected light from the wafer W to the spectrometer  26 , and as a result, an accuracy of the film-thickness measuring operation is improved. In one embodiment, the optical-path selecting mechanism  70  may be configured to optically couple either the light-receiving strand optical fibers  56 ,  57  or the internal optical fiber  72  to the spectrometer  26 . In this case, during polishing of the wafer W, the reflected light is transmitted to the spectrometer  26  through both of the light-receiving strand optical fibers  56 ,  57 . Since intensities of light other than the reflected light from the wafer W are extremely low, it is possible to measure an accurate film thickness by using only light having intensities that are greater than or equal to a threshold value. 
     During polishing of the wafer W, the illuminating fiber  34  directs the light to the wafer W, and the light-receiving fiber  50  receives the reflected light from the wafer W. The reflected light from the wafer W is transmitted to the spectrometer  26 . The spectrometer  26  decomposes the reflected light in accordance with wavelength, measures the intensity of the reflected light at each of the wavelengths over a predetermined wavelength range, and transmits light intensity data obtained to the processor  27 . This light intensity data is an optical signal reflecting a film thickness of the wafer W, and contains the intensities of the reflected light and the corresponding wavelengths. The processor  27  produces, from the light intensity data, the spectral waveform representing the intensity of the light at each of the wavelengths. 
       FIG. 4  is a schematic view illustrating the principle of the optical film-thickness measuring device  25 . In this example shown in  FIG. 4 , a wafer W has a lower film and an upper film formed on the lower film. The upper film is a film that can allow light to pass therethrough, such as a silicon layer or a dielectric film. The light, directed to the wafer W, is reflected off an interface between a medium (e.g., water in the example of  FIG. 4 ) and the upper film and an interface between the upper film and the lower film. Light waves from these interfaces interfere with each other. The manner of interference between the light waves varies according to the thickness of the upper film (i.e., a length of an optical path). As a result, the spectral waveform, produced from the reflected light from the wafer W, varies according to the thickness of the upper film. 
     The spectrometer  26  decomposes the reflected light in accordance with the wavelength and measures the intensity of the reflected light at each of the wavelengths. The processor  27  produces the spectral waveform from the reflected-light intensity data (or optical signal) obtained by the spectrometer  26 . This spectral waveform is expressed as a line graph indicating a relationship between the wavelength and the intensity of the light. The intensity of the light can also be expressed as a relative value, such as a relative reflectance which will be discussed later. 
       FIG. 5  is a graph showing an example of the spectral waveform. In  FIG. 5 , vertical axis represents relative reflectance indicating the intensity of the reflected light from the wafer W, and horizontal axis represents wavelength of the reflected light. The relative reflectance is an index value that represents the intensity of the reflected light. The relative reflectance is a ratio of the intensity of the light to a predetermined reference intensity. By dividing the intensity of the light (i.e., the actually measured intensity) at each wavelength by a predetermined reference intensity, unwanted noises, such as a variation in the intensity inherent in an optical system or the light source of the apparatus, are removed from the actually measured intensity. 
     The reference intensity is an intensity that has been measured in advance at each of the wavelengths. The relative reflectance is calculated at each of the wavelengths. Specifically, the relative reflectance is determined by dividing the intensity of the light (the actually measured intensity) at each wavelength by the corresponding reference intensity. The reference intensity is, for example, obtained by directly measuring the intensity of light emitted from the first optical sensor  61  or the second optical sensor  62 , or by irradiating a mirror with light from the first optical sensor  61  or the second optical sensor  62  and measuring the intensity of reflected light from the mirror. Alternatively, the reference intensity may be an intensity of the reflected light which is measured by the spectrometer  26  when a silicon wafer (bare wafer) with no film thereon is being water-polished in the presence of water, or when the silicon wafer (bare wafer) is placed on the polishing pad  1 . In the actual polishing process, a dark level (which is a background intensity obtained under the condition that light is cut off) is subtracted from the actually measured intensity to determine a corrected actually measured intensity. Further, the dark level is subtracted from the reference intensity to determine a corrected reference intensity. Then the relative reflectance is calculated by dividing the corrected actually measured intensity by the corrected reference intensity. That is, the relative reflectance R(λ) can be calculated by using the following formula (1) 
                     R   ⁡     (   λ   )       =         E   ⁡     (   λ   )       -     D   ⁡     (   λ   )             B   ⁡     (   λ   )       -     D   ⁡     (   λ   )                   (   1   )               
where λ is wavelength, E(λ) is the intensity of the light reflected from the wafer at the wavelength λ, B(λ) is the reference intensity at the wavelength λ, and D(λ) is the background intensity (i.e., dark level) at the wavelength 1 obtained under the condition that light is cut off.
