CZ321196A3 - Novel artificial lamella of silicon carbide - Google Patents

Abstract

The present invention relates to an unsiliconized or siliconized wafer consisting essentially of recrystallized silicon carbide, the wafer having a diameter of at least 150 mm and a thickness of between 0.5 and 2 mm, and having a porosity or free silicon content between 15 v/o and 43 v/o.

Description

Technical field

Typically, manufacturing semiconductor devices such as diodes and transistors requires oxidizing the surfaces of thin silicon wafers, etching the cavities into the wafers' surfaces, and applying some alloying agents to these cavities, such as boron, phosphorus, arsenic, or antimony, and thereby forming transistor contacts. These oxidation and alloying operations include rapid heating and cooling cycles in an electrically heated furnace at temperatures ranging from 1000 ° C to 1350 ° C. After the surface has been etched, the alloying agent is usually supplied as a gas to the tapered lower end of the diffusion process tube housed in the furnace. This gas then diffuses into the etched cavities and settles on their surfaces.

During the oxidation and diffusion steps, the silicon wafers sit on the baths or plates located inside the processing tube. The wafer tray and processing tube are typically made of a material having excellent thermal shock resistance, high mechanical strength, ability to retain its shape during a large number of heating and cooling cycles, and which does not produce gas, i.e. does not bring any furnace atmosphere into the furnace atmosphere. undesirable impurities. One such material that meets these requirements is silicon-saturated silicon carbide.

When the silicon wafers are processed in a bath, it is naturally desirable that each wafers in the bath be subjected to the same gas concentration and temperature waveforms to produce the same product. However, a typical hydrodynamic situation is that the same profiles are found only in the center of the tub, while unequal profiles are often found at the ends of the tubs, resulting in undesired degrees of alloying on the end sheets, making them unusable.

One common way to alleviate the problem with this end effect is to fill the end grooves of the tub with sacrificial silicon wafers. However, silicon wafers have been found to be expensive, highly gassy, collapse at high processing temperatures, flake-shaped particles peel off, and have a short useful life range.

Another common way to alleviate the end-effect problem is to fill the end grooves of the tub with artificial slices. For example, one researcher placed SiC-coated carbon wafers in the end grooves that were exactly the dimensions of adjacent silicon wafers. However, it has been found that these slices break off and contaminate the furnace with released carbon. Another researcher suggested using CVD monolithic silicon carbide as an artificial wafer. However, this material is known to be very expensive.

Japanese Patent Publication 5-283306 discloses an artificial silicon carbide wafer having a silicon-saturated silicon having an alumina / silica coating.

It is therefore an object of the present invention to provide an inexpensive artificial wafer having the dimensional, physical and mechanical properties required of the artificial wafer.

SUMMARY OF THE INVENTION

According to the present invention, a silicon-unsaturated wafer is provided which essentially consists of recrystallized silicon carbide, and the wafer has a diameter of at least 150 mm and a thickness of not more than 2 mm and a porosity between 15 w / o and 43 w / o.

Also provided in accordance with the present invention is a slice of recrystallized silicon carbide having a diameter of at least 150 mm and a thickness of no more than 2 mm and containing between 15 v / o and 43 v / o free silicon and having a silicon carbide CVD coating thereon.

According to the present invention there is also provided a slice of recrystallized silicon carbide having a diameter of at least 150 mm and a thickness of not more than 2 mm and containing between 25 and 40 v / o free silicon, the free silicon comprising raw, interconnected free silicon flakes which have diameters of 5 to 50 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a micrograph of conventional silicon carbide unsaturated silicon, in which the light regions represent silicon carbide and the dark regions represent porosity.

Figure 2 is a photomicrograph of an embodiment of unsaturated silicon according to the present invention, wherein the light regions represent silicon carbide and the dark regions represent porosity.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, v / o refers to volume percent, w / o refers to weight percent, and the commonly manufactured article refers to Si-SiC products manufactured in accordance with US Patent No. 3,951,587 (Alliegro patent). In addition, the expression flatness is considered to be the largest arc height from the center reference plane, where the center reference plane is defined by any diameter on the wafer surface.

