Abstract:
A cage boat having easily replaceable parts utilizes removable spacer rails for holding semiconductor wafers contained within a cage formed from slats removably attached to end pieces perpendicular to the slats.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of semiconductor processing. In one of its more particular aspects, this invention relates to a device for holding wafers of semiconducting materials during thermal processing or coating. In another of its more particular aspects, this invention relates to such a device which can be readily repaired or modified. 
     BACKGROUND OF THE INVENTION 
     In the processing of semiconductors for use in various electronic applications, it is frequently necessary to subject semiconductors to thermal processing or to coat them by such means as chemical vapor deposition. Semiconductors are typically furnished in the form of thin wafers which are treated in special furnaces or coating chambers. In order to prevent injury to the wafers, it has been found desirable to hold them in position during processing by means of slotted racks. The racks are typically placed in semi-open containers for ease in handling and to facilitate coating. One form of container which has been used is a cage boat which is a solid, two-part container provided with slots. The semiconductor wafers are placed within the cage boat, and the cage boat is then placed within the furnace or coating chamber. It is desirable to use a cylindrical configuration for the cage boat, in order to ensure uniformity in the processing of the semiconductor wafers. 
     In the past, racks for holding wafers of semiconducting materials and cage boats containing such racks have been fabricated of quartz. However, quartz cage boats must be handmade, requiring skilled labor and resulting in a relatively expensive device. In addition, quartz cage boats are characterized by poor dimensional tolerances. By their very nature, items fabricated from quartz are fragile. Quartz cage boats were therefore frequently broken during use. Another disadvantage of quartz cage boats is that cleaning with hydrofluoric acid, which is necessary to remove accumulations of silicon dioxide, results in the quartz being attacked by the acid and being devitrified, thereby reducing the strength of the cage boat, changing its dimensions and producing particles on the surface thereof which can contaminate the semiconductor wafers during processing. 
     Recently, cage boats have been produced from silicon carbide. Silicon carbide cage boats are stronger and more resistant to hydrofluoric acid than quartz, but they are expensive, difficult to fabricate and, in general, cannot be made with desired dimensional tolerances. 
     Because of the expense of fabricating cage boats and the need for frequent replacement thereof, it would be desirable to provide a cage boat constructed of easily fabricated material which could be readily disassembled for replacement of damaged parts. 
     SUMMARY OF THE INVENTION 
     The present invention provides a cage boat which has the advantage of being fabricated in such a manner that the parts thereof are readily replaceable. Replacement can be accomplished using common tools and does not require any special skills. The cage boat of the present invention, which is preferably in the form of a substantially closed cylinder, includes end pieces forming the bases of the cylinder and slatted walls which are releasably connected to the end pieces. The cylindrical container is divided into two halves which fit together snugly, one half containing grooved spacer rails for holding the semiconductor wafers. In general, the cage boat of the present invention comprises a pair of end pieces, a plurality of parallel slats perpendicular to said end pieces and removably attached thereto, and one or more grooved spacer rails parallel to said slats and removably positioned thereupon, said end pieces and said slats forming a cage enclosing said spacer rails. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood by reference to the accompanying drawings wherein like numerals refer to like elements, and in which: 
     FIG. 1 is a perspective view of the cage boat of the present invention containing a plurality of semiconductor wafers and partially broken away to show the removability of a slat. 
     FIG. 2 is a perspective view of the cage boat of the present invention showing the two halves thereof and containing a plurality of semiconductor wafers partially broken away to show part of a spacer rail and apertures into which pegs fit for holding the two halves together. 
     FIG. 3 is a cross-sectional view of the cage boat of the present invention taken along line 3--3 of FIG. 1. 
     FIG. 4 is a transverse section of the cage boat of the present invention along line 4--4 of FIG. 1. 
     FIG. 5 is a transverse section of part of the cage boat along line 5--5 of FIG. 3. 
     FIG. 6 is a perspective view of part of another embodiment of the cage boat of the present invention showing one of the spacer rails partially removed from the cage boat. 
     FIG. 7 is a transverse cross-section of part of the embodiment of FIG. 6 along line 7--7 of FIG. 6. 
     FIG. 8 is a cross-section of the embodiment shown in FIGS. 6 and 7 taken along line 8--8 of FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The cage boat of the present invention is fabricated of materials which are sufficiently durable to be readily removable for replacement or modification. It is provided with a plurality of spacer rails which function as racks to hold the semiconductor wafers. The spacer rails are positioned upon slats forming the walls of the cylindrical container. In one embodiment, the rails are held in position by means of slots in the end pieces of the container. Replacement of a rail can be accomplished by removing one slat. In another embodiment, the rails are inserted via through-slots in one end piece and are held in position by means of fasteners which project through the end piece and press firmly against the end of the rails. In this latter embodiment, which is preferred, lateral movement of the rail is prevented. Replacement of a rail in this embodiment merely requires removing the fastener. 
