Patent Publication Number: US-2012024317-A1

Title: Cleaning apparatus and cleaning method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2010-168442, filed on Jul. 27, 2010; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a cleaning apparatus and a cleaning method. 
     BACKGROUND 
     Conventionally, a photolithography is used in a manufacturing process of semiconductor devices. In the dimensional accuracy of a resist pattern formed on a wafer in the photolithography, an exposure amount, an exposure focus position (focus position), and the like at the time of exposure are important factors. Therefore, it is needed to realize an accurate focus position for forming a resist pattern as designed. 
     However, when foreign matter, dust, or the like is adhered to the back surface of a wafer at the time of exposure, the position of a light receiving surface is displaced from a focus position in an optical axis direction, i.e., defocusing occurs, so that a resist pattern is not formed as designed. For removing foreign matter or the like adhered to the surface of a wafer, a cleaning apparatus is used that removes foreign matter or the like on the surface of a wafer by using a brush. However, it is usually not performed to remove protrusions present on the back surface of a wafer. 
     The Chemical Mechanical Polishing (CMP) process is performed to planarize the whole surface of a wafer by grinding the whole surface. However, the CMP process uses abrasive such as slurry, so that the configuration of an apparatus and the process become complicated. 
     Therefore, in the cleaning apparatus and the cleaning method, a technology for efficiently removing protrusions present on the back surface of a wafer is desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a configuration of a cleaning apparatus according to a first embodiment; 
         FIG. 2A  to  FIG. 2C  are cross-sectional views schematically explaining a cleaning method of the back surface of a wafer by the cleaning apparatus according to the first embodiment; 
         FIG. 3A  to  FIG. 3C  are cross-sectional views schematically explaining a cleaning method of the back surface of another wafer by the cleaning apparatus according to the first embodiment; 
         FIG. 4  is a conceptual diagram explaining a case of performing a cleaning process by a brush while maintaining a constant pressure with respect to minor warpage of a wafer in the cleaning apparatus; and 
         FIG. 5A  to  FIG. 5D  are cross-sectional views schematically explaining a cleaning method of the back surface of a wafer according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a cleaning apparatus includes a holding unit capable of holding a semiconductor wafer, a removing unit whose tip portion is harder than a constituent material of a surface layer on a back surface side of the semiconductor wafer and which is configured to clean a semiconductor-wafer back surface to be a process target surface of the semiconductor wafer held by the holding unit, and a moving mechanism that relatively moves the removing unit and the semiconductor wafer in a direction parallel to the semiconductor-wafer back surface. The removing unit grinds and removes a protrusion that is formed of a material same as the surface layer of the semiconductor-wafer back surface, by the moving mechanism relatively moving the removing unit and the semiconductor wafer in the direction parallel to the semiconductor-wafer back surface and causing the tip portion of the removing unit to come into contact with the protrusion. 
     The embodiments of a cleaning apparatus will be explained below in detail based on the drawings. In the drawings illustrated below, the scale of each member is different from a realistic one in some cases for easy understanding. The same thing can be said between the drawings. 
     First Embodiment 
       FIG. 1  is a diagram schematically illustrating a configuration of a cleaning apparatus  1  according to the first embodiment. As shown in  FIG. 1 , the cleaning apparatus  1  according to the present embodiment includes a wafer holding unit  2 , a wafer rotation driving unit  3 , a brush  4 , a brush driving unit  5 , a sensor unit  6 , a sensor driving unit  7 , a control unit  8 , and a cleaning-water supplying unit  9 . 
     The wafer holding unit  2  can hold a semiconductor wafer  10  (hereinafter, wafer  10 ), which is a process target substrate, by gripping the periphery of the wafer  10  with the back surface facing upward. The back surface is a surface on the opposite side of the surface of the wafer  10  on which a semiconductor device is formed. The wafer holding unit  2  grips the periphery of the side surface of the wafer  10  by a wafer gripping unit (not shown) at least at two positions. 
     The wafer rotation driving unit  3  horizontally rotates the wafer holding unit  2  holding the wafer  10  at a given rotation speed in a circumferential direction with the center of the wafer  10  in a plane direction as an axis. The rotation speed of the wafer  10  is appropriately set depending on the condition such as a material of a protrusion on the back surface of the wafer  10  to be a removing target and a material of brush bristles  4   a.  As will be described later, the sensor unit  6  provides length measurement information to the control unit  8  that controls elevation of the brush  4  to keep the distance from the back surface of the wafer  10  approximately constant. At this time, the rotation speed of the wafer  10  can be set to the rotation speed at which length measurement of the sensor unit  6  and control of the control unit  8  follow the rotation of the wafer  10  and does not necessarily need to be high speed. 
