Patent Publication Number: US-2011070813-A1

Title: Method for manufacturing polishing head and polishing apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a method for manufacturing a polishing head for holding a workpiece when a surface of the workpiece is polished and a polishing apparatus provided with the polishing head manufactured by the method, and more particularly to a method for manufacturing a polishing head for holding the workpiece on a rubber film and a polishing apparatus provided with the manufactured polishing head. 
     BACKGROUND ART 
     As an apparatus for polishing a surface of a workpiece such as a silicon wafer, there are a single-side polishing apparatus, in which the workpiece is polished by each side, and a double-side polishing apparatus, in which the both sides of the workpiece are polished at the same time. 
     For example as shown in  FIG. 9 , a common single-side polishing apparatus comprises a turn table  88  onto which a polishing pad  89  is attached, a polishing agent supply mechanism  90 , a polishing head  82  and the like. The polishing apparatus  81  polishes a workpiece W by holding the workpiece W with the polishing head  82 , supplying a polishing agent onto the polishing pad  89  through the polishing agent supply mechanism  90 , rotating the turn table  88  and the polishing head  82  respectively, and bringing a surface of the workpiece W into sliding contact with the polishing pad  89 . 
     As a method for holding the workpiece in the polishing head, for example, there is a method of attaching the workpiece onto a flat disk-shaped plate through an adhesive such as a wax. Other than that, particularly as a holding method of suppressing rise or sag on an outer circumferential portion of the workpiece and of improving flatness of the whole workpiece, there is a so-called rubber-chuck method in which a workpiece holding portion is made of a rubber film, a pressurized fluid such as air is poured into a back face of the rubber film, and the rubber film is inflated by a uniform pressure so as to press the workpiece toward the polishing pad (See Japanese Unexamined Patent Publication (Kokai) No. H5-69310, for example). 
     An example of structure of a conventional polishing head by a rubber-chuck method is schematically shown in  FIG. 8 . An essential part of the polishing head  102  consists of an annular rigid ring  104  made of SUS and the like, the rubber film  103  bonded to the rigid ring  104 , and a mid plate  105  joined to the rigid ring  104 . A sealed space  106  is defined by the rigid ring  104 , the rubber film  103 , and the mid plate  105 . An annular template  114  is provided concentrically with the rigid ring  104  in the periphery of a lower face portion of the rubber film  103 . The pressure of the space is adjusted, for example, by supplying a pressurized fluid by a pressure adjustment mechanism  107  in a center of the mid plate  105 . A pressing means, not shown, for pressing the mid plate  105  in the direction of the polishing pad  109  is provided. 
     As a material of the rubber film  103 , there is a suggestion of various rubber materials, such as a fluorinated rubber, a urethane rubber, a silicon rubber, and an ethylene-propylene rubber, having a rubber hardness ranging from 10 to 100, a tensile strength ranging from 3 to 20 MPa, and a tensile elongation ranging from 50 to 1000% in Japanese Unexamined Patent publication (Kokai) No. 2005-7521. 
     With the polishing head  102  configured as above, the workpiece W is held on a lower face portion of the rubber film  103  through a backing pad  113 , an edge portion of the workpiece W is held with the template  114 , and the workpiece W is polished by bringing it into sliding contact with the polishing pad  109  attached onto an upper face of the turn table  108  by pressing the mid plate  105 . 
     DISCLOSURE OF INVENTION 
     Polishing the workpiece W by using the polishing head  102  in which the workpiece W is held on the rubber film  103 , as described above, may improve flatness and polishing stock removal uniformity of the whole workpiece W in some cases. However, there is a problem such that the flatness and the polishing stock removal uniformity may decrease due to a difference of material of the rubber film  103  or a difference of the manufacturing lot even with regard to the same materials and thus stable flatness of the workpiece W cannot be obtained. 
     The present invention was accomplished in view of the above-explained problems, and its object is to provide a method for manufacturing a polishing head and a polishing apparatus that can stably obtain constant flatness in polishing of the workpiece W. 
