CA2246803A1 - Tape cast silicon carbide dummy wafer - Google Patents
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Abstract
This invention relates to tape casting a silicon carbide slip to eventually produce a silicon carbide wafer having a thickness of between 0.5 and 1 mm and a diameter of at least 100 mm, the wafer preferably having a strength of at least 30 MPa, and a porosity wherein at least 85 % of the pores are no larger than 12 microns.
Description
W O 97/36843 PCT/US97/0~188 TAPE ~AST SILICON CARBIDE DUMMY WAFER
BAC:KGROUND OF THE INVENTION
The manufacture of semi-conducto- devices such as dlodes and ransistors typically requlres the deposit~on of dielectric materials such as polycrystalline sillcon, silLcon r.itride and silicon dioxide on the su.rfaces of thin silicorl wafers. The ~o thi:n layer deposition of these materia s involves rapid heating and cooling cycles in an electrically heated furnace (or "diffusion process tube") at temperatures typically ranging fro:m 250 to 1000 C. When. dielectric precursor gases are fed into a diffusion process tube heated to these temperatures, the gases react and deposit the dielect:ric reaction product on the surface of the silicon wafer.
During the deposition step, the s-licor, wafers are supported in vertical or horizontal. borlts placed within the process tube. The wafer boat and process tube are typically made of a material which has excellent thermal shock resistance, high mechanical strength, an ability to retain its shape through a large number of heatin~ and cooling cycles, and which does not out-gas (i.e., introduce any undeslrable im.purities into the atmosphere of t:he }iln durlng firing operations). One materia.l which meets these requirements is silicon carbide. For the above-mentiolled appllcation, silicon ca.rbide diffusion components such as boats, paddles and process tubes are ty~pically pre-coated with the dielectric se ected for deposition.
When the silicon wafers are pr.oce,sed in ~ boat, lt is na.turally desirable that each wafer n the b3a. be exposed to id.entical gas concentrati.on and temper~ture profiles in order tc produce consistent product. :Howeve-, the t~pical fiuid dynamic situatior is such that conC;istent prof.les are found on.ly in the middlc of the boat whi e _nconsistent prof-les are ofte~ found at thc ends of the boats, -esultir.~ in undesirable degrees of dielectric deposition Lpon -he end-wafers which render them unusa~le.
W 097/36843 PCTrUS97/0~188 One conventl~nal method of mltlgating thls "end-effect"
prGblem is to fil: the end slots of the ~oat with sacrificial ("dummy") wafers made of silicon. However, i- has been found that silicon wafers are expensive, extensivel~l7 out-gas, warp at high process temperatures, flake p~rticles, ar.d have a short useful life span.
Another conventional method of mitigatin~ the "end-effect"
problem is to fil~ the end slots of the boat with dummy wafers maàe of alternative materials. For example, one investigator placed SiC-coated carbon wafers having the exact dimensions of the neighboring s licon wafers in the end slots. However, these wafers were found to break apart, contaminating the furnace with carbon particles. Another investigator proposed using CVD monolithic silicon carbide as a dummy wafer.
However, this material is known to be very expensive. One prior proposal fo~~ producing silicon carbide wafers is a free~e casting approach which produces a green silicon carbide billet having a thicknes, of at least about 25 mm which is recrystallized and then sliced to a commercially useful thickness. However, it has been found~ that the freeze casting process produces significant porosity in the wafer (on the order of 40 v~o, with 15 percent of the pores larger than 25 um). These large pores make it difficult to completely precoat the wafer with the dielectric and make the deposition process very expensive. JP Patent Publication No. 5-283306 ("the Toshiba referencel') discloses forming a commercial~y useful wafer by grinding down a 2 mm thick siliconized silicon carblde disc to a thickness of about 0.625 mm, and then CVD coating the disc with an alum~na-silica coating. However, the grinding, siliconization and CVD steps are expensive, particularly so in the low temperature ~less than 1000 C) applications where silicon infiltration is not required to prevent the oxidation of the silicon carbide.
Therefore, i~ is the object cf the present invention to provide an inexpensive sllicon carbide dummy wafer which possesses the dim~nsional, physical and mechanical properties W 097/36843 PCT~US97/05188 recIuired for use :n appl:Lcations with -empera-::ures less than abc)~t lO00 C.
SI~RY OF THE IN~'7ENTION
In accordance with a preferred embodimen: of the present invention, there :s provided a wa~er consisting essentially of s:ilicon carbide hav~ng a thickness of between 0.5 and l mm, a d:iameter of at let~st lO0 mm, and a strength (as measured by r:ing on ring biax al flexure) of at least 31) MPa (typically bet:ween 50 and 70 MPa), the wafer having a porosity wherein at least 85~ of the pores are no larger than 14 microns.
Preferably, the wafer has a density of at least about 2.15 g,/c:c, more preferably between 2.3 and 2.4 g~cc; its porosity is bet:ween about 14~ and 16~, with at least 85~ cf -ts pores being no larger than 12 microns, and at least 95~o of its pores being larger than 3 mlc~~ons; the average pore size lS between 6 and 11~ um, typically 53 um; the silicon carbide is ln recrystallized form and consists of between 40 and 6C w/o grains having a size o:f between 2 and ; um and between 40 and 60 w/o grains having a s:ize of between about 30 and 200 micrcns; and, the surface of the wafer is unground.
Also in acco-dance with the present nvention, there is p:rovided a preferred process for making a t~in, crack-free g:reen silicon carbide sheet, comprising the steps of:
a) forming a slip comprising a liquid carrier ~preferably, 2s water) and a ceramic powder consisting essentially of silicon carb:ide, b) tape casting the slip to produce a wet green sheet having a thickness of between 0.~ and l.' mm, c) evaporating essentially all the liquid carrier from the wet green sheet to produce a dry green sheet having a thickness which is ~t least 80% of the wet green sheet thickness, d) forming a shape from the ~iecl green sheet to produce a silicon carbide green wafer, e) recrystallizing the sillccn carbide g-een wafer to produce a re~rystallized silicon carbide wafer having a thickness of ().5 to l.0 mm and flatness cf less than 13Q
um and, optionally f) grinding the silicon carbide wafer t~ reduce its thickness by no more than 5~.
In preferred embodiments of this process, the ,illcon carbide powder consists of about 40 to 45 w/o grains having a size of between 2 and 5 um and about 38 to 42 w~o grains having a size of between about 30 and 100 microns (wherein t:he w/o fractions are based upon total slip weight); the water content of the 0 slip is between about 12 to 15 w~o of 1he slip; the total sol.ids content ~silicon carbide plus the solid portion of the binder) of the slip is between about 8'~ and 9C w/o of the slip;
the density of the dry green sheet is ~t least about 2.3 g/cc;
and the slip further comprises between 2 and ~ wJo of a binder having a glass transition temperature of less than 22 C.
Also in accordance with the present inven~ion, there is provided a method of using a silicon ct~rbide dummy wafer, comprising the steps of:
a) providing a silicon wafer diffusion boat having slots for the insertion of wafers, b) inserting into at least one slot of the silicon wafer diffusion boat a silicon carbide wafer of the present invention, said silicon carbide w~fer hav:ing a coating of a dielectric material thereon, 2s c) inserting into at least one ot.ner slot a silicon wafer, and d) depositinc a dielectric material on the surface of the silicon wafer at a temperature of no more than 1000 C.
DETAILED DESCRIPTION OF THE INVENTION:
It has been f:~ound that tape casting an aqueous-based bimodal silicon carbide slip, cutting out a green wafer from the tape, and recrystallizing the green wafer provides an inexpensive, low porosity, silicon carbide wafer suitable for use as a dummy wafer.
The level of porosity produced by tape casting bimodal sili-on carbide s'ips in accordance with the preferred process W 0 97/36843 PCT~US97/05188 oi- the present ln~rention is typicaily on the (,rder of about 14-16 v/o of the recrystallized wafer, with a po~e size dicitribution whereiri at :Least 95% ~f the pores are larger thar.
3 microns, at lea;t 85~ of the pores are no larger than 12 mic:rons (preferab y 10 microns), and the average pore slze is bet:ween 4 and 8 um, typically about 7 um. Because the pores are both infrequent and relatively small, these wafers can be more evenly and less expensively precoated with the dielectric thcln can silicon (arbide wafers from the free~e casting o process. Moreover, because the wafers do not require silicon impregnation ("si iconization"), they are less expensive ~han s:Lliconized silicon carbide wafers.
In addition, because the near net shape casting described abc)ve either reduces or eliminates the need for slicing and grinding, it is believed tape cast dummy wafers produced by the pxesent invention can be produced at â signiflcantly reduced miarlufacturing cos~, regardless of whether the wafer is unsiliconized (fo~ low temperature applications) or siliconized ~Eor high tempera~ure applications). So if an inexpensive si].iconized wafer is desired, the relative absence of very fine pol~es (i.e., pore, less than 3 um) allows for easy siliconization.
Another noveL feature of the present invention is its ability to reliably provide green silicon carbide wafers of the noted thickness which do not crack during dryLng. Typical prior art tape casting of silicon carbide was limited to using non-aqueous based slips to produce much thinner ~i.e., 0.025 to O.L25 mm) green sheets. Because a dummy wafer should be the same thickness as a normal silicon wafer ~i e , about 0.625 mm to ().725 mm), the thinner wafers produced from non-aqueous ba,ed slips would not be suitable as dummy wa~ers. Attempts at ca;ting larger thickness sheets from these norl-aqueous based slips typically resulted in excessive cracklng. Without wishing to be tiei to a theory, it is belleve-~ the excessive ev(~poration rate ~f the organic solverlts in these conventional slips produced a pronounced vertical dispar:t~ rr. the drying W 097136843 PCTrUS97/OS188 rate of the relatlvely thick green sheet, lea~ding to the formation of a dr~ skin on the top surface whose higher packing promoted cracking beneath the skin. In contrast, aqueous-based slips dry at a much slower rate. Since drylnt~ rate is slower, the evaporation profile is more uniform throughout the thickness of the ,heet and formation cf the undesirable dry sk~n is minimized. In addition, one preferred process of the present invention further enhances uniform vertical drying by heating the cast ,heet from below the casting table, thereby lo promoting moisture removal from the bottom of the green sheet.
Moreover, it is believed that tape casting allows for reliable producti~n of larger diameter silicon carbide dummy wafers than was previousiy known. In this regard, it is noted the Toshiba reference discloses a relatively small (150 mm x 150 mm x 2 mm) siliconized tile. lt is also noted that one freeze casting approach which entails freeze casting a 200 mm diameter, 150 mm thick silicon carbide billet and then slicing the billet suffered from excessive cracking in the interior of a large number of the billets. ln contrast, the process of the present invention allows for reliably producing suitable silicon carbide dummy wafers on the order of 200 mm and 300 mm diameters via tape casting.
Preferably, the grain size distribution of the silicon 2s carbide grains used in the present invention LS bimodal. It has been found that using a bimodal distribution ~roduces much less shrinkage in the thin tape cast green sheet (on the order of only about 10~ to 15~) than does a fine unimoda~ distribution (shrinkage on the order of about 85~ to 90%). Since much less shr nkage is involved, the thickness of the tape is more easily controlled. Since the fine unimodal powders were the focus of the tape casting art, this advantage was not suggested in the prior art. Preferably, the bimodal SLC grain distribution comprises between about 38 and 42 w/o coarse SiC grains having 3s a particle size ranging from 10 to 150 microns ("the coarse fraction"), and ~etween about 40 t:o 4~ w~o fine SiC grains WO97/36843 PCT~S97/05188 havlng a particle size ranglng between 2 an(1 ~ mlcrons '"the fine fraction"). More preferably, the fine f~action comprises about 43 wJo of the SiC grain and has an average partil-le size oI about 2-3 microns, while the coarse fractlcn comprises about 40 w/o of the SiC grain and has an average particle size of about 60 microns. ln some embodiments, the f-ne fraction is E277, a silicon carbide powder available from Saint-Gobain/Norton Industrial Ceramics Corporation ("5G~NICC") of ~orcester, MA, ancl the coarse fraction is F,4C;, another silicon carbide powder available from SG/NICC.
