Abstract:
A polishing apparatus capable of correcting an inclination of a polishing head is disclosed. The polishing apparatus includes: a polishing table configured to support a polishing pad thereon; a polishing head configured to press a substrate against the polishing pad; a rotational shaft coupled to the polishing head; a self-aligning rolling bearing that tiltably supports the rotational shaft; a radial rolling bearing that receives a radial load of the rotational shaft; a detector configured to detect an inclination of the rotational shaft; and an inclination adjusting device configured to adjust the inclination of the rotational shaft.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This document claims priority to Japanese Patent Application Number 2014-112479 filed May 30, 2014, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]      FIG. 7  is a schematic perspective view showing a chemical mechanical polishing (CMP) apparatus. As shown in  FIG. 7 , the CMP apparatus is configured to supply slurry from a nozzle  102  onto a polishing pad  101  mounted on a rotating polishing table  100 , while pressing a wafer W against the polishing pad  101  to thereby polish a surface of the wafer W. The wafer W is rotated by a polishing head  105  while being pressed against a polishing surface  101   a  of the polishing pad  101 . The surface of the wafer W is polished by a combination of a chemical action of the slurry and a mechanical action of abrasive grains contained in the slurry. 
         [0003]    During polishing of the wafer W, as shown in  FIG. 8 , flow of the slurry is formed between the wafer W and the polishing pad  101  due to a pumping effect of the wafer W, because the wafer W is being rotated by the polishing head  105 . Such flow of the slurry affects a pressure of the slurry applied to the surface of the wafer W. When a table surface of the polishing table  100  and a wafer holding surface of the polishing head  105  are parallel to each other, a pressure distribution of the slurry is concentric with the center O of the wafer W as shown in  FIG. 9 . 
         [0004]    However, when the polishing head  105  tilts, the flow of the slurry changes, resulting in a change in the pressure distribution of the slurry as shown in  FIG. 10 . If the center of the pressure distribution deviates from the center O of the wafer W, the pressure of the slurry acting on the surface of the wafer W becomes uneven, thus causing uneven polishing rate of the wafer W. 
       SUMMARY OF THE INVENTION 
       [0005]    According to an embodiment, there is provided a polishing apparatus capable of correcting an inclination of a polishing head. 
         [0006]    Embodiments, which will be described below, relate to a polishing apparatus for polishing a substrate, such as a wafer, and more particularly to a polishing apparatus having a mechanism for adjusting an inclination of a polishing head that is to press the substrate against a polishing surface. 
         [0007]    In an embodiment, there is provided a polishing apparatus comprising: a polishing table configured to support a polishing pad thereon; a polishing head configured to press a substrate against the polishing pad; a rotational shaft coupled to the polishing head; a self-aligning rolling bearing that tiltably supports the rotational shaft; a radial rolling bearing that receives a radial load of the rotational shaft; a detector configured to detect an inclination of the rotational shaft; and an inclination adjusting device configured to adjust the inclination of the rotational shaft. 
         [0008]    In an embodiment, the polishing apparatus further comprises a controller configured to operate the inclination adjusting device based on the inclination of the rotational shaft detected by the detector. 
         [0009]    In an embodiment, the controller is configured to emit an alarm signal if the inclination of the rotational shaft does not fall within a predetermined range. 
         [0010]    In an embodiment, the self-aligning rolling bearing is located between the polishing head and the radial rolling bearing, and the inclination adjusting device is coupled to the radial rolling bearing. 
         [0011]    In an embodiment, the radial rolling bearing comprises a self-aligning rolling bearing. 
         [0012]    In an embodiment, the radial rolling bearing comprises a combination of angular contact ball bearings. 
         [0013]    Since the rotational shaft is supported by the self-aligning rolling bearing, the rotational shaft can tilt. Therefore, the inclination adjusting device can adjust the inclination of the rotational shaft to make the rotational shaft perpendicular to a table surface (i.e., a surface on which the polishing pad is supported) of the polishing table. As a result, a substrate, held by the polishing head, becomes parallel to the polishing surface of the polishing pad on the polishing table, and a pressure distribution of the polishing liquid (or slurry) becomes concentric with the center of the substrate. In particular, the controller operates the inclination adjusting device based on the inclination of the rotational shaft detected by the detector, so that the inclination of the rotational shaft can be adjusted automatically during polishing of the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic view of a polishing apparatus according to an embodiment; 
           [0015]      FIG. 2  shows a plan view showing inclination adjusting devices, a bearing housing, and a radial rolling bearing; 
           [0016]      FIG. 3  is a view showing a tilt sensor mounted on a table surface of a polishing table; 
           [0017]      FIG. 4  is a schematic view of the polishing apparatus according to another embodiment; 
           [0018]      FIG. 5  is a schematic view of the polishing apparatus according to still another embodiment; 
           [0019]      FIG. 6  is a schematic view of the polishing apparatus according to still another embodiment; 
           [0020]      FIG. 7  is a perspective view showing a typical chemical mechanical polishing apparatus; 
           [0021]      FIG. 8  a diagram showing flow of slurry formed between a wafer and a polishing pad; 
           [0022]      FIG. 9  a schematic diagram showing a pressure distribution of the slurry on the wafer; and 
           [0023]      FIG. 10  a schematic diagram showing a pressure distribution of the slurry on the wafer. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    Embodiments will be described below with reference to the drawings. 
         [0025]      FIG. 1  is a schematic view of a polishing apparatus according to an embodiment. As shown in  FIG. 1 , the polishing apparatus includes a rotatable polishing table  3  for supporting a polishing pad  1  thereon, a polishing head (or a substrate holder)  5  configured to hold a wafer W, which is an example of a substrate, and press the wafer W against the polishing pad  1 , and a polishing liquid supply nozzle  7  configured to supply a polishing liquid (e.g., slurry) onto the polishing pad  1 . The polishing table  3  has an upper surface that constitutes a table surface  3   a  to which the polishing pad  1  is attached. An upper surface of the polishing pad  1  provides a polishing surface  1   a  for polishing the wafer W. 
         [0026]    The polishing head  5  is secured to a lower end of a rotational shaft  10 , which is coupled to a motor  20  through pulleys  12 ,  13  and a belt  16 . The motor  20  is secured to a head arm  22 . When the motor  20  is set in motion, the rotational shaft  10  and the polishing head  5  are rotated about their own axes. 
         [0027]    Polishing of the wafer W is performed as follows. The polishing table  3  and the polishing head  5  are rotated about their own axes, while the polishing liquid (or slurry) is supplied from the polishing liquid supply nozzle  7  onto the polishing surface  1   a  of the polishing pad  1  on the polishing table  3 . The polishing head  5 , while rotating the wafer W, presses a surface of the wafer W against the polishing surface  1   a  of the polishing pad  1 . The surface of the wafer W is polished by a combination of a chemical action of the polishing liquid and a mechanical action of abrasive grains contained in the polishing liquid. 
         [0028]    The rotational shaft  10  is rotatably supported by a self-aligning rolling bearing  31  and a radial rolling bearing  41 . The self-aligning rolling bearing  31  includes an outer race that has a spherical inner circumferential surface (not shown). A center of curvature of the inner circumferential surface coincides with a center of the self-aligning rolling bearing  31 . The self-aligning rolling bearing  31  includes rolling elements, such as balls or rollers, which are in rolling contact with the inner circumferential surface of the outer race. Therefore, the self-aligning rolling bearing  31  can support the rotational shaft  10  while allowing the rotational shaft  10  to tilt. Examples of the self-aligning rolling bearing  31  include a self-aligning ball bearing and a self-aligning roller bearing. 
         [0029]    The radial rolling bearing  41  is a bearing that can carry a radial load of the rotational shaft  10 . Examples of the radial rolling bearing  41  include a radial ball bearing, an angular contact ball bearing, a self-aligning ball bearing, a radial roller bearing, and a self-aligning roller bearing. The self-aligning rolling bearing  31  and the radial rolling bearing  41  used in this embodiment may be those available on the market. 
         [0030]    The polishing head  5  is secured to the lower end of the rotational shaft  10  by fastening tool (not shown), such as screw or bolt. Therefore, the polishing head  5  is rotatable together with the rotational shaft  10  and is tiltable together with the rotational shaft  10 . As described above, the self-aligning rolling bearing  31  is configured to be able to rotatably support the rotational shaft  10 , while allowing the rotational shaft  10  to tilt. Accordingly, the polishing head  5  and the rotational shaft  10  can tilt around the center of the self-aligning rolling bearing  31 . 
         [0031]    The self-aligning rolling bearing  31  is held by a bearing housing  33  which is in a cylindrical shape. This bearing housing  33  is secured to a head arm  22 . The radial rolling bearing  41  is held by a bearing housing  43  which is in a ring shape. This bearing housing  43  is held by inclination adjusting devices  51  each configured to adjust the inclination of the rotational shaft  10 . 
         [0032]      FIG. 2  shows a plan view showing the inclination adjusting devices  51 , the bearing housing  43 , and the radial rolling bearing  41 . As shown in  FIG. 2 , the inclination adjusting devices  51  are arranged around the rotational shaft  10  at regular intervals. These inclination adjusting devices  51  are secured to the head arm  22  and are further secured to an outer circumferential surface of the bearing housing  43 . Therefore, the inclination adjusting devices  51  are coupled to the radial rolling bearing  41  through the bearing housing  43 . Each inclination adjusting device  51  is configured to push the rotational shaft  10  in a horizontal direction (or a radial direction) through the bearing housing  43  and the radial rolling bearing  41 . The inclination adjusting device  51  may be a combination of a ball screw and a servomotor, a piezoelectric device, or a hydraulic cylinder. 
         [0033]    A tilt sensor  54  is mounted to the rotational shaft  10 . This tilt sensor  54  is a tilt detector for detecting the inclination of the rotational shaft  10 . The polishing apparatus further includes a controller  58  configured to operate the inclination adjusting devices  51  based on an angle and a direction of the inclination of the rotational shaft  10  detected by the tilt sensor  54 . This controller  58  is coupled to the tilt sensor  54  and the inclination adjusting devices  51 . 
         [0034]    When the polishing table  3  is installed, a tilt sensor  56  is mounted on the table surface  3   a  of the polishing table  3  with no polishing pad  1  attached to the polishing table  3  as shown in  FIG. 3 . The polishing table  3  is installed in such a state that the table surface  3   a  is horizontal. Whether the table surface  3   a  is horizontal or not can be detected by the tilt sensor  56  on the table surface  3   a.    
         [0035]    The controller  58  operates (or manipulates) the inclination adjusting devices  51  such that a longitudinal direction (or a central axis) of the rotational shaft  10  is in a vertical direction. More specifically, based on the angle and the direction of the inclination of the rotational shaft  10  detected by the tilt sensor  54 , the controller  58  causes the inclination adjusting devices  51  to push the rotational shaft  10  in the horizontal direction (or in the radial direction) until the rotational shaft  10  becomes in the vertical position. 
         [0036]    The above-described operations of the inclination adjusting devices  51  can keep the rotational shaft  10  perpendicular to the table surface  3   a  of the polishing table  3 . A wafer holding surface (or substrate holding surface) of the polishing head  5  is kept parallel to the polishing surface  1   a  of the polishing pad  1  attached to the table surface  3   a . As a result, the pressure distribution of the polishing liquid existing between the wafer W and the polishing pad  1  becomes concentric with the wafer W. The controller  58  may operate the inclination adjusting devices  51  during polishing of the wafer W or before polishing of the wafer W. The controller  58  is configured to emit an alarm signal if the angle of the inclination of the rotational shaft  10  does not fall within a predetermined range. 
         [0037]    The radial rolling bearing  41  is located above the self-aligning rolling bearing  31 , while the polishing head  5  is located below the self-aligning rolling bearing  31 . In other words, the self-aligning rolling bearing  31  is located between the radial rolling bearing  41  and the polishing head  5 . During polishing of the wafer W, the polishing head  5  receives a horizontal load that is generated due to a friction between the wafer W and the polishing pad  1 . Most part of this horizontal load is received by the self-aligning rolling bearing  31 . Therefore, a radial load applied to the radial rolling bearing  41  is smaller than a radial load applied to the self-aligning rolling bearing  31 . Each of the inclination adjusting devices  5 , which are coupled to the radial rolling bearing  41 , can tilt the rotational shaft  10  with a relatively small force. 
         [0038]      FIG. 4  is a schematic view of the polishing apparatus according to another embodiment. Structures in this embodiment, which are the same as those in the embodiment shown in  FIG. 1 , will not be described particularly and repetitive descriptions thereof are omitted. As shown in  FIG. 4 , a self-aligning rolling bearing is used as the radial rolling bearing  41 . This radial rolling bearing  41 , which is the self-aligning rolling bearing, has the same structure as the self-aligning rolling bearing  31 . The radial rolling bearing  41  is held by a bearing housing  60  which is in a cylindrical shape. This bearing housing  60  is secured to the head arm  22 . Each of the inclination adjusting devices  51  is configured to push the rotational shaft  10  in the horizontal direction through the bearing housing  60  and the radial rolling bearing  41  to thereby adjust the attitude of the rotational shaft  10 . 
         [0039]      FIG. 5  is a schematic view of the polishing apparatus according to still another embodiment. Structures in this embodiment, which are the same as those in the embodiment shown in  FIG. 1 , will not be described particularly and repetitive descriptions thereof are omitted. As shown in  FIG. 5 , a combination of angular contact ball bearings is used as the radial rolling bearing  41 . In this embodiment shown in  FIG. 5 , a combination of two angular contact ball bearings is used. The radial rolling bearing  41  that is constituted by the angular contact ball bearings is held by a bearing housing  65  which is in a cylindrical shape. 
         [0040]    The bearing housing  65  is loosely inserted in a hole  67  formed in the head arm  22 , so that the bearing housing  65  can tilt with respect to the head arm  22 . The bearing housing  65  has a flange  65   a . The inclination adjusting devices  51  are disposed between a horizontal surface (a lower surface) of the flange  65   a  and a horizontal surface (an upper surface) of the head arm  22 . The inclination adjusting devices  51  are configured to push the flange  65   a  in the vertical direction (i.e., in the axial direction) to thereby tilt the entirety of the bearing housing  65 , thus tilting the radial rolling bearing  41 , the rotational shaft  10 , and the polishing head  5 . The tilt sensor  54  is mounted to the bearing housing  65 . This embodiment is advantageous in a case where there is a small installation space in the radial direction for the inclination adjusting devices  51 . 
         [0041]      FIG. 6  is a schematic view of the polishing apparatus according to still another embodiment. Structures in this embodiment, which are the same as those in the embodiment shown in  FIG. 5 , will not be described particularly and repetitive descriptions thereof are omitted. The inclination adjusting devices  51  are disposed between a horizontal surface (an upper surface) of the flange  65   a  and a horizontal surface (a lower surface) of the head arm  22 . The inclination adjusting devices  51  are configured to push the flange  65   a  in the vertical direction (i.e., in the axial direction) to thereby tilt the entirety of the bearing housing  65 , thus tilting the radial rolling bearing  41 , the rotational shaft  10 , and the polishing head  5 . 
         [0042]    The polishing apparatus according to the above-discussed embodiments can, during polishing of the wafer W, keep the polishing head  5  parallel to the polishing surface  1   a  of the polishing pad  1 , and can further reduce a variation between polished wafers. The controller  58  may preferably emit an alarm signal if the inclination of the rotational shaft  10  does not fall within a predetermined range during polishing of the wafer W. 
         [0043]    The previous description of embodiments is provided to enable a person skilled in the art to make and use the present invention. Moreover, various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope as defined by limitation of the claims.