Abstract:
Systems and methods are disclosed for controlling the surface profiles of wafers cut in a wire saw machine. The systems and methods described herein are generally operable to alter the nanotopology of wafers sliced from an ingot by controlling the shape of the wafers. The shape of the wafers is altered by changing the temperature and/or flow rate of a temperature-controlling fluid that comes in contact with the ingot. Different feedback systems can be used to determine the temperature of the fluid necessary to generate wafers having the desired shape and/or nanotopology.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/568,785 filed Dec. 9, 2011, which is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    This disclosure relates generally to wire saw machines used to slice ingots into wafers and, more specifically, to systems for controlling the surface profiles of wafers sliced in the wire saw machines. 
       BACKGROUND 
       [0003]    Semiconductor wafers are typically formed by cutting an ingot with a wire saw machine. These ingots are often made of silicon or other semiconductor or solar grade material. The ingot is connected to structure of the wire saw by a bond beam and an ingot holder. The ingot is bonded with adhesive to the bond beam, and the bond beam is in turn bonded with adhesive to the ingot holder. The ingot holder is connected by any suitable fastening system to the wire saw structure. 
         [0004]    In operation, the ingot is contacted by a web of moving wires in the wire saw that slice the ingot into a plurality of wafers. The bond beam is then connected to a hoist and the wafers are lowered onto a cart. 
         [0005]    Wafers cut by known saws may have surface defects, such as warp, that cause the wafers to have nanotopology that deviates from set standards. In order to ameliorate the deviating nanotopology, such wafers may be subject to additional processing steps. These steps are time-consuming and costly. Moreover, known wire saw machines are not operable to adjust the shape and/or warp of the surfaces of the wafers cut from the ingot by the machines. Thus, there exists a need for a more efficient and effective system to control nanotopology of wafers cut in a wire saw machine. 
         [0006]    This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
       SUMMARY 
       [0007]    One aspect is a system for controlling the surface profile of wafers sliced from an ingot in a wire saw, the wire saw including a wire guide supporting wires. The system includes a containment box positioned vertically beneath the wires and configured to contain a slurry, and a slurry temperature control system configured to circulate slurry through the containment box. 
         [0008]    Another aspect is a method for controlling the surface profile of wafers sliced from an ingot in a wire saw, the wire saw including a wire guide supporting wires. The method includes circulating slurry through a containment box positioned vertically beneath the wires, the slurry circulated using a slurry temperature control system, and immersing at least a portion of the ingot in the slurry after the at least a portion of the ingot passes through the wires. 
         [0009]    Still another aspect is a system for controlling the surface profile of wafers sliced from an ingot in a wire saw, the wire saw including a wire guide supporting wires. The system includes a slurry temperature control system, and at least one nozzle in fluid communication with the slurry temperature control system and configured to spray slurry onto a surface of the ingot to facilitate reducing surface defects in the surface profile of the wafers sliced from the ingot. 
         [0010]    Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above-mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates a wire saw slicing apparatus; 
           [0012]      FIG. 2  illustrates a system for controlling wafer surface profiles; 
           [0013]      FIG. 3  illustrates an alternative system for controlling wafer surface profiles; 
           [0014]      FIG. 4  illustrates an alternative system for controlling wafer surface profiles; 
           [0015]      FIGS. 5A and 5B  are graphs demonstrating the efficacy of the system shown in  FIG. 4 ; and 
           [0016]      FIG. 6  illustrates an alternative system for controlling wafer surface profiles. 
       
    
    
       [0017]    Corresponding reference characters indicate corresponding parts throughout the drawings. 
       DETAILED DESCRIPTION 
       [0018]    The present disclosure is directed to controlling surface profiles of wafers sliced using a wire saw slicing apparatus. An exemplary wire saw slicing apparatus for slicing a single crystal or polycrystalline silicon ingot into individual wafers is illustrated in  FIG. 1 , designated in its entirety by the reference numeral  21 . Any suitable wire saw slicing apparatus may be utilized without departing from the scope of the present disclosure. 
