Patent Publication Number: US-6217411-B1

Title: Polishing apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a polishing apparatus, in particular, a semiconductor wafer polishing apparatus. 
     A typical semiconductor wafer polishing apparatus includes, as shown in FIG. 6, a polishing pad  3  provided on an upper surface of a turn table (not shown), a wafer carrier  2  for carrying a wafer  1  to be polished and a shaft-like support  4  connected to the carrier  2  at the lower end thereof through a universal joint  5 . The support  4  is adapted to be drivingly rotated around its axis by means of a motor (not shown) connected to the support and the universal joint  5  is adapted to transmit a rotational torque from the support to the carrier  2  to rotate the carrier about its axis while allowing the carrier  2  to pivot about the universal joint  5 . 
     In operation, the turn table with the polishing pad  3  is drivingly rotated and the support  4  is positioned so that the wafer  1  carried by carrier  2  is brought into contact with a flat polishing surface  3 ′ of the polishing pad  3  with a degree of pressure F, with the support  4  being simultaneously drivingly rotated, whereby the surface of the wafer  1  contacting the polishing surface of the polishing pad is polished. 
     Accordingly, a friction force f between the polishing pad and the wafer causes an angular moment M which acts on the carrier  2  with the wafer  1  about the universal joint  5  or the connecting point between the carrier  2  and the support  4 , whereby the carrier with the wafer tilts about the universal joint  5  thereby causing an unevenness or gradient in a pressure generated between the contacting surfaces of the wafer and the polishing pad. This causes a deterioration in quality of the polishing operation. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to provide a polishing apparatus which solves the problem discussed above. 
     In accordance with the present invention, a polishing apparatus comprises a polishing member having a polishing surface, a carrier for carrying an article to be polished, a support for supporting the carrier in such a manner that the article carried by the carrier Is engaged with the polishing surface, the carrier and the polishing pad being adapted to be moved relative to each other in such a manner that the article carried by the carrier is moved relative to and on the polishing surface of the polishing pad, a sensor device for sensing a friction force generated between the article and the polishing surface and imposed on the carrier due to the relative movement of the article on the polishing surface, a pressing device operatively associated with the carrier to impose a counter angular moment against a angular moment on the carrier by applying a pressure to at least one point on the carrier towards the polishing surface to generate the counter angular moment, and a control unit for determining a magnitude of the pressure applied to the above-noted at least one point on the carrier on the basis of the friction force sensed by the sensor device so that the angular moment is compensated by the counter angular moment. 
     The above features and advantages of the present invention will be become apparent from the following description and the appended claims taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view of a part of a polishing apparatus in accordance with a first embodiment of this invention showing a positional arrangement of major elements thereof, 
     FIG. 2 is a view taken along a line X—X in FIG. 1, 
     FIG. 3 is a view similar to FIG. 2 showing an influence of an angular moment imposed on a carrier for carrying an article to be polished, 
     FIG. 4 is a block-diagram of an arithmetical and control unit for controlling a pressing device for applying pressures on a carrier towards a polishing surface in a polishing apparatus shown in FIG. 1, 
     FIG. 5 is a view similar to FIG. 2 showing a polishing apparatus in accordance with an other embodiment of this invention, 
     FIG. 6 is a view similar to FIG. 3 showing a relationship between a carrier and a polishing pad in a prior art polishing apparatus in operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, there is shown a polishing apparatus in accordance with the first embodiment of this invention which includes a turn table  11 , a polishing pad  12  provided over the upper surf ace of the turn table  11 , a carrier  13  for carrying an article such as a semiconductor wafer  14  to be polished, and a shaft-like support  20  pivotably connected to the carrier  13  by means of a universal joint  21 . The carrier is in the form of a circular disc having a recess for receiving and carrying the article  14 . The support  20  is connected to the center of the upper surface of the carrier  13 . The support  20  is drivingly connected to a motor (not shown) to be rotated around its axis. The universal joint  21  is adapted to transmit a rotational torque applied to the support by the motor to the carrier  13  to rotate the carrier  13  around its axis while allowing the carrier  13  to pivot about the universal joint  21 . 
