Method of making a silicon carbide product

A method of making a perforated silicon carbide product in a shorter time and with less labor work to achieve a lower cost and high yield. Openings 13a, 13b are previously formed in a mold 11, and inserts 14 are inserted into the openings to project toward the inside of the mold. A slurry 15 composed of a mixture of a silicon carbide powder, an organic binder and water is put in the mold to conduct a slip casting molding method whereby a molded green product with perforation 16 is obtained.

The present invention relates to a method of making a silicon carbide 
product having a perforated structure. In particular, it relates to a 
method suitable for making a silicon carbide product used for a heat 
treatment apparatus for semiconductors. 
In recent years, there has been used silicon carbide of high purity as 
material for members for heat treatment apparatus for semiconductors, 
e.g., a wafer boat, a cantilever (an insert/loading member supporting a 
wafer boat) and so on because the silicon carbide of high purity can 
prevent contamination of a semiconductor by impurities and has excellent 
heat-resistant and corrosion-resistant properties in comparison with 
quartz glass or the like. 
Generally, a silicon carbide product of this type is prepared as follows. A 
slurry obtained by adding an organic binder and pure water to a silicon 
carbide powder of high impurity is poured in a mold to obtain a green 
product; a machining is conducted to the green product if necessary; the 
green product is sintered to obtain a sintered product; molten silicon is 
impregnated to the sintered product according to demand; and a machining 
work is conducted to the sintered product. 
In silicon carbide products, there is a product provided with an opening 
for the purpose of reducing the weight and homogeneous reaction gas flow. 
In a wafer boat as a one-piece molded product, for instance, an opening or 
openings are formed in a side wall in order to reduce the weight and allow 
gases to flow uniformly in a space between wafers. 
In conventional methods of making a silicon carbide product having a 
perforated structure, it was general that a machining work was conducted 
to a green product itself or after the sintering of the green product. 
Accordingly, there was problems that a time and labor work were necessary 
for the machining, and further, breakage was apt to occur during machining 
work, whereby yield of products was decreased and cost for making was 
increased. 
It is an object of the present invention to provide a method of making a 
silicon carbide product having a perforated structure in a shorter time 
and with less labor work to achieve a lower cost and high yield. 
In accordance with the present invention, there is provided a method of 
making a silicon carbide product comprising (1) a slurry preparing step of 
preparing a slurry composed of a mixture of a silicon carbide powder, an 
organic binder and water, (2) a molding step wherein the slurry is put 
into a mold made of a water absorbing material; a part of the slurry in 
contact with the inner surface of the mold is dehydrated and solidified; a 
part of the slurry which is not solidified and remains in a liquid 
suspension state is discharged; and a solidified green product is removed 
from the mold, and (3) a sintering step of sintering the green product, 
said method being characterized in that an opening is previously formed in 
the mold; an insert made of a non-water absorbing material is inserted in 
the opening so as to project in the mold; the slurry is poured into the 
mold to dehydrate and solidify a part of the slurry which is in contact 
with the inner surface of the mold; a part of the slurry which is not 
solidified and remains in a liquid suspension state is discharged; the 
insert is removed; and the solidified green product is removed from the 
mold whereby a perforated green product is obtained.

In accordance with the present invention, an opening is previously formed 
in a mold, and an insert made of material having non-water-absorbing 
properties is inserted in the opening so as to project to the inside of 
the mold. A slurry is poured into the mold. After the slurry is held for a 
while, a part of the slurry which is in contact with the inner surface of 
the mold is dehydrated and solidified due to the water absorbing 
properties of the mold. However, a part of the slurry in contact with the 
insert is not solidified and remains in a slurry state because the insert 
dose not have water absorbing properties. Then, the part in a slurry state 
is discharged along with another part of unsolidified slurry. When the 
insert is removed and a solidified green product is taken out from the 
mold, a green product having a perforated structure is obtainable. The 
shape of the opening formed by this method is the same as the shape of the 
insert projecting toward the inside of the mold. The green product is 
subjected to machining work according to demand, and is sintered whereby a 
sintered silicon carbide product having a perforated structure can be 
prepared with high yield and at a low cost. 
