Abstract:
A wafer boat for accommodating semiconductor wafers comprises two side rods and at least one back rod, the rods being vertically oriented and extending between a top member and a bottom member. The rods comprise vertically spaced recesses formed at corresponding heights, recesses at the same height defining a wafer accommodation for receiving and supporting a wafer in a substantially horizontal orientation, the recesses having an improved shape. The upwardly facing surfaces of the recesses comprise a first flat surface in an inward region of the recess which is horizontal or inclined upward in an outward direction of the recess and a second flat surface in an outer region of the recess which is inclined downward in an outward direction of the recess. The intersection of the first and second surface forming an edge for supporting the wafer. The recesses are easy to machine and prevent damage to the wafer.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a design for a wafer boat for accommodating a vertically spaced stack of semiconductor wafers during processing in a vertical furnace. 
       BACKGROUND 
       [0002]    In vertical furnaces used for semiconductor wafer processing, a wafer boat may be used for accommodating a stack of vertically spaced apart wafers. Typically, as shown in  FIG. 1 , a wafer boat  100  comprises three or more vertical rods  110  which extend between a top member  120  and a bottom member  130 . In the example of  FIG. 1 , three rods  110  are shown: two side rods  110 A and one back rod  110 B. In the rods, vertically spaced apart recesses  150  are formed at corresponding heights, facing inwardly (i.e. generally towards a central axis of the wafer boat  100 ). Each recess serves as a support arranged for engaging a circumferential side portion of a wafer, and recesses at a same height define a wafer accommodation for receiving and supporting a wafer in a substantially horizontal orientation. Wafers can be inserted into and removed from the boat at an open side of the boat, opposite the back rod  110 B. 
         [0003]    A problem with known wafer boats is that damage may occur on the wafer at the point of contact between the wafer and the boat. Due to different thermal expansion of the wafer and the boat, for instance, the wafer may move relative to the recess, resulting in scratches. 
         [0004]    In the art, support members are proposed that have a convex support surface to minimize contact stresses in the wafer at the support point. However, boats having these support members are difficult to manufacture and require elaborate machining. In some proposals the wafer support members are separately machined for ease of machining and then mounted in the wafer boat which is also a complex procedure and construction. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the present invention to provide a wafer boat that minimizes contact stress in supported wafers and is easier to manufacture than known wafer boats. 
         [0006]    To this end, a first aspect of the present invention is directed to a wafer boat comprising two side rods and at least one back rod. In a use orientation of the wafer boat, the rods may be substantially vertically oriented, and extend between a top member and a bottom member. The rods may comprise vertically spaced apart recesses formed at corresponding heights, wherein recesses at a same height may face generally inwards and towards each other to define a wafer accommodation for receiving and supporting a wafer in a substantially horizontal orientation. Each recess may include a lower, upwardly facing surface comprising a first flat surface (portion) and a second flat surface (portion). The first flat surface (portion) may be disposed in an outward region of a respective recess, and extend horizontally or be inclined downward in an outward direction. The second flat surface (portion) may be disposed in an inner region of the recess, and be inclined downward in an inward direction. An intersection of or join between the first and second surfaces may form an edge or ridge for supporting a wafer. 
         [0007]    The first and second flat surfaces or surface portions may thus include an angle. The included angle may typically be an obtuse angle, i.e. an angle in the range of 90-180 degrees. In one embodiment, for instance, the respective angles of inclination of the first and second surfaces/surface portions of a lower surface of a recess may be in a range of 1 to 10 degrees with respect to a horizontal plane; accordingly, the angle included by the respective first and second surfaces may be in the range of (180−(2*10)=)160 to (180−(2*1)=)178 degrees. In a preferred embodiment, the respective angles of inclination of the first and second surfaces/surface portions of a lower surface of a recess may be in a range of 2 to 6 degrees with respect to a horizontal plane; accordingly, the angle included by the respective first and second surfaces may be in the range of (180−(2*6)=)168 to (180−(2*2)=)176 degrees. In a more preferred embodiment, the respective angles of inclination of the first and second surfaces may be about 3 degrees, such that the angle included by the respective first and second surfaces is about (180−(2*3)=)174 degrees, e.g. 174±1 degrees. 
         [0008]    An edge defined by the first and second surfaces/surface portions of a lower surface of a respective recess may preferably extend in a generally horizontal plane, and/or be rounded off to a radius of curvature of at least 1 mm, and more preferably to a radius of curvature of at least 3 mm. 