     To achieve this object, the present invention provides a method for manufacturing a polishing head having at least: an annular rigid ring; a rubber film bonded to the rigid ring with a uniform tension; a mid plate joined to the rigid ring, the mid plate forming a space together with the rubber film and the rigid ring; and a pressure adjustment mechanism for changing pressure of the space, the polishing head holding a back surface of a workpiece on a lower face portion of the rubber film and polishing the workpiece by bringing a surface of the workplace into sliding contact with a polishing pad attached onto a turn table, the method comprising the steps of: performing a tensile test on the rubber film according to JIS K6251 before bonding the rubber film to the rigid ring, and selecting the rubber film having a value of 10 MPa or less of an inclination obtained by a linear approximation of a stress-strain curve within a strain value of 5%; and bonding the selected rubber film having a value of 10 MPa or less of the inclination to the rigid ring to manufacture the polishing head. 
     In this manner, when the method comprises the steps of: performing a tensile test on the rubber film according to JIS K6251 before bonding the rubber film to the rigid ring, and selecting the rubber film having a value of 10 MPa or less of an inclination obtained by a linear approximation of a stress-strain curve within a strain value of 5%; and bonding the selected rubber film having a value of 10 MPa or less of the inclination to the rigid ring to manufacture the polishing head, the polishing head can be manufactured which can suppress a variation in the polishing stock removal uniformity that is generated between material types of rubber film or material lots of the rubber film and which can secure stably good flatness, in polishing of the workpiece. 
     In this case, the workpiece to be held can be a silicon single crystal wafer having a diameter of 300 mm or more. 
     In this manner, even when the workpiece to be held is the silicon single crystal wafer having a large diameter such as a diameter of 300 mm or more, the polishing can be performed with more uniform pressing force over the whole surface of the workpiece and thereby good flatness can be secured, according to the present invention. 
     Furthermore, the present invention provides a polishing apparatus having at least a polishing pad attached onto a turn table, a polishing agent supply mechanism for supplying a polishing agent onto the polishing pad, and a polishing head for holding a workpiece, the polishing apparatus polishing a surface of the workpiece while holding a back surface of the workpiece with the polishing head, wherein the polishing head has at least: an annular rigid ring; a rubber film bonded to the rigid ring with a uniform tension; a mid plate joined to the rigid ring, the mid plate forming a space together with the rubber film and the rigid ring; and a pressure adjustment mechanism for changing pressure of the space, the rubber film is formed by using a rubber material having a value of 10 MPa or less of an inclination obtained by a linear approximation of a stress-strain curve within a strain value of 5%, the stress-strain curve being obtained as a result of performing a tensile test on the rubber film according to JIS K6251, the polishing pad has a young&#39;s modulus of 3.5 MPa or less, the workpiece is polished by bringing a surface of the workpiece into sliding contact with the polishing pad attached onto the turn table with the pressure of the space controlled by the pressure adjustment mechanism. 
     In this manner, when the polishing head has at least: an annular rigid ring; a rubber film bonded to the rigid ring with a uniform tension; a mid plate joined to the rigid ring, the mid plate forming a space together with the rubber film and the rigid ring; and a pressure adjustment mechanism for changing pressure of the space, the rubber film is formed by using a rubber material having a value of 10 MPa or less of an inclination obtained by a linear approximation of a stress-strain curve within a strain value of 5%, the stress-strain curve being obtained as a result of performing a tensile test on the rubber film according to JIS K6251, the polishing pad has a young&#39;s modulus of 3.5 MPa or less, the workpiece is polished by bringing a surface of the workpiece into sliding contact with the polishing pad attached onto the turn table with the pressure of the space controlled by the pressure adjustment mechanism, a variation in the polishing stock removal uniformity which is generated between material types of rubber film or material lots of the rubber film can be suppressed, and the workpiece can be polished with stably good flatness secured. 
     In this case, the workpiece to be polished can be a silicon single crystal wafer having a diameter of 300 mm or more. 
     In this manner, even when the workpiece to be polished is the silicon single crystal wafer having a large diameter such as a diameter of 300 mm or more, the polishing can be performed with more uniform pressing force over the whole surface of the workpiece and thereby good flatness can be secured by holding it with the polishing head according to the present invention to polish. 
