CA2188290A1

CA2188290A1 - Novel silicon carbide dummy wafer

Info

Publication number: CA2188290A1
Application number: CA002188290A
Authority: CA
Inventors: Craig A. Willkens; Normand P. Arsenault
Original assignee: Individual
Current assignee: Saint Gobain Ceramics and Plastics Inc
Priority date: 1995-03-01
Filing date: 1996-02-28
Publication date: 1996-09-06
Also published as: BR9605818A; CN1147806A; CZ321196A3; WO1996026910A1; AU5180496A; EP0758310A1; JP2000513689A; MX9605290A; KR970702830A

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

~ 1 882~
NOVEL SILICON CARBIDE WMMY WAFER
BACKGROllND OF THE INVENTION
The manufacture of semi ~---~-l - ~nr device6 such as diodes and transistors typically requires nYi(li7;ng the surfaces of 5 thin silicon wafers, etching cavities in the surfaces of those wafers, and depositing a dopant ti.e., boron, pho~ullo~ous, arsenic, or antimony) in those cavities, thus forming transistor contact points. The oxidation and doping operations involve rapid heat and cool cycles in an electrically heated furnace at t~ GtUL~s ranging from 1000C to 1350C. After the surface has been etched, the dopant is usually fed as a gas into the necked down end of a diffusion process tube placed in the furnace. The gas then diffuses into the etched cavities and deposits on its surfaces.
During the oxidation and diffusion steps, the silicon wafers sit on boats or plates placed within the process tube.
The wafer boat and process tube are typically made of a material which has excellent thermal 6hock resistance, high -`-nicAl ~,L~.U,Ll., an ability to retain its shape through a 20 large number of heating and cooling cycles, and which does not out-gas (i.e., i.,Llu-luce any undesirable impurities into the G; ~' -re of the kiln during firing operations). One material which meets these requirements is siliconi7ed silicon carbide.
When the silicon wafers are processed in a boat, it is 25 naturally desirable that each wafer in the boat be exposed to identical gas conc~..L~Gtion and temperature profiles in order to produce consistent product. However, the typical llyd~udyl,amic situation is such that consistent profiles are found only in the middle of the boat while inconsistent 30 profiles are often found at the ends of the boats, resulting in undesirable degrees of dopant deposition upon the end-wafers which render them unusable.
One conventional method of mitigating this "end-effect"
problem is to f ill the end slots of the boat with sacrif icial 35 silicon wafers. However, it has been found that silicon wafers are expensive, extensively out-gas, warp at high process temperatures, flake particles, and have a short useful life span .
SUBSTlTUrE SHEET (RULE 26) ~ 21 88~90 Another conventional method of mitigating the "end-effect~
problem is to fill the end 610ts of the boat with "dummy"
wafers. For example, one investigator placed SiC-coated carbon wafers having the exact dimensions of the neighboring silicon 5 wafers in the end slots. However, these wafers were found to break apart, contaminating the furnace with the exposed carbon.
Another investigator ~ osed using CVD monolithic silicon carbide as a dummy wafer. However, this material is known to be very expensive.
lC Japanese Patent Publication 5-283306 discloses a ;cnn;70~1 silicon carbide dummy wafer having an alumina/silica coating.
Therefore, it is the object of the present invention to provide an inexpensive dummy wafer which posCDccDc the tl;- ionql ~ physical and qn;cql properties required for dummy wafers .
6UMMARY OF THE INVEN~ION
In accordance with the present invention, there is provided an lln~il;r~n;7ed wafer consisting essentially of L~_ly~l ,.11; 70'1 silicon carbide, the wafer having a diameter of ~t least 150 mm and a thickness of no more than 2 mm, and having a porosity of between 15 v/o and 43 v/o.
Also in accordance with the present invention, there i8 provided a recrystallized silicon carbide wafer having a diameter of at least 150 mm and a th i rl-nP~c of no more than 2 mm, and comprising between 15 v/o and 43 v/o free silicon, and a CVD silicon carbide coating thereon.
Also in accordance with the present invention, there is provided a recrystallized silicon carbide wafer having a diameter of at least 150 mm and a thickness of no more than 2 mm, and comprising between 25 and 40 v/o free silicon, the free silicon comprising coarse interconnected free silicon pockets having 5 to 50 micron diameters.
DESCRIP~ION OF ~HE FIGURES
Figure 1 is a photomicrograph of conventional unsiliconized silicon carbide, wherein the light regions represent silicon carbide and the dark regions, eplesellL
porosity .

