US 20050123713 A1
Improved methods for manufacturing rings using chemical vapor deposition. Cylindrical tubes are used as deposition substrates and the resulting material deposited on the inside surface of the cylindrical tubes is sliced or cut into the desired ring size and shape. The resulting rings have a crystal growth that is oriented substantially planar to the finished article.
1. A structure formed by chemical vapor deposition having a planar direction and a normal direction, wherein the structure has a dimension in the planar direction that is larger than the dimension in the normal dimension and having grains substantially oriented in the planar direction.
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10. A method of making flat rings by chemical vapor deposition, comprising:
(a) providing a round cross-sectional tube in a deposition zone;
(b) directing precursor gas into the tube;
(c) chemically reacting the precursor gas to form a solid deposit in the shape of the tube on the inside on the tube;
(d) removing the solid deposit; and
(e) dividing the solid deposit into substantially flat rings either before or after removal.
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1. Field of the Invention
The invention relates to articles formed by chemical vapor deposition and methods of forming such articles.
2. Description of Related Art
Chemical vapor deposition (CVD) techniques have been widely used to provide thin films and coatings of a variety of materials on various products. Typically, the process involves reacting vaporized or gaseous chemical precursors in the vicinity of a substrate to result in a material such as silicon carbide (SiC) depositing on the substrate. The deposition reaction is continued until the deposit reaches the desired thickness.
CVD techniques can be used to form relatively thin coatings on the surfaces of pre-existing articles; in this situation, the surface of the article forms the substrate. However, CVD techniques can also be adapted to produce articles that are formed from the deposited material. In this situation, the substrate upon which deposition occurs is a form or mandrel that provides an initial shape to the article. The article, which is removed after sufficient deposition has occurred, has a complimentary surface that corresponds to the form or mandrel. Such articles are called “free-standing” articles herein.
One method by which free-standing SiC articles are formed by CVD includes feeding silicon carbide precursor gases or vapors into a deposition chamber, where they are heated to a temperature at which they react to produce silicon carbide. The precursor gases or vapors react at the surface of a substrate or other structure loaded into or placed in the chamber. The silicon carbide builds up as a shell or a deposit on the substrate. Different articles may require different thicknesses, and thicknesses can range from less than 100 microns to over an inch or two inches thick. The thickness can be controlled by controlling the deposition time and/or other process variables. When the desired deposition thickness is reached, the mandrel or substrate is then removed from the deposition chamber and the deposited silicon carbide is separated therefrom.
In one method for forming SiC articles, methyltrichlorosilane (CH3SiCl3 or MTS), hydrogen (H2), and argon (Ar) gases are introduced into the furnace through an injector. MTS is a liquid at ambient or room temperature, and sufficient vapor can be delivered into the reaction vessel by feeding carrier gas through the MTS liquid or by picking up vapor above the liquid. Gases that are unreacted in the furnace are pumped out by a vacuum pump, filtered, and cleaned in a gas scrubber before being vented to the atmosphere.
Some industries require thinly-formed silicon carbide rings or articles. The current technology of producing monolithic ceramic parts via the chemical vapor deposition process includes producing large sheets of the ceramic material in a CVD furnace. SiC is deposited onto a flat or box-shaped substrate to form the relatively large, flat sheets, from which the final ceramic part or parts are machined. The machining process includes cutting the rough shape out of the large sheet, grinding the piece to near the desired thickness to produce a blank that approximates the final form but with surplus material thickness on each face, and then machining the blank to the dimensions of the final form.
When CVD materials are deposited on large flat substrates, the material exhibits a crystal growth that is perpendicular to the plane of the flat substrate. The material is not necessarily deposited evenly to form a sheet that is uniform in thickness or in microstructure, so that there may be thicker portions in some areas and thinner portions in other areas, instead of a uniform thickness throughout. Because the sheet typically has a thickness profile that is non-uniform, the thickness can vary between rings that are ultimately formed, depending upon where they were cut from the sheet. Moreover, the thickness within an individual ring may vary with position.