     The method according to the present invention comprises the steps of: performing a tensile test on the rubber film according to JIS K6251 before bonding the rubber film to the rigid ring, and selecting the rubber film having a value of 10 MPa or less of an inclination obtained by a linear approximation of a stress-strain curve within a strain value of 5%; and bonding the selected rubber film having a value of 10 MPa or less of the inclination to the rigid ring to manufacture the polishing head. Thereby, the polishing head can be manufactured which can suppress a variation in the polishing stock removal uniformity that is generated between material types of rubber film or material lots of the rubber film and which can secure stably good flatness, in polishing of the workpiece. 
     Moreover, the polishing apparatus according to the present invention has at least the polishing pad attached onto the turn table, the polishing agent supply mechanism for supplying the polishing agent onto the polishing pad, and the polishing head manufactured by the above-described method for manufacturing according to the present invention, the polishing pad has a young&#39;s modulus of 3.5 MPa or less, and the workpiece is polished by bringing a surface of the workpiece into sliding contact with the polishing pad attached onto the turn table. Thereby, a variation in the polishing stock removal uniformity that is generated between material types of rubber film or material lots of the rubber film can be suppressed, and the workpiece can be polished with stably good flatness secured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing an example of the polishing apparatus according to the present invention; 
         FIG. 2  are graphs showing an example of a stress-strain curve obtained in the method for manufacturing a polishing head according to the present invention, in which (A) shows the whole of the stress-strain curve, and (B) shows the stress-strain curve within a strain value of 5%; 
         FIG. 3  are graphs showing the stress-strain curve in the step of selecting the rubber film in Example 1, in which (A) shows the whole of the stress-strain curve, and (B) shows the stress-strain curve within a strain value of 5%; 
         FIG. 4  is a view showing the results of a value of an inclination obtained by a linear approximation of the stress-strain curve within a strain value of 5% in the step of selecting the rubber film in Example 1; 
         FIG. 5  is a view showing the results of the polishing pressure distribution in Example 1 and Comparative Example. 
         FIG. 6  is a view showing the results of the polishing stock removal uniformity in Example 1 and Comparative Example. 
         FIG. 7  is a view showing the results of the polishing stock removal uniformity in Example 2 and Comparative Example. 
         FIG. 8  is a schematic view showing an example of a conventional polishing head; and 
         FIG. 9  is a schematic view showing an example of a conventional single-side polishing apparatus. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, an embodiment of the present invention will be explained, but the present invention is not restricted thereto. 
     In the event that the workpiece is polished while holding the workpiece on the rubber film by using a conventional polishing head, there arises a problem such that a variation in polishing characteristics occurs due to a difference in the type of the rubber, which is a material of the rubber film of the polishing head, or due to a difference in the lot even though there is no difference in the type, and thereby good flatness cannot be obtained. 
     Moreover, such a variation in polishing characteristics is difficult to be predicted based on physical properties such as the hardness of a material lot of the rubber, which is a material of the rubber film, or the tensile strength of the rubber. 
     In view of this, the present inventors keenly conducted experiments and studies of the cause leading to the above-mentioned problem. 
     As a result, the present inventors found out the followings. 
     That is to say, good polishing stock removal uniformity can be obtained by using a rubber material having a small stress for a fine deformation of a strain of 5% or less, regardless of the type, the hardness and the tensile strength of the rubber used as a material of the rubber film. Specifically, when the rubber film having a small stress at the time of finely deforming is used as the rubber film of the polishing head to polish the workpiece, polishing can be performed with uniform pressing force over the whole surface of the workpiece. In addition, when the workpiece is polished by using the polishing head having a template, pressure distribution in an outer circumferential portion of the workpiece that is caused by a small difference between a height of a lower end face of the template and a height of a lower end face of the workpiece can be alleviated, and the polishing stock removal uniformity can be consequently good. 
     Accordingly, the present inventors further keenly conducted experiments and studies to quantify the stress at the time of finely deforming of rubber materials to be used as the rubber film, investigated polishing characteristics of these rubber materials, optimized, and brought the present invention to completion. 
       FIG. 1  shows an example of the polishing head and the polishing apparatus according to the present invention. 
