Abstract:
A method for making ingots, and devices for making ingots are provided. Crucibles are provided that are reusable for forming multicrystalline silicon ingots. Crucibles are provided with features such as multiple components, coefficients of thermal expansion, and coatings that enhance a release of the ingots from the crucibles after cooling. Coatings on crucibles are provided that reduce or eliminate contamination of silicon ingots during formation. Methods of forming composite wafers are provided that produce a low cost wafer without sacrificing performance.

Description:
[0001]    This application claims the benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application Serial No. 60/421,406, filed Oct. 24, 2002, which is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates generally to semiconductor manufacturing. Specifically the present invention relates to processing of multicrystalline silicon wafers for use in integrated circuit chip manufacture.  
         BACKGROUND  
         [0003]    In the production of integrated circuits (IC), a number of chips are typically formed on the surface of a single semiconductor wafer. After the chips are formed, the wafer is cut (diced) into a number of individual chips. Wafer material suitable for semiconductor devices such as transistors is typically a single crystal material. The most common single crystal material is currently silicon. In order to form suitable single crystal wafers, an ingot of single crystal material is typically fabricated, then sliced into a number of individual wafers. Although the semiconductor devices, such as transistors, typically only occupy a very thin portion of the surface of an individual wafer, the wafers are sliced to a larger thickness for reasons such as mechanical integrity. For example, in manufacturing, the wafer must undergo some degree of handling. Wafers need to be thick enough to withstand the handling without a mechanical failure such as cracking or breaking.  
           [0004]    Fabrication of single crystal semiconductor material is an expensive and time consuming process. Because the actual semiconductor devices only occupy a small fraction of the total volume of the wafer, the remaining fraction of the wafer that serves primarily as a carrier is an inefficient use of the expensive single crystal semiconductor material.  
           [0005]    What is needed is a device and method that reduces the inefficient use of single crystal material in semiconductor device fabrication. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1A illustrates a side view of a crucible according to an embodiment of the invention.  
         [0007]    [0007]FIG. 1B illustrates a top view of the crucible in FIG. 1A.  
         [0008]    [0008]FIG. 2 illustrates a side view of a crucible according to an embodiment of the invention.  
         [0009]    [0009]FIG. 3 illustrates a side view of another crucible according to an embodiment of the invention.  
         [0010]    [0010]FIG. 4 illustrates a side view of a crucible and workpiece material according to an embodiment of the invention.  
         [0011]    [0011]FIG. 5 illustrates an isometric view of another crucible according to an embodiment of the invention.  
         [0012]    [0012]FIG. 6 illustrates a block diagram of an ingot forming system according to an embodiment of the invention.  
         [0013]    [0013]FIG. 7 illustrates an ingot according to an embodiment of the invention.  
         [0014]    [0014]FIG. 8 illustrates a wafer according to an embodiment of the invention.  
         [0015]    [0015]FIG. 9 illustrates another wafer according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]    In the following detailed description of the invention reference is made to the accompanying drawings which form a part hereof, and in which are shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and structural, logical, and electrical changes may be made, without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.  
         [0017]    In one embodiment, ingots produced using crucibles and methods described in the present disclosure are used to produce inexpensive wafers for chip manufacture that possess the necessary purity for integrated circuit chip manufacture. In one embodiment, ingots produced using crucibles and methods described in the present disclosure further provide the necessary properties such as mechanical strength to maintain a robust wafer that withstands integrated circuit processing conditions.  
         [0018]    The process of forming silicon ingots presents a number of technical hurdles to overcome. Pure, or substantially pure silicon is very reactive with numerous other element impurities. Selected applications, such as integrated circuit chip applications require a high level of purity in a semiconductor material. The presence or absence of impurities, and the type of impurities in integrated circuit chip wafers can substantially alter electrical properties of the wafers. One desire in equipment such as crucibles used to form ingots for integrated circuit chip wafer is therefore to minimize impurity contamination from the equipment such as crucibles.  