SUBSTITUTE S~IEET tRULE 2~i) ~-~ WO96/26910 2 ~ ~82~a PCT/US96/02880 Figure 2 is a photomicrograph of an unsiliconized --nt of the pre6ent invention, wherein the light regions represent silicon carbide and the dark regions represent porosity .
n~ TT T n DES~:Kl~LlON OF THE INVENTION
For the ~u.~oses of the present invention, "v/o" refers to a volume percent, "w/o" refers to a weight percent and a "conventionally ~Luduced" product refers to si-sic products made in accordance with US Patent No. 3,951,587 ("the Alliegro patent"). In addition, the term "flatness" is considered to be the maximum bow height from a mean datum line, the mean datum line being defined by an arbitrary diameter at the surface of the waf er .
The initial efforts of the present inventors ~YAm; nF~r~
cilir~lni7ec~ silicon carbide, specifically CRYSTAR~ (containing about 15% free silicon and about 85% bimodal sic, manufactured by the Norton Company of Worcester , MA), as the dummy waf er material. However, it was found that the conventional CRYSTAR~
casting process (a bimodal SiC blend slip-cast in a porous pla6ter mold~ could not s~lccPcsfully produce a thick billet .hl~- to slicing. In particular, when the 61ip was slip-cast thicker than about 20 mm in depth, the resulting billet would develop cracks upon drying or f iring due to residual stresses .
It is believed this process failed because water retained in the green body pores after slip-casting turned into e~LL~y~ed steam upon subsequent heating. The internal ~lL~::sr~uL~
buildup generated by the steam forced the cast body to crack and warp. The present inventors noted the conventional slip cast approach ~Loduced only about 15 v/o porosity and pore rhAnnPl c of only about 2 microns (as measured by mercury porosimetry) in the cast body and hypothesized that this level of porosity was not substantial enough to provide continuous rhAnn~.l c suitable for the escape of retained water during 3s conventional drying. They also contemplated that the density gradients pL~.duced by conventional slip casting contributed to the cracking problem, as these gradients produced thermal stresses on heating.