     As shown in  FIG. 1 , the polishing apparatus  1  has the polishing head  2  and the turn table  8 . The turn table  8  is of disk shape, and the polishing pad  9  for polishing the workpiece W is attached onto its upper face. A driving shaft  11  is vertically connected to an lower portion of the turn table  8 . The turn table  8  is configured to be rotated by a turn table rotating motor (not shown) connected to an lower portion of the driving shaft  11 . 
     The polishing head  2  is provided above the turn table  8 . The polishing head  2  has the annular rigid ring  4 , the rubber film  3  that is bonded to the rigid ring  4  with a uniform tension and that has a flat lower face, the mid plate  5  joined to the rigid ring  4 , for example, with bolts. A sealed space  6  is formed by the rigid ring  4 , the rubber film  3  and the mid plate  5 . 
     The polishing head  2  is rotatable about its axis. 
     Here, a material of the rigid ring  4  is not restricted in particular. For example, it can be a rigid material such as SUS (stainless steel). A material and a shape of the mid plate is also not restricted in particular as long as the space  6  can be formed. 
     Moreover, a thickness of the rubber film  3  is not restricted in particular. For example, it may be approximately 1 mm. 
     The rubber film used in the polishing head of the polishing apparatus according to the present invention has the following characteristics. 
       FIG. 2(A)  shows an example of a stress-strain curve of the rubber film, the stress-strain curve which is obtained as a result of performing a tensile test according to JIS K6251.  FIG. 2(B)  is an enlarged graph of a part within a strain value of 5% in the stress-strain curve shown in  FIG. 2(A) . Here, a straight line obtained by performing the linear approximation of the stress-strain curve within a strain value of 5% using the least squares method, as shown in  FIG. 2(B) , is expressed as stress=a×strain+b. 
     In the present invention, the rubber film is used in which a value of the inclination a of this straight line is 10 MPa or less. 
     As shown in  FIG. 1 , the polishing apparatus has a polishing slurry supply means  10  for supplying a polishing slurry to an upper part of the turn table  8 . 
     Moreover, an annular template  14  for holding an edge portion of the workpiece W can be arranged in the periphery of the lower face portion of the rubber film  3  to prevent the workpiece W from coming off during polishing. In this case, the template  14  can be arranged so as to be concentric with the rigid ring  4  and to project downward along an outer circumferential portion of the lower face portion of the rubber film  3 . 
     Here, the height of the lower end face of the template  14  can be the same as the height of the lower end face of the held workpiece W or can be such a height that the lower end face of the template  14  slightly project downward, for example, by approximately 10 μm from the height of the lower end face of the workpiece W. 
     The pressure distribution in the outer circumferential portion of the workpiece can be alleviated by arranging the template  14  as described above, thereby excessive polishing of the outer circumferential portion of the workpiece can be prevented, and the polishing stock removal uniformity of the workpiece can be improved. 
     Moreover, the template  14  can be configured such that its outer diameter is larger than at least an inner diameter of the rigid ring  4  and its inner diameter is smaller than the inner diameter of the rigid ring  4 . 
     By this configuration, the polishing can be performed with more uniform pressing force applied to the whole surface of the workpiece. 
     Here, it is preferable that a material of the template  14  is softer than the workpiece W so as not to contaminate the workpiece W and to give a scratch or an impression and is a high abrasion resistance material that is hard to wear due to sliding contact with the polishing pad  9  during the polishing. 
     Moreover, as shown in  FIG. 1 , the polishing apparatus  1  has the pressure adjustment mechanism  7  for changing the pressure of the space  6  of the polishing head  2 . 
     A through-hole  12  for pressure adjustment communicating with the pressure adjustment mechanism  7  is provided at the center of the mid plate  5 , and the pressure of the space  6  can be adjusted, for example, by supplying a pressurized fluid by the pressure adjustment mechanism  7 . 
     The polishing apparatus also has a means for pressing the mid plate  5  toward the polishing pad  9  (not shown). 
     In this case, a backing pad  13  can be attached to be provided on the lower face of the rubber film  3 . The backing pad  13  is made to contain water so as to attach and to hold the workpiece W on a workpiece holding face of the rubber film  3 . Here, the backing pad  13  can be made of, for example, foamed polyurethane. By providing the above-described backing pad  13  and having it contain water, the workpiece W can be surely held by surface tension of the water contained in the backing pad  13 . 