In preferred embodiments of the slip, water is present in an amount sufficient to produce a slip having from about 80 to 9() w/o solids. The slip may also include conventional additives such as deflocculents, binders and plasticizers. In one preferred embodiment, the slip includes 13 w/o water, 0.Ol w/'o deflocculant such as sodium hydroxide, and between about 1%
and about 10% (preferably between 1% and 5%) of a binder having a glass transition temperature of less than 22 C. The amount of binder used in the present invention is typically less than the amount used in conventional sllicon carbide tape casting, which is usually at least 10% of the slip. The use of a binder wi.th a glass transition temperature below 22 C eliminated the need for a plastio-izer in this formulation. It is believed the lohrer binder fraction yields a more uniform pore size distribution. Typically, the silicon carbide, water and def'locculant components are mixed in a ball mlll evacuated to a VclC'UUm level of between about 27 and 3~ inches Hg, the milling media is removed and the binder is added, and the entire mixture is rolled The tape cast:ing step of the present invention is preferably accomp:ished by draining a reservo~r containing the silicon carbide s ip thrvugh a horizontally-d-sposed slit in the reservoir, the upper surface of sllt belng defined by a doctor blade and 'he lower surface of he s'it being defined by an endless belt which moves under the doctor blade. Typically, the doctor blade s positioned between abou (.4 and l.~~ mm W 097/36843 PCTrUS97/05188 above the endless belt. In some embodiments, ~1 heater is placed underneath the en~iless belt in order to provicie a more uniform drying profile for the cast sheet. The initia:! thickness of the wet cast sheet thus produced is generally between about 0.4 and 1.3 mm. The cast sheet is then dried for 20-30 minutes on the heated belt, after which time the thickness of the cast sheet typically shrinks between about 10% and 15% to provide a green sheet having a density of between about 2.3 g/cc and 2.4 g/cc.
Next, the dr:ed sheets are cut using c.rcular punches to o form green silicon carbide wafers having a diameter of between 100 and 300 mm an(~ a thickness of between about 0.5 and 1.1 mm, preferably between 0.5 and 1 mm, more preferably between about 0.625 and 0.725 mm. When the wafers are in the range of about 0.625 mm and 0.72~ mm, they do not need to be surface ground.
In some embodiments, the scrap material from the dried cut sheet is recycled. Preferably, the scrap is completely dispersed by milling overnight in 25 w/o deionized water; the resulting slip is then mixed with virsin silicon carbide powders and additives; remilled; and water is added to the slip to bring its viscosity to about 30,000 cps at 0.6 rpm. This recycled slip can then be used to make more green sheets, thereby essentially eliminating waste of the silicon carbide raw material.
In some emboiiments, the cast wafers are heat treated prior to recrystallization in order to partiaily remove the binder. It has been found that partial removal of the binder ~i.e., removal of about 15% to 25%) yields a fine pore structure in the sintered body which ~s advantageous for pre-coating. It is believed the partial bllrnout produces small diameter vapor "escape routes" for the remaining binder, thus pre~enting the creation of larger sized pores during the recrystallization process. In prefer-ed embo~iments, this entails exposing the cast wafers to a r at a temperature of 220~C and a pressure of 5 inches Hg fcr 8 hou~s.
Next, the drled green wafers are recrystallized.
Rec~ystallization establishes strength-enhancing neck arowth WO97/36843 PCT~S97/05188 between the SlC grains wi-hout substantial den.~ification. It is generally carried out at about 1900-1950~C under a vacuum of about 0.6 torr n an Ar atmosphere. Ir preferLed embodiments, the wafers are rec:-ystall zed at 1'350~C and ~ torr in argon.
In some embodimen s, the green wafers are s a~ked between surface ground silLcon carbide plales in ~rder to provide flat:ness.
A recrystalli~ed wafer produced lr. ac-o-dance with he bimodal embodiment of the present invention ty-ically exhibits o a bu1k density (as measured by mercury intrusicn porosimetry) of at least about '.15 g/cc, and preferably between 2.3 an~ 2.
g/cc, a total poro,ity of about 14-16~, an average pore size of between 4 and 8 um, with a pore size distribution wherein at least 95~ of the pores are larger than 3 microns and at least 85~ of the pores a e no larger than 12 microns (preferably 10 micl-ons). In preferred embodiments using a bimodal grain dist:ribution, the ~ine fraction grains average about 2 to 5 microns and the co~rse fraction grains average about 30 to 100 microns. It has a measure of flatness of no more than about 13~ um across a di~meter of about 200 ~L. Its strength (as measured by ring on ring biaxial flexuIe) is typically between about 50 MPa and a~out 70 MPa.
Recrystallizei silicon carbide du~Lmy wafers having diameters of at le~st 100 um, preferably between about 150 and about 200 mm; thic~nesses of between about 0.5 and about 1 mm, and preferably abo~t between about 0.6~5 mm and 0.725 mm; and flal-nesses of between about 50 and abo~t 130 mlcrons, preferably less th~n about 100 microns, are obtainable in accordance with this embodiment.
If desired, a~ additional firing step may be undertaken to make the wafer resistant to gas or iiquid atlack in hich te~mperature applications. This typically involves either impregnating the recrystallized wafer with sil~con to eliminate porosity and/or CVD coating it w th an impermeable ceramic such as s_licon carbide. If siliconi~ing l~ selected, it may be carr:~ed out in acc~rdance with US PateIlt No. 3! 9~1~ 587 ("the W O 97/36843 PCT~US97/0~188 Alliegro patent") the specification of which is incorporated herein by referen~e. Therefore, in accordance with the present invention, there is prov1ded a wafer consistirlg essentially of silicon carbide h~ving a thickness of between 0.5 and l mm, a diameter of at le~lst 100 mm, the wafer being infiltrated with silicon so that t~e silicon is present as sil'con pockets and comprises about 1~ to 16 v/o of the wafer, and wherein at least 85~ of the silico~ pockets are no larcrer than 10 microns (preferably 8 microns) and at least 95~ of the silicon pockets o are larger than 3 microns. There is also provided a method of uslng this silico~ized silicon carbide wafer, comprising the steps of:
a) providing a silicon wafer diffusion boat having slots for the insertion of wafers, b) inserting into the slot of the si~icon wafer diffusion boat a silic~n carbide dummy wafer as described in this paragraph, said dummy wafer having a coating of CVD
silicon carbide thereon, c) inserting a silicon wafer into another slot in the boat, and d) oxidizing the surface of the silicon wafer at a temperature of at least 1000 C.
If CVD coating with silicon carb:de is selected, it may be carried out by any conventional CVD S.C method. Likewise, the silicon carbide wafer of the present nvention may be coated with a dielectric material such as po:Lycrystalline silicon, sillcon nitride, or silicon dioxide.
Conventional sandblasting of the siliconized SiC wafer can remove excess free silicon that has exuded to the surface due to the volume expansion of silicon on solidification. Because these wafers possess high strength, they do not break when sub~ected to sanàblasting.
The novel recrystallized silicon carbide wafers of the present invention are preferably used as dummy wafers in slllcon wafer manufacturing. However, they ma~ also find application as rigid discs in computec hard drives; as 1 o W 097/36843 PCTrUS97/05188 substrates for oth~r micro-electronlc appl_cations lncluding acting as setters n single wafer processing and plasma etching; as substr~tes for flat panel ICD dispLays; or as bafE'e plates ln w~fer boats.
Also in accor~ance w th the presert inven~ion, there is prov-ded a preferr~d method of singie wafer processing, co~?rising the ste~s of:
a) providing i silicon carbide disk of the present invention (pr3ferably having a diameter of at least 200 mm 0 and more preferably at least 300 rnm) in a substantially horizontal position, and b) placing a silicon wafer ~preferably having a diameter of at least 200 mm and more prefexably at least 300 mm~
upon the silicon carbide disc, and c) heating the silicon wafer at a rate of at least 100 C
per second.
Also in accordance with the present invention, there is provided a method of cleaning single wafer processing chambers, comprising the steps of:
a) providing a susceptor in a processing chamber, b) placing a silicon wafer upon the susceptor, c) processing the silicon wafer, d) removing the silicon wafer, e) placing a silicon carbide ciisk of the present invention (preferably ~aving a diameter of at least 200 mm and more preferably at least 300 mm) over the susceptor, and f) in-situ c~eaning the processin~ char~er by exposure to free radicals.
Also in accordance with the present invention, there is prc,vided a method of flat panel display processing, compr sing the steps of:
a) providing a silicon carbicle plate of .he present invention (preferably having ~ length c,f at least 165 mm and a width of at least 265 mm) in a substantially horizontal position, and W097/36843 PCT~S97/051~
b) placing a flat glass plate (preferabl~ havlng a length and a width of at least l00 mm) upon the silicon carbide plate, and c) processing the flat glass plat.e by ox dation, dielectric deposition and/or diffusion at: a temperature of no more than 800 C.
Also in accordance with the present invention, there is provided a method of plasma etching si.licon wafers, comprising the steps of:
a) providing a silicon wafer havi.ng a predetermined diameter of ~t least 200 mm, b) placing a silicon carbide ring of the present invention ~having an inner diameter essenti.ally equal to the predetermined diameter of the silicon wafer) around the silicon wafer, and c) plasma et~hing ~preferably~ dry metal plasma etching) the silicon wafer.
Other contemplated uses of the si.licon carbide wafers of the present invention (which could exploit an expected low pressure drop acr~ss the wafer) inclucle gas burner plates, composite substrates and filters.
For the purp~ses of the present lnvention, "v/o" refers to a volume percent, "w/o" refers to a weight percent. In addition, the term "flatness" is cons~dered to be the total spread between the minimum and maximum deflection from a flat granite plate.
EXAMPLE I
A bimodal powder consisting of about 42 w/o fine silicon carbide and about 39 w/o coarse silicon carbide and a deflocculant were mixed with about 8 w/o deionized water, 4 w/o latex binder and 6 w/o plasticizer (P1?G). The resulting slip was milled overnight under vacuum. The visccsity of the slip was found to be about 30,000 to 35,000 cps at 0.6 rpm.
Bubbles were observed in the milled sli~. These bubbles are believed to result in small p-n holes in the green tape cas ing.
A convention~l tape casting +able was used to cast the s:LiF. The unit lncluded a drive controlr mylar carrier, slurry reciervoir, doctor blade, supporting table, drying unit and ta~:e-up drum. The ur.it also had an electric heater below the t~ble so t~at molsture in the slip was removed from the bottom of the tape upwar(~s, thereby preventing a skir from forming at tht-~ surface of the tape.
Eight foot lengths Gf slip were tape CaS at 300 mm per m:inute in widths ~larying between 1~0 and 300 m]m, and at a blade height of 1.25 mm The cast tape was then dried for about 1 hour at 30~C befoIe removal from the t{~ble. The tapes were subsequently allowed to dry overnlght at room temperature to enhance their green strength. The drying resulted in the tapes shrinking essentially exclusively in the thicr.ness dimension so thclt the dried th:.ckness was about 0.94 mm. Wafers were cut f:rom the dried tape in diameters of about 100 m~m, 150 mm and 20() mm.
In preparation for recrystallization, the green wafers were stacked between horizontal dense silicon carbide plates to foI~m a column, wi h graphite paper inserted between each side of the green wafer to prevent sticking. The wafers were then recrystallized at 1950~C and 300 mtorr.
The pin hole, observed in the cast tape were not observed in the recrystallLzed wafers. However, some of the wafers were follnd to be ben~ around the edges and most displayed an imprint on their surfaces. The cause of the bending was believed to be t]he~ silicon carbide plates sliding during recrystallization, wit:h the graphite paper providing lubrication for the sliding.
The cause of the imprint was bel:ieved to be the thermal decomposition of -he graphite paper.
Oxidation of selected recrystallized wafers having the i'mprint did not appreciably remove the imprlnt. The imprint, however, was not leleterious to the er.d produ-t.