In accordance with a preferred embodiment of the present invention, the 
insert is made of resin having elasticity, preferably, a closed-cell type 
foam resin. In a case that the insert is made of resin having elasticity, 
when a part of the slurry in contact with the inner surface of the mold 
slightly shrinks due to dehydration and solidification, a certain degree 
of shrinkage is permitted owing to the elasticity of the insert whereby 
occurrence of a crack in the green product can be prevented. However, even 
in a case of using an insert without having elasticity, occurrence of a 
crack can be prevented to some extent by contriving the shape of the 
insert, e.g., the insert having a convergent taper portion. 
Further, the method of the present invention is preferably applied to a 
method of making a silicon carbide product used for a heat treatment 
apparatus for semiconductors, in particular, a method of making a wafer 
boat. 
The silicon carbide powder as raw material can be prepared by a known 
method such as Acheson method, CVD method, a low-temperature synthesizing 
method or the like. However, powder prepared according to Acheson method, 
which is highly purified by an acid treatment or the like is preferably 
used from the viewpoint mainly of the cost performance of raw material. 
The slurry is prepared by adding and mixing an organic binder and pure 
water to the refined silicon carbide powder of high purity. As the organic 
binder, methyl cellulose, polyvinyl acetate emulsion, acrylic resin 
emulsion, dextrin or the like is preferably used, for example. 
A green product is formed with the slurry by a so-called slip casting 
method. 
FIG. 1 shows an example of shaping steps of such method. FIG. 1a shows a 
state wherein a slurry 15 as raw material is filled in the mold; FIG. 1b 
shows a state wherein a part of the slurry in contact with the inner 
surface of the mold is dehydrated and solidified; FIG. 1c shows a state 
wherein a part of the slurry which is not solidified is discharged and 
inserts are removed; and FIG. 1d shows a green product which is removed 
from the mold. 
In FIG. 1, reference numeral 11 designates a mold made of material having 
water absorbing properties, which is constituted by a lower mold 11a and 
an upper mold 11b. Although plaster is preferably used as the material for 
the mold 11, porous ceramics having an open-cell structure, resin having 
water absorbing properties or the like may be used. The mold 11 has in its 
inside a space 12 whose shape corresponds to the shape of a product to be 
shaped. The lower mold 11a of the mold 11 is provided with an opening 13a 
whose shape corresponds to an opening formed in the lower portion of the 
product to be shaped and openings 13b whose shape corresponds to openings 
formed in a side portion of the product. 
As shown in FIG. 1a, inserts 14 of non-water absorbing properties are 
fitted to the openings 13a, 13b of the mold 11. The material of the 
inserts is such that when the slurry 15 is filled in the mold in which the 
inserts are fitted to the openings of the mold and is held for a while, a 
part of the slurry in contact with the inserts is not substantially 
solidified although a part of the slurry in contact with the mold is 
solidified. Specifically, metal, ceramics and resin are exemplified for 
the material of the inserts. In particular, resin is preferable as the 
inserts because it is easy in handling. As concrete examples, there are 
polypropylene resin, acrylic resin, vinyl chloride resin, polyester resin, 
polyurethane resin, silicone resin, fluorine resin and epoxy resin. It is 
more preferable to use resin having elasticity (e.g. , elastic rubber 
having a hardness of 40 or lower defined in JIS K6253), more preferably, a 
closed-cell type foam resin. Further, the shapes of the openings 13a, 13b 
and the inserts 14 preferably have a convergent taper portion converging 
toward the inside of the mold. Use of the inserts 14 of an elastic 
material which has the above-mentioned tapered shape can permit shrinkage 
of the solidified product when a part of the slurry in contact with the 
inner wall of the mold is dehydrated and solidified. Thus, occurrence of 
crack in the green product is prevented. Further, the inserts 14 can be 
easily removed in the step described hereinafter. 
Then, the slurry 15 prepared by the method described above is filled in the 
mold 11 after the inserts 14 as described above have been fitted to the 
openings 13a, 13b of the mold 11. 
