Patent Publication Number: US-2021187692-A1

Title: Polishing unit, substrate processing apparatus, and polishing method

Description:
TECHNICAL FIELD 
     This application relates to a polishing unit, a substrate processing apparatus, and a polishing method. This application claims priority from Japanese Patent Application No. 2019-233037 filed on Dec. 24, 2019. The entire disclosure including the descriptions, the claims, the drawings, and the abstracts in Japanese Patent Application No. 2019-233037 is herein incorporated by reference. 
     BACKGROUND ART 
     In manufacturing a semiconductor device, a chemical mechanical polishing (CMP) apparatus is used for planarization of a surface of a substrate. The substrate used in the manufacture of the semiconductor device often has a circular plate shape. Not limited to the semiconductor device, a demand for flatness in planarization of a surface of a rectangular substrate, such as a CCL substrate (Copper Clad Laminate substrate), a PCB (Printed Circuit Board) substrate, a photomask substrate, and a display panel, has increased. A demand for planarizing a surface of a package substrate on which electronic devices, such as a PCB substrate, are disposed also has increased. 
     A chemical mechanical polishing device includes a top ring that holds a substrate and a polishing table to which a polishing pad is attached, and is configured to polish the substrate by pressing the substrate against the polishing pad while rotating the top ring and the polishing table. Here, in the polishing of the substrate, the substrate comes off from the top ring to slip outside the top ring in some cases. In contrast, for example, PTL 1 discloses that a photoelectric sensor is used to detect the slip out of the substrate. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent No. 3761673 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the technique disclosed in PTL 1, a light is emitted toward a detection area set on a polishing pad to detect the slip out of the substrate based on a change of an amount of the light reflected from the detection area. That is, since the polishing pad and the substrate are different in light reflectance, the amount of the reflected light from the detection area changes when the substrate comes off from the top ring to appear in the detection area, thereby determining that the substrate has come off from the top ring when the change is detected. PTL 1 also discloses the detection of the slip out of the substrate using a color difference sensor. 
     However, the technique disclosed in PTL 1 still has a room to improve the accuracy of detecting the slip out of the substrate. That is, since the substrate as a process target of the polishing device has a variety in thickness and base material (material) and is large in variation of the reflection-light amount, the photoelectric sensor possibly makes a false detection in the detection method using the light-amount difference. While the polishing device uses a slurry (polishing liquid) to polish the substrate, shavings are generated from the substrate to be mixed in the slurry during the polishing, or the slurry is discolored by a chemical reaction or the like in some cases. This possibly causes the sensor to make a false detection due to flowing of the slurry in which the shavings are mixed or the discolored slurry into the detection area even when the color difference sensor is used. 
     Therefore, this application has one object to improve an accuracy of detecting a slip out of a substrate from a top ring. 
     Solution to Problem 
     One embodiment discloses a polishing unit that includes a polishing table, a top ring, a light emitting member, a slip-out detector, and an elimination mechanism. A polishing pad for polishing a substrate is attached to the polishing table. The top ring holds the substrate to press the substrate against the polishing pad. The light emitting member emits a light to a detection area on the polishing pad. The slip-out detector detects a slip out of the substrate from the top ring based on the light reflected from the detection area. The elimination mechanism eliminates a polishing liquid flowing into the detection area. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view illustrating an overall configuration of a substrate processing apparatus according to one embodiment; 
         FIG. 2  is a perspective view schematically illustrating a configuration of a polishing unit according to the one embodiment; 
         FIG. 3  is a plan view schematically illustrating the configuration of the polishing unit including a slip-out detector and an elimination mechanism; 
         FIG. 4  is a side view schematically illustrating the configuration of the polishing unit including the slip-out detector and the elimination mechanism; 
         FIG. 5  is a side view schematically illustrating the configuration of the polishing unit including the slip-out detector and the elimination mechanism, and illustrates a state where a substrate has slipped out; 
         FIG. 6  is a drawing illustrating an experimental result of slip-out detection based on a color difference; 
         FIG. 7  is a drawing illustrating an experimental result of slip-out detection based on the color difference and a light amount; 
         FIG. 8  is a side view schematically illustrating the configuration of the polishing unit including the slip-out detector and the elimination mechanism, and illustrates a state where the elimination mechanism is used to eliminate a polishing liquid; 
         FIG. 9  is a plan view schematically illustrating a relation between a detection area and an elimination area on a polishing pad; 
         FIG. 10  is a side view schematically illustrating the configuration of the polishing unit including the slip-out detector and the elimination mechanism, and illustrates a state where the elimination mechanism is used to eliminate the polishing liquid; 
         FIG. 11  is a flowchart of the slip-out detection of the substrate; 
         FIG. 12  is a plan view schematically illustrating a configuration of a modification of the polishing unit including the slip-out detector and the elimination mechanism; and 
         FIG. 13  is a plan view schematically illustrating a configuration of a modification of the polishing unit including the slip-out detector and the elimination mechanism. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of a substrate processing apparatus according to the present invention with reference to the attached drawings. In the attached drawings, the same or similar reference numerals are attached to the same or similar components, and overlapping description regarding the same or similar components may be omitted in the description of the respective embodiments. Features illustrated in the respective embodiments are applicable to other embodiments in so far as they are consistent with one another. 
