Abstract:
A device for supporting a semiconductor ingot during growth of the ingot. The device includes a chuck in which is mounted a seed. The seed includes an elongate rod having one end projecting from the chuck for growth of the ingot thereon. A latch pin secures the seed in the chuck in a removable manner. The chuck is suspended in a semiconductor furnace. The chuck, seed and latch pin and interengaged in a manner to reduce forming flaws in the ingot.

Description:
BACKGROUND OF TE INVENTION 
     The process of manufacturing silicon semiconductor wafers is well known in the industry. The principal method of manufacturing them is the Czochralski (CZ) method. Generally the process includes immersing a portion of a seed crystal such as a monocrystalline silicon crystal into molten semiconductor material, such as polycrystalline silicon and, a single crystal ingot with zero dislocations grows from the seed. The seed and growing ingot are slowly moved upwardly and extracted from the molten semiconductor material allowing the crystal to grow. Growth is continued until the proper ingot size is achieved. 
     The CZ method has been very effective at growing semiconductor crystals. However, as crystals have gotten larger in diameter and longer and crystal growing processes are operating at higher temperatures, manufacturing and product problems have been encountered. Further, as price competition for semiconductor wafers has increased, any cost savings that can be achieved are highly desirable. 
     A current method of crystal production includes holding the seed, which is in the form of an elongate rod, in a chuck which is suspended from a cable. The seed is releasably retained in the chuck so when the ingot is completed, it can be easily separated from the chuck for further processing. A typical retention device for retaining the seed in the chuck is a latch pin that is interengaged with a corresponding notch previously formed in the seed. The pin engages a generally planar notch latch surface that is tapered. The degree of taper, as seen in FIG. 8, is such as to provide a taper lock whereby friction locks the seed in place. This system has worked well until the ingots have gotten larger and therefore heavier or as the crystal growing process has been conducted at higher temperatures. The heavier the crystal and the higher the crystal growing process temperature, the higher the probability of a failure. 
     Two modes of failure have occurred. First, the taper lock arrangement can slip and second, the seed can break ruining the crystal. 
     Slippage of the ingot of as little as 0.002″ can result in a scrap crystal. Such slippage results in waves in the molten semiconductor material in the furnace crucible which then creates a flaw in the crystal. Slippage appears to be the result of the formation of a coating on the chuck that holds the seed. This coating is believed to be silicon carbide which forms on the chuck surface engaged by the seed during crystal growth. The coating results in a large disparity between the static and dynamic coefficients of friction between the two parts. If the seed begins movement relative to the chuck, this disparity in the coefficients of friction will allow the ingot to slip more than if they were not as disparate. Movement will continue until the friction is increased by the taper lock effect of the latch pin against the seed which additionally increases the compressive force on the seed. The slippage causes a crystal flaw at least partly because it creates minor waves in the molten semiconductor material. The formation of silicon carbide or other compound on the surfaces currently results in a higher probability of failure requiring more frequent replacement of the chuck to maintain an acceptable risk level. The layer grows more with each use, and thus increases the probability of failure with each additional use. 
     If the seed breaks, the crystal is also ruined since it falls into the molten semiconductor material. A chuck C, latch pin LP and seed S currently used in the art are seen in see FIG.  8 . The seed S is prone to breakage. Breakage is believed to be due at least in part to the small angle A′ that the latch surface LS of the notch is positioned at, which is about 11° from the longitudinal axis of the seed. Such a small angle increases the compressive force applied to the seed S by the latch pin LP. Additionally, the contact between the mating surfaces of the seed and the latch pin may encourage breaking of the seed S under load. In some cases, both are made of materials having a high modulus of elasticity which is now believed to encourage breakage because of a lack of compressibility resulting in a narrow width zone of contact. 
     Two ways of reducing the incident rates of these failure modes is to either make the parts larger and therefore stronger or throw the parts away after fewer or even one use. However, these are expensive alternatives but would lower the probability of failure. Thus, there is need for an improved chuck and seed for producing semiconductor ingots. 
