JP2017220682A - Abrasive pad - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an abrasive pad for use in a polishing device capable of acquiring highly accurate thickness data, in the whole area of a substrate including the center and peripheral portion thereof.SOLUTION: An abrasive pad 22 has a first through-hole 31A and a second through-hole 31B for passing light. The second through-hole 31B is placed on the inside of the first through-hole 31A in the radial direction of the abrasive pad 22. When the first through-hole 31A is under a substrate W held by a top ring 24, the second through-hole 31B is placed at a position not existing under the substrate W held by the top ring 24. The first through-hole 31A is formed at a position facing the central part of the substrate W, and the second through-hole 31B is formed at a position facing the peripheral portion of the substrate W.SELECTED DRAWING: Figure 16

Description

  The present invention relates to a polishing apparatus for polishing the surface of a substrate such as a semiconductor wafer. More specifically, the present invention obtains a film thickness distribution on the entire surface of the substrate including the central portion and the peripheral portion of the substrate during polishing of the substrate. The present invention relates to a polishing pad used in a polishing apparatus that controls a load on a substrate based on a measured film thickness distribution.

  A CMP (chemical mechanical polishing) apparatus is widely known as an apparatus for polishing the surface of a substrate such as a semiconductor wafer. This CMP apparatus polishes the surface of a substrate by pressing the substrate against the polishing pad with a top ring while supplying a polishing liquid to the polishing pad on the rotating polishing table. The CMP apparatus generally includes a film thickness measuring unit that measures a film thickness of the substrate or a signal equivalent to the film thickness, and determines the polishing load on the substrate based on the measured film thickness obtained from the film thickness measuring unit. Control and further determine the polishing end point. As the film thickness measuring unit, an eddy current sensor or an optical sensor is generally used.

  FIG. 1 is a plan view showing a positional relationship between a film thickness measuring unit and a substrate in a conventional CMP apparatus. The film thickness measuring unit 100 is disposed inside the polishing table 102 so as to face the substrate W on the polishing pad 105. Therefore, each time the polishing table 102 rotates, the film thickness measurement unit 100 measures the film thickness at a plurality of measurement points on the substrate W while moving across the substrate W. In the conventional CMP apparatus, the film thickness measuring unit 100 is disposed so as to pass through the center of the substrate W as shown in FIG. This is because the film thickness is measured at a plurality of measurement points distributed in the radial direction of the substrate W as shown in FIG.

  Since a fine circuit pattern exists on the surface of the substrate to be polished, depending on the region on the substrate, data corresponding to the obtained film thickness (for example, an eddy current sensor) The voltage value or current value, and the relative reflectance in the case of an optical sensor may be different. In order to avoid the influence of such a circuit pattern, smoothing is performed on the obtained data.

  The CMP apparatus determines the polishing load to each region (for example, the central portion, the intermediate portion, and the peripheral portion) of the substrate based on the film thickness profile obtained during polishing, and the substrate is arranged so that the film thickness becomes uniform. Grind. However, the conventional CMP apparatus has a problem that an accurate film thickness cannot be obtained because the number of measurement points corresponding to the peripheral edge of the substrate is small. This problem will be described with reference to FIG. FIG. 2 is a diagram showing measurement points on the substrate where film thickness measurement is performed while the polishing table rotates once. The peripheral edge of the substrate is an annular region located on the outermost side, and the width thereof is 10 mm to 20 mm. For this reason, as can be seen from FIG. 2, the number of measurement points on the peripheral portion is reduced.

  The peripheral portion of the substrate is a region that is most susceptible to the influence of polishing load and polishing liquid, and is a region in which the film thickness is likely to change greatly during polishing as compared to other regions of the substrate. Furthermore, the initial film thickness at the peripheral edge of the substrate is often larger than the film thickness in other regions. Therefore, it is necessary to accurately measure and monitor the film thickness at the peripheral edge during polishing of the substrate. However, as described above, since there are few measurement points at the periphery of the substrate, it is difficult to accurately obtain the film thickness of the periphery.

  FIG. 3 is a graph showing changes in the measured value of the film thickness at the central part of the substrate and the measured value of the film thickness at the peripheral part. The vertical axis of the graph shown in FIG. 3 represents the measured value (estimated value) of the film thickness obtained by the optical sensor, and the horizontal axis represents the polishing time. As can be seen from FIG. 3, the film thickness at the center of the substrate (see FIG. 2) gradually decreases with the polishing time, whereas the film thickness at the peripheral edge of the substrate (see FIG. 2) is , Change irregularly. This is because the number of measurement points at the peripheral edge is small and data necessary for the smoothing process cannot be obtained. In particular, when the polishing table rotates at a high speed, the number of measurement points at the peripheral edge of the substrate is further reduced.

  Thus, since it is difficult to acquire highly accurate film thickness data at the peripheral portion of the substrate, a highly accurate film thickness profile of the substrate cannot be acquired during polishing. As a result, it was difficult to obtain a desired film thickness profile by feeding back the film thickness profile to the polishing load.

Japanese Patent Laid-Open No. 2004-154928 Special table 2009-505847

  The present invention has been made to solve the above-described conventional problems, and is used in a polishing apparatus capable of acquiring highly accurate film thickness data over the entire surface including the center and peripheral portions of the substrate. An object is to provide a polishing pad.

  In order to achieve the above-described object, one embodiment of the present invention is a polishing pad against which a substrate held by a top ring is pressed, and the polishing pad includes a first through hole and a first hole for allowing light to pass through. The second through hole is disposed inside the first through hole with respect to the radial direction of the polishing pad, and the first through hole is the top hole. The second through hole is disposed at a position not below the substrate held by the top ring when the substrate is held by the ring, and the first through hole is a central portion of the substrate. The second through hole is formed at a position facing the peripheral edge of the substrate.

In a preferred aspect of the present invention, the first through hole and the second through hole are located on opposite sides with respect to the center of the polishing pad.
In a preferred aspect of the present invention, an angle formed between a line connecting the first through hole and the center of the polishing pad and a line connecting the second through hole and the center of the polishing pad is substantially It is characterized by 180 degrees.
In a preferred aspect of the present invention, an angle formed between a line connecting the first through hole and the center of the polishing pad and a line connecting the second through hole and the center of the polishing pad is substantially It is characterized by 120 degrees.
In a preferred aspect of the present invention, the peripheral portion of the substrate is an annular portion located on the outermost side of the substrate, and the width thereof is 10 mm to 20 mm.
In a preferred aspect of the present invention, a transparent window is provided in each of the first through hole and the second through hole.

