Semiconductor device and manufacturing method thereof

Abstract

There are included: forming element isolation regions in a semiconductor substrate; introducing a first impurity of a first conductivity type, to thereby form a first well and a second well of the first conductivity type; introducing a second impurity of a second conductivity type, to thereby form a third well of the second conductivity type and introducing the second impurity into a region between the first well and the second well, to thereby form a separation well of the second conductivity type; and further introducing a third impurity of the second conductivity type into the region between the first well and the second well.

What is claimed is: 
     
         1 . A semiconductor device comprising:
 element isolation regions that are formed in a semiconductor substrate and demarcate element regions;   a first well of a first conductivity type formed in the semiconductor substrate;   a second well of the first conductivity type formed in the semiconductor substrate;   a third well of a second conductivity type, which is a conductivity type opposite to the first conductivity type, formed in the semiconductor substrate;   a fourth well of the second conductivity type formed in the semiconductor substrate; and   a first separation well of the second conductivity type formed under the element isolation region in a region between the first well and the second well, wherein   the concentration of an impurity of the second conductivity type in the first separation well at a first depth deeper than a bottom surface of the element isolation region and shallower than a bottom surface of the first well is equal to the sum of the concentration of an impurity of the second conductivity type in the third well at the first depth and the concentration of an impurity of the second conductivity type in the fourth well at the first well.   
     
     
         2 . The semiconductor device according to  claim 1 , further comprising:
 a fifth well of the second conductivity type formed in the semiconductor substrate;   a sixth well of the first conductivity type formed in the semiconductor substrate; and   a second separation well of the first conductivity type formed under the element isolation region in a region between the third well and the fifth well, wherein   the concentration of an impurity of the first conductivity type in the second separation well at a second depth deeper than the bottom surface of the element isolation region and shallower than a bottom surface of the third well is equal to the sum of the concentration of an impurity of the first conductivity type in the first well at the second depth and the concentration of an impurity of the first conductivity type in the sixth well at the second depth.   
     
     
         3 . The semiconductor device according to  claim 2 , further comprising:
 a seventh well of the first conductivity type formed in the semiconductor substrate; and   a third separation well of the second conductivity type formed under the element isolation region in a region between the sixth well and the seventh well, wherein   the concentration of an impurity of the second conductivity type in the third separation well at a third depth deeper than the bottom surface of the element isolation region and shallower than a bottom surface of the sixth well is equal to the sum of the concentration of the impurity of the second conductivity type in the third well at the third depth and the concentration of the impurity of the second conductivity type in the fourth well at the third depth.   
     
     
         4 . The semiconductor device according to  claim 2 , further comprising:
 an eighth well of the second conductivity type formed in the semiconductor substrate; and   a fourth separation well of the first conductivity type formed under the element isolation region in a region between the fourth well and the eighth well, wherein   the concentration of an impurity of the first conductivity type in the fourth separation well at a fourth depth deeper than the bottom surface of the element isolation region and shallower than a bottom surface of the fourth well is equal to the sum of the concentration of the impurity of the first conductivity type in the first well at the fourth depth and the concentration of the impurity of the first conductivity type in the sixth well at the fourth depth.   
     
     
         5 . A semiconductor device comprising:
 element isolation regions that are formed in a semiconductor substrate and demarcate element regions;   a first well of a first conductivity type formed in the semiconductor substrate;   a second well of the first conductivity type formed in the semiconductor substrate;   a third well of a second conductivity type, which is a conductivity type opposite to the first conductivity type, formed in the semiconductor substrate;   a fourth well of the second conductivity type formed in the semiconductor substrate; and   a separation well of the second conductivity type formed under the element isolation region in a region between the first well and the second well, wherein   the concentration of an impurity of the second conductivity type in the separation well at a first depth deeper than a bottom surface of the element isolation region and shallower than a bottom surface of the first well is higher than the sum of the concentration of an impurity of the second conductivity type in the third well at the first depth and the concentration of an impurity of the second conductivity type in the fourth well at the first well.