US20020177395A1

US20020177395A1 - Polishing head of a chemical and mechanical polishing apparatus for polishing a wafer

Info

Publication number: US20020177395A1
Application number: US10/153,317
Authority: US
Inventors: Myoung-Sik Han; Chang-ki Hong
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2001-05-23
Filing date: 2002-05-22
Publication date: 2002-11-28
Also published as: KR100437089B1; KR20020090370A

Abstract

Disclosed is a polishing head of a chemical and mechanical polishing apparatus uniformly polishing a wafer. The polishing head has a body defining at least one air passage therein through which air is introduced into and exhausted from the polishing head. The body is movable upward and downward. An air pressure distributing member is mounted to a lower portion of the body for distributing a pressure of the air supplied through the air passage. A membrane is mounted to enclose a lower surface of the air pressure distributing member so as to be expanded and shrunk by the pressure of the air supplied through the air pressure distributing member. A surface of the air pressure distributing member makes contact with a back surface of a wafer. An air pressure compensating member makes uniformly the pressure that is applied to central and edge portions of the wafer which makes contact with the membrane. Since the air pressure compensating member applies to the edge portion of the wafer, the air pressure is compensated so that the uniform air pressure is applied to the wafer. Thus, the wafer is uniformly polished.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a chemical and mechanical polishing apparatus, and more particularly to a polishing head of a chemical and mechanical polishing apparatus capable of uniformly polishing a wafer.

2. Description of the Related Art

In a semiconductor device manufacture process, a surface of a semiconductor wafer is polished, using chemical and mechanical components such as abrasive supplied between the wafer and a polishing pad. This process is commonly known as a chemical and mechanical polishing (CMP).

CMP has developed into an integral component of the manufacture and yield of cost effective semiconductor products. In general, a CMP apparatus for planarizing surface of the wafer includes a polishing head for supporting and pressing the wafer, a polishing platen rotatively operating and having a polishing pad, a slurry supplying device, and a conditioner for conditioning the polishing pad.

When the CMP apparatus performs the polishing process, the surface of the wafer being polished must be uniformly polished throughout. Therefore, the polishing pressure applied to a respective region of the wafer, the amount of slurry, and the condition of the polishing pad which come in contact with the wafer must be regularly controlled.

FIG. 1 is a cross sectional view of a conventional polishing head of a CMP apparatus.

Referring to FIG. 1, when a wafer is polished, a surface of the wafer W contacts with a

polishing pad

10 and a polishing head 12 contacts with the other surface of the wafer for applying a pressure to the wafer W. The polishing pad 10 and the polishing head 12 rotate in the same direction or opposite direction, respectively.

The polishing

head

12 includes a body 14 capable of moving upward and downward. The body 14 includes air passages 16 for receiving and discharging air and a retainer ring 18 for securing the wafer W and preventing it from separating from the body 14.

A

perforated plate

20 is disposed on a lower portion of the body 14 for distributing a pressure of air supplied through the air passages 16. A plurality of thru-holes 20 a are formed in the perforated plate 20 for uniformly distributing the air pressure.

A

membrane

22 is attached to the body 14 to enclose a lower portion of the perforated plate 20.

FIG. 2 is a perspective view showing the

membrane

22 mounted to the polishing head 12 shown in FIG. 1.

The

membrane

22 is resiliently expanded and shrunk by the air pressure supplied through the plurality of thru-holes 20 a (in FIG. 1).

Referring again to FIG. 1 for describing the polishing process of the wafer, the air is exhausted through the

air passages

16 from the body 14, so that the body 14 is in a vacuum state at a certain portion thereof. Due to the vacuum state in the body 14, the wafer W adheres to the lower surface of the membrane 22, which contacts with the polishing pad 10. Air is then introduced through the air passages 16 into the body 14 to expand the membrane 22. The expanded membrane 22 presses the wafer W. A problem with this process is that the membrane 22 is non-uniformly expanded at a center portion and an edge portion thereof.

FIG. 3 is a partially enlarged cross sectional view showing the edge portion A of the expanded membrane of the polishing apparatus shown in FIG. 1.

Referring to FIG. 3, the

membrane

22 is inclined at the edge portion during the expansion. When the membrane 22 is expanded, a gap 22 a is formed between the edge portion of the membrane 22 and the edge portion of the wafer W so that the membrane 22 does not contact with the wafer W at the edge portion thereof. As a result, the pressure is not applied to the edge portion of the wafer W.