  A reference example of the present invention is a polishing apparatus for polishing a substrate, a polishing table for holding a polishing pad, a top ring that presses the surface of the substrate against the polishing pad, and at least one that emits light A light source, a first optical head that irradiates the surface of the substrate with light from the light source, and receives reflected light from the substrate; irradiates the surface of the substrate with light from the light source; A second optical head that receives the reflected light, and a common spectroscope connected to the first optical head and the second optical head.

In a preferred aspect of the above reference example, the polishing table is rotatable about its axis.
In a preferred aspect of the above reference example, the second optical head is arranged outside the first optical head in the radial direction of the polishing table.
In a preferred aspect of the above reference example, the second optical head is disposed inside the first optical head in the radial direction of the polishing table.
In a preferred aspect of the above reference example, an angle formed between a line connecting the first optical head and the center of the polishing table and a line connecting the second optical head and the center of the polishing table is substantially It is characterized by 180 degrees.

In a preferred aspect of the above reference example, the polishing pad has a through hole at a position corresponding to the first optical head and the second optical head.
In a preferred aspect of the above reference example, the at least one light source is one common light source connected to the first optical head and the second optical head.
In a preferred aspect of the above reference example, the one common light source is connected to the first optical head and the second optical head via an optical switch.
In a preferred aspect of the above reference example, the one common spectrometer is connected to the first optical head and the second optical head via an optical switch.

  A reference example of the present invention is a polishing apparatus that polishes a substrate having a film in sliding contact with a polishing pad, the polishing table holding the polishing pad, and holding the substrate. The top ring that presses the surface of the substrate against the polishing pad, at least one light source that emits light, and the surface of the substrate is irradiated with light from the light source, and the reflected light from the substrate is received. A first optical head; a second optical head that irradiates the surface of the substrate with light from the light source and receives reflected light from the substrate; the first optical head and the second optical head; The relationship between the intensity of reflected light and the wavelength is shown from at least one spectrometer for measuring the intensity at each wavelength of the reflected light received by the light, and the intensity at each wavelength of the reflected light measured by the spectrometer. Spectrum And a processing unit for determining the film thickness of the substrate from the spectrum, wherein the first optical head is disposed to face the center of the substrate held by the top ring, and The optical head is arranged so as to face the peripheral edge of the substrate held by the top ring.

In a preferred aspect of the above reference example, the second optical head is arranged outside the first optical head in the radial direction of the polishing table.
In a preferred aspect of the above reference example, the second optical head is disposed inside the first optical head in the radial direction of the polishing table.
In a preferred aspect of the above reference example, the first optical head and the second optical head are arranged at different positions in the circumferential direction of the polishing table.

In a preferred aspect of the above reference example, an angle formed between a line connecting the first optical head and the center of the polishing table and a line connecting the second optical head and the center of the polishing table is substantially It is characterized by 180 degrees.
In a preferred aspect of the above reference example, the second optical head is arranged outside the polishing table.
In a preferred aspect of the above reference example, the top ring has a mechanism for independently pressing the central portion and the peripheral portion of the substrate against the polishing pad, and the polishing apparatus is arranged at the central portion of the substrate. The apparatus further includes a control unit that determines a load of the top ring on the central portion and the peripheral portion of the substrate based on the film thickness and the film thickness at the peripheral portion of the substrate.

  Another reference example of the present invention is a polishing apparatus that polishes a substrate having a film in sliding contact with a polishing pad, the polishing table being rotatable to hold the polishing pad, and the substrate. A first ring thickness sensor comprising: a top ring that holds and presses the substrate against the polishing pad; and a first film thickness sensor and a second film thickness sensor that measure the film thickness of the substrate. Is arranged to face the center of the substrate held by the top ring, and the second film thickness sensor is arranged to face the peripheral edge of the substrate held by the top ring. It is characterized by being.

  Still another reference example of the present invention is a polishing method for polishing a substrate having a film in sliding contact with a polishing pad, the polishing pad holding the polishing pad rotating, and the rotating polishing pad The surface of the substrate is pressed against the substrate, and the first optical head disposed so as to face the center of the substrate irradiates light on the surface of the substrate and receives reflected light from the substrate. The second optical head disposed so as to face the peripheral edge of the substrate irradiates light on the surface of the substrate and receives reflected light from the substrate, and the first optical head and the The intensity at each wavelength of the reflected light received by the second optical head is measured, and a spectrum indicating the relationship between the intensity of the reflected light and the wavelength is generated from the measured intensity, and the film of the substrate is generated from the spectrum. Decide the thickness Characterized in that it.

In a preferred aspect of the above reference example, the first optical head and the second optical head irradiate light on the surface of the substrate at different times and receive reflected light from the substrate. To do.
In a preferred embodiment of the above reference example, the first optical head and the second optical head alternately irradiate light on the surface of the substrate at substantially constant time intervals, and receive reflected light from the substrate. It is characterized by doing.
In a preferred aspect of the above reference example, the peripheral portion of the substrate is an annular portion located on the outermost side of the substrate, and the width thereof is 10 mm to 20 mm.

  According to the present invention, the tip of the second optical head moves along the peripheral edge of the substrate as the polishing table rotates. Therefore, the number of measurement points in the peripheral portion increases, and a more accurate film thickness can be obtained. As a result, a highly accurate film thickness profile (film thickness distribution along the radial direction of the substrate) can be generated during polishing, and a desired film thickness profile can be obtained based on this film thickness profile.

