Device for producing high purity silicon carbide on carbon base by hydrogen reduction of silicon tetrachlorid

Jewell, R. A.

Wakelyn, N. T.

English

NASA Technical Reports Server

Jan. IS, 1 
APPARATUS FOR PROr i N G  HIGH PURITY SILICON CARBIDE CRYSTALS 
BY 
I 
P 
https://ntrs.nasa.gov/search.jsp?R=19700030699 2020-03-17T02:09:49+00:00Z
~ , 2 3 ~ , Q ~ 3  
Patented Jan. 18, 1906 
1 2 
while minimizing the disadvantages thereof. Accord- 
ingly, an object of the present invention is the provision 
of a new and improved method of producing high purity 
SILICON CARBIDE CRYSTALS silicon carbide. 
Another object of the instant invention is the provision 
of an efficient, inexpensive, reliable, and rapid procels 
3,230,053 
APPARATUS FOR PRODUCING HIGH PURITY 
Noel T. Wakelyn and Robert A. Jewel], Hampton, Va., 
assignors to the United States of America as repre- 
Aeronautics for producing high purity silicon carbide crystals. and Space Administration 
Original application Sept. 5, 1962, Ser. No. 221,637, now A further object of the present invention is to provide 
patent NO. 3,174,827. Divided and this application a method of producing high purity silicon carbide from 
Sept. 25, 1963, Ser. No. 320,595 the gaseous phase of the required reactant materials. 
1 Claim. (Cl. 23-277) Another object of the present invention is the provision 
(Granted under Title 35, U.S. Code (1952), sec. 266) of novel apparatus for the production of high purity i l i -  
con carbide crystals. 
The invention described herein may be manufactured According to the present invention, the foregoing and 
and used by or  for the Government of the United States 15 other objects are attained by providing apparatus where- 
of America for governmental Purposes without the Pay- by a hydrogen gas may be saturated with silicon tetra- 
ment of any royalties thereon or therefor. chloride and flowed under pressure through a constricted 
This application is a division of aPPliCatiOn Serial Num- passageway within a high temperature furnace, onto a 
ber 221,637, filed September 5 ,  1962, now US. Patent carbonaceous base or model member. The carbonaceous 
Number 3,174,827. 20 member employed in the present invention extends for- 
This invention relates generally to the production of ward and aft of the constricted furnace area with silicon 
silicon carbide and relates with particularity to the Pro- carbide crystal formation taking place in large quantity 
duction of small quantities of high purity silicon carbide a n  the carbon model only on the portion thereof posi- 
on  a carbon base, by the hydrogen reduction of silicon tioned aft of the constricted gaseous passageway. 
tetrachloride, with crystals being developed from the 25 A more complete appreciation of the invention and 
gaseous phase of the reactive materials. many of the attendant advantages thereof will be readily 
Silicon carbide crystals are becoming of increasing im- appreciated as the same becomes better understood by 
portance in production of high temperature semiconduc- reference to the following detailed description when con- 
tors useful in transistors and the like. Presently avail- sidered in connection with the accompanying drawings 
able transktors are usually made of germanium or silicon 30 wherein: 
which are limited to maximum operating temperatures of FIG. 1 is a schematic, part sectional, view of the ap- 
approximately 70" and 150" C. ,  respectively. Existing paratus employed to produce high purity silicon carbide 
semiconductors employed in transistors are limited to crystals according to the present invention; 
these maximum operating temperatures since the energy FIG. 2 is a cross-sectional view of the apparatus shown 
required to raise an electron in these crystals to a state 35 in FIG. 1 taken along section lines 2-2 of FIG. 1; and 
in which it conducts freely is only 0.7 electron volt for FIG. 3 is a pictorial view of magnified single crystals 
germanium and 1.1 electron volt for silicon. Since the of high purity silicon carbide produced according to the 
electTon gap of silicon carbide is more than twice that present invention. 
of silicon, silicon carbide crystals are desirable for use in Referring now to the drawings, wherein like reference 
manufacturing semiconductors useful at extreme tem- 40 numerals designate like or corresponding parts through- 
peratures such as those anticipated for instrumentation out the several vicws, and more particularly to FIG. 1, 
employed in space vehicles and other anticipated un- there is shown the apparatus, generally designated by 
usual, and extreme conditions. Also, since semicond~c- reference numeral 10, employed in accordance with this 
tors of this type must be reliable and reproducible, it is invention. Apparatus 10 includes a pressurized cylinder 
desirable to manufacture them only from high purity 45 11 of hydrogen gas, having a tube 13 which leads there- 
crystals where controlled trace impurities may be added from through a suitable valve 15 and terminates in a 
to perform the limited amount of conductivity needed. container 17 of liquid silicon tetrachloride 19. Con- 
Thus, there is a definite need in the art for a process of tainer 17 is closed by a suitable stopper 18 through which 
economically producing high purity crystals of silicon passes a conduit 21 terminating above the level of liquid 
carbide. Conduit 21 connects the interior of container 17 
One heretofore process for producing siEcon carbide with the interior of tubular furnace 23 through a con- 
deposits on a carbon base has been successfully accom- ventional T-valve 25 and furnace end closure 26. Fur- 
plishd by applying extreme heat to a carbon rod in an nace 23 is provided with a protective casing 24 there- 
