US 3549847 A
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United States Patent  Inventors Thomas J. Clark Troy;
Howard W. Brown, St. Clair Shores, Mich. [211 App]. No. 631,820  Filed Apr. 18, 1967  Patented Dec. 22, 1970  Assignee General Electric Company a corporation of New York 54 GRAPHITE suscEr'roR Primary ExaminerDavid Klein Attorneys-Harold J. Holt, Frank L. Neuhauser, Melvin M.
Goldenberg and Oscar B. Waddell ABSTRACT: A susceptor for induction heating comprising a body of porous graphite having a surface layer of pyrolytic graphite which penetrates into the pores of the porous body to strongly mechanically interlock the pyrolytic graphite surface layer to the porous body. The pyrolytic graphite surface layer provides a chemically inert impermeable surface which rapidly and uniformly spreads the heat generated in the susceptor, and the interlock assures against separation of the surface layer or cracking from mechanical or thermal shock. The susceptor is particularly useful for the epitaxial growth of semiconductors and integrated circuits. The susceptor is preferably made by pyrolytically depositing the pyrolytic graphite on the porous graphite body first at a relatively low temperature and at low carbonaceous gas pressure and then at increased temperature and carbonaceous gas pressure to complete the deposit. This assures the desired depth of penetration of the pyrolytic deposit to provide the mechanical interlock and also a dense, smooth, continuous outer pyrolytic graphite surface and thereby provides the desired combination of chemical and physical properties for use of the susceptor in epitaxial growth processes.
GRAPHITE SUSCEP'IOR This invention relates to a composite graphite susceptor, particularly useful for the epitaxial growth of semiconductors and the like, and to a method for making same.
In epitaxial growth processes for the manufacture of semiconductors and the like, it is necessary to heat a semiconductor chip, such as a chip of silicon metal, to high temperature to deposit thereon a thin layer or a plurality of thin layers of other materials. To accomplish this it is conventional to heat the chip by supporting it on an electrically conductive body within an evacuated container of quartz or other dielectric material surrounded by an induction heating coil, the conductive body functioning as a susceptor for the induction heating to thereby heat the chip. After the chip is brought to the desired temperature vapor deposition onto the chip can then proceed by admitting the desired vapor to the container.
The susceptor used in such processing must not only be electrically conductive but must also have extremely high heat resistance and must be chemically inert and of extremely high purity to assure against adverse contamination of the semiconductor chip being processed. At the present state of the art it is the common practice to use silicon carbide coated graphite as the susceptor. Such a susceptor has numerous disadvantages. First, there is a significant mismatch between the thermal coefficients of expansion of silicon carbide and graphite in addition to which silicon carbide itself does not have good thermal shock resistance. Hence, such a susceptor cannot be thermal cycled rapidly and even when thermal cycled slowly, cracking frequently occurs. Adding to the problem is the fact that silicon carbide is a rather brittle material and it often occurs that upon cracking the silicon carbide debris is thrown onto the semiconductor chip being processed thereby resulting in scrap losses. Further, silicon carbide is expensive in the pure form and this adds significantly to the expense of such susceptors.
It is the principal object of the present invention to provide a susceptor having improved properties of thermal and mechanical shock resistance, chemical inertness, impermeability, high purity and rapid uniform surface heat conduction. Another object is the provision of a method for manufacturing such susceptors at reasonable cost.
Briefly, these objects are accomplished in accordance with the invention by a susceptor which comprises a body of porous electrographite having a surface layer of pyrolytic graphite which penetrates into the pores of the electrographite body to provide a strong mechanical interlock between the pyrolytic graphite surface layer and the electrographite substrate body. Further in accordance with the invention, such a susceptor is manufactured by pyrolytically depositing the pyrolytic graphite onto the electrographite body first at a temperature of from about l,l C. to 1,600 C. with carbonaceous gas pressure of less than about 1.2 mm. Hg pressure, and then continuing the pyrolytic graphite deposition at a temperature above l,600 C. and at a carbonaceous gas pressure about 1.5 mm. Hg pressure. This assures good depth of penetration of the pyrolytic graphite into the pores of the porous graphite body so that there is a strong mechanical interlock while at the same time providing a smooth dense continuous surface layer of pyrolytic graphite. Susceptors so manufactured demonstrate 7 excellent thermal and mechanical shock resistance by reason of the mechanical interlock between the pyrolytic graphite surface layer and the porous graphite substrate; chemical inertness along with surface impermeability and chemical purity by reason of the characteristics of the pyrolytic graphite; and high efficiency particularly by reason of the excellent surface thermal conductivity of the pyrolytic graphite. The susceptors can be thermal cycled rapidly and can be used repeatedly without damage or significant loss of any of the desired properties. In effect, a susceptor made in accordance with the invention demonstrates a combination of desirable chemical and physical properties comparable to those attainable with a 100 percent pyrolytic graphite body but at greatly reduced cost since the bulk of the susceptor consists of inexpensive electrographite.
