Abstract:
A measurement system installable within a processing equipment and more specifically within the exit station of a polishing machine. The optical scheme of this system includes a spectrophotometric channel, an imaging channel and also means for holding the wafer under measurement.

Description:
CROSS-REFERENCE OT RELATED APPLICATION 
       [0001]    This application is a division of application Ser. No. 11/698,878, filed Jan. 29, 2007, which is itself a continuation of application Ser. No. 10/860,019, filed Jun. 4, 2004, now allowed, itself a continuation of application Ser. No. 09/898,467, filed on Jul. 5, 2001, now U.S. Pat. No. 6,752,689, which is a continuation of application Ser. No. 09/498,926 filed on Feb. 4, 2000, now U.S. Pat. No. 6,368,181, the disclosures of all of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to wafer polishing apparatus in general and to measuring systems incorporated into such apparatus in particular. 
       BACKGROUND OF THE INVENTION 
       [0003]    Wafer polishing systems are known in the art. They polish the top layer of semi-conductor wafers to a desired thickness. To do so, the wafer being polished is immersed in a slurry of water and chemicals during the polishing process. Once the wafer has been polished and washed down, it is placed into an exit station known by some companies as a “water track”, after which the wafer is placed into a cassette of wafers. The cassette is maintained within a water bath until full, after which the entire cassette is brought to a cleaning station to remove any chemicals and slurry particles still remaining on the wafers in the cassette and to dry the wafers. After cleaning, the wafers are brought to a measurement station to determine if the polisher produced the desired thickness of their top layers. 
         [0004]      FIG. 1 , to which reference is now briefly made, illustrates a prior art water track, such as the water track of the #372 Polisher manufactured by IPEC Westech Inc. of Phoenix, Ariz., USA. The water track, labeled  10 , comprises a frame  12  and a base  14 . Frame  12  has jet holes  16  connected to jets (not shown) which emit streams  18  of water through holes  16 . Base  14  has holes  20  connected to bubblers (not shown) which bubble small amounts of water  22  through holes  20 . When a wafer  25  is dropped into water track  10 , pattern-side down, the jets and bubblers are activated. Streams  18 , from the water jets, serve to force the wafer  25  in the direction indicated by arrow  24 . Small streams  22  push the wafer  25  slightly away from the base  14  and ensure that, while the wafer  25  moves through the track, it never rubs against base  14  and thus, the pattern on the wafer is not scratched. 
         [0005]    Other companies produce polishers whose exit stations are formed just of the cassettes. Such a polisher is in the 6DS-SP polisher of R. Howard Strasbaugh Inc. San Luis Obispo, Calif., USA. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to provide a measurement system installable within a polishing machine and, more specifically, within the exit station of a polishing machine. 
         [0007]    In accordance with a preferred embodiment of the present invention, the present invention includes an optical system, which views the wafer through a window in the exit station, and a gripping system, which places the wafer in a predetermined viewing location within the exit station while maintaining the patterned surface completely under water. The present invention also includes a pull-down unit for pulling the measurement system slightly below the horizontal prior to the measurement and returns the measuring system to horizontal afterwards. 
         [0008]    In accordance with a first preferred embodiment of the present invention, the gripping system includes a raisable gate which collects the wafer in a predetermined location, and a gripper which grips the wafer, carries it to the viewing location and immerses the wafer, along a small angle to the horizontal, in the water. The gripper also holds the wafer in place during the measurement operation, after which, it releases the wafer and the raisable gate is raised 
         [0009]    The present invention incorporates the method of immersing an object into water such that very few bubbles are produced on the wafer surface. The method of the present invention preferably includes the step of immersing the object while it is held such that its surface plane is at a small angle to the horizontal. 