Initial experiments by existing inventors tested silicon-saturated silicon carbide, in particular CRYSTAR containing about 15% free silicon and about 85% bimodal SiC, manufactured by the Norton Company of Worcester, MA as an artificial wafer material. However, it has been found that the conventional casting process of CRYSTAR (a bimodal SiC mixed on a casting slurry cast in a porous gypsum mold) could not successfully produce a strong billet capable of slicing. Especially if the casting slurry was cast thicker than about 20 mm in depth, it would develop cracks in the resulting billet due to residual stresses during drying or firing.

This process is believed to have failed because the water remaining in the pores of the raw body after the slurry has been cast has been converted to trapped steam upon subsequent heating. The internal pressure induced by this steam forced the cast body to crack and warp. The present inventors have noticed that the concept of a conventional casting slurry produced only about 15 v / o porosity in the cast body and pore channels of only about 2 microns (measured by mercury porosimetry) and assumed that this level of porosity was not substantially sufficient to provide continuous channels suitable to escape the remaining water during normal drying. They also assumed that the specific gravity gradients created by conventional casting slurry casting contributed to the fracture problem, as these gradients created thermal stresses during heating.

The present inventors have also tried free casting. The free casting method produced a thin sheet having a thickness of about 3 mm (to remember the warping during firing), which was then sanded to the desired thickness of 0.5 to 1.0 mm. The fired product had a porosity of about 15-16 v / o. However, since the desired grinding operation is labor intensive and removes more than half of the wafer material, open casting was considered inadmissible inefficient and expensive. Further attempts to cast the slurry closer to the desired wafer thickness resulted in crude slices that collapsed during drying and peeling.

The present inventors then considered freeze casting of bimodal silicon carbide slurry and unexpectedly found that the freeze casting provided a thin, dimensionally accurate billet that did not collapse or crack during processing, was easily sliced, and after being sliced to provide sufficient strength.

It is believed that the freeze-casting process provides an unburned billet body which is particularly suited to the requirements of mass production of SiC artificial sheets. If the slurry is cast by freeze-casting, the water does not reach a 4% volume increase when ice crystals are formed. Since the freezing casting is carried out in an enclosed space, the expansion of the ice particle has the effect of ramming the SiC particles closer together (compared to the ramming of the SiC slurry) in areas not affected by the ice particles. In addition, it has been observed that the ice crystals formed in the freeze molding are interconnected and thus form pore channels upon drying. Thus, although the freeze-cast body has the same total volume percent solids as the conventional slurry body (i.e., about 72 v / o), the freeze-cast body has both larger interconnected pores and better bond between the particles. Better particle-to-particle bonding not only gives the cast body good strength (despite larger pore size), but also good sintered body strength, as more compressed SiC grains form more readily the necks during recrystallization. Since the interconnected pores provide a vapor escape channel and the bonding of the top particles provides surface strength, it appears that for large-scale production of SiC artificial wafer, freeze-casting avoids the problems encountered in the conventional casting slurry casting process.

Another advantage of the present invention is that its preferred embodiment need not include the vacuum sublimation step required typically during conventional silicon carbide freeze casting. Without wishing to be bound by any theory, it is believed that vacuum sublimation is undesirable because the compaction of the SiC grains during freezing provides a relatively strong, skeletal structure resistant to movement (and thus cracking) when water is removed. In addition, the relatively spacious pore channels formed by the ice crystals provide reduced capillary pressures and reduced drying stresses.

According to one preferred embodiment of the present invention, one SiC-based wafer is produced and used in a process comprising:

(a) mixing silicon carbide powder, water and an ice crystal growth inhibitor to produce a slurry;

b) freezing the casting slurry at approximately -85 ° C for the production of frozen castings,

c) natural drying of the frozen casting for partial water removal,

d) drying the casting at about 200 ° C for approximately 24 hours,

e) vacuum precalination of the body to form a recrystallized billet having a green strength of about 35 MPa;

f) cutting billet into slices,

(g) optional saturation of the wafers with silicon and / or CVD coatings; and

h) placing the petals in the tub.