     A soft polishing pad having a young&#39;s modulus of 3.5 MPa or less is used as the polishing pad  9  attached onto an upper face of the turn table  8 . When the polishing head  2  according to the present invention is used in combination with the soft polishing pad  9  having a young&#39;s modulus of 3.5 MPa or less, contact pressure distribution between the workpiece W and the polishing pad  9  can be more surely alleviated, and the polishing stock removal uniformity of the workpiece W can be more surely improved. 
     It is to be noted that an embodiment of attaching the template  14  directly onto the rubber film  3  is shown in  FIG. 1 , but the present invention does not exclude a case of attaching the template  14  onto the rubber film  3  through the backing pad  13  and the like. 
     With the polishing apparatus  1  configured as described above, the mid plate  5  is pressed toward the polishing pad  9  attached onto the turn table  8  by the means for pressing the mid plate, not shown, and the surface of the workpiece W is polished by bringing it into sliding contact with the polishing pad  9  while the polishing agent is supplied through the polishing agent supply mechanism  10 . Here, the means for pressing the mid plate is preferably able to press the mid plate  5  over the whole surface with a uniform pressure, for example, by using an air cylinder. 
     In this way, when the workpiece W is polished by using the polishing apparatus  1  according to the present invention, the variation in the polishing stock removal uniformity that is generated between the material types of the rubber film  3  or the material lots of the rubber film  3  can be suppressed, and the workpiece W can be polished with stably good flatness secured. 
     In addition, the pressure distribution in the outer circumferential portion of the workpiece W that is caused by a small difference between the height of the lower end face of the template  14  and the height of the lower end face of the workpiece W can be alleviated. The polishing can be thereby performed while the pressing force applied to the workpiece W is kept uniform over the whole surface, even though there are variations in thicknesses of the workpiece W and the template  14  to a certain degree. As a result, the workpiece W can be polished with good polishing stock removal uniformity. 
     In this case, the workpiece to be polished can be a silicon single crystal wafer having a diameter of 300 mm or more. 
     As described above, even when the workpiece to be polished is the silicon single crystal wafer having a large diameter such as a diameter of 300 mm or more, the polishing can be performed with more uniform pressing force over the whole surface of the workpiece by holding it with the polishing head according to the present invention to polish, and thereby good flatness can be secured. 
     Next, the method for manufacturing a polishing head according to the present invention will be explained. 
     For example as shown in  FIG. 1 , the polishing head manufactured by the method for manufacturing according to the present invention is configured to have at least: the annular rigid ring  4 ; the rubber film  3  bonded to the rigid ring  4  with a uniform tension; the mid plate  5  that is joined to the rigid ring  4  and that forms the space  6  together with the rubber film  3  and the rigid ring  4 ; and the pressure adjustment mechanism  7  for changing the pressure of the space  6 . 
     The method for manufacturing a polishing head according to the present invention comprises at least the step of selecting the rubber film  3  as follows. 
     First, the stress-strain curve as shown in  FIG. 2(A)  is obtained by performing a tensile test on the rubber film  3  according to JIS K6251. Then, from this stress-strain curve, the stress-strain curve within a strain value of 5% is extracted as shown in  FIG. 2(B) , and a straight line is obtained by performing the linear approximation of the extracted curve using the least squares method. This straight line is expressed as stress=a×strain+b. 
     Thereafter, the rubber film  3  having a value of 10 MPa or less of the inclination a of the straight line is selected. 
     With the rubber film  3  selected as described above, it is bonded to the rigid ring  4  with a uniform tension. 
     Here, the rubber film  3  can be selected at the stage of a rubber material before forming the rubber film  3  itself, as long as it is selected before bonding rubber film  3  to the rigid ring  4 . 
     As described above, when the polishing head  2  is manufactured by performing a tensile test on the rubber film  3  according to JIS K6251 before bonding the rubber film  3  to the rigid ring  4 , selecting the rubber film having a value of 10 MPa or less of the inclination obtained by the linear approximation of the stress-strain curve within a strain value of 5%, and bonding the selected rubber film  3  having a value of 10 MPa or less of the inclination to the rigid ring  4 , the polishing head  2  can be manufactured which can suppress the variation in the polishing stock removal uniformity that is generated between the material types of the rubber film  3  or the material lots of the rubber film  3  and which can secure stably good flatness, in polishing of the workpiece W. 