 
     The processor  27  performs a Fourier transform process (e.g., fast Fourier transform process) on the spectral waveform to produce a frequency spectrum and determines a film thickness of the wafer W from the frequency spectrum.  FIG. 6  is a graph showing the frequency spectrum obtained by performing the Fourier transform process on the spectral waveform shown in  FIG. 5 . In  FIG. 6 , vertical axis represents strength of a frequency component contained in the spectral waveform, and horizontal axis represents film thickness. The strength of a frequency component corresponds to amplitude of a frequency component which is expressed as sine wave. A frequency component contained in the spectral waveform is converted into a film thickness with use of a predetermined relational expression, so that the frequency spectrum as shown in  FIG. 6  is produced. This frequency spectrum represents a relationship between the film thickness and the strength of the frequency component. The above-mentioned predetermined relational expression is a linear function representing the film thickness and having the frequency component as variable. This linear function can be obtained from actual measurement results of film thickness, an optical film-thickness measurement simulation, etc. 
     In the graph shown in  FIG. 6 , a peak of the strength of the frequency component appears at a film thickness t 1 . In other words, the strength of the frequency component becomes maximum at the film thickness of t 1 . That is, this frequency spectrum indicates that the film thickness is t 1 . In this manner, the processor  27  determines the film thickness corresponding to a peak of the strength of the frequency component. 
     The processor  27  outputs the film thickness t 1  as a film-thickness measurement value to the polishing controller  12 . The polishing controller  12  controls polishing operations (e.g., a polishing terminating operation) based on the film thickness t 1  sent from the processor  27 . For example, when the film thickness t 1  has reached a preset target value, the polishing controller  12  terminates polishing of the wafer W. 
     As described above, the optical film-thickness measuring device  25  directs the light, emitted by the light source  30 , to the wafer W, and determines the film thickness of the wafer W by analyzing the reflected light from the wafer W. However, a quantity of light emitted by the light source  30  is gradually lowered with an operating time of the light source  30 . As a result, an error between a true film thickness and a measured film thickness becomes larger. Thus, in this embodiment, the optical film-thickness measuring device  25  is configured to correct the intensity of the reflected light from the wafer W based on the intensity of light transmitted to the spectrometer  26  through the internal optical fiber  72 , and compensate for the decrease in the quantity of light of the light source  30 . 
     The processor  27  calculates a corrected intensity of the reflected light with use of the following correction formula (2), instead of the aforementioned formula (1). 
                       R   ′     ⁡     (   λ   )       =       [       E   ⁡     (   λ   )       -     D   ⁢           ⁢   3   ⁢     (   λ   )         ]     /     [       [       B   ⁡     (   λ   )       -     D   ⁢           ⁢   1   ⁢     (   λ   )         ]     ×         G   ⁢     (   λ   )       -     D   ⁢           ⁢   3   ⁢     (   λ   )             F   ⁡     (   λ   )       -     D   ⁢           ⁢   2   ⁢     (   λ   )             ]               (   2   )               
where R(λ) represents a corrected intensity of the reflected light, i.e., a corrected relative reflectance, E(λ) represents an intensity of reflected light from the wafer W being polished at a wavelength λ, B(λ) represents a reference intensity at the wavelength λ, D 1 (λ) represents a dark level at the wavelength λ measured under a condition that light is cut off immediately before or immediately after the reference intensity B(λ) is measured, F(λ) represents an intensity of light at the wavelength λ transmitted to the spectrometer  26  through the internal optical fiber  72  immediately before or immediately after the reference intensity B(λ) is measured, D 2 (λ) represents a dark level at the wavelength λ obtained under a condition that light is cut off immediately before or immediately after the intensity F(λ) is measured, G(λ) represents an intensity of light at the wavelength (λ) transmitted to the spectrometer  26  through the internal optical fiber  72  before the intensity E(λ) is measured, and D 3 (λ) represents a dark level at the wavelength λ obtained under a condition that light is cut off before the intensity E(λ) is measured, and immediately before or immediately after the intensity G(λ) is measured.
 
     E(λ), B(λ), D 1 (λ), F(λ), D 2 (λ), G(λ), and D 3 (λ) are measured at each of the wavelengths within a predetermined wavelength range. The light-cut-off environment for measuring the dark levels D 1 (λ), D 2 (λ), and D 3 (λ) can be produced by cutting off the light with a shutter (not shown) installed in the spectrometer  26 . 
