Abstract:
A polishing apparatus comprises a carrier having a pressing surface to be engaged with a platy workpiece to press it against a polishing surface, whereby the workpiece is polished by being subjected to a relative sliding motion relative to the polishing surface while being pressed thereagainst. The pressing surface includes a suction opening provided along an outer peripheral portion of the pressing surface for applying a vacuum to hold the workpiece on the pressing surface during polishing of the workpiece. The carrier further comprises a pressure applying opening provided inside of the suction opening for applying a pressure to press the workpiece against the polishing surface during polishing of the workpiece.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer. 
     In manufacturing high-integration circuit devices and optical devices, elements of these devices, such as semiconductor wafers and optical lenses, are required to be polished to a high degree of uniformity. In recent years, in order to meet this requirement, a so-called CMP (chemical mechanical polisher) has been commonly used as a polishing apparatus for polishing semiconductor wafers. In a CMP, a semiconductor wafer is held by a wafer holder or carrier, which proceeds to lower and press the wafer against a polishing surface comprising a flexible polishing pad of a rotating turntable. The wafer is then subjected to a relative sliding motion relative to the polishing surface of the turntable while, at the same time, an alkali abrasive liquid is supplied to the polishing surface. By using this combination of mechanical and chemical polishing, highly precise polishing of a wafer can be achieved. Since in a polishing operation using a CMP, friction is generated between a wafer and a polishing surface, lateral displacement of the wafer may occur. To avoid displacement of the wafer, a retainer ring is generally employed. In FIG. 5, a retainer ring  1   a  is shown which is formed on a carrier  1  around its outer circumferential edge. In addition to a danger of lateral displacement of a wafer during polishing, there is also a danger that its circumferential edge may be overpolished if the edge is subjected to excessive pressure when the sliding motion is effected while the wafer is pressed against a polishing surface (reference is made, for example, to Unexamined Japanese Patent Application Public Disclosure No. 10-58309). Thus, as shown in FIG. 5, conventionally, a pressure ring  3  is provided outside and separate from the retainer ring l a  on the carrier  1 . During polishing, the pressure ring  3  depresses the flexible polishing pad comprising the polishing surface around the semiconductor wafer  4  by an amount sufficient to prevent the circumferential edge of the wafer to be polished from being subjected to excessive pressure and polishing during a relative sliding motion between the wafer and the polishing surface. Preferably, the pressure ring is positioned as close as possible to the circumferential edge of the wafer held on the carrier. 
     However, in the conventional polishing apparatus in which a retainer ring is positioned between a pressure ring and a wafer, a distance of around 2 mm exists between the pressure ring and the semiconductor wafer and it has been desired to reduce this distance. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a polishing apparatus which enables a reduction in the distance between a circumferential edge of a wafer held on a carrier and a pressure ring. 
     In accordance with the present invention, there is provided a polishing apparatus comprising a carrier having a pressing surface to be engaged with a platy workpiece such as a semiconductor wafer to press the workpiece against a polishing surface, whereby the workpiece is polished by being subjected to a relative sliding motion relative to the polishing surface while being pressed thereagainst, the pressing surface including a suction opening for applying a vacuum to hold the workpiece on the pressing surface during polishing of the workpiece. 
     The pressing surface may include a recessed portion formed at a desired position, which recessed portion has the suction opening and is communicated with a negative-pressure gas source or vacuum source provided outside the carrier, so that a vacuum can be applied to the recessed portion by the vacuum source and the platy workpiece can be securely held on the carrier under the effect of the vacuum. Preferably, the recessed portion extends along an outer peripheral portion of the pressing surface. More preferably, the recessed portion is arranged in the form of an annular groove. 
     Specifically, the carrier comprises a carrier body having a generally disk-like configuration and a backing plate covering the surface of the carrier body facing toward the polishing surface. The surface of the backing plate facing toward the polishing surface provides the pressing surface. This surface of the backing plate includes the recessed portion arranged in the form of an annular groove and a pressure-applying recessed portion formed radially inward of the groove. The pressure-applying recessed portion is communicated with a positive-pressure gas source or fluid pressure source provided outside the carrier. The backing plate may be made of gas-impermeable resilient material. 