     In the brush  4  as a removing unit of foreign matter and protrusions on the wafer  10 , a plurality of the brush bristles  4   a  is supported and arranged to have, for example, a columnar shape on the surface of its main body facing the wafer  10 . The arrangement of the brush bristles  4   a  is not limited to the above-described columnar shape and various forms can be employed, such as a form in which the brush bristles  4   a  are arranged to have a ring shape on the surface facing the wafer  10  and a form in which the brush bristles  4   a  are arranged to have a line shape on the surface facing the wafer  10 . The tip portions of the brush bristles  4   a  are formed of a material whose hardness is higher than the constituent material of the back surface of the wafer  10 . Moreover, the brush bristles  4   a  formed of a plurality of materials can be used. 
     The constituent material of the back surface of the wafer  10 , for example, includes a silicon film, a carbon film, an organic material film, a silicon oxide film, and a silicon nitride film. For surely removing protrusions formed of these films, the hardness of the brush bristle  4   a  is preferably 30 degrees or more. Moreover, a superhard brush with diamond chips can be used. The hardness in this example is a measured value by a type A durometer according to JIS K 6253 (Rubber. vulcanized or thermoplastic-Determination of hardness) or JIS K 7215 (Testing Methods for Durometer Hardness of Plastics). Moreover, when this hardness is 90 degrees or higher, the hardness can be measured by a type D durometer of the same standard. 
     Moreover, when the outermost-layer film of the back surface of the wafer  10  is thin, a lower-layer film under the outermost-layer film of the back surface of the wafer  10  may be present in a protrusion. In this case, it is possible to appropriately select the hardness of the brush bristles  4   a,  for example, for the harder material between the material of the outermost-layer film and the material of the lower-layer film thereunder of the back surface of the wafer  10 . 
     Furthermore, in this example, the brush  4  is illustrated as the removing unit of foreign matter and protrusions on the wafer  10 , however, a deburring cutter for the back surface of the wafer  10  can be used other than the brush  4 . In the cutter, for example, a plurality of cutter blades is arranged on the surface facing the wafer  10  to have a shape such as a columnar shape, a ring shape, and a line shape. The tip portions of the cutter blades, which are in contact with foreign matter and protrusions on the wafer  10 , are formed of a material whose hardness is higher than the constituent material of the back surface of the wafer  10 . The brush bristle  4   a,  although hard, has some flexibility in the tip portion thereof. Therefore, the brush  4  has a responsiveness to warpage of the back surface of the wafer  10  higher than a cutter blade. 
     The brush driving unit  5  raises and lowers the brush  4  to a given height above the wafer  10  held by the wafer holding unit  2 . Moreover, the brush driving unit  5  rotates the brush  4  at a given rotation speed and moves the brush  4  in a predetermined direction over the surface of the wafer  10  held by the wafer holding unit  2 . The brush driving unit  5  can move the brush  4 , for example, in a radial direction of the wafer  10  while horizontally rotating the blush  4 . 
     The sensor unit  6  is composed of a length measurement sensor capable of measuring the distance to the wafer  10 , for example, by instantly calculating the time for light such as laser to be reflected from the wafer  10 . The sensor unit  6  can be raised and lowered above the wafer  10  in the height direction by being driven by the sensor driving unit  7  and can move also in a given direction in the plane direction of the wafer  10 . 
     The control unit  8  performs control of the wafer rotation driving unit  3 , the brush driving unit  5 , the sensor unit  6 , and the sensor driving unit  7 . The cleaning-water supplying unit  9  supplies, for example, cleaning water as cleaning liquid onto the wafer  10  during the cleaning process or after the cleaning process and washes out grinding swarf generated in the cleaning process from the back surface of the wafer  10 . 
     Next, the cleaning process by the cleaning apparatus  1  is explained with reference to  FIG. 2A  to  FIG. 2C .  FIG. 2A  to  FIG. 2C  are cross-sectional views schematically explaining the cleaning method of the back surface of the wafer  10  by the cleaning apparatus  1  according to the first embodiment. 
