Abstract:
A method and apparatus for determining the presence of foreign material on a substrate chuck. The method includes: placing a bottom surface of a substrate on a top surface of the substrate chuck; applying a lateral force in a direction parallel to a top surface of the substrate chuck to the substrate; when the substrate moves partially off or moves completely off the substrate chuck in response to the applying the lateral force then a defect is present between the top surface of the substrate chuck and the bottom surface of the substrate; or when the substrate remains completely on the substrate chuck in response to the applying the lateral force then a defect is not present between the top surface of the substrate chuck and the bottom surface of the substrate.

Description:
BACKGROUND 
       [0001]    The present invention relates to the field of integrated circuit manufacture; more specifically, it relates to an apparatus for detecting foreign material under a resilient film adhesively attached to a substrate chuck and method of using the apparatus. 
         [0002]    The adhesive backed film on the surface of substrate chucks must be removed and replaced periodically. During the replacement process foreign material can become lodged between the surface of the chuck and the adhesive layer causing a bump in the film which will cause the substrate to be held improperly by the chuck causing misprocessing or damage to the substrate and/or devices (e.g. integrated circuit chips) being fabricated on the substrate. Currently no repeatable method exists for detecting foreign material lodged between the surface of the chuck and the adhesive layer. Accordingly, there exists a need in the art to eliminate the deficiencies described hereinabove. 
       BRIEF SUMMARY 
       [0003]    A first aspect of the present invention is a method for determining the presence of foreign material on a substrate chuck, comprising: placing a bottom surface of a substrate on a top surface of the substrate chuck; applying a lateral force in a direction parallel to a top surface of the substrate chuck to the substrate; when the substrate moves partially off or moves completely off the substrate chuck in response to the applying the lateral force then a defect is present between the top surface of the substrate chuck and the bottom surface of the substrate; or when the substrate remains completely on the substrate chuck in response to the applying the lateral force then a defect is not present between the top surface of the substrate chuck and the bottom surface of the substrate. 
         [0004]    A second aspect of the present invention is an apparatus for determining the presence of foreign material on a substrate chuck, comprising: the substrate chuck configured to hold a substrate; means for applying a lateral force in a direction parallel to a top surface of the substrate chuck to the substrate when the substrate is on the substrate chuck; and means for applying a downward force in a direction perpendicular to a top surface of the substrate chuck to the substrate when the substrate is on the substrate chuck. 
         [0005]    These and other aspects of the invention are described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
           [0007]      FIG. 1A  is a cross section through line  1 A- 1 A of the top view of  FIG. 1B  of an exemplary substrate chuck; 
           [0008]      FIG. 2  is a side view of an apparatus for detecting foreign material on the substrate chuck of  FIGS. 1A  and lB according to an embodiment of the present invention; 
           [0009]      FIG. 3  is a top view of an apparatus of  FIG. 2  according to an embodiment of the present invention; 
           [0010]      FIGS. 4 and 5  are cross-section views illustrating the effect of the apparatus of 
           [0011]      FIGS. 2 and 3  on the substrate chuck of  FIGS. 1A and 1B  with no foreign material according to an embodiment of the present invention; 
           [0012]      FIGS. 6 and 7  are cross-section views illustrating the effect of the apparatus of 
           [0013]      FIGS. 2 and 3  on the substrate chuck of  FIGS. 1A and 1B  with foreign material according to an embodiment of the present invention; 
           [0014]      FIGS. 8A, 8B, 8C and 8D  illustrate alternative locations of foreign material; and 
           [0015]      FIG. 9  is a flowchart of the method of using the apparatus of  FIGS. 2 and 3  according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Chemical mechanical polishing (CMP) requires that substrates (e.g., circular semiconductor wafers) be held on a chuck while the wafer is positioned frontside down on the CMP polishing pad. Vacuum, air or water may be applied though the chuck to the backside of the substrate during the CMP process. To protect the wafer and avoid direct contact between the wafer and chuck a pad including an adhesive backed film is applied to the top surface of the chuck (so as to lie between the top surface of the chuck and the backside of the wafer). If foreign material (air or liquid bubbles between the chuck and adhesive backed film or in the adhesive backed film are also defined as foreign material) becomes lodged between the top surface of the chuck and the adhesive layer a bump will be formed in the film over the foreign material. The bump will cause the wafer not to lie flat on the film resulting in the wafer not being held firmly and inconsistent polishing across the surface of the wafer when the chuck is used in a CMP tool. 
