Abstract:
Spacer for a vertical support structure of a heating element coil includes a mating feature including complimentary components on first opposing sides of the spacer, a cavity, open to second opposing sides of the spacer, and an extension offset from an axis intersecting the mating features, the extension including a pocket sized to fit an individual loop of the heating element coil. The spacer can be incorporated into a support structure for a heating element coil interlocking adjacent loops of the coil so that they are retained in a collinear and concentric arrangement while allowing the loops of the coil to move freely inward and outward from the central axis in unison.

Description:
RELATED APPLICATION DATA 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/358,694, filed Jun. 25, 2010, entitled “Support Structure For Heating Element Coil”, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to a support structure for a coiled heating element. In particular, the present disclosure relates to a spacer, having cooperating mating features and arranged in a vertical stack which maintains one or more of the collinearity, concentricity and centering of the heating element coil during thermal expansion of the heating element. The present disclosure also relates to a support structure including such a spacer, such as a support structure in a furnace for processing semiconductor components, and a method of supporting a coiled heating element with such a spacer. 
       BACKGROUND 
       [0003]    In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art. 
         [0004]    Metallic resistance alloy is a dominant material used in the construction in electrical heating element assemblies. Typical FeCrAl alloys achieve their high temperature stability and long life by creating a protective oxide coating on the outer surfaces. This oxide layer contributes to the material&#39;s hot strength as well as protecting the core alloy from the formation of other oxides and nitrides that would rapidly consume the wire. The protective oxide layer is formed via the oxidation of aluminum inclusive in the heating alloy. One of the known properties of the FeCrAl resistance alloy is permanent elongation over time. Elongation is primarily caused when during thermal cycling of the alloy. The wire expands as it is heated, the oxide coefficient of expansion is less than the metal core, tensile stresses are created in the oxide coating and therefore cracks form in the oxide surface. The newly exposed alloy creates more oxide on the exposed areas and “heals” the surface. When the wire is cooled, compressive forces are created from the difference in thermal expansion from the alloy and the oxide. The compressive forces cause some of the oxide to flake or “spall” off of the material. Some portion of the elongation becomes permanent and the effect is cumulative over time. 
         [0005]    Various improvements (such as powdered metallurgy) have been developed to minimize the permanent elongation characteristics of the alloy. It has been found that minimizing the stresses induced in the alloy helps reduce the elongation and generally extends element life. One source of stresses introduced into the wire is the force created when the helical coil of wire expands and pushes against the thermal installation surrounding the element assembly. Various approaches have been taken to attempt to mitigate this situation. Leaving a small space between the wire and insulation provides room for the coils to expand, but these designs do not address the issue of collinearity and concentricity of the coils. These prior art methods generally rely on some form of slot in the ceramic spacer rows that allow for expansion and contraction (as well as permanent elongation), but no mechanism is provided to insure the collinearity and concentricity of the coils. Since these assemblies are vertically mounted, gravity creates a downward force on the coil turns and encouraging the lower portions of the coil to increase in diameter, while the upper turns constrict. This can lead to increased forces applied to the bottom turns prior to the upper portion, leading to accelerated aging in the lower portions. Also, increased forces can be experienced at locations like the power terminals where the coils are somewhat fixed in location and the additional downward force from gravity is exerted. Some prior art attempts to remedy this situation by attaching protrusions to the heating element coils to block them from passing through the spacer assemblies. This can help and mitigate the accumulation of material in the lower part of the assembly but has negative implications to the heating wire temperature uniformity and potential risk of failure. Furthermore, these methods do not address the issue of keeping the coils collinear and centered. There is no constraining mechanism that keeps the coils collinear, therefore one coil can move horizontally relative an adjacent coil leading to irregular distribution of the heating element surface along the vertical axis. This can lead to decreased temperature uniformity within the heating element. Once deformation of the coil is initiated at some point in the assembly, it generally continues to worsen over time at that location. Therefore, the deformation can result in decreased element life as well. 
         [0006]    Temperature uniformity and overall life can be affected by the centering of the coil within the assembly as well. The prior art does not provide a mechanism for maintaining the centering of the coil as well. 
         [0007]    There is a need in the industry for an element assembly that allows the coil to move freely as it expands and contracts during thermal cycling while maintaining concentricity, collinearity and centering of the heating element coil. 