     First, the wafer  10 , which is a process target substrate, is held by the wafer holding unit  2  so that the back surface faces upward. As shown in  FIG. 2A , a first coating  11 , a second coating  12 , and a third coating  13 , which are formed in respective processes for manufacturing a semiconductor device to be formed on the surface of the wafer  10 , are stacked in this order on the back surface of the wafer  10 . This wafer  10  is a wafer in a state before exposure, in which a photoresist film is formed on the semiconductor device surface. The cleaning process by the cleaning apparatus  1  can be performed on the wafer  10  in a state in which a photoresist film is not formed yet on the semiconductor device surface. Moreover, the cleaning process by the cleaning apparatus  1  can be performed in the process using a stacked resist. In this case, for example, after forming a lower-layer resist on the semiconductor device surface of the wafer  10 , the cleaning process by the cleaning apparatus  1  is performed on the back surface of the wafer  10 . Next, the wafer  10  is introduced into an exposure apparatus and exposure is performed after forming an upper-layer resist on the lower-layer resist in the exposure apparatus. 
     A foreign matter  21  is adhered to part of the first coating  11  of the back surface of the wafer  10 . The region over the foreign matter  21  in the second coating  12  and the third coating  13  has a shape conforming to the shape of the foreign matter  21 . Therefore, on the third coating  13  that is the outermost layer of the back surface of the wafer  10 , a protrusion  13   a  is formed in the region over the foreign matter  21 . In other words, the protrusion  13   a  is a protrusion formed of a material same as the third coating  13  that is the surface layer of the back surface of the wafer  10 . In  FIG. 2A , one protrusion  13   a  is focused on, however, a plurality of other protrusions similar to the protrusion  13   a  is present on the surface of the third coating  13 . Moreover, a foreign matter  22 , which is formed of a material different from the third coating  13 , and debris (not shown), which scatters, for example, when the back surface of the wafer  10  is damaged and is formed of a material same as the third coating  13 , are also adhered to the third coating  13 . In this example, foreign matter that is formed on the back surface without being processed or removed through a plurality of manufacturing processes in this manner is called an accumulated foreign matter. 
     Next, the brush  4  is arranged in the center portion of the back surface of the wafer  10  by the brush driving unit  5 . The brush  4  is arranged in a state of floating by a constant separation distance t from the flat surface of the third coating  13  at the height at which the tip portions of the brush bristles  4   a  can come into contact with the protrusion  13   a.  Specifically, the brush  4  is arranged to be separated from the flat surface of the third coating  13  at the height at which the position of the tip portions of the brush bristles  4   a  is lower than the height of the protrusion  13   a  and the tip portions is not in contact with the flat surface of the third coating  13 . The brush  4  is rotated by being driven by the brush driving unit  5 . 
     Next, the wafer rotation driving unit  3  horizontally rotates the wafer holding unit  2 , so that the wafer  10  held by the wafer holding unit  2  rotates. The rotation speed of the wafer  10  and the rotation speed of the brush  4  can be appropriately changed depending on the conditions such as the material of the surface layer of the back surface of the wafer  10  and the material of the brush bristles  4   a . Next, as shown in  FIG. 2B , the brush  4  horizontally moves in the radial direction of the wafer  10  along with driving of the wafer rotation driving unit  3  in a state of being separated from the flat surface of the third coating  13  by approximately the constant separation distance t. Specifically, the brush  4  is arranged in the center portion of the back surface of the wafer  10  and is horizontally moved toward the periphery therefrom while rotating by the brush driving unit  5 . Then, the brush  4  reciprocates over the wafer  10  a plurality of times in the radial direction of the wafer  10 . 
     In this manner, the brush  4  horizontally moves relative to the rotating wafer  10 , so that the brush bristles  4   a  of the brush  4  come into contact with the protrusion  13   a.  Consequently, the protrusion  13   a  is ground by the brush  4  and is removed in a state of leaving a portion with the height corresponding to the distance between the brush bristles  4   a  and the flat surface of the third coating  13 , i.e., the thickness corresponding to the separation distance t, as shown in  FIG. 2C . Moreover, the foreign matter  22  adhered to the surface of the third coating  13  is ground or stripped by the brush bristles  4   a  of the brush  4  coming into contact therewith, thereby being removed from the surface of the third coating  13 . The grinding means to physically remove foreign matter and protrusions on the back surface of the wafer  10  by the brush bristles  4   a  as a grinding unit and includes concepts of polishing and cutting. At this time, cleaning water is supplied onto the wafer  10  from the cleaning-water supplying unit  9 , whereby grinding swarf generated in the cleaning process can be washed out from the back surface of the wafer  10 . Grinding swarf can be removed also by blowing air to the wafer  10 . Moreover, grinding swarf can be removed from the back surface of the wafer  10  by suctioning it. 