         [0010]    In a second embodiment, the measurement system includes a water bath and a gripping system thereabove. The gripping system includes wafer holding elements, which receive the wafer, and a gripper whose initial location is above the expected reception location of the wafer. The gripper is flexibly connected at an angle to a piston such that the wafer is immersed in the water at an angle to the horizontal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
           [0012]      FIG. 1  is a schematic illustration of a prior art water track; 
           [0013]      FIG. 2  is a schematic illustration of a measurement system installable within a polishing machine, the measurement system being constructed and operative in accordance with a preferred embodiment of the present invention; 
           [0014]      FIGS. 3 ,  4 ,  5 ,  6 ,  7  and  8  are schematic, side view illustrations of a gripping system forming part of the measurement system of  FIG. 2  in various stages of operation; 
           [0015]      FIG. 9  is a schematic illustration of an example optical system forming part of the measurement system of the present invention; 
           [0016]      FIG. 10  is a top view of a second embodiment of the measurement system of the present invention; and 
           [0017]      FIGS. 11 ,  12  and  13  are side views of the measurement system during receipt, transfer and measurement of the wafer, respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Reference is now made to  FIG. 2 , which illustrates a measurement unit installable within a polishing machine, such as the IPEC Westech machine, the measurement system being constructed and operative in accordance with a preferred embodiment of the present invention and to  FIGS. 3 ,  4 ,  5 ,  6 ,  7  and  8  which illustrate the operation of a gripping system forming part of the measurement system of  FIG. 2 . Similar reference numerals are utilized to refer to elements of the water track previously discussed. 
         [0019]    The measurement system, labeled  30 , comprises an optical system  32  and a gripping system  34  operative in conjunction with a water track  36 . The optical system  32  can be any optical system which measures the thickness of the top layer of the wafer through water.  FIG. 9  provides one example of such a optical system; other optical systems are also incorporated into the present invention. 
         [0020]    The gripping system  34  comprises a raisable gate  40 , a translatable gripper  42 , a vacuum pad  44  and a vacuum system  46 . Gate  40  is controlled by a lifting mechanism  48  which raises and lowers gate  40  as necessary. Gate  40  has an upper surface  50  with a curved outer edge  52  and a plurality of protrusions  54  extending downward into the water from the upper surface  50 . Protrusions  54  provide a lower surface onto which the gate  40  is lowered while enabling the water to pass through the gate  40 . Curved edge  52  is shaped to match the curved edge of the wafer  25  so that, when gate  40  is in its lowered position, gate  40  will both keep the wafer  25  from passing out of the water track and to hold the wafer  25  in a repeatable location. 
         [0021]    Gripper  42  translates between the wafer collecting position defined by the curved edge  52  and a wafer measuring location indicated in  FIG. 2  by the wafer  25 . Although not visible in  FIG. 2 , the base of the water track at the wafer measuring location has been replaced by a window  60  ( FIGS. 3-9 ) to enable the optical system  32  to view the patterned surface  62  of the wafer  25 . For the purposes of the explanation, the patterned surface  62  is shown exaggeratedly in the Figures. 
         [0022]    Gripper  42  can be translated by any translation system; an example of one such system is provided in  FIG. 2  and labeled  64 . 
         [0023]    The vacuum pad  44  is typically a bellows-shaped pad and is mounted at the end of the gripper  42  and is connected to the vacuum system  46 . The vacuum pad  44  creates a suction so that gripper  42  can raise the wafer  25  and move it from the wafer collecting position to the wafer measuring location. In addition, the vacuum is maintained during the measurement and only released once the measurement is complete. 
         [0024]      FIGS. 3-8  illustrate the operation of the gripping system  34 . Initially, and as shown in  FIG. 3 , the jets, labeled  70 , and the bubblers, labeled  72 , of the water track are operated and the gate  40  is lowered. The polisher (not shown) places the wafer  25  within the water track and the streams  18  from the jets  70  push the wafer  25  towards the gate  40 . The gripper  42  is at the wafer collecting position, shown to the left in  FIGS. 3-8 . 