It is a top view which shows the positional relationship of the film thickness measurement part and board | substrate in the conventional CMP apparatus. It is a figure which shows the measurement point on the board | substrate where a film thickness measurement is performed while a grinding | polishing table rotates once. It is a graph which shows the change of the measured value of the film thickness in the center part of a board | substrate, and the measured value of the film thickness in a peripheral part. FIG. 4A is a schematic diagram for explaining the principle of determining the film thickness based on the spectrum of reflected light from the substrate, and FIG. 4B is a plan view showing the positional relationship between the substrate and the polishing table. FIG. It is a graph which shows the spectrum of the reflected light obtained by simulating based on the interference theory of light regarding the board | substrate of the structure shown to Fig.4 (a). It is sectional drawing which shows the structure of the polishing apparatus which concerns on one Embodiment of this invention. It is a top view which shows arrangement | positioning of the 1st optical head which has a 1st light projection part and a 1st light-receiving part, and the 2nd optical head which has a 2nd light projection part and a 2nd light-receiving part. It is a figure which shows the locus | trajectory on the surface of the board | substrate which the front-end | tip of a 2nd optical head draws. It is a figure which shows the example of the film thickness profile produced by the process part. It is sectional drawing which shows an example of the top ring provided with the several press mechanism which presses the several area | region of a board | substrate independently. It is a graph which shows the film thickness profile of a board | substrate. It is a top view which shows the other example of arrangement | positioning of a 1st optical head and a 2nd optical head. It is a figure which shows the locus | trajectory which the front-end | tip of the 2nd optical head shown in FIG. 12 draws. It is a top view which shows the further another example of arrangement | positioning of a 1st optical head and a 2nd optical head. It is a figure which shows the example provided with the common spectrometer and the common light source about a 1st optical head and a 2nd optical head. It is a top view which shows the further another example of arrangement | positioning of a 1st optical head and a 2nd optical head. It is a top view which shows the further another example of arrangement | positioning of a 1st optical head and a 2nd optical head. It is a top view which shows the further another example of arrangement | positioning of a 1st optical head and a 2nd optical head. It is a top view which shows the example in which the 3rd optical head was provided in addition to the 1st optical head and the 2nd optical head. It is a top view which shows the other example of arrangement | positioning of a 1st optical head, a 2nd optical head, and a 3rd optical head. It is a top view which shows the further another example of arrangement | positioning of a 1st optical head, a 2nd optical head, and a 3rd optical head. It is a top view which shows the further another example of arrangement | positioning of a 1st optical head, a 2nd optical head, and a 3rd optical head. It is a top view which shows other example of arrangement | positioning of a 2nd optical head. It is a top view which shows other example of arrangement | positioning of a 2nd optical head. It is sectional drawing which shows the modification of the polishing apparatus shown in FIG.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 4A is a schematic diagram for explaining the principle of determining the film thickness based on the spectrum of reflected light from the substrate, and FIG. 4B is a plan view showing the positional relationship between the substrate and the polishing table. FIG. As shown in FIG. 4A, a substrate W to be polished includes a base layer (for example, a silicon layer) and a film formed thereon (for example, an insulating film such as SiO ₂ having optical transparency). have. The surface of the substrate W is pressed by the polishing pad 22 on the rotating polishing table 20, and the film of the substrate W is polished by sliding contact with the polishing pad 22.

  The light projecting unit 11 and the light receiving unit 12 are arranged to face the surface of the substrate W. The light projecting unit 11 is connected to the light source 16 and irradiates the surface of the substrate W with light from the light source 16. The light projecting unit 11 projects light substantially perpendicular to the surface of the substrate W, and the light receiving unit 12 receives reflected light from the substrate W. The light emitted from the light source 16 is multi-wavelength light. As shown in FIG. 4B, the surface of the substrate W is irradiated with light every time the polishing table 20 rotates once. A spectroscope 14 is connected to the light receiving unit 12. The spectroscope 14 decomposes the reflected light according to the wavelength, and measures the intensity of the reflected light for each wavelength.

  A processing unit 15 is connected to the spectroscope 14. The processing unit 15 reads the measurement data acquired by the spectroscope 14 and generates an intensity distribution of reflected light from the intensity measurement value. More specifically, the processing unit 15 generates a spectrum (spectral profile) representing the light intensity for each wavelength. This spectrum can be represented as a line graph showing the relationship between the wavelength and intensity of the reflected light. The processing unit 15 is further configured to determine the film thickness of the substrate W from the spectrum, and further to determine the polishing end point. As the processing unit 15, a general purpose or dedicated computer can be used. The processing unit 15 executes predetermined processing steps by a program (or computer software).

  FIG. 5 is a graph showing a spectrum of reflected light obtained by performing a simulation based on the light interference theory for the substrate having the structure shown in FIG. In FIG. 5, the horizontal axis represents the wavelength of light, and the vertical axis represents the relative reflectance determined from the light intensity. This relative reflectance is one index representing the intensity of light, and specifically, is a ratio between the intensity of reflected light and a predetermined reference intensity. Thus, by dividing the intensity of the reflected light (measured intensity) by a predetermined reference intensity, the intensity of the light from which the noise component has been removed can be obtained. The predetermined reference intensity can be, for example, the intensity of reflected light obtained when a silicon wafer on which no film is formed is polished in the presence of water. In addition, you may use the intensity | strength of reflected light itself, without using a relative reflectance.

  The spectrum is an array of light intensities arranged in order of wavelength, and indicates the light intensity at each wavelength. This spectrum varies depending on the film thickness of the substrate. This is a phenomenon caused by interference of light waves. That is, the light irradiated on the substrate is reflected at the interface between the medium and the film and the interface between the film and the layer under the film, and the waves of light reflected at these interfaces interfere with each other. The manner of interference of the light wave changes depending on the thickness of the film (that is, the optical path length). For this reason, the spectrum of the reflected light returning from the substrate varies depending on the thickness of the film as shown in FIG.

  The processing unit 15 is configured to determine the film thickness from the obtained spectrum. A known technique can be used for the method of determining the film thickness from the spectrum. For example, there is a method of estimating a film thickness by comparing a spectrum (measured spectrum) obtained during polishing with a reference spectrum prepared in advance (see, for example, JP-T-2009-505847). In this method, the spectrum at each time point during polishing is compared with a plurality of reference spectra, and the film thickness is determined from the reference spectrum closest to the spectrum. The plurality of reference spectra are prepared in advance by polishing a sample substrate of the same type as the substrate to be polished. Each reference spectrum is associated with the film thickness from which the reference spectrum was acquired. Therefore, the current film thickness can be estimated from a reference spectrum having a shape closest to the spectrum obtained during polishing.

  The processing unit 15 is connected to a control unit 19 that determines polishing conditions such as a polishing load for the substrate. The spectrum generated by the processing unit 15 is transmitted to the control unit 19. The control unit 19 determines an optimum polishing load for obtaining a target film thickness profile based on a spectrum obtained during polishing, and controls the polishing load on the substrate as will be described later.

  FIG. 6 is a cross-sectional view showing a configuration of a polishing apparatus according to an embodiment of the present invention. The polishing apparatus includes a polishing table 20 that supports the polishing pad 22, a top ring 24 that holds the substrate W and presses the polishing pad 22, and a polishing liquid supply mechanism 25 that supplies a polishing liquid (slurry) to the polishing pad 22. It has. The polishing table 20 is connected to a motor (not shown) disposed below the polishing table 20 and is rotatable about its axis. The polishing pad 22 is fixed to the upper surface of the polishing table 20.