         [0019]    Another hurdle in the process of forming silicon ingots includes the high cost of current crucibles due to expensive crucible materials, and the difficulty in re-using crucibles to form multiple ingots from a single crucible. Most crucible materials are brittle and tend to fracture easily. Further, due to coefficient of thermal expansion mismatches, during cooling, a silicon ingot can produce pressure radially outward on crucible walls, which leads to fracture of the crucible. Additionally, silicon frequently interacts with the walls and bottom of the crucible, causing the cooled ingot to bond to the walls of the crucible. If an ingot is bonded to the walls of the crucible, either chemically, or mechanically, etc. then the ingot cannot be removed from the crucible without breaking either the crucible or the ingot.  
         [0020]    Embodiments of crucibles described in the following descriptions address these technical hurdles and produce ingots with the necessary purity for integrated circuit wafer use, in a process that allows re-use of crucibles. Re-using crucibles substantially reduces the manufacturing cost of ingots.  
         [0021]    [0021]FIGS. 1A and 1B show a crucible  100 , having a central axis  101 , according to one embodiment of the invention. In one embodiment, the crucible  100  is used to fabricate an ingot of semiconductor material. In one embodiment, the crucible  100  is used to fabricate an ingot of silicon material. In one embodiment, the crucible  100  is used to fabricate an ingot of polycrystalline semiconductor material.  
         [0022]    The crucible  100  includes a base container  110 . The base container includes a bottom  104  and sides  102 . As shown in FIG. 1A, in one embodiment, the sides  102  form a taper angle  106  in relation to a line perpendicular to the bottom  104 . In one embodiment, the taper angle is between approximately 1 and 3 degrees. A substantially cylindrical crucible  100  is illustrated in FIG. 1, although the invention is not so limited. Other geometries include cubes, rectangles, hemispheres, etc.  
         [0023]    In one embodiment, the base container  110  includes a silicon dioxide base container. Acceptable forms of silicon dioxide material include, but are not limited to, fused silica; slip cast quartz, other grades and processes of quartz, etc. In one embodiment, the base container  110  includes a carbon base container. Acceptable forms of carbon material include, but are not limited to, amorphous graphite, graphite with an oriented grain structure, etc. In one embodiment, graphite material includes purified graphite. In one embodiment, the central axis  101  of the crucible is oriented substantially perpendicular to an orientation of graphite grains. Although specific material examples such as silicon dioxide and carbon are discussed, the invention is not so limited. Other acceptable materials for the base container  110  are within the scope of embodiments of the invention.  
         [0024]    Base containers formed from graphite as described above provide a number of advantages. One advantage includes a coefficient of thermal expansion that is lower that a coefficient of thermal expansion for silicon. Upon cooling, a silicon ingot will therefore shrink faster than the crucible it is contained within. This reduces or eliminates any outward forces that cause destructive fracture of other crucibles using other base container materials. Another advantage of base containers formed from graphite as described above includes utilizing anisotropic coefficients of thermal expansion in oriented grain graphite material. Orienting the central axis  101  to be substantially perpendicular to an orientation of graphite grains further enhances a desired coefficient of thermal expansion for the base container  110  to provide ease of removal of a cooled ingot, and reduces fracture of crucibles.  
         [0025]    A coating layer  120  is further shown in FIG. 1A. In one embodiment, the coating layer  120  includes boron nitride (BN) material. In one embodiment the coating layer  120  includes silicon nitride (Si 3 N 4 ) material. In one embodiment, the coating layer  120  includes a composite coating layer, having portions of two or more different materials. In one embodiment, a composite coating layer includes boron nitride (BN) material and silicon nitride (Si 3 N 4 ) material. In one embodiment, the boron nitride material and the silicon nitride material are both of a high purity of approximately 99.999% pure. High purity reduces a possibility of unwanted contaminants to the ingot process.  