In addition, due to the large size of the SiC sheet from which the SiC rings are typically formed, different microstructures may exist at different areas of the sheet. For example, when a ring is produced from a sheet of deposited material, differences in the material characteristics across the thickness of the deposited sheet may lead to increased tension or stress within the material that can cause a slight axial bow or curve in the machined ring.
Another manifestation of the different microstructures across the large plate is a variation in cosmetic appearance across the final machined parts, e.g. rings. These variations in cosmetic appearance can be further magnified when the finished parts are coated with other vaccuum deposited or vapor deposited coatings, especially CVD deposited coatings such as silicon.
Additionally, because of the thickness variations the CVD process time must be increased to bring the low deposition rate areas up to the minimum thickness requirements for the desired parts. The higher deposition rate areas then cause the pieces cut from the sheet to require more machining time in order to be ground to the required thickness.
Moreover, there is limited flexibility in the geometry of a sheet and the pieces to be cut, so there is material waste due to the layout pattern of the pieces. There is also surplus material in the piece pattern that is cut or ground from between and within the pieces to make the blanks, such as edges and centers of rings, which results in a large quantity of scrap material. For example, the material that is cut out around each ring and between each ring (to form an inner diameter of the ring) is typically wasted, much like the unused cookie dough remaining after cookies are stamped out from a roll of cookie dough.
There are also occasional problems with cracking of a large, CVD-produced plates or sheets of ceramic material during the CVD process, which can reduce the yield significantly. The combination of cracks and the scrap material from the sheet and the material that is ground from the blanks lowers the average raw material-to-product conversion ratio significantly.
An alternate method that has been used to manufacture silicon carbide rings includes mounting disk-like mandrel substrates through their respective centers in a spaced and parallel relationship in grooves on a shaft. The planar surfaces of the disk-like mandrel substrates are oriented perpendicular to the axis of the shaft. During processing, the shaft is rotated as gases are injected into the chamber, such that ring-shaped ceramic parts are formed by deposition on the mandrel substrates.
A different but related method of creating silicon carbide rings is to suspend within a deposition chamber individual, flat graphite ring mandrels having an outside diameter and an inside diameter similar to those desired in the SiC rings. The gas mixture of MTS in hydrogen and argon is fed into the chamber and silicon carbide is deposited on the mandrels to form rings. Once the rings are removed from the graphite ring mandrels, their inner and outer diameter can be machined to the desired dimensions.
One problem related to forming silicon carbide rings from these alternative processes is that the mandrels need to be rotated throughout the formation process to prevent build-up in undesired areas. In many instances, the gases are injected into the reaction chamber such that the gases are not focused on any particular mandrel or surface of interest, but instead are allowed to deposit non-uniformly on all surfaces of the reactor. The rotation or suspension of the substrate in the reaction gas stream is intended to prevent or limit this non-uniformity of deposition.
The specifically-shaped mandrels are also complicated to manufacture. In the embodiment that includes a shaft with disk-shaped mandrels placed in grooves, the grooves must be specially machined for receiving and supporting the disk-shaped mandrel substrates. In the graphite ring embodiment, each ring support axis must have protrusions or tabs that facilitate their suspension or placement in the chamber, and material is often deposited in or around the protrusions or tabs.
Additionally, in each of the above-described methods, the resulting ring has a crystal growth that is oriented axially relative to the ring or finished article, not radially oriented relative to the ring or article. In other words, the microstructure contains grains oriented perpendicular to the plane of the finished part. Further, it is known that as materials are deposited by chemical vapor deposition the size of grains increases as the growth proceeds away from the substrate. The evolution in grain structure from small grains to large grains occurs as grains with lower energy crystallographic orientations grow faster than grains with less preferred orientations, under the particular deposition conditions used. This evolution of grain structure typically produces a gradient in microstructure across the thickness of the material, which in turn causes a gradient in internal stress of the deposited material, resulting in bending or “bow” of the deposited material when it is released from the substrate. This gradient in material microstructure and stress complicates the machining process, and remains in the material even after machining is completed, often resulting in some bow or waviness in the finished part. This bow or waviness is undesirable, especially in parts that require precise tolerances and extremely consistent flatness, such as rings for use in contact with semiconductor wafers. The larger the part, the more significant the problem can become.