     SUMMARY OF THE INVENTION 
     Among the several objects and features of the present invention may be noted the provision of an apparatus for supporting a semiconductor crystal ingot during growth of the crystal that overcomes the aforementioned problems; the provision of such an apparatus that reduces the probability of failure thereby increasing the usable life of the apparatus; the provision of such an apparatus that does not require changing current equipment for its use; the provision of such an apparatus that does not increase the cost of producing semiconductor crystal ingots; the provision of such an apparatus that is simple for an operator to use; the provision of such an apparatus that improves manufacturing efficiency; the provision of a latch pin that reduces the risk of breaking the seed; and the provision of a seed that reduces the risk of slippage of the ingot and breakage of the seed. 
     The present invention is directed to a seed for commencing growth of a semiconductor ingot and supporting the ingot during growth. The seed comprises an elongate rod having a latch end and a growth end and has a notch extending generally transverse to a longitudinal axis of the rod. The notch has a latch surface portion at least partially defining the notch with a contour such that at least a portion of the latch surface portion lies at an angle of at least about 30° from the longitudinal axis of the rod. 
     The present invention also involves the provision of a device for suspending a semiconductor ingot during formation of the ingot. The device includes a chuck with a seed receiving socket extending generally longitudinally of the chuck and opening onto a first end thereof. A latch pin is mounted in the chuck and has a support surface. A seed is positioned in the socket and removably secured therein. The seed has an end thereof projecting from the first end of the chuck and has a transverse notch extending into a side surface. The notch is at least partially defined by a latch surface having at least a portion in engagement with the support surface portion at an angle of contact, the angle of contact between the latch surface and the support surface is such as to prevent locking therebetween. 
     A further aspect of the present invention is the provision of a latch pin for attaching a semiconductor seed crystal to a chuck used to support a semiconductor ingot during ingot growth. The latch pin comprises an elongate shaft having a socket extending into the shaft and opening on an exterior surface of the shaft, the shaft is made of refractory metal. A pillow is removably mounted in the socket and has a first surface extending outwardly from the shaft and is adapted for engagement with a portion of a semiconductor seed. The pillow has a modulus of elasticity of less than about 2.1×10 6  psi. 
     The present invention also involves the provision of a device for suspending a semiconductor ingot during formation of the ingot. The device includes a chuck with a seed receiving socket extending generally longitudinally of the chuck and opening onto a first end thereof. A latch pin is mounted in the chuck and a pillow is mounted to the latch pin and has a support surface portion. A seed is positioned in the chuck socket and is removably secured therein. The seed has an end thereof projecting from the first end of the chuck and also has a transverse notch extending into a side surface of the seed. The notch is at least partially defined by a latch surface having at least a portion in engagement with the support surface portion. The chuck, latch pin, pillow and seed each have a coefficient of thermal expansion with values of the coefficients of thermal expansion having variation therebetween of less than about 50%. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic partial cross section of a furnace used to produce semiconductor ingots from which semiconductor wafers are made; 
     FIG. 2 is an enlarged exploded perspective view of a chuck used for suspending a semiconductor ingot during the manufacturing process with sections broken away to show internal details; 
     FIG. 3 is an enlarged perspective view of a seed; 
     FIG. 4 is an enlarged exploded perspective view of a latch pin; 
     FIG. 5 is an enlarged perspective fragmentary view of a lower portion of the chuck showing details of the chuck, seed and latch pin; 
     FIG. 6 is an enlarged sectional view of a lower portion of the chuck; and 
     FIG. 7 is an enlarged schematic view of chuck, latch pin and seed with details out of proportion to illustrate the engagement of the latch pin and seed. 