SUBSTITUTE SHEET ~ULE 26) W0 96/26910 - ~ 2 1 8 8 2 9 0 The present inventors al60 ~YAllli n~l open-face casting. The open face casting approach produced a thin wafer having a thickness of about 3 mm (to provide for warpage during firing) which wa6 then 6urface ground to the de6ired 0.5 - 1.0 mm 5 thickness. The fired product had a poro6ity of about 15-16 ~
v/o. ~owever, because the required grinding operation is labor inten6ive and removes over half of the wafer stock, open face - ca6ting was con6idered to be prohibitively inefficient and expensive. Further attempts to open face cast the slip closer l~ to the desired wafer thickness resuited in green wafer6 that warped during drying and f iring .
The pre6ent inventor6 then con6idered freeze-casting a bimodal silicon carbide slip and l~n~Yrer~-lly found that freeze casting provided a thick, dimensionally correct billet which 15 did not warp or crack during pror~csin~ was ea6ily sliced, and maintained sufficient ~-L~IlyLll a~ter it was sliced.
It i6 believed that the freeze casting proce6s yields a green body billet which is particularly suited to the requirement6 of large 6cale production of sic dummy wafer6.
20 when a 61ip is freeze cast, the water undergoes a 4~6 volume expansion as it becomes ice crystals. Since freeze casting is performed in a closed volume, the ice particle expansion has the effect of packing the sic particles closer together (when compared to slip cast sic packing) in the regions not taken up 2s by the ice particles. Moreover, it has been observed that the ice crystais formed in freeze casting are interconn~ct~
thereby forming pore r~ nnrll c upon drying. Therefore, although the freeze cast body pOc$F~:5~c the same overall volume percent solids as the conventional slip ca6t body (i.e., about 72 vjo), 30 the freeze cast body has both largér, interr~nn~rt~d pores and better interparticle bonding. The better interparticle bonding provides not only good strength for the cast body (despite the larger pore size) but also good strength for the sintered body, ~s the more highly packed sic grains more readily form necks 3s during recrystallization. Because the interconnected pore6 provide a channel for steam escape and the superior particle bonding provides superior strength, it appears that freeze casting avoids the problems encountered in the conventional SUBSTITUTE SHEET (RULE 2~) ~ WO 961269l0 2 1 8 8 2 q ~
slip casting process for large scale SiC dummy wafer production .
Another advantage of the present invention is that its - preferred process need not include the vacuum sublimation step s typically required during conventional silicon carbide freeze casting. Without wishing to be tied to a theory, it is believed that vacuum sublimation is not required because compaction of the SiC grains during freezing yields a relatively rigid skeletal structure resistant to vc L (and lo therefore cracking) when the water is removed. In addition, the relatively large pore rh;~nn~l c formed by the ice crystals provide reduced capillary ~cs_u,cs and reduced drying L r csses .
In one pref erred ' ' i L of the present invention, a SiC-based wafer is made and used in a process comprising:
a) mixing silicon carbide powder, water, and an ice-crystal growth inhibitor to produce a slip, b) freezing the slip at about -85C to produce a frozen casting, c) air drying the frozen casting to partially remove the water, d) drying the casting at about 200C for about 24 hours, e) vacuum presintering the body to produce a 2s recrys~ l l i 79C~ billet having a green strength of about 3 5 IqPa, f) slicing the billet into wafers, g) optionally, siliconizing and/or CVD coating the wafers, ~nd h) placing the wafers in a boat.
In the above-described ~mho~ir t, the slip typically comprises 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 ("the coarse fraction"), 3s and between about 34 and about 60 v/o fine SiC grains having a particle size ranging between 1 and 4 microns ( "the f ine fraction"). Preferably, the fine fraction comprise~ between about 36 and 42 v/o of the slip and has an average particle size of about 2-3 microns, while the coarse fraction comprises SUBSTTTUTE SHEET (RULE 26) W0 96126910 2 1 8 8 2 0 . r ~
between 33 v/o and 38 v/o of the slip and has an average particle size of about 60 microns. ~ When the coarse SiC
particle size is above about 150 microns, it approaches haif the u L-.Ss s~_-ion of the final wafer and grain pullout during slicing is observed in the f inished waf er .
Water ls generally included in the slip in an amount sufficient to produce a slip having from about 50 to 85 v/o solids. However, othQr solvents amenable to freeze ca6ting (such as glycerol, ethanol, methanol, hexane) may be suitably lo used as the slip's liquid carrier.
The slip also preferably contains an icQ-crystal growth inhibitor. Typical freeze casting techniques create ice crystals as large as 5000-10000 um on both the inside and outside of the frozen casting. Subsequent fteeze drying and firing of these bodies reveal large isolated pores (the remnants of the large ice crystals). These isolated pores act as flaws which degrade both green and final 6trength. The ice-crystal growth inhibitor pL~V~ Ll, large crystal formation by forcing the 51ip to freeze in the form of minute crystals on the order of only 5-50 microns. Typical ice crystal growth inhibitors include ~l~dLU~el~ bond-forming a_ such as glycerol and 2~11 of the ~ ~ similarly identif ied in U. 5 .
Patent No. 4,341,725 ("the Weaver patent"), the entire rreC~ ~ication of which is incu.~uL~ted by reference.