     Next, control of the separation distance t when warpage occurs in the wafer  10  is explained. First, the brush  4  is arranged at the position sufficiently separated from the back surface of the wafer  10  over the wafer  10  by the brush driving unit  5 . 
     Next, the sensor unit  6  measures the distance from the sensor unit  6  to the brush  4  and the distance from the sensor unit  6  to the back surface of the wafer  10  (the third coating  13 ). The sensor unit  6  measures the distance from the sensor unit  6  to the brush  4  and the distance from the sensor unit  6  to the back surface of the wafer  10  (the third coating  13 ) in the peripheral region of the brush  4 , for example, in a vertical direction. The sensor unit  6  is movable in synchronization with the brush  4  by the control unit  8  controlling the sensor driving unit  7  and can perform measurement in accordance with the movement of the brush  4 . Then, the sensor unit  6  sends these length measurement information to the control unit  8 . 
     The control unit  8  stores information on the length from the measurement reference position of the brush  4  (position of the main body of the brush  4 ) to the tips of the brush bristles  4   a,  for example, in a storing unit included in the control unit  8 . The control unit  8  calculates the distance from the tips of the brush bristles  4   a  to the flat surface of the back surface of the wafer  10  based on this information and the length measurement information sent from the sensor unit  6 . The sensor unit  6  and the control unit  8  configure a detecting unit that detects the distance from the tip portions of the brush bristles  4   a  to the flat surface of the back surface of the wafer  10 . Then, the control unit  8  controls the height of the brush  4  so that this distance becomes appropriately the constant separation distance t excluding a portion over the protrusion  13   a  of the wafer  10  when performing the cleaning process by moving the brush  4  in the plane direction of the wafer  10 . Consequently, the cleaning process can be performed while reflecting the warped state of the wafer  10 . 
     The rotation speed of the wafer  10  in the case of controlling while monitoring change in the separation distance t due to the warped state of the wafer  10  in this manner is desirably a lower rotation speed. This is effective, especially, when the tendency of the warpage is uniform from the center to the periphery in the plane of the wafer  10 . Moreover, the rotation speed of the wafer  10  can be increased after obtaining information on the tendency of warpage of the wafer  10  by the sensor unit  6  and the control unit  8 . 
     Moreover, in terms of the distance to the back surface of the wafer  10  (the third coating  13 ) measured by the sensor unit  6 , it is possible to distinguish between change in distance due to the protrusion  13   a  in the third coating  13  and change in distance due to warpage of the wafer  10 , for example, as follows. For example, change in distance is measured at a plurality of points around the brush  4  and the average thereof or the like is calculated to distinguish between the changes. For example, when the distance to the back surface of the wafer  10  (the third coating  13 ) is shorter than the average value by a predetermined distance or more, it is determined that change in distance is due to the protrusion  13   a.  Moreover, when the distance to the back surface of the wafer  10  (the third coating  13 ) is longer than the average value by a predetermined range, it is determined that change in distance is due to warpage of the wafer  10 . 
     The measurement by the sensor unit  6  described above can be performed while performing the cleaning process. Moreover, it is possible to perform the measurement by the sensor unit  6  described above in advance and store the information on warpage of the back surface of the wafer  10  in the storing unit of the control unit  8  before performing the cleaning process. In this case, the control unit  8  can control the distance from the tips of the brush bristles  4   a  to the back surface of the wafer  10  while reflecting the state of warpage of the wafer  10  by using the information on warpage of the wafer  10  stored in the storing unit. It is possible that a function unit related to control of the separation distance t, such as the sensor unit  6 , the sensor driving unit  7 , and the storing unit, is an apparatus separated from the cleaning apparatus  1 . 