Other objects, features and advantages of the invention will appear more clearly from the following detailed description thereof made with reference to the appended drawings in which:
FIG. 1 is a cross-sectional view of apparatus used for manufacture of the susceptors;
FIG. 2 is a cross-sectional view, in enlarged scale, of a portion of the apparatus shown in FIG. 1;
FIG. 3 is a greatly magnified sectional view of a surface portion of a susceptor made in accordance with the invention;
FIG. 4 is a sectional view of apparatus incorporating a susceptor of this invention and used for the manufacture of semiconductors by the epitaxial growth method.
Referring now to FIG. I, the apparatus shown comprises a generally cylindrical casing 10 having a closure plate 12 which is removably secured as by bolts or a suitable hinge and latch. A viewing window 14 enables inspection of the deposition operation within the casing. A body of insulating material 16, such as carbon black, defines an inner cylindrical chamber the walls of which are formed by a graphite cylinder 18 and top and bottom graphite plates 20 and 22 respectively. An induction heating coil 24 surrounds the insulating material 16, the graphite cylinder 18 functioning as a susceptor whereby intense heat is generated within the cylinder 18 by reason of the passage of electric current through the induction coil 24.
Extending through the heating chamberdefined by the cylinder 18 and its end plates is an inner tube 26 of graphite having graphite pegs 28 extending radially inwardly therefrom. The graphite bodies 30 desired to be treated are supported by these pegs as shown in FIG. 2. That is, each graphite body is formed with a small cylindrical opening 32 which is pressed over the conical end of a support peg. If desired, the conical end of the support peg can be formed with a plurality of small slots 34 to assure admission of carbonaceous gas into the opening 32.
An opening in plate 22 accommodates an inlet conduit 36 forthe flow of carbonaceous gas into and through the inner graphite tube 18, the upper end of the tube 18 being open to the interior of the casing whereby the nondeposited products of the pyrolysis of the carbonaceous gas'can exist through the outlet conduit 38. Hence, in operation the carbonaceous gas, such as methane or a mixture of methane and hydrogen, is admitted through tube 36 to the interior of the assembly consisting of graphite tube 18 and the graphite bodies 30 which assembly is intensely heated by the heat generated by the cylinder 16. Pyrolysis of the carbonaceous gas thereby occurs with resultant deposition of the pyrolytic graphite on the intensely heated graphite bodies 30, the hydrogen and other gaseous pyrolysis products being withdrawn from the chamber through the outlet conduit 38.
For optimum properties in the finished articles, the electrographite bodies being processed should have a density not in excess of about 1.9 g./cc. and preferably a density of about 1.7 to 1.9 g./cc. and a pore size distribution which peaks at from I to microns. With smaller pores it is difficult to attain the required pyrolytic graphite penetration into the surface to accomplish the desired strong interlock between the pyrolytic graphite and the substrate body. With a pore size significantly above the aforesaid range an excessively long deposition time and thick pyrolytic deposit is required in order to provide the requisite smooth pyrolytic graphite outer surface.
To manufacture the composite graphite susceptor bodies the chamber is first evacuated to a pressure not in excess of about 0.1 mm. Hg and is heated to from l,l00 to l,200 C. by induction heating of the graphite cylinder 18. Then carbonaceous gas, preferably methane or a mixture of methane and hydrogen, is flowed through the graphite tube 26 at a pressure of from 0.5 to 1.2 mm. Hg while the temperature is maintained at from l,l00to l,600 C; This is continued for about 14 to 20 hours after which the temperature is gradually raised by about 300 to 800 C., to within the range of 1,600 to 2, 100 C., whereby the carbonaceous gas pressure increases by about from 0.5 to 1 mm. Hg, to within the range of 0.8 to
2.5 mm. Hg. The continued deposition at temperature above that used for the initial deposition is continued for a total of about 6 to 10 hours to thereby complete the pyrolytic graphite deposit. Heating and admission of carbonaceous gas are then discontinued, followed by a quiescent cool-down to room temperature over a period of about 12 hours.
During the initial pyrolytic graphite deposition at relatively low temperature and carbonaceous gas pressure, penetration of the pyrolytic deposit well into the porous surface of the electrographite body is accomplished, the remainder of the deposition at higher temperature and carbonaceous gas pressure providing the dense smooth outer surface skin to the pyrolytic deposit. To accomplish the necessary depth of penetration of the pyrolytic graphite deposit into the pores of the electrographite body, at least 50 percent of the total period of deposition should be at the initial relatively low temperature and carbonaceous gas pressure.
