High purity composite useful as furnace components

A Czochralski process furnace component comprises a high purity, semiconductor standard composite including a carbon fiber reinforced carbon matrix having a level of metal impurity below the detection limit of inductively coupled plasma spectroscopy. A process for producing the components includes heat treatment of the carbon fiber and the components.

TECHNICAL FIELD 
The present invention is directed to high purity composites of carbon fiber 
within a carbon matrix and their preparation. More particularly, the 
present invention is directed to high purity composites useful as 
semiconductor material processing components, such as Czochralski furnace 
components and furniture. 
BACKGROUND OF THE INVENTION 
Silicon wafers for use in the semiconductor industry are produced by a 
variety of methods. One of the methods is that referred to as the 
Czochralski or "CZ" technique. In the CZ technique a seed crystal of known 
orientation is immersed in a molten pool of silicon. This triggers 
solidification and precipitation of the silicon. As the crystal is 
mechanically pulled upwardly from the pool, the orientation of the 
solidifying front mimics that of the seed crystal. Silicon wafers can be 
manufactured from the solid ingot by machining and polishing. 
Specifically constructed furnaces are used to accurately control the 
various parameters needed to ensure that high quality crystals are 
produced. Several of the key components in CZ crystal growing furnaces are 
made from graphite. These include various liners, shields, tubes, crucible 
susceptors and the like. Graphite has been the material conventionally 
utilized in such processes due to its high temperature capability and 
relative chemical inertness. 
Disadvantages of graphite include its poor durability brought about by its 
highly brittle nature and its tendency to microcrack when exposed to 
repeated temperature cycles. Such microcracking alters the thermal 
conductivity of the component which in turn makes accurate temperature 
control of the silicon melt difficult. In addition, contamination of the 
silicon melt may occur by the leaching of impurities from the graphite 
components or from particulates generated by the degradation of the 
graphite itself. Semiconductor standards require extremely low levels of 
impurities in the semiconductor processing system, to allow substantially 
no impurities to be incorporated into the semiconductor material, as even 
trace amounts can alter the electronic properties of the semiconductor 
material. 
Further, the deposition of oxides of silicon on graphite parts during the 
production of the silicon crystal occurs to such an extent that parts must 
be cleaned on a regular basis and replaced periodically. Replacing worn 
graphite parts is a time consuming and costly process. 
Therefore, there has been a need for the manufacture of components for CZ 
crystal growing reactors that have the advantages of graphite without the 
disadvantages. Such components would enable the more cost effective 
production of high quality silicon semiconductor wafers. 
There have been attempts made to utilize carbon/carbon composites in 
similar electronic material production processes, in place of graphite 
furnace components and furniture. U.S. Pat. No. 5,132,145 and 
corresponding European Patent application 88401031.5 to Valentian disclose 
a method of making a composite material crucible for use in the Bridgman 
method for producing single crystals of metallic material semiconductors. 
Valentian proposed making a cylindrical crucible for holding a molten 
sample, from a single wall of carbon fibers or silicon carbide fibers 
impregnated with carbon or silicon carbide, and depositing on the inner 
wall of the crucible, a thin inner lining of silicon carbide in 
combination with silica, silicon nitride, and silicon nitride/alumina, or 
in other embodiments, amorphous carbon, boron nitride, titanium nitride or 
diboride, and zirconium nitride or diboride. The thin inner lining is 
required to avoid contamination of the molten sample, to provide a matched 
thermal conductivity with the molten sample, and to avoid crack 
propagation which is a drawback of the bulk material. 
It is therefore an object of the present invention to provide components 
for use in semiconductor processing that are superior in mechanical and 
thermal properties to conventional graphite components. 
It is a further object of the present invention to provide components for 
use in semiconductor processing that are superior in purity 
characteristics to conventional graphite components and to conventional 
carbon/carbon materials. 
SUMMARY OF THE INVENTION 
The present invention provides a high purity carbon/carbon composite 
material consisting of carbon fiber reinforcements within a carbon matrix. 
This material has outstanding thermal capabilities, especially in 
non-oxidizing atmospheres. Before the present invention, use of 
carbon/carbon composite materials in the electronics industry was largely 
restricted due to the inability to produce materials that not only exhibit 
good mechanical properties at high temperature but that are extremely pure 
and will not contaminate sensitive electronic production articles such as 
semiconductor materials or devices, and silicon wafers in particular. 