       FIG. 1  is a plan view illustrating an overall configuration of a substrate processing apparatus  1000  according to one embodiment. The substrate processing apparatus  1000  illustrated in  FIG. 1  includes a loading unit  100 , a conveyance unit  200 , a polishing unit  300 , a drying unit  500 , and an unloading unit  600 . In the illustrated embodiment, the conveyance unit  200  includes two conveyance units  200 A,  200 B, and the polishing unit  300  includes two polishing units  300 A,  300 B. In the one embodiment, these units can be each independently formed. Independently forming these units ensures facilitating to form the substrate processing apparatus  1000  in a different configuration by appropriately combining the number of respective units. The substrate processing apparatus  1000  includes a control device  900 , and each component of the substrate processing apparatus  1000  is controlled by the control device  900 . In the one embodiment, the control device  900  can be configured of a general computer that includes, for example, an input/output device, an arithmetic device, and a storage device. 
     &lt;Loading Unit&gt; 
     The loading unit  100  is a unit for introducing a substrate WF before processes, such as polishing and cleaning, are performed into the substrate processing apparatus  1000 . In the one embodiment, the loading unit  100  is configured to be compliant to a mechanical equipment interface standard (IPC-SMEMA-9851) of Surface Mount Equipment Manufacturers Association (SMEMA). 
     In the illustrated embodiment, a conveyance mechanism of the loading unit  100  includes a plurality of conveyance rollers  202  and a plurality of roller shafts  204  to which the conveyance rollers  202  are mounted. In the embodiment illustrated in  FIG. 1 , the three conveyance rollers  202  are mounted to each roller shaft  204 . The substrate WF is disposed on the conveyance roller  202 , and the substrate WF is conveyed by rotation of the conveyance roller  202 . The conveyance rollers  202  may be mounted to the roller shaft  204  at any positions insofar as they are positions that ensure stable conveyance of the substrate WF. However, since the conveyance rollers  202  contact the substrate WF, the conveyance rollers  202  need to be disposed so as to contact a region without a problem even when the conveyance rollers  202  contact the substrate WF as a process target. In the one embodiment, the conveyance roller  202  of the loading unit  100  can be formed of a conductive polymer. In the one embodiment, the conveyance rollers  202  are electrically earthed via the roller shafts  204  or the like. This is performed to avoid a damage of an electronic device and the like on the substrate WF due to charging of the substrate WF. In the one embodiment, the loading unit  100  may include an ionizer (not illustrated) to avoid the charging of the substrate WF. 
     &lt;Conveyance Unit&gt; 
     The substrate processing apparatus  1000  illustrated in  FIG. 1  includes the two conveyance units  200 A,  200 B. Since the two conveyance units  200 A,  200 B can have the same configuration, they are collectively described as the conveyance unit  200  in the following description. 
     The illustrated conveyance unit  200  includes a plurality of conveyance rollers  202  to convey the substrate WF. By rotating the conveyance rollers  202 , the substrate WF on the conveyance rollers  202  can be conveyed in a predetermined direction. The conveyance rollers  202  of the conveyance unit  200  may be formed of a conductive polymer, or formed of a non-conductive polymer. The conveyance rollers  202  are driven by a motor (not illustrated). The substrate WF is conveyed to a substrate transfer position by the conveyance rollers  202 . 
     In the one embodiment, the conveyance unit  200  includes a cleaning nozzle  284 . The cleaning nozzle  284  is connected to a supply source (not illustrated) of a cleaning liquid. The cleaning nozzle  284  is configured to supply the cleaning liquid to the substrate WF conveyed by the conveyance rollers  202 . 
     &lt;Drying Unit&gt; 
     The drying unit  500  is a device to dry the substrate WF. In the substrate processing apparatus  1000  illustrated in  FIG. 1 , the drying unit  500  dries the substrate WF cleaned by a cleaning unit of the conveyance unit  200  after the polishing by the polishing unit  300 . As illustrated in  FIG. 1 , the drying unit  500  is disposed in the downstream of the conveyance unit  200 . 