EXAMPLE, II
In this exam~le, scrap tape from experlments conducted in substantial accoriance with Example I was usei as a starting W 097/36843 PCT~US97/05188 material. The scrap tape was completely dispersed by milllng overnight in 25 w~o deionized water. The res~llting slip was then mixed with tne silicon carbide pc,wders, deflocculant, binder, and plastLcizer as above, and then miLled overnight under vacuum in a nylon ~ug. Water was then added to the slip to bring its viscosity to about 30,000 cps at 0.6 rpm. Slip comprising up to ~0 w/o scrap producea high quality tapes.
EXAMPLE III
This example attempted to reduce the pin holes observed in o the green bodies of Example I. Since close observation of the slip revealed small, oily bubbles, it was believed the pin holes were the re,ult of incomplete dissolution of either the latex binder or plasticizer. Since the plasticizer (polypropylene glycol) is prone to coagulation at the high shear rates found in milling, it was proposed that the plasticizer be blended with the slip after the water has coated the particles.
Accordingly, a new slip was prepared by vacuum milling the recycled slip of l!.xample II, silicon carbide power, and deflocculant; removlng the milling balls; adding the pre-mixed binder and plasti~izer, and then vacu~ rolling the mixture in a nylon jug overnight. The resulting slip had a significantly lower viscosity (~bout 9900 cps at 0.6 rpm). Upon casting, it was observed that the frequency of pin holes was sharply reduced but not eliminated.
EXAMPLE I~
This example was performed ir, substantia1 accordance with Example I, except that a separate burnout step was incorporated be~ore the recrystallizing step.
The green wafers produced in substantial accordance with Example I were set on recrystallized silicon carbide plates and the plates were stacked with silicon c-arbide spacers in between to form columns. ~hese colu~ns were exposed t~ a r at a temperature of 200~C and a vacuum of c inches Hg for 6 hours to promote burnout of the plasticizer. ~his treatment decomposed the plastici~er and cured the kinder. The removal of the W O 97/36843 PCT~JS97/05188 plasticlzer elimi~ated wafer sticklng and the subsequent need fol- graphite paper and hence the slid ng problem described in Example I.
EXAMPLE ~
Green wafers cast and subject to plastic.zer burr.olt as ln Example IV were si~acked 1n groups of ten between a top and a boltom surface gr~und SiC plate, but withou~ any graphite paper inserted therebet~een. The wafers were ther. ~ecrystallized as in Example I.
It appears t~at the wafers did nc~t slide during recrystallization. Their measure of flatness was less than about 0.005". In addition, sticking between the wafers was minimal, as the wafers could be easil~ pried apart. Breakage from prying apart stuck wafers was only about 0-6%.
EXAMPLE VI
A bimodal po~der consisting of about 43 w/o fine silicon carbide and about 40 w/o coarse silicon carbide and a deflocculant were mixed with about 13 w/o deionized water and 0.01 w/o NaOH solution. The slip was ball milled under vacuum (25 in. Hg) for 12 hours at 200 rpm. About 3 w/o of a latex emulsion binder was added and the slip was mixed without vacuum for 2 hours at 25 rpm. A trace amount of a surfactant (0.1 w/o) was also added to improve the wetting behavior of the slip on the mylar carrier. The total solids content of the slip was between 85 w/o and 90 w/o and the viscosity was about 20,000 cps at 0.6 rpm.
The tape casting table described in Example I was used to tape cast the slip. Tapes with a green thickness of 0.5-1.0 mm were obtained wlth the following cast.ng parameters: a reservoir height of 10 to ~0 mm, carrLer speed of 0.4 to 1.1 m/mln; doctor blade height of 0.4 - 3 7 mm; and under bec hea ing temperat~re of 35-45 C. Depending uFon ~he thickness, 3~ the tapes req~ir~d a total dr~ing time c~ 2~ -- 40 mlnuteC~
res~llting in a l~-15~ shrinkage during drying. The areen tape W O 97/36843 PCTrUS97/05188 was ~ollected fro~ the exit end of the table and sectioned intc, 1 meter long sheets. Wafers were cut from the green tape in diameters of 100~ 00 mm. The green wafers were subject to binder burnout at 200 C for 8 hours in air, and recrystallized as in Example ~. ~'he densities of the recrystàllized wafers were between 2.3 cnd 2.4 g/cc.
The recrysta]lized wafers were then siliconized and sandblasted through conventional means. The densities of the resulting wafers were found to be about between 2.91 and 2.98 o g/cc.
Preliminary studies of pressing the flexible green tapes with a hydraulic press at 8000 psi followed by binder burnout as in Example IV and recrystallization as in Example ~ have demonstrated an increase in density up to 2.5'8 g/cc while the average pore size was reduced to 6 microns. Similar results were obtained by other compaction methods such as cipping at 30,000 psi or passing the tape through a rolling operation.
Although the above SiC ceramic has utility as a dummy wafer, there is a;so interest in making substrates for the deposition of diamond films to be used in thermally enhanced plastic packages or semiconductor devices. For such applications, the SiC substrate must be strong and thin. It is believed that, in order to meet these requirements, the SiC
ceramic should ha~le a thickness of between about 0.1 and 0.6 um, and its strength should be at least 80 MPa. Unfortunately, the 0.5 mm thic~ tape cast SiC wafers described above in Examples I-VI, wh:ch were found to have a DLS pore diameter of about 3 um, a D~s ?ore diameter of 12 um, and a biaxial flexural strength of only about 50-70 MPa , do not meet these req~irements. Therefore, there is a need for a tape cast recrystallized si~icon carbide ceramic (preferably having a thickness of be~ween 0.1 and 0.~ mm) having a strength of at least 80 MPa.
EXAMPLE ~II
In this example, the effect of solely usi~g a fine silicon carblde powaer was examined. In par icular, t~e prccedure of Example I above were substantial_y followed, except that the coarse silicon car:bide f-action was eliminated.
The resultin(~ tape exhibited significant shrinkage and cra.cking during dry ng.
EXAMPLE ~
In this example, both the fine/coarse ratio of the SiC
powder and the sintering temperature were mod~lfied from those ol~ the above Examples (which had about 52 wt~ fines an~ 48 wtCo coa.rse particles, and were sintered at about 1950~C).
The procedure of Example VI above was substantially fol.lowed, except that the three different flne/coarse SiC
powder ratios (35i'65, 50~50, and 65/35) and three different sintering temperatures (1700~C, 1850~C, and 2000~C) were used.
In particular, the coarse grain fraction was first sieved through a 50 um screen to remove its coarse tail. Initial water contents of 15~ was used for the :35~ fines formulation, while l2~; was used for the remaining two formulations. Deflocculation curves were prepared for each formulation in according with ISO
manufacturing procedure, and the optimal deflocculation points were determined. Next, lO00 gram batches of he three formulations were prepared. The amount of B-1035 binder used waci based upon the surface area of the grains, and a conventional non-:onic surfactant was used at a level of 2 wt~
of the binder. 'nhe optimal water fraction for each formulation was determined by mak.ing i.ncremental additions unt:il a slip viscosity o:f 25 ~dia.l) at 0.6 rpm (as measured by a E~rookfield viscometer with a LV-2 s~indle) was attained. The solids loadings for these formulat onc were all between 85 wt~
al1cl about 87 wt!'- 66-68 volgO).
The three fo:-mulations were de-aired in . vacuum bel.l jar al l5"Hg for 15 mLnutes. The batches were ~.a~e cast at a c~rrier speed of .'0"/minute at a b.ade heigkt of 0.0l6" and a tem~erature of l5:)~F. The slurry was pol.red manually ~nto the reC;ervoir~ mainta.ning a reservoir height cf ~.0 cm. The W 097136843 PCTrUS97/05188 average dried green tape thickness for each batch was between about 0.128 and 0 132 inches, and was determined by cutting out 100 mm diameter g~~een wafers and meas~ring the thickness of the four quadrants.
The wafers r:'~Om each batch were cross-roiled (i.e., rolled, rotated 91)~, and rolled again) through spaced 15 cm diameter, hardened steel, primary rollers havlng a roller gap of 0.005". The sLgnificant elastic behavior of the wafers typically results in only about 5-10% compactlon from the green o thickness. The a~erage rolled wafer thickness for each batch was determined by measuring the thickness of the same wafers at the four quadrants.
The wafers were loaded into the sintering furnace on SiC
setter plates. Binder burnout was conducted at a ramp rate 0.2~C/minute and an 8 hour soak at 240''C. The wafers from each batch were divided into three groups and then sintered at the desired temperatu e. Each of the sintering cycles started with a 3~C per minute ramp to 500~C, with an intermediate soak for 1 hour to complete -he burnout cycle. The runs were then ramped at a rate of 5~C/minute to the final soak temperature with a soak time of three hours. Each of the three runs was conducted under Argon at a pressure of 0.6 torr for the entire cycle.
Sintered den,ity and pore si~e distribution was measured by mercury intrusion porosimetry. The biaxiaL flexural strengths were measured by a ring-on-ring configuration.
The sintered densities of the resulting materials are shown in Table I.
W O 97/3~843 PCT~US97105188 Table _ SinteredDensities(~cc) 35%finel65% coarse 50%fine/50%coarse 65%finel35%coarse 17~0~C 2.. -4 2.31 2.21 1~50~C 2.;4 2 41 2.32 2l~)0~C ~ 1 2.4~ 2.36 Examination of Table I reveal.s that dens_ties increase both with increasing coarse fraction and increasing sintering temperature. Increasing the fines content of the formulation requires the use of additional binder and li~uid during green body formation in order to prevent. cracking. The resulting decreased density of the green body leads to a decreased density in the fired body. Similarly, it i.s believed that inrreasing the sintering temperature produces more sintering activity, grain coarsening, and neck formation, and therefore higher densities.
The D50 pore diameters of the resllting materials are shown in Table II.
Table II
D5~,poreDiarneters(um) 35%fine/65% coarse 50%fine/50%coarse 65%fine/35%coarse 17()0~C 2.:~ 1.7 1.8 18;0~C 6.~) 4.0 33 20()0~C 11.0 10.? 7 9 Examination of this table reveals thar pore dlameter increases with sintering temperature and coarse fraction.
The characteristlc flexural strengths ~f the resulting m.aterials, as determined by Weibull analysis, are shown in Table III.
Table III
Flexural Streng:th (MPa) 35%finel65% coarse ~0%fine/50%coarse 65%finel3~%coarse 1700~C 4~ 7 95.0 103 5 1~50~C 31 0 65.4 81 3 2000~C 47 8 49 5 47. 1 Examination of the above table reveal;, the surprising result that lower sintering temperatures and higher fine fractions lead to higher strength wafers.
The results are surprising in two respects. First, it is clear that different phenomenon are occurrlng at different o fine~coarse fractions and different temperatures. In particular, in the Table III studies in which the SiC powder was kept constant at 35~ fines/ 65% coarse, increasing the sintering temperature from 1850~C to 2000~C increases the flexural strength from 31 MPa to 47.B MPa. It is believed, in relatively coarse formulations, the extent of inter-grain necking (which ircreased with sintering temperature) is the dominant factor. In contrast, in the studies in which the powder was kept constant at 65% fines~ 35% coarse, increasing the sintering temperature leads ~o l~wer strength. It is believed that, ir. relatively fine powder formulations, pore size (which increases wlth sintering temperature) becomes the critical flaw.
Similarly, ln the studies in which the sintering temperature was ~ept constant at 2000~C, increasing the fine fra~tion from 35~ to 65~ had relatively no effect on the flexural strength. It is believed that, in high fired embodiments, the large pores created by the grain coarsening are the strength limiting feature. In contrast, in the studies -- in which the sintering temperature was kept constant at 1700~C
increasing the f:ne fraction from 35~O to 65~ increased the flexural strength from ~7.7 to 103.5 MPa.
W O 97/36843 PCT~US97/05188 In sum, these studies do no1 fcllow the traditional expectations that slmply increasing sintering temperature and de(reaslng grain .ize leads to increased strer~gth.