atmosphere of hydrogen gas and silicon tetrachloride; around adapted to receive a quantity of graphite powder 
however, deposits obtained by this process require con- 55 30 as will be further explained hereinafter. A tank 27 
siderable time to produce and are of extremely low containing a pressurizcd inert gas, for example argon, is 
purity. also connected to an inlet of T-valve 25 by way of duct 
Another prior art process for obtaining silicon carbide 29 and "on-off valve 31. Tank 27 also connects with 
crystals involves slow crystallization onto a seed crystal the interior of protective casing 24 by way of valve 32. 
from a melt of silicon or a silicon alloy, contained in a 60 Furnace 23 is of tubular construction, having the 
graphite crucible. By this prior art process. the crucible design'of a conventional carbon tube furnace. It is 
contents are heated to a temperature sufficient to melt resistantly heated, for example, by applying an electrical 
the silicon,- but short of the vaporizing temperature potential to end closure 26 and end stopple 36 by way of 
thereof, and a seed crystal of pure silicon carbide, se- terminals 33 and 53 which lead from a suitable power 
curely attached to a suitable rod, is inserted into the 65 source, not shown. Furnace 23 is vented to the atmos- 
molten mass and slowly and uniformly rotated and with- phere by way of tubulation vent 34 leading from the 
drawn a number of times. After a considerable length downstream end thereof through end stopple 36. Jackets 
of time, for example forty-eight hours, single crystals of 28 and 40 serving to circulate a suitable coolant there- 
high purity silicon carbide crystals may be obtained on through, for example water, are Connected to a suitable 
the seed and on  the graphite crucible walls. 70 central coolant supply, not shown, and adapted to 
The present invention combines the advantageous fea- protect end closure 26 and stopple 36, respectively 
tures of both of the aforementioned prior art processes from the extreme heat present during operation of the 
sented by the Administrator of the 
50 19. 
3,2 30,05 3 
4 
a p p m t u s .  A centriilly perforated model support 35 hav- As the constricted gas flow passes over model 43, a 
ing a plur:ility of minute holes 39 (FIG. 2) formed there- chemical reaction takes place between the gaseous phase 
through, is loos~ly slideable within tubular furnace 23 carbon and the silicon tetrachloride saturated hydrogen 
to divide the furnace interior into an upstream region gas to form a plurality of silicon carbide crystals appear- 
and a downstream region as well as to provide a plurality 5 ing as globules and strands 45 about the surface of model 
of constricted gaseous passageways therethrough and 43. The crystal formation appearance varies from pale 
therearound. Holes 39 may be omitted in some instances yellow to yellow-green and pale green translucency. After 
with the only passageway for the saturated hydrogen ga7 approximately one hour of saturated gas flow through 
flow being between the external surface of model support furnace 23, valve 15 is closed, T-valve 25 is rotated to 
35 and the interior of furnace 23. It is also apparent that 10 open duct 29, and valve 31 is again open& to permit the 
suitable spacers, not shown, may be provided, either ex-  flow of incrt gas from tank 27 for approximately ten min- 
ternally on model support 35 or internally of furnace 23. utcS to purge the system, during which time, the tempera- 
t o  provide specific arca constricted passagcways about ture of fiirnace 23 is raixd and maintained at approxi- 
support 35,within the scope of the preqent invention. ni:itely 4,600” F. After purging, the electric current is 
The central perfaration 41 (PIG. 2) of modcl support 15 shif t  off and the furnace permitted to cool to r m m  tem- 
35 tapers over its length from the upstream furnace perature which requires approximately one hour. At the 
region toward the downstream furnace region and serves end of this cooling period valves 25, 31, and 32 are posi- 
as the sole support for the tapered elongate carbon base tioned to stop the flow of inert gas and crystal group 
or model member 43, slideably received thereby, with 45 may then be removed with a multitude of single 
portions of model 43 extending from each end of model 20 crystals 46 being physically separable from the group 
support 35 and spaced in its entirety from the interior of as shown under microscope lens 48 in FIG. 3. Crystals 
the furnace wall. The tapered configuration described 46 are separable in the form of short thin platelets of 
herein is exemplary only and other suitable shapes may several millimeters’ length, strands of approximately fifty 
obviously be provided for carbonaceous base member millimeters’ length, and globules of several millimeters’ 
43 with any conventional support or retention means 26 length. and are suitable for use as rugged miniature semi- 
therefor within the scope of this invention. conductors in commercial electronic instrumentation. 
The pressure for the hydrogen gas flow from tank 11, 
and that for the inert gas from tank 27, are not considered 
In the operation of the apparatus described herein. critical and may be controlled by the respective valves 
above, model 43 is positioned withir: model support 35 30 15, 31, and 32 such that the pressures thereof slightly 
which is slideably received within tubular furnace 23. 
Furnace 23 is then enclosed by casing 24 which contains The material composition of model support 35 is also 
a volume of finely ground graphite powder 30 suficient not considered critical, although when employing carbon 
