The present invention relates to a semiconductor substrate and a process for producing the same. More particularly, the present invention relates to a semiconductor substrate for use in memory devices and logic devices having an MOS structure, an MOS semiconductor device, and a process for producing the same.
A CZ (Czochralski) silicon substrate taken off from a single crystal silicon grown by the Czochralski method (CZ method) has hitherto been mainly used as a silicon wafer which serves as a substrate for a semiconductor device. Further, an epitaxial wafer prepared by forming a single crystal silicon layer on a CZ silicon substrate by vapor phase epitaxy has also been used as the semiconductor substrate.
When a device having an MOS structure is prepared using the silicon substrate, oxygen contained in the semiconductor substrate is precipitated by heat treatment in the course of the production process, forming precipitation of oxygen, i.e., bulk micro defects (BMD). These bulk micro defects formed in the semiconductor substrate can incorporate therein metal impurities. Therefore, the effect of eliminating electrical failure and the like attributable to metal impurities in the course of device formation, that is, IG (intrinsic gettering) effect (hereinafter often referred to as "gettering effect"), can be expected. Basically, the higher the density of bulk micro defects, the better the IG effect. In order to efficiently attain the IG effect, it is generally considered that the density of bulk micro defects should be not less than 10.sup.8 defects/cm.sup.3.
Formation of bulk micro defects in the semiconductor substrate, however, is disadvantageous in that presence of bulk micro defects in a device forming zone is causative of electrical failure, such as poor gate silicon oxide proof voltage or junction leak failure. For this reason, a DZIG wafer, wherein the device forming zone has been brought to a denuded zone (DZ), is generally used. In this DZIG wafer, bulk micro defects are formed at a density of 10.sup.8 to 10.sup.10 defects/cm.sup.3 in the interior of the semiconductor substrate. This bulk micro defect density of DZIG is convenient for attaining the IG effect.
When a device having an MOS structure is formed using a DZIG substrate having a bulk micro defect density of 10.sup.8 to 10.sup.10 defects/cm.sup.3, however, slipping or dislocation is created in forming zone, resulting in deteriorated mechanical strength of the semiconductor substrate. That is, when the bulk micro defect density of the semiconductor substrate is high, the mechanical strength of the semiconductor substrate is lowered. In this case, for the semiconductor substrate alone, the lowering in mechanical strength can be reduced by controlling the mechanical strength through heat treatment in a temperature region (900.degree. C. or above) where bulk micro defects are grown. On the other hand, when a device is formed on the semiconductor substrate, final heat treatment conditions are automatically determined by the device forming process, making it practically difficult to control the mechanical strength by heat treatment at 900.degree. C. or above.
Thus, the IG effect and the high mechanical strength are contradictory properties, and, at the present time, it is difficult to provide a semiconductor substrate which has a good balance between these properties.