Surface Polishing of Silicon Carbide Electronic Device Substrate Using CEO.sub.2

The present invention relates to an electronic device and, in particular, to a method of surface polishing of a silicon carbide substrate used in an electronic device.

There have been recent attempts to improve the high-frequency and high power performance of silicon-based power devices. The level of performance of such devices, however, is approaching its limit. In addition, power devices are often used in conditions where high temperatures or radiation is present. Silicon devices, however, cannot be used under such conditions. New material must therefore be utilized to provide electronic devices with higher performance.

Since silicon carbide has a wide prohibited band (6H type: 2.93 eV), it can control electric conductivity well and provide high radiation-resistance at high temperatures. Silicon carbide has a dielectric breakdown electric field that is one order of magnitude more than of silicon, which makes it possible to utilize silicon carbide to high voltage-proof devices. In addition, its electron saturation drift velocity, which is twice that of silicon, makes it possible to utilize silicon carbide to high-frequency high power control.

In order to utilize the good material characteristics of silicon carbide in power devices, a substrate of silicon carbide is epitaxially formed and a metal or an oxide film is formed on the substrate to make an electronic device after the surface of a silicon carbide substrate has been finished sufficiently to make it a mirror surface. However, since the silicon carbide substrate has a hardness of nine, it is chemically stable, and is not affected by acid or alkali, diamond grindstones must be used to polish the silicon carbide substrate. Such polishing causes fine flaws or cracks to be created or causes the crystal structure on the surface of the silicon carbide substrate to be disturbed. Such defects must be removed by means of etching. Although etching eliminates the disturbance of the crystal structure, however, it does not remove flaws or cracks. In fact, the number of such defects are increased by etching. If an oxide or a metal film is formed on the silicon carbide substrate, electric fields concentrate on the defective parts (such as flaws or cracks) created by polishing, and cause the characteristics of the electronic device to deteriorate.

Given the above realities, it is an object of the invention to provide a good silicon carbide electronic device by providing a silicon carbide substrate with a smooth surface and a crystal structure free of disturbances.

The above object is achieved by providing an electronic device that uses a silicon carbide substrate which has a silicon carbide substrate and a film formed thereon, wherein the silicon carbide substrate has its surface dry-polished by a polishing means made up of at least one of the following: chromium oxide Cr.sub.2 O.sub.3, cerium oxide CeO.sub.2, and iron oxide Fe.sub.2 O.sub.3. The film is formed on the dry-polished surface of the silicon carbide substrate.

Polishing with chromium oxide Cr.sub.2 O.sub.3, cerium oxide CeO.sub.2 or iron oxide Fe.sub.2 O.sub.3 provides a good polished surface due to the mechanochemical effects of these substances. Because of these mechanochemical effects, frictional heat generated during polishing and the catalysis of these oxides cause silicon carbide to oxidize because of the oxygen in the air, thereafter being mechanically released. As a result, the surface of the silicon carbide substrate is polished, being free of mechanical defects or crystal distortion.

The invention will now be described in greater detail with reference to the accompanying drawings, wherein:

FIG. 1 is a sectional view of a first electronic device made in accordance with the invention;

FIG. 2 is a sectional view of the important part of another electronic device made in accordance with the invention;

FIG. 3 is a photograph showing the grain structure of the polished surface of a silicon carbide substrate in accordance with the invention;

FIG. 4 is a photograph showing the grain structure of the polished surface of the silicon carbide substrate of a conventional electronic device;

FIG. 5 is a histogram illustrating the distribution of the breakdown voltage of one hundred electronic devices formed on a SiC substrate prepared in accordance with the invention; and

FIG. 6 is a histogram illustrating the distribution of the breakdown voltage of one hundred electronic devices formed on a SiC substrate prepared by a conventional method.

A first electronic device formed in accordance with the invention is described below with reference to the drawings. FIG. 1 is a sectional view of an electronic device according to this invention. The electronic device is a Schottky diode. The Schottky diode is manufactured by polishing a silicon carbide substrate 3, forming a silicon carbide epitaxial film 2 on the substrate 3, forming a nickel electrode 4 on the other main surface of the silicon carbide substrate 3, and forming a gold electrode 1, e.g. a Schottky electrode, on the silicon carbide epitaxial film 2.