     The drying unit  500  includes a nozzle  530  to inject a gas toward the substrate WF conveyed on the conveyance rollers  202 . The gas may be, for example, a compressed air or nitrogen. By blowing off water droplets on the conveyed substrate WF by the drying unit  500 , the substrate WF can be dried. 
     &lt;Unloading Unit&gt; 
     The unloading unit  600  is a unit to carry out the substrate WF after the processes, such as the polishing and the cleaning, are performed outside the substrate processing apparatus  1000 . In the substrate processing apparatus  1000  illustrated in  FIG. 1 , the unloading unit  600  receives the substrate after the drying by the drying unit  500 . As illustrated in  FIG. 1 , the unloading unit  600  is disposed in the downstream of the drying unit  500 . In the one embodiment, the unloading unit  600  is configured to be compliant to the mechanical equipment interface standard (IPC-SMEMA-9851) of Surface Mount Equipment Manufacturers Association (SMEMA). 
     &lt;Polishing Unit&gt; 
       FIG. 2  is a perspective view schematically illustrating a configuration of the polishing unit  300  according to the one embodiment. The substrate processing apparatus  1000  illustrated in  FIG. 1  includes the two polishing units  300 A,  300 B. Since the two polishing units  300 A,  300 B can have the same configuration, they are collectively described as the polishing unit  300  in the following description. 
     As illustrated in  FIG. 2 , the polishing unit  300  includes a polishing table  350  and a top ring  302 . The top ring  302  constitutes a polishing head that holds the substrate as an object to be polished to press to a polishing surface on the polishing table  350 . The polishing table  350  is connected to a polishing table rotation motor (not illustrated) disposed via the table shaft  351  therebelow, and rotatable about the table shaft  351 . A polishing pad  352  is attached to a top surface of the polishing table  350 , and a surface  352   a  of the polishing pad  352  constitutes the polishing surface to polish the substrate. In the one embodiment, the polishing pad  352  may be attached via a layer to facilitate detaching from the polishing table  350 . Such a layer includes a silicone layer, a fluorine-based resin layer, and the like, and for example, a layer disclosed in Japanese Unexamined Patent Application Publication No. 2014-176950 may be used. 
     A polishing liquid supply nozzle  354  is disposed above the polishing table  350 , and the polishing liquid supply nozzle  354  is configured to supply the polishing liquid on the polishing pad  352  on the polishing table  350 . As illustrated in  FIG. 2 , a passage  353  is disposed through the polishing table  350  and the table shaft  351  to supply the polishing liquid. The passage  353  is communicated with an opening portion  355  in the surface of the polishing table  350 . The polishing pad  352  is provided with a through-hole  357  at a position corresponding to the opening portion  355  of the polishing table  350 , and the polishing liquid passing through the passage  353  is supplied to the surface of the polishing pad  352  from the opening portion  355  of the polishing table  350  and the through-hole  357  of the polishing pad  352 . The numbers of the opening portions  355  of the polishing table  350  and the through-holes  357  of the polishing pad  352  may be one or more. While the positions of the opening portion  355  of the polishing table  350  and the through-hole  357  of the polishing pad  352  may be any positions, they are provided in the proximity of the center of the polishing table  350  in the one embodiment. 
     While not illustrated in  FIG. 2 , in the one embodiment, the polishing unit  300  includes an atomizer  358  (see  FIG. 1 ) to inject a liquid or a mixture fluid of liquid and gas toward the polishing pad  352 . The liquid injected from the atomizer  358  is, for example, pure water, and the gas is, for example, nitrogen gas. 
     The top ring  302  is connected to a top ring shaft  18 , and the top ring shaft  18  is configured to be moved up and down with respect to a swing arm  360  by an up-and-down motion mechanism  319 . The top ring  302  is configured to be entirely moved up and down with respect to the swing arm  360  by the up-and-down motion of the top ring shaft  18  and positioned. The top ring shaft  18  is configured to be rotated by the driving of a top ring rotation motor (not illustrated). The top ring  302  is configured to be rotated about the top ring shaft  18  by the rotation of the top ring shaft  18 . A rotary joint  323  is mounted to an upper end of the top ring shaft  18 . 
     Various kinds of the polishing pad are available in the market, and for example, SUBA800 (“SUBA” is registered trademark), IC-1000, and IC-1000/SUBA400 (two-layer cloth) manufactured by Nitta Haas Incorporated, and Surfin xxx-5, Surfin 000, and the like (“Surfin” is registered trademark) manufactured by FUJIMI INCORPORATED are included. SUBA800, Surfin xxx-5, and Surfin 000 are nonwoven fabrics made of fibers hardened with urethane resin, and IC-1000 is a rigid polyurethane foam (single layer). The polyurethane foam is porous, and many fine hollows or holes are provided in its surface. 
