Patent Publication Number: US-2010126488-A1

Title: Method and apparatus for cutting wafers by wire sawing

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. 119 to U.S. Provisional Application No. 61/117603, entitled “METHOD AND APPARATUS FOR CUTTING WAFERS BY WIRE SAWING” by Cambridge Energy Resources, Inc., filed on Nov. 25, 2008, which is incorporated herein its entirety by reference. 
    
    
     FIELD OF TECHNOLOGY 
     The present invention relates generally to wire sawing and more particularly relates to method and apparatus of cutting wafers by wire sawing. 
     BACKGROUND 
     Wire saws are extensively used to slice silicon for solar and micro-electronics applications. The wire saws are also used for slicing a variety of other materials including sapphire, gallium arsenide (GaAs), indium phosphide (InP), silicon carbide (SiC), glass, lithium tantalate (LiTaO 3 ) Z-cut crystals, lithium niobate (LiNbO 3 ), lithium triborate (LiB 3 O 5 ), quartz crystals, ceramics like aluminum nitride (ALN) and lead zirconate titanate (PZT), magnetic materials/parts, optical parts and the like material. The wire saws typically use a 120-180 micron diameter steel wire, which is several hundred kilometers long ( FIG. 1 ). The wire is wound around a supply spool  110 , a set of rollers called “wire guides”  130  to make a bed of parallel moving wire, often called “wire web”  140 , and a take-up spool  120  as shown in  FIG. 1 . The wire guides  130  have equally spaced grooves on their outer surface to control spacing between the wires as it goes around the wire guides  130 . The distance between the grooves, called pitch, eventually decides thickness of the wafers. 
     The work piece or the ingot  150 , which needs to be sliced, is first glued to a plate  160  and then mounted on the wire saw. Then the ingot  150  is pressed with a vertical motion (top to bottom or bottom to top) against the horizontally moving wire web  140 . The wire travels at a speed of about 15 meters/sec (or even higher) during slicing of wafers. Abrasive slurry, mainly made up of silicon carbide grains and a lubricant (e.g., polyethylene glycol or mineral oil), is introduced over the wire web  140 . The abrasive slurry  210  coats the wire and travels to the cutting zone as shown in  FIG. 2 . 
     Also, it can be seen in  FIG. 2  that, the abrasive slurry  210  tends to flow downwardly and away from the slicing zone, thereby significantly reducing the efficiency of slicing during the sawing operation. Further, it can be seen that a significant amount of abrasive slurry  210  is wasted by not being used in the slicing operation as the abrasive slurry  210  tends to flow downwardly and away from the cutting zone. Furthermore, it can be seen in  FIG. 2  that, the abrasive slurry  210  flows perpendicular to direction of the horizontally moving wire web  140  ( FIG. 2 ). Also, it can be seen in  FIG. 2  that, majority of the abrasive slurry  210  does not pass through the ingot  150  and instead falls to the ground (bottom) of the wire saw. Further, in the conventional system using low viscosity slurries, risk of particles separating out of the abrasive slurry  210  is high. 
     Typically, slicing is achieved by slowly pushing the ingot  150  against the wire web  140 . Slicing is completed when the ingot  150  completely passes through the wire web  140 . At this point, the wafer stack which is held to the plate  160  is slowly pulled out of the wire web  140 . Then, the stack of wafers is removed from the wire saw and taken for cleaning. 
     The wafers are then cleaned immediately with water and other solvents after slicing is completed to remove the abrasive slurry  210 , otherwise the abrasive slurry  210  may stain the wafers thereby making them unusable in downstream processes. Then the glue which holds the wafer to the plate  160  is softened by heating and wafers are removed individually (mostly manually) for further cleaning and processing. 
     SUMMARY 
     A method and apparatus of cutting wafers by wire sawing is disclosed. According to one aspect of the present invention, a wire sawing apparatus includes a horizontal ingot feeding wire slicing apparatus for slicing wafers, a frame for holding the horizontal ingot feeding wire slicing apparatus, and a control panel for operating the wire sawing apparatus. 
