Abstract:
An apparatus for abrading a substrate including a moveable abrading tool having a bur for abrading the substrate, a stage for supporting the substrate, and a height sensing device in communication with the abrading tool to determine a vertical position of the bur with respect to the substrate. Further disclosed is a method for abrading a substrate using the foregoing apparatus including moving the abrading tool across the substrate so as to abrade the substrate, determining the vertical position of the bur with the height sensing device, and communicating the vertical position of the bur to the abrading tool.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the abrading of substrates and, more particularly, relates to the abrading of ceramic and semiconductor substrates using a small, computer-controlled abrading tool. 
     Ceramic substrates comprising ceramic material and metallization, useful for mounting semiconductor devices, often become nonplanar after sintering due to an uneven distribution of metal and ceramic materials within the ceramic substrate. Ceramic substrates useful for thin films require a planar ceramic substrate so any nonplanarity in the ceramic substrate is removed through polishing of the ceramic substrates. Polishing is typically accomplished by placing the ceramic substrates on a large polishing table wherein a slurry containing an abrasive grit is used as the abrasive medium. 
     As part of the manufacturing process of semiconductor devices, semiconductor wafers are polished by a chemical mechanical polishing (CMP) process, one example of which is disclosed in Trojan et al. U.S. Pat. No. 5,899,798, the disclosure of which is incorporated by reference herein. The uniform removal of material from and the planarity of patterned and unpatterned wafers is critical to wafer process yield. Generally, the wafer to be polished is mounted on a substrate carrier which holds the wafer using a combination of vacuum suction or other means to contact the rear side of the wafer and a retaining lip or ring around the edge of the wafer to keep the wafer centered on the substrate carrier. The front side of the wafer, the side to be polished, is then contacted with a chemically reactive slurry. 
     The amount of material removal is more critical in the planarizing of semiconductor wafers than ceramic substrates. Overpolishing (removing too much) or underpolishing (removing too little) of the wafer results in scrapping or rework of the wafer, respectively, which can be very expensive. To remedy this problem, a number of endpoint detect methods have been devised to detect when the desired endpoint for removal has been reached, and the polishing can be stopped. One such method for endpoint detect in a CMP process is disclosed in Li et al. U.S. Pat. 5,644,221, the disclosure of which is incorporated by reference herein. 
     While the above methods for planarization work well enough, the present invention takes a new approach to planarization of ceramic substrates and semiconductor wafers. Instead of large polishing pads or surfaces which planarize the entire ceramic substrate or semiconductor wafer at the same time, it would be desirable to have a method and apparatus for planarizing a small portion of the ceramic substrate or semiconductor wafer at any given time. This would allow greater versatility in the process, particularly if only small portions of the ceramic substrate or semiconductor wafer need to be abraded or otherwise require material removal. 
     Accordingly, it is a purpose of the present invention to have a method and apparatus for abrading a small portion of the ceramic substrate or semiconductor wafer at any given time. 
     It is another purpose of the present invention to have a method and process for abrading a ceramic substrate or semiconductor wafer which is versatile in use. 
     These and other purposes of the present invention will become more apparent after referring to the following description of the invention considered in conjunction with the accompanying drawings. 
     BRIEF SUMMARY OF THE INVENTION 
     The purposes of the invention have been achieved by providing, according to a first aspect of the invention, an apparatus for abrading a substrate comprising: 
     a moveable abrading tool having at least one bur for abrading the substrate; 
     a stage for supporting the substrate; and 
     at least one height sensing device in communication with the abrading tool to determine a vertical position of the at least one bur with respect to the substrate. 
     According to a second aspect of the invention there is provided an apparatus for abraiding a substrate comprising: 
     a moveable,pneumatically-powered abraiding the substrate; 
     a stage for supporting the substrate; and 
     at least one laser intterferometer in communication with the abraiding tool to determine a vertical position of the at least one bur with respect to the substrate. 
     According to a third aspect of the invention there is provided an apparatus for abrading a substrate with an apparatus comprising a moveable abrading tool having at least one bur for abrading the substrate, a stage for supporting the substrate, and at least one height sensing device in communication with the abrading tool to determine a vertical position of the at least one bur with respect to the substrate, the method comprising the steps of: 
     moving the abrading tool across the substrate so as to abrade the substrate; 
     determining the vertical position of the at least one bur with the at least one height sensing device; and 
     communicating the vertical position of the at least one bur to the abrading tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The Figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is an schematical view of the apparatus for abrading a substrate according to the present invention in which a bur is used to abrade the substrate. 
