Patent Publication Number: US-11376708-B2

Title: Polishing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-170679, filed on Sep. 12, 2018, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a polishing apparatus. 
     BACKGROUND 
     When a semiconductor wafer or a material film on the semiconductor wafer is polished with an almost constant pressure using a polishing apparatus, such as adopting a chemical mechanical polishing (CMP) method or the like, flatness or evenness of the semiconductor wafer and/or thickness of the material film sometimes is irregular. 
     Examples of related art include JP-A-2011-083865, JP-A-09-260316, and JP-A-2001-127925 (U.S. Pat. No. 6,325,696). 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a configuration example of a polishing apparatus according to a first embodiment. 
         FIG. 2  is a cross-sectional view illustrating an example of a configuration of a holder. 
         FIG. 3  is a top plan view of a surface of the holder when viewed in a Z direction. 
         FIG. 4  is a top plan view illustrating another arrangement of vibration sensors. 
         FIG. 5A  is a top plan view illustrating a membrane and the vibration sensor, and  FIG. 5B  is a schematic view illustrating a configuration example of the vibration sensor. 
         FIGS. 6A and 6B  are cross-sectional views taken along line  6 - 6  in  FIG. 5A . 
         FIG. 7  is a flowchart illustrating an example of a polishing method according to the first embodiment. 
         FIG. 8  is a graph illustrating magnitudes of signals from the vibration sensors. 
         FIG. 9  is a flowchart illustrating an example of a polishing method according to a second embodiment. 
         FIG. 10  is a schematic view illustrating a configuration example of a polishing apparatus according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments herein provide a polishing apparatus operable to polish and thus achieve or increase the flatness and evenness of a semiconductor wafer or a material film after polishing, thereby reducing or removing irregularity in the thicknesses of the semiconductor wafer and the material film. 
     In general, according to one embodiment, a polishing apparatus includes a polishing unit configured to polish a target object to be polished. A holder is rotatable while holding the target object to be polished. Multiple elastic members are provided on the holder concentrically around a center of a rotation shaft of the holder and elastically press the target object to be polished against the polishing unit. Multiple vibration sensors are provided in the elastic members and detect vibration from a polishing surface of the target object to be polished. 
     Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present embodiments do not limit the present disclosure. The drawings are schematic or conceptual, and the ratios between portions and the like are not necessarily the same as the actual values thereof. In the specification and the drawings, the same elements, which have been previously described with reference to the previous drawings, are marked with the same reference numerals, and a detailed description thereof will be appropriately omitted. 
     First Embodiment 
       FIG. 1  is a schematic view illustrating a configuration example of a polishing apparatus  1  according to a first embodiment. The polishing apparatus  1  is, for example, a chemical mechanical polishing (CMP) apparatus that polishes a semiconductor wafer W that is the target object to be polished. In addition, the present embodiment is not limited to the CMP apparatus and may be applied to a polishing apparatus that polishes any material to be flat. 
     The polishing apparatus  1  includes a polishing unit  10  (polisher), a holder  20 , a drive unit  30  (driver), a slurry supply unit  40  (slurry supplier), a measurement unit  50  (detector), a calculation unit  60  (calculator), and a control unit  70  (controller). The polishing unit  10  includes a turntable  12  configured to be rotatable (turn about itself) about a shaft  11  in a direction of the arrow A 1 , and a polishing pad  13  provided on the turntable  12 . 
     The holder  20  holds the semiconductor wafer W and is configured to be rotatable (turn about itself) about a shaft  21  in a direction of the arrow A 2  together with the semiconductor wafer W. In addition, as described below with reference to FIGS. and  3 , the holder  20  has film-shaped elastic members (hereinafter, referred to as membranes) and presses the semiconductor wafer W against the polishing unit  10  by introducing air into the membranes. The pressure, which presses the semiconductor wafer W against the polishing unit  10 , may be controlled by a gas pressure in the membrane. 
     The drive unit  30  controls the rotation of the holder  20  and/or the gas pressure in the membrane. The gas pressure in the membrane may be controlled by using a non-illustrated air pump or the like. 
     The slurry supply unit  40  supplies slurry, as a polishing liquid, onto the polishing pad  13 . The slurry includes abrasive grains and is introduced between the semiconductor wafer W and the polishing pad  13  to facilitate the polishing of the semiconductor wafer W. 
     Here, a configuration of the holder  20  will be described. 