     The top ring  302  is configured to hold the rectangular substrate in its lower surface. The swing arm  360  is configured to be turnable about a spindle  362 . The top ring  302  is movable between the substrate transfer position of the conveyance unit  200  described above and a position above the polishing table  350  by the turn of the swing arm  360 . By moving down the top ring shaft  18 , the top ring  302  can be moved down to press the substrate to the surface (polishing surface)  352   a  of the polishing pad  352 . At this time, the top ring  302  and the polishing table  350  are each rotated, and the polishing liquid is supplied on the polishing pad  352  from the polishing liquid supply nozzle  354  disposed above the polishing table  350  and/or from the opening portion  355  provided to the polishing table  350 . Thus, the substrate WF is pressed to the polishing surface  352   a  of the polishing pad  352 , thereby allowing the polishing of the surface of the substrate. The swing arm  360  may be fixed or swung such that the top ring  302  passes through the center of the polishing pad  352  (covers the through-hole  357  of the polishing pad  352 ) during the polishing of the substrate WF. 
     The up-and-down motion mechanism  319  that moves the top ring shaft  18  and the top ring  302  up and down includes a bridge  28 , a ball screw  32 , a support table  29 , and a servo motor  38 . The bridge  28  rotatably supports the top ring shaft  18  via a bearing  321 . The ball screw  32  is mounted to the bridge  28 . The support table  29  is supported by a support pillar  130 . The servo motor  38  is disposed on the support table  29 . The support table  29  that supports the servo motor  38  is secured to the swing arm  360  via the support pillar  130 . 
     The ball screw  32  includes a screw shaft  32   a  connected to the servo motor  38 , and a nut  32   b  with which the screw shaft  32   a  screws. The top ring shaft  18  is configured to move up and down integrally with the bridge  28 . Accordingly, when the servo motor  38  is driven, the bridge  28  moves up and down via the ball screw  32 , thereby moving the top ring shaft  18  and the top ring  302  up and down. The polishing unit  300  includes a distance measuring sensor  70  as a position detection unit that detects a distance to a lower surface of the bridge  28 , that is, a position of the bridge  28 . By detecting the position of the bridge  28  by the distance measuring sensor  70 , the position of the top ring  302  can be detected. The distance measuring sensor  70  constitutes the up-and-down motion mechanism  319  together with the ball screw  32  and the servo motor  38 . The distance measuring sensor  70  may be a laser sensor, an ultrasonic wave sensor, an overcurrent sensor, or a linear scale sensor. The devices in the polishing unit including the distance measuring sensor  70  and the servo motor  38  are each configured to be controlled by the control device  900 . 
     The polishing unit  300  according to the one embodiment includes a dressing unit  356  that dresses the polishing surface  352   a  of the polishing pad  352 . As illustrated in  FIG. 2 , the dressing unit  356  includes a dresser  50 , a dresser shaft  51 , an air cylinder  53 , and a swing arm  55 . The dresser  50  is brought into sliding contact with the polishing surface  352   a . The dresser  50  is connected to the dresser shaft  51 . The air cylinder  53  drives the dresser shaft  51  up and down. The swing arm  55  rotatably supports the dresser shaft  51 . A dressing member  50   a  is held onto the lower portion of the dresser  50 , and needle-shaped diamond particles are electrodeposited to the lower surface of the dressing member  50   a . The air cylinder  53  is disposed on a support table  57  supported by support pillars  56 , and the support pillars  56  are secured to the swing arm  55 . 
     The swing arm  55  is configured to be driven by a motor (not illustrated) to turn about a spindle  58 . The dresser shaft  51  is disposed to be opposed to the polishing pad  352  and rotated by the driving of a motor not illustrated in  FIG. 2 , and the dresser  50  is rotated about the dresser shaft  51  by the rotation of the dresser shaft  51 . The air cylinder  53  moves the dresser  50  up and down via the dresser shaft  51 , and presses the dresser  50  to the polishing surface  352   a  of the polishing pad  352  with a predetermined pressing force. 
     The dressing of the polishing surface  352   a  of the polishing pad  352  is performed as follows. The dresser  50  is pressed to the polishing surface  352   a  by the air cylinder  53 , and the pure water is simultaneously supplied to the polishing surface  352   a  from a pure water supply nozzle (not illustrated). In this state, the dresser  50  rotates about the dresser shaft  51  to bring the lower surface (diamond particles) of the dressing member  50   a  into sliding contact with the polishing surface  352   a . Thus, the polishing pad  352  is scraped off from the dresser  50 , and the polishing surface  352   a  is dressed. 