Second, the strengths recorded in Table III do not 5 correlate well wl h traditional factors typically correlating poc,itively with s rength (i.e., densi_y and -'50 pore d:iameter). For example, the example having 35~ fines and sintered at 1700-C possessed a fairly ~ow strength (47.7 MPa) dec;pite its relat:vely small 2.2 ~Lm Ds pore diameter. Also, lo the fired bodies having the highest densities (i.e., above 2.5 g/c:c) recorded some of the lowest strengths '31.1 MPa and 47.8 MPa~.
The sintered bodies were also analyzed for D85 pore d:iameter, which represents the size which exceeds exactly 85~
of the pores. The D8s pore diameter of the examples are shown in Tàble IV belowj together with the corresponding flexural strength in parentheses:
Table IV
D8s Pore Diameters, in um 'flexural strengths in parentheses) 35%fine/65% coarse S0%finel50%coarse 65%fine/35%coarse 1 J00~C 5 1 (47.7MPa) 2.0 ~95.0 MPa) 2.1 (103.5 MPa) lg50~C 90(31.0M:Pa) 4.8 (6~4MPa) 3.9(81.3MPa) 2()00~C 12 5 (47 8 MPa) 12 .2 (49 5 MPa) 9.8 (47.1 MPa) Comparison o- the D85 pore diameters with the flexural strengths reveals a very strong correlation between D8 pore dklmeter and flexllral strength. For example, only examples h,~ving a D~s pore ~iameter of no more than 4 um possess a flexural strength greater than 80 MPa, and the examples with the smallest D~s p~re diameters (about: 2 u~ nave the largest flexural strength Further, the :low strength (47.7 MPaj of the -example havlng the 2.2 um Dso pore diameter lS now explained by its unexpectedly larger D~s pore s1~e 5.7 um~
Therefore, i- is now clear that Froviding a D8s pore diameter of less han 4 um, and more Freferably less than 2.5 W097/36843 PCT~S97/0~188 um is critical ~o provlding a flexura~ strength of at least 80 MPa, and preferably at least 95 MPa, and that using either :
i) at least ,0c~, fines (preferably at least 60%) and sintering at no more than 1750~C , or ii) at least 60% fines and sintering at r-lo more than 1850~C ~preferably 1750~C) is also critical ~o producing these low D8 pore diameter values which produce the high strengt:h SiC wafers.
Therefore, in accordance with the present invention, there o is provided a ceramic wafer consisting essentially of recrystallized silicon carbide grain and having a 1~5 pore diameter of no more than 4 um, and a biaxiaL flexure strength of at least 80 MP~, wherein between about 50 wt% and 70 wt% of the silicon carbiie comprises fine grains having a grain size of no more than 5 um, and between 30 wt~ and 50 wt~ of the SiC comprises coarse grains having a grain size of at least 20 um .
In some em~o~iments, the wafer has a density of no more than 2.33 g/cc, preferably a D85 pore diameter of no more than 3 um, and a stren~th of at least 90 MPa.
In some embo~iments, the wafer has a density of no more than 2.2~ g/cc an~ a D8s pore diameter of no more than 2.5 um, a strength of at least 100 MPa, and between about 60 wt% and 70 wt% of the silicon carbide comprises fine gra~ns having a grain size of no more than 5 um.
In some emboiiments, the ceramic has a thickness of between 0.1 mm and 0.3 mm, is preferably unground, and more preferably has a D50 pore diameter of at least about 1 um.
In some embodiments, between 30 wt% and 50 wt% of the SiC
comprises coarse grains having a gra:in size ~f at least 30 um.
Also in accordance with the present invention, there is provided a process for making high st ength, tape cast SiC
wafers, comprising the steps of:
a) providing a formulation consi~ting essentially of silicon car~ide powder and water, wherein:
CA 02246803 l998-08-l8 i) betw~en 50 wtC~, and 70 wt:90 ~f hf- SlC powder has d particle size of no more than 5 um . and ii) bet~een 30 wt% and S0 wt~ of t'le SiC powder has a parti~le size of at least 30 ~lm, b~ tape cast;ng the formuiation to produce a tape cast body, c) drying th~ tape cast body to produce ~ green body having thickness of between about 0.1 mm and 0.6 mm, and o d) sintering the green body at a temperature of no more than 1850~C, to form a re-rystallized silicon carbide wafer having a density of no more than 2.3' g/cc and a strength of at leas~ 80 MPa.
In some embol~iments, the sintering temperature is no more than 1750~C, and the recrystallized silicon carbide wafer has a I)~5 pore size of no more than 2.S um and a strength of at least 90 MPa.
In some embodiments, the SiC powder comprises at least 60 Wt1~' SiC particles having a size of no more than 5 um, and the re(rystallized silicon carbide wafer has a D8s pore diameter of no more than 3 um.
In some embodiments, the sintering temperature is no more t]na,r. 1750~C, the ';iC powder comprises at least 60 wt% SiC
p~rticles having l size of no more than 5 um, and the re(-rystallized silicon carbide wafer has Dfls pore size of no mo]-e than 2.5 um, a density of no more than 2.25 g/cc and a s~rength of at l-ast 100 MPa.
In some emboiiments, the sinterirg temperature is no more t!harl 1700~C, the ',iC powder compri es at least 65 wt~ SiC
p,~"-ticles having ~ size of no more thcn 5 um" and the rlecrystallized silicon carbide wafer has a st~ength of at lleast 103 MPa.
In some embo~iments,. the procesC includes the additional step o compactin~ ~he tape cast body to reduce its thickness by at least 5~c (preferably at least 1(~) prio- to sintering.
W 097136843 PCT~US97/05188 EX~MPLE IX:
This example examines the effect cf rol~ ompactlng the tape cast green Si~ wafer.
Example VII~ above was substantia ly followed, except that the ~ormulation comprised 57% fine/43% coarse SiC partic~es, the sintering temperature was 1850~C, and the dried tape cast wafer was not subjected to roll compaction.
The resulting wafers had a density of about 2.36 gicc, a D~5 pore diameter cf about 8.5 um, and ~ flexural strength of o only about 35-45 MPa.
This example can be usefully compared with the 50% fine and 65% fine embodiments of Example V[II above which were roll compacted and sintered at 1850~C, and possesseci flexural strengths of 65.4 MPa and 81.5 MPa respectively. Since each of the previous examples (one of which ha~ slightly less fines and one of which had slightly more fines) had twice the flexural strength, it is clear that roll compacting the tape cast wafer had a slgnificantly positive effect upon the flexural strength of the recrystallized wafer. Without wishing to be tied to a theory, it is believed that roll compacting has the effect of decreasing the interparticle spacing to promote more effective neck formation during recrystallization.
Therefore, also in accordance with the present invention, there is provided a process comprising the steps of:
a~ providing a formulation consisting essentially of a ceramic powd~r, binder and water, b) tape casting the formulation to produce a tape cast body, c) drying the tape cast body to produce a green body, d~ compactinc the green body to reduce its thickness by at least 1~%, and d) sintering the green body.
For the purposes of the present inventlon, po~e diameter is determined by mercury intrusion porosimetry.
BAC:KGROUND OF THE INVENTION
The manufacture of semi-conducto- devices such as dlodes and ransistors typically requlres the deposit~on of dielectric materials such as polycrystalline sillcon, silLcon r.itride and silicon dioxide on the su.rfaces of thin silicorl wafers. The ~o thi:n layer deposition of these materia s involves rapid heating and cooling cycles in an electrically heated furnace (or "diffusion process tube") at temperatures typically ranging fro:m 250 to 1000 C. When. dielectric precursor gases are fed into a diffusion process tube heated to these temperatures, the gases react and deposit the dielect:ric reaction product on the surface of the silicon wafer.
During the deposition step, the s-licor, wafers are supported in vertical or horizontal. borlts placed within the process tube. The wafer boat and process tube are typically made of a material which has excellent thermal shock resistance, high mechanical strength, an ability to retain its shape through a large number of heatin~ and cooling cycles, and which does not out-gas (i.e., introduce any undeslrable im.purities into the atmosphere of t:he }iln durlng firing operations). One materia.l which meets these requirements is silicon carbide. For the above-mentiolled appllcation, silicon ca.rbide diffusion components such as boats, paddles and process tubes are ty~pically pre-coated with the dielectric se ected for deposition.
When the silicon wafers are pr.oce,sed in ~ boat, lt is na.turally desirable that each wafer n the b3a. be exposed to id.entical gas concentrati.on and temper~ture profiles in order tc produce consistent product. :Howeve-, the t~pical fiuid dynamic situatior is such that conC;istent prof.les are found on.ly in the middlc of the boat whi e _nconsistent prof-les are ofte~ found at thc ends of the boats, -esultir.~ in undesirable degrees of dielectric deposition Lpon -he end-wafers which render them unusa~le.
W 097/36843 PCTrUS97/0~188 One conventl~nal method of mltlgating thls "end-effect"
prGblem is to fil: the end slots of the ~oat with sacrificial ("dummy") wafers made of silicon. However, i- has been found that silicon wafers are expensive, extensivel~l7 out-gas, warp at high process temperatures, flake p~rticles, ar.d have a short useful life span.
Another conventional method of mitigatin~ the "end-effect"
problem is to fil~ the end slots of the boat with dummy wafers maàe of alternative materials. For example, one investigator placed SiC-coated carbon wafers having the exact dimensions of the neighboring s licon wafers in the end slots. However, these wafers were found to break apart, contaminating the furnace with carbon particles. Another investigator proposed using CVD monolithic silicon carbide as a dummy wafer.
However, this material is known to be very expensive. One prior proposal fo~~ producing silicon carbide wafers is a free~e casting approach which produces a green silicon carbide billet having a thicknes, of at least about 25 mm which is recrystallized and then sliced to a commercially useful thickness. However, it has been found~ that the freeze casting process produces significant porosity in the wafer (on the order of 40 v~o, with 15 percent of the pores larger than 25 um). These large pores make it difficult to completely precoat the wafer with the dielectric and make the deposition process very expensive. JP Patent Publication No. 5-283306 ("the Toshiba referencel') discloses forming a commercial~y useful wafer by grinding down a 2 mm thick siliconized silicon carblde disc to a thickness of about 0.625 mm, and then CVD coating the disc with an alum~na-silica coating. However, the grinding, siliconization and CVD steps are expensive, particularly so in the low temperature ~less than 1000 C) applications where silicon infiltration is not required to prevent the oxidation of the silicon carbide.
Therefore, i~ is the object cf the present invention to provide an inexpensive sllicon carbide dummy wafer which possesses the dim~nsional, physical and mechanical properties W 097/36843 PCT~US97/05188 recIuired for use :n appl:Lcations with -empera-::ures less than abc)~t lO00 C.
SI~RY OF THE IN~'7ENTION
In accordance with a preferred embodimen: of the present invention, there :s provided a wa~er consisting essentially of s:ilicon carbide hav~ng a thickness of between 0.5 and l mm, a d:iameter of at let~st lO0 mm, and a strength (as measured by r:ing on ring biax al flexure) of at least 31) MPa (typically bet:ween 50 and 70 MPa), the wafer having a porosity wherein at least 85~ of the pores are no larger than 14 microns.
Preferably, the wafer has a density of at least about 2.15 g,/c:c, more preferably between 2.3 and 2.4 g~cc; its porosity is bet:ween about 14~ and 16~, with at least 85~ cf -ts pores being no larger than 12 microns, and at least 95~o of its pores being larger than 3 mlc~~ons; the average pore size lS between 6 and 11~ um, typically 53 um; the silicon carbide is ln recrystallized form and consists of between 40 and 6C w/o grains having a size o:f between 2 and ; um and between 40 and 60 w/o grains having a s:ize of between about 30 and 200 micrcns; and, the surface of the wafer is unground.