       FIG. 2  is a cross-sectional view illustrating an example of a configuration of the holder  20 . The holder  20  has a head unit  22 , a plurality of membranes  23   a ,  23   b ,  23   c , and  23   d , and a retainer ring  24 . The head unit  22  is connected to the rotation shaft  21  and has a surface F 22  that faces the polishing pad  13 . The plurality of membranes  23   a ,  23   b ,  23   c , and  23   d  are provided on the surface F 22  of the head unit  22 . Each of the membranes  23   a ,  23   b ,  23   c , and  23   d  is, for example, a member formed by rolling, in a tubular shape (cylindrical shape), a film made of an elastic material such as resin or rubber, and the membranes  23   a ,  23   b ,  23   c , and  23   d  are configured such that the tubular members are arranged in a ring shape around a center C. In addition, the membrane  23   d  may be a disc-shaped member having the center C as a center thereof. 
     Each of the membranes  23   a ,  23   b ,  23   c , and  23   d  has a hollow cavity H and expands as gas is supplied into the cavity H. In addition, each of the membranes  23   a ,  23   b ,  23   c , and  23   d  is contracted when the supply of the gas into the cavity H is stopped or the gas in the cavity H is drawn out, so that the gas in the cavity H is discharged. In this way, the pressure which presses the semiconductor wafer W against the polishing pad  13  of the polishing unit  10  is controlled by adjusting the gas pressure in the cavities H of the membranes  23   a ,  23   b ,  23   c , and  23   d . In addition, the gas may be, but is not particularly limited to, for example, air, inert gas, and the like. 
     The head unit  22  has supply ports  25  capable of supplying the gas into the membranes  23   a ,  23   b ,  23   c , and  23   d . The drive unit  30  supplies the gas independently into the membranes  23   a ,  23   b ,  23   c , and  23   d  through the supply ports  25 . That is, the gas pressure in the membranes  23   a ,  23   b ,  23   c , and  23   d  may be individually adjusted. Therefore, the membranes  23   a ,  23   b ,  23   c , and  23   d  may press the semiconductor wafer W with different pressures. In addition, a sensor control unit  26 , which serves to control operations of vibration sensors to be described below, is provided in the head unit  22 . 
     The retainer ring  24  is provided along an outer edge of the head unit  22  so as to face a lateral side of the semiconductor wafer W. During the polishing, the retainer ring  24  prevents the semiconductor wafer W from protruding from the holder  20  due to the rotation of the polishing unit  10  or the rotation of the holder  20 . 
       FIG. 3  is a top plan view of the surface F 22  of the holder  20  when viewed in a Z direction. In addition,  FIG. 2  illustrates a cross section taken along line  2 - 2  in  FIG. 3 . In addition, the Z direction is the direction perpendicular to a rotation surface of the holder  20  (direction in which the rotation shaft  21  extends). Each of the membranes  23   a ,  23   b ,  23   c , and  23   d  is formed concentrically around the center C of the rotation shaft  21  of the holder  20 . The disc-shaped membrane  23   d  is provided on the center C, and the membrane  23   c  is disposed outside the membrane  23   d . The membrane  23   b  is disposed outside the membrane  23   c . Further, the membrane  23   a  is disposed outside the membrane  23   b . That is, the membranes  23   d ,  23   c ,  23   b , and  23   a  are arranged in this order progressively further from the center C. In this way, the membranes  23   a ,  23   b ,  23   c , and  23   d  are individually provided in concentric circular areas around the center C, and these areas may press, with different pressures, the semiconductor wafer W against the polishing unit  10 . In addition, in the present embodiment, the four membranes  23   a ,  23   b ,  23   c , and  23   d  are provided in the four areas. However, the number of membranes is not limited to four but may be three or less or five or more. Therefore, the number of areas for controlling the pressing of the semiconductor wafer W may be increased or decreased. 
     As illustrated in  FIGS. 2 and 3 , vibration sensors  100   a ,  100   b ,  100   c , and  100   d  are provided in the cavities H of the membranes  23   a ,  23   b ,  23   c , and  23   d , respectively. Each of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  is a contact vibration sensor; for example, an acoustic emission (AE) sensor. 
     During the polishing of the semiconductor wafer W, the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  are positioned on bottom portions of the membranes  23   a ,  23   b ,  23   c , and  23   d  so as to come into contact with the semiconductor wafer W through the membranes  23   a ,  23   b ,  23   c , and  23   d , and detect vibration from the semiconductor wafer W. The vibration may be detected continuously or intermittently in a certain cycle. 
     The AE sensor uses a piezoelectric element and may detect elastic waves having frequency components (e.g., several kilohertz (KHz) to several megahertz (MHz)) from a low band to a high band that occur on a polishing surface of the semiconductor wafer W (an interface between the semiconductor wafer W and the polishing pad  13 ). 