The resulting pyrolytic graphite deposit and its interlocked relationship to the substrate electrographite body is illustrated in FIG. 3. The pyrolytic graphite deposit 40 extends well into the pores of the porous graphite body but has a smooth impermeable surface layer as shown at 42. The thickness of the surface layer of pyrolytic graphite above the surface of the porous graphite body should be from about 1 to 15 mils and the depth of penetration of the pyrolytic graphite into the pores of the porous graphite body should be from 1 to 10 times the thickness of the surface layer. The pyrolytic graphite deposit is, of course, in the form of laminae extending generally parallel to the surface on which deposited, though this inherent feature of pyrolytic graphite is not shown in the drawings.
The following specific example will serve to further illustrate the process for making the composite graphite susceptor bodies.
The furnace, as shown in FIG. 1, was evacuated to 0.01 mm. Hg and then gradually heated by induction to a temperature of 1,200 C. In the particular furnace here being used the hot zone formed by the graphite tube 26 was 13 inches long with a diameter of 7 inches. With the temperature at l,200 C. a mixture of hydrogen and methane was flowed through the hot zone at a pressure of 0.8 mm. Hg for 16 hours, the inlet flow rate being 6 standard cubic feet per hour hydrogen and 2 stan dard cubic feet per hour methane. Then the temperature was gradually raised, over a period of 6 hours, to 1,800 C., whereby the carbonaceous gas pressure increased to about 1.5 mm. Hg (the inlet flow rate of methane and hydrogen being maintained the same as above) and the deposition was continued at the l,800 C. temperature for an additional 2 hours. The furnace, while under vacuum of about 0.01 mm. Hg, was then cooled to room temperature over a period of 12 hours after which the furnace was brought up to atmospheric pressure by gradual admission of air and the finished composite graphite susceptor bodies removed. The bodies had a smooth continuous surface layer of pyrolytic graphite, such layer having a thickness, above the surface of theporous graphite substrate, of about 4 mils and and a depth of penetration into the surface of the porous body of about 10 mils.
FIG. 4 shows the composite graphite susceptor body 44, made by the process as described'above, incorporated in apparatus for the epitaxial growth of a semiconductor. The chamber 46, generally made of quartz, is connected to a vacuum pump through the opening 48 and to a source of vaporized material for vapor deposition on the semiconductor chips through the opening 50. The composite graphite susceptor body 44 is of flat annular shape with a plurality of circumferentially arranged recesses 52 in its upper surface. The semiconductor chips 54 desired to be treated are positioned in the recesses. The susceptor body is supported on a quartz shelf 56 which rests on the bottom wall of the chamber 46. An induction coil 58 surrounds the chamber for heating of the susceptor body 44, generally to about 1,400 C. The various vaporized materials and the type of layers applied to the chips in the epitaxial growth method arqwell known in the art and form no part of the present invention. The important point is that the composite graphite susceptor body, as described above, provides excellent thermal and shock resistance and hence long life, chemical inertness to assure against contamination of the semiconductors, and excellent efficiency as a susceptor. By reason of the anisotropic properties of the pyrolytic graphite surface layer, there is rapid uniform distribution of the heat generated.
it will be understood that while the invention has been described in detail with reference to a preferred embodiment thereof, various modifications may be made all within the full and intended scope of the claims which follow.
11. A susceptor for use in induction heating apparatus to support and heat material in the manufacture of semiconductor elements in the form of a composite graphite body, said composite graphite body comprising a body of porous graphite with a surface layer of pyrolytic graphite, said pyrolytic graphite layer having a thickness exterior of the surface of said porous body of from about 1 15 mils, and said pyrolytic graphite layer extending into the pores of said porous body to a depth of 1--10 times the thickness of that portion of the layer exterior of said porous body to mechanically interlock said porous layer to said porous body.
2. A composite graphite body as set forth in claim 1 wherein said porous graphite body has an initial density of from about 1.7 to 1.9 and has a pore size distribution which peaks at from about 1 to microns.
3. In combination with an induction heating coil, a susceptor for said induction heating coil comprising a composite graphite body having a substrate of porous electrographite and a surface layer of pyrolytic graphite which extends into the pores of the substrate to thereby mechanically interlock the pyrolytic graphite surface layer with the porous substrate.
4. A method for manufacturing a composite graphite body comprising the steps of depositing pyrolytic graphite on a porous electrographite body by pyrolysis of a carbonaceous gas initially at a temperature of from about 1,100 to 1.600 C. and at a carbonaceous gas pressure of from about 0.5 to 1.2 mm. Hg and then subsequently at a temperature of from about 1,600 to 2,100 C. and at a carbonaceous gas pressure of from about 0.8 to 2.5 mm. Hg.
5. A method as set forth in claim 4 wherein the period for the initial deposition of pyrolytic graphite at from 1,100 to 1,600 C. and at a carbonaceous gas pressure of from about 0.5 to 1.2 mm. Hg constitutes at least 50 percent of the total period of deposition.
6. A method as set forth in claim 5 wherein the period for the initial pyrolytic graphite deposition is from about 14 to 20 hours and wherein the period for the subsequent pyrolytic graphite deposition at a temperature above that used for the initial deposition is about 6 to 10 hours.