The present invention therefore provides a high purity, semiconductor 
standard composite comprising a carbon fiber reinforced carbon matrix 
having a level of metal impurity below the detection limit of inductively 
coupled plasma spectroscopy for the metals Ag, Al, Ba, Be, Ca, Cd, Co, Cr, 
Cu, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sr and Zn. 
The present invention further provides semiconductor processing, such as 
Czochralski crystal growing, furnace components and furniture comprising 
the above high purity, carbon/carbon composite, the composite including a 
carbon fiber reinforced carbon matrix having a level of metal impurity 
below the detection limit of inductively coupled plasma spectroscopy for 
the metals Ag, Al, Ba, Be, Ca, Cd, Co, Cr, Cu, K, Mg, Mn, Mo, Na, Ni, P, 
Pb, Sr and Zn. In one embodiment, the present invention provides a 
semiconductor processing furnace heat shield or furnace tube liner 
comprising the high purity, semiconductor standard composite. In another 
embodiment, the present invention provides a Czochralski process crucible 
susceptor comprising the high purity, semiconductor standard composite. 
The present invention also provides a semiconductor crystal growing 
apparatus comprising at least one high purity, carbon/carbon composite 
components, said composite including a carbon fiber reinforced carbon 
matrix having a level of metal impurity below the detection limit of 
inductively coupled plasma spectroscopy for the metals Ag, Al, Ba, Be, Ca, 
Cd, Co, Cr, Cu, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sr and Zn. 
According to the present invention, therefore, there is provided a 
Czochralski crystal growing process for pulling a semiconductor ingot from 
a semiconductor material melt, such as a silicon ingot from a silicon 
melt, including providing the semiconductor material (such as silicon) 
melt in a quartz crucible, wherein the quartz crucible is isolated from 
contaminant sources by at least one high purity, carbon/carbon composite 
component. In one embodiment, the process includes intimately supporting 
the crucible with the above susceptor. In another embodiment, the process 
includes disposing the furnace heat shield or furnace tube liner between 
the crystal pulling zone and the heating element. 
The present invention also provides a process for the production of a high 
purity, semiconductor standard carbon/carbon composite comprising: 
heating a carbon fiber reinforcement to at least about 2400.degree. C., 
impregnating the carbon fiber with a matrix precursor of high purity 
(semiconductor quality) carbon, 
carbonizing the impregnated fabric to form a carbonized part, 
densifying the carbonized part with high purity carbon to form a component, 
and 
heating the component at a temperature of at least about 2400.degree. C. to 
form a heat treated component, and 
heating the heat treated component at a temperature of at least about 
2400.degree. C. in a halogen atmosphere to form the high purity composite. 
In one embodiment, densifying the carbonized part includes purging a CVD 
processing furnace with an inert gas at a temperature of at least about 
2400.degree. C., and densifying the carbonized part with CVD carbon in the 
purged CVD furnace to form the component. 
I have therefore found that it is possible to produce carbon/carbon 
materials with the desired mechanical, thermal, chemical and physical 
characteristics that make these materials very suitable for use in the 
semiconductor electronics industry, and particularly for use as 
semiconductor processing furnace, such as crystal growing reactor, 
furniture and components.

DETAILED DESCRIPTION OF THE INVENTION 
Carbon fiber reinforced carbon matrix materials, or carbon/carbon 
composites, have thermal stability, high resistance to thermal shock due 
to high thermal conductivity and low thermal expansion behavior (that is, 
thermal expansion coefficient or TEC), and have high toughness, strength 
and stiffness in high-temperature applications. Carbon/carbon composites 
comprise carbon reinforcements mixed or contacted with matrix precursors 
to form a "green" composite, which is then carbonized to form the 
carbon/carbon composite. They may also comprise carbon reinforcements in 
which the matrix is introduced fully or in part by chemical vapor 
infiltration. 