       SUMMARY 
       [0008]    The exemplary embodiments overcome the problems and limitations of the prior art. For example, spacers interlocking the coil along the circumference in a series of columns and limiting the movement relative to adjacent turns in the heating element coil allows the turns of the coil to remain concentric and collinear. At the same time, the interlocked columns of spacers are allowed to slide inward and outward relative to the center of the coil assembly as the coil expands and contracts. This allows the coil to freely expand into the space provided between the outer diameter (OD) of the coil assembly and the inner diameter (ID) of the insulation. 
         [0009]    The supports also can act as guides for the spacer columns, and the supports are preferentially arranged evenly around the circumference while being aligned with the center of the coil assembly. This creates vectors of force that encourage the coil assembly to remain centered within the heating element assembly. 
         [0010]    An exemplary embodiment of a support structure for a heating element coil interlocking adjacent loops of the coil so that they are retained in a collinear and concentric arrangement while allowing the loops of the coil to move freely inward and outward from the central axis in unison comprises a plurality of vertical support column assemblies, each positioned around a circumference of the heating element coil, wherein the vertical support column includes of a plurality of individual spacers having a pitch, the vertical support column residing at least partially inside a vertical channel, and wherein the vertical support column moves slideably within the vertical channel. 
         [0011]    An exemplary embodiment of a spacer for a vertical support structure of a heating element coil comprises a mating feature including complimentary components on first opposing sides of the spacer, a cavity, open to second opposing sides of the spacer, and an extension offset from an axis intersecting the mating features, the extension including a pocket sized to fit an individual loop of the heating element coil. 
         [0012]    An exemplary embodiment of a method of controlling a position relative to a center position of a heating element coil upon heating comprises mounting individual loops of a heating element coil in a column of vertically stacked spacers, wherein an increase in a length of the heating element coil upon heating is accommodated by a radially outward movement of the spacers relative to the center position while cooperation of mating features on adjacent spacers are maintained. 
         [0013]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The following detailed description can be read in connection with the accompanying drawings in which like numerals designate like elements and in which: 
           [0015]      FIG. 1A  is front isometric view of an embodiment of a support structure for a heating element coil. 
           [0016]      FIG. 1B  is a back isometric view of the embodiment of a support structure for a heating element coil shown in  FIG. 1A . 
           [0017]      FIG. 2  is a detailed isometric view of a large-pitch spacer. 
           [0018]      FIG. 3  is a detailed isometric view of a small pitch spacer. 
           [0019]      FIG. 4  is a detailed isometric view of the supporting member. 
           [0020]      FIG. 5  is a side view of an embodiment of a support structure for a heating element coil. 
           [0021]      FIG. 6  is a plan view showing the arrangement of the vertical element support structures arranged around the circumference of a heating coil structure. 
           [0022]      FIG. 7  is a perspective view of the arrangement of the vertical element support structures arranged around the circumference of a heating coil structure. 
           [0023]      FIG. 8  is a diagram depicting the centering force vectors that act on the coil. 
           [0024]      FIG. 9A  is a plan view of an alternate spacer profile and interlocking means on two sides of the vertical channel. 
           [0025]      FIG. 9B  is a plan view of an alternate spacer profile and interlocking means on one side of the vertical channel. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Referring to  FIGS. 1A and 1B , an exemplary embodiment of a spacer assembly  10  includes rows of vertically stacked spacers  12  that provide support for the individual circular loops  14  of the vertically oriented coil. The vertically oriented coil is not shown in its entirety, but rather the individual circular loops  14  thereof are only shown in the area in which they interact with the spacer assembly  10  to allow viewing of the spacer assembly  10 . The vertically stacked spacers  12  form a column  16  and can have various pitch dimensions that allow the spacing between the circular loops  14  of the coil to be adjusted to advantageously distribute the power dissipated by the coil in order to achieve a desired temperature profile characteristic. The lateral movement of any of the individual spacers  16  in the column  12  of vertically stacked spacers  16  is constrained by a vertical channel  18 , for example a channel in a rail  20  or other constraining device, keeping the spacers  16  aligned while still allowing movement inward and outward within the confines of the channel  18 . The vertical channel  18  can be a separate component as illustrated, or could be formed in whole or in part by incorporating a feature into the heater insulation. A spacer column support component  22  distributes the combined weight of the spacers  16  and coil across the supporting surface (not shown) and maintains the orientation of the channel  18  and the column  12  of vertically stacked spacers  16 . A similar spacer column support component (not shown) is located at the top of the column  12  of vertically stacked spacers  16  to constrain the top of the spacer assembly. 