     &lt;Slip-Out Detector&gt; 
       FIG. 3  is a plan view schematically illustrating the configuration of the polishing unit including a slip-out detector and an elimination mechanism.  FIG. 4  is a side view schematically illustrating the configuration of the polishing unit including the slip-out detector and the elimination mechanism. As illustrated in  FIG. 3  and  FIG. 4 , the polishing unit  300  includes a slip-out detector  370  to detect a slip out of the substrate WF from the top ring  302 . At any position on the polishing pad  352 , a detection area  372  for performing a slip-out detection is set. The slip-out detector  370  is mounted to a distal end of a slip-out detection arm  374  extending over the polishing pad  352 , and disposed to be opposed to the detection area  372 . The slip-out detector  370  includes a light emitting member  371  to emit a light to the detection area  372  and a light receiving member  373  to receive a light reflected from the detection area  372 . While this embodiment indicates an example in which the slip-out detector  370  includes the light emitting member  371  and the slip-out detector  370  emits the light while simultaneously receiving the reflected light, the configuration is not limited to this. For example, the light emitting member  371  may be a member independent from the slip-out detector  370 . The slip-out detector  370  is configured to detect the slip out of the substrate WF from the top ring  302  based on the light received by the light receiving member  373 , that is, the light reflected from the detection area  372 . 
     More specifically, the slip-out detector  370  can detect the slip out of the substrate WF from the top ring  302  based on a change of a color of the light reflected from the detection area  372 . That is, the slip-out detector  370  emits the light to the detection area  372  of the polishing pad  352  as a reference before the polishing process of the substrate WF is performed, and registers a reference color of the polishing pad based on the light reflected from the detection area  372 . The slip-out detector  370  compares the color based on the light reflected from the detection area  372  of the polishing pad  352  with the reference color during the polishing process of the substrate WF. 
       FIG. 5  is a side view schematically illustrating the configuration of the polishing unit including the slip-out detector and the elimination mechanism, and illustrates a state where the substrate WF has slipped out. As illustrated in  FIG. 5 , when the substrate WF comes off from the top ring  302  to enter the detection area  372 , the slip-out detector  370  detects a color different from the reference color of the polishing pad  352 , thereby ensuring the detection of the slip out of the substrate WF from the top ring  302 . 
       FIG. 6  is a drawing illustrating an experimental result of the slip-out detection based on a color difference. In  FIG. 6 , the horizontal axis indicates a rotation speed (RPM) of the polishing table  350 , and the vertical axis indicates a sensitivity of color difference based on the reflected light from the detection area  372 . The sensitivity of color difference based on the reflected light from the detection area  372  is a value that indicates a degree of match between the reference color registered in advance and the color based on the reflected light from the detection area  372 . The value of the sensitivity when completely matching the reference color is 999. The slip-out detector  370  can detect the slip out of the substrate WF from the top ring  302  when the sensitivity of color difference based on the reflected light from the detection area  372  becomes less than a threshold value for the slip-out detection. For example, in  FIG. 6 , in the case where the rotation speed of the polishing table  350  is 10 (RPM), the sensitivity of color difference based on the reflected light from the detection area  372  is 996 when the polishing pad  352  is present (when the substrate WF has not entered) in the detection area  372 , and the approximately matching with the reference color is seen. Meanwhile, when the substrate WF is present in the detection area  372 , the sensitivity of color difference based on the reflected light from the detection area  372  is 525. For example, assume that the threshold value for the slip-out detection is 700. In this case, when the substrate WF comes off from the top ring  302  and enters the detection area  372 , the sensitivity ( 525 ) of color difference based on the reflected light from the detection area  372  is less than the threshold value ( 700 ). Therefore, the slip-out detector  370  can detect the slip-out of the substrate WF. 
     Thus, when the sensitivity of color difference based on the reflected light from the detection area  372  is significantly different between the case where the polishing pad  352  is present (the substrate WF is absent) in the detection area  372  and the case where the substrate WF is present in the detection area  372 , the slip-out detector  370  can accurately detect the slip out of the substrate WF from the top ring  302 . As illustrated in the experimental result in  FIG. 6 , by using the color difference for the slip-out detection, the determination is made based on the difference between the color registered as the reference color and the color of the reflected light from the detection area  372 . Therefore, the difference of reflection angle of the emitted/received light due to the difference of the thickness of the substrate WF and the light-amount difference of the reflected light due to the difference of the material of the substrate WF (including the influence of the difference in unevenness of the surface) are less likely to have influences, thus ensuring the effective slip-out detection of the substrate WF. 
     Meanwhile, when the colors of the polishing pad  352  and the substrate WF are close or when a disturbance has an influence, the difference of the sensitivity of color difference based on the reflected light from the detection area  372  possibly decreases between the case where the polishing pad  352  is present (the substrate WF is absent) in the detection area  372  and the case where the substrate WF is present in the detection area  372 . In this case, the slip out of the substrate WF from the top ring  302  possibly cannot be detected regardless of the presence of the substrate WF in the detection area  372  depending on the setting of the threshold value for the slip-out detection. 