The carbon reinforcements are commercially available from Amoco, DuPont, 
Hercules, Celanese and others, and can take the form of fiber, chopped 
fiber, cloth or fabric, chopped cloth or fabric (referred to as moulding 
compounds), yarn, chopped yarn, and tape (unidirectional arrays of 
fibers). Yarns may be woven in desired shapes by braiding or by 
multidirectional weaving. The yarn, cloth and/or tape may be wrapped or 
wound around a mandrel to form a variety of shapes and reinforcement 
orientations. The fibers may be wrapped in the dry state or they may be 
impregnated with the desired matrix precursor prior to wrapping, winding, 
or stacking. Such prepreg and woven structures reinforcements are 
commercially available from BP Chemicals (Hitco) Inc. The reinforcements 
are prepared from precursors such as polyacrylonitrile (PAN), rayon or 
pitch. According to a preferred embodiment of the present invention, the 
reinforcement is in the form of woven cloth. 
Matrix precursors which may be used to form carbon/carbon composites 
according to the present invention include liquid sources of high purity 
(that is, semiconductor quality) carbon, such as phenolic resins and 
pitch, and gaseous sources, including hydrocarbons such as methane, 
ethane, propane and the like. Representative phenolics include, but are 
not limited to, phenolics sold under the trade designations USP 39 and 
91LD, such as supplied by Stuart Ironsides, of Willowbrook, Ill. 
The carbon/carbon composites useful in the present invention may be 
fabricated by a variety of techniques. Conventionally, resin impregnated 
carbon fibers are autoclave- or press-molded into the desired shape on a 
tool or in a die. The molded parts are heat-treated in an inert 
environment to temperatures from about 700.degree. C. to about 
2900.degree. C. in order to convert the organic phases to carbon. The 
carbonized parts are then densified by carbon chemical vapor impregnation 
or by multiple cycle reimpregnations with the resins described above. 
Other fabrication methods include hot-pressing and the chemical vapor 
impregnation of dry preforms. Methods of fabrication of carbon/carbon 
composites which may be used according to the present invention are 
described in U.S. Pat. Nos. 3,174,895 and 3,462,289, which are 
incorporated by reference herein. 
Shaped carbon/carbon composite parts for semiconductor processing 
components can be made either integrally before or after carbonization, or 
can be made of sections of material joined into the required shape, again 
either before or after carbonization. 
Once the general shape of the carbon/carbon composite article is 
fabricated, the piece can be readily machined to precise tolerances, on 
the order of about 0.1 mm or less. Further, because of the strength and 
machinability of carbon/carbon composites, in addition to the shaping 
possible in the initial fabrication process, carbon/carbon composites can 
be formed into shapes for components that are not possible with graphite. 
The high purity carbon/carbon composite according to the present invention 
has the properties of conventionally produced carbon/carbon composites, 
yet has improved purity resulting from the process for the production of a 
semiconductor standard composite of the present invention. 
According to the inventive process, fiber (reinforcement) purity is 
enhanced by the carbon fiber reinforcement, preferably in the form of 
woven fabric, being heat treated in a non-oxidizing (inert) atmosphere to 
a temperature of about 2400.degree. C. (4350.degree. F.) to about 
3000.degree. C. to remove impurities. This heat treatment further sets the 
reinforcements, avoiding shrinkage in later procedures. 
Carbon matrix purity is enhanced by the utilization of high purity matrix 
precursors in the impregnation of the heat treated carbon reinforcement. 
The purity level of the carbon sources should be less than about 50 ppm 
metals. For example, the phenolic resins should contain less than about 50 
ppm metals, should utilize non-metallic accelerators for cure, and 
preferably should be made in a stainless steel reactor. 
The impregnated reinforcements, or prepregs, are staged, laid-up, cured and 
carbonized (or pyrolized) conventionally, except that processing 
conditions are maintained at semiconductor standards. The carbonized part 
is then densified by chemical vapor impregnation or liquid pressure 
impregnation, using the carbon source materials mentioned above. 
In the chemical vapor deposition (CVD) densification of the carbonized 
part, precautions are taken not to introduce any elemental impurities in 
the CVD furnace. Prior to processing the carbonized parts, the furnace is 
purged by running an inert gas, such as argon, helium or nitrogen, through 
it for several heat treat cycles at about 2400.degree. C. to about 
3000.degree. C. 
After the component has been formed by the densification of the carbonized 
part, the component is further heat treated at 2400.degree. C. to about 
3000.degree. C. in a non-oxidizing or inert atmosphere to ensure 
graphitization of the structure and to remove any impurities that may have 
been introduced. The period of time for this procedure is calculated based 
upon graphitization time/temperature kinetics, taking into account furnace 
thermal load and mass. The component may be machined, if desired, to 
precise specifications and tolerances, as discussed above. 