Accordingly, the wafer W has a different polishing rate between the edge portion and portions other than the edge portion during the polishing. Due to the different polishing rate, the wafer W has a non-uniform profile at the edge portion thereof.

U.S. Pat. No. 6,116,992 (issued to Prince on Sep. 12, 2000) discloses a CMP apparatus having a retainer ring of which a bottom surface is projected to help the polishing of the wafer W. However, nonuniformity in a polished wafer still exists in the CMP apparatus, if the membrane has uneven expansion characteristics.

Accordingly, there is a problem in that a process error is found in chips formed on the edge portion of the wafer as the edge portion of the wafer is non-uniformly polished, resulting in decreased yield and reliability of the semiconductor devices.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above-mentioned problem, and accordingly it is an object of the present invention to provide a CMP apparatus capable of uniformly polishing a wafer.

A polishing head of a chemical and mechanical polishing apparatus for polishing a wafer is provided, the polishing head includes: a body having at least one air passage therein, through which air is introduced into the body and exhausted from the body; an air pressure distributing unit mounted to a lower portion of the body for distributing air pressure supplied through the air passage; a membrane for enclosing a lower surface of the air pressure distributing unit, the membrane having a lower surface for making contact with a back surface of the wafer, the membrane being expanded and shrunk by the pressure of the air supplied through the air pressure distributing unit; and an air pressure compensating unit for distributing pressure to selected portions of the surface of the wafer connecting the membrane.

According to an embodiment of the present invention, the selected portions of the surface of the wafer include a center portion and an edge portion of the wafer. The air pressure compensating unit is disposed between the air pressure distributing unit and the membrane and an upper surface and a lower surface of the air pressure compensating unit are respectively fixed to edge of the lower surface of the air pressure distributing unit and edge of an upper surface of the membrane. The air pressure compensating unit has a ring shape, which is adhered to edges of the air pressure distributing unit and the membrane. The air pressure compensating unit has a cross-sectional shape of which an upper side and a lower side are parallel with each other, an inner side is inclined and an outer side is vertical to the upper and lower sides. The inner side of the air pressure compensating unit is inclined in a direction to a center of the membrane so as to compensate a space between the membrane and an edge of the wafer when the membrane is expanded by the air pressure. The air pressure compensating unit has a thickness enough to compensate an expanding difference between the edge portion and the center portion of the membrane when the membrane is expanded by the air pressure. The air pressure compensating unit has a ring shape, disposing under the membrane and an edge of the membrane. The air pressure compensating unit is inclined in a direction to a center of the membrane. The air pressure compensating unit is comprised of resilient material. The resilient material is a rubber or a silicon resin. The air pressure distributing unit is formed as a disc shape having a predetermined diameter, in which a plurality of thru-holes are formed at portions within a range of a predetermined radius from a center of the disc shape.

A polishing head of a chemical and mechanical polishing apparatus for polishing a wafer is also provided, the polishing head includes: a body having a plurality of air passages therein, through which air is introduced into and exhausted from the polishing head, and a retainer ring for securing the wafer, said body being movable upwardly and downwardly; an air pressure distributing plate mounted to a lower portion of the body for distributing air pressure supplied through the air passages; an air cushion connected with an end portion of one of the air passages so as to close the end portion and make contact with an edge portion of an upper surface of the air pressure distributing plate, the air cushion being resiliently expanded and shrunk during introducing and venting of the air through the air passages for applying pressure to the edge portion of the upper surface of the air pressure distributing plate; a membrane mounted to enclose a lower surface of the air pressure distributing unit for being expanded and shrunk by the pressure of the air supplied through the air pressure distributing plate, a lower surface of the membrane making contact with a back side of the wafer; and an air pressure compensating member for uniformly distributing the pressure that is applied at central and edge portions of the wafer which makes contact with the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: [0024]
FIG. 1 is a cross sectional view showing a conventional polishing head of a CMP apparatus; [0025]
FIG. 2 is a perspective view showing a membrane mounted on the CMP apparatus shown in FIG. 1; [0026]
FIG. 3 is a partially enlarged cross sectional view showing an edge of the membrane of the CMP apparatus, in which the membrane is expanded to apply pressure to a wafer; [0027]
FIG. 4 is a cross sectional view showing a polishing head of a CMP apparatus according to a preferred embodiment of the present invention; [0028]
FIG. 5 is a perspective view showing a membrane mounted on the CMP apparatus as shown in FIG. 4; [0029]
FIG. 6 is a plan view showing the membrane mounted on the CMP apparatus shown in FIG. 4; and [0030]
FIG. 7 is a partially enlarged cross sectional view showing an edge of the membrane of the CMP apparatus shown in FIG. 4, in which the membrane is expanded to apply pressure to a wafer.[0031]

