Abstract:
A structural member for use in high temperature environments is disclosed. The structural member has a core encased within a shell material. The core material is formed of a strong material having a melting point well above that of the shell material. The disclosed structural member is particularly useful when forming a boat for heating silicon and the like to temperatures between 900 degrees Celsius and 1500 degrees Celsius. In a preferred embodiment the core material is graphite and the shell is fused silica. Even more preferably, the fused silica pneumatically encases the graphite to thereby prevent inadvertent contamination during use of the structural member. A related method for forming the structural member is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional patent application Ser. No. 60/603,023, filed on Aug. 19, 2004. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to a reinforced structural member for use in extremely high temperature environments such as those found during the processing and manufacture of silicon wafers and the like, a related support structure built therewith, and a related method for fabricating the reinforced structural member.  
       BACKGROUND OF THE INVENTION  
       [0003]     Durable and strong structural members for use in extremely high temperature environments, such as a range between 900 degrees Celsius to 1500 degrees Celsius, are used in a wide variety of applications. For example, in the semi-conductor industry, the manufacture of semi-conductors from silicon frequently requires heating silicon wafers and the like to within this temperature range.  
         [0004]     Usually, the wafers are stacked in a rack-type structure, which is referred to in the industry as a “boat”, and the rack containing the plurality of wafers is placed in a furnace. The structural members forming the rack must be sufficiently strong to hold the wafers, even at these extreme temperatures, without weakening due to the extreme heat. Moreover, it is desirable for the rack to be reusable. Accordingly, the members forming the rack, the stand on which the rack is placed, and the even the furnace structures themselves must be sufficiently durable and strong to withstand numerous heating and cooling cycles.  
         [0005]     Structural members operating within these extreme temperatures must be formed with materials having melting points well above the range of temperatures in which these structural members are expected to operate. Steel and other alloy-based materials commonly used as structural members in lower temperature environments vaporize and/or melt at these extreme temperatures rendering them useless. Accordingly, known materials for constructing structural members used in such extremely high temperature environments are limited.  
         [0006]     Moreover, in cases where a structural member is used in an extremely high temperature to facilitate semi-conductor manufacture, it is important that the structural member limit the amount of impurities released by vaporization during the heating process.  
         [0007]     A particularly favorable material used as a structural member in the construction of boats for use in semi-conductor fabrication is fused silica glass, which is also referred to in the industry as fused quartz and collectively refers to materials containing at least one of a group of minerals that are commonly referred to as the “Si0 2 ” group. This material has a high melting/vaporization point, and can be processed and or selected so as to release few, if any, impurities during the heating process. Moreover, fused silica glass can be formed into structural members, and it can be joined together with other structural members, usually by heat welding, to make a boat or the like.  
         [0008]     Despite the benefits of fused silica glass for use as a structural member, it has several drawbacks. For example, depending on the ultimate temperature in which the boat is operated, the weight of silicon wafers stacked within a boat, can urge the boat&#39;s structural members formed from fused silica glass to “bow” outward during repeated heating and cooling cycles. Accordingly, over time, the effectiveness of the boat can be compromised. Moreover, fused silica glass suitable for use in this environment can be extremely expensive.  
         [0009]     Other materials, such as graphite and the like, can provide an economical and more rigid structure at these high temperatures, even during repeated use. However, these materials tend to be extremely brittle. Accordingly, they can break easily, even with application of an extremely minor impact. Moreover, these materials tend to release an unacceptable level of impurities at high temperature. Accordingly, despite the rigidity offered by these structures, they are not routinely used to form structural members used to hold or process silicon wafers at the like at extremely high temperatures.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, despite the available structural members for use in extremely high temperature environments, there remains a need for an economical thermally resistant, structural member that is more durable than the known structures, particularly during repeated heating and cooling cycles. In addition to other benefits that will become apparent in the following disclosure, the present invention fulfills these needs.  
         [0011]     The present invention is structural member for use in high temperature environments that has a core encased within a shell. The core is formed of a strong material having a melting point well above that of the shell. In a preferred embodiment the core is graphite and the shell is fused silica. Even more preferably, the fused silica pneumatically encases the graphite to thereby prevent inadvertent contamination the heating and cooling process.  
         [0012]     A disclosed method for forming the structural member includes inserting the core within the shell and heat-sealing the shell to the core. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a front, plan view of a structure formed of a plurality of reinforced structural members in accordance with an embodiment of the present invention.  
         [0014]      FIG. 2  is a sectional view of the structure of  FIG. 1  taken along line  2 - 2  of  FIG. 1 .  
         [0015]      FIG. 3  is an enlarged fragmentary view of a portion of the structure of  FIG. 1  taken along line  3  of  FIG. 1 .  
         [0016]      FIG. 4  is an enlarged cross-sectional view of a reinforced structural member of  FIG. 1  taken along line  4 - 4  of  FIG. 3 .  
         [0017]      FIG. 5  is a sectional view of the first alternative reinforced structural member of  FIG. 4  taken along line  5 - 5  of  FIG. 5 .  