In a further purification procedure, the heat treated components are 
further heat treated at 2400.degree. C. to about 3000.degree. C. in a 
halogen atmosphere to remove any remaining metallic elements as the 
corresponding volatile halides. Suitable halogens include chlorine, 
bromine and iodine, with chlorine being preferred. The purification 
treatment may be terminated when no metallic species are detected in the 
off-gas. 
Throughout the production process, great care is taken not to contaminate 
any parts. As discussed above, processing is done to semiconductor 
standards, including the use of laminar air flow in work areas which 
ensure ISO 1000 conditions. 
High purity carbon/carbon composites prepared according to the present 
invention were analyzed by inductively coupled plasma spectroscopy (ICP) 
in comparison with conventional graphite components, the latter of which 
was also analyzed by atomic absorption spectroscopy (AAS), and the results 
are shown in Table I below. 
TABLE I 
______________________________________ 
Detection 
High Purity 
Element (ppm) 
Graphite (1) 
Limit (2) 
C/C Level (2) 
______________________________________ 
Aluminum &lt;0.08 0.1 ND 
Calcium 0.13 0.1 ND 
Chromium &lt;0.07 0.01 ND 
Copper &lt;0.08 0.02 ND 
Iron 0.09 0.04 0.18 
Magnesium &lt;0.02 0.02 ND 
Manganese &lt;0.08 0.01 ND 
Nickel &lt;0.10 0.04 ND 
Potassium &lt;0.10 4 ND 
Sodium &lt;0.05 0.2 ND 
Vanadium &lt;0.07 0.02 .24 
______________________________________ 
(1) by ICP, AAS 
(2) by ICP 
ND Not Detected 
High purity carbon/carbon composites prepared according to the present 
invention were analyzed by inductively coupled plasma spectroscopy in 
comparison with conventional carbon/carbon composites, the latter of which 
was analyzed by high temperature halonization, and the results are shown 
in Table II below. 
TABLE II 
______________________________________ 
Conventional 
Detection 
High Purity 
Element (ppm) 
C/C (1) Limit (2) 
C/C Level (2) 
______________________________________ 
Aluminum 4 0.1 ND 
Calcium 10-30 0.1 ND 
Chromium &lt;0.32 0.01 ND 
Copper &lt;0.06 0.02 ND 
Iron 3-5 0.04 0.18 
Magnesium 3-5 0.02 ND 
Manganese 0.034 0.01 ND 
Molybdenum 1 0.02 ND 
Nickel ND 0.04 ND 
Phosphorous 
5.8 0.02 ND 
Potassium ND 4 ND 
Sodium 4.8 0.2 ND 
______________________________________ 
(1) by High Temperature Halonization 
(2) by Inductively Coupled Plasma Spectroscopy (ICP) 
ND = Not Detected 
As shown in Tables I and II, the high purity carbon/carbon composites of 
the present invention are below the detection limit for inductively 
coupled plasma spectroscopy analysis for the metals Al, Ca, Cr, Cu, K, Mg, 
Mn, Mo, Na, Ni, and P, while these metal impurities are shown to be 
present in graphite, and in conventional carbon/carbon composite materials 
(except in the latter, for nickel and potassium). 
Carbon/carbon composites produced according to the invention were ashed and 
the diluted residue further analyzed by inductively coupled plasma 
spectroscopy for metals content in addition to those metals tested above. 
As demonstrated in Table III below, the concentration of these metals, Ag, 
Ba, Be, Cd, Co, Pb, Sr, and Zn, was also below the detection limit for the 
analytical technique. 
TABLE III 
______________________________________ 
DETECTION LIMIT 
HIGH PURITY C/C 
ELEMENT (PPM) LEVEL 
______________________________________ 
Barium 0.01 ND 
Beryllium 0.01 ND 
Cadmium 0.01 ND 
Cobalt 0.02 ND 
Lead 0.2 ND 
Silver 0.02 ND 
Strontium 0.02 ND 
Zinc 0.02 ND 
______________________________________ 
ND = Not Detected 
Carbon/carbon composites, according to the invention, can be used in 
semiconductor processing without first coating the component, although it 
is preferable to precoat the carbon/carbon composite prior to use, in 
order to lock down any particles which may have formed as a result of the 
composite fabrication or machining process. A coating may be desired in 
the event of a change in the process furnace atmosphere. Carbon/carbon 
composites can readily be coated with a protective refractory coating, 
such as refractory carbides, refractory nitrides, and, particularly with 
regard to the production of gallium arsenide crystals, refractory borides. 