Regardless of these difficulties and costs associated with manufacturing silicon carbide articles, silicon carbide has a unique combination of properties that make it a particularly suitable material for a variety of applications in the semiconductor, optical, electronic and chemical processing fields. Silicon carbide articles produced by CVD processing are recognized to exhibit superior mechanical, thermal, physical and optical properties.
Accordingly, the present inventors have found a way to improve CVD processing for producing articles, in particular planar articles such as rings and discs, and more particularly silicon carbide ring-shaped articles. The resulting articles have a unique microstructural orientation relative to the shape of the article. The invention may also be used to create articles having other shapes. Such articles can be used in fixtures to support silicon and other wafers for processing, susceptor rings for supporting wafers in semiconductor furnaces, and as wafer edge rings.
The invention provides improved methods for manufacturing rings using chemical vapor deposition. In certain embodiments, cylindrical tubes are used as substrates and the resulting material that is deposited on and then removed from the inside surface of the cylindrical tubes can be sliced or cut into the desired ring size and shape. The resulting ring has a diameter in a planar direction and a height in a normal direction. The crystalline grains are oriented in the planar direction, and in certain embodiments, are oriented radially in the planar direction, as opposed to the perpendicular or axial growth orientations seen in the prior art rings. Put another way, the primary direction of crystal growth in materials according to the invention is in the plane of the resulting article; in the case of ring, hoop, or disc shaped articles, the primary direction of crystal growth and grain orientation is in the radial direction.
The invention also relates to methods of forming disk-shaped rings by chemical vapor deposition by
Advantages of this invention include increased space utilization, reduced unit cost of manufacturing the rings, reduced waste during manufacture, and the ability to produce rings of varying diameter, cross-sectional width and thickness depending on the height of cutting (i.e., without varying the deposition time).
The invention is described in more detail below using silicon carbide as a convenient and commercially important example. However, it should be recognized, as it has been by the inventors, that articles according to the invention may be made by practicing the process of the invention using any material that can be deposited by CVD. Illustrative, non-limiting examples include oxide, nitride and carbide ceramic materials including but not limited to aluminum nitride, aluminum oxide, aluminum oxy-nitride, silicon oxide, silicon nitride, silicon oxy-nitride, boron nitride, boron caride, and other materials such as zinc sulfide, zinc selenide, silicon, diamond, diamond-like carbon, and any other material that can be prepared using CVD techniques.
The largest market for SiC sheets is in the semiconductor market, which uses the high stiffness to weight ratio, chemical and physical compatibility and purity to make fixtures to support silicon and other wafers for processing. Many of these fixtures are shaped as rings, typically less than 0.2″ in thickness and up to about 14″ in diameter. The inner diameter is adapted to hold semiconductor wafers.
Although articles made according to the techniques described below are primarily ring-shaped, it is understood that they may have any desired shape, depending upon the size and the shape of the substrate used to form the article. For ease of discussion, however, the below description refers to flat ring-shaped articles and methods of forming the ring-shaped articles using a cylindrical tube as the substrate.
The chemical vapor deposition (CVD) process for silicon carbide (SiC) is generally based upon a thermally induced reaction within a reduced pressure, resistance heated graphite furnace. Certain embodiments of the invention use this equation and these basic parameters:
Instead of using flat sided or four sided boxes for deposited material to form a sheet conforming to the dimensions of the sides of the boxes, the invention provides at least one round cross-sectional tube, the internal dimension of which accommodates the outer diameter of the desired ring size. (If a triangular, oblong, oval, octagonal, or any other shaped article is desired, it is also possible to use tubes having the corresponding cross-section, although it is preferred that instead of sharp corners, the substrates have rounded edges to provide a more uniform distribution and to reduce internal stresses.)