In FIG. 1b, the slurry 15 is dehydrated at a portion in contact with the 
inner wall of the mold 11 due to the water absorbing properties of the 
mold 11; a part of the slurry at the portion is gradually aggregated, and 
a solidified layer 15a is formed along the shape of the inner surface of 
the mold 11. However, such solidified layer 15a is not formed at portions 
where the slurry 15 is in contact with the inserts 14 since the inserts 
have not water absorbing properties. 
Then, an unsolidified portion of the slurry 15 is discharged, and the 
inserts 14 are removed from the opening 13a, 13b whereby a green product 
16 is left and dried as a result of the dehydration and solidification of 
the slurry 15 having a shape corresponding to the shape of the inner 
surface of the mold. 
Finally, the upper mold 11b of the mold 11 is removed, and thus obtained 
green product 16 is taken out from the mold 11. Since the green product 16 
slightly shrinks during the dehydration and solidification, it can be 
easily released from the mold 11. The thus obtained green product 16 has 
an opening 17a at the bottom and openings 17b at the side portion as shown 
in FIG. 1d. The method as described above is an example of application to 
shaping one-piece shell type wafer boat (so-called cassette boat) wherein 
the green product 16 has a bottom portion 18 with an opening 17a, a step 
portion 19 which rises from outer edge of the bottom portion 18 and curves 
outwardly, and an outer peripheral portion 20 with openings 17b, which 
rises upwardly from outer end of the curved portion of the step portion 
19. The free end portion 21 of the outer peripheral portion 20 is slightly 
extended inwardly. Numeral 22 designates portions of the green product 16, 
which oppose in the direction perpendicular to the paper surface in the 
drawing. 
The green product 16 after having been shaped is subjected to cutting 
operations in a green state according to demand. The cutting operations 
are conducted to a portion to which accuracy is not strictly required, in 
a green state of the green product since cutting operations are easy. In 
this specific case, the portions 22 which are needless in the final 
product are cut off from the green product. In this case, the circular 
shape of the openings 17a, 17b formed in the green product is the same as 
the shape of the outer periphery of the inserts. Accordingly, if the outer 
periphery of the inserts is determined to be a desired shape, time and 
labor for cutting operations can substantially be reduced. After the 
cutting operations to the green product 26 have been finished, the green 
product 16 is sintered. Temperature and time for sintering may be those 
usually used for silicon carbide material. For instance, it is preferable 
to sinter the product at 1,500.degree. C.-2,100.degree. C. for 30 min.-400 
min. In sintering, when no sintering aid is added, shrinkage does not 
substantially occur but only bonding between silicon carbide particles 
takes place, and continuous pores of a porosity of about 10%-30% exist. 
Accordingly, when a mechanical strength is required, it is preferable to 
close the pores by impregnating molten silicon of high purity by capillary 
action. 
Then, silicon is impregnated to the sintered product if necessary, and 
machining operations are conducted to it to obtain a final product. In 
this specified case, cutting operations are conducted to an inwardly 
projected corner portion near the step portion 19 and the free end portion 
21 inwardly projected from the outer peripheral portion 20 so that grooves 
23, 24 for inserting and supporting semiconductor wafers W are formed. 
When the impregnation of silicon is conducted, a part of the silicon 
projects from the pores in the outer surface of the sintered silicon 
carbide product. Accordingly, it is necessary to remove the projecting 
silicon by means of sand-blasting or the like. Further, silicon exposed on 
the surface of the sintered silicon carbide product as a result of the 
impregnation of silicon has poor corrosion resistance properties to a 
treatment such as acid cleaning or the like. Accordingly, a thin film of 
silicon carbide may be formed on the outer surface by CVD method if 
necessary. 
A wafer boat 10 prepared by the above-mentioned method has openings 17a, 
17b at its bottom portion 18 and side portion 20. Accordingly, a treatment 
gas spreads even between adjacent wafers W supported by the grooves 23, 24 
whereby conditions for treatment to each of the semiconductor wafers W can 
be uniform. Further, formation of the openings 17a, 17b reduces the weight 
of the wafer boat 10 and provides easy handling. 