     The processor  27  stores therein, in advance, the aforementioned correction formula (2) for correcting the intensity of the reflected light from the wafer W. This correction formula is a function including, as variables, at least the intensity of the reflected light from the wafer W, and the intensity of the light transmitted to the spectrometer  26  through the internal optical fiber  72 . The reference intensity B(λ) is an intensity of light that has been measured in advance at each of wavelengths. For example, the reference intensity B(λ) is obtained by directly measuring the intensity of light emitted from the first optical sensor  61  or the second optical sensor  62 , or by irradiating a mirror with light from the first optical sensor  61  or the second optical sensor  62  and measuring the intensity of reflected light from the mirror. Alternatively, the reference intensity B(λ) may be an intensity of the reflected light measured by the spectrometer  26  when a silicon wafer (bare wafer) with no film thereon is being water-polished in the presence of water, or when said silicon wafer (bare wafer) is placed on the polishing pad  1 . In order to obtain a connect value of the reference intensity B(λ), the reference intensity B(λ) may be an average of multiple values of intensity of the light which have been measured under the same condition. 
     The reference intensity B(λ), the dark level D 1 (λ), the intensity F(λ), and the dark level D 2 (λ) are measured in advance, and inputted as constants into the aforementioned correction formula in advance. The intensity E(λ) is measured during polishing of the wafer W. The intensity G(λ) and the dark level D 3 (λ) are measured before polishing of the wafer W (preferably, immediately before polishing of the wafer W). For example, before the wafer W is held by the polishing head  5 , the processor  27  operates the optical-path selecting mechanism  70  to couple the internal optical fiber  72  to the spectrometer  26  so that the light emitted by the light source  30  is transmitted to the spectrometer  26  through the internal optical fiber  72 . The spectrometer  26  measures the intensity G(λ) and the dark level D 3 (λ), and sends these measured values to the processor  27 . The processor  27  inputs the measured values of the intensity G(λ) and the dark level D 3 (λ) into the aforementioned correction formula. Upon completion of measuring of the intensity G(λ) and the dark level D 3 (λ), the processor  27  operates the optical-path selecting mechanism  70  to couple the light-receiving fiber  50  to the spectrometer  26 . Thereafter, the wafer W is polished, and the intensity E(λ) is measured by the spectrometer  26  during polishing of the wafer W. 
     During polishing of the wafer W, the processor  27  inputs the measured value of the intensity E(λ) into the aforementioned correction formula, and calculates the corrected relative reflectance R′(λ) at each of wavelengths. More specifically, the processor  27  calculates corrected relative reflectances R′(λ) over the predetermined wavelength range. Therefore, the processor  27  can produce a spectral waveform representing a relationship between the corrected relative reflectance (i.e., the corrected intensity of the light) and the wavelength of the light. The processor  27  determines the film thickness of the wafer W based on the spectral waveform according to the method discussed with reference to  FIG. 5  and  FIG. 6 . The processor  27  can determine an accurate film thickness of the wafer W because the spectral waveform is produced based on the corrected intensities of light. 
     According to this embodiment, the reflected light from the wafer W is corrected based on the intensity G(λ), i.e., internal monitoring intensity, which is transmitted to the spectrometer  26  through the internal optical fiber  72  before polishing of the wafer W, instead of calibrating the optical film-thickness measuring device  25  with use of a calibration tool. Accordingly, it is unnecessary to calibrate the optical film-thickness measuring device  25 . 
     The intensity G(λ) and the dark level D 3 (λ) may be measured each time a wafer is polished, or may be measured each time the predetermined number of wafers (for example, twenty-five wafers) are polished. 
     The quantity of light emitted by the light source  30  is gradually lowered with the operating time of the light source  30 . When the quantity of light from the light source  30  is lowered to a certain extent, it is necessary to replace the light source  30  with new one. Thus, the processor  27  is configured to determine a service life of the light source  30  based on the intensity G(λ) of light transmitted to the spectrometer  26  through the internal optical fiber  72  before the wafer W is polished. More specifically, before the wafer W is polished, the processor  27  operates the optical-path selecting mechanism  70  to couple the internal optical fiber  72  to the spectrometer  26  so that the light emitted by the light-source  30  is transmitted to the spectrometer  26  through the internal optical fiber  72 . The spectrometer  26  measures the intensity G(λ) of light transmitted through the internal optical fiber  72 . The processor  27  compares the intensity G(λ) of light with a preset threshold value, and generates an alarm signal if the intensity G(λ) is lower than the threshold value. 