         [0026]    Coating layers including boron nitride provide a number of advantages. One advantage includes release properties with respect to silicon ingots. Liquid silicon does not easily adhere to boron nitride material on the walls  102  and bottom  104  of the base container  110 . Another advantage of embodiments with boron nitride in the coating layer is that if any impurities are introduced to the ingot, the potential impurity is compatible with integrated circuit chip processing. Although in some embodiments, a small amount of boron impurities are introduced into the ingot during formation, boron is typically an acceptable impurity in integrated circuit chip processing. In one embodiment, the presence of boron can be used in wafer design to reduce unwanted conduction.  
         [0027]    [0027]FIG. 2 shows a crucible  200  according to one embodiment of the invention. The crucible  200  includes a base container  210 . The base container includes a bottom  204  and sides  202 . In one embodiment, the base container  210  includes a silicon dioxide base container. In one embodiment, the base container  210  includes a carbon base container. In one embodiment, the base container  210  includes graphite. In one embodiment, a central axis  201  of the crucible is oriented substantially perpendicular to an orientation of graphite grains. Although specific material examples such as silicon dioxide and carbon are discussed, the invention is not so limited. Other acceptable materials for the base container  210  are within the scope of embodiments of the invention.  
         [0028]    A first coating layer  220  is shown forming an interface with the base container  210 . A second coating layer  230  is shown forming an interface with the first coating layer  220 . In one embodiment, the first coating layer  220  and the second coating layer  230  together form a composite coating layer. Although in FIG. 2, the composite coating layer configuration includes two continuous layers formed one over the other, alternative composite coating layers configurations, such as a matrix and dispersed phase composite for example, are also within the scope of the invention.  
         [0029]    In one embodiment, the first coating layer  220  includes boron nitride. In one embodiment, the second coating layer  230  includes silicon nitride. In one embodiment, the first coating layer  220  includes silicon nitride and the second coating layer  230  includes boron nitride. An advantage of using boron nitride as the first coating layer  220  includes the property that boron impurity diffusion into a silicon melt is reduced by substantially covering the boron nitride first coating layer  220  with a second coating layer  230 . An advantage of using silicon nitride as the second coating layer  230  includes the property that silicon nitride has a low reactivity with liquid silicon. Advantages of a composite coating layer including a boron nitride first coating layer  220  and a silicon nitride second layer  230  includes the ability to substantially prevent liquid silicon from contacting walls of the base container  210  while reducing an amount of impurities introduced into the liquid silicon. Another advantage of a composite coating layer including a boron nitride first coating layer  220  and a silicon nitride second layer  230  includes enhanced releaseability of a cooled silicon ingot from coated surfaces such as the walls  202  and bottom  204  of the crucible  200 .  
         [0030]    A silicon nitride coating, when used by itself has problems such as coating porosity that allows liquid silicon to penetrate the silicon nitride coating and come into contact with a base container material. As discussed above, this contact can lead to unacceptable impurities being introduced in to the liquid silicon. Further, silicon will cool into pores of a silicon nitride coating layer and cause a chemical/mechanical bond with surfaces of the silicon nitride coating if there are no additional driving forces to prevent flow into the pores. This can cause difficulty in removing the ingot from the crucible. An advantage of embodiments using boron nitride includes a surface energy force with respect to liquid silicon that keeps liquid silicon out of pores in a silicon nitride coating.  
         [0031]    [0031]FIG. 3 shows a crucible  300  according to one embodiment of the invention. The crucible  300  includes a base container  310 . The base container includes a bottom  304  and sides  302 . In one embodiment, the base container  310  includes a silicon dioxide base container. In one embodiment, the base container  310  includes a carbon base container. In one embodiment, the base container  310  includes graphite. In one embodiment, a central axis  301  of the crucible is oriented substantially perpendicular to an orientation of graphite grains. Although specific material examples such as silicon dioxide and carbon are discussed, the invention is not so limited. Other acceptable materials for the base container  310  are within the scope of embodiments of the invention.  
         [0032]    A first coating layer  320  is shown forming an interface with the base container  310 . A second coating layer  330  is shown forming an interface with the first coating layer  320 . In one embodiment, the first coating layer  320  and the second coating layer  330  together form a composite coating layer. In one embodiment, the first coating layer  320  includes boron nitride. In one embodiment, the second coating layer  330  includes silicon nitride.  