         [0017]    The apparatus according to the embodiments of the present invention detects the presence of a bump in the film by taking advantage of the fact that a test wafer will not lie flat on the chuck but be slightly tilted if foreign material is present under the film or foreign material is present on the surface of the film. A probe tip provides a lateral force to the surface of a test wafer when the probe tip is dragged across the top surface of the test wafer. If foreign material is present the wafer will not sit flat on the film and the wafer will be slid off the chuck as the probe tip is moved. If there is no foreign material present the wafer will sit flat on the film and the wafer will not be slid off the chuck as the probe tip is moved. The probe tip provides frictional contact to the surface of the test wafer and is attached to a spring that presses the probe tip against the wafer surface. The spring and probe tip are dragged along the surface of the test wafer by a rotatable swing arm. 
         [0018]    The apparatus will detect anything that causes the test wafer not to sit flat on the chuck including foreign material between the adhesive and film and foreign material in the adhesive film. The apparatus will detect foreign material on chucks that do not use a resilient film. e.g., chucks where the surface of the chuck contacts the substrate directly. However, while foreign material on the film surface can be detected visually it is more difficult to visually detect foreign material or bubbles under or in the adhesive film. The embodiments of the present invention are not limited to semiconductor wafers but may be adapted to substrate of other materials and shapes. 
         [0019]      FIG. 1A  is a cross section through line  1 A- 1 A of the top view of  FIG. 1B  of an exemplary substrate chuck. In  FIGS. 1A and 1B , a circular substrate chuck  100  includes a chuck body  105  having a fixed ring  110  around the periphery of the chuck and locating notches  115  in a bottom surface  117  of the chuck. Chuck body  105  is provided with channels  120  connected to ports  125  open to a top surface  127  of chuck  100 . Channel  120  is also connected to a backside port  130  in bottom surface  117 . A pad  135  including a film  140  and an adhesive layer  145  is illustrated attached (by the adhesive layer) to top surface  127  of chuck  100 . Holes  150  in pad  135  align with ports  125  in chuck  100 . Holes  150  pass completely through film  140  and adhesive layer  145 . In one example, film  140  is formed from plastic, elastomer or other polymer. In one example, film  140  is resilient. A wafer  155  (dotted lines) is illustrated in place on pad  135 . Wafer  155  is centered on chuck  100 . 