     The intensity of the vibration from the polishing surface of the semiconductor wafer W varies depending on distances between the polishing surface of the semiconductor wafer W and the vibration sensors  100   a ,  100   b ,  100   c , and  100   d . For example, when the distances between the polishing surface of the semiconductor wafer W and the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  are comparatively short (the semiconductor wafer W is comparatively thin), the intensity of the vibration from the polishing surface of the semiconductor wafer W is increased. On the contrary, when the distances between the polishing surface of the semiconductor wafer W and the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  are comparatively long (the semiconductor wafer W is comparatively thick), the intensity of the vibration from the polishing surface of the semiconductor wafer W is decreased. In this way, the thickness of the semiconductor wafer W may be detected based on the intensity of the vibration from the polishing surface of the semiconductor wafer W. Irregularity in the thickness of the semiconductor wafer W represents unevenness of the polishing surface of the semiconductor wafer W. Therefore, the unevenness (flatness) of the polishing surface of the semiconductor wafer W may be detected by detecting the intensity of the vibration from the polishing surface of the semiconductor wafer W. 
     The vibration sensors  100   a ,  100   b ,  100   c , and  100   d  are disposed at optional positions in the membranes  23   a ,  23   b ,  23   c , and  23   d , respectively. For example, in  FIG. 3 , the vibration sensor  100   a  is disposed at a certain position in the membrane  23   a . The vibration sensor  100   b  is disposed, in the membrane  23   b , at a position which is rotated at approximately 90° with respect to the vibration sensor  100   a . The vibration sensor  100   c  is disposed, in the membrane  23   c , at a position which is rotated at approximately 90° with respect to the vibration sensor  100   b  (at approximately 180° with respect to the vibration sensor  100   a ). The vibration sensor  100   d  is disposed, in the membrane  23   d , at a position which is rotated at approximately 90° with respect to the vibration sensor  100   c  (at approximately 270° with respect to the vibration sensor  100   a ). In addition, in  FIG. 3 , the membrane  23   d  is comparatively wide, and thus a plurality of vibration sensors  100   d  are provided in the membrane  23   d . In this way, the positions of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  are arbitrarily set on the surface F 22  of the head unit  22 . For example,  FIG. 4  is a top plan view illustrating another arrangement of vibration sensors. As illustrated in  FIG. 4 , the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  may be arranged approximately rectilinearly in a radial direction of the surface F 22 . 
     During the process of polishing the semiconductor wafer W, the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  are almost stationary at the positions thereof without rotating together with the rotation of the holder  20 . That is, the holder  20  and the membranes  23   a ,  23   b ,  23   c , and  23   d  rotate about the center C, but the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  revolve reversely relative to the holder  20  and the membranes  23   a ,  23   b ,  23   c , and  23   d . Therefore, the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  appear to be almost stationary from the viewpoint of a user (the casing of the polishing apparatus  1 ). 
     In the present embodiment, a linear motor system is used to reversely rotate the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  relative to the rotations of the holder  20  and the membranes  23   a ,  23   b ,  23   c , and  23   d.    
       FIG. 5A  is a top plan view illustrating the membrane  23   a  and the vibration sensor  100   a .  FIG. 5B  is a schematic view illustrating a configuration example of the vibration sensor  100   a . In addition, the other membranes  23   b ,  23   c , and  23   d  and the other vibration sensors  100   b ,  100   c , and  100   d  also have the same configuration as the membrane  23   a  and the vibration sensor  100   a . Therefore, only the configurations of the membrane  23   a  and the vibration sensor  100   a  will be described, and descriptions of the other membranes and the other vibration sensors will be omitted. 
     A pair of magnet rails M 1  and M 2  is provided at both sides in the membrane  23   a . The magnet rails M 1  and M 2  are configured such that N-pole permanent magnets and S-pole permanent magnets are alternately arranged. 
     The vibration sensor  100   a  has electromagnets  101  and  102  disposed at both ends of a main body  105 . When the membrane  23   a  rotates together with the head unit  22 , the electromagnets  101  and  102  are controlled to alternate the N polarity and the S polarity. Therefore, the vibration sensor  100   a  receives a propulsive force along the magnet rails M 1  and M 2 , so that the vibration sensor  100   a  moves relative to the membrane  23   a . When the vibration sensor  100  is rotated in a direction opposite to the direction of the arrow A 2  at a speed approximately equal to a speed of the holder  20 , the vibration sensor  100  appears to be almost stationary when viewed from the main body of the polishing apparatus  1 , by the user, or from the ground surface. In this way, the vibration sensor  100   a  is moved relative to the membrane  23   a  by using the linear motor system. Therefore, the vibration sensor  100   a  appears to be almost stationary when viewed by the user. The vibration sensors  100   b ,  100   c , and  100   d  are also moved relative to the membranes  23   b ,  23   c , and  23   d  by using the linear motor system. 