     Further, the horizontal ingot feeding wire slicing apparatus includes a vertical wire web such that sawing wires of the vertical wire web are located substantially in a vertical plane and move in a substantially vertical direction, at least one top outlet for applying fluid during sawing, wherein the at least one top outlet being located in a top position with respect to at least one work piece, such that the fluid (e.g., an abrasive slurry) flows in a substantially downward vertical direction under a gravitational force, and at least one chute for removing the fluid, such that the at least one chute is located substantially below the at least one work piece for receiving the fluid, in which the at least one work piece is impelled against the vertical wire web by movement in a horizontal direction, and in which the fluid is applied to the top of the at least one work piece and moves in a vertical direction against and into the at least one work piece for slicing wafers. 
     The horizontal ingot feeding wire slicing apparatus further includes at least two wire guide cylinders, such that the sawing wires are stretched between the at least two wire guide cylinders and held substantially in the vertical plane by a defining interval between the sawing wires, a tension control unit for controlling tension of the sawing wires, a support table for carrying the at least one work piece to be sliced, and a power driver for driving the at least two wire guide cylinders. 
     According to another aspect of the present invention, a method for producing wafers includes cutting a work piece including at least one ingot by impelling the work piece into a substantially vertical wire web, such that sawing wires of the substantially vertical wire web are located in a substantially vertical plane and move in a substantially vertical direction and in which the work piece is moved in a substantially horizontal direction into the substantially vertical wire web, and contacting the moving work piece for slicing wafers separately with a fluid including an abrasive slurry, such that the fluid flows in a substantially downward vertical direction under a gravitational force and such that moving the work piece and contacting with the fluid slices the wafers secured at one end to a plate. 
     The methods and apparatuses disclosed herein may be implemented in any means for achieving various aspects. Other features will be apparent from the accompanying drawings and from the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention are illustrated by way of an example and not limited to the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
         FIG. 1  illustrates schematics of a conventional wire saw; 
         FIG. 2  illustrates schematics of the slurry flow in the conventional wire saw through the wafers during cutting; 
         FIG. 3  illustrates an exemplary method of work piece preparation prior to loading on a wire saw apparatus, according to an embodiment of the present invention; 
         FIG. 4  illustrates an exemplary horizontal ingot feeding wire slicing apparatus and a method thereof, according to an embodiment of the present invention; 
         FIG. 5  illustrates an exemplary horizontal ingot feeding wire slicing apparatus, wherein the sliced wafers are removed after slicing process, according to an embodiment of the present invention; and 
         FIG. 6  illustrates an exemplary wire sawing apparatus, according to an embodiment of the present invention. 
     
    
    
     Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows. 
     DETAILED DESCRIPTION 
     A method and apparatus for cutting wafers by wire sawing is disclosed. In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     The terms “slicing”, “sawing”, “watering”, and “cutting” are used interchangeably throughout the document. 
       FIG. 3  illustrates an exemplary method  300  of work piece preparation prior to loading on a wire saw apparatus (e.g., the wire saw apparatus  600  of  FIG. 6 ), according to an embodiment of the present invention. As shown in  FIG. 3 , a work piece  150  is attached to a plate  160 . For example, the work piece  150  may be silicon (Si), sapphire, gallium arsenide (GaAs), indium phosphide (InP), silicon carbide (SiC), lithium tantalate (LiTaO 3 ) Z-cut crystals, lithium niobate (LiNbO 3 ), lithium triborate (LiB 3 O 5 ), quartz crystals, ceramics like aluminum nitride (ALN) and lead zirconate titanate (PZT), magnetic materials/parts, optical parts, or glass. Further, the work piece  150  may be mono-crystalline silicon (i.e., the work piece  150  grown from a single crystal) or multi-crystalline silicon. The plate  160  may be glass, ceramic, plastic, silicon or a like material. 
     In one exemplary implementation, the work piece  150  is attached to the plate  160  by glue  310 . It is appreciated that gluing of the work piece  150  to the plate  160  ensures secured holding of sliced wafers to the plate  160 . One skilled in the art can envision that the work piece  150  can be attached to the plate  160  using other techniques that are well known in the art. 
       FIG. 4  illustrates an exemplary horizontal ingot feeding wire slicing apparatus  400  and a method thereof, according to an embodiment of the present invention. As shown in  FIG. 4 , the horizontal ingot feeding wire slicing apparatus  400  includes a vertical wire web  140 , a top outlet  410  and a chute  430 . 