  The upper surface 22a of the polishing pad 22 constitutes a polishing surface for polishing the substrate W. The top ring 24 is connected to a motor and a lifting cylinder (not shown) via a top ring shaft 28. Thereby, the top ring 24 can be raised and lowered and can be rotated around the top ring shaft 28. The substrate W is held on the lower surface of the top ring 24 by vacuum suction or the like.

  The substrate W held on the lower surface of the top ring 24 is pressed by the top ring 24 against the polishing pad 22 on the rotating polishing table 20 while being rotated by the top ring 24. At this time, the polishing liquid is supplied from the polishing liquid supply mechanism 25 to the polishing surface 22 a of the polishing pad 22, and the surface of the substrate W is polished in a state where the polishing liquid exists between the surface of the substrate W and the polishing pad 22. . A relative movement mechanism for bringing the substrate W and the polishing pad 22 into sliding contact is constituted by the polishing table 20 and the top ring 24.

  The polishing table 20 is formed with holes 30A and 30B that open on the upper surface thereof. The polishing pad 22 has through holes 31A and 31B at positions corresponding to the holes 30A and 30B. The hole 30A and the through hole 31A communicate with each other, and the hole 30B and the through hole 31B communicate with each other. The through holes 31A and 31B are opened at the polishing surface 22a. The holes 30 </ b> A and 30 </ b> B are connected to a liquid supply source 35 via a liquid supply path 33 and a rotary joint 32. During polishing, water (preferably pure water) is supplied from the liquid supply source 35 to the holes 30A and 30B as a transparent liquid. Water fills the space formed by the lower surface of the substrate W and the through holes 31 </ b> A and 31 </ b> B, and is discharged through the liquid discharge path 34. The polishing liquid is discharged together with water, thereby securing an optical path. The liquid supply path 33 is provided with a valve (not shown) that operates in synchronization with the rotation of the polishing table 20. This valve operates to stop the flow of water or reduce the flow rate of water when the substrate W is not positioned over the through holes 31A and 31B.

  The polishing apparatus includes an optical film thickness measurement unit that measures the film thickness of the substrate according to the method described above. The optical film thickness measurement unit includes light sources 16a and 16b that emit light, a first light projecting unit 11a that irradiates the surface of the substrate W with light emitted from the light source 16a, and reflected light that returns from the substrate W. The first light receiving unit 12a that receives the light, the second light projecting unit 11b that irradiates the surface of the substrate W with the light emitted from the light source 16b, and the second light receiving unit that receives the reflected light returning from the substrate W. The spectrum from the measurement data acquired by the spectroscopes 14a and 14b, and the spectroscopes 14a and 14b that measure the intensity of the reflected light over a predetermined wavelength range by decomposing the reflected light from the unit 12b and the substrate W And a processing unit 15 that determines the film thickness of the substrate W based on this spectrum. The spectrum indicates the intensity of light distributed over a predetermined wavelength range, and indicates the relationship between the intensity of light and the wavelength.

  The 1st light projection part 11a, the 1st light-receiving part 12a, the 2nd light projection part 11b, and the 2nd light-receiving part 12b are comprised from the optical fiber. The first light projecting unit 11a and the first light receiving unit 12a constitute a first optical head (optical film thickness measuring head) 13A, and the second light projecting unit 11b and the second light receiving unit 12b are A second optical head (optical film thickness measuring head) 13B is configured. The first light projecting unit 11a is connected to the light source 16a, and the second light projecting unit 11b is connected to the light source 16b. The first light receiving unit 12a is connected to the spectroscope 14a, and the second light receiving unit 12b is connected to the spectroscope 14b.

  As the light sources 16a and 16b, light sources that emit light having a plurality of wavelengths, such as light emitting diodes (LEDs), halogen lamps, and xenon flash lamps, can be used. The first light projecting unit 11a, the first light receiving unit 12a, the second light projecting unit 11b, the second light receiving unit 12b, the light sources 16a and 16b, and the spectroscopes 14a and 14b are arranged inside the polishing table 20. And rotates together with the polishing table 20. The first light projecting unit 11 a and the first light receiving unit 12 a are disposed in a hole 30 </ b> A formed in the polishing table 20, and their respective tips are located in the vicinity of the surface to be polished of the substrate W. Similarly, the second light projecting unit 11b and the second light receiving unit 12b are disposed in a hole 30B formed in the polishing table 20, and the respective tips are located in the vicinity of the surface to be polished of the substrate W. ing.

  The first light projecting unit 11a and the first light receiving unit 12a are arranged perpendicular to the surface of the substrate W, and the first light projecting unit 11a irradiates the surface of the substrate W with light perpendicularly. It has become. Similarly, the second light projecting unit 11b and the second light receiving unit 12b are arranged perpendicular to the surface of the substrate W, and the second light projecting unit 11b emits light perpendicular to the surface of the substrate W. It comes to irradiate.

  The first light projecting unit 11 a and the first light receiving unit 12 a are disposed to face the center of the substrate W held on the top ring 24. Therefore, as shown in FIG. 4A, each time the polishing table 20 rotates, the tips of the first light projecting unit 11a and the first light receiving unit 12a move across the substrate W, and the center of the substrate W Light is irradiated to the region including This is because the first light projecting portion 11a and the first light receiving portion 12a pass through the center of the substrate W to measure the thickness of the entire surface of the substrate W including the thickness of the central portion of the substrate W. is there. The processing unit 15 can generate a film thickness profile (film thickness distribution) based on the measured film thickness data.

  On the other hand, the second light projecting unit 11 b and the second light receiving unit 12 b are arranged to face the peripheral edge of the substrate W held by the top ring 24. The tips of the second light projecting unit 11b and the second light receiving unit 12b move along the peripheral edge of the substrate W every time the polishing table 20 rotates. Therefore, light is applied to the peripheral edge of the substrate W every time the polishing table 20 rotates.

  During the polishing of the substrate W, the substrate W is irradiated with light from the first light projecting unit 11a and the second light projecting unit 11b. The light from the first light projecting unit 11a is reflected by the surface of the substrate W and is received by the first light receiving unit 12a. The light from the second light projecting unit 11b is reflected by the surface of the substrate W and is received by the second light receiving unit 12b. While light is irradiated onto the substrate W, water is supplied to the holes 30A and the through holes 31A, whereby the tips of the first light projecting unit 11a and the first light receiving unit 12a and the surface of the substrate W are supplied. The space between is filled with water. Similarly, while light is irradiated on the substrate W, water is supplied to the holes 30B and the through holes 31B, whereby the tips of the second light projecting unit 11b and the second light receiving unit 12b, and the substrate The space between the surface of W is filled with water.