     The main body  105  of the vibration sensor  100   a  has a communication unit  106  which may communicate with the sensor control unit  26  of the head unit  22 , an electromagnet control unit  107  which controls the electromagnets  101  and  102  based on a control signal from the sensor control unit  26 , and a sensor unit  108  which is disposed on a lower surface of the main body  105 , and a battery  109  which supplies electric power to the respective constituent elements. In addition, the battery  109  may be omitted and electric power may be supplied to the vibration sensor  100   a  from the head unit  22  by using a wireless power transfer technology. 
     Each of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  is a contact sensor such as an AE sensor. Therefore, the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  need to be in contact with the bottom portions of the membranes  23   a ,  23   b ,  23   c , and  23   d  so as to be in indirect contact with the semiconductor wafer W through the membranes  23   a ,  23   b ,  23   c , and  23   d.    
     For example,  FIGS. 6A and 6B  are cross-sectional views taken along line  6 - 6  in  FIG. 5A .  FIG. 6A  illustrates a state where the vibration sensor  100   a  is on standby before or after polishing.  FIG. 6B  illustrates a state where the vibration sensor  100   a  detects vibration during the polishing. In the present embodiment, an electromagnet  110  is provided at a part of the supply port  25  and may attract the vibration sensor  100   a  with magnetic force. 
     During the standby illustrated in  FIG. 6A , the electromagnet  110  functions upon being supplied with power. The vibration sensor  100   a  includes, for example, a magnetic material included in an iron core in the electromagnets  101  and  102 , and as a result, the vibration sensor  100   a  is attracted by the electromagnet  110 . The vibration sensor  100   a  is configured to be fixed to the electromagnet  110  such that the vibration sensor  100   a  is not freely moved in the membrane  23   a.    
     Meanwhile, during the polishing illustrated in  FIG. 6B , the electromagnet  110  is stationary as it is not supplied with power, and the vibration sensor  100   a  is pressed against the bottom portion of the membrane  23   a  by its own weight and/or blasting force (wind pressure) of the gas from the supply port  25 . More specifically, the lower surface (sensor unit  108 ) of the vibration sensor  100   a  is pressed against an upper surface of the bottom portion of the membrane  23   a . Further, during the polishing, as described with reference to  FIG. 5A , the vibration sensor  100   a  moves relative to the membrane  23   a  by using the linear motor system. Therefore, the vibration sensor  100   a  moves according to the linear motor system in the state where the vibration sensor  100   a  is in contact with the bottom portion of the membrane  23   a . When the holder  20  rotates and the vibration sensor  100   a  moves in the reverse direction in the membrane  23   a , it is possible to know the position (height) of the polishing surface in the entire area corresponding to the membrane  23   a . That is, it is possible to know the thickness in the area of the semiconductor wafer W which corresponds to the membrane  23   a . In addition, in the membrane  23   a , the lower surface of the vibration sensor  100   a  and the upper surface of the bottom portion of the membrane  23   a  may be made of a material having a small coefficient of friction. In addition, a lubricant may be supplied between the lower surface of the vibration sensor  100   a  and the upper surface of the bottom portion of the membrane  23   a  in order to reduce friction between the vibration sensor  100   a  and the membrane  23   a.    
     Similarly, the vibration sensors  100   b ,  100   c , and  100   d  also move by the linear motor system in the state where the vibration sensors  100   b ,  100   c , and  100   d  are in contact with the bottom portions of the membranes  23   b ,  23   c , and  23   d . Therefore, it is possible to know positions (heights) of the polishing surface in the entire area which correspond to the membranes  23   b ,  23   c , and  23   d , respectively. 
     The measurement unit  50 , the calculation unit  60 , and the control unit  70  will be described with reference back to FIG.  1 . In some embodiments, the measurement unit  50 , the calculation unit  60  and the control unit  70  may be integrated into a dedicated controller or computer. 
     The measurement unit  50  receives signals which are transmitted from the communication units  106  of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d , through the sensor control unit  26  of the head unit  22 . For example, voltage values of the signals represent intensity (speed) of vibration at each of the membranes  23   a ,  23   b ,  23   c , and  23   d . Therefore, the measurement unit  50  refers to the voltage values of the signals from the vibration sensors  100   a ,  100   b ,  100   c , and  100   d , thereby ascertaining the intensity of the vibration in each of the areas of the semiconductor wafer W where the membranes  23   a ,  23   b ,  23   c , and  23   d  are provided. The measurement unit  50  performs analog-to-digital (AD) conversion on the signals from the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  and outputs the AD-converted signals to the calculation unit  60 . The measurement unit  50  performs AD conversion on signals having a wide frequency range from a low frequency to a high frequency and transmits the digital signals to the calculation unit  60  in real time during the polishing. 