In the above embodiment, the slurry typically consists of a bimodal SiC powder distribution comprising between about 15 and about 41 v / o coarse SiC grains having a particle size ranging from 10 to 150 microns (crude fraction) and between about 34 and about 60 v SiO fine grains having a particle size ranging between 1 and 4 microns (fine fraction). The fine fraction preferably constitutes between about 36 and 42 w / o casting slurry and has an average particle size of about 2-3 microns, while the crude fraction forms between 33 w / o and 38 v / o casting slurry and has an average particle size of about 60 microns. When the raw SiC particle size is above about 150 microns, it is close to half the cross-section of the final wafer and grain peeling is observed in the finished wafer.

Water is generally included in the casting slurry in an amount sufficient to form a casting slurry having from about 50 to about 85 w / o solid particles. However, other freeze-casting solvents such as glycerin, ethanol, methanol, hexane may suitably be used as the liquid carrier of the slurry.

The slurry also preferably comprises an ice crystal growth inhibitor. A typical freeze casting technique produces ice crystals up to 5,000 - 10,000 pm both on the inside and on the outside of the frozen cast. Subsequent sublimation drying and firing of these bodies reveals large isolated pores (residues of large ice crystals). These insulated pores act as cracks that degrade both the green strength and the resulting strength. The ice crystal growth inhibitor prevents large crystalline formations by forcing the casting slurry to freeze in the form of tiny crystals of the order of only 5-50 microns. Typical inhibitors of ice crystal growth include hydrogen bridge forming compounds such as glycerin and all of the compounds similarly identified in US Patent No. 4,341,725 (Weaver's patent), the entire detailed description of which is incorporated by reference. The ice crystal growth inhibitor typically comprises between about 0.2 w / o and about 5 w / o casting slurry, preferably between about 1 w / o and about 1.5 w / o. In more preferred embodiments, the glycerin comprises about 1 w / o casting slurry. Thus, the required amount of ice crystal growth inhibitor depends on the solids content of the slurry, and at high solids content, the slurries require less inhibitor. Other typical casting slurry components include conventional amounts of conventional casting additives. For example, liquefying agents such as NaOH and Na ₂ SO ₃ may be used. Binder may also be present in the range of about 0.25 w / o to 4.0 w / o solids. In preferred embodiments, an acrylic latex binder is used at about 1 w / o solids.

To ensure a homogeneous casting slurry, the casting slurry components are typically mixed in a ball mill exhausted to a vacuum level of between about 27 and 30 inches Hg and rolling for at least about 17 hours.

The freezing step in this embodiment (often called freeze casting) preferably comprises casting the casting slurry into an impermeable mold and lowering its temperature until the liquid carrier freezes, thereby solidifying the casting slurry. Casting freeze δ

The slurry generally means lowering its temperature to between about -20 ° C and -100 ° C for about 30-180 minutes, resulting in a frozen cast body having only small ice crystals, i.e. 5-50 microns. The impermeable mold is preferably made of silicone rubber, which can easily peel off the frozen body.

The natural drying step of the preferred embodiment serves to remove enough free water from the cast to allow it to be placed in a heated dryer without cracking. Natural drying can be accomplished by simply removing the frozen body from its mold and leaving it permanently in the air for about 24 hours. Typical conditions and drying times for natural drying are between 20 and 30 ° C, preferably 25 ° C; between about 0.01 and more atm, preferably at about 1 atm; and between about 18 and about 48 hours, preferably about 24 hours.

The high temperature drying step of the above embodiment is typically carried out at a higher temperature and for a longer time than the natural drying step and removes substantially all of the water absorbed in the casting. Typical conditions and drying times for this step are between 80 ° C and 200 ° C, preferably 140 ° C; between about 0.01 and 1 atm, preferably at about 1 atm; and between about 18 and about 48 hours, preferably about 24 hours. Unexpectedly, it has been found that the body of the freeze cast can be suitably dried at atmospheric pressure under these conditions without cracking. As mentioned above, it has been found that commercially produced SiC slurry bodies burst under high temperature and natural drying conditions. Since the freeze-drying process does not require subsequent vacuum drying, it is significantly less expensive than conventional SiC processing.

