U.S. patent application number 11/809584 was filed with the patent office on 2008-01-03 for apparatus with fillet radius joints.
This patent application is currently assigned to Rohm and Haas Electronic Materials LLC. Invention is credited to Jitendra S. Goela, Thomas Payne, Michael A. Pickering, Jamie L. Triba.
Application Number | 20080000851 11/809584 |
Document ID | / |
Family ID | 38477397 |
Filed Date | 2008-01-03 |
United States Patent
Application |
20080000851 |
Kind Code |
A1 |
Pickering; Michael A. ; et
al. |
January 3, 2008 |
Apparatus with fillet radius joints
Abstract
A wafer holding apparatus including a plurality of rods joined
at opposite ends to endplates by joints having flanges with a
fillet radius. The joints which join the component parts of the
apparatus provide a stable wafer holding apparatus for manual
handling as well as for the harsh processing conditions of
semiconductor wafer processing chambers.
Inventors: |
Pickering; Michael A.;
(Dracut, MA) ; Goela; Jitendra S.; (Andover,
MA) ; Triba; Jamie L.; (Nashua, NH) ; Payne;
Thomas; (Charlton, MA) |
Correspondence
Address: |
John J. Piskorski;Rohm and Haas Electronic Materials LLC
455 Forest Street
Marlborough
MA
01752
US
|
Assignee: |
Rohm and Haas Electronic Materials
LLC
Marlborough
MA
|
Family ID: |
38477397 |
Appl. No.: |
11/809584 |
Filed: |
June 1, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60810461 |
Jun 2, 2006 |
|
|
|
Current U.S.
Class: |
211/41.18 |
Current CPC
Class: |
H01L 21/67303
20130101 |
Class at
Publication: |
211/041.18 |
International
Class: |
A47G 19/08 20060101
A47G019/08 |
Claims
1. An apparatus comprising a plurality of rods secured at opposite
ends to respective endplates by joints having flanges with a fillet
radius.
2. The apparatus of claim 1, wherein the joints are coated with
silicon carbide.
3. An apparatus comprising a plurality of rods secured at opposite
ends to respective endplates by joints having flanges with a fillet
radius, each rod end has a tenon which is inserted into an inside
face of the respective endplates through an elliptical port having
the flange with the fillet radius around its circumference, each
tenon is continuous with a shoulder of the rod at each end of the
rod, each shoulder has a flat surface to abut a top surface of the
flange with the fillet to form an interface between the flat
surface of the should and the top surface of the flange.
4. An apparatus comprising a plurality of rods secured at opposite
ends to respective endplates by joints having a four sided flange
with a fillet radius, each rod end has a tenon which is inserted
into an inside face of the respective endplates through a
rectangular port having the four sided flange with the fillet
radius, each tenon is continuous with a shoulder of the rod at each
end of the rod, each shoulder has a flat surface to abut a top
surface of the flange with the fillet radius to form an interface
between the flat surface of the shoulder and the top surface of the
flange.
5. An apparatus comprises a plurality of rods secured at opposite
ends to respective endplates by joints having a three sided flange
with a fillet radius, each rod has a tenon which is inserted into a
port in a side of the endplate, the three sided flange with the
fillet radius defines the port on an inside surface of the
endplate, each tenon is continuous with a shoulder of the rod at
each end of the rod, each shoulder has a flat surface to abut a top
surface of the flange with the fillet radius to form an interface
between the flat surface of the shoulder and the top surface of the
flange.
6. An apparatus comprising a plurality of rods secured at opposite
ends to respective endplates by joints having a three sided flange
with a fillet radius, each rod has a tenon with the three sided
flange with the fillet radius, each rod is inserted into a port in
a side of the endplate such that each side of the three sided
flange forms an interface with a side of the port.
Description
[0001] The present application claims the benefit of U.S.
provisional application 60/810,461, filed Jun. 2, 2006.
[0002] The present invention is directed to an apparatus for
holding semiconductor wafers where the component parts of the
apparatus are secured by joints having flanges with a fillet
radius. More specifically, the present invention is directed to an
apparatus for holding semiconductor wafers of which the component
parts are secured by joints having flanges with a fillet radius
which can withstand the harsh conditions of semiconductor wafer
processing.