     Next, the mid plate  5  is joined to the rigid ring  4 , and the space  6  is formed by the rigid ring  4 , the mid plate  5  and the rubber film  3  bonded to the rigid ring  4 . The pressure adjustment mechanism  7  is arranged above the mid plate  5 . These steps can be performed as with a conventional method. 
     Here, the annular template  14  for holding the edge portion of the workpiece W can be arranged in the periphery of the lower face portion of the rubber film  3  to prevent the workpiece W from coming off during polishing. In this case, the template  14  can be arranged so as to be concentric with the rigid ring  4  and to project downward along the outer circumferential portion of the lower face portion of the rubber film  3 . 
     Here, the height of the lower end face of the template  14  can be the same as the height of the lower end face of the workpiece W when the workpiece is held or can be such a height that the lower end face of the template  14  slightly project downward, for example, by approximately 10 μm from the height of the lower end face of the workpiece W. 
     The polishing head can alleviate excessive pressure distribution applied to the outer circumferential portion of the workpiece by arranging the template  14  as described above, and thereby can improve the polishing stock removal uniformity of the workpiece. 
     Moreover, the template  14  can be configured such that its outer diameter is larger than at least an inner diameter of the rigid ring  4  and its inner diameter is smaller than the inner diameter of the rigid ring  4 . 
     By this configuration, the polishing head can polish the workpiece with more uniform pressing force applied to the whole surface of the workpiece. 
     Here, it is preferable that the material of the template  14  is softer than the workpiece W so as not to contaminate the workpiece W and to give a scratch or an impression and is a high abrasion resistance material that is hard to wear due to sliding contact with the polishing pad  9  during the polishing. 
     In this case, the backing pad  13 , for example, made of foamed polyurethane can be attached to be provided on the lower face of the rubber film  3 . By providing the above-described backing pad  13  and having it contain water, the polishing head can surely hold the workpiece W by surface tension of the water contained in the backing pad  13 . 
     In this case, the workpiece to be held can be the silicon single crystal wafer having a diameter of 300 mm or more. 
     In this manner, even when the workpiece to be held is the silicon single crystal wafer having a large diameter such as a diameter of 300 mm or more, the polishing can be performed with more uniform pressing force over the whole surface of the workpiece and thereby good flatness can be secured, according to the present invention. 
     Hereinafter, the present invention will be explained in more detail based on Example, but the present invention is not restricted thereto. 
     Example 1 
     The polishing heads as shown in  FIG. 1  were manufactured by the method for manufacturing according to the present invention. Silicon single crystal wafers were polished with the polishing apparatus having each manufactured polishing head, and the polishing stock removal uniformity of each polished workpiece was evaluated. 
     First, in order to select the rubber film, there were prepared two types of lot A and lot B of ethylene-propylene-diene rubber (EPDM) having a hardness standard of 80°, which is a rubber hardness according to JIS K6253, (hereinafter, referred to as EPDM 80° A and EPDM 80° B) and three types of silicone rubber having a hardness standard of 70°, 80°, and 90° (hereinafter, referred to as silicone 70°, silicone 80°, and silicone 90°), as a rubber material of the rubber film. Tensile tests according to JIS K6251 were performed on these 5 types of rubber materials, and the stress-strain curves were measured each. 
     Table 1 shows the results of the tests.  FIG. 3(A)  shows the obtained stress-strain curves. As shown in  FIG. 3(B) , the linear approximation was performed, as stress=a×strain+b, using each stress-strain curve within a strain value of 5% in  FIG. 3(A) , and a value of each inclination a was calculated. 
       FIG. 4  shows the results of the values. As shown in  FIG. 4 , it was revealed that even though the same type of rubber material was used, the values of the inclination a were different due to the difference of the lot, and in the rubber material of EPDM 80° B, silicone 70°, and silicone 80°, the values of the inclination a were 10 MPa or less. 