Preferred refractory coatings are silicon carbide, silicon nitride, boron 
nitride, pyrolytic boron nitride and silicon boride. Graded or layered 
coatings of the carbides, nitrides and borides may also be used. 
Advantages of carbon/carbon (C/C) composites over graphite, particularly 
with regard to semiconductor processing such as in the semiconductor 
crystal growing process furnace, arise from improved mechanical 
properties, namely improved strength, dimensional stability, and impact 
and thermal shock resistance, in part due to the incorporation of the 
reinforcement fibers. Representative graphite components and carbon/carbon 
composite components prepared according to the present invention were 
tested for physical, thermal and mechanical properties, the results for 
which are reported in Table IV. 
TABLE IV 
______________________________________ 
Graphite 
C/C composite 
______________________________________ 
Physical Property 
Density (g/cc) 1.72-1.90 1.64-1.69 
Porosity (%) 9-12 2-15 
Hardness (Shore) 12-80 Off Scale 
Thermal Property 
Conductivity (W/mK) 70-130 100 
TEC (.times. 10.sup.-6 in/in/.degree.C.) 
2.0-3.6 1.4 (in plane) 
6.3 (x-ply) 
Emissivity 0.77 0.52 
Mechanical Property 
Ultimate Tensile Strength (ksi) 
0.9-1.7 35-50 
Tensile Modulus (msi) 
0.8-1.7 3.5-16 
Flexural Strength (ksi) 
1.7-13 16-42 
Compressive Strength (ksi) 
4.4-22 11-30 
Fracture Toughness (Izod Impact ft-lb/in) 
&lt;1 13 
______________________________________ 
Although the properties in Table IV above were tested for composites 
produced according to a preferred embodiment of the invention, the high 
purity, semiconductor standard carbon/carbon composites of the present 
invention can be produced to exhibit a density of about 1.6 to about 2 
g/cc, and a porosity of about 2 to about 25%. These high purity composites 
generally range in tensile strength from about 25 to about 100 ksi, in 
tensile modulus from about 3 to about 30 msi, in flexural strength from 
about 15 to about 60 ksi, in compressive strength from about 10 to about 
50 ksi, and in fractural toughness as measured by Izod impact, about 5 to 
about 25 ft-lb/in. 
Such inventive high purity composites exhibit a thermal conductivity of 
about 20 to about 500 W/mK in plane and about 5 to about 200 W/mK 
cross-ply, thermal expansion coefficients of zero to about 
2.times.10.sup.-6 in/in/.degree.C. in plane and about 6.times.10.sup.-6 
in/in/.degree.C. to about 10.times.10.sup.-6 in/in/.degree.C. cross ply. 
Thermal emissivity of the high purity composites is about 0.4 to about 
0.8. The electrical resistivity of the high purity composites is about 
1.times.10.sup.-4 to about 1.times.10.sup.-2 ohm-cm. 
According to the present invention, the high purity, semiconductor standard 
carbon/carbon composites are formed into components for use in 
semiconductor processing, such as furnace heat shields, furnace tube 
liners, and crucible susceptors. These components are useful in the 
Czochralski crystal growing furnace for producing semiconductor crystals 
or ingots of silicon, as well as other semiconductor materials such as 
gallium arsenide. 
According to the invention therefore, Czochralski process furnace 
components such as heat shields and crucible susceptors have been 
fabricated, comprising a high purity, semiconductor standard composite 
including a carbon fiber reinforced carbon matrix having a level of metal 
impurity below the detection limit of inductively coupled spectroscopy for 
the metals Ag, Al, Ba, Be, Ca, Cd, Co, Cr, Cu, K, Mg, Mn, Mo, Na, Ni, P, 
Pb, Sr and Zn. 