     FIG. 8 is an enlarged schematic view of a prior art chuck, latch pin and seed arrangement. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     As best seen in FIG. 1, a furnace, designated generally as  10 , is provided for the manufacture of semiconductor ingots such as by the Czochralski (CZ) method. This method is well known in the industry and generally involves having a molten semiconductor material  11  such as polycrystalline silicon, hereinafter referred to as silicon, in a crucible  12  in the furnace  10 . A cable  14  is suspended over the crucible  12  and is attached to a winch  15  for slowly moving the cable upwardly. In an alternative embodiment, a rigid shaft could be used in place of the cable  14 . A seed  17  is secured to the cable  14  via a chuck  18 . At the beginning of crystal growth, the seed  17  is partially immersed in the molten semiconductor material  11  such as silicon and when the crystal ingot  20  starts to form, the chuck  18 , seed  17  and the ingot  20  are slowly moved upward with the silicon crystallizing increasing the length of the ingot  20 . Ingot growth and movement is continued until the ingot  20  is complete. After completion of ingot growth and cooling, the ingot  20  and seed  17  are removed from the furnace  10  and chuck  18 . The ingot  20  can then be processed into wafers. The processes of crystal (ingot) growth and wafer production are well known in the industry. 
     The furnace  10  includes a housing  22  with a source of heat (not shown) applied to the crucible  12 . The heat is preferably from an electrically resistive heater which is in heat transfer relation with the crucible  12 . The crucible  12  is suitably mounted for rotation in the housing  22 . Such a furnace  10  that can accommodate crucible  12  is available from Ferrofluidics Corp. as model CZ-150. 
     The chuck  18  is best seen in FIG.  2 . The chuck head  23  is preferably made from a heat resistant graphite material such as grades CZ3L or AI2RL manufactured by SGL Carbon Company. Means is provided to releasably secure the chuck  18  to the cable  14 . As seen in FIG. 2, the chuck  18  includes a chuck head  23  with opposite ends  24 ,  25  with a shank  27  projecting from the end  24 . A cap  29  which preferably is made from fused quartz is suitably removably secured to the shank  27 . The cap  29  includes a longitudinal bore  30  sized and shaped to receive the shank  27  in a snug fit. The cap  29  includes an elongate hole  32  passing transversely thru the cap and opening onto opposite sides thereof The hole  32  opens into the bore  30 . Likewise, the shank  27  has an elongate aperture  33  extending transversely therethrough. As shown, a graphite dowel pin  35  extends through the hole  32  and aperture  33  to removably secure the cap  29  to the chuck head  23 . The cable  14  is preferably of a 7×7×7 tungsten type and has one end  37 , FIG. 1, secured to the winch  15  which is operable to move the chuck  18  vertically, both up and down, upon command. A preferred winch  15  is supplied with the furnace  10  by Ferrofluidics Corp. The other end  38  of the cable is a free end and is secured to the chuck  18  via the cap  29  and hence the chuck head  23 . As shown, the end  38  of the cable  14  extends through a hole  40  into a pocket  41  that opens into the bore  30 . There is a shoulder  42  between the pocket  41  and hole  40 . The end  38  of the cable  14  has a cable clamp  43  secured thereto to engage the shoulder  42  and thus secure the cap  29  to the cable  14 . Insulators  45  are mounted in the bore  30  between a free end  47  of the shank  27  and the clamp  43  for shielding the free end  38  of the cable  14  from damaging thermal loads radiated by the chuck head  23 . 
     The rate of movement of the chuck  18  during crystal formation is in the range of about 0.3 mm/min. through about 2 mm/min. and will depend on several factors. The rate will be determined in large part by the diameter of the ingot  20  to be grown and the thermal conditions present in the furnace  10 . 
     The chuck  18  has a socket or bore  50  opening onto the free end  25 . Preferably, the bore  50  is generally coaxial with the longitudinal axis of the chuck  18  and also the cable  14  above the chuck. It is also preferred that the lower end of the chuck  18  be tapered for unobstructed viewing of an end  60  of the seed  17 . 
     The chuck  18  is also provided with an elongate hole  51  that extends transversely of the longitudinal axis of the chuck. For manufacturing ease, the hole  51  can open onto opposite sides of the chuck  18 . One or both ends of the hole  51  can open into an elongate recessed slot  53  that opens onto the exterior surface of the chuck  18 . Preferably, the slot  53  is defined by at least one, and as shown a pair of generally opposed flat surfaces  55 , FIG.  5 . Preferably, the hole  51  intersects the bore  50  and is positioned such as to have at least about 30% and preferably at least about 50% of its transverse cross sectional area within the bore  50 . By having the hole  51  and bore  50  intersect, a significant portion of the combined cross sectional area of a latch pin  57  and pillow  77  in a plane transverse to the longitudinal axis of the latch pin  57  is exposed in the bore  50  when installed. Preferably the combined exposed cross sectional area is at least about 30% and preferably at least about 50% of the transverse cross sectional area of the latch pin  57 . 
