Abstract:
An edge ring for use in batch thermal processing of wafers supported on a vertical tower within a furnace. The edge rings are have a width approximately overlapping the periphery of the wafers and are detachably supported on the towers equally spaced between the wafer to reduce thermal edge effects. The edge rings have may have internal or external recesses to interlock with structures on or adjacent the fingers of the tower legs supporting the wafers or one or more steps formed on the lateral sides of the edge ring may slide over and then fall below a locking ledge associated with the support fingers. Preferably, the tower and edge ring and other parts of the furnace adjacent the hot zone are composed of silicon.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims benefit of provisional application 60/697,895, filed Jul. 8, 2005 and provisional application 60/721,921, filed Sep. 29, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates generally to batch thermal processing of substrates, especially silicon wafers. In particular, the invention relates auxiliary rings used in wafer support towers.  
       BACKGROUND ART  
       [0003]     Batch thermal processing, in which multiple wafers are simultaneously processed in a furnace, continues to be widely practiced in the semiconductor industry. Most modern batch thermal processing is based on vertical furnaces in which a vertically arranged support tower holds a large number of wafers in a horizontal orientation. The towers are conventionally composed of quartz, especially for processing temperatures under 1000° C. or of silicon carbide, especially for higher processing temperatures, but silicon towers are entering service in commercial use for all temperature ranges.  
         [0004]     One process that utilizes such thermal processing is high temperature oxidation (HTO), in which very thin oxide layers are grown by chemical vapor deposition (CVD) using SiH 4  and N 2 O or NO as precursor gases. Typical CVD temperatures are in the neighborhood of 750° C. The thin oxide may have a thickness in the vicinity of 2.5 nm or less and be used for a tunneling barrier, for example, in flash memories. Other processes are available for growing thin films, such as using O 2  as an oxidizing agent.  
         [0005]     Thickness uniformity of the grown film has, however, been a problem. A thickness profile  12 A is schematically illustrated in the graph of  FIG. 1 . Two peaks  14 A,  16 A in the thickness have been observed near the wafer periphery. The peaks  14 A,  16 A may represent 16% and 33% variation on opposed sides, and, since tunnel current varies exponentially with thickness, a modest thickness variation can produce a large variation in tunneling current and hence the recording performance of flash memories.  
         [0006]     The specific origin of the peaks is not completely understood, but possible causes are believed to include thermal edge effects such as thermal shadowing by the tower legs or proximity to the furnace wall, and by gas flow discontinuities at the wafer periphery. Some have attempted to solve this problem by attaching auxiliary rings to the tower which extend over the edge of the wafer a small distance toward the center. Optimally, the wafer is spaced between the two neighboring edge rings facing its upper and lower faces. Edge rings have been shown to be effective at reducing if not eliminating the peaks.  
         [0007]     The typical design includes a quartz tower and quartz edge rings which are fused with the three or four legs of the tower. This design suffers several problems. Although the quartz is relatively inexpensive, the fusing at so many locations is laborious. If one of the edge rings is broken in service, repair is almost impossible. Either the tower and welded edge rings are discarded or the wafer locations around the broken edge ring are not thereafter used for production wafers. Although quartz is generally accepted for use in thermal support fixtures, advancing technology calls into question whether it has an adequate purity level.  
         [0008]     Accordingly, a better design is desired for edge rings and their support towers.  
       SUMMARY OF THE INVENTION  
       [0009]     A ring tower includes fingers or other projections to support in a vertical stack both wafers and generally annular edge rings which are interleaved between the wafers and preferably extend over a radial band extending outwardly from the periphery of the wafers.  
         [0010]     Both the tower and the edge rings are preferably composed of silicon. The edge rings are more preferably formed of randomly oriented polycrystalline silicon (ROPSi), which may be grown by the Czocharalski method using a polycrystalline seed. The silicon seed may be composed of virgin polycrystalline silicon (electronic grade silicon) grown by CVD or of Czochralski-grown silicon grown from a seed traceable to a virgin polycrystalline silicon seed.  