     The calculation unit  60  determines unevenness (flatness) of the polishing surface of the semiconductor wafer W in accordance with magnitudes of the signals from the vibration sensors  100   a ,  100   b ,  100   c , and  100   d . For example, when the signal from the vibration sensor  100   a  is smaller than the signal from the vibration sensor  100   b , the vibration sensor  100   a  is farther from the polishing surface of the semiconductor wafer W than the vibration sensor  100   b . Therefore, the thickness of the semiconductor wafer W in the area which corresponds to the membrane  23   a  is greater than the thickness of the semiconductor wafer W in the area which corresponds to the membrane  23   b . That is, this means that the polishing surface in the area corresponding to the membrane  23   a  protrudes further than the polishing surface in the area corresponding to the membrane  23   b . On the contrary, when the signal from the vibration sensor  100   a  is larger than the signal from the vibration sensor  100   b , the vibration sensor  100   a  is closer to the polishing surface of the semiconductor wafer W than the vibration sensor  100   b . Therefore, the thickness of the semiconductor wafer W in the area which corresponds to the membrane  23   a  is smaller than the thickness of the semiconductor wafer W in the area of the membrane  23   b . That is, this means that the polishing surface in the area corresponding to the membrane  23   a  is recessed further than the polishing surface in the area corresponding to the membrane  23   b . In this way, an unevenness state (flatness) of the polishing surface of the semiconductor wafer W in the areas corresponding to the membranes  23   a ,  23   b ,  23   c , and  23   d  is ascertained. Therefore, the calculation unit  60  may create an unevenness map for the corresponding polishing surface. 
     The calculation unit  60  may calculate a magnitude of the unevenness of the semiconductor wafer W based on a magnitude of a difference between the signal from the vibration sensor  100   a  and the signal from the vibration sensor  100   b . Alternatively, the calculation unit  60  may calculate the thickness of the semiconductor wafer W based on the magnitude of the signal. 
     The control unit  70  controls the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d  based on the unevenness map for the polishing surface of the semiconductor wafer W. For example, as described above, if the polishing surface in the area corresponding to the membrane  23   a  protrudes further than the polishing surface in the area corresponding to the membrane  23   b , the control unit  70  makes the gas pressure in the membrane  23   a  higher than a gas pressure in a recipe and/or makes the gas pressure in the membrane  23   b  lower than the gas pressure in the recipe. Therefore, the pressure, which presses the semiconductor wafer W against the polishing unit  10 , is increased in the area of the protruding membrane  23   a . Meanwhile, the pressure, which presses the semiconductor wafer W against the polishing unit  10 , may be decreased in the area of the recessed membrane  23   b . Therefore, it is possible to reduce unevenness (irregularity in the thickness) of the semiconductor wafer W and thus polish and flatten the semiconductor wafer W. Here, the recipe is a control sequence which is set in advance in a polishing control program to control the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d.    
     The control unit  70  controls the drive unit  30  to change the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d . The drive unit  30  changes the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d  by operating a non-illustrated air pump or the like based on a command from the control unit  70 . In this way, the control unit  70  may correct the unevenness state (flatness) of the polishing surface of the semiconductor wafer W in real time during the polishing by feedback-controlling the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d . As a result, the polishing apparatus  1  according to the present embodiment may improve flatness of the semiconductor wafer W after the polishing. In addition, when a material film (not illustrated) on the semiconductor wafer W is polished, the polishing apparatus  1  may inhibit irregularity in film thickness of the material film after the polishing. 
     The measurement unit  50 , the calculation unit  60 , and the control unit  70  may be disposed inside the polishing apparatus  1  or may be provided, as separate members, outside the polishing apparatus  1 . When the measurement unit  50 , the calculation unit  60 , and the control unit  70  are separate members provided separately from the polishing apparatus  1 , the measurement unit  50 , the calculation unit  60 , and the control unit  70  may be implemented by, for example, one or a plurality of personal computers. 
     Next, a polishing method according to the present embodiment will be described. 
       FIG. 7  is a flowchart illustrating an example of the polishing method according to the first embodiment. 
     First, the semiconductor wafer W is held by the holder  20 , and the semiconductor wafer W is pressed against the polishing pad  13  (S 10 ). 
     Next, the polishing unit  10  and the holder  20  are rotated while slurry is supplied, so that the semiconductor wafer W begins to be polished (S 20 ). 
