Fig. 6-8 Chemical vapor deposition apparatus for growth of silicon carbide single crystal Raw material of silane and propane gases diluted by hydrogen gas is led to the reaction vessel made of quartz. The reaction vessel is kept at low pressure (1.3 x 104 Pa) and the raw material is blown against the silicon substrate mounted on the graphite table, which is heated up to 1,300 degrees cent. by an induction coil around the vessel. By a thermal reaction of the jet of raw material gases on the surface of silicon substrate, the raw material turns to a cubic single crystal of silicon carbide.

Fig. 6-9 Silicon carbide wafer of cubic single crystal produced Single crystal silicon carbide with 14 micrometer thickness is grown on the whole area of a silicon substrate. The surface is mirror-like and concentric interference rings are observed because of the thin film.

Fig. 6-10 Surface flatness of single crystal silicon carbide In the application of single crystal silicon carbide for a semiconductor device, defects, cracks, and irregularity are a hinderance to use. When the flatness of this crystal is measured by atomic force microscope, small irregularity is observed. This irregularity, however, is very small (one or two atoms in size) and has no effect on the fabrication of electric devices like memory.

The studies on silicon carbide (SiC) single-crystal, which has higher radiation and thermal resistances compared with these of silicon, have been carried out as the material has applications in high performance solar cells and semiconductor memory for artificial satellite. As crystalline SiC has a strong covalent bond, and consists of diatomic materials, there are many crystal structures in SiC, i.e., polytype crystalline. It is, therefore, very difficult to make SiC single-crystal of a large area with few defects, especially in cubic SiC (3C-SiC). We have succeeded the formation of thin 3C-SiC single-crystal with large area and high quality, which is enough to use for memory devices. The 3C-SiC single-crystal film is grown on a silicon substrate at 1,300 degrees cent. by chemical vapor deposition with silane gas (SiH4) and propane gas (C3H8) using hydrogen gas (H2) as assist material. After the trial and error study on the parameters of ratio of the contents in raw materials (SiH4 and C3H8) and hydrogen gas, the flow rate of gases on silicon substrate and the temperature of silicon substrate, growth conditions for high quality and large area single crystal SiC thin film (Fig. 6-9) were found. These are less hydrogen gas and higher flow rates compared with conventional methods. The reason why the smooth surface SiC thin film without defects is obtained can be considered as follows; silane gas reaches the silicon substrate before the molecule is decomposed and the decomposition and reaction takes place on the substrate (Fig. 6-10). We are aiming to realize a semiconductor integrated circuit robust enough to use in severe fields like nuclear and space environments.

Reference M. Yoshikawa et al., Growth of 3C-SiC by Low Pressure Chemical Vapor Deposition with Vertical Reactor, Thin Solid Films (1999). to be published.

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