     In contrast to, the slip-out detector  370  can be configured to detect the slip out of the substrate WF from the top ring  302  based on the change of the amount of the light reflected from the detection area  372  in addition to the change of the color of the light reflected from the detection area  372 . That is, the slip-out detector  370  emits the light to the detection area  372  of the polishing pad  352  as a reference before the polishing process of the substrate WF is performed, and registers a reference amount of the light reflected from the detection area  372 . The slip-out detector  370  compares the amount of the light reflected from the detection area  372  of the polishing pad  352  with the preliminarily registered reference amount of the light during the polishing process of the substrate WF. When the substrate WF comes off from the top ring  302  and enters the detection area  372 , the light amount different from the amount of the reflected light from the polishing pad  352  is detected. Therefore, the slip-out detector  370  can detect the slip out of the substrate WF from the top ring  302 . 
       FIG. 7  is a drawing illustrating an experimental result of the slip-out detection based on the color difference and the light amount. In  FIG. 7 , the horizontal axis indicates a rotation speed (RPM) of the polishing table  350 , and the vertical axis indicates a sensitivity of color difference and light amount based on the reflected light from the detection area  372 . The sensitivity of color difference and light amount based on the reflected light from the detection area  372  is a value that indicates a degree of match between the reference color and reference light amount registered in advance and the color and light amount based on the reflected light from the detection area  372 . The slip-out detector  370  can detect the slip out of the substrate WF from the top ring  302  when the sensitivity of color difference and light amount based on the reflected light from the detection area  372  becomes less than a threshold value for the slip-out detection. As illustrated in  FIG. 7 , in the case of the slip-out detection based on the color difference and the light amount, the sensitivity of color difference and light amount based on the reflected light from the detection area  372  is significantly different between the case where the polishing pad  352  is present (the substrate WF is absent) in the detection area  372  and the case where the substrate WF is present in the detection area  372  regardless of the rotation speed of the polishing table  350 . Accordingly, the slip-out detector  370  can accurately detect the slip out of the substrate WF from the top ring  302 . 
     &lt;Elimination Mechanism&gt; 
     As illustrated in  FIG. 3 , the polishing unit  300  includes an elimination mechanism  380  that eliminates the polishing liquid flowing into the detection area  372 .  FIG. 8  is a side view schematically illustrating the configuration of the polishing unit including the slip-out detector and the elimination mechanism, and illustrates a state where the elimination mechanism is used to eliminate the polishing liquid. As illustrated in  FIG. 8 , the elimination mechanism  380  includes a purge mechanism (purge nozzle)  384  that sprays a fluid  386  to the detection area  372  to eliminate a polishing liquid SL flowing into the detection area  372 . The fluid may be a liquid, such as water or a polishing liquid having the same color as the polishing pad  352 , or a gas, such as air or nitrogen. 
       FIG. 9  is a plan view schematically illustrating the relation between the detection area and an elimination area on the polishing pad. As illustrated in  FIG. 9 , the purge mechanism  384  is configured to spray the fluid  386  from the purge mechanism  384  to an elimination area  382  as an area that includes the detection area  372  and is greater than the detection area  372 . According to the embodiment, the accuracy of detecting the slip out of the substrate WF from the top ring  302  can be improved. That is, when the polishing liquid SL different in color from the polishing pad  352  is used, the slip-out detector  370  possibly makes a false detection when the polishing liquid SL flows into the detection area  372 . Even when the polishing liquid SL having the same color as the polishing pad  352  is used, the slip-out detector  370  possibly makes a false detection when the polishing liquid SL including shavings of the substrate WF generated during the polishing or the polishing liquid SL discolored by a chemical reaction or the like flows into the detection area  372 . In contrast, according to the embodiment, the purge mechanism  384  sprays the fluid  386  to the detection area  372  to avoid flowing of the polishing liquid SL into the detection area  372 , thereby ensuring suppression of the occurrence of false detection by the slip-out detector  370 . For aged deterioration of the polishing pad  352  and stains on the polishing pad  352  caused by the polishing process, a conditioning treatment of the polishing pad  352  may be performed by using at least one of the atomizer  358  and the dresser  50  for each substrate process. The slip-out detector  370  and the elimination mechanism  380  can be controlled to make the slip-out detection of the substrate WF effective only during the polishing of the substrate WF. For example, in a sequence of processes by the polishing unit  300 , the dresser  50  moves across the detection area  372  for the conditioning treatment of the polishing pad  352  in some cases. In this case, operation timings of the slip-out detector  370  and the elimination mechanism  380  can be controlled to avoid the false detection by the slip-out detector  370 . 