The apparatus used in the process of the present invention is a chemical vapor deposition (CVD) chamber designed to deposit ceramic material on the inside of a cylindrical or tubular mandrel.
In this embodiment, the invention uses as a substrate a round or circular cross section cylindrical tube 20, as shown in
In use, reactant gas is admitted at an injector (shown schematically at 30) at the top of chamber 10 (although it could come in from the bottom)and into cylindrical tube 20. The gas is removed by an exhaust port (shown schematically at 32) at the bottom of chamber 10 (although it could be at the top)which connects to the pumping system 14 and effluent treatment system 18.
In this embodiment, the cylindrical tubes 20 are preferably graphite tubes that are attached in a non-rotating manner in chamber 10. Cylindrical tubes 20 may be attached such that they are allowed or caused to rotate, but rotation is not required and is typically not employed.
The graphite cylindrical tubes 20 have a substantially round cross-section, such that the material deposited on the inside of the tubes is formed in a corresponding round cross-sectional shape. The circular radius of cylindrical tubes 20 allows uniform distribution and flow of gases.
A cylindrical tube 20 that forms the substrate for the deposition process is shown in
The tube 20 can have the same cross-sectional dimensions throughout, or the cross-section can be varied. For example, if the deposition tends to be greater at certain areas of the cylinder than at others, the geometry of the cross-section can be varied to accommodate those deposition variations. The cylinder can also be tapered if differently-shaped disks are desired. Alternatively, the cylinder can be terraced or stepped to provide clear divisions between the sections.
During processing, precursor gas is fed into the internal diameter of the cylindrical tube 20 in order to make a tubular monolith of CVD deposited material, such as a ceramic material. In the illustrated embodiment, gases are fed at the top of the chamber through injector 30 and exhausted at a manifold or exhaust port 32 at the bottom of the chamber. Each of the cylindrical tubes 20 has a dedicated, independent injector or precursor gas feed positioned so that the gas enters at one end 26 of cylinder and exits through the other end 28. This allows deposition primarily on the surfaces of interest only, the internal diameter 22 of the tube 20, and not on other surfaces, such as the outside of the cylindrical tube 20. The gas travels in a path along the axis of the tube. This provides a more efficient deposition process by allowing better control of gas conditions. The only asymmetry in thickness that may arise is a thickness gradient from the top to the bottom of the tube, although the thicker areas can be machined to the proper thicknesses.
In certain specific embodiments, the gas feed is a linear, symmetrical chemical vapor deposition system with reaction gas entering the chamber approximately at the center of the diameter such that the gas is deposited substantially evenly along the inside surface 22 of the tube 20. The reaction product is deposited to a set thickness on the internal surface. The SiC builds up on the inner surface of the tube in a substantially radial direction, as shown by
This process is more efficient than the prior art process in which reaction products are deposited throughout the chamber and not at specific locations. The cylinder also does not need to be rotated in order to obtain the desired uniform deposition in a consistent manner, in contrast to many methods of the prior art, although it could be rotated if desired.
Typically, the deposition thickness is approximately 1 inch, although a wide range of material thickness from approximately 0.1 inch to over 2 inches may be obtained by varying gas flow rates and deposition times. Gases from multiple tubes may be evacuated commonly from the chamber though exhaust port 32.
Once the silicon carbide has been deposited on the inside of the tubular mandrel, the deposit is removed from the mandrel by any appropriate technique, including, but not limited to grinding or electrical discharge machining. A release coating or other substance may be applied to the inner surface of tube for ease of removal.
The resulting SiC tube can be cut through the circular cross section to yield a ring or a hoop, as shown in
Rings of varying diameter can be produced by varying the cross-sectional width and thickness of deposition to give the desired internal diameter. Overall height of the ring can be adjusted by varying the height of cutting.
