The present invention relates to an SOS (silicon on sapphire) substrate used for the manufacture of a semiconductor device.
An SOS substrate for a semiconductor device has an SOS structure wherein a thin layer of silicon is formed on a single crystal sapphire plate by hetero-epitaxial growth. The SOS structure allows the complete isolation of elements in the semiconductor device. Furthermore, parasitic capacitance can be greatly decreased. Thus, the SOS structure is receiving great attention in LSI and ultra-LSI techniques.
Generally, the substrate used in the manufacture of the semiconductor device has an outer appearance of disc shape, and has one cut surface (to be referred to as an orientation flat herein after). A case will be described wherein a single crystal silicon substrate (to be referred to as a bulk silicon substrate hereinafter) is used. FIG. 1 is a perspective view showing an example of the bulk silicon substrate. A substantially disc-shaped bulk silicon substrate 1 has a top surface indicated by an index of plane (100) and its orientation flat 2 is formed in the direction of [011]. Generally, the orientation flat 2 is formed perpendicularly to the surface of the bulk silicon substrate 1.
FIG. 1 shows only one example of the bulk silicon substrate. In any bulk silicon substrate, however, its top surface has a predetermined index of plane, and its orientation flat is formed along a predetermined direction due to the following reason. The characteristics of the semiconductor element formed on the surface layer of the substrate substantially depend on the crystal face of the surface and a crystallographic axis along which the semiconductor element is formed. Therefore, in order to form a semiconductor element which has predetermined electrical characteristics, a substrate having a surface of predetermined crystal face must be used. Furthermore, an orientation flat extending along a predetermined crystal orientation is required as a reference to determine the direction of the crystallographic axis on the surface of the substrate.
This is also the case for the SOS substrate. FIG. 2 shows an orientation flat 6 of an SOS substrate 3. Referring to FIG. 2, an epitaxial silicon layer 5 is grown on a single crystal sapphire plate 4. It is known by studies made so far that a crystal face (100) of silicon epitaxially grows on the crystal face (1012) of sapphire to obtain excellent epitaxial growth. Therefore, generally, a conventional SOS substrate 3 is used wherein a silicon layer 5 having a surface with an index of plane (100) is grown on the surface of the sapphire plate 4 which has an index of plane (1012).
In the conventional SOS substrate 3, an orientation flat 6 is formed at an incline of 45.degree. from a projection line [1011] obtained by projecting the C-axis [0001] of the sapphire plate 4 onto the surface of the sapphire plate 4 which has an index of plane (1012). In this case, the direction of the orientation flat 6 corresponds to the direction [011] of the epitaxial silicon layer 5.
On the other hand, when the epitaxial silicon layer 5 is grown on the surface of the sapphire plate 4 which has an index of plane (1012), it is assumed that the cleavage planes of the sapphire plate 4 are nearly parallel or perpendicularly to the orientation flat 6 of the epitaxial silicon layer 5 ("Solid State Technology", April, 1977, PP 81 to 86, J. E. A. Maurits). This assumption supports the formation of the orientation flat of the SOS substrate 3 in the manner as described above.
However, the present inventors have analyzed the conventional SOS substrate 3 in detail. As shown in FIG. 3 which illustrates the rear view of the SOS substrate 3, the ridgeline of the orientation flat 6 is slightly misaligned with the line of intersection between the cleavage plane 7 {0112} of the sapphire plate 4 and the crystal face of the sapphire plate 4 having an index of plane (1012), that is, with the direction indicated by &lt;2201&gt;.
In the process for manufacturing a semiconductor device having an SOS structure, the SOS substrate 3 is diced into chips in the following manner when the wafer process is completed. As shown in FIG. 3, the surface of the sapphire plate 4 of the SOS substrate 3 is scribed by a diamond stylus in directions parallel and perpendicular to the orientation flat 6, and is thus diced into chips. As described above, in the conventional SOS substrate 3, since the direction of cleavage plane 7 on the surface of the sapphire plate 4 does not match the direction of the orientation flat 6, the scribing direction is misaligned from the direction of the cleavage plane 7. Therefore, as compared with dicing along the direction of the cleavage plane, cracking tends to occur. Furthermore, if the scribing depth is not deep enough, dicing cannot be readily performed. In practice, as compared with dicing of the bulk silicon substrate, the number of scribing operations is increased several times. Thus, the wear of the diamond stylus is great.