         [0019]    The apparatus generally includes a frame  23  which mounts four wire guides  25  (two are partially shown) for supporting a wire web  27 . The frame also mounts a movable slide or head  29  which mounts an ingot  30  for movement relative to the frame for forcing an ingot  30  into the web. 
         [0020]    The ingot  30  is typically a single crystal silicon ingot or polycrystalline silicon ingot, more typically a single crystal silicon ingot. Although single crystal silicon is a preferred material for semiconductor-grade wafers, other semiconductor materials may be used. 
         [0021]    The wire guides  25  are generally cylindrical and have a number of peripheral grooves (not shown) that receive respective wire segments making up the wire web  27  and are spaced at precise intervals. The spacing between the grooves determines the spacing between wire segments and thereby determines the thickness of the wafers sliced from the ingot  30 . The wire guides  25  rotate on bearings for moving the wire segments lengthwise or axially. A cutting-slurry is directed onto the wire web  27  by conduits  32 . 
         [0022]    In the wafer slicing operation for producing silicon wafers, the ingot  30  is mounted on an ingot holder  53 , which is held in the wire saw slicing apparatus  21  by a table  51 . The ingot  30  is adhered to a wire saw beam  55 . The surfaces of the ingot holder  53  and the ingot  30  are adhered to the wire saw beam  55  using a suitable adhesive. 
         [0023]    The ingot holder  53  may be constructed from steel or other materials, such as, for example, INVAR (an alloy of iron (64% ) and nickel (36% ) with some carbon and chromium). 
         [0024]    To produce wafers, the ingot  30  is gradually lowered into the wire web  27  of fast moving, ultrathin wire. Cutting action is created by pouring abrasive slurry on the wire web  27 , which is actually a single wire being fed from one spool to another. Immediately after slicing, the “as cut” wafers are cleaned in a series of chemical baths to remove any residual slurry. From here, the wafers are polished and cleaned. 
         [0025]    Numerous mechanisms are believed to affect and/or cause entry marks formed on wafers as they are sliced from the ingot  30  by the wire saw slicing apparatus  21 . For example, wire saw slicing generates frictional heat at the moving front representing the location of the cut. More specifically, during slicing of the ingot  30 , the temperature of the ingot may increase from approximately 20° Celsius to approximately 55° Celsius. 
         [0026]    The majority of this temperature increase occurs during the first part of the cut (i.e., when the wire web  27  initially contacts the ingot  30 ). The increase in temperature causes the ingot  30  to expand in a direction parallel to a longitudinal axis  60  of the ingot  30 , and the expansion may cause variation and defects (i.e., warp) in a surface profile of the wafers produced by the slicing. For example, experimental data shows an expansion of the ingot  30  in the range of 40 nanometers (nm) along the longitudinal axis  60  of the ingot  30 . Using the systems and methods disclosed herein, the temperature of the ingot is monitored and controlled to facilitate reducing expansion of the ingot  30  and to facilitate reducing surface defects in the produced wafers. 
         [0027]    Referring to  FIG. 2 , a system for controlling wafer surface profiles is indicated generally by the reference numeral  100 . In the exemplary embodiment, similar to the wire saw slicing apparatus  21  (shown in  FIG. 1 ) the system  100  includes an ingot  30  and a wire web  27 . The ingot  30  is suspended from an ingot holder  53 . A wire saw beam (not shown) may couple the ingot  30  and the ingot holder  53 . To produce a plurality of wafers, the ingot  30  is lowered into the wire web  27  such that the wires in the wire web  27  slice the ingot  30 . As described above, by controlling the temperature of the ingot  30  during the slicing, surface defects in the sliced wafers may be reduced. 
         [0028]    In the embodiment illustrated in  FIG. 2 , the ingot  30  is preheated to the top temperature it would otherwise reach during the slicing process. That is, before the ingot  30  is lowered into the wire web  27 , the ingot may be heated to approximately 55° Celsius, for example. Accordingly, when the wire web  27  initially contacts the ingot  30 , the temperature of the ingot  30  remains relatively unchanged, any expansion of the ingot  30  is relatively low, and surface defects are reduced. 