         [0011]     Advantageously, the rings are passively interlocked with the tower, for example, by gravitational force. The interlocking can be achieved with recesses formed on the inner or outer periphery of the ring or by steps on its lateral sides. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a thickness profile of oxide grown by a high temperature oxidation process.  
         [0013]      FIG. 2  is an orthographic view of an embodiment of the invention including a tower and edge rings.  
         [0014]      FIG. 3  is an elevational view of the tower and edge rings of  FIG. 2  and also of the wafers.  
         [0015]      FIG. 4  is an exploded orthographic view of one of the legs of the tower of  FIGS. 1 and 2 .  
         [0016]      FIG. 5  is a plan view of an edge ring of the invention.  
         [0017]      FIG. 6  is a elevational view of the wafers and edge rings in areas away from the legs.  
         [0018]      FIG. 7  is an orthographic view of another embodiment of the edge ring.  
         [0019]      FIG. 8  is an orthographic view of a tower leg with which the edge ring of  FIG. 7  may be used.  
         [0020]      FIG. 9  is an exploded view of  FIG. 8 .  
         [0021]      FIG. 10  is an exploded view of a modification of the tower leg of  FIG. 8 .  
         [0022]      FIG. 11  is plan view a yet another embodiment of the edge ring configured for a four-leg tower.  
         [0023]      FIGS. 12 and 13  are orthographic views taken from the front and back side respectively of the engagement between the edge ring of  FIG. 11  and a side leg of the tower of  FIG. 12 .  
         [0024]      FIG. 14  is an orthographic view of a three-leg tower partially loaded with an edge ring which is variant of the edge ring of  FIG. 11 .  
         [0025]      FIG. 15  is partially sectioned plan view of the tower and edge ring of  FIG. 14  and additionally illustrating a wafer.  
         [0026]      FIG. 16  is a cross-sectional side view of a furnace including a liner, injector, and tower. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     A first embodiment of the invention, illustrated in the orthographic view of  FIG. 2  and the elevational view of  FIG. 3 , includes a support tower  10  including two side legs  12 ,  14  and a back leg  16  fixed at their lower ends to a bottom base  18  and at their top ends to a similar unillustrated top base. The legs  12 ,  14 ,  16 , also illustrated in the exploded orthographic view of  FIG. 4 , may be similarly configured and include fingers  22  projecting generally inwardly from axially extending leg stems  24 . The fingers  22  at corresponding axial positions of the three legs  12 ,  14 ,  16  support edge rings  26  on radially outward and lower ring support surfaces  28 . The fingers  22  also support wafers  30  on radially inward and upper wafer support surfaces  32 , which are generally planar and horizontal and defined on their inner sides by ridges  34 . The ridges  34  are positioned to be closely outside the circular wafers  30  supported on the wafer support surfaces  32 , to thereby align the wafers  30  on the tower  10 .  
         [0028]     One edge ring  26 , illustrated in the plan view of  FIG. 5  is a generally annular washer-shaped body generally circularly symmetric about a center  40 , which is intended to coincide with the center of the tower  10  and the centers of the wafers  30 . However, the edge ring is machined to include two side recesses  42 ,  44  and a back recess  46  of generally similar shapes to respectively engage the edge ring  26  on the two side legs  12 ,  14  and back leg  16 . Thin segments  48  in back of the recesses  42 ,  44 ,  46  support the edge ring  26  on the legs  12 ,  14 ,  16 , of the tower  10 . The outside of the segments  48 , particularly on the sides, may be flattened with the side flattening being parallel to the insertion direction to allow a larger outer diameter. The recesses  42 ,  44 ,  46  are disposed at positions extending circumferentially about the center  40  around the back of the ring  36  by an angle sufficiently larger than 180° such that the legs  12 ,  14 ,  16  at similar angular spacings stably support the edge rings  26  but sufficiently small that the side legs  12 ,  14  do not interfere with the insertion of the edge rings  26  (as well as the wafers  30 ) past the side legs  12 ,  14 . For example, the centers of the side recesses  42 ,  44  are displaced a little forward of the ring center  40 .  
