Polishing pad, method and apparatus for treating polishing pad and polishing method

A polishing pad for pressing a work to be polished to a surface thereof, includes a structure obtained by being compressed under a temperature which is higher than an operating temperature for polishing the work and/or under a pressure which is equal or higher than an operating pressure for polishing the work. A method for polishing a includes the step of polishing the work by using the polishing pad.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a polishing pad, a method and an apparatus
 for treating a polishing pad, and a polishing method; particularly, to a
 polishing pad and a polishing method used for polishing a semiconductor
 wafer (hereinafter, it may be simply referred to as "wafer").
 2. Description of Related Art
 In a process for fabricating a semiconductor integrated circuit, with the
 progress of larger scale integration of recent semiconductor devices, the
 requirement for the flatness of a wafer which is a material for the
 devices becomes stricter. Therefore, it is required to improve the
 processing accuracy of a polishing process which influences the flatness
 of the wafer.
 In the polishing process, a wafer which was etched by mixed acid which is
 mixture of hydrofluoric acid, acetic acid and nitric acid or by an aqueous
 solution of sodium hydroxide, potassium hydroxide or the like, that is, an
 etched wafer, is a work to be polished. Because the general flatness of
 the etched wafer is not enough to meet the requirement and the surface
 roughness thereof is large, three staged polishing are often carried out
 in the polishing process. The first polishing step is for mirror polishing
 the etched wafer by removing the waviness thereof, so that the amount of
 removal, that is, the entire polishing stock removal is large. The third
 polishing step is for improving the minute surface roughness of the wafer,
 so that the entire polishing stock removal is small. The second polishing
 step which is an intermediate step between the first and the third steps
 has an intermediate purpose therebetween. Therefore, in each polishing
 step, a polishing is carried out by changing the polishing condition such
 as the type of the polishing pad, the type and supplying amount of the
 polishing slurry, the relative rotation speed between the wafer and the
 polishing pad, and the contacting pressure of the wafer which is contacted
 with the polishing pad, that is, the polishing pressure, according to the
 difference of the purpose of the each polishing step.
 Among various types of polishing conditions, the one with respect to the
 polishing pad has the greatest influence on the flatness of the wafer,
 especially to the waviness which have a cycle or wavelength of several
 millimeters and to the ripples which have a cycle or wavelength of scores
 of microns. That is, the polishing pad has the greatest influence in the
 first polishing step which gives the largest entire polishing stock
 removal.
 The wafer is polished by a polishing machine, for example, the one which is
 shown in FIG. 4. That is, a wafer 1 is held to adhere on the lower surface
 of a holding plate 8 which is attached to a top ring 2 of a polishing
 head. A rotational motion is given to the wafer 1 by a head driving motor
 5, while the wafer 1 is pressed against a polishing pad 9 which is adhered
 on the upper surface of a turn table 4 by an air cylinder 3. On the other
 hand, to the turn table 4, the rotational motion is also given by a turn
 table driving motor 6. Accordingly, a relative motion occurs between the
 wafer 1 and the polishing pad 9. The wafer 1 is polished by supplying
 polishing slurry 7 to a surface of the polishing pad 9 while giving the
 relative motion between the wafer 1 and the polishing pad 9. For the
 polishing slurry in the case, abrasive grains of colloidal silica
 dispersed in alkali aqueous solution is used, so that the wafer 1 is
 polished by the so-called mechano-chemical function which compounds a
 mechanical function and a chemical function.
 In the mechano-chemical function, the speed of removing the wafer material,
 for example, silicon is proportional to the force that the wafer 1
 vertically presses the polishing pad 9, that is, the polishing pressure.
 As a result, when the polishing pressure is non-uniformly distributed
 within the wafer 1, the removal amount of the material is not uniform
 within the wafer 1, so that the wafer 1 is not flatly polished. Therefore,
 in order to obtain the flat wafer 1, it is important to make the
 distribution of the polishing pressure uniform within the wafer 1.
 However, the polishing pad 9 has a visco-elastic property to cause creep
 deformation. That is, when the constant load is continuously applied to
 the polishing pad 9, the polishing pad 9 is pressed to be thinned. The
 amount of the reduced thickness, that is, the amount of displacement is
 increased rapidly right after applying the load and is increased slowly
 thereafter. Then, the thickness of the polishing pad 9 does not return to
 the thickness before the load was applied even if the load is removed, so
 that the displacement will remain forever. Accordingly, under the constant
 load, the amount of the displacement of the polishing pad 9 greatly
 depends on the time during which the polishing pad 9 is subjected to the
 load.
 FIG. 5 shows the relation between the distance from the center 60 of the
 turn table 4 and the applied time of the polishing load which is applied
 to the polishing pad 9 in an automatic single wafer polishing machine
 which polishes the wafer 1. According to FIG. 5, it is understood that the
 applied time of the polishing load is not uniform according to the
 position on the polishing pad 9. The problem of non-uniformity of the
 applied time of the polishing load also occurs when the position of the
 wafer from the center of the turn table varies in the automatic single
 wafer polishing machine and in the batch processing polishing machine
 wherein many wafers are held to adhere on a holding plate.
 When the applied time of the polishing load is not uniform according to the
 position on the polishing pad 9, the amount of displacement of the
 polishing pad 9 depends on the load application time as above-described,
 so that the amount of displacement of the polishing pad 9 which is adhered
 to the turn table 4 is not uniform. The thickness of a polishing pad which
 was used actually has been measured. As a result, a difference in the
 amount of displacement of about 50 .mu.m is observed between a portion to
 which the load was most applied and a portion to which no load was
 applied.
 For the purpose of removing harmful influence on the flatness of the wafer,
 especially waviness and ripple, and avoiding the influence of the
 visco-elastic property of the polishing pad, especially, the creep
 deformation which rapidly progresses right after the polishing pad is
 used, a treatment of the polishing pad, of which a method is similar to
 the polishing method of the wafer is carried out.
 For the treatment, a polishing machine which is used for polishing the
 wafer, that is, for manufacturing the wafer is used. The time when the
 treatment is carried out is right after that a new polishing pad is
 adhered to the turn table. For tools which are used instead of the wafer
 are a ceramic disc on which grooves are formed, and a quartz or a silicon
 disc which is adhered on the surface of the holding plate which is used
 for actual polishing of the wafer.
 However, if the polishing machine which is used for manufacturing the wafer
 is also used for the treatment of the polishing pad, there is a problem
 that the manufacturing of the wafer is required to be stopped during the
 treatment of the polishing pad. For the treatment of the polishing pad, it
 is required to suitably select the temperature of the polishing pad, the
 load applied to the polishing pad or the like. However, there is a problem
 that it is not possible to suitably select the operating conditions except
 the polishing time in the polishing machine. Further, there is a problem
 that it is always required to prepare the tools for the treatment of the
 polishing pad.
 SUMMARY OF THE INVENTION
 The present invention was developed in view of the above-described
 problems. Therefore, an object of the present invention is to provide a
 polishing pad, a method and an apparatus for treating a polishing pad;
 wherein the polishing pad has small creep deformation. high flatness and
 small surf ace roughness; and it is not required to stop a polishing
 operation during a treatment of the polishing pad. Another object of the
 present invention is to provide a polishing method from which a wafer
 having high flatness can be obtained.
 In order to accomplish the above-described object, in one aspect of the
 present invention, a polishing pad for pressing a work to be polished to a
 surface thereof, comprises a structure obtained by being compressed under
 a temperature which is higher than an operating temperature for polishing
 the work and/or under a pressure which is equal or higher than an
 operating pressure for polishing the work. The polishing pad can cause
 little creep deformation during the polishing of the work, because the
 polishing pad is compressed under the above-described condition. The work
 can be a wafer or the like. Therefore, it is possible to obtain a high
 flatness wafer by using the polishing pad.
 The operating temperature of the polishing pad is 15-50.degree. C. and the
 operating pressure thereof is 50-1000 g/cm.sup.2, for example. The
 temperature can be freely selected relatively, however, it may be
 preferable that the temperature is within such a range that the
 deformation progresses quickly and the chemical property of the polishing
 pad does not deteriorate. The polishing pad can have a foamed structure.
 The foamed structure can be continuous or independent. It may be
 preferable that the pressure is within such a range that a basic structure
 of the polishing pad is not destroyed, that is, the foamed structure is
 not crushed to disappear.
 A plurality of grooves can be formed in a surface of the polishing pad. The
 forms of the grooves can be various as far as polishing slurry can be
 spread over the surface of the polishing pad and easily discharged, during
 the polishing of the work.
 In accordance with another aspect of the present invention, a method for
 treating a polishing pad to be adhered to a turn table, for pressing a
 work to be polished to a surface of the polishing pad, comprises the step
 of: compressing the polishing pad under a temperature which is higher than
 an operating temperature for polishing the work and/or under a pressure
 which is equal to or higher than an operating pressure for polishing the
 work.
 According to the method of the present invention, because the polishing pad
 is compressed under the temperature which is higher than the operating
 temperature for polishing the work and/or under the pressure which is
 equal to or higher than the operating pressure for polishing the work, the
 visco-elastic property of the polishing pad is changed. Therefore, because
 a deformation which rapidly progresses right after applying a load is
 progressed as large as possible, it is possible to obtain a polishing pad
 having small creep deformation, high flatness, and small surface
 roughness.
 The compressing step can be carried out before adhering the polishing pad
 on the turn table and the polishing pad can have a foamed structure. The
 temperature, the pressure, and the structure of the polishing pad are the
 same condition as that above-described. Because the compressing step can
 be carried out before adhering the polishing pad on the turn table, the
 polishing pad can cause little creep deformation during a polishing
 process.
 The method can further comprise the steps of: holding the polishing pad
 between two plate-like members to compress the polishing pad and applying
 a fluid pressure to an outer surface of at least one of the plate-like
 members. It may be preferable that the plate-like members are thin and
 easy to bend, such as thin plate of stainless steel to uniformly compress
 the polishing pad. However it is not limited to this, so that another type
 of the plate-like member can be used. According to the method, because the
 fluid pressure can be used, it can be possible to apply the uniform
 pressure to the polishing pad and to obtain the polishing pad having high
 flatness and small surface roughness.
 The method can further comprise the steps of: disposing a hermetic seal
 member between the two plate-like members to have a hermetic space in
 which the polishing pad is disposed; and reducing a pressure of the space
 to draw the plate-like members closer to each other. For the hermetic seal
 member, it can be used various materials through which the gas can not
 pass.
 The method can further comprise the steps of: forming irregularities in an
 inner surface of one of the plate-like members; and transferring the
 irregularities to a surface of the polishing pad. The irregularities
 formed on the plate-like member can be projections to form grooves in the
 surface of the polishing pad. According to the method of the present
 invention, because the grooves can be formed in the surface of the
 polishing pad, it can be possible to obtain the polishing pad which can
 spread the polishing slurry over the surface of the polishing pad and
 easily discharge the polishing slurry.
 In accordance with another aspect of the present invention, an apparatus
 for treating a polishing pad for pressing a work to be polished to a
 surface of the polishing pad, the apparatus comprises: two plate-like
 members to hold the polishing pad between them to compress the polishing
 pad. The two plate-like members can compress the polishing pad by being
 applied a fluid pressure to an outer surface of at least one of the
 plate-like members.
 The polishing pad and the plate-like members can be the same type as that
 above-described.
 The apparatus can further comprise a hermetic seal member between the two
 plate-like members to have a space among the two plate-like members and
 the hermetic seal member, wherein the two plate-like members compress the
 polishing pad disposed in the space by drawing the plate-like members to
 each other by reducing a pressure of the space.
 According to the apparatus of the present invention, because the two
 plate-like members can compress the polishing pad by being applied the
 fluid pressure or reducing the pressure of the space where the polishing
 pad is disposed, the polishing pad is applied an uniform pressure.
 Therefore, it can be possible to obtain the polishing pad having small
 creep deformation, high flatness, and small surface roughness.
 With the apparatus, one of the plate-like members can comprise a structure
 having irregularities formed in an inner surface thereof and the
 irregularities can be transferred to a surface of the polishing pad.
 According to the apparatus of the present invention, it can be possible to
 obtain the polishing pad which can spread the polishing slurry over the
 surface of the polishing pad and easily discharge the polishing slurry,
 during the polishing of the work.
 With the apparatus, the two plate-like members and the hermetic seal
 member, for compressing the polishing pad can be contained in a
 thermostatic chamber.
 In accordance with a further aspect of the present invention, a method for
 polishing a work, comprises the step of polishing the work by using a
 polishing pad comprising a structure obtained by being compressed under a
 temperature which is higher than an operating temperature for polishing
 the work and/or under a pressure which is equal to or higher than an
 operating pressure for polishing the work. The polishing pad can cause
 little creep deformation during the work is polished.
 According to the method of the present invention, because the work, for
 example, a wafer is polished by the polishing pad having small creep
 deformation, high flatness, and small surface roughness, it is possible to
 obtain the wafer having high flatness.
 In the method, the polishing pad can have a plurality of grooves in a
 surface thereof, which is a side contacting with the work during the
 polishing of the work. According to the method, because the polishing pad
 can have the grooves on a surface thereof, it can be possible to spread
 the polishing slurry over the surface of the polishing pad and easily
 discharge the polishing slurry, during polishing of the work.
 As above-described, according to the present invention, it is possible to
 obtain a polishing pad having small creep deformation, high flatness, and
 small surface roughness. Accordingly, it is possible to obtain a flat
 wafer. Further, the treatment is carried out by an apparatus which is
 different from a polishing machine, so that it is possible to rationalize
 operations without stopping manufacturing of a wafer.

PROFFERED EMBODIMENT OF THE INVENTION
 As above-described, the polishing pad has visco-elastic property, so that
 the deformation thereof rapidly progresses right after applying the load,
 then the deformation slowly progresses thereafter. The higher the
 temperature is and the larger the load is, the higher the speed of the
 deformation becomes. There is a difference in the visco-elastic property
 of the polishing pads between the one which was treated by being applied
 with the high temperature and the large load and the other which was
 treated by being not applied by them. That is, the amount of deformation
 of the former is smaller than that of the latter when the applied load is
 the same.
 The present invention uses this phenomenon. In the present invention, the
 polishing pad is left for hours under such environmental circumstances, a
 temperature and/or a pressure of which are higher than that which the
 polishing pad is subjected to during polishing of a work, that is, an
 operating temperature and a pressure of the polishing pad so that the
 visco-elastic property of the polishing pad may change. The polishing pad
 can also be left for hours under such environmental circumstances, a
 pressure of which is equal to the operating pressure; or a temperature of
 which is higher than the operating temperature and a pressure of which is
 equal to the operating pressure. That is, in the present invention, the
 deformation which rapidly progresses right after applying the load is
 progressed as large as possible.
 The operating temperature of the polishing pad is 15-50.degree. C. and the
 operating pressure thereof is 50-1000 g/cm.sup.2, for example. In the
 present invention, the temperature under which the polishing pad is left
 can be freely selected relatively, however, it may be preferable that the
 temperature is within such a range that the deformation progresses quickly
 and the chemical property of the polishing pad does not deteriorate. It
 may be preferable that the pressure and the time of the treatment in the
 present invention are within such a range that a basic structure of the
 polishing pad is not destroyed. That is, these are within such a range
 that the foamed structure is not crushed to disappear, when the polishing
 pad is made of an unwoven cloth having a continuous foamed structure or an
 independent foamed structure. It may be preferable that the treatment for
 compressing the polishing pad is performed before the polishing pad is
 adhered to the turn table of the polishing machine.
 Next, the polishing pad to be treated is explained as follows. For the
 polishing pad which is used in the first and the second polishing steps, a
 polyester unwoven cloth impregnated with polyurethane, having the
 continuous foamed structure, that is, the spaces between fibers are
 continuous, which is called a velour type, is used. For the polishing pad
 which is used in the third polishing step which is for the final
 polishing, a polyester unwoven cloth, on a surface of which the
 independent foamed layers of polyurethane are layered, which is called a
 suede type, is used.
 It is preferable to apply the fluid pressure for compressing the polishing
 pad with the apparatus for treating the polishing pad. This is for
 applying the uniform pressure to the polishing pad. It is preferable that
 the plate-like member for compressing the polishing pad is thin and easy
 to bend so that the polishing pad may be uniformly compressed. Further,
 when the polishing pads are layered through the plate-like members, it is
 possible to treat a plurality of polishing pads at the same time.
 For a polishing pad, the polyester unwoven cloth is impregnated with the
 polyurethane to have the continuous foamed structure, thereafter, sliced
 to a predefined thickness, and ground to have a smooth surface and a
 thickness of 1270 .mu.m, which is marketed by Rodel, Inc. by name of
 "SUBA-600" is used. This polishing pad is applied to the first polishing
 step for polishing a silicon wafer.
 For a treating apparatus for the polishing pad, the apparatus shown in FIG.