         [0029]    The ingot  30  is preheated using a heating device  102 . The heating device heats the surface of the ingot  30  initially, and it takes time for the heat to be transferred to interior regions of the ingot  30 . Accordingly, adequate amounts of heat applied for sufficient amounts of time are used to thoroughly heat the ingot  30 . 
         [0030]    The heating device  102  may include an air gun, one or more resistors, a microwave, and/or any device suitable for preheating the ingot  30 . In one example, the ingot was immersed in 60° Celsius water for four hours to heat the ingot to approximately 52° Celsius. In another example, the ingot was immersed in 60° Celsius water for twenty four hours to heat the ingot to approximately 56° Celsius. 
         [0031]    In the example embodiment, the ingot  30  is not only preheated prior to applying the wire web, but the heating device  102  also heats the ingot  30  at least during the initial slicing by the wire web  27  to ensure the temperature of the ingot  30  remains substantially constant throughout the slicing process. 
         [0032]    In the example embodiment, a temperature probe  104  is coupled to an end face  105  of the ingot  30 . Alternatively, the temperature probe  104  may be coupled at any location within the system  100  that enables the temperature probe  104  to monitor the temperature of the ingot  30 . A controller  106  is communicatively coupled to the temperature probe  104  and the heating device  102 . The controller  106  receives signals from temperature probe  104  that are indicative of the temperature of the ingot  30 . Based on the received signals, the controller  106  can control operation of the heating device  102  such that the ingot  30  remains at a substantially constant temperature throughout the slicing process, reducing surface defects in the produced wafers. 
         [0033]    Referring to  FIG. 3 , an alternative system for controlling wafer surface profiles is indicated generally by the reference numeral  110 . In the embodiment shown in  FIG. 3 , instead of using a heating device, slurry is heated to the top temperature of the ingot  30  (e.g., approximately 55° Celsius), and the heated slurry is then sprayed onto the ingot  30 . To control the temperature of the cutting slurry, system  110  includes a slurry temperature control system  112 . 
         [0034]    In the slurry temperature control system  112 , the slurry is stored in a slurry tank  114 . To raise or lower the temperature of the slurry, a slurry temperature control pump  116  pumps slurry through a heat exchanger  118  and then back into the slurry tank  114 . 
         [0035]    To heat slurry, heat exchanger  118  is configured to transfer heat from a heating fluid (not shown) into slurry. To cool slurry, heat exchanger  118  is configured to transfer heat from slurry into a cooling fluid (not shown). While only one heat exchanger  118  is shown in  FIG. 3 , multiple heat exchangers  118  (e.g., one for heating the slurry and one for cooling the slurry) may be utilized by the temperature control system  112 . Accordingly, using heat exchanger  118 , the temperature of slurry in slurry tank  114  can be raised or lowered. 
         [0036]    A slurry feed pump  120  pumps heated slurry from slurry tank  114  to one or more nozzles  122 , which spray the heated slurry onto the surface of ingot  30 . In this example embodiment, the heated slurry is applied to the surface of the ingot  30  before slicing, and also applied at least during the initial slicing by the wire web  27  to ensure the temperature of the ingot  30  remains substantially constant throughout the slicing process. Alternatively, or additionally, nozzles  122  may also apply the temperature-controlled slurry to the wire web  27  for use as the cutting slurry. 
         [0037]    Similar to system  100  (shown in  FIG. 2 ), an ingot temperature probe  130  is coupled to the end face  105  of the ingot  30  to monitor the temperature of the ingot  30 . Alternatively or additionally, a slurry temperature probe  132  monitors the temperature of the slurry in the slurry tank  114 . A controller  134  communicatively coupled to the heat exchanger  118 , ingot temperature probe  130 , and the slurry temperature probe  132  receives signals from probes  130  and  132  indicative of the temperature of the ingot  30  and the slurry, respectively. The controller  134  can control the temperature of the slurry based on the received signals by controlling operation of the heat exchanger  118 . 
         [0038]    In the system  110 , the temperature of the ingot  30  can also be controlled by adjusting the amount of heated slurry sprayed onto the ingot  30 . Accordingly, in the example embodiment, slurry pumped from slurry feed pump  120  passes through a valve  140  before reaching nozzles  122 . The controller  134  is communicatively coupled to the valve  140 , and can control the valve  140  to adjust a flow rate of the slurry, controlling the temperature of the ingot  30 . 