     FIG. 2 is a schematical view of one device for measuring the height of the bur with respect to the substrate. 
     FIG. 3 is an enlarged view of a tool used for abrading the substrate. 
     FIG. 4 is a block diagram which shows the general layout and feedback CPU of the abrading apparatus according to the present invention. 
     FIGS. 5A and 5B are schematical views of an alternative methodology for measuring the height of the bur with respect to the substrate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the Figures in more detail, and particularly referring to FIG. 1, there is shown a substrate  2  to be abraded, and more preferably, planarized. Substrate  2  may be a semiconductor wafer, a ceramic substrate or other similar article, which collectively hereafter will be referred to as simply a substrate. 
     Apparatus  10  will be used to abrade all or part of substrate  2 . Substrate  2  is placed on stage  12  which can move or remain stationary. Abrading tool  14  has a bur  16  for abrading the substrate. Abrading tool  14  is moveable in the x, y or z directions as well as moveable to make circular, spiral or other patterns on the substrate  2 . As will be understood by those skilled in the art, abrading tool  14  will be connected to additional mechanical or electromechanical apparatus (not shown) through arm  17  which will move the abrading tool  14  in the desired pattern. 
     Apparatus  10  further comprises at least one height sensing device  20  in communication with the abrading tool  14 . The at least one height sensing device determines the vertical position of the bur  16  with respect to the substrate  2 . There may be additional height sensing devices such as height sensing device  28 . 
     In operation, abrading tool  14  and bur  16  move across substrate  2 . The movement of the abrading tool  14  and bur  16  are controlled by cooperation between arm  17  and stage  12 . As noted above, stage  12  can be moveable or stationary. If desired, stage  12  can move in the x, y or z directions, can rotate or can move in some combination of the foregoing. As can be appreciated, the movements resulting from the cooperation of stage  12  and arm  17  are practically infinite in nature. The vertical location of bur  16  is controlled by arm  17  or stage  12  or both in conjunction with height sensing device  20  and/or height sensing device  28 . The height sensing device or devices determines the vertical position of the bur  16  with respect to the substrate  2 . As can be seen in FIG. 1, height sensing device  20  is focused  36  behind bur  16  while height sensing device  28  is focused  38  in front of bur  16 . This information is relayed to arm  17  directly or through an intermediary device such as a tool controller or computer. If stage  12  is moveable, the position information may also be relayed to stage  12 . Thereafter, arm  17  and/or stage  12  move so that bur  16  is in the correct location for abrading. The depth of cut for semiconductor wafers will tend to be shallow, on the order of several microns or less, while the depth of cut for ceramic would probably be much larger. 
     As noted above, apparatus  10  can be used to abrade all or part of substrate  2 . If a completely planarized substrate  2  is desired, apparatus  10  would abrade the entire substrate  2 . In some situations, it may be desirable to abrade only a portion of substrate  2 . As one example, apparatus  10  can be used to selectively expose areas of metallurgy for on-chip capacitors which could then be directly connected to a carrier by wirebond. 
     In one preferred embodiment of the present invention, it is preferred that the at least one height sensing device is a laser interferometer, such as one manufactured by Teletrac, Inc., Goleta, Calif. Referring now to FIG. 2, one such arrangement for a laser interferometer is shown. There, laser interferometer  24  cooperates with mirror  48  and optics  49  to direct its light in the desired direction. The laser light bounces off the substrate  2  and back through optics  49  and mirror  48  to laser interferometer  24 . The height so determined is fed to arm  17  of abrading tool  14  through cable  42 . 
     An alternative methodology for determining the vertical position of the bur  16  with respect to the substrate  2  is illustrated in Figures  5 A and  5 B. As shown in FIG. 5A, a suitable light source  80 , preferably a laser, is shown upon bur  16 . If necessary, light source  80  may be raster scanned in the vertical direction so all of bur  16  is illuminated. Once illuminated, bur  16  casts a shadow having a length  82 . Subsequently, the shadow is measured after the bur  16  has abraded into the substrate  2 . The distance  84  of the shadow after abrading has begun is compared to distance  82  prior to abrading. Knowing the length of bur  16 , the distance “d”, the depth of cut, can be accurately determined. While not shown in FIGS. 5A and 5B, a conventional image capture device such as a camera may be used to measure the length of the shadows  82 ,  84  and bur  16 . The above methodology can also be utilized to determine when the bur  16  contacts substrate  12  by observing when the shadow meets bur  16 . 