     The processor  27  may compare the intensity G(λ) at a predetermined wavelength λ with the threshold value, or may compare an average of the intensities G(λ) [λ=λ1 to λ2] at a predetermined wavelength range (from λ1 to λ2) with the threshold value, or may compare a maximum or a minimum of the intensities G(λ) [λ=λ1 to λ2] at the predetermined wavelength range (from λ1 to λ2) with the threshold value. 
     The intensity G(λ) is an intensity of light that is directly transmitted to the spectrometer  26  through the internal optical fiber  72 , i.e., an internal monitoring intensity. In other words, the intensity G(λ) is an intensity of light that is not affected by the conditions of the wafer W and other optical paths. Therefore, the processor  27  can accurately determine the service life of the light source  30 . 
     The processor  27  operates the optical-path selecting mechanism  70  before polishing of the wafer W to couple the internal optical fiber  72  to the spectrometer  26 , and determines the service life of the light source  30  based on the intensity G(λ) of the light transmitted to the spectrometer  26  through the internal optical fiber  72 . If the intensity G(λ) is lower than the threshold value, the processor  27  generates the alarm signal, and interlocks the polishing head  5  to prevent the polishing head  5  from starting polishing of the wafer W. Such interlock operation can avoid polishing of wafer W while measuring inaccurate film thickness. In this case, the wafer W is not polished, and is returned to a substrate cassette (not shown). 
     As shown in  FIG. 1 , the first optical sensor  61  and the second optical sensor  62  are disposed in the polishing table  3 . A distance from the center of the polishing table  3  to the first optical sensor  61  is different from a distance from the center of the polishing table  3  to the second optical sensor  62 . Therefore, the first optical sensor  61  and the second optical fiber  62  scans different zones of the surface of the wafer W each time the polishing table  3  makes one revolution. In order to properly evaluate the film thicknesses measured at the different zones of the wafer W, the first optical sensor  61  and the second optical sensor  62  are preferably under the same optical conditions. Specifically, the first optical sensor  61  and the second optical sensor  62  preferably illuminate the surface of the wafer W with light having the same intensity. 
     Thus, in one embodiment, the light-source-side ends of the first illuminating strand optical fibers  36  and the light-source-side ends of the second illuminating strand optical fibers  37 , which constitute the first optical sensor  61  and the second optical sensor  62 , are distributed evenly around a center C of the light source  30  as shown in  FIG. 7 . The number of light-source-side ends of the first illuminating strand optical fibers  36  is equal to the number of light-source-side ends of the second illuminating strand optical fibers  37 . Further, an average of distances from the center C of the light source  30  to the light-source-side ends of the first illuminating strand optical fibers  36  is equal to an average of distances from the center C of the light source  30  to the light-source-side ends of the second illuminating strand optical fibers  37 . 
     With such arrangement, the light emitted by the light source  30  travels through the first illuminating strand optical fibers  36  and the second illuminating strand optical fibers  37  evenly, and reaches the first optical sensor  61  and the second optical sensor  62 . Therefore, the first optical sensor  61  and the second optical sensor  62  can emit the light having the same intensity to the different zones of the surface of wafer W. 
     In this embodiment, the internal optical fiber  72  is constituted by one strand optical fiber, and a light-source-side end of the internal optical fiber  72  is located at the center C of the light source  30 . As described above, the internal optical fiber  72  is not used to illuminate the wafer W, and is used for correcting the intensity of the reflected light from the wafer W. Therefore, the intensity of light transmitted to the spectrometer  26  through the internal optical fiber  72  may be relatively low. From this viewpoint, the internal optical fiber  72  is constituted by one strand optical fiber. Since the intensity of light at the center C of the light source  30  is more stable than the intensity of light at an edge of the light source  30 , the light-source-side end of the internal optical fiber  72  is preferably located at the center C of the light source  30  as shown in  FIG. 7 . 
     It is noted that the arrangement and the number of optical fibers  36 ,  37  shown in  FIG. 7  are one example. The arrangement and the number of optical fibers  36 ,  37  are not limited particularly, so long as the light is evenly directed to the first optical sensor  61  and the second optical sensor  62  through the first illuminating strand optical fibers  36  and the second illuminating strand optical fibers  37 , respectively. 
     The previous description of embodiments is provided to enable a person skilled in the art to make and use the present invention. Moreover, various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope as defined by limitation of the claims.