The outer diameter of the ring or hoop will correspond to the inner diameter of the cylindrical tube 20 and the inner diameter of the ring or hoop will depend upon the amount of gases that are fed through the tube and allowed to deposit. The thicker the deposit, the smaller the inner diameter of the ring that can be produced. The diameters of the ring may then be machined, smoothed or otherwise shaped to provide the desired dimensions.
An alternate embodiment that may be used to form rings is shown in
Rings sliced from cylinder deposits of this invention are easier to machine than rings made using the methods of the prior art due to the lower internal or out-of-plane stresses around the ring. Generally, as material is deposited via CVD, there are subtle but unintentional variations in the process parameters that can affect the nature of the material and the crystal growth structure can vary, creating inherent stresses in the material. These stresses can cause bowing or bending in rings that are cut out of a sheet of material with the predominant direction of crystal growth in the direction normal to the plane of the sheet. By contrast, the symmetry provided by using a cylindrical tube mandrel or support according to the invention affords a substantially symmetrical stress gradient around the ring, so that any inherent stress exhibited on one side of the ring is balanced by a similar inherent stress on the opposite side of the ring (i.e., there is no net stress in the plane of the ring). In other words, the inherent stress is directed inwards or outwards in a similar manner, affecting the ring or disk in a substantially symmetrical way.
For SiC that is deposited upon a flat surface, the stress gradient on the lower surface 44 will be different from the stress gradient on the upper surface 46 due to the way the ring is manufactured. This can cause the material to bend or bow, preventing the manufacture of a flat ring.
By contrast, for SiC that is deposited within a cylindrical tube, the stress gradient difference is exhibited between the outer surface 48 and in the inner surface 50, such that any stress gradient “squeezes” the ring symmetrically. Accordingly, rings that are cut from the resulting cylindrical deposition have substantially uniform properties around them due to the uniformity of the deposition parameters across that particular slice. (However, it is possible that properties of slices cut from different positions along the cylinder will be slightly different due to the different deposition parameters along the particular length of the cylinder.)
If the cylindrical tube 20 is provided as a series of sections, the deposit can be removed by disassembling the sections. Even if the resulting deposit fractures or cracks, the cracked areas do not cause a great deal of wasted material because they will typically be across the diameter of the material due to the microcrystalline structure of the material. This is the portion of the ring that will be machined during cutting or slicing.
During the prior art deposition process (whether the ring is formed from a flat sheet or on a ring shaped mandrel), the growth direction of the crystal structure is in the direction of the height 40 of the ring, as shown in
However, rings according to the present invention exhibit the growth direction of the crystal structure in the direction of the length 42 or diameter of the ring, as shown in
In one embodiment of the method of the present invention, free-standing silicon carbide tubes from which the rings are sliced are deposited from a mixture of silicon carbide precursor gases, such as a mixture of methyltrichlorosilane (MTS) and hydrogen, with an optional inert gas, such as argon or helium and optional dopant gas into a deposition chamber heated to a temperature typically between 1300° C. and 1400° C. Deposition pressure is maintained at approximately 100 torr to 300 torr. The relative partial pressure flow ratio of hydrogen to MTS is maintained in the range of approximately 5 to 10.
Reaction gases are delivered into a tubular mandrel through a single injector or an array of injectors positioned symmetrically at either the top or bottom. Under these conditions, silicon carbide is deposited on the inside of the tubular mandrel(s) at a deposition rate of approximately 1 to 2 μm/minute and the deposition process is continued until the desired thickness of SiC material is achieved. Typical deposition periods vary between 50 and 300 hours.
Upon completion of the deposition, the mandrel and deposit are removed from the reaction chamber and with or without first separating the mandrel from the deposited material, the silicon carbide rings are sliced from the tube. The outer and inner dimensions and the thickness of the ring are then machined to specification using diamond grinding methods.
The unique characteristics of the products of the present invention, i.e. low in-plane residual stress, extreme flatness, high control of machinability, allow for the manufacture of very thin (e.g. less than 0.5 inch) hoop-shaped rings, and more complicated rings with or without bulk “drop-offs.”
Using the process of the present invention, 200 mm and 300 mm diameter SiC edge rings for Si wafer processing have been produced and characterized. Rings were made from both electrically insulating CVD-deposited SiC material exhibiting a typical electrical resistivity of about 100 to 10,000 ohm-cm, and electrically conductive SiC with an electrical resistivity of less than 0.01 ohm-cm.
In the following Examples, flexural strength measurements, also known as modulus of rupture (MOR) tests, on the deposited SiC materials were obtained using the 4-point bend test according to ASTM C-1161. The MOR test samples (bars) were cut from rings sliced from the deposited cylinders. The MOR bar samples had the following dimensions: length of 45 mm, width of 4±0.13 mm, thickness of 3±0.13 mm, edge chamfer of 0.12±0.03 mm at an angle of 45°±5° . Chamfers were fabricated by grinding with a 600-grit grinding wheel, while the large planar sides of the MOR bars were finished with a 320-grit wheel. The approximate layout of the MOR bars 9 a cut from the SiC ring blank 9 b is shown in
Electrical resistivity measurements were made with a Jandel Company hand held 4-point probe, consisting of a linear array of 4 points spaced 1 mm apart. The resistivity was calculated assuming an infinite material thickness. Three data sets were obtained at each of four measured positions on each sample ring.
SiC edge rings for handling 300 mm Si wafers for semiconductor processing were fabricated as follows. A graphite tube with nominal inner diameter of 340 mm was manufactured from an isostatically pressed, fine grained grade of graphite with a thermal expansion similar to that of dense CVD SiC. Four tubular sections, each with a length of approximately 13 inches were assembled in a CVD reactor. Optional deposition-prohibiting spacer devices were positioned in between each section to facilitate removal of each tubular section after deposition.
The precursor gas MTS was introduced through a single injector positioned symmetrically at the top of the tubular mandrel assembly at a flow rate of 9.1 liters per minute. Hydrogen was delivered through the same injector at a flow rate of 76 liters per minute. The ratio of hydrogen-to-MTS was 8.4. The CVD reactor pressure was 200 torr and deposition temperature was 1350° C. The total deposition time was 174 hours, and the average deposition rate of the SiC material was 0.05 inch per hour.
After deposition, the reaction chamber was opened and the tubular mandrels, spacer devices and tubular SiC deposits were removed. Each of the tubular deposits was then sliced using a diamond abrasive slitting wheel into ring slices having a thickness of approximately 0.2 inch. These slices were then ground to the final edge ring specification.
The SiC material deposited by this technique had a density of 3.21 g cm31 3, Vickers hardness >2700 kg mm−2 and thermal conductivity between 240 and 270 Wm−1K−1, and an electrical resistivity between 390 and 450 ohm-cm. These measurements are in the same range as Jandel probe resistivity measurements obtained from plate-form SiC material that has been deposited under similar conditions, but on flat graphite sheets.
The flexural strength of rings obtained by this process varied between 40 and 65 ksi (275 to 450 MPa), depending upon where the ring was sliced from the tubular deposit.
For comparison, SiC material deposited in plate form that has been deposited under similar conditions onto flat graphite sheet exhibits a flexural strength in the range of 47 to 70 ksi (320 to 480 MPa). That the rings of the present invention with a SiC grain growth orientation parallel to the plane of the ring had a flexural strength equal to that of rings made from SiC material with grain growth orientation perpendicular to the plane of the ring was surprising. This excellent result was unexpected since in the flexural strength test of the material of the present invention, the SiC material is bent in a direction parallel to the direction of grain growth, i.e. using an analogy to wood, the material is bent “along the direction of the grain.” Therefore, the present invention overcomes prior art deficiencies associated with fabrication of ceramic rings, while maintaining outstanding material strength.