Also in acco-dance with the present nvention, there is p:rovided a preferred process for making a t~in, crack-free g:reen silicon carbide sheet, comprising the steps of:
a) forming a slip comprising a liquid carrier ~preferably, 2s water) and a ceramic powder consisting essentially of silicon carb:ide, b) tape casting the slip to produce a wet green sheet having a thickness of between 0.~ and l.' mm, c) evaporating essentially all the liquid carrier from the wet green sheet to produce a dry green sheet having a thickness which is ~t least 80% of the wet green sheet thickness, d) forming a shape from the ~iecl green sheet to produce a silicon carbide green wafer, e) recrystallizing the sillccn carbide g-een wafer to produce a re~rystallized silicon carbide wafer having a thickness of ().5 to l.0 mm and flatness cf less than 13Q
um and, optionally f) grinding the silicon carbide wafer t~ reduce its thickness by no more than 5~.
In preferred embodiments of this process, the ,illcon carbide powder consists of about 40 to 45 w/o grains having a size of between 2 and 5 um and about 38 to 42 w~o grains having a size of between about 30 and 100 microns (wherein t:he w/o fractions are based upon total slip weight); the water content of the 0 slip is between about 12 to 15 w~o of 1he slip; the total sol.ids content ~silicon carbide plus the solid portion of the binder) of the slip is between about 8'~ and 9C w/o of the slip;
the density of the dry green sheet is ~t least about 2.3 g/cc;
and the slip further comprises between 2 and ~ wJo of a binder having a glass transition temperature of less than 22 C.
Also in accordance with the present inven~ion, there is provided a method of using a silicon ct~rbide dummy wafer, comprising the steps of:
a) providing a silicon wafer diffusion boat having slots for the insertion of wafers, b) inserting into at least one slot of the silicon wafer diffusion boat a silicon carbide wafer of the present invention, said silicon carbide w~fer hav:ing a coating of a dielectric material thereon, 2s c) inserting into at least one ot.ner slot a silicon wafer, and d) depositinc a dielectric material on the surface of the silicon wafer at a temperature of no more than 1000 C.
DETAILED DESCRIPTION OF THE INVENTION:
It has been f:~ound that tape casting an aqueous-based bimodal silicon carbide slip, cutting out a green wafer from the tape, and recrystallizing the green wafer provides an inexpensive, low porosity, silicon carbide wafer suitable for use as a dummy wafer.
The level of porosity produced by tape casting bimodal sili-on carbide s'ips in accordance with the preferred process W 0 97/36843 PCT~US97/05188 oi- the present ln~rention is typicaily on the (,rder of about 14-16 v/o of the recrystallized wafer, with a po~e size dicitribution whereiri at :Least 95% ~f the pores are larger thar.
3 microns, at lea;t 85~ of the pores are no larger than 12 mic:rons (preferab y 10 microns), and the average pore slze is bet:ween 4 and 8 um, typically about 7 um. Because the pores are both infrequent and relatively small, these wafers can be more evenly and less expensively precoated with the dielectric thcln can silicon (arbide wafers from the free~e casting o process. Moreover, because the wafers do not require silicon impregnation ("si iconization"), they are less expensive ~han s:Lliconized silicon carbide wafers.
In addition, because the near net shape casting described abc)ve either reduces or eliminates the need for slicing and grinding, it is believed tape cast dummy wafers produced by the pxesent invention can be produced at â signiflcantly reduced miarlufacturing cos~, regardless of whether the wafer is unsiliconized (fo~ low temperature applications) or siliconized ~Eor high tempera~ure applications). So if an inexpensive si].iconized wafer is desired, the relative absence of very fine pol~es (i.e., pore, less than 3 um) allows for easy siliconization.
Another noveL feature of the present invention is its ability to reliably provide green silicon carbide wafers of the noted thickness which do not crack during dryLng. Typical prior art tape casting of silicon carbide was limited to using non-aqueous based slips to produce much thinner ~i.e., 0.025 to O.L25 mm) green sheets. Because a dummy wafer should be the same thickness as a normal silicon wafer ~i e , about 0.625 mm to ().725 mm), the thinner wafers produced from non-aqueous ba,ed slips would not be suitable as dummy wa~ers. Attempts at ca;ting larger thickness sheets from these norl-aqueous based slips typically resulted in excessive cracklng. Without wishing to be tiei to a theory, it is belleve-~ the excessive ev(~poration rate ~f the organic solverlts in these conventional slips produced a pronounced vertical dispar:t~ rr. the drying W 097136843 PCTrUS97/OS188 rate of the relatlvely thick green sheet, lea~ding to the formation of a dr~ skin on the top surface whose higher packing promoted cracking beneath the skin. In contrast, aqueous-based slips dry at a much slower rate. Since drylnt~ rate is slower, the evaporation profile is more uniform throughout the thickness of the ,heet and formation cf the undesirable dry sk~n is minimized. In addition, one preferred process of the present invention further enhances uniform vertical drying by heating the cast ,heet from below the casting table, thereby lo promoting moisture removal from the bottom of the green sheet.
Moreover, it is believed that tape casting allows for reliable producti~n of larger diameter silicon carbide dummy wafers than was previousiy known. In this regard, it is noted the Toshiba reference discloses a relatively small (150 mm x 150 mm x 2 mm) siliconized tile. lt is also noted that one freeze casting approach which entails freeze casting a 200 mm diameter, 150 mm thick silicon carbide billet and then slicing the billet suffered from excessive cracking in the interior of a large number of the billets. ln contrast, the process of the present invention allows for reliably producing suitable silicon carbide dummy wafers on the order of 200 mm and 300 mm diameters via tape casting.
Preferably, the grain size distribution of the silicon 2s carbide grains used in the present invention LS bimodal. It has been found that using a bimodal distribution ~roduces much less shrinkage in the thin tape cast green sheet (on the order of only about 10~ to 15~) than does a fine unimoda~ distribution (shrinkage on the order of about 85~ to 90%). Since much less shr nkage is involved, the thickness of the tape is more easily controlled. Since the fine unimodal powders were the focus of the tape casting art, this advantage was not suggested in the prior art. Preferably, the bimodal SLC grain distribution comprises between about 38 and 42 w/o coarse SiC grains having 3s a particle size ranging from 10 to 150 microns ("the coarse fraction"), and ~etween about 40 t:o 4~ w~o fine SiC grains WO97/36843 PCT~S97/05188 havlng a particle size ranglng between 2 an(1 ~ mlcrons '"the fine fraction"). More preferably, the fine f~action comprises about 43 wJo of the SiC grain and has an average partil-le size oI about 2-3 microns, while the coarse fractlcn comprises about 40 w/o of the SiC grain and has an average particle size of about 60 microns. ln some embodiments, the f-ne fraction is E277, a silicon carbide powder available from Saint-Gobain/Norton Industrial Ceramics Corporation ("5G~NICC") of ~orcester, MA, ancl the coarse fraction is F,4C;, another silicon carbide powder available from SG/NICC.
In preferred embodiments of the slip, water is present in an amount sufficient to produce a slip having from about 80 to 9() w/o solids. The slip may also include conventional additives such as deflocculents, binders and plasticizers. In one preferred embodiment, the slip includes 13 w/o water, 0.Ol w/'o deflocculant such as sodium hydroxide, and between about 1%
and about 10% (preferably between 1% and 5%) of a binder having a glass transition temperature of less than 22 C. The amount of binder used in the present invention is typically less than the amount used in conventional sllicon carbide tape casting, which is usually at least 10% of the slip. The use of a binder wi.th a glass transition temperature below 22 C eliminated the need for a plastio-izer in this formulation. It is believed the lohrer binder fraction yields a more uniform pore size distribution. Typically, the silicon carbide, water and def'locculant components are mixed in a ball mlll evacuated to a VclC'UUm level of between about 27 and 3~ inches Hg, the milling media is removed and the binder is added, and the entire mixture is rolled The tape cast:ing step of the present invention is preferably accomp:ished by draining a reservo~r containing the silicon carbide s ip thrvugh a horizontally-d-sposed slit in the reservoir, the upper surface of sllt belng defined by a doctor blade and 'he lower surface of he s'it being defined by an endless belt which moves under the doctor blade. Typically, the doctor blade s positioned between abou (.4 and l.~~ mm W 097/36843 PCTrUS97/05188 above the endless belt. In some embodiments, ~1 heater is placed underneath the en~iless belt in order to provicie a more uniform drying profile for the cast sheet. The initia:! thickness of the wet cast sheet thus produced is generally between about 0.4 and 1.3 mm. The cast sheet is then dried for 20-30 minutes on the heated belt, after which time the thickness of the cast sheet typically shrinks between about 10% and 15% to provide a green sheet having a density of between about 2.3 g/cc and 2.4 g/cc.
Next, the dr:ed sheets are cut using c.rcular punches to o form green silicon carbide wafers having a diameter of between 100 and 300 mm an(~ a thickness of between about 0.5 and 1.1 mm, preferably between 0.5 and 1 mm, more preferably between about 0.625 and 0.725 mm. When the wafers are in the range of about 0.625 mm and 0.72~ mm, they do not need to be surface ground.
In some embodiments, the scrap material from the dried cut sheet is recycled. Preferably, the scrap is completely dispersed by milling overnight in 25 w/o deionized water; the resulting slip is then mixed with virsin silicon carbide powders and additives; remilled; and water is added to the slip to bring its viscosity to about 30,000 cps at 0.6 rpm. This recycled slip can then be used to make more green sheets, thereby essentially eliminating waste of the silicon carbide raw material.
In some emboiiments, the cast wafers are heat treated prior to recrystallization in order to partiaily remove the binder. It has been found that partial removal of the binder ~i.e., removal of about 15% to 25%) yields a fine pore structure in the sintered body which ~s advantageous for pre-coating. It is believed the partial bllrnout produces small diameter vapor "escape routes" for the remaining binder, thus pre~enting the creation of larger sized pores during the recrystallization process. In prefer-ed embo~iments, this entails exposing the cast wafers to a r at a temperature of 220~C and a pressure of 5 inches Hg fcr 8 hou~s.
Next, the drled green wafers are recrystallized.
Rec~ystallization establishes strength-enhancing neck arowth WO97/36843 PCT~S97/05188 between the SlC grains wi-hout substantial den.~ification. It is generally carried out at about 1900-1950~C under a vacuum of about 0.6 torr n an Ar atmosphere. Ir preferLed embodiments, the wafers are rec:-ystall zed at 1'350~C and ~ torr in argon.
In some embodimen s, the green wafers are s a~ked between surface ground silLcon carbide plales in ~rder to provide flat:ness.
A recrystalli~ed wafer produced lr. ac-o-dance with he bimodal embodiment of the present invention ty-ically exhibits o a bu1k density (as measured by mercury intrusicn porosimetry) of at least about '.15 g/cc, and preferably between 2.3 an~ 2.
g/cc, a total poro,ity of about 14-16~, an average pore size of between 4 and 8 um, with a pore size distribution wherein at least 95~ of the pores are larger than 3 microns and at least 85~ of the pores a e no larger than 12 microns (preferably 10 micl-ons). In preferred embodiments using a bimodal grain dist:ribution, the ~ine fraction grains average about 2 to 5 microns and the co~rse fraction grains average about 30 to 100 microns. It has a measure of flatness of no more than about 13~ um across a di~meter of about 200 ~L. Its strength (as measured by ring on ring biaxial flexuIe) is typically between about 50 MPa and a~out 70 MPa.
Recrystallizei silicon carbide du~Lmy wafers having diameters of at le~st 100 um, preferably between about 150 and about 200 mm; thic~nesses of between about 0.5 and about 1 mm, and preferably abo~t between about 0.6~5 mm and 0.725 mm; and flal-nesses of between about 50 and abo~t 130 mlcrons, preferably less th~n about 100 microns, are obtainable in accordance with this embodiment.