[0003] Processing of semiconductor wafers involves harsh conditions
such as exposure to corrosive chemicals, high temperatures
exceeding 1000.degree. C. and rapid thermal cycling (RTP). Such
conditions may result in physical damage to the wafer holding
apparatus such as weakening of the apparatus especially at points
where the parts of the apparatus are joined such as at their
connecting joints. The weakening of the joints typically result in
visible cracks along the joint lines, especially where the
component parts of the joint are at right angles to each other.
[0004] The harsh conditions also may cause surface damage to the
apparatus such that particulate matter from the apparatus sloughs
off and contaminates the semiconductor wafers. Contamination caused
by particles sloughing off of the surface of the apparatus is
common especially if the apparatus is coated with a material which
is different than that of the apparatus, such as an apparatus of
quartz coated with silicon or silicon carbide. Such coatings often
crack or form particles under the harsh conditions of semiconductor
processing thus damaging the apparatus and contaminating the wafers
as well as the processing chambers. Cracks in coatings are
especially common at the right angles of joints where component
parts meet. Particulate material also may lodge in spaces between
the components of the joints of the apparatus, especially if the
joints include numerous parts.
[0005] Particle contamination of wafers also may occur if the wafer
holding apparatus is improperly cleaned after use. During wafer
processing the wafers as well as the wafer holding apparatus become
coated with chemical materials such as silicon dioxide, silicon
nitride or polysilicon film. Such materials are difficult to remove
from the apparatus. The cleaning difficulty is compounded when the
apparatus has numerous component parts, especially at the points
where the parts are joined.
[0006] The semiconductor industry has recognized that silicon
carbide can withstand the harsh conditions of semiconductor
processing and that it is a superior material for wafer boats as
opposed to materials such as quartz. U.S. Pat. No. 6,811,040
discloses a wafer holding apparatus composed entirely of
monolithic, chemical vapor deposited silicon carbide. The rods of
the boat which hold the semiconductor wafers during processing are
secured to the end plates by dovetail joints. The apparatus does
not use additional fasteners and parts such as bolts, clamps or
nuts to secure the apparatus components. Optionally, each joint may
be coated with chemical vapor deposited silicon carbide to prevent
any particulate material from lodging in any spaces between the
joint parts.
[0007] Although the silicon carbide apparatus described in U.S.
Pat. No. 6,811,040 is an improvement over many other semiconductor
wafer holding apparatus, the dovetail apparatus is less stable or
rigid in the same plane as the length of the rods than in the other
planes of the apparatus. During semiconductor processing, in
addition to exposure to corrosive chemicals and high temperatures
the harsh conditions of semiconductor wafer processing chambers may
cause wafer holding apparatus to move or vibrate. This is typical
during initial heat up when the temperature of the apparatus is
rapidly raised from room temperature to temperatures exceeding
1000.degree. C. over periods of 15 minutes to 60 minutes. The wafer
holding apparatus absorbs energy and dissipates it as heat and
mechanical energy such as vibration. Such vibration is accentuated
in planes where the apparatus are least stable. Vibration causes a
shearing force where the dovetails and the endplates meet. After
continued use the dovetail joints may loosen which may result in
the rods becoming detached from the endplates. In addition to
vibration, the manual handling of the boat also may cause the
joints to loosen over time.
[0008] Another problem associated with the wafer boat is the
difficulty in machining the dovetail. Silicon carbide is a hard
ceramic material in contrast to many other types of ceramic
materials used for semiconductor wafer apparatus. Machining, even
with diamond tools, presents a challenge. Machining is especially
difficult due to the tapered sides characteristic of dovetails.
[0009] Although there are improved semiconductor wafer holding
apparatus, there is still a need for semiconductor wafer holding
apparatus with joints having improved tolerance of the harsh
conditions of semiconductor wafer processing and which are easier
to machine.
[0010] In one aspect an apparatus is provided including a plurality
of rods secured at their opposite ends to respective endplates by
joints having flanges with a fillet radius.