         [0018]      FIG. 6  is a cross-sectional view of a possible second alternative reinforced structural member in accordance with an embodiment of the present invention.  
         [0019]      FIG. 7  is a sectional view of the second alternative reinforced structural member of  FIG. 7  taken along line  7 - 7  of  FIG. 6 .  
         [0020]      FIG. 8  is a cross-sectional view of a possible third alternative reinforced structural member in accordance with an embodiment of the present invention.  
         [0021]      FIG. 9  is a sectional view of the third alternative reinforced structural member of  FIG. 9  taken along line  9 - 9  of  FIG. 8 .  
         [0022]      FIG. 10  is a cross-sectional view of a possible fourth alternative reinforced structural member in accordance with an embodiment of the present invention.  
         [0023]      FIG. 11  is a cross-sectional view of a possible fifth alternative reinforced structural member in accordance with an embodiment of the present invention.  
         [0024]      FIG. 12  is an exploded, fragmentary, isometric view of the reinforced, structural member of  FIG. 4 .  
         [0025]      FIG. 13  is an isometric view of the reinforced structural member of  FIG. 12 .  
         [0026]      FIG. 14  is a front plan view of an alternative possible support structure formed of reinforced structural members in accordance with an embodiment of the present invention with portions cut away to show internal detail.  
         [0027]      FIG. 15  is a cross sectional view of the support structure of  FIG. 14  taken along line  15 - 15  of  FIG. 14  with portion cut away to show internal detail.  
         [0028]      FIG. 16  is a cross-sectional view of a possible fix alternative reinforced structural member in accordance with an embodiment of the present invention.  
         [0029]      FIG. 17  is a cross-sectional view of a possible seventh alternative reinforced structural member in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]     A reinforced structural member  30  for use in a high temperature environment is disclosed in  FIGS. 1-17 . The structural member  30  preferably includes a reinforced core  32  encircled by a fused silica shell  34 .  
         [0000]     A. Boat Construction  
         [0031]     Preferably and referring to  FIG. 1 , a plurality of structural members  30  are joined together using conventional methods to form a heating boat  36  used to hold silicon wafers  37  ( FIG. 2 ) and the like during high temperature heating in a furnace. The heating boat  36  can include a plurality of elongate structural members  30  aligned substantially parallel to teach other and joined at their respective ends by an upper member  38  and a lower member  40 .  
         [0032]     A plurality of spaced-apart notches  42  is preferably provided along each structural member  30 . Preferably, the notches  42  in each structural member  30  are aligned substantially horizontally to form substantially horizontal rows  44  of like notches  42  within the structural members  30 . Accordingly, a silicon wafer  37  ( FIG. 2 ) may be secured to the heating boat  36  by being placed within one of the rows  44  of notches  42 . More preferably, a plurality of silicon wafers may be secured to the heating boat  36  and spaced-apart from each other by being placed in separate rows  44  of notches  42  on the structural members  30 .  
         [0033]     As best shown in  FIG. 2 , the upper and lower members  38 ,  40  are preferably planar and have a substantially circular shape. Preferably, three structural members  30  are joined to the upper and lower members  38 ,  40  and spaced apart from each other as shown so as to allow a silicon wafer  37  ( FIG. 2 ) to be easily inserted and removed through an open side  46  formed thereby.  
         [0034]     More preferably, the lower side  48  of the lower member  40  includes feet  50  for allowing the heating boat  36  to stand in a furnace. Also and as shown in  FIG. 1 , stabilizing straps  52  can extend between the structural members  30  at defined locations along their longitudinal lengths to reduce the likelihood of the structural members  30  bowing during use.  
         [0000]     B. Reinforced Core  
         [0035]     The core  32  is formed from a material having a higher melting temperature than that of the shell  34 . Preferably, the core  32  is formed of an elongate strip of graphite, which has been machined to have a desired cross section and length. Of course other materials, such as carbon, Monocrystalline Silicon, Polycrystalline Silicon, SiC, AlN, Al2O3, Sapphire, ZrO2, Si3N4, or other material that offer similar strength at elevated temperatures could also be used.  
         [0000]     C. Fused Silica Shell  
         [0036]     The shell  34  is formed of fused silica having a melting point that is higher than the desired range of temperature in which the support member is expected to operate. Preferably, the fused silica shell  34  is one of the SiO2 group.  
         [0000]     D. Method of Fabrication  
         [0037]     The structural members  30  are preferably formed by first machining the core  32  to the desired length and cross-sectional shape. The core  32  can either be a continuous length of material  60  having a constant cross-section there-along as shown in  FIGS. 6 &amp; 7 , or the notches  42  of the finished product can also be reinforced by having a protrusion  62  of core material extending between each notch  42  shown in  FIGS. 8 &amp; 7 . In such case, the core  32  can be a continuous length of material with spaced apart core notches  64  ground therein to form the protrusions  62 .  