     The present invention also provides a polishing apparatus comprising a carrier having a pressing surface for pressing a platy workpiece such as a semiconductor wafer against a polishing surface, and a pressure ring to be positioned outside and adjacent to the workpiece held by the carrier for pressing the polishing surface around the workpiece. The workpiece is polished by being subjected to a relative sliding motion relative to the polishing surface while being pressed thereagainst. The pressure ring and the carrier are capable of rotating relative to one another. Since the pressure ring is provided adjacent to the workpiece, the polishing surface can be depressed to an optimum level relative to the workpiece. Further, since the pressure ring and the carrier are capable of rotating relative to one another, it is possible to avoid a situation that when a lower surface of the pressure ring is undulating, a specific portion of the workpiece is affected by such undulation. This ensures high overall uniformity in the polishing of the workpiece. 
     The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional side view showing a main part of a polishing apparatus of the present invention. 
     FIG. 2 is an enlarged cross-sectional side view showing an essential part of a carrier body of the polishing apparatus of FIG.  1 . 
     FIG. 3 is a cross-sectional side view showing a polishing apparatus according to an embodiment of the present invention. 
     FIG. 4 is a cross-sectional side view showing a polishing apparatus according to another embodiment of the present invention. 
     FIG. 5 is a cross-sectional side view showing a wafer carrier of a conventional polishing apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinbelow, embodiments of the present invention are described. 
     FIG. 1 shows an essential part of a polishing apparatus of the present invention for polishing a semiconductor wafer W. As in the case of the conventional polishing apparatus, the polishing apparatus of the present invention comprises a turntable  14  and a wafer carrier  20  adapted to hold the semiconductor wafer W and press the semiconductor wafer W against a polishing pad  16  provided on an upper surface of the turntable  14 . 
     During polishing, the turntable  14  and the wafer carrier  20  are rotated by rotary drive shafts  22  and  24 , respectively, and a relative sliding motion between the semiconductor wafer W and the polishing pad  16  is effected. At the same time, an alkali abrasive liquid is supplied from a nozzle (not shown) onto the polishing pad  16 . Thus, chemical mechanical polishing of the semiconductor wafer W is conducted by means of the sliding motion in conjunction with the abrasive liquid. 
     As shown in FIG. 1, the wafer carrier  20  comprises a disk-like carrier body  26  connected to the rotary drive shaft  24  for rotation and a backing plate  32  covering a lower surface  30  of the carrier body  26  facing toward the turntable  14 . A pressure ring  34  provided to be separate from the carrier body  26  and the backing plate  32  is provided around the wafer carrier  20  in a manner such that the pressure ring  34  is nearly in contact with an outer circumferential surface of the carrier body  26 . 
     The surface of the backing plate  32  facing toward the turntable  14  includes a wafer-holding groove  40  in an annular form extending along an outer circumferential edge of the backing plate  32  and also includes a pressure-applying recessed portion  42  formed inward of the groove  40 . The pressure-applying recessed portion  42  is in a circular form as viewed from above. The radial width (a width in a transverse direction) of the groove  40  is set to between about 5 mm and about 10 mm. 
     The carrier body  26  and the backing plate  32 , respectively, include through-holes  43  and  44  for communication between the groove  40  and a vacuum source P 1 . The carrier body  26  and the backing plate  32  also include through-holes  46  and  48  for communication between the pressure-applying recessed portion  42  and a fluid pressure source P 2 . 
     The pressure ring  34  is pressed against the polishing pad  16  under a desired pressure F by means of an air cylinder  66  connected to a carrier head  52  (described later) which is provided above the wafer carrier  20  for supporting the wafer carrier  20 . 