[0011] In another aspect the apparatus includes a plurality of rods
secured at opposite ends to respective endplates by joints having
flanges with a fillet radius, each rod end has a tenon which is
inserted into an inside face of the respective endplate through an
elliptical port having the flange with the fillet radius around its
circumference, each tenon is continuous with a shoulder of the rod
at each end of the rod, each shoulder has a flat surface to abut a
top surface of the flange with the fillet radius to form an
interface between the flat surface of the shoulder and the top
surface of the flange.
[0012] In another aspect the apparatus includes a plurality of rods
secured at opposite ends to respective endplates by joints having a
four sided flange with a fillet radius, each rod end has a tenon
which is inserted into an inside face of its respective endplate
through a rectangular port having the four sided flange with the
fillet radius, each tenon is continuous with a shoulder of the rod
at each end of the rod, each shoulder has a flat surface to abut a
top surface of the flange with the fillet radius to form an
interface between the flat surface of the shoulder and the top
surface of the flange.
[0013] In a further aspect the apparatus includes a plurality of
rods secured at opposite ends to respective endplates by joints
having a three sided flange with a fillet radius, each rod has a
tenon which is inserted laterally into a port in a side of the
endplate, the three sided flange with the fillet radius defines the
port on an inside surface of the endplate, each tenon is continuous
with a shoulder of the rod at each end of the rod, each shoulder
has a flat surface to abut a top surface of the flange with the
fillet radius to form an interface between the flat surface of the
shoulder and the top surface of the flange.
[0014] In an additional aspect the apparatus includes a plurality
of rods secured at opposite ends to respective endplates by joints
having a three sided flange with a fillet radius, each rod has a
tenon with the three sided flange with the fillet radius, each rod
is inserted laterally into a port in a side of the endplate such
that each side of the three sided flange forms an interface with a
side of the port.
[0015] The joints having the flanges with the fillet radius provide
for an apparatus having increased strength in contrast to
semiconductor wafer apparatus which do not have such joints.
Additionally, coating reactants applied to apparatus in coating
chambers form thicker and more uniform coatings on apparatus with
joints having flanges with the fillet radius as opposed to joints
with component parts at sharp angles to each other, such as at
right angles. The thicker and more uniform coating adds further
strength to the joints and the apparatus as a whole.
[0016] FIG. 1 is a schematic of a side view of the joint with the
fillet radius;
[0017] FIG. 2 illustrates one embodiment of the semiconductor wafer
holding apparatus showing the rods with teeth and endplates;
[0018] FIG. 3 illustrates one embodiment of the semiconductor wafer
holding apparatus showing the rods joined to an endplate and the
fillet radius at each joint;
[0019] FIG. 4 illustrates the embodiment where the joint has an
elliptical port with an elliptical flange with the fillet radius
and a tenon of the rod;
[0020] FIG. 5 illustrates the embodiment where the rod is secured
in the elliptical port and the fillet radius around the
circumference of the port;
[0021] FIG. 6 illustrates the embodiment where the joint has a
rectangular four sided port with the flange having the fillet
radius and a tenon of the rod;
[0022] FIG. 7 illustrates the embodiment where the joint has a
three sided port with a three sided flange having the fillet radius
and a tenon being inserted laterally into the port;
[0023] FIG. 8 illustrates the embodiment where the flange with the
fillet radius is on the tenon of the rod and the rod is inserted
into a three port laterally;
[0024] FIG. 9A is a photograph of a joint coated with silicon
carbide where the rod is joined to an endplate at a right angle;
and
[0025] FIG. 9B is a photograph of a joint coated with silicon
carbide where the rod is secured to the endplate with a port having
a flange with a fillet radius.
[0026] As used throughout this specification, the following
abbreviations have the following meaning unless the context
indicates otherwise: .degree. C.=degrees Centigrade;
mm=millimeters; cm=centimeters; m=meters; 2.54 cm/inch;
slpm=standard liters per minute; and torr=pressure required to
support 1 mm of mercury at 0.degree. C. All numerical ranges are
inclusive and combinable in any order except where it is logical
that such numerical ranges are constrained to add up to 100%.