In accordance with the method of the present invention, the openings 17a, 
17b are formed by fitting the inserts 14 in the mold at the time of slip 
casting molding wherein the shape of the openings 17a, 17b can be formed 
as desired by determining the shape of the inserts. Accordingly, 
workability is remarkably improves; yield is also increased and 
manufacturing cost can be reduced in comparison with a conventional method 
which requires formation of openings by machining a green product or a 
sintered product, and by processing the edge of the openings formed. 
FIG. 4 shows an example of a cantilever 30 as a shaped product wherein a 
loading zone 32 is thinner than a grip zone 31 whereby the loading zone 32 
is apt to be broken when cutting operations are conducted to form openings 
33 in a green state. However, by utilizing the method of the present 
invention wherein the openings are formed in a final product by inserting 
inserts in the corresponding openings in the mold, openings 33 can be 
formed without a risk of breakage and a requirement of reducing the weight 
can easily be achieved. 
EXAMPLES 
Example 1 
5 parts by weight of methyl cellulose as an organic binder relative to 100 
parts by weight of a silicon carbide powder (average particle diameter: 
about 20 .mu.m) was mixed with 30 parts by weight of pure water to prepare 
a slurry. By using the slurry, a green product of wafer boat was prepared 
according to the method described with reference to FIG. 1. Diametrically 
opposed portion in the side portions of the green product were cut, and 
then the green product was sintered at 2,000.degree. C. for 60 min. to 
obtain a sintered product. 
The sintered product was impregnated with silicon by the method as 
described in U.S. Pat. No. 5,179,049. Then, machining operations were 
conducted to form grooves for inserting wafers to thereby prepare 
one-piece shell type wafer boats (cassette boats) having the shape shown 
in FIGS. 2 and 3. Yield for manufacturing the wafer boats was 98%. A time 
required for machining operations including machining in a green state and 
final machining was 3 hours.times.person per a final product. 
Example 2 
Polyvinyl acetate emulsion was used as an organic binder to form 
cantilevers as green product shown in FIG. 4. The green products were 
sintered at 1,600.degree. C. for 200 min., followed by impregnating molten 
silicon. There was found no crack in the vicinity of the openings. Yield 
for manufacturing was as high as about 95% in comparison with about 50% as 
the conventional method. 
Comparative Example 1 
A raw slurry was prepared in the same manner as in Example 1. Green 
products were formed with the raw slurry by the ordinarily used slip 
casting method wherein each of the green products has the shape as shown 
in FIG. 1 provided it has no opening since no inserts 14 was used. 
Openings were formed in a green state. The green products were sintered 
under the same conditions as in Example 1 to obtain sintered products. The 
sintered products were impregnated with silicon by a known method. Then, 
machining operations were conducted to form wafer boats having the shape 
shown in FIGS. 2 and 3. 
Yield for manufacturing the wafer boats by this method was 55%. A time 
required for machining was 10 hours.times.person per a final product. 
Comparative Example 2 
Wax layers 26a, 26b were formed on portions of the inner surface of a mold 
25 at which openings were intended to form as shown in FIG. 5. Green 
products were obtained in the same manner as in Comparative Example 1. The 
green products were sintered to form wafer boats. The presence of the wax 
layers 26a, 26b in the inner surface of the mold 25 formed openings as a 
result of using the slip casting method. However, edge-like projections 
were formed at the periphery of the openings since the slurry was 
solidified along the periphery of the openings. Accordingly, machining to 
that portions was necessary. Yield for manufacturing the wafer boats was 
70%, and a time required for machining was 7 hours.times.person per a 
final product. 
In accordance with the present invention, openings are previously formed in 
a mold at positions corresponding to those of openings to be formed in a 
final product, and inserts made of material without having no water 
absorbing properties are fitted to the mold so as to project inwardly from 
the inner surface of the mold before a raw slurry is poured in the mold 
according to a slip casting method. Accordingly, a green product with 
openings is obtainable in a desired shape. Accordingly, workability is 
improved, yield for manufacturing is increased with the result that 
manufacturing cost can remarkably reduced in comparison with a 
conventional method wherein openings are formed by machining a green 
product or a sintered product.