  The spectroscope 14a decomposes the reflected light transmitted from the first light receiving unit 12a according to the wavelength, and measures the intensity of the reflected light for each wavelength. Similarly, the spectroscope 14b decomposes the reflected light transmitted from the second light receiving unit 12b according to the wavelength, and measures the intensity of the reflected light for each wavelength. The processing unit 15 generates a spectrum indicating the relationship between the intensity of the reflected light and the wavelength from the intensity of the reflected light measured by the spectrometer 14a and the spectrometer 14b. Further, the processing unit 15 determines the current film thickness of the substrate W from the obtained spectrum by using a known technique as described above.

  FIG. 7 shows a first optical head 13A having a first light projecting part 11a and a first light receiving part 12a, and a second optical head 13B having a second light projecting part 11b and a second light receiving part 12b. It is a top view which shows arrangement | positioning. As shown in FIG. 7, the center of the substrate W is located on the locus drawn by the first optical head 13A, and the peripheral edge of the substrate W is located on the locus drawn by the second optical head 13B. As can be seen from FIG. 7, the second optical head 13 </ b> B moves only across the peripheral edge of the substrate W, and its traveling direction is generally the circumferential direction of the substrate W.

  The first optical head 13 </ b> A and the second optical head 13 </ b> B are arranged along the radial direction of the polishing table 20. Therefore, the angle formed by the line connecting the first optical head 13A and the center O of the polishing table 20 and the line connecting the second optical head 13B and the center O of the polishing table 20 is 0 degrees. The second optical head 13 </ b> B is disposed outside the first optical head 13 </ b> A with respect to the radial direction of the polishing table 20. That is, the distance between the second optical head 13 </ b> B and the center O of the polishing table 20 is longer than the distance between the first optical head 13 </ b> A and the center O of the polishing table 20.

  FIG. 8 is a diagram showing a trajectory on the surface of the substrate W drawn by the tip of the second optical head 13B. More specifically, FIG. 8 shows a locus drawn by the second optical head 13B when the polishing table 20 rotates twice. As can be seen from FIG. 8, the second optical head 13 </ b> B moves along the peripheral edge of the substrate W as the polishing table 20 rotates. As a result, the number of measurement points at the peripheral edge of the substrate W is larger than the number of measurement points in the conventional CMP apparatus shown in FIG. Therefore, the film thickness of the peripheral portion of the substrate W can be accurately determined from more measurement data.

  Here, in this specification, the peripheral part of a board | substrate is an outermost cyclic | annular site | part of a board | substrate, as shown in FIG. 8, The width | variety is 10 mm-20 mm. For example, in the case of a substrate having a diameter of 300 mm, the width of the peripheral edge is about 10 mm. The peripheral edge of the substrate is an area where devices are formed. The peripheral portion of the substrate is a region that is most susceptible to the influence of a polishing load and a polishing liquid during polishing, and is a region in which the film thickness is likely to change greatly during polishing compared to other regions of the substrate. Therefore, highly accurate film thickness monitoring is required during polishing.

  Since the substrate W is polished by the sliding contact between the polishing pad 22 and the substrate W and the chemical action of the polishing liquid, the portion of the polishing pad 22 where the first optical head 13A and the second optical head 13B are disposed. Does not contribute to the polishing of the substrate W. As can be seen from FIG. 8, the second optical head 13B passes only through the peripheral edge of the substrate W and does not pass through other parts. Therefore, the influence on the substrate polishing of the second optical head 13B can be minimized.

In the case where the diameter of the polishing table 20 is larger than the diameter of the substrate W, the lower the rotational speed of the polishing table 20, the longer it takes for the second optical head 13B to pass through the substrate W. Therefore, for example, the rotation speed of the polishing table 20 may be rotated at 50 min ⁻¹ or less during polishing of the substrate W, or the rotation speed of the polishing table 20 is set in advance at predetermined time intervals during polishing of the substrate W. The rotational speed may be lowered to a lower rotational speed than the set rotational speed.

  In the in-situ measurement in which the film thickness of the substrate is measured during polishing, the film thickness measurement may be affected by the polishing liquid. In particular, in an optical film thickness measuring device, light is blocked by the polishing liquid, and there is a case where film thickness measurement with high accuracy is not performed. Therefore, in order to eliminate the influence of the film thickness measurement by the polishing liquid, the substrate is polished (water polished) while supplying pure water to the polishing pad 22 periodically, and the film thickness of the substrate is measured while supplying pure water. You may make it do.

  The processing unit 15 creates a film thickness profile by combining the film thickness value acquired using the first optical head 13A and the film thickness value acquired using the second optical head 13B. FIG. 9 is a diagram illustrating an example of a film thickness profile created by the processing unit 15. As shown in FIG. 9, the film thickness profile is composed of a number of film thickness values determined by the processing unit 15. The film thickness value (denoted by Δ) obtained using the first optical head 13A is assigned to the portion excluding the peripheral edge of the substrate W, and the film thickness value (using the second optical head 13B) ( ) Is assigned to the peripheral edge of the substrate W. Thus, since the film thickness value obtained through the second optical head 13B is used for the part of the film thickness profile corresponding to the peripheral part of the substrate W, the processing unit 15 has a peripheral edge from the center of the substrate W. A film thickness profile with high accuracy can be generated up to the portion.

  The film thickness profile is a film thickness distribution indicating the film thickness in each region of the substrate W being polished. Therefore, by adjusting the polishing load for each region of the substrate W during polishing, a desired film thickness profile or polishing profile (profile indicating the distribution of film removal amount) can be obtained. The top ring 24 has a mechanism capable of independently pressing a plurality of regions including the central portion and the peripheral portion of the substrate W. Hereinafter, the configuration of the top ring 24 will be described with reference to FIG.

  FIG. 10 is a cross-sectional view illustrating an example of the top ring 24 provided with a plurality of pressing mechanisms that independently press a plurality of regions of the substrate. The top ring 24 includes a top ring body 51 that is connected to the top ring shaft 28 via a free joint 50, and a retainer ring 52 that is disposed below the top ring body 51. Below the top ring body 51, a circular flexible membrane 56 that contacts the substrate W and a chucking plate 57 that holds the membrane 56 are disposed. Four pressure chambers (airbags) P1, P2, P3, and P4 are provided between the membrane 56 and the chucking plate 57. The pressure chambers P1, P2, P3, and P4 are formed by a membrane 56 and a chucking plate 57. The central pressure chamber P1 is circular, and the other pressure chambers P2, P3, P4 are annular. These pressure chambers P1, P2, P3, and P4 are arranged concentrically.