     The cage boat of the present invention is preferably fabricated from graphite. Most preferably, all parts, including bolts and screws, are fabricated from graphite to ensure that a tight fit is maintained under processing conditions by using parts having the same coefficient of thermal expansion. Graphite has the advantages of providing the properties desired for the cage boat. In particular, graphite resists chemical attack by hydrofluoric acid, resulting in long life and consistent performance. Graphite can be fabricated to precise dimensions and is dimensionally stable. Since graphite can be purified to contain very low ash levels, i.e., less than about 5 ppm ash, the cage boat of the present invention will not contaminate wafers of semiconducting materials placed within the cage boat. In addition, graphite cage boats are light in weight, graphite having a density of about 1.8 g./cc. and have excellent thermal conductivity. Various grades of graphite are commercially available. A particularly desirable material is a fine particle, high strength, isotropic graphite available from Poco Graphite, Inc., a subsidiary of Unocal Corporation, Decatur, TX which is sold as POCO DFP-2. Other grades of graphite can be used as well. If desired, the components of the cage boat can be coated or infiltrated with pyrolytic carbon to seal the surface, reduce particulates and improve abrasion resistance. 
     Referring to the drawings, FIG. 1 depicts the cage boat of this invention containing a plurality of semiconductor wafers in the form of circular disks. FIG. 1 shows a slat and the fasteners therefor removed from the cage boat. The numeral 10 represents the cage boat, which consists of two halves, a left half 12 and a right half 14, which fit together snugly. Left half 12 contains a top end piece 16 and a bottom end piece 18. Right half 14 contains a top end piece 20 and a bottom end piece 22. Both halves contain slats 24 which are attached to the end pieces by means of screws 26. It can be seen that slats 24 fit together to form a substantially closed cage with openings left between the slats. It is also possible to fit the slats together without leaving openings between the slats using slats having openings in the body of the slat instead. 
     FIG. 1 shows one of slats 24 and the screws 26 normally holding slat 24 in position against top end piece 16 and bottom end piece 18 removed from the left half 12 of cage boat 10. FIG. 1 also shows a number of semiconductor wafers 28 within cage boat 10. 
     In FIG. 2 there are additionally shown pegs 30 projecting from top end piece 16 and bottom end piece 18 of left half 12 of cage boat 10. Pegs 30 fit snugly within apertures 32 in top end piece 20 and bottom end piece 22 of right half 14 of cage boat 10 to secure the two halves together as shown in FIGS. 2 and 3. Also shown in FIGS. 2 and 3 is a spacer rail 34 containing grooves 36. The spacer rails are elongated strips which hold the semiconductor wafers within grooves therein. They rest upon the slats of the cage boat and are held in position by means of slots in the end pieces into which the ends of the spacer rails fit. In the embodiments shown in the drawings, there are three spacer rails. 
     In FIG. 4 spacer rails 34 are seen fitting into slots 38 in top end piece 20 of right half 14. 
     FIG. 5 shows screws 26 securing slats 24 to bottom end piece 18 of left half 12. 
     FIGS. 6, 7 and 8 illustrate a preferred embodiment of the cage boat of the present invention, which differs from the embodiment described above in having a through-slot in one end piece for insertion and removal of the spacer rails and a slot similar to that of the first embodiment in the other end piece. In this embodiment the spacer rails are fixed in position by means of set screws. Referring specifically to FIG. 6, the numeral 40 is used to designate the cage boat. As in the case of the first embodiment, cage boat 40 consists of two halves, left half 42 and right half 44. Cage boat 40 contains top end pieces 46 and 48 on left half 42 and right half 44, respectively. Slats 50 are attached to the top end pieces by means of screws 52. A spacer rail 54 having grooves 56 is shown projecting upwards from top end piece 48 of right half 44 of cage boat 40. Spacer rails 54, only one of which is shown, are inserted into cage boat 40 via through-slot 58 in top end piece 48 in the direction shown by arrow 60. Spacer rails 54 are secured against lateral movement within cage boat 40 by means of set screws 62 which are inserted into apertures 64 in top end piece 48. 
     FIG. 7 shows further details of the arrangement of spacer rails 54 relative to set screws 62 and other parts. 
     FIG. 8 additionally shows bottom end piece 66 containing slot 68 into which one end of spacer rail 54 fits. Set screw 62 presses spacer rail 54 in slot 68 against the edge of end piece 66, preventing lateral movement of rail 54. 
     In operation, the two halves of the cage boat are separated and semiconductor wafers are placed in the grooves of the spacer rails. The two halves are then fitted together. The resulting cage boat containing the semiconductor wafers is then in a form which can be easily picked up, by automated equipment, if desired, and deposited in a furnace or coating chamber. The cage boat is typically used in a horizontal orientation with the semiconductor wafers vertically positioned. 
     While either embodiment described above has the advantage of being constructed of readily removable parts which can be replaced without replacing the entire cage boat, the embodiment providing the through-slots is particularly desirable in that replacement of the spacer rails can be accomplished merely by removing the set screw fixing the spacer rail into position without requiring the removal of the slats or end pieces. Thus, for example, it is possible to remove the spacer rails for cleaning or for replacement upon their becoming worn out. They may also be removed for replacement with another spacer rail of a different size or which may contain fewer or more grooves than the spacer rail being used. In addition to the ready replaceability of the parts of the cage boat of this invention, the fabrication of the parts from graphite makes the various parts readily interchangeable, since the graphite parts can be machined to close tolerances. 
     While particular embodiments of the invention have been described, it will be understood, of course, that the invention is not limited thereto, since many obvious modifications can be made. For example, other shapes than the cylindrical shape of the cage boat of the present invention can be utilized while still providing the advantages of the present invention. It is intended to include any such modifications as will fall within the scope of the appended claims.