         [0039]    Referring to  FIG. 4 , an alternative system for controlling wafer surface profiles is indicated generally by the reference numeral  160 . In the embodiment shown in  FIG. 4 , instead of heating the ingot  30 , heat is transferred from the ingot  30  such that the temperature of the ingot  30  remains substantially constant during the slicing process. 
         [0040]    In the embodiment shown in  FIG. 4 , a slurry channel is defined through the ingot holder  53 . The slurry channel extends from a slurry inlet  162  to a slurry outlet  164 . A slurry temperature control system  168 , including components substantially similar to the slurry temperature control system  112  (shown in  FIG. 3 ), provides cooled slurry to the channel. 
         [0041]    Specifically, cooled slurry is pumped from the slurry tank  114  into the slurry inlet  162  and through the channel in the ingot holder  53 . The slurry exits the ingot holder  53  at the slurry outlet  164 , and in the example embodiment, is channeled back into the slurry tank  114 . The slurry temperature control pump  116  and the heat exchanger  118  control the temperature of the slurry in the slurry tank  114 , as described above. 
         [0042]    As the cooled slurry flows through the channel, heat is transferred from the ingot holder  53  to the slurry, cooling the ingot holder  53 . Through thermal contact with the cooled ingot holder  53 , heat is transferred from the ingot  30  to the ingot holder  53 . Accordingly, by sufficiently cooling the ingot holder  53  with cooled slurry, the temperature of the ingot  30 , which would otherwise increase during the slicing process, may be held substantially constant. As explained above, by keeping the temperature of the ingot  30  substantially constant during the slicing process, surface defects in the produced wafers may be reduced. 
         [0043]    Similar to system  100  (shown in  FIG. 2 ), an ingot temperature probe  170  is coupled to the end face  105  of the ingot  30  to monitor the temperature of the ingot  30 . Alternatively or additionally, an ingot holder temperature probe  172  may be coupled to the ingot holder  53  to monitor the temperature of the ingot holder, and a slurry temperature probe  174  monitors the temperature of the slurry in the slurry tank  114 . A controller  180  communicatively coupled to the heat exchanger  118 , the ingot temperature probe  170 , the ingot holder temperature probe  172 , and the slurry temperature probe  174  receives signals from probes  170 ,  172 , and  174  indicative of the temperature of the ingot  30 , the ingot holder  53 , and the slurry in the slurry tank  114 , respectively. The controller  180  can control the temperature of the slurry based on the received signals by controlling operation of the heat exchanger  118 . 
         [0044]    In the system  160 , the temperature of the ingot  30  can also be controlled by adjusting the amount of cooled slurry flowing through the ingot holder  53 . Accordingly, in the example embodiment, slurry pumped from the slurry feed pump  120  passes through a valve  182  before reaching the ingot slurry inlet  162 . The controller  180  is communicatively coupled to the valve  182 , and can control the valve  182  to adjust a flow rate of the cooled slurry, controlling the temperature of the ingot  30 . In one example, the flow rate of the cooled slurry is 6 liters per minute (l/min). Alternatively, any flow rate that facilitates controlling the temperature of the ingot  30  may be used. 
         [0045]      FIGS. 5A and 5B  are graphs demonstrating the efficacy of the system  160  shown in  FIG. 4 .  FIG. 5A  is a graph  200  plotting a temperature of the ingot holder  53  versus time during the slicing process.  FIG. 5B  is a graph  202  plotting a degree of wafer warp versus time during the slicing process. Prior to a time t 0 , the ingot holder  53  was cooled using cooled slurry. At time t 0 , slurry stopped flowing through the ingot holder  53  (e.g., by closing the valve  182 ). 
         [0046]    As shown in graph  200 , at time t 0 , the temperature of the ingot holder  53  increased, due to the absence of cooled slurry flowing therethrough. Further, as shown in graph  202 , at time t 0 , an increase in the degree of wafer warp was observed, in response to the temperature change of the ingot holder  53 , and consequently, the ingot  30 . Accordingly, channeling cooling slurry through the ingot holder  53  facilitates reducing surface defects, such as warp, in wafers produced from the ingot  30 . 