  Pressurized fluid such as pressurized air is supplied to the pressure chambers P1, P2, P3, and P4 by the pressure adjusting unit 70 through the fluid paths 61, 62, 63, and 64, respectively, or evacuated. ing. The internal pressures of the pressure chambers P1, P2, P3, and P4 can be changed independently of each other, and thereby, four regions of the substrate W, that is, a central portion, an inner intermediate portion, an outer intermediate portion, and a peripheral edge The load on the part can be adjusted independently. Further, by raising and lowering the entire top ring 24, the retainer ring 52 can be pressed against the polishing pad 22 with a predetermined load.

  A pressure chamber P5 is formed between the chucking plate 57 and the top ring body 51. Pressurized fluid is supplied to the pressure chamber P5 through the fluid path 65 by the pressure adjusting unit 70, or vacuuming is performed. It has come to be. As a result, the chucking plate 57 and the entire membrane 56 can move in the vertical direction. The substrate W is surrounded by the retainer ring 52 so that the substrate W does not jump out of the top ring 24 during polishing. An opening is formed in a portion of the membrane 56 constituting the pressure chamber P3, and the substrate W is attracted and held by the top ring 24 by forming a vacuum in the pressure chamber P3. Further, the substrate W is released from the top ring 24 by supplying nitrogen gas, clean air, or the like to the pressure chamber P3.

  The pressure adjustment unit 70 is connected to the control unit 19. The polishing load for each region of the substrate W, that is, the internal pressure of the pressure chambers P1, P2, P3, and P4 is determined by the control unit 19. The control unit 19 is connected to the processing unit 15 described above, and the film thickness profile generated by the processing unit 15 is sent to the control unit 19. The control unit 19 controls the internal pressure of the pressure chambers P1, P2, P3, and P4 of the top ring 24 via the pressure adjustment unit 70 based on the film thickness profile obtained during polishing. That is, the control unit 19 determines the target internal pressure of the pressure chambers P1, P2, P3, and P4 for making the film thickness profile obtained during polishing coincide with the target film thickness profile, and the pressure adjusting unit 70 sets the target internal pressure. Send pressure command signal. The pressure adjustment unit 70 adjusts the internal pressure of the pressure chambers P1, P2, P3, and P4 based on a command signal from the control unit 19. By such an operation, the top ring 24 can press each part of the substrate W with an optimum load. Based on the obtained film thickness profile, it is also possible to control only a certain pressure chamber, for example, only the internal pressure of the pressure chamber P4 corresponding to the peripheral portion of the substrate W. The second optical head 13B is disposed at a position corresponding to the pressure chamber P4.

  FIG. 11 shows the film thickness profile of the substrate polished while feedback controlling the polishing load based on the initial film thickness profile, the target film thickness profile, and the film thickness profile obtained during polishing, and polishing without feedback control. It is a graph which shows the film thickness profile of the board | substrate which carried out. The graph shown in FIG. 11 shows the result of polishing a substrate having an initial film thickness profile in which the film thickness at the peripheral edge of the substrate is larger than the film thickness in other regions. As can be seen from FIG. 11, as a result of polishing the substrate while feedback controlling the polishing load based on the film thickness profile, a film thickness profile close to the target film thickness profile is obtained. On the other hand, it can be seen that a desired film thickness profile is not obtained when feedback control is not performed.

  Generally, the same type of substrate is polished under the same polishing conditions. However, since consumables such as the polishing pad 22 and the retainer ring 52 of the top ring 24 used in the polishing apparatus are gradually worn with the polishing time, the obtained film thickness profile also gradually changes. Such a change in film thickness profile is particularly noticeable at the peripheral edge of the substrate. This is presumably because the polishing load tends to concentrate on the peripheral portion of the substrate, and the peripheral portion of the substrate is easily affected by wear of the retainer ring 52 and the polishing pad 22. According to the present embodiment described above, the film thickness at the peripheral edge of the substrate can be measured with high accuracy, so that it is possible to detect an abnormal polishing caused by wear of the polishing pad 22 and / or the retainer ring 52. it can. That is, the wear of the polishing pad 22 and / or the retainer ring 52 can be detected from the change over time in the film thickness at the peripheral edge of the substrate. For example, the polishing pad 22 and / or the retainer ring 52 are worn when a desired film thickness cannot be obtained at the peripheral edge of the substrate even though the substrate is polished under the same polishing conditions. It can be judged. As described above, the measurement data of the film thickness at the peripheral edge of the substrate can be used not only for real-time feedback control of the polishing load of the substrate but also for detecting wear of consumables such as the polishing pad 22 and the retainer ring 52. .

  FIG. 12 is a plan view showing another example of the arrangement of the first optical head 13A and the second optical head 13B. The arrangement of the first optical head 13A and the second optical head 13B shown in FIG. 12 is basically the same as the arrangement shown in FIG. 7, but the second optical head 13B is more than the first optical head 13A. The difference is that the polishing table 20 is arranged close to the center O of the polishing table 20. That is, in the example shown in FIG. 12, the second optical head 13 </ b> B is located inside the first optical head 13 </ b> A with respect to the radial direction of the polishing table 20. Therefore, the distance between the second optical head 13B and the center O of the polishing table 20 is shorter than the distance between the first optical head 13A and the center O of the polishing table 20.

  FIG. 13 is a diagram showing a trajectory drawn by the tip of the second optical head 13B shown in FIG. 12, and shows a trajectory when the polishing table 20 rotates twice. As can be seen from FIG. 13, the second optical head 13 </ b> B moves along the peripheral edge of the substrate W as the polishing table 20 rotates. Therefore, the film thickness of the peripheral portion of the substrate W can be measured at more measurement points. Further, from the comparison between the trajectory shown in FIG. 8 and the trajectory shown in FIG. 13, it can be seen that the trajectory of the second optical head 13B shown in FIG. 13 is longer than the trajectory of the second optical head 13B shown in FIG. . Therefore, in the arrangement shown in FIG. 12, the film thickness of the peripheral portion of the substrate W can be measured at more measurement points. On the other hand, since the second optical head 13B does not contribute to the polishing of the substrate W, it is preferable that the locus drawn by the second optical head 13B is short from the viewpoint of improving the polishing rate (film removal rate). The arrangement shown in FIG. 7 is preferable from the viewpoint of improving the polishing performance, although the number of measurement points at the peripheral edge of the substrate W is slightly smaller than the arrangement shown in FIG.