     In this embodiment, the polishing apparatus further includes a pressing device which comprises a circular press cover  17  provided on the carrier  13  through bearings  15 ,  16  which enable the carrier  13  to rotate about its axis independently of the circular press cover  17  and four piston-cylinder type pushers Ax 1 , Ay 1 , Ax 2 , Ay 2  arranged along the circular periphery of the press cover  17  at the three o&#39;clock, six o&#39;clock, nine o&#39;clock and twelve o&#39;clock positions on the cover  17  as viewed in FIG. 1, respectively. Each of the pushers Ax 1 , Ay 1 , Ax 2 , Ay 2  includes a cylinder  18  fixed to a stationary frame and a piston-rod  19  the lower end of which is securely connected to the upper surface of the carrier  17 . The piston-rod  19  is adapted to be extruded from the cylinder  18  with a pressure imposed on the piston installed in the cylinder  18  by a fluid supplied into the cylinder  18 . The pressing device further includes pressure sensors Fx and Fy which are fixed on a stationary frame in such a manner that the sensors are nearly in contact with the outer circumferential surface of the press cover  17  at the three o&#39;clock and six o&#39;clock positions of the press cover as viewed in FIG. 1, respectively. The pushers Ax 1 , Ay 1 , Ax 2 , Ay 2  and the support  20  are arranged so that they allow the carrier  13  with the article  14  and the press cover  17  to move slightly horizontally so that when turn table is rotated, the carrier  13  and the press cover  17  are moved towards and press the sensors Fx and Fy. The pressure sensors Fx and Fy include rotatable balls at the tip ends thereof as contact elements with the outer periphery of the press cover  17  to enable the pressure sensors Fx and Fy to sense only horizontal forces transmitted from the press cover  17  in the directions of the lines X—X and X-Y in FIG. 1, respectively, without any angular moments. 
     Furthermore, the pressing device includes a control unit or arithmetic and control unit  22  which receives signals designating values of the pressures or the horizontal forces sensed by the sensors Fx and Fy to determine or compute values Fx 1 , Fy 1 , Fx 2  and Fy 2  of pressures which, as explained below, are imposed on the piston-rods  19  of the pushers Ax 1 , Ay 1 , Ax 2 , Ay 2 , respectively, and thus on the press cover  17 . 
     In operation, upon rotation of the turn table  11  in the direction as designated by an arrow v in FIG. 1, the carrier  13  and the press cover  17  are moved towards and urged against the pressure sensors Fx and Fy by a friction force f generated between the article  14  and the flat surface  12 ′ of the polishing pad  12 . Then, the sensors Fx and Fy sense pressures imposed thereon by the press cover  17  which are X-directional and Y-directional components of the friction force f. The friction force f acts on the article to thereby tilt or pivot the article  14 , the carrier  13  and the press cover  17  as a unit about the universal joint  21  as shown in FIG.  3 . 
     The sensors Fx and Fy sense the components of the friction force f as pressures and transmit signals representing magnitudes of those components to the control unit  22 . As shown in FIG. 4, the control unit determines on the basis of the signals a direction θ of the friction force f and an angular moment M caused by the frictional force f which is calculated from a formula, i.e., M=fh in which h means the distance between the article surface contacting with the polishing pad  12  and the pivotal center A of the universal joint  21  ( 22 - 1 ). Further, the control unit  22  determines a pressure gradient Δp/D in the direction of the friction force f across the article surface contacting with the polishing pad  12  on the basis of the direction θ of the friction force f and the angular moment M ( 22 - 2 ). In the above, “Δp” is a difference between the maximum and minimum pressures imposed on the article from the polishing surface in contact with the article surface and “D” is a diameter of the article. Finally, the control unit  22  determines pressures Fx 1 , Fx 2 , Fy 1 ,Fy 2  ( 22 - 3 ;  22 - 3 ) which the pushers Ax 1 , Ay 1 , Ax 2 , Ay 2  should impose on the press cover  17  to compensate the angular moment thereby making the press cover  17 , the carrier  13  and thus the article surface to be horizontal. This makes it possible for the article surface to be uniformly polished. 
     FIG. 5 shows a polishing apparatus in accordance with the second embodiment of this invention. The polishing apparatus does not include a press cover as in the first embodiment. Instead, the lower ends of the piston-rods  19  of the pushers are provided with rollers  23  which are rotatably engaged with the upper surface of the carrier  13  so that the carrier can rotate independently of the pushers. 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example and were herein described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.