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. [0032]
FIG. 4 is a cross sectional view showing a polishing head of a CMP apparatus according to an embodiment of the present invention. [0033]
Referring to FIG. 4, when a wafer W is polished, a surface of the wafer W is in contact with a [0034] polishing pad 30 and the other surface of the wafer W is in contact with a polishing head 32 so that pressure is applied to the wafer W. A polishing pad 30 and the polishing head 32 rotates in the same direction or opposite direction.
The polishing [0035] head 32 includes a body 34 that is capable of being rotated and moving upwardly and downwardly. The body 34 includes a housing 36, a base 38 for supporting the housing 36, and a retainer ring 40 mounted on edges of a lower portion of the base 38 for securing the wafer W and preventing it from being separated from the body 34.
The [0036] housing 36 is connected with a driving section (not shown), which is upwardly and downwardly moved in a vertical direction to a surface of the polishing pad 30 by the driving section. Further, the housing 36 rotates about a rotation axis that is in a vertical direction to the surface of the polishing pad 30. The housing 36 having upper and lower surfaces in a disc shape includes a plurality of tubes 42 a, 42 b, and 42 c for supplying and exhausting air to/from the CMP apparatus first tube 42 a. The first tube 42 a passes through a center portion from the upper surface to the lower surface. Second and third tubes 42 b and 42 c spaced apart from the first tube 42 a in the radial direction of the housing 36 pass through from the upper surface of the housing 36 to the lower surface of the housing 36. The second and third tubes 42 b and 42 c are preferably disposed symmetricly about the first tube 42 a.
The [0037] base 38 is connected with a certain portion of the lower surface of the housing 36 for supporting the housing 36. The base 38 has upper and lower surfaces in a disc shape. The first tube 42 a of the housing 36 is extended into the base 38 so that the first tube 42 a passes through a center portion of the base 38. Further, the second tube 42 b is also extended to pass through the base 38.
The [0038] housing 36 and the base 38 are connected with each other by means of a connecting member 60. The connecting member 60 includes an outer clamp (not shown) to be connected to the lower surface of the housing 36, an inner clamp (not shown) fixed to an upper surface of the base 38 by means of screws (not shown) and an annular resilient sheet (not shown) connecting the housing 36 to the base 38.
When the [0039] housing 36 is connected with the base 38 by means of the connecting member 60, a first chamber 44 is formed between the housing 36 and the base 38. The first chamber 44 is defined by the housing 36, the base 38, and the connecting member. The third tube 42 c passing through the housing 36 connects with the first chamber 44 and defines a passage for introducing and exhausting air to and from the first chamber 44. Therefore, when the air is introduced into the first chamber 44 through the third tube 42 c, the air pressure increases in the first chamber 44 to downwardly push the base 38. Conversely, when the air is exhausted through the third tube 42 c from the first chamber 44, the first chamber 44 is vacuumed to generate a vacuum pressure. As a result, the base 38 is upwardly moved to the housing 36 by the vacuum pressure.
The [0040] retainer ring 40 has a ring shape, which is fixed along an edge of the lower surface of the base 38. The retainer ring 40 is preferably fixed to the base 38 by screws (not shown) for preventing the wafer W from being departed from the polishing head 32 during the polishing process.
A [0041] perforated plate 46 is mounted on a lower portion of the body 34 for evenly distributing air pressure supplied through the first tube 42 a. Preferably, the perforated plate 46 is mounted on the lower surface of the base 38 and inside of the retainer ring 40. The perforated plate 46 is formed as a disc shape having a predetermined diameter, in which a plurality of thru-holes 46 a are formed in a predetermined range of radius from a center of the perforated plate 46. The air pressure is distributed through the thru-holes 46 a of the perforated plate 46 toward the lower direction. That is, the air is introduced in and exhausted from a space between the base 38 and the perforated plate 46 through the first tube 42 a, and the air is distributed through the thru-holes 46 a of the perforated plate 46 toward the lower direction.
A dividing [0042] plate 48 is further disposed between the base 38 and the perforated plate 46 to support the lower portion of the base 38. The dividing plate 48 is formed as a disc shape having a predetermined thickness, in which a pipe 48 a passes through the center portion of the dividing plate 48 to connect with the perforated plated 46. The pipe 48 a is disposed substantially at the center of the dividing plate 48 and is extended upward.