     The seed  17  can be any suitable seed crystal. Preferably the seed  17  is elongate and in the shape of an elongate rod having a latch end  59  and a growth end  60 . The seed  17  can have any suitable transverse cross sectional shape, e.g., generally round, rectangular, square (FIG.  3 ), etc. The seed  17  is sized and shaped to fit snugly in the bore  50  and has a transverse cross sectional shape corresponding to the transverse cross sectional shape of the bore  50 . The length of the seed  17  is such that when installed in the bore  50 , at least about 110 mm projects outwardly from the free end  25  of the chuck  18 . It is also preferred that at least about 70 mm of seed length be within the bore  50 . 
     As seen in FIGS. 3,  6 , a notch  63  is provided in the seed  17  adjacent the latch end  59 . Preferably the notch  63  is orientated generally transverse to the longitudinal axis of the seed  17  and extends inwardly from the exterior surface toward the center or longitudinal axis of the seed. The notch  63  is defined at least partially by a latch surface  64  positioned at the top end of the notch  63  and adjacent the latch end  59  of the seed  17  and in use faces at least partially downwardly. The latch surface portion  64  is positioned and shaped to engage a portion of the latch pin  57  as hereinafter described. Also, it is preferred that the notch  63  be contoured such as to not have any sharp corners that would produce an undesirable stress concentration factor. Preferably, the transverse shape (FIGS. 3,  6 ) of the latch surface  64  is arcuate and more preferably is an arc of a circle with the latch surface  64  size being in the range of about 45° through about 180° of a circle and having a radius in the range of about 5 mm through about 8 mm and preferably about 7 mm through about 7.5 mm. The latch surface  64  has at least a portion  66 , FIGS. 3,  6 , thereof starting at a position P and extending to the outer surface of the seed  17 . The portion  66  faces generally downwardly having at least a portion thereof at an angle A sufficient of at least about 30°, preferably at least about 45° and most preferably at least about 60° relative to the longitudinal axis of the seed  17 . It is preferred that at least about 50%, preferably at least about 70% and most preferably at least about 85% of the depth of the notch  63  be outward of the position P where the angle A equals or exceeds the aforementioned values. The angle A, on a curved latch surface  64 , would be measured between a line tangent to the latch surface at position P and the longitudinal axis of the seed  17 . 
     The latch pin  57  includes a shaft  67  and a head  68  positioned at on one end of the shaft (FIG.  4 ). The shaft  67  is elongate and includes a socket  69  extending transversely into the shaft  67 . Preferably the socket  69  is defined by an arcuate bottom surface  70  and two generally parallel and spaced apart side surfaces  71 . As shown, the latch pin  57  has a flat  73  on one side that extends longitudinally substantially the entire or the entire length of the shaft  67  and head  68 . The transverse cross section shape of the head  68  corresponds generally to shape of the slot  53  in the chuck  18 . The flat  73  on the head  68  will engage a flat surface  55  of the slot  53  to prevent rotation of the latch pin  57  when installed in the chuck  18 . The free end of the shaft  67  can be tapered to facilitate installation of the latch pin  57  in the chuck  18 . The shaft  67  and the head  68  are generally round, except for the flat  73 , in transverse cross section. The radius of the shaft  67  is in the range of about 5 mm through about 8 mm and preferably in the range of about 6 mm through about 7 mm. Preferably the radius of the shaft  67  is less than the radius of the notch  63  to facilitate installation of the latch pin  57  into the chuck  23  and seed  17 . 
     The latch pin  57  is preferably made of a material that is non-contaminating to the crystal growing process and includes refractory metals such as molybdenum, tungsten, tantalum, etc. A particularly preferred material is molybdenum. 