         [0029]     The inner diameter of the edge ring  26  may be approximately the diameter of the wafer or a little larger, for example, up to 4 to 10 mm larger, for example, 6 mm larger. It is possible to extend the edge ring  26  somewhat inside the wafer diameter, for example, by less than 10 mm for a 200 or 300 mm wafer. In general, the deviance from congruent diameters should not significantly exceed the pitch between wafers  30  in the tower so that a substantial fraction of the solid angle around the wafer edge views wafers  30  or edge rings  26  of the same temperature. Stated differently, the edge of the wafer  30  should not view the furnace walls or liner except through the gap between the two neighboring edge rings  26 , which presents a relatively small viewing angle of the liner. Similarly, the annular width of the edge rings  26  should be greater than the pitch between wafers  30 . The outer diameter of the edge ring  26  should be significantly greater than the wafer diameter to extend the uniform temperature outwardly. The additional diameter may correspond to the location of the peaks  14 A,  16 A in  FIG. 1  from the wafer periphery absent edge rings. As a result, for the most part, the wafer  30  view only other wafers  30  or edge rings  26 , all of which equilibrate to about the same temperature. The largest temperature excursions occur at the outer edges of the edge rings  26  rather than the outer edges of the wafers  30 . The edge rings  26  should move the non-uniform deposition peaks  14 A,  16 A outside of the area of the wafers  30  and onto the edge rings  26 . However, excessively wide edge rings impact the design and use of the oven. Exemplary outer diameters are greater than the wafer diameter by 20 to 40 mm, for example 28 mm. The thickness of the edge ring  26  should be great enough to provide sufficient rigidity to the ring-like structure but thin enough that it not have greatly different thermal capacity than the wafer. Generally, it is preferred that its thickness range from approximately the wafer thickness to about twice the wafer thickness. Present designs utilize thicknesses of 1 to 1.5 mm.  
         [0030]     The edge ring  26  is preferably machined from pure silicon, for example, of randomly oriented polycrystalline silicon (ROPSi), for example, Czochralski-grown silicon pulled from the melt using a randomly oriented silicon seed, for example, a seed of virgin silicon or a seed of polycrystalline silicon traceable to a CVD grown seed. This material and its growth and machining are described in U.S. Provisional Application 60/694,334, filed Jun. 27, 2005 and in U.S. patent application (Express Mail Label EV 291190735 US), filed Jan. 9, 2006, incorporated herein by reference. The fabrication process advantageously includes Blanchard grinding of the surfaces after wire or saw cutting from a silicon ingot in order to generate surface damage on the exposed surfaces to increase the bonding of films deposited thereupon. Ceramic machining techniques are used to fabricate the ring shape from wafer-shaped blanks. In order to remove impurities, especially heavy metals, the rings may be cleaned after machining by techniques used to clean silicon wafers, for example, using a combination of acid or alkaline etchants. After fabrication of the edge ring  26  has been completed, it is advantageous to pre-coat it in a CVD process on all surfaces with a layer of the same material CVD deposited in the oven or deposition process with which it will be used, that is, silicon nitride for a silicon nitride furnace and silicon dioxide for a silicon dioxide furnace. The pre-coat layer will be firmly anchored in the cracks and crevices created as part of the surface damage and will bond well to after-deposited layers of the same material.  
         [0031]     Other types of silicon may be used for the edge rings, for example, monocrystalline silicon. However, Czochralski-grown (CZ) monocrystalline silicon is generally not available in larger diameters at this time needed for 300 mm towers and is further subject to chipping and fracture. Cast silicon is available, which is typically randomly oriented and of adequate size, but its purity and often its strength are generally less than that of randomly oriented CZ polysilicon. It is understood that a silicon material usable according to some aspects of the invention is composed of at least 99 at % elemental silicon although most of the types of silicon mentioned above are much purer.  