If desired, a~ additional firing step may be undertaken to make the wafer resistant to gas or iiquid atlack in hich te~mperature applications. This typically involves either impregnating the recrystallized wafer with sil~con to eliminate porosity and/or CVD coating it w th an impermeable ceramic such as s_licon carbide. If siliconi~ing l~ selected, it may be carr:~ed out in acc~rdance with US PateIlt No. 3! 9~1~ 587 ("the W O 97/36843 PCT~US97/0~188 Alliegro patent") the specification of which is incorporated herein by referen~e. Therefore, in accordance with the present invention, there is prov1ded a wafer consistirlg essentially of silicon carbide h~ving a thickness of between 0.5 and l mm, a diameter of at le~lst 100 mm, the wafer being infiltrated with silicon so that t~e silicon is present as sil'con pockets and comprises about 1~ to 16 v/o of the wafer, and wherein at least 85~ of the silico~ pockets are no larcrer than 10 microns (preferably 8 microns) and at least 95~ of the silicon pockets o are larger than 3 microns. There is also provided a method of uslng this silico~ized silicon carbide wafer, comprising the steps of:
a) providing a silicon wafer diffusion boat having slots for the insertion of wafers, b) inserting into the slot of the si~icon wafer diffusion boat a silic~n carbide dummy wafer as described in this paragraph, said dummy wafer having a coating of CVD
silicon carbide thereon, c) inserting a silicon wafer into another slot in the boat, and d) oxidizing the surface of the silicon wafer at a temperature of at least 1000 C.
If CVD coating with silicon carb:de is selected, it may be carried out by any conventional CVD S.C method. Likewise, the silicon carbide wafer of the present nvention may be coated with a dielectric material such as po:Lycrystalline silicon, sillcon nitride, or silicon dioxide.
Conventional sandblasting of the siliconized SiC wafer can remove excess free silicon that has exuded to the surface due to the volume expansion of silicon on solidification. Because these wafers possess high strength, they do not break when sub~ected to sanàblasting.
The novel recrystallized silicon carbide wafers of the present invention are preferably used as dummy wafers in slllcon wafer manufacturing. However, they ma~ also find application as rigid discs in computec hard drives; as 1 o W 097/36843 PCTrUS97/05188 substrates for oth~r micro-electronlc appl_cations lncluding acting as setters n single wafer processing and plasma etching; as substr~tes for flat panel ICD dispLays; or as bafE'e plates ln w~fer boats.
Also in accor~ance w th the presert inven~ion, there is prov-ded a preferr~d method of singie wafer processing, co~?rising the ste~s of:
a) providing i silicon carbide disk of the present invention (pr3ferably having a diameter of at least 200 mm 0 and more preferably at least 300 rnm) in a substantially horizontal position, and b) placing a silicon wafer ~preferably having a diameter of at least 200 mm and more prefexably at least 300 mm~
upon the silicon carbide disc, and c) heating the silicon wafer at a rate of at least 100 C
per second.
Also in accordance with the present invention, there is provided a method of cleaning single wafer processing chambers, comprising the steps of:
a) providing a susceptor in a processing chamber, b) placing a silicon wafer upon the susceptor, c) processing the silicon wafer, d) removing the silicon wafer, e) placing a silicon carbide ciisk of the present invention (preferably ~aving a diameter of at least 200 mm and more preferably at least 300 mm) over the susceptor, and f) in-situ c~eaning the processin~ char~er by exposure to free radicals.
Also in accordance with the present invention, there is prc,vided a method of flat panel display processing, compr sing the steps of:
a) providing a silicon carbicle plate of .he present invention (preferably having ~ length c,f at least 165 mm and a width of at least 265 mm) in a substantially horizontal position, and W097/36843 PCT~S97/051~
b) placing a flat glass plate (preferabl~ havlng a length and a width of at least l00 mm) upon the silicon carbide plate, and c) processing the flat glass plat.e by ox dation, dielectric deposition and/or diffusion at: a temperature of no more than 800 C.
Also in accordance with the present invention, there is provided a method of plasma etching si.licon wafers, comprising the steps of:
a) providing a silicon wafer havi.ng a predetermined diameter of ~t least 200 mm, b) placing a silicon carbide ring of the present invention ~having an inner diameter essenti.ally equal to the predetermined diameter of the silicon wafer) around the silicon wafer, and c) plasma et~hing ~preferably~ dry metal plasma etching) the silicon wafer.
Other contemplated uses of the si.licon carbide wafers of the present invention (which could exploit an expected low pressure drop acr~ss the wafer) inclucle gas burner plates, composite substrates and filters.
For the purp~ses of the present lnvention, "v/o" refers to a volume percent, "w/o" refers to a weight percent. In addition, the term "flatness" is cons~dered to be the total spread between the minimum and maximum deflection from a flat granite plate.
EXAMPLE I
A bimodal powder consisting of about 42 w/o fine silicon carbide and about 39 w/o coarse silicon carbide and a deflocculant were mixed with about 8 w/o deionized water, 4 w/o latex binder and 6 w/o plasticizer (P1?G). The resulting slip was milled overnight under vacuum. The visccsity of the slip was found to be about 30,000 to 35,000 cps at 0.6 rpm.
Bubbles were observed in the milled sli~. These bubbles are believed to result in small p-n holes in the green tape cas ing.
A convention~l tape casting +able was used to cast the s:LiF. The unit lncluded a drive controlr mylar carrier, slurry reciervoir, doctor blade, supporting table, drying unit and ta~:e-up drum. The ur.it also had an electric heater below the t~ble so t~at molsture in the slip was removed from the bottom of the tape upwar(~s, thereby preventing a skir from forming at tht-~ surface of the tape.
Eight foot lengths Gf slip were tape CaS at 300 mm per m:inute in widths ~larying between 1~0 and 300 m]m, and at a blade height of 1.25 mm The cast tape was then dried for about 1 hour at 30~C befoIe removal from the t{~ble. The tapes were subsequently allowed to dry overnlght at room temperature to enhance their green strength. The drying resulted in the tapes shrinking essentially exclusively in the thicr.ness dimension so thclt the dried th:.ckness was about 0.94 mm. Wafers were cut f:rom the dried tape in diameters of about 100 m~m, 150 mm and 20() mm.
In preparation for recrystallization, the green wafers were stacked between horizontal dense silicon carbide plates to foI~m a column, wi h graphite paper inserted between each side of the green wafer to prevent sticking. The wafers were then recrystallized at 1950~C and 300 mtorr.
The pin hole, observed in the cast tape were not observed in the recrystallLzed wafers. However, some of the wafers were follnd to be ben~ around the edges and most displayed an imprint on their surfaces. The cause of the bending was believed to be t]he~ silicon carbide plates sliding during recrystallization, wit:h the graphite paper providing lubrication for the sliding.
The cause of the imprint was bel:ieved to be the thermal decomposition of -he graphite paper.
Oxidation of selected recrystallized wafers having the i'mprint did not appreciably remove the imprlnt. The imprint, however, was not leleterious to the er.d produ-t.
EXAMPLE, II
In this exam~le, scrap tape from experlments conducted in substantial accoriance with Example I was usei as a starting W 097/36843 PCT~US97/05188 material. The scrap tape was completely dispersed by milllng overnight in 25 w~o deionized water. The res~llting slip was then mixed with tne silicon carbide pc,wders, deflocculant, binder, and plastLcizer as above, and then miLled overnight under vacuum in a nylon ~ug. Water was then added to the slip to bring its viscosity to about 30,000 cps at 0.6 rpm. Slip comprising up to ~0 w/o scrap producea high quality tapes.
EXAMPLE III
This example attempted to reduce the pin holes observed in o the green bodies of Example I. Since close observation of the slip revealed small, oily bubbles, it was believed the pin holes were the re,ult of incomplete dissolution of either the latex binder or plasticizer. Since the plasticizer (polypropylene glycol) is prone to coagulation at the high shear rates found in milling, it was proposed that the plasticizer be blended with the slip after the water has coated the particles.
Accordingly, a new slip was prepared by vacuum milling the recycled slip of l!.xample II, silicon carbide power, and deflocculant; removlng the milling balls; adding the pre-mixed binder and plasti~izer, and then vacu~ rolling the mixture in a nylon jug overnight. The resulting slip had a significantly lower viscosity (~bout 9900 cps at 0.6 rpm). Upon casting, it was observed that the frequency of pin holes was sharply reduced but not eliminated.
EXAMPLE I~
This example was performed ir, substantia1 accordance with Example I, except that a separate burnout step was incorporated be~ore the recrystallizing step.
The green wafers produced in substantial accordance with Example I were set on recrystallized silicon carbide plates and the plates were stacked with silicon c-arbide spacers in between to form columns. ~hese colu~ns were exposed t~ a r at a temperature of 200~C and a vacuum of c inches Hg for 6 hours to promote burnout of the plasticizer. ~his treatment decomposed the plastici~er and cured the kinder. The removal of the W O 97/36843 PCT~JS97/05188 plasticlzer elimi~ated wafer sticklng and the subsequent need fol- graphite paper and hence the slid ng problem described in Example I.
EXAMPLE ~
Green wafers cast and subject to plastic.zer burr.olt as ln Example IV were si~acked 1n groups of ten between a top and a boltom surface gr~und SiC plate, but withou~ any graphite paper inserted therebet~een. The wafers were ther. ~ecrystallized as in Example I.
It appears t~at the wafers did nc~t slide during recrystallization. Their measure of flatness was less than about 0.005". In addition, sticking between the wafers was minimal, as the wafers could be easil~ pried apart. Breakage from prying apart stuck wafers was only about 0-6%.
EXAMPLE VI
A bimodal po~der consisting of about 43 w/o fine silicon carbide and about 40 w/o coarse silicon carbide and a deflocculant were mixed with about 13 w/o deionized water and 0.01 w/o NaOH solution. The slip was ball milled under vacuum (25 in. Hg) for 12 hours at 200 rpm. About 3 w/o of a latex emulsion binder was added and the slip was mixed without vacuum for 2 hours at 25 rpm. A trace amount of a surfactant (0.1 w/o) was also added to improve the wetting behavior of the slip on the mylar carrier. The total solids content of the slip was between 85 w/o and 90 w/o and the viscosity was about 20,000 cps at 0.6 rpm.
The tape casting table described in Example I was used to tape cast the slip. Tapes with a green thickness of 0.5-1.0 mm were obtained wlth the following cast.ng parameters: a reservoir height of 10 to ~0 mm, carrLer speed of 0.4 to 1.1 m/mln; doctor blade height of 0.4 - 3 7 mm; and under bec hea ing temperat~re of 35-45 C. Depending uFon ~he thickness, 3~ the tapes req~ir~d a total dr~ing time c~ 2~ -- 40 mlnuteC~
res~llting in a l~-15~ shrinkage during drying. The areen tape W O 97/36843 PCTrUS97/05188 was ~ollected fro~ the exit end of the table and sectioned intc, 1 meter long sheets. Wafers were cut from the green tape in diameters of 100~ 00 mm. The green wafers were subject to binder burnout at 200 C for 8 hours in air, and recrystallized as in Example ~. ~'he densities of the recrystàllized wafers were between 2.3 cnd 2.4 g/cc.
The recrysta]lized wafers were then siliconized and sandblasted through conventional means. The densities of the resulting wafers were found to be about between 2.91 and 2.98 o g/cc.
Preliminary studies of pressing the flexible green tapes with a hydraulic press at 8000 psi followed by binder burnout as in Example IV and recrystallization as in Example ~ have demonstrated an increase in density up to 2.5'8 g/cc while the average pore size was reduced to 6 microns. Similar results were obtained by other compaction methods such as cipping at 30,000 psi or passing the tape through a rolling operation.
Although the above SiC ceramic has utility as a dummy wafer, there is a;so interest in making substrates for the deposition of diamond films to be used in thermally enhanced plastic packages or semiconductor devices. For such applications, the SiC substrate must be strong and thin. It is believed that, in order to meet these requirements, the SiC
ceramic should ha~le a thickness of between about 0.1 and 0.6 um, and its strength should be at least 80 MPa. Unfortunately, the 0.5 mm thic~ tape cast SiC wafers described above in Examples I-VI, wh:ch were found to have a DLS pore diameter of about 3 um, a D~s ?ore diameter of 12 um, and a biaxial flexural strength of only about 50-70 MPa , do not meet these req~irements. Therefore, there is a need for a tape cast recrystallized si~icon carbide ceramic (preferably having a thickness of be~ween 0.1 and 0.~ mm) having a strength of at least 80 MPa.