Typically, the ice crystal growth inhibitor comprises between about 0.2 w/o and about 5 w/o of the slip, preferably between about 1 w/o and about 1.5 w/o. In more preferred . ~--~i Ls, glycerol comprises about 1 w/o of the slip. The required amount of ice crystal growth inhibitor also depends on the solids content of the slurry, with high solids content slurries requiring less inhibitor. Other typical components of the slip include conventional amounts of conventional casting additives.
For example, deflocculating agents such as NaOH and Na25iO3 may be used. A binder may also be present in the range from about 0.25 w/o to 4.0 w/o solids. In preferred ~ ; -ntS, an acrylic latex binder is used at a level of about 1 w/o of the solids .
In order to insure a homogeneous sIip, the slip components are typically mixed in a ball mill evacuated to a SUE~STITUTE SHEET ~RULE 26) ~ wo 96n69l0 2 1 8 8 ~9 ~ r~
vacuum level of between about 27 and 30 inches Hg and rolled f or at least about 17 hours .
The freezing step of this Pmho~ (often callecl, "freeze casting") preferably includes pouring the slip into an S ; --hlP mold and lowering its t-, ~tUL~: until the liquid carrier freezes, thereby solidifying the slip. Freezing the slip generally entails lowering its temperature to between about -20C and -100C for between about 30-180 minutes, resulting in a freeze-cast body having only small (i.e., 5-50 lo micron) ice crystals. Preferably, the; --hlP mold is made of silicone rubber which can be easily peeled from the frozen body .
The air drying step of the pref erred pmhQ~9; r t serves to remove enough free water from the casting to allow it to be 1S placed in a heated oven without cracking. Air drying can be effectuated by simply removing the frozen body from its mold and letting it stand in air for about 24 hours. Typical conditions and drying times for air drying range between 20 and 30C, preferably 25C; between about 0.01 and several atm 20 pLesDuL?~ preferably 1 atm EJLe:SDULe; and between about 18 and about 48 hours, preferably about 24 hours.
The high temperature drying step of the above ` ;- L
is typically performed at a higher temperature and for a longer duration than the air drying step and removes essentially all 25 the absorbed water in the casting. Typical conditions and drying times for this step range from between 80C and 200C, preferably 140C; between about 0.01 and 1 atm ~ SDULC, preferably 1 atm ~L~:8~UL-'; and between about 18 and about 48 hours, preferably about 24 hours. It was l~npyrprtp~lly found 30 that the freeze cast body can be suitably dried at ~ -ric eDDuLe under these conditions without cracking. As noted above, conventionally processed, slip cast sic bodies were f ound to crack under high temperature, atmospheric drying conditions. Because the freeze drying process does not require 3s subsequent vacuum drying, it is significantly less expensive than conventional sic processing.
The dried casting produced in accordance with this : ` :';- t exhibits a bulk density of at least about 1.8 g/cc and a four point bending strength of at least about 5 MPa. Its SUBSTITVTE SHEET (RULE 26) wo 96n69l0 ~ PCrlU596/02880 , pore ~ize ranges from aoout 5 to 50 micronF . Its average pore size is about 15 microns. In contr2st, the conventional dried SiC casting has an average pore size of only about 2 microns.
The vacuum presintering step of the preferred Pn~ho~
s serves to establish recrystallization ( i . e., neck growth between the SiC grains without densif ication) without cracking .
It may be carried out at about 1900-1950C under a vacuum of about 0 . 5 torr in an Ar atmosphere . Whereas conventional SiC
castings were found to crack under these conditions, it is lo believed the freeze cast bodies of the present invention did not crack because the relatively large pore e hAnn~l c formed by the ice crystals result in low capillary pressures and low "LL-~aes on drying, as well as a uniform density across the part which resists thermal stresses.
The recrystallized billet produced in accordance with this L exhibits a bulk density of at least about 1. 8 g/cc.
Its porosity ranges from 25 v/o to 43 v/o. Its pore size ranges from about 5 to 50 microns. Its average pore size is about 15 microns. In contrast, the conventional recrystallized 20 SiC casting has a porosity of about 16 v/o and an average pore ~ize of about 2 microns. Its strength (as measured by ring on ring biaxial flexure) is at least 30 NPa, typically between 30 and 5 0 MPa .
After presintering, the recrystAlli7~d billet is sliced by 2s conventional processes ( i . e ., a diamond wheel or wire) to its f inal tl i ~ n In contrast to less porous conventional SiC
billets, the recrystAlli7~l SiC billet of this embodiment is easily sliced into thin SiC wafers. The structure of the presintered billet is such that it has sufficient handling 30 strength, but is quickly and easily sliced to a good surface finish and flatness. For example, a 1 mm thick wafer produced in accordance with the present invention may be sliced from a 15 cm diameter billet in only about 5 minutes. In contrast, it is believed that a higher density slip cast SiC billet would 35 require about 60 minutes and a fully dense sic bil1et would require about 120 minutes to slice. Recrystallized silicon carbide dummy wafers having diameters of between about 150 and about 300 mm, thi~knt~cc~c between about 0.5 and about 2 mm, preferably between 0.5 mm and 1.5 mm, more preferably between SUBSTITUTE SHEE~ tRULE 26) - .