SiC edge rings for handling 200 mm Si wafers for semiconductor processing were fabricated according to the process of Example 1, with the following exceptions: the graphite tube had a nominal inner diameter of 240 mm, each tubular graphite section had a length of approximately 9 inches, and the hydrogen-to-MTS flow ratio was 7.4. The average deposition rate was 0.051 inch per hour. The flexural strength of rings obtained varied between 57 and 68 ksi (390 to 465 MPa), again indicating outstanding material strength.
SiC edge rings for handling 200 mm Si wafers for semiconductor processing were fabricated according to the process of Example 2, with the following exceptions: each tubular graphite section had a length of approximately 8.5 inches, and the hydrogen-to-MTS flow ratio was 8.4 and a doping gas was added to produce an electrically conducting SiC deposit. The average deposition rate was 0.065 inch per hour. Rings obtained varied in electrical resistivity from 0.004 ohm-cm to 0.007 ohm-cm.
For comparison, SiC material deposited in plate form that has been deposited under similar conditions onto flat graphite sheet typically exhibits a resistivity between 0.002 ohm-cm and 0.008 ohm-cm. Therefore, the electrical resistivity measurements obtained from the deposit in Example 3 are similar to measurements obtained from plate-form SiC material deposited under similar conditions.
Representative rings from the prior examples have been coated with CVD silicon coatings to reduce opacity to light and to provide a high purity Si surface for contact to Si wafers during wafer processing. Typically, this deposition is conducted at or near atmospheric pressure using trichlorosilane as a source of silicon and a CVD temperature of between about 1000° C. and 1200° C. Other vacuum deposition and vapor deposition methods including but not limited to evaporation, sputtering, ion plating, ion beam deposition and plasma deposition can be used to deposit coatings over the products of the present invention. Because of the orientation and uniform nature of the microstructure of the material of the present invention, coatings applied to the products of the present invention have improved cosmetic appearance compared with coatings applied to prior art CVD products.
In addition to the excellent material properties summarized above, additional unexpected benefits have been found with regard to processing the materials of the present invention. In addition to providing materials having better stress distributions and flatter profiles, the method of the present invention also increases manufacturing capacity significantly by allowing multiple cylindrical deposition chambers inside one CVD furnace, replacing at least one of the conventional squared-sectioned box sites or flat mandrels with at least one cylindrical tube. In addition, other time and cost savings are realized by use of the process of the invention.
First, when compared to fabrication of rings from plate-form CVD SiC material, the fabrication time of product of the present invention is significantly less. Typical fabrication, i.e. cutting or slicing the ring from the deposit, then grinding to final dimensions, is reduced by approximately 35%.
Also, because the rings are flat as sliced from the tubular deposit, grinding to the specified ring flatness of 0.002 inch can be achieved on the first attempt with the product of the present invention. In contrast, plate-form deposited SiC material has a characteristic bow that must be machined out to achieve a final flat part. Multiple processes of alternate side grinding are required with plate-form SiC, and the first grinds are outside of flatness specification be several thousandths of an inch.
It has also been surprisingly found that the process of the present invention produces a material that is very uniform and provides consistent results during grinding, when compared to plate-form material. The plate-form SiC material is often inconsistent in thickness and uniformity across the dimension of a ring, and requires significant machine operator judgment and adjustment to produce finished parts within the desired product specifications. In addition, it is believed that the nonuniform machining of the plate-form material can induce unwanted stresses. Because of the uniform microstructure in the axial direction, and because of the grain orientation of the present invention, the material grinding to final configuration is much more consistent.
Although the specific examples presented above are for CVD silicon carbide materials, similar results are anticipated with other materials that can be deposited by CVD including, but not limited to other oxide, nitride and carbide ceramic materials such as aluminum oxide, aluminum nitride, aluminim oxy-nitride, silicon oxide, silicon nitride, silicon oxy-nitride, boron nitride, boron carbide, and other materials such as zinc sulfide, zinc selenide, silicon and diamond.