Accordingly, the air is introduced in and exhausted from the space between the dividing [0043] plate 48 and the perforated plate 46. Since the dividing plate 48 is disposed between the base 38 and the perforated plate 46, the space for distributing the air which is supplied through the first tube 42 a can be reduced and the air pressure increases in the space.
At an edge of an upper surface of the [0044] perforated plate 46, an air cushion 50 is connected with an end position of the second tube 42 b so as to close an end part of the second tube 42 b. The air is introduced into or exhausted from the air cushion 50 through the second tube 42 b, therefore the air cushion 50 is resiliently expanded or shrunk. Accordingly, when air is introduced into the air cushion 50, the air cushion 50 is expanded to apply the pressure to the edge portion of the perforated plate 46.
The polishing [0045] head 32 further includes a membrane 52 that encloses a substantially lower portion of the perforated plate 46.
FIG. 5 is a perspective view showing the [0046] membrane 52 mounted on the CMP apparatus as shown in FIG. 4. FIG. 6 is a plan view showing the membrane 52 mounted on the CMP apparatus as shown in FIG. 4.
Referring to FIGS. 5 and 6, the [0047] membrane 52 is comprised of a resilient material, an elastic material, or a rubbery material, which can be resiliently expanded and shrunk by the air pressure supplied through the perforated plated 46. According to an embodiment of the present invention, the membrane 52 includes a pressure compensating member 54 at an inside edge of the membrane 52. The details related to the pressure compensating member 54 will be described below.
The [0048] membrane 52 is in contact with a back surface of the wafer W. As the membrane 52 is expanded, the pressure is applied to the back surface of the wafer W during polishing of the wafer W. The membrane 52 continuously encloses the lower surface, the side and the edge of the upper surface of the perforated plate 46 (in FIG. 4) having a disk shape, and the membrane 52 is adhesively fixed to the side of the perforated plate 46 and the edge of the upper surface of the perforated plate 46.
When air is introduced through the [0049] perforated plate 46, the membrane 52 is expanded downwardly due to the air pressure. Furthermore, when air is exhausted through the perforated plate 46, the membrane 52 is in close contact with the perforated plate 46, and some portions of the membrane 52 are sucked into the thru-holes 46 a of the perforated plate 46. During polishing of the wafer W, the membrane 52 is in contact with a back surface of the wafer W.
Hereinafter, an operation of the polishing head according to a preferred embodiment of the present invention will be described in detail. [0050]
Referring again to FIG. 4, air is exhausted through the [0051] first tube 42 a from the space between the dividing plate 48 and the perforated plate 46 to provide vacuum. When the space between the perforated plate and the dividing plate 48 is under vacuum pressure, the back surface of the wafer W is stuck to the lower surface of the membrane 52. Then, the body 34 is moved downwardly so as to allow the front surface of the wafer W to make contact with the polishing pad 30. When the front surface of the wafer W is in contact with the polishing pad 30, the air is introduced between the perforated plate 46 and the dividing plate 48 to expand the membrane 52. The lower surface of the membrane 52 applies the pressure to the wafer W as the membrane 52 is expanded. The polishing pad 30 and the polishing head 32 are respectively rotated to polish the wafer W.
FIG. 7 is a partially enlarged cross sectional view showing an edge B of the [0052] membrane 52, in which the membrane 52 as shown in FIG. 4 is expanded to apply the pressure to the wafer W.
Air introduced through the [0053] first tube 42 a between the perforated plate 46 and the dividing plate 48 applies air pressure to the membrane 52 through the perforated plate 46. Since the perforated plate 46 has the disc shape, in which the plurality of the thru-holes 46 a are formed in the range of the predetermined radius from the center axis thereof, air pressure cannot be uniformly applied to the whole surface of the membrane 52 through the perforated plate 46. That is, the edge portion of the perforated plate 46 has a few thru-holes so that a small amount of air is introduced into the edge portion of the perforated plate 46. Therefore, the membrane 52 is greatly expanded at the center portion rather than the edge portion thereof. In addition, the edge portion of the membrane 52 that is expanded is inclined toward the center portion so that the edge portion of the membrane 52 does not make contact with the wafer W. As a result, the pressure is not applied to the edge portion of the wafer W.