The high purity carbon/carbon composite susceptors have been used in the 
Czochralski crystal growing process for pulling a silicon ingot from a 
silicon melt. In this process, the silicon melt was formed in a quartz 
crucible, which was intimately supported within the furnace by the 
susceptor. Also, a high purity carbon/carbon composite furnace heat shield 
was disposed between the crucible containing the silicon melt and the 
furnace heating elements. 
As shown in the sectional schematic of FIGS. 1 and 1A, a typical 
Czochralski semiconductor processing reactor comprises a furnace 10 having 
a water jacketed stainless steel wall 11 to enclose the processing area. 
Insulation, not shown, protects the wall from the internal heating 
elements 12. Disposed radially inwardly of the heating elements 12 is the 
crystal- or ingot-pulling zone 13, where the semiconductor material is 
melted and processed. 
Within the crystal pulling zone 13, a crucible 14, suitably made of quartz, 
is intimately supported by the high purity composite crucible susceptor 15 
which rests either on a refractory hot surface, insulation, an axle for 
rotation of the crucible susceptor 15, or another furnace component (not 
shown). The semiconductor material is heated within the crucible 14 to 
form a melt 16, from which a crystal or ingot 17 is drawn by conventional 
crystal drawing means 18, such as a weighted pulley. The semiconductor 
material is highly pure, electronic quality silicon or gallium arsenide. 
The crystal pulling zone 13 may be maintained at a subatmospheric 
pressure, by means for evacuating the furnace (not shown). 
As shown in FIG. 1, the heating elements 12 and the crystal pulling zone 13 
is disposed a furnace heat shield or furnace tube liner 19, comprising the 
high purity composite. The crucible susceptor 15, and particularly the 
heat shield or tube liner 19, protect the crystal pulling zone 13 and the 
melt 16 and crystal 17 contained therein from potentially contaminating 
elements. 
These high purity composite components provide a stable thermal environment 
in which the solidification of the crystal or ingot 17 is permitted to 
proceed without non-uniformity causing thermal excursions. The heat shield 
19 as shown in FIG. 1, helps to maintain the crystal pulling zone 13 at an 
optimum temperature for the semiconductor material being processed such as 
about 1450.degree. C. for silicon, even though the outer surface of the 
shield, exposed to the heating elements 12, may experience a much higher 
temperature such as 1500.degree. C. to 2000.degree. C. The crucible 
susceptor 15 intimately supports the crucible 14, which may soften and 
begin to "flow" at operating temperatures. The susceptor 15 maintains the 
structural integrity of the crucible 14 during operation. 
As shown in FIG. 1A, in a smaller furnace design the heat shield 19 can be 
disposed radially outside of a configuration comprising a crucible 14 
within a susceptor 16 in close proximity to the heating elements 12 in 
order to contain heat within the crystal pulling zone 13 and prevent its 
dissipation radially. 
The high purity composites are also resistant to thermal shock and 
heat/cool cycles, offering an improvement over conventional graphite 
components. Other advantageous thermal characteristics are listed in Table 
IV, above. 
As shown in FIGS. 2, 3 and 4, the furnace heat shield or furnace tube liner 
20 can be a generally cylindrical shape, although not being limited to 
that configuration, having a high purity composite wall 21 defining an 
internal opening 22. The crystal pulling zone 13 can be contained within 
the opening 22. 
As shown in FIGS. 5, 6 and 7, the crucible susceptor 30 has a high purity 
composite side wall 31, a top opening 32 and a high purity composite base 
33. The interior of the crucible susceptor 30 is shaped to hold the 
particular crucible design for which it was intended, and thus the base 33 
can be scooped in the form of a bowl, and the side wall 31 can contain a 
ridge 34 such as for nesting the crucible. The side wall 31 may contain 
fixturing holes 35 for mounting the susceptor 30. 
In an alternative embodiment shown in FIGS. 8, 9 and 10, the crucible 
susceptor 40 also has a high purity composite side wall 41, a top opening 
42 and a high purity composite base 43. The base 43 may also be scooped, 
and the side wall 41 can contain one or more ridges 44. Fixturing holes 45 
may be present in the side wall 41. The base 43 can contain a high purity 
composite fitting 46 which defines an engagement zone 47 that may engage 
an axle for rotating the crucible/crucible susceptor assembly, an exhaust 
tubing for lowering the pressure of the furnace interior, or another 
furnace component. The ease of fabrication of the high purity 
carbon/carbon composite materials prior to carbonization, and their 
machinability after carbonization, permits the fabricating the furnace 
components into any desired configuration. 