         [0027]    Now referring to  FIG. 2 , each spacer  16  is constructed such that it has a pocket  30  in which the circular loop  14  of the vertically oriented coil is captured and supported. The spacer also has a mating feature  32   a , 32   b,  such as a protrusion  34  that mates with a recess  36  on an adjacent spacer when placed in the column  12  of vertically stacked spacers  16 . The mating features  32   a , 32   b  in the adjacent spacers work in conjunction with gravity and the weight of the coil to interlock the adjacent spacers in contiguous vertical relationship, such as a column  12 . Other vertical relationships are also possible, including, for example, staggered, alternating and step-wise or stair-wise. The mating features  32   a , 32   b  may simply nest together to facilitate easy assembly, but alternatively the mating feature  32   a , 32   b  could be modified into a more positive locking method like a “dove tail” or can incorporate a fastener if desired without deviating from the spirit of the invention. 
         [0028]    Alternately, the protrusion  34  at the end of a column  12  may mate with a portion of the column support component  22  or the recess  36  may mate with a portion of the opposite column support component. Central cavity  38  traverses at least some, alternatively all, of the width of the spacer and is incorporated to reduce the overall mass of the spacer  16 , which in turn reduces the energy required to heat the spacer  16  and the energy storage in the spacer  16 , which can affect the rate that the spacer  16  cools. 
         [0029]    The spacer  16  depicted in  FIG. 2  is typical of one with a larger pitch dimension. The pitch dimension is defined by the distance from the plane containing the top flat surfaces  42  to the plane containing the bottom flat surfaces  44 , exclusive of the protrusion  34 . The pitch dimension in turn determines the distance between individual circular loops  14  in the coil assembly. 
         [0030]      FIG. 3  depicts another exemplary embodiment of a spacer  16  with a smaller pitch dimension. It consists of the same basic features as the larger pitch spacer  16  depicted and described in connection with the spacer  16  in  FIG. 2 . Namely, these features include a pocket  30 , a mating feature  32   a . 32   b  with a protrusion  34  and a recess  36 , and central cavity  38 . The notable difference in the embodiment of the spacer in  FIG. 3  in comparison to that depicted in  FIG. 2  is that the spacer  16  in  FIG. 3  has a flat base  50  that can be used to mate to the spacer column support component  22 . The flat base  50  provides additional surface area for support of the spacer column and a smooth surface to decrease the friction between the flat base  50  and the spacer column support component  22 . 
         [0031]    The relationships of the components in the spacer support assembly are shown in  FIG. 4 . The spacer column support component  22  includes a guide slot  60  embossed into at least a portion of its top surface. The guide slot  60  aligns the flat base  50  portion of the last (lowest) spacer  16  to the central axis of the spacer column support component  22 . A receptacle  62  is created within the spacer column support component  22  and passes through at least a portion of the spacer column support component  22  and is used to capture the vertical channel  18 , when used, and to maintain the alignment of the spacer column  12  and vertical channel  18 . The opening or void in the spacer column support component  22  also confines the inward lateral movement of the spacer column  12  by capturing the protrusion  34  of the last (lowest) spacer  16  in the column  12 . The outward lateral movement of the spacer column  12  is restricted by the innermost surface of the vertical channel  18 . The interface between the flat base  50  and guide slot  60  can be enhanced by using surface enhancement techniques (like polishing, grinding, selective coating, etc . . . ) in order to minimize friction and therefore allow the spacer support columns  12  to move more freely in the desired axis. Furthermore, small bearings or other structures can be incorporated at this interface to reduce friction even more if desired. 