The polishing of the silicon carbide substrate 3 will now be explained in detail. A polishing plate with chromium oxide Cr.sub.2 O.sub.3 abrasive grains measuring 1 .mu.m distributed in phenol resin is rotated at 100 rpm to polish a specified surface of the substrate 3 (for example a n-type 6H-silicon carbide SiC substrate) while the substrate 3 is inclined at an angle of 5.degree. in a specified direction. Table 1 shows the specified surface and direction:

For the sake of comparison, after polishing with silicon carbide SiC abrasive grains measuring 150 .mu.m, silicon carbide substrates were polished with diamond abrasive grains measuring 30 .mu.m, 3 .mu.m, and 1 .mu.m. Finally, these substrates were buffed with diamond grains measuring 1 .mu.m. Moreover, the substrates were simultaneously etched in a mixed gas made up of hydrogen gas with a flow of 2 slm and hydrogen chloride HCl gas with a flow of 3 sccm at 1200.degree. C. for 5 minutes. After that, they were observed using a Nomarski microscope. No flaws were found on the surfaces polished with chromium oxide Cr.sub.2 O.sub.3 (see FIG. 3), whereas the surfaces polished with diamond abrasive grains of various sizes eventually took on a mirror-surface finish, although groove-like polishing flaws created by the etching process were found (see FIG. 4). In other words, fine flaws that had not been detected by the Nomarski microscope became larger after etching with hydrogen chloride HCl. However, no groove-like etched parts and few flaws were found on the surfaces polished with chromium oxide Cr.sub.2 O.sub.3 and etched with HCl. Many hexagonal etching pits in the figure were created because of transition and were not caused by polishing.

An n-type epitaxial film with a carrier density of 2.times.10.sup.15 cm.sup.-3 and a thickness of 9 .mu.m was formed on a silicon carbide Sic substrate (n-type: carrier density: 1.times.10.sup.18 cm.sup.-3) polished by using the above two methods. As described above, a mixed gas including hydrogen H.sub.2 gas with a flow of 2 slm and hydrogen chloride HCl gas with a flow of 3 sccm at 1200.degree. C. was introduced to etch the surface. Then, a mixed gas made up of hydrogen H.sub.2 gas with a flow of 3 slm, silane SiH.sub.4 gas with a flow of 3 sccm, and propane gas with a flow of 2.5 sccm at 1500.degree. C. for 4 hours was introduced to form the n-type 6H-silicon carbide Sic epitaxial film 2. The epitaxially formed film had surface conditions similar to the surface conditions present after etching, shown in the previous figure, and the epitaxial film formed on the substrate polished with chromium oxide Cr.sub.2 O.sub.3 had fewer flaws and a smoother surface than the film formed epitaxially on the substrate polished with diamond grains.

Ni was vacuum-evaporated on the back surface of the n-type 6H-silicon carbide Sic substrate 3, which was then heated at 1200.degree. C. in an Ar atmosphere for 10 minutes to obtain an ohmic nickel electrode 4. Au was then vacuum-evaporated on the n-type 6H-silicon carbide Sic epitaxial film 2 to obtain a gold electrode 1, that is, a Schottky electrode.

FIG. 5 is a diagram illustrating the distribution of the breakdown voltages for one hundred electronic devices formed on a substrate prepared in accordance with the invention, while FIG. 6 is a diagram illustrating the distribution of the breakdown voltages of one hundred electronic devices formed on a substrate prepared by conventional methods. As can be seen from the figures, the electronic device according to the invention has a breakdown voltage higher than that of a conventional electronic device. This characteristic is also observed when either cerium oxide CeO.sub.2 or iron oxide Fe.sub.2 O.sub.3 is used instead of chromium oxide Cr.sub.2 O.sub.3.

FIG. 2 is a sectional view of the important part of an electronic device according to a different embodiment of the invention. The illustrated device is a MOS diode. An n-type silicon carbide SiC epitaxial film 6 is laminated on a n-type silicon carbide substrate 7, and a silica SiO.sub.2 insulation layer 5 is formed on the epitaxial film 6.

The invention provides an electronic device that uses a silicon carbide substrate which has a silicon carbide substrate and a film formed thereon. The silicon carbide substrate has its surface dry-polished using a polishing means including at least one of the following: chromium oxide Cr.sub.2 O.sub.3, cerium oxide CeO.sub.2, and iron oxide Fe.sub.2 O.sub.3. The film is then formed on the silicon carbide substrate. As a result, the mechanochemical polishing effect of chromium oxide Cr.sub.2 03, cerium oxide CeO.sub.2, or iron oxide Fe.sub.2 O.sub.3 provides a good polished surface free of mechanical defects or crystal distortion, and an epitaxial film can be laminated on the polished surface to manufacture a good silicon carbide electronic device.