[0027] The apparatus is a semiconductor wafer holding apparatus for
the processing of semiconductor wafers. The apparatus includes a
plurality of rods secured at their opposite ends to respective
endplates by joints having flanges with a fillet radius. The fillet
radius provides for a joint having increased strength in contrast
to many conventional joints of semiconductor wafer holding
apparatus. The fillet radius eliminates sharp corners at the
interface of the joint where the rods meet the endplates. The
elimination of sharp corners enables coatings to be deposited on
the joints of the apparatus which are more uniform and thicker than
joints having sharp corners. The more uniform and thicker coatings
on the joints further strengthen the joints. The joints also reduce
or eliminate shearing forces at the joints.
[0028] A fillet radius r of arc shaped flange 10 of joint 20 of the
semiconductor wafer holding apparatus is illustrated in FIG. 1. The
Figure shows rod 30 inserted into endplate 40 forming joint 20 with
fillet radius r of the flange defined by the distance from the tip
of the arrow on a surface of the flange to the imaginary cross
hairs. The dotted circle 42 indicates the best fit sphere.
Typically the fillet radius ranges from 0.25 mm to 20 mm. More
typically the fillet radius ranges from 1 mm to 10 mm. A fillet
radius is a continuously curved concave junction formed where two
surfaces meet. The fillet radius is measured by determining the
surface contour of the fillet radius and then mathematically
determining the best fit sphere to this contour. The radius of this
best fit sphere is the fillet radius. The contour of the fillet
radius surface can be measured using any technique, such as contact
and non-contact profilometers, optical comparators or photographs
that allows one to define the contour mathematically. Such methods
are well known in the art.
[0029] The number of rods for supporting the semiconductor wafers
may vary. Typically the semiconductor wafer holding apparatus
includes three or four rods. More typically the number of rods is
three. The rods include teeth which are separated by spaces where
the semiconductor wafers are placed during processing. The
endplates which secure the rods at their opposite ends may be of
any suitable shape. Such shapes include, but are no limited to
rectangular, elliptical and triangular. Optionally, the endplates
may include holes which allow for the flow of gases across the
wafers held in the apparatus during processing of the wafers.
[0030] FIG. 2 illustrates one embodiment of the semiconductor wafer
holding apparatus. The apparatus 50 includes three rods 60 each
having a plurality of teeth 70 separated from each other by spaces
80. The rods 60 are joined to endplates 90 at their opposite ends.
Each endplate has holes 95 and 97 for the flow of gases across
wafers during semiconductor processing. The rods are joined to the
endplates by joints having arced flanges with a fillet radius. FIG.
3 illustrates the rods joined to the endplates and the flanges with
the fillet radius.
[0031] FIG. 3 is another view of the embodiment shown in FIG. 2.
FIG. 3 shows one end of the apparatus with endplate 90 joined to
rods 60 in ports with three sided flanges 100. The rods and the
flanges join to form interface 105. The flanges are continuous with
the inside face 110 of endplate 90.
[0032] The rods may be any suitable shape. Typically the rods are
elliptical, rectangular or triangular. The rods terminate at their
opposite ends with a tenon for inserting into ports in the
endplates. The tenons are continuous with the rod and have a
smaller diameter or width than the main body of the rod such that
shoulders having flat surfaces are formed where the tenon joins the
main body of the rod. In one embodiment shoulders of the rod meet
with flat surfaces located on top of the arced flanges to form an
interface when the rod is joined to the endplates.
[0033] The tenon may include a bore which is perpendicular to the
length of the rod and passes entirely through the tenon. In another
embodiment a side surface of the endplate has a bore which passes
through the endplate and opens into a channel of the port where the
tenon is inserted into the endplate. A second bore opposite the
bore at the side surface of the endplate opens into the channel of
the port. The tenon is inserted into the port such that the bore of
the tenon is continuous with the bore of the side surface of the
endplate and the second bore which opens into the channel. The
continuous channel formed between the endplate and the tenon
enables a pin to be inserted into the continuous channel to further
secure the rod to the endplate.
[0034] FIG. 4 illustrates one embodiment which includes the pin for
securing the rod to the endplate. Circular rod 115 includes the
main body 120 of the rod and tenon 125. The tenon joins the main
body of the rod at shoulder 130. The tenon 125 is continuous with
the shoulder 130. The tenon includes bore 135 for inserting pin
140. Bore 135 passes through tenon 125. Flange 145, around the
circumference of port 150 on the inside face 155 of endplate 160,
has side 165 which is in the form of an arc and has a fillet
radius. The tenon of the rod is inserted into port 150 such that
the shoulder 130 of the rod meets top surface 170 of the flange and
bore 135 forms a continuous channel with the bore 175 at the side
surface of the endplate and a second bore of the endplate 180 which
opens into port 150 opposite bore 175. Pin 140 is inserted into the
continuous channel to further secure the rod in the port.
[0035] FIG. 5 illustrates the assembled component parts forming
joint 190. The circular rod 115 is inserted into the port to form
interface 195 where the rod and the top surface of flange 145 meet
to form the joint. As illustrated in FIG. 5 Joint 190 does not have
any sharp angles, such as right angles. The arced flange having a
fillet radius eliminates the sharp angles to provide for a joint
having increased strength.
[0036] FIG. 6 illustrates another embodiment where the pin is used
to further secure the rod to the endplate. Rectangular rod 200
includes the main body 205 of the rod and rectangular tenon 210,
which has four faces. The tenon joins the main body of the rod at
shoulder 215, which has four surfaces. The tenon 210 is continuous
with the shoulder 215. The tenon includes bore 220 for inserting
pin 225. Bore 220 passes through tenon 210. Rectangular flange 225
defines the boundaries of port 230 on the inside face 235 of
endplate 240 and has sides 245 which are in the form of an arc and
have a fillet radius. The port has four inner sides 247. The tenon
of the rod is inserted into port 230 such that the shoulder 215 of
the rod meets surface 250 of the flange to form an interface and
bore 220 forms a continuous channel with the bore 255 at the side
surface of the endplate, which opens into port 230 and a second
bore 260 of the endplate which opens into port 230 from an inner
side of the port opposite bore 255. Pin 225 is inserted into the
continuous channel to further secure the rod in the port.
[0037] FIG. 7 illustrates an additional embodiment where the pin is
used to further secure the rod to the endplate. In this embodiment
the rod is inserted laterally into the port in the endplate. The
port of the endplate opens at a side of the endplate. Rectangular
rod 300 includes the main body 305 of the rod and tenon 310. The
tenon joins the main body of the rod at shoulder 315. The shoulder
has three surfaces. The tenon 310 is continuous with the shoulder
315. The tenon includes bore 320 for inserting pin 325. Bore 320
passes through tenon 310. A three sided flange 330 which defines
the boundaries of port 335 has sides 340 which are in the form of
an arc and have a fillet radius. The flange is on the inside face
345 of the endplate. The tenon of the rod is inserted laterally
into port 335 such that the shoulder 315 of the rod meets the
surface 350 of the flange to form an interface. Port 335 has three
inner sides 355. Bore 320 forms a continuous channel with a second
bore 360 which opens into port 335. Pin 325 is inserted into the
continuous channel formed with bore 320 and second bore 360 to
further secure the rod in the port.
[0038] In a further embodiment the flanges having the fillet radius
are on the tenon of the rod instead of defining the boundaries of
the port. FIG. 8 illustrates this embodiment. Rectangular rod 400
includes a main body 405 of the rod and tenon 410. The tenon
includes a flange 415 with four sides 417 and three upper surfaces
420 which are in the form of an arc and have a fillet radius. The
tenon of the rod is inserted laterally into port 425 which opens at
a side of endplate 430. The tenon includes a bore 435 which passes
through tenon 410. The tenon of the rod is inserted into port 425
such that three of the sides 417 of the tenon meet three
corresponding sides 445 of port 425 to form an interface between
the three sides of the tenon and the three sides of the port. A
second bore 450 in a side of the port of the endplate opposite bore
435 opens into port 425 to form a continuous channel with bore 435
in the tenon of the rod when the rod is inserted into the port. Pin
455 is inserted into the continuous channel to further secure the
rod in the port.
[0039] The parts of the apparatus may be composed of any suitable
type of silicon carbide. Typically the parts are composed of
chemical vapor deposited silicon carbide. More typically, the parts
of the apparatus are composed of chemical vapor deposited, cubic
silicon carbide, and most typically the parts are composed of
chemical vapor deposited, cubic .beta.-crystalline silicon carbide.
The cubic form of silicon carbide is most suitable for this
application because the thermal expansion and thermal conductivity
of cubic silicon carbide is isotropic (same in all directions),
thus reducing thermal stresses in the apparatus when it is heated
or cooled. Thermal stresses may lead to distortion of the apparatus
causing damage to the wafers during processing and in severe cases
the stresses may be high enough to cause the apparatus to fail
(fracture).
[0040] The silicon carbide typically is monolithic because it is
oxidation resistant, chemical resistant and thermal shock
resistant. Additionally, such monolithic silicon carbide need not
have any coating thus eliminating the potential for particles to
slough off during semiconductor wafer processing and contaminate
the wafers. The term monolithic means that the silicon carbide is a
solid piece of silicon carbide. Such silicon carbide typically is
formed by chemical vapor deposition where the solid piece is formed
molecule by molecule by depositing the silicon carbide on a
substrate typically referred to as a mandrel. The single piece is
then removed from the mandrel by conventional means and machined to
a desired size and shape. Methods of forming such monolithic
chemical vapor deposited silicon carbide are well known in the art.
Examples of such methods are disclosed in U.S. Pat. No.
5,354,580.
[0041] Minimal machining is employed in preparing the component
parts of the semiconductor wafer holding apparatus. Shaping the
parts of the joint and the grooves of the rods as well as the
endplates involves less time and complexity than machining many
single piece silicon carbide semiconductor wafer holding apparatus.
Further, the joint secures the component parts of the apparatus
without the need for additional mechanical components or
undesirable chemical sealing agents.
[0042] Optionally the joints of the apparatus may be coated with
silicon carbide to further strengthen the joints. Typically the
joints are coated with 1 mm to 5 mm of silicon carbide. The silicon
carbide may be deposited on the joints by conventional methods
known in the art such as physical vapor deposition or chemical
vapor deposition. The joint with the flanges having the fillet
radius provide for a more uniform and thicker coating on the joint
than joints having sharper angles where the component parts meet,
such as at right angles. The more uniform and thicker coatings
further increase the strength of the apparatus. For example, in a
bulk chemical vapor deposition (CVD) process used to manufacture
silicon carbide the CVD reactor is operated in a mass-transport
limiting regime where the flow of chemical reactants across
component surfaces has a great affect on the uniformity of the
coating. A sharp corner, such as a right angle, at a joint causes a
region of deficient flow causing reduced reactant flow and reduced
coating deposition at the joint. By eliminating the shaper corner
at the joint, reactant flow is improved with a more uniform
deposition of coating and a thicker coating as well.
[0043] During semiconductor wafer processing the wafer holding
apparatus along with the wafers in the apparatus are exposed
initially to rapid temperature increase from room temperature to
temperature exceeding 1000.degree. C. over periods of 15 minutes to
60 minutes. Typically the temperature increase from room
temperature to as high as 1450.degree. C. over period of from 20
minutes to 45 minutes. Such rapid temperature increases cause
energy to build up in the wafer holding apparatus at a rapid rate.
The energy build up is dissipated by the apparatus in the form of
heat and mechanical energy such as vibration. Such vibration
typically occurs in the apparatus along its weakest or least stable
planes. Typically this is along the plane or direction of the
length of the rods. The present joint provides a stable joint in
the plane of the rods including in other planes or directions to
reduce or eliminate the vibration or motion.
[0044] The present joint provides sufficient strength and support
for the apparatus such that it does not sag due to the weight of
semiconductor wafers placed in the grooves. Thus, the apparatus of
the present invention may be used to process multiple wafers by
horizontal processes without concern for the problems associated
with horizontal processing. Additionally the silicon carbide
components enable the apparatus to be placed in vertical apparatus
where multiple semiconductor wafers may be processed. Additionally,
the size of the wafer holding apparatus is limited only by the size
of the semiconductor wafer processing chamber employed.
[0045] The following example is intended to illustrate the
invention and is not intended to limit its scope.
EXAMPLE
[0046] Three joints of chemical vapor deposited silicon carbide
were prepared to test their strength using a standard test method
for testing the strength of joints. Each joint included a chemical
vapor deposited silicon carbide endplate section having dimensions
76 mm long.times.76 mm wide.times.6.4 mm thick.
[0047] The endplates were made by using a conventional chemical
vapor deposition method using conventional parameters. Conditions
were optimized for a six triangular box production furnace. The
silicon carbide was made from methyltrichlorosilane (MTS) in an
inert hydrogen (H.sub.2) and argon (Ar) atmosphere. The silicon
carbide deposition conditions in each box of the furnace are in the
table below. TABLE-US-00001 TABLE 1 PROCESS PARAMETERS AMOUNT
Furnace Pressure 200 torr Deposition Temperature 1360.degree. C.
H.sub.2 gas flow rate 50 slpm Ar gas flow rate 52 slpm MTS gas flow
rate 8.3 slpm H.sub.2 partial pressure 91 torr Ar partial pressure
95 torr MTS 14 torr Deposition Rate 1.5 .mu.m/min. H.sub.2/MTS gas
flow ratio 6
[0048] The silicon carbide was deposited on a rectangular graphite
mandrel. After deposition the deposit was removed from the mandrel
and machined using 220 grit diamond impregnated grinding wheels and
tools to form the endplates polished to <1 .ANG. RMS and having
the dimensions described above.
[0049] Three silicon carbide rail beams also were made using the
conventional chemical vapor deposition method as used for the
endplates with the conditions described in table 1. After
deposition the deposits were removed from the graphite mandrels.
The rail beams were 64 mm long, 14 mm wide and 20 mm high. They
were machined and polished by the same method and tools as the
endplates.
[0050] The rail beams were assembled to form joints with the end
plates. One rail beam was joined to the endplate to form a joint
having right angles. The joint was coated with 2.3 mm of chemical
vapor deposited silicon carbide. The other two joints were joints
which had a fillet radius of 3 mm. One had an open back radius
joint as shown in FIG. 7 and the second joint which had a fillet
radius had a closed back radius as shown in FIG. 6. Both joints
were secured with pins and coated with 2.3 mm of chemical vapor
deposited silicon carbide. Chemical vapor deposition was done in a
1.5-m furnace. Deposition conditions are given in the table below.
TABLE-US-00002 TABLE 2 PROCESS PARAMETERS AMOUNT Furnace Pressure
200 torr Deposition Temperature 1360.degree. C. H.sub.2 gas flow
rate 124 slpm Ar gas flow rate 408 slpm MTS gas flow rate 24.5 slpm
H.sub.2 gas partial pressure 44 torr Ar gas partial pressure 147
torr MTS partial pressure 9 torr Deposition Rate 1.5 .mu.m/min.
H.sub.2/MTS gas flow ratio 5
[0051] All three joints were then visually inspected for cracks.
None of the joints showed any visible cracks or flaws. However, the
joint with the rail at right angles to the endplate showed poor
silicon carbide deposition as shown in the photograph of FIG. 9A.
In contrast, the joints with the fillet ratio showed complete
silicon carbide coverage as shown in the photograph of FIG. 9B.
[0052] Each joint was then placed in a standard Instron Mechanical
Tester.TM. to test the amount of load each joint could tolerate
before breaking. The endplate of the joint was secured into a
fixture to hold the endplate with the rail protruding from the
fixture horizontally such that the distance from the endplate to
the point on the rail where the load (force) is applied was 2.5
inches. The Instron Mechanical Tester.TM. head (load cell) was then
set to move at a speed of 0.02 inches/minute pushing down on the
rail section. The load value in pounds and load rate (inches per
minute) were recorded on a conventional chart recorder and the
point at which the joint fractured was identified on the chart
recorder and used to determine the load (force) on the rail that
caused it to fracture.
[0053] The conventional joint with the rail beam at right angles to
the endplate and with poor silicon carbide deposition cracked after
a load of 158 pounds. In contrast, the joint with the open back and
fillet radius and having complete silicon carbide coverage did not
crack or show any flaws at 158 pounds.
[0054] The joint with the open back and fillet radius and having
complete silicon carbide coverage did not fail until the load
applied to it reached 189 pounds. The joint with the closed back
and fillet radius and having complete silicon carbide coverage did
not fail until the load applied to it reached 183 pounds.
Accordingly, the joints having the fillet radius were stronger than
the conventional joint where the rail beam was at a right angle to
the endplate and had poor silicon carbide coverage.
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