         [0032]    A side view of an exemplary spacer column support component  22  is shown in  FIG. 5 , detailing the relationship of the captured protrusion  34  of the last (lowest) spacer  16  in the spacer column  12  and the guide slot  60  in the spacer column support component  22 . A portion  70  of the interlocked spacers  16  resides within the vertical channel  18  keeping the spacers  16  aligned (collinear) and oriented in a preferential direction towards the center of the heating element coil, while still being allowed to move slideably inward and outward on an axis perpendicular to the tangent of the heating element coil diameter and the vertical spacer column  12 . The maximum distance that the spacer  16  may move outward from the center of the heating element coil is defined by the space  72  between the outer surface of the spacer  16  and the inner surface of the vertical support  18 . This maximum movement inward toward the center of the heating element coil is limited by the interference of the innermost surface of the spacer protrusion  34  and the receptacle in the spacer column support component  22 . 
         [0033]    In  FIG. 5 , the wire is supported above the lower surface of the spacer column support component  22  at a distance, D. This allows the wire to be free radiating and not in contact with the surface on which the spacer column support component rests. An example of a suitable distance is 9.35 mm. 
         [0034]    Referring to  FIG. 6 , several columns of vertical element support structures  80 A- 80 H are arranged around the circumference of a heating coil structure  82 . The arrangement is equidistant along the circumference from a central position  84  and in opposing pairs (i.e.  80 A to  80 E,  80 B to  80 F, etc . . . ). The vertical element support structures  80 A- 80 H are seen from the end at which the spacer column support component  22  is located, similar to that shown in  FIG. 4 . 
         [0035]    Referring to  FIG. 7 , vertical element support structures  80 A- 80 H are shown in perspective view arranged around the circumference of a heating coil structure  82 . The view illustrates an example of the coil  82  being held in the pocket  30  of a spacer  16 . The spacers  16  are arranged in a vertical column  12  in the channel  18  of the vertical element support structures  80 A- 80 H. Each of these features is not individually labeled in  FIG. 7  for ease of viewing. 
         [0036]      FIG. 8  schematically represents the forces and movement of the vertical element support structures ( 80 A- 80 H in  FIGS. 6 and 7 ) arranged around the circumference of the heating coil. Movement of the heating coil and the vertical element support structures are represented in idealized manner by arrows  90 A- 90 H. As the temperature of heating element coil  82  increases, the coil length increases causing the coil diameter to increase and the mean diameter to move from a first position  92  to a second position  94 . The vertical spacer columns  12  direct the movement outward relative from the center position  84  while maintaining the concentricity. At the same time, the adjacent coil loops remain interlocked, keeping the coil loops collinear and concentric. When the heating element cools and contracts, the mean diameter decreases from the second position  94  to the first position  92 . The columns  12  of vertically supported spacers  16  direct the movement back to the center of the heating element assembly. Permanent elongation is accommodated in a similar manner where the heating element coil elongates over time, increasing the mean coil diameter. The columns  12  of vertical supported spacers  16  maintain the collinearity, concentricity &amp; centering of the heating element assembly. 
         [0037]    Alternate configurations for the spacer profile and the vertical channel can be employed. Two of these alternates are depicted in plan view in  FIG. 9A  and  FIG. 9B . In  FIGS. 9A and 9B , the spacers  16  fit slideably into vertical channel  18 . The portion  70  of the spacers  16  residing within the vertical channel  18  is of a different width (W) than the rest of the spacer such that it is captured by a feature in the channel, such as a flange edge. In  FIG. 9A , there are two such features, a first flange edge  100   a  and a second flange edge  100   b  and the spacer  16  is symmetrically captured by this features  100   a , 100   b  in the channel  18 , and in  FIG. 9B  there is one such feature  100  and the spacer  16  is asymmetrically captured by this feature  100  in the channel  18 . The feature and the capturing limit the travel of the spacers  16  within the vertical channel  18  in a first direction, i.e., direction Y, in response to changes in diameter and/or position of the heating coil. 
         [0038]    Either alternate configuration can be used in conjunction with or independent of the mechanism described in  FIG. 4  and  FIG. 5 . Employing these alternate configurations has the benefit of reinforcing the maximum inward movement limit of the spacer rows. However, these alternate configurations can require installing the spacers by sliding the vertical channel over the spacers; therefore it is may be more difficult to replace a spacer within the column if it is broken. 
         [0039]    It can be seen from the structure described that several advantageous features are created. Namely, a support structure is presented that allows for expansion and contraction of the heating element coil while keeping the spacer support columns aligned in a collinear arrangement constraining the adjacent loops of the heating element coil and keeping loops collinear, concentric, and maintaining the proper centering of the heating element coil in the assembly. 
         [0040]    Although described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.