       FIG. 3A  to  FIG. 3C  are cross-sectional views schematically explaining the cleaning method of the back surface of another wafer  101  by the cleaning apparatus  1  according to the first embodiment. This wafer  101  is a wafer in a state before exposure, in which a photoresist film is formed on the device surface. The cleaning process by the cleaning apparatus  1  can be performed on a wafer in a state in which a photoresist film is not formed yet on the semiconductor device surface. 
     As shown in  FIG. 3A , the back surface of the wafer  101  itself is damaged, so that protrusions  101   a  are formed on the back surface of the wafer  101 . In other words, the protrusion  101   a  is a protrusion formed of a material same as the back surface of the wafer  101 . The foreign matter  22  formed of a material different from the back surface of the wafer  101  is adhered to the back surface of the wafer  101 . 
     In the similar manner to the case of the cleaning process on the back surface of the wafer  10  described above, the cleaning process is performed on the wafer  101 . In this case again, as shown in  FIG. 3B , the brush bristles  4   a  come into contact with the protrusions  101   a  by relatively moving the brush  4  and the wafer  101 . Consequently, the protrusions  101   a  are ground by the brush  4  and are removed while leaving a portion with the height corresponding the distance between the brush bristles  4   a  and the flat surface of the back surface of the wafer  101 , i.e., the thickness corresponding to the separation distance t, as shown in  FIG. 3C . Moreover, the foreign matter  22  adhered to the back surface of the wafer  101  is ground or stripped by the brush bristles  4   a  coming into contact therewith, whereby the foreign matter  22  is removed. 
     In the manufacturing process of semiconductor devices, the photolithography process is performed, for example, to form a patterned film on a wafer. The photolithography process is largely classified into a photoresist-film forming process of applying a photosensitive film on a film, which is deposited on a wafer and is to be patterned, an exposure process of exposing a pattern on the photoresist film, and a developing process of removing part of the photoresist film by developing the exposed photoresist film and thereby forming a resist pattern. In such a photolithography process, in order to form a resist pattern accurately as designed, especially, in the exposure process, it is necessary to project a mask pattern with no blur on a photoresist film. In other words, it is needed to realize an accurate focus position as designed. 
     Even if foreign matter on the surface side is removed at the time of exposure, when a protrusion due to the accumulated foreign matter formed on the back surface of a wafer or due to damage, such as scratch, on the back surface of a wafer is present, the height of the wafer deviates from a desired set height due to this protrusion. Therefore, the position of a light receiving surface is displaced from the focus position in the optical axis direction, i.e., defocusing occurs. When the defocusing occurs, a resist pattern as designed is not formed, so that another photolithography process (rework) is needed. Performing the rework results in decreasing the production efficiency. Moreover, the photolithography process uses an exposure apparatus, which is expensive, and therefore is a high-cost process. Thus, performing the rework results in increasing the production cost. 
     As described above, foreign matter adhered to the surface of a wafer among protrusions present on the wafer can be removed by a conventionally-used cleaning apparatus using a brush. However, on the other hand, there is a protrusion formed of a material same as a surface layer of the back surface as shown in  FIG. 2A  and  FIG. 3A  on the back surface of the wafer. Such a protrusion is generated, for example, when the surface layer of the back surface of a wafer is damaged or when a coating is formed on the back surface of a wafer which is damaged or to which foreign matter is adhered. Moreover, when a plurality of coatings is stacked on the back surface of a wafer and a coating in the middle is damaged or foreign matter is adhered thereto, such a protrusion is generated. The protrusion, which is formed of a material same as a surface layer on the back surface side of a wafer and is formed on the back surface of a wafer in this manner, cannot be removed by the conventional cleaning apparatus that removes foreign matter by a brush. 
     However, in the cleaning apparatus  1  in the present embodiment, it is possible to grind and remove a protrusion that is formed of a material same as a surface layer of the back surface of a wafer and is formed on the back surface of the wafer. Moreover, in the cleaning apparatus  1 , foreign matter adhered to the back surface of a wafer can be also removed by grinding or stripping it from the back surface of the wafer. Consequently, planarization and cleaning of the back surface of a wafer can be realized. 