wo s6n6sl0 r~

2 1 882~0 about 0.5 and 1.0 mm; and fl~ .ess~s of between about 25 and about 100 microns, preferably les6 than about 50 microns, are obtainable in accordance with this ~mhorl;~ n t, usually after mere diamond saw slicing. If the wafer is subsequently S siliconized, it may need to be rotary ground for a short period to remove a few microns and attain a flatness of less than 100 um.
The final firing step makes the wafer illl~r -hl~ to gases or liquids. It typically involves either impregnating the porous wafer with silicon to eliminate porosity and/or CVD
coating it with an; ~~hle ceramic such as silicon carbide.
If s;licon;7ing is select~d, it may be carried out in accordance with US Patent No. 3,951,587 ("the Alliegro patent" ), the specif ication of which is incorporated by reference. It was ~n~Yr~ct~ly observed that the siliconized wafers had a flatness of about 100 um. In contrast, dimensionally similar conventional "green" sic castings have been found to excessively warp, necessitating a thicker casting and expensive final r--h;nin7 in order to produce the same flat product.
The ~ilic~ln;~ecl wafer of a preferred: '_'tr ~ oE the present invention exhibits a bulk density of at least about 2 . 75 g/cc. Its pockets of free silicon range from about 5 to 50 microns in .1;, t.l:L. It is fully dense. In contrast, a 2s conventionally ~L~,.Iu. ed siliconized SiC wafer has pockets of free silicon that are only about 2 microns in diameter.
The microstructure of this ~mho~ of the present invention appears to have three distinct phases of the material, comprising: a coarse grain SiC phase, a coarse free silicon phase; and a mixed phase comprising fine SiC grains and fine free silicon pockets. D~r~n~lin~ upon whether the SiC
dummy wafer is siliconized, the sic wafer typically comprises:
a) between about 15 v/o and 41 v/o (preferably 33 to 38 v/o) silicon carbide grains having a diameter of between 10 um and 150 um, b) between about 34 v/o and 60 v/o ~preferably 36 to 42 v/o) silicon carbide grains having a diameter of between l um and 4 um, and c) between 25 v/o and 40 v/o free silicon or porosity.