         [0025]    Once the wafer  25  is in the wafer collecting position, as shown in  FIG. 4 , gripper  42  lowers vacuum pad  44  to grab the wafer  25 . It will be appreciated that gripper  42  can be formed of any suitable mechanism, such as a piston, which can move vacuum pad  44  up and down on command. Since bubblers  72  are operating, the small streams  22  maintain the wafer  25  away from the base  14  of the water track. 
         [0026]    The gripper  42  then pulls the wafer  25  out of the water ( FIG. 5 ) and the jets  70  are deactivated. In accordance with a preferred embodiment of the present invention, the axis  74  of symmetry of the vacuum pad  44  is formed at a small angle α from the vertical axis  76 . As a result, a long axis  75  of the wafer  25  is at the same small angle α to the horizontal axis  78 . Angle α is typically in the range of 2-5°. 
         [0027]    Translation unit  64  then moves gripper  42  to the wafer measuring position, shown to the right in  FIGS. 4-8 . At the same time and as shown in  FIG. 6 , a pull-down mechanism slightly lowers the entire water track, gripping and optical system unit (at an angle of 1-3°), about a hinge  80  ( FIGS. 2-8 ), to force the water toward the wafer measuring position. Other methods of forcing the water towards the measuring position are also incorporated in the present invention. 
         [0028]    After the lowering of the water track, gripper  42  lowers the wafer  25  towards the window  60 . Since the vacuum pad  44  is angled, the wafer  25  does not enter the water all at once. Instead, wafer  25  enters the water gradually. Initially, only the side labeled  82  is immersed. As the gripper  42  pushes the vacuum pad  44  further down, more and more of the wafer  25  becomes immersed until the entire wafer  25  is within the water. Vacuum pad  44  is flexible enough to accommodate the changed angle of wafer  25 . 
         [0029]    It will be appreciated that, by gradually immersing the wafer in the water, few, if any, bubbles are created near the patterned surface of the wafer  25 . 
         [0030]    It is noted that the wafer  25  does not rest against the window  60 . Instead, it is held against protruding surfaces  84  such that there is a layer of water  86  between the wafer  25  and window  60 . Due to the gradual immersion of wafer  25 , layer  86  of water has little, if any, bubbles in it and therefore provides a uniform connecting medium between the optical system  32  and the patterned surface  62  of wafer  25 . 
         [0031]    Once the optical system  32  has finished measuring the patterned surface  62  of wafer  25 , gripper  42  returns vacuum pad  44 , with wafer  25  still attached, to its upper position. The pull-down mechanism rotates the water track about hinge  80  to return to its original position, gate  40  is raised, and jets  70  and bubblers  72  are activated. The vacuum system  46  releases the vacuum and the wafer  25  falls into the water track. The flow of water causes the wafer  25  to move toward and under the now raised gate  40 . A sensor  90  determines when the wafer  25  successfully passes out of the water track. The process described hereinabove can now begin for the next wafer. 
         [0032]    Reference is now made to  FIG. 9  which schematically illustrates an example of a suitable optical system  32 . Optical system  32  is a microscope-based spectrophotometer and comprises an objective lens  100 , a focusing lens  102 , a beam splitter  104 , a pin hole mirror  106 , a relay lens  108  and a spectrophotometer  110 . It additionally comprises a light source  112 , a condenser  114 , a charge coupled device (CCD) camera  116  and a second relay lens  118 . 
         [0033]    Light from light source  112  is provided, along an optical fiber  113 , to condenser  114 . In turn, condenser  114  directs the light towards beam splitter  104 . Beam splitter  104  directs the light towards the wafer surface via lenses  102  and  100  and via window  60  and water layer  86 . 
         [0034]    The reflected light from the patterned surface  62  is collected by objective  100  and focused, by lens  102 , onto pin hole mirror  106 . Relay lens  108  receives the light passed through pin hole mirror  106  and focuses it onto the spectrophotometer  110 . 