     It can be seen in  FIG. 4  that, sawing wires of the vertical wire web  140  are located in a vertical plane and move in a substantially vertical direction. In some embodiments, the sawing wires of the vertical wire web  140  are formed by spirally winding between two wire guides cylinders  130 . In these embodiments, the sawing wires are stretched between the two wire guide cylinders  130  and held substantially in the vertical plane by a defining interval between the sawing wires. 
     Further, as shown in  FIG. 4 , the work piece  150  (e.g., including one or more ingots) to be sliced by the horizontal ingot feeding wire slicing apparatus  400  is attached to the plate  160 . In one exemplary implementation, the work piece  150  is attached to the plate  160  by glue  310  (e.g., as shown in  FIG. 3 ). It is appreciated that, the plate  160  is located substantially laterally on a side of the vertical wire web  140  in a substantially vertical plane that is parallel to the plane of the vertical wire web  140 . As shown in  FIG. 4 , a support table  220  of the horizontal ingot feeding wire slicing apparatus  400  carries the work piece  150  attached to the plate  160 . 
     In operation, the work piece  150  is impelled against the vertical wire web  140  by movement in a horizontal direction (e.g., as shown by reference numeral  440 ) for slicing wafers. In one embodiment, the work piece  150  including a plurality of ingots is impelled substantially simultaneously to the vertical wire web  140 . In an alternate embodiment, the work piece  150  including the plurality of ingots is impelled substantially serially to the vertical wire web  140 . It is appreciated that the sawing wires of the vertical wire web  140  are adapted to move in a substantially vertical alternating or continuous direction while impelled against the work piece  150 . 
     Further, in accordance with the above-described embodiments, the top outlet  410  is located in a top position with respect to the work piece  150  for applying fluid  420  during the sawing operation. For example, the fluid  420  is an abrasive slurry. In one exemplary implementation, the top outlet  410  is located and oriented to substantially flow the fluid  420  over the top of the work piece  150  as the work piece  150  is impelled against the vertical wire web  140  and during the slicing of the wafers. It can be seen in  FIG. 4  that, the fluid  420  flows in a substantially downward vertical direction under a gravitational force. 
     Further, as shown in  FIG. 4 , the chute  430  is located substantially below the work piece  150  for removing the fluid  420 . In one exemplary implementation, the fluid  420  is applied to the top of the work piece  150  and flows in a vertical direction against and into the work piece  150  for slicing the wafers, which is finally received by the chute  430 . Moreover, depending on the size, relative value, and speed of motion of the work piece  150 , the fluid  420  can be separately collected and re-used, or can be collected together and discarded. 
     According to the one or more embodiments described above, the method for producing wafers using the above-described horizontal ingot feeding wire slicing apparatus  400  include cutting the work piece  150  including one or more ingots by impelling the work piece  150  substantially into the vertical wire web  140  and contacting the moving work piece  150  for slicing wafers separately with the fluid  420 . Further, moving the ingot and contacting the work piece  150  with the fluid  420  slices the wafers secured at one end to the plate  160 . 
       FIG. 5  illustrates an exemplary horizontal ingot feeding wire slicing apparatus  500 , wherein the sliced wafers are removed after slicing process, according to an embodiment of the present invention. The slicing process is completed when the work piece  150  completely passes through the vertical wire web  140 . 
     As shown in  FIG. 5 , the sliced wafers secured to the plate  160  are slowly pulled out (indicated by a reference numeral  510 ) of the vertical wire web  140 . It is appreciated that thickness of the sliced wafers are separated from each other by sawing gaps due to the defining interval between the sawing wires. In one example embodiment, the thickness of each sliced wafer is less than about 800 microns, less than about 500 microns, less than about 300 microns, less than about 200 microns, less than about 150 microns, less than about 100 microns, or less than about 50 microns. Further, stack of wafers are removed from the horizontal ingot feeding wire slicing apparatus  500  and taken for cleaning and processing, which is well known to a person skilled in the art. 
       FIG. 6  illustrates an exemplary wire sawing apparatus  600 , according to an embodiment of the present invention. Particularly,  FIG. 6  illustrates the wire sawing apparatus  600  which includes the horizontal ingot feeding wire slicing apparatus  400 . It is appreciated that the horizontal ingot feeding wire slicing apparatus  400  is a retrofittable device that is designed to be integrated into the wire sawing apparatus  600 . One can envision that, the horizontal ingot feeding wire slicing apparatus  400  can be integrated into any existing wire sawing apparatus. As shown in  FIG. 6 , the horizontal ingot feeding wire slicing apparatus  400  includes a supply spool  110 , a take-up spool  120 , the wire guide cylinders  130 , the vertical wire web  140 , the top outlet  410 , the chute  430 , a tension control unit  610 , the support table  220  and a power driver  620 . 