The dried casting produced in accordance with this embodiment has a bulk density of at least about 1.8 g / cc and a four-point bending strength of at least about 5 MPa. Its pore size ranges from about 5 to 50 microns. Its average pore size is about 15 microns. In contrast, a commercially dried SiC casting has an average pore size of only about 2 microns.

The pre-vacuum sintering step of this preferred embodiment serves to produce a crack-free recrystallization, i.e., neck growth between the SiC grains without densification. This can be done at about 1900-1950 ° C and under a vacuum of about 0.5 torr in an Ar atmosphere. While conventional SiC castings have been found to burst under these conditions, it is believed that the frost casting bodies of the present invention do not crack because the relatively large pore channels formed by the ice crystals result in low capillary pressures and low drying stresses as well as uniform density across the portion that resists thermal stresses.

The recrystallized billet produced according to this embodiment has a bulk density of at least about 1.8 g / cc. Its porosity ranges from 25 w / o to 43 w / o. Its pore size ranges from about 5 to 50 microns. Its average pore size is about 15 microns. In contrast, a conventional recrystallized SiC casting has a porosity of about 16 v / o and an average pore size of about 2 microns. Its strength, when measured by repeated biaxial bending, is at least 30 MPa, typically between 30 and 50 MPa.

After pre-sintering, the recrystallized billet is cut to its final dimension by conventional techniques, such as diamond disk or wire. Compared to less porous conventional SiC billets, the recrystallized SiC billet of this embodiment is easier to cut into thin SiC slices. The structure of the pre-sintered billet is such that it has sufficient handling strength but is easily cut to good surface quality and flatness. For example, a 1 mm thick slice made in accordance with the present invention can be cut from a billet having a diameter of 15 cm in just 5 minutes. In contrast, it is believed that a SiC billet of higher density gravel would require about 60 minutes to cut and a fully dense SiC billet would require about 120 minutes to cut. In accordance with this embodiment, it is usually possible to obtain artificial silicon carbide slices having a diameter of between about 150 and about 300 mm, a thickness of between about 0.5 and about 2 mm, preferably between 0 and 0, after simply cutting with a diamond saw. 5 mm and 1.5 mm and even more preferably between about 0.5 and 1.0 mm; and a flatness of between about 25 and about 100 microns, preferably less than about 50 microns. If the wafer is subsequently saturated with silicon, it may be necessary to rotate it for a short time to remove a few microns and achieve a flatness of less than 100 µm.

The final firing step makes the wafer impermeable to gases or liquids. This typically involves either impregnating the porous sheet with silicon to remove porosity and / or CVD coating it with some impermeable ceramic material, such as silicon carbide. If silicon saturation is chosen, it can be carried out in accordance with US Patent No. 3,951,587 (Alliegro patent), the disclosure of which is incorporated by reference. Unexpectedly, it was observed that the silicon-saturated slices had a flatness of about 100 µm. On the other hand, it has been found that dimensionally similar conventional SiC raw castings excessively bend, requiring thicker casting and more expensive final machining to produce the same flat product.

The silicon-saturated wafer of the preferred embodiment of the present invention has a bulk density of at least about 2.75 g / cc. Its free silicon flakes range from about 5 to 50 microns in diameter. He's totally dense. In contrast, a conventional silicon-saturated SiC wafer has free silicon flakes having a diameter of only about 2 microns.