     The latch pin  57  includes a pillow or key  77 , as best seen in FIGS. 4,  6 , mounted to the shaft  67 . The pillow  77  is adapted to engage the seed latch surface  64 . As shown, the pillow  77  has a key  78  defined by an arcuate surface  79 , extending arcuately convexly between opposite ends of the pillow  77 , and two parallel side surfaces  80 . The pillow  77  is generally in the shape of a so-called Woodruff key. The key  78  fits into the socket  69  and preferably fits snugly therein. The pillow  77  includes a latch or bearing surface  82  that when the pillow  77  is mounted in the shaft  67 , the bearing surface  82  is positioned above or extends outwardly of the flat  73 . The bearing surface  82  projects above the flat  73  a distance in the range of about 2 mm thru about 3 mm. The chuck hole  51  includes a keyway  84 , FIG. 5, that provides clearance for the bearing surface  82  to pass thru when the latch pin  57  is installed in the chuck head  23 . The contour of the bearing surface  82  is generally complementay the latch surface  64 , arcuate or generally an arc of a circle in transverse cross section. Preferably the radius of the latch surface  64  is larger than the radius of the bearing surface  82  by at least about 1.5% when unloaded and the radius of the bearing surface is in the range of about 7 mm through about 8 mm. 
     The pillow  77  preferably has a modulus of elasticity, as measured at room temperature, less than that of the seed  17  and is less than about 2.1×10 6  psi and preferably in the range of about 1.6×10 6  psi through about 2.1×10 6  psi as measured at room temperature. The modulus of elasticity of the pillow  77  is in the range of about 2 through about 10% of the modulus of elasticity of the seed  17 , also as measured at room temperature. The modulus of elasticity of the seed is preferably in the range of about 21×10 6  psi through about 23×10 6  psi as measured at room temperature. 
     As seen in FIG. 6, the latch pin  57 , through the bearing surface  82 , and seed  17 , through the latch surface  64 , interengage to removably secure the seed  17  in the chuck  18 . In a preferred embodiment, the bearing surface  82  engages the latch surface  64  to mechanically retain the seed  17  in the chuck  18  against relative longitudinal movement therein. Preferably friction between the seed  17  and the wall of the bore  50  does not play a significant role in the securement which would cause the seed  17  to self lock in the chuck head  23 . Rather, it is preferred that the load of the ingot  20  be principally carried by the latch pin  57  in shear, i.e., the seed  17  is mechanically supported thru a combination of shear, bending and compressive stresses. The interengagement of the latch surface  64  and the bearing surface  82  is such as to prevent a taper or friction lock therebetween when the seed  17  is loaded. As shown, the pillow  77  and the latch surface  64  are contoured such that they engage starting at least at a position on the latch surface  64 , and preferably on the bearing surface  82 , at a position CP (FIG. 7) as described above. The contact between the bearing surface  82  and latch surface  64  is at the position CP which is preferably at an angle B of large enough to prevent locking between the surfaces and is at least about 30°, preferably in the range of between about 30° through about 60° and most preferably about 45°. If the latch surface  64  is curved at the position CP the angle B would be measured between a line tangent to the position CP and the longitudinal axis of the seed  17 . 
     In a preferred embodiment of the present invention, the chuck head  23 , seed  17 , latch pin  57  and pillow  77  have similar linear coefficients of thermal expansion. This should reduce stress induced between tight fitting parts throughout the cycling of their temperature thru the broad range of temperatures to which they are exposed during operation of the crystal growing apparatus. It has been found that the coefficients of thermal expansion for a silicon seed  17  is about 5.2×10 −6 /° C., for AI2RL graphite it is about 5.4×10 −6 /° C., for molybdenum it is about 5.4×10 −6 /° C. and for SFG-2 graphite (available from POCO) it is about 7.7×10 −6 /° C. The coefficients of thermal expansion should be maintained for the seed  17 , pillow  77 , latch pin  57  and chuck head  23  within about 50% total variation from one another, preferably within about 20% total variation, more preferably within about 10% and most preferably within about 5% total variation. 
     When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.