     While the Figures show one bur  16  it is within the scope of the present invention to have a plurality of burs  16  acting in unison. This may be accomplished by modifying abrading tool  14  so as to accept move than one bur  16 . Alternatively, a plurality of abrading tools  14  may be provided, each one having a single bur  16 . In this latter case, the plurality of abrading tools  14  would have to be linked mechanically, electrically or by software. 
     Returning to FIG. 1, the abrading apparatus  10  can further include a transducer  18  to audibly determine the load on the abrading tool  14 . When the load on the abrading tool  10  is increased, that is, the bur  16  is meeting increased resistance from the substrate  2 , the sound of the abrading tool will audibly change, which is picked up by the transducer  18 . In a preferred embodiment, transducer  18  is a microphone. This information is relayed back to the abrading tool  14  to speed up or slow down the movement of the abrading tool  14 . 
     Turning now to FIG. 3, an enlarged view of the abrading tool  14  is shown with transducer  18  appended off of abrading tool  14 . Bur  16  has a shaft  50  which is held by abrading tool  14  and abrasive end  52 . The abrasives of abrasive end  52  are preferably diamonds. One preferred abrading tool  14  is a so-called dental handpiece, available from Star Dental (Lancaster, Pa.). The dental handpiece is essentially a pneumatically powered turbine capable of speeds in the neighborhood of 300-500 thousand revolutions per minute. The high RPMs of the dental handpiece lead to a very efficient abrading tool. The dental handpiece would of course have to be suitably modified with arm  17  so that the movement of the dental handpiece can be automated and computer controlled if desired. The burs are commonly available from a number of manufacturers, one of which is Carlisle Labs (England). The burs have a diameter of about 1 mm. 
     Apparatus  10  may further include an image viewer to view the substrate  2  where it has just been abraded. The substrate  2  can be continually monitored for changes in surface features. For example, if one were looking to abrade the surface of the substrate  2  until a metal feature is uncovered, the image viewer could sample the surface until a reflective or shiny surface is located. In this manner, the image viewer functions as an endpoint detect system. Preferably, the image viewer is a camera. 
     The image viewer may be located in the same apparatus that holds the height sensing device  20 . Referring to FIG. 2, body  22  is essentially a microscope which is connected to both the laser interferometer  24  and camera  26 . A separate light source  44  and mirror  46  are provided to illuminate the surface of the substrate  2  and provide reflected light back to camera  26 . Data from camera  26  is transmitted through cable  40 . Height sensing device  28  may be similar to height sensing device  20  in that it could also contain a microscope body  30 , laser interferometer  32  and camera  34 . Camera  34  could be used for the alternative height sensing methodology mentioned with respect to FIGS. 5A and 5B. 
     In a preferred embodiment of the invention, there is provided a feedback means as shown in FIG. 4 which is used to monitor and control the abrading tool  14 . Ideally, the feedback means would sample the apparatus  10  and provide appropriate feedback to the abrading tool  14  on the order of 25-30 times per second. It should be understood that some or all of the components shown in FIG. 4 may be part of the apparatus  10  as explained previously. Height sensing devices  58 ,  64  provide their data to Central Processing Unit (CPU)  56  via cables  66 ,  74 , respectively. The CPU  56  processes the data and determines whether the bur  16  is at the right vertical position with respect to the substrate  12 . If not, CPU  56  communicates by cable  76  to abrading tool  14  and/or stage  12  to bring bur  16  to the right vertical position. 
     Transducer  18  provides its data to CPU  56  via cable  70 . If the sound of abrading tool sounds right, no action is taken. Otherwise, CPU  56  through cable  76 , stage  12  and/or arm  17  causes the movement of abrading tool  14  to speed up or slow down with respect to the substrate  12 , as appropriate. 
     Image viewers  60 ,  62  provide their data to CPU  56  via cables  68 ,  72 , respectively. Recall that image viewers  60 ,  62  may be used for endpoint detect or for determining the vertical position of the bur  16 . For example, image viewer  60  may be used for endpoint detect while image viewer  62  may be used to determine the vertical position of the bur  16  with respect to the substrate  12 . Once CPU  56  processes the data from image viewers  60 ,  62 , the CPU  56  signals appropriate action over cable  76  to abrading tool  14  and/or stage  12 . 
     Lastly, CPU  56  may be part of a computer, a part of another device, or a stand alone tool controller. 
     It will be apparent to those skilled in the art having regard to this disclosure that other modifications of this invention beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.