         [0047]    Referring to  FIG. 6 , an alternative system for controlling wafer surface profiles is indicated generally by the reference numeral  220 . Using the system  220 , instead of channeling cooled slurry through the ingot holder  53 , heat is transferred from the ingot  30  by immersing at least a portion of the ingot  30  in cooled slurry. 
         [0048]    In the embodiment shown in  FIG. 6 , a containment box  230  is located below (i.e., vertically beneath) the ingot  30 . A slurry temperature control system  240 , including components substantially similar to the slurry temperature control system  112  (shown in  FIG. 3 ), provides cooled slurry to the containment box  230 , which functions as a slurry reservoir. That is, the containment box  230  holds a volume of cooled slurry. 
         [0049]    Specifically, cooled slurry is pumped from the slurry tank  114  into a slurry inlet  242  of the containment box  230 . Slurry exits the containment box  230  at a slurry outlet  244 , and in the example embodiment, is channeled back into the slurry tank  114 . The slurry temperature control pump  116  and the heat exchanger  118  control the temperature of the slurry in the slurry tank  114 , as described above. Accordingly, the slurry temperature control system  112  supplies the containment box  230  with a continuous supply of cooled slurry. 
         [0050]    In the example embodiment, enough cooled slurry is provided to the containment box  230  such that a level of the cooled slurry within the containment box  230  reaches a top  250  of the containment box  230 . If the level of the cooled slurry is higher than the top  250 , any excess slurry flows over the top  250  and out of the containment box  230 . Overflow slurry may be collected and returned to the slurry tank  114 . 
         [0051]    In system  220 , the top  250  of the containment box  230  is located just below the wire web  27 . Accordingly, the cooled slurry in the containment box  230  does not contact or otherwise interfere with the wire web  27  slicing the ingot  30 . 
         [0052]    As described above, the ingot  30  is lowered into the wire web  27  during the slicing process. Accordingly, after a portion of the ingot  30  passes through the wire web  27 , that portion is submerged in the cooled slurry in the containment box  230 . Submerging the ingot  30  in the cooled slurry shortly after it passes through the wire web  27  facilitates suppressing the temperature increase that would otherwise occur from the slicing, reducing surface defects in the wafers produced from the slicing process. 
         [0053]    Similar to system  100  (shown in  FIG. 2 ), an ingot temperature probe  260  is coupled to the end face  105  of the ingot  30  to monitor the temperature of the ingot  30 . Alternatively or additionally, a containment box temperature probe  262  may be positioned within the containment box  230  to monitor the temperature of the cooled slurry in the containment box  230 , and a slurry temperature probe  264  monitors the temperature of the slurry in the slurry tank  114 . 
         [0054]    A controller  270  communicatively coupled to the heat exchanger  118 , the ingot temperature probe  260 , the containment box temperature probe  262 , and the slurry temperature probe  264  receives signals from probes  260 ,  262 , and  264  indicative of the temperature of the ingot  30 , the slurry in the containment box  230 , and the slurry in the slurry tank  114 , respectively. The controller  270  can control the temperature of the slurry based on the received signals by controlling operation of the heat exchanger  118 . 
         [0055]    In the system  220 , the temperature of the ingot  30  can also be controlled by adjusting the amount of cooled slurry flowing into the containment box  230 . Accordingly, in the example embodiment, slurry pumped from the slurry feed pump  120  passes through a valve  280  before reaching the slurry inlet  242 . The controller  270  is communicatively coupled to the valve  280 , and can control the valve  280  to adjust a flow rate of the cooled slurry, controlling the temperature of the cooled slurry in the containment box  230 . 
         [0056]    Multiple systems and methods to control (i.e., change, manipulate, adjust) the temperature of an ingot to control its expansion have been disclosed herein. By controlling the expansion of the ingot, it is believed that the defects in the surface of the wafers can eliminated or reduced and/or that the warp or shape of the wafers can be controlled. 
         [0057]    When introducing elements of the present disclosure or the embodiments thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
         [0058]    As various changes could be made in the above without departing from the scope of the present disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.