The microstructure of this embodiment of the present invention appears to have three distinct material phases including: a raw SiC grain phase, a raw free silicon phase, and a mixed phase consisting of fine SiC grains and fine free silicon flakes. Depending on whether the synthetic SiC sheet is saturated with silicon, this SiC sheet typically consists of:

(a) approximately between 15v / o and 41 v / o (preferably 33 to 38 v / o) silicon carbide grains having a diameter between 10 μπι and 150 μπι,

b) between about 34 w / o and 60 w / o (preferably 36 to 42 w / o) silicon carbide grains having a diameter between 1 µm and 4 µm, and

(c) between 25 v / o and 40 v / o free silicon or porosity. The porosity of the unsaturated silicon wafer is characterized by the typical size distribution of the coarse (5-50 pm) and fine pores, while the free silicon wafer of the silicon-saturated wafer is characterized by loose silicon flakes having diameters of 5-50 µm and a scented silicon matrix surrounding fine grains SiC. See Figure 2. In some embodiments, preferably between 35 v / o and 40 v / o free silicon. In comparison, it has been found that prior art microstructures are characterized by a uniform mixed phase structure comprising large SiC grains, small SiC grains and small free silicon flakes or porosity. See Figure 1.

The sanding of a silicon-saturated SiC sheet can remove excess free silicon that has been stripped off the surface due to the solidification of the silicon by solidification, and can be done by conventional sanding procedures. Because these sheets have high strength, they do not break when subjected to sandblasting.

Although the above-described embodiment of the present invention employs freeze-molding to produce a thin, solid SiC wafer, it is also contemplated that useful SiC wafer can be obtained by a variety of alternative processes including: a) hot pressing the SiC billet at 1750 ° C and 3000 psi; b) gel casting and pre-sintering the billet of SiC in accordance with US Patent No. 5,145,908; (c) cold isostatic compression of the billet of SiC; and (d) strip casting or roll molding and then recrystallization of a SiC slurry to produce a fired SiC wafer having a porosity of about 21%.

The novel recrystallized silicon carbide ceramic products of the present invention can be used in conventional silicon-saturated silicon carbide applications or CVD-coated silicon carbide applications, including those disclosed in the Alliegr patent. They can also find use as hard drives of persistent file management programs on a computer, as a pad for other applications in the electrical cabinet, or as reflective boards in sliced tubs. In particular, a silicon carbide disk support for use in a disk drive assembly having a head and disk is provided, the disk comprising a disk support, wherein the disk support comprises a) between 15 w / o and 43 w / o free silicon or porosity, preferably between 25 v / o and 40 v / o; preferably, it has a flatness of between 25 µm and 100 µm; c) preferably has a bimodal grain distribution of SiC, i.e. coarse and fine grains; and

d) preferably has a bimodal distribution of free silicon or pores, i.e. coarse and fine pores. Other intended uses of the highly porous silicon carbide disks of the present invention (which could benefit from a low pressure drop across the disk) include gas burner plates, composite pads and filters.

In some embodiments, the porous sheet of the present invention is optionally coated with either a layer of polysilicon, silicon nitride or silica placed in a diffusion tub in which the silicon wafers are also subsequently treated at a temperature of at least about 600 ° C.

In some embodiments, a silicon-saturated wafer coated with a SiC CVD of the present invention is placed in a diffusion tub, in which silicon wafers are subsequently placed, and these wafers are processed at temperatures above 1000 ° C. It is believed that a coating of CVD SiC is necessary at these temperatures to prevent the oxidation of the SiC grains. Therefore, a process is also proposed which includes:

a) placing a silicon wafer in a diffuser tray having silicon-saturated wafer inserts of the present invention coated with SiC CVD, and

b) processing the silicon wafer at a temperature above about 1000 ° C.

In accordance with the present invention there is also provided a simple wafer processing process comprising the steps of:

a) preparing the silicon carbide wafer of the present invention (preferably having a diameter of at least 200 mm, and more preferably at least 300 mm) in a substantially horizontal position; and

b) placing a silicon wafer (which preferably has a diameter of at least 200 mm, and more preferably at least 300 mm) on the silicon carbide disk; and

c) heating the silicon wafer at a rate of at least 100 ° C per second.

In accordance with the present invention there is also provided a method for cleaning the processing chambers of individual slices, which comprises the steps of:

a) exciter measures in the processing chamber,

b) placing a silicon wafer on the exciter,

c) processing of silicon wafer,

d) silicon wafer removal,

e) placing a wafer of silicon carbide according to the present invention (which preferably has a diameter of at least 200 mm and even more preferably at least 300 mm) on the exciter; and

f) cleaning the processing chamber.