     The cleaning process of the back surface of a wafer by the cleaning apparatus  1  is performed, for example, on a wafer, which is a target to be subjected to the rework, before the exposure process, desirably, immediately before the exposure process. The exposure process is performed on a wafer whose back surface is subjected to the cleaning process by the cleaning apparatus  1 , so that the effect of a minor protrusion on the back surface of a wafer can be reduced and whereby occurrence of another defocusing can be prevented. Moreover, the cleaning process by the cleaning apparatus  1  can be performed on all wafers every time before performing the exposure process in the photolithography process. Furthermore, the cleaning process by the cleaning apparatus  1  can be performed before forming a photoresist film or can be performed after forming a photoresist film. 
     In the above present embodiment, explanation is made for the case of using the brush  4  as the removing unit of foreign matter and protrusions on the back surface of the wafer  10  as an example, however, it is also possible to use, for example, a cutter in which a plurality of cutter blades is arranged to have a columnar shape on the surface facing the wafer  10  instead of the brush  4  and perform control of the separation distance t in the similar manner to the above. 
     In the cleaning apparatus  1  described above, the effect of minor protrusion and recess on the back surface of a wafer at the time of exposure can be reduced by selectively grinding and planarizing only a protruded portion instead of planarizing the whole surface-layer film of a wafer by polishing it as in the conventional CMP technology. Consequently, occurrence of the rework can be suppressed and decrease in the production efficiency and increase in the production cost can be prevented. The cleaning process by such a cleaning apparatus  1  is preferable, particularly, for immersion exposure in which an effect of defocusing at the time of exposure to the accuracy of a resist pattern is large. 
     Moreover, the cleaning apparatus  1  described above does not use abrasive such as slurry as in the CMP, so that a planarizing process can be easily performed by an apparatus with a simple configuration. 
     Furthermore, other than suppression of occurrence of defocusing in the photolithography process, the cleaning process of the back surface of a wafer by the cleaning apparatus  1  has an effect of preventing occurrence of a failure due to minor protrusion and recess on the back surface by performing the cleaning process before a process that is affected by the minor protrusion and recess on the back surface. 
     In the above, explanation is made for the case in which the cleaning process is performed in a state where the tip portions of the brush bristles  4   a  are separated from the flat surface of the back surface of the wafer  10  by approximately the constant separation distance t, however, the cleaning process of the back surface of the wafer  10  can be performed in a state where the separation distance t is zero, i.e., the tip portions of the brush bristles  4   a  are in contact with the flat portion of the back surface of the wafer  10 . The sensor unit  6  and the control unit  8  perform control of setting the separation distance t to zero in the cleaning process of the back surface of the wafer  10 . In this case again, a protrusion formed of a material same as the back surface of the wafer  10  and foreign matter formed of a material different from the back surface of the wafer  10  can be removed from the back surface of the wafer  10  and cleaning of the back surface of the wafer  10  can be performed. 
     Second Embodiment 
       FIG. 4  is a conceptual diagram explaining a case of performing the cleaning process by the brush  4  while maintaining a constant pressure with respect to minor warpage of the wafer  10  in the cleaning apparatus  1 . In this embodiment, a pressure A is applied from above the wafer  10  and a pressure B is applied from below the wafer  10 , with respect to the wafer  10  in which minor warpage occurs. In other words, the pressure A is applied to the back surface of the wafer  10  and the pressure B is applied to the device surface of the wafer  10 . The pressure A is a pressure applied to the brush  4  when the brush  4  comes into contact with the back surface of the wafer  10 . The pressure B is a pressure applied from a not-shown pressurizing mechanism to correct minor warpage of the wafer  10 . The pressurizing mechanism is provided below the wafer  10  to rotate in synchronization with the rotation of the wafer  10 . The pressure B can be applied to the whole surface of the back surface of the wafer  10  or can be applied to the region facing the brush  4  via the wafer  10  during the cleaning process. 
     Then, the control unit  8  performs control so that the pressure A becomes equal to the pressure B, for example, by using an air spring such as an air suspension on the brush driving unit  5  side. By moving the brush  4  in a state of being in contact with the back surface of the wafer  10  while performing such control, a protrusion that is formed of a material same as a surface layer of the back surface side of the wafer  10  and is formed on the back surface of the wafer  10  can be ground and removed without measuring the separation distance t as in the case of the first embodiment. Moreover, foreign matter adhered to the back surface of the wafer  10  can be ground or stripped from the back surface of the wafer  10  to be removed. Consequently, in the similar manner to the case of the first embodiment, only the protruded portion can be selectively ground to be planarized. 