     Between the point in time at which the polishing starts and a predetermined point in time, the calculation unit  60  detects unevenness of the polishing surface of the semiconductor wafer W and creates the unevenness map for the polishing surface (S 30 ).  FIG. 8  is a graph illustrating magnitudes of the signals from the vibration sensors  100   a ,  100   b ,  100   c , and  100   d . The vertical axis indicates voltages of the signals, and the horizontal axis indicates time. A period of time of t 0  to t 1  is the period of time taken to create the unevenness map. A period of time after t 1  is the period of time taken to perform the polishing. In addition, the polishing apparatus  1  may perform the polishing even for the period of time taken to create the unevenness map. In this case, the polishing apparatus  1  continues to perform the polishing after t 1 . The period of time (t 0  to t 1 ) taken to create the unevenness map may be arbitrarily set. 
     The period of time taken to create the unevenness map and the period of time take to perform the polishing may be periodically repeated during the process of polishing one sheet of the semiconductor wafer W. That is, the polishing and the creating of the unevenness map may be repeated, and the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d  may be further controlled while the flatness (unevenness) of the semiconductor wafer W is detected in real time. Therefore, based on the unevenness map, the polishing apparatus  1  may control, in real time, the pressure that presses the semiconductor wafer W against the polishing unit  10 . 
     During the period of time taken to create the map, the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  detect vibration of the semiconductor wafer W. The signals from the vibration sensors  100   a ,  100   b ,  100   c , and  100   d , which are converted by the measurement unit  50 , are processed by the calculation unit  60 . The calculation unit  60  averages the magnitudes of the signals from the vibration sensors  100   a ,  100   b ,  100   c , and  100   d . Further, the calculation unit  60  determines unevenness of the polishing surface of the semiconductor wafer W in the areas corresponding to the membranes  23   a ,  23   b ,  23   c , and  23   d , based on the averaged magnitudes of the signals in respect to the areas corresponding to the membranes  23   a ,  23   b ,  23   c , and  23   d . The determination of the unevenness is as described above. Further, the calculation unit  60  creates the unevenness map that represents flatness between the areas of the semiconductor wafer W which correspond to the membranes  23   a ,  23   b ,  23   c , and  23   d.    
     In the example illustrated in  FIG. 8 , for the period of time of t 0  to t 1  taken to create the map, an average value of the signals is comparatively small for the vibration sensors  100   c  and  100   a  and comparatively large for the vibration sensors  100   d  and  100   b . Therefore, the unevenness map indicates that the polishing surface of the semiconductor wafer W is convex in the areas of the membranes  23   c  and  23   a , and the polishing surface of the semiconductor wafer W is concave in the areas of the membranes  23   d  and  23   b.    
     Referring again to  FIG. 7 , the calculation unit  60  continues to create the unevenness map until a predetermined time passes immediately after the polishing starts (NO in S 40 ). 
     Meanwhile, the creating of the unevenness map ends when the predetermined time has passed immediately after the polishing started (YES in S 40 ), at which time the calculation unit  60  compares a threshold value with a difference in signal between the areas of the membranes  23   a ,  23   b ,  23   c , and  23   d  in the unevenness map (S 50 ). The threshold value is the allowable value, set beforehand. When the difference in signals is small, this means there is almost no unevenness of the polishing surface of the semiconductor wafer W, and unevenness may be a detection error. Therefore, the allowable value is set in advance as the threshold value. 
     When a difference in signal between the areas is smaller than the threshold value (NO in S 50 ), the control unit  70  controls the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d  are depending on the predetermined recipe (S 60 ). 
     However, when a difference in signal between the areas is equal to or larger than the threshold value (YES in S 50 ), the control unit  70  controls the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d  (S 70 ). For example, when a difference in signal between the vibration sensor  100   a  (or  100   c ) in  FIG. 8  and the vibration sensor  100   d  (or  100   b ) is larger than the threshold value, the control unit  70  makes the gas pressures in the membranes  23   a  and  23   c  higher than the gas pressures in the membranes  23   d  and  23   b . The gas pressures in the membranes  23   a  and  23   c  may be increased in accordance with (for example, in proportion to) the magnitude of the difference between the difference in signal and the threshold value. Therefore, the polishing speed on the semiconductor wafer W is made greater in the areas of the membranes  23   a  and  23   c  than in the areas of the membranes  23   d  and  23   b . Alternatively or additionally, the control unit  70  may make the gas pressures in the membranes  23   d  and  23   b  lower than the gas pressures in the membranes  23   a  and  23   c . The gas pressures in the membranes  23   a  and  23   c  may be decreased in accordance with (for example, in proportion to) a magnitude of a difference between the difference in signal and the threshold value. Therefore, the speed at which the semiconductor wafer W is polished is made lower in the areas of the membranes  23   d  and  23   b  than in the areas of the membranes  23   a  and  23   c . In addition, the control unit  70  may increase the gas pressure in the membrane to improve throughput by increasing the speed at which the semiconductor wafer W is polished. 