         [0009]    These and other features and advantages of the invention will be more fully understood from the following detailed description of certain embodiments of the invention, taken together with the accompanying drawings, which are meant to illustrate and not to limit the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0010]      FIG. 1  schematically shows, in a perspective view, an exemplary embodiment of a wafer boat according to the present invention in its entirety; 
           [0011]      FIGS. 2 and 3  show schematically the shape of recesses in two exemplary embodiments of the invention; 
           [0012]      FIGS. 4 and 5  schematically show in more detail the shape of recesses in two exemplary embodiments of the invention in a side view and in a perspective view; 
           [0013]      FIGS. 6 and 7  schematically shows orientations of the rods in two embodiments of the invention in a top view. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The invention will be further explained with reference to the figures. In the figures, similar parts will be referred to with like reference numerals. 
         [0015]      FIGS. 2 and 3  schematically illustrate the shape of recesses  150  in two alternative, exemplary embodiments of the invention; for clarity, each of the recesses  150  is shown to support a wafer  160  at a position along the wafer&#39;s outer circumferential edge. In each embodiment, a lower, upwardly facing surface of a recess  150  comprises two adjacent, flat or substantially flat surfaces/surface portions  210 ,  220  that are inclined relative to each other. That is, the two surfaces  210 ,  220  may include an angle smaller than 180°, while the outward normals of the surfaces  210 ,  220  may diverge from one another. In the context of a wafer boat  100  as shown in  FIG. 1 , the recesses  150  may face generally inward, towards a central axis of the wafer boat  100 . Accordingly, a first  210  of the two surfaces  210 ,  220  may be associated with an inner region of a respective recess  150 , while a second  220  of the two surfaces  210 ,  220  may be associated with an outer region of the respective recess  150 . 
         [0016]    In the embodiment of  FIG. 2 , the first flat/planar surface  210  in the outer region of a recess  150  is horizontal, while the second flat/planar surface  220  in an inner region of the recess is inclined downward in the inward direction. The intersection of the first and second surfaces  210 ,  220  forms an edge  230 . Preferably, edge  230  is rounded off to a radius of curvature of 1 mm or greater and more preferably to a radius of curvature of 3 mm or greater to avoid contact stress in the wafer by a sharp edge. 
         [0017]    When a wafer  160  is supported on a lower, upward facing surface  210 ,  220  of a recess  150  including an edge  230 , a central portion of the wafer  160  sags under its own weight and the wafer  160  shows a radius of curvature, with the wafer edge bending upwards. As a result, the wafer  160  is supported on the edge  230  of the lower, upward facing surface  210 ,  220  of the recess  150 , and the outer region of the wafer  160 , extending beyond edge  230 , is not contacting surface  210 . Also the free hanging part of the wafer  160  at the open end of the wafer boat sags significantly. Preferably, the boat  100  is dimensioned such the support edge  230  supports the wafer  160  at a position not more than 2 mm away from the outer, circumferential edge of the wafer  160 . Preferably the inner edge  240  of the recess  150  is also rounded off. 
         [0018]    In the embodiment of  FIG. 3 , the first flat surface  310  in the outer region of the recess  150  is inclined downward in the outward direction, while the second flat surface  320  in the inner region of the recess  150  is inclined downward in the inward direction. The intersection of the surfaces  310 ,  320  forms an edge  330 . Preferably, edge  330  is rounded off to a radius of curvature of 1 mm or greater and more preferably to a radius of curvature of 3 mm or greater to avoid contact stress in the wafer by a sharp edge. The inclination of the first surface  310  ensures that the outer region of the wafer  160 , extending beyond edge  330 , is not contacting surface  310 , also in case of e.g. small variations in the dimensioning of the boat  100  or the wafer being slightly warped. Preferably, the boat  100  is dimensioned such that the support edge  330  supports the wafer  160  at a position not more than 2 mm away from the edge of the wafer. Preferably the inner edge  340  of the recess is also rounded off. 
         [0019]      FIG. 4  shows the recesses of  FIG. 2  in more detail. And  FIG. 5  shows the recesses of  FIG. 3  in more detail. An angle of inclination relative to a horizontal plane of first surfaces  210 ,  310  is in a range of 0 to 10 degrees, more preferably in a range of 0 to 6 degrees and most preferable about 3 degrees. If inclined, the first surface  210 ,  310  is inclined downward in the outward direction. An angle of inclination relative to a horizontal plane of second surface  220 ,  320  is in a range of 1 to 10 degrees, more preferably in a range of 2 to 6 degrees and most preferable about 3 degrees and the inclination is downward in the inward direction. Edge  230 ,  330  is in about tangential direction of a wafer  160  supported in the boat  100  and are preferably horizontal. In  FIGS. 4 and 5  the angle of inclination relative to the horizontal plane for surfaces  220 ,  310  and  320  is 3 degrees and edge  230 ,  330  is rounded off to a radius of curvature of 3 mm. 