     While the example in which the elimination mechanism  380  includes the purge mechanism  384  is indicate in this embodiment, the configuration is not limited to this.  FIG. 10  is a side view schematically illustrating the configuration of the polishing unit including the slip-out detector and the elimination mechanism, and illustrates a state where the elimination mechanism is used to eliminate the polishing liquid. The elimination mechanism  380  may include a suction mechanism (suction nozzle)  388  that suctions the polishing liquid SL flow into the detection area  372  to eliminate the polishing liquid SL flowing into the detection area  372 . In this case, the suction mechanism  388  can eliminate the polishing liquid SL flowing into the detection area  372  by performing the suction to the elimination area  382 . 
     Next, a description will be given of a procedure of a polishing method including the slip-out detection of the substrate WF according to the embodiment.  FIG. 11  is a flowchart of the slip-out detection of the substrate WF. As illustrated in  FIG. 11 , in the polishing method, first, the top ring  302  receives the substrate WF and moves to a polishing position (Step S 101 ). Subsequently, the polishing method starts the slip-out detection (detecting step S 102 ). Specifically, the polishing method starts emitting and receiving the light by the slip-out detector  370 . Subsequently, the polishing method starts the purge in a slip-out detection area (elimination step S 103 ). Specifically, the polishing method sprays the fluid from the purge mechanism (purge nozzle)  384  to the elimination area  382 . 
     Subsequently, the polishing method starts the polishing of the substrate WF (polishing step S 104 ). That is, the polishing method rotates the polishing table  350  while rotating the top ring  302 , and presses the substrate WF held onto the top ring  302  to the polishing pad  352 . Subsequently, the polishing method determines whether the slip-out of the substrate WF is detected or not (Step S 105 ). When the slip-out of the substrate WF is detected by the slip-out detector  370  (Step S 105 , Yes), the polishing method safely stops the operation of the substrate processing apparatus  1000  (Step S 106 ). 
     Meanwhile, when the slip-out of the substrate WF is not detected by the slip-out detector  370  (Step S 105 , No), and a predetermined polishing time has passed, the polishing method terminates the polishing of the substrate WF (Step S 107 ). Subsequently, the polishing method stops the purge in the slip-out detection area (Step S 108 ). Subsequently, the polishing method stops the emitting and receiving of the light by the slip-out detector  370  to stop the slip-out detection (Step S 109 ). Subsequently, in the polishing method, the top ring  302  delivers the substrate WF to the conveyance unit  200  (Step S 110 ). 
     Subsequently, the polishing method counts a cumulative number of polishing processes in the polishing unit  300  (Step S 111 ). Subsequently, the polishing method determines whether the counted cumulative number of polishing processes has reached a preliminarily set number of polishing processes (threshold value) or not (Step S 112 ). When the counted cumulative number of polishing processes does not exceed the preliminarily set number of polishing processes (threshold value) (Step S 112 , No), the polishing method returns to Step S 101  and repeats the process to the substrate WF as a next process target. Meanwhile, when the counted cumulative number of polishing processes exceeds the preliminarily set number of polishing processes (threshold value) (Step S 112 , Yes), the polishing method resets the reference color of the slip-out detector  370  and automatically reregisters the reference color (Step S 113 ). That is, the slip-out detector  370  assumes the current polishing pad  352  as the reference polishing pad, emits the light to the detection area  372  of this polishing pad  352 , and registers the reference color of the polishing pad based on the light reflected from the detection area  372 . The polishing method returns to Step S 101  after Step S 113 , and repeats the process to the substrate WF as the next process target. Note that the order of the above-described processing steps may be changed, or they may be simultaneously executed. 
       FIG. 12  is a plan view schematically illustrating a configuration of a modification of the polishing unit including the slip-out detector and the elimination mechanism. As illustrated in  FIG. 12 , a plurality of the detection areas  372  may be set on the polishing pad  352 . In this case, a plurality of the slip-out detectors  370  and a plurality of the elimination mechanisms  380  may be disposed corresponding to the respective plurality of detection areas  372  on the polishing pad  352 . According to this embodiment, the slip out of the substrate WF can be detected with high accuracy even when the substrate WF projects from the top ring  302  in various directions. 