         [0032]     It must be emphasized however that many aspects of the inventive edge ring are not limited to silicon rings and towers and may be applied to rings or towers composed of other materials such as quartz, silicon carbide, or silicon-impregnated silicon carbide. Silicon-impregnated silicon carbide can be achieved by either exposing nearly stoichiometric silicon carbide to a silicon melt or by blending controlled amounts of silicon and graphite powder, casting the mixture, and firing the cast to obtain a selected ratio of silicon to carbon.  
         [0033]     Referring specifically to  FIG. 4 , the leg  12 ,  14 ,  16  includes a tendon  50  at both its lower and unillustrated upper end to fit within a corresponding mortise hole in the bottom base  18  or unillustrated top base. The fingers  22  extend radially inwardly in a generally horizontal direction from the leg stem  22  in generally constant thickness and constant width sections  52 , on top of which is formed the ring support surfaces  28 . The fingers  22  then extend farther radially inwardly in a partially upward direction in sloping sections  54 , which may have a constant width but not necessarily so. The fingers  22  then extend farther radially inwardly in a generally horizontal direction but with converging sidewalls  56  in wedge shaped tips, on top of which are formed the wafer support surface  32  bounded on their radially outer sides by the ridges  34 . Sides  58  of the recesses  42 ,  44 ,  46  in the ring  26  of  FIG. 5 , are sloped similarly to the tip sidewalls  56  but are separated by a somewhat greater distance than the separation of the tip sidewalls  56  to allow the edge ring  26  to vertically pass by the tip sidewalls  56 .  
         [0034]     As a result, the edge ring  26  can be manually or robotically inserted into the tower  10  for a set of three corresponding fingers  22  at a level above the top of the ridges  34  for all three legs  12 ,  14 ,  16 . When the edge ring  26  has reached almost the stem  24  of the back leg  16 , the edge ring  26  is lowered, with the recess sidewalls  58  passing the tip sidewall  56 , such that the ring support segments  44  is laid to rest on the ring support surfaces  28  of the legs  12 ,  14 ,  16 . The sloping sections  54  of the fingers  22  help in centering and aligning the edge rings  26  to the  12 ,  14 ,  16 . Once, the edge ring  26  has been placed on the edge support surfaces  28 , it remains there under the force of gravity. However, if desired, the edge ring  26  can be removed in an inverse procedure.  
         [0035]     It is desired that vertical spacing between the wafers  30  and the edge rings  26  be closely controlled. As illustrated in the elevational view of  FIG. 6  taken along a radius not passing through a leg, a top surface  62  of any wafer  30  is separated by a distance A from a bottom surface  64  of the edge ring  26  immediately above and by a distance B from a top surface  66  of the edge ring  26  immediately below. On the other hand, a median plane  68  of the wafer  30  is separated from the bottom surface  64  of the upper edge ring  26  by a distance C and from the top surface  66  of the lower edge ring  26  by a distance D. A first design principle sets the spacings according to A=B. A second design principle sets the spacings according to C=D. Probably the former favors uniformity for transient conditions while the latter favors uniformity for equilibrium. Either design principle determines the vertical separation between the wafer support surface  32  and the edge support surface  28  of each finger  22  taking into account the vertical pitch of the fingers  22  and the thicknesses of the edge rings  26  and the wafers  30 . With either arrangement and with equal thicknesses for wafers and edge rings, the thermal loading averaged between the wafers and edge rings remains substantially constant to well outside the periphery of the wafers. This figure also illustrates that any wafer  30  views equal areas of either other wafers  30  or the edge rings  26 , both sets of which are at substantially the same temperature, thereby reducing the edge effects on the wafers  30 . It is also desirable that the bottom of the wedge-shaped finger tip is approximately at a level of the bottom surface of the edge ring  26  supported on the ring support surface  28 , thereby maximizing clearance for wafer transfer after the edge rings  26  have been placed in the tower  10 . It is understood that other design principles including axially varying spacings are possible.  
         [0036]     After the edge rings  26  have been loaded into the tower  10 , wafers  30  can be inserted into and removed from the tower  10  without interference from the edge rings  26  already located there. The edge rings  26  may remain on the tower  10  during multiple wafer cycles.  