  FIG. 14 is a plan view showing still another example of the arrangement of the first optical head 13A and the second optical head 13B. As shown in FIG. 14, the first optical head 13 </ b> A and the second optical head 13 </ b> B are arranged on opposite sides with respect to the center O of the polishing table 20. More specifically, the angle formed by the line connecting the first optical head 13A and the center O of the polishing table 20 and the line connecting the second optical head 13B and the center O of the polishing table 20 is substantially equal to 180 degrees. In FIG. 14, the first optical head 13 </ b> A faces the center of the substrate W (shown by a solid line), and the second optical head 13 </ b> B faces the peripheral portion of the substrate W (shown by a dotted line). Indicates the state. The second optical head 13B is arranged outside the first optical head 13A in the radial direction of the polishing table 20.

  In the above example shown in FIGS. 7 and 12, the first optical head 13A and the second optical head 13B irradiate light to the substrate W and receive light from the substrate W almost simultaneously, as shown in FIG. In the example, the first optical head 13A and the second optical head 13B apply light to the substrate W at different timings, and receive light from the substrate W.

  As described above, in the arrangement shown in FIG. 14, the film thickness at the center of the substrate W and the film thickness at the peripheral edge of the substrate W are measured at different times. Therefore, it is possible to receive the reflected light from the first optical head 13A and the reflected light from the second optical head 13B with one spectroscope. That is, even if one spectroscope receives the reflected light from the central portion of the substrate W and the reflected light from the peripheral portion of the substrate W, the reflected light does not overlap in the spectroscope. It is also possible to selectively connect one light source to the first optical head 13A or the second optical head 13B. An example including such a common spectroscope and a common light source will be described with reference to FIG.

  As shown in FIG. 15, the first light projecting unit 11a and the second light projecting unit 11b are connected to the light source 16 through the first optical switch 40A. The first optical switch 40A is configured to selectively connect the light source 16 to one of the first light projecting unit 11a and the second light projecting unit 11b. Similarly, the first light receiving unit 12a and the second light receiving unit 12b are connected to the spectroscope 14 via the second optical switch 40B. Here, the optical switch is a device that switches an optical transmission path. A typical optical switch has a configuration including a mirror for changing the traveling direction of light, and is configured to reflect incident light by a mirror and switch a light transmission path. In addition to an optical switch using a mirror, a waveguide type optical switch using a material whose refractive index changes by an input such as heat or electricity may be used. Such a known optical switch can be used as the first optical switch 40A and the second optical switch 40B.

  In the above configuration, when the first optical head 13A moves across the substrate W, the light source 16 and the spectroscope 14 are connected to the first light projecting unit 11a and the first light receiving unit 12a by the optical switches 40A and 40B. Each is connected. On the other hand, when the second optical head 13B moves across the substrate W, the light source 16 and the spectroscope 14 are connected to the second light projecting unit 11b and the second light receiving unit 12b by optical switches 40A and 40B, respectively. The Thus, by using the first optical switch 40A and the second optical switch 40B, the light source 16 and the spectroscope 14 can be alternately connected to the first optical head 13A or the second optical head 13B. .

  Further, in the example shown in FIG. 14, the first optical head 13A and the second optical head 13B are arranged at substantially equal intervals in the circumferential direction of the polishing table 20, so that the first optical head 13A and The second optical head 13B alternately irradiates the substrate W with light at substantially constant time intervals, and receives reflected light from the substrate W. Therefore, the processing unit 15 can secure a sufficient time for processing the measurement data (data including the measurement value of the intensity of the reflected light) sent from the spectroscope 14.

  In the example shown in FIG. 14, the second optical head 13B is arranged outside the first optical head 13A in the radial direction of the polishing table 20, but as shown in FIG. The head 13B may be disposed inside the first optical head 13A with respect to the radial direction of the polishing table 20. That is, the distance between the second optical head 13B and the center O of the polishing table 20 may be shorter than the distance between the first optical head 13A and the center O of the polishing table 20. Even in this case, the same effect as the example shown in FIGS. 14 and 15 can be obtained.

  FIG. 17 is a plan view showing still another example of the arrangement of the first optical head 13A and the second optical head 13B. In the example of FIG. 14 described above, the first optical head 13A and the second optical head 13B are arranged in a straight line, but in the example shown in FIG. 17, the second optical head 13B is polished. The table 20 is disposed at a position shifted from the first optical head 13A in the circumferential direction. FIG. 17 shows a state in which the first optical head 13A faces the center of the substrate W (shown by a solid line), and the second optical head 13B faces the peripheral portion of the substrate W (shown by a dotted line). Indicates the state. In this example, the angle formed by the line connecting the first optical head 13A and the center O of the polishing table 20 and the line connecting the second optical head 13B and the center O of the polishing table 20 is about 120 degrees. is there. Also in this example, since the film thickness of the central portion of the substrate W and the film thickness of the peripheral portion are measured at different times, one light source 16 and one spectroscope 14 are connected to the first as shown in FIG. The optical head 13A and the second optical head 13B can be selectively used.

  In the example shown in FIG. 17, the second optical head 13B is arranged outside the first optical head 13A with respect to the radial direction of the polishing table 20, but as shown in FIG. The head 13B may be disposed inside the first optical head 13A.

  FIG. 19 is a plan view showing an example in which a third optical head 13C is provided in addition to the first optical head 13A and the second optical head 13B. The configuration of the third optical head 13C is the same as that of the first optical head 13A and the second optical head 13B described above, and is connected to a light source and a spectrometer (not shown). As shown in FIG. 19, the first optical head 13A, the second optical head 13B, and the third optical head 13C are arranged along the radial direction of the polishing table 20, and the second optical head 13B. The third optical head 13C is located outside the first optical head 13A.

  The arrangement of the first optical head 13A and the second optical head 13B is the same as that shown in FIG. The third optical head 13C is located between the first optical head 13A and the second optical head 13B. That is, the distance between the first optical head 13A and the third optical head 13C is substantially the same as the distance between the third optical head 13C and the second optical head 13B. The position of the third optical head 13C corresponds to an intermediate part located between the central part and the peripheral part of the substrate. The second optical head 13B is disposed at a position corresponding to the pressure chamber P4 described above, and the third optical head 13C is disposed at a position corresponding to the pressure chamber P2 or the pressure chamber P3. Therefore, a more accurate film thickness profile can be obtained.