     After the rubber materials of the rubber films were selected as described above, three polishing heads were manufactured using EPDM 80° B, silicone 70°, and silicone 80°, which had a value of 10 MPa or less of the inclination a, by the following manner. 
     First, upper parts of the annular rigid rings that were made of SUS and had an outer diameter of 360 mm were covered by the mid plates respectively. The rubber films having a thickness of 1 mm were bonded to the outer circumferences of the rigid rings with a uniform tension, using three types of rubber materials (EPDM 80° B, silicone 70°, and silicone 80°), which had a value of 10 MPa or less of the inclination a. 
     The backing pad was attached to be provided on the workpiece holding face of the rubber film of each polishing head. A commercial template assembly, in which a template made of glass epoxy laminated sheet having an outer diameter of 355 mm and an inner diameter of 302 mm was bonded, was adhered to a surface of the backing pad by double-stick tape. In case of the rubber film formed with silicone rubber, its surface was subjected to coating processing with a thin polyurethane film having a thickness of several μm for the purpose of improving capability for adhering to the double-stick tape. Moreover, the commercial template assembly having a thickness of 787 μm was used as the template so that the position of the lower face of the template slightly project downward from the position of the lower face of the workpiece. 
     With the polishing apparatus having the polishing head manufactured by the method for manufacturing a polishing head according to the present invention as shown in  FIG. 1 , silicon single crystal wafers having a diameter of 300 mm and a thickness of 775 μm, as a workpiece W, were polished. It is to be noted that both surfaces of the used silicon single crystal wafers were subjected to the first polishing in advance, and its edge portions were also subjected to polishing. The turn table having a diameter of 800 mm was used. The polishing pad of the type of containing urethane in a nonwoven fabric was used, and its young&#39;s modulus was 2.2 MPa, which is 3.5 MPa or less. 
     During the polishing, an alkaline solution containing colloidal silica was used as the polishing agent, and the polishing head and the turn table were rotated at 31, and 29 rpm respectively. A polishing load (pressing force) of the workpiece W was set as 15 KPa. The polishing time was 3 minutes. 
     The polishing stock removal uniformity and the polishing pressure distribution during polishing of the workpiece polished as described above were evaluated. It is to be noted that the polishing stock removal uniformity is obtained by measuring the thickness of the workpiece before and after polishing in a region excluding an outermost circumferential portion 2 mm width, as a flatness quality area, with a flatness measurement instrument in a diameter direction of the wafer and by taking a difference in the thickness. It is represented by a formula of polishing-stock-removal-uniformity (%)=(maximum polishing-stock-removal in a diameter direction−minimum polishing-stock-removal in a diameter direction)/average polishing-stock-removal in a diameter direction. 
     The polishing pressure distribution of the wafer in the range of 120 to 148 mm from its center in a diameter direction is shown in  FIG. 5 , the wafer which was polished with the polishing head using EPDM 80° B as the rubber material of the rubber film. The pressure distribution was measured by converting the polishing stock removal at each position into polishing-stock-removal/average polishing-stock-removal×polishing-load (15 MPa) at each position. 
     As shown in  FIG. 5 , it was revealed that a decrease in pressure of the outer circumferential portion of the workpiece W was more suppressed and uniformity of the polishing pressure distribution was better in comparison with the polishing head using the rubber film made of the rubber material having a value of over 10 MPa of an inclination a, which is used in later-explained Comparative Example. 
       FIG. 6  shows the results of the polishing stock removal uniformity. 
     As shown in  FIG. 6 , it was revealed that when polishing was performed by the polishing apparatus having the polishing head using the rubber film made of EPDM 80° B, silicone 70°, and silicone 80° respectively, the polishing stock removal uniformity was improved in comparison with the result of the later-explained Comparative Example, and that it was a good result of 10% or less. 
     Accordingly, it can be confirmed that the variation in the polishing stock removal uniformity that is generated between material types of rubber film or material lots of the rubber film can be suppressed and the stably good flatness can be secured during polishing of the workpiece, by using the polishing apparatus according to the present invention that has the polishing head manufactured by the method for manufactured according to the present invention. 