         [0037]     If an edge ring  26  breaks for whatever reason, it can be removed from the tower  10  and replaced by a new one without needing to build a new tower  10 .  
         [0038]     Another embodiment provides separate leg fingers for the wafer and the edge ring. As illustrated in the orthographic view of  FIG. 7 , an edge ring  70  includes two side recesses  72 ,  74  and a back recess  76 . All the recesses  72 ,  74 ,  76  may be rectangularly cut into the outer periphery of the ring  70  to conform to similarly shaped structure in the legs at angular positions corresponding to the recesses  42 ,  44 ,  46  of  FIG. 5 . A leg  80  illustrated completely in the orthographic view of  FIG. 8  and partially in the exploded orthographic view of  FIG. 9  may be used for any of the legs of the tower of  FIGS. 2 and 3  to support and interlock the ring  70 . The leg  80  includes wafer fingers  82  and ring fingers  84  interleaved with each other and generally extending horizontally radially inwardly from an axially extending stem portion  86 .  
         [0039]     The wafer fingers  82  each include a wafer support area  88 , which may be horizontal or, if desired sloping with a flat support tip area. The back or radially outer side of the wafer support area  88  is defined by a wafer ridge  90 , which aligns the wafers on the wafer support areas  82 . Tapered sidewalls  92  on the outer portion of the wafer finger  82  produce a wedge shaped tip. The ring fingers  84  each include a typically flat and horizontally extending ring support area  94  defined on its front by a finger edge  96  and on its back by a ring ridge  98  positioned slightly in back of the intended periphery of the edge ring  80 . The relative radial and axial positions of the wafer and ring support areas  88 ,  94  may be designed according to the same constraints discussed for the first embodiment.  
         [0040]     Conveniently, the finger edge  96  may be vertically machined at the same radial location as the wafer ridge  90 , which also provides more clearance for the transfer of wafers. A finger step  100  formed in the back of the ring ridge  98  has a width slightly less than the width and a similarly generally rectangular or other shape of the ring recesses  72 ,  74 ,  76 . A passageway  102  between the top of the finger step  100  and the bottom of the wafer finger  82  above is thicker than the thickness of the edge ring  80  to allow the edge ring  80  to pass through it. Thereby, the edge ring  80  can be inserted into the assembled tower by passing or sliding it along the passageway  102  above its intended edge finger  96  of at least the side legs. When the edge ring  80  reaches its intended position, the ring  72 ,  74 ,  76  recesses are positioned around the respective finger step  100  and the edge ring  80  can fall or be lowered with its recesses passing the sides of the finger step  100  until the edge ring  80  rests on the edge support area  94  and is gravitationally interlocked to the finger step  100 . Once the edge rings  80  have been all loaded, they may be left there as sequential sets of wafers are loaded and unloaded from the tower. However, the edge rings  80  are detachable from the legs for maintenance, replacement, or other reasons.  
         [0041]     A recess  104  in back of the wafer support area  88  is typically required for at least the front legs, which are positioned in front of the center of the supported wafer, to allow the total diameter of the wafer to be inserted past the front legs and then lowered onto the wafer support surface. However, the depth of the recess  104  may be reduced, as shown for the leg  106  illustrated in the orthographic view of  FIG. 10 . As a result, the fingers  82 ,  84  are less distinctly separated.  
         [0042]     In a variant of the leg  106  of  FIG. 10 , the separate wafer and ring fingers  82 ,  84  are combined into a single finger by extending the step  90  in back of the wafer support area  88  upwardly to the level of the edge ring support area  94  to merge with the finger edge  96  and to eliminate the recess  104  in back of the wafer support area  88 . The resultant structure is the inverse of the structure of  FIG. 4  for which the fingers  22  extend downwardly and on each finger the wafer support are 32 is below the ring support area  28 . This leg  106  has the advantage of less machining and more mechanical strength but introduces additional leg mass near the edge of the wafer. It is possible for the back leg behind the wafer center to be formed with a two-tier finger with the lower and radially inner tier supporting the wafer and the upper and radially outer tier supporting the edge ring.  