     Next, the cleaning process of the back surface of a wafer according to the second embodiment is explained with reference to  FIG. 5A  to  FIG. 5D .  FIG. 5A  to  FIG. 5D  are cross-sectional views schematically explaining the cleaning method of the back surface of a wafer according to the second embodiment. In  FIG. 5A  to  FIG. 5D , members same as those in  FIG. 2A  to  FIG. 2C  are given the same reference numerals. 
     First, the wafer  10  is held by the wafer holding unit  2  so that the back surface faces upward. As shown in  FIG. 5A , the first coating  11 , the second coating  12 , and the third coating  13 , which are formed in respective processes for manufacturing a semiconductor device to be formed on the front surface of the wafer  10 , are stacked in this order on the back surface of the wafer  10 . On the third coating  13  present in the outermost layer of the back surface of the wafer  10 , the protrusion  13   a  is formed and the foreign matter  22  is adhered. The wafer  10  is a wafer in a state before exposure, in which a photoresist film is formed on the semiconductor device surface. 
     Next, the brush  4  is arranged in the center portion of the back surface of the wafer  10  by the brush driving unit  5 . The brush  4  is arranged in a state where the tip portions of the brush bristles  4   a  are in contact with the flat surface of the third coating  13  by the brush driving unit  5 , i.e., in a state where the separation distance t is zero, to apply the pressure A to the back surface of the wafer  10 . Moreover, on the device surface of the wafer  10 , the predetermined pressure B (constant pressure) is applied to the region facing the brush  4  via the wafer  10  by the not-shown pressurizing mechanism. At this time, in the brush driving unit  5 , the pressure A is set to the pressure same as the pressure B by using an air spring (pressure A=pressure B). Then, the brush  4  rotates by being driven by the brush driving unit  5 . 
     Next, the wafer rotation driving unit  3  horizontally rotates the wafer holding unit  2 , so that the wafer  10  held by the wafer holding unit  2  rotates. Next, as shown in  FIG. 5B , the brush  4  horizontally moves in the radial direction of the wafer  10  along with driving of the wafer rotation driving unit  3  in a state of being in contact with the flat surface of the third coating  13 . Moreover, on the device surface of the wafer  10 , the pressure B is applied to the region facing the brush  4  by the pressurizing mechanism via the wafer  10  in accordance with the movement of the brush  4 . Then, the brush  4  reciprocates on the wafer  10  a plurality of times in the radial direction of the wafer  10 . 
     As shown in  FIG. 5C , when the brush bristles  4   a  of the brush  4  come into contact with the protrusion  13   a,  the brush  4  grinds the protrusion  13   a  while being pressed upward and the pressure (pressure A) applied to the brush  4  increases. When the brush bristles  4   a  pass the protrusion  13   a,  the brush bristles  4   a  become the state of being in contact with the flat surface of the third coating  13 , so that the pressure (pressure A) applied to the brush  4  decreases and the pressure A becomes equal to the pressure B. In the brush driving unit  5 , it is possible to absorb the displacement of the brush  4  in a direction vertical to the back surface of the wafer  10  in synchronization with increase in pressure (pressure A) applied to the brush  4  by an elastic action of an air spring. Moreover, in the brush driving unit  5 , when the brush  4  passes the protrusion  13   a  on the wafer  10  and comes into contact with the flat surface on the outer side of the protrusion  13   a,  the brush  4  can be restored to the original state from the displaced state by an elastic action of an air spring. As the height of the protrusion  13   a  decreases by repeating grinding of the protrusion  13   a,  the rising range of the pressure (pressure A) applied to the brush  4  becomes small. The same thing can be said to the case where the brush bristles  4   a  of the brush  4  come into contact with the foreign matter  22 . 
     Then, when the protrusion  13   a  and the foreign matter  22  disappear as shown in  FIG. 5D , the pressure A becomes always equal to the pressure B (constant pressure) on the back surface of the wafer  10 . Specifically, by performing grinding until the pressure A becomes equal to the pressure B (constant pressure) on the whole surface of the back surface of the wafer  10 , the protrusion  13   a  and the foreign matter  22  can be removed and thus the back surface of the wafer  10  can be planarized. Consequently, only the protruded portion on the back surface of the wafer  10  can be selectively ground to be planarized without controlling the separation distance t. 
     While certain embodiments have been described, these embodiments have been presented by way 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.