     Example 2 
     A polishing head using EPDM 80° B as the rubber film material was manufactured as with Example 1. A silicon single crystal wafer was polished with the polishing apparatus having the manufactured polishing head and the polishing stock removal uniformity was evaluated as with Example 1 except for using the polishing pad having a young&#39;s modulus of 3.2 MPa. 
       FIG. 7  shows the result of the polishing stock removal uniformity. As shown in  FIG. 7 , it was revealed that the polishing stock removal uniformity was improved in comparison with the result of the later-explained Comparative Example, and that it was a good result of 10% or less. 
     Moreover, as shown in  FIG. 7 , it was revealed that when the young&#39;s modulus of the polishing pad was 3.5 MPa or less, the polishing stock removal uniformity was a good result of 10% or less. 
     Comparative Example 
     Silicon single crystal wafers were polished in the same condition as Example 1 except that the polishing heads were manufactured by using the rubber material having a value of over 10 MPa of the inclination a obtained, in Example 1, by the linear approximation was performed, as stress=a×strain+b, using the stress-strain curve within a strain value of 5%, that is, except for using EPDM 80° A and silicone 90° as the rubber film material, and the polishing stock removal uniformity and the polishing pressure distribution during the polishing of each polished wafer were evaluated. 
       FIG. 5  shows the results of the polishing pressure distribution. As shown in  FIG. 5 , it was revealed that the decrease in pressure of the outer circumferential portion of the workpiece was more remarkable and uniformity of the polishing pressure distribution was decreased in comparison with the result of Example 1 in which the rubber film having a value of 10 MPa or less of the inclination a was selected. 
       FIG. 6  shows the results of the polishing stock removal uniformity. As shown in  FIG. 6 , it was revealed that the polishing stock removal uniformity was worse than the results of Example 1. 
     Accordingly, it was confirmed that when the workpiece was polished with a conventional polishing apparatus, the variation in the polishing stock removal uniformity occurred and stable flatness of the workpiece was not obtained due to the differences of the rubber film material and of the manufacturing lot even with regard to the same materials. 
     Next, a silicon single crystal wafer was polished in the same condition as Example 2 except for using the polishing pad having a young&#39;s modulus of 4.5 MPa, and the polishing stock removal uniformity of the polished wafer was evaluated. 
       FIG. 7  shows the result of the polishing stock removal uniformity. As shown in  FIG. 7 , it was revealed that the polishing stock removal uniformity was worse than the result of Example 2. That is to say, the polishing head according to the present invention that selects the rubber film having a value of 10 MPa or less of the inclination obtained by the linear approximation of the stress-strain curve within a strain value of 5% enables the polishing stock removal uniformity to be improved, and when the polishing apparatus is provided with the polishing head to polish, the polishing pad attached onto the turn table needs to have a young&#39;s modulus of 3.5 MPa or less. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 TENSILE 
                 TENSILE 
               
               
                   
                 RUBBER 
                 STRENGTH 
                 ELONGATION 
               
               
                 RUBBER MATERIAL 
                 HARDNESS 
                 (MPa) 
                 (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 EPDM RUBBER80°A 
                 75 
                 10.78 
                 219 
               
               
                 EPDM RUBBER80°B 
                 77 
                 12.57 
                 405 
               
               
                 SILICONE RUBBER70° 
                 73 
                 7.19 
                 127 
               
               
                 SILICONE RUBBER80° 
                 75 
                 7.32 
                 148 
               
               
                 SILICONE RUBBER90° 
                 86 
                 9.53 
                 288 
               
               
                   
               
            
           
         
       
     
     It is to be noted that the present invention is not restricted to the foregoing embodiment. The embodiment is just an exemplification, and any examples that have substantially the same feature and demonstrate the same functions and effects as those in the technical concept described in claims of the present invention are included in the technical scope of the present invention. 
     For example, the polishing head manufactured by the method for manufacturing according to the present invention is not restricted to an embodiment shown in  FIG. 1 , and for example, a shape and the like of the mid plate may be appropriately designed. 
     Moreover, the structure of the polishing apparatus is also not restricted to the embodiment shown in  FIG. 1 , and for example, the polishing apparatus can be provided with a plurality of the polishing head manufactured by the method for manufacturing according to the present invention.