     A degree to which the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d  are adjusted may be calculated by using a maximum value, a minimum value, and an average value of the signal in each of the areas for a predetermined period of time (e.g., a period of time of one loop from S 30  to S 70 ). For example, when increasing the gas pressure in the membrane  23   a , the calculation unit  60  may set the rate of increase in gas pressure in the membrane  23   a  to be the value (1−Smin/Savg) obtained by subtracting from 1 the ratio (Smin/Savg) of the minimum value Smin to the average value Savg of the signal in the area corresponding to the membrane  23   a . Specifically, when Smin/Savg is 0.9, the calculation unit  60  sets 0.1 (10%) to be the rate of increase. The control unit  70  increases the gas pressure in the membrane  23   a  by 10%. For example, when the current gas pressure in the membrane  23   a  is 300 Hpa, the control unit  70  controls and increases the gas pressure by 10% to 330 Hpa. 
     When decreasing the gas pressure in the membrane  23   a , the calculation unit  60  may set the rate of decrease in gas pressure in the membrane  23   a  to be (Smax/Savg−1) obtained by subtracting 1 from the ratio (Smax/Savg) of the maximum value Smax to the average value Savg of the signal in the area corresponding to the membrane  23   a . Specifically, when Smax/Savg is 1.2, the calculation unit  60  sets 0.2 (20%) as the rate of decrease. The control unit  70  decreases the gas pressure in the membrane  23   a  by 20%. For example, when the current gas pressure in the membrane  23   a  is 300 Hpa, the control unit  70  decreases the gas pressure to 240 Hpa. 
     Steps S 30  to S 70  are repeated until the end point is detected (NO in S 80 ). Therefore, the creating of the unevenness map (t 0  to t 1 ) is periodically repeated during the polishing. The polishing ends when the polishing time reaches a predetermined time or when it is detected that the film thickness of the semiconductor wafer W is smaller than a predetermined film thickness. 
     When the endpoint is detected (YES in S 80 ), the polishing process ends. Thereafter, an additional polishing process is performed as necessary, that is, when a residual film remains. 
     As described above, according to the present embodiment, the calculation unit  60  obtains the unevenness map for the polishing surface of the semiconductor wafer W based on the signals from the vibration sensor  100   a  and the like provided in the membranes  23   a  and the like. The vibration sensor  100   a  or the like is a contact sensor, so that the vibration sensor  100   a  may detect, with high precision, vibration from the polishing surface of the semiconductor wafer W which is caused by the polishing. Therefore, the unevenness map indicates flatness of the polishing surface of the semiconductor wafer W with high precision. Further, the control unit  70  feedback-controls the gas pressure in each of the membrane  23   a  and the like based on the unevenness map, and as a result, it is possible to correct the unevenness state (flatness) of the polishing surface of the semiconductor wafer W in real time during the polishing. As a result, the polishing apparatus  1  according to the present embodiment may improve flatness of the semiconductor wafer W or the material film after the polishing, thereby inhibiting irregularity in the thickness. 
     Second Embodiment 
       FIG. 9  is a flowchart illustrating an example of a polishing method according to a second embodiment. In the first embodiment, in step S 50 , the calculation unit  60  compares the difference in signal between the areas of the membranes  23   a ,  23   b ,  23   c , and  23   d  with the threshold value. The polishing apparatus  1  relatively compares the signals between the areas and controls the unevenness of the semiconductor wafer W such that the unevenness of the semiconductor wafer W is equal to or smaller than the threshold value. 
     In contrast, in the second embodiment, in step S 51  as a substitute for step S 50 , the calculation unit  60  compares a difference between a reference value and a signal of each of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  with a threshold value. The reference value is a value obtained by converting a target value of a thickness of the semiconductor wafer W at a certain point in time during the polishing into a signal (voltage) of each of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d . That is, the reference value may represent a target of a thickness of the semiconductor wafer W at each point in time. In addition, the reference value may be applied in common to all of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  in order to flatten the semiconductor wafer W. Alternatively, the reference values may be individually set for the vibration sensors  100   a ,  100   b ,  100   c , and  100   d , respectively, in consideration of differences between the membranes  23   a ,  23   b ,  23   c , and  23   d  and individual difference between the vibration sensors  100   a ,  100   b ,  100   c , and  100   d.    