     As shown in  FIG. 6 , the sawing wires of the vertical wire web  140  are located substantially in a vertical plane and move in a substantially vertical direction. Also, as shown in  FIG. 6 , the sawing wires of the vertical wire web  140  are stretched between the wire guide cylinders  130 . It can be seen in  FIG. 6  that, the sawing wires of the vertical wire web  140  are spirally wound around the supply spool  110 , the two wire guide cylinders  130  and the take-up spool  120 . 
     Further, as shown in  FIG. 6 , the top outlet  410  is located in a top position with respect to the work piece  150 . According to an embodiment of the present invention, the top outlet  410  applies the fluid  420  during sawing, where the fluid flows in a substantially downward vertical direction under a gravitational force. Further, it can be seen in  FIG. 6  that, the top outlet  410  is located and oriented such that the fluid  420  substantially flows over the top of the work piece  150  as the work piece  150  is impelled against the vertical wire web  140  and during slicing of wafers. Further, it can be seen in  FIG. 6  that, the chute  430  is located substantially below the work piece  150  for removing the fluid  420  used in the slicing process. 
     According to the above-described embodiments, the tension control unit  610  controls tension of the sawing wires, the support table  220  carries the work piece  150  to be sliced and the power driver  620  drives the wire guide cylinders  130 . It is appreciated that the support table  220  along with other elements form a horizontal ingot feeding device in the horizontal ingot feeding wire slicing apparatus  400 . In one exemplary implementation, the horizontal ingot feeding device is arranged to maintain, during slicing, partially or completely sliced wafers substantially parallel to each other and such that the width of the sawing gaps is held substantially constant during slicing of the wafers. Further, the horizontal ingot feeding wire slicing apparatus  400  and a method thereof is described in greater detail with respect to  FIG. 4 . The wire sawing apparatus  600  also includes a frame  630  for holding the horizontal ingot feeding wire slicing apparatus  400  and a control panel  640  that may be attached to the frame  630  for operating the wire sawing apparatus  400 , according to the example embodiment illustrated in  FIG. 6 . 
     With reference to the above-described wafer cutting technique, a plurality of zones is envisioned with respect to the work piece  150  and the vertical wire web  140 , in terms of placement of sources of the fluid  420 , and collection of the fluid  420 . Thus, there is a “pre-saw” zone in which the fluid  420  is contacted to the work piece  150 . 
     An embodiment of the present invention, designed to solve the hydrodynamic stress problem without breakage of wafers, provides a design of the wire sawing apparatus in which the work piece is impelled against the vertical wire web by movement in a horizontal direction. Unlike conventional wire saws, in which the fluid is fed over a horizontal wire web, in the present method and apparatus, the fluid is fed downward across the vertical wire web (as illustrated in  FIG. 4 ). As a result, the fluid migrates into the work piece by advantageously responding to the gravitational force as well as the drag force of the fast moving vertical wire web. This arrangement forces the fluid to pass through the cutting zone of the work piece. 
     An advantage of gravity assisted vertical fluid flow in the design of the wire sawing apparatus herein is that, particles do not settle out of the fluid. Rather the entire fluid is forced to pass through the cutting zone of the work piece. Use of the vertical wire sawing apparatus further enables use of low viscosity fluids, which imparts lower stress on the wafers. Further, by use of the above-described wire sawing apparatus, a larger number of abrasive particles are introduced into the cutting zone, and efficiency of the cutting process is thereby significantly increased. Further, the consumable cost of the slicing process is decreased. 
     A skilled person will recognize that many suitable designs of the systems and processes may be substituted for or used in addition to the configurations described above. It should be understood that the implementation of other variations and modifications of the embodiments of the invention and its various aspects will be apparent to one ordinarily skilled in the art, and that the invention is not limited by the exemplary embodiments described herein and in the claims. Therefore, it is contemplated to cover the present embodiments of the invention and any and all modifications, variations, or equivalents that fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein. The contents of all references cited are incorporated herein by reference in their entireties.