According to an embodiment of the present invention, the [0054] pressure compensating member 54 in the polishing head 32 compensates the air pressure at the edge of the membrane 52 so as to make uniform the pressure applied to the wafer W in contact with the membrane 52. The pressure compensating member 54 is provided between the membrane 52 and the perforated plate 46. Preferably, an upper surface of the pressure compensating member 54 is fixedly adhered to the edge of the lower surface of the perforated plate 46 and the lower surface of the pressure compensating member 54 is fixedly adhered to the edge of the inner surface of the membrane 52 that faces the edge of the lower surface of the perforated plate 46. The pressure compensating member 54 has the ring shape, the upper surface and the lower surface of the pressure compensating member 54 are respectively and continuously adhered to the edges of the perforated plate 46 and the membrane 52.
In the cross-sectional view, an upper side and a lower side of the [0055] pressure compensating member 54 are in parallel with each other. Furthermore, an inner side of the pressure compensating member 54 is inclined in a direction to the center of the membrane 46 and an outer side of the pressure compensating member 54 is vertical to the upper side and the lower side of the pressure compensating member 54.
The inner side of the compensating [0056] member 54 has a desired gradient enough to compensate the space between the edge of the wafer W and the membrane 52 when the membrane 52 is expanded due to the air pressure. Preferably, the membrane 52 is inclined at a predetermined angle from the edge to the center axis thereof while being expanded by the pressure of the air supplied through the thru-holes 46 a of the perforated plate 46. Therefore, when the membrane 52 is expanded, the pressure compensating member 54 functions as filling material between the wafer W and the membrane 52, depending on the gradient of the edge of the membrane 51 to uniformly polish the center portion and the edge portion of the wafer W during the polishing of the wafer W. Accordingly, the inner side of the pressure compensating member 52 must have the gradient enough to allow the membrane 52 to make close contact with the wafer W. Further, the pressure compensating member 54 has a thickness enough to compensate for any expansion due to air pressure between the edge portion and the center portion of the membrane 52.
According to an embodiment of the present invention, the [0057] pressure compensating member 54 is also disposed under the membrane 52 and an edge of the membrane 52, and is inclined in a direction to a center of the membrane 52.
According to an embodiment of the present invention, the [0058] pressure compensating member 54 is preferably comprised of resilient material such as a rubber and a silicon resin, of which the edge portion is resiliently bent by the air pressure.
When the [0059] membrane 52 is expanded and presses the wafer W, the pressure compensating member 54 pushes the edge portion of the membrane 52 to remove the space between the edge portions of the membrane 52 and the wafer W. Therefore, the pressure that is supplied for the edge portion and the center portion of the wafer W can be uniform. Accordingly, the edge portion and the center portion of the wafer W are uniformly polished and a polishing profile of the wafer W is improved. Furthermore, since the process failure caused at the edge portion of the wafer W is reduced, the yield and the reliability of the semiconductor device is improved.
Further, the [0060] pressure compensating member 54 uniformly applied pressure to the edge portion and the center portion of the wafer W while preventing the wafer from being dropped downwardly when the wafer W is stuck to the membrane 52. Particularly, when the polishing of the wafer W is finished or temporarily stopped, the wafer W is stuck to the membrane 52 as the air is exhausted through the first tube 42 a from the space between the dividing plate 48 and the perforated plate 46. When the wafer W is stuck to the membrane 52, the wafer W is somewhat bent in a state that the center portion of the wafer W is concaved downwardly as the pressure compensating member 54 is disposed at the edge of the membrane 52. When the wafer W is stuck to the membrane 52 in the state of being bent, the membrane 52 is sucked into the thru-holes 46 a of the perforated plate 46 while the wafer W maintains a close contact with the membrane 52. As a result, the vacuum pressure increases in the space between the membrane 52 and the wafer W so as to prevent the wafer W from being dropped downwardly.
Although preferred embodiments of the present invention have been described, it is understood that the present invention should not be limited to these preferred embodiments but various changes and modifications can be made by one skilled in the art within the spirit and scope of the present invention as hereinafter claimed. [0061]

