SUSCEPTOR

Abstract: The susceptor of the invention includes a carbon substrate and a dense SiC coating formed on the surface of the carbon substrate, the dense SiC coating having a thickness of l0 µm or more and the carbon substrate having a specific electric resistance of from l600 to 5500 µΩcm.

Description: BACKGROUND OF THE INVENTION This invention relates to an improved susceptor for use in epitaxial vapor growth in the production of semiconductor devices, or vapor phase growth for forming various types of insulation coatings, etc. Conventional apparatus for forming epitaxial coatings on semiconductor wafers, include lateral type furnaces, barrel type furnaces and vertical type furnaces. These conventional furnaces employ high frequency heating means or lamps. High frequency heating type epitaxial devices have proven to be particularly useful since such devices can be designed in a simple configuration and the inside of the reaction furnace can easily be kept clean. While the consumption of electric power increases as the size of the semiconductor product becomes larger, an energy savings can be achieved by using solid state oscillators having high conversion efficiency instead of oscillation tubes and by using lower frequencies, which are suitable for larger size furnaces and which give high operation safety. For an induction type heater, a susceptor made of carbon and having a dense SiC coating on its surface is used. However, as the size of the susceptor becomes larger, even if the frequency is lowered to increase the penetrating depth of electric current, the outer surface of the susceptor opposed to the induction coils will be locally heated due to the surface skin effect of eddy current and the interference with the magnetic field. That localized heating results in cracks in the SiC coating due to the difference in thermal expansion and allows impurities contained in carbon to contaminate the semiconductor wafers. Futher, although the generation of cracks can be suppressed by increasing the distance between the induction coils and the susceptor, the energy efficiency is thereby reduced so much that the desired temperature cannot be attained. Further, these problems are acute because the specific electric resistance of the carbon substrate in the conventional susceptor is about l000 µΩcm. SUMMARY OF THE INVENTION In view of the foregoing problems, an object of the invention is to provide a susceptor for high frequency heating with higher heating efficiency and less susceptability to the generation of cracks. Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow. In accordance with the present invention, the specific electric resistance of the carbon substrate used for the susceptor is from l600 to 5500 µΩcm. A dense, nonporous SiC coating having a thickness of at least l0 µm is formed thereon. Since the penetrating depth of the induction current is proportional to the inherent resistance of the carbon substrate used for the susceptor, the electric current flows uniformly within the susceptor, and fewer cracks are generated in the SiC coating formed on the surface of the susceptor. These advantages derive at least in part from the use of a specific electric resistance of l600 - 5500 µΩ cm as compared with about l000 µΩcm in the prior art. Since the specific electric resistance of the susceptor is higher, the electric power consumed in the formation of the susceptor increases. However, the heating efficiency can be improved, by which better workability can be attained, since there is no uneven heat generation in the longitudinal direction of the susceptor, even if the distance between the susceptor and the induction coils is somewhat varied. As previously noted, in the present invention the SiC coating is thicker than l0 µm. It is impossible to prevent impurities contained in the carbon substrate of the susceptor from being heated and dispersed in the semiconductor wafer, if the thickness is less than l0 µm. Further, if the specific resistance of the carbon substrate is less than l600 µΩcm, localized heating will occur due to the surface skin effect of the eddy current so as to cause cracks in the SiC caoting. If it is larger than 5500 µΩcm, the thermal shock resistance of the carbon substance is reduced and cracks are generated in the carbon substrate upon temperature rising in the susceptor. BRIEF DESCRIPTION OF THE DRAWING Fig. l is a perspective view showing a barrel type susceptor produced according to this invention. EXAMPLES Six (6) susceptor substrates, each having a 700 mm outer diameter, a l00 mm inner diameter and a thickness of l5 mm, were prepared by mixing carbon black and coke, with the addition of a binder, and molding, followed by sintering and graphatization. The blending ratio between coke and carbon black was varied to provide the substrates with different specific electric resistances as shown in Table-l. Dense SiC coatings were formed with a thickness of l0 µm or more on the 6 different substrates described above. They were subjected to heating at a frequency of 35 KHz and l50 KW for 30 minutes in a vertical type epitaxial device having spirally wound induction coils of 20 mm × 20 mm in cross sectional size, followed by spontaneous cooling to room temperature. Table-2 shows the highest temperature per unit area at any point on the mounting surface of a 50 mm wafer from the outer circumference to the center of the susceptor, with the distance between the induction coils and the susceptor being 40 mm. The results of repeated heating cycles are reported in the remarks. Since the susceptor for the specimen No. 6 contained a large amount of carbon black blended in and had a high specific electric resistance (6020 µΩcm), it had poor thermal shock resistance and had a short life in cycle use. Further, while the highest temperature could be increased with ease by bringing the susceptors of the specimen Nos. l and 2 closer to the induction coils, temperature scattering at the surface of the susceptors was thereby increased due to the interference of the magnetic field from the coil, the susceptors became more vulnerable to the heat shocks due to the temperature difference and the number of useful cycles was further shortened. Fig. l shows a barrel type susceptor produced according to this invention although the configuration of this susceptor is conventional. According to this invention, since a carbon substrate having the specific electric resistance of from l600 to 5500 µΩcm is used and pore-less SiC coating with a thickness of l0 µm or more is formed on a surface of the carbon substrate, temperature scattering at the surface of the susceptor is eliminated and the susceptor is highly resistant to thermal shocks and free from the effect of thermal shocks due to the temperature difference. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The foregoing embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalents of the claims are therefore intended to be embraced therein.

Claims: 1. A susceptor comprising a carbon substrate and a dense SiC coating formed on said carbon substrate, said dense SiC coating having a thickness of at least about l0 µm and said carbon substrate having a specific electric resistance of from about l600 to 5500 µΩcm. 2. A susceptor as defined in Claim l, wherein the dense SiC coating has no pores. 3. A susceptor as defined in Claim l, wherein the carbon substrate is formed by mixing carbon black and coke with a binder, shaping the mixture, sintering and graphitizing. 4. A susceptor as defined in Claim 3, wherein the carbon black is from about 20 % to 40% and the coke from about 60% to 80%.

Inventors: KATO, SHIGEO OGUNI PLANT TOSHIBA CERAMICS CO. LTD., KON, TOMIO OGUNI PLANT TOSHIBA CERAMICS CO. LTD., NOZAWA, TATSUO OGUNI PLANT TOSHIBA CERAM. CO. LTD., SASAKI, YASUMI OGUNI PLANT TOSHIBA CERAM. CO. LTD.
Applicants: TOSHIBA CERAMICS CO., LTD.
Application year: 1987
Main classification: C30B25
IPC codes: H01L21, C30B25