         [0035]    Pin hole mirror  106  passes light through its hole towards spectrophotometer  110  and directs the light hitting the mirror surface towards CCD camera  116 . Second relay lens  118  receives the light reflected by pin hole mirror  106  and focuses it onto the CCD camera  116 . 
         [0036]    Since the pinhole is placed at the center of the image plane which is the focal plane of lens  102 , it acts as an aperture stop, allowing only the collimated portion of the light beam to pass through. Thus, the pinhole drastically reduces any scattered light in the system. Relay lens  108  collects the light from the pinhole and provides it to spectrophotometer  110 . 
         [0037]    Furthermore, since the pinhole is located at the image plane of the optical imaging system (lenses  100  and  102 ), only that portion of the light, reflected from the surface of wafer  25 , which is the size of the pinhole divided by the magnification will come through the pinhole. Relay lens  118  collects the light and focuses it onto the CCD camera  116 . 
         [0038]    The pinhole serves to locate the measurement spot in the image of the wafer  25 . Since the pinhole allows light to pass through it, rather than being reflected toward the CCD camera  116 , the pinhole appears as a sharp dark point in the image produced by the lens  118 . Thus, when viewing the CCD image, the location of the measurement spot is immediately known, it being the location of the dark spot. 
         [0039]    Reference is now made to  FIGS. 10-13  which illustrate the thickness measuring of the present invention implemented in a polishing machine similar to that produced by Strasbaugh which has no water track. In this embodiment, the polishing machine or an external robot (not shown) brings the wafers  25  to an exit station of the polisher. When the measurement has finished, the robot brings the wafers  25  to their cassette at another exit station.  FIG. 10  is a top view and  FIGS. 11 ,  12  and  13  illustrate the measuring station in three states. 
         [0040]    The measuring station  130  comprises a gripping unit  132 , an optical system  134  and a water bath  136 . The optical system  134  is located beneath the water bath  136  and can be any suitable optical system, such as the one described hereinabove. As in the previous embodiment, the water bath  136  has a window in its bottom surface, labeled  140  in  FIG. 11 , through which the optical system  134  can illuminate the wafer  25 . 
         [0041]    The gripping unit  132  comprises a wafer support  150 , illustrated as being formed of two support elements, a vacuum pad  152 , similar to vacuum pad  44 , and a piston  160 . The polisher places the wafer  25  on the wafer support  150  while the vacuum pad  152  is initially in a position above the support  150 , as shown in  FIG. 11 . Once the wafer support  150  has the wafer in a predefined position, the vacuum pad  152 , which is controlled by piston  160 , moves toward the wafer and grabs it by applying a vacuum. Now that the vacuum pad  152  is holding the wafer, the wafer supports  150  move away, as indicated. 
         [0042]    The piston  160  then pushes the vacuum pad-wafer combination toward the water bath  136 . This is shown in  FIG. 12  which also illustrates that the vacuum pad  152  holds the wafer  25  at a small angle α to the horizontal. The angle α is provided since, as in the previous embodiment, the axis of symmetry of the vacuum pad  152  is formed at a small angle α from the vertical axis. As in the previous embodiment, by immersing the wafer  25  into the water at the angle α, few, if any, bubbles, remain on the undersurface of the wafer after full immersion. 
         [0043]      FIG. 13  illustrates the wafer  25  at its fully immersed, measurement position. Typically, wafer  25  does not directly touch the water surface  163  of the window  140 ; instead, it sits on a measurement support  168 . The result is that there is a water layer  164  between the wafer  25  and the surface  163  of the window. 
         [0044]    Once the measurement process has finished, the piston  160  returns the wafer  25  to its original position and the wafer support elements  150  return to their wafer receiving position. The piston  160  places the wafer  25  on the wafer support elements  150  and releases the vacuum. The external robot can now take the wafer to another exit station where there is a cassette of processed and measured wafers. 
         [0045]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the claims which follow: