Abstract:
A tubular member formed of silicon staves and arranged in a circular pattern to form a central bore in which a wafer support tower can be inserted for batch thermal processing in an oven. The staves are formed along an axis with an interlocking keyway structure in which axially extending hooks engage axially extending catches formed in back of the hooks on neighboring staves. An adhesive, such as a silica-forming agent and silicon powder, coat the keyway structure before assembly and is cured after assembly, so as to bond the staves together. A similar structure may be used to form a plate structure from an array of smaller parts with interlocking structure formed between neighboring parts.

Description:
RELATED APPLICATION 
       [0001]    This application claims benefit of provisional application 60/760,993, filed Jan. 21, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to equipment used in thermal processing of substrates. In particular, the invention relates to large structures used in semiconductor processing such as a tubular liner used in a thermal oven. 
       BACKGROUND OF THE INVENTION 
       [0003]    Batch thermal processing continues to be used for several stages of fabrication of silicon integrated circuits. One low temperature thermal process deposits a layer of silicon nitride by chemical vapor deposition, typically using chlorosilane and ammonia as the precursor gases at temperatures in the range of about 700° C. Other, high-temperature processes include oxidation, annealing, silicidation, and other processes typically using higher temperatures, for example above 1000° C. or even 1350° C. 
         [0004]    For large-scale commercial production, vertical furnaces and vertically arranged wafer towers supporting a large number of wafers in the furnace are typically used, often in a configuration illustrated in the schematic cross-sectional view of  FIG. 1 . A furnace  10  includes a thermally insulating heater canister  12  supporting a resistive heating coil  14  powered by an unillustrated electrical power supply. A bell jar  16 , typically composed of quartz, includes a roof and fits within the heating coil  14 . An open-ended liner  18  fits within the bell jar  16 . A support tower  20  sits on a pedestal  22  and during processing the pedestal  22  and support tower  20  are generally surrounded by the liner  18 . It includes vertically arranged slots for holding multiple horizontally disposed wafers  19  to be thermally processed in batch mode. The diameter of the internal axially extending bore of liner  18  must be great enough to accommodate the wafers  19  and the support tower  20 , that is, significantly greater than 200 mm for processing 200 mm wafers and significantly greater than 300 mm for processing 300 mm wafers. A gas injector  24  is principally disposed between the liner  18  has an outlet on its upper end for injecting processing gas within the liner  18 . An unillustrated vacuum pump removes the processing gas through the bottom of the bell jar  16 . The heater canister  12 , bell jar  16 , and liner  18  may be raised vertically to allow wafers to be transferred to and from the tower  20 , although in some configurations these elements remain stationary while an elevator raises and lowers the pedestal  22  and loaded tower  20  into and out of the bottom of furnace  10 . 
         [0005]    The bell jar  16 , which is closed on its upper end, tends to cause the furnace  10  to have a generally uniformly hot temperature in the middle and upper portions of the furnace. This region is referred to as the hot zone in which the temperature is controlled for the optimized thermal process. However, the open bottom end of the bell jar  18  and the mechanical support of the pedestal  22  causes the lower end of the furnace to have a lower temperature, often low enough that the thermal process such as chemical vapor deposition is not effective. The hot zone may exclude some of the lower slots of the tower  20 . 
         [0006]    Conventionally in low-temperature applications, the tower, liner, and injectors have been composed of quartz or fused silica. However, quartz towers and injectors are being supplanted by silicon towers, liners, and injectors. Silicon towers of somewhat different configurations for various applications and silicon injectors are commercially available from Integrated Materials, Inc. of Sunnyvale, Calif. and are disclosed respectively in U.S. Pat. No. 6,450,346 and U.S. patent application Ser. No. 11/177,808, filed Jul. 8, 2005 and published as U.S. Patent Publication 2006/0185589. Silicon liners present challenges in their fabrication because of their very large diameters and the general unavailability of high-purity silicon in such large sizes. However, Boyle et al. disclose an effective method of fabricating silicon liners from silicon staves in U.S. patent application Ser. No. 10/642,013, filed Sep. 26, 2001 and published as U.S. Patent Publication 2004/0129203, incorporated herein by reference in its entirety. Silicon is available in very high purity in the form of virgin polysilicon (electronic grade silicon) and thus contains very low levels of impurities. However, a silicon member is defined as comprising at least 95 at % and preferably at least 99 at % elemental silicon. 
         [0007]    A silicon liner  30  may be formed by bonding together, as illustrated in the cross-sectional view of  FIG. 2 , sixteen or so silicon staves  32 , which are long and thin, for example, 4 mm thick and 1 m long. Note that the early figures do not accurately portray the thinness of the staves. They are generally rectangular but to conform more closely to the polygonal shape they are somewhat trapezoidal. They are arranged in a closed polygonal (nearly circular) shape about a center  36  and bonded together to form a tubular member having a form similar to that of a wooden wine barrel. To accommodate a tower supporting 300 mm wafer, the liner  30  needs to have an internal diameter of approximately 350 mm. A very effective adhesive for bonding together silicon staves is a composite of a spin-on glass (SOG) and silicon powder, as disclosed by Boyle et al. in U.S. Pat. No. 7,083,694. 
         [0008]    It is perhaps possible that the staves could have flat abutting surfaces. However, the staves must be aligned to each other during the high-temperature curing of the adhesive. Accordingly, the design was developed of a tongue-and-groove joint, illustrated in the sectional view of  FIG. 3 , in which each of two staves  40 ,  42  are formed with a V-shaped tongue  44  and a V-shaped groove  26  with flat areas  48  on opposed sides of the tongue  44  and grooves  46 . The tongue  44  of the first stave  40  faces and mates with the groove  18  of the second stave  44 . The adhesive is applied to the mating surfaces before the staves are assembled together and then annealed at an elevated temperature to cure the adhesive. Such silicon liners have been fabricated, but their assembly is long and difficult and the yield remains low. 
       SUMMARY OF THE INVENTION 
       [0009]    A multi-part structural member formed of bonded parts, particularly a tubular member formed of staves bonded together in a closed pattern, in which the joints are formed with interlocking members extending at least partially transversely to the plane of the parts or staves. A bonding agent may be applied to the joint before its assembly. The interlocking joint inhibits motion across the joint and facilitates alignment. 
         [0010]    One embodiment of the interlocking mechanism includes an axially extending hook on each side of the stave or other part and a catch in back of the hook. The hook of one stave or part engages and interlocks with the catch of the neighboring stave or part. Advantageously, the radius of curvature at a corner of the hook is greater than that of the catch to produce a larger gap at the corner. 
         [0011]    The invention is particularly useful for forming silicon liners and other large silicon tubes used in batch thermal processing furnaces used in the semiconductor industry. The bonding agent for silicon members may be a combination of a spin-on glass and silicon powder. 
         [0012]    For tubular assemblies, the hooks on one stave may extend perpendicularly inward from an outer principal surface to facilitate assembly. 
         [0013]    The invention is also useful for forming planar plates out of smaller members. Interlocking joints for planar assemblies may extend perpendicularly to the principal surfaces of the member or in some applications they are advantageously inclined. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a cross-sectional view of a furnace used for batch thermal processing of wafers and with which a liner of the invention may be used. 
           [0015]      FIG. 2  is schematic cross-sectional view of a liner formed from staves bonded together to form a polygonal tube. 
           [0016]      FIG. 3  is a cross-sectional view of a tongue-and-groove joint between staves. 
           [0017]      FIG. 4  is a graph of the strength of different types of joints including a keyway joint of the invention. 
           [0018]      FIG. 5  is a cross-sectional view of a V-shaped joint between staves. 
           [0019]      FIG. 6  is a cross-sectional view of a keyway joint between two co-planar members. 
           [0020]      FIG. 7  is an orthographic view of a liner formed with keyway joints and including an optional neck. 
           [0021]      FIG. 8  is an exploded orthographic view of the neck of  FIG. 7 . 
           [0022]      FIG. 9  is a cross-sectional view of a liner including one embodiment of the keyway joints. 
           [0023]      FIGS. 10 and 11  are exploded cross-sectional views of two regions of the liner of  FIG. 9  showing two types of staves forming the keyway joints. 
           [0024]      FIG. 12  is a cross-sectional view of a keyway joint in the liner of  FIG. 9 . 
           [0025]      FIG. 13  is another cross-sectional view the keyway joint of  FIG. 12  showing clearances between the staves. 
           [0026]      FIG. 14  is a cross-sectional view of a keyway joint used to assemble a planar sheet. 
           [0027]      FIG. 15  is a cross-sectional view of an inclined keyway joint particularly useful in forming large planar plates and further showing its assembly on a horizontal table. 
           [0028]      FIG. 16  is a cross-section view of an inclined keyway joint and its assembly on a tilted table. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    We have developed a jig to support and align eight staves with the uncured adhesive applied to the joint area. The jig includes at least two sets of T-shaped studs supported at different angles by an arc-shaped base at their bottoms and supporting different ones of the staves at their tops. The staves supported by the jig and sandwiching the uncured adhesive between the staves are then annealed to form a rigid semi-tubular member. The process is then repeated to form the other half and join it to the first half. The gap between the staves in which the adhesive pools and is cured should be kept thin, preferably about 35 μm. We have found it very difficult to maintain both the gap spacing and the proper orientation over the entire length and circumference of the uncured tubular assembly. The required cumulative accuracy for the sixteen staves of a standard design of a liner is about 80 μm and the angular resolution if about ±0.01°. We believe that the angular precision needs to be decoupled from the spatial precision. 
         [0030]    An overall measure of the integrity of a joint is the sheer torque before the joint breaks. A bar chart for sheer torque limits for various joints is presented in  FIG. 4  in units of dyne/cm 2 . For comparison, a solid piece of annealed virgin polysilicon (electronic grade silicon) breaks at about 110,000. For determining the effectiveness of a fusion process, a test stud procedure has been developed in which two rectangular silicon members are fused across a planar interface. We have imposed a standard of about 6000 but have routinely achieved above 60,000 as the process has solidified. The tongue and groove configuration for two co-planar staves, however, regularly fails at about 4000. 
         [0031]    A first approach attempts to emulate a ball-and-socket joint that allows the jig to provide the angular resolution and the joint to provide the spatial resolution. As illustrated in the cross-sectional view of  FIG. 5 , each stave  50  is formed with a convex V-shaped side  52  and a concave V-shaped side  54  which mate with each other with the adhesive filling a gap  56  between them. There is substantially no flat areas on the edges of the V shapes. The test staves were generally rectangular to form a planar assembly to simplify the torque tests. This design allows a substantial angular movement determined by the jig without the gap  56  being made severely non-uniform. The sheer tests displayed in  FIG. 4  showed poor results with breakage occurring around 4000. 
         [0032]    A second approach knocks off the acute end  58  of the convex V-shaped side  52  so that the tip is more rounded. The sheer tests, however, showed even poorer results. 
         [0033]    A preferred third approach uses a keyway design, illustrated in the cross-sectional view of  FIG. 6 . Staves  60 ,  62  are formed with ends having interlocking hook structures. Each stave  60 ,  62  includes a hook  64  and a catch  66  in back of the hook  64  for retaining the hook  64  of the other stave  62 ,  60 . That is, the hooks  64  point in different directions when the two staves  60 ,  62  are assembled together in a pair. The assembled hooks  64  and catches  66  form an interlocking joint between the two staves  60 ,  62  which prevents their separation in a direction parallel to the principal faces of the staves  60 ,  62  away from the joint. In this embodiment, both the hook  64  and the catch  66  have substantially rectangular shapes so that the retaining side is perpendicular to the side along which the staves  60 ,  62  can slide over each other. The hooks  64  and catches  66  are dimensioned such that the two staves  60 ,  62  may be assembled together with a predetermined gap  68  between them, which is pre-filled with the adhesive filling the gap  68 . The gap  68  is typically thinner than as illustrated. In the present designs, the nominal gap is about 35 μm but after completion of machining and surface roughening and cleaning a final gap of about 60 to 70 μm is obtained. It is believed that a final gap of 40 to 100 μm is acceptable. With further developments in the adhesive technology, this gap maybe further decreased. 
         [0034]    The test structure for the third approach was fabricated and fused. The torque tests shown in  FIG. 3  show a strength above 40,000 for the keyway design, that is, substantially in excess of the strengths of the tongue-and-groove joint and the test stud standard and nearly as much as the observed results for advanced test studs. Generally, the test structure showed great rigidity and tends to break in the silicon, presumably in the thin silicon arm in back of the catch  66 . 
         [0035]    We believe, although the invention is not bound by our understanding, that part of the strength of the keyway joint arises from the fusion of the adhesive to silicon in a blind joint  70  separated from the exterior by two right-angle turns on each side of the hook  66 . 
         [0036]    The planar test structure of  FIG. 6  needs to be adapted to the closed polygonal shape of a tube and the need to accurately assemble the staves together. One keylocked tube  80  is illustrated in the orthographic view of  FIG. 7 , its exploded view of  FIG. 8 , and the axial cross-sectional view of  FIG. 9 .  FIGS. 10 and 11  are exploded views of  FIG. 9 , and  FIG. 12  is a further exploded view of a keyway joint of  FIG. 10 . The keylocked tube  80  requires two types of alternating staves although a single type may suffice for other embodiments. Staves  82  have inwardly directed hooks  84 . Staves  86  have outwardly directed hooks  88 . The hooks  84 ,  88  axially extend as ridges along the length of the staves  82 ,  86  and along the central axis of the tube  80  when assembled. Further, both hooks  84 ,  88 , when assembled, extend perpendicularly to the major surface of the stave  82 . The orientations of the hooks and associated catches facilitate the assembly of the last hook-inward stave  82  onto the neighboring two already aligned hook-outward staves  86  to complete the tube if the assembly is done from the outside. Assembly from the interior would be facilitated if the hooks extend perpendicularly to the principal surface of the last assembled stave. 
         [0037]    A further enlarged cross-sectional view of the keyway joint shown in  FIG. 13  illustrates a predetermined small gap  90  between the staves  82 ,  86  around the hooks  84 ,  88  to allow for assembly and for the volume of the adhesive. Additionally, the radius of convex corners  92  of the hooks  84 ,  88  is greater than the radius of corresponding concave corners  94  of the catches so that enlarged corner gaps  96  can accommodate an overflow of the adhesive from the flat portion portions of the gap  90 , which flat portions provide most of the mechanical strength to the keyway joint. 
         [0038]    As is evident in  FIGS. 7 and 8 , the staves  82 ,  86  may be shaped formed to form an optional outer neck  100  on the lower outer side of the liner  80 . The neck  100  is sized such that the liner  80  can be held at its lower end within a circular stainless steel or other type of collar on top of the pedestal  22  of  FIG. 1  used in some types of furnaces. However, other furnaces include support platforms not requiring the neck  100 . The neck  100  maybe formed, as best illustrated in  FIG. 8 , by machining the bottom ends of the staves  82 ,  86  to have two side chamfers  102 ,  104  with a central flat ridge  106  extending from the principal outer surface of the staves  82 ,  86 . The chamfers  102 ,  104  and ridge  106  have equal circumferential widths and are equally angularly oriented with respect to the liner center  36  so that when the liner  80  is assembled the chamfers  102 ,  104  and central flat area  106  approximate a circularly symmetric surface of the neck  100 . The staves  82 ,  86  can be formed into more than three such angularly differentiated portions to better approximate a circle and, if desired, the staves  82 ,  86  maybe machined to have a purely circular neck  100 . 
         [0039]    The structure of tube  80  provides several advantages. There is some angular flexibility between the staves which can be aligned by the jig. As illustrated in  FIG. 13 , a double-blind flat joint  108 , that is, having two acute turns to the exterior, between adjacent hooks  84 ,  88  produces a good fusion between the staves  82 ,  88  through the cured composite adhesive. The size of the gap between the staves  82 ,  88  and hence the thickness of the adhesive can in large part be determined by the initial machining of the staves  82 ,  86 . The interlocking hooks provides some self-assembly and self-alignment in the circumferential as well as radial directions, thus simplifying the assembly and alignment. 
         [0040]    Other designs are possible. Each stave may be formed with hooks facing in opposed directions on the two ends. This design simplifies the fabrication and inventory of staves but presents a challenge in assembling the last, closing stave. Additional hooks and catches maybe added on each end. The hooks and catches do not require a completely rectangular form. 