         [0043]     Because the side recesses  72 ,  74  lock the edge ring  70  of  FIG. 7  to the two front legs, there is no need for a locking mechanism on the back leg. That is, it is possible to eliminate the back recess  76  in the edge ring  70  and to extend backwardly the edge step  98  of the back leg  80 ,  106 , possibly to the leg stem  86 , to align the circular periphery of the edge ring  70 . The reduced machining is offset by the need to separately design and inventory two types of legs.  
         [0044]     Another edge ring  110 , illustrated in the plan view of  FIG. 11 , is configured for a tower having four legs  80 . Two back recesses or notches  112 ,  114  engage two back legs  80 , which are offset at equal and opposite angles from an insertion axis  116  and face the center  40 . Outer sides  118  of the notches  112 ,  114  are cut close to the radius to the center  40  while inner sides  120  are cut parallel to the insertion axis  116  to facilitate loading onto the legs  80 . Inner flats  122 ,  124  are cut parallel to the insertion axis  116  but extend only partially toward the back to form ring steps  126 . Preferably the ring steps  126  are disposed forward of the perpendicular diameter passing through the center  40 . Two side legs  80  are disposed at least partially and preferably completely forward of the center  40  and oriented to face perpendicularly toward the insertion axis  116 .  
         [0045]     When the edge ring  110  is loaded, as illustrated orthographically in the front view of  FIG. 12 , the inner flats  122 ,  124  are aligned to the wafer ridges  90  of the respective side legs  80  and, as illustrated orthographically in the back view of  FIG. 13 , the ring step  126  falls down and is passively and gravitationally locked to the side of the finger step  100  in opposition to the engagement of the edge ring  110  to the back legs  80 . In this position, the edge ring  110  is stably supported by the back legs far in back of the center  40  and by the front legs  80  slightly but completely in front of the center  40 . As illustrated in  FIG. 11 , outer flats  128  may be cut into the lateral sides of the edge ring  110  parallel to the insertion axis  116  to facilitate loading of the edge ring  110  past the side legs  80  and reduce the lateral width of the tower and its legs. A part number and/or serial number  130  may be engraved on a planar surface of the edge ring  110 .  
         [0046]     The configuration of the side flats  122 ,  124  and ring steps  126  can be substituted for the side notches  72 ,  74  in the three-leg ring  70  of  FIG. 7 .  
         [0047]     As has been discussed previously for edge ring  70 , the back recesses  112 ,  114  of the edge ring  110  can be eliminated if the back leg is separately configured to contact the circular periphery of the edge ring  110 .  
         [0048]     The edge ring  110  of  FIG. 11  is designed for a tower having four legs. On the other hand, a tower  140  illustrated in the partial orthographic view of  FIG. 14  and the partially sectioned plan view of  FIG. 15  has only three legs, specifically, two side legs  142 ,  144  and one back leg  146  fixed on their lower ends to a bottom base  148  and at their top ends to an unillustrated top base. As illustrated, the side legs  142 ,  144  are located completely forward of the center  40  of the tower  140 , wafer  30 , and edge rings  152 . Notches  150  are formed in both the back of the back leg  146  and the back of the base  148  to accommodate a thermocouple to measure the temperature close to the wafers. Fingers are formed in the legs  142 ,  144 ,  146  for supporting edge rings  152  and wafers  30  (not illustrated in  FIG. 13 ). The fingers differ between the side legs  142 ,  144  and the back legs  146  to allow the side legs  142 ,  144  to pass the side step of the edge ring  152 . The inner periphery of the edge ring  152  is mostly circular about the center  40  and spaced slightly outwardly of the outer periphery of the wafer  30  except for an inner flat  154  which slightly overhangs the wafer  30 . The outer periphery of the edge ring  152  is mostly circular about the center but includes two side steps and a back notch to passively interlock the edge ring  152  to the legs  142 ,  144 ,  146 .  