         [0033]    In FIG. 3, the second coating layer  330  is shown only covering interior surfaces such as the walls  302  and bottom  304  of the crucible  300 . In one embodiment, all surfaces of the base container  310  are covered by the first coating layer  320 . One advantage of covering external surfaces of the base container  310  includes a suppression of contaminants, such as carbon from a graphite crucible for example, through the atmosphere into the liquid silicon. An advantage of covering only interior surfaces such as the walls  302  and bottom  304  of the crucible  300  with the second coating layer  330  includes cost savings. Only covering the interior surfaces of the crucible  300  with the second coating  330  is effectuve beacause only these surfaces experience direct contact with liquid silicon.  
         [0034]    [0034]FIG. 4 shows a crucible  400  according to one embodiment of the invention. The crucible  400  includes a base container  410 . The base container includes a bottom  404  and sides  402 . In one embodiment, the base container  410  includes a silicon dioxide base container. In one embodiment, the base container  410  includes a carbon base container. In one embodiment, the base container  410  includes graphite. In one embodiment, a central axis  401  of the crucible is oriented substantially perpendicular to an orientation of graphite grains. An amount of liquid material  440  is illustrated in FIG. 4 filling a portion of the crucible  400 . In one embodiment, the liquid material  440  includes liquid silicon. Other semiconductor materials are also within the scope of the invention. As noted above, the liquid material  440  is only in contact with interior surfaces such as the walls  402  and bottom  404  of the crucible  400 .  
         [0035]    [0035]FIG. 5 shows a crucible  500  according to one embodiment of the invention. The crucible  500  includes a first base container portion  512  and a second base container portion  514 . Although two base container portions are illustrated in FIG. 5, other numbers of base container portions are also within the scope of the invention. In one embodiment, the first base container portion  512  and the second base container portion  514  form a bottom (not shown) and sides  502 . A number of fasteners  516  are shown in FIG. 5 to hold the first base container portion  512  and the second base container portion  514  together during melting. In one embodiment, the number of fasteners  516  are removable after cooling of the ingot to facilitate removal of the ingot from the crucible  500 .  
         [0036]    In one embodiment, the first base container portion  512  and the second base container portion  514  include a silicon dioxide material. In one embodiment, the first base container portion  512  and the second base container portion  514  include a carbon material. In one embodiment, the first base container portion  512  and the second base container portion  514  include graphite. In one embodiment, a central axis  501  of the crucible  500  is oriented substantially perpendicular to an orientation of graphite grains. Although specific material examples such as silicon dioxide and carbon are discussed, the invention is not so limited. Other acceptable materials for the first base container portion  512  and the second base container portion  514  are within the scope of embodiments of the invention. A substantially cylindrical ingot form is illustrated in FIG. 5, although the invention is not so limited. Other geometries include cubes, rectangles, hemispheres, etc.  
         [0037]    In one embodiment, surfaces of the first base container portion  512  and the second base container portion  514  include a coating layer similar to embodiments discussed above. In one embodiment, all surfaces of the first base container portion  512  and the second base container portion  514  are coated with boron nitride. In one embodiment, at least the interior surfaces of the first base container portion  512  and the second base container portion  514  are coated with silicon nitride over boron nitride.  
         [0038]    [0038]FIG. 6 shows an ingot system  600 . In one embodiment, the ingot system is used to form silicon ingots. The ingot system  600  includes a crucible  610  as described in selected embodiments above. The ingot system  600  includes a furnace  620  that is adapted to heat the crucible  610  along with its contents such as silicon, to form a silicon ingot.  
         [0039]    In one embodiment, the ingot system  600  includes a cooling system  630 . In one embodiment, the cooling system is used to control a preferred direction of cooling in the crucible  610 . In one embodiment, heat is conducted from a bottom of the crucible preferentially, in contrast to cooling the crucible  610  from all sides at the same time. Cooling preferentially, such as cooling from a bottom of the crucible, forms an anisotropic grain structure in the ingot. Preferential orientation of grains in an ingot such as a silicon ingot leads to desirable properties such as preferential electrical conduction, preferential diffusion characteristics, etc.  