         [0041]    Although the invention is particularly useful for fusing tubular silicon members, it may be applied to other uses. The interlocking mechanism may be applied to planar members that need to be joined together into a larger planar structure of a one- or two-dimensional array. As illustrated in the cross-sectional view of  FIG. 14 , two co-planar silicon plates  110 ,  112  are joined at an interlocking mechanism in which the plates  110 ,  112  includes respective hooks  114 ,  116  and catches  118 ,  120  respectively engaging the hooks  116 ,  114  of the other plate  112 ,  110 . The plates  110 ,  112  are bonded together to form a planar sheet. A double-blind joint promotes a strong adhesive bonding of the two plates  110 ,  112 . A similar interlocking mechanism may be applied to the other side of one or both of the plates  110 ,  112  to form larger sheets or three, four, or more plates. As a result, large silicon sheets can be fused from smaller silicon plates with the interlocking mechanism providing both alignment and a predetermined gap between neighboring ones of the plates. The large bonded sheets can be used to form gas showerheads or liner covers, as disclosed by Cadwell et al. in provisional application 60/765,013, filed Feb. 3, 2006. 
         [0042]    The fusing of the two or more plates  110 ,  112  can be accomplished by coating the keyway joint between the plates  110 ,  112  with the uncured adhesive and assembling the pre-coated plates  110 ,  112  on an assembly table  124  supporting bottom surfaces  126  of the plates  110 ,  112 . A press plate  128  applies pressure to top surfaces  130  of the plates  110 ,  112  to align the plates  110 ,  112  and press excess adhesive out of the joint. After the plates have been bonded together into a sheet with any necessary curing of the adhesive, the sheet may be machined, for example, rounded and bored between its principal surfaces with a plurality of showerhead jet holes or machined to form apertures in the liner cover. 
         [0043]    In the interlocking mechanism of  FIG. 14 , the hooks and catches extended generally perpendicularly to the principal planes of the plates  110 ,  112 . Another interlocking mechanism, illustrated in the cross-sectional view of  FIG. 15 , is particularly useful for assembling plates to form a planar sheet. Two generally planar parts  140 ,  142  are formed with inclined acute hooks  144 ,  146  and corresponding catches  148 ,  150  that have surfaces which are perpendicular to each other but are inclined with respect to opposed principal surfaces  152 ,  154  of the parts  140 ,  142 . After the keyway joints of two or more parts  140 ,  142  have been pre-coated with uncured adhesive, they are assembled vertically with the uppermost part  140  being supported from above by mechanical holding means, including for example the illustrated hangar hook engaged to a fixed support, and with the hooks  144 ,  146  engaging corresponding catches  148 ,  150 . Neither an assembly table nor a press plate is required. If desired, an inclined downward vertical load can be additionally imposed on the bottommost part  142 . The inclined hooks  144 ,  146  and catches  148 ,  150  under gravitational force and the optional downward load align the parts  140 ,  142  and force hooks  144 ,  146  into respective corners  156 ,  158  of the other part  140 ,  142 . The predetermined space between the parts  140 ,  142  filled with the adhesive is not clearly illustrated in  FIG. 15 . A double-blind flat joint  160 , across which the parts  140 ,  142  are pulled, provides for a well fused junction across the cured adhesive. 
         [0044]    Alternatively, as illustrated in the cross-sectional view of  FIG. 16 , the parts  140 ,  142  can be glued and assembled on an assembly table  170  that is tilted at an angle  0  from the horizontal and supports bottom surfaces  172  of the parts  140 ,  142 . The uppermost part  140  is fixed against sliding downwardly on the tilted table  170  and an additional partially downward load can be imposed on the bottommost part  142  to thereby force the parts together and align them on the table  170 . A press plate may be additionally used but is not required. 
         [0045]    The material of the parts assembled joined by the keyway interlocks need not be silicon. The invention is not limited to virgin polysilicon staves or even to silicon staves or other silicon members. Other materials may be used. Further, the method interlocking assembly may be applied to aligning members to be welded by electrical or laser means, particularly into tubular structures such as need for liners. 
         [0046]    The invention thus provides relatively simple means to expedite assembly and assure alignment of parts to be bonded together.