SUBSTITUTE SHEE~ (RULE 26) W0 96126910 , ~ 2 ~ 8 8 2 9 0 PCTIUS96102880 The porosity o~ the l~nci 1 l rnn; 7e~ wafer is characterized by a bimodal size distribution of coarse (5-50 um) pore5 and fine pores, while the free silicon of the slliconized wafer is characterized by free silicon pockets having 5-50 micron S diameters and a free silicon matrix which ~uLLuul-ds fine SiC
grains. See Figure 2. In some `~_~i r ~s, there is preferably between 35 v/o and 40 v/o free silicon. In comparison, prior ~rt microstructures were found to be characterized by a uniform structure of a mixed phase compri6ing large grain SiC, small lo grain sic and small free silicon pockets or porosity. See Figure 1.
Sandblasting of the siliconized sic wafer can remove excess free silicon that has exuded to the surface due to the volu~e expanslon of silicon on solidification, and r~ay be done 15 by conventional sandblasting processe~: . Because these waf ers possess high strength, they do not break when subjected to ~;~n~hl ~cting.
Although the above-d~crr;hed ~ ;r- ~ of the~ present invention exploits freeze casting to produce a thin, strong sic 20 wafer, it is also anticipated that useful sic wafers can be obtained by a number of alternative ~LuueSses~ including: ~) warm pressing a sic billet at 1750C and 3000 psi; b) gel casting and presintering a SiC billet in ~crnr~s~nre with U. 5 .
Patent No. 5,145,908; c) cold isostatic pressing a sic billet, 25 and d) tape casting or roll pressing and then recrystallizing a SiC slip to produce a fired sic wafer having a porosity of about 21%.
The novel recrystallized silicon carbide ceramics of the present invention may be used in conventional 5~1 jrnn;7ed 30 silicon carbide or CVD coated silicon carbide applications, ;nr~ ;ng those applications disclosed in the Alliegro patent.
Tt may also find application as a rigid disc in computer hard drives, as a substrate for other electronic applications, or as baffle plates in wafer boats. In particular, therè is 35 contemplated a silicon carbide disk substrate for use in a disk drive assembly having a head and a disk, the disk comprising the disk substrate, wherein the disk substrate comprises a) between 15 v/o and 43 v/o free silicon or porosity, preferably between 25 v/o and 40 v/o; b) preferably having a flatness of 1~ ~
SUBSTITUTE SHEET ~RULE 261 WO 96126910 2 18 8 2 9 0 r~~
between i5 um and l00 um; c) preferably having a bimodal SiC
grain distribution of coarse and fine grains; and d) preferably having a bimodal free silicon or pore distribution of coarse and f ine pores . Other contemplated uses of the highly porous s silicon carbide discs of the present invention (which could exploit the low pressure drop across the disc) include gas burner plates, composite substrates and filters.
In some ~hoAi- ~s, the porous wafer of the present invention is optionally coated with a layer of either polysilicon, silicon nitride or silicon dioxide, placed in a diffusion boat in which silicon wafers are also s1~hcr-qur-ntly, and the silicon wafers are processed at a tr, ~LUL~ of at least about 600 degrees C.
In some: ` 'i Ls, the cil icrni7r-(~, Sic CVD coated wafer lS of the present invention is placed in a diffusion boat in which silicon wafers are subsequently placed, and the wafers are pL~ce4~ed at t~ lltUL ~s above l000 degrees C. It is believed the CVD SiC coating is necessary at those temperatures to prevent oxidation of the SiC grains. Therefore, there is also provided a process comprising:
a) placing a silicon wafer in a diffusion boat having the siliconized, SiC CVD coated wafer of the present invention placed therein, and b) processing the silicon wafer at a t~ ~tUL~: above 2s about l000 degrees C.
Also in accordance with the present invention, there is provided a method of single wafer processing, comprising the steps of:
a) providing a 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 (preferably having a diameter of at least 200 mm and more preferably at least 300 mm) upon 3s the silicon carbide disc, and c) heating the silicon wafer at a rate of at least l00 C per second .
Also in ac-_vldc,nce with the present invention, there is provided a method of cleaning single wafer processing chambers, SUBSTITUTE S~IEET(RULE 26) wo s6n6sl0 r~
- ` 21 882~0 ` ~
comprising the steps of:
a) providing a susceptor in a processing chamber, b) placing a silicon wafer upon the susceptor, c) proc¢ssing the silicon wafer, S d) removing the silicon wafer, e) placing a silicon carbide wafer of the present invention (preferably having a diameter of at least 200 mm and more preferably at least 300 mm) over the susceptor, and f ) cleaning the processing chamber.
Also in a.ouLd~ e with the present invention, there is provided a method of flat panel display processing, comprising the steps of:
a) providlng a silicon carbide wafers method of the present invention (preferably having a length of at least 165 mm and lS a width of at least 265 mm) in a substantially horizontal positlon, and b) placing a flat glass plate (preferably having and length and width of at least 100 mm) upon the silicon carbide disc, and c) processing the f lat glass plate.
A160 in a~;curdal~.c with the present invention, there i6 provided a method of plasma etching silicon wafers, comprising the steps of:
a) providing a silicon wafer having a pr~ t~rm;n~rl diameter of at least 200 mm, b) placing a silicon carbide ring of the present invention (having an inner diameter essentially equal to the prPrl~t~m;nPd diameter of the silicon wafer) around the silicon wafer, and b) plasma etching ~preferably dry metal plasma etching) the ~ilicon wafer.
EXAMPLE I
~ freeze cast slurry was prepared by mixing the following materials in the quantities shown in Table I, and rolling in a 3s ~ar for 18 hours.
- TAr3LE I -Silicon Carbide (3 micrûn) 4680g Silicon Carbide (lOOF) 4320g Water 1080g SUBSTITUTE Sl iEET ~RULE 26) ~ WO 96/26910 2 1 8 8 2~ 0 PC~IIUS96/02880 BASF Acranol 290D Binder 137g NaOH (lN) 81g Baker Glycerol 90g The slurry was vacuum deaired and poured into a polyvinyl , chloride tube having an inner diameter of 6", an outer diameter of 6.5" and a height of 10". The tube was clamped to a glass 5 plate to prevent leakage and form the bottom surface. The assembly was then placed in a freezer at -85C for 3 hours.
After being fully frozen, the tube was cut away from the billet. The freeze cast billet was initially air dried at about 25 C for 18 hours and final dried at 140C for 48 hours 10 to remove the absorbed water. The billet was then sintered at about 1900C in an argon atmosphere to effect ~ Ly~-L~llization. The porous LC~LY`l~I11 i 7~d billet was dry sliced with a metal bonded diamond saw to a thickness of 0.040". The wafer was infiltrated with molten silicon at about 15 1800C in an argon/nitrogen a, , ^re and then sandblasted with sic grain to remove any excess silicon. The s~n~lhlAqted wafer had a flatness of about 100 microns. Rotary grinding with a diamond abrasive provided a flatness of about 50 microns. It is contemplated that final lapping with a boron0 carbide slurry could produce flatness of 20 microns.
EXAMPL~ II
A l-n;- 1 silicon carbide slip having an average size of 3 microns was hot pressed in a graphite die at about 1850C and 3000 psi for 1 hour. The billet had a 3" diameter, a 4"
2s height, and a density of about 2 . 0 g/cc (about 62% of theoretical density). The billet was dry sliced with a metal bonded diamond wheel to a thickness of 0.75 mm. The wafer was infiltrated with molten silicon at about 1800C in an argon/nitrogen ai ~ . The siliconized wafer was then 30 sandblasted with SiC grain to remove any excess silicon. The sil;ron;~ed wafer had a flatness of about 70 microns. Some of the ~nahl~cted wafers were coated with about 50 microns of SiC
by rh~-~;r~l vapor deposition of methyltrichlorosilane in ll~dLoy~ and argon at about 1100C.