Claims

What is claimed is:

1. A polishing head of a chemical and mechanical polishing apparatus for polishing a wafer, said polishing head comprising:

a body having at least one air passage therein, through which air is introduced into the body and exhausted from the body;

an air pressure distributing unit mounted to a lower portion of the body for distributing air pressure supplied through the air passage;

a membrane for enclosing a lower surface of the air pressure distributing unit, the membrane having a lower surface for making contact with a back surface of the wafer, the membrane being expanded and shrunk by the pressure of the air supplied through the air pressure distributing unit; and

an air pressure compensating unit for distributing pressure to selected portions of the surface of the wafer contacting the membrane.

2. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 1, wherein the selected portions of the surface of the wafer contacting membrane include a center portion and an edge portion of the wafer.

3. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 1, wherein the air pressure compensating unit is disposed between the air pressure distributing unit and the membrane and an upper surface and a lower surface of the air pressure compensating unit are respectively fixed to edge of the lower surface of the air pressure distributing unit and edge of an upper surface of the membrane.

4. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 3, wherein the air pressure compensating unit has a ring shape, which is adhered to edges of the air pressure distributing unit and the membrane.

5. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 4, wherein the air pressure compensating unit has a cross-sectional shape of which an upper side and a lower side are parallel with each other, an inner side is inclined and an outer side is vertical to the upper and lower sides.

6. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 5, wherein the inner side of the air pressure compensating unit is inclined in a direction to a center of the membrane so as to compensate a space between the membrane and an edge of the wafer when the membrane is expanded by the air pressure.

7. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 5, wherein the air pressure compensating unit has a thickness enough to compensate an expanding difference between the edge portion and the center portion of the membrane when the membrane is expanded by the air pressure.

8. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 1, wherein the air pressure compensating unit has a ring shape, disposing under the membrane and an edge of the membrane.

9. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 8, wherein the air pressure compensating unit is inclined in a direction to a center of the membrane.

10. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 1, wherein the air pressure compensating unit is comprised of resilient material.

11. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 10, wherein the resilient material is a rubber or a silicon resin.

12. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 1, wherein the air pressure distributing unit is formed as a disc shape having a predetermined diameter, in which a plurality of thru-holes are formed at portions within a range of a predetermined radius from a center of the disc shape.

13. A polishing head of a chemical and mechanical polishing apparatus for polishing a wafer, said polishing head comprising:

a body having a plurality of air passages therein, through which air is introduced into and exhausted from the polishing head, and a retainer ring for securing the wafer, said body being movable upwardly and downwardly;

an air pressure distributing plate mounted to a lower portion of the body for distributing air pressure supplied through the air passages;

an air cushion connected with an end portion of one of the air passages so as to close the end portion and make contact with an edge portion of an upper surface of the air pressure distributing plate, the air cushion being resiliently expanded and shrunk during introducing and venting of the air through the air passages for applying pressure to the edge portion of the upper surface of the air pressure distributing plate;

a membrane mounted to enclose a lower surface of the air pressure distributing unit for being expanded and shrunk by the pressure of the air supplied through the air pressure distributing plate, a lower surface of the membrane making contact with a back side of the wafer; and

an air pressure compensating member for uniformly distributing the pressure that is applied at central and edge portions of the wafer which makes contact with the membrane.

14. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 13, wherein the air pressure compensating member is disposed between the air pressure distributing plate and the membrane so that an upper surface and a lower surface of the air pressure compensating member are respectively fixed to an edge on the lower surface of the air pressure distributing plate and an edge of an upper surface of the membrane.

15. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 13, wherein the air pressure compensating member has a ring shape, which is adhered to edges of the air pressure distributing plate and the membrane.

16. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 15, wherein the air pressure compensating member has a cross-sectional shape in which an upper side and a lower side are in parallel with each other, an inner side is inclined and an outer side is vertical to the upper and lower sides.

17. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 16, wherein the inner side of the air pressure compensating member is inclined in a direction to a center of the polishing he ad so as to compensate a space between the membrane and an edge of the wafer when the membrane is expanded by the air pressure.

18. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 16, wherein the air pressure compensating member has a thickness enough to compensate an expanding difference between the edge portion and the center portion of the membrane when the membrane is expanded by the air pressure.

19. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 13, wherein the air pressure compensating member has a ring shape, disposing under the membrane and an edge of the membrane.

20. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 19, wherein the air pressure compensating member is inclined in a direction to a center of the membrane.

21. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 13, wherein the air pressure compensating member is comprised of resilient material.

22. The polishing heads of a chemical and mechanical polishing apparatus as claimed in claim 21, wherein the resilient material is a rubber or a silicon resin.

23. The polishing head of a chemical and mechanical polishing apparatus as claimed in claim 13, further comprising a dividing plate between the body and the air pressure distributing member for reducing a space between the body and the air pressure distributing plate.