         [0040]    In one embodiment, the ingot system  600  includes a control gas system  640 . In one embodiment, the control gas system provides a desired gas atmosphere that surrounds the crucible and the liquid material during the melt. In one embodiment, the control gas includes nitrogen. An advantage of including nitrogen is that the presence of an amount of nitrogen suppresses chemical reactions that drive a dissociation of nitrogen containing compounds. For example, nitrides such as boron nitride and silicon nitride tend to not dissociate into boron and nitrogen or silicon and nitrogen if a certain amount of nitrogen is present in the control gas. Reducing these reactions in turn produces fewer undesirable impurities that can contaminate the liquid being melted. In one embodiment, the control gas further includes an amount of inert gas such as argon. In one embodiment, the control gas includes between 5 and 10 percent nitrogen with the remaining gas being argon.  
         [0041]    [0041]FIG. 7 shows an ingot  700  formed using methods, crucibles, and systems as described above. In contrast to common semiconductor wafers, the ingot  700  is a multicrystalline ingot. As discussed above, single crystal wafers are typically used to fabricate devices such as transistors. A radial direction of the ingot  700  is indicated by arrow  702 . Likewise, a longitudinal direction of the ingot  700  is indicated by arrow  704 . In one embodiment, as described above, a grain structure of the ingot  700  is preferential with respect to these ingot coordinate directions. Further shown in FIG. 7 is a multicrystalline wafer  710  that has been cut from the ingot  700 .  
         [0042]    [0042]FIG. 8 shows a composite wafer  800  according to one embodiment of the invention. The composite wafer  800  includes a handling wafer portion  810  and a device portion  820 . In one embodiment, the device portion  820  includes single crystal silicon. In one embodiment, the handling wafer portion  810  includes a multicrystalline wafer formed by methods described above. Also shown in FIG. 8 are a number of individual chips  822  outlined on the device portion  820  of the composite wafer  800 .  
         [0043]    Advantages of the composite wafer  800  include a lower cost material in the handling wafer portion  810  while maintaining a device portion  820  for formation of chips  822  with integrated circuits. Using devices and methods described above, one advantage includes a reduction in manufacturing cost of the handling wafer portion  810  due to a reusable crucible. Another advantage of devices and methods described above includes an ability to form a handling wafer portion  810  with a low impurity concentration that is acceptable to the integrated circuit chip industry. While the handling wafer portion  810  as shown in FIG. 8 is physically separate from the device portion  820 , during semiconductor processing techniques, a contaminated handling wafer portion  810  could introduce unwanted impurities to the device portion  820  through mechanisms such as diffusion. Another advantage of devices and methods described above includes an increased mechanical integrity of the composite wafer  800  at a low cost due to a larger thickness of the composite wafer  800  without the high cost of a completely single crystal wafer.  
         [0044]    [0044]FIG. 9 shows a composite wafer  900  according to one embodiment of the invention. The composite wafer  900  includes a handling wafer portion  910  and a device portion  930 . The composite wafer  900  further includes an isolation layer  920  separating the handling wafer portion  910  from the device portion  930 . In one embodiment, the device portion  930  includes single crystal silicon. In one embodiment, the handling wafer portion  910  includes a multicrystalline wafer formed by methods described above. Also shown in FIG. 9 are a number of individual chips  932  outlined on the device portion  930  of the composite wafer  900 . In one embodiment, the composite wafer  900  operates as a silicon-on-insulator (SOI) wafer. One of ordinary skill in the art, having the benefit of the present disclosure will recognize that SOI wafers possess certain electrical advantages for the design of integrated circuit chips. Similar to embodiments described above and shown in FIG. 8, the composite wafer  900  includes a number of advantages such as low cost, increased mechanical integrity, and low impurity concentrations.  
         [0045]    While a number of advantages of embodiments of the invention are described, the above lists are not intended to be exhaustive. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of embodiments described above. It is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments, and other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention includes any other applications in which the above structures and fabrication methods are used. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.