         [0020]      FIGS. 6 and 7  shows orientations of the rods  110  in two embodiments of the invention in a top view. 
         [0021]      FIG. 6  shows an orientation that is commonly used for wafer boats  100 : the recesses in side rods  110 A are generally facing inwards and towards each other. At a side of each side rod  110 A that is nearest to the centerline of the wafer  160  (the imaginary, interrupted line that extends through the center of the wafer  160  and runs from left to right in the Figure), edge  230 ,  330  extends approximately tangentially to a circumferential edge of a wafer  160 , but at the opposing side of the side rod  110 A a significant deviation from the tangential direction occurs. A disadvantage of this orientation in combination with the sagging of the wafer  160  towards the center and towards the open end of the boat  100  is that the point of contact of the wafer  160  in the recesses  150  of the side rods  110 A will be at positions C, at the side of the rods  110 A, at an end of edges  230 ,  330 . This might not be beneficial for contact stresses in the wafer  160 . Further, when the boat is used in CVD processes, the point of contact at the side of the rods  110 A might be susceptible to sticking to the wafer due to film deposition and particle formation during unloading of the wafer after deposition. In back rod  110 B, the point of contact C will be in the center of the recess  150 , in the center region of the edge  230 ,  330 . 
         [0022]    In  FIG. 7  the orientation of the side rods  110 A has been adapted to mitigate the above-mentioned issues. The adapted orientation of the side rods  110 A may be best described in terms of several geometric lines that relate the orientation of the rods  110 A,  110 B of the wafer boat  100  to a respective wafer accommodation defined by recesses  150  provided therein at a corresponding height. A first geometric line introduced to this end is a (diametrical) side-to-side centerline a that extends through a centerpoint M of the wafer accommodation from one side to another. Another geometric line is a (diametrical) back-to-front centerline b that extends through the centerpoint M of the wafer accommodation from the back to the front, and that is perpendicular to the side-to-side centerline a. A further connection line l may connect the centers of edges  230 ,  330  of opposite side recesses  150  (in side rods  110 A) defining the wafer accommodation, while respective direction lines x and y may extend through the respective centers of (the recesses  150  in) side rods  110 A in a direction substantially perpendicular to the respective recesses  150 , and more in particular to the edges  230 ,  330  provided therein. 
         [0023]    The side rods  110 A may now be positioned to the front of side-to-side centerline a, while the recesses  150  in the side rods  110 A still approximately face the center M of a wafer accommodation. Connection line l may extend parallel to side-to-side center line a. Direction lines x and y may be non-parallel (preferably including an obtuse angle), and intersect back-to-front centerline b in the center of the wafer accommodation M. However, the optimum crossing point/point of intersection of direction lines x and y with back-to-front centerline b may be spaced from center point M, and be located between a point at a distance d in the forward direction from center point M and a point at a distance 2d in backward direction from center point M to achieve that the point of contact C in the recesses of the side rods  110 A will be approximately in the center of the recess, in the center of edges  230 ,  330 . Here, d may equal 7% of a diameter of the wafer accommodation, e.g. the diameter of the inscribed circle of the recesses  150  defining the wafer accommodation. For a wafer accommodation configured to hold a 300 mm wafer, d may thus amount to 21 mm. 
         [0024]    It is an advantage of the present invention that main upwardly facing surfaces of the recesses are flat surfaces that are relatively simple to machine. As discussed before the edge  230 , 330  and the edge  240 ,  340  are preferably rounded off but that is relatively easy to achieve. 
         [0025]    The boat of the present invention can be made of quartz or silicon carbide or silicon or any other temperature resistant ceramic material that does not contaminate the wafers. If the boat is provided with a silicon carbide coating, deposited by Chemical Vapor Deposition, preferably a polishing treatment is applied after deposition of the coating to remove eventual protrusion grown during the coating process. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           100  wafer boat 
           110  rod 
           120  top member 
           130  bottom member 
           150  recess 
           160  wafer 
           210 ,  310  first, outer flat surface 
           220 ,  320  second, inner flat surface 
           230 ,  330  edge formed at intersection of first and second flat surfaces 
           240 ,  340  inner edge of recess 
         C contact position 
         M wafer center/geometric center of wafer accommodation