According to the present invention there is also provided a method of processing a flat panel display comprising the steps of:

a) carrying out the process for the silicon carbide slices of the present invention (which preferably have a length of at least 165 nm and a width of at least 265 mm) in a substantially horizontal plane,

b) placing a flat glass plate (preferably having a length and width of at least 100 mm) on the silicon carbide disk; and

c) processing the flat glass sheet.

In accordance with the present invention there is also provided a method for plasma etching silicon wafers, which comprises the steps of:

a) preparing a silicon wafer having a predetermined diameter of at least 200 mm,

b) positioning the silicon carbide ring of the present invention (having an inner diameter substantially equal to a predetermined silicon wafer diameter) around the silicon wafer; and

c) plasma etching (preferably dry metal etching) of the silicon wafer.

EXAMPLE I

The slurry for freeze-casting was prepared by mixing the following materials in the amounts indicated in Table 1 and rolling in a vessel for 18 hours.

Table 1

silicon carbide (3 microns) silicon carbide (100 F) 4680 g 4320 g * water 1080 g BASF Acranol 290D binder 137 g NaOH (IN) 81 g baker's glycerin 90 g This slurry was degassed in vacuum and poured

a polyvinyl chloride tube having an inner diameter of 6, an outer diameter of 6.5, and a height of 10. This section was secured to the glass plate to prevent leakage and to form the bottom surface. The assembly was then placed in the freezer at -85 ° C for 3 hours. Upon complete freezing, the tube was cut from the billet. The billet made by freeze-casting was initially dried naturally at approximately 25 ° C for 18 hours and finally dried at 140 ° C for 48 hours to remove absorbed water. The billet was then sintered at approximately 1900 ° C in an argon atmosphere to effect recrystallization. The porous recrystallized billet was then dry-cut with a metal-bonded diamond saw to a thickness of 0.040. This slice was infiltrated with molten silicon at approximately 1800 ° C under an argon / nitrogen atmosphere and then blasted with SiC grains to remove excess silicon. The blasted wafer had a flatness of about 100 microns. Rotary grinding with diamond abrasives provided a flatness of about 50 microns. It is believed that the final lapping of the boron carbide slurries could create a flatness of 20 microns.

EXAMPLE II

A silicon carbide unimodal slurry having an average size of 3 microns was hot pressed into a graphite die at approximately 1850 ° C and 3000 psi for 1 hour. The billet had a diameter of 3, a height of 4, and a specific gravity of about 2.0 g / cc (about 62% of the theoretical specific gravity). The billet was cut dry by a diamond bonded metal disc to a thickness of 0.75 mm. The wafer was impregnated with liquid silicon at about 1800 ° C under an argon / nitrogen atmosphere. The silicon-saturated wafer was then blasted with silicon carbide grains to remove excess silicon. The silicon-saturated wafer had a flatness of about 70 microns. Some of the blasted slices were coated with about 50 microns of SiC by chemical precipitation of methyltrichlorosilane vapors in hydrogen and argon at about 1100

Claims (3)