         [0020]      FIG. 2  is a side view of an apparatus for detecting foreign material on the substrate chuck of FIGs. lA and lB according to an embodiment of the present invention. In  FIG. 2 , an apparatus  200  is holding wafer chuck  100  which in turn is holding wafer  155 . Apparatus  200  includes a base  205  having a top  210  surface  210 , a turntable  215  attached to base  205 , a vertical post  220  having a lower end  225  mounted to base  205  by a flange  230  and an upper end  235  bored to accept a first end  240  of a horizontal swing arm  245 . Top surface  210  defines a horizontal reference plane with horizontal directions being parallel to the reference plane and vertical directions and rotational axes being perpendicular to the reference plane. Turntable  215  is rotatable 360° about a vertical axis  247  passing through the center of turntable  215  and chuck  100 . Post  220  is rotatable at least 90° about a vertical axis  248 . A second end  250  of arm is fixed to a slide  255 . Slide  255  is bored to accept a vertical rod  260 . The distance L between post  220  and rod  260  is adjustable by sliding swing arm in or out of the bore in upper end  235  of post  220  and is held in position by screw  265 . The height H of swing arm  245  above top surface  267  of wafer  155  is adjustable by sliding rod  260  up and down in slide  255  and is held in position by screw  270  to prevent up and down and rotational movement of rod  260 . A lower end  275  of rod  260  is attached by to an upper end  280  of a spring  285 . A lower end  290  of spring  285  is attached by connector  295  to a probe tip  300 . The height H is adjusted to spring  285  flexes to push probe tip  300  against top surface  167  of wafer  155 . The amount of force applied by spring  285  is set by adjusting the height H and the spring constant of spring  285 . Probe tip  300  provides a frictional interface to wafer  155 . Probe tip  300  applies a lateral (horizontal) force to wafer  155  when swing arm  245  is swept through a circular arc by rotating post  220 . In one example, probe tip  300  is a sphere. In one example, probe tip  300  is formed from plastic, elastomer or other polymer. In one example, probe tip is resilient. In one example, probe tip is formed from silicone. In one example, chuck  100  is temporally held in position on turntable  215  by locating pins  305 . The combination of swing arm  245 , rod  260 , spring  285  and probe tip  300  apply both lateral and downward forces to wafer  155 . 
         [0021]      FIG. 3  is a top view of an apparatus of  FIG. 2  according to an embodiment of the present invention. In  FIG. 3 , it can be seen that swing arm  245  can rotate about axis  248  so as to swing rod  260  in and arc  310  of length L over chuck  100 . Swing arm  245  is illustrated in a home position. In the example of  FIG. 3 , length L has been adjusted so a center of  315  of rod  260  passes directly over or proximate to axis  247 . Alternatively, the length L can be adjusted so probe tip  300  (see  FIG. 2 ) passes directly over or proximate to axis  247 . The top  210  of table  205  has indicia  315 . Turntable  100  includes two marks  320  and  325  located 90° apart. In use, a wafer is placed on pad  135  and the turntable rotated by a human operator so mark  320  aligns to indicia  315 . Swing arm  245  is in a home position. While holding turntable  100  so it can not rotate, the operator moves swing arm  245  through an arc so probe tip  300  (see  FIG. 2 ) is dragged onto the wafer surface, dragged across the wafer surface and off the wafer past the edge of the wafer. If the wafer is still in position then the swing arm is moved back across the surface in the opposite direction to its home position. If the wafer is still in place on turntable  100  the operator rotates chuck  100  so indicia  315  is aligned with mark  325  and again holding turntable  100  so it can not rotate moves swing arm  245  through an arc so probe tip  300  (see  FIG. 2 ) is dragged onto the wafer surface, dragged across the wafer and dragged off the wafer past the edge of the wafer back to a home position. If the wafer is still in position then the swing arm is again moved back across the surface in the opposite direction to its home position. In total, the probe tip crosses the wafer surface four times, twice each for each of mark to indicia alignment. 
         [0022]      FIGS. 4 and 5  are cross-section views illustrating the effect of the apparatus of  FIGS. 2 and 3  on the substrate chuck of  FIGS. 1A and 1B  with no foreign material according to an embodiment of the present invention. In  FIG. 4 , the entire bottom surface  330  of wafer  330  sits flat on the top surface  335  of film  140 . As probe tip  300  is started to be dragged across the top surface  340  of wafer  155 . In  FIG. 5 , probe tip  300  has been dragged to the opposite side of wafer  155  and wafer  155  remains in place on chuck  100  because a top surface  345  of rim  110  extends above top surface  335  of film  140  more than bottom surface  330  of wafer  155 . The height of ring  110  above the bottom surface  330  of wafer  155  prevents lateral (horizontal) displace of wafer  155  off the chuck. 