     As the polishing pad  16 , it is preferred to use IC1000, IC1000-SUBA400 or Politex (each supplied from RODEL NITTA). An abrasive plate comprising abrasive particles fixed by using a binder may be used; instead of the polishing pad. The backing plate  32  is preferably made of a gas-impermeable elastic material, such as a silicone rubber, a neoprene rubber, a urethane rubber or a fluoro rubber. 
     By using the above-mentioned polishing apparatus, polishing of semiconductor wafers is conducted as follows. First, the wafer carrier  20  is moved outward of the turntable  14  and positioned above the wafer to be polished. A negative pressure (a vacuum) is applied to the groove  40  and/or the pressure-applying recessed portion  42 ; to thereby hold the wafer on the wafer carrier  20  under the effect of the vacuum and transfer the wafer to the polishing pad  16  on the turntable  14 . Subsequently, the turntable  14  and the wafer carrier  20  are rotated by the rotary drive shaft  22  and the rotary drive shaft  24 , respectively, and an abrasive liquid is supplied from the nozzle (not shown) onto the polishing pad  16  and polishing of the wafer is started. During polishing, a pressure-applying fluid is supplied to the pressure-applying recessed portion  42 , to thereby press the semiconductor wafer W against the polishing pad  16 , while the negative pressure is applied to the groove  40 , to thereby securely hold the semiconductor wafer W on the backing plate  32  and hence the wafer carrier  20 . The strength of the vacuum force applied to the wafer during polishing should be sufficient to prevent lateral displacement of the wafer from the wafer carrier  20 , which would otherwise occur due to a lateral frictional force generated between the polishing pad  16  and the wafer during polishing, whereby the wafer is securely held. Specifically, the negative pressure applied to the groove  40  is set to between about −50 Kpa and about −90 Kpa and the pressure applied to the pressure-applying recessed portion  42  is set to between 0 Kpa and 19.6 Kpa (between 0 g/cm 2  and 200 g/cm 2 ). The pressure of the wafer carrier  20  applied to the wafer is set to between about 4.9 Kpa and about 29.4 Kpa (between about 50 g/cm 2  and about 300 g/cm 2 ). The pressure of the pressure ring  34  applied to the polishing pad  16  is set to between 0 Kpa and 49 Kpa (between 0 g/cm 2  and 500 g/cm 2 ). 
     FIG. 3 shows an illustrative example of the polishing apparatus shown in FIGS. 1 and 2. This polishing apparatus comprises the turntable  14  having the polishing pad  16  provided thereon and the wafer carrier  20  for supporting the semiconductor wafer W. The wafer carrier  20  comprises the carrier body  26  and the backing plate  32 . The backing plate  32  includes the groove  40  and the pressure-applying recessed portion  42 . The pressure ring  34  is provided around the wafer carrier  20 . 
     In the present invention, the groove  40  is formed for holding a wafer by application of a vacuum during polishing. Since a groove having a predetermined width such as the groove  40  is formed along the outer circumferential edge of the backing plate  32 , an area for holding a wafer under the effect of vacuum is markedly larger than the total of areas for holding a wafer obtained by small vacuum openings, which are discretely arranged over a back surface of a wafer as is the case in a conventional wafer carrier. Therefore, a large vacuum force can be applied to the wafer. Further, an effect of leakage of vacuum can be suppressed due to the substantial volume of the space in the groove. Consequently, the wafer can be securely held and there is no need to use a retainer ring. 
     The rotary drive shaft  24  is connected to the wafer carrier  20  by means of a universal joint  50 . The rotary drive shaft  24  is adapted to rotated by a motor  56 , which is rotatably supported by the carrier head  52  and connected to the rotary drive shaft  24  through a driving belt  54 . 