EXAMPLE ~II
In this example, the effect of solely usi~g a fine silicon carblde powaer was examined. In par icular, t~e prccedure of Example I above were substantial_y followed, except that the coarse silicon car:bide f-action was eliminated.
The resultin(~ tape exhibited significant shrinkage and cra.cking during dry ng.
EXAMPLE ~
In this example, both the fine/coarse ratio of the SiC
powder and the sintering temperature were mod~lfied from those ol~ the above Examples (which had about 52 wt~ fines an~ 48 wtCo coa.rse particles, and were sintered at about 1950~C).
The procedure of Example VI above was substantially fol.lowed, except that the three different flne/coarse SiC
powder ratios (35i'65, 50~50, and 65/35) and three different sintering temperatures (1700~C, 1850~C, and 2000~C) were used.
In particular, the coarse grain fraction was first sieved through a 50 um screen to remove its coarse tail. Initial water contents of 15~ was used for the :35~ fines formulation, while l2~; was used for the remaining two formulations. Deflocculation curves were prepared for each formulation in according with ISO
manufacturing procedure, and the optimal deflocculation points were determined. Next, lO00 gram batches of he three formulations were prepared. The amount of B-1035 binder used waci based upon the surface area of the grains, and a conventional non-:onic surfactant was used at a level of 2 wt~
of the binder. 'nhe optimal water fraction for each formulation was determined by mak.ing i.ncremental additions unt:il a slip viscosity o:f 25 ~dia.l) at 0.6 rpm (as measured by a E~rookfield viscometer with a LV-2 s~indle) was attained. The solids loadings for these formulat onc were all between 85 wt~
al1cl about 87 wt!'- 66-68 volgO).
The three fo:-mulations were de-aired in . vacuum bel.l jar al l5"Hg for 15 mLnutes. The batches were ~.a~e cast at a c~rrier speed of .'0"/minute at a b.ade heigkt of 0.0l6" and a tem~erature of l5:)~F. The slurry was pol.red manually ~nto the reC;ervoir~ mainta.ning a reservoir height cf ~.0 cm. The W 097136843 PCTrUS97/05188 average dried green tape thickness for each batch was between about 0.128 and 0 132 inches, and was determined by cutting out 100 mm diameter g~~een wafers and meas~ring the thickness of the four quadrants.
The wafers r:'~Om each batch were cross-roiled (i.e., rolled, rotated 91)~, and rolled again) through spaced 15 cm diameter, hardened steel, primary rollers havlng a roller gap of 0.005". The sLgnificant elastic behavior of the wafers typically results in only about 5-10% compactlon from the green o thickness. The a~erage rolled wafer thickness for each batch was determined by measuring the thickness of the same wafers at the four quadrants.
The wafers were loaded into the sintering furnace on SiC
setter plates. Binder burnout was conducted at a ramp rate 0.2~C/minute and an 8 hour soak at 240''C. The wafers from each batch were divided into three groups and then sintered at the desired temperatu e. Each of the sintering cycles started with a 3~C per minute ramp to 500~C, with an intermediate soak for 1 hour to complete -he burnout cycle. The runs were then ramped at a rate of 5~C/minute to the final soak temperature with a soak time of three hours. Each of the three runs was conducted under Argon at a pressure of 0.6 torr for the entire cycle.
Sintered den,ity and pore si~e distribution was measured by mercury intrusion porosimetry. The biaxiaL flexural strengths were measured by a ring-on-ring configuration.
The sintered densities of the resulting materials are shown in Table I.
W O 97/3~843 PCT~US97105188 Table _ SinteredDensities(~cc) 35%finel65% coarse 50%fine/50%coarse 65%finel35%coarse 17~0~C 2.. -4 2.31 2.21 1~50~C 2.;4 2 41 2.32 2l~)0~C ~ 1 2.4~ 2.36 Examination of Table I reveal.s that dens_ties increase both with increasing coarse fraction and increasing sintering temperature. Increasing the fines content of the formulation requires the use of additional binder and li~uid during green body formation in order to prevent. cracking. The resulting decreased density of the green body leads to a decreased density in the fired body. Similarly, it i.s believed that inrreasing the sintering temperature produces more sintering activity, grain coarsening, and neck formation, and therefore higher densities.
The D50 pore diameters of the resllting materials are shown in Table II.
Table II
D5~,poreDiarneters(um) 35%fine/65% coarse 50%fine/50%coarse 65%fine/35%coarse 17()0~C 2.:~ 1.7 1.8 18;0~C 6.~) 4.0 33 20()0~C 11.0 10.? 7 9 Examination of this table reveals thar pore dlameter increases with sintering temperature and coarse fraction.
The characteristlc flexural strengths ~f the resulting m.aterials, as determined by Weibull analysis, are shown in Table III.
Table III
Flexural Streng:th (MPa) 35%finel65% coarse ~0%fine/50%coarse 65%finel3~%coarse 1700~C 4~ 7 95.0 103 5 1~50~C 31 0 65.4 81 3 2000~C 47 8 49 5 47. 1 Examination of the above table reveal;, the surprising result that lower sintering temperatures and higher fine fractions lead to higher strength wafers.
The results are surprising in two respects. First, it is clear that different phenomenon are occurrlng at different o fine~coarse fractions and different temperatures. In particular, in the Table III studies in which the SiC powder was kept constant at 35~ fines/ 65% coarse, increasing the sintering temperature from 1850~C to 2000~C increases the flexural strength from 31 MPa to 47.B MPa. It is believed, in relatively coarse formulations, the extent of inter-grain necking (which ircreased with sintering temperature) is the dominant factor. In contrast, in the studies in which the powder was kept constant at 65% fines~ 35% coarse, increasing the sintering temperature leads ~o l~wer strength. It is believed that, ir. relatively fine powder formulations, pore size (which increases wlth sintering temperature) becomes the critical flaw.
Similarly, ln the studies in which the sintering temperature was ~ept constant at 2000~C, increasing the fine fra~tion from 35~ to 65~ had relatively no effect on the flexural strength. It is believed that, in high fired embodiments, the large pores created by the grain coarsening are the strength limiting feature. In contrast, in the studies -- in which the sintering temperature was kept constant at 1700~C
increasing the f:ne fraction from 35~O to 65~ increased the flexural strength from ~7.7 to 103.5 MPa.
W O 97/36843 PCT~US97/05188 In sum, these studies do no1 fcllow the traditional expectations that slmply increasing sintering temperature and de(reaslng grain .ize leads to increased strer~gth.
Second, the strengths recorded in Table III do not 5 correlate well wl h traditional factors typically correlating poc,itively with s rength (i.e., densi_y and -'50 pore d:iameter). For example, the example having 35~ fines and sintered at 1700-C possessed a fairly ~ow strength (47.7 MPa) dec;pite its relat:vely small 2.2 ~Lm Ds pore diameter. Also, lo the fired bodies having the highest densities (i.e., above 2.5 g/c:c) recorded some of the lowest strengths '31.1 MPa and 47.8 MPa~.
The sintered bodies were also analyzed for D85 pore d:iameter, which represents the size which exceeds exactly 85~
of the pores. The D8s pore diameter of the examples are shown in Tàble IV belowj together with the corresponding flexural strength in parentheses:
Table IV
D8s Pore Diameters, in um 'flexural strengths in parentheses) 35%fine/65% coarse S0%finel50%coarse 65%fine/35%coarse 1 J00~C 5 1 (47.7MPa) 2.0 ~95.0 MPa) 2.1 (103.5 MPa) lg50~C 90(31.0M:Pa) 4.8 (6~4MPa) 3.9(81.3MPa) 2()00~C 12 5 (47 8 MPa) 12 .2 (49 5 MPa) 9.8 (47.1 MPa) Comparison o- the D85 pore diameters with the flexural strengths reveals a very strong correlation between D8 pore dklmeter and flexllral strength. For example, only examples h,~ving a D~s pore ~iameter of no more than 4 um possess a flexural strength greater than 80 MPa, and the examples with the smallest D~s p~re diameters (about: 2 u~ nave the largest flexural strength Further, the :low strength (47.7 MPaj of the -example havlng the 2.2 um Dso pore diameter lS now explained by its unexpectedly larger D~s pore s1~e 5.7 um~
Therefore, i- is now clear that Froviding a D8s pore diameter of less han 4 um, and more Freferably less than 2.5 W097/36843 PCT~S97/0~188 um is critical ~o provlding a flexura~ strength of at least 80 MPa, and preferably at least 95 MPa, and that using either :
i) at least ,0c~, fines (preferably at least 60%) and sintering at no more than 1750~C , or ii) at least 60% fines and sintering at r-lo more than 1850~C ~preferably 1750~C) is also critical ~o producing these low D8 pore diameter values which produce the high strengt:h SiC wafers.
Therefore, in accordance with the present invention, there o is provided a ceramic wafer consisting essentially of recrystallized silicon carbide grain and having a 1~5 pore diameter of no more than 4 um, and a biaxiaL flexure strength of at least 80 MP~, wherein between about 50 wt% and 70 wt% of the silicon carbiie comprises fine grains having a grain size of no more than 5 um, and between 30 wt~ and 50 wt~ of the SiC comprises coarse grains having a grain size of at least 20 um .
In some em~o~iments, the wafer has a density of no more than 2.33 g/cc, preferably a D85 pore diameter of no more than 3 um, and a stren~th of at least 90 MPa.
In some embo~iments, the wafer has a density of no more than 2.2~ g/cc an~ a D8s pore diameter of no more than 2.5 um, a strength of at least 100 MPa, and between about 60 wt% and 70 wt% of the silicon carbide comprises fine gra~ns having a grain size of no more than 5 um.
In some emboiiments, the ceramic has a thickness of between 0.1 mm and 0.3 mm, is preferably unground, and more preferably has a D50 pore diameter of at least about 1 um.
In some embodiments, between 30 wt% and 50 wt% of the SiC
comprises coarse grains having a gra:in size ~f at least 30 um.
Also in accordance with the present invention, there is provided a process for making high st ength, tape cast SiC
wafers, comprising the steps of:
a) providing a formulation consi~ting essentially of silicon car~ide powder and water, wherein:
CA 02246803 l998-08-l8 i) betw~en 50 wtC~, and 70 wt:90 ~f hf- SlC powder has d particle size of no more than 5 um . and ii) bet~een 30 wt% and S0 wt~ of t'le SiC powder has a parti~le size of at least 30 ~lm, b~ tape cast;ng the formuiation to produce a tape cast body, c) drying th~ tape cast body to produce ~ green body having thickness of between about 0.1 mm and 0.6 mm, and o d) sintering the green body at a temperature of no more than 1850~C, to form a re-rystallized silicon carbide wafer having a density of no more than 2.3' g/cc and a strength of at leas~ 80 MPa.
In some embol~iments, the sintering temperature is no more than 1750~C, and the recrystallized silicon carbide wafer has a I)~5 pore size of no more than 2.S um and a strength of at least 90 MPa.
In some embodiments, the SiC powder comprises at least 60 Wt1~' SiC particles having a size of no more than 5 um, and the re(rystallized silicon carbide wafer has a D8s pore diameter of no more than 3 um.
In some embodiments, the sintering temperature is no more t]na,r. 1750~C, the ';iC powder comprises at least 60 wt% SiC
p~rticles having l size of no more than 5 um, and the re(-rystallized silicon carbide wafer has Dfls pore size of no mo]-e than 2.5 um, a density of no more than 2.25 g/cc and a s~rength of at l-ast 100 MPa.
In some emboiiments, the sinterirg temperature is no more t!harl 1700~C, the ',iC powder compri es at least 65 wt~ SiC
p,~"-ticles having ~ size of no more thcn 5 um" and the rlecrystallized silicon carbide wafer has a st~ength of at lleast 103 MPa.
In some embo~iments,. the procesC includes the additional step o compactin~ ~he tape cast body to reduce its thickness by at least 5~c (preferably at least 1(~) prio- to sintering.