PATENT CLAIMS 1. A non-silicon wafer consisting of! essentially from recrystallized silicon carbide, the wafer having a diameter of at least 150 mm and a thickness of between 0.5 and 2 mm and a porosity of between 15 v / o and 43 v / o. A wafer according to claim 1, characterized in that the wafer has a thickness between 0.5 mm and 1.5 mm. The wafer of claim 2, wherein the wafer has a diameter of no more than 300 mm. The wafer of claim 3, v y of n and C at 3 and c í s E t í m that has a porosity between 25 v / o and 40 in/ O. The wafer of claim 3, v y of n and C at 3 and c í s E t í m, the silicon carbide composed that grains carbide silicon. The wafer of claim 4, v y of n and C at 3 and c í s E in that the porosity is characterized by coarse interconnected pores having a diameter of between -5 and 50 µm / µm. 7. The wafer of claim 6, v yznačuj í C and with E t í m, it has a thickness between 0.5 and 1 mm. 8. The wafer of claim 6, v yznačuj í C and with E t í m, that the coarse pores have a diameter around 15 microns. 9. The wafer of claim 4, v yznačuj í C and with E t í m, that SiC has a bimodal size distribution consisting of of coarse grains and fine grains. 10. The wafer of claim 9 a č u j í c and with E t í m, that coarse grains have a diameter between 10 i and 150 microns. 11. The wafer of claim 10 a č u j í c and with E t í m, that the fine grains have a diameter of between and 4 microns. 12. The wafer of claim 11, wherein: a č u j í c and with E t í m, that the coarse grains contain between 15 and 41 in water. 13. The wafer of claim 12 a č u j í c and with E t í m, that the fine grains contain between 34 and 60 v / o water. η A slice of recrystallized silicon carbide having a diameter of at least 150 mm and a thickness of not more than 2 mm and comprising between 15 v / o and 43 v / o free silicon and having a silicon carbide CVD coating thereon. The wafer of claim 14, wherein the wafer has a thickness of between 0.5 mm and 1.5 mm. »» The wafer of claim 15, wherein the wafer has a diameter of no more than 300 mm. 17. The wafer of claim 16 comprising between 25 v / o and 40 v / o free silicon. The wafer of claim 17 having a flatness of between 25 μη and 100 μχη. The wafer of claim 17, wherein the free silicon comprises coarse interconnected free silicon flakes having diameters of 5 to 50 microns. 20. The wafer of claim 19 having a thickness of between 0.5 and 1 mm. The wafer of claim 17, wherein the SiC has a bimodal size distribution comprising coarse grains of SiC and fine grains of SiC. The wafer of claim 21, wherein the coarse SiC grains have a diameter of between 10 and 150 microns. 23. The wafer of claim 22, wherein the fine SiC grains have a diameter of between 1 and 4 microns. The wafer of claim 21, wherein the coarse SiC grains form between 15 and 41 w / o of the wafer. The wafer of claim 21, wherein the fine SiC grains form between 34 and 60 w / w of the wafer. 26. A method comprising: (a) forming a silicon carbide-containing casting slurry; b) freezing the casting slurry to form a frozen casting, c) drying the casting, d) recrystallization of the casting to form billet; and e) cutting a billet to form a plurality of silicon carbide slices according to claim 1. 27. The method of claim 26, further comprising the step of: ⁴ f) saturating the silicon wafer. 28. The method of claim 26, wherein ¹ t in that further comprising the step of: £) coating the CVD silicon carbide slice. 29. The method of claim 26 wherein the casting slurry comprises a silicon carbide powder, water, and a water crystal growth inhibitor. The method of claim 26, wherein the drying step consists essentially of i) air drying the frozen cast to partially remove water and ii) drying the cast at about 200 ° C for about 24 hours. A method of using a silicon carbide wafer comprising the steps of: a) placing a silicon wafer in a diffusion bath in which the wafer of claim 14 is also located. 32. The method of claim 31, further comprising: b) processing the silicon wafer at a temperature above about 1000 degrees C. 33. A recrystallized slice of silicon carbide having a diameter _{t of} at least 150 mm and a thickness of not more than 2 mm and containing between 25 and 40 v / o free silicon, wherein the free silicon i comprises coarse, interconnected free silicon flakes having a diameter of 5 up to 50 microns. 34. The wafer of claim 33 having a diameter of between 150 and 300 mm. 35. The wafer of claim 34 having a thickness of between 0.5 and 1.5 mm. 36. The wafer of claim 35 having a thickness of between 0.5 and 1 mm. A method of using a silicon carbide wafer comprising the steps of: a) placing a silicon wafer in a diffusion tub, in which the wafer of claim 1 is also located, having a coating selected from the group consisting of polysilicon, silica and silicon nitride. t 38. The method of claim 37, further comprising: b) processing the silicon wafer at a temperature above about 600 degrees C. 39. A method of using a silicon carbide wafer comprising the steps of: a) placing a silicon wafer in a diffusion tub in which the wafer of claim 33 is also located. 40. The method of claim 39, further comprising: b) processing the silicon wafer at a temperature above about 1000 degrees C.