     The pressure ring  34  is connected through a radial bearing  60  to a piston-cylinder apparatus  62  provided in the carrier head  52 . The piston-cylinder apparatus  62  comprises an air cylinder  66  fixed to the carrier head  52  and a piston rod  68  extending downward from the air cylinder  66 . A connecting member  70  at a lower end of the piston rod  68  is connected to the pressure ring  34  through the radial bearing  60  and applies the desired pressure F exerted by the air cylinder  66  to the pressure ring  34 . The pressure ring  34  is capable of rotation relative to the connecting member  70  through the radial bearing  60 . Further, the pressure ring  34  is connected through a bevel gear  74  to a motor M attached to an intermediate portion of the piston rod  68 , and adapted to be rotated relative to the connecting member  70  by the motor M. That is, the pressure ring  34  is capable of rotating independently of the wafer carrier  20 . For example, the wafer carrier  20  and the pressure ring  34  can be rotated at different respective speeds by setting the rotation speed of the wafer carrier to 60 rpm, and setting the rotation speed of the pressure ring to 61 rpm. When the wafer carrier  20  and the pressure ring  34  are rotated at the same speed, the positional relationship between the semiconductor wafer W held by the wafer carrier  20  and the pressure ring  34  does not change and therefore, if a lower surface of the pressure ring  34  is undulating, such undulation will adversely affect polishing of the wafer. This can be prevented by rotating the wafer carrier  20  and the pressure ring  34  at different respective speeds. Since the lower surface of the pressure ring  34  is susceptible to wear, it is preferred that the pressure ring  34  be rotated in the same direction as the wafer carrier  20  at a speed slightly lower than that of the wafer carrier  20 . In the present invention, relative rotation between the pressure ring and the wafer carrier is made possible because, as mentioned above, during polishing, a wafer can be securely held on the wafer carrier  20  by application of a vacuum, and contact between the wafer and the pressure ring  34  can be prevented. Reference numeral  78  denotes a piston-cylinder apparatus attached to the carrier head  52 , which is used for moving the rotary drive shaft  24  in a vertical direction relative to the carrier head  52 . 
     FIG. 4 is a modified example of the polishing apparatus of FIG.  3 . In this example, relative rotation between the pressure ring  34  and the wafer carrier  20  is not conducted. The pressure ring  34  is connected to the wafer carrier  20  in a manner such that the pressure ring  34  is capable of vertical movement relative to the wafer carrier  20 . Therefore, members for rotating the pressure ring  34 , such as the motor shown in FIG. 3, are not provided. In FIG. 4, valves R 3  to R 5  are provided in passages for connecting the groove  40  and the pressure-applying recessed portion  42  in the backing plate  32  of the wafer carrier  20  with a vacuum source  80  and a compressed air source  72 . The pressures in the groove  40  and the pressure-applying recessed portion  42  are appropriately controlled by controlling these valves. 
     The polishing apparatus of the present invention is arranged as mentioned above. During polishing, a workpiece such as the semiconductor wafer W is pressed against the polishing pad  16  by means of a pressure-applying fluid supplied to the pressure-applying recessed portion  42 , while a vacuum is applied to the groove  40  so as to securely hold the semiconductor wafer W on the wafer carrier  20 . Therefore, differing from the conventional polishing apparatus, there is no need to provide the retainer ring in the wafer carrier. Since no retainer ring is provided, the distance between the pressure ring  34  and the workpiece can be reduced by the distance corresponding to the retainer ring. Therefore, the polishing pad  16  which is engaged with the workpiece during polishing can be depressed to the same level as the surface of the workpiece to be polished, thus making it possible to avoid a situation that an edge of the workpiece is subject to excessive polishing. In one embodiment of the present invention, the distance between an inner edge of the pressure ring  34  and an outer circumferential edge of the semiconductor wafer, which is at least 2 mm in the conventional polishing apparatus, is reduced to 0.5 mm. 
     Further, since contact between the wafer and the pressure ring  34  during polishing can be prevented, the pressure ring  34  can be rotated relative to the wafer (or the wafer carrier). This avoids a situation such that only a specific portion of the wafer is affected by undulation of a lower surface of the pressure ring  34  during polishing.