W 097136843 PCT~US97/05188 EX~MPLE IX:
This example examines the effect cf rol~ ompactlng the tape cast green Si~ wafer.
Example VII~ above was substantia ly followed, except that the ~ormulation comprised 57% fine/43% coarse SiC partic~es, the sintering temperature was 1850~C, and the dried tape cast wafer was not subjected to roll compaction.
The resulting wafers had a density of about 2.36 gicc, a D~5 pore diameter cf about 8.5 um, and ~ flexural strength of o only about 35-45 MPa.
This example can be usefully compared with the 50% fine and 65% fine embodiments of Example V[II above which were roll compacted and sintered at 1850~C, and possesseci flexural strengths of 65.4 MPa and 81.5 MPa respectively. Since each of the previous examples (one of which ha~ slightly less fines and one of which had slightly more fines) had twice the flexural strength, it is clear that roll compacting the tape cast wafer had a slgnificantly positive effect upon the flexural strength of the recrystallized wafer. Without wishing to be tied to a theory, it is believed that roll compacting has the effect of decreasing the interparticle spacing to promote more effective neck formation during recrystallization.
Therefore, also in accordance with the present invention, there is provided a process comprising the steps of:
a~ providing a formulation consisting essentially of a ceramic powd~r, binder and water, b) tape casting the formulation to produce a tape cast body, c) drying the tape cast body to produce a green body, d~ compactinc the green body to reduce its thickness by at least 1~%, and d) sintering the green body.
For the purposes of the present inventlon, po~e diameter is determined by mercury intrusion porosimetry.
Claims (31)
1. A wafer consisting essentially of recrystallized silicon carbide grains, the wafer having a thickness of between 0.5 and 1 mm, a diameter of at least 100 mm, and a strength of at least 30 MPa [as measured by ring-on-ring biaxial flexure], the wafer having a porosity wherein at least 85% of the pores are no larger than 14 micrometers.
2. The wafer of claim 1 having a density of at least about 2.15 g/cc, wherein the silicon carbide grains are characterized by a bimedal grain size distribution and form a homogeneous microstructure, and the surface of the wafer is unground and has a flatness [as measured by the total spread between the minimum and maximum deflection from a flat granite plate] of less than 130 um.
3. The wafer of claim 1 wherein a least 85% of the pores are no larger than 10 micrometers.
4. The wafer of claim 1 wherein at least 95% of the pores are larger than 3 micrometers.
5. The wafer of claim 1 wherein the silicon carbide consists of grains having a size of between 2 and 5 um in the range of 40 to 60 w/o [weight percent] of the wafer and grains having a size of between about 30 and 200 microns in the range of between 40 and 60 w/o [weight percent] of the wafer.
6. The wafer of claim 1 wherein the silicon carbide consists of grains having a size of between 2 and 5 um in the range of between 50 and 55 w/o [weight percent] of the wafer and between grains having a size of between about 30 and 100 microns in the range of between 45 and 50 w/o [weight percent] of the wafer.
7. The wafer of claim 1 wherein the surface of the water is unground and has a flatness of less than 130 um.
8. The wafer of claim 1 having a thickness of between 0.625 mm and 0.725 mm and a diameter of between 150 mm and 200 mm.
9. A process for making a crack-free green silicon carbide sheet, comprising the steps of:
a) forming an aqueous slip comprising 1-5 w/o [weight percent] binder component, and 80-90 w/o [weight percent]
of a ceramic powder consisting essentially of silicon carbide, b) tape casting the slip to produce a wet green sheet having a thickness of between 0.6 and 1.2 mm and c) evaporating essentially all the water from the wet green sheet to produce a crack-free dry green sheet having a thickness which is at least 80% of the wet green sheet.
a) forming an aqueous slip comprising 1-5 w/o [weight percent] binder component, and 80-90 w/o [weight percent]
of a ceramic powder consisting essentially of silicon carbide, b) tape casting the slip to produce a wet green sheet having a thickness of between 0.6 and 1.2 mm and c) evaporating essentially all the water from the wet green sheet to produce a crack-free dry green sheet having a thickness which is at least 80% of the wet green sheet.
10. The process of claim 9 wherein the density of the green sheet is at least 2.3 g/cc.
11. The process of claim 9 wherein the binder comprises between 3 and 5 w/o [weight percent] of the slip and has a glass transition temperature of less than 22 ~.
12. The process of claim 9 further comprising the steps of:
d) forming a shape from the dried green sheet to produce a silicon carbide green wafer, e) recrystallizing the silicon carbide green wafer to produce an unground recrystallized silicon carbide wafer having a thickness of between 0.625 mm and 0.725 mm and a measure of flatness of less than 130 micrometers.
d) forming a shape from the dried green sheet to produce a silicon carbide green wafer, e) recrystallizing the silicon carbide green wafer to produce an unground recrystallized silicon carbide wafer having a thickness of between 0.625 mm and 0.725 mm and a measure of flatness of less than 130 micrometers.
13. The process of claim 12 further comprising the step of:
f) grinding the silicon carbide wafer to reduce its thickness by no more than 5%.
f) grinding the silicon carbide wafer to reduce its thickness by no more than 5%.
14. A process for making a silicon carbide body, comprising the steps of:
a) forming a slip comprising water and a ceramic powder consisting essentially of silicon carbide particles having bimodal grain size distribution, and b) tape casting the slip to produce a wet green sheet, c) drying the wet green sheet to produce an dry green sheet having a thickness of between about 0.625 and 0.725 mm, d) forming a shape having a diameter of at least 200 mm from the dry green sheet to form a green body, and e) recrystallizing the green body to form a silicon carbide body having a homogeneous microstructure, a diameter of at least 200 mm and a flatness of less than 130 micrometers.
a) forming a slip comprising water and a ceramic powder consisting essentially of silicon carbide particles having bimodal grain size distribution, and b) tape casting the slip to produce a wet green sheet, c) drying the wet green sheet to produce an dry green sheet having a thickness of between about 0.625 and 0.725 mm, d) forming a shape having a diameter of at least 200 mm from the dry green sheet to form a green body, and e) recrystallizing the green body to form a silicon carbide body having a homogeneous microstructure, a diameter of at least 200 mm and a flatness of less than 130 micrometers.
15. Use of a silicon carbide sacrificial wafer, comprising the steps of:
a) providing a silicon wafer diffusion boat having slots for the insertion of wafers, b) inserting into a slot of the silicon wafer diffusion boat a silicon carbide sacrificial wafer as described in claim 1, said sacrificial wafer having a coating of a dielectric material thereon c) inserting a silicon wafer into another slot in the boat, and d) depositing a dielectric material on the surface of the silicon wafer at a temperature of no more than 1000 °C.
a) providing a silicon wafer diffusion boat having slots for the insertion of wafers, b) inserting into a slot of the silicon wafer diffusion boat a silicon carbide sacrificial wafer as described in claim 1, said sacrificial wafer having a coating of a dielectric material thereon c) inserting a silicon wafer into another slot in the boat, and d) depositing a dielectric material on the surface of the silicon wafer at a temperature of no more than 1000 °C.
16. A ceramic wafer consisting essentially of recrystallized silicon carbide grain, the wafer having a D85 pore diameter of no more than 4 um, and a biaxial flexure strength [as measured by ring-on-ring biaxial flexure] of at least 30 MPa, wherein between about 50 wt% and 70 wt% of the silicon carbide comprises fine grains having a grain size of no more than 5 um, and between 30 wt% and 50 wt% of the silicon carbide comprises coarse grains having a grain size of at least 23 um.
17. The ceramic of claim 16 having a density of no more than 2.33 g/cc.
18. The ceramic of claim 17 having a D85 pore diameter of no more than 3 um, and a strength of at least 9) MPa.
19. The ceramic of claim 18 having a density of no more than 2.25 g/cc and a D~5 pore diameter of no more than 2.5 um, and a strength of at least 100 MPa.
20. The ceramic of claim 19 wherein between about 60 wt% and 70 wt% of the silicon carbide comprises fine grains having a grain size of no more than 5 um.
21. The ceramic of claim 16 wherein the wafer is unground.
22. The ceramic of claim 21 having a thickness of between 0.1 mm and 0.6 mm.
23. The ceramic of claim 16 wherein the wafer has a D50 pore diameter of at least about 1 um.
24. The wafer of claim 16 wherein between 30 wt% and 50 wt% of the SiC comprises coarse grains having a green size of at least 30 um.
25. A process for making high strength, tape cast SiC wafers, comprising the steps of:
a) providing a formulation consisting essentially of silicon carbide powder and water, wherein:
i) between 50 wt% and 70 wt% of the SiC powder has a particle size of no more than 5 um, and ii) between 30% and 50% of the SiC powder has a particle size of at least 20um, b) tape casting the information to produce a tape cast body, c) drying the tape cast body to produce a green body having a thickness of between about 0.1 mn and 0.6 mm, and d) sintering the green body at a temperature of no more than 1850°C, to form a recrystallized silicon carbide wafer having a density of no more than 2.35 g/cc and a strength [as measured by ring-on-ring biaxial flexure] of at least 80 MPa.
a) providing a formulation consisting essentially of silicon carbide powder and water, wherein:
i) between 50 wt% and 70 wt% of the SiC powder has a particle size of no more than 5 um, and ii) between 30% and 50% of the SiC powder has a particle size of at least 20um, b) tape casting the information to produce a tape cast body, c) drying the tape cast body to produce a green body having a thickness of between about 0.1 mn and 0.6 mm, and d) sintering the green body at a temperature of no more than 1850°C, to form a recrystallized silicon carbide wafer having a density of no more than 2.35 g/cc and a strength [as measured by ring-on-ring biaxial flexure] of at least 80 MPa.
26. The process of claim 25, wherein the sintering temperature is no more than 1750°C, and the recrystallized silicon carbide wafer has a D85 pore diameter of no more than 2.5 um and a strength of at least 90 MPa.
27. The process of claim 25 wherein the SiC powder comprises at least 60 wt% SiC particles having a size of no more than 5 um, and the recrystallized silicon carbide wafer has a D85 pore diameter of no more than 3 um.
28. The process of claim 25 wherein the sintering temperature is no more than 1750°C, the SiC powder comprises at least 60 w% SiC particles having a size of no more than 5 um, and the recrystallized silicon carbide wafer has D85 pore diameter of no more than 2.5 um, a density of no more than 2.25 g/cc and a strength of at least 100 MPa.
29. The process of claim 25, wherein the sintering temperature is no more than 1700°C, the SiC powder comprises at least wt% SiC particles having a size of no more than 5 um, and the recrystallized silicon carbide wafer has a strength of at least 103 MPa.
30. The process of claim 25 including the additional step of compacting the tape cast body to reduce its thickness by at least 5% prior to the step of sintering.
31. A process for making high strength, tape cast ceramic wafers comprising the steps of:
a) providing a formulation consisting essentially of a ceramic powder, binder and water, b) tape casting the formulation to produce a tape cast body, c) drying the tape cast body to produce a green body having a thickness, d) compacting the green body to reduce its thickness by at least 5%, and e) sintering the green body.
a) providing a formulation consisting essentially of a ceramic powder, binder and water, b) tape casting the formulation to produce a tape cast body, c) drying the tape cast body to produce a green body having a thickness, d) compacting the green body to reduce its thickness by at least 5%, and e) sintering the green body.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/625,383 | 1996-04-01 | ||
| US08/625,383 US5904892A (en) | 1996-04-01 | 1996-04-01 | Tape cast silicon carbide dummy wafer |
| PCT/US1997/005188 WO1997036843A1 (en) | 1996-04-01 | 1997-03-31 | Tape cast silicon carbide dummy wafer |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2246803A1 true CA2246803A1 (en) | 1997-10-09 |
Family
ID=29422954
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2246803 Abandoned CA2246803A1 (en) | 1996-04-01 | 1997-03-31 | Tape cast silicon carbide dummy wafer |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2246803A1 (en) |
-
1997
- 1997-03-31 CA CA 2246803 patent/CA2246803A1/en not_active Abandoned
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