     Here, the target value of the thickness of the semiconductor wafer W will be described. For example, the thickness of the semiconductor wafer W is decreased as time passes after the polishing starts. Further, at the end of the polishing, the thickness of the semiconductor wafer W may become a finally desired film thickness. Therefore, when steps S 30  to S 70  are repeatedly performed, the target value of the thickness of the semiconductor wafer W at each processing point in time in step S 51  is set such that the thickness is gradually decreased from a thickness (initial value) of the semiconductor wafer W when the polishing initially starts to a target value (final target value) of a final thickness of the semiconductor wafer W when the polishing ends. The polishing apparatus  1  may polish the semiconductor wafer W in accordance with the target value, thereby allowing the thickness of the semiconductor wafer W to asymptotically converge on the desired final target value. 
     Actually, to polish the semiconductor wafer W in accordance with the target value, the polishing apparatus  1  polishes the semiconductor wafer W by using the reference value that corresponds to the target value. That is, the polishing apparatus  1  polishes the semiconductor wafer W so that the signals from the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  are suitable for the reference value. Therefore, the polishing apparatus  1  may allow the thickness of the semiconductor wafer W to converge on the desired final target value. In addition, at a certain processing point in time in step S 51 , the reference value of the signals of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  is a value obtained by converting the target value of the thickness of the semiconductor wafer W at that point in time into the signals (voltages) of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d . The reference value is set in advance and stored in a memory (not illustrated) in the calculation unit  60 . 
     In step S 51 , referring to the unevenness map, the calculation unit  60  compares the difference between the reference value and the signal from each of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  with the threshold value (S 51 ). 
     When the difference between the reference value and the signal from any one of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  is smaller than the threshold value (NO in S 51 ), the control unit  70  determines that the signal from the vibration sensor is close to the reference value, and the control unit  70  controls the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d  in accordance with the recipe (S 60 ). The reason is that the thickness of the semiconductor wafer W in the area corresponding to the membrane is considered as almost reaching the target value. 
     However, when the difference (reference value difference) between the reference value and the signal from any one of the vibration sensors  100   a ,  100   b ,  100   c , and  100   d  is equal to or larger than the threshold value (YES in S 51 ), the control unit  70  controls the gas pressures in the membranes  23   a ,  23   b ,  23   c , and  23   d  (S 70 ). For example, when the signal from the vibration sensor  100   a  is larger than the reference value by the threshold value or more, the thickness of the semiconductor wafer Win the area corresponding to the membrane  23   a  is smaller than the target value. Therefore, the control unit  70  makes the gas pressure in the membrane lower than the recipe. Meanwhile, in a case where the signal from the vibration sensor  100   a  is smaller than the reference value by the threshold value or more, the thickness of the semiconductor wafer W in the area corresponding to the membrane  23   a  is larger than the target value. Therefore, the control unit  70  makes the gas pressure in the membrane  23   a  higher than the recipe. The control unit  70  also similarly controls the gas pressures in the other membranes  23   b ,  23   c , and  23   d . In addition, the gas pressure in the membrane may be increased or decreased in accordance with (e.g., in proportion to) a magnitude of the difference between the reference value difference and the threshold value. 
     Steps S 30  to S 70  are repeated until the end point is detected (NO in S 80 ). When the end point is detected (YES in S 80 ), the polishing process ends. In the second embodiment, the polishing is performed such that the thickness of the semiconductor wafer W converges on the final target value. Therefore, hardly any residual film remains, and an additional polishing process is not required. This leads to an improvement of productivity. 
     In this way, the difference between the reference value and the signal from the vibration sensor  100   a  or the like may be compared with the threshold value. The other operations of the second embodiment may be similar to the corresponding operations of the first embodiment. Therefore, the second embodiment may also obtain the same effect as the first embodiment. 
     Third Embodiment 
       FIG. 10  is a schematic view illustrating a configuration example of a polishing apparatus according to a third embodiment. In the first embodiment, the membrane  23   a  or the like has therein the cavity H, and the vibration sensor  100   a  or the like is provided in the cavity H. 
     In contrast, in the third embodiment, a liquid  111  is introduced into the membrane  23   a  or the like. The liquid  111  may be a water-soluble liquid such as water, an oil-based liquid such as oil, or a liquid having viscosity. 
     In this case, the vibration sensor  100   a  or the like may float on the liquid  111 . For example, in addition to the AE sensor, a hydrophone sensor, an ultrasonic sensor, or the like is used as the vibration sensor  100   a  or the like. The vibration sensor  100   a  or the like may detect vibration from the semiconductor wafer W through the liquid  111  and the membrane  23   a  or the like. 
     The other configurations of the third embodiment may be similar to the corresponding configurations of the first embodiment. Therefore, the third embodiment may also obtain the same effect as the first embodiment. In addition, the third embodiment may be combined with the second embodiment. Therefore, the third embodiment may also obtain the same effect as the second embodiment. 
     While certain embodiments have been described, these embodiments have been presented byway of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit, of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.