         [0049]     It is anticipated that after extended operation of a deposition process, the film thickness will build up to a sufficient thickness on both the tower  10  and the edge rings  26 ,  70 ,  110 ,  152  that particle flaking may become a problem. It is also probable by this time that deposited film has glued the edge rings to the tower  10  by bridging between them. There are standard procedures for cleaning films from silicon. Accordingly, both the silicon tower  10  and the attached silicon edge rings can be placed in an etching bath that removes the deposited layer without removing the underlying silicon. For example, HF removes both silicon oxide and silicon nitride from silicon. Silicon parts afford greater selectivity in the cleaning than do quartz parts. It is possible in the case of a broken edge ring that a similar tower and ring etch be performed to remove a broken edge ring having fragments glued to the tower before the fragments are removed.  
         [0050]     It is understood that the shape of the edge ring is not limited to those described above.  
         [0051]     Although a silicon edge ring offers great advantages, other features of the invention including the detachable configuration are also useful even if the tower or the edge rings are composed of other materials, such as quartz, silicon carbide, or silicon impregnated silicon carbide. For all these materials, the simple structure of the rings and towers and the ease of refurbishment can provide significant manufacturing economies.  
         [0052]     The invention is not limited to the described HTO process but may be used for other processes, other process gases if any, other wafers such as silicon-on-insulator wafers or glass or ceramic substrates, and other processing temperatures. Although the invention is most useful for high-temperature processes, it may be applied to lower-temperature processes such as chemical vapor deposition.  
         [0053]     When the edge ring is made of silicon, an all-silicon hot zone is enabled for a furnace useful for large-scale commercial production. A vertically arranged furnace  160  illustrated in the cross-sectional view of  FIG. 16  includes a thermally insulating heater canister  162  supporting a resistive heating coil  164  powered by an unillustrated electrical power supply. A bell jar  166 , typically composed of quartz, includes a roof and fits within the heating coil  164 . A liner  168 , for example, open ended, fits within the bell jar  166 . A support tower  170 , corresponding to the previously described towers, has three or four legs  172  fixed to top and bottom bases  174 ,  176  or supporting both wafers and edge rings not illustrated here. The support tower  170  sits on a pedestal  178  and during processing the pedestal  178  and support tower  170  are generally surrounded by the liner  168 . One or more gas injector  180  having outlet ports at different heights are principally disposed between the liner  168  and the tower  170  and have outlets for injecting processing gas at different heights within the liner  168 . An unillustrated vacuum pump removes the processing gas through the bottom of the bell jar  166 . The heater canister  162 , bell jar  156 , and liner  168  may be raised vertically to allow wafers to be transferred to and from the tower  170 , although in some configurations these elements remain stationary while an elevator raises and lowers the pedestal  178  and loaded tower  170  into and out of the bottom of the furnace  160 .  
         [0054]     The bell jar  168 , which is closed on its upper end, tends to cause the furnace  160  to have a generally uniformly hot temperature in the middle and upper portions of the furnace. This 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  168  and the mechanical support of the pedestal  178  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  170 .  
         [0055]     It is advantageous that not only the edge rings but also the tower, liner, and injectors be composed of silicon so that all materials in the hot zone are of the same silicon material as the silicon wafers being processed and be of nearly equal purity. Silicon baffle wafers are also preferably used, as described in the aforecited provisional application 60/694,334 and its utility application. An all-silicon hot zone provides very low particulate and impurity levels in the processing of silicon wafers. Boyle et al. have described the fabrication of silicon towers in U.S. Pat. No. 6,450,346 and of silicon liners in U.S. patent application Ser. No. 10/642,103, filed Aug. 15, 2003, both incorporated herein by reference. Zehavi et al. have described the fabrication of silicon injectors in U.S. patent application Ser. No. 11/177,808, filed Jul. 8, 2005, incorporated herein by reference. Boyle et al. have described a useful adhesive of silicon powder and spin-on glass for assembling silicon structures in US Patent Application Publication 2004/0213955 A1. All these silicon parts are commercially available from Integrated Materials, Inc. of Sunnyvale, Calif.  
         [0056]     The invention thus provides greatly improved thermal performance and greatly reduced contamination and particles with a structure that is economical to fabricate and easy to maintain.