SUBSTlllJTE SHEEl- ~RULE 263

Claims

We claim:

1. An unsiliconized 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 1 mm, and having a porosity of between 15 v/o and 43 v/o.

2. The wafer of claim 1 having a porosity characterized by an average pore size of 2 um.

3. The wafer of claim 1 wherein the wafer has a diameter of no more than 300 mm.

4. The wafer of claim 1 having a porosity of between 25 v/o and 40 v/o.

5. The wafer of claim 3 wherein the silicon carbide consists of silicon carbide grains.

6. The wafer of claim 4 wherein the porosity is characterized by coarse interconnected pores having a diameter of between 5 and 50 microns um.

7. The wafer of claim 1 having a flatness of between 25 um and 100 um.

8. The wafer of claim 6 wherein the coarse pores have a diameter of about 15 microns.

9. The wafer of claim 4 wherein the SiC has a bimodal size distribution comprising coarse grains and fine grains.

10. The wafer of claim 9 wherein the coarse grains have a diameter of between 10 and 150 microns.

11. The wafer of claim 10 wherein the fine grains have a diameter of between 1 and 4 microns.

12. The wafer of claim 12 wherein the coarse grains comprise between 15 and 41 v/o of the wafer.

13. The wafer of claim 12 wherein the fine grains comprise between 34 and 60 v/o of the wafer.

14. The wafer of claim 1 having a porosity of between 15 v/o and 16 v/o.

15. The wafer of claim 14 having a porosity characterized by an average pore size of about 2 microns.

16. The wafer of claim 1 wherein the silicon carbide is recrystallized at between 1900 C and 1950 C.

17. The wafer of claim 40 comprising between 15 v/o and 41 v/o silicon carbide grains having a diameter of between 10 um and 150 um, and between 34 v/o and 60 v/o silicon carbide grains having a diameter of between 1 um and 4 um.

18. A recrystallized silicon carbide wafer having a diameter of at least 150 mm and a thickness of no more than 2 mm, and comprising between 15 v/o and 43 v/o free silicon, a CVD
silicon carbide coating thereon, and a flatness of between 25 um and 100 um.

19. The wafer of claim 18 wherein the free silicon comprises coarse interconnected free silicon pockets having 5 to 50 micron diameters.

20. The wafer of claim 19 having a thickness of between 0.5 and 1 mm.

21. The wafer of claim 18 wherein the SiC has a bimodal size distribution comprising coarse SiC grains and fine SiC
grains.

22. 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.

24. The wafer of claim 21 wherein the coarse SiC grains comprise between 15 and 41 v/o of the wafer.

25. The wafer of claim 21 wherein the fine SiC grains comprise between 34 and 60 v/o of the wafer.

26. A process comprising:
a) forming a slip comprising silicon carbide, b) freezing the slip to produce a frozen casting, c) drying the casting, d) recrystallizing the casting to produce a billet, and e) slicing the billet to produce a plurality of silicon carbide wafers of claim 1.

27. The process of claim 26 further comprising the step of:
f) siliconizing the wafer.

28. The process of claim 26 further comprising the step of:
f) coating the wafer with CVD silicon carbide.

29. The process of claim 26 wherein the slip comprises silicon carbide powder, water, and an ice-crystal growth inhibitor.

30. The process of claim 26 wherein the drying step consists essentially of i) air drying the frozen casting to partially remove the water and ii) drying the casting at about 200°C for about 24 hours.

31. A method of using a silicon carbide wafer comprising the steps of:
a) placing a silicon wafer in a diffusion boat in which the wafer of claim 14 is also placed.

32. The process of claim 31 further comprising:
b) processing the silicon wafer at a temperature above about 1000 degrees C.

33. A recrystallized silicon carbide wafer having a diameter of at least 150 mm and a thickness of no more than 2 mm, and comprising between 25 and 40 v/o free silicon, the free silicon comprising coarse interconnected free silicon pockets having 5 to 50 micron diameters.

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.

37. A method of using a silicon carbide wafer comprising the steps of:
a) placing a silicon wafer in a diffusion boat in which the wafer of claim 1 having a coating selected from the group consisting of polysilicon, silicon dioxide and silicon nitride is also placed therein.

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 boat in which the wafer of claim 33 is also placed.

40. The method of claim 39 further comprising:
b) processing the silicon wafer at a temperature above about 1000 degrees C.