 1 is used according to the first embodiment of the present invention. The
 treating apparatus 10 for the polishing pad comprises a machine casing 11,
 two compressing plates 12 and 13 in the machine casing 11, which are thin
 plates of stainless steel having a thickness of 0.3 mm, and a soft sealing
 material 14 as a hermetic seal member between peripheral portions of the
 compressing plates 12 and 13 to make a space 15. An opening portion 16
 which leads to the space 15 is formed through the compressing plate 12 and
 connected to an oil rotary vacuum pump 18 through a hose 17. In the space
 15, the polishing pad 9 is disposed. In the space 15, a spacer 20 which is
 slightly thinner than the polishing pad 9 is disposed around the polishing
 pad 9. With the treating apparatus 10 for the polishing pad, the machine
 casing 11, the compressing plates 12 and 13 or the like, which constitute
 a compressing means are contained in a thermostatic chamber 21.
 The treatment for the polishing pad by applying the treating apparatus 10
 for the polishing pad was carried out through the following procedures.
 The polishing pad 9 was disposed in the space 15 of the treating apparatus
 10 for the polishing pad. Thereafter, the compressing means was contained
 in the thermostatic chamber 21 which had been heated to 120.degree. C. in
 advance, then, the oil rotary vacuum pump 18 was driven. The heat capacity
 of the compressing means of the treating apparatus 10 for polishing pad is
 small and the space volume thereof is also small, so that after several
 minutes, the temperature and the pressure of the compressing means reached
 120.degree. C. and -1000 g/cm.sup.2, respectively. Then, it was left for 4
 hours with this condition. Therefore, both surfaces of the polishing pad 9
 are applied the load of 1000 g/cm.sup.2 at 120.degree. C. for 4 hours with
 contacting to the compressing plates 12 and 13.
 It was also carried out the treatment which will be described as follows.
 On a surface of the compressing plate, to which one surface of the
 polishing pad 9 is contacted, projecting portions having a height of 0.5
 mm and width of 1 mm were formed at intervals of 15 mm. Then, the same
 treatment for the polishing pad as above-described was carried out by
 using the compressing plate. The purposes of forming the projecting
 portions are to form grooves on the surface of the polishing pad 9, to
 spread the polishing slurry entirely over the surface of the polishing pad
 by the grooves and to easily discharge the polishing slurry, during the
 polishing process.
 The effect on the visco-elastic property of the polishing pad 9 by the
 present invention was evaluated through the following procedures.
 (1) Initial thickness of the polishing pad 9 was measured.
 (2) After the polishing pad 9 was treated by the treating apparatus 10 for
 the polishing pad, the thickness of the polishing pad 9 was measured.
 (3) A simulated load was applied to the polishing pad 9 for hours
 continuously. The simulated load corresponds to a load which is applied to
 the polishing pad 9 during the polishing of the wafer; the polishing pad 9
 is adhered to the turn table of the polishing machine. Concretely, a
 static load of 1000 g/cm.sup.2 was continuously applied to the polishing
 pad 9 for 15 hours. Then, the thickness of the polishing pad was measured
 right after removing the simulated load.
 (4) The polishing pad 9 was left at room temperature with being applied no
 load for 7.5 hours, then, the thickness of the polishing pad 9 was
 measured.
 In each process, the thickness of the polishing pad 9 was measured by a
 dial gauge after thirty seconds of applying a static load of 0, 200, 400,
 600, 800, and 1000 g/cm.sup.2, respectively.
 The result is shown in FIG. 3B. FIG. 3B shows the thickness 53 of the
 polishing pad 9 before the treatment, the thickness 54 thereof before the
 simulated load is applied, the thickness 55 thereof after the simulated
 load is applied, and the thickness 56 thereof after the polishing pad is
 left for 7.5 hours. The case that the static load was 1000 g/cm.sup.2 will
 be explained as follows. The polishing pad 9 having an initial thickness
 of 1270 .mu.m was compressed by 80 .mu.m which was a reduced amount of
 thickness by the process (2). The polishing pad 9 was further compressed
 by 8 .mu.m by the process (3). The thickness of the polishing pad 9 was
 recovered by 5 .mu.m by the process (4), then, the creep deformation of 3
 .mu.m which was the difference between them was remained.