The following advantages have been realized using the high purity composite 
components of the present invention in the CZ crystal growing apparatus. 
The improved durability of the high purity carbon/carbon composite 
components results in a reduction in furnace downtime. The typical 
lifetime for graphite components in the CZ semiconductor crystal growing 
industry is three to four months, while for the high purity composite 
components, a lifetime of 12 to 15 months can be realized, based on an 
extrapolation of real time in-situ testing. 
The durability of the high purity carbon/carbon composite components is due 
to their superior thermal and mechanical properties. In addition, the 
affinity of silicon oxides for the high purity composite material is 
substantially less than that of graphite, which reduces the need for 
periodic cleaning and replacement. 
The improved purity of the high purity carbon/carbon composite components 
over graphite results in a reduced level of contamination of the silicon 
ingots and wafers. This is evidenced by the time taken for an electrical 
current to flow between contaminating atoms (the Hall Mobility). The 
shorter the time for the current to flow between the contaminating atoms, 
the more "impure" the silicon wafer is. 
Electrical breakdown times for silicon wafers produced from furnaces 
employing graphite and high purity carbon/carbon composite components were 
tested. Electrical breakdown times for silicon wafers produced from 
furnaces utilizing graphite components ranged from 200 to 250 
microseconds. The wafers produced by the high purity carbon/carbon 
composite component-utilizing furnaces are considerably purer, exhibiting 
electrical breakdown times of greater than 300 microseconds. This 
improvement is highly significant to the semiconductor industry. 
In another measurement of impurity concentrations in graphite and the 
inventive material, impurity transfer into silicon was measured by direct 
contact at 550.degree. C. over a period of 12 hours. It was determined 
that the elemental impurities listed in Tables I and II were lower in the 
inventive material than in graphite by a factor of at least one hundred 
(100). 
The use of high purity carbon/carbon composite components in the CZ crystal 
growing reactor results in significant improvements in the yield of 
silicon wafers that are classified as "good for structure". The yield of 
"good for structure" wafers produced with graphite furnace components was 
68%, while the yield of "good for structure" wafers produced with high 
purity carbon/carbon composite furnace components was 72%. It should be 
noted that in the silicon semiconductor wafer manufacturing industry, a 1% 
increase in yield is regarded as extremely financially significant. This 
difference in good for structure yield may be attributable to the superior 
control of thermal conductivity throughout the high purity carbon/carbon 
composite components over time. Very little degradation of thermal 
properties of the inventive materials were observed. 
An additional and unexpected benefit from the use of the high purity 
carbon/carbon composite components over graphite concerned the production 
of large components. The fabrication of large graphite parts is difficult 
due to graphite's low mechanical properties and graphite's inability to 
support its own weight. On the other hand large parts were able to be made 
from high purity carbon/carbon composites with ease, for example, up to 48 
inches in diameter. 
Regarding power consumption, the electrical power required by a CZ furnace 
equipped with high purity carbon/carbon composite components was 
significantly less than that of a similar furnace equipped with 
conventional graphite parts. This is due to the superior thermal 
characteristics of the high purity carbon/carbon composite components, as 
shown above. Furnaces utilizing the high purity composite furnace tube 
liner experienced a 2% to 5% decrease in the amount of power required, 
depending upon the number of components in the furnace. This power savings 
is very significant, in terms of capital requirements as well as operating 
costs. 
Regarding particulation, high purity carbon/carbon composite components 
exhibited outstanding resistance to the generation of dust particles 
relative to conventional graphite, which is described by those skilled in 
the art as mealy. The contamination of silicon wafers produced in furnaces 
with high purity carbon/carbon composite components is substantially 
lessened, as compared to those produced with graphite components. 
Therefore, the objects of the present invention are accomplished by the 
production and use of high purity carbon/carbon composite components for 
use in semiconductor processing. The mechanical and purity advantages of 
the inventive material with respect to graphite, and the purity advantages 
of the inventive material with respect to graphite and conventional 
carbon/carbon composites has been demonstrated, as is shown above. It 
should be understood that the present invention is not limited to the 
specific embodiments described above, but includes the variations, 
modifications and equivalent embodiments that are defined by the following 
claims.