  FIG. 20 is a plan view showing another example of the arrangement of the first optical head 13A, the second optical head 13B, and the third optical head 13C. The arrangement shown in FIG. 20 is basically the same as the arrangement shown in FIG. 19, but the second optical head 13B and the third optical head 13C are the same as the first optical head 13A in the radial direction of the polishing table 20. Is located on the inside.

  21 is a diagram showing a modification of the arrangement of FIG. 19, and FIG. 22 is a diagram showing a modification of the arrangement of FIG. As shown in FIGS. 21 and 22, the third optical head 13C may be disposed closer to the second optical head 13B than the first optical head 13A. According to such an arrangement, the film thickness of the intermediate part closer to the peripheral part of the substrate can be measured using the third optical head 13C.

  FIG. 23 is a plan view showing still another arrangement example of the second optical head 13B. In this example, the second optical head 13 </ b> B is disposed outside the polishing table 20. The position of the first optical head 13A is the same as in the above example. The position of the second optical head 13B is fixed, and is fixed to a support member (not shown). Therefore, the second optical head 13 </ b> B does not rotate with the polishing table 20. In this example, as indicated by an arrow S, the top ring 24 (see FIG. 6) is formed on the polishing table 20 during polishing of the substrate W such that the peripheral edge of the substrate W protrudes from the polishing pad 22 on the polishing table 20. It swings in the radial direction. Therefore, the second optical head 13B can apply light to the exposed peripheral edge of the substrate W and receive the reflected light.

  FIG. 24 is a plan view showing still another arrangement example of the second optical head 13B. In this example, as shown in FIG. 23, the second optical head 13 </ b> B is disposed at the center of the polishing table 20. In this example, the top ring 24 swings in the radial direction of the polishing table 20 as indicated by an arrow T so that the peripheral edge of the substrate W is positioned at the center of the polishing table 20. Therefore, also in this example, the second optical head 13B can apply light to the peripheral edge of the substrate W and receive the reflected light.

  25 is a cross-sectional view showing a modification of the polishing apparatus shown in FIG. In the example shown in FIG. 25, the liquid supply path, the liquid discharge path, and the liquid supply source are not provided. Instead, the polishing pad 22 is provided with transparent windows 45A and 45B. The light projecting units 11a and 11b irradiate the surface of the substrate W on the polishing pad 22 through the transparent windows 45A and 45B, and the light receiving units 12a and 12b receive the reflected light from the substrate W through the transparent windows 45A and 45B. Receive light. Other configurations are the same as those of the polishing apparatus shown in FIG. The transparent window shown in FIG. 25 can be applied to the examples shown in FIGS.

  In each example described above, two optical heads or three optical heads are provided. However, the present invention is not limited to these examples, and at least one optical head is disposed to face the peripheral edge of the substrate. If so, four or more optical heads may be provided. Furthermore, the present invention can be applied not only to an optical film thickness measuring device but also to other film thickness measuring devices such as an eddy current sensor. For example, according to the example shown in FIGS. 7 to 24 described above, the first eddy current sensor that is a film thickness sensor is disposed so as to face the center of the substrate, and the second eddy current sensor is opposed to the peripheral portion of the substrate. You may arrange as follows.

  The embodiment described above is described for the purpose of enabling the person having ordinary knowledge in the technical field to which the present invention belongs to implement the present invention. Various modifications of the above embodiment can be naturally made by those skilled in the art, and the technical idea of the present invention can be applied to other embodiments. Accordingly, the present invention is not limited to the described embodiments, but is to be construed in the widest scope according to the technical idea defined by the claims.

11, 11a, 11b Emitting unit 12, 12a, 12b Light receiving unit 13A, 13B, 13C Optical head 14, 14a, 14b Spectrometer 15 Processing unit 16, 16a, 16b Light source 20 Polishing table 22 Polishing pad 24 Top ring 25 Polishing liquid Supply mechanism 28 Top ring shaft 30A, 30B Hole 31A, 31B Through hole 32 Rotary joint 33 Liquid supply path 34 Liquid discharge path 35 Liquid supply source 40A, 40B Optical switch 45A, 45B Transparent window

The first light projecting unit 11 a and the first light receiving unit 12 a are disposed to face the center of the substrate W held on the top ring 24. Therefore, as shown in FIG. 4B , each time the polishing table 20 rotates, the tips of the first light projecting unit 11a and the first light receiving unit 12a move across the substrate W, and the center of the substrate W Light is irradiated to the region including This is because the first light projecting portion 11a and the first light receiving portion 12a pass through the center of the substrate W to measure the thickness of the entire surface of the substrate W including the thickness of the central portion of the substrate W. is there. The processing unit 15 can generate a film thickness profile (film thickness distribution) based on the measured film thickness data.

FIG. 24 is a plan view showing still another arrangement example of the second optical head 13B. In this example, as shown in FIG. 2 4, the second optical head 13B is arranged in the center of the polishing table 20. In this example, the top ring 24 swings in the radial direction of the polishing table 20 as indicated by an arrow T so that the peripheral edge of the substrate W is positioned at the center of the polishing table 20. Therefore, also in this example, the second optical head 13B can apply light to the peripheral edge of the substrate W and receive the reflected light.

Claims (6)

A polishing pad against which a substrate held by a top ring is pressed,
The polishing pad has a first through hole and a second through hole for allowing light to pass through,
The second through hole is disposed inside the first through hole in the radial direction of the polishing pad,
When the first through hole is under the substrate held by the top ring, the second through hole is disposed at a position not existing under the substrate held by the top ring,
The polishing pad, wherein the first through hole is formed at a position facing the central portion of the substrate, and the second through hole is formed at a position facing the peripheral edge of the substrate. .   The polishing pad according to claim 1, wherein the first through hole and the second through hole are located on opposite sides with respect to a center of the polishing pad.   An angle formed by a line connecting the first through hole and the center of the polishing pad and a line connecting the second through hole and the center of the polishing pad is substantially 180 degrees. The polishing pad according to claim 1.   An angle formed by a line connecting the first through hole and the center of the polishing pad and a line connecting the second through hole and the center of the polishing pad is substantially 120 degrees. The polishing pad according to claim 1.   2. The polishing pad according to claim 1, wherein the peripheral portion of the substrate is an outermost annular portion of the substrate and has a width of 10 mm to 20 mm.   The polishing pad according to claim 1, wherein a transparent window is provided in each of the first through hole and the second through hole.

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