       FIG. 13  is a plan view schematically illustrating a configuration of a modification of the polishing unit including the slip-out detector and the elimination mechanism. As illustrated in  FIG. 13 , the slip-out detector  370  and the elimination mechanism  380  may be mounted to the top ring  302 . That is, while the detection area  372  can be set to any position on the polishing pad  352 , the detection area  372  is preferably set to the proximity of the top ring  302  in the downstream side in the rotation direction of the polishing pad  352 . This is because, since the substrate WF projects from the top ring  302  often to the downstream side in the rotation direction of the polishing pad  352  when the substrate WF comes off from the top ring  302 , the slip out of the substrate WF from the top ring  302  can be instantly detected. However, when the top ring  302  is swung on the polishing pad  352  by the swing arm  360  during the polishing, the position of the detection area  372  also moves. Therefore, a mechanism that moves the slip-out detector  370  and the elimination mechanism  380  corresponding to the move of the detection area  372  is required, thus complicating the configuration of the polishing unit  300 . In contrast, according to the embodiment, since the slip-out detector  370  and the elimination mechanism  380  are mounted to the top ring  302 , the slip-out detector  370  and the elimination mechanism  380  swing in accordance with the swing of the top ring  302 . Accordingly, this embodiment ensures instantly detecting the slip out of the substrate WF from the top ring  302  with the simple structure. 
     The embodiments of the present invention have been described above in order to facilitate understanding of the present invention without limiting the present invention. The present invention can be changed or improved without departing from the gist thereof, and of course, the equivalents of the present invention are included in the present invention. It is possible to arbitrarily combine or omit respective components according to claims and description in a range in which at least a part of the above-described problems can be solved, or a range in which at least a part of the effects can be exhibited. 
     This application discloses, as one embodiment, a polishing unit that includes a polishing table, a top ring, a light emitting member, a slip-out detector, and an elimination mechanism. A polishing pad for polishing a substrate is attached to the polishing table. The top ring holds the substrate to press the substrate against the polishing pad. The light emitting member emits a light to a detection area on the polishing pad. The slip-out detector detects a slip out of the substrate from the top ring based on the light reflected from the detection area. The elimination mechanism eliminates a polishing liquid flowing into the detection area. 
     This application further discloses, as one embodiment, the polishing unit in which the slip-out detector detects the slip out of the substrate from the top ring based on a change of a color of the light reflected from the detection area. 
     This application further discloses, as one embodiment, the polishing unit in which the slip-out detector detects the slip out of the substrate from the top ring based on a change of an amount of the light reflected from the detection area in addition to the change of the color of the light reflected from the detection area. 
     This application further discloses, as one embodiment, the polishing unit in which the elimination mechanism includes a purge mechanism for spraying a fluid to the detection area to eliminate the polishing liquid flowing into the detection area. 
     This application further discloses, as one embodiment, the polishing unit in which the elimination mechanism includes a suction mechanism for suctioning the polishing liquid flowing into the detection area to eliminate the polishing liquid flowing into the detection area. 
     This application further discloses, as one embodiment, the polishing unit in which the light emitting member, the slip-out detector, and the elimination mechanism are disposed to each of a plurality of detection areas on the polishing pad. 
     This application further discloses, as one embodiment, the polishing unit in which the light emitting member, the slip-out detector, and the elimination mechanism are mounted to the top ring. 
     This application further discloses, as one embodiment, a substrate processing apparatus that includes any one of the above-described polishing units that polishes a substrate, a conveyance unit that conveys the substrate, and a drying unit that dries the substrate. 
     This application further discloses, as one embodiment, a polishing method that includes a polishing step of polishing a substrate by flowing a polishing liquid on a polishing pad that polishes the substrate, a detecting step of emitting a light to a detection area on the polishing pad and detecting a slip out of the substrate from the top ring based on the light reflected from the detection area, and an eliminating step of eliminating the polishing liquid flowing into the detection area. 
     This application further discloses, as one embodiment, the polishing method in which the detecting step includes detecting the slip out of the substrate from the top ring based on a difference between a reference color of a light reflected from the detection area of the polishing pad as a reference and a color of the light reflected from the detection area of the polishing pad during a polishing process. 
     This application further discloses, as one embodiment, the polishing method that further includes a step of counting a cumulative number of polishing processes of the substrate and a step of registering the reference color when the cumulative number of polishing processes exceeds a threshold value. 
     REFERENCE SIGNS LIST 
     
         
         
           
               200  . . . conveyance unit 
               300  . . . polishing unit 
               302  . . . top ring 
               350  . . . polishing table 
               352  . . . polishing pad 
               370  . . . slip-out detector 
               371  . . . light emitting member 
               372  . . . detection area 
               373  . . . light receiving member 
               374  . . . slip-out detection arm 
               380  . . . elimination mechanism 
               382  . . . elimination area 
               384  . . . purge mechanism (purge nozzle) 
               386  . . . fluid 
               388  . . . suction mechanism (suction nozzle) 
               500  . . . drying unit 
               1000  . . . substrate processing apparatus 
             SL . . . polishing liquid 
             WF . . . substrate