         [0023]      FIGS. 6 and 7  are cross-section views illustrating the effect of the apparatus of  FIGS. 2 and 3  on the substrate chuck of  FIGS. 1A and 1B  with foreign material according to an embodiment of the present invention. In  FIG. 6 , the entire bottom surface  330  of wafer  330  does not sit flat on the top surface  335  of film  140  because of foreign material  350  between the top surface  127  of body  105  of chuck  100  and adhesive layer  145 . As probe tip  300  is started to be dragged across the top surface  340  of wafer  155  a gap  355  is created (if it does not already exist just due to the presence of foreign material  350 ) on the side of the wafer opposite from probe tip  300 . In  FIG. 7 , probe tip  300  has been dragged further onto wafer  155  and wafer  155  has been dragged by probe tip  300  so the opposite side of wafer  155  from probe tip  300  is coming off chuck  100 . Ring  110  is not sufficiently high above the bottom surface  330  of wafer  155  relative to top surface  127  to prevent lateral (horizontal) displacement of wafer  155  off chuck  100 . Depending upon the size and location of foreign material  350 , probe tip  300  may have to be dragged further onto wafer  155  then illustrated in  FIG. 7  before wafer  155  starts to slide off chuck  100 . Wafer  155  may be slid entirely or only partially off chuck  100  after a complete swing of swing arm  245  (see  FIG. 2  and description supra is completed). Any movement of wafer  155  off chuck  105  or even onto ring  110  is indicative of a failure of the chuck  100 /pad  135  assembly. 
         [0024]      FIGS. 8A, 8B, 8C and 8D  illustrate alternative locations of foreign material. In  FIG. 8A , foreign material  350  is located on the top surface of film  350  causing wafer  155  to tilt relative to the top surface of body  350 . In  FIG. 8B , foreign material  350  is located in film  350  causing wafer  155  to tilt relative to the top surface of body  350 . In  FIG. 8C , foreign material  350  is located in adhesive layer  145  causing wafer  155  to tilt relative to the top surface of body  350 . In  FIG. 8D , foreign material  350  is located on the top surface of body  350  (there is no adhesive film present) causing wafer  155  to tilt relative to the top surface of body  350 . 
         [0025]      FIG. 9  is a flowchart of the method of using the apparatus of  FIGS. 2 and 3  according to an embodiment of the present invention. In step  400 , a film is attached to the surface of a cleaned chuck by an adhesive layer. In step  405 , a test substrate (e.g. semiconductor wafer) is placed on the film on the chuck. In step  410 , a lateral force is applied across the surface of the wafer from a home position in a first direction. Optionally a lateral force may be then applied in a direction opposite to the first direction. If in step  415  the wafer slides completely or partially off the chuck the method proceeds to step  420 . In step  420 , the film and adhesive is removed from the chuck and the chuck cleaned and the method loops back to step  400 . Returning to step  415 , if in step  415  the wafer remains completely on the chuck the method proceeds to step  425 . In step  425 , a lateral force is applied across the surface of the wafer in a second direction different from the first direction. In one example, the angle between the first and second direction is between 45 degrees and 135 degrees. Optionally a lateral force may be then applied in a direction opposite to the second direction. The change from the first direction to the second direction is accomplished by rotating the turntable the chuck is mounted on. If in step  430  the wafer slides completely or partially off the chuck the method proceeds to step  420 . If in step  430  the wafer remains completely on the chuck the method proceeds to step  435 . In step  435  the chuck is released for production. While two tests (steps  410 / 415  and  425 / 430 ) are illustrated in  FIG. 8 , there may be only one test or there may be more than two tests and the direction of the test for multiple tests may be the same or different. The same test may also be repeated. 
         [0026]    Therefore, the embodiments of the present invention provide an apparatus and method for detecting foreign material on a chuck and particularly for detecting foreign material lodged between the surface of the substrate chuck and an adhesive layer attaching a film to the chuck. 
         [0027]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.