to completely engulf the furnace tube. End closure 26 Or graphite as the material for support 35, as well as 
and stopple 36 with water jackets 28 and 40 positioned 35 when using a carbon tube furnace, it would appear that 
thereon are then secured to furnace 23, by suitable clamps the Presence of excess carbon in the vapor phase would 
or threaded connections, not shown, and corduit 21 and aid in the fol‘mation Of silicon carbide crystals 45. 
tubulation vent 34 connected, respectively thereto. Water An example F a d e  of carbon Particularly suitable for 
or other suitable coolant from the central supply, not constrLIctiW model 43 is “AGX” graphite rods commer- 
shown, is then permitted to flow through jackets 28 and 40 cially available from the National Carbon Company, a 
40 and valve 32 is opened to ailow inert gas, for exalnple division of Union Carbide. The use of other commer- 
argon, to flow through the graphite powder 30 within cial grades of carbon or graphite is also apparent to those 
protective casing 24; the gentle gas flow being vented to skilled in the art and, obviously, within the scope of 
the atmosphere by way of tubular vent 38. This flow this invention* 
of inert gas “outgases’’ the graphite powder and permits 45 It is significant to  note that in the above described 
powder 30 to remain in a fluffy condition to thereby corn- Process, globules, and strands of silicon carbide crystals 
p1ete.y protect the exterior of furnace tube 23 from 45 are formed, in quantity, Only on the portions of model 
oxidizing gases during the high temperatures employed 43 positioned aft Of the restricted passageways formed 
during operation of apparatus 10. Valves 25 and 31 are by model support 35, with the remaining surface of model 
also adjusted to allow inert gas from tank 27 to gently 50 43 essentialb’ coated with the light green form of 
flow through furnace 23, and silicon carbide. The formation of silicon carbide CVS- 
for a p e r i d  of approximately ten minutes to purge the tals is thus strictly a gaseous phase reaction and it is 
reaction for removal of any impurities initially believed that as the reactants pass from the center or 
in the furnace which could adversely influence the forma- hottest Part of the furnace into a slightly cooler region 
tion of high purity silicon carbide crystals. Electric cur- 55 that gaseous silicon carbide more readily deposits out in 
rent ir  applied to closure 26 and stopple 36 for an interval solid form, although apPliC2nts do not intend to be limited 
sufficient to preheat furnace 23 and its contents, model to this theory Of ‘peration* 
support 35 and model 43, to  a temperature of approxi- Obviously, many modifications and variations of the 
mately 4,0000 F. and maintained at this temperature Present illvention are possible in the light of the above 
level. For cxample, furnace 23 need not be the 
opened and valve 25 positioned to allow flow from con- c ~ b o n  tube type described and may be heated other than 
tainer 17 while blocking the inert gas flow from tank by the described resistance heating system, such for ex- 
27. The system is calibrated to permit approximately ample induction heating apparatus, when so desired. It 
1.6 liters per minute of hydrogen gas to flow through is therefore to be understood that within the scope of 
tube 13 and bubble through the liquid silicon tetrachlo- 65 the appended claim, the invention may be practiced other- 
ride 19 in container 17. As the hydrogen gas bubbles wise than as specifically described. 
through liquid 19, i t  becomes saturated with silicon tetra- What is claimed and desired to be secured by Letters 
chloride and the saturated gas flows through conduit 21. Patent of the United States is: 
The saturated gas, while flowing through furnace 23, An apparatus for producing recoverable high purity 
passes through the constricted passageways 39, and the 70 silicon carbide crystals on a carbon base in the presence 
space between model support 35 and the interior of the of hydrogen gas saturated with silicon tetrachloride com- 
furnace wall, onto that portion of graphite model 43 prising: 
extending into the downstream region of furnace 23, with 
the excess gas flow being vented to the atmosphere 
through tubulation vent 34. 
Operation 
atmospheric pressure. 
tubulation vent 34, 
Valve 15 from hydrogen cylinder 11 is then 60 IcachiWs. 
an elongate electrically heated furnace, 
a model support slidably positioned within said furnace 
loosely adjacent to the internal surface thereof so 75 
as to form a constricted 
support member and said 
said model support dividing 
3,230,053 
passageway between said 
furnace wall, 
tained silicon tetrachloride into the upstream region 
of said furnace through said constricted passageway 
said elongate furnace into 
an upstream region and a downstream region and 
being provided wjth a substantially central perfora- 
tion tapering in diameter from said upstream furnace 
region to said downstream furnace region, 
said support member slidably receiving an elongated 
tapered graphite rod within said central perfora- 
tion with end portions of said rod extending from 
opposite sides of said support into said upstream and 
said downstream furnace regions, and 
means to effect flow of said hydrogen gas and its con- 
and onto said graphite rod portion within- said 
downstream region of said furnace. 
I 
Reference6 Cited by the Examiner 
UNITED STATES PATENTS 
2,602,033 1/1952 Lander, 
2,677,627 5/1954 Montgomery et al. 
MORRIS 0. WOLK, Primary Examiner. 
JAMES H. TAYMAN, Assistanf Examiner. 
10 


Apparatus for producing high purity silicon carbide crystals Patent

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server