Semiconductor acceleration sensor

1 . A semiconductor acceleration sensor fabricated using a semiconductor substrate, comprising:

an outer frame formed by said semiconductor substrate;

a plurality of beam portions formed by said semiconductor substrate, said plurality of beam portions being connected to said outer frame;

a first mass portion formed by said semiconductor substrate, said first mass portion being connected to said beam portion; and

a second mass portion connected to the end face opposite to the beam portion of said first mass portion, wherein

said second mass portion is formed of material having a higher specific gravity than said first mass portion.

2 . A semiconductor acceleration sensor according to claim 1 , wherein

said second mass portion is formed of metal.

3 . A semiconductor acceleration sensor according to claim 2 , wherein

said metal is of gold, tungsten or nickel.

4 . A semiconductor acceleration sensor according to claim 2 , wherein

said second mass portion is formed by plating.

5 . A semiconductor acceleration sensor according to claim 3 , wherein

said second mass portion is formed by plating.

6 . A three-dimensional semiconductor acceleration sensor fabricated using a semiconductor substrate and detecting acceleration in three axes (X, Y and Z), comprising:

an outer frame section composed of said semiconductor substrate, said outer frame section having through openings in Z-axis direction in the central portion;

a first mass portion composed of said semiconductor substrate, said first mass portion positioned in the interior section with a certain distance from said outer frame in X- and Y-axis directions, said first mass portion having a thickness in Z axis approximately equivalent to that of said outer frame;

four thin beam portions in Z axis composed of said semiconductor substrate, said four thin beam portions being in a plane perpendicular to the Z-axis direction of said outer frame section as well as in almost the same plane as that perpendicular to the Z-axis direction of said outer frame section, said four thin beam portions supporting said mass portion from the interior of said outer frame section in the X- and Y-axis directions, wherein

a second mass portion is stacked onto said first mass portion in a plane perpendicular to the Z-axis direction not connected to said four beams, said second mass portion having a higher specific gravity than said first mass portion.

7 . A three-dimensional semiconductor acceleration sensor according to claim 6 , wherein

said second mass portion is stacked onto said first mass portion by plating.