 On the other hand, the result in the case that the process (2) was omitted
 is shown in FIG. 3A. FIG. 3A shows the thickness 50 of the polishing pad 9
 before the simulated load is applied, the thickness 51 thereof after the
 simulated load is applied, and the thickness 52 thereof after the
 polishing pad is left for 7.5 hours. When the static load before measuring
 was 1000 g/cm.sup.2, the polishing pad 9 was compressed by 28 .mu.m by the
 process (3), the thickness of the polishing pad 9 was recovered by 8 .mu.m
 by the process (4), then, the creep deformation of 20 .mu.m which the
 difference between them was remained.
 Accordingly, when the process (2) of the present invention is carried out,
 the creep deformation of the polishing pad 9 is much smaller compared to
 that when the process (2) of the present invention is not carried out, so
 that the effect of the process (2) is clear. Therefore, the polishing pad
 causes little deformation during the polishing of the wafer. The surface
 and the cross section, of the polishing pad 9 were observed by an electron
 microscope before and after the treatment. As a result, it was found that
 the flatness of the flat surface was improved externally. Further, the
 surface roughness, that is, a center line average roughness was also
 improved from Ra=14 .mu.m to Ra=10 .mu.m. When the compressing plate, on
 the surface of which the projecting portions were formed, was used, the
 grooves were formed on the surface of the polishing pad 9.
 FIG. 2 shows a treating apparatus 30 for a polishing pad according to the
 second embodiment of the present invention, which is different from the
 treating apparatus 10 for the polishing pad, which is used in the first
 embodiment.
 With the treating apparatus 30 for the polishing pad, a recess portion is
 formed in an inner surface of a plate-like apparatus body 31. A rubber
 sheet 32 is adhered to the apparatus body 31 to cover the recess portion,
 so that a pressurizing chamber 33 is formed. A lower compressing plate 35
 is disposed under the apparatus body 31 and a spacer 34 is disposed
 between the peripheral portion of the lower compressing plate 35 and the
 apparatus body 31, so that a space 36 is formed. The pressurizing chamber
 33 is connected to a vacuum pump 38 through a hose 37. In the space 36,
 the polishing pad 9 is disposed on the lower compressing plate 35 and an
 upper compressing plate 39 is disposed on the polishing pad 9. A spacer 40
 which is thinner than the polishing pad 9 is disposed around the polishing
 pad 9.
 The compressing means, such as the compressing plates 35 and 39, the
 apparatus body 31 or the like, of the treating apparatus 30 for the
 polishing pad is contained in a thermostatic chamber 41. The vacuum pump
 38 is driven under the condition of a constant temperature. Then, the
 fluid is applied into the pressurizing chamber 33, so that the rubber
 sheet 32 is inflated downwardly. Accordingly, the upper compressing plate
 39 is moved downwardly, so that the polishing pad 9 is compressed between
 the upper compressing plate 39 and the lower compressing plate 35.
 Accordingly, it is possible to obtain the same advantageous effect as that
 of the first embodiment of the present invention.
 In the above-described embodiments, the polishing pad 9 was subjected to
 the load of 1000 g/cm.sup.2 at 120.degree. C. for 4 hours. However, it was
 possible to obtain the similar effect when the polishing pad 9 was
 subjected to the load of 500-2000 g/cm.sup.2 at 80-100.degree. C. for 1-10
 hours. In these cases, the surface roughness, that is, the center line
 average roughness was also improved from Ra=14 .mu.m to Ra=5-10 .mu.m.
 In the two above-described embodiments, it is described the case that only
 one polishing pad is treated, however, by increasing the number of the
 compressing plates, it is possible to easily treat a plurality of
 polishing pads.
 Then, a wafer is polished with the polishing machine, for example, the one
 shown in FIG. 4 by using the polishing pad obtained by the above-described
 treatments. During the polishing process, little creep deformation of the
 polishing pad causes. When the polishing pad having grooves on the surface
 thereof is used for the polishing process, the polishing slurry is spread
 over the surface of the polishing pad and easily discharged, during the
 polishing process. The wafer obtained by the polishing process has high
 flatness.
 According to the method for polishing a wafer, because the polishing pad
 has small creep deformation, high flatness, and small surface roughness,
 it is possible to obtain a wafer having high flatness.
 From the foregoing description, one skilled in the art can easily ascertain
 the essential characteristics of this invention, and without departing
 from the spirit and scope thereof, can make various changes and
 modifications of the invention to adapt it to various usage and
 conditions.
 The entire disclosure of Japanese Patent Application No. 10-74696 filed on
 Mar. 23, 1998 including specification, claims, drawings and summary are
 incorporated herein by reference in its entirety.