         [0038]     Preferably and as shown in  FIGS. 4, 5 ,  12  and  13 , to allow for thermal expansion and contraction during use, the core  32  is formed of discrete components including an elongate spine  66  which runs the longitudinal length of the structural member  30  and a plurality of notch support members  68 , each having a base portion  70  and a protrusion portion  72 . More preferably, the notch support members  68  are substantially L-shaped. As best shown in  FIG. 12 , the elongate spine  66  preferably includes an elongate recess  74  sized to slidable receive the base portion  70  of the notch support members  68  therein such that protrusion portions  72  extend therefrom. The plurality of L-shaped notch support members  68  is aligned in the elongate recess  74  thereby forming the plurality of spaced apart discrete protrusions  62  within the recess. If desired, the space between the protrusions can be filled with discrete segments of fused silica  80 .  
         [0039]     As shown in  FIG. 12 , the core  32  is inserted into the hollow portion  82  of an elongate fused silica shell  34 . A cap  84  is first fused to one end of the shell  34  thereby sealing that end. A vacuum is preferably applied to the opposite end of the shell  34  while heat having a temperature high enough so as to fuse the silica shell  34  but not so high as to vaporize the core  32  is applied to the fused silica, thereby fusing the shell  34  to the core  32 . A second cap  86  is placed on the free end of the elongate structural member  30  and heat-sealed in place, thereby pneumatically sealing and protecting the brittle core  32  within the fused silica shell  34 .  
         [0040]     Notches  42  are then machined along the elongate structural member  30  using conventional methods.  
         [0041]     Preferably, the elongate structural members  30  are then formed into a heating boat  36  for holding silicon wafers therein using conventional assembly methods, which usually include heat-sealing the structural members to the upper and lower members  38 ,  40 .  
         [0000]     E. Exemplar Cross-Sections  
         [0042]     As shown in  FIGS. 4-11  and  16 - 17 , the core  32  and shell  34  cross-sectional dimensions of the structural member  30  may be selected so as to produce a variety of different cross-sectional shapes for the structural members  30 . For example, the core  32  can have a circular cross-section as shown in  FIGS. 11, 16  and  17 , or the core  32  can have a substantially rectangular cross-section as shown in  FIGS. 6, 8  and  10 . The cross-section of the core  32  can include one or more non-traditional shapes such as that shown in  FIG. 4 . Similarly, the cross sectional shape of the shell  34  can be substantially square as shown in  FIG. 8 , substantially circular as shown in  FIGS. 16 and 17  or a non-traditional shape as shown in  FIGS. 4, 6 ,  10  and  11 .  
         [0043]     One known method for forming the non-traditional shapes of  FIGS. 4, 6 ,  10 , and  11  using commercially available fused silica rods includes heat-sealing a traditional, solid fused silica rod  90  with a reinforced structural member  30  of the present invention. For example, as shown in  FIGS. 10, 11 , and  17 , the strength and durability of a traditional solid fused silica rod  90  has been increased by fusing it with a reinforced structural member  30  of the present invention. Such fusing usually includes positioning the reinforced structural member  30  adjacent to the traditional solid fused silica rod  90  and heating them both above the melting point of the silica but below the melting point of the core  32  material such that the shell  34  of the reinforced structural member  30  fuses with the traditional solid fused silica rod  90 . As shown in  FIG. 17 , a plurality of reinforced structural members  30  may also be fused to a traditional fused silica rod  90 .  
         [0044]     It can be appreciated that the reinforced structural member  30  of the present invention provides a structure with all the strength and durability benefits of graphite without risk of impurities from the graphite contaminating the furnace chamber during use at high temperatures. Moreover, since the majority of the structural support  30  is provided by the graphite, the amount of fused silica used to form the structural member  30  can be reduced, thereby reducing the total material costs of each structural member. Also, encasing the graphite in fused silica protects the brittle graphite from fracturing during a small, inadvertent impact.  
         [0000]     F. Alternative Embodiments  
         [0045]     Having here described preferred embodiments of the present invention, it is anticipated that other modifications may be made thereto within the scope of the invention by individuals skilled in the art. For example, other structures in a heating boat  36  can include the reinforced structural member  30 . In  FIGS. 14 and 15 , the upper and lower members  38 ,  40  include a core  32  encased within a fused silica shell  34 . In such case, the core  32  within the upper and lower members  38 ,  40  can include recesses for operably receiving the core  32  from one or more vertically aligned structural members  30  therein, thereby further securing the upper and lower members  38 ,  40  to the vertically aligned structural members  30 . Stabilizing straps  52  can also be formed of reinforced structural members  30 .  
         [0046]     In addition, the core  32  can be comprised of a plurality of layers of different materials, each having different properties, as shown in  FIG. 11 .  
         [0047]     Similarly, the reinforced structural members  30  can be used in other high temperature environments besides use in the semi-conductor fabrication industry.  
         [0048]     Thus, although preferred, more preferred, and alternative embodiments of the present invention have been described, it will be appreciated that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents as defined in the appended claims.